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Figure 1. RFMD’s pHEMT has two conducting layers: the channel and the cap. It is possible to measure the mobilities in both these layers with a Lakeshore 7612 multi-field Hall system
channel sheet charge (see Figure 1). Keeping an eye on channel mobility is important, because if it falls too low, this can degrade the transistor’s maximum drain current and its drain-source current. However, it is paramount to keep the channel sheet charge close to its target value, because this gives a direct indication of the pinch-off voltage for the FET once the wafers are processed.
The lead author of the paper, Robert Yanka, told Compound Semiconductor that carrier mobility in the channel also provides an indication of the condition of the growth system. “When an MBE system is being qualified following a maintenance cycle, mobility can start off low and will rise as the system cleans up. We have specs that let us know when the material is acceptable for delivery to the fab.”
steps, different staff managed different aspects of the conversion program. By working on smaller projects, those in charge could focus on the finer details.
To keep everyone fully aware of progress, the company held many meetings. Bi-weekly meetings kept senior management informed of progress; various weekly meetings aided internal communication, such as that between factory staff and their managers, or project and program managers; and a daily “9 a.m. 6-inch meeting” helped the company execute efficient production while maintaining project progress. “The number of meetings might seem like overkill, but without them this project wouldn’t have been successful,” says Hafer.
As the project progressed, tools had to be converted back and forth on many occasions between 4-inch and 6- inch processing. If the technicians were unaware of the latest change, they might have tried to run a process on a tool that was now unsuitable for that task, and wasted good material. To prevent this from happening, a continually updated status for every tool was provided to technicians using a web-based approach.
The day of reckoning arrived when Skyworks tested its first devices produced on its 6-inch line. If failure rates were high, the trouble-shooting needed to remedy this could have taken the company beyond its two-year goal for completing the project. But results were excellent: final test yields for transistors from the first two processes run on the new line were 97.2 percent and 98.0 percent.
Measuring mobilities Skyworks’ biggest rival, RFMD, has also been improving its production process by introducing a Lakeshore multi- field Hall system. This tool allows the company to measure non-destructively, rather than destructively, mobilities in the channel and the cap of a pHEMT plus the
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www.compoundsemiconductor.net July 2010
Figure 2. Engineers at RFMD investigated how changes in the excitation current can effect the recorded values of: Channel sheet resistance (Chan Sheet); channel mobility (Chan Mobility); cap sheet resistance (Cap Sheet); and the mobility in the capping layer (Cap Mobility)
Conventional methods for measuring sheet charges and mobilities, such as non-contact sheet resistance approaches and single-field Van der Pauw Hall measurements, are unsuitable for pHEMTs because they fail to distinguish between the high mobility carriers in the channel and the low mobility carriers in the cap. Common ways to determine the sheet charge and carrier mobility in the channel are to either etch away the cap prior to measurement, or to use a specially grown test structure that enables direct measurement of channel properties. Neither approach is ideal. It’s not just the added cost that stems from either destructive testing or the growth of additional structures: if the cap is too thin, the depletion layer extends into the channel, and measurements of channel carrier density are underestimated; and if the caps too thick, it contains residual carriers, leading to an overestimate of the channel carrier density.
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