Advanced Photoresist Technology Jie Sun EE 518 Instructor

Advanced Photoresist Technology Jie Sun EE 518 Instructor: Dr. Jerzy Ruzyllo Apr. 4 2006

Presentation outline Ø Introduction of Photoresists Ø Roadmap of Photoresist Technology Ø Photoresists Solution for Submicron lithography Ø Summary

Introduction of Photoresists Type: Ø Positive: exposed regions dissolve (best resolution) Ø Negative: Unexposed regions dissolve ( Swelling) Photoresists Structure: Resin: a binder that provides mechanical properties (adhesion, chemical resistance) Ø Solvent: used to dissolve the resin, allowing the resin to be applied in a liquid state Ø 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. Ø * 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) Ø Dominant for G-line (436 nm) and I- line (365 nm) exposure and not suitable for very short wavelength exposures Ø Novolac (N): a polymer whose monomer is an aromatic ring with two methyl groups and an OH group. Ø dissolves in an aqueous solution easily Ø Diazoquinone(DQ) Ø 20 -50 % weight Ø Photosensitive Ø DQ UV Carboxylic acid ( dissolution enhancer) Ø *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 (p. H >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 Ø pattern precisely retained. Novolac fairly resistant chemical attack, a good mask for the subsequent plasma etching Ø

Performance of Photoresists Ø Resolution (um) - linearity/ minimum Sensitivity (m. J/cm 2) Ø Focus margin (um) Ø Exposure margin (%) Ø Dry etch resistance(X) Ø Heat resistance Ø Adhesion Ø Standing wave effect (and bulk effect)(um) Ø BARC (bottom anti-reflective coating) compatibility Ø Process margin/stability Ø Photoresists Profile Shelf-life Ø * 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 250 nm, Kr. F (248 nm) marginally acceptable but not Ar. F (193 nm) Photoresist Solution for Submicron Features PMMA Ø PAGs ( Photoacid generator) replace PAC Ø Contrast enhancement layers (CELs) Ø Inorganic resist (Ag-doped Ge-Se) Ø Silicon-containing resists (dry developable) Ø Multi-layer photoresist Ø

PMMA (Ploymethyl methacrylate) Ø Short-wavelength lithography: deep UV, extreme UV, electron-beam lithography Ø Resin itself is photosensitive Advantage: high resolution Disadvantage: Ø Plasma etch tolerance is very low and thick PMMA to protect the thin film Ø Dissociation changes chemistry of the plasma etch and polymeric deposits on the surface of the substrate. Ø 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) Ø CEM photo-bleachable Ø Spun onto the DQN PR after softbake Ø Formed in-situ “conformal contact mask” Ø Enhanced contrast Ø Important for DUV resists with less optical intense and PR radiation absorbtion * http: //www. microsi. com/photolithography/data_sheets/CEM%20365 i. S%20 Data%20 Sheet%

Inorganic Resist Advantage: High contrast γ ≈ 7 Produce fine line Process: Ag-doped Ge-Se Ø Ag plated on sputtered Ge-Se Ø Photodoping create Ag 2 Se after exposure Ø Dissolved in alkaline solution Disadvantage: Ø Require thick planarizing underlayer due to thin film nature Ø Pineholes and defects from Ge-Se *Stephen A. Campbell, “ The Science and Engineering of Microelectronic Fabrication”.

Dry developable: Polysilynes Ø Bi-layer process Ø Silicon-containing resists on top of novolac based resist Ø Highly resistant to plasma process Ø Bleaching under DUV exposure due to cross-linked siloxane network Ø 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 Ø Tri-layer process Ø Thin layer PR + Si. O 2 + thicker planarizing Ø Oxide layer act as hard mask Ø 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 Ø PR chemistry structure changed with wavelength of light source Ø Several PR solutions for DUV application Ø Multi-layer PR replace the single layer for Sub-100 nm features Ø