Patent Description:
The integrated circuit industry requires inspection tools with increasingly higher sensitivity to detect ever smaller defects and particles whose sizes may be a few tens of nanometers (nm), or less. These inspection tools must operate at high speed in order to inspect a large fraction, or even <NUM>%, of the area of a photomask, reticle, or wafer, in a short period of time. For example, inspection time may be one hour or less for inspection during production or, at most, a few hours for R&D or troubleshooting. In order to inspect so quickly, inspection tools use pixel or spot sizes larger than the dimensions of the defect or particle of interest and detect just a small change in signal caused by a defect or particle. Detecting a small change in signal requires a high light level and a low noise level. High speed inspection is most commonly performed in production using inspection tools operating with ultraviolet (UV) light. Inspection in R&D may be performed with UV light or with electrons.

The integrated circuit (IC) industry also requires high precision metrology tools for accurately measuring the dimensions of small features down to a few nanometers or less on semiconductor wafers. Metrology processes are performed on wafers at various points in a semiconductor manufacturing process to measure a variety of characteristics of the wafers such as a width of a patterned structure on the wafer, a thickness of a film formed on the wafer, and overlay of patterned structures on one layer of the wafer with respect to patterned structures on another layer of the wafer. These measurements are used to facilitate process controls and/or yield efficiencies in the manufacture of semiconductor dies. Metrology may be performed with UV light or with electrons.

The semiconductor industry, which is aimed at producing integrated circuits with higher integration, lower power consumption and lower costs, is one of the main drivers of UV optics. The development of powerful UV light sources such as the excimer lasers and frequency multiplied solid state lasers has led to the growth of research and development efforts in the field of UV photon applications.

An optical coating is a layer or several thin layers of material deposited on an optical component such as a mirror or lens, which alters the way in which the optical component reflects and transmits light. One type of optical coating is an antireflection coating (ARC), which reduces undesired reflections from optical surfaces. Optical coatings are ubiquitous in semiconductor inspection and metrology. They are found in most inspection and metrology systems, from reflection and transmission optics such as mirrors, lenses, beam splitters, and prisms, plasma arc lamp coatings, to crystal coatings and laser cavity coatings in deep ultraviolet (DUV) and vacuum ultraviolet (VUV) lasers.

Optical coatings in the DUV (~<NUM> to <NUM>) and VUV (~<NUM> to <NUM>) spectral ranges are challenging. DUV/VUV lasers may have high power levels from several milli-watts (mW) to ten or more watts (W) and high photon energy (for example <NUM>. 5eV at <NUM> and <NUM>. 66eV at <NUM>). Pulsed lasers may have short pulse lengths (ns or less) and high repetition rates (tens of kHz or greater). Optical coatings, in addition to being transparent in the DUV/VUV wavelength ranges, need to withstand these extreme conditions with high optical damage threshold, high hardness and good stability.

There are a few coatings known in the art suitable for DUV and VUV wavelengths. Among them, the most widely and commonly used is a magnesium fluoride (MgF<NUM>) coating. MgF<NUM> is transparent over a wide range of wavelengths. Optical elements such as lenses, mirrors, prisms, windows etc. coated with MgF<NUM> can be transparent from <NUM> (hydrogen Lyman-alpha line) in the VUV to <NUM> in the infrared. MgF<NUM> is used mostly for UV optics and in particular for excimer laser applications. Due to its low refractive index of <NUM>, thin layers of MgF<NUM> are widely used on optical element surfaces as inexpensive anti-reflection coatings. Crystalline MgF<NUM> is usually quite tough and works and polishes well when used to create components such as windows or lenses. However, MgF<NUM> coatings, which are necessarily amorphous, may not be stable as time goes by. Furthermore, they can be slightly porous. Fluorine tends to escape from the coating surfaces and magnesium oxide can be formed on either surface. Furthermore, MgF<NUM> coatings have a low optical damage threshold (~<NUM>. 1GW/cm<NUM>). Other materials such as hafnium oxide (HfO<NUM>), silicon dioxide (SiO<NUM>) and aluminum oxide (Al<NUM>O<NUM>) coatings can also be used as optical coatings, but they are only transparent for wavelengths longer than about <NUM>.

While significant interest in producing stable coating surfaces under VUV and DUV illuminations has existed for several decades, MgF<NUM> coating, despite its disadvantages, remains the only convenient coating especially for the VUV wavelength range. For the present application of high-speed inspection and metrology, optical coatings need to have high optical damage threshold, high hardness and good stability. It is further desired that such coatings have low permeability to diffusion of water and oxygen in order to reduce oxidation of the substrate or underlying layers.

<CIT> discloses a sensor with electrically controllable aperture for inspection and metrology systems
<NPL>.

<CIT> discloses a protective fluorine-doped silicon oxide film for optical components.

What is therefore needed is an optical coating material that overcomes some, or all, of the limitations of the prior art.

The present disclosure is directed to a system as recited in claim <NUM>. The present invention further provides a method of fabricating an optical component as recited in claim <NUM>.

