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Description  |
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FIELD OF THE INVENTION
This invention relates generally to semiconductor manufacture and more
particularly to a method for forming compliant metal contact pins on
active semiconductor dice and silicon interconnects. The contact pins are
particularly suited to testing unpackaged die in the manufacture of known
good die (KGD).
BACKGROUND OF THE INVENTION
Bare or unpackaged semiconductor dice are used extensively in the
manufacture of electronic devices. Known-good-die (KGD) is a collective
term that connotes unpackaged dice having the same quality and reliability
as the equivalent packaged product. The demand for known-good-die has led
to the development of test procedures for testing unpackaged semiconductor
dice.
For test and burn-in of an unpackaged die, a carrier replaces a
conventional single chip package in the testing process. The carrier
typically includes an interconnect element that allows a temporary
electrical connection to be made between the die and external test
circuitry. In addition, such a carrier must allow the necessary test
procedures to be performed without damaging the die. The bond pads on a
die are particularly susceptible to damage during the test procedure.
Recently semiconductor manufacturers have developed carriers for testing
discrete, unpackaged die. Different types of carriers are disclosed in
U.S. Pat. No. 4,899,107 to Corbett et al. and U.S. Pat. No. 5,302,891 to
Wood et al., which are assigned to Micron Technology, Inc., and in U.S.
Pat. No. 5,123,850 to Elder et al., and U.S. Pat. No. 5,073,117 to Malhi
et al., which are assigned to Texas Instruments.
One of the key design considerations for a carrier is the method for
establishing a temporary electrical connection with contact locations on
the die. These contact locations are typically pads such as bond pads or
test pads located on the face of the die. With some carriers, the die is
placed face down in the carrier and biased into contact with an
interconnect. The interconnect contains the contact structure that
physically contacts and forms an electrical connection with the die pads.
Exemplary contact structures include wires, needles, and bumps. The
mechanisms for making electrical contact include piercing the native oxide
of the die pad with a sharp point, breaking or burnishing the native oxide
with a bump, or scrubbing the native oxide with a contact adapted to move
across the die pad.
These different contact structures are designed to establish an electrical
connection with the bond pads of a die under test. Preferably this
connection is low-resistance and ohmic. Low-resistance connotes a
resistance that is negligible. An ohmic connection is one in which the
voltage appearing across the connection is proportional to the current
flowing for both directions of current flow. In the past it has been
difficult to establish a low-resistance ohmic connection with bond pads
while minimizing damage to the pads. A bond pad may only be about 1.mu.
thick and is thus relatively easy to damage. In general, each of the above
noted contact structures will displace the bond pad metallization and
damage the pad.
In addition to forming a low-resistance ohmic connection, it is also
desirable for a contact structure to be compliant in nature. Sometimes an
arrangement of bond pads may present a sloped or irregular surface
topography. This may be due to the mounting arrangement of the die in the
carrier or to the uneven topography of the bond pads across the surface of
the die. It is thus advantageous to form the interconnect with compliant
contacts adapted to conform to the vertical location of the bond pad. In a
similar manner, it is desirable for the contact to possess a resiliency
which permits it to return to its original position for reuse.
OBJECTS OF THE INVENTION
In view of the foregoing, there is a need in the art for improved contact
structures for testing and manufacturing semiconductor dice. Accordingly,
it is an object of the present invention to provide a method for forming
metal contact pins suitable for testing unpackaged semiconductor dice.
It is a further object of the present invention to provide an improved
contact structure that can be used to form contacts for temporary
interconnects and can also be used to form contacts on an active
semiconductor dice.
It is yet another object of the present invention to provide an improved
metal contact pin that is compliant, resilient and adapted to form a
temporarily electrical connection with a bond pad without damage to the
die.
Other objects, advantages, and capabilities of the present invention will
become more apparent as the description proceeds.
SUMMARY OF THE INVENTION
In accordance with the present invention an improved contact structure and
a method for forming the contact structure are provided. The contact
structure, simply stated, comprises metal contact pins attached to a
substrate. In a first embodiment of the invention the substrate is a
silicon interconnect and the contact pins are attached to the interconnect
in electrical communication with conductive traces. In a second
embodiment, the substrate is an active semiconductor die and the contact
pins are attached to die pads (e.g., bond pads, tests pads) on the die. In
either case, the contact pins are suitable for forming an electrical
connection with a mating contact location that is low-resistance and
ohmic.
The contact pins are formed with a compliant structure to permit flexure as
the contact pins are biased against the mating contact location to form an
electrical connection. As an example, an interconnect having contact pins
and a die having flat bond pads may be placed together in a test carrier.
