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SRI 2007: CLS Optical Metrology Facility- Overview
Brian
1
1
Yates ,
Dylan
1
Maxwell
Canadian Light Source Inc., 101 Perimeter Road, Saskatoon, SK S7N 0X4, Canada
Canadian Light Source Metrology Facility Overview:
The CLS Optical Metrology Facility performs beamline ray-tracing/modeling and determines the quality of the optical components that are installed in the CLS beamlines. This requires measurement of a
variety of mirror parameters, to ensure compliance by the vendor to CLS specifications. Typical mirror parameters are surface roughness, calibration of radius of curvature, and figure slope error.
Measurements from three distinct instruments are typically taken to characterize the quality of optical components.
The three instruments are the Ocean Optics Long Trace Profilometer (LTP), the Zygo Verifire AT Fizeau Interferometer (FI), and the Micromap 570 Interferometric Microscope (IM). The LTP directly
measures the slope along a single line on the optical surface. The radius of curvature and slope error is then calculated from this slope profile, which has a spatial period range of approximately 2 mm to
the length of the surface. Typically three tangential and three sagittal slope profiles are acquired for a complete measurement of an optical surface. The FI measures the height map of the optical surface
component. The radius of curvature and slope error can then be calculated from this surface height map, which has a spatial period range of approximately 1 mm to the length and width of the surface.
One measurement is sufficient to characterize the entire surface, but typically at least three measurements are acquired for error analysis purposes. The IM measures the height of the optical surface
over an approximately one millimetre square field-of-view. The surface roughness can then be calculated from this surface height map, which has a spatial period range of approximately 1µm to the
length and width of the field of view. For extremely smooth surfaces, three measurements are required at each sample location, and several locations must be sampled on the optical surface in order to
be fully characterized. Therefore, a single optical component that is fully characterized in the Optical Metrology Facility produces a significant number of data sets, in different formats, requiring analysis
and summarization.
Capabilities
Facility and Instrumentation
1. Class 10,000 Clean Room (± 0.1 degree Celsius temperature stability)
- All metrology instruments mounted on vibration isolation tables (TMC)
to minimize the effects of vibration.
2. 3D surface profiler (IM) from Micromap - Measures surface roughness.
3. Long Trace Profilometer (LTP) from Ocean Optics
- Measures figure “slope error” and radius of curvature.
4. Fizeau Interferometer (FI) from Zygo
- Measures figure “slope error” and radius of curvature using radius
bench measurement and distance measurement interferometer (DMI).
The role of the optical metrology facility is to determine the figure and finish of mirror, crystal and
diffraction grating surfaces, to ensure proper spot sizes at the beamline experimental stations. Figure,
finish and mirror coating determines the photon flux and photon energy range delivered to the
beamline experimental station.
The optical metrology facility instruments are used to determine actual height and slope profiles of
these optical mirror surfaces that are used in x-ray, visible light, and infrared synchrotron beamlines.
The instrument measurement ranges often overlap, giving greater confidence in each of the
measurements.
Removal of the figure curvature (e.g. spherical, cylindrical) from the surface height map or slope map
yields the slope error. A large slope error will result in an undesirable experimental focus, where
photon flux, brightness and spot size have been compromised.
Instrument
3D surface profiler (Micromap 570)
Long Trace Profilometer (Ocean Optics)
Fizeau Interferometer (Zygo VeriFire AT)
3D Surface Profiler (Micromap 570)
3D surface profile
of a smooth
mirror, taken using
a 10X objective
with the Micromap
570 in “smooth
phase” mode,
using filtered white
light (550 nm with
a 25 nm filter
bandwidth). The
image represents
approximately 0.5
x 0.5 mm2 of the
mirror.
