Abstract:
A method of preparing a monolithic structure for scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) and a sample produced by way of the method. In one embodiment, the method includes: (1) aiming a focused ion beam at a location behind or beneath an area of interest in the monolithic structure and (2) employing the focused ion beam to remove at least a portion of an interaction volume of material beneath the area of interest. The area of interest preferably remains substantially intact for the spectroscopy.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to analysis of monolithic structures and, more specifically, to a scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) sample preparation method and a monolithic sample produced by way of the method. 
     BACKGROUND OF THE INVENTION 
     In semiconductor processing today it is often necessary to spectroscopically examine portions of a semiconductor die to determine the results of new or conventional processes. The examination may be to confirm the results of an experimental process, or even to determine the nature of a particular failure or defect in a semiconductor device. Of course, because of the nature of integrated circuits, the examination must often be performed on samples cut from the die in question. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) is frequently used in the determination of the composition of target material in a feature of a semiconductor die. In SEM/EDS, X-rays generated by the primary electron beam&#39;s interaction with the target assist in determining the composition of the target. 
     Referring initially to FIG. 1, illustrated is a cross-sectional view of a conventional SEM sample symbolically showing effects of the SEM process. A primary electron beam  100  with width  180  is directed to a sample  110  having a thickness  111 . The sample  110  may be cut from any orientation within a material of interest. In this embodiment, the beam  100  has a width  180  and a tear shaped interaction volume  120 , with depth  160 . For example, at a given beam energy and in a given material, a beam width  180  of 10 nm can produce an interaction volume with a maximum width  170  of 1 μm. The beam  100  interacts with essentially all of the material in the interaction volume  120 , producing secondary electrons  130 , backscattered electrons  140 , and X-rays  150 . The specific X-rays present in the spectra are traceable by EDS to particular elements present at the site of the SEM examination. 
     As device sizes are made ever smaller, a defect that was insignificant in the past becomes quite significant. However, as the semiconductor features and defects become smaller, it has become increasingly difficult to obtain the lateral resolution needed from EDS for compositional identification of small defects and thin layers. With a decrease in feature size, the feature becomes a smaller percentage of the target, thereby reducing the X-ray emanations from the feature. It is well known that the interaction volume  120  depth  160  and width  170  are dependent upon the accelerating energy of the primary electron beam  100 . However, because the beam  100  interacts with the entire interaction volume  120 , precise data on the nature of a feature of interest  160  at the examination site may be unclear. In order to reduce the interaction volume, the primary beam accelerating voltage has been reduced to account for the reduced feature size. However, several elements commonly used in semiconductor fabrication have overlapping peaks at lower beam energies, and the ability to distinguish between the elements is therefore lost at these low beam energies. At higher beam voltages more peaks in the EDS spectra allow for better discrimination between the elements present. Therefore, reducing beam energy does not effectively improve element discrimination. Of course, tool manufacturers have worked to improve the sensitivity of their tools. Unfortunately, these efforts have met with limited success, and the available sensitivity is still not sufficient for current and projected needs. 
     Referring now to FIG. 2A, illustrated are EDS spectra of a conventional semiconductor SEM sample taken with an incident electron beam of 5 keV. As can be seen, the results make it difficult to resolve between oxygen  210  and titanium  220 . There also appears to be copper  230  present, as well as aluminum  240 , silicon  250  and phosphorous  260 . Referring now to FIG. 2B, illustrated are EDS spectra of the same area as shown in FIG. 2A with an incident electron beam of 10 keV. Comparing FIG. 2B to FIG. 2A, it is clear that oxygen  211 , titanium  221 , aluminum  241 , silicon  251  and phosphorus  261  are all present. It is also now evident that copper is not present. It is readily apparent that the definition of the peaks  211 ,  221 ,  241 ,  251 ,  261  is much better at the higher (10 keV) beam energy. 
     Referring now to FIG. 3A, illustrated are EDS spectra taken at 5 keV of a different semiconductor SEM sample. At this voltage, titanium  310  and possibly silicon  320  appear to be present. However, one who is skilled in the art will readily observe because of the relative magnitudes of the peaks at X-ray energy levels of 0.6 keV and above that it is extremely difficult to discern the presence of any particular element. Referring now to FIG. 3B, illustrated are EDS spectra taken at 10 keV of the same area as shown in FIG.  3 A. One who is skilled in the art will readily appreciate that the spectra of FIG. 3B clearly allows one to differentiate peaks associated with titanium  311 , silicon  321 , and tungsten  331 . Comparing FIG. 3B to FIG. 3A, one can readily understand that the presence of tungsten is simply not recognizable from the spectra of FIG.  3 A. Thus higher beam energies make identification of elements much easier. 
