DE102012209290A1 - Optical arrangement i.e. extreme UV lithography system, has moving unit i.e. linear motor, for selectively introducing filter in beam path or for removal of spectral filter from beam path of light source - Google Patents

Abstract

The arrangement (1) has an extreme UV plasma light source (2) and a spectral filter (11) i.e. window. The filter is utilized for absorption of radiation from the light source in an extreme UV spectral range and for transmission of radiation from the light source in a transmission spectrum region having wavelength above spectral range. A moving unit (12) i.e. linear motor, is utilized for selectively introducing the filter in a beam path (5) or for removal of the spectral filter from the beam path of the light source. An independent claim is also included for a method for spectral separation of radiation of a plasma light source.

Description

Background of the invention

The invention relates to an optical arrangement, in particular an EUV lithography system, with an EUV plasma light source.

It is known that light sources for EUV lithography systems can be designed as plasma light sources. For example, Cymer, Inc. offers a so-called "Laser Produced Plasma" (LPP) EUV plasma light source, in which molten tin droplets are struck with a pulsed CO ₂ high power laser to produce radiation that is present in the EUV Spectral range by about 13.5 nm has a maximum intensity.

However, such an EUV plasma light source generates in addition to the radiation in the EUV spectral range, i. around 13.5 nm, a broad, broadband and quasi-continuous radiation spectrum at longer wavelengths. If an intensity measurement of the EUV radiation generated by the EUV plasma light source is carried out by means of a sensor device, at least one subarea of this quasi-continuous spectrum, which was not filtered on optical elements arranged in the beam path in front of the sensor device, can contribute to the measured total intensity and thus falsify the measurement result of the intensity measurement of the EUV radiation.

Object of the invention

The object of the invention is to provide an optical arrangement and a method which enables in a simple manner a spectral separation of the radiation of a plasma light source.

Subject of the invention

This object is achieved by an optical arrangement comprising: an EUV plasma light source, as well as a spectral filter for absorbing radiation of the EUV plasma light source in an EUV spectral range and for transmitting radiation of the EUV plasma light source in a Transmission spectral range is formed with wavelengths above the EUV spectral range, and a movement device for selectively introducing the spectral filter in the beam path or removing the spectral filter from the beam path of the EUV plasma light source.

When introduced into the beam path spectral filter, an intensity distribution of the radiation of the EUV plasma light source is recorded only in the transmission spectral range of the spectral filter, wherein the radiation is absorbed in the EUV spectral range of the spectral filter. By comparing the intensity distributions, which are measured with and without the spectral filter, the intensity of the EUV radiation can thus be determined by taking into account the contribution of the radiation component within the transmission spectral range. For this purpose, e.g. the intensity distribution, which was measured at the spectral filter arranged in the beam path, is subtracted from the intensity distribution without the spectral filter arranged in the beam path. By using the spectral filter, which can optionally be introduced into the beam path, it is thus possible to dispense with a spectral purity filter on the sensor device itself.

In one embodiment, the spectral filter is formed as a window of a fluoride material, in particular an alkaline earth fluoride material or an alkali fluoride material. Such windows have a transmission spectral range which starts at a minimum wavelength of about 100 nm or about 130 nm and ends at about 1 μm or above, depending on the type of material selected. Such a window represents a particularly simple type of spectral filter and reliably absorbs the radiation in the EUV spectral range. Although the EUV plasma light source also generates radiation in a wider spectral range between the EUV spectral range (about 12 nm to 14 nm) and the transmission spectral range (above about 100 nm or 130 nm), in this wider spectral range However emitted radiation is typically from mirror elements of the optical arrangement, eg Filtered out a collector mirror (parabolic mirror or the like) or other EUV mirrors, so that the spectral filter for this wider spectral range must not be made transparent or transmissive.

In one embodiment, the material of the window is selected from the group comprising: calcium fluoride, barium fluoride, lithium fluoride, magnesium fluoride, and strontium fluoride. These materials all have a transmission spectral range which starts at approximately 100 nm-130 nm and ends at approximately 1 μm, the exact limits of the transmission spectral range being dependent on the particular material chosen. The use of calcium fluoride as a window material has proven to be particularly favorable since it has a transmission wavelength range which extends from approximately 130 nm to well above 1 μm. It is understood that in addition to fluoride materials, other materials can be used for the production of a spectral filter in the form of a window (plane plate), provided that they are transparent to radiation within the limits specified above and absorb radiation in the EUV spectral range.

