Abstract:
The invention is based on the object of providing UV interference filters having improved long-term stability. For this purpose, a method for producing a dielectric transmission interference filter is provided, in which an alternating layer system having the constituents magnesium fluoride, lead fluoride and antimony oxide is produced on a substrate by co-evaporation. After deposition, the substrate is subjected to heat treatment and irradiated with UV light in order to stabilize the layer system.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to interference filters. In particular, the invention relates to interference filters for UV lithography. 
     BACKGROUND OF THE INVENTION 
     One area of use for high-quality UV interference filters is in lithography in the production of semiconductor components, inter alia. By way of example, interference filters are used in so-called steppers in order to filter out as far as possible monochromatic light from the spectrum of the vapor lamps that are usually used. In a stepper, a wafer, or the applied photoresist, is exposed stepwise via a mask projected onto the wafer in demagnified fashion. 
     In many cases the i-line of the mercury of a mercury vapor lamp is used for this purpose. Particular requirements arise here in respect of the filter for filtering out this light. At shorter wavelengths, the tolerances in the optical thickness of the interference layers also decrease correspondingly. The tolerances have to be complied with not just in the case of a newly incorporated filter; stringent requirements are made of long-term stability, too. Here neither the transmission nor the position of the edges of the transmission window or windows should change over a relatively long time. 
     SUMMARY OF THE INVENTION 
     Therefore, the invention is based on the object of providing UV interference filters having improved long-term stability. 
     Accordingly, a method for producing a dielectric transmission interference filter is provided, in which a substrate is disposed in an evaporation chamber and layers having a higher refractive index and a lower refractive index are alternately deposited on the substrate, wherein the layers are in case produced by co-evaporation of at least two of the constituents magnesium fluoride, lead fluoride and antimony oxide, wherein the layers having a high and a low refractive index have different compositions. The substrate coated in this way, after the deposition of the layers, is then subjected to heat treatment at a temperature of at least 50° C. and is irradiated with UV light. In the process there is a decrease in the central wavelength of the transmission window in the transmission of the filter for the central transmission wavelength within a spectral transmission window in the course of the irradiation. 
     In the case of a transmission filter it is expedient here, of course, to select a substrate which is transparent in the transmission window of the layer system. 
     In other words, the method according to the invention consists in depositing, for producing a dielectric interference filter in an evaporation chamber, layers having a higher refractive index and a lower refractive index alternately on a substrate, wherein an alternating layer system is deposited which has alternate first and second layers having a different composition of the constituents magnesium fluoride, lead fluoride and antimony oxide, wherein, after coating, the central wavelength of a transmission or reflection window is shifted toward shorter wavelengths in the direction of the design wavelength by virtue of the coated substrate being subjected to heat treatment and being irradiated with UV light. The shift toward shorter wavelengths is caused by an increase in the refractive index of the relevant layers. The increase in the refractive index is generally also accompanied by an albeit very small widening of the transmission window. 
     Accordingly, what is obtained as a result of the coating is an intermediate product for producing a dielectric UV interference filter, comprising a substrate and, deposited thereon, an alternating layer system having alternate first and second layers, wherein the first layers and the second layers contain a mutually different composition of the constituents antimony oxide, lead fluoride and magnesium fluoride, wherein the layer thicknesses of the layers are a multiple of a quarter of a design wavelength λ, and wherein, by means of heat treatment and/or UV irradiation, it is possible to obtain a shift in the central wavelength of a transmission or reflection window in the direction of the design wavelength proceeding from a longer wavelength than the design wavelength. 
     In particular layers having a lower refractive index are produced by co-evaporation of magnesium fluoride and lead fluoride and layers having a higher refractive index are produced by co-evaporation of lead fluoride and antimony oxide, such that the layers having a lower refractive index contain a mixture of magnesium fluoride and lead fluoride and the layers having a higher refractive index contain a mixture of lead fluoride and antimony oxide. As an alternative, it is also possible for the deposition of the layers having a lower refractive index to comprise the evaporation of magnesium fluoride and antimony oxide and the deposition of the layers having a higher refractive index to comprise the evaporation of lead fluoride and antimony oxide, such that the layers having a lower refractive index contain a mixture of magnesium fluoride and antimony oxide and the layers having a higher refractive index contain a mixture of lead fluoride and antimony oxide. 
     Within the meaning of the invention, central wavelength is understood to be the average wavelength of the plateau in the case of a plateau-shaped transmission maximum, otherwise the wavelength of the maximum transmission within the window. 
     The irradiation can be effected before, during or after the heat treatment. An irradiation after the heat treatment is preferred. 
     It is surprising in this case that, after the heat treatment and in particular after the UV irradiation, the central wavelength then remains stable for a long time, such that the filter has a significantly increased lifetime in comparison with other dielectric transmission filters for the UV range. 
