SiGe heterojunction bipolar transistors with Schottky collector contacts

transistors w i t h S c h o t t k y

collector contacts

P R O E F S C H R I F T

ter verla-ijging v a n de graad v a n doctor aan de Technische U n i v e r s i t e i t D e l f t ,

op gezag v a n de Rector M a g n i f i c u s P r o f . i r . K . C. A . Lnj^ben, v o o r z i t t e r v a n het College voor Promoties,

i n het openbaar t e verdedigen

op dinsdag 9 september 2014 o m 12:30

um-door

G i £ u i p a o l o L o r i t o

D o t t e r e i n Ingegneria E l e t t r o n i c a

v a n U n i v e r s i t a degli S t n d i d i N a p o l i Federico I I , I t a l i ë , geboren t e Salerno, I t a l i ë

D i t proefschi'ift is goedgekeiu'd door de p r o m o t o r : P r o f . dr. i l ' . R. Deld<:er Samenstelling promotiecommissie: Rector M a g n i f i c u s v o o r z i t t e r P r o f . dr. i r . R. Deld<;er p r o m o t o r P r o f . dr. il-. J. W . S l o t b o o m P r o f . ck. P. Steeneken P r o f . d l ' . M . Z e m a n P r o f . dr. R. Jos D r . i r . W . D . v a n N o o r t D r . K . B u i s m a n

Prof. d r . G . Q. Zhang reserveHd

Technische U n i v e r s i t e i t D e l f t Technische U n i v e r s i t e i t D e l f t Technische U n i v e r s i t e i t D e l f t Technische U n i v e r s i t e i t D e l f t Technische U n i v e r s i t e i t D e l f t C h a l m e r s U n i v e r s i t y N a t i o n a l Semiconductor, U S A Technische U n i v e r s i t e i t D e l f t Technische U n i v e r s i t e i t D e l f t Gianpaolo L o r i t o ,

SiGe h e t e r o j i m c t i o n b i p o l a r t r a n s i s t o r s w i t h Schottkj^ coUector contacts P h . D . Thesis, DeUt U n i v e r s i t y o f T e c l m o l o g y ,

v d t h a s i t m m a i y i n D u t c h .

K e y w o r d s :

SiGe h e t e r o j i m c t i o n b i p o l a r transistors, S c h o t t k y collector contacts, Silicon-On-Glass t e c l m o l o g y , l ü t r a s h a l l o w e m i t t e r s , excimer laser annealing, low-temperatiu'e j u n c t i o n f o r m a t i o n , Si C V D e p i t a x y .

I S B N : 978-94-6203-664-2

C o p y r i g h t © 2014 b y Gianpaolo L o r i t o ,

A U r i g h t s reserved. N o p a r t of t h i s p u b h c a t i o n m a y be r e p r o d u c e d , stored i n a r e t r i e v a l system, or t r a n s m i t t e d i n any f o r m or b y any means w i t h o u t t h e p r i o r w i l t t e n permission of t h e c o p y i l g h t owner.

1. Introduction 1

1.1. SiGe H B T teclmolog}^ f o r h i g h fi-equencj^ applications:

state-o f - a r t perfstate-ormance a n d challenges f state-o r n e x t generatistate-on state-of

speed-improved H B T ' s 1 1.2. D I M E S back-wafer c o n t a c t e d Sihcon-On-Glass t e c h n o l o g y 6

1.3. G o a l and oiitUne o f t h e Thesis 7

2. 1-D numerical simulations 11

2.1. I n t r o d u c t i o n n 2.2. S i m u l a t i o n fi-amework 12

2.3. E l e c t r i c a l characteristics of SiGe N P S transistors i n D C

mode. 14 2.4. C u t - o f f h-equency o f SiGe N P S transistors 18

2.5. T r a n s i t t i m e analysis 22 2.5.1. T r a n s i t t i m e d i s t r i b u t i o n i n SiGe N P S transistors 27

2.6. C o m p a r i s o n between SiGe N P S a n d N P N t r a n s i t s t o r s 30 2.6.1. C o m p a r i s o n between SiGe NPS a n d N P N S transistors 32

2.7. Conclusions 34

3 .

Silicon-On-Glass vertical B J T ' s with Schottky collector

contacts

3 5

3 . 1 . I n t r o d u c t i o n 35 3.2. Base leakage a n d i m p a c t i o n i z a t i o n cm-rent i n S O G v e r t i c a l

B J T ' s 36 3.2.1. E x p e r i m e n t a l m a t e r i a l 36

3.2.2. Results a n d discussion 38 A . Base lealcage cm-rent 39 B . I m p a c t i o n i z a t i o n cm-rent 42 3.3. O f f s e t voltage i n v e r t i c a l P N P ' s 44

3.4. 3.3.1. E x p e r i m e n t a l m a t e r i a l 3.3.2. Results a n d discussion Conclusions 44 46 53

4. SiGe H B T ' s with implsinted and laser-annealed emitters 55

4 . 1 . I n t r o d u c t i o n 55 4.2. Device f a b r i c a t i o n 55 4.3. E l e c t r i c a l measm-ements a n d discussion 58

4.4. Conclusions 61

5. C V D ultrashïJlow n-type emitters 63

5.1. I n t r o d u c t i o n 63 5.2. D e t e c t i o n of t h e carrier t r a n s p o r t mechanism i n ultrashaUovi^

j i m c t i o n s : test structm'e f a b r i c a t i o n a n d measm-emeut

m e t h o d 64 5.3. U l t r a s h a U o w n - t y p e emitters b y laser-annealed C V D dopants 69

5.3.1. E l e c t r i c a l measm-ements a n d discussion 70 5.3.2. E x p e r i m e n t a l d e m o n s t r a t i o n of t h e bias-induced

t r a n s i t i o n f ï o m p - S c h o t t k y t o n''"p diode 73 5.4. I n - s i t u doped e p i t a x i a l n - t y p e ultrashaUow mono-emitters 74

5.4.1. E l e c t r i c a l measm-ements a n d discussion 74

5.5. Conclusions 75

6. SiUcon-On-GIass Schottky coUector vertical SiGe H B T ' s 77

6.1. I n t r o d u c t i o n 77 6.2. F a b r i c a t i o n process 77 6.3. E l e c t r i c a l measm-ements a n d discussion 82

6.4. Conclusions 84

7. Conclusions and recommandations 85

7.1. Conclusions 85 7.2. Recommendations 87

A . M E D I C I models and material parameters 89

B . Flowchart of the S O G SiGe H B T process 97

Summary

Samenvatting

List of Publications

Acknowledgements

About the author

I n t r o d u c t i o n

1.1. S i G e H B T technology for high

frequency applications; state-of-art

performance and challenges for next

generation of speed-improved H B T ' s

I n recent years, SiGe H B T technology has gained more and more acceptance as suitable a n d lowcost technologjr for integi'ating h i g h -performance bipolar transistors i n advanced C M O S modules. Indeed B i C M O S processes have been ah-eady d e m o n s t r a t e d featming SiGe H B T ' s w i t h values o f t h e device figmes o f m e r i t (i.e. cm'rent g a i n a n d E a r l y voltage p r o d u c t P X V A , t r a n s i t fi-eciuency a n d brealcdown voltage p r o d u c t f^xBVcEo a n d m a x i m u m o s c i l l a t i o n fi'equency f^^^^) comparable t o t h e I I I - V c o m p o i m d semiconductor based transistors [1,2]. Moreover, i t has been p r e d i c t e d t h a t b o t h a n d f„^^ above 1 T H z can be reached at r o o m temperatm-e w i t h t a i l o r e d designs of t h e v e r t i c a l profiles a n d l a t e r a l dimensions of SiGe H B T ' s [3,4].

However, t h e ongoing d e m a n d f o r higher a n d liigher f^ (and f^,^.,) poses serious t e c l m o l o g i c a l challenges since i t imposes such a n aggressive v e r t i c a l device d o w n s i z i n g t h a t t h e t o t a l distance fi'om t h e e m i t t e r t o collector c o n t a c t should be reduced t o o n l y f e w tens o f nanometers i n order t o m i n i m i z e t h e t o t a l t r a n s i t t i m e TEC = l/27ifT o f t h e chcU'ge carriers. I n a d d i t i o n , t h e d o p i n g c o n c e n t r a t i o n i n t h e v e r y t h i n p - t y p e SiGe base layer m u s t be a r o i m d t h e b o r o n ( B ) solid s o l u b i h t y t o reduce t h e base sheet resistance t o f e w k O / a a n d t h u s increase f,,,^.,. A l o w base resistance is also r e q r ü r e d f o r m i n i m i z i n g t h e device noise figm-e [5]. A n example of fe-ont-end-of-Uue ( F E O L ) d o p i n g profiles a n d cross section of

SiGe heterojimction bipolar transistors w i t h Schottky collector contacts

a state-of-art h i g h speed SiGe:C h e t e r o j u n c t i o n b i p o l a r t r a n s i s t o r is shown i n F i g . 1.1 [6].

B

F i g u r e 1.1 1-D Schematic, F E O L SIMS profiles and SEM pictme of the cross section of the conventional SiGe N P N transistor w i t h ohmic coUector contact from [6].

