Polymers for Microelectronics - American Chemical Society

Chapter 10

Acid-Sensitive Pyrimidine Polymers for Chemical Amplification Resists Yoshiaki Inaki, Nobuo Matsumura, and Kiichi Takemoto

Downloaded by FUDAN UNIV on December 16, 2016 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch010

Department of Applied Fine Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan

Pyrimidine bases such as uracil and thymine are known to form photodimers with exposure to U V light (1). It seemed interesting to explore the photochemical reaction of the pyrimidine bases to the photolithographic process in microelectronics fabrication technology. We have studied the synthesis and application of a series of polymers containing pendent pyrimidine bases, and their application to negative-type photoresists, as well as polymers containing pyrimidine photodimers in the main chain and their applications to positive-type photoresists (2-8). Nucleic acid bases exist as mixtures of two or more rapidly interconvertible isomers of tautomeric forms. Tautomers, at least in principle, can be separated at low temperatures where the rate of interconversion is low. The keto-enol equilibrium is the classic example of tautomerism. The enol is present in small amount, since it is usually less stable than the keto form. Uracil (1) is one of the nucleic acid bases, that exists as mixture of the tautomeric keto and enol forms (Scheme 1). The ratio of the enol tautomer to the keto tautomer of uracil, however, is small. The existence of the enol form of Scheme 1 is the basis for referring to uracil as dihydroxypyrimidine. The enol form is readily formed from the keto tautomer by virtue of the fact that hydrogen atoms attached to nitrogen atoms that are immediately adjacent to carbonyl groups are acidic. Although the enol tautomer of uracil is difficult to isolate, 2,4-dialkoxypyrimidine that is thought to be a tautomeric form of uracil, can be easily prepared from dichloropyrimidine (4) (Scheme 2). The 2,4-dialkoxypyrimidine is very sensitive to acids forming the more stable uracil. Di-tert-butoxypyrimidine (2) is a stable and water insoluble compound. In the presence of a catalytic amount of an acid this compound rearranges to give uracil which is soluble in alkaline aqueous solution (Scheme 1). In this reaction, a proton attacks the nitrogen atom and the alkyl group leaves to give the alkyl cation and the keto tautomer. The driving force for this reaction is the tautomerism of uracil.

0097-6156/94A)537-0142$06.75/0 © 1994 American Chemical Society Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

Acid-Sensitive Pyrimidine Polymers


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POLYMERS FOR MICROELECTRONICS

This paper deals with the application of the tautomerism of uracil to a chemical amplification photoresist. Polyethers containing 2,4-dialkoxypyrimidine units (3) were prepared. These polymers were found to be very sensitive to an acid and to be applicable to chemical amplification photoresists (Scheme 3). EXPERIMENTAL


Materials Preparation of the model compound (Scheme 2). 2,4-Di-te/f-butoxypyrimidine (2) was prepared by the reaction of potassium f-butoxide with 2,4-dichloropyrimidine (4) which was obtained by the reaction of uracil (1) and phosphorus oxychloride. Another model compound (5) was prepared from the potassium salt of 2,5-dimethyl-2,5-hexanediol with 2,4-dichloropyrimidine. 2, 4-Di-tert-butoxypyrimidine (2). Uracil (1) was reacted with phosphorus oxychloride to give 2,4-dichloropyrimidine (4) (9). Potassium (0.78 g, 20 mmol) was added to ferf-butanol (20 ml) and reacted under reflux for 1 hour to give a potassium tert-butoxide solution. To the solution, 2,4-dichloropyrimidine (1.5 g, 10 mmol) was added at room temperature, and the mixture was stirred under reflux for 1 hour. After the reaction, the precipitated KC1 was removed by filtration, and the solvent was removed by evaporation. The residue was dissolved in diethyl ether, and washed with 30% aqueous K O H and dried over sodium sulfate. After evaporation of the solvent, the product was obtained by distillation under reduced pressure. Yield 1.1 g (49%). IR: 1580 and 1155 cm" . ' H - N M R (CDC1 , TMS, 8): 1.4 (18H, s), 6.0 (1H, d), and 7.9 (1H, d). 1

