Chapter 9
Synthesis and Polymerizations of N-(tertButoxy)maleimide and Application of Its Polymers as a Chemical Amplification Resist Kwang-Duk Ahn and Deok-Il Koo Polymer Chemistry Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea N-(tert-Butoxy)maleimide, t-BuOMI was synthesized as a new kind of a protected acid-labile monomer. Its radical copolymerizations were performed and the thermal deprotection behavior of its copolymers were investigated. t-BuOMI was readily copolymerized with styrene derivatives (X-St) to give copolymers, P(t-BuOMI/X-St) in high conversions. The t-BuOMI units of the tert-butyl (t-Bu) protected copolymers were converted into the N-hydroxymaleimide (HOMI) units by heating at about 280 °C releasing 2-methylpropene and the facile deprotection of the side-chain t-Bu groups resulted in a large polarity change in the polymer structure. The deprotected copolymers, P(HOMI/X-St) have very high glass transition temperatures higher than about 250 °C and showed solubilities in aqueous base solutions whereas the protected polymers are soluble only in organic solvents. Acidolytic deprotection of P(t-BuOMI/St) was observed at 130 °C or lower temperatures in the presence of catalytic acids. Resist solutions of P(t-BuOMI/St) containing triphenylsulfonium salts as a photoacid generator were prepared and the films were imagewise exposed to 260 nm light by a contact mode. The films were post-exposure baked at 130 °C and were followed by development with 2.38 wt% TMAH solution to obtain positive tone images.
The design and synthesis of protected polymers having acid-labile groups are considered to be of challenging research for obtaining imageable polymers which can serve high sensitivity resist materials for fabricating microelectronic devices (i). Adequately protected polymers bring a remarkable change in polarity, thereof considerable differentiation in solubilities, when the side-chain protecting groups of polymers are deprotected by thermolysis or acidolysis. Those protected polymers are best suited for the application as chemically amplified resists to achieve high sensitivity (2). A typical acid-labile protecting group isterf-butoxycarbonyl(t-BOC) group which is known to be readily deprotected in the presence of catalytic acids. Poly[p-(r-butoxycarbonyloxy)styrene] has been investigated in detail as a prototype t-BOC protected polymer and successfully applied in the manufacture of VLSI devices (3, 4). In this case, the acid-labile t-BOC group is utilized to
0097-6156/94/0537-0124$06.00/0 © 1994 American Chemical Society
9. A H N AND K O O
Synthesis of N-(tert'Butoxy)maleimide
125
protect the phenol functionality of poly( /^hydroxystyrene) which is then regenerated to give a large polarity change after deprotection of t-BOC side-chains. One of required properties placed on resist polymers is thermal stability of patterned resist images for the use in advanced lithographic processes such as plasma etching and ion implantation. Thus the resist polymers which have high glass transition temperature (T ) above 200 °C have been searched. One of common synthetic techniques to improve thermal stability of resist polymers is the incorporation of a maleimide unit into copolymers (5, 6). Recently we reported the synthesis and polymerizations of a new t-BOC protected maleimide monomer, 7V-(te/t-butyloxycarbonyl)maleimide, t-BOCMI (7). t-BOC protected maleimide copolymers were found to be applicable as deep U V resist materials having superior thermal stability along with high sensitivity by chemical amplification (5). Upon deprotection of the t-BOC groups of the copolymers, the deprotected maleimide copolymers exhibit very high thermal stability with T s of about 250 °C because of the maleimide (MI) backbone structure. The abovementioned t-BOC protected polymers are readily deprotected by loss of carbon dioxide and isobutylene to corresponding polymers with phenol or maleimide functionality, thermally at about 150 to 180 °C and acidolytically at about 100 °C in the presence of catalytic acids. The previously mentioned t-BOC protected polymers are considered to have rather low thermal deprotection temperatures at about 150 to 180 °C for practical application (9). Therefore our objective was to make acid-labile polymers based on MI backbone structures which have protecting groups rather than t-BOC units to obtain much higher deprotection temperatures. Now we report the synthesis and polymerizations of AMterf-butoxy)maleimide (t-BuOMI) as a novel protected maleimide monomer while thermal deprotection of its copolymers occurs at higher than 200 °C. Characteristic properties of t-BuOMI copolymers along with the capability of facile deprotection to iV-hydroxymaleimide (HOMI) structure at higher temperatures were investigated and resist applicaton of the copolymers in the deep U V region are also discussed. g
