Positioning two elements using an alignment target with a designed offset

An alignment system for aligning two elements includes an alignment target with periodic patterns on each element. The alignment target includes two locations, at least one of which has a designed in offset. If desired, both locations may have designed in offsets of the same magnitude but in opposite directions. The diffraction patterns produced at the two locations are compared. If the difference between the patterns is at a minimum, the elements are properly aligned. When an alignment error is introduced, however, the calculated difference can be used to determine the error. In another embodiment, bands in the moiré fringes from the different locations may be compared to determine the alignment error. The two elements may then be moved relative to each other to minimize the alignment error. Thus, the alignment target may advantageously be used in any alignment system, such as an exposure tool.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to alignment control, and in particular to an alignment system that uses an alignment target that includes at two locations having designed in offsets.

2. Discussion of the Related Art

Semiconductor processing for forming integrated circuits requires a series of processing steps. These processing steps include the deposition and patterning of material layers such as insulating layers, polysilicon layers, and metal layers. The material layers are typically patterned using a photoresist layer that is patterned over the material layer using a photomask or reticle. As the integrated circuit feature sizes continue to decrease to provide increasing circuit density, it becomes increasingly difficult to align one layer with another.

During processing, the substrate is moved from one location to the next so that different areas, e.g., dies, on the substrate can be exposed. The alignment system, e.g., the exposure tool, typically uses an alignment target to properly align the substrate during exposure.FIG. 1shows a conventional alignment system50, which includes a diffraction grating52on the substrate and a second diffraction grating54that is stationary, e.g., is fixed to the lens on the exposure tool. A light source56produces coherent light that passes through a beam splitter58and is incident on the diffraction grating52after passing through a lens60. The light is diffracted by diffraction grating52and passes through lens60back to beam splitter58. The coherent light beam from source56is also reflected off beam splitter58, passes through lens62and is incident on diffraction grating54. The light diffracted by diffraction grating54passes back through lens62to beam splitter58. At beam splitter58the light diffracted from diffraction gratings52and54is combined and the combined diffraction patterns is received by light detectors64.

Alignment system50provides an accuracy of approximately 15 nm. One disadvantage of alignment system50is that coherent light is used. Thus, if the diffraction grating52on the sample absorbs the particular frequency used, alignment system50cannot provide an accurate measurement. While multiple coherent light sources may be used to avoid this disadvantage, the use of multiple light sources adds complexity and cost.

Thus, there is a need in the semiconductor industry for an improved alignment system.

SUMMARY

In accordance with the present invention, an alignment system is used for aligning two elements, such as a substrate and/or a reticle with a reference mask, two stages, or any two items in general. The alignment system includes an alignment target that has at least one periodic pattern, e.g., diffraction gratings, on each of the two elements to be aligned. The alignment target includes at least two locations with at least one having a designed in offsets between the periodic patterns on each of the elements. If desired, the alignment target may have a designed in offset at each measurement location of the same magnitude but in opposite directions. Alternatively, the locations may have a designed in offset of different magnitudes or in non-parallel directions. For example, the two locations on the alignment target may be two separate overlay patterns, e.g., sets of periodic patterns, that are mirror images of each other. A single set of periodic patterns may be used, with one periodic pattern on one element and another periodic pattern on the other element. The periodic patterns may have different pitches. In such an embodiment, the length of the pattern is sufficient to allow two locations that have a designed in offset of the same magnitude but opposite direction.

To determine the alignment error, radiation is incident on and reacts with the two locations in the alignment target. The radiation is then detected and compared, e.g., the difference between the radiation may be calculated. Advantageously, the calculated difference is extremely sensitive to any alignment error. When the difference between the patterns is at a minimum, the elements are properly aligned. If the elements are not aligned, the difference will be increased. A slight alignment error will produce a relatively large, and easily detected, calculated difference. By moving the elements into alignment the difference will converge on approximately zero. The alignment target may be used, e.g., on an exposure tool to pre-align with a substrate prior to exposure or any other system in which precise alignment between two elements is desirable.

It should be understood that the designed in offset is an intended offset between the periodic pattern on one element relative to the periodic pattern on the other element. In practice, when the two elements are not in alignment, the offset between the periodic patterns on the two elements will not be the designed in offset, but will be the combination of the designed in offset and the overlay error.

In one embodiment of the present invention, a method of positioning a first element with respect to a second element includes providing an alignment target on the first element and the second element, the alignment target having at least one periodic pattern on the first element and at least one periodic pattern on the second element, the alignment target having at least two locations, at least one of which has a designed in offset between the periodic pattern on the first element and the periodic pattern on the second element. In one embodiment, both locations have a designed in offset of the same magnitude but in opposite directions. In another embodiment, the designed in offsets may be different magnitudes or in different directions. The method includes illuminating the at least two locations on the alignment target with incident radiation and detecting the radiation after it interacts with the alignment target. The detected radiation from each location is then compared to determine the alignment of said first element with respect to said second element. The method includes moving one element with respect to the other. The movement of the elements may be in response to the comparison, i.e., to minimize the alignment error.

The first element may be a substrate and the second element may be a reference mask. The method may then include moving at least one of the substrate and the reticle to minimize the alignment error based on the comparison between the measured light diffracted by the at least two locations.

The method may include receiving the radiation by a two dimensional detector that receives the spectra dispersed along one dimension and the spectra associated with different overlay patterns separated along a direction perpendicular to the spectral dispersion.

The detected radiation may be diffracted light or may be an image of the different locations of the alignment target in the form of moiré fringes. The bands of the moiré fringes can then be detected and compared to determine alignment of the two elements.

In accordance with another embodiment, a method of aligning two elements includes determining whether a first element is aligned with a second element using at least two locations of an alignment target. The alignment target has at least one periodic pattern on the first element and at least one periodic pattern on the second element, where at least one of the two locations has designed in offset between the periodic pattern on the first element and the periodic pattern on the second element. The method includes moving the first element relative to the second element.

Another embodiment of the present invention is an apparatus for positioning a first element with a second element. The apparatus includes an alignment target having at least one periodic pattern on the first element and at least one periodic pattern on the second element, the alignment target including at least two locations, at least one of which has a designed in offset between the periodic patterns. If both locations have a designed in offset the offsets may be of the same magnitude in opposite directions. The apparatus includes a light source for producing light to be incident on the at least two locations of the alignment target and a light detector for detecting light after reacting with the two locations of the alignment target. The apparatus further includes a processor with a computer-usable medium having computer-readable program code embodied therein for causing the processor to calculate the difference between the detected light from the two locations and using the difference to determine if the first element and the second element are aligned. The apparatus includes a stage that is controlled by the processor and that moves one of the elements with respect to the other element to minimize the alignment error.

DETAILED DESCRIPTION

In accordance with an embodiment of the present invention, an alignment system aligns two elements using an alignment target that includes at least one periodic pattern on one element that is aligned relative to another periodic pattern on the other element. The alignment target includes at least two locations, i.e., areas of the alignment target, at least one of which has a designed in offset between the periodic patterns. For example, both locations can have a designed in offset that is equal in magnitude but opposite in direction. Alternatively, the magnitudes may differ and/or the directions may be non-parallel. The alignment target may be used to determine the alignment error between any two elements, e.g., between the lens and the substrate and/or reticle in an exposure tool, any two stages, or any other items to be aligned. Advantageously, the alignment target of the present invention is relatively insensitive to unintentional rotation of the top element with respect to the bottom element.

