Importance of DonorâAcceptor Reactions for the ... - ACS Publications

Chapter 7

Importance of Donor—Acceptor Reactions for the Photogeneration of Acid in Chemically Amplified Resists Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 10, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch007

Nigel P. Hacker Research Division, IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099

The photophysical interactions between the polymer and photoinitiator in resist systems can play an important role in the photogeneration of acid in chemically amplified resists. If there is no photophysical interaction acid is generated by direct photodecomposition, but if the polymer can act as an excited state electron donor an electron transfer can occur. Fluorescence spectroscopy and photoproduct analyses are used as mechanistic process for electron transfer reactions.

The invention of new photoresists using photochemically generated acid to catalyze reactions in polymer films has traditionally been a vertical process. The process involves the synthesis of a new polymer that can undergo an acid catalyzed chemical reaction. A requirement of the acid catalyzed reaction is a change in polarity of the polymer, e.g. a hydrophobic to hydrophilic reaction, that renders the polymer more soluble in the development solvent. Once the polymer dissolution properties are optimized to give good contrast, i.e. is good dissolution inhibitor before, and gives enhanced dissolution after the acid catalyzed process, the cationic photoinitiator is added. The role of the cationic photoinitiator is to make the resist system photosensitive. The chemistry of the cationic photoinitiator was considered to be innocuous because the photoinitiator concentration is low, c.a. 1-2 wt %. For example the changes in dissolution properties of a formulation that may be caused by a chemical change to an additive at 1 wt % loading in a high contrast

0097~6156/94/0579-0093$08.00y0 © 1994 American Chemical Society

Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 10, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch007

94

POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS

resist, are expected to be minimal. Although the role of the photoinitiator for changing the wall profiles of sub-micron features of resists has been observed, the mechanism is not well understood. It has always been appreciated, however, that the U V absorption of the photoinitiator should be optimized such that the initiator absorbs all of the incident light. Also under these conditions the polymer should have zero absorbance. In most deep U V resists the polymer has aromatic pendent groups that absorb some of the incident light and it is generally assumed that light absorption by the polymer is wasted and thus is detrimental to resist performance. In this paper the photochemical and photophysical properties of the polymer and photoinitiator will be described and the consequences of these properties on the photoinitiation reaction will be discussed. The goal is to better understand photoinitiation processes in chemically amplified resists and clarify the roles played by both polymer and photoinitiator. This knowledge, coupled with learning physical properties, e.g. dissolution properties, solubility and photophysics, of the individual components should lead to a horizontal approach for the development of new photoresists. Photochemistry The solution photochemistry of triarylsulfonium salts has been studied in detail. Direct photolysis of triphcnylsulfonium salts (TPS) gives 2-, 3- and 4-phenylthiobiphenyls (PTB), diphcnylsulfidc, and acid. (/ - 4) The PTB's are formed by in-cage fragmentation-recombination reactions, whereas diphenylsulfide formation is a cage-escape process. The general trend that the yield of PTB isomers increased relative to diphenylsulfide in more viscous solvents, presented strong evidence for the cage-vcrsus-cscape reactivity from direct photolysis of TPS. Accompanying the change in relative yields of PTB's is a decrease in quantum yield in more viscous solvents. This is because a process that regenerates sulfonium salt, fragmentation followed by recombination on sulfur, becomes predominant. Figure 1 plots the decrease in relative quantum yield and the increase in ratio of PTB's : diphenylsulfide versus increasing viscosity. Also the viscosity effect is observed in polymers where it was found that the ratio of PTB's : diphcnylsulfidc were higher in films of poly(methyl methacrylate) and polyvinyl alcohol) than in non-viscous solutions. (3) 1

Triphenylsulfonium salts have a triplet energy of 74 kcal mole' and undergo triplet sensitized photolysis to give diphenylsulfide, benzene and acid by a cage-escape reaction. (5) In contrast to direct photolysis, which occurs



HACKER

Donor—Acceptor Reactions for Photogeneration of Acid

10-, Δ relative quantum yield

8H

•

•

2H

0-1 1E~01

1 1E+00

phenylthiobiphenyls : diphenylsulfide

• 1 1E4-01

ι 1E+02

Viscosity (cp)

1 1E+03

1 1E+04

Figure 1. Product yield and distribution versus viscosity from photolysis of triphenylsulfonium salts at λ = 254 nm.


