Advanced Interconnects for ULSI Technology

Mikhail R. Baklanov IMEC, Leuven, Belgium Paul S. Ho Laboratory for Interconnect and Packaging, University of Texas at Austin, Austin, Texas, USA Ehrenfried Zschech Fraunhofer Institute for Nondestructive Testing, Dresden, Germany

About the Editors xv List of Contributors xvii Preface xxi List of Abbreviations xxv Section I Low-k Materials 1 1 Low-k Materials: Recent Advances 3 Geraud Dubois and Willi Volksen 1.1 Introduction 3 1.2 Integration Challenges 5 1.2.1 Process-Induced Damage 6 1.2.2 Mechanical Properties 9 1.3 Processing Approaches to Existing Integration Issues 10 1.3.1 Post-deposition Treatments 11 1.3.2 Prevention or Repair of Plasma-Induced Processing Damage 14 1.3.3 Multilayer Structures 15 1.4 Material Advances to Overcome Current Limitations 16 1.4.1 Silica Zeolites 16 1.4.2 Hybrid Organic Inorganic: Oxycarbosilanes 19 1.5 Conclusion 22 2 Ultra-Low-k by CVD: Deposition and Curing 35 Vincent Jousseaume, Aziz Zenasni, Olivier Gourhant, Laurent Favennec and Mikhail R. Baklanov 2.1 Introduction 35 2.2 Porogen Approach by PECVD 37 2.2.1 Precursors and Deposition Conditions 37 2.2.2 Mystery Still Unsolved: From Porogens to Pores 41 2.3 UV Curing 42 2.3.1 General Overview of Curing 42 2.3.2 UV Curing Mechanisms 43 2.4 Impact of Curing on Structure and Physical Properties: Benefits of UV Curing 49 2.4.1 Porosity 49 2.4.2 Chemical Structure and Mechanical Properties 50 2.4.3 Electrical Properties 56 2.5 Limit/Issues with the Porogen Approach 57 2.5.1 Porosity Creation Limit 58 2.5.2 Porogen Residues 59 2.6 Future of CVD Low-k 62 2.6.1 New Matrix Precursor 62 2.6.2 Other Deposition Strategies 64 2.6.3 New Deposition Techniques 66 2.7 Material Engineering: Adaptation to Integration Schemes 68 2.8 Conclusion 70 3 Plasma Processing of Low-k Dielectrics 79 Hualing Shi, Denis Shamiryan, Jean-Francois de Marneffe, Huai Huang, Paul S. Ho and Mikhail R. Baklanov 3.1 Introduction 79 3.2 Materials and Equipment 80 3.3 Process Results Characterization 82 3.4 Interaction of Low-k Dielectrics with Plasma 85 3.4.1 Low-k Etch Chemistries 85 3.4.2 Patterning Strategies and Masking Materials 87 3.4.3 Etch Mechanisms 88 3.5 Mechanisms of Plasma Damage 92 3.5.1 Gap Structure Studies 93 3.5.2 Effect of Radical Density 95 3.5.3 Effect of Ion Energy 96 3.5.4 Effect of Photon Energy and Intensity 99 3.5.5 Plasma Damage by Oxidative Radicals 103 3.5.6 Hydrogen-Based Plasma 105 3.5.7 Minimization of Plasma Damage 108 3.6 Dielectric Recovery 112 3.6.1 CH4 Beam Treatment 112 3.6.2 Dielectric Recovery by Silylation 113 3.6.3 UV Radiation 119 3.7 Conclusions 121 4 Wet Clean Applications in Porous Low-k Patterning Processes 129 Quoc Toan Le, Guy Vereecke, Herbert Struyf, Els Kesters and Mikhail R. Baklanov 4.1 Introduction 129 4.2 Silica and Porous Hybrid Dielectric Materials 130 4.3 Impact of Plasma and Subsequent Wet Clean Processes on the Stability of Porous Low-k Dielectrics 134 4.3.1 Stability in Pure Chemical Solutions 134 4.3.2 Stability in Commercial Chemistries 135 4.3.3 Hydrophobicity of Hybrid Low-k Materials 138 4.4 Removal of Post-Etch Residues and Copper Surface Cleaning 141 4.5 Plasma Modification and Removal of Post-Etch 193 nm Photoresist 146 4.5.1 Modification of 193 nm Photoresist by Plasma Etch 146 4.5.2 Wet Removal of 193 nm Photoresist 153 Section II Conductive Layers and Barriers 173 5 Copper Electroplating for On-Chip Metallization 175 Valery M. Dubin 5.1 Introduction 175 5.2 Copper Electroplating Techniques 176 5.3 Copper Electroplating Superfill 177 5.3.1 The Role of Accelerator 177 5.3.2 The