Materials for Microlithography - American Chemical Society

14 Soluble Polysilane Derivatives: Interesting New Radiation-Sensitive Polymers R. D. M I L L E R , D. H O F E R , D. R. McKEAN and C. G . W I L L S O N IBM Research Laboratory, San Jose, C A 95193

R. W E S T and P. T. T R E F O N A S III

Downloaded by CORNELL UNIV on May 10, 2017 | http://pubs.acs.org Publication Date: July 1, 1985 | doi: 10.1021/bk-1984-0266.ch014

Department of Chemistry, University of Wisconsin Madison, WI 53706

Organopolysilanes (i.e., high molecular weight polymers which contain only silicon in the backbone) are old materials which have renewed interest because of improvements in synthetic and characterization techniques. The first reported aromatic organosilane polymers were described by Kipping in 1924 (1). Twenty-five years later, Burhard reported the preparation of permethylated polysilane (2). These materials were, however, highly crystalline, insoluble white solids which evoked little scientific interest until recently when it was discovered that silane polymers could be used as thermal precursors to β-silicon carbide fibers (3-5). In this regard, Yajima and co-workers reported that poly(dimethyl)silane could be converted by the two -step process shown below to β-silicon carbide, a structural material of considerable industrial importance.

Although the chemistry of the initial thermal transformation is obviously quite complex, it was determined that considerable carbon insertion into the Si-Si bonds occurs, resulting in an intermediate carbosilane which can be drawn into fibers. A t this point, brief oxidation results in the formation of a surface oxide which imparts dimensional stability, and subsequent heating to 1300°C produces silicon carbide fibers (3,5). The observation that more soluble, less crystalline materials containing larger alkyl substituents (6) or aromatic groups (7,8) could be synthesized suggested additional applications for polysilane derivatives. In this regard, West and co-workers have reported that a soluble copolymer 1 produced by the 0097-6156/84/0266-0293$06.00/0 © 1984 American Chemical Society

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


294

MATERIALS FOR MICROLITHOGRAPHY

co-condensation of dimethyl and methyl phenyl dichlorosilane (7) could be used as an impregnating agent for ceramic materials (9,10). In addition, they found that the same copolymer could be doped to a semiconducting level by treatment with oxidizing agents such as arsenic pentafluoride (7). We were intrigued by the observation that 7, unlike the related carbon polymer polystyrene, absorbs strongly from 200-350 nm. Furthermore, this material was described as photosensitive, and irradiation produced insoluble material which indicated that cross-linking had occurred (7). The unique spectral properties of 7, coupled with the reported photosensitivity, suggested that soluble polysilane derivatives might comprise a new class of radiation sensitive polymers with lithographic potential. In particular, it seemed that materials of this type, which have a relatively high S i / C ratio and can be imaged in an efficient manner, could be useful in dry etch image transfer processes utilizing oxygen reactive ion etching ( 0 - R I E ) (77). In addition, the strong adsorbance from 200 to 350 nm suggested multilayer lithographic applications which are currently popular as a technique for generating high resolution images where the dimensions of chip topography approach the imaged feature sizes (12). 2

I

Ph

\

Si

-4-

3

+

— Si \

/

CH \

C

H

3 /

\

C

H

h

3

Synthesis and Characterization We have recently synthesized and characterized a variety of soluble, substituted polysilane homopolymers by the condensation of appropriately substituted methylsilyl dichlorides with sodium dispersion as shown below and in Table I (75). Me RMeSiCl

2 3 4 5

R R R R

= =

2

2Na(5% xs) > Toluene Δ

Ph p-Tolyl 0-Phenethyl n-Propyl

I

_ Si

+ 2NaCl

I

R 6 R 7R 8 R 9R

= =

n-Butyl n-Hexyl n-Dodecyl Cyclohexyl


(2)

14.

295

Soluble Polysilane Derivatives

MILLER ET AL.

Table I. Yields and Molecular Weights (Relative to Polystyrene) for Both Fractions of the Bimodal Distribution from Gel Permeation Chromatography in T H F of Organosilane Polymers. M x 10" n


Polymer

0

M x 10"

R*

% Yield

z

3

x 10"

3

3

M /M w

n

(2)

107 3.3

193 5.6

313 9.9

1.81 1.69

0.72

55

(3)

66 4.7

213 5.9

421 7.0

3.23 1.26

0.56

25

(4)

134 3.3

286 4.4

489 6.2

2.13 1.36

2.30

35

(5)

297 7.4

644 13.3

1160 22.1

2.17 1.79

0.27

32

(6)

50 4.4

110 5.9

218 7.6

2.19 1.36

1.50

34

(7)

281 14.6

524 20.5

811 27.2

1.86 1.40

2.40

11

(β)

172

483

881

2.81

b

8

(9)

300 3.2

804 4.5

1419 5.9

2.67 1.40

8.7

10

a.

