EUV mask

Abstract

Disclosed herein is a mask for patterning radiation in a lithographic apparatus, the mask comprising: a patterned surface arranged to impart a pattern to radiation; and a waveguide structure disposed on the patterned surface; wherein the patterned surface comprises: a first reflective region; and a second reflective region arranged such that radiation reflected from the second reflective region is phase shifted with respect to radiation reflected from the first reflective region.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 23161667.3 filed on March 14, 2023 and EP application 23174812.0 filed on May 23, 2023, which are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention relates to masks for EUV radiation. Masks according to embodiments have improved imaging properties over known masks for EUV radiation. This can both improve feature fabrication by EUV lithography processes and reduce the required dose of EUV radiation. Background Art

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate (typically onto a target portion of the substrate). A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device (alternatively referred to as a mask or reticle) can be used to create the circuit pattern to be formed on a single layer of the IC. This pattern can be transferred to a target portion (e.g., a portion comprising a die, a die, or several dies) on a substrate (e.g., a silicon wafer). The pattern is typically transferred by imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. Typically, a single substrate will contain a grid of consecutively patterned adjacent target portions.

[0005] Photolithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and / or structures. However, as the size of features manufactured using photolithography becomes smaller, photolithography becomes a more critical factor in enabling the manufacture of miniature ICs and other devices and / or structures.

[0006] The theoretical estimate of the limit of pattern printing can be given by the Rayleigh criterion of resolution, as shown in equation (1):

[0007] (1)

[0008] where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process-dependent adjustment factor (also known as the Rayleigh constant), and CD is the characteristic size (or critical dimension) of the printed feature. According to equation (1), a reduction in the minimum printable size of a feature can be achieved in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA, or by reducing the value of k1.

[0009] To shorten the exposure wavelength and thus reduce the minimum printable size, it has been proposed to use an extreme ultra violet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm (for example within the range of 13-14 nm). It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed soft x-ray radiation or extreme ultraviolet radiation. Possible sources include laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

[0010] A lithographic apparatus includes a patterning device (e.g., a mask or a reticle). Radiation is provided through or by the patterning device to form an image on a substrate.

[0011] It is generally desirable to improve known masks for EUV radiation. SUMMARY

[0012] According to a first aspect of the application, there is provided a mask for patterning radiation in a lithographic apparatus, the mask comprising: a patterning surface arranged to impart a pattern to radiation; and a waveguide structure arranged on the patterning surface; wherein the patterning surface comprises: a first reflective region; and a second reflective region arranged such that radiation reflected from the second reflective region is phase shifted relative to radiation reflected from the first reflective region.

[0013] According to a second aspect of the application, there is provided a lithographic apparatus comprising a mask according to the first aspect.

[0014] According to a third aspect of the application, there is provided a method of designing a reflective mask for patterning radiation in a lithographic apparatus, wherein the mask comprises a first reflective region and a second reflective region for imparting different phase shifts on the impinging radiation, and the mask comprises a waveguide structure on the first and / or second reflective region, the method comprising: determining a combined width of the first and second reflective regions of the mask according to a user defined pitch of features to be printed by the mask patterned radiation; determining a material and a location of each waveguide structure; and determining a size of each waveguide structure, a width of the first reflective region and a width of the second reflective region such that the radiation patterned by the mask has a desired characteristic.

[0015] According to a fourth aspect of the application, there is provided a method for performing lithography, the method comprising in an exposure operation projecting a beam of radiation onto a substrate through a low pressure environment; wherein the beam of radiation is reflected from a mask located in the low pressure environment; and the mask is a mask according to the first aspect.

[0016] According to a fifth aspect of the application, there is provided a mask for patterning radiation in a lithographic apparatus, the mask comprising a patterned surface arranged to impart a pattern to radiation; wherein: the patterned surface comprises a first reflective region and a second reflective region; the second reflective region comprises an embedded phase shift structure arranged such that radiation reflected from the second reflective region is phase shifted relative to radiation reflected from the first reflective region; the first reflective region comprises a main region and at least one sub-region at an end of the first reflective region; the at least one sub-region of the first reflective region has different reflective properties to the main region of the first reflective region; the second reflective region comprises a main region and at least one sub-region at an end of the second reflective region; and the at least one sub-region of the second reflective region has different reflective properties to the main region of the second reflective region.

[0017] According to a sixth aspect of the application, there is provided a lithographic apparatus comprising a mask according to the fifth aspect.

[0018] According to a seventh aspect of the application, there is provided a method for performing lithography, the method comprising in an exposure operation projecting a beam of radiation onto a substrate through a low pressure environment; wherein the beam of radiation is reflected from a mask located in the low pressure environment; and the mask is a mask according to the first aspect. BRIEF DESCRIPTION OF DRAWINGS

[0019] Embodiments of the application will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0020] Figure 1 A lithographic apparatus according to an embodiment of the application is depicted;

[0021] Figure 2 A more detailed view of a lithographic apparatus;

[0022] Figure 3a A known EUV mask is schematically depicted;

[0023] Figure 3b A spatial image of a known EUV mask is schematically depicted;

[0024] Figure 4 An EUV mask according to a first embodiment of the application is schematically depicted;

[0025] Figure 5 schematically depicts an EUV mask according to a second embodiment of the present invention;

[0026] Figure 6 Another known EUV mask is schematically depicted;

[0027] Figure 7a schematically depicts, in side view, an EUV mask according to a third embodiment of the present invention; and

[0028] Figure 7b An EUV mask according to a third embodiment of the present invention is schematically depicted in plan view. DETAILED DESCRIPTION

[0029] Figure 1 A lithographic apparatus 100 comprising a source collector module SO according to an embodiment of the present invention is schematically depicted. The apparatus 100 comprises:

[0030] - an illumination system (or illuminator) IL configured to condition a radiation beam B (eg EUV radiation).

[0031] a support structure (e.g., mask table) MT configured to support a patterning device (e.g., mask or reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;

[0032] a substrate table (eg wafer stage) WT configured to hold a substrate (eg a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and

[0033] - a projection system (eg, a reflective projection system) PS configured to project the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (eg, comprising one or more dies).

[0034] The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping or controlling the radiation.

[0035] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions (e.g., whether the patterning device is held in a vacuum environment). The support structure MT may use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, e.g., fixed or movable as desired. The support structure MT may ensure that the patterning device MA is in a desired position, e.g., relative to the projection system PS.

[0036] The term“patterning device” should be interpreted in the broadest sense as referring to any device capable of imparting a pattern to a beam of radiation B so as to create a pattern in a target portion C of the substrate W. The pattern imparted to the beam of radiation B can correspond to a particular functional layer in a device being created in target portion C, such as an integrated circuit.

