JP5860494B2 - Illumination system for illuminating a mask in a microlithographic projection exposure apparatus - Google Patents

Description

  The present invention relates generally to an illumination system for illuminating a mask in a microlithographic projection exposure apparatus. More specifically, the present invention relates to a system that includes an array of reflective elements that can be implemented as a microelectromechanical system (MEMS).

  Microlithography (also called photolithography or simply lithography) is a technique for processing integrated circuits, liquid crystal displays, and other microstructured devices. More specifically, microlithographic processing is used in conjunction with etching processing to pattern features in a thin film stack formed on a substrate, eg, a silicon wafer. In each layer of processing, the wafer is first coated with a photoresist, which is a radiation sensitive material such as deep ultraviolet (DUV) light. Next, the wafer having the photoresist thereon is exposed to projection light in a projection exposure apparatus. This apparatus projects a mask containing a pattern onto the photoresist such that the photoresist is exposed only at certain locations determined by the mask pattern. After the exposure, the photoresist is developed, and an image corresponding to the mask pattern is generated. An etching process then transfers this pattern into a thin film stack on the wafer. Finally, the photoresist is removed. Repeating this process with different masks results in a multilayer microstructured component.

In general, a projection exposure apparatus includes an illumination system for illuminating a mask, a mask stage for aligning the mask, a projection objective, and a wafer alignment table for aligning the photoresist-coated wafer. The illumination system illuminates a field of view on the mask, which can have the shape of an elongated rectangular slit, for example.
In current projection exposure apparatus, a distinction can be made between two different types of apparatus. In one type, each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one shot, and such an apparatus is commonly referred to as a wafer stepper. In the other type of device, commonly referred to as a sequential scanning device or a scanner, each target portion progressively scans the mask pattern under a projection beam in a predetermined reference direction while simultaneously parallel or parallel to this direction. Irradiation is performed by scanning the substrate table synchronously in non-parallel. The ratio of the speed of the wafer to the speed of the mask is usually less than 1 and equal to the magnification of the projection objective, for example 1: 4.

  It should be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning means. In general, the mask used includes a transmissive or reflective pattern and can be, for example, of binary, alternating phase shift, attenuated phase shift, or various hybrid mask types. However, there are also active masks, for example masks implemented as programmable mirror arrays. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. More information about such mirror arrays can be gleaned from, for example, US 5,296,891, US 5,523,193, US 6,285,488B1, US 6,515,257B1, and WO 2005 / 096098A2. A programmable LCD array can be used as an active mask as described in US 5,229,872. For the sake of simplicity, the remainder of this specification may relate specifically to an apparatus including a mask and a mask stage, but the general principle described in such an apparatus is the patterning means described above. Should be viewed in a broad sense.

  As technology for manufacturing microstructured devices advances, there is a constantly increasing demand for lighting systems. Ideally, the illumination system illuminates each point of the illumination field on the mask with projection light having an appropriately defined irradiance and angular distribution. The term angular distribution describes how the total light energy of a light beam that converges towards a particular point in the mask plane is distributed among the various directions in which the light rays that make up the light beam propagate. The angular distribution in the mask plane is often simply referred to as the illumination setting.

  Usually, the angular distribution of the projection light incident on the mask is adapted to the type of pattern projected on the photoresist. For example, a relatively large size feature may require a different angular distribution than a small size feature. The most commonly used angular distributions of projection light are called conventional illumination settings, annular illumination settings, dipole illumination settings, and quadrupole illumination settings. These terms refer to the irradiance distribution in the pupil plane of the illumination system. In the annular illumination setting, for example, only the annular region in the pupil plane is illuminated. Accordingly, there is only a small angle range in the angular distribution of the projection light, i.e. all light rays are incident on the mask at a similar angle with an inclination.

  Different means are known in the art for modifying the angular distribution of the projection light in the mask plane to obtain the desired illumination setting. In the simplest case, an aperture stop (diaphragm) containing one or more apertures is positioned in the pupil plane of the illumination system. Since the position in the pupil plane is converted into an angle in the field plane having a Fourier relationship such as the mask plane, the size, shape, and position of the aperture in the pupil plane determine the angular distribution in the mask plane. . However, any change in lighting settings requires a change of aperture. This makes it difficult to finally adjust the lighting settings because a large number of apertures with slightly different sizes, shapes, or positions of apertures are required. Apart from this, such an aperture absorbs a large amount of light. This absorption reduces the processing function of the entire projection exposure apparatus.

  Thus, many common illumination systems include adjustable elements that allow at least a certain degree to continuously change the illumination of the pupil plane. Conventionally, a zoom axicon system that includes a zoom objective and a pair of axicon elements is used for the purposes described above. Axicon elements are refractive lenses that have a conical surface on one side and are usually flat on the opposite side. By providing a pair of such elements, one having a convex conical surface and the other having a complementary concave conical surface, the light energy can be shifted radially. This shift is a function of the distance between the axicon elements. The zoom objective makes it possible to change the size of the illumination area in the pupil plane.

  However, with such a zoom axicon system, only conventional and annular illumination settings can be generated. For other illumination settings, such as dipole or quadrupole illumination settings, an additional stop or optical raster element is required. The optical raster element produces an angular distribution corresponding to a certain illumination area in the far field at each point on its surface. In many cases, such optical raster elements are realized as diffractive optical elements, in particular computer generated holograms (CGH). By positioning such an element in front of the pupil plane, optionally with an additional condenser lens between them, almost any arbitrary intensity distribution in the pupil plane can be generated. Additional zoom axicon systems can be used to alter the illumination distribution generated in the pupil plane by the optical raster element to some limited extent.

