A SCANNING TUNNELING MICROSCOPE FOR SURFACE MODIFICATION

HAL Id: jpa-00225708

https://hal.archives-ouvertes.fr/jpa-00225708

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A SCANNING TUNNELING MICROSCOPE FOR SURFACE MODIFICATION

M. Mccord, R. Pease

To cite this version:

M. Mccord, R. Pease. A SCANNING TUNNELING MICROSCOPE FOR SURFACE MODIFICA- TION. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-485-C2-491. �10.1051/jphyscol:1986274�.

�jpa-00225708�

JOURNAL DE PHYSIQUE

C o l l o q u e C 2 , s u p p l k m e n t au

n03,

Tome 47,

mars

1986 page c2-485

A SCANNING TUNNELING MICROSCOPE FOR SURFACE MODIFICATION

M.A. McCORD and _R.F.W. PEASE

Solid State Electronics Laboratory, Stanford University,

S t a n f o r d

, CA 94305, U.S.A.

ABSTRACT

With t h e r e c e n t s u c c e s s f o r t h e s c a n n i n g t u n n e l i n g microscope (STM) h a s come t h e q u e s t i o n of w h e t h e r t h e i n s t r u m e n t might be u s e d f o r m a t e r i a l m o d i f i c a t i o n s and l i t h o g r a p h y . We embarked on t h e d e s i g n of an STM w i t h t h i s a p p l i c a t i o n i n mind. Its f e a t u r e s i n c l u d e a wide X-y p i e z o e l e c t r i c s c a n r a n g e of 1 0 o r more m i c r o n s , p l u s c o a r s e mechanical motion i n t h e X d i r e c t i o n t o a new r e g i o n on t h e sample w h i l e i n vacuum. Coarse motion i n t h e z d i r e c t i o n i s accomplished w i t h a micrometer d r i v e , w h i l e f i n e movement i s done w i t h a p i e z o e l e c t r i c t r a n s d u c e r . Both t h e sample and t h e t i p can be q u i c k l y changed. F i n a l l y , t h e i n s t r u m e n t i s mounted on t h e s t a g e of a s c a n n i n g e l e c t r o n microscope which a l l o w s r e a l - t i m e o b s e r v a t i o n w h i l e i t i s o p e r a t i n g .

Using t h i s i n s t r u m e n t we were a b l e t o p a t t e r n sub-micron g o l d l i n e s on s i l i c o n s u b s t r a t e s by s p u t t e r e t c h i n g t h r o u g h a c o n t a m i n a t i o n r e s i s t p a t t e r n l a i d down by t h e t u n n e l i n g e l e c t r o n s p o l y m e r i z i n g o r g a n i c m o l e c u l e s p r e s e n t on t h e t a r g e t . We were a l s o a b l e t o e x p o s e l i n e s u s i n g a Langmuir-Blodgett f i l m , which c o n s i s t s of s e v e r a l m o l e c u l a r monolayers of a m a t e r i a l known t o a c t a s a n e l e c t r o n beam r e s i s t d e p o s i t e d i n d i v i d u a l l y on t h e s u r f a c e of t h e s u b s t r a t e .

INTRODUCTION

The s c a n n i n g t u n n e l i n g microscope (STM) h a s shown i t s e l f t o be a remarkable i n s t r u m e n t c a p a b l e t o imaging i n d i v i d u a l atoms on c o n d u c t i n g s u r f a c e s [ l ] . How- e v e r , t h e p o s s i b i l i t i e s of t h e i n s t r u m e n t f o r modifying s u r f a c e s r a t h e r t h a n j u s t l o o k i n g a t them a r e o n l y b e g i n n i n g t o be e x p l o r e d . I n p r e v i o u s work we have shown t h a t when o p e r a t e d i n t h e f i e l d e m i s s i o n mode i t c a n g e n e r a t e a n i n t e n s e submicron beam of low e n e r g y e l e c t r o n s [ZJ. I n the- t h e o r e t i c a l s t u d y we modelled t h e e m i t t e r t i p a s a s p h e r e and s t u d i e d t h e beam d i a m e t e r which r e s u l t e d from d i f f e r e n t g e o m e t r i e s . It t u r n e d o u t t h a t f o r any g i v e n v o l t a g e t h e r e i s an optimum t i p d i a m e t e r which r e s u l t s i n a minimum beam d i a m e t e r ( F i g u r e 1 ) .

