Nikon I9c i-Line stepper (NA 0.58, Sigma 0.6) is a widely used in 6’’ wafer IC manufacture. Mark V from TEL is an automatic track for resist spin-coating, baking and development processes. The thickness of photoresist film thickness was measured by F50 from Filmetrics. CD-SEM S8840 from Hitachi is used to measure top-down CD of features. Hitachi S4800 was used for cross-section profiles observation. UV-Vis spectrophotometer from Labtech was used for UV absorbance of polymer at 365nm wavelength.

Poly (4-hydroxyl styrene-co-t-butyl-acrylate) (PHS-PtBA)

Poly (4-hydroxyl styrene-co-styrene-co-t-butyl-acrylate) (PHS-PS-PtBA)

Ethyl vinyl ether blocked PHS (EVE-PHS)

Ethyl vinyl ether blocked novolak (EVE-novolak)

PAG-1(triflic acid generator)

PAG-2(toluene sulfonic acid)

PAG-3(hydrochloric acid)

Propylene glycol monomethyl ether acetate (PGMEA)

Acetonitrile

The ICA resist was prepared by adding polymer, PAG, base, and additives into PGMEA. Then the mixed solution went through a 0.2um PTFE filter. After spin-coating on a Si wafer, the 4.2um thick resist process conditions were as follows: pre-apply bake (PAB) at 100~130 ℃ for 180s, exposure on Nikon I9c i-Line stepper, post-exposure bake (PEB) at 90 ℃ for 60s, and then development in a 0.26N TMAH aqueous solution for 60s. The target feature size was 2um L/S.

The CD was measured by Hitachi CD-SEM S8840. For each wafer, 9 points were measured and the average CD and standard deviation are reported in this work.

The profiles of features were obtained using an Hitachi X-SEM S4800.

The absorbance of polymers was determined by UV-Vis spectrophotometer. The PHS type polymers were dissolved in acetonitrile at 10wt%, and the EVE-Novolak was dissolved in acetonitrile at 0.1wt%. Then UV absorbance at 365nm of both solutions was measured and normalized to 10wt% concentration.

An ICA resist with the high Ea polymer, PHS-PtBA, and triflate PAG-1 was evaluated with a bake temperature matrix using 150mJ/cm2 exposure dose. First PEB was set at 90 ℃ and PAB temperature varied from 100 ℃ to 130 ℃. From Table 1, we can see that CD increased as PAB temperature increased. This was caused by a denser film at higher PAB temperature, which limited the diffusion of acid and gave a small reaction diffusion length. The dense film made it difficult for the developer to penetrate the resist. As shown in Figure 1, at 130 ℃ PAB, the space could not open at the same exposure dose.

Then PAB temperature was set at 110 ℃, and PEB temperature varied from 90 ℃ to 120 ℃. As shown in Table 2, CD measured by CD SEM shows a slight decrease as PEB temperature increases. This is because high PEB temperature gives faster deblocking reactions and thus smaller CD. As shown in Figure 2, with the increase of PEB temperature, the bottom CDs were getting smaller, while the top CDs didn’t change much. The shape turned into a reverse trapezoid when the PEB temperature rose above 120 ℃. Formation of the reversed trapezoid pattern is complicated. It involves acid distribution during exposure, acid redistribution during PEB and the deblocking reaction during PEB. For the formulation that is shown, triflic acid was generated during exposure. Its small size and strong acidity make it easy to diffuse within the resist film and easy to evaporate from the top of the resist to the environment. At low PEB temperatures, there was a mild evaporation of acid from the top of the film, and diffusion within the film and the deblocking reaction had similar rates, so that the total results of diffusion and reaction was a vertical sidewall. But at high PEB temperature, there was more evaporation of acid from the top of the film, and, since the reaction was faster, the acid diffusion length was smaller, thus the total results of diffusion and reaction formed a reverse trapezoid pattern. To get a more vertical profile, the polymer/PAG interaction needs to be fine-tuned during exposure and PEB.