As the interconnect and die are biased together in the carrier, the
contact pins on the interconnect form an electrical connection with the
bond pads on the die. The compliant structure of the contact pins permits
flexure to accommodate variations in the mating topographies. The
compliant structure can include a flexible spring segment for the contact
pins or an angled mounting arrangement for the contact pins. In addition,
the contact pins can include a contact ball on the end to facilitate the
formation of an electrical connection.
There are two preferred methods for attaching the contact pins to the die
pads of a semiconductor die or to the substrate of an interconnect. These
attachment methods are wire bonding and welding. During the attachment
process, the contact pins are formed with a compliant structure, such as a
spring segment, using localized heating and bending. Localized heating and
bending can be accomplished with a laser working in conjunction with a
wire bonder or a welding tool. A heated capillary wire feed system can
also be used to heat and bend the contact pins during the attachment
process. The contact ball is preferably formed on an end of each contact
pin at the completion of the attachment process.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1E are schematic cross sectional drawings illustrating the steps
in forming contact pins on a semiconductor die in accordance with the
invention using a wire bonding process;
FIG. 2 is a schematic cross sectional drawing of contact pins wire bonded
to a semiconductor die at an angle to provide a compliant contact
structure;
FIG. 3A is a schematic view illustrating a contact pin having a compliant
structure formed as a spring segment;
FIG. 3B is a schematic view illustrating a heating and shaping process for
forming the contact pin of FIG. 3A which uses a laser and wire bonding
tool;
FIG. 3C is a schematic view illustrating a heating and shaping process for
forming the contact pin of FIG. 3A which uses a heated capillary tool;
FIGS. 4A and 4B illustrate alternate embodiment contact pins having
compliant structures formed as spring segments;
FIGS. 5A-5E are schematic cross sectional drawings illustrating steps in
forming contact pins on a semiconductor die in accordance with the
invention using a welding process;
FIG. 6A is a perspective view of an interconnect having contact pins formed
in accordance with the invention;
FIG. 6B is an enlarged view of a portion of FIG. 6A;
FIG. 6C is an enlarged cross sectional view of a contact pin mating with a
flat bond pad on a semiconductor die to establish an electrical
connection;
FIG. 7A is a schematic drawing illustrating a portion of an interconnect
having contact pins formed in accordance with the invention with a spring
segment;
FIG. 7B is a schematic drawing illustrating a portion of an interconnect
having contact pins formed in accordance with the invention and mounted at
an angle with respect to a substrate of the interconnect to provide a
compliant structure; and
FIG. 8 is a perspective view of an interconnect having two different
embodiments of contact pins formed in accordance with the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIGS. 1A-1E the method of the invention is illustrated for
forming contact pins on an active semiconductor die 10. As shown in FIG.
1A, the semiconductor die 10 is a conventional die that includes a silicon
substrate 12 on which integrated circuits have been formed. Bond pads 14
are in electrical communication with the integrated circuits. During the
manufacturing process the die 10 is fabricated on a wafer with a large
number of other dice. Each die 10 on the wafer can subsequently be
singulated by saw cutting.
The bond pads 14 of the die 10 are embedded in a passivation layer (not
shown). The bond pads 14 are typically polygonal (e.g., square) metal
pads, about 100 .mu.m on a side, which are separated by a space of about
50 to 100 .mu.m. Typically, the bond pads 14 are formed of aluminum (Al)
using a metallization process. Metals other than aluminum are sometimes
used to form the bond pads on a semiconductor die but aluminum is most
prevalent. The aluminum bond pads 14 are used for establishing an
electrical connection between the integrated circuits on the die and the
outside world. Depending on the application, an electrical connection to
the bond pads is made using various methods such as wire bonding, tape
automated bonding or flip chip bonding. The present invention allows this
connection to be made using a contact pin formed with a compliant
structure.
Initially, as shown in FIG. 1B, a layer of photoresist 16 is spun onto the
wafer containing the die 10. The layer of photoresist 16 is exposed,
developed and etched to form a mask having openings 18. The openings 18
are in alignment with the bond pads 14 and have the same peripheral
configuration as the bond pads 14.
