1.1 Silicon Crystal Structure - University of California ...

1.1 Silicon Crystal Structure - University of California ...

Section 2: Lithography Jaeger Chapter 2 Litho Reader EE143 Ali Javey Slide 5-1 The lithographic process EE143 Ali Javey Slide 5-2

Photolithographic Process (a) (b) (c) (d) (e) (f) (g) EE143 Ali Javey 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 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 m 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 EE143 Ali Javey

Slide 5-7 Contact Printing hv Photo Mask Plate photoresist wafer Resolution R < 0.5m mask plate is easily damaged

or accumulates defects EE143 Ali Javey Slide 5-8 Proximity Printing hv Photoresist g~20m wafer exposed

R is proportional to ( g ) 1/2 ~ 1m for visible photons, much smaller for X-ray lithography EE143 Ali Javey Slide 5-9 Projection Printing hv De-Magnification: nX lens

10X stepper 4X stepper 1X stepper focal plane P.R. wafer ~0.2 m resolution (deep UV photons) tradeoff: optics complicated and expensive EE143 Ali Javey Slide 5-10 Diffraction

EE143 Ali Javey Slide 5-11 Aerial Images formed by Contact Printing, Proximity Printing and Projection Printing EE143 Ali Javey Slide 5-12 Photon Sources

EE143 Ali Javey Slide 5-13 Optical Projection Printing Modules Optical System: illumination and lens Resist: exposure, post-exposure bake and dissolution Mask: transmission and diffraction Wafer Topography: scattering

Alignment: 14 Optical Stepper field size increases with future ICs scribe line 1 2 wafer

Image field Translational motion EE143 Ali Javey Slide 5-15 Resolution in Projection Printing f = focal distance d = lens diameter

Minimum separation of a star to be visible. 16 Resolution limits in projection printing EE143 Ali Javey Slide 5-17 Depth of Focus (DOF)

point EE143 Ali Javey Slide 5-18 EE143 Ali Javey Slide 5-19 Example of DOF problem Photo mask

Field Oxide Different photo images EE143 Ali Javey Slide 5-20 Tradeoffs in projection lithography (1) lm 0.6

NA want small lm ( 2) DOF 2 2 NA want large DOF (1) (1)and and(2)

(2)require requireaacompromise compromisebetween between and and NA NA!! EE143 Ali Javey Slide 5-21 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 5-22 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 = 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 5-23 Image Quality Metric: Contrast Contrast is also sometimes referred as the Modulation Transfer Function (MTF) EE143 Ali Javey Slide 5-24 Questions: How does contrast change as a function of feature size?

How does contrast change for coherent vs. partially coherent light? 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 resist resist Finite contrast resist

substrate resist 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 Two Resist Types Negative Resist Composition: 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 N2 => Alkali Soluble Acid Develops by etching - No Swelling.

EE143 Ali Javey Slide 5-29 Positive P.R. Mechanism Photons deactivate sensitizer dissolve in developer solution polymer + photosensitizer

EE143 Ali Javey Slide 5-30 Positive Resist hv mask exposed part is removed 100% (linear

scale) P.R. Resist contrast resist thickness remaining Q E 10 Q E

fT exposure photon energy (log scale) Qf log Q EE143 Ali Javey

Slide 5-31 Negative P.R. Mechanism hv % remaining mask after development QET f

Q E1 0 photon energy 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 (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 EE143 Ali Javey

Slide 5-35 Rotational / Translational Errors (2) Translational Error image Al n+ p (3) Rotational Error EE143 Ali Javey

referrer Slide 5-36 Overlay implications: Contacts Al SiO2 ideal SiO2 n+ p-Si

Al SiO2 Alignment error SiO2 n+ short, ohmic contact p-Si Solution: Design n+ region larger than contact hole

Al SiO2 SiO2 n+ EE143 Ali Javey Slide 5-37 Overlay implications: Gate edge Ideal Fox

S/D implant Electrical n+ short With alignment error poly-gate Solution: Make poly gate longer to overlap the FOX EE143 Ali Javey

Slide 5-38 Total Overlay Tolerance 2 total i 2

i i = std. deviation of overlay error for ith masking step total = std. deviation for total overlay error Layout design-rule specification should be > total EE143 Ali Javey Slide 5-39 Standing Waves hv Higher Intensity

Faster Development rate Lower Intensity Slower Development rate Positive Photoresist substrate Positive Photoresist.

After development substrate EE143 Ali Javey Slide 5-40 Standing waves in photoresists x d P.R.

SiO2/Si substrate x d m 2n m = 0, 1, 2,... Intensity = maximum when x d m 4n m = 1, 3, 5,...

Intensity = minimum when n = refractive index of resist 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 (imaging layer) 2: thin develop-stop layer, used for pattern transfer to 3 (etch stop) 3: thick layer of hardened resist (planarization layer) EE143 Ali Javey Slide 5-43 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 10s 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 EE143 Ali Javey

Slide 5-46 The Proximity Effect EE143 Ali Javey Slide 5-47 Richard Feynman Dip Pen Nanolithography Dip-Pen Nanolithography: Transport of molecules to the surface via water meniscus.

Dip-pen Lithography, Chad Mirkin, NWU Patterning of individual Xe atoms on Ni, by Eigler (IBM)

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