Patent Application
20080100816
The present invention relates to a lithographic apparatus and method.
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist).
Instead of a mask, the patterning device may comprise a patterning array that comprises an array of individually controllable elements. An advantage of such a system compared to a mask-based system is that the pattern can be changed more quickly and for less cost.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic apparatus typically comprises an illuminator to provide a conditioned illumination beam of radiation. In some circumstances it may be desirable to change the angular intensity distribution of a propagating illumination beam, in order to control the spatial intensity distribution in the cross section of the illumination beam. In order to change the angular intensity distribution of the illumination beam it is known to provide one or more diffractive optical elements within the illuminator. The diffractive optical element causes different parts of the illumination beam to be diffracted at different angles, and thus changes the shape of what is known as the pupil plane of the illumination beam. Alternatively, it is known to provide an array of individually controllable elements, such as a programmable mirror array, arranged to selectively redirect portions of the illumination beam to control the angular intensity distribution of the illumination beam. Since an array of individually controllable elements are used, the angular distribution of the illumination beam can be readily changed from one angular distribution to another. However, an illuminator which uses an array of individually controllable elements to control the angular intensity distribution of the illumination beam may have a larger footprint than an illuminator that does not use an array of individually controllable elements. Space in and around a lithographic apparatus may be valuable, and an illuminator with a larger footprint reduces the amount of available space.
According to an aspect of the invention, there is provided an illuminator for a lithographic apparatus, the illuminator comprising:
an array of individually controllable reflective elements capable of changing the angular intensity distribution of an incident illumination beam of radiation,
wherein the array of individually controllable reflective elements is provided on a curved support structure, or the array of individually controllable reflective elements is arranged to serve as a curved reflective surface.
According to an aspect of the invention, there is provided a method of conditioning an illumination beam of radiation using an illuminator, the method comprising:
illuminating an array of individually controllable reflective elements with the illumination beam of radiation, the array of individually controllable reflective elements being capable of changing the angular intensity distribution of the illumination beam of radiation; and
controlling the position or orientation of the reflective elements by providing the array of individually controllable reflective elements with an input signal to cause the array to serve as a curved reflective surface.
According to an aspect of the invention, there is provided a method of correcting for imperfections in an optical apparatus used in an illuminator, the illuminator comprising an array of individually controllable reflective elements capable of changing the angular intensity distribution of an incident illumination beam of radiation, the array of individually controllable reflective elements being provided on a curved support structure, or the array of individually controllable reflective elements being arranged to serve as a curved reflective surface, the method comprising:
illuminating the array of individually controllable reflective elements with the illumination beam of radiation; and
controlling the position or orientation of the reflective elements to correct for imperfections in the optical apparatus used in the illuminator.
According to an aspect of the invention, there is provided a lithographic apparatus, comprising:
an illuminator configured to condition a beam of radiation, the illuminator comprising an array of individually controllable reflective elements capable of changing the angular intensity distribution of an incident illumination beam of radiation, the array of individually controllable reflective elements being provided on a curved support structure, or the array of individually controllable reflective elements being arranged to serve as a curved reflective surface;
a support structure configured to hold a patterning device, the patterning device configured to impart the beam with a pattern in its cross-section;
a substrate table configured to hold a substrate; and
a projection system configured to project the patterned beam onto a target portion of the substrate.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:
a and 3b depict an array of individually controllable reflective elements as employed in an embodiment of the invention;
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the illumination apparatus and lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
A patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
The support structure holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure may use mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device”.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more support structures). In such “multiple stage” machines the additional tables (and/or support structures) may be used in parallel, or preparatory steps may be carried out on one or more tables (and/or support structures) while one or more other tables (and/or support structures) are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of a projection system.
an illumination system (illuminator) IL configured to condition a beam PB of radiation (e.g. UV radiation);
a support structure (e.g. a mask table) MT configured to hold a patterning device (e.g. a mask) MA and connected to a first positioning device PM to accurately position the patterning device with respect to item PL;
a substrate table (e.g. a wafer table) WT configured to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioning device PW to accurately position the substrate with respect to item PL; and
a projection system (e.g. a refractive projection lens) PL configured to image a pattern imparted to the beam PB by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.
As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above).
The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be integral part of the apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.
The illuminator IL may comprise an adjusting device AM configured to adjust the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL generally comprises various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam of radiation PB having a desired uniformity and intensity distribution in its cross-section.
In accordance with an embodiment, the illuminator IL further comprises a programmable mirror array 1 arranged to modulate the beam PB, as will be described in more detail below.
The beam PB is incident on the patterning device MA, which is held on the support structure MT. Having traversed the patterning device MA, the beam PB passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and position sensor IF (e.g. an interferometric device), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the beam PB. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in
The depicted apparatus can be used in one or more of the following modes:
1. In step mode, the support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.
2. In scan mode, the support structure MT and the substrate table WT are scanned synchronously while a pattern imparted to the beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure MT is determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.
3. In another mode, the support structure MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array of a type as referred to above.
Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.
