DE102011086513A1 - Projection exposure method for exposure of semiconductor wafer with image of pattern of reticle for manufacturing semiconductor component, involves heating mask corresponding to two-dimensional heating profile by heating device - Google Patents

Abstract

The method involves holding a radiation-sensitive substrate (W) so that a radiation-sensitive surface of the substrate is arranged in a region of an image plane (IS) of a projection lens (PO), where the image plane is optically conjugated to an object plane (OS). An illuminating region of a reflective mask (M) is illuminated with an illuminating radiation. A part of a pattern lying in the illuminating region is imaged on the substrate by the lens. The mask is heated corresponding to a two-dimensional heating profile by a heating device (HD), which is separate from an illuminating system (ILL). An independent claim is also included for a projection exposure system.

Description

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a projection exposure method for exposing a radiation-sensitive substrate having at least one image of a pattern of a mask according to the preamble of claim 1 and to a projection exposure apparatus suitable for carrying out the method according to the preamble of claim 13.

Description of the Prior Art

For the production of semiconductor components and other finely structured components, predominantly microlithographic projection exposure methods are used today. In this case, masks (photomasks, reticles) are used, which carry the pattern of a structure to be imaged, z. B. a line pattern of a layer (layer) of a semiconductor device. The mask is positioned in a projection exposure apparatus in the beam path between an illumination system and a projection objective such that the pattern lies in the region of the object plane of the projection objective. A substrate to be exposed, for example a semiconductor wafer coated with a radiation-sensitive layer (resist, photoresist), is held in such a way that a radiation-sensitive surface of the substrate is arranged in the region of an image plane of the projection lens that is optically conjugate to the object plane. In an exposure process, the pattern is illuminated by means of the illumination system, which forms from the radiation of a primary radiation source an illumination radiation directed to the pattern, which is characterized by certain illumination parameters and impinges on the pattern within an illumination field of defined shape and size. The radiation changed by the pattern passes through the projection lens as projection radiation, which images the pattern onto the substrate to be exposed coated with a radiation-sensitive layer. Microlithographic projection exposure methods can also be used to make masks (reticles).

In order to be able to produce ever finer structures, different approaches are pursued. For example, the resolution of a projection lens can be increased by increasing the image-side numerical aperture (NA) of the projection lens. Another approach is to work with shorter wavelengths of electromagnetic radiation.

Attempting to improve the resolution by increasing the numerical aperture may present problems in that as the numerical aperture increases, the depth of focus (DOF) achievable decreases. This is disadvantageous because, for example, for reasons of achievable flatness of the substrates to be patterned and mechanical tolerances, a depth of field in the order of at least 0.1 microns is desirable.

For this reason, among other things, optical systems for microlithography have been developed which operate at moderate numerical apertures and which achieve magnification of resolving power essentially through the short wavelength of the extreme ultraviolet (EUV) electromagnetic radiation used, particularly at working wavelengths in the range of 5 nm and 30 nm. In the case of EUV lithography with working wavelengths around 13.5 nm, for example, at image-side numerical apertures of NA = 0.3, theoretically a resolution of the order of 0.03 μm can be achieved at typical depths of field in the order of about 0.15 μm.

Extreme ultraviolet (EUV) radiation can not be focused or guided by refractive optical elements because the short wavelengths are absorbed by the known optical materials transparent at higher wavelengths. Therefore, mirror systems are used for EUV lithography. The masks used are reflective masks.

A reflective mask for EUV lithography has a substrate that carries on its front side an EUV radiation-reflective structured coating that forms the pattern. Such coatings are designed as multi-layer arrangements (multilayer), which have many pairs of layers with alternating low-refractive and high-refractive layer material and act like Bragg reflectors. For structuring, d. H. for the formation of the pattern, suitable absorbers are used. A not inconsiderable proportion of the incident radiation energy of the illumination radiation is absorbed on or in the mask, which can lead to a thermal load and thus to thermally induced deformations of the mask.

In order to minimize thermally induced deformations of the reflective mask, materials with a relatively low coefficient of thermal expansion are frequently used to construct the mask substrate (see, for example, US Pat. WO 2010/020337 A1 ).

There are also suggestions, one caused by thermal stress or otherwise Counteract mask deformation by suitable manipulators.

In the patent US 7,675,607 For example, a projection exposure apparatus is described that has a support for supporting a patterning device, wherein the structuring device may be a reflective mask. The carrier has a clamping device (support clamp) with which the structuring device can be clamped to the carrier. With an additional bending mechanism, a bending moment can be exerted on the clamped structuring device without changing the clamping force exerted on the structuring device. This should, inter alia, be able to counteract thermally induced deformations of the structuring device.

The patent US 7,187,432 B2 describes a projection exposure apparatus for EUV microlithography, in which a holding means for a reflective mask is provided. In order to counteract gravity-induced deflection of the mask, in addition to some holding contacts engaging the mask, the holding means has an electrostatically-acting attraction portion which non-contactly acts on and attracts a portion of the mask. The size of a gap between the attraction section and the mask is measured, this measure serves as the basis for controlling the attraction.

In the JP 70-50245 A It is proposed to suppress the deformation of a mask by setting a defined pressure difference between the front side and the back side of the mask.

In the JP 10-097967 A a vacuum chuck for a mask will be described. The vacuum fixture has suction sections at the front and at the back of the mask. The suction pressures of the suction sections can be adjusted to correct the deformation of the mask.

Also known is the problem that masks sag due to their own weight under the influence of gravity between support points on the mask edge. A gravitational mask deflection may occur as an alternative or in addition to thermally induced deformations (see eg. US 2008/0252987 A1 or US 2003/133087 A1 ).

SUMMARY OF THE INVENTION

It is an object of the invention to provide a projection exposure method which ensures a high quality of the structures produced on the substrate under different operating conditions. It is a further object of the invention to provide a projection exposure apparatus suitable for carrying out the projection exposure method.

This object is achieved by a projection exposure method having the features of claim 1 and by a projection exposure apparatus having the features of claim 13. Advantageous further developments are specified in the dependent claims. The wording of all claims is incorporated herein by reference.

