Abstract:
Metallographic samples are cross-sectioned to inspect internal features of the samples and to determine the cause of component failures. In order to reach an area of interest that is to be inspected, the cross-sections undergo grinding. Conventional grinding techniques require that a metallographic sample be removed from the grinding apparatus and visually inspected in order to determine whether the area of interest has been reached. An improved grinding apparatus and method of grinding, images the sample while it is being ground so that grinding does not have to be interrupted in order to determine how far grinding has progressed. Real-time monitoring of the grinding process allows precision control of grinding of metallographic samples.

Description:
TECHNICAL FIELD 
     This invention generally relates to the field of sample preparation for cross-section analysis. More particularly, the present invention pertains to an improved method of preparing a metallographic sample for inspection, by grinding a surface of the sample. 
     BACKGROUND OF THE INVENTION 
     The following disclosure describes how the present invention applies to the field of semiconductor devices. However, the invention is not limited to semiconductor device applications, but applies to any sample to be cross-sectioned where an externally observable feature identifies the desired location of the cross-section. Other applications include, but are not limited to metals, ceramics, glass, plastics, composites, etc. 
     Semiconductor devices comprise a plurality of features formed on a semiconductor wafer. Semiconductor devices typically comprise a plurality of layers made up of conductive and insulative patterns, vias, and trenches. In order to function properly, the semiconductor device layers must be accurately aligned with each other, and sound electrical contacts must be formed with the conductive patterns. Inspections are performed on semiconductor devices as both part of routine quality control, and when trouble shooting to determine the cause of a semiconductor device failure. Because semiconductor devices comprise a plurality of layers, features internal to the semiconductor device are not readily observable by visual inspection. In order to inspect internal features, a cross-section of the semiconductor device is viewed. 
     Internal features that are inspected include flip chip/package solder bonds, feature and layer thicknesses, microstructure characterization, and the alignment of conductive layers and interconnects and contacts. Possible failure mechanisms that need to be inspected include the presence of voids in welds and solder bonds; layer separation, i.e., delamination or debonding of layers; and misregistration of device features. 
     The term semiconductor devices as used herein is not be limited to the specifically disclosed embodiments. Semiconductor devices as used herein include a wide variety of electronic devices including flip chips, flip chip/package assemblies, transistors, capacitors, microprocessors, random access memories, etc. In general, semiconductor devices refer to any electrical device comprising semiconductors. 
     Typically, to inspect the interior of a semiconductor device, a section of the semiconductor device, containing an area of interest that is to be inspected, is cut from the semiconductor device. The section cut from the semiconductor device can be cut using a metallographic saw, such as a wire saw, diamond impregnated blades, silicon carbide blades, or other abrasive saws. A margin of semiconductor device surrounding the area of interest is left after cutting so that the cutting does not damage the area of interest that is to be inspected. The section of the semiconductor device containing the area of interest is often mounted on a suitable holder, such as a stub, which is supported by a chuck. Then the margin surrounding the area of interest is removed by grinding. A grinding wheel or belt with a suitable grinding media is used to grind the sample. As the margin is ground away and the grinding wheel approaches the area of interest, the grinding media is successively changed to a finer grit material. In the final stages of grinding, polishing of the sample is performed. As used in the instant specification and claims the term “grinding” includes polishing. 
     The section of the semiconductor device being inspected may be mounted on a metallic or plastic stub with either two-sided tape or an adhesive, such as a thermal adhesive. The stub is mounted in a chuck, which supports the sample while it is undergoing grinding. 
     A sample can also be encased or potted within a transparent polymer resin. When potted, the sample can be held manually or clamped in a sample fixture when grinding. 
     In the prior art method of grinding semiconductor device samples, the grinding has to be stopped frequently, the sample removed from the chuck, and visually inspected with a microscope to determine whether the area of interest has been reached. The danger exists that grinding can proceed too far and either damage or grind right through the area of interest. The prior art process is inefficient and time consuming because the grinding process has to be interrupted each time the sample is inspected to determine whether grinding is complete. 
