Microelectronics Processing - American Chemical Society

3 Dopant Profiles by the Spreading Resistance Technique Robert G. Mazur Downloaded by CORNELL UNIV on June 7, 2017 | http://pubs.acs.org Publication Date: January 28, 1986 | doi: 10.1021/bk-1986-0295.ch003

Solid State Measurements, Inc., Monroeville, PA 15146

Spreading resistance profiles are made by stepping a pair of specially-conditioned "point"-contact probes across the bevelled surface of a sample. The nearzero-bias resistance between the probes is measured at each point. Measurement accuracy depends very strongly on properly calibrating the probes with knownresistivity samples. In addition, a theoreticallyderived correction factor must be calculated and applied to each raw data point to account for the effects of PN junctions and other boundaries in the sample. At its current level of development, the spreading resistance technique can provide detailed dopant density profiles on essentially all silicon structures of interest, including as-grown crystals and diffused, ion-implanted, and epitaxial wafers. For silicon, there are no limits on dopant density and essentially no limits on the depth resolution. A spatial resolution as low as 1 nanometer per point has been reported and layers with a thickness as little as 20 nanometers have been profiled. This paper details the current state-of-the-art of the spreading resistance technique and presents a number of typical examples. D i e t e r Schroder has j u s t p r e s e n t e d a comprehensive r e v i e w of t h e e l e c t r i c a l t e c h n i q u e s used i n c h a r a c t e r i z i n g semiconductor m a t e r i a l s ( 1 ) . One p o i n t t h a t I ' d l i k e t o develop f u r t h e r i s t h e need f o r h i g h s p a t i a l r e s o l u t i o n dopant p r o f i l i n g . As you know, i n t h e w o r l d of m i c r o e l e c t r o n i c s , the emphasis i s always on t h e " m i c r o " ; e v e r y t h i n g i s f a n t a s t i c a l l y s m a l l . T h i s i s because both the o p e r a t i n g speed and t h e m a n u f a c t u r i n g y i e l d o f i n t e g r a t e d c i r c u i t s improve as i n d i v i d u a l d e v i c e elements a r e made s m a l l e r and more c h i p s a r e put on a w a f e r . T h i s m i n i a t u r i z a t i o n p r o c e s s generates a need f o r h i g h s p a t i a l r e s o l u t i o n dopant p r o f i l i n g , because making d e v i c e s s m a l l e r means making t h e doped l a y e r s o f which t h e y ' r e composed t h i n n e r . A t the p r e s e n t t i m e , many d e v i c e s u s e 0097-6156/86/0295-0034$06.00/0 © 1986 American Chemical Society Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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Dopant Profiles by the Spreading Resistance Technique

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Downloaded by CORNELL UNIV on June 7, 2017 | http://pubs.acs.org Publication Date: January 28, 1986 | doi: 10.1021/bk-1986-0295.ch003

i o n - i m p l a n t e d , d i f f u s e d , o r e p i t a x i a l l a y e r s o n l y a few tens o r hundreds o f nanometers t h i c k . To get t h e dopant c o n c e n t r a t i o n p r o f i l e s needed i n p r o c e s s development and c o n t r o l o f these t h i n l a y e r s , we must s e c t i o n and e v a l u a t e them w i t h a s p a t i a l r e s o l u t i o n on t h e o r d e r o f one nanometer. One method t h a t has t h e r e q u i r e d s p a t i a l r e s o l u t i o n i s t h e s p r e a d i n g r e s i s t a n c e t e c h n i q u e ( 2 , 3 ) • T h i s t e c h n i q u e i s based on measuring t h e c o n t a c t r e s i s t a n c e o f s p e c i a l l y - p r e p a r e d p o i n t - c o n t a c t diodes. I n t h i s paper, I w i l l f i r s t d e s c r i b e t h e s p r e a d i n g r e s i s t a n c e t e c h n i q u e as i t e x i s t s today and p r e s e n t some t y p i c a l examples. Then I ' l l t r y t o g i v e you a p e r s p e c t i v e on s p r e a d i n g r e s i s t a n c e by d i s c u s s i n g i t i n r e l a t i o n t o SIMS. I ' l l f i n i s h by i n d i c a t i n g t h o s e s i t u a t i o n s i n which t h e s p r e a d i n g r e s i s t a n c e t e c h n i q u e i s most e f f e c t i v e l y used f o r c h a r a c t e r i z i n g semiconductor materials. The

