A multigigahertz probe for testing electrical micro connections or wafers. A body has a top, bottom and a testing tip at one end, and the one end slants upwardly and inwardly from the bottom towards the top. At least one coaxial cable is carried by the body and includes a first connector end and a second testing end forming testing contacts. The testing end extends to the intersection of the bottom and the slanting one end of the body.

BACKGROUND OF THE INVENTION 
A multigigahertz probe is used as a wafer probe for microwave frequencies 
for performing on wafer measurements of electrical components such as 
transistors and integrated circuits. 
A probe sold by Cascade Microtech performs well in the transmission of test 
signals. However, the probe tip's contact surfaces are attached to a thin 
ceramic support which is stiff and fragile. This inhibits the probe's 
ability to conform to surface irregularties in the wafer and the tip may 
be easily broken. The probe uses coplanar wave guide transmission lines 
which are typically sensitive to geometry variations and is expensive to 
manufacture. 
The present invention is directed to a Probe which is compliant and can 
withstand small deflections and impacts without breaking and can conform 
and make good contact with slightly irregular pad heights and which is 
inexpensive. The present probe utilizes a micro-coaxial cable or cables 
with simple non-lithographic fabrication techniques to create a probe with 
pads (signal/ground) with good high frequency performance. 
SUMMARY 
The present invention is directed to a multigigahertz probe for testing 
electrical microconnections on wafers and includes a body having a top, 
bottom and testing tip at one end in which the one end slants upwardly and 
inwardly from the bottom towards the top. At least one coaxial cable is 
carried by the body and has a first connector end and a second testing 
end. The testing end extends to the intersection of the bottom and the 
slanting one end of the body for engaging contacts on a wafer and 
performing tests of the electrical components on the wafer. 
Still a further object is wherein the coaxial cable includes a circular 
shield and a center conductor forming testing contacts positioned at the 
intersection of the bottom and the slanting one end of the body. 
Yet a still further object of the present invention is wherein the testing 
end of the coaxial cable substantially forms a half circle on the slanting 
one end of the body. 
Still a further object of the present invention is wherein the body is 
formed from a material consisting of thermosetting polymers, thermoplastic 
polymers and elastomers. Preferably, the body is a polyester resin. 
Still a further object of the present invention is wherein a gold contact 
is electroformed on the ends of the center conductor and the shield. 
Yet a still further object of the present invention is wherein the length 
of the coaxial cable is short enough to satisfy the maximum percent rise 
time degradation and maximum reflective losses. 
Yet a still further object of the present invention is wherein the plane of 
the slanting end is approximately 35.degree. from the plane of the bottom 
for optimizing the cable profile. 
Still a further object of the present invention is wherein the tip of the 
probe has at least two mils of deflection under a probe load of 10 grams 
of force. 
Other and further objects, features and advantages will be apparent from 
the following description of presently preferred embodiments of the 
invention, given for the purpose of disclosure and taken in conjunction 
with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, and particularly to FIGS. 1, 2, 4, 5 and 6, 
the reference numeral 10 generally indicates the probe of the present 
invention and generally includes a body 12 and at least one coaxial cable 
14. 
The body 12 includes a top 16, a bottom 18, a tip 20 at one end, in which 
said one end 20 slants upwardly and inwardly from the bottom 18 towards 
the top 16. 
The coaxial cable 14 includes a circular outer shield 22 and a center 
conductor 24 which are separated by an insulation 26 such as a plastic 
sold under the trade name Teflon. The coaxial cable 14 has a first end 
connected to a connector 28 and a second testing end extending to the 
intersection of the bottom 18 and the slanting one end 20. The probe 10 
carries a test signal through the middle conductor 24 and one ground 
signal through the circular shield 22. 
The probe 10 is used for microwave frequencies for on wafer measurements of 
electrical components such as transistors and integrated circuits in which 
a standard SMA connector 28 is connected to a time domain reflectometer 
(TDR) such as a Tekronix 7854 TDR with a S-52 generating head and a 
Hewlett-Packard 8510 network analyzer. The SMA connector 28 may be 
connected perpendicularly or in line with the connector end of the Coaxial 
cable 14, here shown as being perpendicular. This requires that the 
multigigahertz signal is taken from the testing instrument which uses the 
standard SMA connector's 70 mil centers to the probe testing end in which 
the spacing between the circular shield 22 and the center conductor 24 may 
be quite small, for example, 4.0 mils. The micro-coaxial cable 14 provides 
the means of achieving this reduction in the transmission line size. The 
advantages of micro-coaxial cables are: (1) micro-coaxial cables are 
relatively inexpensive, (2) such cables are available with SMA connections 
preattached, (3) coaxial cables maintain a good transmission line 
environment up to the point of contact, (4) micro-coaxial cables may be 
tuned to give the best transmission line response, (5) such cables have 
frequency carrying capacities in the multigigahertz range for short 
lengths, and (6) a design which joins several of these cables together 
would be highly extendable. Coaxial cables, suitable for small probes, in 
the range of 0.008 and 0.141 inches in diameter are available. One 
satisfactory cable used in the present probe 10 which was 13 mils in 
diameter was that sold by Micro-Coax Inc. as model UT-13. However, in 
satisfying the desired maximum percent of rise time degradation required 
and the maximum reflective losses, it was found that the coaxial cable 14 
should be no longer than one inch based on cable sample tests. Of course, 
longer lengths could be used in other applications. The overall length of 
the probe was set at 0.6 inches as the extra 0.4 inches of cable was 
needed for the SMA connector 28. In addition, the coaxial cable can 
maintain the required 50 ohm transmission line environment up to the point 
of contact in order to meet the performance specifications. 
