Abstract:
Methods of etching a semiconductor structure using ion milling with a variable-position endpoint detector to unlayer multiple interconnect layers, including low-k dielectric films. The ion milling process is controlled for each material type to maintain a planar surface with minimal damage to the exposed materials. In so doing, an ion beam mills a first layer and detects an endpoint thereof using an optical detector positioned within the ion beam adjacent the first layer to expose a second layer of low-k dielectric film. Once the low-k dielectric film is exposed, a portion of the low-k dielectric film may be removed to provide spaces therein, which are backfilled with a material and polished to remove the backfill material and a layer of the multiple interconnect metal layers. Still further, the exposed low-k dielectric film may then be removed, and the exposed metal vias polished.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to methods and systems for unlayering multi-layer structures incorporating low-k dielectric materials.  
       BACKGROUND OF THE INVENTION  
       [0002]     Advanced semiconductor designs typically incorporate planarized multilevel structures including alternating layers of insulating materials supporting dual damascene and single damascene metal interconnections. Exemplary structures include alternating layers of insulating films, for example low-k dielectric films, with alternating chemical-mechanical hardmask endstop layers, for example silicon nitride and/or high density plasma oxide. Damascene metal can comprise, for example, copper.  
         [0003]     Selective unlayering of these multilayer structures is often necessary for purposes of manufacturing rework or recovery of wafers, to perform defect yield analysis, and/or for electrical characterization or physical failure analysis of wafers, wafer fragments, individual dies, or packaged dies to perform reliability defect root cause analysis. Regular unlayering of these multi-layer structures can also be done in the course of automated pattern recognition inspection of defects in comparison with electrical test maps.  
         [0004]     Unlayering multi-layer structures including low-k dielectric materials is problematic using known layer removal techniques. In particular, the fragile nature of the low-k dielectric materials cause them to react poorly to processes effective for oxides. For example, the lower modulus of low-k dielectric films are susceptible to damage when exposed to conventional chemical-mechanical polish processes. Wafer delayering for manufacturing rework or recovery of wafers cannot employ conventional delayering processes using plasma, or reactive ion etching or chemical-mechanical polish removal for planar deprocessing low-k films without damaging underlying films and undercutting metal layers. Still other techniques involving incident gallium ion beams can result in undesirable implantation of gallium into the low-k films or produce beam interactions (i.e. chemical bond breakdown in organic components present in some low-k films) producing unwanted electrical leakage paths and electrical shorts.  
         [0005]     Further, conventional processes used to remove overlying metal layers, particularly copper single damascene and copper dual damascene metal, can result in damage to the underlying low-k dielectric layers. For example, conventional chemical-mechanical removal of copper layers can easily scratch, or embed polishing media or slurry into, underlying low-k films, by compromising hardmask endstop materials. Attempts to unlayer multilevel structures using conventional reactive ion etching can produce non-planar etch results due to the presence of porous regions within the low-k film as well as the in-homogenity of the low-k films, themselves.  
         [0006]     Conventional reactive ion etching of copper requires elevated temperatures, producing nonvolatile species that can contaminate low-k dielectric films. Further, reactive ion etch removal of overlying insulating films can result in undercutting of underlying copper metal layers resulting in non-uniform etch removal of underlying low-k dielectric films.  
         [0007]     New and improved processes are thus desirable which facilitate the selective planar de-processing of metal layers, hardmask materials and chemical-mechanical endstop materials over low-k dielectric films without damaging the underlying low-k dielectric layers.  
       SUMMARY OF THE INVENTION  
       [0008]     Two proposed methods involving deprocessing low-k structures with copper metallurgy/copper interconnections are described.  
         [0009]     First, a new and improved apparatus and methods involving collimated Argon ion beam milling/Chemical assisted Argon Ion beam etching is provided for unlayering multi-layer structures including low-k dielectric layers with mininal damage to the low-k dielectric films while maintaining planarity of the unlayered surface. Such methods and apparatus are applicable, for example, to unlayering back-end multilevel metallurgy found in semiconductor devices.  
         [0010]     In accordance with one embodiment of the invention, there is provided a method of ion beam etching a semiconductor structure including a first layer of material overlaying a second layer of low-k dielectric film, comprising the steps of: ion beam-milling the first layer of material; and detecting, with an optical detector positioned in the ion beam adjacent the first layer of material, an endpoint of the first layer; whereby to expose the second layer of low-k dielectric film.  
