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Description  |
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FIELD OF THE INVENTION
This invention relates generally to the burn-in and/or testing of
integrated circuit devices, and more particularly to the burn-in and/or
testing of integrated circuit devices in the form of wafers before they
have been diced to form integrated circuit chips.
In the manufacture and testing of integrated circuit chips, a common
technique is to form a wafer of semiconductive material such as silicon,
and to form in and on the wafer the necessary circuitry for a multiplicity
of integrated circuit chips. The wafer is formed with all of the circuitry
necessary for the functioning of each of the chips, including I/O contacts
for connecting to I/O signals, power and ground. The contacts may be for
either wire bond connections or compressive bonding connections or for
controlled collapse chip connect (C4) bonds which utilizes solder balls
for flip chip bonding.
Whatever type of contact is to be made to the chip, it is necessary for the
chip to be tested and a determination made as to whether the chip is good.
This testing normally includes what is known as a "chip burn-in." During
burn-in, the chips are heated to an elevated temperature above which they
are rated to function, and stressed by the application of voltage and
current, also often in excess of what the chip is rated to withstand. In
the past, this burn-in has normally taken place on the individual chips
after the wafer has been diced to form the individual chips, often after
the chips have been packaged. Thus, in the past, it was necessary to form
the chips, individually handle the chips and test the chips individually,
and only then were bad chips detected. An example of testing such chips is
shown in commonly assigned U.S. Pat. No. 5,420,520.
While this type of testing does work well, it requires a significant amount
of processing of the wafer following the formation of the circuitry on and
in the wafer before a determination is made that a chip is indeed bad.
Moreover, the device of U.S. Pat. No. 5,420,520, has a direct connection
between the tester head and the chip. When the tester head connection is
worn, the entire head must be discarded, or at least significantly
reworked.
It is therefore a principal object of the present invention to provide a
technique for testing and stressing the circuitry on an integrated circuit
device while at the wafer level and before the wafer has been diced to
provide individual chips. It is a related object of this invention to
provide a technique for testing integrated circuit devices at any level,
wherein inexpensive contact is provided between the chip and the test
device, which contact can be easily replaced as wear occurs.
SUMMARY OF THE INVENTION
According to the present invention, a technique for testing/stressing
integrated circuit devices, and especially wafers, having a plurality of
contacts on one face thereof arranged in a predetermined pattern is
provided. The technique includes providing an interposer, with the
interposer including a dielectric substrate having a device contact face
and a tester contact face. A first plurality of releasable connectors is
provided on the device contact face arranged in the same predetermined
pattern, and a second plurality of releasable connectors is provided on
the tester contact face, also arranged in the same predetermined pattern.
Conducting vias connect the corresponding connectors of the first and
second releasable connectors respectively. A test head is provided having
a plurality of contact pads also arranged in the same predetermined
pattern. Circuitry is provided on the test head to connect each of the
contact pads thereon with external leads extending to provide signal
contact to each of the contact pads on the test head. The interposer is
positioned between the IC device and the test head, with the contacts on
the IC device in contact with the first plurality of connectors and the
contact pads on the test head in contact with the second plurality of
connectors. Signals are provided to the connector pads from the electrical
leads for performing testing and/or burn-in of the integrated circuit
device. The test head can be heated to provide the necessary heat for
testing at elevated temperatures, although preferably the entire assembly
of the test head, interposer and IC device is placed in an oven which is
heated to a controlled temperature, at which point the electrical testing
signals are provided from an external source. The oven itself may, in some
instances, be provided with outlets to connect with the electrical leads,
thus connecting the leads to the exterior of the oven for the reception of
the electrical signals.
