Patent Publication Number: US-2013250293-A1

Title: Method and Apparatus for Actively Monitoring an Inductively-Coupled Plasma Ion Source using an Optical Spectrometer

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to methods and structures for active monitoring of an inductively coupled plasma (ICP) ion source in a focused ion beam (FIB) system. 
     BACKGROUND OF THE INVENTION 
     Focused ion beam systems are used in a variety of applications in integrated circuit manufacturing and nanotechnology to create and alter microscopic and nanoscopic structures. Focused ion beams can use a variety of sources to produce ions. Liquid metal ion sources can provide high resolution (a small spot size) but typically produce a low current and are limited in the types of ions available. Different ion species have different properties, which make some ion species more preferable than others for specific applications. For example, whereas helium ions are useful for imaging or light polishing, xenon ions provide higher milling rates that are useful for bulk processing. To produce a very narrow beam for high resolution processing, the current in a beam from an LMIS must be kept relatively low, which means low etch rates and longer processing times. As the beam current is increased beyond a certain point, the resolution rapidly degrades. 
     Plasma ion sources ionize gas in a plasma chamber and extract ions to form a beam that is focused on a work piece. Plasma ion sources may consist of a single gas species or many different types of gases to provide different species of ions. When compared to LMIS sources, plasma ion sources have larger virtual source sizes and are much less bright. An ion beam from a plasma source, therefore, cannot be focused to as small of a spot as the beam from an LMIS, although a plasma source can produce significantly more current. 
     Plasma sources, such as a duoplasmatron source described by Coath et al., “A High-Brightness Duoplasmatron Ion Source Microprobe Secondary Ion Mass Spectroscopy,” R EV . S CI . I NST.  66(2), p. 1018 (1995), have been used as ion sources for ion beam systems, particularly for applications in mass spectroscopy and ion implantation. Inductively coupled plasma (ICP) sources have been used more recently with a focusing column to form a focused beam of charged particles, i.e., ions or electrons. 
     An inductively coupled plasma source is capable of providing charged particles within a narrow energy range, which allows the particles to be focused to a smaller spot than ions from a duoplasmatron source. ICP sources, such as the one described by Keller et al., for “Magnetically enhanced, inductively coupled plasma source for a focused ion beam system,” U.S. Pat. No. 7,241,361, which is assigned to the assignee of the present invention and incorporated herein by reference, includes a radio frequency (RF) antenna typically wrapped around a ceramic plasma chamber. The RF antenna provides energy to maintain the gas in an ionized state within the chamber. Because the virtual source size of a plasma source is much larger than the virtual source size of an LMIS, the plasma source is much less bright. 
     Unfortunately, drift and instabilities in plasma source emission are common. The causes and possible corrective actions for these types of instabilities are often difficult to discern. Various means for monitoring plasma conditions are known, but these methods have a number of disadvantages. Monitoring gas purity or species composition is possible using a mass spectrometer, but the equipment required is expensive, especially for high atomic mass gas species such as xenon (131 AMU). Monitoring plasma density can be achieved using a Langmuir probe, but this is also quite expensive and difficult to implement in a 30 kV plasma. As a result, the active monitoring of plasma conditions for adjustment and control of the source is not done at present. 
     At present, it is also quite difficult to monitor, control and optimize the distribution of ion species (whether for singly- or multiply-ionized or multi-atomic ions) emitted by ICP ion sources. Monitoring of the ion species would be desirable to ensure the proper species of ions and the desired quantity of said species is being emitted, whether a single species or multiple species is desired. Such monitoring would also be desirable to detect the presence of unwanted contaminant species in plasma reaction chamber. If a mass filtering system is malfunctioning, inadequate, or not employed, such contaminants may travel down the focusing column along with the desired beam species. 
     What is needed is a method and apparatus for active monitoring of plasma conditions of an inductively coupled plasma (ICP) ion source, both for adjustment and control of the source and to detect the presence of unwanted contaminant species in the plasma reaction chamber. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method and apparatus for actively monitoring conditions of a plasma source for adjustment and control of the source and to detect the presence of unwanted contaminant species in the plasma reaction chamber. 
     In accordance with preferred embodiments, spectral analysis is used to quantify the components of the plasma and relay other useful information, such as the relative intensity and the composition of ion species within the source tube. Feedback control loops use this information to regulate and monitor the source and other components of the charged particle beam apparatus to ensure proper compliance. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an example spectrum from a xenon plasma source; 
         FIG. 2  is a schematic diagram of a particle optical apparatus with ICP ion source embodying the light-optical spectrometer monitoring and feedback control of the present invention; 
         FIG. 3  is a flowchart representing the initial start-up sequence for the ICP ion source according to the invention; 
         FIG. 4  is a flowchart corresponding to a procedure for oxygen plasma cleaning of the interior of the source tube; 
         FIG. 5  is a flowchart corresponding to the procedure for bringing the ICP source and FIB column on-line for processing of a sample; and 
         FIG. 6  is a flowchart corresponding to an optional procedure for endpoint detection using a second spectrometer which collects light emitted from the sample due to the impact of the focused ion beam with the sample surface. 
     
