Resumen de Wide Bandgap Semiconductors for MEMS and Related Process Technologies
Marcel Placidi
The work here presented was oriented to the study and development of technologies to process novel materials, the wide bandgap (WBG) semiconductors, Silicon Carbide (SiC), and III-nitrides, Aluminum Nitride (AlN) and Gallium Nitride (GaN), in order to further design and fabricate MEMS and related devices. The characterisation of the processed mechanical structures allowed evaluating the quality of the material, mainly grown on Si substrate, providing direct feedback to the growth research groups in order to improve their material growth process. The main advantage of using Si substrates is that it opens the door to large area WBG wafers but also possible integration of Si electronics with WBG devices. A more complete characterisation also allowed comparing the performances of these devices with equivalent devices processed on Silicon materials. The main contributions of the work are the following: 1. Single crystal SiC grown on Si-substrates for SiC resonators was first used in this work. Its outstanding mechanical properties and low stress have been demonstrated. Therefore, this starting material is appropriate to fabricate SiC-based MEMS. The resulting Young modulus was in the range of 395-530 GPa. The resonance frequencies and quality factors of SiC resonators were higher than those equivalents on Si (around 60%). 2. Innovative SiC suspended devices, thicker than 1 µm, widths in the range of 0.8-2.5 µm and lengths up to 500 µm, were successfully fabricated. SiC on semi-Insulated Silicon (SIS) and SiC on SOI (Silicon On Insulator), were also considered as alternative substrates for SiC electrostatic resonators fabrication. The results showed that the bulk isolation is critical to avoid the leakage, in reducing the required voltage to actuate the SiC resonators and for sensing an electrical output signal. Operation of resonant devices in the temperature range of 25-200 ºC was demonstrated. Therefore, this approach makes possible the integration of SiC MEMS together with control electronics either on SiC or on Si, for applications able to work in harsh environment. 3. III-nitrides, and especially AlN, have been considered as an alternative material to SiC for resonator fabrication. The insulator properties of the epitaxied AlN were first investigated, by thermally oxidizing AlN in O2, and compared with sputtered AlN. The epitaxied AlN has been probed to be a good dielectric on Si with a relatively low interface state density. Besides, two different approaches for AlN resonator fabrication have been investigated. The first approach was based on etching the AlN layer using dry chlorine-based plasma, while the second one relies on Si etching techniques. The resulting Young moduli were in the range of 272-286 GPa. Moreover, the high tensile stress of the AlN layer allows increasing the resonance frequency of bridge-based resonators, thus increasing the sensitivity. 4. The different required technological steps for GaN MOSFET or MEMS fabrication were also considered: the surface cleaning, the ohmic contact formation both on implanted p-type GaN (for MOSFETs) and AlGaN (for HEMTs), and the dielectric gate deposition using SiO2... Contact resistivity (¿C) in the range of (1-5)×10-5 ¿.cm2 was obtained on Si-implanted p-type GaN contacts using the Ti/Al metallic scheme, and around 10-3 ¿.cm2 on AlGaN contacts using the Ti/Al/Ti/Pt scheme. Concerning the dielectrics on GaN, an interface state density of 6×1010 cm-2 has been obtained for the silane-based SiO2. Furthemore, GaN MOSFETs with channel mobilities in the range of 10-20 cm2/V.s have been fabricated.