The alignment system uses incident light that interacts at the two locations of the alignment target, e.g., is diffracted or creates moiré fringe patterns. The difference in the light that interacts with the alignment target at the two locations is extremely sensitive to alignment error. Thus, for example, if the alignment error is approximately zero, the difference will be a minimum, e.g., approximately zero plus noise. If, however, a slight alignment error is present, the difference will be relatively large. The presence of a small alignment error may be easily and accurately determined, which is advantageous in an exposure tool or any other device in which an accurate alignment between two elements is desired.

To determine the presence of an alignment error, the difference in the light that interacts with the alignment target at the two locations, e.g., the spectra from the two locations, is calculated. When the difference is at a minimum, ideally zero, the alignment error is minimized. If the difference is non-zero, one element may be moved relative to the other element and the difference recalculated. The difference will converge on approximately zero as the elements are moved into alignment.

Alternatively, an image of the resulting moiré fringe patterns produced by the gratings may be used. The bands of the moiré fringes from one location are detected and compared to the bands of the moiré fringes from the other location. When the bands of the moiré fringes are in the same relative positions the alignment error is minimized.

The present invention may be used to control the alignment error down to a fraction of a nanometer, while the current industry standard is approximately 15 nm. Thus, the present invention provides a large improvement compared to current technology.

Alignment target is used generally to mean a target that aids in the alignment process, e.g., in an exposure tool, as well as an overlay target, which is used to measure the overlay error on a substrate. It should be understood that the use of the alignment target of the present invention is not limited to aligning two elements in an alignment process, but may be used to measure an overlay error on a device, such as an overlay error between two layers on a substrate. The use of the alignment target to measure an overlay error is described in more detail in U.S. patent application entitled “Alignment Target with Designed in Offset” by Weidong Yang, Roger R. Lowe-Webb, John D. Heaton, and Guoguang Li, Ser. No. 10/116,863, and in U.S. patent application entitled “Measuring An Alignment Target With Multiple Polarization States” by Weidong Yang, Roger R. Lowe-Webb, Ser. No. 10/116,798; and in U.S. patent application entitled “Encoder with Alignment Target”, by John D. Heaton, Weidong Yang, and Roger R. Lowe-Webb, Ser. No. 10/116,855, all of which are filed herewith and have the same assignee as the present application and are incorporated herein by reference.

To assist in the description of an alignment system using the alignment target, it is helpful to first describe the alignment target.

FIG. 2shows a cross-sectional view of an alignment target100in accordance with an embodiment of the present invention in which there are two locations with a designed in offset of the same magnitude but different directions. The two locations in alignment target100are referred to as overlay patterns102and104, respectively. Each of the overlay patterns102,104includes a bottom periodic pattern, referred to as diffraction gratings106,116, and a top periodic pattern, referred to as diffraction gratings108,118. Alignment target100is described as being produced on different layers on a substrate. It should be understood, however, that the alignment target100may also be used in an alignment system, in which case, the top and bottom diffraction gratings are movable relative to each other, as will be discussed in more detail in reference toFIG. 15.

Each overlay pattern102,104is similar to the overlay pattern described in the U.S. Ser. No. 09/960,892 entitled “Spectroscopically Measured Overlay Target”, filed Sep. 20, 2001, which has the same assignee as the present disclosure and which is incorporated herein in its entirety by reference. However, only one overlay target is described in U.S. Ser. No. 09/960,892, whereas two mirror imaged overlay patterns102,104are used as the alignment target in accordance with the present invention. While it is possible to determine the overlay error by measuring the reflectance spectrum of one grating target and modeling the spectrum using, e.g., RCWA, as described in U.S. Ser. No. 09/960,892, the accuracy suffers from systematic errors such as imperfections in the model. The effect of systematic error on the absolute accuracy can be expressed as shiftmesasured=shiftreal+errorsystematic. The present invention, advantageously, increases the accuracy by canceling out the systematic error term.

The overlay patterns102,104are produced on a substrate101, which is e.g., a silicon substrate. The substrate may be a semiconductor wafer, flat panel display or any other structure in which accurate alignment of successive layers is important. Of course, there may be layers between substrate101and the bottom diffraction gratings106,116. It should be understood that whileFIG. 2shows overlay patterns102and104as separate patterns, overlay patterns102and104are included on the same substrate, preferably near to each other and together form alignment target100.

The bottom diffraction gratings106,116are produced, for example, by providing a layer of appropriate material, such as a 200 nm layer of polysilicon, followed by a layer of photoresist. The desired image including the diffraction gratings106,116is exposed in the photoresist, which is then developed. The polysilicon is then etched away leaving diffraction gratings106,116. A layer103, such as a dielectric layer, is then deposited over diffraction gratings106,116.

As shown inFIG. 2, an overlaying layer105, which may be, e.g., a 1000 nm layer of silicon oxide, is deposited over the bottom diffraction gratings106,116. The top diffraction gratings108,118are then produced using, e.g., photoresist, in a manner similar to the bottom diffraction grating102, where the top diffraction gratings108,118are separated from the bottom diffraction gratings106,116by the thickness of layer105, e.g., 1000 nm. For example, an 800 nm layer of the photoresist is deposited over layer105. The desired image including the diffraction gratings108,118is exposed in the photoresist layer. The photoresist is then developed, thereby leaving diffraction gratings108,118.

It should be understood that the processing steps used to produce overlay patterns102and104are exemplary. Different or additional layers may be included between the substrate101and the bottom diffraction gratings106,116or between the bottom diffraction gratings106,116and the top diffraction gratings108,118. Additionally, fewer layers may be included between the bottom diffraction gratings106,116and the top diffraction gratings108,118, i.e., layer105may not be used. In fact, if desired, the top diffraction gratings108,118may be on the same layer as the bottom diffraction gratings106,116, in which case, top and bottom refers to the order in which the gratings are produced, i.e., the bottom diffraction grating being first and the top diffraction grating being second. Thus, the alignment target100is used to ensure that a first pattern produced on a first layer on the substrate is aligned with a second pattern produced on the same layer on the substrate.

In an alignment system, the bottom periodic patterns are on one element and the top periodic patterns are on a second element, where the first element and second element are movable with respect to each other. Further, there is a small separation between the top and bottom periodic patterns, with the ambient environment between the periodic patterns.

The dimensions of the patterns and the thicknesses of the layers may also be altered. For example, the bottom diffraction gratings106,116need not extend to the top of layer105.

Moreover, latent images may be used, and thus, the photoresist need not be developed to form diffraction gratings108,118, because the exposed photoresist has a different index of refraction than the unexposed photoresist.