    
    
     The accompanying drawings are intended to aid in understanding the present invention and, unless otherwise indicated, are not drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention employ information available from the optical emission of the IC plasma source to control aspects of both the source and column operation, such as gas switching, gas mixing, cleaning the source tube of contaminants, setting the mass filter selection, and gating the source and column. Applicants have discovered that the ICP source emits sufficient light to allow the optical emissions to be sampled by fiber optic cable and spectrally analyzed. During spectral analysis, the constituent parts of the plasma can be quantified, and both the absolute and relative intensities of the spectral lines used to provide meaningful information that can be used to control parts of the source and/or system. The spectra can easily be measured with a low-cost spectrometer, such as the USB650 Red Tide Spectrometer available from Ocean Optics. In this invention, the intensity of a multitude of characteristic plasma photon wavelengths is measured in real-time and used in one or more feedback loops regulating the source and/or column, for example, by controlling system operations such as blanking, gas switching, gas-mixing, RF power, cleaning, plasma replenishing, etc. 
     Preferred embodiments of the present invention thus use the detection and analysis of characteristic photons emitted from a plasma source to assist in the maintenance of optimal plasma conditions for the extraction of charged particles. As seen in a prior art inductively coupled plasma ion source, when xenon is used as the source gas species, a green light is emitted from the plasma and passes through a translucent Al 2 O 3  plasma source cell wall. When argon is used as the source gas species, the emitted light has a pink color. Furthermore, it is known that the apparent intensity and color of the plasma also changes due to various input factors such as plasma cell pressure and power transmitted into the plasma. The observed colors are in fact a mixture of characteristic energy photons corresponding to the energy transitions in the atomic levels of the plasma. Although the discrete photon frequencies cannot be distinguished by the human eye, these discrete photon frequencies can be easily distinguished by a typical prior-art spectrometer. In preferred embodiments of the present invention, the relative intensity of a multitude of characteristic plasma photon frequencies is periodically quantified and used as a metric for one or more feedback loops that regulate the plasma conditions for stable, optimized performance. Feedback parameters regulated based upon the intensity of certain photon frequencies could include but are not limited to plasma cell pressure and source power transmitted from the RF generator. 
       FIG. 1  shows an exemplary spectrum from a xenon plasma source. Applicants have discovered that the intensity of the plasma can be quantified by the intensity ratio of the peaks on the spectrum. For example, the intensity ratio of peak  102  at an approximate wavelength of 700 nm to peaks  104 ,  106 , and/or  108  above a wavelength of 800 nm provides a reliable indicator of the plasma intensity, which can then be used in feedback loops to monitor the plasma source and the components of the ion beam column. Parameters such as plasma cell gas pressure and RF power can then be adjusted, either automatically or by an operator, to achieve a desired optimum source spectrum. 
     In preferred embodiments of the present invention, the following parameters are examples of the parameters that can be monitored in real time and used to regulate/optimize the plasma conditions:
         1) The existence of a plasma within the source tube of the ICP ion source, as evidenced by the collection of light emitted from within the source tube, which may be combined with information on the reflected power returning to the RF power supply;   2) The intensity of plasma generation within the source tube, as evidenced by the intensity of the light emitted from within the source tube;   3) The elemental composition of the plasma within the source tube in cases where multiple gas species are being simultaneously ionized, as evidenced by a spectrometric analysis of the light emitted from within the source tube;   4) The ion species composition in cases of single gas species being ionized—e.g., for ionization of oxygen, the distribution of O + , O 2   + , O 2+ , etc., as evidenced by a spectrometric analysis of the light emitted from within the source tube;   5) The combined ion species composition in gases where multiple gas species are being simultaneously ionized—this is a combination of 3) and 4), above, as evidenced by a spectrometric analysis of the light emitted from within the source tube; and   6) The particle-sample reactions (for example, chemical reactions and non-chemical reactions such as cathodoluminescence and x-ray emission) proceeding at or near the surface of a sample being processed by the ion beam produced by the ICP ion source, as evidenced by the spectrometric analysis of the light emitted from the surface or near the surface.       

