Abstract:
A method is described that adjusts the position of a item and sets a tilt angle for each of a plurality of micro-mirrors of a digital micro-mirror device. The setting of the tilt angles is to establish a filter within the optical channel of an inspection tool that inspects the item. The filter is to reduce noise received at an optical detection device. The tilt angle settings are a function of the position. The method also includes comparing information from the optical detection device that describes an inspected region of the item&#39;s surface against an expected version of the information.

Description:
FIELD OF INVENTION  
       [0001]     The field of invention relates generally to wafer inspection, and, more specifically, to wafer inspection with a customized reflective optical channel component.  
       BACKGROUND  
       [0002]     In the semiconductor arts multi-layer structures are created upon semiconductor wafers that comprise electrically conductive lines and vias surrounded by one or more electrically insulating dielectric layers. The lines, vias and dielectric layers may be said to be “pattered” into specific structures that implement specific circuitry. Because the dimensions of these patterns can be extremely small (e.g., current mass production technologies can reduce features to as small as 90 nm across), the impact of a manufacturing defect is becoming more important as is the difficulty in detecting it.  
         [0003]     In order to detect manufacturing defects optically, a spot of light is focused on a region of the wafer&#39;s patterned structures. The reflected image of the patterned structure is captured through an optical channel and resolved on an optical detection device such as a photodetector or charge coupled device (CCD). Data produced by the optical detection device is then compared against “expected data”; where, the expected data corresponds to the data from the optical detection device if the manufactured patterns illuminated by the focused spot of light were properly manufactured.  
         [0004]      FIGS. 1   a  and  1   b  demonstrate a problem that exists with respect to the optical detection of a manufacturing defect.  FIG. 1   a  shows an example of a “noiseless” optical signal that is resolved to the optical detection device; and,  FIG. 1   b  shows an example of a “noisy” optical signal that is resolved to the optical detection device. Spot  101   a  corresponds to the illuminated spot of light that is focused on the wafer&#39;s structural patterns. As a simple example, the wafer&#39;s structural patterns are shown to include a repeating pattern of conductive lines  102   1  through  102   5  separated by a dielectric material region (which is drawn in  FIG. 1   a  as a shaded region).  
         [0005]     Looking along axis  104 , a one-dimensional signal  107  should be resolved upon the optical detection device. Assuming the conductive lines have higher reflectivity of the focused spot of light than the dielectric material, enhanced optical intensity should be observed at the detector for the conductive lines as compared to the dielectric regions. Signal  107  indicates as much through optical intensity spikes  105   1  through  105   5  which are meant to correspond to conductive lines  102   1  through  102   5 , respectively.  
         [0006]     A manufacturing defect  103  is also observed in the image  101   a  of  FIG. 1   a . This manufacturing defect may be the result of a thin layer of “spilled” conductive material, a “pit” or “void”, etc. Whatever its form, the defect ultimately reproduces on the optical detection device as part of signal  107  with an intensity  106  that is less than the intensity of the spikes  105   1  through  105   5  that are associated with the conductive lines  102   1  through  102   5 .  
         [0007]      FIG. 1   b  shows an image  101   b  of the same region of the same wafer as depicted in  FIG. 1   a , but with noise attributed to complications that arise from the optical processing of the reflected image from the wafer. Specifically, because the patterned structures on the wafer are “three dimensional” in the sense that the conductive lines  102   1  through  102   5  have “edges” that determine the lines&#39; thickness, the light that reflects off of the wafer does not reflect uniformly off of the wafer surface. For example, a ray of light that impinges directly upon a flat portion of a conductive line may reflect perpendicular to the surface of the wafer while a ray of light that impinges at an edge of a conductive line may reflect at an angle other than perpendicular with the wafer surface (i.e., the angle of reflection is different for rays that impinge upon flat conductive line surfaces as opposed to conductive line edges).  
         [0008]     As processed by the optical channel between the wafer and the optical detection device, the various components of light that reflect off of the wafer at varying angles depending on surface topography (and intensity depending on reflectivity) may constructively or de-constructively interfere so as to create bright or dark “noise” spots in the signal that is resolved on the optical detection device.  FIG. 1   b  shows such a bright noise spot  108  (also referred to as a “lobe”) that might result, for instance, from the constructive interference of light reflected off of the edges of the conductive lines.  