In addition, it should be understood that either diffraction gratings108,118or106,116may actually be a continuous diffraction grating. Thus, for example, bottom diffraction gratings106and116may be one long pattern with top diffraction gratings108and118disposed over the top. Alternatively, top diffraction gratings108and118may be one continuous pattern disposed over the top of bottom diffraction gratings106,116. Because the present invention uses separate locations on the alignment target, however, the use of a single or separate periodic patterns does not matter. Consequently, it should be understood that discussion of two diffraction gratings, e.g., patterns106and116, is intended to include two separate areas of a single continuous diffraction grating.

As can be seen inFIG. 2, overlay pattern102includes a designed in offset of +D between the top diffraction grating108and bottom diffraction grating106. Thus, during processing, if the top diffraction grating108is produced over the bottom diffraction grating106with an offset exactly +D, the top diffraction grating108and the bottom diffraction grating106are perfectly aligned. If the top diffraction grating108is produced over the bottom diffraction grating106with some deviation from the offset +D, the deviation is the amount of the overlay error.

Overlay pattern104is similar to overlay pattern102but is mirror imaged, i.e., has an offset of −D. Because overlay patterns102and104are similar, the top diffraction gratings108,118are designed to have the same line width (LWT), height (HT) and pitch (PT), while the bottom diffraction gratings106,116are likewise designed to have the same line width (LWB), height (HB) and pitch (PB). In one exemplary embodiment, diffraction gratings106,116may have a line width (LWB) of 200 nm, a line height (LHB) of 200 nm and a pitch (P) of 600 nm, while diffraction gratings108,118may have a line width (LWT) of 200 nm, a line height (LHT) of 800 nm and a pitch (PT) of 600 nm. It should be understood that the alignment target100may be produced using various materials and the dimensions optimized for the materials used. Thus, the dimensions of the alignment target100may be altered to maximize sensitivity based on the types of materials used.

Because overlay pattern102and104are produced at the same time, during the same processing steps, any variation between the intended structure and the actual structure should be the same for both overlay pattern102and104. Because overlay pattern102and overlay pattern104have the same amount of designed in offset D, but in opposite directions, the diffraction patterns produced by overlay patterns102and104will be the same. Consequently, if there is no overlay error, the detected light patterns from overlay pattern102and overlay pattern104will be the same.

However, because overlay pattern102has an offset of +D and overlay pattern104has an offset of −D, any overlay error will alter the offset in the two patterns differently, i.e., the symmetry between overlay pattern102and overlay pattern104is broken. For example, an overlay error of +e will increase the magnitude of the offset for overlay pattern102while decreasing the offset for overlay pattern104.

FIG. 3shows an alignment target100′, which is similar to alignment target100inFIG. 2, except that an overlay error of +e is introduced into the alignment target100′. The overlay error of +e results in overlay pattern102′ having an offset vector with an absolute magnitude of D+e, and overlay pattern104′ having an offset vector with an absolute magnitude of D−e.

Accordingly, by calculating the difference between the measurements of diffracted light from overlay pattern102and overlay pattern104, the amount of overlay error e can be accurately determined. For example, if there is no overlay error, the difference between the measurements will be zero (plus any system noise) due to the symmetry between overlay pattern102and overlay pattern104. However, if there is an overlay error e, by calculating the difference in the diffraction measurements, the magnitude of the overlay error vector is functionally doubled i.e., |D+e|−|−D+e|=2e. Moreover, because the diffraction measurements from one overlay pattern is subtracted from the diffraction measurements from the other overlay pattern, systematic errors will be cancelled.

It should be understood that if desired, only one overlay pattern, e.g., overlay pattern102may be used and measured in two different positions. Thus, for example, an initial measurement is made, the top and bottom elements are then moved relative to one anther by a known amount D, and a second measurement is made. Thus, functionally, two measurements with designed in offsets have been made using only one overlay pattern. Alternatively, a library of spectra at different alignment error positions can be pre-generated and used to determine the alignment shift based on a measurement with the single overlay pattern.

To understand why the accuracy of the alignment target100is improved, reference is made toFIG. 4, which is a sensitivity map for a diffraction pattern produced by a pattern similar to overlay pattern102, but with no designed in offset. The sensitivity map ofFIG. 4shows the sensitivity of each wavelength to a relative grating shift, i.e., the top grating moves relative to the bottom grating, where the x axis shows the relative normalized grating shift of the diffraction gratings from 0 to 1, and the y axis is the wavelength of light from 500 nm to 700 nm. The sensitivity of each wavelength of light for each relative grating shift was calculated using the following equation:

S⁡(λi,φj)=R⁡(λi,φj)-R⁡(λi,φj-1)φj-φj-1=∂R∂φeq.⁢1
where R(λi, φj) is the calculated reflectance at a particular relative grating shift for a particular wavelength and R(λi, φj-1) is the calculated reflectance at the previous relative grating shift at the same wavelength, and the sensitivity S(λi,φj) is in arbitrary units ranging from 0 to 14.

As can be seen inFIG. 4, the sensitivity of an overlay pattern102is symmetrical about 0 and about 0.5, which correspond to perfect alignment between the top diffraction grating106and bottom diffraction grating104and a shift of 50% of the pitch, respectively.

FIG. 5is a graph showing the overlay sensitivity of a diffraction grating, such as overlay pattern102, where the x-axis is the relative grating shift and the y-axis is the sensitivity in arbitrary units. The sensitivity for each grating shift was averaged over the wavelength variable according to the following equation:

〈S⁡(φ)〉λ=1N⁢∑i=1N⁢S⁡(λi,φ)eq.⁢2
where N is the number of discrete wavelengths comprising the sensitivity spectrum. The slight shift of the curves inFIG. 5to the right on the x axis is due the comparison of the reflectance for a current grating shift with the previous grating shift in equation 1. As can be seen inFIG. 5, maximum sensitivity lies between approximately 5% to 40% (and 60% to 95%) of the pitch P. While the maximum of the curves appears at approximately 34% and 66% of the pitch, there is a roll off between approximately 40% and 60%. Moreover, from inspection of the sensitivity map ofFIG. 4it can be seen that the various wavelengths are less sensitive to change when the relative grating shift is close to 50%.

Thus, in one embodiment, the designed in offset D in overlay patterns102,104lies in the range of maximum sensitivity, as shown inFIG. 5. For example, the offset ±D may be approximately ±5% to 40% of the pitch, and more particularly ±95% of the pitch. Of course, the amount of offset may be altered to optimize sensitivity for the materials used and the dimensions of the patterns.

FIG. 6shows the spectral reflectance from alignment target100inFIG. 2when there is no overlay error. The overlay pattern102has an offset of +D, while the overlay pattern104has an offset of −D. As can be seen inFIG. 6, as well as inFIGS. 4 and 5, when the same magnitude but different direction offset D is used, the resulting diffraction measurement from the two overlay patterns will be the same. It should be understood, that whileFIG. 6shows the spectral reflectance, any diffraction measurement, i.e., a measurement of any one or more of the diffractive orders, including the 0thorder, may be used. Moreover, the measurement may be made using a single wavelength, a plurality of discrete wavelengths, or a continuum of wavelengths.