[0037] The patterning device can be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays and programmable liquid crystal display (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. Programmable mirror arrays are an example of static mask technology. Programmable mirror arrays have a matrix of small mirrors on a matrix substrate that can be individually tilted so as to steer the beam of radiation traveling on the plane of the matrix substrate. The tilted mirrors impart a pattern in the beam of radiation, which is reflected from the matrix substrate.

[0038] Like the illumination system IL, the projection system PS can include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It is also possible to use immaterial means as the vacuum for EUV radiation, since other gases can absorb too much radiation. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0039] As shown herein, the lithographic apparatus 100 is reflective (e.g. employs a reflective mask).

[0040] The lithographic apparatus 100 can be of a type having two (dual stage) or more substrate tables WT (and / or two or more support structures MT). In such “multi-stage” lithographic apparatuses, additional substrate tables WT (and / or additional support structures MT) can be used in addition to the substrate table WT (and / or support structure MT) described above. An additional substrate table WT (and / or additional support structure MT) can be used in addition, or instead, of one or more substrate tables WT (and / or one or more support structures MT) described above. Such further substrate table(s) WT (and / or such further support structure(s) MT) can be used to hold a substrate W that is to be exposed, that is being exposed, or that has been exposed (for example, while one or more other substrates W are being exposed).

[0041] With reference to Figure 1 The illumination system IL receives a beam of radiation from a source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state, wherein one or more elements of the material emit radiation in the EUV range. In one such method, often termed laser-produced plasma (“LPP”), a fuel, such as a droplet, a stream or a cluster of material having a desired element, is irradiated by a laser beam. A source collector module SO can be any suitable source that provides a beam of radiation within the EUV range. A source collector module SO can be reflective, refractive, or a combination thereof.Figure 1 part of the EUV radiation system (not shown) for providing the laser beam that excites the fuel. The resulting plasma emits output radiation, e.g. EUV radiation, which is collected by a radiation collector, optimized to collect as much as possible of the radiation generated by the plasma. The laser and the source collector module SO can be separate entities, for example when a CO2laser is used to provide the laser beam for excitation of the fuel. In other cases, the source can be an integral part of the source collector module SO, for example when the source is a discharge produced plasma EUV generator, often termed as DPP source.

[0042] In this case, the laser is not considered to form part of the lithographic apparatus 100, and the radiation beam B passes from the laser to the source collector module SO with the aid of a beam delivery system comprising, for example, suitable beam directing mirrors and / or a beam expander. In other cases the source can be an integral part of the source collector module SO, for example when the source is a discharge produced plasma EUV generator, often termed as DPP source.

[0043] The illumination system IL can include an adjuster for adjusting the angular intensity distribution of the radiation 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 illumination system IL can be adjusted. In addition, the illumination system IL can include various other components, such as facetted field and pupil mirror devices. The projection system PS can be used to focus the radiation beam B onto a target portion C of the substrate W. The projection system PS can be any projection system, for example an optical projection system, a catadioptric projection system, or a mirror projection system. The projection system PS can be a refractive projection system, in which case it can include in order of succession, a pupil plane, an exit pupil plane and an entrance pupil plane. A mirror projection system has, in order of succession, an exit pupil plane, a pupil plane, and an entrance pupil plane. The projection system PS can be a refractive projection system, in which case it can include in order of succession, a pupil plane, an exit pupil plane and an entrance pupil plane. A mirror projection system has, in order of succession, an exit pupil plane, a pupil plane, and an entrance pupil plane. The illumination system IL and / or the projection system PS can be designed to have a uniform illumination distribution at their respective input or output, in order to balance out optical aberrations induced by the optics.

[0044] The radiation beam B is incident on the patterning device (e.g., mask) MA held by the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. The patterned beam B then passes through the projection system PS, which focuses the beam B onto a target portion C of the substrate W. The substrate table WT is accurately positionable, for example, using a second positioner PW and position sensor PS2 (e.g., an interferometer device, a linear encoder, or a capacitance sensor), so as to position different target portions C in the path of the beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (e.g., mask) MA in the path of the beam B. The patterning device (e.g., mask) MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

[0045] The controller 500 controls the overall operation of the lithography apparatus 100, and particularly, carries out the operational procedures described further below. The controller 500 can be embodied as a suitably programmed general purpose computer comprising a central processing unit, volatile and non-volatile memory means, one or more input and output devices such as a keyboard and screen, one or more network connections, and one or more interfaces to various parts of the lithography apparatus 100. It will be appreciated that a one-to-one relationship between a control computer and a lithography apparatus 100 is not essential. In embodiments of the application, one computer can control multiple lithography apparatuses 100. In embodiments of the application, multiple networked computers can be used to control one lithography apparatus 100. The controller 500 can also be configured to control one or more associated process devices and substrate handling devices in a lithographic cell or cluster of which the lithography apparatus 100 forms part. The controller 500 can also be configured to be subordinate to a supervisory control system for the lithographic cell or cluster and / or to an overall control system for the factory.

[0046] Figure 2 The lithography apparatus 100 is shown in more detail in Figure 1. The lithography apparatus 100 includes a source collector module SO, an illumination system IL, and a projection system PS. The EUV radiation emitting plasma 210 can be formed by a plasma source. EUV radiation can be produced by a gas or vapor, for example Xe gas, Li vapor, or Sn vapor in which the radiation emitting plasma 210 is formed to emit radiation in the EUV range of the electromagnetic spectrum. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0047] Radiation emitted by the radiation emitting plasma 210 enters the collector chamber 212 from the source chamber 211.

[0048] The collector chamber 212 can include a radiation collector CO. Radiation that passes through the radiation collector CO can be focused in a virtual source point IF. The virtual source point IF is often referred to as the intermediate focus, and the source collector module SO is arranged such that the virtual source point IF is located at or near an opening 221 in the enclosure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

[0049] Subsequently, the radiation traverses the illumination system IL, which can include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of the unpatterned beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the unpatterned beam 21 at the patterning device MA, held by the support structure MT, a patterned beam 26 is formed, and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the substrate table WT.

[0050] Typically, there may be more elements in the illumination system IL and the projection system PS than shown. Furthermore, there may be more mirrors than shown in the figures, e.g. Figure 2 As shown in FIG, there may be 1 to 6 additional reflective elements in the projection system PS.

[0051] Alternatively, the source collector module SO may be part of an LPP radiation system.

[0052] like Figure 1 As shown, in an embodiment, a lithographic apparatus 100 includes an illumination system IL and a projection system PS. The illumination system IL is configured to emit a radiation beam B. The projection system PS is separated from a substrate table WT by an intervening space. The projection system PS is configured to project a pattern imparted to the radiation beam B onto a substrate W. The pattern is EUV radiation from the radiation beam B.

