Semiconductor Lithography : Principles, Practices, and Materials

Semiconductor lithography is one of the key steps in the manufacturing of integrated silicon-based circuits. In fabricating a semiconductor device such as a transistor, a series of hot processes consisting of vacuum film deposition, oxidations, and dopant implantation are all patterned into microscopic circuits by the wet processes of lithography. Lithography, as adopted by the semiconductor industry, is the process of drawing or printing the pattern of an integrated circuit in a resist material. The pattern is formed and overlayed to a previous circuit layer as many as 30 times in the manufacture of logic and memory devices. With the resist pattern acting as a mask, a permanent device structure is formed by subtractive (removal) etching or by additive deposition of metals or insulators. Each process step in lithography uses inorganic or organic materials to physically transform semiconductors of silicon, insulators of oxides, nitrides, and organic polymers, and metals, into useful electronic devices. All forms of electromagnetic radiation are used in the processing. Lithography is a mUltidisciplinary science of materials, processes, and equipment, interacting to produce three-dimensional structures. Many aspects of chemistry, electrical engineering, materials science, and physics are involved. The purpose of this book is to bring together the work of many scientists and engineers over the last 10 years and focus upon the basic resist materials, the lithographic processes, and the fundamental principles behind each lithographic process.

1. Introduction.- 1-1. Semiconductor Device Manufacturing.- 1-2. Lithography Processing and Materials.- 1-3. Lithography Costs and Process Equipment.- 1-4. Organization of the Book.- 2. Positive Photoresists.- 2-1. Introduction.- 2-2. Diazoquinone Photoresists.- 2-2-1. General Performance.- 2-2-2. General Photochemistry and Solubility Properties.- 2-2-3. Spectral Absorbance.- 2-2-4. Resins for DQ Resists.- 2-2-5. Chemical Mechanisms in DQN.- 2-3. Shelf Life and Quality Control of DQN.- 2-4. Summary and Outlook on DQN Resists.- 2-5. Deep-UV Exposure.- 2-5-1. Optical Systems.- 2-5-2. Resist Sensitometry.- 2-5-3. Deep-UV Resists.- 2-6. PMMA and PMIPK.- 2-6-1. PMMA.- 2-6-2. PMIPK.- 2-6-3. Summary.- 2-7. Nitroaldehyde Resists.- 2-8. Photocatalyzed Positive Resists.- 2-9. Miscellaneous Positive Deep-UV Resists.- 2-10. Outlook.- 3. Positive Radiation Resists.- 3-1. Introduction.- 3-1-1. Tool Contributions.- 3-1-2. Resist Considerations.- 3-1-3. High-Energy Radiation Absorption.- 3-1-4. Radiation Chemical Reactions.- 3-2. X-Ray Positive Resists.- 3-2-1. Basic Exposure Tool.- 3-2-2. X-Ray Positive Resists.- 3-3. Positive Ion-Beam Resists.- 3-4. Electron-Beam Positive Resists.- 3-4-1. Electron-Beam Tool and Process Considerations.- 3-4-2. One-Component Positive Radiation Resists.- 3-4-3. Pre-cross-linked PMMA Resist.- 3-4-3-1. Plasma Stability of PMMA.- 3-4-3-2. Processing.- 3-4-3-3. Outlook for PMMA.- 3-5. Polyolefin Sulfones and Other Electron-Beam Positive Resists.- 3-6. Practical Positive Resists.- 3-7. Outlook.