Wafer Probe

The present invention relates to integrated circuits (IC) measurement generally, and particularly to wafer type probes for measurement of electrical characteristics of small planar devices (IC elements) fabricated on semiconductive or dielectric wafers.

Wafer probes provide temporary electrical contacts between test equipment and the very small terminal points (bonding pads) of IC elements on semiconductive wafers. Use of wafer probes permits operation and testing of IC elements prior to separating, bonding and packaging the individual IC elements on the wafer.

A major problem with most existing wafer probes is their inability to permit accurate measurements of the electrical characteristics of the devices when contacting low impedances at high frequencies (e.g., when signal frequencies above approximately 2 GHz are applied). These inaccuracies arise from high frequency characteristics of the probes themselves. At high frequencies, excessive probe inductance or changing probe radiation impedance can greatly reduce the accuracy of the tests made. Although the use of computercorrected measurements may reduce the adverse effect of excess inductance to some extent, the radiation is generally not repeatable and, therefore, not correctable.

These inaccuracies were significantly reduced by the improved wafer probe described and claimed in prior application, Ser. No. 318,084 filed Nov. 4, 1981. The present invention is a further improvement in wafer probe technology alleviating another source of non-repeatable measurement innacuracy which becomes more apparent when inaccuracies due to exccessive probe inductance and changing probe radiation impedance are otherwise reduced.

This additional source of measurement inaccuracy arises in existing wafer probes when some of the energy transmitted or reflected from the device under test excites the transmission line mode resulting from the shield or ground of the probe transmission line acting as one conductor, and the wafer stage, wafer or other nearby conductors acting as the other conductor. This "outer conductor" mode energy typically propagates up the probe board, reflects off the probe mounting block or other discontinuities, and then propagates back down the board. Upon reaching the device under test, some of the "outer conductor" mode energy can couple back into the normal modes, while the remainder is dissipated in the device under test, and/or reflected back up the probe. Thus a resonator results from the energy alternately reflecting from the device under test and from the probe mounting area. While the "outer conductor" mode resonates on the probe board, some of its energy is also radiated.

This radiation from the "outer conductor" mode resonance is particularly undesirable since the radiated energy reflects back from the conductors in the vicinity of the wafer stage and these conductors typically move in relation to the wafer, creating non-reproducible measurements.

What is needed and would be useful, therefore, is a low excess inductance wafer probe capable of making accurate on-wafer measurements by minimizing errors due to excessive probe inductance and changing probe radiation impedance and by minimizing radiation due to transmission line mode resonance along the probe ground.

In accordance with the present invention a wafer probe is described incorporating microwave absorbing material positioned to attentuate microwave energy propagating along the ground plane of the wafer probe. The addition of this microwave absorbing material significantly reduces transmission line mode resonance along in the probe ground conductors and thereby reduces the resultant radiation.

In a particular embodiment of the present invention, microwave absorbing material is provided in combination with a wafer probe of the type generally disclosed in the aforementioned prior application, Ser. No. 318,048. The improved probe comprises a transmission line assembly and a coaxial cable connector installed on a mounting block and layers of absorber material generally surrounding the transmission line assembly. The transmission line assembly, comprising ground and signal conductors deposited on a tapered alumina substrate, conducts signals between the bonding pads of the device under test and the connector from which a coaxial cable may carry signals between the probe and test equipment. The sizing and arrangement of the ground and signal conductors of the transmission line are adjusted to match the characteristics impedance of the coaxial cable. This arrangement extends the (e.g., 50 ohm) environment of the coaxial cable to the bonding pads of the device under test, thus significantly reducing errors resulting from excessive probe impedance at high test frequencies, from impedance mismatching, and from changing probe radiation impedance.

The microwave absorbing material, which generally surrounds the transmission line assembly, and which may be in contact with the probe ground, is provided to absorb energy propagating up the ground plane of the probe, thereby keeping this energy from resonating, radiating or re-exciting the normal transmission line mode.