[0053] The space between the projection system PS and the substrate table WT may be at least partially evacuated.The intervening space may be defined at the location of the projection system PS by a solid surface through which the employed radiation is directed towards the substrate table WT.

[0054] The lithographic apparatus 100 can include a dynamic airlock. The dynamic airlock includes a diaphragm 80. In an embodiment, the dynamic airlock includes a hollow portion covered by the diaphragm 80 positioned in an intervening space. The hollow portion is positioned around the radiation path. In an embodiment, the lithographic apparatus 100 includes a blower configured to blow air into the interior of the hollow portion. The radiation travels through the diaphragm before striking the substrate W.

[0055] As mentioned above, the lithographic apparatus 100 may comprise a reflective mask for patterning the radiation beam.A known type of reflective mask for EUV radiation is described in EP 1 260 862 A1.The mask comprises a stack of substrates, wherein adjacent layers in the stack have different refractive indices.

[0056] Another known type of EUV mask is a reflective absorber-based mask. The illuminated surface of this mask includes regions that absorb incident EUV radiation and regions that reflect it. When the mask is illuminated by EUV radiation, the EUV radiation reflected from the mask is patterned according to the pattern of reflective and absorptive regions of the mask. A problem with this absorber-based mask is that the intensity of the EUV radiation can be significantly reduced by the mask. Consequently, the intensity of the EUV radiation that illuminates the substrate is low.

[0057] An alternative to absorber-based masks is a hard phase shift mask (HPSM). HPSMs differ from absorber-based masks in that they include an illuminated surface that is designed to reflect EUV radiation only. HPSMs do not include regions that are designed to absorb EUV radiation. Due to the increased reflectivity of HPSMs, the intensity of EUV radiation illuminating a substrate is higher than in the case of using absorber-based masks.

[0058] Figure 3a A known HPSM 300 for EUV radiation is schematically illustrated.

[0059] The HPSM 300 is a reflective mask. The upper surface of the mask is illuminated by EUV radiation and the EUV radiation reflected from the mask is patterned by the mask. The mask includes a plurality of material layers that provide reflective regions. To provide reflection, each material layer can have a different refractive index from its adjacent layers in accordance with known techniques for providing a reflective surface for EUV radiation. The HPSM 300 includes raised structures 304 on a base 305. The raised structures 304 can be made by selectively etching regions in a multi-layer structure, where each trough between adjacent raised structures 304 is a region of the multi-layer structure that is removed by the etching process. It should be noted that the actual number of layers in the HPSM 300 is typically much greater than the number of layers shown in Figure 3. For example, each raised structure 304 can include about 50 layers and the base 305 can include about 50 layers.

[0060] The upper surface of each raised structure 304 defines a first reflective region R1. The upper surface of the base 305 that is not covered by the raised structures 304 defines a second reflective region R2. The raised structures 304 extend a height 302 above the base 305. Each raised structure 304 can have a width 303 and is spaced apart from an adjacent raised structure 304 by a width 301 of each R2. Figure 3a The length of each raised structure 304, i.e. the length in a direction orthogonal to the height 302 and the width 303, is not shown in Figure 3. The height 302 of each raised structure 304, the width 303 of each raised structure 304, the width 301 of each R2 and the length of each raised structure 304 are design parameters of the HPSM 300 that define the pattern that the HPSM 300 imparts to incident EUV radiation. The height 302 is a spatial difference in the reflective surface of the HPSM 300. The height 302 can be formed by each raised structure 304 on the substrate 305 including about 10 to 30 substrate layers. The required number of substrate layers will depend on the particular properties of the substrate layers. The height 302 can be set so that the EUV radiation reflected from R1 has a desired phase difference from the EUV radiation reflected from R2. The desired phase difference can be about 180 degrees. The width 301 of each R2 can be set so that the EUV radiation reflected from R2 has a desired phase difference from the EUV radiation reflected from R1. The desired phase difference can be about 180 degrees. The width 303 of each raised structure 304 can be set so that the EUV radiation reflected from R1 has a desired phase difference from the EUV radiation reflected from R2. The desired phase difference can be about 180 degrees. oHowever, other phase differences can be used. The corner 306 defines the border between R1 and R2.

[0061] Figure 3b A spatial image 307 of the EUV radiation that has been patterned by reflection from the HPSM 300 is shown. The spatial image 307 includes dark lines at each border between R1 and R2. The critical dimension CD of the feature printed on the substrate depends on the width 308 of the dark lines. The width 303 of R1 and the width 301 of R2 define the separation distance between the dark lines in the spatial image 307. Thus, the pitch of the feature printed on the substrate depends on the width 303 of R1 and the width 301 of R2. Due to demagnification of the optics of the lithography system, the image properties at the HPSM 300 can be about 4 times the image properties at the substrate. It is the image properties at the substrate that define the actual printed feature. Thus, when the width 308 of the dark lines is 56 nm, the CD of the printed feature on the substrate can be 14 nm.

[0062] The problem is that the CD of the printed feature on the substrate is highly dependent on the pitch of the printed feature on the substrate. When the width 303 of R1 and the width 301 of R2 at the HPSM 300 are both about 112 nm, the width 308 of the dark lines at the HPSM 300 that define the CD can be about 56 nm. However, when the width 303 of R1 and the width 301 of R2 at the HPSM 300 are increased to about 200 nm, the width 308 of the dark lines at the HPSM 300 can increase to about 88 nm and thus increase the CD of the printed feature. This variation of CD with pitch cannot be prevented by changing the design parameters of the known HPSM 300. Thus, there is no design freedom that allows compensation of the variation of the CD. Another problem of this known HPSM 300 for EUV radiation is that a 180 o The height 302 required for the phase difference inevitably deviates from the imaging performance of a perfectly flat mask. There is also no design parameter that can be freely changed in order to improve aspects of the imaging performance. Due to these problems, the known HPSM 300 is difficult to use with EUV radiation.

[0063] The first and second embodiments provide a new HPSM for EUV radiation that improves the known HPSM 300 described above.

[0064] Figure 4 A HPSM 400 for EUV radiation according to the first embodiment is shown schematically.

[0065] HPSM 400 is a reflective mask. The upper surface of HPSM 400 is illuminated by EUV radiation, and EUV radiation reflected from HPSM 400 is patterned by HPSM 400. HPSM 400 comprises a plurality of material layers that provide reflective regions. To provide reflection, each material layer can have a different refractive index from its neighboring layers according to known techniques of providing a reflective surface for EUV radiation. HPSM 400 comprises raised structures 304 on a base 305. Raised structures 304 can be made by etching, as described for known HPSM 300 shown in FIG. 3. It should be noted that the actual number of layers in HPSM 400 is typically much more than the number of layers shown. Figure 4 For example, each raised structure 304 can comprise about 50 layers, and base 305 can comprise about 50 layers.

