Patent Description:
To enable high volume manufacturing with extreme ultraviolet (EUV) lithography (EUVL), a pellicle is needed to protect the reticle from any fall-on particles. An EUVL pellicle comprising a carbon nanotube (CNT) membrane is a promising pellicle solution, being transparent enough to limit the imaging impact while robust enough to survive handling and capable of stopping particles. A considerable major challenge remaining for realizing a CNT-based pellicle however is to make it withstand the hydrogen plasma environment of the EUV scanner during a large number of exposures, e.g. preferably on the order of tens of thousands or more. It has been proposed to coat a CNT membrane to make it less sensitive to exposure to hydrogen plasma. However, due to the interconnected web-like structure of CNT membranes and the relatively inert CNT surfaces, achieving a coated CNT membrane with a sufficiently maintained high and uniform EUV transmission is not trivial. <NPL> discusses a method for forming an EUVL pellicle by coating a carbon nanotube (CNT) membrane by using atomic layer deposition, and mounting the CNT membrane to a pellicle frame. <CIT> discusses the manufacture of an EUVL pellicle by coating a silicon membrane with a pre-coating seed layer and a capping layer, and mounting the silicon membrane to a pellicle frame.

An objective of the present inventive concept is to address this challenge and provide a method of forming an EUVL pellicle with a CNT-based pellicle membrane coated in an improved manner. Further and alternative objectives may be understood from the following.

According to an aspect of the present inventive concept there is provided a method for forming an EUVL pellicle, the method comprising:.

Atomic layer deposition (ALD) is a process allowing formation of atomically thin films or layers with a high degree of control and excellent uniformity. However, the small number of active sites for thin film anchoring and nucleation at the initial growth stage provided by CNTs (typically only provided by defect sites of the CNT membrane), ALD applied to CNTs tend to result in formation of localized islands resulting in CNT partially uncoated, even if a large number of ALD cycles are performed.

However, as realized by the inventors, preceding the ALD with an act of pre-coating the CNTs (i.e. outer surfaces thereof) with a seed material allows the number of active sites available for the ALD process to be increased to such an extent that an outer coating with a sufficient degree of coverage and uniformity (for EUVL applications) may be obtained. Preferably, the outer coating is formed to completely encapsulate the pre-coated CNTs.

As may be appreciated, the membrane may comprise bundles of CNTs. Accordingly, pre-coating CNTs with a seed material may comprise pre-coating of exposed outer surfaces of the CNTs bundles. Correspondingly forming an outer coating on the pre-coated CNTs may comprise depositing the coating material on exposed outer surfaces of the CNT bundles and on pre-coated outer surfaces of the CNT bundles (i.e. CNT surfaces covered with seed material).

Pre-coating by a seed material enables the above-described functionalization to be achieved in a benign manner, compared to other techniques for functionalizing CNTs to reduce the surface inertness (e.g. ozone and plasma treatments to name a few). It is desirable that the coating process does not cause significant structural damage to the CNTs. Damaging the CNTs would undermine the mechanical stability and integrity of the CNT membrane, which is of considerable importance for the mechanical reliability and particle stopping function of the pellicle.

The CNT membrane may advantageously be a free-standing CNT membrane. The CNTs may be single-walled (SWCNT) or multi-walled (MWCNT). The term MWCNT as used herein is intended to encompass also double-walled CNTs (DWCNTs).

The CNT membrane may be pre-coated using a non-ALD deposition technique. A number of advantageous deposition techniques for depositing the seed material include physical vapor deposition (PVD) methods such as thermal or e-beam evaporation, remote plasma sputtering; and alternatively electrochemical deposition (ECD) and electroplating. A common denominator for these deposition techniques is that they are benign to the CNTs and allow forming of a seed material pre-coating which is both thin and provides a degree of coverage sufficient to allow a sufficiently uniform outer coating by ALD. Any of these techniques alone would not be able to produce a coating on a CNT membrane meeting the aforementioned design targets. However, combining seed material pre-coating step with an outer coating ALD step provides a synergistic effect of enabling a coating on a CNT membrane providing a reliable protection with a minimum impact on EUV transmission.

