APPARATUS AND METHOD FOR REDUCING DIFFERENTIAL SPUTTER RATES

Abstract: The invention relates to reducing Focused Ion Beam (FIB) differential sputter rates in crystalline materials. In integrated circuits, copper-based interconnects are beginning to replace aluminum-based interconnects. Unfortunately, FIB sputtering of copper is more difficult than that for known aluminum alloys, because of large changes in milling rates based upon grain orientation. In order to meet the need for an improved milling technique that will lead to more uniform milling of such elements, it is proposed according to the invention to apply a sacrificial layer onto a surface of a crystalline structure followed by the projection of an ion beam onto the applied sacrificial layer, preferably in a repetitive process until the crystalline structure is removed or reduced to a predetermined level, thus increasing uniformity of the removal of the atoms from the surface of the crystalline structure during the projection of the ion beam.

Description: Apparatus and method for reducing differential sputter rates FIELD OF THE INVENTION This invention relates generally to sputter rates in crystalline materials and, in particular, to apparatus and methods used for reducing differential sputter rates in crystalline materials. BACKGROUND OF THE INVENTION Focused Ion Beam (FIB) microscope systems have been produced commercially since the mid 1980's, and are now an integral part of rapidly bringing semiconductor devices to market. FIB systems produce a narrow, focused beam of charged particles, and scan this beam across a specimen in a raster fashion, similar to a cathode ray tube. Unlike the scanning electron microscope, whose charged particles are negatively charged electrons, FIB systems use charged atoms, hereinafter referred to as ions, to produce their beams. In most commercial FIB systems, the ions used are positively charged gallium ions (Ga + ) from liquid metal ion sources, however beams of other ions can be produced. For example, materials such as silicon, indium, cesium or even gases such as argon, krypton or oxygen can be utilized as ion sources. Current instruments such as the 2500 FIB system produced by FEI Company/Micrion of Hillsboro, Oregon can produce a beam of gallium ions as narrow as approximately 5 nm in diameter. One can increase the current of ions in the beam to operate the FIB as an "atomic scale milling machine", selectively removing materials wherever the beam is placed, and at the same time imaging the sample by correlating the known beam position with signals given off as the incident beam interacts with the specimen. One skilled in the art would understand the operation of this well-known procedure. Semiconductor devices such as microprocessors can be made up of millions of transistors, each interconnected by thin metallic lines branching over several levels and isolated electrically from each other by layers of dielectric materials. When a new semiconductor design is first produced in a semiconductor fabrication facility, hereinafter referred to as a "fab", it is typical to find that the design does not operate exactly as expected. It is then necessary for the engineers who designed the device to test their design and "rewire" it to achieve the desired functionality. Due to the complexity of building a semiconductor device in the fab, it typically takes weeks or months to have the re-designed device produced. Further, the changes implemented frequently do not solve the problem or expose a yet further difficulty in the design. Iterating through the process of testing, re- designing and re- fabrication can significantly lengthen the time to market of new semiconductor devices. Over the past decade, techniques have been developed to allow FIB systems to reduce the time required for this procedure of perfecting a design. FIB instruments were first used to "cut" metal lines, typically comprised of alloys of aluminum and/or tungsten, on prototype devices, thus allowing for design verification in simple cases. Further, techniques have been developed using special gas chemistries in the FIB system to permit selective deposition of thin metallic lines to connect two or more conductors, selective removal of dielectric insulators but not metallic interconnects, and selective removal of metal interconnects without removing the dielectric insulators. Techniques have yet further been developed that allow the deposition of insulating materials. Hence, these advances in FIB system technologies now allow the cutting of metal interconnect lines, the insulating of these metal interconnect lines from their surroundings and the re-wiring of the lines to another location. Essentially, these capabilities now permit prototyping and design verification in a matter of days or hours rather than weeks or months as re-fabrication would require. This FIB "rapid prototyping" is frequently referred to as "FIB device modification" or "microsurgery". Due to its speed and usefulness, FIB microsurgery has become crucial to achieving the rapid time-to-market targets required in the competitive semiconductor industry. Until recently, the typical metals used for metallic interconnects were primarily alloys of aluminum and/or tungsten. The above described advances in FIB system techniques for cutting and depositing metal interconnects were specifically designed for these metal alloys and their particular physical characteristics. Polycrystalline aluminum interconnect lines are composed of small, contiguous grains of aluminum. Within each grain, the atoms share a regular array-like order, but the relative position of the arrays of atoms can vary from grain to grain. This alignment of the arrays of atoms is known as the "crystallographic orientation" of a given grain. Differences in crystallographic orientation can cause grains milled with a simple ion beam, hereinafter referred to as "sputtering", to be removed at different rates, depending on the given orientation. For aluminum though, the difference between most of the slowest sputtering orientations compared to the fastest sputtering orientations is not particularly significant and hence is not a key factor in the techniques to cut and/or remove aluminum interconnects. Additionally, for aluminum, chemistries have been developed that selectively attack aluminum, causing grains of any orientation to sputter much more quickly in the presence of the gas than with just the ion beam alone. This process is well-known within the art and is commonly referred to as Gas Assisted Etching (GAE). In one particular well- known technique that can be used with the FEI Company/Micrion 2500 FIB systems, chlorine gas is used to perform GAE of aluminum interconnects, cleanly removing aluminum grains, with little regard to their individual crystallographic orientation. Recently, copper-based interconnects have begun to replace aluminum-based interconnects in state-of-the-art devices due to increased transmission speeds achievable with the use of copper. Unfortunately, FIB sputtering of copper is more difficult than that for aluminum alloys. Firstly, aluminum atoms have a lower atomic mass and less "stopping power" than copper atoms, and simple ion beam milling of the copper atoms is less effective than the equivalent milling of aluminum. Further, a gas chemistry that permits GAE of copper has not yet been successfully developed. And yet further, the relative sputter rate between grains of different crystallographic orientations of copper can differ by a large factor, this factor being approximately 360% in some experimental tests. Some of the difficulties that can occur when attempting to sputter copper interconnects that have different crystallographic orientations will now be described by example with reference to Figure 1 and Figures 2A through 21. Figure 1 illustrates three grains in a typical section of copper interconnect, sections 2, 3, and 4. The grains at each end section 2 and 4, in this example, have similar orientations, while the grain in the centre section 3 is quite different. Consider a situation where the grains at each end section 2 and 4 are "slow milling", whereas the grain in the centre section 3 is "fast milling." Figures 2A through 21 illustrate these three grain sections 2, 