Abstract:
A method for determining process-specific data of a vacuum deposition process, in which a substrate is coated in a vacuum chamber by a material detached from a target connected to a magnetron, an optical emission spectrum being recorded and process-significant data of the vacuum deposition process being determined therefrom for further processing in measurement or regulating processes, is optimized to minimize errors in the determination of process-significant data. At least three intensities of spectral lines of at least two process materials are determined from the optical emission spectrum. From these, single and multiple intensities are mathematically correlated with and to one another and a process-significant datum, which is used in subsequent measurement or regulating processes, is determined from the relation results by a further mathematical relation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority of German applications 10 2011 003 260.6 filed on Jan. 27, 2011, 10 2011 004 513.9 filed on Feb. 22, 2011, and 10 2011 017 583.0 filed on Apr. 27, 2011, all of which are hereby incorporated by reference herein in their entirety. 
       BACKGROUND ART  
       [0002]    The invention relates to a method for determining process-specific data of a vacuum deposition process, in which a substrate is coated in a process space in a vacuum chamber by means of a material detached from a target connected to a magnetron while applying a target voltage provided by a regulated voltage source between the target and a back electrode and while introducing a process gas into the vacuum chamber, an optical emission spectrum being recorded and process-significant data of the vacuum deposition process being determined therefrom for further processing in measurement or regulating processes. 
         [0003]    In what follows, intensity is intended to mean the value of the intensity of a spectral line of a material. When reference is made to a plurality of intensities of a material, this means that a plurality of spectral lines are defined from a spectrogram, in their height i.e. the value of the intensity of the respective spectral line is determined and processed further as an intensity. 
         [0004]    A process gas as referred to below is used inter alia to set the pressure in the vacuum space. It may consist of a working gas which is inert i.e. does not chemically influence the process, for example argon, krypton or xenon. For reactive processes, however, the process gas may also consist of a reactive gas, for example oxygen, in order to initiate chemical reactions during the layer deposition, for example oxygen for oxidation. The process gas may also consist of a mixture of working gas and reactive gas. 
         [0005]    The process gas, in particular the working gas and the reactive gas, are materials involved in the coating process, also referred to as process materials for brevity in the context of the invention. 
         [0006]    Another process material is the target material of which a target of a magnetron consists, for example aluminium or zinc. 
         [0007]    In order to ensure the deposition of a layer with consistent parameters, it is necessary to keep the working point of the coating process constant over a long time during which the target material is consumed. In particular, homogeneity is to be maintained in a long-term stable manner with respect to the layer thickness, the layer composition (doping) and further properties, such as sheet resistance. The progressive consumption of the target material makes this difficult. Primarily, the positional relationships between the target surface, magnetic field and gas inflow change geometrically as a result of the consumption. 
         [0008]    By means of first calibration (so-called trimming), gas pressure distributions of working and reactive gases (for these terms, see below) can be carried out. 
         [0009]    The pressures of the gases and the target voltage, which are also referred to as process parameters, are readjusted in the course of the process. The rotational speed in the case of a rotating magnetron is also another process parameter. 
         [0010]    Because of the substrate passing through, it is also necessary to automatically readjust the process parameters rapidly. One solution, in which the ratio of two intensities is used for the regulation, has already been described in DE 10 2009 053 903 B2. 
         [0011]    The checking of longterm stability is based on optical emission spectroscopy (OES) by means of two lines from the plasma which is formed over the target surface when the target voltage is applied in a vacuum. In this case, intensity lines at discrete wavelengths provide information about states of materials involved in the coating process, i.e. process materials as mentioned above. 
         [0012]    The plasma is observed, and process parameters are readjusted in order to ensure constant layer parameters, in particular a constant sheet resistance of the growing layer. 
