Temperature effects on electrical resistivity of selected ceramics for high-temperature packaging applications

We report the results of the characterization of the electrical resistivity of selected ceramic materials, namely aluminum nitride (AlN), silicon nitride (Si₃N₄), beryllium oxide (BeO), yttrium oxide (Y₂O₃), and non-ceramic silicon dioxide (SiO₂), which are candidates for packaging silicon carbide (SiC) sensors and electronics that can operate at temperatures beyond 800°C. Samples of these materials were thermally cycled twice to 1200°C in nitrogen while the bulk and surface resistivities were measured in situ. Although the resistivity values of all samples dropped with increasing temperature, BeO maintained the relatively highest bulk and surface resistivity values at 1200°C of 28 and 34 kΩ cm, respectively, while AlN had the lowest values at 6.8 and 0.64 kΩ cm. In terms of viability for packaging SiC sensors for >800°C operation, AlN and Si₃N₄ are the choice candidates due to their lowest mismatches in the coefficient of thermal expansion with SiC. However, the relative resistivity stability of Si₃N₄ over AlN makes it a promising candidate, even with its lower thermal conductivity relative to AlN.     

High-temperature electronics - a role for wide bandgap semiconductors?

The fact that wide bandgap semiconductors are capable of electronic functionality at much higher temperatures than silicon has partially fueled their development, particularly in the case of SiC. It appears unlikely that wide bandgap semiconductor devices will find much use in low-power transistor applications until the ambient temperature exceeds approximately 300/spl deg/C, as commercially available silicon and silicon-on-insulator technologies are already satisfying requirements for digital and analog VLSI in this temperature range. However practical operation of silicon power devices at ambient temperatures above 200/spl deg/C appears problematic, as self-heating at higher power levels results in high internal junction temperatures and leakages. Thus, most electronic subsystems that simultaneously require high-temperature and high-power operation will necessarily be realized using wide bandgap devices, once they become widely available. Technological challenges impeding the realization of beneficial wide bandgap high ambient temperature electronics, including material growth, contacts, and packaging, are briefly discussed.     

Stable Electrical Operation of 6H–SiC JFETs and ICs for Thousands of Hours at 500 $^{circ}hbox{C}$

The fabrication and testing of the first semiconductor transistors and small-scale integrated circuits (ICs) to achieve up to 3000 h of stable electrical operation at 500degC in air ambient is reported. These devices are based on an epitaxial 6H-SiC junction field-effect transistor process that successfully integrated high-temperature ohmic contacts, dielectric passivation, and ceramic packaging. Important device and circuit parameters exhibited less than 10% of change over the course of the 500degC operational testing. These results establish a new technology foundation for realizing durable 500degC ICs for combustion-engine sensing and control, deep-well drilling, and other harsh-environment applications.     

Characterization of highly doped n- and p-type 6H-SiCpiezoresistors

Highly doped (~2×10¹⁹ cm^-3) n- and p-type 6H-SiC strain sensing mesa resistors configured in Wheatstone bridge integrated beam transducers were investigated to characterize the piezoresistive and electrical properties. Longitudinal and transverse gauge factors, temperature dependence of resistance, gauge factor (GF), and bridge output voltage were evaluated. For the n-type net doping level of 2×10¹⁹ cm^-3 the bridge gauge factor was found to be 15 at room temperature and 8 at 250°C. For this doping level, a TCR of -0.24%/°C and -0.74%/°C at 100°C was obtained for the n- and p-type, respectively. At 250°C, the TCR was -0.14%/°C and -0.34%/°C, respectively. In both types, for the given doping level, impurity scattering is implied to be the dominant scattering mechanism. The results from this investigation further strengthen the viability of 6H-SiC as a piezoresistive pressure sensor for high-temperature applications     

Electrical characterization of annealed Ti/TiN/Pt contacts onN-type 6H-SiC epilayer

We report results of the electrical characteristics of in vacuo deposited Ti/TiN/Pt contact metallization on n-type 6H-SiC epilayer as function of impurity concentration in the range of 3.3×10¹⁷ cm^-3 to 1.9×10¹⁹ cm^-3. The as-deposited contacts are rectifying, except for the highly doped sample. Only the lesser doped remains rectifying after samples are annealed at 1000°C between 0.5 and 1 min in argon. Bulk contact resistance ranging from factors of 10^-5 to 10^-4 Ω-cm² and Schottky barrier height in the range of 0.54-0.84 eV are obtained. Adhesion problems associated with metal deposition on pre-processed titanium is not observed, leading to excellent mechanical stability. Auger electron spectroscopy (AES) reveals the out diffusion of Ti-Si and agglomeration of Ti-C species at the epilayer surface. The contact resistance remains appreciably stable after treatment in air at 650°C for 65 h. The drop in SBH and the resulting stable contact resistance is proposed to be associated with the thermal activation of TiC diffusion barrier layer on the 6H-SiC epilayer during annealing