Patent Description:
Miniaturization in semiconductor manufacturing technology is advancing year by year. Technologies for advancing miniaturization have been improved in many processes in semiconductor manufacturing. Among them, extreme ultraviolet (EUV) light having a wavelength of <NUM> has begun to be used in the exposure process in place of conventional ArF exposure having a wavelength of <NUM>. The wavelength is reduced to <NUM>/<NUM> or less by changing from ArF light to EUV light, and the optical properties of EUV light are completely different from those of ArF light. Therefore, a lot of new technological developments are required. In particular, development of a pellicle having a high EUV transmittance as a pellicle which serves as a particle adhesion-preventing membrane of, for example, a photomask (reticle) has been awaited. Currently, there is no pellicle having a high transmittance for EUV light, and in a case where existing pellicle is used, there are problems such that the EUV light intensity greatly decreases when EUV light passes through the pellicle, and the throughput decreases due to a longer exposure time, or the pellicle absorbs the EUV light and becomes hot and degrades in a short time.

Therefore, a pellicle membrane has been developed. For example, Patent Literature <NUM> (<CIT>) discloses a pellicle membrane including a core layer that contains a material substantially transparent for EUV radiation such as (poly-)Si and a cap layer that contains a material absorbing IR radiation. However, the poly-Si based pellicle membrane is not yet practical because the EUV transmittance cannot reach the target value of <NUM>% in a case where the thickness is set to maintain the membrane strength. A carbon nanotube (CNT)-based pellicle membrane has also been developed (Patent Literature <NUM> (<CIT>)), which is expected to have higher EUV transmittance. A further pellicle membrane comprising a beryllium core is disclosed in <CIT>.

Although the pellicle membrane may be damaged during use, it is also required to take measures so as not to affect pattern transfer even in such a case. For example, Patent Literature <NUM> (<CIT>) discloses a pellicle membrane including a breakage region which is configured to preferentially break during normal use in a lithographic apparatus, whereby a position on the pellicle membrane which breaks first is determined in advance. This literature also discloses a method of detecting breakage of a pellicle membrane, a method of preventing breakage from developing in the case of detecting the breakage, and a method of causing a pellicle membrane to roll up due to difference in tension in the thickness direction of the pellicle membrane when the pellicle membrane is damaged.

For example, the CNT-based pellicle membrane, however, does not have durability against a pellicle use environment (low-pressure hydrogen atmosphere). When the pellicle membrane is coated with metal to provide durability, the EUV transmittance decreases, making it impossible to achieve a practical level of transmittance. For this reason, device manufacturers have no choice but to make their products by EUV exposure without using a pellicle membrane. In particular, since Patent Literature <NUM> does not disclose any specific material, thickness, stress, and other characteristics of the pellicle membrane, the state of the pellicle membrane at the time of breakage varies depending on the properties of the pellicle membrane to be used. Therefore, it is desired to appropriately control the properties of the pellicle membrane to be used so as not to affect pattern transfer even when the pellicle membrane is damaged. Thus, there are various performances required for a pellicle membrane, such as high EUV transmittance, in-plane uniformity of EUV transmittance, and durability in a low-pressure hydrogen atmospheric environment. In particular, it is especially desirable that the pellicle membrane does not generate particles even when damaged. In case the pellicle membrane is damaged and particles are generated, the particles adhere to a reticle, mirror, or other object to cause a pattern transfer error. If particles adhere to the reticle, the photomask needs to be cleaned, and if particles adhere to the inside of an exposure system, the exposure system needs to be cleaned. When the exposure system is cleaned, the device manufacturer suffers significant damage, such as long-term system stoppage and high costs. Therefore, there is a demand for an EUV transmissive membrane as a pellicle membrane that is less likely to generate particles even when damaged.

The present inventors have recently found that by combining a main layer composed of metallic beryllium and a pair of surface layers containing fluoride, it is possible to provide an EUV transmissive membrane that is less likely to generate particles in case the membrane is damaged by any possibility.

Accordingly, an object of the present invention is to provide an EUV transmissive membrane that is less likely to generate particles in case the membrane is damaged by any possibility.

The present invention provides the following aspects:.

An EUV transmissive membrane comprising:.

The EUV transmissive membrane according to Aspect <NUM>, wherein the surface layer comprises beryllium fluoride.

The EUV transmissive membrane according to Aspect <NUM> or <NUM>, wherein the surface layer comprises beryllium fluoride nitride.