         [0013]    Conventional (economical) spectrometers and their arrangement in the vicinity of the process present disadvantages with respect to the measured intensities or their absolute values (accuracy, deposits, variations). The sometimes unsharp resolution as a function of wavelength means that compromises have to be made in relation to readily identifiable (usable in control technology) intensity lines. In some cases, the lines are also very close together. 
         [0014]    DE 103 41 513 B4 “Method for regulating the reactive gas flow in reactive plasma-enhanced vacuum coating processes” has already described observation of two lines of the OES signal and a solution for regulating the reactive gas flow in reactive plasma-enhanced vacuum coating processes, in which a controlled variable, which is determined by a plasma of the vacuum coating process, is recorded from the vacuum chamber as a controlled system by means of optical spectroscopy in a measuring element and the amount of a reactive gas supplied to the vacuum coating process is adjusted as a manipulated variable. The controlled variable is in this case employed as a value calculated from a measurement value of the intensity of a spectral line of the coating material involved in the process and a measurement value of the intensity of a spectral line of the reactive gas, or as a value calculated from a value to be determined of the corresponding intensities. In the arrangement likewise disclosed therein, the measuring element contains an acousto-optical spectrometer comprising a control input which is connected to a regulator output. 
         [0015]    Although the intensities of two lines were correlated with one another and used as a controlled variable in this known solution, the reactive gas flow was however used as a manipulated variable, which does not sufficiently ensure consistency of the layer parameters, for example a constant sheet resistance, of the growing layer with progressive target erosion. 
         [0016]    EP 1 553 206 A1 describes a magnetron sputtering method comprising working point regulation. In this case, the ratio of two intensities of spectral lines of materials involved in the coating process is used as a controlled variable for the regulation. In this regulation, the target voltage serves as a manipulated variable. With the invention, it has been found that the effect of such working point regulation can be improved. 
       BRIEF SUMMARY OF THE INVENTION  
       [0017]    It is now an object of the invention to minimize errors in the determination of process-significant data, which are caused by the measurement position and/or by the spectrometer, in order to render subsequent measurement or regulating processes more reliable. 
         [0018]    According to the invention, the object is achieved in that at least three intensities I 1  . . . I 3  of spectral lines of at least two process materials are determined from the optical emission spectrum. A first relative intensity R 1  is calculated from one pair of the intensities I 1  . . . I 3  by a first mathematical relation. A second relative intensity R 2  is calculated from another pair of the intensities I 1  . . . I 3  by a second mathematical relation. Finally, an intensity relation IV is calculated as a process-significant datum from the first relative intensity R 1  and the second relative intensity R 2  by a third mathematical relation. This process-significant datum is then used in subsequent measurement or regulating processes, so that their accuracy and reliability are increased. 
         [0019]    In one configuration of the method, at least four intensities I 1  . . . I 4  of at least two process materials are determined. The first relative intensity R 1  is calculated respectively from two of the intensities I 1  . . . I 4  which do not derive from the same process material. The second relative intensity R 2  is calculated respectively from two others of the intensities I 1  . . . I 4  which do not derive from the same process material. 
         [0020]    In this way, it is possible to calibrate the intensities of one material with respect to another material. 
         [0021]    By the inventive use of a plurality of lines for the process control and for ascertaining the properties of the growing layer, account is taken of the fact that the line intensities naturally depend on the excitation conditions. For example, the ratio of a Zn line to an O line varies as a function of the pressure because the interaction cross sections depend differently on the electron temperature (i.e. on the pressure). 
         [0022]    A fundamental advantage of the invention, irrespective of its use, is that by virtue of the mathematical relations, it no longer uses the absolute values of the intensities which are susceptible to error, or simple relative intensities whose error still remains high, but instead a third relative intensity obtained from two relative intensities whose error is then largely freed of perturbing variables. The nature of the mathematical relations, the choice of the intensities and the materials from which these intensities are obtained, are also determined by the use of the process-significant data, as will be explained in more detail below. 
