Abstract:
A cathode current control system employing a current thief for use in electroplating a wafer is set forth. The current thief comprises a plurality of conductive segments disposed to substantially surround a peripheral region of the wafer. A first plurality of resistance devices are used, each associated with a respective one of the plurality of conductive segments. The resistance devices are used to regulate current through the respective conductive finger during electroplating of the wafer. Various constructions are used for the current thief and further conductive elements, such as fingers, may also be employed in the system. As with the conductive segments, current through the fingers may also be individually controlled. In accordance with one embodiment of the overall system, selection of the resistance of each respective resistance devices is automatically controlled in accordance with predetermined programming.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of U.S. Ser. No. 08/933,450, filed Sep. 18, 1997, and entitled “Cathode Current Control System for a Wafer Electroplating Apparatus”. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0003]    Most inorganic and some organic chemical compounds, when in a molten state or when dissolved in water or other liquids, become ionized; that is, their molecules become dissociated into positively and negatively charged components, which have the property of conducting an electric current. If a pair of electrodes is placed in a solution of an electrolyte, or an ionizable compound, and a source of direct current is connected between them, the positive ions in the solution move toward the negative electrode and the negative ions toward the positive. On reaching the electrodes, the ions may gain or lose electrons and be transformed into neutral atoms or molecules, the nature of the electrode reactions depending on the potential difference, or voltage, applied.  
           [0004]    The action of a current on an electrolyte can be understood from a simple example. If the salt copper sulfate is dissolved in water, it dissociates into positive copper ions and negative sulfate ions. When a potential difference is applied to the electrodes, the copper ions move to the negative electrode, are discharged, and are deposited on the electrode as metallic copper. The sulfate ions, when discharged at the positive electrode, are unstable and combine with the water of the solution to form sulfuric acid and oxygen. Such decomposition caused by an electric current is called electrolysis.  
           [0005]    Electrolysis has industrial applicability in a process known as electroplating. Electroplating is an electrochemical process for depositing a thin layer of metal on, usually, a metallic base. Objects are electroplated to prevent corrosion, to obtain a hard surface or attractive finish, to purify metals (as in the electrorefining of copper), to separate metals for quantitative analysis, or, as in electrotyping, to reproduce a form from a mold. Cadmium, chromium, copper, gold, nickel, silver, and tin are the metals most often used in plating. Typical products of electroplating are silver-plated tableware, chromium-plated automobile accessories, and tin-plated food containers.  
           [0006]    In the process of electroplating, the object to be coated is placed in a solution, called a bath, of a salt of the coating metal, and is connected to the negative terminal of an external source of electricity. Another conductor, often composed of the coating metal, is connected to the positive terminal of the electric source. A steady direct current of low voltage, usually from 1 to 6 V, is required for the process. When the current is passed through the solution, atoms of the plating metal deposit out of the solution onto the cathode, the negative electrode. These atoms are replaced in the bath by atoms from the anode (positive electrode), if it is composed of the same metal, as with copper and silver. Otherwise they are replaced by periodic additions of the salt to the bath, as with gold and chromium. In either case equilibrium between the metal coming out of solution and the metal entering is maintained until the object is plated.  
           [0007]    Recently recognized applications of electroplating relate to the electroplating of a semiconductor wafer. The electroplated metal is used to provide the interconnect layers on the semiconductor wafer during the fabrication of integrated circuit devices. Due to the minute size of the integrated circuit devices, the electroplating process must be extremely accurate and controllable. To ensure a strong and close bond between the wafer to be plated and the plating material, the wafer is cleaned thoroughly using a chemical process, or by making it the anode in a cleaning bath for an instant. To control irregularities in the depth of the plated layer, and to ensure that the grain at the surface of the plated layers is of good quality, the current density (amperes per square foot of cathode surface) and temperature of the wafer must be carefully controlled.  
           [0008]    The present inventors have recognized this need for controlling irregularities in the depth of the plated layer across the surface of the wafer. The present invention is directed, among other things, to a solution to this problem.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    A cathode current control system employing a current thief for use in electroplating a wafer is set forth. The current thief comprises a plurality of conductive segments disposed to substantially surround a peripheral region of the wafer. A first plurality of resistance devices are used, each associated with a respective one of the plurality of conductive segments. The resistance devices are used to regulate current through the respective conductive finger during electroplating of the wafer.  
           [0010]    Various constructions are used for the current thief and further conductive elements, such as fingers, may also be employed in the system. As with the conductive segments, current through the fingers may also be individually controlled. In accordance with one embodiment of the overall system, selection of the resistance of each respective resistance devices is automatically controlled in accordance with predetermined programming.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic block diagram of an electroplating system constructed in accordance with one embodiment of the invention.  
         [0012]    FIGS.  2 - 6  illustrate various aspects of the construction of a rotor assembly and current thief constructed in accordance with one embodiment of the present invention.  
