Patent Description:
The lithographic tool is a machine that is used to print a desired pattern onto substrate. The tool is used to transfer a pattern from a mask to an individual layer of the integrated circuit, printed on a wafer. The transfer is typically carried out via imaging onto a sensitive layer, termed the resist. As the targeted critical dimension (CD) of the pattern elements shrinks, the imaging process window shrinks, which results in a smaller process window in terms of depth of focus (DOF). In order to control the printed pattern uniformity, it is necessary to measure the parameters of the lithographic tool and especially the parameters of the focus. For example, the advanced node requires very tight focus control, e.g., 3σ <<NUM> across the wafer.

<NPL>), teach using, for focus measurements, asymmetric targets for which the asymmetry, i.e., the difference between the effective side wall angles (SWA) at the left and the right edges, changes monotonically through focus (see refrence1). The SWA difference shows up as an intensity difference between +<NUM> and -<NUM> diffractions orders in the measured scatterometry signal. However, the target pitch taught by Wong et al. is at least four times the product pitch, which makes the target sensitive to lithographic tool aberration. Another issue is that when the SWA angle asymmetry is small the signal difference becomes small, leading to inaccurate results.

<NPL>) and <NPL>) teach using, for focus measurements, Focus Dose Pattern and Process Monitor Grating, in which targets are designed for higher sensitivity to dose and focus variation. The targets are made more sensitive by using scattering bar techniques, end of line techniques and forbidden pitch. However, the targets are disadvantageously with respect to their small depth of field (DOF) and the printability of the patterns in a whole range of process window. The measurement method uses the scatterometry model base approach which makes it sensitive to model errors.

<CIT> discloses a segmented mask including a set of cell structures. Each cell structure includes a set of features having an unresolvable segmentation pitch along a first direction. The unresolvable segmentation pitch along the first direction is smaller than the illumination of the lithography printing tool. The cell structures have a pitch along a second direction perpendicular to the first direction. The unresolvable segmentation pitch is suitable for generating a printed pattern for shifting the best focus position of the lithography tool by a selected amount to achieve a selected level of focus sensitivity.

<CIT> determines whether an exposure apparatus is outputting the correct dose of radiation and whether a projection system of the exposure apparatus is focusing the radiation correctly. A test pattern is used on a mask for printing a specific marker onto a substrate. This marker may be measured by an inspection apparatus, such as, for example, a scatterometer to determine whether errors in focus, dose, and other related properties are present. The test pattern is arranged such that changes in focus and dose may be easily determined by measuring properties of a pattern that is exposed using the mask. The test pattern of the mask is arranged so that it gives rise to a marker pattern on the substrate surface. The marker pattern contains structures that have at least two measurable side wall angles. Asymmetry between side wall angles of a structure is related to focus (or defocus) of the exposure radiation from the exposure apparatus. The extent of defocus may thereby be determined by measuring an asymmetry in side wall angle of the printed marker pattern structures.

Said application discloses a target design comprising:a periodic structure having a plurality of recurring elements which are arranged based on a first pitch in a first direction, wherein the plurality of recurring elements are periodic and are arranged based on a second pitch along a second direction that is perpendicular to the first direction, wherein the plurality of recurring elements include a plurality of focus-insensitive elements arranged in a focus-insensitive pattern, wherein the second pitch includes a spatial distance between adjacent focus-insensitive elements of the plurality of focus-insensitive elements, wherein the plurality of recurring elements include a plurality of focus-sensitive elements arranged in a focus-sensitive pattern, wherein the plurality of focus-sensitive elements are positioned between adjacent focus-insensitive elements of the plurality of focus-insensitive elements, wherein the focus-sensitive pattern is segmented at a sub-resolution pitch.

<CIT> relates to method and system for performing a focus test in a lithographic apparatus. According to the method, a substrate is provided with a layer of radiation sensitive material. The substrate is illuminated in the lithographic apparatus using a first focus sensitive feature and a second focus feature, and the substrate is analyzed to provide results of the focus test using the first focus sensitive feature as imaged on the substrate. The focus test is performed on a regular production wafer, and the results of the focus test are allowed if predetermined statistic properties associated with the second focus feature as imaged on the substrate are within predetermined limits. The second focus feature may be focus sensitive or focus insensitive.

