Abstract:
A line image acquisition apparatus suitable for being added onto a line-scan wafer macro-inspection system which incorporates oblique incidence illumination and detection, both for brightfield and for darkfield, which incorporates double darkfield observation capability, which incorporates broadly tunable angle of incidence illumination and tunable angle of detection, which incorporates multi-channel detection into a line-scan macro-inspection system, and which is an add-on feature compatible with current line-scan macro-inspection systems.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application corresponds to US Provisional Application No. 60,782,048, filed Mar. 14, 2006, and claims priority therefrom. 

   FIELD OF THE INVENTION 
   This invention relates to integrated circuit technology, and in particular to macro inspection of integrated circuit wafers. 
   BACKGROUND OF THE INVENTION 
   When performing macro inspection of integrated circuit wafers, several factors contribute to its effectiveness under varying circumstances. Included among these factors are the illumination angle, illumination intensity, angle of detection, polarization. The illumination and detection angles in particular affect observation in many ways. Brightfield observation occurs when the detected light is specularly reflected from the sample surface, i.e., when the angle of illumination and the angle of detection are the same. Brightfield inspection is useful for inspecting patterned regions with shallow topography, i.e., not much z variation. On the other hand, dark field inspection detects scattered and diffracted light, rather than specularly reflected light. Dark field inspection is often more effective when small changes are being detected, since a small change causes a larger percentage change of the less intense scattered darkfield signal. 
   In the case of integrated circuit inspection, the metal lines on the circuit in general approximate a grating configuration, so that scattered light usually forms a diffraction pattern. The grating equation applies:
 
sin(θ)+sin(θ i )= nλ/D  
 
where λ, is the wavelength of incident light, D is the grating spacing, θ i  is the angle of incidence of light with respect to a surface normal, θ is the angle of detection of light, and n is the diffraction order.  FIG. 1   a  illustrates a resulting diffraction pattern in side view. Incident light beam  100  from illumination source  105  is at angle θ i  from normal onto sample  110 . Zeroeth order diffracted light beam  115 , i.e., specularly reflected beam is shown at angle θ i , as well as first order diffracted beam  120  centered at angle θ 1  and second order diffracted beam  125  centered at angle θ 2 . The diffracted beams can be thought of as narrow “lobes” centered at the respective angles. The lobes typically decrease in intensity as the diffraction order increases. For detecting small changes in the sample structure, e.g. defocus effects, it is preferable to observe the higher order diffraction lobes, which will show the largest percentage change.
 
   As the grating spacing D decreases, i.e. integrated circuit dimensions decrease and packing density increases, the diffraction lobes separate and spread out. As D approaches λ, it is necessary to have a larger angle of incidence in order to see higher order diffraction lobes. Also, the higher order diffraction lobes will appear at larger angles away from normal as D decreases, therefore the detection angle must also increase. This can be demonstrated as follows: 
   Assume that D=2λ, and assume normal incidence light, i.e., θ i =0. Then the grating equation becomes sin(θ)+sin(0)=n/2, or sin(θ)=n/2. In this case, shown in  FIG. 1   b , the first order diffraction lobe  130  (n=1) will be centered at a 30 degree angle from normal to the sample surface, and the second order diffraction lobe  135  will extend parallel to the sample surface, nearly impossible to detect. Assuming grazing incidence light, i.e., θ i =90 degrees, then the grating equation becomes sin(θ)+sin(90)=n/2, or sin(θ)=n/2−1. In this case, shown in  FIG. 1   c , the first order diffraction lobe  140  will be at a −30 degree angle from normal to the surface, (i.e., in the same quadrant as the incident light), the second order diffraction lobe  145  will be normal to the surface, the third order diffraction lobe  150  will be at a +30 degree angle from normal, and the fourth order diffraction lobe  155  will extend parallel to the sample surface. 
   Clearly, then, using oblique incidence light, and being able to tune the angle of detection to be a variable off-normal angle, yield advantages for darkfield sample inspection when higher order diffraction lobes are preferable, as in the case when inspecting for defocus defects. The ability to vary both the angle of incidence and angle of detection of light would provide the maximum flexibility to optimize inspection according to the details of the sample. The use of oblique incidence additionally yields a strong polarization dependence which is not present for normal incidence light. This polarization dependence, which increases as the angle of incidence increases away from normal, is further described in commonly authored and owned U.S. patent application Ser. No. 10/829,727, filed Apr. 22, 2004, issued as U.S. Pat. No. 7,142,300, on Nov. 28, 2006, which is hereby incorporated by reference. This effect provides substantial background suppression, allowing improved inspection of small changes in signal. 
