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Micro-fabrication

Micro-fabrication. Photolithography-Outline. History Methods and Theories of Photolithography Preparation and Priming Spin-Coating Photoresists Soft-baking Mask Alignment and Exposure Developing Hard-baking. History.

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Micro-fabrication

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  1. Micro-fabrication

  2. Photolithography-Outline • History • Methods and Theories of Photolithography • Preparation and Priming • Spin-Coating • Photoresists • Soft-baking • Mask Alignment and Exposure • Developing • Hard-baking

  3. History • Historically, lithography is a type of printing technology that is based on the chemical repellence of oil and water. • Photo-litho-graphy: latin: light-stone-writing • In 1826, Joseph Nicephore Niepce, in Chalon, France, takes the first photograph using bitumen of Judea on a pewter plate, developed using oil of lavender and mineral spirits • In 1935 Louis Minsk of Eastman Kodak developed the first negative photoresist • In 1940 Otto Suess developed the first positive photoresist. • In 1954, Louis Plambeck, Jr., of Du Pont, develops the Dycryl polymeric letterpress plate

  4. Photolithography • In photolithography, the pattern is created photographically on a substrate (silicon wafer) • Photolithography is a binary pattern transfer: there is no gray-scale, color, nor depth to the image • This pattern can be used as a resist to substrate etchant, or a mold, and other forms of design processes • The steps involved are wafer cleaning, photoresist application, soft baking, mask alignment, and exposure and development

  5. Preparation and Priming • Prepare the substrate : • Wash with appropriate solvent to remove any matter and other impurities • TCE, Acetone, MeOH • Dry in Oven at 150°C for 10 min. • Place on hotplate and cover with petri dish, let temp. stabilize at 115°C. • Deposit Primer (optional) • Chemical that coats the substrate and allows for better adhesion of the resist TCE = trichloroethylene, MeOH = methanol

  6. Spin-Coating the Resist • Spin on the photoresist to the suface of the wafer • Standard methods are to use high spin coaters • rpm: • Time: • Produces a thin uniform layer of photoresist on the wafer surface. • Exposure Use UV light at this stage

  7. Photoresist • Photoresist is an organic polymer which changes its chemical structure when exposed to ultraviolet (UV) light. • It contains a light-sensitive substance whose properties allow image transfer onto a printed circuit board. • There are two types of photoresist: positive and negative

  8. Positive/negative resist

  9. Diagram: • Exposure to UV light causes the resist to polymerize, and thus be more difficult to dissolve • Developer removes the unexposed resist • This is like a photographic negative of the pattern • Exposure to UV light makes it more soluble in the developer • Exposed resist is washed away by developer so that the unexposed substrate remains • Results in an exact copy of the original design

  10. Soft-Baking • Put on hotplate, or in oven • Temperature；Time • Removes volatile solvents from the coating • Makes photoresist non-sticky • Hardens to amorphous solid • Be careful not to over bake and destroy the sensitizer

  11. Masking and Exposure

  12. Mask fabrication process

  13. Mask Alignment and Exposure • Photomask is a square glass plate with a patterned emulsion of metal film on one side • After alignment, the photoresist is exposed to UV light • Three primary exposure methods: contact, proximity, and projection

  14. Exposure Methods

  15. Photoresist Developer • Highly-pure buffered alkaline solution • Removes proper layer of photoresist upon contact or immersion • Degree of exposure affects the resolution curves of the resist

  16. Hard Baking • Final step in the photolithographic process • Not always necessary; depends on the resist • Hardens the photoresist • Improves adhesion of the photoresist to the wafer surface

  17. Etching • Etching type Wet etching Dry etching • Etching steps Oxidation Reaction Remove products

  18. Factors in Wet Etching • Limited • • Reaction limited • • Diffusion limited • • Factors • • Concentration • • Temperature • • Stirring

  19. HNA • mixture of nitric (HNO3), hydrofluoric (HF) and acetic (CH3COOH) acids • HNO3 oxides Si, HF removes SiO2, repeat… • high HNO3:HF ratio (etch limited by oxide removal) • low HNO3:HF ratio (etch limited by oxide formation) • dilute with water or acetic acid (CH3COOH) • acetic acid is preferred because it prevents HNO3 dissociation

  20. Anisotropic Wetting Etching

  21. Anisotropic Etch

  22. Anisotropic Etchants

  23. Wet anisotropic etching

  24. Etch the material

  25. Wet Etching vs Dry Etching

  26. Etch Mechanism

  27. Gas Phase Etch

  28. RF-Plasma-Based Dry Etching

  29. Dry Etch Chemistries

  30. Methods of Dry Etching

  31. Types of Dry Etching Processes

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