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Description  |
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FIELD OF THE INVENTION
This invention relates to the field of semiconductor device testing. More
specifically, a method and apparatus for testing an unpackaged
semiconductor die is described.
BACKGROUND OF THE INVENTION
Many types of semiconductor devices are made using similar manufacturing
procedures. A starting substrate, usually a thin wafer of silicon, is
doped, masked, and etched through several process steps, the steps
depending on the type of devices being manufactured. This process yields a
number of die on each wafer produced. Each die on the wafer is given a
brief test for full functionality, and the nonfunctional die are
mechanically marked or mapped in software. This brief test is only a gross
measure of functionality, and does not insure that a die is completely
functional or has specifications that would warrant its assembly in a
package.
If the wafer has a yield of grossly functional die which indicates that a
good quantity of die from the wafer are likely to be fully operative, the
die are separated with a die saw, and the nonfunctional die are scrapped
while the rest are individually encapsulated in plastic packages or
mounted in ceramic packages with one die in each package. After the die
are packaged they are rigorously electrically tested. Components which
turn out to be nonfunctional or which operate at questionable
specifications are scrapped or devoted to special uses.
Packaging unusable die only to scrap them after testing is a waste of time
and materials, and is therefore costly. Given the relatively low profit
margins of commodity semiconductor components such as dynamic random
access memories (DRAMs) and static random access memories (SRAMs), this
practice is uneconomical. However, no thorough and cost effective method
of testing an unpackaged die is available which would prevent this
unnecessary packaging of nonfunctional and marginally functional die.
It is proposed that multiple integrated circuit devices be packaged as a
single unit, known as a multi chip module (MCM). This can be accomplished
with or without conventional lead frames. This creates two problems
compared to conventional test methods. Firstly, discrete testing is more
difficult because the conventional lead frame package is not used.
Furthermore, when multiple devices are assembled into a single package,
the performance of the package is reduced to that of the die with the
lowest performance. In other words, the ability to presort the individual
dice is limited to that obtained through probe testing. Secondly, the
packaging may have other limitations which are aggravated by burn-in
stress conditions so that the packaging becomes a limitation for burn-in
testing.
The practice of packaging die only to find the component must be scrapped
can especially adversely affect yields on multi-chip modules (MCMs). With
MCMs, several unpackaged die are assembled into a single component, then
the component is tested as a single functional unit. If a single die is
nonfunctional or operates outside of acceptable specifications, the entire
component fails and all die in the package are scrapped or an attempt is
made to "re-work" the MCM. There is presently no cost-effective way to
reclaim the functioning die. Statistically, the yields of MCMs decrease in
proportion to the increasing number of die in each module. The highest
density modules have the lowest yields due to their increased total
silicon surface area. For discretely packaged parts, if the product yield
of good parts from preliminary testing to final shipment (probe-to-ship)
is, for example, 95%, one would not be particularly concerned with
packaging costs for the failed parts, if packaging costs are 10% of the
product manufacturing costs. Where packaging costs are considerably
higher, however, as in ceramic packaged parts, testing a die before
packaging is economical when the cost of packaging divided by the package
yield is equal to or greater than the cost of testing:
C.sub.DIE .times.(C.sub.PACKAGE /Package Yield)=C.sub.DIE
.times.C.sub.ADDL.KGD
where
C=cost
C.sub.DIE =manufacturing cost of functional site
C.sub.ADDL.KGD =additional cost of testing unpackaged die in order to
produce known good die (KGD)
Note that in the case of discretely packaged parts, the cost of the die
(C.sub.DIE) is essentially not a factor since it is the same on both sides
of the equation. This changes in the case of MCMs having more than one
part type, for example memory and a microprocessor. Scrapping (or
reclaiming) the microprocessor after packaging because of malfunctioning
memory is much more costly than scrapping a module containing only memory:
(C.sub.DIE).times.(Number of Die/Die Yield).times.C.sub.PACKAGE =C.sub.DIE
.times.C.sub.ADDL.KGD
The above equation must be modified to account for varied costs and yields
of die in modules with mixed part types. With MCMs, the cost of packaging
a failed part is proportional to the number of die in the module. In the
case of a memory module having 16 die, where probe-to-ship yield of the
die is 95%, the costs are:
16/0.95.times.C.sub.PACKAGE =C.sub.ADDL.KGD
so the additional costs of testing for known good die (KGD) may be 16 times
the cost of testing an unrepairable module and still be economical. This,
of course, is modified by the ability to repair failed modules.
