Correlation of the Strength of Photogenerated Acid with the Post

Chapter 6

Correlation of the Strength of Photogenerated Acid with the PostExposure Delay Effect in Positive-Tone Chemically Amplified Deep-UV Resists 1

Downloaded by AUBURN UNIV on December 27, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch006

F. M. Houlihan, E. Chin, O. Nalamasu, J. M. Kometani, and R. Harley

AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 The changes in clearing doses (D ) for deep UV chemically amplified resists formulated with poly(4-t-butoxycarbonyloxystyrene-sulfone) were correlated with changing the photo-acid generator (PAG) component using a variety of 2-trifluoromethyl-6-nitrobenzyl benzenesulfonates. These changes inD 'swere also monitored under different environmental conditions, before post-exposure bake (PEB), to determine their sensitivities to airborne contaminants. It was found that if airborne contamination is eliminated through using a protective overcoat, then D decreased with increasing reactivity of the photo-released acid. Also, for most of the resist formulations, the sensitivity to airborne contamination increased with the strength of the photo-released acid. However, sensitivity to airborne contaminants was lessened in the presence of PAG's with substituents that can associate strongly with other molecules. Combining these effects, the order of resistance of resist formulations to airborne contaminants as a function of substituents on PAG on the aryl sulfonate moiety was 4-OCH >4-NO > 4-Cl > > 3-SO R > > 4-CH ≥ 4-H > 2-CF ≥ 3CF ≥ 4-CF > 2,4-diF > 2-NO > 3,5-diCF . p

p

p

3

3

2

3

3

2

3

3

3

Scheme 1 depicts chemical amplification through the acidolysis reaction during post-exposure bake (PEB) initiated by photo-released acid in a typical tbutoxycarbonyl (ί-BOC) based deep UV resist (/). One serious processing problem with such materials is the occurrence in exposed areas of surface depletion of photo-released acid by basic airborne contaminants (Scheme 2). This results in an ineffective removal of dissolution inhibiting /-BOC groups during PEB. This depletion of acid manifests itself as a developer insoluble surface layer which leads to T-shaped lines (T-topping) resulting in loss of resolution of small features (2,5). Although airborne contamination may occur at any time during processing, its effect is most readily seen by increasing the time between exposure 1

Current address: Stanford University, Stanford, CA 94305 0097-6156/95/0614-0084$13.50/0 © 1995 American Chemical Society

Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


6. HOULIHAN ET AL.

Photogenerated Acid and Post-Exposure Delay Effect

and PEB; in this circumstance, it is called the post-exposure delay (PED) effect. Methods have been described in the literature to reduce the PED effect by using protective overcoats (2),filteringthe impuritiesfromthe air (5), and annealing away free volumefromthe polymer (4). We will describe here work that we have done to better understand the part the chemistry of the photo-acid generators (PAG's) plays in the PED effect. This information would be helpful in the rational design of PAG's less susceptible to the PED effect. In our previous work on PAG's based on the 2-nitrobenzyl chromophore, we have shown that Hammett plots were useful in predicting the relative thermal stability of resist films in which PAG's of differing structures were added (5-8). This supported the reaction mechanism for thermal decomposition depicted in Scheme 3. Hammett plots should also be useful in predicting lithographic properties such as lithographic sensitivity. However, to accomplish this one must first try to establish a relationship between lithography and chemistry. Previously, we have shown (la) that there is a reciprocal relationship between the clearing dose (D ) (as defined in (9)) and the product of absorbance/μπι (ABS/μπι), quantum yield (Φ), and the catalytic chain length for the removal of /-BOC groups (Chain Length). This relationship was improved by using the fraction of light absorbed per micron (%ABS/μπι), (Equation 1) which is a better measure of the number of photons absorbed by the resist film (Figure 1), especially in more absorbant films. p

Dp oc l/(%ABS^m*0*Chain Length) (1) Figure 1 allows us to link empirically the relative values of Chain Length for /-BOC removal for a resist system toitsD as shown in Equation 2. p

Chain Length oc l/(%ABS^m*0*D ) p

(2)