0.~1

T i p Radius

R /

^,

0.21--

/

Separation S

Beam 0 . 1

Radtus r

\,

T I P P O T E N T I A L ( V o l t s )

F i g u r e 1. Minimum beam r a d i u s f o r a f i e l d e m i s s i o n t i p i n c l o s e p r o x i m i t y t o a t a r g e t p l a n e

.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986274

J O U R N A L D E PHYSIQUE

Because of absence of space charge and l e n s a b e r r a t i o n s , t h e STM can produce submicron beams of a few hundred v o l t s o r l e s s w i t h c u r r e n t s much h i g h e r t h a n a r e p o s s i b l e u s i n g more conventional t e c h n o l o g i e s . We f e l t t h a t beams of a t l e a s t a few eV would be needed i n o r d e r t o e f f e c t v a r i o u s chemical and p h y s i c a l changes on t h e s u r f a c e of a sample, although r e c e n t work i n d i c a t e s t h a t 0.1 v o l t s is capable of some e f f e c t [31

.

The requirements f o r an STM intended mainly f o r s u r f a c e m o d i f i c a t i o n a r e q u i t e d i f f e r e n t from those r e q u i r e d of an instrument capable of imaging atoms.

Most important is a l a r g e s c a n f i e l d . F i e l d s l e s s than s e v e r a l microns on a s i d e a r e extremely d i f f i c u l t t o l o c a t e w i t h a microscope ( o p t i c a l o r e l e c t r o n ) once t h e sample has been removed from t h e STM. The o t h e r requirement i s t h e a b i l i t y t o vary t h e beam v o l t a g e and c u r r e n t over s e v e r a l o r d e r s of magnitude.

Since t h e instrument i s n ' t n e c e s s a r i l y r e q u i r e d t o r e s o l v e atoms, t h e d e s i g n can be much s i m p l e r i n s e v e r a l ways. Much h i g h e r n o i s e l e v e l s can be T o l e r a t e d , and an u l t r a - h i g h vacuum i s not r e q u i r e d . Since t h i s instrument was intended mainly f o r u s e a t h i g h e r v o l t a g e s and t h u s a l a r g e r gap between t h e t i p and sample, a much h i g h e r v i b r a t i o n l e v e l can be t o l e r a t e d .

SCANNER DESIGN

A p i e z o e l e c t r i c scanner s i m i l a r t o t h o s e i n o t h e r STMs, was chosen a s t h e most p r a c t i c a l way of a c h i e v i n g three-dimensional, e l e c t r o n i c a l l y - c o n t r o l l a b l e motion on a nanometer s c a l e . I n o r d e r t o provide a l a r g e s c a n a r e a , one d e s i g n shown i n F i g . 2a used orthogonal one i n c h PZT tubes d r i v e n with a 900 v o l t scan s i g n a l , s u f f i c i e n t t o provide a 10x10 micron s c a n range.

PIEZOELECTRIC TUBES

PIEZOELECTRIC TUBE

Figure 2 .