Typically, the windows of the above materials have a thickness of 1 mm or less. This material thickness is usually sufficient to effectively absorb the EUV radiation.

In one embodiment, the spectral filter is transparent to radiation in a transmission spectral range between 130 nm and 1 μm. As described above, e.g. Calcium fluoride and other alkali or alkaline earth fluoride materials meet these requirements. Above a wavelength of about 1 .mu.m, the intensity of the radiation emitted by the EUV plasma light source is very low, so that the spectral filter or the window for radiation does not necessarily have to be transparent in this wavelength range.

In one embodiment, the optical arrangement comprises at least one EUV mirror element which is designed to reflect the radiation of the plasma light source in the EUV spectral range and to absorb the radiation of the plasma light source in an absorption spectral range whose lower end is connected to the EUV Spectral region connects and whose upper end adjoins the transmission spectral range or partially overlaps with this. The mirror element that absorbs radiation in the absorption spectral range is typically an EUV mirror, i. around a mirror, on whose surface a multilayer coating is applied, which is optimized for the reflection of EUV radiation at about 13.5 nm. The EUV mirror may, for example, be a collector mirror, which serves to focus the radiation of the EUV plasma light source in an intermediate focus.

However, other EUV mirrors of the optical arrangement can cause, in addition to the deflection of the radiation in the EUV spectral range, absorption of radiation outside the EUV spectral range. Typical EUV mirrors have, for example, a plurality of alternating layers of silicon and molybdenum, the number of layers and the layer thicknesses being matched to one another such that they have a sharp intensity maximum at the operating wavelength of typically about 13.5 nm. However, of such conventional Si / Mo layer stacks, radiation above a certain wavelength, which is typically in the range of about 130 nm, is no longer absorbed but also reflected.

Due to the filtering properties of the mirror elements in the absorption spectral range, it is not necessary for the spectral filter to be transparent to the entire spectral range above the EUV spectral range, i. the entire radiation emitted by the EUV plasma light source outside the EUV spectral range is detected. Rather, it suffices if the transmission spectral range adjoins the absorption spectral range within which the radiation is absorbed by the mirror elements or if the transmission spectral range partially overlaps the absorption spectral range.

In a further embodiment, the optical arrangement comprises a sensor device for (broadband) detection of the intensity of the radiation of the plasma light source, and an evaluation device for determining the intensity of the EUV radiation by comparing the detected intensity of the sensor device with and without the spectral filter in the beam path. As has been shown above, the difference distribution between the intensity distributions measured with and without the spectral filter can be used to determine the intensity distribution of the radiation emitted in the EUV spectral range by the EUV plasma light source.

In the case of an optical arrangement in the form of an EUV lithography system, the proportion of unfiltered radiation (so-called "false light") which is not in the EUV spectral range is of particular interest in comparison to EUV radiation in the region of the wafer plane, since the unfiltered Radiation with wavelengths outside the EUV spectral range disturbs the exposure of the wafer-level photosensitive layer (resist) as a structureless background and leads to a loss of contrast.

The mirror elements of the optical assembly may be removed by suitable means, e.g. the provision of additional absorbent layers e.g. made of silicon carbide or silicon nitride so that they are also for radiation at wavelengths above about 130 nm absorbing. Therefore, it is of interest to observe the time course of the relationship between the intensity of the EUV radiation and the intensity of the unwanted light in the wafer plane, e.g. to detect a degradation of the layers absorbing the false layer and, if necessary, be able to take countermeasures.

In a further development, the sensor arrangement is preferably arranged in a wafer plane and designed to determine the intensity of scattered EUV radiation (also) outside the beam path or the image field, for example by dimensioning the sensor surface appropriately large. Since the wavelengths of the EUV radiation and the radiation not filtered by the EUV mirrors (in the UV or VIS range) differ by a factor> 10, a significantly higher proportion of the EUV radiation typically becomes within one range scattered outside the beam path than this for the unfiltered radiation in the UV or VIS range is the case.

In the wafer plane, virtually no stray light is present outside the beam path or the image field, while a higher stray light fraction is present within the image field. If one wishes to determine the scattered light component of the EUV radiation in the wafer plane, it is therefore necessary to first determine the intensity of the EUV radiation within the image field (and to eliminate the intensity component of the false light) in order to determine the intensity in the manner described above EUV radiation within the image field can be compared with the intensity of EUV radiation outside the image field.

