Abstract:
A method for making connections to conductors buried under dielectrics layers using a focused ion beam and an etch-assisting gas is described. The method uses a halogenated hydrocarbon, such as 2, 2, 2-trifluoroacetamide, to enhance etching of the dielectric while attenuating etching of the conductor once expose. The method thereby allows a via to be milled to contact the conductor without substantial etching and degrading the conductor.

Description:
This application is a continuation-in-part of Ser. No. 09/169,566 Oct. 9, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to chemically enhanced ion beam etching, and in particular, to using a focused ion beam to selectively etch inter layer dielectrics deposited during integrated circuit fabrication. 
     Integrated circuits are fabricated by growing, depositing, diffusing, and etching thin layers of conductors, insulators, and semiconductors onto a substrate of a semiconductor material, such as silicon or gallium arsenide wafer. To keep the fabrication processes operating properly, or to diagnose and correct the process when a defect does occur, process engineers must be able to quickly examine the various processed layers. 
     A primary tool used for examining, analyzing, and repairing processing layers is a focused ion beam (FIB) system. FIB systems improve manufacturing yields by identifying and analyzing defects on in-process wafers, allowing the source of defects to be located and corrected. For example, layers can be sputter-etched by an FIB system to expose underlying layers for observation and testing, or cross sections can be cut to expose the edges of multiple layers to observe layer thickness, uniformity, and inclusions. FIB systems can also form images of microscopic features and can be used to repair or test integrated circuits by depositing conductive or insulating material. 
     The processing layers exposed by the removal of covering material using FIB etching can be examined either using the imaging capability of the FIB system, or using a scanning electron microscope (SEM). The electron beam of an SEM causes less sample damage than does the ion beam of an FIB system, and the SEM is typically capable of forming a higher resolution image. SEMs are often available within the same vacuum chamber as an FIB system, such as in the DualBeam™ family of FIB Systems from FEI Company, the assignee of the present invention. In such a system, a cross section of the processing layers can be milled and then observed within the same vacuum chamber, with little or no movement of the sample. Such a system is particularly well suited to process control applications, where specimens must be analyzed quickly to provide feedback to a production line. 
     Many of the layers in an integrated circuit are composed of relatively non-conductive materials that are used to separate conductive layers or as passivation and protection layers for the chip. Such layers are known as interlayer dielectrics (ILDs). ILDs include deposited oxides of various densities, thermal oxides, spun on glass, and nitrides. When ILDs are cross-sectioned with a focused ion beam and viewed, it is often impossible to distinguish among them. Thus, individual layer thickness cannot be determined and process engineers cannot isolate defects to a particular layer. 
     To distinguish between different ILD layers, it has been necessary to remove the specimen from the vacuum chamber and etch it in a bath of wet chemicals, such as ammonium fluoride (NH 4 F) and hydrofluoric acid (HF), or a combination of NH 4 F, HF, and acetic acid. The wet etching process etches the various layers slightly differently, so that upon rinsing, cleaning, and re-inserting into a vacuum chamber, the different layers can be viewed. Unfortunately, the time required to perform the multitude of steps involved in this process makes it unsuitable for real-time process control. Moreover, the etching of a chemical bath cannot practically be limited to the area of interest; the entire wafer must be etched to increase the contrast in a cross section of a single device of interest. 
     It has also been found that plasma etching using gases such as CF 4  and C 4 F 8 , enhances the contrast between the layers that were exposed by focused ion beam milling. Plasma etching is performed in a plasma chamber associated with a plasma-generating device. As in the wet chemical process described above, it is necessary to remove the specimen from the FIB vacuum chamber, place it in the plasma chamber for etching, and then place it in another high vacuum imaging instrument, such as a scanning electron microscope, for observation. The time required to switch between machines makes the plasma etching process for contrast enhancement unsuitable for production support when process engineers need answers quickly to keep a fabrication line running smoothly. 
     FIB systems are also useful in the design stage of an integrated circuit. When a prototype integrated circuit is fabricated and tested, it is often found that changes to the circuit design are necessary. An FIB system can modify an integrated circuit, allowing changes to be implemented and tested without having to modify the photolithography masks and create a new prototype. Such changes are called “device edits.” The FIB system can sever electrical connections by etching through conductors or create new connection by the selective deposition of conductive materials. 