     Accordingly, what is needed in the art is a sample preparation method that minimizes the interaction volume while retaining the feature of interest. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, the present invention provides a method of preparing a monolithic structure for scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) and a sample produced by way of the method. In one embodiment, the method includes: (1) aiming a focused ion beam at any location beneath an area of interest of any size or depth in the monolithic structure and (2) employing the focused ion beam to remove at least a portion of an interaction volume of material beneath the area of interest. The area of interest preferably remains substantially intact for the subsequent spectroscopy. 
     The present invention therefore introduces the broad concept of employing a focused ion beam to remove at least some of the interaction volume beneath an area of interest that would otherwise generate unwanted background noise during subsequent SEM/EDS analysis. For purposes of the present invention, a “monolithic structure” is defined as a structure formed in or on a body of material. A monolithic structure may be a semiconductor wafer (such as a silicon or gallium arsenide wafer), but can be composed of any material. For purposes of the present invention, an “area of interest” is defined as any portion of the monolithic structure that is to be the subject of analysis. 
     In one embodiment of the present invention, the area of interest is a portion of a surface of the monolithic structure. Alternatively, the area of interest could be located above or below the surface of the monolithic structure. 
     A preferred embodiment of the present invention comprises employing the focused ion beam to remove an entirety of the interaction volume. Of course, removal of only a portion of the interaction volume may be advantageous in some applications. 
     In one embodiment of the present invention, the employing comprises employing the focused ion beam to remove at least the portion of the interaction volume such that the area of interest becomes membranous. The membranous area of interest may, but need not, have a thickness comparable to samples employed in conventional transmission electron microscope (TEM)/EDS analysis. 
     In one embodiment of the present invention, the monolithic structure comprises a silicon wafer. Alternatively, the monolithic structure could be a gallium arsenide wafer or a wafer or body of another suitable composition. 
     In one embodiment of the present invention, the monolithic structure comprises at least one layer located on a silicon substrate. Alternatively, the monolithic structure could comprise a layer formed within the silicon substrate, or the substrate itself. 
     In one embodiment of the present invention, the method further includes performing the energy dispersive spectroscopy on the area of interest at an energy greater than 5 keV. In an embodiment to be illustrated and described, the energy dispersive spectroscopy is performed at an energy of about 10 keV. Those skilled in the pertinent art will understand, however, that the present invention is not limited to a particular energy level or range thereof. 
     The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a cross-sectional view of a conventional SEM sample symbolically showing effects of the SEM process; 
     FIG. 2A illustrates EDS spectra of a conventional semiconductor SEM sample taken with an incident electron beam of 5 keV; 
     FIG. 2B illustrates EDS spectra of the same area as shown in FIG. 2A with an incident electron beam of 10 keV; 
     FIG. 3A illustrates EDS spectra taken at 5 keV of a different semiconductor SEM sample; 
     FIG. 3B illustrates EDS spectra taken at 10 keV of the same area as shown in FIG. 3A; 
     FIG. 4 illustrates an isometric view of a semiconductor die having a feature of interest; 
     FIG. 5 illustrates a sectional view of the SEM sample of FIG. 4 symbolically showing the effects of the SEM/EDS process; 
     FIG. 6A illustrates an isometric view of a semiconductor die with an embedded feature of interest to be sampled by an alternative embodiment of the present invention; 
     FIG. 6B illustrates the semiconductor die of FIG. 6A with a stair step trench cut in the top surface; 
     FIG. 6C illustrates the semiconductor die of FIG. 6B with a small, parallelepiped-shaped trench cut in the die to form a rear face of the feature; 
     FIG. 6D illustrates the semiconductor die of FIG. 6C with the feature of interest on what is now identifiable as a sample; 
     FIG. 6E illustrates the semiconductor die of FIG. 6D tilted to permit the FIB to cut part way up a second side; and 
     FIG. 6F illustrates the semiconductor die of FIG. 6E returned to an erect position for further thinning of the sample and cutting through the second side. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 4, illustrated is an isometric view of a semiconductor die  400  having a feature of interest  410 . In the present case, the feature of interest  410  is located on a surface  411  of the die  400 . The die  400  may be a monolithic structure as is common in semiconductor manufacturing and may be at the semiconductor wafer stage, or any subsequent stage wherein one or more layers of material has been deposited on the substrate. The die  400  has been initially prepared by cutting so that the feature of interest  410  is proximate an edge  412  of the die  400 . Of course, if the feature of interest  410  is initially located within the die  400 , it may be necessary to cut the die  400  so as to expose the feature of interest  410 . One who is skilled in the art is familiar with methods for cutting and polishing semiconductor dies to a prescribed depth. 
