Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1999-02-23
2002-12-10
Le, N. (Department: 2858)
Photocopying
Projection printing and copying cameras
Illumination systems or details
Reexamination Certificate
active
06493066
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure apparatus for transferring a circuit pattern drawn on a mask onto a substrate coated with a photosensitive agent and, more particularly, to an exposure apparatus which uses a laser as a light source and with which good exposure amount control is possible when the oscillation wavelength of the laser source is to be changed.
2. Description of the Related Art
Conventionally, in the process of manufacturing a semiconductor element, e.g., an LSI or a VLSI, which is formed of a very fine pattern, a reduction projection exposure apparatus for reducing and projecting a circuit pattern drawn on a mask onto a substrate coated with a photosensitive agent and printing it is used. As the integration density of the semiconductor elements increases, a finer pattern is required. Along with a progress in the resist process, the exposure apparatus must cope with a finer pattern.
As a means for improving the resolving power of the exposure apparatus, a method of changing the exposure wavelength to a shorter one and a method of increasing the numerical aperture (NA) of the projection optical system are available. It is generally known that a resolving power is proportional to the exposure wavelength and inversely proportional to the NA. Attempts have been made to keep the depth of focus of the projection optical system while improving the resolving power. Generally, the depth of focus is proportional to the exposure wavelength and decreases to be in an inverse proportion to the square of the NA, and to improve the resolving power and to keep the depth of focus are contradictory issues. In order to solve this problem, the phase shift reticle method, the FLEX (Focus Latitude Enhancement Exposure) method, and the like are proposed.
Regarding the exposure wavelength, a KrF excimer laser having an oscillation wavelength near 248 nm has become the mainstream recently to replace a 365-nm i-line. As the next-generation exposure light source, an ArF excimer laser having an oscillation wavelength near 193 nm is under development.
From the viewpoint of the manufacturing cost of the semiconductor element, a further increase in throughput in the exposure apparatus has been made. For example, a method of shortening the exposure time per shot by increasing the output from the exposure light source, a method of increasing the number of elements per shot by increasing the exposure area, and the like are proposed as the attempts aiming at this increase in throughput.
In recent years, in order to cope with an increase in chip size of the semiconductor element, a step-and-repeat type, so-called stepper, which sequentially prints the mask patterns and moves them in the step manner is shifting to a step-and-scan type exposure apparatus which performs scanning and exposure while keeping the mask and wafer in the synchronized state and sequentially moves them to the following shot. This step-and-scan type exposure apparatus has a slit-like exposure field and can accordingly increase the exposure area without increasing the size of the projection optical system.
As the resolving power increases by increasing the NA of the projection optical system and decreasing the wavelength of the light source described above, an exposure amount must be given to the photosensitive material (photoresist) applied to the wafer at high precision, and an increase in precision in exposure amount control has been made.
FIG. 5
is a conceptual view of an exposure apparatus for exposing a circuit pattern image onto a wafer. Referring to
FIG. 5
, a beam emitted by an excimer laser
1
is shaped into a predetermined beam shape through a beam shaping optical system
2
and becomes incident on an optical integrator
3
formed by two-dimensionally aligning a plurality of small lenses. The optical integrator
3
forms a secondary source near its exit surface
3
a
. A light beam from the secondary source is focused by a first focusing lens
4
. A blind (not shown) for regulating the illumination range is arranged near a plane perpendicularly intersecting an optical axis including a focal point
6
of the first focusing lens
4
. A light beam from the first focusing lens
4
uniformly illuminates the pattern surface of a mask
8
through a second focusing lens
7
. The pattern of the mask
8
is reduced and projected by a projection optical system
9
to a wafer
10
coated with a photosensitive material. A half mirror
5
is placed between the first focusing lens
4
and focal point
6
to branch part of the light. The photoelectric conversion surface of a sensor
11
is placed near a focal point
6
a
of the branched light. Hence, the surface of the wafer
10
, the pattern surface of the mask
8
, and the plane perpendicularly intersecting the optical axis including the focal point
6
are conjugate, and accordingly, the sensor
11
detects the illuminance at a position equivalent to the pattern surface of the mask
8
.
A signal from the sensor
11
is amplified by an amplifier
12
and connected to an integrated exposure amount control unit
13
including a CPU (not shown). The integrated exposure amount control unit
13
is connected to the excimer laser
1
to control oscillation of the laser on the basis of the signal from the sensor
11
.
Exposure amount control in the step-and-repeat method in the above arrangement will be described with reference to FIG.
6
.
FIG. 6
is a flow chart of exposure amount control done by the CPU (not shown) included in the integrated exposure amount control unit
13
. When exposure is started for a certain shot, in step
100
, an emission pulse count m is set to 0 and a remaining exposure amount Ja(m−1) is set to an initial value (target exposure amount Ja). In step
101
, the target exposure amount Ja is divided by a standard exposure amount Js per pulse to calculate a total number P of pulses required for exposure. A remainder L of this division is a theoretical number of insufficient pulses (0≦L<1) obtained when exposure is performed under the above conditions.
In step
102
, the target exposure amount Ja is divided by the total number P of pulses to calculate a preset energy Je of the first pulse. Theoretically, when exposure is performed with this preset energy Je for the number P of pulses, the target exposure amount Ja is obtained.
In step
103
, the preset energy Je for one pulse is set for the excimer laser (excimer laser
1
in
FIG. 5
) through the integrated exposure amount control unit (integrated exposure amount control unit
13
in FIG.
5
). In step
104
, a 1-pulse emission instruction is made through the integrated exposure amount control unit (integrated exposure amount control unit
13
in FIG.
5
).
In step
105
, the emission count of the excimer laser (excimer laser
1
in
FIG. 5
) is incremented. In step
106
, an energy Jt per pulse is detected through the sensor (sensor
11
in
FIG. 5
) and the amplifier (amplifier
12
in FIG.
5
).
In step
107
, a current remaining exposure amount Jam is calculated from the previous remaining exposure amount Ja(m−1) and the current energy Jt per pulse.
In step
108
, the preset energy Je for the next pulse is calculated from the remaining exposure amount Jam, the total number P of pulses, and the emission pulse count m from the start of exposure.
In step
109
, the total number P of pulses required for exposure and the emission pulse count m from the start of exposure are compared. If P=m, the flow advances to exposure at the next shot position; if p>m, the flow returns to step
103
.
In this manner, the steps
103
to
108
are repeated until the emission pulse count m reaches the total number P of pulses.
Regarding exposure amount control of the step-and-scan method, for example, as shown in Japanese Patent Laid-Open No. 7-254559, a method is proposed by the present applicant, in which exposure amount control is performed by adjusting the light amount of a pulse beam to irradiate next in accordance with the integrated li
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Le N.
LeRoux Etienne P
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