Large-Sized PolySiC Wafers

Ruth DeJule, Associate Editor -- Semiconductor International, 1/1/1999

Though previously thought to be unachievable, polycrystalline silicon carbide (SiC) wafers have demonstrated electrical characteristics comparable to single-crystal SiC. Using a unique version of chemical vapor deposition (CVD) developed by Sullivan & Company (Indianapolis), silicon carbide can be rapidly deposited at atomospheric pressure without porosity or 'micropipes.' The result is that large size wafers measuring up to 150 mm have been grown. It is expected that economies of scale in the material making process will produce prices competitive with volume production silicon wafers up to 400 mm in diameter.

Silicon carbide has demonstrated significant advantages over silicon as a wafer material. Researchers at Purdue University in Layfayette, Ind., for example, have used SiC to form a novel accumulation-channel MOSFET (ACCUFET) having an off-mode blocking voltage (V_B) of 1400 V and an on-mode resistance (R_ON) of 15.7 mV/cm². For this SiC device, the figure-of-merit, V_B²/R_ON is 125 MW/cm², 25X higher than the theoretical limit for silicon power MOSFETs.

Silicon carbide is typically classified according to crystal orientation, single crystal alpha-phase (4H and 6H) and beta-phase (3C), which can be either single or polycrystalline. The material grown at Sullivan is 3C polycrystalline SiC. The wide bandgap (2.4 eV compared to 1.12 eV for silicon) provides high thermal stability and can be used at 600°C while maintaining semiconductivity. Another characteristic of the 3C material is a high breakdown electric field of 2 x 10⁶ V/cm at 1000 V compared to 2.5 x 10⁵ V/cm for silicon, an 8X difference. The saturated electron drift velocity of 3C SiC, high compared to silicon, can reduce potential electrical interference and enable device operation at high frequencies. For example, in a 200 kV/cm electric field, the saturated electron drift velocity of SiC is ~1.8 x 10⁵ m/sec compared to ~1.0 x 10⁵ m/sec for silicon. At 300 kV/cm, SiC is in a class of its own with ~1.9 x 10⁵ m/sec drift velocity.

The Johnson's Figure of Merit for power applications rates SiC at 2533 compared to nine for silicon, indicating that SiC is potentially 281X better than silicon for high-frequency, high-power wafer applications. Similarly, the Keye's Figure of Merit for device density gives silicon carbide a 90.3 figure compared to 13.8 for silicon, indicating that silicon carbide is potentially 6.5X better for integrated circuits. Electronic devices formed on silicon carbide wafers can be used at higher temperatures, at higher power levels and under more radiation intense environments than those on silicon wafers. Die sizes for typical devices can be reduced as much as 25X.

Currently only single crystal SiC wafers are produced with either 4H/6H material or with epitaxially grown 3C. Despite potentially higher mobilities from single crystal 4H SiC, production devices are limited to small regions free of holes and poor surface morphology. Epitaxial 3C on 4H or 6H SiC substrates offer the greatest foundry capability and economic advantage; however, lattice mismatch and thermal expansion coefficient issues still pose problems. PolySiC Datanite wafers developed at Sullivan offer one solution. These wafers are oriented so that only a single crystal orientation is exposed on the wafer face. The effect is a polycrystalline material that acts like a single crystal.

PolySiC, unlike other semiconductor materials, has the same carrier mobility as single-crystal, 370 cm²/Vsec at 7 x 10¹³/cc doping. The reason may be that SiC is a covalently bonded material with high surface energy due to the availability of d orbitals in the valence shell and the distance of valence electrons from the nucleus. Valence electrons being weakly held by the Si nucleus are available when a field is applied. A close bonding of individual crystals in the polycrystalline silicon carbide structure also facilitates electron mobility. With these advantages and the potential for larger wafers, silicon carbide is anticipated to make inroads into the semiconductor industry.