RF probe contacts have traditionally been made from beryllium-copper (BeCu). However, the early development of RF probe technology was vastly different from today’s advanced tools. Over time, engineers made significant breakthroughs in probe design, establishing the fundamental requirements and operational principles that define modern RF probes.
Radio frequency (RF) probes are essential throughout the entire life cycle of RF products, from initial research and development to model parameter extraction, design verification, and final production testing. These probes enable accurate measurement of RF components at the wafer level, significantly reducing R&D time and lowering the cost of new product development.
In just 30 years, RF probe technology has evolved dramatically, expanding from low-frequency measurements to commercial solutions for a wide range of applications. This includes impedance matching in high-frequency and high-temperature environments up to 110 GHz, multi-port, differential, and mixed-signal measurement devices, high-power measurements up to 60W in continuous wave mode, and even terahertz applications reaching 750GHz.
The earliest RF probes used a 50-Ω microstrip line that connected to a short wire tip through the probe substrate. A small hole made contact with the device under test (DUT) pad. At the time, the main challenge was achieving repeatable measurements above 4GHz. Although calibration could reduce the impact of inductance, movement of the wafer holder caused variations in radiation resistance. High-frequency probe tips differ from those used for DC or low-frequency measurements, requiring a 50-Ω environment as close as possible to the DUT contact point.
Engineers later developed more reliable designs, leading to key advancements in RF probe technology:
1. The 50-Ω planar transmission line must directly touch the DUT without contacting the conductor. Microstrip and coplanar designs use small metal balls for reliable contact.
2. Probes need to 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 on-chip standards and calibration methods.
4. Highly repeatable contact allows precise calibration, moving the reference plane closer to the probe tip, while minimizing losses and reflections from the probe wire to the coaxial connector.
5. Due to their small size, planar standards can be modeled as purely lumped elements, making it easier to predict parameters based on geometry.
As probe designs shifted from microstrip lines to coplanar waveguides (CPW), fabrication became simpler. Tektronix transformed the probe from a DIY tool into a true commercial product, marking the start 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 launched 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 reached 50 GHz, supporting the rapid development of monolithic microwave integrated circuits (MMICs). V-band and W-band probes followed in 1991 and 1993. In 1988, Cascade introduced the 26.5GHz Series of Tip-Replaceable Probes (RTPs), allowing quick ceramic tip changes without moving the probe body. Despite its contributions, the WPH had limitations, particularly with fragile ceramic CPW lines.
In 1988, GGB Industries patented a micro-coaxial cable-based RF probe, offering mechanical improvements, easier repair, better electrical performance, simplified manufacturing, and reduced costs. By 1999, their probes reached 220 GHz, and by 2012, they achieved 500 GHz. With collaborations like Karl Suss, GGB became a major player in the global RF market.
Meanwhile, Cascade introduced the 40-GHz air-coplanar probe (ACP) in 1994, eventually reaching 110 GHz and 140 GHz. Engineers preferred ACP for its soft, non-destructive contact with gold pressure points.
In 2000, Rosenberger introduced the ∣Z∣-probe, a new concept for PCB applications. It featured a direct transition from coaxial to air-insulated coplanar lines, fabricated using UV-LIGA technology for precision and repeatability.
With increasing RF demands and smaller contact points, Cascade introduced a new round crystal probe in 2002, using a soft polyimide film substrate. This allowed detection of very small contact points with excellent consistency and low crosstalk.
Cascade continued to expand its offerings, introducing waveguide-based probes for 220 GHz and 325 GHz in 2005 and 2007, and Infinity probes for 500 GHz in 2009.
New entrants like DMPI and Allstron entered the market, targeting sub-THz and cost-sensitive applications. Allstron’s traditional micro-coaxial design offered affordable solutions for frequencies below 110 GHz.
Modern RF wafer probes convert signals from 3D media (coaxial or waveguide) to 2D coplanar interfaces. This requires careful handling of impedance and electromagnetic energy conversion. While the input is standardized, the output allows various design concepts, introducing discontinuities that must be managed to maintain signal integrity.
Traditional RF probes consist of several components: tester interface, transition to micro-coaxial cable, transition to planar waveguide, and the coplanar interface for the DUT. Some designs combine steps or eliminate the micro-coaxial cable.
The lifecycle of a probe technology is typically around 12 years, driven by the need for higher accuracy in high-end applications and lower testing costs for mainstream uses. New suppliers and service providers continue to innovate across various frequency ranges.
For example, the MP series coaxial probes support DC-20GHz measurements with ultra-low insertion and return loss. They offer flexible configurations such as GSG and GS, with pitches ranging from 0.8mm to 2.5mm.
Advantages include easy detection without soldering, compatibility with pogo pins for non-planar structures, longer probe life, and faster testing. These probes are widely used in RF module testing, high-frequency PCB analysis, and high-speed digital circuit evaluation.
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