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*Corresponding authors (email: gqyang@iccas.ac.cn; g1704@iccas.ac.cn)

• ARTICLES • doi: 10.1007/s11426-014-5122-y

Outgassing analysis of molecular glass photoresists under EUV irradiation

CHEN Li1, XU Jian1, YUAN Hua1, YANG ShuMin2, WANG LianSheng2, WU YanQing2, ZHAO Jun2, CHEN Ming2, LIU HaiGang2, LI ShaYu1, TAI RenZhong2,

WANG ShuangQing1* & YANG GuoQiang1*

1Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

2Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China

Received January 22, 2014; accepted February 19, 2014

A device was designed and assembled to analyze the outgassing of molecular glass (MG) photoresists under extreme ultravio-let (EUV) exposure. The outgassing of the photoresists with different components and different concentrations of tert-butoxycarbonyl (t-Boc), photo-generated acid (PAG), and acid quencher was systematically investigated. Based on ex-periments, some solutions for reducing the outgassing of MG photoresists were proposed.

photoresist, EUV lithography, molecular glass, outgassing

1 Introduction

Extreme ultraviolet (EUV) lithography with 13.5 nm irradi-ation is most likely to be used for the next-generation li-thography, and it is a key technology for the continuation of Moor’s Law [1]. The most important challenges in EUV lithography include the EUV lithography system, mask, and photoresist [2]. For the EUV lithography system, ASML has made great progress towards high-volume production of semiconductor devices [3]. Its second-generation EUV li-thography system NXE 3300 is at the final stage of com-missioning, which is expected to expose 125 wafers per hour. For EUV mask, Intel has installed its pilot production line and has made some progress in manufacturing, fixing, and detecting [4]. As for the photoresist, great efforts need to be made in improving its resolution, line width roughness (LWR), and sensitivity [5–7]. Molecular glass (MG) photo-resist, as one of the promising photoresists for the EUV

lithography, has been widely studied because of its high sensitivity and low LWR [8–11]. However, the outgassing of MG photoresist under EUV irradiation has not been sys-tematically reported. Outgassing may break high vacuum, pollute the lens and masks, and reduce the productivity, even damage the expensive lithographic system. Therefore, it is important to evaluate the outgassing and find out a way to control the outgassing [12–14].

In this paper, a device was designed and assembled for analyzing the outgassing of photoresists in both quantitative and qualitative studies. The average outgassing rates and the outgassing species were recorded. To find a way to reduce outgassing, the outgassing of MG photoresists based on FPT-8Boc and FPT-4Boc was studied and discussed.

2 Experiments

2.1 The photoresist outgassing analysis system

To analyze the outgassing of MG photoresists under high

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vacuum condition, a device was designed and assembled [15]. Figure 1 shows the configuration of the photoresist outgassing analysis system. It includes a quadruple mass spectrometer (QMS), a vacuum system, a sample transition system, and a central controlled telecommunication system. QMS can detect the residual gas and the pressure change of the system under the high vacuum of 108 mbar. The sample transition system is used for sample transfer between the fore-vacuum cylinder and the detection chamber. The tele-communication system is used for the control of mass spec-trometer, data recording, and analysis. With the device, the total outgassing amount and species during the irradiation can be obtained.

The device was installed at the BL08U1B beamline of Shanghai Synchrotron Radiation Facility, China. The irradi-ation was proceeded in the EUV energy range of 13.5 nm (91.84 eV), and the irradiation power was 0.48 mW/cm2 (3.3 × 1013 photons/(s cm2)).

2.2 Materials and processes

Two new MG compounds, named FPT-4Boc and FPT-8Boc, were synthesized [16]. Their molecular structures are shown in Figure 2. Generally, chemical-amplified photoresists in-clude resistant material, photo-acid generator (PAG), acid quencher, and solvent [17]. In our photoresists, FPT-4Boc or FPT-8Boc was used as the resistant material, triphen-ylsulfonium perfluoro-1-butanesufonate as PAG, tri-n-oct-

Figure 1 The configuration of the resist outgassing system.

ylamine (TOA) as the acid quencher, and propylene glycol 1-monomethyl ether 2-acetate (PGMEA) as the solvent. To study the influence of the components of EUV photoresists on outgassing, a series of photoresists with different pre-scriptions were prepared.

3 Results and discussion

3.1 Outgassing qualification based on the pressure rise

A method deriving from Ref. [18] was used to evaluate the outgassing of photoresists. The method was based on the pressure rise. In this method, two equations were inferred from the modified ideal gas equation. Eq. (1) was used to evaluate the outgassing amount from the start of exposure, and Eq. (2) was used to evaluate the average outgassing rate [15, 18].