g
EXPERIMENTAL Materials and Instruments Furane, JV-hydroxymaleimide (HOMI), styrene (St), p-methylstyrene (MeSt), and /?-chlorostyrene (CISt) were purchased from Aldrich Chemical Co. Maleic anhydride and hydroxylamine hydrochloride were obtained from Kanto Chemical Co. /7-Trimethylsilylstyrene (SiSt), p-(fe/f-butoxycarbonyloxy)styrene (tBOCSt), and p-acetoxystyrene (AcOSt) were kindly donated by Korea Kumho Petrochemical Co., Eastman Kodak Co., and Hoechst Celanese Corp., respectively. The styrene monomers (X-St) and solvents were purified by distillation with standard procedures. Other chemicals were purified by conventional methods. The radical initiators, N,N'-azobis(isobutyronitrile) (AIBN) and dicumyl peroxide (DCP) were purchased from Aldrich and used after recrystallization. As photoacid generators (PAG), triphenyl sulfonium hexafluoroantimonate and
126
POLYMERS FOR MICROELECTRONICS
triflate were prepared according to the known procedures or obtained by kind donation. The resist solvent, cyclohexanone was purchased from Aldrich and used without purification. The aqueous base developer, 2.38 wt% tetramethylammonium hydroxide (TMAH) solution was kindly donated from Hoechst Korea Ltd. and Tokyo Ohka Co. Proton N M R spectra were taken on a JEOL Model PMX-60 SI (60MHz) spectrometer or a Varian Gemini 300MHz spectrometer in deuteriochloroform using TMS as an internal standard. Carbon-13 N M R spectra were also obtained with a Varian Gemini spectrometer in deuteriochloroform. Infrared spectra were recorded on an Polaris FT-IR spectrophotometer of Mattson Instrument Co. and U V absorption spectra with a Shimadzu UV-240 spectrophotometer, and mass spectra with a JEOL JMS-DX 303 spectrometer. Elemental analysis was done with a Perkin-Elmer Model 240C elemental analyzer. Thermal analysis was carried out on a Du Pont Model 910 DSC and Model 951 T G A at a heating rate of 10 °C per min under nitrogen atmosphere. Solution viscosities of polymers were measured with a Cannon-Fenske viscosity tube (No. 50) or an Ubbelohde viscometer tube mounted to an automatic measuring apparatus of Schott-Gerate G M B H at 25 °C in THF as a solvent. Preparation of JV-(ferf-Butoxy) maleimide, t-BuOMI The furan/JV-hydroxymaleimide (furan/HOMI) adduct i . e., N-hydroxy3,6-epoxy-l,2,3,6-tetrahydrophthalimide, was prepared by a reaction of 3,6epoxy-l,2,3,6-terahydrophthalic anhydride and hydroxylamine in a high yield of 81% according to the known procedure (10). The melting point of the adduct is 185 °C [lit., (10) 187 °C]. To a solution of the furan/HOMI adduct (80.00 g, 0.44 mol) in 500 ml of methylene chloride in a pressure reactor added were 1.0 ml of concentrated sulfuric acid and 200 ml of isobutylene, and the mixture was stirred for 8 days at 110 °C. After finishing the reaction, the insoluble impurities were filtered off and the filtrate was washed with distilled water to remove acid residue, and the volatiles were evaporated under reduced pressure. The yellow powdery JV-terf-butoxy adduct (furan/t-BuOMI), i.e., N-(terf-butoxy)-3,6-epoxy1,2,3,6-tetrahydrophthalimide was obtained in a yield of 82.2% (86.10 g) with mp of 145 °C and used for the next procedure. furan/t-BuOMI: H N M R (60MHz, CDC1 ) (ppm) 1.30 (s, 9H, t-Bu), 2.70 (s, 2H, 2 -CO-CH-), 5.20 (s, 2H, 2 -O-CH-), 6.30 (s, 2H, 2 = CH-); IR (KBr) (cm ) 2990 (t-Bu), 1790 and 1720 (imide), 1370 (t-Bu), 1190 and 1150 (ether). In a sublimation apparatus 10.44 g of the furan/t-BuOMI adduct was placed and pyrolyzed at 140-150 °C for l h under reduced pressure. The solid product was obtained as a white crystal by sublimation in a yield of 74.6% (5.55 g). Recrystallization from a solution of methylene chloride and n-hexane (1:10 by vol) gave 4.26 g (yield 57.3%) of the desired monomer t-BuOMI in needle crystal with mp 92 °C. t-BuOMI: H N M R (60MHz, CDC1 ) (ppm) 1.30 (s, 9H, t-Bu), 6.56 (s, 2H, 2 = CH-); IR (KBr) (cm ) 3100 (olefinic CH), 2980 (t-Bu), 1730 (imide), 1370 (t-Bu), 1150 (ether); C N M R (300MHz, CDC1 ) (ppm) 27.11 (Me), 85.73 (t-Bu), 132.35 (C = C), 167.85 (carbonyl of MI); MS 169.40 (M), 154 !