Semiconductor inspection tools must operate at high speed in order to inspect a large fraction, or even <NUM>%, of the area of a photomask, reticle, or wafer, in a short period of time. For example, inspection time may be one hour or less for inspection during production or, at most, a few hours for R&D or troubleshooting. In order to inspect so quickly, inspection tools use pixel or spot sizes larger than the dimensions of the defect or particle of interest, and detect just a small change in signal caused by a defect or particle. High speed inspection is most commonly performed in production using inspection tools operating with ultraviolet (UV) light. High precision metrology tools are required for accurately measuring the dimensions of small features down to a few nanometers or less on semiconductor wafers. Metrology processes are performed on wafers at various points in a semiconductor manufacturing process to measure a variety of characteristics of the wafers such as a width of a patterned structure on the wafer, a thickness of a film formed on the wafer, and an overlay offset of patterned structures on one layer of the wafer with respect to patterned structures on another layer of the wafer. These measurements are used to facilitate process controls and/or yield efficiencies in the manufacture of semiconductor dies. High-speed inspection and metrology require high light levels and a stable signal. Optical coatings that reduce the reflection losses of transmissive optics and increase the reflection of reflective optics increase the light available at the sensor and thus allow for higher sensitivity and higher speed. Coatings that do not degrade, or degrade more slowly than existing coatings, can result in a more stable signal making it easier to detect small changes in signal. Such coatings also can reduce the operating cost of an inspection or metrology tool by reducing the frequency of replacement of optical components.

The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which:.

Although the claimed subject matter will be described in terms of certain embodiments, other embodiments, including embodiments that do not provide all of the benefits and features set forth herein, are also within the scope of this disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the disclosure. Accordingly, the scope of the disclosure is defined only by reference to the appended claims.

The following description is presented to enable one of ordinary skill in the art to make and use the disclosure as provided in the context of a particular application and its requirements. As used herein, directional terms such as "top," "bottom," "over," "under," "upper," "upward," "lower," "down," and "downward" are intended to provide relative positions for purposes of description and are not intended to designate an absolute frame of reference. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present disclosure is not intended to be limited to the particular embodiments shown and described but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.

<FIG> illustrates an exemplary inspection or metrology system <NUM> configured to inspect or measure a semiconductor-fabrication-related sample <NUM>, such as a silicon wafer, a reticle, or photomask. System <NUM> generally includes an illumination (light) source <NUM>, a sensor <NUM> and a stage <NUM>.

Illumination source <NUM> is preferably configured to generate (emit) deep UV (DUV) and/or vacuum UV (VUV) incident light (radiation) LIN having a wavelength in the range of <NUM> to <NUM> but may be configured to generate light having wavelengths greater than <NUM>. In some embodiments illumination source <NUM> utilizes one or more lasers and one or more optical components (e.g., a frequency converter <NUM>-<NUM>) to generate incident light LIN. In one embodiment, illumination source <NUM> may be a continuous source, such as an arc lamp, a laser-pumped plasma light source, or a continuous wave (CW) laser. In another embodiment, illumination source <NUM> may be a pulsed source, such as a mode-locked laser, a Q-switched laser, or a plasma light source pumped by a mode-locked or Q-switched laser. Suitable light sources that may be included in illumination source <NUM> are described in <CIT>et al. , <CIT>, and <CIT>.

Stage <NUM> is configured to receive sample <NUM> and to facilitate movement of sample <NUM> relative to optical system <NUM> (i.e., such that optical system <NUM> focuses incident light LIN on different regions of sample <NUM>). Stage <NUM> may comprise an X-Y stage or an R-θ stage. In one embodiment, stage <NUM> can adjust the height of sample <NUM> during inspection to maintain focus. In another embodiment, optics <NUM> can be adjusted to maintain focus.

Optical system (optics) <NUM> comprises multiple optical components and other optical components that are configured to direct and focus incident light LIN onto sample <NUM>, and to direct reflected (including scattered) light LR/S from the sample <NUM> to sensor <NUM>. The optical components of optical system <NUM> illustrated in <FIG> includes an illumination tube lens <NUM>-<NUM>, an objective lens <NUM>-<NUM>, a collection tube lens <NUM>-<NUM>, a condensing lens <NUM>-<NUM> and a beam splitter <NUM>-<NUM>.

During the operation of system <NUM> incident light LIN leaving illumination source <NUM> is directed by condensing lens <NUM>-<NUM> and illumination tube lens <NUM>-<NUM> to beam splitter <NUM>, which directs incident light LIN downward through objective lens <NUM>-<NUM> onto sample <NUM>. Reflected light LR/S represents the portion of incident light LIN that is reflected and/or scattered in an upward direction into objective lens <NUM>-<NUM> by the surface features of sample <NUM>, and is directed by objective lens <NUM>-<NUM> and collection tube lens <NUM>-<NUM> to sensor <NUM>. Sensor <NUM> generates an output signal/data based on the amount of reflected light LR/S received from sample <NUM>. The output of sensor <NUM> is provided to a computing system <NUM>, which analyzes the output. Computing system <NUM> is configured by program instructions <NUM>, which can be stored on a carrier medium <NUM>. In one embodiment, computing system <NUM> controls the inspection or metrology system <NUM> and sensor <NUM> to inspect or measure a structure on sample <NUM>. In one embodiment, system <NUM> is configured to illuminate a line on sample <NUM> and to collect reflected/scattered light in one or more dark-field and/or bright-field collection channels. In this embodiment, detector assembly <NUM> may include a time delay and integration (TDI) sensor, a line sensor or an electron-bombarded line sensor.

