Environmentally stable integrated optic chip mount

A wedge-shaped block of pyrolytic graphite or other suitable anisotropic material cut along certain predetermined angular dimensions is used as a thermal expansion coefficient transformer to match the anisotropic thermal expansion coefficients of a lithium niobate or lithium tantalate IO chip to an isotropic substrate material such as 316 stainless steel or aluminum, or to match the isotropic thermal expansion coefficients of a silicon IO chip to an isotropic substrate material, the wedge being used as an intermediate block mounted between the IO chip and the substrate, the anisotropic material used as the thermal transformer having a thermal expansion coefficient which is greater than the value of the thermal expansion coefficient of the IO chip material in one direction, and having a thermal expansion coefficient which is lesser than the thermal expansion coefficient of the IO chip material in another direction.

DESCRIPTION 
1. Technical Field 
This invention relates to integrated optics, and more particularly to an 
environmentally stable mounting system for an integrated optic chip. 
2. Background Art 
Integrated optic (IO) chips fabricated from X-cut lithium niobate have two 
different thermal expansion coefficients in the plane of the device and, 
subsequently, in the surface of the chip which is mounted to a substrate. 
These expansion coefficients are 15.4.times.10.sup.-6 / .degree.C. in the 
X and Y directions, and 7.5.times.10.sup.-6 / .degree.C. in the Z 
direction. The anisotropic thermal expansion of lithium niobate creates a 
difficult mounting problem, so far as producing an exact thermal expansion 
match between the chip and the substrate. A poor thermal expansion match 
between the chip and substrate will produce excessive stress gradients in 
the bonding material, initiating bond line failures as well as inducing 
stress in the lithium niobate. The stress changes the optical properties 
of the lithium niobate and may cause cracking of the chip at extreme 
temperatures. 
The prior art is limited to use of a substrate that matches the average 
thermal expansion coefficient of the lithium niobate. The prior art has 
also been limited to laboratory applications where the environmental 
extremes are limited. 
DISCLOSURE OF INVENTION 
Objects of the present invention include the provision of a wedge of 
anisotropic material which matches the anisotropic thermal expansion 
coefficients of a lithium niobate or lithium tantalate IO chip to a 
substrate, typically an isotropic substrate. Further objects include the 
provision of a wedge of anisotropic material which matches the isotropic 
thermal expansion coefficients of a silicon IO chip to a substrate, 
typically an isotropic substrate. 
According to a first aspect of the present invention, a wedge-shaped block 
of pyrolytic graphite or other suitable anisotropic material cut along 
certain predetermined angular dimensions is used as a thermal expansion 
coefficient transformer to match the anisotropic thermal expansion 
coefficients of a lithium niobate or lithium tantalate IO chip to an 
isotropic substrate material such as 316 stainless steel or aluminum, the 
wedge being mounted between the IO chip and the substrate, the anisotropic 
material used as the thermal transformer having a thermal expansion 
coefficient which is greater than the value of the thermal expansion 
coefficient of the IO chip material in one direction, and having a thermal 
expansion coefficient which is lesser than the value of the thermal 
expansion coefficient of the IO chip material in another direction. 
According to a second aspect of the present invention, a wedge-shaped block 
of pyrolytic graphite or other suitable anisotropic material cut along 
certain predetermined angular dimensions is used as a thermal expansion 
coefficient transformer to match the isotropic thermal expansion 
coefficients of a silicon IO chip to an isotropic substrate material such 
as 316 stainless steel or aluminum, the wedge being mounted between the IO 
chip and the substrate, the anisotropic material used as the thermal 
transformer having a thermal expansion coefficient which is greater than 
the value of the thermal expansion coefficient of the IO chip material in 
one direction, and having a thermal expansion coefficient which is lesser 
than the value of the thermal expansion coefficient of the IO chip 
material in another direction. 
The invention exactly matches the thermal expansion coefficients of the 
lithium niobate, lithium tantalate, or silicon and the substrate material 
by the use of the wedge which serves as a thermal expansion coefficient 
transformer. This thermal coefficient matching device makes possible an IO 
chip mounting system that will exceed the -55.degree. to +125.degree. C. 
environment typically required for electronic devices. 
These and other objects, features and advantages of the present invention 
will become more apparent in light of the detailed description of a best 
mode embodiment thereof, as illustrated in the accompanying drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 is a perspective illustration of an integrated optic (IO) chip 10 
fabricated from X-cut lithium niobate. Lithium niobate is an orientation 
dependent (anisotropic) device that has different thermal expansion 
coefficients in two different directions, e.g., along the X,Y axes, and Z 
axis, respectively. The coefficients, .alpha..sub.x and .alpha..sub.y, in 
the X and Y directions are 15.4.times.10.sup.-6 / .degree.C., while the 
coefficient, .alpha..sub.z, in the Z direction is 7.5.times.10.sup.-6 / 
.degree.C. 
FIG. 2 is a perspective illustration of a block 14 of pyrolytic graphite 
from which the thermal expansion coefficient transformer of the present 
invention is cut. As seen in greater detail with respect to FIG. 3, the 
transformer is cut in the shape of a wedge 18. The pyrolytic graphite's 
anisotropic thermal expansion coefficients are 1.times.10.sup.-6 / 
.degree.C. in both the X and Y directions (.alpha..sub.x, .alpha..sub.y), 
and 25.times.10.sup.-6 / .degree.C. in the Z direction (.alpha..sub.z). 
