Patent Description:
Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.

Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. A number of metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, overlay, film thicknesses, composition and other parameters of nanoscale structures.

Traditionally, measurements are performed on targets consisting of thin films and/or repeated periodic structures. During device fabrication, these films and periodic structures typically represent the actual device geometry and material structure or an intermediate design. As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty. For example, modern memory structures are often high-aspect ratio, three-dimensional structures that make it difficult for optical radiation to penetrate to the bottom layers. Optical metrology tools utilizing infrared to visible light can penetrate many layers of translucent materials, but longer wavelengths that provide good depth of penetration do not provide sufficient sensitivity to small anomalies. In addition, the increasing number of parameters required to characterize complex structures (e.g., FinFETs), leads to increasing parameter correlation. As a result, the parameters characterizing the target often cannot be reliably decoupled with available measurements.

In one example, opaque, high-k materials are increasingly employed in modern semiconductor structures. Optical radiation is often unable to penetrate layers constructed of these materials. As a result, measurements with thin-film scatterometry tools such as ellipsometers or reflectometers are becoming increasingly challenging.

In response to these challenges, more complex optical metrology tools have been developed. For example, tools with multiple angles of illumination, shorter illumination wavelengths, broader ranges of illumination wavelengths, and more complete information acquisition from reflected signals (e.g., measuring multiple Mueller matrix elements in addition to the more conventional reflectivity or ellipsometric signals) have been developed. However, these approaches have not reliably overcome fundamental challenges associated with measurement of many advanced targets (e.g., complex 3D structures, structures smaller than <NUM>, structures employing opaque materials) and measurement applications (e.g., line edge roughness and line width roughness measurements).

X-Ray based metrology systems have shown promise to address challenging measurement applications. However reliable soft X-ray illumination sources suitable for x-ray based metrology technologies such as reflective small angle x-ray scatterometry (SAXS), coherent diffractive imaging (CDI), and other x-ray based imaging and overlay based techniques remain under development.

In some other examples, illumination light may be provided directly by a laser. One approach has been the harmonic upconversion of longer wavelength sources to shorter wavelengths. However, this approach has yet to yield a practical soft x-ray illumination source.

In some examples, illumination light may be provided by a light source pumped by a continuous wavelength laser (e.g., laser sustained plasma). Laser sustained plasmas are produced in high pressure bulbs surrounded by a working gas at lower temperature than the laser plasma. While substantial radiance improvements are obtained with laser sustained plasmas, the temperature of these plasmas is generally limited by the photophysical and kinetic processes within these lamps. Pure atomic and ionic emission in these plasmas is generally confined to wavelengths longer than <NUM>. Excimer emission can be arranged in laser sustained plasmas for wavelength emission at <NUM> (e.g., xenon excimer emission), but these sources are typically narrow band, limited in power, and limited in radiance. Excimer emission at <NUM> nanometers optimizes at low pressures (e.g., <NUM> bar and below), and the power of <NUM> emission is greatly diminished at higher pressures needed for high radiance. As a consequence, a simple gas mixture in a high pressure bulb is only able to sustain wavelength coverage above <NUM> with sufficient radiance and average power to support high throughput, high resolution metrology. In some examples, solid electrode targets are employed, but low repetition rate, electrode erosion, and large plasma size result in low brightness and short lifetime, limiting their effectivity for x-ray based semiconductor metrology.

Development efforts in the area of extreme ultraviolet (EUV) lithography are focused on light sources that emit narrowband radiation (e.g., +/-<NUM>) centered at <NUM> nanometers at high power levels (e.g., <NUM> watts of average power at the intermediate focus of the illuminator). Light sources for EUV lithography have been developed using a laser droplet plasma architecture. For example, xenon, tin, and lithium droplet targets operating at pulse repetition frequencies of approximately <NUM> are pumped by C02 coherent sources. The realized light is high power (e.g., <NUM> watts of average power at the intermediate focus of the illuminator is the goal for lithography tools at <NUM> nanometers). However, the materials that comprise a semiconductor wafer exhibit practically no reflectivity to narrowband light at <NUM> nanometers.

Experiments have been performed to provide broadband, soft x-ray illumination from a gas jet based laser produced plasma. Additional details are described by <NPL>). The use of a gas jet results in a large plasma (e.g., several hundred micrometers). Such a large plasma spot size severely limits the effectivity of such an illumination source for practical semiconductor metrology applications.

Broadband, soft X-ray illumination sources with the required radiance and average power for metrology applications are desired.

<NPL> discloses generation of radiation from a non-metallic target excited with sub-ns laser pulses.