         [0009]     The portion  111  of the resolved signal  110  that corresponds to the lobe  108 , being a deviation from the noiseless signal  107 , corresponds to an item of noise that makes detection of the defect difficult or impossible. For example, with the lobe  108  being positioned around the defect itself, the resolved signal superimposes the intensity spikes  112 ,  113  that correspond to conductive line  1024  and defect  103 , respectively. At a minimum, owing to the intensity  111  of the lobe, it will more difficult to detect the signal from the defect  113  in  FIG. 1   b  as compared to the signal from defect  106  observed in  FIG. 1   a . In an extreme case, if the lobe&#39;s intensity is beyond the saturation level  114  of the optical detector device, the defect will be impossible to detect (because the data from the optical detection device will clip at level  114 ).  
     
    
     FIGURES  
       [0010]     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:  
         [0011]      FIG. 1   a  shows a noiseless image for wafer defect inspection and a corresponding signal along a particular axis;  
         [0012]      FIG. 1   b  shows an exemplary noisy image for the wafer defect inspection of  FIG. 1   a  and a corresponding signal along the same axis;  
         [0013]      FIG. 2  shows a design for a wafer inspection tool;  
         [0014]      FIG. 3   a  shows an aperture wheel;  
         [0015]      FIG. 3   b  shows a liquid crystal array;  
         [0016]      FIG. 4  shows a wafer inspection tool that employs a digital micromirror device (DMD) as a filter;  
         [0017]      FIG. 5   a  shows a first perspective of a DMD;  
         [0018]      FIG. 5   b  shows a second perspective of a DMD;  
         [0019]      FIG. 6  shows a methodology that can be performed by the wafer inspection tool of  FIG. 4 .  
     
    
     DESCRIPTION  
       [0020]     In order to address noise problems that result from the varying of reflective angles and reflectivity from a three dimensional patterned structure, engineers have attempted to impose filters in the optical processing channel used for optical defect detection.  FIG. 2  shows a depiction of an imaging system for detecting defects on a wafer  206 . A light source (e.g., a laser)  201  emits light that is collected by a lens  202 . The collected light travels through a filter  203  which filters the light as described in more detail further below.  
         [0021]     The filtered light impinges upon a beam splitter  204  which directs the light toward the wafer  206  surface. The light from the beam splitter  204  is focused into a “spot” on the wafer  206  by a focusing lens  205 . The patterned features of the wafer  206  that are illuminated beneath the spot correspond to the features that are being inspected for a defect. Typically, the wafer  206  can be moved beneath the spot so that after a first region of the wafer is inspected, second and subsequent wafer regions can be inspected by adjusting the wafer position for each region to be inspected.  
         [0022]     The light that is reflected from the wafer surface is collected and directed toward the optical detection device  209  (e.g., photodetector, charge coupled device (CCD) array, etc.). Along the optical channel between the wafer  206  and the optical detection device  209 , another filter  207  may exist (or, filter  207  may exist and filter  203  may not exist). The filters are configured to affect the light so as to reduce the noise that may be resolved onto the optical detection device  209 .  
         [0023]     For example, considering the problem discussed above with respect to  FIG. 1   b , filter  203  may be configured to filter out specific regions of light that that impinge on a conductive line edge and constructively interfere with other light (i.e., an attempt is made to filter out light that contributes to lobe  108 ,  111  prior to focusing onto wafer  206 ). Likewise, filter  207  may be configured to filter out specific regions of light that would otherwise constructively interfere with one another to form lobe  108 ,  111  upon the optical detector  209 . By filtering out light that contributes to noise, the resolved signal at the detector will approach a “noiseless” signal (e.g., as depicted in  FIG. 1   a ) so as to make defect inspection easier.  
         [0024]     Because each different region of the wafer that is inspected is expected to have “its own” patterned structure, and because the noise produced from reflected light is expected to exhibit differing features for differing patterned structures, the noise to be filtered out “changes” as a function of wafer position. For example, a first wafer region having conductive lines oriented along a “z” axis might produce a lobe at first position within the reflected image; while, a second wafer region having conductive lines oriented along an “x” axis might produce a lobe at a second position within the reflected image. The filtering profile of filters  203 ,  207  must therefore be specially configured for each wafer position.  
         [0025]     Filters  203 ,  207  are therefore ideally capable of imposing different filtering characteristics that can be precisely specified.  FIG. 2  indicates that a computing system  210  which controls the wafer position is also used to set the filtering profile of each of the filters  203 ,  207 . Here, the expected patterned structure beneath the focused light—and its corresponding noise—is known prior to the testing of the wafer  206 . As such, the computing system  210  can be programmed with the proper filtering profiles for each wafer position so that the noise that is created at each wafer position can be diminished at least to some degree.  
         [0026]      FIGS. 3   a  and  3   b  show different types of filters that have been used for the filters  203 ,  207  discussed above.  FIG. 3   a  shows an aperture wheel  301  and  FIG. 3   b  shows a liquid crystal array  302 . The aperture wheel has a collection of different apertures around its circumference that each act as a filter having a specific filtering profile. Light is passed through a specific aperture by rotating the wheel such that the specific aperture is rotated into the light path.  
