Using soft lithography to fabricate GaAs/AlGaAs heterostructure field ...
Using soft lithographyto fabricateGaAs/AlGaAsheterostructurefield effect transistors K. D. Maranowski,b) JunminHu, R. G. Beck,")Tao Deng, R. M. Westervelt,") A. C. Gossard,b)and George nll.Whitesides") Department of Chemistry and Chemical Biology, Harvard Universit;. Camhridge, Massachusetts02138
(ReceivedI July 1997;acceptedfor publication5 August 1997) This letter describesthe fabrication of functional GaAs/AlGaAs field effect transistorsusing micromolding in capillaries-a representative soft lithographictechnique.The fabricationprocess involved three soft lithographicstepsand two registrationsteps.Room temperaturecharacteristics of thesetransistorsresemblethose of field effect transistorsfabricatedby photolithography.The fabricationof functional microelectronicdevicesusing multilayer soft lithographyestablishesthe compatibilityof thesetechniqueswith the processingmethodsusedin devicefabrication,and opens the door for their developmentas a techniquein this area. O 1997AmericanInstituteof Physics.
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Soft lithography-a set of techniquesfor nonphotolithographicpatterntransferbasedon contactprinting and polymer molding-is now establishedas an alternativeto photolithography in microfabrication.r*aWhile it has been demonstratedthat soft lithographyhas obvious applications in optical5-7and microanalyticalsystems,sits potential for applicationto microelectronicdevicesremainsto be established.Fabricationof such devicesrequireshrgh reproducibility in patterntransfer,accurateregistrationbetweenlar'ers,low distortionof the elastomericmolds.and little surfacc contaminationor damageduring processing.Thc oh.jectirc of the work reportedhere is to begin to addresstheseissues by usingsoft lithographicprocesses to fabricatesimple.electrically functionaldevices,and by establishing the characteristics of theseprocesses. In this letter we describethe fabricationof field effect transistors(FETs)on a GaAs/AlGaAsheterostructure usinga representativesoft lithographictechnique:micromolding rn capillaries(MIMIC).' The fabricationprocessrequiredthree molding stepsand two registrationsteps.The FETs har,'ean o ve rall s iz e of I m mx 0 .5 5 m m w i th g a te l e n g th(/-) and gate width (Z) both ranging from about 20 to -50ptm. Their characteristicsare similar to those of FETs fabricatedusing conventionalphotolithographictechniques.These dimensionsare not lower limits; we have made simple test structureswith dimensionsdown to 30 nm by soft lithography.e'10 Rather,theseFETs establishthat soft lithographyis compatible with multilayerfabricationof functionalmicroelectronic devices,and set a benchmarkagainstwhich to measurefurther developmentin this area. Figure I shows the schematicFET structure.The FET was made on a molecularbeam epitaxially (MBE)-grown GaAs/AlGaAs heterostructure.Figure l(a) is a cross sectional view of the wafer, composedof GaAs, Alg.-jGqruAs, a n d a S i E dopedlay e r n :3 .8 x l 0 r2 c m -2 4 0 0 A from rhe interface.The two-dimensionalelectron gas (2DEG) is lo')Dept. of Physicsand Division of Engineeringand Applied Sciences,Harvard University,Cambridge,MA 02138. brMaterials Dept.. University of California.SantaBarbara.CA 93106. ''Author to whom correspondence should be addressed.Electronic mail: gwhitesides @gmwgroup.harvard.edu
Appl.Phys.Lett.71 (14),6 October1997
the cated 1500 A below the surtace.At room temperature, measured 2D E G densi tyand mobi l i tyw ere 3.9x l0lr cm - 2 and 4.0x 103cm2/Vs. In Fig. l(b) the sourceand drain are AuNiGe ohmic contacts,the channelis defined by a mesa etch, and the gate is a CrlAu Schottkycontact. Figure2 illustratesthe processusedto fabricatethe FET. W e usedMIMIC i n al l threefabri cati on steps.In M I M I C, an recessed regionsis put elastomericrnold with interconnected i n confi rrmalcontactu i th the substrate. C onti nuouchannels s are tirrrled br thc reccsscdregionson the mold and the substrate.A l rqui dprepol rrneri s appl i edto the openen dsof t he y capillar it y. channel sand ti l l s the channel sautomati cal lby The prepolymeris cured eitherthermallyor by long wavelengthultraviolet(UV; light, and the mold is then removed. The elastomericmolds (or stamps)were made by casting (P D MS . S yl gard 184, D ow- Cor ning. pol ydi methyl si l oxane A :B -- l :15) on mastersgenerated by a rapi d prot ot yping techniquebasedon patternsprintedusing a high resolution
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Gate width Z = 20pm-50pm
FIG. I. (a) Schematiccross sectionalview of the field effect transistor (FET) to show the growth profile of the GaAs/AlGaAs material.(b) Schematic diasram of GaAs/AlGaAs heterostructure FET.
