SiC high voltage device development 2006. 11. 30 KERI Power Semiconductor Group
outline 1. Device design & simulation for power devices 2. SiC power diode process development Ion implantation & activation optimization SiC PiN diode SiC JBS diode Process optimization & results 3. Oxide research for SiC power MOSFETs 4. SiC epitaxy
1200V device Simulation Guard Ring effect BV dependence on number of FLRs
Ion implantation condition sample I/I condition [scheme] Annealing [ o C] JJ6 JBS=Al19, FLR=Al17 1600 JJ7 JBS=Al19, FLR=Al17 1700 JA6 JBS=Al19, FLR=Al17 1600 JA7 JBS=Al19, FLR=Al17 1700 JB6 JBS=Al19, FLR=B17 1600 JB7 JBS=Al19, FLR=B17 1700 JBS : Al ion, surface doping concentration(implanted) 5 10 19 cm -3, junction doping concentration 5 10 18 cm -3 ( Al19) FLR : Al or B ion, surface doping concentration 5 10 17 cm -3, junction doping concentration 5 10 16 cm -3 (Al17, B17)
Ion implantation mask design Side implantation when the SiO 2 mask is removed by wet etching SiC I/I mask (SiO 2 /Ti/Au) SiO 2 mask Bad aspect ratio Side implantation effect Trench formation Au mask Peeling-up problem Additional SiO 2 layer : suppress reaction between Ti & SiC Protect side implantation Suppress trench formation Suppress Au Peeling-up Stable at high temp.
High-temperature activation of Ion Implanted SiC SiC dummy wafer SiC sample Argon atmosphere graphite Al,B: 1500-1750 N : 1100-1300 (Argon pressure ~600 torr)
Current Density [A/cm 2 ] 3.5x10 1 1600 o C 3.0x10 1 1700 o C 2.5x10 1 2.0x10 1 1.5x10 1 1.0x10 1 5.0x10 0 Activation annealing condition dependence Current[A] 4.0x10-4 3.0x10-4 2.0x10-4 1.0x10-4 1600 o C 1700 o C 0.0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Forward Voltage[V] 0 200 400 600 800 1000 1200 1400 Reverse Voltage[V] Device annealed at 1700 Too long diffusion length decrease of schottky area R on increase Device annealed at 1600 is not fully activated Failure at metal layer not at pn junction
0.6um High R on due to diffusion? 2.6um 3.4um FESEM image of devices after 1700oC annealing Width of JBS increased by 0.4 um decreasing schottky area Theoretical analysis of R on with schottky area variation Lateral diffusion of implanted ions reduces the schottky area increase of R on
Interface layer formation during annealing interface layer changes the metal/sic contact property from schottky to ohmic harsh environment produce more interface layer the interface layer could be conductive
Interface layer formation Good adhesion Extreme case the interface layer deteriorate the metal/sic adhesion
Interface layer formation mechanism and its proposed solution High temperature annealing Removal of interface layer by dry etching C-rich layer was formed due to preferred sublimation of Si-species during high temperature annealing Non-stoichiometric interface layer provide leakage current path Using ICP-RIE with low damage condition, stable SiC devices were produced without interface layer
Surface treatment for removing an interface layer ICP-RIE system for SiC etching (Oxford ICP65 system) 1. SF 6 /O 2 or CF 4 /O 2 2. 300W RF power generator, 600W ICP power generator Etch condition for removing interface layer Metal mask (Ni, Al) Selectivity > 10:1 (metal mask: SiC) Etch rate ~100nm/min Surface roughness: 1.6nm (RMS)
Interface layer analysis: reverse characteristics Surface treatment using ICP removed the interface layer formed during high temperature, leading to lower leakage current and higher breakdown voltage
Current[A] 4.0x10-4 3.0x10-4 2.0x10-4 1.0x10-4 0.0 Breakdown voltage of JBS diode space 3um space 9um BV decrease with schottky area increase higher trap density lower schottky barrier -1.0x10-4 0 200 400 600 800 1000 1200 1400 Reverse Voltage[V] BV with variation of space between JBS lines
