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THERMAL BEHAVIOUR OF GRAIN BOUNDARIES IN ALUMINIUM NITRIDE CERAMICS
J. Sachet, J. Laval, F. Lepoutre, A. Boccara
To cite this version:
J. Sachet, J. Laval, F. Lepoutre, A. Boccara. THERMAL BEHAVIOUR OF GRAIN BOUNDARIES
IN ALUMINIUM NITRIDE CERAMICS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-617-
C1-622. �10.1051/jphyscol:1990197�. �jpa-00230366�
COLLOQUE DE PHYSIQUE
COllOque C l , s u p p l 6 m e n t a u n o l , Tome 5 1 , j a n v i e r 1 9 9 0
THERMAL BEHAVIOUR OF GRAIN BOUNDARIES I N ALUMINIUM NITRIDE CERAMICS
J . P . S A C H E T * - * * , J . Y . L A V A L * * , F. LEPOUTRE*** and A.C. BOCCARA"""
*centre des Materiaux P.M. Fourt, ENSMP, BP. 87, F-91003 Evry Cedex, rzance
Laboratoire des Microstructures CNRS-ESPCI, 10 Rue Vauquelin, F-75231 :?$is Cedex 05, France
Laboratoire d l O p t i q u e Physique CNRS-ESPCI, 10 Rue Vauquelin, F-75231 Paris Cedex 05, France
Resume
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Nous avons m i s au p o i n t une technique de p h o t o r e f l e c t i o n basbe s u r l e s v a r i a t i o n s du c o e f f i c i e n t d e r e f l e c t i o n o p t i q u e i n d u i t e s p a r un f a i s c e a u l a s e r . C e t t e t e c h n i q u e a permis d e m e t t r e e n evidence l ' i m p o r t a n c e d e s phases i n t e r g r a n u l a i r e s s u r l a d i s s i p a t i o n thermique dans l e s ceramiques B base de n i t r u r e d'aluminium. On t r o u v e une bonne c o r r e l a t i o n e n t r e l ' e x p e r i e n c e e t l e c a l c u l a d a p t 6 aux d i f f b r e n t s t y p e s d ' i n t e r f a c e s .A b s t r a c t
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A p h o t o r e f l e t a n c e t e c h n i q u e based on t h e d e t e c t i o n of t h e v a r i a t i o n s of t h e o p t i c a l r e f l e c t i o n c o e f f i c i e n t was developed. T h i s t e c h n i q u e p i n p o i n t s t h e i n f l u e n c e of t h e i n t e r g r a n u l a r phases on t h e h e a t d i s s i p a t i o n i n AlN-based ceramics. There e x i s t s a good c o r r e l a t i o n between e x p e r i m e n t a l and t h e o r e t i c a l d a t a which were a d j u s t e d f o r d i f f e r e n t i n t e r f a c e s .1
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INTRODUCTIONS i n g l e c r y s t a l s o f Aluminium n i t r i d e a r e c h a r a c t e r i z e d by a h i g h thermal c o n d u c t i v i t y of 320 W/m0K which i s c l o s e d t o t h e t h e o r e t i c a l v a l u e / l / and remarkable e l e c t r i c a l p r o p e r t i e s . I t i s why an important e f f o r t h a s been c a r r i e d o u t on t h e thermal c o n d u c t i v i t y of p o l y c r y s t a l l i n e aluminium n i t r i d e which i s one o f t h e hopeful c a n d i d a t e s f o r s u b s t r a t e s i n semi-conductor d e v i c e s . U n f o r t u n a t e l y . i n such s u b s t r a t e s t h e thermal p r o p e r t i e s a r e reduced by t h e presence of i m p u r i t i e s such a s oxygen and t r a n s i t i o n metal i o n s i n t h e s t a r t i n g powder, and d e f e c t s such a s d i s l o c a t i o n s , p o r o s i t y and f i n a l l y g r a i n b 0 u n d a r i e s . A ~ a m a t t e r o f f a c t , t h e s e g r a i n boundaries can c o n t a i n a second phase. S i n c e i t i s a compound having a s t r o n g c o v a l e n t bond and a high m e l t i n g p o i n t , A 1 N i s d i f f i c u l t t o s i n t e r . Consequently p o l y c r y s t a l l i n e A 1 N i s prepared by adding a v a r i a b l e amount o f Y,03 t o form a dense s i n t e r e d p r o d u c t with i n t e r g r a n u l a r phases such a s Y,Al,O,, and Y,A1,0,
.
