The present invention discloses a maleimide-alt-trialkylsilylstyrene copolymer having the formula: ##STR1## wherein: R.sub.1 is H or methyl; PA1 R.sub.2 is C.sub.1 -C.sub.3 alkyl; and PA1 n is an integer such that the copolymer has a weight average molecular weight of 25,000-120,000. The copolymer of formula (I) is useful in the preparation of a photoresist composition. A process for preparing the copolymer of formula (I) is also disclosed.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to new and useful Si-containing copolymers 
and photoresist compositions containing same, in particular, to a 
maleimide/silylstrene copolymer having a structure of alternating 
maleimide and silylstyrene units. 
BACKGROUND OF THE INVENTION 
A typical positive photoresist is a generally two-component material 
consisting essentially of an alkaline soluble polymer and a 
radiation-sensitive dissolution inhibitor such as diazonaphthoquinone 
sulfonate (DNS), wherein the alkaline soluble polymer is rendered 
insoluble in aqueous alkaline solutions through addition of the 
radiation-sensitive dissolution inhibitor. 
The most widely used positive photoresist is a novolac resin (a 
condensation polymer of formaldehyde and phenol) containing DNS. To date, 
several advanced IC manufactures require images that are stable up to 
200.degree. C. or even higher. The novolac-based photoresists are unable 
to perform very well in this temperature range. Thus, some other phenolic 
polymers have been developed to serve as replacements for the conventional 
novolace-based photoresists. Example of these phenolic polymers include 
poly(p-hydroxystyrene) disclosed in U.S. Pat. Nos. 4,139,384 and 
4,439,516; poly(p-hydroxy-.alpha.-methylstyrene) disclosed by H. Ito, et 
al, Macromolecules, 16, 513 (1983); and copolymers of 
N-(4-hydroxyphenyl)maleimide and various olefins disclosed by S. R. 
Turner, et al, Polym. Eng. Sci., 26, 1096 (1986). In general the alkaline 
soluble polymers contain carboxylic acids or maleimides as well as 
phenolic units. For example, poly(p-benzoic acid), disclosed by H. Ito, et 
al in "Advances in Resist Technology and Processing IV", SPIE, 771, 24 
(1987), and maleimide/styrene copolymer, disclosed by C. E. Osuch, et al 
in "Advances in Resist Technology and Processing III", SPIE, 531, 68 
(1986), have been employed to provide positive imaging of high temperature 
polymers. In addition, a maleimide/allyltrimethylsilane copolymer, 
disclosed by R. Sezi, et al, Polym. Eng, Sci., 29, 891 (1989), was used as 
the top imaging layer in a bilayer resist process because of its excellent 
durability during oxygen-plasma etching. 
G. N. Taylor, et al (Solid State Technol., 27, 145 (1984) reported that 
small amounts of silicon (about 10 wt %) can drastically lower the 
oxygen-plasma etching rate of organic polymers. 
We, in an article entitled "Preparation and Properties of Si-Containing 
Copolymer for Near-UV Resist. I. 
Poly(N-(4-hydroxyphenyl)maleimide-alt-p-trimethylsilylstyrene 3", J. 
Polym. Sci., part A., 29, 399 (1991), and in a pending U.S. application 
Ser. No. 07/781,616, filed Oct. 23, 1991, disclosed a method to synthesize 
poly(N-(4-hydroxyphenyl)maleimide-alt-p-trimethylsilylstyrene) (PHTMSS). 
The co-inventors of the present invention also, in an article entitled 
"Preparation and properties of silicon-Containing copolymers for Near-UV 
Resist II", Die Angewandte Makromolekulare Chemie, 205, 75-90 (1993), 
disclosed a method to synthesize 
poly(N-(4-hydroxyphenyl)maleimide-alt-p-trimethylsilyl-.alpha.-methylstyre 
ne) (.alpha.-PHTMMS), and apply them as the top imaging layers. In the 
.alpha.-PHTMMS copolymer so disclosed, the N-(4-hydroxyphenyl)maleimide, 
which acted as a highly thermally stable unit, and the p-silylstyrene, 
which acted as an oxygen-plasma etching resistent unit, were alternatingly 
copolymerized. However, in both disclosures the silicon content of the 
PHTMSS and a .alpha.-PHTMMS copolymer is about 7 wt %. 
