Browse Prior Art Database

Extended Focal Depth Optical Microlithography

IP.com Disclosure Number: IPCOM000035125D
Original Publication Date: 1989-Jun-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 63K

Publishing Venue

IBM

Related People

Starikov, A: AUTHOR

Abstract

It is proposed to improve the operation of optical lithography tools used in semiconductor manufacturing operations so as to minimize the defocus effect caused by the vertical topography on the wafers. The proposal suggests that the wafer be moved through the focus to lessen the defocus effects. This will serve to compensate for the shallow depth of focus of the tool and the resulting problem of image size variation.

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Extended Focal Depth Optical Microlithography

It is proposed to improve the operation of optical lithography tools used in semiconductor manufacturing operations so as to minimize the defocus effect caused by the vertical topography on the wafers. The proposal suggests that the wafer be moved through the focus to lessen the defocus effects. This will serve to compensate for the shallow depth of focus of the tool and the resulting problem of image size variation.

This proposal will become increasingly useful since the limited depth of focus (DoF) of microlithography tools promises to be aggravated in the newer tools being developed. The classical definition of the optical depth of focus is g CD DoF ~ ------ ~ -- NA2 NA where g = illumination wavelength NA =
numerical aperture of the tool CD = smallest dimension printed

(Image Omitted)

With newer tools, the wavelength is anticipated to decrease and the numerical aperture to increase making the DoF more limited than at present. Meanwhile, as chip sizes and topography increase it is increasingly difficult to maintain an in-focus condition across the chip.

It is proposed to extend the practical depth of focus by varying the position of the wafer surface 1 in Fig. 1 from well outside the conventional depth of focus 2 into focus and out to the other extreme. This motion 3 changes the position of the wafer relative to the reticle 4 and the lens 5 of the tool. This motion and exposure may be either continuous or in steps.

There are two proposed modes of operation, one utilizing dry lithography and one using a typical photoresist. With the dry method, areas exposed in a suitable recording material on the wafer simply ablate, thereby forming a transferred pattern. Examples of an in-focus image and an out-of-focus image of a narrow line are given in Figs. 2A and 2B, respectively, where I(x) is normalized image intensity image distribution. When the image is in focus, the line is printed LW (~ g /NA) in size. As the image goes out of focus and with ablation threshold properly set it prints less than LW in size at 7 for the same value of illumination intensity at the reticle. For I(x) above the...