Polymers for Advanced Technologies Volume 4, pp. 302-308

Photosensitive Polyimides Consisting of Simple Mixtures of 4-Substituted Diazonaphthoquinones and Polyamic Acids Shuzi Hayase’, Yukihiro Mikogami, Kei Takano, Yoshihiko Nakano and Rumiko Hayase Chemical Laborato y, Research and Development Center, Toshiba Corporation, Komukai-Toshiba-cho, Saiwa-ku, Kawasaki 210, Japan

ABSTRACT

Polyimide resists that can be developed with a basic aqueous solution were produced by simple mixtures of conventional polyamic acids and naphthoquinone di- azides in which sulfonate groups are substituted at the 4- position. The diazonaphthoquinones bearing electron- withdrawing groups gave negative tone resists. On the other hand, those bearing electron-donating groups gave positive tone resists. The difference in the resist behav- iors depending on the photoactive structure was explained by crosslinking caused by photogenerated sulfonic acids. The thermal stability of polyimides pre- pared from the resists was almost the same as that of conventional polyimides.

KEYWORDS: Photosensitive, Polyimides, Naphthoquinone diazides, Resists, Basic aqueous solution, Development, Polyamic acids, Diazonaphthoquinines

INTRODUCTION Polyimides have many desirable properties that are useful in the microelectronics industry. They are one of the best high-temperature resistant polymers. The Polyimides feature good planarization capabilities as well as electrical insulating properties [l] .

Photosensitive polyimides, including photo- sensitive polyimide precursors, have attracted great

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interest, because they simplify processing by elimi- nating the need for separate resist application and removal steps [2, 31.

Negative working polyimides are quite common [2, 31. They have photosensitive acrylic acid moieties in the side chains. During UV irradiation, the acrylic moieties react to give crosslinking products that are insoluble in organic solvents. Therefore, they need organic solvents to develop and rinse the resist to give the resist patterns. This is not compatible with existing photoresist processing technologies in microelectronics, where aqueous solutions contain- ing dilute organic bases are used [4].

Some resist systems have been developed with aqueous basic solutions. Khanna and Mueller have reported [5-81 on the use of polyimides bearing hydroxy groups for positive working diazonaphthoquinone-sensitized resists [5, 61. Positive polyimide resists, in which diazonaphtho- quinones are bound to the hydroxy groups on the polyimide backbone have been also reported [7, 81. These resist systems provide excellent resist patterns: however, hydroxy groups have had to be incorpor- ated either in the polymer backbones or in the side chains, which has made the polymer syntheses com- plicated.

Diazonaphthoquinones act as dissolution inhibi- tors to base-soluble binder polymers, such as phenol novolac resins [9]. Considering that polyamic acids, which are precursors to polyimides, are soluble in dilute basic aqueous solutions, diazonaphtho- quinones might also act as dissolution inhibitors to the polyamic acids. However, this has not yet been

Received 7 August 1992 Revised 30 October 1992

Polyimides Consisting of Simple Mixtures I303

TABLE 1.

0

R H

- 1 3 0 - 2 2.5 0. 5 - 3 2 1 - 4 1 . 5 1.5

O H

- 7 N C e O R

9 - Po 6 R

CH 3

11 e~-R I -

- 13 R-0-B u

8 - 0

CH 3

dH 3

12 - dH

CF 3

proved for conventional polyamic acids prepared from benzophenone tetracarboxylic acid dianhyd- ride and diaminodiphenylether. This is because they are so soluble in the basic solutions that both exposed and unexposed areas are usually washed away during development.

Resists consisting of simple mixtures of polyamic acids and naphthoquinone diazides have the poten- tial to give polyimides with excellent properties because the parent polyamic acids are the same as conventional polyamic acids which give excellent polyimide films after imidization [l]. Certain specific polyamic acids have been synthesized as positive working resists [lo, 111. These polyamic acids are partially imidized during soft bake processing at 135°C before exposure, so as to adjust the polymer solubility in the basic aqueous solutions [lo, 111. The resist consisting of these polyamic acids and diazonaphthoquinone works as a positive tone [lo, 111.

