2. Description of the Related Art Semiconductor devices are processed in levels. Materials are formed in layers and patterned, typically using lithographic processes. To build devices and components on semiconductor devices, layers of materials are employed.

These layers must be properly aligned so that patterns and components on different layers line up and function correctly once fabricated. Lithographic alignment on prior levels is critical to ensure proper overlay. Lithographic alignment typically includes providing a buffet and target arrangement where the bullet is an alignment mark to be aligned against a target alignment mark.

These alignment marks may include features with sharp edges, for example, trenches or plateaus formed on a layer of the semiconductor device. A photomask includes features (e. g., alignment marks) corresponding to the features (e. g., alignment marks) formed in a previous level of the semiconductor device. The features of the photomask are aligned to the features of the semiconductor device to ensure proper overlay.

Alignment of these marks is typically performed by an optical system.

Either bright field or dark field broad band illumination is used to detect the prior level alignment marks (e. g., the alignment marks formed on the device). Broad band illumination is employed to make a plurality of wavelengths of light available to detect the prior level alignment marks. Light is incident on a surface and is reflected back in different areas based on the thickness of different features. For example, if light is directed normal to a surface having a plateau formed thereon, light would be reflected from a top surface of the plateau and from surfaces adjacent to the base of the plateau. The reflected light from both surfaces includes the same wavelengths, but are out of phase by the thickness of the plateau. This causes an interference condition either constructive or destructive depending on the thickness and the wavelength of light employed.

Since a broad band spectrum is employed many wavelengths are available to perform this measurement and improve contrast of the feature or alignment mark, in this case a plateau.

Although broad band illumination is used to enhance the contrast of the alignment marks, there are still instances, where the properties of the stack on the alignment marks and of the surrounding area have very low contrast. For example, small thickness variations can cause the complete loss of contrast. For dark field illumination, the angle of light incidence or detection can lead to image misplacement.

Therefore, a need exists for a system and method for improving the contrast of alignment features when broad band illumination is not enough to decipher the alignment features.

SUMMARY OF THE INVENTION A method for providing contrast for determining an edge in alignment systems, in accordance with the present invention includes propagating light for irradiating an alignment mark and a background portion surrounding the alignment mark of a semiconductor wafer. Wavelengths of the propagated light are modulated, and reflectance of the wavelength-modulated light is measured for the alignment mark and for the background portion. A largest change between the reflectance of the alignment mark and the reflectance of the background portion is determined such that a position where the largest reflectance change occurs indicates an edge of the alignment mark.

Another method for providing contrast for determining an edge in alignment systems, includes the steps of providing a semiconductor wafer including a stack of layers formed thereon, the stack of layers including an alignment mark on one layer and a background portion on a same layer or on a different layer of the stack, propagating light onto the semiconductor wafer for irradiating the alignment mark and the background portion surrounding the alignment mark, modulating wavelengths of the propagated light, measuring reflectance of the wavelength modulated light for the alignment mark and for the background portion, the wavelength modulated light being reflected from the semiconductor wafer, storing the measured reflectance as a function of wavelength and determining changes between the reflectance of the alignment mark and the reflectance of the background portion from the stored measured reflectance such that positions where the largest reflectance changes occur indicate edges of the alignment mark.

In other methods, the step of modulating wavelengths of the propagated light may include the steps of providing a filter head having a plurality of filters each permitting a different wavelength of light to pass therethrough, and rotating the filter head through a broad band light beam to modulate the wavelength of light. The filter head may include two band pass filters each permitting a different wavelength to pass therethrough. The step of measuring reflectance of the wavelength modulated light for the alignment mark and for the background portion may include the step of plotting the reflectance for the alignment mark and for the background portion versus wavelength of the light.

In still other methods, the step of determining a largest change may include the steps of differentiating a plotted reflectance curve for the alignment mark and a plotted reflectance curve for the background portion with respect to wavelength to determine the slopes of the curves and determining a largest difference between the slopes of the curves to indicate the edge between the background portion and the alignment mark. The step of determining a largest change may include the steps of plotting the reflectance for the alignment mark and for the background portion versus wavelength of the light to create a first set of curves, differentiating the first set of curves with respect to wavelength to determine slopes of the curves, and plotting the differentiated first set of curves to create a second set of curves. The method may further include the step of comparing the first set of curves and the second set of curves to determine edges of an alignment mark by determining the largest change between the reflectance of the alignment mark and the reflectance of the background portion.

The alignment mark may include a trench formed in a layer or a plateau formed in a layer.

