Abstract:
A compositionally modulated material electroplated film is deposited by using at least two source metal anodes in an electroplating apparatus, and changing at least one contact area between an anode an electrolyte. In order to obtain sharp boundaries between successive layers of the film, voltage can be switched from an electroplating substrate to a dummy electrode immediately before the contact area is changed, in order to allow the electrolyte to equilibrate at a new set of solute concentrations, before electroplating on the substrate is recommenced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to the inventor&#39;s applications “METHOD AND APPARATUS FOR ELECTROPLATING ALLOY FILMS”, Ser. No. 09/671,223, and “METHOD AND APPARATUS FOR ELECTROPLATING ALLOY FILMS”, Ser. No. 09/671,230, which were filed on the same day as the present invention pending. These related applications are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains to a method and apparatus for depositing metal alloy films. More particularly the invention pertains to a method and apparatus for electroplating alloy metal films. 
     BACKGROUND OF THE INVENTION 
     In the semiconductor art alloys such as Aluminum-Copper are used to produce interconnect metallizations. 
     For various composition material films it is important to strictly control the proportions of constituent materials in the compound. In the case of solder alloys, for example, the proportions of constituent metals effect critical properties including melting point temperature, and adhesion. 
     One conventional method for composition film deposition is sputtering. In sputtering, a source material target having a predetermined composition is arranged in a vacuum chamber opposite a substrate onto which a film having a composition corresponding to the predetermined composition is to be deposited. The chamber is charged with a small amount of reactive or inert gas. A high voltage DC and/or high frequency discharge is struck in the gas to cause ions formed from the gas to collide with the target and knock off particles which are deposited on the substrate. Unfortunately, sputtering requires vacuum processing, complex power supplies, and is generally only suited to single substrate processing. While sputtering machines can be scaled up to handle multiple substrates, although there is little gained in economies of scale by doing so. 
     Another method for depositing alloy films is electroplating. In one conventional method, electroplating is carried out using inert electrodes, and an electrolyte solution bearing a plurality of solutes compounds including metals to be deposited. However, in electroplating with inert electrodes, solutes need to be constantly replenished in order to sustain continued plating. Moreover, in the case of alloy plating, solutes corresponding to different constituents of the alloy may become depleted from the electrolyte solution at different rates and this leads to uncontrolled variations in the alloy composition obtained. 
     In another conventional method, electroplating is carried out using an anode made of an alloy, corresponding to the alloy film which is desired. However, in practice, it has been found that the proportions of constituents in the alloy film do not, in fact, correspond to the constituents of the alloy anode. Moreover, during the course of plating, the electrolyte tends to shift towards one of the metals of the alloy in an uncontrolled manner. 
     An additional disadvantage of using an alloy anode is that alloy anodes tend to require more power to sustain a given rate of dissolution, compared to pure metal electrodes made of any of the alloy constituent metals. 
     It is also possible to produce an alloy electroplated film by using electrolyte comprising compounds (e.g., salts) of different metals which are included in the alloy, and a pure metal anode. However, as the electrolyte is used during plating, its composition shifts toward the metal of the pure metal anode. 
     In addition to the desirability of depositing homogeneous alloy films, it is desirable, for certain applications such as high corrosion resistance films, and thin film microelectronic devices, to produce Compositionally Modulated Multilayers (CMM), which are also know as Functionally Graded Materials (FGM). CMM or FGM films, are characterized by constituent element proportions which vary in a predetermined manner as a function of depth in the film. 
     One conventional method for obtaining multilayer metal films is to repeatedly transfer a substrate between a plurality of plating baths each of which deposits a film of a particular composition on the substrate or an underlying film. However, using multiple plating baths is complicated, and such a process is also hampered by the problem of contamination and oxidation during transfer of the substrate which can passivate a surface layer on the substrate and create difficulty for further plating. The oxidation might create another layer between the two desired layers rendering the substrate unsuitable for its application 
     It would be desirable to have a method and apparatus for depositing CMM films that avoids interfacial contamination and oxidation between layers of the CMM. 
     It would also be desirable to have a system and method which overcomes the above mentioned shortcomings. 