         [0011]     In another embodiment of the invention, there is provided a system for delayering a semiconductor structure including a first layer of material overlaying a second layer of low-k dielectric film, comprising: a processing chamber; an ion beam milling source in the processing chamber for generating a beam of ions to mill the semiconductor structure; a platen in the processing chamber for supporting the semiconductor structure in the beam of ions; a sapphire crystal endpoint detector in the processing chamber; a photospectrometer outside of the processing chamber; means for connecting the sapphire crystal endpoint detector to the photospectrometer; and means for positioning the sapphire crystal endpoint detector proximate the platen to monitor milling of the semiconductor structure.  
         [0012]     Secondly, a set of processes for deprocessing spun-on low-k structures with copper metallurgy/copper interconnections is described. 
     
    
     DESCRIPTION OF THE DRAWING FIGURES  
       [0013]     These and other objects, features and advantages of the invention will be apparent from a consideration of the Detailed Description of the Invention when read with consideration of the drawing Figures, in which:  
         [0014]      FIG. 1  is a cross-sectional view of a semiconductor device including a back-end, multilevel metal contact structure exemplary of one type to which the invention is applicable; and  
         [0015]      FIGS. 2-7  are cross-sectional views of the multilevel metal contact structure of  FIG. 1  showing successive processing steps in accordance with one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0000]     Methodology for Reworking/Recovering Whole Wafers/Wafer Fragments/Individual Die:  
         [0016]     With reference now to  FIG. 1 , there is shown a semiconductor structure  16  including a silicon device front-end  20  overlain by a multilevel metal back-end structure  18 . Front end  20  comprises a conventional silicon-on-insulator (SOI) construction including a device-bearing, doped semiconductor layer  26  overlying an insulator layer  24  over a single-crystal silicon layer  22 . Semiconductor devices formed by implantation and processing in layer  26  are contacted through back-end structure  18  in the manner described below.  
         [0017]     Continuing with reference to  FIG. 1 , back-end structure  18  is seen to comprise a first-level connector layer including an insulator layer  28 A, such as an oxide, having a metal-filled via  28 B for contacting semiconductor devices on doped silicon layer  26 . Metal-filled via  28 B contacts a source/drain region on the surface of doped silicon layer  26  adjoining a gate structure  28 C of a semiconductor field-effect transistor (FET). If the gate material is silicon oxide, this device is further identified as a metal-oxide FET, or MOSFET.  
         [0018]     A series of four layers of a low-k dielectric material, indicated at  30 A,  32 A,  34 A and  36 A, respectively, overlay layer  28 A, each low-k layer in turn overlain by an insulator layer  30 B,  32 B,  34 B and  36 B. Insulator layers  30 B- 36 B each comprises a hardmask, chemical-mechanical endstop, such as a nitride, which will be used, in a manner described below, as an etching mask in a process for exposing the underlying low-k dielectric layer. Each series of low-k film/insulator layers  30 A/B- 36 A/B includes a metal-filled via interconnect extending there through, the metal-filled vias indicated respectively at  30 D,  32 D,  34 D and  36 D. A series of metal connector layers, indicated respectively at  30 C,  32 C,  34 C and  36 C, are interposed between adjacent vias.  
         [0019]     Two oxide layers, indicated respectively at  38  and  40 , overlie insulator layer  36 , each oxide layer including a metal-filled via  38 D,  40 D and overlying metal layer  38 C,  40 C. A respective silicon nitride chemical-mechanical etch stop layer,  38 A,  40 A, overlies each oxide layer  38 , 40 .  
         [0020]     It will be appreciated that back-end structure  18  illustrates a conventional multilevel metal structure with a first interconnect  28 B, for example comprising tungsten, overlaid by six, and optionally more, levels of metal vias and connectors graphically represented by  30 C/D,  32 C/D,  34 C/D,  36 C/D,  38 C/D and  40 C/D. Typically, these metal interconnections comprise dual damascene copper and single damascene copper. In the described embodiment of the invention, the tungsten metal wiring level identified as  28 B combined with the successive overlying copper metal lands identified as  30 C,  32 C,  34 C,  36 C,  38 C and  40 C would comprise what is known in the art as ‘seven level metal.’ Various methods and materials are known in the art for fabricating this type of structure.  