     Modern circuits can use as many as twelve or more conductive metal layers, separated by insulators. In debugging a circuit design, it may be necessary to create connections between buried connectors. This can be done by FIB milling a hole, or “via,” through the insulating layers above a conductor to expose the underlying conductor. When the underlying conductors are deeply buried, however, the material sputtered at the bottom of the hole during milling tends to redeposit on the side walls of the hole. Thus, it becomes impossible to mill a hole having a high aspect ratio, that is, a deep hole much deeper than it is wide. It is necessary, therefore, to mill a wide hole  2 , as shown in FIG. 11, to expose a deep conductor  3 . Unfortunately, with the dense packing of modern integrated circuits, a wide hole may damage circuitry on other layers, such as conductor  4 . 
     To FIB mill a high aspect ratio hole  5  as shown in FIG. 12, XeF 2  gas can be used to enhance the FIB etching of the inter layer dielectric. Unfortunately, XeF 2  is highly toxic and very corrosive. Moreover, XeF 2  etches copper, which is becoming widely used as a conductor in the fabrication of integrated circuits, because of its high conductivity. It is difficult to etch through an IDL using XeF 2  to a copper conductor without etching and significantly degrading the exposed copper conductors, which then exhibit increased resistivity and can render the rewiring of the circuit ineffective. FIG. 12 shows that by using XeF 2 , conductor  4  is undamaged, but conductor  3  is inadvertently etched by the XeF 2  gas and significantly reduced in the cross section, and therefore increased in resistivity. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved method and apparatus to selectively etch materials using a charged particle ion beam. 
     It is an object of the present invention to provide an improved method and apparatus to selectively etch ILD materials using a charged particle beam. 
     It is another object of the invention to provide additional compounds for charged particle beam etching by modifying etchant compounds to increase their adsorption onto the surface. 
     It is a further object of the invention to provide an improved method and apparatus to distinguish ILD layers in an integrated circuit cross section milled by a focused ion beam. 
     It is yet another object of the present invention to provide such a method and apparatus that does not require the specimen to be removed from the vacuum chamber. 
     It is still another object of the present invention to provide an improved method and apparatus for defect analyses in semiconductor integrated circuits. 
     It is yet a further another object of the present invention to provide an improved method and apparatus for process control in integrated circuit semiconductor manufacturing. 
     It is still a further object of the invention to provide rapid analysis of semiconductor processing steps by selectively delineating or removing dielectric layers. 
     It is yet a further object of the invention to facilitate device editing of integrated circuits and, in particular, device editing of integrated circuits including copper conductors. 
     It is still a further object of the invention to provide a method of milling a high aspect ratio via terminating at a copper conductor without significantly degrading the copper conductor. 
     It is yet a further another object of the present invention to provide a gas to enhance the etching of dielectric layers that is less corrosive and toxic than XeF 2 . 
     In accordance with the invention, molecules of an etch-assisting gaseous compound are adsorbed onto the surface of a specimen in a charged particle beam system. The gas causes different materials on the specimen to be etched at different rates in the presence of the charged particle beam. Such selective etching provides an observer with a sharp, clean cross section that allows the various layers in the cross section to be distinguished by an observer. The selective etching also allows the removal of some materials without significantly affecting other materials on a sample. 
     A molecule of the etch-assisting gaseous compound preferably includes an etching portion and a functional group to increase the stickiness of the molecule and enhance adsorption. It is believed that the gas is adsorbed onto the surface of the exposed layers and the charged particle bombardment provides energy to initiate a reaction of the adsorbed gas molecule with the surface material to be etched. The reaction produces volatile products that dissipate in the vacuum chamber, thereby removing material from or etching the specimen. 
     The etch rate is thought to vary for different materials because the strength of the etch reaction may vary with different materials, the sticking coefficient of the gas may be different for different materials, and the reaction products may be different and have different degrees of volatility. The gas may inhibit the etching of some materials by producing a reaction product that is not volatile and that forms a protective film over the second layer. 
     A preferred gaseous compound for practicing the invention comprises a halogenated hydrocarbon with an added functional group to enhance adsorption. For example, 2, 2, 2-trifluoroacetamide selectively etches ILD layers so that they can be distinguished by an observer using SEM or FIB imaging, yet forms a protective film that inhibits further etching on silicon, either single- or poly-crystalline, and metallic layers. 
     In one preferred application, a cross section of the various layers of an integrated circuit is exposed using a liquid metal gallium focused ion beam. After the cross section is exposed, the specimen is tilted and the exposed cross section is ion-beam etched in a second etch step while a gas, such as 2, 2, 2-trifluoroacetamide, is directed at the surface. The gas preferentially assists the ion beam etching, thereby increasing the contrast between or delineating the ILD layers. The second etch step is typically briefer and uses a lower current density that the first etch step. 
