U.S. patent application number 12/214001 was filed with the patent office on 2008-10-16 for membrane probing system.
This patent application is currently assigned to Cascade Microtech, Inc.. Invention is credited to Clarence E. Cowan, Mike P. Dauphinais, Martin J. Koxxy, Kenneth R. Smith, Paul A. Tervo.
Application Number | 20080252316 12/214001 |
Document ID | / |
Family ID | 26880533 |
Filed Date | 2008-10-16 |
United States Patent
Application |
20080252316 |
Kind Code |
A1 |
Tervo; Paul A. ; et
al. |
October 16, 2008 |
Membrane probing system
Abstract
A membrane probing assembly includes a probe card with
conductors supported thereon, wherein the conductors include at
least a signal conductor located between a pair of spaced apart
guard conductors. A membrane assembly includes a membrane with
contacts thereon, and supporting at least a signal conductor
located between a pair of spaced apart guard conductors. The guard
conductors of the probe card are electrically interconnected
proximate the interconnection between the probe card and the
membrane assembly. The guard conductors of the membrane assembly
are electrically interconnected proximate the interconnection
between the probe card and the membrane assembly.
Inventors: |
Tervo; Paul A.; (Vancouver,
WA) ; Smith; Kenneth R.; (Portland, OR) ;
Cowan; Clarence E.; (Newberg, OR) ; Dauphinais; Mike
P.; (Beaverton, OR) ; Koxxy; Martin J.;
(Hillsboro, OR) |
Correspondence
Address: |
CHERNOFF, VILHAUER, MCCLUNG & STENZEL
1600 ODS TOWER, 601 SW SECOND AVENUE
PORTLAND
OR
97204-3157
US
|
Assignee: |
Cascade Microtech, Inc.
|
Family ID: |
26880533 |
Appl. No.: |
12/214001 |
Filed: |
June 16, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11509176 |
Aug 23, 2006 |
7403025 |
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12214001 |
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11144852 |
Jun 3, 2005 |
7148711 |
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11509176 |
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10909229 |
Jul 29, 2004 |
6930498 |
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11144852 |
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09637527 |
Nov 29, 2000 |
6838890 |
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10909229 |
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60184851 |
Feb 25, 2000 |
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Current U.S.
Class: |
324/754.1 |
Current CPC
Class: |
G01R 1/06772 20130101;
G01R 31/2889 20130101; G01R 1/0735 20130101 |
Class at
Publication: |
324/754 |
International
Class: |
G01R 1/067 20060101
G01R001/067 |
Claims
1. A probing assembly comprising: (a) a card with conductors
supported thereon, including: (i) an elongate card signal conductor
including an end portion and a first portion having a first edge
and a second edge; and (ii) a card guard conductor comprising a
first portion spaced apart from said first edge of said first
portion of said card signal conductor, a second portion spaced
apart from said second edge of said first portion of said signal
conductor and a third portion interconnecting said first and said
second portions of said guard conductor proximate to, but spaced
apart from said end portion of said card signal conductor; and (b)
an assembly including a membrane supporting: (i) an elongate
flexible signal conductor including an end portion and a first
portion having a first edge and a second edge; and (ii) a guard
conductor comprising a first portion spaced apart from said first
edge of said first portion of said signal conductor, a second
portion spaced apart from said second edge of said first portion of
said signal conductor and a third portion interconnecting said
first and said second portions of said guard conductor proximate
to, but spaced apart from said end portion of said signal
conductor.
2. The probing assembly of claim 1 wherein said signal conductor
includes a portion in conductive contact with said card signal
conductor and said guard conductor includes a portion in conductive
contact with said card guard conductor.
3. The probing assembly of claim 2 wherein said portion of said
guard conductor in contact with said card guard conductor contacts
said card guard conductor at a location nearer said first edge of
said first portion of said card signal conductor than said second
edge of said first portion of said card signal conductor.
4. The probing assembly of claim 1 wherein at least one of said
first portion and said third portion of said guard conductor
further comprises a contact region arranged for conductive contact
with a portion of said guard conductor, said contact region having
a first edge proximate said card signal conductor and a second edge
more remote from said card signal conductor than either edge of
said first portion of said card guard conductor.
5. The probing assembly of claim 1 further comprising: (a) a second
elongate card signal conductor supported by said card, said second
card signal conductor terminating proximate to, but spaced apart
from said end portion of said card signal conductor and comprising
a portion interposed between, and spaced apart from, said second
edge of said card signal conductor and said card guard conductor;
and (b) a second elongate signal conductor supported by a flexible
material, said second signal conductor terminating proximate to,
but spaced apart from said end portion of said signal conductor and
comprising a portion interposed between, and spaced apart from,
said second edge of said signal conductor and said guard
conductor.
6. The probing assembly of claim 5 wherein said signal conductor
includes a portion in conductive contact with said card signal
conductor; said second signal conductor includes a portion in
conductive contact with said second card signal conductor; and said
guard conductor includes a portion in conductive contact with said
card guard conductor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/509,176, filed Aug. 23, 2006, which is a
continuation of U.S. patent application Ser. No. 11/144,852, filed
Jun. 3, 2005, now U.S. Pat. No. 7,148,711, which is a continuation
of U.S. patent application Ser. No. 10/909,229, filed Jul. 29,
2004, now U.S. Pat. No. 6,930,498, which is a continuation of U.S.
patent application Ser. No. 09/637,527, filed Nov. 29, 2000, now
U.S. Pat. No. 6,838,890, which claims the benefit of U.S.
Provisional App. 60/184,851, filed Feb. 25, 2000.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to probe assemblies of the
type commonly used for testing integrated circuits (IC).
