Method for measuring a coated substrate

Abstract

A method for measuring a coated substrate for the EUV wavelength range is proposed, comprising the following steps: a) Identification of a time t1 at which the coated substrate leaves a vacuum, b) Identification of a time t2 at which the coated substrate is introduced into a further vacuum, c) Performing an EUV reflectivity measurement on the coated substrate in a wider vacuum, d) Evaluation of the EUV reflectivity measurement taking into account the results of an EUV reflectivity measurement series carried out on samples from substrates with the same coating, whereby the samples were introduced from one vacuum to another vacuum at different times. This method can be integrated into the production of reflective optical elements for the EUV wavelength range, where, after a coating process, the correspondingly coated substrate is measured in order to evaluate the EUV reflectivity measurement more accurately.

Description

[0001] The present invention relates to a method for measuring a coated substrate for the EUV wavelength range and to a method for producing a reflective optical element for the EUV wavelength range and to a reflective optical element for the EUV wavelength range produced in this way.

[0002] In EUV lithography devices, mirrors for the extreme ultraviolet (EUV) wavelength range (e.g., wavelengths between approximately 5 nm and 20 nm), such as photomasks or mirrors based on multilayer systems, are used for the lithography of semiconductor components. Since EUV lithography devices typically have multiple reflective optical elements, these must exhibit the highest possible reflectivity to ensure sufficiently high overall reflectivity. Reflective optical elements for the EUV wavelength range are coated in a vacuum during manufacture and are also measured in a vacuum for control purposes. A key step in the manufacture of a reflective optical element is the deposition of one, several, or a multitude of layers on a substrate. Of particular importance is the measurement of the EUV reflectivity and the thickness of the coating, or the thickness of individual layers of the coating.Between coating and EUV reflectivity measurement, the reflective optical elements are transported or stored outside of a vacuum, resulting in contamination on their surfaces. This can include carbon-containing contaminants, particularly long-chain hydrocarbon compounds, which deposit on the surfaces, or oxidative contamination from atmospheric oxygen. Under operating conditions, such as in an EUV lithography device under the influence of EUV radiation and hydrogen residues in the operating vacuum, the contamination recedes, changing the EUV reflectivity of the respective reflective optical element. Since this change can be wavelength- and angle-dependent, it can affect not only the transmission of an optical system with one or more reflective optical elements, but also imaging properties such as apodization.Thus, imaging simulations of an optical system with one or more reflective optical elements based on these EUV reflectivity measurements may exhibit systematic errors due to contamination. To remedy this, the EUV reflectivity measurement can be performed on the fully or partially coated substrate without breaking the vacuum. However, this requires considerable equipment and operational effort. Furthermore, the reflective optical element can be cleaned of contamination immediately before measuring the EUV reflectivity. However, this places significant demands on the measuring device. Furthermore, there is a risk of distorting the EUV reflectivity measurement due to excessive cleaning and thus damaging the coating. The additional irradiation required for cleaning with discharge or plasma radiation sources can also lead to metallic contamination.

[0003] It is an object of the present invention to provide an alternative method for reducing the systematic error with reduced equipment expenditure.

[0004] This task is solved by a method for measuring a coated substrate for the EUV wavelength range with the following steps: a) Identification of a time t1 at which the coated substrate leaves a vacuum, b) Identification of a time t2 at which the coated substrate is placed in a further vacuum, c) Carrying out an EUV reflectivity measurement on the coated substrate in a further vacuum, d) Evaluation of the EUV reflectivity measurement taking into account the results of a series of EUV reflectivity measurements carried out on samples from substrates with the same coating, whereby the samples were moved from one vacuum to another vacuum at different times.

[0005] This approach is based on the realization that while the coated substrate is not in a vacuum prior to measurement, contamination forms on it, which negatively influences the measurement result of the EUV reflectivity measurement and tends to increase with longer exposure of the coated substrate.By recording the duration of exposure of the coated substrate outside a vacuum and comparing the measured EUV reflectivity with the results of a series of EUV reflectivity measurements performed on samples from substrates with the same coating and which were transferred from one vacuum to another at different times, an estimate of the contamination present on the coated substrate during the EUV reflectivity measurement can be made. This allows this contamination to be taken into account in the evaluation of the EUV reflectivity measurement, making it more meaningful than if the contamination were not taken into account. In particular, when measuring various reflective optical elements that may be integrated together into an optical system, systematic errors resulting from different exposure times outside a vacuum can be reduced.

[0006] It is particularly advantageous to determine the EUV reflectivity of the measured coated substrate without contamination by evaluating the EUV reflectivity measurement in order to then be able to make the most accurate possible prediction of the optical properties of a reflective optical element with the measured coated substrate as well as the optical properties of an optical system that would have this reflective optical element.

