4.3.5 · HinglishSemiconductor Fabrication
Photoresist (positive and negative)
4.3.5· Hardware › Semiconductor Fabrication
Photoresist exist kyu karta hai?
Poora point yeh hai: ek optical pattern ko ek physical chemical barrier mein convert karo.
Photoresist kis cheez se bana hota hai? (do types)
Chemistry kaise kaam karti hai? (first principles se derive karo)
Positive resist — light ise dissolve kyu kar deta hai
Classic positive resist = DNQ / novolac system.
- Novolac ek phenolic resin hai. Apne aap yeh ek alkaline (basic) developer jaise TMAH mein dheere dissolve hota hai, kyunki phenol –OH groups base ke saath react kar sakti hain.
- DNQ (diazonaphthoquinone) add karo, jo PAC hai. DNQ hydrophobic hota hai aur physically block karta hai base ko novolac pe attack karne se → resist ek inhibitor ban jaata hai, kahin bhi barely soluble hota hai.
- UV shine karo. DNQ ek photon absorb karta hai aur Wolff rearrangement se guzarta hai, N₂ kho deta hai aur phir water ke saath react karke ek carboxylic acid (indene carboxylic acid) banata hai.
- Carboxylic acid base mein bahut soluble hota hai. Toh exposed regions fast dissolve ho jaate hain; unexposed regions (jo abhi bhi DNQ inhibitor se bhare hain) reh jaate hain.
Negative resist — light ise harden kyu kar deta hai
Classic negative resist = polymer + cross-linking PAC (jaise bis-azide, ya acid-catalyzed epoxy).
- Base polymer (aam taur pe organic) developer mein soluble hota hai.
- UV PAC ko activate karta hai, jo reactive species (nitrenes / radicals) banata hai jo neighbouring polymer chains ko ek 3-D network mein cross-link kar dete hain.
- Ek cross-linked network dissolve hone ke liye bahut bada/tangled hota hai → exposed region rehta hai; unexposed region wash away ho jaata hai.
Positive vs Negative — trade-off (80/20 core)
| Property | Positive | Negative |
|---|---|---|
| Exposed region | Dissolve ho jaata hai | Rehta hai |
| Resolution | Zyada (swelling nahi hoti) | Kam (swelling edges blur kar deti hai) |
| Adhesion | Kamzor | Zyada mazboot |
| Cost / sensitivity | Zyada expensive, kam sensitive | Sasta, zyada sensitive (faster) |
| Modern high-density ICs | Dominant | Purane / non-critical layers |
Worked Examples
Common Mistakes (Steel-manned)
Flashcards
Positive resist mein, exposed region ka kya hota hai?
Yeh zyada soluble ho jaata hai aur developer mein dissolve ho jaata hai (exposed area remove ho jaata hai).
Negative resist mein, exposed region ka kya hota hai?
Yeh cross-link ho jaata hai, insoluble ho jaata hai, aur rehta hai; unexposed region wash away ho jaata hai.
Positive resist mein UV exposure pe DNQ mein kya chemical change hota hai?
Wolff rearrangement → N₂ kho deta hai → indene carboxylic acid banta hai (base-soluble).
DNQ (unexposed) region dissolve hone se kyu resist karta hai?
DNQ ek dissolution inhibitor ki tarah kaam karta hai, base ko novolac resin pe attack karne se rok ke.
Negative resist ki resolution worse kyu hoti hai?
Organic developer cross-linked polymer ko swell kar deta hai, fine features ko distort kar ke.
Leading-edge IC fabrication mein kaun sa resist type dominant hai aur kyu?
Positive — aqueous development mein minimal swelling ki wajah se higher resolution.
Kya exposure akele pattern create karta hai?
Nahi — exposure sirf solubility change karta hai; development soluble part remove karke pattern banata hai.
Positive resist behavior ke liye mnemonic?
"Positive → exposed Perishes."
Positive resist ko overexpose karne ka line width pe kya effect hota hai?
Surviving resist feature shrink ho jaata hai (zyada area soluble ho jaata hai aur remove ho jaata hai).
Positive (DNQ/novolac) resist ke liye kaunsa developer typically use hota hai?
Aqueous base jaise TMAH (tetramethylammonium hydroxide).
Recall Feynman: ek 12-saal ke bacche ko explain karo
Socho tum apne wafer pe ek special jelly se paint kar rahe ho jo light pe react karta hai. Upar ek stencil rakhte ho aur lamp jalate ho. Positive jelly ke saath, light melt kar deta hai jo parts usne touch kiye, toh rinse karne pe woh parts wash ho jaate hain — tum jelly wahan bacha lete ho jahan stencil ne light block ki. Negative jelly ke saath, light harden kar deta hai jo parts usne touch kiye jaise glue sukh jaaye, toh rinse shadowed parts ko wash away kar deta hai. Phir tum bacha hua jelly shield ki tarah use karte ho silicon ko sirf open spots mein scratch ya color karne ke liye. Positive aur negative bas do aisi jellies hain jo light ke saath bilkul ulti cheezein karti hain.
Connections
- Photolithography — resist use karke overall pattern-transfer process.
- Etching — resist ek mask ki tarah kaam karta hai jo etch survive karta hai.
- Ion Implantation — resist masked regions mein dopants ko block karta hai.
- Optical Masks and Reticles — mask polarity resist type ke hisaab se choose ki jaati hai.
- Diffraction Limit and Resolution — swelling/resolution kyu matter karta hai.
- Spin Coating — resist film kaise uniformly apply hoti hai.
- Developer Chemistry (TMAH) — aqueous base jo soluble resist remove karta hai.