Strain release contact system for integrated circuits

The problem of stress transmission from the outside of an integrated circuit package into the interior of the semiconductor has been significantly reduced by placing a micro-spring between the external solder ball and the interior tab. The process for manufacturing such a structure begins with a fully completed integrated circuit on whose surface freestanding metal posts are formed, each post being in contact with an I/O pad. Using a leveling plate at elevated temperature, the posts are given a permanent tilt relative to the surface and are then encapsulated in an elastomer. This subprocess may then be repeated as many times as desired with the direction in which the posts lean being changed by 90 degrees at each iteration. This results in the formation of an orthogonal spiral which acts as a coil spring to absorb stress originating at the solder ball.

FIELD OF THE INVENTION 
The invention relates to the general field of integrated circuit packaging 
with particular reference to the absorption of stress at the contacts. 
BACKGROUND OF THE INVENTION 
In the macro world (measured in fractions of an inch) spring loaded 
contacts are widely used because of their ability to absorb stresses 
associated with pressing two surfaces together. This is of particular 
importance if either, or both, surface is fragile and subject to damage if 
the contact force were to be transmitted to it. Contacting systems of this 
type are commonly formed by seating a ball in a cylindrical cavity, behind 
an opening that is slightly smaller than the diameter of the bail, and 
mounting a coil spring between the ball and the other end of the cylinder. 
Similar stresses are associated with contacting systems that are used in 
the micro world (measured in microns and fractions thereof) but, to the 
best of our knowledge, spring loaded contacts similar to those described 
above have not yet been developed. A particular example of a micro system 
in which a fragile set of surfaces need to make and maintain good contact 
with a second set of surfaces, is a packaged integrated circuit where 
contact pads that are part of a chip need to make contact with solder 
balls located on the outside of the package. These solder balls will be 
used later to attach the package to the next packaging level (for example 
a printed circuit board) and it is important that the associated thermal 
and mechanical stresses not be transmitted down to the chip level. 
The reason that spring loaded contacts have thus far not been used in the 
micro world is due, we believe, to the difficulty of fabricating a truly 
three dimensional object such as a coil spring (as opposed to a 
multi-layer structure with vertical connections between layers). 
The closest prior art to (though still substantially different from) the 
present invention that we have found are a pair of patents by Little (U.S. 
Pat. No. 5,663,596--structure) and (U.S. Pat. No. 5,665,648--method). In 
Little's invention, chip contacting pads are located on the vertical edges 
of the integrated circuit chip. The latter is attached to a substrate 
through spring contacts that have a base portion joined to a line on the 
substrate and a spring portion that presses against the edge of the chip. 
To fabricate the spring, a release layer is first laid down. Then a metal 
film is deposited under conditions known to induce high tensile stress so 
that, when the release layer is etched away, the freed metal curls up to a 
sufficient degree to allow the chip to be pushed against its underside, 
thereby forming a spring contact. 
While Little's structure is not a coil, coil-like structures have been 
described in the prior art for use as micro inductors. These structures 
are, however, not truly three dimensional and comprise multiple layers of 
open rings that are then connected in series by means of vertical vias. An 
example of this is described by Lee (U.S. Pat. No. 5,831,331). Such 
structures, while working as inductors, could not be used to provide 
stress relief as each ring is separated from its neighbors by a rigid 
layer. Even if the latter was made to be flexible, absorption of stress 
would be limited to the vias which have very little flexibility so that 
any, except very small, stresses would be transmitted between contacting 
surfaces. 
SUMMARY OF THE INVENTION 
It has been an object of the present invention to provide a structure 
capable of absorbing stress between an integrated circuits package and the 
semiconductor. 
Another object of the invention has been to provide a process for 
manufacturing said structure. 
A still further object of the invention has been that said process be fully 
compatible with existing manufacturing techniques for integrated circuits. 
These objects have been achieved by introducing a micro-spring between the 
solder ball and the contact tab. The process for manufacturing such a 
structure begins with a fully completed integrated circuit on whose 
surface freestanding metal posts are formed, each post being in contact 
with an I/O tab. Using a leveling plate at elevated temperature, the posts 
are given a permanent tilt relative to the surface and are then 
encapsulated in an elastomer. This subprocess may then be repeated as many 
times as desired with the direction in which the posts lean being changed 
by 90 degrees at each iteration. This results in the formation of an 
orthogonal spiral which acts as a coil spring that absorbs stress 
originating at the solder ball.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
We will disclose the present invention through a description of the process 
for its manufacture. This will also serve as a description of the 
structure of the invention. 
Referring now to FIG. 1, the process of the present invention begins with 
the provision of silicon wafer 1 which has reached the end of the IC 
manufacturing cycle so that it contains at least one fully completed 
integrated circuit, including a final passivation layer (if used) and, 
particularly, contact pads on the surface that connect to various points 
inside the integrated circuit. A blanket layer 5 of metal such as gold, 
copper, or silver is deposited on the surface of wafer 1 to a thickness 
between about 500 and 7,000 Angstroms. This is then coated with 
photoresist; layer 2 to a thickness between about 5 and 200 microns. This 
particular thickness is carefully controlled since it will determine the 
lengths of the various posts (or rods) that make up the structure. Using 
standard photolithographic techniques, layer 2 is then processed to form a 
mask that is present everywhere except for holes, such as 3, over the 
contact pads. The diameter of these holes, which is typically between 
about 5 and 100 microns, is also carefully controlled since this will 
determine the diameters of posts. Electrical contact is made to the 
blanket layer 5 so that, by means of electroplating, holes 3 can be filled 
with metal. Our preferred metal for this has been gold but other metals 
such as copper, solder, silver, or aluminum could also have been used. 
