Electron beam exposure system

An electron exposure system comprises an electron source for emitting an electron beam in a path along a predetermined optical axis, a mask carrying a plurality of apertures corresponding to a pattern that is to be written on an object, a first deflector unit provided at a side of the mask closer to the electron source for shifting the electron beam from the optical axis by causing a deflection of the beam such that the electron beam passes through one of the apertures in the mask means in a direction substantially perpendicular to the mask, a second deflector unit provided at a side of the mask away from the electron source for shifting the electron beam such that the electron beam travels again in a path coincident with the optical axis, a focusing system for focusing the electron beam on the object, and a third deflector unit for deflecting the focused electron beam onto the object, wherein the mask further includes a calibration part having a size corresponding to an area within which the electron beam is shifted by the first deflection means. The calibration part includes a plurality of apertures of a predetermined size and disposed relatively to each other at a common, predetermined interval for calibrating the deflection of the electron beam caused by the first and second deflector units.

BACKGROUND OF THE INVENTION 
The present invention generally relates to electron beam exposure systems 
and more particularly to an electron beam exposure system for writing a 
pattern on an object by an electron beam. 
With the requirement of increased integration density and reduced device 
size, use of the electron beam exposure systems is spreading in the 
fabrication of semiconductor integrated circuits. In the electron beam 
exposure system, an electron beam of rectangular or other suitable cross 
sections is used for writing a pattern on a semiconductor wafer while 
changing the size of the beam spot. Such an electron beam exposure system 
is particularly advantageous for patterning minute semiconductor devices, 
as the electron beam exposure system is capable of writing submicron 
patterns on the object. On the other hand, the electron beam exposure 
system has suffered from a problem of low throughput because of the 
feature pertinent to such a system that the pattern has to be written 
consecutively step by step by a single electron beam. 
In order to avoid the foregoing problem of low throughput, a technique 
called block exposure has been proposed (IEEE TRANS. ON ELECTRON DEVICES, 
vol.ED-26, p.633, 1979), wherein a patterned mask is placed in the path of 
the electron beam between the electron gun and the object. The mask 
carries a number of patterned apertures that correspond to various basic 
patterns of semiconductor devices, and deflector systems are disposed at 
both sides of the mask. Thereby, the deflector at the upstream side of 
electron beam causes a deflection of electron beam away from an optical 
axis and directs the same to one of the patterned apertures. On the other 
hand, the deflector at the downstream side causes a deflection of the 
electron beam back to the original optical axis. Upon passage through the 
selected aperture, the electron beam is shaped as desired and hits the 
surface of the object, such as a semiconductor wafer, at a predetermined 
position after focusing and deflection caused by the usual electron lens 
and deflection systems. 
FIG. 1 shows such an electron exposure system that carries out the block 
exposure. 
Referring to FIG. 1, the electron exposure system includes an electron gun 
14 that produces an electron beam 16. The electron beam 16 travels along a 
predetermined optical axis and experiences shaping upon passage through an 
aperture 18. After passing through the aperture 18, the electron beam is 
focused by an electron lens 20 at a point Pl located on the optical axis. 
At the point P1, there is provided an electrostatic deflector 22 that 
causes a deflection of the electron beam 16 away from the optical axis in 
response to a control signal applied thereto. At the downstream side of 
the deflector 22, there is provided a mask 10 in which a number of 
patterned apertures 12-1 - 12-5 are formed, and the deflector 22 causes 
the deflection of the electron beam 16 to one of the apertures on the mask 
10. Upon passage through the selected aperture, the electron beam 16 
experiences shaping for the second time and thus, the electron beam 
exiting from the mask 10 has a desired cross section. 
In correspondence to the mask 10, there is provided an electron lens 28 
such that the optical axis of the lens 28 coincides with the optical axis 
of the electron beam, and the lens 28 focuses the electron beam passing 
therethrough. More specifically, the electron beam 16 that has exited from 
the mask 10 is deflected and focused on a point P2 located on the original 
optical axis of the beam 16. Further, the electron beam is demagnified by 
a lens 30 and deflected by the deflectors 34 and 36 provided immediately 
above a semiconductor wafer 38. Thereby, the electron beam 16 hits the 
predetermined part of the surface of the wafer 38 with a desired cross 
section. 
