Abstract:
A confocal three dimensional inspection system, and process for use thereof, allows for inspecting of bumps and other three dimensional (3D) features on wafers and other semiconductor substrates. The sensor eliminates out of focus light using a confocal principal to improve depth response. This process and system creates multiple parallel confocal optical paths along a line. The out of focus light is eliminated by placing an aperture at a plane which is a conjugate focal plane to the surface of the sample. The result is that the sensor produces a signal only when the sample surface is in a narrow focal range.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application cross references and incorporates by reference U.S. Provisional Patent Application No. 60/232,443 filed on Sep. 12, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a system, and process for use thereof, for inspecting wafers and other semiconductor substrates, and specifically for inspecting three dimensional (3D) features thereon such as bumps. Specifically, the present invention relates to a confocal optical system for inspecting bumps and other 3D features on wafers or like substrates, and a process of using such system. 
     2. Background Information 
     Over the past several decades, the semiconductor has exponentially grown in use and popularity. The semiconductor has in effect revolutionized society by introducing computers, electronic advances, and generally revolutionizing many previously difficult, expensive and/or time consuming mechanical processes into simplistic and quick electronic processes. This boom in semiconductors has been fueled by an insatiable desire by business and individuals for computers and electronics, and more particularly, faster, more advanced computers and electronics whether it be on an assembly line, on test equipment in a lab, on the personal computer at one&#39;s desk, or in the home via electronics and toys. 
     The manufacturers of semiconductors have made vast improvements in end product quality, speed and performance as well as in manufacturing process quality, speed and performance. However, there continues to be demand for faster, more reliable and higher performing semiconductors. 
     One process that has evolved over the past decade plus is the semiconductor inspection process. The merit in inspecting semiconductors throughout the manufacturing process is obvious in that bad wafers may be removed at the various steps rather than processed to completion only to find out a defect exists either by end inspection or by failure during use. In the beginning, wafers and like substrates were manually inspected such as by humans using microscopes. As the process has evolved, many different systems, devices, apparatus, and methods have been developed to automate this process such as the method developed by August Technology and disclosed in U.S. patent application Ser. No. 09/352,564. Many of these automated inspection systems, devices, apparatus, and methods focus on two dimensional inspection, that is inspection of wafers or substrates that are substantially or mostly planar in nature. 
     One rapidly growing area in the semiconductor industry is the use of bumps or other three dimensional (3D) features that protrude outward from the wafer or substrate. The manufacturers, processors, and users of such wafers or like substrates having bumps or other three dimensional desire to inspect these wafers or like substrates in the same or similar manner to the two dimensional substrates. However, many obstacles exist as the significant height of bumps or the like causes focusing problems, shadowing problems, and just general depth perception problems. Many of the current systems, devices, apparatus, and methods are either completely insufficient to handle these problems or cannot satisfy the speed, accuracy, and other requirements. 
     SUMMARY OF THE INVENTION 
     The inspecting of semiconductors or like substrates, and specifically the inspection of three dimensional features, such as bumps, on their surface is accomplished by the present invention, which is a confocal sensor system to improve depth response by eliminating out of focus light thereby resulting in the sensor producing a signal only when the surface being inspected is in a narrow focal range. The result is an accurate height determination for a given point or area being inspected such that the cumulation of a plurality of height determinations from use of the confocal sensor system across a large surface allows the user to determine the topography thereof. 
     In sum, this system and process creates multiple parallel confocal optical paths along a line whereby the out of focus light is eliminated by placing an aperture at a plane which is a conjugate focal plane to the surface of the sample. The result is that the sensor produces a signal only when the sample surface is in a narrow focal range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiment of the invention, illustrative of the best mode in which applicant has contemplated applying the principles, are set forth in the following description and are shown in the drawings and are particularly and distinctly pointed out and set forth in the appended claims. 
     FIG. 1 is drawing of one embodiment of the present invention; 
     FIG. 2 is an illustration of the microlens portion of the present invention; 
     FIG. 3A is a drawing of a second embodiment of the present invention; 
     FIG. 3B is a drawing of another embodiment of the present invention; and 
     FIG. 4 is a drawing of another embodiment of the present invention. 