3, and 4 in the cross-section of a semiconductor device with two levels of copper interconnect. The three grain sections from Figure 1 are represented in copper layer 5 in Figures 2A through 21. As in a "real" device, they are covered with a protective dielectric glass 6. They are also isolated, in the vertical dimension, from lower level conductors by a dielectric glass 7. A second, lower layer of conductive interconnect is represented by a copper layer 8. Subsequent lower levels of the device containing transistors, etc. are not shown for clarity. Additionally, one skilled in the art would realize that copper layers 5 and 8 would extend to the left and right of the figure, and be intended to carry electrical signals necessary to the functioning of the device. For purposes of illustration, Figures 2A through 21 show the case where it is necessary to perform microsurgery to modify the device by "cutting" copper layer 5 (milling away all the conductive copper to produce an "open circuit" along this metal line) so as to verify a design change. Figures 2A and 2B illustrate the first steps, that is dielectric glass 6 being removed by bombardment of ions 9 to expose copper layer 5. Dielectric glasses typically are "amorphous", meaning they possess no crystal structure, .and tend to mill very uniformly in the FIB. In Figure 2C, the incident Ga + ion beam has cleanly and uniformly removed all of dielectric glass 6, and begins to mill copper layer 5. As discussed above with reference to Figure 1, copper layer 5 is composed of the three grain sections 2, 3, and 4 of copper, each end grain section 2 and 4 possessing a "slow" milling orientation, while the central grain section 3 has a "fast" milling orientation. Figure 2D illustrates the result of the difference in milling speeds, that is the central "fast milling" grain section 3 is almost completely removed before even 25% of the thickness of the end grain sections 2 and 4 have been milled away. Figure 2E illustrates the point where the central "fast" milling grain of copper layer 5 has been completely removed by FIB milling, while the "slow" milling grains at either end still retain more than 50% of their initial thickness. At this point, one could assert that the initial requirement to "cut" copper layer 5 to provide an open circuit appears to have been accomplished. However, due to the three dimensional nature of the grains (not shown in Figures 2 A through 21, as the third dimension would be into the page), it is frequently necessary to proceed beyond this point to remove surrounding grains that would still be making connections ("closed circuit") in the third dimension of the metal line. Figures 2F and 2G show the continuation of the milling process. As the central grain of copper layer 5 is now gone, dielectric glass 7 begins to be milled away by the incident ion beam in the region below the now-removed "fast" milling central grain section 3 of copper layer 5. In a short time, this portion of dielectric glass 7 is also milled away. In Figure 2H, the now exposed portion of copper layer 8 is now milled by the ion beam. If this portion also happens to contain a "fast milling" orientation of copper, it too is rapidly removed. Figure 21 shows the point where the last of copper layer 5 is finally milled away by the ion beam. Unfortunately, during this time, sufficient milling has occurred to the "fast" milling copper grains in copper layers 5 and 8 that the center of copper layer 8 has also been milled away, inadvertently cutting the interconnect line of copper layer 8, causing an unwanted open-circuit in this signal as well. There are numerous other situations where this orientation induced difference in relative sputter rates in copper poses other difficulties. If FIB milling stopped at the point represented in Figure 2G above, copper layer 8 is still intact, but now exposed. Subsequent deposition of conductive material in the FIB microsurgery process could lead to an unwanted short-circuit between the FIB deposited conductive layer and the now exposed copper layer 8. Hence, there is a need for an improved technique to perform milling of copper, and more generally there is a need for an improved technique for milling elements that have large changes in milling rates based upon grain orientation. This improved milling technique will preferably lead to more uniform milling of such elements when being performed by an FIB. SUMMARY OF THE INVENTION The embodiments of the present invention are directed to apparatus and methods for reducing differential sputter rates of crystalline structures and, preferably, to FIB microsurgery directed at copper-based crystalline structures. The method of the preferred embodiments of the present invention comprises applying of a sacrificial layer onto a surface of a crystalline structure followed by the projection of an ion beam onto the applied sacrificial layer. This application of the sacrificial layer increases uniformity of the removal of the atoms from the surface of the crystalline structure during the projection of the ion beam. Preferably, this process of applying a sacrificial layer and directing the ion beam in fact is a repetitive process until the crystalline structure is removed or reduced to a predetermined level. In other embodiments, the method of the preferred embodiment can be utilized to improve the sputtering of other materials that might have grains with different crystallographic orientations having different sputtering rates. According to one embodiment of the present invention, the method further includes applying of a plurality of primer atoms prior to the applying of the sacrificial layer, a primer atom enhancing the effectiveness of the sacrificial layer. According to another embodiment of the present invention, the method further includes applying of a plurality of de-conductive atoms that operate to diminish the conductivity of any residue left over from the sputtering operation. For instance, this de- conductive atom could comprise water vapour, oxygen molecules and/or other oxidizing agents. According to another embodiment of the present invention, the method further includes applying the ion beam in one or more steps at first dwell points or pixels spaced further apart than the beam diameter and then, to produce smooth finish, at second dwell points spaced closer together than the beam diameter, the sacrificial layer preferably being applied optionally while milling at the first dwell points and optionally, though not preferably, while milling at the second dwell points. Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiment of the present invention is described with reference to the following figures, in which: Figure 1 illustrates a three-dimensional view of a piece of copper split into three sections of different grain orientations; Figures 2A through 2H illustrate cross-sectional views of a semiconductor device being milled at different stages according to a well-known microsurgery technique; FIG. 3 shows schematically a typical focused ion beam system used in a preferred embodiment of the invention. Figure 4A illustrates the atomic structure for Face Centered Cubic (FCC) copper unit cell; Figures 4B through 4D illustrate copper atoms on different atomic planes of three orientations of copper; Figures 5A and 5B illustrate a FIB milling process into a densely packed crystallographic orientation and a sparsely packed crystallographic orientation respectively; Figure 6 illustrates a cross-sectional transmission electron microscope (TEM) image of a FIB produced sample through the bottom of a sputter crater in (110) copper; Figure 7 illustrates a Convergent Beam Electron Diffraction (CBED) pattern from the single crystal substrate in Figure 6; Figure 8 illustrates a CBED pattern from the gallium rich layer in Figure 6; Figure 9 illustrates a small scale cross-sectional TEM image of the sample of Figure 6; Figure 10 illustrates a cross-sectional view of a crystal having a dechanneling layer according to the preferred embodiment of the present invention; Figures 11 A through I IP illustrate cross-sectional views of a copper crystalline structure at various stages within a sputtering operation according to a preferred embodiment of the present invention; Figure 12 illustrates two samples of FIB microsurgery according to preferred embodiments of the present invention for polycrystalline copper samples; Figure 13 illustrates a TEM image of an array of disrupted spots formed by a FIB; Figure 14 is a flowchart showing the steps of an embodiment of the invention; and Figures 15 A- 15F show samples after a series of milling operations in accordance with the method of FIG. 14. FIG. 15A shows the results of applying a first milling box to the sample. FIG 15B shows the application of a second milling box to the sample from FIG. 15 A, FIG. 15C shows the application of a third milling box to the sample from FIG. 15B, and so forth; and FIG. 16 A shows the results of a conventional raster milling, and FIG 16 shows the results of milling in accord. nce with the invention of Figure 14 using a dose of 5.5 nC/mm 2 in each box. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Preferred embodiments of the present invention are directed to apparatus and methods used for milling of crystalline materials including copper-based metal lines within semiconductor devices. Although the following description is directed at copper milling and copper milling within semiconductor devices specifically, it should be recognized that the apparatus and methods of the present invention could equally be utilized in the milling of other crystalline structures, by itself or in combination with other techniques. Hence, the scope of the present invention should not be limited to simply the milling of copper-based crystalline structures. FIG. 3 shows a typical focused ion beam system 10 used to implement the present invention. Focused ion beam system 10 includes an evacuated envelope 11 having an upper neck portion 12 within which are located a liquid metal ion source 14 and a focusing column 16 including extractor electrodes and an electrostatic optical system. Ion beam 18 passes from source 14 through column 16 and between electrostatic deflection means schematically indicated at 20 toward sample 22, which comprises, for example, a semiconductor device positioned on movable X-Y stage 24 within lower chamber 26. An ion pump 28 is employed for evacuating neck portion 12. The chamber 26 is evacuated with turbomolecular and mechanical pumping system 30 under the control of vacuum controller 32. The vacuum system provides within chamber 26 a vacuum of between approximately lxlO "7 Torr and δxlO "4 Torr. If an etch-assisting, an etch retarding gas, or a deposition precursor gas is used, the chamber background pressure may rise to about lxl 0 "5 Torr. High voltage power supply 34 is connected to liquid metal ion source 14 as well as to appropriate electrodes in focusing column 16 for forming an approximately 1 keV to 60 keV ion beam 18 and directing the same downwardly. Deflection controller and amplifier 36, operated in accordance with a prescribed pattern provided by pattern generator 38, is coupled to deflection plates 20 whereby beam 18 may be controlled to trace out a corresponding pattern on the upper surface of sample 22. In some systems the deflection plates are placed before the final lens, as is well known in the art. The source 14 typically provides a metal ion beam of gallium, although other ion sources, such as a multicusp or other plasma ion source, can be used. The source typically is capable of being focused into a sub one-tenth micron wide beam at sample 22 for either modifying the sample 22 by ion milling, enhanced etch, material deposition, or for the purpose of imaging the sample 22. An charged particle multiplier 40 used for detecting secondary ion or electron emission for imaging is connected to video circuit and amplifier 42, the latter supplying drive for video monitor 44 also receiving deflection signals from controller 36. The location of charged particle multiplier 40 within chamber 26 can vary in different embodiments. For example, a charged p.article multiplier 40 can be coaxial with the ion beam and include a hole for allowing the ion beam to pass. A scanning electron microscope 41, along with its power supply and controls 45, are optionally provided with the FIB system 10. A fluid delivery system 46 extends into lower chamber 26 for introducing and directing a gaseous vapor toward sample 22. U.S. Pat. No. 5,851,413 to Casella et al. for "Gas Delivery Systems For Particle Beam Processing," assigned to the assignee of the present invention, describes a suitable fluid delivery system 46. Another gas delivery system is described in U.S. Pat. No. 5,435,850 to Rasmussen for a "Gas Injection System," also assigned to the assignee of the present invention. A door 60 is opened for inserting sample 22 onto stage 24, which may be heated or cooled, and also for servicing an internal gas supply reservoir, if one is used. The door is interlocked so that it cannot be opened if the system is under vacuum. The high voltage power supply provides an appropriate acceleration voltage to electrodes in ion beam column 16 for energizing and focusing ion beam 18. When it strikes sample 22, material is sputtered, that is physically ejected, from the sample. Focused ion beam systems are commercially available, for example, from FEI Company, Hillsboro, Oregon, the assignee of the present application. Prior to the description of preferred embodiments of the present invention, a description of the development that led to the present invention is now provided for background. Initially, a study with bulk single crystal specimens of copper, each with a known crystallographic orientation, was conducted to determine the cause of the difference in FIB milling rates of various orientations of copper grains. Figure 4A shows schematically the crystalline structure, normally formed by copper atoms. The structure is base upon a "Face Centered Cubic" (FCC) unit cell 70, where the atoms 72 are grouped as if forming a cube, with an atom 72 at each of eight corners, plus six additional atoms 72, one at the centre of each face of the cube. A single grain of copper contains an identical, repetitive array in three dimensions of these unit cells. The unit cell can be orientated in many different directions relative to the observer, or the incident beam of the FIB. Materials scientists frequently refer to a particular orientation in FCC materials by describing the atomic "plane" within the unit cell whose normal vector is parallel to the direction of observation. Figures 4B through 4D show the relative locations of atoms on three different planes within the unit cell, these planes being referred to as (100), (110) .and (111) respectively. One skilled in the art would understand that these copper grains with different planes have different distributions of atoms, and subsequently different atomic densities. Among the single crystal copper samples obtained for the study, three of these samples could be oriented such that the (100), (110) and (111) planes, respectively, were normal to the incident ion beam in the FIB. Next, using standard milling conditions in a FEI Company/Micrion 2500 FIB system (having a 5 nm resolution FIB column with 50 kV Ga + ions as the primary beam), experiments were performed where a known dose of gallium ions (5 nC of gallium per square μm) was delivered over a number of areas, each of 10 μm x 10 μm on each sample. The resultant depth of milling for this known dose was then measured using the standard FIB technique of sectioning and imaging the specimen. It is noted that a protective layer of FIB deposited tungsten was laid down prior to sectioning to preserve the original surface, following well-known techniques. Results from these experiments are shown in Table 1 below: Table 1 From these experiments, it was determined that a "fast" milling orientation such as (111) mills approximately 3.6 times faster than a "slow" milling orientation such as (110). There are a number of theories that can be generated from this experiment. From a metal physics standpoint, given the relatively "open" lattice in the (110) crystalline structure compared to the densely packed (111) crystalline structure (see Figures 4C and 4D respectively), the incident ion beam may be "channeling," that is, slipping in between planes of atoms normal to the ( 110) surface, implanting itself deeper into the copper. Ions