Spreading Resistance

Technique

F i g u r e 1 i l l u s t r a t e s t h e e x p e r i m e n t a l procedure used i n making s p r e a d i n g r e s i s t a n c e measurements. Two probes a r e c a r e f u l l y a l i g n e d and then stepped a c r o s s t h e b e v e l l e d s u r f a c e of a semiconductor sample; a t each p o i n t , t h e probes a r e lowered onto t h e sample s u r f a c e and t h e r e s i s t a n c e between t h e two probes i s measured and p l o t t e d . The t e c h n i q u e i s r e f e r r e d t o as t h e s p r e a d i n g r e s i s t a n c e t e c h n i q u e because t h e dominant r e s i s t a n c e o f a p o i n t c o n t a c t d i o d e o c c u r s i n a v e r y s m a l l volume beneath t h e probe, where t h e c u r r e n t r a p i d l y spreads out i n t o t h e sample. S p r e a d i n g r e s i s t a n c e p r o f i l e s are u s u a l l y computer-processed t o y i e l d r e s i s t i v i t y o r dopant concentration p r o f i l e s . F i g u r e 2 shows a t y p i c a l a u t o m a t i c s p r e a d i n g r e s i s t a n c e system. I t c o n s i s t s o f a m e c h a n i c a l a p p a r a t u s t o o p e r a t e t h e probes and s t e p them a c r o s s a b e v e l l e d t e s t c h i p and an e l e c t r o n i c s sub-system t o a c q u i r e , p r o c e s s and p l o t t h e d a t a . The s p r e a d i n g r e s i s t a n c e t e c h n i q u e i s c h a r a c t e r i z e d by f o u r major f e a t u r e s : 1) s p e c i a l probes and m e c h a n i c a l apparatus t o make t h e contacts; 2) t h e use of a v e r y low a p p l i e d v o l t a g e d u r i n g t h e r e s i s t a n c e measurements; 3) a c a l i b r a t i o n procedure u s i n g samples of known r e s i s t i v i t y ; 4) a m u l t i - l a y e r c o r r e c t i o n procedure t o c o r r e c t f o r boundary effects. P r o b e s . The most i m p o r t a n t p a r t s o f a s p r e a d i n g r e s i s t a n c e system a r e t h e probes and t h e m e c h a n i c a l a p p a r a t u s t h a t o p e r a t e s them. F i g u r e 3 i s a c l o s e - u p v i e w of a p a i r o f s p r e a d i n g r e s i s t a n c e probes mounted on g r a v i t y - l o a d e d probe arm a s s e m b l i e s . Note t h a t t h e probe arms a r e m e c h a n i c a l l y m a s s i v e , thus p r o v i d i n g a s t a b l e and r i g i d mounting f o r t h e probe p i n s . Each probe arm i s s u p p o r t e d by a k i n e m a t i c - d e s i g n b e a r i n g system w i t h a t o t a l o f f i v e p o i n t c o n t a c t s . T h i s c o n s t r u c t i o n g i v e s t h e probe arm o n l y one d e g r e e - o f - f r e e d o m , a r o t a t i o n around a h o r i z o n t a l a x i s . When t h e probe arm i s p i v o t e d t o

Casper; Microelectronics Processing: Inorganic Materials Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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MICROELECTRONICS PROCESSING: INORGANIC MATERIALS CHARACTERIZATION

1. R

= T— ( s i n g l e probe)

2. R

= f ( p , radius c o n d u c t i v i t y type, o r i e n t a t i o n , surface f i n i s h , e t c . )

F i g u r e 1. A s c h e m a t i c i l l u s t r a t i o n of dopant p r o f i l i n g w i t h t h e spreading r e s i s t a n c e technique. Reproduced w i t h p e r m i s s i o n from R e f . J3. C o p y r i g h t 1984 American S o c i e t y f o r T e s t i n g and Materials.

F i g u r e 2.

An ASR-100C/2 A u t o m a t i c S p r e a d i n g

Resistance

Probe.