The purpose o f t h i s i n v e s t i g a t i o n i s t o determine t h e e f f e c t o f g r a i n boundaries with o r without second phases and d e f e c t s on t h e h e a t d i s s i p a t i o n . The d e t e c t i o n of t h e s e thermal b a r r i e r s n e c e s s i t a t e s a l a s e r induced thermal wave d e t e c t i o n technique. A c o r r e l a t i o n between experimental r e s u l t s and c a l c u l a t e d d a t a , o b t a i n e d by a t h e o r e t i c a l model t o p r o v i d e a q u a n t i t a t i v e measurement o f thermal b a r r i e r s i s r e p o r t e d .
2
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EXPERIMENTAL METHODS 2 . 1 - Sample p r e p a r a t i o nHigh d e n s i t y AlN-Y203 samples were prepared by t h e f o l l o w i n g procedure : 2 and 10 w t % Y203 -added A 1 N powder, which c o n t a i n s 2 . 5 w t % 0 2 , was mixed i n b u t y l a l c o h o l ; a f t e r d r y i n g , t h e mixed powder was u n i a x i a l l y p r e s s e d a t l t / c m 2 ; A1N green b o d i e s were p r e s s u r e l e s s s i n t e r e d i n a BN c r u c i b l e between 1800°C and 1970'C i n n i t r o g e n atmosphere f o r one hour. Then specimens were p o l i s h e d w i t h alumina powder f o r m i c r o s t r u c t u r a l o b s e r v a t i o n s and photothermal experiments.
2 . 2
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Photothermal experimentThe p r i n c i p l e o f a photothermal experiment i s t o d e t e c t t h e very s m a l l p e r i o d i c
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990197
Cl-618 COLLOQUE DE PHYSIQUE
v a r i a t i o n s o f temperature induced by t h e o p t i c a l a b s o r p t i o n of a modulated pump beam i n c i d e n t on t h e sample. T h i s s m a l l p e r i o d i c temperature a c t s i n t h e sample a s a thermal wave p r o p a g a t i n g i n t h e sample. Such a wave i s a b l e t o d e t e c t d e f e c t s i . e . thermal b a r r i e r s which p e r t u r b t h e homogeneous h e a t d i f f u s i o n . Furthermore t h i s wave is s t r o n g l y damped and t h e c h a r a c t e r i s t i c damping l e n g t h P can be c o n t r o l l e d by t h e modulation frequency, f , of t h e pump :
where a i s t h e thermal d i f f u s i v i t y o f t h e sample. I n a good h e a t d i f f u s e r (a
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1 0 - 4 m 2 / s ) p w i l l be o f t h e o r d e r of some pro f o r f e q u a l t o some MHz. I f i n a d d i t i o n t h e pump beam diameter is a l s o o f t h e o r d e r o f 1 W, w e w i l l b e a b l e t o t h e r m a l l y c h a r a c t e r i z e t h e sample a t a s c a l e of 1 pro.CIR 1 dR
The measured modulated r e f l e c t a n c e i s
-
=- -
AT ( 2 ) where AT i s t h e v a r i a t i o n of R, R, dTt h e temperature o f t h e sample s u r f a c e from Toand l / R o dR , t h e t e m p e r a t u r e c o e f f i c i e n t of r e f l e c t i v i t y . T h e whole s i g n a l c o l l e c t e d by t h e ~ ~ h o t o m u l t i p l i c a t o r c o n t a i n s two o p t i c a l i n f o r m a t i o n s which must b e s u b s t r a c t e d by u s i n g a simultaneous a c q u i s i t i o n through a computer : t h e v a r i a t i o n of t h e o p t i c a l r e f l e c t i o n o f t h e He-Ne l a s e r between g r a i n s and g r a i n boundaries and t h e Ar+-ion l a s e r power which a l t e r s t h e l o c a l a b s o r p t i o n of t h e m a t e r i a l . F i n a l l y , s i n c e t h e a b s o r p t i o n o f t h e m a t e r i a l i s v e r y low, i t i s n e c e s s a r y t o make a complementary g o l d c o a t i n g (5 nm t h i c k ) .
