Silicon Nitride Substrates for Power Electronics Ulrich Voeller, Bernd Lehmeier
Table of content Si 3 N 4 1 2 3 4 Material characteristics Technology - comparison AMB/DBC Interfacial structure and chemistry Performance- metallized substrate 5 Summarized 2
Mechanical characteristics Thermal conductivity [W/mK] Bending strength [MPa] Fracture toughness [MPa / m] Al 2 O 3 96% AlN ZTA (9%) (HPS9%) Si 3 N 4 24 180 28 90 450 450 700 650 3,8-4,2 3-3,4 4,5-5 6,5-7 Advantage: high reliability (thermo shock resistance) besides good thermal performance 3
Electrical characteristics Ceramic material Thickness [mm] ε r (50Hz) tan δ (50Hz) *10-3 Breakdow n voltage [kv] Normed Breakdow n voltage [kv/mm] Partial discharge E=14kV/mm Si 3 N 4 0,32 7,62 2,26 ZTA 9% (HPS 9%) 0,32 10,32 1,67 AlN 0,63 8,7 1,76 14,85 ± 4,46% 11,86± 3,27% 21,16 ± 2,82% 46,4 <5pC 37,1 <5pC 33,6 <5pC Average values of N=10 Partial discharge free 4
Si 3 N 4 Technology Active Metal Brazing Direct Bond Copper Copper Brazing-solder Si 3 N 4 Copper Si 3 N 4 Screen printing Oxidation Coating Tempering Brazing process DBC process Masking, Copper etching Masking, Copper etching 2nd Etching Laser, Plating (Ni,Au/ Au/ Ag ) Laser, Plating (Ni,Au/ Au/ Ag ) 5
Interfacial structure Si 3 N 4 DBC Cross section: Cu/Si 3 N 4-100x: Sample preparation: Ion beam etching Cross Section Polishing Cu Si 3 N 4 Analysis of cross section: HR (high resolution)-sem 6
Interfacial structure Si 3 N 4 DBC Cross section: Cu/oxidation-layer/Si 3 N 4-20kx: Explanation: Oxidation layer Si 3 N 4 Passivation of the Si 3 N 4 surface Activation of the surface for DBC process Cu thickness: 1-2µm chemical adhesion: Copper silicate 7
Interfacial structure Si 3 N 4 AMB Cross section: Cu/Brazing/Si 3 N 4-100x / 10kx Si 3 N 4 Brazing Copper chemical adhesion: Intermetallic compound 8
Performance: Reliability Reliability [-55-150 C] Weibull - 95% CI Arbitrary Censoring - ML Estimates Comparison ceramics: ΔT=205K (-55-150 C) Percent 99 90 80 70 60 50 40 30 20 10 5 3 2 1 10 100 Cycles 2000 5000 Ceramic material AlN: 35 cycles Al2O3: 55 cycles HPS9%: 110 cycles Si3N4 DBC: 2300 cycles Si3N4 AMB: 5000 cycles Table of Statistics Shape Scale AD* 3,85392 35,16 1,243 4,66993 56,94 1,159 8,06937 109,99 15,624 5,13231 2370,46 1,846 d(cu)=0,3mm; d(al 2 O 3 ;HPS9%;Si 3 N 4 ) =0,32mm d(aln)=0,63mm same Layout no Dimple- Design Best Perfomance: Si 3 N 4 DBC (Cu 0,3): 2300cycles Si 3 N 4 AMB (Cu 0,5): >5000cycles 9
Failure mode: DBC Si 3 N 4 : Start 500 cycles 1000 cycles 1500 cycles 2000 cycles HPS9%: Start 25 cycles 50 cycles 75 cycles 100 cycles gradual delamination of Copper, Si 3 N 4 ceramic is staying undamaged! conchoidal fracture in HPS 9% ceramic 10
comparison thermal cycling Combination Copper Layoutside [mm] Ceramic [mm] Copper Backside [mm] Thermal Cycles [1] Al 2 O 3 DBC 0.3 0.38 0.3 55 HPS 9% (ZTA) DBC 0.3 0.32 0.3 110 AlN DBC 0.3 0.63 0.3 35 Si 3 N 4 DBC 0.3 0.32 0.3 2300 Si 3 N 4 AMB 0.5 0.32 0.5 5000 DBC - Al 2 O 3 cycled substrate after 50 cycles conchoidal fracture DBC Si 3 N 4 cycled substrate after 2300 cycles delamination 11
Performance: Breakdown strength (DBC) d(cu)=0,3mm ; d(si 3 N 4 )=0,32mm N=9 Testing condition: continuous rise of voltage until breakdown is registered Breakdown strength [kv] 95% CI for the Mean 12 Breakdown strength [kv] 11 10 9 8 7 8,9 9,0 9,1 Breakdown strength: >6,5 kv @ 0,3mmCu/ 0,32mm Si 3 N 4 6 IX VII Lot X 12
Performance: Partial discharge free (DBC) Partial discharge free : d(cu)=0,3mm ;d(si 3 N 4 )=0,32mm N=12 Testing condition: 5 Boxplot of Si3N4 Lot1; Si3N4 Lot2 4 partial discharge [pc] 3 2 1 0 Si3N4 Lot1 Si3N4 Lot2 Proof voltage: 5kV @ 10sec Target value: Partial discharge free: <10pC 13
Performance: R th comparison Rth [K/W] 0,38 0,36 0,359 0,34 Rth [K/W] 0,32 0,30 0,28 0,26 0,24 0,275 0,271 0,22 0,20 0,197 0,202 0,201 1_Al2O3_0,63mm P el 2_Al2O3_0,32mm 3_HPS 9%_0,32mm Material 4_AlN_0,63_mm 5_Si3N4_AMB_0,32mm 6_S i3n4_dbc_0,32mm P el = 50 W = const. T 1 = 100 ± 0,1 C = const. Solder: SnAg3,7Cu0,7 (d=0,05mm) 14
Summary Process Technology Direct Bonded Copper (DBC) Reliability 2300 cycles >5000 cycles Active Metal Brazing (AMB) Material combination (Cu/Si 3 N 4 /Cu) 0,3/0,32/0,3 0,5/0,25/0,5 0,5/0,32/0,5 Partial discharge free Design Rules ++ ++ Standard DBC Design Rules: min spacing: 0,5mm @ 0,3mm Cu Cost +20% AMB Design Rules: min. spacing: 1mm @ 0,5mm Cu 15
comparison summary The increasing demand for longer life time cycles and higher thermal performance in power modules can be realized with high strength Si 3 N 4 insulating material. The investigation showed that depending on the metallization method, the reliability of Si 3 N 4 was better by a factor of 20 using DBC (direct bond copper) technology and by a factor of 50 using AMB (active metal brazing) technology compared to conventional DBC ceramic materials. The higher mechanical properties of Si 3 N 4 ceramic, especially its very high fracture toughness (K 1C ), contribute to its enhanced reliability. Furthermore, Si 3 N 4 s higher strength enables the use of a thinner cross section resulting in a comparable thermal performance to AlN. 16
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