Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
  • G03F7/70591—Testing optical components
  • G03F7/706—Aberration measurement

Definitions

• the invention relates to a measuring method and a measuring system for measuring the imaging quality of an optical imaging system.
• the preferred field of application is the measurement of projection objectives for microlithography.
• Microlithographic projection exposure systems - are used for the production of semiconductor components and other finely structured components.
• a pattern of a mask or a reticle is imaged with the aid of a projection lens on a substrate covered with a light-sensitive layer, for example a wafer.
• the finer the structures to be imaged the more the quality of the products produced is determined and limited by imaging errors in the optical imaging systems used. These aberrations have an influence, for example, on the line widths shown and the position of the images of the structures shown.
• a highly precise determination of imaging errors is a crucial step in the manufacturing process of optical imaging systems in order to be able to provide systems with minimal imaging errors by means of suitable adjustment. Interferometric measurement methods are often used for this.
• a device for wave front detection working in the manner of a Shea ng interferometer, which enables fast, highly precise measurement of high-resolution projection objectives, is described in German patent application DE 101 09 929 (corresponding to US 2002001088 A1). described.
• a measuring mask to be illuminated with incoherent light is arranged in the object plane of the imaging system to be checked in order to form the coherence of the emerging radiation.
• This can have a transparent support, for example made of quartz glass, on which a mask structure is applied, for example by coating with chrome.
• Typical structure dimensions of radiation-transmissive areas of this mask structure can be large compared to the wavelength of the measurement radiation used.
• This is also referred to here as a two-dimensional or two-dimensionally extended mask structure.
• a reference structure designed as a diffraction grating is arranged in the image plane of the imaging system. Due to the superposition of the waves generated by diffraction, an intensity distribution in the form of an interferogram is created behind the diffraction grating, which is electronically recorded with the aid of a position-resolving detector and evaluated with the aid of an evaluation device connected to the detector. Image errors of lower and higher orders can be determined from the wavefront aberrations.
• Another class of devices for wavefront measurement are the point diffraction interferometers, which work structures with openings in the order of the measurement light wavelength used or below.
• an object pattern is arranged in the object plane of the test object, which for example comprises a plurality of parallel lines that form an object structure.
• Typical structural dimensions of object patterns which can be switched, for example, in the manner of gratings, are large compared to the wavelength of the measurement radiation used, so diffraction effects are usually negligible.
• a reference structure similar to the object structure is arranged in the image plane. The object structure and the reference structure are coordinated with one another in such a way that when the object structure is mapped onto the reference structure with the aid of the imaging system, an overlay pattern (an intensity distribution) is created in the form of a moiré pattern with moiré strips.
• Imaging parameters in particular for the distortion of the imaging system, can be determined from the intensity distribution of the stripe pattern, which can be detected electronically with a spatially resolving detector.
• Moire processes are known, for example, from US Pat. No. 5,767,959 or US Pat. No. 5,973,773 or EP 0 418 054.
• US Pat. No. 5,828,455 describes a measuring method which allows in-situ wavefront measurement of projection objectives.
• the measuring method is based on a Hartmann test and requires a complex special reticle with a perforated plate with several holes and an aperture plate attached behind it.
• the structures of the special reticle are exposed on a wafer coated with photoresist.
• the construction of the reticle means that a local tilting of the wavefront is converted into distortion in the image plane.
• the exposed wafer is evaluated by Measure the structures outside the projection exposure system with a scanning electron microscope (SEM) or other microscope-based inspection devices.
• SEM scanning electron microscope
• the measuring light of the method is provided by the lighting systems of the projection exposure system.
• the measuring method offers sufficient measuring accuracy for most applications. However, since a large part of the illuminating light is blocked out at the special reticle, extremely long exposure times for the wafer result.
• the evaluation of the exposed wafer is complex in terms of equipment and time.
• the deformations that can be measured on the depicted object with respect to the ring diameter and ring shape in a focus series are recorded with a scanning system of the highest resolution and subjected to a Fourier analysis, from which Zernike coefficients can then be derived.
• the process is time consuming.
• the accuracy of the results depends on the underlying model assumptions.
• the invention is based on the object of providing a measuring method and a measuring system which allow highly precise measurement of optical imaging systems at their place of use with little expenditure of time and little expenditure on equipment.
• a quick and precise measurement of Projection lenses in microlithography is based on the object of providing a measuring method and a measuring system which allow highly precise measurement of optical imaging systems at their place of use with little expenditure of time and little expenditure on equipment.
• the invention provides a measuring method with the features of claim 1 and a measuring system with the features of claim 24.
• Advantageous further developments are specified in the dependent claims. The wording of all claims is incorporated by reference into the content of the description.
• Imaging quality of an optical imaging system includes the following
• Steps Providing a mask structure in the area of an object area of the
• Pupil surface of the imaging system Capture the intensity distribution or an image of the
• the invention makes use of the fact that the original information required for the determination of aberration parameters or the like is present in a spatial intensity distribution, which is stored latently or permanently in the recording medium during the measurement process.
• This spatial intensity distribution is also referred to below as an overlay pattern.
• Intensity distribution or the overlay pattern (or an image thereof) with the aid of the recording medium is also briefly referred to below as “recording” of the intensity distribution or overlay pattern or as “recording”.