3

2,4-Di-(l,l,4,4-tetramethyl-4~hydroxybutoxy)pyrimidine (5). Potassium (0.78 g, 20 mmol) was added to a solution of 2,5-dimethyl-2,5-hexanediol (2.8 g, 20 mmol) in dioxane (20 ml), and the solution was refluxed until potassium completely reacted. The reaction mixture was cooled to room temperature, 2,4-dichloropyrimidine (1,5 g, 10 mmol) was added slowly to the solution and the mixture was refluxed for 1 hr. The solution was filtered to remove KC1 and the solvent was removed by evaporation. From the residue, excess 2,5-dimethyl-2,5-hexanediol was removed by sublimation at 60 °C under reduced pressure to give a glassy product. Yield 2.23 g (60%). The product was identified by IR and *H-NMR spectra. IR: 1580, and 1560 cm" . H - N M R (CDC1 , TMS, 8): 1.0(12H, s), 1.4(12H, s), 1.9(4H, t), 2.2(4H, t), 6.1(1H, d), and 7.9(1H, d). 1

a

3

Preparation of the polymers (Scheme 4) Polyethers (3) containing dialkoxypyrimidine units in the main chain were prepared from dichloropyrimidine (4) with various diols. Potassium (0.78 g, 20 mmol) was added to a solution of a diol (11 mmol) in dioxane (50 ml), and the mixture was refluxed until potassium completely reacted. After the reaction mixture was cooled to room temperature, 2, 4-dichloropyrimidine (1.5 g, 10


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mmol) was added to the mixture. The mixture was again heated to reflux for 20 min, and the solvent was removed by evaporation. Water (100 ml) was added to the residue, and the mixture was stirred overnight. The polymer was isolated as a solid by filtration. The polymers were purified by dissolution in chloroform and precipitation with methanol. The results of the polymerization are tabulated in Table 1. IR (3c): 1580 cm" . *H-NMR (CDC1 , TMS, 8) (3c): 5.3(4H, s), 6.35(1H, d), 7.4(4H, b), and 7.9(1H, d). 1

3


Measurements IR spectra of polymer films spin-coated on undoped silicon wafers were recorded on a JASCO IR-810 infrared spectrophotometer. U V spectra were recorded on a JASCO U-best-30 U V / V i s spectrophotometer. T G A and DSC analysis were carried out using a SEIKO SSC/580DS T G A / D S C instrument at a heating rate of 10.0 deg/min. in nitrogen, from 50 to 300 °C. The molecular weight of the polymers was determined by gel permeation chromatography using Toyo Soda H L C CP8000 with a thermostated column: Cosmosil GPC100 and GPC300 with chloroform as the eluent, and a U V detector operating at 254 nm. The molecular weight was obtained by compairing retention times to those of polystyrene standards. Acid hydrolysis of the polymer in solution To a solution of polymer (0.1 g) in chloroform (5 ml), trifluoromethanesulfonic acid (3c: 0.1 g, 3d-e: 0.01 g) was added, and the mixture was stirred overnight at room temperature. The precipitate was separated by a centrifugation, and the product (3c: 0.05 g, 3d: 0.02 g, 3e: 0.04 g) was analyzed by IR and N M R spectra. Photoacid-catalyzed reaction in solid film Two-component resist solutions were prepared from polymer (0.09 g), triphenylsulfonium trifluoromethanesulfonate (0.01 g), and chloroform (1 ml) as a solvent. The resist solutions were spin-coated onto silicon wafers and baked in an oven at 100 °C (at 60 °C for 3e) for 10 min. The resist films were exposed by monochromatic light at 250 ± 1 nm using a JASCO CRM-FA spectro-irradiator equipped with a 2 kW Xenon-Arc lamp and with a exposed energy integrator. The films were then heated at 60 °, 100 °, or 150 °C for 10 min, and the IR spectra were measured. Solubility measurements The resist solutions of polymer (0.09 g) with photoacid generator (triphenylsulfonium trifluoromethanesulfonate, 0.01 g) in chloroform (1 ml) were cast on a quartz plate, and heated in an oven at 100 °C (at 60 °C for 3e) for 10 min. The exposed film was post-exposure baked in an oven at 100 °C for 10 min, and


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developed in tetramethylammonium hydroxide (TMAH) aqueous solution (0.1%) for 15 seconds. The film thickness was measured by U V spectra at 270 nm.