     Using a focused ion beam (FIB)  425 , a volume  430  of material (removal volume) is cut from the die  400  beneath the feature of interest  410  creating a cavity  431  and forming a sample  440 . The use of the term “beneath” is relative to the position of the feature of interest  410  when placed for examination by SEM/EDS. The removal volume  430  is all of but the surface of what would have constituted an interaction volume  120  as in FIG. 1 if it were still present when the feature  410  is examined with SEM/EDS. By eliminating the removal volume  430 , a primary electron beam  450  cannot interact with the material removed. A thickness  441  of the sample  440  is such that it may be considered to be membranous, or electron transparent. The feature  410  may, depending upon the tool capabilities, be examined as it remains attached to the die  400 . Alternatively, the sample  440  may be cut free from the die  400  and mounted for examination. Alternatively, the FIB beam  425  and the primary electron beam  450  can be interchanged so the material is removed from behind rather than beneath the feature of interest. 
     Referring now to FIG. 5, illustrated is a sectional view of the SEM sample of FIG. 4 symbolically showing the effects of the SEM/EDS process. The primary electron beam  450  of about 10 nm in width and greater than 5 keV is directed to the sample  440  having thickness  441 . As can be seen, the beam  450  has an interaction volume  420  that is significantly less than (perhaps only 5 percent of) the SEM interaction volume  120  of FIG.  1 . As a result, of the smaller interaction volume  420 , secondary emission of electrons, backscattered electrons and X-rays  455  are essentially eliminated except for those occurring in the feature of interest  410 . Because there is no material below the sample  440 , higher beam energies, e.g., ≧10 keV, may be used. Therefore, better test results, including improved lateral resolution, are available using these thinned samples and higher beam energies. As can be seen by comparing FIG. 1 to FIG. 5, the difference in sample thicknesses ( 111  vs.  441 ) between SEM and thinned samples is significant. 
     Referring now to FIG. 6A, illustrated is an isometric view of a semiconductor die  600  with an embedded feature of interest  610  to be sampled by an alternative embodiment of the present invention. In this instance, the feature of interest  610  is embedded in the die  600  rather than on an exposed surface  601 . The semiconductor die  600  is initially prepared by having a layer of metal  620  deposited on the surface  601  of the die  600  to protect the surface  601 . Using a FIB  605 , a stair step or sloped trench  630  is cut in the surface  601  up to a front face  635  of the feature  610  as is shown in FIG.  6 B. FIG. 6C shows the semiconductor die  600  of FIG. 6B with a small, parallelepiped-shaped trench  640  cut in the die  600  to form a rear face  645  of the feature  610 . FIG. 6D shows the feature  610  on what is now identifiable as a sample  650  having been further thinned with the FIB  605  and one side  651  cut through from the top surface  601 . In FIG. 6E, the die  600  is tilted to permit the FIB  605  to undercut the sample  650  at its base  652 , and part way up a second side  653 . In FIG. 6F, the die  600  has been returned to its erect position for further thinning of the sample  650  and cutting through the second side  653 . The sample  650  may then be removed by a micromanipulator (not shown) and mounted for examination. Examination of the sample  650  and feature  610  is by conventional SEM/EDS methods. However, these FIB lift-out samples may also be used in TEM, AFM, AES and SIMS systems. 
     Therefore, methods have been described for the preparation of samples from a semiconductor die using focused ion beam technology to create electron transparent specimens. The removal of essentially all of the interaction volume below a feature of interest eliminates extraneous X-rays from occurring and clouding EDS examination. The method may be used to prepare semiconductor die samples at any stage of semiconductor manufacturing. 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.