         [0027]      FIG. 3   b  shows an array  302  of liquid crystals. Each liquid crystal in the array  302  can have its optical transmissivity individually set (e.g., to pass light or to not pass light). For example as depicted in  FIG. 3   b , the upper left liquid crystal in the array  302  is configured to not pass its incident light whereas the lower left liquid crystal in the array  302  is configured to pass its incident light.  
         [0028]     A problem with the aperture wheel is that the number of apertures, and therefore the number of different filtering profiles that can be effected, is limited. The liquid crystal array  302 , although capable of a multitude of different filtering profiles owing to the discrete transmission control of its constituent liquid crystals, is expected to be less and less “workable” as the wavelength of the light from light source  201  is reduced in the coming years into the deep ultra violet (DUV) spectrum (so as to enable the detection of smaller wafer pattern features). Presently known liquid crystals are either highly absorptive or reflective in the DUV spectrum and therefore do not posses the transmission qualities for effecting a transmission filter as described just above with respect to  FIG. 3   b.    
         [0029]     A solution to these problems is to use a digital micro-mirror device (DMD) as a noise filter in an inspection tool. A DMD is an arrangement (e.g., an array) of small mirrors (“micro-mirrors”), where the tilt angle of each micro-mirror in the arrangement is capable of being individually set with digital information that is directed toward the DMD. Examples of DMDs include DLP™ DMDs from Texas Instruments, Inc. Because many materials are known to be reflective in the DUV spectrum, a DMD coated with one or more materials that are suitably reflective in the DUV spectrum should not experience a functional roll-off in the DUV spectrum as with liquid crystal arrays.  
         [0030]     Moreover, because the tilt angle can be individually set for each mirror in the arrangement, the reflectivity of the DMD surface as a whole can be configured into a number of different filtering profiles. As such, the filtering profile variability associated with liquid crystal arrays is at least somewhat preserved but with workable functionality at least in the DUV spectrum. Even though aperture wheels are workable in the DUV spectrum, the limited selection of filtering profiles that is attainable with an aperture wheel should make a DMD based filter a better solution for filtering out the different types of noise profiles that may need to be filtered over the lifetime of the inspection tool.  
         [0031]      FIG. 4  shows a wafer inspection tool that employs DMD filters  403 ,  407 . A light source (e.g., a laser, a lamp, etc.)  401  emits light that is collected by a lens  402 . The collected light travels through a DMD filter  403  which filters the light as described in more detail further below.  
         [0032]     The filtered light impinges upon a beam splitter  404  which directs the light toward the wafer  406  surface. The light from the beam splitter  404  is focused into a “spot” on the wafer  406  by a focusing lens  405 . The patterned features of the wafer  406  that are illuminated beneath the spot correspond to the features that are being inspected for a defect. Like the system shown on  FIG. 2 , the wafer  406  can be moved beneath the spot so that after a first region of the wafer is inspected, second and subsequent wafer regions can be inspected by adjusting the wafer position for each region to be inspected.  
         [0033]     The light that is reflected from the wafer surface is collected and directed toward the optical detection device  409 . Along the optical channel between the wafer  406  and the optical detection device  409 , another filter  407  may exist (or, filter  407  may exist and filter  403  may not exist). Like the system in  FIG. 2 , the filters are configured to affect the light so as to reduce the noise that may be resolved onto the optical detection device  409 . However, unlike the system of  FIG. 2 , the filters  403 ,  407  use reflection—rather than transmission—as the mechanism by which light is passed through the channel.  
         [0034]      FIGS. 5   a  and  5   b  explore this aspect in more detail.  FIG. 5   a  shows the surface of a DMD device  501 . Note that the DMD surface is pixilated similar to that of a liquid crystal display. However, each pixel corresponds to a reflective mirror rather than a transmissive liquid crystal. Because the tilt angle of each mirror can be individually set, a first tilt angle can be used to reflect light so that it continues into the optical channel that is used to inspect the wafer; while, a second tilt angle can be used to reflect light so that it is reflected out of the optical channel that is used to inspect the wafer. Therefore, in order to establish a specific filtering profile, the mirrors that receive the regions of light that are intended to be filtered out are set to the second tilt angle; and, the mirrors that receive the regions of light that are not intended to be filtered out are set to the first tilt angle.  
