• 专利附录
  • 专利说明
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    • 专利摘要:
    A lithographic apparatus comprising an illumination system for providing a beam of radiation; a support structure for supporting a patterning device, the patterning device serving to impart the radiation beam with a pattern in its cross-section; a substrate table for holding a substrate; and a flat lens, wherein the flat lens is positioned between the support structure and the substrate table, and comprises at least four cells, wherein each cell consists of a first layer and a second layer of material, and wherein the first layer is made of a material with permittivity such that the real part of permittivity is negative and the imaginary part of permittivity is positive, and the second layer is made of a material with permittivity such that the real part of permittivity is positive and the imaginary part of permittivity is positive.
    公开号:NL2019925A
    申请号:NL2019925
    申请日:2017-11-16
    公开日:2018-06-19
    发明作者:Cornelis Maas Ruben；Van Zwol Pieter-Jan
    申请人:Asml Netherlands Bv；
    IPC主号:
    专利说明:

    Lithographic Apparatus and Method
    FIELD
    [0001] The present invention relates to a lithographic apparatus and a device manufacturing method.
    BACKGROUND
    [0002] 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). 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.
    [0003] In a lithographic apparatus, the major cost driver is the optical train, and more particularly the projection lens. As an empirical observation the lens cost scales usually roughly as (FS*NA)3, where FS is field size and NA is numerical aperture. In lithography the numerical aperture NA and the field size FS are both large. There are also further requirements, such as strict aberration, NA, field size and wavelength requirements. As a consequence, the optical path length, the number of elements, the size and cost of the lens system has increased as the resolution of lithographic apparatus has increased. An optical train can make up as much as 1/3 to 1/2 of the cost of a lithographic apparatus.
    [0004] It may be desirable to provide a lithographic apparatus which is less complex and therefore cheaper.
    SUMMARY
    [0005] According to an aspect of the invention, there is provided a lithographic apparatus comprising: an illumination system for providing a beam of radiation: a support structure for supporting a patterning device, the patterning device serving to impart the radiation beam with a pattern in its cross-section; a substrate table for holding a substrate; and a flat lens, wherein the flat lens is positioned between the support structure and the substrate table, and comprises at least four cells. wherein each cell consists of a first layer and a second layer of material, and wherein the first layer is made of a material with permittivity such that the real part of permittivity is negative and the imaginary part of permittivity is positive and the second layer is made of a material with permittivity such that the real part of permittivity is positive and the imaginary part of permittivity is positive.
    [0006] The optical train of such a lithographic apparatus is less complex. The optical path is shorter and the number of optical elements is lower. The projection lens may be not provided. Therefore, the apparatus is more cost effective. While the complexity and the number of elements of the optical train are reduced, the apparatus still provides a good working distance and a good resolution.
    [0007] In some embodiments, one or more of the following features may be used.
    [0008] The first and second layer of each cell may be positioned so that the radiation incident on the flat lens first passes through the first layer and then through the second layer.
    [0009] The first and second layer of each cell may be positioned so that the radiation incident on the flat lens first passes through the second layer and then through the first layer.
    [0010] The lithographic apparatus may further comprise a patterning device, the patterning device being positioned between the support structure and the flat lens.
    [0011] The flat lens may be fixed to the patterning device.
    [0012] A dielectric layer may be present between the flat lens and the patterning device.
    [0013] The thickness of the first layer may be between 10 nm and 65 ran, and the thickness of the second layer may be between 15 nm and 50 nm.
    [0014] The real part of the permittivity of the first layer may be between 0 and -10. The real part of the permittivity of the first layer may be between -1 and -3.
    [0015] The size of the flat lens may be at least 26 mm x 33 mm. The size of the flat lens may be at least 150 mm x 150 mm.
    [0016] The overall thickness of the flat lens may be between 0.5 pm and 10 pm. The overall thickness of the flat lens may be between 0.5 pm and 5 pm. The overall thickness of the flat lens may be between 0.5 pm and 1.5 pm.
    [0017] The imaginary part of the refractive index of the second layer may be 0.1 or less.
    [0018] The refractive index of the second layer may be between 1.5 and 2.5.
    [0019] According to a further aspect of the invention, there is provided a method comprising: providing a substrate; providing a beam of radiation using an illumination system; using a patterning device to impart the radiation beam with a pattern in its cross-section; providing a flat lens, wherein the flat lens is positioned between the patterning device and the substrate and comprises at least four cells, wherein each cell consists of a first layer and a second layer of material, and wherein the first layer is made of a material with permittivity such that the real part of permittivity is negative and the imaginary part of permittivity is positive, and the second layer is made of a material with permittivity such that the real part of permittivity is positive and the imaginary part of permittivity is positive; and projecting the patterned radiation beam onto a target portion of the substrate.
    [0020] With this method, the image quality is sufficient while the costs are reduced and the through-put is increased. Some optical aben-ations are reduced or even eliminated.
    [0021] In some embodiments, one or more of the following features may be used.
    [0022] The working distance between the patterning device and the flat lens may be between 0.5 pin and 10 pm. The working distance between the patterning device and the flat lens may be between 0.5 pm and 5 pm. The working distance between the patterning device and the flat lens may be between 0.5 pm and 1.5 pm. The working distance between the patterning device and the flat lens may be equal to the thickness of the flat lens, or the working distance between the patterning device and the flat lens may be smaller than the thickness of the flat lens.
    [0023] The whole area of substrate may be exposed to the patterned radiation in one step. An area of a single field of the substrate may be exposed to the patterned radiation in one step. An tire a of a defined fraction of the shape and area of the substrate may be exposed to the patterned radiation in one step.
