Section 2: Lithography Jaeger Chapter 2 EE143 Ali Javey Slide 5-1
The lithographic process EE143 Ali Javey Slide 5-2
Photolithographic Process (a) (b) (c) (d) (e) (f) (g) Substrate covered with silicon dioxide barrier layer Positive photoresist applied to wafer surface Mask in close proximity to surface Substrate following resist exposure and development Substrate after etching of oxide layer Oxide barrier on surface after resist removal View of substrate with silicon dioxide pattern on the surface EE143 Ali Javey Slide 5-3
Photomasks - CAD Layout Composite drawing of the masks for a simple integrated circuit using a four-mask process Drawn with computer layout system Complex state-of-the-art CMOS processes may use 25 masks or more EE143 Ali Javey Slide 5-4
Photo Masks Example of 10X reticle for the metal mask - this particular mask is ten times final size (10 μm minimum feature size - huge!) Used in step-and-repeat operation One mask for each lithography level in process EE143 Ali Javey Slide 5-5
Lithographic Process EE143 Ali Javey Slide 5-6
Printing Techniques Contact printing Proximity printing Projection printing Contact printing damages the mask and the wafer and limits the number of times the mask can be used Proximity printing eliminates damage Projection printing can operate in reduction mode with direct step-onwafer, eliminating the need for the reduction step presented earlier EE143 Ali Javey Slide 5-7
Contact Printing hv Photo Mask Plate wafer photoresist Resolution R < 0.5μm mask plate is easily damaged or accumulates defects EE143 Ali Javey Slide 5-8
Proximity Printing hv g~20μm Photoresist wafer exposed R is proportional to ( λ g ) 1/2 ~ 1μm for visible photons, much smaller for X-ray lithography EE143 Ali Javey Slide 5-9
Projection Printing hv lens De-Magnification: nx 10X stepper 4X stepper 1X stepper focal plane P.R. wafer ~0.2 μm resolution (deep UV photons) tradeoff: optics complicated and expensive EE143 Ali Javey Slide 5-10
Aerial Images formed by Contact Printing, Proximity Printing and Projection Printing EE143 Ali Javey Slide 5-11
Optical Stepper scribe line field size increases with future ICs Image field 1 2 wafer Translational motion EE143 Ali Javey Slide 5-12
Excimer Laser Stepper Light is in pulses of 20 ns duration at a repetition rate of a few khz. About 50 pulses are used. EE143 Ali Javey Slide 5-13
Photon Sources Hg Arc lamps 436(G-line), 405(H-line), 365(I-line) nm Excimer lasers: KrF (248nm) and ArF (193nm) Laser pulsed plasma (13nm, EUV) Source Monitoring Filters can be used to limit exposure wavelengths Intensity uniformity has to be better than several % over the collection area Needs spectral exposure meter for routine calibration due to aging EE143 Ali Javey Slide 5-14
Resolution limits in projection printing EE143 Ali Javey Slide 5-15
Resolution limits: Bragg condition EE143 Ali Javey Slide 5-16
Image Degradation by lens Mask + 2 φ +1 0 lens wafer plane parallel optical beam L P - 2-1 grating with spatial frequency 1/P L θ P=2L P sin φ = nλ n = 0, ± 1, ± 2,... sin θ = NA of lens EE143 Ali Javey Slide 5-17
Mask Intensity The λ/na limit I max The lens has to collect at least the n=1 diffracted beams to show any spatially varying intensity on wafer. Therefore P m ( = 2 l m ) λ/ NA O Image on wafer optical system x x EE143 Ali Javey Slide 5-18
Resolution and the need for higher order terms n=0 n=0 + n=1 n=0 + n=1 + n=3 n=0 + n=1 + n=3 + n=5 EE143 Ali Javey Slide 5-19
Depth of Focus (DOF) off point best EE143 Ali Javey Slide 5-20
Example of DOF problem Photo mask Field Oxide Δ Different photo images EE143 Ali Javey Slide 5-21
Tradeoffs in projection lithography () 1 l 06. ( 2) DOF =± l m m λ NA 2 want small l λ NA ( ) 2 m want large DOF (1) (1) and (2) (2) require a compromise between λ and NA!! EE143 Ali Javey Slide 5-22
Sub-resolution exposure: Phase Shifting Masks Pattern transfer of two closely spaced lines (a) Conventional mask technology - lines not resolved (b) Lines can be resolved with phase-shift technology EE143 Ali Javey Slide 5-23
