Browse Prior Art Database

Microoptics in a Mask

IP.com Disclosure Number: IPCOM000081249D
Original Publication Date: 1974-Apr-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 27K

Publishing Venue

IBM

Related People

Dicke, RH: AUTHOR

Abstract

High-resolution optical imaging on photoresist can normally be achieved only by employing a strong convergence of radiation at the focus. One disadvantage associated with the strong convergence is the accompanying short depth of focus. One advantage is the elimination of most of the interference fringes inside the photoresist, caused by the use of normally incident radiation.

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Microoptics in a Mask

High-resolution optical imaging on photoresist can normally be achieved only by employing a strong convergence of radiation at the focus. One disadvantage associated with the strong convergence is the accompanying short depth of focus. One advantage is the elimination of most of the interference fringes inside the photoresist, caused by the use of normally incident radiation.

Focusing properties can be built into a mask by correctly shaping the walls of a groove, to induce a focus to a line along the center of the groove.

It is evident that the appropriate parabolic shape in the walls of the groove could induce such a focus, see Fig. 1. This may invoke a picture of a small optician lavishing great care in grinding and polishing the walls of this groove to a high precision, but matters are much simpler.

First, it should be noted that ray-optics theory is inadequate when the characteristic dimensions of the optical system are comparable with a wavelength. The tolerance required in shaping the groove is of the order of magnitude of a fraction of a wavelength, independent of the dimensions of the groove in the mask. Thus, the narrower the groove, i.e., the smaller the optical system, the less fractional precision is required in shaping the groove. Fig. 2 shows what might be an adequate approximation to a parabolic shape for a narrow groove.

Secondly, the interference of the straight-through unscattered wave with the wave reflected from the walls of the groove must be considered. Due to this interference effect, very little intensity in the reflected wave can produce a...