• the recording medium can be, for example, a film, photographic paper, photoresist or another radiation-sensitive registration medium in which the image information of the intensity distribution is stored via chemical or chemical-physical processes. It would also be conceivable to use light-sensitive, spatially resolving memory chips as the recording medium.
• the recording medium can, for example, comprise a film or a layer with homogeneously premagnetized ferromagnetic material, for example in the form of embedded ferromagnetic crystals. Similar to tape or video material, the magnetization of the material can be mostly the same direction before recording. Due to the incident radiation (photo effect) and / or local, radiation-induced heating (absorption), the Change the orderly state of the material, ie the degree of alignment of the elementary magnets, locally depending on the amount of radiation.
• a readout device can be used in the evaluation process, which reads out the orderly state of the recording medium analogously to a magnetic read head of a video recorder and converts it into an analogue or digital signal. If the transfer function is known, the intensity distribution can be constructed.
• the recording medium can consist of a stretched plastic film that only allows a selected portion of the radiation in a specific direction of polarization to pass.
• the order status of the film can change locally due to the photo effect and / or radiation absorption, so that the degree of polarization also changes locally in accordance with the amount of radiation.
• a spatial intensity distribution can be encoded.
• the evaluation can take place, for example, with the aid of a light table or the like, the exposed recording medium being overlaid with a suitable analyzer, for example a second polarizing film, in such a way that its preferred orientations are rotated relative to one another, for example by 90 °.
• the "exposed”, ie depolarized, zones can then appear bright and the "unexposed” zones can appear dark.
• This pattern can be digitized using optoelectronic means and used for further evaluation.
• a suitable lithographic photoresist photoresist
• measuring methods such as the multi-stripe method, in which local distortions or phase deviations as Strip deflections, that is to say registered as a lateral offset from an ideal strip position or as local strip deflection, can also be used with resist materials which do not resolve gray levels, but rather work essentially “digitally” or “binaryly” and thus only the states “bright” and “ dark "register.
• the recording medium is normally placed behind the reference structure in the radiation path.
• the reference structure is integrated in the recording medium.
• a mirror layer can be provided, which is attached at a distance behind the reference structure on the back of a transparent substrate.
• the recording medium can be arranged in the region of the reference structure.
• the recording medium After the recording medium has been "exposed" during the measuring process or during part of the measuring process, it can be removed from the recording position and evaluated outside of the optical arrangement to be checked.
• the basic information for the evaluation is therefore not transported exclusively by wire, for example electronically, but includes the removal of the recording medium from the recording position can be carried out promptly or at a greater distance from the actual measuring process.
• the recording medium can be removed from the area of the image area of the imaging system so that it can be used again for its original task, for example the exposure of wafers.
• the mask structure and the reference structure are arranged “in the region” of surfaces that are optically conjugated to one another. This means that a structure is arranged exactly in the corresponding surface or slightly axially offset from it, that is to say at a suitable distance near the surface (defocused) The optimum position depends on the desired method variant.
• the surfaces which are optically conjugated to one another and which are generally flat surfaces are also referred to below as "object surface” or object plane and as "image surface” or image plane
• the object area is the area in the area of which the mask structure is arranged during the measurement, while the reference structure is located in the area of the image area.
• the object area of the measurement can be identical to the object area when the imaging system is used as intended, but it can also the image area at b comply with the intended use.
• the measurement direction in the measurement method according to the invention can correspond to the radiation direction during use, but it can also run in the opposite direction.
• the reference structure depends on the desired measurement method.
• reference structures are normally used which act as diffraction gratings for the measuring radiation.
• suitable Lattice constants can be selected depending on the desired diffraction angle, which in turn determines the spatial resolution of the method.
• Typical dimensions can be in the range of the wavelength of the measuring radiation or up to an order of magnitude or more.
• typical structure dimensions can be significantly larger than the measuring radiation wavelength, or possibly smaller.
• the reference structure In the case of point diffraction interferometry, generally comprises at least one quasi-point-shaped “hole” for generating a reference spherical wave, as well as significantly larger pass areas for a test specimen wave. The diameter of the hole or the transmissive area is typically smaller than the measurement light wavelength.
• the typical structure dimensions of the mask structure can differ depending on the measurement method.
• mask structures are preferably used in which typical dimensions of radiation-transmissive areas are large compared to the wavelength of the radiation used.
• Such mask structures are also referred to as “two-dimensional” mask structures. Accordingly, two-dimensional wavefront sources are formed, which are composed of a large number of individual spherical waves, the sources of which are infinitesimally close to one another in a passband of the mask structure.
• the typical structural dimensions are also large against the measuring light wavelength It is also possible to use mask structures in which at least some of the radiation-permeable areas have typical structure dimensions in the area of the used
• a sensor unit which comprises the reference structure and the recording medium with correct spatial allocation to one another. If the sensor unit is arranged in such a way that the reference structure essentially coincides with the image surface, the recording medium is also arranged in the correct position at the same time, for example at a distance behind the reference structure parallel to it.
• the sensor unit can be dimensioned and shaped in such a way that it can be inserted into a holder provided for this object instead of an object to be exposed, such as a wafer.
• the sensor unit can essentially have the shape of a wafer and can be installed in its place in a wafer stage or removed again after the measurement.
• a projection exposure system can easily be switched between the production configuration (for wafer exposure) and the measurement configuration at its place of use.
