polysilanes for semiconductor fabrication

23
Polysilanes for semiconductor fabrication Shuzi Hayase * Corporate Research and Development Center, Toshiba Corporation, Komukai-toshiba-cho, Saiwai-ku, Kawasaki 212-8585, Japan Received 5 November 2001; revised 26 May 2002; accepted 10 June 2002 Abstract Polysilanes have various interesting properties and many applications have been proposed. However, low durability at elevated temperature and that toward light exposure on polysilanes have limited their application fields. The most promising application is lithography for LIS fabrication. The unusual photo-reactivity and high etching durability render polysilanes the candidate of most promising material for the LSI lithography in future, where new materials have been required in order to fabricate micro-patterns less than 0.2 mm. In this report, application of polysilanes to semiconductor fabrication, particularly, bilayer resists and anti- reflection layers with high etching properties, is reviewed. This review includes synthesis of new polysilanes, photo-reactivity, amplification of the photo-reactivity, bilayer resists developable with aqueous solutions, pattern fabrication with all dry process, UV absorptions, anti-reflection properties, durability toward plasma etchings and the pattern transfer. q 2002 Published by Elsevier Science Ltd. Keywords: Polysilane; Lithography; Bilayer; Resist; Anti-reflection; Photo-reactivity; Amplification; Electron transfer; Benzophenone; Pattern transfer Contents 1. Introduction ................................................................... 360 2. Problems that current LSI lithography technologies are facing .............................. 360 3. Bilayer resist systems ............................................................ 360 3.1. Polysilane resists developed with dry processes ..................................... 361 3.2. Wet development with base aqueous solutions...................................... 362 4. Anti-reflection layers and problems to be improved ...................................... 365 4.1. Syntheses ................................................................ 367 4.2. UV absorption ............................................................. 367 4.3. Relationship between n, k values and polysilane structures ............................. 369 4.4. Polysilane etching behaviors ................................................... 370 4.4.1. Relationship between etching selectivity toward resists and polysilane structures....... 370 4.4.2. Oxygen atoms in polysilane backbones ..................................... 371 4.4.3. Effects of pressures in an etching chamber .................................. 371 4.4.4. Reaction mechanism................................................... 372 4.5. Etching durability of polysilanes during SiO 2 etching ................................ 373 5. Pattern fabrication .............................................................. 374 5.1. Resist patterns on anti-reflection layers ........................................... 374 5.2. Patterning of polysilane layers by dry etching ...................................... 375 6. Conclusion .................................................................... 378 Acknowledgements ................................................................ 378 References ....................................................................... 378 0079-6700/03/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S0079-6700(02)00034-5 Prog. Polym. Sci. 28 (2003) 359–381 www.elsevier.com/locate/ppolysci * Present address: Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 1-1 Hibikino Wakamatsu-ku, Kitakyushu 808-0196, Japan. Tel.: þ81-93-695-3479; fax: þ 81-93-695-3470. E-mail address: [email protected] (S. Hayase).

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Page 1: Polysilanes for semiconductor fabrication

Polysilanes for semiconductor fabrication

Shuzi Hayase*

Corporate Research and Development Center, Toshiba Corporation, Komukai-toshiba-cho, Saiwai-ku, Kawasaki 212-8585, Japan

Received 5 November 2001; revised 26 May 2002; accepted 10 June 2002

Abstract

Polysilanes have various interesting properties and many applications have been proposed. However, low durability at elevated

temperature and that toward light exposure on polysilanes have limited their application fields. The most promising application is

lithography for LIS fabrication. The unusual photo-reactivity and high etching durability render polysilanes the candidate of most

promising material for the LSI lithography in future, where new materials have been required in order to fabricate micro-patterns

less than 0.2 mm. In this report, application of polysilanes to semiconductor fabrication, particularly, bilayer resists and anti-

reflection layers with high etching properties, is reviewed. This review includes synthesis of new polysilanes, photo-reactivity,

amplification of the photo-reactivity, bilayer resists developable with aqueous solutions, pattern fabrication with all dry process,

UV absorptions, anti-reflection properties, durability toward plasma etchings and the pattern transfer.

q 2002 Published by Elsevier Science Ltd.

Keywords: Polysilane; Lithography; Bilayer; Resist; Anti-reflection; Photo-reactivity; Amplification; Electron transfer; Benzophenone; Pattern

transfer

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360

2. Problems that current LSI lithography technologies are facing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360

3. Bilayer resist systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360

3.1. Polysilane resists developed with dry processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361

3.2. Wet development with base aqueous solutions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362

4. Anti-reflection layers and problems to be improved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365

4.1. Syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

4.2. UV absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

4.3. Relationship between n, k values and polysilane structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

4.4. Polysilane etching behaviors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370

4.4.1. Relationship between etching selectivity toward resists and polysilane structures. . . . . . . 370

4.4.2. Oxygen atoms in polysilane backbones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

4.4.3. Effects of pressures in an etching chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

4.4.4. Reaction mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372

4.5. Etching durability of polysilanes during SiO2 etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373

5. Pattern fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374

5.1. Resist patterns on anti-reflection layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374

5.2. Patterning of polysilane layers by dry etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

0079-6700/03/$ - see front matter q 2002 Published by Elsevier Science Ltd.

PII: S0 07 9 -6 70 0 (0 2) 00 0 34 -5

Prog. Polym. Sci. 28 (2003) 359–381

www.elsevier.com/locate/ppolysci

* Present address: Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 1-1 Hibikino Wakamatsu-ku,

Kitakyushu 808-0196, Japan. Tel.: þ81-93-695-3479; fax: þ81-93-695-3470.

E-mail address: [email protected] (S. Hayase).

Page 2: Polysilanes for semiconductor fabrication

1. Introduction

Polysilanes are high molecular weight polymers contain-

ing consecutive silicon main chains synthesized first in

1980s [1]. The consecutive silicon chains are expected to be

completely different from the corresponding consecutive

carbon chains. As results of intensive researches, various

properties such as liquid crystal properties, thermochro-

mism, piezochromism, photo-reactivity, non-linear optical

properties, emission properties and semiconductive proper-

ties have become clear, some of which are very important

for future electronics. For example, non-linear optical

properties, emission properties and semiconductive proper-

ties are suggested to be useful for swift optical switches and

light emission displays [2–4].

Low durability at elevated temperatures and toward light

exposures are serious problems for polysilanes to be

successfully applied to these electronic devices. Promising

applications would be to use polysilanes as materials for

fabrications of patterns, because polysilanes are easily

changed to materials having completely opposite properties

when these are exposed to lights [5] and do not remain in

these devices. In LSI (large scale integrated circuit)

fabrication processes, patterns are fabricated by differences

in etching rates of various materials. For example, SiO2 and

Si layers are dry-etched faster than organic polymers by

using fluorine and chlorine gases. Therefore, SiO2 and Si

patterns can be fabricated by using these organic polymers

as masks. SiO2 layers are dry-etched faster than Si layers by

using fluorine gases, which makes it possible to fabricate

SiO2 layers by using Si layers as masks. Similarly, dry

etching rates for polysilane layers are different from the

corresponding polysiloxane layers which are formed by

irradiating polysilanes. The dry etching rates can be

controlled by the exposure doses. This makes polysilanes

attractive as lithography materials. At the end of the

lithography process, these polysilanes are removed and do

not remain in these devices. Durability for long time is not

required for the purpose. Another advantage for polysilanes

to be useful for LSI manufacturing processes is that silicon

atoms contained in polysilanes are familiar with LSI

manufacturing processes and are not recognized to be

impurities. Other metal atoms are apt to cause serious

contamination problems.

In this paper, first of all, problems that LSI lithography

technologies are facing are discussed. Then, potential

applications to bilayer resist and anti-reflection layers are

reviewed. Bilayer photoresists developed with organic

solvents and laser ablation have been extensively investi-

gated since 1980s and there are many reviews [5–14].

Therefore, they are omitted. Bilayer resists, which can be

developed with dry etchings, are added in this review

because these results are closely related to the fabrication of

anti-reflection layers.

2. Problems that current LSI lithography technologies

are facing

KrF and ArF excimer laser lithography has been

investigated for geometry down 0.2 mm, but the lithography

process windows become narrower as the design rule

reaches below the wavelength. Precise critical dimension

(CD) controls become very important in order to achieve

high performance of devices and good yields. Reflection

between resist/air interface or resist/substrate interface

causes two serious problems. One is line-width variation,

and the other is standing wave as shown in Fig. 1.