     SUMMARY OF THE INVENTION 
     There exists a heed in the metallographic sample inspection art to eliminate the problem of over-grinding a sample. There exists a need in this art to perform real-time monitoring of the grinding process to determine how fast grinding is progressing and to determine when the area of interest is reached. There further exists a need in this art to determine when to change the grit media to finer media without having to remove the sample from the chuck and perform a visual inspection of the sample. 
     These and other needs are met by the embodiments of the present invention, which provide an arrangement for grinding a metallographic sample comprising a metallographic sample, containing an area of interest, with first and second opposing major sides. An imaging arrangement is positioned so as to generate images of the first major side of the semiconductor device sample while the sample is undergoing grinding. A grinding wheel is provided for grinding a surface of the sample. 
     The earlier stated needs are also met by another embodiment of the instant invention which provides a method of real-time monitoring of the grinding of a metallographic sample comprising: providing a metallographic sample, containing an area of interest, with first and second opposing major sides. The sample is positioned so that a surface approximately normal to the opposing major sides can be ground. An imaging arrangement is positioned to image the first side of the sample while the sample is being ground. A side of the sample approximately normal to the opposing major sides undergoes grinding to approach the area of interest in the semiconductor device. The first side of the sample is imaged while the sample is undergoing grinding to monitor grinding progress. 
     The earlier stated needs are further met by another embodiment of the instant invention which provides an apparatus for monitoring the grinding of a metallographic sample comprising an imaging arrangement mounted on one surface of a substantially transparent substrate. The imaging arrangement comprises a lens and video camera located along a common optical path with the substantially transparent substrate. 
     The present invention provides real-time monitoring of the grinding of a metallographic sample by imaging the sample being ground. The imaging arrangement includes a video camera for imaging the sample. In certain embodiments, the substantially transparent substrate provides support for both the imaging arrangement and the sample, and allows the video camera to be positioned away from the grinding area. To prevent damage to the camera, fiber optic tapers, and fiber optic lines comprising fiber optic tubes or cables can be used to further remove the video camera from the grinding area. 
     To improve image resolution, an artificial light source is used in certain embodiments to illuminate the sample being ground. The light source can either be located in the optical path of the imaging arrangement or it can be a remote light source. The video camera records the image and can either send the output to a video monitor for real-time display or transmit the data to a computer, which captures the image and stores the image data. 
     Some advantages of the instant invention include the ability to perform real-time monitoring of the grinding process. The sample being ground does not need to be removed and visually inspected to determine how far grinding has progressed. The apparatus and method of the present invention provide a more efficient inspection process. Use of real-time monitoring prevents over-grinding of the sample and the resulting loss of the area of interest. Inspection of metallographic samples is a labor intensive, time-consuming process. The present invention is more efficient because the grinding process is not interrupted to check the sample to see how far grinding has progressed and sample loss because of over-grinding is eliminated. 
     The foregoing and other features, aspects, and advantages of the present invention will become apparent from the following detailed description of the present invention when taken in conjunction with accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts an embodiment of the present invention where the camera is remote from the grinding area and sample illumination is directed through the optical path of the imaging arrangement. 
     FIG. 2 depicts an embodiment of the present invention where the light source is remote from the optical path of the imaging arrangement. 
     FIG. 3 depicts an embodiment of the present invention where the camera is attached to a prism. 
     FIG. 4 depicts an embodiment where a dual prism support is provided. 
     FIG. 5 depicts an embodiment of the present invention where LEDs are used to illuminate the sample and the camera is supported by a chuck. 
     FIG. 6 depicts an embodiment of the present invention in which the camera is supported by a chuck and a light ring is used to illuminate the sample. 
     FIG. 7 depicts the sample mounted on a stub. 
     FIG. 8 depicts an embodiment in which the sample and imaging arrangement are attached to a parallelepiped substrate. 
     FIG. 9 depicts an embodiment in which the camera is positioned to image the sample across an open space. 
     FIG. 10 depicts a top view of an embodiment where the camera is positioned to image the sample across an open space. 
     FIG. 11 depicts an embodiment in which a fiber optic taper is positioned across an open space from the sample. 
     FIGS.  12 A and  12 B—depicts the effect that a dual prism has on light refraction from an outside light source. 
     FIGS.  13 A and  13 B—depicts a detailed view of an embodiment of the invention and the image as recorded by the camera. 