( b )

P i e z o e l e c t r i c tube s c a n n e r , b ) p i e z o e l e c t r i c bimorph

Tubes p r o v i d e good mechanical s t i f f n e s s a s w e l l a s l i g h t w e i g h t . An a l t e r n a t e d e s i g n u s i n g p i e z o e l e c t r i c bimorphs i s shown i n F i g . 2b. Bimorphs a r e made from two s t r i p s of o p p o s i t e l y poled p i e z o e l e c t r i c m a t e r i a l joined such t h a t one s i d e c o n t r a c t s w h i l e t h e o t h e r expands when an e l e c t r i c f i e l d i s a p p l i e d , w i t h t h e r e s u l t t h a t t h e s t r i p bends r e l a t i v e l y l a r g e d i s t a n c e s . Using t h i s design a compact scanner could be b u i l t w i t h a range of hundreds of microns. Our d e s i g n used .5 i n c h s t r i p s and gave a 30 micron range f o r a 150 v o l t s c a n s i g n a l . A PZT tube c o n t i n u e s t o provide motion i n t h e z d i r e c t i o n . An e a r l i e r v e r s i o n used a bimoPph i n t h e z a s w e l l a s t h e X and y d i r e c t i o n s . This d e s i g n s u f f e r e d from s e v e r e mechanical resonance a t a frequency of 1.2 k i l o h e r t z

.

The scanner could be made u s a b l e by adding damping c o n s i s t i n g of a t h i c k f i l m of vacuum g r e a s e , but i t s performance was s t i l l i n f e r i o r t o t h e o t h e r d e s i g n s . E m i t t e r t i p s were made by e l e c t r o c h e m i c a l l y e t c h i n g t u n g s t e n w i r e t o a s h a r p p o i n t w i t h a r a d i u s of c u r v a t u r e of s e v e r a l hundred t o s e v e r a l thousand angstroms. T i p s were mounted on t h e s e s c a n n e r s u s i n g a s m a l l metal block w i t h a s e t screw, h o l d i n g them s e c u r e l y w h i l e allowing them t o be q u i c k l y changed, u s e f u l s i n c e t h e t i p s a r e e a s i l y d e s t r o y e d .

SAMPLE MOUNT

The sample mount shown i n F i g . 3 i s a s p r i n g loaded s t e e l s t r i p connected t o a micrometer d r i v e . T h i s provides a c o a r s e motion i n t h e z d i r e c t i o n t h a t a l l o w s t h e sample t o be brought w i t h i n t h e range of t h e z p i e z o e l e c t r i c t r a n s d u c e r . This design i s much s i m p l e r than t h e "louse" o r "walker" used i n o t h e r STMs although i t makes i t d i f f i c u l t t o p r o v i d e v i b r a t i o n i s o l a t i o n i n s i d e t h e vacuum chamber. The sample i s h e l d on t h e sample mount by s p r i n g c l i p s which a l s o provide e l e c t r i c a l c o n t a c t t o t h e f r o n t s u r f a c e of t h e sample. This allows f o r simple and q u i c k sample changes. The sample mount a l s o provides c o a r s e motion i n t h e X d i r e c t i o n by means of a l i n e connected between a second micrometer d r i v e and t h e end of t h e s t e e l s t r i p , which p i v o t s a t t h e o t h e r e n d . This allows s e v e r a l d i f f e r e n t exposures t o be made on a sample w i t h o u t having t o b r i n g t h e system up t o atmospheric p r e s s u r e between each exposure.

COARSE D R M

COARSE X DRNE

F i g u r e 3 . STM c o n f i g u r a t i o n

.

SYSTEM DESIGN

'IC

Since n o i s e and s u r f a c e c l e a n l i n e s s were not s e v e r e c o n s t r a i n t s , an u l t r a - high vacuum was not r e q u i r e d and i t appeared t h a t a vacuum of a few times 10E-5 t o r r should be s u f f i c i e n t [ 4 ] . Thus t h e system was designed t o be placed on t h e s t a g e of a scanning e l e c t r o n microscope. This not only s a t i s f i e d t h e vacuum requirements, but a l s o allowed i n s i t u examination of t h e t i p and t h e sample, and

