HAL Id: jpa-00225948
https://hal.archives-ouvertes.fr/jpa-00225948
Submitted on 1 Jan 1986
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.
ATOM-PROBE STUDY OF METAL-SiC INTERFACES
S. Nakamura, Y. Hasegawa, T. Hashizume, T. Sakurai
To cite this version:
S. Nakamura, Y. Hasegawa, T. Hashizume, T. Sakurai. ATOM-PROBE STUDY OF METAL-SiC INTERFACES. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-309-C7-314.
�10.1051/jphyscol:1986753�. �jpa-00225948�
JOURNAL DE PHYSIQUE
Colloque C7, supplément au n o 11, Tome 47, Novembre 1986
ATOM-PROBE STUDY OF METAL-Sic INTERFACES
S. NAKAMURA", Y. HASEGAWA, T. HASHIZUME'" and T. SAKURAI
The Institute for Solid State Physics (ISSP), the University of Tokyo, Minato-ku, Tokyo 106, Japan
' ~ h e Institute of Scientific and Industriaf Research, Osaka University, Ibaraki, Osaka, Japan
* * A T & T. Bell Laboratory, Murray Hill, N. J. 07974, U.S.A.
Abstract - Good quality field ion images of 2H-Sic and B -Sic whisker were obtained using a He imaging gas at 20K. The compositional depth profiles of annealed specimens of SiC showed that the surface region was enriched in C and that the top layer consisted of carbon atoms only. In contrary, the Si concen- tration at the surface was found to increase when S i c was annealed in the presence of Oz or H+O gas. In our present study on the Ti/SiC system, we conclude; (1) Ti did not mix with Sic at al1 below room temperature, in cont- rast to the case of the Ti/Si system. (2) Annealing at higher temperatures (-v700°c) resulted in intermixing of Ti, Si and C at the interface. The thick- ness of the interface was estimated to be approximately three atomic layers.
In the Pd/SiC system, interrnixing at the interface was noticed after annealing at lower temperature than the Ti/SiC system.
1 - INTRODUCTION
ïhe cubic form of silicon carbide has been the object of increased research since the developement of a Chemical Vapor Deposition (CVD) method for the production of single crystal films. [l] ïhe properties of this material has been considered as a good candidate for various applications, such as high temperature elechonic devices which could be operated at temperatures in excess of 600°c. ïhe material is also hard, inert under various chemical enviroments and mechanically strong.
Several studies of the surface of Sic have been reported. [Z, - 41 An attempt to apply an atom-probe for compositional analysis of A-Sic was made by Nakamura et al.
in 1978 [ 5 ] . The new atom-probe at ISSP has superb features such as, a 100% detec- tion efficiency [6] and laser-pulse mode analysis and this is applicable for the detailed study of Sic. Ln the present work, a well defined surface of a Sic tip and interface of Ti/SiC or Pd/SiC system obtainable by heat treatment have k e n identified by the ToF Atom Probe FIM at both in a pulse laser mode or in a pulse voltage mode. We have found that ToF atom-probe yields a stoichetrically correct composition using the high-resistivity Sic specimens in a laser pulse mode [7].
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986753
JOURNAL DE PHYSIQUE
The Sic specimens were prepared £rom three different types; (1) whisker, (II) fila- ment, and (III) powder. However, only type 1 yielded successful results in the present study. A 2H-Sic or h' -Sic whisker was inserted into a smll Pt tube which was spot-welded at the center of a Mo-U shaped loop. A tip was chemically etched into sharp needle using molton KOH solution in a Ni crucible. Good quality field ion images were obtained using the He image gas at 20K (E=5.1 V/A). After deposi- tion of the metals on the tip surface, the tip was heated to achieve an equibriated interface by joule heating of the Mo loop in-situ of the main chamber. The tempe- rature was measured by an optical pyrometer.
III - RESULTS AND DISCUSSION
'Ihe -Sic whiskers were heat-treated at 6 0 0 " ~ for 10 sec in a UHV condition (ph 10"
Torr). Atom-probe measurments in a valtage pulse mode were carried out keeping the tip at room temperature in presence of HZ gas (p~lxl~-9~orr). Tne mass histograms of the Sic tip was simple showing only Sizf, Cf , CL+ and H' ions. lhere were no complex ions or unknown elements (~ig 1-a). In Fig 1-b, the cumulative numbers of the detected Si and C ions were plotted against the total nwiber of signais, show- ing the depth profiles. Tne top surface layer of the ~i~(111) surface should only C, which should be taken as a graphite surface. Fig 1-b reveals that the thickness of a Si deplated region was approximately 10 atomic layers estimated £rom the total number of evaporated Si and C atoms. The temperature of the heat-treatment was too low for Si to be thermaly evaporated £rom near surface region [2]. One of the SIC tips was exposed to the residual gas (mainly Hz 0 and O a up to 10-'Torr) for 2hr.