In the foregoing conventional apparatus, however, there exists a problem in 
that, associated with the focusing action of the electron lens 28 that 
deflects back the electron beam to the point P2 on the optical axis, the 
electron beam tends to experience deformation by various aberration 
effects that are caused by the electron lens. It should be noted that the 
path length of the electron beam changes depending on what aperture on the 
mask 10 is selected. The effect of aberration appears strongly 
particularly when the aperture which is far from the optical axis is 
selected since as the electron beam is deflected by a large angle and then 
deflected back by also a large angle in such a case. 
In order to eliminate the foregoing problems, the applicant of the present 
invention has previously proposed an electron beam exposure system wherein 
an incidence side electron lens and an exit side electron lens are 
disposed respectively at the upstream side and the downstream side of the 
mask, and the electron beam passes through the aperture in the mask with 
an angle perpendicular to the plane of the mask. Between the incidence 
side electron lens and the mask, there is provided an incidence side 
deflector system for shifting the path of the electron beam parallel to 
the optical axis. Further, between the mask and the exit side electron 
lens, there is provided an exit side deflector system for shifting back 
the electron beam back to the original optical path. 
FIG. 2 shows the electron beam exposure system described above. 
Referring to FIG. 2, there is provided an electron gun 68 that produces an 
electron beam 70. The electron beam 70 travels along a predetermined 
optical axis 90 and passes through a beam shaping aperture 72. Thereby, 
the electron beam 70 experiences beam shaping and passes through an 
electron lens 74 subsequently. The electron lens 74 focuses the electron 
beam 70 on a point P1 located on the optical axis 90 and a minute 
adjustment of the electron beam is achieved by a deflector 76. 
The electron beam is then passed through another electron lens 78 where it 
is converted to a parallel electron beam, and the parallel electron beam 
thus formed hits a mask 40 substantially perpendicularly. The mask 40 is 
formed with a number of patterned apertures in correspondence to the mask 
10 of the system of FIG. 1, and the electron beam is shaped as desired 
upon passage through a selected aperture. The electron beam, thus passed 
through and shaped by the mask 40, is then received by another electron 
lens 92 located at the downstream side of the electron beam 40 and focused 
on a point P2 located on the optical axis 90. The mask 40 carries a large 
number of patterned apertures that which differ from each other and which 
are arranged in aperture groups, and these aperture groups are selectively 
placed into the area through which by the electron beam passes by a mask 
drive unit 104 which moves the mask 40 in a direction perpendicular to the 
optical path 90. 
In order to effect the desired shifting of the electron beam, the system of 
FIG. 2 employs a pair of deflectors 80 and 82 provided between the lens 78 
and the mask 40 wherein the deflector 80 deflects the electron beam away 
from the optical axis 90 and the deflector 82 deflects the electron beam 
thus deflected back to a path parallel to the original optical path 90. 
Further, there are provided deflectors 84 and 86 between the mask 40 and 
the lens 92 such that the electron beam, after passing through the mask 
40, is deflected toward the optical axis 90 by the deflector 84 and the 
electron beam thus deflected by the deflector 84 is further deflected by 
the deflector 86 such that the electron beam returns to the original 
optical path 90. The foregoing lens 92 focuses the electron beam, after 
experiencing the deflection of the deflector 86, on the point P2. Further, 
there is provided a control unit 88 which supplies control signals to the 
deflectors 80, 82, 84 and 86, and in response to the control signals, the 
foregoing deflection of the electron beam occurs. 
The electron beam thus obtained at the point P2 is shaped according to the 
pattern of the selected aperture on the mask 40 and is focused on the 
surface of a wafer 102 after passing through the usual electron optical 
system that includes lenses 94 and 98 as well as deflectors 100. Between 
the lens 94 and lens 98, there is provided an aperture or pinhole 96 for 
proper alignment of the electron optical system. More specifically, the 
pinhole 96 has a limited diameter of about 100 .mu.m and allows the 
passage of an electron beam there through only when the electron beam has 
traveled along the optical axis 90 through the electron lenses 94 and 98. 
