Scanning ion beam etch (SIBE) process have been used in fabrication of large-aperture diffractive optical elements (DOEs). We have demonstrated an on-line etch depth detection method based on fringes of equal thickness of a wedge plate during SIBE process. In the detection system, a piece of transparent wedge plate illuminated by extended He-Ne laser is used as sensitive element of etch depth, and half of the element is protected by a thin graphite plate when it is crossing through the ion beam. Then the relative movement of the fringes of equal thickness can be used to calculate the etch depth.

The experimental results show that the depths detected by this on-line system correspond very well with those measured by a surface profiler after etch process, and the difference between the two kinds of results is less than 10 nanometers. Several phase DOEs, such as phase Ronchi gratings, were successfully fabricated with this method. This on-line etch depth detection method can be reliably used in end-point determination for the SIBE process of transmission DOEs.

The following figures show (a) the schematic diagram of scanning etching and on-line diffractive intensity measurement system, and (b) The .

ion beam                                                       Sample                                          Wedge plate

(a) Schematic diagram of scanning etching and on-line measurement system.

He-Ne laser                                       d     　                                        h                                                                         d

(b)

　

For more information on this on-line interferometric measurement method, please see: