Silicon Nitride Substrates for Power Electronics

Ulrich Voeller, Bernd Lehmeier

2

Table of content Si3N4

Material characteristics1

2

3

4

Technology - comparison AMB/DBC

Interfacial structure and chemistry

Performance- metallized substrate

5 Summarized

3

Mechanical characteristics

Al2O3 96% AlN ZTA (9%)(HPS9%)

Si3N4

Thermal conductivity[W/mK] 24 180 28 90

Bending strength[MPa] 450 450 700 650

Fracture toughness[MPa / ] 3,8-4,2 3-3,4 4,5-5 6,5-7m

Advantage:

high reliability (thermo shock resistance) besides good thermal performance

4

Electrical characteristics

Ceramic material

Thickness [mm]

ε r (50Hz)tan δ

(50Hz)*10-3

Breakdown voltage

[kV]

Normed Breakdown voltage [kV/mm]

Partial discharge

E=14kV/mm

Si3N4 0,32 7,62 2,26 14,85 ±4,46% 46,4 <5pC

ZTA 9% (HPS 9%) 0,32 10,32 1,67 11,86±

3,27% 37,1 <5pC

AlN 0,63 8,7 1,76 21,16 ±2,82% 33,6 <5pC

Average values of N=10 Partial discharge free

5

Si3N4 Technology

Si3N4Copper

Coating

Tempering

Laser, Plating (Ni,Au/ Au/ Ag )

Copper Brazing-solder

Screen printing

Brazing process

Masking, Copper etching

2nd Etching

Si3N4

Active Metal Brazing Direct Bond Copper

DBC process

Oxidation

Masking, Copper etching

Laser, Plating (Ni,Au/ Au/ Ag )

6

Interfacial structure Si3N4 DBC

Cross section: Cu/Si3N4-100x:Sample preparation:Ion beam etching„Cross Section Polishing“

Analysis of cross section:

HR (high resolution)-SEM

Cu Si3N4

7

Interfacial structure Si3N4 DBC

Cross section: Cu/oxidation-layer/Si3N4 -20kx:

Si3N4

Cu

Oxidation layer

• thickness: 1-2µm• chemical adhesion: Copper silicate

Explanation:

Passivation of the Si3N4surface

Activation of the surface forDBC process

8

Interfacial structure Si3N4 AMB

Cross section: Cu/Brazing/Si3N4- 100x / 10kx

Si3N4

Brazing

Copper

chemical adhesion: Intermetallic compound

5000200010010

99

90807060504030

20

10

5

32

1

Cycles

Perc

ent

3,85392 35,16 1,2434,66993 56,94 1,1598,06937 109,99 15,6245,13231 2370,46 1,846

Shape Scale A D*Table of Statistics

AlN: 35 cyclesAl2O3: 55 cyclesHPS9%: 110 cyclesSi3N4 DBC: 2300 cycles

Ceramic material

Reliability [-55-150°C]

Arbitrary Censoring - ML EstimatesWeibull - 95% CI

9

Performance: Reliability

Si3N4 AMB: 5000 cycles

Comparison ceramics:

ΔT=205K (-55-150°C)

d(Cu)=0,3mm; d(Al2O3;HPS9%;Si3N4)

=0,32mm d(AlN)=0,63mm same Layout no Dimple- Design

Best Perfomance: Si3N4 DBC (Cu 0,3):

2300cycles Si3N4 AMB (Cu 0,5):

>5000cycles

10

Failure mode: DBC

Start 75 cycles50 cycles25 cycles 100 cycles

HPS9%:Start 1000 cycles 2000 cycles1500 cycles500 cycles

Si3N4:

gradual delamination of Copper, Si3N4 ceramic is staying undamaged! conchoidal fracture in HPS 9% ceramic

11

comparison – thermal cyclingCombination Copper

Layoutside [mm]Ceramic

[mm]Copper Backside

[mm]Thermal Cycles

[1]

Al2O3 DBC 0.3 0.38 0.3 55HPS 9% (ZTA) DBC 0.3 0.32 0.3 110AlN DBC 0.3 0.63 0.3 35Si3N4 DBC 0.3 0.32 0.3 2300Si3N4 AMB 0.5 0.32 0.5 5000

DBC - Al2O3 cycled substrate after 50 cycles conchoidal fracture

DBC – Si3N4 cycled substrate after 2300 cycles delamination

d(Cu)=0,3mm ; d(Si3N4)=0,32mm N=9 Testing condition:continuous rise of voltage until breakdown is registered

12

Performance: Breakdown strength (DBC)

XVIIIX

12

11

10

9

8

7

6

Lot

Brea

kdow

n st

reng

th [

kV]

9,19,08,9

95% CI for the Mean Breakdown strength [kV]

Breakdown strength:

>6,5 kV@ 0,3mmCu/ 0,32mm Si3N4

13

Performance: Partial discharge free (DBC)Partial discharge free :d(Cu)=0,3mm ;d(Si3N4)=0,32mm N=12 Testing condition:

Si3N4 Lot2Si3N4 Lot1

5

4

3

2

1

0

part

ial d

isch

arge

[pC

]

Boxplot of Si3N4 Lot1; Si3N4 Lot2

Proof voltage: 5kV @ 10sec

Target value: Partial discharge free: <10pC

14

Performance: Rth comparison

6_S i3N

4_DB

C_0,32

mm

5_Si3

N4_AMB_

0,32m

m

4_AlN_

0,63_

mm

3_HPS

9%_0

,32mm

2_Al2

O3_0,3

2mm

1_Al2O

3_0,6

3mm

0,38

0,36

0,34

0,32

0,30

0,28

0,26

0,24

0,22

0,20

Material

Rth

[K/

W]

0,2010,2020,197

0,2710,275

0,359

Rth [K/W]

PelPel = 50 W = const. T1 = 100 ± 0,1 °C = const. Solder: SnAg3,7Cu0,7

(d=0,05mm)

15

Summary

ProcessTechnology

Direct Bonded Copper (DBC) Active Metal Brazing (AMB)

Reliability 2300 cycles >5000 cycles

Materialcombination(Cu/Si3N4/Cu)

0,3/0,32/0,3 0,5/0,25/0,50,5/0,32/0,5

Partial dischargefree

++ ++

Design Rules Standard DBC Design Rules: min spacing: 0,5mm @ 0,3mm Cu

AMB Design Rules: min. spacing: 1mm @ 0,5mm Cu

Cost +20%

16

comparison – summary

The increasing demand for longer life time cycles and higher thermal performance in power modules can be realized with high strength Si3N4 insulating material.

The investigation showed that depending on the metallization method, the reliability of Si3N4 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 Si3N4 ceramic, especially its very high fracture toughness (K1C), contribute to its enhanced reliability.

Furthermore, Si3N4‘s higher strength enables the use of a thinner cross section resulting in a comparable thermal performance to AlN.

Thank you for your attention