Optical Lithography. Here Is Why. Burn J. Lin SPIE PRESS. Bellingham, Washington USA

Transcription

1 Optical Lithography Here Is Why Burn J. Lin SPIE PRESS Bellingham, Washington USA

2 Contents Preface xiii Chapter 1 Introducing Optical Lithography /1 1.1 The Role of Lithography in Integrated Circuit Fabrication The Goal of Lithography The Metrics of Lithography The Contents of This Book 6 Chapter 2 Exposure Systems / Proximity Printing Projection Printing and a Comparison to Proximity Printing Full-Wafer Field Step and Repeat Step and Scan Reduction and 1X Systems X Mask Fabricated with a Reduction System Summary 25 References 25 Chapter 3 Image Formation / The Aerial Image Effects of a spherical wavefront and deviations from it Spherical wavefront The effect of a finite numerical aperture on the spherical wavefront Deviation from a spherical wavefront The Seidel aberration coefficients The Zernike aberration coefficients Imaging from a mask pattern Coherent imaging from a mask pattern Incoherent imaging from a mask pattern Partial coherent imaging from a mask pattern Spatial frequencies Spatial frequencies of an isolated line opening Spatial frequencies of line-space pairs Angular spectrum Imaging results Reflected and Refracted Images 60 V

3 vi Contents Methods to evaluate the reflected and refracted image from a mask Impact of multiple reflections on DOF The Latent Image The Resist Image The А В, С coefficients The lumped parameters ß and n From Aerial Image to Resist Image The Transferred Image Isotropic etching Anisotropic etching Lift off Ion implantation Electroplating 91 References 92 Chapter 4 The Metrics of Lithography / The Resolution and DOF Scaling Equations Determination of/ci and k$ Based on Microscopy Determination of k b k 2, and /c 3 Based on Lithography E-D branches, trees, and regions E-D window, DOF, and exposure latitude Determination of k u k 2, and /c 3 using E-D windows /CL к г, and k 3 as Normalized Lateral and Longitudinal Units of Dimension The E-D Tools Construction of E-D trees E-D tree construction from E-D matrix linewidth data E-D tree construction from E-D matrix edge data E-D tree construction from intensity distribution Importance of log scale in the exposure axis Elliptical E-D window EL-versus-DOF tradeoff Incorrect elliptical E-D window CD-centered versus full-cd-range E-D windows E-D window and CD control Application of E-D tools Combination of feature types Combination of feature sizes Combination of cuts for 2D features Combination of CD tolerances Combination of resist processing tolerances Combination of image field positions Setting the mask-making tolerance 127

4 Contents vii Effects of phase-shifting mask errors Comparison of experiment and theory 130 References 132 Chapter 5 Components in Optical Lithography/ Light Source Mercury arc lamp Excimer laser Operation principle Bandwidth narrowing Spatial coherence Maintenance, safety, and lifetime of excimer lasers Illuminator Köhler illumination system Off-axis illumination Masks Mask substrate and absorber Pellicles Critical parameters for masks CD control Placement accuracy Mask transmission and thermal expansion Mask reflectivity Mask flatness Physical size Defect level Phase-shifting masks Operating principle Un-flat BIM is not a PSM PSM types and mechanisms of imaging improvement PSM configurations Imaging Lens Typical lens parameters Numerical aperture Field size Reduction ratio Working distance Telecentricity Lens configurations Dioptric systems Reflective systems Catadioptric systems Lens aberrations Lens fabrication Lens Maintenance 166

5 viii Contents 5.6 Photoresists Classifications Polarity Working principle Imaging configurations Light interactions with a photoresist Wavelength compression Light absorption Resist bleaching or dyeing Resist outgassing Multiple reflections Profile of resist images Antireflection Coatings Wafer Wafer Stage Alignment System Off-axis alignment and through-the-lens alignment Field-by-field, global, and enhanced global alignment Bright-field and dark-field alignments 207 References 208 Chapter 6 Processing and Optimization / Optimization of the Exposure Tool Optimization of NA Optimization of illumination Exposure and focus DOF budget Components in DOF requ i red Focus monitoring Exposure tool throughput management Resist Processing Resist coating Defects Resist adhesion Resist thickness Resist uniformity Saving resist material Resist baking Precoating bake Postapply bake (pre-exposure bake) Postexposure bake Hard bake Resist developing Aspect ratio of resist image Environmental contamination 252