In the method, the mask is heated by means of a heater separate from the illumination system. The heating device can heat the mask according to a prescribable two-dimensional heating profile. The term "heating profile" here stands for a spatial (local) heat energy distribution, in which locally different amounts of heat energy are introduced into the mask in the heated area and / or generated in the mask if required. A location-dependent heating of the mask achievable by the heating device leads to an uneven thermal expansion of the heated material and thus to thermally induced surface deformations of the patterned mask surface in accordance with the thermal expansion coefficient of the heated material.

This creates a thermal mask deformation manipulator with which certain surface deformations on the mask can be generated by thermal means within certain limits.

With the aid of the heating device, different surface deformation profiles, which can be defined by the distribution of the heat energy, of the pattern-bearing mask surface can be produced. Since this mask surface is arranged in the immediate area of the object plane of the projection lens and the object plane is a field plane of the projection objective, it is possible to use the heater to set locally different degrees of deformation in the area of a field plane and thus field curves of surface deformations. This creates a near-field thermal mask deformation manipulator that can be used for different purposes during the operation of the projection exposure apparatus.

For example, a thermally induced surface deformation profile can be generated with which undesired deformations of the mask can be corrected partially or completely by way of compensations. These undesirable deformations can be due to different causes. For example, the mask can locally heat up by irradiation of the illumination radiation and a partial absorption of the illumination radiation associated with it, so that a mask deformation occurs due to thermal expansions induced by the illumination radiation. With the help of the heater, the resulting deformations can be partially or completely compensated.

Also known is the problem that masks sag due to their own weight under the influence of gravity between support points on the mask edge. A gravitational mask deflection may occur as an alternative or in addition to thermally induced deformations.

While z. B. on the field constant spherical Zernike can be corrected in principle by means of simple axial displacement of the mask (by so-called z-manipulation), a correction of higher-wave field variations of aberrations has not or only insufficiently possible. In contrast, with the aid of a thermally induced mask deformation, it is generally possible to at least partially correct even higher-wave field characteristics of one or more aberrations (aberrations) of the projection objective. For example, it is possible to correct higher-field field characteristics of spherical Zernikes. The aberrations which can be influenced with the aid of the thermal mask deformation manipulator include, in particular, higher-wave field characteristics of all wavefront aberrations and distortion errors (distortion). The term "higher-wave aberration field" in this context means that the extent of the respective aberration varies significantly from field point to field point.

Since the thermally deformable mask surface is located directly in a field plane, it is also possible to correct any errors in the surface topography of the substrate to be exposed whose surface lies in the image plane of the projection objective that is optically conjugate to the object plane of the projection objective.

In some embodiments, the mask is heated by a backside of the mask facing away from the pattern. This is particularly advantageous in projection exposure apparatuses for EUV lithography, because in such a projection exposure apparatus typically the patterned front mask surface of a reflective mask is located in an evacuated area of the projection exposure apparatus while the opposite rear surface is exposed without interference with the sensitive environment (miniature). Environment) of the pattern can be used for purposes of manipulation.

Although a heater may be provided that is in heat-conductive, physical contact with the mask, in some embodiments it is contemplated that the mask may be formed by introducing electromagnetic radiation energy, i. H. by irradiation, contactless (i.e., without contact) is heated. In this case, since no external mechanical forces or moments are exerted on the mask, no modifications in the region of the mask holding device, which is provided for holding the mask in the intended mask position, are generally required.

In some embodiments, the mask is irradiated by a heater having a radiator array with a plurality of independently controllable radiant heat sources. It is thus possible to irradiate a plurality of laterally offset heating regions of the mask simultaneously or with a time offset with heating radiation, wherein for each heating region the irradiation duration and / or the irradiation intensity can be controlled independently of other heating regions. As a result, if necessary, a location-dependent varying deformation of the mask with high spatial resolution can be achieved. Another advantage is that a radiator array can optionally be constructed without moving parts.

It is also possible that the heater one or more movable, z. B. has pivotable radiator for emitting heat radiation. A heating jet, similar to a scanner, can be moved in accordance with a predeterminable path profile over a region of the mask to be heated and heat successive heating regions along the path. The residence time and / or the irradiation intensity of the heating beam can be predetermined by a controller.

In order to achieve the most efficient heating of the mask via heating radiation, it is provided in some embodiments that the mask has an absorber layer which consists of an absorber material which has a high absorption coefficient in a wavelength range of the heating radiation. This may be greater by one or more orders of magnitude than the corresponding absorption coefficient of the material from which the mask or the pattern-carrying part of the mask is made, ie at least ten times or at least one hundred times as large. As absorber material, for example Carbonaceous materials such as graphite or graphene and various heat stable, non-gassing plastics into consideration.

An absorber layer should have the strongest possible absorption, but poor area conductivity, ie poor thermal conductivity in the lateral direction. A flat poor thermal conductivity can be achieved, for example, by segmenting the absorber layer into individual absorber zones. It may be advantageous if the individual absorber zones are relatively localized, ie if their area is small compared to the area of the entire absorber layer. In particular in these cases metals such as copper, nickel, silver, chromium or tungsten or alloys with one or more of these metals can be used as absorber material.

In some embodiments it is provided that the mask is heated by a front side of the mask having the pattern. For this purpose, in one embodiment, a diaphragm with a passage opening for the illumination radiation directed onto the mask and a passage opening for the radiation which is reflected by the mask is arranged at a distance in front of the mask, the first side facing the mask being arranged on the mask Aperture a heater is arranged to heat the front of the mask. By way of example, the heating device can have a radiator array with a multiplicity of radiant heat sources which can be controlled independently of one another. The diaphragm may have two separate passage openings, optionally the illumination radiation and the reflected radiation may be guided through one and the same passage opening in the diaphragm. The heating from the front can act directly on near-sample areas of the mask, so that surface deformations with high spatial resolution can be generated.

Whereas reflective masks are provided in projection exposure apparatuses which work with extreme ultraviolet radiation, projection masks which use longer-wavelength ultraviolet radiation, for example from the deep ultraviolet range (DUV) or vacuum ultraviolet range (VUV), generally prefer transmitting masks (transmission masks), since materials with sufficient transparency are available for this wavelength range for the mask substrate, and thus the rays running from the illumination system to the projection lens can pass through the mask perpendicularly or at least at an acute angle to the mask normal. As a result, inter alia shading problems on the three-dimensionally structured region of the pattern can be avoided.