3
1
!
3
1
1 3
3
9. A H N AND K O O
Synthesis of N-(ttrt-Butoxy)maleimide
127
(M - C H , 3), 112 (1), 57 (2-methyl propene, 100). Anal. Calcd for C H N 0 : C, 56.79; H , 6.55; N , 8.28. Found: C, 56.60; H , 6.53; N , 8.14. 3
8
n
3
Polymerization All the polymerizations were carried out in ampoules sealed after freezethaw cycles. The t-BuOMI monomer and the styrene comonomers (X-St) were copolymerized in 1 to 1 molar feed ratio in dioxane using AIBN as a radical initiator. The radical polymerizations were conducted under conditions described in Table I. The copolymers were obtained by precipitating into methanol, whereas the homopolymers P(t-BuOMI) were precipitated into a methanol-water solution. The structure of obtained polymers was fully characterized by spectroscopy and the thermal deprotection of the side-chain f-butyl groups was investigated by T G A and DSC analysis. As a representative (^polymerization, t-BuOMI and styrene was copolymerized according to the following procedure to obtain P(t-BuOMI/St). To a pyrex ampoule placed were 3.38 g (0.02 mol) of t-BuOMI, 2.09 g (0.02 mol) of styrene and 131.3 mg (0.80 mmol, 2 mol%) of AIBN in 5.50 ml of dioxane. The copolymerization was proceeded for 3h at 55 °C and instantaneous polymer formation was observed at the initial stage. The product was diluted with dioxane and precipitated into 2 L of methanol. The white powdery alternating copolymer P(t-BuOMI/St) was obtained in a conversion of 90% (4.90 g) after drying in vacuo at 40 °C. The inherent viscosity of the copolymer was determined to be 1.18 dl/g in THF at 25 °C. Photoimaging of P(t-BuOMI / St) Resist solutions by 20 wt% in cyclohexanone were prepared by dissolving P(t-BuOMI/St) with various molecular weights and sulfonium salt such as triphenylsulfonium hexafluoroantimonate (TPSHFA) or triflate (TPSOTfi. The spun films with 2000 to 3000 rpm on HMDS-treated silicon wafers were softbaked at 90 °C for 1 min on a hot plate and imagewise exposed to deep U V by a contact mode. The exposure of the resist films was made on a Hybralign Series 400 Exposure Systems of Optical Associates Inc. equipped with a light source of a 500W short arc Hg-Xe lamp and optics tuned to 260 nm with light intensity of 15mW/cm . Then the exposed resist films underwent post-exposure bake (PEB) at a given temperature on a hot plate followed by development with commercial 2.38 wt% T M A H aqueous solution such as NMD-3 and A Z 700MIF developer for 1 min. 2
RESULTS AND DISCUSSION Synthesis of N-(tert-Butoxy) maleimide A maleimide monomer, N-(terf-butoxy)maleimide, t-BuOMI was prepared by means of a retro-Diels-Alder reaction which is a useful procedure to synthesize reactive maleimide derivatives (10) as described in Scheme I.
128
POLYMERS FOR MICROELECTRONICS
Table I. Radical Copolymerizations of AMtert-Butoxy)maleimide P(t-BuOMI/X-St)
a
AIBN
b
(mol%) P(t-BuOMI)
time
conver-
(g/ml)
(hr)
sion (%) viscosity*
M/S
c
inherent 1
0.5
1.00
48
84
0.06
P(t-BuOMI)
6
1
77
0.25
P(t-BuOMI)
1
— —
24
6
3
56
0.17
P(t-BuOMI/St)
1
1.00
3
89
1.02
P(t-BuOMI/St)
2
1.00
3
90
1.18
P(t-BuOMI/St)
4
0.50
2
90
0.75
P(t-BuOMI/St)
6
0.33
3
92
0.59
P(t-BuOMIA-BOCSt)
4
1.00
3
79
1.00
P(t-BuOMVSiSt)
4
0.62
2
79
0.86
P(t-BuOMI/AcOSt)
2
1.00
2
84
1.23
P(t-BuOMI/MeSt)
2
1.00
2
87
0.83
P(t-BuOMI/aSt)
2
1.00
2
82
0.45
4
1.00
3
69
0.21
2
1.00
5
71
0.24
P(HOMI/St)
f
P(HOMI/SiSt)
g
All the t-BuOMI copolymers have alternating structures. Copolymerizations were carried out in 1 to 1 molar feed ratio at55°C in dioxane: St, styrene;
t-BOCSt, /?-(f-butoxycarbonyloxy)styrene; SiSt, p-trimethyl-
silylstyrene;
AcOSt, /?-acetoxystyrene; MeSt,
p-methylstyrene;
/7-chlorostyrene; HOMI, A^-hydroxymaleimide.
