Photoresist (positive and negative)
WHY does photoresist exist?
The whole point: convert an optical pattern into a physical chemical barrier.
WHAT is photoresist made of? (the two types)
HOW does the chemistry work? (derive it from first principles)
Positive resist — why light dissolves it
Classic positive resist = DNQ / novolac system.
- Novolac is a phenolic resin. On its own it dissolves slowly in an alkaline (basic) developer like TMAH, because the phenol –OH groups can react with base.
- Add DNQ (diazonaphthoquinone), the PAC. DNQ is hydrophobic and physically blocks the base from attacking the novolac → the resist becomes an inhibitor, barely soluble anywhere.
- Shine UV. DNQ absorbs a photon and undergoes the Wolff rearrangement, losing N₂ and then reacting with water to form a carboxylic acid (indene carboxylic acid).
- Carboxylic acid is very soluble in base. So exposed regions dissolve fast; unexposed regions (still full of DNQ inhibitor) stay.
Negative resist — why light hardens it
Classic negative resist = polymer + cross-linking PAC (e.g. bis-azide, or acid-catalyzed epoxy).
- Base polymer is soluble in the (usually organic) developer.
- UV activates the PAC, which forms reactive species (nitrenes / radicals) that cross-link neighbouring polymer chains into a 3-D network.
- A cross-linked network is too big/tangled to dissolve → exposed region stays; unexposed region washes away.
Positive vs Negative — the trade-off (80/20 core)
| Property | Positive | Negative |
|---|---|---|
| Exposed region | Dissolves | Stays |
| Resolution | Higher (no swelling) | Lower (swelling blurs edges) |
| Adhesion | Weaker | Stronger |
| Cost / sensitivity | More expensive, less sensitive | Cheaper, more sensitive (faster) |
| Modern high-density ICs | Dominant | Older / non-critical layers |
Worked Examples
Common Mistakes (Steel-manned)
Flashcards
In positive resist, what happens to the exposed region?
In negative resist, what happens to the exposed region?
What chemical change does DNQ undergo on UV exposure in positive resist?
Why does the DNQ (unexposed) region resist dissolving?
Why does negative resist have worse resolution?
Which resist type dominates leading-edge IC fabrication and why?
Does exposure alone create the pattern?
Mnemonic for positive resist behavior?
Effect of overexposing positive resist on line width?
What developer is typically used for positive (DNQ/novolac) resist?
Recall Feynman: explain to a 12-year-old
Imagine painting your wafer with a special jelly that reacts to light. You put a stencil on top and shine a lamp. With positive jelly, the light melts the parts it touches, so when you rinse, those parts wash away — you're left with jelly everywhere the stencil blocked the light. With negative jelly, the light hardens the parts it touches like glue drying, so the rinse washes away the shadowed parts instead. Then you use the leftover jelly as a shield to scratch or color the silicon only in the open spots. Positive and negative are just two jellies that do exactly opposite things to light.
Connections
- Photolithography — the overall pattern-transfer process using resist.
- Etching — resist acts as the mask that survives the etch.
- Ion Implantation — resist blocks dopants from masked regions.
- Optical Masks and Reticles — mask polarity is chosen based on resist type.
- Diffraction Limit and Resolution — why swelling/resolution matters.
- Spin Coating — how the resist film is applied uniformly.
- Developer Chemistry (TMAH) — the aqueous base that removes soluble resist.
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Photoresist ek light-sensitive polymer coating hai jo wafer pe lagayi jaati hai. Iska kaam hai ek pattern (aapka circuit) ko mask se wafer tak transfer karna. Aap UV light mask ke through daalte ho, aur jahan light padti hai wahan resist ka chemistry change ho jaata hai. Phir developer daal ke kuch part ghul ke nikal jaata hai, aur jo bacha rehta hai wo etching ya doping ke time protection deta hai — bilkul stencil ki tarah.
Do types hote hain. Positive resist mein jahan light padti hai wahan resist ghul jaata hai (soluble ho jaata hai). Yaad rakho: "Positive → exposed Perishes." Negative resist mein ulta hai — jahan light padti hai wahan polymer cross-link ho ke hard ho jaata hai aur ruk jaata hai, aur baaki (bina light wala) part wash ho jaata hai. Isliye same mask se dono opposite pattern dete hain.
Chemistry samjho: positive (DNQ/novolac) mein DNQ ek inhibitor hai jo resist ko ghulne se rokta hai. Light padte hi DNQ react karke carboxylic acid ban jaata hai jo base developer mein easily ghul jaata hai — isliye exposed part nikal jaata hai. Negative mein light se cross-linking hoti hai, molecular welding jaisa, aur welded plastic ghulta nahi.
Practical baat: aaj ke advanced chips mein positive resist zyada use hota hai kyunki uski resolution better hai. Negative resist develop karte time swell ho jaata hai (sponge ki tarah paani soak karta), jisse fine lines blur ho jaati hain. Positive sasta nahi hai par sharp lines deta hai — aur chhote features ke liye sharpness sabse important hai.