that are channeled mill away very little sample material compared to ions that expending their energy knocking into densely packed copper atoms normal to the (111) orientation and thereby mill away these densely packed copper atoms. Figures 5A and 5B illustrate schematically this possible phenomenon for the (111) and (110) copper orientations respectively with gallium ions or atoms 74 being implanted among copper atoms 72. To investigate this further cross-sectional TEM specimens from the milled regions of the single crystal copper samples were examined, one such TEM image for the (110) copper sample being illustrated in Figure 6. It is noted that the TEM specimens for imaging were generated with a thin protection layer of gold and a thicker protection layer of FIB deposited tungsten, prior to the FIB beam being used to mill the region of interest until it was thin enough to be transparent to the electrons used in a Philips 200 kV Field Emission CM20 TEM. As can be seen in Figure 6, in the case of the (110) copper, a layer of copper material rich in gallium was found, approximately 85 nm in thickness, at the surface of the sputter crater. Next, the electron beam in the TEM was focused to a fine spot and Convergent Beam Electron Diffraction (CBED) patterns were obtained from the substrate copper and the gallium rich layer at the crater surface. Standard techniques to identify these diffraction patterns confirmed that the substrate material was copper as shown in Figure 7, and revealed the unknown surface layer to be a copper-gallium intermetallic phase with the composition of Cu 3 Ga as shown in Figure 8. The crystallographic parameters of this phase can be obtained by searching the Joint Committee on Powder Diffraction Standards (JCPDS) powder diffraction database for compounds containing copper and gallium, and correspond with the description of Cu 3 Ga found on JCPDS powder diffraction file PDF 44-1117. With this TEM image it seems that not only was the (110) copper crystal slow to mill because of channeling, but also enough gallium was implanting into the sample to transform the surface of the crater to Cu 3 Ga, which resists sputtering. After further examination of the edge of the sputter crater in the TEM, it was determined that the Cu 3 Ga phase was not evident until the normal to the surface of the crater became very close to parallel to the incident ion beam. This effect is illustrated by the white, double headed arrows in the TEM image of Figure 9, Figure 9 being a high magnification TEM image of Figure 6. From this experiment, it was theorized that the Cu 3 Ga phase is only produced when the incident ion beam was effectively parallel to the (110) surface normal. As discussed previously, these are the same conditions where substantial channeling would be expected into the pure copper. In summary, the difficulty with sputtering of copper is that, depending on the crystallographic orientation of the copper, the rate of removal of material for a given dose of gallium ions can vary by at least a factor of 3.6. Thus, because a copper conductor is typically made up of many crystal grains having random orientations, it was thought to be necessary to remove the crystallographic effect in order to produce more uniform sputter rates. One possible solution to accomplish this would be to disturb the surface of the grains sufficiently that they lost all of their crystallographic order. Another solution would be to send the incident beam down to the crystalline structure such that each ion in the beam came from a "random" direction. Since, for an FIB microsurgery implementation, tilting the sample to a different orientation for each incident ion or tilting the beam to a new, random angle for each incident ion is not practical, another solution was required. Preferred embodiments of the present invention, as will be discussed herein below, comprise techniques for reducing the difference in sputter rates between different crystallographic orientations of copper grains. With the basis of the above described analysis, it was determined that the dynamic deposition of a thin layer of an essentially amorphous or nanocrystalline, sufficiently dense material during ion milling could be tailored to eliminate the non-uniform milling if at least one of two criteria were met. Firstly, this layer preferably should be such that it can be driven into the copper lattice by the ion beam, thus disrupting the lattice sufficiently to provide an effectively "random" crystallographic texture for each incident ion. Secondly, the layer preferably should be sufficient to deflect incoming ions, prior to the ions hitting the copper, by relatively random angles, such that the resulting direction of each ion would be varied compared to the ion before; the deflections generated effectively giving each grain a different orientation relative to each incident ion. The approach of using an additional layer, as will be described during specific examples below, is hereinafter referred to as the dynamic production of a sacrificial layer, the sacrificial layer being used to at least one of deflect the incoming ions so as to change the angle of incidence between the ions and the copper atoms, and disrupt the crystalline orientations of the copper atoms when bombarded with an ion beam. A sacrificial layer satisfying the second criteria is illustrated within Figure 10, in which a plurality of ions are colliding with the sacrificial layer, subsequently resulting in these ions being deflected into the copper substrate. In one preferred embodiment, as will be described in a specific implementation below, a tungsten hexacarbonyl gas delivery system is used to apply the sacrificial layer. To generate this sacrificial layer, the parameters of gas pressure, gallium beam dwell time, ion beam current, pixel size and pixel spacing are selected so that, rather than building up a layer of deposited tungsten hexacarbonyl as conventionally occurs, just a thin layer is allowed to be absorbed at the surface of the copper layer before the ion beam converts the gas into a tungsten based sacrificial layer by driving off some of the carbonyl compounds. This conversion leaves FIB deposited tungsten that is a fine grain, possibly with some gallium and carbon incorporated. After the conversion, the ion beam further sputters away the thin tungsten layer. Preferably, there would be a sufficient quantity of tungsten hexacarbonyl gas present at a location each time the ion beam makes a pass such that the tungsten layer would last throughout the time the ion beam dwelled at the particular location, but be thin enough such that it would not substantially suppress the ion beam from penetrating through to the copper layer below or prevent sputter removal of the copper, or result in significant quantities to be left on the copper so as to cause an ongoing accumulation of tungsten. One particular example of a set of procedures according to the present invention and predetermined parameters that can be used to successfully sputter copper with relative uniformity are now described. Initially for this procedure, a semiconductor component with copper metallizations is placed in an FEI Company/Micrion 2500 FIB system with a gallium ion beam accelerated to 50 kV, being tuned to a 672 pA beam current using a 150 um aperture and being aligned using standard procedures. The base pressure in the FIB chamber starts in the order of lxlO "7 Torr. Subsequently, a 10 μm x 10 μm region of copper metallization is selected and exposed using standard procedures. Next, the tungsten hexacarbonyl gas nozzle is inserted and, by opening the nozzle valve, the pressure in the chamber is raised to about lxl 0 "5 Torr. using standard procedures. In software, a "mill box" is prepared to cut the copper using a spacing between subsequent pixels in the box of 0.01 μm in both "x" and "y" dimensions, and a dwell time for the beam of 2 μsec. At this point the refresh and retrace times are set at the default value of 10 μs and, for this particular example, the box should be set for a total dose of 2 nC/μm 2 to cut approximately 0.75 μm deep. After all the preliminary preparations are complete, the mill is initiated. During this procedure, the tungsten hexacarbonyl gas is continuously flowing