Testing of unpackaged die before packaging would be desirable as it would
result in reduced material waste, increased profits, and increased
throughput. Using only known good die in multichip modules would increase
yields significantly.
SUMMARY OF THE INVENTION
An object of the invention is to provide a means for testing and burning in
a semiconductor die before it is packaged.
This and other objects of the invention were realized by temporarily
placing a die in a ceramic package, then testing the assembled package. To
temporarily package the die, a ceramic housing was manufactured having
input/output leads electrically coupled with contacts on the interior of
the package. In one embodiment, gold alloy was chosen as a material from
which to manufacture the bumps because of its soft, conductive properties,
but other workable materials are possible.
The back (noncircuit) side of a discrete semiconductor die was mounted to a
lid, and bond pads on the die were aligned with the contacts on the
interior of the package. The bond pads were then contacted with the
contacts to form a temporary electrical contact therebetween. Since the
semiconductor package had the form of a conventional package, conventional
functional and parametric testing means was possible and completed. After
testing the package, the lid was removed from the ceramic package body,
and the gold bumps yielded without resulting damage to the die bond pads.
The known good die is then available for packaging in a multichip module,
for assembly in a conventional package, or for use in other systems using
unpackaged die.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross section of a first embodiment of the invention having a
die mounted to a lid;
FIG. 2 is a top view of the FIG. 1 embodiment;
FIG. 3 is a cross section of a second embodiment of the invention in which
a vacuum is used to secure a die to the lid during placement into a
carrier;
FIG. 4 is a cross section of a third embodiment of the invention having a
die mounted to the package housing; and
FIG. 5 is a cross section of a fourth embodiment of the invention in which
metal contacts couple an interconnect with a lower housing member.
It should be emphasized that the drawings of the instant application are
not to scale but are merely schematic representations and are not intended
to portray the specific parameters or the structural details of the
invention, which can be determined by one of skill in the art by
examination of the information herein.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a cross section, and FIG. 2 is a top view, describing an
embodiment of the present invention. A housing 10 was manufactured from
ceramic having a bottom shelf 12 for receiving a substrate insert 14 and a
pair of "bond" shelves 16. Upon the bond shelves 16 were housing
connection points 18 coupled with traces (not shown), the traces running
through the housing 10 and terminating externally to the housing 10. In
the instant case the traces manufactured within the housing comprised a
gold alloy and the housing 10 was manufactured using standard technology
used in the art of ceramic packaging of semiconductor die. Other materials
may function adequately for the housing 10 and the traces.
The bottom shelf 12 of the housing 10 received the insert interconnect
substrate 14, the insert 14 being permanently attached to the bottom shelf
14 of the housing 10. This attachment is preferably accomplished with a
room temperature vulcanizing silicone rubber (RTV, not shown) or with an
adhesive epoxy, although other attachment technology or materials can be
used. The insert 14 comprised a nonconductive support structure 14A, in
the instant case manufactured from ceramic, having electrically conductive
gold interconnection circuitry 14B, although other conductive materials
may be used. Precision contacts 20, manufactured in the instant case from
an alloy of gold, platinum, and palladium allowed for coupling with the
bond pads 22 on the semiconductor die 24 to be tested. The contacts 20
were approximately 0.001" in diameter, and extended 0.001" to 0.002" high
above the insert trace circuitry 14B. The contacts 20 were all coplanar to
within 10% across the contact pattern. An insert 14 thickness of
approximately 0.015" was sufficient, with outside dimensions being
slightly smaller than the cavity in the bottom of the housing 10. The
contacts 20 mirrored the bond pads 22 on the die 24, and were spaced so as
to contact the bond pads 22 on the semiconductor die 24 directly. The
contacts 20 were coupled with the insert substrate electrical traces 14B,
the insert traces 14B running from the contacts 20 to the edge of the
insert substrate 14A. The insert traces 14B terminated near the edge of
the insert substrate 14A.