         [0023]    One use of the invention relates to a method for regulating vacuum deposition processes in which spectra of materials that are involved in the process are recorded in situ, a plurality of intensities of process materials are determined therefrom and are mathematically correlated with one another, and the result of the mathematical relation is used as a controlled variable of a control loop which sets a process parameter as a manipulated variable so that the result of the mathematical relation tracks a reference variable. 
         [0024]    In order to ensure a high layer quality, it is necessary to avoid variation of layer parameters due to increasing target erosion during the coating process by ensuring longterm stabilization of the working point. 
         [0025]    The invention may then be aimed in particular at longterm stabilization of the layer quality in deposition processes, and in this context particularly at the development of a long-term stable reactive process for depositing Zn:O as TCO. In this case, a substrate is coated in a process space by means of a material detached from a target connected to the magnetron while applying a target voltage provided by a regulated voltage source between the target and a back electrode and while introducing a process gas into the vacuum chamber, the power or the discharge current being regulated by means of an oxygen flow. 
         [0026]    For such a use of the invention:
       at least four intensities I 1  . . . I 4  of at least two process materials are determined, the intensity I 1  of a first spectral line of a process material and the intensity I 2  of a second spectral line of a process material being measured at a first position in the process space and the first relative intensity R 1  being formed therefrom by the first mathematical relation f 1 (I 1 ,I 2 ),   an intensity I 3  of the first spectral line and an intensity I 4  of the second spectral line are measured at the second position in the process space different from the first position, and the second relative intensity R 2  is formed therefrom by the second mathematical relation f 2 (I 3 ,I 4 ),   and the third mathematical relation f 3  is formed from the first relative intensity R 1  and the second relative intensity R 2  by f 3 ={f 1 (I 1 ,I 2 ), f 2 (I 3 ,I 4 )}, the result of which is used as an intensity relation IV as a controlled variable in the regulating process.       
 
         [0030]    This regulation may be carried out by tracking the target voltage U T  and/or the speed of a relative movement between the magnet system and the target as a manipulated variable of the regulation so that the intensity relation IV as a controlled variable of the regulation is kept constant at a setpoint value IV S  of the intensity relation IV which is set as a reference variable. 
         [0031]    Control of the target voltage and/or the speed of the relevant movement can be carried out with relatively little outlay. Speed regulation or voltage regulation are provided in any case, in order to keep the values constant in the course of operation. These regulations may then be used to set the voltage and/or speed so that the intensity relation is kept at a constant value. 
         [0032]    In one configuration of the method, the second mathematical relation is of the same type as the first mathematical relation (f 1 =f 2 ). 
         [0033]    Preferably, a spectral line of the target material may be selected as the first spectral line and a spectral line of a reactive gas may be selected as the second spectral line. 
         [0034]    It is desirable to use spectral lines which are significant as possible, in order to increase the accuracy of the method according to the invention. To this end, the spectral lines of different materials have been indicated above. The significance can furthermore be increased by selecting at least one of the spectral lines as an emission line which is attributable not to the neutral material state but to the excited material state (for example an ionized zinc line). 
         [0035]    The basis of the method according to the invention is that a unique association of layer properties, voltage value of the target voltage, speed of the relative target movement and the intensity of spectral lines can be established. In this case, it is furthermore to be noted that perturbing influences on this unique association can be excluded by forming an intensity relation of two intensities. 
         [0036]    The intensities may preferably be obtained by implementing the first and second mathematical relations in the form of ratio formation: f 1 =I 1 /I 2  and f 2 =I 3 /I 4 . 
         [0037]    The third mathematical relation may be implemented in the form of ratio formation f 3 =f 1 (I 1 ,I 2 )/f 2 (I 3 ,I 4 ) or averaging f 3 =(f 1 (I 1 ,I 2 )+f 2 (I 3 ,I 4 ))/2. 
         [0038]    The aforementioned association may then expediently be used to define the setpoint value, by establishing the intensity relation IV for a value a i  of a layer property a to be achieved from a function IV=f(a). 
