Abstract:
A glow discharge source, in particular for the analysis of solid specimens by means of glow discharge, with an anode and a cathode and with means for the direct or indirect cooling of a specimen, and at least one Peltier element provided as the cooling means.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a glow discharge source, in particular for the analysis of solid specimens by means of glow discharge, with an anode and a cathode and with means for the direct or indirect cooling of a specimen. Glow discharge sources are known, inter alia, in the form of ion sources for mass spectrometric analyses. In the glow discharge source, the surface of a specimen is removed and ionized by a plasma. The ions originating from the specimen are discharged from the source and fed to a mass spectrometer. 
   The solid specimen is heated up by the plasma. Cooling of the specimen is advantageous, to avoid melting. This applies in particular to thin specimens or systems of layers. A constant specimen temperature is also advantageous for the accuracy and reproducibility of the measurement results. Finally, the stability of the removal of the specimen surface sputter process is to be ensured. 
   In known devices, the cooling of the specimen takes place with the aid of water. In this case, only temperatures near freezing can be achieved without additives. For specimens with a low melting point, this is possibly not adequate, for example for gallium (Ga). With this type of cooling, the rate of temperature change is also low. 
   Finally, it is advantageous to be able to heat up the specimen after cooling to avoid condensation. This requires additional technical measures. 
   BRIEF SUMMARY OF THE INVENTION 
   With the present invention it is intended to improve the cooling of the specimen in the region of the glow discharge source. 
   The glow discharge source according to the invention is characterized in that at least one Peltier element is provided as the cooling means. When a voltage is applied to the Peltier element, one side is cooled, while the opposite side of the element is heated up. The heat is accordingly transported from one side to the opposite side. If the voltage at the Peltier element is reversed, the direction of heat flow changes correspondingly. 
   With the Peltier element, rapid temperature changes are possible and relatively low temperatures, even below 0° C., can be achieved. For heating the specimen to avoid condensation, it is merely necessary to reverse the voltage. 
   According to a further idea of the invention, the Peltier element is arranged between the anode and the cathode of the glow discharge source. In this case, one of the two parts is cooled and the other is heated up. The Peltier element is preferably formed as an insulator, for instance with ceramic surfaces, so that there is good electrical insulation between the anode and the cathode. 
   According to a further idea of the invention, it is provided that the Peltier element lies against the cathode and cools it, and the cathode lies against the specimen. The Peltier element absorbs the heat of the cathode and the latter absorbs the heat of the specimen until there is a state of equilibrium that can be controlled by the Peltier element. By reversing the voltage at the Peltier element, heating up of the specimen is also possible in a simple way. 
   According to a further idea of the invention, means for cooling the anode are provided. The thermal energy of the cathode is transferred to the anode through the Peltier element. The said anode can be correspondingly cooled and, for this purpose, preferably has channels for a flowing cooling medium to flow through. Cooling water or some other cooling liquid is preferred. Gas cooling is also possible. The anode is advantageously at earth potential, so that a flowing coolant is unproblematical in this region. 
   According to a claimed idea of the invention that is also independent of the use of the Peltier element, the cathode of the glow discharge source consists of a material, that has a great hardness with at the same time good thermal and electrical conductivity. The high-grade steel that is usually used does not have particularly good properties in this respect. The mechanical hardness is also important, because the specimen lies against the cathode and, if the cathode does not have adequate hardness, its surface may scratch and influence the electrical and thermal transfer between the specimen and the cathode, and consequently also the subsequent measurement results. 
   In particular, the cathode material has the property a) and at least one of the subsequent properties b), c):
         a) the Vickers hardness (HV) of a surface facing the specimen is at least 120,   b) the electrical conductivity is at least 14% ICAS, the ICAS value usually being normalized to the electrical conductivity of copper (100%),   c) the thermal conductivity is at least 80 W(mK).       

   The cathode materials used preferably have all three stated properties a) to c). 
   The cathode is preferably produced from materials with the following properties:
         a) the Vickers hardness (HV) of a surface facing the specimen is at least 120, preferably at least 180, in particular at least 210,   b) the electrical conductivity is at least 14% ICAS, preferably 20% ICAS, in particular at least 30% ICAS, and   c) the thermal conductivity is at least 80 Wm −1 K −1 , preferably at least 100 Wm −1 K −1 , in particular at least 120 Wm −1 K −1 .       