  However, in a zoom axicon system, the possibilities for adjusting the lighting settings are limited. For example, only one of the four poles in a quadrupole illumination setting cannot be displaced along any direction. For this purpose, another optical raster element specially designed for this particular intensity distribution in the pupil plane must be used. The design, manufacture, and transport of such optical raster elements is a time consuming and expensive process and therefore lacks the flexibility to adapt the light intensity distribution in the pupil plane to the requirements of the operator of the projection exposure apparatus.

In order to increase the flexibility of generating different angular distributions in the mask plane, it has been proposed to use a mirror array that illuminates the pupil plane.
In EP 1,262,836 Al, such a mirror array is realized as a microelectromechanical system (MEMS) comprising more than 1000 fine mirrors. Each of the mirrors can be tilted in two different planes perpendicular to each other. That is, radiation incident on such a mirror device can be reflected in (substantially) any desired direction of the hemisphere. A condenser lens arranged between the mirror array and the pupil plane converts the reflection angle generated by the mirror into a position in the pupil plane. An optical coupler such as a quartz rod is used to make the beam uniform. This known illumination system allows the pupil plane to be illuminated with a plurality of circular spots, each associated with one specific fine mirror, which can be moved freely across the pupil plane by tilting these mirrors To.
Similar lighting systems are known from other patent documents such as US 2006/0087634 A1 and US 7,061,582 B2.

  WO 2005/026843 A2 discloses an illumination system in which diffractive optical elements are arranged in the beam path between the mirror array and the pupil plane of the illumination system. The diffractive optical element generates an additional angular distribution superimposed on the angular distribution generated by the mirrors of the mirror array. In this case, the intensity distribution in the pupil plane can be described as a convolution of the intensity distribution generated by the mirror array and the intensity distribution generated by the diffractive optical element. An optical coupler such as a fly-eye lens or a quartz rod is disposed between the mirror array and the mask. The optocoupler ensures uniform illumination of the mask and at least approximately forms the illumination field geometry.

US5, 296, 891 US5, 523, 193 US6, 285, 488B1 US6, 515, 257B1 WO2005 / 096098A2 US 5,229,872 EP1,262,836Al US2006 / 0087634A1 US7, 061, 582B2 WO2005 / 026843A2 PCT / EP2007 / 001267

  An object of the present invention is to provide an illumination system for illuminating a mask in a microlithographic projection exposure apparatus. The illumination system shall allow the operator of the projection exposure apparatus to set a wide range of different illumination settings, but the system complexity is low.

  According to the invention, this object is achieved by an illumination system comprising an objective system having an object plane, at least one pupil plane and an image plane in which a mask can be placed. The illumination system further includes a beam deflection array of reflective or transmitted beam deflection elements, each beam deflection element adapted to deflect incident light by a deflection angle that can be changed in response to a control signal. According to the invention, the beam deflection element is arranged at or close to the object plane of the objective system.

  In the illumination system according to the invention, the beam deflection array illuminates the mask “directly”, ie without an intermediate optical coupler positioned at or in the immediate vicinity of the pupil plane. Therefore, the angular distribution of the light incident on a specific point on the mask mainly depends on the deflection angle at which the incident light beam is deflected by the respective beam deflection element. Thus, at a given time during the scanning operation, each point on the mask is illuminated only by light having a very limited angular distribution. However, during scanning operations, each point on the mask can be illuminated by a plurality of different beam deflection elements, thereby producing a variety of different angular distributions. Thus, the angular distribution obtained after completion of the scanning operation is obtained by integrating the angular distributions generated by the individual beam deflection elements. That is, the illumination system according to the invention can even generate a field-dependent angular distribution, i.e. different points on the mask can be illuminated with different angular distributions.

The illumination system according to the invention can have a very simple overall configuration since it only requires an objective that conjugates the beam deflection array to the mask plane as an indispensable component.
A field-forming raster element can be arranged between the light source of the illumination system and the beam deflection array. The field-forming raster element at least partly determines the shape of the field illuminated on the beam deflection array and thus also on the mask as a result of optical conjugation between the beam deflection array plane and the mask plane. Generate a dimensional far-field intensity distribution. A collector may be placed between the field-forming raster element and the beam deflection array to improve system performance and reduce the overall length of the illumination system.

  The angular distribution of light incident on the mask is determined by the deflection angle associated with each beam deflection element. To further enhance the variability in manipulating the angular distribution, additional pupil forming raster elements can be provided that are arranged in a field plane that is optically conjugate with the image plane of the objective. This field plane can be the objective plane of the objective, the intermediate image plane of the objective, or the field plane in front of the objective. In the case of a field plane in front of the objective, an additional optical system is required that conjugates this field plane with the objective plane of the objective.

The pupil forming raster element can include an array of microlenses or diffractive structures. In the case of an array of diffractive structures, it is even possible to form the pupil forming raster element directly on the surface of the beam deflection element.
When a pupil forming raster element is used, the two-dimensional far field intensity distribution in the pupil plane of the objective system is the far field intensity distribution generated by the beam deflection array and the far field intensity generated by the pupil forming raster element. Convolution with the distribution.

Each beam deflection element can be adapted to be in either an “on” state or an “off” state, where the “on” state is determined such that the deflected light beam passes through the pupil plane. . The “off” state is determined such that the deflected light does not pass through the pupil plane. With such a configuration of the beam deflection element, the illumination dose, ie the total light energy received by a particular point on the mask after the completion of the scanning operation, is used to illuminate this particular point on the mask. One or more of the contributing beam deflection elements can be adjusted by simply switching on and off.
A field stop can be provided to achieve a sharp edge of the illumination field of the mask along at least one direction. The field stop can be located in the object plane or any other plane conjugate thereto, for example in or close to the intermediate image plane of the objective.

  Since the beam deflection elements are inevitably separated by gaps, no light will be incident on the mask where the images of these gaps are formed. Therefore, measures must be taken to ensure uniform illumination of the mask. One measure is to stagger the beam deflection elements so that at least one beam deflection element is illuminated on any arbitrary line extending between the ends of the beam deflection array parallel to the scanning direction. . This ensures that there are no points on the mask that do not receive any light during the scanning operation. Ideally, the gaps are evenly distributed perpendicular to the scan direction so that all points on the mask “see” the same number of gaps during the scan operation.

