Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-07-08
2001-02-27
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S394000
Reexamination Certificate
active
06194103
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods of fabrication of a photomask and more particularly to methods of fabrication of an attenuating phase shifting photomask using a modulated electron beam to expose a layer of resist.
2. Description of the Related Art
As photolithography advances to 0.35 microns and below, new technologies are required to provide accurate photolithographic patterns. One such new technology involves the use of attenuating phase shifting materials to form mask images. The attenuating phase shifting material both shifts the phase of light passing through the material, usually by 180°, and provides a partial absorption of the light passing through the material. Attenuating phase shifting masks often use opaque material, usually chrome, in conjunction with the attenuating phase shifting material to prevent unwanted exposure of photoresist in border regions or corner regions of masks where multiple exposure steps can result in unwanted exposure of photoresist.
U.S. Pat. No. 5,738,337 to Tzu et al. describes a method of fabricating a attenuating phase shifting photomask with a chrome border using two exposure doses of a single electron beam.
U.S. Pat. No. 5,480,747 to Vasudev describes an attenuated phase shifting mask using buried absorbers.
U.S. Pat. No. 5,503,951 to Flanders et al. describes an attenuating phase shifting mask and fabrication method for a mask having recessed attenuating phase shifting regions.
U.S. Pat. No. 5,565,286 to Lin describes a structure and fabrication method for a phase shifting mask.
A paper by G. Owen and P. Rissman, Journal of Applied Physics, Vol. 54, No. 6, June 1983, pages 3753-3581 describes proximity effect and methods of correcting for proximity effect.
SUMMARY OF THE INVENTION
Attenuating phase shifting masks are fabricated using electron beam exposure to form patterns in a layer of resist on a mask blank. The mask blank comprises a transparent mask substrate with a layer of attenuating phase shifting material formed thereon. A layer of opaque material is formed on the layer of attenuating phase shifting material. A layer of resist is then formed on the layer of opaque material.
FIG. 1
shows a diagram of a conventional pattern layout of a mask pattern which will be used to form via holes in an integrated circuit wafer. First pattern regions
10
will be used to form the via holes and will be transferred to the mask blank. The field boundaries
12
are indicated by dashed lines in FIG.
1
and represent the regions within which the electron beam can be deflected without moving the mask blank relative to the electron beam apparatus. After each field is exposed the mask blank and electron beam apparatus must be moved before the next field is exposed.
Special care must be taken to insure that pattern accuracy is achieved at the edge of the field boundaries of the mask pattern. The pattern at the edge of a field boundary is subject to inaccuracies due to problems such as stitching effect, caused by multiple exposures at the field boundaries, and proximity effect, caused by back scattering of electrons used to expose a layer of resist.
It is a principle objective of this invention to provide a method of forming a mask using a single modulated electron beam which has good pattern accuracy at the field and pattern boundaries.
This objective is achieved by using a modulated electron beam to expose a layer of resist on a mask blank. The resist will be exposed to form a first number of first pattern regions, the first number of second pattern regions, the first number of third pattern regions, and a fourth pattern region. Each of the second pattern regions surrounds one of the first pattern regions and provides a first width of unexposed resist around each of the first pattern regions. Each of the third pattern regions surrounds one of the second pattern regions and the fourth pattern region surrounds all of the first pattern regions, all of the second pattern regions, and all of the third pattern regions forming a border region of the mask.
FIG. 2
shows a diagram of the mask pattern layout of this invention for a mask which will be used to form via holes in an integrated circuit wafer. First pattern regions
10
will be used to form the via holes. The field boundaries
12
are indicated by dashed lines in FIG.
2
. The second pattern regions
14
are shown in FIG.
2
and are regions where the resist is not exposed by the electron beam.
The modulated electron beam exposes the first number of first pattern regions with a first exposure level and the first number of third pattern regions with a second exposure level without exposing the first number of second pattern regions or the fourth pattern region. The second exposure level is less than the first exposure level. The first pattern regions are the part of the mask pattern which is to be transferred to a layer of photoresist on an integrated circuit wafer. The first width of unexposed resist surrounding the first pattern regions prevents stitching effect caused by poor electron beam shot connections on the third pattern regions from occurring. Therefor, the first width of unexposed resist provides good pattern accuracy at the field boundaries. The second exposure on the third pattern regions compensates for proximity effect. Conceptually, this is like the ghost technique in that the first exposure on the first pattern regions serves as the “pattern exposure” and the second exposure on the third pattern regions serves as a “correction exposure”.
The layer of resist is then developed to expose the layer of opaque material in the first pattern regions. The opaque material in the first pattern regions is then etched away using wet etching exposing the layer of attenuating phase shifting material in the first pattern regions. The attenuating phase shifting material in the first pattern regions is then etched away using dry anisotropic etching. The remaining part of the layer of resist except the fourth pattern region, or border region, is then removed using oxygen plasma etching and that part of the layer of opaque material not covered by the resist in the border region is etched away using wet etching. The remaining resist is then removed and the mask is cleaned.
REFERENCES:
patent: 5480747 (1996-01-01), Vasudev
patent: 5503951 (1996-04-01), Flanders et al.
patent: 5565286 (1996-10-01), Lin
patent: 5783337 (1998-07-01), Tzu et al.
patent: 5849437 (1998-12-01), Yamazaki et al.
patent: 6057066 (2000-05-01), Hanawa
G. Owen et al., “Proximity Effect Correction for Electron Beam Lithography by Equalization of Background Dose”, Journal of Applied Physics, vol. 54, No. 6, Jun. 1983, pp. 3753-3581.
Chiu Chinq-Shiun
Tzu San-De
Ackerman Stephen B.
Prescott Larry J.
Rosasco S.
Saile George O.
Taiwan Semiconductor Manufacturing Company
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