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Alternative Substrates

Y-C Jung,S-H Won, D.G.

Ast

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The biggest strength of OLEDs is that they do not require a backlight and can be made thinner than any other technology used today. A 2 mm thick OLED is a reality today where the thinnest LCD is 3 mm”

Sharp

http://www.sharpsma.com/lcd/lcdguide/Technologies/Tech_index.php

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The US Army provided the core funding of $43.7 M to establish the Flexible Display Center at ASU.

www.asu.edu/ia/photogallery/fdc/1.htm

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Outline

1 Overview

2 Polymer Substrates

3. Flexible Glass Substrates

4. Processing Corning Microsheet

5. Results

6. Summary

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CTE PET 65 • 10-6/oC) CTE 00 MS 7.4 • 10-6/oC)

-Si:H

LTO

α-Si:H

T < 300 °C T< 600 °C

Polymers Borosilicate Glass

ELA poly-Si + 1m SiO2 LPCVD poly-SiMILC siliconELA poly-Si

CTE Si 2.6 • 10-6/oC)

LTO, Annealed LTO

CTE Si 2.6 • 10-6/o

C)

1. Overview

Better match: Glass !

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2. Polymer Substrates

Uncoated : Oxygen, Water Transmission: 1… 100 g/m2/ day

Coated with 4 inorganic layer (Vitex)

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“Due to accumulated internal stresses, optimisation of composition, stress and adhesion for each layer is needed in order to avoid warping of the substrate and cracking of the layers during deposition and/or laser annealing. Such stresses arise, for the most part, from difference in CTE of the plastic substrate and inorganic layers”.

SID 03 Digest. Paper 47.1 High Performance Plastic Substrates for Active Matrix Flexible FPD. Simone Angiolini, Mauro Avidano, Roberto Bracco, Carlo Barlocco Specialty Materials, Ferrania Imaging Technologies, Italy ; Nigel D. Young, Michael Trainor Philips Research Laboratories, UK; Xiao-Mei Zhao Electronic Materials, Promerus LLC, USA]

Elaborate Adhesion and Stress Management required

Up to 13 inorganic layers with CTE 1/30’th of polymer may have to be deposited on polymer!

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The most difficult step of poly-Si on plastics Electronics

PECVD SiO2 must be 5 to 10 times thicker than channel

www.FlexIcs.com

)/(2: SiOSiaSiaMeltPolymer ddTT

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Intel-funded plastic IC firm folds, assets up for sale

Plastics on semiconductor technology is not new ..but difficult to manufacture and commercialize

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3. Flexible Glass Substrates

• Solve most of CTE induced stress problems

• Permit process temperatures in excess of 600 oC

• Can use sintered semiconductors (e.g. inkjet printed nano-slurries)

• Are available from two rivaling glass companies

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Corning 0211 Microsheet Borosilicate Glass

~50…~100m (00,0)

CTE : 7.4….8.4 x 10-7/oC

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Corning Microsheet Schott D263

SiO2 (65wt%) SiO2 (64%), Al2O3 (2wt%)…… dopant Al2O3(4%), B2O3 (9wt%)…….. Dopant B2O3(8%), Na2O (7wt%)……. GB collapse Na2O 6%, K2O (7wt%) …….. GB collapseZnO (7wt%) ……. Deep state: 0.3,0.6eV3% not listed 18% not listed

Commercial Products

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Mechanical Properties (Schott D 263)

Influence of edge and surface coating on strength. R=30mm; pfail <1%

Fracture strength of glass A upper edge (initial scribing) and B lower edge. Radii of 30 mm; failure < 1%

Armin Plichta, Andreas Habeck, Silke Knoche, Anke Kruse, Andreas Weber, Norbert Hildebrand in Ch. 3 “Flexible Glass Substrates” in “Flexible Flat Panel Displays”, Wiley-SID Series in DisplayTechnology, Ed.: Gregory P. Crawford, ISBN: 9780470870488 Online ISBN: 9780470870501;

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4 Processing Microsheet

1. Anodic Bonding to a Si wafer, release via release a la surface MEMS

2. Peripheral Anodic Bonding to Si wafer, release by cutting, sacrificing edge

3. Anodic bonding to flexible Si structures, e.g. pillars, springs

4. Floating on liquid metal

5. Minature version of susceptor slot, created on carrier wafer

4.1 Options to process flexible, ultra-thin glass in a conventional line

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Anodic Bonding, 350oC, 10N, 1000V

Failure mode: Delayed fracture, driven by 0.2% residual tensile strain

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GaxIn1-x bonding

Failure mode: Microsheet edge lift of during CNF required RCA clean

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Rectangular substrate, diamond cut, good edge finish

“Pocket” carrier system, fabricated on 4” Si wafer:

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4.2.1 Furnace

Mixed (580oC) precursor + 600…620oC Furnace Anneal (Oxidized 4” Si)

4.2.2 RTA

Mixed (580oC) precursor + 650oC Rapid Thermal Annealin

4.2.3 ELA

Amorphous (500 oC) precursor + Excimer Laser Annealing

4 Processing Microsheet

4.2 Processes Used to fabricate TFTs on oxidized Si and Microsheet

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Poly-Si TFT Structure

Baseline on oxidized Si wafer Barrier coated Microsheet

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4.1. Mixed amorphous/x-stalline 580oCprecursor

620oC, 24 hr Anneal

4. Results

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Mixed Precursor/Furnace (or RTA)

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2 theta (degree)

10 20 30 40 50 60 70

Co

un

t (#

)

20

30

40

50

60

70

80

90

100

Mixed a/x-stalline Si Precursor: Structure

<111> peak of Poly Si

Mixed Si 150 nm deposited at 580oC

LPCVD (silane)Temp : 580 oCTime : 30 min

<220>

<311>

TEM

XRD

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TEM of mixed amorphous, crystalline precursor,

To be converted (620oC furnace, 650oC RTA) to fully poly-Si film.