     SUMMARY OF THE INVENTION 
     Briefly, one embodiment of the present invention provides, an apparatus for depositing a composition film comprising a first metal and a second metal on a substrate. The apparatus includes a container for containing an electrolyte including a first dissolved disassociated compound including the first metal, and a second dissolved disassociated compound including the second metal. Additionally there is a support for an first anode which includes the first metal, a support for a second anode which includes the second metal, and a servo for changing a contact area between the first anode and the electrolyte. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings. 
     FIG. 1 is a schematic of an electroplating apparatus according to a first embodiment of the present invention. 
     FIG. 2 is a partial schematic of an electroplating apparatus having a different power supply arrangement in accordance with a second embodiment of the invention. 
     FIG. 3 depicts an electroplating anode geometry according to one preferred embodiment of the present invention. 
     FIG. 4 is a flow diagram of an open loop control process performed by a control computer for controlling an anode contact area in an electroplating apparatus according to another preferred embodiment of the present invention. 
     FIG. 5 is a flow diagram of an open loop control process performed by a control computer for controlling the power supply setting in an electroplating apparatus according to yet another preferred embodiment of the present invention. 
     FIG. 6 is a flow diagram of a closed loop control process performed by a control computer for controlling a power supply setting in an electroplating apparatus according to another embodiment of the present invention. 
     FIG. 7 is a flow diagram of a closed loop control process performed by control computer for controlling an anode to electrolyte contact area in an electroplating apparatus according to still another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It should be understood that the embodiments presented below are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in the plural and vice versa with no loss of generality. 
     FIG. 1 is a schematic of an electroplating apparatus  100  according to a first embodiment of the invention. 
     As shown, the electroplating apparatus  100  includes a control computer  102 , which can for example be an IBM personal computer. The control computer  102  is electrically coupled to a digital-to-analog converter  104  (e.g., through an ISA card slot of the computer  102 ). The digital-to-analog converter is electrically coupled to the control inputs of a first variable power supply  106 , and a second variable power supply  108 . The power supplies  106  and  108  are for example switch mode power supplies, in which a voltage received at a control input is used to determine a power switch duty cycle. 
     An anode terminal of the first power supply  106  is electrically coupled through a first ammeter  112 , to a first electroplating source metal anode  116  (e.g., nickel). An anode terminal of the second power supply  108  is electrically coupled through a second ammeter  114  to a second electroplating source metal anode  118  (e.g., Iron). The first and second ammeters  112  and  114  have signal outputs coupled to an analog-to-digital converter  120 . The analog-to-digital converter  120  is in turn electrically coupled to the control computer  102  (e.g., through an ISA card slot). The control computer receives readings from the ammeters  112 , and  114  through the analog-to-digital converter  120 . Although two anodes  116 ,  118  are shown, more than two anodes can be provided in order to produce electroplate alloy coatings with more than two metals. 
     Two negative terminals of the variable power supplies  106  and  108  are coupled to an input pole of a single pole double throw relay  110 . A control input of the relay  110  is electrically coupled to the digital-to-analog converter  104 , so that the control computer  102  can operate the relay  110  through the digital-to-analog converter  104 . An amplifier (not shown) such as a Darlington pair can be interposed between the digital-to-analog converter and the control input of the relay  110 . A first output of the relay  110  is electrically coupled to an electroplating substrate (working electrode)  122 . A second output of the relay  110  is coupled to dummy electrode  124 . 
     In general the single pole double throw relay  110  can be any type of electrical device (e.g., switch, switch network) for selectively coupling between an input terminal and either of two output terminals, (i.e., coupling between the negative terminals of the power supplies  106 ,  108  and either the substrate  122  or the dummy electrode  124 ). 
     During operation, the first and second power supplies  106 ,  108  apply first and second voltages between the anodes,  116  and  118  and either the substrate  122 , or the dummy electrode  124 . 
