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BACKGROUND OF THE INVENTION
The present invention relates to microwave wafer probes, and particularly
to such probes having replaceable probe tips.
Microwave wafer probes are well known to the art as exemplified by probe
cards marketed by Tektronix Inc. of Beaverton, Oreg., and by cantilevered
probes marketed by Cascade Microtech, Inc. of Beaverton, Oreg. and shown
in U.S. Pat. No. 4,697,143. Most existing wafer probes have nonreplaceable
transmission line probe tips permanently connected to connector assemblies
which in turn detachably connect to external transmission lines linked to
test equipment. However, wafer probes having cantilevered tips which are
detachably connected to such connector assemblies are also known, thereby
enabling replacement of a damaged or worn probe tip without replacing the
entire probe head. Prior examples of replaceable probe tips include a
coplanar probe tip detachably interconnected with a coaxial connector
assembly in a probe marketed by Design Technique International, Inc. of
Chatsworth, Calif. Also, Cascade Microtech, Inc. of Beaverton, Oreg. has
previously marketed a wide probe having a coplanar probe tip detachably
connected to a microstrip "fanout" assembly which in turn is connected to
multiple coaxial connectors, as shown in U.S. Pat. No. 4,764,723.
It is often desirable that the wafer probe be equipped with passive
electrical circuit components such as capacitors, and/or active components
such as amplifiers, to accomplish predetermined probing functions. Such
components have previously been mounted directly on probe tips of either
the permanent or replaceable type. Unfortunately, in either case, damage
or wear to the probe tip requires replacement not only of the tip but of
these expensive circuit components as well. This cost cannot feasibly be
avoided by placing the components in a remote location relative to the
probe tip because such location results in undesirable phase shifts and
signal losses.
Accordingly, what is needed is a probe tip system which makes it possible
to replace a damaged or worn probe tip economically without requiring
replacement of the aforementioned passive or active electrical circuit
components, while compatibly enabling the circuit components to be
connected in very close proximity to the probe tip.
SUMMARY OF THE INVENTION
The present invention satisfies the above-identified need by interposing a
planar controlled-impedance transmission line circuit board, which
includes the above-mentioned components or to which the components are
closely connected, operatively between the probe's external connector
assembly and a replaceable, planar, controlled-impedance transmission line
probe tip. Preferably, both the circuit board and the probe tip comprise
respective coplanar transmission lines, or waveguides, which may be
detachably joined together by an overlapping interconnection which is
secured by a selectively releasable compression member holding the
overlapping portions of the two elements forcibly together. Alternatively,
the probe tip and/or circuit board could employ multiplanar printed
transmission lines such as microstrip or stripline structures.
A significant problem to be overcome with such a replaceable planar
transmission line probe tip is the problem of misalignment of the tip with
the circuit board. Any variations in alignment of two transmission lines
causes variations in the characteristic impedance of their interface,
preventing a constant characteristic impedance and causing undesirable
reflections and distortions of high-frequency signals. In the present
invention, this problem is solved preferably by providing overlapping end
portions of the corresponding conductors of the circuit board and probe
tip which are shaped to permit variations in transverse and/or
longitudinal alignment without causing substantial changes in the
characteristic impedance of the interface.
Another problem to be overcome is the reduction in length of a planar probe
tip necessary to interpose a transmission line circuit board between the
tip and the external connector. Even though the tip's dielectric substrate
is resilient, the shortened length adds stiffness to the tip making it
less likely to conform its plane to that of the device under test when
contact is made, thus risking insufficient contact with some of the pins
of the device. In the present invention this problem is solved by making
the tip's substrate of a different rigidity than that of the circuit
board's substrate, and/or by providing a resilient compressive member at
the extremity of the tip to forcibly conform the tip's plane to that of
the device under test.
The foregoing and other objectives, features and advantages of the present
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 DRAWINGS
FIG. 1 is an exploded perspective view of an exemplary embodiment of a
wafer probe constructed in accordance with the present invention.
FIGS. 2 and 3 are enlarged top views of different exemplary embodiments of
planar circuit boards which could be employed in the probe of FIG. 1.
FIG. 4A is an enlarged top view of the interconnecting ends of a circuit
board and replaceable probe tip, prior to their interconnection, showing
an exemplary configuration which is tolerant to transverse misalignment of
the two elements.
FIGS. 4B and 4C are side and top views, respectively, of the circuit board
and probe tip of FIG. 4A in interconnected overlapping relationship.
FIG. 5A is an enlarged top view of an alternative exemplary configuration
of the interconnecting ends of the circuit board and probe tip,
respectively, which is tolerant to both transverse and longitudinal
misalignment.
FIGS. 5B and 5C are side and top views, respectively, of the circuit board
and probe tip of FIG. 5A in overlapping connected relationship.