The interference between the upper and lower resist

interfaces strongly affects the energy absorbed in the resist

film, and the line-width of the resist patterns varies with the

changes in the thickness of substrates [15,16]. Standing

waves appear strong in high resolution resists, because the

high resolution resists have high dissolution contrast and

small acid diffusion length. These lithographic problems can

be solved by using bilayer resist systems or anti-reflective

layers (ARCs).

3. Bilayer resist systems

Bilayer resist processes using polysilanes provide a

solution to the topography and the reflectivity. The top layer

acts as the photo-imaging layer and a hard mask for the

etching of the underlying substrate. The bottom layer acts as

an ARC as shown in Fig. 2 [5–14,17–38]. The following

properties are required. (1) High sensitivity, (2) high

resolution, (3) development without using organic solvents,

(4) high Tg, (5) high oxygen-reactive ion etching (O2-RIE)

durability toward underlying organic layers, and (6) able to

be removed easily from the substrate. The high sensitivity

relates to the through-put, and the exposure dose under

100 mJ/cm2 is needed. Developments using much solvent

should be avoided from the environmental point of view.

The high Tg is needed in order to avoid pattern deformation

Fig. 1. Problems caused by multi-reflections. ARC: anti-reflection

layers; standing waves: see Fig. 24.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381360

Page 3: Polysilanes for semiconductor fabrication

during O2-RIE. Commonly, the thickness of the imaging

layer and the bottom organic layer are around 0.1–0.2 and

1.0 mm, respectively. Therefore, while the bottom layer is

etched, the etching rate for the bottom layer should be more

than 20 times higher than that for the imaging layer. The

etching rate has been reported to increase with an increase in

the amount of silicon atoms contained in the imaging layer

[39].

Their unique photo-reactivity and high silicon content,

facilitating excellent pattern transfer, make polysilanes

ideally suited for this process. Polysilane layer patterns are

fabricated by its exposure to a deep UV light, followed by

wet development. Then, the organic polymer layer under the

polysilane layer is etched with oxygen-reactive ions. In this

process, SiO2 layers formed at the surface of the polysilane

during the O2-RIE, act as the hard mask.

Recent reports have focused on the development process

which does not use organic solvents. Conventional alkyl or

aryl substituted polysilane resists had to be developed with

organic solvents, such as alcohol/methylethylketone mix-

tures [41–43]. The development is not compatible with

current production lines, where dilute basic aqueous

solutions have been employed. Fig. 3 summarizes poly-

silane resists in terms of development methods.

Two methods have been presented to solve the problem.

One is a dry development and the other is a development

with aqueous solutions.

3.1. Polysilane resists developed with dry processes

The dry development is carried out using Cl2 or HBr gas.

The unexposed polysilane area is etched faster than the

photo-oxidized polysilane area, providing the negative

image [18,20,21,24,26,27,44].

Surface-imaged polysilane layers for 193 nm excimer

laser lithography has been reported [21]. Penetration depth

of lights is apt to become crucial problems for lithography

using shorter wavelength, because conventional organic

resists have inevitable UV absorptions in the wavelength

region below 200 nm. The lithography using surface-

imaging methods is an ideal process. The oxidized polymer

exposed to 193 nm light inhibits bromine-initiated etching,

however, the parent polysilane film was etched to yield a

negative-tone image. Linear and branched polysilanes are

evaluated for the surface-imaging lithography. Patterns of

0.2 mm were fabricated with 50 mJ/cm2 exposure. The

addition of triazine as the sensitizer increased the sensitivity

up to 20 mJ/cm2. The etching rate of the unexposed area was

three times higher than that of the exposed area when

cyclohexylsilyne–butylsilyne copolymer (2:8) was used.

The etching selectivity increased up to 30 when bromine and

hydrogen gases were added to HBr gas. A 100 nm thick

polysilyne film was patterned and the pattern was trans-

ferred to the underlying organic polymer layer by O2-RIEs.

The extent of photo-oxidation is linear with dose, and the

latent image depth is controlled by altering the degree of the

photo-bleaching. During the etching by use of HBr-based

plasma, non-linearity was observed with respect to the

oxidation degree, yielding the development contrast of more

than five. Polybutylsilyne had highest sensitivity toward

HBr etching. Large substituents such as phenyl and

cyclohexyl groups decreased the sensitivity [21].

Polymethylsilyne is expected to have highest sensitivity

toward the HBr dry etching. However, the polymethylsilyne

has problems with solubility and oxidation. Therefore, it

was difficult to deal with and make a high quality film.

There are a few reports on the fabrication of polysilynes

by chemical vapor deposition (CVD). For example, the film

has been fabricated from hexamethylsilazane, hexamethyl-

disilane, tetramethylsilane [20] and methyltrihydrosilane

[24,44]. The polymethylsilyne film (thickness: 120–

170 nm) was plasma-polymerized by using methyltrihydro-

silane gas on a 700 nm layer of a hard-baked photoresist

[44]. After the film was irradiated with 248 nm light, the

film was exposed to chlorine plasma to remove unexposed

regions selectively. The etching selectivity as low as three

between exposed area (100 mJ/cm2) and unexposed area

was obtained. The etching selectivity has been reported to

be sufficient to achieve good lithographic process latitude.

Due to their high silicon content and efficient photo-

oxidation chemistry, the polymethylsilyne film was

readily patterned to give thin-oxide-like-hard masks.

Patterns of 200 nm have been successfully fabricated

without phase shift masks. The results show the high

potential of the polymethylsilyne film for 248 nm

lithography.

A new negative-tone dry development process has been

proposed [45]. First of all, a polysilane layer is exposed to

soft X-rays in order to oxidize the polysilane layer.

Unoxidized areas are removed by KrF laser ablation.

Unexposed areas of the polysilane film had a high ablation

rate while exposed areas cannot be ablated. A bilayer resist

using a polysilane film as a surface-imaging layer has been

patterned using this dry development process. A 0.12 mm

thickness of poly(methylhexyl)silane was coated on a

Fig. 2. Process flow for bilayer resist process.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381 361

Page 4: Polysilanes for semiconductor fabrication

hard-baked novolac resin layer with 1 mm thickness. The bi-

layer resist was exposed to X-rays with a proximity gap of

0.15 mm. The X-ray exposure dose was 59 J/cm2. After the

exposure, the resist was irradiated in air by KrF laser with a

pulse intensity of 39 mJ/cm2. Total irradiated dose was

120 mJ/cm2. Patterns of 0.4 mm have been reported to be

replicated (Fig. 4).

3.2. Wet development with base aqueous solutions

Polysilane films developable with basic aqueous sol-

utions have been trying to synthesize, because developments

with organic solvents are apt to be avoided in production

lines. Polysilane structures developable with aqueous

solutions are summarized in Fig. 5.

Fig. 3. Polysilane resists and development methods.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381362

Page 5: Polysilanes for semiconductor fabrication

Polysilanes could be developed with basic aqueous

solutions when hydrophilic substituents such as hydroxy and

carboxylic acid groups are introduced into polysilanes

[33–35]. Polysilanes with phenol groups synthesized for the

first time by us were evaluated as resist materials for KrF

lithography. The polysilane with a phenol group had little

solubility in an aqueous solution of tetramethylammonium-

hydroxide (TMAH), a common developer in recent LSI

lithography. Carboxylic acid groups were successfully

introduced into the polysilanes bearing phenol groups. The

polysilane was proved to be developable with the TMAH

aqueous solution [33,34].

A new copolysilane with carboxylic acid was syn-

thesized by hydrosilylation reaction of poly(phenylhydro)-

silane with vinyl acetic acid [35,36]. This polysilane could

be developed with an aqueous solution of sodium bicar-

bonate. It has been reported that the introduction of t-butyl

groups on phenyl groups improve the sensitivity. The

sensitivity was 540 mJ/cm2 to fabricate patterns with

1.5 mm thickness. The increase in the sensitivity is due to

the t-butyl group blocking the aryl ring from the attack by

silyl radicals making it more difficult to form cross-linking

[35,36].

These resists have hydrophilic substituents in the side

chains to promote the dissolution to basic aqueous solutions.

Poly(phenylhydrosilane) not bearing the corresponding

hydrophilic substituents was found to be developed with

the TMAH aqueous solution [37,38]. This has an advantage

that the introduction of these hydrophilic substituents are

not needed. The poly(phenylhydro)silane has been reported

to be synthesized directly from phenylhydrodichlorosilane

by using zirconocene complexes [46].