In particular, for these cases it can be provided that a heating device has a plurality of independently controllable radiant heat sources, which are arranged laterally outside of the light path leading to the projection lens beam path and irradiate the mask. The radiant heat sources can be arranged, for example, in the form of a rectangular, polygonal or partially or completely round ring around the beam path. If the heat radiation sources are arranged in the region between the illumination system and the mask, irradiation of the heating power can take place obliquely on the rear side of the mask. In an arrangement in the region between the mask and the projection lens, the heating radiation can obliquely from the front of the mask meet on this.

In some embodiments, the mask is heated by lateral irradiation of heating radiation, wherein preferably at least a portion of the heating radiation penetrates into the mask substrate substantially parallel to a mask plane via lateral boundary surfaces. A corresponding heating device may have a plurality of independently controllable radiant heat sources, which are distributed over the circumference of the mask in an area adjacent to the mask. For example, an annular arrangement of radiant heat sources can be provided, which are arranged substantially in the mask plane and can radiate heating radiation into the mask substrate from at least two sides which are at an angle, in particular perpendicular to one another.

The irradiation is preferably carried out by a plurality of points, d. H. from at least two points, each angularly resolved in a plurality of at least two beam segments. If necessary, the beam intensity in the angles of incidence can be varied discretely or continuously. If necessary, the edge of the mask can be structured to achieve certain optical effects, for example in the form of a Fresnel optic. Alternatively or additionally, auxiliary optics can be provided for beam injection into the mask.

As an alternative to or in addition to a radiant heater, an electrothermal heater may be provided. Preferably, such an electrical heating device has an array with a plurality of independently controllable electrothermal elements, which are in heat-conducting contact, in particular in touching contact or solid contact with the mask, in particular the back of the mask. These may be, for example, resistance heating elements. The heating device may be designed in the manner of a "heated rear window", wherein electrothermal elements in the form of Conductor tracks can be provided from Widerstandsheizmaterial. This makes it possible that the mask is heated by means of independently controllable electrothermal elements, which are in heat-conducting contact, in particular in touching contact with the mask.

From the US 2009/0257032 A1 are constructed with interconnects electrothermal heating devices for the heating of optical elements of a projection lens, in particular for curved mirrors or lenses known. In the present case, inter alia, the use of such heaters for the spatially-resolving heating of masks is proposed. The disclosure of the US 2009/0257032 A1 concerning the structure and control of electrothermal heating devices constructed with printed conductors, the contents of the present description are incorporated herein by reference.

Tracks can z. B. applied to the mask or integrated into the mask material. In such embodiments, very little heating power is required. However, the power supply may need to be reconnected during a mask change. Conductor tracks may also be arranged on and / or in an electrothermal heating element separate from the mask. When traces are applied to a separate heating element which is attached to the mask, the mask can be changed quickly and only one heating element with wires or other leads is needed.

In some embodiments, contrary to the effect of the heating, thermal energy is actively removed by cooling from the heated mask. By using a cooling in addition to a heating bidirectional manipulation of the mask is possible. By cooling, a faster manipulation of the mask shape is possible if required. The mask can, for. B. have a certain uneven temperature profile. Now a contrary temperature profile should be set. If the mask is cooled, the old temperature profile is neutralized faster and the new temperature profile is set faster. In addition, with the help of active cooling, if appropriate, the surface deformation can be achieved with a higher spatial resolution.

By active cooling, the mask can be stabilized if necessary with respect to a temperature zero point system. The term "temperature zero point position" here denotes the ideal lens temperature. If the mask temperature is in the "temperature zero" position, the mask will be perfect for an ideal lens, i. H. imaged on the substrate without aberrations.

In order to achieve active cooling, a heat sink with thermal contact with the mask may be provided, wherein a mean temperature of the heat sink is less than an average temperature of the mask. This results in a heat flow from the mask in the direction of the colder heat sink, which heat energy can be deducted.

It is possible to realize contactless cooling. In some embodiments, this is achieved by passing a stream of a cooling fluid, in particular of a cooling gas, onto the mask. The flow of cooling fluid can sweep the entire side of the mask to be cooled so that it is cooled substantially uniformly. It is also possible to achieve different locally strong cooling effects by possibly several streams of cooling gas generated and passed to different locations of the side to be cooled of the mask.

Active cooling in addition to heating the mask can in many cases be beneficial, but is not mandatory. The following considerations can be taken into account in the design. If, for example, you want to set a new temperature profile in the mask, then this process can be mentally divided into two steps.

First, the mask can be heated so that sets a substantially constant temperature over the entire mask as before heating. Then a new temperature profile with locally different temperatures can be "written". The mean temperature of the mask may increase from cycle to cycle. If the change in temperature profiles takes place relatively quickly, this can lead to an increasing heating of the mask and, as a result, also to higher temperatures in the entire projection exposure apparatus. This can be unfavorable with regard to the thermal budget, for example, of the projection lens. Therefore, relatively slow change cycles in the heating are generally advantageous. If active cooling is used, there may be more degrees of freedom for the achievable temporal changes of heating profiles. For example, an active cooling can be provided in which always has to be heated to achieve a balanced temperature in the range of the temperature zero point position. In this case, temperature profiles can be changed faster, if necessary, without fear of overheating. Active cooling can thus be advantageous, for example, with regard to relatively rapid temperature profile changes. By contrast, active cooling can often be dispensed with if relatively slow change cycles are provided.

If a heating and a simultaneous active cooling are provided, this can be done both from the same side of the mask. It is also possible to heat locally from one of the sides, for example from the front side, and to remove heat from the opposite rear side, that is to say to cool it. A locally uniform cooling is usually easier to implement than a likewise possible locally varying cooling.

The operation of the heater can be performed based on control data stored in a controller (open-loop control). It is also possible to integrate the heating device into a closed-loop control. For this purpose, in one embodiment, a deformation of the surface of the mask is measured. From this, surface deformation data is generated, e.g. For example, describe the extent and local distribution of surface deformation as appropriate. The control of the heater is then performed in response to the surface deformation data. As a result, a particularly precise adjustment of a desired surface deformation profile is possible even with varying operating conditions.

Alternatively or additionally, it is also possible to measure the local current heat distribution of the mask and derive therefrom control commands for the heating device. A temperature distribution measuring device can either face the structured side of the mask (ie, the pattern) or be located on the rear side of the mask facing away from the pattern.

Alternatively or additionally, it is also possible to measure the overall optical effect of the mask and of the projection objective during operation and to carry out the control of the thermal mask deformation manipulation device on the basis of these measurement results.

The invention also relates to a projection exposure apparatus for exposing a radiation-sensitive substrate to at least one image of a pattern of a mask. The projection exposure apparatus is configured to perform the method. Accordingly, it may include means of the kind described above and explained in more detail below to produce a targeted thermal mask deformation manipulation.

The above and other features are apparent from the claims and from the description and from the drawings, wherein the individual features are realized individually or in each case in the form of sub-combinations in an embodiment of the invention and in other fields and advantageous and can represent protectable embodiments. Embodiments of the invention are illustrated in the drawings and explained in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

1 shows components of an EUV microlithography projection exposure apparatus according to an embodiment of the invention;

2 shows an embodiment of a heater that in the EUV microlithography projection exposure system 1 can be used;

3 shows another embodiment of a heater that in the EUV microlithography projection exposure system 1 can be used;

4 shows an embodiment of a heating device, which is associated with a cooling device for actively cooling the mask by means of cooling gas;

5 shows an embodiment of an electrothermal heater with electrothermal cooling device;

6 shows a heater for heating a transmission mask with oblique irradiation of heating radiation in the back of the mask; and

7 shows in 7A a view directed parallel to the optical axis of the projection exposure apparatus and in FIG 7B a side view of an embodiment of a heater having an annular array of radiant heat sources, which radiate heating radiation via lateral boundary surfaces of a transmission mask in the mask substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1 shows components of an EUV microlithography projection exposure apparatus WSC for exposing a radiation-sensitive substrate W disposed in the region of an image plane IS of a projection objective PO with at least one image of a pattern of a reflective mask M. arranged in the region of an object plane OS of the projection objective.

The projection exposure apparatus is operated with the radiation of a primary radiation source RS. An illumination system ILL serves to receive the radiation of the primary Radiation source and for shaping directed to the pattern illumination radiation. The projection objective PO serves to image the structure of the pattern onto the photosensitive substrate W.

The primary radiation source RS may be, inter alia, a laser-plasma source or a gas-discharge source or a synchrotron-based radiation source. Such radiation sources generate radiation RAD in the extreme ultraviolet region (EUV range), in particular with wavelengths between 5 nm and 15 nm. In order for the illumination system and the projection objective to be able to work in this wavelength range, they are constructed with components reflective to EUV radiation.

The radiation RAD emanating from the radiation source RS is collected by means of a collector C and directed into the illumination system ILL. The illumination system comprises a mixing unit MIX, a telescope optics TO and a field-shaping mirror FFM. The illumination system forms the radiation and thus illuminates an illumination field which lies in the object plane OS of the projection objective PO or in its vicinity. The shape and size of the illumination field thereby determine the shape and size of the object field OF which is actually used in the object plane OS. The illumination field is usually slot-shaped with a high aspect ratio between width and height.

In the object level OS, a reflective mask M is arranged during operation of the system (cf. 2 to 5 The projection lens PO here has six mirrors M1 to M6 and images the pattern of the mask on a smaller scale in the image plane in which the substrate to be exposed, z. B. a semiconductor wafer is arranged.

The mixing unit MIX consists essentially of two facet mirrors FAC1, FAC2. The first facet mirror FAC1 is arranged in a plane of the illumination system which is optically conjugate to the object plane OS. It is therefore also called a field facet mirror. The second facet mirror FAC2 is arranged in a pupil plane of the illumination system, which is optically conjugate to a pupil plane of the projection objective. It is therefore also referred to as a pupil facet mirror.

With the aid of the pupil facet mirror FAC2 and the imaging optical subassembly following in the beam path, which includes the telescope optics TO and the grazing incidence field-forming mirror FFM, the individual specular facets (individual mirrors) of the first facet mirror FAC1 are imaged into the object field.

The spatial (local) illumination intensity distribution at the field facet mirror FAC1 determines the local illumination intensity distribution in the object field. The spatial (local) illumination intensity distribution at the pupil facet mirror FAC2 determines the illumination angle intensity distribution in the object field OF.

A device RST for holding and manipulating the mask M (reticle) is arranged so that the PAT pattern arranged on the mask (cf. 2 to 4 ) lies in the object plane OS of the projection objective PO, which is also referred to here as the reticle plane. The mask can be moved in this plane for scanner operation in a scan direction (y-direction) perpendicular to the reference axis AX of the projection objective (z-direction) by means of a scan drive SCM.

The substrate W to be exposed is held by a device WST comprising a scanner drive SCW for moving the substrate in synchronism with the mask M perpendicular to the reference axis AX in a scanning direction (y-direction). Depending on the design of the projection objective PO, these movements of mask and substrate can take place parallel to one another or counterparallel to one another.

The device WST, which is also referred to as "wafer days", and the device RST, which is also referred to as "reticle days", are part of a scanner device which is controlled via a scan control device, which in the embodiment is transferred to the central control device CU the projection exposure system is integrated.

All optical components of the projection exposure apparatus WSC are housed in an evacuatable housing H. The operation of the projection exposure apparatus takes place under vacuum.

EUV projection exposure systems with a similar basic structure are z. B. from the WO 2009/100856 A1 or WO 2010/049020 A1 the disclosure of which is incorporated herein by reference.

The area around the mask M is in 2 shown schematically in detail. The mask M has a mask substrate MS with a structured, macroscopically substantially planar first mask surface MS1 and an opposite planar second mask surface MS2, which is aligned substantially parallel to the first mask surface. On the first mask surface is a reflective for EUV radiation multilayer film arrangement, the z. B. alternating layers of molybdenum and silicon or ruthenium and silicon. The PAT pattern of the mask is formed by one or more structured absorber layers, the z. B. from a tantalum-based absorber material can exist. In the illustrated installed state, the PAT pattern is at the front of the mask M, while the second mask surface is at the back. The front side faces the illumination system ILL and the projection objective PO such that the illumination radiation ILR provided by the illumination system strikes the pattern obliquely, ie at an angle to the surface normal N of the first mask surface, is changed by the pattern and then obliquely in as projection radiation PR the projection lens PO is irradiated.