Thus, the microwave wafer probe of the present invention allows accurate on-wafer measurements of very small planar devices such as transistors, inductors, capacitors, resistors, IC's, SAW filters and other such devices at frequencies from DC to 18 GHz. By connecting a network analyzer to the probe and calibrating the system using impedance standards much smaller than a wavelength, chip measurements of substantial accuracy can be attained. The improved accuracy is due to the elimination of irreproducible parasitics of fixturing, packaging and bond wires, and to the elimination or substantial reduction of radiation due to transmission line mode resonance along the probe ground.

The probe of the present invention permits quick simultaneous measurements at one or more bonding pads on a chip, without dicing, mounting and bonding the devices. The probe also allows selection of die from RF performance before being bonded and allows automated trimming of microwave characteristics of integrated circuit devices in the wafer state.

It is accordingly an object of the present invention to provide an improved apparatus for providing temporary electrical contact between test equipment and the bonding pads of small planar devices fabricated on semiconductive wafers, the improved apparatus being capable of permitting accurate measurement of the high frequency electrical characteristics of the devices through reduction of adverse effects due to excessive or changing probe impedance, to impedance mismatching, and to transmission line mode resonance along the probe ground.

The subject matter which we regard as our invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings wherein like reference characters refer to like elements.

FIG. 1 is an exploded perspective view of the wafer probe of the present invention;

FIG. 2 is a plane view of the wafer probe in relation to a wafer element to be tested;

FIG. 3 is a side view of the wafer probe of FIG. 2;

FIG. 4 is a sectional view (4--4) of the wafer probe of FIG. 2;

FIG. 5 is sectional view (5--5) of the wafer probe of FIG. 2;

FIG. 6 is a bottom view of the tip of the probe board of the wafer probe of FIG. 2 showing a coplanar transmission line arrangement of the ground and signal conductors;

FIG. 7 is a sectional view of an alternate embodiment of the present invention wherein the wafer probe is adapted to simultaneously probe a plurality of signal points on a single wafer; and

FIG. 8 is a bottom view of the probe board of the wafer probe in FIG. 7 showing a plurality of coplanar transmission line conductors.

Referring now to FIG. 1, there is shown a wafer probe 11 of the present invention. The probe comprises probe mounting block 17, transmission line assembly 19, metal channel 35, upper absorber layer 21, intermediate absorber layers 23, lower absorber layer 25 and connector 27. In FIGS. 2 and 3, probe 11 is shown in its intended use engaging minutely spaced bonding pads 13 of a planar device 15 on wafer 16 for testing selected circuit components of planar device 15.

As shown in FIG. 1, transmission line assembly 19 comprises a signal conductor 29 and ground conductors 31 mounted on tapered probe board 33, made of a dielectric material such as alumina (A1.sub.2 O.sub.3) approximately 10 mils thick. Signal conductor 29 and ground conductors 31 are strips of conducting material such as gold deposited on the alumina substrate of probe board 33. As seen in FIG. 6, conductors 29 and 31 converge toward probe board tip 41 to align above the bonding pads of the device being tested. Contact with the device bonding pads is made through, for example, small gold contact pads 37 mounted at the ends of signal conductor 29 and ground conductors 31.

As seen in FIG. 6, signal conductor 29 width and distance from ground conductors 31 is held constant up to a short distance from probe board tip 41. Then signal conductor 29 and ground conductors 31 are tapered down, for example, from a 10-mil width to a 1-mil width at probe board tip 41. The gaps between the signal conductor 29 and ground conductors 31 are correspondingly tapered near probe board tip 41 such that the transmission line formed by signal conductor 29 and ground conductors 31 is of a constant characteristic impedance (e.g., fifty ohm) through the length of transmission line assembly 19 to the points of contact with the bonding pads (e.g., pads having a surface area of approximately 2 mils square, of similar width to conductors 29 and 31 at probe board tip 41, and center spaced approximately 4 mils apart) of the device being probed.

