Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-06-26
2003-07-15
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000
Reexamination Certificate
active
06593040
ABSTRACT:
FILED OF THE INVENTION
The present invention relates generally to a method for correcting defects formed on a photomask, more specifically, to a method for correcting defects formed on a alternating phase shift photomask.
BACKGROUND OF THE INVENTION
The attempt to increase integration density of semiconductor circuits has resulted in the significant size reduction of patterns used to form the semiconductor circuits. Photomasks are utilized by the semiconductor circuit industry to produce the minute patterns onto semiconductor materials or wafers through photolithographic processing during the manufacturing of these integrated semiconductor circuits.
FIG. 1
depicts a sectional view of a portion of a standard photomask
52
. Referring to
FIG. 1
, a typical mask has a metal light shielding film or layer
54
of a prescribed pattern formed on a mask substrate
56
. When the metal light shielding film
54
is formed through known techniques an opaque defect (excess metal film)
72
may be generated.
FIGS. 2-4
illustrate one example the generation of an opaque defect
72
on a mask
52
in the manufacturing process of a standard mask
52
.
FIG. 2
depicts one of the initial stages in the generation of a mask
52
. A layer of light shielding film
54
is disposed over the entire surface of mask substrate
56
. Mask substrate is normally formed of a transparent silica, quartz or other material well known in the art. Light shielding film
54
is typically formed of elements and compounds consisting of Cr, Mo, F, Si, Zr, O, N. A layer of photo sensitive polymer resist
58
is deposited over the entire surface of layer of light shielding film
54
. After resist
58
is coated over the metal light shielding layer
54
, a prescribed portion of resist
58
is exposed to a radiation source
64
. If foreign material
66
is present in or on resist
58
at the time of exposure, in a region of mask
52
to be exposed, the portion of resist positioned under foreign material
66
will not be exposed resulting in a mask defect. Referring to
FIG. 3
, if the region to be originally exposed is thus not exposed due to foreign material
66
, unnecessary resist artifacts
68
are formed when resist
58
is developed. Referring to
FIG. 4
, when unnecessary resist artifact
68
is formed, and the metal light shielding film
54
is subjected to etching, utilizing resist
58
as an etching pattern, a metal light shielding film opaque defect
72
is formed. Alternatively, due to incorrect exposure or an error in the design pattern, other areas of resist
58
also may not be exposed resulting in similar defects in mask
52
.
FIG. 5
depicts a standard method to correct opaque defects
72
in metal light shielding film
54
utilizing an Nd:YAG laser beam or alternatively a focus ion beam (FIB)
74
. FIB
74
is irradiated onto opaque defect
72
. Thus, opaque defect
72
is evaporated or sputtered and removed, as shown in FIG.
6
. An Nd:YAG laser beam typically used has a wavelength of about ~532 nm and irradiated for about 200 mJ-300 mJ, while a typical FIB
74
uses a dose of approximately 400-600 &mgr;J.
The prior art methods for correcting the opaque defect
72
of metal light shielding material have provided satisfactory results in correcting opaque defects
72
in standard masks. However, when using a laser to remove the metal light shielding material, the metal light shielding material is not removed, but is displaced or redeposited in the surrounding areas causing material swelling in the mask. Additionally, the laser spot size is large, thus, limiting the ability to repair highly integrated patterns. Further, the use of lasers tends to remove a portion of the substrate under the defect depending on the defect size. Because of the heat required to ablate the opaque defect
72
of light shielding material, the underlying substrate
56
is usually partially ablated or melted through thermal conduction, thus, reducing the effectiveness and precision of mask
52
.
The effectiveness of an FIB
74
in the removal of opaque defect
72
is, to a large extent, based on the fact that a contrast distinction exists between the defect
72
and the transmitting window
78
. Because FIB devices utilize secondary charged particles for imaging (i.e. secondary electrons or ions), a contrast between the defect and the correctly etched areas must be present to properly determine the size of the defect and the amount of light shielding material to be removed. With a standard mask
52
, the contrast is large, the defect consists of a metal light shielding material while the correctly etched area is a transparent material. Thus, an FIB device can determine the size and amount of correction. However, without a significant contrast, an FIB device would not be able to accurately determine the defect size nor the degree of correction needed to accurately correct the defect or to avoid further damage to the surrounding mask.
The need to increase integration density has forced the positioning of the light transmitting windows or regions
76
a
and
76
b
closer together, as shown in FIG.
7
. However, light transmitted through transmitting windows
76
will overlap if transmitting windows are positioned too close together. The light intensity
78
of the light transmitted through transmitting windows
76
a
and
76
b
is shown in FIG.
8
. Light intensity
78
a
transmitted through window
76
a
will overlap light intensity
78
b
transmitted through window
76
b.
Thus, a total light intensity
80
results producing an in inaccurate integrated circuit.
In order to achieve further increased integration density a photomasking technique has been developed which provides for an alternating phase shift in the light which is projected through the mask. This type of photomasking is known as alternating phase shift photomasking or Levenson phase shift masking.
FIG. 9
depicts a sectional view of a portion of an alternating phase shift photomask or Levenson phase shift mask
120
. Phase shift mask
120
includes a transparent silica substrate
122
, including a phase shift region or layer
124
formed in substrate
122
, and a radiation shielding film
126
. For simplicity, this specification assumes the radiation used to form an integrated circuit pattern is light, although other types of radiation are also suitable. When radiation, for example from a light source, is shown or exposed onto mask
120
, light passes through the regions of mask
120
which are not covered by light shielding film
126
. The light which passes through mask
120
will be affected by the phase shift layer
124
. Because of differing heights of phase shift layer
124
, the light passing through it will exit mask
120
with different phases. The transparent regions or phase shift areas
130
a
and
130
b
are specifically arranged in an alternating pattern such that a first phase shift layer thickness (for example at
130
a
) is positioned next to a second phase shift layer thickness (
130
b
) such that light passing through transparent regions
130
a
and
130
b
will exit at alternating phases thus producing a cancelling effect. The height differences of the phase shift layer
124
are specifically calculated according to the exposure lambda (&lgr;) and index of refraction (n) of the substrate step depth (d), defined by: d=&PHgr;/&pgr;&lgr;/(n−1).
In
FIG. 10
, the electric field of the transmitted light passing through mask
120
is shown. The light transmitted through region
130
a
of mask
120
produces a negative electric field
142
due to the lack of phase shift layer
124
. The light transmitted through region
130
b
of mask
120
, and thus phase shift layer
124
, results in a positive electric field
144
and is thus 180° out of phase with the light transmitted through region
130
a.
Due to the placement of the alternating transparent regions, the light transmitting through transparent regions
130
a
and
130
b
will produce a cancelling or destructive interference on those regions between the transparent regions
130
a
and
Bachand Edward N.
Dorsey & Whitney LLP
Metron Technology
Rosasco S.
LandOfFree
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