The mask is in the area of a "mini-environment", i. H. in a sealed area with special requirements regarding vacuum, purity, etc.

Between the region of the mask and the optical systems (illumination system and projection objective) of the projection exposure apparatus, there is a diaphragm D with a passage opening DO, through which the illumination radiation focused in the region of the mask and the projection radiation reflected by the mask pass. An essential task of this diaphragm is to reduce the scattered radiation level within the optical system.

The mask substrate MS has a thickness of approximately 6.35 mm (0.25 inches) and consists essentially of quartz. The mask substrate has a multilayer structure. At the front there is a first plate P1 made of quartz, whose plane free surface forms the first mask surface M1 and carries the coating with the PAT pattern. At the back there is a second plate P2, which is also made of quartz. Between the first plate and the second plate, an absorber layer ABS is arranged, which consists of an absorber material which has a high absorption coefficient for infrared radiation. The absorber layer may, for example, contain graphite or graphene or consist of these carbon materials. Certain heat-stable, non-gassing plastic materials can also be used for the absorber layer.

The mask is held in the installed state by means of a mask holder MH in the desired mask position. The mask holder is shown here only schematically and acts on edge regions of the mask outside of the patterned area substantially. The mask is held in a horizontal orientation so that gravity is substantially parallel to the surface normal N of the mask surfaces.

For the mounting of the mask different principles are possible. In some embodiments, the mask holder has holding members for mechanically clamping the mask. A pneumatic mounting of the mask by means of a vacuum holding device is also possible. Furthermore, a non-contact or contactless holder by means of magnetic forces is possible (see, eg WO 2005/026801 A2 ). Particularly with this type of support, which operates without mechanical attack on the mask, contactless thermal manipulation can be used to great advantage, as any asymmetrical mechanical engagement with the mask for deformation could affect the position of the mask as a whole.

The projection exposure machine is equipped with a thermal mask deformation manipulator. An important component of this device is a heater HD, which is arranged at a defined distance from the second mask surface at the back of the mask. The heater is constructed and configured so that the mask M can be heated from the back side according to a prescribable two-dimensional heating profile. For this purpose, with the aid of the heating device, a corresponding heat energy distribution is generated at and / or in the mask or the mask substrate. The heating device of the exemplary embodiment has a carrier SUP whose lateral dimensions essentially correspond to the lateral dimensions of the free-floating region of the mask. On the mask-facing front side of the carrier SUP a two-dimensional array (array) is mounted with a plurality of heat radiation sources HS, which may be arranged in a regular or irregular arrangement at a distance from each other. The radiant heat sources may, for example, be light-emitting diodes (LEDs) which emit concentrated heat radiation HR in the infrared range. It may also be laser diodes or other heat radiation sources. Furthermore, additional optics may be provided to shape the radiant heat emitted by the radiant heat sources and / or to direct or redirect to certain target areas of the mask.

With the aid of a control device HC of the heating device connected to the central control device CU of the projection exposure apparatus, the radiation intensity can be controlled in a time-dependent manner for each radiant heater HS independently of the other radiant heaters. Each radiant heater irradiates a spatially limited heating area HR1, HR2 of the mask so that laterally offset heating areas of the mask can be irradiated with heating radiation, wherein for each Heating range, the irradiation time and / or the irradiation intensity can be controlled independently of the other heating areas.

Due to the spatially and / or temporally inhomogeneous irradiation of the mask with infrared radiation, thermal energy is introduced into the mask according to a predeterminable spatial pattern and generated on and / or in the mask. A particularly high efficiency is achieved with the help of the absorber layer, in which a relatively high proportion of the radiation energy of the heating radiation is converted into heat energy. With the aid of the infrared radiant heater of the heating device, which radiate into a wavelength range suitable for the absorber material, an inhomogeneous temperature profile is generated, above all in the region of the rear side of the mask and in the region of the absorber layer. The local heating of the mask leads, via the thermal expansion of the heated materials, to an uneven expansion of the material (s) and thus to certain, desired surface deformations of the first mask surface MS1 carrying the PAT pattern.

Since the irradiation of heating power is performed by the rear side of the mask opposite the illumination system and the projection objective, no disturbing scattered light can enter the projection objective.

During scanner operation, the mask is moved perpendicular to the reference axis of the projection lens in a scanning direction (y-direction). However, in order to achieve the desired thermal surface deformation in the mask, there are different possibilities. In one variant, the part of the heating device HD with the carrier SUP and the heat radiators HS attached thereto is moved together with the mask. For this purpose, the carrier SUP may be mechanically coupled to the mask holder MH. It is also possible that the heater is mounted stationary and does not move with the mask. In this case, the desired thermal energy distribution in the mask can be adjusted by means of rapidly switchable stationary radiant heaters by synchronizing the activation of the radiant heaters with the movement of the mask such that the back of the mask has substantially the same spatial distribution in each phase of its movement the irradiation intensity.

The heating of the mask from the rear side facing away from the projection lens is advantageous in many cases since the mask-near parts of the heating device can not be arranged in the region of the vacuum passing through the EUV radiation, but outside the evacuated region.

However, it is also possible to contactlessly heat the mask from the front bearing the pattern PAT by introducing electromagnetic radiation energy. This shows 3 an embodiment. The same or corresponding components bear, for reasons of clarity, the same reference designations as in the embodiment in FIG 2

As already mentioned, the beam path in the region of the mask is relatively strongly focused in comparison with the beam path within the projection objective PO. Therefore, the diaphragm D, which is disposed opposite to the front side of the mask between the optical systems (illumination system and projection lens) of the projection exposure apparatus and the mask M, can have relatively small passage openings. The aperture D in 3 has a first passage opening DO1 for passing the illumination radiation ILR in the direction of the mask M and a separate second diaphragm opening DO2 for passing the projection radiation PR reflected by the mask in the direction of the projection objective. On the side of the diaphragm facing the mask M, a heating device HD is arranged, whose carrier SUP is fastened to the diaphragm BL. The radiant heat sources HS are located on the side of the heating device facing the mask, so that the heat radiation bundles HR emitted by the individual radiant heaters HS strike the mask from the front side provided with the pattern PAT. Accordingly, in this arrangement pattern-near regions of the mask substrate are heated particularly strongly, so that a very accurately targeted spatially resolving surface deformation of the first surface MS1 can be produced.