This new idea for positive tone resists does not suit conventional polyamic acids, because diazo compounds in general decompose rapidly above 135°C. Conventional polyamic acids are not con- verted to the imide structure at all when the soft bake process is carried out below 135°C.

Recently, the authors found another approach, High-Temperature-Post-Exposure Bake (HIT-PEB), which produces negative working resists [12, 131. The resist consists of conventional polyamic acids and naphthoquinone diazides, which are bound to a benzophenone group through sulfonate linkage at the 4-position on the naphthoquinone diazide (4- NAC). Naphthoquinone diazides on which a sulfo- nate group is substituted at the 5-position (5-NAC) did not work even when the HIT-PEB process was applied.

This paper discusses the relationship between the chemical structures of the photoactive com- pounds and the resist properties provided by the HIT-PEB process.

EXPERIMENTAL All of the photoactive compounds used in this study (see Table 1) were purchased from Toyo Gousei and were used without any purification.

Polyamic acid (Pl) was synthesized by the reac- tion of 0.3 mol of pyromellic acid anhydride and 0.7 mol of benzophenone tetracarboxylic acid anhyd- ride with 0.9 mol of diaminodiphenyl ether and 0.1 mol of di(aminopropy1)tetramethyldisiloxane.

304 I Hayase et al.

The intrinsic viscosity was 0.65 (dl/g) (0.5 g in 100 ml of NMP) at 30°C. All of the resists were prepared by dissolving 30 g of the photoactive com- pound and 70 g of the P1 in NMP (N-methyl-2- pyrrolidone) and filtering before use. R1-R14 repre- sent resists containing P1 and the photoactive com- pounds 1-14, respectively.

Ultraviolet exposure was carried out by using a CA 800 (Cobilt contact printer) equipped with a Xe lamp.

A model compound of polyamic acid was pre- pared by reacting 93 g of aniline in 100 ml of THF with 148 g of phthalic acid anhydride in 100 ml of THF. After the reaction mixture had been stirred for 4 hr at room temperature, the solvent was removed under reduced pressure at room temperature to give a white powder. F1-F14 represent resists containing the model compound and the photoactive com- pounds 1-14, respectively.

UV absorption was monitored with a Hitachi 330 Model spectrometer. Thermal analysis was carried out using a Daini Seikosha model SSCl560GH.

The HIT-PEB Process

The HIT-PEB process is shown in Scheme 1. Resists consisting of the polyamic acids and naphtho- quinone diazides were spin-coated onto Si wafers and each wafer was soft baked at 90°C for 10 min, followed by exposure. Each wafer was baked again at 140-150°C for 2-5 min, developed with tetramethyl- ammonium hydroxide aqueous solution (TMAH) and rinsed with deionized water. Detailed con- ditions are described in each figure.

The dissolution rates of the resists were calcu- lated from the change in the film thickness before and after immersion in the basic solution. In all sensitivity curves, the film thickness after the post-

P O L Y A M I C A C I D

D I A Z O N A P H T H O Q U I N O N E

+ S P I N C O A T I N G

P R E B A K E

C . l o r n i n ) U V E X P O S U R E

P O S T E X P O S U R E

( 1 4 0 C - 1 6 0 C. 1 - 5 r n i n )

D E V E L O P M E N T

( T e t r a r n e t h y I A m m o n i u m H y d r o x i d e A q u e o u s S o l u t i o n )

P A T T E R N

SCHEME 1. The HIT-PEE3 process.

exposure baking was normalized to unity. The film thickness was measured with a Taly Step (Taylor- Hobson Model).

RESULTS AND DISCUSSION The HIT-PEB process features high-temperature post-exposure baking at above 140°C. This tempera- ture is surprisingly high because diazonaptho- quinones start to decompose at around 130°C [lo, 111. The decomposed diazo compound usually causes crosslinking as well as the formation of car- boxylic acid [14, 151. Therefore, this high- temperature process could not be applied to the conventional resists. However, in the case of poly- amic acids, we found that the high-temperature post- exposure baking process gave exceptionally good results.

