Browse Prior Art Database

PUPIL PLANE FILTER FOR EUV LITHOGRAPHY

IP.com Disclosure Number: IPCOM000008911D
Original Publication Date: 1999-Jan-01
Included in the Prior Art Database: 2002-Jul-23
Document File: 4 page(s) / 211K

Publishing Venue

Motorola

Related People

Scott Hector: AUTHOR

Abstract

Placing a phase and or amplitude filter in the pupil plane of an optical projection lithography sys- tem has been shown to increase resolution and depth of focus1,2,3,4. The pupil filter sharpens the image intensity slope at the wafer by modifying the spatial frequency spectrum of the image. The pupil plane of a projection imaging system contains a spatial frequency spectrum of the image. A filter in this plane can tailor the spatial frequency spectrum to sharpen the image.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 36% of the total text.

Page 1 of 4

0 M-

Technical Developments

PUPIL PLANE FILTER FOR

EUV LITHOGRAPHY

by Scott Hector

INTRODUCTION

  Placing a phase and or amplitude filter in the pupil plane of an optical projection lithography sys- tem has been shown to increase resolution and depth of focus1,2,3,4. The pupil filter sharpens the image intensity slope at the wafer by modifying the spatial frequency spectrum of the image. The pupil plane of a projection imaging system contains a spatial frequency spectrum of the image. A filter in this plane can tailor the spatial frequency spectrum to sharpen the image.

PLACEMENT OF A DIFFRACTIVE OPTICAL ELEMENT IN A REFLECTIVE PROJECTION LITHOGRAPHY SYSTEM

  In the present conception of an extreme ultravio- let lithography (EUVL) system5, the pupil plane is directly accessible since the imaging system is a set of mirrors spaced apart. (A reflective 157 nm wave- length projection lithography system could also be designed with an accessible pupil). At EUV wave- lengths, most materials are quite opaque, but there are some materials that are either relatively transpar- ent or have a large ratio of phase shift to absorption on transmission. These materials can be used for complex amplitude filters placed in the pupil plane of the projection optics system or elsewhere in the illumination system.

  a. Specifically, a semi-transparent membrane can be used to support a patterned film of a material with appropriate ratio of phase shift to absorption to form a diffractive optical element (DOE) that can be placed in the pupil plane of the optical system. The proper distribution of phase and amplitude of the pupil plane filter to sharpen the image intensity slope can be determined from aerial image simula- tions of the optical projection system and iterative methods such as simulated annealing. The distribu- tion of phase and amplitude may be different for dif- ferent patterns being imaged on the wafer.

b. Such DOE's could also be placed in the illu- minator assembly to modify the spatial coherence (i.
e. the pupil fill) of the illumination. Ray tracing cal- culations can be run iteratively to optimize the filter complex amplitude transmittance to make the desired change in pupil fill.

  Figure 1 shows a schematic representation of a reflective imaging system and locations where dif- fractive optical elements could be placed to enhance imaging with an EUV lithography system. The dif- fractive optical element could either be transmissive as shown, or if the design of the reflective imaging were such that the pupil plane was at or near a reflecting surface, the complex reflectance of that surface could be modified to impart the desired Cl- tration of the image frequency distribution.

53 Motorola. Inc. ,999 26 January 1999

[This page contains 14 pictures or other non-text objects]

Page 2 of 4

0 M

MO7VROLA Technical Developments

1 orm

1 or more ima= 4

    Fig. 1 Schematic of a reflective EUV projection lithography system. The mirrors before the mask form the...