Self-aligned alternating phase shift mask patterning process

Details Self-aligned alternating phase shift mask patterning process Self-aligned alternating phase shift mask patterning process

2002-06-05

2004-11-30

Mohamedulla, Saleha R. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

Reexamination Certificate

active

06824932

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for making phase shift masks used in the manufacture of semiconductor microcircuits and other electronic components to transfer a circuit pattern onto a workpiece, and, in particular, to a self-aligned alternating phase shift mask wherein etching of an anti-reflective coating used on the opaque patterns of the mask is avoided.
2. Description of Related Art
In the manufacture of circuit patterns on electronic components such as the manufacture of integrated circuits on semiconductor substrates photomasks are used to transfer the desired circuit pattern onto the substrate workpiece. Typically, a photomask comprises a patterned metal film such as chromium, nickel or aluminum in a thickness of about 1,000 Å deposited on a transparent base such as glass or quartz. The photomask is generally manufactured by depositing a thin film of the metal on the surface of the transparent substrate, coating an anti-reflective coating on the metal, coating with a photoresist layer, exposing a pattern on the photoresist coating, developing the resist coating, and removing the metal from the unprotected areas of the film by etching leaving patterned metal film on the substrate. Mask blanks are typically purchased containing the metal film, anti-reflective coating and photoresist layer.
The pattern contained in the photomask is reproduced onto the surface of a workpiece typically by placing the mask over the workpiece and irradiating a radiation-sensitive resist material on the workpiece. The varieties of radiation sources for lithography that have been used or proposed for phase shift masks include visible light and ultraviolet light with optical wavelengths of 365 nm, 248 nm, 193 nm and 157 nm. When illuminated by the radiation, the metal pattern on the photomask serves to selectively block portions of the radiation beam while allowing other portions to be transmitted therethrough. In this manner, very complex geometries having very narrow line widths can be reproduced allowing the economical production of very large scale integrated circuits and other devices.
Optical photolithography has been widely used in the semiconductor industry in connection with the formation of a wide range of structures in integrated circuit (IC) chips. As the device density on IC chips has increased, the size of the structures making up the devices has approached the wave length (around 0.25 micron) of the light used in optical photolithography processes. Future structures approach 90 nm images with a 160 nm pitch (line & space) at 193 nm wavelength. Optical diffraction makes it difficult to form images smaller than the wavelength of light when conventional masks and illumination techniques are used. Future density increases in IC devices may be difficult to achieve absent the development of alternative lithographic technologies.
Phase-shift lithography was developed to enhance the resolution of conventional optical photolithography. Phase-shift lithography is based on opposite phase destructive interference of the waves of incident light. By shifting the phase of one region of the incident light waves 180°, relative to an adjacent region of incident light waves, a sharply defined dark zone is created in the projected image of the phase shift mask. This zone defines an interface with greater edge contrast than can be achieved in the interface between a clear area and an opaque absorber in a conventional mask. Such a phase shift mask is well-known in the art and is termed an alternating phase shift mask (altPSM). The alternating regions that are etched to 180° phase are defined by a write operation on the mask. To facilitate proper alignment of the mask pattern to the chrome-on-glass (COG) mask these phase patterns are slightly oversized.
The anti-reflective coating is used on the surface of the blocking (chrome) layer to reduce the scattering of stray light in the optical lithographic process. Stray light in the optical lithography process reduces image contrast by scattering into areas that should be dark. Unfortunately, during the quartz etching process to form the phase shift pattern, exposed portions of the anti-reflective coating resulting from the oversize write operation may become degraded and lost to the quartz etch. This is shown in
FIGS. 2A-2E
which show a prior art method for making a self-aligned alternating phase shift mask patterning process using a chrome-on-glass mask which is etched to form the phase shift pattern.
Referring to
FIG. 2A
, a COG mask is shown generally as
10
and comprises a transparent substrate
12
, patterned chrome regions
13
on the surface of the substrate
12
and an anti-reflective coating
14
on the surface of each of the chrome regions
13
. A photoresist
15
is shown overlying the coated chrome patterns
13
as well as the upper surface
12
a
of the substrate.
A write operation using an energy beam
18
is directed onto the resist
15
to form the regions
19
which are to be etched to form the 180° shift.
Referring to
FIG. 2B
, after exposure of the mask
10
, portions
19
of the photoresist
15
were exposed corresponding to the write operation. It can be seen that the exposed areas
19
extend over the edge of the chrome pattern
13
and overlying anti-reflective coating
14
. This is the conventional process used to form a slightly oversized opening in the fabrication of alternating phase shift masks.
In
FIG. 2C
, the exposed areas
19
are developed forming openings
20
in the resist layer
15
. Again it can be seen that the openings
20
overlap the chrome pattern
13
and overlying anti-reflective coating
14
.
In
FIG. 2D
the mask
10
is etched forming phase shift recesses
21
in the substrate
12
. The recesses
21
are formed where there are openings
20
in the resist
15
formed from the imaging process. The recesses are formed typically to provide a 180° phase shift. It will be noted that part of the anti-reflective coating
14
is also etched as shown at
22
so that the remaining anti-reflective coating
14
does not cover the entire chrome pattern
13
surface. The remaining resist
15
is then removed leaving the final phase shift mask as shown in FIG.
2
E. As can be seen, the anti-reflective coating
14
does not completely cover the chrome pattern design
13
and thus reduces the effectiveness of the mask when used to transfer the pattern onto a workpiece. In addition to causing more reflection in the lithography tools, the missing anti-reflective coating that is formed around every phase opening appears as a bright halo in the reflective image capture during mask inspection. The concept of the enlarged phase opening is to provide overlay insensitivity and this halo can be severely misaligned relative to the chrome pattern and cause inspection problems. Recesses
21
are formed to provide typically a 180° phase shift in the substrate
12
.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide a method and apparatus to make phase shift masks and, in particular, self-aligned alternating phase shift masks having an anti-reflective coating on the opaque patterns of the mask.
A further object of the present invention is to provide phase shift masks made by the method and apparatus of the invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
SUMMARY OF THE INVENTION
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method for making a phase shift mask comprising the steps of:
supplying a transparent substrate mask having an upper surface and a lower surface wherein the upper surface has open regions, opaque pattern regions and an overlying coating on the opaque pattern regions, with the mask being ready for etching the open regions to form the phase shift mask;
applying

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