The mounting system in accordance with the present invention employs a 
wedge 18 of pyrolytic graphite as an intermediate block that is mounted 
between the lithium niobate and the substrate material 20 (FIG. 4) of 
choice. In the best mode embodiment of the present invention, the 
substrate material 20 is 316 stainless steel. However, other isotropic 
materials such as aluminum may be used if desired. By cutting the 
pyrolytic graphite at specific angles into a wedge, the anisotropic nature 
of the pyrolytic graphite adjusts the thermal expansion coefficient of the 
wedge's resulting mounting surfaces 22,24 to match the thermal expansion 
coefficients of both the lithium niobate IO chip and the substrate. 
The pyrolytic graphite has both higher and lower in magnitude thermal 
expansion coefficients as compared to the lithium niobate (i.e., higher 
than the value of the thermal expansion coefficient in the Z direction and 
lower than the value of the thermal expansion coefficient in the X,Y 
directions). Thus, the graphite can be cut at an angle of beta.sub.1 
(.beta..sub.1) in the Y/Z plane to produce an apparent thermal expansion 
coefficient, .alpha..sub.a1, along the resulting inclined plane, as 
described by Eq. 1 and illustrated in FIG. 2. 
EQU .alpha..sub.al =[(.alpha..sub.z *sin .beta..sub.1).sup.2 +(.alpha..sub.y 
*cos .beta..sub.1).sup.2 ].sup.1/2 (Eq. 1) 
As illustrated in FIG. 3, this angled cut technique is applied to the top 
of the block of pyrolytic graphite of FIG. 2, to achieve the appropriate 
angled dimensions of the wedge. By solving Eq. 1 for .beta..sub.1, the 
resulting wedge angle is 38.0 degrees in the Y/Z plane. 
In a similar manner, the graphite can also be cut at an angle of beta.sub.2 
(.beta..sub.2) in the X/Z plane to produce the apparent thermal expansion 
coefficient, .beta..sub.a2, along the resulting inclined plane, as 
described by Eq. 2 and illustrated in FIG. 2. 
EQU .alpha..sub.a2 =[(.alpha..sub.z *sin .beta..sub.2).sup.2 +(.alpha..sub.x 
*cos .beta..sub.2).sup.2 ].sup.1/2 (Eq. 2) 
By solving Eq. 2 for .beta..sub.2, the resulting wedge angle is 17.3 
degrees in the X/Z plane. The two angled cuts result in the graphite 
block's top surface having two different thermal expansion coefficients 
which exactly match the thermal expansion coefficients of the lithium 
niobate. 
The 316 stainless steel substrate material is isotropic in nature and has a 
thermal expansion coefficient of 16.2.times.10.sup.-6 / .degree.C. 
Equations 1 and 2 are used to determine the resulting angled cut, 
.beta..sub.3, to the block of graphite to match the thermal expansion 
coefficients of the graphite and the 316 stainless steel. The resulting 
value of .beta..sub.3 is 40.4 degrees. 
FIG. 4 is a perspective illustration of the wedge 18 of FIG. 3 sandwiched 
between the lithium niobate IO chip 10 of FIG. 1 and the substrate 
material 20. 
As described hereinbefore, the material used for the thermal matching 
transformer is pyrolytic graphite. However, it is to be understood that 
other materials may be used without deviating from the broadest scope of 
the present invention. These alternative materials include calcite, which 
has thermal expansion coefficients of 25.times.10.sup.-6 / .degree.C. in 
the X and Y directions, and -5.8.times.10.sup.-6 / .degree.C. in the Z 
direction. Other anisotropic materials which may be used in different 
applications include crystal quartz or barium titanate. It suffice for the 
present invention that the thermal matching material chosen have a thermal 
expansion coefficient which is greater than the value of the thermal 
expansion coefficient of the IO chip material in one direction, and have a 
thermal expansion coefficient which is lesser than the value of the 
thermal expansion coefficient of the IO chip material in another 
direction. 
Also, the IO chip material has been described as comprising lithium 
niobate. However, other suitable materials may be used if desired, such as 
lithium tantalate, which has a thermal expansion coefficient of 
16.1.times.10.sup.-6 / .degree.C. in a first direction, and a thermal 
expansion coefficient of 4.1.times.10.sup.-6 / .degree.C. in a second 
direction. 
Further, the IO chip material has been described as being anisotropic in 
nature. However, in accordance with a second aspect of the present 
invention, it is to be understood that the IO chip material may have 
isotropic thermal expansion coefficients. An example of such material is 
silicon which has an isotropic thermal expansion coefficient of 
2.6.times.10.sup.-6 / .degree.C. Thus, FIG. 1 may be interpreted to 
represent silicon having equal thermal expansion coefficients 
.alpha..sub.x, .alpha..sub.y and .alpha..sub.z in the X, Y and Z 
directions, respectively. It follows that Equations 1 and 2 are used to 
determine the resulting angles .beta..sub.1, .beta..sub.2, and 
.beta..sub.3, at which the block of anisotropic material of FIG. 2 is cut 
to match the isotropic thermal expansion coefficients of the silicon IO 
chip to the graphite, and to match the anisotropic thermal expansion 
coefficients of the graphite to the substrate. 
Although the invention has been illustrated and described with respect to a 
best mode embodiment thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions, and 
additions in the form and detail thereof may be made without departing 
from the spirit and scope of the invention.