Lecture 2: Resists
Technology for Micro- and Nanostructures
Micro- and Nanotechnology
Peter Unger
mailto: peter.unger @ uni-ulm.de
Institute of Optoelectronics
University of Ulm
http://www.uni-ulm.de/opto
Copyright 2012 by Peter Unger
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 1/40
Outline of Lecture 2: Resists
Positive and Negative Resists
Requirements for Resists
One-Component Polymer Resists
Self-Developing Resists
Two-Component Novolak–Diazide Photoresist
Chemically Amplified Resist
Silylated Resists
Multilayer Resist Systems
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 2/40
Positive and Negative Resists
Resist:
Masking Material, which can be Patterned by Irradiation (and
Development)
Ultraviolett Light
Electron Beams
X-Rays
Ion Beams
Positive Resist:
Resist is Removed in the Exposed Areas
Negative Resist:
Resist is Removed in the Areas which are not Exposed
Exposure Dose D:
D [J/cm2 ] for X-Ray and UV Exposure
D [C/cm2 ] for Electron and Ion Exposure
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 3/40
Positive and Negative Resists
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 4/40
Requirements for Resists
Contrast
Resolution
Sensitivity
Resistance to Etching Processes
Adhesion
Homogeneity
Ease of Removal (Solvants, Plasma)
Topographical Coverage
Temperature Stability
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 5/40
Comparison of Photography and Lithography
Photography: Gray-Shade Image
Resolution ⇐⇒ Sensitivity
Lithography: Binary Process =⇒ High Contrast
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 6/40
One-Component Polymer Photoresists
Crosslinking Polymers:
Irradiation Causes Crosslinking of Polymer Chains
=⇒ Negative Resist
e.g. Polychloro Methylstyrene
One Crosslink per 20 eV of Absorbed Energy
Chain-Scission Polymers:
Ionizing Radiation Causes Scission of Polymer Chains
=⇒ Positive Resist
e.g. Polymethyl Methacrylate (PMMA)
One Broken Chain per 5 eV of Absorbed Energy
Developer is a mixture of:
Solvent: e.g. Methylisobutyl Ketone (MIBK)
Non-Solvent: e.g. Isopropyl Alcohol (IPA)
Solubility in Developer is a Strong Function of Molecular Weight
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 7/40
Crosslinking and Chain-Scission Polymers
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 8/40
Molecular Weight Distribution
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 9/40
Definition of Resist Contrast
Contrast of Negative Resist
Contrast of Positive Resist
γp =
1
lg(Dp0 /Dp1 )
γn =
Dose: 5 × 10−5 –5 × 10−4 C/cm2
1
lg(Dn1 /Dn0 )
Dose: 5 × 10−3 C/cm2
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 10/40
Polymethyl Metacrylate (PMMA)
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 11/40
Contrast of PMMA
E-Beam Exposure
X-Ray Exposure
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 12/40
Electron-Beam Lithography at 100 keV
Resist:
PMMA/MAA
Substrate:
Silicon
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 13/40
Normalized Resist Thickness
after Development (%)
Contrast of Commercially Available Resists
D1
100
x
x
x
x
x
x
x
x
50
x
AZ 2400
AZ 1350, AZ1450
HPR 204
ODUR 1013
PMMA
x
λ = 313 nm
λ = 248 nm
x
0
10
1
x
2
10
10
3
D0
2
Dose (mWs/cm )
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 14/40
Swelling and Collapsing of Polymers
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 15/40
20 nm Structures in PMMA by E-Beam Lithography
STEM Image
Min. Width:
20 nm
Substrate:
Si3 N4
Membrane
Resist:
PMMA
Transfer into
15 nm-thick
Ti Layer
500 nm
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 16/40
Self-Developing Resists
Nitrocellulose (UV and Electron Exposure)
C6 H7 N3 O12 −→ N2 , CO2 , H2 O
Alkali Halides and other Ionic Salts
NaCl, AlF3 , LiF(AlF3 )
Advantages:
Extremely High Resolution
Disadvantages:
Very Insensitive
Bad Resistance to Subsequent Processing
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 17/40
Self-Developing Nitrocellulose Resist
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 18/40
Self-Developing E-Beam Lithography in NaCl
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 19/40
Lithium Floride as Self-Developing E-Beam Resist
Structures with 37 nm Pitch
Gratings with 5.5, 10, 15, 20 nm
Pitch
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 20/40
Sensitivity versus Resolution for Resists
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 21/40
Two-Component Novalak–Diazide Photoresists
Base Resin (Novalak), Acidic Polymer
Inhibitor (Naphthoquinone Diazide)
5–30 % Weight
Strong UV Absorber
Undergoes Photochemical Decomposition and Bleaching
H2 O Must be Present (No Vacuum)
Developer:
Basic Solutions (Buffered KOH, Organic Base) which Etches the
Base Resin.
