Reflectometer system and method for determining a reflectance

Abstract

A reflectometer system (1, 10) for analyzing a sample (2) comprising a measurement radiation system comprising a measurement radiation source (3) for generating a measurement beam (30); a reflectance sensor (4) for detecting an intensity of the measurement beam (30) after reflection by the sample (2), when present; an alignment radiation source (5) for generating an alignment beam (50); optical elements (61, 62) configured for directing said measurement beam (30) and / or said alignment beam (5) towards the sample (2), when present; and a position sensor (7) for measuring a position of the alignment beam (50) on said position sensor (7) after reflection by the sample (2), wherein the reflectometer system (1, 10) is adapted for determining an alignment of the system (1, 10) relative to the sample (2) based on the measured position of the alignment beam (50), wherein a beam width of the alignment beam (50) is smaller than a beam width of the measurement beam (30), wherein the reflectometer system (1, 10) is configured so that, when an alignment of the reflectometer system (1, 10) is optimized, at least part of a propagation path of the measurement beam (30) is substantially parallel to at least part of a propagation path of the alignment beam (50).

Description

[0001] Reflectometer system and method for determining a reflectance

[0002] Technical field of the invention

[0003] The present invention relates to a reflectometer system and a method for determining the reflectance of a sample.

[0004] Background of the invention

[0005] Reflectometry is a widely used technique for analyzing the properties of a sample by measuring the intensity of light reflected from the sample. This method is critical in various industries, including semiconductor manufacturing, materials science, and optics, where precise measurements of reflectance can provide valuable information about the composition, structure, and quality of a material.

[0006] In the field of reflectometry, accurate alignment of the sample with respect to the measurement beam is important for obtaining reliable and reproducible results. Misalignment can lead to significant errors in the measurement of reflectance, as the angle of incidence and the position at which the light beam strikes the sample surface can greatly affect the general or spectral intensity of the reflected light.

[0007] One of the challenges in the field is the difficulty in achieving and maintaining precise alignment, especially when a range of samples are to be measured, and when dealing with a variety of sample sizes, shapes, and types. The alignment process can be time-consuming and complex, especially when a high accuracy is wanted.

[0008] Despite advancements in reflectometry technology, these challenges highlight the need for further improvements in the field. There is a continuous demand for more efficient, accurate, and user-friendly systems that can provide reliable reflectance measurements while minimizing the potential for errors due to sample misalignment. Therefore, there is still a need for further advancements in the field to address these challenges.

[0009] Summary of the invention

[0010] It is an object of embodiments of the present invention to provide a good system and method for reflectometry, which may provide accurate alignment of a sample for reflectance measurements by utilizing a narrow alignment beam in conjunction with a position sensor.

[0011] This objective is accomplished by a reflectometer system and a method for determining a reflectance of a sample. It is an advantage of embodiments of the present invention that a precise and accurate determination of the alignment, and, in turn, precise and accurate alignment, of a sample relative to a reflectometer system can be achieved. Said precise and accurate determination of the alignment may be achieved through the use of a narrow alignment beam, which allows for an accurate determination of the sample's position relative to the measurement beam. As the diameter of the alignment beam may be significantly smaller than that of the measurement beam, a highly accurate determination of a location of the alignment beam on the position sensor may be achieved. Thus, embodiments of the present invention may facilitate accurately determining whether the reflectometer system is well aligned, e.g., whether the system is aligned within predetermined specifications. Furthermore, this may, in turn, result in a more precise alignment process, which can be advantageous for accurate reflectance measurements.

[0012] In a first aspect, the present invention relates to a reflectometer system for analyzing a sample. The reflectometer system comprises a measurement radiation system comprising a measurement radiation source for generating a measurement beam. The reflectometer system comprises a reflectance sensor for detecting an intensity of the measurement beam after reflection by the sample, when present, an alignment radiation source for generating an alignment beam, a position sensor for measuring a position of the alignment beam on said position sensor after reflection by the sample, and optical elements configured for directing said measurement beam and / or said alignment beam towards the sample, when present, or for directing said alignment beam towards the position sensor. The reflectometer system is adapted for determining an alignment of the system relative to the sample based on the measured position of the alignment beam. A beam width of the alignment beam is smaller than a beam width of the measurement beam. Said beam widths may be determined that at the sample, and / or that at the position sensor.

[0013] Said reflectometer system for analyzing the sample may be for determining a reflectance of the sample, or for determining properties, such as the composition or surface properties, of the sample based on the determined reflectance of the sample, or properties of a coating being present at the surface of the sample.

[0014] The reflectometer system is configured so that, when an alignment of the reflectometer system is optimized, at least part of a propagation path of the measurement beam is substantially parallel to at least part of a propagation path of the alignment beam. Preferably, when the alignment of the reflectometer system is optimized, the propagation path of the measurement beam and the propagation path of the alignment beam are at least substantially parallel at the sample. For example, both beams may be substantially parallel when incident onto and after reflection by the sample, or the alignment beam incident onto the sample may be substantially parallel to the measurement beam after reflection by the sample. This configuration facilitates proper and straightforward alignment, or determination of the alignment, of the reflectometer system, as it may ensure that both beams interact with the sample in a similar manner.

[0015] In embodiments, the propagation path of the alignment beam may substantially overlap with the propagation path of the measurement beam over at least part of their propagation paths towards the sample, when said alignment is optimized. This overlapping of the beams' paths can further enhance the precision of the alignment, or of determining the alignment, by ensuring that the beams follow a common path over a portion of their propagation path towards the sample. These embodiments may further ensure that both beams interact with the sample in a similar manner, thus further improving the reliability of the determined alignment and of the alignment process.

[0016] In embodiments, the measurement beam and the alignment beam may be directed at an angle of at most 16°, preferably at most 12°, and typically close to 8° with respect to normal incidence onto the sample, when present, when said alignment is optimized. In preferred embodiments, the measurement beam and the alignment beam may be directed at substantially normal incidence onto the sample, when present, when said alignment is optimized. This configuration may further facilitate determining the alignment of the reflectometer system, as it may ensure that both beams interact with the sample in a similar manner. Furthermore, in this configuration, the alignment or the exact position of the sample along a direction of propagation of the beams may be not very critical for the measured reflectance.