       FIG. 2  is a schematic diagram of a particle optical apparatus  200  with an inductively coupled plasma (ICP) ion source embodying the light-optical spectrometer monitoring and feedback control of the present invention. A gas manifold  202  may comprise one or more gas supplies  204  each with its own flow and/or pressure regulator  206 . The gas supplies  204  can comprise a variety of different gases or can also be multiple sources of the same gas. Each of the gas supplies  204  may be independently selected by automatic opening of their respective flow valves  208 . Activation of a single flow valve corresponds to the case where a single gas species is desired, while activation of more than one flow valve simultaneously corresponds to the case where a gas mixture is desired. The single gas, or gas mixture, is then regulated by a final flow valve  210  leading to gas feed tube  212  to plasma source tube  220 , within which a plasma is generated. The plasma source tube can alternatively be called a plasma chamber. A pressure gauge  214  monitors the gas pressure in the gas feed tube  212 . The gas feed tube  212  may be evacuated by source turbopump  266  through vent valve  268 . Source turbopump  266  can also pump unwanted contaminants, for example, particles from a formed coating, from the source tube  220 . 
     RF power is provided by RF power supply  223  and is fed to the plasma tube  220  through a match box  222  which is connected to an RF antenna  224  surrounding the source tube  220 . A split Faraday shield (not shown) is preferably positioned between the antenna  224  and plasma tube  220  to reduce capacitive coupling, thereby reducing the energy spread of the extracted charged particles, and allowing the particles to be focused to a smaller spot. RF power supply  223  could also apply power to the antenna in a “balanced” manner, as described in U.S. Pat. No. 7,241,361, to further reduce energy spread of the particles in the extracted beam. Near the bottom of the plasma source tube  220 , a source electrode  226  is biased to a high voltage to apply a high voltage to ions from the plasma. A biasing source power supply  264  is connected to the source biasing electrode  226  and the plasma is initiated by igniter  258 . The source biasing electrode can be biased to a large negative voltage when an electron beam or beam of negative ions is to be generated or to a high positive voltage when a beam of positive ions is to be generated. For example, the plasma is typically biased to about positive 30 kV when ions are extracted for ion beam milling; the plasma is typically biased to between about negative 20 kV and negative 30 kV when electrons are extracted for EDS analysis or when negative ions are extracted for secondary ion mass spectrometric analysis; and the plasma is typically biased to between about negative 1 kV and negative 10 kV when electrons are extracted for forming a scanning electron beam image. Use of an ICP with a split faraday shield and a balanced antenna to reduce the beam energy spread facilitates the production of a higher resolution beam which is suitable for some applications in which a large spot size would not be adequate. 
     An extractor electrode  228  in focusing column  250  is biased by a power supply  260 , referenced to the output voltage of the biasing source power supply  264 . The extractor electrode power supply  260  may be a bipolar power supply. A condenser electrode  230  in the column is biased by a power supply  262 , also referenced to the output voltage of the biasing power supply  264  and also possibly a bipolar power supply. 
     Ions are extracted from the plasma contained in the source tube  220  due to the high electric field induced at the lower end of the source tube  220  by the bias voltage on the extractor electrode  228  relative to the voltage on the source biasing electrode  226 . The ions extracted from the source tube  220  emerge downwards through the opening, or aperture, in the source-biasing electrode  226 , forming a charged particle beam (not shown) which enters the optical column  250 . 
     Two electrostatic einzel lenses  232  and  234  within the column are shown for focusing the ion beam onto sample  280  mounted on stage  282 , which is controlled by stage control  286  at the bottom of sample chamber  284 ; however the exact number and type of lenses is not part of the present invention. A sample chamber turbopump  286  ensures a vacuum is maintained within the chamber  284  and column  250  by removing excess gas particles. Turbopump  286  can also be used to remove contaminants from the column  250  and/or sample chamber  284 . Electrostatic lenses  232  and  234  are controlled by high voltage power supplies  233  and  235 , respectively. The lenses  232  and  234  can be also bipolar lenses. A mass filter  240  controlled by mass filter control  242  may be located within the column for selection of a single ion species out of a multiplicity of ion species which may be produced by the ICP ion source. This multiplicity of ion species may comprise various types of ions of a single element, such as O + , O 2   + , or O 2+ , or in the case of multi-element ionization (e.g., O 2 +Xe), this multiplicity of ion species may comprise, for example, O + , O 2   + , or O 2+ , combined with Xe + . The mass filter  240  typically comprises a deflection in the beam path or a chicane, which is used to separate ions. The particles can be deflected, for example, by a magnetic deflection element such as a dipole or a quadrupole by an E×B mass filter. The position of the mass filter within the column can be varied, for example, to be positioned before the first electrostatic lens  232 . 
