Patent Publication Number: US-2013248357-A1

Title: Glow discharge milling apparatus and glow discharge milling method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-64219, filed on Mar. 21, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a glow discharge milling apparatus and a glow discharge milling method for milling a sample surface by using sputtering associated with glow discharge. 
     BACKGROUND 
     A technique is known that a sample surface serving as a target of observation or analysis is milled by using glow discharge (for example, see Japanese Patent Publication No. 4143620. At that time, the gas used for sputtering is an inert gas or alternatively a mixed gas obtained by combining inert gases. On the other hand, a technique is known that for the purpose of forming a thermal head used in a thermal printer, sputtering is performed by using a mixed gas consisting of argon and oxygen (for example, see Japanese Patent Application Laid-Open No. H10-305603). 
     Meanwhile, the present inventor has found that when gas obtained by mixing oxygen gas into inert gas is used in sputtering, a high smoothness is obtained in the sample surface after processing. Nevertheless, an increase in the mixing ratio of oxygen gas causes a decrease in the etching rate. On the other hand, when the mixing ratio of oxygen gas is reduced, although the etching rate is increased, the smoothness in the sample surface after processing is reduced. Thus, a suitable mixing ratio of oxygen gas used in sputtering depends on the kind of sample. 
     SUMMARY 
     According to an aspect of the embodiments, a glow discharge milling apparatus milling a sample by using glow discharge, includes: a glow discharge tube in which in an atmosphere of mixed gas supplied through a pipe, a voltage is applied between an internal electrode and a sample placed opposite to the electrode so that glow discharge is generated; a reception part receiving a mixing ratio by which inert gas and oxygen gas are to be mixed with each other; a control part, in accordance with the mixing ratio received by the reception part, controlling the amounts of supply of the inert gas and the oxygen gas; and a supply unit mixing the inert gas and the oxygen gas with each other in accordance with the amounts of supply controlled by the control part and then supplying the mixed gas to said glow discharge tube through said pipe. 
     According to an aspect of the glow discharge milling apparatus, receives a mixing ratio by which inert gas and oxygen gas are to be mixed with each other, and then controls the amounts of supply of the inert gas and the oxygen gas in accordance with the received mixing ratio. The glow discharge milling apparatus mixes the inert gas and the oxygen gas with each other in accordance with the controlled amounts of supply and then supplies the mixed gas to the glow discharge tube through the pipe. Then, in an atmosphere of the supplied mixed gas, the glow discharge milling apparatus generates glow discharge by applying a voltage between the electrode located inside the glow discharge tube and the sample and thereby mills the sample. This realizes that the partial pressure of oxygen in the mixed gas consisting of inert gas and oxygen gas is set up appropriately in accordance with the sample. 
     The object and advantages of the invention will be realized and attained by the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the embodiment, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating outlines of an exemplary hardware configuration of a glow discharge milling apparatus. 
         FIG. 2  is a flow chart illustrating an example of a procedure of glow discharge milling processing. 
         FIGS. 3A and 3B  represent examples of scanning electron microscope photographs each obtained, respectively, before and after milling processing using argon gas alone. 
         FIGS. 4A and 4B  represent examples of scanning electron microscope photographs each obtained, respectively, before and after milling processing using mixed gas consisting of argon gas and oxygen gas. 
         FIGS. 5A and 5B  represent examples of scanning electron microscope photographs each obtained, respectively, before and after milling processing using mixed gas consisting of argon gas and oxygen gas. 
         FIG. 6  is an explanation diagram illustrating an example of record layout of an auto-bias voltage mixing-ratio table. 
         FIG. 7  is a flow chart illustrating an example of a procedure of glow discharge milling processing. 
         FIG. 8  is a flow chart illustrating an example of a procedure of glow discharge milling processing. 
         FIG. 9  is an explanation diagram illustrating an example of record layout of a kind-of-sample mixing-ratio table. 
         FIG. 10  is a flow chart illustrating an example of a procedure of glow discharge milling processing. 
         FIG. 11  is a flow chart illustrating an example of a procedure of glow discharge milling processing. 
         FIG. 12  is a block diagram illustrating outlines of an exemplary hardware configuration of a glow discharge milling apparatus according to Embodiment 4. 
         FIG. 13  is a flow chart illustrating an example of a procedure of glow discharge milling processing. 
         FIG. 14  is a flow chart illustrating an example of a procedure of glow discharge milling processing. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A glow discharge milling apparatus according to the present embodiment is used for generating a sample surface to be observed by an observation instrument or alternatively a sample surface to be analyzed by an analyzing instrument. Observation instruments employable here include an optical microscope, a scanning electron microscope, a transmission electron microscope, a scanning transmission electron microscope, and an X-ray microscope. Analyzing instruments employable here include an X-ray photoelectron spectrometer, an Auger electron spectrograph, a SIMS (Secondary Ion-microprobe Mass Spectrometer), and an EPMA (Electron Probe Micro Analyzer). 
     The gas used in the glow discharge milling apparatus according to the present embodiment is a mixed gas consisting of inert gas and oxygen gas. Inert gases used here include helium gas, neon gas, argon gas, krypton gas, xenon gas, and mixed gas containing these gases. In the following description, the glow discharge milling apparatus performs sputtering of a sample surface by using mixed gas consisting of argon and oxygen. 