[0017] In embodiments, the alignment radiation source may be located adjacent to the reflectance sensor such that the alignment beam is directed in a substantially opposite direction with respect to a direction of the measurement beam that is incident onto the reflectance sensor after reflection by the sample, when present. For example, in these embodiments, the alignment radiation source, such as a laser, may be located adjacent to the reflectance sensor, e.g., a CCD detector. For example, in these embodiments, an optical fiber of the alignment radiation source for providing the alignment beam may be located adjacent to an optical fiber of the reflectance sensor for receiving the measurement beam. This arrangement can prevent the alignment beam from interfering with the measurement beam by preventing the alignment beam from being detected by the reflectance sensor, thus improving the reliability of the reflectance measurements.

[0018] In embodiments, the alignment radiation source may be located adjacent to, or may be incorporated in, the measurement radiation source such that the alignment beam and measurement beam may follow similar paths. In these embodiments, the alignment radiation source and the measurement radiation source may have similar interactions with all optical elements and with the sample. This arrangement may allow realizing equivalent beam paths for the alignment beam and the measurement beam, while the position sensor may measure the position of the alignment beam on the position sensor, e.g., by filtering out or removing the signal of the measurement beam based on different radiation properties for the alignment beam than for the measurement beam (by having, e.g., a higher intensity for the alignment beam than for the measurement beam, and / or a different spectral content, and / or by pulsing alternatingly, ...).

[0019] In embodiments, the measurement beam and the alignment beam may be directed at non-normal incidence onto the sample, when present, when said alignment is optimized. In embodiments, the measurement beam and the alignment beam may be directed at an angle of larger than zero, preferably of from 1° to 20°, more preferably of from 3° to 10°, with respect to normal incidence onto the sample, when present, when said alignment is optimized. In these embodiments, the beam path of the measurement beam and of the alignment beam may assume a V-shape, and the beam path of the beams towards the sample typically do not overlap with the beam path of the same beams after reflection on the sample. It is an advantage of these embodiments that the number of beam-splitters may be limited, so that measurement times may be limited. The inventors have found that in such a reflectometer system, reproducibility and accuracy of measurements may be particularly good. Preferably, the beam paths are parallel, or substantially overlap over at least part of their propagation paths. For example, the beam path of the alignment beam towards the sample may be substantially parallel, or substantially overlap, with the beam path of the measurement beam after reflection on the sample, or, in other words, the alignment beam and the measurement beam may propagate in opposite directions along substantially a same path along at least part of their propagation paths. In these embodiments, a beam-splitter may be used for directing the alignment beam towards the position sensor.

[0020] In embodiments, the diameter of the alignment beam on the sample, when present, may be less than half the diameter of the measurement beam on the sample. This smaller diameter of the alignment beam can lead to a more precise determination of the sample's alignment. In embodiments, the diameter or width of the alignment beam on the sample, when present, may be less than 20%, preferably less than 10%, of the diameter or width of the measurement beam on the sample. This further reduction in the alignment beam's diameter can provide even greater accuracy in determining the alignment.

[0021] In embodiments, when said alignment is optimized, the alignment beam and / or the measurement beam may be, at least at the sample, substantially collimated. In embodiments, when said alignment is optimized, the alignment beam and / or the measurement beam may have, at least at the sample, a beam divergence having a magnitude, i.e., absolute value, of at most 16°, preferably at most 12°, more preferably at most 8°. In embodiments, when said alignment is optimized, the alignment beam and / or the measurement beam may have, at least at the sample, a beam convergence having a magnitude, i.e., absolute value, of at most 16°, preferably at most 12°, more preferably at most 8°.

[0022] The alignment source may be any type of light source, such as a light emitting diode or a laser. In preferred embodiments, the alignment radiation source may be a laser. The use of a laser can provide an alignment beam with a small width, which can be advantageous for achieving precise alignment.

[0023] In embodiments, the measurement radiation source may be a diffuse light source. A diffuse light source can provide a uniform illumination of the sample, which can be important for accurate reflectance measurements. For example, when a diffuse light source is used, the intensity of the measurement beam may be substantially uniform along the beam width. This type of light source is often used in reflectometer systems to provide uniform illumination over a wide area of the sample. Examples of diffuse light sources include frosted bulbs, fluorescent lamps, and light-emitting diodes with diffusing optics (including both lenses and mirror systems that may be continuous or patterned). In embodiments, the measurement radiation source may comprise an Ulbricht sphere. An Ulbricht sphere can help to create a highly diffuse and uniform light source, which can further improve the quality of the reflectance measurements. An Ulbricht sphere, also known as an integrating sphere, is often used in conjunction with a light source to produce diffuse illumination for reflectance measurements in a reflectometer system.

[0024] The use of a laser as the alignment radiation source is advantageous for accurate alignment, while the use of a diffuse light source, such as e.g. an Ulbricht sphere, for the measurement radiation source is advantageous for generating uniform illumination over the sample's surface, which is desirable for accurate reflectance measurements. In embodiments, the position sensor may comprise at least two pixels. In embodiments, the position sensor may comprise a two-dimensional photodetector, such as a photodetector comprising a two-dimensional array of pixels. The use of a two-dimensional photodetector can allow for more detailed spatial information to be captured regarding the position of the alignment beam, which can be beneficial for fine-tuning the alignment. In embodiments, the photodetector may be selected from a charged coupled device and a complementary metal-oxide-semiconductor sensor. The incorporation of a two-dimensional photodetector, such as a charged coupled device or a complementary metal-oxide- semiconductor sensor, as the position sensor is advantageous because it enables the detection of the alignment beam's position on the position sensor, typically with a high accuracy and resolution. In embodiments, the photodetector may be selected from position sensitive devices (PSD) or optical position sensors (OPS), allowing for measuring the position of a light spot in one or two dimensions on a sensor surface. Such devices may only need a limited number of connector pins (e.g., only 5) while still being able to determine the (one- or two- dimensional) position of a light spot.

[0025] In embodiments, the diameter of the alignment beam on the position sensor may be less than half the diameter of the measurement beam on the position sensor. This smaller diameter of the alignment beam can lead to a more precise determination of the sample's alignment. In embodiments, the diameter or width of the alignment beam on the position sensor may be less than 20%, preferably less than 10%, of the diameter or width of the measurement beam on the position sensor. This further reduction in the alignment beam's diameter can provide even greater accuracy in determining the alignment.

[0026] In embodiments, the photodetector may have a photosensitive area that is larger than a beam width of the alignment beam at the photodetector. These embodiments may ensure that the entire alignment beam is captured by the photodetector, which is important for accurate position measurement. Furthermore, this may allow for detecting larger deviations or misalignments of the sample.