Sensitivity measurements The resist solution of polymer 3d (0.09 g) with photo acid generator (triphenylsulfonium trifluoromethanesulfonate, 0.01 g) in ethylene glycol monomethyl ether acetate (0.9 g) was spin-coated (1000 rpm) onto silicon wafers and baked on a hot-plate at 100 °C to give a 0.35 fim film. In a case of polymer 3e, the resist solution of polymer (0.09 g) with photo acid generator (triphenylsulfonium trifluoromethanesulfonate, 0.01 g) in chloroform (0.9 g) was spincoated (500 rpm) onto silicon wafers and baked on a hot-plate at 100 °C to give a film of 0.45 fim thickness. The resist films were exposed using a Hg-Xe lamp with a 251.5 nm interference filter and post-exposure baked at 100 °C for 60 seconds. The film thickness was measured using a Sloan D E K T A K 3030 after development in a 0.1% T M A H aqueous solution for 15 seconds. A positive image was obtained with contact optical exposure at 250 nm (6 mJ/cm ) and development for 15 seconds in a 0.1% T M A H aqueous solution. 2

EB sensitivity In the case of EB resist, exposures were carried out using a point electron beam system (ELIONIXJELS-3300) at 20 kV. After exposure, the film was baked 100 °C for 5 minutes (3d) or 1 minute (3e), and developed in 0.1% T M A H aqueous solution for 15 seconds to give the positive image. RESULTS AND DISCUSSION Preparations of Polymers Polyethers (3) containing dialkoxypyrimidine units in the main chain were prepared from dichloropyrimidine (4) with various diols (Scheme 4). Table 1 shows the result of the polymerization, molecular weight of the polymers, and elementary analysis for five polymers, prepared from alicyclic and aromatic primary, secondary, and tertiary diols.

Table 1. Yield and Analytical Data of Polyethers Containing Pyrimidine (3)

Yield (%)

Mol. Wt X10

H

a b c d e

86 86 96 95 99

8.5 3.0 2.3 2.7 1.3

7.32 6.29 4.71 5.82 6.71

3

Anal (Calcd.) C N 65.43 62.49 67.28 69.41 71.09

12.72 14.57 13.08 11.56 10.36

H

Anal (Found) N C

7.14 6.13 4.83 5.84 7.14

63.71 58.76 65.21 68.51 70.65


12.26 13.87 11.81 11.17 8.47

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Se0O SCF 3


polyether containing 2,4-dialkoxypyrimidme and f-Alkoxy Units

3

(3e)

H uracil

Scheme 3.

CI

Cr

6

HO-R-OH

N

(3)

(4)

CH

a:

c:

CH —

—CH

2

d:

o

CH

CH

e:

k

CH-

-CH

r

—

2

/=\

T"

3

3

AmetseaBnGbemfcal Society Library 1155 16th St.. N.W.

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Thermal analysis of the polymers D T A and T G A curves for the polymer 3c-e are shown in Figures 1-3. Polymer 3c was stable to 300 °C. However, polymer 3e decomposed at around 200 °C, and the residue after heating was identified by IR spectrum as uracil. Tg of this polymer was about 120-130 °C. These polymers are soluble in chloroform and insoluble in water.


Tautomerization by an acid in solution Acid decomposition of the 2,4-dialkoxypyrimidine derivatives was studied in solution by addition of an acid. Figure 4 shows the U V spectra of 2,4-di-terfbutoxypyrimidine (2) in acetonitrile. Addition of acid (CF S0 H) caused the change of U V spectrum as shown in the Figure 4. The spectrum after the reaction was found to be the same as the spectrum of uracil in the presence of acid. This result suggested that the 2,4-dialkoxypyrimidine compounds can easily tautomerize to uracil by addition of acid. Addition of a small amount of the acid to the solution of polymers in chloroform caused precipitation of a solid that was identified as uracil from its IR and N M R spectra. In the case of the polymer (3c) prepared from the primary diol, however, it was difficult to rule out the possibility of a rearrangement shown in Scheme 5. The attack of proton on the nitrogen at N3 of pyrimidine a may cause tautomerization of the pyrimidine base to give b. At this step, the attack of another proton on the nitogen at N l of pyrimidine b may cause tautomerization of the pyrimidine base to give c. However, the leaving cation can attack the unprotonated nitrogen of the pyrimidine b to give the N-substituted uracil derivative d. The product d could not be identified by IR spectra, but was identified by N M R spectra as the N-substituted uracil derivative. This fact suggests that in addition to the main elimination reaction, the primary polyether underwent rearrangement as a side reaction. 3