<CIT> discloses a system, method and apparatus for droplet catcher for prevention of backsplash in a EUV generation chamber.

<CIT> describes a differential evacuation system.

<CIT> discloses high-efficiency, low-debris short-wavelength light sources.

A first aspect of the present invention provides a laser produced plasma light source as recited in claim <NUM>. A second aspect of the present invention provides a metrology system as recited in claim <NUM>. A third aspect of the present invention provides a method as recited in claim <NUM>.

Reference will now be made in detail to background examples and some embodiments of the invention, examples of which are illustrated in the accompanying drawings.

Methods and systems for x-ray based semiconductor metrology utilizing a broadband, soft X-ray illumination source are described herein. More specifically, a laser produced plasma (LPP) light source generates high brightness (i.e., greater than <NUM><NUM> photons/(sec-mm<NUM>-mrad<NUM>)), broadband, soft x-ray illumination (i.e., including wavelengths in a range of <NUM> nanometer to <NUM> nanometers). To achieve such high brightness, the LPP light source directs a highly focused, short duration laser source to a non-metallic droplet target in a liquid or solid state. The interaction of the focused laser pulse with the droplet target ignites a plasma. Radiation from the plasma is collected by collection optics and is directed to a specimen under measurement.

<FIG> depicts an x-ray based metrology system <NUM> including a laser produced plasma (LPP) light source <NUM> in at least one novel aspect. LPP light source <NUM> includes a plasma chamber <NUM>, droplet generator <NUM>, and a pulsed laser illumination source <NUM>. Plasma chamber <NUM> includes one or more walls that contain a flow of buffer gas <NUM> within the plasma chamber. Droplet generator <NUM> dispenses a sequence of droplets of a non-metallic feed material <NUM> into plasma chamber <NUM>.

In one aspect, the droplets generated by droplet generator <NUM> are in a solid or liquid state in plasma chamber <NUM>. In some embodiments, the non-metallic feed material is Xenon, Krypton, Argon, Neon, Nitrogen, or any combination thereof. In some embodiments, each droplet of non-metallic feed material <NUM> is less than <NUM> micrometers in diameter. In a preferred embodiment, each droplet of non-metallic feed material <NUM> is less than <NUM> micrometers in diameter. In some embodiments, droplet generator <NUM> is a high frequency fluid dispenser based on commercially available ink jet technology. In one example, droplet generator <NUM> dispenses a sequence of nominally <NUM> micron droplets of feed material <NUM> at a rate between <NUM> and <NUM> kilohertz.

Pulsed laser <NUM> generates a sequence of pulses of excitation light. Each pulse of excitation light is directed to a droplet of feed material <NUM>. The excitation light is focused by illumination optics <NUM> onto the droplets over a very small spot size. In some embodiments, the excitation light is focused onto the droplets with a spot size of less than <NUM> micrometers. In some embodiments, the excitation light is focused onto the droplets with a spot size of less than <NUM> micrometers. In a preferred embodiment, the excitation light is focused onto the droplets with a spot size of less than <NUM> micrometers. As the spot size of the excitation light decreases, the spot size of the induced plasma decreases. In a preferred embodiment, the spot size of plasma <NUM> is less than <NUM> micrometers. In some embodiments, pulsed laser <NUM> is a Ytterbium (Yb) based solid state laser. In some other embodiments, pulsed laser <NUM> is a Neodymium (Nb) based solid state laser.

In a further aspect, the duration of each pulse of excitation light is less than one nanosecond. In some embodiments, the duration of each pulse of excitation light is less than <NUM> nanoseconds.

The interaction of a pulse of excitation light with one or more droplets of the feed material causes the droplet(s) to ionize to form a plasma <NUM> that emits an illumination light <NUM> with very high brightness. In a preferred embodiment, the brightness of plasma <NUM> is greater than <NUM><NUM> photons/(sec)·(mm2)·(mrad2). The illumination light comprises broadband light in a spectral region from about <NUM> nanometer to about <NUM> nanometers.

The illumination light <NUM> is collected by collector <NUM> and focused onto specimen <NUM> under measurement. In the embodiment depicted in <FIG>, collector <NUM> gathers illumination light <NUM> emitted by plasma <NUM> and directs illumination light <NUM> through window <NUM>. In some embodiments, window <NUM> is an x-ray filter configured to be transparent to x-ray radiation within a range of wavelengths of interest (e.g., between <NUM> nanometer and <NUM> nanometers), and substantially absorb radiation outside of the range of wavelengths of interest.