[0027] In embodiments, a width of the pixels of the photodetector may be smaller than a beam width of the alignment beam at the photodetector. This pixel size configuration can ensure that the photodetector is capable of resolving the small spot size of the alignment beam, which may be preferred for accurate alignment detection.

[0028] In embodiments, the position sensor is adapted for distinguishing the alignment beam from the measurement beam. In embodiments, a wavelength of the measurement beam may be different from a wavelength of the alignment beam. This difference in wavelengths can help to distinguish between the two beams, reducing the likelihood of cross-talk and improving the accuracy of the measurements. In embodiments, the reflectance sensor is adapted for separating or filtering out the wavelength of the alignment beam. For example, the reflectance sensor may comprise a spectral filter for filtering out the wavelength of the alignment beam. For example, the reflectance sensor may have a higher sensitivity towards the wavelength of the measurement beam than towards the wavelength of the alignment beam. In embodiments, the wavelength of the measurement beam may be separated by the position sensor. Preferably, the measurement beam may be refracted by a grating, and the refracted measurement beam may be projected on a one- or two-dimensional CCD, so that the measurement beam may be spectrally separated from the alignment beam, enabling detecting the measurement beam separately from the alignment beam. For example, the position sensor may be adapted for sensing light having the alignment beam's wavelength and for not sensing light having the measurement beam's wavelength, such as by using a band pass filter adapted for passing the alignment beam and for blocking the measurement beam. Furthermore, a laser or a light emitting diode may be selected that has a narrow wavelength bandwidth that can be easily separated from the wavelength content of the measurement radiation source or that has a wavelength content that is outside of the sensing wavelength range of the reflectance sensor.

[0029] In embodiments, pulses of the measurement beam and / or pulses of the alignment beam on the one hand and / or pulses on the reflectance sensor and / or pulses on the position sensor on the other hand may be alternated. In embodiments, the measurement radiation source may be configured for generating pulses of the measurement beam and / or the alignment radiation source may be configured for generating pulses of the alignment beam, and, simultaneously or synchronized with the source, the reflectance sensor and / or the position sensor may be configured for pulsedly detecting the measurement and / or alignment beam. Preferably, the measurement beam is continuous (i.e., not pulsed) so as to prevent changes in the spectral content of the measurement beam that may occur due to said pulsing. The reflectance sensor may be adapted for pulsedly measuring the (e.g., continuous) measurement beam, wherein the alignment beam is pulsed inbetween the pulses or periods during which the reflectance sensor measures the measurement beam. In embodiments, the alignment radiation source may be adapted for providing a pulsed alignment beam and the reflectance sensor may be adapted for separating or filtering out the alignment beam. Alternating the pulses can allow for temporal separation of the measurement beam and the alignment beam, so that the position sensor and / or the reflectance sensor may distinguish between the measurement beam and the alignment beam. This may reduce potential interference and improve the overall accuracy of the system.

[0030] In embodiments, the reflectance sensor is adapted for detecting the intensity of the measurement beam after it has been reflected by the sample. The signal of the reflectance sensor may be used to quantify the amount of light reflected, which is related to the sample's reflectivity. Examples of reflectance sensors include photodiodes, photomultiplier tubes, and spectrophotometers.

[0031] In embodiments, the reflectometer system may further comprise a beam splitter for separating part of the measurement beam before reflection by the sample, when present, and a reference sensor for detecting the intensity of the separated measurement beam. Typically, a beam splitter is an optical element with limited absorptance and having a fraction of incident radiation that is transmitted and a fraction of the incident radiation that is reflected. Said fraction may be any value between typically 5% and 95%. If the absorptance is low, the sum of the fractions of transmittance and reflectance will approach close to 100%. Said reference sensor may, for example, be a photodiode, a photomultiplier tube, or a spectrophotometer. This feature can provide a reference signal for the intensity of the measurement beam, which can, thus, be used to calibrate the system and ensure accurate reflectance measurements.

[0032] In embodiments, the reflectometer system comprises a controller for receiving the determined position from the position sensor. The controller may be adapted for comparing the determined position with a predetermined area or range, and when the determined position falls within the predetermined area or range, the alignment may be considered to be optimized.

[0033] In embodiments, the reflectometer system may comprise a sample holder for holding the sample, wherein the sample holder is adapted for displacing, e.g., rotating, the sample for optimizing said alignment based on the measured position of the alignment beam on the position sensor. This capability can allow for mechanical adjustment of the sample's orientation to achieve optimal alignment, which can be critical for accurate reflectance measurements.

[0034] In embodiments, the reflectometer system may further comprise steering means for changing the configuration of the optical elements for improving an alignment of the reflectometer system based on the measured position of the alignment beam on the position sensor. This feature can enable dynamic adjustment of the optical path to correct for misalignments, enhancing the system's flexibility and accuracy. Optical components of which the configuration could be adapted for improving the alignment typically contain any optical components on which, when the alignment is optimized, both the alignment beam is reflected on its propagation path from the alignment radiation source towards the position sensor, and the measurement beam is reflected on its propagation path from the measurement radiation source towards the reflectance sensor.

[0035] In embodiments, the reflectometer system may further comprise a controller for automated control of the steering means based on the measured position of the alignment beam on the position sensor. Automation of the alignment process can increase the efficiency and repeatability of the system, reducing the need for manual intervention.

[0036] In embodiments, at least three selected from, preferably each of, the measurement radiation system, the alignment radiation source, the reflectance sensor, and the position sensor may be located in a same housing, wherein the reflectometer system is configured for analyzing the sample located outside of said housing. In embodiments, the reflectometer system may be configured for analyzing the sample located at a distance of said housing that is larger than the beam width of the measurement beam, e.g., at least 5 times the beam width, or even more than 10 times the beam width. These embodiments may facilitate flexibility and compatibility between the reflectometer system and the different types of samples that can be analyzed.

[0037] The same housing may refer to a single physical enclosure or chassis that contains said at least three components of the reflectometer system. Said components being located in a same housing may indicate that at least three of the measurement radiation system, the alignment radiation source, the reflectance sensor, and the position sensor are all integrated within the same structural unit, which may enhance the stability and portability of the reflectometer system.

[0038] In a second aspect, the present invention relates to a setup comprising the reflectometer system of embodiments of the first aspect, and a vacuum system, e.g., a vacuum sputtering system, comprising a vacuum chamber comprising an optical window. The optical window may be provided in an interface flange. The reflectometer system is located outside of the vacuum chamber, and is arranged so that the measurement beam and the alignment beam propagate through said optical window for analyzing a sample, when present, in the vacuum chamber.