Role of Suppressor 178 5.3.3 The Role of Leveler 180 5.4 Alternative Cu Plating Methods 182 5.4.1 Electroless Plating 182 5.4.2 Direct Plating 182 5.5 Electroplated Cu Properties 184 5.5.1 Resistivity 184 5.5.2 Impurities 184 5.5.3 Electromigration 185 5.6 Conclusions 186 6 Diffusion Barriers 193 Michael Hecker and Rene Hubner 6.1 Introduction 193 6.1.1 Cu Metallization, Barrier Requirements and Materials 193 6.1.2 Barrier Deposition Techniques 195 6.1.3 Characterization of Barrier Performance 196 6.2 Metal-Based Barriers as Liners for Cu Seed Deposition 198 6.2.1 Ta-Based Barriers 198 6.2.2 W-Based Barriers 209 6.2.3 Ti-Based Barriers 210 6.2.4 Further Systems 211 6.3 Advanced Barrier Approaches 212 6.3.1 Barriers for Direct Cu Plating 212 6.3.2 Metal Capping Layers 214 6.3.3 Self-Forming Diffusion Barriers 216 6.3.4 Self-Assembled Molecular Nanolayers and Polymer-Based Barriers 218 6.4 Conclusions 221 Section III Integration and Reliability 235 7 Integration and Electrical Properties 237 Sridhar Balakrishnan, Ruth Brain and Larry Zhao 7.1 Introduction 237 7.2 On-Die Interconnects in the Submicrometer Era 237 7.3 On-Die Interconnects at Sub-100 nm Nodes 240 7.4 Integration of Low-k Dielectrics in Sub-65 nm Nodes 241 7.4.1 Degradation of Dielectric Constant during Integration 243 7.4.2 Integration Issues in ELK Dielectrics Due to Degraded Mechanical Properties 246 7.5 Patterning Integration at Sub-65 nm Nodes 248 7.5.1 Patterning Challenges 249 7.6 Integration of Conductors in Sub-65 nm Nodes 252 7.6.1 Narrow Line Copper Resistivity 253 7.6.2 Integrating Novel Barrier/Liner Materials and Deposition Techniques for Cu Interconnects 254 7.6.3 Self-Forming Barriers and Their Integration 256 7.6.4 Integration to Enable Reliable Copper Interconnects 257 7.7 Novel Air-Gap Interconnects 258 7.7.1 Unlanded Via Integration with Air-Gap Interconnects 258 7.7.2 Air-Gap Formation Using Nonconformal Dielectric Deposition 259 7.7.3 Air-Gap Formation Using a Sacrificial Material 260 8 Chemical Mechanical Planarization for Cu Low-k Integration 267 Gautam Banerjee 8.1 Introduction 267 8.2 Back to Basics 268 8.3 Mechanism of the CMP Process 268 8.4 CMP Consumables 271 8.4.1 Slurry 271 8.4.2 Pad 273 8.4.3 Pad Conditioner 274 8.5 CMP Interactions 276 8.6 Post-CMP Cleaning 281 8.6.1 Other Defects 286 8.6.2 Surface Finish 286 8.6.3 E-Test 287 8.7 Future Direction 287 References 288 9 Scaling and Microstructure Effects on Electromigration Reliability for Cu Interconnects 291 Chao-Kun Hu, Rene Hubner, Lijuan Zhang, Meike Hauschildt and Paul S. Ho 9.1 Introduction 291 9.2 Electromigration Fundamentals 293 9.2.1 EM Mass Flow 293 9.2.2 EM Lifetime and Scaling Rule 294 9.2.3 Statistical Test Method 296 9.2.4 Effect of Current Density on EM Lifetime 298 9.3 Cu Microstructure 300 9.3.1 X-ray Diffraction (XRD) 300 9.3.2 Electron Backscatter Diffraction in the Scanning Electron Microscope 302 9.3.3 Orientation Imaging Microscopy in the Transmission Electron Microscope 304 9.4 Lifetime Enhancement 306 9.4.1 Effect of a Ta Liner 306 9.4.2 Upper-Level Dummy Vias 308 9.4.3 Plasma Pre-clean and SiH4 Soak 310 9.4.4 CVD and ECD Cu and the Effect of Nonmetallic Impurities 311 9.4.5 Cu Alloys 314 9.4.6 CoWP Cap Near-Bamboo and Polycrystalline Cu Lines 319 9.5 Effect of Grain Size on EM Lifetime and Statistics 321 9.6 Massive-Scale Statistical Study of EM 326 9.7 Summary 329 10 Mechanical Reliability of Low-k Dielectrics 339 Kris Vanstreels, Han Li and Joost J. Vlassak 10.1 Introduction 339 10.2 Mechanical Properties of Porous Low-k Materials 340 10.2.1 Techniques to Measure