     According to the current level of knowledge, using various analytical methods, practically no differences in the layer system before and after the UV irradiation can be established per se. The morphology of the layers appears to be unchanged for example when a break edge is viewed with a scanning electron microscope. 
     It is assumed that the effect is based primarily on a change in the composition of the layers. MgF 2  and in particular PbF 2  dissociate during vapor deposition and recombine during deposition. In this case, incomplete recombination can then occur since fluorine is more readily volatile, such that substochiometric layers are obtained. 
     The UV irradiation evidently brings about here a post-loading with oxygen, for instance on account of UV-excited oxygen migration, which then leads to a change in the optical density of the layers here, on the one hand, and chemical stabilization, on the other hand. If a heat treatment and UV treatment are not carried out after the coating, water may possibly be incorporated at the oxygen defects, which then permanently prevents chemical stabilization. 
     A UV transmission filter is understood to be, in particular, such a filter in which the layer thicknesses of the layers are coordinated such that the transmission window has, after the UV irradiation, a spectral position whose longer-wave edge lies at a wavelength of less than 450 nanometers. The particularly preferred application of the invention is in the production of a transmission filter for the i-line of mercury in order to use the filter in an exposure device for semiconductor lithography. Accordingly, for this purpose the layer thickness of the layers of the coating is chosen such that the transmission window includes a wavelength of 365 nanometers in particular after the irradiation with UV light. 
     A good narrowband nature is generally required for line filters. Preferably, therefore, the layer system or the number and layer thickness of the layers are chosen such that the transmission maximum has a full width at half maximum of at most 20 nanometers, preferably at most 15 nanometers. Layer systems having a total of at least 15 layers, preferably at least 20 layers, are suitable for this purpose. 
     The irradiation with UV light for shifting and stabilizing the design wavelength is preferably performed for a duration of at least five hours, preferably for a duration of approximately 24 to 48 hours. In particular, a mercury vapor lamp or a lamp which likewise emits UV light at a wavelength of 365 nanometers is suitable as UV light source. 
     In general, a shift in the central wavelength of the transmission window toward shorter wavelengths by at least 0.1 nanometer, preferably at least 0.2 nanometer, is achieved by means of the irradiation and/or the heat treatment. Customary values are a shift in the range of 0.2 and 0.5 nanometers. In the case of the narrowband nature generally required, however, such a shift already leads to considerable transmission increases for lines of vapor lamps since the typical line widths also only lie in the range of a few nanometers. 
     The heat treatment is preferably carried out at moderate temperatures of less than 250° C. The duration of the heat treatment is furthermore preferably at least twelve hours. By way of example, a heat treatment of 24 hours at 100° C. has proved to be suitable. 
     In a particularly advantageous manner, large-area long-term-stable filters can also be produced by means of the method. Thus, a substrate area having a lateral dimension or a diameter of at least 100 millimeters can be coated and aftertreated according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail below with reference to the accompanying drawings, in which: 
         FIG. 1  shows a cross-sectional view through an interference filter such as can be produced according to the invention, 
         FIG. 2  shows a spectrum of a mercury vapor lamp with a superimposed excerpt from the spectral transmission of an interference filter, 
         FIG. 3  shows an illustration of the transmission characteristic of a filter produced according to the invention, before and after the aftertreatment by means of heat treatment and UV irradiation, and 
         FIG. 4  shows a schematic view of a stepper for the exposure of wafers with a filter such as is illustrated by way of example in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically shows a dielectric transmission interference filter  1  for filtering out the i-line of a mercury vapor lamp. The filter comprises a substrate  3  having sides  31 ,  32 , which substrate is transparent to the i-line, or to light having a wavelength of 365 nanometers. Glass, for example, is suitable as a material for the substrate. 
     An interference alternating layer system  5  having alternate layers  51 ,  52  is deposited on the substrate. In this case, layers  51  and layers  52  respectively alternate, wherein the layers  51  have a higher refractive index than the layers  52  and are referred to as layers having a high refractive index for the sake of simplicity below. 
     The layers  51  having a high refractive index are produced by co-evaporation of Sb 2 O 3  and PbF 2 , and the layers  52  having a low refractive index are produced by co-evaporation of PbF 2  and MgF 2 , wherein a dedicated source is used for each of the materials. Generally, electron beam evaporation, in particular, is suitable for depositing the materials having a high melting point. One or a plurality of layers can be provided as spacer or resonator layers in the layer system. The example shown in  FIG. 1  has one such layer  53 . The one or the plurality of resonator layers have an optical thickness having an odd-numbered multiple of half the design wavelength λ. The optical thickness of the other layers is in each case an odd-numbered multiple of λ/4. 