[0039] In a third aspect, the present invention relates to a method for determining a reflectance of a sample in a reflectometer system, comprising directing an alignment beam towards the sample for reflecting the alignment beam, measuring a position of the reflected alignment beam on a position sensor, determining an alignment of the reflectometer system based on the measured position, directing a measurement beam, having a beam width larger than that of the alignment beam, towards the sample for reflecting the measurement beam, and measuring an intensity of the reflected measurement beam for determining said reflectance. The reflectometer system is configured so that, when an alignment of the reflectometer system is optimized, at least part of a propagation path of the measurement beam is substantially parallel to at least part of a propagation path of the alignment beam.

[0040] Determining the alignment of the reflectometer system may be followed by optimizing the alignment of the reflectometer system, based on said determined alignment. Said optimizing the alignment may be performed before said measuring of the intensity of the reflected measurement beam for determining said reflectance. Alternatively, the alignment beam and the measurement beam may be directed at the sample at the same time, and determining, and optionally optimizing, the alignment may be performed during said measuring of the intensity of the reflected measurement beam for determining said reflectance. Still alternatively, no optimization of the alignment is performed. For example, the determined position may be compared to predetermined area or range, and if the determined position falls within said predetermined area or range, the measured reflectance may be assumed to be accurate while if the determined alignment falls outside of said predetermined area or range, the measured reflectance may be assumed to be inaccurate.

[0041] Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

[0042] The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

[0043] Brief description of the drawings

[0044] FIG. 1 is a schematic view of a reflectometer system according to embodiments of the present invention. FIG. 2 is a schematic view of the reflectometer system with an optimized sample alignment according to embodiments of the present invention.

[0045] FIG. 3 is a schematic view of a reflectometer system with integrated components within a housing according to embodiments of the present invention.

[0046] FIG. 4 is a schematic view of a position sensor used in the reflectometer system according to embodiments of the present invention.

[0047] FIG. 5 is a schematic view of a reflectometer system according to embodiments of the present invention wherein the beams are incident on a sample at non-normal incidence.

[0048] FIG. 6A is a schematic view of a reflectometer system according to embodiments of the present invention comprising a sample, a reference sample, and a shutter, wherein the reflectometer system is aligned on the sample.

[0049] FIG. 6B is a schematic representation of a cross-sectional view of a bifurcated fiber containing six strands emitting the alignment beam, and a single, central strand for receiving the measurement beam of the reflectometer system of FIG. 6B.

[0050] FIG. 7 is a schematic view of the reflectometer system of claim 6A according to embodiments of the present invention comprising a sample, a reference sample, and a shutter, wherein the reflectometer system is aligned on the reference sample.

[0051] FIG. 8 is a schematic view of a reflectometer system of claim 6A and 7 according to embodiments of the present invention comprising a sample, a reference sample, and a shutter, wherein the reflectometer beams are blocked by the shutter.

[0052] In the different figures, the same reference signs refer to the same or analogous elements.

[0053] Description of illustrative embodiments

[0054] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[0055] Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0056] Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

[0057] It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. The term "comprising" therefore covers the situation where only the stated features are present and the situation where these features and one or more other features are present. The word "comprising" according to the invention therefore also includes as one embodiment that no further components are present. Thus, the scope of the expression "a device comprising means A and B" should not be interpreted as being limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

[0058] Similarly, it is to be noticed that the term "coupled" should not be interpreted as being restricted to direct connections only. The terms "coupled" and "connected", along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression "a device A coupled to a device B" should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled" may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

[0059] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0060] Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

[0061] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0062] Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

[0063] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0064] The following terms are provided solely to aid in the understanding of the invention.

[0065] As used herein, and unless otherwise specified, the term "beam width" refers to the diameter or size of a beam of light or other electromagnetic radiation at a specified location along the propagation path of the beam, e.g., measured at a surface of the sample or at the position sensor. The width may be the full width at half maximum or half-power beam width, or alternatively the 1 / e2width, which are parameters well-known in the art for determining the width of a beam. In case of an asymmetric (e.g., non-circular) beam profile, the width may be the smallest width at a specified location along the propagation path of the beam. The beam width is an important parameter in determining the resolution and precision of the reflectometer system. The term "smaller beam width" indicates a narrower beam, which is typically used for alignment purposes, while a "larger beam width" indicates a wider beam, which is typically used for measurement purposes.

[0066] As used herein, and unless otherwise specified, the term "beam divergence" refers to the angle over which a beam of light or other electromagnetic radiation spreads out as it travels from its source or after passing through an optical system. As used herein, and unless otherwise specified, the term "beam convergence" refers to the angle over which a beam of light or other electromagnetic radiation narrows or comes together as it travels towards a focal point. The angle of the beam convergence or divergence may, for example, be calculated by the magnitude, or absolute value, of 2xarctan((wl-w2) / (2x|)), with wl and w2 the beam widths at two different locations along the beam propagation path, and I the distance between said two different locations along the beam propagation path.

[0067] As used herein, and unless otherwise specified, a collimated beam refers to a beam of light or other electromagnetic radiation that has parallel rays and thus does not diverge or converge over a certain distance. When it is, for example, said that the alignment beam is "substantially collimated" at least at the sample, this may indicate that the alignment beam maintains a substantially consistent beam width as it travels towards and strikes the sample.

[0068] The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the technical teaching of the invention, the invention being limited only by the terms of the appended claims.

[0069] In a first aspect, the present invention relates to a reflectometer system for analyzing a sample. The reflectometer system comprises a measurement radiation system comprising a measurement radiation source for generating a measurement beam. The reflectometer system comprises a reflectance sensor for detecting an intensity of the measurement beam after reflection by the sample, when present, an alignment radiation source for generating an alignment beam, a position sensor for measuring a position of the alignment beam on said position sensor after reflection by the sample, and optical elements configured for directing said measurement beam and / or said alignment beam towards the sample, when present, or for directing said alignment beam towards the position sensor. The reflectometer system is adapted for determining an alignment of the system relative to the sample based on the measured position of the alignment beam. A beam width of the alignment beam is smaller than a beam width of the measurement beam. Said beam widths may be determined that at the sample, and / or that at the position sensor. The reflectometer system is configured so that, when an alignment of the reflectometer system is optimized, at least part of a propagation path of the measurement beam is substantially parallel to at least part of a propagation path of the alignment beam.