96


by initial heterolysis of the carbon-sulfur bond, the triplet sensitized reaction proceeds by homolytic cleavage to give the phenyl radical - diphenylsulfinyl radical cation, triplet radical pair which react with solvent to give escape products faster than undergoing spin inversion to the singlet radical pair for recombination. Triphcnylsulfonium salts are good electron acceptors (E d = -1.2 V) and react with excited state electron donors by an electron transfer reaction. For example the singlet excited state for anthracene reacts with TPS to give anthracene radical cation and triphenylsulfur radical, this pair of intermediates fragments to give the triad of anthracene radical cation, phenyl radical and diphenylsulfide. (6) Acid is produced from the latter triad by an in-cage recombination reaction to give 1-, 2- and 9-phenylanthracenes or by reaction with solvent to give cage-escape products. Triplet energy transfer from the excited state of anthracene will not occur because the energy of the triplet excited state of anthracene, 42 kcal mole" , is too low for energy transfer. Electron transfer from the singlet excited state of anthracene is exothermic by 23 kcal mole" . Further evidence for electron transfer from the singlet excited state of anthracene comes from fluorescence spectroscopy. Figure 2 shows the effect on the fluorescence spectrum of anthracene by adding triphcnylsulfonium salt. The wavelengths of the anthracene fluorescence peaks are not shifted but do decrease in intensity upon addition of sulfonium salt. A Stcrn-Volmer plot of this fluorescence quenching reveals that the reaction occurs at close to the diffusion controlled rate in acctonitrile solution. In contrast to the triplet energy sensitized reaction, the electron transfer reaction gives diphenylsulfide as a by-product from both in-cage recombination and cage-escape reactions.


re

1

1

Evidence for the intermediates described from the above product studies for the direct, triplet sensitized and photo-induced electron transfer reactions of TPS salts has been also presented using pboto-CÏDNP and nanosecond flash photolysis techniques. (7) Interaction between Polymer and Photoinitiator The photochemistry of TPS in poly[4-[ (terf-butoxycarbonyl)oxy] styrene] (poly-TBOC) was studied to better understand the photochemistry of chemically-amplified resists. (8, 9) From detailed product studies in solution and in films, it was determined that the polymer was sensitizing the photodecomposition of TPS because considerably less PTB relative to diphenylsulfide was produced in the presence of poly-TBOC. Table 1 shows the effects the polymers can have on the yields of PTB's and diphenylsulfide



7.

HACKER

97 Donor—Acceptor Reactions for Photogeneration of Acid

Wavelength (nm) Figure 2. Quenching of anthracene fluorescence by triphenylsulfonium salt.


98


Table 1: Photoproduct Distribution from Irradiation of Triphenylsulfonium Salts (Concentration χ 10 M) 5