         [0021]     As described above, low-k dielectric layers are identified at  30 A- 36 A. Typically, these low-k dielectric layers comprise materials having a k factor of about 2.85 or less. Such films typically comprise PECVD-deposited SiCOH films and compounds thereof, PECVD-deposited carbon-doped oxides and other organic polymers and porous oxides. Commercially available low-k dielectric products and materials include Dow Corning&#39;s SiLK™ and porous SiLK™, Applied Materials&#39; Black Diamond™, Novellus&#39; Coral™, Honeywell&#39;s HOSP™, and Trikon Technologies FLOWFILL™, among others.  
         [0022]     Low-k dielectric films have desirable electrical characteristics effective to significantly reduce the lateral and inter-level capacitive effects between closely spaced electrical conductors of multilevel metal. Such conductors include, for example, the dual damascene-formed copper conductors described above.  
         [0023]     As noted above, however, these low-k dielectric materials possess certain chemical and mechanical characteristics that make them prone to damage from certain common semiconductor processes. They are, for example, soft, pliable and porous as well as susceptible to damage through the use of typical semiconductor processes. Processes typically used to remove or unlayer low-k dielectric materials, such as mechanical unlayering, reactive ion etching (RIE), focused ion beam (FIB) techniques, chemical-mechanical polishing and wet chemical removal processes may damage the low-k or cause conductive leakage paths within the dielectric layers themselves.  
         [0024]     It will be understood that the terms “layer” and “film,” and variants thereof, are used interchangeably herein to describe the thin, conformal sheets of semiconductor, insulator and conductive materials that comprise a semiconductor device structure. It will be further understood that the terms “low-k dielectric material” and “SiLK,” and variants thereof, are used interchangeably herein to described the low-k dielectric materials described above.  
         [0025]     With reference now to  FIG. 2 , there is shown semiconductor structure  16  having hardstop layer  40 A and oxide layer  40  removed down to via  40 D. Partial oxide layer  40  may be removed using one of many known processes including, for example, chemical-mechanical polishing or reactive ion etching.  
         [0026]     As further shown in  FIG. 2 , structure  16  is situated in a processing chamber  48 , comprising, for example, one of many commercially known vacuum chambers available with a vacuum feedthrough and a tungsten filament ion beam source or a filament-less source. Processing chamber  48  supports structure  16  on a liquid-cooled platen  46  capable of full rotation and full tilt and of cooling in a range of −15 to +35 degrees centigrade.  
         [0027]     In the described embodiment, a cooling media of 50% di-ionized water and 50% ethylene glycol-antifreeze is used to control the temperature of structure  16 , preventing damaging overheating from incident ion beam energy by heat exchange cooling of platen  46 . Structure  16  is attached to platen  46 , for example, using heat dissipating grease. Alternatively, a cooling helium gas can be metered into a fixture (not shown) holding structure  16  for incident ion beam delayering. Processing chamber  48  accommodates a line  50  for introducing a focused input of a controlled mixture of a selected processing gas(es)  51  into the chamber. Processing chamber  48  further supports an argon ion beam milling source  52 .  
         [0028]     In the described embodiment, ion beam milling source  52  is manufactured by milling tool manufacturer VEECO instruments. The system has been modified with the addition of a gas nozzle for introducing gases into the chamber. Further included are four individual mass flow controllers for metering separate gases controlled via an external computer (not shown) having a conventional RS232 interface to monitor gas flow rates.  
         [0029]     In the described embodiment, milling source  52  comprises a 400 watt pulse-mode 30 kHz switchable DC power supply, avoiding the risk of charge-inducted damage that may be associated with conventional 13.56 mHZ RF generators typically used in RIE/PLASMA etching systems. Milling source  52  preferably includes a circumferential charge neutralizing filament  53  surrounding the collimated Argon beam. Milling source  52  is capable of currents in the range of 300 eV-3 KeV for a full range of incident beam energies.  
         [0030]     While the invention is described with respect to Argon ion beam milling, any inert noble gas species found in the same column of the periodic table as Argon, including helium, neon, xenon, and radon would work. Noble gases ionize in a plasma to form non reactive, inert ions. Argon gas is typically selected for all plasma processes because it is inexpensive and requires the least energy to disassociate into ions and form a plasma.  