         [0013]    [0013]FIG. 7 is an exemplary cross-sectional view of a printed circuit board forming a part of the current thief of FIGS.  2 - 6  and showing the connection between a resistive element and its corresponding conductive segment.  
         [0014]    [0014]FIG. 8 illustrates one manner of implementing and controlling a resistive element connected to a respective segment.  
         [0015]    FIGS.  9 - 14  are schematic drawings illustrating one embodiment of a current control system that may be used in the system of FIGS.  1 - 7 .  
         [0016]    [0016]FIGS. 15 and 16 are schematic drawings illustrating one embodiment of a stator control system that may be used in the system of FIGS.  1 - 7 .  
         [0017]    [0017]FIGS. 17 and 18 illustrate a further embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    [0018]FIG. 1 is a schematic block diagram of a plating system, shown generally at  50 , for electroplating a metallization layer, such as a patterned copper metallization layer, on, for example, a semiconductor wafer  55 . The illustrated system generally comprises a vision system  60  that communicates with a main electroplating control system  65 . The vision system  60  is used to identify the particular product being formed on the semiconductor wafer  55  before it is placed into an electroplating apparatus  70 . With the information provided by the vision system  60 , the main electroplating control system  65  may set the various parameters that are to be used in the electroplating apparatus  70  to electroplate the metallization layer on the wafer  55 .  
         [0019]    In the illustrated system, the electroplating apparatus  70  is generally comprised of an electroplating chamber  75 , a rotor assembly  80 , and a stator assembly  85 . The rotor assembly  80  supports the semiconductor wafer  55 , a current control system  90 , and a current thief assembly  95 . The rotor assembly  80 , current control system  90 , and current thief assembly  95  are disposed for co-rotation with respect to the stator assembly  85 . The chamber  75  houses an anode assembly  100  and contains the solution  105  used to electroplate the semiconductor wafer  55 .  
         [0020]    The stator assembly  85  supports the rotor assembly  80  and its associated components. A stator control system  110  may be disposed in fixed relationship with the stator assembly  85 . The stator control system  110  may be in communication with the main electroplating control system  65  and may receive information relating to the identification of the particular type of semiconductor device that is being fabricated on the semiconductor wafer  55 . The stator control system  110  further includes an electromagnetic radiation communications link  115  that is preferably used to communicate information, to a corresponding electromagnetic radiation communications link  120  of the current control system  90  used by the current control system  90  to control current flow (and thus current density) at individual portions of the current thief assembly  95 . A specific construction of the current thief assembly  95 , the rotor assembly  80 , the stator control system  110 , and the current control system  90  is set forth in further detail below.  
         [0021]    In operation, probes  120  make electrical contact with the semiconductor wafer  55 . The semiconductor wafer  55  is then lowered into the solution  105  in minute steps by, for example, a stepper motor or the like until the lower surface of the semiconductor wafer  55  makes initial contact with the solution  105 . Such initial contact may be sensed by, for example, detecting a current flow through the solution  105  as measured through the semiconductor wafer  55 . Such detection may be implemented by the stator control system  110 , the main electroplating control system  65 , or the current control system  90 . Preferably, however, the detection is implemented with the stator control system  110 .  
         [0022]    Once initial contact is made between the surface of the solution  105  and the lower surface of the semiconductor wafer  55 , the wafer  55  is preferably raised from the solution  105  by a small distance. The surface tension of the solution  105  creates a meniscus that contacts the lower surface of the semiconductor wafer  55  that is to be plated. By using the properties of the meniscus, plating of the side portions of the wafer  55  is inhibited.  
         [0023]    Once the desired meniscus has been formed at the plating surface, electroplating of the wafer may begin. Specific details of the actual electroplating operation are not particularly pertinent to the use or design of present invention and are accordingly omitted.  
         [0024]    FIGS.  2 - 7  illustrate the current thief assembly  95  and rotor assembly  80  as constructed in accordance with one embodiment of the present invention. As shown, the current thief assembly  95  comprises a plurality of conductive segments  130  that extend about the entire peripheral edge of the wafer  55 . In the illustrated embodiment, the conductive segments  130  are formed on a printed circuit board  135 . Each segment  130  is associated with a respective resistive element  140  as shown in FIG. 7. In the illustrated embodiment, the resistive elements  140  are disposed on the side of the printed circuit board opposite the segments  130 . The resistive element  140  respectively associated with each segment may take on various forms. For example, the resistive element  140  may be a fixed or variable resistor. The resistive element  140  also may be constructed in the form of a plurality of fixed resistors that are selectively connected in circuit to one another in a parallel arrangement to obtain the desired resistance value associated with the respective segment. The switching of the individual resistors to or from the parallel circuit may ensue through a mechanical switch associated with each resistor, a removal conductive trace or wire associated with each resistor, or through an automatic connection of each resistor. Further details with respect to the automatic connection implementation are set forth below.  