The reaction mechanism was explained as follows:

Poly(phenylhydrosilane) photodecomposes to form silyl

compounds having Si–OH groups, which are acidic and

become soluble in an aqueous base solutions as shown in

Fig. 6. The parent poly(phenylhydrosilane) is stable in the

aqueous solution. Therefore, the polysilane film provides

positive-tone patterns. The results proved that poly(phenyl-

hydrosilane) has potentials to give the positive tone patterns

with aqueous base solutions. However, the sensitivity and

the pattern shape had to be improved. The dose to make

patterns reached about 1 J/cm2.

It was found that the addition of 3,30,4,40-tetra (t-

butylperoxycarbonyl)benzophenone (EA1) increases

specifically the resist sensitivity to 100–200 mJ/cm2. The

increase in the resist sensitivity was discussed in terms of

photolysis rates and dissolution rates into aqueous base

solutions. The increase in the photolysis occurs through

photo-induced electron transfers from polysilanes to

electron acceptors.

The electron transfer was analyzed by employing Stern-

Volmer kinetics, which is often followed by purely

diffusional quenching of fluorescent materials by electron

acceptors [47]

F0=F ¼ 1 þ Ksv½Q� Ksv ¼ Kqðp0Þ

Kq is the bimolecular quenching constant; F0, the fluorescenceFig. 5. Polysilane structures developable with base aqueous

solutions. PR1: Refs. [33,34]; PR2: Ref. [35]; PR3: Refs. [37,38].

Fig. 4. Process flow for polysilane pattern fabrication by flat KrF

exposure. Reprinted with permission from Jpn J Appl Phys 1995;34:

6800. q1995 Japan Society of Applied Physics [45].

Fig. 6. Dissolution mechanism of poly(phenylhydrosilane) film into

base aqueous solution Poly(phenylhydrosilane) itself is hydro-

phobic and is insoluble in base aqueous solution. But after exposure

to KrF laser, compounds bearing SiOH groups form. These low

molecular weight oligomers or monomers bearing SiOH are soluble

in base aqueous solution. because SiOH moieties are acidic.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381 363

Page 6: Polysilanes for semiconductor fabrication

in the absence of electron acceptors; F, the fluorescence in

the presence of electron acceptors; Ksv, the Stern-Volmer

constant; p0 is the fluorescence life time of poly(methyl-

phenylsilane), 86ps [48,49].

The fluorescence intensity of poly(methylphenylsilane)

should decrease when electrons transferred from photo-

excited poly(methylphenylsilane) to electron acceptors.

The decrease in the polysilane fluorescence implies the

ease of electron transfer reactions.

Fig. 7 shows the relationship between F0=F and electron

acceptor (quencher) concentration. Poly(methylphenylsi-

lane) was used instead of poly(phenylhydrosilane), because

the fluorescence of poly(methylphenylsilane) was easy to

measure. EA1, EA2, EA4 and EA7 were used as electron

acceptors, where EA4 and EA7 have been reported to be

efficient photosensitizers for polysilane photo-decompo-

sitions [48,49]. The Stern-Volmer plot shows slightly

positive deviation from linearity at higher acceptor

concentration. Therefore, we fitted it at low acceptor

concentration by using the method of least squares in

order to obtain Ksv and Kq. (Kq)s for various electron

acceptors are summarized in Table 1. Fig. 7 clearly shows

that the poly(methylphenylsilane) is quenched more

efficiently by the addition of EA1 and EA2, compared

to other electron acceptors. EA1 and EA2 show

anomalous behavior for fluorescence quenching of

poly(methylphenylsilane). (Kq)s of the polysilane in

the presence of EA1 and EA2 were one order higher

than expected [48,49]. We assume that EA1 and EA2

may locate close to polysilanes with configurations

suitable for electron transfers by the weak interaction,

Fig. 7. Relationship between F0=F and quencher concentration.

Reprinted with permission from Macromolecules 1998;31:8794.

q1998 American Chemical Society [38].

Table 1

Kq values for polysilane fluorescence quenching in the presence of

various electron acceptors

Electron acceptor Kq ( £ 10210) (M21 s21)

EA2 212

EA4 17

EA1 221

EA5 12

EA7 20

EA6 6

EA3 12

Fig. 8. Relationship between decrease in UV absorption and

irradiation time for poly(methylphenylsilane) film in the presence of

electron acceptors. UV exposure, 365 nm; observation, 340 nm.

Reprinted with permission from Macromolecules 1998;31:8794.

q1998 American Chemical Society [38].

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381364

Page 7: Polysilanes for semiconductor fabrication

such as hydrophobic interactions between a benzophe-

none structure and polysilane structures or hydrophilic

interactions between a benzophenon carbonyl and Si–

OHs of the polysilane chain ends.

The fluorescence quenching rate of poly(phenylhydrosi-

lane) in the presence of EA1 was almost the same as that in

the presence of benzophenone (EA2). EA1 did increase both

photolysis rates and resist sensitivities, but EA2 did not.

Fig. 8 shows the decrease in the UV absorption for a

poly(methylphenylsilane) film when 340 nm light are

irradiated. The decrease in the UV absorption corresponds

to polysilane photo-decomposition. The difference between

EA1 and EA2 was discussed in terms of back-electron

transfer reactions as shown in Fig. 9. When polysilanes are

photo-excited, electrons shift from polysilanes to acceptors

to form polysilane radical cations and acceptor radical

anions. The radical anion should exist as a counteranion of

the polysilane radical cation (R2 in Fig. 9). Radical anions

of EA1 have peroxide moieties and should decompose to

carboxylate anions and tert-butyloxy radicals (R5 in Fig. 9).

Therefore, the electron does not get back to the polysilane

radical cation (R6 in Fig. 9), leading to Si–Si bond

cleavages, where nucleophilic attacks may be needed from

the viewpoint of the energy diagram [38]. In case of EA2,

because the EA2 radical anion should be very stable, the

polysilane radical anion would have a chance to obtain an

electron from the EA2 radical anion to form the parent

polysilane by back-electron transfers. Miller and co-workers

have reported that the photolysis of poly(methylphenylsi-

lane) was showed to be stabilized in the presence of

perchloropentacyclododecane in spite of the fact that

effective fluorescence quenching of poly(methylphenylsi-

lane) was observed in the presence of it. They implied that

this may be due to rapid back-electron transfer [48,49].

This may provide a useful method to stabilize polysilane

films against UV irradiations. We found that poly(methyl-

phenylsilane) was strongly quenched by C60. The Kq was

300 times larger than that of EA1. It has been reported that

photo-induced electron transfers actually occur from photo-

excited polysilanes to C60 [50–52]. When poly(methyl-

phenylsilane) was exposed to a 254 light in the presence of

C60 (0.5 wt% to polysilanes), we found that the photolysis

rate actually decreased, compared with that in the absence of

C60. This would also be explained by back-electron

transfers from very stable C60 radical anions to polysilane

radical cations. The C60 radical anion has been reported to

survive for more than 1.5 s [53].

In case of EA1, the cleavage of peroxide bonds on the

benzophenone moiety would effectively inhibit the back-

electron transfer to the polysilane radical cation. In addition,

EA1 photodecomposes to form compounds bearing car-

boxylic acids which also facilitate the dissolution of the

resist layer into aqueous base solutions. Fig. 10 shows the

contrast curves for polysilane resist (poly(phenylhydrosi-

lane)) in the presence of EA1 and EA2. The development

was carried out by using a 2.38% tetramethylammonium

hydroxide aqueous solution. The figure shows clearly that

the addition of EA1 increased the sensitivity.

4. Anti-reflection layers and problems to be improved

It has been reported that bottom ARCs can eliminate

lithographic problems discussed before. ARCs are deposited

by spin-coating [54], by CVD [55] and sputtering. CVD

ARCs have advantages of highly conformal coatings and

minimize the amount of over-etch. Recently, plug vias and

damascene metalization process are used and pattern

formations on chemical mechanical polished (CMP)

dielectric films are required. As semiconductor topographies

become more planar by using the CMP process, the high

conformality obtained from CVD ARCs becomes less of an

advantage. Spin-coating type ARCs are superior to CVD

ARCs in terms of their simplicity.