         [0031]     Processing chamber  48  further includes a fiber optic photospectrometer endpoint detector system  54 . Endpoint detector system  54  includes a sapphire tip detector  56  positioned within the argon ion beam near the region of interest on semiconductor structure  16 . Sapphire tip detector  56  is connected, through a ferro-fluidic vacuum feed-through accommodating a stainless steel tube housing an appropriate fiber optic cable, the tube and cable assembly indicated at  57 , to a personal-computer based charge-coupled device (CCD) CCD UV-VIS array spectrophotometer  58  located outside of chamber  48 . In the described embodiment, a standard PCI card is used for interfacing spectrophotometer  58  with a computer (PC)  60 . The tube/cable  57  is preferably motor operated in a conventional manner by an electromechanical positioning device  59  so that detector  56  can be repositioned at various process steps to be proximate regions of interest on semiconductor substrate  16 .  
         [0032]     In a manner known in the art, spectrophotometer  58  can be tuned to various peak sensitivities, including copper peak sensitivity, tantalum liner peak sensitivity, carbon peak sensitivity (for such organic low-k dielectric films as SiLK), silicon peak sensitivity (for PECVD silicon oxide and LPCVD/HDP silicon nitride dielectrics), as well as tungsten peak sensitivity for tungsten interconnection, to detect different types of endpoint materials. This spectrophotometer may, for example, be controlled using PC  60  running Visual C++ software code.  
         [0033]     As is further described below, the above-described equipment is used in an argon ion beam milling process to etch back end structure  18 , including the various insulator, metal and low-k dielectric film layers. It will be seen that the etching process results in highly controllable, essentially planar delayering, while avoiding the damage to low-k materials typically associated with prior art processes, including: damage caused by FIB gallium beam induced charge implantation, undesirable anisotropic etch profiles associated with reactive ion etching (RIE) etch profiles, oxygen and H2O uptake in SiLK low-k films, solvent uptake in low-k dielectric films as well as susceptibility to scratching/slurry particle embodiment damage by chemical-mechanical processing.  
         [0034]     Applicant has developed the following, exemplary operating parameters for planarizing low-k dielectric films:  
         [0035]     For planarizing Dow Corning Porous SiLK (porous methyl silsequioxane; k=2.2) 
        Argon ion beam currents in the range of 100 mA/cm 2  to 150 mA/cm 2  (maximum of 300 mA/cm 2 )     Accelerating voltages in the range of 300 eV to 400 eV (maximum of 650 eV)     Angle of Incidence of Argon Beam to stage/sample surface in the range of 7 degrees up to 21 degrees (93 degrees from normal to 79 degrees from normal);     Chuck temperature control in the range of 0 degrees centigrade to 15 degrees centigrade (maximum)     Heat transfer/heat conduction to liquid cooled chuck; Silicone based grease such as MUX for minimum out-gassing and greatest heat dissipation     Stage rotation: about 10 rpm     Charge Neutralizing Current: about 300 uA (maximum)     Argon Magnet Voltage Level: in the range of about 0.80 mV to 0.85 mVolts 
 
 These parameters will result in an etch rate in the range of 250 A/minute to 440 A/minute, depending on incident angle and beam current. 
       
 
         [0044]     For planarizing Dow Corning SiLK (k=2.65) 
        Beam Currents in the range of 150 mA/cm 2  to 250 mA/cm cm 2       Accelerating voltages in the range of 400 eV to 500 eV (maximum of 650 eV)     Angle of Incidence of Argon Beam to stage/sample surface in the range of 7 degrees up to 21 degrees (93 degrees from normal to 79 degrees from normal);     Chuck temperature control in the range of 0 degrees centigrade to 15 degrees centigrade (maximum)     Heat transfer/heat conduction to liquid cooled chuck; Silicone based grease such as MUX for minimum out-gassing and greatest heat dissipation     Stage rotation; about 10 rpm     Charge Neutralizing Current: about 300 uA (maximum)     Argon Magnet Voltage Level; in the range of about 0.80 to 0.85 mV 
 
 These parameters will result in an etch rate in the range of 225 A/minute to 400 A/minute, depending on incident angle and beam current. 
