PROBE FOR TESTING A DEVICE UNDER TEST
BACKGROUND OF THE INVENTION The present invention relates to probe measurement systems for
measuring the electrical characteristics of integrated circuits or other microelectronic
devices at high frequencies.
There are many types of probing assemblies that have been developed
for the measurement of integrated circuits and other forms of microelectronic devices.
One representative type of assembly uses a circuit card on which the upper side are
formed elongate conductive traces that serve as signal and ground lines. A central
opening is formed in the card, and a needle-like probe tip is attached to the end of
each signal trace adjacent the opening so that a radially extending array of
downwardly converging needle-like tips is presented by the assembly for selective
connection with the closely spaced pads of the microelectronic device being tested.
A probe assembly of this type is shown, for example, in Harmon U.S. Patent Number
3,445,770. This type of probing assembly, however, is unsuitable for use at higher
frequencies, including microwave frequencies in the gigahertz range, because at such
frequencies the needle-like tips act as inductive elements and because there are no
adjoining elements present to suitably counteract this inductance with a capacitive
effect in a manner that would create a broadband characteristic of more or less
resistive effect. Accordingly, a probing assembly of the type just described is
unsuitable for use at microwave frequencies due to the high levels of signal reflection
and substantial inductive losses that occur at the needle-like probe tips.
In order to obtain device measurements at somewhat higher frequencies than are possible with the basic probe card system described above,
various related probing systems have been developed. Such systems are shown, for
example, in Evans U.S. Patent Number 3,849,728; Kikuchi Japanese Publication No.
1-209,380; Sang et al. U.S. Patent Number 4,749,942; Lao et al. U.S. Patent Number
4,593,243; and Shahriary U.S. Patent Number 4,727,319. Yet another related system
is shown in Kawanabe Japanese Publication No. 60-223,138 which describes a probe
assembly having needle-like tips where the tips extend from a coaxial cable-like
structure instead of a probe card. A common feature of each of these systems is that
the length of the isolated portion of each needle-like probe tip is limited to the region
immediately surrounding the device-under-test in order to minimize the region of
discontinuity and the amount of inductive loss. However, this approach has resulted
in only limited improvement in higher frequency performance due to various practical
limitations in the construction of these types of probes. In Lao et al., for example, the
length of each needle-like tip is minimized by using a wide conductive blade to span
the distance between each tip and the supporting probe card, and these blades, in turn,
are designed to be arranged relative to each other so as to form transmission line
structures of stripline type. As a practical matter, however, it is difficult to join the
thin vertical edge of each blade to the corresponding trace on the card while
maintaining precisely the appropriate amount of face-to-face spacing between the
blades and precisely the correct pitch between the ends of the needle-like probe tips.
One type of probing assembly that is capable of providing a controlled- impedance low-loss path between its input terminal and the probe tips is shown in
Lockwood et al. U.S. Patent Number 4,697,143. In Lockwood et al., a ground-signal- ground arrangement of strip-like conductive traces is formed on the underside of an
alumina substrate so as to form a coplanar transmission line on the substrate. At one
end, each associated pair of ground traces and the corresponding interposed signal
trace are connected to the outer conductor and the center conductor, respectively, of a
coaxial cable com ector. At the other end of these traces, areas of wear-resistant
conductive material are provided in order to reliably establish electrical connection
with the respective pads of the device to be tested. Layers of ferrite-containing
microwave absorbing material are mounted about the substrate to absorb spurious
microwave energy over a major portion of the length of each ground-signal-ground
trace pattern. In accordance with this type of construction, a controlled high-
frequency impedance (e.g., 50 ohms) can be presented at the probe tips to the device
under test, and broadband signals that are within the range, for example, of DC to 18
gigahertz can travel with little loss from one end of the probe assembly to another
along the coplanar transmission line formed by each ground-signal-ground trace
pattern. The probing assembly shown in Lockwood et al. fails to provide satisfactory
electrical performance at higher microwave frequencies and there is a need in
microwave probing technology for compliance to adjust for uneven probing pads.