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b r i n g the probe i n t o c o n t a c t w i t h the t e s t sample, the probe p i n i t s e l f makes the s i x t h p o i n t c o n t a c t between the probe arm assembly and the r e s t o f the a p p a r a t u s , e l i m i n a t i n g a l l s i x degrees-of-freedom. T h i s i s i m p o r t a n t , because i t means t h a t t h e probe doesn't move l a t e r a l l y ( i . e . , " s c r u b " ) when i t makes c o n t a c t w i t h the sample. Because the probe doesn't " s c r u b , " probe t i p wear i s e s s e n t i a l l y e l i m i n a t e d and damage t o the semiconductor s u r f a c e i s minimized. F i g u r e 4 i s a s c a l e drawing o f a s p r e a d i n g r e s i s t a n c e probe t i p and the nominal c o n t a c t s i z e f o r a 10 gram probe l o a d . The probe t i p s are a h a r d tungsten-osmium a l l o y ; they have a p r e c i s e l y - c o n t r o l l e d shape and are massive i n r e l a t i o n t o the c o n t a c t spot d i a m e t e r . Because o f t h e i r r e l a t i v e s i z e and h a r d n e s s , the probes do not undergo g r o s s d e f o r m a t i o n d u r i n g measurements. When s e t down, they deform o n l y e l a s t i c a l l y ; thus they make v e r y r e p r o d u c i b l e mechanical c o n t a c t s . E l e c t r i c a l l y , we have t o " c o n d i t i o n " the probes i n a s p e c i a l way t o get a good c o n t a c t . T h i s i s n e c e s s a r y because a l l s i l i c o n samples are covered w i t h a tough n a t i v e o x i d e some 2 t o 3 nanometers t h i c k . S i n c e the probes don't " s c r u b " on c o n t a c t , they cannot break t h r o u g h the o x i d e l a y e r the way t h a t o t h e r probes w o u l d . I n s t e a d , we a c h i e v e an e l e c t r i c a l c o n t a c t w i t h the r i g h t c h a r a c t e r i s t i c s by c o n t r o l l i n g the micro-topography o f the probe t i p s such t h a t t h e c o n t a c t a r e a c o n s i s t s o f a l a r g e number o f m i c r o s c o p i c p r o t r u s i o n s , o r " a s p e r i t i e s . " These a s p e r i t i e s are s u f f i c i e n t l y s m a l l t h a t they f r a c t u r e the o x i d e by p r e s s u r e a l o n e , g e n e r a t i n g a m i c r o - c o n t a c t under each a s p e r i t y . Because o f the c o n t r o l l e d shape o f the probe t i p , the r e s u l t a n t c l u s t e r o f m i c r o - c o n t a c t s i s c l o s e l y enough grouped t h a t i t a c t s e l e c t r i c a l l y as a s i n g l e c o n t a c t . I n r e c e n t y e a r s we've l e a r n e d t o c o n d i t i o n probes so t h a t we get a l a r g e number o f m i c r o - c o n t a c t s , thus r e d u c i n g the p r e s s u r e a t each o f them. T h i s m i n i m i z e s probe p e n e t r a t i o n and produces good p r o f i l e s , even on the e x t r e m e l y t h i n l a y e r s now common i n s i l i c o n t e c h n o l o g y . As an example o f c u r r e n t c a p a b i l i t y , one s p r e a d i n g r e s i s t a n c e probe u s e r r e c e n t l y r e p o r t e d p r o f i l i n g an i o n - i m p l a n t e d l a y e r j u s t 20 nanometers t h i c k . The improvement i n s p r e a d i n g r e s i s t a n c e p r o f i l e s o b t a i n e d t h r o u g h c o n t r o l l i n g probe p e n e t r a t i o n i s i l l u s t r a t e d i n F i g u r e 5. The s t r u c t u r e i s an NPNN" t r a n s i s t o r . The l o w e s t p l o t was done s e v e r a l y e a r s ago w i t h 20 gram p e n e t r a t i n g probes; the m i d d l e p l o t shows the improvement o b t a i n e d a t t h a t time by u s i n g a G o r e y - S c h n e i d e r probe g r i n d e r t o c o n t r o l probe p e n e t r a t i o n ( 4 ) . The uppermost p r o f i l e was measured r e c e n t l y , u s i n g c o n t r o l l e d l o w p e n e t r a t i o n 10 gram probes, a l s o c o n d i t i o n e d w i t h a G o r e y - S c h n e i d e r probe g r i n d e r , but done a c c o r d i n g t o our c u r r e n t p r o c e d u r e s and c r i t e r i a (5). A f u r t h e r example o f the depth r e s o l u t i o n p o s s i b l e w i t h p r o p e r l y c o n d i t i o n e d s p r e a d i n g r e s i s t a n c e probes i s shown i n F i g u r e 6. T h i s i s a narrow-base NPN t r a n s i s t o r , p r o f i l e d w i t h probes l o a d e d t o about 5 grams and w i t h a v e r y s h a l l o w b e v e l a n g l e t o o b t a i n a depth increment o f j u s t 2.6 nanometers per p o i n t ( F i g u r e 6 a ) . Note t h a t i t i s p o s s i b l e t o a c h i e v e even f i n e r r e s o l u t i o n by s i m p l y g o i n g t o a s m a l l e r h o r i z o n t a l s t e p on the b e v e l s u r f a c e , 1-



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F i g u r e 3. A p a i r of s t a n d a r d s p r e a d i n g r e s i s t a n c e probe arm a s s e m b l i e s w i t h probe p i n s a t t a c h e d , shown i n measurement p o s i t i o n on a b e v e l l e d t e s t sample.

F i g u r e 4. S c a l e drawing of a s p r e a d i n g r e s i s t a n c e probe t i p and t y p i c a l m i c r o c o n t a c t c l u s t e r s produced a t v a r i o u s degrees of probe c o n d i t i o n i n g . Reproduced w i t h p e r m i s s i o n from Ref • 5. Copyright 1984 A m e r i c a n S o c i e t y f o r T e s t i n g and M a t e r i a l s .



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e o

39

1983 Low p e n e t r a t i o n probes (10 g)

00

c cd u

cu

CO

/1981 C o n t r o l l e d low penetration f probes (20 g)"\

1981 P e n e t r a t i n g probes (20 g)

Depth (vim). 1

F i g u r e 5. S p r e a d i n g r e s i s t a n c e p r o f i l e s o f an NPNN"" t r a n s i s t o r s t r u c t u r e , as measured w i t h p e n e t r a t i n g probes and w i t h c o n t r o l l e d low p e n e t r a t i o n probes. Reproduced w i t h p e r m i s s i o n from R e f . _5. C o p y r i g h t 1984 American S o c i e t y f o r T e s t i n g and M a t e r i a l s .



10

6

-1550

A N 435A

10


3

§

5

10*

10

3

Sf Probe load - 5g Bevel angle: 3.5 mil Tangent of angle: 0.00102 Step increment: X = 2.5 Z

26A

10 DEPTH -

-439A P

-I

BLOW-UP OF NARROW P-BASE Probe load: - 5 g Bevel angle: 3.5 mil Step increment: X = 1 /um Z = 10^A

DEPTH-

F i g u r e 6. S p r e a d i n g r e s i s t a n c e p r o f i l e s of a narrow base NPN transistor.