FIG. 6 is an enlarged side view of the forward extremity of the probe tip
in contact with a device under test.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 1, an exemplary embodiment of the wafer probe of the
present invention comprises a probe body 10 having a connector assembly 12
mounted thereon for connecting the probe, by an external coaxial cable or
other type of external transmission line (not shown), to appropriate
testing equipment such as a network analyzer (not shown). A coplanar
transmission line probe tip 14 of trapezoidal shape is provided for
temporarily contacting a device under test by means of downwardly facing
ground lines 16 and a central signal line 18 mounted on a resilient
substrate 20 of a suitable dielectric material such as Kaptonp198 .
Microwave absorbers 22 and 24 are preferably provided to absorb energy
propagating along the probe ground lines, although absorber 24 may be
deleted to provide space for electrical circuit components. In the
foregoing respects, the probe is similar in principle to that disclosed in
U.S. Pat. No. 4,697,143, which is incorporated herein by reference.
The salient difference between the present invention and that disclosed in
the above-mentioned patent is the presence of a coplanar transmission line
circuit board 26, interconnected operatively between the connector
assembly 12 and the probe tip 14, having upwardly facing coplanar ground
lines 28 and a central signal line 30 mounted on a dielectric substrate
27. These lines join in a longitudinally overlapping connection with the
overlying, downwardly facing, coplanar ground lines 16 and signal line 18
of the probe tip 14. Such interconnection is an easily detachable one by
virtue of the fact that the overlapping portions of the probe tip 14 and
circuit board 26 are clamped together between the probe body 10 and tip
absorber 22 by a suitable releasable structure such as a pair of screws 32
(only one of which is shown) engaging threaded sockets 34 in the probe
body 10 when the circuit board 26 and probe tip 14 are matingly nested in
overlapping alignment within a receptacle 36 in the probe body 10. A
resilient Poron.TM. or other suitable dielectric compressor bar 38 ensures
a tight compressive electrical contact between the overlapping portions of
the probe tip and circuit board when the screws 32 are tightened. Studs 40
protruding from the probe body are insertable in mating sockets (not
shown) in the bottom of the tip absorber 22 for alignment purposes. The
opposite end of the circuit board 26 is either permanently or detachably
connected to the coaxial connector assembly 12 in a conventional manner.
The dielectric substrate 20 of the probe tip is preferably less rigid than
the substrate 27 of the circuit board in order to preserve the tip's
compliant resilient contacting capability despite its short length.
Alternatively, a less rigid circuit board substrate could be employed in
those cases where a more rigid tip is required, which would likewise
provide compliance of the tip if a movable interconnecting structure
between the board and tip is provided. A rigid extension 22a of the tip
absorber 22, overlying a resilient rubber or other dielectric compression
member 42, may also be provided to overlie the tip 14 at its forward
extremity. As shown in FIG. 6, the compression member 42, upon contact of
the tip with the device under test, forces the plane of the tip to conform
to the plane of the device to insure proper contact.
The advantage of having the planar circuit board 26 interposed between the
connector assembly 12 and the detachable probe tip 14 is illustrated by
the exemplary alternative embodiments of the circuit board shown in FIGS.
2 and 3. For example, in FIG. 2, circuit board 26a is equipped with a
conventional bias tee assembly consisting of a capacitor 44 bridging a gap
cut in the signal line 30, and an inductor 46 interconnecting the signal
line 30 with a capacitor chip 48 mounted on a ground line 28 which, in
turn, is connected to a bias supply. FIG. 3, on the other hand, shows an
alternative circuit board 26b having a series of capacitors 50
interconnecting one ground line 28 with the other ground line through
conductors 52, thereby enabling the ground lines 28 to function as low
impedance power lines. Other passive or active electrical circuit
components which could be incorporated into the circuit board include such
elements as directional couplers, directional bridges, amplifiers,
distributed micro-bolometers, varactor-loaded multiplier lines, tuned
matching networks, Lange couplers, and so forth. These components could
either be added to the circuit board or incorporated therein originally as
part of a printed circuit. In any case, damage to the probe tip 14
requiring its replacement will not likewise require the replacement of
such expensive passive and active electrical circuit components.
Nevertheless, such components are mounted in extremely close proximity to
the probe tip so that the benefits of minimized phase shift and low signal
loss, normally obtainable only through direct mounting of such components
on the probe tip, are obtained despite the absence of such direct
mounting.