The EUV transmissive membrane according to Aspects <NUM> to <NUM>, wherein the surface layer comprises beryllium fluoride oxide.

The EUV transmissive membrane according to Aspects <NUM> to <NUM>, wherein the surface layer comprises beryllium fluoride nitride oxide.

The EUV transmissive membrane according to Aspects <NUM> to <NUM>, wherein the surface layer further comprises at least one selected from the group consisting of beryllium nitride, beryllium nitride oxide, and beryllium oxide.

The EUV transmissive membrane according to Aspect <NUM>, wherein the surface layer comprises beryllium fluoride and beryllium nitride.

The EUV transmissive membrane according to Aspect <NUM> or <NUM>, wherein the surface layer comprises beryllium fluoride and beryllium nitride oxide.

The EUV transmissive membrane according to Aspects <NUM> to <NUM>, wherein the surface layer comprises beryllium fluoride and beryllium oxide.

The EUV transmissive membrane according to Aspects <NUM> to <NUM>, wherein the surface layer comprises beryllium fluoride, beryllium oxide, and beryllium nitride.

The EUV transmissive membrane according to any one of Aspects <NUM> to <NUM>, wherein a thickness of the surface layer on the first surface is <NUM> times or more of a thickness of the surface layer on the second surface.

The EUV transmissive membrane according to any one of Aspects <NUM> to <NUM>, wherein a fluorine atom concentration at an outermost surface of the surface layer on the first surface is <NUM> times or more of a fluorine atom concentration at an outermost surface of the surface layer on the second surface.

The EUV transmissive membrane according to any one of Aspects <NUM> to <NUM>, wherein a thickness of the surface layer on the first surface is greater than a thickness of the surface layer on the second surface, and a fluorine atom concentration at an outermost surface of the surface layer on the first surface is higher than a fluorine atom concentration at an outermost surface of the surface layer on the second surface,
wherein a ratio of the thickness of the surface layer on the first surface to the thickness of the surface layer on the second surface, denoted by A, and a ratio of the fluorine atom concentration at the outermost surface of the surface layer on the first surface to the fluorine atom concentration at the outermost surface of the surface layer on the second surface, denoted by B, satisfy A × B ≥ <NUM>.

<FIG> illustrates a schematic cross-sectional view of an EUV transmissive membrane <NUM> according to an embodiment of the present invention. The EUV transmissive membrane <NUM> includes a main layer <NUM> and a pair of surface layers <NUM> provided on both sides of the main layer <NUM>. In other words, the main layer <NUM> has a first surface 12a and a second surface 12b, and the surface layer <NUM> is provided on each of the first surface 12a and the second surface 12b. The main layer <NUM> is composed of metallic beryllium, while the surface layer <NUM> contains at least one fluoride selected from beryllium fluoride, beryllium fluoride nitride, beryllium fluoride oxide, and beryllium fluoride nitride oxide. In particular, a preferred aspect of the surface layer <NUM> is to contain a lot of fluoride on the outermost surface. In this way, by combining the main layer <NUM> composed of metallic beryllium and a pair of the surface layers <NUM> containing fluoride, it is possible to provide the EUV transmissive membrane <NUM> that is less likely to generate particles, in case the membrane is damaged by any possibility.

In other words, there are various performances required for a pellicle membrane, such as high EUV transmittance and durability in a low-pressure hydrogen atmospheric environment. In particular, it is especially desirable that the pellicle membrane does not generate particles even when damaged. In case the pellicle membrane is damaged by any possibility and particles are generated, the particles adhere to a reticle, mirror, or other object to cause a pattern transfer error. If particles adhere to the reticle, the photomask needs to be cleaned, and if particles adhere to the inside of an exposure system, the exposure system needs to be cleaned. When the exposure system is cleaned, the device manufacturer suffers significant damage, such as long-term system stoppage and high costs. All these problems are successfully solved by the EUV transmissive membrane <NUM> of the present invention. The mechanism of particle generation at the time of breakage of the pellicle membrane will be described in detail as follows. First, the size of the EUV transmissive membrane is typically about <NUM> × <NUM>. In this case, since the gap between the reticle and the EUV transmissive membrane is only <NUM> to <NUM>, depending on the state of the EUV transmissive membrane after breakage, the EUV transmissive membrane may generate particles from the damaged portion of the EUV transmissive membrane by coming into contact with the reticle, and the particles may unfortunately adhere to the reticle. After a laminated membrane to be the EUV transmissive membrane <NUM> is formed on a substrate (e.g., a Si substrate) used at the time of deposition, for example, a free-standing membrane as the EUV transmissive membrane <NUM> of the present invention (or the conventional EUV transmissive membrane <NUM>) can be typically fabricated by removing an unnecessary portion of the Si substrate through etching with XeF<NUM> to form the free-standing membrane. In the subsequent exposure process, as illustrated in <FIG>, the substrate used at the time of deposition remains as a border <NUM> at the outer edge of the EUV transmissive membrane <NUM> or <NUM>, and a reticle <NUM> is positioned on a border <NUM> opposite to the EUV transmissive membrane <NUM> or <NUM>. As illustrated in <FIG>, the conventional EUV transmissive membrane <NUM> is damaged, the damaged portion of the EUV transmissive membrane <NUM> moves in a state of fluttering and comes into contact with the reticle <NUM>, thereby generating particles P.

