Interferometer characterizes laser-textured steel

A white-light interferometer combining vertical-scanning with phase-shifting measurement techniques can profile rough surfaces such as laser-textured steel with accuracies of 3 Å rms. Laser-textured strip steel offers advantages over smooth-surfaced steel for a variety of applications. In precision metal drawing and forming applications, the uniform texture creates microscopic reservoirs to hold lubricant. In bonding applications, the increased surface area of textured steel creates tighter

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Interferometer characterizes laser-textured steel

Paul Caber

A white-light interferometer combining vertical-scanning with phase-shifting measurement techniques can profile rough surfaces such as laser-textured steel with accuracies of 3 Å rms. Laser-textured strip steel offers advantages over smooth-surfaced steel for a variety of applications. In precision metal drawing and forming applications, the uniform texture creates microscopic reservoirs to hold lubricant. In bonding applications, the increased surface area of textured steel creates tighter, stronger contacts between bonded surfaces. Laser-textured steel is also known for how well it accepts paint.

Cold Metal Products Inc. (CMP; Youngstown, OH) manufactures textured cold-rolled steel as a specialty strip product. A computer-controlled, 1.6-kW carbon dioxide laser operating continuous-wave at 10.6 µm textures the work rolls that impart the rough surface to the steel. A chopper produces pulsed output with a 40-kHz repetition rate. Each pulse applied to the work roll creates a 18-µm-diameter crater, and arrays of craters with varying geometric configurations can be produced. After inspection, the work roll is installed in the rolling mill, where it imprints strip steel with the cratered texture.

Performing metrology on such a surface is more complex than it would appear--a contact profilometer, for example, can generate misleading results. If the stylus falls between a row of craters on the uniform surface pattern of the steel, the profilometer will register a smooth surface. If the stylus traces down a row of craters, it will read a rough and uneven surface.

Optical metrology techniques offer a solution to these problems, providing a noncontact, real-time method for process control. At CMP, engineers evaluate work-roll and strip-steel surfaces with the Wyko RollScope, a portable surface-profiling interferometer designed for use on cylindrical parts, and the Wyko RST 500, the same instrument configured for laboratory use (see Fig. 1). Both instruments are designed and manufactured by Veeco Process Metrology (Tucson, AZ).

System design

The instruments are white-light interferometers that combine vertical-scanning and phase-shifting techniques to produce high-resolution measurements on rough surfaces. Depending on the interferometric objective used, the instrument can be configured as a Michelson or a Mirau interferometer (see Fig. 2). In white-light interferometry, fringe contrast, or modulation, reaches a maximum at the minimum optical-path difference (OPD) between the reference and measurement beams. For a rough surface, the minimum OPD, and hence maximum fringe modulation, varies from point to point. In the vertical-scanning systems, a piezoelectric transducer (PZT) scans the interferometric objective containing the reference mirror a distance of approximately 1000 µm, in steps on the order of 0.05 to 0.1 µm.

A linear variable differential transformer position encoder controls the motion of the PZT to minimize measurement errors introduced by nonlinearities and hysteresis. A low-noise 480 ¥ 740-pixel charge-coupled-device (CCD) array detector with 8-bit digital output captures the fringe modulation at each step. Digital-signal-processing hardware demodulates the fringe data in real time, using communications techniques. Vertical-scanning interferometry alone achieves surface height resolutions of approximately 3 nm rms. To increase system resolution to 3 Å rms, the systems incorporate phase-shifting techniques.

In conventional phase-shifting interferometers, the phase of the measurement beam is shifted relative to that of the reference beam by a known amount, and the phase of the interference fringes is evaluated. This approach provides fast, three-dimensional profiles of surfaces, but surfaces with large point-to-point height variation introduce errors into the measurement. If a white-light source is used, however, the system measures the degree of fringe modulation, or coherence, in the transmitted beam, and the process is independent of surface-height discontinuities.

To permit phase-shifting measurements, the interferometers incorporate a narrow bandpass filter in the optical path. At each major increment in vertical scan position, the system acquires six intensity frames of data. Between each of these frames, the PZT introduces a phase shift by moving the reference mirror a minor increment of l0/8, where l0 is the center wavelength of the bandpass filter. The fringe pattern captured by the CCD detector is digitized and processed to yield surface height data with resolutions of 3 Å rms.

Data displays include 2-D slicing, 3-D variable view, color contour, and height distribution. The data-analysis package calculates volume, volume/wear curve, bearing-area curve, and a complete set of roughness parameters based on the 1995 ASME B46.1 standard. A full 3-D surface analysis with the RollScope takes less than one minute. In comparison, a stylus profilometer can require five to ten minutes to measure a surface, while yielding fewer data than the optical instrument.

The RollScope using the 20X objective provides a 612 ¥ 468-mm field of view (FOV), while the 10X objective provides a minimum FOV of 1.22 ¥ 0.94 mm. Available objectives for the RST500 range from 1.5X (8.5 ¥ 6.4-mm FOV) to 50X (0.14 ¥ 0.10-mm FOV).

The combination of vertical-scanning and white-light, phase-shifting techniques has produced a high-resolution interferometer for 3-D evaluation of surfaces with roughnesses ranging from 1 Å to 20 µm rms. Hardware-based algorithms perform real-time analysis for rapid measurement. The system has applications in the manufacture of products such as magnetic recording media and rolls used in the printing industry. o

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FIGURE 1. White-light vertical-scanning, phase-shifting interferometer can measure surfaces as rough as 20 µm rms with vertical resolutions of 3 Å rms.

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FIGURE 2. Piezoelectric transducer vertically shifts objective assembly of interferometer (containing the reference mirror) to change optical path length between the reference and measurement paths and introduce appropriate phase shifts for high-resolution interferometry.

PAUL CABER is profiler products manager, Veeco Process Metrology, 2650 E. Elvira Rd., Tucson, Arizona 85706.

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