• it is only necessary to bring a suitable measurement structure into the area of the object plane for example by exchanging the reticle used for wafer exposure with the useful pattern for a measurement mask that bears the two-dimensional mask structure of the measurement system.
• a platform-independent measuring system is thus created.
• the overlay patterns differing in that there are relative phase levels (phase shift) between the mask structure and the reference structure.
• phase shift phase shift
• a relative shift between mask structure and Reference structure carried out in a direction of displacement perpendicular to the optical axis of the imaging system in order to obtain several overlay patterns with different phase positions.
• the overlay patterns or images of the overlay patterns are preferably recorded with the aid of the recording medium in such a way that the individual evaluation patterns are offset from one another in the recording medium and in particular do not overlap.
• An “evaluation pattern” is the form of spatial intensity distribution (of the overlay pattern) present in the recording medium, for example a latent or direct image. Overlay patterns generated in succession are thus converted into evaluation patterns which are spatially offset from one another. Depending on the type of evaluation, these can in turn be successively in time or if necessary also be evaluated in parallel to one another.
• a common displacement of the reference structure and the recording medium is provided relative to the mask structure perpendicular to the optical axis, the displacement path being an integral multiple of a periodicity length p of the reference structure plus a fraction ⁇ of the periodicity length is.
• a large lateral shift is thus provided. Different areas of the reference structure are used each time a transmission pattern is recorded.
• Recording medium relative to the reference structure perpendicular to the optical axis made.
• the same area of the reference structure can always be used for the measurement.
• the exposure of adjacent, non-overlapping areas of the recording medium can take place, for example, analogously to the exposure of a film in a 35 mm camera.
• phase shifting methods it is also possible to use suitable multi-strip methods for the measurement.
• the basic principles of the multi-strip method are known per se and can be found, for example, in the specialist book "Optical Shop Testing" by D. Malacara.
• the multi-strip method can be used, for example, for distortion measurement using Moire technology.
• the spatial resolution plays an important role in the detection of spatial intensity distributions of an overlay pattern, since the phase positions are calculated from the relative positions of the strip positions of strips.
• the phase information is thus encoded as a lateral offset or lateral offset of strips.
• the information encoded in the overlay pattern is recorded with a suitable spatially continuous recording medium, for example a suitable film or a photoresist layer, resolutions of 400 lp / mm or more can also be easily achieved even with typical standard materials.
• a suitable spatially continuous recording medium for example a suitable film or a photoresist layer
• resolutions of 400 lp / mm or more can also be easily achieved even with typical standard materials.
• the higher spatial resolution and the lack of discretization of the information (non-pixelation) of film material and other continuous recording media is just for that Multi-strip methods are an advantage over information acquisition using a CCD camera.
• the measuring system can have a very compact, simple structure in the area of the reference structure. All the parts required here can be combined in one sensor unit, which comprises a reference substrate for carrying the reference structure and a recording medium for carrying and / or supporting the recording medium.
• the reference substrate can be a plate made of a transparent material, in which the reference structure is attached to or in the vicinity of a plate surface.
• the recording medium can also be a plate made of a transparent material and can support and / or support the recording medium on one of its plate surfaces.
• the reference substrate and the recording medium can be formed by a single common plate of suitable thickness, which can essentially have the shape of a wafer.
• the reference substrate and the recording medium can be separate elements, for example two plates, which, if necessary, can be brought into optical contact with one another along complementary contact surfaces, for example by being wrung, and can be separated from one another.
• This embodiment enables a method variant in which after the imaging system has been measured with the aid of the sensor unit, the recording medium is separated from the reference substrate. While the reference substrate with the possibly sensitive reference structure can remain in place, the recording medium can be brought to an evaluation device and the recording medium can be evaluated there. This reduces the risk of damage to the possibly expensive and sensitive reference substrate in different process steps and this can be reused several times.
• the recording medium carrying the recording medium is generally less sensitive and can be provided inexpensively.
• the recording medium can be a flexible film which carries the recording medium. The film can be pressed onto a flat or curved support surface for recording, glued or fixed in some other way and removed after the recording.
• the recording medium can be firmly connected to the recording medium, for example by gluing, vapor deposition, spin coating, laminating or another type of coating.
• the recording medium can e.g. be designed as a positive film or negative film.
• the recording medium can be formed by a photoresist layer applied directly to a transparent substrate.
• the recording medium in such a way that the evaluation pattern is present in the recording medium in a form that can be further processed immediately after exposure. It is also possible for a development step to be interposed between the acquisition of the evaluation pattern and the subsequent evaluation, for example in order to convert a latent image into an evaluable image.
• the recording medium can be permanently connected to the recording medium. It is also possible for the recording medium to be designed for releasable attachment to a recording medium.
• a displacement device for displacing the recording medium relative to the recording medium e.g. be assigned along a support surface of the record carrier, for example in order to guide a film or the like along a disk surface.
• At least one auxiliary structure can be provided in addition to the reference structure and / or in addition to the pattern structure, which together with the
• Overlay pattern is imprinted in the recording medium.
• Auxiliary structures include, for example, registration marks for the correct arrangement of the recording medium and / or gray value gradients for checking or normalizing the resolved gray levels and / or line or cross gratings for image control using moiré technology, as well as combinations of these structures.
• the evaluation of the evaluation pattern present in or on the recording medium or a development product thereof includes, in a preferred method variant, an opto-electronic recording of the evaluation pattern or a development product of the evaluation pattern for generating digitally processable evaluation data and a computer-aided evaluation of the evaluation data to determine at least one representative of the image quality imaging parameter.