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y i e l d i n g a p o i n t - t o - p o i n t depth increment o f j u s t 1.02 nanometers (Figure 6b). T h i s d i s c u s s i o n of the m e c h a n i c a l and e l e c t r i c a l p r o p e r t i e s o f s p r e a d i n g r e s i s t a n c e c o n t a c t s makes i t c l e a r t h a t the probes used d i f f e r from any o t h e r s p r e v i o u s l y used i n e i t h e r p o i n t - c o n t a c t d i o d e work o r i n semiconductor e l e c t r i c a l measurements. System E l e c t r o n i c s . I n a d d i t i o n t o u s i n g s p e c i a l i z e d c o n t a c t s and mechanical apparatus, spreading r e s i s t a n c e p r o f i l i n g a l s o r e q u i r e s s p e c i a l i z e d e l e c t r o n i c s . I n p a r t i c u l a r , we use an ohmmeter w i t h a v e r y low a p p l i e d v o l t a g e o f 5 mV (2.5 mV a c r o s s each p r o b e ) . This low v o l t a g e m i n i m i z e s s e v e r a l e f f e c t s n o r m a l l y a s s o c i a t e d w i t h p o i n t c o n t a c t d i o d e s ; e.g., excess c a r r i e r i n j e c t i o n . The ohmmeter's s i n g l e l o g a r i t h m i c range spans r e s i s t a n c e s from 1 ohm t o 1 0 ohms, a l l o w i n g measurements on m a t e r i a l w i t h a r e s i s t i v i t y o f from l e s s t h a n 1 0 " ohm-cm on up t o the i n t r i n s i c l e v e l . y


6

C a l i b r a t i o n . The t h i r d key f e a t u r e o f the s p r e a d i n g r e s i s t a n c e t e c h n i q u e i s i t s use of c a l i b r a t i o n c u r v e s . Because measurements are made on r e a l s i l i c o n s u r f a c e s , the r e l a t i o n s h i p e x p e c t e d on the b a s i s o f s i m p l e t h e o r y doesn't h o l d ( F i g u r e 1, e q u a t i o n 1 ) . I n s t e a d , the measured r e s i s t a n c e , R , depends not o n l y on the sample r e s i s t i v i t y p and the c o n t a c t r a d i u s a, but a l s o on the sample's c o n d u c t i v i t y t y p e , c r y s t a l l o g r a p h i c o r i e n t a t i o n , and s u r f a c e f i n i s h (see F i g u r e 1, e q u a t i o n 2 ) . T h e r e f o r e , we make r e s i s t i v i t y measurements by f i r s t g e n e r a t i n g c a l i b r a t i o n c u r v e s on k n o w n - r e s i s t i v i t y samples o f the same t y p e , o r i e n t a t i o n , and s u r f a c e f i n i s h as the t e s t specimens t o be profiled. C a l i b r a t i o n c u r v e s are g e n e r a t e d f o r a p a r t i c u l a r p a i r o f probes a t a p a r t i c u l a r t i m e , u s i n g k n o w n - r e s i s t i v i t y samples of t h e h i g h e s t q u a l i t y a v a i l a b l e . This c a l i b r a t i o n procedure i s a p a r t i c u l a r l y noteworthy c h a r a c t e r i s t i c o f the s p r e a d i n g r e s i s t a n c e technique. I t means t h a t s p r e a d i n g r e s i s t a n c e i s a comparison method, and t h a t i t s u l t i m a t e a c c u r a c y i s t h e r e f o r e l i m i t e d o n l y by the c a l i b r a t i o n m a t e r i a l a v a i l a b l e . F o r t u n a t e l y , i t ' s now p o s s i b l e t o o b t a i n complete s e t s o f c a l i b r a t i o n samples from the N a t i o n a l Bureau o f S t a n d a r d s . m

Multilayer Corrections. F i n a l l y , the s p r e a d i n g r e s i s t a n c e t e c h n i q u e , as we use i t today, i s a l s o c h a r a c t e r i z e d by the use o f a m u l t i l a y e r c o r r e c t i o n p r o c e d u r e . C o r r e c t i o n s t o the raw d a t a are n e c e s s a r y because of the e f f e c t s o f b o u n d a r i e s , such as PN o r l o w - h i g h j u n c t i o n s , i n the v i c i n i t y o f the p r o b e s . These c o r r e c t i o n s are made u s i n g a method based on a p o i n t - b y - p o i n t s o l u t i o n o f the L a p l a c e e q u a t i o n , t r e a t i n g each p o i n t on the s p r e a d i n g r e s i s t a n c e p r o f i l e as a s e p a r a t e s u b - l a y e r i n the s t r u c t u r e ; hence the term " m u l t i l a y e r . " A c o n s i d e r a b l e amount o f work has been done on t h e s e c o r r e c t i o n s over the l a s t f i f t e e n y e a r s , b e g i n n i n g w i t h the o r i g i n a l m u l t i l a y e r c a l c u l a t i o n s o f Schumann and Gardner a t IBM ( 6 ) , and c o n t i n u e d by D'Avanzo, Rung, and D u t t o n a t S t a n f o r d U n i v e r s i t y (7) and by S. C. Choo and h i s co-workers a t t h e




