Electrical Characterization of Semiconductor Materials and Devices

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Microelectronics Processing: Inorganic Materials Characterization Downloaded from pubs.acs.org by TUFTS UNIV on 11/05/16. For personal use only.

Electrical Characterization of Semiconductor Materials and Devices Dieter K. Schroder Center for Solid State Electronics Research, Arizona State University, Tempe, AZ 85287 A few selected techniques that are representative of recent advances are described as examples of the much broader field of s e m i c o n d u c t o r electrical characterization. In particular, resistivity, carrier concentration, junction depth, generation/recombination l i f e t i m e , deep level transient spectroscopy and MOSFET mobility measurements are discussed. The importance of non-contacting methods is stressed and recent trends in this direction are outlined. This paper serves as an introduction to some of the following papers in this volume. Most semiconductor d e v i c e s o p e r a t e w i t h e l e c t r i c a l i n p u t a n d o u t p u t signals. Some, such as l i g h t e m i t t i n g d i o d e s , use e l e c t r i c a l i n p u t s to g i v e o p t i c a l o u t p u t s , w h i l e o t h e r s , l i k e p h o t o d e t e c t o r s , use o p t i c a l i n p u t s t og i v e e l e c t r i c a l o u t p u t s . The e l e c t r i c a l c h a r a c t e r i s t i c s o f a l l o f these d e v i c e s a r e t h e r e f o r e i m p o r t a n t , a n d e l e c t r i c a l characterization i s the ultimate test. There a r e , o f c o u r s e , n o n - e l e c t r i c a l c h a r a c t e r i z a t i o n t e c h n i q u e s i n use. They a r e , however, l a r g e l y employed as v a r i o u s types o f process m o n i t o r s d u r i n g device f a b r i c a t i o n o r asc h a r a c t e r i z a t i o n t o o l s during failure analysis. C h a r a c t e r i z a t i o n t e c h n i q u e s f a l l i n t o t h r e e main c a t e g o r i e s • process monitors • failure analysis • d e v i c e performance i n d i c a t o r s . N o n - e l e c t r i c a l methods f a l l , by and l a r g e , i n t o t h e f i r s t t w o c a t e g o r i e s , w h i l e e l e c t r i c a l methods f a l l i n t o a l l t h r e e . The b a s i c mechanisms i n t h e m a t e r i a l o r d e v i c e t h a t a r e u t i l i z e d by t h e v a r i o u s c h a r a c t e r i z a t i o n t e c h n i q u e s a r e g e n e r a l l y w e l l known. Much p r o g r e s s has been made i n t h e l a s t few y e a r s i n t h e r e s o l u t i o n and s e n s i t i v i t y o f t h e equipment used. The d r i v i n g f o r c e has been a n i n c r e a s i n g c o m p l e x i t y o f c i r c u i t s and d e v i c e s , more q u a l i t y c o n t r o l , as w e l l a s c o m p u t e r a u t o m a t i o n o f t e s t e q u i p m e n t , f o r m e r l y under 0097-6156/ 86/ 0295-0018506.00/ 0 © 1986 American Chemical Society

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Electrical Characterization of Semiconductor Materials

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manual c o n t r o l . T h i s h a s g e n e r a l l y meant more e x p e n s i v e e q u i p m e n t , but t h e h i g h e r c o s t o f m a t e r i a l g r o w t h , c i r c u i t d e s i g n and m a n u f a c t u r i n g o p e r a t i o n s has j u s t i f i e d t h e i n v e s t m e n t s i n e x p e n s i v e c h a r a c t e r i z a t i o n equipment. I w i l l l i m i t m y s e l f h e r e t o a d i s c u s s i o n o f some r e c e n t t r e n d s i n e l e c t r i c a l c h a r a c t e r i z a t i o n b y u s i n g examples o f a few t e c h n i q u e s . N o n - e l e c t r i c a l methods a r e d i s c u s s e d i n s u b s t a n t i a l d e t a i l i n some o f the f o l l o w i n g chapters.


