MASK FOR LITHOGRAPHIC APPARATUS AND METHODS OF INSPECTION

MASK FOR LITHOGRAHIC APPARATUS AND METHODS OF INSPECTION

FIELD

The present invention relates to a mask for use in a lithographic apparatus, methods of forming such a mask, a method and apparatus for inspecting the mask, and to a lithographic apparatus incorporating the mask.

This application claims the benefit of US provisional application 61/625,947, filed on April 18^th, 2012 which is incorporated herein in its entirety by reference.

BACKGROUND

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a mask (alternatively referred to as a reticle or more generally referred to by a patterning device) may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation- sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

Lithography is widely recognized as one of the key steps in the manufacture of

ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given by the

Rayleigh criterion for resolution as shown in equation (1):

CD = k *—

NA (1) where λ is the wavelength of the exposure radiation used, NA is the numerical aperture of the projection system used to print the pattern, k_\ is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k_\.

In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of about 5-20 nm, for example within the range of 13-14 nm, or example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

EUV radiation may be produced using a plasma. A radiation system for producing

EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material such as tin. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.

The EUV radiation is then directed by an optical system onto a mask that patterns the radiation beam before the patterned radiation beam is then directed by the optical system onto a resist bearing substrate, which resist is exposed by the patterned radiation beam.

EUV radiation is generally strongly absorbed by condensed matter. For this reason a reflective mask is commonly used for EUV lithography. A reflective mask surface generally contains reflective portions and absorbing portions arranged in a pattern. When light of a selected wavelength is applied to the mask, the light is reflected off only the patterned reflective portions.

Conventionally, the mask is formed from an ultra-low expansion substrate on which is provided a multilayer mirror (typically alternating layers of Mo and Si). A patterned absorber material, for example TaN or TaBN though other materials are also possible, is then deposited on the multilayer mirror with a desired pattern that is to be printed onto the resist on the wafer by the EUV radiation.

[0010] A difficulty with such conventional prior art is that inspecting the mask for particle contamination is extremely challenging because of the difficulty in distinguishing between contaminant particles and the absorber pattern. While light scattering techniques offer very fast inspection times and high capture rates on flat surfaces, they are ineffective on patterned surfaces. This means that it is necessary to use more time consuming and inefficient inspection techniques that rely on imaging and comparison or actinic inspection, and these techniques are expensive, slow and difficult to integrate into existing lithographic apparatus. These techniques are also more prone to false positives and missed particles.

SUMMARY OF THE INVENTION

[0011] According to an aspect of the invention there is provided a mask for use in a lithographic apparatus, said mask comprising:

a substrate,

a first reflective multilayer structure deposited on said substrate, said first reflective multilayer structure having a top surface

a pattern formed in said first reflective multilayer structure, said pattern having a depth and a top surface, the top surface of said pattern being generally at the same height from said substrate as the top surface of said first reflective multilayer structure, and

a second structure arranged above the top surface of the first reflective structure and the top surface of the pattern, wherein the second structure has a planar top surface and wherein the second structure is chosen so as to minimize the transmission through the second structure of light of an given inspection wavelength.

Preferably the second structure is chosen so as to minimize the transmission through the second structure at a given angle of incidence and a given polarization of the light of the inspection wavelength.

[0012] Preferably the second structure is a multi-layer structure, which may comprise sub-layers of molybdenum, ruthenium and silicon. Preferably the pattern comprises absorber material (preferably a material that absorbs EUV radiation in the range of from 5nm to 20nm) deposited in said first reflective multilayer structure. Alternatively the pattern may comprise areas of local deformation of said first reflective multilayer structure.

Preferably the first reflective multilayer structure comprises alternating layers of molybdenum and silicon.

According to another aspect of the present invention there is provided a method of forming a mask comprising the steps of:

(a) depositing a first reflective multilayer structure on a substrate, said first refelective multilayer structure having a top surface,

(b) forming a pattern in said first reflective multilayer structure, said pattern having a depth and a top surface and the top surface of said pattern being formed to be generally at the same height from said substrate as the top surface of said reflective multilayer structure, and

(c) depositing a second structure on the top surface of the first reflective multilayer structure and the top surface of the pattern, such that the second structure has a planar top surface and wherein the second structure is chosen so as to minimize the transmission through the second structure of light of a given inspection wavelength.

Preferably the second structure is chosen so as to minimize the transmission through the second structure at a given angle of incidence and a given polarization of the light of the inspection wavelength.

[0016] Preferably the second structure is deposited as a multilayer structure.

Preferably the method comprises forming said pattern of absorber material.