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i n r a . g e - s e t t i nsgv s t e n t . " T h e t h i c k n e s so f ' t h e P D M S l l o l d s L r s c di n F E T l i r b r i c u t i o l . l\ \ ' a s t v p i c u l l \ , - . ,1 n t n t u ' h i l c t h c r e l i e f s t r u c t u r c sh u c l l 0 p r n t s t c p s . A s s l r o u r r i n F i g s . 2 ( a ) . 2 ( d ) . a n d 2 ( g ) . t h e r c c c s s c dr e g i ( ) n s( ) l l t l ' ) cr n o l r l s\ \ c r c i n t c r . c o n n c c t e cul n c lh a d o p e n e n d s . P D I \ { S i s i t g o o c le h o i c c f i r r t h c n r o l c l n r a t e r i a l b e c a u s e i t u ' c t s t h c s a n r p l e s L u ' l : r c ca n r l h c c a L r s ei t i s t r a n s p a r e l t t o t h c U V l i g h t L r s c dt o c u r - ct l i c p r - c p o l v n r e r . T h e t r a n s r . n i s s i o nc o e f l i c i e n t o f a l - n r r n - t l r i c k P D M S b l o c k w a s 9 l % a t \ : 3 6 5 n r l . S a r n p l e s\ \ ' e r c c l e a n e c l i n t r i c h k r r ' o e t h v l c n ei .r c e t o n c .a n d n r e t h u n o lb c f i r r c c a c h i r l l p l i c a t i o no l ' t h e P D M S n r o l c l . F i g L r r e sl ( a ) - l ( c ) c l c s c r i b ct h e f i r s t l n o l d i n g s t e p : t h e f ' a b r i c a t i o no l ' o l r n r i c c o n t a c t s . T h e o h r n i c c o n t i l c t s . s o L r r c e ancl clrilin. werc 2(X) pntX2(X) 4.rn sqLluresscpar-atecl bi' l-50 #.nr.Two alignrlent lnarks of'size l-50 prntX 150 g.nt \\'ere a l s o d e f i n e c il n t h c f i r s t s t e p f o r a l i - r n i n gt h e e t c h t r e n c h e sa n c l t h c - u a t c .T h e p r e p o l v r l e r . p o l y u r e r h a n e( P U ) N O A 7 3 ( N o r l a n d P r o c l L r c t s )h. a d a l o w y i s c o s i t y ( r 7 : 1 1 0 c p s ) . P U r e q u i r e d a b o L r t2 0 r n i n r o f i l l a 6 - r n r n - l o n - sc h a n n e l [ F i - u .2 ( a ) ] and wlrs cllred bv exposr-rreto a ,150 W UV lantp (CanradH a n o v i a I m m c r s i o n L a r n p . M o d e l 7 8 2 5 - 3 ; 1 )l i i r I h . T h e P D M S m o l d w a s t h e n r e m o v e d [ F i g . 2 ( b ) J .O t i e n a r h i n l a y e r o f P U ( < 0 . 1 p r n ) d i f f u s e d i n t o t h e r e c e s s e dr e g i o n s o n t h e sample: this layer was removed using &fl Or reactive ion etch . A p p l .P h y s .L e t t . V , o l .7 1 , N o . ' 1 4 6 O c t o b e r1 9 9 7
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a r e a i l t ' t h e s t r i p h c u t c r ' ( + 0 n r n r l ) u s e c l i r r u n n e r r l i n gT. h e r e g i s t r a t i o n\ \ ' r . rrsi i t h i n l l 4 n r o \ c r i l J 0 r r r n r lu r e u .- f h c n u r i n f i t c t c l r st h a t a l ' f ' c c t c cr le g i s t n r t i o nu c l ' c t h c r l i s t o r t i o n l r n r l t h c v a r i a t i o ni n t h i c k n c s so t ' t h c c l u s t o r n c r i cn r o l d . \ \ c h l r ic l r l s o noticed that locul sagginu ot' the PDMS rnoltl coultl ocelu. d u r i n g r e g i s t r a t i o n' ' h c n o n e s i d e o f t h c p l ) N l s r n o l c l u ' a s s i g n i t i c a n t l ' rt' h i c k c r t h a n t h e o t h e r - . \ / ? - l ( X ) p t r n . T h e s a - u . u i n gc o u l d h o u c r e r h e n r i n i n r i z e dw i t h s p e c i a lc a r c i n b r i n - r r i n g t h e r n o l d a n c l t h c s u n r p l ei n t o c o n t a c t . T h e f i r s t M I M I C
i n g t h i s r i i t h t h c r c g i s t n r t i o nu e h a v c a c h i c r . , e h c lc r e ( a p p r . o x i t t l r t c l r I I p r r n o \ c t ' i . l 1 0 1 1 1 1 1l u1'ic l )r s l t o L r l ccl c r t a i n l y a l l o w l l t c l t r l ' r r rl e r t i O r ro l t c s t i t t ' l ' r .\ t -t\r l ' t r l r r t s i s t o r si . r t t col t h e r n t i c r o c l c et r o n i e e( ) l n l ) o l t c l t t\\\ i t h r n i n i n t u n rf ' c l r t r r rsci z e s- < l 0 p r . r r i . T h c u L r t h o r '\\\ ( ) u l t l l i k c t o t l t r n k X i a o - M c i Z h t t o . S t e v c S h c p p a r c l .a n r l I ) u r i c l f ' u r t o ' l i r r h e l p t i r l c l i s c L r s s i o n sT.h i s 'urorkwus sLrpportccl in parl by APO Grant No. DAAH0-1-950102. NSF Clrant No. DMR-9-l-00396. and AFOSR Grant No. F--lcXrl0-| -0 l -stJ.
