The technology for 193-nm lithography is developing rapidly to satisfy the requirements that envision printing features of 0.16 μm and below. However, high performance resists suitable for these exposure tools must be designed before they can be put into practical application. 193-nm photoresists should possess optimum range of transparency at 193 nm, high imaging property and etching resistance, and good adhesion property.
Alicyclic polymers have the necessary transparency at 193 nm and etch resistance, but their incorporation gives rise to poor adhesion, cracks in resist film, and lowers resist sensitivity. Therefore, hydrophilic moiety such as hydroxyl group should be incorporated in the polymer. However, it was found that the resist formulated with the polymer containing hydroxyl group showed “foot” profiles, and this is suggested that the hydroxyl group causes cross-linking during a lithographic process. It was also found that hydroxyl group gave an undesirable effect on storage stability of polymers, especially when the monomer containing hydroxyl group copolymerized with maleic anhydride.
In this study, we investigated which functional groups raise cross-linking reactions during lithographic process. 2-(2-Methoxyethoxy)ethyl ester, and 2-acetoxyethyl groups were introduced into side chains of the matrix polymer, respectively, in order to improve adhesion to a silicon substrate without causing cross-linking and storage stability. 2-(2-Methoxyethoxy)ethyl 5-norbornene-2-carboxylate (MENC) was synthesized and polymerized with t-butyl 5-norbornene-2-carboxylate (BNC), norbornene (NB), and maleic anhydride (MA). 2-Acetoxyethyl 5-norbornene-2-carboxylate (AENC) was also synthesized and polymerized with BNC, NB, and MA. These polymers showed comparable adhesion property to the polymer containing 2-hydroxyethyl 5-norbornene-2-carboxylate (HNC), and better storage stability than the polymer. Sub-micron line and space patterns were obtained with the resists formulated with these polymers, respectively using ArF excimer laser stepper and a conventional developer.
Now, to meet the upcoming demand of next generation lithography, new chemically amplified resist materials should be developed that can perform at the limit where the image feature size is on the order of molecular dimensions. All commercial resists make use of linear polymers in the formation of chemically amplified resist systems. However, non-polymeric material has several advantages over linear polymers as patterning features become smaller as follows. The limit of resolution can be expanded to molecular level since the building block of image feature shrinks to small molecule. The small molecular resist helps to dissolve more uniformly during development and decrease the line edge roughness because it does not exhibit strong intermolecular interactions such as chain entanglement due to the short chain length, the small radius of gyration, and the high density of sterically congested peripheral groups if it has star-shape structures.
In this study, 1,3,5-cyclohexanetricarboxylic acid tri-t-butyl cholate ester $(Ch-TBC_3)$, pentanedioic acid 2-(4-t-butyl cholate oxycarbonylbutyryloxy)-1-(4-t-butyl cholate oxycarbonylbutyryloxymethyl)ethyl t-butyl cholate ester $(Buty-TBC_3)$, pentanedioic acid 3,5-bis(4-t-butyl cholate oxycarbonylbutyryloxy)cyclohexyl t-butyl cholate ester $(ChButy-TBC_3)$, t-BOC β -cyclodextrin inclusion complex with 1-adamantanecarboxylic acid $(t-BOC \beta -CD IC)$, and octa(2-t-butyl cholate ethyl) octamethyl siloxane (Si8TBC) were synthesized as low molecular weight amorphous resist materials. $Ch-TBC-3$ is amorphous, but due to its rigid structure, it was hard to be spin coated on substrate. Thus it was utilized as additive to improve etch resistance and transparency at 193 nm of poly(t-butyl methacrylate) (PTBMA). To make a material that has more flexible core, $Buty-TBC_3$, and $ChButy-TBC_3$ were synthesized and it was successful to spin coat these materials. Both these molecular resist materials have good transparency at 193 nm and adhesion property. Using Hg-Xe lamp in a contact mode printing and diluted TMAH developer, patterns were delineated. New chemically amplified resist system based on the inclusion complex of β -cyclodextrin, t-BOC β -CD IC, also acted as molecular resist material, and sub-micron patterns were delineated with this material using KrF stepper and conventional 2.38 wt% developer. The silicon containing resist material, Si8TBC, was able to work in bilayer process as an imaging layer as well as single layer process.
반도체 선폭을 0.16 μm 이하로 낮추기 위하여 193-nm 리소그라피 기술은 빠르게 발전하고 있다. 그러나, 193-nm 리소그라피용 노광 장비에 알맞은 고성능 레지스트가 개발이 선행되어야 한다. 193-nm 리소그라피용 포토레지스트는 193 nm에서 적정한 투과도를 가져야 하며, 이미징 성질이 좋고, 접착력이 우수해야 한다.