In place of a mask table MT and a mask MA, there may be provided a patterning device PD (e.g. an array of individually controllable elements) that modulates the beam PB. Generally, the pattern created on the target portion of the substrate will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or a flat panel display (e.g., a color filter layer in a flat panel display or a thin film transistor layer in a flat panel display). Examples of such a patterning device include, e.g., programmable mirror arrays, laser diode arrays, light emitting diode arrays, grating light valves, and/or LCD arrays. A patterning device whose pattern is programmable with the aid of electronic means (e.g., a computer), such as a patterning device comprising a plurality of programmable elements that can each modulate the intensity of a portion of the radiation beam, (e.g., all the devices mentioned in the previous sentence), including an electronically programmable patterning device having a plurality of programmable elements that impart a pattern to the radiation beam by modulating the phase of a portion of the radiation beam relative to adjacent portions of the radiation beam, is referred to herein as a “contrast device”. In an embodiment, such a patterning device comprises at least 10 programmable elements, e.g. at least 100, at least 1000, at least 10000, at least 100000, at least 1000000, or at least 10000000 programmable elements Embodiments of several of these devices are discussed in some more detail below:
A programmable mirror array. This may comprise a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident radiation as diffracted radiation, whereas unaddressed areas reflect incident radiation as undiffracted radiation. Using an appropriate spatial filter, the undiffracted radiation can be filtered out of the reflected beam, leaving only the diffracted radiation to reach the substrate; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. It will be appreciated that, as an alternative, the filter may filter out the diffracted radiation, leaving the undiffracted radiation to reach the substrate. An array of diffractive optical MEMS devices may also be used in a corresponding manner. A diffractive optical MEMS device is comprised of a plurality of reflective ribbons that may be deformed relative to one another to form a grating that reflects incident radiation as diffracted radiation. A further alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which may be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuator. Once again, the mirrors are matrix-addressable, such that addressed mirrors reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam may be patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing may be performed using suitable electronic means. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891, U.S. Pat. No. 5,523,193, U.S. Pat. No. 7,088,468, and PCT patent application WO 98/33096.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872.
The lithographic apparatus may comprise one or more patterning devices, e.g. one or more contrast devices. For example, it may have a plurality of arrays of individually controllable elements, each controlled independently of each other. In such an arrangement, some or all of the arrays of individually controllable elements may have one or more common illumination systems (or parts of illumination systems), a common support structure and/or a common projection system (or part of the projection system).
As noted above, the illuminator IL may comprise a programmable mirror array 1, or any other suitable array of individually controllable reflective elements arranged to modulate or change the angular intensity distribution of the illumination beam. The programmable mirror array 1 selectively reflects portions of the illumination beam in different directions in order to change the angular intensity distribution of the illumination beam. That is, the programmable mirror array 1 is arranged to modulate the spatial intensity distribution at the pupil plane of the illumination beam IB.
The programmable mirror array 1 within the illuminator is similar to a programmable mirror array used as a patterning device to impart the pattern to the beam to be projected onto a target portion of the substrate, as described above for the lithographic apparatus of
Being able to modulate the illumination beam that is incident upon the array of individually controllable elements may be desirable for one or more embodiments of a lithographic apparatus in which it is desirable to be able to rapidly switch between different cross sections of an illumination beam. Additionally or alternatively, such a controllable array may be useful in that it is relatively cheap and flexible in providing any desired illumination setting. For instance, for a particular lithographic apparatus, it may be necessary to switch between different lithographic patterning devices in order to project different patterns onto a target area of the substrate. Each patterning device may itself require an illumination beam with a different mode (i.e. angular intensity distribution). As noted above, a lithographic apparatus may provide varying modes (i.e. varying angular intensity distributions) for the illumination beam by providing a diffractive optical element in the illuminator that can be changed between exposures of the substrate. However, it can be time consuming to change the illuminator mask, for instance when the patterning device is switched. Therefore the ability to rapidly and controllably change the cross section of the illumination beam by controlling an array of individually controllable elements may be advantageous.
After the illumination beam IB has been expanded, its angular intensity distribution is then controlled by the programmable mirror array 100 by selectively reflecting parts of the illumination beam IB in different directions. This is achieved by tilting individual mirrors within the programmable mirror array 100. The programmable mirror array 100 thus changes the spatial intensity distribution at the pupil plane of the illumination beam IB. The illumination beam IB is reflected towards another plurality of lenses 103 which are used to reduce the size of the beam to a desired extent. The illumination beam IB is then reflected off a second mirror 104 which may be used to direct the illumination beam IB to other lenses 105 or other equipment which may be used to further condition the illumination beam IB. In this Figure, the mirror array 100 is larger than the pupil plane, but it will be understood that this is not essential.
