Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-12-20
2002-04-30
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C382S144000, C348S087000, C356S392000, C356S396000, C356S397000
Reexamination Certificate
active
06379848
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor fabrication. More particularly, the invention relates to a method for inspecting a reticle for use in photolithography, a method for making a reticle for use in photolithography, and a reticle for use in photolithography.
2. Description of the Related Art
As is well known to those skilled in semiconductor fabrication, photolithography involves selectively exposing regions of a resist-coated silicon wafer to a radiation pattern, and developing the exposed resist to either protect or expose regions of underlying wafer layers (e.g., regions of substrate, polysilicon, or dielectric). In the fabrication of semiconductor chips, one of the problems experienced during photolithography is that the features defined in the wafer layer are subject to corner rounding. One source of this problem is the rounding of features that occurs during the manufacturing of the reticle, which includes the photomask in which a pattern corresponding to features at one layer of an integrated circuit (IC) design is defined.
FIG. 1A
shows a conventional stepper apparatus
10
used in photolithography. Stepper apparatus
10
includes radiation source
12
, reticle
14
, and focusing lens
16
. As shown in
FIG. 1A
, stepper apparatus
10
is disposed above resist-coated wafer
18
, which is divided into a plurality of dies
20
. In operation, radiation
22
(e.g., light) from radiation source
12
is projected onto the surface of resist-coated wafer
18
. After passing through reticle
14
and focusing lens
16
, radiation
22
contacts the surface of resist-coated wafer
18
within one of dies
20
and defines IC design pattern
24
′ in the die surface. This process is typically repeated until an IC design pattern has been defined in each of dies
20
by stepping stepper apparatus
10
in two dimensions above resist-coated wafer
18
. Thereafter, conventional development, etching, and stripping operations are typically performed to define the features at one layer of the IC design.
FIG. 1B
shows a detailed view of conventional reticle
14
shown in FIG.
1
A. As shown in
FIG. 1B
, reticle
14
includes transparent glass plate
26
on which photomask
28
is formed. Photomask
28
, which is formed of chromium or other suitable light-blocking material, has IC design pattern
24
defined therein. IC design pattern
24
corresponds to features at one layer in an IC design. When radiation
22
from radiation source
12
is directed toward reticle
14
, radiation (e.g., light) passes through pattern
24
, which corresponds to the portion of transparent glass plate
26
not covered by photomask
28
, and projects onto resist-coated silicon wafer
18
(see
FIG. 1A
) disposed below the reticle.
Reticles are presently manufactured by depositing a chromium photomask on a transparent glass plate, coating the photomask with a resist, defining a pattern in the resist using a pattern generator, developing the resist, and subjecting the photomask to chemical processing to remove everything but the desired pattern from the glass plate. In the operation in which the pattern is defined in the resist, the pattern generator directs an electron beam to define the desired features in the resist.
FIG. 2A
shows an exemplary ideal IC design pattern
24
that may be defined in a photomask. The ideal pattern
24
has well-defined outside corners
30
and inside corners
32
. The corners
30
and
32
form sharp, 90 degree angles. Unfortunately, when the pattern is defined in the photomask, these corners are subject to rounding due to electromagnetic wave effects and chemical processing effects.
FIG. 2B
illustrates the corner rounding that may occur during preparation of a photomask in which ideal IC design pattern
24
shown in
FIG. 2A
is to be defined. As shown in
FIG. 2B
, actual IC design pattern
24
′ has rounded outside corners
30
a
and rounded inside corners
32
a
. These corners do not form sharp, 90 degree angles and, consequently, the actual IC design pattern
24
′ does not have the desired shape of ideal IC design pattern
24
. This is undesirable because the discrepancy in shape of desired features, e.g., metallization features, may render numerous devices on a wafer inoperable and thereby decrease the manufacturing yield.
The deleterious effects of corner rounding become more problematic at smaller feature sizes. Thus, as the feature sizes in modem IC designs continue to decrease, it becomes increasingly more important to monitor closely all potential sources of corner rounding to avoid excessive yield losses. As noted above, one source of corner rounding is the rounding of corners that occurs in the process of manufacturing the reticle.
To date, several approaches have been used to inspect reticles for corner rounding. In one approach, an optical inspection device is used to compare the actual features of the photomask to corresponding data on a data tape used to prepare the photomask. This approach is undesirable because it requires complex equipment that is not only expensive, but also may be difficult and time consuming to install and operate. In another approach, an operator inspects the reticles under a microscope to determine visually whether the degree of corner rounding of features defined in the photomask is acceptable. This approach suffers from the disadvantage that it may be inconsistent or unreliable because it relies upon the operator's subjective judgment to determine the acceptability of the reticle.
In view of the foregoing, there is a need for an inexpensive, consistent, and reliable method of inspecting reticles to determine whether the degree of corner rounding is within acceptable limits.
SUMMARY OF THE INVENTION
Broadly speaking, the present invention fills these needs by providing a photomask having a test pattern and a reference marker defined therein. When a reticle having the photomask formed thereon is inspected under a microscope, the crosshair of the microscope may be used in conjunction with the reference marker to evaluate the degree of corner rounding of a feature of the test pattern.
In one aspect of the invention, a method for inspecting a reticle to evaluate the degree of corner rounding of a feature of a test pattern is provided. In this method a reticle having a photomask formed thereon is placed under a microscope. The photomask has a pattern corresponding to features of a semiconductor chip design defined therein. In addition, the photomask further has a test pattern and a crosshair orientation mark defined therein. The test pattern has at least one test corner for evaluating a degree of corner rounding when the test pattern is defined in the photomask. The crosshair orientation mark is defined in the photomask to orient a crosshair of the microscope relative to the test pattern. Once the crosshair of the microscope is aligned with the crosshair orientation mark, the crosshair of the microscope is used to evaluate the degree of rounding of the test corner of the test pattern.
In another aspect of the invention, a method for inspecting a reticle to determine the pass/fail status of the reticle is provided. In this method, a reticle having a photomask formed thereon is placed under a microscope. The photomask has a pattern corresponding to features of a semiconductor chip design defined therein. The photomask further has a test pattern and a crosshair alignment mark defined therein. The test pattern has at least one test corner for determining a pass/fail status of the reticle. The crosshair alignment mark is defined in the photomask to orient a crosshair of the microscope relative to the test pattern. Once the crosshair of a microscope is aligned with the crosshair alignment mark, the crosshair of the microscope is used to determine the pass/fail status of the reticle.
In yet another aspect of the invention, a method for making a reticle is provided. In this method, a glass substrate is first provided. A photomask having a pattern corresponding to fea
Baxter Janet
Lee Sin J.
Philips Electronics No. America Corp.
Zawilski Peter
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