     The first and second anodes  116  and  118  are supported in an electrolyte solution  126 , which is held in a container  128 . The electrolyte solution  126  includes a first dissolved and disassociated compound (e.g., a salt) that includes a metal included in the first anode  116 , and a second dissolved and disassociated compound including a metal included in the second anode  118 . The electrolyte solution  126  can also include other solutes such as organic solutes known in the electroplating art as for decreasing the porosity of deposited films and for achieving fine surface finish. 
     The first anode  116  is supported by a preferably electrically insulating tether  130 , which mechanically connects it to a first linear motion servo  134 . Likewise, the second anode  118  is supported by a tether  132  by which it is connected to a second linear motion servo  136 . The linear motion servos  134  and  136  are supported by a support  140 . The linear motion servos  134  and  136  control depths of insertion of the first and second anodes  116  and  118 , and thus operate to change the contact areas between the first and second anodes  116  and  118  and the electrolyte solution  126 . The first and second linear motion servos  134  and  136  are electrically coupled to a servo controller  138  and the servo controller  138  is electrically coupled to the control computer  102 . The control computer, under program control, drives the servo controller  138  to change the areas of the first and second anodes  116 ,  118  which are in contact with the electrolyte solution. 
     As an alternative to the linear motion servos  134 ,  136 , other arrangements can be provided for varying the contact areas between the anodes  116  and  118  and the electrolyte solution  126 . For example, stepper motors that are mechanically coupled to the anodes  116 ,  118  can rotate the anodes into the electrolyte solution  126  to varying degrees. As a further alternative, an area of contact between a stream of electrolyte solution in a circulation circuit and an anode in the circulation circuit can be altered, (e.g., by deflecting the stream or moving the anode). A computer readable medium  142  can be used to load software onto the control computer  102 , for configuring it to control the electroplating apparatus  100  in order to deposit the CMM films in the manner described above and below. 
     In operation, a setting of one of the variable power supplies  106  and  108  or settings of the two variable power supplies, or a contact area between the electrolyte  126  and one of the anodes  116  and  118 , or two contact areas between the two anodes  116  and  118  and the electrolyte  126 , is varied during the plating process in order to change the composition of the film being plated onto the substrate  122  in order to deposit a CMM film. If the relay  114  is maintained so as to supply power to the substrate  122  while the voltages and or areas are changed, then a composition of the deposited CMM will vary gradually through the thickness of the film because the electrolyte composition changes gradually. 
     In order to obtain a sharp transition in the composition between a first layer of a CMM and an adjacent second layer of the CMM film, the electroplating apparatus can be operated for a first period with the relay  110  set to supply power to the substrate  122  with a first set of power settings of the power supplies  106  and  108  and with a first set of contact areas of the anodes  116  and  118 . The first set of settings comprises a setting (e.g.,voltage or current) of the first power supply  106  and a setting (e.g., voltage or current) of the second power supply  108 . During the first period, a first set of concentrations of the first and second dissolved disassociated compounds will be obtained in the electrolyte  126 , and an electroplate film layer of a first predetermined composition will be deposited. At the end of the first period, the relay  114  is switched to supply power to the dummy electrode  124 . 
     During a second period in which power is supplied to the dummy electrode, at least one voltage of the power supplies  106 ,  118 , and/or at least one contact area between at least one of the anodes  116  and  118  and the electrolyte  126  is changed. During the second period, a second set of concentrations of the first and second dissolved disassociated compounds will be obtained in the electrolyte. The second period is preferably sufficient in duration to allow the electrolyte to equilibrate at the second set of concentrations. At the end of the second period, the relay  110  is switched to again supply power to the substrate  122 . Electroplating of the CMM then continues, preferably with the second set of concentrations, for a third period. By repeating this process more than two distinct CMM layers can be built up. In general plating is performed with a set of concentrations, the relay switches to the dummy electrode  124 , the concentrations are changed, and the relay switches back to the substrate electrode  122 . 
     Accordingly, a CMM film with one or more sharp composition transitions is obtained. During the course of the process, the substrate  122  remains submersed in the electrolyte, so that contamination and formation of interfacial native oxides are prevented. 