         [0039]    To this end, the function IV=f(a) may be recorded during a calibration coating process by measuring values a i  of the layer property and, if a current value a n  does not match the values a i , modifying the target voltage and/or the speed of the relative movement between the magnet system and the target until a subsequent value a n+x  corresponds to the value of the intended layer property, and using the intensity relation IV thereby to be determined as a setpoint value IV S  and setting it as a reference variable. 
         [0040]    It is also possible to configure the method in such a way that a calibration coating process as presented above, which leads to highly reproducible results and therefore increases the accuracy of the method but is wide-ranging, can be reduced in terms of its outlay. It is therefore proposed that the setpoint value IV S  be determined for a value a i  of a layer property a to be achieved by measuring values a i  of the layer properties during a coating process and, if a current value a n  does not match the values a i , modifying the target voltage and/or the speed of the relative movement between the magnet system and the target until a subsequent value a n+x  corresponds to the value of the intended layer property, and using the intensity relation IV thereby to be determined as a setpoint value IV s  and setting it as a reference variable. It is therefore possible to generate not a set of characteristic curves from which various parameters can be read, but instead merely to determine the one setpoint value relevant to the value of the layer parameter. 
         [0041]    The first alternative of the solution, namely varying the target voltage U T , may be used for sputtering processes in devices with a static arrangement between the target and the magnet system as well as with a dynamic arrangement, and the second alternative for dynamic arrangements in which, however, both alternatives may be employed. 
         [0042]    If the invention is used in the case of a planar magnetron, a relative movement may be carried out by moving the plasma generated over the target relative to the target surface. This may, for example, be achieved by a mobile magnet system below the target. However, the planar magnetron itself may also be moved relative to the substrate. In a particular configuration of the method according to the invention, the speed of these two relative movements may be controlled so as to keep the intensity relation constant. 
         [0043]    The invention is also, and in particular, suitable for use in the case of a tubular magnetron. The tubular magnetron has an elongate magnet system preferably lying transversely to the transport direction of the substrate, around which a tubular target is rotatably arranged. Therefore, inter alia, more uniform target erosion is achieved and the target material yield is increased. In the present invention, the rotational movement may be considered as a relative movement of the tubular target relative to the substrate, the rotational speed of which can be controlled. 
         [0044]    In practice, it has been found that the intensities of spectral lines vary during a target revolution. In order to exclude the influence of such a variation on the method according to the invention, it is preferable for the intensity relation to be generated as an average value over at least one revolution of the tubular magnetron. 
         [0045]    The method presented above is preferably suitable for a single magnetron inside a vacuum chamber. Two magnetrons may influence one another via the plasma and different burning voltages. For this reason according to a preferred embodiment, in the case of two magnetrons arranged in a vacuum chamber, the regulation is respectively carried out separately for each magnetron. The separation of the two regulations can be reinforced, and the mutual influence minimized, by using at least one intensity of a different spectral line from the other respective magnetron for each magnetron. Thus, different intensity relations are used in the two regulations. 
         [0046]    In another configuration, which is useful for the regulation, four intensities I 1  . . . I 4  are determined from three process materials. The first intensity I 1  is determined from a first process material, the second intensity I 2  is determined from a second process material, and the third intensity I 3  and the fourth intensity I 4  are determined from a third process material. The first intensity I 1  is correlated with the third intensity I 3  by means of the first mathematical relation to form the first relative intensity R 1 , the second intensity I 2  and the fourth intensity I 4  are correlated by means of a second mathematical relation to form the second relative intensity R 2 . The intensity relation IV is determined from the first relative intensity R 1  and the second relative intensity R 2  by means of a third mathematical relation and used as a controlled variable in the control loop. 
         [0047]    Here, it is expedient for the target voltage process parameter to be used as a manipulated variable in the control loop. 
         [0048]    In the case of reactive deposition processes, the reactive gas flow process parameter may be used as a manipulated variable. 