Horizontal Spatial Period
Range
Vertical Dynamic
Range
Approximate Measurement Limit
3D Surface Profiler
(Micromap 570)
(0.0002 – 1.25) mm
(0.010 – 150000) nm
0.03 nm RMS Height
Long Trace Profilometer
(LTP) (Ocean Optics)
(2 – 1500) mm
(0.014 – 10000) µrad
10 nm RMS Height
0.45 µrad RMS Slope Error
Fizeau Interferometer
(Zygo Verifire AT)
(0.039 – 150) mm (normal)
(0.39 - 1500) mm (grazing)
(5 – 1000) nm
20 nm RMS Height
0.45 µrad RMS Slope Error
The Micromap 570 3D surface profiler (shown to the far left) is a microscope-based instrument that utilizes visible light
interferometry to measure the surface finish (i.e. surface roughness) of reflective optical surfaces. A CCD detector mounted
on the microscope is used to measure an area (on the order of ~ 1 x 1 mm2, which is dependent on the microscope objective
used) with a height resolution of approximately 0.03 nm.
Four different objectives are currently available: 5X, 10X, 20X, and 50X, which enable us to cover a wide range of spatial
periods (0.0002 - 1.25) mm or spatial frequencies (0.0004 - 2.5) µm-1.
Three measurement modes are available in software – smooth phase mode (interferometry phase measurement), wave
mode (white light interferometry phase scan) and focus mode (interferometry focus scan). Smooth phase mode is ideal for
very smooth surfaces (0.02 – 100) nm Sq roughness, but is limited to a maximum height measurement of 4 µm (focus depth).
Wave mode is ideal for smooth surfaces with heights and steps <5 µm. Focus mode is ideal for rough surface textures and
real surface heights >5 mm (maximum height measurement is 150 µm).
Long Trace Profilometer (LTP) from Ocean Optics (shown to the far left) measures the slope and curvature of
Long Trace Profilometer (LTP) (Ocean Optics) The
mirror surfaces along one dimension using a zero-path difference interferometer. A solid state HeNe laser source is
used to probe the mirror under test. The CLS LTP can measure optical surfaces up to 1.5 meters in length, covering
the range of spatial periods from (2 – 1500) mm or spatial frequencies of (0.00033 – 0.25) mm-1.
Surfaces of virtually any shape can be measured in situ, as long as the surface slope change is within the ± 5
milliradian acceptance angle of the LTP optical system, with a sensitivity of 0.1 µrad and 10 nm in height. Dr. Peter
Takacs and staff developed the LTP at Brookhaven National Laboratory (BNL).
Height profile of a “reference” spherical
mirror – the edges are raised ~20 µm.
The instrument is mounted on a vibration isolation table to minimize the effects from external vibrations, and
shrouded in a plastic curtain normally while measurements are taken (not shown in photograph to the far left), in
order to reduce air turbulence effects on the optical head. The bearing has recently been upgraded from a crossedroller bearing to a more precise linear air bearing. A further upgrade is planned in the near future to an even flatter
ceramic beam.
collimated HeNe laser beam (l=632.8 nm, 150 mm diameter)
The Zygo VeriFire AT Fizeau interferometer (shown to the far left) measures the
Return flat
Mirror under test
Fizeau Interferometer (Zygo Corp.)
height deviation of an optic under test from a reference surface. It accomplishes
this using phase shifting interferometry. The figure error of optical surfaces up to
150 mm in diameter at normal incidence, or larger optics at grazing incidence
angle (<1500 mm), can be measured with this instrument.
A picture of a grazing incidence measurement, in double pass mode is shown to
the left (middle position). A collimated HeNe laser beam (l=632.8 nm, 150 mm
diameter) is output from the Fizeau Interferometer through a reference
transmission flat (left), reflected off the mirror under test (right) and then reflected
back with a return flat (top right) to the detector in the interferometer. Double pass
refers to the fact that the collimated beam is reflected twice off the test mirror.
Transmission Flat
Fizeau Interferometer
Radius bench measurement/DMI Laser
Our Operating
Funding Partners
45° Grazing incidence measurement,
in so-called “double pass” mode.
3D height profile of a transmission flat,
measured in “single pass” normal incidence
mode.
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