Solder joint

A joint for joining a silicon disc (21) to a molybdenum disc (22) comprises a layer of titanium silicide (20) and a layer of aluminium-silicon solder (23). The titanium silicide is formed by depositing a layer of titanium on the silicon disc and heating the silicon disc and the titanium to around 550.degree. C. to encourage the formation of titanium silicide. A solder disc is then compressed between the silicon and molybdenum discs at about 690.degree. C. to fuse the solder to the titanium silicide layer and the molybdenum disc. The layer of titanium silicide protects the silicon disc from dissolution during soldering, so that diffused-in features in the silicon are not damaged.

BACKGROUND OF THE INVENTION 
The present invention relates to solder joints between silicon members and 
members comprising a refractory metal, and a process for forming such 
joints. The invention may be employed, for example, in the manufacture of 
semiconductor devices. 
In the manufacture of semiconductor devices, particularly high-power 
semiconductor devices, it is often desired to connect silicon 
semiconductor material to a heat sink of a high conductivity metal, for 
example copper. To help relieve thermal stress which may occur at such a 
connection it is conventional to join an intermediate member comprising a 
refractory metal, for example molybdenum, between the silicon 
semiconductor material and the high conductivity metal heat sink. 
Conventionally, the silicon and the refractory metal members are 
disc-shaped. 
Aluminium has long been used as the solder metal for joining silicon and 
molybdenum discs together, the soldering being accomplished somewhat above 
the eutectic temperature of aluminium and silicon (577.degree. C.). The 
aluminium may initially contain a proportion of silicon, typically the 
eutectic composition, 11.7% Si by weight, although pure aluminium may be 
used. In each case, material is normally dissolved from the surface of the 
silicon disc during the soldering process, although more is dissolved if 
pure aluminium is used as the solder than if the eutectic composition is 
used. 
Another effect involves a reaction which occurs between the silicon content 
of the aluminium-silicon alloy and the molybdenum surface, and results in 
the formation of an interfacial layer of molybdenum disilicide. This 
denudes the aluminium-silicon alloy of some of its silicon content at the 
interface and the consequent concentration gradient causes transport of 
silicon from the silicon disc surface towards the molybdenum surface. Such 
further dissolution of the surface of the silicon disc is generally 
undesirable, especially as there is often a tendency for it to occur 
non-uniformly, resulting in stepped or spiked erosion of the silicon 
surface. 
Further, aluminium, being a dopant in silicon, forms a specifically p-type 
contact with the silicon unless the total concentration of n-type 
counter-dopants (e.g. phosphorus) is sufficient to maintain approximately 
ohmic properties at the contact, through degeneracy of the silicon 
electronic band structure. This requirement imposes a combination of 
constraints on permissible dopant profiles and on alloying erosion 
(dissolution) effects, as described above, at the silicon to 
aluminium-silicon boundary. In particular, where the silicon dopant 
distribution includes small and critical features, it is vital that these 
shall not be lost by dissolution into the aluminium-silicon solder at the 
site of an irregularity (spike) in the alloying front. 
FIG. 1 shows a typical section through a joint which has been formed using 
such a prior art method. A silicon disc 1 is shown attached to a 
molybdenum disc 2 by a layer of aluminium-silicon solder 3. It can be seen 
that the penetration of the solder layer 3 into the diffused silicon disc 
1, is irregular as shown by reference numeral 4. In some small areas, the 
depth of penetration caused by the dissolution of silicon may cause 
diffused-in features in the silicon disc, such as n.sup.+ type region 5, 
to be seriously eroded or even annihilated, with the most undesirable 
consequences. A layer of molybdenum disilicide 6 several microns in 
thickness forms between the solder 3 and the molybdenum disc 2. 
A variety of techniques have been proposed to improve uniformity and reduce 
the silicon erosion at such soldering joints, for example producing the 
required bonding at the lowest possible temperature or coating the surface 
of the molybdenum component to inhibit the formation of molybdenum 
disilicides. Such techniques have often been less successful than might 
have been hoped. Other methods of forming the bond, such as diffusion 
soldering [Jacobson, D.M. and Humpston, G., High Power Devices: 
Fabrication Technology and Developments, Metals and Materials, December 
1991] claim success but at the cost of some complexity. 
An alternative approach, that of forming a barrier layer at the silicon 
surface in order to prevent the dissolution or modification of the dopant 
structure within the silicon has been proposed, mainly in connection with 
thin deposition films and VLSI technology, e.g. Babcock, S.E. and Tu, 
K.N., Journal of Applied Physics, vol. 59, No. 5, pp 1599-1605, March 
1986. The technique therein disclosed does not, however, extend to the 
necessary area nor does it provide for the attachment of a heavy 
molybdenum supporting electrode, such as would be required by a large area 
power device extending up to 100 mm or so in diameter. 
As prior art, there may also be mentioned GB-A-2 238 267 which discloses a 
process for brazing a silicon body to a metal body, the silicon body being 
provided with an adherent oxide film. The oxide film is coated with a 
metal layer structure which provides a brazable surface. Attack of the 
silicon surface by braze alloy is prevented by the oxide film. The metal 
layer structure comprises titanium, molybdenum and nickel. The braze is a 
silver/copper alloy. 
SUMMARY OF THE INVENTION 
According to the present invention from one aspect there is provided a 
joint structure which joins a silicon member to a member comprising a 
refractory metal, the joint comprising a layer of titanium silicide 
adjacent the silicon member and a layer of solder, containing aluminium as 
the principal constituent, lying between the layer of titanium silicide 
and the member comprising a refractory metal. 
According to the present invention from another aspect there is provided a 
method for joining a silicon member to a member comprising a refractory 
metal, the method comprising providing the silicon member with a surface 
layer of titanium silicide and providing a layer of solder containing 
aluminium as the principal constituent between the layer of titanium 
silicide and the member comprising a refractory metal. 
Advantageously, the member comprising a refractory metal contains 
molybdenum as the principal constituent. 
The solder suitably contains silicon in the range from 10 to 15% by weight, 
preferably about 11.7% by weight (the eutectic composition). 
Preferably, the silicon member is provided with the layer of titanium 
silicide by coating the silicon member with a layer of titanium and 
heating the silicon member and the layer of titanium to encourage the 
formation of titanium silicide. Preferably, the silicon member and the 
layer of titanium are heated to a temperature in the range from 
500.degree. to 700.degree. C., more preferably in the range from 
500.degree. to 600.degree. C. and most preferably about 550.degree. C. 
Preferably, the titanium silicide is principally composed of titanium 
monosilicide. The thickness of the layer of titanium is preferably about 1 
.mu.m. 
Preferably, the layer of solder is provided by compressing a solder member 
between the titanium silicide layer and the member comprising a refractory 
metal and heating the solder member to fuse it to the titanium silicide 
layer and to the member comprising a refractory metal. Preferably, the 
fusing of the solder is accomplished at a temperature in the range from 
577.degree. to 760.degree. C., more preferably in the range from 
660.degree. to 700.degree. C. and most preferably about 690.degree. C. The 
thickness of the layer of solder is preferably less than 50 .mu.m and most 
preferably about 30 .mu.m.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following example, a silicon disc which is to be joined to a 
molybdenum disc is provided with a layer of titanium silicide on the 
surface where the joint is to be made, and is joined to the molybdenum 
disc by a layer of aluminium-silicon solder. 
Such a structure is shown in FIG. 2. Referring to FIG. 2, a silicon disc 21 
is provided with a coating 20 of titanium silicide. The silicon disc is 
joined, via that coating, to a molybdenum disc 22 by a layer of 
aluminium-silicon solder 23. 
The process by which the structure of FIG. 2 may be achieved will now be 
described. The silicon disc 21, typically 50 mm in diameter and 600 .mu.m 
thick, which may contain dopants to give desired diffused-in features 25 
and 25', is prepared so as to be free from surface contamination, 
especially oxides. It is placed in a high vacuum evaporation apparatus in 
which titanium is vaporised by heating with a high energy electron beam so 
that titanium vapour condenses on to at least that surface of the silicon 
disc to which it is intended to attach a molybdenum support electrode. The 
titanium is typically deposited to a thickness of 1 .mu.m. The interface 
between the titanium and the silicon is next modified by heating the disc 
to a temperature of at least 500.degree. C. for a period in the range from 
20 to 30 minutes in an atmosphere of nitrogen, although other suitably 
inert gases (e.g. hydrogen) would serve as well. Experiments showed that a 
satisfactory titanium-silicon reaction could be obtained with a 
temperature in the range from 500.degree. to 700.degree. C., the 
preference being for 500.degree. to 600.degree. C. and most preferably 
about 550.degree. C. At higher temperatures, increasing amounts of the 
disilicide TiSi.sub.2 are believed to be formed. TiSi.sub.2 appears to 
have inferior resistance to attack by aluminium compared to the 
monosilicide TiSi formed predominantly in the 500.degree. to 600.degree. 
C. range. Similarly, to avoid the formation of TiSi.sub.2, the reaction 
time should not be unduly prolonged. 
After this temperature treatment, the silicon disc is attached by its 
titanium-coated face to a molybdenum disc of similar diameter and 
typically with a thickness in the range from 2 to 3 mm using a 30 .mu.m 
thick solder disc of aluminium-silicon eutectic composition (11.7% silicon 
by weight) in a suitably inert or reducing atmosphere, e.g. nitrogen or 
hydrogen by compressing the solder disc between the silicon and molybdenum 
discs, at a moderate pressure, e.g. 300 pascals, and raising their 
temperature to 690.degree. C. for a period of 10 to 20 minutes to fuse the 
solder disc to each of the other discs. Thereafter, the fused assembly is 
cooled, slowly through the range 570.degree. down to 300.degree. C. for 
reasons of stress relief. The fusing of the solder could be accomplished 
between 577.degree. C. (the aluminium-silicon eutectic temperature) and 
about 700.degree. C. but about 690.degree. C. has been found to be 
preferable. 
During the soldering process, penetration of the aluminium-silicon solder 
23 into the silicon disc 21 at its face 24 is impeded by the coating of 
titanium silicide 20, which has sufficiently low solubility in aluminium 
that it is not significantly eroded and acts as a membrane-like filter 
moderating the dissolution of the silicon. The integrity of the 
diffused-in features 25 and 25' is thereby better preserved. The 
dissolution of the silicon surface 24 by the aluminium-silicon solder 23 
is not totally prevented by the titanium silicide barrier, but occurs to a 
lesser extent and quite uniformly in contrast to the irregular and 
non-uniformly deep dissolution found with the prior art. Possibly, the 
titanium silicide barrier acts to allow access of aluminium to the silicon 
surface but to prevent passage of silicon in the opposite direction, i.e. 
towards the molybdenum surface. Thereby, the thickness of any molybdenum 
disilicide layer 26 formed between the aluminium-silicon solder 23 and the 
molybdenum disc 22 during soldering is moderated by the limitation in 
availability of silicon at that interface as a result of the action of the 
titanium silicide barrier. After the operation is completed the 
membrane-like layer of titanium silicide is found, as shown in FIG. 3, to 
reside within the aluminium-silicon solder, somewhat closer to the silicon 
surface than to the molybdenum, its motion with time (at the alloying 
temperature) appearing to be in the direction of the molybdenum. 
The precise thickness of the solder disc is not critical, the minimum being 
dictated by the need to accommodate any deviations from flatness in the 
faces to be joined, but it should not be greatly thicker than 50 .mu.m. 
The dimensions of the silicon and the molybdenum discs are given by way of 
illustration only and may differ to a significant extent as may be 
required by other considerations. 
The silicon, molybdenum and aluminium-silicon solder members of the joint 
structure need not be in the form of discs but could be of any appropriate 
shape. 
The joint structure and the process for forming it may be applied to joints 
between silicon and members comprising other refractory metals than 
molybdenum, for example chromium. 
The resulting fused joint assembly has been found to have a mechanical 
strength similar to that resulting from the prior art method while being 
free from the problems caused by deep and/or irregular erosion and/or deep 
and/or irregular dissolution of the silicon disc. The method described 
requires only simple additional steps to the method used in the prior art 
which may therefore conveniently continue in use for less critical 
applications concurrently with the method of the invention.