Device

Characteristics

One o f t h e e a s i e s t w a y s t o v i s u a l i z e t h e m a t e r i a l a n d d e v i c e c h a r a c t e r i s t i c s t h a t n e e d t o be m e a s u r e d i s t o c o n s i d e r a semiconductor d e v i c e . F o r t h i s I have chosen i n F i g u r e 1 a m e t a l oxide-semiconductor f i e l d - e f f e c t t r a n s i s t o r (MOSFET) a s r e p r e s e n t a t i v e o f a t y p i c a l semiconductor d e v i c e . I n d i c a t e d on i t a r e t h e i m p o r t a n t m a t e r i a l a n d d e v i c e p a r a m e t e r s t h a t n e e d t o be measured. O n l y some o f them a r e a d d r e s s e d i n t h i s c h a p t e r . Other d e v i c e s c o u l d have been chosen, but t h e MOSFET i n c o r p o r a t e s most o t t h e p a r a m e t e r s o f i n t e r e s t and i s t h e most common i n t e g r a t e d c i r c u i t dev i c e . The c h a r a c t e r i s t i c s a r e : •

•

•

• •

•

•

Resistivity • Substrate • Source and d r a i n • Gate ( p o l y - s i l i c o n , s i l i c i d e , m e t a l ) Carrier Concentration • Source and D r a i n • Implanted C h a n n e l , Channel Stop Insulator • Charge D e n s i t y • Interface State Density Mobility • Channel Resistance • Source and D r a i n • Contact Impurities • Deep L e v e l I m p u r i t i e s • Oxygen and Carbon • Generation/Recombination L i f e t i m e • S t r u c t u r a l Imperfections (dislocations, faults) P h y s i c a l Dimensions • J u n c t i o n Depth • E p i t a x i a l F i l m Thickness • Channel Length and W i d t h • Oxide T h i c k n e s s

stacking

F o r o t h e r d e v i c e s t h e r e w i l l be m i n o r v a r i a t i o n s i n t h e s e parameters. A d e t a i l e d d i s c u s s i o n o f a l l o f t h e s e f a c t o r s i s beyond t h e scope o f t h i s a r t i c l e . I w i l l u s e some e x a m p l e s t o i l l u s t r a t e recent trends i n e l e c t r i c a l c h a r a c t e r i z a t i o n . Some o f these t r e n d s a r e :


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MICROELECTRONICS PROCESSING: INORGANIC MATERIALS CHARACTERIZATION

F i g u r e 1 . A MOSFET u s e d h e r e as a r e p r e s e n t a t i v e s e m i c o n d u c t o r d e v i c e . I n d i c a t e d a r e t h e v a r i o u s p a r a m e t e r s t h a t n e e d t o be characterized.

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• • • •


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Measurement o f i n c r e a s i n g l y s m a l l e r f e a t u r e s Instrumentation with increasing s e n s i t i v i t y N o n - c o n t a c t i n g measurement t e c h n i q u e s Computer data a c q u i s i t i o n and d i s p l a y .