    [0024] According to a further aspect of the invention, there is provided a flat lens for a lithographic apparatus, wherein the flat lens is positioned between the patterning device and the substrate, and comprises at least four cells, wherein each cell consists of a first layer and a second layer of material, and wherein the first layer is made of a material with permittivity such that the real part of permittivity is negative and the imaginary part of permittivity is positive, and the second layer is made of a material with permittivity such that the real part of permittivity is positive and the imaginary part of permittivity is positive.
    [0025] The flat lens, as described above, contributes to the above-described advantages of the lithographic apparatus and method.
    [0026] Features of different aspects of the invention may be combined with each other.
    [0027] For the sake of brevity, a material with permittivity such that the real part of permittivity is negative and the imaginary part of permittivity is positive will be called a metal material here below. Similarly, a layer of such material will be called a metal layer. A material with permittivity such that the real part of permittivity is positive and the imaginary part of permittivity is positive will be called dielectric material, and a layer of such material will be called a dielectric layer.
    BRIEF DESCRIPTION OF THE DRAWINGS
    [0028] 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:
    Figure 1 depicts a lithographic apparatus according to an embodiment of the invention;
    Figures 2a and 2b are schematic depictions of tw'o alternative optical systems for a lithographic apparatus comprising a flat lens;
    Figure 3 is a schematic depiction of a flat lens;
    Figure 4a is a schematic depiction of a single cell of a flat lens;
    Figure 4b is a schematic depiction of a flat lens according to an embodiment of the present invention;
    Figure 5 shows the dependence of certain optical indicators on metal layer thickness and on dielectric layer thickness for a first example;
    Figure 6 shows the dependence of optical contrast and maximum NILS on the line/spaces (L/S) pitch for 50/50 L/S for the first example;
    Figure 7 shows the dependence of certain optical indicators on metal layer thickness and on dielectric layer thickness for a second example;
    Figure 8 shows the dependence of optical contrast and maximum NILS on the L/S pitch for 50/50 L/S for the second example;
    Figure 9 shows the dependence of certain optical indicators on metal layer thickness and on dielectric layer thickness for a third example;
    Figure 10 shows the dependence of optical contrast and maximum NILS on the line/spaces (L/S) pitch for 50/50 L/S for the third example;
    Figure 11 shows the dependence of certain optical indicators on metal layer thickness and on dielectric layer thickness for a fourth example;
    Figure 12 shows the dependence of optical contrast and maximum NILS on the L/S pitch for 50/50 L/S for the fourth example;
    Figure 13 shows a calculated aerial image for a 50/50 line/spaces patterning device for four different line/space pitches, the aerial image being calculated for the first example;
    Figure 14 show's a calculated aerial image for a 50/50 line/spaces patterning device for four different line/space pitches, the aerial image being calculated for the second example;
    Figure 15 shows a calculated aerial image for a 50/50 line/spaces patterning device for four different line/space pitches, the aerial image being calculated for the third example;
    Figure 16 shows a calculated aerial image for a 50/50 line/spaces patterning device for four different line/space pitches, the aerial image being calculated for the fourth example.
    DETAILED DESCRIPTION
    [0029] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of lCs, it should be understood that the 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, metrology (measuring specific aspects of the printed features on the substrate), 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, lor example in order to create a multi-layer 1C, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
    [0030] The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (LTV) radiation (e.g. having a wavelength of 365, 248, 193, 157, 146 or 126 nm).
    [0031] The term “patterning device” used herein should be broadly interpreted as referring to a device that can be used to impart a radiation 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 radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
    [0032] A patterning device may be transmissive. Examples of patterning device include masks 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.
    [0033] The support structure holds the patterning device. It 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 can 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”.
    [0034] Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.
    [0035] 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, collecti vely or singularly, as a 'lens”.
    [0036] 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 may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
    [0037] 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 techniques are well known in the art for increasing the numerical aperture of projection systems.
    [0038] Figure 1 schematically depicts a lithographic apparatus according to a particular embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL to condition a beam PB of radiation (e.g. UV radiation). a support structure MT to support a patterning device (e.g. a mask) MA and a flat lens FL, positioned directly under the patterning device MA; and a substrate table (e.g. a wafer table) WT for holding a substrate (e.g. a resist coated wafer) W.
    [0039] As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask).
    [0040] 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. In such case, the beam delivery system BD is used to optimize the illumination conditions of the patterning device MA. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.
    [0041] The illuminator IL may comprise adjusting means AM for adjusting the angular intensity distribution of the beam. Generally, at least the outer and/or inner radial extent (commonly referred to as □-outer and □-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, polarization and intensity distribution in its cross section.
    [0042] The radiation beam PB is incident on the patterning device (e.g. mask) MA, which is held on the support structure MT. Having traversed the patterning device MA, the beam PB passes through a flat lens FL, which projects the beam onto the substrate W. The flat lens FL has a flat, even surface, and can be positioned directly under the patterning device MA. meaning that there is no further element, and no gap, between the flat lens FL and the patterning device MA. In an embodiment, the flat lens FL is fixed to the patterning device MA. In an embodiment, the flat lens may be deposited directly onto the patterning device MA, after the patterning device MA has been planarized using e.g. spin-on-glass (SOG).
    [0043] The depicted apparatus can be used in the following mode. The support structure MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the beam PB by the patterning device MA is projected onto the substrate W in one go (i.e. a single static exposure). The substrate is then moved before the next exposure hikes place (so called stepping exposure). Variations on the above described mode of use or entirely different modes of use may also be employed.