   The presented variations of the various properties can be combined with one another as desired. Of course, a material that has the maximum values for all the stated properties is best. 
   The cathode is preferably produced from at least one, in particular precisely one, of the following materials: 
   W75Cu25, 
   WCu, 
   CrZrCu, 
   CoBeCu, 
   WAg, 
   W90NiCu, 
   CuBe2, 
   WNiCu, 
   CuNiBe, 
   CuCoNiBe, 
   CuNiCrSi, 
   CuCr, 
   WCAg. 
   At least in the case of a cathode constructed from a number of paths, the various materials may also be combined with one another. 
   According to a further idea of the invention, the specimen is formed as a pin and is inserted in a conducting manner with part of its length in a corresponding recess in the cathode and protrudes with another part of its length into a recess in the anode, without touching the latter. In the case of this embodiment, the cathode acts as a pin holder or specimen holder. The pin-shaped specimen is inserted in the cathode in a clamping manner. In this case, the cathode is of a multipart form, with an annular part, into which the anode partly protrudes, and with a substantially disc-shaped or block-shaped part for receiving the specimen and at the same time for covering the annular cathode part. 
   According to a further idea of the invention that is independent of the use of the Peltier element and the special cathode material, a covering of the cathode is provided in such a way that the specimen is completely covered and the covering has a peripheral sealing edge with respect to the cathode, it being possible for a volume between the covering and the specimen to be extracted by suction and, for this purpose, the covering has a connection for suction extraction. Inside the glow discharge source there is a vacuum or a pressure of approximately 1 mb (0.1 to 10 mb). A pressure-tight arrangement of the specimen at the cathode, for instance with a (very flat) sealing ring lying in between, has previously been customary. This hinders the electrical and thermal transfer between the cathode and the specimen. With the solution according to the invention, that of the covering described, there is no need for the sealing between the specimen and the cathode. 
   According to a further idea of the invention, the cathode may be formed in a divided manner, a part near the specimen being removable together with the specimen and the covering from a part of the cathode remote from the specimen. This measure makes particularly simple changing of the specimen possible. A new specimen can be fixed on the part of the cathode near the specimen outside the glow discharge source and then subsequently placed together with it onto the part of the cathode remote from the specimen. A vacuum seal is correspondingly provided between the two parts of the cathode. 
   Further features of the invention can be taken from the remaining description and the claims. Advantageous exemplary embodiments are explained in more detail below on the basis of drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a section along a centre axis of a first embodiment of the glow discharge source according to the invention, 
       FIG. 2  shows an embodiment similar to  FIG. 1 , but with a covering over the specimen, 
       FIG. 3  shows an embodiment similar to  FIG. 2 , but with a covering and a divided cathode, 
       FIG. 4  shows a section along the line A-B in  FIGS. 1-3 , 
       FIG. 5  shows a perspective representation of the glow discharge source, 
       FIG. 6  shows a section along a centre axis of a further embodiment of the glow discharge source according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The figures show a glow discharge source  10  of the Grimm type. The anode  11  and the cathode  12  are formed in a substantially annular manner, with a common centre axis  13 . Provided between the anode  11  and the cathode  12  is a gap  14 , which is partly filled by a substantially disc-shaped insulator  15 . The gap  14  in this case runs perpendicular to the centre axis  13 . 
   Opposite from the gap  14 , a specimen  15  is held on the cathode  12  by devices not shown in any more detail. A good electrical and thermal transfer is to be ensured between the specimen  16  and the cathode  12 . 
   Extending along the centre axis  13  is a free volume  17  with a cathode fall  18  near the specimen  16 . The cathode  12  has as a rule a much larger inside diameter than the anode  11 . Furthermore, the sleeve-like continuation  19  of the anode  11  extends into the cathode  12  and in the direction of the specimen  16 . 
   Between the sleeve-like continuation  19  and the relatively outer cathode  12  there is formed an annular volume  20 , which is in communication with the glow discharge zone  18  via a radially directed volume  21 . In this case, the radial volume  21  is delimited in the axial direction on the one hand by the specimen  16  and on the other hand by the continuation  19 . The latter has in its region facing the specimen  16  an outwardly directed thickening  22 , so that the annular volume  20  is subdivided into a wide portion  23  near the gap  14  and a narrow portion  24  at the level of the thickening  22 . 
   A substantially sleeve-shaped insulator  25  is provided on a circumferential inner side  26  of the cathode  12 . In this case, the insulator  25  extends from the insulator  15  to the specimen  16 , so that there is no “visible clearance” between parts of the anode  11  and of the cathode  12 . 
   In the region of the insulator  15 , a number of Peltier elements  27 , that is six in this case, are arranged between the anode  11  and the cathode  12  in the circumferential direction, see also  FIG. 4 . These lie against the anode  11  and the cathode  12  on the upper side and underside in such a way that good heat transfer is ensured. At the same time, the Peltier elements  27  are produced from ceramic material in order to ensure electrical insulation. They are preferably Peltier elements each with 30 watts, it being intended that the total output of 180 watts is greater than or equal to the output of the glow discharge. Peltier elements of this kind are, for example, the high-temperature elements PF-127-10-13 (silicone-sealed) from Telemeter Elektronik GmbH with I max  3.9 amperes, U max  16.4 volts, P cmax  35.6 watts, δT: 72° Celcius. The dimensions of the parts arranged around the Peltier elements  27  are such that the Peltier elements  27  lie against the anode  11  and the cathode  12  without a gap or via intermediate layers and there are good heat transfers. 
   The Peltier elements  27  are connected in a way not shown in any more detail to an electrical voltage source and cool the cathode  12  directly, and consequently cool the specimen  16  indirectly. At the same time, the anode  11  is directly heated up. A voltage reversal at the Peltier elements  27  is possible. This allows, for example, the specimen  16  to be heated up after carrying out the measurement in order to avoid condensation forming after the vacuum is eliminated in the region of the specimen. 