Advantageously, the diaphragm BL prevents infrared scattered light from reaching the projection objective PO to a disturbing extent. For this purpose, the diaphragm openings DO1 and DO2 can be designed in this way and the radiant heaters HS can be designed and arranged on the carrier SUP in such a way that as little infrared light as possible reaches the projection lens through the diaphragm openings. Any remainder can be absorbed by suitable absorber elements, such as frosted checker plates or the like, in the projection lens and thus rendered harmless. A reduction of the level of possibly interfering infrared scattered light can alternatively or additionally also be achieved by suitable filter elements, for example a spectral filter arranged in the second passage opening DO2, which filters out the infrared light of the heating radiation and ideally passes the remaining beam portions more or less unhindered. For the EUV sector, for example, so-called "spectral purity filters" are suitable, such as they z. In US 7,372,623 B2 are described for another application.

With this arrangement, since the shutter BL is stationary while the mask is moved in the scanning direction during scanner operation, the heating control HC ensures that the local distribution of the heating power with the moving mask moves with it.

In order to be able to predetermine the shape and size of the heating area irradiated thereby for each radiant heater, in some embodiments a ballast optics is provided for the radiant heaters.

As an alternative to the direct heating shown, it is also possible to direct the used for heating or entry of thermal energy electromagnetic radiation by means of optical fibers to the mask, for example by means of optical fibers, which may be summarized, for example, in a fiber bundle.

In 4 an embodiment of a non-contact, thermal mask deformation manipulator is shown, which operates spatially resolving and bidirectional and allows to set a defined temperature zero point position so that the mask temperature does not or only slightly deviates from the temperature of the projection lens. This is achieved in that, contrary to the effect of the heating, thermal energy can be actively removed by cooling from the heated mask. Similar to the embodiment of 1 is at a distance from the back of the mask M, a heater HD arranged with a two-dimensional array of radiant heaters HR, which can be controlled via a heating control HC individually and independently from each other for the delivery of heat radiation HR. Corresponding components bear the same reference identifications as in 2 ,

In addition to the heating device HD, however, a cooling device CD is provided here, which allows the mask to be cooled over a large area from its rear side. For this purpose, a stream of cooling gas CG is introduced into the space between the heater HD and the mask M. The cooling device CD has for this purpose a nozzle device ND, which is connected to a cooling gas pump CP and a plurality of nozzle openings N, via which the cooling gas, such as cooled air, can be introduced into the space between the heater and mask.

By virtue of the active cooling of the mask, thermal deformations can be produced which act in opposition to those thermal deformations which can be achieved by the active heating. This results in additional degrees of freedom with respect to the surface deformations that can be set on the mask. The cooling effect, so to speak, a restoring force for the deformations generated by heating.

Active cooling in addition to heating also creates a relatively fast mask deformation manipulator, which makes it possible to set several new temperature profiles in relatively short time intervals. A "fast" mask deformation manipulation is given here in particular if the targeted mask deformation state can be changed at least once during the exposure of a wafer or even during the exposure of a single target area using the mask deformation manipulator.

5 shows a schematic oblique perspective view of an embodiment in which an electrothermal heater HD is provided in combination with a flat cooling device CD. At the front side MS1 of the mask substrate, the pattern is arranged, which converts the incident EUV illumination radiation ILR into structured projection radiation PR. On the opposite rear side of the mask substrate, that is to say on the second mask surface MS2, a two-dimensional, grid-shaped arrangement of crossing resistance heating elements in the form of conductor tracks L1, L2 made of a metallic material having a relatively high electrical resistance is applied. Individual traces or groups of traces may be energized independently of other traces or groups of traces so that the locally generated amount of thermal energy may be controlled via the heater controller HC to produce a two-dimensional heating profile at the back of the mask substrate. As a result, the first mask surface MS1 carrying the pattern can be specifically thermally deformed.

The planar cooling device CD is, unlike in the schematic representation, relatively close, that is, mounted at a slight distance from the rear side of the mask or in point or area contact contact with the mask and is controlled by means of an electric cooling control CC. The cooling device CD may, for example, contain a multiplicity of Peltier elements with which the cooling device can be lowered to temperatures below the temperature of the heated mask M. Optionally, a spatially resolving cooling is possible, that is, a cooling in which the cooling capacity and thus the local temperature over the surface of the cooling device is selectively varied. By using the cooling device is a Bidirectional thermal manipulation of mask deformation possible.

Based on the embodiments, in the 6 and 7 are schematically illustrated, devices and methods are now described which are particularly suitable to be used in conjunction with transmission masks, but may also be used in reflective masks.

A transmission mask, which can also be referred to as a transmitting reticle, generally has a mask substrate MS which consists of a material which has a high degree of transmission or absorption for the ultraviolet radiation used in the projection exposure apparatus for projection exposure, such that the mask of FIG the radiation is radiated loss. Deep Ultraviolet (DUV) or Vacuum Ultraviolet (VUV) radiation often uses synthetic quartz glass mask substrates.

The mask M is spatially positioned between the illumination system ILL and the projection objective PO in the area of the object surface OS of the projection lens such that the illumination radiation ILR emanating from the illumination system ILL first strikes the back side of the mask facing the illumination system, penetrates the latter with little loss, by means of the illumination Front of the mask formed pattern is influenced and finally enters the projection lens PO as projection radiation PR.