s t e p t y p i c a l l y h a c l u r i c l d o l ' ) t ) 8 % . T h e d e f ' e c t sa r o s c f r o n r r e g i o n s n e a r t h c o p c r r c n d s o f ' t l i c P D M S n r o l c l .T h e s L r b s e q L r e n tM I N , { l C s t c p s h u c l a y , , i e l do t ' - 9 5 % . T h e a d d i t i o n a l d c f - e c t sc a n r e f l ' o n r l o c a l s a g g i n g o f t h e P D M S n r o l c l sa s s t r c i l u t e cw l i t h t h e r a r i a t i o n i n t h e t h i c k n e s s< l l 't h e p D M S n r o l c l s . FigLrreJ shou's thc rrieasureddc perfilrrlance ot' a rcpr e s c n t a t i r eF E T w ' i t l i L : 3 0 # . n r a n d Z : 2 l p t r n a t r o o r n t c r r . r p e r a t L r r cF. i g u r c - l ( a ) s h o w s t h e c l r a i n - s o u l . cceL u - r e n1t 1 r \r ' c r s u s d n r i n - s o l r r c ev o l t a g e v l ; q c h a r a c t e r i s t i c so l ' t h i s F E T f i l r a s e r i c s o f g a t e r o l t a g c s . T h e s e 1 , r * l / 1 , qc h a r a c t er i s t i c s w e r e s i r n i l a r t o t h o s c o f F E T s f - a b r i c a t e du s i n . 9c o n v c - n t i u n apl h o t . l i t h o g r a p h i c t e c h n i q u e s . ' lT h e c h a n n e l r e s i s t a r c ea t v r ; s : 0 r v a s R c t * L n r c tI:7 . - 5k O . t h e t r a n s c o n d u c t a n c e a t 1 1 ; q -8 0 r r A u ' a s , g , , r 0 : 6 3 p r S .a n c lt h e b r e a k d o w n v o l t a g e o f t h c F E T n ' a s B V r , r s r i -l ( X ) V . T h e t r a n s f ' e rc h a r a c t c r i s t i c sw e r c r n e a s u r e da t V l r q : . 1 V . a n d a r e s h o w n i n F i g . - l ( b ) . A s e x p e c t e c l . 1 1 D ql ' a r i e s l i n e a r l y w i t h V 1 ; s . s i r n i l a r t o t h a t o l - a h e t e r o strLlctureFET.lt Th. pinch-off l'oltage was cleternrinedfronr t h e t r a n s f - ecr h a r a c t e r i s t i c sa. n d w a s V , , : 2 ) V . T h e s e c h a r acteristicsresemble the comesponding values for the heterostructure FETs of p h o t o l i t h o g r a p h yI I. 2022
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. l. I . . \ \ ' i l b r r r ', . \ . K r r n t r r rl :. K i n t . r r n t l( i . \ \ ' h i t c s r c l cAs r. l r . \ l u t c r . 6 . 6 0 0 { 199-1i t t r . K ' r n .\ ' \ i r r . r r n t(l i . \ l \ \ h i r c s i t l c s\ l.r t r r r c( l - o r . r t k r n ) 3 -7568. 1( 1 9 9 . i ) . '\.-\1. l l r r r o .\ ' . \ i r r . r i n t l( i \ l \ \ l r i r t ' : i r l c s, {. c l r . N l r r t c rl .t . l J . l 7( I ( ) 9 ( r ) '\'. Xiir. t:. Kirn. \.-\1. Zhir.. .1..\. I{.gcrr. \1. [)rc.ntisu s .' r l ( i . \ i . \ \ ' h i t c s i t l c sS . cicne c 2 7 1 . l l 7 r I t ) 9 ( rr 'X.-\1. Z h u o . , \ S t o r l t l l r rSt .. P S r n i t h f. : . K i n t . \ ' . X i l t . l \ l. p r e n t i s su.n c (l i \ 1 . \ \ ' h i t c s r t l c s, ..\ t l r .\ l a t c r . t l . l l 0 ( 1 9 9 6 ) . " . 1 . . . \R . t ' g c r . rR. . . l . . l r r c k r n l tOn . . 1 . . . \ .S c h u e l l c u r .n t l ( i . N , l .\ \ ' h i t c s i c l c s . (l(X)6) O p t 6 6 + l 3 5 . _Appl. D . Q i n . \ ' . X i a . r r n t l( i . \ l \ \ ' h i r c s i d e sA.c l r . \ l l r e r . g . - + 0 j ( l r ) 9 7 r . 'c'. s . I r l l c n h u u s cG r .. . 1 .N 1 .B r u i n . , \ . P u L r l Lar sn.c l \ { . E h r a t .i n p r o e c c r l i n s s o l t h c l l t t l I t t t c r l l t t i o r t uSl _nr r 1 . l t l : i u n o r r N , { i n i a t u r i z cTcol t a l , , \ n u l sr i s S r s t r ' r n s .i - r T . \ S 9 ( r[.J i r s c l .1 9 9 6 .p . l 2 J . " Y . X i a . . l . . l .l \ l c ( ' l c l l a r r cRl . ( i u p r r . D . e i n . X . - N I .Z h a o .L . I _ .S o h n .R . J . ( - e l o t t l . a n r l G . \ 1 . \ \ ' h i t e s i r l c s.., \ c l r .N , l a t c r9. . l 4 i ( 1 9 9 7 ) r"X.-i\{. Z h a o .Y . X i u . . n d G . \ 1 . \ \ ' h i t e s i c l c s .N . l ,. l a r e C r .h e ' r . 7 . 1 0 6 9 I 1 9 9 7) . D . Q i n . Y . X i a . a n r lC i . \ { . \ \ ' h i t c s i c l eA s .c l ' uM . u r e r .t J . 9 l 7 ( l r ) L ) 6 ) . 'rs. M. S z e . . ! c r l i t o t t d t r t ' r oI )rc t ' i t ' L ' .Ps /. r r ' . r r r 'u, r t t lr t , t h t t o l o c r ' ( w i l e v .N e w , Y o r k . 1 9 8 5) . '*T. NIi"r,-rr,r. S . H i l ' a r n i z uT. . F L r j i ia. n d K . N a n b u J. p n .J . A p p l . p h 1 s . .p a r t I 1 9 .L l l - s ( l 9 t t 0 ) .