Mechanical Properties of Thin Films 340 10.2.2 Effect of Porosity on the Stiffness of Organosilicate Glass Films 342 10.2.3 Hybrid Dielectrics Containing Organic/Inorganic Bridging Units 344 10.2.4 Effect of UV Wavelength and Porogen Content on the Hardening Process of PECVD Low-k Dielectrics 349 10.3 Fracture Properties of Porous Low-k Materials 352 10.3.1 Adhesion Measurement Methods 352 10.3.2 Fracture Toughness Measurement Techniques 354 10.3.3 Effect of Porosity and Network Structure on the Fracture Toughness of Organosilicate Glass Films 355 10.3.4 Effects of UV Cure on Fracture Properties of Carbon-Doped Oxides 357 10.3.5 Water Diffusion and Fracture Properties of Organosilicate Glass Films 359 10.4 Conclusion 361 11 Electrical Breakdown in Advanced Interconnect Dielectrics 369 Ennis T. Ogawa and Oliver Aubel 11.1 Introduction 369 11.1.1 Dual-Damascene Integration of Low-k Dielectrics 370 11.1.2 Low-k Types and Integrating Low-k Dielectrics 373 11.2 Reliability Testing 378 11.2.1 Measurement of Dielectric Degradation 378 11.2.2 Reliability Analysis 390 11.3 Lifetime Extrapolation and Models 397 11.4 Future Trends and Concerns 403 Section IV New Approaches 435 12 3D Interconnect Technology 437 John U. Knickerbocker, Lay Wai Kong, Sven Niese, Alain Diebold and Ehrenfried Zschech 12.1 Introduction 437 12.2 Dimensional Interconnected Circuits (3DICs) for System Applications 438 John U. Knickerbocker 12.2.1 Introduction 438 12.2.2 System Needs 441 12.2.3 3D Interconnect Design and Architecture 444 12.2.4 3D Fabrication and Interconnect Technology 446 12.2.5 Trade-offs in Application Design and Product Applications 464 12.2.6 Summary 466 12.3 Advanced Microscopy Techniques for 3D Interconnect Characterization 467 Lay Wai Kong, Sven Niese, Alain Diebold and Ehrenfried Zschech 12.3.1 Scanning Acoustic Microscopy 467 12.3.2 IR Microscopy 473 12.3.3 Transmission X-ray Microscopy and Tomography 474 12.3.4 Microstructure Analysis 480 12.4 Summary 486 13 Carbon Nanotubes for Interconnects 491 Mizuhisa Nihei, Motonobu Sato, Akio Kawabata, Shintaro Sato and Yuji Awano 13.1 Introduction 491 13.2 Advantage of CNT Vias 492 13.3 Fabrication Processes of CNT Vias 493 13.4 Electrical Properties of CNT Vias 496 13.5 Current Reliability of CNT Vias 498 13.6 Conclusion 501 14 Optical Interconnects 503 Wim Bogaerts 14.1 Introduction 503 14.2 Optical Links 505 14.2.1 Waveguides 507 14.2.2 Waveguide Filters and (De)multiplexers 510 14.2.3 Transmitter: Light Source 513 14.2.4 Transmitter: Modulators 514 14.2.5 Receiver: Photodetector 517 14.2.6 Power Consumption and Heat Dissipation 517 14.2.7 Different Materials 518 14.2.8 Conclusion 519 14.3 The Case for Silicon Photonics 519 14.3.1 Waveguides and WDM Components 519 14.3.2 Modulators, Tuners and Switches 523 14.3.3 Photodetectors 526 14.3.4 Light Sources 526 14.3.5 Conclusion 527 14.4 Optical Networks on a Chip 528 14.4.1 WDM Point-to-Point Links 529 14.4.2 Bus Architecture 529 14.4.3 (Reconfigurable) Networks 530 14.5 Integration Strategies 532 14.5.1 Front-End-of-Line Integration 533 14.5.2 Backside Integration 535 14.5.3 Back-End-of-Line Integration 535 14.5.4 3D Integration 536 14.5.5 Flip-Chip Integration 537 14.5.6 Conclusion 537 14.6 Conclusion 538 15 Wireless Interchip Interconnects 543 Takamaro Kikkawa 15.1 Introduction 543 15.2 Wireless Interconnect Technologies 547 15.2.1 Figure of Merit for Wireless Interconnects 547 15.2.2 Capacitively Coupled Wireless Interconnects 549 15.2.3 Inductively Coupled Wireless Interconnects 550 15.2.4 Antennas and Propagation 553 15.3 Conclusion 561 References 561 Index