     FIGS.  14 A- 14 F—depicts the progression of the grinding process and the corresponding images as recorded by the camera. 
     FIGS.  15 A- 15 C—depicts the effect of different prism/grinding wheel angles, and prism/sample angles. 
     FIG.  16 —depicts a clamped sample cast in resin. 
     FIG.  17 —depicts a clamped sample and prism cast in resin. 
    
    
     DETAILED DESCRIPTION 
     The present invention allows real-time monitoring of metallographic sample grinding. This invention allows the progress of the grinding process to be monitored without having to remove the sample from the grinding assembly and visually inspect the sample under a microscope. This is accomplished by positioning the sample, such as a cross-section of a semiconductor device, in proximity to a grinding surface, such as a grinding wheel. A camera or other optical device is positioned to image a first major side of the sample. An image of the sample is recorded while it is undergoing grinding. The image is displayed on a video monitor or stored in a computer. When an area of interest of the metallographic sample is reached the sample can be removed from the grinding assembly for more detailed inspection. The detailed inspection can include visual, microscopic, or x-ray radiography inspection of the sample. 
     FIG. 1 shows one embodiment of the present invention. A cross-sectioned metallographic sample  12  is obtained from a larger sample, such as a semiconductor device (not shown). The sample section contains an area of interest  10 , which is to be visually inspected. The sample  12  can be cut from the semiconductor device using conventional means, such as a diamond impregnated blade or a wire saw. 
     The sample  12  is securely mounted on a stub  29  using an adhesive  33  such as two-sided tape or a thermal adhesive, as shown in FIG.  7 . The stub  29 , in turn, is held by a chuck  18 . The sample  12  is mounted so that the portion of the sample  12  containing the area of interest  10  extends beyond the edge  21  of chuck  18 . Stub  29  typically comprises either aluminum or stainless steel and typically sample  12  rises about 3 millimeters above the stub surface  35 . 
     The sample  12  can also be securely mounted in a resin, such as an epoxy resin, rather than mounting on a stub. FIG. 16 shows sample  12 , cast in a resin  112  and positioned by a clamp  114  against grinding wheel  20 . Alternatively, the entire sample  12 /prism  14  combination can be cast in resin  112  as shown in FIG.  17 . 
     The sample section contains two major opposing sides. The second side  13  is attached to the stub, while the first side  11  is adhesively attached to one surface  15  of a substantially transparent substrate  14  using an optical adhesive  16 . In this embodiment, the substantially transparent substrate is a right angle prism  14 . Optical adhesives  16  are commercially available adhesives such as Norland Optical Adhesives available from Edmund Scientific, Barrington, N.J. Optical adhesives are either cured with ultraviolet light or are thermally cured. 
     The grinding arrangement  100  of the embodiment of FIG. 1 further includes an imaging arrangement  102  which is adhesively mounted using optical adhesive  16  to second surface  17  of prism  14 . Prism surface  17  is normal to first surface  15  upon which the sample  12  is mounted. The hypotenuse  19  of the prism forms a reflective surface for reflecting illuminating light onto the first side of the sample  11  and for reflecting the image of sample side  11  to the imaging arrangement  102 . 
     The sample is ground with grinding wheel  20 . Initially, the grinding process starts with a relatively course grit grinding media. As grinding progresses and the area of interest  10  is approached the grinding media is successively changed to finer grit grinding media. The final stage of grinding is polishing the sample surface. As used herein, the term grinding includes polishing. Typically, SiC and diamond media is used for rough grinding at the start of the grinding process. For grinding intermediate size features ranging from 30 microns to 0.1 microns, diamond media is used. Diamond grinding media includes diamond paste suspended diamonds, and diamond impregnated sheets. The final stage of grinding, polishing, may be performed with Al 2 O 3  (alumina) grit. Alumina is used for polishing fine surface features such as those ranging between 1 μm to 0.05 μm. 
     The substantially transparent substrate  14  comprises either a glass or a polymer composition. The polymer composition should be a clear polymer with suitable optical properties. Suitable polymer compositions include acrylic resins and polycarbonate resins. 
