conference report technology
Optimizing BiHEMT epitaxy Despite the poor state of the global economy, handset sales are rising again, with the biggest gains in the Smartphone sector. These feature-rich mobile devices incorporate multiple bands and multiple modes, leading to high levels of power consumption and a high chip count. To minimize battery drain and GaAs real estate, some chipmakers have developed processes to unite HBTs with either FETs or pHEMTs. This includes Hitachi Cable, which has been developing a BiHEMT process to unite HBTs and pHEMTs since 2003.
At this year’s CS-Mantech, Junichiro Takeda and his co- workers detailed one of the problems that they uncovered when developing the growth process for this product: a reduction in the carrier mobility in the InGaAs channel of their pHEMT from 7000 cm2/Vs to 5800 cm2/Vs when this transistor was inserted in their BiHEMT structure (see Figure 4 for details of the device architecture).
Figure 3. The rapid feedback provided by the Lakeshore 7612 multi-field Hall system makes it an effective tool for process control. Engineers at RFMD used measurements from this instrument to adjust the temperature of the silicon cell, and bring the pinch-off voltage back into the center of the spec. Note the following: the pinch-off voltage and channel sheet resistance values have been normalized; in both cases the data presented is a five-wafer rolling average; and the sign of the pinch-off voltage has been flipped during the normalization process
Multi-field Hall measurements can overcome these issues. The paper presented by Yanka and his co-workers details measurements made on 6-inch wafers produced in a series of runs using magnetic field strengths from 3kG to 15kG and spring-loaded probe pins tinned with indium, which can be “blasted” with a voltage pulse to form an ohmic contact (see Figure 2).
RFMD’s engineers use the Lakeshore tool for qualification of MBE reactors following a maintenance cycle. Historical calibrations get the MBE system close to target using multi-field Hall characterization, before a series of three wafers with high, low and nominal doping are then sent for processing to provide the data necessary to finalize the target sheet charge. Growth of product follows, with electrical channel characteristics monitored via Hall measurements on three or four wafers per day.
By monitoring the channel sheet charge density and the pinch-off voltage of processed devices, it is possible to fine-tune the temperature of the silicon effusion cell and produce wafers well within spec (figure 3). Thanks to this feedback, RFMD greatly reduced the number of off-target wafers dispatched for processing.
To expose the cause of this drop in mobility, engineers used a variety of measurements to compare the pHEMT in the BiHEMT structure with a standalone device. Photoluminescence (PL) measurements produced a peak with the same energy in both structures, indicating that the channel in both pHEMTs had an identical thickness and indium concentration, a conclusion subsequently confirmed by X-ray diffraction measurements.
The PL intensity in the BiHEMT structure was weaker than that from the standalone pHEMT, indicating either an increase in the density of non-radiative recombination centers, or a fall in carrier concentration in this trench. Hall measurements revealed which of these two possible scenarios was to blame: The channel carrier concentration was unchanged, implying that the fall in mobility was probably caused by an increase in defects or impurities in the channel.
Figure 4. Engineers at Hitachi Cable studied a BiHEMT structure with silicon-doped layers
July 2010
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