[0003] The trend in electronic production has been toward
increasingly smaller geometries particularly in integrated circuit
technology wherein a very large number of discrete circuit elements
are fabricated on a single substrate or "wafer." After fabrication,
this wafer is divided into a number of rectangular-shaped chips or
"dice" where each die presents a rectangular or other regular
arrangement of metallized contact pads through which input/output
connections are made. Although each die is eventually packaged
separately, for efficiency sake, testing of the circuit formed on
each die is preferably performed while the dies are still joined
together on the wafer. One typical procedure is to support the
wafer on a flat stage or "chuck" and to move the wafer in X, Y and
Z directions relative to the head of the probing assembly so that
the contacts on the probing assembly move from die to die for
consecutive engagement with each die. Respective signal, power and
ground lines are run to the probing assembly from the test
instrumentation thus enabling each circuit to be sequentially
connected to the test instrumentation.
[0004] One conventional type of probing assembly used for testing
integrated circuits provides contacts that are configured as
needle-like tips. These tips are mounted about a central opening
formed in a probe card so as to radially converge inwardly and
downwardly through the opening. When the wafer is raised beyond
that point where the pads on the wafer first come into contact with
these tips, the tips flex upwardly so as to skate forwardly across
their respective pads thereby removing oxide buildup on the
pads.
[0005] The problem with this type of probing assembly is that the
needle-like tips, due to their narrow geometry, exhibit high
inductance so that signal distortion is large in high frequency
measurements made through these tips. Also, these tips can act in
the manner of a planing tool as they wipe across their respective
pads, thereby leading to excessive pad damage. This problem is
magnified to the extent that the probe tips bend out of shape
during use or otherwise fail to terminate in a common plane which
causes the more forward ones of the tips to bear down too heavily
on their respective pads. Also, it is impractical to mount these
tips at less than 100 micron center-to-center spacing or in a
multi-row grid-like pattern so as to accommodate the pad
arrangement of more modern, higher density dies. Also, this type of
probing assembly has a scrub length of the needle tips of 25
microns or more, which increases the difficulty of staying within
the allowed probing area.
[0006] In order to reduce inductive losses, decrease pad wear, and
accommodate smaller device geometries, a second type of probing
assembly has been developed that uses a flexible membrane structure
for supporting the probing contacts. In this assembly, lead lines
of well-defined geometry are formed on one or more plies of
flexible insulative film, such as polyimide or MYLAR.TM.. If
separate plies are used, these plies are bonded together to form,
for example, a multilayered transmission line structure. In the
central portion of this flexible structure or membrane, each
conductive line is terminated by a respective probing contact which
is formed on, and projects outwardly from, an outer face of the
membrane. These probing contacts are arranged in a predetermined
pattern that matches the pattern of the device pads and typically
are formed as upraised bumps for probing the flat surfaces
conventionally defined by the pads. The inner face of the membrane
is supported on a supporting structure. This structure can take the
form, for example, of a truncated pyramid, in which case the inner
face of the center portion of the membrane is supported on the
truncated end of the support while the marginal portions of the
membrane are drawn away from the center portion at an angle thereto
so as to clear any upright components that may surround the pads on
the device.
[0007] With respect to the membrane probing assembly just
described, excessive line inductance is eliminated by carefully
selecting the geometry of the lead lines, and a photolithographic
process is preferably used to enable some control over the size,
spacing, and arrangement, of the probing contacts so as to
accommodate higher density configurations. However, although
several different forms of this probing assembly have been
proposed, difficulties have been encountered in connection with
this type of assembly in reducing pad wear and in achieving
reliable clearing of the oxide layer from each of the device pads
so as to ensure adequate electrical connection between the assembly
and the device-under-test.
[0008] One conventional form of membrane probing assembly, for
example, is exemplified by the device shown in Rath European Patent
Pub. No. 259,163A2. This device has the central portion of the
sheet-like membrane mounted directly against a rigid support. This
rigid support, in turn, is connected by a resilient member
comprising an elastomeric or rubber block to the main body of the
assembly so that the membrane can tilt to match the tilt of the
device. Huff U.S. Pat. No. 4,918,383 shows a closely related device
wherein radially extending leaf springs permit vertical axis
movement of the rigid support while preventing it from tilting so
that there is no slippage or "misalignment" of the contact bumps on
the pads and further so that the entire membrane will shift
slightly in the horizontal plane to allow the contacts to "scrub"
across their respective pads in order to clear surface oxides from
these pads.
[0009] In respect to both of these devices, however, because of
manufacturing tolerances, certain of the contact bumps are likely
to be in a recessed position relative to their neighbors and these
recessed bumps will not have a satisfactory opportunity to engage
their pads since they will be drawn away from their pads by the
action of their neighbors on the rigid support. Furthermore, even
when "scrub" movement is provided in the manner of Huff, the
contacts will tend to frictionally cling to the device as they
perform the scrubbing movement, that is, there will be a tendency
for the pads of the device to move in unison with the contacts so
as to negate the effect of the contact movement. Whether any
scrubbing action actually occurs depends on how far the pads can
move, which depends, in turn, on the degree of lateral play that
exists as a result of normal tolerance between the respective
bearing surfaces of the probe head and chuck. Hence this form of
membrane probing assembly does not ensure reliable electrical
connection between each contact and pad.
[0010] A second conventional form of membrane probing assembly is
exemplified by the device shown in Barsotti European Patent Pub.
No. 304,868A2. This device provides a flexible backing for the
central or contact-carrying portion of the flexible membrane. In
Barsotti, the membrane is directly backed by an elastomeric member
and this member, in turn, is backed by a rigid support so that
minor height variations between the contacts or pads can be
accommodated. It is also possible to use positive-pressure air,
negative-pressure air, liquid or an unbacked elastomer to provide
flexible backing for the membrane, as shown in Gangroth U.S. Pat.
No. 4,649,339, Ardezzone U.S. Pat. No. 4,636,772, Reed, Jr. et al.
U.S. Pat. No. 3,596,228 and Okubo et al. U.S. Pat. No. 5,134,365,
respectively. These alternative devices, however, do not afford
sufficient pressure between the probing contacts and the device
pads to reliably penetrate the oxides that form on the pad
surfaces.
[0011] In this second form of membrane probing assembly, as
indicated in Okubo, the contacts may be limited to movement along
the Z-axis in order to prevent slippage and resulting misalignment
between the contacts and pads during engagement. Thus, in Barsotti,
the rigid support underlying the elastomeric member is fixed in
position although it is also possible to mount the support for
Z-axis movement in the manner shown in Huff U.S. Pat. No.
