NL2013557A - Membranes for use within a lithographic apparatus and a lithographic apparatus comprising such a membrane. - Google Patents

Description

MEMBRANES FOR USE WITHIN A LITHOGRAPHIC APPARATUS AND A LITHOGRAPHIC APPARATUS COMPRISING SUCH A MEMBRANE

Field

[0001] The present invention relates to membranes for use within a lithographic apparatus, and more specifically to EUV transmissive membranes which can form part of pellicle or optical filter components within the apparatus, and a lithographic apparatus comprising such a membrane.

Background

[0002] 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 patterning device, which is alternatively referred to as a mask or a reticle, 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.

[0003] 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.

[0004] A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):

(1) where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, kl 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 kl.

[0005] 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 5-20 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 extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring or based on free electron laser.

[0006] Extreme ultraviolet 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 apparatus 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 (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. 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 apparatus 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.

[0007] One application of an EUV radiation source is in lithography. 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 patterning device, which is alternatively referred to as a mask or a reticle, may he 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.

[0008] In order to reduce the minimum printable size, imaging may be performed using radiation having a short wavelength. It has therefore been proposed to use an EUV radiation source providing EUV radiation within the range of 13-14 nm, for example. 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-fO nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation.

[0009] Thin transmissive EUV membranes are often required in EUV lithographic apparatus for a number of reasons. One such reason may be to protect, for example, reticles and/or lithographic components from contamination by particles (with a grain size ranging from nm to qm). Another reason may be to spectrally filter out unwanted wavelengths from the generated EUV radiation.

[0010] These transmissive EUV membranes are required to be highly transparent to EUV radiation, and therefore need to be extremely thin. Typical EUV membranes have a thickness of 10 to 100 nm, to minimize absorption of EUV radiation.

[0011] EUV membranes may comprise a free-suspended membrane comprising a material such as polysilicon (poly-Si), produced by etching of a silicon wafer. EUV membranes may also comprise one or more layers of protective coatings on one or both surfaces to prevent hydrogen (H, H+, H2+and/or H3+) EUV-induced plasma etching.

[0012] Although absorption of EUV radiation by EUV membranes is low, it is not zero and absorption of residual EUV radiation results in an increase in temperature of the EUV membrane. Should the temperature of an EUV membrane exceed a damage threshold (for example, about 500-700° C), damage to the EUV membrane may occur. Damage can also occur, or be exacerbated, when there are large temperature gradients within the EUV membrane. Where such damage is severe, the EUV membrane may break, leading to damage/contamination of an unprotected reticle, or photoresist exposure to undesired non-EUV wavelength radiation, leading to a significant manufacturing process downtime.

[0013] It is apparent that maintaining the temperature of the EUV membrane below the damage threshold, as well as minimizing temperature gradients, can increase the EUV membrane lifetime.

SUMMARY

[0014] It is desirable to improve the thermal characteristics of EUV membranes, such as improved cooling and/or minimization of temperature gradients within the EUV membranes.

[0015] In a first aspect of the invention there is provided a membrane transmissive to EUV radiation, comprising: one or more high doped regions where said membrane is doped with a high dopant concentration, and one or more low doped regions where said membrane has no doping or a low dopant concentration; wherein a high dopant concentration is defined as dopant concentration greater than 101 2 cm"3 and a low dopant concentration is defined as a dopant concentration less than 101 cm"3.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0016] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. Embodiments of the invention are described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 depicts schematically a lithographic apparatus having reflective projection optics;

Figure 2 is a more detailed view of the apparatus of Figure 1;

Figure 3 illustrates an EUV membrane according to a first embodiment of the invention being used as a pellicle for a reticle;

Figure 4 illustrates an EUV membrane according to a second embodiment of the invention;

Figure 5 illustrates an EUV membrane according to a third embodiment of the invention;

Figure 6 illustrates an EUV membrane according to a fourth embodiment of the invention;

Figure 7 is a graph of expected temperature distribution against distance L across an EUV membrane, for a flat E4UV membrane and for the EUV membrane depicted in Figure 6; and

Figure 8 illustrates an EUV membrane according to a fifth embodiment of the invention.