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RESULTS AND DISCUSSIONThe p r i n c i p l e o f t h e r e f l e c t a n c e technique a s i t i s s e e n , i s based on t h e p e r i o d i c v a r i a t i o n s of t h e complex r e f r a c t i v e index of t h e sample from t h e absorbed i n t e n s i t y which i s modulated by t h e pump beam. However t h e s e v a r i a t i o n s c a n r e s u l t from non-thermal phenomena. For i n s t a n c e , i n m a t e r i a l s w i t h low energy gap such a s semiconductors, t h e r e a r e c o n t r i b u t i o n s t o t h e p h o t o r e f l e c t a n c e s i g n a l s a r i s i n g from t h e p r e s e n c e of f r e e c a r r i e r s which a r e g e n e r a t e d by t h e pump beam a t t h e sample s u r f a c e . The aluminium n i t r i d e band gap i s t o o high (Eg 6.3 eV) t o a l l o w f o r d i r e c t c a r r i e r s photogeneration. However s i n c e t h e laboratory-made s u b s t r a t e s ( A1N-2), c o n t a i n a n o t i c e a b l e amount of oxygen due t o a s u b s t i t u t i o n p r o c e s s i n t h e aluminium n i t r i d e l a t t i c e / 3 / . i t i s p o s s i b l e , by means of i n d i r e c t t r a n s i t i o n s t o c r e a t e photogenerated c a r r i e r s .
To t h e s e c o n s i d e r a t i o n s must b e added t h e g r a i n s i z e , t h e second phase d i s t r i b u t i o n , s t r u c t u r a l d e f e c t s , p o r o s i t y , e t c . . . w h i c h complicate t h e i n t e r p r e t a t i o n . I t i s why a n a l y s e s were c a r r i e d o u t on o t h e r m a t e r i a l s t o o b t a i n comparative i n f o r m a t i o n s : a second p h a s e - f r e e A I N (AlN-l), w i t h a s m a l l e r g r a i n s i z e ( W 4 pm) and a v e r y s m a l l oxygen c o n t e n t (< 100 ppm) and a z i r c o n i a with a l a r g e g r a i n s i z e ( W 50 pm) and without i n t e r g r a n u l a r phase b u t with poor thermal p r o p e r t i e s c o n t r a r y t o t h e two o t h e r s .
Experimental r e s u l t s have been compared w i t h a t h e o r e t i c a l model based on a method proposed by MC Donald and a 1
/ h / .
Assume an i n f i n i t e medium with a g r a i n boundary ( f i g . 1 ) i n t h e X-z p l a n e ( y = 0 ) c h a r a c t e r i z e d by a thermal r e s i s t a n c e R,. I f a u n i t p o i n t s o u r c e i s on t h e y a x i s t h e temperature may be w r i t t e n f o r y = 0 , a s :with m: = p2 + j
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W w = 2nf and G t h e temperature f a r from t h e b a r r i e r OL,Thus t h e v a l u e o f R,can be deduced from t h e measurements o f T. T h i s measurement o f R, can be compared with a v a l u e c a l c u l a t e d from a model d e s c r i b e d i n f i g . 2. Suppose a g r a i n w i t h a thermal r e s i s t i v i t y l g / k g ( lg : mean g r a i n s i z e and k g t h e thermal c o n d u c t i v i t y of a s i n g l e c r y s t a l ) . we have :
c interface
Fig. 1 : Schematic diagram f o r photothermal experiment a c r o s s a g r a i n boundary.
