Probes and Fixtures Ease Microwave Testing

Although they are often viewed as accessories, RF/microwave wafer probes and test fixtures provide the means to characterize many devices that might otherwise be difficult to test.

Characterizing microwave semiconductor performance is reliant upon high-performance signal sources and analyzers, but many of those measurements would not be possible without microwave probes and test fixtures. Ideally, these test "accessories" should be electrically invisible but provide a practical means of injecting and extracting RF/microwave signals to and from a semiconductor device of interest, whether on wafer or in a package.

Test fixtures and probes, in concept, are simply adapters that make it possible to move test signals to and from a device under test (DUT). Often that DUT may be in a tiny surface-mount package, where finding a way to make connections to signal and DC power pins can be nerve-wracking and overly time consuming. A properly designed test fixture can not only make such measurements easily possible, but also greatly assist the accuracy and repeatability of such measurements by providing a uniform test surface. Similarly, on-wafer probes, if the probe-tip dimensions are properly sized to the requirements of a device's contact pads, can greatly contribute to measurement accuracy and repeatability.

What are the things to look for in a microwave test fixture? In addition to accommodating a package of interest (and the devices within), a test fixture must provide adequate bandwidth for characterizing a DUT. In some cases, this may be a frequency range that is well beyond the moding frequency of a given device's package, and not simply the operating frequency range of the DUT. Electrically, a test fixture should be invisible, with low insertion loss, but it should also provide the flexibility to adapt to as many test scenarios as possible. One of the better-known names in microwave test fixtures, Inter-Continental Microwave, offers an adjustable mainframe test fixture for measurements from DC to 40 GHz. It is available with interchangeable transitions and a variety of connector options, including 2.4-mm, APC-7 (7-mm), and APC-3.5 (3.5-mm) connectors. The test fixture is suitable for S-parameter, noise figure, and power measurements on packaged transistors, can handle temperatures from -55 to +125C, and can be supplied with midsections fitted for liquid cooling, air cooling, or a combination of air and liquid cooling for characterizing high-power transistors.

A somewhat lesser-known name in test fixtures, Electro-Phonics, offers its model FRFS-0352 test fixture for small-signal measurements from DC to 12.4 GHz. It also has swappable midsections that can be selected for different size components. In keeping with being electrically invisible, the test fixture exhibits typical insertion loss of 0.5 dB at 3.5 GHz and 1.7 dB at 12.4 GHz. The VSWR is typically 1.10:1 from DC to 3.5 GHz and 1.25:1 from 3.5 to 12.4 GHz. This fixture can also be used from -45 to +125C and is supplied with female SMA connectors.

Companies such as Focus Microwaves and Maury Microwave Corp. that specialize in load-pull test systems also offer test fixtures for use with their load-pull systems. Focus Microwaves, for example, supplies lines of microstrip and coaxial transistor test fixtures specially designed for load-pull applications. In both series of fixtures, a DUT is clamped in place. The firm, well known for its accurate load-pull and source-pull measurement systems, supports its impedance tuners with fixtures based on extremely short electrical signal paths with low loss and low VSWR. Both series of text fixtures can be used through 18 GHz.

Maury Microwave Corp. recently introduced a series of test fixtures for use with its load-pull systems and for other measurement applications, including the model MT964 (Fig. 1). It is available with heatsinks and fans, as well as with water cooling as an option, for performing high-power measurements. The fixture can be supplied with characteristic impedances of 50 or 10 to 12 O for use from 0.1 to 18.0 GHz. Power-handling capabilities vary with the type of connectors, with fixtures having 2.4- and 3.5-mm connectors rated for 25 W CW maximum RF/microwave power and a model MT964A2 with 7-mm connectors capable of handling 250 W CW power. These high-performance test fixtures boast less than 1-dB insertion loss at 18 GHz.

Test-equipment manufacturers, notably those who produce vector network analyzers (VNAs), often also provide broadband test fixtures for use with their instruments. Anritsu Co., for example, offers the 3680 series of universal test fixtures for applications from DC to 60 GHz. The fixtures feature spring-loaded jaws to ensure 0.1-dB measurement repeatability on circuit substrates with thicknesses of 5 to 75 mils. In additional to a variety of test fixtures for evaluating the performance of chip and surface-mount devices, Agilent Technologies produces the model 16451B material test fixture for making dielectric measurements on materials. It can be used to measure capacitance, dielectric constant, and dissipation factor on circuit materials. This test fixture can be used at frequencies of 30 MHz and higher and with materials of 10 mm thickness or more.

For those who wish to characterize their devices over temperature, Khoury Industries offers their model KI1000 Universal RF/microwave thermal test fixture which can work with an thermal source and has an operating temperature range of -90 to +225C. It employs a "clam-shell" design with interface to standard testers or ATE systems, a pair of DB9 connectors, 10 SMA feedthrough connections, and 2 thermocouples to monitor and control the ambient temperature and the case temperature of a DUT. This fixture supports multiple DUTs simultaneously.

In terms of on-wafer measurements using probes, Cascade Microtech has offered innovative solutions to meet the ever-evolving needs of testing ICs and on-wafer devices at fast digital speeds and high microwave frequencies. The firm recently announced its InfinityQuad multicontact probe head, which is capable of probing a wide range of different type probe pads, including aluminum, copper, and gold, with dimensions as small as 30 x 50 m.

The new wafer probe uses a unique lithographically defined probe tip to probe small pads without damage. The probes can be equipped with as many as 25 contacts for ground, power, and test signals at frequencies to 110 GHz, in custom configurations. Pads as fine as 30 x 50 m can be probed at temperatures from -40 to +125C. Michael Burger, President and CEO of Cascade Microtech, notes: "The unique design of the InfinityQuad allows our customers to stay ahead of the technology curve for testing complex device geometriesAs the market moves to smaller and smaller footprints for high-frequency test, precision measurement becomes critical. Cascade Microtech is pleased to offer our customers a truly innovative, high-performance multi-contact probe solution to meet the challenges of device test today and in the future."

Cascade also has developed its long-life series of |Z| Probe products for testing from DC to 50 GHz. The patented technology in these probes yields low contact resistance with tightly controlled impedance and extremely long operating lifetime, with guaranteed useful lifetime of at least 1,000,000 contact cycles under standard operating conditions. Multiple probe contacts can move independently of each other, with minimal crosstalk.

In addition to its PicroProbe test fixture for measurements from DC to 18, 26, or 40 GHz, GGB Industries offers its model 40A microwave probe for testing from DC to 40 GHz. It features individually spring-loaded, low-inductance contacts, in a user-defined mix of RF and DC contacts. The model 40A probe is actually a pair of separate probes mounted on a single holder (Fig. 2). Each probe can be separately configured, and each probe allows for independent motion in support of versatile on-wafer measurements. Model 40A exhibits insertion loss of less than 0.8 dB through 40 GHz and return loss of better than 18 dB through 40 GHz.

Of course, to make any test fixture or microwave probe work effectively, it must be properly calibrated, which requires proper calibration standards. The calibration process ideally removes the electrical presence of a test fixture or probe. For a test fixture or wafer probe, this requires the use of calibration standards, such as short and open circuits, as well as well-behaved transmission lines also known as through or "thru" circuits, and standards with known impedances or loads. GGB Industries, for example, supplies calibration substrates that are usable from DC to 220 GHz, in support of signal-ground (SG), ground-signal (GS), and ground-signal-ground (GSG) test configurations and numerous calibration approaches, such as short, open, load, through (SOLT), line-reflect-match (LRM), and line-reflect-line (LRL) calibration methods.

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