Patent Description:
Controlling the focal distance between a photomask and imaging optics during photomask inspection is important for defect-inspection sensitivity and repeatability. Focal-distance control is especially important for inspection of extreme ultraviolet (EUV) photomasks. Furthermore, different defects can exhibit different through-focus behaviors.

To control focus variation, the shape of the photomask is mapped before inspection. This process, which is referred to as focal mapping, produces a focal map that provides a trajectory to be followed during inspection to control the focal distance. For example, a servo control system may be used to adjust the imaging optics so that they follow the trajectory during inspection.

Focal mapping of modern photomasks (e.g., EUV photomasks), however, presents significant challenges. Traditional focal-mapping processes have used candidate positions in non-patterned areas on a photomask. The small pattern features and high pattern densities on modern photomasks make it difficult to find suitable non-patterned candidate positions. And three-dimensional electromagnetic effects associated with the high pattern densities cause focal offsets measured for patterned areas on photomasks to be incorrect, resulting in incorrect focal maps and trajectories.

<CIT> discloses an auto focus system for reticle inspection. <CIT> describes an interferometry apparatus for determining characteristics of an object surface, with spatially coherent illumination.

Accordingly, there is a need for accurate and quick focal-mapping techniques.

A first aspect of the invention provides a method as recited in claim <NUM>.

A second aspect of the invention provides a photomask-inspection system as recited in claim <NUM>.

A third aspect of the invention provides a non-transitory computer-readable storage medium stores one or more programs for execution by one or more processors of a photomask-inspection system that further includes a broadband light interferometer and photomask-inspection optics as recited in claim <NUM>.

For a better understanding of the various described implementations, reference should be made to the Detailed Description below, in conjunction with the following drawings.

Like reference numerals refer to corresponding parts throughout the drawings and specification.

Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings.

<FIG> shows a broadband light interferometer <NUM> used to image (i.e., generate interferograms for) a photomask <NUM> to determine heights of respective areas on the surface of the photomask <NUM>, in accordance with some embodiments. Patterning and bowing of the photomask <NUM> cause different areas on the surface of the photomask <NUM> to have different heights. The broadband light interferometer <NUM> has a low numerical aperture (NA). For example, the NA for the broadband light interferometer <NUM> is less than <NUM> (e.g., equals <NUM> or <NUM>). The broadband light interferometer <NUM> performs Mirau interferometry. Alternatively, another type of broadband light interferometer (e.g., a Michelson interferometer) may be used to image the photomask <NUM>.

The broadband light interferometer <NUM> includes a broadband light source <NUM>. The use of broadband light reduces temporal coherence of the light for fringe-nulling purposes. In some embodiments, the broadband light source <NUM> provides visible light. In some embodiments, the wavelength band (e.g., color) of the light provided by the broadband light source <NUM> is adjustable. For example, light-emitting diodes (LEDs) and/or filters in the broadband light source <NUM> may be changed to change the wavelength band. In another example, the broadband light interferometer <NUM> may have multiple broadband light sources <NUM> that can be multiplexed into the optical path, each of which provides a different wavelength band.

A condenser lens <NUM> focuses light from the broadband light source <NUM> onto an objective <NUM>, which is an Mirau-type objective in the example of <FIG>. A beamsplitter <NUM> is situated between the condenser lens <NUM> and the objective <NUM> in the optical path to reflect the light from the broadband light source <NUM> onto the objective <NUM>. The objective <NUM> includes a lens <NUM>, mirror <NUM>, and beamsplitter <NUM> in sequence. The beamsplitter <NUM> illuminates the surface of the photomask <NUM> with a portion of the light from the broadband light source <NUM> while reflecting, in conjunction with the mirror <NUM>, another portion of the light from the broadband light source <NUM>. The reflected portion serves as the reference light for the broadband light interferometer <NUM>. The objective <NUM> collects light from the photomask <NUM>. The collected light and the reference light, which interfere, are collimated by the lens <NUM> and transmitted through the beamsplitter <NUM> to a tube lens <NUM>, which focuses them onto a camera <NUM> (e.g., a digital camera).