  Another measure is to use a field stop having a stop edge extending at least substantially along the X direction perpendicular to the scanning direction. This edge has depressions, each depression corresponding to a gap between adjacent beam deflection elements. These depressions increase the amount of light incident on the mask at the conjugate point and can thus compensate for the effect of the gap. Such a field stop can be constituted by a stop edge formed by a plurality of blades, and the shape and / or position of at least some of the blades can be adjusted using a manipulator.

Yet another measure to improve illumination uniformity is to deflect the beam when A _min is the shortest distance from the object plane (50) that can completely illuminate the usable field in the object plane with the deflecting element. The element is placed at a distance A where | A |> _Amin . On the other hand, the beam deflection element should not be placed too far away from the object plane. Preferably, | A | <A _max , and the distance A _max is the shortest distance from the object plane where two light beams emitted from both edges of the deflection element intersect.
The beam deflection element may be a transmissive element that deflects light rays passing through the element. Such a transmissive element can be realized as an electro-optic or acousto-optic element. However, preferably the deflection element is a mirror that can be tilted with respect to the object plane.

In order to reduce the distortion caused by tilting illumination by the beam deflection array, the object plane and the image can be tilted with respect to each other. In this case, the optical axis of the objective system must be tilted with respect to both the normal on the object plane and the normal on the image plane. Qualitatively, this is an essential part of what is usually referred to as a Scheimpflug condition.
Various features and advantages of the present invention can be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which:

1 is a schematic perspective view of a projection exposure apparatus according to the present invention. FIG. 2 is a meridional section through an illumination system housed in the projection exposure apparatus shown in FIG. 1. It is a perspective view of the mirror array accommodated in the illumination system of FIG. It is sectional drawing through the mirror array of FIG. It is a top view on the pupil plane accommodated in the objective system of the illumination system shown in FIG. FIG. 6 is a schematic view of a mirror array (or an image on its mask) in a bipolar illumination setting. FIG. 7 is similar to FIG. 6 but for an annular illumination setting. FIG. 6 is a meridional section through an illumination system according to another embodiment in which the objective plane of the objective and the image plane are not parallel. FIG. 6 is a meridional section through the objective of the illumination system showing imaging on a mask of the gap formed between adjacent mirror elements. FIG. 6 is a schematic top view of an adjustable field stop. FIG. 6 is a cross-sectional view of two mirror elements arranged off the object plane of the objective system. FIG. 12 is a view similar to FIG. 11 but with a larger distance between the object plane of the objective and the mirror element. FIG. 6 is a meridional section through an illumination system according to yet another embodiment of the invention with a field stop located in the intermediate image plane. FIG. 6 is a meridional section through an illumination system according to yet another embodiment provided with additional pupil shaping elements. FIG. 3 is an intensity distribution obtained in the pupil plane of the objective as a result of convolution of two far-field intensity distributions.

1. General Structure of Projection Exposure Apparatus FIG. 1 is a schematic perspective view of a projection exposure apparatus 10 used for manufacturing integrated circuits and other fine structure components. The projection exposure apparatus includes an illumination system 12 that houses a light source for generating projection light and an illumination optical instrument that converts the projection light into a projection beam having carefully defined characteristics. The projected light beam illuminates the field of view 14 on the mask 16 that houses the microstructures 18. In this embodiment, the illumination field 14 has a generally ring segment shape. However, other shapes of the rectangular illumination field 14 are also contemplated.

  The projection objective 20 images the structure 18 in the illumination field 14 onto a photosensitive layer 22, for example a photoresist, applied on a substrate 24. A substrate 24, which can be formed by a silicon wafer, is placed on a wafer stage (not shown) so that the upper surface of the photosensitive layer 22 is accurately located in the image plane of the projection objective 20. The mask 16 is positioned in the object plane of the projection objective 20 using a mask stage (not shown). Since the projection objective 20 has a magnification smaller than 1, for example 1: 4, a reduced image 14 ′ of the structure 18 in the illumination field 14 is formed on the photosensitive layer 22.

2. Illumination System FIG. 2 is a more detailed meridional section through the first embodiment of the illumination system 12 shown in FIG. For clarity, the diagram of FIG. 2 is highly simplified and not to scale. This means in particular that different optical units are represented by very few optical elements only. In reality, these units can include a significant number of lenses and other optical elements.
The illumination system 12 includes a housing 28 and a light source implemented as an excimer laser 30 in the illustrated embodiment. The excimer laser 30 emits projection light having a wavelength of about 193 nm. Other types of light sources and other wavelengths such as 248 nm or 157 nm are also contemplated.

  In the illustrated embodiment, the projection light emitted by the excimer laser 30 is incident on a beam expansion unit 32 where the light beam is expanded without changing its geometrical optical flux. The beam expansion unit 32 can include several lenses, as shown in FIG. 2, or can be realized as a mirror array. The projection light is incident on the field-forming optical raster element 34 after passing through the beam expanding unit 32.

  The field-forming optical raster element 34 can be configured as an optical coupler that generates a plurality of secondary light sources. In general, such optical couplers include one or more microlens arrays that produce a well-defined angular distribution when illuminated by a substantially parallel light beam. If the optical coupler includes at least two orthogonal arrays of cylindrical microlenses, different angular distributions can be generated along the direction in which the microlenses extend. For further details, reference is made to the international patent application PCT / EP2007 / 001267 assigned to the present applicant.

Alternatively, the field-forming optical raster element 34 can be configured as a diffractive optical element (DOE), eg, a computer generated hologram (CGH). Diffractive optical elements have the advantage that almost any arbitrary angular distribution, and thus almost any desired far-field intensity distribution, can be generated.
In general, the field-forming optical raster element 34 can be implemented by any optical element that increases the geometric optical flux. This means more specifically that any optical element that increases the divergence of any light beam incident on any surface area of this element is suitable.

  The angular distribution generated by the field-forming optical raster element 34 is converted into an intensity distribution that varies locally in the far field. In the far field, the distance over which the intensity distribution generated by the optical raster element is observed is large compared to the general width of the structures contained within this element. In the case of a diffractive optical element, the far-field intensity distribution depends on a Fourier transform of a complex function that accounts for the effect on the wavefront of the diffractive structure contained within the diffractive optical element.

  A condenser lens 36 is arranged behind the field-forming optical element 34 mainly to reduce the vertical dimension of the illumination system 12. For simplicity, FIG. 2 shows the condenser lens 36 as a single lens. In an actual system, a lens system that includes a plurality of individual lenses rather than a single lens can be used. The condensing lens 36 is disposed such that its front focal plane corresponds to the plane on which the field-forming optical element 34 is disposed.

  The condenser lens 36 converts the divergent light beam exiting from the field-forming optical element 34 into a substantially parallel beam 38 having a spatial intensity distribution determined by the field-forming optical element 34. When the divergence introduced by the field-forming optical element 34 is different in the orthogonal directions X and Y perpendicular to the optical axis OA, the cross section of the parallel beam 38 emitted from the condenser lens 36 has a rectangular slit shape. Can have. The aspect ratio of the slit is determined by the divergence generated by the field-forming optical element 34 along the X and Y directions.

The collimated beam 38 is incident on a mirror array 40 that includes a plurality of rectangular mirror elements M _ij in the illustrated embodiment. Figure 3 is a perspective view of a mirror array 40, the mirror elements M _ij have shown how to how form a rectangular reflective surface is shielded only by narrow gaps between adjacent mirror elements M _ij. Each individual mirror element M _ij can be tilted independently of each other by two tilt axes, preferably aligned perpendicular to each other. In Figure 3 illustrates by dashed lines 42, 44 the tilting axis with respect to a single mirror element M _35. By tilting the individual mirror elements M _ij , the incident light can be guided in any arbitrary direction of the direction range limited by the tilt range of the mirror elements M _ij .

FIG. 4, which is a schematic side view of a portion of the mirror element M _ij , shows the parallel beam 38 into a plurality of individual partial beams that can be guided in various directions by tilting the mirror element M _ij. It shows how it is subdivided by the mirror element M _ij .
In order to tilt each individual mirror element M _ij about two orthogonal tilt axes 42, 44, two actuators (not shown) are connected to each individual mirror element M _ij . The actuator is connected to a control unit 46 that generates appropriate control signals towards the actuator. Accordingly, the control unit 46 determines the tilt angle of the mirror elements M _ij, whereby the mirror elements M _ij is also determined angle for deflecting the incident light. The control unit 46 is connected to an overall system controller 48 that coordinates various operations of the projection exposure apparatus 10.

The reflecting surface of the mirror array 40, or more precisely the mirror element _Mij , is arranged on or in the immediate vicinity of the objective surface 50 of the objective system 52, which in the schematic diagram of FIG. 2 includes two lenses 52a and 52b. As will be described below with reference to FIG. 12, an objective system 52 that can have one or more intermediate image planes has an image plane 54 that is optically conjugate to the object plane 50. . Image plane 54 corresponds to the mask plane on which mask 16 is positioned during the exposure process. The objective system 52 has at least one pupil plane 60 having a Fourier relationship with adjacent field planes, in this case the optical plane 50 and the image plane 54.

Since the mirror array 40 is arranged at or close to the object plane 50 of the objective system 52, an image of the mirror element M _ij is formed on the mask 16 in the image plane 54. Therefore, there is a one-to-one relationship between each point on the mirror element M _ij and a conjugate point in the image plane 54. In FIG. 2, this conjugate property is emitted from two points 62 a and 64 a on the mirror element M _ij , respectively. It is illustrated by

Since the mirror elements in which the object points 62a and 64a are located have different inclination angles, the light beams 62 and 64 intersect the pupil plane 60 at different positions. Accordingly, the image points 62b and 64b on the mask 16 are illuminated with different angular distributions. In the configuration shown in FIG. 2, the light beams 62 and 64 illuminate the image points 62b and 64b with inclination from the opposite direction.
At least as long as these object points are illuminated in exactly the same manner by the collimated beam 38, the same idea applies correspondingly to all object points on a particular mirror element M _ij . Thus, all light reflected from a particular mirror element M _ij intersects the pupil plane 62 at the same spot. This also means that the angular distribution at all image points of a particular mirror is substantially the same at a given time.

Nevertheless, slight deviations in the angular distribution may be observed if all mirror elements M _ij have equal tilt angles. This deviation is due to the parallel beam 38 tilting and illuminating the mirror array 40, so that object points are not illuminated in exactly the same manner. In order to avoid such deviations, the field-forming optical element 34 can be modified so that exactly the same illumination conditions are established on all mirror elements M _ij .

In the configuration shown in FIG. 2, the mirror element M so that all light beams reflected from the mirror element M _ij pass through the same circular area in the pupil plane 60, as shown in the schematic diagram of FIG. Assume _ij is tilted. These two areas, referred to below as poles P ₁ and P ₂ , are arranged mirror-symmetrically with respect to the XZ plane along the Y direction. Such illumination of the pupil plane characterizes an effective bipolar illumination setting, especially for imaging structures aligned along the X direction. The diameters of the poles P ₁ and P ₂ are determined by the residual divergence of the beam 38 incident on the mirror array 40. This residual divergence can be generated by the light source 30 and the field-forming optical element 34.

