Nonswelling Negative Resists Incorporating Chemical Amplification

Chapter 5

Nonswelling Negative Resists Incorporating Chemical Amplification The Electrophilic Aromatic Substitution Approach 1

1

1,3

Jean M . J . Fréchet , Stephen Matuszczak , Harald D. H. Stöver , C. Grant Willson , and Berndt Reck 2

2

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch005

1

2

Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301 IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099

Photoinduced crosslinking of polymers through electrophilic aromatic substitution has been achieved with a family of styrenic polymers or copolymers containing both latent electrophiles and activated aromatic groups. These polymers can be used in combination with a photoacid generator to design a non-swelling negative multipurpose resist which can be used for deep-UV, X-ray or Ε-beam imaging and has a very high sensitivity. For example, in the deep-UV, sensitivities of less than 1 mJ/cm are obtained with very high contrasts. Irradiation of the two-component resists results in the generation of strong acid which, upon baking, activates the latent electrophile to a carbocationic species that couples to neighboring activated aromatic moieties in a crosslinking process. Vinyl-phenol units are incorporated in the copol ymer formulation to provide activated aromatic sites, solubility of the resist in aqueous base, and lack of swelling during image develop ment. Alternate three-component formulations in which the latent electrophile is separate from the activated aromatic moiety are also suitable. 2

A number of new resist materials which provide very high sensitivities have been developed in recent years [1-3]. In general, these systems owe their high sensitivity to the achievement of chemical amplification, a process which ensures that each photoevent is used in a multiplicative fashion to generate a cascade of successive reactions. Examples of such systems include the electron-beam induced [4] ringopening polymerization of oxacyclobutanes, the acid-catalyzed thermolysis of poly mer side-chains [5-6] or the acid-catalyzed thermolytic fragmentation of polymer main-chains [7]. Other important examples of the chemical amplification process are found in resist systems based on the free-radical photocrosslinking of acrylated polyols [8]. The seminal work on deep-UV resist materials which incorporate chemical amplification was started at IBM San Jose's Research Laboratory in 1979 when Frechet and Willson first prepared poly(4-t-butyloxycarbonyloxy styrene) and endcapped copolymers of o-phthalaldehyde and 3-nitro-l,2-phthalic dicarboxaldehyde. 3

Current address: Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4M1, Canada 0097-6156/89/0412-0074$06.00/0 ο 1989 American Chemical Society

Reichmanis et al.; Polymers in Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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Nonswelling Negative Resists

