Cell projection using an electron beam

Details Cell projection using an electron beam Cell projection using an electron beam

2000-05-17

2003-11-18

Lee, John R. (Department: 2881)

Radiant energy

Irradiation of objects or material

Ion or electron beam irradiation

C250S492100, C250S492200, C250S492220, C250S492230

Reexamination Certificate

active

06649920

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cell projection technique using an electron beam and, more particularly, to forming a fine pattern on a semiconductor wafer with a high accuracy in the cell projection process.
2. Description of a Related Art
With the development of semiconductor devices typically known as Large Scale Integrated-Circuits (LSIs) finer patterning technique is rapidly advanced in the fabrication procedure of the semiconductor devices. In a lithographic process in the fabrication, an electron beam exposure technique is expected to afford finer patterning compared to the conventional optical exposure, achieving as low as 0.25 &mgr;m rule or less suited for next generation semiconductor devices.
FIG. 1
schematically shows a typical electron beam exposure system using a cell projection (block exposure) technique, together with some details of the exposure system. In the exposure system, an electron beam
50
ejected by an electron gun
101
is shaped using a first mask plate
103
having a single rectangular aperture
103
A, and then patterned or further shaped by a second mask plate
106
having a plurality of rectangular apertures
106
A to
106
E and a single rectangular aperture
106
F. The electron beam
50
B passed by the second mask plate
106
generally has plural beam shapes corresponding to one of the patterns formed in the apertures
106
A to
106
E. The electron beam
50
B is incident onto a photoresist film formed on a semiconductor wafer
11
, thereby forming a fine pattern thereon.
The electron beam
50
consecutively passes a blanking electrode
102
, the first aperture
103
, a shaping lens
104
, a shaping deflector
105
, the second aperture
106
, a demagnification lens
107
, a main deflector
108
, a sub deflector
109
and an objective lens
110
to reach the semiconductor wafer
11
mounted on a sample stage
12
.
The first mask plate
103
having the aperture
103
A shapes the electron beam
50
to form a rectangular electron beam
50
A. The second mask plate
106
having apertures
106
A to
106
E, which are generally called cells or cell apertures, passes the rectangular electron beam
50
A at selected one of the cell apertures
106
A to
106
E. Thus, as exemplified in the drawing, an electron beam
50
B having a pattern shape provided by one of the apertures
106
A to
106
E is incident onto a photoresist film formed on the semiconductor wafer
11
. The patterns on the cell apertures
106
A to
106
E include a contact-hole pattern and line-and-space pattern, for example, defined by design data (or Computer Assisted Design (CAD) data) for the semiconductor device.
The pattern data is generally converted into a format suited for the electron beam exposure system and then stored in a storage device
15
from outside the exposure system. A CPU
14
reads the pattern data through a bus
13
from the storage device
15
, temporarily stores the same in a pattern data memory
17
, and conducts processing to the pattern data such as expanding or sorting of the pattern data. The pattern data thus processed is transferred through a control unit
16
to the blanking electrode
102
, shaping deflector
105
, main deflector
108
and sub deflector
109
, thereby irradiating the electron beam
50
B or
50
C having desired patterns to the desired location of the semiconductor wafer
11
. The main deflector
108
and the sub deflector
109
function for shifting the image of the cell apertures with respect to the semiconductor
11
. The shaping deflector
105
shifts the electron beam
50
A on the mask plate
106
for aligning a selected one of the cell apertures
106
A to
106
E and the variably shaped beam aperture
106
F to the electron beam
50
B for cell projection and variably shaped beam
50
C on the semiconductor wafer
11
.
The second mask plate
106
has aperture
106
F having a single rectangular thereon and used for a variably shaped shot which is attained by narrowing the sectional area of the electron beam
50
A. Thus, an electron beam
50
C having desired dimensions may be incident onto the semiconductor wafer
11
instead of the electron beam
50
B having plural patterns therein, as illustrated in FIG.
1
.
The cell projection technique as described above affords reduction of the number of exposure shots down to {fraction (1/10)} to {fraction (1/100)} of the number of exposure shots used in the conventional variably shaped electron-beam exposure system. Thus, the cell projection technique improves the throughput in the conventional variably shaped electron beam exposure system. In addition, the dimensions of the electron beam
50
B after passing through the second mask plate
106
are stable and accurate due to the dimensions of the apertures
106
A to
106
E, whereby a reliable pattern having accurate dimensions can be obtained.
The electron beam
50
B having a desired pattern shape therein is deflected by the main deflector
108
and the sub deflector
109
and incident onto desired location of the semiconductor wafer
11
.
For a DRAM, each of the cell apertures
106
A to
106
E may have a basic unit pattern, which is consecutively transferred onto different locations of the semiconductor wafer
11
to define a single overall pattern in the wafer
11
. In semiconductor devices, especially in the case of DRAM, if a misalignment occurs at the stitching boundary between adjacent shot areas due to a poor deflection by the deflectors
108
and
109
in the exposure system, the resulting overall pattern has a defect at the stitching boundary such as a distortion, disconnection or overlapping in the pattern. These defects may cause a functional error in the resultant semiconductor devices, and must be eliminated by using a suitable offset value for a reliable operation.
The term “offset value” as used herein means a correction amount to be applied for correcting the deviation at the stitching boundary between adjacent shot areas. In fact, if the adjacent areas are exposed by using the CAD data as it is, a variety of factors cause deviations at the stitching boundary between the shot areas in the resultant pattern.
Patent Publication JP-A-11-40482 proposes for solving the above problem, wherein a basic unit pattern in the CAD data is modified so that a single exposure shot does not include divided patterns which are included in a single-piece pattern. This is achieved by shifting the exposure area in the mask plate to include the whole single-figure pattern in the exposure area while maintaining the size of the exposure area.
The proposed technique may be effective to eliminate deviation at the stitching boundary so long as the single-piece pattern is concerned. However, this technique does not afford elimination of the deviation between the exposed area and the non-exposed area. Conventional techniques for solving such a deviation problem include one using a mask plate
106
, such as shown in
FIG. 2
, including adjustment apertures
106
G to
106
K each, having an adjustment pattern thereon used for correction of the shot location using a corresponding one of the cell apertures
106
A to
106
E. The pattern includes a first adjustment section for Y-direction and a second adjustment section for X-direction.
FIG. 3
shows an exposure process using the mask plate
106
of FIG.
2
. In the exposure process, the mask plate
106
is first mechanically shifted to the area of the adjustment apertures
106
G to
106
K (step S), followed by a trial exposure through the adjustment apertures
106
G to
106
K to a test wafer (step S
2
) by using a cell projection technique and a variably shaped technique
106
F.
FIG. 4
shows the result of the trial exposure, wherein master scales
21
(hatched portions) are formed by the cell projection shot and the vernier scales
22
(stained portions) are formed by the variably shaped
106
F shot. The offset value (Sx, Sy) is then calculated based on the dimensions of the vernier scales
22
with respect to the master scales
21
(step S
3
). The mask plate
106

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