Wide lithographic margin could be obtained by thin

Fig. 10. Contrast curves of polysilane resist (poly(phenylhydrosi-

lane)) in the presence of EA1 and EA2. Exposure, 254 nm;

development, 2.38% tetramethylammonium hydroxide aqueous

solution for 1 min. Reprinted with permission from Macromol-

ecules 1998;31:8794. q1998 American Chemical Society [38].Fig. 9. Electron transfer from polysilanes to electon acceptor EA1.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381 365

Page 8: Polysilanes for semiconductor fabrication

resist systems, because they prevent these resist pattern from

collapsing [55]. Thick ARCs with moderate absorbance

have a good anti-reflection ability. However, conventional

organic RACs cannot be used as a thick film, because resist

thickness loss occurs during the RAC etching. Spin-coatable

ARCs with high etching selectivity toward resists are

strongly desired.

We would like to propose a potential use of polysilanes

for ULSI fabrication, where the polysilane acts as an ARC

as well as a hard mask for fabricating underlying substrates.

The multi-layer is composed of a resist layer, a polysilane

layer and a substrate. These structures and the process flow

are summarized in Fig. 11.

An organic resist is spin-coated on a polysilane layer and

is exposed to UV lights through photo-masks. The

polysilane layer absorbs the UV light passing through the

resist layer and prevents reflections from the underlying

substrate. After the resist pattern is fabricated, the multi-

layer is exposed to Cl2 plasma. Because the polysilane layer,

consisting of consecutive Si–Si bonds, is etched faster than

the resist layer, the resist profile is transferred exactly to the

polysilane layer. This process has two advantages. Firstly,

high resolution organic resists can be used freely, without

the synthetic limitations experienced when using silicon-

containing resists. Secondly, the thickness of the resist for

this process can be thinner than that for a single layer resist

process, which improves the process latitude during

exposure. Anti-reflective properties are attainable by

employing conventional organic polymers not bearing

silicon atoms. However, it is difficult to etch the organic

anti-reflective layer (ORGARC) selectively using the resist

mask, because both the anti-reflective polymer and the resist

polymer have aliphatic hydrocarbon structures.

In order to complete the ARC process successfully, a

number of features are important, for example, the solubility

properties of the polysilane as it is necessary, to prevent the

polysilane layer mixing with the resist layer. We require the

ability to make a planar film, high glass transition

temperatures (Tg), anti-reflective properties, a high etching

selectivity toward resists, and a high etching durability

toward underlying substrates. These requirements are also

summarized in Fig. 11.

In this paper, we focus on clarifying the following

relationships.

1. The relationship between polysilane structures and UV

absorptions and n, k values at 248 nm.

2. The relationship between polysilane structures and

etching selectivity toward resist polymers.

Fig. 11. Process flow for polysilane anti-reflection layer and

requirements for each step.

Fig. 12. Abbreviations for linear polysilanes, cross-linked poly-

silanes and branched polysilanes.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381366

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3. The relationship between polysilane structures and

etching selectivity toward SiO2 layers.

Finally, SiO2 patterns are fabricated by using polysilane

anti-reflection layers to discuss the potential.

In order to fulfill these requirements, a few new

polysilanes were synthesized. Before discussing properties

of anti-reflection layers, we want to describe the synthesis.

4.1. Syntheses

Polysilanes have been reported to be synthesized by

Wurt coupling reaction [68], reducing metals less active

than sodium [56–58], electrochemical reactions [59,60],

ring-opening reactions [61–65], masked disilane methods

[66,67], and so on. Among three kinds of polysilanes,

namely, linear polysilanes, polysilanes cross-linked with

ethylene units, and branched polysilanes, as shown in Fig.

12, linear polysilanes were prepared by conventional Wurtz

coupling reaction [68]. Polysilanes cross-linked with

ethylene units were also synthesized in the same ways.

Instead of dichlorosilane monomers, 1,2-bis (dichloro-

methyl)ethane were employed for the syntheses of PX1,

PX2, PX3 and PX4. Poly(phenylhydrosilane) PB1 was

synthesized according to the method reported by Li et al.

[69].

Some of the branched polysilanes have Q units (Si units

bonding to four silicon atoms). Polysilanes containing Q

units have been synthesized by reacting mixtures of organic

halides, trichlorosilane and tetrachlorosilane with Na, or by

reacting tetrachlorosilane with Mg under ultrasonic

irradiation, followed by adding organic halides [57,58,

70–72]. Polysilanes containing Q units are apt to make gels

during the syntheses. We synthesized these polymers by a

modified method as follows.

In a four necked round bottomed flask equipped with a

dropping funnel and a condenser, 100 ml of dimethox-

yethane (DME), 13.3 g of 15-crown-5-ether, 0.1 g of CuCl,

48.63 g of Mg and a small amount of iodine were placed.

Then 84.9 g of tetrachlorosilane in 30 ml DME was added

dropwise. After exothermic reaction started, 370 ml of DME

was added in the flask at once. The reaction mixture was

stirred for 20 h, followed by stirring the mixture for 2 h for

75 8C. Butylbromide of about 137 g in 20 ml DME was

added dropwise to the mixture for 60 min and the mixture

was stirred for 1 h at 60 8C. Then toluene was added to the

reaction mixture in order to precipitate salts. The mixture

was filtered under vacuum and the filtrate was washed with

water. The organic layer was separated and dried over

anhydrous MgSO4. The supernatant solution was separated

and solvents were removed under vacuum. The residue was

resolved in 50 ml of toluene and the mixture was poured into

500 ml of ethanol to obtain polymer PB3. Yield, 33%.

Polysilanes cross-linked with ethylene units were also

synthesized for the first time. PX2 was synthesized as

follows. In a four necked round bottomed flask equipped

with a dropping funnel and a condenser, 130 ml of xylene,

20 ml of anisol, 0.35 g of CuCl and 20.6 g of Na were

placed. The sodium was dispersed by stirring the mixture

vigorously at 120 8C. Then, diphenyldichlorosilane

(63.38 g, 0.25 mol) and 1,2-bis(dichloromethylsilyl)ethane

(15.37 g, 0.06 mol) were added for 20 min and stirred for

2 h at 120 8C. After the reaction mixture was cooled down to

room temperature, 500 ml of toluene was added. After the

precipitate was removed under an Ar atmosphere, the

solvent in the filtrate was removed under a reduced pressure.

The residue was again dissolved in 100 ml of toluene. Then

the polysilane was precipitated from 500 ml of ethyllactate.

The polymer was purified three times in the same manner,

and dried at 90 8C for 1 day. PX2: Yield, 33%.

4.2. UV absorption

The reasons why these three structures are selected are as

follows. Linear polysilanes have unique UV absorptions in

the region from 300 to 400 nm assigned to s–sp transitions.

These UV absorptions vary, depending on substituents on

silicone atoms, the main chain conformation, the branching

of the main chain, and molecular weights [73–86].

The introduction of phenyl substituents on silicon atoms

has been reported to shift the UV absorption peak up to

around 400 nm, and to increase the UV absorbance at

around 250–300 nm assigned to the p–pp transition of the

phenyl group [37–41,46]. Linear polysilanes were syn-

thesized in order to know the effectiveness of these aromatic

groups on the UV absorbance at 248 nm. The distortion of

the silicon main chains disturbs the overlap of the silicon s

orbital, and shifts the UV absorption peak to shorter

wavelengths [44,45]. Distorted polysilanes were given by

the cross-linking of polysilane chains by ethylene units. The

branching of the silicon main chain and the decrease in the

number of the consecutive silicon atoms also shift the UV

absorption peak to shorter wavelengths [39–41,47–52]. Me

(SiMe2)5Me has a UV absorption peak at 250 nm. However,

it was difficult to make the high quality flat film with the

polydimethylsilane oligomers. Therefore, the introduction

of aromatic substituents, branched structures and distorted

structures was taken into account to increase the UV

absorbance at 248 nm.

Fig. 13. Representative UV absorptions for linear polysilanes, cross-

linked polysilanes and branched polysilanes.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381 367

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Anti-reflective properties of polysilanes were compared

each other, by measuring the UV absorbance of their films

with 0.1 mm thickness. The UV absorbance was monitored

at 248 nm, the wavelength employed for KrF excimer laser

lithography. Details for the anti-reflective property of

polysilanes are described later.

UV absorption peaks assigned to the silicon s–sp

transition were clearly observed for linear polysilanes

from PS1 to PS6 at around 300–400 nm, but those for

other polysilanes were observed as shoulders or were

overlapped with other UV absorptions at around 250–

300 nm. This leaded to the increase in the UV

absorbance at 248 nm. Representative UV absorption

spectra for the linear polysilane (PS1), the cross-linked

with ethylene units (PX2) and the polysilanes with

branched structures are shown in Fig. 13. These spectra

imply that the introduction of the branching and the

chain distortion is effective for the increase in the UV

absorption at 248 nm.