CISt,
mol% of the initiator C
AIBN to the total amounts of two monomers used. M/S is the ratio of the total
weight
of two monomers to the volume of
dioxane solvent,
inherent viscosities in dl/g were measured at a concentration of 0.20 g/dl e
in T H F at 25°C. Homopolymerization with DCP as an initiator in bulk f
at 110°C. P(HOMI/St) was obtained by copolymerization of HOMI and 8
styrene in 1 to 1 molar feed ratio. P(HOMI/SiSt) was obtained copolymerization of HOMI and SiSt in 1 to 1 molar feed ratio.
by
9. A H N AND K O O
Synthesis of N-(tert-Butoxy)maleimide
129
The furan/HOMI adduct was prepared in a quantative yield by two steps, firstly, a Diels-Alder reaction and secondly a reaction with hydroxylamine. TTie terf-butyl (t-Bu) group was introduced to the furan/HOMI adduct by a reaction with isobutylene in a pressure reactor and the resulting furan/t-BuOMI adduct was obtained as a yellow powder in a high yield. Theterf-butoxy(t-BuO) adduct was pyrolyzed at about 145 °C and the desired monomer, t-BuOMI was collected by sublimation in a yield of 75%. Recrystallization gave the colorless needle crystal of t-BuOMI with melting point of 92 °C. In an N M R spectrum, t-BuOMI shows only two singlet peaks at 6.56 ppm for two olefinic protons and at 1.30 ppm for nine protons of the t-Bu groups. The structure of t-BuOMI was confirmed by *H and C NMR, IR, mass spectra, and elemental analysis. 13
Polymerization The radical copolymerizations of the t-BuOMI monomer with various comonomers were carried out and the results are summarized in Table I. t-BuOMI was readily copolymerized with styrene derivatives (X-St) such as styrene (St), p-(/err-butoxycarbonyloxy)styrene (t-BOCSt), p-trimethylsilylstyrene (SiSt), /?-acetoxystyrene (AcOSt), p-methylstyrene (MeSt), and /?-chlorostyrene (CISt) in high conversions in the presence of a radical initiator within three hours (Scheme II). However, the homopolymerization of t-BuOMI were rather sluggish and usually obtained were the low molecular weight polymers with some amounts of the free iV-hydroxymaleimide (HOMI) unit by adventitious deprotection of t-Bu groups during the polymerization, particularly, in solution. This homopolymerization behavior of t-BuOMI is ascribable to the bulky side-chain of the t-Bu group along with 1,2-disubstitution. In contrast to the poor homopolymerization behavior of the t-BOC protected MI monomer, t-BOCMI (7), the new monomer t-BuOMI rendered homopolymers in quite high conversions above 77% by a polymerization at 110 °C in bulk with dicumyl peroxide (DCP) as a radical initiator, but still in low molecular weights. Molecular weights of the t-BuOMI copolymers were controlled by the amount of the initiator and dioxane solvent used in copolymerizations. To obtain a large amount of copolymers suitable molecular weights, some of the copolymerizations were conducted by using large quantities of the A I B N initiator and the copolymers were used for characterizing the resist properties. The copolymerizations of t-BuOMI with styrene monomers were performed in 1 to 1 molar feed ratio and the obtained copolymers, poly(t-BuOMI-co-X-St), i.e., P(tBuOMI/X-St) were confirmed to have 1 to 1 molar composition ratio from the proton N M R spectral analyses. Thus the polymers P(t-BuOMI/X-St) are expected to be of alternating structure because it is well known that nearly alternating structure is formed when an electron-poor monomer (t-BuOMI) and an electron-rich monomer (styrene) are copolymerized (11)Thermal Deprotection and Structural Change In thermogravimetric analysis (TGA), the copolymer P(t-BuOMI-alt-St), i.e., P(t-BuOMI/St) was found to undergo rapid thermal deprotection of the t-Bu
POLYMERS FOR MICROELECTRONICS
t-BuOMI
X-St
P(t-BuOMI/X-St)
where X-St: H (St); C H (MeSt); CI (CISt); SiMe (SiSt); OAc (AcOSt); 0-C0 -Bu-t (t-BOCSt). 3
3
2
Scheme II.