• An image-capturing camera for example, can be used for opto-electronic recording, with which many areas of the evaluation pattern can be captured simultaneously in a large area by means of image capturing. It is also possible to use a scanner with which the evaluation pattern is recorded in succession along lines and fed to further evaluation. With magnetic recording media, a reader with one or more magnetic reading heads can be used.
• the invention can be used with different measurement techniques. For example, if a reference structure is provided that the
• Object structure is adapted such that when the
• any Moire method can be carried out in the manner according to the invention.
• the recording medium is arranged in the vicinity of the image area or in an area conjugated to the image area. If an arrangement in the surface of the reference structure is not possible and optical imaging between the reference structure and the recording medium is to be dispensed with, it is preferred if the recording medium is arranged in the region of a Talbot surface of the reference structure. As is known, depending on the wavelength and structure dimensions, a self-mapping of the structure takes place behind a lattice structure in the so-called Talbot distance. This fact can be used for the generation of overlay patterns with only slight blurring of the location information.
• Typical periodicity lengths are in the range of the wavelength ⁇ of the measuring light used, e.g. at 1 - 20 ⁇ or above In this case, it is preferable to set a distance between the reference structure and the recording medium in
• Direction of radiation is dimensioned so that the
• Recording medium is arranged in the optical far field of the reference structure.
• the diffraction grating in the far field creates a coherent overlay of laterally displaced pupils and thus an interferogram as an overlay pattern.
• an optical system between the reference structure and the recording medium for imaging a pupil surface of the imaging system on the recording medium.
• Such arrangements are also possible for point diffraction interferometry.
• FIG. 1 is a schematic illustration of a first embodiment of a measuring system which operates in the manner of a shearing interferometer
• FIG. 2 is a schematic illustration of a first embodiment of a sensor unit for the one shown in FIG. 1
• FIG. 3 is a schematic illustration of a second embodiment of a sensor unit for the measuring system shown in FIG. 1;
• Fig. 4 is a schematic representation of a third
• Fig. 5 is a schematic representation of a
• Recording medium with a plurality of interferograms arranged side by side;
• FIG. 6 is a schematic illustration of a section of an exposed recording medium with an interferogram and several auxiliary structures exposed with the interferogram;
• FIG. 7 is a schematic illustration of an embodiment of an evaluation device with one to one
• Image processing computer connected digital camera and a computer-controlled X / Y shift table for a recording medium to be evaluated;
• FIG. 8 is an exemplary embodiment of a reference structure with an internal checkerboard grid and external line grids for checking the relative position and phase levels between the mask structure and the reference structures;
• FIG. 9 is a schematic example of an evaluation pattern which can be generated with the aid of structures according to FIG. 8;
• Fig. 10 shows different moiré patterns, which are superimposed by
• Fig. 1 1 is a schematic representation of a second
• FIG. 12 is a schematic illustration of a first exemplary embodiment of a sensor unit for use in a measuring system according to FIG. 11;
• FIG. 13 is a schematic illustration of a second embodiment of a sensor unit for use with a
• FIG. 14 shows an example of a binary pre-structured recording medium
• Fig. 15 shows an example of a sinusoidally pre-structured recording medium
• FIG. 16 shows a sensor unit with a sinusoidally structured cover layer over a recording medium
• 17 shows a schematic illustration of an embodiment of a measuring system for point diffraction interferometry.
• FIG. 1 schematically shows a projection objective 10 designed for imaging with ultraviolet light, which is installed in a (not shown) projection exposure system in the form of a wafer stepper at the production site of a semiconductor chip manufacturer.
• the projection objective 10 serves to image a pattern of a reticle provided with a useful pattern in its object plane 11 into the image plane 12 conjugated to the object plane on a reduced scale without an intermediate image.
• Several lenses two of which are shown with dashed lenses, and a pupil plane 13, in which an aperture diaphragm 14 is arranged, lie between the object plane and the image plane.
• the reticle is carried by a reticle holder 15 and the wafer by a wafer holder 16.
• Computer-controlled scanner drives are assigned to the reticle holder and the wafer holder, in order to synchronize the wafer with the reticle perpendicular to the optical axis 17 during scanning of the projection lens to move in opposite directions.
• the ultraviolet light for the projection is provided by an upstream lighting system 18.
• Fig. 1 shows the projection objective 10 in a measurement configuration in which it is in-situ, i.e. using an embodiment of a measurement system according to the invention. at its place of use in the installed state, can be measured interferometrically.
• the measuring system comprises a measuring mask, which has a mask structure 20 and can be arranged in the reticle holder 15 in exchange for the reticle provided with a useful pattern such that the mask structure essentially coincides with the object plane 11.
• the measuring system comprises a sensor unit 21, shown enlarged in FIG. 2, which essentially has the round disk shape of a wafer and can be inserted into the wafer holder 16 in a precise manner in exchange for a wafer.
• the mobile sensor unit 21 of the exemplary embodiment comprises a substrate 22 made of synthetic quartz glass in the form of a plane-parallel plate. On the flat upper side of the quartz wafer 22 there is a reference structure 23 in the form of a matched to the mask structure
• Diffraction grating made of chrome lines.
• an areal, radiation-sensitive recording medium 24 is applied, which is also referred to below as the registration medium and is firmly connected to the substrate 22.