As a result of an extensive investigation to develop a photoresist which is 
more thermally stable and more resistant to oxygen-plasma etching than 
prior art photoresists, a series of novel maleimide/silylstyrene 
copolymers have been newly synthesized in the present invention. 
Accordingly, one object of the present invention is to provide new and 
useful maleimide/silylstyrene copolymers. 
Another object of the present invention is to provide photoresist 
compositions containing the new maleimide/silylstyrene copolymers. 
A further object of the present invention is to provide a process for 
preparing the new maleimide/silylstyrene copolymers. 
SUMMARY OF THE INVENTION 
In order to achieve the above-mentioned objects, a 
maleimide-alt-trialkylsilylstyrene copolymer having the following formula 
is disclosed: 
##STR2## 
wherein: R.sub.1 is H or methyl; 
R.sub.2 is C.sub.1 -C.sub.3 alkyl, preferably, methyl; and 
n is an integer such that the copolymer has a weight average molecular 
weight of 25,000-120,000, preferably 30,000-40,000. 
In the synthesis of the copolymer of formula (I), 
p-trialkylsilyl(-.alpha.-methyl)styrene is used to replace the 
allyltrimethylsilane unit of the maleimide/allyltrimethylsilane copolymer 
disclosed by R. Sezi, et al to enhance the thermal stability. For 
increasing the silicon content of the copolymer (I), 
N-(4-hydroxyphenyl)maleimide units of PHTMSS and .alpha.-PHTMMS copolymer 
disclosed in our pending U.S. Ser. No. 07/781,616 patent application and 
Die Angewandte Makromolekulare Chemie, 205, 75-90 (1993) are replaced by 
maleimide. 
A photoresist composition is also prepared in the present invention 
comprising 20:1:100 to 10:10:100 weight ratio of the copolymer of formula 
(I), diazonaphthoquinone sulfonate, and a solvent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is related to a maleimide-alt-trialkylsilylstyrene 
copolymer having the formula: 
##STR3## 
wherein: R.sub.1 is H or methyl; 
R.sub.2 is C.sub.1 -C.sub.3 alkyl, preferably, methyl; and 
n is an integer such that the copolymer has a weight average molecular 
weight of 25,000-120,000. 
Preferably, the copolymer of formula (1) has a weight average molecular 
weight of 30,000-40,000, and a silicon content of about 10 wt %. 
The chain-stiffening effects of the maleimide and the styrene group of the 
copolymer of formula (I) are responsible for high thermal stabilities, and 
the silicon content of about 10 wt % thereof renders the copolymer of 
formula (I) more resistant to an oxygen-plasma etching process. 
A process suitable for preparing the copolymer of formula (I) is by 
solution free-radical copolymerization, which comprises the steps of 
reacting p-trialkylsilyl(-.alpha.-methyl)styrene monomer with equal moles 
of maleimide monomer in an anhydrous solvent and in the presence of a 
thermal initiator. An example of suitable anhydrous solvent to be used in 
the reaction is dry tetrahydrofuran (THF). In general, many conventionally 
used azo-type initiators, such as 2,2'-azobisisobutyrontirile (AIBN), can 
be used as the thermal initiator in the reaction. The reaction can be 
represented by the formula as follows: 
##STR4## 
wherein R.sub.1 and R.sub.2 have the same definitions as in formula (I). 