FIGS. 4A, 4B and 4C depict an exemplary configuration for the end portions
of the conductive lines of a coplanar transmission line circuit board 26c
and probe tip 14a which permit the circuit board and probe tip to be
detachably interconnected in an overlapping fashion without causing any
significant variation in the characteristic impedance of the overlapping
interface even though the circuit board and probe tip may be transversely
misaligned (i.e. misaligned in a direction transverse to their respective
axes). Normally the overlapping end portions of corresponding conductors
of joined coplanar transmission lines have identical transverse dimensions
which, in the case of the signal lines, are somewhat reduced relative to
the normal signal line width so as to reduce signal line-to-ground line
capacitance at the overlapped interface. This normally compensates for the
increase in capacitance at the interface caused by the longitudinal
overlap of the two dielectric substrates, thereby maintaining the
characteristic impedance of the transmission lines constant at the
interface if the two transmission lines are not misaligned transversely or
longitudinally. But, if transverse misalignment occurs, the overall
transverse dimension of the overlapping end portions of the signal lines
increases, while the transverse spaces between the overlapping signal line
end portions and the overlapping ground line end portions decrease. Both
of these changes in dimensions cause an increase in capacitance of the
interface, which reduces the impedance thereof so that it no longer
matches that of the interconnected transmission lines. However, the
configurations of the end portions of the conductors shown in FIG. 4A are
tolerant of transverse misalignment (but not longitudinal misalignment) of
the probe tip and circuit board. This tolerance to transverse misalignment
is achieved by the fact that each of the end portions of the ground and
signal lines 16a, 18a of the probe tip has a respective transverse
dimension which is greater than that of the corresponding end portion of
the ground and signal lines 28c, 30c of the circuit board 26c. Thus, each
of the end portions of the lines of the probe tip, when overlapping a
narrower corresponding end portion of a line of the circuit board, has
excess conductive material extending parallel to the plane of the probe
tip beyond the conductive material of the narrower corresponding
overlapped end portion. Therefore, if the probe tip and circuit board are
transversely misaligned, the transverse dimension 56 (FIG. 4C) of the
combined overlapped signal line end portions, and the transverse spaces 58
between the overlapped signal line end portions and the ground line end
portions, remain constant within reasonable limits of misalignment.
Accordingly, impedance variations at the interface of the probe tip and
circuit board are minimized despite variations in transverse alignment
thereof.
Longitudinal misalignment of the probe tip and circuit board can likewise
cause a change in the impedance of the interface. Misalignment in a
direction causing excessive longitudinal overlap increases the capacitance
of the interface by increasing the overlap of the dielectric substrates,
while insufficient longitudinal overlap decreases the capacitance (or
increases the inductance) of the interface. Accordingly, FIG. 5A shows a
further embodiment comprising a circuit board 26d and probe tip 14b which
are effective to minimize variations in impedance resulting both from
transverse and from longitudinal variations in alignment of the probe tip
and circuit board. In this embodiment, each of the end portions of the
ground lines 16b and signal line 18b of the probe tip 14b has a greater
transverse dimension than the end portion of the corresponding line 28d,
30d of the circuit board 26d so as to minimize variations in impedance of
the interface due to variations in transverse alignment in accordance with
the principles of the embodiment of FIGS. 4A, 4B and 4C. The excess
conductive material of the wider end portions of the probe tip lines keeps
the transverse dimension 56 of the overlapped signal line end portions
(FIG. 5C), and the transverse spaces 58 between the overlapped signal line
end portions and the overlapped ground line end portions, constant despite
variations in transverse alignment. Concurrently, each of the overlapping
end portions of the signal lines has a transverse dimension which
increases in magnitude in a direction toward the other transmission line,
and each is foreshortened relative to its respective dielectric substrate
20 or 27, respectively. Consequently, their combined area parallel to the
planes of the respective transmission lines and within the overlap of the
dielectric substrates increases as the circuit board and probe tip are
pulled apart, thereby increasing the capacitance between the signal line
end portions and the ground line end portions to compensate for the
decrease in capacitance which normally would result from pulling the probe
tip and circuit board apart. A corresponding opposite compensation takes
place if the probe tip and circuit board are pushed together
The exact sizes and shapes of the geometric arrangements of the foregoing
embodiments will vary with the characteristic impedance of the
transmission lines and the dielectric constants of the respective
overlapping substrates. Although the figures show the case of overlapping
substrates having similar dielectric constants, such constants could be
different. In general the structure most tolerant to longitudinal
misalignment, and thus requiring the least geometric compensation, is one
where the dielectric constants are minimized. Also, such geometric
arrangements are equally applicable to planar transmission lines having
different numbers of ground and signal lines. As used herein, the term
"ground lines" encompasses comparable lines used for other purposes.
The term "wafer probe" as used herein is intended also to encompass probes
employed for other purposes, such as probing IC packages, thin or thick
film hybrid circuits, interconnect structures, chips, and other planar and
non-planar devices.
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.
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