Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-11-01
2002-12-31
Berman, Jack (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492100, C250S492230, C250S370070, C716S030000, C430S030000, C430S022000, C430S311000
Reexamination Certificate
active
06501083
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle, onto a sensitive substrate) using an energy beam such as a charged particle beam (e.g., electron beam or ion beam). Microlithography in general is a key process used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, the invention pertains to methods for determining proximity effects encountered with certain microlithography techniques, especially charged-particle-beam (CPB) microlithography. The methods involve determinations of cumulative exposure energy (exposure dose) at specific loci on a sensitive substrate. The results of such determinations are used for, inter alia, designing reticles that are less subject to proximity effects during use.
BACKGROUND OF THE INVENTION
All conventional wafer-processing methods include at least one microlithography step, in which a pattern, defined on a mask or reticle, is transferred onto a sensitive substrate such as a semiconductor wafer. Typically, multiple “chips” are formed on each wafer, and multiple microlithography steps are performed to form various patterned layers of the chips. As an energy medium for making the pattern transfer, a beam of electromagnetic radiation (e.g., light, X-rays) or of charged particles (e.g., electron beam, ion beam) is used. To be imprinted with the pattern, the substrate is coated with a suitable “resist.” With a “positive” resist, regions that receive a cumulative exposure energy (dose) exceeding a threshold value are removed by developing the resist. With a “negative” resist, regions that receive a cumulative exposure dose exceeding a threshold value are left on the wafer after development. Hence, in order to form a pattern properly on the substrate, it usually is necessary to calculate whether the cumulative exposure dose at each region of the substrate is higher than the specified threshold value. It also is necessary to configure the reticle so that the respective shapes of the regions where energy accumulates above the threshold value will form pattern elements having the intended profiles.
In charged-particle-beam (CPB) microlithography, “proximity effects” frequently are encountered, in which the respective doses of exposure energy received at any of various loci on the wafer surface vary according to the respective distribution and configuration of pattern elements in the vicinity of the loci. I.e., the distribution and configuration of nearby pattern elements affect the manner in which charged particles of the beam are scattered as they encounter the substrate. Scattered charged particles can cause unintentional development of nearby regions of the resist in various ways, resulting in pattern elements being formed on the substrate having profiles that deviate substantially from the intended profiles of the elements. I.e., the profiles of the pattern elements as formed on the wafer unintentionally have different profiles than the corresponding elements as defined on the reticle.
In optical microlithography, proximity effects as summarized above generally do not occur. However, errors in the pattern as transferred to the substrate can occur due to light diffraction. Hence, in both optical and CPB microlithography, unavoidable differences can arise between the shapes of pattern elements as defined on the reticle versus the shapes of corresponding pattern elements as formed on the substrate.
Various methods have been considered for solving this problem. The methods generally involve changing and adjusting localized cumulative amounts of radiation dose received at the substrate so as to obtain the desired distribution of cumulative exposure energy at the surface of the substrate. For example, certain methods involve localized deformation of certain pattern elements as defined on the reticle.
A conventional method for calculating local cumulative exposure dose, leading to corresponding deformations of pattern elements as defined on the reticle to correct proximity effects in CPB microlithography, is discussed below, referring to FIGS.
20
(
a
)-
20
(
c
). In FIG.
20
(
a
), item
101
is a pattern element having a profile that is to be transferred to the substrate with minimal profile change. For convenience, the transfer magnification is unity. The edge of the pattern element
101
is divided into N segments S
1
-S
N
as shown in FIG.
20
(
b
). The element as defined on the reticle is deformed deliberately by moving each of these segments a tiny amount in a direction perpendicular to the direction in which the edge of the segment extends. This is shown in FIG.
20
(
c
), which depicts a magnified view of a portion of the pattern element
101
after moving certain segments. Specifically, FIG.
20
(
c
) shows segments S
1
, S
2
, and S
N
. The desired outline of the pattern element to be formed on the substrate is denoted by the line
102
. In FIG.
20
(
c
), segments S
N
and S
2
have been moved inward relative to the line
102
(thereby forming “indents”), and the segment S
1
is moved outward (thereby forming a “projection”). A
1
, A
2
, A
N
denote respective points on the line
102
corresponding to centers of the respective edges of“pre-move” segments S
1
, S
2
, S
N
. The points A
1
, A
2
, A
N
are points at which evaluations of cumulative exposure dose are made.
Referring further to FIG.
20
(
c
), D
1
, D
2
, and D
N
are respective vectors indicating the magnitude and direction of movement of the edge of each respective segment S
1
, S
2
, S
N
. A respective vector D
1
-D
N
is assumed for each segment S
1
-S
N
in this manner, and the change in cumulative exposure dose occurring at each evaluation point A
1
-A
N
is calculated in accordance with the change in position of the edge of each segment according to the respective vector. That is, cumulative exposure energy (dose) is changed at each evaluation point A
i
(i=1, 2, . . . , N) by moving the edge of the respective segment S
1
-S
N
according to the respective vector D
j
(j=1, 2, . . . , N), and the change in cumulative exposure dose received at each evaluation point A
i
(i=1, 2, . . . , N) can be determined from the respective vector.
Before actually moving each segment S
1
-S
N
, the cumulative exposure dose that would be received by the entire respective pattern element (e.g. as shown in FIG.
20
(
a
)) after moving the segments is determined. Desirably, the total exposure dose received by the element after moving the segments is unchanged from the total exposure dose that otherwise would be received by the pattern element before moving the segments. Hence, for each element, the vectors D
j
(j=1, 2, . . . , N) are re-determined in an iterative manner as required until the total exposure dose received by all the evaluation points A
i
(i=1, 2, . . . , N) of the element is equal to the total that otherwise would be obtained if the respective pattern element were unchanged from that shown in FIG.
20
(
a
).
In the method summarized above, the evaluation points A
i
(i=1, 2, . . . , N) are positioned on the sensitive substrate to correspond to the center points of each “pre-move” segment S
1
-S
N
. Alternatively, the distribution of cumulative exposure dose may be evaluated along respective lines intersecting each segment at an angle, as described in U.S. Pat. No. 5,698,859, incorporated herein by reference. For example, the distribution of cumulative exposure dose can be evaluated along a line
103
perpendicularly intersecting segment S
2
(FIG.
20
(
c
)).
FIGS.
21
(
a
)-
21
(
b
) show a second conventional method for calculating cumulative exposure dose, termed a “representative figure” method. This method achieves simplification of pattern-element figures by reducing the number of pattern-element figures referred to for calculating cumulative exposure energy (dose). I.e., the complex and fine pattern-element figures of, e.g., an LSI pattern are replaced (for analysis purposes) with simpl
Berman Jack
Fernandez Kalimah
Klarquist & Sparkman, LLP
Nikon Corporation
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