Preferably, the releasable connectors are formed of dendrites. Also, if the
connection mode of the chips is to be through solder balls, these solder
balls preferably are applied to the IC device prior to testing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a wafer after the circuitry has been formed
thereon for integrated circuit devices, but before it has been diced to
form individual chips;
FIG. 2 is a side elevational view of a wafer, test head, interposer and
platens in position for testing; and
FIG. 3 is a longitudinal, sectional exploded view of a portion of a test
head and interposer positioned for testing of a wafer according to this
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, and for the present to FIG. 1, a silicon
wafer 10 which has been circuitized to form IC devices is shown before the
wafer 10 has been diced to form individual integrated circuit chips. The
outline of where the dicing will take place to form the various chips 12
is shown in dotted lines. The circuitry includes a plurality of I/O pads
14 arranged in a predetermined pattern on one face 15 of the wafer 10. In
this case, the overall pattern is comprised of the patterns and all of the
chip pads. In the disclosed embodiment, the devices are being formed for
C4 connection and, hence, solder bumps or solder balls 16 are formed on
the I/O pads 14 (see FIG. 3). Signals must be conducted through the solder
bumps 16 and I/O pads 14 to provide the burn-in function and testing
function of the circuitry in the various chips 12 formed on the wafer 10.
Referring now to FIGS. 2 and 3, to provide the test signals, a test head 20
is provided which has a substrate 22 having contact pads 24 formed on a
contact pad face 26 thereof which are arranged in the same pattern as the
I/O pads 14 on the wafer 10. Conducting plated through holes 28 extend
internally into the substrate 22 and connect with internal wiring 29 to
provide the electrical leads or signal planes necessary for supplying
electrical signals to and reading electrical signals from the wafer 10.
The testing head also includes ground planes 30 and voltage planes 31, as
well as the signal planes 29. In the preferred embodiment, the test head
is formed from a plurality of flexible circuits 32 which are laminated or
otherwise joined at their central position to provide the connection to
the wafer. The flexible cover circuits 32 are maintained separately at
their apparent ends as shown in FIGS. 2 and 3 and are provided with
electrical connections 33 which connect to the circuitry 29. Indeed, the
circuitry 29 is a part of the flexible circuits 32. In one embodiment, a
base plate 34 is laminated to the bottom mount flexible circuit 32 which
can be used to provide the ground or power input.
An interposer 36 is provided which is formed of a dielectric material, as
will be described presently, which has a device contact face 38 and a test
head contact face 40. A set of connectors 42 is formed on the device
contact face 38, and a set of connectors 44 is formed on the tester
contact face 40. Conducting vias or through holes 46 extend from the
connectors 42 to the connectors 44.
Each of the connectors in the sets of connectors 42 and 44 are formed with
dendrites 48 on their surfaces to provide for releasable connection. The
provision of dendrites to form releasable connections is well known in the
art, as are the techniques for forming dendrites. Preferably, the
dendrites are formed from palladium (Pd). The forming of dendritic
connections is described in commonly assigned U.S. Pat. No. 5,137,461 to
Bindra, et al., entitled "Separable Electrical Connection Technology."
This patent describes separable and reconnectable electrical connections
for electrical equipment in the form of dendrites characterized by
elongated, cylindrical acicular morphology. The U.S. Pat. No. 5,137,461
also describes a technique for forming the dendrites.
In order to control the coefficient of thermal expansion (CTE) of the
interposer 36, it is preferred that a low CTE core 50, which can be formed
of either copper-invar-copper (CIC), molybdenum or invar, be provided for
controlling of the CTE of the interposer as will be explained presently.
To perform a test and burn-in function, the interposer 36 is positioned
between the wafer 10 and the test head 20, with the I/O pads 14 and
contact pads 24, connectors 42 and 44 all in alignment, and pressure is
asserted by a fixture including platens 52 and 54 as indicated by the
arrows in FIGS. 2 and 3 to squeeze the wafer 10, interposer 36 and test
head 20 together. The dendrites 48 on the connectors 42 engage and make
electrical contact with the solder bumps 16, and the dendrites 48 on the
connectors 44 engage and make contact with the contact pads 24 on test
head 20 providing a good electrical path between the electrical leads of
the flex cable 32 and the I/O pads 14 on the wafer 10. In the embodiment
depicted, the platens 52 and 54 are conventional platens in a laminating
press, and hence are heated as shown schematically by electric heating
element 56. The elements 56 are then heated in a controlled manner to heat
the wafer to the desired test temperature which conventionally is between
about 100.degree. C. and about 190.degree. C. The connectors 33 are
attached to a computer controlled signal generator (not shown) to generate
the test signals in a well known manner. The computer is programmed in a
well-known manner to perform the tests and burn-in functions.