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U n i v e r s i t y of Singapore (8) • As a r e s u l t of t h i s work, we now have programs t h a t enable us t o do a r a t h e r e f f e c t i v e j o b of making spreading r e s i s t a n c e c o r r e c t i o n s . More r e c e n t l y , a d d i t i o n a l work t o improve the a c c u r a c y of c a l c u l a t e d c o r r e c t i o n f a c t o r s has been done by B e r k o w i t z and Lux a t F o r t Monmouth ( 9 ) , by P i e s s e n s , V a n d e r v o r s t , and Maes at the U n i v e r s i t y of Leuven i n B e l g i u m ( 1 0 ) , and, a g a i n , by Choo and h i s group i n Singapore ( 1 1 ) • These newer procedures and programs are now b e i n g implemented; they o f f e r the promise of g i v i n g us even b e t t e r c o r r e c t i o n s i n the f u t u r e . F i g u r e 7 i s an example of a s p r e a d i n g r e s i s t a n c e p r o f i l e c o r r e c t e d w i t h a m u l t i l a y e r procedure. I t shows a s h a l l o w boron i o n i m p l a n t i n t o a P-type s u b s t r a t e , a l o n g w i t h the c a r r i e r c o n c e n t r a t i o n p r o f i l e d e r i v e d from the s p r e a d i n g r e s i s t a n c e d a t a , and a t h e o r e t i c a l curve ( s o l i d l i n e ) , c a l c u l a t e d w i t h the program SUPREM I I . N o t w i t h s t a n d i n g the f a c t t h a t we don't know whether the e x p e r i m e n t a l p r o f i l e or the t h e o r e t i c a l c a l c u l a t i o n i s the more c o r r e c t , the r e l a t i v e l y good agreement between them g i v e s us some i d e a of the a c c u r a c y of the s p r e a d i n g r e s i s t a n c e - d e r i v e d concentration p r o f i l e . F i g u r e 8 i s a more r e c e n t s p r e a d i n g r e s i s t a n c e and c a r r i e r c o n c e n t r a t i o n p r o f i l e of a low dose boron i o n - i m p l a n t i n t o an N-type s u b s t r a t e . The peak i n the s p r e a d i n g r e s i s t a n c e p r o f i l e a t about 9 x 1 0 ohms i n d i c a t e s the p o s i t i o n of the PN j u n c t i o n . The c a l c u l a t e d c a r r i e r c o n c e n t r a t i o n r i s e s from a v a l u e of 1 0 cm~^ a t the s u r f a c e t o a maximum of about 2 x 1 0 cm"^ b e f o r e f a l l i n g t o a low v a l u e a t the PN j u n c t i o n a t a depth of 1.2 urn. T h i s p r o f i l e was done w i t h 10 gram, c o n t r o l l e d low p e n e t r a t i o n probes, u s i n g a nominal 17 b e v e l a n g l e and a s t e p increment a l o n g the b e v e l s u r f a c e of 5 urn t o g i v e a p o i n t - t o - p o i n t depth increment of 210A or 21 nanometers• b

1 7

1 7

1

Spreading Resistance P r o f i l e

Accuracy

An obvious q u e s t i o n a t t h i s p o i n t i s "how a c c u r a t e are the c o n c e n t r a t i o n p r o f i l e s d e r i v e d from s p r e a d i n g r e s i s t a n c e measurements ?" I n our day-to-day p r o f i l i n g a t SSM, we depend p r i m a r i l y on comparisons w i t h t h e o r y (as i n F i g u r e 7 ) , or on comparisons between t h e known i o n i m p l a n t dosage and a dosage c a l c u l a t e d by i n t e g r a t i n g the dopant p r o f i l e . F o r example, i n F i g u r e 8, the known i m p l a n t f l u e n c e was 2 x 1 0 cm~2 w h i l e the dosage c a l c u l a t e d from the c a r r i e r c o n c e n t r a t i o n p r o f i l e was 1.5 x 1 0 cm"^ • We a l s o f r e q u e n t l y compare the measured sheet r e s i s t a n c e of a l a y e r w i t h a P v a l u e c a l c u l a t e d from the measured r e s i s t i v i t y p r o f i l e . We expect t o get a b e t t e r i d e a of s p r e a d i n g r e s i s t a n c e p r o f i l e a c c u r a c y from a "round r o b i n " t e s t now i n p r o g r e s s i n the ASTM F - l committee. T h i s t e s t i s b e i n g done i n v a r i o u s l a b o r a t o r i e s throughout the w o r l d and w i l l r e s u l t i n an e x p e r i m e n t a l d e t e r m i n a t i o n of the p r e c i s i o n of the s p r e a d i n g r e s i s t a n c e t e c h n i q u e , i n support of a p u b l i s h e d ASTM s t a n d a r d on s p r e a d i n g resistance p r o f i l i n g (12). 1 6

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F i g u r e 7. S p r e a d i n g r e s i s t a n c e , c a r r i e r c o n c e n t r a t i o n and SUPREM I I p r o f i l e s o f a boron i o n - i m p l a n t . Reproduced w i t h p e r m i s s i o n from Ref. 4. C o p y r i g h t 1981 S o l i d S t a t e Technology.