     Prism  14  is commercially available from Edmund Scientific, Barrington, N.J. Prism  14  can either be uncoated or the hypotenuse  19  is aluminized overcoated with inconel and black paint to improve the reflective properties of the prism  14 . Although the prism does not need to be metallized, an improved image can be provided by a metallized surface. The prism reflective surface is at an approximate 45° angle to the grinding wheel surface. Prism  14  undergoes grinding at the same time as sample  12 . It is desirable to match the grinding rate of prism  14  with the grinding rate of the material of sample  12 . Prism  14  is disposable after grinding a sample, so that a subsequent sample to be ground would then be adhered to a new prism. 
     In addition to prisms, the substantially transparent substrate is alternatively, substantially a parallelepiped. FIG. 8 illustrates grinding arrangement  800 , wherein the sample  12  is attached to a first side  86  of a substantially transparent parallelepiped  84 . A fiber optic taper  22  of imaging arrangement  802  is attached to a second side  88  of the parallelepiped  84 , directly across from the sample. The image of the first side  11  of sample  12  passes directly through the parallelepiped substrate  84  to the fiber optic taper  22 . To prevent damage to the fiber optic taper  22  in this arrangement  800 , the grinding wheel  20  grinds only a portion of the lower surface  94  of the substrate  84 , not the entire lower surface  94 . 
     After reflecting, off of surface  19 , in FIG. 1, the image of sample  12  is directed towards fiber optic taper  22 . Fiber optic taper  22  is adhesively attached by means of an optical adhesive  16  to prism surface  17 . Fiber optic taper  22  takes a square image from the prism and optically tapers it down to a circular cross-section so that the image can be sent through fiber optic line  24 , comprising a fiber optic tube or cable, to fiber optic taper  26  that converts the circular cross-section image back to a square image format, which can be captured by a camera  36 . Camera  36  can be a charge coupled device (CCD) or an optical camera. The use of the fiber optic taper  22 ,  26  and the fiber optic line  24  allows camera  36  to be located at a remote location from the grinding wheel  20 . This eliminates difficulties associated with rigidly mounting camera  36 . Because the camera  36  is remote from the grinding area the risk of damaging the camera is reduced, and a better, high resolution camera  36  can be used to image the sample  12  instead of a cheaper, lower resolution camera. Fiber optic tapers are commercially available from Edmund Scientific. 
     In other embodiments, the risk of damaging the camera during the grinding process is reduced by positioning the camera  72  across an open space  96  from the sample  12  and grinding wheel  20 . FIGS. 9 and 10 show arrangement  900 , wherein imaging arrangement  902  comprises a camera  72  mounted in chuck  80  directly across an open space  96  from sample  12 . The image is not obscured by dust generation during grinding, because the lubricants typically applied to the sample during grinding inhibit dust formation. Conventional grinding lubricants include water, diamond grinding paste, and oil. 
     A fiber optic taper  22  and lens  28  can also be positioned directly across an open space  96  from the first major side of the sample to be ground, as shown by arrangement  1100 , illustrated in FIG.  11 . The fiber optic taper  22  and lens  28  can be positioned using conventional supports (not shown), such as clamps. 
     As shown in FIG. 1, the image from fiber optic taper  26  is magnified by lens  28 , and then the image passes through a filter  30 . Filter  30 , can be any filter for enhancing the image, including a polarizing filter  30 , which reduces reflections and glare. 
     Beam splitter  32  permits the image to pass through to camera  36  while allowing light from light source  50  to be reflected through the imaging arrangement  102  optical path to the prism  14  and onto the sample  12 . The sample  12  can be illuminated with either white light or infrared light. 
     Camera  36  outputs the image data through the video output  52  to the video monitor  40 . The grinding can be viewed in real-time on the video monitor  40 . In addition, the video output can be further transmitted through a video output  54  to a computer  44  which captures and stores the video images for later viewing. The video image, if desired, is transmitted to a plurality of computer workstations through a network connection  46 . Power lines,  34 ,  38 ,  42 , and  48  provide power to the camera, video monitor, computer, and light source. The light source  50 , video monitor  40 , and computer  44  are all conventional, commercially available devices. The camera  36  can be a visible light camera or an infrared camera depending on the source of illumination used. A high-resolution color DSP (digital signal processor) microboard camera, available from Edmund Scientific, is suitable for use in this invention. 