[0070] In a second aspect, the present invention relates to a setup comprising the reflectometer system of embodiments of the first aspect, and a vacuum system comprising a vacuum chamber comprising an optical window. The reflectometer system is located outside of the vacuum chamber, and is arranged so that the measurement beam and the alignment beam propagate through said optical window for analyzing a sample, when present, in the vacuum chamber.

[0071] In a third aspect, the present invention relates to a method for determining a reflectance of a sample in a reflectometer system, comprising directing an alignment beam towards the sample for reflecting the alignment beam, measuring a position of the reflected alignment beam on a position sensor, determining an alignment of the reflectometer system based on the measured position, directing a measurement beam, having a beam width larger than that of the alignment beam, towards the sample for reflecting the measurement beam, and measuring an intensity of the reflected measurement beam for determining said reflectance. The reflectometer system is configured so that, when an alignment of the reflectometer system is optimized, at least part of a propagation path of the measurement beam is substantially parallel to at least part of a propagation path of the alignment beam.

[0072] Reference is made to FIG. 1, which is a schematic representation of a reflectometer system (1) in accordance with embodiments of the present invention for analyzing a sample's (2) reflectance with high precision. In FIG. 1, the sample (2) is not correctly aligned with the reflectometer system (1).

[0073] The system (1) is designed to optimize the alignment of the sample (2) to ensure accurate measurements. The reflectometer system (1), as depicted in FIG. 1, comprises a measurement radiation system with a measurement radiation source (3) that generates a measurement beam (30). The measurement beam (30) propagates through optical elements that may comprise two beam splitters (61, 62), towards the sample (2). The reflectometer system (1) may comprise further optical elements, such as mirrors or lenses, for directing the measurement beam (30) towards the sample (2).

[0074] The measurement beam (30) is, after reflection by the sample (20), reflected by the first beam splitter (61) towards a reflectance sensor (4) when the alignment is optimized. The reflectance sensor (4) is provided for detecting the intensity of the measurement beam (30) after it reflects off the sample (2). As may be observed, in FIG. 1, the measurement beam (30) is not incident onto the reflectance sensor (4), as the sample (2) is not aligned with the reflectometer system (1).

[0075] In the example illustrated, the second beam splitter (62) separates part of the measurement beam (30) before it reflects off the sample (2) towards a reference sensor (33), as shown in FIG. 1. Alternatively, the system (1) may use the first beam splitter (61), or may comprise a still different beam splitter, for separating part of the measurement beam (30) before it reflects off the sample (2) towards a reference sensor (33). The reference sensor (33) detects the intensity of this separated measurement beam (32), providing a reference signal for calibration of the measurement beam (30) intensity detected by the reflectance sensor (4). In the example illustrated, the controller (8) may receive signals from the reference sensor (33) and from the reflectance sensor (4) and may determine a reflectance of the sample (2) based on these signals.

[0076] The system (1) also includes an alignment radiation source (5) that produces an alignment beam (50) with a smaller beam width than the measurement beam (30). The alignment radiation source (5) may be any kind of radiation source (5), such as a laser diode or LED. In the example illustrated, the alignment beam (50) is reflected by the first beam splitter (61) to direct the alignment beam (50) towards the sample (2). The first beam splitter (61) is arranged so that the alignment beam (50) incident onto the sample (2) and the measurement beam (30) incident onto the sample (2) are substantially parallel. In the example illustrated, after the alignment beam (50) is reflected back by the sample (2), it propagates through the first beam splitter (61) and is reflected by the second beam splitter (62) towards a position sensor (7), which measures the position of the alignment beam (50) onto the position sensor (7) after said reflection onto the second beam splitter (62). As the alignment beam (50) incident onto the sample (2) and the measurement beam (30) incident onto the sample (2) are substantially parallel, the beams (30, 50) are also substantially parallel after reflection by the sample (2). The reflectometer system (1) uses the information on the location of the alignment beam (50) onto the position sensor (7) to determine the sample's (2) alignment. The position sensor (7) may comprise any type of position-sensitive device. The position sensor (7) may comprise, for example, a ID or 2D array with at least two pixels, any type of camera, or any type of position sensitive device. The position sensor (7) may comprise a two-dimensional photodetector (7), such as a charged coupled device or a complementary metal-oxide- semiconductor sensor, preferably with a pixel width smaller than the beam width of the alignment beam (50). Furthermore, the position sensor (7) may comprise, for example, a position sensitive device (PSD) or optical position sensor (OPS), allowing for measuring the position of a light spot in one or two dimensions on a sensor surface.

[0077] This ensures that the photodetector (7) can resolve the small spot size of the alignment beam (50) for accurate alignment detection.

[0078] The alignment beam's (50) smaller diameter allows for a more precise determination of the sample's (2) alignment.

[0079] The controller (8) may receive information on a location of the alignment beam (50) on the position sensor (7). The alignment beam (50) may be differentiated from the measurement beam (30) in several ways. In the example illustrated, the alignment beam (50) and the measurement beam (30) are parallel but spatially separated from each other, so that the position sensor (7) may detect the alignment beam (50) separately from the measurement beam (30). Alternatively, the alignment beam (50) may have a different spectral content, or wavelength, than the measurement beam (30). The position sensor (7) may be adapted to distinguish the wavelength of the alignment beam (50) from the wavelength of the measurement beam (30), or the system (1) may contain a spectral filter for allowing the alignment beam (50) towards the position sensor (7) but that blocks the measurement beam (30) from propagating towards the position sensor (7). Still alternatively, pulses from the measurement beam (30) may be alternated with pulses from the alignment beam (50), so that the position sensor (7) may be able to distinguish between the beams (30, 50) because of the different time at which the beams (30, 50) are incident onto the position sensor (7).

[0080] Based on the determined location of the alignment beam (50) onto the position sensor (7), e.g., by comparing said determined location with a predetermined location, or determining whether the determined location falls within a predetermined area or range, the alignment of the reflectometer system (1) may be optimized. In particular, in the example illustrated, the sample (2) may be moved, e.g. rotated. For example, the sample (2) may be positioned in a sample holder (not shown) that may be induced by the controller (8) to move, e.g. rotate the sample (2).

[0081] In FIG. 2, the reflectometer system (1) is shown after said rotation of the sample (2) so that the alignment is optimized, for example, so that the alignment beam (50) is incident within a predetermined area onto the position sensor (7). In the example illustrated, when the alignment is optimized, the measurement beam (30) and the alignment beam (50) are incident at normal incidence onto the sample (2).

