--- title: "Steps of photolithography process" subject: "Embedded System" chapter: 8 key: "semester_6/embedded_system:photolithography-steps" source: https://www.bctengineer.com/q/semester_6/embedded_system/photolithography-steps site: BCT Engineering --- # Steps of photolithography process Embedded System · Chapter 8: IC Technology - Asked in: 2069 Bhadra, 2+6 marks; 2072 Asoj, 5 marks; 2075 Baisakh, 4 marks; 2078 Chaitra (asked 4 times) - Video: [Photolithography: Step by step](https://www.youtube.com/watch?v=oBKhN4n-EGI) — Jae-Hwang Lee - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/embedded_system/photolithography-steps - Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-8 ## Question Define photolithography. Explain the various steps involved in photolithography (with diagrams). ## Short answer Photolithography transfers a mask pattern to a wafer’s photoresist for subsequent etching or doping. The sequence involves wafer cleaning, resist coating, pre-bake, alignment & UV exposure, development, post-bake, pattern etching, and resist stripping. ## Answer **Photolithography** (optical lithography) is the process of transferring a circuit pattern from a mask onto a light-sensitive layer (photoresist) on the silicon wafer, so that selected regions can then be etched, doped or coated. It is repeated for each layer of the IC. #### Steps ```text 1 Clean + oxide 2 Spin resist 3 Expose (UV) [ SiO2 ] [= resist =] UV UV UV [ Si ] [ SiO2 ] [##] [##] mask [ Si ] [= resist =] 4 Develop 5 Etch oxide 6 Strip resist [=] [=] resist [=] [=] resist [oxide] [oxide] [oxide gap oxide] [ Si windows ] [ Si wafer ] ``` 1. **Wafer cleaning and preparation:** remove dust, organics and moisture; grow or deposit the layer to be patterned (SiO2, poly-Si or metal). A dehydration bake and an adhesion promoter (HMDS) are applied. 2. **Photoresist coating:** a few drops of liquid resist are placed on the wafer, which is spun at 3000-6000 rpm to give a uniform film about 1 µm thick. 3. **Soft bake (pre-bake):** about 90-100 °C removes solvent and hardens the resist. 4. **Alignment and exposure:** the mask (reticle) is aligned to marks already on the wafer and the resist is exposed to UV light through it. 5. **Development:** a developer dissolves the exposed resist (positive resist) or the unexposed resist (negative resist), leaving the pattern. 6. **Hard bake (post-bake):** about 120 °C increases the resist's resistance to etching. 7. **Etching (pattern transfer):** wet (HF) or dry (plasma) etching removes the layer where it is not protected by resist. 8. **Resist stripping and inspection:** the remaining resist is removed with solvents or oxygen plasma and the pattern is inspected; then doping or the next layer follows. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is photolithography? A. Growing the crystal B. Transferring a mask pattern onto photoresist on the wafer C. Doping directly D. Cutting wafers Answer: B. Then etch or dope. (Key point: Definition) 2. How is resist applied? A. Spun at 3000-6000 rpm to a ~1 um film B. Brushed on C. Sprayed thick D. Evaporated Answer: A. Uniform film. (Key point: Spin coat) 3. Positive resist development? A. Unexposed dissolves B. Nothing C. Resist hardens D. Exposed resist dissolves Answer: D. Pattern equals mask. (Key point: Development) 4. What follows development? A. Spin coating B. Slicing C. Hard bake, etching, then resist stripping D. Crystal growth Answer: C. Pattern transfer then strip. 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