[0007] For the EUV reflectivity measurement series, the EUV reflectivity of the samples is advantageously measured continuously at defined time points to obtain a temporal progression of the measured EUV reflectivity. This allows the accumulated contamination to be modeled more precisely.

[0008] Advantageously, the coated substrate is stored and / or transported under defined conditions during time interval I1, and the evaluation of the EUV reflectivity measurement is carried out taking into account the contamination that occurred during time interval I1 between t1 and t2, depending on the storage and / or transport conditions. This allows the evaluation of the EUV reflectivity measurement on the coated substrate to be further refined.

[0009] In a preferred embodiment, a series of EUV reflectivity measurements is performed on substrates with the same coating that were stored and / or transported under controlled conditions for different time intervals. The evaluation of the EUV reflectivity measurement on the coated substrate is carried out taking into account the results of the EUV reflectivity measurement series. Through systematically acquired knowledge about the development of contamination under controlled and thus defined conditions over time, the resulting contamination can be taken into account particularly precisely when evaluating the EUV reflectivity measurement on the coated substrate.

[0010] Particularly in cases where a coated substrate is already measured in a partially coated state and is subsequently further coated, the following additional steps are preferably carried out in order to increase the accuracy of the measurement evaluation with regard to contamination that has occurred outside of a vacuum: e) transferring the measured and coated substrate into a coating chamber with a second further vacuum for applying at least one further layer and identifying a time t3 at which the coated substrate has left the further vacuum and a time t4 at which the coated substrate is introduced into the second further vacuum of the coating chamber, f) applying a further coating to the coated substrate, g) Repeat steps a) to d), whereby the evaluation of the EUV reflectivity measurement also takes into account the contamination that occurred in the time interval I2 between t3 and t4.

[0011] When measuring the EUV reflectivity of the coated substrate, the EUV reflectivity of the coated substrate is preferably measured. The influence of any contamination is particularly high in EUV reflectivity measurements, so it is particularly important to consider the contamination that occurred during exposure of the coated substrate outside of a vacuum when evaluating the EUV reflectivity measurement.

[0012] Furthermore, the object is achieved by a method for producing a reflective optical element for the EUV wavelength range, comprising the steps - Coating a substrate in a vacuum - Measuring the coated substrate according to the previously proposed measuring method.

[0013] Using the proposed approach, process variation in the production of a plurality of reflective optical elements can be reduced, particularly due to different contamination times between coating and EUV reflectivity measurement on the coated substrate. Furthermore, the problem is solved by a reflective optical element manufactured using this method.

[0014] The present invention will be explained in more detail with reference to preferred embodiments. Fig. 1 a schematic representation of an exemplary implementation of the measuring or manufacturing process; Fig. 2 a schematic representation of a coated substrate; Fig. 3 a schematic representation of the increase in contamination over time; Fig. 4 a schematic representation of the decrease in EUV reflectivity as a function of time; and Fig. 5 a schematic representation of the change in EUV reflectivity for different levels of contamination.

[0015] In Fig. 1 schematically shows a preferred embodiment of a method for producing a reflective optical element for the EUV wavelength range, into which a preferred embodiment of the method proposed here for measuring a coated substrate for the EUV wavelength range is integrated.

[0016] In a first step 101, a substrate is coated in the vacuum of a coating chamber. Any chemical and / or physical vapor deposition process can be used for this purpose, optionally with plasma and / or particle assistance. A complete or initially only a partial coating can be applied.Particularly in the case of multilayer coatings, in which alternating layers of a material with a higher refractive index at an operating wavelength in the EUV wavelength range and of a material with a lower refractive index are applied and are widely used as a reflective coating for reflective optical elements for the EUV wavelength range, it can be advantageous to interrupt the coating after the application of a partial coating in order to carry out a quality control by measuring the coated substrate so far, especially if the partial coatings have different functionalities such as radiation protection of the substrate, compensation of layer stresses, removal of interference radiation, protection of the reflective coating, influencing the EUV reflectivity profile.

[0017] An example of a reflective optical element for the EUV wavelength range is shown in Fig. 2. The reflective optical element 200 has a coating 203 on a substrate 201, which can be divided into a protective layer 211 and a reflective coating 205. The reflective coating 205 is based on a multilayer system with alternating layers of a material with a higher real part of the refractive index at the operating wavelength, at which, for example, the lithographic exposure is carried out (also called spacer 209) and a base material with a lower real part of the refractive index at the operating wavelength (also called absorber 207), with an absorber-spacer pair forming a stack. This simulates, to a certain extent, a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection occurs.Typically, reflective optical elements for an EUV lithography device or optical system are designed such that the respective wavelength of maximum reflectivity essentially coincides with the operating wavelength of the lithography process or other applications of the optical system. The protective layer 211 can be designed in multiple layers and protects the reflective coating from chemical and mechanical influences, while minimizing the negative impact on optical properties such as EUV reflectivity.

