Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1999-07-12
2003-03-25
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S068000
Reexamination Certificate
active
06538723
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a scanning exposure method and a scanning-type exposure apparatus and, more particularly, to a scanning exposure method and a scanning-type exposure apparatus used in a photolithographic process for manufacturing, for example, semiconductor devices, liquid crystal display devices, image pick-up devices (including CCDs), thin-film magnetic heads, and so on.
When manufacturing a semiconductor device or the like using a photolithographic technique, a projection exposure apparatus is used to transfer the pattern on a reticle, which functions as a mask, via a projection optical system onto the respective shot areas of a substrate (i.e., a wafer or a glass plate) coated with a photoresist layer. One of the essential functions of such a projection exposure apparatus is exposure control, i.e., keeping the integral exposure dose on each point on the wafer within an appropriate range.
In a projection exposure apparatus of a collective-exposure type, such as a stepper, a continuous light source (e.g., a super high-pressure mercury-vapor lamp) or a pulsed laser light source (e.g., an excimer laser) is generally used. A collective-exposure type exposure apparatus transfers the circuit pattern of a reticle as a whole onto the shot areas of a wafer, while keeping the wafer stage used for supporting the wafer at rest, unlike a scanning-type exposure apparatus. With either kind of light source, a cut-off control is generally employed as an exposure control device, in which a portion of the exposure light is extracted and guided to a photoelectric detector, called an integrator sensor, during the process of exposing a wafer which is coated with a photosensitive material (photoresist). The integrator sensor detects the amount of light energy to which the wafer is exposed. If a laser light source is used, laser beam emission is continued until the integral value of the exposure energy detected exceeds a predetermined critical level (hereinafter, referred to as the designated amount of exposure dose) required for that photosensitive material. If a continuous light source is used, the shutter is closed when the integral value of the exposure dose detected exceeds the critical level.
With a pulsed laser light source, there is a small variation or error in the energy of the pulsed laser beams. In order to overcome this, the wafer is exposed to a number of pulses greater than the minimum number of exposure pulses to achieve desired exposure-control accuracy. However, if, for example, a wafer coated with a high-sensitivity resist whose designated amount of exposure dose is considerably small is used, the intensity of each pulse of laser beam is high relative to the sensitivity of the resist. If the wafer is exposed to a number of pulses greater than the minimum number, the total amount of exposure dose would exceed the designated amount of exposure dose for that resist. Therefore, in the case in which a material whose designated amount of exposure energy is relatively small is used, an attenuator or the like is positioned in the optical path to attenuate each pulse of the laser beam so as to keep the number of laser pulses emitted to the wafer greater than the minimum number while preventing the exposure dose from exceeding the designated amount.
In recent years, scanning-type projection exposure apparatus, including a step-and-scan exposure apparatus, have been developed, in which a reticle and a wafer are synchronously scanned with respect to the projection optical system so as to transfer a larger circuit pattern imprinted on a reticle onto the shot areas of a wafer with high accuracy. The cut-off control method cannot be used in scanning-type exposure apparatus because the cut-off control method focuses on only one point on the wafer to detect the exposure dose. Therefore, two other control methods have been used for scanning-type exposure apparatus.
The first control method used in the scanning-type exposure apparatus is to simply integrate the quantity of each pulse of the illumination light beam to control the exposure dose. This method is called open exposure control. The second control method is the individual pulse control method, which is disclosed in, for example, Japanese Patent Application Laid-open No. 6-252022. In the individual pulse control method, the exposure dose of a plurality of sub-areas within slit-like exposure areas on the wafer is measured in real time to calculate an integral amount of exposure dose for each pulse of the beam, and the exposure dose of the next pulse of the beam is determined based on the previously calculated integral exposure dose. This method requires a complicated algorithm.
In the first control method, the energy of the pulsed light must be controlled so that the number of laser pulses emitted onto each point of the wafer is an integer in order to achieve a good linearity in the exposure control. The designated exposure amount is expressed as:
(designated amount of exposure dose)=(the number of pulses)×(average energy of a pulse) (1)
The average energy of a pulse is determined immediately before the exposure process from the values detected by the integrator sensor. To this end, an energy modulator is provided in the optical path.
FIGS.
28
(A) and
28
(B) illustrate examples of conventional energy modulators. The double-grating modulator shown in FIG.
28
(A) comprises a pair of grating plates
72
and
74
that are positioned in the optical path of the emitted laser beam LB. Grating
72
is a fixed grating in which transparent portions and light-blocking portions are alternately formed at a predetermined interval. Grating
74
is a movable grating that moves in the direction of the interval. The position of the movable grating
74
relative to the fixed grating
72
is changed to vary the transmittance with respect to the laser beam LB. The modulator shown in FIG.
28
(B) comprises a pair of glass plates
76
and
78
that are tilted in the optical path so that the inclination angle &THgr; is variable. An antireflection coating is applied to both surfaces of the glass plates
76
and
78
. The transmittance of the glass plates
76
and
78
changes according to the incident angle of the laser beam LB. Taking advantage of this property, the inclination angle &THgr; is controlled to regulate the transmittance with respect to the laser beam LB. As still another means of modulation, the energy of the laser source itself may be modulated.
In addition to equation (1), the scanning-type exposure apparatus must also satisfy equation (2):
V=Ws/N×f
(2)
where V is the scanning speed of the wafer (or the wafer stage) during scanning exposure, Ws is the width of a slit-like exposure area on the wafer, which is referred to as a slit width, N is the number of pulses emitted onto each point of the wafer, and f is the oscillation frequency of the laser.
In the exposure process, the total exposure dose (i.e., the designated amount of exposure dose) required for the wafer is determined first. Then, the average energy of the pulsed light is measured, and the number of pulses emitted onto each point of the wafer is calculated. Finally, the scanning speed V is determined using the slit width Ws and the oscillation frequency f as constants.
In the conventional technique, the oscillation frequency of the laser is fixed at the maximum oscillation frequency f
0
based on the maximum scanning speed, which is defined by the capabilities (including mechanical function) of the stage control system of the exposure apparatus.
In scanning exposure, the slit width Ws is a fixed value determined by the optical design of the system, and the oscillation frequency f of the laser is also a fixed value f
0
, which corresponds to the maximum scanning speed V
max
determined by the capabilities of the stage control system. When the number of pulses emitted to the wafer is the minimum number N
min
(N=N
min
), the scanning speed V is set to V
max
based on equation (2).
As in the coll
Hagiwara Shigeru
Ozawa Ken
Adams Russell
Kim Peter B.
Nikon Corporation
Nixon & Vanderhye P.C.
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