Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-09-14
2001-05-22
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C382S144000
Reexamination Certificate
active
06235435
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to photolithography and, more particularly, to phase shift photo masks used in photolithography processes.
BACKGROUND OF THE INVENTION
Photolithography is an essential process in semiconductor device and integrated circuit fabrications. Because of their high resolution, phase shift photo masks are often used in fabricating semiconductor devices and integrated circuits of fine size.
FIG. 1
shows a schematic cross sectional view of a phase shift photo mask
10
suitable for a photolithography process in the fabrication of fine size semiconductor devices and integrated circuits. Phase shift photo mask
10
is formed from an optically transparent substrate
11
. Opaque light blocking structures
14
,
16
,
17
, and
19
are formed on a major surface
12
of substrate
11
. Typically, substrate
11
is made of quartz glass and light blocking structures are chromium structures. Structures
14
,
16
,
17
, and
19
define a pattern to be transferred to a semiconductor wafer in the photolithography process. Substrate
11
is selectively etched to form depressions or cavities
15
and
18
. Cavity
15
changes the optical thickness of substrate
11
there under. Consequently, the lights transmitted through substrate
11
near two opposite sides of light blocking structure
14
have a phase shift of 180 degrees relative to each other. This 180° phase shift creates an interference pattern on the semiconductor wafer and significantly increases the image resolution of light blocking structure
14
on the semiconductor wafer. Cavity
15
also increases the image resolution of light blocking structure
16
. Likewise, cavity
18
increases the image resolutions of light blocking structures
17
and
19
. Cavities
15
and
18
are also referred to as phase shifters.
A way of achieving high quality, reliability, and yield of the photolithography processes is inspecting photo mask
10
, and repairing or discarding photo mask
10
if it has defects. Photo mask
10
is usually inspected using an optical microscope. Because quartz glass is colorless and transparent to light over a wide spectrum, the light transmittance of substrate
11
is nearly 100 percent and substantially independent of its thickness. Therefore, determining the existence, location, and dimension of cavities
15
and
18
on substrate
11
is difficult and time consuming. The inspection of photo mask
10
is complicated and often fails to accurately reveal the status and condition of phase shifters
15
and
18
on photo mask
10
.
Accordingly, there exists a need for a phase shift photo mask that can be easily and accurately inspected for possible defects. There is also a need for methods of making and inspecting such a phase shift photo mask. It is desirable for the methods to be simple and cost efficient. It is also desirable for the phase shift photo mask to be compatible with existing photolithography processes.
SUMMARY OF THE INVENTION
A general advantage of the present invention is providing a phase shift photo mask that can be easily and accurately inspected for possible defects. A specific advantage of the present invention is providing a photo mask having phase shifting patterns that can be easily identified during an inspection of the photo mask. Another advantage of the present invention is providing a method for forming such a photo mask. Yet another advantage of the present invention is providing a process for inspecting the photo mask and identifying the phase shifting patterns thereon.
These and other advantages of the present invention are achieved through a dichroic phase shift photo mask formed by forming a dichroic film over a major surface of a transparent substrate and selectively etching the dichroic film to form the phase shifting patterns of the photo mask. Typically, the photo mask also includes light blocking structures over the transparent substrate. The dichroic film is etched on one side of a light blocking structure. The dichroic film preferably has high transmittance to a light used in a photolithography process that transfers the pattern on the photo mask to an object such as, for example, a semiconductor wafer. During the photolithography process, the phase shifters generate an interference pattern on the semiconductor wafer and significantly increase the image resolution of light blocking structures on the semiconductor wafer. On the other hand, the dichroic film has a low transmittance to a light used for inspecting the photo mask. The inspection process of the photo mask includes illuminating the photo mask with a light beam, and identifying the phase shifting patterns by observing the light transmitted through the photo mask. Specifically, an area through which a high intensity of light is transmitted is identified as an area where the dichroic film is etched to form phase shifters. On the other hand, an area through which a low intensity of light is transmitted is identified as an area where the dichroic film is not etched. This inspection process for identifying phase shifters on the photo mask is simple, accurate, and cost efficient.
REFERENCES:
patent: 5508132 (1996-04-01), Komatsu
patent: 5538816 (1996-07-01), Hashimoto et al.
patent: 5631109 (1997-05-01), Ito
patent: 5691090 (1997-11-01), Isao et al.
patent: 5721075 (1998-02-01), Hashimoto et al.
patent: 5725975 (1998-03-01), Nakamura et al.
patent: 5770335 (1998-06-01), Miyake et al.
patent: 5804338 (1998-09-01), Lim et al.
patent: 6103430 (2000-08-01), Yang
patent: 6110623 (2000-08-01), O'Grady
International Business Machines - Corporation
Rosasco S.
Schmeiser Olsen & Watts
Walter, Jr. Howard J.
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