• the radiation-sensitive material of the recording medium 24 designed as a photoresist layer is sensitive to the ultraviolet light of the illumination system 18, but is essentially insensitive to light from the visible wavelength range.
• the thickness of the substrate 22 is dimensioned such that the reference structure 23 in the case of a sensor unit 21 inserted in the wafer holder essentially coincides with the image plane 12 of the projection lens and the recording medium is arranged in a recording position which lies in the direction of light at a distance behind the reference structure in the far optical field of the diffraction grating 23.
• the mask is designed as a shadow mask with a symmetrical distribution of holes, the extent of which is large compared to the wavelength used.
• suitable two-dimensional mask structures are described in DE 101 09 929. The disclosure content of this publication is incorporated by reference into the content of this description.
• the mask structure 20 acts as a wavefront source for generating wavefronts which pass through the optical imaging system 10 and are normally distorted by this imaging system with the generation of wavefront aberrations before they strike the diffraction grating 23.
• the optical system 10 maps the structure of the wavefront source 20 onto the diffraction grating 12.
• the spatial structure of the wavefront source is used to form the spatial coherence of the wavefront.
• the interferograms differing in that relative phase steps (phase shift) lie between the mask structure 20 shown and the diffraction grating 23.
• the sensor unit 21 is shifted step by step with the drive of the wafer stage 16 between successive recordings perpendicular to the optical axis 17.
• the mask structure can also be moved using the reticle stage.
• the lateral movements of the sensor unit 21 are carried out perpendicular to the optical axis 17 with the aid of the wafer holder 16 in such a way that the interferograms 25, 25 ', 25 "and 25'” recorded in succession do not overlap and a relative phase step is additionally introduced.
• the displacement path x between successive recordings i • p + n • ⁇ , where i is an integer, n is the number of the phase step and ⁇ is the amount of the phase step
• the phase position ⁇ of grid patches can also be adapted in this way, as indicated in FIG.
• corresponding interferograms 25 to 25 '" can be generated at different locations of the recording medium at adjacent locations of the recording medium.
• this method by shifting the entire sensor unit, both the recording medium 24 as well as the reference structure 23 are shifted, a different location of the reference structure 23 is used for the measurement for each measurement, which places high demands on the accuracy of the production of the Reference structure 23, which can be produced, for example, by microlithography.
• FIG. 3 shows another embodiment of a sensor unit 121, in which the same lattice position of the reference structure 123 can always be used to generate adjacent interferograms 125 in a recording medium 124.
• This embodiment comprises a quartz substrate 122 formed as a plane-parallel plate, on the upper side of which the reference structure 123 is attached in a relatively small area, which lies in the area of the optical axis 17 when the sensor unit is installed.
• the recording medium 124 which may be in the form of a self-supporting flexible film, is guided along the opposite flat back of the substrate, this back guiding the recording medium so that it is aligned parallel to the reference structure.
• a displacement device 127 is provided with a supply roll 128 and a take-up roll 129, which are rotated step by step during the measuring process by a drive, not shown, to successively different locations of the recording film 124 in the region of the optical axis 17 below to bring the reference structure 123.
• the supply roll, the drive for the film transport and, if applicable, a non-wired power supply are so compactly designed and arranged that the entire sensor unit 121 can be inserted into any commercially available wafer holder instead of a wafer.
• This embodiment can be implemented very simply and inexpensively, in particular for areas of application with lower demands on the measurement accuracy, by retrofitting a 35 mm camera or its film transport mechanism.
• the entire sensor unit can remain stationary during the phase shift.
• the phase shift can by sliding the mask structure with the help of the reticle holder.
• the recording medium can be applied directly, that is to say without an intermediate carrier, to a suitable surface of the substrate.
• a layer of photoresist can be spun on or a light-sensitive silver layer can be evaporated.
• the recording medium can be applied in such a way that it can be easily removed after the evaluation, for example by washing with a solvent. These steps can be performed without damaging the reference structure. Recyclable substrates are thus possible, so that the measurement can be carried out inexpensively.
• a single substrate in the form of a quartz wafer which serves as a reference substrate for supporting the reference structure and at the same time as a carrier or substrate or support surface for the recording medium.
• the third embodiment of a sensor unit 221 according to FIG. 4 has a two-part substrate, which comprises a reference substrate 222 'carrying the reference structure 223 and a film-carrying substrate 222 "serving as recording medium.
• the two substrates 222' and 222" each have the Form of thin, wafer-shaped quartz plates and are detachably connected to one another by wringing along flat contact surfaces with optical contact with one another.
• the thickness of the substrates 222 ', 222 " is dimensioned such that the axial distance between the reference structure 223 and the recording medium 224 essentially corresponds to the thickness of the substrate 22 according to FIG. 1.
• the plates 222', 222" are in the variant shown with a additional clamp fixed to each other on the peripheral area, this clamp can be omitted.
• the overall shape of the sensor unit 221 corresponds to the shape of a wafer, so that the sensor unit 221 can be placed in a wafer holder 16 in exchange for a wafer.
• the two-part, detachable design of reference substrates 222 ' and recording medium 222 "reduces the risk of damage to the expensive and sensitive grid substrate 222'. This can be used again and again in different process steps.