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F i g u r e 8. S p r e a d i n g r e s i s t a n c e and c a r r i e r c o n c e n t r a t i o n p r o f i l e s of a boron i o n - i m p l a n t i n t o an N-type s u b s t r a t e as measured w i t h c o n t r o l l e d low p e n e t r a t i o n , 10 gram probes.


3.

MAZUR


45

I n a d d i t i o n , a number of a u t h o r s have r e c e n t l y p u b l i s h e d comparisons o f s p r e a d i n g r e s i s t a n c e and SIMS p r o f i l e s . F o r example, Marek P a w l i k o f the GEC R e s e a r c h L a b o r a t o r i e s i n England and J i m E h r s t e i n o f the N a t i o n a l Bureau o f Standards both gave papers comparing s p r e a d i n g r e s i s t a n c e and SIMS a t the F e b r u a r y , 1984 Symposium on Semiconductor P r o c e s s i n g i n San J o s e ( 1 3 , 1 4 ) . J i m E h r s t e i n ' s paper was a c t u a l l y a three-way comparison o f boron i m p l a n t s u s i n g s p r e a d i n g r e s i s t a n c e , SIMS, and a N e u t r o n Depth P r o f i l i n g (NDP) method w h i c h i s now under i n v e s t i g a t i o n a t the NBS and which w i l l be d e s c r i b e d i n a paper t o be g i v e n l a t e r a t t h i s meeting by R. G. Downing o f the N a t i o n a l Bureau o f S t a n d a r d s . Comparison o f S p r e a d i n g R e s i s t a n c e and SIMS. F i g u r e 9 i s t a k e n from J i m E h r s t e i n ' s paper a t the San Jose Symposium. I t shows the dopant c o n c e n t r a t i o n p r o f i l e s o f t h r e e i o n i m p l a n t s o f B i n t o an N-type s u b s t r a t e , as measured a f t e r a n n e a l i n g by SIMS, NDP, and s p r e a d i n g r e s i s t a n c e . The i m p l a n t s were done a t 70 keV, w i t h f l u e n c e s of 1 x 1 0 , 4 x 1 0 , and 1 x 1 0 c n T . These p r o f i l e s show t h r e e major d i f f e r e n c e s between the two p h y s i c a l p r o f i l i n g methods and the s p r e a d i n g r e s i s t a n c e t e c h n i q u e . F i r s t , SIMS and NDP both show an i n c r e a s e i n boron c o n c e n t r a t i o n a t the s u r f a c e , due t o a r e d i s t r i b u t i o n o f boron i n t o a t h i n o x i d e grown d u r i n g the a n n e a l i n g p r o c e s s . T h i s boron i s n o t e l e c t r i c a l l y a c t i v e and i s t h e r e f o r e not seen by the s p r e a d i n g r e s i s t a n c e probe. Second, t h e r e i s a hump i n the boron c o n c e n t r a t i o n a t about 0.2 um i n both the SIMS and NDP p r o f i l e s w h i c h i s not seen i n the s p r e a d i n g r e s i s t a n c e p r o f i l e because i t i s due t o u n d i s s o l v e d boron w h i c h i s a l s o not e l e c t r i c a l l y a c t i v e . F i n a l l y , t h e r e i s a s i g n i f i c a n t d i f f e r e n c e i n the c o n c e n t r a t i o n of boron measured by s p r e a d i n g r e s i s t a n c e and by SIMS a t boron c o n c e n t r a t i o n s below about 1 0 cm~^. A t the p r e s e n t t i m e , we do not have a l o g i c a l mechanism t o e x p l a i n t h i s d i s c r e p a n c y . E h r s t e i n et a l suggest t h a t the d i s c r e p a n c y may be caused by a " s p i l l - o v e r " o f c a r r i e r s from the PN j u n c t i o n . However, they a l s o r e p o r t t h a t t h e j u n c t i o n s were c o p p e r - s t a i n e d and t h a t the s t a i n agreed c l o s e l y w i t h the j u n c t i o n p o s i t i o n i n d i c a t e d by the s p r e a d i n g r e s i s t a n c e p r o f i l e s . Thus, i t i s not c l e a r whether we're s e e i n g an a r t i f a c t o f the s p r e a d i n g r e s i s t a n c e t e c h n i q u e , whether t h e r e i s s t i l l some problem w i t h SIMS measurements a t low c o n c e n t r a t i o n , o r whether the d i s c r e p a n c y i s m a t e r i a l s - r e l a t e d due t o some a s - y e t unknown mechanism. S i n c e the o t h e r d i s c r e p a n c i e s between SIMS and s p r e a d i n g r e s i s t a n c e a r e c l e a r l y r e l a t e d t o the f a c t t h a t the s p r e a d i n g r e s i s t a n c e t e c h n i q u e measures o n l y e l e c t r i c a l l y - a c t i v e i m p u r i t i e s , i t i s t e m p t i n g t o suppose t h a t the cause o f the low c o n c e n t r a t i o n d i s c r e p a n c y i s the same. T h i s h y p o t h e s i s gets some support from Marek P a w l i k ' s paper on i o n - i m p l a n t e d s i l i c o n - o n - s a p p h i r e ( 1 4 ) , i n w h i c h he r e p o r t s a s i m i l a r d i s c r e p a n c y between SIMS and s p r e a d i n g r e s i s t a n c e a t low boron c o n c e n t r a t i o n s . P a w l i k a s c r i b e s t h e d i s c r e p a n c y t o the i n t e r s t i t i a l d i f f u s i o n of boron, due t o d e f e c t s i n the d e p o s i t e d SOS f i l m . T h i s suggests the p o s s i b i l i t y t h a t , i n


