Patent Number: 
Section: description

Reference is now made in detail to a specific embodiment of the present invention which illustrates the best mode presently contemplated by the inventors for practicing the invention. Initially, reference is made to the article xe2x80x9cFilter Optimization for X-ray Inspection of Surface-Mounted ICsxe2x80x9d, pages 377-379, by Richard C. Blish II, Susan Xi, and David Ledtonen, published April, 2002 at IEEE 40th Annual International Reliability Physics Symposium, Dallas, Texas, which material is herein incorporated by reference. FIG. 6 is similar to FIG. 1, but further including a filter 100 as will now be described As shown in FIG. 6 (rotated 90 degrees clockwise from a conventional orientation of elements therein), a semiconductor device 120 is placed on an inspection tray 122 of for example polyimide material. This typical semiconductor device 120 includes a silicon body 124 having a protective coating 125 of molding compound (in FIG. 6 shown lying on the tray 122), the silicon body 124 having an active region 124A and an inactive region 124B secured to a substrate 126 by a silver-organic material adhesive 128 (wire bonds connecting silicon body 124 and substrate 126 not shown). The substrate 126 includes organic portions (for example dielectric layers 130, 132) and patterned copper layers (one shown at 134), which copper layer 134 communicates with the active region 124A of the silicon body 124 (approximately 1 xcexcm in thickness and oriented most adjacent the tray 122) and solder balls 136 (commonly lead/tin but which may consist of a lead-free composition, usually tin-rich) which connect to a layer of copper traces 138 on an organic material (for example polyimide, epoxy, polyethylene, or glass fiber) printed circuit board 140. As previously described, it will be understood that the particular configuration of the semiconductor device 120 is for purposes of illustration, and that such device 120 may be configured in a wide variety of ways, including for example a number of levels of copper layers 134 and dielectric layers 130, 132, with appropriate vias connecting the copper layers. As in the previous example, the following typical thicknesses are given: In the present embodiment, a filter 100 in the form of a zinc foil is positioned between a source of x-rays 142 and the tray 122, such plate 100 having a thickness of for example 300 xcexcm. FIG. 7 is a graph showing x-ray absorption coefficient vs. x-ray energy level for silicon, copper, tin and lead (similar to FIG. 2), but also showing x-ray absorption coefficient vs. x-ray energy level for zinc. Zinc has an atomic number of 30, one greater than the atomic number of copper, which results in the K edge of zinc lying to the right of the K edge of copper, i.e., at a higher energy level (FIG. 7). The importance of this feature will be described further on. During x-ray inspection, x-rays including a wide range of energy levels are provided from the source 142 through the tray 122 and into and through the semiconductor device 120. FIGS. 8 and 9 are graphical representation of the structure of FIG. 6 for x-ray energy levels of 3 KeV and 9 KeV respectively. With reference to FIG. 8, for x-ray energy at the 3 KeV level, i.e., that x-ray energy level wherein silicon has a high coefficient absorption, the zinc filter 100 causes the intensity of the x-ray bean to drop significantly prior to passing through the tray 122 and reaching the silicon body 124. Thus, the intensity of the x-ray beam presented to the silicon body 124 is substantially lower th in the prior art, and the change in intensity of the x-ray beam through the silicon body 124, corresponding to the absorption of x-ray energy by the silicon body 124, is substantially lower than in the prior art, due to the inclusion of the zinc filer 100 (compare FIG. 3 and 8). With this low level of absorption of x-ray energy by the silicon body 124 as compared to the prior art, the problem of ionization of the silicon body 124 in the active region 124A, as described above, is overcome. Meanwhile, the thin adhesive 128 and dielectric layer 130, having low absorption, allow significant x-ray intensity to reach the high absorption copper layer 134. After a high degree of absorption by the copper layer 134, x-ray energy passes through the dielectric layer 132 (low absorption), solder balls 136 (high absorption), copper layer 138 (high absorption) and printed circuit board 140 (low absorption), so that the copper layers and solder balls are properly imaged as a radiograph at the image detector 144 at the 3 KeV x-ray energy level. FIG. 9 is a graphical representation similar to that of FIG. 8, but for the 9 KeV x-ray energy level. At this energy level, absorption by the zinc filter 100 is lower than as shown in FIG. 8 but is still considerable (see absorption coefficients of zinc for various x-ray energy levels in FIG. 7). Meanwhile, the silicon body 124 is substantially less absorbing at this energy level (again see FIG. 7), so that the problem of ionization of silicon is again avoided. Meanwhile, because the atomic number of the zinc filter 100 is greater than the atomic number of copper (in this case one atomic number greater), resulting in the offset in the K lines of copper and zinc in the region of 9 KeV of x-ray energy as shown, the copper is substantially more energy absorbing than the zinc at this energy level, so that the zinc filter 100 transmits a significant amount of x-ray energy to the copper (which at this energy level has significant absorption), providing for highly effective imaging of the copper layers. An important factor in being able to properly image a layer (copper in the examples given) is the selection of filter material with an atomic number greater than a specified layer to be imaged. This determines the offset between the K lines of the layer to be imaged and filter material (greater difference in these numbers determining greater offset between these K lines, and vice versa), which provides a xe2x80x9cwindowxe2x80x9d between these K lines at approximately the 9 KeV energy level, so that proper imaging at this energy level is achieved. It is to be noted that this is achieved along with proper shielding of the silicon body by the filter material as described above. It will be understood that this approach is not limited to copper and zinc, but is highly useful in imaging a wide variety of materials, wherein the material of the filter has a slightly higher atomic number than the material to be imaged. For example, in the situation where copper is to 4he be imaged, filter 100 may with advantage be made up of or include a material having an atomic number ranging from 30 through 35 inclusive, i.e. zinc (atomic number 30), gallium (atomic number 31), germanium (atomic number 32), arsenic (atomic number 33), selenium (atomic number 34), or bromine (atomic number 35). While this approach provides proper imaging of the copper, which is normally difficult or impossible to achieve, imaging of the thick solder balls, which are an import target in the imaging process, is easily accomplished in the normal approach described above, i.e., regardless of the presence or absence of a filter of a particular atomic number. The foregoing description of the embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Other modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill of the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.