     The beam acceptance angle (and thus the total beam current) can be varied by selection of one beam-acceptance aperture (BAA)  236  out of a multiplicity of apertures which may be mechanically positioned onto the beam axis by BAA Actuator  238 . In some embodiments, a beam blanker  244 , which deflects the beam of charged particles, is controlled by a blanker control  246  and is implemented within the column  250 . A Faraday cup  249  can be placed within the column, such that the blanker  244  deflects the beam onto the cup, and an electrometer  248  measures the current of the particles within the beam. 
     Spectrometer  252  is configured to collect and analyze the light emitted from the plasma (not shown) within the source tube  220 . A fiber-optic cable  254  is shown conducting the light emitted through the walls of the source tube  220  (preferably fabricated from Alumina or another translucent ceramic or quartz) to the spectrometer  252 . Even though a fiber optic cable  254  is depicted, any light collection means to conduct the light from the source tube  220  to the spectrometer  252  is suitable. In some embodiments, parallel collection of multiple wavelengths of the dispersed light may enable efficient monitoring of the plasma emission spectrum. In other embodiments, a single light collector may be repeatedly scanned across the dispersed light spectrum to produce a time-dependent spectrum signal. Spectrometer  252  therefore provides information that allows the monitoring of the plasma source such that the source remains free of contaminants and such that the desired ion species compositions are employed in the ion beam being focused onto the sample  280 . 
     In preferred embodiments, a second spectrometer  290  collects light emitted from the sample  280  as a result of the impact of the charged particle beam with the sample. The signal from spectrometer  290  may, for example, be used for precise endpoint detection during sample processing. A fiber optic cable  292  can similarly be used to conduct light and transfer the light to spectrometer  290 . Although a fiber optic cable  292  is depicted, any light collection means to conduct the light from the sample  280  to the spectrometer  290  is suitable. 
     A programmable logic controller (PLC)  256  provides control and automation for the RF power source  223  and the igniter  258 . The Plasma Source Controller  270  controls the various voltages of the ICP ion source, ion extraction optics, and optionally other components in the system including spectrometer  252 , and PLC  256 . Plasma Source Controller  270  can also provide feedback to Focused Ion Beam (FIB) System Controller  272  from the PLC  256  and the spectrometer  252 . The FIB System Controller  272  has overall responsibility for the proper operation of charged particle apparatus  200 , as illustrated by the various control links shown schematically in  FIG. 2 . 
     The FIB System Controller  272 , for example, is capable of controlling BAA Actuator  238 , high-voltage lens power supplies  233  and  235 , mass filter control  242 , blanker control  246 , electrometer  248 , spectrometer  290 , stage control  286 , and Plasma Source Controller  270 , which in turn controls other components as described above. The user can input a certain set of operating parameters, such as a pre-defined intensity level or a pre-defined max power, to FIB System Controller  272  or Plasma Controller  270 , and these controllers can vary certain system parameters such as RF power, flow, and pressure settings, so that the properties of the plasma match the set of predetermined parameters. The user can also input desired operating parameters such as beam current, lens voltages, beam acceptance angle, mass filter deflection angles, electrode voltages, and beam blanker deflection angles such that the FIB System Controller  272  will monitor and adjust the parameters of the apparatus to match the desired parameters. Skilled persons will recognize that this input can easily be accomplished with a computer or directly to the system controller through another input device. 
     Preferred embodiments of the system include program memory  294  and data memory  296 , which stores data from feedback loops, such as the intensity or composition information as measured from the spectrometer. The collected information from feedback loops can also be displayed, for example, on a computer (not shown) to direct and instruct the user to further action. Information from the feedback loops is used to monitor the conditions of the plasma source and/or the focusing column. In some embodiments, certain operating parameters are adjusted automatically based on analysis of the information from the feedback loops. 
     The controllers and the memory components can provide automated processes, such as autotuning, which automatically adjusts system parameters such as RF power, pressure, or flow settings based on spectrum feedback from the spectrometer. Autotuning can be used to ensure that the correct ion composition is present. Autostop programs can also be used such that when a certain process reaches its endpoint, that process automatically stops. 
     Skilled persons will understand that the components of the particle-optical apparatus, i.e. its lenses, mass filter, etc., can be re-arranged and configured differently without compromising its function Skilled persons will also understand that the operating parameters of the plasma source and the focusing column are typically optimized for the type of particles extracted from the plasma source. Table I shows typical operating parameters for operating with xenon ions and positive oxygen ions. Skilled persons understand that an einzel lens can be operated as an “acceleration lens” or a “deceleration lens,” depending on the voltage on the lens, and also that other electrostatic and magnetic lens configurations can perform the same function in place of an einzel lens. In some embodiments, it is preferable to operate the first and second lenses as accelerating lenses when focusing low energy charged particles and as a decelerating lens when focusing high-energy charged particles. 
     
