Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1991-04-22
1993-03-30
Turner, Samuel A.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356346, G01B 902
Patent
active
051988720
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates generally to a wavelength detecting apparatus which is employed for detecting the wavelength of a laser light. More particularly, the present invention relates to a wavelength detecting apparatus which is preferably employable for detecting the wavelength of an excimer laser light especially in the case where an excimer laser light unit is installed as a light source for an equipment for projecting and exposing an image in a reduced scale wherein the equipment is installed for producing semiconductor devices.
BACKGROUND ART
Attention has been paid to employment of an excimer laser light unit as a light source for an equipment for projecting and exposing an image in a reduced scale (hereinafter referred to as a stepper) wherein the stepper is installed for producing semiconductor devices. The reason for employment of the excimer laser light unit consists in that many significant advantages can be expected from the employment of the excimer laser light unit based on the fact that an excimer laser light has a short wavelength (e.g., about 248.4 nm in the case of a KrF laser light), one of the foregoing advantages being the presence of a possibility that a limit of light exposure is extensively set to the range shorter than 0.5 micron, other one being such that the unit has a deep focus compared with the conventional mercury lamp including a g line and an i line on the assumption that they have a same resolution, respectively, another one being such that the unit has a small number of lens apertures (NA), another one being such that the exposure range can be enlarged and further another one being such that the unit can generate a large magnitude of power.
Since the excimer laser light has a short wavelength of 248.4 nm, only a quartz, CaF.sub.2, MaF.sub.2 or the like material are available as a material employable for permitting the laser light having the foregoing wavelength to permeate therethrough. In this connection, it should be added that the quartz only can be employed as a raw material for lenses due to various restrictions associated with optical uniformity, working accuracy and others. This makes it very difficult to design a lens employable for projecting an image in a reduced scale with its chromatic aberration corrected properly. For the reason, in the case where the excimer laser light unit is used as a light source for the stepper, there is a need of restricting the use of a laser light outputted from the excimer laser light unit within the narrow-band range to such an extent that the chromatic aberration may be neglected and moreover stably controlling the wavelength of an output laser light within the narrow-band range with a high accuracy.
A monitor etalon has been heretofore used for measuring or detecting the wavelength of an output light from an excimer laser light unit adapted to oscillate within the narrow-band range. The monitor etalon is constructed by using an air gap type etalon comprising a pair of partially refractive mirrors arranged opposite to each other with a predetermined gap therebetween. Here, the wavelength of a light to permeate through the air gap type etalon is expressed by the following equation. partially refractive mirrors constituting the etalon, n designates a refractive index between the partially refractive mirrors and .theta. designates an angle defined by a normal line of the etalon and an optical axis of the incident light.
As is apparent from the equation, the angle .theta. varies corresponding to variation of the wavelength, provided that n, d and m are constant, respectively. The monitor etalon is used to detect the wavelength of a light to be detected by utilizing the characteristic nature as mentioned above. With respect to the above-described monitor etalon, it has been found that as the pressure within the air gap and the environmental temperature vary, the angle .theta. varies correspondingly even through the wavelength is kept constant. Thus, in the case where the monitor etalon is used, the wavelength
REFERENCES:
patent: 4914662 (1990-04-01), Nakatani et al.
Kobayashi Yukio
Kowaka Masahiko
Wakabayashi Osamu
Fish Ron
Kabushiki Kaisha Komatsu Seisakusho
Turner Samuel A.
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