Optical mask Comparator (LMS 2020)
PTB uses a mask metrology system LMS 2020 (manufacturer: Leica) for coordinate measurement on photomasks, which was modified according to special PTB requirements.
In this instrument the photomask is mounted on top of an x-y-stage, which moves on air bearings and is driven by friction rods. The position of the stage is measured and controlled by means of reference plane mirror interferometry. See drawing! The coordinate microscope allows simultaneous detection of line edge positions in x- and y-direction. The high-NA microscope objective has to be constantly kept in focus, which is realized by an integrated laser autofocus system measuring the z-profile of the substrate surface.
To obtain highest measurement precision, careful adjustment of the comparator as well as application of sophisticated correction procedures of systematic error contributions is necessary. The mask comparator is mounted on a vibration isolating support and housed within a separate clean room cabin which offers temperature stabilities of a few 0,01 K. The wavelength of the laser light used for diplacement interferometry in air is continously measured by means of a tracking refractometer. Wavelength comparisons in reference to an iodine stabilized HeNe-laser can be performed regularly without removing the laser head from the mask comparator.
By all these measures and accompanied by different comparison measurements, traceable calibrations of microstructure coordinates on standard 6 inch masks can be performed with uncertainties of u95 = 35 nm. More details are given in table Calibration services.