JOURNAL

DE

PHYSIQUE

enabled u s t o watch a s t h e sample was brought up t o t h e t i p u s i n g t h e c o a r s e z motion. The SEM a l s o provides a s i n g l e s t a g e of v i b r a t i o n i s o l a t i o n which proved t o be adequate f o r v o l t a g e s down t o 1 v o l t and l e s s . Figure 4 shows t h e c o n t r o l c i r c u i t r y . The beam v o l t a g e , VB, i s s e t w i t h a r e g u l a t e d v a r i a b l e DC s u p p l y . The c u r r e n t i s sensed a c r o s s a r e s i s t o r i n s e r i e s with t h e t i p and compared with a r e f e r e n c e s i g n a l t o produce an e r r o r s i g n a l . The e r r o r s i g n a l i s f i l t e r e d t o remove f r e q u e n c i e s around t h e resonance of t h e s c a n n e r . The e r r o r s i g n a l and i t s i n t e g r a l a r e t h e n f e d through v a r i a b l e g a i n a m p l i f i e r s t o t h e z p i e z o e l e c t r i c t r a n s d u c e r , forming a P I c o n t r o l l e r t h a t keeps t h e beam c u r r e n t c o n s t a n t and t b u s s e t s t h e t i p a c o n s t a n t h e i g h t above t h e sample.

PIEZOELECTRIC TRANSDUCER

E M I T T E R T I P

l

^{L y jTARGET}

I

Figure 4 . STM c o n t r o l c i r c u i t r y .

PRELIMINARY RESULTS

Beam v o l t a g e s from 1 t o 200 v o l t s a t c u r r e n t s from 1 t o s e v e r a l thousand nA have been produced. C a r e f u l s e t t i n g of t h e g a i n parameters k l and k2 i s i m p o r t a n t , s i n c e t o o l i t t l e g a i n l i m i t s t h e response time and t h u s t h e maximum scan speed and a l s o i n c r e a s e s n o i s e , while t o o much g a i n causes o s c i l l a t i o n s . A t beam v o l t a g e s above 20 v o l t s t h e c u r r e n t was noisy and u n s t a b l e , p o s s i b l y due t o s p u t t e r e t c h i n g of t h e t i p . A s t h e v o l t a g e was reduced below 20 v o l t s t h e c u r r e n t became much l e s s n o i s y , presumably due not only t o a r e d u c t i o n i n t h e energy of s p u t t e r i n g i o n s but a l s o t o a t r a n s i t i o n from t h e f i e l d emission regime t o t h e MVM t u n n e l i n g regime.

The f i r s t a t t e m p t a t l i t h o g r a p h y used contamination r e s i s t where hydrocarbons p r e s e n t i n t h e vacuum a r e polymerized on t h e s u r f a c e of t h e sample by t h e beam [ 5 ] . Figure 5a shows a micrograph of l i n e s of contamination on a 100 nm gold

111

f i l m on s i l i c o n . The exposure was done with a 10 v o l t , 60 nA beam and a scan speed of .25 microns/second. The sample was then s p u t t e r etched t o remove t h e gold r e v e a l i n g l i n e s of gold shown i n F i g . 5b. Lines were a l s o made by exposing a Langmuir-Blodgett f i l m of docosenoic a c i d [ 6 ] . This i s a f i l m s e n s i t i v e t o an e l e c t r o n beam d e p o s i t e d a s i n g l e monolayer a t a time. The f i l m was 10 nm t h i c k on t o p of 100 nm of aluminum on s i l i c o n . I t was exposed with a 25 v o l t 15 nA beam a t a scan speed of .5 microns/second, developed i n a l c o h o l and t h e aluminum wet e t c h e d away. The r e s u l t was c o a r s e aluminum l i n e s shown i n F i g . 6 .

[ l ] These r e s u l t s were presented a t t h e 29th I n t e r n a t i o n a l Conference on E l e c t r o n , Ion, and Photon Beams

.

Figure 5 . a ) Lines of contamjnation on g o l d . Dark squares a r e a d d i t i o n a l con- t a m i n a t i o n caused by t h e SEM t a k i n g t h e micrograph, b ) a f t e r s p u t t e r e t c h i n g t h e g o l d .

JOURNAL

DE

PHYSIQUE

F i g u r e 6 . Lines of aluminum on s i l i c o n formed by exposure of a Langmuir- B l o d g e t t f i l m .