4,980,637. Pad damage is likely to occur with this type of design,
however, because a certain amount of tilt is typically present
between the contacts and the device, and those contacts angled
closest to the device will ordinarily develop much higher contact
pressures than those which are angled away. The same problem arises
with the related assembly shown in European Patent Pub. No.
230,348A2 to Garretson, even though in the Garretson device the
characteristic of the elastomeric member is such as to urge the
contacts into lateral movement when those contacts are placed into
pressing engagement with their pads. Yet another related assembly
is shown in Evans U.S. Pat. No. 4,975,638 which uses a pivotably
mounted support for backing the elastomeric member so as to
accommodate tilt between the contacts and the device. However, the
Evans device is subject to the friction clinging problem already
described insofar as the pads of the device are likely to cling to
the contacts as the support pivots and causes the contacts to shift
laterally.
[0012] Yet other forms of conventional membrane probing assemblies
are shown in Crumly U.S. Pat. No. 5,395,253, Barsotti et al. U.S.
Pat. No. 5,059,898 and Evans et al. U.S. Pat. No. 4,975,638. In
Crumly, the center portion of a stretchable membrane is resiliently
biased to a fully stretched condition using a spring. When the
contacts engage their respective pads, the stretched center portion
retracts against the spring to a partially relaxed condition so as
to draw the contacts in radial scrub directions toward the center
of the membrane. In Barsotti, each row of contacts is supported by
the end of a respective L-shaped arm so that when the contacts in a
row engage their respective pads, the corresponding arm flexes
upwardly and causes the row of contacts to laterally scrub
simultaneously across their respective pads. In both Crumly and
Barsotti, however, if any tilt is present between the contacts and
the device at the time of engagement, this tilt will cause the
contacts angled closest to the device to scrub further than those
angled further away. Moreover, the shorter contacts will be forced
to move in their scrub directions before they have had the
opportunity to engage their respective pads due to the controlling
scrub action of their neighboring contacts. A further disadvantage
of the Crumly device, in particular, is that the contacts nearer to
the center of the membrane will scrub less than those nearer to the
periphery so that scrub effectiveness will vary with contact
position.
[0013] In Evans et al. U.S. Pat. No. 5,355,079 each contact
constitutes a spring metal finger, and each finger is mounted so as
to extend in a cantilevered manner away from the underlying
membrane at a predetermined angle relative to the membrane. A
similar configuration is shown in Higgins U.S. Pat. No. 5,521,518.
It is difficult, however, to originally position these fingers so
that they all terminate in a common plane, particularly if a high
density pattern is required. Moreover, these fingers are easily
bent out of position during use and cannot easily be rebent back to
their original position. Hence, certain ones of the fingers are
likely to touch down before other ones of the fingers, and scrub
pressures and distances are likely to be different for different
fingers. Nor, in Evans at least, is there an adequate mechanism for
tolerating a minor degree of tilt between the fingers and pads.
Although Evans suggests roughening the surface of each finger to
improve the quality of electrical connection, this roughening can
cause undue abrasion and damage to the pad surfaces. Yet a further
disadvantage of the contact fingers shown in both Evans and Higgins
is that such fingers are subject to fatigue and failure after a
relatively low number of "touchdowns" or duty cycles due to
repeated bending and stressing.
[0014] Referring to FIG. 1, Cascade Microtech, Inc. of Beaverton,
Oreg. has developed a probe head 40 for mounting a membrane probing
assembly 42. In order to measure the electrical performance of a
particular die area 44 included on the silicon wafer 46, the
high-speed digital lines 48 and/or shielded transmission lines 50
of the probe head are connected to the input/output ports of the
test instrumentation by a suitable cable assembly, and the chuck 51
which supports the wafer is moved in mutually perpendicular X, Y, Z
directions in order to bring the pads of the die area into pressing
engagement with the contacts included on the lower contacting
portion of the membrane probing assembly.
[0015] The probe head 40 includes a probe card 52 on which the
data/signal lines 48 and 50 are arranged. Referring to FIGS. 2-3,
the membrane probing assembly 42 includes a support element 54
formed of incompressible material such as a hard polymer. This
element is detachably connected to the upper side of the probe card
by four Allen screws 56 and corresponding nuts 58 (each screw
passes through a respective attachment arm 60 of the support
element, and a separate backing element 62 evenly distributes the
clamping pressure of the screws over the entire back side of the
supporting element). In accordance with this detachable connection,
different probing assemblies having different contact arrangements
can be quickly substituted for each other as needed for probing
different devices.
[0016] Referring to FIGS. 3-4, the support element 54 includes a
rearward base portion 64 to which the attachment arms 60 are
integrally joined. Also included on the support element 54 is a
forward support or plunger 66 that projects outwardly from the flat
base portion. This forward support has angled sides 68 that
converge toward a flat support surface 70 so as to give the forward
support the shape of a truncated pyramid. Referring also to FIG. 2,
a flexible membrane assembly 72 is attached to the support after
being aligned by means of alignment pins 74 included on the base
portion. This flexible membrane assembly is formed by one or more
plies of insulative sheeting such as KAPTON.TM. sold by E.I. Du
Pont de Nemours or other polyimide film, and flexible conductive
layers or strips are provided between or on these plies to form the
data/signal lines 76.
[0017] When the support element 54 is mounted on the upper side of
the probe card 52 as shown in FIG. 3, the forward support 66
protrudes through a central opening 78 in the probe card so as to
present the contacts which are arranged on a central region 80 of
the flexible membrane assembly in suitable position for pressing
engagement with the pads of the device under test. Referring to
FIG. 2, the membrane assembly includes radially extending arm
segments 82 that are separated by inwardly curving edges 84 that
give the assembly the shape of a formee cross, and these segments
extend in an inclined manner along the angled sides 68 thereby
clearing any upright components surrounding the pads. A series of
contact pads 86 terminate the data/signal lines 76 so that when the
support element is mounted, these pads electrically engage
corresponding termination pads provided on the upper side of the
probe card so that the data/signal lines 48 on the probe card are
electrically connected to the contacts on the central region.
