1. Field of the Invention
The present invention relates generally to the field of optical lithography and, more particularly, to a design of phase shift mask (PSM) capable of forming ultra-small, isolated hole pattern on a photo resist layer.
2. Description of the Prior Art
In the circuit making processes, lithographic process has not only been a mandatory technique but also played an important role in limiting feature size. By lithographic process, a wafer producer can precisely and clearly transfer a circuit pattern onto a substrate. In a lithographic process, a designed pattern, such as a circuit pattern or a doping pattern, is created on one or several photo mask, then the pattern on the mask is transferred by light exposure, with a stepper and scanner, onto a substrate. Recently, the most mature lithographic technique is optical lithographic technique, of which the light sources including KrF laser (248 nm), ArF laser (193 nm) and F2 laser (157 nm) . . . etc., among which KrF laser and ArF laser light exposure techniques are arguably the most developed.
While the traditional technologies continue to advance at breakneck speed they are becoming quite costly and are no longer able to provide the resolution and depth of focus (DOF) with an acceptable process window on their own. This is a direct result of the Sub-Wavelength environment. Fortunately there is an additional knob the industry can adjust to improve the overall system performance. The field of low k1 lithography includes such techniques as optical proximity correction (OPC), phase shift masks (PSM), off-axis or modified illumination, spatial filters and high contrast resists. These techniques, collectively referred to as Resolution Enhancement Techniques (RETs), work in conjunction with the traditional techniques of decreasing wavelength and increasing NA to extract the highest level of performance possible from the advanced lithography systems.
Central to the resolution enhancement techniques are PSMs. The principle PSMs deployed in the industry include attenuated (usually used for contacts and metal layers) and alternating aperture (used for CD control for gates). However, the conventional designs of the PSMs are still not providing satisfactory results when facing ultra-small, isolated hole patterns.
It is therefore the primary object of the present invention to provide a novel PSM design for ultra-small hole patterning.
According to the claimed invention, a phase shift mask comprises a light transparent substrate having a main surface; an opaque material layer coated on the main surface of the light transparent substrate, wherein the opaque material layer has an window opening exposing a light transparent area of the light transparent substrate; a cruciform first phase shifting region of the exposed light transparent area; and a second phase shifting region of the exposed light transparent area except the cruciform first phase shifting region. Light passing through the cruciform first phase shifting region has a phase shift of 180 degrees relative to light passing through the d second phase shifting region.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.
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The present invention is further characterized in that the light transmission area 140, which is, in accordance with one preferred embodiment, rectangular, includes a cruciform first phase shifting region 142 that is etched into the light transparent substrate 100 to a first substrate thickness t1. The rectangular light transmission area 140 further includes a second phase shifting region 144, which is just the rest of the exposed light transmission area 140 except the cruciform first phase shifting region 142. As shown in
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Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.