Next, as shown in FIG. 1C, an intermediate pad 20, formed of a conductive
material such as copper, is deposited in the openings 18 and onto the bond
pads 14. The intermediate pad 20 is on the order of 1 .mu.m to 50 .mu.m in
thickness and is approximately the same size as the bond pads 14. The
purpose of the intermediate pad 20 is to allow a solder pad 22 (FIG. 1D),
formed of a solderable material such as nickel, to be attached to the
aluminum bond pad 14. Although the solder pad 22 can be deposited directly
to the aluminum bond pad 14, the intermediate pad 20 formed of a material
such as copper, bonds more readily to the aluminum bond pad 14.
The intermediate pad 20 is deposited using an electroless electrolytic
plating process. Various metal films including copper (Cu), nickel (Ni),
gold (Au) and palladium (Pd) can be deposited through electroless
deposition using aqueous solutions comprising metal ions and reducing
agents. Electroless plating is often used to form contacts by filling a
via with a deposited metal. As an example, in the present application for
depositing Cu on Al, the aqueous base solution contains Cu++ ions and
ascorbic acid reducing agents. The intermediate pad 20 is plated onto the
bond pad 14 to a thickness that is less than the thickness of the layer of
photoresist 16. This permits the same opening 18 to be used for a
subsequent plating process.
Next, as shown in FIG. 1D, the solder pad 22 is deposited on the copper pad
20. The solder pad 22 may also be deposited using an electroless plating
process. As an example, an aqueous solution containing Ni+ ions and
dimethy aminoborane reducing agents can be used to deposit a nickel solder
pad 22. The solder pad 22 has a thickness of from 1 .mu.m to 50 .mu.m and
an outer peripheral configuration that matches the intermediate pad 20 and
the aluminum bond pad 14. The solder pad 22 is formed of a material that
can be easily soldered such as nickel, nickel alloys or gold.
Next, as shown in FIG. 1E, a contact pin 24 is attached to the solder pad
22. The contact pin 24 is formed of wire having a thickness of about 1 to
5 mils. Suitable materials for the contact pin 24 include copper, gold,
nickel alloys and spring steel alloys. The contact pin 24 is attached to
the solder pad 22 using a wire bonding apparatus. The contact pin 24 may
be bonded directly to the solder pad 22 or a solder material 25 may be
used during the wire bonding process.
Wire bonding apparatus are well known in the art. Wire bonding apparatus
are manufactured by Kulicke and Soffa Industries, Inc., Horsham, Pa. and
Mitsubishi Denki, Japan. U.S. Pat. No. 3,894,671 to Kulicke, Jr. et al.
and U.S. Pat. No. 4,877,173 to Fujimoto et al. disclose representative
wire bonding apparatus.
During the wire bonding process the end of the wire which forms the base 25
(FIG. 1E) of the contact pin 24, is heated by an electrical discharge or a
hydrogen torch to a molten state. This forms a ball of molten metal on the
base 25 of the contact pin 24. The molten ball is then pressed by a
bonding tool against the solder pad 22. Ultrasonic vibrations are also
applied to the molten ball as it is pressed against the solder pad 22.
This mechanically bonds the wire to the solder pad 22. The wire is then
electrically opened or sheared to form the contact pin 24.
Following the wire bonding process the layer of photoresist 16 can be
stripped from the substrate 12. Depending on the photoresist formulation
this can be accomplished using a suitable wet chemical stripper. In
addition, a protective layer (not shown) can then be applied to the
substrate 12 to protect the surface of the substrate and limit the
stresses applied to the contact pin substrate interface from the motion of
the contact pins 24.
With reference to FIG. 2, a compliant structure can be achieved by
positioning the contact pin 24A at an acute angle with respect to the
plane of the substrate 12. The contact pin 24 is thus able to flex as
indicated by arrows 26 as it is biased against a mating component (e.g., a
contact on a printed circuit board). The contact pin 24A may be positioned
at an angle by appropriate manipulation of the bonding tool of the wire
bonding apparatus.
With reference to FIGS. 3A and 3B, a technique for forming another
compliant contact structure for a contact pin 24B is shown. As shown in
FIG. 3A, the contact pin 24B includes a compliant structure formed as a
spring segment 34. In addition, the contact pin 24B includes a contact
ball 32 on the end. The contact ball 32 facilitates electrical connection
of the contact pin 24B with a mating contact location, such as a pad,
formed on an interconnect or other substrate (e.g., circuit board).