[0066] The upper surface of each raised structure 304 defines a first reflective region Rl. The upper surface of base 305 that is not covered by raised structures 304 defines a second reflective region R2. Raised structures 304 extend a height 302 above base 305. Each raised structure 304 can have a width 303 and is spaced apart from an adjacent raised structure 304 by a width 301 of each R2. Figure 4 The length of each raised structure 304, i.e. the length in a direction orthogonal to height 302 and width 303, is not shown in FIG. 4. Height 302 is a spatial difference in the reflective surface of HPSM 400. Height 302 can be set such that EUV radiation reflected from Rl has a 180 o phase difference from EUV radiation reflected from R2. Corners 306 define a boundary between Rl and R2. Rl and R2 define a patterned surface of HPSM 400.

[0067] HPSM 400 according to the present embodiment differs from known HPSM 300 in that it further comprises waveguide structures 401. Each waveguide structure 401 can be a cuboid in shape. Waveguide structures 401 can be provided on the side of each raised structure 304. Each waveguide structure 401 can have a height 403 above base 305 that is the same as height 302 of raised structure 304 above base 305. Thus, the upper surface of each waveguide structure 401 can be coplanar with the upper surface of raised structure 304. Each waveguide structure 401 has a width 402. There is a spacing distance 404 between two waveguide structures 401 arranged between adjacent raised structures 304. Since the combined width 402 of waveguide structures 401 between adjacent raised structures 304 is less than width 301 of each R2, a channel is defined between waveguide structures 401. The width of the channel is spacing distance 404, and one end of the channel is closed by the surface of base 305.

[0068] The waveguide structures 401 are waveguides for illuminating the HPSM 400 with EUV radiation. The waveguide structures 401 and the channels are guiding structures for modes of the EUV radiation. The formation and shape of such waveguide modes are described in: Investigation of waveguide modes in EUV mask absorbers, Hazem Mesilhy et al., Journal of Micro / Nanopatterning, Materials, and Metrology, Volume 20, Issue 2, 021004 (May 2021), https: / / www.spiedigitallibrary.org / journals / journal-of-micro-nanopatterning-materials-and-metrology / volume-20 / issue-2 / 021004 / Investigation-of-waveguide-modes-in-EUV-mask-absorbers / 10.1117 / 1.JMM.20.2.021004.full (as of February 21, 2023).

[0069] There are a variety of suitable materials for each waveguide structure 401. Preferred materials are ruthenium and molybdenum, other possible materials are listed in Table 1 below.

[0070] In the HPSM 400 according to the present embodiment, the design parameters include all of the height 302 of each protrusion structure 304, the width 303 of each protrusion structure 304, the width 301 of each R2, the length of each protrusion structure 304, the material of each waveguide structure 401, the width 402 of each waveguide structure 401, and the height 403 of each waveguide structure 401. These design parameters of the HPSM 400 define the pattern that the HPSM 400 imparts to the EUV radiation. As such, the use of waveguide structures 401 increases the design freedom, as the characteristics of the pattern that the HPSM 400 imparts to the EUV radiation depend on the material and dimensions of each waveguide structure 401.

[0071] As described with reference to Figures 3A and 3B, the width 303 of each protrusion structure 304 and the width 301 of R2 define the pitch 303, 301 of the printed features on the substrate W. In the present embodiment, the width 303 of each protrusion structure 304 and the width 301 of R2 can be set so that the printed features on the substrate W have a user-defined pitch. The user-defined pitch depends on the specific design of the product being manufactured by the lithographic process. The minimum possible value of the user-defined pitch will depend on the numerical aperture NA of the lithographic system. For a 0.33 NA system, the minimum user-defined pitch printed at the substrate can be about 28 nm, and for a lithographic system with a higher NA, the minimum user-defined pitch can be smaller. The user-defined pitch printed at the substrate can increase to a value of 150 nm or more to print features with different pitches. Thus, the required user-defined pitch can vary over a wide range of values.

[0072] The material of each waveguide structure 401 can be selected, and the width 402 and height 403 of each waveguide structure 401 determined, so that for a user-defined pitch of the printed features on the substrate W, the aerial image has the desired properties. Thus, the use of waveguide structures 401 allows the desired CD of the printed features at the substrate W (which can be 14 nm) to be achieved over a wide range of different pitches of the printed features at the substrate W. This is a significant advantage compared to the known HPSM 300 shown in Figure 3, which has no way of preventing the CD from varying significantly as the pitch varies.

[0073] In the present embodiment, both the width 402 and height 403 of each waveguide structure 401 are design parameters that can be varied. However, in a preferred implementation of the present embodiment, only the width 402 of each waveguide structure 401 is varied to change the properties of the patterned radiation. The height 403 of the waveguide structure 401 is constant, and is the same as the height 302 of each protrusion structure 304. This simplifies the manufacture of the HPSM 400.

[0074] It should be noted that although the design of the waveguide structure 401 can be determined so as to maintain the same CD over the entire pitch range, the use of waveguide structures 401 can provide other benefits. For example, other advantages can include one or more of improved CD uniformity, reduced pattern shift, improved contrast, and improved CD control. Embodiments also include determining the material and dimensions of the waveguide structure 401 so as to improve any property of the patterned EUV radiation. For example, the material and dimensions of the waveguide structure 401 can be determined so that the patterned EUV radiation has improved contrast.

[0075] Simulations have shown that when the waveguide structures 401 are made of ruthenium or molybdenum, a CD of 14 nm at the substrate can be achieved at the substrate for all pitches between 28 nm and 120 nm. To achieve a CD of 14 nm at the substrate over the entire pitch range, all of the width 303 of each protrusion 304, the width 301 of R2, and / or the width 402 of each waveguide structure 401 can be adjusted. The width 303 of each protrusion 304 and the width 301 of R2 can be varied according to the constraint that the sum of the width 303 of each protrusion 304 and the width 301 of R2 equals twice the pitch-defining separation distance in the aerial image at the HPSM 400. By making the height 403 of the waveguide structures 401 the same as the height 302 of each protrusion structure 304 at all pitches at the substrate, a CD of 14 nm at the substrate can be achieved over the entire pitch range.

[0076] Embodiments include making each waveguide structure 401 of a plurality of different materials. A non-exhaustive list of suitable materials for each waveguide structure 401 is provided in Table 1. It should be noted that some of the refractive index values in Table 1 have been determined experimentally and the actual refractive index values can vary.

[0077]

[0078]

[0079] The material of each waveguide structure 401 preferably has a refractive index of 0.97 or less. For materials with a higher refractive index, it can not be possible to maintain a constant CD over a wide pitch range by changing the width 402 of the waveguide structures 401, R1, and R2 alone.

