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Lithography fabrication ppt

  1. Lithography Fabrication 1 Jadhav Avinash J  2K13E11  Savitri Bai Phule Pune University
  2. What is Lithography? 2  Lithography(Greek word) means printing is done on stone.  Photo-litho-graphy: light-silicon wafer-printing.  Components in photolithography: (1)Mask (2)Photoresist (3)UV exposure system
  3. Overview of the Photolithography Process 3
  4. Steps Used in Photolithography  Surface cleaning  Spin coating with photoresist  Soft baking  Mask alignment  Exposure  Development  Hard baking  Plasma Etch-Or Add Layer  Post process cleaning  Final Inspection 4
  5. Surface Cleaning 5  Typical contaminants that must be removed prior to photoresist coating: dust from scribing or cleaving (minimized by laser scribing)  photoresist residue from previous photolithography (minimized by performing oxygen plasma ashing)  atmospheric dust (minimized by good clean room practice)  bacteria (minimized by good DI water system)
  6. 6  films from other sources: -solvent residue -H2 O residue -photoresist or developer residue -silicone  For particularly troublesome grease, oil, or wax stains: Start with 2-5 min. soak in 1,1,1- trichloroethane (TCA) or trichloroethylene (TCE) with ultrasonic agitation prior to acetone
  7. Spin coating with photoresist 7  Wafer is held on a spinner chuck by vacuum and resist is coated to uniform thickness by spin coating.  Typically 3000 - 6000 rpm for 15-30 seconds.  Resist thickness is set by: primarily resist viscosity secondarily spinner rotational speed  Most resist thicknesses are 1-2 μm for commercial Si processes.
  8. 8  Resist thickness is given by t =square of( kp)/root of(w1), where, k = spinner constant, typically 80-100 p = resist solids content in percent w = spinner rotational speed in rpm/1000
  9. Stages of Resist Coating 9
  10. Soft baking 10  Used to evaporate the coating solvent.  Typical thermal cycles:90-100°C for 20 min. in a convection oven 75-85°C for 45 sec. on a hot plate  Microwave heating or IR lamps are also used  Optimizes light absorbance characteristics of photoresist
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  12. Mask alignment & Exposure 12 Transfers the mask image to the resist-coated wafer Activates photo-sensitive components of photoresist Three types of masking (1) Contact printing (2) Proximity printing (3) Projection printing
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  14. 14  Contact printing capable of high resolution but has unacceptable defect densities. May be used in Development but not manufacturing.  Proximity printing cannot easily print features below a few mm in line width. Used in nano-technolgy.  Projection printing provides high resolution and low defect densities and dominates today.They print » 50 wafers/hour.
  15. Positive resist & Negative resist 15
  16. Development 16  Soluble areas of photoresist are dissolved by developer chemical  Visible patterns appear on wafer  Quality measures:  –line resolution  –uniformity  –particles & defects
  17. Hard baking 17  Used to stabilize and harden the developed photoresist prior to processing steps that the resist will mask.  Postbake removes any remaining traces of the coating solvent or developer.  Higher temperature than soft bake (120-150 degree)
  18. Plasma Etch-Or Add Layer 18
  19. Post process cleaning 19  Plasma etching with O2 (ashing)  Simple solvents are generally sufficient for non-postbaked photoresists:  Positive photoresists: aceton trichloroethylene (TCE)  Negative photoresists: methyl ethyl ketone methyl isobutyl ketone 
  20. Final Inspection 20  Photoresist has been completely removed  Pattern on wafer matches mask pattern (positive resist)  Quality issues: –defects –particles –step height –critical dimensions
  21. Any Questions??? 21