6 Contents ix 6.3/d Reduction Phase-shifting masks Alternating phase-shifting mask (AltPSM) Attenuated phase-shifting mask (AttPSM) Off-axis illumination Working principle of OAI Conceptual illustration D illumination on 2D geometry Analytic forms Simulation results OAI and AltPSM Comparison of imaging performance Other considerations Combination of OAI and AltPSM Scattering bars Imaging improvement from scattering bars Complications Restricted pitch D features Mask-making concerns Full-size scattering bar Hollow subresolution scattering bars and subresolution assist PSM Optical proximity correction The proximity effect Edge corrections Rule-based OPC Model-based OPC Local-dose OPC Full-depth OPC Correction to AEI Hot-spot checking CD Uniformity CDNU analysis Linear model for CDU contributions Geometrical decomposition Physical decomposition CDU summation CDU improvement Active compensation with exposure tools Active compensation on tracks 317 References 320

7 x Contents Chapter 7 Immersion Lithography / Introduction Resolution and DOF Wavelength reduction and spatial frequencies Resolution and DOF scaling equations Improving resolution and DOF with an immersion system DOF in Multilayered Media Transmission and reflection in multilayered media Effects of wafer defocus movements Diffraction DOF Required DOF Available DOF Preferred refractive index in the coupling medium Tradeoff between resolution and DOF dif f rac Polarization-Dependent Stray Light Imaging at different polarizations AAA Recombination of spatial frequency vectors in the resist Polarized refraction and reflection at the resist surface Different effects of polarized illumination Stray light System stray light Stray light from recombination of spatial frequency vectors inside the resist Stray light from reflection off the resist surface Incorporating polarization effects to E-D windows Simulation results with PDS Immersion Systems and Components Configuration of an immersion system The immersion medium The immersion lens Bubbles in the immersion medium The mask Subwavelength 3D masks The photoresist Impact on Technology Simulation for an immersion system Polylayer Contact layer Metal layer Recommendation for the three technology nodes Practicing Immersion Lithography Printing results Defect reduction Monitoring the immersion hood and special routing 391

8 Contents xi Other defect-reduction schemes Wafer and equipment cleanliness Wafer seal ring Results Extension of Immersion Lithography High-index materials Solid-immersion mask Polarized illumination Double exposures and pitch splitting Pack-unpack Overcoming the throughput penalty with double imaging Conclusion on Immersion Lithography 408 References 410 Chapter 8 Outlook and Successors to Optical Lithography / Outlook of Optical Lithography Optical lithography galaxy for logic gates Optical lithography galaxy for contact holes Optical lithography galaxy for equal lines and spaces EUV Lithography Introduction EUV sources Wall-power requirement of EUV systems EUV masks Configuration of EUV masks Random phase shifting Effects of oblique incidence Overlay mismatch Pattern shadowing Overlay error from mask flatness EUV mask fabrication Absence of mask pellicle EUV projection optics Wall-power consumption EUV resist EUV OPC Summary of EUVL Massively Parallel E-beam Maskless Imaging Introduction to e-beam imaging MEB ML2 systems proposed MAPPER MEB ML2 system LV system with individual sources HV MEB ML2 system MBS MEB ML2 system Reflected e-beam lithography 451

9 xii Contents Comparison of the different systems Data volume and the rate of DW systems Power consumption of MEB ML Extendibility of MEB ML2 systems Comparison of 4X mask writing to 1X wafer writing Applications for MEB ML Summary of MEB ML Outlook of lithography Double patterning EUV lithography MEB ML Nanoimprint lithography Conclusions 462 References 465 Index / 469