Etching (wet vs dry - plasma) — Hinglish notes

4.3.10 · Hardware › Semiconductor Fabrication

Etching HAI kya?

Anisotropy matter KYUN karta hai? (Scratch se derivation)

Socho ek mask opening hai width $w$ ki, ek film ke upar jo thickness $t$ ki hai. Etch material ko neeche (rate $R_{v}$ ) aur mask ke neeche sideways (rate $R_{l}$ ) dono taraf remove karta hai.

Film clear karne ke liye hum time $τ$ tak etch karte hain jab tak vertical cut bottom tak pahunche: $R_{v} τ = t \Rightarrow τ = \frac{t}{R _{v}}$

Usi time mein, etch sideways bhi ghus jaati hai ek amount jise undercut $u$ kehte hain: $u = R_{l} τ = R_{l} \cdot \frac{t}{R _{v}} = \frac{R _{l}}{R _{v}} t$

Ye step kyun? Undercut rates ke ratio se set hota hai, film thickness se scale hota hai — zyada thick films usi anisotropy ke liye hamesha zyada undercut karengi.

Ab anisotropy define karo $A = 1 - R_{l} / R_{v}$ . Substitute karo $R_{l} / R_{v} = 1 - A$ : $u = (1 - A) t$

Perfectly isotropic ( $R_{l} = R_{v}$ ): $A = 0 \Rightarrow u = t$ . Aap dono sides mein ek poora film-thickness ki width kho dete ho!
Perfectly anisotropic ( $R_{l} = 0$ ): $A = 1 \Rightarrow u = 0$ . Seedhi walls.

Wet etching isotropic KYUN hoti hai

Mechanism (3 steps, purely chemical):

Reactant surface tak diffuse hota hai.
Surface reaction film ko dissolve karti hai (e.g. $SiO_{2} + 6 HF \to H_{2} SiF_{6} + 2 H_{2} O$ ).
Products diffuse ho jaate hain.

Kyunki ye chemical hai, wet etching bahut selective hoti hai (aap chemistry chunte ho jo mask ko ignore kare) lekin isotropic hoti hai aur diffusion/temperature se rate-limit hoti hai (Arrhenius: $R \propto e^{- E_{a} / k T}$ ).

Dry/plasma etching anisotropic KYUN ho sakti hai

Property	Wet	Dry / Plasma
Mechanism	Chemical (liquid)	Chemical + physical (ions)
Anisotropy $A$	~0 (isotropic)	high (near 1 with RIE)
Selectivity	bahut high	moderate
Feature size	sirf badi (undercut)	sub-micron ✔
Cost / throughput	sasta, batch	mehenga vacuum tools
Damage	koi nahi	ion-induced surface damage
Waste	chemical disposal	toxic gases

Worked examples

Common mistakes

Recall Ek 12-saal ke bachche ko samjhao (Feynman)

Socho aapne ek cake pe stencil tape kiya aur uncovered parts ko carve out karna chahte ho.

Wet etching aise hai jaise cake ko acid syrup mein dip karo — wo cake ko har jagah pighalta hai jahan touch karta hai, tape ke neeche sideways bhi, isliye aapka carved shape mota aur blurry ho jaata hai.
Dry/plasma etching aise hai jaise ek super-fine sandblaster seedha neeche shoot kare — wo sirf wahan khoda karta hai jahan aim karo, isliye walls sharp aur thin rehti hain. Wo seedha-neeche trick hi reason hai ki aapke phone mein billions of tiny transistors ho sakte hain.

Flashcards

Etching step ka purpose kya hai?

Patterned mask ke through exposed film material ko remove karna, mask pattern ko film mein transfer karna.

Etching mein selectivity define karo.

Target film ki etch rate ka mask/underlayer se ratio,

S = R_{f i l m} / R_{ma s k}

Anisotropy define karo.

A = 1 - R_{l} / R_{v}

; measure karta hai ki etch kitna vertical hai (1 = perfectly vertical, 0 = isotropic).

Undercut per side ka formula.

u = (1 - A) t

jahan

t

film thickness hai.

Wet etching isotropic kyun hoti hai?

Liquid etchant har direction mein equally react karta hai (

R_{l} \approx R_{v}

), isliye mask ko undercut karta hai.

Kaun sa etch type highly selective hai lekin small features ke liye poor hai?

Wet etching.

Kaun sa etch sub-micron features enable karta hai aur kyun?

Dry/plasma (RIE) — vertical ion bombardment high anisotropy deta hai.

RIE kya combine karta hai?

Radicals dwara chemical etching + directional physical ion bombardment (synergy).

Isotropic etch (A=0) ke liye undercut equal hai?

Dono sides mein poora film thickness

t

Hum overetch kyun karte hain?

Film ko thickness/rate non-uniformity ke bawajood clear karne ke liye; high selectivity chahiye taaki mask/underlayer survive kare.

Wet etch rate ki temperature dependence?

Arrhenius:

R \propto e^{- E_{a} / k T}

Dry etching ke main drawbacks?

Ion-induced surface damage, toxic gases, mehenga vacuum equipment, lower selectivity.

Connections

Photolithography — wo mask banata hai jo etching transfer karti hai.
Photoresist — etch-protecting stencil; selectivity iske against measure hoti hai.
Thin-Film Deposition — wo film dalti hai jo etching baad mein remove karti hai.
Plasma Physics — sheath fields ions ko vertically accelerate karte hain (anisotropy ka source).
Critical Dimension & Feature Scaling — undercut minimum line width set karta hai.
Arrhenius Equation — wet etch rate ki temperature dependence govern karta hai.