N o n - C o n t a c t i n g Measurements Most e l e c t r i c a l c h a r a c t e r i z a t i o n t e c h n i q u e s r e q u i r e p h y s i c a l c o n t a c t s between t h e w a f e r and t h e measuring instrument. They c a n be nonpermanent c o n t a c t s ( e . g . f o u r - p o i n t p r o b e ) o r p e r m a n e n t c o n t a c t s ( e . g . evaporated metal). F o r some a p p l i c a t i o n s s u c h p e r m a n e n t c o n t a c t s a r e n o t p e r m i s s i b l e . They may, f o r e x a m p l e , c r e a t e damage o r l e a v e r e s i d u e s t h a t a r e d e l e t e r i o u s d u r i n g subsequent p r o c e s s i n g . N o n - c o n t a c t i n g methods a l l o w complete i n s p e c t i o n o f a l l w a f e r s because no p h y s i c a l c o n t a c t i s made. Various p r i n c i p l e s a r e u t i l i z e d . R e s i s t i v i t y i s m e a s u r e d by i n s e r t i n g t h e w a f e r b e t w e e n two c o i l s c o n n e c t e d t o an o s c i l l a t o r . The time v a r y i n g m a g n e t i c f i e l d s , s e t up b y t h e o s c i l l a t o r , induce eddy c u r r e n t s i n t h e w a f e r . T h e s e eddy c u r r e n t s d i s s i p a t e energy s u p p l i e d by t h e o s c i l l a t o r . A m e a s u r e o f t h e power r e q u i r e d t o s t a b i l i z e t h e o s c i l l a t o r i s a n a c c u r a t e measure o f t h e r e s i s t i v i t y . By c o m b i n i n g t h i s t e c h n i q u e w i t h c a p a c i t i v e c o u p l i n g o r u l t r a s o u n d r e f l e c t i o n , w a f e r t h i c k n e s s and w a f e r f l a t n e s s i n f o r m a t i o n i s a l s o obtained. A f u r t h e r step i s t o wafer-map t h e d a t a . Using o p t i c a l s c a n n i n g , s u r f a c e d e f e c t maps a r e generated (1) and i n s u l a t o r t h i c k n e s s v a r i a t i o n s a r e measured e l 1 i p s o m e t r i c a l l y a n d d i s p l a y e d . As d i s c u s s e d f u r t h e r o n , r e c o m b i n a t i o n l i f e t i m e maps c a n a l s o be generated by n o n - c o n t a c t i n g methods. The t r e n d i n n o n - c o n t a c t i n g c h a r a c t e r i z a t i o n i s i t s e x t e n s i o n t o m a t e r i a l o r d e v i c e p a r a m e t e r s , measured by c o n t a c t i n g t e c h n i q u e s i n the p a s t . F u r t h e r m o r e , i t i s b e i n g combined more and more w i t h twoo r t h r e e - d i m e n s i o n a l data d i s p l a y . Resistivity The f o u r - p o i n t probe i s commonly used f o r r e s i s t i v i t y m e a s u r e m e n t s (2). I t i s w e l l u n d e r s t o o d and, u n t i l r e c e n t l y , was l a r g e l y m a n u a l l y o p e r a t e d w i t h a l i m i t e d number o f d a t a p o i n t s p e r w a f e r . Computer a d v a n c e s have added a d i m e n s i o n t o t h i s technique that g i v e s very useful information. By p l a c i n g t h e m e c h a n i c a l p r o b e s t e p p i n g a s w e l l a s t h e d a t a a c q u i s i t i o n under computer c o n t r o l , i t i s e a s y t o g a t h e r many d a t a p o i n t s p e r w a f e r and d i s p l a y t h e d a t a i n a way f o r c e r t a i n t r e n d s t o become o b v i o u s . F o r example, i n F i g u r e 2 such d a t a p o i n t s a r e p l o t t e d a s c o n t o u r s o f e q u a l r e s i s t a n c e ( 3 ) . These p l o t s c l e a r l y show n o n - u n i f o r m i t i e s l i k e gas f l o w p a t t e r n s d u r i n g e p i t a x i a l g r o w t h or d i f f u s i o n . They have a l s o been s u c c e s s f u l l y used t o c h a r a c t e r i z e i m p l a n t p a t t e r n s o f i o n i m p l a n t e r s (1) . A s y s t e m a t i c v a r i a t i o n o f t h e i m p l a n t e d d o s e a c r o s s a w a f e r , w h i c h may be t h e r e s u l t o f t h e i m p l a n t beam o r s u b s t r a t e h o l d e r m o t i o n o f t h e i m p l a n t e r , g i v e s r i s e t o t h r e s h o l d v o l t a g e v a r i a t i o n s o f MOSFETs a c r o s s t h a t w a f e r . Doping u n i f o r m i t y mapping t e c h n i q u e s a r e now b e i n g a c c e p t e d by i m p l a n t e r vendors f o r t h e c h a r a c t e r i z a t i o n and q u a l i f i c a t i o n o f t h e i r systems.