CC-
Fig. 2 : Thermal r e s i s t i v i t y modeli a t a g r a i n boundary.
..
...>....,... : : : / . . : :" . . . : : ... ..-.-...
.... . . . .... .... . ..... . . . . . . . . .
: ; . . . .
. . . .
... . . . . . . . . . . ....
amplitude
'"
phase .mFig.
4
: Amplitude and phase thermal scans a t t h e v i c i n i t y of a g r a i n boundary i n z i r c o n i aa) experimental d a t a b) t h e o r e t i c a l d a t a
Fig.
3
: O p t i c a l micrograph of zirconiashowing a set of g r a i n boundaries s t u d i e d Fig. 5 : Bright f i e l d TEM i n A ~ N - I
by the photoreflectance technique. ( oxygen content
<
100 ppm).
COLLOQUE DE PHYSIQUE
with k t g : macroscopic h e a t c o n d u c t i v i t y ; kg was found i n r e f / l / while k t g was d i r e c t l y measured on o u r samples.
The calc,ulated thermal r e s i s t i v i t i e s of t h e 3 samples a r e then deduced from r e l a t i o n ( 3 ) and summarized i n t a b l e 1.
TABLE 1
3.2
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Thermal behaviour of z i r c o n i aFig. 3 shows t h e analyzed m i c r o s t r u c t u r e . The experimental d a t a o f t h e p h o t o r e f l e c t a n c e t e c h n i q u e a r e r e p o r t e d i n f i g . 4a. These r e s u l t s i n d i c a t e t h a t t h e r e i s a good c o r r e l a t i o n with t h e o r e t i c a l c a l c u l a t i o n ( f i g . 4b) : t h e amplitude i s n o t reproduced by t h e t h e o r y as w e l l a s t h e phase, b u t t h e r e s u l t which must b e emphasized, i s t h e l a r g e v a l u e of RT(10-5m2'Cw-') o b t a i n e d i n very good agreement w i t h t h e d a t a of t a b l e 1. I t i s t h i s l a r g e v a l u e which e x p l a i n s t h e v e r y s m a l l h e a t c o n d u c t i v i t y k V g of t h e sample d e s p i t e of t h e very l a r g e g r a i n s i z e .
3 . 3
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Thermal behaviour o f AlN-based m a t e r i a l sExperimental and t h e o r e t i c a l r e s u l t s show t h a t i t i s n e c e s s a r y t o t a k e i n t o account a m i c r o s t r u c t u r a l parameter i n o r d e r t o e x p l a i n t h e thermal behaviour d i f f e r e n c e s between t h e two t y p e s o f A1N. T h i s parameter i s h e r e t h e i n t e r g r a n u l a r phase. TEM c h a r a c t e r i z a t i o n s do n o t r e v e a l any second phase i n A1N-1 ( f i g . 5 ) w h i l e t h i s phase i s very i m p o r t a n t when a high Y203 amount is added ( f i g . 6b) f o r t h e A1N-2. These i n t e r g r a n u l a r phases correspond e s s e n t i a l l y t o Y3A15 0,
,