       
 
         [0053]     Still with reference to  FIG. 2 , assembly  57  is used to position sapphire detector  56  proximate the upper surface of remaining oxide layer  38 . Spectrophotometer  58  is adjusted to sense nitride, by optimizing the signal to detect silicon nitride. A selected gas  51 , for example a mixture of CF4 and oxygen, is introduced into chamber  48  and the parameters of ion beam milling source  52  and platen  46  are adjusted in a conventional manner to optimize the etching of oxide and copper. The mixture of CF4 gas and oxygen will preferentially etch oxide and nitride without attacking copper. Ion beam milling would mill copper at one rate while ionized gas would preferentially etch oxide and nitride. Milling source  52  is operated to remove the remaining portion of oxide layer  40  along with the copper in copper-filled via  40 . When photospectrometer  58  detects nitride from nitride layer  38 , the process is terminated.  
         [0054]     With reference now to  FIG. 3 , photospectrometer  58  is adjusted to sense oxide by optimizing the signal to sense oxygen or silicon but NOT nitride, and the parameters of gas  51 , milling source  52  and platen  46  are optimized in a conventional manner to etch nitride and copper. Nitride layer  38 A is thus removed along with copper conductor  38 C, exposing the upper surface of remaining oxide layer  38  and copper filled via  38 D as shown in  FIG. 3 .  
         [0055]     It will be understood that following each etching step, assembly  54  and platen  46  are manipulated to place semiconductor structure  16  and endpoint detector  56  in the optimum physical position for the next etch step. Endpoint detector  56  can be placed directly within the ion beam of milling source  52  proximate the areas of interest being etched. The temperature, angle and position of platen  46  are adjusted to optimize the etch process for the materials being etched.  
         [0056]     With reference now to  FIG. 4 , photospectrometer  58  is adjusted to sense a low-k dielectric film such as SiLK by optimizing for sensing a carbon peak, the principal elemental component of an organic low-k film. By sensing the appropriate principal elemental component, i.e. for conventional oxides, this would be oxygen or silicon; for silicon nitride, this would be nitrogen, the above-described etching processes are used to remove the remaining portion of oxide layer  38 , nitride layer  36  and the copper connectors and copper-filled vias there through, exposing low-k dielectric layer  36 A and copper-filled via  36 D as shown in  FIG. 4 .  
         [0057]     It will be appreciated from a consideration of the process description and drawings that the etching systems and processes that are the subject of the present invention yield highly planar exposed surfaces, regardless of the materials being etched. Further, the processes can be controlled to accurately etch very thin films and very small depths within films, thus enabling the exposure of substantially any desired layers or features within a semiconductor structure.  
         [0058]     With reference now to  FIGS. 5 and 6 , structure  16  is shown with the above-described process steps repeated to expose low-k dielectric layers  34 A ( FIG. 5 ) and  32 A ( FIG. 6 ), respectively. The etching processes used to remove the intermediate nitride layers and copper metallurgy are substantially identical to those described above. Endpoint detector  56  and platen  46  are repositioned intermediate each etching step, in the manner described above, to optimize both the etching process and the endpoint detection.  
         [0059]     With reference now to  FIG. 7 , semiconductor structure  16  is shown with all of back-end structure  18  removed excepting a remaining portion of layer  30 A overlying layer  28 A, and the associated metallurgy  30 D,  28 B and FET gate  28 C.  
         [0000]     Methodology for Unlayering/Reworking Individual Die and Wafer Fragments with Low-K Dielectric Films Using Mechanical Polish/Removal of Metal/Metal Interconnects Combined with RIE or Plasma Deprocessing Steps:  
         [0060]     The following methods allow for unlayering/deprocessing of Low-K spun-on dielectric films where each low k dielectric level has been built (ie. Hardmask, CMP endstop layer is intact) before unlayering/deprocessing is initiated.  
         [0000]     Method (a)  
         [0061]     Using a RIE etch (ie. CF4 process gas) or wet etch, remove nitride cap layer and hardmask (ie. Applied Materials BLOK™). Next remove spun-on Low-K Dielectric with Oxygen Plasma Next deposit an oxide or tetraothrosilicate (TEOS) CVD layer to permit mechanical polish removal of metal/metal interconnection through the underlying SiLK layer to the hardmask or cap nitride layer. Next, deposit replacement cap layer. Resume normal spun on low k Dielectric film deposition.  