Preferably the absorber material is a material that absorbs EUV radiation in the range of from 5nm to 20nm.

In one embodiment of the invention the method comprises partially forming said first reflective multilayer structure, depositing said absorber material on said partially formed first reflective multilayer structure, depositing a sacrificial layer on said absorber material, depositing further layers of said partially formed first reflective multilayer structure to complete said first reflective multilayer structure, and removing said sacrificial layer and any layers deposited thereon. In another embodiment of the invention the method comprises etching said first reflective multilayer structure to define a pattern of trenches, and depositing said absorber material in said trenches.

In another embodiment of the invention the method comprises forming said pattern by locally deforming said first reflective multilayer structure.

According to another aspect of the present invention there is provided apparatus for inspecting a mask of a lithographic apparatus, said mask comprising a planar top surface, said apparatus comprising means for directing light onto said planar surface of said mask, and means for detecting light scattered from said planar surface, wherein the apparatus is configured to direct light of an inspection wavelength that will have minimum transmission through said planar top surface.

Preferably the apparatus further comprises means for directing the light of the inspection wavelength onto said planar surface of said mask at an angle of incidence, and means for controlling the polarization of the light of the inspection wavelength, and is the apparatus configured to direct the light of the inspection wavelength at an angle of incidence and with an polarization that will have minimum transmission through said planar top surface.

According a still further aspect of the present invention there is provided a method for inspecting a mask, said method comprising directing light at a planar top surface of said mask and detecting light scattered from said mask, wherein at least one of the wavelength of the light, the polarization of the light, and the angle of incidence of the light directed on said planar top surface are chosen to give minimum transmission of said light through said planar top surface of said mask.

According to a still further aspect of the present invention there is provided lithographic apparatus comprising: a radiation source for generating a beam of radiation at an exposure wavelength, a support and means for holding a mask having a planar top surface, and an optical system for directing said beam of radiation onto the planar top surface of the mask, and mask inspection means comprising means for directing light onto said planar top surface of said mask, and means for detecting light scattered from said planar top surface, wherein the mask inspection means is configured to direct light of an inspection wavelength that will have minimum transmission through said planar top surface. Preferably the mask inspection means in the lithographic apparatus further comprises means for directing the light of the inspection wavelength onto said planar surface of said mask at an angle of incidence, and means for controlling the polarization of the light of the inspection wavelength, and wherein the mask inspection means is configured to direct the light of the inspection wavelength at an angle of incidence and with an polarization that will have minimum transmission through said planar top surface.

[0025] Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] 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:

[0027] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention;

[0028] Figure 2 shows the apparatus of Figure 1 in more detail;

Figures 3(a) and 3(b) schematically depict two embodiments of the invention;

Figures 4(a)-(e) schematically depict a method of forming a mask according to an embodiment of the invention;

[0031] Figures 5(a)-(f) schematically depict an alternative method of forming a mask according to an embodiment of the invention;

[0032] Figures 6(a)-(d) schematically depict an alternative method of forming a mask according to an embodiment of the invention;

[0033] Figures 7(a)-(d) schematically depict an alternative method of forming a mask according to an embodiment of the invention; and

[0034] Figure 8 is a plot illustrating an example of a relationship between wavelength of the inspection light and transmittance for an embodiment of the invention.

[0035] The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.

DETAILED DESCRIPTION

[0036] This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the present invention. The scope of the present invention is not limited to the disclosed embodiment(s). The present invention is defined by the claims appended hereto.

The embodiment(s) described, and references in the specification to "one embodiment," "an embodiment," "an example embodiment," etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

Figure 1 schematically depicts a lithographic apparatus 100 according to an embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a exposure radiation beam B (e.g., EUV radiation); a support structure (e.g., a mask table) MT constructed to support a mask MA and connected to a first positioner PM configured to accurately position the mask; a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and a projection system (e.g., a reflective projection system) PS configured to project a pattern imparted to the exposure radiation beam B by mask MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

The illumination system 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 radiation. The support structure MT holds the mask MA in a manner that depends on the orientation of the mask, the design of the lithographic apparatus, and other conditions, such as for example whether or not mask is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the mask. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the mask is at a desired position, for example with respect to the projection system.

The term mask should be broadly interpreted as referring to any device that can be used to impart an exposure radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the exposure radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

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. The projection system, like the illumination system, may 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 beam being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps. Some gas may be provided in some parts of the lithographic apparatus, for example to allow gas flow to be used to reduce the likelihood of contamination reaching optical components of the lithographic apparatus.

As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask).