         [0049]    Furthermore, in the case of reactive deposition processes, it is possible for the first to fourth intensities I 1 -I 4  to be determined from the process materials: working gas, reactive gas and target material. 
         [0050]    An attempt may thus be made, for example, to “calibrate” the intensities of lines of the layer elements with the respect to intensities of the lines of the working gas. 
         [0051]    This could, for example, be of the form: 
         [0000]      [I(Zn)/I(Ar,1)]/[I(O)/I(Ar,2)] 
         [0000]    where
   I(Zn) is the intensity of a zinc line,   I(O) is that of an oxygen line,   I(Ar,1) is the intensity of a first argon line and   I(Ar,2) is the intensity of a second argon line.   
 
         [0056]    The most expedient relation may also have a different mathematical form, since the pure ratio is a good approximation only in a particular range. 
         [0057]    According to another possible use of the method according to the invention, namely measuring the doping, in the case of coating with two target materials, three intensities I 1  . . . I 3  are determined from three process materials. The first intensity I 1  is determined from a first target material, the second intensity I 2  is determined from a second target material and the third intensity I 3  is determined from a third target material. The first intensity I 1  is correlated with the second intensity I 2  by means of the first mathematical relation to form the first relative intensity R 1 , the second intensity I 2  and the third intensity I 3  are correlated by means of a second mathematical relation to form the second relative intensity R 2 . The intensity relation IV is determined from the first relative intensity R 1  and the second relative intensity R 2  by means of a third mathematical relation and transmitted as a process-significant datum of a measurement for doping of the deposited layer with one or other target material. 
         [0058]    This method may be used particularly in the case of an aluminium zinc oxide (AZO) coating. In this case, the first relative intensity R 1  is determined from an intensity of the target material aluminium and from an intensity of the target material zinc, and the second relative intensity R 2  is determined from an intensity of the reactive gas oxygen and the intensity of the target material aluminium or the intensity of the target material zinc. 
         [0059]    One possibility consists in determining the first relative intensity by R 1 =I 1 /I 2 , the second relative intensity by R 2 =I 3 /I 1 , and the third intensity relation by IV=R 1 /R 2 . 
         [0060]    In this way, it is possible to obtain somewhat more information about the doping concentration. At least 3 lines would then be needed a priori in the case of AZO: zinc, oxygen and aluminium (relation for example [I(Zn)/I(Al)]/I(O)/I(Zn)]; optionally also with different Zn lines). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES  
         [0061]    The invention will be described in more detail below with the aid of four exemplary embodiments. In the appended drawings: 
           [0062]      FIG. 1  shows the method according to the invention in a first exemplary embodiment, illustrated with reference to a control loop having the target voltage as a manipulated variable and 
           [0063]      FIG. 2  shows the method according to the invention in a second exemplary embodiment, illustrated with reference to a control loop having the rotational speed as a manipulated variable, 
           [0064]      FIG. 3  shows the position of the spectral lines and their intensities for the working gas (AG) reactive gas (RG) and target materials (TM) in the spectrogram, 
           [0065]      FIG. 4  shows the time profile of the absolute values of the intensities of the process materials: working gas (AG) reactive gas (RG) and target material (TM), 
           [0066]      FIG. 5  shows the representation of the determination of the intensity relation IV from the four intensities of the three process materials: working gas (AG) reactive gas (RG) and target material (TM), 
           [0067]      FIG. 6  shows the time profile of the absolute values of the intensities of the three process materials: first target material (TMa), second target material (TMb) and reactive gas (RG) and 
           [0068]      FIG. 7  shows the representation of the determination of the intensity relation IV from the four intensities of the three process materials: first target material (TMa), second target material (TMb) and reactive gas (RG). 