To achieve improved spatial conformance between the tip conductors
of a probe and an array of non-planar device pads or surfaces, several high-frequency
probing assemblies have been developed. Such assemblies are described, for
example, in Drake et al. U.S. Patent Number 4,894,612; Coberly et al. U.S. Patent
Number 4,116,523; and Boll et al. U.S. Patent Number 4,871,964. The Drake et al. probing assembly includes a substrate on the underside of which are formed a
plurality of conductive traces which collectively form a coplanar transmission line.
However, in one embodiment shown in Drake et al., the tip end of the substrate is
notched so that each trace extends to the end of a separate tooth and the substrate is
made of moderately flexible nonceramic material. The moderately flexible substrate
permits, at least to a limited extent, independent flexure of each tooth relative to the
other teeth so as to enable spatial conformance of the trace ends to slightly non-planar
contact surfaces on a device-under-test. However, the Drake et al. probing assembly
has insufficient performance at high frequencies.
With respect to the probing assembly shown in Boll et al, as cited
above, the ground conductors comprise a pair of leaf-spring members the rear portions
of which are received into diametrically opposite slots formed on the end of a
miniature coaxial cable for electrical connection with the cylindrical outer conductor
of that cable. The center conductor of the cable is extended beyond the end of the
cable (i.e., as defined by the ends of the outer conductor and the inner dielectric) and
is gradually tapered to form a pin-like member having a rounded point. In accordance
with this construction, the pin-like extension of the center conductor is disposed in
spaced apart generally centered position between the respective forward portions of
the leaf-spring members and thereby forms, in combination with these leaf-spring
members, a rough approximation to a ground-signal-ground coplanar transmission
line structure. The advantage of this particular construction is that the pin-like
extension of the cable's center conductor and the respective forward portions of the
leaf-spring members are each movable independently of each other so that the ends of
these respective members are able to establish spatially conforming contact with any
non-planar contact areas on a device being tested. On the other hand, the transverse- spacing between the pin-like member and the respective leaf-spring members will
vary depending on how forcefully the ends of these members are urged against the
contact pads of the device-under-test. In other words, the transmission characteristic
of this probing structure, which is dependent on the spacing between the respective tip
members, will vary in an ill-defined manner during each probing cycle, especially at
high microwave frequencies.
Burr et al, U.S. Patent No. 5,565,788, disclose a microwave probe that
includes a supporting section of a coaxial cable including an inner conductor coaxially
surrounded by an outer conductor. A tip section of the microwave probe includes a
central signal conductor and one or more ground conductors generally arranged
normally in parallel relationship to each other along a common plane with the central
signal conductor so as to form a controlled impedance structure. The signal conductor
is electrically connected to the inner conductor and the ground conductors are
electrically connected to the outer conductor, as shown in FIG. 1. A shield member is
interconnected to the ground conductors and covers at least a portion of the signal
conductor on the bottom side of the tip section. The shield member is tapered toward
the tips with an opening for the tips of the conductive fingers. The signal conductor
and the ground conductors each have an end portion extending beyond the shield
member and the end portions are able to resiliently flex, despite the presence of the
shielding member, relative to each other and away from their common plane so as to
permit probing devices having non-planar surfaces.
In another embodiment, Burr et al. disclose a microwave probe that
includes a supporting section of a coaxial cable including an inner conductor coaxially
surrounded by an outer conductor, as shown in FIGS. 2 A, 2B, and 2C. A tip section of the microwave probe includes a signal line extending along the top side of a dielectric substrate connecting a probe finger with the inner conductor. A metallic
shield may be affixed to the underside of the dielectric substrate and is electrically
coupled to the outer metallic conductor. Ground-connected fingers are placed
adjacent the signal line conductors and are connected to the metallic shield by way of
vias through the dielectric substrate. The signal conductor is electrically connected to
the inner conductor and the ground plane is electrically connected to the outer
conductor. The signal conductor and the ground conductor fingers (connected to the
shield via vias) each have an end portion extending beyond the shield member and the
end portions are able to resiliently flex, despite the presence of the shielding member,
relative to each other and away from their common plane so as to permit probing
devices having non-planar surfaces. While the structures disclosed by Burr et al. are
intended to provide uniform results of a wide frequency range, they unfortunately tend
to have non-uniform response characteristics at high microwave frequencies.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an existing probe.