Referring to Figure 1, the illuminator IL receives an extreme ultra violet (EUV) exposure radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP"), the desired plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in Figure 1 , for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a C0₂ laser is used to provide the laser beam for fuel excitation.

The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the exposure 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 illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator may be used to condition the exposure radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

The exposure radiation beam B is incident on the mask MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the mask. After being reflected from the mask MA, the exposure radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the exposure radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the mask MA with respect to the path of the exposure radiation beam B. Mask MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

Figure 2 shows the apparatus 100 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 220 of the source collector module SO. An EUV radiation emitting plasma 210 may be formed by a discharge produced plasma (DPP) source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the very hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. As will be discussed in more detail below in the case of a laser produced plasma (LPP) source the very hot plasma 210 is created by configuring laser LA to emit a beam of laser radiation 205 that is focused on target area 211 to which is supplied a fuel droplet, e.g., a droplet of tin (Sn). The laser generates a plasma of Sn vapour, which emits EUV radiation as is known in the art.

The source module SO further includes a radiation collector CO that collects the generated EUV radiation and focuses the EUV radiation at a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module is arranged such that the intermediate focus IF is located at or near an opening 221 in the enclosing structure 220. The virtual source point IF is an image of the radiation emitting plasma 210.

Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device 22 and a facetted pupil mirror device 24 arranged to provide a desired angular distribution of the exposure radiation beam 21, at maskMA, as well as a desired uniformity of radiation intensity at the mask MA. Upon reflection of the beam of radiation 21 at the mask 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 wafer stage or substrate table WT.

Referring to Figures 3a and 3b there are shown in cross-section two mask structures in accordance with embodiments of the invention. In both Figures 3 a and 3b the mask comprises a substrate 1 formed of an ultra-low expansion material, which may incorporate for example silica, titania, lithia and/or alumina. On top of the substrate 1 is formed a first reflective multilayer structure 2, for example formed of alternating layers of Mo and Si as is known in the art. Other materials may be used to form the pairs of thin films such as molybdenum/beryllium, niobium/beryllium, ruthenium/beryllium, rhodium/beryllium or silicon/ruthenium. It may also be possible to combine thin films of multiple materials and not just pairs of thin films. A pattern is formed on the mask either by the use of absorber material 3a (as shown in Figure 3a) or by localised deformation 3b of the first reflective multilayer structure 2 (as shown in Figure 3b) to give low or no EUV reflection. When an absorber material is used the layers are formed in such a way that the absorber material is embedded in the first reflective multilayer structure such that the top surface of the first reflective multilayer structure is generally at the same height from the substrate as the top surface of the absorber pattern. Above the top surface of the absorber material 3a, or the localised deformations 3b, and above the top surface of the first refelective multilayer structure reflective for EUV radiation, there is provided a second structure being a single layer or a plurality of layers (i.e., a multilayer) 4 leaving a planar top surface (i.e., a flat upper surface) of the mask.

The absorber material may be chosen from known absorber materials such as chromium, tungsten, tantalum, aluminium, germanium, silicon or copper as well as from oxides, borides, nitrides and/or silicides of these materials.

The planar top surface of the second structure represented by the layers 4 in

Figures 3 s and 3b, permit the mask to be inspected by light scattering techniques as there is no longer any danger of confusion between contaminant particles on the mask and absorber material below layers 4. For optimum performance the materials for the second structure, the wavelength of the light used for inspection, and the angle of incidence and polarization of the inspection light are preferably chosen so as to minimise transmission of the inspection light through the second structure 4. In such a manner it is possible to maximize reflection such that the inspection light does not penetrate the second structure (e.g., overgrown layers 4) to a depth such that the inspection light reaches the absorber material (or localised deformation) and therefore does no suffer sub-surface scattering from the buried absorber material (or localised deformation).

The layer(s) forming the second structure having a planar top surface are made of materials that have high reflectance for the inspection wavelength (typically in a range from 193nm to 400nm) , especially so at shallow angles (typically in a range of from 70° to 89° relative to a line perpendicular to the planar top surface). At large angles of incidence of more than 70°, preferably more than 80°, these layers provide a large reflectance of the inspection wavelength. Preferably the calculated grazing incidence transmittance is less than 0.01, more preferably less than 0.001, even more preferably less than 0.0001 at angles of more than 70°, more preferably at angles of more than 80°, and most preferably at angles of more than 85°. The grazing incidence transmittance can be determined from the known complex index of refraction of the materials using the Fresnel equations and taking into account the multilayer structure by, for example, using a transfer matrix method. The thickness of the top layer is preferably a few nanometers, in the range for example of 4 to 25nm.

