NanoGrid and NanoScale are high precision encoder products that can
exceed the performance of laser interferometers within a typical
manufacturing environment for lower cost. Unique in a number of
features, most particularly in the high degree of interpolation
provided, both work on the same principle of measurement: NanoScale in
one axis, NanoGrid in two orthogonal axes.
A key NanoGrid design
innovation is the simultaneous measurement of motion in two separate
orthogonal axes using a grid pattern illuminated by a single
laser. The XY encoder, or grid, has a basic period of 10 microns
in both directions, and the metrology system generates a measurement
period of 5 microns. The NanoGrid sensor and associated
electronics provide either 8 or 14 bits of interpolation, corresponding
to measurement resolutions of either 19.53nm or 0.305nm.
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Principle of Measurement
1. Fringe Formation
The optical layout to the right illustrates the optical system used to
form interference fringes from the separate horizontal & vertical
rulings of the grid. The fringes can be understood either as
spatially filtered images of the grating, or as interference fringes
between ±1-order diffracted beams. In either event, fringes with
precisely defined spacings are formed as shown. The fringe
spacing is independent of laser diode wavelength.
2. Fringe Phase Measurement
Each of the two fringe patterns described above is imaged onto a
90-element triple detector array. These detector arrays generate
signals which can be processed to make very accurate phase
measurements, as illustrated to the right (only a portion of the
detector array is shown). When the grid encoder moves relative to
the sensor head, the fringes move across the detector arrays,
generating R, S and T signals which are 120º apart in phase, Φ.
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| Technical Background - Positioning Metrology |
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Three signals are needed because of the three variables (I, J and Φ)
which define an interferometric measurement. Processing signals
in this manner makes the resulting measurements independent of the
laser power, the reflectivity of the grid, and the relative intensities
of the ±1 diffracted orders.
This approach makes possible shot-noise-limited phase resolution of 1
part in 2e14, corresponding to a measurement resolution of 0.3nm.
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