     In a second preferred application, a layer of a dielectric material, such as silicon oxide, can be removed to expose a layer of underlying material, such as a polysilicon conductor. The dielectric layer is etched by exposure to an ion beam with a gas such as 2, 2, 2-trifluoroacetamide directed at the area of impact of the gallium ions. The dielectric layer is removed by the etch, whereas the underlying layer is essentially unaffected, thereby exposing the underlying layer for further analysis. The gas reacts with some underlying materials to form a protective film that inhibits further reaction with the underlying material. 
     In another example of removing a dielectric material to expose a conductor, a high aspect ratio via is milled in a layer of dielectric material using a halogenated hydrocarbon gas such as 2, 2, 2-trifluoroacetamide to expose a copper conductor. The preferred gas is less corrosive and toxic than XeF 2 . and reduces the rate at which the ion beam etches the copper conductor after it has milled through the dielectric. The exposed conductor can be electrically connected to another circuit element by depositing an electrically conductive material in the via and electrically connecting, such as by FIB assisted deposition of another conductive, the material in the via to the other circuit element. The other circuit element could be, for example, another conductor-filled, high aspect ratio hole electrically connected to different buried conductor. 
     The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic representation of a focused ion beam system for etching in accordance with the present invention; 
     FIG. 2 is a partial cross-sectional view of a gas containment apparatus employed for injecting gas toward a substrate inside the FIB system of FIG. 1; 
     FIG. 3 is a partial, enlarged side view, broken away, of the gas injection system of FIG. 1; 
     FIG. 4 is a further enlarged schematic side view of the gas injection nozzle of FIG. 3; 
     FIG. 5 is a flow chart showing the steps of a preferred embodiment of a the present invention; 
     FIG. 6 is a schematic representation of a trench milled in an integrated circuit using the apparatus of FIG. 1; 
     FIG. 7 is a schematic representation of the trench of FIG. 6 after etching in accordance with the invention; 
     FIG. 8 is a flow chart showing the steps of another application of the present invention; 
     FIG. 9 is a schematic representation of a defective trench capacitor; 
     FIG. 10 is a schematic representation of the trench capacitor of FIG. 9 after being etched in accordance with the invention; 
     FIG. 11 shows a via milled by an FIB, without using an etch enhancing gas, to expose a buried conductor; 
     FIG. 12 shows a via milled to expose a buried conductor by an FIB using XeF 2  gas, which degrades the conductor; 
     FIG. 13 shows a device edit made in accordance with a preferred embodiment of the present invention; and 
     FIG. 14 is a flow chart showing the preferred steps for the device edit of FIG.  13 . 
    
    
     DETAILED DESCRIPTION 
     The system according to a preferred embodiment of the present invention includes a charged particle beam system that includes a gas injection system for injecting a gaseous component toward the area of the specimen surface impacted by the beam. 
     Referring to FIG. 1, illustrating a focused ion beam system for carrying out the present invention, an evacuated envelope  10  includes an upper neck portion  12  within which are located a liquid metal ion source  14  and a focusing column  16  which includes extractor electrode means 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 suitably comprises a semiconductor device positioned on movable X-Y stage  24  within lower chamber  26 . Components for generating, focusing, and directing the ion beam are referred to collectively as ion beam generator  29 . 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 . 
     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 30 keV ion beam  18  and directing the same downwardly. Deflection controller and amplifier  36 , operated in accordance with a prescribed pattern such as a raster 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 . 
     The source  14  typically provides a metal ion beam of gallium (although other metallic ions can be used, for example indium or aluminum). The source is capable of being focused into a sub 0.1 micron width beam at sample  22  for either modifying the surface  22  by providing an insulating layer or a metal layer or for the purpose of imaging the surface  22 . An electron multiplier  40  used for detecting secondary 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 . 
     Evacuated envelope  10  preferably also includes a scanning electron microscope (SEM)  43  that can be used to view the results of operations performed by the focused ion beam or that can perform electron beam processing. SEM  43  includes an electron beam generator  41  and associated power supply and controls  45 . A preferred focused ion beam system that includes a SEM is the DualBeam™ XL860 model from FEI Company, the assignee of the instant invention. 