FIGS. 2A-2C illustrate another existing probe.
FIG. 3 illustrates one embodiment of a probe.
FIG. 4 illustrates a side view of a portion of the probe of FIG. 3.
FIG. 5 illustrates a bottom view of a portion of the probe of FIG. 3.
FIG. 6 illustrates another embodiment of a probe.
FIG. 7 illustrates yet another embodiment of a probe. FIG. 8 illustrates still another embodiment of a probe.
FIG. 9 illustrates a force versus vertical probe deformation graph.
FIG. 10 illustrates probe pre-loading.
FIG. 11 illustrates a force versus vertical probe deformation graph for probe
pre-loading.
FIG. 12 illustrates a probe contact.
FIG. 13 illustrates a modified probe contact.
FIG. 14 illustrates contact resistance.
FIG. 15 illustrates contact resistance.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present inventors considered the co-planar fingered probing
devices disclosed by Burr et al., including the co-planar finger configuration and the
microstrip configuration with fingers extending therefrom. In both cases,
electromagnetic fields are created during probing between the fingers. These
electromagnetic fields encircle each of the fingers, electrically couple the signal finger
to the ground fingers, and electrically couple the ground fingers one another. While
the probing device is being used for probing, the resulting electromagnetic fields
surrounding the fingers interact with the wafer environment. While probing in
different regions of the wafer, the interaction between the electromagnetic fields
around the fingers and the wafer change, typically in an unknown manner. With a
significant unknown change in the interaction it is difficult, if not impossible, to accurately calibrate out the environmental conditions while probing a device under test.
When multiple probes are being simultaneously used for probing the
same area of the wafer, the probe tips come into close proximity with one another and
result in additional coupling between the probes, normally referred to as cross-talk. In
addition, the region between the support for the fingers, such as a dielectric substrate,
and the extended portion of the fingers results in a significant capacitance, which
impedes high frequency measurements.
The present inventors were surprised to determine that the microstrip
structure disclosed by Burr et al. further does not calibrate well on calibration test
substrates at very high frequencies, such as in excess of 70 GHz. This calibration is
independent of potential interaction with a wafer at a later time during actual probing
of a device under test. After examination of this unexpected non-calibration effect the
present inventors speculate that an energy is created in an "undesired mode", other
than the dominant field modes, at such extreme frequencies. This "undesired mode"
results in unexpected current leakages from the signal path thus degrading the signal
integrity. The present inventors further speculate that this "undesired mode" involves
resonating energy in the ground plane as a result of discontinuities in the ground path,
including for example, the connection between the ground plane and the external
portion of the cable, and the inductance in the ground plane. This ground plane
resonant energy results in unpredictable changing of the energy in the signal path to
the device under test, thus degrading performance. This degradation wasn't apparent
at lower operating frequencies, so accordingly, there was no motivation to modify
existing probe designs in order to eliminate or otherwise reduce its effects.
Referring to FIG. 3, a semi-rigid coaxial cable 40 is electrically connected at its rearward end to a connector (not shown). The coaxial cable 40 normally includes an inner conductor 41, a dielectric material 42, and an outer
conductor 43. The coaxial cable 40 may likewise include other layers of materials, as
desired. The forward end of the cable 40 preferably remains freely suspended and, in
this condition, serves as a movable support for the probing end of the probe.
A microstrip style probe tip 80 includes a dielectric substrate 88 that is
affixed to the end of the coaxial cable 40. The underside of the cable 40 is cut away
to form a shelf 85, and the dielectric substrate 88 is affixed to the shelf 85.
