HAL Id: jpa-00223149
https://hal.archives-ouvertes.fr/jpa-00223149
Submitted on 1 Jan 1983
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
PRODUCTION OF SMALL OHMIC CONTACTS BY LASER PROCESSING FOR Si AND GaAs DEVICES
H. Kräutle, M. Schröder, H. Beneking
To cite this version:
H. Kräutle, M. Schröder, H. Beneking. PRODUCTION OF SMALL OHMIC CONTACTS BY LASER PROCESSING FOR Si AND GaAs DEVICES. Journal de Physique Colloques, 1983, 44 (C5), pp.C5- 439-C5-443. �10.1051/jphyscol:1983564�. �jpa-00223149�
JOURNAL DE PHYSIQUE
Colloque C5, suppldment au nO1O, Tome 44, octobre 1983 page C5-439
PRODUCTION OF SMALL OHMIC CONTACTS BY LASER PROCESSING FOR Si AND GaAs DEVICES
H. K r a u t l e , M. SchrGder and H. Beneking
I n s t i t u t e of Semiconductor Electronics, Technical University, Aachen, F.R.G.
R6sum6 - Le r e c u i t l o c a l de contacts de p e t i t e dimension s u r une couche m e t GaAs a &ti! r e a l i s 6 a' l ' a i d e d'un l a s e r c o n t i n u f o c a l i s 6 (0=3 vm, 0,5W) sans a f f e c t e r l e s contacts Schottky s i t u g s quelques pm a' cot6 des parties i r r a d i g e s . Des contacts a' base de T i / P t s u r du s i li c i u m du type N o n t montri! une r e s i s t a n c e i n f e r i e u r e a' 10-6km2, des rgsistances de c o n t a c t du Ti/AuGe/P 1 . 5 . 1 0 - ~ Rcm J . s u r une couche de GaAs du type N s o n t de 1 'o r d r e de Les r 6 s u l t a t s s o n t comparables ceux obtenus pour des contacts r e c u i t p a r une msthode conventionelle.
A b s t r a c t - Small c o n t a c t p a t t e r n s on Si and GaAs have been a l l o y e d l o c a l - l y w i t h a focused cw l a s e r beam (B=3~m, 0,5W), w i t h o u t a f f e c t i n g Schottky contacts few m a p a r t t h e i r r a d i a t e d areas. Contact r s i s nces o f l e s s
5 5 9
than 10-6 Qcm f o r T i / P t contacts on n-Si and 1.5.10- a c m f o r Ti/.AuGe/,Pt on contacts on n-GaAs have been obtained. Conventional a l l o y e d contacts show s i m i l a r values.
Local h e a t i n g o f s u b s t r a t e surfaces can be obtained w i t h a focused e l e c t r o n o r l a s e r beam. This has been used f o r w e l d i n g o r the d r i l l i n g o f holes b u t can a l s o be used f o r the annealing o f implanted semiconductors /1/ o r a l l o y i n g metal contacts/2/ .The aim o f t h i s work i s t o show t h a t ohmic contacts w i t h low c o n t a c t r e s i s t a n c e can be formed near Schottky contacts on S i and GaAs substrates.
The temperature i n a s u b s t r a t e , heated w i t h a focused l a s e r beam, w i t h a gaussian po- wer d i s t r i b u t i o n , has been c a l c u l a t e d b y Lax /3/. Taking t h e n o n - l i n e a r thermal con-
d u c t i v i t y i n t o account /4/, t h e temperature g r a d i e n t i s even steeper and allows l o - c a l h e a t i n g o f patterns i n the um range w i t h o u t a f f e c t i n g areas some vm away.
A c t i v e areas about 0.3 vm t h i c k w i t h c a r r i e r concentrations o f 1017-1020cm-3 were produced by i o n i m p l a n t a t i o n . For r e s i s t i v i t y measureme t s , 4 c o n t a c t p a t t e r n s (shown i n F i g . 8a) were evaporated across the SOxZSO pmq implanted s t r i p e . F o r n-Si, a m e t a l l i z a t i o n o f 10 nm Ti + 50 nm P t and f o r n-GaAs, 10 nm T i + 30 nm AuGe + 50 nm P t was deposited. To evaluate the i n f l u e n c e o f t h e l a s e r beam d u r i n g annealing on the a d j a c e n t m e t a l l i z e d areas p a t t e r n s used f o r FET p r o d u c t i o n were evaporated on implanted areas o f 200x200 (see Fig. 7). I n t h i s s t r u c t u r e , o n l y the gate s t r i p e was i r r a d i a t e d . This a1 lows t o i n v e s t i g a t e the i n f l u e n c e o f the h e a t treatment on the r e c t i f y i n g behaviour o f the adjacent D and S areas.
For t h e annealing procedure, a cw l a s e r (Pmax = 1W) was used (see Fig.1). The power was c o n t r o l l e d w i t h photodiodes behind the m i r r o r s which had a transmission o f about
1%. The beam was focused onto the s u b s t r a t e using a conventional microscope objec- t i v e . I n a d d i t i o n , the s u r f a c e o f the s u b s t r a t e was i l l u m i n a t e d w i t h I R LEDs. The s t r u c t u r e s on the surface and the p o s i t i o n o f the l a s e r beam was monitored w i t h a TV camera (see Fig. 1 ) .