T h e i m p r o v e m e n t s i n chemical-vapor-deposition ( C V D ) e p i t a x y teclnnciues over t h e last yeais have made i t possible t o r e l i a b l y gTow t l i i n defect-free B-doped SiGe layers, w i n c h are w e l l c o n t r o l l e d b o t h i n thickness ( d o w n t o less t h a n 10 m i i ) a n d i n d o p i n g level (up t o about 10^° cm"^, depending o n t h e Ge c o n c e n t r a t i o n ) . However, t h e real processing challenge is t o a v o i d any o u t - d i f f u s i o n of t h e B spike-profile i n t h e post-epi process. T h e spread of t h e b o r o n b e y o n d t h e SiGe base layer w o u l d create parasitic barriers i n t h e c o n d u c t i o n b a n d at b o t h E - B and C - B j i m c t i o n s , w l i i c h results i n a severe degi-adation of t h e D C a n d A C device o p e r a t i o n [7,8]. A s any other d o p a n t i m p m i t i e s i n sihcon, b o r o n diffuses b y an i n t e r m i t t e n t process where B atoms reside most of the t i m e i n s u b s t i t u t i o n a l sites a n d occasionally are converted i n a fast-m o v i n g d o p a n t species b y i n t e r a c t i n g w i t h a p o i n t defect. I n t h e case of b o r o n , t h e p o i n t defects t h a t assist t h e d o p a n t d i f f u s i o n are Si self-i n t e r s t self-i t self-i a l s . T h e phenomenon self-is k n o w n as "Idck-out" mechanself-ism [9]. A s e l f - i n t e r s t i t i a l traveUing i n between adjacent stable locations i n t h e c r y s t a l encoimters a n d reacts w i t h a s u b s t i t u t i o n a l B a t o m . T h e result of such i n t e r a c t i o n is a m o b i l e d o p a n t species (either a B a t o m a n d Si i n t e r s t i t i a l p a n or a single i n t e r s t i t i a l B a t o m ) w h i c h can m i g r a t e easily tln-ough i n t e r s t i t i a l spaces f o r some distance before dissolving i n another s u b s t i t u t i o n a l site [10].

T h e c o n c e n t r a t i o n o f Si i n t e r s t i t i a l s increases due t o several processing steps such as Irigh temperatm-e t r e a t m e n t s . Si o x i d a t i o n a n d d o p a n t i m p l a n t a t i o n s . T h e i o n - i m p l a n t - i n d u c e d d i f f u s i o n o f b o r o n is k n o w i i as transient-enhanced d i f f u s i o n ( T E D ) . Therefore, t h e m a i n concern f o r o i i t - d i f f i i s i o n of t h e B s p i k e - p r o f ü e i n t h e SiGe base comes fi-om t h e T E D t h a t m i g h t occm- as a conseciuence of t h e base contact a n d e m i t t e r i m p l a n t a t i o n s a n d t h e subseciiient anneaUng step necessary f o r a c t i v a t i n g t h e dopants [11].

I n t h i s regard, t h e i n c o r p o r a t i o n of a s m a l l a m o u n t of c a r b o n (C) i n t o t h e SiGe base a n d t h e use o f r a p i d t h e r m a l amiealing ( R T A ) steps have p r o v e n t o be beneficial f o r c o u n t e r a c t i n g t h e b o r o n T E D [11,12]. T h e suppression of t h e B d i f f u s i o n i n C-em-iched regions is r el a t ed t o t h e p r o p e r t j ^ o f Si i n t e r s t i t i a l s t o f o r m mobUe pans w i t h C atoms m o r e e f f i c i e n t l y . T l i i s means t h a t i n such regions t h e c o n c e n t r a t i o n of Si i n t e r s t i t i a l s available f o r t h e " I d c k - o i i t " mechanism is s i g n i f i c a n t l y reduced. However, as t h e B d o p i n g i n t h e SiGe base increases, t h i s s o l u t i o n becomes less and less u s e f u l since t h e r e q i m e d higher percentage of C necessary t o guarantee t h e complete suppression o f T E D d m i n g t h e c o m m o n l y used R T A steps wiU increase t h e base leakage due t o t h e a m o i m t o f C i n t e r s t i t i a l s i n t h e E - B d e p l e t i o n region [13,14].

SiGe tieteroj miction bipolar transistors w i t h Schottky collector contacts

A n o t h e r issue t h a t m-ises i n b i p o l a r transistors t h a t are operated at frequencies i n t h e himdi-eds of gigahertz, is t h a t t h e e m i t t e r series resistance, a s i g n i f i c a n t component of w h i c h is t h e contact resistance, w i l l start t o U m i t t h e speed. I n t h i s regard, t h e r e l a t i v e ^ large series resistance of t h e c o m m o n l y used i n - s i t u doped p o l y - or m o n o - c r y s t a l l i n e emitters becomes l i m i t i n g [15,16,17,18].

C o n c e r n i n g t h e collector region, coUector designs f o r h i g h speed SiGe H B T ' s have been d e m o n s t r a t e d consisting of a f e w tens o f nanometers t h i c k fuUy-depleted n - t y p e coUector r e g i o n w i t h a sharp t r a n s i t i o n t o a r e g i o n w i t h a d o p i n g c o n c e n t r a t i o n above 10"° cm'^ [19]. T h e c o n v e n t i o n a l b m i e d coUector layer design is n o t suitable f o r t h e f a b r i c a t i o n of such a steep a n d h i g h l y doped coUector p r o f i l e , since t h e h i g h d o p i n g c o n c e n t r a t i o n i n t h e coUector c a n be o b t a i n e d oirly i n c o m b i n a t i o n w i t h a v e r y s m o o t h d o p i n g t r a n s i t i o n [2]. T h i s is due t o t h e f a c t t h a t t h e d o p a n t s o f t h e b m i e d coUector layer are i n t r o d u c e d at t h e b e g i n n i n g of t h e f a b r i c a t i o n process, a n d hence are subjected t o a considerable o u t - d i f f u s i o n w h i c h can easily be as m u c h as a f e w hundreds of nanometers d m i n g t h e l i i g l i t e m p e r a t m e steps t h i ' o u g h o u t t h e whole process fiow. T h i s d r a w b a c k is reduced b y i n t r o d u c i n g a selectively i m p l a n t e d coUector ( S I C ) . However, i t is v e r y d i f f i c i ü t w i t h t h i s a p p r o a c h t o f r d f i l t h e r e q u f r e m e n t of h a v i n g a t h i n f u U y depleted sub-coUector a n d at t h e same t i m e a sharp t r a n s i t i o n t o d o p i n g levels i n t h e order of 10^° cm"^. I n recent yeai's, coUector modnles based o n selective e p i t a x y have been developed w h i c h aUow f o r t h e f a b r i c a t i o n of d o p i n g profiles closer t o t h e desfred ones b u t at t h e expense of a n e x t r a e p i t a x i a l step i n t h e process flow [2,17,20].

Consequently, a l t h o u g h S i / S i G e : C h e t e r o j i m c t i o n b i p o l a r processes have been e f f e c t i v e l y used t o f a b r i c a t e devices i n t h e 300/500 G H z fT/fma.x range [1], t h e real cliaUenge f o r fmther steps towaids a C M O S -c o m p a t i b l e SiGe H B T te-chnology f o r appU-cations i n t o t h e t e r a h e r t z fi-equency range is t o develop process m o d r ü e s w i t h m i n i m m n a d d i t i o n a l technological e f f o r t w h i c h aUow t h e f o r m a t i o n of higher-doped, m o r e a b r u p t a n d shaUower d o p i n g profiles.

W i t h respect t o t h e coUector-profUe downscaling, t h e goal is t o f a b r i c a t e an i n t r i n s i c coUector r e g i o n w i t h l o w series resistance, Rc, a short t r a n s i t t i m e , TC, a n d such t h a t t h e onset o f fx degi'adation w i t h coUector c m r e n t a n d t h e avalanche b r e a k d o w n at t h e C - B j u n c t i o n is s h i f t e d t o higher coUector cm-rent densities and C - E voltages, respectively. A n a l t e r n a t i v e coUector architectm-e w h i c h has been p r e d i c t e d t o meet these requirements is reaUzed b y replacing t h e

c o n v e n t i o n a l n / n + i n t r i n s i c coUector w i t h a Schottkj^ j i m c t i o n [21], as i U u s t r a t e d schematically i n F i g 1.2. Such a S c h o t t k y collector contact has been demonstrated i n I I I - V H B T technology [22].

R

S c h o t t k y c o n t a c t \ n + _p

_{I P+i}

_p -n + _p

_{I P+i}

_p -10 \ 10^'

f

1 0

-I

10'^ ro O 10^' O 10"' O) .E 10'^ % 10'^ 10^^ : ! 1 1 1 1 1 1 — • ultra-shallow 1 1 1 1 r 1 T 1 m f — 1—1 — r CVD emitter t - - ' Ge

-: n+

_{l i '}

-P

-

V, 1 Sctioltky -- 1\ collector

-^\ /

• ! 1 1-1 1 _ t •

1

P -• -•- -• 1 • • 0.05 0.10 0.15 0.20 0.15

I

'o 2 0.10

2

n F 0.05 § O 0,20

distance [|Lim]

F i g u r e 1.2 1-D scliematic and dopmg profries of the novel SiGe NPS transistor w i t h Schottky coUector contact. The conventional n/n"^ collector is replaced w i t h a Schottky jrmction on the lowly doped p-type collector region.