A system for providing contrast for determining an edge in alignment systems, in accordance with the present invention, includes illumination means for irradiating an alignment mark and a background portion surrounding the alignment mark of a semiconductor wafer with propagated light, means for modulating wavelengths of the propagated light, means for measuring reflectance of the wavelength modulated light for the alignment mark and for the background portion, and means for determining a largest change between the reflectance of the alignment mark and the reflectance of the background portion such that a position where the largest reflectance change occurs indicates an edge of the alignment mark.

In other embodiments, the means for modulating wavelengths may include a filter head having a plurality of filters each permitting a different wavelength of light to pass therethrough such that when the filter head is rotated through a broad band light beam the wavelength of light is modulated. The filter head may includes two band pass filters each permitting a different wavelength to pass therethrough. The means for measuring reflectance of the wavelength modulated light may include a sensor. The means for determining a largest change may include a program module for differentiating a plotted reflectance curve for the alignment mark and a plotted reflectance curve for the background portion with respect to wavelength to determine the slopes of the curves, and for determining a largest difference between the slopes of the curves to indicate the edge between the background portion and the alignment mark.

In still other embodiments, the means for determining a largest change may include a program module for plotting the reflectance for the alignment mark and for the background portion versus wavelength of the light to create a first set of curves, differentiating the first set of curves with respect to wavelength to determine slopes of the curves and plotting the differentiated first set of curves to create a second set of curves.

The program module may further include means for comparing the first set of curves and the second set of curves to determine edges of an alignment mark by determining the largest change between the reflectance of the alignment mark and the reflectance of the background portion.

These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS This disclosure will present in detail the following description of preferred embodiments with reference to the following figures wherein: FIG. 1 is a block/flow diagram for a system/method for adjusting contrast in accordance with the present invention; FIG. 2 is a schematic diagram showing an adjustable aperture employed for adjusting the angle of incidence distribution of a lens to adjust contrast in accordance with the present invention ; FIG. 3 is a block diagram showing a system for adjusting contrast in accordance with the present invention; FIG. 4 is a perspective view of a filter head employed for changing filters in accordance with the present invention; FIG. 5 is a cross-sectional view of an illustrative semiconductor device employed to make contrast comparisons between the prior art and the present invention; FIG. 6 is a plot of contrast of an edge between layers of FIG. 5 in broad band illumination with insufficient contrast to define the edge in accordance with the prior art; FIG. 7 is a plot of contrast in narrow band illumination showing sufficient contrast to define the edge in FIG. 5 in accordance with the present invention; FIG. 8 is a block/flow diagram for a system/method for determining edges of patterns or alignment marks with little or no contrast; FIGS. 9 and 10 depict illustrative cross-sections of a semiconductor wafer having alignment marks formed therein; FIG. 11 is an illustrative plot of reflectance versus wavelength for an alignment mark and a background portion in accordance with the present invention; and FIG. 12 is an illustrative plot of reflectance difference (the slope of the reflectance plot) versus wavelength for an alignment mark and a background portion in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention includes a method and system for adjusting an interference condition for improving contrast in illumination systems. In one embodiment, contrast is improved by adjusting the illumination spectrum during exposure or detection. In another embodiment, contrast is improved by employing a short focal length lens or optic and adjusting the aperture during the alignment process.

In yet another embodiment, reflectance measurements are made for wavelength modulated light. Reflectance measurements are made for both a background layer and an alignment mark. In situations where little or no contrast exists between the alignment mark and the background, differences in reflectance curve slopes are employed to substitute for contrast measurements to provide a position against which alignment processes may be performed.

These embodiments may be combined in a single method to provide the most flexibility. The present invention provides tools for determining alignment marks or patterns in a pattern recognition system. In situations where alignment marks are difficult to find due to lack of contrast, the present invention is particularly useful.

The present invention provides an adjustable contrast enhancement method which preferably employs using wavelength filters, which can be changed during alignment techniques and/or a variable aperture which may be employed in conjunction with a short focus lens system. By changing the filters, the spectrum of the illumination can be changed or wavelengths modulated, and the color variation on the alignment mark stack as well as the surrounding area can be controlled dynamically. The same effect can be achieved by changing the aperture in a short focal length illumination system. By changing aperture, the distribution of the angle of incidence of the illuminating light changes and the alignment mark contrast can be controlled.

It should be understood that the elements shown in FIGS. 1,3 and 8 may be implemented in various forms of hardware, software or combinations thereof.

