Wideband Probing Techniques For Planar Devices: A Review

Eric W. StridCascade Microtech, Inc.Abstract: A review of high- frequency wafer -probing techniques which usecontact to the device under test is given. 50 -ohm probes have recently beendeveloped for frequencies up to 50 GHz. On -wafer impedance standards andcalibration accuracies have also improved. Probes for high -speed MSI circuitsrequire low- impedance power -supply connections. Probes for non -intrusivemeasurement of internal IC nodes are also reviewed.I. INTRODUCTIONAs the speeds of integrated circuits, microwave integrated circuits (MICs),and monolithic microwave integrated circuits (MMICs) increase, so must thetest equipment bandwidths and speeds increase. Since the most economicalmanufacturing strategy is to test and reject parts as early in themanufacturing process as possible, wafer probing of devices and /orpartially -completed devices or test structures is often used. Many devices canbe sorted from DC measurements alone, but more complex circuits tend torequire test at full operating speed. Another application of wafer testing atspeed is to grade or bin the parts for maximum speed, minimum noise figure,etc., before the dice are mounted into expensive packages.There are two classes of high- frequency probe applications. The first is touse the probe as a substitute for the I/O and power connections normallyprovided by the package, for purposes of testing the terminal characteristicsof the device under test (DUT). This application requires controlled- impedancesignal lines, low- inductance ground contacts, and low- impedance powercontacts. This paper first reviews approaches to controlled- impedance probesand the state of the art in 50 -ohm probes. Common -lead connections (powers andgrounds) are very important, so the need and typical performances oflow- impedance probes is discussed separately in the next section.The second class of probe application is to measure the terminai or internalnodes of the DUT without disturbing the DUT's operation. The DUT may beoperating in its intended system or subsystem application, or be operatingwith power, stimulus, and loading supplied by probes as mentioned above. Thusthe two classes of probes may be used alone or with the other. Probes fornon -invasive measurements must present a high impedance to the DUT node beingmeasured, but high impedances are of course difficult to realize at very highfrequencies. The final section will review means of measuring nodes with highbandwidth and minimal loading.II. 50 -Ohm ProbesA. Typical ImplementationsThe basic idea of a 50 -ohm probe is to control the characteristic impedancedown to the wafer connections as well as possible. Some of the approacheswhich have been used include sharpening the center conductors of microcoaxlines, attaching very short probe needles to microstrip lines, and definingcoplanar contacts with microstrip or coplanar waveguide feeds on flexible orspringy substrates.Figure 1 illustrates an implementation of a high- frequency probe card [1],wherein a low- frequency card's dominant electrical parasitics were scaled downto accomodate higher frequencies. The signals converge to a hole in themicrostrip substrate, where short needles reach down to contact the deviceunder test. A ground contact is made by shorting that needle to a ground viaor wraparound to the ground plane. A power contact is made by connecting acapacitor between the power needle and a ground via.