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.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

2

Figure 1 schematically depicts a lithographic apparatus 100 including a source module SO according to one embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. EUV radiation).

a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device; 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 radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0019] 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.

[0020] 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, such as for example whether or not the patterning device is held in a vacuum environment. The support stmcture can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. 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 patterning device is at a desired position, for example with respect to the projection system.

[0021] The term “patterning device” should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0022] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilled so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0023] 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 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 other 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.

[0024] As here depicted, the apparatus is of a reflective type (e.g. employing a reflective mask).

[0025] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0026] Referring to Figure 1, the illuminator IL receives an extreme ultra violet radiation beam from the source 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 required 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 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 module. The laser and the source module may be separate entities, for example when a C02 laser is used to provide the laser beam for fuel excitation.

[0027] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source module with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0028] The illuminator IL may comprise 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 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 radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0029] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (e.g. mask) MA, the 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 radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B. Patterning device (e.g. mask) MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0030] An EUV membrane, for example a pellicle PE, is provided to prevent contamination of the patterning device from particles within the system. Such pellicles may be provided at the location shown and/or at other locations. A further EUV membrane SPF may be provided as a spectral purity filter, operable to filter out unwanted radiation wavelengths (for example DUV). Such unwanted wavelengths can affect the photoresist on wafer W in an undesirable manner. The SPF may also optionally help prevent contamination of the projection optics within projection system PS from particles released during outgassing (or alternatively a pellicle may be provided in place of the SPF to do this). Either of these EUV membranes may comprise any of the EUV membranes disclosed herein.

[0031] The depicted apparatus could be used in at least one of the following modes: 1. In step mode, the support structure (e.g. mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. In scan mode, the support structure (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support stmcture (e.g. mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0032] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0033] Figure 2 shows an embodiment of the lithographic apparatus in more detail, including a radiation system 42, the illumination system IL, and the projection system PS. The radiation system 42 as shown in Figure 2 is of the type that uses a laser-produced plasma as a radiation source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma is created by causing an at least partially ionized plasma by, for example, optical excitation using CCF laser light. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. In an embodiment, Sn is used to create the plasma in order to emit the radiation in the EUV range.

[0034] The radiation system 42 embodies the function of source SO in the apparatus of Figure 1. Radiation system 42 comprises a source chamber 47, in this embodiment not only substantially enclosing a source of EUV radiation, but also collector 50 which, in the example of Figure 2, is a normal-incidence collector, for instance a multi-layer mirror.

[0035] As part of an LPP radiation source, a laser system 61 is constructed and arranged to provide a laser beam 63 which is delivered by a beam delivering system 65 through an aperture 67 provided in the collector 50. Also, the radiation system includes a target material 69, such as Sn or Xe, which is supplied by target material supply 71. The beam delivering system 65, in this embodiment, is arranged to establish a beam path focused substantially upon a desired plasma formation position 73.

[0036] In operation, the target material 69, which may also be referred to as fuel, is supplied by the target material supply 71 in the form of droplets. A trap 72 is provided on the opposite side of the source chamber 47, to capture fuel that is not, for whatever reason, turned into plasma. When such a droplet of the target material 69 reaches the plasma formation position 73, the laser beam 63 impinges on the droplet and an EUV radiation-emitting plasma forms inside the source chamber 47. In the case of a pulsed laser, this involves timing the pulse of laser radiation to coincide with the passage of the droplet through the position 73. As mentioned, the fuel may be for example xenon (Xe), tin (Sn) or lithium (Li). These create a highly ionized plasma with electron temperatures of several 105 K. Higher energy EUV radiation may be generated with other fuel materials, for example Tb and Gd. The energetic radiation generated during de-excitation and recombination of these ions includes the wanted EUV which is emitted from the plasma at position 73. The plasma formation position 73 and the aperture 52 are located at first and second focal points of collector 50, respectively and the EUV radiation is focused by the normal-incidence collector mirror 50 onto the intermediate focus point IF.