onto the exposed copper surface. Control software run on the FIB system positions the ion beam onto the "x=0, y=0" pixel and leaves it there for 2 μsec, that being the dwell time. This can be illustrated in the cross-sectional views of Figures 11 A through 11C (though these cross- sections illustrates the sputtering after a portion of the underlying material has already been removed). During the first fraction of this dwell time, the ion beam converts the adsorbed tungsten hexacarbonyl gas to a tungsten based crystalline structure while individual ions are being somewhat deflected by the tungsten based compounds, driving some of these compounds into the top few layers of the copper. Eventually, as seen in Figures 11 A through 1 ID, the tungsten layer is sputtered away along with some copper material that the deflected ions collide with. Ideally, when the dwell time is complete, there is no tungsten remaining on the surface of the copper at that pixel. After the 2 μs dwell time expires, the control software moves to the next pixel and once again dwells for 2 μs as shown in Figures 1 IE through 11H. The result is identical to that described above for this pixel, as well as the dwell time of the third pixel illustrated in Figures 111 through 11L. This step-dwell-step-dwell process continues until all pixels in the mill box have been visited by the beam, whereupon the process starts over with the beam once more directed to the first pixel (x=0, y=0) as can be seen in Figures 11M through 1 IP. Although Figures 11 A-l IP show material being removed evenly across the pixel, skilled persons will recognize that the current density within the ion beam, and therefore the actual etching profile, is typically is Guassian shaped. Also, although Figures 11 A-l IP show pixels as contiguous and non-overlapping, skilled persons will recognize that pixels spacing is typically less than the beam diameter, so adjacent pixels overlap. As described in more detail below, it is preferable in some embodiments to use pixels that are spaced apart further than the beam diameter. As discussed above, in this example implementation, the tungsten hexacarbonyl gas continuously is being applied to the surface of the exposed copper and so, by the time the ion beam returns to the first pixel, a new layer of tungsten hexacarbonyl has adhered to the target surface at the pixel. Any tungsten hexacarbonyl remaining on the sample surface when milling is complete will not result in tungsten deposition because the decomposition to tungsten occurs only during ion beam bombardment. In this implementation, while the beam is dwelling at each point, copper is being sputtered away as discussed above. This sputtered material contains particles that may be detected, either using the standard detectors in the FIB system, a detector specifically designed for the purpose, or a signal not based on detected particles but another phenomena such as the current being absorbed by the sample. By examining the signal from a detector, correlated with the known beam position at that point in time, it is possible to view a virtual "image" of the process. Analyzing this image visually, or performing some other analysis of the signal permits "endpoint determination"; that is to say, determining when all the copper has been removed from the vicinity of the mill box, exposing the underlying dielectric gas. A standard FIB system provides a method to determine "how much" ion beam has been delivered to a particular site, this "how much" value hereinafter being referred to as the dose. One can either decide to stop the milling procedure at a given dose, when a sufficient end-point signal has been detected or a combination of the two. Once milling has been completed, the gas flow is stopped, and the chamber pressure allowed to return to a value nearer its base pressure. Correct application of the technique described above will result in a flat bottomed crater with little or no evidence that differential sputtering on a grain-by-grain basis has occurred. The above sample procedure was generated by experiments to optimize conditions on single crystal samples. Using this technique, the depths of sputter for a given dose ranged from a slow sputtering (110) at 0.68 μm depth and fast sputtering (111) at 2.45 μm depth to values of approximately 1.8 μm depth for both (111) and (110) crystallographic orientations. Further experiments were performed on poly crystalline copper semiconductor devices from a commercial manufacturer and high quality results, in terms of relative uniform removal of a number of copper grains, were obtained. Figure 12 illustrates two such test craters. As mentioned, the above description is for a specific implementation and it should be understood that the adjusting or "tweaking" of parameters to other systems and circumstances may be required to apply the method of the preferred embodiment successfully to a separate example. For instance, it was found that when using a FEI Company/Micrion 9500 FIB system equipped with a special power supply that generates approximately 2,000 pA of beam current using the 150 μm aperture, the default gas settings did not raise the " chamber pressure to the required lxl 0 "5 Torr., but rather 7x10 "6 Torr. Attempting to use identical parameters as those described above resulted in differential sputtering and, as a result, some grains of copper were not removed cleanly while others were. It was theorized that the higher beam currents and lower gas pressure resulted in a consumption of the sacrificial tungsten-based layer too quickly. This problem was rectified by reducing the dwell time from 2 μsec to approximately 0.2 μsec and increasing the pixel spacing from 0.01 μm to 0.05 μm. The key parameters when using a FIB system can be considered the current density which includes the ion beam current and the pixel size as variables; the application rate of the sacrificial layer which is based upon the gas pressure; and the dwell time. Experiments have shown the technique of preferred embodiments of the present invention can be optimized to give relatively uniform removal of copper grains regardless of orientation over a range of typical instrument operating parameters. Though, it should be noted that other systems that are capable of performing the method of the present invention could use parameter values that are greater than or less than these parameters and so the present invention should not be limited to these cases. There are a number of undesirable situations that can occur in cases that the parameters are not optimized. For instance, the sacrificial layer, such as deposited tungsten, could accumulate over time if not all of the tungsten is sputtered during the dwell time. This accumulation could eventually cause a build up to such a thickness that none of the underlying material is removed, as the ions are not simply deflected, but essentially blocked from reaching the bottom layer. In this case, a higher beam current, a longer dwell time or a reduced gas pressure could be employed, for example, to slow the rate of deposition. One skilled in the art, after observing a particular deviation from the desired uniform milling rate, should be capable of readily adjusting his/her parameters to achieve the desired result. Other undesirable situations that occurred in some experiments with the present invention indicate that further changes to the standard milling algorithm may be required in some circumstances. For example, in certain experiments, while the copper was removed cleanly and uniformly across the crater, right at the edge of the crater walls, an excess of copper was removed, resulting in some "troughing" at the edges. Creation of such a trough can be prevented by modifying the dose that is applied near the edges of the crater. Yet another undesirable situation may occur if the sacrificial layer is formed using conductive metal based gas such as tungsten hexacarbonyl as described above. In this case, some residual conductive material may be left at the end of the mill process, possibly resulting in short circuits within the device. This residue could remain from within the areas in which the milling took place and could also be generated with the sputtering off at one site possibly ricocheting and