Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-03-30
2002-01-08
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S071000
Reexamination Certificate
active
06337734
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an exposure control method for controlling the amount of exposure of a photosensitive substrate in an exposure apparatus used in a lithographic process for manufacturing, e.g., a semiconductor element, a liquid crystal element, an image sensing device (CCD, etc.) or a thin-film magnetic head, as well as to an exposure apparatus and device manufacturing method capable of employing this control method. The present invention is applicable not only to a batch exposure type exposure apparatus but also in a case where the amount of exposure is controlled in a step-and-scan scanning exposure type projection exposure apparatus in which part of the pattern on a mask (reticle) is projected onto a photosensitive substrate and the mask and substrate are then scanned synchronously with respect to a projection opticals unit, whereby the mask pattern is transferred to shot areas on the substrate to expose these areas to the mask pattern.
BACKGROUND OF THE INVENTION
FIG. 2
illustrates a projection exposure apparatus according to the prior art. The apparatus includes a light source
1
such as a high-voltage mercury-vapor lamp which emits illuminating light. The light from the light source
1
is condensed to a point by a condensing mirror
2
and impinges upon a fly-eye lens
4
through an opticals unit
3
. There are instances where a laser or the like may be used as the source of illuminating light, in which case the condensing mirror
2
is unnecessary and the light from the laser need only impinge upon the fly-eye lens
4
through the opticals unit
3
.
The fly-eye lens
4
is a bundle of rod lenses the entrance and exit surfaces of which have their focal points on each other's surface. A group of light beams that impinge upon the rod lenses at an identical angle are condensed at the exit surfaces and form a number of points of condensed light on the exit surface of the fly-eye lens.
Utilizing the group of condensed points of light formed on the exit surface of the fly-eye lens
4
, the opticals unit
5
uniformly illuminates the position of a diaphragm
6
, which controls an illuminated area at a position that is conjugate with the plane of a mask
8
. An opticals unit
7
is for forming the image of the position of the uniformly illuminated diaphragm
6
on the mask surface
8
. Uniform illumination of the mask surface
8
is achieved by forming the image of the position of the uniformly illuminated diaphragm
6
on the mask surface
8
. It should be noted that the position of mask
8
, the position of diaphragm
6
and the entrance surface of the fly-eye lens
4
are located at conjugate points.
The apparatus further includes a projection opticals unit
9
for forming the image of the pattern of mask
8
on a substrate
11
. A photosensitive agent that has been applied to the substrate
11
is exposed to the mask pattern by the illuminating light from the illuminating opticals unit. The projection opticals unit
9
is a telecentric unit in which projection magnification does not change even if the position of the mask
8
or the position of the substrate
11
shifts along the optical axis. The arrangement is such that a principal ray which passes through the center of the projection unit at the position of a diaphragm
10
perpendicularly intersects the surface of the mask
8
and the substrate
11
.
It should be noted that the diaphragm
10
of the projection opticals unit
9
and the exit surface of the fly-eye lens
4
are located at conjugate points.
The apparatus further includes a movable stage
12
on which the substrate
11
and an exposure sensor
15
are mounted. The exposure sensor
15
can be moved over the illuminated area when the amount of exposure at a position identical with that of the substrate
11
is measured with stepping movement for exposing a plurality of shots on the substrate
11
.
In such a projection exposure apparatus used in the manufacture of semiconductor devices and the like, it is required that the substrate be subjected to a proper amount of exposure, which depends upon the photosensitive agent that has been applied to the substrate and the pattern possessed by the mask
8
, in order that the mask pattern will be transferred to the substrate in optimum fashion. If the amount of exposure is less than the proper amount in a case where a positive pattern and a negative resist are used, for example, the photosensitive agent will not be sensitized sufficiently and the lines of the pattern may become too fine and be rendered discontinuous at points. If the amount of exposure is too large, on the other hand, the photosensitive agent will be sensitized excessively and the lines of the pattern may become so thick that neighboring lines will contact each other. Further, if the amount of exposure is less than the proper amount in a case where a negative pattern and a positive resist are used, the photosensitive agent will not be sensitized sufficiently and the lines of the pattern may become so thick that neighboring lines will contact each other. If the amount of exposure is too large, on the other hand, the photosensitive agent will be sensitized excessively and the lines of the pattern may become too fine and be rendered discontinuous at points. In any case, when exposure is carried out with an improper amount of exposure, a suitable pattern cannot be formed on the substrate. This invites a decline in yield when semiconductor devices or the like are manufactured.
Control of the amount of exposure to which the substrate
11
is subjected must be controlled in order to obtain the proper amount of exposure. However, the amount of exposure being applied to the substrate
11
cannot be measured directly during the transfer of the pattern of mask
8
to the substrate
11
. If the amount of exposure is measured along the optical path of the exposing light, the shadow of the exposure sensor has an influence when the mask pattern is transferred to the substrate
11
. For this reason, the amount of exposure is controlled upon measuring the amount of exposure at a position which is at a conjugate point with the substrate
11
and offset from the optical path of the exposing light.
Use is made of a half-mirror
13
, which has a very low reflectivity, inserted into the optical path of the exposing light in order to produce a position which is at a conjugate point with respect to the substrate
11
and offset from the optical path of the exposing light. That is, the half-mirror
13
produces a position which is at a point conjugate with the substrate
11
and offset from the optical path of the exposing light at the position of an exposure sensor
14
. The sensor
14
is placed directly in front of the point conjugate with the substrate
11
and at an inclination relative to the optical axis of the exposing light for the purpose of measuring the amount of exposure from the light diverted to it by the half-mirror
13
.
The exposure sensor
14
is so adapted as to be capable of measuring an amount of exposure that corresponds to the amount of exposure exactly at the center of the illuminated area, namely at the position of the substrate
11
on the optical axis. Before the substrate
11
is exposed, the exposure sensor
15
mounted on the stage
12
is moved to the center of the illuminated zone, trial exposure is carried out and the relationship between the amount of exposure at the position measured by the exposure sensor
14
and the amount of exposure on the substrate
11
is found, thereby making it possible to estimate the amount of exposure on the substrate
11
from the output of the exposure sensor
14
.
A controller
16
is for controlling the amount of exposure. On the basis of the output of the exposure sensor
14
, and in accordance with a predetermined control program, the controller
16
controls the amount of exposure by controlling the opening and closing of a shutter
17
, the transmittance of beam attenuating means
18
the transmittance of which is variable, and the input to the light source
1
.
As
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