D

s

e

S

t

ii t a

PS tN

NA RT

(1)

D

sM

D S

e

( )

t

ii t a

PS tN

NA t t RT

(2)

where NS (molecules/cm2) is the outgassing amount for 12 s from the start of exposure. Hence, tD-tS is equal to 12. This is because the exposing dose of 12 s is enough for FPT-4Boc or FPT-8Boc. NM (molecules/(cm2 s)) is the av-erage outgassing rate; ∆Pi (Pa) is the pressure rise; Se (m

3/s) is the pump exhaust velocity equal to 180 m3/s; Na is the Avogadro constant equal to 6.02 × 1023 mol1; A is the ex-posure area, which is equal to 1.80 cm2 in our device; R (Pa/(m3 K mol)) is the gas constant equal to 8.314 Pa/(m3 K mol); T (K) is the room temperature equal to 298 K.

3.2 The influence of different components in photore-sists

Outgassing analysis based on pressure rise

To investigate the influence of different components of photoresist on outgassing, four formulas of MG photoresists were prepared. All photoresists were based on FPT-8Boc.

Figure 2 The structures of FPT-4Boc and FPT-8Boc.

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Figure 3 Pressure rise characteristics of photoresists based on FPT-8Boc with different components. (a) FPT-8Boc; (b) FPT-8Boc + TOA (0.25 wt% of FPT-8Boc); (c) FPT-8Boc + PAG (5 wt% of FPT-8Boc); (d) FPT-8Boc + PAG (5 wt% of FPT-8Boc) + TOA (0.25 wt% of FPT-8Boc).

The formulas of Figure 3(a–c) contained FPT-8Boc, FPT- 8Boc and TOA, and FPT-8Boc and PAG, respectively, whereas formula of Figure 3(d) contained all three com-ponents of FPT-8Boc, PAG, and TOA. It was observed that the photoresists of Figure 3(a) which contained only FPT-8Boc produced the smallest amount of outgassing. Its average outgassing rate was 1.78 × 1014 molecules/(cm2 s). That was because the EUV light was so powerful to de-compose some of the resistant material. Even if the TOA was added to formula (a), the outgassing amount was the same. That was to say that TOA had no effect on FPT-8Boc. However, when PAG was added into formula (a), the outgassing increased quickly. Its average outgas-sing rate was 9.14 × 1014 molecules/(cm2 s), which was about five times of formula (a). This phenomenon indi-cated that the PAG generated acid under EUV exposure, and the acid promoted the leaving of the acid-sensitive group t-Boc. Comparing with formula (c), the outgassing of formula (d) was reduced because of the addition of TOA. Its average outgassing rate was about 6.49 × 1014 molecules/(cm2 s). Thus, it can be concluded that TOA restricts the diffusion of the acid and reduces the outgas-sing. This is because TOA is a weak base, which can be reacted with the acid generated by PAG. Hence, the addi-tion of TOA can reduce the outgassing.

Characterization and analysis of outgassing species

Figure 4(a) shows that the outgassing species of the photo-resist containing only FPT-8Boc were CH3 (15 amu), CH4 (16 amu), H2O (18 amu), CO (28 amu), C3H3 (39 amu), C3H5 (41 amu), CO2 (44 amu), C4H8 (56 amu), and C4H9 (57 amu). These species were outgassed because of the decom-posing of the protecting group t-Boc [19]. Figure 4(b, c) shows that C6H5 (77) and C6H6 (78) appeared in the outgas-sing species. This is because both photoresists contain PAG.

Comparing Figure 4(c) with Figure 4(b), the amount of the outgassing species from t-Boc is reduced, which can be ra-tionalized to the existence of TOA in the photoresist. Therefore, we can conclude that TOA limits the diffusion of PAG, consequently, reducing the outgassing. The addition of TOA is a very important way to reduce the outgassing of the photoresists.

Because there was no species of C6H5 (77) or C6H6 (78) in Figure 4(a), it can also be concluded that the skeleton of tetraphenylthiophen is very stable under the EUV exposure. The skeleton section of our MG compounds containing no weak bond may reduce the outgassing.