and Y4 A1, O9 phases which a r e l o c a l i z e d a t t h e t r i p l e j u n c t i o n s when o n l y 2wt% Y203 i s added. For 10wt% Y20g t h e s e phases a r e s u r r o u n d i n g t h e g r a i n s . The macroscopic thermal c o n d u c t i v i t y and e l e c t r i c a l r e s i s t i v i t y i n c r e a s e w i t h t h e Y2O3 c o n t e n t a s w e l l / 5 / . The second parameter w e have t o c o n s i d e r i s t h e oxygen content and i t s d i s t r i b u t i o n s i n c e i t may have a g r e a t e f f e c t on h e a t d i s s i p a t i o n . S l a c k and a 1 /6/ have s t u d i e d i t s i n f l u e n c e and shown t h a t thermal c o n d u c t i v i t y d e c r e a s e s when oxygen i n c r e a s e s i n t h e A 1 N l a t t i c e . The oxygen d i s t r i b u t i o n w i t h i n g r a i n s i s obtainedby cathodoluminescence /7.8/ f o r AlN-2. For 10wt% Y203 t h e r a d i a t i v e recombinations occur a t t h e p e r i p h e r y of g r a i n s ( f i g . 7 ) . The photothermal experiment shows o f f more s i g n i f i c a n t v a r i a t i o n s a t t h e p e r i p h e r y of g r a i n s with h i g h e r oxygen c o n c e n t r a t i o n than i n t h e i r c e n t e r , namely i n t h e amplitude mode t h e s i g n a l is 4 times h i g h e r a t t h e periphery. For t h e A l N - 1 t h e oxygen c o n t e n t i s too low within g r a i n s t o produce r a d i a t i v e recombinations ; consequently v a r i a t i o n s i n amplitude and phase mode a r e n o t v i s i b l e .From d a t a c a l c u l a t e d i n t a b l e 1 f o r A l N - 1 R,
=
10-9m2"Cw-1, i t i s seen t h a t t h e experimental l i n e s c a n is i n good agreement with t h e c a l c u l a t i o n s between g r a i n s which p r e s e n t h i g h o p t i c a l c o n t r a s t ( f i g . 8 ) . For g r a i n s showing l i t t l e c o n t r a s t , t h e r e a r e n e i t h e r phase nor magnitude v a r i a t i o n s . These r e s u l t s imply t h a t t h e phonon s c a t t e r i n g is h i g h e r f o r h i g h l y m i s o r i e n t a t e d g r a i n s (which p r e s e n t a h i g h c o n t r a s t ) . For AIN-2 a thermal r e s i s t i v i t y R," 1 0 - ~ m' 'Cw-l h a s been found, which corresponds t o a thermal b a r r i e r w i t h a h a r d l y n o t i c e a b l e second phase ; t h i s v a l u e i m p l i e s a s i m i l a r thermal behaviour t h a n f o r A l N - 1 b u t with a h i g h e r phase change (10 d e g r e e s a t 2 MHz i n s t e a d of 2 d e g r e e s f o r A l N - 1 ) . One n o t e t h a t a t t h e c e n t e r o f t h e g r a i n b o u n d a r i e s , i n t h e t h e o r e t i c a l c a l c u l a t i o n , t h e amplitude i s vanishing. I n o r d e r t o t a k e i n t o account t h e beam s p o t s i z e , i t would be n e c e s s a r y t o do a convolution whereas t h e probe beam i s considered a s a p o i n t f o r t h e c a l c u l a t i o n .Fig. 6a: Bright field
TPI
of98
wt% ~ i g . 6b: Bright fieldTEM
of90
wt%*IN
-
Zwt% Yz O3.
AIN-
10wt;Y Y20J.F i g . 7a: Cathodoluminescence of grains Fig. 7b: Cathodoluminescence of grains
in 98wt% A1N
-
2wt% Y203. in gOwt% A1N-
lOwt% Y2O3.2 MHZ
1
a
'-6m
a) experimental data
Fig. 8 : Comparison of experimental and theoretical thermal data in AlN-1.
...
.... ...
...
.... ...
.... ....