       
         
           
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                 Operating parameter 
                 Xenon ions 
                 Oxygen ions 
               
               
                   
               
             
            
               
                 Gas pressure in plasma 
                 .01 mbar to 0.4 mbar 
                 .005 mbar to 0.3 mbar 
               
               
                 chamber 
               
               
                 Plasma bias voltage 
                 +1 kV to +30 kV 
                 +1 kV to +30 kV 
               
               
                 Extractor electrode 
                 −2 kV to −15 kV 
                 −2 kV to −15 kV 
               
               
                 voltage (relative to 
               
               
                 beam voltage) 
               
               
                 Deflection voltage range 
                 −150 V to +150 V 
                 −150 V to +150 V 
               
               
                 First lens voltage 
                 0 V to +30 kV 
                 0 V to +30 kV 
               
               
                 Second lens voltage 
                 −15 V to +30 kV 
                 −15 V to +30 kV 
               
               
                 RF Power 
                 25 W to 1000 W 
                 200 W to 1000 W 
               
               
                 Gas pressure in specimen 
                 5 × 10 −7  mbar to 1 × 
                 5 × 10 −7  mbar to 
               
               
                 chamber 
                 10 −7  mbar 
                 1 × 10 −7  mbar 
               
               
                   
               
            
           
         
       
     