[0080] Figure 5 A HPSM 500 for EUV radiation according to a second embodiment is schematically illustrated. The HPSM 500 of the second embodiment is also a reflective mask having waveguide structures 501.

[0081] The HPSM 500 of the second embodiment differs from the HPSM 400 of the first embodiment in that the waveguide structures 501 are arranged in a different position. The HPSM 500 of the second embodiment can otherwise be identical to the HPSM 400 of the first embodiment. In particular, the protrusion structures 304 and the base 305 can be as described for the first embodiment. R1 and R2 define the patterned surface of the HPSM 500. The waveguide structures 501 of the second embodiment can be made of the same materials as the waveguide structures 401 of the first embodiment.

[0082] As Figure 5As shown, in the second embodiment, each waveguide structure 501 is located on the upper surface of each raised structure 304. The shape of each waveguide structure 501 can be a cuboid. The waveguide structure 501 can be positioned at each edge of the raised structure 304 such that the side surface of the waveguide structure 501 is coplanar with the side surface of the raised structure 304. Each waveguide structure 501 has a height 503 and a width 502. There is a separation distance 504 between adjacent waveguide structures 501 on each raised structure 304.

[0083] In the second embodiment, the waveguide structure 501 also defines a waveguide for illuminating the HPSM 500 with EUV radiation. The waveguide structure 501 and the channels are guiding structures for a mode of EUV radiation. This mode can be formed in the channels defined between adjacent waveguide structures 501 on adjacent raised structures 304, where the width of the channel is the width 301 of R2. If the separation distance 504 is much larger than the width 301 of R2, then this can be the primary or only guiding structure. However, if the separation distance 504 is similar to or smaller than the width 301 of R2, then a mode can also be formed in each channel defined between adjacent waveguide structures 501 on the same raised structure 304. The width of the channel for these modes is the separation distance 504.

[0084] The second embodiment is similar to the first embodiment in that the material, height 503 and width 502 of each waveguide structure 501 are variable design parameters that affect the characteristics of the pattern that the HPSM 500 imparts to the EUV radiation. As such, the use of waveguide structures 501 increases the design freedom because the pattern that the HPSM 500 imparts to the EUV radiation depends on the material and dimensions of each waveguide structure 501. Therefore, the advantages of the second embodiment are the same as those described for the first embodiment.

[0085] For the second embodiment, simulation experiments have shown that a CD of 14 nm can be achieved at the substrate for all pitches at the substrate between 28 nm and 120 nm when the waveguide structure 501 is made of ruthenium or molybdenum. To achieve a CD of 14 nm at the substrate for the entire range of pitches, all of the width 303 of each raised portion 304, the width 301 of R2, the width 502 of each waveguide structure 501 and the height 503 of each waveguide structure 501 can be adjusted. The width 303 of each raised portion 304 and the width 301 of R2 can be varied according to the constraint that the sum of the width 303 of each raised portion 304 and the width 301 of R2 is equal to twice the separation distance of the pitch that defines the printed feature in the aerial image at the HPSM 500. The width 502 of each waveguide structure 501 is constrained to the extent necessary to accommodate the waveguide structure 501 on the upper surface of each raised structure 304. The height 501 of each waveguide structure can be less than about 100 nm.

[0086] The HPSMs 400, 500 of the first and second embodiments are both superior to the known HPSM 300 due to the increased design freedom offered by the waveguide structures 401, 501. The HPSMs 400, 500 of the first and second embodiments are also superior to known absorber-based masks for EUV radiation because they have a higher reflectivity. Thus, the embodiments reduce the required dose of EUV radiation.

[0087] According to a third embodiment, a technique is provided to improve another known type of HPSM for EUV radiation. Figure 6 A known type of HPSM 600 for EUV radiation is shown, which can be referred to as an embedded phase shifter HPSM 600.

[0088] The HPSM 600 is a reflective mask. The upper surface of the mask is illuminated by EUV radiation, and the EUV radiation reflected from the mask is patterned by the mask. In a similar way to the previously described HPSMs 300, 400, 500, the HPSM 600 includes a plurality of material layers that provide reflective regions. To provide reflection, each material layer can have a different refractive index to its adjacent layers, in accordance with known techniques for providing a reflective surface for EUV radiation. Figure 3a 、 Figure 4 and Figure 5 The HPSM 600 includes a phase shift structure 606. The phase shift structure 606 is covered by a plurality of layers 607 and is thus embedded within the HPSM 600. The phase shift structure 606 reflects incident EUV radiation and imparts a phase shift to it. Figure 6 The HPSM 600 is similar to the known HPSM 300 as described with reference to

[0089] The HPSM 600 includes a phase shift structure 606. The phase shift structure 606 is covered by a plurality of layers 607 and is thus embedded within the HPSM 600. The phase shift structure 606 reflects incident EUV radiation and imparts a phase shift to it.

[0090] The HPSM 600 is similar to the known HPSM 300 as described with reference to Figure 3a

[0091] Each R1 does not include an embedded phase shift structure 606. Each R1 includes a plurality of layers 604. The layers 604 of each R1 can extend a height 602 above the substrate 605 of the HPSM 600. The height 602 can be formed by approximately 10 to 30 layers 604 on the base 605. The required number of layers 604 will depend on the specific properties of the layers. The layers 604 can be substantially the same as the layers 304 of the HPSM 300 shown in Figure 3a Each R1 has a width 603. Figure 6 ​The length of each R1, i.e. in a direction orthogonal to the height 602 and the width 603, is not shown in the figure.

[0092] Each R2 includes an embedded phase shift structure 606. Each R2 includes a plurality of layers 607. The layers 607 of each R2 can extend a combined height 608 of the layers 607 and a height of the embedded phase shift structure 606 above the substrate 605 of the HPSM 600. The height 608 can be formed by about 10 to 30 layers 607. The required number of layers 607 will depend on the specific properties of the layers 607 and the phase shift structure 606. The layers 607 can be formed of the same material as the substrate 605. Figure 3a The layers 304 of the HPSM 300 shown are substantially the same as the layers 604 of the HPSM 600. Each R2 has a width 601. Figure 6 The length of each R2, i.e. in a direction orthogonal to the height 608 and the width 601, is not shown in the figure. Each R1 can be spaced apart from an adjacent R1 by the width 601 of an R2.