In order to examine the effect of the aromatic group, the

UV absorbance for polysilane films with 0.1 mm thickness

at 248 nm was plotted against the percentage of the aromatic

group (Fig. 14). In case of methylphenylsilane–diphenylsi-

lane copolymers (PS2–PS6, Area A in Fig. 14), the UV

absorbance at 248 nm increased with increasing the

percentage of the phenyl group.

The UV absorbance of polysilanes cross-linked at the

ethylene group (PX1, PX2 and PX3) was found to be higher

than that of linear polysilanes (Area B in Fig. 14),

suggesting that partial cross-linking increases UV absor-

bance at 248 nm. The UV absorbance increased in the order

of PX1, PX3 and PX2. PX3 had almost the same UV

absorbance with PX2, even though the percentage of the

phenyl group of PX3 (35%) was much less than that of PX2

(65%). The introduction of the silicon branched structure in

PX3 may well explain the increase in the UV absorbance.

Fig. 14 shows that branched polysilanes are in the region

(Area C) above polysilanes cross-linked with ethylene units

(Area B). For example, the UV absorbance of PB2 was higher

than that of the linear polysilane, PS3, even though the

percentage of the phenyl group of PS3 is the same as that of

PB2. In each area, there was the tendency that the UV

absorbance increases with increasing the amount of the

aromatic group with a few exceptions. PB1 was in the region

under Area C. This is explained as follows: Hydrogen atoms

of PB1 have been reported to be partially removed to make

lower branched silicon backbones during the synthesis [56,

69]. The degree of the branching for PB1 should be lower

Fig. 14. Relationship between UV absorbance for polysilane films and percentage of aromatic groups. 0.1 mm thickness at 248 nm.

Fig. 15. Relationship between UV absorbance for polysilane films and Si content. 0.1 mm thickness at 248 nm.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381368

Page 11: Polysilanes for semiconductor fabrication

than that for PB2. Therefore, PB1 has linear polysilane

structures and lower branched structures. This explain the

results that PB1 is in the region between A and C areas. PB4

has large UV absorbance even though it has no phenyl

groups. PB4 has 42% of Q structures, the structure that a

silicon atom is bonded to four Si atoms. This may increase the

UV absorbance, instead of phenyl groups. Fig. 14 shows that

UV absorbance increased with increasing the amount of

aromatic groups for all three kinds of polysilanes, and the UV

absorbance increased in the order of linear polysilanes,

polysilanes cross-linked with ethylene units and branched

polysilanes.

Since the etching selectivity of polysilanes toward resists

is expected to increase with the increase in the Si content, it

is important to know how the UV absorbance changes,

depending on the Si content. Fig. 15 shows the relationship

between them.

The UV absorbance decreased with increasing the Si

content for all three kinds of polysilanes. The tendency is

shown as circles D, E and F in Fig. 15. When compared at

the same Si content, the UV absorbance increased in the

order of linear polysilanes, polysilanes cross-linked with

ethylene units and branched polysilanes. PX3 has a higher

UV absorbance than other polysilanes cross-linked with

ethylene units. This may be explained by the specific

structure containing both cross-linking and branched

structures. These relations imply that branched polysilanes

should be the best structure in order to obtain both high Si

content and UV absorbance.

In conclusion, the introduction of aromatic groups, the

chain distortion and the branching of the silicon main chain

turned out to be effective for the increase in the UV

absorption of the polysilane film at 248 nm.

4.3. Relationship between n, k values and polysilane structures

Refractive index is defined as follows:

N ¼ n 2 ik

N is the complex index of refraction; n, the refractive index;

k is the extinction coefficient.

The k means the decrease in the wave amplitude when

the wave moves in a media by one wavelength. Both n and k

correlate with a (absorbance) as follows

a ¼ 4pnk=l

The intensity reflectance, R, is defined by the following

equation [87]

R ¼ ððnr 2 npÞ2 þ ðkr 2 kpÞ

2Þ=ððnr þ npÞ2 þ ðkr þ kpÞ

nr is the refractive index of resist; np, the extinction

coefficient of resist; kr, the refractive index of polysilane; kp

is the extinction coefficient of polysilane.

This equation shows that R decreases when nr is close to

np and kr is close to kp. R also decreases when nr, np, kr and kp

have large values.

Fig. 16 shows the relationship between the weight

percentage of phenyl groups and n and k values for linear

polymers, PS2–PS6. Simple copolymers were employed in

order to eliminate other factors associated with n and k

values. The n and k values at 248 nm increased with

increasing the weight percentage of the phenyl group.

Fig. 17 shows the relationship between Si content and n

and k values for polysilanes having Q units and cross-linked

structures. The values of n and k increased with increasing

Si content. The values of n for PB4 was lower than expected.

This may be explained by the fact that the n value for

Fig. 16. Relationship between weight percentage of phenyl groups

and n, k values for model polymers, PS2–PS6.

Fig. 17. Relationship between Si content and n, k values for cross-linked and branched polysilanes.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381 369

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amorphous silicon is 1.74. PB4 has Q unit structures similar

to the amorphous silicon. The n and k values for

conventional resists are 1.78 and 0.02. The reflectance

should vary, depending on the n and k values for both resists

and polysilanes. The reflectivity should decrease when the n

value is close to 1.78 and the k value increases. The actual

reflectivity will be discussed later.

Table 2 summarizes n and k values for each layer of

multi-layers. One of the advantages of polysilane layers is

that the n value is close to that of resist. This is one of the

requirements in order to decrease the reflection as

mentioned earlier.

4.4. Polysilane etching behaviors

4.4.1. Relationship between etching selectivity toward

resists and polysilane structures

During etching of polysilane layers by chlorine plasma,

the polysilane layer must be etched anisotropically. The

anisotropic etching conditions of polysilane films with a

large aspect ratio have not been previously reported in

detail. Fluorine-containing gases, Cl2 and HBr gases have

been used for inorganic silicon etching [88]. The silicon

layer is etched by chemical reactions and physical

sputtering. The latter may contain chemical reactions

assisted with ion bombardments [88]. The chemical reaction

tends to etch the silicon layer isotropically while the

physical sputtering tends to etch it anisotropically [89].

It has been reported that fluorine-containing gases etch

silicon layers isotropically, because atomic fluorine in the

plasma reacts with the silicon layer spontaneously. On the

other hand, the process using Cl2 and HBr gases provides

anisotropic etching because spontaneous reactions of these

gases with the silicon layer are very few and the etching occurs

in the presence of ion bombardments. The difference between

fluorine and chlorine gases has been explained by the boiling

point of silicon halides and the ease with which the atomic

halides can penetrate the silicon surface [89]. The boiling point

of SiF4 is 178 K and is particularly volatile, while SiCl4 and

SiBr4 have boiling points of 330 and 427 K, respectively.

Therefore, SiCl4 and SiBr4 are less desorbed from the silicon

surfaces: the desorption is accelerated by bombardment with

ions which attack the silicon surfaces anisotropically with the

aid of a self-bias potential. The isotropic etching observed for

fluorine gas has also been explained by the small size of the

fluorine atom, 0.7 A, making it possible to penetrate 10–20 A

in depth from the silicon surface. The covalent radius of Br is

1.1 A and is comparable in size to a silicon atom (1.2 A).

Therefore, in this study, Cl2 gas was employed for the

anisotropic etching of polysilane layers.

Fig. 18 shows the relationship between the silicon

content in the polymer and the etching selectivity of the

polysilane toward the resist (R1) containing polyvinylphe-

nol polymers. The etching selectivity increased with

increasing silicon content for linear polysilanes having

methyl and phenyl groups, such as PS1, PS2, PS3, PS4, PS5

and PS6 (Area G in Fig. 18). This result is expected from the

results of inorganic silicon etchings.

The etching selectivity of polysilanes partially cross-

linked at the ethylene unit, such as PX1, PX2 and PX3, was

not as large as that for linear polysilanes with comparative

silicon content (Area H in Fig. 18). Branched polysilanes

were also in Area H. Etching selectivity for both cross-

linked polysilanes and branched polysilanes tended to

increase with increasing Si content. The etching selectivity

reached 5.2 for PB3, highly branched polysilanes bearing

methyl groups.

A significantly large deviation was observed for PX1.