   The anode  11  is provided with devices for cooling. In the present example, the anode  11  has cooling channels  28 , which extend in particular in the circumferential direction, receive a flowing cooling medium and can be connected in a way not shown in any more detail to an external cooling unit. 
   Argon flows into the glow discharge source  10  as the process gas, here through at least one radially directed channel  29 , which opens out into the free volume  17  and extends in the anode  11  between the cooling channels  28  (lying in a radial plane) and the Peltier elements  27 . 
   In a corresponding way, the cathode  12  has at least one radially directed outflow channel  30 , which is connected to the annular volume  20  or to the wide portion  23  of the same, and for this purpose penetrates through the insulator  25 . 
   The process gas ionizes in the region of the free volume  17  and ions detach particles from the surface of the specimen  16 , which are taken away from the specimen  16 , in the direction of the arrow  31  along the free volume  17  and fed to a mass spectrometer (not shown). 
   The cathode  12  is produced from a particularly hard and at the same time electrically and thermally conductive material, preferably from a tungsten-copper alloy with a tungsten content of 75% and, correspondingly, a copper content of 25%. 
   During operation, a pressure of approximately 0.1 to 10 mb prevails in the glow discharge zone  18 . The cooling provided allows specimens at temperatures well below 0° Celcius to be analyzed, for example down to 70 Kelvins below the temperature of the anode, which is cooled by cooling water. 
   The temperature of the Peltier elements or the specimen can be kept constant by means of a control circuit (not shown). What is important in this connection is that the output of the Peltier elements is made to match the thermal output occurring in the glow discharge source  10 . 
   The arrangement of Peltier elements may also be provided at some other location, for instance directly for cooling the specimen. Likewise, removal of the heat to the anode  11  is not mandatory. 
   In the present case, the anode  11  is at earth potential, while the cathode  12  and the specimen  16  are under voltage. 
     FIG. 2  shows a further exemplary embodiment. Here, the specimen  16  is covered by a covering, that is a housing  32 , which takes the form of a cover with a peripheral seal  33  at the edge. The said seal lies against the cathode  12  at a distance from the specimen  16 . The housing  32  has approximately at the centre and opposite the specimen  16  a connecting piece  34  for a vacuum line. An interior space  35  of the housing  32  is largely evacuated, preferably with a residual pressure which corresponds approximately to the pressure in the glow discharge source  10  or, if appropriate, is somewhat higher. Holding devices for the specimen are present but not depicted. 
   The particular advantage of the housing  32  is that the specimen  16  does not have to be arranged in a vacuum-tight manner with respect to the cathode  12 . Special sealing means between the cathode  12  and the specimen  16  can therefore be avoided. 
   Finally,  FIG. 3  shows a further embodiment. Here, the housing  32  is likewise provided. By contrast with the embodiment shown in  FIG. 2 , however, the cathode  12  is formed in a two-part manner, with a part  36  near the specimen (removable part) and a part  37  remote from the specimen (fixed part) of the cathode. The part  36  near the specimen is preferably formed with a smaller outside diameter than the part  37  remote from the specimen. The housing  32  extends here over the part near the specimen up to the part  37  remote from the specimen. The peripheral seal  33  provides a sealing effect in particular with respect to the part  37  remote from the specimen, but also with respect to the part  36  near the specimen, and is arranged in the angle between the parts  32 ,  36 ,  37 . 
   For removing the specimen  16 , the housing  32  is also removable, along with the part  36  near the specimen and the specimen  16 , from the glow discharge source  10 . Subsequently, an already prepared new housing with another specimen can be fitted. The succession of a number of measurements can therefore be speeded up significantly. The insulator  25  is preferably inserted only in the cathode  12  or the parts  36 ,  37  and kept there by static friction. The channel  30  runs in the part  37 . A guiding tube  38  may be fitted in the free volume  17 , and similarly in the other embodiments of the glow discharge source  10 . 
     FIG. 5  shows a simplified perspective representation of the glow discharge source  10  with an analyzer. Of the latter, only the housing wall  39  is indicated here. 
     FIG. 6  shows a variation of the glow discharge source according to  FIG. 1 . Instead of a substantially disc-shaped specimen, a pin-shaped specimen, namely a pin  41 , is shown in  FIG. 6 . This pin is held in a corresponding recess  42  of a holder  43 . The pin  41  thereby extends along the centre axis  13 , to be precise with part of its length within the recess  42  and with another part of its length into the free volume  17  or into the continuation  19 . 
   The holder  43  lies at the electric potential of the cathode  12  and, for this purpose, lies partly against the cathode  12 . To this extent, the holder  43  is a component part of the cathode  12 . At least at the edge, the holder  43  otherwise lies directly against the cathode  12 , so that a good thermal and electrical transfer is ensured. 
   The recess  42  is formed in the wall  44  of the holder  43  that lies against the cathode  12 . Also provided in the wall  44 , substantially concentrically in relation to the recess  42 , is a relatively shallow recess  45 , in which an insulator  46 , for example made of ceramic, lies flush and the free outer side  47  of which lies in part opposite the free volume  17  or the annular volume  20  and with another part lies against the insulator  25  and against the cathode  12 . The aim here is to shield the holder  43  from the interior of the glow discharge source. Only the pin  41  lying at cathode potential protrudes into the free volume  17 . 
   On account of the arrangement of the pin  41 , the continuation  19  also has a special geometry at its free end  48 . The free end  48  forms a constriction with a conically narrowing section and with the smallest diameter in the region of an opening  49 , which is arranged near the insulator  46  but still at a distance from it. 
   The pin  41  extends through the opening  49  into the free volume  17 . 
   The cathode material (including holder  43 ) is intended to have the best possible thermal and electrical conductivity, while at the same time the greatest possible surface hardness. A tungsten-copper alloy (WCu), or some other alloy with similar properties, for instance copper-chromium (CuCr), tungsten-silver (WAg) or tungsten-carbon-silver (WCAg), is preferred as the material. A tungsten-copper alloy with a tungsten content of 60 to 90% is preferred, in particular W75Cu25. 
   