   A manual inspection system incorporating a tiltable, rotatable table for mounting the sample on so as to provide a wide range of incidence angles for brightfield inspection is described in U.S. Pat. No. 5,096,291, issued Mar. 17, 1992. An inspection system using diffracted light is described in U.S. Pat. No. 5,777,729 (assigned to Nikon Corp.), issued Jul. 7, 1998. As described therein, (and as implemented in the Nikon AMI inspection system), the wafer to be inspected is mounted on a tiltable, rotatable plate so as to be able to tune the inspection angle. The wafer motion during inspection causes problems in matching images. Furthermore, the large parabolic mirrors used for collecting outgoing light have a large focal length, resulting in a very large inspection machine, which costs valuable fab space. 
   An extension of darkfield detection known as “double darkfield” occurs when the incident light beam and the scattered beam which is detected are not co-planar. In the most pronounced case, illustrated in  FIG. 2 , the xz plane  205  formed by the incident beam  210  and the sample normal  212  is perpendicular to the yz plane  215  formed by the detected scattered beam  220  (by detector  225 ) and the sample normal. In this case, the detected light has been scattered in two directions, which provides for a very low background intensity upon which small changes are quite pronounced. 
   A method of incorporating oblique incidence illumination and double darkfield capability, as well as broad tunability of angle of incidence and angle of detection, into a line scan macro-inspection system such as the Viper system by KLA-Tencor (described in U.S. Pat. No. 5,917,588 by Addiego, which is hereby incorporated by reference), would be a significant advancement in macro-inspection technology. (The current Viper system has limited oblique incidence and detection for brightfield, but with angle limited to being fairly close to normal, i.e., approximately + or −30 degrees, so as to not interfere with the motion stage). The additional ability to incorporate multi-channel detection would allow the most complete inspection according to multiple parameter variations. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to provide a method and apparatus for incorporating oblique incidence illumination and detection, both for brightfield and for darkfield, into a macro-inspection system such as a line-scan macro-inspection system. 
   It is a further object of this invention to provide a method and apparatus for incorporating double darkfield observation capability into a macro-inspection system such as a line-scan macro-inspection system. 
   It is a further object of this invention to provide a method and apparatus for incorporating broadly tunable angle of incidence illumination and tunable angle of detection into a macro-inspection system such as a line-scan macro-inspection system. 
   It is a further object of this invention to provide a method and apparatus for incorporating multi-channel detection into a macro-inspection system such as a line-scan macro-inspection system. 
   It is a further object of this invention to provide the above-mentioned improvements to a line-scan macro-inspection system as an add-on feature which is compatible with current line-scan macro-inspection systems. 
   These objects are met by the method and apparatus disclosed herein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  shows a side view of diffracted rays from a diffraction grating. 
       FIG. 1   b  shows the diffracted rays for normal incidence light when λ=D. 
       FIG. 1   c  shows the diffracted rays for grazing incidence light when λ=D. 
       FIG. 2  illustrates double-darkfield detection. 
       FIG. 3  shows a configuration used to test the sensitivity of defocus defects to detection under oblique incidence illumination, using a two dimensional detector. 
       FIG. 4  shows an image of a test wafer using oblique incidence illumination, oblique detection, and P polarization. 
       FIG. 5  shows an embodiment of the invention using a configuration similar to that of  FIG. 4  which can be incorporated into the architecture of a line-scan macro-inspection system. 
       FIG. 6  is an illustration of an embodiment of the invention employing multiple stages as in  FIG. 5 . 
       FIG. 7  illustrates an embodiment of the invention which incorporates double darkfield analysis into the Viper line scan architecture. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  shows a configuration used to test the sensitivity of defocus defects to detection under oblique incidence illumination. Sample  305  is illuminated from oblique angle α by illuminator  310 , which is shown in this case to be a fluorescent light box. The light box has a heavy diffuser in front of the fluorescent tubes to provide uniform, diffuse illumination. The spectrum of the fluorescent illuminator consists primarily of the narrow spectral lines of Hg. Detector  315  is a conventional 2-dimensional color camera, in this case a commercial SLR Digital camera. The lens  320  and the detector  315  are tilted with respect to the sample surface according to the Scheimpflug configuration which allows the sample to stay in focus across the sample with oblique incidence illumination. The Scheimpflug geometry is discussed in U.S. Pat. No. GB 1196/1904, which is hereby incorporated by reference. A further analysis of the Scheimpflug geometry is found at http://www.trenholm.org/hmmerk/SHSPAT.pdf. Adjustable polarizer  325  is positioned between sample  305  and detector  315 . The angle of detection was adjusted to be the Brewster angle for silicon, i.e., the angle for which the reflectivity of p-polarized incident light goes to zero. At the Brewster angle in P polarization, the wafer “goes dark” and it becomes much easier to see small signal changes. 
   Under these conditions, a test wafer was imaged using P polarization. The wafer contained diagonal rows of variable focus and variable exposure dies. The wafer image was subtracted from that of a wafer without the anomalies, and was rescaled, stretched in y and re-contrasted for illustration purposes. The resultant image is shown in  FIG. 4 . The diagonal lines  405  clearly show the focus/exposure effects. Color variation was also seen which gives a “direct readout” of the defocus or exposure variation. The image could be improved further by using a microscope setup to image the resist pattern in at least 2 points, and rotating and translating the wafer to accurately register the pattern itself. If this process is followed, the bright edges remaining in the image of  FIG. 4  would largely vanish, leaving mainly only the defects of interest. One problem with this configuration is the lack of telecentricity of the imaging lens, which results in a top/down and left/right variation of base intensities. 
