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The Radiation Sensitivity of Select Metal Chelate Polymers: Mechanistic Chgulesat VUnchifi e

1 PASQN AVN n;Hardiman, Christopher John; and Lee, Annabel Y.

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17 COAII ODES!a SE T.TERMS (Cantr n # on re arse if necessary an identify Oy blo~k number)rIL O~PaM ion sensi Lve povr mea coordination oolymersuranyl polymers dioxouranium(VI) polymerspositive resists negative resists

19 ABSTRACT (Continue on reverse of Recellsary and ientity by block nunmber)

Based on uranyl photochemnistry in the visible/ulatrviolet region, uranyl carboxylates are knownto undergo CO2 evolution however, with gamma radiation (cesium-137, 662 key) a sizable nu'7berof uranyldicarboxylate polymers crosslink with high efficiency and without CO2 evolution. Onthe other hand several dicarboxylates with sulfur in thie oroanic backbone exhibit very highscission efficiency. G values in excess of normal organi c resi sts is related to thie heavy atom-content of these metal coordination polymers.

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TECHNICAL REPORT NO. 11

THE RADIATION SENSITIVITY OF SELECT METAL CHELATE POLYMERS:MECHANISTIC CHANGES AT HIGHER ENERGIES

by

Ronald D. Archer, Christopher J. Hardiman and Annabel Y. LeeDepartment of Chemistry, University of Massachusetts

Amherst, Massachusetts 01003

Accepted for Publicationin

Photochemistry and Photophysics of Coordination CompoundsH. Yersin and A. Vogler, editors

Springer-Verlag, Berlin, Publishers

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THE RADIATION SENSITIVITY OF SELECT METAL CHELATE POLYM7PS:

MECHANISTIC CHANGES AT HIGHER ENERGIES §

R. D. Archer, C. J. Hardiman, and A. Y. Lee

Department of Chemistry, Universityr of Massachusetts, Armherst, .,.

01003, United States of America

INTRODUCTION

Our interest in the radiation sensitivity of heavy metal polymers was

first sparked by the goal of enhanced miniaturization of integrated

circuit (IC) chip features. As the features of IC chins get closer

and closer together at the submicron level, microlithographic tech-

niques must use higher energy photons (or electrons, etc.) than are

currently used. 1 Other,;ise, diffraction effects by the photons whicn

pass through adjacent features blur the features and allow short c--

cuits. The thin organic polymer films, which are normally used as

lithogranhic resists, suffer from an inability to absorb all of the

higher energy radiation. The unabsorbed radiation backscatters from

the oxide layer which is normally between the resist and the semi-

conductor.

Heavy atom-containing polymer films should have an enhanced sensitivi-

ty to applied radiation because of the atomic number (Z) dependence

of the photoelectric effect which has an atomic absorption coefficient

which is Z4 or greater (Evans 1972) whereas photon scattering by heavy2 2

atoms only has a Z dependence. To test this concept, we synthesized

and tested a number of uranv dicaroxvuate nolvmers for ga.mma-ra'.

1 Visible and ultraviolet radiation c-assed through masks or orc ectedonto the surface orovide the mass croducticn necessary for the lo-;cost devices currently on the market. Ai'houan electron beat litho-graphy can provide hiaher resolution, tne tnrougnput rate and back-scattering problems suggr-est that x-ray or low-energw ca-mma-rav met'.-odology is needed. For more details see either of tw,;o recent reviewvolumes (Thompson 1983, 1984)

2 Polymers with main groun heavy elements nave shown increased sensi-tivity using the empirical method of determining solubilities beforeand after radiation (Webb 1979; Haller 1979). Such sensitivities arebased on the dose per unit area required to dissolve the irradiatedpolymer with 1/2 of the unirradiated polymer still on the substrate(for a positive resist and vice versa for a negative resist). Theyare solvent and laboratory denendent, cf. Thomrson (1983, Iq841 , andare not as fundamental as I value s.

sensitivity. From the uxt2n s v - i t-,raturi on ur in, 1 carbcxv,,3a, rr mto-

chemistry, cf. Burrows (1974), we anticipated CO 2evolution, oossibl/

mixed with hydrogen abstraction (Rehorek 1982). Much to our surorise,

a sizable number of the uranyl polymers show no CO 2 loss during 137Cs

irradiation, but they crosslink with vel, high eff cienc-' instead.

