碳化硅热氧化

Thermal oxidation of silicon carbide (SiC) surface is based on the formation of either an active oxide (SiO,layer (Gate) for MOSFET fabrication or a passive (protective) oxide film for passivation and edge terminationThe thermal oxidation kinetics and oxide-SiC interface are poorly understood in comparison to that of siliconFurthermore, the quality of the Si0-SiC interface is also inferior to that of Si0,/Si interface. Different thermaloxidation techniques are proposed to increase the efticiency as well as oxide and interface quality formanufacturing reliable power devices such as diodes and MOSFETs.This paper deals with the recent developments in SiC technology and its thermal oxidation techniques aiminga better understanding of the electrical behavior of SiC power devices and seeking for more preferment devices.

Introduction

Silicon carbide (SiC) is an IV-IV compound thathas distinguished characteristics and features ascompared to other semiconductor compounds. Iihas excellent physical and electrical properties thatallow fabrication of devices operating at significanthigher temperature as compared to devicesproduced on silicon or GaAs. Silicon carbide hasseveral advantageous properties such as chemicalstability, higher thermal conductivity, as well ascoefficient of thermal expansion and highersaturated electron velocity. A larger SiC bandgap(3 eV) leads to low leakage currents and higherbreakdown voltages. Due to these electronic andphysical properties, SiC has been chosen tofabricate high temperature, high frequency andhigh power electronic devices. In addition, it hasbeen recognized as an ideal materialforapplications, where superior attributes such ashardness and stiffness, strength and oxidationresistance at elevated temperatures, as well asresistance to wear out and abrasion are of primaryimportance.There are various SiC polytypeshaving slightly different characteristics. The mainpolytypes are the 4H-SiC and 6H-SiC. The choiceof a polytype relies on the SiC-SiO, interface andcarrier mobility.Fabrication of devices usually requires thickoxide layer growth during processing. Oxidation ofSiC surface can be thermal or chemical depositionIn this work, we are concentrating on thermal oxidation, which is considered to be a breakthroughin SiC-MOS technology. Thermaloxidation isbased on the formation of either an active oxidelayer (Gate) or a passive (protective) SiO2 film.Oxidation can be used to passivate the surface andto reduce the surface polishing damage producedby the etching of the oxide layer. The SiO,/SiCinterface is a critical element of many potentialincluding MOSFETs.charge-coupleddevices,devices (CCD), bipolar transistors (as a passivationlayer), nonvolatile memories, and others.

The electrical quality of the thermally oxidizedSiO_/SiC-MOSinterfacedepends on variousfactors.These are thesurface preparation,oxidation ambience (wetor dry),oxidationtemp erature,substratedopanttypeandconcentration, orientation, substrate polytype, andpost-oxidation annealing conditions. Generally, thegrowth rate of thermal oxidation on SiC, even athigher temperatures, is small compared to siliconThis does not allow adopting the selectiveoxidation, such as the local oxidation of silicon(LOCOS) used for device isolation in integratedcircuits. The SiC oxidation rate depends on the SiCterminal face, ie., oxidation on C-face is fasterthan on Si-face.

It is known [l] that the oxidation kinetics andoxide-SiC interface are poorly understood incomparison to that of silicon The oxidation rate ofSiC is lower than that of Si by a factor greater than10. Furthermore, the quality of the SiO/SiCinterface is also considerably inferior to that ofSiO,/Si interface. This problem causes low channelmobility in SiC-MOSFETs due to the high density of interface states that exist in the band gap nearthe conduction band edge. Refinements [2-4] incleaning and oxidation methods have improved theinterface quality of SiO/SiC, but producingreliable devices will still require additional improvements.

In general, the oxidation procedure should becarried out following certain steps and schemesThe basic procedures are designed and developedby the CREE research. After a slow wafer load ataround 800°C, a pre-oxidation in dry oxygen is firstdone at a temperature of about (800-900°C)Slowly, the ramp temperature is increased to therequired value. It is desirable that SiC wafer surfacefaces the gas inlet. The oxidation should end by adry oxidation followed by argon annealing underthe same temperature. This annealing is meant forevacuation of all oxidation generated gases (mainlyinside the oxide). A post-oxidation process (dry Oflow) is then achieved at 900 C. Then, thetemperature is decreased with the same rate as itwas increased prior to the main oxidation step. Thispost-oxidation process is found to lower theinterface satedensity. As a laststep,thetemperature should be decreased at the same ratebefore the sample is removed. Under the aboveconditions, layers of 1  m thickness are grownafter 4 hours of dry oxidation in 1150 Cenvironment(10].