FIG. 7shows the spectral reflectance from alignment target100′ inFIG. 3, i.e., when the alignment target includes a non-zero alignment error. For the purpose ofFIG. 7, the overlay error is 2 nm, whereas the pitch is 400 nm, top line width is 100 nm, bottom line width 100 nm, the height HB is 200 nm and the height HT is 500 nm. As can be seenFIG. 7, when there is an overlay error, the spectral reflectance from overlay pattern102′ is different than that for overlay pattern104′.

FIG. 8shows the difference in the reflectances shown inFIGS. 6 and 7, i.e., the reflectances for overlay patterns when there is no overlay error and when there is a 2 nm overlay error. As can be seen inFIG. 8, the difference in spectral reflectance between the overlay patterns102and104when there is no overlay error, is equal to approximately zero (except for a slight amount of noise that can be seen). However, there is a large difference in spectral reflectance between the overlay patterns102′ and104′, when there is small overlay error of 2 nm, particularly within wavelengths 400 to 600 nm. Thus, by calculating the difference between the measured diffraction of the two overlay patterns, small overlay errors may be observed.

FIG. 9is a graph showing the spectral reflectances from an overlay pattern, such as overlay pattern102inFIG. 2, with different offsets. The overlay pattern used to generate the data forFIG. 9has a bottom diffraction grating height of 200 nm and line width of 100 nm and a top diffraction grating height of 500 nm and line width of 100 nm. The separation between the top and bottom diffraction gratings is 500 nm and the pitch of diffraction gratings is 400 nm.FIG. 9shows the spectral reflectance for an offset of between 80 nm to 120 nm in 2 nm increments.

FIG. 10is a graph showing the delta shift from 4 nm to 40 nm in spectral reflectances ofFIG. 9. In other words,FIG. 10shows the difference between each spectral reflectance inFIG. 9and the spectral reflectance for 80 nm. Thus,FIG. 10shows the difference between the spectral reflectances for offsets of 80 nm and 84 nm, 80 nm and 88 nm, 80 nm and 92 nm, etc. . . . BecauseFIG. 9shows the spectral reflectances for offsets of 80 nm to 120 nm at every 4 nm, the delta shift shown inFIG. 10is from 4 nm (84 nm–80 nm) to 40 nm (120 nm–80 nm). As can be seen inFIG. 10, as the delta shift increases, the resulting difference in spectral reflectance increases.

FIG. 11is a graph showing the difference in spectral reflectances for neighboring offsets ofFIG. 9, i.e., 84 nm–80 nm; 88 nm–84 nm, 92 nm–88 nm, etc. . . . Thus,FIG. 11shows the difference between spectral reflectances for offsets ranging from 80 nm to 120 nm, where each difference is based on a difference in offset of 4 nm. As can be seen inFIG. 11, the differences in spectral reflectances are nearly identical, which indicates that the difference in the spectral reflectances is highly linear.

Alignment target100may be measured using several types of metrology devices, such as a normal incidence reflectometer, a non-normal incidence reflectometer, ellipsometer, scatterometer or other such device.FIG. 12A, for example, shows a block diagram of a normal incidence polarized reflectance spectrometer that may be used to measure alignment target100. Spectrometer120is discussed in detail in the U.S. patent application entitled “Apparatus and Method for the Measurement of Diffracting Structures,” filed Sep. 25, 2000, having Ser. No. 09/670,000, and the U.S. patent application entitled “Measurement Of Diffracting Structures Using One-Half Of The Non-Zero Diffracted Orders” filed Apr. 27, 2000, having Ser. No. 09/844,559, both of which have the same assignee as the present disclosure and are incorporated herein by reference. Spectrometer120may use rigorous coupled wave analysis (RCWA) as described in Ser. No. 09/670,000, or folded rigorous coupled wave analysis as described in Ser. No. 09/844,559 to measure overlay target100.

As shown inFIG. 12A, spectrometer120is similar to a reflectometer, which is well known in the art. Spectrometer120includes a polychromatic light source122that generates a light beam that is partially reflected by beam splitter124along the optical axis123. The light beam is directed towards a sample126having an overlay target, such as target100, to be measured. Sample126may be, e.g., a semiconductor wafer or flat panel display or any other substrate, and is supported by a stage128, which may be a polar coordinate, i.e., R-θ, stage or an x-y translation stage. Spectrometer120includes a rotatable polarizer130and a lens132(or series of lenses) to polarize and focus the light beam onto the sample126at normal incidence. The beam is reflected off sample126and the reflected light is transmitted through lens132and polarizer130. A portion of the reflected light is transmitted through beam splitter124and is received by a spectrophotometer134. Spectrophotometer134is coupled to processor136, which analyzes the data provided by spectrophotometer134. Processor136is e.g., a computer with a computer-usable medium having computer-readable program code embodied therein for causing the computer to determine the overlay error based on the light that is diffracted from the alignment target. Thus, the computer-readable program code causes the computer to calculate the difference between the diffracted light from the two locations and use the difference to determine if the alignment target is aligned. One of ordinary skill in the art can program code necessary to determine alignment in accordance with the present invention in view of the present disclosure. For more information on the general operation of a normal incidence polarized reflectance spectrometer, the reader is referred to Ser. Nos. 09/670,000 and 09/844,559, which are incorporated herein by reference.

If desired, other measurement routines may be used to measure overlay target100including ellipsometry and scatterometry.FIGS. 12B and 12Cshow block diagrams of a spectroscopic ellipsometer140and spectroscopic scatterometer145. Ellipsometer140includes a broadband radiation source141and a spectrophotometer142, which is connected to a processor143. Spectroscopic scatterometer145includes a broadband radiation source146and a spectrophotometer147, which is connected to a processor148. As indicated by the arrows inFIG. 12C, one or both of broadband radiation source146and spectrophotometer147are adjustable to alter the angle of incidence. If a non-normal metrology device, such as an ellipsometer or scatterometer is used with the alignment target of the present invention, the azimuthal angle is preferably perpendicular to the lines of the diffraction grating. Alternatively, the azimuthal angle may be oriented at an arbitrary angle with respect to the grating lines. The same azimuthal angle should be used for the measurement at both locations. If desired, different azimuthal angles may be used for the measurement of each location on the alignment target if the angles are perpendicular or anti-parallel to each other. The operation of an ellipsometer140and spectroscopic scatterometer145is well known to those skilled in the art. Processor143and148may be similar to processor136shown inFIG. 12A.

It should also be understood, that the present invention may use a single wavelength, a plurality of discrete wavelengths, or a continuum of wavelengths. For example, referring toFIG. 8, it can be seen that the measurement is extremely sensitive at wavelengths of approximately 410 nm. Thus, single wavelength measurements at, e.g., 410 nm may be particularly useful.

In one embodiment, the spectrometer apparatus detects the spectra from multiple targets at the same time. For example, as shown inFIG. 13A, a spectrometer800in which multiple targets802aand802bon a sample801are illuminated simultaneously. The spectrometer800includes a broadband illumination source804that produces light through a number of circular apertures806a,806b. As shown inFIG. 13A, two beams are produced that reflect off a beam splitter808and are focused by an objective lens810to selectively illuminate desired locations, i.e., the two targets802a,802b, on the sample801. The two targets are, e.g., the overlay patterns102and104shown inFIG. 2.

