Conoscopic holography measures submicron distances
Andrew Wilson
As semiconductor-manufacturing tolerances continue to tighten and minimum feature dimensions shrink, process-control systems for the in-line monitoring of product dimensional consistency and quality control need to provide at least an order of magnitude better precision. Unfortunately, standard interferometers are limited to measuring distances from optically flat and specularly reflecting surfaces and to absolute distance measurements of more than 1 mm. In contrast, conoscopic holographic-based probes can measure diffuse reflecting surfaces, do not require an optically rigid platform, and are adaptable for operation at measurement scales covering several orders of magnitude.
"In conoscopic holography, a ray from an incoherent monochromatic point-light source falls on a birefringent crystal," says Freddy Paz, vice president of marketing and operations at Optimet (Peabody, MA). "Within the crystal, the incident ray is divided into two separate rays (ordinary and extraordinary) that propagate at different velocities along practically identical geometrical paths. The two rays are characterized by separate wavefronts that emerge from the crystal with distinct relative phase and cross-polarization angles," he adds. Polarizing plates align the directions of the electrical field of the rays, which are then divided and recombined to create an interference fringe pattern at the output.
"As in classical interference, the fringe spacing is proportional to the distance from the point of reflection," says Paz. To take advantage of conoscopic holography, Optimet has developed a point probe that registers the precise range of more than 4000 depth points with 10-mm accuracy. The probe operates at a mean working distance of 100 mm and a data-acquisition rate of 1000 points/s. Working distance and depth of field can be adjusted by substituting a single lens on the front face of the instrument, according to Paz.
Optimet has also developed a variety of preproduction prototype devices, including rangefinders, surface profilers, and roughness gauges. "Our general-purpose conoscopic probe is specified with 10-mm accuracy and 40-mm working range," says Paz. "And we believe the device should match the requirements of many OEMs as a noncontact alternative to conventional mechanical distance probes. For example, used as a scanning surface profiler, the collinear configuration of the conoscopic system faithfully maps complex topologies with gradients to 85, such as for tiny screw holes. And, with special front-lens configurations, a standoff of more than 3 m can be obtained.
Optimet is also readying a conoscopic microscope probe for surface-inspection applications. Designed to fit the camera end of a standard unmodified optical microscope, the probe`s precision and depth of field are adjustable by rotating the microscope`s objective barrel. According to Paz, the nominal distance measurement accuracy for a 20X objective is 50 nm over a 100-mm working range.