In order to illuminate only the poles P ₁ and P ₂ in the pupil plane 60, each or at least part of the mirror elements M _ij should be individually controlled by the control unit 46. The control unit 46 reflects each mirror element M _{ij so} that the part of the parallel beam 38 incident on the mirror element M _ij passes through the poles P ₁ and P ₂ in the pupil plane 60. Guarantee. Usually this will require that all mirror elements M _ij have different tilt angles. The control unit 46 can include a look-up table in which the tilt angles for all mirror elements M _ij are stored for a particular lighting setting. Typically, the selection of illumination settings is performed manually in the system controller 48 taking into account the particular configuration of structures contained within the projected mask 16.

However, it should be noted that at a given point in time, each particular image point on the mask 16 is illuminated only by a light beam emanating from a conjugate object point on one of the mirror elements M _ij of the mirror array 40. Should. In the case of a dipole illumination setting, this means that at a given point during the scanning operation of the projection exposure apparatus 10, each point on the mask 16 is illuminated by light passing through the pole P ₁ or the pole P _2. It is either illuminated by light passing through.

As a result of the scanning operation, the mask 16 is moved through the illumination field along the scanning direction Y. Therefore, exposure of a specific point on the mask 16 can be obtained by integrating the contribution in the illumination field from all the object points through which the specific point on the mask 16 passes. For example, if a point on the mask is illuminated from _one side (Pole P ₁ ) through the first half of the scan operation and from the other side (Pole P ₂ ) through the other half of the scan operation, Bipolar illumination is obtained in which specific points on the mask are illuminated symmetrically from both sides after completion of the scanning operation.

This is illustrated in FIG. 6, which shows in the upper half a part of the rows R ₁ , R of the mirror array 40 in which the mirror elements M _ij are tilted so that the reflected light passes through the pole P _{1. 2} ,. . . Is shown. The pupil plane 60 is schematically shown on the left of FIG. In the lower part of FIG. 6, it is assumed that the mirror elements M _ij in the other rows R _i are tilted so that all reflected light passes through the pole P ₂ .
Since the mirror array 40 is imaged on the mask 16 by the objective system 52, the grating shown in FIG. 6 can also be considered to represent the image of the mirror array 40 in the image plane 54. Thus, at a particular point during the scan operation, all points on the mask 16 positioned in the upper half of the grid are illuminated from _one side (pole P ₁ ) and positioned in the remaining half of the grid. All the points are illuminated from the opposite side (pole P ₂ ). If a point on the mask 16 moves through the illumination field along the scanning direction Y represented by the arrow 70 in FIG. 6, this point is initially illuminated only by light that has passed through the pole P ₁ , Next, it is illuminated only by the light passing through the pole P _2. After completion of the scanning operation, certain points on the mask 16 are illuminated from both sides (poles P ₁ and P ₂ ), as shown below the left “=” symbol.

From this description, it is possible for the illumination system 12 to change not only the total intensity of light that every particular point on the mask 16 will receive after completion of the scanning operation, but also the total angular distribution almost continuously. It becomes clear that. For example, if it is desirable to illuminate the mask more strongly from _one side (pole P ₁ ) than the other side (pole P ₂ ), one of the rows R _i of mirror elements M _ij extending along the X direction. One or more can easily be switched to the off state where no reflected light passes through the pupil plane 60 at all. Alternatively, the tilt angle of these mirror elements can be changed so that the reflected light does not pass through pole P ₁ but passes through pole P ₂ .

It is even possible to obtain different intensities and different angular distributions at different positions on the mask 16. In this case, a row C _j of mirror elements M _ij extending along the Y direction gives a different total intensity and / or a different angular distribution at every point on the mask illuminated by this row C _j of mirror elements M _ij. As controlled by the control unit 46.
For example, individual mirror elements M _ij in this row can be turned off, or other areas in the pupil plane 60 are illuminated by one or more of the mirror elements M _ij in this row. be able to. Further, consider changing the tilt angles of the mirror elements contained in a particular row C _j continuously or abruptly so that the points on the mask 16 illuminated by this particular row are subjected to different angular distributions. Even is possible.

From the above, it should be clear that almost any arbitrary angular distribution of the projection light can be obtained on the mask 16. FIG. 7 shows in a similar representation to FIG. 6 the configuration of the mirror element M _ij such that an annular illumination setting is achieved. More specifically, each row R _i of the mirror array 40 reflects incident light to pass through a specific area in the pupil plane 60 indicated by P _i on the left hand side. The areas P _i that can be partially overlapped are combined to form _a substantially annular pattern Pa, as shown below the “=” symbol in the lower left corner of FIG.

If a point on the mask 16 is illuminated continuously by a mirror element in a particular row C _j , then this point will be incident on this point from a different solid angle, but at an angle during the scanning operation. It will be exposed to projection light. After the scanning operation is completed, the above point, will have been exposed to the projection light from all directions associated with the annular region P _a which is shown in the lower left portion of FIG.
Again, the angular distribution can be corrected by tilting the individual mirrors M _ij of the mirror array 40. Apart from this point, an annular illumination setting is achieved at a point on the mask that is illuminated by one or more columns C _{j in} the manner described above and is illuminated by mirror elements M _ij of different columns C _j ′. Different illumination settings can be achieved at other points on the mask, for example, dipole or quadrupole illumination settings.

  The possibility of illuminating different points on the mask 16 with different angular distributions can be used to improve the imaging of different structures on the mask 16. Another application is the compensation of field-dependent disturbances in the angular distribution generated by other components of the illumination system 12. In the latter case, the goal is to achieve the same angular distribution for all points on the mask 16, but certain optical components may have an adverse effect on this uniformity. is there.

3. Alternative Embodiments It should be appreciated that various alternative embodiments are still contemplated that still fall within the scope of the invention.