DISCUSSION

We have demonstrated tenth-micron l i t h o g r a p h y u s i n g t h e STM d e s c r i b e d above.

Other p o s s i b l e u s e s f o r t h e i n s t r u m e n t i n c l u d e l o c a l i z e d h e a t i n g o r m e l t i n g f o r m i c r o f a b r i c a t i o n o r e x t r e m e l y dense mass s t o r a g e . Another i n t e r e s t i n g p o s s i b i l i t y would be t o r e v e r s e t h e a p p l i e d v o l t a g e and o p e r a t e a s a l i q u i d o r gaseous i o n s o u r c e . One s e r i o u s problem t h a t an i o n s o u r c e would need t o overcome would be f i e l d e m i s s i o n of e l e c t r o n s from t h e sample back t o t h e s o u r c e .

F u t u r e p l a n s i n c l u d e moving t h e STM t o a n u l t r a - h i g h vacuum system t o lower n o i s e , reduce u n d e s i r e d c o n t a m i n a t i o n , and t o a l l o w s t a b l e o p e r a t i o n a t h i g h e r v o l t a g e s . Experiments w i l l i n c l u d e a t t e m p t s t o expose n o v e l r e s i s t systems i n c l u d i n g m e t a l h a l i d e s a s w e l l a s more o r d i n a r y polymeric r e s i s t s .

Before t h i s could e v e r become a commercial system, a g r e a t d e a l of engi- n e e r i n g i s needed t o a l l o w r e a s o n a b l e w r i t i n g s p e e d s . Both t h e bandwidth of t h e feedback l o o p and s u r f a c e f l a t n e s s must be g r e a t l y improved i n o r d e r f o r t h e t i p t o f o l l o w s u r f a c e topography a t h i g h e r s p e e d s . However, modern d i s c mass s t o r a g e technology employing head f l y h e i g h t s of a few thousand angstroms o r l e s s o f f e r s hope t h a t t h i s might n o t be a n u n r e a l i s t i c g o a l .

ACKNOWLEDGMENTS

T h i s work was s u p p o r t e d j o i n t l y by t h e N a t i o n a l Science Foundation through Grant ECS 82-03296 and by DARPA C o n t r a c t N00014-84-K-0624. M . A. McCord i s t h e h o l d e r of a N a t i o n a l Science Foundation F e l l o w h i p . The a u t h o r s a r e g r a t e f u l t o P . L. Maccagno f o r p r o v i d i n g t h e Langmuir-Blodgett f i l m sample and p r o c e s s , t o T.

H . Newman f o r p r o v i d i n g t h e s c a n n i n g e l e c t r o n microscope, and t o A . Bryant, D . P Smith, and C. F. Q u a t e f o r p r o v i d i n g a d v i c e and components f o r t h e s c a n n i n g t u n n e l i n g microscope.

1. G . B i n n i g and H . R o h r e r , H e l v . P h y s . A c t a .

2,

726 ( 1 9 8 2 ) .

2 . M . A. McCord and R . F . W . P e a s e , J . Vac. S c i . T e c h n o l .

E,

1 9 8 (1985).

3 . M . R i n g g e r , H . R . H i d b e r , R. ~ c h l i j g l , P . O e l h a f e r , and A. J. GGntherodt, Appl. Phys

.

^{L e t t}

. ^%,

832 ( 1 9 8 5 ) .

4 . C . N . Coenraads, 2 4 t h Annual Conference on P h y s i c a l E l e c t r o n i c s , MIT 107 ( 1 9 6 4 ) .

5 . A. N . B r o e r s , W . W . Molzen, J . J . Cuomo, and N . D . W i t t l e s , Appl. Phys.

L e t t

. ^9,

596 ( 1 9 7 6 ) .

6 . M . Pomerantz i n J

.

^{G .} Dash and J

.

Ruvalds ( e d s ) Phase T r a n s i t i o n s i n S u r f a c e F i l m s , Plenum, NY 1980, p . 3 1 7 .