     In designing a specific process for depositing a specific CMM film, voltages of the power supplies  106  and  108  and contact areas, for the anodes  116  and  118  are to be determined experimentally. For a given predetermined composition, corresponding to a given depth in a desired CMM film, appropriate concentrations of the first and second dissolved, disassociated compounds are determined experimentally by trial and error using different concentrations, guided by basic electrochemistry knowledge. 
     A set of voltages for the first and second power supplies are then chosen so that appropriate concentrations will be maintained. The latter can be done by trying different voltages and repeatedly testing concentrations of the first and second dissolved, disassociated compounds to see that they are maintained. The concentrations can for example be tested using an Inductively Coupled Plasma (ICP) spectroscopy instrument. 
     Rather than optimizing the voltages to maintain the concentrations, rough voltage settings can be preselected, and then a set of areas of contact between the anodes  116 ,  118 , and the electrolyte  126  are optimized by trial and error to determine contact area values which result in the appropriate concentrations being maintained. The process of determining the voltages, or areas is repeated for each of the different compositions in the CMM film. 
     In the process described above in which the relay  110  is operated to obtain sharp transitions, each composition corresponds to a discrete layer. 
     In order to minimize the aforementioned second period the volume of the tank  128  should be minimized. An electrolyte agitator (not shown) can also be provided to reduce the second period, and to maintain a homogeneous electrolyte composition, and thus predictable film composition. 
     A method of forming a CMM electroplated film according to an embodiment of the present invention can comprise the steps of: providing an electrolyte  126 , establishing a first predetermined contact area between a first anode  116  and the electrolyte  126 , establishing a second predetermined contact area between a second anode  118  and the electrolyte  126 , placing an electroplating substrate  122  in contact with the electrolyte  126 , placing a dummy electrode  124  in contact with the electrolyte  126 , applying a first voltage between the first anode  116  and the electroplating substrate  122  during a first period, applying a second voltage between the second anode  118  and the electroplating substrate  122  during the first period, establishing a third predetermined contact area between a first anode  116  and the dummy electrode  124  during a second period, establishing a fourth predetermined contact area between a second anode  118  and the electrolyte  126  during the second period, applying a third voltage between the first anode  116  and the dummy electrode  124  during the second period, applying a fourth voltage between the second anode  118  and the dummy electrode  124  during the second period, applying a fifth voltage between the first anode  116  and the electroplating substrate  122  during a third period; and applying a sixth voltage between the second anode  118  and the electroplating substrate  122  during the third period. During the third period the third predetermined contact area is preferably maintained between a first anode  116  and the electrolyte, and the fourth predetermined contact area is preferably maintained between the second anode  118  and the electrolyte  126 . 
     The fifth voltage applied during the third period is preferably equal to the third voltage applied during the second period, and the sixth voltage applied during the third period is preferably equal to the fourth voltage applied during the second period. In other words, it is preferable to maintain the voltages used in the second period, in the third period. Alternatively the difference between the voltage used during at least a portion of the second period and the voltage used during the first period, can be greater than the difference between the voltages used during the first and third periods, in order to obtain faster change of concentrations between the concentrations used in the first period and the concentrations used during the third period. 
     Settings of the first and second power supplies  106 ,  108  and contact areas set by linear motion transducers  134  and  136  can be controlled in an open loop manner. Alternatively, a set of currents delivered to the anodes  116 ,  118  can be read using ammeters  112 ,  114  and used for closed loop control of the power supplies  106 ,  108  and/or the linear motion servos  134 ,  136 . 
     In open loop control, the electroplating apparatus  100  is operated based on a first predetermined schedule for the position of at least one of the linear motion transducers  134  and  136  as a function of time, and/or for a voltage settign of at least one of the power supplies  106  and  108  as a function of time. A first computer program is provided that sets up a time function (e.g., using a software construct such as time delay loops or through a system clock), and reads data from the first predetermined schedule, and adjusts at least one voltage applied to the power supplies  106 ,  108  through the digital-to-analog converter according to the first predetermined schedule, and/or adjust the position of at least on of the linear motion servos  134  and  136  through the servo control  138  according to the first predetermined schedule. 