[0018] Typical substrate materials for reflective optical elements for EUV lithography are silicon, silicon carbide, silicon-infiltrated silicon carbide, quartz glass, titanium-doped quartz glass, glass, and glass ceramic. Particularly with such substrate materials, a layer made of a material with high absorption for radiation in the EUV wavelength range can be additionally provided between the multilayer system 203 and the substrate 201. This layer is used during operation of the reflective optical element 200 to protect the substrate 201 from radiation damage, such as unwanted compaction. Furthermore, the substrate can also be made of copper, aluminum, a copper alloy, an aluminum alloy, or a copper-aluminum alloy.One or more layers or layer systems can also be arranged between substrate 201 and multilayer system 205, which perform functions other than optical ones, for example, the compensation or reduction of layer stresses induced by a multilayer system 205 forming a reflective coating. Furthermore, an adhesion promoter layer can also be provided between substrate 201 and multilayer system 205. Frequently used reflective optical elements for the EUV wavelength range, for example, EUV mirrors or masks for EUV lithography, have silicon as the spacer material and molybdenum as the absorber material for an operating wavelength of around 13.5 nm, while protective layers 211 with at least one ruthenium layer are used.The thicknesses of the individual layers 207, 209 as well as the repeating stacks can be constant across the entire multilayer system 205 or can vary across the surface or the total thickness of the multilayer system 205, depending on the desired spectral or angle-dependent reflection profile or maximum reflectivity at the operating wavelength. Furthermore, additional layers can also be provided as diffusion barriers between the spacer and absorber layers 209, 207.

[0019] To measure the coated substrate, it is removed from the coating chamber and leaves its vacuum at an identified time t1 (step 103). To be measured, the coated substrate must be placed in a corresponding measuring chamber at an identified time t2, where a further vacuum prevails (step 105). Between these two times, in the present example, the coated substrate is transported under controlled conditions from the coating chamber to the measuring chamber and, if necessary, temporarily stored. Advantageously, the two times t1 and t2 are firmly linked to the coated substrate or to the resulting reflective optical element, for example, via the manufacturing documentation of the respective reflective optical element. The transport and, if applicable, storage conditions can also be documented there.In the example shown here, the reflectivity of the coated substrate is measured at the planned operating wavelength in the EUV wavelength range.

[0020] If the coated substrate is not in a vacuum, but in contact with an atmosphere such as air or a protective atmosphere of nitrogen, carbon contamination may grow under the influence of a carbon-containing residual gas atmosphere. If the surface of the coated substrate is exposed to an oxygen-containing residual gas atmosphere, contamination may occur due to oxidation of the material on the surface. In any case, the contamination negatively affects the EUV reflectivity of the coated substrate. Fig. Figure 3 schematically illustrates the growth of a carbonaceous contamination layer over time in arbitrary units. Especially when leaving the vacuum, the contamination layer grows rapidly onto the coated substrate, while saturation occurs over time and the thickness increases only slightly. The corresponding influence on the EUV reflectivity is shown in arbitrary units in Fig. 4. For example, at a working wavelength in the EUV wavelength range, the EUV reflectivity initially decreases sharply at a fixed angle of incidence, only to subsequently decrease very slowly. Even if the coated substrate is transferred from the coating chamber to the measurement chamber as quickly as possible, the subsequent EUV reflectivity measurement can be significantly distorted by contamination during transport and, if applicable, storage.

[0021] Therefore, the proposed procedure for evaluating the EUV reflectivity measurement takes into account the results of a series of EUV reflectivity measurements carried out on samples with the same coating and treatment ( Fig. 1, step 107). In particular, the identical treatment is based on the fact that the storage and / or transport conditions are identical or at least comparable for both the coated substrate and the samples of the EUV reflectivity measurement series. The respective contamination is thus determined using data obtained by conducting an EUV reflectivity measurement series on substrates with the same coating that were stored and / or transported under controlled conditions for different time intervals. Advantageously, the controlled storage and transport conditions for the EUV reflectivity measurement series and the specific coated substrate to be measured are essentially the same. Since the samples have the same coating as the respective coated substrate, it can be assumed that the same contamination grows on them.It should be noted that it is not necessary to record EUV reflectivity measurement series for all possible parameter combinations, but contamination growth can also be modeled on the basis of some measurement data and the evaluation can be carried out based on the corresponding model data.