In one embodiment, illumination tube lens <NUM>-<NUM> is configured to image illumination pupil aperture <NUM> to a pupil stop within objective lens <NUM>-<NUM> (i.e. illumination tube lens <NUM>-<NUM> is configured such that the illumination pupil aperture <NUM> and the pupil stop are conjugate to one another). Illumination pupil aperture <NUM> may be configurable, for example, by switching different apertures into the location of illumination pupil aperture <NUM>, or by adjusting a diameter or shape of the opening of illumination pupil aperture <NUM>. In this way, sample <NUM> may be illuminated by different ranges of angles depending on the measurement or inspection being performed under control of computing system <NUM>.

In one embodiment, collection tube lens <NUM>-<NUM> is configured to image the pupil stop within objective lens <NUM>-<NUM> to collection pupil aperture <NUM> (i.e. collection tube lens <NUM>-<NUM> is configured such that the collection pupil aperture <NUM> and the pupil stop within objective lens <NUM>-<NUM> are conjugate to one another). Collection pupil aperture <NUM> may be configurable, for example, by switching different apertures into the location of collection pupil aperture <NUM>, or by adjusting a diameter or shape of the opening of collection pupil aperture <NUM>. In this way, different ranges of angles of light reflected or scattered from sample <NUM> may be directed to detector assembly <NUM> under control of computing system <NUM>.

Either, or both, of illumination pupil aperture <NUM> and collection pupil aperture <NUM> may comprise a programmable aperture such as one described in <CIT>, or to one described in <CIT>. Methods of selecting an aperture configuration for wafer inspection are described in <CIT>, and <CIT>.

According to an aspect of the invention that is described in additional detail in the specific embodiments provided below, one or more optical components utilized in illumination source <NUM> and/or in optics <NUM> includes at least one optical material layer formed on at least one surface of the components' substrate structure, where the optical material layer consists essentially of strontium tetraborate (i.e., at least <NUM>% of the optical material layer is SrB<NUM>O<NUM>). For example, in an exemplary specific embodiment at least one of frequency converter <NUM>-<NUM> of illumination source <NUM> and optical components <NUM>-<NUM> to <NUM>-<NUM> of optical system <NUM> includes a single strontium tetraborate optical material layer (e.g., as described below with reference to <FIG>). In another embodiment at least one of frequency converter <NUM>-<NUM> of illumination source <NUM> and optical components <NUM>-<NUM> to <NUM>-<NUM> of optical system <NUM> includes a multiple optical material layers, at least one of which consisting essentially of strontium tetraborate (e.g., as described below with reference to <FIG> and <FIG>). Exemplary materials for the optical material layers other than the SrB<NUM>O<NUM> layer(s) include, for example, hafnium oxide, aluminum oxide, aluminum fluoride and magnesium fluoride. In another embodiment at least one of frequency converter <NUM>-<NUM> of illumination source <NUM> and optical components <NUM>-<NUM> to <NUM>-<NUM> of optical system <NUM> includes an SrB<NUM>O<NUM> optical material layer that forms a continuous encapsulating structure that entirely surrounds each optical component's substrate structure (e.g., as described below with reference to <FIG>). As described with reference to the specific embodiments provided below, the thickness of the SrB<NUM>O<NUM> layer may be chosen such that destructive interference occurs for a reflected beam when light is incident on the coated optical component, and thus reflectance of the component is reduced. Alternatively, the thickness of the SrB<NUM>O<NUM> layer may be chosen such that constructive interference occurs for a reflected beam, and thus reflectance of the component is enhanced. The overall light throughput of system <NUM> may be improved by appropriately coating one or more optical components. The lifetime of key optical components may also be improved by the SrB<NUM>O<NUM> coating.

Additional details of various embodiments of inspection or metrology system <NUM> are described in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, and <CIT>.

<FIG> illustrates an exemplary optical component <NUM> including a single SrB<NUM>O<NUM> optical material layer (coating) <NUM> formed on a top (light receiving) surface 201T of a substrate <NUM> according to a first embodiment of the present disclosure. Substrate <NUM> is depicted in a generalized form for descriptive purposes and may comprise any suitable material and have any form required to implement any optical component found in inspection and metrology systems (e.g., any of components <NUM>-<NUM> to <NUM>-<NUM> shown in <FIG>). That is, optical component <NUM> may be of any type capable of implementing reflection or transmission optics such as a mirror, a lens, a beam splitter, a prism, a plasma arc lamp envelope, a nonlinear crystal, and a laser crystal. In one embodiment, SrB<NUM>O<NUM> optical material layer <NUM> has a thickness T-<NUM> in the range between <NUM> and <NUM>.