[0037] The beam of radiation emanating from the source chamber 47 traverses the illumination system IL via so-called normal incidence reflectors 53, 54, as indicated in Figure 2 by the radiation beam 56. The normal incidence reflectors direct the beam 56, via pellicle PE, onto a patterning device (e.g. reticle or mask) positioned on a support (e.g. reticle or mask table) MT. A patterned beam 57 is formed, which is imaged by projection system PS via reflective elements 58, 59 onto a substrate carried by wafer stage or substrate table WT. More elements than shown may generally be present in illumination system IL and projection system PS. For example there may be one, two, three, four or even more reflective elements present than the two elements 58 and 59 shown in Figure 2. Radiation collectors similar to radiation collector 50 are known from the prior art.

[0038] As the skilled reader will know, reference axes X, Y and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various components, and the radiation beams 55, 56, 57. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. The Z axis broadly coincides with the direction of optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source module (apparatus) 42, the X axis coincides broadly with the direction of fuel stream (69, described below), while the Y axis is orthogonal to that, pointing out of the page as indicated. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X

axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram Figure 2, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0039] In addition to the wanted EUV radiation, the plasma produces other wavelengths of radiation, for example in the visible, UV and DUV range. There is also IR (infrared) radiation present from the laser beam 63. The non-EUV wavelengths are not wanted in the illumination system IL and projection system PS and various measures may be deployed to block the non-EUV radiation. As schematically depicted in Figure 2, an EUV membrane filter in the form of a spectral purity filter SPF may be applied upstream of the virtual source point IF, for IR, DUV and/or other unwanted wavelengths. In the specific example shown in Figure 2, two spectral purity filters are depicted, one within the source chamber 47 and one at the output of the projection system PS. In a practical embodiment it is likely that only one spectral purity filter SPF is provided, which may be in either of these locations or elsewhere between the plasma formation position 73 and wafer W.

[0040] Disclosed is an EUV membrane for transmission of EUV radiation, having improved thermal characteristics compared to present EUV membranes. Such EUV membranes may comprise, for example poly-Si EUV membranes. The membranes may be comprised within a spectral purity filter (SPF) or a pellicle. SPFs and/or pellicles may be provided at many locations within a lithographic system, as already described.

[0041] In absorbing radiation during use, the EUV membranes heat up. Should their temperature increase too high or the temperature gradients within the membrane be too great, the EUV membranes can be damaged. Therefore it is desirable to minimize temperature and temperature gradients within the EUV membrane. As the EUV membranes will be used in very low pressure (vacuum) environments, the only means of cooling is radiation. It is therefore desirable to increase thermal emissivity of the EUV membrane for the wavelengths (for example 1 to 10 pm) at which most energy is radiated when the temperature of the EUV membrane ranges from few hundred to about 1000 ° C, and more specifically at moderate temperatures (less than 500 ° C). In these conditions pure silicon (Si) material presents a low thermal emissivity, since all free carriers are still bound.

[0042] To increase emissivity in an EUV membrane comprising a semiconductor material, the EUV membrane material may be doped to increase the number of free carriers within the material. This increases the radiation absorption coefficient of the doped membrane, which leads to an increase in the emissivity. The skilled reader will know that doping of semiconductor materials with donors and/or acceptors modifies the free carrier concentration (electrons and/or holes) at moderate temperatures.

[0043] The concentration of impurity to be doped into the semiconductor membrane should be higher than 1017 cm"3 for a significant effect. Concentrations may preferably be higher than 1018cm"3, 1019cm"3 or 1020cm"3. It can be shown that absorption coefficients can increase by a factor of 1000 at wavelengths greater than 1.2pm when the dopant concentration is increased from 1017cm"3 to 1020cm"3 This applies equally to doping with p-dopants and n-dopants.

[0044] However, adding dopants tends to reduce the strength of semiconductor material such as polysilicon. This is particularly a problem from EUV membranes due to their need to be particularly thin in order to transmit the EUV radiation with the minimum amount of loss. Consequently a number of solutions are proposed to address this.

[0045] Figure 3 is a schematic diagram of a EUV membrane 300 which is positioned in front of the patterned area of a reticle MA. EUV membrane 300 is shown here as forming part of a pellicle designed to keep particles D off the patterned area of reticle MA, while allowing transmission of EUV radiation beam 305. In such an example EUV membrane 300 may comprise an EUV membrane within a pellicle frame (not shown). The EUV membrane 300 may further comprise (for example) securing elements for attaching the pellicle to the reticle (not shown). EUV membrane 300 may be placed out of the focal plane, at some distance from reticle MA, such that contaminants are not imaged onto the wafer.