[0082] In particular when the position sensor (7) is farther away from the sample (2) than the reflectance sensor (4) for measuring the reflected source signal, high accuracy on said alignment may be achieved. Indeed, the length of the propagation path of the alignment beam (50) from the sample (2) to the position sensor (7) is preferably larger than the length of the propagation path of the measurement beam (30) from the sample (2) to the reflectance sensor (4) (when the alignment of the system (1) is optimized).

[0083] In the example illustrated, the alignment radiation source (5) is positioned adjacent to the reflectance sensor (4) to prevent interference between the alignment beam (50) and the measurement beam (30). Indeed, when the alignment is optimized, the alignment beam (50) propagates in a substantially opposite direction with respect to the measurement beam (30), and the alignment beam (50) is unlikely to be incident onto the reflectance sensor (4). Furthermore, the beams (30, 50) are parallel to each other at least at the sample (2), which may facilitate proper alignment of the reflectometer system (1) based on the detection of the alignment beam (50) by the position sensor (7). More in particular, in the example illustrated, when the alignment is optimized such as in FIG. 2, the alignment beam (50), from the alignment radiation source (5) via the first beam splitter (61) to the sample (2), and the measurement beam (30), from the sample (2) via the first beam splitter (61) to the reflectance sensor (4), propagate along substantially parallel paths.

[0084] In the example illustrated, the method for determining a sample's (2) reflectance involves directing the alignment beam (50) towards the sample (2), measuring the position of the reflected alignment beam (50), determining the system's (1) alignment based on this measurement, and directing the measurement beam (30) towards the sample (2) to measure the intensity of the reflected measurement beam (30). The directing of the alignment beam (50) and of the measurement beam (30) towards the sample (2) may overlap in time. Although the alignment of the system (1) may be optimized based on the determined alignment, this is not essential. For example, instead, the determined alignment may be used to calibrate the measured reflectance. Alternatively, the measured reflectance may be deemed acceptable or sufficiently accurate when the determined position of the alignment beam (50) on the position sensor (7) falls within a predetermine range.

[0085] FIG. 3 introduces a variation on the reflectometer system (1), wherein the measurement radiation source (3), the alignment radiation source (5), the reflectance sensor (4), and the position sensor (7) are all located within the same housing (9). In the example illustrated, the controller (8) is adapted to control the position of the first beam splitter (61). The orientation of the first optical element (61) may be adapted by a combination of rotating and translating the beam splitter (61). The system (1) is aligned when the alignment beam (50) hits a predetermined location on the position sensor (7), ensuring that the measurement beam (30) is incident on the reflectance sensor (4). The reflectometer system (1) is configured so that, when the position of the alignment beam (50) is incident at a predetermined location of the position sensor (7), the reflectometer system (1) is aligned so that the measurement beam (30) is incident onto the reflectance sensor (4). In the example illustrated, when the alignment is optimized (as shown in FIG. 3), the beams (30, 50) are substantially parallel at the sample (2). In the example illustrated, when the alignment is optimized (as shown in FIG. 3), the beams (30, 50) may be incident onto the sample (2) at an angle different from normal incidence. This may be a result of optimizing the alignment not by optimizing the alignment of the sample (2) but of different optical components, in particular, as in the example illustrated, the first beam splitter (61). Indeed, as both the alignment beam (50) is reflected on its propagation path from the alignment radiation source (5) towards the position sensor (7), and the measurement beam (30) is reflected on its propagation path from the measurement radiation source (3) towards the reflectance sensor (4), realignment of the first beam splitter (61) realigns both the propagation path of the alignment beam (50) towards the position sensor (7) and of the measurement beam (30) towards the reflectance sensor (4). Thereby, optimization of the alignment of the alignment beam (50) may go together with optimization of the alignment of the measurement beam (30), or in other words, the propagation direction of both beams (30, 50) may be adapted at the same time so that when the position of the alignment beam (50) on the position sensor (7) is optimized, the alignment of the measurement beam (30) with respect to the reflectance sensor (4) is optimized as well.

[0086] In the example illustrated, the sample (2) is located outside of the housing (9). The beams (30, 50) may leave the housing (9) through an opening towards the sample (2) and, after reflection by said sample (2) the beams (30, 50) may enter the housing (9) back through said same opening (90). Therefore, in the example illustrated, it may be difficult to align the sample (2) by, e.g., rotating the sample (2). This configuration may, however, have the advantage that the housing (9) and the parts of the system (1) located therein, may be straightforwardly installed, while the alignment of the system (1) may still be optimized independent of the particular orientation of the housing (9) with respect to the sample (2).

[0087] FIG. 4 illustrates an exemplary position sensor (7) that comprises a two-dimensional array (70) of pixels (71), e.g., in the example illustrated, of 12x12 pixels (71). The narrow spot (51) for the alignment beam provides high accuracy for determining the alignment of the reflectometer system (1), as opposed to the large spot (31) for the measurement beam. The different spot sizes result from the different widths of the beams. Determining a location of a center of the broad spot (31) onto the position sensor (7) may result in a large error for the determined location. Furthermore, in practice, a size of the larger spot (31) for the measurement beam may be larger than the photosensitive area (70) of the position sensor (7).

[0088] In the example illustrated, the alignment of the sample or of the system is deemed to be optimized when the narrow spot (51) for the alignment beam is incident onto the center of the position sensor (7). Hence, in the example illustrated, the alignment is deemed optimized.

[0089] Instead, when the alignment is not optimized, the position sensor (7) may detect the alignment beam spot (51) at a location away from the center of the position sensor (7). For example, such as is the case in the example of FIG. 1, the sample (2) may be slightly tilted or misaligned. The alignment of the system may, in that case, be optimized, wherein the alignment is deemed to be optimized when the narrow spot (51) for the alignment beam is incident onto the center of the position sensor (7). Alternatively, the signal from the reflectance sensor may be accepted within given boundaries of deviation of the narrow spot (51) on the position sensor (7), the signal from the reflectance sensor may be corrected depending on the position sensor (7) reading signal, or the signal from the reflectance sensor may be ignored when the alignment is deemed to be not optimized.

[0090] Reference is made to FIG. 5, which is a schematic representation of another reflectometer system (10) in accordance with embodiments of the present invention for analyzing a sample's (2) reflectance with high precision.