The light diffracted from the targets802a,802bis focused by objective lens810onto the spectrometer entrance apertures812aand812bafter passing through beam splitter808. The light is incident on an imaging spectrometer grating814and is collected by a two-dimensional array detector816, such as a CCD detector. Thus, the light scattered by targets802aand802bis selectively collected. The spectra for each target are dispersed along one dimension of the detector816. If additional overlay patterns are used, additional beams may be produced with additional circular apertures806and with additional spectrometer entrance apertures812.

FIG. 13Bshows a front view of the detector816, where the X-axis is wavelength of the spectra and the Y-axis is the position on the sample for each spectrum, i.e., the spectra produced by the first, second, to n targets. The spectra associated with specific locations on the sample are separated along the direction perpendicular to the direction of the spectral dispersion by sufficient distance that light from one location does not leak into the spectrum of another location.

As shown inFIG. 13A, spectrometer800works well with an X-Y stage. If an R-θ stage, additional optics such as an image rotator, e.g., a dove prism, may be inserted below the beamsplitter, allowing the image of the sample to be rotated to match the spectrometer and illumination apertures.

FIGS. 14A and 14Bshow another embodiment of a spectrometer900that may be used to detect the spectra from multiple targets at the same time. As shown inFIG. 14A, spectrometer900is similar to spectrometer800, except that an illumination slit906is used in place of a plurality of circular apertures806a,806binFIG. 13A. Consequently, more area of sample901will be illuminated. Spectrum isolation is achieved, as shown inFIG. 14B, by electronically binning only the rows of the detector that correspond to each target on the sample. All other light collected is not used.

The measurement of all locations simultaneously is particularly advantageous in an alignment process when the top element and bottom element are moving with respect to each other. If the measurements are not made simultaneously, the top and bottom elements should be held stationary while the measurement are made.

As discussed above, the alignment target may be used advantageously to assist in the alignment process, i.e., aligning two separate elements. Thus, for example, the present invention may be used to assure substrate to reticle registration when the substrate is on the exposure tool during processing. The present invention, of course, is not limited to exposure tools, but may be used to assist in the precise alignment of any separate elements.

FIG. 15is a block diagram of an exposure tool600with which the present invention may be used. Exposure tool600includes X and Y substrate stages602and604that hold the substrate601. The exposure tool600also includes X and Y reticle stages606and608that hold the reticle610. Exposure tool600may include two sets of stages, one set for large motions and another set for fine motions. For sake of simplicity, X and Y stages602,604,606, and608may be used for both large motion and fine motion.

A control system612controls the motion of the stages. A lens614or other suitable optics is positioned between the substrate601and the reticle610and is used to focus light from light source616that is transmitted through reticle610onto substrate601. The operation and control of exposure tools is well known in the art.

A reference mask618extends from the lens614by way of a low thermal expansion arm619. Spectrometers620are positioned above reference masks618. As shown inFIG. 15, a plurality of reference masks618may be used, each having an associated spectrometer620.

FIG. 16shows a perspective view of substrate601and reticle610with lens614and four reference masks618disposed between the substrate601and reticle610. The spectrometers620are not shown inFIG. 16. As can be seen inFIG. 16, a number of separate alignment targets660are used, where the top diffraction gratings662is on the reference masks618and bottom diffraction gratings664is on the substrate601.

Referring back toFIG. 15, the spectrometers620may be coupled to the lens614as shown inFIG. 15, or may be connected to another stationary object. The spectrometers620communicate with the control system612. The control system612includes, e.g., a computer-usable medium having computer-readable program code embodied therein for causing the control system to calculate the difference between the diffracted light from the two locations and using the difference to determine if the first element and the second element are aligned. The control system612is coupled to the stage to adjust the location of the substrate601in response to the signals provided by spectrometer620until the starting position of the substrate601is precisely aligned with the lens614. Once the substrate601is aligned, the control system612can move the stages to perform the desired exposure operation.

FIG. 17shows a schematic view of a spectrometer620that may be used in the present invention. Spectrometer620includes an illumination source622that produces radiation that is reflected off beam splitter624. The radiation passes through an aperture626and lens628and is focused on the reference mask618and the substrate601. The incident radiation reacts with, e.g., is diffracted by, the alignment target, which includes the periodic pattern on the reference mask and the periodic pattern on the substrate. After reacting with the alignment target, the radiation passes through lens628, aperture626, and beam splitter624and is focused by another lens630onto the detector632.

Illumination source622produces radiation, e.g., polychromatic light, that will not expose the photoresist on substrate. Thus, for example, light in the visible spectrum may be used, where the photoresist is UV sensitive. The distance between the reference mask618and the substrate601should be small, e.g., between 1 and 10 μm. Moreover, the objective lens628should have a small NA, e.g., between 0.02 and 0.005, such as 0.01. A small NA minimizes errors that may be caused by a non-zero angle of incidence.

FIG. 18Ashows a cross-sectional view of alignment target660. As discussed in reference toFIG. 2, alignment target660includes two locations, referred to as overlay patterns661,663, where there is a designed in offset of equal magnitude but in opposite directions. Alignment target660includes a periodic pattern on each element, e.g., a diffraction grating662on the reference mask618and diffraction grating664on the substrate601(which includes some layer on the substrate). The top diffraction gratings662may include two separate diffraction gratings662aand662bor may be a single continuous grating. The bottom diffraction gratings664may also include two separate diffraction gratings664aand664bor may be a single continuous diffraction grating.

When properly aligned, top diffraction grating662awill be offset by an amount +D from the bottom diffraction grating664aat one location, i.e., at the end of the periodic pattern, and the top diffraction grating662bwill be offset by an amount −D from the diffraction grating664bat another location, i.e., at the other end of the periodic pattern. Thus, as can be seen inFIG. 18A, alignment target660is similar to alignment target100, and thus, the alignment error between the top diffraction gratings662and the bottom diffraction gratings664can be easily determined as discussed above. For example, the spectra at the two overlay patterns can be measured. The control system612may calculate the difference between the spectra. If the substrate601and reference mask618are mis-aligned, there will be a non-zero difference. As the substrate601is moved by the stage into alignment with the reference mask618, however, the difference between the spectra will converge on approximately zero (some noise may be present). Thus, once the difference is approximately zero, the substrate601and reference mask618will be in alignment.

To align the two elements, the radiation that reacts with the periodic patterns at the two locations on the alignment target is detected and the difference is calculated. If the difference is non-zero, the two elements are moved relative to each other and the difference is recalculated. If the difference is less than the previous measurement, the two elements are moved again in the same direction. The process continues until the difference is minimized. If the difference is increased, the two elements are moved relative to each other again, but in the opposite direction. The process is continued until the difference is minimized.

The process can be optimized using a referencing technique, similar to that described in U.S. patent application entitled “Alignment Target with Designed in Offset” by Weidong Yang, Roger R. Lowe-Webb, having Ser. No. 10/116,863, which is incorporated herein by reference. The reference technique, for example, may include moving the two elements with respect to one another over a distanced and recording the change in the differential spectrum caused by the movement. The change in the differential spectrum as the alignment between the top element and the bottom element changes acts as a reference to determine the precise amount of alignment error.

