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Advanced Photoresist Technology
Jie Sun
EE 518
Instructor: Dr. Jerzy Ruzyllo
Apr. 4 2006
Presentation outline

Introduction of Photoresists

Roadmap of Photoresist Technology

Photoresists Solution for Submicron
lithography

Summary
Introduction of Photoresists
Photoresists Type:
 Positive: exposed regions dissolve (best resolution)
 Negative: Unexposed regions dissolve ( Swelling)
Photoresists Structure:
Resin: a binder that provides mechanical properties (adhesion, chemical resistance)
 Solvent: used to dissolve the resin, allowing the resin to be applied in a liquid
state
 Photoactive Compound (PAC): Act to inhibit or promote the dissolution of the
resin in the developer. PAC inhibits dissolution in positive resists before light
exposure. After exposure the PAC promotes dissolution of the resin.

* George Tech, “Photoresists and Non-optical Lithography”
Photoresists Chemistry (1)
Positive Photoresist
Two-component DQN resists:
DQN, corresponding to the photo-active compound, diazoquinone (DQ) and resin,
novolac (N)
 Dominant for G-line (436nm) and I- line (365nm) exposure and not suitable for very
short wavelength exposures
 Novolac (N): a polymer whose monomer is an aromatic ring with two methyl groups
and an OH group.

dissolves in an aqueous solution easily


Diazoquinone(DQ)
 20-50 % weight
 Photosensitive
 DQ
UV
Carboxylic acid ( dissolution enhancer)
*Stephen A. Campbell, “ The Science and Engineering of Microelectronic Fabrication”.
Photoresists Chemistry (2)
Positive Photoresist
1.
DQ molecule will not dissolve in a base developer
solution (pH >7).
2.
UV light breaks the nitrogen molecule off forming
an unstable molecule
3.
To “stabilize” itself, one of the 6 carbon atoms in the
ring pops out of the ring (leaving 5)
4.
Once exposed to water (a developer /water mixture),
an OH group attaches to the carbon atom, forming
an acid.
5.
The acid can then react and dissolve with the basic
developer solution.
Advantage:

Unexposed areas unchanged by the presence of the developer, line width and shape of a
pattern precisely retained.

Novolac fairly resistant chemical attack, a good mask for the subsequent plasma etching
Performance of Photoresists

Resolution (um) - linearity/ minimum

Sensitivity (mJ/cm2)

Focus margin (um)

Exposure margin (%)

Dry etch resistance(X)

Heat resistance

Adhesion

Standing wave effect (and bulk effect)(um)

BARC (bottom anti-reflective coating) compatibility

Process margin/stability

Shelf-life
Photoresists Profile
* Han Ku Cho, Samsung Electronics Co., Ltd, “Lithography technology review of what it is and what to be”, March 2003
Roadmap of Photoresists Technology
* Han Ku Cho, Samsung Electronics Co., Ltd, “Lithography technology review of what it is and what to be”, March 2003
Deep UV Photoresist
Limitation of Novolac based Photoresist
Strongly absorb below 250nm, KrF (248nm) marginally acceptable but not ArF
(193nm)
Photoresist Solution for Submicron Features

PMMA

PAGs ( Photoacid generator) replace PAC

Contrast enhancement layers (CELs)

Inorganic resist (Ag-doped Ge-Se)

Silicon-containing resists (dry developable)

Multi-layer photoresist
PMMA (Ploymethyl methacrylate)

Short-wavelength lithography: deep UV, extreme UV, electron-beam lithography

Resin itself is photosensitive
Advantage: high resolution
Disadvantage:

Plasma etch tolerance is very low and thick PMMA to protect the thin film

Dissociation changes chemistry of the plasma etch and polymeric deposits on the
surface of the substrate.

Low sensitivity: Add PAG (chemically reactive dissociating) or elevate exposure
temperature
*Stephen A. Campbell, “ The Science and Engineering of Microelectronic Fabrication”.
Contrast enhance layers (CELs)

CEM photo-bleachable

Spun onto the DQN PR after softbake

Formed in-situ “conformal contact mask”

Enhanced contrast

Important for DUV resists with less optical intense and PR radiation absorbtion
* http://www.microsi.com/photolithography/data_sheets/CEM%20365iS%20Data%20Sheet%
Inorganic Resist
Advantage:
High contrast γ ≈ 7
Produce fine line
Process: Ag-doped Ge-Se

Ag plated on sputtered Ge-Se

Photodoping create Ag2Se after exposure

Dissolved in alkaline solution
Disadvantage:

Require thick planarizing underlayer due to
thin film nature

Pineholes and defects from Ge-Se
*Stephen A. Campbell, “ The Science and Engineering of Microelectronic Fabrication”.
Dry developable: Polysilynes

Bi-layer process

Silicon-containing resists on top of novolac based resist

Highly resistant to plasma process

Bleaching under DUV exposure due to cross-linked siloxane network

Etch silicon selectively to silicon dioxide in HBr plasma
* Roderick R. Kunz, et al, “193 nm Resists and Lithography”, Polymers for Advanced Technologies, Volume 5, p p.12-21
Multi-layer Resists and Hard mask

Tri-layer process

Thin layer PR + SiO2 +
thicker planarizing

Oxide layer act as hard mask

Oxide layer: Dry etching
resistant layer
*E.Ong and E.L.Hu, “Multilayer Resists for Fine Line Optical Lithography,” Solid State Technol.
Process Comparison for SLR, BLR and MLR
* Han Ku Cho, Samsung Electronics Co., Ltd, “Lithography technology review of what it is and what to be”, March 2003
Summary

Photoresists technology: Basic and key technology in lithography

PR chemistry structure changed with wavelength of light source

Several PR solutions for DUV application

Multi-layer PR replace the single layer for Sub-100nm features