Ph S

Ph-PhSPh

0.95 1.92 13.24 110.9

1.95 6.75 10.42 82.8

0.75

121.5 221.4

137.3 75.0

1.13 0.34

0.70 1.28 8.96 3.70 14.43

0.55

0.78

2

Cage/Esc


λ = 254 nm

1 2 3 4

1.0%, TBOC, Film 1.0%, ΡΜΜΛ, Film 1.0%, TBOC, C U C N 10.0%, TBOC, C I I 3 C N

5

0.01 M , C I Ï 3 C N

6

0.01 M , C I Ï 3 C N

3

2.04 3.51 0.79

+ 0.1 M Anisole λ = 300 nm

7

1.0%, TBOC, Film

8 1.0%, TBOC, C I Ï 3 C N 9 10.0%, TBOC, CH3CN 10 0.01 M , CH3CN 11 0.01 M , CII3CN -H 0.1 M Anisole

trace 0.37 1.29 0.79

—

0.04 0.35 0.06

in solution and the solid stale. Entries I and 2 show that relatively more PTB's are formed after photolysis of TPS salts at 254 nm in poly(methylmethacrylate) (PMMA) than in poly-TBOC. The change in PTB formation may be due to a sensitization process which occurs in poly-TBOC and not P M M A . Alternatively if (he micro viscosity of P M M A is larger than poly-TBOC, then a similar trend is expected for the sulfide photoproducts. To eliminate the microviscosity effect, TPS / poly-TBOC formulations were dissolved in acetonitrile and irradiated. Entries 3 and 4 show that the relative yield of PTB's markedly decrease to less than from direct photolysis of TPS in the absence of polymer (entry 5). These results suggest that a sensitization reaction is occurring. To further probe the sensitization reaction, TPS formulations were irradiated at 300 nm where the polymer absorbs more of the incident light than the photoinitiator. The presence of poly-TBOC significantly lowers the relative yield of PTB's (entries 7-10) in both films and solution. For example the ratio of PTB : diphcnylsulfidc (entry 9) is 0.04 in the presence of poly-TBOC whereas without the polymer the ratio is 0.35 under


7.

HACKER

Donor—Acceptor Reactions for Photogeneration of Acid 99

similar conditions (entry 10). Experiments using anisole (entries 6 and 11) seem to mimic the poly-TBOC influence on TPS photochemistry in solution, suggesting anisole is a good candidate as a model monomeric compound for poly-TBOC. If anisole is used as a model, energy transfer from the triplet excited state of anisole (Εχ = 80.8 kcal mole" ) to TPS (Εχ = 74 kcal mole" ) is viable from energetic arguments. Similarly an electron transfer reaction from either the singlet or triplet excited state of anisole to TPS (- 44 and -22 kcal mole" respectively) is also thermodynamically favorable. To determine the nature of the sensitization reaction the fluorescence spectroscopy of the pol ymers was studied. Figures 3 and 4 show the fluorescence spectra of a number of substituted poly(styrcncs) in solution. All of these polymers emit in the 280 - 450 nm region. In particular poly(4-hydroxystyrene) (poly-HOST), poly(4-methoxystyrene) (poly-MOST), and poly-TBOC all fluoresce in the 300 - 350 nm region in both solution and as films. Addition of TPS to solutions of 4-oxystyrene polymers does not shift the emission peaks but results in a decrease in the emission intensities. In solution the fluorescence from these polymers is quenched by TPS to give linear Stern-Volmer plots (Figure 5). This quenching is at close to diffusion controlled rate based on the lifetimes of model monomers and an experimentally obtained value for poly-TBOC (Table 2).


1

1

1

Table 2: Quenching constants for TPS in 4-oxystyrene polymers 1

k T (M" ) q

Gradient'

Polymer

Estimai c

1

b

poly-MOST

166

166

poly-HOST

80

40 - 148

poly-TBOC

90

50

e

d

a. from Stern-Volmer plots (Φο / Φ = Ιο /1 = 1 + k T [ Q ] (for acetonitrile K = 2 χ 10 L s" ). b. for anisole, a model monomer, Τ = 8.3 ns. c. for phenol, a model monomer, Τ = 2.1 - 7.4 ns. d. for poly-TBOC, Τ = 2.5 ns. q

10

1

q




Ο

δ

Ο

ΗΝ

η

ο

Η

r Μ ο

Η

ο

*3

ο

g

Ο

Η

η

Μ δ

r;

*a

h-1 Ο Ο


7.

HACKER

300


350

400

Wavelength (nm) Figure 4. Fluorescence spectra of poly(4-oxystyrene) derivatives.


450

102


C H 2

t

H— poly-HOST C H — poly-MOST Ο

-H„

3

(CH ) COC—


3

3

poly-TBOC

0.01 0 02 0.03 TPS.Sb Concentration (M)

0.04

Figure 5. Plot of I /I versus molar concentration for emission of poly(4-oxystyrene) derivatives in acctonitrile solution in the presence of triphenylsulfonium salt. 0

Ρ + Ph S*X" 3

+

[Ph S XT 3

[ΡΓ + Ph S*X" 3

P*' + Ph S* + X" 3

P*' + Ph* + X"

+

+

[P]* + P h S X " + [Ph S X~]* + Ρ 3

3

PhPhSPh + Ph S + HX 2

- Ρ*· + Ph S* + X" 3

+

Ρ · + Ph* + Ph S + X" 2

P-Ph + HX

where Ρ = poly [4-[(tert-butoxycarbonyl)oxy]styrene] Figure 6. Dual photoinitiation mechanism for triphenylsulfonium salts in poly-TBOC resist.