     Samples to be milled by the glow discharge milling apparatus according to the present embodiment include inorganic substances, organic substances, and mixtures of inorganic substances and organic substances. Such samples include metallic materials, organic materials, ceramics, glass materials, plastics, stones, minerals, cosmetics, bones, organic single crystal semiconductors, and hybrid multilayer samples consisting of metals/glass materials/organic substances. 
     Embodiment 1 
     Embodiment 1 relates to a mode that the partial pressure of oxygen in mixed gas of argon and oxygen is manually changed in accordance with the property of the sample. 
       FIG. 1  is a block diagram illustrating outlines of an exemplary hardware configuration of a glow discharge milling apparatus  1 . The glow discharge milling apparatus  1  includes a glow discharge tube  2 , a sample pressing member  3 , a power supply part  4 , a gas supply discharge part  5 , and a computer  6 . 
     The glow discharge tube  2  generates glow discharge on a sample S to be milled. The glow discharge tube  2  includes an anode (electrode)  21  and an O-ring  22 . 
     The anode  21  is located at a position biased from a substantial center of the glow discharge tube  2  toward the sample S. The anode  21  includes a disk part and a cylindrical part. The surface of the disk part is substantially in parallel to the surface of the sample S to be milled. Then, the cylindrical part protrudes in a direction extending from the center of the disk part to the sample S. The sample-S-side end of the cylindrical part reaches the vicinity of an opening provided in one end surface of the glow discharge tube  2  opposite to the sample S. Here, the anode  21  is grounded. 
     The O-ring  22  is provided in the surroundings of the above-mentioned opening. The sample S is arranged such that the to-be-milled surface is oriented toward the glow discharge tube  2  and abuts against the O-ring  22 . When the sample S is placed, a space K surrounded by the O-ring  22  forms a closed space. In the space K, the supplied mixed gas is changed into plasma. 
     The sample pressing member  3  presses the sample S toward the glow discharge tube  2 . Further, the sample pressing member  3  serves also as an electrode through which a voltage is applied to the sample S. In the glow discharge milling apparatus  1 , the sample S on which the voltage is applied serves as a cathode. 
     The power supply part  4  is connected to an AC power supply AC. The power supply part  4  generates a high-frequency power. Then, by virtue of the generated high-frequency power, a high-frequency voltage is applied between the anode  21  and the sample S through the sample pressing member  3 . Two modes are available in the high-frequency voltage applied between the anode  21  and the sample S from the power supply part  4 . The two modes are a continuous mode and an intermittent mode. In the continuous mode, the power supply part  4  generates a continuous high-frequency power during a certain period of time and thereby applies a continuous high-frequency voltage between the anode  21  and the sample S. On the other hand, in the intermittent mode, the power supply part  4  generates an intermittent high-frequency power during a certain period of time and thereby applies an intermittent high-frequency voltage between the anode  21  and the sample S. The power supply part  4  is connected to the computer  6  and thereby switches the voltage-application mode in response to an instruction from the computer  6 . 
     The power supply part  4  includes a generator (a detection part)  41  and a matching box  42 . When the voltage-application mode is of intermittent mode, the generator  41  generates a high-frequency power corresponding to the supply frequency and the duty ratio instructed from the computer  6 . The generator  41  detects an output value Pf indicating the power value of the forward wave travelling toward the sample pressing member  3  and a reflection value Pr indicating the power value of the reflected wave generated by reflection of the forward wave in the sample S. Further, the generator  41  detects a voltage (referred to as an auto-bias voltage Vdc) generated on the surface of the sample S onto which the high-frequency voltage is applied. The impedance value of the sample S varies as milling progresses. Then, the generator  41  adjusts the output value Pf such that the difference (Pf−Pr) between the output value Pf and the reflection value Pr is maintained constant. 
     The matching box  42  contains a variable capacitor whose electric capacity is changed by driving by a motor. When the voltage-application mode is of continuous mode, the matching box  42  adjusts the module and the phase by using the variable capacitor and thereby minimizes the reflection value Pr of the reflected wave returned from the sample S. In this manner, when the voltage-application mode is of continuous mode, the matching box  42  maintains constant the output value Pf associated with the high-frequency power generated by the generator  41 . 
     The gas supply discharge part  5  includes a vacuum suction device  51 , an argon gas cylinder  52 , an oxygen gas cylinder  53 , MFCs (control parts)  54 , and a mixer (a supply unit)  55 . Here, the argon gas cylinder  52  and the oxygen gas cylinder  53  may be excluded in the constituting parts of the glow discharge milling apparatus  1 . 
     The vacuum suction device  51  is a vacuum pump for generating vacuum in the space K of the glow discharge tube  2 . For example, the vacuum suction device  51  is constructed from a rotary pump, a diffusion pump, and a turbo-molecular pump. The vacuum suction device  51  and the glow discharge tube  2  are connected to each other through a pipe. In the glow discharge tube  2 , an exhaust hole is provided that penetrates through between the pipe and the space K. The vacuum in the space K is generated by the vacuum suction device  51  through the pipe and the exhaust hole. The vacuum suction device  51  is connected to the computer  6  and operated in response to an instruction from the computer  6 . 
     Here, a user may manually control the vacuum suction device  51  directly. 
     The argon gas cylinder  52  and the oxygen gas cylinder  53  contain argon gas which is an inert gas and oxygen gas, respectively. 
