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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of exposing a peripheral part of wafer
which is used for a fine pattern formation process in the processing of
ICs, LSIs or other electronics elements. More particularly, this invention
relates to remove, in a development step, an unnecessary portion of a
photoresist coated on a semiconductor substrate, typically a silicon
wafer, or a substrate consisting of a dielectric, a metal or an insulator.
2. Description of the Prior Art
In the manufacture of ICs and LSIs, for forming a fine circuit pattern of a
photoresist pattern is formed by coating a photoresist on the surface of a
silicon wafer or the like and exposing and developing the coated
photoresist. The photoresist pattern thus formed is used as a mask to
effect ion implantation, etching, lifting-off and other steps.
Usually, the photoresist is coated by spin coating. The spin coating
process is disclosed in Japanese Un-examined Patent Publication("KOKAI
KOHO" in Japan) 61-79227 and 61-73330. In the spin coating process, the
wafer is spun while pouring photoresist onto the center position of the
right side of the wafer and the poured photoresist is coated on the right
side of the wafer by centrifugal forces. In this spin coating process,
however, the photoresist gets off the peripheral part of the wafer and is
brought to the wrong side of the wafer.
The photoresist, which is coated on the peripheral part of wafer and
brought to the wrong side of the part, is not exposed in a exposing
process for forming circuit pattern. Therefore, these photoresists remain
unnecessary after development.
The remaining unnecessary photoresist coated on a peripheral part of wafer
gives rise to the following problem. The wafer with the photoresist coated
is conveyed in various steps and by various systems or units. A peripheral
part of wafer thus is sometimes mechanically chucked and sometimes rubs
walls of wafer cassette or like wafer accommodation means. In such cases,
the unnecessary photoresist portion on the peripheral part of wafer is
liable to be removed and re-attached to the pattern formation part of the
wafer. In this case, correct pattern formation can no longer be obtained,
thus reducing the yield.
The fact that the unnecessary photoresist remaining on a peripheral part of
wafer becomes "refuse" to reduce the yield poses significant problems
particularly in view of the recent trend for higher functional level and
finer pattern in Integrated Circuit. So, for removing the unnecessary
photoresist remaining on a peripheral part of wafer, a technique of
removing the photoresist by a solvent spray method is in practical use.
In this method, the solvent is sprayed on the wrong side of a peripheral
part of wafer, solving and removing the unnecessary photoresist.
SUMMARY OF THE INVENTION
This invention has an object of providing a method of exposing a peripheral
part of wafer to remove unnecessary photoresist, which can permit exposure
of a peripheral part of wafer covering a fixed distance from the wafer
edge with high accuracy and high efficiency.
In this invention, there is provided a method of exposing a peripheral part
of wafer with turning a wafer on which photoresist coated, exposing a
peripheral part of the wafer to light guided by optical fiber lightguide,
detecting the edge of the wafer by sensor, and controlling the light
emission end of the optical fiber lightguide to expose a fixed distance
from the edge of the wafer according to signal from the sensor.
With the method for exposing a peripheral part of wafer according to the
invention, the photoresist portion on a peripheral part of wafer covering
a fixed distance from the wafer edge can be exposed with high accuracy,
and also it is possible to dispense with a mechanism of centering the
wafer with respect to the center of turning of the stage and mechanism of
detecting, positioning and exposing the orientation flat section.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a sectional view showing photoresist coated on a peripheral part
of wafer.
FIG. 2 is a view showing the shape of an exposed circuit pattern on the
wafer.
FIG. 3(a)-3(c) are views for explaining a prior art wafer edge exposure
method.
FIG. 4(a) is a schematic view for explaining one embodiment of the method
of exposing a peripheral part of wafer according to the invention. FIG.
4(b) is a view for explaining the relation between the portion of
detecting wafer edge and the position of exposure.
DESCRIPTION OF THE PREFERRED EMBODIMENT
This invention will be described concretely hereinafter on the basis of
embodiments shown in accompanying drawings.
FIG. 1 is a cross sectional view showing photoresist coated on a peripheral
part of wafer. In the Figure, reference numeral 1 designates a wafer, 1p a
peripheral part of wafer 1, 1a a photoresist portion on a pattern
formation part of the wafer, 1b a photoresist portion on a peripheral part
1P, and 1c a photoresist portion brought to the wrong side of the wafer 1
from the edge. The photoresist portion 1c brought to the wrong side of the
wafer is not exposed in an exposure step for pattern formation, and if it
is a positive type photoresist, it remains after the development.