The role of the pinhole 96 will be described later in relation to the 
present invention. 
In this electron exposure system, it should be noted that the electron beam 
passes through the aperture on the mask 40 in the direction substantially 
perpendicular to the plane of the mask. Further, the electron beam passes 
only through the central part of the lenses 78 and 92. Thus, one can 
eliminate the problem of aberration caused by the electron lens by using 
this electron exposure system. 
In such an electron exposure system known commonly as the block exposure 
system, it is necessary to calibrate the deflectors 80 and 82 at the 
upstream side of the mask 40 as well as the deflectors 84 and 86 at the 
downstream side. Such a calibration includes two types of calibrations, 
i.e., a) calibration about the mutual relationship of the deflection 
angles caused by the deflectors 80-86; and b) calibration about the 
absolute magnitude of deflection caused by the deflectors 80-86. Here, the 
calibration a) establishes a mutual relationship between the control 
signals applied to the deflectors 80-86 such that the electron beam 
deflected away from the optical axis 90 by the deflector 80 returns to the 
optical axis 90 again after deflection by the deflector 86. The 
calibration b) on the other hand determines the absolute magnitude of the 
control signals that are supplied to the deflectors 80-86 for the desired 
deflection. Such a calibration process is generally time consuming and 
decreases the productive throughput of the electron exposure. 
SUMMARY OF THE INVENTION 
Accordingly it is a general object of the present invention to provide a 
novel and useful electron beam exposure system wherein the foregoing 
problems are eliminated. 
Another and more specific object of the present invention is to provide an 
electron beam exposure system that uses a plurality of deflectors at both 
sides of a mask for shaping the electron beam, said plurality of 
deflectors deflecting the electron beam to address i.e. pass through, a 
selected aperture on the mask for beam shaping, wherein the calibration of 
the deflectors is achieved easily. 
Another object of the present invention is to provide an electron beam 
exposure system having a mask for shaping the electron beam, said mask 
carrying a plurality of patterned apertures for shaping the beam, said 
electron beam exposure system including a plurality of deflectors provided 
for addressing the apertures on the mask, wherein there is provided a 
first patterned aperture that is used for establishing a mutual 
relationship between the deflection caused by the plurality of deflectors, 
and wherein there is provided a second patterned aperture that is used for 
calibrating the absolute magnitude of the deflection caused in the 
electron beam. According to the present invention, the deflection of the 
electron beam is controlled such that the path of the electron beam is 
shifted parallel to the original path by the deflectors by an exact, 
desired distance and passes through the selected aperture. After passing 
through the selected aperture, the electron beam returns exactly to the 
original path, and thus coinciding with the optical axis of the electron 
beam exposure system. With the use of the first and second patterned 
apertures, the calibration of the deflectors is established efficiently. 
Other objects and further features of the present invention will become 
apparent from the following detailed description when read in conjunction 
with the attached drawings.

DETAILED DESCRIPTION 
Hereinafter, a first embodiment of the present invention will be described 
with reference to FIG. 3, which shows a the mask used in combination with 
the system of FIG. 2 and FIG. 4, which shows a cross section of the FIG. 
23 view of mask 40. In the description below, those parts which are 
described previously with reference to FIG. 2 are designated by the same 
reference numerals and the description thereof will be omitted. 
Referring to FIG. 3, the mask 40 carries a number of patterned apertures 
that are classified into several types. For example, there is a pattern 
group 42 that is divided into areas 56, 58, 60 and 62 each including 
patterned apertures such as 48, 50, 52 and 54 in correspondence to a 
pattern which is to be written on the wafer 102 by a single shot. 
Typically, each of the areas 56, 58, 60, 62, . . . has an edge dimension 
of about 500 .mu.m. There are a number of different pattern groups 42 on 
the mask 40, each including a respective different set of apertures. 
