Progress in understanding flatness measurementsTom Diehl November 24, 2005Draft as of 11/24/05

Micro-Epsilon Opto-NCDT 2400We need to control the flatness of the focal plane.We need to measure components without touching their surfaces.Using a DEMO version Jim Fast determined that this device could measure the distance to the surface of a CCD mounted in the test dewar through the dewars quartz window.100 micron spot diameter24 mm measurement range at 222 mm distance~1 micron resolution

Micro-Epsilon Opto-NCDT 2400

The MachineryLabview controls X and Y dimensionStages w/ ~1 micron precisionOn the scale of our devicesMicro-epsilonimagerSample

Silicon on a pedestalExample of a mm x mm scan of a part with very reflective surface. Non-flat surface features are evident.

Silicon Piece 6 cm x 3 cmGlued to aluminum nitride substrateTaped to a piece of unistrut. The device overlaps the unistrut by ~ 1 cm on each side.Surface has observable grind marks from a thinning process.

2 Imagesof Same Device mm x mm grid Offset by 0.1 mm in Y direction illustrates the problem. One can see the warp in the surface but only thatgeneral feature matches.Individual points do not.

Systematic UncertaintyThe measurements of a point on the surface hold within about 1 micron.If I move off and move back on the measurement repeats.If I move by off by 1/10 mm, the measurement is more different than what I think is the height of the typical surface feature. Spurious reflections from the grind marks trick the system?A systematic uncy is determined from 2 measurements offset by 1/10 mm.deltaZ=delta(Za-Zb)/sqrt(2)~15 microns

Gregs Device 6 cm x 3 cmAverage the measurements in cm x cm array. The total uncertainty on mean in each of the 72 regions is ~ 1 micron.13/72 regions gt 10 microns

The PlanA few more improvements to the Labview programAutomation is pretty good.Surface feature identification and concentration is nearly doneBring into the CCD testing Laboperate through a windowIncorporate results into the DES CCD databaseDevelop a flatness grade for each CCDDevelop a way to test the DES Focal plane assembly