7.

HACKER


The results from fluorescence spectroscopy indicate that the sensitization reaction is a photoinduced electron transfer reaction from the singlet excited state of the polymer to TPS salts. However the photoproduct studies show that PTB's, products expected from direct photolysis of TPS salts, are formed, albeit in lower than expected yields. The combined photoproduct and fluo rescence spectroscopy studies suggest that acid formed by both direct photolysis and photoinduced electron transfer reaction in the poly-TBOC/TPS resist and a Dual Photoinitiation Mechanism (DPM) is proposed for acid formation (Figure 6). At 1-10 wt % TPS loadings both the polymer and the photoinitiator absorb the incident light. The light absorbed by TPS generates acid by the direct photolysis mechanism whereas the light absorbed by the polymer also produces acid by an electron transfer reaction from singlet excited state of poly-TBOC to TPS. The latter reaction occurs by mechanism similar to the reaction described for anthracene sensitization of TPS salts. Excited State Polymer and Ground State Photoinitiator Reactions The ability of the polymer to generate acid by a photoinduced electron transfer reaction with the cationic photoinitiator is not exclusive to TPS derivatives. Figure 7 shows the UV absorption spectra of non-ionic photo-acid generators. The concentrations of each initiator are adjusted for maximum absorbance. The pyrogallol and succinimidoyl derivatives have extinction coefficients of 10 M " at 250 nm, about an order of magnitude less than substituted polystyrenes (ε = I0 M " cm" ). As the photoacid generator is only 1-2 wt % of the formulation in chemically-amplified systems, it can be concluded that the polymer absorbs 99 % of the incident light in resists formulated with these initiators. The Stern-Volmer plots for quenching of poly(4-hydroxystyrene) fluorescence by non-ionic photoinitiators are shown in Figure 8. The pyrogallol sulfonate derivative exhibits similar quenching behavior to TPS salts, i.e. quenching is close to the diffusion controlled rate. While the succinimidoyl derivative is 2-3 times slower than TPS salts, sub stituted derivatives of these imides can exhibit more efficient quenching. The relatively weak absorbances of these photoinitiators in the deep UV and their ability to efficiently quench the polymer fluorescence suggest that the polymer absorbs the incident photon and sensitizes the decomposition of the initiator. Kasai has reported that excitation of the D-line of sodium atoms in argon matrices with imidoyl triflates results in a dissociative electron transfer reac tion. (10) A similar reaction occurs in resists. The singlet excited state of the polymer donates an electron to the photoinitiator which dissociates and gen2

1

3

1

1


1



104


7.

HACKER


CH o=s=o


3

1 \ II N—O—S—R

Ù ο

.0000

11

Κ

ο

.0040

Δ

.0080 .0120 .0160 Photoinitiator Concentration (M)

.0200

Figure 8. Plot of I /I versus molar concentration for quenching the fluorescence from poly(4-hydroxystyrene) in acetonitrile solution in the presence of cationic photoinitiators. 0


106


erates acid. There is no direct photolysis component, which is observed with TPS salts, because of the weak absorbanccs of the non-ionic photoinitiators at 250 nm. This rationalizes the observation of quantum yields > 10 for pyrogallol sulfonate derivatives resist formulations by Ueno who concluded that a sensitization reaction must occur. (//)