Immersion Lithography A liquid with index of refraction n>1 is introduced between the imaging optics and the wafer. Advantages 1) Resolution is improved proportionately to n. For water, the index of refraction at λ = 193 nm is 1.44, improving the resolution significantly, from 90 to 64 nm. 2) Increased depth of focus at larger features, even those that are printable with dry lithography. EE143 Ali Javey Slide 5-24
Image Quality Metric: Contrast EE143 Ali Javey Slide 5-25
Image Quality metric: Slope of image * simulated aerial image of an isolated line EE143 Ali Javey Slide 5-26
The need for high contrast Optical image Infinite contrast Finite contrast resist resist resist resist substrate substrate Position x EE143 Ali Javey Slide 5-27
Resists for Lithography Resists Positive Negative Exposure Sources Light Electron beams Xray sensitive EE143 Ali Javey Slide 5-28
Negative Resist Two Resist Types 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 N 2 => Alkali Soluble Acid Develops by etching - No Swelling. EE143 Ali Javey Slide 5-29
Positive P.R. Mechanism Photons deactivate sensitizer less cross-linking dissolve in developer solution polymer + photosensitizer EE143 Ali Javey Slide 5-30
Positive Resist mask hv 100% (linear scale) resist thickness remaining exposed part is removed E T = resist sensitivity Resist contrast P.R. log 10 1 ΕΤ Ε 1 E 1 E T exposure photon energy (log scale) EE143 Ali Javey Slide 5-31
Negative P.R. Mechanism hv % remaining mask after development E T E 1 photon energy γ 1 1 log E E T hv => cross-linking => insoluble in developer solution. EE143 Ali Javey Slide 5-32
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 EE143 Ali Javey Slide 5-33
+ + Overlay Errors alignment mask wafer + + photomask plate Alignment marks from previous masking level EE143 Ali Javey Slide 5-34
Thermal Run-in/Run-out errors R= r ( ΔT ) m α ΔT α m si si run-out error wafer radius ΔT, ΔTsi = change of mask and wafer temp. α m m, α si = coefficient of thermal expansion of mask & Si EE143 Ali Javey Slide 5-35
Rotational / Translational Errors (2) Translational Error Al image p n + (3) Rotational Error referrer EE143 Ali Javey Slide 5-36
Overlay implications: Contacts SiO 2 SiO 2 Al n + p-si ideal Alignment error SiO 2 SiO 2 Al n + p-si short, ohmic contact Δ Solution: Design n+ region larger than contact hole Al SiO 2 SiO 2 n + EE143 Ali Javey Slide 5-37
Overlay implications: Gate edge Ideal Fox S/D implant n + Electrical short With alignment error poly-gate Solution: Make poly gate longer to overlap the FOX EE143 Ali Javey Slide 5-38
Total Overlay Tolerance σ σ 2 2 total = i i σ i = std. deviation of overlay error for i th masking step σ total = std. deviation for total overlay error Layout design-rule specification should be > σ total EE143 Ali Javey Slide 5-39
hv Standing Waves Higher Intensity Lower Intensity Faster Development rate Slower Development rate Positive Photoresist substrate After development Positive Photoresist. substrate EE143 Ali Javey Slide 5-40
Standing waves in photoresists x P.R. d Intensity = minimum when Intensity = maximum when λ x = d m 2n x n = refractive index of resist SiO 2 /Si substrate m = 0, 1, 2,... λ = d m m = 1, 3, 5,... 4n EE143 Ali Javey Slide 5-41
Proximity Scattering EE143 Ali Javey Slide 5-42
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 2: thin develop-stop layer, used for pattern transfer to 3 3: thick layer of hardened resist (planarization layer) (imaging layer) (etch stop) EE143 Ali Javey Slide 5-43
Electron-Beam Lithography 12.3 λ = V Angstroms for V in Volts Example: 30 kv e-beam => λ = 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. EE143 Ali Javey Slide 5-44
Types of Ebeam Systems EE143 Ali Javey Slide 5-45
Resolution limits in e-beam lithography resolution factors beam quality ( ~1 nm) secondary electrons ( lateral range: few nm) performance records organic resist PMMA ~ 7 nm inorganic resist, b.v. AlF 3 ~ 1-2 nm EE143 Ali Javey Slide 5-46
The Proximity Effect EE143 Ali Javey Slide 5-47