Physical vapor deposition (PVD - sputtering)

4.3.13 · Hardware › Semiconductor Fabrication

PVD KYA hai?

CVD se comparison: Chemical Vapor Deposition mein film gaseous precursors ki chemical reaction se banti hai. PVD mein film ke atoms directly aur physically ek solid source se aate hain — yahi defining difference hai.

Isko kyun use karte hain?

Hum chips par metals aur barriers deposit karne chahte hain: aluminum/copper interconnects, TiN/Ta barrier layers, W contacts, aur kuch dielectrics bhi. Metals ke liye PVD ke dominant hone ke do reasons hain:

Metals ko CVD se deposit karna mushkil hai — volatile aur safe metal precursors ki zaroorat padegi, jo aksar exist nahi karte ya toxic/expensive hote hain.
PVD se achha adhesion aur controllable stress milta hai kyunki atoms kinetic energy ke saath aate hain aur tightly pack hote hain.

Sputtering kaise kaam karta hai — first principles se

Step 1: Plasma create karo

Wafer aur ek target (cathode) ko chamber mein rakho. High vacuum tak pump karo, phir ek inert gas — usually argon — se kuch mTorr tak backfill karo. Target par ek bada negative voltage apply karo. Stray electrons accelerate karte hain, Ar atoms se collide karke unhe ionize karte hain: $Ar + e^{-} \to Ar^{+} + 2 e^{-}$ Yeh chain reaction ek self-sustaining glow discharge (plasma) create karta hai.

Step 2: Ion bombardment → momentum transfer

Positive $Ar^{+}$ ions negative target ki taraf accelerate hote hain aur usmein jam ke lagte hain. Agar ek incoming ion kaafi momentum carry karta hai, to wo ek surface target atom ko eject kar sakta hai. Yeh ek billiard-ball process hai.

Step 3: Sputter yield threshold derive karna

Sputter yield $Y$ = atoms ejected per incident ion define karo.

Collision ko ion (mass $M_{1}$ , energy $E$ ) aur ek surface atom (mass $M_{2}$ ) ke beech ek head-on elastic hit ki tarah model karo. Elastic collision mein maximum energy transfer: $E_{transfer} = γ \frac{4 M _{1} M _{2}}{( M _{1} + M _{2} ) ^{2}} E$

$γ$ derive karo: 1-D elastic collision ke liye momentum aur kinetic energy conserve karo. Stationary $M_{2}$ se hit karte projectile $M_{1}$ ke liye, transfer hone wali energy ka fraction $γ = \frac{4 M _{1} M _{2}}{( M _{1} + M _{2} ) ^{2}}$ hai (standard result — maximize hota hai jab $M_{1} = M_{2}$ ).

Ek atom tab escape karta hai jab use mili energy surface binding energy $U_{s}$ se zyada ho. Isliye ek threshold energy hoti hai: $E_{t h} = \frac{U _{s}}{γ} = \frac{( M _{1} + M _{2} ) ^{2}}{4 M _{1} M _{2}} U_{s}$

Step 4: Transport aur condensation

Ejected atoms chamber ke paar travel karte hain. Few-mTorr pressures par mean free path thodi chhoti hoti hai, isliye atoms scatter karte hain — yeh conformal-ish lekin kuch directional coverage deta hai. Wo wafer par land karke ek film mein condense ho jaate hain.

Magnetron sputtering (practical upgrade)

Figure — Physical vapor deposition (PVD - sputtering)

DC vs RF sputtering

Worked examples

Common mistakes

Recall Feynman: ek 12-saal ke baache ko explain karo

Socho ek wall hai jo tiny magnetic balls se bani hai (woh hai target). Tum us par marbles (argon ions) bahut tez fire karte ho. Har marble ek ball ko loose knock kar deta hai, aur loose balls room ke paar float karke tumhare toy (wafer) par gently chipak jaate hain, use ek shiny layer mein coat karte hain. Tumne kuch melt nahi kiya — tumne sirf cheezein zyada hit karke knock off ki. Wall ke peeche magnets lagaane se "gun" kaafi tez fire karta rehta hai, isliye coating jaldi grow hoti hai.

Flashcards

Sputtering mein atoms ko eject karne ka physical mechanism kya hai?

Energetic ion bombardment se momentum transfer (ek collision cascade), heat nahi.

Kaun sa gas typically use hota hai aur kyun?

Argon — inert hai (target/film ke saath react nahi karega) aur itna heavy hai ki achha momentum transfer kar sake.

Sputter yield Y define karo.

Ek incident ion per target atoms ki ejected sankhya.

Elastic collision mein max energy-transfer fraction γ ka formula?

γ = 4·M₁·M₂ / (M₁+M₂)², maximize hota hai jab M₁ = M₂.

Sputtering ke liye threshold energy?

E_th = U_s / γ, jahan U_s surface binding energy hai.

Magnetron deposition rate kyun badhata hai?

Magnets electrons ko ek E×B racetrack mein trap karte hain, isliye har electron bahut zyada Ar atoms ionize karta hai → denser plasma → lower pressure/voltage par higher ion flux.

Insulating targets ko DC ki jagah RF se kyun sputter karna padta hai?

DC positive ions ko insulator surface charge karne deta hai, aur ions ko repel karke plasma quench kar deta hai; RF electrons se charge neutralize karta hai alternately.

Film quality ke liye low chamber pressure kyun prefer kiya jaata hai?

Lamba mean free path → kam scattering → energetic, directional atoms → denser, better-adhesion films.

PVD aur CVD mein key difference kya hai?

PVD: film atoms physically ek solid source se aate hain. CVD: film gaseous precursors ki chemical reaction se banti hai.

Metal interconnects ke liye PVD kyun prefer kiya jaata hai?

CVD ke liye volatile safe metal precursors scarce hain; PVD Al/Cu/TiN/Ta ke liye achha adhesion aur controllable stress deta hai.

Connections

Chemical Vapor Deposition — films ke liye chemical-reaction alternative.
Thin Film Deposition — parent umbrella topic.
Plasma Physics — glow discharge, ionization, sheath voltage.
Mean Free Path — sets karta hai kyun pressure film quality control karta hai.
Interconnect Metallization — jahan Al/Cu/TiN barriers use hote hain.
Sputter Etching — same physics, reversed intent (deposition nahi, removal).