FIG. 3B illustrates a process for forming the shaped contact pin 24B with a
compliant spring segment 34. In an automated wire bonding apparatus,
movement of a wire bonding tool 26 in the x, y and z directions is under
computer control. Using appropriate software, the wire bonding tool 26 can
be moved in a path to manipulate a length of wire to form the spring
segment 34 (FIG. 3A). The spring segment 34 includes a flat s-shaped
segment as shown, or may be formed with multiple coils twisted into a
spiral. At the same time that the wire bonding tool 26 is moving a length
of wire through a specified path, a laser 28 focused through a lens 30
initiates localized heating of the wire. This softens the wire material so
that it can be formed into the desired shape, which upon cooling will be
permanent. The laser 28 or an electrical discharge arrangement or separate
shearing member, is then operated to clip the wire. The contact ball 32
(FIG. 3A) is formed using the laser 28 to heat the end of the contact pin
24B and form a molten ball. Alternately, the electrical discharge
arrangement or a hydrogen torch can be used to form the contact ball 32
(FIG. 3A) at the end of the contact pin 24B.
FIG. 3C illustrates another technique for heating and shaping a length of
wire using a heated capillary wire feed system. This technique is
essentially the same as that previously explained for FIG. 3B. In this
case, a capillary tool 33 having a heating element 31 is moved in a path
to manipulate the length of wire for shaping. One suitable heated
capillary tool 33 is manufactured by Kulicke and Soffa Industries, Inc.,
507 Prudential Road, Horsham, Pa 19044 and is a component of a wire
bonding apparatus.
FIGS. 4A and 4B illustrate alternate embodiment contact pins 24C and 24D.
In FIG. 4A, the contact pin 24C is formed with a spring segment 38 that
has multiple s-shaped segments. In FIG. 4B a contact pin 24D is formed
with a spring segment 40 having multiple s-shaped segments that are wider
at the base. In each case the spring segment 38 may be flat, as shown or
formed with a spiral twist.
Referring now to FIGS. 5A-5E an alternate embodiment process for forming
contact pins on the semiconductor die 10 is shown. The alternate
embodiment process attaches contact pins 24W (FIG. 5E) to the bond pads 14
of the die 10 using welding rather than wire bonding.
Initially a shorting layer 36 is blanket deposited over the substrate 12
and bond pads 14 of the semiconductor die 10. The shorting layer 36 is
preferably formed of a highly conductive material such as copper. Other
suitable materials include nickel or aluminum. The shorting layer is
deposited using a suitable metallization process such as electrolytic
plating, chemical vapor deposition or sputtering. The shorting layer 36
functions as an electrical path for a welding current. This helps to shunt
the welding current away from the integrated circuits which may be
adversely affected by the.
Next, as shown in FIG. 5B, a layer of photoresist 42 is deposited on the
shorting layer 36. The layer of photoresist 42 is developed and etched to
form a mask having a pattern of openings 44. Each opening 44 extends
through the layer of photoresist 42 to the shorting layer 36 and is
aligned with a bond pad 14 of the die 12.
Next, as shown in FIG. 5C weld pads 46 are deposited in the openings 44 and
onto the shorting layer 36. The weld pads 46 are deposited using an
electroless deposition process as previously described. The weld pads 46
are formed of a material that can be easily welded. Suitable materials
include nickel, gold and copper.
Next as shown in FIG. 5D, contact pins 24W are welded to the weld pads 46
using a welding process. In some embodiments the weld pads 46 are not
required as the contact pins 24W can be welded directly to the shorting
layer. Welding apparatus for semiconductor manufacture are similar in
construction to wire bonding apparatus and are used for the same purposes.
Suitable welding apparatus are manufactured by the previously identified
wire bonder manufactures. With minor modifications to the electrical
circuitry, flame-off components and grounding of the workholder, such a
wire bonding apparatus can be modified to perform the functions previously
outlined in FIGS. 3D and 3C (i.e., heating and shaping).
In general, a welding apparatus uses an electric current to generate the
heat necessary to form molten material at the interface of two metals. For
the present application the welding current is passed through the contact
pins 24W and to the weld pads 46. The shorting layer 36 provides a closed
loop to carry the current through all of the weld pads 46 formed on the
die 10. The welding current forms molten metal at the interface of each
contact pin 24W and its respective weld pad 46 and bonds these elements
together.
Next as shown in FIG. 5E, the layer of photoresist 42 is stripped. In
addition, the shorting layer 36 is patterned and etched so that base pads
48 are formed. This forms a stacked structure on each aluminum bond pad 14
which includes the base pad 48, the weld pad 46 and the welded contact pin
24W. In addition, a protective layer (not shown) can be applied to protect
the surface of the substrate 12 and limit the stresses applied to the
contact pin-weld spot interface from the motion of the contact pin 24W.