Collector <NUM> may be any suitable shape to gather illumination light generated from plasma <NUM>. Suitable examples include elliptical collectors and collectors with multiple surface contours. Exemplary techniques for collecting light emitted from a plasma are described in <CIT>.

In the embodiment depicted in <FIG>, illumination light <NUM> exits plasma chamber <NUM> via window <NUM> and is redirected toward specimen <NUM> by mirror <NUM>. In addition, illumination optics <NUM> are employed to further shape illumination light <NUM> incident on specimen <NUM> over measurement area <NUM>. Illumination optics <NUM> may include a hollow optical homogenizer or a reflective light tube to efficiently transmit illumination light to a specimen. In some other embodiments, an illumination and collection objective may be employed. In these embodiments, illumination optics <NUM> transmit illumination light to the objective.

The illumination of specimen <NUM> over measurement area <NUM> causes light to be scattered from specimen <NUM>. Scattered light <NUM> is detected by detector <NUM>. Detector <NUM> generates signals <NUM> indicative of the scattered light incident on the active area(s) of detector <NUM>. Detector <NUM> communicates signals <NUM> to computing system <NUM> for analysis. Computing system <NUM> determines properties of the specimen <NUM> based at least in part on the acquired signals <NUM>.

In another further aspect, computing system <NUM> is configured to generate a structural model (e.g., geometric model, material model, or combined geometric and material model) of a measured structure of a specimen, generate a x-ray scatterometry response model that includes at least one geometric parameter from the structural model, and resolve at least one specimen parameter value by performing a fitting analysis of x-ray scatterometry measurement data with the x-ray scatterometry response model. The analysis engine is used to compare the simulated x-ray scatterometry signals with measured data thereby allowing the determination of geometric as well as material properties such as electron density of the sample. In the embodiment depicted in <FIG>, computing system <NUM> is configured as a model building and analysis engine configured to implement model building and analysis functionality as described herein.

<FIG> is a diagram illustrative of an exemplary model building and analysis engine <NUM> implemented by computing system <NUM>. As depicted in <FIG>, model building and analysis engine <NUM> includes a structural model building module <NUM> that generates a structural model <NUM> of a measured structure of a specimen. In some embodiments, structural model <NUM> also includes material properties of the specimen. The structural model <NUM> is received as input to x-ray scatterometry response function building module <NUM>. X-ray scatterometry response function building module <NUM> generates a full beam x-ray scatterometry response function model <NUM> based at least in part on the structural model <NUM>. In some examples, the x-ray scatterometry response function model <NUM> is based on x-ray form factors, <MAT> where F is the form factor, q is the scattering vector, and ρ(r) is the electron density of the specimen in spherical coordinates as described in equation (<NUM>). The x-ray scattering intensity is then given by equation (<NUM>). <MAT> X-ray scatterometry response function model <NUM> is received as input to fitting analysis module <NUM>. The fitting analysis module <NUM> compares the modeled x-ray scatterometry response with the corresponding measured data to determine geometric as well as material properties of the specimen.

In some examples, the fitting of modeled data to experimental data is achieved by minimizing a chi-squared value. For example, for x-ray scatterometry measurements, a chi-squared value can be defined as <MAT> where, <MAT> is the measured x-ray scatterometry signals <NUM> in the "channel" j, where the index j describes a set of system parameters such as diffraction order, energy, angular coordinate, etc. <MAT> is the modeled x-ray scatterometry signal Sj for the "channel" j, evaluated for a set of structure (target) parameters v<NUM>,. ,VL, where these parameters describe geometric (CD, sidewall angle, overlay, etc.) and material (electron density, etc.). σSAXS,j is the uncertainty associated with the jth channel. NSAXS is the total number of channels in the x-ray metrology. L is the number of parameters characterizing the metrology target.

Equation (<NUM>) assumes that the uncertainties associated with different channels are uncorrelated. In examples where the uncertainties associated with the different channels are correlated, a covariance between the uncertainties, can be calculated. In these examples a chi-squared value for full beam x-ray scatterometry measurements can be expressed as <MAT>
where, VSAXS is the covariance matrix of the SAXS channel uncertainties, and T denotes the transpose.

In some examples, fitting analysis module <NUM> resolves at least one specimen parameter value <NUM> by performing a fitting analysis on x-ray scatterometry measurement data <NUM> with the x-ray scatterometry response model <NUM>. In some examples, <MAT> is optimized. In the embodiment depicted in <FIG>, the determined values <NUM> are stored in memory <NUM>.