Additionally, if desired, bottom diffraction gratings664a,664band top diffraction gratings662a,662bmay have difference pitches. For example, bottom diffraction gratings664a,664bhave a pitch of P1, while the top diffraction gratings662a,662bhave a pitch P2that is equal to pitch P1plus an additional term δ, i.e., P2=P1+δ. The difference in pitch may be, e.g., 10 percent, or any other appropriate amount.

In operation, the incident radiation reacts with the alignment target at the two locations. After reacting with the alignment target, the radiation is detected. Because the top diffraction gratings666,668and the bottom diffraction gratings662,664have different pitches, a moiré fringe pattern will be produced.

The use of moiré fringe patterns in alignment devices is described in U.S. Pat. No. 5,216,257, which is incorporated herein by reference. As described in U.S. Pat. No. 5,216,257, however, a single top diffraction grating and single bottom diffraction grating was used. U.S. Pat. No. 5,216,257 teaches that after deposition and development, the single resultant moiré fringe pattern can be observed and measured on the wafer structure. The overlay error can then be determined by the displacement of the centerline of the fringe from the centerline of the grating structure.

In accordance with the present invention, however, the pre-alignment of the substrate601may be accomplished in exposure tool600using alignment targets660prior to exposure and development. At least two moiré fringes are produced, e.g., one by both overlay patterns601,603in alignment target660, and are received by detector603. The two moiré fringes are compared to each other to determine if the elements are in alignment. When there is no alignment error, the moiré fringes produced by overlay patterns601and603will be the same because the overlay patterns are symmetrical. However, if there is an alignment error, the error will create an asymmetry between overlay patterns601and603. For example an alignment error +e will change the magnitude of the offset of overlay pattern601by |D+e|, while changing the magnitude of the offset of overlay pattern603by |D−e|. Thus, the moiré fringes produced by the asymmetrical overlay patterns will be different. By calculating the difference between the moiré fringes produced by overlay patterns601and603, the presence and general magnitude of any alignment error may be quickly and accurately determined. Thus, the control system612, which may include some intermediary processor, receives the light detection signals from detector603and calculates the difference between the moiré fringes created by overlay patterns661and663(as well as the moiré fringes created by the overlay patterns in alignment target650). Control system612can then move the substrate stage in the appropriate manner and distance for precise pre-alignment.

FIG. 18Bshows an example of moiré fringes672,674produced by overlay patterns601and603when the reference mask618and substrate601are in alignment. As can be seen, the bands in the moiré fringes are aligned with each other. Alternatively, the bands may be viewed as being in the same relative position with respect to a reference point, e.g., center lines673,675.FIG. 18Cshows an example of moiré fringes672,674produced by overlay patterns601and603when the reference mask618and substrate601are not in alignment. As can be seen, the bands in the moiré fringes672,674are no longer aligned with respect to each other, or alternatively, the bands do not have the same relative position with respect to the reference point, e.g., center lines673,675. As the reference mask618and the substrate601approach alignment, the bands in the moiré fringes672,674will approach each other, as indicated by the arrows inFIG. 18C.

It should be understood that in accordance with the present invention, the position of the bands may be determined relative to each other or relative to some reference point. The reference point need not be a center line.

Thus, by detecting the bands in the moiré fringes and determining the relative positions of the bands, it may be determined if the substrate601and the reference mask618are in alignment. If the substrate601and the reference mask618are out of alignment, the control system612can move the stage holding the substrate601until the bands of the moiré fringes are in the same relative position, thus, assuring that the substrate601and the reference mask618(and therefore the lens614) are in alignment. Once the substrate601is pre-aligned, the substrate601may then be processed by the exposure tool600.

FIG. 19Ais another example of an alignment target680that includes a designed in offset and that may be used to produce moiré fringe patterns. As shown inFIG. 19A, the top periodic patterns684aand684bare offset from the bottom periodic pattern682by an angle, i.e., the top periodic patterns684aand684bare rotated with respect to the bottom periodic pattern682. The top periodic patterns684aand684bare rotated in opposite directions. As shown inFIG. 19A, the top periodic patterns684,684bmay be touching or if desired there may a separation between them.

In operation, when the top periodic patterns684a,684bare properly aligned with the bottom periodic pattern682, moiré fringe patterns are generated that are aligned. However, if there is an alignment offset between the top and bottom periodic patterns, the moiré fringe patterns will separate, as shown inFIG. 19B. For example, if the top periodic patterns684aand684bmove to the left over bottom periodic pattern682, as illustrated by the arrow inFIG. 19A, the moiré fringe patterns produced by the periodic patterns684aand682will move down, while the moiré fringe patterns produced by the periodic patterns684band682will move up. Thus, by moving the top element with respect to the bottom element, the moiré fringe patterns from patterns684aand684bwill move in opposite directions. By imaging the moiré fringe patterns and observing when the bands are aligned, the top and bottom element can be placed into proper alignment. Moreover, by measuring the separation between the moiré fringe patterns, the amount of the alignment error may be determined.

It should be understood that the reticle610may be similarly pre-aligned with the lens614. Thus, where both the reticle610and substrate601are pre-aligned with the lens614, the reticle610and substrate601will be aligned with respect to each other.

It should also be understood that the present invention is not limited to pre-alignment, but may be used with the exposure tool600during exposure. For example, with judicious placement of a plurality of bottom diffraction gratings604a,604bon substrate601, for example, in the scribe lines around each die or neighboring dies on the substrate601, alignment may be performed prior to or during exposure for each die. Further, other alignment device embodiments may be possible using the alignment target of the present invention. For example, in an exposure tool, the top diffraction grating may be placed on the reticle610. The reticle610and substrate601may then be aligned with respect to each other. In such an embodiment, for example, high orders of scattered light may be detected and used to determine alignment after the incident light travels through the top diffraction gratings on the reticle and lens, and is scattered off the bottom diffraction gratings on the substrate. The collected higher orders may be used to from images and moiré patterns.

It should also be understood, that while the present embodiment is described in terms of two overlay patterns in each alignment target660, if desired the alignment target may use three or four overlay patterns as described in U.S. patent application entitled “Alignment Target with Designed in Offset” by Weidong Yang, Roger R. Lowe-Webb, and John D. Heaton, Ser. No. 10/116,863, which is incorporated herein in its entirety. The use of additional overlay patterns may provide the ability to calculate the amount of misalignment, which may eliminate the need to use a feedback mechanism to determine if the relative movement of the elements is decreasing or increasing the difference in the detected radiation from each location on the alignment target.

Moreover, while the present embodiment is described in terms of an exposure tool used in the semiconductor industry, the present invention may be used in any apparatus or process in which precise alignment between two elements is required. For example, the present invention may be used advantageously in technologies in which small elements must be aligned, such as biotechnology or nanotechnology, or in technologies in which very precise alignment of larger elements, e.g., a reticle and stage, is desirable.

