Patent Description:
Such apparatuses are used for producing extremely fine structures, in particular on semiconductor components or other microstructured component parts. The operating principle of said apparatuses is based on the production of very fine structures down to the nanometre range by way of generally reducing imaging of structures on a mask, a so-called reticle, on an element to be structured, a so-called wafer, that is provided with photosensitive material. The minimum dimensions of the structures produced are directly dependent on the wavelength of the light used. Recently, light sources having an emission wavelength in the range of a few nanometres, for example between <NUM> and <NUM>, in particular in the region of <NUM>, have increasingly been used. The wavelength range around <NUM> is also referred to as the EUV range. Apart from EUV systems, the microstructured component parts are also produced using commercially established DUV systems, which have a wavelength of between <NUM> and <NUM>, in particular <NUM>. As a result of introducing the EUV range, and hence the possibility of being able to produce even smaller structures, there has also been a further increase in the demands in respect of the optical correction of the DUV systems with a wavelength of <NUM>. Additionally, there is an increase in the throughput of each new generation of projection exposure apparatuses, independently of the wavelength, so as to increase the profitability; this typically leads to a greater thermal load and hence to more imaging aberrations caused by the heat. To correct the imaging aberrations, use can be made of optical assemblies, such as manipulators, for example, which alter the position and alignment of the optical elements or else influence the imaging properties of the optical elements, in particular mirrors, by deforming the optical effective surfaces. In this case, an optical effective surface should be understood to mean that part of a surface of an optical element which is impinged on by used light for the imaging of structures. Alternatively, the optical elements of the optical assemblies which are embodied as mirrors can be temperature-regulated by means of direct water cooling. For this purpose, the mirrors comprise cutouts which are embodied as cooling channels and which are arranged in one or more planes at different distances from the optical effective surface and through which temperature-regulated water flows. An accurate measurement of the surface temperature is advantageous for both approaches.

Exemplary solutions for temperature measurement are disclosed in several publications.

For example, US patent publication <CIT> discloses an arrangement for mirror temperature measurement in a microlithographic projection exposure system. In this case, the temperature is measured through an access channel from the rear side of the mirror.

<CIT> describes a temperature measurement on an optical element using a special coating or a Bragg reflector.

International Patent Application <CIT> discloses the monitoring of a collector mirror of a source using infrared sensors.

German Patent Application <CIT>discloses the temperature monitoring of a mirror by means of infrared sensors from its rear side. The patent applications <CIT> and <CIT> disclose a projection exposure apparatus comprising an infrared camera, which detects the temperature of the surface of a mirror. That has the disadvantage, however, that the infrared camera detects not only the radiation emitted from the surface of the mirror but also the radiation of the thermal radiation reflected from the surroundings at the mirror surface and, as a result, a differential between the temperature of the mirror surface and the surroundings cannot be determined with a sufficient accuracy.

It is an object of the present invention to provide a device which eliminates the above-described disadvantages of the prior art.

This object is achieved by a device having features of the independent claim. The dependent claims relate to advantageous developments and variants of the invention. A projection exposure apparatus for semiconductor lithography according to the invention comprises at least one optical element and at least one temperature recording device for detecting a temperature on an optical effective surface of the optical element by means of the thermal electromagnetic radiation emitted or reflected from the optical effective surface of the optical element. According to the invention the device comprises a polarisation filter for filtering the electromagnetic radiation.

The filter can be arranged between the temperature recording device and the optical element. By virtue of the fact that the radiation used for detecting the temperature is filtered before it reaches the temperature recording device, disturbing radiation that cannot be used for determining the temperature of the surface or would corrupt the measurement can be masked out, with the result that the accuracy of the temperature measurement is improved.

According to the invention, the filter is embodied as a polarisation filter. This makes use of the fact that a predominant proportion of the disturbing radiation does not originally originate from the surface of the optical element, but rather emanates from other constituent parts of the projection exposure apparatus and has only been reflected from the surface of the optical element in the direction of the temperature recording device. A partial or complete polarisation occurs, however, when electromagnetic radiation is reflected at interfaces, that is to say that portions of radiation that are polarised perpendicularly to the plane of incidence of the radiation are predominantly reflected by the surface of the optical element. The interface thus acts in the manner of a polariser in a polariser/analyser arrangement. The portions of radiation mentioned can then be filtered out by the polarisation filter arranged upstream of the temperature recording device, said polarisation filter acting as an analyser in this case. As a result, the radiation emitted by the surroundings, that is to say for example the housing surrounding the optical element, and reflected by the optical effective surface can be partly or completely suppressed, such that primarily the unpolarised radiation emitted by the optical effective surface itself or considerable portions of this radiation impinge(s) on the temperature recording device and can be detected by the latter.