METHODOLOGY

The irradiation studies were conducted with a cesiuum-137 gamma ohoton

source (662 keV) with dose rates of 0.022 to 0.073 Mrad/hr based on

Fricke dosimetry (Hine 1956; Getoff 1962). Corrections for different

mass absorption cross sections between the dosimeter and the uranyl

polymers were made subsequently using literature data (Mann 1960) in-

terpolated when necessary. SamDles were irradiated in vacuo using

sealed Pyrex or quartz tubes. Molecular weight analysis of the poly-

mers before and after irradiation were conducted via gel permeation

(or size exclusion) chromatoaraphy (CPC) in N-methvlnvrrolidone (NMP)solutions on Ultrastyragel columns, which had been calibrated withpolystyrene standards (in NMP). Gas chromatography-mass spectroscopy

(GCMS), electron spin resonance (ESR), Fourier transform nuclear mag-netic resonance (NMR) and infrared (IR), and ultraviolet-visible (U%'A)spectroscopies were used to help elucidate the products.

G values were determined using the method of Charlesby (1954, 1960,

1964) and reiterated by Dole (1973):

n n + (C-_x) a

where n o is the number-average molecular weiaht before irradiation,Mn ' is the number-avera.-e molecular weight after irradiation,G is the number of chemical scissions per 100 elpctron volts,

G is the numb.rer cf crcsslinks eer 1OO <c:cron volts,is the radiation dose in etctron s -et ram, and

N is Avoca"rc'a-Converted to Me rara ds (r L e7-o (C .. t-s']:: , O :

These e-uations are based or. ; 7-in and crosslinkinq nrocessos,random initial molecular weif .. -.... s (. . - 2) , and Gwith Cand G dose independent. Es'mf,-r .. ns, they provide the

r n'n- proid t'best guide possible to the ma(7nttud, cf the radiation effects, and the

dose dependence is apparent from p"ots of 1 ": vs dose in Mrad.n

The synthesis and characterization of the uranyl d _a2r x'at- S is d,_I -

tailed elsewhere (Archer 1987; Hardiman 1987) . The essential synthetic

reaction is

U02 (O2 CCH 3 ) 2.(H20) 2 + HO 2 CRCO2i C2 H-6 so UO 2 (O2CRCO 2 )Jn + CH 3CO 2 H [3J

The molecular weights have been determined by GPC, viscosity, and NM

end-group analysis. Solid-state 13C-NMR has also been used (Archer 1937)

to confirm the coordination modes of the carboxylate ligands to the

uranyl ion. The results are as anticipated from IR results (Hardiman

1987) ; that is, both monodentate and bidentate carboxylate coordination

occur such that 7-coordination predominates in polymers with either one

or two moles of solvent coordinated to the uranyl ion.

URANYL POLYMERS WHICH APPEAR TO EXCLUSIVELY CROSSLINK

As anticipated, the uranyl dicarboxylate polymers are very sensitive to

gamma-ray irradiation. The data for four branched alkyl, one alkene,

and one aromatic dicarboxylates which appear to exclusively crosslink,

with no evidence of CO2 evolution, are summarized in Table 1. Whereas

the entire absorption for the lighter elements (0.257, 1.54, 2.04, and

4.09 barns/atom for H, C, 0, and S, respectively) is of the Compton

type, for uranium 50% is photoelectric absorption (47.7 barns/atom

total). Thus, the uranium atom constitutes from 50 to 58% of the ab-

sorption in the unit at this energ,,, (662 keY), but only modifies the

mass absorption coefficient for the oolvmer by about 13 to 16% relative

to water, the absorber of the Fricke dosimeter. Organic nolymers are

typically lower than water; e.g., polv(methyl met.acry-late), PMMA, is

0.083 cm2 /g using the same numbers, or 17 to 20% lower than the uranvi

polymers. However, on a reneatinu unit basis, the uranvl polymers a --

sorb fr.m 600 to 700% as much rau3aticn as PNA (, = 1.3) and cra

nolymers with G values as h"c as 10 are kncwn (T'71omoson 1934.