According to the EUV transmissive membrane <NUM>, in contrast, the EUV transmissive membrane <NUM> can be rolled up on the first surface side (opposite to the reticle side) so that it does not come into contact with the reticle <NUM>, as illustrated in <FIG>. The reason why the EUV transmissive membrane <NUM> of the present invention moves as described above will be described below. First, metallic beryllium having high EUV transmittance is used as a material of the EUV transmissive membrane <NUM> for the main layer. However, since metallic beryllium has high reactivity with a XeF<NUM> gas used for etching, a cap layer is formed on both sides (the first surface 12a and the second surface 12b) of the main layer <NUM> in order to suppress the reactivity. At the time of etching, the XeF<NUM> gas reacts with the cap layer, and at least a part of the cap layer is fluorinated to form fluoride, whereby the aforementioned pair of surface layers <NUM> are formed on both sides of the main layer <NUM>. Since the surface layer <NUM> thus contains fluoride, by controlling the stress of the pair of surface layers <NUM> and making a difference in stress on both sides of the EUV transmissive membrane <NUM>, the EUV transmissive membrane can be rolled up in a case where the EUV transmissive membrane is damaged by utilizing the difference in stress. Thus, it is possible to provide the EUV transmissive membrane <NUM> that has high EUV transmittance but is less likely to generate particles in case the membrane is damaged by any possibility.

The main layer <NUM> is composed of metallic beryllium. However, the main layer <NUM> does not need to be completely composed of metallic beryllium. Preferably <NUM>% by weight or more, more preferably <NUM>% by weight or more, and even more preferably <NUM>% by weight or more of the main layer <NUM> may be composed of metallic beryllium, whereby an EUV transmissive membrane has high EUV transmittance at a practical level can be fabricated. From this viewpoint, the thickness of the main layer <NUM> is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, and even more preferably <NUM> to <NUM>.

The pair of surface layers <NUM> are designed to protect the main layer <NUM>, a metallic beryllium layer, while preventing the EUV transmissive membrane <NUM> from rolling up and coming into contact with the reticle <NUM> to generate particles P in a case where the membrane is damaged by any possibility. Accordingly, the surface layer <NUM> is provided on the first surface 12a and the second surface 12b of the main layer <NUM>, thereby making it possible to control stresses on both sides of the main layer <NUM>. The surface layer <NUM> typically contains at least one selected from the group consisting of beryllium nitride, beryllium nitride oxide, and beryllium oxide as a main component. From the viewpoint of stress control, however, the surface layer <NUM> contains at least one fluoride selected from beryllium fluoride, beryllium fluoride nitride, beryllium fluoride oxide, and beryllium fluoride nitride oxide. Among them, the surface layer <NUM> preferably contains beryllium fluoride, beryllium fluoride nitride, beryllium fluoride oxide, or beryllium fluoride nitride oxide. Here, the "main component" in the surface layer <NUM> means a component that accounts for <NUM>% by weight or more, preferably <NUM>% by weight or more, more preferably <NUM>% by weight or more, and even more preferably <NUM>% by weight or more of the surface layer <NUM>. The fluorine atom content at the outermost surface of the surface layer <NUM> is preferably <NUM> to <NUM> atom%, more preferably <NUM> to <NUM> atom%, and even more preferably <NUM> to <NUM> atom%.