     Two MFCs (mass flow controllers)  54  are respectively connected to the argon gas cylinder  52  and the oxygen gas cylinder  53  through pipes. The two MFCs  54  controls the mass flow rates of the argon gas and the oxygen gas in accordance with the mixing ratio specified from the computer  6 . Further, one MFC  54  is arranged in a middle of a supply pipe (a pipe) SP connecting the mixer  55  to the glow discharge tube  2 . The two MFCs  54  and the one MFC  54  are connected to the computer  6 . 
     The MFCs  54  control the mass flow rates of fluids. However, in place of the MFCs  54 , volume flow control devices controlling the volume flow rates of fluids may be employed. In this case, the volume flow control devices control the volume flow rates of the argon gas and the oxygen gas. 
     The mixer  55  is connected to the two MFCs  54  and the one MFC  54  respectively through the pipe and the supply pipe SP. The mixer  55  mixes uniformly the argon gas and the oxygen gas whose flow rates are respectively controlled by the two MFCs  54 . The mixer  55  supplies the uniformly mixed gas to the one MFC  54  through the supply pipe SP. Then, the one MFC  54  supplies to the glow discharge tube  2  the mixed gas having undergone flow control. The glow discharge tube  2  is provided with a gas supply hole penetrating through between the space K and the supply pipe SP connected to the mixer  55 . That is, the mixer  55  supplies the mixed gas to the space K through the one MFC  54 , the supply pipe SP, and the gas supply hole. 
     The computer  6  includes a CPU (a retrieval part and an identification part)  61 , a RAM (Random Access Memory)  62 , a ROM (Read Only Memory)  63 , a hard disk (a storage part)  64 , a timer  65 , a display part  66 , an operation part (a reception part)  67 , and an interface  68 . The individual constituting parts of the computer  6  are connected to each other through a bus  6   b.    
     The CPU  61  is a processor controlling the power supply part  4  and the gas supply discharge part  5 . The CPU  61  reads into the RAM  62  a program  1 P stored in the hard disk  64 , and then executes the program  1 P. 
     The RAM  62  temporarily stores working variables, data, and the like necessary in the course of processing performed by the CPU  61 . Here, the RAM  62  is an example of a main storage device. Thus, in place of the RAM  62 , a flash memory, a memory card, or the like may be employed. 
     The ROM  63  is a read-only storage medium, for example, a nonvolatile semiconductor memory or a memory other than semiconductor. The ROM  63  stores a BIOS (Basic Input/Output System), firmware, and the like executed by the CPU  61  at the time of startup of the computer  6 . 
     The hard disk  64  stores the program  1 P executed by the CPU  61 . The hard disk  64  may be built in the computer  6  and may be located outside the computer  6 . Here, the hard disk  64  is an example of an auxiliary storage unit and may be substituted by a flash memory capable of recording a large size of information or by an optical disks such as a CD (Compact Disc), a DVD (Digital Versatile Disk), or a BD (Blu-ray Disc, registered trademark). In this case, the computer  6  includes a disk drive reading information from the optical disk and recording information onto the optical disk. 
     The hard disk  64  stores an auto-bias voltage mixing-ratio table  1 T and a kind-of-sample mixing-ratio table  2 T. The auto-bias voltage mixing-ratio table  1 T stores the auto-bias voltage Vdc and the mixing ratio of argon gas and oxygen gas in the mixed gas, in correspondence to each other. The kind-of-sample mixing-ratio table  2 T stores the kind of sample and the mixing ratio of argon gas and oxygen gas in the mixed gas, in correspondence to each other. 
     The timer  65  performs time counting of date and time, and then transmits a signal of the time counting result to the CPU  61 . 
     The display part  66  has a screen such as a liquid crystal display and an organic electroluminescence (Electro-Luminescence) display, and displays various kinds of screens concerning the program  1 P in response to an instruction from the CPU  61 . 
     The operation part  67  includes input devices such as a keyboard and a mouse used by the user for performing various kinds of inputs. On the basis of the operation performed by the user, the operation part  67  generates an input signal. The generated input signal is transmitted through the bus  6   b  to the CPU  61 . 
     The interface  68  includes devices, circuits, and connectors for connecting the power supply part  4  and the gas supply discharge part  5  to the computer  6  so as to establish transmission and reception of information. 
     Next, operation of the glow discharge milling apparatus  1  is described below. 
       FIG. 2  is a flow chart illustrating an example of a procedure of glow discharge milling processing. The procedure of  FIG. 2  is used when a mixing ratio of argon gas and oxygen gas suitable for the sample S is known. 
     The CPU  61  displays on the display part  66  an input screen (not illustrated) used for inputting various kinds of parameters such as a voltage-application mode, a frequency, a duty ratio, a milling time, and a mixing ratio for the mixed gas (step S 101 ). The user inputs various kinds of parameters to the input screen through the operation part  67  (step S 102 ). The user sets the sample S in the glow discharge tube  2  (step S 103 ). 
     The CPU  61  causes the vacuum suction device  51  to perform vacuum suction of the glow discharge tube  2  (step S 104 ). The CPU  61  outputs an instruction to each MFC  54  such that the flow rates of the argon gas and the oxygen gas are controlled in accordance with the received mixing ratio (step S 105 ). The mixer  55  mixes uniformly the argon gas and the oxygen gas whose flow rates are respectively controlled by the MFCs  54 , and then supplies the uniformly mixed gas into the glow discharge tube  2  (step S 106 ). The CPU  61  outputs to the power supply part  4  the received contents of setting (step S 107 ). 