As described hereinbefore, the fitting of x-ray scatterometry data is achieved by minimization of chi-squared values. However, in general, the fitting of full beam x-ray scatterometry data may be achieved by other functions.

The fitting of x-ray scatterometry metrology data is advantageous for any type of x-ray scatterometry technology that provides sensitivity to geometric and/or material parameters of interest. Specimen parameters can be deterministic (e.g., CD, SWA, overlay, etc.) or statistical (e.g., rms height of sidewall roughness, roughness correlation length, etc.) as long as proper models describing x-ray scatterometry beam interaction with the specimen are used.

In general, computing system <NUM> is configured to access model parameters in real-time, employing Real Time Critical Dimensioning (RTCD), or it may access libraries of pre-computed models for determining a value of at least one specimen parameter value associated with the specimen <NUM>. In general, some form of CD-engine may be used to evaluate the difference between assigned CD parameters of a specimen and CD parameters associated with the measured specimen. Exemplary methods and systems for computing specimen parameter values are described in <CIT>.

In some examples, model building and analysis engine <NUM> improves the accuracy of measured parameters by any combination of feed sideways analysis, feed forward analysis, and parallel analysis. Feed sideways analysis refers to taking multiple data sets on different areas of the same specimen and passing common parameters determined from the first dataset onto the second dataset for analysis. Feed forward analysis refers to taking data sets on different specimens and passing common parameters forward to subsequent analyses using a stepwise copy exact parameter feed forward approach. Parallel analysis refers to the parallel or concurrent application of a non-linear fitting methodology to multiple datasets where at least one common parameter is coupled during the fitting.

Multiple tool and structure analysis refers to a feed forward, feed sideways, or parallel analysis based on regression, a look-up table (i.e., "library" matching), or another fitting procedure of multiple datasets. Exemplary methods and systems for multiple tool and structure analysis is described in <CIT>.

In one further aspect, metrology tool <NUM> includes a computing system (e.g., computing system <NUM>) configured to implement beam control functionality as described herein. In the embodiment depicted in <FIG>, computing system <NUM> is configured as a beam controller operable to control any of the illumination properties such as intensity, divergence, spot size, polarization, spectrum, and positioning of the incident illumination beam <NUM>.

As illustrated in <FIG>, computing system <NUM> is communicatively coupled to detector <NUM>. Computing system <NUM> is configured to receive measurement data <NUM> from detector <NUM>. In one example, measurement data <NUM> includes an indication of the measured response of the specimen (i.e., intensities of the diffraction orders). Based on the distribution of the measured response on the surface of detector <NUM>, the location and area of incidence of illumination beam <NUM> on specimen <NUM> is determined by computing system <NUM>. In one example, pattern recognition techniques are applied by computing system <NUM> to determine the location and area of incidence of illumination beam <NUM> on specimen <NUM> based on measurement data <NUM>. In some examples, computing system <NUM> communicates a command signal (not shown) to illumination optics <NUM> to select the desired illumination wavelength and redirect and reshape illumination beam <NUM> such that incident illumination beam <NUM> arrives at the desired location and angular orientation with respect to specimen <NUM>. In some other examples, computing system <NUM> communicates a command signal <NUM> to wafer positioning system <NUM> to position and orient specimen <NUM> such that incident illumination beam <NUM> arrives at the desired location and angular orientation with respect to specimen <NUM>. In some other examples, computing system <NUM> communicates command signals <NUM> and <NUM> to LPP light source <NUM> to select the desired illumination wavelength and redirect and reshape illumination beam <NUM> such that incident illumination beam <NUM> arrives at the desired location and angular orientation with respect to specimen <NUM>.

In some embodiments, it is desirable to perform measurements at different orientations. This increases the precision and accuracy of measured parameters and reduces correlations among parameters by extending the number and diversity of data sets available for analysis to include a variety of large-angle, out of plane orientations. Measuring specimen parameters with a deeper, more diverse data set also reduces correlations among parameters and improves measurement accuracy. For example, in a normal orientation, x-ray scatterometry is able to resolve the critical dimension of a feature, but is largely insensitive to sidewall angle and height of a feature. However, by collecting measurement data over a broad range of out of plane angular positions, the sidewall angle and height of a feature can be resolved.