In this arrangement, special precautions must be taken so that elements of the thermal mask deformation manipulation devices do not protrude into the beam path and interfere with the projection exposure. Therefore, it is provided in the embodiment that all elements of the heater are mounted outside of the optically used volume between the illumination system and the projection lens. 6 shows an embodiment in which the heater HD connected to a heating control HC has a rectangular support frame SUP, which is arranged in the axial region between the outlet of the illumination system ILL and the mask M. The annular support encloses a rectangular passage opening, the dimensions of which are each greater perpendicular to the optical axis than the corresponding transverse dimensions of the optically used region on the mask. On the underside of the support SUP, which faces the mask, numerous heat radiation sources HS in the form of infrared lamps or infrared diodes are arranged, which are oriented in such a way that they can emit a heat radiation HR directed at the back of the mask. Since the carrier frame and the heat radiation sources mounted thereon lie in the transverse direction outside the beam path, the heating radiation strikes the mask back substantially obliquely to the optical axis with the center of gravity directed inwards, with each heating beam heating an associated heating area HR1, HR2. The location-dependent control of the incoming heating power leads in the manner already described above to a thermally induced mask deformation.

In order that an arbitrary temperature profile can also be generated on the mask during a scanning operation, fast-switching infrared heating lamps, for example light-emitting diodes, are preferably used, possibly with a suitable ballast optics. Alternatively, a light supply by means of glass fibers is possible.

The wavelength range of the infrared heating radiation is far outside the ultraviolet wavelength range used for the projection exposure and for which the material of the mask is highly transparent. Preferably, heating radiation is used from a wavelength range for which the mask material has a very high absorption coefficient, so that even with relatively thin masks, the heat radiation striking from behind is absorbed almost completely in the mask substrate. It can, for. B. infrared radiation from the wavelength range around 10.6 microns can be used. If, however, residual parts of the heating radiation pass through the mask in the direction of the projection objective, the oblique irradiation ensures that the heating radiation can not or only to a negligible extent enter the projection objective.

In the embodiment of 7 the heating device HC connected to the heater HD in turn has a support frame SUP, the internal dimensions are transverse to the optical axis AX significantly larger than the outer dimensions of the mask M. The support frame and attached to the inside of heat radiation sources HS are substantially in the area of the object plane OS Projection lens PO attached, where the pattern PAT of the transmitting mark M is arranged.

Adjacent to each of the four mask edges is a row of several heat radiation sources HS, for example in the form of a linear laser diode array. With the heat radiation sources arranged annularly around the mask at the edge of the mask, it is possible to generate almost any desired temperature profile within the mask and, via thermal expansion of the mask material, gives a targeted two-dimensional To cause mask deformation. The heating radiation can be radiated in principle from all edge points of the mask and in almost any direction perpendicular to the optical axis in the mask substrate or slightly obliquely with respect to this vertical direction. The absorption of the mask material for the heating radiation leads to locally different heating and thus to locally different thermal expansions. As a result, the mask surface bearing the pattern is locally deformed to different degrees, whereby a precise wavefront correction is possible.

There is an unambiguous, mathematically describable relationship between the spatial and temporal distribution of the irradiated heating energy, the local distribution of the heating power achievable thereby and the resulting mask deformation within the mask. Similar to image reconstruction using the radon method, as in the case of tomographic imaging examination methods, the relationship between the desired mask deformation and the requisite control of the discrete heat radiation sources arranged around the mask edge can be described by an inverse application of the radon method. The functional principle can therefore also be referred to as "inverse tomography" or as "reticle deformation" by tomographic heating from the mask edge ". For the mathematical basics of image reconstruction with the radon method, reference is made to the corresponding specialist literature.

The non-contact lateral coupling of the infrared heating radiation is possible in all cases in which the mask or the reticle has an edge overflow, since only the optically used area of the mask (indicated by dashed lines) must be covered by means of heating. The radiant heat sources are stationary with respect to the edge of the mask, which can be achieved, for example, by a mechanical connection between the support SUP carrying the radiant heat sources and the mask M (see schematic connecting elements V). The field of radiant heat sources may therefore move synchronously with the mask during a scan.

Preferably, the infrared laser diodes are adjusted so that they get no laser radiation from the opposite laser diodes. In order to prevent radiated heating radiation emerging uncontrollably from the mask, for example, mirrors may be provided which reflect the exiting heating radiation back into the mask. Alternatively or additionally, suitably arranged absorbers, for example in the form of corrugated sheets or the like, may be provided in order to intercept stray light as radiation traps.

The basis of the 6 and 7 explained embodiments of heaters are particularly suitable for the use of transmission masks. It should also be noted that in principle they can also be used for spatially resolving heating of reflective masks. This is especially true for those related to 7 explained reticle deformation by tomographic heating from the mask edge forth for all cases in which the reflective reticle is a mask substrate made of a heat radiation sufficiently permeable and at the same time sufficiently absorbent material.

Numerous variants and alternatives to the embodiments described in detail and partially illustrated are possible. For example, as an alternative to the embodiment of FIG 2 possible to use a mask without a separate absorber layer. In this case, the material of the mask substrate is directly heated. To still allow enough entry of heat energy by heating radiation, here are z. B. substrate materials with integrated absorbers advantageous. For example, the use of an OH-rich glass may be advantageous, for example with an OH content of more than 40 ppm. In particular, the OH content may be 100 ppm or more or even 300 ppm or more. When selecting a suitable concentration range of absorbers, for example OH groups, the thickness of the suitable substrate can also be taken into account. An optimal OH content may also change depending on the requirement profile. In general, a relatively high OH content will ensure that a local heat input already takes place at a greater distance from the pattern, which may result in a certain spatial blurring of the thermal profile. If work is carried out with relatively low OH contents, only relatively little heat energy resulting from absorption can be used to set the temperature profile, and it may be necessary to drive expenditures in disposing of residual heat. But it is possible a higher spatial resolution.

