LaserTrac Autofocus0 pages

AXSYS TECHNOLOGIES

LASLRIRAC AUTOFOCUS

The LaserTrac AutoFocus was designed and has been applied to the most demanding applications. Inspection systems processing semiconductor wafers, read-write heads, hard disks,and even razor blades benefit from the use of this unique instrument. In accomplishing these tasks, the LaserTrac provides increased (in some cases 300%) throughput and greatly reduced operator fatigue. Whether used for automated or manual based inspection, LaserTrac ensures the target image remains in focus over the entire inspection area. The design of the LaserTrac AutoFocus introduces a focused light source (750-780nm) into the optical path. A dichroic mirror reflects the infrared laser down to the inspection surface. The mirror is highly reflective in the infrared and highly transmissive in the visible light range. Therefore, the microscope image is not degraded by the introduction of the mirror and the laser energy is almost entirely blocked. Laser light returning from the inspection surface is directed by the mirror into the AutoFocus module. There, it is imaged onto two photodetectors. Internal electronics compare the differential intensity. By monitoring this reflected beam, the LaserTrac AutoFocus is able to disseminate above or below focus conditions and generate an error signal to continuously keep the system in focus. This allows for fast and repeatable focus even when the sample is being moved.

The speed (or convergence) of the system to an "in focus" condition is affected by many factors. The more fundamental is the bandwidth of the focusing mechanism. Through the use of tractionwheels, direct drive systems, Piezo electric elements, and voice coil type actuators, Axsys has been able to obtain bandwidths of 10 to greater than 200Hz, depending on the application. With the bandwidth range available, the LaserTrac AutoFocus can be integrated into numerous inspection applications.

The use of the LaserTrac AutoFocus has been instrumental in providing OEM's with the ability to increase the throughput of their products and processes.

The AutoFocus was designed specifically to project a line image onto the object being studied. The line has a distinct advantage of integrating the focus information over the topology. This line integration reduces hunting and is far less confused by high-relief wafers. Spot-based systems lack this important capability.