9. A H N AND K O O
131
Synthesis of N-(tert-Butoxy)maleimide
groups at about 280 °C to yield poly(JV-hydroxy- maleimide-alt-styrene), P(HOMI/St) by removing 2-methylpropene as shown in Figure 1. P(tBuOMI/SiSt) and P(t-BuOMI/MeSt) are deprotected at higher temperatures to the corresponding copolymers P(HOMI/SiSt) and P(HOMI/MeSt), respectively, and the thermograms are compared with P(t-BuOMI/St) in Figure 1. The protected copolymers, P(t-BuOMI/X-St) are converted into P(HOMI/X-St) by thermal deprotection of t-Bu groups above 270 °C as shown in Scheme III. The weight loss of the alternating copolymer P(t-BuOMI/St) in a T G A thermogram of Figure 1 was estimated to be of 21% which is the same amount as the theoretically calculated weight loss due to the release of 2-methylpropene from the copolymer and is listed in Table II. The DSC thermograms of P(tBuOMI/St) in Figure 2 reveal the glass transition (T ) at 200 °C and an endothermic event corresponding to the deprotection of t-Bu groups at 283 °C (T ) in the first run, and T of the deprotected polymer P(HOMI/St) at 245 °C and onset decomposition at 340 °C with some kinds of reactions due to HOMI units in the second run. In DSC measurments, the first run was recorded to 300 °C, then the same sample was cooled down to room temperature and the second run was conducted to the main-chain decomposition. Therefore T of the protected P(t-BuOMI/St) is 200 °C before deprotection and the observed glass transition temperature in the second run corresponds to that of the deprotected polymer and was confirmed to be identical to that of P(HOMI/St) obtained by the direct radical copolymerization of the corresponding monomers HOMI and St. T s of P(t-BuOMI/AcOSt) and P(t-BuOMI/ClSt) were found to be of 210 °C whereas P(t-BuOMI/MeSt) and P(t-BuOMI/SiSt) did not show T . In the particular case, the copolymer of t-BuOMI and t-BOCSt, P(tBuOMI/t-BOCSt) exhibits two-step thermal deprotection behavior by DSC and T G A analysis as shown in Figure 3, since the copolymer has two kinds of protecting groups in both the repeating units. In the first step the deprotection of t-BOCSt units to /?-hydroxystyrene (HOSt) units occurs at 185 °C measured by DSC. P(t-BuOMI/t-BOCSt) is converted to P(t-BuOMI/HOSt) evolving isobutylene and carbon dioxide. In the second deprotection at 252 °C, P(tBuOMI/HOSt) is converted to P(HOMI/HOSt) which does not show T before its main-chain decomposition. The two-step deprotection behavior of P(tBuOMI/t-BOCSt) is also confirmed by mass loss in a T G A analysis in Figure 3 and the amount of mass loss in each step is described in Table II. The thermal deprotection temperatures (T ) of the t-Bu protected polymers P(t-BuOMI/X-St), T s of the deprotected polymers P(HOMI/X-St), and the onset decomposition temperatures (T ) of the deprotected polymers were measured in nitrogen atmosphere and are summarized in Table II. The amounts of weight loss of the protected copolymers during the thermal deprotection measured by T G A agreed well with the calculated amounts. A l l the t-BuOMI copolymers are deprotected at about 270 °C or above and the deprotected polymers P(HOMI/X-St) having HOMI units showed high T s of about 245 °C or no T observed. In the case of the homopolymer P(t-BuOMI), the deprotection was observed at somewhat low temperature of 247 °C and a smaller amount of weight loss than the theoretical amount was found due to adventitious deprotection during g
d
g
g
g
g
g
dp
g
dc
g
g
132
POLYMERS FOR MICROELECTRONICS
0
100
200
300
400
500
600
Temperature (°C)
Figure 1. T G A thermograms of P(t-BuOMI/St), P(t-BuOMI/SiSt) and P(tBuOMI/MeSt) in a nitrogen stream at the heating rate of 10 °C/min.
