RF probe contacts have traditionally been made using beryllium-copper (BeCu). However, the early use of RF probe technology was vastly different from the advanced tools available today. Over time, engineers made significant breakthroughs in probe technology, establishing the fundamental requirements and operational principles that define modern RF probes.
RF probes play a crucial role throughout the entire lifecycle of RF products, from initial research and development to design verification, commissioning, small-scale production testing, and final product testing. These probes allow for precise measurement of RF components at the wafer level, significantly reducing R&D time and development costs. In just 30 years, RF probe technology has evolved dramatically, expanding from low-frequency measurements to commercial solutions for high-frequency, high-temperature environments up to 110 GHz. It now supports multi-port, differential, and mixed-signal measurements, as well as high-power tests up to 60W in continuous wave mode and terahertz applications reaching 750GHz.
The earliest RF probes used a 50-Ω microstrip line connected to a short wire tip through the probe substrate. A small hole allowed contact with the device under test (DUT) pad. At that time, achieving repeatable measurements above 4GHz was a major challenge. Although calibration could reduce the impact of series inductance, movement of the wafer holder often caused large variations in radiation resistance. High-frequency probe tips were designed differently from those used for DC or low-frequency measurements, requiring the 50-Ω environment to be as close as possible to the DUT contact point.
Engineers later refined probe designs, leading to the determination of key requirements for RF probes:
1. The 50-Ω planar transmission line must make direct contact with the DUT without touching the conductor.
2. Probes must be tilted to simultaneously touch both signal and ground points on the DUT, a process known as "planarization."
3. Probe contact repeatability is much better than that of coaxial connectors, enabling the development of probe tips and on-chip calibration methods.
4. Highly reproducible contact allows accurate probe calibration and shifts the measurement reference plane closer to the probe tip.
5. Due to their small size, planar standards can be modeled as purely lumped elements, making it easier to predict model parameters from their geometry.
As probe designs evolved from microstrip lines to coplanar waveguides (CPW), fabrication became simpler. Tektronix transformed the probe from a DIY tool into a commercial product, marking the beginning of wafer-level RF measurement.
In the early 1980s, Tektronix introduced the first commercial RF wafer probe, the TMP9600, along with the sapphire calibration substrate CAL96. Eric Strid and Reed Gleason later founded Cascade Microtech and introduced the WPH probe. Both companies provided similar RF probes until Tektronix exited the market in the early 1990s, allowing Cascade to become a leading supplier in the industry.
By the late 1980s, the WPH probe had reached 50 GHz, meeting the growing demand for monolithic microwave integrated circuits (MMICs). V-band and W-band probes followed in 1991 and 1993, respectively. Cascade also introduced the 26.5GHz Series of Tip-Replaceable Probes (RTPs), allowing quick ceramic tip changes without moving the probe body from the test bench. Despite its contributions, the WPH probe had limitations, such as fragile ceramic CPW lines that could crack under minimal pressure.
In 1988, GGB Industries introduced a micro-coaxial cable-based RF probe, offering significant mechanical and electrical improvements. This technology extended probe life, simplified repair, improved performance, and reduced costs. By 2012, GGB’s probes had reached 500 GHz, solidifying its position in the global RF market.
Meanwhile, Cascade introduced the 40-GHz air-coplanar probe (ACP) in 1994, which eventually reached 140 GHz. Engineers favored the ACP for its soft, non-destructive contact, especially when testing gold pressure points.
In 2000, Rosenberger launched the ∣Z∣-probe, designed for PCB applications and capable of operating up to 40 GHz. This probe used a direct transition from coaxial to air-insulated coplanar lines, fabricated using UV-LIGA technology for high precision.
As silicon became more prevalent in RF applications, challenges arose due to the need for non-oxidizing materials like tungsten for aluminum contact points. The ∣Z∣-probe addressed this by using nickel tips, which offered better contact properties with aluminum and gold. Other suppliers soon adopted similar designs.
Cascade continued to innovate, introducing the Infinity probe in 2002 for thin-film technology. This probe featured a microstrip line on a polyimide film, allowing detection of very small contact points. The Infinity probe demonstrated excellent consistency and low crosstalk.
By 2009, Cascade offered Infinity probes up to 500 GHz, while other companies like DMPI and Allstron entered the market, targeting sub-THz and cost-sensitive applications, respectively.
Modern RF wafer probes convert signals from three-dimensional media (coaxial or waveguide) to two-dimensional planar interfaces. This requires careful handling of impedance and electromagnetic field conversion. Traditional probes include a tester interface, transitions to micro-coaxial cables, and planar waveguide interfaces.
The natural lifespan of probe technology is about 12 years, driven by the need to improve accuracy in high-end applications and reduce costs in mainstream ones. New suppliers and service providers continue to offer products for various frequency ranges, ensuring ongoing innovation in the RF probe industry.
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