- 4. Negative Photoresists.- 4-1. Introduction.- 4-1-1. Chemical Reactions.- 4-1-2. Exposure and Contrast.- 4-1-3. Photosensitizers.- 4-1-4. Commercial Resist Compositions.- 4-2. Polyvinylcinnamate.- 4-3. Azide-Sensitized Resists.- 4-3-1. Azide-Activated Formulations.- 4-3-2. Photochemistry of Azides.- 4-3-3. Rubber and Phenolic Based Systems.- 4-3-4. Photospeed of Rubber Azide.- 4-3-5. Oxygen Effect in Azide Resists.- 4-4. Miscellaneous Radical-Based Photoresists.- 4-5. High-Temperature Negative Photoresists.- 4-6. Photopolymerization.- 4-7. Charge Transfer Resists.- 4-8. Inorganic Resists.- 4-9. Summary and Outlook.- 5. Negative Radiation Resists.- 5-1. Introduction.- 5-2. X-Ray Resists.- 5-3. Ion-Beam Resists.- 5-4. Electron-Beam Resists.- 5-4-1. Styrene-Based Resists.- 5-4-2. Epoxy-Based Resists.- 5-4-3. Allyl-Based Resists.- 5-4-4. Charge Transfer Resists.- 5-4-5. Silicone Negative Resists.- 5-4-6. Miscellaneous Resists.- 5-5. Two-Component Resists.- 5-6. Practical Use of Negative Electron-Beam Resists.- 5-7. Outlook.- 6. Surface Preparation and Coatin.- 6-1. Introduction.- 6-2. Cleaning the Wafer Surface.- 6-2-1. Contamination and General Consequences.- 6-2-2. Causes and Mechanisms of Surface Contamination.- 6-2-3. Removal of Contaminants.- 6-2-3-1. Clean Rooms.- 6-2-3-2. Liquid Contamination.- 6-2-3-3. Removal of Contaminants on the Wafer Surface.- 6-2-4. Detection of Contaminants on the Wafer Surface.- 6-3. Storage of Cleaned Wafers.- 6-4. Adhesion Promoters.- 6-5. Physical Chemistry of Spin Coating.- 6-5-1. Solvents for Spin Coating.- 6-5-2. Practical Aspects of Spin Coating.- 6-6. Other Resist Coating Techniques.- 6-6-1. Langmuir-Blodgett Films.- 6-6-2. Gas-Phase Deposition.- 6-7. Resist Film Thickness Measurements.- 6-8. Pinholes in Resist Films.- 6-8-1. Causes of Pinholes.- 6-8-2. Measurement of Pinholes in Resist Films.- 6-8-3. Reduction and Control of Pinholes in Resist Films.- 6-9. Summary and Outlook of Resist Coatings.- 7. Prebake (Softbake).- 7-1. Introduction.- 7-2. Kinetics of Prebaking.- 7-3. Physical Changes in Prebaking.- 7-3-1. Residual Stress Strain.- 7-3-2. Measurement of Residual Solvent in Prebaked Films.- 7-4. Prebaking Positive Resists.- 7-5. Prebake Equipment and Processes.- 7-6. Summary.- 8. Optical Exposure.- 8-1. Introduction.- 8-2. Optical Principles.- 8-3. Contact and Proximity Print.- 8-4. Projection Print.- 8-5. Overlay.- 8-6. Miscellaneous Exposure Techniques.- 8-7. Photomasks.- 8-8. Sensitometry.- 8-9. Outlook.- 9. Radiation Exposure.- 9-1. Introduction.- 9-2. Electron-Beam Performance.- 9-3. Electron-Beam Exposure Equipment.- 9-4. Electron-Beam-Resist Interaction.- 9-4-1. Charging of Resist.- 9-4-2. Resist Outgassing.- 9-5. Registration.- 9-6. Proximity Effects.- 9-7. Radiation Damage.- 9-8. X-Ray and Ion-Beam Exposure.- 9-8-1. X Rays.- 9-8-2. Ion Beams.- 9-9. Outlook.- 10. Developing Resist Images.- 10-1. Introduction.- 10-2. General Mechanisms.- 10-3. PMMA Developer Sensitivity.- 10-4. Enhancement of R and R0 of PMMA.- 10-5. Diazoquinone-Novolak Resists.- 10-5-1. Introduction.- 10-5-2. Modeling DQN Development.- 10-5-2-1. Photokinetics of Exposure.