     In some preferred embodiments of the present invention, the relative intensity of certain characteristic plasma photon frequencies can be used to detect the presence of unwanted contaminant species in plasma reaction chamber. This can be especially valuable, for example, when a different ion species is introduced for another purpose, such as when a cleaning plasma is used. During operation of the ICP system, the source tubes in the ICP ion sources tend to accumulate a coating on their inner walls which reduces the intensity of light collected outside the source. This intensity reduction sometimes causes the control software to (incorrectly) determine that the plasma is absent or below the desired set-level. 
     To clean the interiors of source tubes in situ, oxygen cleaning-plasmas have been proposed. To date, however, there has been no means for determining when these cleaning processes are complete. Excessive exposure to oxygen plasmas in sources configured for operation with more inert gases (e.g., Xe, Ar, or Ne) may reduce the source lifetime through erosion of the source electrode at the exit of the source tube. Thus, a method is needed to determine the level of proper of exposure to reactive oxygen plasmas in order to minimize these complications of excessive exposure. In preferred embodiments of the present invention, the relative intensity of certain characteristic plasma photon frequencies can be used to both determine when cleaning of the source is appropriate and also when the source has been sufficiently cleaned. 
       FIGS. 3-5  are flowcharts showing the steps of operating an ICP ion source according to preferred embodiments of the present invention.  FIG. 3  is a flowchart  300  representing the initial start-up sequence for the ICP ion source according to the invention. First, in step  302 , the gas supply is turned on. As described in  FIG. 2 , the gas flowing into the source tube  220  of the ICP ion source may comprise one or more different gases, either from separate gas containers or from one or more gas containers which may comprise multi-gas mixtures. In step  304 , a pre-defined gas flow and/or pressure set-point must be reached before turning on the RF supply  223  in step  306 . Once the RF supply  223  has been turned on, step  308  includes tuning the match box  222 . Step  310  includes monitoring the reflected power back into the RF supply  223  until the reflected power has dropped below a pre-defined maximum reflected power. If the reflected power is not less than a pre-defined max value, the matchbox  222  is re-tuned. This re-tuning of the matchbox can be performed automatically. 
     Next, step  312  includes activating spectrometer  252 , and, in step  314 , the light emitted from a plasma within the source tube  220  is acquired as illustrated in  FIG. 2 . At first, in step  316 , the total emission intensity is analyzed by, for example, determining intensity ratios of its peaks in a certain range, and then, in step  318 , the total emission current is compared with a pre-defined minimum light intensity. The specific minimum light intensity for a particular ICP source design can be found empirically, for example by doubling the light intensity of the extinction-threshold plasma condition, the extinction-threshold condition being reached by slowly decreasing the RF power until the point below which plasma is no longer self-sustaining. The minimum light intensity need not be the same for all plasma species, and so should be determined severally as appropriate. If the light intensity falls below this minimum value, the process proceeds with step  320 , in which the system operator is informed through the FIB system operator interface, for example through the Plasma Source Controller  270  or FIB System Controller  272 , and the system subsequently waits for a response from the operator in step  322 , leading to flowchart  400  in  FIG. 4 . If the light intensity exceeds the minimum value, then step  324  includes analyzing the spectrum, and then the start-up procedure moves on to the process shown in flowchart  500  in  FIG. 5 . 
       FIG. 4  is a flowchart  400  corresponding to a procedure for oxygen plasma cleaning of the interior of the source tube. The use of this cleaning process is predicated on the assumption that the reason the detected light emission from the source tube falls below the minimum intensity is that a coating has formed on the inner wall of the source tube, the coating arising from decomposition of either process gases or possibly parts of the source tube and electrode assemblies. This coating is presumed to be of a material, for example, carbon or hydrocarbons which may be oxidized into volatile compounds (e.g., CO, CO 2 , H 2 O, etc.) which may then be pumped away by the Source Turbopump  266 . Flowchart  400  proceeds from step  322  of  FIG. 3 . Step  402  includes determining if any light is detected. It is assumed that the coating never becomes so opaque that no light at all may be collected and analyzed by spectrometer  252 , thus the decision block  402 , in the event that no light is detected, proceeds to step  404 , in which the operator is informed that there is no plasma. Subsequently, step  412  effectively involves the operator taking other corrective action, such as replacing one or more of the gas reservoirs  204 . Even in a situation where the reflected power back to the RF power supply  223  is below the threshold shown as seen in step  310  in  FIG. 3 , it is possible for other parts of the ICP source to absorb the RF power even in the absence of a plasma. This scenario is most likely a highly undesirable situation which should be terminated as soon as possible through either operator intervention or an automatic RF power cutoff to prevent excessive heating of portions of the ICP source (especially components such as o-rings which may overheat and outgas or burn up). 
     When some light is detected, step  406  includes informing the system operator that the source tube may need cleaning. Step  408  includes waiting for operator approval to clean the source tube. Next, step  410  includes the decision step of approving the oxygen tube cleaning. If the operator does not approve source tube cleaning, the process proceeds with step  412 , in which flowchart  400  is exited, and the operator then is free to take whatever corrective actions may be deemed necessary, the same as for the case of no light being detected. If the operator does approve the use of oxygen plasma cleaning on the source tube interior, step  414  includes turning on the cleaning oxygen supply. In step  416 , a pre-defined gas flow and/or pressure set point must be reached before turning on the RF supply  223  in step  418 . Once the RF supply  223  has been turned on, step  420  includes tuning the match box  222 . 
