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Light Scattering Measures Subangstrom Roughness

Photonics Spectra
Sep 1999
Laurel M. Sheppard

Hard disk drive manufacturers are looking for ways to increase storage capacity on the same hard drive. One way to accomplish this is to manufacture disks with ultrasmooth surfaces, placing requirements on the hard disk of the future for a radial roughness of 1.5 Å and circumferential roughness of 0.8 Å. However, conventional surface measurement technologies are slow, require extensive operator training and have difficulty measuring roughness levels below 10 Å.

Schmitt Measurement Systems Inc. has addressed this problem by developing an advanced laser-based technology that measures the surface of each disk at levels of 0.2 to 1000 Å. The company says this method also measures radial and circumferential surface roughness simultaneously, which is not possible with conventional atomic force microscopes, interferometers or profilometers.

Schmitt has incorporated this technology into its TMS-2000RC system. Using proprietary light-scattering techniques, the system directs a Class II 670-nm laser at a disk surface to create a reflected specular beam and scattered light. The scattered light is collected by an optic over a hemisphere above the surface and across spatial frequency bands to determine the total integrated scatter. A segmenting optic receives the scattered light from the collector and segments it to isolate anisotropic from isotropic roughness. Detectors measure and detect the intensity of each segment.

More throughput

The system measures several surface roughness parameters, including optical properties such as total integrated scatter, diffuse reflectance, total reflectance and specular reflectance. It can take 100 measurements per second and has a resolution of 0.01 Å. Repeatability is 0.1 Å for the same sample and same machine, and 0.25 Å or 0.5 percent (whichever is greater) for the same sample and different machines.

The system can quadruple production throughput, according to John Ferland, product sales manager. Minimal training is required, and the system can be correlated to other measurement systems.

The method also has applications in other industries. The company is adding it to its semiconductor product line and developing a similar system for measuring the circumference of roll grinders that are precision ground for use in the brass and steel industries, Ferland said.

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