Page 1© Oxford Instruments 2019 CONFIDENTIAL
Mark Dineen, Ph.D.
Oxford Instruments Plasma Technology
Advances in Plasma Processing for
WBG Power Electronics
Page 2© Oxford Instruments 2019 CONFIDENTIAL
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Page 3© Oxford Instruments 2019 CONFIDENTIAL
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Oxford Instruments Plasma Technology
Page 4© Oxford Instruments 2019 CONFIDENTIAL
• OIPT currently provides production process solutions for D-
mode GaN devices
• Key challenges for E-mode GaN HEMTs
• Recess gate etch
• Device passivation/isolation
• OIPT’s process solutions for E-mode GaN devices
• Atomic Layer Etching
• Atomic Layer Deposition
Introduction
Page 5© Oxford Instruments 2019 CONFIDENTIAL
E-mode for Power Supply Applications
Cascode mode
Enhancement mode
• Power supply applications have fail safe
requirements
• Fail safe operation can be achieved using:
• Normally on GaN HEMT in cascode configuration
• Normally off GaN HEMT
• Cascode produces enhanced performance
compared to Si SJ MOSFET
• Cascode performance is limited by Si based
gate driver
• E-mode devices are preferred for simpler
circuitry and better performance at low voltage
and high frequency
Page 6© Oxford Instruments 2019 CONFIDENTIAL
GaN HEMT Manufacturing
Epitaxy
Mesa isolation
PECVD +
ICPRIE
Ohmic
contact
Gate
deposition
Passivation
PECVD / ICP
CVD + ICP RIE
Gate recess
and isolation
ALE + ALD
Passivation
PECVD/ ICP
CVD + ICP RIEBond pads
GaN HEMT
AlGaN
GaN
Gate
Substrate
Source Drain
Buffer
Page 7© Oxford Instruments 2019 CONFIDENTIAL
• Cyclical ALE process
enables precise control and
low damage etching
• Plasma can be on
throughout cycle (or be
switched on/off)
What is Atomic Layer Etching?
Chlorinating dose
gas,e.g. Cl2
Ar etch gas removes
the chlorinated layer
Oxidation is
self-limiting
AlGaN
Chlorinated surface
AlGaN
Chlorinated surface
AlGaN
AlGaN
Chlorinated surface
Cl+ Cl+
Ar Ar Ar
1
2
3
4
Page 8© Oxford Instruments 2019 CONFIDENTIAL
ALE of GaN HEMT
• ALE required to form recessed gate for E-mode device
• Required controlled etch of AlGaN to GaN surface without 2DEG
damage
High mobility
through 2D electron
gas (2DEG) at
AlGaN/GaN
interface
Selective etch by ALE
through AlGaN to form
recessed gate
AlGaN
GaN
Substrate
Source Drain
Buffer
Page 9© Oxford Instruments 2019 CONFIDENTIAL
AlGaN/GaN ALE results: Ar/Cl2
• Etch rate 1.5-3.0 Å/cycle• up to 18 Å/min
• Uniformity <±5% over 200mm
• Added roughness <<1nm
ALE of AlGaN
AlGaN surface roughness 200 cycles:
Before etching (top)
After etching (bottom)AlGaN etching rate per cycle
0
1
2
3
4
5
6
7
0 10 20 30 40
AlG
aN E
PC
(Å/c
ycle
)
DC bias (V)
with Cl2
No Cl2
Etc
h r
ate
per
cycle
(Å
/cycle
)
DC bias (V)
Page 10© Oxford Instruments 2019 CONFIDENTIAL
• Recess etched in GaN HEMT at OIPT
• Device fabrication at University of Glasgow
• Vth shifts from -5 to +0.15V by recess etching
From D-mode to E-mode HEMT
Before recess etch
Normally ON (D-mode)
After recess etch
Normally OFF (E-mode)0
50
100
150
200
250
300
350
400
-6 -4 -2 0 2
Id (
mA
/mm
)
Vg(V)
Page 11© Oxford Instruments 2019 CONFIDENTIAL
PEALD for Production
• Plasma pre-treatment improves GaN surface and device performance
• Remote PEALD offers excellent film properties with low substrate damage
Pace • Fast cycle times enable high throughput with low damage remote PEALD
• Throughput of 10,000 WPM of PEALD 20 nm Al2O3 with low CoO
Performance
Productivity
PEALD:
plasma-
enhanced ALD
Page 12© Oxford Instruments 2019 CONFIDENTIAL
• Plasma pre-treatment required to remove native oxide and
restore GaN surface
• ALD required for deposition of conformal layer to passivate
surface and electrically isolate GaN from gate
ALD for GaN HEMT
AlGaN
GaN
Substrate
Source Drain
Buffer
Plasma surface pre-
treatment to improve
interface
Passivation by ALD for
low leakage
Page 13© Oxford Instruments 2019 CONFIDENTIAL
• OIPT ALD systems used in production: 25 wafer cassettes; 24/7
• Meeting power semiconductor requirements of 20 nm Al2O3 with
throughput of 10,000 WPM
• High-quality PEALD Al2O3 films deposited at low CoO
PEALD Al2O3 for GaN HEMT Passivation
Material Al2O3
Precursor TMA (Al(CH3)3)
Co-reactant O2 plasma
Deposition temperature 50-400 °C
Thickness uniformity (150 mm) < 0.5%
Thickness uniformity (200 mm) < 1.0%
Refractive Index @ 632.8nm >1.63
-10 -5 0 5 10-10
-5
0
5
10
Y (
cm
)
X (cm)
51.7
51.8
51.9
52.0
52.1
52.2
52.3
52.4
52.5
52.6
Thickness
(nm)
Thickness Uniformity
Page 14© Oxford Instruments 2019 CONFIDENTIAL
• Use plasma treatments to form
high-quality nitride interlayer
Nitride interlayer as GaN pre-treatment
Yang et al., IEEE Electron Device Lett. 34, 1497 (2013)
Chen et al., Phys. Status Solidi A (2014) / DOI 10.1002/pssa.201431712
Strong reduction of Ga-O bonds
with plasma pre-treatment
• Small sub-threshold swing: 64 mV/dec
• Low hysteresis: 0.09 V
• Low Dit: 1-6×1012 cm-2 eV-1
Page 15© Oxford Instruments 2019 CONFIDENTIAL
• Epitaxial growth of AlN by PEALD
• ALD Al2O3 gate dielectric
Epitaxial AlN for GaN transistor
Liu et al., IEEE Electron Device Lett. 34, 1106 (2013)
Liu et al., Phys. Status Solidi C 11, 953 (2014)
Epitaxial 4 nm AlN on GaN
• Low subthreshold swing
• ∼85 mV/decade
• Low-field mobility
• ∼27 cm2/(V·s))
Page 16© Oxford Instruments 2019 CONFIDENTIAL
• Global install base of ALD systems for GaN device
processing
• Production customers
• R&D customers
• Key collaboration with University of Glasgow and Kelvin
Nanotechnology for GaN HEMT fabrication and
characterisation
OIPT’s ALD Experience with GaN devices
Page 17© Oxford Instruments 2019 CONFIDENTIAL
• ALE required for accurate AlGaN etching to form recessed gate
for E-mode devices
• Plasma ALD provides a process solution for reliable
manufacturing of E-mode devices
• Proven ALD process solutions and worldwide install base
• Ease of process transfer from R&D to HVM for GaN HEMT
customers
Conclusions
OIPT delivers Atomic Precision on a Production Scale
Page 18© Oxford Instruments 2019 CONFIDENTIAL
Thank you
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