The reaction product is isolated by precipitation of the viscous polymer 
solution into ether, instead of methanol, which was used in our pending 
U.S. Ser. No. 07/781,616 patent application. Because maleimide monomer 
shows good solubility in methanol (1:20), the hot viscous polymerization 
solution is readily soluble in methanol. On the other hand, 
polysilylstyrene is easily soluble in ether. For this reason, ether is 
used not only as the precipitator but also as the purification agent of 
the silicon-containing copolymer of formula (I). 
The p-trialkylsilyl(-.alpha.-methyl)styrene used in reaction (II) can be 
prepared by reacting p-chloro(-.alpha.-methyl)styrene with magnesium under 
anhydrous conditions in a donor solvent to form a Grignard reagent, and 
then adding chlorotrialkylsilane to react with the Grignard reagent. A 
typical reaction for preparing p-trialkylsilyl(-.alpha.-methyl)styrene can 
be represented by the formula: 
##STR5## 
wherein R.sub.1 and R.sub.2 are defined as in formula (I). 
In reaction (II), the electron-rich styreneic monomers tend to undergo 
alternating copolymerization with electron-poor maleimide monomer. This 
classic behavior is well documented for similar systems, such as 
maleimide/styrene, maleimide/allyltrimethylsilane, and 
N-(4-hydroxyphenyl)maleimide/p-trimethylsilylstyrene copolymers described 
in the previous section on Background of the Invention. Because the 
prepared copolymers have alternating structure units, sufficient alkaline 
solubilities are obtained at 50 mol % maleimide units. Although Y. 
Ohnishi, et al in "Advances in Resist Technology and Processing II", 539, 
62 (1985) report that the solubility of novolac resin in a base solution 
decreased greatly by introducing trialkylsilyl group, the copolymer of 
formula (I), which is prepared by introducing trialkylsilyl group into 
maleimide/(.alpha.-methyl) styrene copolymer, is readily soluble in 
aqueous base solutions such as aqueous sodium hydroxide or 
tetramethylammonium hydroxide (TMAH). In addition, the copolymer of 
formula (I) shows good solubility in polar solvents such as 
dimethylsulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), dimethylformamide 
(DMF), dimethylacetamide (DMAc), pyridine, THF and acetone. 
The copolymer of formula (I) is useful in preparing a photoresist coating 
composition. A suitable preparation process comprises the steps of 
dissolving 10-20 parts by weight of the copolymer, and 1-10 parts by 
weight of diazonaphthoquinone sulfonate (DNS) as an alkali insoluable 
sensitizer, in 100 parts by weight of a solvent. Ordinarily, after being 
exposed to radiation, the DNS would bring about carbene formation. Through 
the Wolff rearrangement, ketene was formed and then reacted with water to 
form a base soluble indenecarboxylic acid that no longer inhibited 
dissolution of the copolymer in an aqueous alkaline solution wash, thereby 
facilitating development of a positive image. 
In the two-component positive photoresist composition, the copolymers are 
applied as binders for DNS photochemical reaction, positive images should 
be obtained after the radiation exposure and alkaline bath. However, a 
negative image is obtained when the copolymer of formula (I) used in the 
photoresist composition is maleimide-alt-trialkylsilylstyrene, i.e. 
R.sub.1 is H. This unusual phenomenon has also been reported in our 
previous articles entitled "Preparation and Properties of Si-Containing 
Copolymer for Near-UV Resist. I. 
Poly(N-(4-hydroxyphenyl)maleimide-alt-p-trimethylsilylstyrene", J. Polym. 
Sci., part A., 29, 399 (1991) and "Preparation and properties of 
silicon-containing copolymers for Near-UV Resist II" Die Angewandte 
Makromolekulare Chemie, 205, 75-90 (1993), details thereof are 
incorporated by reference. 
Suitable solvents for using in the photoresist coating composition include, 
but not are limited to, acetone, THF, toluene, xylene, ethyl 
2-ethoxyacetate, ethyl 2-methoxyacetate and a mixture of any two solvents 
thereof. 