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Drain Voltage (V)

-10 0 10 20 30 40I D

Cu

rren

t (A

)0.0

2.0e-5

4.0e-5

6.0e-5

8.0e-5

1.0e-4

1.2e-4

Gate Voltage (V)

-20 0 20 40 60 80

Lo

g I

D C

urr

ent

(A)

1e-10

1e-9

1e-8

1e-7

1e-6

1e-5

1e-4

1e-3 Vg(V)40

302010

Vd(V)10 5

0.1

Transfer Characteristics

* W/L = 55um/8um* Channel Mobility 7 cm2/Vs

* a/x-stal mixture active layer: 580°C, 100nm* Gate oxide (LTO): 400°C, 100nm

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SQRT TIME (hr)

0.0 0.5 1.0 1.5 2.0 2.5

Vth

Sh

ift

0

10

20

30

40

Hydrogenation

- H2 PECD System 300 oC, 80 sccm, 600 mTorr, 350W

580oC precursor; 620oC, 24 hrs, Reference wafer

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Failure Mode: Shorts

CTE increases > 5500C

Cool down - Cracks close - undetectable

B.L. deposition BL at 620oC Anneal

Doremus: Glass Science. Jon Wiley and Sons

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Boron Profile after 24hr , 620oC Anneal

MicrosheetSiNxSiO2poly

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Failure Mode Summary

• Devices failed as shorts (100%)

• Traced to non-linear CTE of MS glass approaching Tg

• Failure by CTE mismatch to barrier layer - not device silicon

• A more SiO2 rich substrate (not commercially available) would . solve problem

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Mixed amorphous/x-stalline 580oC precursor

650oC, 100 Pulse RTA Anneal

4. Results (continued)

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TFT on Microsheet

* a/x-stal precursor: 580°C, 100nm* Gate oxide (LTO): 400°C, 100nm

- 650°C, 100 Pulse - Pulse time: 5 sec

Low yield

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Excimer Laser Processing

4. Results (continued)

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ELA Process

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XRD of ELA annealed 500oC a-Si)

124~361 mJ/cm2, single pulse

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Vg(V)40

302010

Vd(V)10 5

ELA; 500oC a-Si ; 274 mJ/cm2 ; Oxidized Si

* a-Si active layer: 500°C, 100nm* Gate oxide (LTO): 400°C, 100nm* Annealing (500 °C, 4hrs.)

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Gate Voltage (V)

-20 -10 0 10 20 30 40 50

I D C

urr

ent

(A)

1e-13

1e-12

1e-11

1e-10

1e-9

Drain Voltage (V)

0 10 20 30 40

I D C

urr

ent

(A)

0.0

2.0e-10

4.0e-10

6.0e-10

8.0e-10

1.0e-9

1.2e-9Vd(V)10 5

Vg(V)45

35

25

* a-Si active layer: 500°C, 100nm* Gate oxide (LTO): 400°C, 100nm* Annealing (500 °C, 4hrs.)

ELA ; 500oC a-Si ; 274 mJ/cm2 ELA; Microsheet

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Failure Mode

Open - indicating insufficient doping activation

Channel Resistivity ~ 106 cm indicative of undoped poly-Si

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Summary

1. Three methods to process Microsheet carry Microsheet in a line explored:

a) Anodic bonding (full and partial) to 4” wafer

b) InGa bonding to 4” wafer

c) a MEMS fabricated carrier on 4” Si.

2. Three TFT processes investigated on two substrates (Microsheet, Si):

- Amorphous Si with x-stal nuclei + 600 to 620 °C Furnace anneal

- Amorphous Si with x-stal nuclei + 650 RTA anneal, Gla

- Amorphous Si + Excimer Laser Annealing (ELA)

3. The upper temperature at which TFTs can be fabricated by cw (furnace) o

r (RTA) anneal limited by CTE difference between MS and the SiO2/SiNx

barrier layer. Above about 550 °C, barrier layer fails in tension, permitting

the out-diffusion of Boron which in turn shortens the TFT source to drain.

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Summary (continued)

4. Poly-Si TFTs can be fabricated on Microsheet with ELA (Excimer L

aser Annealing using power densities of about 280 mJ/cm2)

5. Future work ELA fabricated TFTs with hydrogen passivation from P

ECVD Nitride and simultaneous LTO densification using dummy abs

orber

Acknowledgements

This investigation was carried out with the financial support of Corning Inc. at the Cornell Nanofabrication Center, an NSF

supported node in NSF NNUN network.

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BACK UP

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Finished 580oC precursor; 620oC Si TFT reference wafer