1. Automated Characterization of Optical Image Quality
SFR Workshop
May 24, 2001
Andrew R. Neureuther, Garth Robins (SRC/DARPA),
Kostas Adam
Berkeley, CA
2001 GOAL: Complete initial simulations and experiments sufficient to design polarization masks and multi-parameter test structures 9/30/2001.
5/24/2001

2. Motivation
Lithography at low k1, high NA and with phase-shifting masks is adversely affected by small residual aberrations in lenses.
Test targets are needed:
As ‘canaries’ that are more sensitive than product features
To quantify individual aberrations to 0.01 waves rms
The introduction of polarizing features allows an as yet an untapped degree of freedom for lithography.
A combination of theoretical image analysis, simulation and experimentation can be used to gain insight and understanding to assure small feature reproducibility.
Combining automatic (optical and/or SEM) wafer inspection with simulation interpretation gives leverage.
5/24/2001

3. Programmed-Probe Based Aberration Targets: Concept
0.5 l/NA
The 0.4 l/NA by 0.4 l/NA programmed 180o phase defect provides an interferometric like reference electric field with magnitude 0.43 and phase of 180o compared to the clear area field.
0.4 l/NA 180o Sq. Defect
0.6 l/NA Line
Sidelobe
Coma Sidelobe
-0.1 wave => 0.40
0.0 wave => 0.30
0.1 wave => 0.21
Detect Sidelobe by over exposing 2.5x and using automatic wafer inspection
5/24/2001

4. Programmed-Probe Based Aberration Targets: Layout
Dark Field Patterns
Grid is 0.1 l/NA
Probe
Astigmatism
Spherical
1 at 90o
Trefoil
Coma
1 at 0o
1 at 180o
5/24/2001

5. Evolution of the Program-Probe Targets
Improvement
Sensitivity = Δ I / Δ l
Target
0.1λ Coma(cos)
-0.74
-0.93
1.74
0.1λ Coma(sin)
-0.08
-0.07
-0.05
0.1λ Trefoil(cos)
-0.47
-0.72
-0.20
0.1λ Trefoil(sin)
-0.10
-0.22
5/24/2001

6. First Demonstration of Separation of Spherical and Defocus90
180
5/24/2001

7. NEW Theoretical Foundation for PPBAT
Approach
Represent target diffraction as sum of Zernike’s in pupil
Approx. exp[jOPD] as 1 + jOPD
Result
Orthogonality allows each aberration to be excited alone
Target is the Fourier Transform of that particular Zernike
Implications
Diameter of target goes inversely with radial order
Phase variation on target is similar to Airy function
The center is a natural location for the probe because the weighted area of Zernike is zero and gives a null in the center.
5/24/2001

8. Tolerance Assessment of PPBAT
Mask Dimensions
Probe size – High 10% –Calibrate via isolated probe => 1%
Feature Linewidth –Low – 0.8%
Alignment
Probe to feature – High 4% or with raster scan Low 1%
Phase to feature – Moderate 1.5%
Phase Process
Phase – Low 0.25%
Etch Bias – Moderate 1.5% – Fix: use flavors => 0.5%
Profile shape – Low 1% (can be checked with TEMPEST)
Ga stain – Low 0.1%
Use to monitor mask quality?
RSS Combined ~ 4.8% => 2.5%
5/24/2001

9. 2002 and 2003 Goals Aberration Targets Electrical Test
Measure with CD SEM
Electrical Test
Measure with Electroglass
Haolin Zhang and Costas Spanos
Conduct and quantitatively interpret polarization masks, and multi-parameter test structures, by 9/30/2002.
Define apparatus, specify testing procedures, and interpret data for polarization masks, and multi-parameter test structures, by 9/30/2003.
5/24/2001