When implemented in a host inspection and metrology system, optical component <NUM> is fixedly maintained within illumination source <NUM> or optics <NUM> with its substrate <NUM> positioned to intercept a light beam portion <NUM> propagating through external medium <NUM>, which is typically air, an inert purge gas such as nitrogen or argon, or a vacuum. Referring to <FIG>, depending upon where optical component <NUM> is implemented, light beam portion <NUM> may be either incident light LIN or reflected/scattered light LR/S. In either case, optical component <NUM> is oriented such that light beam portion <NUM> strikes upper surface 202T of SrB<NUM>O<NUM> layer <NUM> at an angle of incidence θ<NUM> relative to a normal (depicted by the dashed-dotted line) to surface 202T, and a portion of light beam portion <NUM> passes through SrB<NUM>O<NUM> layer <NUM> before reaching top (light receiving) surface 201T of substrate <NUM>. The portion of incident light beam <NUM> that enters SrB<NUM>O<NUM> layer <NUM> is refracted by the boundaries (at 202T and 201T) of SrB<NUM>O<NUM> layer <NUM>, as shown by beams <NUM> and <NUM>. In addition to refraction, some part of incident light beam <NUM> and refracted light beam <NUM> is also reflected by these boundaries, as shown by reflected beams <NUM> and <NUM>. Beam <NUM> is in turn reflected and refracted at the material boundary (top surface of substrate) 201T, as shown by beams <NUM> and <NUM> respectively. Beam <NUM> is in turn reflected and refracted at the material boundary at 201T, as shown by beams <NUM> and <NUM>. Beam <NUM> is reflected and refracted at 202T and so on, leading to an infinite series of internal reflections within SrB<NUM>O<NUM> layer <NUM> (the first few of which are shown as <NUM>, <NUM>, <NUM>, <NUM> and <NUM>), an infinite series of downward traveling transmitted beams in substrate <NUM> (the first few of which are shown as <NUM>, <NUM> and <NUM>) and an infinite series of upward traveling transmitted beams in external medium <NUM> (the first few of which are shown as <NUM>, <NUM> and <NUM>). Note that the refraction/reflection angles and layer thicknesses shown in <FIG> are not to scale but are merely used to demonstrate that a single incident light beam can be refracted and reflected multiple times at the boundaries of substrate <NUM> and SrB<NUM>O<NUM> layer <NUM>. Thus, other refractions and reflections can occur based on beam <NUM>, for example, but are not shown for simplicity. In the embodiments of the inventions disclosed herein, a width of incident beam <NUM> would typically be many times greater than the thickness T-<NUM> of layer <NUM> (for example, the width of incident beam might be in the range of several microns to tens of mm, where T-<NUM> might be in the range of tens to hundreds of nm), hence the multiple beams are all substantially spatially overlapped and appear as one reflected beam equal to the sum of all upward travelling beams in medium <NUM> (i.e. the sum of reflected beam <NUM> and transmitted beams such as <NUM>, <NUM> and <NUM>), and one transmitted beam traveling into substrate <NUM> equal to the sum of all the transmitted beams (i.e. the sum of beams such as <NUM>, <NUM> and <NUM>).

In one embodiment optical material layer <NUM> is configured to minimize a reflectivity of optical component <NUM>, thereby minimizing an amount of received light portion <NUM> that is directed away from the top surface 202T, i.e. minimizing the total power of beams <NUM>, <NUM>, <NUM>, <NUM> etc. That is, optical material layer <NUM> is configured such that the sum of the upward-travelling secondary beams in external medium <NUM> (e.g. beams <NUM>, <NUM>, and <NUM>) is substantially opposite in phase (i.e. a phase difference of substantially <NUM>° or any odd integer multiple of <NUM>°) to reflected beam <NUM>. In one embodiment, the amplitude of reflected beam <NUM> is approximately equal in amplitude to (e.g. within a range of about <NUM>% of the amplitude to about <NUM>% of the amplitude of) the sum of upward-traveling secondary beams in external medium <NUM> (e.g. beams <NUM>, <NUM> and <NUM>), but opposite in phase. When outgoing beam <NUM> is substantially cancelled by the outgoing secondary beams <NUM>, <NUM> and <NUM> etc., destructive interference occurs and the reflectance of the system is considerably reduced, thereby improving the light transmission of optical component <NUM> and reducing light losses in a system incorporating optical component <NUM>.

It is well known that the amplitude reflectivities rp and rs (i.e. complex reflectivities of the electric fields for p and s polarizations respectively) at a wavelength λ of a single layer film on a substrate are given by the expressions: <MAT> where <MAT> the phase change when traversing optical material layer <NUM> once, <MAT> the Fresnel reflectivity for p polarized light incident on the interface between layers j and k from the j side, <MAT> the Fresnel reflectivity for s polarized light incident on the interface between layers j and k from the j side, Tj represents the thickness of layer j (i.e. T<NUM> = T-<NUM> in <FIG>), nj represents the refractive index of layer j, θj represents the angle of incidence on the boundaries of layer j (e.g. θ<NUM> and θ<NUM> in <FIG> for the external medium <NUM> and the optical material layer <NUM> respectively), and layer indices j and k = <NUM>, <NUM> and <NUM> refer respectively to the external medium <NUM>, the optical material layer <NUM>, and the substrate <NUM>. See, for example <NPL>. Angles of incidence θ<NUM> in optical material layer <NUM> and θ<NUM> in substrate <NUM> can be calculated from θ<NUM> (the angle of incidence on surface 202T) by Snell's law.