During the welding process, the contact pins 24W can be angled to form a
compliant structure as previously described. In addition, compliant
structures comprising spring segments may be formed substantially as
previously described by simultaneously heating and moving a length of wire
which forms the contact pin 24W.
Referring now to FIGS. 6A and 6B, an interconnect 50 having contact pins 56
formed in accordance with the invention is shown. The interconnect 50
includes a substrate 52 and a pattern of circuit traces 54 formed on the
substrate 52 as an aid in constructing and subsequently using the
interconnect 50. The circuit traces 54 can be formed by depositing and
patterning a conductive metal. A pattern of alignment fiducials 53 is also
formed on the substrate 52.
A preferred material for the substrate 52 is silicon which will have the
same coefficient of thermal expansion as a silicon die. Other suitable
materials include ceramic and silicon on sapphire. The interconnect 50 is
adapted for use with a carrier for testing bare or unpackaged
semiconductor dice. The interconnect 50 is adapted to establish temporary
electrical communication between a semiconductor die retained in the
carrier and external test circuitry. U.S. Pat. No. 5,302,891 to Wood et
al. entitled "Discrete Die Burn-In For Nonpackaged Die" discloses a
carrier that uses an interconnect.
In use, the interconnect 50 is placed within a test carrier, along with a
die, and the contact pins 56 formed on the interconnect 50 are placed in
contact with the bond pads on the die to establish an ohmic contact. The
circuit traces 54 of the interconnect are then placed in electrical
communication with external test circuitry. As an example, the circuit
traces 54 may be wire bonded to external connectors on the carrier which
connect to the test circuitry.
As shown in FIG. 6B, the contact pins 56 are formed on the circuit traces
54. The circuit traces 54 are formed of a conductive material such as a
thick film metal which is patterned as it is deposited on the substrate 52
(e.g., screen printing). As shown in FIG. 6C, the contact pin 56 includes
a c-shaped segment 64. In addition, the contact pin 56 includes a base
portion 68 that attaches to the circuit trace 54 and a tip portion 66
adapted to contact a bond pad 70 of a die under test. The contact pins 56
may be formed substantially as previously described using a wire bonding
process or a soldering process. Exemplary dimension for the contact pins
include a diameter of 1 to 5 mils and a height of from 3 to 10 mils.
Referring now to FIGS. 7A-7C, alternate embodiment interconnects having
contact pins formed in accordance with the invention are shown. In FIG.
7A, an interconnect 50A includes a silicon substrate 52A, a circuit trace
54A and a contact pin 60. The contact pin 60 is formed on the circuit
trace 54A using a wire bonding process substantially as previously
described. The contact pin 60 includes a base portion 76 attached to the
circuit trace 54A, a spring segment 72 and a contact ball 62 formed on the
end for contacting a bond pad of a die under test. The ball 62 is formed
using a gas flame (e.g., hydrogen gas) or electrical discharge as
previously described.
In FIG. 7B an interconnect 50B includes a silicon substrate 52B, a circuit
trace 54B and contact pin 60B. In addition, the contact pin 60B includes a
base portion 78 and a contact ball 62B for contacting the bond pad of a
die under test. In this case, a compliant structure is achieved by
attaching the contact pin 60B at an acute angle to the substrate 52B. In
other words, the axis of the contact pin 60B is situated at an acute angle
with respect to the plane of the substrate 52B. Furthermore, contact pin
60B rather than being attached directly to the circuit trace 54B, attaches
to the circuit trace via a bond site 74 formed on the circuit trace 54B.
The bond site 74 is a conductive layer, preferably a metal such as nickel,
that is plated or deposited on the circuit trace 54B. As before, the
contact ball 62B is formed using a gas flame or electrical discharge.
In FIG. 8, an interconnect 50C includes a silicon substrate 52C, circuit
traces 54C formed on the substrate 52C and two types of contact pins 56
and 58 formed on the circuit traces 54C. The contact pins 56 have been
previously described. Contact pins 58 are straight pins adapted for
connection to a socket or female member. In use of the interconnect 50C in
a test carrier for a die under test, the contact pins 56 are placed in
contact with contact locations on the die. Contact pins 58 are placed in
contact with external test circuitry.
Thus the invention provides a method for forming contact pins suitable for
use as contacts on active semiconductor dice or contacts on interconnects
useful for testing unpackaged semiconductor dice. The contact pins
preferably include a compliant structure to permit flexure during the
formation of an electrical connection with a contact location on a mating
component.
Although the invention has been described in terms of preferred
embodiments, it is intended that alternate embodiments of the inventive
concepts expressed herein be included within the scope of the following
claims.
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