Canadian Light Source Metrology Facility Overview:

1. SRI 2007: CLS Optical Metrology Facility- Overview Brian Yates1, Dylan Maxwell1 1 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. Facility and Instrumentation Capabilities 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. 3D surface profiler (Micromap 570) Long Trace Profilometer (Ocean Optics) Fizeau Interferometer (Zygo VeriFire AT) 3D Surface Profiler (Micromap 570) 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). 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. Long Trace Profilometer (LTP) (Ocean Optics) The Long Trace Profilometer (LTP) from Ocean Optics (shown to the far left) measures the slope and curvature of 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). 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 crossed-roller bearing to a more precise linear air bearing. A further upgrade is planned in the near future to an even flatter ceramic beam. Height profile of a “reference” spherical mirror – the edges are raised ~20 µm. collimated HeNe laser beam (l=632.8 nm, 150 mm diameter) Fizeau Interferometer (Zygo Corp.) The Zygo VeriFire AT Fizeau interferometer (shown to the far left) measures the 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. Return flat Mirror under test Transmission Flat Fizeau Interferometer 3D height profile of a transmission flat, measured in “single pass” normal incidence mode. 45° Grazing incidence measurement, in so-called “double pass” mode. Radius bench measurement/DMI Laser