FIG. 2 is a view showing a circuit pattern formed by exposure on the wafer.
Each area labeled T corresponds to one circuit pattern. In a peripheral
part of wafer, a correct circuit pattern can not be formed in many cases,
or it can be formed with an inferior yield. The photoresist portion on
peripheral part of wafer is substantially unnecessary in the circuit
pattern formation. But, it cannot be helped that the photoresist is coated
on the peripheral part of wafer in spin coating, as shown in FIG. 2.
In prior-art method, the solvent spray method, although the photoresist
portion 1c as shown in FIG. 1 can be removed, the unnecessary photoresist
portion 1b on the surface of the peripheral part of wafer can not be
removed. Further, if it is arranged such that the solvent is sprayed on
the right side of the wafer 1 for removing the unnecessary photoresist
portion 1b, not only there arises the problem caused by scattering of the
solvent, it is impossible to remove only the unnecessary photoresist
portion 1b with high controllability and providing for a sharp borderline
between the unnecessary photoresist portion 1b on the peripheral part of
the wafer and photoresist portion 1a on the pattern formation part of the
wafer which is necessary as a mask layer for a subsequent etching or ion
implantation process.
Recently, it has been in practice to expose a peripheral part of wafer so
as to remove unnecessary photoresist in the development process, in
addition to the exposure for pattern formation.
FIGS. 3(a) and 3(b) are views for explaining the prior art peripheral
exposure method for such unnecessary resist removal.
A wafer 70 is set on a turning stage 71 by a conveyor system(not shown),
and vacuum chucked on the stage 71. Then, the stage 71 begins to turn, and
an orientation flat section 73 of the wafer 70 is detected by a sensor 72.
When the orientation first section 73 is brought to a substantially
predetermined position, the wafer 70 is stopped. Then, the centering of
the wafer 70 and positioning of the orientation flat section 73 are done
by the pushing board 74a and the pushing pins 74b to 74d are done.
Subsequent to the centering and the positioning, a peripheral part of the
wafer 70 is exposed to light from a light emission end 75 of the optical
fiber lightguide, while the wafer 70 turning. By the exposure of the
peripheral part with the turning motion(in the direction of arrow 76a),
the orientation flat section 73 is not completely exposed. Therefore, it
is necessary to stop the orientation flat section 73 at a predetermined
position, and move the light emission end 75 in a direction parallel to
the orientation flat section 73(in the direction of arrow 76b) to effect
exposure.
Although in the above exposure method the exposure is done with linear
motion to the orientation flat section and turning motion to other section
of peripheral part, sometimes a follow cam and a follow roller are also
used to move the light emission end 75 along the peripheral part of the
wafer 70.
With this peripheral exposure method, a borderline between the unnecessary
photoresist portion 1b (in FIG. 1) on the peripheral part of wafer and
photoresist portion 1a (FIG. 1) on the pattern formation part that is
necessary as a mask layer for a subsequent ion implantation or like step
can be removed sharply and with high controllability. Thus, this method is
superior to the solvent spray method.
To this end, however, it is necessary to expose with high accuracy only an
area covering a fixed distance from the wafer edge, which is
predetermined, as unnecessary photoresist portion. In the prior art
exposure method, it is necessary to set the center of wafer on the center
of the turning stage, to detect and position the orientation flat section
of the wafer.
The process, therefore, takes a long time. Further, it is difficult to
obtain sufficient accuracy because the centering and the positioning is
done mechanically. For example, if the centering could be obtained with an
error of 0.2 mm, this error will appear as an exposure width error of
double the value, i.e., 0.4 mm, when the exposure is done by rotating the
wafer. Further, the fluctuation of diameters of wafers is directly
reflected on the exposure width. Further, where wafers having different
diameters or substrate having various shapes are processed, the mechanical
control is very cumbersome.
Further, where the follow cam and follow roller are also used, the accuracy
of exposure is determined in dependence on the accuracy of the cam. So, it
is necessary to make the accuracy higher, but the accuracy has a
limitation. Besides, if it is intended to increase the speed of the
follow, the accuracy is deteriorated.
Accordingly this invention provides a method for exposing a peripheral part
of wafer which can permit exposure of a wafer portion covering a fixed
distance from the wafer edge with high accuracy and high efficiency.
Now, an embodiment of this invention will be described.
FIG. 4(a) is a schematic view for explaining an embodiment of the method or
exposing a peripheral part of wafer according to this invention, and FIG.
