Preparation of a Novel Silicone-Based Positive Photoresist and Its

Chapter 11

Preparation of a Novel Silicone-Based Positive Photoresist and Its Application to an Image Reversal Process Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 16, 2016 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch011

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Akinobu Tanaka , Hiroshi Ban , and Saburo Imamura 1

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NTT LSI Laboratories, Morinosato, Atsugi-shi, Kanagawa 243-01, Japan NTT Basic Research Laboratories, Midoricho, Musashino-shi, Tokyo 180, Japan

We have developed a novel silicone-based positive photoresist (SPP) for two-layer resist systems. SPP is composed of an acetylated poly(phenylsilsesquioxane) (APSQ) and diazonaphthoquinone sensitizer. SPP can be developed with alkaline aqueous solutions, because the matrix resin, APSQ, is alkali-soluble due to the presence of silanol groups formed during synthesis of APSQ. SPP is useful not only for near UV lithography (positive mode), but also in negative mode using high energy sources for exposure. Negative process (image reversal) is capable of sub-halfmicron resolution using electron beam (EB), X-ray, and deep UV exposures. Resist sensitivities of SPP to EB, X-rays and deep UV are 5 μC/cm , 80-160 mJ/cm and 10 mJ/cm , respec tively. We suggest that a coupling of APSQ and the sen sitizer occurs during EB and X-ray exposures, but it is absent during near UV exposures. This coupling reaction and the generation of indenecarboxylic acid are compet ing processes during the deep UV exposures. 2

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Silicon-containing r e s i s t s have been proposed as top imaging l a y e r s i n two-layer r e s i s t (2LR) systems for high r e s o l u t i o n l i t h o g r a p h y . U As they have high resistance to oxygen reactive ion etching (0 RIE), fine patterns formed i n a very t h i n top r e s i s t can be transferred into a thick bottom organic polymer layer by 0 RIE. Recently, alkali-developable silicon-containing positive photoresists have a t t r a c t e d much a t t e n t i o n " ' due to t h e i r com p a t i b i l i t y with p r a c t i c a l VLSI fabrication processes using novolacdiazonaphthoquinone p o s i t i v e photoresists (AZ-type r e s i s t s ) . We have synthesized an acetylated poly(phenylsilsesquioxane) (APSQ), and prepared an a l k a l i - d e v e l o p a b l e s i l i c o n e - b a s e d positive photoresist (SPP) ' composed of APSQ and a diazonaphthoquinone compound as a photosensitizer for near UV lithography. 2

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0097-6156/89A)412-0175$06.00/0 ο 1989 American Chemical Society

Reichmanis et al.; Polymers in Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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POLYMERS IN MICROLITHOGRAPHY

However, the continuing drive towards minimization i n pattern sizes has created a demand for lithographic resolution higher than can be achieved with near UV lithography. Since one useful method for improving resolution is to use higher energy sources, we have a p p l i e d SPP to EB, X - r a y , deep UV l i t h o g r a p h y . Although SPP also exhibits a positive action when exposed to EB, X-rays and deep UV, the positive pattern requires a high dosage that i s not acceptable for a p r a c t i c a l use except for deep UV lithography. An image revers a l process of AZ-type r e s i s t s has been r e p o r t e d " ' to have several advantages such as improvement i n resolution, s e n s i t i v i t y , and thermal s t a b i l i t y . We have also found that an image reversal of SPP dramatically increases the s e n s i t i v i t y . Therefore, we have app l i e d SPP to an image r e v e r s a l process using high energy sources such as EB, X-rays and deep UV. This paper describes the preparation of SPP and i t s a p p l i c a tion to an image reversal process, as well as the chemistry of the SPP image r e v e r s a l .

Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 16, 2016 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch011

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Synthesis and characterization of APSQ APSQ was synthesized by acetylation of PSQ in the presence of a F r i e d e l - C r a f t s catalyst. A solution of poly(phenylsilsesquioxane) (PSQ) i n acetyl chloride (AcCl) was reacted with a solution of anhydrous A I C I 3 i n AcCl below 20°C. After s t i r r i n g for 90 min, the solution was poured into ice water to obtain APSQ. The details of this process are described e l s e w h e r e . ' An attempt at conventional elemental analysis for APSQ f a i l e d because s i l i c o n c a r b i d e was i r r e g u l a r l y produced d u r i n g the measurement. Therefore, the molecular structure of APSQ was determined by NMR and IR. The IR data i n d i c a t e that APSQ molecular s t r u c t u r e i s fundamentally s i m i l a r to PSQ except the a d d i t i o n of a c e t y l (1650 cm" ) and hydroxy (3400 cm" ) groups. S i NMR spectra are shown i n Figure 1 together with assignments of Si chemical s h i f t s . ' PSQ purchased from Petrarch Systems Inc. and O w e n s - I l l i n o i s Co. were used. Although S i NMR spectra of these PSQ s d i f f e r , the APSQ s obtained from them have almost the same spectra. The t o t a l reactions are described in equation 1. 11

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2 9

2 9

1 2

2 9

T

f

(1)

R'=C H , C H C 0 C H , OH 6

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An i n t e r e s t i n g issue i s the simultaneous i n t r o d u c t i o n of s i l a n o l groups during the a c e t y l a t i o n of phenyl groups. As i n d i cated by S i NMR, some of the Si-phenyl bonds and framework siloxane bonds in PSQ are cleaved and chlorinated in the presence of Lewis a c i d s . S i - C bonds have r e l a t i v e l y low resistance to e l e c t r o p h i l i c attack and can be substituted by S i - C l bonds in the presence of Lewis acids. **' Although framework siloxane bonds are r e l a t i v e l y strong and stable, they also undergo c h l o r i n a t i o n . The 2 9

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11. TANAKAETAL.

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Novel Silicone-Based Positive Photoresist

S i - C l groups produced are hydrolyzed by water to Si-OH groups during work-up. APSQ is soluble in a dilute aqueous solution of tetramethylammonium hydroxide (TMAH). When APSQ was treated with t r i m e t h y l s i l y l c h l o r i d e (TMSC1), the s o l u b i l i t y of APSQ i n the TMAH s o l u t i o n decreased, because s i l a n o l groups were terminated with TMS groups. This indicates that APSQ is a l k a l i - s o l u b l e due to the presence of s i l a n o l groups. The s o l u b i l i t y depended on the s i l a n o l content, which can be controlled by synthesis conditions or appropriate t e r mination of s i l a n o l groups. APSQ obtained from Owens-Illinois PSQ was more soluble in TMAH solutions than that from Petrarch Systems, probably due to lower molecular weight (Mw). We used the former (Mw = 1,500) in this study. Preparation of SPP and Application to near UV lithography SPP was prepared by d i s s o l v i n g a novolac r e s i n diazonaph thoquinone sulfonyl ester (DNQ) and APSQ i n methyl isobutyl ketone. A concentration of 20 w U DNQ r e l a t i v e to APSQ i s s u f f i c i e n t for i t to act as a dissolution i n h i b i t o r . Figure 2 shows an SEM photograph of a 0.4 #m l i n e and space pattern on a substrate with topographic features using the SPP 2LR system. A 0.2 /zm t h i c k SPP layer was spun onto a 1.5 #m t h i c k bottom p l a n a r i z i n g l a y e r . The r e s i s t was exposed with a g - l i n e (436nm) stepper equipped with a high numerical-aperture reduction lens (NA=0.6) and then dip-developed i n a 1.6 wt% TMAH aqueous solution for 60 s at 25 °C. The pattern formed i n the SPP layer was transferred to the bottom layer by 0 RIE. The 0 RIE etching rate of SPP was less than 3.5 nm/min, whereas that of the bottom layer was more than 90 nm/min. The s e l e c t i v i t y of SPP to the bottom layer was more than 26. 2