I n t i l l s Thesis a f n s t i n s i g h t i n t o t h e i m p a c t o n t h e device performance of the S c h o t t k y coUector design i n Si a n d SiGe b i p o l a r transistors is presented. T h e i n v e s t i g a t e d devices are f a b r i c a t e d b y using t w o - s i d e d c o n t a c t i n g SiUcon-On-Glass technologj^ developed i n D I M E S , w l i i c h is described s l i o r t l j ' i n t h e next Section.

SiGe lieteroj miction bipolar transistors w i t h Schottk}' collector contacts

1.2. D I M E S back-wafer contacted

Silicon-On-Glass technology

Substrate t r a n s f e r processes are g a i n i n g acceptance as possible l o w -cost technologies enabling high-performance l o w - p o w e r R F integTated circuits w i t h on-chip integi-ation of R F active devices a n d h i g h - q u a l i t y passive components [ 2 3 , 2 4 ] . T h e D I M E S back-wafer c o n t a c t e d Silicon-On-Glass ( S O G ) p r o c e s s e s a Substrate-Transfer-Technology ( S T T ) where a n almost fttUy processed SiUcon-On-Instdator ( S O I ) wafer is t r a n s f e r r e d t o glass b y using a n adhesive process based o n U V - l i g l i t cm-ing. T h e Si substrate is t h e n r e m o v e d b y a n e t c h i n g step selective t o t h e b m i e d oxide ( B O X ) . T h e o r i g i n a l P h i l i p s v e r s i o n of t l i i s t r a n s f e r process was developed i n t h e b e g i n n i n g of t h e 1 9 9 0 ' s t o reduce t h e N P N substrate parasitics a n d t o m i n i m i z e t h e coUector-base capacitance of a n o v e l l a t e r a l S O I N P N t r a n s i s t o r b y using t h e B O X i s o l a t i o n [25]. A t D I M E S t h e S O G process itself was developed fm-ther i n t o a t w o - s i d e d c o n t a c t i n g technology b y a d d i n g o n t h e backside of t h e device layer process modules f o r t h e f a b r i c a t i o n of neai--ideal diodes a n d l o w - o h m i c contacts aligned w i t h t h e f r o n t s i d e l a y o u t . T h e first f a b r i c a t e d Si device was a n N P N B J T featm-ing 25 G H z f^. I n such a t r a n s i s t o r t h e coUector-base capacitance a n d collector series resistance were m i n i m i z e d hy p l a c i n g t h e coUector c o n t a c t dh-ectly o n t h e i n t r i n s i c device r e g i o n m i d e r n e a t h t h e e m i t t e r [26]. T h e double-sided c o n t a c t i n g w i t h aligned emitter-coUector contacts malces t h e D I M E S S O G b i p o l a r technolog}^ v e r y a t t r a c t i v e t o investigate a l t e r n a t i v e device arcbitectm-es a n d designs since t h e device performance approaches t h e ideal 1 - D i n t r i n s i c characteristics.

However, t h e c i r c u i t realizations i n S O G technology were hampered b y t h e e x t r e m e l y l i i g h t h e r m a l resistance o f t h e transistors themselves as a d n e c t consequence of t h e almost perfect electrical a n d t h e r m a l dielectric-isolation o f t h e devices. T h e t h e r m a l resistance R X H of a n S O G device can be as m u c h as 1 0 0 times t h e t h e r m a l resistance o f a compai-able brrUc-Si device. T h e e l e c t r o t h e r m a l i n v e s t i g a t i o n o f these devices has p r o d u c e d new insights i n t h e e l e c t r o t h e r m a l behavior o f b i p o l a r transistors [27]. However, f o r rehable i n t e g i - a t e d - c i r c i ü t reaUzations i t is obviously i m p e r a t i v e t o have an effective t h e r m a l management at device level. Therefore, f o r manj^ S O G devices heatspreading a n d h e a t s i n k i n g s t r u c t m e s become indispensable, even at v e r y l o w power d i s s i p a t i o n levels [28]. Replacing t h e bnUc-Si b y copper ( C u ) can be advantageous f o r reducing t h e t h e r m a l resistance. F o r large area devices, t h i c k C u p l a t i n g and d n e c t smface m o u n t i n g t o a p r i n t e d

-c i r -c u i t b o a r d -can p r o v i d e t h e ne-cessarj^ heatsiiddng [29]. F o r small d i m e n s i o n devices, such as liigh-frequency bipolar transistors, heat t r a n s p o r t fi-om t h e active area is more effective i f accomphshed b y t h e r m a l l y c o n d u c t i v e dielectric layers deposited d n e c t l y o n t h e device i t s e l f For tlris pm-pose physical-vapor-deposition ( P V D ) A I N t h i n - f i l m s have been developed and integi-ated w i t h success as heatspreaders at device level [30]. T h e residts have d e m o n s t r a t e d t h a t t h e c o m b i n e d use of m i c r o n - t h i c k layers o f A I N o n t h e fi-ont- a n d back-wafer and C u blocks o u backside can s l i i f t t h e onset of t h e t h e r m a l i n s t a b i l i t y o f SOG devices t o acceptable values of dissipated power [31].

1.3. Goal and outline of the Thesis

T h e goal of t h i s Thesis is t o characterize t h e S c h o t t k y collector design as a p o t e n t i a l coUector arcliitectm-e suitable f o r next generation o f SiGe liigh-fi-equency H B T ' s . T h e order o f t h e Chapters foUows t h e sequence of t h e developments a n d technology' i m p r o v e m e n t s w h i c h , s t a r t i n g from t h e 1-D s i m i ü a t i o n s , resulted i n t h e f a b r i c a t i o n of t h e fti-st p - S c h o t t k y coUector SiGe h e t e r o j i m c t i o n b i p o l a r transistors ( N P S ) . T h e D I M E S S O G bipolar technolog-j^ is the n a t m a l c a n d i d a t e f o r t h e f a b r i c a t i o n of such devices due t o i t s large design a n d process f l e x i b U i t y i n c o m b i n a t i o n w i t h t h e s t r o n g r e d u c t i o n of t h e device parasitics as a result o f t h e double-side c o n t a c t i n g .

M o r e specificalty:

• T h e A C a n d D C chai-acteristics o f several N P S devices o b t a i n e d b y v a r y i n g coUector parameters, such as t h e tlnckness a n d d o p i n g o f coUector region a n d t h e S c h o t t k y barrier height at t h e coUector contact, are i n v e s t i g a t e d b y means of 1-D n m n e r i c a l s i m i ü a t i o n s i n C h a p t e r 2. N P S transistors are also compared t o c o n v e n t i o n a l SiGe N P N devices w i t h o h m i c coUector contact w i t h r e g a r d t o t h e device performance i n terms of c u t - o f f frequency a n d open-base brealcdown voltage p r o d u c t .

• C h a p t e r 3 coUects t h e r e s i ü t s of t h e e x p e r i m e n t a l c h a r a c t e r i z a t i o n of t h e effect o f S c h o t t k y coUector contacts o n t h e base lealtage a n d brealcdown voltage o f n - t j ' p e a n d p-tj^pe v e r t i c a l B J T ' s f a b r i c a t e d i n t h e s t a n d a r d D I M E S S O G b i p o l a r process. Also, several S c h o t t k y coUector P N P ' s wdth d i f f e r e n t

SiGe heterojimction bipolar transistors w i t h Schottky coUector contacts

coUector tlnckness and d o p i n g are i n v e s t i g a t e d w i t h special focus o n t h e offset voltage.

• T h e i m p l a n t e d a n d fmnace-annealed 200 nm-deep A s enritter of t h e s t a n d a r d f u U y i m p l a n t e d D I M E S S O G bipolax technology is n o t suitable f o r SiGe H B T processes. I n f a c t t h e reqithed l i i g h processing t e m p e r a t m e wiU cause t h e o u t -cliffusion o f t h e boron peak i n t h e t l i i n i n - s i t u doped e p i t a x i a l SiGe base layer a n d t h e consequent d e g r a d a t i o n o f t h e device performance. Therefore, another issue t h a t needed t o be t a c k l e d was t h e development of l o w - t e m p e r a t m e n - t y p e e m i t t e r modides. I n C h a p t e r 4 t h e D C a n d A C chai-acteristics of 60 G H z fx SiGe H B T ' s f a b r i c a t e d i n b i ü k - S i technology vidth A s e m i t t e r s made b y i m p l a n t a t i o n a n d laser anneaUng a c t i v a t i o n o f t h e d o p a n t ai-e presented. I t is s h o w n t h a t t h e I ¬ V chai-acteristics of these devices are n o t a f f e c t e d b y t h e d e t r i m e n t a l effects related t o t h e T E D of t h e b o r o n spike i n t h e SiGe base. However, t h e device performance stUl suffers f r o m residual i m p l a n t a t i o n - i n d u c e d damage w h i c h is n o t r e m o v e d b y t h e laser anneaUng. • I n C h a p t e r 5 a s t r a i g h t f o r w a r d test s t r u c t m e is i n t r o d u c e d w h i c h aUows f o r v e r i f i c a t i o n of t h e a c t u a l c o u n t e r - d o p i n g of t h e Si sm-face d m i n g t h e f a b r i c a t i o n of a n ultrashaUow j u n c t i o n . T w o d i f f e r e n t n - t y p e e m i t t e r s , where t h e f o r m a t i o n of t h e n+ r e g i o n is based o n C V D processes at t e m p e r a t m e s n o t higher t h a n 7 0 0 ° C , are characterized b y means o f simple D C measm-ements. A n i n t e r e s t i n g development of t h i s i n v e s t i g a t i o n is t h e e x p e r i m e n t a l s t u d y of t h e electrical behavior of idtrashaUow j u n c t i o n s where t h e d e p t h a n d d o p i n g o f t h e counter-doped r e g i o n at t h e smface ai-e such t h a t t h i s r e g i o n is f t d l y depleted at t h e r m o d y n a m i c e q u i U b r i m i i . T h i s i n t e r m e d i a t e case between t h e c o n v e n t i o n a l S c h o t t k y - a n d p n - j u n c t i o n s has been so f a r o n l y i n v e s t i g a t e d b y a n a l y t i c a l models a n d n u m e r i c a l simulations [32].