subsequently converting the tungsten hexacarbonyl in another area to tungsten even before the primary beam of gallium ions is received. One possible solution is to use standard clean-up procedures already well-known in the art such as a quick cleaning mill with XeF . Yet another undesirable situation that can occur is continuing to mill past the bottom the crystalline material over part or all of the area being milled. Depending upon the material beings milled and the material located below the material being milled, it is sometimes possible to introduce a gas, typically near the end of the milling process, that selectively inhibits the etching of the material below the material being milled. For example, when milling a copper material positioned on a dielectric material, such as a silicon oxide, an oxygen containing gas, such as water vapor or oxygen, can be introduced to selective inhibit the etching of the oxide layer over the copper material, thereby promoting a smooth floor to the milled area. The use of water vapor in focused ion beam etching is described in U.S. Pat. No. 5,958,799 to Russell, et al. for "Method for Water Vapor Enhanced Charged-Particle- Beam Machining." It should be recognized that one could replace tungsten deposition from tungsten hexacarbonyl gas, as is described above, for another gas that has sufficient properties to allow, once being deposited and projected with an ion beam, to deflect ions and/or disrupt the copper array. Some examples of replacements, that are not meant to be limiting, include many metallorganic gases such as platinum-based and gold-based molecules; silicon and oxygen based molecules such as siloxane and oxygen which can be used to produce a silicon dioxide layer on the surface; and even a non-solid could possibly be used such as a noble gas. For each of these new sacrificial layer substances a new set of parameters based on experimentation would be required to find the optimal conditions for the sputtering. Another approach that can be used with or without the sacrificial layer described above is to "damage" the top surface of the copper at regular intervals so as to disrupt the regular array of lattice planes, hence reducing the effects of channeling. This approach comprises creating an array of disrupted regions using the ion beam to create very small depressions each surrounded by implantation damage. Another approach to reduce the problem of differential sputtering is done by leaving the beam at regular locations in the disrupted region for a longer period, not only damaging the array under the beam, but creating a substantial hole. Figure 13 shows a Transmission Electron Microscope (TEM) image of an array of disrupted regions prepared with the first approach. Another novel technique that has been found useful for uniform etching entails milling a target first at a series of non-contiguous pixels, and then performing a final milling, preferably at overlapping pixels, to smooth the floor of the milled area. The milling of noncontiguous pixels can be performed in a single step or in several steps, with the pixel spacing progressively decreasing in each subsequent step. Successful milling of copper using this technique has been achieved with as few as two and as many as ten milling steps, depending on specific copper characteristics such as thickness and grain structures. This technique can be used with or without applying a sacrificial layer as described above. This technique inhibits the formation of etch-resistant areas, perhaps by reducing localized heating of the copper conductor that may contribute to the formation of the gallium copper compounds described above. Milling non-contiguous locations may allow energy imparted by the impact of the ion beam at one location to dissipate tlirough the specimen before the beam returns to that area, thereby preventing the accumulation of energy necessary to cause formation of the etch resistant region. The propagation of etch resistant areas may also be reduced because the inventive method creates holes that isolate the copper into segments, and the etch resistant areas may be unable to propagate across these holes. This technique not only prevents formation of the etch resistant area, it also increases the milling rate of the copper conductor in general. Because of the finite edge resolution of the ion beams, the sides of the milled holes are not exactly perpendicular to the surface. This increases the exposed surface area of the target and causes some of the impacting ions to strike part of the target surface at a non-normal angle. Both the increased surface area and the non-normal angle of incidence are thought to increase the etch rate. In accordance with a system embodying this aspect of the invention, signals applied to deflection controller and amplifier 36 cause the focused ion beam to move within a target area to be milled in a novel pattern that increases the milling rate and produces more uniform material removal. FIG. 14 is a flowchart showing the steps involved in a preferred embodiment of the present invention. In step 100, a target area to be milled is located on sample 22 in lower chamber 26. Step 101 shows that a precursor gas, as described above, for depositing a sacrificial layer upon bombardment of an ion beam is optionally directed to the sample. In step 102, the target area is milled using a pixel spacing greater than the beam spot size, defined as the diameter from edge to edge of the ion beam at the target surface, the edge of the beam being considered the point where the beam intensity drops to one tenth of its maximum value. The ion beam typically has a spot size for milling of between 0.05 μm and 0.15 μm, although, depending upon the target to be milled, beams as small as 0.005 μm are readily attainable and can be used, as can beams as large 0.8 μm or larger. In step 104, the target area is milled again, using a pixel spacing smaller than that used step 102. Depending upon the material being milled, the size of the area being milled, the characteristics of the ion beam being used, step 104 is repeated, with the pixel spacing being reduced, until, as shown in step 106, the pixel spacing is sufficiently small to produce a smooth, planar floor on the target area. Optionally, a selectively etch retarding gas can be applied in step 108, preferably near the completion of the milling operation, to the decrease the etch rate of the underlying material to further reduce any unintentional etching of the underlying substrate. A sacrificial layer can be deposited and etched as described above during any of the milling steps 102 through 106, although the sacrificial layer is preferably not deposited during the final milling step, thereby avoiding leaving deposited material on the target. Depending upon the material being removed, an etch enhancing gas can also be applied during the entire milling process to either increase the etch rate of the milled thin-film metal or inhibit the etch rate of the lower level dielectric under the film. In one embodiment, referred to as Variable Pixel Milling (VPM), the invention uses a series of "milling boxes." A milling box is defined as a group of milling locations or pixels spaced apart by specified amounts in the x and y directions. An average ion dose, expressed as charge per unit area, is specified for each milling box. The ion beam moves between milling locations in the box, milling at the milling locations until each milling location in the box has received approximately the designated average dosage specified for the box. At each milling location, a hole having a diameter roughly equivalent to the beam spot size is milled. In applying the dose, the ion beam typically visits each point in the box multiple times, rather than delivering to each point the entire dose for the box during a single dwell period. After the first milling box is completed, the sample surface will have multiple holes separated by unmilled material. Multiple milling boxes can be applied, each having successively closer pixels. The 'nested' boxes are positioned over the desired feature of interest and the milling is performed serially, with subsequent boxes having closer pixel spacing. The result is a series of segregated copper islands that appears to