As shown in FIG. 1, transmission line assembly 19 is generally surrounded by absorber layers 21, 23 and 25. These absorber layers are made of a microwave absorbing material having a relative characteristic impedance close to that of free space (.vertline.z.vertline./.vertline.z.sub.o .vertline.=0.5 to 0.7) but with a high magnetic loss coefficient. Material of this characteristic impedance will not reflect microwave energy or produce dielectric transmission line modes. Typical examples of suitable absorber material are iron or ferrite-containing Emerson & Cuming, Inc. "Eccosorb" brand models GDS, MF-S, MF, and CR-S. As shown in FIG. 5, upper absorber layer 21, grooved to provide spatial separation of the absorber from signal conductor 29, is mounted on the upper side of probe board 33. Intermediate and lower absorber layers 23 and 25 are mounted below probe board 33 in contact with metal channel 35. Metal channel 35, contacting ground conductors 31 and spanning signal conductor 29, maintains absorber spacing around signal conductor 29, improves structural integrity of transmission line assembly 19, electrically bonds opposing ground conductors 31, and provides shielding for signal conductor 29. Transmission line assembly 19 and connector 27 are attached to mounting block 17 as best shown in FIG. 4. In the preferred embodiment, mounting block 17 is also of a microwave absorbing material with characteristics similar to that of the absorber layers. Signal conductor 29 is terminated on connector 27 by conducting tab 39. Conductor 27 provides means for connecting an external cable (e.g., a fifty ohm coaxial cable, not shown) to wafer probe 11 for carrying signals to external test equipment.

The geometry of the wafer probe can be modified to correspond to the geometry of the device to be measured by the probe. While a probe having only a single signal conductor has been discussed, a similar probe may be constructed having a plurality of signal and ground conductors capable of simultaneously probing a plurality of planar device bonding pads. For example, FIGS. 7 and 8 show an alternative embodiment of the present invention, multi-signal wafer probe 43, wherein a plurality of signal conductors 29 and the ground plane probe conductors 31 are combined in a single trapezoidal-shaped wafer probe board 33. FIG. 7 is a sectional view (analagous to FIG. 5) of multi-signal wafer probe 43 showing upper, intermediate and lower absorber layers 21, 23 and 25, respectively, generally surrounding probe board 33. Metal channel 35 connects all ground conductors 29 while spanning all signal conductors 29. Connectors 27 provide external cable access to each signal conductor 29. FIG. 8 depicts tip 41 of the underside of probe board 33 for the multi-signal wafer probe 43. On the narrow end of probe board 33 conductors 29 and 31 taper to align above the device bonding pads (e.g. 4-mil on center). In FIG. 7, signal conductors 29 are spaced (e.g. 400-mil on center) at the wide end of probe board 33 to align below cable connectors 27 mounted on mounting block 17. As in the case of the single signal conductor probe, signal and ground conductor width and spacing in the multi-signal wafer probe are selected to maintain constant transmission line characteristic impedance throughout the length of probe board 33.

While in the preferred embodiments transmission line assembly 19 is shown in FIG. 5 as generally surrounded by absorber material, it is noted that upper absorber layer 21, in the absence of intermediate absorber layers 23 and lower absorber layer 25, alone provides most of the reduction in undesireable transmission line mode resonance in the ground plane of the transmission line assembly providing significant attenuation of microwave radiation from the probe. The addition of intermediate and lower absorber layers 23 and 25 provides a more modest improvement in probe performance. Thus, in an alternative embodiment of the present invention, intermediate absorber layers 23 and lower absorber layer 25 may be omitted with some, but not a major, loss of probe accuracy. The absorber layers are taperd to produce a gradual absorbtion of a wave propagating up probe ground conductors allowing little reflection. While the absorber material surrounding the transmission line assembly is shown constructed of four distinct layers, alternative embodiments may incorporate other absorber material arrangements.

Although the embodiments discussed illustrate probes employing transmission lines having a coplanar waveguide arrangement, it is understood that performance of other probes incorporating coaxial, non-coplanar metallic strip or other transmission line arrangements may be improved by installation of suitable microwave absorbing material in appropriate locations about the probe.

While we have shown and described several preferred embodiments of our invention, it will be apparent to those skilled in the art that many other changes and modifications may be made without departing from our invention in its broader aspects. We therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.