In Fig. 1 are shown the sensitivity curves for resists in which photoactive compounds bearing a benzophenone moiety are contained. Compound 9, which does not have a benzophenone group, is shown for comparison. Rl-R4 gave almost the same sensitivity: however, the sensitivity and the film thickness of the resists after development decreased as the number of diazo groups bound to the benzo- phenone groups decreased. R9 also gave a negative tone curve: however, the resist property was inferior in sensitivity to those of Rl-R4. This result implies that the benzophenone moiety plays an important role for the resist properties.

The sensitivity curves of resists in which the diazo compound is bonded to a bis-phenol group are shown in Fig. 2. These structures are similar to those of 1-4. The sensitivity curves for resists containing 8 and 1 are also shown in this figure for comparison. R8, R6 and R5 gave negative features. However, surprisingly, R14 and R10 gave positive features. The photoactive compounds differ in the moieties between the two phenyl groups. The diazo com- pounds bearing -CO- or -SO2- groups gave negative resists, and those having -C(CH&- or -C(CF&- groups gave positive resists.

NORMALIZED THICKNESS

R1 0. 8

0. 0 50 100

FIGURE 1. Sensitivity curves for resists containing naphthoquinone diazides bearing a benzophenone group. Rl-R9 represent resists containing P1 and 1-9 respectively. HIT-PEB, 140"C, 5 min, development, 1.79% TMAH aqueous solution. P1: diazo compound (7:3, wt/wt).

5 10 EXPOSURE DOSE (mJ/cmZ)

Polyimides Consisting of Simple Mixtures / 305

NORMALIZED TH I CKNESS

". " 5 10 50 100 EXPOSURE DOSE (mJ/cmZ>

FIGURE 2. Sensitivity curves for resists containing naphthoquinone diazides bearing a bis-phenol group. R1, R5, R6, R8, R10 and R14 represent resists containing P1 and 1, 5, 6, 8, 10 and 14, respectively. R1, HIT-PEB, 1 40°C, 5 min, development, 1.79% TMAH aqueous solution; R5, HIT-PEB, 140"C, 5 min, development, 2.38% TMAH aqueous solution; R6, HIT-PEB, 150"C, 5 min, development, 2.38% TMAH aqueous solution; R8, HIT-PEB, 150°C, 3 min, development, 2.38% TMAH aqueous solution; R10, HIT-PEB, 140"C, 5 min, development, 0.79% TMAH aqueous solution; R14, HIT-PEB, 140"C, 5 min, development, 0.79% TMAH aqueous solution. P1: diazo compound (7: 3, wt/wt).

NORMALIZED TH I CKNESS

1 . 0

0. 8 - 0. 6 - 0. 4

0. 2 -

0. 0 I- 1 l l . 5 10 50 100 EXPOSURE DOSE (mJ/cmz)

FIGURE 3. Sensitivity curves for resists containing diazonaphthoquinone bearing a phenyl group. R7, R8 and R12 represent resists containing P1 and 7, 8 and 12, respectively. R7, HIT-PEB, 150"C, 3 min, development, 2.38% TMAH aqueous solution; R8, HIT-PEB, 150"C, 3 min, development, 2.38% TMAH aqueous solution; R12, HIT-PEB, 140"C, 3 min, development, 0.79% TMAH aqueous solution. P1: diazo compound (7: 3, wt/wt).

It is implied in Figs 1-3 that diazo compounds bearing electron-withdrawing groups give negative tone features, and that those having electron- donating groups give positive tone resists. In Fig. 4 is shown the relationship between Ahv/Adark and Hammett's value 0 for each substituent appearing in Fig. 3. Here, A hv and Adark represent the dissolution rates in exposed and unexposed areas, respectively. If the value of Ahv/Adark exceeds one, the resist shows a positive tone feature. The value of Ahv/Adark de- creased with an increase in the (J value.