The former material was designed for its ability to lose its t-BOC protecting groups under a variety of conditions thereby affording a free phenolic polymer and imaging by differential dissolution. The second, designed as a self-developing imaging system, owed its activity to the presence of photocleavable o-nitrobenzyl groups which allowed its partial depolymerization upon exposure to U V light due to a ceiling temperature phenomenon. These interesting new approaches were later extended and improved by Ito et al. [5] with the introduction of appropriately chosen triarylsulfonium or diaryliodonium salts as the photoprecursors of catalytic amounts of strong acid for t-BOC group removal or cleavage of the poly(phthalaldehyde). The use of phenolic polymers in photocrosslinkable systems usually involves multicomponent systems which incorporate polyfunctional low molecular weight crosslinkers. For example, Feely et al. [9] have used hydroxy methyl melamine in combination with a photoactive diazonaphthoquinone which produces an indene carboxylic acid upon irradiation to crosslink a novolac resin. Similarly, Iwayanagi et al. [10] have used photoactive bisazides in combination with poly(p-hydroxy-styrene) to afford a negative-tone resist material which does not swell upon development in aqueous base. Preparation of the Resists and Lithographic Evaluation. Design of the Resist Material. Our approach to resists that operate via electrophilic aromatic substitution is outlined in Scheme I. The reaction sequence which is used can be summarized as follows: In the first step acid is generated by photolysis of a triarylsulfonium salt. Subsequent reaction with a latent electrophile, such as a substituted benzyl acetate, produces a carbocationic intermediate while acetic acid is liberated. The carbocationic intermediate then reacts with neighboring aromatic moieties in a coupling reaction which liberates a proton thus ensuring that the overall process is catalytic and that chemical amplification is achieved. Several approaches are possible as the latent electrophile and the activated aromatic compound may be part of the same or of different molecules. However, it is necessary that at least one of the components of the mixture be a polymer with good coating, solubility, and optical properties and that the different components of the mixture be compatible. The paragraphs below will describe first an approach in which both the latent electrophile and the electronrich aromatic components are part of the same polymer, then a second approach in which non-polymeric difunctional latent electrophile is used with a phenolic polymer. In both cases the source of photogenerated acid is a triarylsulfonium salt, other sources of photogenerated acid are available [5]. Preparation of Copolymers Containing Both Electrophilic and Nucleophilic Groups. Our first implementation of this reaction scheme involved the preparation of a series of copolymers incorporating both a latent electrophile and an electron-rich aromatic moiety which, being phenolic, also provides access to swelling-free development in aqueous medium. The copolymers are prepared as shown in Figure 1 by copolymerization of 4-t-butyloxycarbonyIoxy-styrene with 4-acetyloxymethyl-styrene. A l though the reactivity ratios of these two monomers are different [11], our study of this system has confirmed that they copolymerize essentially in random fashion. Removal of the t-BOC protecting groups from the copolymer is best done by refluxing in glacial acetic acid, a process which does not affect the acetoxymethyl pendant groups or the molecular weigh distribution of the final polymer. Figure 2 shows the GPC trace for a copolymer containing 80% free phenolic groups and 20% 4-acetoxymethyl groups (80/20 copolymer). Curve (a) shows the polymer before deprotection with M = 62,000 and M = 28,000 (polydispersity = 2.2), while curve (b) shows the same polymer after deprotection with M = 45,000 and M = 20,000 w

n

w


n

76

POLYMERS IN MICROLITHOGRAPHY

1) 0 S 3

SbF '

+

6

CH —O—C—CHo


2)

CH

2

+ ?

+ HO—C—ChU

, C H 3 )

R

'"

H

(^^~

O

H

+

Scheme I.

R

"^(^^~

C H

2

2

- ^ ^ - R

R'-YQV-OH

+

+ H

+

Resist design based on electrophilic aromatic substitution.

-(CH —ÇH>^~(CH —ÇH>^ 2

©•Ό Ο ο

2

û Ç

AIBN

CH

Ο

2

CH I Ο

Λ

ο Ο Ο

ο

2

Λ

'HOAc

HCH -CH> (CH -CH^ 2

φφ r

OH

Figure 1.

2

CHo I Ο

Preparation of the copolymers of 4-vinylbenzyl acetate and 4-vinylphenol.



5.

FRECHET ET AL.

10

6

10

5

NonsweUing Negative Resists

10

4

10

3

Molecular Weight Figure 2. G e l Permeation Chromatogram of the copolymer (a) before and (b) after removal of the t-BOC protecting groups


