Materials for Microlithography - American Chemical Society

16 Positive-Working Electron-Beam Resists Based on Maleic Anhydride Copolymers K. U. P O H L and F. R O D R I G U E Z School of Chemical Engineering, Olin Hall

Downloaded by TUFTS UNIV on June 4, 2018 | https://pubs.acs.org Publication Date: July 1, 1985 | doi: 10.1021/bk-1984-0266.ch016

Y . M. N. NAMASTE and S. K . O B E N D O R F Department of Design and Environmental Analysis Martha Van Rensselaer Hall Cornell University, Ithaca, N Y 14853

Many studies of structure versus radiation sensitivity have led to certain generalizations regarding possible candidates for positive-working electron-beam resists. One such generalization is that polymers bearing hydrogens on adjacent chain carbons are not suitable since they are likely to cross-link (1). However, the generalization is based mainly on observations of vinyl and vinylidene structures. Maleic anhydride and its derivatives provide polymer structures in which adjacent chain carbons have only one hydrogen each. It has been found that this structure does not lead to cross-linking, and, in fact, undergoes chain scission, at least in the polymers reported in the present study. Maleic Anhydride Maleic anhydride is a five membered ring anhydride (structure I) containing an

olefinic double bond. It was commonly believed until the early 1960s that this monomer would not homopolymerize. Since then it has been shown that maleic anhydride can be polymerized with both gamma and U V radiation, free-radical initiators, pyridine-type bases, electrochemically, and under shock-waves (2). However, the yields are generally poor and the molecular weights are low. Copolymerization, on the other hand, is very easy with maleic anhydride. It copolymerizes by a free-radical reaction with a wide variety of monomers and many of the copolymers are perfectly alternating. This tendency of MA to form alternating copolymers derives from the participation of a donor-acceptor complex formed by the two reacting monomers. The term is used to describe 0097-6156/84/0266-0323$06.00/0 © 1984 American Chemical Society

Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

324

MATERIALS FOR MICROLITHOGRAPHY

an intermolecular complex between maleic anhydride, the acceptor, and the other monomer, the donor. The interaction of the donor and the acceptor can be summarized by the equilibrium equation:


D +A

λ

^

DA

(1)

The magnitude of the equilibrium constant, K, is a measure of the strength of interaction between the two molecules. The equilibrium constant has been determined for a variety of complexes (3). The rate of formation of this complex is usually much higher than the rate of polymerization. The polymerization proceeds by adding donor-acceptor units to the growing chain. It is generally accepted that a good positive resist should have a high G (scission) value, the G (s) value being a measure of the number of main chain scissions per 100 eV of absorbed energy. There are some empirical rules for polymer structures according to which one can predict degradation versus cross-linking of a given polymer. Polymers of the general structure II (where ι - C H

2

- C -

II

R and R are substituents other than hydrogen) are expected to degrade predominantly upon electron beam radiation. Copolymers containing M A do not conform with this structure since M A contains one hydrogen on each olefinic carbon. X

2

Characterizing Sensitivity A variety of techniques have been used in the present work to establish the relative sensitivity of positive electron-beam resists made from copolymers of maleic anhydride (Table I). The term sensitivity is used rather loosely at times. In the most practical sense, sensitivity is a comparative measure of the speed with which an exposure can be made. Thus, the exposure conditions, film thickness, developing solvent and temperature may be involved. Most often, the contrast curve is invoked as a more-or-less objective measure of sensitivity. The dose needed to allow removal of exposed film without removing more than about 70% of the unexposed film can be a measure of sensitivity. The initial film thickness and the developing conditions still must be specified so that this measure is not, strictly speaking, an intrinsic property of the polymeric material. Another measure of sensitivity can be obtained from the rate of dissolution of exposed versus unexposed film at various doses. The amount of energy needed to obtain some arbitrary ratio of rates or the exponent of the dissolution rate ratio versus dose at high doses may be used. In the present study, both a contrast curve and a solubility rate ratio versus dose data are reported for the copolymer of maleic anhydride with alphamethylstyrene. However, these tests are burdensome when many materials are to be screened


16.