     The power supply part  4  starts voltage application in accordance with the contents of setting based on the instruction from the CPU  61  (step S 108 ). Thus, milling of the surface of the sample S is performed under the mixed gas atmosphere with which the space K is filled (step S 109 ). On the basis of time counting obtained from the timer  65 , when the received milling time has elapsed, the CPU  61  outputs to the power supply part  4  an instruction of stopping voltage application (step S 110 ). When receiving from the CPU  61  the instruction of stopping voltage application, the power supply part  4  stops voltage application (step S 111 ) and then terminates the processing. 
       FIGS. 3A and 3B  represent examples of scanning electron microscope photographs obtained, respectively, before and after milling processing using argon gas alone. The sample S employed in  FIGS. 3A and 3B  is stainless steel.  FIG. 3A  is an example of a scanning electron microscope photograph obtained before milling processing. The scale bar located at the lower right corner in  FIG. 3A  indicates 3 μm.  FIG. 3B  is an example of a scanning electron microscope photograph obtained after milling processing. The scale bar located at the lower right corner in  FIG. 3B  indicates 500 nm. 
     In  FIG. 3A , it is recognized that the sample S is of polycrystalline substance. On the other hand, in  FIG. 3B , for example, a fine texture in two mutually crossing directions is clearly recognized in the crystal grain in the lower part. Further, a deposited substance is recognized in the grain boundary near the center of  FIG. 3B . Nevertheless, although the sample S surface is observed in  FIG. 3B , the situation of the deposited substance on the deeper side than the sample S surface is unobserved. When sputtering of the sample is performed further by using argon gas alone, the sample S surface becomes rough and hence observation of texture, deposited substance, and inclusion in the sample S becomes unachievable. 
       FIGS. 4A ,  4 B,  5 A, and  5 B represent examples of scanning electron microscope photographs each obtained before and after milling processing using mixed gas consisting of argon gas and oxygen gas. The sample S employed in  FIGS. 4A ,  4 B,  5 A, and  5 B is of stainless steel. Further, in the cases of  FIGS. 4A ,  4 B,  5 A, and  5 B, the ratio of argon gas to the entire mixed gas is 5% by weight. 
       FIG. 4A  is an example of a scanning electron microscope photograph obtained with an Out lens before milling processing. The scale bar located at the lower right corner in  FIG. 4A  represents 10 μm.  FIG. 4B  is an example of a scanning electron microscope photograph obtained with an Out lens after milling processing. The scale bar located at the lower right corner in  FIG. 4B  indicates 200 nm. 
       FIG. 5A  is an example of a scanning electron microscope photograph obtained with an In lens before milling processing. The scale bar located at the lower right corner in  FIG. 5A  indicates 10 μm.  FIG. 5B  is an example of a scanning electron microscope photograph obtained with an In lens after milling processing. The scale bar located at the lower right corner in  FIG. 5B  indicates 200 nm. 
     The acceleration voltage employed in the photographing of  FIGS. 4A ,  4 B,  5 A, and  5 B is 1.00 kV. The working distance is 3 mm, and the grid voltage is 999 V. The MAG mode in the photographing of  FIGS. 4A and 5A  is ×1.01K. The MAG mode in the photographing of  FIGS. 4B and 5B  is ×37.91K. 
     Also in  FIGS. 4A and 5A , it is recognized that the sample S is polycrystalline. In  FIGS. 4B and 5B , the fine texture observed in  FIG. 3B  is not observed. However, the smoothness of the sample S surface in  FIGS. 4B and 5B  is much higher than the smoothness of the sample S surface in  FIG. 3B . Deposited substance is recognized in the grain boundary near the center of  FIGS. 4B and 5B . When milling of the sample S is performed with mixed gas obtained by mixing oxygen gas into argon gas, the milling rate is slow. Thus, starting at the state of  FIGS. 4B and 5B , when milling further progresses, the sample S surface at several stages in the depth direction is allowed to be observed until the deposited substance near the center of  FIGS. 4B and 5B  is sputtered completely. 
     In the present embodiment, adjustment of the mixing ratio of argon gas and oxygen gas through the MFCs  54  has been performed by the computer  6 . However, adjustment of the mixing ratio of argon gas and oxygen gas through the MFCs  54  may be performed directly by the user operating the MFCs  54 . 
     Further, in the present embodiment, the mixed gas has been generated from the gases in the argon gas cylinder  52  and the oxygen gas cylinder  53  and then the generated mixed gas has been supplied to the glow discharge tube  2 . Instead, a mixed gas having a desired mixing ratio may be contained in a gas cylinder in advance and then the mixed gas may be directly supplied from the gas cylinder to the glow discharge tube  2 . 
     In this case, a plurality of gas cylinders respectively containing a plurality of types of mixed gas having different mixing ratios may be prepared in advance. Then, the plurality of gas cylinders may be connected to the glow discharge tube  2  through pipes. A bulb is provided in each pipe connecting each of the plurality of gas cylinders to the glow discharge tube  2 . Thus, when a corresponding bulb is opened, the desired mixed gas alone is supplied to the glow discharge tube  2 . 