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78



(polydispersity = 2.2) confirming the clean nature of the deprotection process. Figure 3 shows the U V spectrum of a 1 jim thick film of a resist consisting of 90 wt% of the same 80/20 copolymer and 10 wt% of triphenylsulfonium hexafiuoroan timonate; it is seen that the film is suitable for imaging at 254nm since the absorb ance of the resist does not exceed 0.6 per micrometer of film thickness. Sensitivity and Contrast Measurements. Imagewise exposure of films of the various copolymers containing from 5-10% triphenylsulfonium hexafiuoroantimo nate to U V light at 254 nm resulted in the crosslinking of the exposed areas as shown in Figure 4. The characteristic curves [1] for the various copolymers were determined at a constant loading of 10 wt% sulfonium salt using lum thick films and exposure through a narrow bandwidth Hg-line filter to varying doses of 254 nm radiation. The characteristic curve shows the thickness of the insolubilized regions of the film remaining after development as a function of log[exposure dose]. These measure ments provide access to Dg or gel dose, the minimum dose required to observe the formation of an insoluble residue, as well as Dg the minimum dose required to produce an insolubilized film of thickness equal to that of the starting film (lum in this instance). The characteristic curve of a 65/35 copolymer is shown in Figure 5. This Figure shows that the lithographic sensitivity of the resist material based on a 65/35 copolymer having M = 22,000 and M = 46,000 is approximately 0.6 mJ/cm while its contrast (slope of the curve) is close to 4. Measurements of the lithographic characteristics of a series of copolymers having different compositions and essentially the same molecular weights and polydispersities are summarized in Table 1. It can be seen in Table 1 that the lithographic sensitivity of the copolymers blended with 10% sulfonium salt increases as the percentage of latent electrophile (vinylbenzyl acetate) units is increased. For a 50/50 copolymer the lithographic sensitivity is approximately 0.5 mJ/cm with a very high contrast of over 4. It should be noted however that aqueous development is no longer possible for the 50/50 copolymer for which some isopropanol must be added to the aqueous base developer. n

w

2

2

Table 1: Lithographic sensitivity and contrast data for various copolymers Copolymer 95/5 90/10 80/20 65/35 50/50 100/0

a

M

w

44,000 45,000 45,000 46,000 41,000 39,000

M

n

20,000 21,000 20,000 22,000 21,000 20,000

M /M w

2.2 2.1 2.2 2.1 2.0 1.9

D n

D

g

0.68 0.56 0.56 0.45 0.51 0.86

°g

Y

1.0 1.0 1.0 0.85 0.6 1.2

>4 ~ 4 ~ 4 3.6 >4 >4

^Copolymer composition x/y: the first number χ indicates the mole % of 4hydroxystyrene units and the second number y indicates the mole % of 4acetoxy methyl styrene units in the copolymer. All measurements were made at a con stant 10 wt% loading of triphenylsulfonium hexafiuoroantimonate. Use of a Difunctional Crosslinker. An alternate approach to chemically amplified imaging through electrophilic aromatic substitution is shown in Figure 6 below. In this approach a polyfunctional low molecular weight latent electrophile is used in a three component system also including a photoactive triaryl sulfonium salt and a phenolic polymer. In this case again crosslinking of the polymer is observed upon



5. FRECHET ET AL.

79


hi

(ο

SbF.

,CH

2

OH

OH

Protons are regenerated with each addition .

.

.

,.

Γ

».

CROSSLINKED

Process incorporates chemical amplification R » OH, C H O A c , etc... 2

Figure 4. Crosslinking process via electrophilic aromatic substitution.


+H

80



1.0

^

C H

H

2-? >7I65^

C H

2~9H>Ô3

5

JEi Ε — Ê l 0.5 •ο Û

CH OAc

OH

2

03 ^

Ε

ro

0.0

0.25

0.5

1.0

2.0

4.0

Log Dose [mJ/cm ] 2

Figure 5.

A

X

V

^ p \ J ^ "

Characteristic curve for sensitivity measurement on the 65/35 copolymer.

CHjOAc

Υ..

γ

CH,OAc

Δ

».

^ OH

Protons are regenerated with each addition Process incorporates chemical amplification Figure 6.

Crosslinking via a non-polymeric multifunctional latent electrophile.


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81

irradiation. Sensitivity measurements for such systems are still in progress. Another example of this approach involves the use of a novolac as the phenolic component [12]. Imaging experiments The resist material was prepared using 90 wt% of the 80/20 copolymer and 10 wt% of the triphenylsulfonium hexafiuoroantimonate. After spin-coating onto silicon wafer to Ιμτα thickness and baking 5 min at 105°C the wafer was exposed to

Nonswelling Negative Resists Incorporating Chemical Amplification

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