suppress the formation of a highly channelling region that may appears black on the display screen. By suppressing the formation of this region, the milling produces a flat, uniform surface. In addition, the increased surface area achieved by the formation of these copper islands enhances overall milling efficiency by a factor of about 1.25 to about 4.0, even without the application of a sacrificial layer. Some of the holes milled in subsequent mill boxes will typically overlap previously milled holes. Upon finishing the complete set of milling boxes, however, the amount of material removed from each point in the sample is approximately equal to produce a uniform, planar floor over the entire area milled. The last milling box typically uses conventional pixel spacing, that is, the pixel spacing is smaller than the beam spot size to produce a smooth, uniformly milled area. In some applications, only two milling boxes are required: a first box having a pixel spacing greater than the beam diameter and a second mill box having a pixel spacing less than the beam diameter. In one preferred embodiment for milling a copper interconnect using a 150 μm aperture in the FIB column and a beam current of 2.04 nA, six milling boxes are used. The ion beam has a beam spot size of between 0.05 μm and 0.10 μm. The parameters of each box are shown below in Table 2 for a copper thickness of 1.0 μm. Table 2 The first five of the milling boxes of Table 2 use a pixel spacing that is significantly larger than those used for standard default milling. Only the last milling box uses the conventional default scan spacing. Within each milling box, the beam makes multiple circuits, that is, each pixel is milled multiple times, until it has received approximately the average dose designated for the box. The beam typically dwells at each pixel for approximately 5 μsec, resulting in a dose at each pixel of about 0.001 nC during each dwell period. Although six milling boxes are shown in Table 2, in other embodiments, two milling boxes are used, the pixels in the first milling box being non-contiguous and separate, for example, by 50 μm, and the pixels in the second milling box overlapping, for example separated by 0.05 μm. FIGS. 15A-15F shows the sample after each of the milling boxes of Table 2 is sequentially applied, with FIG. 15F showing a sample after the last milling box is complete. FIG. 15F shows the sample having a smooth bottom surface with no residual copper evident. FIG. 16A shows a sample copper interconnect milled using convention milling, and FIG. 16B shows for comparison a similar copper interconnect milled using, without a sacrificial layer, first non-contiguous pixels and subsequently overlapping pixels. Mill boxes have been successfully scaled over a wide range of sizes which extend from greater than 20 μm x 20 μm to 1 μm x 1 μm using a 150 μm aperture. The boxes are simply expanded or contracted to the desired area, maintaining the same pixel spacing and dosages. Use of apertures that are either larger or smaller than 150 μm requires careful scaling of the pixel spacing for the successful execution of this procedure. Applicants have scaled milling boxes over an aperture range of 50 μm to 150 μm. For example, to maintain the milling time to as short a period as possible for areas larger than 20 μm x 20 μm, a FIB operator can choose a larger aperture which provides both an increase in spot size and greater beam current. As the spot size increases, the pixel spacing in the multiple boxes must be correspondingly increased, so that the pixels are not contiguous in the first few boxes. Skilled persons will be able to determine without undue experimentation a set of boxes to use for any aperture size guided by the example provided above. Applicants have also found that the milling specifications, that is, the pixel spacing, dose, ion energy, beam size, and other parameters, of each box, as well as the number of different boxes used, may be varied to optimize milling depending on the copper characteristics, such as the copper grain structure and thickness of the material being milled. Applicants have successfully milled copper using as few as two and as many as ten milling boxes. The specification of each milling box and the number of boxes can be determined experimentally using the example and information provided above as a guide. Although in the patterns described above, the milling is being performed in rectangular arrays of fixed pixel spacings within each box, the invention is not limited to that embodiment. For example, the pixel spacing can be gradually reduced, rather than being reduced in discrete steps, and the milling locations do not need to be distributed in a rectangular or regular pattern, so long as the pixels are spaced widely enough to prevent the growth of etch-resistant areas and the amount of material eventually removed from each point is about the same to produce a flat, smooth, planar floor. The pixel spacing could even remain unchanged, with the pixel locations changing in subsequent scans to produce a flat, smooth, planar floor. Applicants have also found that a reduced beam energy can also prevent the formation of the etch resistant region. A preferred reduced beam energy provides sufficient energy to sputter the copper at an acceptable rate, yet provides insufficient excess energy to form etch-resistant regions. For example, the accelerating voltage of the focused ion beam could be reduced from the typical milling range of 30 keV to 50 keV to impart less into the target per ion. A preferred beam energy is less than about 20 keV and more preferably less than about 10 keV. A preferred minimum energy is about 1 keV. Applicants have found an energy of about 5 keV to be suitable for some copper targets. As mentioned previously, although the detailed description herein above is specific to copper, it should be recognized that other crystalline structures could benefit from the sputtering technique of the present invention. For instance, some examples of structures that could benefit include gold based, platinum based, tungsten based, nickel based, and chromium based crystalline structures. Note that this list is not meant to limit the scope of the present invention, as it could apply to any crystalline structure that could benefit from the effect of the different crystallographic orientation being reduced. It is noted that although the continuous flow of tungsten hexacarbonyl is described herein above for the preferred embodiment of the present invention, this is not meant to limit the scope of the present invention. Merely a sufficient flow that allows for appropriate levels of the sacrificial layer to be absorbed at each pixel location before the ion beam returns to that location is sufficient. Although the description of the present invention above is mainly directed at a method of sputtering, it should be recognized that an apparatus performing the operation of this method would further be within the scope of the present invention. Yet further, it should be recognized that embodiments of the present invention could be implemented within control logic (software and/or hardware) or even as a control software program within a computer readable storage medium. To assist skilled persons to better understand the process described herein, applicants have described above theoretical mechanisms that account for the success of the invention. The invention has been shown to work empirically, and its utility is independent of the validity of theories of the underlying mechanism. The preferred embodiment uses a focused ion beam, although the method of the present invention may also be useful for electron beam processing. The invention has been shown to solve the problem of milling copper interconnects, but can be applied to any charged particle milling including plasma gas ion sources and is not limited to any specific material or application. Persons skilled in the art will appreciate that there are yet more alternative implementations and modifications possible for implementing the present invention, and that the above implementations are only illustrations of certain embodiments of the invention. The scope of the invention, therefore, is only to be limited by the claims appended hereto.

