From epitaxy to backside process: Reproducible AlGaN/GaN HEMT technology for reliable and rugged power devices
W. Bronner, P. Waltereit, S. Müller, M. Dammann, R. Kiefer, P. Dennler, F. van Raay, M. Mußer, R. Quay, M. Mikulla, and O. Ambacher
phone: ++49 761 5159822, fax: +49 761 515971822, e-mail: wolfgang.bronner@iaf.fraunhofer.de
Fraunhofer Institute for Applied Solid State Physics, Tullastrasse 72, 79108 Freiburg, Germany

Abstract:

Group III nitrides have found to be well suited for high temperature and high power applications, due to their wide band gap, high breakdown field, current density and saturated velocity. We report on a powerful technology well-matched to the requirements to a large variety of applications ranking from RF to millimeter wave frequencies for both large power bar high electron mobility transistors (HEMTs) as well as monolithic microwave integrated circuits (MMICs).
AlGaN/GaN heterostructures are grown on semi-insulating SiC substrates by metal organic chemical vapor deposition with average sheet resistance non-uniformity of 1.5%. Up to twelve 3-inch SiC wafers can be loaded into one epi-run batch. The growth procedure is optimized for a highly insulating buffer as well as low trap densities.
Processing is performed in microstrip line technology consisting of frontside processing, substrate thinning down to 100 μm, and backside processing including front-to-back substrate via holes. Standard gate lengths of our HEMTs are 0.25 μm and 0.50 μm, depending on the specific application, in combination with thin film resistors, high-voltage capacitors and inductors for impedance matching to a 50 Ω environment. Standard processing techniques are used involving electron-beam and optical lithography: stepper alignment for frontside and contact mask alignment for backside device definition. The whole technology sequence is subject of a continuous improvement process with the main focus on reduction of leakage currents and low frequency dispersion, increase of breakdown voltages and simultaneously improving the overall RF performance and reliability. This is mainly achieved by a combination of epitaxial growth optimization as well as modifications in the passivation nitride and the gate module. For the backside process, the thermal management has been a subject of development to avoid a performance degradation of the devices. The whole process technology exhibits good uniformity across a single wafer as well as high reproducibility from wafer to wafer.
Power HEMTs have both high PAE (power added efficiency) and low leakage currents, thus demonstrating that we have successfully achieved high isolation and low trap densities. Continuous wave loadpull power sweep wafer mapping of a 0.8 mm gate width structure without intentional harmonic matching across all 21 cells on an entire 3-inch wafer (Figure 1) yields a PAE of (68±2)% per single wafer measured at 50 V. The high level of reproducibility is demonstrated in a comparison of these values on a total of 22 engineering batches, where more than 100 wafers are involved (Figure 2).
Dynamic evaluation circuits (DECs) are used to monitor the high frequency parameters of our HEMTs. As an example, figure 3 shows the frequency dependent small signal properties of a DEC operating at 28 V, which has been fabricated in 0.25 μm gate length technology with a total gate width of 600 μm (8 x 75 μm). The measurement shows a good performance over the bandwidth from 16 GHz to 20 GHz and indicates an excellent homogeneity of the active device as well as of the passive matching network across a complete 3-inch wafer.
The reliability of our MMIC technology has been investigated using another DEC monitor with 0.25 μm gate length and a total gate width of 1 mm (8 x 125 μm) which has been measured in a RF package at a frequency of 10 GHz. The reliability is very promising. Figure 4 is showing an Arrhenius plot of the DEC measured at 42 V in 2 dB compression. The failure criterion has been -1 dB degradation of the output gain. This leads to an activation energy of Ea = 1.7 eV. The extrapolated life time is calculated to 5x105 h at a channel temperature of 200°C.
Keywords: GaN, AlGaN, HEMT, MMIC, reliability, reproducibility

Paper 4.3.pdf