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DEPTH (pm) F i g u r e 9. S p r e a d i n g r e s i s t a n c e , SIMS, and NDP ( N e u t r o n Depth P r o f i l i n g ) p r o f i l e s of a B i o n - i m p l a n t i n t o an N-type s u b s t r a t e . Reproduced w i t h p e r m i s s i o n from R e f . 13. C o p y r i g h t 1984 American S o c i e t y f o r T e s t i n g and M a t e r i a l s . 1 0


3.

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Dopant Profiles by the Spreading Resistance Technique 1

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E h r s t e i n s work, t h e B may a l s o have d i f f u s e d i n t e r s t i t i a l l y , perhaps as a r e s u l t o f d e f e c t s r e m a i n i n g from t h e i o n - i m p l a n t a t i o n damage• The end r e s u l t s of t h e s e comparisons between SIMS and s p r e a d i n g r e s i s t a n c e a r e t h a t t h e two t e c h n i q u e s agree w e l l a t h i g h concentrations, but that there i s a s i g n i f i c a n t , unresolved d i s c r e p a n c y a t low c o n c e n t r a t i o n s . Furthermore, there i s a d e v e l o p i n g consensus t h a t t h e two t e c h n i q u e s a r e complementary r a t h e r than c o m p e t i t i v e . T h i s i s due t o t h e p r i m a r y d i f f e r e n c e between them; t h a t i s , SIMS i s s p e c i e s s e l e c t i v e , whereas s p r e a d i n g r e s i s t a n c e sees o n l y t h e net c a r r i e r c o n c e n t r a t i o n , r e g a r d l e s s o f the c h e m i c a l n a t u r e o f t h e i m p u r i t i e s c o n t r i b u t i n g t h e c a r r i e r s . C l e a r l y , when used i n c o m b i n a t i o n , SIMS and s p r e a d i n g r e s i s t a n c e g i v e more u s e f u l i n f o r m a t i o n than e i t h e r does by i t s e l f .


Spreading Resistance

Applications

I c a n suggest some s i t u a t i o n s i n which s p r e a d i n g r e s i s t a n c e measurements a r e p a r t i c u l a r l y u s e f u l . F o r i n s t a n c e , i f you need t h e u l t i m a t e i n range o f a p p l i c a t i o n , y o u ' l l f i n d t h a t t h e s p r e a d i n g r e s i s t a n c e technique i s v i r t u a l l y u n l i m i t e d , a t l e a s t f o r s i l i c o n . S p r e a d i n g r e s i s t a n c e measurements a r e r o u t i n e l y made on m a t e r i a l w i t h dopant c o n c e n t r a t i o n s r a n g i n g from g r e a t e r t h a n 1 0 cm"-* t o near i n t r i n s i c m a t e r i a l ( < 1 0 c m " * ) . A l s o , j u s t about any d e v i c e s t r u c t u r e c a n be p r o f i l e d , w i t h o u t r e g a r d t o t h e number of d i f f e r e n t l a y e r s o r t o t h e l a y e r c o n d u c t i v i t y types o r t h i c k n e s s e s . Even p o l y c r y s t a l l i n e s i l i c o n i s e a s i l y and a c c u r a t e l y p r o f i l e d . The s p r e a d i n g r e s i s t a n c e t e c h n i q u e w i l l a l s o s e r v e you w e l l when you need v e r y h i g h s p a t i a l r e s o l u t i o n . S p r e a d i n g r e s i s t a n c e p r o f i l e s a r e done w i t h a r e s o l u t i o n as l i t t l e as one nanometer p e r p o i n t — a n d t h i s i s not a fundamental l i m i t . The t e c h n i q u e i s s t i l l b e i n g d e v e l o p e d , w i t h s m a l l e r , l i g h t e r , and l e s s - p e n e t r a t i n g probes a d e f i n i t e p o s s i b i l i t y . A l s o , s i n c e s p r e a d i n g r e s i s t a n c e probes c a n be spaced as c l o s e t o g e t h e r as about 15 t o 20 urn, v e r y s m a l l p a t t e r n e d s t r u c t u r e s o r d e v i c e s can be p r o f i l e d . S p r e a d i n g r e s i s t a n c e p r o f i l i n g i s c u r r e n t l y t h e most a c c u r a t e method a v a i l a b l e f o r depth measurements of PN j u n c t i o n s o r o t h e r f e a t u r e s i n s i l i c o n , because t h e probe arm a s s e m b l i e s and t h e t e s t specimen a r e mounted on p r e c i s i o n m i c r o - p o s i t i o n e r s . A l s o , b e v e l a n g l e s can be measured t o w i t h i n +2% w i t h an o p t i c a l d e v i c e f i t t e d t o t h e m i c r o s c o p e used i n s p r e a d i n g r e s i s t a n c e measurements. T h i s c o m b i n a t i o n o f p r e c i s e a n g l e measurements and t h e l a t e r a l p o s i t i o n a c c u r a c y p r o v i d e d by the micrometers means t h a t s p r e a d i n g r e s i s t a n c e p r o f i l e s c a n g i v e j u n c t i o n depths a c c u r a t e t o about 2 o r 3%. S p r e a d i n g r e s i s t a n c e p r o f i l e s a r e generated q u i c k l y , so t h e t e c h n i q u e i s u s e f u l when speed i s e s s e n t i a l . Most s i l i c o n s t r u c t u r e s c a n be b e v e l l e d and measured and t h e raw d a t a then p r o c e s s e d and p l o t t e d as a r e s i s t i v i t y o r c a r r i e r c o n c e n t r a t i o n p r o f i l e i n l e s s than t h i r t y m i n u t e s . F i n a l l y , s p r e a d i n g r e s i s t a n c e i s r e l a t i v e l y i n e x p e n s i v e . The c a p i t a l equipment i n v o l v e d c o s t s w e l l under $100,000, and many z l