Alternatively, the dielectric substrate 88 may be supported by an upwardly facing shelf cut away from the cable or the end of the cable without a shelf. Referring also to
FIG. 4, a conductive shield 90, which is preferably planar in nature, is affixed to the
bottom of the substrate 88. The conductive shield 90, may be for example, a thin
conductive material (or otherwise) that is affixed to the substrate 88. By using a
generally planar conductive material having a low profile the shield 90 is less likely
to interfere with the ability to effectively probe a device under test by accidently
contacting the device under test. The conductive shield 90 is electrically coupled to
the outer conductor 43 to form a ground plane. The other conductor 43 is typically
connected to the ground, thought the outer conductor 43 may be provided with any
suitable voltage potential (either DC or AC). The conductive shield 90 preferably
covers all of the lower surface of the substrate 88. Alternatively, the conductive
shield 90 may cover greater than 50%, 60%, 70%, 80%, 90%, and/or the region
directly under a majority (or more) of the length of a conductive signal trace on the opposing side of the substrate 88.
One or more conductive signal traces 92 are supported by the upper surface of the substrate 88. The conductive traces 92, may be for example, deposited
using any technique .or otherwise supported by the upper surface of the substrate. The
conductive trace(s) 92 is electrically interconnected to the inner conductor 41 of the
coaxial cable 40. The inner conductor 41 of the coaxial cable 40 and the conductive
trace(s) 92 normally carries the signal to and from the device under test. The
conductive trace(s) 92 together with the shield layer 90 separated by a dielectric
material 88 form one type of a microstrip transmission structure. Other layers above,
below, and/or between the shield layer 90 and the conductive trace 92 may be
included, if desired.
To reduce the effects of the aforementioned unexpected high frequency
signal degradation, the present inventors determined that the signal path may include a
conductive via 94 passing through the substrate 88. The conductive via 94 provides a
manner of transferring the signal path from the upper surface of the substrate to the
lower surface of the substrate. The conductive via 94 avoids the need for using a
conductive finger extending out from the end of the substrate 88 that would otherwise
result in a significant capacitance between the extended finger and the end of the
substrate 88. The conductive via 94 provides a path from one side of the substrate 88
to the other side of the substrate 88 in a manner free from an air gap between the
conductive via 94 and the substrate 88 for at least a majority of the thickness of the
substrate 88. In addition, the shield layer 90 preferably extends beyond the via 94 to
provide additional shielding.
Referring also to FIG. 5, the lower surface of the substrate 88
illustrates a contact bump 100 electrically connected to the via 94 and the trace 92 extending below the lower surface of the substrate 88 and the shield 90 which may be
used to make contact with the device under test during probing. The conductive
shield 90 may include an "patterned" section around the contact "bump" 100 so that
the shield layer 90 and the signal path are free from being electrically interconnected
(e.g., the shield layer 90 may be greater than 50%, 75%, or laterally surrounding all of
the contact at some point). It is to be understood that the contact may take any
suitable form, such as for example a bump, a patterned structure, a conductive
structure, a needle structure, or an elongate conductor. The conductive shield 90 may
laterally encircle the conductive bump which increases the resistance to external
electromagnetic fields. Also, the conductive shield 90 extending beyond the
conductive bump 100 reduces the crosstalk from other probes. For some probing
applications, one or more shield 90 contacts 102 may be provided, if desired. The
shield layer and the conductive trace are normally constructed to provide a microstrip
transmission line controlled impedance structure. While typically the signal line has a
test signal and the shield has a ground potential, the two conductive paths may
likewise be any other configuration, such as balanced inputs which vary with respect
to ground.
Referring to FIG. 6, the probe may employ an outer cone shaped upper
shield 110. The outer conductor 43 of the coaxial cable is comiected to the upper
shield 110 is therefore electrically connected to ground. This design provides a
smooth transition between the coaxial cable and the end of the probe. The probe is
therefore shielded as it transitions to the tip of the cone portion.
