Use coplanar waveguide probes for accuracy and repeatability of RF measurements
As consumer devices like cell phones, laptop computers and PDAs get smaller, users are now expecting more functions in a single piece of equipment. They want that one little powerhouse to be easy to use, extremely reliable and smaller than a bar of soap. Consequently, highly specialized components and assemblies used in these and other demanding applications are operating at higher frequencies and must meet tighter performance specifications.
Thus, 0201s are now favored because they are 75% smaller and occupy 66% less board space than the 0402s they typically replace. With packages, typical SMD lead pitches and micro BGA bump pitches are as small as 0.5 mm. Much smaller interconnects on boards and assemblies, with conductor traces less than 4 mils (0.1016 mm), also challenge the test milieu.
As for frequency, commodity commercial products are at 6 GHz, CPUs on board interfaces have 10 GHz bandwidth, and telecommuni-cations devices operate at 10 Gb, 20 Gb and 40 Gb. Increased performance is also expected in SOICs, for example, where more functions are demanded from the same area. This means more electrical contacts to effectively stimulate, and a higher mix of signal types, and some of these are always at a higher bandwidth.
With these changes in electronic components and assemblies, quality engineers feel the dual pressures of working with objects that are too small to see and touch yet are simultaneously held to higher standards for precision and reliability. To meet these challenges, precise, affordable probe stations have been created to handle the more difficult test regimens required by the changes mentioned above.
These probe stations also have microwave test accessories available, as well as test cables specifically designed for microwave microprobe testing, and precise thin film network (TFN) adapters for coplanar waveguide to microstrip circuits and MMIC sub-assemblies. Some manufacturers offer a wide range of sizes, features and prices that fit the needs of any size lab. Before offering a brief summary of microprobe testing equipment, however, there are issues to consider regarding fixturing and setup of several specific types of components.
Issues to consider
The use of coplanar and coaxial microprobes has made many microwave measurements easier and more accurate. However, there are many products, including FETs, MMICs, chip capacitors, chip resistors, and chip inductors that are designed for microstrip applications. None of these products have the required signal and ground pad orientation and the required spacing to allow microprobing. These devices will generally be wire bonded into a circuit, so that the wire bond becomes one of the circuit elements.
Consequently, it is desired that the measured S-parameters of this device also include the bond wire response. For example, a low-noise GaAs FET die will generally be die attached to a metalized ceramic substrate, and the gate, drain, and source are bond wired, using short double bonds, to the specified pads on the substrate. Note that the bond wire lengths of the test samples must be identical with the specific application.
Until recently, these measurements have been difficult and tedious. Now, a new set of adapters and associated calibration technique makes these measurements straightforward. The adapters, shown in Figure 1, adapt a coplanar probe to a microstrip, which connects to the device under test (DUT) with bond wires.
Double bonds, as used in the actual application, connect the gate and drain metallization to the adapter microstrip. The source metallization is wire bonded to the ground plane, which is common to the entire setup. Any gold metalized conductor works well for the carrier. The DUT dice and the adapter substrates are either attached with silver epoxy or eutectic solder, as required.
Having looked at this particular concern that affects microprobe testing of some component types, three brief case histories will be presented, followed by a summary of the range of equipment available for microprobe testing.
Three brief case histories
These case histories
After a broadband bandpass filter in the Q band was designed, the circuit was tested using a test fixture. Several poles in the in-band response of the filter and the high insertion loss, caused by the double coaxial-to-microstrip transition, were observed. In order to eliminate all inaccuracies of the test method that masked the actual behavior of the circuit, and to obtain an accurate measurement up to 50 GHz, a coplanar to microstrip transition connected to a coplanar probe station was used. The circuit is shown in Figure 2a and its behavior in Figure 2b. Using the coplanar to microstrip transition to test the filter, the tested results were quite close to the simulations.