—< I O-Bu-t
£
P(t-BuOMI/X-St)
P(HOMI/X-St)
where X = H, C H , CI, SiMe OAc, O-COrBu-t; when X=0-C0 -Bu-t, X=OH (HOSt) after deprotection. 3
3f
2
Scheme III.
9.
Synthesis of H-(tert-Butoxy)maleimide
AHNANDKOO
Table II. Thermal Properties of the t-BuOMI Copolymers P(HOMI/St)*
solvent
P(t-BuOMI/St)
acetone
++
—
chloroform
++
-
hexane
-
-
toluene
++
-
chlorobenzene
++
anisole
++
-
cyclohexanone
++
methyl isobutyl ketone
++
-
2-ethoxyethyl acetate
++
-
AT^V-dimethylformamide
++
++
tetrahydrofuran
++
+
dioxane
++
+
methanol 0.7 N K O H (aq) 1.0 N NaOH (aq) 0.3 N TMAH**(aq)
-
—
+ +
-
Remark:++, very soluble; +, soluble; *The alternating
-
++ ++ insoluble.
copolymer P(HOMVSt) was obtained by thermal
deprotection of P(t-BuOMI/St) at 280°C. Tetramethylammonium hydroxide (2.38 wt%).
133
134
POLYMERS FOR MICROELECTRONICS
I
50
i
100
i
i
i
i
1
150
200
250
300
350
Temperature (°C)
Figure 2. DSC analysis of P(t-BuOMI/St) in a nitrogen stream at the heating rate of 10 °C/min: the first run for the deprotection of t-Bu side-chains and the second run for the deprotected polymers.
0 I 0
1
100
1
200
"
1
300 400 Temperature (°C)
1
500
1
600
Figure 3. Thermograms of T G A and DSC for the two-step deprotection of P(t-BuOMI/t-BOCSt).
9. A H N AND K O O
135
Synthesis of N-(tert-Butoxy)maleimide
the homopolymerization. The observed mass loss of the obtained P(t-BuOMI) was 21 to 30% depending on the polymerization conditions whereas the theoretically calculated amount is 33%. In addition, P(t-BuOMI) revealed no observable T even after deprotection in the second run of DSC. As a similar polymer, Crivello and his coworkers (22) investigated the deprotection of poly(4-terfbutoxystyrene) to poly( p-hydroxystyrene) thermally and acidolytically, and positive image formation was achieved. The tendency of facile, complete thermal deprotection of t-Bu groups is more prominent in the case of t-BuOMI copolymers when compared with that of poly(4-f-butoxystyrene). The thermal deprotection behavior was easily followed by infrared spectral change using a film of P(t-BuOMI/St) as shown in Figure 4. The starting protected polymer has absorption bands at 2980 and 1370 cm" for t-Bu groups, at 1790 and 1730 c m for imide carbonyls and at 1190 cm" for ethers, whereas the deprotected polymer P(HOMI/St) obtained by heating above 285 °C shows absorption bands at 3200 cm* for imino groups, 1780 and 1710 cm" for imide carbonyl groups, and 1220 cm' for N-hydroxy groups due to the deprotection of the side-chain t-Bu groups. Furthermore the characteristic absorption peak at 1370 cm' due to symmetric bending of methyl groups in t-Bu groups disappeared in the deprotected copolymer P(HOMI/St). The IR spectrum of the deprotected polymer was found to be identical to that of P(HOMI/St) which was directly obtained by a radical copolymerization of the corresponding monomers. g
1
1
1
1
1
1
1
Solubility Change by Deprotection All the t-BuOMI copolymers were white powders having a good film forming property. The t-BuO protected copolymers show considerable change in solubility after deprotection due to the large polarity change. P(t-BuOMI/St) is very soluble in common organic solvents such as acetone, chloroform, toluene, anisole, and DMF, but insoluble in aqueous alkaline solutions and methanol as described in Table III. Instead, the deprotected polymer P(HOMI/St) is soluble in aqueous base solutions, dioxane, and DMF, but insoluble in common organic solvents. Other t-BuOMI copolymers are also showed the similar solubility behavior before and after deprotection. The solubility of the deprotected polymers in aqueous base solutions is of the utmost importance for practical application as positive resist materials. Lithographic Evaluation of P(t-BuOMI / St) The t-BuO protected MI polymers, P(t-BuOMI/X-St) appeared to have very low absorption in deep U V region like the t-BOC protected MI polymers (5). P(t-BuOMI/St) has an absorbance of 0.15//tm at 248 nm and an U V absorption spectrum is showed in Figure 5. P(t-BuOMI/SiSt) was found to have an absorbance of 0A3/fim at 248 nm. This low absorption in deep U V region is quite comparable to the tetrahydropyranyl-protected methacrylate copolymers for resist application (25). The acidolytic deprotection of t-Bu groups of P(tBuOMI/St) was detected at 130 °C or lower temperatures in the presence of /7-toluenesulfonic acid as shown in Figure 6 that indicates capability of the
136
POLYMERS FOR MICROELECTRONICS
9.