FIG. 20is another alignment target400that may be used in accordance with an embodiment of the present invention. Alignment target400includes a bottom periodic pattern, e.g., diffraction grating402and a top periodic pattern, e.g., diffraction grating404. The bottom diffraction grating402has a pitch P1. The top diffraction grating404has a pitch P2that is equal to pitch P1plus an additional term δ, which is a small fraction of pitch P1, i.e., P2=P1+δ. The linear dimension L perpendicular to the grating lines of alignment target400should be sufficient to ensure that all phases between 0 and 360 degrees is included, i.e.,

L=(P1δ+1)·P1.eq.⁢3
Consequently, the overlay pattern of alignment target400is mirror imaged around a line406due to the variation in pitch. In other words, the alignment of the top diffraction grating404with the bottom diffraction grating402is designed to be mirror imaged at the same lateral distance on either side of line406. Thus, similar to alignment target400, if an overlay error e is inserted, the variation in alignment at the locations on the alignment target400at the same lateral distance on either side of line406will be2e.FIG. 20shows line406in the center of alignment target400. It should be understood, however, that the length L of alignment target400need not be longer than is necessary to include all phases between 0 and 360 degrees, in which case, the line406may not be located in the center of the lateral dimension of alignment target400.

Any appropriate metrology instrument, such as those described inFIGS. 12A–12C, may be used to measure at least two locations on the alignment target400. The diameter of the probe spot used to measure alignment target400, however, should have a lateral dimension, i.e., in the direction perpendicular to the grating lines, that is sufficiently small that the variation in the lateral offset between the lines of the bottom diffraction grating402and the top diffraction grating404within the probe spot is small compared to pitch P1. The measurement of diffracted light, i.e., the 0thorder and/or any number of higher order diffracted beams, is made at least at two locations on the alignment target400, where there is a designed in offset of approximately equal magnitude but in opposite directions. Thus, for example, if the length L of the pattern includes a 360 degree cycle of phases, the two locations on the alignment target400are measured at equal distances from the respective ends, or alternatively from the center of the pattern. Alternatively, a plurality of lateral positions along the alignment target400may be measured. If desired, a third location on the pattern may be measured as a reference offset to calculate the alignment error, as described in U.S. patent application entitled “Alignment Target with Designed in Offset” by Weidong Yang, Roger R. Lowe-Webb, and John D. Heaton, Ser. No. 10/116,863, which is incorporated herein in its entirety. Alternatively, multiple locations may be measured along the pattern.

The measured diffraction at the two locations is then compared, e.g., subtracted. Variations in the diffraction as a function of lateral position along the alignment target400are characteristic of the lateral offset error between the top diffraction grating404and the bottom diffraction grating406.

It should be understood that there may be some separation between the bottom diffraction grating402and the top diffraction grating404, particularly when the alignment target400is used in an alignment system, such as that described in reference toFIG. 15.

FIG. 21is another alignment target450, similar to alignment target400shown inFIG. 20, but that includes two overlay patterns452and462, in accordance with an embodiment of the present invention. Overlay pattern452includes a bottom periodic pattern, e.g., diffraction grating454, and a top periodic pattern, e.g., diffraction grating456. The bottom diffraction grating454has a pitch P1. The top diffraction grating456, similar to alignment target400inFIG. 20, has a pitch P2that is equal to pitch P1plus an additional term δ, which is a small fraction of pitch P1, i.e., P2=P1+δ. The linear dimension L perpendicular to the grating lines of alignment target450for overlay pattern452should be sufficient to ensure that all phases between 0 and 360 degrees are present.

Overlay pattern462includes a bottom diffraction grating464and a top diffraction grating466. The bottom diffraction grating464also has a pitch P1. The top diffraction grating466has a pitch P3that is equal to pitch P1minus the additional term δ, i.e., P3=P1−δ. The linear dimension L perpendicular to the grating lines of alignment target450for overlay pattern462should be sufficient to ensure that all phases between 0 and 360 degrees.

Similar to alignment target400, any appropriate metrology instrument, such as those described inFIGS. 12A–12C, may be used to measure the alignment target400, but the diameter of the probe spot should have a lateral dimension that is sufficiently small that the variation in the lateral offset between the lines of the bottom diffraction gratings454,464and the top diffraction gratings456,466within the probe spot is small compared to pitch P1. The diffraction from the overlay pattern452and overlay pattern454is measured, either simultaneously or separately. The resulting measured diffraction as a function of lateral position of overlay pattern452and462is then compared, e.g., subtracted to provide the lateral offset error.

FIG. 22is another alignment target500, that includes overlay pattern502and overlay pattern512, which is a mirror image of overlay pattern502. Overlay pattern502includes a bottom diffraction grating504and a top diffraction grating506. The bottom diffraction grating504has a pitch P1and the top diffraction grating506has a pitch P2that is equal to pitch P1plus an additional term δ, which is a small fraction of pitch P1, i.e., P2=P1+δ. The linear dimension L perpendicular to the grating lines of alignment target500for overlay pattern502should be sufficient to ensure that all phases between 0 and 360 degrees. The second overlay pattern512of alignment target500includes a bottom diffraction grating514that has pitch P2and a top diffraction grating516that has pitch P1. The measurement of the overlay error using alignment target500is similar to that described in reference to alignment target450.

In addition, it should be understood that two dimensional periodic patterns, referred to herein as bi-gratings, may be used with the present invention.FIG. 23shows a top view of an alignment target700that is two bi-gratings702,704that may be used in accordance with the present invention. Instead of a series of lines that extend in one direction, overlay pattern700includes a series of squares that extend in two directions. The solid squares inFIG. 23are, e.g., the bottom diffraction bi-gratings, while the empty squares are the top diffraction bi-gratings. Overlay pattern700, includes a designed in offset ±D1in the X direction and a designed in offset ±D2in the Y direction. The magnitude of offsets D1and D2may be the same or different. If desired, the bi-grating may be formed using other shapes besides squares, e.g., circles or polygons. If desired, bi-gratings may be used as the top periodic pattern, the bottom periodic pattern or both.

FIGS. 24A and 24Bshows a top view of two exemplary arrangements of periodic patterns in an alignment target of the present invention. As can be seen inFIG. 24A, the periodic patterns for the X axis and the Y axis may all be linearly arranged.FIG. 24Bshows the periodic patterns in a matrix arrangement. With the proper arrangements of circular apertures or slits, as described in reference toFIGS. 13A and 14A, all of the periodic patterns inFIGS. 24A and 24Bmay be simultaneously measured if desired.

In addition, it should be understood that the alignment target used in the present invention does not need to have a symmetrical designed in offset at the measurement locations. For example, the magnitude may vary and/or the direction of offset may be at an angle from one measurement location to the other.