The mask design of embodiments of the present invention is amenable to optical inspection for particle contamination. Because the surface is substantially flat (e.g., with a root mean square roughness below lnm) it can easily be inspected for defects and particles when compared to conventional reflective masks having the absorber pattern on the top of the reflective multilayer. Furthermore, because the incident inspection light is reflected by the planar top layer(s), the shadowing effect present for conventional masks having the pattern on the top of the reflective multilayers is in this case non-existent.

Inspection of EUV masks requires an inspection apparatus having a high magnification and a large field of view at the image plane. The inspection apparatus for inspecting masks according to the present invention may include elements such as a light source directed to an inspected surface having a specific inspection wavelengths. Furthermore the inspection apparatus may comprise optics or other means for directing the inspection light onto the inspected surface at a given angle of incidence, means for controlling the polarization of the inspection light. The inspection apparatus further includes a detector for detecting light scattered from the inspected surface, and an optic configuration for directing the light from the inspected surface to the detector. In addition, the apparatus for inspecting a mask being formed with a planar surface may be configured to direct light of a wavelength and polarisation that will have a minimum transmission through the second structure of the mask at the given incidence angle. In particular, the detector can include a plurality of sensor modules. Inspection of masks with such an inspection apparatus may be done inside or outside of a lithographic apparatus.

The same elements as used for the apparatus for inspecting a mask according to embodiments of the invention may also be incorporated in a lithographic apparatus as to provide the lithographic apparatus with a built-in inspection apparatus where for instance in-situ analysis of the mask is envisaged. The lithographic apparatus comprises a support and means to fix and release the mask according to the invention, such that the mask may be loaded and unloaded from the apparatus; and the apparatus is configured such as to direct an inspection light of an inspection wavelength that will have a minimum transmission through the second structure of the mask. In practice the location of the inspection apparatus within the lithographic apparatus may be such that control of the angle of incidence may be limited and it may therefore be advantageous to control the multilayer overgrown layers forming the second structure and the inspection wavelength of the inspection light to give a minimum transmission through the second structure at a particular angle of incidence. This is illustrated in Figure 8 that shows a plot of transmittance T against wavelength λ for an overgrown layer structure comprising a 2nm thick layer of Ru, a l lnm thick layer of Si and a 2.76nm thick layer of Mo with a fixed angle of incidence of 88°. As can be seen from the plot forming Figure 8 with this combination the transmittance has a minimum at about 288nm and this is therefore the inspection wavelength that ideally should be chosen for the inspection light, though constraints in the light sources available may dictate that a non-optimum inspection wavelength needs to be chosen at the expense of higher inspection light transmission through the second structure.

Figure 4 illustrates a first possible method for forming a mask in accordance with an embodiment of the invention. A substrate 1 is provided of an ultra-low expansion material. As shown in Figure 4(a) deposited on the substrate 1 is a first reflective multilayer structure 2, which may - for example - be formed of alternating layers of molybdenum (Mo) and silicon (Si). The first reflective multilayer structure 2 is deposited in a conventional manner but to a height that is 40nm to lOOnm less than would be normal in a conventional prior art mask. Referring to Figure 4(b) a pattern of absorber material 3 is then deposited on the first refelective multilayer structure 2 and before lift off of the absorber material 3 a sacrificial layer 5 is deposited. At this point in the process pattern repair can be performed if necessary.

Once the pattern of absorber material has been deposited, and any necessary repairs to the pattern have been made, further layers of the first refelective multilayer structure 2 are grown until the height of the first reflective multilayer structure 2 matches as closely as possible the top of the absorber material 3 (excluding the sacrificial layer 6). This is shown in Figure 4(c). The sacrificial layer 5 - together with the multilayer structure 2 that will in the previous step have been deposited on the sacrificial layer 5 - is then removed (Figure 4(d)).

Finally, as shown in Figure 4(e) the overgrown top layers 4 forming the second structure are deposited. Alternatively the second structure may be fabricated in any other suitable manner so long as a smooth top surface may be provided. Excessive overgrowth followed by planarization may be performed to guarantee flatness of the top layer. Planarization may be performed, for example, by chemical-mechanical planarization or by ion beam polishing. Once a suitable roughness level has been achieved by planarization, extra layers may be grown to obtain a final flat surface according to required specifications. No planarization may be necessary if the height mismatch between the top of the absorber material 3a and the top of the reflective multilayer structure 2 is small enough (e.g., less than about lnm).