The image (i.e., interferogram) produced in the camera <NUM> includes interference effects resulting from interference between the collected light from the photomask and the reference light. The interference effects vary as a function of the height of the objective <NUM> above the photomask <NUM>. The height of the objective <NUM> above the photomask <NUM> is adjustable. For example, the objective <NUM> may be on a z-scanning stage that can move the objective <NUM> up and down along the z-axis (i.e., adjust the z-height). The z-height can also be adjusted by moving the photomask under the objective with a z-scanning stage. Multiple images (i.e., interferograms) of the photomask <NUM> may be taken with the objective <NUM> at different heights (i.e., z-values) above the photomask <NUM>, and the heights of respective areas on the surface of the photomask <NUM> may be determined by comparing the images, using interferogram analysis (e.g., known three-, four-, or five-step interferogram-based analysis algorithms).

The heights measured for patterned areas on the surface of the photomask will be incorrect due to the three-dimensional electromagnetic effects, however, and therefore cannot be directly used to produce a focal map for defect inspection. For example, assuming NA = <NUM> and a central wavelength of <NUM> for the broadband light source <NUM> (which is an extended spatially incoherent light source), the smallest pitch that the broadband light interferometer <NUM> can resolve is λ/(<NUM>*NA) = <NUM>. This value is far above the pitch for patterns on photomasks for modern deep-submicron semiconductor devices. Below this pitch limit, the broadband light interferometer <NUM> observes an average effect of the dense patterns on the photomask <NUM> without resolving the features, such that only the zeroth-order diffracted light from the photomask <NUM> will interfere with the reference light. This effect results in incorrect height measurements. The error in the measured height varies as a function of the fill factor (e.g., defined as the area percentage not covered by absorber material <NUM>, <FIG>) for a patterned area on the photomask <NUM>. The following discussion describes techniques for determining corrections to apply to the incorrect height measurements, to produce height values that may be used in a focal map of defect inspection. The corrections may be determined based on the fill factors. In some embodiments, the produced height values for the focal map are substantially pattern-independent: they depend on the fill-factor correction but not on the details of the patterns.

<FIG> is a side cross-sectional view of a patterned area <NUM> on the surface of the photomask <NUM>. A portion of the patterned area <NUM> is covered by an absorber material (Ab) <NUM>, which absorbs ultraviolet light (e.g., extreme ultraviolet light). The absorber material <NUM> is situated above a multilayer material (ML) <NUM>. The remainder of the patterned area <NUM> is not covered by the absorber material <NUM>, such that the multilayer material <NUM> is exposed. The multilayer material <NUM> (imperfectly) reflects light.

In some embodiments, the photomask <NUM> is for EUV (e.g., <NUM>) photolithography. The absorber material <NUM> absorbs the EUV (e.g., <NUM>) light and the multilayer material <NUM> partially reflects the EUV (e.g., <NUM>) light. The multilayer material <NUM> includes alternating layers of molybdenum (Mo) and silicon (Si) above a substrate (e.g., a blank photomask), with a capping layer covering the alternating layers of Mo and Si. The capping layer may be ruthenium (Ru) or boron (B). Each pair of adjacent Mo and Si layers is called a MoSi bilayer. The Mo layer thickness may be <NUM>, the Si layer thickness may be <NUM>, and the capping layer thickness may be <NUM>. The number of MoSi bilayers in the multilayer material <NUM> may be <NUM> or more, or <NUM> or more. The absorber material <NUM> includes a tantalum boron nitride (TaBN) layer with a tantalum boron oxide (TaBO) capping layer above the TaBN layer. The TaBO capping layer has a thickness of <NUM>. The thickness of the TaBN layer <NUM> may be <NUM>-<NUM>. These are merely respective examples of absorber materials and materials underlying the absorber materials (i.e., underlying materials that are exposed where the absorber material is absent). Other absorber materials and/or underlying materials can also be used in accordance with some embodiments.

<FIG> is a plan view of a pattern <NUM> on the surface of the photomask <NUM>, illuminated by a spot <NUM> of light from the broadband light interferometer <NUM> (<FIG>), in accordance with some embodiments. The pattern <NUM> includes regions in which the multilayer material <NUM> is exposed and regions in which the absorber material <NUM> covers the multilayer material <NUM>. The pattern <NUM> is periodic. Focal-mapping techniques described herein do not require periodic patterning, however, and may also be used for photomasks with non-periodic (e.g., random) patterning.