FIG. 25shows another embodiment of an alignment target1000that has three measurement locations, referred to as overlay patterns1002,1004, and1006. Each of the overlay patterns has a bottom periodic pattern on element1001, e.g., diffraction gratings1002a,1004a,1006aand a top periodic pattern on element1003, e.g., diffraction gratings1002b,1004b,1006b. Each of the measurement locations, i.e., overlay patterns1002,1004,1006, on the alignment target1000have a designed in offset between the bottom periodic pattern and the top periodic pattern. However, as can be seen inFIG. 25, the designed in offsets at each location are not equal in magnitude. For example, overlay pattern1002has a designed in offset of +D, while overlay pattern1004has a designed in offset of −(D+a) and overlay pattern1006has a designed in offset of −(D−b), where a and b may or may not be the same. An alignment error in alignment target1000may then be determined by measuring each location, e.g., producing light that is incident on each location and detecting the light from each location after interacting with the location, and comparing the detected light from each location. The alignment error e in alignment target1000may be calculated as follows:

FIG. 26Ashows another embodiment of an alignment target1100that has measurement locations, referred to as overlay patterns1102and1104. Again, each of the overlay patterns has a bottom periodic pattern on element1101, e.g., diffraction gratings1102aand1104aand a top periodic pattern on element1103, e.g., diffraction gratings1102band1104b. Each of the measurement locations, i.e., overlay patterns1102and1104, on the alignment target1100have a designed in offset between the bottom periodic pattern and the top periodic pattern. Overlay pattern1102has a designed in offset of +D while overlay pattern1104has a designed in offset of −(D+a), where a is a known amount. When the difference between the detected radiation, e.g., diffracted light, from the two overlay patterns1102and1104, is at a minimum, the alignment error will be skewed by a/2. Alignment target1100is particularly useful in an alignment system, as once the difference between the detected radiation is minimized, the alignment system can simply move the two elements1101and1103a distance a/2 relative to each other to place the two elements in alignment. To determine the amount of the alignment error, a modeling process, or a reference measurement location, as described above, may be used.

FIG. 26Bshows alignment target1100′, which is equivalent to alignment target1100inFIG. 26Awith a shift of a/2. Alignment target1100′ has symmetrical designed in offsets of +D+a/2 on overlay pattern1102′ and −(D+a/2) on overlay pattern1104′. Thus, it can be seen that the asymmetric designed in offset is equivalent to the symmetric designed in offset. Consequently, any of the embodiments that may be used for symmetrical designed in offsets may also be used for asymmetrical designed in offsets, with an appropriate correction, e.g., a/2.

Thus, as can be seen, the magnitude of the designed in offset between the measurement locations on the alignment target need not be equal. In addition, if desired, the two measurement locations on the alignment target may include an arbitrary angle between the direction of the periodic patterns.

FIGS. 27A and 27Bare top views showing alignment targets1200and1250, which includes two measurement locations having designed in offsets. The two measurement locations are referred to as overlay patterns1202,1204and1252,1254, where the bottom periodic patterns are shown as solid blocks and the top periodic patterns are shown as empty blocks. The X and Y axes are shown for reference inFIGS. 27A and 27B. As can be seen, the periodic patterns of overlay patterns1202and1252form an angle θ with the Y axis and periodic patterns of overlay patterns1204and1254form an angle −θ with the Y axis. Thus, overlay patterns1202and1204are arranged at an angle 2θ with respect to each other, as are overlay patterns1252and1254. Consequently, the designed in offset at the two overlay patterns1202and1204in alignment target1200(and overlay patterns1252and1254in alignment target1250) may have the same magnitude, e.g,. D, but it is not in opposite directions. The direction of the designed in offsets in the overlay patterns are at an arbitrary angle with respect to each other. It should be understood that the angles used in alignment targets1200and1250may differ if desired.

Alignment target1200inFIG. 27Ais sensitive to alignment error along the X axis but not the Y axis because of the arrangement of the bottom periodic patterns (shown as solid blocks) and the top periodic patterns (shown as empty blocks). In other words, if an alignment error occurs along the Y direction, both overlay patterns1202and1204will change in the same manner, and thus, alignment target1200is insensitive to the error. If however, an alignment error occurs along the X direction, the overlay patterns1202and1204will vary by different amounts. Thus, alignment target1200is sensitive to alignment errors in the X direction.

Alignment target1250inFIG. 27B, on the other hand, is sensitive to alignment error along the Y axis but not the X axis because of the arrangement of the bottom periodic patterns (shown as solid blocks) and the top periodic patterns (shown as empty blocks). As can be seen inFIG. 27B, if an alignment error occurs along the X direction, both overlay patterns1252and1254will change in the same manner, but if an alignment error occurs along the Y direction, the overlay patterns1252and1254will vary by different amounts. Thus, alignment target1250is sensitive to alignment errors in the Y direction and insensitive to alignment errors in the X direction.

FIG. 28is a top view showing an alignment target1300, which is similar to alignment targets1200and1250combined. Alignment target1300includes overlay patterns1302,1304, which are similar to overlay patterns1202and1204. In addition, alignment target1300includes a third measurement location, referred to as overlay pattern1306, where the bottom periodic patterns are shown as solid blocks and the top periodic patterns are shown as empty blocks. The periodic patterns of overlay pattern1302forms an angle θ with the Y axis and periodic patterns of overlay pattern1304forms an angle −θ with the Y axis. The periodic patterns of overlay pattern1306also forms an angle θ with the Y axis as shown inFIG. 28. As described above in reference toFIGS. 27A and 27B, overlay patterns1302and1304are sensitive to alignment error along the X axis, while overlay patterns1304and1306are sensitive to alignment error along the Y axis.

FIG. 29shows an alignment target1350, similar to alignment target1300, like designed elements being the same, except that overlay pattern1304and overlay pattern1356are sensitive in a direction along a different coordinate system. Thus, a coordinate system conversion must be performed to determine the alignment error in the Y direction.

FIG. 30is a top view showing an alignment target1400, which includes three measurement locations, referred to as overlay patterns1402,1404, and1406, where the bottom periodic patterns are shown as solid blocks and the top periodic patterns are shown as empty blocks. As can be seen inFIG. 30, the overlay patterns1402,1404, and1406are at arbitrary angles with respect to one another. Moreover, the magnitude of the designed in offsets between the top periodic patterns and the bottom periodic patterns in the overlay patterns1402,1404, and1406may vary.

For example, designed in offsets of d1, d2, and d3may be used for overlay patterns1402,1404, and1406, respectively. Without losing generality, overlay pattern1402can be set along the y direction. Overlay pattern1404is separated from overlay pattern1402by an angle θ1, and overlay pattern1406is separated from overlay pattern1404by an angle θ2. Thus, with an alignment error of (x, y), the new offset for the three overlay patterns1402,1404, and1406are a1, a2, and a3, respectively. If a careful arrangement of incidence angles, azimuthal angles, and polarization states is used, spectra from the three overlay patterns will become identical when a1=a2=a3. This occurs when the following conditions are satisfied:

A special case is θ1=120°, θ2=−120°, and the conditions are reduced to:

Another special case is θ1=90°, θ2=−90°, and the conditions are reduced to:

x=d2-d32⁢⁢y=d2+d3-2⁢d12.eq.⁢11
Thus, it can be seen that designed in offsets in the overlay patterns can have various magnitudes and angles with respect to each other.

Although the invention has been described with reference to particular embodiments, the description is only an example of the invention's application and should not be taken as a limitation. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.