The NanoGrid Principle of Measurement
The NanoGrid metrology system is unique in a number of its features, but particularly in the high degree of
interpolation that it provides. The XY encoder, or grid, has a basic period of 10 microns in both the X and Y
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.
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 the ±1-order diffracted beams. In either
event, fringes with precisely defined spacings are
formed as shown. The fringe spacing is independent
of the 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 120°
apart in phase. These three signals are electronically
processed to provide an unambiguous measurement of
the phase, φ. 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 in the ±1 diffracted orders.
This approach makes possible shot-noise-limited phase
resolution of 1 part in 214, corresponding to a
measurement resolution of 0.3nm.