- 10-5-2-2. Kinetics of Dissolution.- 10-6. Development of Negative Resists.- 10-7. Dry Development.- 10-7-1. Direct Image Formation.- 10-7-2. Plasma-Developed Resists.- 10-7-2-1. Monomer Matrices for Plasma-Developable Resists.- 10-7-2-2. Polymer Matrices for Plasma Development.- 10-7-3. Plasma Etching without Development.- 10-7-4. Outlook for Dry Development.- 10-8. Practical Development of Resists.- 11. Postbake.- 11-1. Introduction.- 11-2. Physical Chemistry of Postbake.- 11-2-1. Adhesion.- 11-2-2. Melting.- 11-3. Chemical Reactions in Postbake.- 11-4. Other Methods of Hardening DQN.- 11-5. Practical Postbaking.- 11-6. Summary and Outlook.- 12. Additive Processes.- 12-1. Introduction.- 12-2. Single-Layer Resist for Resist Processes.- 12-3. DQN for Lift-off.- 12-4. Lift-off with PMMA Films.- 12-5. Single-Film Lift-off with Shallow-Profile Resists.- 12-6. Summary and Outlook of Single Films.- 12-7. Double-Layer Resist.- 12-7-1. Developer Transfer Bilayer Resist.- 12-7-2. Optical Dual-Layer Resist.- 12-7-3. Dual-Layer Plasma Etch Transfer.- 12-8. High-Temperature Lift-off.- 12-9. Summary and Outlook of Double-Layer Resists.- 12-10. Trilayer Resists.- 12-10-1. Planarizing Layers.- 12-10-2. Barrier Layers.- 12-10-3. Image Layer.- 12-11. Practical MLR Processing.- 12-12. Metal Deposition and Lift-off.- 12-12-1. Resist Profile and Lift-off.- 12-12-2. Resist Thickness and Lift-off.- 12-13. Lift-ofT Process Control.- 12-14. Miscellaneous Additive Processes.- 12-15. Electroplating with Resists.- 12-16. Ion Implantation Resist Masking.- 13. Subtractive Etching.- 13-1. Introduction.- 13-1-1. General Principles of the Rate of Etching.- 13-1-2. Phenomenological Mechanisms of Etching.- 13-2. Liquid Etching.- 13-2-1. SiO2 Liquid Etching.- 13-2-2. Wetting, Adhesion, and Undercutting.- 13-2-3. Surface Primers and Treatments.- 13-2-4. Stress in Resist Films.- 13-3. Passivity in Etching SiO2.- 13-4. Silicon Etching.- 13-5. Sandwich Etching.- 13-6. Aluminum Etching.- 13-6-1. Etchants for Aluminum.- 13-6-2. Electroetching.- 13-6-3. Summary.- 13-7. Practical Aspects of Wet Etching.- 13-8. Troubleshooting Etching.- 13-8-1. Spray versus Static Etching.- 13-8-2. Other Etching Considerations.- 13-9. Modeling of Wet Etching.- 13-10. Outlook of Wet Etching.- 13-11. Introduction to Gas Etching.- 13-12. Aluminum Etching.- 13-12-1. Introduction.- 13-12-2. Mechanism of A1 Etching.- 13-12-3. Selectivity of A1 Etching.- 13-12-4. Residue and Postetch Corrosion.- 13-12-5. Resist Hardening.- 13-12-6. Practical A1 Etching.- 13-12-7. Summary.- 13-13. Silicon Etching.- 13-13-1. Introduction.- 13-13-2. Gas-Phase Etching.- 13-13-2-1. Thermal Gas-Phase Etching.- 13-13-2-2. Plasma Etching.- 13-13-2-3. Plasma Etch Rates of Radical Systems.- 13-13-2-4. Plasma Ion Etching.- 13-13-2-5. Role of Plasma-Deposited Polymers.- 13-13-2-6. Gas-Phase Recombination Model.- 13-13-2-7. Role of Electrode in Etching.- 13-13-2-8. Summary of Plasma Etching Mechanisms.- 13-13-3. Selectivity.- 13-13-4. Etch Profiles and Image Bias.- 13-13-4-1. Etch Profile Factors.- 13-13-4-2. Ion Transport Model.- 13-13-5. Uniformity of Plasma Etching.- 13-13-6. Plasma Etching Contamination.