Ground State Polymer and Excited State Photoinitiator Reactions The UV spectrum of the naphthalimidoyl inflate shows a long wavelength absorbance at 300 - 360 nm (Figure 7). The fluorescence spectrum at room temperature and phosphorescence spectrum at 77 Κ arc shown in Figure 9. The observation of fluorescence from a photoinitiator at room temperature is unusual because most photoactive compounds e.g. TPS salts photodecompose rather than fluoresce. However if phenol is added to acetonitrile solutions of naphthalimidc triflate, the fluorescence intensity de creases (Figure 10). If the quenching is diffusion controlled, a fluorescence lifetime of 0.75 ns is estimated from the Stern-Volmer plot. It is known that naphthalimide triflate is good electron acceptor and undergoes dissociative electron transfer reactions in the presence of an electron donor. (10) If phenol is considered as a model compound for poly-HOST, it is proposed that the excited state of naphthalimidc photoinitiator accepts an electron from the ground state polymer and dissociates to generate acid. Wallraff has reported that naphthalimide triflate is a poor photoacid generator in poly(alkyl acrylate) resists relative to onium salts. (12) Acrylatcs arc poor electron donors and thus will not sensitize the photodecomposition of photoinitiators. Onium salts decompose by the direct photolysis mechanism in acrylate pol ymers and do not require the presence of an electron donor to generate acid. Conclusion From these studies it can be concluded that light absorption by the polymer can result in acid formation from the photoinitiator in chemically amplified resists. Onium salts arc capable of generating acid by direct ab sorption of light and also by electron transfer sensitization from the polymer excited state. Pyrogallol and succinimidoyl sulfonate derivatives are weakly absorbing and generate acid by a photoinduced electron transfer reaction from the singlet excited state of the polymer. In contrast while naphthalimidoyl sulfonates do absorb the incident light, photogeneration of acid occurs by an electron transfer of the polymer ground state to singlet excited state of the photoinitiator. From these studies it is concluded that the photochemistry



Figure 9. Fluorescence and phosphorescence spectra of naphthalimidoyl triflate in ethanol/methanol.




C/2

Ο

H

r

π

ο 2

PC

H

ο w r w η

Π

Ο

H

η

S

M

Ο

*d

h-* Ο 00

7.

HACKER


and photophysics of both the polymer and cationic photoinitiator need to be optimized for maximum performance from a new photoresist design.


References 1. N. P. Hacker, in New Aspects of Radiation Curing in Polymer Science and Technology, Volume 2, J. P. Fouassier and J. F. Rabek, Eds. Elsevier Science, 1993, 473; N. P. Hacker, in Photopolymers, Mechanisms, Devel opment and Applications A. D. Trifunac and V. V. Krongauz, Eds. Chapman and Hall, in press. 2. Dektar, J. L.; Hacker, N. P. J. Chem. Soc., Chem. Commun. 1987, 1591. 3. Hacker, N. P.; Dektar, J. L. Polym Prepr. 1988, 29, 524. 4. Dektar, J. L.; Hacker, N. P. J. Am. Chem. Soc. 1990, 112, 6004. 5. Dektar, J. L.; Hacker, N. P. J. Org. Chem. 1988, 53, 1833. 6. Dektar, J. L. ; Hacker, N. P. J. Photochem. Photobiol., A. Chem., 1989, 46, 233. 7. Welsh, Κ. M.; Dektar, J. L.; Garcia-Garibaya, Μ. Α.; Hacker, N. P.; Turro, N. J. J. Org. Chem., 1992, 57, 4179. 8. Hacker, N. P.; Welsh, Κ. M.; Macromolecules 1991, 24, 2137. 9. Hacker, N. P.; Welsh, Κ. M. Structure-Property Relations in Polymers: Spectroscopy and Performance, ACS Advances in Chemistry Series No. 236, Urban, M. W.; Claver, C. D. Eds.; American Chemical Society, Washington D. C. 1993, 557. 10. Kasai, P. H. J. Am. Chem. Soc. 1992, 114, 2875. 11. Schlegel, L.; Ueno, T.; Shiraishi, H; Hayashi, N.; Iwayanagi, T. Chem. Mater., 1990, 2, 299. 12. Wallraff, G.; Allen, R.; Hinsberg, W.; Larson, C.; Johnson, R. DiPietro, R.; Breyta, G.; Hacker, N.; Kunz, R. R. J. Vac. Sci. Technol. B, 1993, 11, 2783. RECEIVED September 13, 1994


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