    [0044] The flat lens FL focuses the radiation using negative refraction of energy. The effect of this focusing is analogous to a stepping lithographic apparatus (a component which is widely used in lithography). By using the flat lens in proximity to the wafer, the mask pattern is focused in a way analogous to a reduction lithographic apparatus (but without any magnification or reduction of the image).
    [0045] As mentioned above, the radiation beam PB, after leaving the radiation source SO and passing tlirough the illuminator IL, is incident on the patterning device MA. The radiation produced by the illuminator IL may be distributed substantially evenly over the whole size (area) of the patterning device MA, without substantial loss of intensity at the edges of the patterning device MA.
    [0046] An example of the patterning device can be seen in more detail in Figure 2a. In one example, the patterning device MA is a mask which comprises a glass or silica substrate 102 and a mask pattern 104, e.g. formed in a chrome layer on the substrate. The mask pattern 104 is provided on a lowermost surface of the substrate 102. The substrate 102 is provided for mechanical stability and supports the patterned chrome layer. The substrate 102 also acts as a pathway for heat (due to optical absorption) to dissipate the heat, and protects the underlying layers, in particular the mask pattern 104. In an embodiment, the mask is e.g. conventional size (6 inches x 6 inches, i.e. approx. 15.2 cm x 15.2 cm). The mask 104 may be of a type known in the art, as described above. The substrate W may be a conventional size, e.g. 300mm diameter. In an embodiment, the mask may be of approximately the same dimensions, shape and area as the substrate W onto which the mask pattern 104 is projected.
    [0047] The mask 104 may be positioned so that its lowermost layer is in direct contact with the uppermost layer of the flat lens FL. Such a position is beneficial, because any possible gap (separation) between the mask 104 and the flat lens FL reduces working distance WD (explained below).
    [0048] In an alternative arrangement, shown on Figure 2b, there may be a superstate 106, encompassing mask pattern 104 (formed e.g. as a chromium binary mask pattern) and positioned, together with the mask pattern 104, directly below the glass substrate 102. This superstate 106 may be made of transparent dielectric, e.g. spin-on-glass (SOG), and acts to planarize the interface with the flat lens FL and as protection of the mask pattern 104. The flat lens FL is positioned directly under the glass superstate 106. In an embodiment, the regions where the non-transparent substance (e.g. chromium) is removed may be filled with a transparent dielectric, for example spin-on glass (the same material as the superstate 106. The mask pattern 104 is therefore covered with a protective layer of transparent dielectric. The thickness of the transparent dielectric (superstrate 106) may be between 20-50 nm. In this way, the mask pattern is isolated from the flat lens and thus better protected, while the impact on the working distance WD is negligible. The mechanical stability of the flat lens FL is improved. Excessive heat may be transported by the superstrate 106 away from the flat lens FL. As discussed above, the superstrate 106 compensates for uneven surface of the mask pattern 104 (which is normally not completely flat). This allows the flat lens FL to be more stably positioned relative to the mask pattern 104 with the superstrate 106. The superstrate 106 may alternatively be referred to as a layer.
    [0049] After passing through the patterning device MA, the radiation passes through the flat lens FL. The flat lens FL is a flat element with a uniform thickness across its whole area. It can be positioned directly under the mask 104. The flat lens FL is an element w’hich comprises at least eight layers. At least four of the layers are made of a material with permittivity such that the real part of permittivity is negative and the imaginary part of permittivity is positive. For the sake of simplicity, such material will be called a metal material here below'. At least one of the layers is made of a material with permittivity such that the real part of permittivity is positive and the imaginary part of permittivity is positive. For the sake of simplicity, such material will be called a dielectric material The function and benefits of the flat lens FL and in particular the layered structure, where some layers are made of the metal material, i.e. the material with the real part of permittivity being negative, and some layers are made of the dielectric material, i.e. the material with the real part of permittivity positive, are described below.
    [0050] Once the radiation beam passes through the mask 104 and the flat lens FL, it passes through a separation gap 108 and is projected onto the substrate W. In an embodiment, there are no lenses or other optical elements between the flat lens FL and the substrate W. Alternatively, there may be e.g. a suitable liquid, such as water, which has a higher refractive index than air, and therefore allows higher quality images to be formed (commonly known as immersion lithography).
    [0051] In the example embodiments of Figures 2a and 2b, the substrate W comprises a photoresist layer 110 and a silicon wafer 112. These components are conventional, [0052] The size of the separation gap 108 is the distance between the lowermost surface of the flat lens FL and the uppermost surface of the substrate W (that is, the uppermost surface of a photoresist coating 110 in the example of Figures 2a and 2b). This distance depends on the thickness of the flat lens FL. In an embodiment, the size of the gap 108 is the same as the thickness of the flat lens FL. In an alternative embodiment, the size of the gap 108 is equal to the thickness of the flat lens FL minus the size of any gap between the mask 104 and the flat lens FL. Examples of the flat lens FL composition, thickness and other properties are given below. For example, in the below described example of the 1020 nm thick flat lens FL, the size of the gap 108 is 1020 nm.
    [0053] The above-described arrangement with a flat lens FL positioned beneath the mask 104 and above the substrate W is beneficial, because in certain applications it can replace the conventional lithographic lens. In other words, in certain applications (e.g. applications where a resolution of 500 nm is sufficient), the use of the flat lens FL means that no further lenses and other optical elements have to be used. Therefore, compared to conventional lithographic apparatus, the optical path is significantly shorter (reduced from metres to micrometres), and the volume of the optical elements is reduced from cubic metres to cubic centimetres. The cost of the system is therefore also significantly reduced.