[0018] A feature of the probing assembly 42 is its capability for
probing a somewhat dense arrangement of contact pads over a large
number of contact cycles in a manner that provides generally
reliable electrical connection between the contacts and pads in
each cycle despite oxide buildup on the pads. This capability is a
function of the construction of the support element 54, the
flexible membrane assembly 72 and their manner of interconnection.
In particular, the membrane assembly is so constructed and
connected to the support element that the contacts on the membrane
assembly preferably wipe or scrub, in a locally controlled manner,
laterally across the pads when brought into pressing engagement
with these pads. The preferred mechanism for producing this
scrubbing action is described in connection with the construction
and interconnection of a preferred membrane assembly 72a as best
depicted in FIGS. 6 and 7a-7b.
[0019] FIG. 6 shows an enlarged view of the central region 80a of
the membrane assembly 72a. In this embodiment, the contacts 88 are
arranged in a square-like pattern suitable for engagement with a
square-like arrangement of pads. Referring also to FIG. 7a, which
represents a sectional view taken along lines 7a-7a in FIG. 6, each
contact comprises a relatively thick rigid beam 90 at one end of
which is formed a rigid contact bump 92. The contact bump includes
thereon a contacting portion 93 which comprises a nub of rhodium
fused to the contact bump. Using electroplating, each beam is
formed in an overlapping connection with the end of a flexible
conductive trace 76a to form a joint therewith. This conductive
trace in conjunction with a back-plane conductive layer 94
effectively provides a controlled impedance data/signal line to the
contact because its dimensions are established using a
photolithographic process. The backplane layer preferably includes
openings therein to assist, for example, with gas venting during
fabrication.
[0020] The membrane assembly is interconnected to the flat support
surface 70 by an interposed elastomeric layer 98, which layer is
coextensive with the support surface and can be formed by a
silicone rubber compound such as ELMER'S STICK-ALL.TM. made by the
Borden Company or Sylgard 182 by Dow Corning Corporation. This
compound can be conveniently applied in a paste-like phase which
hardens as it sets. The flat support surface, as previously
mentioned, is made of incompressible material and is preferably a
hard dielectric such as polysulfone or glass.
[0021] In accordance with the above-described construction, when
one of the contacts 88 is brought into pressing engagement with a
respective pad 100, as indicated in FIG. 7b, the resulting
off-center force on the rigid beam 90 and bump 92 structure causes
the beam to pivot or tilt against the elastic recovery force
provided by the elastomeric pad 98. This tilting motion is
localized in the sense that a forward portion 102 of the beam moves
a greater distance toward the flat support surface 70 than a
rearward portion 104 of the same beam. The effect is such as to
drive the contact into lateral scrubbing movement across the pad as
is indicated in FIG. 7b with a dashed-line and solid-line
representation showing the beginning and ending positions,
respectively, of the contact on the pad. In this fashion, the
insulative oxide buildup on each pad is removed so as to ensure
adequate contact-to-pad electrical connections.
[0022] FIG. 8 shows, in dashed line view, the relative positions of
the contact 88 and pad 100 at the moment of initial engagement or
touchdown and, in solid-line view, these same elements after
"overtravel" of the pad by a distance 106 in a vertical direction
directly toward the flat support surface 70. As indicated, the
distance 108 of lateral scrubbing movement is directly dependent on
the vertical deflection of the contact 88 or, equivalently, on the
overtravel distance 106 moved by the pad 100. Hence, since the
overtravel distance for each contact on the central region 80a will
be substantially the same (with differences arising from variations
in contact height), the distance of lateral scrubbing movement by
each contact on the central region will be substantially uniform
and will not, in particular, be affected by the relative position
of each contact on the central region.
[0023] Because the elastomeric layer 98 is backed by the
incompressible support surface 70, the elastomeric layer exerts a
recovery force on each tilting beam 90 and thus each contact 93 to
maintain contact-to-pad pressure during scrubbing. At the same
time, the elastomeric layer accommodates some height variations
between the respective contacts. Thus, referring to FIG. 9a, when a
relatively shorter contact 88a is situated between an immediately
adjacent pair of relatively taller contacts 88b and these taller
contacts are brought into engagement with their respective pads,
then, as indicated in FIG. 9b, deformation by the elastomeric layer
allows the smaller contact to be brought into engagement with its
pad after some further overtravel by the pads. It will be noted, in
this example, that the tilting action of each contact is locally
controlled, and the larger contacts are able, in particular, to
tilt independently of the smaller contact so that the smaller
contact is not urged into lateral movement until it has actually
touched down on its pad.
[0024] Referring to FIGS. 10 and 11, the electroplating process to
construct such a beam structure, as schematically shown in FIG. 8,
includes the incompressible material 68 defining the support
surface 70 and the substrate material attached thereon, such as the
elastomeric layer 98. Using a flex circuit construction technique,
the flexible conductive trace 76a is then patterned on a
sacrificial substrate. Next, a polyimide layer 77 is patterned to
cover the entire surface of the sacrificial substrate and of the
traces 76a, except for the desired location of the beams 90 on a
portion of the traces 76a. The beams 90 are then electroplated
within the openings in the polyimide layer 77. Thereafter, a layer
of photoresist 79 is patterned on both the surface of the polyimide
77 and beams 90 to leave openings for the desired location of the
contact bumps 92. The contact bumps 92 are then electroplated
within the openings in the photoresist layer 79. The photoresist
layer 79 is removed and a thicker photoresist layer 81 is patterned
to cover the exposed surfaces, except for the desired locations for
the contacting portions 93. The contacting portions 93 are then
electroplated within the openings in the photoresist layer 81. The
photoresist layer 81 is then removed. The sacrificial substrate
layer is removed and the remaining layers are attached to the
elastomeric layer 98. The resulting beams 90, contact bumps 92, and
contacting portions 93, as more accurately illustrated in FIG. 12,
provides the independent tilting and scrubbing functions of the
device.
