Bade scale pe reticle banana aasaan hota hai — reticle pe size d ka ek defect wafer pe d/4 ho jaata hai, isliye mask ki tolerances 4× relax ho jaati hain.
Lekin iske liye price chukani padti hai: wafer pe printed field har dimension mein 4× chhota hota hai (isliye area mein 16× chhota), toh tumhe wafer pe bahut zyada baar step karna padta hai.
Hum Rayleigh resolution ko first principles se derive karte hain.
Step 1 — Diffraction sets a minimum angle.
Pitch p (line + space) ka ek grating light ko diffract karta hai. First diffraction order angle θ par aata hai jo is equation se milta hai:
psinθ=λ
Why this step? Yeh grating equation hai wave interference se: first order ki constructive interference ke liye path difference exactly ek wavelength λ chahiye.
Step 2 — The lens must capture that order.
Pattern ko reconstruct karne ke liye lens ko kam se kam 0th aur 1st diffraction orders gather karne chahiye. Lens maximum half-angle θmax tak ke rays accept kar sakta hai. Numerical aperture define karo:
NA=nsinθmax
jahan n last lens aur wafer ke beech ke medium ka refractive index hai (n=1 air mein; n≈1.44immersion lithography mein paani ke liye).
Why this step? NA quantify karta hai "lens diffracted light ka kitna wide cone collect kar sakta hai." Bada cone = zyada higher orders capture = sharper image.
Step 3 — Combine.
Sabse chhoti resolvable pitch ke liye sinθ≤sinθmax chahiye, yaani λ/p≤NA (n=1 ke saath), jisse minimum pitch pmin=λ/NA milti hai. Sabse chhota feature (half-pitch / critical dimension) ideal case mein iska aadha hota hai, aur ek process-dependent factor k1 real-world losses (partial coherence, resist, imperfect capture) ko account karta hai:
WHY the tension: CD shrink karne ke liye NA badhao — lekin DOF 1/NA2 ki tarah girta hai, isliye high-NA systems mein focus tolerance razor-thin ho jaati hai. Chhota λ CD aur shrink karta hai aur DOF ke liye bhi gentle hota hai, yahi wajah hai ki industry ne march kiya 436nm→365→248→193nm→ EUV 13.5nm.
Mask koi bhi patterned quartz/chrome plate hai jo light ko stencil karta hai; ek reticle ek field/die contain karta hai aur demagnification ke saath wafer par step-and-repeat hota hai.
Projection systems 4× demagnification kyun use karte hain?
Bade reticle features write/inspect karna aasaan hota hai aur mask defects wafer pe 4× shrink ho jaate hain; cost hai chhota printed field → zyada stepping.
Rayleigh resolution formula batao aur har term define karo.
psinθ=λ aur lens sinθmax=NA/n tak capture karta hai; isliye pmin=λ/NA, half-pitch = CD.
26×33 mm reticle field 4× demag ke saath kaunsa wafer field print karta hai, aur 300 mm wafer pe kitne fit hote hain?
Wafer field = 6.5×8.25 mm ≈ 53.6 mm²; ~1300 exposures per 300 mm wafer (area ~70 686 mm²).
Recall Feynman: explain to a 12-year-old
Imagine karo tumhare paas ek tiny drawing hai jo tum chip pe stamp karna chahte ho, baal se bhi bahut chhoti. Itna chhota draw karna possible nahi, isliye tum ise ek glass plate pe bada draw karte ho, phir usme se light shine karte ho aur ek magnifying lens ulta use karte ho taaki shadow chip pe shrink ho jaye. Chip pe ek special goo lagi hai jo light hit karne par harden hoti hai. Problem: light wobbly hoti hai (yeh spread out hoti hai — yahi diffraction hai), isliye agar do lines bahut paas hain to shadows smear ho jaati hain. Smear fix karne ke liye ya tum "bluer" light use karte ho jisme chhoti wave ho, ya ek moti lens use karte ho jo spreading light zyada catch kare. Bas yahi poora game hai: drawing shrink karo, light pakdo, blur ko harao.