Basic knowledge of RF test probe
2019-05-254720
Traditionally, the contacts of RF probes are made of beryllium-copper (BeCu). And the earliest use of RF probe technology is very different from today's tools. Afterwards, engineers made breakthroughs in probe technology before determining the basic requirements and working principles of RF probes.
Radio frequency (RF) probes play an important role in almost every stage of the RF product life cycle: from technology development, model parameter extraction, design verification and debugging all the way to small-scale production testing and final production testing. By using RF probes, it is possible to measure the true characteristics of RF components at the wafer level. This can reduce research and development time and greatly reduce the cost of developing new products.
In just thirty years, RF probe technology has made amazing progress, from low-frequency measurement to commercial solutions for a variety of applications: such as impedance matching at 110GHz high-frequency and high-temperature environments, multi-port, differential and Mixed-signal measurement devices, high-power measurements up to 60W in continuous wave mode, and terahertz applications up to 750GHz can all be seen with RF probes.
The earliest use of RF probe technology was very different from today's tools. Early probes used a 50-Ω microstrip line that gradually converged with a short wire tip (wireTIp). The small hole is in contact with the pad of the device under test (DUT). At this time, its technical difficulty lies in how to achieve repeatable measurement when it breaks through 4GHz. Although it is possible to eliminate the influence of a relatively large series inductance of a contact line tip through the calibration process, when the wafer holder is moved, the radiation impedance of the line tip will change greatly. The high-frequency measurement uses a different tip design than that used for DC and low-frequency measurements, and the 50-Ω environment must be as close to the DUT voltage point as possible.
After that, engineers made breakthroughs in probe technology. Determined the basic requirements and working principle of the RF probe:
1) The 50-Ω flat transmission line of the probe should be in direct contact with the DUT pressure point without touching the wire. For microstrip lines and subsequent coplanar probe designs, the contact of the probe is achieved with a small metal ball, which should be large enough to ensure reliable and repeatable contact.
2) In order to be able to access the signal pressure point and ground pressure point of the DUT at the same time, it is necessary to tilt the probe. This process is called "planarization of the probe".
3) The contact repeatability of the probe is much better than the repeatability of the coaxial connector. Facilitates the development of probe tip and on-chip standards and dedicated calibration methods.
4) Highly repetitive contact allows accurate calibration of the probe and moves the measurement reference plane towards its pole tip. The loss and reflection of the probes from the probe wires and the transition to the coaxial connector are offset by similar errors made by the RF cable and the connector.
5) Due to its small geometric size, one can assume that the equivalent model of a flat standard is purely lumped. In addition, one can easily predict model parameters from the geometry of the standard part.
In the early 1980s, Tektronix introduced the earliest RF wafer probe model TMP9600 and sapphire calibration substrate CAL96. Eric Strid and Reed Gleason, the lead developers of the probe, founded Cascade Microtech in 1983 and launched the WPH probe. The two companies have provided very similar RF probes for several years, until Tektronix finally withdrew from the wafer probe business in the early 1990s. With such an opportunity, CascadeMicrotech has become the leading supplier of RF probes in the industry by virtue of its good relationship with Hewlett Packard.
The frequency of WPH probes was expanded to 26 GHz in a short period of time, and reached 50 GHz in 1987 to meet the needs of rapidly developing monolithic microwave integrated circuits (MMICs). V-band and W-band probes appeared in 1991 and 1993, respectively. In 1988, Cascade introduced the 26.5GHz series of extremely sharp replaceable probes (RTPs) for mass production applications. Now, people can quickly change ceramic pole tips without moving the probe body from the test bench. WPH probes contributed to the development of microwave technology in the 1980s and 1990s, but there were several technical limitations. The most critical limitation is the fragile ceramic CPW wire. Even applying a minimum force higher than the recommended value (for example, for better contact) can damage the probe. Many engineers call this moment the "voice of death." The cracking sound of ceramic probes usually pushes the entire project to an end, because probes are very expensive for universities and small research laboratories. Although the RTP series was introduced, ceramic probes were pushed out of the market by other technologies.
1988When GGB Industries applied for a patent for RF probes based on micro-coaxial cables, 1988 was another milestone. The benefits of using a micro-coaxial cable as an intermediate transition medium are:
1) Significant mechanical improvements have extended the life of the probe.
2) Damaged probes can be retaped in a relatively easy and inexpensive way.
3) Electrical characteristics have been improved.
4) Simplify the manufacturing process.
5) Reduce costs.
1993In 1993, GGB introduced the W-band probe at the International Microwave Annual Conference (IMS) of the IEEE Theoretical and Technical Association. In 1999, their probes reached 220 GHz, expanded to 325 GHz in 2006, and reached 500 GHz in 2012. Coupled with close cooperation with suppliers such as Karl Suss (later SUSS MicroTech), GGB Industries has become one of the most influential companies in the RF market worldwide.
Picoprobe Probe from GGB Industries