The seed material may advantageously be deposited to form a seed layer with an average thickness in a range of <NUM> to <NUM>, preferably <NUM> to <NUM>, even more preferably <NUM> to <NUM>. As may be understood from the above, the pre-coating may not result in formation of a uniform and continuous layer. However, depositing seed material onto the CNTs until a seed layer with an average thickness in this range is obtained provides a sufficient number of nucleation sites to facilitate the subsequent ALD of the coating material, while keeping the seed layer thin to limit the total thickness of the coating (seed layer thickness plus outer coating thickness). By average thickness is hereby meant an average thickness of the seed layer (which may be continuous or be formed of a plurality of discrete seed layer portions) over all exposed outer CNT surfaces (bundled or not) of the membrane.

The coating material may be deposited to form an outer coating with an average thickness in the range of <NUM> to <NUM>, preferably <NUM> to <NUM>. An outer coating in these thickness ranges allows a reliable protection for the CNTs against the hydrogen plasma environment, while limiting impact on EUV transmission.

A deposition rate of <NUM>Ångström/s or lower has been identified to allow seed material pre-coating of a sufficient quality, also for thin layers.

The seed material may be selected from the group of: C, Zr, ZrN, Hf, HfN, B, B<NUM>C, BN, Y, YN, La, LaN, SiC, SiN, Ti, TiN, W, Be, Au, Ru, Al, Mo, MoN, Sr, Nb, Sc, Ca, Ni, Ni-P, Ni-B, Cu, Ag. These materials may provide a seeding function for a broad class of materials which may be deposited by ALD and which may form an outer coating with sufficient reliability and EUV transmission.

The coating material may be selected from the group of Zr, Al, B, C, Hf, La, Nb, Mo, Ru, Si, Ti or Y; or carbides, nitrides or oxides thereof.

Advantageously, the seed material and the coating material may be selected from the group of: Zr seed material and ZrO<NUM> coating material; B seed material and ZrO<NUM> coating material, HfO<NUM> coating material or Al<NUM>O<NUM> coating material; B<NUM>C seed material and ZrO<NUM> coating material, HfO<NUM> coating material or Al<NUM>O<NUM> coating material; Zr seed material and Al<NUM>O<NUM> coating material or ZrAlOx coating material; Mo seed material and ZrO<NUM> coating material.

According to some embodiments, pre-coating the CNTs with seed material comprises depositing seed material by PVD. The seed material may advantageously be deposited by thermal evaporation or e-beam evaporation, or by remote plasma sputtering. Common for these deposition techniques is that they allow material to be deposited in thin films on CNTs, at relatively low temperatures and without introducing significant damage to the CNT structure.

Pre-coating the CNTs with seed material using PVD, such as any of the afore-mentioned techniques, may comprise depositing seed material (e.g. by evaporation or remote plasma sputtering) from a first side of the membrane and depositing seed material (e.g. by evaporation or remote plasma sputtering) from a second opposite side of the membrane. Thereby, seed material may first be deposited on a first side / first main surface of the membrane and then be deposited on a second side / second main surface of the membrane. This facilitates providing a pre-coating with a sufficient degree of coverage and uniformity for the subsequent ALD process.

The act of pre-coating the CNTs of the membrane may be performed in a deposition tool comprising a substrate holder supporting the membrane. Depositing seed material from the first side of the membrane may comprise mounting the membrane to the substrate holder with the first side / the first main surface of the membrane facing towards a seed material vapor flux. Correspondingly, depositing seed material from the second side of the membrane may comprise mounting the membrane to the substrate holder with the second side / the second main surface of the membrane facing towards a seed material vapor flux. As may be appreciated, orienting a side/main surface of the membrane towards the seed material flux implies orienting the side/main surface towards a crucible of the deposition tool holding the seed material source (i.e. the target material to be vaporized).

The act of pre-coating the CNTs of the membrane may be performed in a deposition tool comprising a substrate holder supporting the membrane. For a more uniform seed material deposition, the deposition of seed material may be performed during continuous rotation of the substrate holder. By rotating the substrate holder the membrane may be rotated in relation to a seed material vapor flux, or equivalently, in relation to a crucible of the deposition tool holding the seed material source (i.e. the target material to be vaporized target material). This enables a more even distribution of the seed material.

According to some embodiments, pre-coating the CNTs with seed material comprises depositing seed material by electrochemical deposition or electroplating. Common for these deposition techniques is that they allow material to be deposited in thin films on CNTs at relatively low temperatures while limiting a risk of damaging the CNT membrane.

The coating material may be deposited by thermal ALD or plasma enhanced ALD (PEALD). Thermal ALD may be even more benign to the CNTs than PEALD and may hence in some cases be preferable.