3.1. Scheimpflag Array FIG. 8 shows an alternative embodiment in a more simplified representation similar to FIG. In FIG. 8, the components corresponding to those shown in FIG. 2 are represented by the same reference numbers plus 100, and most of these components will not be described in detail again. To.
In the embodiment illustrated in FIG. 8, the object plane 150 and the image plane 154 of the objective system 152 are not parallel, but are inclined with respect to each other. In order to form a sharp image of the tilted object plane 150 on the image plane 154, the objective system 152 has an optical axis OA that is both normal to the object plane 150 and normal to the image plane 154. Should be arranged to form a certain angle.

Such a configuration usually follows what is referred to as a Scheimpflug condition. This condition requires that the object plane and the object main plane of the objective system indicated by H _o in FIG. 8 intersect at least approximately along a straight line. The same should be applied to the image side principal plane H _i and the image plane 154. When the Scheimpflug condition is satisfied, the tilted objective surface 150 is clearly imaged on the image plane 154. The image of the mirror array 140 will be distorted in the image plane 154, but this distortion has no adverse effect on either the intensity or angular distribution of the light incident on the mask 16.
Satisfaction of the Scheimpflug condition has the advantage that the angle between the object plane 50 on which the mirror array 40 is arranged and the optical axis of the condenser lens 36 is reduced. Thereby, the distortion of the illumination of the mirror array 40 by the parallel beam 38 is reduced.

3.2. Gap between mirror elements When the mirror element M _ij is accurately placed in the object plane 50 of the objective system 52, the inevitable gap between the mirror elements M _ij is clearly imaged on the mask 16. become. This is illustrated in the approximate meridional section of FIG. 9, which shows _two light fluxes B ₂ and B ₃ that are reflected from mirror elements M ₂ and M ₃ and incident on mask 16, respectively. As clearly seen in FIG. 9, the gap between adjacent mirror elements M ₂ , M ₃ is imaged at 72 on the mask 16. The width of the gap image depends on the magnification of the objective system 52.
If such gaps 72 extend perpendicular to the scanning direction Y, these gaps can be scarcely due to the integration effect obtained by the scanning operation. However, a gap 72 (referred to herein as a Y gap) that extends parallel to the scan direction Y may result in non-uniform illumination of the mask 16, which is ultimately undesirable on the wafer 24. It will be converted into a structure size.

One solution to this problem is to ensure that the Y gap is not aligned along the scanning direction Y and is distributed somewhat evenly along the X direction over the illumination field.
If the Y gap is not evenly distributed along the X direction, there will be multiple stripes that receive only low light energy without other compensation measures extending along the Y direction. However, when appropriate compensation measures are applied, a uniform energy distribution during the scanning operation can be achieved despite the above. Such compensation measures can include, for example, intensity reduction in the remaining area between these stripes. For this purpose, the individual mirror elements M _ij can be turned off.

  Another way to reduce the intensity in the remaining area between the stripes is to place a special field stop at the object plane 50, image plane 54, or intermediate image plane. FIG. 10 shows in a schematic top view a suitable field stop device 74 that includes a plurality of adjacent blades 76 that can be moved along the Y-direction using an actuator 78. The short edge 80 of the blade 76 determines the size of the illumination field along the Y direction. These short edges 80 are combined to form the two long sides of the rectangular slit. In the position where the short edge 80 is retracted, these long sides have depressions. These depressions having a large distance between the opposing blades 76 correspond to positions where the Y gap image exists. It guarantees the desired compensation.

Another approach to avoiding dark stripes extending along the scanning direction Y is to place the mirror element M _ij slightly off the object plane 50. This approach takes advantage of the fact that the light beam reflected from the mirror element M _ij has at least a small divergence. For example, when the mirror element M _ij is arranged behind the object plane 50, the reflected light appears to be emitted from the completely bright object plane 50. If the mirror element M _ij is arranged in front of the object plane 50, the diverging light beams are superimposed in the object plane 50, so that the object plane 50 is completely illuminated.
The case where the mirror element M _ij is arranged in front of the object plane 50 is illustrated in FIG. 11, which shows two mirror elements M ₁ and M ₂ in cross section. For simplicity, the reflective surfaces 821 and 822 are assumed to be arranged in a plane 83 that is separate from the object plane 50 of the objective system 52 by (at least approximately) the distance A _min. The width of the gap between _two adjacent mirror elements M ₁ and M ₂ is represented by D.

In FIG. 10, the reflected divergent light beam is illustrated by a cone 84. As shown by the broken line in the mirror element M ₂ , when the mirror elements M ₁ and M ₂ are tilted, the reflected light beam 84 changes its direction, but due to the limited maximum tilt angle, It still remains within a cone or solid angle 85 with an opening angle α. Thus, light rays emanating from point 86 on mirror surface 822 can pass along any line within cone 85.

In the center of FIG. 10, two cones 851, 852 are shown associated with two points 861, 862 on the adjacent edges of mirror elements M ₁ and M ₂ respectively. At the shortest distance A _min from the plane 83, the rays emanating from the points 861, 862 can intersect so that there is no unilluminated area of the object plane 50 near the gap. Preferably, to ensure that the object plane 50 is completely illuminated by the divergent beam 84 at all tilt angles, the distance A between the object plane 50 and the plane 83 of the mirror surfaces 821, 822 is A Must exceed _min .