     A gas source  46  is located inwardly of the side of chamber  26  by translation device  48  adapted for positioning said source via support means within bellows  52 . Source  46  includes a reservoir  50  and a heater  54 , which may comprise a membrane type heater and which may be used for raising the temperature of a compound within reservoir  50  to a temperature for providing a suitable vapor pressure as hereinafter more fully disclosed. A transfer tube or nozzle  56  comprising a capillary tube provided by a hypodermic needle extends from reservoir  50  and is connected thereto via control valve  58  adapted for releasing gaseous vapor. The nozzle is extended and translated in orthogonal directions substantially perpendicular to its axis employing translation apparatus  48 , so that gaseous vapor can be aimed directly toward a region on the top surface of sample  22 . 
     A door  60  is opened for inserting sample  22  on stage  24  which may be heated, and also for servicing the reservoir  50 . The door is interlocked so that it cannot be opened if the temperature in reservoir  50  is substantially above room temperature. A gate valve, schematically illustrated at  62 , is closed before door  60  can be opened to seal off the ion source and focusing column apparatus. 
     When reservoir  50  is raised to a desired temperature for vaporizing the compound within reservoir  50 , valve  58  may be opened by withdrawing actuator rod  150  (FIG. 2) from outside the apparatus to open and regulate the position of valve plunger  40 , while the nozzle  56  is directed towards the desired area of the sample as shown enlarged in FIG.  3  and further enlarged in FIG.  4 . Bellows  52  accommodates movement of the nozzle assembly and reservoir relative to the sample without affecting the vacuum within chamber  26 . 
     The vacuum control system along with the heater of gaseous vapor source  46  are operated to provide an appropriate vapor pressure condition for establishing a gaseous vapor flux in chamber as directed toward substrate  22  for etching or depositing material. To establish a given gaseous flux, the reservoir is heated to a predetermined temperature. 
     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 the sample having condensed gaseous vapor adhered thereupon, the ion beam provides energy for initiating a reaction between the etch-enhancing gaseous compound and the substrate and for sputter etching the sample. 
     Deflection controller and amplifier  36  causes the ion beam to be deflected in a desired pattern but wherein deflection of the ion beam is at a rate slow enough for etching sample  22 . Considerations regarding deflection speed, loop time, etc. are well understood by those skilled in the art. 
     As hereinbefore mentioned, the vacuum system provides a vacuum of between approximately 1×10 −6  Torr and 5×10 −4  Torr within chamber  26 . With emission of gaseous vapor, the chamber background pressure is suitably about 1×10 −5  Torr. In an exemplary embodiment, the gaseous source  46  is heated to a temperature for providing a gaseous flux of roughly 1×10 16  to 1×10 17  molecules per second per square centimeter via the capillary tube of the hypodermic needle, while the metal ion source and focusing column are suitably controlled for generating a flux of 1×10 13  to 1×10 15  charged particles per second per square centimeter within the rastered area. Skilled persons can readily determine appropriate pressures and gas flows for any particular application. 
     U.S. Pat. No. 5,435,850 to Rasmussen for a “Gas Injection System” assigned to the assignee of the present invention discloses an apparatus for introducing and directing gaseous vapor toward sample  22 . Referring to FIG. 2 of the present application, the vapor source comprises the reservoir  50  within which the compound to be vaporized is received, the lower end of the reservoir being provided with nozzle  56  in the form of hypodermic needle  56  providing a capillary tube having a small orifice for directing gas toward substrate  22 . The hypodermic needle is attached to the threaded lower end of reservoir  50  by lock fitting  100 . Upper flange  76  of reservoir  50  is secured to the periphery of sealing chamber  78 , the latter depending ultimately from support tube  80 . Support tube  80  is attached with screws to the lower end of bellows  52  as well as to positioning mechanism (not shown) within the bellows. 
     Reservoir  50  comprises a solid metal block elongated in a direction longitudinal of hypodermic needle  56  and provided with a central cylindrical passage  84  through which gas passes to the hypodermic needle. At its lower end, the longitudinal passage  84  narrows at  86 , forming a shoulder for receiving O-ring valve seal  88  that cooperates with the tapered end of valve plunger  90  for regulating the flow of gas from passage  84  to nozzle  56 . Plunger  90  is located at the lower end of actuator  98 , the latter comprising a rod disposed coaxially within passage  84  and extending back through the passage. The outer diameter of actuator  98  is less than the inside diameter of passage  84  in order to form a channel for the delivery of gas. 
     Around central passage  84  in reservoir  50  are disposed a plurality of elongated cylindrical chambers  92  parallel to and in substantially surrounding relation with cylindrical passage  84 , each chamber  92  comprising a longitudinal bore in the reservoir block  50  adapted to receive vaporizable material  94 , such as 2, 2, 2-trifluoroacetamide as hereinafter further described. The upper end  96  of the reservoir is open to sealing chamber  78  wherein gas generated within the reservoir chamber is communicated to central passage  84 . 