    [0054] Figure 3 shows schematically the principle of functioning of a flat lens. Compared to normal dielectric materials, rays undergo negative refraction of energy upon entering the flat lens. This is demonstrated by the rays being refracted by the flat lens on the same side of the normal on entering the material. This is illustrated by one particular beam 3a, which is refracted on the same side of the normal 3b. The flat lens does not amplify evanescent waves. A flat lens is therefore not a perfect lens, and does not provide a near field resolution enhancement. A flat lens, due to its geometry, also does not create a magnified image. Therefore, the image 2 of the object 1 shown in Figure 3 is a 1:1 image. The flat lens 4 consists out of a multilayer stack of alternating metal and dielectric thin films. The negative refraction of energy observed in the flat lens 4 is a consequence of radiation-matter interactions between the incident radiation and the nanoscale structure of the multilayer of which the flat lens 4 is composed. The multi-layer structure is further discussed below.
    [0055] The multilayer structure of the flat lens 4 shown in Fig 3 is designed to achieve negative refraction of energy up to a certain maximum angle. The surrounding medium has a refractive index n = 1. In the example of Figure 3, the refraction angle is -1 times the incident angle, and the thickness L of the element 4 is chosen such that L = di + do, where di is the distance between an image 2 of an object 1 and an adjacent surface 4a of the element 4, and do is the distance between the object 1 and an adjacent surface 4b of the element 4. Therefore, the rays 3 (e.g. the beam 3a) from the object 1 positioned above the flat lens 4 and incident on the adjacent surface 4b of the element 4 are bent by the element 4 in the way shown in Figure 3, in the same half-plane created by the normal (e.g. normal 3b) to the surface 4b of the element 4. The same happens when the rays 3 leave the element 4 through the surface 4a, which is opposite to the surface 4b. The rays 3 are focused to form the image 2 of the object 1.
    [0056] The structure of the flat lens FL is as shown in Figures 4a and 4b. Figure 4a shows a single unit cell 200 (described below), which may also be referred to as a cell. In an embodiment, a plurality of cells 200 forms the flat lens FL (shown in Figure 4b). For example, the flat lens FL may be formed by four cells 200. The arrangement of four cells has a particular benefit that it does not excessively absorb radiation and yet provides an increased tliickness of the lens and focal length of the lens, increased working distance, and improved resolution compared to a conventional proximity printing or a flat lens formed of a single cell.
    [0057] In other examples, the number of cells 200 may be any number between 5 and 30. For example, there may be more than 10 cells, more than 15 cells, more than 20 cells or more than 25 cells 200 forming the flat lens FL. There may be between 11 and 21 cells. The number of cells 200 may be e.g. 11, 13, 15 or 21. The optimal number of the cells 200 depends on the material used to form the cells 200, wavelength of the radiation used, the desired thickness of metal and dielectric layers 202 and 204 (described in detail below), the desired transmission, and the desired thickness of the flat lens FL, as will be apparent from the description below. Examples of number of unit cells for different wavelengths and different configurations are shown in Fig 5b. 7b, 9b and 1 lb (described in detail below).
    [0058] The material permittivity and layer thickness of the metal and dielectric layers, and more generally of all the layers forming a flat lens, may be chosen so that for the flat lens, the refraction angle is -1 times the incident angle up to a maximum angle. Further information about the maximum angle may be found in a paper “Planar metal/dielectric single-periodic multilayer ultraviolet flat lens” (Optica 2016, open access). The maximum angle depends on the materials used and the wavelength of the radiation used. In practice, the maximum angle used may be less than 60 deg., for example approximately 55 degrees. The thickness of the layers of the metal and dielectric material of the flat lens may be chosen so that the focal distance is equal to the thickness of the flat lens. The benefits of such arrangement are described below. Alternatively, the focal distance may be less than the thickness of the flat lens, e.g. when the superstrate 106 is provided (as described above).
    [0059] Figure 4a shows a single unit cell 200. The cell 200 comprises a metal layer 202 and a dielectric layer 204. In the description below, examples of cells 200 with the metal layer 202 being positioned above the dielectric layer 204 are described. It should be understood that the reverse order is also possible, i.e. the cell 200 may have the dielectric layer 204 positioned above the metal layer 202.
    [0060] In an example embodiment, the metal layer 202 is made of a suitable metal, and the dielectric layer 204 is made of a suitable dielectric. The tliickness of the metal layer 202 may be between 10 nm and 40 nm. The thickness of the dielectric layer 204 may be also between 10 nm and 40 nm. The thicknesses of the metal layer 202 and the dielectric layer 204 depend on the wavelength of the radiation used in the lithographic system, and on the refractive indices of the respective materials. The thickness of the dielectric layer 204 may also depend on the thickness of the metal layer 202.
    [0061] The metal layer 202 may be made of metal, metal alloy, or other suitable material which has a negative real part of the permittivity. The real part of the permittivity of the material of the metal layer 202 may be below 0. The real part of the permittivity of the material of the metal layer 202 may be as low as -10. Preferably, the real part of the permittivity of the metal layer 202 is between -1 and -3. The imaginary part of the permittivity is positive, and preferably less than 0.5. Since permittivity of metal varies as a function of wavelength, different metals may be used for different wavelengths of radiation beam (e.g. to find a metal for a given wavelength which provides the above permittivity values).
    [0062] The material of the dielectric layer 204 may have a low optical absorption, but relatively high refractive index. The value of the refractive index of the material of the dielectric layer 204 may be e.g. between 1.5 and 2.5. Preferably, the value of the refractive index is around 2. The imaginary part of the refractive index of the dielectric layer 204 may be around 0.1, preferably less than 0.1. Since permittivity of dielectric varies as a function of wavelength, different dielectrics may be used for different wavelengths of radiation beam (e.g. to find a dielectric for a given wavelength which provides the above permittivity values).