In this case, it may be advantageous if the temperature recording device is arranged in such a way that the radiation reflected at the surface and incident on the temperature recording device is reflected at an angle close to the Brewster angle. The Brewster angle is the angle at which the reflected light, when radiation is incident on an interface between two media, is polarised maximally perpendicularly to the plane of incidence of the radiation. The Brewster angle is dependent on the refractive indices of the two media and the frequency of the electromagnetic radiation and is <NUM>° for the interface between air and SiO2, for example.

Furthermore, the polarisation filter can be embodied as a rotating polarisation filter. As a result, a known frequency can be impressed on the intensity of the reflected, polarised thermal radiation, said intensity being detected by the temperature recording device. In this way, the signal portion attributed to reflected (and unwanted) radiation can easily be identified by the temperature recording device and is taken into account in the determination of the surface temperature of the optical element.

To improve the measurement accuracy further, the device can comprise a lock-in amplifier. A lock-in amplifier amplifies a weak electrical signal which is modulated with a known frequency and phase. In the case of a projection exposure apparatus, the radiation used for imaging a structure onto a wafer is usually clocked. In this regard, by way of example, an optical element embodied as a mirror can be irradiated with a clocking of <NUM> second radiation on and <NUM> second radiation off. In the case described, this can have the effect that the bulk material of the mirror reaches a temperature of approximately <NUM>° Celsius, but the topmost layers of the mirror reach a surface temperature of between e.g. approximately <NUM>° Celsius and <NUM>° Celsius which changes cyclically over time. This cyclically hot topmost layer is very thin and its proportion of the total thermal radiation is likewise small as a result; it may correspond in particular only to <NUM>% of the total thermal radiation of the mirror. However, exactly this clocked signature of the radiation can be amplified, detected and determined with the aid of the lock-in technique.

The lock-in technique can also make it possible to dispense with the use of a filter. The same applies to the variants described below. Each of these variants by itself or else in combination with one or more other variants can also be realised without the use of a filter.

Furthermore, at least one element of the projection exposure apparatus whose radiation impinges on the temperature recording device as a result of reflection at the optical element can be configured to be temperature-regulated. What is achieved thereby is that radiation which reaches the temperature recording device and which does not originate directly from the surface - of interest for the measurement - of the optical element is emitted at least by an element whose temperature and thus whose thermal radiation can be controlled. By means of the measure according to the invention, the error contribution of the thermal radiation reflected on the surface of the optical element can be managed better than has been possible hitherto according to the prior art.

Additionally or alternatively, the surface of the optical element can comprise a coating having an emissivity for the wavelength range detected by the temperature recording device of <NUM>, preferably greater than <NUM>, and particularly preferably of greater than <NUM>. The surfaces of the optical element usually have coatings for reflecting electromagnetic radiation having a wavelength of between <NUM> and <NUM>. An additional layer, which does not reduce the reflectivity in the range of <NUM> to <NUM> and can increase the emissivity of the surface for a wavelength of e.g. <NUM> to <NUM> to the values described further above, can therefore be formed on the surface. It goes without saying that it is also conceivable to increase the emissivity for wavelengths outside the range mentioned.

In one advantageous variant of the invention, at least one element of the projection exposure apparatus whose radiation impinges on the temperature recording device as a result of reflection at the optical element can be embodied in such a way that the emissivity for the wavelength range detected by the temperature recording device is greater than <NUM>, preferably greater than <NUM>, and particularly preferably greater than <NUM>. The element can likewise be coated, wherein a functional layer, as in the case of the optical element for semiconductor lithography, does not have to be taken into account here. In this case, the element acts like a beam trap for thermal radiation emanating from the surroundings since the high emissivity mentioned is also accompanied by a high absorbance.

Furthermore, the temperature recording device can comprise an IR camera.

In particular, the temperature recording device can be designed to detect a partial region of the surface of the optical element. By limiting the field of view of the camera, the radiation detected by the IR camera as a result of reflection at the optical element, which radiation represents disturbing parasitic radiation for the detection of the surface temperature of the optical effective surface, can be reduced to a minimum. In the case of a plurality of cameras, the cameras can detect different partial regions of the optical element.

Furthermore, the temperature recording device can be configured to detect in a scanning fashion the radiation emitted or reflected by the optical element.

In particular, the temperature recording device can be movable in such a way that the radiation emitted or reflected by the optical element is detected in a scanning fashion.