A patterned area, such as the patterned area <NUM> (<FIG>) or an area in the pattern <NUM> (<FIG>) has a fill factor a equal to the fraction of the area <NUM> that is not covered by the absorber material <NUM> (i.e., a is the fill factor for the exposed multilayer material <NUM>). As shown in <FIG>, <MAT> is the reflected electric field off the absorber material <NUM> (which is not a perfect absorber, such that <MAT> is non-zero) and <MAT> is the reflected electric field off the multilayer material <NUM> (which is not a perfect reflector). The zeroth-order diffracted light off of the patterned area <NUM> is: <MAT>.

The wavelength-dependent phase factor of <MAT> is: <MAT>.

Assuming that w(λ) is the spectrum of light from the broadband light source <NUM> (<FIG>), with λ extending across the wavelength band of the light, the measured height (i.e., the effective height obtained through broadband light interferometry, which is different from the actual height) is: <MAT>.

In equation <NUM>, 4π accounts for the double-pass of the light reflected by the surface of the photomask <NUM>. Manipulating the zeroth-order diffracted light, the following equation is obtained: <MAT> where t is the height (i.e., thickness) of the absorber material <NUM> and φ<NUM> is the sudden phase change difference between reflection of light from the multilayer material <NUM> and from the absorber material <NUM>.

The absorber material <NUM> and multilayer material <NUM> are chosen to absorb and reflect light, respectively, at the appropriate photolithographic wavelength (e.g., at <NUM> for EUV photolithography). The absorber material <NUM> and multilayer material <NUM> are dispersive, however, and their reflectivity varies as a function of wavelength. <FIG> is a graph <NUM> showing measured reflectivity dispersion curves <NUM> and <NUM> (i.e., curves showing reflectivity versus wavelength) for the multilayer material <NUM> and the absorber material <NUM>. In the example of <FIG>, the multilayer material <NUM> and absorber material <NUM> are for <NUM> photolithography. The multilayer material <NUM> includes MoSi bilayers as described above. The absorber material <NUM> includes TaBN with a TaBO capping layer, as described above.

<FIG> is a graph <NUM> showing calculated height-correction curves in accordance with some embodiments. The height correction curves show height corrections, which are to be applied to photomask heights measured using broadband light interferometry, versus the fill factor a (i.e., the ML <NUM> fill factor). The height correction curves are calculated in accordance with equations <NUM> and <NUM>, and vary as a function of the phase change of light upon reflection by the multilayer material <NUM>. <FIG> shows a first curve <NUM> for a phase change of -<NUM>°, a second curve <NUM> for a zero-degree phase change, and a third curve <NUM> for a phase change of +<NUM>°. The actual phase change may be measured offline (e.g., using a calibration photomask), and the height-correction curve for the actual phase change calculated accordingly. In the example of <FIG>, the height corrections are defined as negative and may be subtracted from the photomask heights measured using broadband light interferometry to provide a focal map that gives a trajectory for focusing the inspection optics (e.g., photomask-inspection optics <NUM>, <FIG>) on the top of the absorber material <NUM>. The height corrections may alternatively be defined as positive and added to the measured photomask heights. In still other examples, the height corrections may be correction factors by which the measured photomask heights are multiplied or divided. Application of the height corrections may produce a focal map that gives a trajectory for focusing the inspection optics on top of the absorber material <NUM>, on top of the multilayer material <NUM> (i.e., at the bottom of the absorber material <NUM>), or on any predetermined surface in between these two surfaces.

