The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Air Force Office of Scientific Research contract No. F49620-85K-0016.
The invention pertains to the field of testing and characterization of millimeter-wave and very high speed devices and integrated circuits such as those fabricated on gallium arsenide. More particularly, the invention pertains to the field of generating millimeter-wave frequencies for test purposes and guiding these test signals to a particular point on a device or an integrated circuit to be characterized.
Conventional millimeter-wave sources have their outputs in waveguide configuration. This creates a great deal of complexity and inconvenience in trying to inject signals from a waveguide at particular points of the microscopic structure of a millimeter-wave or an ultra-fast device or integrated circuit. With waveguide output of conventional millimeter-wave test signal sources, it is necessary to plumb the waveguide to the location of the integrated circuit and use suitable adaptors and test fixtures to couple the energy out of the waveguide and onto the integrated circuit. This must be done with a minimum of losses caused by scattering of energy into higher order modes and with a minimum of reflection back toward the input of energy propagating in the desired mode at discontinuities. Because of the awkwardness and in- convenience of using waveguide output signal sources, high frequency coaxial cable and connectors and wafer probes having a planar configuration using coplanar waveguides have been developed for up to 50 gHz. A tapered wafer probe with a planar configuration can more easily be used to contact pads on the device or integrated circuit.
Cascade Microtech Inc. currently makes a passive coplanar waveguide probe for use up to 50 gHz to contact devices or integrated circuit pads. This probe has a coaxial cable input to receive a test signal at a frequency below 50 gHz to be injected and couples energy from this test signal into a coplanar waveguide printed on the surface of the insulating substrate of the probe. This substrate tapers to a point where contact pads coupled to the coplanar waveguide are formed. Nickel plated probe tips are placed in contact with the contact pads on the integrated circuit to be tested to inject the test signal.
One difficulty with using such a probe is that high frequency test sources capable of generating test signals at approximately 100 gHz with coaxial outputs are not available. Another difficulty is that at least the coaxial section of the probe is not usable at frequencies above 50 gHz. This is because coaxial cable and connectors are not currently available for operation at frequencies above 50 gHz. Coaxial cable cannot be used at these high frequencies because the dimensions of the cable would have to be so small as to preclude a practical structure. This restriction on dimensions is a result of the solutions to Maxwell's gate equations. It is desirable to propagate energy in only one mode i.e., the lowest order or the fundamental mode. The cutoff frequency for the higher order modes for coaxial cable is set by the circumference. of the outer conductor. When this dimension approaches within the wavelength of the signal to be guided through the coaxial cable, scattering of energy into higher order modes occurs which is undesirable. Thus, to guide higher frequencies with coaxial cable requires the dimensions of the cable to be made increasingly smaller. However, this shrinking of the cable must be made proportionally to all dimensions to maintain an industry standard characteristic impedance of 50 ohms. Because of mechanical and electrical constraints which have yet to be overcome, at the current time, no coaxial cable exists which can be used above 50 gHz.
Signal sources which can provide output test signals at frequencies above 50 gHz are available, but the output configuration of these systems is waveguide instead of coaxial. One difficulty with using a planar probe in connection with a waveguide configuration output from a high frequency millimeter-wave source is that an adapter must be made for use between the waveguide configuration and the coaxial configuration at the input of the probe. Another difficulty is that the coaxial part of the probe cannot be used above 50 gHz with today's technology. Because the shapes of the rectangular waveguide at the output of the test source and the coaxial cable at the input of the probe are mismatched, making a suitable adapter is difficult and causes losses from the resulting discontinuity as will be understood by those skilled in the art. Coaxial systems can currently be used to guide millimeter-wave energy at or below approximately 50 gHz to planar configuration IC passive probes such as the Cascade Microtech Inc. probe. However, to extend the frequency of
sources beyond approximately 50 gHz requires the use of frequency multipliers to raise the frequency and requires the above-described waveguide-to-coax fixtures to couple energy from the waveguide output of the multiplier into the chip under test.
Needle probes which were in use before the introduction of the planar configuration probe by Cascade Microtech Inc. are unacceptable at the very high frequencies prevalent in the microwave and millimeter-wavelength bands. The reason for this is that the inductance of the needle probe wire presents a very high impedance to the test signal microwave and millimeter-wavelength frequencies. This causes difficulty in coupling sufficient amounts of energy to the chip under test because of the impedance mismatch between the 50 ohm line coaxial cable and the impedance of the wire probe. This impedance mismatch causes reflection of power at the interface thereby limiting the amount of power which is available for coupling to the chip under test.
Recently, Cascade Microtech Inc. probes with 2.4 millimeter coaxial connectors have been made publicly available. These probes allow single probe coaxial test operations to be made up to 50 gHz. However, such probes still cannot be used at frequencies above 50 gHz, the frequency range which many new fast technologies are operating.
An alternative approach would be to generate millimeter-wave signals closer to the input of the device or integrated circuit on the wafer under test. For example, the signal could be generated directly on the wafer by mixing two optical beams which are separated in frequency by the desired millimeter-wave test frequency in a nonlinear device on the wafer. The disadvantage of this approach is the need for two complex optical systems and the need for an appropriate nonlinear device formed on the wafer under test. This makes the approach unattractive for many users.
Therefore, a need has arisen for a system which can generate millimeter-wave frequencies above 50 gHz easily with low loss and with single mode operation for use in testing and characterization of millimeter-wave and ultra-fast devices and integrated circuits. Such a system must have a configuration to allow easy coupling of the energy to the integrated circuit and must not require any special constructions or device formation on the wafer under test. Further, the system must have a simple interface with conventional microwave sources and must be efficient in coupling large amounts of the millimeter-wave power generated to the chip under test.
Further, it is important to be able to take high frequency signals off the device or the integrated circuits under test and step them down in frequency to a frequency compatible with the vast majority of test equipment currently available. Therefore, a need has arisen for an integrated, planar probe structure in the form of a harmonic mixer which can perform such a step down function.
Finally, it is essential to be able to characterize two-port devices or integrated circuits operating at very high frequencies in terms of their S-parameters at the actual operating frequencies. To do this requires an S-parameter active probe which injects signals with frequencies on the order of 100 gHz to the device under test on-wafer and which can guide a known amount of 100 gHz reflected power to an output port for measurement while simultaneously stepping the reflected signal frequency down to a frequency compatible with existing test equipment. Further, this probe must also guide a portion of the incident 100 gHz signal back to a test output for measurement while simultaneously stepping it down to a lower frequency capable of measurement.
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