F i g u r e 2. Examples of r e s i s t a n c e p l o t s u s i n g w a f e r mapping. C o u r t e s y of D.S. P e r l o f f .

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SCHRODER



Doping and C a r r i e r

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Concentrations

The d o p i n g c o n c e n t r a t i o n i s o f t e n c o n f u s e d w i t h t h e c a r r i e r concentration. I n u n i f o r m l y and m o d e r a t e l y doped s u b s t r a t e s t h e two a r e v i r t u a l l y i d e n t i c a l a t room t e m p e r a t u r e . T h i s i s no l o n g e r t r u e when t h e s u b s t r a t e i s h e a v i l y doped o r when a d i f f u s e d o r i o n i m p l a n t e d l a y e r i s measured. E v e n i f a l l t h e d o p a n t atoms a r e e l e c t r i c a l l y a c t i v e , thec a r r i e r concentration i n heavily-doped m a t e r i a l i s l o w e r t h a n t h e d o p i n g c o n c e n t r a t i o n , a s d e s c r i b e d by Fermi-Dirac s t a t i s t i c s ( 4 ) . This i sf u r t h e r aggravated i f , f o r e x a m p l e , t h e i o n - i m p l a n t e d l a y e r i s not w h o l l y a c t i v a t e d o r i f t h e r e i s a steep doping g r a d i e n t . I t i s i m p o r t a n t t h a t one i s a w a r e o f these d i f f i c u l t i e s . Various techniques are used f o r t h e s e measurements. The most p o p u l a r a r e c a p a c i t a n c e - v o l t a g e (C-V) p r o f i l i n g , s p r e a d i n g r e s i s t a n c e and secondary i o n mass s p e c t r o s c o p y ( S I M S ) . SIMS i s n o t a s t r i c t l y e l e c t r i c a l c h a r a c t e r i z a t i o n t e c h n i q u e , b u t i s i n c l u d e d h e r e because i t i s r o u t i n e l y used t o measure the d o p a n t atom d i s t r i b u t i o n . The b a s i s f o r these three techniques are very d i f f e r e n t and I w i l l b r i e f l y d e s c r i b e them. C-V

Profiling

C-V p r o f i l i n g r e q u i r e s a r e v e r s e - b i a s e d s p a c e - c h a r g e r e g i o n I t can be implemented w i t h any one o f these d e v i c e s : •

• •

(scr).

Schottky b a r r i e r contact • permanent m e t a l c o n t a c t • temporary mercury c o n t a c t • temporary l i q u i d c o n t a c t pn j u n c t i o n MOS c a p a c i t o r o r t r a n s i s t o r

In the conventional approach, t h ec a r r i e r c o n c e n t r a t i o n i s d e t e r m i n e d b y m e a s u r i n g t h e c a p a c i t a n c e as a f u n c t i o n of r e v e r s e b i a s e d v o l t a g e ( 5 ) . The c a r r i e r c o n c e n t r a t i o n p and t h e d e p t h W a r e g i v e n by 2

2

p = -2/(qK e A d(l/C )/dV)

(1)