     Center rod extension  138  is threadably engaged at  140  by the end of actuator  98  whereby the center of membrane  122  is sealingly disposed between flange  134  and head  142  of center rod extension  138 . Metal bellows  174  separates region  126  above membrane  122  from atmospheric pressure within support tube  80 . The bellows  174  extends between rings  170  and  176 , the former being locked between spacer ring member  120  and heat sink  110 , while the latter is secured to the upper end of center rod extension  138  proximate the end of cavity  184  of sink  110  within which it slides as rod  150  is moved against the bias of spring  154  to open and close the valve comprising plunger  90  and O-ring  88 . 
     Membrane  122  defines the upper wall of sealing chamber  78  and a lower wall of region  126  which is vented to chamber  26 . Actuator  98  includes a radial flange  134  within chamber  78  for centrally engaging the membrane  122  which is peripherally held, while portion  136  of the actuator passes through a central aperture in membrane  122  and into a recess within the head end of center rod extension  138 . Actuator  98  has a threaded portion  140  adapted to engage a mating thread in center rod extension  138 . 
     The center rod extension  138  is provided with an upper internal threaded portion  144  mating with threads  14  at the lower end of actuating rod  150 . Rod  150  is adapted to receive linear motion under the control of means within the positioning mechanism inside bellows  52  in FIG. 1 or therebeyond. Upper cavity  152  in heat sink  110  houses spring  154  acting between the heat sink and the upper end of center rod extension  138  so that the center rod extension and attached parts including actuator  98  are normally biased in a direction for closing plunger  90  against O-ring  88  to close off the flow of gas. However, when rod  150  is pulled upwardly (by means not shown) the valve is opened as center rod extension  138  and ring  176  slide within lower recess  184  in heat sink  110 . The membrane  122  flexes with movement of the actuator. 
     Upper end portion  158  of heat sink  110  is of reduced cylindrical diameter and receives therearound a band heater  159  provided electrical current by means not shown, the heater being covered and held in place by shrink band  160 . A thermistor  162  is embedded within portion  158  of the heat sink, and when electrical current is supplied to band heater  159 , the thermistor  162  provides feedback to a control circuit for regulating the temperature of the heat sink at a desired elevated level for heating the reservoir  50  and the material therewithin. The heater and control therefor are conveniently located outside the vacuum region of chamber  26  eliminating electrical feedthroughs, but the heat generated is conducted via the vacuum wall to the reservoir. 
     The gas injection system  46  forms a housing providing an enclosure for generating and containing gas therewithin, the enclosure including chambers  92  and central passage  84  of reservoir  50  as well as sealing chamber  78  surrounded by the lower end of sealing member  102 . The gas tight enclosure additionally comprises the flexible rubber membrane  122  clamped between sealing member  102  and spacer ring member  120  at the periphery thereof, while also being centrally clamped in sealing relation to actuator  98  between actuator flange  134  and the head  142  of center rod extension  138  as previously mentioned. 
     Another type of gas delivery system is described in U.S. Pat. No. 5,149,974 to Kirch et al. for “Gas Delivery For Ion Beam Deposition and Etching.” This gas delivery system introduces a gas into a cylinder positioned above the specimen and co-axial with the ion beam. The cylinder has apertures for the ion beam to enter and exit, and the gas migrates to the specimen surface through the bottom aperture. The cylinder may also include a deflection means for applying an electric or magnetic field to deflect secondary particles out of the cylinder for detection. 
     A preferred system for milling high aspect ratio holes is similar to that described by Kirch et al., but uses a needle co-axial with the ion beam as the cylinder. A co-axial needle gas delivery system has been found to be useful for milling holes having an aspect ratio of approximately 16:1. 
     FIG. 5 is a flow chart showing the steps of a typical defect analysis application of the present invention. Step  200  shows that an engineer locates the defect on the integrated circuit sample. The defective element may be located, for example, by its failure during electrical testing. Step  202  shows that a trench  210 , as shown in FIG. 6, is milled in an integrated circuit  214  on a silicon substrate  216  to expose a cross-sectional face  218  of ILD layers  220  and metal layers  226 . The edges of ILD layers  220  and metal layers  226  are exposed in cross sectional face  218 . The trench is milled relatively quickly using a beam of preferably about 5 nanoamps, depending on the size of the trench being milled, to provide a current density of approximately 2 mCoul/mm 2 . 