<CIT> relates to a lithographic apparatus which includes an alignment system for aligning a substrate. The alignment system comprises an illuminator system configured to illuminate an alignment mark on the substrate with an illumination spot, the alignment mark comprising a plurality of lines and spaces. The system also includes a combiner system configured to transfer two images of the illuminated alignment mark without spatial filtering of the images, rotate the images <NUM>° relatively to each other, and combine the two images; and a detection system configured to detect an alignment signal from the combined images and to determine a unique alignment position by selecting a specific one of extreme values in the detected alignment signal.

One aspect of the present invention provides a target design as recited in claim <NUM>. A second aspect of the present invention provides a method as recited in claim <NUM>. Preferred embodiments of the invention are laid down in the dependent claims.

Prior to the detailed description being set forth, it may be helpful to set forth definitions of certain terms that will be used hereinafter.

The term "focus-insensitive pattern" as used in this application refers to a region of an element in a target design which is continuous and not subdivided, and is characterized by a uniform critical dimension. The term "focus-sensitive pattern" as used in this application refers to a region of an element in a target design which either subdivided (in any direction, e.g., segmented or including gaps) and/or is characterized by a non-uniform critical dimension.

<FIG> is a level schematic illustration of target designs <NUM> on the mask, produced targets <NUM> and scatterometry measurements 96A, 96B thereof, which is not part of the invention.

Target designs <NUM> comprise a periodic structure having a plurality of recurring elements <NUM> characterized by a first pitch <NUM> in a first direction (x). Elements <NUM> are themselves periodic with a second pitch <NUM> along a second direction (y) that is perpendicular to the first direction. Elements <NUM> are characterized in the second direction by alternating, focus-sensitive and focus-insensitive patterns <NUM>, <NUM>, respectively, with second pitch <NUM>. Focus-insensitive pattern <NUM> may have a first critical dimension (CDa) upon production from an elements width <NUM> on the mask. Focus-sensitive pattern <NUM> may exhibit a second critical dimension (CDb) upon printing from the mask. In certain embodiments, second critical dimension CDb depends upon the focus during production of target <NUM>. In certain embodiments, second critical dimension CDb may equal first critical dimension CDa upon production on the wafer only when specified focus requirements are satisfied.

Produced target <NUM>, with produced elements <NUM>, is characterized by a pitch Px in the first direction (x) and possibly a pitch Py in the second direction (y), in case CDb differs from CDa (illustrated in an exaggerated manner). Pitch Py may be design to be several time larger than pitch Px (which may be similar to device pitch) and enable scatterometry measurements of the ±first diffraction orders in addition to the zeroth diffraction order (96B) by a metrology tool <NUM>. The scatterometry measurements may thus be used to estimate the focus with which targets <NUM> were produced.

<FIG> are high level schematic illustrations of target designs <NUM>. <FIG> represent target designs <NUM> on a respective photolithography mask used to produce targets <NUM> on the wafer. It is noted that, as explained above, the actual produced form <NUM> of target designs <NUM> on the wafer may differ from target designs <NUM> on the mask depending on production parameters such as focus and dose. <FIG> schematically illustrate non-limiting possibilities of target designs <NUM>, which may be modified according to the principles which are presented below.

Target designs <NUM> comprise a periodic structure having a plurality of recurring elements <NUM> characterized by first pitch <NUM> in a first direction (x), which are also periodic with second pitch <NUM> along a second direction (y) that is perpendicular to the first direction. Elements <NUM> are characterized in the second direction by alternating, focus-sensitive patterns <NUM> and focus-insensitive patterns <NUM>, which alternate with second pitch <NUM>.

In certain embodiments, upon production, first, focus-insensitive pattern <NUM> may have a first critical dimension marked CDa, and second, focus-sensitive pattern <NUM> may have a second critical dimension marked CDb. In certain embodiments, the latter, focus-sensitive pattern <NUM> may exhibit the first critical dimension (CDa) upon production on a wafer only when specified focus requirements are satisfied. Upon inappropriate focus a different critical dimension, e.g., between CDb and CDa may be produced.