None o t-e scecies in Tahle 1 show an, tencencv fr scission tn..,

the G s-G x value is essentially -G x . With increased dose levels, a-'

exhibit lower G values--a locical consecuence of fewer effective c-t

links. Extra crosslinks between. the same chains do not increase

molecular weight any furtner. The values in the table are for 2 MraJ

doses (or 3 Mrad for the succinate and ohthalate derivatives as no

curvature was observed until above 3 Mrad for these two snecies. .e

extremely high G value for the tetranimelate derivative must be r-

lated to the mobility of the lon-er alkyl chain counled with stabii::ed

tertiary radicals counl,-d with effcient en.riv t>"nsfer.

Tab it' 1 . Ganrna- r vi S,'n:;~ viti 0 ..... v 'i of a,....

Empirical Formula Unit a n s x c / d -G

(bridging carboxylate)

0oCC(C 3 )2 C11 C0 (C H SO) 9,oo01 -14- 0.1OO -Y

JO 23 2 2~ 6(2,2-dimethylsuccinate)

UO2 O2 CC(CH3 )2 CH2 CH2CO 2 (C2H6 SO) 48,OOf -3.f 0.100 -2.7

(2,2-dimethylglutarate)

UO2 O 2CCH 2 C(CH3) 2CH 2Co 2 (C2H6 SO) 12,OOO -8.O g 0.10 or

(3,3-dimethylglutarate)

UO2 0 2CC(CH 3 )2 (CH2 )3C(CH3) 2CO 2 (C2H 6S0) 12,0001 -43 g 0.099 - -

(2, 2,6,6-tetramethylplnelate)

UO2 (Z-O 2 CH=CHCO2 ) (C 2 H6 SO13 9 3-0 -3.3 0.09 -2.

(maleate)

(70 CC H 1O,o -3.77 0.097 -3.3UO2 - 2 6 4 CO 2 6 (C2H6SO' 2

(phthalate)

aRepeating unit of polymer cna n including solvation

bM of samples not irradiated; based on GPC in NYP with ,nolxst,renen

standards, or by NMR end-croup anal"'sis if so indicatedCMeasured net G value usina equation 2 and GPC after 3 Cs irrad'_::dMass absorntion coefficients in cm2 / for 662 keV irradiation

Corrected G values relative tc Fricke dosimeter(0.086 cm>/g for

Polymer has poor M distribution and poor agreement with end-'rou, d tn

g,; values for this poymer--end-9rou-) calibrated by N.MR

URA;YTIL POL YME R S ?H1 PT2SRI 11S3::;TI

.n .e ct.e 'hand, .ranvL to .:-rs .. . .-,;I *t*: -*

s an _ f-a1 saow rr a va a I .s 3 7

noted in T a L 1e 2. Th-e cle_-anest one .,s the'.--'.0 r% " : -!:: -

poo_ *ire, 'ic s cws no .- ndc:v, towar' o;

rjroa§'y a tru--e -easre of G. The "PC's _f so

r. - .nat v, toe': are so - r,

'aI X ....re.... .. ... .

• N :iy '': , : .. . , : , ,<. i e d: r ' :.n'r: , ., ' :.- . - ..

i, b 1' . Garnna-ra,., Sens it!,I*iyo y ' .';t UranVi Dol',MvrS

Empirlical Formula Unit3 a .MG1 - ., G s -Gx

(bridging carboxylate)

1-0 (O2)CCffSCH2)CO2 ) (C H 5SO: 24,Cj06 0.0J97 5o

(thiodiglycolate)

U0 2 ( 2 CCH 2 SSCH 2 o2)H ( 2 H 6SO) 15 1 2,0 0 0 g-j 30O'J_ 0.097 28D7-

(dithiodiglycolate)

U02 (0 CCH SCH SCH CO ) (C H SO)., 1 5 , 0 0 0 g,h I D3g. 0 D. 096 93 '7 ,

(methylenebis thioglycolate

U02 (E-0 CCH=CH-CO ) (C H SO), 2 6 ,0 0 0h~ 6 3h~ O.oD9

(furnarate)

a-e Sm as Table 1

f G values based on filrst 0.5 to 0.6 Mrad or.,-,

9M adjusted to aaree with NM'.R end-carcu-) a-al-.s,,sh Poiymer undergoes both scission and crosslin,'cino7

