Formation of Structural Defects in AlGaN/GaN High Electron Mobility - - PowerPoint PPT Presentation

Formation of Structural Defects in AlGaN/GaN High Electron Mobility Transistors under Electrical Stress Transistors under Electrical Stress

Prashanth Makaram1, Jungwoo Joh2, Carl V. Thompson1 Jesús A. del Alamo2 and Tomas Palacios2

1Material Processing Center 2Microsystems Technology Laboratories

g y gy Massachusetts Institute of Technology, Cambridge, MA, USA

( ) Acknowledgements: ARL (DARPA WBGS program) ONR (DRIFT‐MURI) TriQuint Semiconductor

Introduction Introduction

• GaN HEMT Reliability: big concern

GaN HEMT Reliability: big concern

– RF power degradation I decrease R increase I increase V change – ID decrease, RD increase, IG increase, VT change…

• Goal: understand degradation mechanism

RF stress

• 0.4
• 0.2

(dB)

10 GHz,VD=28 V IDQ=150 mA/mm Pin=23 dBm P =33 7 dBm

• 0.8
• 0.6

∆Pout

2

Pout=33.7 dBm

• 1

5 10 15 Time (hr)

High Voltage Degradation in GaN HEMTs HEMTs

V

1 E 00 1.E+01 1 15 1.2

OFF-state, VGS=-10 V G S D AlGaN

VGS VDS

1.E-02 1.E-01 1.E+00 1.05 1.1 1.15 A/mm) 0), R/R(0)

RS RD

=‐10 V

2DEG

1 E 05 1.E-04 1.E-03 0 9 0.95 1 |IGoff| (A IDmax/IDmax(0

IDmax IG ff

Joh, EDL 2008

GaN

1.E-06 1.E-05 0.85 0.9 10 20 30 40 50 I V (V)

IGoff Vcrit

ID, RD, and IG start to degrade beyond critical voltage (Vcrit)

IDmax: VDS=5 V, VGS=2 V IGoff: VDS=0.1 V, VGS=‐5 V

VDGstress (V)

3

D D G crit

(+ trapping behavior – current collapse) Common physical origin in ID and IG degradation

Material Degradation around V it Material Degradation around Vcrit

VDG=0 V VDG=16 V~Vcrit

50 30 40 50 egradation (%) 10 20 ermanent IDmax De

Joh ROCS 2010

VDG=25 V VDG=37 V

2 4 6 8 Pe Pit depth (nm)

Joh, ROCS 2010

Good correlation between pit depth and electrical degradation

4

Initial dimple followed by deeper pitting and cracking. degradation

Plan View Approach Plan View Approach

• Limitation of TEM: costly extremely local

Limitation of TEM: costly, extremely local hi k

• This work:

– Removal of SiN passivation and gate

• SiN passivation: HF etch (1:10 HF: H2O)
• Contact and gate metals: aqua regia (3:1 HCl: HNO3) at

80 ˚C for 20 minutes 80 C for 20 minutes

• Surface cleaning: piranha solution (H2SO4: H2O) for 5

minutes at 115 ˚C

– Plan view imaging through SEM and AFM

5

SiN and Gate Removal SiN and Gate Removal

Source Gate Drain G

6

Unstressed (high T storage) Stressed (> Vcrit)

Experimental Experimental

• OFF‐state step stress

OFF state step stress

– VGS=‐7 V V stepped from 5 to 8 12 20 35 50 V (1V/min) – VDS stepped from 5 to 8, 12, 20, 35, 50 V (1V/min) – Tbase=150C

D t il d d i h t i ti

• Detailed device characterization:

– DC device parameters: IDmax, RS, RD, VT… – Trap characterization: current collapse

• Removal of passivation and gate metal
• SEM and AFM plan view imaging

7

Electrical Degradation Electrical Degradation

1.E+01 12

OFF‐state step stress

1.E+00 8 10 m) dation (%) e (%)

OFF state step stress

1.E‐02 1.E‐01 6 8

• ff| (mA/mm

nent Degrad nt Collapse IGoff CC 1.E‐03 2 4 |IGo

Dmax Perman

Curre IDmax

Vcrit

1.E‐04 10 20 30 40 50 60 ID V (V)

Fresh

Current collapse: 1s pulse V 0 V 10V

VDGstress (V)

Typical critical behavior beyond 19 V

VDS=0, VGS=‐10V

8

Structural Degradation Structural Degradation

200 nm 200 nm 200 nm

Unstressed VDG=15 V VDG=19 V

DG DG

(Vcrit)

200 nm 200 nm

VDG=42 V VDG=57 V

Initial continuous groove formation Deeper pit formation along the groove

9

Pit Cross Section Area Pit Cross Section Area

Drain Source

100 120 140 m2) 0.5 nm)

Gate

60 80 100 ge Pit Area (n 1 5

• 1
• 0.5

ge Pit Depth ( 20 40 Avera 0 2 0 4 0 6

• 2.5
• 2
• 1.5

Averag

VDGstress=57 V

10 20 30 40 50 60 Stress Voltage VDGstress (V) 0.2 0.4 0.6 5 x (m)

Drain side pit area also shows critical behavior.

10

Correlation between Electrical and l d

12

)

12

)

Structural Degradation

8 10 12

t Collapse (%)

8 10 12

gradation (%)

2 4 6

tress Current

2 4 6

nent IDmax Deg

2 50 100 150

Post‐St f (

2)

2 50 100 150

Perman f (

2)

Average Defect Area (nm2) Average Defect Area (nm2)

Good correlation between electrical degradation and pit area Good correlation between electrical degradation and pit area

11

Time Evolution Time Evolution

OFF‐state stress VDGstress=50 V (>Vcrit), Tbase=150 C

S id f ti Source side groove formation Pits grow in density and merge with each other.

12

Symmetric Stress (VDS=0) Symmetric Stress (VDS 0)

Stress conditions: Stress conditions: ‐ VDS=0, VGS=‐50 V (stressed on both sides) 40 min

Drain

‐ 40 min. ‐ Room temperature

VGS

Gate

G S D AlGaN 2DEG

Source

GaN 2DEG

Grooves and pits on both sides of the gate

13

Degradation Mechanisms Degradation Mechanisms

• Consistent observation in TEM and plan‐view

Co s ste t obse at o a d p a e

– Grooves and pits are not by‐product of etching

• Groove formation

– Field induced oxidation? – Electrochemical etching?

• Pit formation

– Degradation is E‐field driven (Little current is needed) – Field/stress induced diffusion of material away from gate?

• In any event mass transport is involved
• In any event, mass transport is involved.

14

Summary Summary

• Developed a simple process for plan‐view

e e oped a s p e p ocess o p a e assessment of structural degradation

• Evolution of structural damage:

g

– Below Vcrit: shallow continuous groove formation at gate edge Ab V l l i f i l h – Above Vcrit: local pit formation along the groove – Number of pits increases with Vstress and time Number of pits increases with Vstress and time and pits merge

• Field induced mass transport is involved

in GaN HEMT degradation

15