Optically aligning an object with a reference mark

1,134,373. Optical alignment apparatus. INTERNATIONAL BUSINESS MACHINES CORP. July 20, 1967 [July 21, 1966], No.33299/67. Heading G2J. Apparatus for optically aligning an object such as a semi-conductor wafer 1 with a reference mark 6 comprises an optical deflecting system 15, 16 which transmits a beam of radiation from a reflecting reference mark 5 on wafer 1 towards mark 6, control devices 18, 19 causing deflecting systems 15, 16 to scan the beam over the plane 17 containing mark 6 in accordance with a predetermined raster, a photo-cell 20 interrupting the scanning operation when the beam is incident on mark 6. Printed circuit patterns 22 are to be projected successively on to the photo-sensitized surface of wafer 1, and the wafer is first roughly aligned with the projection axis by screws 3, 4. The optical axis is then finally adjusted relative to the wafer using a subsidiary monochromatic light source 8 to which the wafer surface is insensitive, and a light switch 7. Light polarized at 11 passes via beam-splitter 12, lenses 13, 4 and devices 7, 15, 16 on to the area containing mark 5 and is reflected back through the system on to reference mark plane 17. When alignment of the projection axis is achieved, radiation from source 8 is cut off by switch 7 and the wafer is exposed to the pattern 22 which may be in hologram form using polarizing laser 24. Light switch 7 comprises an electro-optical crystal (31), Fig. 2 (not shown), to which a voltage (V) may be applied whereby the plane of polarization of a monochromatic light beam is rotated through 90 degrees, a birefringent plate (32) and a reflecting plate (33). Polarizer (34) passes light polarised in direction (41) only which is that of light beam (40) from source 8. Beam (44) from the laser is polarized at 90 degrees to direction (41) and is therefore only passed when crystal (31) is energized. Beams (40), (44) correspond to the extraordinary and ordinary beams for plate (32). Deflection units 15, 16 operating in X and Y directions are similar to switch 7 and comprise five electro-optical elements each followed by a birefringent element through which light may pass, depending on the state of the electro-optical elements, either as an ordinary or a deflected extraordinary beam. Each birefringent element is twice the thickness of its preceding element. The scanning deflections of the beam are obtained by activating the electro-optical elements in succession through five inter-connected bi-stable devices