[0025] Another suitable technique of the construction of a membrane
probe is disclosed in co-pending U.S. patent application, Ser. No.
09/115,571, incorporated by reference herein. However, for the
inventions described herein, the present inventors have no
preference as to the particular construction of the contacting
portion of the membrane assembly nor the general structure of the
membrane or membrane assembly itself.
[0026] While providing an improved technique for effective
scrubbing action is significant, the present inventors determined
that excessive noise still remains in the signals sensed by the
measurement device.
BRIEF SUMMARY OF THE INVENTION
[0027] The present invention overcomes the aforementioned drawbacks
of the prior art by providing a membrane probing assembly with a
probe card that includes conductors supported thereon, wherein the
conductors include at least a signal conductor located between a
pair of spaced apart guard conductors. A membrane assembly includes
a membrane with contacts thereon, and supporting at least a signal
conductor located between a pair of spaced apart guard conductors.
The guard conductors of the probe card are electrically
interconnected proximate the interconnection between the probe card
and the membrane assembly. The guard conductors of the membrane
assembly are electrically interconnected proximate the
interconnection between the probe card and the membrane
assembly.
[0028] The foregoing and other objectives, features, and advantages
of the invention will be more readily understood upon consideration
of the following detailed description of the invention, taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0029] FIG. 1 is a perspective view of a membrane probing assembly
bolted to a probe head and a wafer supported on a chuck in suitable
position for probing by this assembly.
[0030] FIG. 2 is a bottom elevational view showing various parts of
the probing assembly of FIG. 1, including a support element and
flexible membrane assembly, and a fragmentary view of a probe card
having data/signal lines connected with corresponding lines on the
membrane assembly.
[0031] FIG. 3 is a side elevational view of the membrane probing
assembly of FIG. 1 where a portion of the membrane assembly has
been cut away to expose hidden portions of the support element.
[0032] FIG. 4 is a top elevational view of an exemplary support
element.
[0033] FIGS. 5a-5b are schematic side elevational views
illustrating how the support element and membrane assembly are
capable of tilting to match the orientation of the device under
test.
[0034] FIG. 6 is an enlarged top elevational view of the central
region of the construction of the membrane assembly of FIG. 2.
[0035] FIGS. 7a-7b are sectional views taken along lines 7a-7a in
FIG. 6 first showing a contact before touchdown and then showing
the same contact after touchdown and scrub movement across its
respective pad.
[0036] FIG. 8 is a schematic side view showing, in dashed-line
representation, the contact of FIGS. 7a-7b at the moment of initial
touchdown and, in solid-line representation, the same contact after
further vertical overtravel by the pad.
[0037] FIGS. 9a and 9b illustrate the deformation of the
elastomeric layer to bring the contacts into contact with its
pad.
[0038] FIG. 10 is a longitudinal sectional view of the device of
FIG. 8.
[0039] FIG. 11 is a cross sectional view of the device of FIG.
8.
[0040] FIG. 12 is a more accurate pictorial view of the device
shown in FIGS. 10 and 11.
[0041] FIG. 13 is partial plan view of a membrane assembly and a
probe card.
[0042] FIG. 14A is a partial pictorial view of the traces of a
membrane assembly.
[0043] FIG. 14B is a partial plan view of the interconnection
between a membrane assembly and a probe card.
[0044] FIG. 14C is a partial sectional side view of the
interconnection between the membrane assembly and the probe card of
FIG. 14B.
[0045] FIG. 15 is a partial sectional view of a probe card
illustrating the leakage currents from the end portions of the
signal and guard conductors.
[0046] FIG. 16 is a partial sectional view of a probe card
illustrating the interconnecting of a pair of guard conductors
together with a signal conductor therebetween.
[0047] FIG. 17 is a partial plan view of a portion of a probe card
illustrating power conductors, signal conductors, force conductors,
sense conductors, removed interconnecting portions, and
interconnected guard conductors.
[0048] FIGS. 18A-18D are a partial plan view of a portion of a
membrane assembly illustrating signal conductors, force conductors,
sense conductors, and interconnected guard conductors.
[0049] FIG. 19 is a partial plan view of a probe card and a
membrane assembly suitable for a Kelvin connection.
[0050] FIG. 20 is a partial plan view of a probe card illustrating
different geometries for the interconnection to a membrane
assembly.
[0051] FIG. 21 is a partial plan view of a membrane assembly
illustrating a guard conductor looping around a respective probing
device.
[0052] FIGS. 22A-22C are a plan view of a "pogo-pin" probe card
constructed in accordance with aspects of the present invention,
where the connections to the probe card are normally electrical
contacts from a probe ahead positioned above the probe card.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0053] With particular regard to probe cards that are specially
adapted for use in measuring ultra-low currents, probe card
designers have been concerned with developing techniques for
controlling (e.g., minimizing) leakage currents. Unwanted currents
that flow into a particular cable (or conductor)from surrounding
cables (or conductors) may distort the current measured in that
particular cable (or conductor). For a given potential difference
between two spaced apart conductors, the amount of leakage current
that will flow between them will vary depending upon the volume
resistivity of the insulating material that separates the
conductors. In other words, if a relatively lower-resistance
insulator is used, this will result in a relatively higher leakage
current. Thus, a designer of low-current probe cards will normally
avoid the use of rubber-insulated single-core wires on a
glass-epoxy board since rubber and glass-epoxy materials are known
to be relatively low-resistance insulators through which relatively
large leakage currents can flow.
[0054] One technique that has been used for suppressing
inter-channel leakage currents is positioning the signal conductor
between a pair of guard conductors, where each guard conductor is
maintained at the same potential as the signal conductor by a
feedback circuit in the output channel of the test instrument.
Because the voltage potentials of the guard conductors and the
respective signal conductor are made to substantially track each
other, negligible leakage current will flow from the signal
conductor to the corresponding guard conductors. Although leakage
current can still flow between different sets of guard conductors,
this is typically not a problem because the guard conductors,
unlike the signal conductors, are at low impedance. By using this
guarding technique, significant improvements may be realized in the
low-level current measuring capability of certain probe card
designs by reducing the capacitance between signal and guard, and
increasing the resistance between signal and guard.
