Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
2003-05-19
2004-02-17
Hoff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S005000, C430S396000
Reexamination Certificate
active
06692900
ABSTRACT:
TECHNICAL FIELD
The invention pertains to methods of patterning radiation, methods of forming radiation-patterning tools, and to radiation-patterning tools themselves.
BACKGROUND OF THE INVENTION
Photolithography is commonly used during formation of integrated circuits on semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation-patterning tool and onto a semiconductor wafer. The radiation-patterning tool can be, for example, a photomask or a reticule, with the term “photomask” being sometimes understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticule” being sometimes understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticule” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting the claims that follow, the terms “photomask” and “reticule” will be given their historical distinction such that the term “photomask” will refer to a patterning tool that defines a pattern for an entirety of a wafer, and the term “reticule” will refer to a patterning tool that defines a pattern for only a portion of a wafer.
Radiation-patterning tools contain light restrictive regions (for example, totally opaque or attenuated/half-toned regions) and light transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel-spaced conductive lines on a semiconductor wafer. The wafer is provided with a layer of photosensitive resist material commonly referred to as photoresist. Radiation passes through the radiation-patterning tool onto the layer of photoresist and transfers the mask pattern to the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implantation or etching relative to materials on the wafer proximate the photoresist.
Advances in semiconductor integrated circuit performance have typically been accompanied by a simultaneous decrease in integrated circuit device dimensions and a decrease in the dimensions of conductor elements which connect those integrated circuit devices. The demand for ever smaller integrated circuit devices brings with it demands for ever-decreasing dimensions of structural elements on radiation-patterning tools, and ever-increasing requirements for precision and accuracy in radiation-patterning with the tools.
An exemplary prior art radiation-patterning tool
12
is shown in FIG.
1
. Radiation-patterning tool
12
comprises a substrate
14
which is at least partially transparent to radiation which is to be patterned, and a structure
16
joined to substrate
14
and formed of a material which is less transparent to the radiation than is substrate
14
. Substrate
14
typically comprises fused silica (for example, quartz), and structure
16
typically comprises chrome.
FIG. 1
further illustrates radiation
18
being directed toward radiation-patterning tool
12
, and shows a plot
20
of radiation intensity exiting from radiation-patterning tool
12
. Plot
20
illustrates that structure
16
has attenuated the radiation intensity. Specifically, plot
20
comprises a region
22
of decreased intensity where radiation
18
has been at least partially blocked by structure
16
, and higher intensity regions
24
where radiation
18
has not been blocked by structure
16
. In particular embodiments of the prior art, structure
16
will comprise a material substantially opaque to radiation
18
(for example, chrome can be opaque relative to ultraviolet light), and substrate
14
will be substantially transparent to the radiation (for example, quartz can be transparent to ultraviolet light).
A problem associated with the radiation-patterning described with reference to
FIG. 1
can be in accurately and reproducible forming the dip in radiation intensity shown at region
22
of plot
20
. Specifically, if radiation
18
is slightly defaced from an optimal focus position, the depth of region
22
(i.e., the change in intensity between region
22
and regions
24
) can be altered, which can cause variation in a critical dimension of openings ultimately patterned into photoresist. Also, the shape of the intensity profile in graph
20
can be less precise than is desired. Specifically, it would be ideal if the intensity profile of plot
20
exactly mirrored the pattern defined by structure
16
(i.e., if the intensity profile had sharp corners at transitions between regions
24
and
22
, and if region
22
had a flat bottom with a width corresponding to that of structure
16
).
An improved prior art radiation-patterning tool
12
a
is described with reference to FIG.
2
. In referring to
FIG. 2
, similar numbering is utilized as was used in referring to
FIG. 1
, with the suffix “a” used to indicate structures shown in FIG.
2
. Radiation-patterning tool
12
a
is similar to the patterning tool
12
of
FIG. 1
in that it comprises a substrate
14
a
which is at least partially transparent to incoming radiation
18
a
, and a structure
16
a
which is less transparent to radiation
18
a
than the substrate. However, radiation-patterning tool
12
a
differs from the patterning tool
12
of
FIG. 1
in that subresolution assist features
30
are provided adjacent structure
16
a
. Subresolution assist features
30
are formed of an identical material as structure
16
a
(which simplifies processing, as a single material can be formed over substrate
14
a
and patterned to form features
30
and structures
16
a
). Features
30
are referred to as subresolution assist features because intensity variations caused by features
30
are not resolved from intensity variations caused by structures
16
a
at the resolution provided by the particular wavelength of incoming radiation
18
a
. This is shown in the intensity graph
20
a
. Specifically, graph
20
a
shows a dip
22
a
corresponding to a region wherein an intensity variation is caused by structure
16
a
, and shoulders
32
corresponding to regions wherein intensity variation is caused primarily by features
30
. Since the intensity variations caused by features
30
are shoulders
32
along region
22
a
, rather than distinctly resolved elements, such intensity variations are subresolution variations.
Subresolution assist features
30
can alleviate some of the problems described above as being associated with the radiation-patterning tool
12
of FIG.
1
. Specifically, subresolution assist features
30
can stabilize an intensity difference between non-blocked regions
24
a
and blocked region
22
a
relative to subtle variations in focus of radiation
18
a
. Further, subresolution assist features
30
can improve the overall shape of blocked region
22
a
in the intensity profile
20
a
relative to the shape of region
22
in intensity profile
20
of FIG.
1
. Specifically, subresolution assist features
30
can flatten a bottom of region
22
a
, and sharpen the transition at corners of region
22
a
, such that region
22
a
has a width which better approximates a width of structure
16
a
than the width of
FIG. 1
region
22
approximates a width of structure
16
.
A problem associated with the formation of subresolution assist features is that as the dimension of semiconductor devices becomes smaller the desired dimension of subresolution assist features also becomes smaller. It is therefore becoming increasingly difficult to form satisfactory subresolution assist features as integrated circuit device dimensions decrease. It would accordingly be desirable
Hoff Mark F.
Mohamedulla Saleha
Wells St. John P.S.
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