The TV camera allows o n l i n e c o n t r o l o f the i r r a d i a t e d area and accurate alignment of the l a s e r s p o t onto t h e small patterns. The s u b s t r a t e mounted on a heatable t a b l e can be moved w i t h step motors w i t h speeds from 10-3 t o 10-2 cm/s. For most of the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983564
C 5 - 4 4 0 JOURNAL DE PHYSIQUE
exqeriments, t h e d i s t a n c e between the scan traces was lpm which guarantees t h a t each p a r t was heated a t l e a s t two times by t h e l a s e r beam. I n comparison, a s e t o f e l e - ments were annealed i n a conventional way i n a furnace. The 4 p a t t e r n s allowed the use o f a m o d i f i e d 4 p o i n t probe technique / 5 / . The i n f l u e n c e o f the r e s i s t a n c e o f the probe neddle and o f the metal p a t t e r n s c o u l d be o m i t t e d by using two d i f f e r e n t probes on d i f f e r e n t p a r t s opposite o f each c o n t a c t f o r c u r r e n t and v o l t a g e measure- ments.
Detector n TV-Camera
v Filter
Mirror Prism
IR- LED Shutter splitter
L a s e r beam
heatable Sampleholder
Fig. 1: Laser anneal system Contacts on S i
20 3
The c o n t a c t r e s i s t a n c e o f a T i / P t c o n t a c t on n-Si ( n = l . 1 - 1 0 cm- ) as a f u n c t i o n o f i n c i d e n t l a s e r power i n a 3pm beam s p o t i s given i n F i g . 2. A t room temperature, the power was n o t s u f f i c i e n t t o reach t h e temperature necessary f o r good c o n t a c t forma- t i o n . C a l c u l a t i o n s showed t h a t the dwell time does n o t i n f l u e n c e the temperature o f the heated spot, because the temperature reached n e a r l y a steady s t a t e a t a l l dwell times i n v e s t i g a t e d / 4 / . Therefore the s u b s t r a t e was a l s o heated. The back temperature can be regarded as o f f s e t i n temperature. The temperatures(T ) c a l c u l a t e d f o r a l a s e r power o f 500 mW i s given i n F i g . 3, t a k i n g the tempera?%e dependence o f the thermal c o n d u c t i v i t y i n t o account.
F i g . 2: The dependence o f the c o n t a c t r e s i s t a n c e o f T i / P t contacts on n-Si on l a s e r power i n a 3pm l a s e r s p o t .
I n Fig. 3 t h e i n f l u e n c e on dwell time i s seen. As w i t h conventional a l l o y e d contacts /6/, the c o n t a c t r e s i s t a n c e f i r s t l y decreases w i t h time and i n c r e a s q a g a i n f o r l o n g e r times o f h e a t treatment. However, the time s c a l e i s f a r s m a l l e r f o r t h i s r a t h e r h i g h temperature process. Contacts t h a t were simultaneously t h e r m a l l y an- nealed a t 4500C i n a N2 atmosphere f o r more than 5 min showed values o f 1 ss than 10-~11cm~ f o r T i / P t c o n t a c t p a t t e r n s on n-Si w i t h doping l e v e l s o f l.4.102gcn-3 and about ~ O - ~ R C ~ ~ f o r doping l e v e l s o f 1018 cm-3.
F i g . 3: Contact r e s i s t a n c e o f T i / P t contacts on n-Si versus dwell time o f a 3pm, 0.5W cw-Laser beam.
The comparison o f the c o n t a c t r e s i s t a n c e , obtained f o r T i / P t contacts on n-Si w i t h d i f f e r e n t doping l e v e l s , shows good agreement w i t h t h e o r e t i c a l data of Chang e t a1 /7/ f o r the given values o f the T i S i 2 / S i b a r r i e r ( F i g . 4 ) .
Fig.
c:q@, =O.ZeV
. experiment
i n 7 ."
1 ola 10'9 lo2'
Contact r e s i s t a n c e o f T i / P t contacts on n-Si w i t h l e v e l s comparedwi t h c a l c u l a t e d values o f Chang e t Contacts on GaAs
f f e r e n t
/ 7/.
doping
18 - 3
Ohmic contacts on GaAs (n=3.5'10 cm ) are produced using the same s t r u c t u r ~ s as w i t h S i . F o r the Ti/AuGe/Pt m e t a l l i z a t i o n , no a d d i t i o n a l h e a t i n g o f the s u b s t r a t e was
necessary t o reach t h e a l l o y i n g temperature, due t o t h e lower h e a t c o n d u c t i v i t y of
JOURNAL DE PHYSIQUE
o f GaAs 141.
F i g . 5 shows the c o n t a c t r e s i s t a n c e versus l a s e r power. F o r a dwell time o f 100 ns and l a s e r power P 190mW, t h e contacts do n o t show ohmic c h a r a c t e r i s t i c s and f o r PL
320mW, the m e t a k l i z a t i o n i s damaged by t h e l a s e r beam.