Assuming that the different photo-acids diffuse equally, then differing substituents on the arylsulfonate moiety of the PAG's may affect the Chain Length for /-BOC acidolysis through reactions which change directly the concentration of 'free' (10) H* or affect this concentration indirectly by changing the concentration of the intermediate /-butyl cation. Moreover, these reactions must be reversible to account for the observation that in our 2-nitrobenzylsulfonate based resists increasing PEB time also increases the Chain Length (7a). Reversible, interceptions of either 'free' H*or /-butyl cation would account for this. Scheme 4 illustrates one possible scenario in which increased nucleophilicity and/or basicity of the sulfonate anion decreases 'free' H*. The nucleophilicity (11) and the basicity (12,13) of arylsulfonates are known to decrease 2-3 of magnitude with increasing σ. Because arylsulfonic acids are know to be more poorly dissociated in non-polar environments (14), substituents effects could greatly affect the equilibrium in Scheme 4, changing the concentration of 'free' H* available in the relatively non-polar polymer matrix. Similarly, binding of nucleofiigal anions (even triflate) in carbocationic-like processes is an accepted phenomenon which has been reviewed in the literature (15). This binding would be expected to proceeds well under SNI or SN conditions especially in a non-polar environment such as the resist matrix. As an example, tosic acid cannot be used effectively in the cationic Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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MICROELECTRONICS TECHNOLOGY

86

Scheme 1 (CHT-CH) (SO )


2

(CHÎ-ÇH) (SO ) 2

CU

3

CH —C—CH 3

Δ

CH2=C

3

N

èH

3

CH

Scheme 3


3


6. HOULIHAN ET AL.


8

Figure 1 Plbtof l/(%ABSfym**D)) oc p

ρ

σ

(4)

These results indicated that the increased reactivity of the acid (decreased basicities and/or nucleophilicities of arylsulfonate such as depicted in Schemes 4 and 5) was being fiinneled into some other reaction pathway. At the time, we proposed (7a) that one pathway might be the PED effect caused by airborne basic contaminants (2,5). We report here further investigation to demonstrate conclusively the link between the PED effect and the reactivity of the acid employed in photoimaging by implementing control of PED conditions. We have chosen for this study the 2-trifluoromethyl-6-nitrobenzyl chromophore, instead of the 2,6-dinitrobenzyl chromophore, because of the large enhancement in thermal stability (7a) it provides to PAG's. This allowed us to study resists formulated with PAG which gave stronger photo-acids without competitive thermal generation of acid during PEB. The polymer matrix, poly(4-/-butoxycarbonyloxystyrene-sulfone) (PTBSS), was chosen for this study because it yields upon acidolytic cleavage the very hydrophilic poly(hydroxystyrene-sulfone). Poly(hydroxystyrene-sulfone) has an extremely fast rate of dissolution and cannot be significantly dissolution inhibited. This minimized contributions to the rate of dissolutionfromthe PAG's differing hydrophobicities. In this work we have found that by reducing the PED effect with an overcoat (2), the behavior expected from Equation 4 was observed. This confirmed that without basic contaminants resist sensitivity is governed by the reactivity of the photoacid due to 'free' H \ Moreover, when resists are exposed to airborne contamination under controlled conditions, most of the PAG's show a PED effect which increases with the reactivity of the photo-acid. A few of the PAG's which have polar, Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


8


d. HOULIHAN ET AL.

Figure 2 A) Plot of Log(Chain Length) versus the Hammett sigma values for substitution at the benzenesulfonyl moiety of 2-6-dinitrobenzyl be sulfonate PAG's inaPTBSS resist matrix. B) Plot of Log(l/(D *%ABS/pm*0.2

0

02

0.4

0.6

0.8

1

weaker acid

Ο

Figure 6 Plots of Log(Chain Length) versus the Hammett sigma values for substitution at the benzenesulfonyl moiety of 2-rifluoromethyl-6-nit benzenesulfonate inaPTBSS resist matrix. Resists processed either with an overcoat and no PED or without an overcoat and with a 30 mi 3.5

' \ ' -0.4

1

weaker acid

1

1 ' ' -0.2

1

I ' 0

1

1

I ' ' 0.2

1

\ ' 0.4

"

1

I • • 0.6

1

a

c

ι

OS i

. 1

stronger d

Figure 7 Plot of LogfAChain Length ) versus the Hammett sigma values for s ubstitution at the benzenesulfonyl moiety of2-triftuoromethyl-6-nitro benzenesulfonate inaPTBSS resist matrix.