[0046] In other embodiments, EUV membrane may form part of a pellicle for use in another location within a lithographic apparatus, or an SPF.

[0047] The EUV membrane 300 may comprise a number of layers. These layers may include the main substrate layer 310, cover layers 311, 312, and intermediate layers 313, 314. The main substrate layer 310 may be, for example, a poly-Si layer. This arrangement is shown by way of example only, and other combinations of the layers shown are possible. For example, the EUV membrane 300 may comprise cover layers 311, 312 without any intermediate layers. In another exemplary alternative, there may be only one cover layer on just one surface of the main substrate layer (with or without an intermediate layer between cover layer and substrate layer). There could also be more than two layers on one or both surfaces of the main substrate layer.

[0048] Typically, cover layers 311, 312 are made of an inert material to resist any etching or reacting agents that can harm the main substrate layer 310, e.g., O and H radicals, H2 and EUV. Examples of such a material include M0S12, S13N4, C3N4, ZrN, SiC. Such materials typically have a wide forbidden energy zone and are similar in properties to ceramics. Consequently, such materials have high emissivity even at moderate temperatures, for example less than 500 C. Moreover these materials are produced from elements with low absorption of EUV, which is comparable with pure Si absorption. Therefore, provided that the cover layers 311, 312 have a much smaller thickness than main substrate layer 310, they do not significantly increase overall EUV absorption of EUV membrane 300. The cover layers 311, 312 should also not place too great a stress on the main substrate layer 310, so as to preserve its mechanical properties.

[0049] Intermediate layers 313, 314 may be provided to reduce the stress. For example intermediate layers 313, 314 may comprise material having an intermediate lattice size between the main substrate layer 310 and cover layer 311, 312. Intermediate layers 313, 314, like the cover layers 311, 312, should be highly transparent to EUV.

[0050] In an embodiment, the covers layers 311, 312, and/or the intermediate layers 313, 314 (if present) may be doped to increase the concentration of free carriers, as already described. In this way the covers layers 311, 312, and/or the intermediate layers 313, 314 form high doped regions within the membrane. The main substrate layer 310 may be formed as a low doped region to maintain strength. The doping of one or more of the other layers 311, 312, 313, 314 significantly increases emissivity of the EUV membrane 300 as already described.

[0051] High doped regions have a dopant concentration of at least 1017 cm 3, while low doped regions have a dopant concentration less than 1017 cm"3 Doping levels of the high doped regions may be any of those described above, in relation to the doping of the semiconductor membrane, and as such may be higher than 10 cm" , higher than 10 cm" or higher than 102()cm"a for example. Doping levels of low doped regions, such as the main substrate layer, may be less than 1016 cm"3, less than 1016 cm"3, or less than 1014 cm'3, for example. Low doped regions may be undoped and therefore have no (intentional) added dopants.

[0052] Figure 4 shows an alternative embodiment showing EUV membrane 400 having the same layer structure as EUV membrane 300, but also comprising additional cover layers 411, 412 placed on cover layers 311, 312, as shown in Figure 4. These additional cover layers 411, 412 may be high doped regions instead of (or in addition to) the cover layers 311, 312. The doping concentrations of the additional cover layers 411, 412 may be any of those mentioned in the previous paragraph.

[0053] By doping only the cover layers 311, 312, 411, 412 or intermediate layers 313, 314, rather than the main substrate layer 310, the weakening effects of the doping are mitigated and the overall EUV membrane 300 is stronger as a result.

[0054] Figure 5 illustrates another embodiment. It shows an EUV membrane 500, which may comprise only a single main substrate layer, or alternatively may comprise cover/intermediate layers, such as layers 311, 312, 313, 314 and possibly also layers 411, 412. In this embodiment, one or more of: the main substrate layer, and (where present) the cover/intermediate layers comprises doping (which may be at the concentrations already described), but where the high doped regions is limited to only a central region 510 of the layer doped. The periphery 520 of this doped layer is a low doped region, where it may be held by a frame. This increases the strength of the EUV membrane 500 at its periphery, which is subject to greater stresses due to holding by the frame. It should be appreciated that the peripheral area 520 transmits little or no EUV, as this is mostly or completely transmitted through the central region 510. Consequently the peripheral area 520 is subject to little heating and its thermal characteristics are less important.