As illustrated in <FIG>, metrology tool <NUM> includes a specimen positioning system <NUM> configured to both align specimen <NUM> and orient specimen <NUM> over a large range of out of plane angular orientations with respect the scatterometer. In other words, specimen positioning system <NUM> is configured to rotate specimen <NUM> over a large angular range about one or more axes of rotation aligned in-plane with the surface of specimen <NUM>. In some embodiments, specimen positioning system <NUM> is configured to rotate specimen <NUM> within a range of at least <NUM> degrees about one or more axes of rotation aligned in-plane with the surface of specimen <NUM>. In some embodiments, specimen positioning system is configured to rotate specimen <NUM> within a range of at least <NUM> degrees about one or more axes of rotation aligned in-plane with the surface of specimen <NUM>. In some other embodiments, specimen positioning system <NUM> is configured to rotate specimen <NUM> within a range of at least one degree about one or more axes of rotation aligned in-plane with the surface of specimen <NUM>. In this manner, angle resolved measurements of specimen <NUM> are collected by metrology system <NUM> over any number of locations on the surface of specimen <NUM>. In one example, computing system <NUM> communicates command signals <NUM> to motion controller <NUM> of specimen positioning system <NUM> that indicate the desired position of specimen <NUM>. In response, motion controller <NUM> generates command signals to the various actuators of specimen positioning system <NUM> to achieve the desired positioning of specimen <NUM>.

By way of non-limiting example, as illustrated in <FIG>, specimen positioning system <NUM> includes an edge grip chuck <NUM> to fixedly attach specimen <NUM> to specimen positioning system <NUM>. A rotational actuator <NUM> is configured to rotate edge grip chuck <NUM> and the attached specimen <NUM> with respect to a perimeter frame <NUM>. In the depicted embodiment, rotational actuator <NUM> is configured to rotate specimen <NUM> about the x-axis of the coordinate system <NUM> illustrated in <FIG>. As depicted in <FIG>, a rotation of specimen <NUM> about the z-axis is an in plane rotation of specimen <NUM>. Rotations about the x-axis and the y-axis (not shown) are out of plane rotations of specimen <NUM> that effectively tilt the surface of the specimen with respect to the metrology elements of metrology system <NUM>. Although it is not illustrated, a second rotational actuator is configured to rotate specimen <NUM> about the y-axis. A linear actuator <NUM> is configured to translate perimeter frame <NUM> in the x-direction. Another linear actuator (not shown) is configured to translate perimeter frame <NUM> in the y-direction. In this manner, every location on the surface of specimen <NUM> is available for measurement over a range of out of plane angular positions. For example, in one embodiment, a location of specimen <NUM> is measured over several angular increments within a range of -<NUM> degrees to +<NUM> degrees with respect to the normal orientation of specimen <NUM>.

In general, specimen positioning system <NUM> may include any suitable combination of mechanical elements to achieve the desired linear and angular positioning performance, including, but not limited to goniometer stages, hexapod stages, angular stages, and linear stages.

In a further aspect, plasma chamber <NUM> is filled with a buffer gas <NUM>. Optical elements such as laser illumination window <NUM>, collector <NUM>, and x-ray filter <NUM> are sensitive to material deposition from plasma <NUM>. Buffer gas <NUM> absorbs very little of the soft X-ray radiation generated by the plasma, but thermalizes fast ions generated by plasma <NUM>. In this manner buffer gas <NUM> protects illumination window <NUM>, collector <NUM>, and x-ray filter <NUM> from contamination by material generated by the plasma <NUM>. In some embodiments, the buffer gas <NUM> is helium, hydrogen, or a combination thereof. Both helium and hydrogen are transparent to soft x-ray radiation at wavelengths of interest (i.e., wavelengths between <NUM> nanometer and <NUM> nanometers). In a preferred embodiment, helium is employed as the buffer gas because it is inert, and thus inherently safer than hydrogen. In some embodiments, the distance between plasma <NUM> and the optical elements of plasma chamber <NUM> (e.g., windows <NUM> and <NUM> and collector <NUM>) is at least ten centimeters. In preferred embodiments, the flow of buffer gas <NUM> through plasma chamber <NUM> is maintained at relatively low pressure (e.g., between <NUM> and <NUM> torr).

In another further aspect, LPP light source <NUM> includes a gas separation system <NUM> that separates feed material (e.g., Xenon) from the buffer gas (e.g., Helium) and provides the separated feed material back to the droplet generator. As depicted in <FIG>, gas separation system <NUM> receives a flow <NUM> of buffer gas from plasma chamber <NUM>. Flow <NUM> includes both buffer gas <NUM> and non-metallic feed material <NUM> in a gaseous state (i.e., after heating by plasma <NUM>). Gas separation system <NUM> separates the non-metallic feed material <NUM> from buffer gas <NUM>. The recovered non-metallic feed material <NUM> is transported to droplet generator <NUM> to be reused. In addition, the recovered buffer gas <NUM> is transported back into plasma chamber <NUM>.