[0055] To further improve low-current measurement capability, the
membrane assembly is constructed so as to likewise minimize leakage
currents between the individual probing devices. Typically, this
minimization involves the selection of membrane materials and
likewise providing limited guarding of the signal conductor by a
pair of guard conductors to a location proximate the probing
device. Referring to FIG. 13, to provide the guarded path to a
location proximate the probing devices each respective signal
conductor 200 is located between a pair of respective guard
conductors 202, 204 on the probe card 52, and the membrane assembly
72 likewise has a matching set of signal conductors 206 and guard
conductors 208, 210. It is thought that this arrangement provides
continuous sets of signal conductor/guard conductors to a location
proximate the probing devices in a manner to achieve low leakage
along nearly its entire length. However, even with the guarding of
the signal conductors on the probe card 52 and the membrane
assembly 72, the leakage current levels remain unacceptable for
low-current low-noise measurements.
[0056] In other probe card designs, efforts have been directed
toward systematically eliminating low-resistance leakage paths
within the probe card and toward designing extensive and elaborate
guarding structures to surround the signal conductors along the
signal path. For example, in one design, the entire glass-epoxy
main board is replaced with a board of ceramic material which
presents a relatively high resistance to leakage currents. However,
the ceramic material used in these newer designs is relatively more
expensive than the glass-epoxy material it replaces. Another
problem with ceramic materials is that they are relatively
susceptible to the absorption of surface contaminants such as can
be deposited by the skin during handling of the probe card. These
contaminants can decrease the surface resistivity of the ceramic
material to a sufficient extent as to produce a substantial
increase in the leakage current levels. In addition, the more
extensive and elaborate guarding structures that are used in these
newer designs has contributed to a large increase in design and
assembly costs.
[0057] It should be noted that there are other factors unrelated to
design that can influence whether or not the potential of a
particular probe card for measuring low-level currents will be
fully realized. For example, if less special care is taken in
assembling the probe card, it is possible for surface contaminants,
such as oils and salts from the skin or residues left by solder
flux, to contaminate the surface of the card and to degrade its
performance (due to their ionic character, such contaminants can
produce undesirable characteristics). Furthermore, even assuming
that the card is designed and assembled properly, the card may not
be suitably connected to the test instrument or the instrument may
not be properly calibrated so as to completely null out, for
example, the effects of voltage and current offsets. The probe card
or the interconnecting lines can serve as pickup sites for ac
fields, which ac fields can be rectified by the input circuit of
the test instrument so as to cause errors in the indicated dc
values. Thus, it is necessary to employ proper shielding procedures
for (1) the probe card, (2) the interconnecting lines, and (3) the
test instrument in order to shield out these field disturbances.
Due to these factors, when a new probe card design is being tested,
it can be extremely difficult to isolate the causes of undesirable
background current in the new design due to numerous and possibly
interacting factors that may be responsible.
[0058] The present inventors reconsidered a seemingly improbable
source of noise, namely, the interconnection between the probe card
52 and the membrane assembly 72, which from initial considerations
would appear to be effective at providing a guarded signal path to
the probe device because of the "continuous" signal path upon
interconnection. However, upon further consideration the present
inventors determined that there is in fact significant unguarded
and/or unshielded leakage paths existing in the region proximate
the interconnection. Referring to FIG. 14A, each conductive path of
the membrane is normally encapsulated within at least one layer of
material (FIG. 14A illustrates multiple conductive paths without
additional membrane materials). This provides a structure for
routing conductive paths, such as the signal and guard conductors,
to a location proximate the probing device without being on the
exterior (lower surface) of the membrane assembly which may result
in inadvertent contact with the device under test. Referring to
FIGS. 14B-14C, the signal and guard lines are actually
interconnected between the probe card 52 and the membrane assembly
72 by conductive structures 220 that pass through the outer layer
222 of the membrane assembly 72 to the interior conductive paths
206, 208, 210 of the membrane assembly 72. To form the electrical
connection, the probe card 52 and membrane assembly 72 are
mechanically aligned, and accordingly respective conductive
structures 220 of the membrane assembly 72 are interconnected with
the conductors 200, 202, 204 of the probe card 52. It is normally
undesirable for the membrane assembly 72 interconnection to
electrically connect at the absolute end of the conductors 200,
202, 204 (e.g., signal conductor and guard conductors) of the probe
card 52 because then the tolerances for the interconnection would
be extremely small, requiring nearly perfect alignment and
extremely accurate fabrication. Accordingly, normally the signal
and guard conductors supported by the probe card 52 extend beyond
the region of electrical interconnection.
[0059] After further consideration, the present inventors came the
realization that this extension of the signal and/or guard
conductors beyond the location of electrical connection results in
significant additional leakage paths. Referring to FIG. 15, the
region 216 beyond the end of the guard conductors provides for
surface leakage paths 218, which are primarily DC in nature with
the characteristic of an added resistance between the respective
conductive paths. This surface leakage path from a signal conductor
around the end of the adjacent guard conductors reduces the
accuracy of measurements by increasing the leakage currents. Also,
the present inventors likewise realized that the region 216 beyond
the end of the guard conductors provides for a bulk leakage path,
which is primarily AC (e.g., not DC) in nature with the
characteristic of an added capacitance, between the signal
conductor and the conductors beyond the adjacent guard conductors.
This bulk leakage path from the signal conductor around the end of
the adjacent guard conductors reduces the accuracy of measurements
by increasing the leakage currents. It is to be noted that the
guard conductors, in effect, impose a guard voltage into the bulk
of the probe card in a region generally underneath the respective
guard conductor. This reduces the bulk capacitive leakage currents
from the interposed signal conductor in regions with an adjacent
guard conductor.