• the record carrier 222 ′′ can be detached from the grid-bearing substrate 222 ′ and brought to the evaluation device. For a further measurement, a recording medium with a still unexposed
• Recording medium can be sprinkled onto the reference substrate in the manner shown to form a sensor unit 221 which can be used for a new measurement.
• the film-carrying substrate 222 'with the recording medium 224 can be handled much more simply when coating the substrate with the recording medium than a substrate provided with a sensitive reference structure, so that the coating process can be carried out quickly and inexpensively, for example by spin coating or the like.
• a large number of interferograms which correspond to the different phase levels of the relative displacement are arranged next to one another.
• 5 shows, by way of example, the recording medium 24 with a plurality of interferograms 25, 25 ' , 25 " lying next to one another, which are arranged in a regular, square grid. Interferograms with a plurality of phase stages in the x direction and a plurality of phase stages are shown in the y direction.
• the evaluation of the image information contained in the exposed recording medium can be carried out as follows.
• the recording medium with the measurement information contained therein is removed from the recording position in the wafer holder, with the entire sensor unit normally being removed for this purpose. If the measuring mask is also removed from the reticle holder, the projection exposure system is ready for further production.
• the evaluation pattern can be in a form that can be directly evaluated, for example in the form of a stripe pattern. If necessary, the evaluation pattern latently present in the recording medium still has to be developed chemically or in another way. If the image information is present in the recording medium in an optically evaluable form, it is brought into an evaluation position outside the projection exposure system and evaluated there.
• the measurement system considered here includes an evaluation device 40, shown schematically in FIG. 7.
• the camera is connected to a computer 42 which, in addition to devices for image acquisition, contains an evaluation program which is configured to determine at least one imaging parameter representing the imaging quality of the optical imaging system.
• a monitor 43 connected to the computer can be provided to display the images captured by the camera 41 and, if appropriate, to display data serving the operator guidance and information.
• a shift table 44 is also connected to the computer 41 and is used to bring an interferogram to be detected into the image field of the camera 41 by movements in the x or y direction, which can be adjusted along a vertical z direction for focusing is.
• the camera can also be displaced in the xy direction, so that an immovable storage surface can be used to hold the recording medium 24.
• the interferograms are read in according to their assignment to the field point, phase level and phase shift direction and evaluated with the aid of the evaluation program. The evaluation is not part of this invention and is therefore not explained in detail. Possible evaluation routines are described, for example, in the specialist book "Optical Shop Testing" by B. Malacara, 2nd edition, John Wiley & Sons Inc. (1992).
• auxiliary structures are present in the recording medium in addition to the interferograms (cf. FIG. 6). These can either be introduced by the exposure itself or in some other way. Corresponding auxiliary structures can be assigned to the mask structure and / or the reference structure for generating the auxiliary structures by the exposure itself, the effect of which will be explained later in connection with FIGS. 8 to 10.
• the auxiliary structures can be, for example, registration marks or reference marks 45, which allow the various evaluation patterns to be offset with one another in a precise position.
• auxiliary structures can be provided which make it possible to record effects of geometric distortions which can arise, for example, when processing recording media.
• Gray wedges or the like which can either be stepped (gray wedge 46) or continuous (gray wedge 47), can also be imaged for checking or normalizing the resolved gray levels and the exposure.
• These structures can improve the measurement accuracy that can be achieved with the method. 8 to 10 explain how, in preferred embodiments, by providing additional auxiliary structures in addition to the mask structure and / or the reference structure, a check and, if necessary, arithmetical correction of phase step errors during phase shifting is possible.
• These structures can be designed such that both the phase steps and a possible relative rotation of the mask structure and the reference structure can be detected and taken into account.
• the structures or their superimposition patterns can be imprinted in the recording medium when generating the interferograms and recorded during evaluation and used to correct evaluation errors.
• 8 shows an embodiment of a mask structure 420 in the form of a square checkerboard grid. Line grids 426 extending in the x and y directions are arranged outside the mask structure.
• the structured surface of an associated sensor unit has a similar structure with an internal checkerboard grille and external line grids.
• FIG. 9 shows an example of an intensity pattern generated in a recording medium 424 which arises when the mask is imaged on the reference structure.
• An interferogram 425 is created in the circular central region as the overlay pattern. The overlaying of the line gratings results in moiré patterns 419 which extend in the x and y directions and lie outside the overlay pattern 425.
• FIG. 10 For an explanation of the information content of moiré patterns, reference is first made to FIG. 10.
• sub-picture (a) two superimposed line gratings of the same period are shown, which run parallel to one another and therefore do not produce any moire stripes.
• sub-figure (b) the line gratings aligned parallel to one another have different periodicity lengths, so that a sinusoidal stripe pattern is produced.
• the partial figure (c) shows the moiré stripes, though a slight relative lateral displacement of the two line gratings shown in FIG. (b) has occurred parallel to the longitudinal direction of the gratings. In this case, the position of the moire stripes shifts, the spacing of which remains unchanged.
• Partial figure (d) shows the result of a relative rotation of two line gratings against each other.
• the resulting moire pattern is a stripe pattern perpendicular to the line direction of the line grid.
• Sub-figure (e) finally explains a moiré pattern which arises when line gratings of different periodicity lengths (see (b)) are rotated relative to one another.
• a moire pattern is created with oblique stripes, the line spacing of which is a measure of the relative rotation.
• the image information shown in FIG. 9 can be interpreted as follows.