As described above, the surface layer <NUM> preferably contains at least one selected from the group consisting of beryllium nitride, beryllium nitride oxide, and beryllium oxide (typically as a main component). More preferably, the surface layer <NUM> contains (i) beryllium fluoride and beryllium nitride, (ii) beryllium fluoride and beryllium nitride oxide, (iii) beryllium fluoride and beryllium oxide, or (iv) beryllium fluoride, beryllium oxide, and beryllium nitride. The term "beryllium nitride" as used herein means a comprehensive composition that allows not only a stoichiometric composition such as Be<NUM>N<NUM> but also a non-stoichiometric composition such as Be<NUM>N<NUM>-x, wherein <NUM> < x < <NUM>. The same is also applicable to other compound names such as beryllium nitride oxide, beryllium oxide, beryllium fluoride nitride, beryllium fluoride oxide, and beryllium fluoride nitride oxide.

In the exposure process, from the viewpoint of stress control, it is preferable that the surface layer <NUM> on the first surface 12a is provided on the side opposite to the reticle <NUM> side in the main layer <NUM>, and the surface layer <NUM> on the second surface 12b is provided on the reticle <NUM> side in the main layer <NUM>. Consequently, in case the EUV transmissive membrane <NUM> is damaged by any possibility, the EUV transmissive membrane <NUM> can be rolled up on the first surface side (opposite to the reticle side) without coming into contact with the reticle <NUM>, whereby the particles P are less likely to be generated, as illustrated in <FIG>, for example. In order to preferably realize such a state when the EUV transmissive membrane <NUM> is damaged, the thickness of the surface layer <NUM> on the first surface 12a is preferably <NUM> times or more, more preferably <NUM> to <NUM> times, and even more preferably <NUM> to <NUM> times the thickness of the surface layer <NUM> on the second surface 12b. The fluorine atom concentration at the outermost surface of the surface layer <NUM> on the first surface 12a side is preferably <NUM> times or more, more preferably <NUM> to <NUM> times, and even more preferably <NUM> to <NUM> times the fluorine atom concentration at the outermost surface of the surface layer <NUM> on the second surface 12b side. Alternatively, when the thickness of the surface layer <NUM> on the first surface 12a is greater than the thickness of the surface layer <NUM> on the second surface 12b, and the fluorine atom concentration at the outermost surface of the surface layer <NUM> on the first surface 12a side is higher than the fluorine atom concentration at the outermost surface of the surface layer <NUM> on the second surface 12b side, It is preferable to satisfy A x B ≥ <NUM>, wherein A is the ratio of the thickness of the surface layer <NUM> at the first surface 12a to the thickness of the surface layer <NUM> at the second surface 12b, and B is the ratio of the fluorine atom concentration at the outermost surface of the surface layer <NUM> on the first surface 12a side to the fluorine atom concentration at the outermost surface of the surface layer <NUM> on the second surface 12b side, more preferably <NUM> ≥ A × B ≥ <NUM>, and even more preferably <NUM> ≥ A x B ≥ <NUM>. Here, the "outermost surface" of the surface layer <NUM> means the surface of the surface layer <NUM> opposite to the surface on the main layer <NUM> side (first surface 12a or second surface 12b). In this way, it is possible to control the stress more effectively on both sides of the EUV transmissive membrane and to preferably achieve the aforementioned effects by making the thickness and/or the fluorine atom concentration different between the surface layer <NUM> on the first surface 12a and the surface layer <NUM> on the second surface 12b.

The surface layer <NUM> on the first surface 12a preferably has a thickness of <NUM> to <NUM>, more preferably <NUM> to <NUM>, and even more preferably <NUM> to <NUM>. The surface layer <NUM> on second surface 12b is typically thinner than surface layer <NUM> on first surface 12a and preferably has a thickness of <NUM> to <NUM>, more preferably <NUM> to <NUM>, and even more preferably <NUM> to <NUM>.

The fluorine atom concentration at the outermost surface of the surface layer <NUM> on the first surface 12a side is preferably <NUM> to <NUM> atom%, more preferably <NUM> to <NUM> atom%, and even more preferably <NUM> to <NUM> atom%. The fluorine atom concentration at the outermost surface of the surface layer <NUM> on the second surface 12b side is preferably lower than the fluorine atom concentration at the outermost surface of the surface layer <NUM> on the first surface 12a side, and is preferably <NUM> to <NUM> atom%, more preferably <NUM> to <NUM> atom%, and even more preferably from <NUM> to <NUM> atom%.