     FIG. 3 depicts another embodiment of the present invention. Throughout the figures like reference numbers depict like features. The grinding arrangement  300 , comprising optical arrangement  302 , is illustrated in FIG.  3 . In this embodiment the lens  28 , filter  30 , and camera  36  are mounted directly on the second surface  17  of prism  14 . Light source  68  provides light through fiber optic line  70  to either beam splitter or light emitter  37 . The light then passes through a filter  30  and lens  28  and is reflected off the hypotenuse  19  of prism  14  to illuminate the first side  11  of sample  12 . An image of sample  12  is then reflected back up through magnifying lens  28  and filter  30  to camera  36 . Because camera  36  is attached to prism  14  and is adjacent to the grinding area, a small, relatively inexpensive camera  36  would be suitable for this configuration. 
     The grinding arrangement  400  depicted in FIG. 4 comprises a dual prism  23  sample support and optical arrangement  402 . In this embodiment a second prism  25  is joined with prism  14 . The prisms are joined together at their respective hypotenuses  19  to form a parallelepiped. In this embodiment, the dual prism presents a constant surface area grinding surface  27 . In the other embodiments, which have one prism, the surface area of the prism/sample increases throughout the grinding process. The constant surface area grinding surface  27  provides for even grinding throughout the grinding process. The two prisms can be attached to each other using the same optical adhesives used to attach a sample to the prism. 
     An additional improvement is also featured in FIG.  4 . First prism  14  has a truncated edge  31  adjacent to the grinding wheel. This truncated edge  31  provides a space between the first side  11  of sample  12  and the reflective surface  19  of the first prism  14  and allows the image at the end of the sample  12  to be reflected nearer to the center of the field of camera  36 . This space ensures that the camera records the entire first side  11  of sample  12 , and that the end of sample  12  is not outside of the camera&#39;s  36  field. 
     The dual prism arrangement is also useful when an external light source is used, as shown in FIG.  2 . In grinding arrangement  200 , light source  60  is remote from the optical path of the imaging arrangement  202 . Light is transmitted from light source  50  through a fiber optic line  62 , comprising a fiber optic tube or cable, to a variable angle or variable rotation lens  64 . Lens  64  can be adjusted to provide light at an optimal location or angle. In this embodiment the illuminating light is not transmitted through the optical path and there is no need for a beam splitter. The dual prism  23  prevents the light rays  92  emanating from the lens  64  from being refracted by the hypotenuse  19  of the first prism  14  which could lead to shadows at the position of grinding, as shown in FIGS. 12A and 12B. 
     In alternative embodiments, the camera  72  is held by chuck  80  as shown in FIG.  5 . Grinding arrangement  500  utilizes light emitting diodes (LEDs)  76  to illuminate sample  12 . In this embodiment, the imaging arrangement  502  comprises camera  72 , LEDs  76 , and magnifying lens  28 . The LEDs are positioned on the outside of magnifying lens  28 . The LEDs  76  are powered by power line  78  and comprise either visible or infrared LEDs. Camera  72  is either a visible camera or an infrared camera depending on the choice of LEDs  76 . This embodiment provides a compact grinding assembly. 
     In the embodiment illustrated by FIG. 6, the grinding arrangement  600  comprises imaging arrangement  602  wherein the camera  36  is mounted on chuck  80 . In this embodiment the light is supplied to the sample  12  by light projector  82 . Light projector  82  is an annular shaped light ring which projects light around the outside of the filter  30  and magnifying lens  28 . The reflected image from the sample  12  is directed up through magnifying lens  28  and filter  30  through the center of the light ring  82 . 
     FIGS. 13A and 13B illustrate an example of an embodiment of the invention and an image recorded by the camera. FIG. 13A illustrates a detailed view of an embodiment of the invention. FIG. 13B illustrates the image recorded by the camera of the arrangement in FIG.  13 A. Detail A is a view of the face  11  of sample  12 . All points of the face  11  are in focus, as each ray of light travels the same distance from the sample face  11 , to the reflective surface  19 , and up to the camera, through lens  28 . B is the edge of the sample  12  being ground. C is the edge of the grinding surface and detail D is the feature of interest. 