Hu et al.
(ReceivedI July 1997;acceptedfor publication5 August 1997) This letter describesthe fabrication of functional GaAs/AlGaAs field effect transistorsusing micromolding in capillaries-a representative soft lithographictechnique.The fabricationprocess involved three soft lithographicstepsand two registrationsteps.Room temperaturecharacteristics of thesetransistorsresemblethose of field effect transistorsfabricatedby photolithography.The fabricationof functional microelectronicdevicesusing multilayer soft lithographyestablishesthe compatibilityof thesetechniqueswith the processingmethodsusedin devicefabrication,and opens the door for their developmentas a techniquein this area. O 1997AmericanInstituteof Physics.
r (e7)03 840-0] [s0003-6e5
Soft lithography-a set of techniquesfor nonphotolithographicpatterntransferbasedon contactprinting and polymer molding-is now establishedas an alternativeto photolithography in microfabrication.r*aWhile it has been demonstratedthat soft lithographyhas obvious applications in optical5-7and microanalyticalsystems,sits potential for applicationto microelectronicdevicesremainsto be established.Fabricationof such devicesrequireshrgh reproducibility in patterntransfer,accurateregistrationbetweenlar'ers,low distortionof the elastomericmolds.and little surfacc contaminationor damageduring processing.Thc oh.jectirc of the work reportedhere is to begin to addresstheseissues by usingsoft lithographicprocesses to fabricatesimple.electrically functionaldevices,and by establishing the characteristics of theseprocesses. In this letter we describethe fabricationof field effect transistors(FETs)on a GaAs/AlGaAsheterostructure usinga representativesoft lithographictechnique:micromolding rn capillaries(MIMIC).' The fabricationprocessrequiredthree molding stepsand two registrationsteps.The FETs har,'ean o ve rall s iz e of I m mx 0 .5 5 m m w i th g a te l e n g th(/-) and gate width (Z) both ranging from about 20 to -50ptm. Their characteristicsare similar to those of FETs fabricatedusing conventionalphotolithographictechniques.These dimensionsare not lower limits; we have made simple test structureswith dimensionsdown to 30 nm by soft lithography.e'10 Rather,theseFETs establishthat soft lithographyis compatible with multilayerfabricationof functionalmicroelectronic devices,and set a benchmarkagainstwhich to measurefurther developmentin this area. Figure I shows the schematicFET structure.The FET was made on a molecularbeam epitaxially (MBE)-grown GaAs/AlGaAs heterostructure.Figure l(a) is a cross sectional view of the wafer, composedof GaAs, Alg.-jGqruAs, a n d a S i E dopedlay e r n :3 .8 x l 0 r2 c m -2 4 0 0 A from rhe interface.The two-dimensionalelectron gas (2DEG) is lo')Dept. of Physicsand Division of Engineeringand Applied Sciences,Harvard University,Cambridge,MA 02138. brMaterials Dept.. University of California.SantaBarbara.CA 93106. ''Author to whom correspondence should be addressed.Electronic mail: gwhitesides @gmwgroup.harvard.edu
Appl.Phys.Lett.71 (14),6 October1997
the cated 1500 A below the surtace.At room temperature, measured 2D E G densi tyand mobi l i tyw ere 3.9x l0lr cm - 2 and 4.0x 103cm2/Vs. In Fig. l(b) the sourceand drain are AuNiGe ohmic contacts,the channelis defined by a mesa etch, and the gate is a CrlAu Schottkycontact. Figure2 illustratesthe processusedto fabricatethe FET. W e usedMIMIC i n al l threefabri cati on steps.In M I M I C, an recessed regionsis put elastomericrnold with interconnected i n confi rrmalcontactu i th the substrate. C onti nuouchannels s are tirrrled br thc reccsscdregionson the mold and the substrate.A l rqui dprepol rrneri s appl i edto the openen dsof t he y capillar it y. channel sand ti l l s the channel sautomati cal lby The prepolymeris cured eitherthermallyor by long wavelengthultraviolet(UV; light, and the mold is then removed. The elastomericmolds (or stamps)were made by casting (P D MS . S yl gard 184, D ow- Cor ning. pol ydi methyl si l oxane A :B -- l :15) on mastersgenerated by a rapi d prot ot yping techniquebasedon patternsprintedusing a high resolution
-
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Gate width Z = 20pm-50pm
FIG. I. (a) Schematiccross sectionalview of the field effect transistor (FET) to show the growth profile of the GaAs/AlGaAs material.(b) Schematic diasram of GaAs/AlGaAs heterostructure FET.