    [0063] As mentioned above, the flat lens FL may comprise more than one cell 200, for example 5-15 cells 200, 10-25 or 15-30 cells 200, or any other suitable number of cells 200. This multi-layer (multi-cell) arrangement has the following advantages. The flat lens acts as an optical element that creates an image of an object (in the form of a flat lens). This flat lens replaces projection optics used in current lithography apparatus. These projection optics have very strict requirements with respect to material quality and alignment in modem lithography apparatus, and are the most expensive component of the lithography apparatus. Replacing the projection optics with the flat lens allows a much more compact and lighter design of the lithography apparatus, significantly reducing costs. The working distance of the flat lens FL is generally similar to the thickness of the flat lens. From calculations of the flat lens performance (as shown in figures 5-12) it is concluded that fabricating a flat lens with a thickness of 1 pm is feasible. This distance is much larger than the current working distance in modern lithography apparatus, and therefore making the separation between the flat lens and the substrate equal to this working distance is relatively straightforward. Typical height variations of the sample substrate over the size of one typical exposure field (26 x 33 mm) are much smaller than the working distance of 1 pm. Therefore entire fields may be exposed in a stepper-type process. More than one typical exposure field may be exposed in a stepper-type process, thereby enabling higher throughput.
    [0064] Due to the absorption characteristics, the exact geometry (type of material and layer thickness of the metal layer 202 and the dielectric layer 204) is obtained by: (1) optimizing the negative refraction of energy as a function of angle of incidence (finding the configuration in which the negative refraction of energy can be realised with good absorption and other parameters for sufficiently broad range of angle of incidence), and (2) a trade-off between ensuring a sufficient working distance (e.g. more than or equal to I pm), and at the same time maintaining enough transmission (e.g. more than 5%).
    These criteria, combined with a specific wavelength, lead to a combination of a metal and dielectric material, and their layer thicknesses, which result in an optimal metal/dielectric multilayer stack which acts as a flat lens. In a particular example, the thickness of the metal layer 202 may be less than 65 nm. Preferably, the thickness of the metal layer 202 is between 20 nra and 60 nm. However, a 65 nm thick metal layer 202 only provides a 65 nm working distance, and the negative refraction of energy will not be equal to -1 times the angle of incidence. To provide a flat lens with a greater thickness (and therefore greater working distance), and to improve the angular response of the negative refraction of energy, it has been found that a layer of dielectric material 204 may be added directly above or directly below the metal layer 202. Further information may be found in a paper “Planar metal/dielectric single-periodic multilayer ultraviolet flat lens" (Optica 2016, open access).
    [0065] The thickness of the dielectric layer 204 may be less than 55 nm. Preferably, the thickness of the dielectric layer 204 is between 15 nm and 50 nm, and may depend on the thickness of the metal layer 202. Thus, a cell 200 is formed. To further increase the thickness of the flat lens and improve the angular response of the negative refraction of energy, it has been found that instead of increasing the thickness of the metal layer 202 and/or the thickness of the dielectric layer 204, it is beneficial to provide more cells 200, positioned directly above each other. For example, a layered structure with more than one cell 200 and a thickness of approximately 1 pm provides a good combination of thickness of the flat lens, increased working distance, as well as at least 5% transmission. If losses higher than 95% are acceptable, the flat lens thickness, and therefore the working distance, could be increased to more than 1 pm (for example, 1.5 pm, 5 pm, 10 pm, or any suitable value between these two values). On the other hand, if lower losses are required, the flat lens thickness and therefore the working distance can be reduced to less than 1 pm, for example about 0.5 pm.
    [0066] Belov.', specific examples of combinations of layer thickness, refractive index and permittivity are given. The choice of material and corresponding parameters depends on wavelength.
    [0067] In a first example, the wavelength of the radiation used is 365 nm. The metal layer is silver (Ag), and the dielectric layer is tantalum pentoxide (TaeO.-s). The thicknesses of the metal layer dmouü and the dielectric layer daia are as follows: dmctui = 47.3 nm and duiei = 38.5 nm. The permittivity of the metal layer is emetai = -1-76 + 0.27*1, and the permittivity of the dielectric layer is ea«f]=5.29+0.0025*i, wherein i is imaginary unit, in this particular example, there are 11 cells. The thickness of the flat lens is 943 nm.
    [0068] Figure 5 relates to the first example and show's: in the top left corner as Figure 5a, the dependence of the normal incidence transmission on the metal and dielectric layer thickness; in the top right corner as Figure 5b, the number of unit cells required for a total flat lens thickness approximately equal to 1 μιη, for changing metal and dielectric layer thicknesses; in the bottom left comer, the dependence of optical contrast of the aerial image of a 250 ran wide space mask (i.e. an opaque chromium layer with a single open slit which is 250 nm wide), formed by the flat lens on metal and dielectric layer thickness, and in the bottom right comer as Figure 5d, the dependence of the maximum normalized image log slope (NILS) of the aerial image of a mask with 250 nm wide lines spaced apart by 250 nm, formed by the flat lens on metal and dielectric layer thickness. NILS is a metric frequently used in photolithography to quantify the quality of an optical (aerial) image, and where NILS is calculated using NILS = CD* l/l(x)*dl(x)/dx, where I(x) is the aerial image intensity distribution, and CD is the critical dimension (which for line/spaces the CD typically equals half the pitch).