In certain embodiments, first pitch <NUM> may be produced to yield Px that is close to a product pitch and the produced first critical dimension (CDa) may be less than half the produced pitch Px. In certain embodiments, second pitch <NUM> may be produced to yield Py that may be 1½-<NUM> times the first critical dimension (CDa). In certain embodiments, the second critical dimension (CDb) may be ½-<NUM> times the first critical dimension (CDa). In certain embodiments, Py may be <NUM>-<NUM> times Px. In certain embodiments, target design <NUM> may be configured to produce Px that yields a zeroth diffraction order signal 96A and produce Py that yields zeroth as well as ±first diffraction order signals 96B upon scatterometry measurements.

The second, focus-sensitive pattern <NUM> is segmented at a sub-resolution pitch. Focus-sensitive pattern <NUM> may be designed in different ways, some of which are illustrated in <FIG>. For example, focus-sensitive pattern <NUM> comprises horizontal elements <NUM> and vertical elements <NUM>. The term "horizontal" in this context is understood to be along the x direction (i.e., along first pitch <NUM> and respectively along the short dimension of element <NUM>) while the term "vertical" in this context is understood to be along the y direction (i.e., along second pitch <NUM> and respectively along the long dimension of element <NUM>). Either horizontal element(s) <NUM> or vertical element(s) <NUM> alone may define focus-sensitive pattern <NUM> (see e.g., <FIG>) or horizontal element(s) <NUM> and vertical element(s) <NUM> may be combined to form focus-sensitive pattern <NUM> (<FIG>). Horizontal element(s) <NUM> and/or vertical element(s) <NUM> may be periodic (<FIG> for horizontal element(s) <NUM>, though horizontal element(s) <NUM> does not necessarily have to be periodic; and <FIG> for vertical element(s) <NUM> with differing directions of periodicity) and may vary in length within focus-sensitive pattern <NUM> (e.g., <FIG>). The dimensions of sub-resolution elements <NUM>, <NUM> may be optimized according to production and measurements tools and parameters as well as according to required focus sensitivity. Exemplary feature dimensions comprise lengths <NUM>, <NUM> and width <NUM> of horizontal elements <NUM>, length (not designated) and widths <NUM>, <NUM> of vertical elements <NUM>, depending on the specific designs.

Lithographic tool focus offset may be measured using target designs <NUM>. Target designs <NUM> may be configured to be robust, sensitive to focus, and correlate to product. The measurement approach may be configured to use zeroth and first diffraction order signals or parts thereof (e.g., +<NUM> and/or -<NUM> orders). The pitch of produced target designs <NUM> (Px) and/or the critical dimension of produced target designs <NUM> (CDa) may be close to product pitch. In a non-limiting manner, line patterns are used to illustrate target designs <NUM>, and CDa is illustrated to be about or less than half Px. In certain embodiments, CDa may be close to the product CD and/or may be e.g., <NUM>% to <NUM>% of Px.

The enhanced sensitivity of produced targets <NUM> to focus parameters may be achieved along a direction that is different from the main measurement direction (along which Px is designed), typically a direction perpendicular thereto. Thus, along that direction, a secondary pitch Py is formed by producing alternating patterns <NUM>, <NUM> along element <NUM>. It is noted that while only one focus-sensitive pattern <NUM> is illustrated in <FIG>, multiple alterations and multiple focus-sensitive patterns <NUM> may be designed along element <NUM>, depending on its length. Some or all elements <NUM> in target design <NUM> may comprise alternating focus-sensitive and focus-insensitive patterns <NUM>, <NUM> (respectively), and different elements <NUM> may comprise different pattern <NUM> and/or <NUM>, yielding different pitches Py and/or different critical dimensions CDb.

In certain embodiments, secondary pitch <NUM> may be larger than first pitch <NUM>, e.g., up to four to six times larger, and be, for example between <NUM>-<NUM>. The first critical dimension (CDa) may be close to product CD and may be e.g., <NUM>% to <NUM>% of Px The second critical dimension (CDb) may be e.g., <NUM>% to <NUM>% of Py.

Second, focus-sensitive pattern <NUM> may be patterned by sub-resolution features <NUM>, <NUM>, i.e., features which are not necessarily reproduced in produced target <NUM>, yet influence the critical dimension of the respective part of produced element <NUM>. Target design <NUM> may hence be printed as a periodic structure lacking the distinction between first and second patterns <NUM>, <NUM>, at least in cases of correct photolithographic parameters such as focus and dose, and may indicate, by deviation from such expected periodic structure, the use of inappropriate photolithographic parameters such as focus and dose, e.g., parameters beyond specified tolerance regions. The details of focus-sensitive pattern <NUM> may be used and designed to define and adjust the sensitivity region and tolerance regions.