<FIG> shows a plan view of a height image of a photomask <NUM> and a corresponding curve showing heights in a cross-sectional profile <NUM> of the photomask <NUM>, in accordance with some embodiments. The height image is generated by performing broadband light interferometry (e.g., using the broadband light interferometer <NUM>, <FIG>) to generate interferograms with the objective (e.g., objective <NUM>, <FIG>) at multiple respective heights (i.e., z-positions) above the photomask <NUM>, and analyzing the interferograms to determine measured heights. In the example of <FIG>, the photomask <NUM> includes an unpatterned area <NUM> in which the multilayer material <NUM> is covered by absorber material <NUM>, an area <NUM> (e.g., a target area) in which the absorber material <NUM> is absent and the multilayer material <NUM> is exposed, and patterned areas <NUM> in which portions of the multilayer material <NUM> are covered by absorber material <NUM>. Ignoring patterning, height variation across the photomask <NUM> is primarily due to bowing of the photomask <NUM>. For example, the height variation in the unpatterned area <NUM> is primarily due to bowing. The height (i.e., z-component) of each side of the area <NUM> corresponds to the height (i.e., thickness) of the absorber material <NUM>. Heights in the patterned areas <NUM> similarly should step up and down by an amount equal to the height (i.e., thickness) of the absorber material <NUM>, since the absorber material <NUM> is either present or absent at each point in the patterned areas <NUM>. Broadband light interferometry, however, is not able to resolve the features in the patterned areas <NUM> (e.g., because the patterning pitch is less than the resolution limit). As a result, the measured heights in the patterned areas <NUM> include intermediate values, which are incorrect. Raw heights as measured through broadband light interferometry therefore cannot be used for focal mapping. Once height corrections (e.g., the height corrections of <FIG>) have been applied to the raw heights, then the corrected heights may be used for focal mapping. In <FIG>, application of the height corrections in the patterned areas <NUM> for the profile <NUM> results in a trajectory <NUM> that can be used for the cross-section of the profile <NUM> through the patterned areas <NUM> in a focal map. The focal map allows the inspection optics (e.g., photomask-inspection optics <NUM>, <FIG>) to focus on the top of the absorber material <NUM> during defect inspection.

To obtain height corrections from a height-correction curve (e.g., one of the curves in <FIG>), the fill factors for respective photomask areas are first determined. In some embodiments (e.g., for die-to-database inspection), the database of the design for the photomask is available and the fill factors are determined from the database (e.g., as in step <NUM> of the method <NUM>, <FIG>).

In other embodiments (e.g., for die-to-die inspection) (e.g., in which the database of the design for the photomask is not available), the fill factors can be determined based on average reflectivities. Each z-position (i.e., z-height) for the objective (or photomask) corresponds to a distinct phase index i. Taking interferograms with the objective at different z-positions corresponds to stepping through phase indices i, where the interferogram intensity is: <MAT> where Δ is the phase (i.e., height) for the sample (i.e., photomask area) of interest, and coefficients a and b are related to the reflected light from the reference surface and the sample (i.e., photomask) surface in the broadband light interferometer. The coefficients a and b may be obtained through known analysis of the interferograms. Having obtained a and b, the ratio |rr/rt| of the reference-surface reflectivity rr and the testing-surface reflectivity rt (i.e., the sample-surface reflectivity, which is the reflectivity of an area of the photomask surface) is deduced, again through known interferometric analysis. The reference-surface reflectivity rr is a known property of the broadband light interferometer, and the amplitude (i.e., magnitude) of the testing-surface reflectivity rt is determined accordingly.

<FIG> is a graph <NUM> showing curves of average reflectivity amplitude versus fill factor a (i.e., versus the multilayer-material <NUM> fill factor) for different wavelength bands (e.g., colors), in accordance with some embodiments. The curves of the graph <NUM> are calculated and thus prophetic. The curves include a first curve <NUM> for a first wavelength band in which the ratio of the reflectivity of the multilayer material <NUM> to the reflectivity of the absorber material <NUM> is <NUM> (i.e., RML/RAb = <NUM>) and a second curve <NUM> for a second wavelength band in which the ratio of the reflectivity of the multilayer material <NUM> to the reflectivity of the absorber material <NUM> is <NUM> (i.e., RML/RAb = <NUM>). The curves <NUM> and <NUM> are calculated assuming a zero-degree phase change upon reflection by the multilayer material <NUM> (e.g., as for the curve <NUM>, <FIG>). Similar curves may be calculated for other phase changes.

As the curves <NUM> and <NUM> show, the reflectivity for a single wavelength band does not specify a single fill factor: the correlation between reflectivities and fill factors is not one-to-one. In the curves <NUM> and <NUM>, the correlation between reflectivities and fill factors is one-to-two (i.e., a respective reflectivity value corresponds to two fill factors). Multiple (e.g., two) different wavelength bands (e.g., colors), however, can be used together to determine a unique fill factor for measured reflectivities. For example, the broadband light interferometer <NUM> may be configured to generate interferograms using each of the two different wavelength bands (e.g., by changing LEDs in the broadband light source <NUM>, changing filters in the broadband light source <NUM>, or multiplexing multiple broadband light sources <NUM>). Given the resulting reflectivity data, unique fill factors may be identified for respective photomask areas, using the curves <NUM> and <NUM>.