In determining what material would be suitable for the body 12 for 
supporting the coaxial cable 14, the criteria was based upon its 
moldability, durability and flexibility. Plastics and rubbers were 
considered for their good dielectric properties and strengths. While 
thermosetting resins, thermoplastic polymers and elastomers are 
satisfactory, polyester resin 32-032 sold under the trademark Polylite was 
chosen as polyester resin can be easily cast and is low in cost. The 
plastic body is easily machined and clear which allows for the 
non-destructive observation of the embedded cable 14 and the placement of 
the cable 14 can be easily controlled. Probes 10 were manufactured in 
which the mold was first spread with dry mold release agent and the cables 
were clamped down to insure that they remained in place. Polyester resin 
was mixed with hardener (one part hardener/100 parts resin) and poured 
into the mold and allowed to set in an oven at 200.degree. for thirty 
minutes. The material for the body 12 was selected to provide that the tip 
of the probe 10 should experience at least two mils of deflection when 
applied with a load of 10 grams of force in order to insure good contact 
with the test pads and to accommodate slightly irregular test pad heights. 
As one example, the probe body 12 using cast polyester resin 32-032 had a 
thickness of approximately 0.04 inches, a length of approximately, 0.5 
inches, a width of 0.25 inches, a tip deflection of 0.00209 inches with a 
load of ten grams and the bottom 18 had an angle of approximately 
11.degree. to the horizontal. 
In grinding the slant end 20 and bottom 18, it was noted that a small ball 
of material would form on the end in the grinding direction which would 
cause problems. The procedure which yielded the best result took two 
steps. First, the slanted end 20 of the probe, as best seen in FIG. 3, was 
ground upwardly as indicated by the directional arrow 30 with course sand 
paper followed by fine sand paper. Then the bottom 18 of the probe was 
ground as indicated in the directional arrow 32 in FIG. 4 with the course 
paper again moving away from the edge. The grinding divided the cable in 
half as best seen in FIGS. 4 and 5 wherein the testing end of the coaxial 
cable substantially forms a half circle on the slanting end 20 of the body 
12. This allowed the ground and signal contacts on the shield 22 and 
conductor 24 to be aligned on a wafer through the use of an optical 
microscope. That is, the contacts 22 and 24 are then visible for making 
contact with pads below the probe 10. The angle of the slanting end 20 
relative to the bottom 18 was optimized. If the angle with respect to the 
bottom surface 18 of the probe was too small, a large amount of the center 
conductor 24 would be exposed. On the other hand, if the angle was too 
large, the profile of the cable 14 would become more circular thus 
bringing the grounded outer shield 22 closer to the center conductor 24. 
The grounding outer shield 22 would then get close enough to the center 
conductor to risk contacting more than one testing pad at a time thereby 
short circuiting the device under test. Both an optimum cable profile and 
reduced separation occurred at an angle of approximately 35.degree.. 
Again, other angles could be used in other applications. 
Preferably, noble metal connections are needed for circuit tips to guard 
against corrosion. For example, as best seen in FIG. 6, gold tips 32 are 
connected to the shield 22 and gold tip 34 is connected to the center 
conductor 24. While the gold contact forming may be performed by any 
suitable process, electroforming was found to be satisfactory. This 
process requires no special machinery, the materials needed are readily 
available and inexpensive, the contacts which are electroformed adhere 
well to a copper substrate, and the atom-by-atom deposition produces pad 
heights with great accuracy. As shown in FIG. 6, the probe 10 is in 
position on a test wafer 36 having a signal line 38 which is placed in 
contact with the center conductor 24 and ground leads 40 which are placed 
in contact with the ground shield 22. However, as best seen in FIGS. 7 and 
8, the probe can be provided with additional coaxial cables to provide a 
multitude of test points. Thus, in FIG. 7, the probe 10a includes two 
coaxial cables. The first cable 22a has an outer ground shield 22a and an 
inner center conductor 24a while the second coaxial cable has an outer 
ground shield 22b and an inner center conductor 24b. And as best seen in 
FIG. 8, a probe 10b is provided having a plurality of coaxial cables 14c, 
14d and 14e. 
As previously described, prior art probes were very stiff and fragile and 
the excessive stiffness inhibited the probe's ability to conform to slight 
surface irregularites in the wafer, making complete contact difficult. The 
probe 10 is more compliant and can withstand small deflections and impacts 
without breaking and is able to conform and make good contact with 
slightly irregular pad heights. Both the type of material used for the 
body 12 and the structure of the coaxial cable 14 act to provide these 
advantages. 
FIG. 9 illustrates the deflection of the tip end 20 in an upward and 
downward direction for allowing the testing end to be moved into and out 
of contact with a testing wafer without damage. It is to be noted that 
this flexing provides an advantageous wiping action for cleaning the 
contacts. FIG. 10 illustrates the ability of the probe 10 to make good 
contact with testing pads in which the testing pads are in a plane at an 
angle to the plane of the bottom of the probe 10. 
FIG. 11 illustrates the ability of the center conductor 24, which has been 
present downwardly a slight amount, to make contact with an offset testing 
pad. 
Two series of tests were performed on the probe 10. The probe's electrical 
performance and contact characteristics were determined by testing the 
probe in air and on a testing substrate. A first set of measurements was 
taken without contacting a wafer substrate. The probe was mounted on the 
end of a 50 ohm air line and pulsed with a 55 ps step input. A 1.8% Trd 
and 1.75 Db reflection losses at 20 GHz were observed for a signal 
degradation by the probe 10. 
The present invention, therefore, is well adapted to carry out the objects 
and attain the ends and advantages mentioned as well as others inherent 
therein. While presently preferred embodiments of the invention have been 
given for the purpose of disclosure, numerous changes in the details of 
construction and arrangement of parts will be readily apparent to those 
skilled in the art and which are encompassed within the spirit of the 
invention and the scope of the appended claims.