The intensity or power reflectivity coefficients, Rp and Rs, are each equal to the square of the modulus of the amplitude reflectivity coefficient for that polarization: <MAT>.

In one embodiment, SrB<NUM>O<NUM> layer <NUM> is formed such that its thickness T-<NUM> operably generates the desired destructive interference at wavelength λ-<NUM> of the received light portion <NUM>. For example, if r<NUM>,p and r<NUM>,p are real numbers with the same sign (i.e. all materials are substantially non-absorbing at wavelength λ-<NUM> and the phase change at both interfaces is equal), then layer <NUM> is generated with thickness T-<NUM> approximately equal to one-quarter of the wavelength of the light in optical material <NUM> (i.e. T-<NUM> should be approximately equal to <MAT>), but if r<NUM>,p and r<NUM>,p have opposite signs (i.e. all materials are substantially non-absorbing at wavelength λ-<NUM> and the phase change at one interface is <NUM>° relative to that at the other interface), then layer <NUM> is generated with thickness T-<NUM> approximately equal to one-half of the wavelength of the light in optical material <NUM> (i.e. T-<NUM> should be approximately equal to <MAT>). Since SBO has a refractive index that is higher than that of most commonly used DUV and VUV substrate and non-linear crystal materials (such as fused silica, CaF<NUM> and CLBO), r<NUM>,p and r<NUM>,p will typically have opposite signs (and similarly for r<NUM>,s and r<NUM>,s), thickness T-<NUM> will need to be equal to approximately a half-wave when SBO is used to coat such materials.

In another embodiment, optical component <NUM> may be configured to function as a mirror by configuring optical material layer <NUM> to maximize an amount of received light portion <NUM> that directed away from the top surface 201T. In this embodiment, thickness T-<NUM> of SrB<NUM>O<NUM> layer <NUM> may be chosen such that constructive interference can take place so as to enhance the reflectivity of optical component (e.g., layer <NUM> is generated with thickness T-<NUM> such that at wavelength λ-<NUM>, one round trip such transmitted beam <NUM>, the phase change due to reflection at 201T and reflected beam <NUM> arrive back at surface 202T substantially in phase with reflection <NUM>). This too will reduce light losses in a system incorporating optical component <NUM> configured as a mirror. If substrate <NUM> has minimal absorption, i.e. any imaginary part of its refractive index n<NUM> is negligible, then the amplitude reflection coefficients r<NUM>,p and r<NUM>,s will be substantially real and the phase shifts upon reflection at this interface will be <NUM>° or <NUM>° depending on the signs of r<NUM>,p and r<NUM>,s, and a <NUM>° or <NUM>° phase shift due to thickness T-<NUM> can be chosen as appropriate to give constructive interference at surface 202T to match the phase shifts of r<NUM>,p and r<NUM>,s. Aluminum is a convenient substrate for DUV and VUV mirrors intended for use over a broad range of wavelengths as it has high reflectivity (for example about <NUM>% or higher) throughout the DUV and VUV spectrum. If substrate <NUM> comprises a metal, such as aluminum, the refractive index of the substrate n<NUM> will be complex. The above equations can be used to calculate the reflectivity, but the amplitude reflectivity coefficients f<NUM>,p and f<NUM>,s will be complex, i.e. the reflection results in a phase change which is neither <NUM>° or <NUM>°. An appropriate thickness T-<NUM> can be chosen to create constructive interference at surface 202T.

Note that an optical component such as a lens or mirror may have a curved surface. Any radius of curvature will be much larger than the thickness T-<NUM> of layer <NUM>, so the reflectivity at any one location on the surface can be calculated with good accuracy by the above equations. Because of the curvature, the angle of incidence θ<NUM> of light <NUM> will vary with location on the surface of the optical component. It may not be possible to achieve minimum or maximum reflectivity at every location on the surface of the optical component. In such a case, the thickness T-<NUM> may be chosen to minimize or maximize, as appropriate, the average reflectivity of the component.

In yet another embodiment, optical component <NUM> may be configured as a beam splitter. In this embodiment, the thickness of the SrB<NUM>O<NUM> layer may be chosen such that, for example, approximately <NUM>% of the incident light is reflected and approximately <NUM>% transmitted. In another example of a beam splitter, the thickness of the SrB<NUM>O<NUM> layer may be chosen such that one polarization state of the incident light is substantially reflected, and an orthogonal polarization may be substantially transmitted. Other relationships between transmission and reflection of a beam splitter may be chosen depending on the desired application of the beam splitter.

SrB<NUM>O<NUM> crystallizes in the orthorhombic system, Pnm<NUM><NUM>, with approximate unit cell dimensions a = <NUM>. 43Å, b = <NUM>. 71Å, and c = <NUM>. All boron atoms are coordinated tetrahedrally and an oxygen atom is common to three tetrahedra. Despite the three-dimensional network of tetrahedral, the borate network appears as a layer-like structure since there are relatively fewer links in the c direction of the unit cell.