Delay between different lithography processes is common in IC fabs, especially during troubleshooting or equipment maintenance. ICA resist is sensitive to environmental contaminates, such as base and acid. In this section, two types of process delay, delay between PAB and exposure, and delay between exposure and PEB were evaluated. For the first type of delay, the wafers with resist films were not exposed right after the PAB process, but preserved in the clean room without airborne contamination control for 1h and 2h. Then the wafers went through the rest of the lithography processes and formed patterns after development. The top-down CD increased by 12.9% to 2.332um for a 1h delay, and by 19.4% to 2.466um (Table 3) for a 2h delay. This indicated that the resist film absorbed airborne amine during the delay time. The amine neutralizes H+ generated during exposure, which hinders the reaction between H+ and blocked PHS. The longer the delay time, the larger the CD as outlined below (Figure 3).

For the second type of delay, wafers with resist were delayed between exposure and PEB. The wafer was kept in the clean room for 1h and 2h, then went through the rest of the lithographic process. Compared to pre-exposure delay, post-exposure delay had more influence on feature size. The top-down CD increased by 28% to 2.644um for a 1h delay, and by 60% to 3.305um (Table 4) for a 2h delay. In post-exposure delay, the H+ directly reacts with airborne amine. The CD and profiles increased and small T-top structures were observed at the tops of patterns (Figure 4).

To optimize the overall performance of ICA photoresist, several different types of blocked polymer were investigated, such as PHS-PS-PtBA, EVE-PHS and EVE-novolak. Compared to PHS-PtBA, High Ea polymer PHS-PS-PtBA was harder to deblock by H+ catalysis; and even after full deblocking, there were fewer -COOH groups generated, thus giving a small Rmax, so it was not suitable for a thick film application. Shallow trenches were formed in resist film even the exposure dose was raised to 1000mJ/cm2. On the other hand, Low Ea polymer EVE-PHS polymer formed patterns at exposure doses under 30mJ/cm2. Severe undercuts and standing waves were observed along with the trapezoid sidewalls. This can be explained in the following mechanisms:

1) EVE-PHS is more transparent than EVE-novolak at 365nm, which gives a relatively high exposure dose at the bottom of the resist film;

2) The substrate used is bare silicon, which has strong reflection of 365nm wavelength. Combined with the high exposure dose at the bottom, this gives a reversed trapezoid pattern;

3) EVE-PHS can easily deblock when acid is present, even at room temperature, thus the acid generated during exposure does not have enough time to diffuse, which induces strong standing waves.

Meanwhile, the low E_a polymer EVE-novolak showed tapered profiles due to the high absorbance of novolak at 365nm (Figure 5).

CA resist uses H+ generated from PAG during exposure to catalyze deblocking reactions of polymers. The PAG is critical for resist lithography performance. Three different types of PAGs were evaluated in the high Ea PHS-PtBA platform. The anion type affects the strength and diffusion length of H+, which will affect the photospeed and profile of resists. PAG-1 generates triflic acid with strong acidity and long diffusion length. Periodic Line/Space patterns were obtained with catalysis of triflate acid under 150mJ/cm2 exposure dose. The acidity of toluene sulfonic acid from PAG-2 is weaker than triflic acid. Shallow trenches were observed at the surface of resist with PAG-2, even though the exposure dose was raised to 1200 mJ/cm2 (Figure 6). The hydrochloric acid from PAG-3 is the weakest acid and it hardly catalyzed the deblocking reaction. There was no pattern formed in the resist with PAG-3.

By optimizing the polymer structures and PAG/base choices, an environmentally stable ICA resist was developed by Kempur. C9005 has dramatic airborne amine resistance. It could maintain a stable CD uniformity after 2h pre-/post- exposure delay (Table 7). The rectangular profiles of C9005 were barely changed after 2h delay (Figure 7).

Table7. CD change of ICA resist with different pre-/post –exposure delay.

Due to high transparency of polymer used, C9005 showed a good resolution (Figure 8). We obtained 0.75um L/S features in 4.2um resist thickness. The aspect ratio reached 5.6.

C9005 has a broad process window (Figure 9). It can achieve 6um of depth of focus and 46.7% of exposure latitude for +/- 10% of CD variation. It can be utilized for different layers and applications.