Preface ................................................... XIII
Glossary .................................................... XV
1 Introduction ................................................. 1
1.1 Organic Electronics: History and Market Opportunities ... 3
1.1.1 Large-Area Displays .............................. 4
1.1.2 Rollable Displays ................................ 5
1.1.3 Radio Frequency Identification (RFID) Tag ........ 6
1.1.4 Technological Challenges ......................... 6
1.1.4.1 Device Performance ...................... 7
1.1.4.2 Device Manufacture ...................... 8
1.1.5 Scope and Organization ........................... 8
References .................................................. 10
2 Organic Thin Film Transistor (OTFT) Overview ................ 13
2.1 Organic Semiconductor Overview ......................... 14
2.1.1 Basic Properties ................................ 15
2.1.2 Charge Transport ................................ 17
2.1.3 Microstructure and Molecular Alignment .......... 18
2.1.4 Material Development and Classifications ........ 21
2.1.4.1 Small Molecules ........................ 22
2.1.4.2 Polymers ............................... 24
2.1.4.3 n-Type Semiconductors .................. 25
2.1.5 Sensitivity to Environmental Influences ......... 26
2.2 OTFT Operation and Characteristics ..................... 27
2.2.1 OTFT Parameter Extraction ....................... 30
2.2.2 Contact Resistance Extraction ................... 32
2.2.3 Desirable OTFT Characteristics .................. 34
2.3 OTFT Device Architecture ............................... 34
2.3.1 Top-Contact and Bottom-Contact OTFTs ............ 35
2.3.2 Top-Gate, Bottom-Gate, and Dual-Gate OTFTs ...... 37
2.4 OTFT Device Material Selection ......................... 38
2.4.1 Organic Semiconductor ........................... 39
2.4.2 Gate Dielectric ................................. 41
2.4.3 Electrodes/Contacts ............................. 43
2.4.4 Substrate ....................................... 45
2.4.5 Encapsulation Strategies ........................ 47
2.5 Summary ................................................ 49
References .................................................. 49
3 OTFT Integration Strategies ................................. 55
3.1 Technological Challenge in OTFT Integration ............ 55
3.2 Overview of Processing and Fabrication Techniques ...... 58
3.2.1 Deposition Methods for Organic Semiconductors ... 61
3.2.1.1 Vacuum Evaporation ..................... 61
3.2.1.2 Solution-Processed Deposition .......... 61
3.2.2 Patterning by Shadow Mask ....................... 62
3.2.3 Patterning by Photolithography .................. 64
3.2.3.1 Photolithography Basics ................ 65
3.2.3.2 Photolithography Considerations for
OTFTs .................................. 66
3.2.4 Patterning by Inkjet Printing ................... 68
3.2.4.1 Inkjet Printing of OTFTs ............... 71
3.2.4.2 Improved Resolution by Surface-Energy
Assisted Inkjet Printing ............... 72
3.2.4.3 Printing Peripheral Circuit: Vias and
Interconnects .......................... 73
3.2.5 Microcontact Printing ........................... 73
3.2.6 Other Deposition Methods ........................ 75
3.3 OTFT Fabrication Schemes ............................... 76
3.3.1 Basic One-Mask Processing Scheme for Bottom-
Gate OTFT ....................................... 76
3.3.2 Photolithography Scheme for Fully-Patterned
and Fully-Encapsulated Bottom-Gate OTFT ......... 78
3.3.2.1 Directly Patterned OTFTs ............... 79
3.3.2.2 Indirectly Patterned OTFTs ............. 81
3.3.3 Hybrid Photolithography-Inkjet Printing Scheme
for Fully-Patterned Bottom-Gate OTFT ............ 83
3.3.4 Photolithography Scheme for Top-Gate and Dual-
Gate OTFTs ...................................... 85
3.3.4.1 Top-Gate OTFT .......................... 87
3.3.4.2 Dual-Gate OTFT ......................... 90
3.3.4.3 Analysis ............................... 92
3.3.5 Fabrication Scheme Comparisons .................. 93
3.4 Summary and Contributions .............................. 94
References .................................................. 96
4 Gate Dielectrics by Plasma Enhanced Chemical Vapor
Deposition (PECVD) ......................................... 101
4.1 Overview of Gate Dielectrics .......................... 101
4.1.1 Organic Dielectrics ............................ 101
4.1.2 Inorganic Dielectrics .......................... 104
4.2 Experimental Details and Characterization Methods ..... 105
4.2.1 Deposition Conditions of PECVD Silicon
Nitride (SiNx) ................................. 105
4.2.2 Thin Film Characterization Methods ............. 105
4.2.2.1 Fourier Transform Infrared
Spectroscopy (FTIR) ................... 106
4.2.2.2 Ellipsometry .......................... 106
4.2.2.3 X-Ray Photoelectron Spectroscopy
(XPS) ................................. 106
4.2.2.4 Atomic Force Microscopy (AFM) ......... 107
4.2.2.5 Contact Angle Analysis ................ 107
4.3 Material Characterization of PECVD SiNx Films ......... 108
4.3.1 Bulk/Structural Characterization ............... 108
4.3.1.1 FTIR Spectroscopy ..................... 108
4.3.1.2 Refractive Index ...................... 111
4.3.1.3 [N]/[Si] Ratio ........................ 113
4.3.2 Surface Characterization ....................... 114
4.3.2.1 Contact Angle ......................... 114
