High-resolution littrow spectrometer and method for the quasi-simultaneous determination of a wavelength and a line profile

Technical field

The invention relates to a spectrometer comprising a dispersing element that between at least two positions is movable and in the first position in the easily dispersed radiation of a selected wavelength directly in the incident beam falls back.

The invention further relates to a method for measuring radiation from a Spektalbereich with different spectral resolution in which the beam from a Imaging optics is directed onto a dispersing element and of the Dispersion element back to the imaging optics. The invention relates to specifically the use of such a spectrometer.

State of the art

The spectrometer described in the preamble of claim 1 are also known under the term Littrow spectrometer. In a Littrow arrangement falls of Beam of a selected wavelength according to the dispersion at a grating or prism approximately back into itself. This allows the same optical Components are used as imaging and camera optics. A complete Superposition of the incident and dispersed radiation is practically impossible, since the entrance slit detector can not be arranged. Departure is in generally only for small angles tolerable since the aberrations increase.

High-resolution spectrometer with spectral bandwidths, which are narrow up to 15 fm, In particular, when measuring the spectral intensity distribution of lasers used. In addition to the measurement of the spectral intensity distribution, it may also be desirable, the exact spectral position of the maximum intensity or the Line centroid to determine. These must have a precise relationship between the position of the Detectors, or the image points on the detector to the respective wavelengths done.

There are Echelle spectrometer known which mesh with a step-like Section included. By stages like structure with a suitable Blaze Angle produces a diffraction pattern, the intensity in very high order z. B. achzigster to one hundredth order concentrated. These orders can be - Depending on incident wavelength - overlie.

In some of such Echelle spectrometers the orders are therefore again perpendicular to the dispersion plane dispersed to occurring the various separate orders. This gives a two-dimensional spectrum with row or surface detectors can be detected.

The publication "Novel metrology for measuring spectral purity of KrF lasers for deep UV lithography "AI Ershov, GG Padmabandu, J. Tyler, P. Palash, in http://www.cymer.com 17.3.1999 remain Echelle spectrometer for laser are known in which a collimated beam in a Littrow arrangement is guided on a grating. In the Littrow arrangement, the beam of a specific wavelength is at an angle reflected on the grid, he nearly falls back into at least. The dispersed Beam is directed by a mirror on a line detector. between collimator Lens and the grating is a partially transparent mirror. Thereby, a part of light reflected by the first dispersion at the grating in the partially transmitting mirror and again dispersed in the lattice. The partially transparent mirror is arranged such that the normal it forms a small angle with the beam. As a result, a slight achieved angular displacement of beams with single and plural dispersion, so that the peaks are at different locations on the detector.

The known arrangement uses an intensity debilitating partially reflecting mirror, which must be traversed by the single-dispersion beam twice. at multiple passes, the intensity is reduced even more. For measurements at single passage without such a reduction must be made of the partially transparent mirror the beam path to be removed.

From US 5,771,252 is a hybrid assembly of a Littrow / Littman configuration is known in which a grating and a reflection prism connected to each other and together are pivotally as an element around a rotational axis. The arrangement is for Laser resonators. The beam always remains in this arrangement in a plane.

Disclosure of the Invention

The invention has for its object to provide a high-resolution spectrometer, with which both the spectral intensity distribution of an emission line relative to Line center and the absolute wavelength position of line with gravity high accuracy and sensitivity can be determined.

According to the invention the object is achieved by a spectrometer according to claim. 1

In such an arrangement, the dispersing element set once be that the beam is immediately returned. Through a change in position z. B. a simple rotation of the dispersing element, the beam can also a reflective element are conducted, from which it returns to the dispersing element is passed. Then the beam passes twice through the dispersing element. Accordingly, a higher spectral resolution is achieved. There the beam is not divided, a good signal remains at a plurality of passages get to noise ratio. Only reflection and efficiency losses reflecting and dispersing element will reduce the intensity.

For the separation of the radiation according to the number of passes for the agent can be provided from the dispersing plane deflection of the beam. They are sized, that the single-dispersion beam in a different plane than the runs repeatedly dispersed beam. A mirror, which is inclined about an axis parallel to the Dispersion plane and perpendicular to the incident beam, this is particularly suitable. The deviation can be achieved even with a prism.