102


After the polymer, the PAG is the second most important component (6 % mole) of the resist. Even in exposed regions 90-96% of the PAG remains unaltered. Thus, basic airborne contaminants will likely meet with undissociated PAG before free sulfonic acid. Therefore, if the PAG associates strongly with the airborne basic contaminants, this will slow the rate of the reaction with acid by slowing diffusion of base into the film. The importance of the diffusion rate of airborne basic impurities into the resist film has been established by others (22). In our case, all three of the PAG's which are resistant to airborne contaminants have polar 4-substituents susceptible to dipolar interactions. Conversely, most of the materials showing poor resistance to the PED effect either have groups that are weakly polarizable (F, CF ) or ones with poor dipoles (H, CH ) and would consequently form poor dipolar interactions. One exception, the PAG with a 2-N0 substituent—unlike the 4N 0 substituted material— suffers from a vicinal interaction with the S0 R moiety which would make interactions with contaminants difficult. The only remaining exception, the 3-S0 R substituted PAG, does show an enhanced resistance to PED effect if the delay time is shortened (Figure 4C). According to Scheme 7, this entails that although the 3-S0 R substituent can interact with airborne contaminants it does so less effectively than the other polar substituents. A more detailed discussion of polarizability and its possible significance in explaining the PED effect will be done at the end of this section. A second possible mechanism is that photo-acids bearing polar polarizable group could also undergo interaction with the resist matrix limiting its diffusion towards absorbed basic impurities (Scheme 8). Thus, using arguments similar to those described above, this model also explains the differences in the magnitude of the PED effect observed due to the differing structure of the PAG resist component. A third possibility is that the 4-MeO, 4-C1 and 4-N0 substituents, which all have lone pairs of electrons, are acting as basic moieties and are competing with airborne contaminants for the 'free' H* thereby slowing their rate of reaction with these and suppressing the turnover (Scheme 9). Although this may account for the behavior of the PAG with a 4-MeO (Scheme 6) substituent (we attempted to correct for this by employing σ') the other substituents bearing lone pairs, 4-C1 and 4-N0 are unlikely to be protonated. For example, the pK of protonated nitrobenzene is only -12 far less than a protonated carbonyl group —7 (23). Moreover, there should not be a very big pK difference between 2-nitrobenzenesulfonate and 4-nitrobenzenesulfonate protonated PAG's (or the corresponding acids) but nonetheless resists formulated with these two materials show very different PED behavior as described above. However, hydrogen-bond formation decreasing the accessibility of 'free' H* cannot be discounted as a possibility since N0 , MeO, CI are hydrogen bond acceptors (24) and sulfonic acids are potent hydrogen bond donors (25). The intermolecular hydrogen bonding of 2-nitrobenzenesulfonic acid, would be sterically hindered by the vicinal S0 H group. Correlation Between Molecular Polarizability and the 'Anomalous' PED Substituent Effect. Molecular polarizability governs many physical properties of materials. For instance, Debye interactions (dipole-induced-dipole interactions) explain the mutual solubility of many polar and polarizable molecules (26). Debye interactions also explain why polarizable aromatic molecules are easily extract by N methypyrilidone from oil (26). Similarly, N-methylpyrilidone from the air would be expected to have a greater affinity for PAG's with polar polarizable substituents (i.e. 4MeO, 4-N0 and 4-C1). Increasing the polarizability of alkyl groups on amines 3


3

2

2

3

3

3

2

2

a

a

2

3

2


HOULIHAN ET AL.


Scheme 7


-JW

POLYMER

Scheme 8 HOH

RESIST

RESIST

RESIST

w

BASE

BASE—Η Q - | - ^ ^

Scheme 9

4ΦΑ -

4-^.5 BASE

BASE—H

Q~I~^^)