It may also be advantageous if an optical unit of the temperature recording device can be moved in such a way that the radiation emitted or reflected by the optical element is detected in a scanning fashion. The scanning detection of the radiation has the advantage that infrared cameras having a narrow field of view can be used. The scanning movement can also be embodied in such a way that the angle of the detected radiation is always close to the Brewster angle even in the case of concave or convex surfaces of the optical element. In this context, close to the Brewster angle is taken to mean that the angle is reached with a deviation of e.g. a maximum of <NUM>°.

Exemplary embodiments and variants of the invention are explained in more detail below with reference to the drawing. In the figures:.

<FIG> illustrates an exemplary projection exposure apparatus <NUM> in which the invention can be used. The projection exposure apparatus <NUM> serves for the exposure of structures on a substrate which is coated with photosensitive materials, and which generally consists predominantly of silicon and is referred to as a wafer <NUM>, for the production of semiconductor components, such as computer chips.

In this case, the projection exposure apparatus <NUM> substantially comprises an illumination device <NUM> for illuminating an object field <NUM> in an object plane <NUM>, a reticle holder <NUM> for receiving and exactly positioning a mask provided with a structure and arranged in the object plane <NUM>, said mask being a so-called reticle <NUM>, which is used to determine the structures on the wafer <NUM>, a wafer holder <NUM> for mounting, moving and exactly positioning precisely said wafer <NUM>, and an imaging device, namely a projection optical unit <NUM>, having a plurality of optical elements <NUM>, which are held by way of mounts <NUM> in a lens housing <NUM> of the projection optical unit <NUM>.

The basic functional principle in this case provides for the structures introduced into the reticle <NUM> to be imaged on the wafer <NUM>, the imaging generally reducing the scale.

A light source <NUM> of the illumination device <NUM> provides a projection beam <NUM> in the form of electromagnetic radiation, said projection beam being required for the imaging of the reticle <NUM> arranged in the object plane <NUM> onto the wafer <NUM> arranged in the region of an image field <NUM> in an image plane <NUM>, said electromagnetic radiation being in a wavelength range of between <NUM> and <NUM>, in particular. A laser, a plasma source or the like can be used as source <NUM> for this radiation, also referred to hereinafter as used light. The radiation is shaped by means of optical elements <NUM> in an illumination optical unit <NUM> of the illumination device <NUM> in such a way that the projection beam <NUM>, when incident on the reticle <NUM> arranged in the object plane <NUM>, illuminates the object field <NUM> with the desired properties with regard to diameter, polarisation, shape of the wavefront and the like.

An image of the reticle <NUM> is generated by way of the projection beam <NUM> and, after having been correspondingly reduced by the projection optical unit <NUM>, is transferred to the wafer <NUM> arranged in the image plane <NUM>, as has already been explained above. In this case, the reticle <NUM> and the wafer <NUM> can be moved synchronously, so that regions of the reticle <NUM> are imaged onto corresponding regions of the wafer <NUM> virtually continuously during a so-called scanning process. The projection optical unit <NUM> has a multiplicity of individual refractive, diffractive and/or reflective optical elements <NUM>, such as for example lens elements, mirrors, prisms, terminating plates and the like, wherein said optical elements <NUM> can be actuated for example by means of one or a plurality of actuator arrangements, not illustrated separately in the figure.

<FIG> shows by way of example the basic construction of a microlithographic EUV projection exposure apparatus <NUM> in which the invention can likewise find application. The construction of the projection exposure apparatus <NUM> and the principle of the imaging of a structure on a reticle <NUM> arranged in the object plane <NUM> onto a wafer <NUM> arranged in the image plane <NUM> are comparable to the construction and procedure described in <FIG>. Identical component parts are designated by a reference sign increased by <NUM> relative to <FIG>, i.e. the reference signs in <FIG> begin with <NUM>. In contrast to a transmitted-light apparatus as described in <FIG>, in an EUV projection exposure apparatus <NUM> only optical elements <NUM>, <NUM> embodied as mirrors can be used for imaging and/or for illumination on account of the short wavelength of the EUV radiation <NUM> used as used light in the range of <NUM> to <NUM>, in particular of <NUM>.