The i n f l u e n c e o f t h e dwell time on the c o n t a c t r e s i s t a n c e f o r t h r e e d i f f e r e n t l a s e r powers i s seen i n Fig. 6. The r e s i s t a n c e drops s l i g h t l y w i t h annealing time.
Fig. 5: The dependence o f the c o n t a c t F i g . 6: Contact r e s i s t a n c e o f Ti/AuGe/
r e s i s t a n c e o f T i /AuGe/Pt contacts P t contacts on n-GaAs versus on n-GaAs on l a s e r power i n a 3pm dwell time o f a 3pm cw-Laser l a s e r spot. The scan speed i s beam w i t h d i f f e r e n t power.
3x10-3cm/s.
For low power, a minimum dwell time i s necessary t o o b t a i n ohmic behavior. The depen- dence on laser-power shows t h a t the c o n t a c t formation i s more s e n s i t i v e t o tempera- t u r e , which i s p r o p o r t i o n a l t o the laser-power, than t o time. This i s probably due t o t h e exponential dependence o f the d i f f u s i o n c o e f f i c i e n t on temperature. The b e s t values are comparable w i t h the b e s t values o f c o n v e n t i o n a l l y a l l o y e d contacts.
rectifying \ \
'\ 0 0
k
A 0 p.180 mW p=240mW p-270mW- 1
%I c \
104.
lo-'
To e v a l u a t e the i n f l u e n c e o f t h e l a s e r processing o f a 4 pm wide p a t t e r n on the me- t a l p a t t e r n 6 vm a p a r t , the area G i l l u s t r a t e d i n Fig. 7 was l a s e r processed. This f i g u r e shows a l s o the I - V c h a r a c t e r i s t i c s between G and S (e.g. G and D) a f t e r l a s e r i r r a d i a t i o n w i t h PL = 210mW ( 2 ) and PL = 250mW ( 3 ) as w e l l as before i r r a d i a t i o n (1).
150 200 250 3% 0 50 100 150 200
p/m W __C
tdW,,/f"S
a] c \
8 l o p -
recti-//
fying
- 7
- 1 0-5
Laserbeam
1 as evaporated 2 PL= 210 rnW 3 PL=250 mW
F i g . 7: I - V c h a r a c t e r i s t i c s o f Ti/AuGe/Pt p a t t e r n s on n-GaAs before and a f t e r l a s e r processing o f the G - s t r i p e (beam p o t 3pm 10, speed 0.01 cmls).
S i m i l a r experiments f o r S i showed no i n f l u e n c e o f the D and S p a t t e r n s 1 um and 2 pm a p a r t from a 1 pm s t r i p e a f t e r l a s e r processing o f the s t r i p e .
The morphology has n o t changed a f t e r l a s e r i r r a d i a t i o n (Fig. 8b). I n c o n t r a s t t o conventional a l l o y e d contacts (Fig. 8c), no b a l l i n g up o f the metal i s seen except f o r GaAs, when the laser-beam touches t h e GaAs, then the edge and the GaAs
d e t e r i o r a t e s , o r the l a s e r power exceeds a c e r t a i n l i m i t , s u f f i c i e n t t o damage the whole l a y e r .
F i g . 8: a ) Test s t r u c t u r e f o r r e s i s t i v i t y measurements ; SEM p i c t u r e s o f Ti/F,uGe/Pt p a t t e r n s on GaAs, b ) l a s e r annealed (PL=280mW, v~=0.014cm/s),
c ) furnace annealed (450°C, I s ), d) l a s e r annealed s t r i p e (P =210mW, vL=O.O1 cm/r ), e ) l a s e r annealed s t r i p e (PL=Z40mW, vL-0.01 cm/s\.
I n summary, i t has been shown t h a t ohmic contacts can be formed l o c a l l y w i t h o u t a f f e c t i n g Schottky contacts pm a p a r t . The c o n t a c t r e s i s t a n c e on S i and GaAs ob- t a i n e d w i t n a X6 l a s e r beam o f 0 = 36m and power o f 0.5 W i s comparable w i t h values obtained w i t h conventional a l l o y i n g process.
References
/l / P.M. Tsien, H. Ryssel, D. R'oschenthaler, I. Ruge, J. AppT. Phys. 52 (1981), 4775
/ G. Eckhardt, Laser and E l e c t r o n Beam Proc. o f Mat. ed. by C.W. White, P.S. Peery, Acad. Press (1980), 467
/3 / M. Lax, Appl. Phys. 48 (1977), 3819
/ 4 / Y . I . Nissim, A. L i c t o i l a , R.B. Gold and J.F. Gibbons, J. Appl. Phys. - 5 1 (1980), 2 74
/ 5 / H.M. Nagnib, J. Electrochem. Soc. 2 (1977), 573
/6 / K. Heime, U. Konig, E. Kohn, A. Wortmann, S o l i d - S t a t e Electron. Jl (1974) 835
/ 7 / C.Y. Chang, Fang, Sze, S o l i d - S t a t e Electron. 2 (1971) 541