The CNT membrane is preferably coated prior to being mounted to the pellicle frame. Prior to the coating, the CNT membrane may be assembled with a membrane border.

<FIG> schematically illustrates the basic structures and steps for fabrication of a EUVL pellicle <NUM> and of a reticle system <NUM> comprising the pellicle <NUM> and a reticle <NUM>, i.e. a "pelliclized reticle". The structures in <FIG> are shown in a schematic cross-sectional side-views.

The pellicle <NUM> comprises a pellicle membrane <NUM>, or shorter "membrane". The membrane <NUM> is a CNT membrane and may accordingly be formed by one or more layers of a CNT film, either of SWCNTs or MWCNTs. A SWCNT may be described as a cylindrical or tubular molecule of a single graphene sheet. SWCNTs may have a diameter in the range of <NUM>-<NUM>. MWCNTs may have a diameter in the range of <NUM>-<NUM>.

The membrane <NUM> may advantageously be formed as a free-standing CNT membrane. Properties of, as well as fabrication techniques for, free-standing CNT membranes are as such known in the art and will therefore not be discuss in detail herein. For sake of completeness it is however noted that a CNT membrane being free-standing or self-supporting is capable of supporting its own weight when being suspended by e.g. a pellicle frame. In other words, a free-standing CNT pellicle membrane is capable of supporting its own weight when having a size being relevant for use in lithography, without any appreciable sagging.

The CNT pellicle membrane may comprise a plurality of CNT films arranged on top of each other in a stacked fashion. The membrane <NUM> may for example comprise <NUM>, <NUM>, <NUM>, <NUM> or more CNT films just to give a few non-limiting examples. Each CNT film may include a random or regular web or grid of CNTs. The CNT films may be bonded together so as to form the CNT pellicle membrane.

The CNTs may also form bundles, wherein a CNT film may be formed of a web of CNT bundles. By way of example, a CNT bundle may include for instance <NUM>-<NUM> individual CNTs. In a CNT bundle, individual CNTs may be aligned and joined along their longitudinal directions. CNTs of a bundle may also be joined end-to-end such that the length of the CNT bundle is greater than the length of the individual CNTs. The CNTs may typically be joined by van der Waals forces.

Still with reference to <FIG>, the membrane <NUM> is assembled with a border <NUM> (as indicated by step S10). The membrane <NUM> may be attached to the border <NUM> along its edges. The membrane <NUM> may be attached to the border e.g. by an adhesive, by cold-welding, or in any other conventional manner known in the art.

The membrane <NUM> is coated (as indicated by step S12). As will be disclosed in greater detail below, the coating process comprises a pre-coating step and a subsequent second coating step to form a coated membrane <NUM> having a coating <NUM> with a degree of coverage and uniformity desirable for EUVL applications. A portion of the coating <NUM> is formed on a first side or first main surface 10a of the membrane <NUM>. A portion of the coating <NUM> is formed on a second side or second main surface 10b of the membrane <NUM> opposite the first side/first main surface.

The coated membrane <NUM> and the border <NUM> are assembled with a pellicle frame <NUM> (as indicated by step S14) to form the pellicle <NUM>. The border <NUM> may be attached to the frame <NUM> for instance by an adhesive, by cold-welding or by some mechanical fixation structures such as clamps. The frame <NUM> may for instance be formed by Si, SiN, SiO<NUM> or quartz. However other materials are also possible, such as metal, plastic or ceramic materials.

Step S16 depicts assembly of the reticle system <NUM> comprising the pellicle <NUM> mounted over the reticle <NUM>. The frame <NUM> may be attached to the reticle <NUM> using an adhesive. The reticle may be formed as a reflective reticle defining a pattern which is to be transferred to a wafer.

The membrane <NUM> may have a rectangular shape, although other shapes such as circular, oval or polygonal shapes also are conceivable. The border <NUM> and the frame <NUM> may have a shape corresponding to the shape of the membrane <NUM>.

Assembling the membrane <NUM> and the border <NUM> prior to the coating process of step S12 as disclosed above may facilitate handling of the membrane <NUM> during the coating process. However, it is also possible to first coat the membrane <NUM> and thereafter assemble the coated membrane <NUM> with the border <NUM>. It is also an option to omit a border and instead directly assemble a pellicle membrane with a pellicle frame, for instance by directly attaching the membrane to the frame using an adhesive.