The surface layer <NUM> preferably has a nitrogen concentration gradient region where nitrogen concentration decreases as the surface layer becomes closer to the main layer <NUM>. In other words, the composition of nitride to be contained in the surface layer <NUM> may include form the stoichiometric composition such as Be<NUM>N<NUM> to the non-stoichiometric composition such as Be<NUM>N<NUM>-x, wherein <NUM> < x < <NUM>, as described above. In a case where the nitride is contained in the surface layer <NUM>, the nitride preferably has a gradient composition that approaches a beryllium-rich composition as the surface layer <NUM> becomes closer to the main layer <NUM>. As a result, it is possible to improve the adhesion between the surface layer <NUM> and the main layer <NUM> (i.e., metallic beryllium layer) and relieve stress caused by the difference in thermal expansion between the two layers. That is, it is possible to improve the adhesion between the two layers to suppress delamination, and to make delamination difficult as a thermal expansion relaxation layer between the two layers in the case of absorbing EUV light and becoming a high temperature. The thickness of the nitrogen concentration gradient region is preferably smaller than that of the surface layer <NUM>. In other words, the entire thickness of the surface layer <NUM> does not need to be in the nitrogen concentration gradient region. For example, it is preferable that only a part of the thickness of the surface layer <NUM>, for example, a region of preferably <NUM> to <NUM>% and more preferably <NUM> to <NUM>% of the thickness of the surface layer <NUM> is the nitrogen concentration gradient region.

In the EUV transmissive membrane <NUM>, the main region for transmitting EUV is preferably in a form of the free-standing membrane. In other words, it is preferable that the substrate (e.g., Si substrate) used at the time of deposition remains as a border only at the outer edge of the EUV transmissive membrane <NUM>. That is, no substrate (e.g., Si substrate) remains in the main region other than the outer edge. In shoer, the main region preferably consists of the main layer <NUM> and the surface layer <NUM>.

The EUV transmissive membrane <NUM> may have high EUV transmittance at a practical level, and preferably has an EUV transmittance of <NUM>% or more, more preferably <NUM>% or more, and even more preferably <NUM>% or more. Since a higher EUV transmittance is desirable, the upper limit is not particularly limited. Although ideally <NUM>%, the EUV transmissive membrane <NUM> can typically have a EUV transmittance of <NUM>% or less, more typically <NUM>% or less, and even more typically <NUM>% or less.

After a laminated membrane to be an EUV transmissive membrane is formed on a Si substrate, the EUV transmissive membrane according to the present invention can be fabricated by removing an unnecessary portion of the Si substrate through etching with XeF<NUM> to form a free-standing membrane while fluorinating the membrane. Accordingly, the main portion of the EUV transmissive membrane is in the form of the free-standing membrane in which no Si substrate remains as described above.

First, a Si substrate for forming a laminated membrane thereon is prepared. After the laminated membrane composed of the main layer <NUM> and the surface layer <NUM> is formed on the Si substrate, the main region (i.e., a region to be a free-standing membrane) other than the outer edge of the Si substrate is removed by etching with XeF<NUM>. Accordingly, it is desirable to reduce the thickness of the Si substrate in the region to be formed into the free-standing membrane in advance in order to perform the etching efficiently in a short time. Therefore, it is desirable that a mask corresponding to the EUV transmission shape is formed on the Si substrate by employing a normal semiconductor process, and the Si substrate is etched by wet etching to reduce the thickness of the main region of the Si substrate to a predetermined thickness. The wet-etched Si substrate is cleaned and dried to prepare a Si substrate having a cavity formed by wet etching. The wet etching mask may be made of any material that is corrosion resistance to Si wet etchant, for example, SiO<NUM> is suitable for use. In addition, the wet etchant is not particularly limited as long as it is capable of etching Si. For example, TMAH (tetramethylammonium hydroxide) is preferred because it can be used under appropriate conditions, and very good anisotropic etching can be performed on Si.