• the same phase position of opposing moiré patterns 419 means that the mask structure and the reference structure have no relative rotation, that is to say they are perfectly adjusted to one another.
• the phase position of the diffraction grating relative to the coherence-forming mask can be determined with high precision from the phase position of the moiré pattern.
• the focus can be checked via the contrast of the moire pattern. The greatest contrast is achieved when the mask structure and the reference structure or the associated line gratings lie exactly in conjugate planes. Tilting the contrast curve would indicate that the mask and diffraction grating were not aligned parallel to the object plane or image plane. Based on this additional information, the evaluation pattern can be evaluated with the greatest accuracy.
• the substrate thickness or the axial distance between the line grating should be essentially one in addition to the reference structure and the recording medium Talbot spacing of the grid can be adjusted, in which the grid is self-mapped and thus blurring of location information can be minimized.
• a diffusing screen and / or a fluorescent layer to reduce the spatial coherence can optionally be dispensed with.
• separate measures can be provided to protect the recording medium in order to protect this layer from mechanical damage, for example scratches, and / or from optical damage, e.g. by external exposure.
• Protective layers can be provided for mechanical protection, but they must not impair the evaluation.
• the recording medium can be encapsulated by suitable cassettes or the like. It is particularly useful if the material used is essentially only or predominantly sensitive to the useful wavelength used in the measurements (typically in the ultraviolet range) and insensitive in other wavelength ranges, for example in visible ranges. This is particularly advantageous when used in wafer steppers, since light-optical path length and positioning measuring systems are often used, which often work with laser light (e.g. 633nm wavelength).
• the areas between the partial grids can be provided with a protective layer or a closed chrome layer for measurement over the entire surface.
• a barrier layer e.g. a bandpass filter, attached to a surface in front of the recording medium to protect against false light and to simplify handling in ambient light.
• the measurement method is carried out here, for example, using a measurement with a single measurement channel for a single field point.
• the measuring system comprises a measuring mask with a mask structure 520, which is to be arranged in the object plane 11, and a sensor unit 521 with a reference structure 523, which is to be arranged in the image plane 12 of the projection objective 10.
• the mask structure 520 is typically a line grid or a parquet pattern.
• the associated reference structure 523 is a similar grid with the same pattern, which is matched to the imaging scale of the projection objective.
• the line widths typically correspond approximately to the resolution of the optical imaging system and can be in the micrometer range or less when measuring microlithographic projection objectives.
• an overlay pattern is created, which typically has the form of a stripe pattern. This is done directly or with the interposition of a frequency converter layer captured by a corresponding recording medium 524.
• the distortion can be determined from the shape of the moire pattern created by superimposition.
• a phase shift method can be used, similar to the interferometric method described, in order to obtain overlay patterns or evaluation patterns with different phase positions.
• the recording can be carried out with two lattice structures that are preferably oriented orthogonally to one another or with two-dimensional lattice structures.
• the measuring system has the mask with the mask structure 520 and a sensor unit 521 with the reference structure 523 and the recording medium 524.
• the sensor unit 521 has the flat disk shape of a wafer. The movements of the mask structure and / or reference structure required for the phase shift are carried out by the movable holders 15 and 16 of the projection exposure system.
• the sensor unit comprises a relatively thin substrate 522 made of quartz glass, on one plate surface of which the reference structure 523 and on the opposite plate surface the recording medium 524 is applied in the form of a thin film of light-sensitive material.
• a mechanically stable, thicker quartz plate 526 In other embodiments, this plate can also consist of non-transparent material, for example silicon.
• the thin substrate 522 is transparent to the measuring light, but it can also have a scattering effect and / or have frequency-converting properties. For example, it can consist of cerium-doped quartz glass. With the moire It is important in technology that the recording medium 524 is as close as possible to the reference structure or to a conjugate plane. A high-contrast overlay can be achieved despite the distance from the reference structure if, as in the embodiment shown, the recording medium is arranged at a Talbot distance from the reference structure.
• the recording medium 524 need not be attached to the thin reference substrate 522. It is also possible to mount the recording medium on the stable carrier plate 526 and to place the reference substrate only on the recording medium. Separate elements can optionally be provided on the edge area of the sensor unit 521 for fixing. It is also possible that the recording medium is not firmly attached to any of the substrates 522, 526.
• An embodiment which enables the recording medium to be displaced relative to the reference structure is shown schematically in FIG. 13.
• the spatial sequence of reference structure 623, reference substrate 622, recording medium 624 and stable carrier plate 626 corresponds to the structure in FIG. 12.
• FIG. 3 Analogously to the embodiments according to FIGS.
• the entire sensor unit 521 is moved step by step with the aid of the wafer holder between successive measurements, and different grating regions of the reference structure are used in succession.
• the embodiment according to FIG. 13 permits an immovable arrangement of the sensor unit 621, since only the film-like, flexible recording medium 624 has to be moved relative to the reference structure.
• the same area of the reference structure is always used here, which can be designed to be correspondingly small.
• a recording medium exposed with the embodiment according to FIG. 12 can in principle be constructed as shown in FIG. 5. In phase shifting, for example, 2 * 8 phase steps in the x direction and the same number of phase steps in the y direction can be detected. With an image diameter of approx.
• An evaluation device analogous to the evaluation device 40 shown in FIG. 7 can be used to record and evaluate the evaluation pattern, with a different work program having to be used when evaluating moiré patterns.
• a variant known as a multi-strip method can also be used.
