Measuring Microholes by Measuring Shadows

Hank Hogan

Measuring microscopic holes may benefit from a light touch, according to researchers from the National Institutes of Standards and Technology (NIST) in Gaithersburg, Md., and the University of North Carolina at Charlotte. They have devised a method that quantifies hole diameters with three times the accuracy of other methods and that can be used with holes that are up to 5 mm deep but only 100 µm wide. They achieve this performance by tracking the flexing of an optical fiber.

A new technique measures the interior dimension of microscale holes, such as this optical ferrule, with an uncertainty of 35 nm at depths of up to 5 mm. Courtesy of National Institute of Standards and Technology.

Bala Muralikrishnan, a guest researcher at NIST from the university, noted that, in a variety of applications, the geometry of 1- to 10-mm-deep microscopic holes is desired. Fiber optic ferrules, injection syringes, ink-jet cartridges, wire dies and fuel injection nozzles have millimeter-deep holes that measure microns across. Gauging such holes is challenging and must be performed with little force.

Commercial products have appeared recently that measure microholes with an uncertainty of some 100 to 500 nm -- too large for the standards work performed at NIST. Moreover, the commercially available methods do not work well, or at all, for deep microholes. To address these problems, the researchers came up with their own approach, which Jack A. Stone, a NIST physicist, said grew out of some pioneering efforts at the German equivalent of the agency.

In their work, the NIST-led group bonded a 75-µm-diameter microsphere onto the tip of a 50-µm-diameter, 20-mm-long fiber. The researchers then fired two lasers, one for the X-coordinates and another for the Y-coordinates, through the fiber approximately 5 mm below the tip. They tracked the fiber's location using standard cameras and optics.

"The fiber actually acts as a cylindrical lens to focus the light passing through it to a line," Stone explained. "The bright line stands out in sharp contrast to the surrounding shadow."

Positioning the fiber directly below a hole to be probed, the researchers pushed the microsphere into the hole and moved the ball around so that it touched the hole's sides while the fiber did not. By tracking the fiber deflection, they could measure the hole's dimensions to an accuracy of 35 nm at depths of up to 5 mm.

Planned improvements include adding another light source, which will enable three-dimensional measurements. At present, Stone said, the technique will not be commercialized. Instead, it will be used for internal calibration purposes at NIST.

The group reported on the work in May at the Society of Manufacturing Engineers' MicroManufacturing Conference that took place in Minneapolis.