[0093] The height 602 of each R1, the width 603 of each R1, the height 608 of each R2, the width 601 of each R2, the length of R1 and R2, and the properties of the phase shift structure 606 are all design parameters of the HPSM 600 that define the pattern that the HPSM 600 imparts to the incident EUV radiation. The design parameters can be set such that the EUV radiation reflected from R1 has a desired phase difference from the EUV radiation reflected from R2. The desired phase difference can be about 180 degrees. However, other phase differences can also be used. o

[0094] The EUV radiation patterned by reflection from the HPSM 600 has a spatial image similar to the spatial image shown in Figure 3B. The spatial image includes dark lines at each boundary between R1 and R2. The critical dimension CD of the features printed on the substrate depends on the width of the dark lines. The width 603 of R1 and the width 601 of R2 define the separation distance between the dark lines in the spatial image. Thus, the pitch of the features printed on the substrate depends on the width 603 of R1 and the width 601 of R2. Due to demagnification by the optics of the lithography system, the image properties at the HPSM 600 can be about 4 times the image properties at the substrate W. It is the image properties at the substrate W that define the actual printed features. Thus, when the width 308 of the dark lines is 56 nm, the CD of the printed features on the substrate can be 14 nm.

[0095] ​The problem is that the CD of printed features on a substrate is highly dependent on the pitch of the printed features on the substrate. When the width 603 of R1 and the width 601 of R2 at the HPSM 600 are both about 112 nm, the width of the dark line at the HPSM 600 that defines the CD can be about 56 nm. However, when the width 603 of R1 and the width 601 of R2 at the HPSM 600 are increased to about 200 nm, the width of the dark line at the HPSM 600 can increase to about 88 nm and thus increase the CD of the printed features. This variation of CD with pitch cannot be prevented by changing the design parameters of the known HPSM 600. Thus, there is no design freedom that allows to compensate for the variation of CD.

[0096] A third embodiment provides a new HPSM for EUV radiation that improves the known HPSM 600 described above.

[0097] Figure 7a A side view of a HPSM 700 with embedded phase shifters according to the third embodiment is schematically shown. Figure 7b A plan view of the HPSM 700 according to the third embodiment is schematically shown.

[0098] The HPSM 700 is a reflective mask. The upper surface of the HPSM 700 is illuminated by EUV radiation and the EUV radiation reflected from the HPSM 700 is patterned by the HPSM 700.

[0099] The HPSM 700 is similar to the known HPSM 600 described Figure 6 The known HPSM 600 described is similar in that it comprises a first reflective region R1 and a second reflective region R2. Each R1 comprises a plurality of layers 604, 704, a majority of which are not arranged above the embedded phase shift structure 702. Each R2 comprises a plurality of layers 607, 705, a majority of which are arranged above the embedded phase shift structure 702.

[0100] The HPSM 700 according to the present embodiment differs from the known HPSM 600 in that the end portions of each R1 are adapted to comprise an embedded phase shift structure 701a, 701b and the end portions of each R2 are adapted to not comprise an embedded phase shift structure.

[0101] Each R1 comprises a main region having a width 603b, the main region providing a majority of the width of R1. The main region can be substantially as described for the known HPSM 600 in Figure 6

[0102] ​At the first end of each R1, there is a first sub-region of R1 that includes a plurality of layers 704 on a first sub-resolution phase patch 701a. The first sub-resolution phase patch 701a is an embedded phase shift structure. The width 603a of the first sub-region of R1 can be much smaller than the width 603 of R1. The height 703a of the layers 704 in the first sub-region of R1 can be a combination of the height of the extending layers 704 and the height of the embedded phase shift structure 701a above the base 605 of the HPSM 700. As a result, the height of the first sub-region of R1 is greater than the height 602 of the main region of R1. The length of the first sub-region of R1 can be the same as the length of the main region of R1.

[0103] At the second end of each R1, there is a second sub-region of R1 that also includes a plurality of layers 704 on a second sub-resolution phase patch 701b. The second sub-resolution phase patch 701b is an embedded phase shift structure. The width 603c of the second sub-region of R1 can be much smaller than the width 603 of R1. The height 703b of the layers 704 in the second sub-region of R1 can be a combination of the height of the extending layers 704 and the height of the embedded phase shift structure 701b above the substrate 605 of the HPSM 700. As a result, the height of the second sub-region of R1 is greater than the height 602 of the main region of R1. The length of the second sub-region of R1 can be the same as the length of the main region of R1.

[0104] The first sub-resolution phase patch 701a can be the same as the second sub-resolution phase patch 701b. The first sub-resolution phase patch 701a and the second sub-resolution phase patch 701b can include the same type of phase shift structure 606 used in the R2 of the known HPSM 600. The height and width 603a, 603c of the first and second sub-regions of R1 can be the same as each other. The height of the first and second sub-regions of R1 can be the same as the height of the main region of each R2.

[0105] Each R2 includes a main region having a width 601b that provides most of the width of R2. The main region can be substantially as described for the known HPSM 600 in Figure 6 except with a smaller width 601b. The phase shift structure 702 can be substantially the same as the phase shift structure 606 in the HPSM 600 except with a smaller width 601b. The layers 607 can have the same height 608 and length as the layers in the HPSM 600.

[0106] At the first end of each R2, there is a first sub-region of R2 that includes a plurality of layers 705 that are not disposed on the phase shift structure 702. The width 601a of the first sub-region of R2 can be much smaller than the width 601 of R2. The height of the layers 705 in the first sub-region of R2 can be the same as the height 602 of the layers 604 in the main region of R1. Thus, the height of the first sub-region of R2 is less than the height of the main region of R2. The length of the first sub-region of R2 can be the same as the length of the main region of R2.

[0107] At the second end of each R2, there is a second sub-region of R2 that includes a plurality of layers 705 that are not disposed on the phase shift structure 702. The width 601c of the second sub-region of R2 can be much smaller than the width 601 of R2. The height of the layers 705 in the second sub-region of R2 can be the same as the height 602 of the layers 604 in the main region of R1. Thus, the height of the second sub-region of R2 is less than the height of the main region of R2. The length of the second sub-region of R2 can be the same as the length of the main region of R2.

[0108] The height and width 601a, 601c of the first and second sub-regions of R2 can be the same as each other.

[0109] Thus, in this embodiment, the HPSM 700 differs from the known HPSM 600 in the sub-regions at the ends of each R1 and R2. The width 603a, 603c, 601a, 601c of all the sub-regions can be much smaller than the width 603 of R1 and the width 601 of R2. For example, when the width 603 of R1 of the HPSM 700 is in the range of about 112 nm to 500 nm, and the width 601 of R2 of the HPSM 700 is in the range of about 112 nm to 500 nm, the width 603a, 603c, 601a, 601c of each sub-region can be in the range of 1 nm to 21 nm. Due to their narrow width, each sub-region can be a sub-resolution part of the HPSM 700. Thus, when the HPSM 700 is illuminated with EUV radiation, some of the illuminating light will exit the imaging numerical aperture NA due to the presence of the sub-regions and the different properties of the sub-regions from the main regions. The effect of the sub-regions is to effectively reduce the intensity of the printed feature. Thus, when the HPSM 700 is designed to have a small pitch, the use of the sub-regions can increase the CD, and also reduce the variation of the CD with pitch.