W = K e A/C s o

(2)

g

w h e r e q = 1.6x10

o

-1 9

c o u l , K e i st h e semiconductor d i e l e c t r i c so c o n s t a n t , A i s t h e d e v i c e a r e a , C i s t h e measured c a p a c i t a n c e and V i s the a p p l i e d v o l t a g e . By v a r y i n g t h e a p p l i e d v o l t a g e , t h e s c r w i d t h i s i n c r e a s e d and a p l o t of p vs. W i s obtained. There a r e two l i m i t s ( i ) how c l o s e t o t h e s u r f a c e , a n d ( i i ) how deep i n t o t h e w a f e r c a n t h e d e v i c e be profiled. L i m i t ( i ) i s s e v e r a l Debye l e n g t h s f r o m t h e s u r f a c e a n d l i m i t ( i i ) i s s e t by the v o l t a g e breakdown o f t h e d e v i c e ( 6 ) . Frequently i t i s desirable to p r o f i l e deeper than v o l t a g e breakdown a l l o w s . T h i s i s e s p e c i a l l y i m p o r t a n t f o r h e a v i l y doped l a y e r s where the breakdown l i m i t a l l o w s o n l y a n e x t r e m e l y shallow p r o f i l e measurement. A clever solution is theelectrochemical


24


p r o f i l e r shown i n F i g u r e 3 (_7) • The S c h o t t k y b a r r i e r i s c r e a t e d by a l i q u i d - s e m i c o n d u c t o r c o n t a c t , e l i m i n a t i n g d e v i c e p r o c e s s i n g and making t h e measurement v e r y e a s y . A f i x e d voltage coupled w i t h a c a p a c i t a n c e m e a s u r e m e n t g i v e s one datum p o i n t on a p-W p l o t . The a p p l i c a t i o n o f a c u r r e n t t o t h e e l e c t r o l y t e causes t h e s e m i c o n d u c t o r t o be e t c h e d , w i t h t h e e t c h depth c o n t r o l l e d by t h e d u r a t i o n o f t h e c u r r e n t f l o w . Both p and W a r e remeasured f o r t h e next p o i n t a n d s o on. An e n t i r e p-W p l o t i s g e n e r a t e d i n t h i s manner w i t h no depth limitation. An e x a m p l e o f s u c h a p l o t i s shown i n F i g u r e 3, w h e r e t h e increment between e x p e r i m e n t a l d a t a p o i n t s i s 3 5A, m a k i n g t h i s a n extremely high r e s o l u t i o n instrument. I t i s preferable to a c o n v e n t i o n a l C-V p r o f i l e r because i t has no depth l i m i t a t i o n and uses w a f e r s w i t h o u t t h e need t o f a b r i c a t e d e v i c e s . I t i s destructive, however, by l e a v i n g a n a p p r o x i m a t e l y 1 mm d i a m e t e r h o l e i n t h e w a f e r . Spreading

Resistance

Profiling

I n t h e 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 r ( S R P ) , two m e t a l p r o b e s c o n t a c t t h e s e m i c o n d u c t o r (_8) . A s m a l l v o l t a g e o f t y p i c a l l y a few mV, i s a p p l i e d between t h e c o n t a c t s and t h e r e s i s t a n c e i s m e a s u r e d . It is t h e sum o f s e v e r a l c o m p o n e n t s , w i t h t h e d o m i n a n t one b e i n g t h e s p r e a d i n g r e s i s t a n c e under t h e p r o b e s . The s u c c e s s o f t h i s m e t h o d r e s t s o n two c h i e f f e a t u r e s : ( i ) a mechanical arrangement that allows the probes t o c o n t a c t t h e semiconductor s u r f a c e i n a w e l l c o n t r o l l e d m a n n e r , and ( i i ) a c a l i b r a t i o n o f t h e measured s p r e a d i n g resistance to calibrated standard wafers. I f these standards a r e c a l i b r a t e d i n d o p i n g r a t h e r t h a n c a r r i e r c o n c e n t r a t i o n s , t h e SRP a c t u a l l y g i v e s d o p i n g c o n c e n t r a t i o n as a f u n c t i o n o f d i s t a n c e . F o r s h a l l o w i m p l a n t e d o r d i f f u s e d l a y e r s , t h e semiconductor i s a n g l e - b e v e l l e d and t h e p r o b e moved a l o n g t h e b e v e l l e d s u r f a c e . A s h a l l o w angle coupled w i t h s h o r t probe s t e p s , a l l o w s e q u i v a l e n t s h a l l o w depth s t e p s . Examples a r e g i v e n i n (_8) w i t h depth r e s o l u t i o n of 200X. The a c c u r a c y i s l i m i t e d by t h e c o r r e c t i o n f a c t o r s i n t h e a l g o r i t h m used i n t h e d a t a r e d u c t i o n . Secondary I o n Mass