     According to the glow discharge milling apparatus  1 , the partial pressure of oxygen in the mixed gas consisting of inert gas and oxygen gas is set up appropriately in accordance with the property of a sample. 
     In the mixed gas, an optimum mixing ratio optimizing the milling rate and the smoothness of the sample S surface is present for each sample S. For example, the mixing ratio may be expressed by the partial pressure of oxygen in the mixed gas. When oxygen gas is introduced in the sputtering gas, oxygen radicals are generated in the plasma. The oxygen radicals form an oxide film in the sample S surface by virtue of a strong oxidization force. This oxide film has an effect of suppressing the sputtering of the sample S achieved by argon ions. Thus, the introduction of oxygen gas in the sputtering gas reduces the milling rate and hence has an effect of making the sample S surface more smooth after processing. 
     For example, when EBSD analysis (crystal orientation analysis) is to be performed with a scanning electron microscope, it is important for the sample S surface of a target of measurement to have a roughness-free surface. According to the glow discharge milling apparatus  1 , a roughness-free surface is always generated in the direction of milling of the sample S. This permits 3-D analysis in EBSD analysis. 
     Further, when the element distribution or the chemical composition of the sample S surface is to be analyzed, the sample S surface needs be smooth. According to the glow discharge milling apparatus  1 , a smooth sample S surface suitable for the analyzing instrument is generated in a short time. 
     Embodiment 2 
     Embodiment 2 relates to a mode that the mixing ratio in the mixed gas of argon and oxygen is automatically changed on the basis of the auto-bias voltage Vdc. In Embodiment 2, the auto-bias voltage mixing-ratio table  1 T is used. 
       FIG. 6  is an explanation diagram illustrating an example of record layout of the auto-bias voltage mixing-ratio table  1 T. The auto-bias voltage mixing-ratio table  1 T contains columns of auto-bias voltage and mixing ratio. The auto-bias voltage Vdc is a voltage generated on the surface of the sample S onto which the high-frequency voltage is applied, and is detected by the generator  41 . The auto-bias voltage Vdc varies depending on the impedance values of the sample S, and hence is different for each kind of sample. In the column of auto-bias voltage in the auto-bias voltage mixing-ratio table  1 T, a voltage value range such as a voltage value 1 to a voltage value 2 is stored in each row. 
     The mixing ratio indicates the mixing ratio of argon gas and oxygen gas. For example, the mixing ratio is expressed by the weight-percentage of oxygen gas relative to the entire mixed gas. The auto-bias voltage Vdc and a suitable mixing ratio are determined in advance for each of samples S having various impedance values, and then the auto-bias voltage mixing-ratio table  1 T is prepared. 
     Next, operation of the glow discharge milling apparatus  1  based on the auto-bias voltage Vdc is described below. 
       FIGS. 7 and 8  are flow charts illustrating an example of a procedure of glow discharge milling processing. The procedure of  FIGS. 7 and 8  is used when the kind of sample for the sample S and the mixing ratio for the mixed gas are unknown. 
     The CPU  61  displays on the display part  66  an input screen (not illustrated) used for inputting various kinds of parameters such as a voltage-application mode, a frequency, a duty ratio, and a milling time (step S 201 ). The user inputs various kinds of parameters in the input screen through the operation part  67  (step S 202 ). The user sets the sample S in the glow discharge tube  2  (step S 203 ). 
     The CPU  61  causes the vacuum suction device  51  to perform vacuum suction of the glow discharge tube  2  (step S 204 ). The CPU  61  outputs an instruction to each MFC  54  such that the flow rates of the argon gas and the oxygen gas are controlled in accordance with a mixing ratio of initial setting (step S 205 ). Here, the mixing ratio of initial setting is stored in advance in the hard disk  64 . Then, the CPU  61  acquires from the hard disk  64  the mixing ratio of initial setting. The mixer  55  uniformly mixes the mixed gas having the mixing ratio of initial setting consisting of argon gas and oxygen gas whose flow rates are controlled by the MFCs  54 , and then supplies the uniformly mixed gas into the glow discharge tube  2  (step S 206 ). The CPU  61  outputs to the power supply part  4  the received contents of setting (step S 207 ). The power supply part  4  starts voltage application in accordance with the contents of setting based on the instruction from the CPU  61  (step S 208 ). 
     The generator  41  detects the auto-bias voltage Vdc and then outputs the detected auto-bias voltage Vdc to the CPU  61  (step S 209 ). Here, during the detection of the auto-bias voltage Vdc, the generator  41  continues outputting the continuously detected auto-bias voltage Vdc to the CPU  61 . On the basis of the auto-bias voltage Vdc acquired from the generator  41 , the CPU  61  retrieves a mixing ratio from the auto-bias voltage mixing-ratio table  1 T (step S 210 ). The CPU  61  outputs an instruction to each MFC  54  such that the flow rates of the argon gas and the oxygen gas are controlled in accordance with the retrieved mixing ratio (step S 211 ). 
     The mixer  55  mixes uniformly the argon gas and the oxygen gas whose flow rates are controlled by the MFCs  54 , and then supplies the uniformly mixed gas into the glow discharge tube  2  (step S 212 ). Thus, milling of the surface of the sample S is performed under the mixed gas atmosphere with which the space K is filled (step S 213 ). On the basis of the time counting obtained from the timer  65 , the CPU  61  determines whether the received milling time has elapsed (step S 214 ). 
