Spatial Atomic Layer Deposition: A Path to High-Quality Films on Continuous Substrates t David H. Levy, Roger S. Kerr, Shelby F. Nelson, Lee W. Tutt, and Mitchell Burberry Eastman Kodak Company Rochester, NY
Agenda Atomic Layer Deposition (ALD) as a process Spatial ALD Approach Performance Devices and patterning using Spatial ALD Working demonstrations of film quality Effective film patterning with ALD
Atomic Layer Depositions (ALD) ALD: process where a substrate is exposed to reactive gases one by one Precursor (I) Precursor (II) Cycle is repeated Film growth occurs layer by layer High quality Conformal Low temperature Eastman Kodak Company 3
Atomic Layer Deposition Uses Barrier layers Very conformal and dense coating Prevent moisture and oxygen transmission Thin layers (100 200 Å) are effective Thin, high-performance dielectrics New generation silicon chips: 25 A layers with low electrical leakage Many other applications Coating of high aspect ratio structures Transparent conductors Oxide and other binary/ternary semiconductors
Spatial Atomic Layer Deposition (S-ALD) Chamber ALD Substrate Exposure Time Inert (I) (II) Q Spatial ALD S-ALD Head Substrate Exposure (Point Q) Time Spatial Process Steady-state gas flows Can be open air Suitable for large or continuous substrates
Isolating the Reactive Gases Gas confinement is key There is a variety of proposed p systems for gas confinement Gas regions Inert (I) (II) Source and exhaust slots
The ALD Coating Head P P Large Gap Small Gap Large gap to substrate Low pressure gradients Gas will mix across many channels Small gap Substrate floats (gas bearing) High pressure to drive from source to exhaust: Good Isolation Excellent control of substrate position Very small chamber
Equipment Design Current work is on a laboratory scale unit 2" wide coating width Used with discrete 2.5" square substrates Process demonstrations Gas isolation ALD film growth and saturation Open air operation extendability to long substrates 8
Isolation of Precursor Gases How good is the gas separation? Measure by using a tag gas (NH 3 ) in the metal channels Look for crossover of this gas to the oxygen channels P Results metal exhaust Pure NH 3 oxy- exhaust ppm level NH 3 detector Stationary operation: No detectable mixing At our current maximum velocity (0.26 m/s): ~23 ppm mixing Gas phase reaction minimal factor Eastman Kodak Company 9
Saturation Behavior for TMA/Water Relative residence times hard wired by head design However Constant flow: accurate control over chemistry levels 1.0 Very sharp chemistry 0.8 changes 06 0.6 Growth h/cycle (Å) Clear saturation 0.4 behavior 0.2 Saturation near 1.2 Å/cycle for TMA/Water 1.4 200 C 1.2 0.0 P TMA P Water (mtor (mtor r) r) 300 170 30 170 300 17 0 500 1000 1500 2000 2500 30 17 Residence Time (msec) Eastman Kodak Company 10
Equipment Development (underway) Objectives Increased coating width Web handling 6 inch rigid Phase A Coating head migration to 6 width DEMO:Ability to construct wider heads DEMO: Uniform delivery of gas 6 DEMO: Uniform delivery of gas Short Pass s Web Phase B Short pass 6 web unit DEMO:Ability to handle free standing webs
Throughput Empirical model can be constructed for a given reaction system Growth per Cycle GPC (A) 1.400 1.200 1.000 0.800 0.600 0.400 0.200 GPC GPC-Calc Required Thickness Slot spacing gth Reactor Len 0.000 0.000 1.000 2.000 3.000 Residence (sec) Residence Time Model Web Speed Currently have good data on Al 2 O 3 and ZnO system 12
Throughput for Al 2 O 3 or ZnO Zone Length for 100 00A (m) 30 25 20 15 10 5 0 Al 2 O 3 ZnO 230 mtorr Water 5.9 Torr Water 0 5 10 Speed (m/min) Growth Per Cycle Residence Time To date: Small to no effects when not in complete saturation The material system matters Slot spacing is a weaker dependence Longer spacing Longer residence more deposition per cycle Easier head assembly Example: 200 Å Film 5 m/min web speed 1.6 m zone
S-ALD ZnO Thin-Film Transistors (TFT) TFTs: the drive element for flat displays Laptop screen: a-si TFTs with mobility ~1 cm 2 /V-s To drive an OLED Higher mobility is needed to handle the pixel current Higher stability is needed to continuously supply the pixel ZnO is a promising alternative V d i Evaporated Al Contacts 250 Å ZnO By the Spatial ALD Process 1100 Å Al 2 O 3 V g Glass Side View, Schematic Eastman Kodak Company 14 ITO Gate Layer on Glass (commercially obtained)
Typical Device Performance W/L = 600/50 mm T ox = 1100 Å ITO Gate Shadow mask Al contacts (A) Drain Current 1E-02 1E-03 1E-04 1E-05 1E-06 1E-08 1E-09 1E-10 1E-11 Le eakage (A/cm m2) Al 2 O 3 Dielectric Leakage 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 DF14 1-6 - D 6 C- ID- [ V A =0 ] DF141-4-D1E-ID-[VA=0] 4 D1E DF141-2_D2C-ID-[VA=0] 150 C 200 C 1E-07 250 C 1E-12-10 0 10 20 30 Gate Voltage (V) High on/off ratio >10 8 Low gate leakage <2.5 10-8 A/cm 2 Mobility: ~15 cm 2 /V-s I g 0 2 4 6 8 10 Applied Field (MV/cm) Additional Characteristics Stability: Comparable to a-si. 2.3 MHz ring oscillator circuits: Fast (J. Sun, et al., IEEE Electron Device Lett.)