- 13-13-6-1. Sputtered Metals.- 13-13-6-2. Insulation Deposits.- 13-13-6-3. Etch Residues.- 13-13-6-4. Radiation Damage.- 13-13-7. Applications of Plasma Processing.- 13-13-8. Loading Effects.- 13-13-9. Etch Rate and Endpoint Monitor.- 13-14. Summary and Outlook of Etching.- 14. Stripping of Resists.- 14-1. Introduction.- 14-2. Liquid Stripping Formulations.- 14-2-1. Organic Solvent Strippers.- 14-2-1-1. Simple Solvent Strippers.- 14-2-1-2. Multiple Component Strippers.- 14-2-1-3. Surface Wetting Agents.- 14-2-2. Typical Liquid Strippers.- 14-3. Water-Based Strippers.- 14-3-1. Acidic Strippers.- 14-3-2. Alkaline Strippers.- 14-4. Practical Aspects of Stripping.- 14-5. Plasma Gas Stripping of Resists.- 14-5-1. Reactants of Plasma Stripping.- 14-5-2. Polymeric Reactants.- 14-5-3. Oxygen Plasma.- 14-5-4. Plasma Radiation Field.- 14-6. Kinetics of Stripping.- 14-6-1. Gas-Solid Interactions.- 14-6-2. Activation Energy of Stripping.- 14-6-3. Monitoring Plasma Stripping.- 14-6-4. Resist Stripping Rates and Polymer Structure.- 14-7. Side Effects of Plasma Stripping.- 14-7-1. Detection of Damage.- 14-7-2. Prevention and Control of Damage.- 14-8. Plasma Stripping Apparatus.- 14-9. Miscellaneous Stripping Techniques.- 14-10. Applications of Oxygen Plasma to Semiconductor Lithography.- 14-11. Outlook.- 15. Process Controls.- 15-1. General Process Considerations.- 15-2. Lithographic Process Automation.- 15-3. Resist Material Controls.- 15-4. Resist Processing Controls.- 15-4-1. Exposure Controls.- 15-4-2. Development Process Controls.- 15-4-3. Etching Process Controls.- 15-5. Process Troubleshooting.- 15-6. Summary.- 16. Nonresist Processe.- 16-1. Introduction.- 16-1-1. Elementary Considerations.- 16-1-2. Chemical and Thermal Energy Requirements.- 16-1-3. Laser Sources.- 16-2. Gas-Solid Photon-Induced Reactions.- 16-2-1. Deposition.- 16-2-2. Subtractive Etching and Removal.- 16-2-3. Summary of Gas-Solid Reactions.- 16-3. Liquid-Solid Reactions.- 16-3-1. Deposition Including Oxidation.- 16-3-2. Etching at Liquid-Solid Interface.- 16-4. Solid-Solid Reactions.- 16-4-1. Sublimography.- 16-4-2. Fill-and-Fire Resists.- 16-4-3. Metal Silicides.- 16-4-4. Metal Salt Decomposition.- 16-4-5. Generation of Etchants from Solid Precursors.- 16-5. Direct Laser Treatments.- 16-6. Summary of Photoassisted Processes.- 16-7. Electron-Beam-Assisted Patterning.- 16-7-1. Deposition by Electron Beams.- 16-7-2. Electron-Beam-Induced Etching.- 16-8. Ion-Beam Processes.- 16-9. Summary and Outlook of Resistless Direct Patterning Lithography.- 17. Summary and Future Processes.- 17-1. Critical Lithography.- 17-2. Resist Materials.- 17-2-1. Positive Photoresists.- 17-2-2. Negative Photoresists.- 17-2-3. Positive Radiation Resists.- 17-2-4. Negative Radiation Resists.- 17-2-5. Complementary Lithographic Materials.- 17-2-6. Overall Rating of Resists.- 17-3. Process Lithography.- 17-3-1. Coating Techniques.- 17-3-2. Prebake and Postbake.- 17-3-3. Expose.- 17-3-4. Develop.- 17-3-5. Subtractive Processing.- 17-3-6. Additive Processes.- 17-3-7. Stripping of Resists.- 17-3-8. Resistless Processes.- 17-3-9. Process Controls.- 17-4. Postlude.