[0055] Optionally, the doping can be graded, such that doping increases towards the center. In such arrangements, the gradient may occur over the full radius of the EUV membrane, or layer thereof (i.e. doping starts at the membrane edge and increases towards the center). Alternatively doping may only begin at the edge of the central region 510 and increase towards the center, with the peripheral region 520 having no doping. Or the doping grading may occur for only an intermediate section between a peripheral region having no doping and a central region having high doping.

[0056] Using a similar principle to that described in the previous paragraph, doping can be introduced to any layer in the form of spot doping. Spot doping comprises a plurality of high doped (high emissivity) regions, separated by regions of no or low doping (and therefore greater strength). Again, this concept can apply to an EUV membrane 500, comprising only a single main substrate layer, or to EUV membrane 500 comprising additional layers, such as cover layers and/or intermediate layers, in which case the doping can be introduced to any one or more of these layers. In an example, the high doped regions may be separated from one another by approximately 1 pm to 5 pm. It should be appreciated that the heal flux to the highly doped regions is by phonons with comparable or even longer wavelengths than this. Heat is transferred by two mechanisms: radiation (photons) and heat conduction (oscillation of atoms within the lattice, phonons). When the distance between where power is deposited (undoped region) and where power is removed (high doped region) is close, the power is transferred significantly faster; close may be defined as being comparable to wavelength of a phonon with a typical energy (defined by temperature, such a wavelength is in the region of a few microns).

[0057] Of course the concepts described in the three previous paragraphs may be combined such that the spot doping is confined only to a central region 510 of an EUV membrane, or layer thereof, with no doping in the peripheral region 520. And the doping concentration may be graded such that high doped regions nearer the periphery are less highly doped than those nearer the center. This can help control thermally induced stress and the cooling rate (both of which are a function of dopant concentration). This can also help to control deformations such as wrinkles or folds being formed. When the temperature of the EUV membrane is increased, the material of which it is comprised expands. The flat plane, which is the nominal shape of an EUV membrane, cannot accommodate the expanded material, and folds or wrinkles are formed. EUV radiation absorption by the folds is higher as EUV radiation crosses the EUV membrane at an angle, and thus the effective absorption path is longer. The folds may have a transverse scale of about 10 micrometers or larger (across) and will be imaged on the wafer. Using spot doping, the typical scale of the folds is defined by the geometry and scale of high doped and low doped regions due to the combined effect of temperature profile control and mechanical properties control. Where the temperature increases, the angles of the folds in a spot-doped membrane are the same, but the transverse size is decreased and therefore such folds are no longer imaged.

[0058] Previous studies have shown that, for example, photon tunneling and surface polaritons may play a key role in near-field radiative energy transfer when separating distances between radiating objects are smaller than dominant thermal wavelengths. For example, a study by B. Liu et al, Phys. Rev. B 87, 115403, (2013), has demonstrated that near-field radiative heat transfer of some materials can exceed the blackbody radiation limit by few orders of magnitude due to energy transfer through evanescent waves. The studied material supported surface polaritons in the 1R region (for example, doped Si materials, SiC, BN or any suitable material that might be used as candidate materials for cover layers 510 and 514).

[0059] A graph comparing a near-field radiative heat transfer between two semi-infinite plates made of SiC and gold as function of distance d can also be found in B. Liu et al. (Fig.l). Distance d represents the vacuum gap size between the two plates. As can be seen in Fig.l of B. Liu et al., near-field radiative heat transfer between plates made of SiC and gold is three orders of magnitude less than the heat transfer between two SiC plates.

[0060] Consequently, in order to further improve transverse radiative heat transfer along pellicles, in an embodiment it is proposed to provide a plurality of additional features. These additional features can be grown or formed during the etching process. The additional features may be of any suitable shape. In one example the additional features comprise periodic or aperiodic wires or thin walls or ribs extending normal from the EUV membrane surface. The additional features may comprise doped Si or Si-based materials or any suitable cover layer material, such as any of the materials, having any of the dopant concentrations and arrangements disclosed herein. The feature size of each additional feature should be significantly smaller than the size of the area bounded by the features. It can be shown that, if the distance between additional features is < 1 pm, the radiative heat transfer is expected to be 10-10000 times higher than the blackbody limit.