Mapping Electrical Characteristics Deposited film on Si (shows thickness steps) Map of Linear Differential Mobility Thinner dielectric and semiconductor Measurement region: Central area Eastman Kodak Company 16
TFTs with Shadow-masked Al Contacts Linear Mobility Vth 12.3 ± 06cm 0.6 2 /V-s 468± 4.68 ± 004V 0.04 240 devices Eastman Kodak Company 17
Bias Stability Initial observations Stability depends on Gate Bias (not current flow) Mobility shows little change Conditions Typically stress time = 10,000 s Bias applied Vg = 10 V (for gate dielectric thickness = 50 nm) Relatively high field (2 10 6 V/cm) For W/L = 500/50 Linear: Vd = 0.25 V, drain current ~50 μa Saturation: Vd = 10 V, drain current ~0.9 ma Eastman Kodak Company 18
Passivated TFT Passivation with alumina in Spatial ALD system 200 C process Thickness = 50 nm 1.0E-04 1.0E-05 Al ZnO Al t (A) 1.0E-06 Al 2 O 3 Chromium Gate Drain Curren 1.0E-07 1.0E-08 1.0E-09 1.0E-10 t = 0 s t = 10000 s Glass 1.0E-11-10 -5 0 5 10 Vg (V) Normalized Curre ent 1.0 0.9 08 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1 10 100 1000 10000 100000 Stress Duration (s) Low movement of threshold or current Eastman Kodak Company 19
Patterning and R2R ALD Typical Semiconductor Processing Photolithographic process Layers applied, then patterned with photoresist + etching Large-area processing: A new landscape Material Process Printed Patterning Direct Print Functional Materials Physical Vapor Deposition Chemical Vapor Deposition ALD? Selective Area
Selective Area Deposition Reaction inhibition If precursors cannot react with the substrate, the film does not grow Advantages Thin inhibitor layers Inhibitors can be printed After ALD, film is ready for next layer
Characterization of Growth Inhibition ½ Sample No Inhibitor ½ Sample Inhibitor O.D. (~thickne ess) (inhibition) Inhibitor spun on sample ALD Cycles During ALD growth, sample removed periodically No inhibitor: Normal surface growth Inhibitor side: Reduced/eliminated d/ i d growth Growth of ZnO characterized with 355 nm optical density
PMMA: a Good Inhibitor PMMA solutions spun on borosilicate glass 950,000000 MW (Microchem 950-PMMA-A4) A4) Thicknesses by ellipsometry on silicon controls (3000 rpm) 9 Å (0.025% solution) 18 Å (0.05% 05% solution) 38 Å (0.1% solution) Inhibition results Strong inhibition even for 9 Å film 40 Å suitable for most applications Thinness: Quick inhibitor removal for inline process O.D. @ 355 nm 1.4 1.2 1 0.8 0.6 0.4 0.2 0 38 A PMMA 18 A PMMA 9 A PMMA Bare 0 500 1000 1500 lf i li ALD Cycles
TFT Structure Completely by Selective Area ALD 1000 Å Doped ZnO 1100 Å Al 2 O 3 300 Å Intrinsic ZnO 1000 Å Doped ZnO Stamp Stamp Stamp Stamp PDMS Based Stamping Result: Working transistor with mobility ~3 cm 2 /V-s All layers by ALD All patterning by selective area deposition 1.0E-03 1.0E-04 1.0E-05 1.0E-06 Drain Current (Vd=10V) Drain Current (Vd=20V) Gate Leakage (Vd=10V) Gate Leakage (Vd=20V) 1.0E-12-10 0 10 20 30 Transparent, too! 1.0E-07 1.0E-08 1.0E-09 1.0E-10 1.0E-11 Eastman Kodak Company 24
Conclusions Spatial ALD Approach Open air performance demonstrated on rigid substrates Scaleup and flexible work underway Applications High performance semiconductor / dielectrics Accessible patterning Eastman Kodak Company 25