   In an embodiment of the present invention, a configuration similar to that of  FIG. 3  is implemented which can be incorporated into the architecture of a line-scan macro-inspection system such as the Viper system made by KLA-Tencor, with a broad variation in angles of incidence and detection. This embodiment is illustrated in  FIG. 5 . Wafer  505  is oriented perpendicular to the plane of the paper. Illuminator  510  is a fiber-optic light line with fanned fibers, and collimating illumination optics  515  are placed in front of illuminator  510  to provide telecentricity perpendicular to the scan direction, which is into the plane of the paper. Analyzer  520  and imaging optics  525  are positioned between sample  505  and line-scan sensor  530 . The sensor and imaging optics are oriented to the sample according to the Schiempflug geometry so as to keep the entire sample in focus. Note that the dimension of the imaging and illumination optics may be smaller than the 300 nm wafer diameter due to the oblique angle of observation. The imaging and illumination optics may be curved mirrors, by way of example. Polarizer  535  may be positioned on the illumination side to permit cross-polarization observation. The configuration as shown is effectively a one-dimensional imaging ellipsometer. The analyzer could be replaced with a polarizing beam splitter splitting the line image onto two line sensors, each observing with a different polarization. Alternatively, dichroic beam splitters could split the images into more sensors so as to provide spectral information, as follows: Dichroic beam splitters allow transmission of wavelengths above a critical wavelength, and reflects those below. Using a dichroic beam splitter, the outgoing light from the sample can be split by wavelength, therefore separate measurements can be made on each wavelength range. Alternately, a 2D sensor having a grating or prism could be used to spread light be wavelength into multiple adjacent lines. 
   Because of the scan direction  518  with respect to the illumination and optics, the geometry resembles a car wash whereby the wafer may pass sequentially through multiple stages (with a single stage as shown in  FIG. 5 ), with multiple illuminators  510 ,  610 , and multiple analyzers  520 ,  620 .  FIG. 6  is an illustration of another embodiment of the invention. For example, laser line generators can be used, and each stage  605  could utilize a different wavelength of illumination. Alternatively or additionally, different polarizations could be analyzed, and different angles of incidence of illumination could be employed. The configuration, with both the illuminators and the detectors moved to the side so as not to interfere with the scan motor, also gives sufficient space to facilitate the broad tuning of the angle of incidence and angle of detection. This tuning could be accomplished by mounting the illuminator  510  and sensor  530  on circular rails  640 , allowing the selection of illumination and/or observation angles. The illuminators or detectors could be moved in a motorized fashion using lead screws, rack-and-pinion mechanisms, or piezomotors. Whereas the available space is limited for any one position along the scan direction, by adding the capability of arranging illuminators, analyzer, and sensors at varying positions along the scan direction, a great deal of potential space is made available. 
   Another embodiment, with one possible configuration illustrated in  FIG. 7 , incorporates double darkfield analysis into the Viper line scan architecture. If the incident illumination vector  705  is not coplanar with the outgoing light vector  710  entering the analyzer and sensor, scattering in two directions has occurred before detection, resulting in very low background signal. As illustrated in the figure from a top view, illuminator  715  which may be a fiber optic fan illuminates a line  720  across the wafer  721  which is scanning in direction of arrow  722 . A possible illuminator configuration is described by commonly owned U.S. Pat. No. 6,796,697 (shown in FIG. 4), which is hereby incorporated by reference. Line sensor  725  employs Scheimpflug optics  730 . Note that the illuminator and detector positions could be exchanged, using a conventional line sensor imager as a camera. 
   The inventive embodiments as described above can be added to an existing Viper macro-inspection system as an additional acquisition channel. The pixel size in the scan direction (defined as the x-direction) is given by the sampling rate of the sensor, and can be small, consistent with the current Viper configuration. The resolution in the direction perpendicular to the scan, but parallel to the sample surface (defined as the y-direction) will be reduced by the oblique detection angle. The line scan configuration disclosed herein can be implemented using standard Viper components, i.e., sensors and illuminators, thus potentially yielding a short time-to-market. The system as described is modular and can be kept almost completely independent of the existing Viper system, thereby easing the software development effort. 
   It is not intended that the invention be restricted to the exact embodiments disclosed herein. It should be apparent to one skilled in the art that certain changes and modifications can be made without departing from the inventive concept. For example, other types of analyzers may be incorporated in additional stages, such as: point measuring sensors, 2D sensors, 2D sensors with gratings in front for multispectral observations. Other types of illuminators may be used, such as short laser-line generators for monochromatic observations. The scope of the invention should be construed in view of the claims.