The upper shield 110 has a tapered cylindrical portion whose forward
end is a tapered tip and whose rear end has a contour that is in continuous contact with the outer coaxial conductor along its circumference so that there is no gap between the
outer conductor and portions of the shield that could create fringing fields that could
effect probe measurements. Likewise, anyother shape may be used for the shield 110, as desired. In addition, the forward end preferably extends past the via and forms a
substantially closed region so that there is reduced fringing fields at the forward end.
The shield reduces parasitic coupling to any external structure and the construction of
the shield as a single piece of metal reduces complexity of assembly. The shield is
preferably made of a thin foil and is capable of being formed by a fabrication process.
The sheild may also be deposited or made of other material.
The lower shield member 90 extends underneath the conductive trace
between the fingers and the chuck holding the device under test. The shield therefore
helps to block the generation of ground plane resonant modes that can otherwise
interfere with and degrade the signal from the device under test.
Referring to FIG. 7, in an alternative embodiment a conductive finger
112 or other elongate conductive element may be provided that is electrically
interconnected to the via. One or more additional ground fingers 114 may be
electrically com ected to the lower shield material. If desired, each respective finger
may include a cantilevered portion that extends down away from the substrate. The
cantilevered portions are preferably arranged in transversely spaced apart relationship
to each other so as to cooperatively form a controlled impedance transmission line in
order that a low loss transition can be made between the respective conductors on the
cable and the respective pads on the device-under test.
While the use of an upper shield 110 that includes a curved surface
provides an improvement to signal integrity, the changes in the structure of the upper
shield tend to introduce some limitations into the signal integrity at high frequencies, thus impeding performance. For example, the changes in the height of the upper
shield changes the electromagnetic field pattern along the length of the signal
conductor. In addition, increased manufacturing complexity occurs with the upper
shield. Furthermore, in most cases microwave microstrip transmission structures are
enclosed in a housing, such as a conductive case, and accordingly there is reduced
motivation to include an upper shield structure.
To further increase the performance at high frequencies the present
inventors considered the effects of the substrate material. In many cases the dielectric
constant of the dielectric substrate material is high, such as Al2O3 which has a 9.9
dielectric constant. Materials with a high dielectric constant have a tendency to
concentrate the electromagnetic fields therein, thus decreasing the electromagnetic
fields susceptible to influence by other devices. In addition, the thickness of the
substrate is typically 250-500 microns to provide mechanical stability. Thus the fields
tend to concentrate within the substrate.
Referring to FIG. 8, while considering such substrates the present
inventors came to the realization that the flexible membrane substrate may be substituted for the more rigid substrate 88. An example of membrane material is
described in U.S. Patent No. 5,914,613, incorporated by reference herein together will
all other references cited herein incorporated by reference herein. In general,
membrane based probes are characterized by a flexible (or semi-flexible) substrate
with traces supported thereon together with contacting portions being supported
thereon. The membrane portion of the probe may be constructed from a sacrificial
substrate into which is created a depression. Into this depression is located conductive
material, traces are located thereon if desired, and flexible dielectric material is
located on or under the traces. Thereafter, the sacrificial substrate is removed leaving
the probe tip, traces, and membrane material. The contacting portions come into contact with the device under test and the traces are normally on the opposing side of
the membrane connected to the contacting portions using vias. In many cases, the
membrane technology may be significantly thinner than ceramic based substrates,
(see, e.g., substrate 88) such as 40, 30, 20, 10, 5, or 3 microns or less. Normally the
dielectric constant of the membrane material is 7 or less, sometimes less than 6, 5, or
4 depending on the particular material used. While normally using a lower dielectric
constant substrate is unsuitable, using a significantly thinner substrate together with a
lower dielectric constant substrate raises the theoretical frequency range of effective
signal transmission to 100's of GHz. The significantly thinner substrate material
permits positioning the lower shield material significantly closer to the signal traces
than the relatively thick ceramic substrate, and therefore tends to more tightly confine
the electromagnetic fields there between. With a tight confinement of the electric
fields in the membrane material, the present inventors determined that the high
frequency performance of the membrane material may be increased by locating an
upper shield material above the membrane material. Moreover, the upper shield
material should likewise be correspondingly close to the signal path, so the curved
upper shield material positioned at a significant distance from the signal trace
previously used may not normally be sufficient. Accordingly, the shield material
should be patterned on the top of the membrane material with a dielectric between the
signal trace and the upper shield material. In many cases, the distance between the
signal trace and the upper shield directly above the signal trace should be no more
than 10 times the distance between the signal trace and the lower shield material.
