Sub-angstrom surface metrology with a virtual reference interferometer

Non-contact, 3D optical interferometric profilers provide detailed topography measurements of super-smooth surfaces such as hard disk substrates and super-polished optics. However, the contribution of the interferometer system to the measurement can be significant for surfaces with an RMS roughness of one Angstrom and below. Special care must be taken to minimize random noise as well as to remove the systematic instrument error from the measured data. While the random noise can be addressed by low-noise design and averaging of measurements, the systematic instrument error is more difficult to eliminate. In this paper an interferometer configuration is presented that eliminates the mid to higher spatial frequencies from the reference beam. This configuration is called a virtual-reference interferometer, since there is no physical surface in focus conjugate to the test surface. This provides very smooth systematic instrument errors with essentially no contribution in the mid to high spatial frequencies of surface waviness and roughness. The virtual-reference interferometer has a midsize measurement area of 20x20 mm, is fully compensated for white light, extended source illumination, and enables data acquisition for both phase shifting and coherence scanning modes. Current performance data show a residual systematic tool waviness error of < 0.2 Angstrom RMS, with potential for improvement. Efficient stitching of subaperture measurements accommodates high resolution roughness and waviness maps of test surfaces up to 150 mm x100 mm.