R is the ratio of the high M W fraction of the bimodal distribution to the low M W fraction from the G P C elution profile.

b.

This polymer displayed a broad, monomodal distribution.

c.

See Equation 2 for specific structure.

The polymers were produced in yields which ranged from 8-55%. It was observed in most cases that the highest molecular weights were obtained when the sodium dispersion was added to the monomer in toluene (4:1 toluene/monomer) in what we term "inverse addition." Normal addition (i.e., addition of the monomer to the sodium) usually resulted in slightly improved yields of lower molecular weight material. In either case, it is advantageous to avoid a large excess of sodium in the reaction mixture, since this usually results



296


in the formation of small amounts of toluene insoluble gels which are difficult to filter, thus complicating the polymer purification. Most of the polymers isolated using the "inverse addition" technique show distinct bimodal molecular weight distributions (see Table I). In this regard, 8 is anomalous, as the material isolated in the usual fashion has a broad "monomodal distribution." A l l of the polymers are soluble in toluene, although 9 was considerably more soluble in a 3:1 mixture of toluene-ethylcyclohexane. The polymers containing an aromatic substituent (2-4) as well as the cyclohexyl derivative 9 are hard, brittle, high melting solids when completely solvent free. The other materials are generally soft, sticky or elastomeric. The spectral and analytical data for these polymers are as expected and are described in detail in Reference 13. The hard brittle materials 2-4 and 9 have glass transition temperatures in excess of 75 °C as measured by differential scanning calorimetry (DSC) or by thermomechanical analysis ( T M A ) . Thermal gravimetric analysis (TGA) shows that the polysilanes, in general, are quite stable in a nitrogen atmosphere and suffer little weight loss at temperatures below 325 °C. Examination of Table I reveals another interesting feature of the anionic polymerization process. As the steric bulk of the substituents in the monomer increases, the isolated yields of high polymer drop precipitously. The remaining material is, in such cases, isolated as a mixture of low molecular weight materials including cyclic oligomers. We felt that the poor yields of high polymer might be due to a slow propagation reaction between the sterically hindered growing anionic chain and the tertiary silyl electrophile, a feature that would be exacerbated by the presence of a nonpolar solvent such as toluene. To test this hypothesis, the polymerization medium was modified by the incorporation of varying quantities of diethyleneglycol dimethyl ether (diglyme) in hope of facilitating the propagation step. The results of this procedure for the preparation of 9 are shown in Table II. The addition of diglyme (-—25%) results in an approximately three-fold increase in the yield of high polymer. A t the same time, this modification allows the control of molecular weight, since the increased solvent polarity results in an early precipitation of polymer. The successful solvent modification for the preparation of 9 suggests possible utility for the generation of polymers from very sterically hindered monomers. Spectral Properties Examination of the absorption spectra of the new polysilane materials reveals a number of interesting features (14). As shown in Table III, simple alkyl substituted polymers show absorption maxima around 300-310 nm. Aryl substitution directly on the silicon backbone, however, results in a strong bathochromic shift to 335-345 nm. It is noteworthy that 4, which has a pendant aromatic side group that is buffered from the backbone by a saturated spacer atom, absorbs in the same region as the peralkyl derivatives. This red shift for the silane polymers with aromatic substituents directly bonded to the backbone is reminiscent of a similar observation for phenyl substituted and terminate silicon catenates relative to the corresponding permethyl derivatives


14.

MILLER ET AL.

Table II.

Entry

297

Solvent Effect on Yields of Poly(Cyclohexyl Methyl) Silane.

Na/Si (molar ratio)

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2.05

Solvent b

% Yield 14

M x 10" n

x 10~ 840

3

2.0

b

11

1.101

3.2

4

2.0

d

31

16.5

25% Diglyme 75% Toluene

d.

75% Diglyme 25% Toluene

2.27

2.08

23.1

c.

1.40

11.1

30

Toluene

2.67

1.33

c

b.

486

n

4.6

2.0

Bimodal Distribution

a

w

6.1 3

a.