    [0069] It is apparent from Figure 5a that there is a preferred region in which the normal incidence transmission is relatively high, and a region in which the normal incidence transmission is close to zero or is zero. The thickness of the metal layer and dielectric layer should be selected accordingly, so that a positive normal incidence transmission (the radiation region on Figure 5a) is achieved. The most beneficial configurations of the metal and dielectric thicknesses are those where the normal incidence tr ansmission is close to 40% (regions showing in white in Figure 5a).
    [0070] As can be seen from Figure 5c, there is a preferred region in which the optical contrast is relatively high, e.g. close to 1. It is beneficial to have the highest possible value of optical contrast, because the higher the contrast, the better the quality of the image. With higher contrast, it is possible to print details which cannot be printed with lower values of contrast.
    [0071] Figure 5d shows that there is a relatively narrow region in which normalized image log slope (NILS) is positive and relatively high, and regions in which NILS is close to zero or is zero. In lithography, NILS measures quality of the image - the higher NILS is, the better quality image is achievable. The most beneficial configurations of the metal and dielectric thicknesses are those where NILS is higher than 6, or even higher than 8.
    [0072] Figure 6 also relates to the first example and shows the dependence of optical contrast and maximum NILS on the line/space (L/S) pitch for 50/50 L/S (where 50/50 L/S means that the line/space ratio is 50% lines, 50% line-free space). It is desirable for optical contrast as well as maximum NILS to be as high as possible. Contrast is calculated as (lmax-Imin)/(Imax+lmm), wherein and Imin refer to the maximum and minimum value of the aerial image intensity distribution. However, in modern substrates, NILS is the most important parameter. It is beneficial for NILS to be at least 2.0, such value ensures a robust lithography process.
    [0073] In a second example, the wavelength of the radiation used is 365 nm. In this example, immersion lithography is used. The medium between the flat lens and the substrate (image plane) is water (which has refractive index n = 1.33). The metal layer is silver (Ag), and the dielectric layer is TajOs. The thickness of the metal layer dmetai is between 20 nm and 60 nm; the thickness of the dielectric layer daw is between 40 nm and 60 nm. In a specific example, the thicknesses of the metal layer and the dielectric layer are dmeu.i = 55.0 nm and dausi = 30.0 nm. The permittivity of the metal layer is Emetai = -1.76 + 0.27*i, and the permittivity of the dielectric layer is Eaioi=5.29+0.0025*i, wherein i is imaginary unit. The refractive index of the metal layer is nmctai = 0. l+1.33*i, and the refractive index of the dielectric layer is naid ~ 2.3+0.00054*1. The thickness of the flat lens with dmctai = 55.0 nm and ddk-i = 30.0 nm is 935 nm. There are 11 cells.
    [0074] Figure 7 relates to the second example. Figures 7 a to 7d provide information analogous to Figures 5a to 5d. Figure 8 also relates to the second example, and provides information analogous to Figure 6. As can be seen from Figures 7-8, the NILS is significantly increased in the second example due to the presence of water between the flat lens and the substrate.
    [0075] Comparing Figures 7 and 8 (relating to the second example) to Figures 5 and 6 (relating to the first example), it can be seen that the contrast and the NILS are higher in the second example (immersion lithography using water) than in the first example. This shows that replacing air separating the flat lens FL and the image by water is beneficial, because the quality of the image is improved.
    [0076] In a third example, the wavelength of the radiation used is 146 nm. The metal layer is aluminium (Al), and the dielectric layer is silicon dioxide (SiCh). The thicknesses of the metal layer dmeiai and the dielectric layer datet are as follows: dmetai = 25.0 nm and datei = 21.0 nm. The permittivity of the metal layer is Emetai = -1.68 + 0.195*i, and the permittivity of the dielectric layer is «^1=3.179+0.0114*1, wherein i is imaginary unit. The refractive index of the metal layer is n,no,.d = 0.075+1.3*i, and the refractive index of the dielectric layer is rwi = 1.783+0.0032*1. The thickness of the flat lens is 976.5 nm, and the number of unit cells is 21.
    [0077] hr general, the absorption of the radiation with wavelength of 146 nm (as described in the third example) is significantly increased. The transmission properties for the above-given wavelength of 146 nm, or for any other wavelength used in general, may be improved with antireflection coatings provided on the top and bottom of the flat lens, or by changing the lens thickness.
    [0078] Figure 9 relates to the third example. Figures 9a to 9d provide information analogous to Figures 5a-5d and 7a-7d. Figure 10 also relates to the third example, and provides information analogous to Figures 6 and 8.
    [0079] In a fourth example, the wavelength of the radiation used is 193 nm. The metal layer is magnesium (Mg), and the dielectric layer is silicon dioxide (S 1()2). The thicknesses of the metal layer dmaai and the dielectric layer daw are as follows: dmetai = 37.5 nm and daw = 35.0 nm. The permittivity of the metal layer is Emeui = -1.89 + 0.55*i, and the permittivity of the dielectric layer is 6aieF=2.43+6.15*10"7*i, wherein i is imaginary unit. The refractive index of the metal layer is nmeui = 0.197+l.39*i, and the refractive index of the dielectric layer is nt)id = 1.56+l.97*10"7*i. In this particular example, the thickness of the flat lens is 942.5 nin. i.e. there are 13 cells.
    [0080] In the fourth example, the metal layer could alternatively be an alloy of silver (Ag) with aluminium (Al) or silver (Ag) with magnesium (Mg). It is also foreseen that different metals with a high plasma frequency (e.g. beryllium (Be) or aluminium (Al)) may be used. More generally, these materials could be used with other wavelengths as well - their use is not exclusively limited to 193 nm.
    [0081] Figure 11 relates to the fourth example. Figures 11 a to lid provide information analogous to Figures 5a-5d, 7a-7d and 9a-9d. Figure 12 also relates to the third example, and provides information analogous to Figures 6, 8 and 10.
    [0082] Figures 5-12, described above, are based on models. Figures 13 to 16 show' a calculated aerial image for a 50/50 line/spaces mask, for lour different pitches, for the first, second, third and fourth example, respectively. Such aerial images were used to calculate the optical contrast and NILS shown in Figures 6, 8, 10 and 12, where pitch of the line/spaces is varied. As can be seen; the contrast and NILS increase with pitch.
    [0083] The above combinations of materials, as described in the examples, show good combinations of working distance and absorption. They also show a negative refraction angle which matches the angle of incidence up to a certain maximum angle of incidence (discussed above; further information can be found in a paper “Planar metal/dielectric single-periodic multilayer ultraviolet flat lens” (Optica 2016, open access).
    [0084] As discussed above, the number of the cells 200 is preferably 4 or more. Depending on materials of the metal layer and the dielectric layer, a good number of cells 200 may be e.g. 11, 13, 15 or 21, A particular number of cells 200 may depend on the thickness of the layers 202, 204, required working distance and on absorption of the layers 202, 204. The working distance is less than or equal to the thickness of the flat lens FL. It may be desirable to increase the w'orking distance, thereby increasing the thickness of the flat lens FL. How'ever, the thicker the flat lens FL is, the more radiation it absorbs. Therefore, the particular number of layers 202, 204 may be chosen with regard to balance betw'een increased working distance and acceptable absorption.
    [0085] The flat lenses described above are designed for specific given wavelengths. The materials of the layers of the lenses were chosen for their permittivity values at that w'avelength. The layer thicknesses were then optimized to achieve the negative refraction of energy, absorption and w'orking distance required for the particular flat lens. Such a procedure may be performed for a very broad range of wavelengths; the only requirement is that there is a metal material and a dielectric material. The layer thicknesses are then determined througli optimizing the stack. The layer thicknesses usually scale with the wavelength.
    [0086] In a conventional proximity printing, the effect of diffraction is the main factor limiting the resolution, and a cause of the resolution of proximity printing being not better than approx. 1-2 μιη. Compared to conventional proximity printing, if a flat lens is used, the effect of diffraction is reduced, and the resolution is therefore improved. Depending on working distance, the resolution can be improved by a factor of 2. e.g. to approx. 500 nm. There is no need for further focusing the radiation or for corrections for an error caused by diffraction. This leads to a lower complexity and thus lower cost systems, with resolution sufficient for certain applications.
    [0087] Due to the flat surfaces, absence of optical axis and short optical path, certain aberrations are naturally smaller for flat lens than for curved refractive lenses. For example, Petzval, spherical or coma aberrations affect flat lenses much less than a conventional curved refractive lenses. These aberrations may even not be present at all due to the translational symmetry of the flat lens. This is beneficial when compared to a curved lens, where multiple lenses are needed to correct the aberrations and therefore the optical system gets more complex, with longer optical path.
    [0088] Due to less sensitivity to chromatic aberration, there are less strict requirements on the radiation source, in particular, the linewidth may be e.g. 1 nm (instead of 1 pm required by the conventional lithography).
    [0089] Due to the absence of optical axis, the field size is limited only by the area of the flat lens. As discussed above, the cost of a conventional lithographic system scale roughly as (FS*NA)3. With the flat lens, the cost scales with area of the lens instead of its volume, leading to significant cost reduction. In particular, the flat lens is cheap enough to be considered disposable in case it gets damaged by particles.
    [0090] Due to the low cost of flat lens FL and increased working distance between the flat lens and the substrate, the mask and the flat lens could be of the same dimensions, shape and area as the substrate W onto which the mask 104 is projected. In particular, the substrate is not perfectly flat. There are portions of the substrate with higher elevation, and portions of the substrate with lower elevation. The differences are usually in the order of nanometres, tens of nanometres or around 100 nm. If the working distance is about 1 pm, i.e. it is greater than these differences, it allows the mask to be of the same shape and area as the substrate, and the substrate to be exposed in one go. Therefore, there is no need to use a stepping process to project the mask multiple times onto the substrate. This is beneficial because it increases throughput of the lithographic apparatus.
    [0091] The use of the flat lens FL is further beneficial because it provides flexibility to choose anything between single field exposures and full wafer exposures. In other words, any field size may be chosen. The size of the flat lens may be equal to the size of a single field (26 mm x 33 mm), or a single mask (depending on the mask size and/or dimensions). The optimum flat lens size may be determined by finding balance between required alignment accuracy and throughput.
    [0092] When using flat lenses, in some embodiments, vacuum is not required to be provided in the lithographic apparatus. This allows reducing the overall costs of the lithographic apparatus.
    [0093] The flat lens, as described above, could be used when the space between the substrate and the flat lens is filled with air. Alternatively, the flat lens could work with immersion lithography, i.e. where the substrate is immersed in a suitable liquid. This could allow a more precise adjustment of the required optical properties of the system.
    [0094] Other requirements on the lithographic system, such as depth of focus (DOF), angle control, alignment etc. are similar to the current requirements on a conventional lithographic system. Therefore, a lithographic apparatus with the flat lens can benefit from off-axis illumination, mask assist features, and source-mask optimisation.
    [0095] Apart from or in addition to the above described embodiments, the flat lens can be used in the following beneficial ways.
    [0096] For a better alignment between the patterning device MA and the substrate W (i.e. for a better depth of field control), the flat lens can be scanned in sync with the wafer.