3.3 Outgassing dependence on the concentrations of t-Boc, PAG, and TOA

To explore the influence of the amount of the t-Boc group, PAG, and TOA on the outgassing, a series of experiments were carried out. The pressure rise characteristics of the photoresists based on FPT-4Boc and FPT-8Boc are shown in Figure 5(a). For these two compounds, their skeleton structures are the same. They all have the structure of tetra-phenylthiophene, but different numbers of the acid sensitive group t-Boc. For these two photoresists, the optimal expos-ing time was examined to be 12 s with the exposure inten-sity of 0.48 mW/cm2. Thus, NS and NM were calculated for 12 s from the start of exposure. The outgassing amount NS (7.65 × 1015 molecules/cm2) and the average outgassing rate NM (6.38 × 1014 molecules/(cm2 s)) of FPT-8Boc was 1.6 times that of FPT-4Boc. Obviously, the maximum pressure peak and the outgassing rate increases with increasing the number of acid-sensitive group. That is, decreasing the number of acid-sensitive group can reduce the outgassing of photoresists.

Figure 5(b) shows the pressure rise characteristics of the FPT-8Boc-based photoresist with different PAG concen-trations (2.5, 5, 10, and 15 wt% of FPT-8Boc). With the increasing of the PAG concentration, the maximal pressure of the outgassing increases. NM of the photoresists con-taining 15 wt% PAG (18.30 × 1014 molecules/(cm2 s)) is about 3.6 times that of the photoresists containing 2.5 wt% PAG (5.06 × 1014 molecules/(cm2 s)). Thus, the concen-tration of PAG is another important factor to affect the outgassing of the photoresists. We conclude that decreas-ing the concentration of PAG can reduce the outgassing of MG photoresists.

Figure 5(c) shows the pressure characteristics of the FPT-8Boc-based photoresists with varied TOA concentra-tions (2.5, 5, 10, and 15 wt% of PAG). The outgassing de-creases with the concentration of TOA increasing from 2.5 wt% to 10 wt% of PAG. But the average outgassing rate of photoresists containing 15 wt% TOA is equal to that of the photoresists containing 10 wt%. This indicates that TOA can prevent the diffusion of the acid and there is an optimal con-centration of TOA to reduce the outgassing. For FPT-8Boc,

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Figure 4 The outgassing species of photoresists based on FPT-8Boc with different components. (a) FPT-8Boc; (b) FPT-8Boc + PAG (5 wt% of FPT-8Boc); (c) FPT-8Boc + PAG (5 wt% of FPT-8Boc) + TOA (0.25 wt% of FPT-8Boc).

Figure 5 Pressure-rise characteristics of photoresists with different concentrations of t-Boc (FPT-4Boc + PAG (5 wt% of FPT-8Boc) + TOA (0.25 wt% of FPT-8Boc) and FPT-8Boc + PAG (5 wt% of FPT-8Boc) + TOA (0.25 wt% of FPT-8Boc)) (a), FPT-8Boc + PAG (2.5, 5, 10, and 15 wt% of FPT-8Boc) + TOA (0.25 wt% of FPT-8Boc) (b), and FPT-8Boc + PAG (5 wt% of FPT-8Boc) + TOA (2.5, 5, 10, and 15 wt% of PAG) (c).

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it is about 10 wt% of PAG.

3.4 Lithography pattern

Based on the outgassing results, we selected FPT-8Boc + PAG (5 wt% of FPT-8Boc) + TOA (10 wt% of PAG) as a photoresist for lithography. Figure 6 shows its SEM pattern of lithography. The resolution of this photoresist reaches 32 nm and the line width roughness (LWR) is less than 2.5 nm. All the results indicate that the photoresist with a relatively low outgassing possesses a very good performance in li-thography.

4 Conclusions

A device was designed and assembled to evaluate the out-gassing of photoresists under the EUV exposure. QMS was used to collect the data of pressure rise of the system and the outgassing species. The outgassing amounts and average rates of MG photoresists were calculated based on the pres-sure rise. From the comparison of the outgassing of the photoresists with different components, it can be concluded that TOA could prevent the diffusion of acid and reduce the outgassing of the MG photoresists. The comparison of the concentrations of t-Boc, PAG, and TOA illustrates that de-creasing the amount of t-Boc and PAG can reduce the out-gassing. There is an optimal concentration of TOA which is about 10 wt% of PAG. Also, based on the study of the out-gassing species, it is verified that the outgassing is primarily from the protecting group t-Boc. The skeleton of tetra-phenylthiophene was very stable with no outgassing under the EUV exposure. The skeleton segment of MG compounds

Figure 6 SEM pattern of photoresist (FPT-8Boc + PAG (5 wt% of FPT-8Boc) + TOA (10 wt% of PAG)).

containing no weak bond can reduce the outgassing. The photoresist with a relatively low outgassing could be a good candidate for high-resolution EUV lithography.

This work was financially supported by the National Natural Science Foundation of China (21373240, 91123033, 21233011) and the National Science and Technology Major Project of China (2011ZX02701).

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