Spectroscopy

I n t h e SIMS t e c h n i q u e , an oxygen o r cesium i o n beam i n c i d e n t o n t h e s a m p l e , s p u t t e r s atoms f r o m t h e s u r f a c e . E i t h e r n e g a t i v e l y or p o s i t i v e l y charged i o n s a r e mass a n a l y z e d and t h e i r d e n s i t y d i s p l a y e d as a f u n c t i o n o f s p u t t e r t i m e . By u s i n g c a l i b r a t i o n s t a n d a r d s , t h e d e n s i t y i s c a l i b r a t e d as c o n c e n t r a t i o n / c m , and by m e a s u r i n g t h e s p u t t e r c r a t e r depth/ t h e time a x i s i s c o n v e r t e d t o a d i s t a n c e a x i s , g i v i n g a dopant c o n c e n t r a t i o n v s . depth p l o t . T h i s t e c h n i q u e g i v e s t h e t o t a l dopant c o n c e n t r a t i o n , n o t j u s t t h e e l e c t r i c a l l y a c t i v e p o r t i o n . We m e n t i o n i t h e r e , e v e n t h o u g h i t i s n o t a n e l e c t r i c a l c h a r a c t e r i z a t i o n t e c h n i q u e i n t h e sense t h a t t h e o t h e r s a r e , because i t i s r o u t i n e l y used t o c h a r a c t e r i z e t h e d o p a n t c o n c e n t r a t i o n a n d d e p t h o f i o n - i m p l a n t e d and d i f f u s e d l a y e r s . When the dopant atoms a r e e l e c t r i c a l l y a c t i v e , t h e n SIMS i s f o u n d t o g i v e r e s u l t s v e r y c l o s e t o those obtained from spreading r e s i s t a n c e measurements (_9) . When e l e c t r i c a l a c t i v a t i o n i s n o t c o m p l e t e , t h e n t h e r e w i l l be s i g n i f i c a n t d e v i a t i o n s b e t w e e n SIMS and SRP o r C-V data.

2.

SCHRODER

25



-Wafer

Light /ww^J (for n-type)

Electrolytesemiconductor Schottky / contact

J/

-Electrolyte

J

I

I

I 0.2

I

I

I

I

L_ J 0.4

I

I

L 0.6

Depth (]im) F i g u r e 3. A schematic o f t h e e l e c t r o c h e m i c a l p r o f i l e r and a c a r r i e r c o n c e n t r a t i o n v s . depth p l o t . The example i s a Zn d i f f u s i o n i n t o GaAs. C o u r t e s y o f R . J . R o e d e l .

26



J u n c t i o n Depth A l l of t h e t h r e e p r e c e d i n g t e c h n i q u e s a r e u s e d t o m e a s u r e j u n c t i o n depth. T h e r e i s g e n e r a l agreement between t h e t h r e e t e c h n i q u e s f o r deep j u n c t i o n s w i t h j u n c t i o n d e p t h s o f t h e o r d e r o f 1-2 jxm o r greater. H o w e v e r , f o r s h a l l o w j u n c t i o n s i t i s b e l i e v e d t h a t SIMS g i v e s t h e most a c c u r a t e r e s u l t s . T h i s q u e s t i o n i s under a c t i v e i n v e s t i g a t i o n a t p r e s e n t b e c a u s e j u n c t i o n s w i t h depths of

Electrical Characterization of Semiconductor Materials and Devices

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