    
    
     Similar numerals refer to similar parts throughout the drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The three dimensional (3D) inspection system of the present invention is indicated generally at  20  as is best shown overall in FIG.  1  and is used in one environment to view, inspect, or otherwise optically observe three dimensional features, such as bumps, on an otherwise flat or planar substrate. The 3D inspection system includes a light source  22 , a sensor system  26 , and a camera system  28 . It is also contemplated that in certain configurations a mirror  24  may be used. 
     The light source  22  is any source of light that provides sufficient illumination for the measurement system. Examples of the light source include, but are not limited to, white light sources such as halogen or arc lights, lasers, light emitting diodes (LEDs) including white LEDs or any of the various colored LEDs, fluorescent lights, or any other type of light source. In one embodiment as is shown in the Figures, the light source is preferably an arc lamp or halogen lamp. 
     The light source may be positioned in any position relative to the sensor so long as it provides the necessary light to the sensor  26  and sample S to be viewed, inspected or otherwise optically observed. In the embodiment shown in the Figures, the light source is offset, not axial, to the sensor system  26 , and uses mirror  24  to orient the light in the desired axial direction within the sensor system  26 . This offset light source  22  with mirror  24  is used because the mirror, which is a cold mirror, reflects visible light into the optics  30  as is needed and defined below while allowing undesirable infrared light to pass or transmit through the mirror and out of the system. Such undesirable infrared light is usually emitted from the light source and removed from the system by passing through the mirror. In the alternative, an infrared filter could be used to accomplish this task. Another alternative is a light source  22  could be used that emits only light in a controlled frequency, i.e. the visible frequency that is needed by the system and no other light, thereby eliminating the need to filter or reflect only desirable light. An example of this is a laser. 
     The light source  22  may include a reflector as is well known in the art to focus or direct the maximum light in the preferred direction. 
     In at least some embodiments, the system  20  further includes a lens or other optics  30  in between the light source  22  and the mirror  24 , or if no mirror is needed then the sensor  26 , for gathering or collecting maximum light from the light source  22  and directing it at the mirror  24  or sensor  26 . The optics  30 , in the embodiment shown, are a condenser lens located to collect light from the light source and direct it toward the mirror  24  or the sensor  26  if no mirror is used. 
     The light emitted from the light source  22  and directed by the mirror  24 , if needed, enters sensor system  26  whereby it is manipulated, focused, split, reflected, refracted, collimated, recollimated, filtered, or otherwise processed and allowed to pass through if certain qualities or properties are met. The light that passes through the entire sensor system  26  is received by camera  28  and then processed by a computer or the like to calculate or determine the topography of the sample including the location, size, shape, contour, roughness, and/or metrology of the bumps or other features thereon. 
     The camera  28  may be any line scan camera, area scan camera, combination of multiple line scan cameras, time delay integration (TDI) line scan camera or other camera or cameras as one of skill in the art would recognize as functionally operational herewith. In the embodiment shown in FIG. 1, the camera  28  is a line scan camera. In the embodiment shown, the camera system  28  includes a telecentric camera lens  36  and a line camera  28 . 
     In more detail and in the embodiment shown in the Figures, the sensor  26  includes collimating optics  40 , an optional linear polarizer  41 , a polarizing beam splitter  42 , a microlens line array  44 , a telecentric imaging lens  46 , and a ¼ wave plate  48 . 
     The collimating optics  40  may be any system or device that collimates light, that is makes the light rays from the light source  22  parallel to each other. The collimating optics  40  must collimate light from the light source into a high quality light beam. In the embodiment shown in the Figures, collimating optics  40  includes a pair of lenses  60  and  62  with a pin hole aperture  64  therebetween, all of which acts as a spatial filter. The overall function of this spatial filter is to remove noncollimated light by blocking it with the aperture. Lens  60  functions to direct and focus light toward aperture  64 , whereby focused light passes through aperture  64  and is recollimated by lens  62 . 
     An optional linear polarizer (not shown) is provided between the light source  22  and the polarizing beam splitter  42  to allow for light intensity control. Essentially all light not in line with the polarizer&#39;s preferential direction is blocked. This linear polarizer can be rotated relative to the polarizing beam slitter&#39;s polarization axis to provide light intensity control which is needed for sample to sample intensity control. Alternately, this linear polarizer provides additional polarization to the polarizing beam splitter  42  where and if one polarizing device is insufficient, that is not capable of providing 100% polarization, or where precaution is desired to make sure sufficient polarization occurs. 
