ECSE 6300 IC Fabrication Laboratory Lecture 3 Photolithography Prof. James J. Q. Lu Bldg. CII, Rooms 6229 Rensselaer Polytechnic Institute Troy, NY 12180 Tel. (518)276 2909 e mails: luj@rpi.edu http://www.ecse.rpi.edu/courses/s18/ecse 6300/index.html 3-1 Lecture Outline Photolithography Practice Photoresist Chemistry Contact and Proximity Printing Standing Wave Problem Geometric Optics Future Trends Topics Not Covered Note: The lecture slides were prepared based on the original materials written by Profs. T.P. Chow and J.-Q. Lu 3-4
Device Lithography Step 3-5 Exposure Techniques for Pattern Definition 3-6
Positive vs. Negative Photoresist 3-7 Photoresist Chemistry 3-8
Photoresist Development Dose = Time x Light Intensity 3-9 Mask to Wafer Alignment Particularly challenging for a dark field mask Use of iron oxide (semitransparent) mask helps Global vs. die by die alignment 3-10
Contact and Proximity Printing 3 Modes: Hard Contact (Vacuum needed) Soft Contact Proximity Printing g 20 to 50m Diffraction Minimum feature size resolved: W min kg k~1: constant 3-11 Q Parameter If << g <<W 2 / : 2 Q W g, g Q more faithful image Dotted curve: Diverging half angle = 1.5 /W rad. 3-12
Proximity Printing with Collimated and Uncollimated Light 3-13 Example of a Proximity Printer F: M: L: Filter Mirror Fly s eye lens 3-14
Standing Wave Problem Solution: Post exposure bake (PEB) Anti reflective Coating (PARC and BARC) http://www.imicromaterials.com/technical/reflectivity-control-in-lithography http://www.imicromaterials.com/technical/lithography-process-overview 3-15 Geometric Optics D N.A = n sin(α) = D/2f 3-16
Resolution: CD k1 NA (0.6 < k 1 < 0.8, depending on resist) Depth of Focus: DoF k 2 Numerical Aperture and Depth of Focus NA 2 NA = n sin(α) = D/2f 3-17 Transfer Function Comparison Spatial coherence of the system: Light Source Diameter S Condenser Lens Diameter Lens A NA = 0.28 S = 0.75 = 436 nm mask I MTF I max max - I I min min Lens B (Quartz) NA = 0.38 S = 1 = 248 nm 1 Feature size Lens B is capable of much higher resolution 3-18
Future Trend Photolithography has been counted out many times before, but.. Stable UV sources and mask cost are the two most formidable challenges Wrong Prediction*! *R.K. Watts, in VLSI Technology, 1988. 3-19 Optical Projection Photolithography Step and Repeat Exposure Light source Reticle Reduction Optics Thermal Stability Alignment & Stage Stepping Throughput Light sources Simplified Stepper System 3-20
Light Sources for photolithography 546 nm e line 436 nm g line 405 nm h line 365 nm i line Mercury Lamp Output Intensity Kr = Krypton 3-21 What are the challenges for NGL? Technical challenges Resolution (<45 nm) Throughput (80 120 W/H) CD control (10% of nominal CD) Overlay (30% of the node) Resist issues Pellicles issues Economical challenges Mask cost and fabrication delay $100 350K / mask (1st Year in production) and 3 5 months Manufacturing Cycle Times (<3 year/cycle)!! Equipment cost (>$10M for 193 nm, >$1M for lens) Equipment in time to market 3-22
Optical Engineering Optical Proximity Correction (OPC) Phase Shift Mask (PSM) 3-23 Immersion Lithography Lens 193 nm immersion Medium n Eqvl H 2 O 1.47 131 nm Lens n Stage Water Wafer 157 nm dry CD k1 NA DoF k2 NA N 2 1.0 157 nm 2 NA = n sin(α) = D/2f http://www.eweek.com/c/a/it Infrastructure/New Features of AMDs 45nm Shanghai Processor 3-24
EUV Lithography EUV Development: Forever Delayed? = 13.5 nm The world s first extreme ultraviolet (EUV) Alpha Demo Tool a $65 million tool from ASML Source: College of Nanoscale Science and Engineering (CNSE) of UAlbany https://www.youtube.com/watch?v=l_67fp1uhim 3-25 Topics Not Covered Projection lithography: e beam, X ray and ion beam Stable UV light sources (e.g., KrF) Pellicles Bi and Tri layer Resist Contrast Enhanced Photolithography Master Mask Generation Nano imprinter 3-26