It is to be noted that the relative dimensions of the elements shown in <FIG>, such as the thickness dimensions and height/dimensions of the membrane <NUM> and the coating <NUM> relative to e.g. the border <NUM> and the frame <NUM>, is merely schematic and may, for the purpose of illustrational clarity, differ from a physical structure.

<FIG> depicts a flow chart for a coating process <NUM> for forming a coating on a pellicle membrane, such as the coating <NUM> of the membrane <NUM> in <FIG>. The coating process <NUM> comprises a pre-coating step S102 followed by a step S104 of forming an outer coating on the pre-coated membrane by ALD.

In more detail, the pre-coating step S102 may comprise depositing a seed material on CNTs of the membrane <NUM> by e-beam evaporation. <FIG> schematically depicts an e-beam evaporation device <NUM>. The device <NUM> comprises a crucible <NUM> containing the target material <NUM>. For the purpose of the pre-coating process the target material <NUM> is the seed material. The device <NUM> further comprises an e-beam source <NUM> and a substrate holder or deposition stage <NUM>. The substrate holder <NUM> is adapted to support the pellicle membrane during the pre-coating process. The substrate holder <NUM> may as indicated be adapted to support more than one membrane <NUM> to allow pre-coating of plural membranes in parallel. The crucible <NUM> and the substrate holder <NUM> may be accommodated in a vacuum chamber <NUM> of the device <NUM>. The vacuum chamber <NUM> may be evacuated by a vacuum pump.

In use, the e-beam source <NUM> emits a beam of electrons towards the target material which thereby evaporates to form a vapor flux <NUM> of the target/seed material towards the substrate holder <NUM>. The arrows indicate line of sights from the crucible <NUM> (or equivalently from the target/source material <NUM>) towards the substrate holder <NUM>, wherein the arrow z denotes a major axis of the flux <NUM>. The vapor will coat exposed surfaces of the membrane(s) <NUM> supported by the substrate holder <NUM>. Thus, the target/seed material may be deposited to form a seed layer on the CNTs of the membrane <NUM>.

To improve the uniformity of the seed layer deposition, the pre-coating step <NUM> may comprise a first sub-step S102a of pre-coating the first side 10a of the membrane <NUM> and a second sub-step S102b of pre-coating the second side of the membrane 10b. That is, the membrane <NUM> may first be arranged on the substrate holder <NUM> with the first side 10a facing towards the crucible <NUM>, such that the first side 10a may face towards the vapor flux. Seed material may then be evaporated onto the membrane <NUM>. The pre-coating process may then be interrupted wherein the membrane <NUM> may be reoriented on the substrate holder <NUM> with the second side 10b facing towards the crucible <NUM>, such that the second side 10b may face towards the vapor flux.

A further measure to improve the uniformity of the seed layer deposition may be to deposit the seed material during continuous rotation of the substrate holder <NUM>. This may be performed during both the first and the second pre-coating sub-steps 102a, 102b.

A further measure to improve the uniformity of the seed layer deposition is to orient the membrane <NUM> such that the major flux axis z deviates from normal incidence (i.e. deviates from an angle of incidence of <NUM>°) onto (a plane parallel to) the main surface of the membrane <NUM> facing the vapor flux <NUM>. This may be achieved as indicated in <FIG> by an angled substrate holder <NUM>. The angle x indicated in <FIG> corresponds to the major flux axis z deviation from normal incidence. The angle x may for example be in a range of <NUM>-<NUM>° degrees. A non-zero angle of incidence, especially if combined with a continuous rotation during deposition, may allow CNT surfaces otherwise obscured by CNTs closer to the crucible along the line of sight to be exposed to the vapor flux <NUM> and thus be pre-coated with the seed material.

The seed material may generally be selected as any material which may provide a seeding function for the subsequent ALD step. The seed material may for example be selected from the following group: C, Zr, ZrN, Hf, HfN, B, B4C, BN, Y, YN, La, LaN, SiC, SiN, Ti, TiN, W, Be, Au, Ru, Al, Mo, MoN, Sr, Nb, Sc, Ca, Ni, Ni-P, Ni-B, Cu, Ag.

Seed material may be evaporated onto the membrane <NUM> until a seed layer has been formed with an average thickness in a range of e.g. <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. In practice an average thickness of the seed layer may be estimated by calculating an average thickness based on thickness measurements at a plurality of (randomly) selected measurement locations on the membrane <NUM>. A seed layer quality may be improved by limiting a deposition rate. Experiments indicate that a seed material deposition rate of <NUM>Ångström/s or lower may yield a satisfying pre-coating quality. During evaporation, the gas flow across the membrane <NUM> may be controlled to ensure that the membrane <NUM> can withstand the pump/vent cycles.

