High Spatial Resolution Thermal Conductivity investigation of SiC Wafers

Abstract

ABSTRACTSilicon carbide (SiC) is a material with very attractive properties for high power/high temperature electronic devices. Its mechanical strength, high thermal conductivity ( κ), large bandgap, and extreme chemical inertness are a few of the characteristics making SiC interesting for semiconductor electronics. Due to the significant head load generated over large areas in high power devices, it is desirable forthe thermal properties of the substrate to be uniform and optimal. Scanning thermal microscopy (SThM), which provides nondestructive, absolute measurements of the thermal conductivity with a spatial/depth resolution in the 2-3 μm range, was used to examine the room temperature κ as a function of position of four 2” diameter SiC wafers. Wafers of 4H and 6H polytype were fabricated with carrier concentrations in the(1-3)×1018 cm−3 and (6-9)×1017 cm−3 ranges, respectively. A radial distribution of the thermal conductivity was determined for all the investigated samples. For a radius r < r1 (r1 ∼ 0.3”) and r > r2 (r2 ∼ 0.7”) highest thermal conductivity values were measured in the range of (3.8-3.9) W/cm-K, comparable to the highest κ reported for this material [D. Morelli et al., Inst. Phys. Conf. Ser. 137, 313 (1993); E.A. Burgemeister,.et al., J. Appl. Phys. 50, 5790 (1979)]. For r1 < r < r2 the thermal conductivity drops to about (2.85-3.25) W/cm-κinterval. Atomic force microscopy (AFM) investigation revealsthat the influence of surface roughness effects on κ is negligible. The κ dip may arise from a higher basal plane defect density in this region that could be associated with the presence of super screw dislocations, or “micropipes” [M. Dudley et al., J. Phys. D: Appl. Phys. 28, A63 (1995)]. The implications of these findings for device applications and design are considered.