Alicyclic polymer는 193 nm에서 적정한 투과도와 내에칭성을 가지고 있으나, 접착력이 떨어지는 단점이 있다. 이 문제점을 해결하기 위하여 히드록시기와 같은 친수성기가 도입된 폴리머를 개발하였으나, 이러한 폴리머는 리소그라피 과정에서 가교 반응을 일으켜 “foot” profile을 만드는 원인이 된다. 특히 히드록시기를 가진 단분자와 말레산 무수물이 공중합되었을 경우에 레지스트의 저장안정성에도 좋지 않은 영향을 미친다.
본 연구에서는 여러 가지 작용기를 가지는 폴리머들을 합성하여 리소그라피에서 어떤 작용기들이 가교 반응을 일으키는지 연구하였다. 2-(2-Methoxyethoxy)ethyl ester 기 혹은 2-acetoxyethyl 기를 도입한 폴리머는 접착성 뿐 아니라 저장안정성도 좋은 것으로 판명되었다. 2-(2-Methoxyethoxy)ethyl 5-norbornene-2-carboxylate (MENC)를 합성하여 t-butyl 5-norbornene-2-carboxylate (BNC), norbornene (NB), 그리고 maleic anhydride (MA)와 공중합한 폴리머를 합성하였다. 또한 2-Acetoxyethyl 5-norbornene-2-carboxylate (AENC)를 합성하여 BNC, NB, 그리고 MA와 공중합한 폴리머도 합성하였다. 이러한 폴리머들은 2-hydroxyethyl 5-norbornene-2-carboxylate (HNC)를 포함한 폴리머와 비교했을 때 대등한 접착 특성을 나타내었으며, 저장 안정성면에서 더 좋은 결과를 나타내었다. 리소그라피 평가를 하였을 때 ArF 엑시머 레이저를 노광원으로 하고 2.38 wt% 현상액을 사용하여 sub-micron 패턴을 얻을 수 있었다.
최근, 차세대 리소그라피는 이미지 형상의 크기가 분자 레벨로 작아지고 있기 때문에, 이러한 요구를 만족시키기 위하여는 레지스트 물질 자체가 새롭게 개발되어야 한다. 대부분의 상업적인 레지스트는 선형고분자를 사용하고 있으나, 저분자 물질을 사용하였을 경우에 다음과 같은 장점을 생각해 볼 수 있다. 빌딩 블록이 분자 단위로 작아지기 때문에 해상도의 한계가 분자 단위로 작아질 수 있다. 또한 저분자 물질은 현상 과정 중에 균일하게 용해되어 line edge roughness가 크게 완화된다. 저분자 물질은 분자 길이가 짧고, 회전 반경이 짧고, 특히 star-shape 구조일 경우에는 입체적으로 혼잡한 구조를 가지고 있기 때문에 사슬 엉킴과 같은 분자간 상호작용이 없기 때문이다.
본 연구에서는 1,3,5-cyclohexanetricarboxylic acid tri-t-butyl cholate ester $(Ch-TBC_3)$, pentanedioic acid 2-(4-t-butyl cholate oxycarbonylbutyryloxy)-1-(4-t-butyl cholate oxycarbonylbutyryloxymethyl)ethyl ester $(Buty-TBC_3)$, pentanedioic acid 3,5-bis(4-t-butyl cholate oxycarbonylbutyryloxy)cyclohexyl ester t-butyl cholate ester $(ChButy-TBC_3)$, 1-adamantanecarbolic acid를 게스트로 사용한 t-BOC β -cyclodextrin inclusion complex $(t-BOC \beta -CD IC)$, 그리고 octa(2-t-butyl cholate ethyl) octamethyl siloxane (Si8TBC)와 같은 저분자 레지스트 물질을 합성하였다. $Ch-TBC_3$ 는 비결정이지만 중심부가 너무 단단한 구조를 가지므로 기판 위에 코팅이 되지 않았다. 따라서 PTBMA의 내에칭성과 193 nm에서 투과도 향상을 위하여 첨가제로 사용하였다. 좀더 유연성이 있는 물질을 만들기 위하여 $Buty-TBC_3$ 와 $ChButy-TBC_3$ 를 합성하였다. 이 두 물질들은 193 nm에서 적정한 투과도와 좋은 접착성능을 나타내었다. 콘택트 프린팅 방식의 Hg-Xe 램프와 희석된 TMAH 현상액을 사용하여 패턴을 형성할 수 있었다. t-BOC b-CD IC는 KrF 스텝퍼와 2.38 wt% TMAH 현상액을 사용하여 sub-micron의 패턴을 형성할 수 있었다. 실리콘이 포함된 저분자 레지스트 물질인 Si8TBC는 단층 레지스트 물질로 뿐 아니라 이층 레지스트의 이미징층으로 사용하여 리소그라피 평가를 수행하였다.