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F I G . l . S c h e r r t a t i ci l l t r s t r a t i o no f t h c p n r c c r l u r ct i r r F E J ' l ' a h r i c u t i o n .( u ) F i r s t \ ' l l N 4 l C t t s i t t up o l r t t r c t h l u t c( P I r i t o t l c f i n c t h c o l r n t i e e ( ) n t i t ( ' t l\r n t l u l i r n n t e r r t r n a r k s . ( h ) C ' u r c ' l ) L ' .p c e l o l l t h c l i r ' : t I ) [ ) \ l S n t o l t l . r u t t l r e n t r , r f t i t r ' l ) [ c . l i l t r , l l I ) [ . . r n t i r t n n e l r1] , , L t t t t L ' r l i t ruc s| i n g o - R I I r . r g 1 [ 1 1 1 1 1 . 1 1\ ri 1 r \;is( [i e t i t r r t t t h c o l t t t t i e e o n t l r et \ l i r l t h c \ ( ) u r ' ! c l r n t l t l r l i i l r . r r l r [ l t L r s t c r . . s c r ' t r r ] r l N { l N 'Il C t o c l c t j n c c t e h l r . c n c l t c s (. c ) ( ' r r r c P L . p e c l o i l t l t c , c t . r r l t r lI r l ) \ l S r t t o l t i .a n t l r c l l l ( ) \ L 't h c L r r t c l c r l l r r Lu'\ri n ! O . R I I r . t l ' ) I - t c l t i n u i t r i c u ei t l l r n t l h v c h ' o g c np c r o r i r l c s o l L r t i o nt ( ) r c n r ( ) \ c t h c I D H C i i n t h c c t c h t r c n e h c s . ( g ) R c g i s l c r ' .t l r i r d l \ l I \ l l ( ' t o c l c f i n e r h c g u r t - ' (. h ) c u r c P L ' . p c c l o l ' l ' t h c t h i l c l P D \ 4 S t t t o l c l .l t t t c lr c r t t ( ) \ ' ct h e L r n r l c r l l r l c rr r s i n g O . R I E . ( i ) F . r u l t o t r t c ( ' r l A L r a n t l l i t t o l ' l ' P L rt o t i r r n t t h c s u t e . { j ) C l c u r c t \ \ ( ) c l l ( l \ t ( ) r e l l ( ) \ c t h c l I ) L , ( i i n t l t e ' s er e g i o n s .
i n r a . g e - s e t t i nsgv s t e n t . " T h e t h i c k n e s so f ' t h e P D M S l l o l d s L r s c di n F E T l i r b r i c u t i o l . l\ \ ' a s t v p i c u l l \ , - . ,1 n t n t u ' h i l c t h c r e l i e f s t r u c t u r c sh u c l l 0 p r n t s t c p s . A s s l r o u r r i n F i g s . 2 ( a ) . 2 ( d ) . a n d 2 ( g ) . t h e r c c c s s c dr e g i ( ) n s( ) l l t l ' ) cr n o l r l s\ \ c r c i n t c r . c o n n c c t e cul n c lh a d o p e n e n d s . P D I \ { S i s i t g o o c le h o i c c f i r r t h c n r o l c l n r a t e r i a l b e c a u s e i t u ' c t s t h c s a n r p l e s L u ' l : r c ca n r l h c c a L r s ei t i s t r a n s p a r e l t t o t h c U V l i g h t L r s c dt o c u r - ct l i c p r - c p o l v n r e r . T h e t r a n s r . n i s s i o nc o e f l i c i e n t o f a l - n r r n - t l r i c k P D M S b l o c k w a s 9 l % a t \ : 3 6 5 n r l . S a r n p l e s\ \ ' e r c c l e a n e c l i n t r i c h k r r ' o e t h v l c n ei .r c e t o n c .a n d n r e t h u n o lb c f i r r c c a c h i r l l p l i c a t i o no l ' t h e P D M S n r o l c l . F i g L r r e sl ( a ) - l ( c ) c l c s c r i b ct h e f i r s t l n o l d i n g s t e p : t h e f ' a b r i c a t i o no l ' o l r n r i c c o n t a c t s . T h e o h r n i c c o n t i l c t s . s o L r r c e ancl clrilin. werc 2(X) pntX2(X) 4.rn sqLluresscpar-atecl bi' l-50 #.nr.Two alignrlent lnarks of'size l-50 prntX 150 g.nt \\'ere a l s o d e f i n e c il n t h c f i r s t s t e p f o r a l i - r n i n gt h e e t c h t r e n c h e sa n c l t h c - u a t c .T h e p r e p o l v r l e r . p o l y u r e r h a n e( P U ) N O A 7 3 ( N o r l a n d P r o c l L r c t s )h. a d a l o w y i s c o s i t y ( r 7 : 1 1 0 c p s ) . P U r e q u i r e d a b o L r t2 0 r n i n r o f i l l a 6 - r n r n - l o n - sc h a n n e l [ F i - u .2 ( a ) ] and wlrs cllred bv exposr-rreto a ,150 W UV lantp (CanradH a n o v i a I m m c r s i o n L a r n p . M o d e l 7 8 2 5 - 3 ; 1 )l i i r I h . T h e P D M S m o l d w a s t h e n r e m o v e d [ F i g . 2 ( b ) J .O t i e n a r h i n l a y e r o f P U ( < 0 . 1 p r n ) d i f f u s e d i n t o t h e r e c e s s e dr e g i o n s o n t h e sample: this layer was removed using &fl Or reactive ion etch . A p p l .P h y s .L e t t . V , o l .7 1 , N o . ' 1 4 6 O c t o b e r1 9 9 7
