EE143 – Ali JaveySlide 5-1 Section 2: Lithography Jaeger Chapter 2 Litho Reader

EE143 – Ali JaveySlide 5-2 The lithographic process

EE143 – Ali JaveySlide 5-3 Photolithographic Process (a)Substrate covered with silicon dioxide barrier layer (b)Positive photoresist applied to wafer surface (c)Mask in close proximity to surface (d)Substrate following resist exposure and development (e)Substrate after etching of oxide layer (f)Oxide barrier on surface after resist removal (g)View of substrate with silicon dioxide pattern on the surface

EE143 – Ali JaveySlide 5-4 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 JaveySlide 5-5 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 JaveySlide 5-6 Lithographic Process

EE143 – Ali JaveySlide 5-7 Printing Techniques 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-on- wafer Contact printing Proximity printing Projection printing

EE143 – Ali JaveySlide 5-8 Contact Printing wafer hv photoresist Resolution R < 0.5  m mask plate is easily damaged or accumulates defects Photo Mask Plate

EE143 – Ali JaveySlide 5-9 Proximity Printing wafer hv g~20  m exposed Photoresist R is proportional to ( g ) 1/2 ~ 1  m for visible photons, much smaller for X-ray lithography

EE143 – Ali JaveySlide 5-10 Projection Printing hv lens wafer P.R. focal plane ~0.2  m resolution (deep UV photons) tradeoff: optics complicated and expensive De-Magnification: nX 10X stepper 4X stepper 1X stepper

Diffraction EE143 – Ali JaveySlide 5-11

EE143 – Ali JaveySlide 5-12 Aerial Images formed by Contact Printing, Proximity Printing and Projection Printing

EE143 – Ali JaveySlide 5-13 Photon Sources

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

EE143 – Ali JaveySlide 5-15 Optical Stepper

16 Resolution in Projection Printing Minimum separation of a star to be visible. f = focal distance d = lens diameter

EE143 – Ali JaveySlide 5-17 Resolution limits in projection printing

EE143 – Ali JaveySlide 5-18 point Depth of Focus (DOF)

EE143 – Ali JaveySlide 5-19

EE143 – Ali JaveySlide 5-20 Field Oxide Photo mask Different photo images Example of DOF problem

EE143 – Ali JaveySlide 5-21 (1) and (2) require a compromise between and NA ! Tradeoffs in projection lithography

EE143 – Ali JaveySlide 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 JaveySlide 5-23 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. Immersion Lithography

EE143 – Ali JaveySlide 5-24 Image Quality Metric: Contrast Contrast is also sometimes referred as the Modulation Transfer Function (MTF)

Questions: EE143 – Ali JaveySlide 5-25 How does contrast change as a function of feature size? How does contrast change for coherent vs. partially coherent light?

EE143 – Ali JaveySlide 5-26 * simulated aerial image of an isolated line Image Quality metric: Slope of image

EE143 – Ali JaveySlide 5-27 The need for high contrast

EE143 – Ali JaveySlide 5-28 Resists for Lithography Resists –Positive –Negative Exposure Sources –Light –Electron beams –Xray sensitive

EE143 – Ali JaveySlide 5-29 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 N 2 => Alkali Soluble Acid –Develops by “etching” - No Swelling.

EE143 – Ali JaveySlide 5-30 Positive P.R. Mechanism Photons deactivate sensitizer polymer + photosensitizer dissolve in developer solution

EE143 – Ali JaveySlide 5-31 Resist contrast Q f           log Positive Resist Q Q f Q 0

EE143 – Ali JaveySlide 5-32 Negative P.R. Mechanism hv => cross-linking => insoluble in developer solution. Q 0 Q f

EE143 – Ali JaveySlide 5-33 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 JaveySlide 5-34 Overlay Errors Alignment marks from previous masking level wafer alignment mask photomask plate

EE143 – Ali JaveySlide 5-35 run-out error wafer radius (1) Thermal Run-in/Run-out errors

EE143 – Ali JaveySlide 5-36 (2) Translational Error referrer image n+n+ Al p Rotational / Translational Errors (3) Rotational Error

EE143 – Ali JaveySlide 5-37 Overlay implications: Contacts

EE143 – Ali JaveySlide 5-38 Overlay implications: Gate edge

EE143 – Ali JaveySlide 5-39 Total Overlay Tolerance  i = std. deviation of overlay error for i th masking step  total = std. deviation for total overlay error Layout design-rule specification should be >  total

EE143 – Ali JaveySlide 5-40 Standing Waves

EE143 – Ali JaveySlide 5-41 P.R. Intensity = minimum when x d m = 0, 1, 2,... Intensity = maximum when m = 1, 3, 5,... n = refractive index of resist SiO 2 /Si substrate Standing waves in photoresists

EE143 – Ali JaveySlide 5-42 Proximity Scattering

EE143 – Ali JaveySlide 5-43 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 (imaging layer) (etch stop) (planarization layer)

EE143 – Ali JaveySlide 5-44 Electron-Beam Lithography Angstroms for V in Volts Example: 30 kV e-beam => = 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.

EE143 – Ali JaveySlide 5-45 Types of Ebeam Systems

EE143 – Ali JaveySlide 5-46 Resolution limits in e-beam lithography

EE143 – Ali JaveySlide 5-47 The Proximity Effect

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)