137
Synthesis of N-(tert-Butoxy)maleimide
AHNANDKOO
Table III. Solubility of P(t-BuOMI/St) and P(HOMI/St) P(t-BuOMI/X-St)
a
a
b
P(HOMI/X-St) weight loss (%)
Td
P(HOMI)
P(t-BuOMI/St)
P(HOMI/St)
21- 30 33
Tg
d
Tdc
°C
°C
240- 247 —
335
found calc. P(t-BuOMI)
c P
°C
21
21
283
245 340
P(t-BuOMl/t-BOCSt) P(t-BuOMI/HOSt) 25
26
185
—
f
e
-
P(HOMI/HOSt)
13
14
252
275
340
P(t-BuOMI/SiSt)
P(HOMl/SiSt)
15
16
300
335
P(t-BuOMIZAcOSt)
P(HOMI/AcOSt)
16
17
270
— —
P(t-BuOMI/MeSt)
P(HOMI/MeSt)
19
20
290
250 340
P(t-BuOMI/ClSt)
P(HOMI/aSt)
18
18
270
245
330
-
335
— —
P(HOMI/St) P(HOMI/SiSt) a
After
deprotection of t-Bu groups of
BuOMI/X-St),
340
- - -
-
the original
copolymers P(t-
340
t-BuOMI units are converted into A^-hydroxymaleimide b
(HOMI) units in the deprotected copolymers P(HOMI/X-St). Measured in wt % by T G A and theoretical calculation, °Tdp is the deprotection temperature measured in the
first run of DSC (cf. Fig. 2). ^Tg is the
glass transition temperature of the deprotected copolymer measured in the second run of DSC (cf. Fig. 2).
^dc
is
the onset decomposition f
temperature of main-chains measured by TGA. t-BOCSt units are converted to p-hydroxystyrene (HOSt) units by releasing carbone dioxide and 2-methylpropene (ref. 3).
138
POLYMERS FOR MICROELECTRONICS
220
240
260
280
300
Wavelength (nm)
Figure 5. An U V absorption spectrum of P(t-BuOMI/St) in 1.4 um thick film.
0
100
200
300
400
Temperature (°C)
Figure 6. Comparison of T G A thermograms of (a) P(t-BuOMI/St) and (b) P(t-BuOMI/St) containing 10 wt% of p-toluenesulfonic acid.
9. A H N AND K O O
Synthesis of N'(tert'Butoxy)maleimide
139
polymer as a chemically amplified resist. It is evident that the acidolytic deprotection slowly proceeds even at temperatures below 130 °C in the solid state despite of its high thermal deprotection temperature of 283 °C In the preliminary resist evaluation of P(t-BuOMI/St) containing onium salts as a PAG, positive patterns were obtained by deep U V or electron beam irradiation followed by post-exposure bake (PEB) and development with aqueous base solutions. Two kinds of triphenylsulfonium salts, hexafluoroantimonate (TPSHFA) and triflate (TPSOTf) were used more than 15 wt% to generate proper positive tone images even high temperature PEB treatment at about 150 °C. The resist films of P(t-BuOMI/St) containing TPSHFA were imagewise exposed to 260 nm light by a contact mode and followed by PEB treatment at 130 °C for lmin, and developed with 2.38 wt% T M A H solution for lmin. Thus the SEM photographs of positive tone images were obtained as shown in Figure 7. The inherent viscosity of the polymer was 1.0 dl/g for (a) and 0.5 dl/g for (b)
Figure 7. SEM photographs of coded 2.2 fim L / S patterns formed in 0.6 /im thick resist of P(t-BuOMI/St) containing TPSHFA: exposure to 260 nm light for 5 sec by a contact mode; PEB at 130 °C for 1 min. (a) 20 wt% TPSHFA, (b) 16.7 wt% TPSHFA.