When using OH-rich glass material for the mask substrate, a heating radiation having wavelengths in the range of absorption peaks of the OH groups in the glass material should be selected. Especially when using glasses containing OH, wavelengths in the range around 1450 nm and / or 2200 nm and / or 2700 nm can be advantageous. If, on the other hand, one wishes to work with an absorber layer, then it is generally advantageous to select heating radiation from a wavelength range which, although strongly absorbed in the material of the absorber layer, shows only slight absorption in the other substrate material. As a result, it can be achieved that the heat generation can be concentrated substantially on the region of the absorber layer, whereby desired Temperature profiles can be set very precisely.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2010/020337 A1 [0008]
• US 7675607 [0010]
• US 7187432 B2 [0011]
• JP 70-50245 A [0012]
• JP 10-097967A [0013]
• US 2008/0252987 A1 [0014]
• US 2003/133087 A1 [0014]
• US 2009/0257032 A1 [0036, 0036]
• WO 2009/100856 Al [0069]
• WO 2010/049020 A1 [0069]
• WO 2005/026801 A2 [0075]
• US 7372623 B2 [0084]

Claims (21)

Projection exposure method for exposing a radiation-sensitive substrate to at least one image of a pattern of a mask, comprising the following steps: Holding the mask between an illumination system and a projection objective of a projection exposure apparatus such that the pattern is arranged in the region of the object plane of the projection objective; Holding the substrate such that a radiation-sensitive surface of the substrate in the region of an object plane to the optically conjugate image plane of the projection lens is arranged; Illuminating an illumination region of the mask with illumination radiation provided by the illumination system; Imaging a part of the pattern lying in the illumination area onto the substrate with the aid of the projection objective; marked by Heating the mask according to a predetermined two-dimensional heating profile by means of a heating device separate from the illumination system. A projection exposure method according to claim 1, wherein the masking induces mask deformation such that a higher-wave field pattern of one or more aberrations of the projection lens is at least partially corrected. A projection exposure method according to claim 1 or 2, wherein the mask is contactlessly heated by irradiation of electromagnetic radiation energy. A projection exposure method according to any one of the preceding claims, wherein the mask is irradiated by a heater having a radiator array with a plurality of independently controllable radiant heat sources. A projection exposure method according to any one of the preceding claims, wherein a plurality of laterally offset heating areas of the mask are irradiated with heating radiation, wherein for each heating area, the irradiation time and / or the irradiation intensity is controlled independently of other heating areas. A projection exposure method according to any one of the preceding claims, wherein the mask is heated by the backside of the mask facing away from the pattern. A projection exposure method according to any one of the preceding claims, wherein the mask is heated by a patterned face of the mask. Projection exposure method according to one of the preceding claims, wherein the mask is heated by lateral irradiation of heating radiation, wherein at least a portion of the heating radiation penetrates into the mask substrate substantially parallel to a mask plane via lateral boundary surfaces, wherein preferably from at least two sides at an angle sides heating radiation is irradiated into the mask substrate. A projection exposure method according to any one of the preceding claims, wherein the mask is heated by means of independently controllable electrothermal elements which are in thermally conductive contact, in particular in touching contact, with the mask. A projection exposure method according to any one of the preceding claims wherein, contrary to the effect of the heating, thermal energy is actively withdrawn from the heated mask by cooling, preferably providing a heat sink with thermal contact with the mask, wherein a mean temperature of the heat sink is less than a mean mask temperature , Projection exposure method according to claim 10, wherein bidirectional manipulation of the mask is effected by use of the cooling, wherein preferably the mask is stabilized by active cooling with respect to a temperature zero point system. Projection exposure method according to one of the preceding claims, characterized by the following steps: Measuring a deformation of the surface of the mask to generate surface deformation data; Control of the heater as a function of the surface deformation data. A projection exposure apparatus for exposing a radiation-sensitive substrate to at least one image of a pattern of a mask, comprising: an illumination system (ILL) for receiving primary radiation of a primary radiation source and for generating illumination radiation (ILR) directed to the mask (M); a projection objective (PO) for generating an image of the pattern (PAT) in the region of an image plane (IS) of the projection objective; a mask holder (RST) for holding the mask between the illumination system and the projection lens such that the pattern is located in the region of an object plane (OS) of the projection lens; a substrate holding device (WST) for holding the substrate such that a radiation-sensitive surface of the substrate in the region of the object plane optically conjugate image plane (IS) of the projection lens is arranged; characterized by a separate from the illumination system heater (HD) for heating the mask according to a predetermined two-dimensional heating profile. Projection exposure apparatus according to claim 13, wherein the heating device (HD) comprises a radiator array with a plurality of independently controllable radiant heat sources (HS), which are preferably arranged such that a plurality of laterally offset heating regions of the mask can be irradiated with heating radiation, wherein for each Heating range, the irradiation time and / or the irradiation intensity is controlled independently of other heating areas. A projection exposure apparatus according to claim 14, wherein the emitter array is disposed opposite to a backside of the mask (MS) facing away from the pattern (PAT) such that the mask (M) is heatable from the backside of the mask. A projection exposure apparatus according to any one of claims 13 to 15, wherein the mask (M) comprises an absorber layer (ABS) consisting of an absorber material having a high absorption coefficient in a wavelength region of the heating radiation, wherein preferably the absorption coefficient of the absorber material is larger by at least an order of magnitude as an absorption coefficient of the material from which the pattern-carrying part of the mask is made. A projection exposure apparatus according to any one of claims 13 to 16, wherein the heating means (HD) comprises a plurality of independently controllable radiant heat sources (HS) arranged laterally outside the beam path leading from the illumination system (ILL) to the projection lens (PO) and to the Irradiate mask, wherein the heat radiation sources (HS) are preferably arranged in the form of a ring around the beam path around. Projection exposure apparatus according to claim 17, wherein an annular arrangement of radiant heat sources (HS) is provided, which are arranged substantially in the object plane (OS) of the projection lens and at least two at an angle, in particular perpendicular, mutually facing sides heating radiation in the mask substrate (MS ) can radiate. A projection exposure apparatus according to any one of claims 13 to 18, wherein the heating means comprises a plurality of independently controllable electrothermal elements in heat-conductive contact with the mask, in particular the back of the mask, preferably electrothermal elements in the form of resistive heating material tracks. Projection exposure apparatus according to one of Claims 13 to 19, characterized by a cooling device (CD) for active cooling of the mask against the effect of heating by the heating device (HD), wherein the cooling device preferably has a heat sink with thermal contact with the mask, wherein an average temperature the heat sink is smaller than an average temperature of the mask. Projection exposure apparatus according to one of claims 13 to 19, characterized by: a measuring device for measuring a deformation of the surface of the mask for generating surface deformation data; and a controller for controlling the heater in response to the surface deformation data.

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