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Application to image reversal process a)Electron beam lithography A 0.3 fim thick SPP layer was exposed to a 20 kV electron beam followed by a flood exposure using near UV r a d i a t i o n with an i n tegrated dose of over 500 mJ/cm . Such a dose was s u f f i c i e n t to convert the remaining DNQ to indenecarboxylic a c i d . The r e s i s t was then dip-developed in an aqueous TMAH solution for 60 s at 25°C. Figure 3 shows the s e n s i t i v i t y curves for SPP ( s o l i d l i n e s ) compared with that of a novolac-based r e s i s t (dashed l i n e ) . From these curves, we obtained the maximum c l e a r i n g dose (DQ), the dose for 50% thickness remaining ( D ) , and lithographic contrast (γvalue). These r e s i s t characteristics are summarized in Table I . 2

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Table I Characteristics of SPP and novolac-based Resist

TMAH ) (wt%) (uc/ cm ) SPP 0.65 0 .9 SPP 0.70 5 .0 SPP 0.80 13 .5 Novolac-based 1.20 6 .2 * 7 -value = l/2(log(D / D ) )

D

50 ο (/zC/cm ) 1.7 6.6 16.5 14.5

r-value *

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resist

1.8 4.1 5.7 1.4

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Ô -20 -40 -60 -80 -100 -120 -140 Chemical Shift (ppm) Figure 1. 39.7 MHz ^Si NMR spectra of PSQ and APSQ obtained from PSQ-B in acetone-^ . Chromium acetylacetonate was used as a relaxation reagent, and transients were 5000. PSQ-A ( M = 900) and PSQ-B (M = 9500) were purchased from OwensIllinois and Petrarch Systems, respectively. 6

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w

Figure 2. SEM photograph of 0.4-μπι line-and-space pattern on a substrate with topographic features using SPP two-layer resist system. A 0.2-pm-thick SPP layer was exposed with a g-line stepper (NA = 0.6) at 350 mJ/cm and then dip-developed in a 1.6 wt% TMAH solution for 60 s at 25 * C. The pattern formed in the SPP top layer was transferred to 1.5-pm-thick bottom layer by 0 RIE. 2

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11. TANAKAET AL»

Novel Silicone-Based Positive Photoresist

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A higher s e n s i t i v i t y of SPP can be obtained using a more d i l u t e TMAH solution, but at the expense of lower contrast. A solu t i o n more d i l u t e than 0.6 wt% cannot completely d i s s o l v e the r e s i s t . The SPP exhibited a higher s e n s i t i v i t y and contrast than the novolac-based r e s i s t . Figure 4 shows an SEM photograph of 0.3 μ m l i n e and 0.5 μ m space pattern delineated in an SPP 2LR system with a dose of 5 // C / c m . The combination of t h i s SPP image r e v e r s a l process and EB direct wafer-writing technology represents a promising approach for achieving sub-halfmicron resolution. Downloaded by UNIV OF TENNESSEE KNOXVILLE on March 16, 2016 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch011

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b) X-ray lithography A 0.4 /zm thick SPP layer was exposed to X-rays followed by a f l o o d exposure using near UV r a d i a t i o n . The r e s i s t was then d i p developed in a 0.8 w U TMAH solution for 60 s at 25 °C. We used two x-ray exposure systems to evaluate the characteristics of the SPP r e s i s t . One i s S R - 1 ' which has a source composed of a molybdenum rotating anode with a 0.54 nm Mo-La c h a r a c t e r i s t i c l i n e . The ex posure was carried out i n a i r . The other has a synchrotron r a d i a tion source with a central wavelength of 0.7 nm (KEK Photon Factory Beam L i n e , BL-1B). The exposure was c a r r i e d out i n vacuum (

Preparation of a Novel Silicone-Based Positive Photoresist and Its

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