CCD_Rasnik: Principle of the coding of the mask.
A mask is generated by stacking position coded basic building blocks.
We will call such a block a B3. The horizontal and vertical codes give
the position of the basic building block in the total mask. Counting starts
at 0,0 in the lower left corner of the mask. A startbit is used to make
recognition and decoding easier. It is the basepoint of the building block.
The ones in the binary position code invert the location color. The startbit
is also inverted in respect to the basic chessboard. The code for the horizontal
position is in the vertical codelines and visa versa.The size of one spot
(field in the chessboard) is in the order of 100 microns, which covers
roughly 10 times 10 CCD camera pixels.
Coding example
(click to enlarge)
Example of a coded mask, with B3's of 11 by 9.
The codebits of the example above.
These
are roughly 10 times 10 B3's. The vertical code is 10 bits long and the
horizontal code 8 bits. So theoretically one can stack 1024 B3's vertically
and 256 B3's horizontally.
Glass mask
The actual mask is a metal on glas mask, as used for integrated circuit
production (or a contact copy of such a mask). The anti-reflective coated
side should be pointing to the projecting lens.These masks are produced
by commercial firms, taking Gerber files as input. Postscript files, plotted
on high resolution machines, turned out to be disappointing. Since postscript
interpreters work with floats, rather then integers, it is very hard to
define the data in such way that rounding off is done in the proper way.Also
tests were performed with gray code locations instead of inverting them.
In practice gray has to be defined as fine lines of black and white and
the results are unpredictable.
go back to the code page
august 1995