<FIG> depicts a simplified illustration of gas separation system <NUM> in one embodiment. As depicted in <FIG>, gas separation system <NUM> includes a cryogenic chamber <NUM> and a distillation column <NUM>. Valves <NUM> and <NUM> control the flow of gas into and out of cryogenic chamber <NUM>. Computing system <NUM> controls the state of valves <NUM> and <NUM> via command signals <NUM> and <NUM>, respectively. In one example, computing system <NUM> communicates command signal <NUM> that causes valve <NUM> to open and allow unseparated gas from gas flow <NUM> to fill cryogenic chamber <NUM>. The unseparated gas is chilled until the non-metallic feed material freezes and separates from the buffer gas, which remains in gaseous form. The separated buffer gas <NUM> is evacuated from the cryogenic chamber <NUM>. After the separated buffer gas <NUM> is evacuated, the separated feed material <NUM> is heated to a gaseous state within cryogenic chamber <NUM>. The separated feed material <NUM> is then evacuated from the cryogenic chamber <NUM> into distillation column <NUM>. Computing system <NUM> communicates command signal <NUM> that causes valve <NUM> to open and allow separated feed material <NUM> from cryogenic chamber <NUM> to distillation column <NUM>. The separated feed material <NUM> is again chilled within distillation column <NUM>. Distillation column <NUM> maintains a temperature gradient from the top of the column (e.g., warmer) to the bottom of the column (e.g., cooler). As the separated feed material <NUM> is cooled, it condenses into a liquid state and settles at the bottom of distillation column <NUM>. The condensed feed material <NUM> is drained from distillation column <NUM> and transported to droplet generator <NUM>. In addition, residual buffer gas <NUM> is recovered from distillation column <NUM>. In the depicted embodiment, separated buffer gas <NUM> and residual buffer gas <NUM> are transported to a refinement system <NUM> to further increase the purity of the recovered buffer gas before transport to plasma chamber <NUM>. In general, refinement system <NUM> is optional. In some embodiments, the separated buffer gas <NUM> and the residual buffer gas <NUM> are sufficiently pure and no additional processing is needed before reintroduction into plasma chamber <NUM>. In some other embodiments, the buffer gas is not reused; rather the recovered buffer gas <NUM> and the residual buffer gas <NUM> are discarded and not reintroduced into plasma chamber <NUM>.

In general, gas separation system <NUM> may include multiple cryogenic chambers. Additional valves may be employed to control gas flows through each cryogenic chamber. For example, while one cryogenic chamber is chilling down an incoming gas flow, another cryogenic chamber may heat the frozen feed material to transport the feed material to distillation column <NUM>.

As depicted in <FIG>, system <NUM> includes a single laser focused directly on a droplet to generate plasma <NUM>. However, system <NUM> may include more than one laser with each laser configured differently or the same. For example, the lasers may be configured to generate light having different characteristics that can be directed to a droplet at the same or different times. In another example, the lasers may be configured to direct light to a droplet from the same or different directions. Exemplary techniques for directing excitation light to a target are described in the aforementioned <CIT>, the entirety of which is incorporated herein by reference.

<FIG> depicts a reflective small angle x-ray scatterometry (SAXS) system in one embodiment. However, other x-ray based metrology systems employing a LPP light source as described herein may be contemplated within the scope of this patent document. In some examples, a coherent diffractive imaging (CDI) based metrology system includes a LPP light source as described herein. In other examples, a LPP light source may be employed as part of an imaging x-ray metrology system.

In some embodiments, an imaging objective directs collected light to a detector. In some embodiments, illumination light generated by light source <NUM> is transmitted by illumination optics <NUM> to an objective. In some embodiments the illumination optics <NUM> and the imaging objective may be comprised of primarily the same elements and be substantially the same. In some other embodiments, illumination light generated by light source <NUM> is transmitted by illumination optic <NUM> directly to specimen <NUM> without first being directed through the elements of the imaging objective. In response to the illumination light incident on specimen <NUM>, light from specimen <NUM> is collected, magnified, and directed to a detector by an imaging objective.

In some embodiments, an imaging objective designed with an adequate field of view is employed. The light path through the objective should preferably include a minimum number of interactions with reflective surfaces to minimize absorption losses associated with each interaction. Exemplary designs for an objective with all reflective components using a four mirror, four pass design are described in <CIT> In addition, exemplary designs for an objective with all reflective components using a four mirror, six pass design is described in <CIT>.