Preferably, these elements are implemented on one or more appropriately programmed general purpose digital computers having a processor and memory and input/output interfaces. These elements may also by implemented on specially designed chips or software modules to provide the functionality in accordance with the present invention.

Referring now in specific detail to the drawings in which like reference numerals identify similar or identical elements throughout the several views, and initially to FIG. 1, a flow diagram is shown for a method for improving contrast in accordance with the present invention. In block 100, a light illumination system is provided. The light illumination system preferably includes a broad band illumination system, such as a commercially available broadband system. A broad band system is preferable to provide as many available wavelengths of light as possible. Broad band systems may include, for example, wavelengths between about 400 nm and about 800 nm. These systems are modified as described herein to provide the contrast adjustment capabilities as described in accordance with the present invention. In block 102, a work piece, such as a semiconductor device, or object with a pattern to be recognized is provided. The work piece may include an alignment mark against which a photomask (with alignment marks) is to be aligned. In a preferred embodiment, the work piece includes a semiconductor wafer with an alignment mark or pattern fabricated thereon. The alignment pattern may be aligned to photomasks, etch masks, etc.

The illumination system provided in block 100 includes an exposure system and a detection system. In block 104, the work piece is illuminated by the exposure system by supplying broad band light on the work piece. In block 106, the detection system is employed to determine the locations of the alignment marks on the work piece. This is performed by determining constructive or destructive interference in areas of the alignment marks. This picks up the edges of the alignment marks (e. g., trenches or plateaus). Contrast is employed to determine the edges. However, if insufficient contrast is available, the alignment marks will not be determined.

In block 108, contrast is adjusted in accordance with the present invention. Block 108 includes steps for adjusting contrast for determining alignment marks or patterns when employed with broad band illumination systems. The steps of block 108 may be combined to provide flexibility in improving contrast in accordance with the present invention. In block 110, wavelength filtering is provided. Wavelength filtering is provided in either the exposure system of the detection system. In one embodiment, a plurality of wavelength filters are provided for the illumination system. Wavelength filtering may also be performed by diffraction gratings and/or prisms. These filters may be employed to filter out predetermined wavelengths of light. In one example, these filters permits the following wavelength ranges: 450-550 nm, 550-650 nm and 650-750 nm if the visible light spectrum is employed. The above example is illustrative only. Filter wavelengths may be broken up into smaller wavelength ranges or in unequal wavelength ranges. For example, a set of filters may pass wavelength ranges of 250-450 nm, 450-475 nm and 475-800 nm. In this application, the 450-475 nm filter is employed since the highest contrast is determined in this range. Other configurations are contemplated.

Although employing the visible spectrum is preferred the broadband illumination may include ultraviolet spectrum or beyond.

These filters may be automatically or manually switched in front of the exposure source, in front of the detection sensor of the illumination system, or at any other convenient point in the light path. These filters may be selected from a plurality of filters, based on the best contrast enhancement achieved. In a preferred embodiment, an adjustable contrast enhancement is provided by introducing automatically exchangeable filters (or grating/prism arrangements) in a bright field broad band illumination system.

Due to light interference effects, the color variation in the detected image depends on the spectrum of the illumination as well as a film stack from which the light is reflected from. For different film stacks, the color variation changes differently with variations in the illumination spectra. Changes of the illumination spectra may mean a change in the illumination or detection light path. By image processing, the spectrum in the illumination or detection can be adjusted for optimum contrast. For example, an image processing method may include taking a greyscale of the image and optimize the spectral region of interest for a maximum difference between the alignment mark and surrounding area.

In block 112, contrast may also be adjusted by providing and employing a variable aperture. Referring to FIG. 2 an illustrative schematic diagram is depicted to show a variable aperture implementation. A work piece 202 may include a semiconductor wafer having alignment marks formed thereon for providing an alignment reference for a photomask, for example. Work piece 202 is illumination with light (from the exposure system) which is reflected back to the detection system. The detection system includes a lens 204 for receiving reflected light 206. Reflected light 206 is reflected at a plurality of different angles relative to a surface 208 of work piece 202. A variable aperture 210 is provided which may be adjusted in accordance with the invention to improve contrast of image light 212. By changing the aperture size, the distribution for the angle of incidence of reflected light 206 will change, which will lead to a contrast change in the image. This technique would also work with a single wavelength light source, such as, for example, a laser beam.

Again, for different film stacks, the color variation changes differently with variations in the aperture. By image processing, the spectrum in the illumination or detection can be adjusted for optimum contrast. By changing the angle of incidence, the interference conditions will change dependent on the film stack.