       
    
    
     DETAILED DESCRIPTION 
       [0069]    In the following exemplary embodiments, it will be assumed that a substrate transported in the longitudinal direction in a vacuum coating apparatus is coated using a tubular magnetron arranged transversely to the transport direction. A layer which has various layer properties is in this case deposited. In parallel with the regulations according to the invention as presented here, a regulation known per se regulates the oxygen flow by means of the power. This regulation is not represented in detail in the figures. 
         [0070]    In this exemplary embodiment, which relates to a reactive process for the deposition of ZnO:Al, the resistivity p is considered—as generic example for all other possible layer properties a—which is intended to have a particular value and should in particular be constant and homogeneous over the length of the substrate. 
         [0071]    As shown in  FIG. 1 , the intensities I 11 , I 21 , I 12  and I 22  of the first and second spectral lines are respectively measured at the first and second positions in the process space by means of one or more optical emission spectrometers as measuring elements  4 . A first mathematical relation f 1 (I 11 ,I 21 ) and a second mathematical relation f 2 (I 12 ,I 22 ) are then formed therefrom. In this case, the second mathematical relation is of the same type as the first mathematical relation (f 1 =f 2 ) this means that the second mathematical relation is likewise produced by ratio formation when the first mathematical relation is carried out as ratio formation. 
         [0072]    By means of a third mathematical relation f 3 , an intensity relation IV is formed from the first and second mathematical relations f 3 ={f 1 (I 11 , I 21 ), f 2 (I 12 ,I 22 )}. Their result is used as a controlled variable of the regulation. 
         [0073]    From a prior calibration coating process, the value pairs {IV i ,ρ i } are now available for a value a i  of an i th  measurement of a layer property a, for example with ρ i  as the resistivity thereby determined. 
         [0074]    If a particular resistivity p is now intended to be set, then the corresponding IV value is taken from the corresponding value pair and used as a setpoint value IV S . The control deviation ΔIV is then calculated from the actual value IV and the setpoint value IV S , and delivered to a regulator  5 . The regulator  5  and the calculation represented here are implemented in a process computer  6 . The latter also determines the corresponding value of a control voltage U st  which is delivered to the voltage-regulated generator  7  as a controlling element, from which a target voltage U T  is set in the latter as an output voltage which is applied to the target in the vacuum chamber  8 , which can be considered as a controlled system. 
         [0075]    Another possibility for keeping the intensity relation IV constant is to vary the target rotational speed N, the target voltage being kept constant by means of the oxygen flow. 
         [0076]    As shown in  FIG. 2 , intensities I 11 , I 21 , I 12  and I 22  of the first and second spectral lines are again measured respectively at the first and second positions in the process space by means of one or more optical emission spectrometers as measuring elements  4 . A first mathematical relation f 1 (I 11 ,I 21 ) and a second mathematical relation f 2 (I 12 ,I 22 ) are then formed therefrom. In this case, the second mathematical relation is likewise of the same type as the first mathematical relation (f 1 =f 2 ). This means that the second mathematical relation is likewise produced by ratio formation when the first mathematical relation is carried out as ratio formation. 
         [0077]    By means of a third mathematical relation f 3 , an intensity relation IV is formed from the first and second mathematical relations f 3 ={f 1 (I 11 ,I 21 ), f 2 (I 12 ,I 22 )}. Their result is used as a controlled variable of the regulation. 
         [0078]    From a prior calibration coating process, the value pairs {IV i ,ρ i } are now available for a value a i  of an i th  measurement of a layer property a, for example with ρ i  as the resistivity thereby determined. 
         [0079]    If a particular resistivity p is now intended to be set, then the corresponding IV value is taken from the corresponding value pair and used as a setpoint value IV S . The control deviation ΔIV is then calculated from the actual value IV and the setpoint value IV S , and delivered to a regulator  5 . The regulator  5  and the calculation represented here are likewise implemented in a process computer  6 . The latter also determines the corresponding value of a speed of rotation n which is delivered to the voltage-regulated generator  7  as a controlling element, from which the latter sets a target rotational speed N that determines the relative speed between the target and the substrate in the vacuum chamber  8 , which can be considered as a controlled system. 