The laminated membrane may be formed by any deposition method. An example of the preferred deposition method is a sputtering method. In the case of fabricating a laminated membrane with a three-layer structure of beryllium nitride/beryllium/beryllium nitride, it is preferable that the beryllium membrane as the main layer <NUM> is fabricated by sputtering using a pure Be target and the beryllium nitride membrane as the surface layer <NUM> is fabricated by reactive sputtering. The reactive sputtering can be performed, for example, by adding nitrogen gas to the chamber during sputtering using a pure Be target, whereby beryllium and nitrogen react to each other to generate beryllium nitride. As another method, beryllium nitride can be produced by forming a beryllium membrane and then irradiating the membrane with nitrogen plasma to cause a nitriding reaction of beryllium, thereby generating beryllium nitride. In any case, synthetic methods for beryllium nitride are not limited thereto. Although it is preferable to use different beryllium targets for forming the beryllium nitride membrane and the beryllium membrane, it is also possible to use the same target for forming the beryllium nitride membrane and the beryllium membrane. The beryllium nitride and beryllium membranes may be formed in a one-chamber sputtering apparatus as in Examples described later, or a two-chamber sputtering apparatus may be used to form the beryllium nitride and beryllium membranes in separate chambers.

In the case of forming a nitrogen concentration gradient region, when depositing metallic beryllium from the beryllium nitride membrane, the introduction of nitrogen gas may be stopped and switched to metallic beryllium deposition while continuing a sputtering. In this way, a region is formed in which the nitrogen concentration in the film-deposited membrane decreases in the thickness direction as the concentration of nitrogen gas decreases. On the other hand, in the case of switching the metallic beryllium to the beryllium nitride, the nitrogen concentration gradient region can be formed by starting the introduction of nitrogen gas in the middle of the process while sputtering is performed, contrary to the above. The thickness of the nitrogen concentration gradient region can be controlled by adjusting the time for which the nitrogen gas concentration is changed.

An unnecessary portion of the Si substrate other than the outer edge of the Si substrate where the composite membrane is formed, which is left as a border, is removed by etching with XeF<NUM> to make the composite membrane free-standing. In this etching step, when the etching of Si in the cavity portion progresses and Si disappears, the back side of the composite membrane (the surface on the cavity side which has been in contact with the Si substrate) is exposed and a fluorination reaction occurs, thereby forming a surface layer on the second surface of the main layer. On the other hand, the front side of the composite membrane (the side opposite to the Si substrate that is exposed from the beginning) also undergoes a fluorination reaction, thereby forming the surface layer on the first surface of the main layer. However, since the front side is exposed from the beginning, the fluorination reaction occurs from the beginning of the etching step. In other words, since the exposure time to XeF<NUM> differs between the front and back sides of the composite membrane, the front side has longer contact time with XeF<NUM> and more fluoride. Accordingly, the fluorine atom concentration of the surface layer on the first surface side is higher than the fluorine atom concentration of the surface layer on the second surface side. Although the type of fluoride varies depending on the material of the beryllium layer that reacts with XeF<NUM>, reaction temperature, reaction time, and other factors, it has been confirmed that in a case where beryllium nitride is formed at the outermost layer of the composite membrane, beryllium fluoride, beryllium fluoride oxide, beryllium fluoride nitride, and beryllium fluoride nitride oxide are produced.

The present invention will be described in more detail with reference to the following examples.

In order to confirm how the stress of a beryllium composite membrane (a laminated membrane with a three-layer structure of beryllium nitride/beryllium/beryllium nitride) changes when fluorinated, a composite membrane was formed on a Si substrate, and the stress change in the composite membrane before and after fluorination was examined. First, a SiO<NUM> membrane was formed on one side of the Si substrate by thermal oxidation to prevent the one side from being etched by XeF<NUM> used for fluorination of the composite membrane. A beryllium composite membrane was formed on the surface of the Si substrate opposite to the surface on which the SiO<NUM> membrane was formed. Specifically, the Si substrate with the SiO<NUM> membrane formed was first set in a sputtering apparatus, and a pure Be target was attached thereto. A chamber was evacuated, the flow ratio of argon gas and nitrogen gas was adjusted to <NUM>:<NUM> to carry out reactive sputtering at an internal pressure of <NUM> Pa, and the reactive sputtering was terminated at the time when <NUM> of beryllium nitride was layer-deposited. Subsequently, sputtering was performed only with argon gas without introducing nitrogen gas, and the sputtering was terminated at the time when <NUM> of beryllium was layer-deposited. Thereafter, reactive sputtering was performed while introducing nitrogen gas again in the same manner as in the first step, and the reactive sputtering was terminated at the time when <NUM> of beryllium nitride was layer-deposited. In this manner, a beryllium composite membrane with <NUM> of beryllium nitride (Be<NUM>N<NUM>)/<NUM> of metallic beryllium (Be)/<NUM> of beryllium nitride (Be<NUM>N<NUM>) was formed. When a warp amount of the Si substrate was measured before and after the formation of the composite membrane, the warp amount of the Si substrate after the formation of the composite membrane was larger than that of the Si substrate before the formation of the composite membrane, thus confirming that tensile stress was present on the composite membrane. Subsequently, the Si substrate was exposed to XeF<NUM> gas to fluorinate beryllium nitride in the composite membrane. When a warp amount of the Si substrate was measured before and after fluorination, the warp amount of the Si substrate after fluorination was larger than that of the Si substrate before fluorination, thus confirming that tensile stress of the composite membrane was increased after fluorination. The results indicate that stress is inherent in the beryllium composite membrane and that the stress can be controlled by fluorination. Accordingly, fluorination can make a difference in stress on both sides of the EUV transmissive membrane (e.g., beryllium composite membrane), and the difference in stress can be used to allow the EUV transmissive membrane to roll up when the EUV transmissive membrane is damaged.