R


104


>

generally increases their gas phase basicity (27) (t-butyiNH > i-JProNH MeNH > NH ). Thus the PAG's with polar polarizable groups would also be expected to interact better with strong polarizable airborne bases such as (Me) N and (Me) NH. Measurements of both the 'anomalous' PED substituent effect and the relative polarizabilities of the PAG's are needed to see if there is indeed any correlation between these two properties. The PAG's with polar substituents gave lower Log(A Chain Length) values than expectedfromthe regression line found for the other PAG's in the plot of Log(A Chain Length) versus σ (Figure 7). The difference, ALog(AChain Length), between the observed value of Log(A Chain Length) and the value expected from the regression line established by the * non-anomalous' substituents can be used as a measure of the effect of the polar polarizable groups on the PED effect. Physical properties such are the refractive index are often used a measure the polarizability of molecules. Unfortunately, refractive indexes of suitable model compounds could not be found. However, another example of a physical property governed by molecular polarizability is the nematic to isotropic (N-I) transition temperature in nematic liquid crystals (28). The N-I transition in liquid crystals is know to increase in the order 4-MeO >4-N0 >4-Cl > 4-CH which is thought to indicate the increasing order of molecular polarizability (28). This substitution effect parallels the in the difference in PED effect we have observed (Δ log(A Chain Length) in the PAG's with polar polarizable 4-subsituents. This is demonstrated by a plot of this difference against a measure of the relative polarizability induced by the 4-substituents (the N-I transition temperature of a typical 4-substituted aryl nematic liquid crystal (28)) (Figure 8). This indicates that PAG's which have substitution patterns that induce better polarizability tend to give less of a PED effect. This correlation between 'polarizability' and the unusual PED behavior tends to point towards either thefirstor second mechanism (Scheme 7 and 8) where the polarizability reduces the tendency for photo-acid to encounter base by either 'tyingup' base, photo-acid or both and preventing themfromdiffusing towards each other. However, a competition between the hydrogen bonding of the photo-acids and their reaction with airborne base contaminants is still a possibility. 2

2

2

3


3

2

2

3

0.7

CH0 3

0· 150

160

170

180

190

200

210

220

N-I °C 'Moe lcua lr Ρ^ίαΜφ·

Figure 8 Plot of ALogfA Chain Length) versus T-N transition temperature Polarizability of substituents. ' Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


6. HOULIHAN ET AL.

Photogenerated Acid and Post-Exposure Delay Effect 105

The 'Anomalous' PED Effect and Lithographic Imaging of Small Features. Further evidence for the PAG induced improvement in resistance to the PED effect was found by looking at SEM cross-sections. This was done by comparing the resist formulation with 2-trifluoromethyl-6-nitrobenzyl 4methoxybenzenesulfonate to that with a strong acid PAG (σ > 0.54) such as 2trifluoromethyl-6-nitrobenzyl 2,4-difluorobenzenesulfonate. As can be seen in Figure 9 A, B, the resist with the strong acid PAG, even with a protective overcoat, gives considerable T-topping of 0.5 μιη lines and spaces with a 30 minute PED. In contrast, the resist with the 4-methoxybenzenesulfonate PAG showed no T-topping (0.35 μιη lines and spaces) after 30 minute PED (Figure 10 A, B). Furthermore, T-topping is not observed even when a protective overcoat is omitted, although some rounding of features is noticeable. This is probably caused by volatilization of acid. Although the other resistant PAG's also offer some protection against T-topping, as expected, the effect decreases in the order 4-CH 0>4-N0 > 4-C1. 3

2

Conclusion We have established the following decreasing order of resistance to the PED effect: 4-OCH >4-NO > 4-C1 > >3-SO R > > 4-CH > 4-H >2-CF > 3CF > 4-CF > 2,4-diF> 2-NO*> 3,5-diCF^ These trends have been rationalized as follows: It has been shown that when airborne contamination is minimized, the resist sensitivity increases with the reactivity of the acid. Several mechanisms have been postulated to account for this increase in reactivity (changing acidity, nucleophilicity). For PAG's that have substituents that do not interact well with other functionalities, the sensitivity of the resist to the PED effect increases with the reactivity of the acid. Several mechanisms have been postulated to account for this: It may be a kinetic effect, governed only by how quickly basic impurities come across photo-acid in the viscous resist matrix during PED. It may be a leveling of the 'free' H* concentration, which would increase with the difference in pK 's between the photo-acid and the protonated basic impurities. Finally, the weaker sulfonic could be reacting less with the impurities because they are unable to dissociate sufficiently rapidly during the initial stages of the PEB to compete with base volatilization. The magnitude of the PED effect appears to be lessened in the presence of PAG's which have polar-polarizable substituents that can interact well with other moieties. Three possible mechanisms have been outlined to explain this effect: The undissociated PAG interacting with airborne bases, photo-acids interacting with the resist matrix and "free" If's or hydrogen bonded photo-acids interacting with the substituents on the PAG. All three mechanisms could account for the effect, however, the correlation between the expected order of polarizability and the lessening of the PED effect tends to favor thefirsttwo possibilities. The poor basicity expected to be induced by the lone pairs of the 4-C1 and 4-N0 substituents is not a strong argument in favor of the third mechanism. However, one cannot discount the possibility a hydrogen bond interaction of the photo-acid (or some other H* complex). 3

