1. Technical Field
The invention relates to a spectrometer with a dispersing element, which is movable between at least two positions and where in the first position the radiation of a selected wavelength being dispersed once is reflected immediately into the incident light path.
The invention also relates to a method for measuring radiation from a spectral range with different spectral resolutions wherein the light beam is directed onto a dispersing element by an imaging optical system and from the dispersing element back to the imaging optical system. In particular the invention is related to the use of such spectrometers.
2. State of the Art
The spectrometers are also known as Littrow-spectrometers. In a Littrow arrangement the beam of a selected wavelength is at least approximately reflected into the incident light path after dispersion by a grating or a prism. Thereby the same optical components can be used as an imaging and camera optical system. A complete superposition of the incident and the dispersed beams is practically not possible, because a detector cannot be positioned in the entrance slit. A deviation is tolerable only for small angles, because the image detortions increase.
High resolution spectrometers with spectral bandwidths as small as 15 fm are used especially for measuring the spectral intensity distribution of lasers. Additionally to the spectral intensity distribution it can be desireable to determine the exact spectral position of the maximum of the intensity or of the line center. In order to achieve this an exact relationship between the position of the detectors or the picture points of the detector, respectively, and the respective wavelengths has to be determined.
Echelle-spectrometers are known, which comprise a grating with a stair-like cross-section. A diffraction pattern is generated by the stair-like structure with a suitable Blaze-angle, that concentrates the intensity in a very high order, for example in 80th to 100th order. These orders can overlap depending on the incident wavelengths.
In some of such Echelle-spectrometers the orders are dispersed once more perpendicular to the dispersion plane in order to separate the different occurring orders. In such a way a two-dimensional spectrum is obtained which can be detected with a linear or an array detector.
From the publication xe2x80x9cNovel metrology for measuring spectral purity of KrF lasers for deep UV lithographyxe2x80x9d by A. I. Ershov, G. G. Padmabandu, J. Tyler, P. Palash, in http://www.cymer.com on 17.3.1999 furthermore Echelle-spectrometers for lasers are known in which a collimated beam in a Littrow arrangement is directed to a grating. In the Littrow-arrangement the beam of a certain wavelength is reflected with such an angle, that it overlaps at least approximately with the incident beam.
The dispersed beam is directed to the linear detector by means of a mirror. Between the collimator lens and the grating a partially transparent mirror is arranged. Thereby a portion of the light is reflected at the mirror after dispersion at the grating and again dispersed at the grating. The partially transparent mirror is arranged such that its normal forms a small angle with the beam. Thereby a small angular shift of the beams with single and plural dispersion is achieved so that the peaks can be found on different locations on the detector.
The known arrangement makes use of an intensity reducing partially transparent mirror through which also the beam which is once dispersed must pass twice. Accordingly, with several passages the intensity is reduced even more. For measuring with a single passage without such a reduction the partially transparent mirror has to be removed from the light path.
It is the object of the invention to provide a high resolution spectrometer which is suitable for determining the spectral distribution of the intensity of an emission line relative to the line center and also the absolute wavelength position of the line center with high accuracy and sensitivity.
According to the invention this object is achieved in that in the second position the dispersed radiation of the selected wavelength falls on a reflecting element which is arranged such that the radiation passes at least once more over the dispersing element and then into the incident light path.
In such an arrangement the dispersing element can be switched in a first turn such that the beam is immediately returned back. By changing the position, for example by simply rotating the dispersing element, the beam can also be directed to the reflecting element from which it is reflected back to the dispersing element. In this case the beam passes the dispersing element twice. Accordingly a higher spectral resolution is achieved. As the beam is not divided a good signal-to-noise-ratio is maintained even with a plurality of passages. Only reflection and efficiency losses at the reflecting and dispersing element will reduce the intensity.
In order to separate the radiation according to the amount of passages means for deviating the beam from the dispersing plane can be provided. They are chosen such that the single-dispersion beam runs in a different plane than the multiple-dispersion beam. A mirror which is inclined about an axis which is parallel to the dispersion plane and perpendicular to the incident beam is particularity suitable for this purpose. However, the deviation can be achieved by a prism also.
In a preferred embodiment the dispersing element is a grating, which in a especially preferred embodiment is an Echelle-grating. Preferably the Blaze-angle of the Echelle-grating is at least 45xc2x0. Then the measurements operate in a high order with a large incident angle, whereby a high resolution is achieved according to the diffraction equation.
An Echelle spectrometer according to the invention in which the positions of the grating are determined by the angle relative to the incident beam has the advantage that both positions can be achieved by a simple rotation of the grating. The rotational axis is the same as the axis about which the grating has to be rotated to adjust the wavelength.