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325

Positive- Working Electron-Beam Resists

Table I. Methods and Hardware used in Characterizing the Sensitivity and Radiation Properties of the Polymers Exposure tools:

6 0

Gamma radiation from C o source Flood exposure to 50 keV electrons Pattern exposure to 20 keV electrons

Measures of change:

Viscosity measurements Intrinsic viscosity Viscosity-average molecular weight


Molecular weight distribution, G P C Solubility Rates Thickness changes on development

as resist candidates. The change in molecular weight on exposure to measured doses of electron radiation can be used to calculate G(s), the number of chain scissions per 100 eV of absorbed energy. Plotting l/M versus incident dose yields a straight line with a slope which is proportional to G (s) (4). A "depthdose function" is used to convert incident dose to absorbed dose (5). In the present work a 3-inch diameter silicon wafer (oxide-coated) is spin-coated with a polymer solution and baked. This step requires adjustment of viscosity and concentration to give a consistent final film thickness of about a micron. Also, not every solvent yields a homogeneous film without cracks or holes. The solvent must also be removable at some reasonable baking temperature. A wafer coated in this manner bears about 5 mg of polymer. Flood exposure of the wafer surface to the calibrated, 50 keV, defocused beam of electrons in a conventional transmission electron microscope ( R C A Model E M V - 3 ) results in an adequate amount of exposed polymer for a number of molecular weight measurements. The preferred technique is gel permeation chromatography, G P C , using a high performance liquid chromatograph, H P L C , since this gives a complete molecular weight distribution from which M and M can be derived. Once again, the polymer being tested has to be soluble in the chosen eluting solvent, in our case, tetrahydrofuran, T H F . Moreover, the polymer must not exhibit any tendency to associate with the column packing lest it provide false, low values of molecular weight (long elution times) or, worse yet, not elute at all. Several copolymers of maleic anhydride do show these unfortunate tendencies. Another exposure tool is available in gamma radiation. While the correlation is not always perfect, there is a high degree of similarity in the response of polymers to the radiation from C o and from 50 keV electrons. Because of the penetrating nature of gamma rays, the exposure is not restricted to thin films or small amounts of polymer. Also, the absorbed dose is not complicated by the depth-dose function which must be used when electronn

n

w

6 0


326


beam radiation absorption is used. Since films do not have to be spun, it is not necessary to work out a spinning solvent. Gamma radiation can be used with macroscopic amounts of polymer. This is particularly welcome when polymers are not compatible with the G P C technique. Larger samples can be characterized by viscosity changes, usually measured in dilute solutions. A l l that is needed is a suitable solvent. If the Mark-Houwink parameters are known, it is possible to calculate viscosityaverage molecular weight, M , from dilute solution viscosities. However, even the raw viscosity-concentration data in terms of the reduced viscosity may be enough to indicate the sensitivity of a given polymer in qualitative terms. The reduced viscosity at concentrations c is v /c where η = (solution viscosity — solvent viscosity)/solvent viscosity. v

sp

8ρ


Copolymer with Alpha-Methylstyrene There are two generalizations often made concerning poly(alphamethylstyrene), ( P A M S ) . The first is that it degrades by random chain scission when exposed to high energy radiation and is in contrast to the cross-linking which predominates when polystyrene is so exposed. The second is that, because of the derealization of energy by phenyl rings, P A M S is inherently more stable than any non-aromatic, carbon-chain counterpart. The radiationchemical yield cited (6) most often is G (s) =0.25 scissions/100 eV. In the present work, values of 0.23 and 0.25 were obtained by gamma radiation and electron beam radiation (50 keV), respectively. A copolymer of M A and A M S was made by free-radical polymerization, see Table II. Seymour and Garner (7) have shown that the alternating copolymer is invariably obtained below polymerization temperatures of 80 °C although random copolymers are obtained above 100°C. CH I - C - C H - CH - C H 3

2

Ô

P(AMS-MA)

C

/Λ /w 0

0

0

Table II. The Effect of Maleic Anhydride ( M A ) and the Methyl Half-Ester of Maleic Acid ( M M ) on Alphamethylstyrene ( A M S ) Polymer Sensitivity Yield

PAMS

P(AMS-MA)

P(AMS-MM)

G(s), scissions per 100 eV

0.25

0.62

1.59

Relative yield

1.0

2.5

6.4



16.

POHL ET AL.