<FIG> is a flowchart showing a method <NUM> for inspecting photomasks, in accordance with some embodiments. The method <NUM> may be performed by a photomask-inspection tool (e.g., photomask-inspection system <NUM>, <FIG>) that includes a broadband light interferometer (e.g., the broadband light interferometer <NUM>, <FIG>). In the method <NUM>, heights on a surface of a photomask (e.g., photomask <NUM>, <FIG>) are measured (<NUM>) using broadband light interferometry (e.g., using Mirau interferometry or Michelson interferometry). The heights include heights of patterned areas of the photomask (e.g., areas patterned with an absorber material <NUM> situated above a multilayer material <NUM>, <FIG>). In some embodiments, the broadband light interferometry uses visible light. In some embodiments, a height image (e.g., as shown in <FIG>) for the photomask is generated (<NUM>) using the broadband light interferometry. The height image may include measured heights for the entire photomask or a portion thereof. For example, the height image may include measured heights for a cross-section of the photomask (e.g., for the profile <NUM>, <FIG>).

In some embodiments, fill factors (e.g., fill factors a for multilayer material <NUM>) for the patterned areas are calculated (<NUM>) based on a database of the design for the photomask. For example, the database is a gds file or includes design data that was provided in a gds file. The design data in the database specifies where absorber material (e.g., absorber material <NUM>, <FIG>) is present and absent on the photomask, thus allowing fill factors to be calculated. This calculation of fill factors is performed, for example, for die-to-database inspection, in which the subsequent defect inspection of step <NUM> involves comparing results from inspecting the photomask to results from simulated inspection of the design in the database.

Fill factors (e.g., fill factors a for multilayer material <NUM>) may alternatively be determined without using a database of the photomask design. In some embodiments, reflectivities (e.g., average reflectivities) of the patterned areas are determined (<NUM>) based on the broadband light interferometry (e.g., in accordance with equation <NUM>). The broadband light interferometry includes respective instances of broadband light interferometry performed using respective wavelength bands (e.g., colors) of a plurality of wavelength bands (e.g., two wavelength bands), and the reflectivities are determined for each of the respective wavelength bands. The fill factors are determined (<NUM>) based on the reflectivities, using predetermined correspondences between reflectivities and fill factors for the plurality of wavelength bands (e.g., correspondences that are not one-to-one). For example, the fill factors are determined using correspondences like those shown in <FIG>. The plurality of wavelength bands may include a first color and a second color. The predetermined correspondences may include a first correspondence between reflectivities and fill factors for the first color and a second correspondence between reflectivities and fill factors for the second color, with neither the first correspondence nor the second correspondence being one-to-one. This determination of fill factors based on reflectivities is performed, for example, for die-to-die inspection, in which the subsequent defect inspection of step <NUM> involves comparing results from inspecting a die area on the photomask to results from inspecting a reference photomask die area.

A focal map is produced (<NUM>) from the measured heights on the surface of the photomask. Producing the focal map includes adjusting the measured heights of the patterned areas based on fill factors for the patterned areas. In some embodiments, the height image is adjusted (<NUM>) to offset the measured heights of the patterned areas (e.g., including measured heights in a cross-section of the photomask, such as the cross-section for the profile <NUM>, <FIG>), based on the fill factors. Height corrections for respective measured heights of the patterned areas may be determined (<NUM>) based on the fill factors, using a predetermined correspondence between measured heights and fill factors (e.g., using a correspondence like those shown in <FIG>). The height corrections may be applied (<NUM>) to the respective measured heights (e.g., to produce a trajectory <NUM>, <FIG>). The resulting focal map may be substantially pattern-independent (e.g., such that the light used for photomask inspection in step <NUM> is focused on the top of absorber material <NUM>, on top of the multilayer material <NUM>, or on any predetermined surface in between these two surfaces).