More preferably, the aforementioned distance would be preferably 7, 5, 4, or 2 times.
When the membrane based probe comes into contact with a device
under test, as in most probes, it tends to skate across the pad as additional pressure is
exerted. This skating is the result of the angled probe and/or co-axial cable flexing
while under increasing pressure against the test pad. A limited amount of skating is
useful to "scrub" away oxide layers, or otherwise, that may be built up on the contact
pad, which results at least in part from a suitable amount of pressure and/or skating.
In many cases the test pad is typically relatively small and excessive skating from
slightly too much pressure being applied results in the probe simply skating off the
test pad. In addition, if excessive pressure is exerted damage to the probe and/or
contact pad may result. Accordingly, there is an acceptable range of pressure and
skating that should be maintained.
Referring to FIG. 9, for purposes of illustration the force applied by the
probe versus vertical deformation of the probe as a result of the force being applied is
shown. Line 400 is for a low stiffness probe and line 402 is for a high stiffness probe.
Vertical line 404 illustrates the maximum skating distance before the probe is likely
off the contact pad, and accordingly the greatest distance of over travel of the probe
after contact with the contact pad. Vertical line 406 illustrates the minimum generally
acceptable skating distance of the probe to ensure sufficient scrubbing distance of the
oxide layer or otherwise that may be built up on the contact pad, and accordingly the
minimum generally acceptable distance of over travel of the probe after contact with
the contact pad. Typically the range of useful over-travel is approximately 50 to 200
microns. Horizontal line 408 illustrates the maximum acceptable force that the probe
may apply so that damage to the probe and/or contact pad is minimized. Horizontal line 410 illustrates the minimum acceptable force that the probe should apply so that
sufficient pressure is exerted to break through the oxide layer or otherwise that may be built up on the contact pad.
It may be observed that there is a rectangular region (in this example)
within which acceptable over-travel and force is applied by the probe to the contact
pad. For the low stiffness probe 400 a range of 420 is shown where acceptable
probing takes place. It may be observed that this distance uses less than the maximum
range between vertical lines 404 and 406, and thus the over-travel must be carefully
controlled by the operator. For the high stiffness probe 402 a range of 422 is shown
where acceptable probing takes place. It may be observed that this distance uses less
than the maximum range between vertical lines 404 and 406, and thus the over-travel
must be carefully controlled by the operator. Accordingly, the stiffness of the probe
needs to be carefully controlled, which is difficult, in order to establish an acceptable
operating region. Further, it is noted that there is some relationship between skate to
over-travel which may be controlled also. To little skate is problematic because some
scrubbing action improved contact resistance and lateral motion provides visual
confirmation (through the microscope) that contact has been made. To much skate is
problematic because then the probe tip slides across and off the pad before getting
enough force for good contact resistance. The pre-load provides the opportunity to tune that ratio by varying the curvatures of the probe and the pre-load location.