327

DOSE (/xC/cm ) 2

Figure 1. Sensitivity curve for a perfectly alternating copolymer of maleic anhydride with a-methylstyrene. A 50 keV beam was used. The copolymer definitely is more sensitive to electron beam radiation than the homopolymer of A M S (Figure 1). There is, of course, a certain hazard in comparing polymers made by different initiating systems. For example, the thermal stability of poly(methylmethacrylate) is known to vary with the initiator used in its polymerization (#). However, in the present work, the circumstances dictate such a comparison because A M S does not homopolymerize by free-radical initiation and the alternating copolymer is most conveniently prepared this way. There can be little doubt that the predominant effect of radiation is chain scission with a G(s) = 0.62. This result indicates that M A enhances the scissioning tendency of the copolymer despite the fact that the M A unit does not belong to the category of polymers of the general structure II discussed above. Whether or not there is a small amount of crosslinking is difficult to pin down. The slope of the (M )~ curve (Figure 1) is 1.9 times that of the corresponding (M )~ curve. According to the equations of Saito and others (9) the ratio of the slopes can be used to ascertain the relative l

n

l

w


328


amounts of scissioning and cross-linking. The slope of the (M ) is proportional to

1

n

curve, σ(/ι),

G(s) - C O c ) The slope of the ( M )

_ 1

w

(2)

curve, σ(>ν), is proportional to G(s)/2-2G(x)

(3)

at least when the initial distribution is the "most probable." It follows that the ratio of the slopes is σΟι) σ(νν)

=

1

q

=

G(s) -G(x) G(s)/2-2G(x)

=

G(s)/G(x) - 1 G(*)/G0c)-4

(

Thus, it would seem that GGc) is essentially zero. The initial M /M for the copolymer was 1.60 and the ratio increased only to a maximum value of 1.78 (Figure 1). Random chain scission is expected to cause the M /M to approach a value of two. Films for lithographic evaluation were cast from methyl cellosolve acetate, and prebaked at 120°C in a vacuum oven for one hour. Patterns were developed using mixtures of ethyl cellosolve acetate and methyl cellosolve acetate. Areas exposed at a dose of 80 μΟ/οπι (20 keV) were developed with about 10% thinning of the unexposed resist. For development at 100 μΟ/cm , a contrast of 2.1 was observed (Figure 2), and the resolution at this dose was limited to about one micron. Superior sensitivity and resolution were obtained using a M A - A M S copolymer formed on the wafer by prebaking a copolymer of the methyl half ester of maleic acid with alphamethylstyrene. This latter resist system is discussed a little later. The conventional wisdom predicts cross-linking on the basis of radiation studies of ordinary vinyl polymers. On the other hand, there seems to be no previous mention of the radiation sensitivity of maleic anhydride polymers in the literature. There is some reason to expect that conversion of the anhydride to a half-ester might reduce the sensitivity of the copolymers. Hiraoka (10) determined the relative sensitivities of P M M A , P M A (polymethacrylic acid) and P M A A N (polymethacrylic anhydride) by measuring the gaseous products (CO, C 0 , and H ) given off when these polymers were exposed to electron beam radiation of 2.5 keV at 297 °K. He found that the G values (number of chemical events produced per 100 eV of absorbed radiation) for the removal of side groups are 2.0, 7.4 and 16 for P M M A , P M A and P M A A N , respectively. Anderson (//) found a similar relative order of sensitivity. For copolymers of methylmethacryate with 25% dimethylitaconate, 25% monomethyl itaconate or 25% itaconic acid (or anhydride) the G (s) values were 1, 2, 3, respectively. For the copolymer of alpha-methylstyrene and monomethyl maleate, on the other hand, we find an increase in sensitivity by a factor of 2.5 over the corresponding anhydride as described below. The copolymer of A M S with the methyl-ester of M A , M M , was prepared by refluxing a previously-obtained A M S - M A copolymer in methanol w


,

n

w

n

2

2

2

2


16.


POHL ET AL.

1

ι

\

329

ι

\

\

\

7 = 2.1

\ \ LU

\

ω ω Downloaded by TUFTS UNIV on June 4, 2018 | https://pubs.acs.org Publication Date: July 1, 1985 | doi: 10.1021/bk-1984-0266.ch016

LU

ο

\

y 0.5 —

λ

\\

\

û LU N

\\ \\

Materials for Microlithography - American Chemical Society

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