ELECTRONIC DEVICES & MATERIALS GROUP Selection of Combinations of Semiconductor/Dielectric Metal Oxides Deposited without Substrate Heating for Transparent Thin Film Transistors A. J. Flewitt, F. M. Li, M. Mann, R. Waddingham, A. Kiani, N. S. Staack, S. M-L. Pfaendler and W. I. Milne Electrical Engineering Division, Cambridge University CIKC CAMBRIDGE INTEGRATED KNOWLEDGE CENTRE
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ELECTRONIC DEVICES & MATERIALS GROUP
Selection of Combinations of Semiconductor/Dielectric Metal Oxides Deposited without Substrate Heating for Transparent Thin Film Transistors A. J. Flewitt, F. M. Li, M. Mann, R. Waddingham, A. Kiani, N. S. Staack, S. M-L. Pfaendler and W. I. Milne
Electrical Engineering Division, Cambridge University
CIKC CAMBRIDGE INTEGRATED KNOWLEDGE CENTRE
ELECTRONIC DEVICES & MATERIALS GROUP
Outline
• Technology Selection
• Materials vs. patterning vs. deposition
• HiTUS Deposition Technology
• Materials
• Dielectrics and n- and p-type semiconductors
• TFTs with Varying Materials Combinations
• Metrics for selection
• Conclusions
ELECTRONIC DEVICES & MATERIALS GROUP
Large Area Electronic Materials
AMORPHOUS SILICON
Metal Oxides
Other Silicons
Organic
ELECTRONIC DEVICES & MATERIALS GROUP
Understanding the Incumbent a-Si:H Technology
Known technology
• Silicon is a known technology that is reproducible, physically (if not electronically) stable and non-toxic
• Range of dielectrics available
Amorphous structure
• Excellent homogeneity over 10 m2 areas
Low temperature processing
• Reasonable performance as low as ~150 °C
Low carrier mobility
• a-Si:H TFTs cannot be used for drive circuitry
Metastable
• Defects are created when a gate bias is applied to a TFT
Patterning costs
• Photolithography and etching used for patterning
ELECTRONIC DEVICES & MATERIALS GROUP
Understanding Metal Oxide Technology
Unknown technology
• Metal oxides are difficult with developing consensus on the physics
• Chemical stability, electrical stability and reproducibility are all challenging
Amorphous structure
• Excellent homogeneity possible
Low temperature processing
• Evidence of excellent device performance with processing < 100 °C
High carrier mobility
• 10 cm2 V-1 s-1 is a conservative estimate of what should be possible
? Stable
• Evidence of instability under certain conditions, but management may be possible
? Patterning costs
• Photolithography and etching used for patterning, but alternatives are possible due to low deposition temperature
ELECTRONIC DEVICES & MATERIALS GROUP
Material Class Material Sets
N-type Semiconductors
• ZnO, IZO, GIZO, HIZO
• SnO2
• ZTO
P-type semiconductors
• Cu2O
• SnO
• Acceptor-doped ZnO
Dielectrics
• Si3N4, SiO2
• HfO2, Al2O3
• HfxSiyO
Material Technologies
ELECTRONIC DEVICES & MATERIALS GROUP
Deposition Methods
Features
ALD
Excellent quality films
Slow deposition rates (0.1 – 1 nm/min)
High-temperature deposition or annealing
RF Magnetron Sputtering
Up to 5 nm/min at high RF powers
…but ion-induced damage
Targets need frequent changing and systems frequent cleaning
Sputtering systems seen in standard manufacturing lines (e.g. ITO layers)
PLD
Very good quality films
Relatively slower deposition rates (~1 nm/min)
High-temperature deposition or annealing
Deposition Technologies
ELECTRONIC DEVICES & MATERIALS GROUP
Patterning Technologies
• Photolithography is the dominant patterning technology
• High resolution
• High cost
• Wet chemical etching of metal oxides can be difficult
• Lift-off processing possible but reduced yield
• Low deposition temperature does allow for alternative patterning technologies • Printing-based patterning
• Alternative masking
• Marrying a high-rate, reproducible, low cost vacuum deposition technology with a low cost patterning technology is key
ELECTRONIC DEVICES & MATERIALS GROUP
Technology Interdependence
Material technology
Deposition technology
Patterning technology
ELECTRONIC DEVICES & MATERIALS GROUP
Transparent Electronics
5
12
3
4
1
2
3
4Corning 7059 glass substrate
ITO gate
Hafnium oxide gate insulator
Zinc oxide channel
ITO source/drain metallisation5
Transparent TFT
ELECTRONIC DEVICES & MATERIALS GROUP
HiTUS Sputtering System
High target utilisation
High deposition rate
Independent control of plasma density and sputtering ion energy Excellent control of material
properties including stress
Sample removed from sputtering plasma Reduced sample ion
bombardment
Ar Gas99.999%
O Gas99.999%
2
5 6
7
78
8
8
9
10
1
3
3
8 8
2
4
11
12
13
14
1
2
3
4
5
6
Sputter target
Earth shield
Gas ring
Rotating sample stage
13.56 MHz rf power supply
Matching network
Mass flow controllers
Valves
7
8
9
10
Turbo pump
11
12
13
14
15
15 Shutter
ELECTRONIC DEVICES & MATERIALS GROUP
Sputtering Processes
• Deposition of metal species
• Deposition of metal oxide species
• Oxidation
• Gas phase oxidation and thermalisation due to collisions
• Sputtering of substrate material by Ar+ ions
• Oxidation of target surface (poisoning)
Ar Gas99.999%
O Gas99.999%
2
5 6
7
78
8
8
9
10
1
3
3
8 8
2
4
11
12
13
14
1
2
3
4
5
6
Sputter target
Earth shield
Gas ring
Rotating sample stage
13.56 MHz rf power supply
Matching network
Mass flow controllers
Valves
7
8
9
10
Turbo pump
11
12
13
14
15
15 Shutter
ELECTRONIC DEVICES & MATERIALS GROUP
Polycrystalline Zinc Oxide
• A sharp transition occurs as a function of O2 flow rate between highly resistive and highly conductive material
• Deposition rate ~50 nm min-1
• Polycrystalline structure
ELECTRONIC DEVICES & MATERIALS GROUP
InZnO
• Sputtering a binary oxide is more difficult due to additional stoichiometry control