         [0080]    In a spectrogram  10 ,  FIG. 3  represents a first spectral line  11  of the working gas, in this case argon (Ar), a second spectral line  12  of the working gas, a spectral line  13  of the reactive gas, in this case oxygen (O 2 ), a spectral line  14  of a first target material, in this case aluminium (Al), and a spectral line  15  of a second target material, in this case zinc (Zn). 
         [0081]    In an exemplary embodiment according to  FIG. 4  and  FIG. 5 , a measure of the energetic excitation states of the electrons in the plasma space, and therefore a measure of the electron temperature, is determined with the aid of line intensities from multiple intensities. On the basis of this measure of the electron temperature, the single intensities are evaluated in order to derive controlled variables for setting the layer properties. 
         [0082]    Overall, at least four intensities I 1 -I 4  of the spectral lines  11  to  14  are measured as output variables and processed respectively for three of the process materials: working gas (AG), reactive gas (RG) and target material (TM). In this case, one intensity—single intensity—is respectively determined for each of two process materials (AG and TM) and at least two intensities—multiple intensity—are determined for the third process material (AG). 
         [0083]    For the regulation, a single intensity is respectively first correlated with (mathematically related to) a multiple intensity, from which two controlled variables are obtained which, when correlated with (mathematically related to) one another, give the final controlled variable. 
         [0084]    To first approximation, as known from the prior art, it is sufficient for the regulation when the single intensities for the process materials are taken into account. Carrying out the measurement in the vicinity of the target and the substrate further improves the regulation. 
         [0085]    According to the invention, however, a further controlled variable is derived from two or more line intensities for the same material (multiple intensity). By forming the ratio of intensities, variations in the sensitivity of the spectrometer can be compensated for (for example also due to deposition on the collimator), as can be seen in  FIG. 5 , in which case the regulation according to the prior art would be insensitive. 
         [0086]    In the example, the working gas argon is mentioned for the measurement of multiple intensities. The invention may, however, also be used for the other process materials. Likewise, the mathematical relations are indicated here only by way of example. Other mathematical relations, for example by forming differences or ratios, can also lead to practicable determination of the controlled variable. 
         [0087]    For example, a first relative intensity R 1  is determined from an intensity of the target material I TM  and from a first intensity I AG1  of the working gas by 
         [0000]        R   1   =I   TM   /I   AG1 . 
         [0088]    A second relative intensity R 2  is determined from an intensity I RG  of the reactive gas and from a second intensity I AG2  of the working gas by 
         [0000]        R   2   =I   RG   /I   AG2 . 
         [0089]    The intensity relation IV, which is finally used as a controlled variable, is determined from 
         [0000]        IV=R   1   /R   2 . 
         [0090]    Improved accuracy is achieved by the concepts presented above, so that in another exemplary embodiment the doping concentration may also be determined. 
         [0091]    With the aforementioned condition that other variables are also employed for the multiple intensity determination, for example a first relative intensity R 1  is determined from an intensity of a spectral line  14  of a first target material I TM a  (for example Al) and from an intensity of a spectral line  15  of a second target material I TM b  (for example Zn) by 
         [0000]        R   1   =I   TM a   /I   TM b . 
         [0092]    A second relative intensity R 2  is determined from an intensity I RG  of a spectral line  13  of the reactive gas and from the intensity I TM a  of the spectral line  14  of the first target material by 
         [0000]        R   2   =I   RG   /I   TM a . 
         [0093]    As an alternative, the second relative intensity R 2  may be determined from a first intensity I AG1  of the working gas and from a second intensity I AG2  of the working gas by 
         [0000]        R   2   =I   AG1   /I   AG2 . 
         [0094]    The intensity relation IV, which is finally used as a measure for the doping concentration, is determined from 
         [0000]        IV=R   1   /R   2 .