In this embodiment, the interconnect insert 14 is a separate component
which is bonded to the bottom shelf 12 of the housing 10. It is also
possible to construct the invention with an interconnect structure
incorporated into the housing 10, rather than as a separate component.
The insert traces 14B which terminated near the edge of the insert
substrate 14A were wire bonded 26 to the connection points 18 within the
housing 10, and were therefore electrically coupled with leads 28 attached
to the traces (not shown) on the exterior of the housing 10. The leads 28
were coupled to the package using side brazing, but other coupling means
may be available. The leads 28 allow for coupling of the package with a
test fixture.
A package lid structure 30 was used upon which was mounted the
semiconductor die 24. The back (noncircuit) side of the die 24 was
attached to the lid 30 with a flat, soft sheet of a flexible adhesive
polymer material 32 that performed as both an adhesive tape and a force
loading material, although the use of such an adhesive material may not be
necessary. A semi-cured silicone material was used, and in the instant
case was silicone elastomer, from Gel Pak Corporation of Stanford, Calif.
The polymer allowed for the removal of the die 24 when desired, but
provided a sufficiently firm attachment to keep the die from shifting once
it was attached. The adhesive properties of the polymer, therefore, were
relatively strong in shear but relatively weak in tension and allowed for
separation from the die with no residual adhesive material left on the
back of the die. This property also allowed the polymer sheet to be
removed from the lid and replaced, since the material may cure and lose
its adhesive properties over time, or with elevated temperatures which may
be present during the testing phase of the assembled package. Curing of
the silicone material to a point that it is no longer sufficiently
flexible and soft can cause problems with cushioning the semiconductor die
and force loading of the insert substrate contacts.
The lid 30 was a commercially available, low cost metal item. A lid
approximately 0.01" in thickness was found to be sufficient, although any
reasonable lid thickness would function sufficiently, and in the
embodiment, the lid was a 0.045" thick austenitic stainless steel. Other
materials may function adequately.
Once the die 24 was attached to the lid 30, the lid 30 was positioned over
the insert 14. The bond pads 22 on the die 24 were aligned with the insert
contacts 20 on the insert 14. In the instant case, an alignment system,
available from Research Devices of Piscataway, N.J., was used to
vertically align the bond pads 22 on the die 24 with the contacts 20 on
the insert 14. The alignment system is usually used for flip chip die
attachment, but functions sufficiently in the inventive capacity. After
vertical alignment, the alignment system lowered the lid assembly to
contact the bond pads 22 on the die 24 with the contacts 20 on the insert
14. The "bumps" that made up the contacts 20 contacted the metal bond pads
22 of the die 24 sufficiently to ensure a good electrical contact.
Sufficient contact force must be maintained to insure good electrical
contact between the insert contacts 20 and the die bond pads 22. Excessive
contact force, however, will cause damage to the die bond pads 22 (usually
manufactured from aluminum) and underlying die circuitry. The contact
force must be sufficient to push the insert substrate contacts 20 through
the layer of aluminum oxide (not shown) which typically forms on the
aluminum bond pads 22. Penetration of the aluminum oxide layer is
necessary for good electrical contact, since aluminum oxide is a poor
electrical conductor. A force of about 80 grams per contact was found to
be sufficient for contacts and bond pads as described above.
After mating the bond pads 22 with the contacts 20, metal clips 34 held the
lid 30 in place to prevent shifting of the lid 30, and therefore the die
24, during testing. Removably "tacking" the lid 30 to the housing 10, for
example with solder or epoxy, may also be sufficient as long as the lid 30
can be removed from the housing 10 without damage to the lid 30 or housing
10.
Since the assembled package was similar to a conventional ceramic
semiconductor package, a conventional test sequence, including burning in,
was used to ensure functionality of the die. After the die was tested, the
lid was removed from the package by removing the clips, and the die was
removed from the lid. Nonfunctional die can be discarded while the known
good die can be packaged or shipped directly to customers. In any case,
packaging only known good die increases the yield of packaged devices. As
long as the die is not damaged before packaging, any malfunctions of the
packaged die results from an improperly packaged device. Poor wire bonds,
cracked packages, and improper die attachments are the most likely causes
of device malfunctions using such die.