above room temperature heated by the ceramic heater installed underneath the sample holder. The atom-probe mass spectnim showed mainly sif , siZf with a significant
level residual gas ions (H+ H ~ O + 0' and 0: ) (~ig 2-a). The depth profiles of Si and C concentrations in this case are shown in Fig 2-b. We noticed the suprisingly high concentration of Si atoms in the near surface. It is unfortunate that the
field evaporation voltage was too high in this case to continue probing to recovere the stoichiometrically correct composition of the bulk region. m e n the Sic tip
was annealed-in the residual gases- (p-l~-p~orr) at ~2550OC for a few min,
00
u. (a) the depth profile in the near h a c e
-
Ua
$2 200 0
Fig 1 (a) Mass histogram and (b) Depth profile of the surface of A -Sic upon heating at 600°C for 10 sec
i O 20 30 in 10-'O~orr. Probing at room
YASS TO CHARGE PATIO (m/n) temperature in presence of lxlO-'
Torr HZ gas .
region was essentially the same as those shown in Fig 2.
600
S L C
o ixl0-'Torr Y. room temp C
focusrng. hrgh v o l t a g e
s L
m 3 Z
(b)
NUMBER OF S L + C 1 000
400
vac. room temp. 6)
m
3 Z
O
O NUMBER OF Si+C 500
Although it is preliminary we have obtained the depth profile of the Ti deposited Si tip mixed layers of Ti and Ti0 have been observed in the near surface, followed by the Ti + Si region identified as the Tisi phase (~ig 3-a). 'Ihe interface between the silicide and Si substrate was recognized clearly with a sharp boundary. Oxygen atoms in the case of Ti deposit ion appear to k sweeped out to the Ti rich surface region during formation of Ti silicide. Mter annealing at 550'~ for 3 min, the substrate Si atoms were diffused into the Ti over layer to f o m the TiSiz phase which also has a sharp boundary with the substrate (Fig 3-b).
400
T L deposirion as-deprisil
.A
( I O O ) . v a c . lOOK l a s e r . focusing wl
O NUMBER OF S i + T i 600 200
.2
Ti depos i t ion
F annealed a t 550 % f o r 3min
\ .-. I ~ l D - ~ T o r r H,.40K
rB LL O M W m
x 3 Z
O
O NUMBER OF S i + T i 300
Fig 2 (a) Mass histogram and (b) Depth profile of -Sic upon heatin in presence of residual gases $mainly H i O, O r ). Ihe probing conditions were in vacuum (p-lO-'O) at room tempe- rature and voltage pulse mode.
Fig 3 (a) Depth profile of the interface of as-deposited Ti/Si surface probing in vacuum at 100'~ using laser pulse mode, (b) Depth profile of the inter- face of Ti on Si upon heating at 550°C for 3 min. probing at 40°K in presence of lxl~-~Torr Hz gas.
C7-312 JOURNAL DE PHYSIQUE
1 ,
!iTOuk 6 : ' ' d0 ' 1 0 0 I
MASS TO CHARGE RATIO (m/n)
Fig 5 ( a ) Mass histogram and (b) Depth p r o f i l e of the interface of Ti deposited on -Sic upon heating a t 700°C f o r 30 sec.
The t h i c h e s s of intermixing layer i s about 3 atomic layers.
+ .A
When Ti was deposited on the Sic t i p surface a t room temperature, there was no mixed layer a t the boundary between the Ti over layer and S i c substrate (Fig 4-a). When the Ti/SiC interface was annealed i n vacuum a t approximate by 700°C f o r 30 sec the reaction s t a r t e d t o take place and the new phase was observed ( ~ i g 5). But t h i s phase was very thin, estimated t o be approximately three atomic layers. The concen- t r a t i o n r a t i o of S i and C atoms i n t h i s new interface layer deviated £rom the s t o i - chiometric correct Sic substrate. T i atoms did not penetrate i n t o the Sic subst- rate. ïhere were no molecular ions detected i n the fonn of Tic o r Ti s i l i c i d e .
When the Ti/SiC interface was heated
a a t T>700°C f o r several minutes, the
30 (a) surface region resulted apparently a
W +
W new phase with Ti0 a s an impunity i n
O contrast with Ti/Si system i n Fig 3.
Z 20
O It was not possible t o the f i e l d
evaporate u n t i l the phase boundary.
O 10 m x 2 Z
O Fig 4 (a) Mass histogram and (b)
MASS TO CHARGE RATIO (./R) Depth p r o f i l e of the interface
O NUMBER OF S i + C + T i 60.0
150 of Ti/SiC without any heat
R d e p o s i t i o n v a c . room temp.
treatment. The probing condi-
'. o f o c u s i n g . h i g h v o l t a g e
L
03 L L O
(b) t i o n was i n vacuum a t r o m temperature using voltage pulse mode.
O NUMBER OF Si+C+Ti 300
W
It may be possible to predict the reaction products by the calculated Ti-Si-C ternary phase diagram shown in Fig 6 [8]. l h e tie lines in the ternary-phase diag- ram connect phases in stable equilibrium, and are generated by comparing one tie line against another crossing tie line to determine which pair of end products has the lowest AG. Since there is no Ti-Sic tie line we expect a reaction with produ- cts, Tic and Ti silicide. However to continue the Ti-Sic reaction Si or carbon atom should be decomposed £rom Sic and diffuse across the interface layer. Our data show that the Si concentration was twice as much as C in the thin interface (Fig 5).
The deviation £rom the stoichiometric concentration of silicon carbide suggests that Ti starts to react with Sic substrate and form not only Tic but TijSij in the interface. When Pd was deposited on the Sic tip surface which was kept at or below room temperature, the depth profile showed that Pd remained on the surface without any reaction taking place. When the Pd/SiC interface was annealed in vacuum at approximately 600°C for 1 min, the reaction took place and several new phases developed (~ig 7). Top surface was covered with Si atom only. As soon as the Si overlayer evaporated, b t h of Si and Pd atoms were detected. It seems to form a Pd silicide phase. Continuing the field evaporation, C atoms began to be detected.
The interface between Pd silicide phase and substrate was sharp. But a few Pd signals were still detected below this
c interface. Unlike Ti/SiC, Pd/SiC is
an example of reactive interface even at low annealing temperature. Pd/SiC might have good adhesion as already be pointed out by Bermudez h o has
studied by AES and E E S technique [9].
Fig 6 Ternary-phase diagram for the Si, Ti, and C system at 600'~
with Table; Free energy formation data for elements and compounds in the Ti-Si-C system (Kal/mol) .
(ref. 8)
Ti Ti55i3 Tisi T i s i Z Si
S i"
C' c2'
S 'i pd2*
P d
di!.!.. .
O 50 I O 0
MASS T O CHARGE R A T I O ( m / ~ )
_
1 Pd deposit iona. annealed at 590'C for l m i n
\ I x l O-'~orr Y. 70K focusing. high v o l t a g e
LO /Si
Fig (b)
7 (a) Mass histogram and (b) Depth profile of the interface of Pd deposited on A-s~c upon heating at 600°C for 1 min.
The formation of paradium silicide seems to be evident.
O NUMBER OF Si+C+Pd 600
JOURNAL DE PHYSIQUE
IV - CONCLUSION
1) When the Sic was heated in a vacuum at 600°C, the surface region was enriched in graphite monolayer. In contrary, the Si concentration at the surface was found to increase after annealing in the presence of OZ or HrO gases.
2) The interfacial reactions of Ti/Si, Ti/SiC and Pd/SiC were studied by an atom probe technique. ïhe reaction products in the interface were discussed £rom the concentration depth profile in atomic scale.
We wish to thank Dr. Setaka, National Research Institute for Inorganic Materials, for providing Sic whiskers.
/1/ S.Nishino, J.A.Powel1, and H.A.Wi.11, Appl. Phys. Lett. 42(5) (1983) 460.
/2/ A. J.V.Bormne1, J.E.Crombeen and A.V.Tooren, Surf. Sci. 48 (1975) 463.
/3/ M.Dayan, J. Vac. Sci. Technol. A3(2) (1985) 361.
/4/ J.J.Bellina Jr., Appl. Surf. ci: 25 (1986) 380.
/5/ S.Nakamura and T.Kuroda, Surf. ~ci.70 (1978) 452.
/6/ T.Sakurai, T.Hashizume and A.Jimbo, S v . Sci. Instrum. 57 (1986) 236.
/7/ S .Nakamura, T.Hashizume, Y.Hasegawa and T.Sakurai, Surf .%i. accepted for publication.
/8/ M.A.Taubenblatt and C.R.Helms, J. Appl. Phys. 59(6) (1986) 1992.
/9/ V.M.Bermudez, Appl. Surf. Sci., (1983) 12. -
