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Optical Etch-Point Detection Using Chromatic Aberration

IP.com Disclosure Number: IPCOM000047372D
Original Publication Date: 1983-Nov-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 2 page(s) / 40K

Publishing Venue

IBM

Related People

Hopf, G: AUTHOR [+3]

Abstract

A surface 1 to be etched to a depth w is illuminated by a laser 4, beam 3 of which has two different wavelengths g and g' and is focussed at F and F' by means of a lens system 2 having a well-defined chromatic aberration and separating the two foci by the desired etch depth w. The light reflected from surface 1 is coupled out by semitransparent mirrors 5, 6, passing to pinhole diaphragms 7, 8 to reach detectors 10, 11, the output difference of which is amplified by difference amplifier 12. Semitransparent mirrors 5, 6 and diaphragms 7, 8 are oriented such that the images of foci F and F' are positioned symmetrically in front and behind diaphragms 7 and 8, respectively.

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Optical Etch-Point Detection Using Chromatic Aberration

A surface 1 to be etched to a depth w is illuminated by a laser 4, beam 3 of which has two different wavelengths g and g' and is focussed at F and F' by means of a lens system 2 having a well-defined chromatic aberration and separating the two foci by the desired etch depth w. The light reflected from surface 1 is coupled out by semitransparent mirrors 5, 6, passing to pinhole diaphragms 7, 8 to reach detectors 10, 11, the output difference of which is amplified by difference amplifier 12. Semitransparent mirrors 5, 6 and diaphragms 7, 8 are oriented such that the images of foci F and F' are positioned symmetrically in front and behind diaphragms 7 and 8, respectively. During operation, either only one wavelength is used at a time or there are two wavelengths which are polarized orthogonally to each other and separated by polarizers 9 in front of detectors 10, 11. Equal output signals are generated by detectors 10, 11 only if focus F coincides with surface 1, i.e., after the desired etch depth has been reached. For calibration, focus F' is kept at the original height of surface 1, for example, by laterally shifting the optical system (following arrow 13) across a non-etched part of surface 1 (position A in the figure). This surface part can be protected against etching by transparent layer 14, whose influence on the position of focus F' has to be taken into account. The proposed arrangement for etch end-poin...