r \ l r . r ' , , s l r P l t ' o l [ ; [ : ' f s r i i t h r a r i o t r s g u t c l c n g t h s 1 _u t r r lc a t c u i d t h s Z lrtbr.itrttch r lr : o l l I r t h o s r l r p l t , '{.u ) 1 - - 1 6 t r r r na n t l 1 . - l 6 g . p r : ( h ) 1 - - 3 - l 4 n r I t r r r l. Z . 3 0 p i r l : ( c ) 1 - - - l - l p r n ' a n t lZ : - l l f r n t : ( d ) 1 - - 5 2 p r 1 u ; r d 1 , - 1 J unl. I l(;
(R IE ).r A uN i Ge contactsw ere then deposi r edby t her r nal evaporati on. the P U w as l i fi ed off i n rncthvl enchlor e ide.and the contactsw eretherrnal l vanneal ed to ntakco hr niccont act t o t h e2 D E G . F i g L r r el (sr l ) l t l ' t o u t l i n et h c s c c o n dn t o l c l i n sc t e p :n t a k i n g c t eh l r c n el t c : t o l i r r n t t h c e l u r n n c lR . c - r i s t r a t i oonf t h c l l l c \ i . tc t t l t l l r r c l ' t o l l t c r o i t t ' e ct u t t l d r i t i n c o n t i t c t s\ \ ' i l s i l c c o l n p l i : h c t l u . i n s l t K u r l S u s s . \ l i g n c r 'T. I t e n t o l d \ \ ' i l sn t o u n t e do n t h c u l i g n c r b r p l a c i n c i t s f l a t s u r f a c c o n a c l u a r t zs l i d e [ F i - u . 2 ( c l ) l l i ' h i c h . i n t u r n . w i , t sn t o l l n t e do n t h e r n a s k h o l c l c ro f t h e K a r l S u s s . F i n e r e g i s t r a t i o nw a s a c h i c v e c lb y a l i g n i n g i r c r o s s o n t h c s e c o n c lr l o l c l u i t h t l t e l e f i s q L r a r eo n t h e s a n t p l e .O n c e t h c r n o l d a n d t h c s a r n p l e\ \ ' e r e w c l l a l i g n e c l .t h e y w e r c c a r c f u l l y ' b r o u g h t i r r t oc o r r t L r cT t . h e P D M S s t a y e c li n c o n t a c t w i t h t h e s a r n p l ca s i " r e s u l t o f V a n d e r W a a l s i n t e r a c t i o n s W . e then n r o v e d t h e r n o l c la n d t h e s a r n p l ef n r n t t h e n t a s k h o l c l c r a n d u s e d t h e s a n t e M I M I C p r o c e d u r ea s t h a t u s e d t i r r t h e f i t b r i c l r t i o n o t ' o h r n i c c o n t a c t st o o b t a i n t h c P U s t r u c t u r es h o w n i n F i g . 2 ( e ) . C h c n r i c a l c t c h i r r gi n l 0 : l s o l u r i o n o f - 5 0 % c i r r i c ruciclancl .10(rlhvdroselt pcroricle etched uw,ay,l8(X) A GaAs/ . \ l ( i l A s t o r l c l i n c t h c c h a n n e lI F i g . 2 ( f ) ] F i g u r c sl t g t - 2 t i ) d e s c r i b ct h e t h i r d r l o l d i n g s t e p : f a h r i c a t i n c t h c ! u t c . J - h cr c c i s t r u r t i oun, a sa c h i e r e c bl v a l i g n i n gt h c c r o s s o n t h c t h i r c l n r o l r l w i t h t h c r i - g h ts c l r a r eo n t h e s a n r p l c . D u e t o t h c c l u s t o r n e r i cn a t L r r co l ' t l r e n r o l d . i t c o L r l dr e l i a b l y transf'erthc pitttern cvcn o\ cl' r.tonplanarsurfaces.We dcposi t e c ll ( x ) A o l ' C r a n c l3 ( x x ) A o l ' A u b 1 ,t h e r r r a l e v a p o r a t i o n a n r l l i l i e c lo l ' l ' t h e P U t o f i r r n t t h c g a t c . F i n a l l v . a s s h o r r n i n F i g . l ( - i ) . t h e t w ' o e n d s o f t h e s a n r p l ew e r e c l e a r e c lt o r e n r o \ c thc ll)E(i irt llre:c rcgrons. Fi-uure3 shor,rs l'epl'r\enti.rtir c intages of' the FETs firbric a t e d b y M I M I C . T h e s u r f ' a c e so f t h c s o l l r c c a n d d r a i n w c r e r o u - u ha s a r e s u l t o t ' t h e r r n a l a n n c a l i n - uB. o t h t h e e d g e r o u g h (-10 pnt) ot'these FET r t c s s( - - 2 p n ' r )a n d t h e r e s o l u r t i o n structures 'ulercdeterntined by the lltasters.Thc values titr L and Z shown in the figr-rrecaption were nteilsLlredfiont the micro-eraphsof the FETs. A typical sample with patterned area of 40 rnmr had about 40 FETs. The number of FETs that could be fabricated at one time was limited bv the usable Hu et al.