• I n C h a p t e r 6 t h e z e r o i m p l a n t a t i o n lowtemperatme t w o -sided c o n t a c t e d Schottky-coUector S O G SiGe H B T t e c h n o l o g y is demonstrated. I n such a process t h e emitter-to-coUector distance is a r o u n d 440 r m i , less t h a n h a l f o f t h e same distance i n t h e o r i g i n a l D I M E S S O G bipolar process. T h e coUector r e g i o n tliickness is 350 n m a n d is m a i n l y d e f i n e d hy t h e thickness of t h e Si t o p layer of t h e i n i t i a l S O I wafer. F o r

terahertz-fx performance a coUector w i d t h of about 50 n m is reqiih-ed. A c h i e v i n g t i n s tliickness w i t h t h e proposed technology w o i d d o n l y be U m i t e d b y t h e avaUabiUty of S O I wafers w i t h a high-cjuaUty siUcon t o p layer o f t h i s thickness. C h a p t e r 7 coUects aU t h e conclusions a n d recommendations f o r f m t h e r developments.

1-D n u m e r i c a l simulations

2.1. Introduction

I n t h i s C h a p t e r t h e electrical p e r f o r m a n c e of SiGe n-tj^pe b i p o l a r transistors w i t h p - S c h o t t k y coUector c o n t a c t ( N P S ) are investigated b y means of 1-D m m i e r i c a l simulations. Several N P S devices ai-e o b t a i n e d b y v a r y i n g coUector parameters, such as t h e thickness a n d t h e d o p i n g o f coUector r e g i o n a n d t h e S c h o t t k y barrier height at t h e contact. I t is s h o w n h o w these technology-related parameters influence t h e D C characteristics and t h e c u t - o f f frequency f j . Moreover, the d i f f e r e n t c o n t r i b u t i o n s t o t h e t o t a l emitter-to-coUector t r a n s i t t i m e are s h o w n t l n o u g h t h e d i s t r i b u t i o n o f t h e i n t e r n a l delay times r e s r ü t i n g f r o m t h e t r a n s i t t i m e analysis. FinaUy, t h e performance i n t e r m s of c u t - o f f frequency a n d open-base brealcdown voltage p r o d u c t , f-rXEVcEO) o f t h e c o n v e n t i o n a l SiGe N P N devices w i t h ohmic coUector contacts a n d t h e S c h o t t k y coUector transistors are compared.

T h e s i m i d a t i o n f r a m e w o r k is i n t r o d u c e d i n Section 2.2. T h e simrdated D C a n d A C characteristics o f d i f f e r e n t N P S devices are discussed i n Sections 2.3 and 2.4, respectively. Section 2.5 shows t h e results o f t h e t r a n s i t t i m e analysis. T h e comparison o f t h e high-fi-equency N P S t r a n s i s t o r t o t h e c o n v e n t i o n a l SiGe N P N device is presented i n Sections 2.6. T h e m a i n conclusions ai-e t h e n coUected i n Section 2.7.

SiGe lieteroj miction bipolar transistors w i t h Schottky collector contacts

2.2. Simulation framework

A l l t h e s i m i ü a t i o n s are p e r f o r m e d w i t h M E D I C I s i m t ü a t o r (version W - 2 0 0 4 . 0 9 ) . T h e adopted models a n d values of m a t e r i a l parameters axe r e p o r t e d i n a p p e n d i x A . N o t e t h a t t h e non-local m o d e l self-consistently i n c l u d e d i n t h e solutions has been chosen f o r t h e i m p a c t i o n i z a t i o n cm-rent. N o seU-heating a n d b a l l i s t i c t r a n s p o r t models have been considered i n t h e s i m i ü a t i o n s .

Figm-e 2.1 shows t h e d o p i n g c o n c e n t r a t i o n a n d thickness o f t h e device regions f o r t h e N P S transistors. A U t h e s i m t ü a t e d devices have t h e same e m i t t e r tlnckness (dE=30 n m ) a n d d o p i n g ( N E = 5 X 1 0 ^ ° c i n ^ ) . T h e Si a n d SiGe base thickness a n d p - t y p e d o p i n g a p p r o x i m a t e t h e S I M S measm-ements i n F i g m e 2.2, w i n c h refers t o t h e f a b r i c a t e d N P S SiGe t r a n s i s t o r presented i n C h a p t e r 6 ( N g i B ^ l x l O ^ ^ c n i ^ , d s i 3 = 2 5 m n , NsiGe,B=4.3xlO'^ c n i ^ , dsiGe,B=8 n m ) . Also, t h e Ge box-Uke p r o f i l e a n d m o l e fi-action IUSQ^ have been d e f i n e d i n accordance t o t h e same S I M S measm-ements.

Several S c h o t t k y coUector designs are i m p l e m e n t e d b y v a r j d n g t h e p -t y p e d o p i n g of -t h e l o w l y doped coUec-tor ( N L d , c = [ l x l O ' ' , I x l O ' ' , I x l O ' ^ IxlO^**] cm"^) a n d i t s thickness (dLd,c=[350,150,60] n m ) . A l s o t w o values of t h e S c h o t t k y b a r r i e r height at t h e coUector contact have been considered ((I)p=[0.55,l] e V ) . N o t e t h a t these tln-ee technolog-j^-related parameters can be s t r a i g h t f o r w a r d l y t i m e d i n t h e S O G b i p o l a r process developed i n D I M E S .

O ro 1— c (ij o e O Ü cn Q. O XI

n+

Schottky contact 0,20 | 0 1 5 § O ro 0,10 ^ O E CD 0,05 (T)

distance

F i g u r e 2.1 Doping and Ge profiles of the simulated NPS transistors. The Ge percentage in the p+ base is 20% (mf^,^=0.2). Np;=5xl0''' cm'', d,.=30 nm, Na,u=lxlO'^ cm-', dsi,Q=25 nm, Nsi,„,,=4.3xl0'" cnr', d„a^„=8 imi, N,,,,c=[lxlO''', 1x10'^ 1x10'^ 1x10'°] cnr', dL„c=[350,150,60] nm, cD|=[0.55,l] eV.

F i g u r e 2.2 SIMS measurement of the B doping throughout the epi layer and the Ge profile i n the base region.

SiGe lieterojunction bipolar transistors with Scliottlcy collector contacts

2.3. Electrical characteristics of S i G e N P S

transistors in D C mode

Several s i m u l a t i o n s have been p e r f o r m e d to evaluate the influence of the collector d o p i n g NL,I,C, the collector thickness dL,i,c and the S c h o t t k y barrier height at the collector contact O,, on the D C electrical characteristics of the NPS transistors.

T h e G u m m e l plots i n f o r w a r d mode at V C B = 0 f o r N P S devices w i t h = l eV and d i f f e r e n t values of NLH.C and dL,,,c are shown i n F i g u r e 2.3. C o m m o n e m i t t e r o u t p u t characteristics i n base-current-controlled mode are shown i n F i g u r e 2.4 ( a ) , ( b ) for N P S devices w i t h d i f f e r e n t O,,, NLd.c and

dLd.c-As can be seen, f o r a 6 0 n m t h i n collector region the electrical characteristics show a, negligible dependence on the collector d o p i n g , irrespectively of the S c h o t t k y barrier height at the contact (see Figure 2 . 4 ( b ) ) . O n the other h a n d , for devices w i t h dLd,c=350 n m , the collector c u r r e n t reduces as NLH.C increases. T h e S c h o t t k y barrier height at the contact influences p r i m a r i l y the c u r r e n t gain, the b r e a k d o w n voltage BVcEo and almost l i n e a r l y the offset voltage, V„ffset, w h i c h is the collector-e m i t t collector-e r voltagcollector-e VcE at w h i c h I ^ is zcollector-ero for a forccollector-ed IQ i n c o m m o n - collector-e m i t t collector-e r c o n f i g u r a t i o n . N,^ =1x10''cm"'' N,^^=1x10 ' c m ^ N,^^=1x10''cm"'^ Ld.C N,,^=1x10'*'cm ' L d , C 1,0

F i g u r e 2.3 Simulated Gummel plots in forward mode at ¥ ^ ^ = 0 for different collector thickness d,,|,cand doping N,,,,cof NFS devices with 0 , = 1 eV,

2.5x10' 2.0x10" 15x10" " 1.0x10 o 5.0x10"-tOp=1 eV I' -0 ' ' ' ' I I ' ' ' ' I ' ' • d^^^=350 nm N^^^=1x10''cm"' I ' • ' • T • . . I . . . . r J|,=200riA/|.im" J^=100nA/).im'

•••'••''r'^v;'-'-'-

j|.=50n A/|.im /.:.:.ii•-l•.vVA^^"^".'l'!^"-'J•-'!^'vr^vlvv•ï'lVl'rJ^ •••l^:--'-'-- J =10nA/nm'' • • • (a) 0 0.5 1.0 1.5 2.0 2.5 3,0 3.5 4.0 4.5 5.0 2.5x10"' 2.0x10' 1.5x10' ^ l OxlO"' Ü 5,0x10"' -I—I—I—\—I—I—1—I— I-d,^^=60 nm ...N,^^=1x10' cm Ld C t---N^^^=1x10"cm"' 0 Ui -I—'—I—'—I—I—r Op= 0.55 eV-cpp=1 eV J^=200nA/um" J„=100nAynm' / I y J|.=50nA/^lm' / / ' / • ' Jg=10nA/nm'

::::::;!?;;'==-- I— ' —I—I—I—I—I—I—I — . — I — 1 I I I , I • I (b) 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 v . . [ V ]

F i g u r e 2.4 Simulated common-emitter output characteristics for different values of base current density for different collector doping N^,,,,;: and Schottky barrier height at collector contact O,, of NPS devices with (a) d,,,,c =350 nm and (b) d,,|,c =60 nm. Tire vertical lines are used to group the output characteristics of the device w i t h a given Oj,.

