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Poly(tetrafluorochloropropyl methacrylate) as Positive Electron Resist B. BEDNÁŘ, J. DEVÁTÝ, J. KRÁLÍČEK, and J. ZACHOVAL Department of Polymers, Prague Institute of Chemical Technology, 166 28 Prague 6, Czechoslovakia Poly/tetrafluoro-chloropropyl methacrylate/ (PFCPM) prepared by solution free radical polymerization was tested as a potential candidate for electron positive resist. Thermal s t a b i l i t y of the polymer was stu died by means of TGA and DSC methods. The sensitivity of PFCPM to electron beam ran ged from 7µC/cm2to 2 µC/cm . The sensiti vity is as high as more than an order of magnitude larger than that of PMMA with the same molecular weight. PFCPM can be employed as a positive electron resist at the dose less than 50 µC/cm2. At the dose of more than 50 µC/cm it can be employed as a negative electron resist because cross linking predominates in this dose region. The influence of polymer chain structure on scission and crosslinking reactions i s discussed. 2
2
The fabrication of LSI c i r c u i t s , and of VLSI circuits i n particular, requires patterns of micron and submicron dimensions, and consequently polymer resists with a high degree of resolution (1). So far the most frequently used positive electron resist has been poly(methyl methacrylate) (PMMA), which affords a high resolution power together with a relatively good thermal s t a b i l i t y (2-4)· A serious limitation of PMMA with respect to the efficiency of the electron lithography system i s i t s low sensitivity to electron irradiation ( 10-5 - 10-4-c/cm ). Por the preparation of polymer electron resists i t i s desirable that the resist should possess such sensitivity that doses of the order of luC/cm* could be used during exposure* This requirement can be easily met i n the preparation of negative electron resists (5-6)· Of the positive electron resists used so far, only the PBS resists 0097-6156/ 84/ 0242-0129506.00/ 0 © 1984 American Chemical Society 2
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(copolymers of olefins with S0 ) meet this reguirement (7), and recently some substituted methacrylate polymers, the most successful of which appears to be the halogen-substitued methacrylates (8-10)· One of the above mentioned polymers i s poly(tetrafluoro-chloropropyl methacrylate) (PPCPM), some lithographic properties of which w i l l be presented i n the present paper. p
Experimental Polymers used i n the study were prepared by solution, free r a d i c a l - i n i t i a t e d polymerisation. TGA,DTGA and DSC measurements were performed using the DuPont 951 and DuPont DSC-910 devices connected to an electronic device DuPont TA-990. The device was not c a l i brated to precise values of heat effect. The determination of the lithographic properties was made using a modified scanning microscope JSM 35· To determine the sensitivity always 4 areas 1x1 mm large were exposed with l i n e scanning of the density of 200 lines per mm with a 1 ^urn electron beam diameter. Results and Discussion The polymerization conditions and some properties of the prepared PPCPM are presented i n Table I. Polymerizations of samples of PPCPM-1, PPCPM-2 and PPCPM-3 were carried out i n the presence of 1056 tetrafluoro-chloropropanol used f o r the preparation of the monomer. As seen from the data presented i n Tabl e I, the presence of alcohol does not substantially affect the rate of polymerization, but the Mn values of the prepared polymers are lower than those obtained i n the polymerization of a pure monomer. The decrease i n molecular weight values i s apparently due to the transfer of the tetrafluoro-chloropropanol during polymerization. TGA, DTGA and DSC measurements have shown that the thermal s t a b i l i t y i s not markedly affected by the molecular weight of the polymer. In contrast with PMMA, the TGA ourve can be divided into two main regions (Fig. 1), the f i r s t decrease i n weight occuring at 270°C, and the other change occurring at 355°C. The Tg values determined by DSC measurements are not the same f o r a l l the PFCPM, which cannot be attributed to the dependence on molecular weights, but more probably to different amounts of impurities present (e.g. solvent). Similarly as with other chlorosubstituted methacrylates the crosslinking of the polymer takes place during thermal treatment at about 200°C, but at ambient temperature the
I
4,4 14,7 22,3 4,0
PPCPM-1 PF0PM-2 PFCPM-3 PPCPM-4 PPCPM-5 PPCPM-6
- i n i t i a t o r ΑΒΓΚΓ,
6,9 21,6
[ l ] x10 mol/1
Polymer
3
26,5 46,4
50,5 22,0
23,5 42,2
Conversion 4
15,2 10,6 51,4 35,8 27,8
-
g/mol
V ™ ~
93 85 85
-
°c
- data from membrane osmometry
6 6 6 9 9 9
Polym. time hrs.