It was found that the test package 10, 14, lid 30, and polymer 32 could be
used to test a plurality of semiconductor die 24. The substrate contacts
20 flattened out slightly during the first use, but it was found that if
the pressure between the lid assembly and the housing 10 are controlled,
an adequate electrical contact would be maintained during subsequent die
testing.
As shown in FIG. 3 embodiment, a lid 40, for example of a 0.045" thick
austenitic stainless steel, had a hole 42 therethrough to aid in securing
the die 24 in alignment with the lid 40 during a flip chip process. To
attach the die 24 to the lid 40, a vacuum device (not shown) picked up the
lid 40 with the vacuum device placed over the hole 42. The vacuum was
sufficient to hold the lid 40. The die 24 was then aligned with the lid
40, and the vacuum also picked up the die 24 thereby holding the die 24 in
aligned contact with the lid 40. The lid 40 and die 24 were then lowered
onto the insert 14, thereby permitting the alignment of the bond pads with
the insert contacts 20. This vacuum arrangement was found to preclude the
need for the adhesive polymer which was used in the previous embodiment.
A third embodiment of the invention, described in FIG. 4, may also be used
to test a discrete semiconductor die 24. In this embodiment, a housing 10
with traces within as described for a first embodiment was formed. A layer
of adhesive polymer 32 was placed on the bottom shelf 12 of the housing
10, and the noncircuit side of a die 24 is attached to the housing 10 with
the polymer 32.
Next, a lid 30 was formed and an insert 14 of ceramic or other workable
material was permanently attached thereto with epoxy or other material
(not shown). The insert 14 had traces and contacts 20 thereon as
previously described for the embodiment of FIGS. 1 and 2. To electrically
couple the bond pads of the die 24 to the input/output leads 28 that are
attached to the traces (not shown) running through the housing 10, the
contacts 20 on the insert 14 were aligned with the bond pads of the die
24. The lid 30 was lowered, and the contacts 20 on the insert 14 contacted
the bond pads on the die 24, forming an electrical connection therewith.
The insert 14 was larger than the insert of the first embodiment, and
extended past the edge of the bond shelf 16. The ends of the traces 44 on
the insert 14 were aligned with the contact points 18 on the bond shelf
16, and electrical contact was made using a sheet of Z-axis conductive
polymer 46, available from A.I. Technology, of Princeton, N.J. Other
coupling means are possible, and an alternative is described below. The
conductive polymer 46 was interposed between the contact points 18 on the
bond shelf 16 and the insert traces 44 which extended to the edge of the
insert 14. As long as a minimum force was maintained between the lid 30
and the housing 10, the electrical contact between the insert traces 44
and the contact points 18 was maintained. An electrical pathway extends
from the bond pads on the die 24, through the contacts 20 on the insert
14, through the traces 44 on the insert 14, through the Z-axis conductive
polymer 46 to the contact points 18 on the bond shelf 16, through the
traces (not shown) within the housing 10, and finally to the leads 28
attached thereto.
After assembly, the lid 30 was attached to the housing 30, for example by
using clips, by solder tacking the lid 30 to the housing 10, or by other
workable means. Any means used, however, must allow for the removal of the
lid without damage to either the lid or the housing.
As shown in FIG. 5, a number of folded or curved metal contacts 48 were
supported around an elastomer (not shown) and were attached to the contact
points 18 on the housing bond shelf 16. The folded contacts 48 made
electrical connection with the insert interconnect traces (not shown) when
contacts 20 on the insert 14 were brought down into contact with the bond
pads on the die 24.
While this invention has been described with reference to illustrative
embodiments, this description is not meant to be construed in a limiting
sense. Various modifications of the illustrative embodiments, as well as
additional embodiments of the invention, will be apparent to persons
skilled in the art upon reference to this description. It is therefore
contemplated that the appended claims will cover any such modifications or
embodiments as fall within the true scope of the invention. For example,
contacts can be manufactured from another material other than the gold,
palladium, and platinum alloy described, and indeed other means for
coupling the bond pads with the traces on the insert may be contemplated.
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