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CHAP 2 : Lithography The Photolithographic Process

What is the lithgraphy process?

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Steps of the photolithographic process (Fig. 2.1)

We will discuss the above steps in detail.

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Drawings of wafer through the various steps of the lithography process (Fig. 2.2)

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(d:PR develop, e:SiO2etching, f:PR removal)

Final pattern after completing a photo-lithographic process.

Compare to the pattern on the mask.

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Clean room is needed for IC fabrication. Clean room Construction (one of examples)

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Cleanroom Class

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Step-by-Step Discussion for “Lithography” (1) Wafer Cleaning

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Typically RCA cleaning method is used.

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(2) Barrier Layer (i.e., thin film to be addressed to the lithographic process) Formation

(3) Photoresist Application

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(4) Soft Baking (or Prebaking) : Used to improve adhesion and remove solvent from the photoresist (PR).Annealing in an air or N2 atmosphere at 80-90oC for 10-30 min.

(5) Mask Alignment

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(6) Photoresist Exposure and Development

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(7) Hard Baking : Used to harden the photoresist. Annealing in an oven at 120-180oC for 20-30 min.

(8) Etching

(9) PR Removal Wet Dry (Resist Ashing) : Remove resist by oxidizing

(burning) it in an oxygen plasma system.

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Etching Techniques

Wet Chemical Etching Dry Etching

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Isotrophic etching was acceptable in the past.

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However, a narrow design rule require anisotrophic etching.

Lost the pattern

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For anisotrophic etching, we need to understand the plasma magic: However, it is beyond the scope of the current course.

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Use both the ion energy and the chemical reaction

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Example of dry etching; Deep Trench

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Photomask Fabrication

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Mask Fabrication : (a) Composite computer graphics plot of all masks for IC

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(b) 10x reticle of metal-level mask

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(c)Final-size emulsion mask with 400 copies of the metal level of the IC in (a)

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Various Printing Techniques