Illumination direction affects how a structure on a wafer is resolved by a metrology system such as metrology system <NUM>. In some embodiments, optical configurations discussed may have non-uniform optical properties in one of more of the reflective elements that are specifically optimized for illumination purposes. For example, coatings may be optimized to increase the coating durability due to the high exposure energy in the illumination path.

<FIG> illustrates a method <NUM> suitable for generating broadband, soft x-ray illumination light for x-ray based metrology in accordance with at least one inventive aspect. It is recognized that data processing elements of method <NUM> may be carried out via a preprogrammed algorithm stored as part of program instructions <NUM> and executed by one or more processors of computing system <NUM>. While the following description is presented in the context of system <NUM> depicted in <FIG>, it is recognized herein that the particular structural aspects of system <NUM> do not represent limitations and should be interpreted as illustrative only.

In block <NUM>, a sequence of droplets of a non-metallic feed material in a solid or liquid state is dispensed into a plasma chamber. The plasma chamber includes at least one wall to contain a flow of buffer gas within the plasma chamber.

In block <NUM>, a pulse of excitation light is generated and directed to a droplet of the feed material in the plasma chamber. The pulse of excitation light has a duration of less than one nanosecond. The interaction of the pulse of excitation light with the droplet of the feed material causes the droplet to ionize to form a plasma that emits an illumination light. The illumination light comprises broadband light in a spectral region from about <NUM> nanometer to about <NUM> nanometers and is useable to illuminate a specimen under measurement.

In block <NUM>, in response to the illumination light an amount of light is detected from the specimen.

In block <NUM>, a value of at least one parameter of interest of the specimen under measurement is determined based at on the amount of detected light.

It should be recognized that the various steps described throughout the present disclosure may be carried out by a single computer system <NUM> or, alternatively, a multiple computer system <NUM>. Moreover, different subsystems of the system <NUM>, such as the specimen positioning system <NUM>, gas separation system <NUM>, droplet generator <NUM>, laser <NUM>, and detector <NUM>, may include a computer system suitable for carrying out at least a portion of the steps described herein. Therefore, the aforementioned description should not be interpreted as a limitation on the present invention but merely an illustration. Further, the one or more computing systems <NUM> may be configured to perform any other step(s) of any of the method embodiments described herein.

In addition, the computer system <NUM> may be communicatively coupled to the specimen positioning system <NUM>, gas separation system <NUM>, droplet generator <NUM>, laser <NUM>, and detector <NUM> in any manner known in the art. For example, the one or more computing systems <NUM> may be coupled to computing systems associated with the specimen positioning system <NUM>, gas separation system <NUM>, droplet generator <NUM>, laser <NUM>, and detector <NUM>, respectively. In another example, any of the specimen positioning system <NUM>, gas separation system <NUM>, droplet generator <NUM>, laser <NUM>, and detector <NUM>, may be controlled directly by a single computer system coupled to computer system <NUM>.

The computer system <NUM> of the system <NUM> may be configured to receive and/or acquire data or information from the subsystems of the system (e.g., specimen positioning system <NUM>, gas separation system <NUM>, droplet generator <NUM>, laser <NUM>, and detector <NUM>, and the like) by a transmission medium that may include wireline and/or wireless portions. In this manner, the transmission medium may serve as a data link between the computer system <NUM> and other subsystems of the system <NUM>.

Computer system <NUM> of the system <NUM> may be configured to receive and/or acquire data or information (e.g., modeling inputs, modeling results, etc.) from other systems by a transmission medium that may include wireline and/or wireless portions. In this manner, the transmission medium may serve as a data link between the computer system <NUM> and other systems (e.g., memory on-board system <NUM>, external memory, or external systems). For example, the computing system <NUM> may be configured to receive measurement data (e.g., signals <NUM>) from a storage medium (i.e., memory <NUM>) via a data link. For instance, measurement results obtained using detector <NUM> may be stored in a permanent or semi-permanent memory device (e.g., memory <NUM>). In this regard, the measurement results may be imported from on-board memory or from an external memory system. Moreover, the computer system <NUM> may send data to other systems via a transmission medium. For instance, parameter values <NUM> determined by computer system <NUM> may be stored in a permanent or semi-permanent memory device (e.g., memory <NUM>). In this regard, measurement results may be exported to another system.

Computing system <NUM> may include, but is not limited to, a personal computer system, mainframe computer system, workstation, image computer, parallel processor, or any other device known in the art. In general, the term "computing system" may be broadly defined to encompass any device having one or more processors, which execute instructions from a memory medium.