M /M

300

a

4.5 2

3

6.04

2.73

(15). In the model compounds, this red shift has been ascribed to a combination of σ-ττ mixing of the H O M O of the silicon backbone with the πorbitals of the aromatic substituent coupled with a decrease in the L U M O energy due to ττ -(σ , d) interactions (15,16). Further examination of the data in Table III shows that the absorption maximum of the cyclohexylmethyl derivative, 9, is also somewhat red-shifted relative to the other alkyl polymers suggesting that the steric bulk of the substituents and/or conformational effects may also influence the polysilane absorption spectrum. Earlier work has shown that materials with catenated Si-Si bonds and only alkyl substituents absorbed strongly from 200-300 nm (16,17). This transition has been described as either a σ-σ* or a a-3dw ^ transition which rapidly shifts to longer wavelengths with increased catenation until it Sl

Sl


298

MATERIALS FOR M I C R O L I T H O G R A P H Y

Table III. Ultraviolet Absorption Properties of High Molecular Weight Poly(organosilanes). Polymer

a

λ

M

b 2


"max

[PhMeSi]„ (2)

341

193,000

[(p-toly) (Me) Si]„ ( 3 )

337

75,000

[(/3-phenethyl) (Me) Si]„ (4)

303

286,000

[(n-Pr) (Me) Si]„ (5)

306

644,000

[(η-Bu) (Me) Si]„ (6)

304

110,000

[(η-Hex) (Me) Si]„ (7)

306

524,000

[(n-dodecyl) (Me) Si]„ (8)

309

483,000

[(Cyc) (Me) Si]„ (9)

326

804,000

a.

U V of 2-8 was measured in spectrograde T H F . U V of 9 was measured in cyclohexane.

b.

M

w

determined by G P C and are relative to polystyrene.

approaches a limiting value in the region of 20-24 monomer units (18). We have extended the range of this study using photodegraded samples of 8 (vide infra) of known molecular weight and the results are shown in Figure 1. Since the X of polysilane derivatives approaches a limiting value at a relatively low degree of polymerization, the spectral characteristics of high polymers of different molecular weights can be directly compared with relatively little error. Pitt and co-workers have discussed the theoretical aspects of this spectral leveling phenomenon (79). Figure 2 displays a plot of the molar absorptivities per Si-Si bond versus the chain length for two typical polysilanes, 2 and 8. In a manner similar to that observed for the absorption maxima, the molar absorptivities of both polysilanes increase rapidly with molecular weight, but ultimately approach a limiting value. It is interesting that not only does 2 absorb at longer wavelengths than 8, but it is also more strongly absorbing. The behavior of the molar absorptivities with molecular weight is somewhat unexpected based on a related study involving linear permethyl silanes M e ( M e S i ) M e , η < 24 which indicated that although the total absorptivity increased with n, the absorptivity/Si-Si actually decreased (17,18). m a x

2

n


14.

MILLER ET AL.

299


300

275 ο

ε


250

225

200 1350 100 150 200 CHAIN LENGTH, η Figure 1. Plot of UV absorption maxima versus chain length(n) for poly (alkyIsilane). The (+) represent Me(Me Si) and the (*) represent [(n-dodecyl) (Me)Si] . 50

2

n

n

Photochemistry The dependence of both the X and the molar absorptivity on the degree of polymerization has interesting consequences for any radiation induced process which significantly lowers the molecular weight. This is dramatically demonstrated in Figure 3 which shows the effect of continued photolysis on the absorption spectrum of a solid film of 2. The continuous shift in the absorption maximum to shorter wavelengths and the attending decrease in the peak intensity strongly suggests that the polymer is undergoing a significant reduction in molecular weight. This fact was confirmed by G P C examination (micro-styragel column, polystyrene standards) of T H F solutions of the photodegraded polymer films and is particularly interesting in light of the report that the copolymer I undergoes extensive cross-linking upon exposure, particularly in the solid-state (7). In an effort to quantify the effect of photolysis, polysilanes 2 and 8 were selected as typical models. For this experiment, samples of 2 were fractionated by repeated precipitation from toluene using isopropanol. No additional m a x



300


Figure 2. Plot of absorptivity per Si-Si bond at M versus chain length n. The circles represent [PhMeSi] and the squares represent [(n -dodecyl) (Me)Si] . m a x

n

n

10.0

I

I

I

I

I

-

9.0 8.0 7.0

ο ζ 6.0 < m cr 5.0 ο ω 4.0

UJ

Materials for Microlithography - American Chemical Society

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