This will lead to a color variation or, in the case of a single wavelength illumination, to a intensity variation.

Referring again to FIG. 1, the techniques of blocks 110 and 112 may be implemented for prior level lithography alignment as well as in pattern recognition systems. Blocks 110 and 112 may be employed independently or used in combination to provide more flexibility in the contrast measurement. Contrast measurement may be an iterative process in which the detected image is captured and checked to determine if enough contrast exists. If more contrast is needed to determine the location of, for example, alignment marks, the filters may be switched for the illumination light (or the detected light), the aperture may be adjusted or a combination of both may be performed to enhance the contrast.

In block 114, readjustments in accordance with the invention may be performed in real time while viewing the detected image or may be performed automatically by employing image processing software, known in the art. Imaging software is preferably integrated into a stepper system by the stepper manufacturer.

Stepper systems are known in the art. The steps of the present invention may be iteratively performed until sufficient contrast is available for alignment or identification of a feature.

Referring to FIG. 3, a system for improving contrast in images is shown in accordance with the present invention. An illumination system 300 may include a broadband illumination tool or single wavelength illumination tool (for use with aperture adjustment only). System 300 include an exposure or illumination system 302 for providing light to illuminate a work piece 314. Work piece 314 may include a semiconductor wafer where alignment is needed between a prior layer and a new layer to be formed of the prior layer. Work piece may alternately include an object or device where pattern recognition is needed. Exposure system 302 propagates light onto a surface of work piece 314. Light from exposure system 302 passes through a filter module 306 either before hitting the target (e. g., work piece 314) or after being reflected from the target. An additional filter module 306'may be included at a detection system 304 to filter light, which has been reflected from the target. The additional filter module 306' may be included in addition to or instead of the filter module 306. Filter modules may both be included to increase the number of available filters or to provide additional filtering.

A plurality of color filters are included in color filter module 306 (or 306'). Color filter module 306 (or 306') may include a plurality of color filters (or diffraction gratings, prisms, etc.) for filtering out predetermined wavelengths of light. In one embodiment, color filter module 306 (or 306') includes a rotatable head 400 having a plurality of windows 402 (see FIG. 4) which comprise the filters. Upon activation of a control 318, by a user or by a computer 318, filter head 400 rotates to another filter window 402, thereby permitting contrast enhancement.

When filter modules 306 are employed, broad band illumination from a light source in exposure system 302 is preferred.

Detection system 304 includes sensors for detection of light reflected back from the target. Exposure and detection systems 302 and 304 may be the type commonly provided on illumination tools. An adjustable or variable aperture 312 is provided. Aperture 312 is adjusted to limit the angle of incidence distribution on a lens 310, this aperture feature is preferably employed with narrow band (filtered) light from system 302 or a single wavelength of light (e. g., a laser source included in exposure system 302). Aperture 312 is employed with lens 310, which collects reflected light from the target.

After collecting light from the target, an image is displayed on a display 322 to permit a user to control contrast if necessary, or computer 318 is employed to automatically adjust contrast if contrast criteria are not met. Computer 318 preferably controls adjustment of aperture 312, and adjustments of filters in filter modules 306 (or 306') in accordance with a user or an image processing program 320 stored on computer 318. Computer 318 and display 322 may be included in system 300. Computer 318 also preferably controls a stage 316, which adjusts the position of work piece 314, as is known in the art.

Referring to FIG. 4, a filter head 400 is shown in accordance with one embodiment of the present invention. Filter head 400 includes a plurality of windows 402. Each window filters out different spectrum bands from light propagated therethrough. Filters 404 in filter windows 402 may include diffraction gratings, prisms or other devices known in the art. Windows 402 are rotated into a light beam 406 (e. g., indicated as a circle and directed into the plane of the page) to filter out selected wavelengths of light. Head 400 is rotated to switch the filter 404 that the light beam 406 passes through.

An example of the benefits of the present invention will now be described.

Referring to FIG. 5, a semiconductor memory device which employs deep trench capacitor technology includes an alignment mark 502 formed from an oxide glass in a silicon substrate 504. A nitride layer 506 (pad nitride) is formed over substrate 504. A boro-phospho silicate glass (BPSG) layer 508 is to be patterned. BPSG fills trench 503, which represents the alignment mark 502. An anti-reflection coating (ARC) 510 is formed followed by a photoresist layer 512.