• a carrier frequency By rotating the grating orientation, a carrier frequency can be impressed on the moiré pattern so that this method can be used.
• the advantage is that the phase distribution can be calculated from a single overlay pattern (Moire image), so no phase shift is required. Suitable evaluation methods are described in, for example.
• the recording medium is arranged in the light path at a distance behind the reference structure, the distance being adapted to the respective measurement method.
• the reference structure is integrated into the evaluation medium in such a way that the evaluation medium and the reference structure have no or only a very small distance from one another. For example, by pre-structuring the recording medium with a grid pattern, the recording medium can be brought directly into the plane of the reference structure. In this case, it is not necessary to destroy the spatial coherence.
• the moiré image is not created here by coherent superposition of diffraction orders behind the grating, but solely by adding intensity in the plane of the reference structure.
• the reference structure can be patterned differently, corresponding structure lines or grid lines can have different intensity profiles and different types of production of such structured recording media are possible.
• Typical basic patterns are, for example, line grids, cross grids, parquet grids or checkerboard grids.
• Other lattice shapes, for example combinations of the lattice types mentioned, are also possible.
• the intensity profiles can be binary, ie abrupt, or designed in grayscale. As an example, FIG.
• a binary pre-exposed or pre-structured registration medium 724 for example a photoresist or a film, in which there is a rectangular light-dark curve perpendicular to the lines.
• the material of the recording medium can, for example, be fully exposed or removed in the light areas until saturation and can only be sensitive to radiation at the unexposed spaces.
• Intensity profiles in grayscale are also possible, for example the sinusoidal intensity profile of a registration medium 824 shown in FIG. 15.
• Structured recording media of this type can, for example, be pre-exposed in the grating pattern Recording material are generated. It is also possible to produce a contact copy of a master template, which can be formed by a chrome lattice or by writing processes.
• Exposed areas for example of a photoresist, can remain or be removed from the recording medium.
• conventional techniques can also be used in lithography, for example coating the layer to be structured with a binary lacquer, exposing it, developing and etching the structure.
• lithography for example coating the layer to be structured with a binary lacquer, exposing it, developing and etching the structure.
• exposure can be carried out with targeted blurring or with the aid of low-pass filtering of the image.
• Imaging by a projection lens would also be possible, the numerical aperture of which would have to be adapted accordingly to the structural dimensions. This has the advantage, among other things, that the geometry errors of the inscribed grating would be precisely known, since distortion errors of the objective and errors of the grating template can be determined in advance.
• Sinusoidal linear gratings of high accuracy can also be produced holographically by coherent superposition of plane waves.
• a pre-structured recording medium or a recording medium in the immediate vicinity of a reference structure can also be produced by a thin, structurable or already structured reflection or absorber layer 940 e.g. on a layer of the recording medium 924. is applied by lamination.
• FIG. 17 schematically shows the structure of a mobile, phase-shifting point diffraction interferometer.
• a lighting optic 919 following the lighting device 18 is used for light focusing on a Mask structure serving perforated mask 920, which is arranged in the object plane 11 of a projection objective 10 to be measured.
• the diameter of the hole in the mask structure is smaller than the wavelength of the measuring light and thus serves to generate a spherical wave (solid line) by diffraction.
• a diffraction grating 921 between shadow mask 920 and projection objective 10 serves to generate a second wave (shown in dashed lines) coherent with the first spherical wave and for phase shifting which may be used.
• the diffraction grating can also be arranged between the projection objective and its image plane.
• the reference structure 973 to be arranged in the image plane 12 of the projection objective is likewise designed as a shadow mask. It has at least one quasi-punctiform hole 976, which is used to generate a reference spherical wave by diffraction. Its diameter is smaller than the measuring light wavelength. Next to it there is (at least) a larger hole 977, the diameter of which is significantly larger than the measurement light wavelength and which serves as a spatial limitation of the test specimen wave shown by solid lines.
• the reference structure 973 is arranged on a flat upper side of a transparent substrate 972.
• a radiation-sensitive recording medium 974 is applied to the opposite side of the sensor unit 971, for example in the form of a lacquer layer made of photoresist.
• the axial distance between the reference structure 973 and the recording medium 974 is dimensioned such that the recording medium is located in an area in which an interference pattern (superimposition pattern) is produced by the superimposition of the reference wave coming from the hole 976 and the test specimen wave passing through the hole 977, which information about the imaging quality of the projection lens contains.
• the interference pattern is stored in layer 974 and can be recorded and evaluated analogously to the manner described above after removal of sensor unit 921 by a camera and the like.
• the invention provides possibilities, for example, to carry out high-precision wavefront measurements by means of shearing interferometry or by means of point diffraction interferometry or highly precise measurements using moiré technology on projection lenses which are installed in a projection system at their place of use.
• the measurements are possible regardless of the type of projection exposure system and therefore platform-independent.
• Mobile sensor units are preferably used for this purpose, which comprise a reference structure and a recording medium and can be inserted into the wafer stages instead of a wafer. These manipulation devices which can be moved with high precision can be used for any necessary displacements of the reference structure, if necessary without modification.
• the measurement technology on the projection exposure system does not require any optoelectronic image capture devices which work, for example, with a CCD camera and, if appropriate, imaging optics.
• a universal measuring system has thus been created which, despite the simple structure of its components, permits extremely precise measurements.

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• 2002
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