     When it is determined that the received milling time has not yet elapsed (step S 214 : NO), the CPU  61  returns the procedure to step S 210 . When it is determined that the received milling time has elapsed (step S 214 : YES), the CPU  61  outputs to the power supply part  4  an instruction of stopping voltage application (step S 215 ). When receiving from the CPU  61  the instruction of stopping voltage application, the power supply part  4  stops voltage application (step S 216 ) and then terminates the processing. 
     In Embodiment 2, the mixing ratio has been obtained by using the auto-bias voltage mixing-ratio table  1 T. Instead, the mixing ratio may be obtained by substituting the auto-bias voltage Vdc value into an approximation formula prepared in advance. When this approach is to be adopted, first, an auto-bias voltage Vdc and a suitable mixing ratio are determined for each of samples S having various impedance values. Then, an approximation formula representing the correlation between the auto-bias voltage Vdc and the mixing ratio is generated and stored into the hard disk  64 . 
     Then, at step S 210  in  FIG. 8 , on the basis of the auto-bias voltage Vdc acquired from the generator  41 , the CPU  61  calculates the mixing ratio from the above-mentioned approximation formula. 
     According to the glow discharge milling apparatus  1 , at the time of milling the sample S, even when the kind of sample of the sample S and the mixing ratio for the mixed gas are unknown, the mixing ratio is set up automatically. 
     The glow discharge milling apparatus  1  automatically controls the mixing ratio on the basis of the detected auto-bias voltage Vdc. Thus, in addition to the case of a sample consisting of a single kind, even in the case of a sample S obtained by stacking several different kinds of materials and the case of a sample S in which deposited substance is buried, the mixing ratio is changed successively on the basis of the detected auto-bias voltage Vdc. Thus, milling is performed always with a suitable mixing ratio. That is, in the course of the processing of milling the sample S, when the kind or material to be milled varies, the mixing ratio is automatically changed in accordance with the kind or material having varied. 
     The present Embodiment 2 is as described above. The other points are similar to those in Embodiment 1. Thus, like parts are designated by like reference numerals, and their detailed descriptions are omitted. 
     Embodiment 3 
     Embodiment 3 relates to a mode that when the kind of sample is known, the mixing ratio in the mixed gas of argon and oxygen is automatically set by specifying of the kind of sample. In Embodiment 3, the kind-of-sample mixing-ratio table  2 T is used. 
       FIG. 9  is an explanation diagram illustrating an example of record layout of the kind-of-sample mixing-ratio table  2 T. The kind-of-sample mixing-ratio table  2 T contains columns of the kind of sample, mixing ratio, and milling rate. The kind of sample is the kind of the sample S. For example, in a case that samples S are composed of a metallic material, the samples S are recognized as mutually different with respect to the kind of sample when their metal textures of the metallic material are different from each other. Further, when the metallic materials of samples S are in mutually different phases in terms of crystallographical or thermodynamical properties, the samples S are recognized as mutually different with respect to the kind of sample. 
     The mixing ratio indicates the mixing ratio of argon gas and oxygen gas. For example, the mixing ratio is expressed by the weight-percentage of oxygen gas relative to the entire mixed gas. 
     The milling rate indicates the amount of progress of milling per unit time. For example, the employed unit is nm/s. 
     A suitable mixing ratio and a milling rate in the mixing ratio are measured in advance for each known sample S, and then the kind-of-sample mixing-ratio table  2 T is prepared. 
     Next, operation of the glow discharge milling apparatus  1  based on the kind of sample is described below. 
       FIGS. 10 and 11  are flow charts illustrating an example of a procedure of glow discharge milling processing. In the following description, milling processing is performed on a sample S which is a stacked material obtained by stacking several different kinds of materials in the milling direction. Further, the kind of sample, the order of stacking, and the layer thickness of each layer constituting the sample S are assumed to be known. 
     The CPU  61  displays on the display part  66  an input screen (not illustrated) used for inputting various kinds of parameters such as a voltage-application mode, a frequency, and a duty ratio, as well as the kind of sample, the order of stacking, the layer thickness, and the milling time for each layer (step S 301 ). The user inputs various kinds of parameters in the input screen through the operation part  67  (step S 302 ). The user sets the sample S in the glow discharge tube  2  (step S 303 ). 
     On the basis of the inputted values for the kind of sample and the order of stacking of each layer concerning the sample S, the CPU  61  searches the kind-of-sample mixing-ratio table  2 T so as to retrieve a mixing ratio and a milling rate to be used in milling each layer (step S 304 ). The CPU  61  divides the inputted layer thickness of each layer by the retrieved milling rate of each layer so as to obtain a milling time for each layer (step S 305 ). The CPU  61  stores a list of the kind of sample, the mixing ratio, and the milling time for each layer of the sample S corresponding to the time series of milling processing, in the form of an array, into the RAM  62  (step S 306 ). The CPU  61  causes the vacuum suction device  51  to perform vacuum suction of the glow discharge tube  2  (step S 307 ). 
     Then, the CPU  61  repeats the processing from step S 308  to step S 316  in a number of times equal to the number of layers in the sample S. In the following description of the procedures, generalization is adopted by using the expression “the n-th layer”. 