     The ILD layers may include, for example, silicon oxides, silicon nitrides, low dielectric constant (k) dielectrics, spun-on glasses, polymers, and other similar materials. Etching other dielectric materials, such as passivation layers, are also within the scope of the invention and the term ILD used herein can include such other layers. Skilled persons will recognize that each of these materials can be deposited using various methods that impart to the layer properties that are required for the functioning of that particular layer. For example, a thin silicon oxide layer used as part of a gate in a transistor (a gate oxide) will typically be deposited by a different process and be much denser than a thicker, less dense layer of PE CVD oxide used between two metallic conductors. 
     After trench  210  is initially milled, step  230  shows that the sample is tilted approximately 45 degrees to present face  218  to the focused ion beam. Step  234  shows that face  218  is etched at a lower beam current in the presence of an etch-enhancing gaseous compound that selectively etches exposed layers  220  so that an observer can distinguish among the layers. FIG. 7 illustrates in an exaggerated manner how etching face  218  in accordance with the invention allows ILD layers  220  to be distinguished by a viewer. 
     The ion-beam etching step  234  is relatively brief lading approximately two minutes, and removes only a few nanometers of material. A typical beam current, which will vary with the size of the cross section, is 11 picoAmps, which produces a beam current density of approximately 15 to 20 nCoul/mm 2 . Higher current densities could be used in step  234 , but currents as high as 25-30 nCoul/μm 2  begin to cause distortion in the cross-section face  218 . 
     The energy in the ion beam is typically between30 keV and 50 keV, although ion beam energy of less than 30 keV could be used. Ion beams at 30 keV result in less sputtering than higher energy ions, thereby reducing the non-selective removal of material and increasing the contribution of the gas to the removal of surface material. Skilled persons can readily adjust the etch time, gas flow, and ion beam characteristics to suit the particular materials and size of the cross section being exposed. 
     Step  236  shows that the cross section face  218  is viewed, preferably using SEM  43 . Cross section face  218  can also be viewed using the imaging capability of the FIB system. The inventive process provides a sharp, clear image of the cross section face  218  so that a user can identify defects or irregularities in the exposed layers. 
     Optional step  238  shows that before imaging the cross-sectional face  218  by SEM  43 , the cross section face  18  is coated with a thin conductive coating to improve the image by reducing beam-induced charging. The coating, which can include, for example, carbon or a metal such as platinum, and is preferably deposited using charged particle beam assisted deposition. For example, after the chemically assisted etching to delineate the ILDs, a needle  36  connected to a reservoir  50  containing a platinum compound, such as methylcyclopentadienyl trimethyl platinum, could be inserted into lower chamber  26 , while the cross section is bombarded again with the ion beam to deposit a few atomic thicknesses of the conductor. Alternatively, the conductive coating can be deposited by charged particle beam assisted deposition using the electron beam of SEM  43 , which would damage the substrate less than would ion-beam induced deposition. 
     The etch-assisting gas used in the invention comprises a gaseous compound that will etch the different layers in a different manner so as to allow the layers to be distinguished upon observation. The gaseous compound may etch the different layers at a different etching rates or may merely produce different surface appearances or textures on different materials. To expedite transport of the compound into the vacuum chamber and to the substrate surface, the etch-assisting compound is preferably in the gas phase at or near room temperature and at the pressures present in the focused ion beam vacuum chamber. The gas preferably combines with the material to be etched to produce volatile compounds that will not remain on the surface of the substrate. The volatile reaction products will migrate into the vacuum chamber and be removed. 
     The gaseous compound preferably is characterized by a sticking coefficient that is sufficiently high to ensure that molecules will adhere to the substrate surface in sufficient concentrations to react with the surface molecules in the presence of the ion beam. One method of ensuring an adequate sticking coefficient is to attach a functional group onto a molecule that etches the substrate layers in the presence of the ion beam. 
     A preferred gas, 2, 2, 2-trifluoroacetamide (CF 3 CONH 2 ),                           
     includes the functional amido group that is believed to enhance the stickiness of the compound. 2, 2, 2-trifluoroacetamide is a solid at room temperature and is heated to approximately 30 degrees Celsius in a gas injection system. The CF 3  portion of the molecule or the fluorine that is liberated in the reaction with the ion beam is believed to be responsible for the etching. Other similar compounds, such as trifluoroacetic acid (CF 3 COOH)                           
     and pentafluoropropionic acid (CF 3 CF 2 COOH),                           
     can also be used to implement the present invention. Also thought to be usable are compounds such as trifluoroacetyl fluoride (CF 3 COF)                           
     3, 3, 3-trifluorolactic acid (CF 3 COHCOOH)                           
     and hexafluoroacetone (CF 3 C[O]CF 3 )                           
     The gaseous compound preferably has little or no reaction with the substrate in the absence of the charged particle beam. The pressure of the gaseous compound at the substrate where the ion beam impinges is preferably about 10 −3  mBar, which is thought to be insufficient pressure for a gas phase interaction with the ion beam. The gas flow will depend upon the application, but in many applications the flow is preferably around 2.5×10 −7  moles per second. Thus, it is assumed that the molecules are adsorbed onto the specimen surface and react in the solid state when energy is provided by the ions impacting the specimen. 