In order to make the most efficient use of the programmable mirror array 100, the illumination beam IB is expanded, which requires the use of a plurality of lenses 102. Once the angular intensity distribution of the illumination beam IB has been controlled, lenses 103 are required to reduce the beam width (e.g., diameter) to the required extent. The lenses 102, 103 required to expand and then reduce the beam width take up a lot of space. The implication of this is that an illuminator using a flat programmable mirror array 100 to modulate the illumination beam IB has a larger footprint (i.e. is bigger) than an illuminator that does not use a flat programmable mirror array to modulate the illumination beam IB. An illuminator using a flat array of individually controlled elements may be up to 500 millimetres greater in size than an illuminator that does not use a flat array of individually controlled elements. Of course, the same is true of an illuminator provided with any suitable array of individually controllable reflective elements. Since space within and around a typical lithographic apparatus is valuable, an increase in size in the illuminator may have a corresponding increase in terms of cost. It is therefore desirable to keep the illuminator as small as possible.
The programmable mirror array 100 of
a and 3b illustrate side views of programmable mirror arrays according to an embodiment of the invention.
Referring now to
The use of a Fresnel mirror generally reduces the quality of a reflected image in comparison to a continuous lens which the Fresnel lens has been constructed to behave like. The quality of the image is reduced due to the irregular nature of the surface of the Fresnel mirror. Boundaries between parts of the surface of the Fresnel mirror are not able to reflect radiation in a desired direction, thus reducing the quality of the reflected image. When a mirror array is used to condition the illumination beam, the array will have areas between the mirrors which cannot reflect radiation. This means that when the array is used as Fresnel mirror (and therefore has areas which cannot reflect radiation in a desired direction), the reduction in quality of a reflected image is less pronounced, since the continuous (or flat) array already had areas which could not reflect radiation.
It can be seen from
a shows the curved mirror array 200 as being curved in one-dimension (e.g. it is as an elongate U-shape, or the like). It will be appreciated that the curved mirror array may be curved in two-dimensions (e.g. such that the array is bowl shaped, or the like), in which case the mirrors 201 will be angled towards the center of the array. Curvature of the curved mirror array 200 in one-dimension only may be sufficient if the illumination beam IB incident upon the curved mirror array 200 is non-symmetric, and/or if divergence of the illumination beam IB only needs to be compensated for in one-dimension. In this case, the mirror elements 201 of the mirror array 200 may be directed towards another center of the array 200, specifically the center (or mid-point) of the arc which defines the curvature of the array 200. This center will extend as an imaginary line along and across the array 200 (e.g. along the bottom of the elongate U-shaped array). One dimensional curvature of the mirror array 200 may desirable if the illumination beam IB incident upon it is rectangular in cross section. Similarly, if the mirror array 300 of
In contrast to the flat mirror array 100 of
From a comparison of
As described previously, space within and around a lithographic apparatus is valuable, and so a reduction in the size of the illuminator by using a curved mirror array may reduce cost as well as saving space. Additionally, lenses used in lithography are often expensive due to the strict requirements often associated with lithography. For example, lenses often need to be extremely smooth, have a very low birefringence and have a very low thermal expansion coefficient. If these expensive lenses are not required due to the incorporation of a curved mirror array, the cost of the lithographic apparatus, or the illuminator of the lithographic apparatus, may be further reduced. Furthermore, lenses may be heavy, so the less lenses that are required, the lighter the illuminator and/or lithographic apparatus is. This may reduce transport costs, etc. It is also well known that the intensity of a radiation beam reduces each time it passes through a lens. By using a curved mirror array (or a mirror array configured as a Fresnel mirror), less lenses may be required, and so the reduction in intensity of the radiation beam may be less than in a prior art illuminator.
As described above, a programmable mirror array is not essential. For example, any suitable array of individually controllable reflective elements may be used. The elements of the array may be provided on a curved supporting surface. Alternatively, elements of the array may be arranged so that the array of reflective elements serves as a curved reflective surface, such as a Fresnel mirror. The curved array or array arranged to serve as a curved reflective surface may be, for example, spherical or aspherical. The curvature of the array may be concave or convex, and the curvature may depend on whether the beam incident on the array is diverging or converging.
It may be easier and less expensive to manufacture an array of individually controllable reflective elements which are provided on a flat supporting structure, and which are angled toward the center of the array to behave as a Fresnel mirror. Elements within the array may be constructed so that they, by default, lie at an angle to the supporting structure (i.e. the elements may be provided on the array at the correct angles). Alternatively or additionally, elements within the array may, by default, lie parallel to the supporting structure, and the Fresnel mirror effect may be introduced by manipulating the angles at which different elements within the array lie to the supporting structure (for example by establishing electrostatic fields between the elements and the supporting structure using signals provided by an array control apparatus).
The array of individually controllable reflective elements may be used to correct for imperfections in optical apparatus used in the illuminator. For example, the position or orientation of elements of the array may be controlled to correct for imperfections in one or more of the lenses used in the illuminator. Correcting for the imperfections of a lens may result in that lens not being required, further reducing the cost, size and/or complexity of the illuminator. For example, the position or orientation of elements of the array may be controlled such that the array serves as an aspherical reflective surface, which may be useful for optimizing the optical properties of the illuminator.
Referring back to
It will be appreciated by one skilled in the art that the above embodiments have been described by way of example only. It will be appreciated that various modifications may be made to these and indeed other embodiments without departing from the scope of the invention, as defined by the claims that follow.
