Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-08-08
2001-09-18
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06291115
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of integrated circuit fabrication, and more specifically to a method for repairing bump and divot defects in a phase shifting mask that does not require the classification of the defect as either a bump or divot defect.
2. Description of the Related Art
In the manufacture of semiconductor wafers, microlithography is used to pattern various layers on a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and a template such as a mask or reticle (as used herein, “mask” shall refer to templates of any kind including masks and reticles). During the exposure process a form of radiant energy, such as ultraviolet light, is directed through the mask to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, thereby forming a resist mask on the wafer. The resist mask can then be used to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes.
An individual reticle can cost up to $20,000 and typically requires up to two weeks to manufacture. Mask production likewise involves substantial time and expense. The complete circuit patterning for a modern IC will typically require 10 to 20 or more reticles, and the ability to repair a reticle or mask quickly saves turnaround time and cost.
An example of a traditional mask
10
is shown in FIG.
1
. The mask
10
is formed on a transparent substrate
12
such as quartz. Opaque material
14
such as Chromium (Cr) is deposited on the substrate
12
to form a pattern of alternating opaque areas
14
and clear areas
13
. As shown in
FIG. 1
, the width W of the clear areas
13
is equal to the width W of the opaque areas
14
. The minimum width W of the alternating areas
13
,
14
is referred to as the line width or feature size.
As the semiconductor manufacturers attempt to decrease the size of integrated circuits, the line width naturally continues to shrink. One known problem with conventional masks is that diffraction causes the light pattern transmitted throughout the mask to “blur.” This problem is particularly acute as the line width reaches submicron levels. This problem has led to the use of phase shifting masks.
Phase shifting masks shift (usually by 180 degrees) the phase of light transmitted through every other clear area. The phase shift is accomplished by providing phase shift areas, or wells, in every other clear area. The wells may be provided in different ways. For example,
FIG. 2
illustrates a phase shifting mask
100
including a substrate
12
into which phase shifting wells
16
have been etched. The phase shifting wells
16
correspond to every other clear area
13
. Thus, the light transmitted through the mask
100
on one side of an opaque area
14
is phase-shifted with respect to the light transmitted through the mask
100
on the other side of the opaque area
14
. As used herein, the term “non-phase shift area” refers to a clear area that is not in a phase shift well, while the terms “phase shift area” and “phase shift well” refer to a clear area that is recessed with respect to non-phase shift areas.
FIG. 3
illustrates a second type of phase shifting mask
200
. Rather than etching phase shifting wells into the substrate
12
, a transparent material
18
is provided on top of the substrate
12
in all areas except every other clear area
13
. Thus, each opaque area
14
and every other clear area
17
are on the transparent material
18
, while the remaining clear areas
13
are formed in phase shifting wells
16
in which no transparent material
18
is present. The discussion herein will focus on the type of phase shifting mask
100
illustrated in
FIG. 2
, although those of skill in the art will readily recognize that the discussion and invention are equally applicable to the type of phase shifting mask
200
illustrated in FIG.
3
.
FIG. 4
illustrates bump and divot defects in a phase shifting mask
100
. It should be noted that the defects that are being discussed herein (sometimes referred to as clear area defects) are different from defects in the opaque areas
14
such as the clear and opaque defects discussed in my co-pending application entitled “Method for Removing the Carbon Halo Caused by FIB Clear Defect Repair of a Photomask,” Ser. No. 09/190057, filed Nov. 12, 1998.
FIG. 4
illustrates a phase shift well
16
with a bump defect
20
, as well as a phase shift well
16
with a divot defect
22
.
FIG. 4
also illustrates a divot defect
22
in non-phase shift clear area
17
. Bump defects do not occur in the non-phase shift area in the type of phase shifting mask
100
illustrated in
FIGS. 2 and 4
, but may occur in the type of phase shifting mask
200
illustrated in
FIG. 3
depending upon the flatness of the coated transparent material areas
18
.
Methods of correcting bump and divot defects in phase shifting masks are taught in U.S. Pat. No. 5,382,484 (the “'484 patent”). The methods taught therein, however, suffer from drawbacks which make their use relatively difficult and expensive in practice. First, the methods taught by the '484 patent require the identification of the type of defect (i.e. bump or divot) in addition to identifying the existence of the defect. Although the presence of a clear area defect can often be detected with an optical mask inspection tool, in practice the identification of the type of defect often requires the use of a device such as a SEM or an AFM. Second, the methods taught by the '484 patent require tools such as FIBs (focused ion beam) and laser tools. Use of both of these tools adds to the cost and complexity of repairing defects in masks, as well as adding the risk that the mask substrate or opaque areas will be damaged by the FIB or laser tool used in the repair process.
What is needed is a method of repairing bump and divot defects in phase shifting masks that can be performed without requiring the identification of the type of defect and without requiring the u se of expensive and complex tools which may damage the mask.
SUMMARY OF THE INVENTION
The present invention provides a method for repairing bump and divot defects in phase shifting masks that does not require the identification of the type of defect (i.e. bump or divot) or the use of special tools, such as FIBs or lasers, which may damage the substrate. In a preferred embodiment, the method is carried out with the aid of an optical microscope equipped with a light source that produces light at a wavelength that does not expose resist (yellow in the preferred embodiment), and light that exposes resist (ultraviolet in the preferred embodiment), a first filter for filtering all light other than non-exposing light from the light source, a second filter for filtering all light other than exposing light from the light source, and an adjustable field aperture. Preferably, the microscope includes a shutter to facilitate changing from the first filter to the second filter. The defect may be positioned to a desired location by a computer file containing desired coordinates. If defects are present only in phase shift wells or only in non-phase shift areas, all defects may be repaired in one iteration of the process described below. However, if defects are present in both phase shift wells and non-phase shift areas, two iterations will be necessary.
The method is performed by first identifying the locations of all defects. In the preferred embodiment, this step is accomplished with a standard automated mask inspection tool that creates a computer file that lists all defect locations. Next, the type of area (i.e. phase shift area or non-phase shift area) each defect is in is determined. This step is performed using an optical microscope in the preferred embodiment. Preferably, the computer file that holds the defect locations is modified to incl
Dickstein , Shapiro, Morin & Oshinsky, LLP
Micro)n Technology, Inc.
Rosasco S.
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