2
4,9
-
2,5 5,0
-
uC/cm
Q
Table I· Characteristics of poly(tetrafluoro-chloropropyl methacrylate)
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polymer i s stable. The sensitivity of the FFCPM-3, PFCPM-4 and FFCPM-6 polymers to electron irradiation has shown a r e l a t i v e l y small dependence on the mole cular weight values (Table I) but, similarly as with some previous methacrylates (9)» at higher i r r a d i a tion doses the crosslinking of PPCPM takes place, the polymers thus changing from positive resists to nega tive ones (Fig. 2). On the basis of the present expe rimental results, however, i t i s impossible to decide whether the change i n the ratio of the crosslinking and the degradation reactions with the increasing irradiation dose i s not affected i n a decisive way by the reactions induced by increased temperature during irradiation. The change i n the ratio of the crosslin king and the degradation reactions was not observed during irradiation of the polymers with -radiation or with accelerated electrons (without any temperatu re change), not even at extremely high doses (11).The sensitivity values (Table I) indicate that PPCPM i s one of the most sensitive positive electron r e s i s t s . The s e n s i t i v i t y value f o r the positive electron re s i s t , however, i s always affected to a considerable degree by the developer employed ( i n our case i t was a mixed solvent, 1,4-dioxane/80 v o l . % η-heptane), so that by choosing a proper developer the sensitivity w i l l most probably be increased. The region of doses i n which the crosslinking of PFCPM takes place starts with the dose of 4·5x10-5 C/cm (from this value on ward the samples were developed i n pure 1,4-dioxane) for PFCPM-3, increasing s l i g h t l y with the increasing value of molecular weight, which i s a course opposite to that to be expected. This can be attributed to the degradation reactions preceding the crosslinking and consequently changing the values of molecular weights and their distribution, and to the thermal degrada tion during the crosslinking. The resolution power of PFCPM permits patterns of submioron dimensions to be prepared. Their r e s i s tance to wet etching i s very good, i n plasma etching being lower than that of PMMA. but substantially better than that of PBS. 2
11.
BEDNAR FT AL.
PFCPM
as Positive Electron Resist
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F i g u r e 1. R e s u l t s o f t h e t h e r m a l t r e a t m e n t , ( a ) DTG c u r v e s , (b) DSC c u r v e s . Key: 1, PMMA; and 2, PFCPM-6.
1,0 ι — ι
1
r
'/ΐο
β
1.10"
s
1.10
4
2
1.10" [Ccii ]
F i g u r e 2. Dependence o f r e d u c e d r e s i s t f i l m t h i c k n e s s a f t e r d e v e l o p m e n t on t h e r a d i a t i o n dos^e f o r PFCPM-3. Development c o n d i t i o n s :(up t o 3 χ 10 C/cm ) l ^ m i n , m i x t u r e 1 , 4 - d i o x a n e / 8 0 % v o l . h e p t a n e ; and ( f r o m 5 χ 10 C/cm ) 2 m i n , 1 , 4 - d i o x a n e .
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Literature cited 1. Thompson, L.F.; Kerwin, R.E. Ann.Rew.Mater.Sci. 1976, 6, 267. 2. Hatzakis, M. J.Electrochem.Soc. 1969, 116, 1033. 3. Chang, H.P.; Hatzakis, M.; Wilson. A.D.; Speth, A.J.; Kern, A. Electron. 1977, 8, 51. 4. Zeitler, H.V.; Hieke, E.K. J.Electrochem.Soc. 1979, 126, 1430. 5. Thompson, L.F.; Stillwagon, L.E.; Joerries, E.M. J.Vac.Sci.Technol. 1978, 15, 938. 6. Thompson, L.F.; Van, J.; Doerrie, E.M. J.Electrochem.Soc., 1979, 126, 1703. 7. Bowden, M.J.; Thompson, L . F . J. Electrochem.Soc. 1974, 12, 1620. 8. Tada, T. J. Electrochem.Soc. 1979, 126, 1829. 9. Tada, T. J. Electrochem.Soc. 1979, 126, 1635. 10. L a i , J.H.; Helbert, J.N.; Cook, C.F.; Pittman, C.V. J.Vac.Sci.Technol. 1979, 16, 1992. 11. Dole, M. in "The radiation Chemistry of Macromolecules", (Dole, M. Ed.), Academic Press, New York 1973. RECEIVED December
19, 1983