     The collimated light enters the polarizing beam splitter  42 . The polarizing beam splitter  42  may be any system or device that separates linear polarized light of one polarization from polarized light of the orthogonal polarization by transmitting one polarization and reflecting the orthogonal. In the embodiment shown in the Figures, the polarizing beam splitter  42  polarizes the collimated light transmitted from the collimating optics  40  and separates the linearly polarized light of one polarization from the orthogonally polarized light by allowing the transmission of the linearly polarized light while reflecting the orthogonally polarized light. 
     The linearly polarized light that transmitted through the polarized beam splitter  42  is directed at the microlens array  44  or other array structure such as a multiple microlens. The microlens line array  44  may be any system or device that orients collimated light into a controlled line or area array or like pattern. 
     In the embodiment shown in the Figures, the microlens line array  44  is a line of microlenses. Each microlens has an opaque mask material deposited onto the back surface thereof. This opaque mask material  50  has small apertures  51  therein that are coincident to each microlens focal point and these small apertures  51  are sized to allow only the passage of light through the central lobe of the focal point of each microlens  52  as is illustrated in FIG.  2 . In design, each microlens  52  in the array  44  is constructed such that the focal point of each microlens  52  in the array  44  corresponds to the thickness of the substrate  53  on which the microlenses are made. 
     Returning to FIG. 1, light passing through the aperture line formed by the multiple apertures aligned in the microlens array  44  is then collected by the telecentric imaging lens  46 . The telecentric imaging lens  46  creates a reduced image of the pinhole array on the sample S surface. 
     The magnification of this telecentric lens is determined by the desired effective pixel size. For example, if the microlens diameter is 50 microns, and a micron pixel size is desired, the magnification for the telecentric imaging lens must be 0.1×. 
     This telecentric imaging lens  46  also contains the ¼ wave plate  48 . This ¼ wave plate changes the polarization of the light passing through the telecentric lens  46  from linearly polarized to circularly polarized. This circularly polarized light is transmitted onto the sample S surface. 
     Light reflecting from the sample S surface incurs a phase change of 180 degrees. This phase change alters the circularly polarized light such that the reflected light is still circularly polarized, but in the opposite direction. This reflected light is returned back through the telecentric imaging lens  46  where the ¼ wave plate  48  changes the circularly polarized light back to linearly polarized light. However, the direction of the linear polarization is orthogonal to the original direction so as to cause it to be reflected by, not transmitted through, the polarizing beam splitter  42  as discussed below. 
     The light from the “in focus” portions of the sample S will pass back through the small apertures and be recollimated by the microlenses  44 . All “out of focus” light will be blocked by the opaque mask around the apertures. The light which is passed back through the small apertures will now be reflected by the beam splitter  42  and then collected by the camera lens in the camera system  28 . 
     This camera lens creates an image of the microlens array onto the camera sensor. The magnification of the camera lens is selected so that each microlens is imaged onto one sensor pixel. For example, if the microlens size is 50 microns, and the sensor pixel size is 10 microns, then the camera lens magnification must be 0.2×. The camera sensor is a linear photosensor with the number of pixels matching the number of microlenses. 
     In an alternative sensor arrangement, the camera may be multiple line cameras (shown at  70  in FIG. 3A) or an area camera (shown at  80  in FIG. 3B) used in multi-line mode. If such alternative embodiment is used, the microlens will consist of multiple linear arrays of lenses which focus light correspondingly onto the multiple array of linear photo sensors. 
     The scanning process for the above invention is as follows: The line of focal points or spots of the microlens array  44  will be oriented 90 degrees to the scanning direction. After the stage begins to move, the camera line starts integration at the rising edge of the scanning axis encoder clock. The stage moves distance of encoder counts equal to the horizontal optical size of the sensor. The camera ends integration on the falling edge of the last encoder count, transfers the data to a storage register, and begins the next integration period. For example, if the optical X-Y pixel size is 5 um, and the scanning axis encoder resolution is 50 nm, the camera is line triggered every 100 encoder counts to create square pixels. This line will then be scanned at a fixed height above the sample S surface either by moving the camera  28  or moving the sample S. This scan will generate one height location on a depth response curve for each pixel location on the sample S. The sensor will then return to its original starting position, be incremented in height, and the scan will be repeated to generate a second height location of the depth response curve for each pixel location on the sample S. This can then be repeated any number of times desired for the interpolation method used (typically at least two or three scans, although more are certainly contemplated and will assure accuracy). The multiple locations on the depth response curve are then interpolated for each pixel to generate a map of the surface height under the scan. The scans can then be repeated such that a height map is generated for the entire sample S surface. 