Buhr, Grunwald and co-workers have reported [14, 151 that in an image reversal process in resist technology, crosslinking of phenol novolac resins with crosslinker was accelerated by sulfonic acid photogenerated from 4-NAC. The acceleration was remarkable in using 1-4; in other words, the struc- ture bearing a benzophenone moiety. On the other hand, the crosslinking rate of the resist in which diazonaphthoquinones is bonded to the bisphenol-A moiety was more than ten times lower, compared to that containing a benzophenone moiety.

This has been explained as follows (Scheme 2). Naphthoquinone diazides photodecompose to form indene carboxylic acid derivatives. Then the photol- ysis compounds further decompose to form com- pounds with both carboxylic acids and sulfonic acids. The presence of carbonyl groups in the indene carboxylic acid derivatives facilitates proton accept- ance and thus promotes Lewis base behavior. Therefore, diazonapthoquinones bearing carbonyl groups produce more sulfonic acids, facilitating crosslinking by acid catalysis. However, diazonaphthoquinones which do not have proton acceptors, such as a bis-phenol-A group, do not release the acids. This is why the crosslinking is not carried out effectively.

The idea of acid catalysis may be useful in order to explain the dependency of the resist behavior on the photoactive compounds in the HIT-PEB process.

The dissolution rate of P1 that was baked at 150°C for 10 min was almost the same as that of P1 baked following the same procedure in the presence of p-toluene sulfonic acid (Fig. 5). Grunwald et al.

In order to examine the effect of substituents on benzene rings, the sensitivity curves of resists con- taining photoactive compounds 7, 8 and 12 are compared with each other (see Fig. 3). Photoactive compound 12, bearing a methyl group, gave a posi- tive resist: however, photoactive compounds 7 and 8, bearing cyano and acetyl groups, respectively, gave negative tone features. The resist patterns were washed away completely at the end of the develop- ment, with the diazo compound bonded either to butanol (13) or an aniline derivative (11).

-0. 5 0 0. 5 Q VALUE

FIGURE 4. The relationship between Ahv/A.dorkand Hammett's u. R7, R8 and R12 represent resists containing P1 and 7, 8 and 12, respectively. HIT-PEB, 1 50"C, 5 min, development, 2.38% TMAH aqueous solution. P1: diazo compound (7:3, wt/wt).

306 I Hayase et al.

0 E s 0 2 s 01

I O A r O A r O A r

s 0 2 s 01 I I

O A r O A r O A r

O F r H

SCHEME 2.

[14] have reported that thermal imidization with a decrease in the solubility prevents the polyamic acids from swelling in the developers or detaching from a Si wafer surface during the development process. Our results suggested that the imidization of polyamic acids was not accelerated when catalytic amounts of sulfonic acids were added. Therefore, this suggests that the negative tone features were not produdced by imidization catalyzed by photogener- ated sulfonic acids.

D I S S O L U T I O N R A YICRG,., ULI.

1 2 0 1 3 0 1 4 0 1 5 0 B A K E T E M P E R A T U R E

('C)

FIGURE 5. Comparison of dissolution rates for resists. Prebaking at 90"C, for 10 min. Development, TMAH, 2.38% aqueous solution. A, R12; B, R12+5% p-toluene sulfonic acid; C, R5; D, R5 + 5% p-toluene sulfonic acid; E, P1 only; F, P1+5% p-toluene sulfonic acid.

A large difference in the dissolution rates was seen when the photoactive compounds were present with P1 (Fig. 5). The dissolution rate of R5 (the mixture of P1 and 5) was ten times lower when baked in the presence of p-toluene sulfonic acid (D) than in the absence of the sulfonic acid (C). A similar result was also seen in the case of diazo compound 12 (A and B in Fig. 5): however, the decrease was small, compared with 5.

The thermal stability of photoactive compounds in the presence and absence of an amic acid is shown in Fig. 6. A reaction product of phthalic acid anhyd- ride and aniline was used as the model compound of the polyamic acids. In both cases, no significant difference in the thermal stability was seen. All of the photoactive compounds started to decompose at around 130"C, and the decomposition was completed at around 160°C; with the exception that 6 started to decompose at a temperature which was 20°C higher. However, 12 and 14 seemed to complete their decomposition at a. slightly lower temperature.