In certain embodiments, focus-sensitive and focus-insensitive patterns <NUM>, <NUM> may have similar sensitivity to dose changes while patterns <NUM>, <NUM> may differ in their focus sensitivity, i.e., under dose changes CDb may stay approximately equal to CDa, while under focus changes CDb may diverge from CDa, depending on the extent of the focus deviation. In certain embodiments, a uniformity of elements <NUM> or a symmetry between produced patterns <NUM>, <NUM> within elements <NUM> may serve as a metric for estimating the focus deviation, or the focus correctness.

<FIG> schematically illustrate simulation results for producing elements <NUM>, according to some embodiments of the invention. Using PROLITH® as the simulation tool, <FIG> illustrates the dependency of CDa-CDb of resist structures <NUM> produced by the illustrated target design (similar to <FIG>) on focus changes having the values -<NUM>, <NUM> and +<NUM> and on dose changes having the values -1mJ/cm<NUM>, <NUM>, +1mJ/cm<NUM>. <FIG> depict the results for each CDa, CDb, for focus changes (<FIG>) and dose changes (<FIG>). In the illustrated embodiments, CDb is not equal to CDa, and the difference CDa-CDb clearly depends on, and hence may be used to indicate focus deviations. It is noted that CDa and CDb depend on the dose in a similar manner and thus the focus parameter may be measured independently of the dose parameter.

<FIG> is a high level schematic illustration of measuring the focus using targets <NUM> on wafer <NUM> produced using target designs <NUM>, according to some embodiments of the invention. An illumination source <NUM> is used to derive diffraction signals <NUM> from target <NUM>. Diffraction signals <NUM> are schematically illustrated on the right hand side of <FIG>, showing the zeroth order signal as function of wavelengths (along the x direction) 96A and both zeroth and ±first diffraction orders signal along the y direction 96B.

<FIG> are high level schematic illustrations of target designs <NUM>, according to some embodiments of the invention. Certain embodiments comprise target designs <NUM> having multiple sub-targets 100A, 100B comprising similar elements <NUM> and separated into distinct groups at a pitch <NUM> which is different from first pitch <NUM> (<FIG>). Certain embodiments comprise target designs <NUM> having two or more types of elements <NUM>, e.g., elements 110A, 110B with different focus-sensitive patterns <NUM>. In a non-limiting example, illustrated in <FIG>, elements 110A, 110B are of the types presented in <FIG>, respectively, composed of horizontal elements 121A in the former and of horizontal elements 121B and vertical elements 122B in the latter. Target designs <NUM> are hence characterized by two distinct horizontal pitches, pitch <NUM> between adjacent elements 110A, 110B and another pitch <NUM> (=<NUM>·pitch <NUM>) between adjacent similar elements 110A (or similar elements 110B). The difference between focus-sensitive patterns <NUM> in elements 110A, 110B may be used to enhance the accuracy of the focus measurements derived therefrom. Exemplary feature dimensions comprise lengths <NUM>, <NUM> and widths <NUM>, <NUM> of horizontal elements <NUM>, 121A, 121B, length (not designated) and widths <NUM>, <NUM> of vertical elements <NUM>, 122B depending on the specific designs.

Certain embodiments comprise target designs <NUM> having two or more sub-targets 100A, 100B with elements <NUM> that differ in their focus-sensitive patterns <NUM>. Using multiple focus sensitive patterns <NUM> enhances the accuracy of the focus measurements derived therefrom. <FIG> are high level schematic illustration of target designs <NUM>, according to some embodiments of the invention. In the non-limiting examples, sub-target 100A is similar in the three designs to <FIG> (with horizontal elements 121A), while sub-target 100B is modified according to patterns introduced in <FIG>, respectively (with horizontal elements 121B and vertical elements <NUM> in <FIG> and <FIG>). Any combination of two or more sub-target designs may be used as target design <NUM>. In the illustrated cases, sub-targets 100A, 100B are characterized by first pitch <NUM> while sub-targets 100A, 100B are designed at a larger pitch <NUM>. Exemplary feature dimensions comprise lengths <NUM>, <NUM> and widths <NUM>, <NUM>, <NUM> of horizontal elements 121A, 121B, length (not designated) and widths <NUM>, <NUM> of vertical elements <NUM>, depending on the specific designs.