SrB<NUM>O<NUM> exhibits unique optical and mechanical properties. The transparency range of SrB<NUM>O<NUM> is <NUM>-<NUM> in wavelength. This broad transmission window makes SrB<NUM>O<NUM> a good candidate for optical coating material especially for DUV and VUV wavelength ranges. The refractive indices of SrB<NUM>O<NUM> are high compared with other coating materials suitable for VUV wavelengths such as MgF<NUM>. For example, the refractive indices at <NUM> are <NUM> in the x direction, <NUM> in the y direction and <NUM> in the z direction. Note that the differences among these refractive indices are small; thus, phase matching processes of second harmonic generation and sum-frequency generation in SrB<NUM>O<NUM> are not possible. The optical damage threshold is very high (<NUM>. 7GW/cm<NUM>) compared with other VUV-transmissive materials such as MgF<NUM>. The microhardness of SrB<NUM>O<NUM> is also high (<NUM>/mm<NUM> in the x direction, <NUM>/mm<NUM> in the y direction and <NUM>/mm<NUM> in the z direction). The high optical damage threshold and microhardness allow SrB<NUM>O<NUM> coatings to withstand extreme conditions when exposed to DUV and VUV radiation. DUV and VUV lasers may have high power levels from several milli-watts (mW) to <NUM> watts (W) or more, and high photon energy (for example <NUM>. 5eV at <NUM> and <NUM>. 66eV at <NUM>). Pulsed lasers may have short pulse lengths (ns or less) and high repetition rates (tens of kHz or greater).

<FIG> and <FIG> illustrate exemplary optical components 300A and 300B including multi-layer coatings formed on a top surface 301T of a substrate <NUM>, wherein at least one of the optical material layers forming each multi-layer coating consists essentially of SrB<NUM>O<NUM>. As mentioned above with reference to <FIG>, substrate <NUM> is depicted in a generalized form for descriptive purposes and may comprise any suitable material and form required to implement any optical component found in inspection and metrology systems (e.g., any of components <NUM>-<NUM> to <NUM>-<NUM> shown in <FIG>). Substrate <NUM> can be part of any optical component found in inspection and metrology systems, from reflection or transmission optics such as mirrors, lenses, beam splitters, and prisms, plasma arc lamp coatings, to crystal coatings and laser cavity coatings in DUV and VUV lasers.

<FIG> illustrates an exemplary optical component with two-layer coating including a lower optical material layer 302A disposed on top surface 301T of substrate <NUM>, and an upper optical material layer 303A disposed on an upper surface 302T of lower layer 302A. As mentioned above, at least one of layers comprises or consists of SrB<NUM>O<NUM>. For example, in one embodiment lower optical material layer 302A consists essentially of SrB<NUM>O<NUM>, and upper optical material layer 303A consists of a conventional optical material (e.g., hafnium oxide, aluminum oxide, silicon dioxide or magnesium fluoride). In another embodiment, lower optical material layer 302A consists of a conventional optical material and upper optical material layer 303A consists essentially of SrB<NUM>O<NUM>. The thicknesses of each layer 302A and 303A may be chosen to achieve a desired reflectivity or reflectivity bandwidth using skills known in the art. The optical damage threshold and microhardness of the multi-layer coating may be reduced compared with a single SrB<NUM>O<NUM> layer but may still be acceptable for optical components used where the UV power density is not too high, such as components that are not in close proximity to the UV light source. The two-layer coating shown in <FIG> may be preferred over a single-layer coating (e.g., as shown in <FIG>) when the two-layer coating can more closely achieve the desired reflectivity or reflectivity bandwidth, and the reduction in damage threshold is acceptable for that component's intended use.

<FIG> illustrates an exemplary optical component 300B having a multi-layer coating including a lowermost optical material layer 302B formed on top surface 301T of substrate <NUM>, a lower-intermediate optical material layer 303B formed on an upper surface of lowermost layer 302B, a upper-intermediate optical material layer <NUM> formed on lower-intermediate layer 303B, and an uppermost optical material layer <NUM> formed on upper-intermediate layer <NUM>. The coating may consist of four layers as shown, or it may include more than four layers (not explicitly shown). At least one of layers 302B, 303B, <NUM> and <NUM> consists essentially of SrB<NUM>O<NUM>, with the remaining layers consisting of a conventional optical material (e.g., hafnium oxide, aluminum oxide, silicon dioxide or magnesium fluoride). In an exemplary embodiment, uppermost layer <NUM> consists essentially of SrB<NUM>O<NUM> to enhance protection of underlying layers 302B, 303B and <NUM> and substrate <NUM>. The thicknesses of the layers may be chosen to achieve a desired reflectivity or reflectivity bandwidth. The optical damage threshold and microhardness may be reduced compared with a single SrB<NUM>O<NUM> layer, but they may still be acceptable for optical components that are not in close proximity to the UV light source or are otherwise subject to low UV power density. A multi-layer coating may be preferred over a single-layer or two-layer coating when the multi-layer coating can more closely achieve the desired reflectivity or reflectivity bandwidth, and the reduction in damage threshold is acceptable for that component.

