latter approach remedies the erosion of the non etching surface problem it requires the introduction of foreign materials which may act to contaminate the product being manufactured it is therefore an object of the present invention to provide an improved in situ rate and depth monitoring method and apparatus for monitoring the etching or deposition of material as it undergoes thickness change it is a further object of the present invention to provide an improved rate and thickness monitoring method and apparatus for monitoring the etching of material which is opaque to light it is yet a further object of the present invention to provide an improved method and apparatus for rate and depth monitoring of thickness changes in film materials with high resolution and accuracy it is yet still a further object of the present invention to provide an in situ rate and thickness monitor and method which is simple in design and easy to install and maintain it is yet another object of the present invention to provide an improved in situ method and apparatus for monitoring and measuring etch or deposition rate and changes in film thickness with improved resolution and signal to noise ratio it is yet still another object of the present invention to provide an improved in situ method and apparatus for monitoring and measuring etch or deposition rate and changes in film thickness of silicon layers.