1. Field of the Invention
The present invention relates to interference in polarized light and to etching of optics and more specifically, it relates to the use of polarized light interference for real time measurements of thickness changes in optics being etched.
2. Description of Related Art
It is desirable to improve on the accuracy of techniques for etching structures into SiO2 to a desired depth by monitoring the etch depth in real time. It has been observed that the SiO2 etch rate in Buffered HF vary from 14 to 17 nm per minute. For a 384 nm etch depth, this can become an 82 nm etch depth uncertainty. It is desirable to reduce this uncertainty to less than 10 nm.
Previously, the present inventors have etched to approximately 80% of the desired depth, removed the mask in the region of interest, measured the current etch depth and recalculated an expected end point at the current etch rate. The SiO2 etch was then completed and its depth was measured at a different inspection point, as was done for the 80% point, and then again to confirm the expected etch depth result. This proved to be a time-consuming and labor-intensive process requiring extensive handling of the part. It is desirable to eliminate these steps and etch to a consistent, repeatable endpoint by monitoring the etch depth in real time.
All currently available step height measuring instruments require dry first surface access. It is preferable that a real time monitor be provided that can view the surface being etched from the back side. No instrument maker has done this to date. Also, the reflectivities of the surfaces being etched are not large due to the effective index-matching of water and optical surfaces.
An instrument is desired that will reduced the work required to insure SiO2 etch depths are done accurately to within +/xe2x88x925 nm, while improving process yield and reducing fluid consumption and the concomitant hazardous waste disposal costs. Slow degradation of the etchant solution as it is consumed is not of consequence if the etch rate is monitored, and thus the etchant remains viable for many cycles and waste disposal costs are significantly lowered.
It is an object of the present invention to provide an optical monitoring instrument designed to monitor etch depth and etch rate for controlling a wet-etching process.
It is another object of the invention to provide optical monitoring instrument designed to monitor etch depth and etch rate including means for viewing through the back side of a thick optic onto a nearly index-matched interface.
Still another object of the invention provides a method for use of linear translation of a Wollaston prism for phase stepping in a real time optical monitoring instrument.
A phase stepping interferometric microscope, with a Wollaston prism that provides image shearing, monitors etch depths in real time during fabrication of etched structures in optical materials. Linear motion of a Wollaston prism for phase stepping, optical baffling, and proper surface preparation are keys to this instruments success. The interface to monitor is nearly index matched and presents a challenge due to unwanted reflections and scattered light.
This unique optical monitoring instrument is designed to monitor etch depth and etch rate for controlling a wet-etching process. The instrument provides means for viewing through the back side of a thick optic onto a nearly index-matched interface. Optical baffling and the application of a photoresist mask minimize spurious reflections to allow for monitoring with extremely weak signals. Phase unwrapping occurs while etching proceeds to provide a smooth measurement of actual etch depth.
The optical monitor must be useable on a machine that will vibrate and move. To minimize sensitivity to vibrations, a common path monolithic interferometer construction, using the same surface as signal and reference, is chosen as a starting point. The design features of the present invention can be realized with a shearing interferometer. Polarization based interferometers use crystal birefringence to shear the image. The relative intensity between the two images can be adjusted by aligning linear polarizers relative to the optical axis of the crystal.