     The CPU  61  outputs instructions to each of the MFCs  54  so as to control the flow rates of the argon gas and the oxygen gas in accordance with the mixing ratio corresponding to the n-th layer stored in the RAM  62  (step S 308 ). The mixer  55  mixes uniformly the argon gas and the oxygen gas whose flow rates controlled by the MFCs  54 , and then supplies the uniformly mixed gas into the glow discharge tube  2  (step S 309 ). The CPU  61  outputs to the power supply part  4  the received contents of setting (step S 310 ). The power supply part  4  starts voltage application in accordance with the contents of setting based on the instruction from the CPU  61  (step S 311 ). Thus, milling of the n-th layer of the sample S is performed under the mixed gas atmosphere with which the space K is filled (step S 312 ). 
     On the basis of the time counting obtained from the timer  65 , the CPU  61  determines whether the milling time corresponding to the n-th layer has elapsed (step S 313 ). When it is determined that the milling time corresponding to the n-th layer has not yet elapsed (step S 313 : NO), the CPU  61  returns the procedure to step S 313 . When it is determined that the milling time corresponding to the n-th layer has elapsed (step S 313 : YES), the CPU  61  outputs to the power supply part  4  an instruction of stopping voltage application (step S 314 ). When receiving from the CPU  61  the instruction for stopping voltage application, the power supply part  4  stops voltage application (Step  315 ). 
     The CPU  61  determines whether any remaining layer to be milled is present (step S 316 ). When it is determined that any layer to be milled remains (step S 316 : YES), the CPU  61  returns the procedure to step S 308 . When it is determined that no more remaining layer to be milled is present (step S 316 : NO), the CPU  61  terminates the processing. 
     Here, in  FIGS. 10 and 11 , voltage application start and voltage application stop have been repeated every time one layer is milled. Instead, voltage application may be started before milling of the first layer and then voltage application may be stopped after completion of milling of the last layer. This shortens the necessary processing time. 
     Further, in  FIGS. 10 and 11 , processing for each layer has been started at step S 308 . Instead, the processing may be started at step S 307 . In this approach that processing for each layer is started at vacuum suction, milling is started in a state that the space K is filled up with the mixed gas reliably having the target mixing ratio even when the mixing ratio largely varies depending on the stacked layers. 
     In the above-mentioned examples, a sample S which is a stacked material obtained by stacking different kinds of materials has been employed. However, even in the case of a sample S composed of a single substance, when the kind of sample is known, the kind of sample of the sample S serving as a target of milling may be inputted to the glow discharge milling apparatus  1 . Then, a mixing ratio is retrieved from the kind-of-sample mixing-ratio table  2 T and then milling is executed with a mixed gas having the retrieved mixing ratio. 
     According to the glow discharge milling apparatus  1 , a suitable mixing ratio is automatically determined in accordance with the kind of sample. Further, even in a case that the sample S is composed of several different kinds of materials, after the kind of sample, the order of stacking, and the layer thickness of each layer are set up, the glow discharge milling apparatus  1  automatically executes milling processing. 
     The present Embodiment 3 is as described above. The other points are similar to those in Embodiments 1 and 2. Thus, like parts are designated by like reference numerals, and their detailed descriptions are omitted. 
     Embodiment 4 
     Embodiment 4 relates to a mode that an analyzing instrument analyzing the element distribution or the element composition of the sample S is built in the glow discharge milling apparatus  1 . The kind of sample of the to-be-analyzed sample S is identified from the element distribution or the element composition of the sample S obtained by the analyzing instrument and then, on the basis of this, a mixing ratio for argon gas and oxygen gas is determined from the kind-of-sample mixing-ratio table  2 T. Here, the apparatus in which such an analyzing instrument is built in the glow discharge milling apparatus  1  may be recognized as a glow discharge light-emission spectroscopy apparatus. 
       FIG. 12  is a block diagram illustrating outlines of an exemplary hardware configuration of a glow discharge milling apparatus  10  according to Embodiment 4. The glow discharge milling apparatus  10  includes a glow discharge tube  2 , a sample pressing member  3 , a power supply part  4 , a gas supply discharge part  5 , a computer  6  and a spectroscope (a measuring instrument)  7 . The configuration and the function of the glow discharge tube  2 , the sample pressing member  3 , the power supply part  4 , and the gas supply discharge part  5  are the same as those illustrated in  FIG. 1 . Thus, their detailed description is omitted. 
     The spectroscope  7  is a measuring instrument measuring the composition of elements contained in the sample S. The spectroscope  7  is connected to the glow discharge tube  2 . The spectroscope  7  acquires from the glow discharge tube  2  an emission line spectrum specific to each element generated when atoms, molecules, and the like constituting the sample S under sputtering are excited in the plasma and then return to the ground states. The spectroscope  7  is connected to the computer  6  through a connection cable bundle constructed from a bundle of a plurality of cables. However, in  FIG. 12 , the connection cable bundle is represented by and illustrated as a single connection line. The spectroscope  7  outputs to the computer  6  electric signals indicating the presence or absence and the intensity of the emission line spectrum corresponding to the content percentage of element constituting the sample S. 