     The size of the “in focus” region is determined by the telecentric imaging lens  36 . If this lens has a larger numerical aperture (˜ratio of the focal length to diameter) the focus range will be small, and conversely if the lens  36  has a low numerical aperture the focus range will be large. The best “in focus” range is dependent on the height range that needs to be measured. The profile of each sensor element&#39;s response as the focus plane is translated through a sample surface is shown below: 
     The peak intensity of the profile corresponds to the location of the sample surface. Existing confocal microscopes can generate a 3D profile of a sample by stepping the sensor in very small increments of height, and storing the location of the maximum intensity. This is a slow process because many steps must be taken with very small step sizes to achieve high resolution and accuracy. The sensor of one embodiment of the present invention will not operate in the slow standard mode, but will take advantage of the fact that the depth profile remains constant for different sample types and light levels. Therefore, because the shape of the response can be known, it is possible to measure at only two or three height locations and accurately interpolate the location of the sample S surface. 
     As noted above, an alternative embodiment is contemplated using multiple lines of microlenses with either multiple line cameras or an area camera used in multiline mode. Each line of microlenses and its corresponding imaging camera or pixels serves as a separate confocal microscope. These microscopes are then adjusted such that each one has a different sample to sensor separation and therefore acquires confocal images at different heights. The sample to sensor separation may be adjusted by tilting the sample, by tilting the microlens array, or by placing glass plates or wedges (shown at  90  in FIG. 3A) between the microlens array and sample. These confocal microscopes running in parallel will acquire data at multiple heights, and therefore multiple locations on the depth response curves, during one acquisition of data. By measuring multiple locations on the depth response curve in a single scan, the overall speed of the system can be improved manyfold. 
     It is contemplated by the present invention that the sensor  26  ins a broader embodiment may be described as including the following: a collimating subsystem for making the light rays from the light source  22  parallel to each other, a desirable versus non-desirable light separator subsystem for separating the desired light from the non-desired light, a light gate subsystem for collecting light and only passing through to the sample S that light in focus, a filtering subsystem for filtering out non-focused light that reflects off of the sample S a light collector for collecting light that passed through the filtering subsystem, and an interpolator for gathering light intensity measurements taken at different heights from the light collector for a plurality of points and interpolating the measurements to define a topographic map of the sample S. 
     Another-embodiment of the present invention is shown in FIG. 4 as system  20 A. This embodiment is substantially similar to the first embodiment of FIGS. 1-2 as described above except the microlens array  44  of the first embodiment is replaced with an array of apertures or pinholes  44 A and the camera system  28  includes a TDI camera  38 A. It is also preferable that the light source  22  be a halogen light. 
     In sum, the present invention is a unique sensor and analysis technique which incorporates various novel, useful and nonobvious inventions including using a line-scan camera in conjuction with multiple, parallel confocal apertures, lenses, microscopes or other arrays of specially designed light passages to quickly acquire area confocal images at a particular sample/sensor separation and then, by using interpolation of multiple images at several sample/sensor separations, estimating the height of certain features. 
     Accordingly, the invention as described above and understood by one of skill in the art is simplified, provides an effective, safe, inexpensive, and efficient device, system and process which achieves all the enumerated objectives, provides for eliminating difficulties encountered with prior devices, systems and processes, and solves problems and obtains new results in the art. 
     In the foregoing description, certain terms have been used for brevity, clearness and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirement of the prior art, because such terms are used for descriptive purposes and are intended to be broadly construed. 
     Moreover, the invention&#39;s description and illustration is by way of example, and the invention&#39;s scope is not limited to the exact details shown or described. 
     Having now described the features, discoveries and principles of the invention, the manner in which it is constructed and used, the characteristics of the construction, and the advantageous, new and useful results obtained; the new and useful structures, devices, elements, arrangements, parts and combinations, are set forth in the appended claims.