In addition to the pattern group 42, there is provided a first calibration 
part 44 which includes a cutout 64 formed in the mask 40. The cutout 64 
may for example have a size of 5 mm for each edge in correspondence to the 
maximum amplitude of beam deflection. By providing the cutout region 64 
with such a size, one can deflect the electron beam freely without being 
interrupted when the mask 40 is set such that the electron beam passes 
generally at the center of the pattern 44. As shown in the elevational 
cross section of FIG. 4 taken along the line 4-4' of FIG. 3, the cutout 64 
penetrates through the mask 40 and passes the electron beam there through 
freely. 
Further, there is provided a second calibration part 46 wherein a number of 
patterned apertures 66 are formed in a row and column formation. Here, 
each patterned aperture 66 is separated from adjacent apertures in the 
same part 46 by an equal distance that may be 500 .mu.m, for example. When 
the electron beam hits the aperture 66, the mask 40 allows the electron 
beam to pass there through freely, while when the electron beam misses the 
appropriate aperture 66, in calibration part 46 the beam is interrupted. 
Next, the operation of calibration in the electron beam exposure system of 
FIG. 2, achieved by using the mask 40 of FIG. 3, will be described. 
Referring to FIG. 2, the mask drive unit 104 is energized initially such 
that the mask 40 is moves to a position at which the optical axis 90 of 
the electron beam 70 is located generally at the center of the cutout 64. 
Next, the control unit 88 applies various control voltages to the 
deflectors 80-86 until the electron beam passes through the pinhole 96. 
More specifically, a predetermined control signal is applied to the 
deflector 80 and the control signals to the rest of the deflectors 82-86 
are changed until the electron beam reaches the wafer 102 after passing 
through the pinhole 96. In order to detect the passage of the electron 
beam through the pinhole 96, a current detector 103 is used. The magnitude 
of the current that is detected by the detector 103 is on the order of 
several microamperes. As the pinhole 96 has a reduced size of 100 .mu.m or 
less as described previously, a minute deviation of the electron beam from 
the optical axis is sufficient for causing the interruption of the 
electron beam and thereby preventing if from reaching the substrate 102. 
As a result of the calibration, the relative relationship between the 
respective magnitudes of the control signals to the deflectors 80-86 is 
established. 
When the aforesaid relationship between the control signals to the 
deflectors 80-86 is thus established, the mask drive unit 104 moves the 
mask 40 again to a position at which the optical axis of the electron beam 
is located at the center of the second calibration part 46. Under that 
condition the control unit 88 the control signals while maintaining the 
aforesaid relationships, as established in the preceding step. During this 
process, the current is monitored by the detector 103. Thus, when the 
detector 103 detects the current, it means that the electron beam hits one 
of the apertures 66 in the second calibration part 46. As the apertures 66 
are displaced by a common interval, one can calibrate the control signals 
as a function of the deflection of the electron beam. It should be noted 
that the object 102 is not necessarily a semiconductor wafer but any other 
object may be used during this calibration procedure. When the object is 
an insulating material, a thin conductive coating may be applied. 
When the calibration of the deflectors 80-86 is thus completed, the writing 
of the pattern on the semiconductor wafer 102 is started. Thereby, the 
electron beam selectively hits one of the areas 56 of the pattern group 42 
and the electron beam having the desired shaped cross section is formed 
upon passage therethrough. As already described, there are a number of 
pattern groups 42 on the mask 40, and the addressing of any selected 
aperture is made with precision as a result of the calibration achieved 
previously. 
FIG. 5 shows a second embodiment of the present invention. Referring to 
FIG. 5, a mask selection unit 116 is provided instead of the mask drive 
unit 104, and the unit 116 selectively drives a plurality of masks 
105-108. Here, the mask 105 carries thereon a number of basic patterns 110 
corresponding to the pattern group 42, the mask 106 carries thereon a 
first calibration part 112 corresponding to the cutout 64, and the mask 
108 carries thereon a second calibration part 114 corresponding to the 
apertures 66 of the part 46. In this embodiment, the mask 110 alone is 
used during the calibration for the mutual relationship of the control 
signals, the mask 112 alone is used during the calibration for the 
absolute magnitude of the control signals, and the mask 114 alone is used 
in the actual exposure of the semiconductor wafer. 
Further, the present invention is not limited to the embodiments described 
heretofore, but various variations and modifications may be made without 
departing from the scope of the invention.