     When the preferred etch-assisting gaseous compound 2, 2, 2-trifluoroacetamide etches layers of silicon oxides in the presence of an ion beam, the oxygen in the oxide material is thought to form a volatile compound, such as CO 2  or CO, with the carbon freed as the etch assisting gas compound reacts with the silicon oxide, thereby removing the carbon from the surface. Similar volatile compounds, such as H 2 0, NH 2 , and OCNH 2 , are formed during the reaction of the gaseous compound with the substrate. When etching nitride layers, volatile products include SiF 2 , SiF 4  and N 2 . The volatile compounds are eventually evacuated by the vacuum pump. 
     Sample materials that do not provide oxygen or another material with which to form volatile compounds to liberate the carbon have deposited upon them a film including the carbon and fluorine. Such materials include single crystal silicon, poly-crystalline silicon, and metals. The deposited film inhibits further etching of the silicon, thereby greatly enhancing the selectivity of etching silicon oxides over silicon and metals. The process can be use to deposit a protective film onto silicon or similar materials. Other compounds, such as perfluorooctanoic acid, (CF 3 ) 8 COOH, are efficient at depositing carbon bearing films, because of the large amount of carbon present in each molecule. To avoid depositing films, it preferable to have a simple etching portion of the molecule, such as a single CF 3 . 
     FIG. 8 is a flowchart showing a process of defect analysis for a trench capacitor. FIG. 9 and 10 illustrate a trench capacitor  250  before and after processing in accordance with the invention. The defect analysis process of FIG. 8 uses the selectivity of the gas-enhanced charged particle beam etch to remove a first material while minimally affecting a nearby second material. 
     Step  254  shows that trench capacitor  250  is exposed by removing layers above it, preferably by ion beam milling. FIG. 9 shows trench capacitor  250  formed by trenches that are filled with a conductive polysilicon material  262  and that are electrically isolated from the silicon substrate  256  by a silicon oxide dielectric  266 . FIG. 9 shows a defect  270  in dielectric layer  266 . When trench capacitor  250  is used, defect  270  will cause electrical charge to leak from the capacitor, causing it to fail. 
     Step  274  shows that silicon oxide dielectric  266  is etched in accordance with the invention, using a focused ion beam and an etch-enhancing gas such as 2, 2, 2-trifluoroacetamide. FIG. 10 shows trench capacitor  250  with silicon oxide dielectric  260  removed. The selectivity of the invention causes the oxide dielectric  266  to be etched at a much higher rate than polysilicon material  262 . The silicon oxide material below defect  270  will not be removed, because polysilicon material  262 , which is essentially unetched, will mask and therefore prevent the ion beam from etching the material beneath it. Step  278  shows that trench capacitor  250  is inspected and defect  270  is readily apparent to the viewer. Using the gaseous compound described above, it is possible to rapidly etch dielectric oxide layer  266  with little or no effect on the nearby polysilicon. 
     The prior art used a gas, such as XeF 2 , to etch the oxide, but XeF 2  etched polysilicon conductors faster than it etched silicon oxide dielectric layers. When etching a dielectric layer to expose an underlying polysilicon layer, it was difficult, yet vital, to stop the etching process at the precise moment that polysilicon conductors was exposed to prevent etching of the polysilicon. With the selectivity of the invention, such precise timing is no longer necessary. 
     The gaseous compounds described above are illustrative and do not limit the scope of the invention. Other compounds will be useful in different applications. For example, chlorinated hydrocarbons, such as 2,2,2-trichloroacetamide                           
     may be used for etching aluminum oxides. Chlorides are preferable for etching aluminum oxides because the etch product of fluorine and aluminum, aluminum fluoride, is not sufficiently volatile to evaporate from the substrate and be removed by the vacuum system. Tri-bromides may also be useful for etching other oxides of materials, such as Al 2 O 3  or GeO 4 . Each of these compounds may include, if necessary, functional groups that increase the sticking coefficient and therefore the adsorption of the molecules to the substrate surface. 