Particular DNS suitable to be used in the photoresist coating composition 
are: 
##STR6## 
In the following examples two compounds of formula (I) are synthesized and 
identified, and the properties of the photoresist compositions containing 
these two compounds are also examined. These examples are only meant to 
illustrate the present invention not for limiting the scope thereof. 
EXAMPLE 1 
a) synthesis of para-trimethylsilyl-.alpha.-methylstyrene (.alpha.-TMMS) 
Placing 2 g (83 mmol) of dry magnesium into a 250 mL flask equipped with a 
condenser, a dropping funnel and a magnetic stirrer, the reaction was 
carried out in a nitrogen atmosphere. The magnesium was initially 
activated by adding 1 mL of p-chloro-methylstyrene (Aldrich, Germany) with 
0.5 g iodine in 5 mL THF at 50.degree. C. for a period of 30 minutes. Into 
the mixture a solution of 11 g (72 mmol) of p-chloro-.alpha.-methylstyrene 
in 50 mL THF was dropped over a period of 12 hours under reflux. The 
reaction mixture was cooled to 40.degree. C., then 7.8 g (72 mmol) of 
chlorotrimethylsilane (Fluka, Japan) in 30 mL of THF were added dropwise 
and reacted for 24 hours. After the reaction was completed, a large amount 
of water was added and the reaction mixture was extracted with ether. The 
ether layer was dried over magnesium sulfate, and distilled under reduced 
pressure, bp 66.degree.-67.degree. C./2 mmHg. Yield, 56%. The colorless 
liquid crude product was purified by column chromatography, which was 
performed on silica gel using n-hexane as an eluent. The product was 
identified by IR and .sup.1 H-NMR spectroscopy. 
b) synthesis of para-trimethylsilylstyrene (TMSS) 
The procedures of a) were repeated except that the crude product was 
obtained by distillation at bp 84.degree. C./4.2 mmHg. Yield, 38%. 
EXAMPLE 2 
a) synthesis of poly(maleimide-alt-p-trimethylsilyl-.alpha.-methylstyrene 
(.alpha.-PMTMMS) 
Under a nitrogen atmosphere, 50 mL polymerization ampoule was charged with 
0.97 g (0.01 mole) of maleimide (Tokyo Kasei Kogyo co., Japan), 1.90 g 
(0.01 mole) of .alpha.-TMMS, 15 mL of dry THF and 0.02 g of 
azobisisobutyronitrile (AIBN) (Wako, Japan). The ampule was quenched by a 
dry ice-acetone and sealed. Copolymerization was carried out in a 
60.degree. C. bath for 24 hours. The copolymer so obtained was isolated by 
precipitation of the viscous polymerization solution with ether. After 
drying under vacuum at 60.degree. C. for 24 hours., 85 wt % of copolymer 
was obtained. Viscosity, measured in THF at 30.degree. C. with Ubbelohde 
viscometer, gave intrinsic viscosity [.eta.]=0.85. GPC analysis in THF 
gave Mw=37,800; Mn=19,300, and the calculated polydispersity=1.96. 
Tg=230.degree. C., melting point=312.degree. C. 
Anal. Calcd. for C.sub.16 H.sub.21 NO.sub.2 Si (1:1 structure): C, 66.90%; 
H, 7.32%; N, 4.88%. Found: C, 66.85%; H, 7.34%; N, 4.84%. 
b) synthesis of poly(maleimide-alt-p-trimethylsilylstyrene (PMTMSS) 
The procedures of a) synthesis of .alpha.-PMTMMS were repeated to obtain 
PMTMSS. Yield, 92%. The intrinsic viscosity [.eta.]=0.55, measured in THF 
at 30.degree. C. with Ubbelohde viscometer. GPC analysis in THF gave 
Mw=31,800; Mn=18,100, and calculated polydispersity=1.71. Tg=217.degree. 
C., melting point=275.degree. C. 