4.3.2.2 Surface Morphology and Roughness ...... 115
4.3.2.3 Chemical Composition .................. 116
4.3.3 Electrical Characterization .................... 117
4.3.3.1 I-V Measurements ...................... 117
4.3.3.2 C-V Measurements ...................... 120
4.3.4 Summary ........................................ 121
4.4 Electrical Characterization of OTFTs with PECVD Gate
Dielectric
4.4.1 300°C SiNx Gate Dielectrics .................... 125
4.4.2 150°C SiNx Gate Dielectrics .................... 129
4.4.3 Stacked SiNx Gate Dielectrics .................. 131
4.4.4 200°C SiOx Gate Dielectrics .................... 134
4.4.5 OTFTs on Plastic Substrates .................... 136
4.5 Summary and Contributions ............................. 139
References ................................................. 142
5 Dielectric Interface Engineering ........................... 147
5.1 Background ............................................ 148
5.1.1 Self Assembled Monolayer (SAM) ................. 148
5.1.2 Oxygen Plasma Treatment ........................ 151
5.1.2.1 Basics of Plasma Processing
(Etching) ............................. 152
5.2 Experimental Details .................................. 154
5.3 Impact of Dielectric Surface Treatments ............... 155
5.3.1 Electrical Characterization .................... 156
5.3.2 Interface Characterization ..................... 158
5.3.2.1 Contact Angle ......................... 159
5.3.2.2 Surface Roughness ..................... 161
5.3.2.3 Chemical Composition .................. 163
5.3.3 Analysis ....................................... 164
5.4 Impact of Oxygen Plasma Exposure Conditions ........... 166
5.4.1 Electrical Characterization .................... 167
5.4.1.1 Impact of Exposure Duration ........... 167
5.4.1.2 Impact of Exposure Power .............. 169
5.4.2 Interface Characterization ..................... 170
5.4.2.1 Contact Angle ......................... 170
5.4.2.2 Surface Roughness ..................... 172
5.4.2.3 Chemical Composition .................. 173
5.4.2.4 XPS Depth Profile Analysis ............ 177
5.4.3 Analysis and Discussion ................. 178
5.5 Summary and Contributions ............................. 181
References ................................................. 182
6 Contact Interface Engineering .............................. 185
6.1 Background ............................................ 186
6.1.1 Charge Injection ............................... 186
6.1.2 Alkanethiol SAM on Metals ...................... 189
6.2 Experimental Details .................................. 190
6.3 Impact of Contact Surface Treatment by Thiol SAM ...... 192
6.3.1 Electrical Characterization .................... 192
6.3.2 Interface Characterization ..................... 197
6.3.2.1 Contact Angle ......................... 197
6.3.2.2 Surface Roughness ..................... 198
6.3.2.3 Chemical Composition .................. 198
6.3.3 Analysis ....................................... 199
6.4 Impact of Execution Sequence of Surface Treatment ..... 201
6.4.1 Electrical Characterization .................... 202
6.4.2 Interface Characterization ..................... 203
6.4.2.1 Contact Angle ......................... 203
6.4.2.2 Surface Roughness ..................... 203
6.4.2.3 Chemical Composition .................. 204
6.5 Summary and Contributions ............................. 205
References ................................................. 206
Further Reading ............................................ 207
7 OTFT Circuits and Systems .................................. 209
7.1 OTFT Requirements for Circuit Applications ............ 209
7.1.1 Speed .......................................... 210
7.1.2 Leakage ........................................ 212
7.1.3 Current Drive Capacity ......................... 212
7.1.4 Stability ...................................... 213
7.2 Applications .......................................... 213
7.2.1 Displays ....................................... 213
7.2.2 RFID Tags ...................................... 214
7.3 Circuit Demonstration ................................. 218
7.3.1 Fabrication Schemes ............................ 218
7.3.2 Inverters ...................................... 219
7.3.3 Current Mirrors ................................ 223
7.3.4 Ring Oscillators ............................... 225
7.3.5 Display Pixel Circuits ......................... 226
7.3.5.1 Conventional 2-TFT Pixel Circuit ...... 226
7.3.5.2 Compensating 2-TFT Pixel Circuit ...... 229
7.3.5.3 4-TFT Current Mirror Pixel Circuit .... 231
7.4 Summary, Contributions, and Outlook ................... 232
7.4.1 Active-Matrix Backplane Integration ............ 233
7.4.2 Back-End Process Integration: Bonding and
Packaging ...................................... 234
References ................................................. 235
Further Reading ............................................ 237
8 Outlook and Future Challenges .............................. 239
8.1 Device Performance .................................... 240
8.2 Device Manufacture .................................... 241
8.3 Device Integration .................................... 242
References ................................................. 243
Index ......................................................... 245
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