For a small cone angle α, the shortest distance A _min can be determined from the gap width D and the angle α by the following equation.
A _min > D / α

On the other hand, the distance between the plane 83 of the reflecting surfaces 821 and 822 and the object surface 50 should not be so large. If the mirror array 40 is positioned too far away from the object plane 50, the direction of the reflected beam is not correctly converted to the desired angular distribution on the mask 16.
The effective upper limit of the distance A can be such that the light beams emitted from the single mirror element M _ij do not intersect within the object plane 50. This is illustrated in FIG. 12, which shows two mirror elements M ₁ ′ and M ₂ ′ of width L. The opening angle of the light beam emitted from the points 821a ′ and 821b ′ on both edges of the mirror element M ₁ ′ is represented by δ. At a distance A _max , the luminous flux 88
4a ′ and 884b ′ intersect. The distance A _max is determined by the following equation from the width L and the opening angle δ.
A _max <L / δ

3.3 Field Stop The embodiment illustrated in FIGS. 1-7 assumes that the illumination field 14 on the mask 16 has a generally ring segment shape. Such a geometry can be obtained, for example, by a field-forming optical element 34 configured as a suitable diffractive optical element. However, it is difficult to obtain a sharp edge within the far field. In general, in the illumination field 14, at least the edge extending along the scanning direction Y must be clear.

  Field stop elements 94, 96 covering portions of the mirror array 40 that should not be imaged on the mask 16 in order to obtain a sharp edge at least along the scanning direction Y (see FIGS. 2 and 3). Can be used. If the edge extending perpendicular to the scanning direction Y should also be imaged sharply on the mask 16, an additional field stop element can be provided. In FIG. 3, such a further field stop element is indicated by a broken line and indicated by 98. Also, the more complex field stop device 74 shown in FIG. 10 can be positioned in the object plane 50 of the objective 52. This is particularly useful when the mirror array 40 is positioned at a distance A from the object plane 50 as described above with reference to FIGS.

FIG. 13 shows an alternative embodiment of a lighting system according to the invention in a meridional section similar to FIG. Components corresponding to those shown in FIG. 2 are represented by the same reference numbers plus 300, and most of these components will not be described again.
In the illumination system 312, the objective system 352 has an intermediate image plane 391 on which a field stop 374 is arranged. The field stop 374 can be of an adjustable type, as described above with reference to FIG. The additional intermediate image plane 391 provides greater freedom to install larger field stop devices within the illumination system 312.

3.4 Transmission Deflection Element FIG. 14 shows another alternative embodiment of the illumination system according to the invention in a meridional section similar to FIG. Components corresponding to those shown in FIG. 2 are represented by the same reference numbers plus 400, and most of these components will not be described again.
The illumination system 412 differs from the illumination system 12 shown in FIG. 2 mainly in two respects.

First, the mirror array 40 is replaced by a refractive array 440 that includes a plurality of transmissive refractive elements T _ij . These transmission and refraction elements T _ij can be configured, for example, as electro-optic or acousto-optic elements. For such elements, the refractive index can be altered by exposing the appropriate material to ultrasound or an electric field, respectively. These effects can be used to create a refractive index grating that deflects incident light in various directions. The direction can be changed in response to an appropriate control signal.

3.4 Additional pupil-forming elements Another difference is that the field-forming optical element 434 does not directly illuminate the refractive array 440 and is in a plane 450 ′ that is optically conjugate to the object plane 450 by the objective system 499. Illuminate through positioned pupil forming optical element 493. A pupil forming optical element 493, which can be configured as an optical raster element such as a microlens array or a diffractive optical element, produces an angular distribution that is imaged onto the refractive array 440 by the objective system 499. As a result, the direction of the rays of light outgoing from the refractive elements T _ij is not only the deflection angle produced by the individual refractive elements T _ij, also depends on the angular distribution produced by the pupil forming element 493. The pupil forming element can include sections associated with individual refractive elements T _ij , which generate different far-field intensity distributions. This can be used, for example, to provide a common or different fixed offset angle for a given transmission element T _ij . Such an offset angle can be particularly useful when mirror elements are used instead of refractive elements T _ij .

The intensity distribution in the pupil plane 460 of the objective 452 can then be described as a convolution of the far field intensity distribution generated by the pupil forming element 493 and the far field intensity distribution generated by the refractive array 440. .
This is illustrated in FIG. 15, which shows an exemplary far-field intensity distribution D _PDE generated by the pupil forming element 493 above it. In this case, it is assumed that the far-field intensity distribution has a regular hexagonal shape. The middle portion of FIG. 15 shows an exemplary far field intensity distribution D _RA generated by the refractive array 440. The far field intensity distribution D _RA is created by a plurality of individual far field intensity distributions D _ij , each generated by a single refractive element T _ij . These individual far-field intensity distributions D _ij are small spots.

The convolution of these two far-field intensity distributions D _PDE and D _RA represented by the symbol CONV in FIG. 15 is a far-field intensity distribution D _{tot in} which a plurality of hexagonal distributions D _PDE are made according to the point distribution D _RA Produce. In this exemplary case, the above results in two opposite poles P ₁ ′, P ₂ ′ illuminated in the pupil plane 460. These hexagons can be made so that no gaps remain between them, so that the poles P ₁ ′, P ₂ ′ are illuminated almost continuously. By changing the deflection angle generated by the refractive element T _ij , the hexagonal far-field intensity distribution D _PDE can be changed to other geometric shapes, for example differently arranged at different positions or with different total intensities. Can be made to the pole.

  Also, of course, additional pupil forming elements can be used in connection with the mirror array. Apart from this, the pupil forming element can be placed in a separate field plane. For example, in the embodiment shown in FIG. 13, the intermediate field plane 391 would be ideally suited for this purpose. Apart from this, a transparent reflective or refractive element with a diffractive structure that produces the same effect as the pupil-forming optical raster element 494 can be provided.

  The above description of the preferred embodiment has been given by way of example. From the disclosure provided, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the disclosed structures and methods. Accordingly, applicants are urged to include all such changes and modifications as fall within the spirit and scope of the invention as defined by the appended claims and equivalents thereof.