[0061] Figure 6 shows an EUY membrane 600 comprising a plurality of additional features 620 (e.g. formed by periodic or aperiodic wall or wire structures 620). The additional features 620 may be located on the lower side of the EUV membrane 600 (the side exposed to EUV radiation). The side of the EUV membrane facing the reticle may be flat to maintain purity. Radiative heat transfer is symbolized by vertical arrows 630. Horizontal arrows 640 symbolize a transverse radiative heat transfer generated by the additional features 620. Note that illuminating EUV radiation (not shown) propagates almost normal to pellicle P. Therefore, the additional features 620 (in the form shown here, i.e., wires or ribs) cast a minimal shadow on reticle MA and/or wafer W.

[0062] Transverse temperature gradients in the EUV membrane are believed to cause as much damage to the membrane as high temperatures by themselves. While all the embodiments described herein significantly reduce temperature gradients in the EUV membrane during exposure to EUV radiation, the embodiment depicted in Figure 6 is particularly effective since transverse heat conduction is increased compared to a flat membrane case (where temperature is only transferred by phonons) by adding another mechanism: radiation heat transfer. It is believed that heat transfer from EUV membrane to an additional feature 620 is not limiting, since the typical scale is small. An efficient transverse heat transfer would minimize these temperature gradients and extend lifetime of the pellicle.

[0063] Figure 7 is a graph of expected temperature distribution against distance L across the EUV membrane. Line Peuv represents the EUV radiation power distribution across the pellicle. Line TA represents the temperature distribution of a flat EUV membrane. Line TB represents the temperature distribution across the EUV membrane depicted in Figure 6. As can be seen from Figure 7, temperature gradients across the EUV membrane are reduced for the Figure 6 example, compared to a flat EUV membrane.

[0064] Figure 8 shows a further embodiment of an EUV membrane 800, comprising a refinement to the embodiment depicted in Figure 6. In this embodiment, the additional features 820 comprise a shape and/or formation which mimics that of an echelette grating. In the specific example, the additional features comprise repeated groups of wires or ribs 820, with the individual wires/ribs 820 of each group descending (or increasing) progressively in height as shown. The result is an approximation of an echelette grating, which is illustrated by the dotted line. An echelette grating-like structure helps to direct unwanted radiation 830, originating from scattering of EUV radiation by each wire/rib 820 individually, away from orders (e.g. 0 and 1st orders) of the EUV radiation 840 during transfer of a pattern from reticle MA to the wafer.

[0065] In all the above embodiments, doping materials may be limited to those transparent for EUV, and which have the smallest mismatch with Si lattice (e.g. carbon, boron and nitrogen) for the sake of strength and reliability. In other embodiments, dopants which are not transparent for 13.5 nm but are transparent to other EUV/BUV wavelengths can be used, where the wavelength is appropriate for the lithographic system. These dopant materials may include: S, Te, As, O, Al, Sn, Sb, In, Ga, Br, Cl, I, C, B, N.

[0066] In summary, this disclosure provides simple and robust examples for increasing EUV membrane performance, and therefore performance of EUV pellicles and SPFs. EUV membrane temperatures, and temperature gradients across the EUV membrane, are reduced. As a consequence the lifetime of the EUV membrane and tolerance to EUV radiation power is improved. Additionally, high EUV membrane robustness is achieved without decreasing EUV radiation intensities (deteriorating the manufacturing system performance).

[0067] 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 artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), 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 multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0068] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0069] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The 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 clauses set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A membrane transmissive to EUV radiation, comprising: one or more high doped regions where said membrane is doped with a high dopant concentration, and one or more low doped regions where said membrane has no doping or a low dopant concentration; wherein a high dopant concentration is defined as dopant concentration greater than 1017 cm"3 and a low dopant concentration is defined as a dopant concentration less than 1017 cm"3.