     The number of layers is preferably greater than in the example shown in  FIG. 1 , in order that a specific UV line of the spectrum of a vapor lamp can be filtered out as selectively as possible with sufficient narrowband nature. By way of example, the layer system can have 29 layers  51 ,  52  including three resonator layers  53 . A transmission window around the mercury i-line having a full width at half maximum of 12 nanometers can be obtained with such a layer system. Twenty seven layers  51 ,  52  including two resonator layers  53  are provided in accordance with a further example. It is thus possible to achieve a transmission window having a full width at half maximum of 6 nanometers around the design wavelength of 365 nanometers, in accordance with the wavelength of the mercury i-line. 
       FIG. 2  shows a spectrum of a mercury vapor lamp for clarification purposes. The i-, h- and g-lines of the mercury is spectrum are identified by the corresponding letters. The i-line is particularly suitable for lithography. Accordingly, the g- and h-lines at the wavelength of 436 nanometers and the wavelength of 405 nanometers, respectively, should be shielded as far as possible by the filter. A correspondingly narrowband filtering around a design wavelength in accordance with the i-line is favorable for this purpose. On the other hand, this is associated with the fact that the light intensity of an arrangement having a mercury vapor lamp and such a filter depends very sensitively on the spectral position of the transmission window of the filter. 
     This also becomes clear from the excerpt from the spectral transmission with the transmission window  9  as depicted in  FIG. 2 . The window has a full width at half maximum of approximately 12 nanometers, in accordance with the abovedescribed example of a filter having 29 layers. If the central wavelength of the window, that is to say the maximum thereof, and the i-line no longer coincide exactly, a decrease in the light intensity and spectral widening occur very rapidly on account of the folding of the emission line with the form of the window  9 . This effect is even more pronounced, of course, in the case of an even narrower band filter with a window having a full width at half maximum of 6 nanometers. 
       FIG. 3  shows an illustration of the transmission characteristic of a filter produced according to the invention, before and after the aftertreatment by means of heat treatment and UV irradiation. In this case, the solid curve is the spectral transmission profile of the filter not subjected to aftertreatment, and the curve depicted in a dashed fashion shows the transmission profile of a filter that was subjected to heat treatment and irradiated with UV light. The curves were calculated in each case on the basis of the refractive index changes occurring in the layers having a high refractive index and having a low refractive index. The refractive index in the layers having a high refractive and having a low refractive index increases in different ways in this case. Specifically, an increase by Δn=0.003 was assumed in the Sb 2 O 2 +PbF 2  layer having a high refractive index and an increase by Δn=0.01 was assumed in the MgF 2 +PbF 2  mixed layer having a low refractive index. 
     The reason for the different increase in the refractive index is that substochiometric lead fluoride Pb X F Y  is converted into Pb a O b F c  in the course of the heat treatment and in particular the UV irradiation and the refractive index increases. This effect also occurs in the magnesium fluoride. Since the material having a high refractive index contains less fluorine, that is to say the migration effect of oxygen to the lead oxide will not be as pronounced as in the case of the material having a low refractive index, the shift in refractive index is also greater. 
     This relative change in the refractive indices that takes place to different extents possibly also gives rise to a smaller refractive index difference between layer having a high refractive index and layer having a low refractive index, whereby the full width at half maximum of the filter increases somewhat, which should also be observed in practice. 
     The transmission increase to be observed in practice was simulated in the calculation of the curves shown in  FIG. 3  by a reduction of the absorption coefficient after the heat treatment and the UV irradiation. 
     As can be discerned on the basis of the transmission profiles, the spectral position of the central transmission wavelength—here the average wavelength of the plateau-shaped transmission maximum—is shifted toward longer wavelengths by approximately 0.2 nanometer on account of the refractive index changes of the individual layers. 
     In the case of the layer thicknesses of the layers, therefore, in general without restriction to the example of  FIG. 3 , a corresponding margin is set such that the desired central transmission wavelength is achieved after the heat treatment and the UV irradiation. In the example shown in  FIG. 3 , and also in a preferred embodiment of the invention, this is a wavelength of 365 nanometers, or the i-line of the spectrum of a mercury vapor lamp. 
       FIG. 4  shows a schematic view of a wafer stepper for which the interference filters are produced according to the invention find a preferred use. The stepper comprises a mercury vapor lamp  10 , which emits light with a spectrum as illustrated in  FIG. 2 . A concave mirror  11  reflects back light emitted on the rear side with respect to the optical path. The light is filtered by means of an interference filter  1  produced according to the invention, such that essentially only light of the i-line of the mercury is present downstream of the interference filter  1 . 
     The light is collected by means of a condenser lens  12  and passes through a mask  13  having the pattern to be transferred to the photoresist layer on a wafer  15 . By means of a demagnifying lens  14 , the image of the mask  13  is then imaged on the wafer  15  in demagnified fashion. By means of a stepper unit  16 , the wafer  15  is moved stepwise in the image plane, such that different regions of the wafer  15  are exposed successively. 
     It is evident to the person skilled in the art that the invention is not restricted to the merely illustrative exemplary embodiments described above, but rather can be varied in diverse ways. In particular, the individual features of the exemplary embodiments can also be combined with one another.