Referring now to FIG. 2, the photoresist is now removed, leaving behind 
free standing metal posts 21. Next, blanket layer 5 is removed by etching 
in Kl/l.sub.2 solution (in the case of gold) just long enough to not 
affect posts 21 (typically between about 1 and 20 minutes). Note that the 
posts have a slightly thickened appearance at their base. This is 
achievable through appropriate use of the negative photoresist. Although 
not explicitly illustrated, each post 21 is now attached to a single 
contact pad. 
A key feature of the invention is illustrated in FIG. 3. After heating the 
structure to a temperature between about 100 and 450.degree. C., leveling 
plate 35 is lowered onto the structure so that its under-surface is in 
uniform contact with the top ends of all metal posts 21. A leveling plate 
needs to have a melting point greater than that of the post metal and must 
not react with it. Typically a stainless steel plate about 2 cm. thick, 
having a surface that is optically flat, has been preferred. Through 
robotic control, a uniform force is applied to the top surface 35. This 
force (symbolized by arrows 36) is at an angle of approximately 45.degree. 
to the horizontal, resulting in posts 21 becoming tilted at an angle of 
between about 15 and 75 degrees relative to the substrate. Since the 
chosen temperature is above the point at which plastic flow of the metal 
begins, once tilted and then cooled down again, posts 21 retain their 
orientation relative to the surface after the leveling plate 35 is 
removed. 
Once the sloping posts have been formed, with the leveling plate still in 
position they are encapsulated in elastomer layer 42, as shown in FIG. 4, 
after the leveling plate has cooled down. The structure is placed in a 
moderate vacuum (around 0.1 torr), elastomer material, such as silicone 
elastomer, is dispensed along the wafer edge, following which the pressure 
is returned to atmospheric, allowing the elastomer to be sucked into the 
gap. Once the elastomer has filled all empty space between the leveling 
plate and the wafer surface, the former is removed. Thus, while the posts 
are fully covered, their free ends are left uncovered. Our preferred 
elastomer material has been silicone elastomer, but other materials such 
as polyimides or benzocydobutene (BCB) could also have been used. 
Referring now to FlG. 5, the above process is repeated so that posts 22 are 
formed, each member of 22 being attached to a free (uncovered) end of a 
post 21. As before, a leveling plate is used on metal posts 22 to tilt 
them away from the vertical. Although the angle of tilt is the same as 
before, the direction in which the posts are pushed is such that posts 22 
end up pointing in a direction that is orthogonal to that to which posts 
21 point. 
This is schematically illustrated in FIG. 6 where posts 21 have been drawn 
with broken lines to signify that are pointing in a direction that is 
normal to the plane of the figure while posts 22 are drawn with solid 
lines to signify that they lie in the plane of the figure. If desired, the 
process of the present invention could be terminated at this point. If 
this option is elected, then, following the introduction of second 
elastomer layer 43, contacts to the outer world are formed at the top 
surface of 43. First, pads 61 of an underlayer barrier metal such as 
chromium/chrome-copper/copper or similar copper metal systems are formed 
over all uncovered ends of posts 22 following which solder balls 62 are 
grown over and around pads 61 by means of electroplating or screen 
printing. Our preferred material for the solder balls has been eutectic 
solder but other compositions including high temperature solders, Sn.sub.5 
Pb.sub.95, or other no lead solder, such as indium-tin could also have 
been used. The solder balls had diameters between about 10 and 500 microns 
and are spaced, on average, a distance of between about 10 and 1,000 
microns apart. 
In the likely event that it was elected to not terminate the process after 
only two iterations, the above described steps may be repeated as many 
times as desired (in most cases this would be up to about 4 additional 
times), In FIG. 7 we illustrate a schematic cross-section of a structure 
in which 4 iterations were performed. As in FIG. 6, broken lines signify 
that posts are pointing into (or out of) the plane of the figure. Note 
that although posts 24 have been drawn with solid lines, they are not 
actually in the plane of the figure but in a plane parallel to it. 
FIG. 8 is an isometric view of the structure of the present invention. Five 
iterations of post formation are shown in this case. Top surface barrier 
pads and solder balls are not shown in the figure and the successively 
formed layers of elastomer are shown with broken lines although, in 
practice, they would form a single uniform layer in the final structure. 
The illustration shows how, through application of the present invention, 
a structure, functionally equivalent to a coil spring has been formed. We 
refer to this as an `orthogonal spiral` since it is made up of straight 
line segments that are orthogonally disposed relative to one another. 
In FIG. 9 we show an alternative optional embodiment of the invention. As 
in the previously described structures, that of FIG. 9 shows a series of 
posts (rods) 21 through 24 joined to form an orthogonal spiral. They have 
been encapsulated in elastomer through successive application of layers 
42-45. The key feature of this embodiment is the presence of joint 
strengthening discs 91 between the rods. These discs were formed, in each 
iteration, directly after the introduction of the elastomer. At that point 
in the process a layer of joint strengthening metal such as gold, copper, 
or nickel was deposited onto the elastomer. It was then patterned and 
etched to form joint strengthening discs 91 which were symmetrically 
disposed on and around the uncovered ends of the rods. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made without departing from the spirit and scope of the invention.