TEST & MEASUREMENT

Paul Pulaski, Daniel R. Neal, and T. D. Raymond

Sur face-measurement
stitching technique scales
to all sizes of optics
A novel surface-measurement technique uses a reflective Shack-Hartmann
wavefront sensor to scan optical surfaces and then uses software to stitch
individual measurements together, resulting in high-resolution accuracy and high
Tdynamic range with the ability to scale up or down to any size optic.
he competing requirements of accuracy, ^{Fiber-coupled}
spatial resolution, dynamic range, and ^{light source}

Collimating lens large field of view limit many optical measurement techniques. To address ^Telescope these limitations, WaveFront Sciences has developed a technique that scans and stitches together sub- ^Beamsplitter aperture data to enable surface topology measurements with Shack-Hartmann high resolution, accuracy, and dynamic range, along with wavefront sensor a large field of view determined by the scan range. Using a

Shack-Hartmann wavefront sensor that measures the re- ^{Wafer surface} _{Wafer chuck}
flective optical phase from a light beam incident on the sur-
face, the optical phase can be directly related to the surface-
height information that is mapped in two dimensions across _{FIGURE 1. A Shack-Hartmann wavefront sensor is the basis}
the surface. After scanning the surface—either by a lateral for a noninterferometric surface-metrology instrument that
scan of the reflective wavefront sensor system across the measures semiconductor wafers. A fiber-coupled 635-nm
surface or by keeping the sensor fixed and laterally translat- laser-diode light source is collimated with a lens and directed
ing the surface relative to the sensor—individual measure- onto the silicon wafer surface. The wafer is mounted on a wa-
_{ments are stitched together by sophisticated soft ware algo-} fer chuck attached to a two-axis translation stage that can
_{rithms that scale over a range of dimensions from 100-mm} position the wafer so the surface can be analyzed one patch
_{wafers up to mirrors larger than 1 m in diameter.} at a time. The phase of the reflected light is imprinted with
the height variations on the wafer surface. Reflected light is
^{As an alternative to interferometric techniques for sur-} directed via a beamsplitter and imaged by a simple 1:1 relay-
^{face topology measurements, the Shack-Hartmann wave-} imaging telescope onto a Shack-Hartmann wavefront sensor
front sensor does not require any vibration isolation and _{for analysis of the phase, and thus, the wafer surface.}
alignment is fairly straightforward. A metrology device
to measure the nanotopography of silicon wafer surfaces wafer, the Shack-Hartmann sensor measures the slope
uses this wavefront sensor to optically probe the wafer directly, permitting high-speed measurement via the
surface and nondestructively measure topography and stitching technique.
site flatness with measurement sensitivity comparable to
interferometric techniques (see Fig. 1).¹ Unlike an inter- The Shack-Hartmann wavefront sensor
ferometer, which requires a long settling time for the The basic elements of the Shack-Hartmann sensor are a
microlens array, a mounting housing, and a megapixel
digital CCD camera.² The microlens array is produced
on a fused-silica substrate using gray-scale photo-
lithography and plasma etching.³ Each lenslet in the

Translation stage

PAUL PULASKI is optical engineer, DANIEL R. NEAL is technical director, and T. D. RAYMOND is research scientist at WaveFront Sciences, 14810 Central Ave. SE, Albuquerque, NM 87123-3932; e-mail: pdpulaski@wavefrontsciences.com.

Laser Focus World www.laserfocusworld.com October 2005 S11