[0091] The reflectometer system (10) comprises a measurement radiation source (3) for generating a measurement beam (30) and an alignment radiation source (5) for generating an alignment beam (50). Herein, the diameter of the alignment beam (50) is smaller than the diameter of the measurement beam (30), e.g., at least at the sample. In the illustrated example, when the reflectometer system (10) is aligned, the measurement beam (30) and the alignment beam (50) are incident onto the sample (2) at non-normal incidence. As a result, the beam paths of the beams (30, 50) assume a V-shape. In the example illustrated, the measurement beam (30) and the alignment beam (50) follow substantially parallel beam paths (but in opposite directions) over at least part - in the example illustrated, over largest part - of their propagation paths. As a result, the effect of the orientation of the sample (2) on both beams (30, 50) is similar, which may facilitate alignment. However, parallel beam paths are not essential, and instead, the beam paths may be non-parallel, which may limit cross-talk.

[0092] In the example illustrated, the reflectometer system (10) comprises a position sensor (7). The position sensor (7) measures a position of the alignment beam (50) on the position sensor (7) after reflection by the sample (2), which may be used to determine whether the reflectometer system (10) is correctly aligned. Furthermore, the reflectometer system (10) comprises a sensor (33) for detecting the intensity of the separated measurement beam (32). The reflectometer system (10) further comprises a reflectance sensor (4) for detecting an intensity of the measurement beam (30) after reflection by the sample (2).

[0093] It is an advantage of the illustrated reflectometer system (10), wherein the beams (30, 50) are incident at non-normal incidence, that the number of beam splitters (62) may be limited. In the example illustrated, a single beam splitter (62) is used, for diverting the alignment beam (50) towards the position sensor (7), and for diverting part of the measurement beam (30) towards a sensor (33) for detecting the intensity of the separated measurement beam (32). Instead, in the other examples above, a further beam splitter was used for diverting the measurement beam (30) after reflection by the sample (2) from the path of the measurement beam (30) before said reflection, and for directing the alignment beam (50) towards the sample (2) at normal incidence. The less beam splitters (62) are used, the higher the signal at the sensors (4, 7) may be, and the shorter the measurement times may be needed to achieve accurate and reproducible measurements. In the current embodiment, it may be preferable that the transmittance level of the beam splitter (62) is larger than its reflectance level.

[0094] Reference is made to FIG. 6A, which is a schematic representation of another exemplary reflectometer system (10) according to embodiments of the present invention, that may switch between detecting a sample (2), a reference sample (21), and a shutter (22).

[0095] In FIG. 6A, the reflectometer system (10) is aligned on the sample (2) for analyzing the sample (2), e.g., for determining a reflectance of reflectivity of the sample (2). The reflectometer system (10) comprises a measurement radiation source (3), which may, for example, be an Ulbricht sphere or an integrating sphere, for generating a measurement beam (30) and an alignment radiation source (5) for generating an alignment beam (50). A beam splitter (62) is configured for diverting the alignment beam (50), after reflection on the sample (2), towards a position sensor (7). The beam splitter (62) is also configured for diverting part of the measurement beam (30) towards a sensor (33) for detecting the intensity of the separated measurement beam (32). The reflectometer system (10) comprises a sensor (33) for detecting the intensity of the separated measurement beam (32). The reflectometer system (10) further comprises a reflectance sensor (4) for detecting an intensity of the measurement beam (30) after reflection by the sample (2).

[0096] The position sensor (7) measures a position of the alignment beam (50) on the position sensor (7) after reflection by the sample (2), which may be used to determine whether the reflectometer system (10) is correctly aligned. In the example illustrated, the reflectometer system (10) contains a lens (72) for focusing the reflected alignment beam (50) on the position sensor (7), which may result in improved sensitivity and accuracy for the alignment.

[0097] In the reflectometer system (10) of the present example, the reflectance sensor (4) and the alignment radiation source (5) comprise optical strands, contained in a bifurcated optical fiber, for receiving the measurement beam (30) and for providing the alignment beam (50), respectively. FIG. 6B is a schematic cross-sectional representation of the bifurcated fiber, containing, in the example illustrated, six strands (5) for emitting the alignment beam (50), encircling a single, central strand (4) for receiving the measurement beam. However, instead, a single strand may be used for providing the alignment beam (50), or more than six strands (5) could be provided, and the stands (5) may be provided in a different configuration than encircling the central, measurement beam strand (4), the invention not being limited thereto.

[0098] In the present example, a lens (54) is provided or arranged for coupling the measurement beam into the optical strand of the reflectance sensor (4). The lens (54) is further arranged for providing a substantially parallel or substantially collimated alignment beam (50). The measurement beam strand (4) may be optically coupled to a detector for sensing the measurement beam signal. The alignment beam stands (5) may be optically coupled to any source, e.g., to a laser or LED, configured for generating the alignment beam (50). The sample (2) may be analyzed, e.g., a signal for the reflectance or reflectivity of the sample (2) may be detected. The sensor (33) may detect the intensity of the separated measurement beam (32), which may be used for calibrating said signal. The reflectometer system (10) may comprise a lens (34) for focusing the separated measurement beam (32) onto the sensor (33). Alternatively, the sensor (33) may comprise an optical fiber and the lens (34) may be for coupling the separated measurement beam (32) into the optical fiber. The optical fiber may be coupled to a detector of the sensor (33) for detecting the separated measurement beam (32).

[0099] In FIG. 7 , the sample (2) has been removed from the propagation path of the beams (30, 50) to expose the reference sample (21) on which the beams (30, 50) are now incident. In the example illustrated, the reflectometer system (10) is now aligned on the reference sample (21) for detecting the reflection of the reference sample (21). Due to the different location of the reference sample (21) than the sample (2), compared to FIG. 6A, a re-alignment of the reflectometer system (10) may have been performed, which may be - similar as for the other examples - be based on the signal measured by the position sensor (7). Preferably, said realignment is minimized. Furthermore, for consistency and accuracy, in view of the angledependence of reflectivity on a surface, it may be preferred that the angle of incidence of the measurement beam (30) onto the reference sample (21), when analyzing the reference sample

[0100] (21), is substantially the same as the angle of incidence of the of the measurement beam (30) on the sample (2) (as in FIG. 6A), when analyzing the sample. The reference sample (21) may be analyzed, e.g., a signal for the reflectance or reflectivity of the reference sample (21) may be detected. This reference measurement may be used for calibrating the signal detected for the sample (2).