amplitude X t - l phase "l-t b) theoretical data
Cl-622 COLLOQUE DE PHYSIQUE
On a g r a i n boundary with an i n t e r g r a n u l a r phase, t h e experimental r e s u l t s i n d i c a t e a higher v a r i a t i o n i n amplitude and phase mode. The thermal c h a r a c t e r i s t i c s of t h e second phase a r e very d i f f e r e n t from t h e matrix with ks
=
10 w/m'K and as= 5.10-7m2/s ; t h e s e d a t a imply a R, about 10-5m2*Cw-1similar t o t h e one obtained i n z i r c o n i a . The c a l c u l a t i o n i n d i c a t e s a phase change of about 40 degrees which agrees with t h e experimental r e s u l t s . Variations of 100 degrees can be obtained f o r lower thermal c h a r a c t e r i s t i c s o r l a r g e r second phases.3.4
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DiscussionThe i n v e s t i g a t i o n of t h e s e 3 m a t e r i a l s , with very d i f f e r e n t thermal p r o p e r t i e s , proves t h a t t h e p h o t o r e f l e c t a n c e s i g n a l is mainly due t o a thermal e f f e c t . The l a r g e r a r e t h e thermal p r o p e r t i e s changes from p a r t t o p a r t o f t h e m a t e r i a l , t h e g r e a t e r a r e t h e v a r i a t i o n s of temperature phase changes. This behaviour can be a t t r i b u t e d i n t h e c a s e of A1N-2 t o t h e i n t e r g r a n u l a r phase. This second phase produces a high phonon s c a t t e r i n g , while t h e m i s o r i e n t a t i o n between g r a i n s involves a s m a l l e r e f f e c t which agrees with S l a c k ' s observations 161. However t h i s e f f e c t is more important f o r t h e A 1 N which c o n t a i n s a second phase because of t h e amount of oxygen which i s higher i n t h i s m a t e r i a l and most probably because of t h e p o s s i b l e thin-second phase which i s hard t o r e v e a l with t h e o p t i c a l microscope. Consequently t h e amount of second phase can i n c r e a s e t h e thermal r e s i s t i v i t y and then modify t h e experimental r e s u l t s . This behaviour may be connected with t h e oxygen d i s t r i b u t i o n observed i n cathodoluminescence which i s more important a t the g r a i n ' s periphery f o r high Y203 contents. This e f f e c t is due t o the trapping of oxygen by Y 0 which l e a d s t o t h e
2 ?
formation of the i n t e r g r a n u l a r phase and which induces higher v a r i a t i o n s i n magnitude and phase of t h e temperatuw near g r a i n boundaries. Thus i t i s necessary t o add a s u f f i c i e n t Y203amount t o promote t h e l i m i t a t i o n of aluminium vacancies i n A1N l a t t i c e during t h e s i n t e r i n g s t e p which i s c a r r i e d o u t i n N, atmosphere. The l i m i t a t i o n w i l l induce an augmentation of t h e thermal conductivity by i n c r e a s i n g t h e mean f r e e path of phonons and a l s o improve t h e e l e c t r i c a l r e s i s t i v i t y .
For z i r c o n i a , t h e thermal e f f e c t s i n amplitude can be concealed by p o r o s i t y and a l s o by t h e oxygen d i s t r i b u t i o n i n s i d e g r a i n s , b u t t h e thermal change i s v i s i b l e i n the phase mode.
I V
-
CONCLUSIONThe modulated r e f l e c t a n c e technique has allowed thermal b a r r i e r s t o be shown up i n m a t e r i a l s with d i f f e r e n t thermal p r o p e r t i e s . The amount of t h e i n t e r g r a n u l a r phase and t h e oxygen content a r e t h e most important parameters we must consider .to o b t a i n a good thermal c o n d u c t i v i t y . The i n t e r g r a n u l a r phase i n c r e a s e s t h e thermal r e s i s t i v i t y and t h e m i s o r i e n t a t i o n between g r a i n s a l s o decreases t h e h e a t s i n k b u t i n lower proportions. Moreover t h i s behaviour i s enhanced by t h e oxygen content : consequently i t is necessary t o c o n t r o l accurately the chemical composition of t h e s t a r t i n g powders i n o r d e r t o l i m i t t h e oxygen and impurities content. This condition w i l l l e a d t o optimizing t h e Y203 amount t o promote a second phase and reduce l o c a l thermal b a r r i e r s . Moreover, t h e reduction of A 1 vacancies near t h e g r a i n boundaries w i l l obviously c o n t r i b u t e t o improve t h e e l e c t r i c a l r e s i s t i v i t y .
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This research has been supported b y the French Government (Ministere de l a Recherche e t de l a Technologie, convention 86-22-062).
