Section 2: Lithography. Jaeger Chapter 2 Litho Reader. The lithographic process

Transcription

1 Section 2: Lithography Jaeger Chapter 2 Litho Reader The lithographic process

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 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

3 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 Lithographic Process

4 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 Contact Printing hv Photo Mask Plate wafer photoresist Resolution R < 0.5μm mask plate is easily damaged or accumulates defects

5 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 Projection Printing hv lens De-Magnification: nx 10X stepper 4X stepper 1X stepper focal plane wafer P.R. ~0.2 μm resolution (deep UV photons) tradeoff: optics complicated and expensive

6 Diffraction Aerial Images formed by Contact Printing, Proximity Printing and Projection Printing

7 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 Optical Projection Printing Modules Optical System: illumination and lens Resist: exposure, post-exposure bake and dissolution Mask: transmission and diffraction Wafer Topography: scattering Alignment:

8 Optical Stepper scribe line field size increases with future ICs Image field 1 2 wafer Translational motion Resolution in Projection Printing f = focal distance d = lens diameter Minimum separation of a star to be visible.

9 Resolution limits in projection printing =n.sinθ, where n is the index of refraction Depth of Focus (DOF) point

10 Example of DOF problem Photo mask Field Oxide Δ Different photo images

11 Tradeoffs in projection lithography λ () 1 lm 06. want small lm NA λ ( 2) DOF =± want large DOF 2 2 ( NA) (1) (1) and (2) (2) require a compromise between λ and NA!! 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

12 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. Image Quality Metric: Contrast Contrast is also sometimes referred as the Modulation Transfer Function (MTF)

13 Questions: How does contrast change as a function of feature size? How does contrast change for coherent vs. partially coherent light? Image Quality metric: Slope of image * simulated aerial image of an isolated line

14 The need for high contrast Optical image Infinite contrast Finite contrast resist resist resist resist substrate substrate Position x Resists for Lithography Resists Positive Negative Exposure Sources Light Electron beams Xray sensitive

15 Negative Resist Composition: Two Resist Types Polymer (Molecular Weight (MW) ~65000) Light Sensitive Additive: Promotes Crosslinking Volatile Solvents Light breaks N-N in light sensitive additive => Crosslink Chains Sensitive, hard, Swelling during Develop Positive Resist Composition Polymer (MW~5000) Photoactive Dissolution Inhibitor (20%) Volatile Solvents Inhibitor Looses N 2 => Alkali Soluble Acid Develops by etching - No Swelling. Positive P.R. Mechanism Photons deactivate sensitizer dissolve in developer solution polymer + photosensitizer

16 Positive Resist mask hv 100% (linear scale) resist thickness remaining exposed part is removed P.R. EQ Q 1 E exposure 0 ft photon energy (log scale) Resist contrast log 10 1 Q f Q0 Negative P.R. Mechanism hv % remaining mask after development QE ft QE 01 photon energy hv => cross-linking => insoluble in developer solution.

17 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 + + Overlay Errors alignment mask wafer + + photomask plate Alignment marks from previous masking level

18 (1) 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. m m α, α si = coefficient of thermal expansion of mask & Si Rotational / Translational Errors (2) Translational Error Al image p n + (3) Rotational Error referrer

19 Overlay implications: Contacts SiO 2 SiO 2 ideal Al n + p-si SiO 2 SiO 2 Al Alignment error n + Δ short, ohmic contact p-si Solution: Design n+ region larger than contact hole Al SiO 2 SiO 2 n + 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

20 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 hv Standing Waves Higher Intensity Lower Intensity Positive Photoresist Faster Development rate Slower Development rate substrate After development Positive Photoresist. substrate

21 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 λ d m 4n m = 0, 1, 2,... = m = 1, 3, 5,... Proximity Scattering

22 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) Electron-Beam Lithography 12.3 λ = V Angstroms for V in Volts Example: 30 kv e-beam => λ = 0.07 Angstroms NA = Resolution < 1 nm But beam current needs to be 10 s of ma for a throughput of more than 10 wafers an hour.

23 Types of Ebeam Systems 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

24 The Proximity Effect

25 Richard Feynman

26

27 Dip Pen Nanolithography Dip-Pen Nanolithography: Transport of molecules to the surface via water meniscus.

28 Dip-pen Lithography, Chad Mirkin, NWU

29 Patterning of individual Xe atoms on Ni, by Eigler (IBM)