Thin Film Metrology Using Wavefront Thin Film Metrology Using - - PowerPoint PPT Presentation

Photon 04, September 2004

Thin Film Metrology Using Wavefront Thin Film Metrology Using Wavefront Sensing. Sensing.

D M Faichnie, A H Greenaway, I Bain*

Physics, Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, Scotland, EH14 4AS *Scalar Technologies Ltd, 9 Cochrane Square, Brucefield Industrial Park, Livingston, Scotland, EH54 9DR

Photon 04, September 2004

Acknowledgements. Acknowledgements.

OMAM Collaborators. Funding Institutions.

Photon 04, September 2004

Overview Of Presentation. Overview Of Presentation.

• What is Adaptive Optics and how can we use it for metrology.
• Project aims.
• Analysis of measurement and experimental set-up.
• Results To Date and System Calibration.
• Future Work and sensor design.
• Conclusions.

Photon 04, September 2004

Typical AO Vs Metrology. Typical AO Vs Metrology.

AO System. Metrology Using AO

Wavefront Sensor and Science Camera Feedback Control System L i g h t F r

• m

O b j e c t Sample Under Test

Photon 04, September 2004

Project Aims. Project Aims.

• Produce robust industrial thickness monitor.
• Sensor must be able to be used in-line for control feedback.
• Sensor should be able to measure coloured films.
• Sensor should be able to cope with rough sample surfaces.
• Sensor should be able to be used with multiple layer laminates.
• Accuracy dependent on application, could be as low as 10nm.

Photon 04, September 2004

Analysis Of Current Measurement. Analysis Of Current Measurement.

Lens Detector Plane Lens Film, n1 Air, n0 Substrate, n2 Virtual Source 1 Virtual Source 2 Point Source Input

Photon 04, September 2004

Experimental Set Experimental Set-

• Up.

Up.

Variable ND Filter Single Mode Fibre Sample HeNe Laser Microscope Objective Translation Stage Variable Aperture Point Source Input Lens 1 Lens 2 CCD Array

Photon 04, September 2004

Results To Date. Results To Date.

Photon 04, September 2004

System Calibration. System Calibration.

• Our optical system requires calibration before we can relate

pixel separation to actual thickness.

• Separation will change with angle of illumination, magnification
• f optical system, thickness, refractive index and surface tilt.
• Number of options :
• Absolute calibration with prism + model of refraction.
• Relative calibration with calibrated sample set.
• Calibration with system model.
• Choice will be dependent on application.

Photon 04, September 2004

Example Of Relative Calibration. Example Of Relative Calibration.

• Results from four samples ranging from 250 to 500 microns.
• Lets make 500 micron sample our calibration sample.

Actual thickness 480 microns, use pixel separation to get calibration value to gain measure of pixels/micron for system.

Sample Nominal Thickness (µm) Measured Thickness (µm) 450 Micron 426 +/- 5 µm 423.6 +/- 7.7 µm 350 Micron 378 +/- 4.47 µm 377.3 +/- 6.3 µm 250 Micron 234 +/- 5.47 µm 235.8 +/- 5.48 µm

Photon 04, September 2004

Theory Of Future Measurement. Theory Of Future Measurement.

• Phase Diversity Wavefront sensor used to measure aberrations

introduced by the film structure.

• Addition of diffractive optical element to current set-up.
• Use of generalised phase diversity to optimise DOE design

for thickness measurement.

• Design sensor to be most sensitive to aberrations expected.
• Tilt, Defocus, Astigmatism, Coma, Spherical, Trefoil

Photon 04, September 2004

Future Sensor Design. Future Sensor Design.

Pigtailed Laser Diode CCD Point Source Input DOE Control Signal Output On Board Programmable Electronics

• Small compact sensor design
• Can be mounted on industrial scanner

S a m p l e

Photon 04, September 2004

Future Work. Future Work.

• Model wavefront sensor system using plane wave

decomposition.

• Use model results to design appropriate DOE for sensor.
• Incorporate DOE into current system and test.
• Investigate application to curved and textured surfaces.
• Investigate simultaneous measurement of thickness and

refractive index.

Photon 04, September 2004

Conclusions. Conclusions.

• Current measurement can be used for films 8mm to 12 microns.
• Technique should be able to work down to below 3 microns.
• Experimentally shown linear relationship between virtual source

position and thickness.

• Shown that relative calibration approach can be accurate.
• Shown that sensor should be compact and will be able to

be used in-line in industrial processes.