'~distribution coor; results very aonroximate

3Polymer very sensitive to radiation--nonirradiatiod samoles chan7edistribution sio7nificarntlv durinq irradiation o4 other samroles

end-group deter-mined weigohts. Snecificallv, tb-iodilvooilate, d~~

d_,~vc~a-eand methvlenebis thioalvcolate were orio7inaliv calcuIlated

+- o _ys tyreno -e 7,ui -.-a et callbrations. These no-sab-sorb fro

about,: 5 to 7times as much enera':. cr ool'.mer untas an oroanic -c-v-

77e, as w020 'a Mca7e tA' C,;u a S0 eo foo~2 u;m X

a~-. r. Cc' . A- S: ~ . A.

a T k-3 1 , .'7 -

3 3 c' 2t

polymers. Teeand similar VO 1-diketonates bridqf-d *-.itn s'iI'4ir

atoms (Archer 1986) show radiation effects which are both .'l-nt

and state/solvent dependent. For example, instead of reduction to co-

balt(II) , gammua irradiation produces facile C-S scission in thecoa-

(IPI tolvmers. Soace limitat'ions rnrecludes furth"er detalls.

ACKNOWLEDGEM.ENTS

The financial suuoort of the U.S. Office of Naval Research and hei-nfu-l

interactions with Professors J.C.W. Chien (Univ. Massa chuse tts) and N.

Getoff (Univ. Wien) are gr-atefully acknowledged.

RE FEIRE NCE S

Archer RD, Tramontano VJ, Lee AY, Grybos R (1986) Thodithic- and-sulfoxo-bis-f3-d-,ketonate metal chelate polymners. Inte--rnational Conf-,'erence on Coordination Comocunds, Athens, Greece

Archer RD, Dickenson LC, Lee AY, Och~ava I;O, Chien jC7W, (1937) Monoden-tate and bidentate carbonvl coordination in coordination nolvm-,ers o'uranium (to be cublished)

Burrows HD, Kem-D TJ (1974) The nhotochemistrv of the uranvi ion. ThemSoc Rev 3: 139-165

Charlesby A (195"4) Molecular weight changes in degradation onln-chain polymers. Proc Royal Soc London, Ser A 224: 120

Charlesbv. A (1960) Atomic radiation and nolvmers. Derqamon, OxfordCharlesby A, Moore N (1964) Comojarison of ga.mm-a and ultra-violet radca-

tion effects in co 1-.me thvmethacrv late at hihrtemrnerature. Int JAcol. Radiat Isot 15: 70--3-703

Dole M (1973) The radiation chemistry of macromolecules, Vol 2. AcademicPress, New York

Evans RD (1972) Amer inst physics hdbk, 3rd ed. Mcorw-iil NY, secspoGetoff N (1962) Fortschritte der Strahlenchem-ie w~i3ric-er LZbsuneen.

Osterr Th-em Z--, 63: 91-98Haller I, Feder R, atzakis M, Soiller EA (1979) Conol,'eoers of- me-tnvi,

methacr-ylate and methacr-zlic acid and their metal salts as radiation-sensitive resists. J El1ectrochem Soc 126: 154-161

Hardiman CJ, Archer RD (1937) iornimn crbx.asrevr:syntnesis and cnaracterizat-Ion of trac7table ront:ev:idence for ritd,- rod conformatin. Macrrm

Hine GJ, 3 row nel C1''L (eds (1956) R ad iat d'es Ie r-: A'-~m: T- 0N ew Yocr'.-Ma7 a: -.: 7aa r a c ross s (,ct--:n data. -a

Prooulsion D)ent, Cincinatt--Rehorek, D, Pua,:.: Jn (1982) Formation of rd2'

of uranyl salts in alinhatic cro'i cLett 52: 29-33D

Thompson LF, Willson CG, Bowden IMJ (eds) (19() Tt1rc

lit-hograohv: theory, materiai-s & rocessinef 'v' m:c219. American Chemical Society, Dahi"t ,

Thompson LF, Wills Dn CC, Frechet JMJ (ods~) i ~ aradiation sensitive polymers, ACS S.'mno)'S-nM s es ~ ~ aChemical Society,, Washington DC

WIebb DJ, Hatzakis M (1979) Metal methacrvlt~s a~S,,n(met'.' mehac~lae) lectron resists-. 3 j

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