Several microstrip broadband attenuators were designed with a topology that provides constant attenuation over broad bandwidth with good input and output matching, which requires a reliable test fixture. The use of coplanar transitions provided accurate measurements. One attenuator of 3 dB is shown in Figure 3a and the attenuation and matching tested are depicted in Figure 3b.
In a circuit design, it is important to have an accurate model or characterization of the active device. Before designing some amplifiers in the Q band, an EC2612 GaAs pHEMT transistor was tested using the coplanar to microstrip transitions as shown in Figure 4a. The chip has internal via holes for the source connections to ground. The drain and gate were bonded with 17.5 micron gold wires and length 200 µm. The reflection S-parameters of the transistor for -10 dBm input power, drain voltage 2 V and a drain-to-source current 10 mA, are shown in Figure 4b.
The issues for testing the new sizes and frequencies in electronics are handling, fixturing and vision. Handling is a problem because human hands do not have the dexterity called for by the smallest electronic components and assemblies. Fixtures that are “hand-sized,” with their larger contacts and thereby looser tolerances, generate parasitics that hinder good quality testing. As for vision, the human eye is not calibrated to easily distinguish discrete pieces as small as 0201 component, for example, with enough clarity to perform tests on them.
A rule of thumb is that repeatable measurements require 1 mil (0.0254 mm) contact placement accuracy at 10 GHz. A general solution to the previously mentioned test issues that enables an engineer to achieve this rule of thumb accuracy involves:
CPW probes that provide an electrical reference plane at a precise point in space with precise planar contact, and are capable of quick and easy calibration.
The DUT holder must be secure, maneuverable and easy to load or unload.
The probe holding fixture should be rigid, repeatable and allow the flexible placement of probes.
The best CPW probes offer a wide range of planar contact styles, such as GSG, GS and SG. They offer a broad range of pitches from 75 µ to 2500 µ, controlled impedance, traceable calibration and relatively low cost. These probes are defined by bandwidth, e.g., 18 GHz, 40 GHz, up to 220 GHz and should be relatively low priced so that labs can afford to have probes with planar contacts precisely suited to their testing needs.
CPW probe calibration is most efficient when there is a standard calibration kit available with standard calibration procedures. These procedures work best when they are internal to the test equipment and offer open-short-load-through (OSLT), line-reflect-match (LRM) and through-reflect-line (TRL) calibration, are software controlled with the capability of using SOLR, multiline (NIST) and other protocols.
Probe stations/DUT holders
A good probe station is flexible, portable, and low-cost and has options available that allow lab personnel to meet the specific testing demands of their customers' components and assemblies. The stations listed in the following paragraphs are capable of handling a wide range of applications, including, but not limited to testing semiconductor devices, microwave packages, MIC components and doing small sample failure analysis.
A basic, low-cost probe station that meets the needs of many engineers and scientists is capable of movement in the X, Y and Z axes. This grade of instrument will typically enable one inch of travel in the X and Y axes and 50 mils of vertical movement in the Z axis. The station will have a platen approximately 7" × 12" (178 mm × 305 mm) supporting a stage of approximately 2" × 2" (51 mm × 51 mm). A vacuum hold down secures components on the platen without placing stress in any plane. Optics on this type of probe station are in the 10x+ power range, and a fixed-intensity fluorescent ring illuminator provides adequate lighting for many applications.
For the engineer or scientist who requires higher precision in a probe station that fits well in a personal workspace, the instrument of choice would be a compact manual probe station. This instrument, sometimes called a personal probe station, expands the features of the basic probe station. The stage size increases to 4.5 inches (114.3 mm) and travel in the X axis is 2.5 inches (63.5 mm) and in the Y axis is 4 inches (102 mm). An added feature of this class of probe station is travel up to 180° rotation in either direction. Standard optics have 7x-112x capability, and lighting is a fixed-intensity fluorescent ring illuminator.