After consideration of the limitations seemingly inherent with the
stiffness of the probe, the present inventors came to the realization that by using a
relatively flexible probe together with pre-loading a portion of the force to be applied
by that probe, a modified force-distance profile may be obtained that is more readily
within the acceptable region. The modified force-distance profile may include more
of the acceptable probing range, namely a wider probing range within the rectangular
region, than otherwise achieved if the probe were not otherwise modified. Referring
to FIG. 10, this pre-loading may be achieved by using a string 440 or other support member to upwardly flex the probe. If the low stiffness probe 400 is used, then a modified force profile 444 (see FIG. 11) may be obtained. It is noted that the lower curved portion 446 is as a result of the pre-loading of the probe. The upper portion 448 is a result of the probe itself and generally has the same force slope as the probe without pre-loading. It may be observed that in this manner a probe profile that has a relatively low slope that is suitable to extend across more of the useful probing range while maintaining reasonable forces may be used. The pre-loading raises the initial force to a range near the minimum generally acceptable force. The height of the profile 444 may be modified by adjusting the pre-loading. Also, the slope of the profile 444 may be lessened by selecting a more flexible probe or otherwise modifying the orientation of the probe in relation to the contact pad. This pre-load system, while especially useful for membrane type probes, is likewise useful with other probing technologies.
When making probing measurements the contact resistance between the probe and the device under test is an important consideration. The tip of the probe may be designed in such a manner as to achieve a low contact resistance while permitting effective viewing of the area to be probed with an associated microscope. The probe tip 438 (see FIG. 12) is typically constructed in such a manner that the resulting structure has a pair of opposing inclined surfaces 450 and 452. The tip 454 of the probe is preferably extended from the inclined surfaces 450 and 452. The construction of the probe tip may be done using a sacrificial substrate into which is created a depression. Into this depression is located conductive material, traces are located thereon if desired, and flexible dielectric material is located on or under the
traces. See U.S. Patent No. 5,914,613, incorporated herein by reference, together with
all references cited herein. Thereafter the sacrificial substrate is removed leaving the
probe tip, traces, and membrane material. The probe tip 438 is acceptable, however, it
is difficult to see the region proximate the tip 438 when contacting the device under
test because of the inclined surfaces 450 and 452.
To improve the visibility of the tip 438 during probe it has been
determined that the probe 454 may be ground back or otherwise a portion of the probe
removed,, as illustrated in FIG. 13. By removal of a portion of the probe a greater
visibility may be achieved during probing of the device under test as illustrated in
FIG. 13. It is also to be understood that the probe may be constructed in a manner
such that a portion of the probe does not need to be removed. The tip portion 454 is
preferably approximately 12 μm x 12 μm, with about 2-3 mills of vertical over-travel
resulting in about 1 mil of longitudinal tip scrub. The probe may likewise retain a lip
460 that provides additional structural support for the tip 454. The backside 462 of
the probe may even be undercut with respect to the plane of the base 464 of the probe.
Alternatively, the backside 462 of the probe may be within 30 degrees of vertical
(undercut or not) with respect to the plane of the base 464 of the probe.
The contact resistance resulting from the described structure turns out
to be exceedingly low, especially in comparison to other types of probing systems like
Tungsten Probes. Referring to FIG. 14, the contact resistance on un-patterned
aluminum is less then 30 mΩ over 5000 contact cycles, which is considerably lower than conventional tungsten probes where the contact resistance is approximately 130
mΩ. Referring to FIG. 15, with the probe being held in contact with the aluminum
pads the contact resistance is shown as a function of time. As illustrated in FIG. 15,
only 10 mΩ of variation was observed during a 5-hour interval. In a similar test, conventional tungsten probes show significant changes over the same period, typically
the contact resistance varies from 35 mΩ to 115 mΩ.
Another consideration in the design of the probe is the characteristics
of the different transmission structures. The coaxial cables provide good high
frequency transmission characteristics. Within the membrane structure, connected to
the coaxial cables, the micro-strip structure provides good high frequency
characteristics. Connected to the membrane structure includes a set of contacts, such
as a signal contact and a pair of ground contacts. The contacts provide a co-planar
transmission structure which has less bandwidth capability than the coaxial cable and
the micro-strip structure. To achieve acceptable bandwidth the present inventors have
determined that the contacts should be no longer than 150 microns. More preferably
the contacts should be no longer (e.g., height from planar surface) than 100 microns, or no longer than 75 microns, or no longer than 55 microns.