In a preferred embodiment, wherein the dispersing element a grille that, in a particularly preferred embodiment, an echelle grating is. Preferably the blaze angle of the echelle grating is at least 45 °. Then is measured in the high-order at a large angle of incidence, which after Grating equation is obtained a high resolution.

An inventive echelle spectrometer, in which the positions of the grating by the angles are determined to the incident beam, has the advantage that the two Positions can be accomplished by a simple rotation of the grating. The axis of rotation is the same as the axis around which rotated the grating for wavelength adjustment must become.

As reflective element may according to the invention, a mirror as a plane mirror is organized or serve a one-sided mirrored prism. The mirror or the Prism preferably simultaneously serves for beam deflection of the beam from the Dispersion plane. This preferably takes place at a level around by tilting a very small angle about an axis that is parallel to the dispersion plane and perpendicular to the incident on the mirror beam. Thereby, the multiple dispersed beam deflected slightly up or down and appears to the single-dispersion beam offset in height in the image plane, at the usually a detector is arranged.

Advantageously, the reflecting element is arranged such that the angle which the the normal of the dispersing element and the dispersed beam is smaller than the angle formed by the incident beam. Thereby the parallel Ray bundle, which falls on the grating has a smaller diameter than the bundle, incident on the reflective element. The reflective element, eg. As the Plane mirror is then formed larger than the collimator mirror, which is generally is expensive. But there are also arrangements in which the reverse arrangement is more advantageous.

There are preferably means for controlling the angle at which the incident beam incident on the dispersing element provided at the spectrometer. This means may be formed by a stepper motor, with which a lever is movable with the rotation of the dispersing element is effected.

There are preferably also means for controlling the angle at which the dispersed Beam leaves the dispersing element. There are further provided means with which the positions of the components can be controlled from a computer. Then, without further modification of the spectrometer from the computer the grid rotated and hence the wavelength and the number of grid crossings set will.

In the spectrometer is preferably adjacent to the dispersing element exclusively reflective optical components are provided. Thereby avoid chromatic aberrations and the spectrometer can in each Wavelength range are used. An additional magnification z. B. by two Cylindrical mirror effect only in the direction of the dispersion level, that the resolution of Device is not limited by the finite width of the detector elements.

According to the invention, for measuring the radiation from a Spektalbereich with different spectral resolution of the beam of an imaging optical system on a dispersing element is passed back and forth from the dispersion element again the imaging optics. The beam is then optionally through a position change the dispersing element, z. B. rotation, on a reflective element conducted, from which it is directed back to the dispersing element.

The invention also relates to a method according to claim 18th

If the light to the second position of the dispersing element of a Reference light source is coupled into the spectrometer, then one can the smaller use resolution with large inspection area to a wavelength allocation to make the detector elements by means of a reference light source. The Reference light source, for. Example, a low-pressure hollow-cathode lamp, is generally significantly less intensity than about a laser. If the line of radiation then reference light source on too many detector elements distributed, is obtained for each Detector element a bad signal to noise ratio. To determine the Line peak or line focus extends u. U. a lower resolution of. It therefore makes sense here to measure with easy passage at the grating.

In the measurement of a line profile of a laser, however, the intensity is sufficient and the line can be spread over many detector elements. Here, the grating to be adjusted to that measured at multiple grid passage can. Of course is possible to measure the reference radiation.

An inventive spectrometer is particularly suitable for determining the spectral properties of an excimer laser for the photolithography. Thus, the serve spectral characteristics of the excimer laser to its control and regulation. In the photolithography can be out of the line profile of the laser used, the determine laser quality. If z. B. the spectral half width at a KrF Excimer laser is greater than a threshold value, the gas mixture must be replaced. Peak-to-peak variation of the laser pulses can be monitored and evaluated , so that only those pulses are actually conducted on the wafer, certain Fulfill criteria. With the spectrometer according to the invention, a resolution can be achieved which allows for the simultaneous measurement and thus a line profile control and control of the lithography process due to the spectral characteristics of the Laser.

Embodiments of the invention are subject of the subclaims. below are Some embodiments of the invention with reference to the accompanying Drawings. therein show

<b>Fig.</b> 1 The spectrometer according to the invention with double passage at the grating, sideways shifted exit slit and fixed mirror positions

<b>Fig.</b> 2 The composition of an Echelle grating.