According to the procedures illustrated in <FIG> and <FIG>, a composite free-standing membrane (EUV transmissive membrane) with a three-layer structure of surface layer/main layer/surface layer was fabricated as follows.

A Si wafer <NUM> having a diameter of <NUM> inches (<NUM>) was prepared (<FIG>). A SiO<NUM> membrane <NUM> having a thickness of <NUM> was formed on both sides of the Si wafer <NUM> by thermal oxidation (<FIG>). Resist was applied to both sides of the Si wafer <NUM>, and a resist mask <NUM> for SiO<NUM> etching was formed by exposure and development so that a <NUM> × <NUM> resist hole was created on one side (<FIG>). An exposed portion of the SiO<NUM> membrane <NUM> was etched and removed by wet-etching one side of the substrate with hydrofluoric acid to fabricate a SiO<NUM> mask 22a (<FIG>). The resist mask <NUM> for SiO<NUM> etching was removed with an ashing apparatus (<FIG>). Si was then etched with a TMAH solution. This etching was performed only for an etching time to obtain a target Si substrate having a thickness of <NUM> with an etching rate measured in advance (<FIG>). Finally, the SiO<NUM> membrane <NUM> formed on the surface not subjected to Si etching was removed and cleaned with hydrofluoric acid to prepare a Si substrate <NUM> (<FIG>). The Si substrate outline may be diced with a laser <NUM>, if necessary (<FIG>), to achieve the desired shape (<FIG>). In this way, a <NUM> × <NUM> cavity <NUM> was provided at the center of the <NUM>-inch (<NUM>) Si wafer <NUM> to prepare the Si substrate <NUM> having a Si thickness of <NUM> in the cavity <NUM> portion.

On the Si substrate <NUM> including the cavity <NUM> obtained in (<NUM>) above, a composite membrane with a three-layer structure of beryllium nitride/beryllium/beryllium nitride was formed as follows (<FIG>). First, the Si substrate <NUM> was set in a sputtering apparatus, and a pure Be target was attached thereto. A chamber was evacuated, the flow ratio of argon gas and nitrogen gas was adjusted to <NUM>:<NUM> to carry out reactive sputtering at an internal pressure of <NUM> Pa, and the reactive sputtering was terminated at the time when <NUM> of beryllium nitride was layer-deposited. Subsequently, sputtering was performed only with argon gas without introducing nitrogen gas, and the sputtering was terminated at the time when <NUM> of beryllium was layer-deposited. Thereafter, reactive sputtering was performed while introducing nitrogen gas again in the same manner as in the first step, and the reactive sputtering was terminated at the time when <NUM> of beryllium nitride was layer-deposited. In this manner, a composite membrane <NUM> with <NUM> of beryllium nitride (Be<NUM>N<NUM>)/<NUM> of metallic beryllium (Be)/<NUM> of beryllium nitride (Be<NUM>N<NUM>) was formed.