A further aspect of the invention relates to a method for the spectral separation of the radiation of an EUV plasma light source, comprising: introducing a spectral filter into a beam path of the plasma light source for filtering radiation in the EUV spectral range and for transmitting radiation in a transmission spectral range with wavelengths above the EUV spectral range.

In a variant of the method, a spectral filter in the form of a window made of a fluoride material, in particular an alkaline earth fluoride material or an alkali fluoride material, is introduced into the beam path. As has been shown above, these materials have a transmission spectral range between approximately 100 nm and approximately 1 μm and are therefore suitable for the entire spectral range of interest from the EUV plasma spectra of interest here, which is not or not completely filtered by the EUV mirrors. Light source emitted radiation transparent.

Further features and advantages of the invention will become apparent from the following description of embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims.

The individual features can be realized individually for themselves or for several in any combination in a variant of the invention.

drawing

Embodiments are illustrated in the schematic drawing and will be explained in the following description. It shows

1 a schematic representation of an optical arrangement with an EUV plasma light source,

2 a schematic representation of three different wavelength ranges, in the optical arrangement of 1 relevant, as well as

3 a representation of a wafer plane of an optical arrangement with an image field to be exposed and a surrounding area and the radiation intensity in the wafer plane.

1 is a schematic representation of an optical arrangement 1 which is an EUV plasma light source 2 in the form of a so-called LPP light source and an EUV mirror element 3 in the form of a collector mirror, which is that of the EUV plasma light source 2 generated radiation to an intermediate focus 4 focused, in which a diaphragm is arranged. In the beam path 5 the EUV plasma light source 2 after the intermediate focus 4 is a sensor device 6 in the form of a CCD chip, which provides a (broadband) spatially resolved detection of the intensity of the EUV plasma light source 2 generated radiation allows. An evaluation device 7 is with the CCD chip 6 connected by signal technology and serves the evaluation of the intensity signals supplied by this.

The EUV plasma light source 2 not only generates radiation in an EUV spectral range 8th which is typically between about 12 nm and 14 nm (cf. 2 ), but over a broadband spectral range. The outside of the EUV spectral range 8th lying radiation component is in the optical arrangement 1 undesirable. To suppress this proportion is the collector mirror 3 provided with a layer stack of alternating Mo / Si layers, which leads to radiation in an absorption spectral range 9 (see. 2 ) which adjoins the EUV spectral range 8th connects and typically at wavelengths between about 14 nm and about 130 nm, is absorbed. Radiation in a transmission spectral range 10 between about 130 nm and about 1 micron, however, is from the collector mirror 3 not absorbed and therefore coincides with the radiation in the EUV spectral range 8th on the CCD chip 6 , The radiation component in the transmission spectral range 10 is also referred to as a misleading below.

To the intensity distribution of the radiation in the EUV spectral range 8th without the stray light component in the far field of the EUV plasma light source 2 to determine is a spectral filter 11 in the form of a calcium fluoride window (flat plate with a thickness of about 1 mm) in the optical arrangement 1 provided, which by means of a direction indicated by a double arrow movement means 12 (eg linear motor) optionally in the beam path 5 in the area of the intermediate focus 4 can be inserted and removed from this.

The intensity distribution of the EUV radiation on the CCD chip 6 can be determined by first a first intensity measurement without the in the beam path 5 introduced spectral filter 11 is carried out. Here, the whole of the EUV plasma light source 2 generated and not on the collector mirror 3 filtered radiation, ie it becomes the sum of the intensities of the radiation in the EUV spectral range 8th and in the transmission spectral range 10 determined. Subsequently, a second intensity measurement is in the beam path 5 introduced spectral filter 11 performed, ie it is only the intensity of the radiation in the transmission spectral range 10 detected. By subtraction between the two intensity distributions can in the evaluation 7 (eg computer) the intensity of the EUV radiation without the proportion of stray light be determined.

Alternatively to introducing the spectral filter 11 in the beam path 5 in the area of the intermediate focus 4 and the attachment of the sensor device 6 after the intermediate focus 4 it is also possible to use the spectral filter 11 elsewhere in the optical arrangement 1 for example, within a lighting system (not shown) when the optical arrangement 1 For example, is an EUV lithography system. In this case, the sensor device 6 For example, in the area of an object plane, in which an object field has a

Mask is arranged, which by means of a projection system on a photosensitive substrate (wafer) in the region of a wafer plane 13 is pictured in 3 is shown.

