Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-11-04
2001-07-03
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06255023
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photolithographic technique. More particularly, the present invention relates to a method of manufacturing a binary phase shift mask.
2. Description of the Related Art
Photolithography plays an important part in the manufacturing of semiconductors. Many processing operations such as etching and doping must go through photolithographic operations. In fact, the ultimate quality of most semiconductor devices depends on the resolution of light and depth of focus (DOF) in photolithography.
In the fabrication of semiconductors having a line width greater than 0.18 &mgr;m, a binary photomask consisting of a quartz plate with a plated chromium film pattern thereon is often used. In general, a moderate to high-quality pattern can be transferred to a photoresist layer using this type of photomask.
However, when semiconductor products having a line width smaller than 0.18 microns are fabricated, light diffraction due to the reduction of hole diameters and line width in the mask pattern dominates. To minimize diffraction, a phase shifting layer is added to the binary photomask. Utilizing the positive and negative interference of light through the phase shift layers, resulting resolution of the photomask is improved. This type of photomask that also has an added phase shift layer is called a phase shift mask (PSM). The advantage of a phase shift mask is that resolution is improved without the need to use a new light source. Only changes in the photomask are required. A half-tone PSM is a photomask formed using phase-shifting material capable of shifting the phase of light by 180°. In addition, the phase-shifting material is semi-transparent, and permits at most 30% of the light to pass through. In general, a portion of the pattern on the half-tone PSM is transparent. In other words, there is no phase shift through that area. Most phase shift materials contain molybdenum silicon oxy-nitride (MoSi
z
O
x
N
y
) and chromium oxide. However, a pattern to be transferred to a photoresist layer generally contains some small and some large openings. Moreover, some of the small openings may come very close to some large ones, for example, the layout of local interconnects. Consequently, side-lobe effects are a common occurrence.
FIG. 1A
is a schematic cross-sectional view of a conventional half-tone phase shift mask. As shown in
FIG. 1A
, a phase shift layer
110
is formed over a quartz substrate
100
. The phase shift layer
110
has a smaller opening
120
and a larger opening
130
.
FIG. 1B
is a graph showing the variation of luminosity on a photoresist layer during light exposure for a mask having cross-section as shown in FIG.
1
A. As shown in
FIG. 1B
, the photoresist layer receives more light from the larger opening
130
. In addition, intensity of light coming from the edge of the opening
130
is also greater (that is, diffracted light leaking out from the edge of the opening
130
). Conversely, the photoresist layer receives less light from the smaller opening
120
and corresponding intensity from the edge of the opening
120
is smaller.
Since diffracted light coming from the edge of the opening
120
is relatively small, phase-reversed light from the phase shift layer
110
has sufficient intensity to cancel out the diffracted light. However, diffracted light coming from the edge of the larger opening
130
is more intense; hence phase-shifted light from the phase shift layer
110
is only able to cancel a portion of the diffracted light. Therefore, a swath of light is still emitted from the edge of the opening
130
. Residual light from diffraction causes the exposure of photoresist in unwanted areas. This phenomenon is the so-called side-lobe effect.
This is a difficult situation because neither increasing nor decreasing light exposure helps. If the period of exposure is decreased, the photoresist layer receives too little light from smaller openings such as the opening
120
. On the other hand, if the period of exposure is increased so that the photoresist layer receives sufficient light from all the smaller openings, side-lobe effects occur in all the larger openings such as the opening
130
.
SUMMARY OF THE INVENTION
The present invention is to provide a binary phase shift mask. The mask includes a transparent substrate, a phase shift layer and a mask layer. The phase shift layer is above the transparent substrate. The phase shift layer is a pattern with a plurality of first openings and a plurality of second openings. Both the first openings and the second openings expose a portion of the transparent substrate. The mask layer is above the phase shift layer. The mask layer is mainly a layer of mask material that surrounds the edges of all the first openings. All first openings occupy an area greater than a preset minimum area while the second openings all occupy an area smaller than the preset minimum area.
The invention provides a method of manufacturing a binary phase shift mask. A transparent substrate is first provided. A phase shift layer and a mask layer are sequentially formed over the transparent substrate. The mask layer and the phase shift layer are patterned to form a plurality of first openings and a plurality of second openings that expose a portion of the transparent substrate. The mask layer is patterned to form a layer of mask material that surrounds the edges of all first openings. All first openings occupy an area greater than a preset minimum area while the second openings all occupy an area smaller than the preset minimum area.
The binary phase shift photomask of the invention has a line of mask material next to the edges of the larger first openings. Hence, intensity of diffracted light passing through the edges of a large opening is greatly reduced. Therefore, lobe-side effects resulting from large first openings can be ameliorated so that all the advantages of a phase shift mask can be maintained.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 5900337 (1999-05-01), Lee
Huang Chien-Chao
Huang Michael W C
Wu Juan-Yuan
Rosasco S.
Thomas Kayden Horstemeyer & Risley LLP
United Microelectronics Corp.
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