Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-05-26
2003-08-05
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S069000, C355S053000
Reexamination Certificate
active
06603533
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an irradiation control method and irradiation control apparatus for a pulsed light source for irradiating a pulsed beam on an illumination subject. More specifically, the present invention relates to an irradiation control method and irradiation control apparatus for a pulsed light source for an exposure apparatus which is used in photolithography process for fabricating a micro device, such as a semiconductor device, a liquid crystal display device, an image pickup device (CCD or the like) or a thin-film magnetic head.
Conventionally, fabrication of devices, such as semiconductor devices, uses an exposure apparatus which transfers and exposes the pattern of a reticle as a mask on a wafer (or a glass plate) on which a photosensitive material, such as a photoresist, is coated. The recent miniaturization of circuit patterns demands that such an exposure apparatus have a higher resolution. A high resolution may be acquired by shortening the wavelength of exposure light or increasing the numerical aperture (N.A.) of a projection optical system. Increasing the numerical aperture of the projection optical system however makes the depth of focus shallower. Therefore, a practical solution is to employ, for example, an excimer laser which has a short wavelength.
When an excimer laser is used as a pulsed light source, the integrated amount of exposure may vary from one point to another in each shot area on a wafer. At the time of exchanging a wafer or at the time of measuring alignment of a reticle, wafer or the like, for example, the emission interval of laser beams becomes long. In such a case, laser learning control is halted such that the state of the gas in the laser chamber, for example, differs from that at the time of the last laser emission before the halt. As a result, the laser beam output varies. The energy of the pulsed beam emitted from the pulsed light source may go significantly above or below the set energy until the pulsed light source returns to a stable oscillation state since the initiation of emission of the laser beam.
Recently, modified illumination for irradiating exposure light from an illumination optical system onto a reticle via an annular aperture or a diaphragm with a small aperture has been adopted in order to cope with the requirement for higher resolution. When the normal illumination is switched to the modified illumination, however, the intensity of the energy of exposure light that reaches the top of a wafer changes. To cope with such a change in the energy of the laser beam, conventional exposure apparatus is equipped with an exposure dose control capability which sets the aforementioned integrated amount of exposure (integrated exposure energy) within a proper range.
Exposure apparatus includes a full wafer exposure apparatus and scanning exposure apparatus. The full wafer exposure apparatus projects and exposes the pattern of a reticle on the shot areas on a wafer when a wafer stage on which the wafer is placed is still. When a pulsed light source is used, the full wafer exposure apparatus basically employs an exposure dose control method based on cutoff control.
According to this cutoff control, part of the exposure light directed toward a wafer branches and is led to a photodetector called an integrator sensor. The integrator sensor indirectly detects the amount of exposure on the wafer. The integrated value of this detection result is checked to determine if is exceeds a predetermined level (critical level) corresponding to the set exposure amount. When the integrated value exceeds the critical level, the laser emission from the light source is stopped.
The scanning exposure apparatus scans a reticle and a wafer with a step and scan system or the like in synchronism with the projection optical system to thereby sequentially transfer the pattern of the reticle onto the individual shot areas on the wafer. For example, Japanese Unexamined Patent Publication No. 8-8160 and its corresponding U.S. Pat. No. 5,677,754 disclose a scanning exposure apparatus which integrates the energy of a laser beam prior to exposure of individual shot areas and controls the amount of exposure based on the integrated value.
Specifically, the integrated value is measured based on the amount of exposure detected by the integrator sensor, and the average energy E of one pulse of a laser beam is computed based on the integrated value. To acquire the linearity of the desired exposure dose control, the average energy E of the laser beam is adjusted such that the number of exposure pulses of the laser beam becomes an integer using the following equation (1):
So=N×E
(1)
where So is the set amount of exposure and N is the number of exposure pulses per point.
In the case of a scanning exposure apparatus, the following equation (2) in addition to the equation (1) should be satisfied.
V=Ws/N×f
(2)
where V is the scanning speed on a wafer or a wafer stage, Ws is the width (slit width) of a slit-like exposure area on the wafer surface in the scanning direction and f is the oscillation frequency of the pulsed light source.
In the exposure sequence of the conventional exposure apparatus, first, the set exposure amount So is set and the pulsed light source emits a laser beam having the maximum oscillation frequency fmax and a predetermined oscillation output. Then, the integrator sensor measures the average energy E of the laser beam and computes the number N of exposure pulses per point in each shot area on the wafer. Exposure parameters, such as the transmittance of a light reducing unit, the oscillation output of the pulsed light source, the exposure pulse number N and the oscillation frequency f of the pulsed light source, at the time of scanning exposure are determined based on the computed pulse number N, the maximum oscillation frequency fmax and the predetermined slit width Ws. In addition to those exposure parameters, the scanning speed V is also determined based on the equation (2).
Note that the conventional exposure apparatus measures the energy of a laser beam which has the maximum oscillation frequency fmax only once before exposing a wafer. The pulsed light source is designed to ensure the most stable energy state at the rated maximum oscillation frequency fmax. When the oscillation frequency f in the exposure parameters differs from the maximum oscillation frequency fmax, therefore, the actual energy of the laser beam that has been emitted at the oscillation frequency f differs from the energy measured first at the maximum oscillation frequency fmax. This makes it difficult to carry out accurate exposure on a wafer.
When an ArF excimer laser which has a short wavelength is used, particularly, the transmittances of the illumination optical system and projection optical system change with time due to moisture present in the exposure apparatus, an organic material such as a silicon-based material, or the like. In this case, the difference between the actual energy and the energy measured first becomes larger.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an irradiation control method and apparatus for a pulsed light source which can reduce a difference between the measured energy of a pulsed beam and the actual energy.
In a first aspect of the present invention, there is provided an irradiation control method for a pulsed light source for irradiating a pulsed beam on an illumination subject. In this method, the energy of a pulsed beam having a predetermined oscillation frequency emitted from the pulsed light source is measured first. Next, the oscillation frequency of the pulsed light source is determined in accordance with the number of pulses of the pulsed beam which is determined based on a relationship between the measured energy and a predetermined illumination energy. Then, the energy of the pulsed beam is adjusted when the determined oscillation frequency differs from the predetermined oscillation frequency.
In a secon
Nguyen Henry Hung
Nikon Corporation
Synnestvedt & Lechner LLP
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