The dissolution rate of R5 decreased more rapidly with an increase in the baking time than did that of R12 in the exposed area. On the contrary, in the unexposed area the dissolution rate of R5 did not decrease as much as that of R12, as shown in Fig. 7. This result suggests that in the exposed area, pho- toactive compounds bearing electron-withdrawing groups or groups acting as Lewis bases photodecom- pose to form sulfonic acids. In the course of post- exposure baking, the polyamic acids and remaining naphthoquinone diazides crosslink, catalyzed by the photogenerated acids. In the unexposed area, certain thermal decompositions and thermal crosslinking may take place: however, the extent would be small because not all of the diazo compounds decompose, as shown in Fig. 6.

On the other hand, photoactive compounds bearing electron-donating groups or groups which

Polyimides Consisting of Simple Mixtures f 307

ENDOTHERM.

I I I I I I I

100 120 140 160 180 200 TEMPERATURE ( " C )

ENDOTHERM.

100 120 140 160 180 200 TEMPERATURE ('C)

FIGURE 6. DSC curves for photoactive compounds and resists.

do not act as Lewis bases photodecompose to form carboxylic acids [14, 151. The release of the sulfonic acids is supposed to be slow, according to the results of Buhr et al. [15]. Therefore, the crosslinking would not be accelerated as much as in the case of the former photoactive compounds. Furthermore, the generation of carboxylic acids may increase the dis- solution rate. In the unexposed area, because of crosslinking due to the thermal decomposition of the photoactive compounds [14, 151, the dissolution rate decreases. The dissolution rate of R5 was higher than that of R12 in the unexposed area. The lower decomposition temperature of R12 than of R5 may explain the smaller dissolution rate.

Photograph shows the polyimide patterns de- veloped with tetramethylammonium hydroxide (TMAH) aqueous solution. 0.3 pm patterns were fabricated with vertical walls.

The thermal stability of the polyimides is shown in Fig. 8. The thermal stability of cured R1 was a little lower, compared with cured P1. This shows that the heat resistance of the polyimide prepared from R1

l o - '

DISSOLUTION RATE (I I CRON/SEC. ) l o - *

10-3

l o - '

L

R5-U

R12-E R5-E

5 10 BAKE TIME (MIN. )

FIGURE 7. The solubilities of the resist films in exposed and unexposed areas after the HIT-PEE process. HIT-PEE temperature 150°C; development, 2.38% TMAH aqueous solution; R5-E and R12-E represent the solubilities of R5 and R12 in exposed areas. R5-U and R12-U represent the solubilities of R5 and R12 in unexposed areas.

WE I GHT (%I

50 t I I 1 I I I

100 200 300 400 500 TEMPERATURE ("C)

FIGURE 8. Thermal stabilities for polyimides. For abbreviations, see Fig. 1 and the experimental section. Cure process: 150°C for 1 hr, 250°C for 1 hr and 320°C for 1 hr.

308 I Hayase et al.

A polyimide pattern fabricated by the HIT-PEB process. Photoactive compound: 4-naphthoquinone diazides bonded to 2,3,4,4'-tetrahydroxy benzophenone through sulfonate linkages. Three out of four OH groups are substituted with the diazo groups. P1: photoactive compound = 7 : 3 (wt/wt); HIT-PEB, 1 40°C, 5 min, development, TMAH, 1.19%, 20 sec, 20°C; initial thickness, 1.75 pm, 3 pm line and space pattern.

was almost the same as that of conventional poly- imides.

CONCLUSIONS Polyimide resists that can be developed with a basic aqueous solution were produced by simple mixtures

of conventional polyamic acids and naphthoquinone diazides in which sulfonate groups were substituted at the 4-position, by using HIT-PEB process. This presented a novel method of providing positive and negative photosensitive polyimide resists.

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