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Disclosed is a structure for a mask for X-ray Lithography which has areas transparent to visible light for mask wafer registration using visible light.
English (United States)
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X-Ray Lithography Mask With Visible Light Transmitting Areas
Disclosed is a structure for a mask for X-ray Lithography which has areas
transparent to visible light for mask wafer registration using visible light.
A silicon wafer 1 shown in cross section in Fig. 1 is coated with a silicon
dioxide coating 2 to make a boron diffusion mask. The front surface of the wafer
3 has oxide dots 4 laid down in the positions 7 where fiducial markers are
desired. After boron is diffused approximately 3 microns deep into the silicon to
form a doped silicon layer 9, and the oxide stripped off, a thin layer of silicon
germanium boron alloy 8 is grown by low temperature ultra high vacuum
chemical vapor deposition on surface 3 and the wafer is etched in an anisotropic
etch such as KOH or pyrocatechol. Low temperature, ultra high vacuum CVD
has been shown to grow boron doped silicon with etch rate ratios over undoped
silicon of millions to one, and the low temperatures used ensure that the boron
will not diffuse and change the doping profile attained by the previous boron
The mask 5 is delineated on the surface 3 of the wafer in precise relation to
the alignment marks 6 aligned with areas 7 defined by the oxide dots 4 of the
preceding steps to give the structure sketched in Fig. 2. The thin area carrying
the fiducial marks can be lined up with normal optical means.
The thickness of the silicon germanium boron layer 8 can be controlled
exactly to have a thickness such...