In-Process Measuring System for U1tra-Precision Diamond Turned Surface

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Several non-contact optical techniques have been investigated for the measurement of fine surface roughness. Disadvantages of these techniques are (1) systematic errors of measurements occur, (2) it is rather difficult to apply to in-process measurement and (3) it requires a long time to perform the test.A new optical method based on the Fraunhofer diffraction theory is proposed, in which it is possible to estimate quantitatvely the fine surface roughness on the order of nanometeres of the surfaces generated by ultra-precision diamond turning without disadvantages mentioned above.The results obtained in this paper are summarized as follows :(1) A conventional equation which can accurately estimate the fine diamond turned surface roughness of less than 200nmRmax by measuring the intensity values of Oth-3rd order diffractions is derived from theoretical analysis based on the Fraunhofer diffraction. The Fraunhofer diffraction roughness R corresponding to the surface roughness (Rmax) can be determined from the following equation<<numerical formula>>where I_0-I_3 are intensities of the principal maximum in order of 0-3 diffractions, lambda is light wavelength.(2) From the measurements of surface roughness of a magnetic memory disk as well as various workpieces produced by diamond turning, this proposed Fraunhofer diffraction method makes it possible to measure the surface roughness from 1nm to the maximum 200nmRmax within an error range of 10%.(3) It is possible to evaluate the texture of a magnetic memory disk surface from the distribution map of the average Fraunhofer diffraction roughness during rotation, so that this method will be applied to in-process measurement of the ultra-fine surface roughness during turning operation.(4) This method can be applied to estimate quantitatively both the depth and the width of fine scratches of less than submicron in mirror surfaces, such as a silicon wafer surface.