[0110] Advantageously, the sub-regions provide the HPSM 700 of the present embodiments with additional design freedom over the known HPSM 600, and this allows at least some of the problems of the known HPSM 600 to be overcome. In particular, when the HPSM 700 is used in a lithographic apparatus, the width and height of each sub-region, and the type of phase shift structure used in some sub-regions, are design parameters that can be tuned in order to control the properties of the printed features.

[0111] Embodiments also include many modifications and variations of the above described techniques.

[0112] In the first and second embodiments described above, the waveguide structures 410, 501 are cuboids in shape. Embodiments include alternative shapes for the waveguide structures 410, 501. For example, each waveguide structure 401, 501 can instead have a triangular cross-section along its length.

[0113] In addition to the waveguide structures 501 according to the second embodiment, the first and second embodiments also include the use of waveguide structures 401 according to the first embodiment. Thus, the waveguide structures 401, 501 can be provided on the upper surface of the raised structures 304, and also in the regions between adjacent raised structures 304.

[0114] Embodiments also include adapting other types of EUV mask to include waveguide structures. In particular, embodiments include adapting a reflective absorber based mask for EUV radiation to include waveguide structures adjacent to and / or on the absorber material. The material, height and width of each waveguide structure can be varied to change the properties of the pattern imparted to the EUV radiation by the mask.

[0115] In the third embodiment, the sub-region of the first reflective region is a sub-resolution region of the first reflective region having different reflective properties to the main region of the first reflective region. The different reflective properties can be provided by any difference between the sub-region and the main region, and are not necessarily provided by the use of embedded phase shift structures. For example, the number and / or type of layers in the sub-region can be different to the main region. Similarly, the sub-region of the second reflective region is a sub-resolution region of the second reflective region having different reflective properties to the main region of the second reflective region. The different reflective properties can be provided by any difference between the sub-region and the main region, and are not necessarily provided by the non-use of embedded phase shift structures. For example, the number and / or type of layers in the sub-region can be different to the main region, or a different type of embedded phase shift structure can be used.

[0116] Although embodiments have been described with reference to masks for EUV radiation, masks according to embodiments can optionally be used for DUV radiation.

[0117] Although embodiments have been described with reference to an EUV lithography system, embodiments can be applied to any type of lithography system, such as a DUV lithography system.

[0118] Although specific reference can be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein can have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memory, flat-panel displays such as liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The substrates referred to herein can be processed, before or after exposure, in for example, one or more of the steps of a track (a tool that typically applies a resist layer to a substrate and develops the exposed resist), a metrology tool, and / or an inspection tool. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrates can be processed more than once, for example, to create a multi-layer IC, so the term substrate as used herein can also refer to a substrate that already contains a number of processed layers.

[0119] Although specific embodiments of the application have been described above, it will be appreciated that the application can be practiced otherwise than as described. For example, individual photoresist layers can be replaced by non-photoresist layers performing the same function.

[0120] Embodiments include the following numbered clauses:

[0121] 1. A mask for patterning radiation in a lithographic apparatus, the mask comprising:

[0122] a patterning surface arranged to impart a pattern to radiation; and

[0123] a waveguide structure arranged on the patterning surface;

[0124] wherein the patterning surface comprises:

[0125] a first reflective region; and

[0126] a second reflective region arranged such that radiation reflected from the second reflective region is phase shifted relative to radiation reflected from the first reflective region.

[0127] 2. The mask of clause 1, wherein:

[0128] the first reflective region is a planar surface;

[0129] the second reflective region is a planar surface; and

[0130] the planes of the first and second reflective regions are parallel to each other and spaced apart from each other.

[0131] 3. The mask of clause 1 or 2, wherein, in use, radiation reflected from the second reflective region is phase shifted relative to radiation reflected from the first reflective region by about 180 o .

[0132] 4. The mask of any of the preceding clauses, wherein the waveguide structure defines one or more waveguides for the radiation.

[0133] 5. The mask of any of the preceding clauses, wherein the waveguide structure is arranged on the first reflective region of the patterned surface.

[0134] 6. The mask of any of the preceding clauses, wherein the waveguide structure is arranged on the second reflective region of the patterned surface.

[0135] 7. The mask of any of the preceding clauses, wherein the waveguide structure has a refractive index of 0.97 or less.

[0136] 8. The mask of any of the preceding clauses, wherein the waveguide structure comprises ruthenium and / or molybdenum.

[0137] 9. The mask of any of the preceding clauses, wherein the mask comprises a multi-layer stack of substrates; and

[0138] the first reflective region comprises more substrate layers than the second reflective region.

[0139] 10. The mask of any of the preceding clauses, wherein the mask is arranged to pattern EUV radiation in an EUV lithography apparatus.

[0140] 11. A lithographic apparatus comprising a mask according to any of the preceding clauses.

[0141] 12. A method of designing a reflective mask for patterning radiation in a lithographic apparatus, wherein the mask comprises first and second reflective regions for imparting different phase shifts on illuminating radiation, and the mask comprises waveguide structures on the first and / or second reflective regions, the method comprising:

[0142] determining a combined width of the first and second reflective regions of the mask according to a user defined pitch of features to be printed by the radiation patterned by the mask;

[0143] determining a material and location of each waveguide structure; and

[0144] determining the dimensions of each waveguide structure, the width of the first reflective region and the width of the second reflective region such that radiation patterned by the mask has a desired characteristic.

[0145] 13. The method of clause 12, wherein the desired characteristic is a desired critical dimension.

[0146] 14. The method of clause 12 or 13, wherein the mask is the mask of any of clauses 1 to 10.

[0147] 15. A method for performing lithography, the method comprising in an exposure operation projecting a beam of radiation onto a substrate through a low pressure environment;

[0148] wherein the beam of radiation is reflected from a mask located in the low pressure environment; and

[0149] the mask is the mask of any of clauses 1 to 10.

[0150] 16. The method of clause 15, wherein the beam of radiation is a beam of EUV radiation.

[0151] 17. A mask for patterning radiation in a lithographic apparatus, the mask comprising a patterned surface arranged to impart a pattern to radiation;

[0152] wherein:

[0153] the patterned surface comprises a first reflective region and a second reflective region;

[0154] the second reflective region comprises an embedded phase shift structure arranged such that radiation reflected from the second reflective region is phase shifted relative to radiation reflected from the first reflective region;

[0155] the first reflective region comprises a main region and at least one sub-region at an end of the first reflective region;

[0156] at least one sub-region of the first reflective region has different reflective properties to the main region of the first reflective region;

[0157] the second reflective region comprises a main region and at least one sub-region at an end of the second reflective region; and

[0158] at least one sub-region of the second reflective region has different reflective properties to the main region of the second reflective region.