SiGe lieterojunction bipolar transistors w i t l i Scliottky collector contacts

I n view of the f o l l o w i n g i n v e s t i g a t i o n on the speed performance of N P S devices, and considering the w e l l - k n o w n t r a d e - o f f between fx and BVcEo (the Johnson l i m i t ) , i t is useful to plot B V C E O as f u n c t i o n of V B E (see F i g u r e 2 . 5 ( a ) , ( b ) ) . T h e value of B V C E O for a given VQE has been o b t a i n e d f r o m the collector-emitter voltage VCE at w h i c h J B = 0 . F r o m Figures 2 . 5 (a) and 2 . 5 (b) i t is clear t h a t increasing results i n a r e d u c t i o n of BVCEQ- Also, the dependence of B V C E O on the collector d o p i n g reduces as the collector region shrinks and i t becomes negligible for dL,i,c = 6 0 n m .

N o t e t h a t the B V C E O for dL,i,c = 3 5 0 n m shows a r e d u c t i o n a r o u n d V B E = 0 . 7 5 - 0 . 8 V . T h e devices w i t h duLc = 1 5 0 n m e x h i b i t the same behavior, a l t h o u g h less pronounced, at higher values of VQE. T h i s k i n k i n the B V C E O curves is unexpected since B V C E O is supposed to increase for those values of V B E due to the r e d u c t i o n of the current gain.

>

o 3 ijj ^ o

>

Ld.C d^^ =60 nm N,^^=1x10''cm"' N,, =1x10''cm L d , C • '

O =0.55 eV

p N^^ =1x10''cm"' N ' > 1 x 1 0 " c m - ' Ld,C 0.6 0.7 0.8 0.9 6 O -i LU O

>

I d,^^=350 nm L d . C ' I ' ' ' I I I

0

=^

eV

p V c = 1 5 0 nm d, ,.=60 nm L d . C I T -a - - - N , , ^ = 1 x 1 0 ' cm"' - - - N , , =1x10''cm"' N,^^=1x10"cm''' L d , C o -N,,^=1x10''cm"' 0.6 0.7 0.8 0.9 (b)

F i g u r e 2.5 Simulated emitter-collector breakdown voltage at open base, BVci^o, as function of "V,,,, for different collector thickness d,,|,cand doping N | , | c o f NPS devices with (a) (I),=0.55 eV and (b) cD|,=l eV. For a given V,,,,, BVCE,, is obtained as the value of "Vc;,.; at which Ju=0.

SiGe lieterojunction bipolar transistors w i t h Scliottky collector contacts

2.4. Cut-off frequency of S i G e N P S

transistors

For a given D C bias c o n d i t i o n , the f u n c t i o n i n g of a bipolar transistor is the result of a corresponding d i s t r i b u t i o n of charges across the device. These charges are f i x e d ions i n the emitter-base and collector-base depletion regions and free carriers, electrons and holes, t r a v e l l i n g t h r o u g h the device.

A time-variable small-signal p e r t u r b a t i o n on the D C base c u r r e n t or voltage w i l l produce an a m p l i f i e d p e r t u r b a t i o n on the o u t p u t D C collector current i f the m o d u l a t i o n t i m e of the charges stored i n the device is small enough t o f o l l o w the applied signal p e r t u r b a t i o n . T h e parameter used to q u a n t i f y this device performance is the c u t - o f f frequency, fx, w h i c h is defined as the frequency at w h i c h the modulus of the small-signal current gain, h j i , is one. For a given D C bias c o n d i t i o n , this parameter is the upper l i m i t of the frequencies of an i n p u t oscillating signal base current ii3(t) t h a t generates an a m p l i f i e d o u t p u t small-signal collector current i c ( t ) .

T h e m o d u l a t i o n t i m e of the i n t e r n a l charges depends on device parameters, such as d o p i n g and Ge profile, thickness of e m i t t e r , base and collector regions, and varies w i t h the biasing p o i n t . T h e fx e x h i b i t s i n f a c t a bell-shaped dependence on Jq. A t the low current regime the mobile charge is relatively low and the speed-limiting factor is expected t o be the charge v a r i a t i o n of the fixed ions due to the m o d u l a t i o n of the boundaries of the space charge regions ( S C R ) . T h i s effect reduces as the collector c u r r e n t increases, when the m o d u l a t i o n of the free carrier d i s t r i b u t i o n s dominates the i n t e r n a l delays. A t the h i g h current regime, the fx performance degrades p r i m a r i l y due to high i n j e c t i o n effects i n the collector region ( K i r k effect and base push-out).

A C small-signal simulations have been performed i n order to evaluate the c u t - o f f frequency of the investigated N P S devices. For each D C s o l u t i o n , the A C s i m u l a t i o n calculates the Y parameters for a sinusoidal p e r t u r b a t i o n w i t h a chosen frequency and a m p l i t u d e applied to the base voltage. T h e fx value is t h e n determined by s t r i c t l y a p p l y i n g the d e f i n i t i o n w i t h o u t any e x t r a p o l a t i o n (see Figure 2 . 6 ) . However, note t h a t the h j i curves show the expected theoretical decay of 2 0 d B / d e c a d e .

T h e results are shown i n Figures 2 . 7 for V C ; B = 0 as f u n c t i o n of the collector current density JQ i n the case of NPS device w i t h (a) O j , = 0 . 5 5 eV and (b) %=1 eV.

:

NPS

r N, =lxlo'° cm' cl.p=1 eV 10 V_|^=0 ,7 V

10

I

1 10 10' 10' 10' 10' 10' 10' 10° i o ' i o ' ° i o " i o

freq [Hz]

12

F i g u r e 2.6 Simulated modulus of the smaU-signal current gain h^, versus oscillation frequency for different values of Vu,, for NPS device w i t h N , , , ( . = l x l 0 ' " cm-', d|,,,c=60 nm, $,,=1 eV. The cut-off frequency f^ is the frequency at whichjh^i = 1 .

I t is i n s t r u c t i v e t o analyze the fx curves i n c o m b i n a t i o n w i t h the B V C E O plots presented i n the previous Section. F i r s t , the r e d u c t i o n of the collector thickness dL,i,c results i n a higher f^ while the B V C E O is reduced. T h i s is expected since the t h i n n e r the collector region the shorter the distance the electrons i n j e c t e d i n t o the collector need to t r a v e l i n order to reach the contact. O n the other hand the electric field i n the collector depletion region becomes higher as dL.ic shrinks, and hence the r e d u c t i o n

of B V C E O .

I n the low current regime the collector parameters have a negligible effect on fV. T h i s indicates t h a t for the investigated N P S the m a i n c o n t r i b u t i o n to the i n t e r n a l delay i n this current regime comes f r o m the emitter-base capacitance. T h e n , for a given dLd.c, increasing O,, at the collector contact causes a r e d u c t i o n of B V C E O and an increase i n f^.

SiGe lieterojunction bipolar transistors w i t h Schottky collector contacts N I

10'

10'

IO--10^

O =0.55 eV

p

10"

dLd,c=60 nm - - - N , , ^ = 1 x 1 0 cm L d . C N,^^^=1x10^^ cm"' N , , ^=1x10''cm"' Ld C i i i i i i I I i i i n l I 1 i i l l ! 10"-

10"'

10"'

10

-3

10-'

10"

(b)

10

^ 10' N X

10"

10

I I r I M i l ] r

V =0

08

O =1 eV

p

11 1—I I 11 r r i | I — I I r 1 i i i TIIT| d _ = 6 0 n m ^ dLa,c=150nm d,,^=350 nm L d . C . . . N , , , = 1 x 1 0 " c m ' ' N , , , = 1 x 1 0 ' ' cm"' L d . C Ayl -1 L d . C l e T . - . N , , ^ = 1 x 1 0 ' ' c m - . -N , , ^ = 1 x 1 0 ' ' c m " ' L d . C L d . C l l 1 I I . I I I m i l l l

10"' 10"^ 10"' 10"' 10"'

[A/iirn^]

10"'

10"

F i g u r e 2.7 Simulated cut-off freciuency f j as function of collector current density J,;; at V(;B=0 for different collector thickness and doping of NPS devices with (a) %=0.55 eV and (b) cD^, = 1 eV.