     Step  422  includes monitoring the reflected power back into the RF supply  223  until the reflected power has dropped below a pre-defined maximum reflected power. If the reflected power is not less than a pre-defined max value, the matchbox  222  is retuned until the reflected power is less than the pre-defined maximum power. Once this has been accomplished by adjustments to the settings on the matchbox  222 , the method proceeds with step  424 , in which an oxygen emission spectrum from the light emitted by the plasma within the source tube  220  is acquired. In step  426 , the intensity of this emitted light is then analyzed and monitored in step  428  as an oxygen plasma is maintained within the source tube until the intensity of the emitted light (at a constant plasma power) gradually increases to exceed a predefined intensity level. This analysis of the intensity of the oxygen spectrum provides an endpointing method that determines when cleaning of the plasma tube is complete. This endpointing method is advantageous as overexposure to certain cleaning methods, such as oxygen cleaning plasmas, can be harmful to the plasma source. Once this has been accomplished, step  430  includes turning off both the RF power and the flow of oxygen to the source tube  220 . At this point, feedback system of plasma source controller  270  has identified the crucial endpoint for oxygen cleaning of the source tube, such that potential damage from over-cleaning is avoided. The system should then be ready for use and the flowchart of  FIG. 3  is re-entered at the start block  302 . 
       FIG. 5  is a flowchart  500  corresponding to the procedure for bringing the ICP source and FIB column on-line for processing of a sample. Flowchart  500  is entered after the plasma has been initiated in flowchart  300  in  FIG. 3 , up to the point of an adequate total light emission (“Yes” exiting from block  318  to block  324 ) from the source tube  220 . In step  502 , the spectral data acquired from the source tube  220  is now analyzed more fully, to determine the distribution of ion species within the source tube and to compare this to a predefined desired ion species distribution. If the spectrum does not match, then step  504  includes the system determining whether it has been set to Autotune mode. The composition of source tube can be autotuned, for example, by using automation from the PLC  256  and data from the Plasma Source Controller  270 . If not, step  508  includes notifying the operator that the ion species distribution does not match the predefined distribution and the system then waits for the operator to take other actions. 
     If the system is in Autotune mode, the process proceeds with step  506 , in which various system parameters (e.g., RF power, flow and/or pressure settings for each of the process gases, the total pressure in the gas feed tube, etc.) are systematically varied based on predetermined plasma optimization results. These parameters can be varied automatically based on a predetermined set of input values. As this is done in real-time, spectrometer  252  is continuously monitoring and analyzing the light emission from the source tube until the observed emission spectra matches the predefined spectra for the desired ion species. At this point, the decision block leads to step  510 , in which the BAA for the desired ion beam current is selected. The BAA actuator may be used to select a larger aperture if the beam current is too low or a smaller aperture if the beam current is too high. 
     Step  512  is a decision block to determine if a gas mixture is being ionized. If a gas mixture is being ionized, for example, ions of more than one element are being produced by the ICP ion source, then an additional step  514  is required to set-up the mass filter to select the ion species of interest out of the multiplicity of ion species being produced by the ICP ion source. It is important to note that the multiplicity of ion species can be from different elements with different charge and molecular states. Once the mass filter electric and/or magnetic fields have been set to transmit only the ion species of interest or if there is not a gas mixture being ionized, a beam blanker  244  may be used in step  516  to deflect the ion beam into a Faraday cup  249 , enabling the beam current which would be focused onto the sample  280  to be precisely measured in step  518 . The measured current can be monitored by the system controller  272  and displayed for the system operator in step  520 . 
     In some preferred embodiments of the present invention, a second spectrometer  290  can be used for endpointing during processing of sample  280  by the focused charged particle beam.  FIG. 6  is a flowchart  600  corresponding to an optional procedure in which a second spectrometer  290  collects light emitted from the sample  280  due to the impact of the focused beam with the sample surface and uses the spectral information obtained to determine when sample processing is complete. The procedure would be followed after the successful completion of the procedures illustrated in  FIGS. 3 and 5 , that is, after step  520 . If the procedure described in  FIG. 4  is necessary, it would be completed and then the system would return to complete flowchart  300  in  FIG. 3  and then flowchart  500  in  FIG. 5 , before flowchart  600  of  FIG. 6  could be followed. 
     In the process of  FIG. 6 , spectrometer  290  can have a fiber optic  292  or similar light collecting device directed at the sample surface where the focused ion impacts. Step  602  includes activating spectrometer  290 , and subsequently, in step  604 , spectral information is acquired and analyzed. Two spectra would be predefined for this endpointing process: 1) an initial spectrum characteristic of the light which would be expected to be emitted by the sample when first impacted by the focused charged particle beam, and 2) a “final” spectrum characteristic of the light which would be expected to be emitted by the sample after processing by the focused beam. Examples of these spectra might be:
         1) The spectrum of silicon dioxide, corresponding to the insulating material of a metallization stack on an integrated circuit.   2) The spectrum of copper, corresponding to the conductors within the metallization stack. Normally, these copper conductors would be buried beneath layers of insulating material and the FIB would be used to form high aspect ratio openings to reach these conducting layers in order to perform circuit edit, for example.       