A photomask (not shown) must be aligned with alignment mark 502 (e. g., Active Area-to-Deep trench alignment mark structure) to correctly locate the photomask to pattern the resist layer 512. For a 256 Mbit DRAM, the alignment mark 502 may be about 150 nm in width. Pad nitride 506 thickness, in this film stack, due to polishing (e. g., CMP) processes, can have as much as 20% variation from lot to lot. In the case of the product in this example, variations from 100 nm to 160 nm with a target thickness of 130nm can be experienced.

The structure of FIG. 5 was employed in bright field contrasts, which are simulated with two different illumination spectra. An edge 510 is to be detected or deciphered between the deep trench and the pad nitride. Displayed in FIGS.

6 and 7 are the bright field contrast simulation results with broadband (450nm- 750nm) (FIG. 6) and narrow-band (450-550nm) (FIG. 7) illumination sources, respectively. FIG. 6 represents a prior art alignment mark contrast plot using broad band illumination (wavelengths between 450 nm and 800 nm). As indicated in FIG. 6, at around 140 nm of pad nitride thickness there is no contrast between the pad nitride region (506 in FIG. 5) and deep trench region (alignment mark 502).

As shown in FIG. 7, the present invention was employed to filter wavelengths to pass a only narrow band of illumination (wavelengths between 450 nm and 550 nm). The narrow-band illumination provides excellent contrast throughout the pad nitride thickness variation range 702, as shown in FIG. 7. The image processing may be performed in the following way, e. g. using a detector, which is just sensitive to intensity (no color). From the picture, one or more lines are selected, which run through the alignment mark and the intensity on the detector is plotted as a function of the location. One would see a change in intensity at the interface between an alignment mark area and the surrounding area. The higher this change is, the higher the contrast is. One way to determine the edge would be to define the edge as the point with the highest change in signal (intensity) across the line (s). Other processing method may also be employed.

Referring to FIG. 8, in another embodiment of the present invention, reflectance measurements are made by employing modulated wavelength illumination. In FIG. 8, a flow diagram is shown for a method for improving contrast in accordance with the present invention. In block 800, a light illumination system is provided, for example, the light illumination systems as described above with reference to FIG. 1 may be employed. The light illumination system preferably includes a broad band illumination system. These systems are modified as described herein to provide the contrast adjustment capabilities as described in accordance with the present invention. In block 802, a work piece, such as a semiconductor device, or object with a pattern to be recognized is provided. The work piece may include an alignment mark against which a photomask (with alignment marks) is to be aligned. In a preferred embodiment, the work piece includes a semiconductor wafer with an alignment mark or pattern fabricated thereon. The alignment pattern may be aligned to photomasks, etch masks, etc.

The illumination system provided in block 800 includes an exposure system and a detection system. In block 804, the work piece is illuminated by the exposure system by supplying broad band light on the work piece.

In block 806, a modulated wavelength of light is provided from the illumination system. Wavelength modulation may be performed in a plurality of ways. In one example, the filter wheel 400 (FIG. 4) may be employed to rotate with an angular velocity, w, to modulate the wavelength of light therethrough.

Other wavelength modulation techniques may also be employed. For example, temperature variations and/or stress variations of the work piece may employed to modulate the light. In block 808, by employing the modulated wavelength of light, reflectance is measured for both a background portion or layer and an alignment mark. The reflectance data is measured and stored or a plot is generated for the measured reflectance data (see, e. g., FIG. 11) In block 810, reflectance data is compared to determine changes in slope. This may be performed by reviewing the plot of block 808. Alternately, a derivative may be taken of the reflectance curve to determine slope changes (for the background and the alignment mark) and comparing the derivatives to determine the difference in the slope of the curves. If the slopes between the derivative of the reflectance curve of the background and the derivative of the reflectance curve of the alignment mark differ by a threshold amount or indicate a maximum or minimum or other location where a significant change in slope occurs, then an edge of the alignment mark is determined. In the example of a semiconductor device having alignment marks, a maximum value of the slope difference may be determined. Advantageously, this edge (between the background and the alignment mark) is discovered although no contrast is present in the visible image of the alignment mark/background interface.

Optionally, in block 812, a reflectance difference plot plotting derivatives of the reflectance curves may be employed to determine edges of alignment marks.

For example, the derivative of the reflectance curve of the background and the derivative of the reflectance curve of the alignment mark may be plotted and the maximum difference between the two plots may be determined. This maximum difference indicates transitions or edges of the alignment mark or pattern. These reflectance curves provide an indication of edges where no contrast exists. This process may be iterative and repeated until an appropriate edge is located.