16 Mask 40 Beam deflection array 50 Object plane 52 Objective system 54 Image plane 60 Pupil plane M _ij Beam deflection element

Claims (29)

a) an object plane (50; 150; 250; 350; 450), at least one pupil plane (60; 160; 360; 460) and an image plane (54; 154;) on which a mask (16) can be placed. 354; 454) and an objective system (52; 152; 252; 352; 452), and b) each beam deflection element (Mij; Tij) deflects the incident beam by a deflection angle that is variable in response to a control signal. A beam deflection array (40; 240; 340; 440) of reflective or transmitted beam deflection elements (Mij; Tij) adapted to
An illumination system for illuminating a mask (16) in a scanning microlithographic projection exposure apparatus (10) comprising:
A beam deflection element (Mij; Tij) is at or near the object plane (50; 150; 250; 350; 450) of the objective system (52; 152; 252; 352; 452) at a conjugate position of the image plane. is disposed, an image of the beam polarization element, it to is formed on the image plane of the objective system,
A system characterized by that.   A light source (30; 330; 430) and a field-forming raster element (34; 134; 334;) disposed between the light source (30; 330; 430) and the beam deflection array (40; 240; 340; 440). 434). The illumination system of claim 1, wherein:   The field-forming raster element (34; 334; 434) generates a two-dimensional far-field intensity distribution that at least partially determines the shape of the field (14) illuminated on the mask (16). The lighting system according to claim 2.   4. The illumination system of claim 3, wherein the far-field intensity distribution generated by the field-forming raster element has a rectangular or curved slit geometry.   The illumination system according to claim 4, wherein the slit has a length that is at least twice as long as a width of the slit.   6. Illumination system according to any one of claims 2 to 5, wherein the field-forming raster element (34; 134; 334; 434) is a diffractive optical element.   6. Illumination system according to any one of claims 2 to 5, wherein the field-forming raster element (34; 134; 334; 434) comprises at least one array of microlenses. Featuring a concentrator (36; 136; 336; 436) disposed between the field-forming raster element (34; 134; 334; 434) and the beam deflection array (40; 140; 340; 440). The illumination system according to any one of claims 2 to 7.   9. The pupil forming raster element (493) arranged in a field plane optically conjugate with the image plane (454) of the objective (452). Lighting system. A two-dimensional far-field intensity distribution (Dtot) on the pupil plane (460) of the objective system (452) is obtained;
The distribution (Dtot) is a convolution of the far field intensity distribution (DRA) generated by the beam deflection array (440) and the far field intensity distribution (DPDE) generated by the pupil forming raster element (493). is there,
The illumination system according to claim 9.   The pupil forming raster element (493) includes n = 2, 3, 4,. . . 11. A lighting system according to claim 9 or 10, characterized in that it generates a far-field intensity distribution (DPDE) having at least a substantially polygonal shape with n corners.   Each beam deflecting element (Mij; Tij) is in an “on” state in which the deflected light beam is determined to pass through the pupil plane (60; 160; 360; 460), or the deflected beam is in the pupil plane (60; 160; 360; 460) in any one of the “off” states determined not to pass through the lighting system. .   13. Illumination system according to any one of the preceding claims, characterized by a field stop (94, 96, 98; 374).   14. Illumination system according to claim 13, characterized in that the field stop (94, 96, 98) is arranged on the object plane (50).   14. The illumination system according to claim 13, wherein the objective system (352) has an intermediate image plane (391) on which the field stop (374) is arranged.   The beam deflection elements are arranged in a staggered manner so that at least one beam deflection element is illuminated on any arbitrary line extending parallel to the scanning direction between the ends of the beam deflection array. The illumination system according to any one of claims 1 to 15. The field stop (74) has an aperture edge, and the aperture edge is
a) extending at least substantially along the X direction perpendicular to the scanning direction, and b) having indentations each corresponding to a gap between adjacent beam deflection elements,
The illumination system according to any one of claims 13 to 15, wherein The aperture edge is formed by a plurality of blades (80),
The shape and / or position of at least a portion of the blade (80) is adjustable with the aid of an actuator (78);
The lighting system according to claim 17 . The beam deflection elements (M1, M2) are arranged at a distance A that | A |> Amin from the object plane (50),
The distance Amin is the shortest distance from the object plane (50) at which the usable field of view at the object plane can be completely illuminated by the beam deflection element,
The illumination system according to any one of claims 1 to 18, wherein The beam deflection elements (M1 ′, M2 ′) are arranged at a distance A <Amax from the object plane (50),
The distance Amax is the shortest distance from the object plane (50) where two light beams (884a ′, 884b ′) exiting from both edges of the deflection element (M1 ′) intersect.
The lighting system according to any one of claims 1 to 19, wherein   21. Illumination according to any one of the preceding claims, wherein the beam deflecting element is a transmissive element (Tij), the transmissive element deflecting light rays passing through the transmissive element. system.   22. Illumination system according to claim 21, characterized in that the transmissive element (Tij) is an electro-optic or acousto-optic element.   21. The beam deflection element according to any one of claims 1 to 20, characterized in that the beam deflection element is a mirror (Mij) that can be tilted with respect to the object plane (50; 150; 350; 450). The lighting system described.   24. Illumination system according to claim 23, characterized in that the mirror (Mij) can be tilted by two tilt axes (42, 44) forming an angle therebetween.   The illumination system according to claim 24, wherein the angle is 90 degrees.   26. The illumination system according to any one of claims 1 to 25, wherein the object plane (150) and the image plane (154) are inclined with respect to each other.   The objective system (152) has an optical axis (OA) tilted with respect to both a normal on the object plane (50) and a normal on the image plane (54). Item 27. The illumination system according to Item 26. The object plane (150) and the object side principal plane (Ho) of the objective system (152) intersect at least substantially along a first straight line;
The image plane (154) and the image-side main plane (H1) of the objective system (152) at least substantially intersect along a second straight line;
The lighting system according to claim 27. A lighting system (12; 412; 512) according to any one of claims 1 to 28;
A projection objective (20) for imaging the mask (16) on the photosensitive layer (22);
A microlithographic projection exposure apparatus comprising:

Images