Claims: CLAIMS: 1. A method of removing material from an area of a target, the method comprising: forming a layer over the area of the target; and directing an ion beam toward the area of the target to remove material, the formed layer interacting with the ion beam to increase the uniformity of the material removal by the ion beam. 2. A method according to claim 1 in which directing an ion beam toward the area of the target includes removing a substantial portion of the layer as the ion beam removes the material from the area of a target. 3. A method according to claim 1, wherein the material to be removed includes atoms positioned in crystal planes and wherein the layer deflects the ions in the ion beam to vary the angles of incidence of the ions with respect to the crystal planes, thereby reducing the effects of the orientation of the crystal planes on the milling rate. 4. A method according to claim 1, wherein the material to be removed includes atoms arranged in a crystalline structure and wherein the ion beam causes atoms in the layer to disrupt the crystal structure, thereby reducing the effects of the crystal structure on the milling rate. 5. A method according to claim 1 in which forming a layer includes directing a precursor gas toward the area of the target concurrently with directing the ion beam toward the area of the target, the precursor gas decomposing in the presence of the ion beam to form the layer. 6. A method according to claim 5, wherein: the precursor gas adheres to the target surface; and the ion beam dwells for dwell times at multiple dwell points in the area of the target, the dwell time at each dwell point being sufficient to decompose substantially all of the precursor gas adhered to target at the dwell point. 7. A method according to claim 6, wherein the ion beam is directed to each dwell point multiple times, the time interval between successive dwells at the same point being sufficiently long to adhere sufficient precursor molecule to the dwell point to form a new layer. 8. A method according to claim 5, wherein the precursor gas comprises metallorganic molecules. 9. A method according to claim 8, wherein the metallorganic gas molecules comprise tungsten, platinum, or gold. 10. A method according to claim 1, wherein the ion beam has a beam diameter and wherein directing an ion beam toward the area of the target comprises: directing the ion beam to first dwell points within the area of the target, the first dwell points spaced further apart than the beam diameter; and directing the ion beam to second dwell points within the area of the target, the second dwell points spaced closer together than the beam diameter, thereby producing a smooth floor on the area of the target to which the ion beam is directed. 11. A method according to claim 10, further comprising after directing the ion beam to the first dwell points and before directing the ion beam to the second dwell points, directing the ion beam to additional dwell points or sets of dwell points within the area of the target, the additional dwell points or sets of dwell points being spaced closer together than the first dwell points are spaced and further apart than the second dwell points are spaced. 12. A method according to claim 1 , wherein the layer comprises noble gas atoms, silicon atoms, or oxygen atoms. 13. . A method according to claim 1 , wherein the material to be removed includes copper atoms. 14. A method according to claim 1 , wherein the material to be removed includes gold, platinum, tungsten, nickel or chromium atoms. 15. A method according to claim 1 further comprising applying of a plurality of primer atoms prior to forming the layer, the primer atoms facilitating the formation of the layer. 16. An ion beam system for removing material from a target, comprising: an ion source for providing ions; ion optical elements for forming ions from the ion source into a beam having a diameter and directing the beam toward an area on a target; a gas injection system for directing a precursor gas toward the target to form under bombardment of the ion beam a sacrificial layer, the gas injection system being adjusted to deliver precursor gas at a rate sufficient to make etching of the material substantially uniform over the area on the target but not sufficient to result in a significant built-up of decomposition products. 17. The system of claim 16 in which the ion source and ion optical elements produce a beam characterized by beam parameters and an impact point, and in which the ion optical elements include deflection electrodes for changing the positioned of the impact point on the target surface, the rate at which the gas injection system delivers the precursor gas being dependent on the beam parameters and the scheme by which the deflection electrodes change the position of the impact point. 18. The system of claim 16 further comprising a controller for controlling the ion beam, the controller directing the ion beam to first dwell points within the area of the target, the first dwell points spaced further apart than the beam diameter and subsequently the controller directing the ion beam to second dwell points within the area of the target, the second dwell points spaced closer together than the beam diameter, thereby producing a smooth floor on the area of the target to which the ion beam is directed.

Inventors: LI JIAN, PHANEUF MICHAEL W, LI, JIAN, PHANEUF, MICHAEL, W.
Applicants: FEI CO, FEI COMPANY
Application year: 2001
Main classification: H01L21
IPC codes: B23K15, C23F4, G03F1, H01J37, H01L21, H01L23