Figures 5(a)-(f) illustrate an alternative method for forming a mask according to an embodiment of the invention. Beginning with Figure 5(a) a first reflective multilayer structure 2 is deposited on an ultra- low expansion substrate 1. In the embodiment of Figure 5, however, unlike that of Figure 4, the first reflective multilayer structure 2 is deposited to the normal height that would be expected in the prior art. A sacrificial masking layer 5 defining the negative image of the intended absorber pattern 3 is then formed on the top surface of the first reflective multilayer structure 2 (Figure 5(b)) and at this stage pattern repair may be carried out if necessary on the sacrificial masking layer 5. An etching process is then carried out (Figure 5(c)) in which portions of the first reflective multilayer structure 2 not protected by the sacrificial masking layer 5 are etched to produce trenches of a predetermined depth. The etching process may - for example - comprise reactive ion etching. Any errors produced by particles masking the etchant can be corrected at this stage.

Referring now to Figure 5(d) the trenches are filled by depositing absorber material 3 to a depth that matches the depth of the trenches such that the top of the absorber material 3 deposited in the trenches is at the same height as the top of the first reflective multilayer structure 2. During the step shown in Figure 5(d) absorber material will also be deposited on the sacrificial masking layer 6 and as shown in Figure 5(e) this unwanted additional absorber material is removed along with the sacrificial masking layer 5. Finally, as shown in Figure 5(f) the overgrown top layers 4 forming the second structure may be deposited and subject to planarization as described with reference to Figure 4(e). It should be noted here that other methods for depositing the layers 4 are possible.

Figures 6(a)-7(d) show a method of manufacturing a mask according to an embodiment of the invention using an etch stop layer. In this embodiment as shown in Figure 6(a) the first reflective multilayer structure 2 is deposited on an ultra-low expansion substrate 1 in a conventional manner except that at a desired location within the first reflective multilayer structure there is deposited an etch stop layer 7 formed, for example, of ruthenium. A resist 5 is then deposited on the top surface of the first refelective multilayer structure 2 in a negative image of the desired absorber pattern. The first reflective multilayer structure is then etched down to the etch stop layer 7 leaving only those portions of the first refelective multilayer structure 2 above the etch stop layer 7 that are protected by the resist 5 (Figure 6(b)) and which define trenches down to a depth limited by the etch stop layer 7. A layer of absorber material 3 is then deposited which fills the trenches formed in the previous step to a depth such that the top of the absorber material 3 is at the same height as the top of the first reflective multilayer structure 2 as well as being deposited on resist 5 (Figure 6(c)). The resist 5 (and the absorber material 3 deposited on top of the resist material 5) is then removed by a lift-off process leaving the structure shown in Figure 6(d) in which an absorber material 3 is deposited in patterned trenches formed in the first reflective multi-layer structure 2 down to a depth defined by the etch stop layer 7 with the top surfaces of the absorber material being in the same plane as the top surface of the first reflective multilayer structure. A second structure of the type previously disclosed is then deposited on top of the first reflective multilayer structure 2 with embedded absorber material 3.

Figures 7(a)-7(d) show a further method of manufacturing a mask according to an embodiment of the invention using two etch stop layers. As shown in Figure 7(a) a first reflective multilayer structure 2 is formed on an ultra-low expansion substrate 1 and a first etch stop layer 7' is formed at a desired location within the first reflective multilayer structure 2 (e.g., at a depth of 50-70nm). A second etch stop layer 7" is then deposited on the top surface of the first reflective multilayer structure 2. Both etch stop layers 7' and 7" may be formed of ruthenium. As is further shown in Figure 7(a) a patterned sacrificial layer such as a resist layer 5 is deposited on a top surface of the second etch stop layer 7' ' as a negative of a desired absorber pattern. An etch is then performed that etches through the first reflective multilayer structure 2 down to the first etch stop layer 7' except for those regions protected by the resist layer 5. The resist layer 5 is then removed (Figure 7(b)). An absorber material 3 is then deposited over the whole surface so that it occupies the trenches formed in the first reflective multilayer structure 2 by the previous etch and covers the regions of the first reflective multilayer structure 2 previously covered by resist 5 (Figure 7(c)). Planarisation may be carried out at this point. The absorber material 3 is then etched until the etch reaches the second etch stop 7" at which point the absorber material 3 is only present embedded in the trenches formed in the top surface of the first reflective multilayer structure 2 (Figure 7(d)). A second structure, such as overgrown layers 4, is then deposited as previously described.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled person 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), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multilayer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, or example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.

While the terms "inspection light" and the like have been used in the above description, it will be understood that non-visible electromagnetic radiation could also be used for the inspection and the term light should therefore be understood broadly.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practised otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.