[0060] In many embodiments, the opening 230 into which the membrane
assembly 72 is supported includes a conductive surface 232 therein
(e.g., guard, shield, ground) to further isolate the membrane
assembly 72 from the probe card 52. Unfortunately, the conductive
surface 232 results in significant fringe fields 234 (on the
surface and in the bulk of the probe card 52) at the end of the
signal conductors 200 and guard conductors 202, 204. These fringe
fields 234 appear to the measuring device as an additional parallel
capacitance and resistance. This fringe leakage path at the end of
the guard and signal conductors 200, 202, 204 reduces the accuracy
of measurements by increasing the leakage currents. The cumulative
result of the additional bulk leakage currents, additional surface
leakage currents, and additional fringe capacitance and resistance
(leakage currents), appears to the measuring device as a
capacitance and resistance lumped together with the measurements of
the actual device under test. It is difficult, if not nearly
impossible, to calibrate such additional leakage currents out of
any measurements so that the true measurement of the device under
test is obtained. Further, the additional capacitance results in an
increase in the settling time of signals thereby increasing the
time required to obtain a set of accurate measurements.
[0061] It is desirable to maximize the number of interconnections
available between the probe card 52 and the membrane assembly 72 in
order to provide the capability of probing an increasingly greater
number of devices under test. While increasing the size of the
membrane assembly 72 to provide a greater circumferential edge may
be employed, it remains desirable to limit the size of the membrane
assembly 72 to minimize the length of the conductive paths to
reduce leakage currents.
[0062] To increase the number of interconnections available between
the membrane assembly 72 and the probe card 52, the width of the
conductors of the membrane assembly 72 and the probe card 52 may be
decreased together with the spacing between the conductors. While
decreasing the size of the conductor increases the number of
interconnections for a given circumferential edge, this
unfortunately results in an increased difficultly of aligning the
respective conductive traces together. Further, the greater density
increases the manufacturing expense of the device.
[0063] In general, the membrane assembly 72 is suitable for a
higher density of conductive paths than the probe card 52.
Accordingly, the initial limit to the number of interconnects is
the ability to fabricate an increasingly greater number of
conductive traces on the probe card 52.
[0064] Referring to FIG. 16 the present inventors came to the
realization that the preferred solution to overcome the
aforementioned drawbacks of the presently accepted techniques is to
interconnect the guard conductors around the end of the signal
conductor, in contrast to the apparent solution of merely
decreasing the feature size of the interconnects. The
interconnecting portion 240 for each respective pair of guard
conductors (effectively one electrical conductor) is preferably on
the same plane, such as the top surface of the probe card 52,
together with the guard conductors and signal conductors. The
interconnecting portion 240 reduces the surface leakage path from
the signal conductor by interposing a guarded path around the end
of the signal conductor. In addition, the interconnecting portion
240 likewise decreases the bulk leakage path from a signal
conductor by imposing a guard voltage in a region of the bulk of
the probe card completely enclosing the end of the signal
conductor. Also, the fringe leakage path to the central conductive
surface 232 from the end of the signal conductor is substantially
reduced, or otherwise eliminated, by providing the guarded
interconnecting portion 240 around the end of the signal conductor.
Reducing the leakage currents by including the additional
interconnecting guard portion 240 results in the measurements made
from the measuring device to be more accurate because less leakage
currents are erroneously included in the measurements. In addition,
a decrease in the settling time of the signals is achieved which
reduces the time required to obtain a set of accurate measurements.
One or more of the aforementioned drawbacks and/or advantages may
be present and/or achieved depending upon the particular device and
implementation.
[0065] With the interconnecting portion 240 electrically
interconnecting together a pair of guard conductors 202, 204
another benefit is more achievable, namely, increasing the number
of potential interconnections, without necessarily changing the
size of the membrane assembly 72, without necessarily changing the
geometry of the individual conductors, and without necessarily
decreasing the spacing between adjacent conductors. Referring to
FIG. 17, the contacting region 250 for the contacts 220 of the
membrane assembly 72 on the probe card 52 are provided on at least
one side of the interconnected guard conductor 202, 204, 240. This
permits easier alignment of the membrane assembly 72 and the probe
card 52. The width of the guard conductor on the side generally
opposite the contacting region may be considerably thinner because
there is no contact by the membrane assembly 72 with that portion
of the guard conductor. The different widths of the guard
conductors proximate the end of the signal conductor permits a
greater density of conductors to be achieved, if desired, without
decreasing the mechanical tolerances required. A pair of contacts
(one on either side of the signal conductor) may be used, if
desired. As a result, the density of the interconnect between the
probe card 52 and the membrane assembly 72 is closer to the
capability of the membrane assembly 72.
[0066] Referring to FIG. 18, to provide a single contact between
the pair of guard conductors on the probe card 52 and a respective
pair of guard conductors of the membrane assembly 72, the guard
conductors of the membrane assembly 72 preferably include an
interconnecting guard portion 260 with the inderdisposed signal
conductor, in a manner similar to the interconnecting guard portion
240. The interconnecting membrane guard portion 260 provides many
of the same advantages as described above with respect to the
interconnecting probe guard portion 240. By including the
interconnecting membrane guard portion 260, only a single
conductive structure 220 needs to be provided between the membrane
assembly 72 and the probe card 52 for each set of guard
conductors.
[0067] Ideally in a two lead conductor system a "true Kelvin"
connection is constructed. This involves using what is generally
referred to as a force signal and a sense signal. The signal
conductor from one of the two conductors is considered the force
conductor, while the signal conductor from the other of the two
conductors is considered the sense conductor. The force conductor
is brought into contact with a test pad on the wafer. The force
conductor is a low impedance connection, so a current is forced
through the force conductor for testing purposes. The sense
conductor is a high impedance connection and is also brought into
contact with the same test pad (or a different test pad) on the
wafer, preferably in close proximity to the sense conductor, in
order to sense the voltage. As such the current versus voltage
characteristics of the test device can be obtained using the force
and sense conductors.