3

Tautomerization of the model compound in solid state The model compound (5) with triphenylsulfonium trifluoromethanesulfonate (10 wt%) in the solid film was irradiated at 250 nm. Photolysis of the model compounds was carried out in the solid film by exposure to monochromic U V light from a spectro-irradiator. After post-baking of this film, IR spectra were observed as shown in Figure 5. The absorbance at 1580 cm" assigned to the C = N and C - C stretching vibrations of alkoxypyrimidine (10) decreased with an increase in energy dose. At the same time, a new peak at 1700 cm" assigned to the C = O streching vibration of uracil increased. The spectrum after 75 mJ/cm irradiation was the same as that of uracil suggesting a complete reaction. In Figure 6, the relative absorbances of 1580 and 1710 cm" were plotted against exposure dose. In this figure, the decrease in 1580 cm" absorbance occurrs in parallel with the decrease of the 1710 cm" band. 1

1

2

1

1

1

Photoacid-catalyzed reactions of the polymers in the solid film Photolysis of the 2,4-dialkoxypyrimidine polymer (3) in the presence of a photo-acid generator was carried out in the solid film. Thin films of these Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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100.0


87.5

75.0

62.5

-300.0 50.0

100.0

150.0

200.0

250.0

50.0 300.0

Temp. °C

Figure 1. DTA and T G A curves for polyether 3c. 100.0

-300.0 50.0

100.0

150.0

200.0

250.0

300.0

Temp. °C

Figure 2. D T A and T G A curves for polyether 3d.


150



r

200

250 300 Wavelength (nm)

350

Figure 4. U V spectra of 2,4-di-terr-butoxypyrimidine (2) in acetonitrile. Dotted line: without acid. Solid line: with C F S 0 H . 3

3




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152


1900

1600

1300 cm"

1

Figure 5. IR spectra of the model compound 5 with triphenylsulfonium trifluoromethanesulfonate (10 wt%) in solid film. Exposure: 250 nm. PEB: 100 °C for 10 min.

Figure 6. Acid decompositions of the model compound 5 with triphenylsulfonium trifluoromethanesulfonate (10 wt%) in solid film. Exposure: 250 nm. PEB: 100 °C for 10 min.


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polymers were obtained from their chloroform solutions by spin coating on silicon substrates and prebaking at 100 °C for 5 minutes. The films were exposed to the monochromatic light (250 nm) until the reaction was complete. The reaction was followed by IR spectrophotometer at 1580 cm" and 1710 cm" , which are the absorption bands characteristic of the 2,4-dialkoxypyrimidine and the regenerated uracil, respectively (Figures 7-9). The changes of the IR spectra of the polymers 3a and 3b were small, because these polymers were prepared from primary and secondary alicyclic diols. On the other hand, the change of IR spectra of the polymers 3c, 3d, and 3e was remarkable because these polymers have a benzyloxy group. Figure 7 shows IR spectral change of polymer 3c obtained from a primary alcohol in the presence of 10% acid generator. The peak at 1580 cm" of the original unirradiated polymer (i in Figure 7) is assigned to alkoxypyrimidine. After excess exposure of U V light at 250 nm in the presence of acid generator and post baking at 100 °C, the peak at 1580 cm" disappeared, and new peak at 1710 cm" appeared (ii in Figure 7). The peak at 1710 cm" is assigned to uracil. Therefore, this change in IR spectrum was caused by the tautomerization of the alkoxypyrimidine to uracil. However, postbaking at 150 °C was necessary for the complete conversion to uracil (iii in Figure 7). Without the acid generator, this polymer was stable even after exposure to U V light and postbaking at 200 °C. Figure 8 shows the IR spectra of the polymer 3d prepared from the secondary alcohol, after no irradiation (i in Figure 8), 25 mJ/cm (ii in Figure 8), and 100 mJ/cm irradiation at 250 nm followed by postbaking at 100 °C (iii in Figure 8). After 100 mJ/cm irradiation, this polymer decomposed completely. In the case of polymer 3e from the tertiary alcohol, however, the reaction was complete with 20 mJ/cm irradiation (at 250 nm) and postbake at 60 °C (Figure 9). These IR spectra of polymer 3c-e on silicon wafer in the presence of triphenylsulfonium triflate during irradiation at 250 nm indicated tautomeric change of the alkoxypyrimidine to uracil. From the results of IR spectra, sensitivities of these polymers were calculated (Figure 10). The highest sensitivity was obtained for the polymer 3e. For the complete reaction of the polymer 3e, the U V dose of 10 mJ/cm and a post-bake at 60 °C are sufficient. Polymer 3d from the secondary diol shows a sensitivity of 100 mJ/cm . However, the sensitivity of polymer 3c, prepared from the primary diol was very low even after postbaking at 100 °C. Excellent sensitivity of polymer 3e with terf-alcohol units suggests the acid catalyzed reaction as shown in Scheme 6. 1