     The spectroscope  7  includes a slit  71 , a diffraction grating  72 , and a plurality of photomultiplier tubes (a photo-multiplexer)  73 . The slit  71  limits the width of light transmitted from the glow discharge tube  2  to the diffraction grating  72 . The diffraction grating  72  causes interference in the light so as to achieve spectroscopy of the emission line spectrum. The photomultiplier tubes  73  convert into electric signals the intensities in the emission line spectrum obtained by the spectroscopy by the diffraction grating  72 . The photomultiplier tubes  73  outputs the converted electric signals to the computer  6  through the connection cable bundle. 
     An analysis program  2 P used for analyzing the element composition of the sample S on the basis of the measured values obtained from the spectroscope  7  is installed in the hard disk  64  of the computer  6 . The CPU  61  receives the electric signals outputted from the spectroscope  7  and then, on the basis of the received electric signals, analyzes the element composition of the sample S. The CPU  61  identifies the kind of sample of the sample S on the basis of the analysis result. On the basis of the identified kind of sample, the CPU  61  retrieves from the kind-of-sample mixing-ratio table  2 T a mixing ratio for the mixed gas to be outputted to the MFCs  54 . 
       FIGS. 13 and 14  are flow charts illustrating an example of a procedure of glow discharge milling processing. In the following description, milling processing is performed on a sample S obtained by stacking several different kinds of materials in the milling direction. Further, the kind of sample, and the layer thickness of each layer constituting the sample S are assumed to be unknown. 
     The CPU  61  displays on the display part  66  an input screen (not illustrated) used for inputting various kinds of parameters such as a voltage-application mode, a frequency, a duty ratio, and a milling time (step S 401 ). The user inputs various kinds of parameters in the input screen through the operation part  67  (step S 402 ). The user sets the sample S in the glow discharge tube  2  (step S 403 ). 
     The CPU  61  causes the vacuum suction device  51  to perform vacuum suction of the glow discharge tube  2  (step S 404 ). The CPU  61  outputs an instruction to each MFC  54  such that the flow rates of the argon gas and the oxygen gas are controlled in accordance with the mixing ratio of initial setting (step S 405 ). Here, the mixing ratio of initial setting is stored in advance in the hard disk  64 . Then, the CPU  61  acquires from the hard disk  64  the mixing ratio of initial setting. The mixer  55  uniformly mixes the argon gas and the oxygen gas whose flow rates are controlled by the MFCs  54 , and then supplies into the glow discharge tube  2  the uniformly mixed gas having the mixing ratio of initial setting (step S 406 ). The CPU  61  outputs to the power supply part  4  the received contents of setting (step S 407 ). The power supply part  4  starts voltage application in accordance with the contents of setting based on the instruction from the CPU  61  (step S 408 ). 
     The spectroscope  7  performs spectroscopy of the emission line spectrum and then outputs to the CPU  61  the electric signals indicating the intensities of the light under spectroscopy (step S 409 ). Here, during the spectroscopy of the emission line spectrum, the spectroscope  7  continues outputting to the CPU  61  the electric signals indicating the intensities of the light under spectroscopy. On the basis of the electric signals acquired from the spectroscope  7 , the CPU  61  analyzes the element composition of the sample S (step S 410 ). Then, on the basis of the analysis result, the CPU  61  identifies the kind of sample of the sample S (step S 411 ). On the basis of the identified kind of sample, the CPU  61  retrieves a mixing ratio from the kind-of-sample mixing-ratio table  2 T (step S 412 ). The CPU  61  outputs instructions to the MFCs  54  such that the flow rates of the argon gas and the oxygen gas are controlled in accordance with the retrieved mixing ratio (step S 413 ). 
     The mixer  55  mixes uniformly the argon gas and the oxygen gas whose flow rates controlled by the MFCs  54 , and then supplies the uniformly mixed gas into the glow discharge tube  2  (step S 414 ). Thus, milling of the surface of the sample S is performed under the mixed gas atmosphere with which the space K is filled (step S 415 ). On the basis of the time counting obtained from the timer  65 , the CPU  61  determines whether the received milling time has elapsed (step S 416 ). 
     When it is determined that the received milling time has not yet elapsed (step S 416 : NO), the CPU  61  returns the procedure to step S 410 . When it is determined that the received milling time has elapsed (step S 416 : YES), the CPU  61  outputs to the power supply part  4  an instruction for stopping voltage application (step S 417 ). When receiving from the CPU  61  the instruction for stopping voltage application, the power supply part  4  stops voltage application (step S 418 ) and then terminates the processing. 
     According to the glow discharge milling apparatus  10 , even when the kind of sample of the sample S and the mixing ratio for the mixed gas to be employed at the time of milling the sample S are unknown, the mixing ratio is set up automatically. 
     On the basis of the kind of sample identified by analysis, the glow discharge milling apparatus  10  automatically controls the mixing ratio. Thus, in addition to the case of a sample consisting of a single kind, even in the case of a sample S obtained by stacking several different kinds of materials and the case of a sample S in which deposited substance is buried, the mixing ratio is changed successively on the basis of the kind of sample identified by analysis. Thus, milling is performed always with employing a suitable mixing ratio. That is, in the course of the processing of milling the sample S, when the kind of sample serving the target of milling varies, the mixing ratio is automatically changed in accordance with the kind of sample having varied. 
     The present Embodiment 4 is as described above. The other points are similar to those in Embodiments 1 to 3. Thus, like parts are designated by like reference numerals, and their detailed descriptions are omitted. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.