T h e influence of coUector d o p i n g o n f^- varies w i t h t h e coUector thickness. F o r devices w i t h dLd,c=60 n m i t is negUgible b u t i n devices w i t h a 350 n m t h i c k coUector, r e d u c i n g t h e Nta.c residts i n a n increase o f peak i j , w h i c h is imexpected since at t h e same t i m e also B V C E O increases. T h e e x p l a n a t i o n o f t l i i s effect c o i d d be t h e presence i n these devices o f a n i m d e p l e t e d pai-t o f l o w l y doped p - t y p e coUector at t h e base side, w h i c h gives a n a d d i t i o n a l i n t e r n a l delay due t o t h e chaige stored i n t o i t . W h e n t h e coUector d o p i n g reduces, t h i s p a r t becomes t h i n n e r . O n t h e other h a n d , t l i i s causes a r e d u c t i o n of t h e electric f i e l d peak i n t h e depleted coUector a n d hence t h e increase i n B V C E O - T h i s effect is also present b u t less significant i n devices w i t h d ^ c = 1 5 0 n m , w i n c h malces sense since t h e i m d e p l e t e d collector region w o u l d be smaUer f o r such NPS's.

T o have a n idea h o w t h e f^ can be increased b y increasing V C B i n case of large coUector thickness, t h e f j c m v e at V C B = 3 V f o r a device w i t h NLd,c=1^10'' cm-3, dLd,c=350 n m , %=0.55 eV is shown i n F i g m e 2.8.

F i g u r e 2.8 Simulated cut-off freciuency f^ as fimction of coUector cm-rent density for a NPS device w i t h NL<I,C=1X10^^ cm-3, dL,i,c=350 mn, Op=0.55 eV at VcB=0 and VCB=3 V .

SiGe heteroj miction bipolar transistors w i t h Scliottkj' coUector contacts

2.5. Transit time analysis

T h e cliargecontrol t h e o r y states t h a t , under t l i e c o n d i t i o n of a l o w -fi-ec(uenc3' cm-rent g a i n m u c h larger t h a n 1, t h e c u t - o f f fi-eciuency fx c a n be expressed i n t e r m s of t h e t o t a l emitter-to-coUector signal-delay t i m e

TEC as [33]:

/ r = : r ^ = (2.1)

where AQx is t h e v a r i a t i o n o f t h e t o t a l chai-ge per u n i t area i n t h e w h o l e b i p o l a r t r a n s i s t o r r e s u l t i n g fi-om t h e p e r t m - b a t i o n AJc of t h e coUector cmrent density f o r constant e m i t t e r c o l l e c t o r voltage ( d V c E = 0 )

-T h e t e r m AQx c a n be expressed as t h e s u m o f t h e c o n t r i b u t i o n s fi-om a l l device quasi-neutral a n d space charge regions:

A a = X ^ e , „ „ + E ^ ö . c « , A . (2-2)

I n a q u a s i - n e u t r a l region:

An(x) = Ap{x)

V x (2.3)

where x is t h e p o s i t i o n c o o r d i n a t e along t h e axis fi-om e m i t t e r t o coUector, An(A-) a n d A p ( x ) ai-e t h e vai-iation of t h e e l e c t r o n a n d hole concentrations i n p o s i t i o n x, respectively, caused b y A J Q .

U s i n g charge conservation, t h e foUo-wing r e l a t i o n c a n be d e r i v e d f o r t h e space chai-ge region:

.Y" .V"

^QscR = 9 j M^)dx = qj Ap(x)dx (2.4)

.Y' .Y'

where x' a n d . Y " are t h e boundaries of t h e space charge r e g i o n a n d q is t h e electron charge.

T h e coordinates x' a n d x" are bias dependent. B y p l o t t i n g f o r each biasing c o n d i t i o n A n ( x ) a n d A p ( x ) , t h e boundaries o f a space charge r e g i o n c a n be i d e n t i f i e d b y considering t h a t t h e var-iation of t h e m a j o r i t y

carrier concentration is m u c h larger t h a n the v a r i a t i o n of m i n o r i t y carriers i n the v i c i n i t y of the edges, t h a t is:

A / 7 ( x ) » A/7(x) or A / ? ( x ) » A t t ( x ) ( 2 . 5 )

Based on equations ( 2 . 3 ) and ( 2 . 4 ) , the regional p a r t i t i o n of the device can be done for each D C bias b y means of numerical simulations e x p l o i t i n g the f o l l o w i n g a l g o r i t h m [ 3 4 ] :

a p o i n t X belongs to a quasi-neutral region i f b o t h the conditions

.V .V

A r t ( x ) = A/7(x) and ÏAn{z)dz = \Ap(z)dz ( 2 . 6 )

0 0

are verified, where A-=0 is the position of the e m i t t e r contact; otherwise x is a p o i n t of the space charge region. N u m e r i c a l l y the conditions ( 2 . 6 ) are i m p l e m e n t e d by imposing t h a t the absolute value of the difference of the two terms of each equation is smaller t h a n a chosen a r b i t r a r i l y - l o w threshold. T h i s method provides an unambiguous regional p a r t i t i o n of the device by i d e n t i f y i n g for each biasing c o n d i t i o n the 4 parameters XE, '•^'D.E! .-'^'D.C) -YC as exemplified i n F i g u r e 2 . 9 for a SiGe N P N transistor at

peak-fx for V c n = 0 . T h e i n t r i n s i c d o p i n g profile of such device is shown i n F i g u r e 2.10.

SiGe lieterojunction bipolar transistors with Scliottky collector contacts 3x10 2x10 c o ro 1x10 != a) o c o O -1x10 0 10 0 1§^.^ d i s t a n c e [|,im] 0,20

F i g u r e 2.9 Regional partition of SiGe N P N transistor for V|,|i=0.81 V and VcB=0. Tlie doping profile is shown in Figure 2.10.

10'

f

1 0

-I

10'^ ^ 10'^ § 1 0 " -g 1 0 " 10 - I 1 I I f—

n+

. 1 I I 1_ ohmic contact

n+

0,05 0,10 0,15

distance

[)im]

0.20 0,15

I

O ro 4= 0.10 o E (U 0,05 CD 0 0,20

F i g u r e 2.10 Doping profile of the simulated SiGe N P N referred to in Figure 2.9, The Ge percentage in the p+ base is 20% (nifc„=0.2).

G i v e n such a device p a r t i t i o n , i t is s t r a i g h t f o r w a r d t o calcidate t h e regional delaj^ times, also called "transit times". T h e d e l a j ' t i m e associated w i t h a quasi-neutral region is caused b y t h e m o d i d a t i o n of t h e n e u t r a l charge stored i n t l i i s region due t o the cm-rent p e r t m b a t i o n A J Q . Instead, w i t l r i n a space charge region we need t o talce i n t o consideration t w o types of charges. Free carriers traveUing across t h e space charge region create a compensated and uncompensated charge. T h e m o d i d a t i o n o f t h e l a t t e r is related t o t h e charging a n d discharging o f t h e j u n c t i o n d e p l e t i o n capacitance.

T h e t r a n s i t t i m e s associated t o t h e d i f f e r e n t types o f charge can be calcidated as foUows:

t r a n s i t t i m e i n c|iiasi-neiitral e m i t t e r region:

•V, _£

C

0

(2.7)

t r a n s i t t i m e i n E B SCR due t o compensated charges:

(2.8)

t r a n s i t t i m e i n E B SCR due t o imcompensated charges:

X, B.E

T

EBJ

[Ap{x) - }7i{x)]dx

AJ.

' [An{x)-m(x)]clx (2.9)

t r a n s i t t i m e i n c^uasi n e u t r a l base region:

(2.10)

t r a n s i t t i m e i n C B SCR due t o compensated charges:

SiGe lieterojmiction bipolar transistors w i t h Schottky coUector contacts

t r a n s i t t i m e i n C B S C R due t o uncompensated charges:

.Vc .Vc

^Bcj

= T V

I

[ A « C A - ) - ' " W ] ^ * ^ A -

= -^ j

[Ap{x)-m{x)\clx

(2.12) , A J c /

where m ( . v ) = m i n { A n ( x ) , A p ( x ) } .

I n p r a c t i c a l cases t h e delay i n t h e quasi n e u t r a l coUector is negligible. T h i s means t h a t i n equations (2.11) a n d (2.12) t h e coordinate of t h e coUector contact L c a n be used instead of Xq. F i g m e 2.11 shows t h e t r a n s i t t i m e s f o r t h e simrdated SiGe N P N device at peak-fx f o r V C B = 0 . T h e figme also gives a n idea o n t h e r e l a t i v e c o n t r i b u t i o n f o r each device r e g i o n t o t h e t o t a l enritter-to-coUector delay t i m e .

N o t e t h a t t h e f^ d e t e r m i n e d fi-om t h e s i m r d a t e d h j i according t o t h e d e f i n i t i o n gives a v a l u e o f 100.6 G H z f o r t h e above SiGe N P N i n t h e considered biasing c o n d i t i o n ( V B E = 0 . 8 1 V a n d V C B = 0 ) , w h i c h translates ttn-ough ec[uation (2.1) i n t o a t o t a l emitter-to-coUector signal-delay t i m e TEC of 1-582 ps. T h e integi-ation o f t h e t r a n s i t t i m e d i s t r i b u t i o n s f o r electrons a n d holes as o b t a i n e d from t h e pei-tm-bation m e t h o d gives T E C , n = l - 5 5 9 ps a n d TEC.P =1-553 ps, respectively. T h e f a c t t h a t t h e values

of TECU a n d TEC.P are practicaUy t h e same a n d have a verj^ good c o r r e l a t i o n

w i t h TEC calcidated fi-om t h e simrdated h j i is an i n d i c a t i o n o f t h e consistency a n d correctness of t h e s i m i d a t i o n fi-amework used f o r t h e t r a n s i t t i m e analysis.