<b>Fig.</b> 3 The spectrometer according to the invention with double passage at the grating, inclinable parabolic mirror and sideways shifted exit slit.

<b>Fig.</b> 4 The spectrometer according to the invention with single passage at the grating and perpendicular to the plane of dispersion shifted exit slit.

<b>Fig.</b> 5 The spectrometer according to the invention with double passage at the grating and perpendicular to the plane of dispersion shifted exit slit.

<b>Fig.</b> 6 The spectrometer according to the invention with verkipptem reflector.

<b>Fig.</b> 7 The spectrometer according to the invention with reduced external dimensions and additional magnification.

<b>Fig.</b> 8 a detector signal of a completely resolved doublett at twice Grid passage.

<b>Fig.</b> 9 A detection signal of the not fully resolved doublett from <b>Fig.</b> 8th with a simple grid passage.

Description of Embodiments

In <b>Fig.</b> 1, numeral 10 designates a spectrometer. The light is via an entrance slit<b>12</b> in the spectrometer <b>10</b> coupled. At an off-axis parabolic mirror<b>14</b> is the divergent beam to a parallel beam is collimated and an echelle grating <b>16</b> With led large Blaze angle of at least 45 degrees.

An echelle grating is in <b>Fig.</b> 2 again in detail. The echelle grating includes stepped grating grooves. The grating grooves have a distance d, which the lattice constant equivalent. The incident angle α is the angle between the incident beam <b>20</b> and the perpendicular <b>22</b> on the grating <b>16</b>, The incident angle is β the angle between the reflected beam <b>24</b> and the perpendicular <b>22</b> on the grating <b>16</b>, As blaze angle θ_B is the angle between the normal <b>22</b> on the grating <b>16</b> and the perpendicular to the grating grooves <b>18</b> designated.

A high blaze angle the intensity of the diffraction pattern is on high Diffraction orders in the area between achzigster and hundertster order concentrated. A high diffraction order causes a high resolution. Likewise causes a large diffraction angle high resolution. The echelle grating has a high line count of 50 to 100 strokes per millimeter to the diffraction-limited make resolution as large as possible. Due to the large angle of incidence, the grid must be correspondingly long, when all of the radiation to be incident on the grating.

In <b>Fig.</b> 1 is shown with an axis of rotation 26. The grid<b>16</b> is around this axis <b>26</b> rotatably disposed, as illustrated by an arrow labeled 28 becomes. The rotation can be achieved by the grid on a turntable (not shown) is attached, which is connected to a lever. Using stepper motors can from a computer the lever moves and thus a defined rotation be effected. Over the angle of rotation of the grating, the wavelength to be measured, or the wavelength range can be selected in a particularly simple manner.

The grid <b>16</b> is adjusted so that the dispersed beam for the selected Wavelength on a plane mirror <b>30</b> falls. This is indicated by the arrow<b>32</b> shown. Of the plane mirror <b>30</b> is such that the reflected beam falls back directly back in and again on the grating <b>16</b> applies. This is indicated by the arrow<b>34</b> shown. Thereby, a double passage causes thereby achieved an increased spectral resolution becomes. When the set angle of the grid<b>16</b> falls the beam <b>38</b> back into itself. This is indicated by the arrow <b>36</b> shown. Here, the beam falls<b>38</b> not complete in itself back, but only as far as the dimensions of the entrance slit <b>12</b> Allow to allow the Detector can still be placed next to it.

The beam is of the parabola <b>14</b> refocused and just beside the entrance slit <b>12</b> shot past where the spectrum in an exit plane <b>40</b> with a linear array of a is detectable detector. As a detector can be used a CCD line, it is However, any other detector line or any other suitable detector array, that a having sufficiently small pixel size.

In <b>Fig.</b> 3, the spectrometer <b>10</b> out <b>Fig.</b> 1 with a modification. The grid<b>16</b> is arranged such that the dispersed beam by an arrow <b>44</b> is shown is directly reflected back into itself, ie in the incident beam passing through an arrow <b>42</b> is shown, and not on the mirror <b>30</b> falls. The spectrometer<b>10</b> Has a smaller spectral resolution than with a grid position in the a double passage is effected.