[0068] Referring to FIG. 19, one potential technique to achieve a
Kelvin connection with the membrane probing system is to design the
probe card 52 to include multiple sets of a force conductor, a
sense conductor, and a corresponding pair of guard connectors on
opposing sides of the force/sense conductors (preferably with the
interconnection portion). The membrane assembly 72 likewise
includes corresponding sets of a force conductor, a sense
conductor, and guard conductors (preferably with the
interconnecting portion). This provides a potential technique for
achieving a Kelvin connection but unfortunately this wastes
interconnection space on the probe card 52 in the event that a
Kelvin connection for any particular device under test is not
necessary. Alternatively, the probe card 52 may be redesigned for
each membrane probing assembly, which is typically unique for each
application. However, redesigning the probe card 52 for each
application is expensive and not generally an acceptable
solution.
[0069] While considering how to maintain one or more standard probe
cards 52, together with providing Kelvin connections for each line,
the present inventors initially observed that the probe card 52 has
more available surface area for routing the conductors further from
the interconnection between the probe card 52 and the membrane
assembly 72. With the additional surface area at regions not
proximate the interconnection between the probe card 52 and the
membrane assembly 72, a pair of conductive traces 280, 282 are
easily routed, the pair being located between a pair of guard
conductors 284, 286, to a location generally proximate the
interconnection (see FIG. 17). For non-Kelvin measurements, one of
the conductors may be used as the signal line with the remaining
interconnected conductor not used. If desired, the interconnection
270 between the two interconnected signal conductors may be removed
(open-circuited) for low noise measurements. However, with the two
signal conductors (e.g. force and sense) normally interconnected it
is a simple matter to break the interconnection 270 by removing a
portion of conductors at region 290. In the event of "quasi-Kelvin"
connections, the interconnection portion may be maintained and one
of the pair of conductors 280, 282 would be used as a force
conductor and the other conductor of the pair would be used as a
sense conductor. Quasi-Kelvin connections are generally formed by
the interconnection of a sense conductor and a force conductor at a
point before the device under test.
[0070] To accomplish effective probing with the membrane assembly
72, typically low impedance power conductors 300 are provided on
the probe card 52 to supply power to the probing devices of the
membrane assembly 72. The present inventors determined that the
interconnection 270 between the pair of conductors may be removed
and the force conductor 280 may be jumpered with a wire bond 302
(or any other suitable technique) to an unused power conductor on
the probe card 52. Each of the power conductors 300 on the probe
card 52 are preferably conductive members within the bulk of the
probe card 52, electrically connected to the surface of the probe
card 52 by using a set of vias 304, 306. Each power conductor is
routed to a location proximate the interconnection between the
probe card 52 and the membrane assembly 72. The power conductor is
normally a low impedance conductor. Because the force conductor is
a low-impedance connection designed to carry significant current it
is preferable to locate the force conductor outside of the guards
284, 286 of its corresponding sense conductor. In addition, because
the force conductor is a low-impedance path carrying significant
(non-negligible) current levels it does not necessarily require the
guarding provided by the guard conductors 284, 286 on opposing
sides of the sense conductor 282.
[0071] The power conductors, to which force conductors may be
interconnected with, are preferably routed within the bulk of the
probe card 52 in a region directly underneath the corresponding
sense conductor. The conductive power conductor provides additional
protection for the sense conductor to minimize leakage currents.
Alternatively, the power conductor may be routed on the top surface
(or bottom surface) of the probe card, if desired.
[0072] The power conductor is preferably routed to a point
"interior" to the end of the corresponding signal conductor using a
"via" 306 to the upper surface of the probe card 52. Accordingly,
the power conductor is available at a location suitable for
interconnection to the membrane assembly 72, if desired, while
likewise being available for interconnection as a force conductor.
In this manner, the same power conductor may be used to provide
power to the device under test, while likewise providing a force
connection, both of which in a manner that maintains the density of
the interconnection of the interface between the probe card 52 and
the membrane assembly 72. The actual use of the power conductors
depends on the application and the particular design of the
membrane assembly 72.
[0073] Another technique suitable to provide a greater density of
interconnects, and their corresponding interconnecting regions
(normally having a greater surface area for contact to the membrane
assembly 72) is to align the interconnects of the probe card 52 in
a non-linear fashion (e.g., some closer and some farther from the
edge of the probe card 52) around the circumference of the membrane
assembly 72, as shown in FIG. 20. A further technique suitable to
provide a greater density of interconnects, is to align the
interconnecting regions in an overlapping manner with respect to a
direction perpendicular to the adjacent membrane assembly 72. The
membrane assembly 72 would likewise have corresponding structures
suitable to interconnect to the two-dimensional structure of the
conductors of the probe card 52.
[0074] The present inventors came to the realization that the
membrane assembly is susceptible to absorption of moisture which
increases the leakage currents within the membrane assembly.
Referring to FIG. 21, another structure suitable to reduce leakage
currents for the probing devices is shown. Preferably, the guarded
conductors 310 of the membrane assembly 72 encircle the end of the
probing device 312, with the signal conductor connected thereto
314. Preferably, the guarded conductors 310 are within the bulk of
the membrane assembly 72 to prevent their inadvertent contact with
the device under test. Providing the guarded probing devices
significantly reduces the effects of leakage currents between the
probing devices, especially due to the effects of humidity.
However, the present inventors determined that the surface leakage
currents between adjacent probing devices may be reduced by
removing at least a portion of the membrane material (dielectric)
316 in a location proximate a portion of the guard conductors 310
and between the probing devices 312. In this manner, a portion of
the guard conductor 310 will be exposed to the surface, albeit
somewhat recessed from the surface of the membrane assembly 72,
thereby impeding the passage of surface leakage between probing
devices 312.
[0075] Referring to FIG. 22, in one embodiment of the present
invention a pogo pin probe card includes guarded signal paths and
is suitable for receiving a progo pin probe head for connection
thereto.
[0076] The terms and expressions which have been employed in the
foregoing specification are used therein as terms of description
and not of limitation, and there is no intention, in the use of
such terms and expressions, of excluding equivalents of the
features shown and described or portions thereof, it being
recognized that the scope of the invention is defined and limited
only by the claims which follow.
* * * * *