[0159] 18. The mask of clause 17, wherein:

[0160] the first reflective region comprises a first sub-region at a first end of the first reflective region and a second sub-region at a second end of the first reflective region; and

[0161] each sub-region of the first reflective region has different reflective properties to the main region of the first reflective region.

[0162] 19. The mask of clause 17 or 18, wherein:

[0163] the second reflective region comprises a first sub-region at a first end of the second reflective region and a second sub-region at a second end of the second reflective region; and

[0164] each sub-region of the second reflective region has different reflective properties to the main region of the second reflective region.

[0165] 20. The mask of any of clauses 17 to 19, wherein the main region of the first reflective region provides a majority of the first reflective region.

[0166] 21. The mask of any of clauses 17 to 20, wherein the main region of the second reflective region provides a majority of the second reflective region.

[0167] 22. The mask of any of clauses 17 to 21, wherein each sub-region of the first reflective region comprises a phase shift structure.

[0168] 23. The mask of any of clauses 17 to 22, wherein each sub-region is a sub-resolution region of the mask.

[0169] 24. The mask of any of clauses 17 to 23, wherein the mask is arranged to pattern EUV radiation in an EUV lithographic apparatus.

[0170] 25. A lithographic apparatus comprising the mask of any of clauses 17 to 24.

[0171] 26. A method for performing lithography, the method comprising in an exposure operation causing a beam of radiation to be projected onto a substrate through a low pressure environment;

[0172] wherein the beam of radiation is reflected from a mask located in the low pressure environment; and

[0173] the mask is the mask of any of clauses 17 to 24.

[0174] 27. The method of clause 26, wherein the beam of radiation is a beam of EUV radiation.

[0175] The above description is intended to be illustrative and not restrictive. Thus, it will be apparent to those skilled in the art that modifications can be made to the described embodiments without departing from the scope of the claims set forth below.

Claims

1. A mask for patterning radiation in a lithographic apparatus, the mask comprising: a patterning surface arranged to impart a pattern to radiation; and a waveguide structure arranged on the patterning surface; wherein the patterning surface comprises: a first reflective region; and a second reflective region arranged such that radiation reflected from the second reflective region produces a phase shift relative to radiation reflected from the first reflective region.

2. The mask of claim 1, wherein: the first reflective region is a planar surface; the second reflective region is a planar surface; and the planes of the first and second reflective regions are parallel to each other and spaced apart from each other.

3. A mask according to claim 1 or 2, wherein in use, radiation reflected from the second reflective region produces a phase shift of about 180° relative to radiation reflected from the first reflective region. o .

4. The mask of any of the preceding claims, wherein the waveguide structure defines one or more waveguides for the radiation.

5. The mask of any of the preceding claims, wherein the waveguide structure is arranged on the first reflective region of the patterning surface.

6. The mask of any of the preceding claims, wherein the waveguide structure is arranged on the second reflective region of the patterning surface.

7. The mask of any of the preceding claims, wherein the waveguide structure has a refractive index of 0.97 or less.

8. The mask of any of the preceding claims, wherein the waveguide structure comprises ruthenium and / or molybdenum.

9. The mask of any of the preceding claims, wherein the mask comprises a multi-layer stack of substrates; and the first reflective region comprises more layers of the substrate than the second reflective region.

10. The mask of any of the preceding claims, wherein the mask is arranged to pattern EUV radiation in an EUV lithographic apparatus.

11. A lithographic apparatus comprising a mask according to any of the preceding claims.

12. A method of designing a reflective mask for patterning radiation in a lithographic apparatus, wherein the mask comprises a first reflective region and a second reflective region for imparting different phase shifts to impinging radiation, and the mask comprises a waveguide structure on the first and / or second reflective regions, the method comprising: determining a combined width of the first and second reflective regions of the mask according to a user-defined pitch of features to be printed by the radiation patterned by the mask; determining a material and location of each waveguide structure; and determining a size of each waveguide structure, a width of the first reflective region and a width of the second reflective region such that the radiation patterned by the mask has a desired characteristic.

13. The method of claim 12, wherein the desired characteristic is a desired critical dimension.

14. The method of claim 12 or 13, wherein the mask is according to any of claims 1 to 10.

15. A method for performing lithography, the method comprising projecting a beam of radiation through a low pressure environment onto a substrate in an exposure operation; wherein the radiation beam is reflected from a mask located in the low pressure environment; and the mask is according to any one of claims 1 to 10.

16. The method of claim 15, wherein the radiation beam is an EUV radiation beam.

17. A mask for patterning radiation in a lithographic apparatus, the mask comprising a patterned surface arranged to impart a pattern to radiation; wherein: the patterned surface comprises a first reflective region and a second reflective region; the second reflective region comprises an embedded phase shift structure arranged such that radiation reflected from the second reflective region produces a phase shift relative to radiation reflected from the first reflective region; the first reflective region comprises a main region and at least one sub-region at an end of the first reflective region; at least one sub-region of the first reflective region has different reflective properties to the main region of the first reflective region; the second reflective region comprises a main region and at least one sub-region at an end of the second reflective region; and at least one sub-region of the second reflective region has different reflective properties to the main region of the second reflective region.

18. The mask of claim 17, wherein: the first reflective region comprises a first sub-region at a first end of the first reflective region and a second sub-region at a second end of the first reflective region; and each sub-region of the first reflective region has different reflective properties to the main region of the first reflective region.

19. The mask of claim 17 or 18, wherein: the second reflective region comprises a first sub-region at a first end of the second reflective region and a second sub-region at a second end of the second reflective region; and each sub-region of the second reflective region has different reflective properties to the main region of the second reflective region.

20. The mask of any one of claims 17 to 19, wherein the main region of the first reflective region provides a majority of the first reflective region.

21. The mask of any one of claims 17 to 20, wherein the main region of the second reflective region provides a majority of the second reflective region.

22. The mask of any one of claims 17 to 21, wherein each sub-region of the first reflective region comprises a phase shift structure.

23. The mask of any one of claims 17 to 22, wherein each sub-region is a sub-resolution region of the mask.

24. The mask of any one of claims 17 to 23, wherein the mask is arranged to pattern EUV radiation in an EUV lithographic apparatus.

25. A lithographic apparatus comprising the mask of any one of claims 17 to 24.

26. A method for performing lithography, the method comprising projecting a radiation beam through a low pressure environment onto a substrate in an exposure operation; wherein the radiation beam is reflected from a mask located in the low pressure environment; and the mask is according to any one of claims 17 to 24.

27. The method of claim 26, wherein the beam of radiation is an EUV beam of radiation.

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