1


1

1

1

1

2

2

2

2

2

2

Development Polyethers containing 2,4-dialkoxypyrimidine in the main chain were hydrophobic and insoluble in water. Uracil, however, is soluble in an alkaline aqueous solution. Therefore, the solubility of the polyether changes following photoacid tautomeriszation. Figure 11 shows U V spectra of the polymer 3d in the presence of 10 wt% photoacid generator in the solid film on a quartz plate, prepared by casting from chloroform solution.



154


1900 1800 1700 1600 1500 1400 cm'

1

Figure 7. IR spectra of polymer 3c with triphenylsulfonium trifluoromethanesulfonate (10 wt%) in solid film. Exposure: 250 nm.



Figure 8. IR spectra of polymer 3d with triphenylsulfonium trifluoromethanesulfonate (10 wt%) in solid film. Exposure: 250 nm.




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lafn Absorb!ince (1580 cm"


10.

1.0

157

i \3c

a>

3el

& .01

.1

1

10

100

1000

10000 100000

Energy (mJ/cm ) 2

Figure 10. Relative absorbance versus exposed energy of polymers (3c-e), postbaked at 100 °C (3c, d) and 60 °C (3e), photoacid generator 10 wt%.

Scheme 6.




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The original polymer film after prebaking has the spectrum (i) in Figure 11. Spectrum (ii) is for the polymer film after development of the unirradiated polymer in 0.1% triethylammonium hydroxide aqueous solution. No decrease in absorbance suggestes insolubility of polymer 3d in the alkaline aqueous solution. After exposure to U V light and postbaking, spectrum (i) in Figure 11 changed to spectrum (iii), corresponding to the reaction of pyrimidine to uracil. After development of (iii) in Figure 11, products were completely dissolved in the developer as shown by spectrum (iv). These results suggest that polymer 3d can be used as positive type chemical amplification photoresist. Figure 12 shows U V spectra of polymer 3e in the presence of a 10 wt% photoacid generator in the film spin-coated on a quartz plate: spectrum (i in Figure 12) is the original polymer film after prebaking and (ii in Figure 12) the polymer film after development by 0.1% aqueous triethylammonium hydroxide. No decrease in absorbance suggested insolubility of polymer 3e in the alkaline aqueous solution. After exposure to U V light (100 mJ/cm at 250 nm) and postbaking, spectrum (i in Figure 12) changed to spectrum (iii). Development of the 100 mJ/cm postbaked film gives spectrum (iv in Figure 12). In this case, however, slight residue after development was observed and is possibly due to contamination of the polymer. These results suggest that polymer 3e can be used as a highly sensitive positive type chemical amplification photoresist after some improvement of preparation method of the polymer. 2

2

Sensitivity of the Resist Polyethers containing 2,4-dialkoxypyrimidine were evaluated as a chemical amplification photoresist. Figure 13a shows relative film thickness after development against exposed energy for the polymer 3d containing photoacid generator (10 wt% triphenylsulfonium triflate). A positive image on silicon wafer was obtained from contact exposure (600 mJ/cm ) and post-expose bake at 100 °C for 10 min. Figure 13b shows relative film thickness after development against exposed energy for the polymer 3e containing photoacid generator (10 wt% triphenylsulfonium triflate). In this case, only 4 mJ/cm exposure caused complete reaction to give products which were soluble in alkaline aqueous solution. Therefore, this system is a highly sensitive chemical amplification photoresist. 2

2

Electron-Beam Radiation of Polymers Polyethers 3d-e were evaluated as chemical amplification electron-beam resists. Figure 14a shows relative film thickness after development against exposed energy for the polymer 3d containing acid generator (10 wt% triphenylsulfonium triflate) after development with 0.1% aqueous triethylammonium hydroxide. Sensitivity of this polymer as E B resist was about 300 /zC/cm . A positive image was obtained by E B irradiation using polymer 3d where line & space was 0.5 fim (Figure 15). Figure 14b is a sensitivity data for polymer 3e with 10 wt% photoacid generator. Sensitivity of this resist was about 7 fiC/cm . A positive image was 2

2



Figure 12. U V spectra of polymer 3e in the presence of 10 wt% photoacid generator in solid film on a quartz plate.