By rotating the parabolic mirror <b>14</b> about an axis <b>46</b>, Indicated by an arrow <b>48</b> in <b>Fig.</b> 3 is shown, the beam may be shifted a little from the plane. Then the spectrum does not appear next to, but above or below the entrance slit <b>12</b>, This Shift occurs in a simple grid passage, as in <b>Fig.</b> represented 4 is, as well as a multiple passage at the grating, as in <b>Fig.</b> is shown fifth Of the returning beam, represented by an arrow <b>50</b>, That extends above the incident beam, represented by an arrow <b>52</b>,

There are grid positions at which the corresponding wavelength in an order directly on the parabolic mirror <b>14</b> running back, in a different order but just to the mirror <b>30</b>, The latter are reflected back again in the manner described above and run again back on the grid. Then one finds in the image plane of the primary image <b>40</b> two different signals from different orders. At least One of these signals is undesirable.

This problem can be solved by the mirror <b>30</b> about an axis <b>54</b> tilted is, which is parallel to the dispersion plane and perpendicular to the incident beam. Of the Tilt angle can be very low. Thereby, the resulting Aberrations minimized. However, it must be such that the<b>image</b> 58 of entrance slit <b>12</b> on the primary image plane in a multiple-dispersion beam or under the <b>image</b> 60 of the entrance slit <b>12</b> is a single-dispersion beam. In<b>Fig.</b> 6 is the <b>image</b> 58 of the multiple-dispersion beam below the <b>image</b> 60 of single-dispersion beam.

Instead of an inclined plane mirror <b>30</b> is also a correspondingly arranged Littrow Suitable prism (not shown). By unilaterally mirrored prism correspondingly small prism angle of the beam is somewhat out of the dispersion plane the grid <b>16</b> out bent. The<b>image</b> 58 and 60, the entrance slit <b>12</b> is then also displaced in the exit plane at the detector.

In <b>Fig.</b> 7 is a spectrometer according to the invention is shown in which the outer Dimensions were reduced. The light is by a light guide<b>62</b> in the entrance slit <b>12</b> passed. Thereby, the light source positioned at any point will. plane mirror<b>64</b> and <b>66</b> in the beam path cause folding of the Beam path. Also, the beam path by means of cylindrical mirror<b>68</b> and <b>70</b> folded. The cylindrical shape causes an optical magnification of the image in the direction of dispersion while the dimensions of the image remain the same perpendicular to the dispersion direction.

The peak is then distributed to more pixels of the detector and the Picture elements are no longer limit the resolution.

In the described arrangement, by a simple rotation of the grating <b>16</b> without further changes two different measurements with different resolutions and different intensities are made to a detector element. at the first grid position is dispersed the beam twice, then runs back only on the detector <b>74</b>, Thus a large resolution is achieved and the intensity of Light is distributed over many detector elements. This setting is therefore suitable in particular for the measurement of line profiles of intensity strong light sources, as laser light sources. A signal obtained with this setting<b>80</b> is in <b>Fig.</b> 8th shown. One sees two distinctly separated peaks<b>82</b> and <b>84</b>,

In a second grid position of the single-dispersion beam directly to the detector <b>74</b> returned. This gives a somewhat lower resolution and distributes the light on less detector elements <b>72</b>, The signal to noise ratio at each detector element is thereby greater. This setting is particularly to the emission spectrum of low-intensity light sources, such as from to measure low pressure hollow cathode lamps. Although Then perhaps the Line profile of the emission peaks are not completely dissolved, the resolution ranges but still made to the line of gravity or the intensity maximum to determine.

A signal obtained with this setting <b>86</b> the same light source as in <b>Fig.</b> 8 in <b>Fig.</b> shown. 9 One sees a double peak at a significantly reduced Background noise. The smaller noise is a fact that the same distributed intensity due to the lower resolution to less detector elements and secondly to the fact that the reflection losses at the mirror and particularly the grid not occur. The signal, however, is sufficient to z. B. a line center with a good to determine accuracy.

Combining the two measurements described above offers particular advantages:With high resolution the line profile can be measured. With the low Resolution, the wavelength of a reference light source, such as a Hollow cathode lamp are determined and so, the wavelength calibration of the Detector elements are made.

About a computer the grid position is relative to the position of the off-axis Parabolic mirror can be determined. The grid<b>16</b> may be rotatably mounted and means a lever to be rotated by a stepping motor. In this way, can also be Set smallest angle changes with sufficient accuracy. Similarly Example, a positioning of the parabolic mirror to be made.