a

2




Figure 9: SEM's of 0.50 mm lines and spaces obtained with a resist formu with a strong acid PAG (s > 0.54 ) such as 2 -trifluoromethyl-6-nitrobenzyl 2,4-difluorobenzenesulfonate. A) Resist with overcoat, no PED, Dp=52mJ/cm2. B) Resistwith overcoat, 30 minute PED, Dp = 86mJ/cm2


Photogenerated Acid and Post-Exposure Delay Effect 10


6. HOULIHAN ET AL.

Figure 10 S EM*s of 0.35 mm lines and spaces obtained with a resist formu with 2-trifluoromethyl-6-nitrobenzyl 4-methoxybenzenesulfonate. A) Resist with overcoat, no ΡED, Dp= 162 mJ/cm2. B) Resist with overcoat, 30 minute PED, Dp = 162mJ/cm2.


108



To minimize the PED effect, an ideal PAG would both generate a less reactive photo-acid (small σ) and have a polarizable substituents that can interact well with other molecules. However, since a weaker acid would reduce sensitivity (unless PEB temperature or time is increased) then a better compromise might involve a PAG that generates a reactive photo-acid but is substituted with a polarizable functionalities that can hinder the PED effect.

References 1. Willson, C. G.; Frechet J. M. J.; Tessier, T; Houlihan, F.M., J. Electrochem. Soc. 1986, 133(1), 181: Ito, H.; Proc.ΚTIMicroelectronics Seminar, 1988, 81: Reichmanis, E.; Houlihan, F. M.; Nalamasu, O.; Neenan, T.X., Chem. of Mat. 1991, 3, 397: Lamola, Α. Α.; Szmanda, C.R.; Thackeray, J. W; Solid State Technol. 1991, 34(8), 53. 2. Nalamasu, O.; Reichmanis, E.; Cheng, M.; Pol, V.; Kometani, J. M.; Houlihan, F. M.; Neenan, T.X.; Bohrer, M.P.; Mixon, D.A.; Thompson, L. F.; Proc. Soc. Photo-Opt Instr. Eng. 1991, 1466, 13. 3. MacDonald, S.A.; Clecak, N.J.; Wendt,, H. R.; Willson, C. G.; Snyder, C. D.; Knors, C.J; Peyoe, N.; Maltabes, J. G.; Morrow, J.; Mc Guire, A.E.; and Holmes, S. J., Proc. Soc. Photo-Opt. Instr. Eng. 1991, 1466, 2. 4. Ito H.; England, W.P.; Clecak, N.J.; Breyta, G.; Lee, H.; Yoon, D.Y.; Sooriyakumaran, R.; and Hinsberg, W.D., Proc. Soc. Photo-Opt. Instr. Eng., 1925, Proc. Soc. Photo-Opt. Instr. Eng. 1993, 1925, 65. 5. Houlihan, F.M.; Shugard,, Α.; Gooden,, R.; Reichmanis, E.; Macromolecules, 1988, 21, 2001. 6. Neenan, T.X.; Houlihan, F.M.; Reichmanis, E.; Kometani, J. M.; Bachman, B.J.; Thompson, L. F.; Macromolecules, 1990, 23, 145. 7. a) Houlihan, F.M.; Neenan, T.X.; Reichmanis, E.; Kometani, J. M . ; Chin, T., Chem. Mater., 1991, 3, 462: b) Houlihan, F. M.; Neenan, T. X.; Reichmanis, E.; 1993, Us Patent 5,200,544. 8. Houlihan, F.M.; Chin, E.; Nalamasu, O.; Kometani,, J.M.;, Neenan, T. X.; Pangborn, Α., J. Photopolym. Sci. Techn., 1993, 6(4), 515. 9. Thompson, L.F.; Bowden, M.J.; In Introduction to Microlithography; Editors, Thompson, L.F., Willson, C.G., Bowden, M.J.; ACS Symposium Series 219; Washington DC; 1983; Chapter 4; pp 170. 10. The authors do not imply that actual 'free' H exists in the resist matrix but employ this euphemism to refer to acid which is dissociated from the sulfonate moiety and loosely associated with some other components of the resist matrix. 11. Furukawa, N., Fujihara, H.; in The Chemistry of Sulphonic Acids, Esters and thei Derivatives; Editors, Patai, S., Rappoport, Z.; The Chemistry of Functional Groups; John Wiley and Son; New York, NY; 1991; Chapter 7; pp 266. 12. King J.F., In The Chemistry of Sulphonic Acids, Esters and their Derivati Editors, Patai, S., Rappoport, Z.; The Chemistry of Functional Groups; John Wiley and Son; New York, NY; 1991; Chapter 6; pp 250. 13. Perrin, D.D., Dempsey, B.; In pK Prediction for Organic Acids and Bases; Chapman and Hall; New York, NY; 1981; pp 128. +