In some embodiments, the optical component 300B can be coated with alternating high and low refractive index materials, for example first layer 302B and third layer <NUM> may comprise a high index material, and second layer 303B and fourth layer <NUM> may comprise a low index material, or alternatively 302B and <NUM> may comprise a low index material and 303B and <NUM> may comprise a high index material. The high index material may comprise SrB<NUM>O<NUM>. The low index material may comprise MgF<NUM> or other material with a refractive index lower than SrB<NUM>O<NUM>. One of ordinary skill would understand how to choose the number of layers and layer thicknesses in order to achieve a desired reflectivity. Although multi-layer coatings are well known in the art, heretofore no efficient multi-layer coating has been possible for VUV wavelengths because of a lack of a high-index material with low absorption and high damage threshold over a broad range of VUV wavelengths.

Additional coating layers may be placed on top of the optical component 300B. A multi-layer coating may comprise <NUM>, <NUM>, <NUM>, <NUM> or more layers. Although coatings with alternating pairs of high and low index layers are convenient for making high-reflectivity and low-reflectivity surfaces, other configurations are possible and are within the scope of this invention.

<FIG> shows a typical transmission curve of SrB<NUM>O<NUM> (<NPL>)). As shown in transmission curve <NUM>, the transparency range of SrB<NUM>O<NUM> is very broad, namely from about <NUM> to about <NUM>, which covers VUV, DUV, visible, and near infrared (IR) wavelength ranges. The VUV and DUV ranges are of particular interest to semiconductor inspection and metrology. Various coating designs can be implemented as described above in relation to <FIG>, <FIG> and <FIG>. It is also noted that the transmittance is high. For instance, the transmittance exceeds <NUM>% from about <NUM> to about <NUM>. This high transmittance makes SrB<NUM>O<NUM> a good candidate for optical coating materials especially for the UV wavelength range.

<FIG> illustrates an electron-beam sputtering coating chamber with SrB<NUM>O<NUM> as source material in accordance with the present disclosure. Electron-beam technology is widely used for coating substrates. In coating chamber <NUM>, source material or coating material <NUM> comprising SrB<NUM>O<NUM> is placed in a crucible <NUM>. Alternatively, SrB<NUM>O<NUM> can placed into a "pocket" in an electron gun. Electron gun <NUM> is located close to the crucible <NUM>. Power supply <NUM> is applied to the electron gun <NUM> to cause electrons to bombard coating material <NUM> as indicated by arrow <NUM> such that SrB<NUM>O<NUM> molecules <NUM> are released. Some of these molecules <NUM> travel toward the substrate <NUM> as indicated by arrows <NUM> and are deposited onto the substrate <NUM>. The stream of electrons produced by electron gun <NUM> may be preferably steered by a series of electromagnets (not shown) onto the coating material SrB<NUM>O<NUM> <NUM>. To improve uniformity, the substrate <NUM> can be rotated as illustrated by arrow <NUM>. Two or more substrates may be placed in the coating chamber <NUM> if there are multiple parts to be coated. To improve durability, the coating chamber <NUM> can be heated, and an ion beam gun <NUM> may be added, which is directed at the substrate <NUM> to increase coating density. Electron-beam systems are usually versatile and can be reconfigured from one coating type to another simply by changing the source material. When a multi-layer coating is desired, coating chamber <NUM> may include multiple crucibles (not shown) similar to crucible <NUM>. Each crucible may contain a different material. A controller (not shown) may uncover different crucibles at different times in the coating process, for example by moving a shutter (not shown), or may otherwise direct the electron beam to the appropriate crucible at the appropriate time in order to achieve the desired coating structure on substrate <NUM>.

<FIG> illustrates another optical component <NUM> having a non-linear optical crystal (substrate) <NUM> coated with SrB<NUM>O<NUM> over its entire outer peripheral surface. In an embodiment of inspection or metrology system <NUM> shown in <FIG>, illumination source <NUM> includes a laser that generates a deep UV wavelength such as a wavelength shorter than <NUM>. For example, illumination source <NUM> may include a laser configured to generate a wavelength close to <NUM> by generating a fourth harmonic of approximately <NUM> wavelength light generated by a solid-state or fiber laser. The laser may generate a second harmonic of the <NUM> light by using an appropriately configured lithium triborate (LBO) crystal. LBO is not hygroscopic and may be operated in air, even when laser power levels of tens of Watts are used. The fourth harmonic may be generated using a CLBO crystal configured to double the frequency of the second harmonic light. CLBO is hygroscopic and must be kept in a very low humidity environment at all times, whether the laser is operating or not. For high power operation, such as a fourth harmonic power of about 10W or higher, it may be desirable to lower the oxygen level of the environment of the crystal well below that of the atmosphere in order to minimize surface damage to the crystal. By way of other examples, illumination source <NUM> may include a laser configured to generate a wavelength close to <NUM> or close to <NUM> by using harmonic generation and frequency summation in multiple non-linear optical crystals, as represented by frequency conversion crystal (optical component) <NUM>-<NUM>. CLBO and CBO are the most useful materials for frequency conversion at such wavelengths, but both materials are hygroscopic. <FIG> illustrates a frequency conversion crystal <NUM> comprising a substrate <NUM> consisting of one of CLBO and CBO and configured to generate a deep UV wavelength, for example configured to double the frequency of light having a wavelength near <NUM>, or configured to sum the frequencies of light having wavelengths near <NUM> and near <NUM> to generate light having a wavelength near <NUM>.