     In another example, the invention is used to mill a high aspect ratio via through a dielectric material to provide a conductive path to a buried conductor. FIG. 13 shows a cross section of a typical integrated circuit  290  fabricated on a substrate  294 . Integrate circuit  290  includes multiple conductors  300 , including a conductor  304  and a conductor  306 . The conductors are separated by a dielectric material  314 . FIG. 13 shows a via  342  filled with an electrically conductive material  352  to provide at the surface of substrate  294  an electrical contact to conductor  304 . A via  344  filled with electrically conductive material  352  similarly provides an electrical contact to conductor  306 . Filled vias  342  and  344  are connected by a conductor  364  to establish an electrical contact between conductors  304  and  306 . 
     The conductors preferably comprise copper. The high electrical conductivity of copper compared to, for example, aluminum allows the conductors to be smaller, thereby increasing the number of devices per area and increasing the speed of the devices. The increased density of the copper conductors also permit a circuit to be implemented with a reduced number of layers. The increased density of the conductors requires that vias, such as via  342  and via  346  milled to connect to buried conductors, be of relatively small diameter to avoid etching away nearby conductors  300  on intermediate levels. Such small diameter, high aspect ratio vias require an etch-enhancing gas to prevent redeposition of the sputter material in the bottom of the hole. 
     It will be understood that dielectric  314  is typically composed of several different layers, although with regard to exposing buried conductors, dielectric layer  314  will be considered as a single material to be etched. The dielectric material is preferably an ultra low k dielectric, such as a porous silica xerogel comprised of fluorinated silicon oxide. Integrated circuits using xerogel dielectrics and copper conductors operate at higher speeds than do circuits using aluminum conductors and conventional dielectric layers having higher dielectric constants. Although the invention provides a great advantage over the prior art with regard to integrated circuits using copper conductors and ultra low k dielectrics, the invention is not limited to any particular type of conductor or oxide. 
     FIG. 14 is a flow chart showing the preferred steps used to create the electrical connection shown in FIG. 13 between buried conductors  304  and  306 . Step  330  shows that an etch-enhancing gas, such as 2, 2, 2-trifluoroacetamide, is directed to an area above conductor  304  by gas injection system  46 . Step  332  shows that a focused ion beam is directed from ion beam generator  29  to the area toward which the etch-enhancing gas is directed above conductor  304 . The FIB mills via  342  through dielectric  314  to expose a part of conductor  304 . The milling operation is preferably performed as described above in the previous embodiments. 
     If conductor  304  is to be electrically connected to another buried conductor, such as conductor  306 , the process is repeated to expose the other buried conductor. Step  334  shows that the jet of etch-enhancing is optionally redirected, by moving either the jet or substrate  294 , to an area above the second conductor. Because the jet of etch-enhancing gas is significantly wider than the ion beam, it may be unnecessary to redirect the etch-enhancing gas when milling the second hole. Via  346  is milled in step  348  in the same manner that via  342  was milled in step  332 . 
     The FIB etch rate of copper conductors in the presence of 2,2,2-trichloroacetamide is approximately one half the etch rate of the copper conductor by the FIB alone. The reaction by-products of the gas and the copper are thought to be CuF 2  and carbon, which are not highly volatile at the temperatures of the vacuum chamber, and are thus thought to reduce the etch rate of the FIB. 
     Step  350  shows that vias  342  and  344  are filled with a conductive material  352 , preferably using FIB-enhanced deposition. For example, vias  342  and  344  could be filled with tungsten by irradiating the via with the FIB as a stream of a tungsten containing compound, such as tungsten hexacarbonyl, is directed toward the substrate surface. Step  360  shows that when the vias are filled, the conductive materials in the vias are connected by conductor  364 , for example, a platinum conductor, deposited using FIB assisted deposition with a platinum containing compound, such as methylcyclopentadienyl trimethyl platinum. Such metal deposition processes are known in the art. Although FIG. 13 shows the two conductors being electrically connected as being in the same metal layer and near each other, the invention can be used to connect conductors in different layers and on different parts of the substrate. 
     Although the invention has a particularly useful application in selectively etching ILDs on semiconductors, the etch selectivity of the invention is useful in a wide variety of materials and applications, and is not limited to the applications and materials described above. Moreover, although the invention has been described with the use of a liquid metal focused ion beam, skilled persons will recognize that other charged particle beams, such as electron beams, may also be used without departing from the scope of the invention. 
     While a preferred embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.