2. A membrane as claimed in clause 1 comprising a plurality of layers which include a main substrate and one or more additional layers, wherein: said main substrate has a low dopant concentration and forms a low doped region; and said high doped regions are comprised within some or all of said additional layers.

3. A membrane as claimed in clause 2 wherein said additional layers comprise one or more cover layers for the protection of the membrane from etching or reacting agents, and said doped regions are comprised within said cover layers.

4. A membrane as claimed in clause 2 wherein said additional layers comprise one or more cover layers and one or more intermediate layers, arranged such that an intermediate layer is located between a cover layer and the main substrate; said cover layers being for the protection of the membrane from etching or reacting agents material, and said intermediate layers having an intermediate lattice size between that of the main substrate and the cover layer so as to reduce stress within the membrane; and wherein said high doped regions are comprised within said cover layers and/or said intermediate layers.

5. A membrane as claimed in clause 2, 3 or 4 wherein said main substrate is comprised of a poly-Si material.

6. A membrane as claimed in any preceding clause wherein said membrane, or a layer thereof, comprises a central region and a peripheral region around said central region, wherein said high doped region comprises said central region and said low doped region comprises the peripheral region.

7. A membrane as claimed in any preceding clause wherein said membrane, or a layer thereof, comprises a plurality of said high doped regions separated by said low doped regions.

8. A membrane as claimed in clause 7 wherein the separation between adjacent high doped regions is between 1 pm and 5 pm.

9. A membrane as claimed in any preceding clause wherein the doping concentration is graded, and increases towards the center of the membrane, or a layer thereof.

10. A membrane as claimed in any preceding clause wherein the high doped regions are doped with a dopant concentration greater than 1018 cm"3.

11. A membrane as claimed in any of clauses 1 to 9 wherein the high doped regions are 19 -3 doped with a dopant concentration greater than 10 cm .

12. A membrane as claimed in any of clauses 1 to 9 wherein the high doped regions are doped with a dopant concentration greater than 1020cm~3.

13. A membrane as claimed in any preceding clause wherein the low doped regions are doped with a dopant concentration less than 1016 cm"3.

14. A membrane as claimed in any of clauses 1 to 12 wherein the low doped regions are doped with a dopant concentration less than 1015 cm"3.

15. A membrane as claimed in any of clauses 1 to 12 wherein the low doped regions are doped with a dopant concentration less than 1014 cm"3.

16. A membrane as claimed in any preceding clause wherein the membrane has a thickness less than lOOnm.

17. A membrane as claimed in any preceding clause comprising a plurality of additional features on one or both surfaces of the membrane which are operable to increase transverse heat transfer.

18. A membrane as claimed in clause 17 wherein said additional features comprise ribs or wires extending normal from the membrane surface.

19. A membrane as claimed in clause 17 or 18 wherein the distance between additional features is < 1 pm.

20. A membrane as claimed in clause 17, 18 or 19 wherein the additional features are configured to resemble an echelette grating.

21. A membrane as claimed in clause 20 wherein said additional features comprise repetitive groups of wires or ribs, with each group comprising wires/ribs progressively descending or increasing in height.

22. A membrane as claimed in clause 6, 7 or 8 comprising only a single layer.

23. A membrane as claimed in any preceding clause wherein said high doped regions are doped with a dopant material comprising one or more of: S, Te, As, O, Al, Sn, Sb, In, Ga, Br, Cl, I, C, B and N.

24. A lithographic apparatus comprising one or more membranes as claimed in any preceding clause.

25. A lithographic apparatus as claimed in clause 24 wherein at least one of said membranes operates as a pellicle protecting a component from contamination.

26. The lithographic apparatus as claimed in clause 25 comprising a support constructed to support a patterning device, the patterning device being capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam; wherein at least one of said membranes operates as a pellicle protecting said patterning device from contamination.

27. The lithographic apparatus as claimed in clause 25 or 26 comprising a projection system operable to project a patterned radiation beam onto a wafer, wherein at least one of said membranes operates as a pellicle protecting optical components within said projection system from contamination· 28. A lithographic apparatus as claimed in any of clauses 24 to 27 wherein at least one of said membranes operates as a spectral filter for blocking unwanted wavelengths of radiation.

Claims (1)

A lithography device comprising: an illumination device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.