140
POLYMERS FOR MICROELECTRONICS
in Figure 7. The sensitivities of the resists are not enough compared with the known chemical amplification resists (2, 13). The p K values of W-hydroxysuo cinimides are reported to be as high as 6 to 7 due to the strongly acidic JV-hydroxy functionality (14) and are higher than those values of the imino protons of succinimides and phenolic protons which are known to be about 10. However, the high deprotection temperatures of the t-Bu protected HOMI copolymer, namely P(t-BuOMI/St), rendered rather low sensitivity as a resists material compared with t-BOC protected polymers. a
CONCLUSIONS
As a new protected monomer, A^tert-butoxy) maleimide (t-BuOMI) was synthesized by a retro-Diels-Alder reaction in a high yield and the radical copolymerizations of t-BuOMI with styrene monomers (X-St) were carried out to obtain a new kind of t-BuO protected polymers based on the maleimide structure. The protected maleimide copolymers with styrene monomers, P(t-BuOMI/X-St) were completely converted into the free N-hydroxymaleimide copolymers P(HOMI/Y-St) by thermal deprotection of the side-chain t-Bu groups at about 270 to 300 °C. The deprotected polymers P(HOMI/X-St) have very high T s at 245 °C or above along with good solubilities in aqueous base solutions. The acidolytic deprotection of t-Bu groups from P(t-BuOMI/St) was occurred at 130 °C or lower temperatures in the presence of catalytic acids despite of its original high deprotection temperature. t-BuO protected maleimide copolymers were found to possess specific requirements such as alkaline solubility, high T , low U V absorption and facile deprotection for application as thermally stable, sensitive deep U V resist materials based on the chemical amplification concept. The full accounts of synthesis and polymerizations of t-BuOMI and further evaluation of its polymers as chemical amplification resists will be published elsewhere. g
g
ACKNOWLEDGMENT
The authors are deeply grateful to Korea Ministry of Science and Technology for the financial support on the research project of advanced resist materials. REFERENCES
1. Willson, C. G.; Ito, H.; Fréchet, J. M. J.; Tessier, T. G.; Houhihan, F. M.; J. Electrochem. Soc. 1986, 133, 181. 2. Reichmanis, E.; Houlihan, F. M.; Nalamasu, O.; Neenan, T. X. Chem. Mater. 1991, 3, 394; references cited therein. 3. Ito, H.; Willson, C. G. In Polymers in Electronics; ACS Symposium Series, No. 242; Davidson, T., Ed.; American Chemical Society: Washington, D C , 1984; p 11. 4. Maltabes, J. G.; Holmes, S. J.; Morrow, J. R.; Barr, R. L.; Hakey, M.; Reynolds, G.; Brunsvold, W. R.; Wilson, C. G.; Clecak, N.; MacDonald, S.; Ito, H . Proc. SPIE 1990, 1262, 2.
9. A H N AND K O O
Synthesis of N-(tert-Butoxy)maleimide
141
5. Turner, S. R.; Arcus, R. A.; Houle, C. G.; Schleigh, W. R. Polym. Eng. Sci. 1986, 26, 1096. 6. Turner, S. R.; Ahn, K.-D.; Willson, C. G. In Polymers for High Technology: Electronics and Photonics; ACS Symposium Series, No. 346; Bowden, M . J.; Turner, S. R., Eds.; American Chemical Society: Washington, DC, 1987; p 200. 7. Ahn, K.-D.; Lee, Y . H.; Koo, D.-I. Polymer 1992, 33, 4851. 8. Ahn, K.-D.; Koo, D.-I.; Kim, S.-J. J. Photopolym. Sci. Technol. 1991, 4, 433. 9. Brunsvold, W.; Conley, W.; Crockatt, D.; Iwamoto, N. Proc. SPIE 1989, 1086, 357. 10. Narita, M.; Teramoto, T.; Okawara, M . Bull. Chem. Soc. Jpn. 1971, 44, 1084. 11. Turner, S. R.; Anderson, C. C.; Kolterman, K. M . J. Polym. Sci., Polym. Lett. 1989, 27, 253. 12. Conlon, D. A.; Crivello, J. V.; Lee, J. L.; O'Brien, M . J. Macromolecules 1989, 22, 509. 13. Taylor, G. N.; Stillwagon, L . E.; Houlihan, F. M.; Wolf, T. M.; Sogah, D. Y.; Hertler, W. R. Chem. Mater. 1991, 3, 1031. 14. ames, D. E.; Grey, T. E . J. Chem. Soc. 1955, 631. RECEIVED December 30, 1992
