Pattern-transfer methods and masks

Details Pattern-transfer methods and masks Pattern-transfer methods and masks

1999-01-14

2001-02-27

Rosasco, S. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Radiation modifying product or process of making

Radiation mask

C430S022000

Reexamination Certificate

active

06194102

ABSTRACT:

FIELD OF THE INVENTION
The invention pertains to pattern transfer methods, apparatus, and masks for transferring patterns to a substrate such as a semiconductor wafer.
BACKGROUND OF THE INVENTION
Charged-particle-beam (CPB) pattern-transfer methods (such as electron-beam microlithography) take advantage of the high-resolution imagery available with electron beam optical systems. However, the throughput achieved with conventional CPB methods is low.
So-called “hybrid” pattern-transfer methods, known as “cell-projection,” “character-projection,” or “block exposure” are useful for transferring patterns containing large numbers of repeating subunits (each measuring, for example, about 5 &mgr;m by 5 &mgr;m). In hybrid pattern-transfer methods, one or more of the subunit patterns is defined by a mask and repeatedly transferred onto the wafer with a projection-exposure process. Such methods are particularly well suited for pattern transfer for the highly repetitive patterns of high-density memory chips (e.g., DRAMs). Hybrid pattern-transfer methods used in the production of a semiconductor integrated circuit (such as a DRAM) permit throughputs that are about one tenth the throughput achievable using other pattern-transfer methods.
Unfortunately, hybrid pattern-transfer methods improve throughput only for transferring patterns having significant repetition. For non-repetitive patterns, throughput is not improved because of the long times required to transfer the non-repetitive pattern portions to the wafer.
In so-called “divided-field” pattern-transfer methods, an entire circuit pattern for a semiconductor die is defined by a mask. An electron beam (or other CPB) irradiates an area of the mask and an electron-optical system projects a demagnified image of the irradiated area onto a wafer. The electron-optical system is typically a two-stage projection-lens system. Because the electron beam does not irradiate the entire circuit pattern simultaneously, the circuit pattern is divided into a number of fields or “stripes”, and each stripe is further divided into sub-fields. The sub-fields are sequentially irradiated with the electron beam and imaged onto the wafer. The patterns from the sub-fields must be accurately connected (“stitched”) on the wafer so that the entire die pattern is transferred. To control aberrations, the electron-optical system is adjusted for each sub-field. This method is discussed in, for example, U.S. Pat. No. 5,260,151 and Japanese laid-open patent document Hei 5-160012. Divided-field pattern-transfer methods permit excellent aberration control, but the commercial application of such methods in the fabrication of ULSI semiconductor devices is limited by their throughput.
A particularly important problem for these divided-field pattern-transfer methods is the precise stitching of the images of the mask sub-fields on the wafer. One method for connecting sub-field patterns is the “gray-splicing” method described in Japanese laid open patent document Sho 63-1032. In the gray-splicing method, mating edges of two sub-fields projected adjacent on the wafer have the same patterns and the patterns are projected to overlap on the wafer. The area of the wafer in which the patterns from the two sub-fields overlap receives two exposures, corresponding to the exposures of each of the two sub-fields. The dose (total charge per unit area) delivered to the wafer in each exposure is adjusted so that the total dose of the two exposures in the area of overlap equals the dose delivered to single-exposed areas of the wafer. Unfortunately, the gray-splicing method is applicable only to pattern-transfer methods using a variably shaped CPB or a focused CPB beam.
Idesawa et al. describe tapering the dose distribution of exposures made in a feature-connection area located between two adjacent sub-fields on the wafer. See Idesawa et al., “Discontinuity Reduction Method in Pattern Connection,”
J. vac. Sci, Technol.
, 19 (4), November/December 1981, p. 983. This technique produces a constant dose over the entire wafer with inconspicuous stitching of patterns between the mating sub-fields. Unfortunately, however, this technique is applicable only when the beam shape is variable. In addition, tapering the dose distribution is difficult.
Another problem of the divided-field method is the difficulty of accurately stitching together the images of adjacent patterns on the wafer in the presence of distortion in the pattern-transfer apparatus optical system. This distortion problem is illustrated in FIG.
7
. Images
498
,
499
of respective square sub-fields on the wafer exhibit pincushion distortion caused by the optical system of the pattern-transfer apparatus. (The images can also exhibit barrel distortion.) Respective corners
498
a
,
499
a
of the distorted images
498
,
499
overlap as imaged on the wafer, and the overlapping region is therefore overexposed by the pattern-transfer apparatus. In addition, because the images
498
,
499
are distorted, a region
497
between the images
498
,
499
remains unexposed. The unexposed region
497
can cause circuit defects such as short circuits or open circuits while the double exposure at the corners
499
a
,
499
b
can produce circuit features that are larger or smaller than intended.
One solution to these problems is described in Japanese laid open patent document Hei 7-2098550 and corresponding U.S. Pat. No. 5,523,580. With reference to
FIG. 8
, pattern line segments
500
,
510
are defined by patterns in respective sub-fields. The pattern line segments terminate with respective end portions
501
,
511
. The pattern line segments
500
,
510
and the respective end portions
501
,
511
are imaged onto the wafer so that the end portions
501
,
511
overlap and a continuous line is formed.
The end portions
501
,
511
, define a checkerboard pattern with transmitting sections such as exemplary squares
501
a
,
501
b
(shown in
FIG. 8
as squares with hatch marks) and absorbing sections such as exemplary squares
501
c
,
501
d
(shown in
FIG. 8
without hatch marks). When the images of the two adjacent line patterns are projected on the wafer, the end portions
501
and
511
overlap on the wafer, so that the transmitting sections of the end portion
501
coincide with the absorbing sections of the end portion
511
, and vice versa. Thus, the checkerboard patterns formed on the end portions
501
and
511
are complementary. If the images of the sub-fields are accurately projected onto the wafer without distortion, the images form a single continuous straight line with no overexposed or underexposed regions because of the patterns of the end portions
501
,
511
are complementary Another prior-art solution is illustrated in FIG.
9
. Pattern lines
600
,
610
are defined by respective sub-fields and include triangular end portions
601
,
611
. (The hatched areas in
FIG. 9
represent transmitting sections) The pattern lines
600
,
610
are ideally projected to form a single straight line.
These prior-art methods have several disadvantages. Individual squares such as exemplary absorbing squares
501
c
,
501
d
of the checkerboard pattern
501
are almost completely surrounded by transmitting sections such as squares
501
a
,
501
b
. Thus, the transmitting section
501
a
is not adequately supported and additional support must be provided. For example, a two-layer mask construction can be used in which an absorbing material is supported by a transmitting layer. However, fabrication of such a two-layer mask is difficult and the exposure contrast obtained with such a mask is less than that of a single-layer mask. In addition, the transmitting supporting layer is heated by the charged-particle beam.
Referring again to
FIG. 9
, the triangular end portions
601
,
611
are not complementary. Accordingly, if the images of pattern lines
600
,
610
(and the triangular end portions
601
,
611
) are offset due to distortion or misalignment, short circuits, open circuits or other defects arise. Even if the images are precisely connected,

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