[0022] Of particular importance is the fact that not only does the maximum EUV reflectivity decrease with increasing contamination, but also that the EUV reflectivity curve changes as a function of the wavelength at a fixed angle of incidence or as a function of the angle of incidence at a fixed wavelength in such a way that the EUV reflectivity decreases to a different extent on one side of the maximum EUV reflectivity than on the other side of the maximum EUV reflectivity. Fig.Figure 5 schematically shows the EUV reflectivities as a function of wavelength at a fixed angle of incidence, with contamination increasing in the direction of the arrow, in arbitrary units. Thus, contamination can not only cause a reduction in transmission in a reflective optical element, which is multiplicatively amplified in optical systems with two or more reflective optical elements. In such an optical system, the varying degree of EUV reflectivity decrease also changes the apodization.

[0023] Overall, the contamination comparison data can be used to determine the expected contamination of the coated substrate being measured, depending on the time spent outside of a vacuum and the prevailing conditions. Using this contamination thickness and a suitable layer model, the EUV reflectivity or the change in the EUV reflectivity curve for the respective coated substrate without contamination can be determined. This condition can be achieved after cleaning the coated substrate prior to use. This allows the expected EUV reflectivity during operation in an optical system to be described.

[0024] Depending on whether the entire coating or only a partial coating has already been applied to the substrate, in the first case the reflective optical element can be completed (step 113) or the coated substrate can be removed from the further vacuum of the measuring chamber for further coating at an identified time t3 (step 109) and, after appropriate transport and, if necessary, storage under controlled conditions, introduced into a second further vacuum of the same coating chamber as before or a different coating chamber at an identified time t4 (step 111). Steps 101 to 107 are then repeated, whereby the contamination that occurred between the newly determined times t1 and t2 is now taken into account when evaluating the EUV reflectivity measurement.The accuracy of the evaluation can be increased by additionally considering the contamination that occurred between times t3 and t4. For this purpose, the results of a series of EUV reflectivity measurements are taken into account. These measurements were performed on samples from substrates with the same coating. These samples were also transferred from the additional vacuum into the second additional vacuum of a coating chamber, also under controlled transport and, if applicable, storage conditions that are identical or at least comparable to the conditions to which the coated substrate is exposed. Optionally, before a new coating step 101, the surface of the partially coated substrate can be cleaned of existing contamination, as under operating conditions, for example, with hydrogen in combination with EUV irradiation.In this case, the contamination previously created on the surface between times t3 and t4 as well as between the original times t1 and t2 can be omitted.

Claims

[1] Method for measuring a coated substrate for the EUV wavelength range with the steps: a) Identification of a time t1 at which the coated substrate leaves a vacuum, b) Identification of a time t2 at which the coated substrate is placed in a further vacuum, c) Carrying out an EUV reflectivity measurement on the coated substrate in a further vacuum, d) Evaluation of the EUV reflectivity measurement taking into account the results of a series of EUV reflectivity measurements carried out on samples from substrates with the same coating, whereby the samples were moved from one vacuum to another vacuum at different times. [2] Method according to claim 1, characterized by that the EUV reflectivity of the measured coated substrate is determined without contamination by evaluating the EUV reflectivity measurement. [3] Method according to claim 1 or 2, characterized by that for the EUV reflectivity measurement series, the EUV reflectivity of the samples is measured continuously at defined times. [4] Method according to one of claims 1 to 3, characterized by that the coated substrate is stored and / or transported under defined conditions between time t1 and time t2 and the evaluation of the EUV reflectivity measurement is carried out taking into account the results of a series of EUV reflectivity measurements carried out on samples from substrates with the same coating, which were also stored and / or transported under controlled conditions over different time intervals. [5] Method according to one of claims 1 to 4, with the additional steps e) transferring the measured and coated substrate into a coating chamber with a second further vacuum for applying at least one further layer and identifying a time t3 at which the coated substrate has left the further vacuum and a time t4 at which the coated substrate is introduced into the second further vacuum of the coating chamber, f) applying a further coating to the coated substrate, g) Repeating steps a) to d), whereby the evaluation of the EUV reflectivity measurement takes into account the results of a series of EUV reflectivity measurements carried out on samples from substrates with the same coating, which were also introduced from the further vacuum into the second further vacuum of a coating chamber. [6] Method for producing a reflective optical element for the EUV wavelength range, comprising the steps - Coating a substrate in a vacuum - Measuring the coated substrate according to one of claims 1 to 5. [7] Reflective optical element for the EUV wavelength range, manufactured by the method according to claim 6.

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