Anal. Calcd. for C.sub.15 H.sub.19 NO.sub.2 Si (1:1 structure): C, 65.93%; 
H, 6.96%; N, 5.13%. Found: C, 65.81%; H, 6.93%; N, 5.12%. 
EXAMPLE 3: PHOTORESIST PROCESSING AND PROPERTIES OF THE DEVELOPED IMAGES 
Resist solutions were prepared by dissolving 5 wt % of the prepared 
copolymer, 3 wt % of benzophenone-tridiazonaphoquinone sulfonate 
(BP-t-DNS) as an alkali insoluble sensitizer, in 92 wt % of THF. The 
solution were filtered through a 0.5 .mu.m Millipore filter. PH-1 and PH-2 
represent the resist solutions prepared by using .alpha.-PMTMMS and PMTMSS 
as the copolymer respectively. 
Resists were spin-coated (4000 rpm) on a 4 inch silicon wafers to yield 
1.2-1.6 .mu.m film thickness using a spinner (Silicon Valley Group Inc., 
model SVG-8026). The resist films were prebaked in a conventional oven at 
50.degree. C. for 30 min. 
Near UV exposures (350-450 nm) were carried out using a Canon Contact 
Aligner PLA-501F. After exposure, the wafers were soaked in 5 wt % 
tetramethylammonium hydroxide (HUNT Co., U.S.) for 12 seconds with an 
agitation developer (SVG-8132) to develop positive images, or continuously 
rinsed in THF for 15 seconds to develop negative images. The film 
thickness was measured with a TENCO .alpha.-step 200 instrument. The 
exposure response curves of resists are shown in FIG. 1, which shows that 
the normalized film thickness of PH-1 resist decrease with increase of the 
dose that is a positive-type resist (sensitivity: 222.6 mJ/cm.sup.2). 
Conversely, PH-2 resist is a negative-type resist (sensitivity: 47.7 
mJ/cm.sup.2). 
The positive and negative images developed with 220 and 47 mJ/cm.sup.2 
dosages, respectively, were evaluated by the adhesion test and found that 
neither the positive nor the negative images were stripped after the 
adhesion test. This advantage is attributed to the silicon proton on the 
phenyl group which provides good adhesion to the silicon wafer. In 
addition, the developed positive images of PH-1 were postbaked in a 
conventional oven at 220.degree. C. for 30 min and found highly resistant 
to thermal deformation; the developed negative images of PH-2 were 
postbaked in a conventional oven at 300.degree. C. for 30 min and found 
highly resistant to thermal deformation, wherein the patterns of the 
developed positive and negative images were not deformed by the post bake. 
The oxygen plasma etching behavior of the resists prepared by 
.alpha.-PMTMMS, PMTMSS, and several other polymers was determined and 
shown in Table 1. The resists were prepared and developed as above, and 
then hard baked for 30 min. The oxygen plasma condition had an RF level 
power of 1300 W, and chamber pressure of 0.4 torr. 
TABLE 1 
______________________________________ 
Slicon 
content Etching rate 
Etching 
polymer wt % Mw .ANG./min. 
selectivity 
______________________________________ 
.alpha.-PHTMMS 
7.4 48,000 70 1:6 
PHTMSS 7.7 126,000 60 1:7 
.alpha.-PMTMMS 
9.8 37,800 40 1:10.5 
PMTMSS 10.3 31,000 35 1:12 
HPR-204* 
-- -- 420 1:1 
______________________________________ 
*HPR-204 is a positivetype resist and available from HUNT Co., U.S. 
It can be seen from the data in Table 1 that the etching selectivity 1:6 of 
.alpha.-PHTMMS containing 7.3 wt % silicon is increased to 1:12 of PMTMSS 
containing 10.3 wt % silicon. The etching selectivity of 1:12 good enough 
for PMTMSS to be used as a top imaging layer for transferring patterns to 
a bottom HPR-204 layer in a bilayer resist system.