The Si substrate <NUM> with the composite membrane <NUM> prepared in (<NUM>) above was set in a chamber of an XeF<NUM> etcher capable of processing an <NUM>-inch (<NUM>) substrate. The chamber was sufficiently evacuated. At this time, if moisture remains in the chamber, the moisture reacts with the XeF<NUM> gas to generate hydrofluoric acid, and corrosion of the etcher or unexpected etching occurs. Therefore, the sufficient evacuation was performed. If necessary, vacuuming and nitrogen gas introduction were repeated in the chamber to reduce residual moisture in the chamber. When the sufficient evacuation was achieved, a valve between a XeF<NUM> material tank and a preliminary space was opened. As a result, XeF<NUM> was sublimated, and XeF<NUM> gas was also accumulated in the preliminary space. When the XeF<NUM> gas was sufficiently accumulated in the preliminary space, the valve between the preliminary space and the chamber was opened to introduce the XeF<NUM> gas into the chamber. The XeF<NUM> gas was decomposed into Xe and F, and F reacted with Si to generate SiF<NUM>. Since the boiling point of SiF<NUM> was -<NUM>, SiF<NUM> generated was rapidly evaporated, causing a reaction of F with the newly exposed Si substrate. When the Si etching proceeded and F in the chamber decreased, the chamber was evacuated, and the XeF<NUM> gas was introduced into the chamber again to perform the etching. In this manner, the evacuation, the introduction of the XeF<NUM> gas, and the etching were repeated, and the etching was continued until the Si wafer <NUM> corresponding to the portion to be formed into the free-standing membrane disappeared. When the Si substrate of the unnecessary portion disappeared, the etching was terminated. In this way, a composite free-standing membrane having a border made of Si is obtained as the EUV transmissive membrane <NUM> (<FIG>). In the etching, F also reacts with a beryllium layer of the first surface (the surface opposite to the Si wafer <NUM>) and a beryllium nitride layer of the second surface (the surface on the Si wafer <NUM> side) in the composite membrane <NUM>. Thus, a pair of surface layers in the EUV transmissive membrane <NUM> thus obtained contains fluoride.

Samples of EUV transmissive membrane cut into cross sections by focused ion beam (FIB) processing were observed with a transmission electron microscope (TEM) (JEM-2100F, manufactured by JEOL Ltd. ) at <NUM> kV and elements were analyzed with an EDS analyzer (JED-2300T, manufactured by JEOL Ltd. ) at an acceleration voltage of <NUM> keV for elemental mapping and point analysis mode. As a result, it was found that the main layer had a thickness of <NUM>, the surface layer at the first surface had a thickness of <NUM>, and the surface layer at the second surface had a thickness of <NUM>. Since a small amount of fluorine was detected only at the outermost surface of the surface layer on the second surface, it was concluded that beryllium fluoride nitride was produced at the outermost surface of the surface layer on the second surface. On the other hand, fluorine was detected deeper in the surface layer on the first surface than in the surface layer on the second surface, thus indicating that more beryllium fluoride nitride was produced in the surface layer on the first surface. In addition, nitrogen and a small amount of oxygen were also detected from a portion where fluorine was detected in the surface layers on the first and second surfaces. From the above, it was found that the surface layer on the first surface of the EUV transmissive membrane had a higher fluorine atom concentration than the surface layer on the second surface, and that the fluoride generated on the surface layers (first and second surfaces) contained beryllium fluoride nitride as the main component and a small amount of beryllium fluoride nitride oxide and/or beryllium fluoride oxide.

In the evaluation in (<NUM>) above, the fluorine atom concentration at the outermost surface of the surface layer on the first surface side was <NUM> atom% and at the outermost surface of the surface layer on the second surface side was <NUM> atom%. Therefore, A, the ratio of the thickness of the surface layer on the first surface to the thickness of the surface layer on the second surface, is <NUM>, and B, the ratio of the fluorine atom concentration at the outermost surface of the surface layer on the first surface side to the fluorine atom concentration at the outermost surface of the surface layer <NUM> on the second surface side, is <NUM>. It was thus found that A x B = <NUM>.

When the center of the EUV transmissive membrane was pierced with a needle, the torn free-standing membrane rolled up on the first surface side, and no particle generation was observed. In other words, the state of the EUV transmissive membrane after breakage was as illustrated in <FIG>.

Claim 1:
An EUV transmissive membrane (<NUM>) comprising:
a main layer (<NUM>) composed of metallic beryllium that has a first surface (12a) and a second surface (12b); and
a pair of surface layers (<NUM>) provided on the first surface (12a) and the second surface (12b) of the main layer (<NUM>), characterized in that each surface layer (<NUM>) comprises at least one fluoride selected from beryllium fluoride, beryllium fluoride nitride, beryllium fluoride oxide, and beryllium fluoride nitride oxide.