E 1 . 5 x 1 0 ' ° =1 o OJ c O 1.0x10" 3 § 5.0x10"" "w E 0 tn 2 -5.0x10"

NPN

T '

r-V^^=0.81V

V =0

0.05 0.10 0.15

distance [|am]

0.20

F i g u r e 2.11 Transit time distribution and associated regional delay times for the simulated SiGe NPN at peak-f,. for VcLi=0. The inset zooms on the emitter region.

2 . 5 . 1 . T r a n s i t t i m e d i s t r i b u t i o n i n S i G e N P S t r a n s i s t o r s

T h e above t r a n s i t t i m e analysis is e x p l o i t e d i n this Section t o provide an insight i n t o the mechanism of charge storage for NPS devices. T h e r e s u l t i n g i n f o r m a t i o n on the local c o n t r i b u t i o n to the t o t a l e m i t t e r t o -collector t r a n s i t t i m e allows the i d e n t i f i c a t i o n of the H m i t i n g factors for the s i m u l a t e d devices w i t h S c h o t t k y collector designs.

Figures 2.12 (a) and 2.12 (b) show the t r a n s i t t i m e d i s t r i b u t i o n for electrons (black hne) and holes (red line) at l o w - c u r r e n t regime and at peak-fx, respectively, f o r the N P S device w i t h NLrt,c=lxlO"' c r n ^ dLd,c=60 n m , ( D , = l eV. N o t e t h a t f o r this device the t o t a l t r a n s i t t i m e of the holes is very close t o TEC calculated f r o m the s i m u l a t e d h2i, while the electrons t r a n s i t t i m e has a discrepancy of about 15%. M o r e specifically, at the low c u r r e n t regime TEC,„=9.27 ps and TEC,„=10.86 ps w h i l e TEC=11.05 ps. A t peak fx, TEc,„=h82 ps and TEC,P=1.59 ps against a calculated igc of 1.60 ps.

SiGe lieterojunction bipolar transistors w i t h Schottl<:y collector contacts -5.0x10 (a) 0.05 0.10 distance [\.im] 0.15 E 1.5x10-''' O ^ 1 0 x 1 0 ' " Ö ; § 5.0X10" h E 0 2 -5.0X10'"

Jp=8.6 xio '' A/'[.inn

(b)

NPS

N|^,i,_=1x10"' c m ' d ,^=60 nm Ld.C <]\=1 eV 0.05 0.10

distance [|.im]

0.15

F i g u r e 2.12 Transit time distribution for electrons (black line) and holes (red line) at low-current regime (a) and peak-f^ (b) for N P S device w i t h NL,,,C-=1-^10"' cm-'', d,,,,,c:=60 nm, (I)„=l eV.

Figure 2.12 (a) c o n f i r m s t h a t at the low current regime the i n t e r n a l delays are m a i n l y related to charge m o d u l a t i o n i n the base e m i t t e r j u n c t i o n , w h i c h is expected since i n this regime there is no influence of the collector parameters on the f j curves. A t peak-f^ (see F i g u r e 2.12 (b)), the i n t e r n a l delays i n t r o d u c e d by the charge m o d u l a t i o n i n the E - B and C - B space charge regions are comparable and d o m i n a t i n g over the base t r a n s i t t i m e .

T h e t r a n s i t t i m e analysis is also useful to c o n f i r m the e x p l a n a t i o n given i n Section 2.4 of the simultaneous increase of I'T and B V C E O for N P S device w i t h t h i c k collector when the collector d o p i n g decreases. F i g u r e 2.13 shows indeed t h a t i n the case of devices w i t h dL,i,c=350 n m and (I),,= l eV, a larger base t r a n s i t t i m e and an a d d i t i o n a l delay i n the collector region close to the base is observed f o r a comparable Jq and higher NL,,,C- Note t h a t f o r these devices the discrepancy between the T^ECu! TECP and TEC f r o m the h^i is w i t h i n 7%.

1.5x10'' O 1.0x10' 'J •9 'i— ^ 5.0x10 TD E c 0.1 0.2 0.3

distance [jiim]

F i g u r e 2.13 Transit time distribution for electrons (black lines) and holes (red lines) for NPS device w i t h d|,,c=350 nm and Op=l eV. The inset zooms on the emitter-base region.

SiGe lieterojunction bipolar transistors w i t l i Schottky collector contacts

2.6. Comparison between S i G e N P S and

N P N transistors

T h e f u n c t i o n i n g of the bipolar j u n c t i o n transistor is based on the f u n d a m e n t a l p r i n c i p l e t h a t the m i n o r i t y carriers i n j e c t e d i n t o the base f r o m the e m i t t e r d r i f t to the collector contact b y an electric f i e l d . I n c o n v e n t i o n a l Si(Ge) N P N transistor this is achieved by f o r m i n g a space charge region t h r o u g h the difference i n d o p i n g between the base and collector regions. Instead, i n the investigated N P S device such electric f i e l d is created by the depletion region o f the S c h o t t k y j u n c t i o n at collector contact. T h e question arises t h e n , how this new manner o f i m p l e m e n t i n g the bipolar j u n c t i o n transistor influences the device performance i n terms of f'x and

BVCEO-T o answer t h i s question, the f^ and B V C E O curves o f the N P S devices are compared w i t h t h a t of a c o n v e n t i o n a l SiGe N P N w i t h n - t y p e o h m i c c o n t a c t e d collector w i t h the same e m i t t e r and base regions. T h e collector region o f the N P N device consists of a n - t y p e l o w l y doped layer (NLd,c=1^10'" cin^) a n d a h i g h l y doped layer (NHH,C=1X10'''' cm"') at the contact side. T w o thicknesses for each o f such layers have been considered, specifically dL,i,c= [60,75] n m and dH,i,c= [50,20] n m . T h e d o p i n g p r o f i l e of the SiGe N P N devices is depicted i n F i g u r e 2.14. T h e comparison w i t h the N P S transistor i n terms o f f^ a n d B V C E O is shown i n F i g u r e 2.15.

0.05 0.10 0.15

d i s t a n c e [).tm]

F i g u r e 2.14 Doping profile of the simulated SiGe N P N devices. Tire thicknesses of the lowly and higWy doped collector regions are d,,,e=[60,75] nm and dH.i,c=[50,20] nm, respectively. The Ge percentage in the p+ base is 20% (mfc„=0.2). N I 10' 10' 10' 10° 10'

—r I iiinij—1 rTTin^—r i imm—i i iim^

/

1 iMimi 1 « '•\ W ' > / / r / _/ / : / ^ 1-5 \A

/

/

f 1.3 / / i 06 07 oa 03

:

NPS

...-d^,j^=60 nm. il>p=i eV

NPN

d ^ „ ^ = 6 0 n m , d „ ^ , = 5 0 n m .... d. .=60 nm, d.. . =20 nm .... d, ,.=75 nm, d„ .=50 nm J J J J 3'' 10'" 10"' 10"' 10 ' 10"

Jc [A

/|4m1

F i g u r e 2.15 Simulated fj- and (inset) BVCKO curves of NPN devices w i t h di,,i,c=[60,75j nm and dH,,,c=[50,20] nm as compared to those of a NPS transistor w i t h N _L<1,C =1x10'" cnr-', d,,,c=60 nm, 0 = 1 eV.

SiGe lieterojunction bipolar transistors with Scliottky collector contacts

I t can be seen t h a t the use of c o n v e n t i o n a l ohmic contact in the investigate devices does not i m p r o v e the f^ b u t s l i g h t l y reduces the BVcEOi independently of the thickness of the h i g h l y doped collector region. T h e same B V C E O as the N P S device is achieved for the N P N by increasing the thickness of the l o w l y doped collector by 25%, w h i c h results i n a r e d u c t i o n of the peak-f^ of about 10%. Therefore, i t can be concluded that the S c h o t t k y collector design seems t o p e r f o r m s l i g h t l y better t h a n the conventional ohmic contacted collector i n terms of fx and B V C E O p r o d u c t .

2.6.1. C o m p a r i s o n b e t w e e n S i G e N P S a n d N P N S

t r a n s i s t o r s

Previously i t has been shown t h a t for N P S transistors the t h i n n e r the collector region the lower the influence of the collector d o p i n g level on f j and BVcEo- For dL,i,c=60 n m b o t h f^ and B V C E O are i n practice independent of the collector d o p i n g (see Figures 2.5 and 2.7). For the sake of completeness, the influence of the type of collector d o p i n g on f^ and B V C E O has also been investigated for these t h i n collector N P S transistors. A new device ( N P N S ) has been s i m u l a t e d where the 60 n m l o w l y doped collector region is n-type (see Figure 2.16).

T h e results are shown i n F i g u r e 2.17, and indicate t h a t even the type of the collector d o p i n g has a negligible effect on f^ and B V C E O for S c h o t t k y contacted devices w i t h a t h i n collector region.