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s t n r t c .w c b c l i c r c t h u t t h c s cc l c t c c t sc a n h e n r i n i r n i z c ci ln t h e l ' r - r t u rbcr o p t i r n i z i n gt h c R I E t r c u t n t e n t . T h i s f ' a b r i c u t i o rorl ' f ' u n c t r o n uF l E T s L r s i n gM I M I C d e n t onstrates that sof't lithograph)' can be usecl to firbricatc rnic r o e l e c t r o n i c d e v i c c s h a r , i n c s a t i s f a c t r t r yp e r f b r r n a n c e .a n d that sofi lithography can be used firr fhbrication of nricrostnrcturesin registrationfbr proce\ses requirins nrLrltiple. r e g i s t e r e dl e i ' c l s . T h e a p p l i c a t i o no f r n u l t i l a v e r s o f t l i t h o g r a p h r i s n o t l i n r i t e d t o r n r c n r e l e c t r o n idc e v i c e s .I t s h o u l d b e l u p p l i c l r b l ci n r n l u r r o t l r c r u r e a s n ' h e r c -s ( ) l l L -r e s i s t r a t i o n i s t l c c t l c t l \. u c h r t \ t h r c c - t lni t c n s i o t u rol p t i c l r ls e l t s o r sr.l L r l tlia v e r \ltr\lS.
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[ ' r i o : c r r s ( ) l l' u \ .t ( l l ) u t l l r n c l d i s p l a rs . T h e n t r n n . n L l r n l c u t u r c : r z ci n t h c s r \ l c l n \ \ c h u r c L r s c di s 2 0 p r n . b L r t h i s f ' e a t L r r cs i z c i s c l c t c r r n i n e d b y t h e r a p i d p r o t o t y p i n g t c c h n i q u e l r u s e c l t o n r a k e t h c r n a s t e r sl b r t h e e l a s t o n r e r i c s t a r n p s .W e a r c c o n f i c l c n t h a t M I M I C c a n b e e a s i l y e x t e n d e d t o s n r a l l e rd i n r c n s i o n sw ' i t hh i g h r e s o l u t i o nm a s t e r s C . onrbin-
a r e a i l t ' t h e s t r i p h c u t c r ' ( + 0 n r n r l ) u s e c l i r r u n n e r r l i n gT. h e r e g i s t r a t i o n\ \ ' r . rrsi i t h i n l l 4 n r o \ c r i l J 0 r r r n r lu r e u .- f h c n u r i n f i t c t c l r st h a t a l ' f ' c c t c cr le g i s t n r t i o nu c l ' c t h c r l i s t o r t i o n l r n r l t h c v a r i a t i o ni n t h i c k n c s so t ' t h c c l u s t o r n c r i cn r o l d . \ \ c h l r ic l r l s o noticed that locul sagginu ot' the PDMS rnoltl coultl ocelu. d u r i n g r e g i s t r a t i o n' ' h c n o n e s i d e o f t h c p l ) N l s r n o l c l u ' a s s i g n i t i c a n t l ' rt' h i c k c r t h a n t h e o t h e r - . \ / ? - l ( X ) p t r n . T h e s a - u . u i n gc o u l d h o u c r e r h e n r i n i n r i z e dw i t h s p e c i a lc a r c i n b r i n - r r i n g t h e r n o l d a n c l t h c s u n r p l ei n t o c o n t a c t . T h e f i r s t M I M I C
i n g t h i s r i i t h t h c r c g i s t n r t i o nu e h a v c a c h i c r . , e h c lc r e ( a p p r . o x i t t l r t c l r I I p r r n o \ c t ' i . l 1 0 1 1 1 1 1l u1'ic l )r s l t o L r l ccl c r t a i n l y a l l o w l l t c l t r l ' r r rl e r t i O r ro l t c s t i t t ' l ' r .\ t -t\r l ' t r l r r t s i s t o r si . r t t col t h e r n t i c r o c l c et r o n i e e( ) l n l ) o l t c l t t\\\ i t h r n i n i n t u n rf ' c l r t r r rsci z e s- < l 0 p r . r r i . T h c u L r t h o r '\\\ ( ) u l t l l i k c t o t l t r n k X i a o - M c i Z h t t o . S t e v c S h c p p a r c l .a n r l I ) u r i c l f ' u r t o ' l i r r h e l p t i r l c l i s c L r s s i o n sT.h i s 'urorkwus sLrpportccl in parl by APO Grant No. DAAH0-1-950102. NSF Clrant No. DMR-9-l-00396. and AFOSR Grant No. F--lcXrl0-| -0 l -stJ.
