Materials for Microlithography - American Chemical Society

13 The Photo-Fries Rearrangement and Its Use in Polymeric Imaging Systems T. G . T E S S I E R and J. M. J. F R E C H E T

Downloaded by NORTH CAROLINA STATE UNIV on September 8, 2013 | http://pubs.acs.org Publication Date: July 1, 1985 | doi: 10.1021/bk-1984-0266.ch013

Department of Chemistry, University of Ottawa Ontario, KIN-9B4, Canada

C. G . W I L L S O N and H. ITO IBM Research Laboratory, Dept. K42-282 San Jose, C A , 95193

The photo-Fries rearrangement which was first reported (1,2) in the early 1960's has some potential in the design of photoresist materials since it involves the transformation of molecules such as phenolic esters into free phenols, thereby providing a route to selective image development by differential dissolution. Initial interest in the photo-Fries rearrangement of aromatic polyesters was mainly due to the fact the reaction is accompanied by the formation of o-hydroxy aromatic compounds which possess great photostability. This interesting property of the rearranged products can be used to design polymers containing photostabilizing ortho-hydroxy aromatic groups (3-6); the photostabilizing action of such compounds has been recently reviewed (7-8). Scheme 1 shows the reaction which occurs when an aromatic polyester such as [I] is subjected to U V irradiation (5). The polymer first undergoes main-chain cleavage with subsequent rearrangement to polymer [II] which is photostable and can be used as a thin coating to protect efficiently other substrates which are normally photodegradable. A considerable amount of attention has also been paid to the photo-Fries rearrangement of polymer pendant groups. For example, the rearrangement of poly(phenyl acrylate) (10,11) in solution or in the solid-state, is usually incomplete and results in the formation of both the ortho and the para -hydroxyphenone rearranged products in amounts which vary with the conditions of the photolysis. A concurrent side-reaction, which we term the Fries degradation, also results in the liberation of small amounts of phenol (Scheme 2). Similar results have been obtained with poly(phenyl methacrylate) and other substituted aryl acrylates (4,9,12). The enhanced stability of the photo-Fries rearrangement products was again confirmed in a thorough study by Guillet and co-workers (13) who attributed it to the high extinction coefficient of both the ortho and the para photoproducts and to their ability to dissipate the absorbed energy by nonphotochemical pathways. 0097-6156/84/0266-0269S07.25/0 © 1984 American Chemical Society

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

270

MATERIALS FOR MICROLITHOGRAPHY

R = A L K Y L OR H

LR


LR

II SCHEME 1

hi/

c=o

c=o

SCHEME 2. Although the photo-Fries rearrangement and the concurrent degradation of a number of polymers has been reported in the literature (3-13), essentially no previous attempts at using this reaction in lithographic processes have been reported. The lithographic potential of the photo-Fries reaction rests on the ability to selectively dissolve either the exposed or the unexposed areas of a polymer film. Typically, it is expected that in the case of polymers containing phenolic ester groups, the photoproducts, being substituted phenols, should dissolve in aqueous base while the unchanged starting polymer should remain undissolved. Similar results should also be within reach using the photo-Fries reaction on polymers containing aromatic amide groups. In addition, the photodegradation component of the photo-Fries reaction which often results in a decrease in the molecular weight of the irradiationed polymer, would also be expected to contribute to the solubility difference between exposed and unexposed areas of an aromatic polyester or polyamide film. Although it is


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TESSIER ET A L .

271

Photo-Fries Rearrangement

anticipated that, in some cases, the photostabilizing effect of the rearrangement products might have a deleterious effect on the imaging characteristics of the polymers, it is nevertheless likely that imaging of thin films should remain possible. The polymers used in this study included poly(/?-acetoxystyrene) [III], poly(/?-formyloxystyrene) [IV], poly(p-acetamidostyrene) [V], poly (phenyl methacrylate) [VI], and poly(methacryl anilide) [VII].


Preparation of the Polymers The /7-acetoxystyrene monomer, precursor of polymer III, is prepared from phydroxyacetophenone using the procedure of Corson et al. (14) which involves acetylation of the phenolic group followed by catalytic hydrogénation of the ketone and dehydration of the resulting benzylic alcohol as shown in Scheme 3.

OCOCH

OCOCH3

3

SCHEME 3 . This reaction sequence is satisfactory although the overall yield is approximately 50%. A different route for the preparation of poly(pacetoxystyrene) involves the direct acetylation of poly(/?-hydroxystyrene) with acetic anhydride. The main problem with this approach is the lack of commercial availability of high purity poly(/?-hydroxystyrene). Similarly, poly(/?-formyloxystyrene) (IV) can be prepared by formylation of poly(/?-hydroxystyrene) using formic acid-acetic anhydride mixture as a formylating agent (Scheme 4). The formylation reaction is best

£cH -CH^J 2

HCOOH, T H F (CH C0) 0 3

2

F

^

IV

PYRIDINE OCHO

S C H E M E 4.


272

MATERIALS FOR MICROLITHOGRAPHY

carried out in the presence of a small amount of pyridine and is almost complete. Model experiments with phenol have shown that up to 98% formylation is obtained while no acetylated material is obtained. Alternately, the /7-formylstyrene monomer can be prepared by formylation of phydroxystyrene using the same reagent mixture and polymer IV is obtained by free-radical polymerization (Scheme 5). The infrared spectrum of poly(/?C H = CH

C H = CH

0

2

CH P (C H ) Br, N r| (CH C0) Downloaded by NORTH CAROLINA STATE UNIV on September 8, 2013 | http://pubs.acs.org Publication Date: July 1, 1985 | doi: 10.1021/bk-1984-0266.ch013

3

6

5

THF, ( C H ) 3

OH

3

3

2

3

COK

2

0/HCOOH^X

PYRIDINE (CAT.)' OH

^-^J OCHO

A ,

AIBN

SCHEME 5. formyloxystyrene) shows characteristic twin carbonyl absorptions at 17381761 c m which are due to the presence of both possible s-cis and s-trans conformations of the formate group in the polymer. The partial double bond character of the ester C-O bond due to electron derealization is responsible for this phenomenon (17) which has also been observed with numerous carboxylic acids and amides (18). In contrast, the N M R spectrum of the polymer shows only a single formyl signal at room temperature due to the rapid interconversion between the conformers. Poly(/?-acetamidostyrene) (V) is prepared from p-nitrobenzyl bromide as shown in Scheme 6. Homopolymer V has very little solubility in common organic solvents and thus it is difficult to use; attempts at increasing the solubility of V by incorporation of up to 40% styrene units in copolymers such as Va do not result in any significant improvement in solubility. Poly (phenyl methacrylate) (VI) and poly(methacryl anilide) (VII) are prepared from the corresponding monomers according to literature procedures (19-20). - 1

Photochemical Studies There are a number of difficulties in studying the photochemical modification of polymers, the most significant of which is that, unlike low molecular weight materials, the polymeric photoproducts cannot be separated from unreacted moieties for purification. Thus, if a photochemical reaction only reaches 50% conversion, the final product is a polymer which incorporates equal amounts of modified and unmodified units. In addition, side-reactions give rise to small


13.

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Photo-Fries Rearrangement

TESSIER ET AL.

® CH P(C H )

CH Br

2

2

6

Materials for Microlithography - American Chemical Society

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