They contain a propyl group. The etching selectivity for

PX4, having a cyclohexyl and a naphthyl group, was also

much less than that for PX1, not shown in Fig. 18 because

the etching condition was slightly different from the others

Table 2

n and k values for each layer of multi-layers

Resist Organic ARC Polysilane SiO2 Si

n 1.8 1.5 1.8–2 1.52 1.5

k 0.01–0.03 0.42 0.2–0.4 0.02 3.42

Fig. 18. Relationship between silicon content and etching selectivity of polysilane toward the resist (R1). Etching selectivity: etching rate of

polysilane films/etching rate of resist. Etching gas: C12.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381370

Page 13: Polysilanes for semiconductor fabrication

shown in Fig. 18. This also supports the fact that alkyl

substituents longer than a methyl group decrease the etching

selectivity.

Highly branched silicon backbones or cross-linking

through ethylene groups increased (Tg)s of polysilanes.

This increase in (Tg)s is needed in order to prevent

polysilane patterns from being deformed thermally during

reactive ion etchings. For example, the Tg of 128 8C for PS3

increased to 155 8C for PX2 by the introduction of partial

cross-linking by ethylene groups. Although the introduction

of the cross-linking decreased the etching selectivity, the

decrease was much less than that brought about by the

introduction of large alkyl groups. Large or bulky alkyl

groups should therefore be avoided. As far as etching

selectivity is concerned methyl groups would be the best

choice, however, phenyl groups are needed to make the

polymers soluble in organic solvents and to fabricate

uniform films.

Fig. 19 shows the relationship between etching selectiv-

ity toward the resist and UV absorbance at 248 nm. If

polysilanes have linear structures and are substituted with

methyl and phenyl groups, the etching selectivity increases

and the UV absorbance decreases with an increase in the

silicon content. Therefore, it is difficult to find a linear

polysilane structure with both a high etching selectivity and

high UV absorbance. UV absorption groups, except for

phenyl groups, have to be incorporated into the polysilane

structure in order to satisfy the high etching selectivity and

high UV absorption criteria. Highly branched polysilanes,

such as PB2, PB3 and PB4, are such candidates.

4.4.2. Oxygen atoms in polysilane backbones

The presence of oxygen atoms in the polysilane structure

was found to be crucial. Etching conditions of polysilane

layers referred to silicon wafer etching conditions. There-

fore, the etching gasses are selected so as to enlarge the

etching selectivity of consecutive silicon atoms against

organic resists. The introduction of other atoms into these

consecutive silicon chains have to be avoided. The most

probable case is the insertion of oxygen atoms into these

chains. In the case of PB1 containing hydrogen groups,

oxidation occurs while the film is baked unless this process

is carried out under argon. Oxidation may occur during

synthesis for all polysilanes. The etching selectivity for PB1

decreased to 80% when 60% of Si–Si bonds were oxidized

to Si–O–Si bonds, and decreased to 40% when all of the

Si–Si bonds were oxidized. PB1 has Si–H groups and is

easily oxidized. The oxidation of polysilanes should be

avoided during their synthesis and the film preparation.

4.4.3. Effects of pressures in an etching chamber

The dependence of etching pressure on etching rates

provides the information on etching mechanisms. As ion

energies are lowered by surrounding molecules, it is

expected that at lower pressures, etching will be dominated

by physical processes, while the etching at higher pressures

emphasizes chemical processes [76]. The etching rate for

the resist film R1 and the PX2 film decreased with

increasing pressure. However, the etching selectivity of

the PX2 film against the resist film R1 increased with an

increase in the pressure as shown in Fig. 20. This implies

that the etching behavior of the PX2 film is similar to that of

the organic resist R1 but the etching of the PX2 film is a

little more pronounced as a result of the chemical reaction,

probably because Si–Si bonds are more chemically reactive

than C–C bonds.

Fig. 19. Relationship between etching selectivity toward resist and UV absorbance at 248 nm. See Fig. 18 for etching conditions.

Fig. 20. Dependence of etching pressure on etching selectivity of

PX2 film against resist film R1. See Fig. 18 for etching conditions.

Etching selectivity: etching rate for PX2/etching rate for R1.

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4.4.4. Reaction mechanism

Etching rates of silicon substrates by Cl2 gas have been

discussed in terms of their reactivity with chlorine radicals,

the volatility of reaction products, their reactivity towards

radicals assisted by ion bombardments, and physical

sputtering [89]. The physical sputtering is affected mainly

by the film density. The film density of polysilane films was

almost the same as that of the organic resist film R1, namely,

1.2–1.4 g/cm3. Therefore, we assumed that the etching of

polysilane films by physical sputtering should be almost the

same as that for the resist R1.

Polysilanes have Si–Si, Si–C and C–C bonds, among

which, Si–C and Si–Si bonds are characteristic to

polysilanes. In order to elucidate the etching mechanism,

the polysilane surface was analyzed by X-ray photoelectron

spectroscopy (XPS). The PS6 film has a peak at 101 eV

assigned to the Si 2p for polysilane main chain silicon

atoms. When the PS6 film was exposed to Cl2 gas, the peak

did not change. The peak shifted to 103 eV and became

broader after the S6 film was exposed to Cl2 plasma. The

shift in the binding energy due to one Si–Cl has been

estimated to be about 1.1 eV [89,93]. Therefore, the peak

shift of about 2 eV and peaks overlapped with broad peaks

in the lower energy region could be assigned to silicon

atoms bearing a few Cl groups [89,93]. We are not able to

exclude the possibility that the peak may contain com-

ponents attributed to Si–O linkages which should be formed

as a result of the reaction of Si–Cl with H2O. A similar peak

shift has been observed previously in the case of fluorine and

HBr-etched silicon surfaces when silicon–fluorine species

and silicon–bromine species were formed [90–92]. Actu-

ally, SiCl4 gas was detected by mass spectroscopy in the

exhaust gases from the etching chamber. These results

support the reaction mechanism that during polysilane

etchings, cleavages occur at Si–Si linkages predominantly

to produce Si–Cl groups.

Reaction products have to be removed from film surfaces

swiftly in order to promote dry etchings. Therefore, the

volatility of reaction products has been reported to be one of

the factors determining the dry etching rates [89]. When PS3

is exposed to chlorine plasmas, a number of small molecules

containing one silicon atom, e.g. tetrachlorosilane (Si1),

methyltrichlorosilane (Si2), phenyltrichlorosilane (Si3), and

methylphenyldichlorosilane (Si4) can result. Their boiling

points are 57, 66, 80 8C/10 mm Hg, and 55 8C/1 mm Hg,

respectively. Since etchings were carried out at 80 8C under

a reduced pressure, Si1 and Si2 would be removed

effectively. Si3 and Si4 may need further bond cleavages

for volatilization. The boiling point of propyltrichlorosilane

(Si5) which is characteristic of PX1 decomposition, is

123 8C. This is higher than that of Si2 containing a methyl

group. This may partially explain the phenomena that the

etching rate for polysilanes bearing a propyl or cyclohexyl

group is extremely low. In addition, this phenomena may be

explained by the fact that long alkyl groups bonded to

polysilane backbones are easily cross-linked by ion attacks,

resulting in non-volatile. For poly(di-n-hexylsilane), cross-

linking had been reported to occur by ion beam irradiation,

containing H þ , He þ , and He2 þ ions [94].

Let me explain the reason why polysilane films are

etched faster than organic resist layers by chlorine plasmas.

The anisotropic etching behaviors suggest that sputtering

mechanisms are predominant. When Si radicals formed by

Fig. 21. Supposed etching mechanism.

Fig. 22. Relationship between etching selectivity of polysilane films toward SiO2 layers and Si content. Etching selectivity: etching rate for SiO2

layer/etching rate for polysilane layer; etching gas: C4H8/CO/O2/Ar.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381372

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sputtering of charged atoms, the Si radicals react with other

Si radicals to form polysilane structures again, or react with

Cl radicals to form Si–Cl as shown in Fig. 21. Because Si

radicals are more stable than C radicals, Si radicals may

have larger chances to react with Cl radicals than C radicals.

In case of C radicals, recombination reactions between two

C radicals to form C–C linkages are more likely than those

between two Si radicals. The fact that the boiling point of

SiCl4 (57 8C) was lower than that of CCl4 (77 8C) may have

something to do with the difference in selective etching

rates.