In another embodiment, heated platens are not used. Rather, any clamping
device can be used to exert pressure on planes of the platens, and the
assembly of the wafer 10, interposer 36 and test head 20, together with
whatever device is clamping them together, is then put into a furnace and,
if the furnace has internal leads, the leads from the flex cable 32 are
inserted therein; otherwise the flex cable 32 extends from the furnace and
is attached to whatever device such as a computer that is used to provide
signals to the test head 20. Such connections and devices are well known
in the art and need not be shown or described further. The assembly of the
tester, interposer and wafer is then heated to the required temperature,
which as indicated previously normally is in the range of about
100.degree. C. to 190.degree. C. and the necessary burn-in and test
functions are performed at these conditions.
Because of the heating at elevated temperatures for testing, there is a
significant amount of thermal expansion which takes place in the various
elements of the structure being heated; and, as is well known, the larger
the object being heated, the more expansion takes place at the outer edges
thereof. Thus, while in the case of a chip, which is relatively small,
thermal expansion may not be extremely significant; nevertheless, in the
case of a wafer which is so much larger, thermal expansion can and does
play a significant role. Thus, it is desired to design and construct the
interposer 36 and tester to have CTE's which are close enough to that of
the wafer 10 so as to not disrupt testing at these elevated temperatures.
The CTE of silicon is about 2.5 ppm/.degree. C. Thus, the interposer should
be constructed to have a relatively close CTE. To this end, the dielectric
material of the interposer 36 can be either polytetrafluoroethylene (PTFE)
or polyimide, with the core 50 being either copper-invar-copper,
molybdenum or invar. If the core 50 is invar and the dielectric is either
PTFE or polyimide, the composite CTE of the interposer will be around 3;
and if the core 50 is made of CIC, the interposer 36 will have a CTE of
about 6 ppm/.degree. C. For many applications, silica filled PTFE is the
preferred material.
With the tester 20 being formed as shown and described, the signal planes
29 can be either shielded or unshielded. With shielded planes, the CTE
will be about 10 ppm/.degree. C., and with unshielded planes, the CTE will
be about 8.50 ppm/.degree. C. In any event, whatever the choice is, the
CTE's of all of these various elements should be reasonable close to the
other so that, upon heating, contact is maintained at all connections.
Preferably, the CTE of the interposer is between that of the wafer 10 and
the test head 50.
Following the testing, either the platens are disengaged or the device is
removed from the furnace, whichever the case may be; the assembly cooled
down; the wafer 10 and interposer 34 are removed from the test head; and
the wafer 10 is diced into chips saving those chips which are good and
discarding those which have been determined to be bad during the testing.
One of the advantages of the present invention is the use of an interposer
between the tester and the IC device being tested. The interposer is a
relatively inexpensive device and if, as often happens, after multiple
uses, the dendrites become worn or do not function as well as when they
were new, it is relatively easy to discard the interposer and provide a
new one. Whereas, if dendritic connectors were provided on the test head,
when the dendrites lose their efficacy, it would be necessary to discard,
or at least substantially rework, the entire test head. Thus, a
significant cost savings can be realized. Moreover, the interposer can be
customized to any particular wafer configuration, thus also resulting in a
significant cost savings over having to fashion a test head specific to
each configuration of wafer contacts.
Because of these features, it is also possible to use an interposer
effectively in testing individual chips, even though the thermal expansion
consideration is not as stringent.
Accordingly, the preferred embodiment of the present invention has been
described. With the foregoing description in mind, however, it is
understood that this description is made only by way of example, that the
invention is not limited to the particular embodiments described herein,
and that various rearrangements, modifications, and substitutions may be
implemented without departing from the true spirit of the invention as
hereinafter claimed.
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