     In closed loop control, the electroplating apparatus  100  is operated based on a second predetermined schedule for a current supplied to one of the anodes  116  and  118 , or currents supplied to both anodes  116  and  118 . A second computer program is provided which reads the second predetermined schedule, reads one or both of the ammeters  112 ,  114  through the analog-to-digital converter  118 , and adjusts at least one voltage applied to power supplies  106 ,  108  through the digital-to-analog converter  104 , and/or at least one contact area between an anode  116  and/or  118  and the electrolyte solution  126 , in order to make the current supplied to one or both of the anodes nominally equal to a current specified in the second predetermined schedule for a particular instant of time. In one specific embodiment the second computer program can implement a proportional integral differential (PID) feedback control algorithm to control the power supplies  106  and/or  108 . A PID algorithm may result in there being a residual difference between a target current specified in the second predetermined schedule and an actual current obtained. For that reason, the term ‘nominally equal’ is used. 
     In the case of contact area control, the second computer program operates the control computer  102 , servo controller  138 , and analog-to-digital controller  118  to function as a feedback control system. In the case of voltage control, the second computer program operates the control computer  102 , digital-to-analog converter  104 , and analog-to-digital converter  118  to function as a feedback control system. Current passing through an anode  116  or  118  increases as the contact area between the anode  116  or  118  and the electrolyte increases, and increases as the voltage of the power supply  106 ,  108  coupled to the anode  116  or  118  increases. 
     In order to obtain a specific CMM film the predetermine schedules are designed to vary at least one contact area and/or at least one voltage setting, so that different compositions are obtained at different depths in the deposited film. According to some embodiments, voltage and current can be controlled independently with the help of potentiostats. 
     Further, in order to obtain CMM films with abrupt composition transitions, the state of the relay  110  is changed to switch between supplying power to the substrate  122 , and the dummy electrode  124 . 
     The aforementioned first computer program, second computer program, first predetermined schedule, and second predetermined schedule can be stored on computer readable medium  142 . 
     FIG. 2 is a partial schematic diagram of an electroplating apparatus having a different power supply arrangement according to a second embodiment of the invention. 
     In FIGS. 1 and 2 like elements are denoted by the same reference numerals. These elements are described above in connection with FIG. 1, and some are not described again below. 
     Referring to FIG. 2, a single power supply  202  is connected to the two anodes  116 ,  118  through an impedance network  204 . The impedance network  204  divides a current supplied by the power supply  202  between the two anodes  116 ,  118 . The impedance network can be fixed or can include variable impedance elements, (e.g., rheostat or resistors). The combination of a power supply  202 , and a variable impedance element is one example of a variable power supply. 
     FIG. 3 shows an electroplating anode geometry according to a preferred embodiment of the invention. 
     The anode  300  comprises an upper edge E 2 , a lower edge opposite to the upper edge E 2 , a right side edge E 1 , and left side edge opposite to the right edge E 1 , a front face F 1 , and a back face opposite to the front face F 1 . The sum of the surface area of the front face F 1  and the back face is preferably approximately equal to a total surface area of all the faces and edges of the anode  300 . In such a case, the surface area is relatively stable as the mass of the anode is reduced in the course of electroplating. This is especially preferable in the case of open loop control of the electroplating apparatus. 
     FIG. 4 is a flow diagram of an open loop control process  400  performed by a control computer for controlling an anode contact area in an electroplating apparatus according to a preferred embodiment of the present invention. In process block  402 , a contact area setting corresponding to the current time is read from a first predetermined schedule. In process block  404 , data is written to the servo controller  138  to cause adjustment of a contact area between an anode  116 ,  118  and the electrolyte  126 , by a servo  134 ,  136  in accordance with the setting read in process block  402 . In process block  406 , the schedule is read to determine if the process is complete. Process block  408  is a decision block the outcome of which depends on whether the first predetermined schedule, and the process  400  for CMM film plating is complete. If not complete, then the process  400  loops back to process block  402  after a delay  410 , to process block  402 . If complete then the process terminates. Other control functions can be added to those shown in FIG. 4 including but not limited to altering power supply  106 ,  108  settings at different points in the process, and controlling the state of relay  110 . 