The photomask is inspected (<NUM>) for defects, using the focal map (i.e., the photomask-inspection optics focus the light used to inspect the photomask in accordance with the focal map). In some embodiments, ultraviolet (UV) light is used (<NUM>) to inspect the photomask. For example, extreme ultraviolet (EUV) (e.g., <NUM>) light or deep ultraviolet (DUV) (e.g., <NUM>) light is used. EUV is a common, well-known and well-understood technical term that refers to light with wavelengths in the range of <NUM> down to <NUM>. DUV is a common, well-known and well-understood technical term that refers to light with wavelengths in the range of <NUM> down to <NUM>.

<FIG> is a block diagram of a photomask-inspection system <NUM> in accordance with some embodiments. The photomask-inspection system <NUM> has optics <NUM>, including a broadband light interferometer (BLI) <NUM> (e.g., the broadband light interferometer <NUM>, <FIG>) for measuring heights on photomasks (e.g., for generating interferograms that are used to create height images, such as the height image of <FIG>) and photomask-inspection optics <NUM> for inspecting photomasks for defects. In some embodiments, the broadband light interferometer <NUM> uses a visible wavelength band. The wavelength band used by the broadband light interferometer <NUM> may be variable (e.g., by multiplexing different light sources, using filters, or changing LEDs). For example, the broadband light interferometer <NUM> may be configured to use multiple visible wavelength bands. In some embodiments, the photomask-inspection optics <NUM> include ultraviolet optics, with corresponding ultraviolet light being used for defect inspection. For example, the photomask-inspection optics <NUM> may include EUV optics (e.g., for <NUM> light) or DUV optics (e.g., for <NUM> light).

The photomask-inspection system <NUM> also includes a computer system with one or more processors <NUM> (e.g., CPUs), optional user interfaces <NUM>, memory <NUM>, and one or more communication buses <NUM> interconnecting these components and the optics <NUM> (and other components of the photomask-inspection system <NUM> that are not shown, such as photomask-handling robotics). The user interfaces <NUM> may include a display <NUM> and one or more input devices <NUM> (e.g., a keyboard, mouse, touch-sensitive surface of the display <NUM>, etc.). The display may show height images, focal maps, and defect-inspection data and may report the status of the photomask-inspection system <NUM> (e.g., the status of the method <NUM>, <FIG>).

Memory <NUM> includes volatile and/or non-volatile memory. Memory <NUM> (e.g., the non-volatile memory within memory <NUM>) includes a non-transitory computer-readable storage medium. Memory <NUM> optionally includes one or more storage devices remotely located from the processor(s) <NUM> and/or a non-transitory computer-readable storage medium that is removably inserted into the computer system of the photomask-inspection system <NUM>. In some embodiments, memory <NUM> (e.g., the non-transitory computer-readable storage medium of memory <NUM>) stores the following modules and data, or a subset or superset thereof: an operating system <NUM> that includes procedures for handling various basic system services and for performing hardware-dependent tasks, a broadband light interferometry (BLI) module <NUM> for controlling the broadband light interferometer <NUM>, a focal-map production module <NUM>, a defect-inspection module <NUM> for controlling photomask defect inspection using the photomask-inspection optics <NUM>, and a reporting module <NUM> for reporting results from the modules <NUM>, <NUM>, and/or <NUM>. The memory <NUM> (e.g., the non-transitory computer-readable storage medium of the memory <NUM>) includes instructions for performing all or a portion of the method <NUM> (<FIG>). Each of the modules stored in the memory <NUM> corresponds to a set of instructions for performing one or more functions described herein. Separate modules need not be implemented as separate software programs. The modules and various subsets of the modules may be combined or otherwise re-arranged. In some embodiments, the memory <NUM> stores a subset or superset of the modules and/or data structures identified above.

Claim 1:
A method (<NUM>), comprising:
measuring heights on a surface of a photomask using broadband light interferometry, the heights comprising heights of patterned areas of the photomask (<NUM>);
producing a focal map from the measured heights on the surface of the photomask (<NUM>), comprising adjusting the measured heights of the patterned areas based on fill factors for the patterned areas to thereby produce a corrected focal map; and
inspecting the photomask for defects with photomask-inspection optics, using the corrected focal map (<NUM>).