     
       
             
           
             
             
             
             
             
           
         
             
                 
             
             
               List of designations: 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               10 
               glow discharge source 
               34 
               connecting piece 
             
             
                 
               11 
               anode 
               35 
               interior space 
             
             
                 
               12 
               cathode 
               36 
               part near the specimen 
             
             
                 
               13 
               centre axis 
               37 
               part remote from the 
             
             
                 
               14 
               gap 
                 
               specimen 
             
             
                 
               15 
               insulator 
               38 
               guiding tube 
             
             
                 
               16 
               specimen 
               39 
               housing wall 
             
             
                 
               17 
               free volume 
               40 
             
             
                 
               18 
               cathode fall 
               41 
               pin 
             
             
                 
               19 
               continuation 
               42 
               recess 
             
             
                 
               20 
               annular volume 
               43 
               holder 
             
             
                 
               21 
               radial volume 
               44 
               wall 
             
             
                 
               22 
               thickening 
               45 
               recess 
             
             
                 
               23 
               wide portion 
               46 
               insulator 
             
             
                 
               24 
               narrow portion 
               47 
               free outer side 
             
             
                 
               25 
               insulator 
               48 
               free end 
             
             
                 
               26 
               inner side 
               49 
               opening 
             
             
                 
               27 
               Peltier elements 
             
             
                 
               28 
               cooling channels 
             
             
                 
               29 
               channel 
             
             
                 
               30 
               outflow channel 
             
             
                 
               31 
               arrow 
             
             
                 
               32 
               housing 
             
             
                 
               33 
               seal