     FIG. 5 is a flow diagram of an open loop control process  500  performed by control computer for controlling power supply setting in an electroplating apparatus according to another preferred embodiment of the present invention. 
     In process block  502  a power setting corresponding to the current time is read from a second predetermined schedule. 
     In process block  504  data is written to the digital-to-analog converter  104  to cause adjustment of at least one of the power supplies  106  and  108  in accordance with the power setting read in process block  502 . 
     In process block  506 , the second predetermined schedule is read to determine is the process is complete. Process block  508  is a decision block, the outcome of which depends on whether the second predetermined schedule and the CMM plating process is complete. If not then the process  500  loops back through a delay to process block  502 . If complete then the process  500  terminates. 
     FIG. 6 is a flow diagram of a closed loop control process  600  performed by control computer  102  for controlling power supply setting in an electroplating apparatus  100  according to a yet another embodiment of the invention. 
     In process block  602 , a target current to be supplied to at least one of the anodes at the current time is read from the second predetermined schedule. Preferably, the second predetermined schedule is read from computer readable medium  142 . The second predetermined schedule includes a sequence of entries, each of which specifies a beginning time, an ending time, an anode current value for each of the anodes,  116 ,  118 , and a field which specifies whether current is to be supplied to the electroplating substrate  122 , or the dummy electrode  124 . The current time is a relative time measured from a start of the process  600 . In carrying out process block  602 , the current time will be checked against the beginning times and ending times of the sequence of entries to determine which entry applies to the current time. In process block  604 , an actual current being supplied to the anode is read by the control computer  102 , through the analog-to-digital converter  120 , from an ammeter  112  or  114 . In process block  606 , a difference between the target current and the actual current is computed. In process block  608 , a power setting (e.g. voltage) of a power supply  106  or  108  is adjusted in accordance with the difference. In electroplating, anode current increases with increasing voltage. The dependence is approximately but not exactly exponential. The exact functional relationship between anode current and anode voltage is may vary in different implementations of the invention, and is not critical to the invention. A PID feedback control algorithm is preferably used in determining an adjustment to be made to the power setting based on the difference computed in process block  606  in order to bring the actual current closer to the target current. A detailed description of PID control algorithms can be found in  Basic Feedback Control Systems,  Alternate 2 nd  Ed, New Jersey, Prentice Hall 1991, which is incorporated herein by reference. PID control algorithms are available as a software library from National Instruments of Austin, Tex. under the trade name of The Componentworks PID Toolkit. 
     In process block  610 , the second predetermined schedule is read to determine if current is to be supplied to the substrate  122  or the dummy electrode  124  at the current time. Process block  612  is a decision block, the outcome of which depends on whether current is to be supplied to the substrate  122 . If, according to the second predetermined schedule, current is to be supplied to the substrate  122 , then in process block  614  current is supplied to the substrate. If current is to be supplied to the dummy electrode  124 , then in process block  616  current is supplied to the dummy electrode  124 . Selection of either the substrate  122  or the dummy electrode  124  is carried out by control computer by operating the relay  110  through the digital-to-analog converter  104 . 
     From process blocks  614  or  616  the process  600  continues with process block  618 , in which it is determined, based on the second predetermined schedule if the electroplating process is complete. In carrying out process block  618  a current time can be checked against an ending time in a last entry in the sequence of entries of the second predetermined schedule. Process block  620  is a decision block, the outcome of which depends on whether, as determined in process block  618 , the process  600  is complete. If complete, then the process terminates. If not complete, then the process  600  loops back through a delay  622  to process block  602 . 
     In this process the power settings of both of the power supplies  106 ,  108  are preferably adjusted in order to maintain each anode current in conformance with the second predetermined schedule. 
     FIG. 7 is a flow diagram of a closed loop control process  700  performed by a control computer  102  for controlling a contact area between an anode  116 ,  118  and the electrolyte  126  in the electroplating apparatus  100  according to yet another embodiment of the present invention. 
     As indicated by the commonality of reference numerals, process  700  inherits from process  600  all process blocks except for process block  708 . In process block  708  a contact area between an anode  116 ,  118  is adjusted in accordance with the difference obtained in process block  608 , in order to bring the actual current measured in process block  604  closer to the target current measured in process block  606 . A PID feedback control algorithm can be used in determining an adjustment made in process block  708 . 
