Formation of Structural Defects in AlGaN/GaN High Electron Mobility Transistors under Electrical StressTransistors under Electrical Stress
Prashanth Makaram1, Jungwoo Joh2, Carl V. Thompson1
Jesús A. del Alamo2 and Tomas Palacios21Material Processing Center 2Microsystems Technology Laboratoriesg y gy
Massachusetts Institute of Technology, Cambridge, MA, USA
( )Acknowledgements: ARL (DARPA WBGS program)ONR (DRIFT‐MURI)TriQuint Semiconductor
IntroductionIntroduction
• GaN HEMT Reliability: big concernGaN 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 mechanism0
RF stress -0.4-0.2
0
(dB
)
10 GHz,VD=28 VIDQ=150 mA/mmPin=23 dBmP =33 7 dBm
-0.8-0.6
∆Pou
t
2
Pout=33.7 dBm -10 5 10 15
Time (hr)
High Voltage Degradation in GaN HEMTsHEMTs
V 1 E 00
1.E+01
1 15
1.2OFF-state, VGS=-10 V
GS D
AlGaN
VGSVDS
1.E-02
1.E-01
1.E+00
1.05
1.1
1.15
A/m
m)
0), R
/R(0
)
RS
RD
=‐10 V
2DEG
1 E 05
1.E-04
1.E-03
0 9
0.95
1
|I Gof
f| (A
I Dm
ax/I D
max
(0IDmax
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 itMaterial Degradation around VcritVDG=0 V VDG=16 V~Vcrit
50
30
40
50
egrada
tion
(%)
10
20
erman
ent I
DmaxDe
Joh ROCS 2010VDG=25 V VDG=37 V 0
0 2 4 6 8
Pe
Pit depth (nm)
Joh, ROCS 2010
Good correlation between pit depth and electrical degradation
4Initial dimple followed by deeper pitting and cracking.
degradation
Plan View ApproachPlan View Approach
• Limitation of TEM: costly extremely localLimitation 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 minutes80 C for 20 minutes
• Surface cleaning: piranha solution (H2SO4: H2O) for 5 minutes at 115 ˚C
– Plan view imaging through SEM and AFM5
SiN and Gate RemovalSiN and Gate RemovalSource
Gate
DrainG
6Unstressed (high T storage) Stressed (> Vcrit)
ExperimentalExperimental
• OFF‐state step stressOFF 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 imaging7
Electrical DegradationElectrical Degradation
1.E+0112 OFF‐state step stress
1.E+00
8
10
m)dation
(%)
e (%
)OFF state step stress
1.E‐02
1.E‐01
6
8
off| (m
A/m
m
nent Degrad
nt Collapse
IGoffCC
1.E‐032
4 |IGo
DmaxPerm
anCu
rre
IDmaxVcrit
1.E‐040
0 10 20 30 40 50 60
I D
V (V)
FreshCurrent collapse:
1s pulseV 0 V 10V VDGstress (V)
Typical critical behavior beyond 19 V
VDS=0, VGS=‐10V
8
Structural DegradationStructural Degradation
200 nm200 nm200 nmUnstressed VDG=15 V VDG=19 V DG DG(Vcrit)
200 nm200 nmVDG=42 V VDG=57 V
Initial continuous groove formationDeeper pit formation along the groove 9
Pit Cross Section AreaPit Cross Section Area
DrainSource
100
120
140
m2 )0
0.5
nm)
Gate
60
80
100
ge Pit Area (n
1 5
-1
-0.5
ge P
it D
epth
(
0
20
40
Avera
0 0 2 0 4 0 6-2.5
-2
-1.5
Ave
rag
VDGstress=57 V0 10 20 30 40 50 60
Stress Voltage VDGstress (V)
0 0.2 0.4 0.65
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
grad
ation (%
)
2
4
6
tress Cu
rren
t
2
4
6
nent I D
maxDeg
0
2
0 50 100 150Po
st‐St
f ( 2)
0
2
0 50 100 150
Perm
an
f ( 2) Average Defect Area (nm2)Average Defect Area (nm2)
Good correlation between electrical degradation and pit areaGood correlation between electrical degradation and pit area
11
Time EvolutionTime EvolutionOFF‐state stress VDGstress=50 V (>Vcrit), Tbase=150 C
S id f tiSource side groove formationPits 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 minDrain ‐ 40 min.
‐ Room temperatureVGS
Gate GS D
AlGaN
2DEG
Source
GaN
2DEG
Grooves and pits on both sides of the gate13
Degradation MechanismsDegradation Mechanisms
• Consistent observation in TEM and plan‐viewCo 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
SummarySummary
• Developed a simple process for plan‐view e e oped a s p e p ocess o p a eassessment of structural degradation
• Evolution of structural damage:g– Below Vcrit: shallow continuous groove formation at gate edgeAb V l l i f i l h– Above Vcrit: local pit formation along the groove
– Number of pits increases with Vstress and timeNumber of pits increases with Vstress and time and pits merge
• Field induced mass transport is involved in GaN HEMT degradation
15