4.5. Etching durability of polysilanes during SiO2 etching

After polysilane patterns are fabricated, the underlying

SiO2 layer has to be etched by using the patterned polysilane

layer as a hard mask. Mixtures of C4F8, CO, O2 and Ar gases

were used for the etching. The gas mixture was employed

commonly in order to etch SiO2 layers. The C4F8 gas etches

the SiO2 layer. The CO and O2 gases eliminate residues

deposited on the etched surface of the SiO2 layer during the

etching, which retards the swift etching. The C4F8 gas

deposits on polymer layers or Si layers selectively to make

fluoride carbon polymer layers which protect these layers

from the etching. The deposition occurs selectively on

layers whose oxygen content is lower. This is because

fluorocarbon polymers deposit selectively on polymer films

and Si layers [95]. Since polysilane films do not contain

oxygen atoms ideally, high etching durability was expected.

Fig. 22 shows the relationship between Si content and

etching selectivity, where the etching selectivity means the

value of etching rates of SiO2 layers/the etching rate of

polysilanes layers. The etching selectivity defined here is

opposite to that defined in case of polysilane etchings, where

the etching selectivity is defined as etching rates of

polysilane layers/the etching rate of resist layers. The

etching selectivity of conventional polymer anti-reflection

layers is about 5 and that of resists is around 6. The etching

selectivity of polysilane films is higher that that of organic

resist films. Polysilane films are easily etched with chlorine

gas plasmas, however, they are hardly etched with fluorine

gas plasmas. This may be surprising, considering that both

are halogen gases. This difference can be explained by the

selective deposition during the etching mentioned earlier. In

case of polysilane etchings, chlorine gases are used and

deposition does not occur. But, in case of SiO2 etching,

decomposed fluoride gases deposit on polysilane layers to

protect them. As shown in Fig. 21, the etching selectivity

depends on the polysilane structures. The etching selectivity

did not change among PB4, PB5 and PB6 when silicon

content varied. PH2 had higher etching selectivity than the

others. We were not able to elucidate the relationship

between Si content and the etching selectivity from their

limited number of the results. Probably, the etching

mechanism associated with the deposition may make the

explanation difficult. Further works on the relationship

between oxygen content in polysilane films and the etching

selectivity would be needed. Fig. 23 shows the relationship

between the etching selectivity during polysilane etchings

and that during SiO2 etchings. The upper right area in Fig.

23 is the best position. PX2 and PB4 are candidates to fulfill

the requirement.

Fig. 23. Relationship between polysilane etching selectivity toward resist film during polysilane etching and SiO2 etching selectivity toward

polysilane films during SiO2 etching. Etching selectivity during polysilane etching: etching rate for PX2/etching rate for R1; gas: Cl2; etching

selectivity during SiO2 etching: etching rate for SiO2 layer/etching rate for polysilane layer; etching gas: C4H8/CO/O2/Ar.

Table 3

Etching selectivity for anti-reflection layer

Constitution Etching gas Etching selectivity

Conventional ORGARC/resist Fluorocarbon/O2 1.2

This time Polysilane/resist Cl2 2.4

ARC, anti-reflection layer; etching selectivity, etching rate for ARC/etching rate for resist layer.

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381 373

Page 16: Polysilanes for semiconductor fabrication

Tables 3 and 4 summarize the etching selectivity during

anti-reflection layers and that during SiO2 layers. During the

etching of ARCs, polysilane layers are etched more than two

times faster than that of conventional ORGARCs. On the

other hand, polysilane films have durability toward SiO2

layer etchings. Etching rates of polysilane films are 1.2–1.5

times slower than that of conventional organic layers. These

data show the advantages of polysilanes against conven-

tional ORGARCs.

5. Pattern fabrication

5.1. Resist patterns on anti-reflection layers

The effectiveness of the polysilane ARC was verified by

using actual lithography processes [96–98].

PX2 in anisole was spin-coated on the substrate and

baked on a hot plate at 190 8C for 60 s. The resist (R1) for

KrF excimer laser lithography consisting polyvinylphenol

as the main polymer was spin-coated on the polysilane layer

and dried at 100 8C for 90 s. The film was insoluble in

common resist solvents such as ethyl lactate, butyrolactone

and so on. Therefore, any mixing layers between the resist

and the polysilane layer were not observed. The resist film

was exposed by a KrF scanning stepper (numerical aperture

(NA) ¼ 0.6, s ¼ 0:75), followed by developed with a

2.38% tetramethylammoniumhydroxide aqueous solution

(TMAH) using a 60 s puddle process. Fig. 24 shows the

resist patterns on the polysilane PX2 layer. The multi-layer

consists of a resist layer (470 nm)/a polysilane layer

(n ¼ 2.0, k ¼ 0.23) (240 nm)/a SiO2 layer (240 nm)/a Si

substrate. For comparison, the resist pattern on a commer-

cially available organic anti-reflective material (n ¼ 1.55,

k ¼ 0.42) (80 nm) (ORGARC) on the SiO2 layer (150 nm)

is shown in Fig. 25. Good profiles are obtained without any

footing or residue on the polysilane ARCs. Standing waves

are observed on the pattern wall on the ORGARC layer,

however, the resist wall on the PX2 layer was flat.

The anti-reflective performance of the PX2 layer was

simulated and compared with that of the ORGARC layer.

The reflectivity was calculated by an in-house software,

considering the multi-player interference effect. Optical

parameters of stacked films for the calculation are listed

below.

Thickness: resist (470 nm)/ARC/SiO2 (150–300 nm)/Si;

Anti-reflective layer: PX2, 240 nm; ORGARC, 80 nm;

Refractive index at 248 nm. Resist: n ¼ 1:78; k ¼ 0:02;

SiO2: n ¼ 1:52; k ¼ 0:02;

Table 4

Etching selectivity for SiO2 etching

Constitution Etching gas Etching selectivity

Conventional SiO2/ORGARC C4F8/CO/O2/Ar ,5

Conventional SiO2/resist C4F8/CO/O2/Ar ,6

This time SiO2/Polysilane C4F8/CO/O2/Ar ,8

Etching selectivity: etching rate for SiO2 layer/etching rate for ARC layer.

Fig. 24. 0.18 mm L=S resist patterns on polysilane layer. Consti-

tution: resist (470 nm)/PX2 (240 nm) (n ¼ 2.00, k ¼ 0.23)/

SiO2(150 nm). Reprinted with permission from J Vac Sci Technol

1999;17(6):3398. q1999 American Vacuum Society [96].

Fig. 25. 0.18 mm L=S resist patterns on conventional ORGARC.

Constitution: resist (470 nm)/ORGARC(80 nm) (n ¼ 1.55,

k ¼ 0.42)/SiO2(150 nm). Reprinted with permission from J Vac Sci

Technol 1999;17(6):3398. q1999 American Vacuum Society [96].

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381374

Page 17: Polysilanes for semiconductor fabrication

Si: n ¼ 1:50; k ¼ 3:42; PX2: n ¼ 2:00; k ¼ 0:23;

ORGARC: n ¼ 1:55; k ¼ 0:42:

Fig. 26 shows the reflectivity at the resist/ARC interface

when the SiO2 thickness was varied from 150 to 300 nm.

When the thickness of the ARC is thin, the reflectivity

varied, depending on the thickness of anti-reflection and

SiO2 layers. For thicker films, the intrinsic refractive index

governs the total reflectivity because the ARC absorbs all

lights. The intrinsic reflectivity of the ORGARC layer was

1.7% and that of the PX2 film was 0.7%. The reflectivity less

than 1% is required. In the case of the PX2 film, much

thicker film can be employed than that of the ORGARC film

because of the larger etching selectivity. The etching

selectivity of the ORGARC layer was only 0.70. If thicker

ORGARC layers were used as the ARC, organic resist

layers should be lost before the ORGARC layer was etched.

An advantage of polysilane ARCs is that thick films can be

employed because of the larger etching selectivity. The

reflectivity variation for the thick polysilane film is small

even though the thickness of anti-reflection layers and the

underlying SiO2 layers varied during actual manufacturing

processes.

Fig. 27 shows the relationship between the CD variation

and the thickness of SiO2 layers. The CD variation means

the change of pattern widths. The CD variation of the resist

pattern on the polysilane film was 4% which was smaller

than that on the ORGARC film (8%) even though the SiO2

thickness changed.

Polysilane anti-reflection layers exhibit good litho-

graphic performances because the high etching selectivity

toward resist makes it possible to use thick anti-reflection

layers which decrease the deflection itself and increase

latitudes for variations of each layer thickness.