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14. Izutsu, K.; In Acid-Base Dissociation Constants in Dipolar Aprotic Solven Blackwell Scientific Publications; Boston, Mass; 1990; pp 131. 15. Zefirov, N.S.; Koz'min, A.S., Acc. Chem. Res; 1985; 18; 154. 16. Aim. R.; Modern Paints and Coatings; 1980; V 70; No 10; 88. 17. Tarascon, R. G.; Reichmanis, E.; Houlihan, F. M.; Shugard, Α.; Thompson, L.F., Polym. Eng. Sci. 1989, 29(3), 850. 18. This would occur if the effective pK of the arylsulfonic acid is high enough in the non polar polymer matrix so that -d(pK )/dT = (pK + 0.052ΔS°)/T (takenfromreference 13, pp 7) predicts a decrease in pK with temperature. For example, assuming a pK of 6.4 (toluenesulfonic acid in propylene carbonate taken from reference (14)) and a ΔS ~ -100 Jdeg mole , d(pK )/dT = -0.004, increasing temperture by 100 deg will decrease the pK by ~0.4. A similar argument can be made for the dissociation from the BH 'conjugate acid' of the resist polymer. In this instance, since no change of charge occurs, the entropy change is far less ~-17 mJ/cm giving, -d(pK )/dT = (pK - 0.9)/T , d(pK )/dT = 0.018. This predicts a much greater dependance on temperature, thus assuming that the pK of this conjugate acid is 6.4 then increasing the temperature by 100°C will decrease it ~1.8. 19. Nalamasu, O.; Vaidya, S.; Kometani, J.M.; Reichmanis, E.; Thompson, L.F., Proc. Reg. Tech. Conf. on Photopolymers, Ellenville, NY Oct 28-30, 1991 225. 20. Mc Kean, D. R.; Schaedeli, J.; MacDonald, S.A., In Polymers in Microlithography ACS Symposium Series No 412; Reichmanis, E„ MacDonald, S.A., Iwayanagi, T. Eds American Chemical Society; Washington DC 1989, 27. 21. There is also probably some residual spinning solvent, but the amount of this should also remain unchanged since the processing conditions for the different resists are the same. 22. Hinsberg, W.; MacDonald, S.A.; Clecak, N.; Snyder, C., J. Photopolym. Sci. Tech, 1993, 6(4), 535. 23. Lowry, T.H. Richardson, K.S.; In Mechanism and Theory in Organic Chemistry; Harper and Row; New York, NY; 1976; pp149. 24. Ferguson, L.N.; In Organic Molecular Structure; Willard Grant Press; Boston, Mass.; 1975; pp 19. 25. Reference (11) pp 271. 26. Grant, D. J.W.; Higuchi, T., In Solubility Behavior of Organic Compounds; Editor, Saunders Jr. W. H.; Techniques of Chemistry, Volume 21; Wiley Interscience; New York, N.Y.; 1990; pp 69. 27. Reference (24) pp 79. 28. Coates, D.; In Liquid Crystals Applications and Uses; Editor Bahadur, B.; World Scientific; New Jersey; 1990; Vol 1; pp 103. 29. Hansch, C., and Leo, Α., In Substituent Constants for Correlation Analysis in Chemistry and Biology; Wiley-Interscience; New York, 1979, pp1-69. 30. Steric Effects in Organic Chemistry, Newman, M . S., John Wiley and Son, Chapter 13 "Separation of Polar, Steric, and Resonance Effects in Reactivity," 1956, pp 556. a

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Correlation of the Strength of Photogenerated Acid with the Post

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