    [0097] A conventional i-line mercury lamp could be used.
    [0098] A ilex-ray illuminator may be used to obtain resolution enhancement in the same way as with a flat lens system.
    [0099] As discussed above, the projection optics box of the flat lens may be smaller than a conventional one (cubic centimetres compared to cubic metres). An illuminator may comprise a UV LED, a simple condenser and a monochromator. Combined with such an illuminator, the patterning device and projection optics box can be made much smaller and lighter than the substrate table.
    [00100] Actuators may be used to move the mask and flat lens instead of the substrate table. This has benefits for motor size and shifts the resonance frequencies up, making electromechanical control easier. In other words, instead of adjusting the wafer substrate height, it may be cheaper, more accurate and faster to adjust the position of the mask MA together with the flat lens FL with respect to the substrate. This is enabled by the fact that the mask MA together with the flat lens FL may be lighter than the substrate W and the substrate table WT.
    [00101] As the field of view is no longer limited by the radius of curvature of the lens, but rather by the lateral size of the flat lens, very large images may be printed directly. The fabrication of multilayer stacks has improved dramatically over the last decades. Therefore, fabricating lenses which have an area of (tens or hundreds of) square centimetres is possible. This is beneficial because the throughput scales linearly with field size. Therefore, greater area of the flat lens means increased throughput.
    [00102] Useful information about flat lenses and optical properties of layered materials may be found e.g. in Ruben Maas, Radiation propagation in multilayer metamaterials, PhD thesis (2015), and in a paper “Planar metal/dielectric single-periodic multilayer ultraviolet flat lens” (Optica 2016, open access).
    [00103] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention. Other aspects of the invention are set out as in the following numbered clauses: 1. A lithographic apparatus comprising: an illumination system for providing a beam of radiation; a support structure for supporting a patterning device, the patterning device serving to impart the radiation beam with a pattern in its cross-section; a substrate table for holding a substrate; and a flat lens, wherein the flat lens is positioned between the support structure and the substrate table, and comprises at least four cells, wherein each cell consists of a first layer and a second layer of material, and wherein the first layer is made of a material with permittivity such that the real part of permittivity is negative and the imaginary part of permittivity is positive, and the second layer is made of a material with permittivity such that the real part of permittivity is positive and the imaginary part of permittivity is positive. 2. Lithographic apparatus as in clause 1, wherein the first and second layer of each cell are positioned so that the radiation incident on the flat lens first passes through the first layer and then through the second layer. 3. Lithographic apparatus as in clause 1, wherein the first and second layer of each cell are positioned so that the radiation incident on the flat lens first passes through the second layer and then through the first layer. 4. Lithographic apparatus as in any one of the preceding clauses, further comprising a patterning device, the patterning device being positioned between the support structure and the flat lens. 5. Lithographic apparatus as in clause 4, wherein the flat lens is fixed to the patterning device. 6. Lithographic apparatus as in clause 4, wherein between the flat lens and the patterning device, there is a dielectric layer. 7. Lithographic apparatus as in any one of the preceding clauses, wherein the thickness of the first layer is between 10 nm and 65 nm, and the thickness of the second layer is between 15 nm and 50 nm. 8. Lithographic apparatus as in any one of the preceding clauses, wherein the real part of the permittivity of the first layer is between 0 and -10. 9. Lithographic apparatus as in any one of the preceding clauses, wherein the overall thickness of the flat lens is between 0.5 pm and 10 pm. 10. Lithographic apparatus as in any one of the preceding clauses, wherein the imaginary part of the refractive index of the second layer is 0.1 or less. 11. A method comprising: providing a substrate; providing a beam of radiation using an illumination system; using a patterning device to impart the radiation bean with a pattern in its cross-section; providing a flat lens, wherein the flat lens is positioned between the patterning device and the substrate, and comprises at least four cells, wherein each cell consists of a first layer and a second layer of material, and wherein the first layer is made of a material with permittivity such that the real part of permittivity is negative and the imaginary part of permittivity is positive, and the second layer is made of a material with permittivity such that the real part of permittivity is positive and the imaginary part of permittivity is positive; and projecting the patterned radiation beam onto a target portion of the substrate. 12. Method as in clause 11 wherein the working distance between the patterning device and the flat lens is between 0.5 pm and 10 pm 13. Method as in clause 11 or clause 12 wherein the working distance between the patterning device and the flat lens is equal to the thickness of the flat lens, or wherein the working distance between the patterning device and the flat lens is smaller than the thickness of the flat lens. 14. Method as in any one of clauses 11 to 13, wherein the whole area of substrate is exposed to the patterned radiation in one step. 15. A flat lens for use in a lithographic apparatus wherein the flat lens comprises at least four cells, wherein each cell consists of a first layer and a second layer of material, and wherein the first layer is made of a material with permittivity such that the real part of permittivity is negative and the imaginary part of permittivity is positive, and the second layer is made of a material with pennitti vity such that the real part of permittivity is positive and the imaginary part of permittivity is positive.
    权利要求:
    Claims (1)
    [1]
    A lithography device comprising: an illumination device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being able to apply a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.
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    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    JP2001308002A|2000-02-15|2001-11-02|Canon Inc|Method of forming pattern by use of photomask and pattern-forming device|
    KR20120071640A|2010-12-23|2012-07-03|한국전자통신연구원|Aspheric high resolution lens|
    KR102252049B1|2014-08-04|2021-05-18|삼성디스플레이 주식회사|Mask for photolithography, method of manufacturing the same and method of manufacturing substrate using the same|
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    申请号 | 申请日 | 专利标题
    EP16204385|2016-12-15|
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