5.2. Patterning of polysilane layers by dry etching

In order to fabricate deep trenches, a fairly thick resist is

needed because of aggressive etching conditions. The thick

resist reduces lithographic process windows. Polysilane ARC

is one candidate to solve this problem. The PX2 was coated on

the substrate and the following constitution was fabricated

R1 resistð430 nmÞ=PX2 filmð240 nmÞ=SiO2 layerð700 nmÞ=

£ SiN layerð220 nmÞ=Si substrate

The deep trench has a twin hole pattern separated by

100 nm lines. The hole width is 300 nm. The SiO2 and SiN

layers act as a hard mask when the Si substrate is etched.

Fig. 28 shows the deep-trench pattern (0.2 mm design rule)

of the resist R1 before etching. Any standing waves on the

resist R1 wall were not observed. The PX2 film was dry-etched

through the resist pattern mask by using Cl2 gas. During

the etching of PX2 film (t: 240 nm), the resist thickness

decreased by 96 nm. The etching selectivity of the PX2 film

Fig. 26. Relationship between reflectivity at resist/ARC interface and thickness of anti-reflection layers. (A) ORGARC, constitution: resist

(470 nm)/ORGARC (80 nm) (n ¼ 1.55, k ¼ 0.42)/SiO2 thickness was varied from 150 to 300 nm. (B) Polysilane anti-reflection layer.

Constitution: resist (470 nm)/PX2 (240 nm) (n ¼ 2.00, k ¼ 0.23)/SiO2. Reprinted with permission from J Vac Sci Technol 1999;17(6):3398.

q1999 American Vacuum Society [96].

Fig. 27. Relationship between CD and SiO2 thickness; Polysilane

anti-reflection layer (PX2). Constitution: resist (470 nm)/PX2

(240 nm) (n ¼ 2.00, k ¼ 0.23)/SiO2 (l50 nm); ORGARC. Consti-

tution: resist (470 nm)/ORGARC (80 nm) (n ¼ 1.55, k ¼ 0.42)/SiO2

(150 nm). Reprinted with permission from J Vac Sci Technol

1999;17(6):3398. q1999 American Vacuum Society [96].

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381 375

Page 18: Polysilanes for semiconductor fabrication

toward the resist R1 film was 3.75 in the deep trench, which

was much higher than that in the flat surface (the etching

selectivity: 2.1). The resist pattern turns out to be transferred to

the polysilane layer with exactly the same dimension as shown

in Fig. 29.

The transfer of the deep trench pattern to the Si substrate

was conducted by a dry etching process with C4H8/CO/O2/Ar

gas mixtures. For comparison, the pattern was transferred by

using conventional ORGARCs. The etching condition was

as follows

R1 resistð430 nmÞ=ORGARC filmð80 nmÞ=

SiO2 layerð700 nmÞ=SiN layerð220 nmÞ=Si substrate

Figs. 30 and 31 show top-down profiles of SiO2/SiN

patterns when PX2 and ORGARC films were employed

for ARCs. The residual resist and ARCs were removed by

etching and striping, respectively. The polysilane layer

was removed by O2 asher followed by washing with a

dilute fluoric acid solution. When the ORGARC layer was

used as the ARC, striations around the twin holes were

observed. The PX2 anti-reflective film improved this

striation. It has been reported that the significant rough-

ness in the resist surface during etchings is responsible for

the striations, because the roughness is transferred to

SiO2/SiN patterns. Polysilane may prevent the resist

roughness from transferred to SiO2/SiN layers [99,100].

An advantage of polysilane ARCs is that thinner resist

layers can be employed. The thinner resists make the

exposure latitude wider.

Fig. 32 shows surface profiles measured by AFM for films

of PB4, PS2 and resist R1 after exposure to C4H8/CO/O2/Ar

gas plasma. While PS2 and resist R1 were denatured, the P1

film exhibited a smoother surface. The peak to valley heights

of PS2, resist 1 and PB4 films were 105, 25.4 and 6.5 nm,

respectively. The phenomena can be explained as follows.

The denatured surface is formed by ion bombardment during

etching, because relatively high rf power is applied for the

etching. The hardness of PB4 was 740 N/mm2, which is much

harder than that of PS2, 260 N/mm2. The hardness brought

about by the tightly cross-linked Q structures should be

responsible for the roughness improvement.

The roles for each part of polysilane molecules are

summarized in Fig. 33. Solubility is controlled by organic

substituents so as to fabricate flat films by spin-coating and

Fig. 28. Resist pattern of a deep trench with a 0.20 mm design rule on PX2 anti-reflection layer. Constitution: resist R1

(430 nm)/PX2(240 nm)/SiO2(700 nm)/SiN(220 nm)/Si. Further explanation: see text. Reprinted with permission from J Vac Sci Technol

1999;17(6):3398. q1999 American Vacuum Society [96].

Fig. 29. Resist and PX2 patterns of a deep trench with a 0.20 mm design rule after the PX2 layer was etched. Constitution: resist

R1(430 nm)/PX2(240 nm)/SiO2(700 nm)/SiN (220 nm)/Si. Etching gas: Cl2, for further explanation: see text. Reprinted with permission from J

Vac Sci Technol 1999;17(6):3398. q1999 American Vacuum Society [96].

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381376

Page 19: Polysilanes for semiconductor fabrication

Fig. 30. Top-down profile of SiO2/SiN patterns when PX2 film was employed for ARCs. Constitution: resist R1(430 nm)/PX2(240 nm)/SiO2

(700 nm)/SiN (220 nm)/Si. Etching gas for PX2: Cl2; etching gas for SiO2/SiN layer: C4F8/CO/O2/Ar. The remaining resist and PX2 layers were

removed with an asher using O2/CF4 gas mixtures. Reprinted with permission from J Vac Sci Technol 1999;17(6):3398. q1999 American

Vacuum Society [96].

Fig. 31. Top-down profile of SiO2/SiN patterns when ORGARC film was employed for ARCs. Constitution: resist R1(430 nm)/ORGARC

(60 nm)/SiO2(700 nm)/SiN(220 nm)/Si. Etching gas for ORGARC: CF4/O2/Ar gas mixtures; etching gas for SiO2/SiN layer: C4F8/CO/O2/Ar.

The remaining resist and ORGARC layers were removed with an asher using O2 gases. Reprinted with permission from J Vac Sci Technol

1999;17(6):3398. q1999 American Vacuum Society [96].

Fig. 32. AMF profiles of various films after C4H8/CO/CO2/Ar gas RIE. (A) PB4, (B) PS2, (C) resist 1. Area measured: 1 mm £ 1 mm. The peak

to valley height of the roughness for PB4, PS2 and resist 1 were 6.5, 105, and 25.4 nm, respectively. Reprinted with permission from Jpn J Appl

Phys 2000;39:6781. q2000 Japan Society of Applied Physics [97].

S. Hayase / Prog. Polym. Sci. 28 (2003) 359–381 377

Page 20: Polysilanes for semiconductor fabrication

not to make mixing layers. UV absorptions and etching

selectivity relate to Q units and phenyl silyl groups. High Tg

is brought about by cross-linking and branched structures.

Optimizations of polysilane molecular structures are needed

to cope with all requirements.

6. Conclusion

We discussed the potential application of polysilanes for

LSI fabrication process. Since LSI manufacturing processes

dislike metal impurity, materials employed are restricted.

One of the silicon polymers, polysilanes, should be accepted

for the process without any difficulty. The most interesting

property for polysilanes is that etching rates can be changed

with oxidations of UV-exposures. This makes it possible to

fabricate fine patterns by anisotronic-dry etching without

other resist materials. Bi-layer resists using polysilanes

would become important for deep UV exposures, the

wavelength shorter than 200 nm, because this would

eliminate the problem on penetration depth of the light.

Anti-reflection layer for deep UV lithography is another

interesting application. Polysilanes were found to have a

unique property on dry etchings. Polysilane films are etched

faster than organic resists by using chlorine gas plasmas.

But, the polysilane were etched slower than organic resists

and SiO2 layers by using fluorine gas plasmas. The anti-

reflective property was better than that of conventional

ORGARCs. Advantages for polysilane ARC are that the

resist thickness can be reduced and the ARC thickness can

be increased. Both increased the lithography process

latitude. The etching selectivity and optical properties

make polysilanes the most promising anti-reflection film

for future lithography.

Acknowledgements

The author would like to thank Y. Sato, E. Shiobara,

S. Miyoshi, A. Asano, H. Matsuyama, Y. Onishi, and

T. Ohiwa at Microelectronics Engineering Laboratory,

Toshiba Corporation, and Y. Nakano, S. Yoshikawa,

H. Ohota at Corporate Research and Development Center,

Toshiba Corporation, for their help during this work.

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