Wavelength (nm)



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0.0 H .1

•

•—> . . . . . i 1

.

• •

I 10

Energy (mJ/cm ) 2

Figure 13. Normalized thickness against exposed energy at 250 nm for (a) polymer 3d and (b) polymer 3e. Photoacid generator: 10 wt% triphenylsulfonium triflate. Prebake: 100 °C for 1 min. Postbake: 100 °C for 1 min. Development: 0.1% T M A H aqueous solution for 15 sec.


162



0.0 H .1

• — •

• • • •" i

•—•

• •

10

1

Dose (iiC/cm ) 2

Figure 14. Normalized thickness versus EB(20kV) dose for (a) polymer 3d and (b) polymer 3e. Photoacid generator: 10 wt% triphenylsulfonium triflate. Prebake: 100 °C for 1 min. Postbake: 100 °C for 5 min (polymer 3d) and 1 min. (polymer 3e). Development: 0.1% T M A H aqueous solution for 15 sec.



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Figure IS. Scanning electron micrograph of positive images in polymer 3d containing 10 wt% of triphenylsulfonium triflate, exposed to 100 fiC/cm of electron beam(20kV), postbaked at 100 °C for 10 min, and then developed with TMAHaq(0.1%) for 15 sec. Line & space: 0.5 fim. 2


164


also obtained for polymer 3e with 10 wt% photoacid generator. In this case, sensitivity was high (10 /zC/cm ) and postbaking temperature was low (70 °C for 30sec). However, slight residue after development (0.1% TMAHaq) was observed. Further preparation of the polyether containing tertiary alkyl unit is in progress. 2


CONCLUSION Tautomerism of uracil was applied to a chemical amplification photoresist system. Polymers containing 2,4-dialkoxypyrimidine derivatives, an enol form of uracil, were highly sensitive to acid, and rearranged to uracil with photo generated acid. The sensitivity of the polymer containing tertiary alkyl unit after irradiation at A = 250 nm was about 4 mJ/cm . In conclusion, we found that polyethers of a pyrimidine containing tertiary alcohol were highly sensitive deep-UV, and EB chemical amplification resists that gave positive images. 2

ACKNOWLEDGMENTS The authors wish to acknowledged Manufacturing Development Laboratory, Mitsubishi Electric Corp., for photolithographic evaluation of the materials. REFERENCES 1. Wang, S. Y. Photochemistry and Photobiology of Nucleic Acids ; Academic: New York, 1976. 2. Inaki, Y.; Moghaddam, M . J.; Takemoto, K. In Polymers in Microlithography; Reichmanis, E.; MacDonald, S.; Iwayanagi,T., Eds, ; ACS Symposium Series No. 412; American Chemical Society: Washington, DC, 1989; p 303. 3. Moghaddam, M . J.; Hozumi, S.; Inaki, Y.; Takemoto, K. J. Polymer Sci. Polymer Chem. Ed., 1988, 26, 3297. 4. Moghaddam, M . J.; Hozumi, S.; Inaki, Y.; Takemoto, K. Polymer J. 1989, 21, 203. 5. Moghaddam, Kanbara, K.; M . J.; Hozumi, S.; Inaki, Y.; Takemoto, K. Polymer J. 1990, 22, 369. 6. Moghaddam, M . J.; Inaki, Y.; Takemoto, K. Polymer J, 1990, 22, 468. 7. Inaki, Y.; Horito, H.; Matsumura, N.; Takemoto, K. J. Photopolym. Sci. Technol., 1990, 3, 417. 8. Horito, H.; Inaki, Y.; Takemoto, K. J. Photopolym. Sci. Technol., 1991, 4, 33. 9. Bhat, C. C.; Munson, H . R. In Synthetic Procedures in Nucleic Acid Chemistry; Zorbach, W. W.; Tipson, R. S. Eds.; Interscience: New York, 1968, Vol. 1, p 83. 10. Tsuboi, M.; Kyougoku, Y. In Synthetic Procedures in Nucleic Acid Chemistry; Zorbach, W. W.; Tipson, R. S. Eds.; Interscience: New York, 1973, Vol. 2, p 215. RECEIVED May 17,

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Polymers for Microelectronics - American Chemical Society

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