The illumination device <NUM> of the projection exposure apparatus <NUM> comprises, besides a light source <NUM>, an illumination optical unit <NUM> for the illumination of the object field <NUM> in an object plane <NUM>. The EUV radiation <NUM> in the form of optical used radiation generated by the light source <NUM> is aligned by means of a collector, which is integrated in the light source <NUM>, in such a way that it passes through an intermediate focus in the region of an intermediate focal plane <NUM> before it is incident on a field facet mirror <NUM>. Downstream of the field facet mirror <NUM>, the EUV radiation <NUM> is reflected by a pupil facet mirror <NUM>. With the aid of the pupil facet mirror <NUM> and an optical assembly <NUM> having mirrors <NUM>, the field facets of the field facet mirror <NUM> are imaged into the object field <NUM>. Apart from the use of mirrors <NUM>, the construction of the downstream projection optical unit <NUM> does not differ in principle from the construction described in <FIG> and is therefore not described in further detail.

<FIG> shows a detail view of a projection exposure apparatus <NUM>, <NUM> as described in <FIG> or <FIG>, in which an optical element embodied as a mirror <NUM> and arranged in a housing <NUM> is illustrated. In the example shown, the housing <NUM> comprises a cutout <NUM> closed off by a window <NUM>, thereby avoiding gas exchange with the surroundings through the cutout <NUM>. A temperature recording device embodied as an infrared camera <NUM> in the example shown detects, through the window <NUM>, partly or completely the optical effective surface <NUM> irradiated with used light <NUM>, <NUM> as that part of the surface of the mirror <NUM> which is under consideration in this case. The window <NUM> is transmissive to the thermal radiation <NUM>, <NUM> which is detected by the infrared camera <NUM> and which is relevant to the temperature determination. In other embodiments of the invention, the temperature recording device can also be situated elsewhere, in particular also within the housing. A window would not be necessary in this case. The mechanical links of the mirror <NUM> and of the infrared camera <NUM> to the housing <NUM> are not illustrated for reasons of clarity. The illustration likewise does not show one or a plurality of optional temperature sensors in the mirror material, which can be used for calibration purposes, in particular. The optical effective surface <NUM> of the mirror is heated as a result of absorption of used light <NUM>, <NUM>, wherein the temperature on the optical effective surface <NUM> is not constant on account of the non-constant distribution of the used light <NUM>, <NUM> on the optical effective surface <NUM> and different heat flows in the mirror <NUM>. Actuators can compensate for possibly unwanted rigid body movements of the mirror. In order to compensate for the temperature differences by way of the optical effective surface <NUM> and the deformation effected thereby, the mirror <NUM> can be temperature-regulated by a temperature-regulating device (not illustrated) and/or can be deformed by actuators (not illustrated), which can be arranged at the rear side of the mirror <NUM>, for example, in such a way that the deformations formed as a result of the heating of the mirror <NUM> are compensated for. To that end, the surface temperature of the optical effective surface <NUM> is determined by the infrared camera <NUM> comprising a filter <NUM> and a controller <NUM>. The controller <NUM> is connected to the infrared camera <NUM> via a line <NUM>, wherein in the case where the mirror <NUM> is cooled and/or deformed, the temperature-regulating device and/or the actuators are/is likewise connected to the controller <NUM>. The infrared camera <NUM> sees only the thermal radiation <NUM> emitted by the mirror <NUM> and the thermal radiation <NUM> emitted by a part <NUM> of the housing <NUM> and reflected by the mirror <NUM>. Depending on the ratio of the thermal radiation <NUM> originally emitted by the irradiated surface <NUM>, which thermal radiation is relevant to the determination of the temperature of the irradiated surface <NUM>, and the thermal radiation <NUM> emitted by the part <NUM> of the housing <NUM> that is detected by way of reflection, the determination of the surface temperature of the optical effective surface <NUM> can be possible with a sufficient accuracy of below ± <NUM>°K. In the example shown, the controller <NUM> comprises a lock-in amplifier <NUM>, which can enable an improved determination of the thermal radiation <NUM> emitted by the optical effective surface <NUM> against the background of the thermal radiation <NUM> emitted by the housing part <NUM>. This can make use of the fact that the thermal radiation <NUM> of the irradiated surface <NUM> comprises a constant portion and a cyclic portion caused by the cyclic exposure of the mirror <NUM> during the operation of the projection exposure apparatus. The optical effective surface <NUM> is irradiated for example with a clocking of <NUM> seconds "light on" and <NUM> seconds "light off". In this example, the bulk material of the mirror <NUM> would have a temperature of approximately <NUM>° Celsius but the topmost layers of the mirror <NUM> would have a surface temperature of between e.g. approximately <NUM>° Celsius and e.g. approximately <NUM>° Celsius that changes cyclically over time. This topmost layer is small and its proportion of the total thermal radiation is likewise small as a result; it may correspond in particular only to <NUM>% of the total thermal radiation of the mirror surface. However, this clocked signature of the thermal radiation <NUM> can be amplified, detected and determined with the aid of the lock-in technique. In the case where the lock-in amplifier <NUM> is used, the controller <NUM> is connected by a line <NUM> to the illumination or the illumination controller (neither being illustrated) of the projection exposure apparatus, via which the clocking of the used light <NUM>, <NUM> is communicated to the controller <NUM>. Furthermore, the ratio of the thermal radiation <NUM> emitted by the housing part <NUM> and the thermal radiation <NUM> emitted by the optical effective surface <NUM> can also be increased by means of the setting of the emissivities of the housing part <NUM> imaged onto the infrared camera <NUM> and of the optical effective surface <NUM> for the wavelength range detected by the infrared camera <NUM>. In this case, the emissivity at least for the imaged part of the housing <NUM> is reduced and that of the irradiated surface <NUM> is increased, which will be described below with reference to <FIG>, <FIG>.