Referring to <FIG>, an entire peripheral surface of non-linear crystal (CLBO or CBO) substrate <NUM> is covered (surrounded) by a layer <NUM> consisting essentially of SrB<NUM>O<NUM>. In the depicted example, crystal (CLBO or CBO) substrate <NUM> has a rectangular prism (cuboid) shape with opposing top and bottom surfaces 601T and 601B, opposing side surfaces <NUM> S1 and <NUM> S2, and opposing end surfaces 601E1 and 601E2, which respectively form light input and light output surfaces of frequency conversion crystal <NUM>. In this example SrB<NUM>O<NUM> layer <NUM> includes an unbroken cuboid structure having portions that extend over all six surfaces 601T, 601B, 601S1, 601S2, 601E1 and 601E2. <FIG> shows an exemplary frequency conversion crystal 600A in which an SrB<NUM>O<NUM> layer 602A forms a single-layer coating directly on peripheral surface 601P (e.g., top and bottom surfaces 601T and 601B and end surfaces 601E1 and 601E2) of substrate <NUM>. In other embodiments, the SrB<NUM>O<NUM> layer may be part of a multi-layer encapsulating structure including one or more conventional optical material layers. For example, <FIG> shows another exemplary frequency conversion crystal 600B including a multi-layer coating structure in which a conventional optical material layer 603B is formed on the peripheral surface of substrate <NUM>, and an SrB<NUM>O<NUM> layer 602B is formed on layer 603B. <FIG> shows another exemplary frequency conversion crystal 600C with an alternative multi-layer coating structure in which SrB<NUM>O<NUM> layer 602C is formed on the peripheral surface of substrate <NUM>, and a conventional optical material layer 603C is formed on layer 602C. In each example the thickness of SrB<NUM>O<NUM> layer <NUM> is chosen to slow or prevent at least water from penetrating into crystal substrate <NUM>. The thickness of SrB<NUM>O<NUM> layer <NUM> may be approximately <NUM> or thicker, or approximately <NUM>. The thickness of SrB<NUM>O<NUM> layer <NUM> need not be uniform as long as it is sufficiently thick to slow or prevent diffusion of at least water, and as long as the layer is free of pinholes. The thicknesses of the SrB<NUM>O<NUM> layer on input surface 601E1 and output surface 601E2 may be further chosen to reduce reflectivity of one or more wavelengths used or generated in the frequency conversion. As indicated in <FIG>, one or both of input surface 601E1 and output surface 601E2 may be coated with additional layers on top of or below the SrB<NUM>O<NUM> layer to reduce reflectivity at one or more wavelengths of interest. The uniformity of the coatings on end surfaces 601E1 and 601E2 should be controlled to avoid too much variability in an optical property, such as a reflectivity, of those surfaces. Even if the SrB<NUM>O<NUM> coating is not completely impervious to water and oxygen, if it slows diffusion of these molecules sufficiently, it may allow storage and/or operation of a hygroscopic non-linear crystal in an environment with higher water and oxygen content than an uncoated crystal, and therefore can reduce filtration and operating costs. Since crystal substrate <NUM> must be held during the coating process, in one embodiment, two coating operations are used to coat the entire surface.

Various modifications to the described embodiments will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Although it is expected that the optical coating material disclosed herein will be particularly useful in semiconductor inspection and metrology systems, it is also envisioned that these coatings and materials may be useful in other applications where VUV and DUV radiation are present, such as in an optical lithography system, and where visible or IR radiations are present, such as in an IR camera system.

Claim 1:
A system (<NUM>) including:
a stage (<NUM>) configured to support a sample (<NUM>);
a light source (<NUM>) configured to generate incident light having a wavelength in a range between <NUM> and <NUM>;
a sensor (<NUM>); and
an optical system (<NUM>) configured to direct said incident light onto the sample, and to direct reflected light from the sample to the sensor,
wherein at least one of the light source and the optical system includes at least one optical component (<NUM>-<NUM>) comprising:
a substrate (<NUM>) positioned to receive a light portion such that said received light portion is directed toward a top surface (201T) of the substrate, said received light portion comprising one of the incident light and the reflected light; and
a first optical material layer (<NUM>) disposed on the substrate over the top surface and configured such that a portion of the received light portion passes through the first optical material layer to the top surface of the substrate, and
wherein first optical material layer consists essentially of strontium tetraborate (SrB<NUM>O<NUM>);
wherein the substrate (<NUM>) has an outer peripheral surface including said top surface and a bottom surface disposed opposite to the top surface, and
wherein the first optical material layer includes a first portion disposed over the top surface and a first portion disposed over the bottom surface; and
wherein the optical component comprises a frequency conversion crystal and said substrate comprises a hygroscopic non-linear optical material , and
wherein the first optical material layer forms a continuous encapsulating structure that entirely surrounds the outer peripheral surface of said substrate.