Optical Proximity Correction Mask Wafer *Auxiliary features added on mask 1
Overlay Errors + + alignment mask wafer + + photomask plate Alignment marks from previous masking level 2
(1) Thermal run-in/run-out errors R r Tm m Tsi si run-out wafer error radius Tm, Tsi change of mask and wafer temp. m, si coefficient of thermal expansion of mask & Si 3
run-out (2) Translational Error Al n+ p image referrer 4
(3) Rotational Error 5
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Characterization of Overlay Errors T O + y x R L O + + O C + O O + R O =optical image + =alignment marks on wafer wafer B 7
Example m x y T 0.0 0.7 R 0.7 1.0 C 0.5 0.5 L 0.3 0.0 B 1.0 0.3 * Center of wafer has only translation error Terror = (0.5, 0.5) After subtracting Terror, 8
T -0.5 0.2 x y T R 0.2 0.5 C 0 0 B 0.5-0.2 y x 0.5 R L L -0.2-0.5 0.2 B wafer 9
Run out error 0.2 m Rotational error 0.5 m [counter clockwise] If wafer diameter is 4" D 10 cm Rotational error 5 10 radians y R 10
Optical image of mask alignment marks Reference marks on wafer With thermal run-out, the alignment error is 1/2 of the image/reference difference [best scenario] 11
Total Overlay Tolerance 2 i total 2 i th masking step = std. deviation of overlay error for i i total = std. deviation for total overlay error Layout design-rule specification should be > total 12
Example: Contact to source/drain of MOSFET. SiO2 SiO2 Al ideal n+ p-si SiO2 Alignment error between oxide opening and n+ pattern SiO2 n+ Al short, ohmic contact p-si 13
Solution: Design n+ region larger than contact hole Al SiO2 SiO2 n+ 14
Two Resist Types Negative Resist Polymer (Molecular Weight (MW) ~65000) Light Sensitive Additive Promotes Crosslinking Volatile Solvents Light breaks N-N => Crosslink Chains Sensitive, hard, Swelling during Develop Positive Resist Polymer (MW~5000) Photoactive Inhibitor (20%) Volatile Solvents Inhibitor Looses N2 => Alkali Soluble Acid Develops by etching - No Swelling. 15
Positive Resist hv mask exposed part is removed 100% (linear scale) E1 P.R. ET = resist sensitivity Resist contrast LOG TO BASE 10 log resist thickness remaining ET exposure photon energy (log scale) ~ 5 to 10 Note: In the 143 Reader, is defined as natural log 16
Positive P.R. Mechanism Photons deactivate sensitizer less cross-linking dissolve in developer solution polymer + photosensitizer 17
diazide Positive Resist Exposure Reaction ketene - a = C = 0 group The ketene is shortlived intermediate PAC carboxylic acid moisture The carboxylic acid can react with the alkaline solution (the soluble ester developer) to form a soluble ester. 18
Chemical Amplified Resist (CAR) For reference only Photo-Acid generator 19 N Cheung EE243 s2010 L
Negative P.R. Mechanism hv % remaining mask after development ET 1 E1 log ET E1 photon energy hv => cross-linking => insoluble in developer solution. Log to base 10 20
Why High-Contrast Resist is desirable? Optical image Infinite contrast resist resist substrate Finite contrast resist resist substrate Position x 21
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Positive vs. Negative Photoresists Positive P.R.: higher resolution aqueous-based solvents less sensitive Negative P.R.: more sensitive => higher exposure throughput relatively tolerant of developing conditions better chemical resistance => better mask material less expensive lower resolution organic-based solvents 23
Standing Waves *Photoresist has a finite thickness hv Higher Intensity Faster Development rate Lower Intensity Slower Development rate Positive Photoresist substrate Positive Photoresist. After development substrate 24
Standing reflecting wave effect surface Air Photoresist E E E Resist profile and energy deposition depend on oxide thickness underneath (see handout for derivation) 4 3 E 1 Oxide 2 x=0 x=d r x=d T 1 I23 (x) = (E (x)+e3(x))2dt T 2 I23 (max) 0 1 = (E2 - E3)2 + 2E2E3sin2[k(d-x)] 2 1 1 I23 (max) = (E2 + E3)2 ; I 23(min) = (E2 - E3)2 2 2 I23 (min) x 2 n In ten sity m in im a occu r at : (d -x) = 0,, 2,... 2 n In ten sity m a x im a occu r a t : (d -x ) = /2, 3, 5,... d 25
x P.R. d SiO2/Si substrate Intensity = minimum when x d m Intensity = maximum when x d m n = refractive index of resist 2n 4n m = 0, 1, 2,... m = 1, 3, 5,... 26
Simulated Resist Cross-section as function of development time 27
Proximity Scattering 28
Approaches for Reducing Substrate Effects Use absorption dyes in photoresist Use anti-reflection coating (ARC) Use multi-layer resist process 1: thin planar layer for high-resolution imaging (imaging layer) 2: thin develop-stop layer, used for pattern transfer to 3 (etch stop) 3: thick layer of hardened resist (planarization layer) 29
Electron-Beam Lithography 12.3 V Angstroms for V in Volts Example: 30 kv e-beam => = 0.07 Angstroms NA = 0.002 0.005 Resolution < 1 nm But beam current needs to be 10 s of ma for a throughput of more than 10 wafers an hour. 30
Low Throughput for both raster and vector scanning (Serial Process) Variable Beam-shape EBL Stencil Mask EBL 31
The Proximity Effect Monte Carlo simulation of electron trajectories 32
e-beam lithography resolution factors beam quality ( ~1 nm) secondary electrons ( lateral range: few nm) performance records organic resist PMMA ~ inorganic resist, b.v. AlF3 ~ 7 nm 1-2 nm 33
Immersion Lithography A liquid with index of refraction n>1 is introduced between the imaging optics and the wafer. liquid = air /n With water, the index of refraction at = 193 nm is 1.44, improving the resolution significantly. 34
Phase-Shifting Mask For resolution enhancement. Example shown is an alternating PSM
EUV Lithography =11.2 nm 36
Schematic for EUV Litho Mo-Si Reflective Mask reflectivity 37 N Cheung EE243S05 Lec
Nanoimprinting 38 N Cheung EE243S05 Lec
Why photolithography? High throughput Empirical : Resolution (in Å) ~ 23 Areal Throughput (in um2/hr) 0.2 39 N Cheung EE243S05 Lec
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Hands On Exploration of Images http://cuervo.eecs.berkeley.edu/ A web browser-based simulator of lithography 41