A full-featured manual probe station offers the greatest range of capabilities to deal with the most demanding applications. This instrument gives the engineer or scientist a choice of a 6.5 inch (165 mm) or 8.5 inch (216 mm) stage. Added weight and size give this model more stability during tests. Movement in the X and Y axes is a full 6 inch (152.4 mm), 0.25 inches (6.4 mm) Z lift, and 180° rotation in the . Zoom optics have a 0.7x-4x objective lens providing magnification of 42x-270x for probe placement and DUT alignment with the standard 0.5x auxiliary lens. Removal of the auxiliary lens changes the range of magnification to 84x-540x for inspection and fine geometry probing. There is a 2x relay lens and a 0.5x or 1.0x (no lens) objective multiplier.
Many electronics devices can be tested in the different probe station models. Some of these applications fit one model better than another, but in general, the following devices can all be tested in a good quality probe station.
First-order devices (do not require fixturing):
Semiconductors — GaAs or any other advanced IC, II-VI or III-V devices for process control monitoring, RF performance or pulsed IV performance.
Surface-mount devices (SMD) — Micro BGAs, leadless carriers or leaded carriers such as a standard SOIC, upside down on either a conductive or non-conductive chuck.
SMD passive devices — Standard or custom calibration of hybrid couplers using a custom chuck.
MMIC packages — Capable of standard or custom calibration, often with a custom probe configuration and a custom device under test (DUT) holder.
Interconnect structures — Test high-performance PCBs for signal integrity, transition, impedance and parasitic elements.
Second-order devices (require fixturing with thin film adapter substrates):
- Transistors — Modeling and evaluation.
- Diodes, single-port devices.
- Packages that cannot be tested with standard CPW.
While a probe station usually comes equipped with standard features that are adequate for a majority of applications, sometimes it is necessary to add options that enable testing components with unusual requirements. Some of the options available for probe stations are:
Manipulator/probe holder — these are available in different sizes, prices and features.
The basic model has a magnetic-mount positioner with dovetail slides for dc and general-purpose ac microprobing use, and features X, Y and Z travel. It usually comes with several needles that can be used to secure parts for test. This entry-level model has a magnetic mount.
A slightly higher-priced model is available in which the slides are mounted on roller bearings for smoother X-Y travel and longer life. The engineer or scientist who buys this class of manipulator/probe holder can expect a 40 or 80 turns-per-inch positioner knob for precise placement of parts being tested, as well as somewhat greater travel capability in all axes than on the basic model. Knob planarity adjustment is another feature that is standard on this model. As with the basic model, a magnetic mount attaches it to the probe station.
Full-featured manipulator/holders have all the features listed in the previous models, but are bolt-mounted rather than magnet-mounted, giving them greater part holding strength and stability. They also offer greater movement in all axes.
Upgraded microscope system;
magnification range of 6.7x -168x with optional 1.5x objective;
temperature control range of -5 °C to +125 °C;
vacuum DUT hold down; and
PCB holder — allows the testing of double-sided PCBs with bottom-side clearance.
Using CPW probes for precise measurements raises the bar. Having quality microwave transitions removes the uncertainty of how accurate the test data is because they improve the test contacts' integrity and the methods for micro-component measurements. CPW adapter substrates expand the applications that are possible. Microstrip devices become testable. A standardized calibration procedure assures that the measurement data is precise, repeatable and there is cross-facility data correlation.
Eduardo Artal, Beatriz Aja, Rocio Garcia, Marco Detratti, Luis Angel Rodriguez, “Measurement of Active and Passive Millimetre Wave Devices Using Microstrip to Coplanar Line Transitions,” Departamento de Ingenieria de Comunicaciones ETSI Telecomunicacion, Universdidad de Cantabria, Santander, Spain.
ABOUT THE AUTHOR
Jerry Schappacher is executive advisor, at J microTechnology Inc. He graduated with a BSEE from Virginia Tech. His career has included a variety of assignments and opportunities in the field of RF, microwave and optics including design, manufacture and test of semiconductor devices, hybrid circuit modules, subsystems and systems. He is a member of the IEEE, written numerous articles and presented at workshops on these subjects. He founded J microTechnology in 1993.
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