         [0000]     Method (b)  
         [0062]     Using a RIE etch (ie. CF4 process gas) or wet etch, remove nitride cap and hard mask (ie. Applied Materials BLOK™). Partially remove spun on low-K Film to top level of metal interconnection. Deposit oxide or CVD tetraorthosilicate insulator film in gap formerly occupied by low k spun-on dielectric film. Mechanically polish/remove via level/metal interconnection and metal level itself. Deposit CVD nitride cap layer. Resume normal spun-on low k dielectric film (ie. Dow Corning SiLK) film deposition.  
         [0000]     Method (c)  
         [0063]     Mechanically polish to remove the nitride cap and hardmask layer overlying metal and low-K spun on dielectric film. Remove spun-on low K dielectric film with oxygen plasma process. Deposit oxide layer or CVD tetraorthosilicate (TEOS) dielectric film in the gap formerly occupied by spun-on Low-k dielectric film. Mechanically polish to remove metal level along with deposited oxide/TEOS layer through underlying nitride cap and stop on hardmask. Deposit new CVD nitride cap layer. Resume normal spun-on low k dielectric film (ie. Dow Corning SiLK) deposition.  
         [0000]     Method (d)  
         [0064]     Mechanically polish to remove nitride cap and hardmask layer overlying metal and low-K spun on dielectric film. Partially remove spun-on low K dielectric film with oxygen plasma process to top of metal interconnection level. Deposit oxide layer or CVD tetraorthosilicate (TEOS) dielectric film in gap formerly occupied by spun-on Low-k dielectric film. Mechanically polish/remove metal level lines along with deposited oxide/TEOS layer. Deposit new CVD nitride cap layer. Resume normal spun-on low k dielectric film (ie. Dow Corning SiLK) deposition.  
         [0000]     Method (e)  
         [0065]     Use RIE (ie. CF4 process gas) or wet etch to remove nitride cap and hardmask layer (ie. BLOK ™). Alternatively, mechanically polish to remove nitride cap layer and hardmask layer (ie. BLOK™). Immerse sample in copper wet etch bath in order to remove metal and metal interconnection levels. Next, wet etch barrier liner (ie. Ta/TaN) material. Next, Plasma etch with oxygen process gas to remove low-k spun on dielectric film. Mechanically polish/remove residual metal interconnection layer and stop at hardmask material. Deposit replacement nitride cap layer. Resume normal spun-on low k dielectric film (ie. Dow Corning SiLK) deposition. Complete BEOL build cycle.  
         [0000]     Method (f)  
         [0066]     Use RIE (ie. CF4 process gas) or wet etch to remove nitride cap and hardmask layer (ie. BLOK ™). Alternatively, mechanical polish removal of the cap nitride and hardmask could be substituted, instead. First, expose wafer to a slow copper etchant to controllably remove the copper metal wiring levels and copper interconnection without etching into the underlying copper metal lands. Once the copper lands and copper interconnection levels are etched away (using a timed etch process), employ a different chemical etchant to remove the barrier liner metallurgy (typically a tantalum/tantalum nitride material) without attacking the underlying copper metal wiring. Next, perform an oxygen plasma etch to remove the spun on low-k dielectric film (ie. Dow Corning SiLK film) which will selectively stop on the underlying silicon nitride cap layer. Mechanically polish off the cap nitride along with any residual copper metallurgy and tantalum/tantalum nitride liner material. Redeposit CVD nitride cap layer. Resume normal spun-on low k dielectric film (ie. Dow Corning SiLK) deposition. Complete BEOL build cycle.  
         [0067]     There have thus been described systems and methods using ion beam etching with endpoint detection for selectively etching multi-layer structures on semiconductor devices. The systems and methods of the present invention enable highly selective and controllable planar removal of different types of materials, including oxides, nitrides, metals and low-k dielectrics such as SiLK. They further permit such removal without damaging the soft, fragile, low-k dielectric materials. There have further been a set of processes for deprocessing spun-on low-k structures with copper metallurgy/copper interconnections.  
         [0068]     The present invention can be used to facilitate, for example, the reworking and/or evaluation of semiconductor devices, particularly those incorporating low-k dielectric films. The invention thus has application in the manufacture, rework and analysis of semiconductor devices.