<FIG> shows a detail view of the mirror <NUM> and of the housing <NUM>. The thermal radiation <NUM> emitted by the housing part <NUM> is initially not polarised. As a result of the reflection at the optical effective surface <NUM> of the mirror <NUM>, said thermal radiation is polarised primarily perpendicularly for angles of incidence of greater than <NUM>°. The thermal radiation <NUM> arriving at the infrared camera <NUM>, said thermal radiation being emitted by the partial housing <NUM> and reflected at the mirror <NUM>, is thus oriented primarily perpendicular to the plane of incidence of the radiation <NUM> on the mirror <NUM>. This effect is particularly pronounced in particular if the thermal radiation <NUM> is reflected at an angle of reflection close to the Brewster angle. In this context, close should be understood to mean a deviation of the angle of incidence from the Brewster angle of less than <NUM>°. The filter <NUM> arranged between the mirror <NUM> and the infrared camera <NUM> blocks the perpendicular polarisation direction, however, such that only the parallel polarisation direction, that is to say the portions of the thermal radiation <NUM> which are oriented parallel to the plane of incidence, impinge on the infrared camera <NUM>. As a result, the ratio of reflected thermal radiation <NUM> of the housing part <NUM> and the thermal radiation <NUM> of the mirror <NUM> is decreased further and the determination of the temperature of the optical effective surface <NUM> is additionally simplified as a result. The filter <NUM> can also be embodied as a rotating polarisation filter, whereby a known frequency is impressed on the intensity of the reflected, polarised thermal radiation, said intensity being detected by the IR camera <NUM>. In this way, the signal portion attributed to reflected (and unwanted) radiation is easily identified by the IR camera <NUM> and taken into account in the determination of the surface temperature of the optical element. A combination of the filter <NUM> and the lock-in technique is also possible. Furthermore, it is also possible for a plurality of infrared cameras <NUM> to be directed at the optical effective surface <NUM> and to detect different regions of the optical effective surface <NUM>. In addition, the infrared camera <NUM> itself or only an optical unit <NUM> can be mounted in a movable fashion, such that the optical effective surface <NUM> can be scanned.

<FIG> shows a diagram in which the spectral radiance in W/(m<NUM>mSr), referred to hereinafter just as radiance, is plotted against the wavelength in a double logarithmic plot. The wavelength is plotted on the abscissa and the radiance is plotted on the ordinate. The range bounded by the points A and B on the abscissa represents the wavelength range in which the infrared camera detects thermal radiation, which range is between <NUM> and <NUM> in the example shown. Curve I shows the radiance of the housing part <NUM> temperature-regulated to -<NUM>° Celsius, said housing part being illustrated in <FIG> and <FIG>. Curves II and III show the radiance of an irradiated optical effective surface <NUM> temperature-regulated to <NUM>° Celsius, said optical effective surface being illustrated in <FIG> and <FIG>. In this case, curve II represents the radiance for an emissivity of <NUM> and curve III represents the radiance for an emissivity of <NUM>, that is to say for a black body.

Claim 1:
Projection exposure apparatus (<NUM>,<NUM>) for semiconductor lithography, comprising
- at least one optical element (<NUM>),
- at least one temperature recording device (<NUM>) for detecting a temperature on an optical effective surface (<NUM>) of the optical element (<NUM>) by means of electromagnetic radiation (<NUM>) emitted or reflected from the optical effective surface (<NUM>) of the optical element (<NUM>),
characterised in that
the temperature recording device (<NUM>) comprises a polarisation filter (<NUM>) for filtering the electromagnetic radiation (<NUM>).