Measurement set-up for measuring the pico-ampere-level leakage current Reverse Leakage Current [A] 0.0-2.0x10-6 -4.0x10-6 -6.0x10-6 -8.0x10-6 -1.0x10-5 -1200-1000 -800-600 -400-200 0 Reverse Bias Voltage [V] -1200-1000 -800-600 -400-200 0 Reverse Leakage Current [A] 10-6 10-7 10-8 10-9 10-10 Si FRD SiC Cree(600V) SiC Cree(1200V) Reverse Bias Voltage [V] Si FRD SiC Cree(600V) SiC Cree(1200V) Measured Reverse leakage characteristics of commercially available SiC SBDs and Si FRD
Measurement set-up for measuring the pico-ampere-level leakage current Measured reverse recovery characteristics of commercially available SiC SBD and Si Diodes < 20ns <15ns (a) Si FRD (b) SiC SBD (600V) > 640ns (c) Si Diode time scale: 40ns/div for (a) and (b), 80ns/div for (c) channel 3 (purple): current flowing through DUT in 1A/V
Thermal analysis of packaged device power devices
Distribution of Interface-Trap Density power devices
Comparison of critical Electric field Nitrided thermal oxide showed ideal critical electric field
Dependence of device proces parameter (RTA process, D it ) The interface properties of thermally grown nitrided oxide was not deteriorated ded after device process
Initial growth (on-axis substrate) 1600 o C, 3min deposition Optical microscopy reflective 50μm Crossed-polar transmission circular-shaped etch pits were formed in the vicinity of micropipes. Some of them shows spiral structure which is thermodynamic equilibrium shape, especially in the growth mode.
H 2 etching (on-axis substrate) 1600 o C, 3min Optical microscopy reflective 20μm Crossed-polar transmission The region near micropipes was etched with non-circular shape. There is another driving force for etching the SiC surface.
Typical etch pit morphology No step bunching Well developed step bunching Bright region is low doped grown SiC epitaxial film and the dark region is high n-doped substrate. It is clear that the region near micropipes was etched not deposited. Two types of etched micropipe were found. the reason was not cleared, yet.
Shapes of micropipe etch pits 30μm Etch pit revealed by KOH etching 220nm 160nm Etch pit revealed at initial growth stage The etch pits formed during initial growing stage showed bellybutton-shape, however the pits formed by KOH etching shows hexagon. The size of the micropipe is small(~200nm in diameter) with respect to usual one (~ several mm in diameter).
Spiral shape in the vicinity of micropipes (a) (b) Spirals (a) etched surface and (b) grown surface. The spiral on the grown surface shows more clear steps, i. e., higher step heights
Nearly on-axis substrates Nearly on-axis substrates provide wide enough terraces for spiral formation. Circular-shaped sprial etch pits formed in the vicinity of micropipe
Off-axis substrates The surface steps, which is formed parallel each other, break the continuous step of spiral. Consequently, the spiral is hard to exist on the off-axis substrates
Growth rate & thickness uniformity Epi-layer substrate 1660 o C, 11min SiH 4 : C 3 H 8 = 15:5 1.6um (9um/hr) 1600 o C, 30min SiH 4 : C 3 H 8 = 12:3 3.2um (6.5um/hr) - high speed growth rate achieved (G max = 1um/min) - utilized up to 1inch substrate - adaptation of 2 inch substrate is remained project
Summary 1. design optimization 2. 2kV PiN, 1.2kV JBS diode was achived 1. Further research : development of devices for higher current capability 3. Obtaining high quality thermal oxide was achieved which can be utilized to SiC power MOSFETs 1. Further research : development of high speed switching devices 4. Home made SiC epitaxy layer will be used for power devices