     The progression of grinding is illustrated in FIGS. 14A-14F, which shows a detailed view of the grinding process as feature of interest, detail D, is approached. FIGS. 14B,  14 D, and  14 F are the images recorded by the camera at grinding stages  14 A,  14 C, and  14 E, respectively. Grinding is completed when B reaches D. As illustrated, the invention allows for continuous, real-time monitoring of the grinding process. 
     Generally, an isosceles right angle prism is desired in the practice of the invention. As shown in FIG. 15A, the 45° angle of the prism  14  allows the entire face  11  of the sample  12  to be in focus. The lengths of optical paths ABC, DEF, and GHJ are all the same length, so the sample always stays in focus. There is no need to refocus throughout the grinding process. 
     One of the difficulties encountered with an isosceles right angle prism is that the sample surface  11  near the grinding wheel may become shadowed because of uneveness in the prism surface  31 . For example, if the prism  14  grinds at a faster rate than the sample  12 , dishing occurs on the prism surface  31 . Surface imperfections, such as dishing, distort the sample image as it passes through the prism  14 . 
     To overcome the limitations of surface imperfections in the prism, a prism  14  can be used that makes an angle with the sample  12  of less than 45°, so that the angle the reflective surface  19  makes with the grinding wheel  20  is greater than 45°. As shown in FIG. 15B, the optical path for point J, at the grinding wheel  20 /sample  12  interface, angles upward toward point H, away from any surface imperfections at the prism  14 /grinding wheel  20  interface. However, the use of a prism with a reflective surface 19 /grinding wheel  20  angle of greater than 45° has its own shortcomings. Optical paths ABC, DEF, and GHJ are all different lengths, thus the entire sample surface  11  is not in focus at one time. Refocusing of the optical arrangement is required during grinding. 
     In order to obtain a clear image of the sample  12 /grinding wheel  20  interface and to keep the entire sample surface  11  in focus at the same time, an isosceles right angle prism is used and the entire sample/prism/imaging arrangement is tilted with respect to the grinding wheel  20 , as shown in FIG.  15 C. The imaging arrangement remains approximately perpendicular to the sample surface  11 , but is tilted with respect to the grinding wheel  20 . Detail J, at the grinding wheel  20 /sample  12  interface is not obscured by surface imperfections in either the grinding wheel  20  or prism  14 . In this arrangement the entire sample surface  12  is in focus at the camera, as optical paths ABC, DEF, and GHJ are all the same length. 
     Several embodiments in the invention have been described in the present disclosure. It is understood by one of ordinary skill in the art that other embodiments of the invention are within the scope of the inventive concept as expressed herein. For example the imaging arrangement, in alternative embodiments, is removably adhered to the prism surface. While the prisms are disposable, the lenses, filters, and cameras are reusable. 
     In order to ensure that the camera and sample are rigidly held in place, in addition to supporting the sample with a chuck, in another embodiment the imaging arrangement, comprising the camera, lens, and filter, are also supported by a support fixture. In other embodiments, a fixture is attached to the second surface of the prism, which contains an opening for the camera lens to fit in. The camera lens can be held in place by a variety of fastening means, including setscrews. 
     The arrangement for grinding a metallographic sample, the process of real-time monitoring of grinding metallographic samples, and the apparatus for monitoring the grinding of a metallographic sample of the present invention provide an improvement over the prior art grinding apparatuses and methods. Real-time monitoring of the grinding process avoids the time consuming prior art steps of removing the sample from the grinding apparatus and visual inspection with a microscope to determine the state of grinding. The approach of the grinding wheel to the area of interest can be continuously monitored on a video display and the grinding halted once the area of interest is reached. The present invention also prevents over-grinding of the sample. Thus, valuable information about possible failure mechanisms of the semiconductor device is not lost. In addition, technician time is not wasted having to redo quality control inspection tests. 
     The embodiments illustrated in the instant disclosure are for illustrative purposes only and should not be construed to limit the scope of the claims. As is clear to one of ordinary skill in this art, the instant disclosure encompasses a wide variety of embodiments not specifically illustrated herein.