4(b) is a view for explaining the relation between the wafer edge
detection position and exposed light pattern. Referring to FIG. 4(a)
reference numeral 10 designates a mercury lamp, 11 an oval converging
mirror, 12 a flat mirror, 13 a shutter, 14 a shutter actuator, e.g., a
rotary solenoid, 8 an optical fiber lightguide, 8a a light emission end of
the lightguide 8 provided with a focusing lens(not shown), 8a-1 a servo
motor, 21 a light emitter, 22 a light receiver, 1 a wafer, lp a peripheral
part of the wafer, 6 a stage, 20 a system controller, and 15 a wafer
conveyor unit. Referring to FIG. 4(b), reference numeral or symbol 1p
designates a peripheral part of wafer, 1d the wafer edge, .sub.0 a wafer
edge detection position by a photo-sensor, and S exposed light pattern.
Referring to FIG. 4, when the wafer 1 is brought by the wafer conveyor unit
to a position above the stage 6, the stage 6 is raised so that the wafer 1
is set on the stage 6. Then, the wafer 1 is fixed on the stage 6 with an
operation of a vacuum chuck mechanism provided for the stage 6.
Thereafter, a servo mechanism as will be described later is operated to
move the light emission end 8a of the optical fiber lightguide 8 that has
been at a retreated position to a position to expose the peripheral part
1p. The turning of the stage 6 is started, and the system controller 20
supplies a shutter "open" signal to the shutter actuator 14 to open the
shutter 13, whereby the peripheral exposure in this embodiment of the
invention is effected. As the mercury lamp 10, a superhigh pressure
mercury lamp of short-arc type is used in this embodiment. Further, the
arc position of the mercury lamp 10 is set to a first focal point of the
oval projection mirror 11, and the light incidence surface of the optical
fiber lightguide 8 is set at a second focal point of the mirror 11. Thus,
light from the mercury lamp 10 is efficiently led through the lightguide 8
to expose the peripheral part 1p.
In FIG. 4(a), the light emitter 21 and light receiver 22 constitute a
transmission type photo-sensor, which can detect the wafer edge ad by
detecting a change in the amount of received light. Portion of the light
emitter 21 and light receiver 22 are driven by the servo mortar 8a-1 in
unity with light emission end 8a so that the photo-sensor can detect the
wafer edge 1d at all time. The light emission end 8a is controlled by
controller 20 according to the signal from the photo-sensor in order that
it can expose a fixed distance from the wafer edge 1d. The predetermined
width of the peripheral part 1p can be changed by changing this distance
as desired. According to the invention, accurate and efficient exposure of
a peripheral part of wafer can be obtained without need of any wafer
centering mechanism and orientation flat section detection/positioning
mechanism.
This embodiment adopts an arrangement, in which the wafer edge detection
point O and exposed light pattern S overlap. However, the edge detection
point O and the exposed light pattern S need not overlap, and the former
may be located in front of or behind the latter in a range, in which the
necessary exposure accuracy can be obtained.
In this embodiment, the exposed light pattern S has a rectangular shape
because the light emission end 8a of the optical fiber lightguide has a
rectangular shape. This is so because the rectangular shape make the
integral of exposed light amount substantially fixed in the peripheral
part 1p in the circumferential direction of the wafer 1.
Further, in this embodiment light from the light emission end 8a of the
optical fiber lightguide 8 is focused by a focusing lens(not shown) on the
wafer 1, so that the exposed light pattern S has a sharp contour, and
photoresist can be removed sharply after the development. The photoresist
portion 1c that has been brought from the edge of the wafer to the wrong
side thereof as shown in FIG. 2, may be removed by spraying a solvent
against the wrong side of the wafer as in the prior art, or it may be
removed in the development step by providing another optical fiber
lightguide exposing the wrong side of the wafer 1.
Further, while in this embodiment the sensor for detecting the edge 1d of
the wafer 1 is a transmission type photosensor consisting of the light
emitter 21 and light receiver 22, this is by no means limitative; for
example, it is possible to use a reflection type photo-sensor or an
electrostatic type sensor.
Furthermore, the scope of utility of the invention is by no means limited
to semiconductor wafers of silicon, gallium arsenide, etc., and the
invention is also applicable to the manufacture of surface elastic wave
elements of tantalum lithiumate, tantalum niobate, etc., bubble memory
elements, and other electronics elements, liquid crystal elements,
magnetic recording elements and other electronics elements. Further, the
wafer shape is not limited to circular shapes, but it may be rectangular.
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