Ai

American Chemical Society Library 1155 16th St., N.W. Washington, D.C. 20036


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systems are s a t i s f a c t o r i l y o p e r a t e d by w e l l - t r a i n e d t e c h n i c i a n s or hourly operators. The s p r e a d i n g r e s i s t a n c e t e c h n i q u e does take a b i t of l e a r n i n g , p r a c t i c e , and p a t i e n c e . However, when i t ' s done r i g h t , t h e r e i s r e a l l y no match to the s e n s i t i v i t y and d e t a i l i n the r e s u l t a n t profiles. T h a t ' s why the s p r e a d i n g r e s i s t a n c e t e c h n i q u e has made and w i l l c o n t i n u e to make a s i g n i f i c a n t c o n t r i b u t i o n t o the a c t i v i t y t h a t w e ' r e a l l concerned w i t h — t h e n e v e r - e n d i n g t a s k of c h a r a c t e r i z i n g semiconductor m a t e r i a l s . Acknowledgments


The a u t h o r would l i k e t o thank D r . James R . E h r s t e i n of the N a t i o n a l Bureau of Standards f o r p r o v i d i n g F i g u r e s 6a and 6b and D r . Marek P a w l i k of the GEC R e s e a r c h L a b o r a t o r i e s f o r s u p p l y i n g the boron i m p l a n t p r o f i l e d i n F i g u r e 8.

Literature Cited 1. Schroder, D. K. Symposium on Materials Characterization in Microelectronics Processing, American Chemical Society, 1984. 2. Mazur, R. G.; Dickey, D. H. J. Electrochem. Soc. 1966, 113, 255-9. 3. Ehrstein, J. E. "Non-destructive Characterization of Semiconductor Materials"; Zemel, J . , Ed.; NATO Advanced Institute, Plenum Press: 1978; Chap. 1. 4. Mazur, R. G.; Gruber, G. A. Solid State Technology 1981, 24, 69. 5. Mazur, R. G. In "Semiconductor Processing"; Gupta, D. C., Ed.; ASTM STP 850, American Society for Testing and Materials: Philadelphia, 1984. 6. Schumann, P. A.; Gardner, E. E. J. Electrochem. Soc. 1969, 116, 87. 7. D'Avanzo, D. C.; Rung, R. D.; Dutton, R. W. Stanford Electronics Laboratories; Technical Report No. 5013-2; 1977. 8. Choo, S. C.; Leong, M. S.; Hong, H. L.; Li, L.; Tan, L. S. Solid-State Electronics 1978, 21, 769-74. 9. Berkowitz, H. L.; Lux, R. A. J. Electrochem. Soc. 1981, 128, 1137-41. 10. Piessens, R.; Vandervorst, W. B.; Maes, H. E. J. Electrochem. Soc. 1983, 130, 468-74. 11. Choo, S. C.; Leong, M. S.; Sim, J. H. Solid-State Electronics, 1983, 26, 723-30. 12. ASTM Standard F 672-80, 1982 Annual Book of ASTM Standards, Part 43, American Society for Testing and Materials. 13. Ehrstein, J. R.; Downing, R. G.; Stallard, B. R.; Simons, D. S.; Fleming, R. F. In "Semiconductor Processing"; Gupta, D. C., Ed.; ASTM STP 850, American Society for Testing and Materials: Philadelphia, 1984. 14. Pawlik, M. In "Semiconductor Processing"; Gupta, D. C., Ed.; ASTM STP 850, American Society for Testing and Materials: Philadelphia, 1984. RECEIVED

August 12, 1985


Microelectronics Processing - American Chemical Society

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