A mirror which can be a plane mirror or a prism which is coated with a reflecting layer on one side can serve as a reflecting element. The mirror or the prism preferably simultaneously serves to deflect the beam from the dispersing plane. This is achieved preferably by inclining the mirror by a very small angle about an axis which is parallel to the dispersing plane and perpendicular to the beam incident on the mirror. Thereby the multiple-dispersion beam is deflected a little bit up or down and occurs shifted in height in the image plane where usually the detector is positioned relative to the single-dispersion beam.
Advantageously the reflecting element is arranged such that the angle between the normal of the dispersing element and the dispersed beam is smaller than the angel the incident beam. Thereby the parallel bundle of rays falling on the grating has a smaller diameter than the bundle falling on the reflecting element. The reflecting element, for example the plane mirror, is larger in this case than the collimating mirror which is generally more expensive. However, there may be arrangements, where the opposite arrangement is more advantageous.
Preferably there are means provided in the spectrometer for controlling the angle with which the incident beam falls on the dispersing element. These means can be formed by a step motor moving a lever effecting a rotation of the dispersing element.
Preferably there are also means provided for controlling the angle with which the dispersed beam leaves the dispersing element. Furthermore means are provided for controlling the positions of the components with a computer. The grating can be rotated without further modification of the spectrometer by the computer and thereby the wavelength and the amount of grating passages can be set.
In a preferred embodiment of the invention an entrance slit is provided and means by which the beam is deflectable in such way that the primary image of this entrance slit at a selected wavelength is shifted sideways relative to the entrance slit in the dispersion plane. This sideways shift can be achieved by simple rotation of the grating by a small angle. The dispersion plane generally is defined as the plane perpendicular to the grooves of the grating, if the dispersing element is a grating. If the dispersing element is a prism, the dispersing plane is the plane which is perpendicular to the roof edge of the prism.
In an alternative embodiment of the invention means are provided by which the beam is deflectable in such way that the primary image of this entrance slit at a selected wavelength is shifted upwards or downwards relative to the entrance slit in the dispersion plane. Such a deflection can be achieved by inclining one of the imaging elements, for example a mirror.
Preferably only reflecting optical components are provided in the spectrometer apart from the dispersing element. Chromatic image distortions can thereby be avoided and the spectrometer can be used in every wavelength range. An optical arrangement can be provided for the magnification of the primary image only in the direction of the dispersion plane, for example by two cylinder lenses, by which it is achieved that the resolution of the arrangement is not limited by the finite width of the detector elements.
According to the invention the light beam is directed onto a dispersing element by an imaging optical system and from the dispersing element back to the imaging optical system to measure the radiation from a spectral range with different spectral resolutions. The light beam can then be directed to a reflecting element by a change of the position of the dispersing element, i.e. by rotation, from which reflecting element is is directed back to the dispersing element.
The following method can be applied:
(a) positioning of the dispersing element such that the dispersed light beam of a selected wavelength is reflected into the incident light path
(b) measuring of the signal at the detector,
(c) positioning of the dispersing element such that the dispersed light beam of a selected wavelength is reflected towards the reflecting element, and
(d) measuring of the signal at the detector.
If after the second positioning the light of a reference light source is coupled into the spectrometer the larger inspection range with smaller resolution can be used to provide a wavelength calibration of the detector elements by means of a reference light source. The reference light source, for example a low pressure hollow cathode lamp, generally has substantially less intensity than a laser. If the radiation emitted by a line of the reference light source is distributed on too many detector elements a poor signal-to-noise ratio is obtained. In order to determine the peaks of the line or the center of the line a lower resolution can be sufficient. It therefore is obvious to measure in this case with a single passage at the grating.
However, the intensity is sufficient for the measuring of the line profile of a laser and the line can be distributed on many detector elements. Then the grating can be rotated in a suitable manner such that measurment is carried out with several passages at the grating. Of course it is possible to measure the reference radiation first.
The spectrometer is particularity suitable for the determining of the spectral characteristics of an excimer laser used for the photolitography. The spectral characteristics of the excimer laser can be used for its adjustment and automatic control. In photolitography the quality of the laser can be determined from the line profile of the used laser. For example, if the spectral half width value of a KrF excimer laser is larger than a threshold value the gas mixture has to be exchanged. Peak-to-peak variations of the laser pulse can also be observed and evaluated in order to pass only such pulses to the waver which fulfill certain criteria. With the spectrometer according to the invention a resolution can be achieved which allows the simultaneous measurment of a line profile and therefore the adjustment and automatic control of the litography process due to the spectral characteristics of the laser.
Further embodiments are subject matter of the subclaims. Some embodiments of the invention are described hereinbelow with reference to the accompanying drawings. In which is shown:
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