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Laser Alignment Overlay Tool for Proximity Lithography

IP.com Disclosure Number: IPCOM000051208D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 4 page(s) / 64K

Publishing Venue

IBM

Related People

Herd, HH: AUTHOR [+2]

Abstract

A lithographic proximity printing mask may be aligned with a microcircuit wafer by first aligning orthogonally oscillating laser spots to the wafer and then by aligning the mask with respect to the same oscillating laser spots.

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Laser Alignment Overlay Tool for Proximity Lithography

A lithographic proximity printing mask may be aligned with a microcircuit wafer by first aligning orthogonally oscillating laser spots to the wafer and then by aligning the mask with respect to the same oscillating laser spots.

A pair of lasers with associated beam-expanders and imaging objectives rearranged to form two small light spots of gaussian intensity cross-section on a substrate such that they are separated by the distance between conventional alignment marks on a matched mask and wafer. These alignment marks are formed by contrast of reflectivity or depth. Let the line joining the two alignment marks be designated the x-axis, and the orthogonal direction the y-axis. The beams are caused to oscillate parallel to the x and y directions by electromagnetic drivers moving the carriage on which the laser optics are affixed. The frequency of oscillation is fixed, and the amplitude is comparable to the dimensions of the laser light spots.

The return light is sensed by the lasers themselves using optical feedback, or by separate sensors at the rear of the lasers, or by diverting via beam-splitters to photodetectors. Fig. 1 illustrates the optical feedback concept. Consider just one of the lasers. The fraction of the light that is returned to the laser cavity is dependent upon the defect of focus at the reflecting plane.

In particular, the fraction is a maximum when the spot is at perfect focus on the reflecting plane, and falls off on either side away from focus. Variations in the position of the reflecting plane result in power variations or modulation of the return light, which in turn brings about a modulation in the current, voltage of power in the laser cavity, and this effect is externally measurable. If the spatial or mechanical modulation is on one side of the maximum (Fig.
1.2), the feedback signal (Fig. 1.3) relative to the mechanical oscillation (Fig. 1.1) has phase opposite to that of the feedback signal (Fig. 1.11) associated with DC displacement on the opposite side of maximum (Fig. 1.10). When the mechanical oscillation is centered about focus (Fig. 1.6), the laser signal (Fig. 1.7) exhibits twice the mechanical oscillation frequency.

While the effect of optical feedback has been demonstrated for axial displacement from focus, exactly the same effect is observed when the spot is oscillated across a small, contrast region, such as an alignment mark.

When the laser spot is on one side of the alignment mark, the phase of the return light signal relative to the oscillator driver will be opposite with respect to When the spot is on the other side of the alignment mark. When the spot is centered on the alignment mark, the return light signal will have the frequency of twice that of the oscillator driver. The same will be true in each of two optical legs. The x and y signals from one laser can be used to drive x and y displacements to center that spot on one alignment...