     During the process described in  FIG. 6 , as the FIB first mills material away, the emission spectrum from the sample is expected to be that of the initial sample, called “Spectrum (initial)” in step  608 . Step  608  loops while this is still the case. Once the analyzed spectrum starts to differ from Spectrum (initial), the procedure moves to the next decision block  610 , which compares the analyzed real-time spectra with “Spectrum (final).” For a certain period, determined by the beam processing rate and material thicknesses, the analyzed spectrum will differ from both Spectrum (initial) and Spectrum (final). Once a sufficient amount of break-through using the beam has occurred, the analyzed spectrum will be sufficiently close to Spectrum (final), and in step  612 , the system operator will be notified that the process has reached endpoint. The determination of when an analyzed spectrum is sufficiently close to Spectrum (final) will, in practice, be made by the operator either by trial-and-error or by special knowledge of which spectrum types provide adequate endpointing performance in the process underway. Step  614  includes determining if the system is in Autostop mode. If the system has been set in Autostop mode, the beam will be automatically blanked in step  616 . If the system has not been set in Autostop mode, it will proceed to step  618  and just wait for the system operator to manually respond by presumably blanking the beam. In addition to etching, this endpointing method can also be applied to other sample processing methods such as deposition. 
     Although the description of the present invention above is mainly directed at an apparatus, it should be recognized that a method of using the claimed apparatus would further be within the scope of the present invention. Further, it should be recognized that embodiments of the present invention can be implemented via computer hardware or software, or a combination of both. The methods can be implemented in computer programs using standard programming techniques—including a computer-readable storage medium configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner—according to the methods and figures described in this Specification. Each program may be implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the programs can be implemented in assembly or machine language, if desired. In any case, the language can be a compiled or interpreted language. Moreover, the program can run on dedicated integrated circuits programmed for that purpose. 
     Further, methodologies may be implemented in any type of computing platform, including but not limited to, personal computers, mini-computers, main-frames, workstations, networked or distributed computing environments, computer platforms separate, integral to, or in communication with charged particle tools or other imaging devices, and the like. Aspects of the present invention may be implemented in machine readable code stored on a storage medium or device, whether removable or integral to the computing platform, such as a hard disc, optical read and/or write storage mediums, RAM, ROM, and the like, so that it is readable by a programmable computer, for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. Moreover, machine-readable code, or portions thereof, may be transmitted over a wired or wireless network. The invention described herein includes these and other various types of computer-readable storage media when such media contain instructions or programs for implementing the steps described above in conjunction with a microprocessor or other data processor. The invention also includes the computer itself when programmed according to the methods and techniques described herein. 
     Computer programs can be applied to input data to perform the functions described herein and thereby transform the input data to generate output data. The output information is applied to one or more output devices such as a display monitor. In preferred embodiments of the present invention, the transformed data represents physical and tangible objects, including producing a particular visual depiction of the physical and tangible objects on a display. 
     The invention has broad applicability and can provide many benefits as described and shown in the examples above. The embodiments will vary greatly depending upon the specific application, and not every embodiment will provide all of the benefits and meet all of the objectives that are achievable by the invention. To the extent that any term is not specially defined in this specification, the intent is that the term is to be given its plain and ordinary meaning. The accompanying drawings are intended to aid in understanding the present invention and, unless otherwise indicated, are not drawn to scale. 
     In the discussion herein and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Further, whenever the terms “automatic,” “automated,” or similar terms are used herein, those terms will be understood to include manual initiation of the automatic or automated process or step. The term “FIB” or “focused ion beam” is used herein to refer to any collimated ion beam, including a beam focused by ion optics and shaped ion beams. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.