     In this process contact areas between the electrolyte  126 , and both of the anodes  116  and  118  are preferably adjusted in order to maintain the current of each in conformance with the second predetermined schedule. It is also possible to use a process in which both power supply settings, and contact areas are adjusted in order to maintain anode currents in conformance with the second predetermined schedule. Current is expected to have a nearly linear (in fact, nearly proportional) dependence on contact area for a fixed anode voltage. In a practical system some nonlinearities can be expected to occur. Small nonlinearities do not defeat the feedback control algorithm. 
     The following is an exemplary format for the predetermined schedule for obtaining a two layer CMM with a sharp interface between two successive film layers. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 BEGIN 
                 END 
                 ANODE 116 
                 ANODE 118 
                 CATHODE 
               
               
                 TIME 
                 TIME 
                 CURRENT 
                 CURRENT 
                 SELECTION 
               
               
                   
               
             
             
               
                 T1 
                 T2 
                 la 
                 lb 
                 SUBSTRATE 
               
               
                 T2 
                 T3 
                 lc 
                 ld 
                 DUMMY 
               
               
                 T3 
                 T4 
                 lc 
                 ld 
                 SUBSTRATE 
               
               
                   
               
             
          
         
       
     
     In this exemplary process, a first film layer is deposited on the substrate  122  during a first period between T 1  and T 2 . The first film layer is a first alloy of two metals of the two anodes  114 ,  116  in relative percentages that depend on currents Ia and Ib. The values of Ia and Ib are experimentally determined to obtain the first alloy. During the first period there is a first set of concentrations of the two dissolved disassociated compounds including metals from the two anodes  116 ,  118 . 
     A second film layer is deposited on the substrate  122  during a third period between T 3  and T 4 . The second film layer is a second alloy of two metals corresponding to the two anodes  114 ,  116  in relative percentages that depend on currents Ic and Id. The values of Ic and Id are experimentally determined to obtain the second alloy. During the third period there is a second set of concentrations of two dissolved disassociated compounds including respectively elements from the two anodes  116 ,  118 . The second set of concentrations corresponds to the currents Ic and Id. In other words with currents maintained at Ic, Id the second set of concentrations will be maintained. 
     During a second period between T 2  and T 3  concentrations of the two dissolved disassociated compounds gradually changes from the first set of concentrations to the second set of concentrations. During the second, period the relay  110  is switched to supply current to the dummy electrode in lieu of the substrate. During the second period, a film with a smoothly varying composition is deposited onto the dummy electrode  124 , but not onto the substrate. 
     The transition in between the second alloy and the first alloy in the film deposited onto the substrate  122  will be abrupt. 
     The arrangement of the algorithms discussed in connection with FIGS. 6 and 7 should be taken as exemplary, not limiting. 
     Software for carrying out the processes depicted in FIGS. 4-7 may be loaded onto the control computer  102  from the computer readable medium  142 . The predetermined schedules read in the process shown in FIGS. 4-7 can be loaded onto the control computer  102  from the computer readable medium. 
     Power supplies  106 ,  108  can be self regulating such that they have internal feedback and control circuits for maintaining current or voltage at a predetermined setting. A voltage regulating power supply would be particularly useful in a process in which the contact area between the electrolyte  126  and anodes  116 ,  118  is controlled in order to control the composition of a deposited film. A current regulating power supply is particularly useful in a process in which a power supply setting (current in this case) is controlled to control the composition of a deposited film. If a power supply with an external control input, an internal current regulating circuitry is used, it is possible to dispense with ammeters  112 ,  114  an analog-to-digital converter  120 . 
     Future developments in computational electrochemistry may reduce or eliminate the need for experimentation in determining voltages and contact areas. It may be possible to determine correct power and contact area settings based on a mathematical model. 
     While there has been illustrated and described what are presently considered to be the preferred embodiments of the present invention, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the present invention. Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Furthermore, an embodiment of the present invention may not include all of the features described above. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims.