Zero stress bonding of silicon carbide to diamond

Details Zero stress bonding of silicon carbide to diamond Zero stress bonding of silicon carbide to diamond

1998-07-10

2001-02-20

Turner, Archene (Department: 1775)

Metal fusion bonding

Process

Bonding nonmetals with metallic filler

C428S408000

Reexamination Certificate

active

06189766

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention generally relates to an apparatus and method which achieve a bond between two or more materials of a composite structure, with differing thermal coefficients of expansion. In particular, the apparatus and method of the present invention produce a stress-free composite structure of silicon carbide and diamond.
High power RF transistor needs are increasingly being upgraded so that they are required to produce more power in less area. This places a greater emphasis on heat removal since these devices are made of semiconductor materials which exhibit degraded performance when their temperatures increase. In addition, many of the conventional high power RF devices are operated in a pulse mode which requires not only good thermal conductivity (for steady-state heat removal), but also requires very high thermal diffusivity to quickly spread the heat from the heat generating area. Table 1 shows the thermal properties of common semiconductor materials and other heat spreading materials.
TABLE 1
Heat
Thermal
Thermal
Capacity
Conductivity
Diffusivity
Density
Material
(J/gmc
0
)
(w/cm
0
K)
(cm
2
/sec)
(g/cm
3
)
GaAs
0.3
0.46
0.248
5.3
Si
0.7
1.5
0.93
2.3
SiC
0.628
4.5
2.23
3.2
Cu/W(15%)
0.29
2.55
0.54
16.4
Beryllium
1.05
0.25
0.084
2.85
Oxide
Diamond
0.51
15 to 20
9.47
3.52
To achieve these higher powers and greater heat dissipation, the assembled transistor package must have a small thermal resistance between the actual power absorbing volume and the exterior surface in contact with some form of air flow cooling. This application would be applicable for any number of active or passive devices which generate or collect energy (heat). In addition, the material closest to the actual power absorbing volume should have a very high thermal diffusivity so that high transient thermal pulses can be spread rapidly away from the heat creating sites. A candidate to accomplish both of these is shown in FIG.
1
.
FIG. 1
illustrates a SiC/diamond stack
100
, which includes a diamond layer
102
with a thickness L, bonding material
104
, with a thickness that is virtually zero, and a silicon carbide die
106
with heat creating sites
108
formed on an upper portion thereof. Although not required, in this example the silicon carbide die
106
has a thickness approximately equal to the diamond layer
102
.
The thermal conductance of this SiC/Diamond stack
100
is 17 times better than a comparable Si/BeO stack. In addition, the diamond layer
102
provides a 4.2× improvement in thermal diffusivity over the SiC itself. The problem with such a solution is that the stack
100
cannot conventionally be bonded together. Stresses which result from the different coefficients of thermal expansion (CTE) either tear the bonding material
104
or fracture the stacking materials when cooled to operational temperatures. The bonding materials of choice are metals rather than epoxies because of the need for low thermal resistance. The metal chosen, for example, titanium or titanium nickel, must also have a solidification temperature higher than the highest operating temperature. The assembly of the stack
100
is thus done at temperatures up to 100° C. higher than the upper operation temperature at the material interfaces. The maximum CTE stresses which result when cooled to the lowest operating temperature result in fracture or debonding. It is this problem that is addressed in this disclosure.
SUMMARY OF THE INVENTION
It is an object of the present invention to achieve a bond between two or more materials of a composite structure, with different thermal coefficients of expansion, such that the composite structure does not fracture or debond.
It is further another object of the present invention to provide a multi-layer structure including a layer of silicon carbide and a layer of diamond, bonded together, which does not fracture or debond.
The present invention achieves these objects by providing a multi-layer structure, comprising: a first layer; a second layer, with a coefficient of thermal expansion different than a coefficient of thermal expansion of said first layer, a bulk modulus different than a bulk modulus of said first layer and a thermal conductivity different than a thermal conductivity of said first layer; a bonding layer, having an isostatic pressure versus temperature curve with one of said first layer and said second layer which is substantially similar to an isostatic pressure versus temperature curve of said first layer and said second layer, such that a substantially stress-free bond is formed between said first layer and said second layer; wherein said first layer, said second layer, and said bonding layer are arranged as a sandwich, with the bonding layer in between said first layer and said second layer.
The present invention also achieves these objects by providing a process for bonding together a silicon carbide layer and a diamond layer, yielding a composite structure which is substantially stress-free at a selectable reference temperature and reference isostatic pressure, comprising the steps of: (a) providing the silicon carbide layer and the diamond layer; (b) determining a critical line for the silicon carbide layer and the diamond layer in a pressure-temperature plane wherein a location of the critical line depends on the selectable reference temperature and reference isostatic pressure and depends on coefficients of thermal expansion and bulk moduli material constants of the silicon carbide layer and the diamond layer, wherein the critical line sets forth a plurality of temperature-pressure pairs at which the composite structure will be substantially stress-free; (c) controlling a temperature and an isostatic pressure during bonding such that the temperature and the isostatic pressure represent a point on the critical line; (d) bonding the silicon carbide layer and the diamond layer at the temperature and the isostatic pressure in said step (c); and (e) returning to the selectable reference temperature and reference isostatic pressure after bonding is completed by following a path along the critical line for the silicon carbide layer and the diamond layer which avoids imposing disruptive stresses on the composite structure.
Finally, the present invention achieves these objects by providing an apparatus for bonding together a silicon carbide layer and a diamond layer, yielding a composite structure which is stress-free at a selectable reference temperature and reference isostatic pressure, comprising: (a) means for supporting the silicon carbide layer and a diamond layer; (b) means for determining a critical line for the silicon carbide layer and the diamond layer in a pressure-temperature plane, wherein a location of the critical line depends on the selectable reference temperature and reference isostatic pressure and depends on coefficients of thermal expansion and bulk moduli material constants of the silicon carbide layer and the diamond layer, wherein the critical line sets forth a plurality of temperature-pressure pairs at which the composite structure will be substantially stress-free; (c) means for controlling a temperature and isostatic pressure during bonding such that the temperature and the isostatic pressure represent one or more points on the critical line; and (d) means for bonding the silicon carbide layer and the diamond layer at the temperature and the isostatic pressure in said step (c); (e) means for returning to the selectable temperature and reference isostatic pressure after bonding is complete by following a path along the critical line for the silicon carbide layer and the diamond layer which avoids imposing disruptive stresses on the composite structure.
As described above, there may exist a set of points defining a line where these locus of points represent a selection of pressures and temperatures. Movement between any two points on this P-T line will result in an identical change in area of both of the two components. This line, sometimes a curved line, is referred to in this application as a critical line.

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