s t e p t y p i c a l l y h a c l u r i c l d o l ' ) t ) 8 % . T h e d e f ' e c t sa r o s c f r o n r r e g i o n s n e a r t h c o p c r r c n d s o f ' t l i c P D M S n r o l c l .T h e s L r b s e q L r e n tM I N , { l C s t c p s h u c l a y , , i e l do t ' - 9 5 % . T h e a d d i t i o n a l d c f - e c t sc a n r e f l ' o n r l o c a l s a g g i n g o f t h e P D M S n r o l c l sa s s t r c i l u t e cw l i t h t h e r a r i a t i o n i n t h e t h i c k n e s s< l l 't h e p D M S n r o l c l s . FigLrreJ shou's thc rrieasureddc perfilrrlance ot' a rcpr e s c n t a t i r eF E T w ' i t l i L : 3 0 # . n r a n d Z : 2 l p t r n a t r o o r n t c r r . r p e r a t L r r cF. i g u r c - l ( a ) s h o w s t h e c l r a i n - s o u l . cceL u - r e n1t 1 r \r ' c r s u s d n r i n - s o l r r c ev o l t a g e v l ; q c h a r a c t e r i s t i c so l ' t h i s F E T f i l r a s e r i c s o f g a t e r o l t a g c s . T h e s e 1 , r * l / 1 , qc h a r a c t er i s t i c s w e r e s i r n i l a r t o t h o s c o f F E T s f - a b r i c a t e du s i n . 9c o n v c - n t i u n apl h o t . l i t h o g r a p h i c t e c h n i q u e s . ' lT h e c h a n n e l r e s i s t a r c ea t v r ; s : 0 r v a s R c t * L n r c tI:7 . - 5k O . t h e t r a n s c o n d u c t a n c e a t 1 1 ; q -8 0 r r A u ' a s , g , , r 0 : 6 3 p r S .a n c lt h e b r e a k d o w n v o l t a g e o f t h c F E T n ' a s B V r , r s r i -l ( X ) V . T h e t r a n s f ' e rc h a r a c t c r i s t i c sw e r c r n e a s u r e da t V l r q : . 1 V . a n d a r e s h o w n i n F i g . - l ( b ) . A s e x p e c t e c l . 1 1 D ql ' a r i e s l i n e a r l y w i t h V 1 ; s . s i r n i l a r t o t h a t o l - a h e t e r o strLlctureFET.lt Th. pinch-off l'oltage was cleternrinedfronr t h e t r a n s f - ecr h a r a c t e r i s t i c sa. n d w a s V , , : 2 ) V . T h e s e c h a r acteristicsresemble the comesponding values for the heterostructure FETs of p h o t o l i t h o g r a p h yI I. 2022
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. l. I . . \ \ ' i l b r r r ', . \ . K r r n t r r rl :. K i n t . r r n t l( i . \ \ ' h i t c s r c l cAs r. l r . \ l u t c r . 6 . 6 0 0 { 199-1i t t r . K ' r n .\ ' \ i r r . r r n t(l i . \ l \ \ h i r c s i t l c s\ l.r t r r r c( l - o r . r t k r n ) 3 -7568. 1( 1 9 9 . i ) . '\.-\1. l l r r r o .\ ' . \ i r r . r i n t l( i \ l \ \ l r i r t ' : i r l c s, {. c l r . N l r r t c rl .t . l J . l 7( I ( ) 9 ( r ) '\'. Xiir. t:. Kirn. \.-\1. Zhir.. .1..\. I{.gcrr. \1. [)rc.ntisu s .' r l ( i . \ i . \ \ ' h i t c s i t l c sS . cicne c 2 7 1 . l l 7 r I t ) 9 ( rr 'X.-\1. Z h u o . , \ S t o r l t l l r rSt .. P S r n i t h f. : . K i n t . \ ' . X i l t . l \ l. p r e n t i s su.n c (l i \ 1 . \ \ ' h i t c s r t l c s, ..\ t l r .\ l a t c r . t l . l l 0 ( 1 9 9 6 ) . " . 1 . . . \R . t ' g c r . rR. . . l . . l r r c k r n l tOn . . 1 . . . \ .S c h u e l l c u r .n t l ( i . N , l .\ \ ' h i t c s i c l c s . (l(X)6) O p t 6 6 + l 3 5 . _Appl. D . Q i n . \ ' . X i a . r r n t l( i . \ l \ \ ' h i r c s i d e sA.c l r . \ l l r e r . g . - + 0 j ( l r ) 9 7 r . 'c'. s . I r l l c n h u u s cG r .. . 1 .N 1 .B r u i n . , \ . P u L r l Lar sn.c l \ { . E h r a t .i n p r o e c c r l i n s s o l t h c l l t t l I t t t c r l l t t i o r t uSl _nr r 1 . l t l : i u n o r r N , { i n i a t u r i z cTcol t a l , , \ n u l sr i s S r s t r ' r n s .i - r T . \ S 9 ( r[.J i r s c l .1 9 9 6 .p . l 2 J . " Y . X i a . . l . . l .l \ l c ( ' l c l l a r r cRl . ( i u p r r . D . e i n . X . - N I .Z h a o .L . I _ .S o h n .R . J . ( - e l o t t l . a n r l G . \ 1 . \ \ ' h i t e s i r l c s.., \ c l r .N , l a t c r9. . l 4 i ( 1 9 9 7 ) r"X.-i\{. Z h a o .Y . X i u . . n d G . \ 1 . \ \ ' h i t e s i c l c s .N . l ,. l a r e C r .h e ' r . 7 . 1 0 6 9 I 1 9 9 7) . D . Q i n . Y . X i a . a n r lC i . \ { . \ \ ' h i t c s i c l eA s .c l ' uM . u r e r .t J . 9 l 7 ( l r ) L ) 6 ) . 'rs. M. S z e . . ! c r l i t o t t d t r t ' r oI )rc t ' i t ' L ' .Ps /. r r ' . r r r 'u, r t t lr t , t h t t o l o c r ' ( w i l e v .N e w , Y o r k . 1 9 8 5) . '*T. NIi"r,-rr,r. S . H i l ' a r n i z uT. . F L r j i ia. n d K . N a n b u J. p n .J . A p p l . p h 1 s . .p a r t I 1 9 .L l l - s ( l 9 t t 0 ) .
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