<FIG> is a high level flowchart illustrating a method <NUM>, which is not part of the invention. Design stages in method <NUM> may be carried out at least partially by a computer processor.

Method <NUM> may comprise designing a periodic target to have a first pitch in a first direction and elements having a second pitch along a perpendicular direction that have alternating, focus-sensitive and focus-insensitive patterns (stage <NUM>). In certain embodiments, method <NUM> may comprise segmenting the focus-sensitive pattern at a sub-resolution pitch (stage <NUM>).

Method <NUM> may further comprise designing the focus-insensitive pattern to have upon production a first critical dimension, and the focus-sensitive pattern to have upon production a second critical dimension that equals the first critical dimension only when specified focus requirements are satisfied (stage <NUM>).

For example, method <NUM> may comprise designing the second pitch to be <NUM>-<NUM> times the first pitch and the second critical dimension to be ½-<NUM> times the first critical dimension (stage <NUM>).

Method <NUM> may comprise configuring the target to be measured by scatterometry, wherein the first pitch is configured to yield zeroth order signal and the second pitch is configured to yield zeroth and at least one first order signals (stage <NUM>).

Method <NUM> may comprise producing the designed target (stage <NUM>) and verifying a tool's focus by measuring the produced focus-sensitive patterns (stage <NUM>).

Method <NUM> may further comprise preparing a FEM (focus exposure matrix) wafer from the designed targets with varying critical dimensions and/or pitches (stage <NUM>) and deriving a focus parameter from a comparison of a measured target signal with signals measured from the FEM wafer (stage <NUM>).

Method <NUM> may further comprise deriving a model from scatterometry measurements of designed targets with varying critical dimensions and/or pitches (stage <NUM>) and deriving a focus parameter from a measured target signal according to the model (stage <NUM>). The model may be derived by the methods described in <CIT>, disclosing process variation-based model optimizations for metrology.

Advantageously, target designs <NUM> and methods <NUM> provide both higher sensitivity to scanner focus than the prior art as well as good target printability. Relations between CD variation, scatterometry signal parameters and focus are used to derive focus deviations. In certain embodiments, produced targets <NUM> are periodic with a main pitch (Px) close to a product pitch and comprise a perpendicular structure having at least one repeating focus-sensitive pattern having a pitch Py larger than the illumination wavelength used by the scatterometer tool. Target designs <NUM> may comprise two or more sub-targets having different focus sensitivities. A signal model or reference targets may be used to derive the focus from respective diffraction signals (e.g., zeroth, +first and/or - first orders). Multiple targets or sub-targets may be used to de-correlate focus and dose measurements and/or deviations.

In certain embodiments, film pad targets may be used for under layer de-correlation. The diffraction order signal (e.g., zeroth order) may be extracted from measurements of the film pad target. The signal measured using the film pad target may be fed-forward to the measurement on the grating with the sensitive focus target, described above. In certain embodiments, under layer de-correlation using film pad targets may increase the accuracy of the focus measurement.

Claim 1:
A target design (<NUM>) comprising:
A periodic structure having a plurality of recurring elements (<NUM>), which are arranged based on a first pitch (<NUM>) in a first direction, wherein the plurality of recurring elements (<NUM>) are periodic and are arranged based on a second pitch (<NUM>) along a second direction that is perpendicular to the first direction,
wherein the plurality of recurring elements (<NUM>) include a plurality of focus-insensitive elements arranged in a focus-insensitive pattern (<NUM>), wherein the second pitch (<NUM>) includes a spatial distance between adjacent focus-insensitive elements of the plurality of focus-insensitive elements,
wherein the plurality of recurring elements include a plurality of focus-sensitive elements arranged in a focus-sensitive pattern (<NUM>), wherein the plurality of focus-sensitive elements are positioned between adjacent focus-insensitive elements of the plurality of focus-insensitive elements,
wherein the focus-sensitive pattern (<NUM>) is segmented at a sub-resolution pitch,
wherein the focus-sensitive pattern (<NUM>) comprises horizontal elements (<NUM>) which extend along the first direction and vertical elements (<NUM>) which extend along the second direction with the vertical elements being narrower in width along the first direction than the horizontal elements, and wherein the vertical elements are positioned between adjacent horizontal elements.