In some cases, both plotted curves (from block 808 and block 812) may be needed to determine the edges of the alignment mark. This may be performed in optional block 814. Now, alignment marks may be employed to align, for example, a photomask thereto.

Referring again to FIG. 3, the system as described above is adapted for locating edges or patterns in images where little or no contrast exists in accordance with the present invention. Illumination system 300 may include a broadband illumination tool or single wavelength illumination tool (for use with aperture adjustment only). System 300 includes an exposure or illumination system 302 for providing light to illuminate a work piece 314. Work piece 314 may include a semiconductor wafer where alignment is needed between a prior layer and a new layer to be formed of the prior layer. Work piece 314 may alternately include an object or device where pattern recognition is needed. Exposure system 302 propagates light onto a surface of work piece 314. Light from exposure system 302 passes through filter module 306 either before hitting the target (e. g., work piece 314) or after being reflected from the target.

In this embodiment, a filter wheel (see FIG. 4) is rotated at frequency w, to modulate the wavelength of light transmitted to or reflected from a surface of the work piece. Additional filter module 306'may be included at a detection system 304 to filter light, which has been reflected from the target. The additional filter module 306'may be included instead of the filter module 306.

A plurality of color filters are included in color filter module 306 (or 306'). Color filter module 306 (or 306') may include a plurality of color filters (or diffraction gratings, prisms, etc.) for filtering out predetermined wavelengths of light. In one embodiment, color filter module 306 (or 306') includes a rotatable head 400 having a plurality of windows 402 (see FIG. 4) which comprise the filters. The filters 402 of filter head 400 preferably include band-pass filters.

Upon activation of a control 318, by a user or by a computer 318, filter head 400 rotates to another filter window 402 and so on at a constant frequency w, thereby permitting wavelength modulation. Other modulation techniques may also be employed. In one embodiment, head 400 includes only two filters.

Detection system 304 includes sensors for detection of light reflected back from the target. Exposure and detection systems 302 and 304 may be the type commonly provided on illumination tools. An adjustable or variable aperture 312 is optional. Image processing software 320 employed by computer 318 is employed to collect data develop reflectance plots. (See, e. g. FIG. 11).

As shown in FIGS. 9 and 10, illustrative cross-sections of structures 900 and 901 are shown to depict alignment marks 902 and 903 against backgrounds 904 and 905. For simplicity these cross-sections provide the alignment marks 902 and 903 in a same layer as the backgrounds 904 and 905. In other applications, stacks of layers are employed having backgrounds and alignment marks on different layers.

Referring to FIGS. 11 and 12, a reflectance plot and reflectance difference plot are shown for either of structures 900 and 901. Reflectance of a film layer may be expressed as: Ri = sin (4, nldl) (EQ. 1) where n, is the refractive index of the layer, d, is its thickness, 8 is the wavelength and N is the phase shift. By modulating the wavelength, changes in n, d or both are determined as changes in reflectance. In addition, by modulating the wavelength of light, a slope difference may be employed to provide a contrast edge. This may be achieved by taking the derivative of EQ. 1 with respect to 8: - (4) (EQ. 2) In FIG. 11, a reflectance plot is shown with reflectance being illustratively plotted against 1/8 for the mark (902 or 903) and the background (904 or 905).

Reflectance measurements may be made by employing, for example, a phase- lock-in detection method. Arrows 1101 and 1102 correspond to positions 906 and 907 of FIGS. 9 and 10. Arrows 1101 and 1102 indicate locations of maximum slope change. Arrow 1101 points to position 1103. Position 1103 indicates a large difference in slope change between the mark curve and the background curve. No contrast is present in the image, but the edge can be detected in accordance with the present invention. The locations pointed to by arrows 1101 and 1102 indicate positions or edges 906 and 907 (see FIGS. 9 and 10) of an alignment mark and determine the edges where little or no contrast could be detected previously. Although maximum slope positions are apparent in FIG. 11, many instances exist where several positions may be candidates for edges. A reflectance difference plot (FIG. 12) may be employed to determine a maximum reflectance difference position. Reflectance difference is plotted against 1/8 for both the mark and the background. The plot of FIG. 12 represents a plot of the slopes of the curves in FIG. 11. These curves may be employed in conjunction with the curves in FIG. 11 to determine edges of the alignment mark or pattern. A maximum reflectance difference at arrows 1201 and 1202 indicates the edges 906 and 907 of the alignment mark (see FIGS. 9 and 10).

Having described preferred embodiments for contrast enhancement for lithography alignment mark recognition and pattern recognition (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.