Functioning of the Resist:
Inhibitor Prevents Resin from Being Etched by the Developer.
Etching Takes Place where Inhibitor is Destroyed by Irradiation.
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 22/40
Chemistry of the Inhibitor Component
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 23/40
Photo-Bleaching of the Inhibitor Component
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 24/40
Standing Waves during Optical Exposure
Long Exposure Time
Short Exposure Time
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 25/40
Two-Component Novalak–Diazide Photoresists
Advantages:
More Sensitive than One-Component Resists
Bleaching (No Reflection on Wafer Surface)
Very Stable, No Swelling
Disadvantages:
Resolution is not as Good as for One-Component Resists
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 26/40
Chemically Amplified Resists (CARs)
Three Components:
Binder Matrix: Novolak Resin
Electron- or Photon-Sensitive Acid Generator
Acid Catalyzed Converter (Inhibitor)
Exposure Process:
Photons or Electrons Release Acid
R–H −→ R− + H+
Acid Causes either
(a) Hydrolysis (Positive Resist)
or
(b) Crosslinking (Negative Resist)
of the Inhibitor
Catalytic Process =⇒ One H+ can Catalyze Many Events
=⇒ Chemical Amplification
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 27/40
Functioning of a Chemically Amplified Resist
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 28/40
Chemically Amplified Resists (CARs)
Advantages:
Very Sensitive (Chemical Amplification)
Resolution ⇐⇒ Sensitivity Can be Controlled
Electron, X-Ray and UV Resist, No H2 O Needed
Disadvantages:
Requires Controlled Handling
Resists with Two Components have Better Resolution
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 29/40
Hexamethyldisilizane (HMDS)
Chemical Structure:
HMDS Works as Adhesion Promoter:
HMDS Converts the Surface from Hydrophilic to Hydrophobic.
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 30/40
Silylated Resist System
Functioning of the Silylated Resist:
Exposure Frees O–H
HMDS Diffuses into the Resist and Bonds to Exposed Sites
(Silylation Process)
Silylated Resist Forms a Mask for Reactive Ion Etching (RIE)
using Oxygen.
Advantages:
Mask is very Resistant to Dry-Etching Processes
Lithography at the Surface with Highly Absorbing Resists
e.g. Using a Dye as Absorber
Disadvantages:
Mask is not Easy to Remove
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 31/40
Process Sequence of a Silylated Resist
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 32/40
Reactive Ion Etching (RIE) System
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 33/40
Multilayer Resist Systems
Functioning of a Multlayer Resist System:
Bottom Polymer with Good Resistivity and Planarization
Anorganic Intermediate Layer (Ti, SiO2 )
Thin Electron- or Photon-Sensitive Top Resist
Pattern Transfer into the Bottom Polymer by Two Reactive Ion
Etching (RIE) Processes
Advantages:
High Resolution
Good Planarization
Surface Lithography
Good Dimensional Control
Disadvantages:
Many Process Steps (Two Vacuum Processes)
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 34/40
Trilevel Resist System
11111111 00
00000000
11 11
00111
000
00000000
11111111
00
11
11
00
000
111
Top Photoresist
00000000
11111111
00 11
11
00111
000
Intermediate Layer
11111111 00000000
00000000
11111111
Bottom Polymer
Substrate
Layer Sequence
After Photolithography
11
00
000
111
000
111
00111
11
000111
000
11111111 00000000
00000000
11111111
After CF4 RIE
After O2 RIE
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 35/40
Submicrometer Structures Using Trilevel Resist
Lithography
Pattern Transfer
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 36/40
Comparison to Single-Layer Photoresists
Single-Layer Photoresist
Multilayer Resist System
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 37/40
Stability Limit of Trilevel Resist Structures
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 38/40
Stability Limit of Trilevel Resist Structures
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 39/40
Further Reading
Marc J. Madou
Fundamentals of Microfabrication, 2nd edition
CRC Press, Boca Raton 2002
Henry I. Smith
Submicron- and nanometer-structures technology, 2nd edition
Lecture 10, Resists
NanoStructures Press, 437 Peakham Road, Sudbury, MA 01776, USA, 1994
L.F. Thompson, C.G. Willson, and M.J. Bowden
Introduction to Microlithography, 2nd edition
ACS Professional Reference Book, American Chemical Society, 1994
Peter Unger, Technology for Micro- and Nanostructures — Lecture 2: Resists, Version of October 25, 2012 – p. 40/40