Although the above described deposition tool <NUM> is an e-beam evaporator tool, it should be noted that a pre-coating step may be performed in a corresponding manner using also other PVD techniques, such as thermal evaporation (hereby intended to encompass non-e-beam techniques such as thermal evaporation e.g. by resistive heating evaporation, as well as molecular beam epitaxy) or remote plasma sputtering.

Subsequent to the pre-coating step S102, the membrane(s) <NUM> may be removed from the tool <NUM> and transferred to an ALD tool (not shown). The ALD tool may be of a conventional type such as a thermal ALD tool or a PE-ALD. If a PE-ALD is to be used a remote PE-ALD may be favorable due to the lower risk of damaging the CNTs of the pre-coated membrane <NUM> with the plasma. In the ALD tool, a coating material may be deposited / epitaxially grown on the pre-coated CNTs of the membrane <NUM> to form an outer coating covering the same.

In line with the above discussion, the seed material / seed layer pre-coated on the CNTs of the membrane <NUM> increases the number of active sites available for the ALD process. For example, the outer coating may be formed to completely encapsulate the pre-coated CNTs of the membrane <NUM>.

The coating material may generally be selected as any material which may be deposited by ALD, provide sufficient protection of the CNTs against the hydrogen atmosphere, and enable an outer coating with sufficient EUV transmission. The coating material may be selected from the group of: Zr, Al, B, C, Hf, La, Nb, Mo, Ru, Si, Ti or Y. Carbides, nitrides or oxides of the aforementioned species are also possible coating materials.

The coating material may be deposited to form an outer coating with an average thickness in the range of for example <NUM> to <NUM>, or <NUM> to <NUM>. Increasing the thickness further may result in a too low EUV transmission of the membrane <NUM>.

Combinations of seed material and coating material which may be suitable for the intended EUVL application include:.

<FIG> is a schematic depiction of the above described coating process showing a SWCNT <NUM> which may represent a CNT of a membrane, such as the membrane <NUM> in <FIG>, prior to pre-coating (left), subsequent to pre-coating with a seed material forming a seed layer <NUM> (center), and finally forming an outer coating by ALD <NUM> (right). The seed layer <NUM> and the outer ALD coating <NUM> forms a combined coating <NUM> (corresponding e.g. to the coating <NUM> in <FIG>). <FIG> show SEM images of a portion of a SWCNT membrane coated in a corresponding manner: The CNTs have been pre-coated with a B<NUM>C seed layer of an average thickness of <NUM>. An outer coating has been formed on the B<NUM>C pre-coated CNTs by ALD ZrO<NUM> (<NUM> cycles). As may be seen a uniform outer coating encapsulating the CNTs has been obtained.

<FIG> is a schematic depiction of a comparative coating process where a SWCNT <NUM> is coated by ALD without a preceding pre-coating step. As may be seen the result is an incomplete and non-uniform coating <NUM>. <FIG> show SEM images of a SWCNT membrane coated in a corresponding manner, i.e. by ALD without a preceding pre-coating step. As may be seen a non-uniform in-complete coating has been obtained.

<FIG> show SEM images of a MWCNTs coated by ALD ZrO<NUM> (<NUM> cycles) without (<FIG>) and with (<FIG>) a preceding e-beam pre-coating by Zr. <FIG> shows a comparably non-uniform and particle-like coating. In comparison, <FIG> shows a uniform coating. An average thickness of the outer coating in <FIG> is approximately <NUM>.

<FIG> show SEM images of different ALD coatings on SWCNTs pre-coated by e-beam evaporation. Again the uniformity of the coating is apparent.

In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.

Claim 1:
A method for forming an EUVL pellicle, the method comprising:
coating a carbon nanotube, CNT, membrane, and
mounting the CNT membrane to a pellicle frame,
wherein coating the CNT membrane comprises:
pre-coating CNTs of the membrane with a seed material, wherein the seed material is deposited at a rate of <NUM>Ångström/s or lower,
and
forming an outer coating on the pre-coated CNTs, the outer coating covering the pre-coated CNTs, the forming of the outer coating comprising depositing a coating material on the pre-coated CNTs by atomic layer deposition.