         [0035]      FIG. 5   b  shows an example. The DMD device  501  of  FIG. 5   a  is shown oriented in  FIG. 5   b  at an angle that conforms to the position of DMD filter  403  of  FIG. 4 . Here, a tilt angle of 0° relative to the plane  504  of the DMD device  501  (i.e., a micro-mirror with its reflective surface lying along plane  504 ) corresponds to a micro mirror being positioned to reflect its light into the optical channel. More specifically, referring to  FIGS. 4 and 5   b , the plane of DMD filter  403 ,  501  is oriented 45° relative to the z axis. As a consequence, any micro mirror having a tilt angle of 0° relative to the plane  504  of the DMD filter  403 ,  501  will cause light  502   a  that is directed along the +y axis from light source  401  to be reflected along the +z axis  502   b  into beam splitter  404 . As such, a 0° tilt angle relative to the plane  504  of the DMD filter  403 ,  501  corresponds to the “first” tilt angle defined above for those regions of light that are not supposed to be filtered out of the optical channel.  
         [0036]     By contrast, micro mirrors oriented so as to face more toward the xz plane than the 0° tilt angle faces toward the xz plane will cause light  503   a  that is directed along the +y axis from light source  401  to be reflected  503   b  in the −y direction and out of the optical channel. As such, an orientation that faces more toward the xz plane than the 0° tilt angle corresponds to the “second” tilt angle described above for those regions of light that are supposed to be filtered out of the optical channel. The same analysis discussed above for DMD filter  403  also applies to DMD filter  407 .  
         [0037]     Therefore specific filtering profiles can be established simply by setting to a fist tilt angle those micro-mirrors that receive light that is not supposed to be filtered out; and, setting to a second tilt angle those micro-mirrors that receive light that is supposed to be filtered out.  
         [0038]     Similar to the discussion provided above with respect to the system shown in  FIG. 2 , because each different region of the wafer that is inspected is expected to have “its own” patterned structure, and because the noise produced from reflected light is expected to exhibit differing features for differing patterned structures, the noise to be filtered out “changes” as a function of wafer position. For example, a first wafer region having conductive lines oriented along the “z” axis might produce a lobe at first position within the reflected image; while, a second wafer region having conductive lines oriented along an “x” axis might produce a lobe at a second position within the reflected image. The filtering profile of filters  403 ,  407  must therefore be specially configured for each wafer position.  
         [0039]     Because each of the micro-mirrors of DMD filters  403 ,  407  are capable of being individually adjusted the DMD filters  403 ,  407  are capable of imposing different filtering characteristics that can be precisely specified.  FIG. 4  indicates that a computing system  410  which controls the wafer position is also used to set the filtering profile of each of the DMD filters  403 ,  407  through the individual adjustment of their respective micro-mirrors (e.g., for each of the DMD filters  403 ,  407 , a first group of micro mirrors are set to the above described “first” tilt angle and a second group of micro mirrors are set to the above described “second” tilt angle.  
         [0040]     Here, as with the system in  FIG. 2 , the expected patterned structure beneath the focused light—and its corresponding noise—is known prior to the testing of the wafer  406 . As such, the computing system  410  can be programmed with the proper filtering profiles for each wafer position so that the noise that is created at each wafer position can be diminished at least to some degree.  
         [0041]      FIG. 6  shows a wafer inspection process that can be executed by the system observed in  FIG. 4 . The wafer inspection process may be implemented, at least in one embodiment, as program code that is executed upon the processing core of the computing system  410 . According to the process of  FIG. 6 , a set of micro-mirror positions are configured for a particular (nth) position on the wafer onto which the inspection light is focused  601 . If the system comprises two DMD filters (as observed in  FIG. 4 ), the set includes the positions for both of the DMD filters.  
         [0042]     In an embodiment, a first group of the set are configured to a first tilt angle that keeps light within the optical channel and a second group of the set are configured to a second tilt angle that reflects light out of the optical channel. The first DMD filter&#39;s first group and second group combination defines the first DMD filter&#39;s filtering profile. The second DMD filter&#39;s first group and second group combination defines the second DMD filter&#39;s filtering profile.  
         [0043]     The filtering should at least reduce to some degree the noise that is resolved to the optical detection device. The data from the optical detection device is collected  602  and, at some point, compared to data that represents the expected image at the nth wafer position. Deviations from the expected image and the collected data image are used to flag manufacturing defects. At least after the data is collected for the nth position wafer position, the wafer position is changed so that the focused light impinges upon the next wafer at a next wafer position  604  out of N total wafer positions  603 .  
         [0044]     It should be noted that although the context of the above description has been directed to patterned semiconductor wafer inspection, the principles described herein can be applied to any item whose surface is to be inspected.  
         [0045]     It is also to be understood that because embodiments of the methods of the present teachings may be implemented as one or more software programs, embodiments of the present teachings may be implemented or realized upon or within a machine readable medium. A machine readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine readable medium includes read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc.  
         [0046]     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.