Program instructions <NUM> implementing methods such as those described herein may be transmitted over a transmission medium such as a wire, cable, or wireless transmission link. For example, as illustrated in <FIG>, program instructions stored in memory <NUM> are transmitted to processor <NUM> over bus <NUM>. Program instructions <NUM> are stored in a computer readable medium (e.g., memory <NUM>). Exemplary computer-readable media include read-only memory, a random access memory, a magnetic or optical disk, or a magnetic tape.

Metrology techniques as described herein may be used to determine characteristics of semiconductor structures. Exemplary structures include, but are not limited to, FinFETs, low-dimensional structures such as nanowires or graphene, sub <NUM> structures, thin films, lithographic structures, through silicon vias (TSVs), memory structures such as DRAM, DRAM 4F2, FLASH and high aspect ratio memory structures, such as 3D-NAND structures. Exemplary structural characteristics include, but are not limited to, geometric parameters such as line edge roughness, line width roughness, pore size, pore density, side wall angle, profile, film thickness, critical dimension, pitch, and material parameters such as electron density, crystalline grain structure, morphology, orientation, stress, strain, elemental identification, and material composition.

In some embodiments, the techniques described herein may be implemented as part of a fabrication process tool. Examples of fabrication process tools include, but are not limited to, lithographic exposure tools, film deposition tools, implant tools, and etch tools. In this manner, the results of the temperature measurements are used to control a fabrication process.

Various embodiments are described herein for a semiconductor processing system (e.g., an inspection system, a metrology system, or a lithography system) that may be used for processing a specimen. The term "specimen" is used herein to refer to a wafer, a reticle, or any other sample that may be processed (e.g., printed or inspected for defects) by means known in the art.

As used herein, the term "specimen" generally refers to a wafer. However, it is to be understood that the methods and systems described herein may be used to provide illumination of any other specimen known in the art.

As used herein, the term "wafer" generally refers to substrates formed of a semiconductor or non-semiconductor material. Examples include, but are not limited to, monocrystalline silicon, gallium arsenide, and indium phosphide. Such substrates may be commonly found and/or processed in semiconductor fabrication facilities. In some cases, a wafer may include only the substrate (i.e., bare wafer). Alternatively, a wafer may include one or more layers of different materials formed upon a substrate. One or more layers formed on a wafer may be "patterned" or "unpatterned. " For example, a wafer may include a plurality of dies having repeatable pattern features.

A "reticle" may be a reticle at any stage of a reticle fabrication process, or a completed reticle that may or may not be released for use in a semiconductor fabrication facility. A reticle, or a "mask," is generally defined as a substantially transparent substrate having substantially opaque regions formed thereon and configured in a pattern. The substrate may include, for example, a glass material such as quartz. A reticle may be disposed above a resist-covered wafer during an exposure step of a lithography process such that the pattern on the reticle may be transferred to the resist.

One or more layers formed on a wafer may be patterned or unpatterned. For example, a wafer may include a plurality of dies, each having repeatable pattern features. Formation and processing of such layers of material may ultimately result in completed devices. Many different types of devices may be formed on a wafer, and the term wafer as used herein is intended to encompass a wafer on which any type of device known in the art is being fabricated.

Claim 1:
A laser produced plasma light source (<NUM>), comprising:
a plasma chamber (<NUM>) having at least one wall that is configured to contain a flow of buffer gas (<NUM>) within the plasma chamber;
a droplet generator (<NUM>) that dispenses a sequence of droplets of a non-metallic feed material (<NUM>) in a solid or liquid state into the plasma chamber;
a pulsed laser (<NUM>) that generates a pulse of excitation light directed to a droplet of the feed material in the plasma chamber, the pulse of excitation light having a duration of less than one nanosecond, wherein the interaction of the pulse of excitation light with the droplet of the feed material causes the droplet to ionize to form a plasma (<NUM>) that emits an illumination light (<NUM>), wherein the illumination light comprises broadband light in a spectral region from about <NUM> nanometer to about <NUM> nanometers, wherein the illumination light is useable to illuminate a specimen under measurement;
a collector (<NUM>) that gathers an amount of the illumination light emitted by the plasma and directs the amount of illumination light through a window (<NUM>) of the plasma chamber;
a gas recycling system (<NUM>) configured to separate an amount of the feed material from the buffer gas and provide the amount of feed material to the droplet generator; the gas recycling system comprising:
at least one cryogenic chamber (<NUM>) to separate the amount of feed material from a portion of the buffer gas; and
a distillation column (<NUM>) to separate the amount of feed material from a residual portion of the buffer gas.