In the area of the wafer level 13 is a picture frame 13a formed in the radiation from the beam path 5 the EUV plasma light source 2 to the wafer level 13 incident. A the picture box 13a surrounding area 13b ("Out-of-field" area), however, is only hit by stray light, the passage of the radiation of the EUV plasma light source 2 through the optical arrangement 1 was generated. To a scattered light measurement in the area of the wafer level 13 perform the sensor device 6 be arranged there, wherein the (spatially resolved) measurement is both on the image field 13a and on the area surrounding this 13b extends. The comparison of the measured intensity values provides the scattered light component of the EUV radiation, which provides information about the state of the optical arrangement 1 gives.

In the scattered light measurement occurs in the present case, however, the problem that the stray light portion in the field of view 13a itself from that of the area surrounding this 13b differs, cf. the intensity bars in the graph in 3 down depending on the position X in the wafer plane 13 , While within the image field 13a the false light portion FL is clearly different from zero, this is true in the surrounding area 13b not too, because there is the mischief share (almost) at zero. The ratio between the overall intensities of the image field 13a and in the surrounding area 13b incident radiation thus provides no accurate information about the scattered light component of the EUV radiation in the wafer plane 13 ,

In order to carry out the scattered light measurement for the EUV radiation, it is therefore necessary to refer to the 1 described manner the intensity of the EUV radiation in the image field 13a be determined. This can then be combined with the EUV radiation in the surrounding area 13b be compared in order to determine the scattered light component of the EUV radiation. It is understood that such a scattered light measurement several times
During the life of the optical arrangement can be performed to draw conclusions, for example, on the degree of soiling of the EUV mirror 3 to be able to pull.

Claims (10)

Optical arrangement ( 1 ) comprising: an EUV plasma light source ( 2 ), and a spectral filter ( 11 ) used to absorb radiation from the EUV plasma light source ( 2 ) in an EUV spectral range ( 8th ) and the transmission of radiation of the EUV plasma light source ( 2 ) in a transmission spectral range ( 10 ) with wavelengths above the EUV spectral range ( 8th ), and a movement device ( 12 ) for selectively introducing the spectral filter ( 11 ) in the beam path ( 5 ) or removing the spectral filter ( 11 ) from the beam path ( 5 ) of the EUV plasma light source ( 2 ). Optical arrangement according to Claim 1, in which the spectral filter is used as a window ( 11 ) is formed of a fluoride material, in particular an alkaline earth fluoride material or an alkali fluoride material. An optical arrangement according to claim 2, wherein the material of the window ( 11 ) is selected from the group comprising: calcium fluoride, barium fluoride, lithium fluoride, magnesium fluoride and strontium fluoride. Optical arrangement according to claim 2 or 3, in which the window ( 11 ) has a thickness of 1 mm or less. Optical arrangement according to one of the preceding claims, in which the spectral filter ( 11 ) is transparent to radiation in a transmission spectral range between 130 nm and 1 μm. Optical arrangement according to one of the preceding claims, further comprising: at least one EUV mirror element ( 3 ), which is used to reflect the radiation of the plasma light source ( 2 ) in the EUV spectral range ( 8th ) and for absorbing the radiation of the plasma light source ( 2 ) in an absorption spectral range ( 9 ) whose lower end adjoins the EUV spectral range ( 8th ) and its upper end to the transmission spectral range ( 10 ) of the spectral filter ( 11 ) or partially overlaps it. Optical arrangement according to one of the preceding claims, further comprising: a sensor device ( 6 ) for detecting the intensity of the radiation of the plasma light source ( 2 ), as well as an evaluation device ( 7 ) for determining the intensity of the radiation in the EUV spectral range ( 8th ) by comparing the detected intensity of the sensor device ( 6 ) with and without the in the beam path ( 5 ) introduced spectral filter ( 11 ). Optical arrangement according to Claim 7, in which the sensor device ( 6 ) preferably in a wafer plane ( 13 ) and for determining the intensity of scattered radiation outside the beam path ( 5 ) is trained. Method for the spectral separation of the radiation of a plasma light source ( 2 ), comprising: introducing a spectral filter ( 11 ) in a beam path ( 5 ) of the EUV plasma light source ( 2 ) for filtering radiation in the EUV spectral range ( 8th ) and the transmission of radiation in a transmission spectral range ( 10 ) with wavelengths above the EUV spectral range ( 8th ). Method according to Claim 9, in which a spectral filter in the form of a window ( 11 ) of a fluoride material, in particular an alkaline earth fluoride material or an alkali fluoride material in the beam path ( 5 ) is introduced.

Images