[0101] FIG. 8 is a schematic representation of the reflectometer system (10), wherein the shutter (22) has been positioned in the propagation path of the beams (30, 50). The shutter

[0102] (22) is typically adapted for preventing that the measurement beam (30) is detected by the reflectance sensor (4). The shutter (22) may also be adapted for preventing that the alignment beam (50) is detected by the position sensor (7). The shutter (22) is preferably optically absorbing towards the measurement beam (30). The shutter (22) is preferably optically absorbing towards the alignment beam (50). When the measurement beam (30) is blocked by the shutter, a dark measurement may be performed by the reflectance sensor (4). This dark measurement may be used for correcting the signal detected for the sample (2) and for the reference sample (21) for background signals. For example, this dark measurement signal may be subtracted from the measured reflectance signal for the sample (2). For example, this dark measurement signal may be subtracted from the measured reflectance signal for the reference sample (21).

[0103] Although, in the above examples, the measurement beam and the alignment beam are generally directed into a substantially opposite direction along at least part of their propagation paths, the invention is not limited thereto. For example, instead, the measurement beam and the alignment beam may directed into a substantially same direction along at least part of their propagation paths. It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope of this invention. Steps may be added or deleted to methods described within the scope of the present invention.

Claims

Claims1.- A reflectometer system (1, 10) for analyzing a sample (2) comprising: a measurement radiation system comprising a measurement radiation source (3) for generating a measurement beam (30), a reflectance sensor (4) for detecting an intensity of the measurement beam (30) after reflection by the sample (2), when present, an alignment radiation source (5) for generating an alignment beam (50), and a position sensor (7) for measuring a position of the alignment beam (50) on said position sensor (7) after reflection by the sample (2), and wherein the reflectometer system (1, 10) is adapted for determining an alignment of the system (1, 10) relative to the sample (2) based on the measured position of the alignment beam (50), wherein a beam width of the alignment beam (50) is smaller than a beam width of the measurement beam (30), wherein the reflectometer system (1, 10) is configured so that, when an alignment of the reflectometer system (1, 10) is optimized, at least part of a propagation path of the measurement beam (30) is substantially parallel to at least part of a propagation path of the alignment beam (50).2.- The reflectometer system (1, 10) of claim 1, wherein, when said alignment is optimized, the propagation path of the alignment beam (50) substantially overlaps with the propagation path of the measurement beam (30) over at least part of their propagation paths towards the sample (2).3.- The reflectometer system (1) of claim 1 or 2, wherein, when said alignment is optimized, the measurement beam (30) and the alignment beam (50) are directed at an angle of at most 16°, preferably at most 12°, such as typically 8° with respect to normal incidence onto the sample (2), when present.4.- The reflectometer system (1, 10) of any of claims 1 to 3, wherein the alignment radiation source (5) is located adjacent to the reflectance sensor (4) such that the alignment beam (50) is directed in a substantially opposite direction with respect to a direction of the measurement beam (30) that is incident onto the reflectance sensor (4) after reflection by the sample (2), when present.The reflectometer system (1, 10) of any of the previous claims, wherein the diameter of the alignment beam (50) on the sample (2), when present, is less than half the diameter of the measurement beam (30) on the sample (2). The reflectometer system (1, 10) of any of the previous claims, wherein the position sensor (7) comprises a two-dimensional photodetector. The reflectometer system (1, 10) of any of the previous claims, wherein a wavelength of the measurement beam (30) is different from a wavelength of the alignment beam (50). The reflectometer system (1, 10) of claim 7, wherein the reflectance sensor (4) is adapted for separating or filtering out the wavelength of the alignment beam (50). The reflectometer system (1, 10) of any of the previous claims, wherein pulses of the measurement beam (30) and / or pulses of the alignment beam (50) on the one hand and / or pulses on the reflectance sensor (4) and / or pulses on the position sensor (7) on the other hand are alternated. The reflectometer system (1, 10) of any of the previous claims, wherein the alignment radiation source (5) is adapted for providing a pulsed alignment beam (50) and the reflectance sensor (4) is adapted for separating or filtering out the alignment beam (50). The reflectometer system (1, 10) of any of the previous claims, further comprising a beam splitter (62) for separating part of the measurement beam (30) before reflection by the sample (2), when present, and a sensor (33) for detecting the intensity of the separated measurement beam (32). The reflectometer system (1, 10) of any of the previous claims, comprising a sample holder for holding the sample (2), wherein the sample holder is adapted for displacing the sample (2) for optimizing said alignment based on the measured position of the alignment beam (50).13.- The reflectometer system (1, 10) of any of the previous claims, further comprising optical elements (61, 62) configured for directing said measurement beam (30) and / or said alignment beam (5) towards the sample (2), when present, and / or for directing said alignment beam (5) towards the position sensor (7).14.- The reflectometer system (1, 10) of claim 13, further comprising steering means for changing the configuration of the optical elements (61, 62) for improving an alignment of the reflectometer system (1, 10) based on the measured position of the alignment beam (50) on the position sensor (7).15.- The reflectometer system (1, 10) of claim 14, further comprising a controller (8) for automated control of the steering means based on the measured position of the alignment beam (50) on the position sensor (7).16.- The reflectometer system (1, 10) of any of the previous claims, wherein at least three selected from the measurement radiation system (1, 10), the alignment radiation source (5), the reflectance sensor (4) and the position sensor (7) are located in a same housing (9), wherein the reflectometer system (1, 10) is configured for analyzing the sample (2) located outside of said housing (9).17.- The reflectometer system (1, 10) of claim 16, wherein the reflectometer system (1, 10) is configured for analyzing the sample (2) located at a distance of said housing (9) that is larger than the beam width of the measurement beam (30), e.g., at least 5 times the beam width, or even more than 10 times the beam width.18.- A setup comprising the reflectometer system (1, 10) of any of the previous claims, and a vacuum system comprising a vacuum chamber comprising an optical window, wherein the reflectometer system (1, 10) is located outside of the vacuum chamber, and is arranged so that the measurement beam and the alignment beam propagate through said optical window for analyzing a sample (2), when present, in the vacuum chamber.19.- A method for determining a reflectance of a sample (2) in a reflectometer system (1,10), comprising:directing an alignment beam (50) towards the sample (2) for reflecting the alignment beam (50), measuring a position of the reflected alignment beam (50) on a position sensor (7), determining an alignment of the reflectometer system (1, 10) based on the measured position, directing a measurement beam (30), having a beam width larger than that of the alignment beam (50), towards the sample (2) for reflecting the measurement beam (30), and measuring an intensity of the reflected measurement beam (30) for determining said reflectance, wherein the reflectometer system (1, 10) is configured so that, when an alignment of the reflectometer system (1, 10) is optimized, at least part of a propagation path of the measurement beam (30) is substantially parallel to at least part of a propagation path of the alignment beam (50).

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