10-- 10-- 1 0 ^ f 10^'

I

10" ^ 10'

8

10" cn % 10" 10 • 1 • mf' ' • • i n +

h

1 1

h

1 1 i Schottky p i p-^ 1 contact ! 1 1 I . I " 1 1 . i . 1 1 0 0.05 0.10 0.15 0.20 0,15 § 0.10 ^ O E 0.05 d i s t a n c e [(.im]

F i g u r e 2.16 Doping profüe of the simulated SiGe NPNS device. The collector parameters are Nj.ic—lxlO'" cm-'' (n-type), d,

percentage in the p+ base is 20% (mfc,,=0.2).

=60 nm, (D,=l eV. The Ge 10' 10 N I 10 10" 10° ^ 10' r 1 r i i m j — r n r m q — j i m i i ^ i i i i i n ^ -r

V =0

: CB 1 i i i i i i ^ 1,8 \ 1 7 / / / / / / \ 1 0 / r / i S • s / / / 1,4 / / / 1.3 / : / / / / [Vj

; NPS

NPNS

! . . . d , =60 nm. <|) =1 eV L d C •* d,^ =60 n m, t l J =1 eV L d . C r 10-' 10"' IQ-' 10'' 10'' 10'' [ A/ | i m 1

F i g u r e 2.17 Simulated fp and (inset) BV^po curves of the NPNS device as compared to those of the NPS transistor with the same collector parameters (N,,„f,=lxl0"' cm-', du_(.=60 nm, ( ^ = \ eV).

SiGe lieterojunction bipolar transistors w i t h Scliottlty collector contacts

2.7. Conclusions

T h e D C and A C performance of n-type bipolar transistors w i t h S c h o t t k y collector contacts has been investigated w i t h a special focus on the influence of the collector design on the f - p X E V c E o p r o d u c t . I n such devices the electric field w h i c h collects the m i n o r i t y carriers i n j e c t e d i n t o the base f r o m the e m i t t e r is created by the depletion region induced by the S c h o t t k y barrier at the collector contact instead of the conventional base-collector p n j u n c t i o n . A s expected, the t h i n n e r the collector region the higher the f^. T h e thinnest collector for the s i m u l a t e d devices is 60 n m . Such devices e x h i b i t fx and B V C E O independent of the t y p e and value of the collector d o p i n g . O n the other hand, f j increases w i t h the S c h o t t k y barrier height at the contact O^. T h e l a t t e r has a strong i m p a c t on the offset voltage V„ff„,t as well. T h e lower the O,, the larger the V.ffe^. F o r the s i m u l a t e d devices, the t r a n s i t t i m e analysis shows t h a t the base t r a n s i t t i m e is negligible at peak-f^ as compared t o the delay times associated t o the emitter-base and collector space charge regions. A t the low c u r r e n t regime, the transit t i m e is d o m i n a t e d by the i n t e r n a l delay due to the charge stored i n the emitter-base j u n c t i o n .

S c h o t t k y collector devices w i t h t h i n collectors have been compared w i t h c o n v e n t i o n a l SiGe N P N transistors where the collector is contacted t h r o u g h a h i g h l y doped layer. T h e base and e m i t t e r regions are the same i n a l l devices. T h e s i m u l a t i o n results show t h a t the use of the S c h o t t k y collector does not reduce the device performance i n terms of fx and

B V C E O

-I n conclusion, the S c h o t t k y collector design simplifies the whole collector module to j u s t a m e t a l deposition on a l o w l y doped siUcon region and does not degrade the device speed. Technologically this means t h a t due t o the low processing temperature required the collector module could be executed d u r i n g the back-end-of-line ( B E O L ) process. T h i s makes i t a n a t u r a l candidate for the collector module i n the D I M E S SOG b i p o l a r technology when a i m i n g t o the f a b r i c a t i o n of high frequency transistors w i t h reduced v e r t i c a l dimensions.

Silicon=On=Glass v e r t i c a l

B J T ' s w i t h S c h o t t k y

collector contacts

3.1. Introduction

I n t h e D I M E S SiUcon-On-Glciss ( S O G ) bipolar process l o w - o h m i c coUector contacts are f a b r i c a t e d d n e c t l y u n d e r n e a t h t h e enritter regions a f t e r t h e on-giass transfer o f t h e S O I wafer a n d t h e Si substrate r e m o v a l . Such back-wafer contacts are f o r m e d hy liigh-dose l o w - e n e r g j ' i m p l a n t a t i o n s i n t h e contact w i n d o w s opened i n t o t h e exposed b m i e d oxide ( B O X ) f o l l o w e d b y high-energj^ excimer laser anneaUng ( E L A ) t o a c t i v a t e t h e dopants [35]. I n t h i s i n n o v a t i v e low-parasitic collector design, t h e mechanical stabiUty of t h e adhesive b o n d i n g layer malces i t impossible t o use a high-temperatm-e t h e r m a l process t o anneal t h e i m p l a n t a t i o n - i n d u c e d damage resrdting fi-om t h e back-wafer contact i m p l a n t a t i o n . Tlris damage consists of Si i n t e r s t i t i a l s , w l i i c h ai'e i n j e c t e d i n t o t h e silicon d m i n g t h e i m p l a n t a t i o n [36]. Since t h e i n t e r s t i t i a l s can easily d i f f u s e i n t h e siUcon, t h e y are n o t coirfined t o t h e a c t u a l i m p l a n t e d c o n t a c t region, a n d t h u s t h e y are n o t annealed b y t h e laser anneaUng, w h i c h o i d y recrystallizes a f e w tens of nanometers t h i n r e g i o n at t h e sm-face [37]. A previous w o r k o n S O G v a r a c t o r diodes has s h o w n t h a t i n these devices t h e arsenic-implanted back-wafer contact induces defects at least as f a r as 0.2 | r m away f r o m t h e c o n t a c t [38]. A comparable residt f o r B F 2 + i m p l a n t a t i o n has been r e p o r t e d i n [39] where t h e defect d i s t r i b u t i o n created hy t h e i m p l a n t a t i o n is analyzed b y p o s i t r o n a n n i l d l a t i o n D o p p l e r broadeiring. Moreover, i n t h e above case of S O G varactors, i t has been

SiGe heterojimction Ijipolar transistors w i t l i Schottky collector contacts

observed t l i a t t h e device b r e a k d o w n voltage decreases i f t h e d e p l e t i o n r e g i o n extends i n t o t h e defect region.

I n t h e fii'st p a r t of t l i i s C h a p t e r (Section 3.2), t h e r e U a b i l i t j ' o f S O G v e r t i c a l N P N ' s a n d P N P ' s is i n v e s t i g a t e d i n connection w i t h t h e collector c o n t a c t i n g m e t h o d . I n p a r t i c t d a r , a l t e r n a t i v e back-wafer contact designs are presented i n order t o reduce t h e Si l a t t i c e damage ( a n d t h e associated d e t r i m e n t a l effects) created i n t h e area s m r o t m d i n g t h e contact itself. T h e proposed coUector contacts are f o r m e d b y t U t e d i m p l a n t a t i o n of t h e dopants or b y using Schottkj^ back-wafer contacts, w h i c h c o m p l e t e ^ eUminate t h e i m p l a n t a t i o n .

I n t h e second pai't of t h e C h a p t e r (Section 3.3), t h e D C performance of v e r t i c a l P N P ' s w i t h d i f f e r e n t back-wafer collector contacts, i n c l u d i n g ohmic p"*" regions a n d p- or n - t y p e A l / S i ( l % ) S c h o t t k } ' j i m c t i o n s , is characterized. T h e i n f l u e n c e of t h e coUector design o n t h e offset voltage Voffset) w h i c h is t h e emitter-coUector voltage at zero coUector c m r e n t f o r a f o r c e d base c u r r e n t i n c o m m o n - e m i t t e r configm-ation, is i n v e s t i g a t e d as weU.

3.2. Base leakage and impact ionization

current in S O G vertical B J T ' s

3 . 2 . 1 . E x p e r i m e n t a l m a t e r i a l

T h e basic D I M E S S O G back-wafer contacted c o m p l e m e n t a r y bipolar' process is iUustrated i n F i g m e 3 . 1 . T h e .starting m a t e r i a l is a S O I wafer w i t h 0.34 p m i n t r i n s i c mono-crystaUine Si o n a 0.4 p m B O X laj^er. T h e t o p Si layer thickness is increased b y e p i t a x y t o 0.94 p m a n d t r e n c h e t c h i n g is used t o define islands electrically-isolated b y Si02. I n such siUcon islands t h e f u U y i m p l a n t e d transistors are f a b r i c a t e d . T h e f r o n t -wafer process ( F W P ) consists o f t h e i m p l a n t a t i o n s f o r t h e l o w l y doped coUector, t h e enritter and base regions, metaUization a n d d e f i n i t i o n of t h e base a n d e m i t t e r contacts. A 400 ° C aUoying step i n f o r m i n g gas Itydrogen-passivates aU oxide-silicon interfaces. T h e n , t h e f r o n t - w a f e r is covered w i t h 1 p m P E C V D oxide a n d glued t o a glass substrate. N e x t , t h e silicon b u l k is r e m o v e d b y selective wet etcliing using t h e B O X as etch-stop layer. I n t h e back-wafer process ( B W P ) t h e ohmic contacts t o t h e coUector are made b y i m p l a n t a t i o n of As^*- and BFj"*" f o r N P N ' s a n d P N P ' s , respectively, i n t o c o n t a c t w i n d o w s opened cUrectly below t h e e m i t t e r regions. T h e i m p l a n t a t i o n s are p e r f o r m e d at 5 k e V w i t h a dose of