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(Invited) Electrothermal Performance Optimization of III-Nitride HEMTs Capped with Nanocrystalline Diamond

Monday, 30 May 2016: 10:50
Aqua 310 B (Hilton San Diego Bayfront)
M. J. Tadjer, T. J. Anderson, T. I. Feygelson (U.S. Naval Research Laboratory), K. D. Hobart (Naval Research Laboratory), M. G. Ancona (U.S. Naval Research Laboratory), A. D. Koehler (Naval Research Laboratory), J. K. Hite, V. D. Wheeler, B. B. Pate (U.S. Naval Research Laboratory), F. J. Kub (Naval Research Laboratory), and C. R. Eddy Jr. (U.S. Naval Research Laboratory)
AlGaN/GaN high electron mobility transistors (HEMT) capped with nanocrystalline diamond (NCD) have been demonstrated in the past to outperform electrically and thermally their SiN-passivated counterparts [1, 2]. However, a major process limitation for the integration of a diamond heat spreader has been the O2-plasma damage in the gate opening associated with etching the diamond cap. To reduce plasma damage, Anderson et al. have demonstrated a two-step diamond etch process, where the plasma power is reduced towards the end of the etch to minimize damage to the AlGaN surface [3]. While this process has enabled improved electrical performance of NCD-capped HEMTs, optimal passive heat spreading requires NCD films thicker than 0.5 µm which challenges the two-step etch approach [4]. Further complications such as on-wafer variations in NCD thickness have motivated the search for a more robust gate process.

     Recently, we have proposed the use of a sacrificial gate (SG) prior to diamond growth [5, 6]. The SG stack consisted of a 10 nm thick ALD-grown Al2O3 layer followed by a 100 nm thick PECVD-grown SiNx layer. The SiNx layer served as an etch stop for the NCD layer, and the Al2O3 served as an etch stop for the SiNx layer. The Al2O3/SiNx stack was patterned with the dimensions of the gate and a blanket 500 nm thick CVD NCD film was conformally-grown over the feature. The gate feature was then realigned and a high power O2-plasma was used to clear the NCD film in the gate region. This resulted in a much more reliable gate opening since the window for NCD over-etch was much larger. The sacrificial SiNx finger was subsequently etched away using a SF6-ICP process. The Al2O3 layer was removed in dilute HF solution with no measureable degradation to the rest of the device. Finally, the Ni/Au gate metal stack was lifted off.

     On HEMTs with a SG, the addition of a NCD cap did not cause any significant degradation in mobility, carrier density, or sheet resistance. By contrast, when a SG process was not used to protect the AlGaN surface, the NCD-capped HEMT experienced a decrease in mobility and an increase in sheet resistance. For the SG NCD-capped HEMT, the off-state current increased by less than an order of magnitude and threshold voltage shifted by about 1 V. However, on-state drain current levels remained comparable and degradation in dynamic on resistance (RON,DYN) upon an off-state drain bias stress was improved, indicating potential improvement in AlGaN surface passivation by the NCD film [2, 3]. Such improvement is also corroborated by the improvement in breakdown voltage in NCD-capped HEMTs with a conventional gate opening design.

     This presentation will review progress in simulation, process integration, channel temperature mapping, and electrical characterization of diamond-capped devices. We will also review several novel NCD concepts, such as in-plane control of NCD thermal conductivity, as well as a diamond-gated AlGaN/GaN HEMT [7, 8]. 

[1] M.J. Tadjer, et al., IEEE Electron Dev. Lett., vol. 33, no. 1, pp. 23-25, 2012.

[2] D.J. Meyer, et al., IEEE Electr. Dev. Lett., vol. 35, no. 10, pp. 1013, 2014.

[3] T.J. Anderson et al., 70th Dev. Research Conf. Proc., pp. 155-156, 2012.

[4] A. Wang, M.J. Tadjer, and F. Calle, Semicond. Sci. Technol., vol. 28, pp. 055010 (2013).

[5] T.J. Anderson et al., CS Mantech Conf. Digest, pp. 205-208, 2013.

[6] M.J. Tadjer, et al., Phys. Status Solidi A, 2015, in press.

[7] T.J. Anderson, et al., IEEE Electr. Dev. Lett., vol. 34, no. 11, pp. 1382, 2013.

[8] J. Anaya et al., Acta Materialia 103 (2016) 141-152.