         [0025]    In each instance, the resistive element has a first lead  150  in electrical contact with the segment  130  and a second lead  155  for connection to cathode power. As such, the resistive elements  140  provide an electrical connection between the conductive segments  130  and, for example, a cathodic voltage reference  160  (See FIG. 1). In the disclosed embodiment, the voltage reference is a ground and is established through a brush connection between the rotor assembly  80  and the stator assembly  85  which is itself connected to ground. During electroplating of the semiconductor wafer  55 , the resistive element  140  associated with each segment  130  controls current flow through the respective segment. The resistance value used for each of the resistive elements  140  is dependent on the current that the respective segment  130  must pass to ensure the uniformity of the plating over the portions of the wafer surface that are to be provided with the metallization layer. Such values may be obtained experimentally and may vary from segment to segment and from product type to product type.  
         [0026]    A still further resistive element that may be used to control current flow through each respective segment  130  is shown in FIG. 8. Here, the resistive element is comprised of a pair of FETs  170  and  175 . The gate terminals of each FET  170  and  175  are connected to be driven by the output of a comparator  180  which is part of the feed-forward portion of a feedback control system shown generally at  185 . The source terminals of the FETs  170 ,  175  are connected to the cathode power while the drain terminals of the FETs are connected to a respective segment (or, as will be set forth below, a respective finger).  
         [0027]    In the feedback system  185 , a current monitor circuit  190  monitors the current flowing through the respective segment  130  and provides a signal indicative of the magnitude of the current to a central processing unit  195 . The control processing unit  195 , in turn, provides a feedback signal to a bias control circuit  200  that generates an output voltage therefrom to the inputs of comparator  180 . Comparator  180  uses the signal from the bias control circuit  200  and, further, from a plating waveform generator  205  to generate the drive signal to the gate terminals of the FETs  170  and  175 .  
         [0028]    The central processing unit  195  is programmed to set the individual set-point current values for each of the segments  130  of the current thief assembly  95 . If the measured current exceeds the set-point current value, the control processing unit  195  sends a signal to the bias control circuit  200  that will ultimately control the drive voltage to the FETs  170 ,  175  so as to reduce the current flow back to the set-point. Similarly, if the measured current falls below the set-point current value, the control processing unit  195  sends a signal to the bias control circuit  200  that will ultimately control the drive voltage to the FETs  170 ,  175  so as to increase the current flow back to the set-point for the respective segment.  
         [0029]    The current thief assembly  95  is disposed for co-rotation with the rotor assembly  80 . With reference to FIG. 6, the printed circuit board  135  is attached on a surface of a hub  210  of the rotor assembly  80 . The board  135  is spaced the hub  210  by an insulating thief spacer  215  and secured to the spacer  215  using a plurality of fasteners  220 . The spacer  215 , in turn, is secured to the hub  210  of the rotor assembly  80  using fasteners  220  that extend through securement apertures  225  of both the spacer  215  and hub  210 .  
         [0030]    The hub  210  of the rotor assembly  80  is also provided with a plurality of support members for securing the wafer  55  to the rotor assembly  80  during the electroplating process. In the illustrated embodiment, the support members comprise insulating projections  230  that extend from the hub surface and engage a rear side of the wafer  55  and, further, a plurality of conductive fingers  235 . The fingers  235  are in the form of j-hooks and contact the surface of the wafer that is to be plated. Preferably, each of the fingers  235  may be respectively associated with a resistive element  140  such as described above in connection with the segments  130  of the current thief assembly  95 . The current flow through each of the fingers  235  and its respective section of the wafer  55  may thus be controlled. Still further, conductive portions of the fingers  235  that contact the electroplating solution during the electroplating process may also perform a current thieving function and, accordingly, control current density in the area of the fingers. To this end, the amount of exposed metal on each of the fingers  235  may vary from system to system depending on the amount of current thieving required, if any, of the individual fingers  235 .  
         [0031]    The conductive fingers  230  may be part of a finger assembly  240  such as the one illustrated in FIGS. 5A and 5B. As shown, the finger assembly  240  is comprised of an actuator  250  including a piston rod  255 . The piston rod  255  engages the finger  235  at a removable interconnect portion  260  for ease of removal and replacement of the finger  235 . Further, the actuator  255  is biased by springs  265  so as to urge the fingers against the wafer  55  as shown in FIG. 5. The fingers  235  may be urged to release the wafer  55  by applying a pressurized gas to the actuator  250  through inlet  270 . Application of the pressurized gas urges the fingers  235  in the direction shown by arrow  275  of FIG. 5 thereby facilitating removal of the wafer  55  from the rotor assembly  80 .  
         [0032]    As shown in FIG. 4, the hub  210  is connected to an axial rod assembly  280  that extends into rotational engagement with respect to the stator assembly  85 . The axial rod  280  is coaxial with the axis of rotation of the rotor assembly  80 . The brush connection used to establish the reference voltage level with respect to the anode assembly  100  used in the electroplating process may be established through the axial rod.  
         [0033]    FIGS.  9 - 14  illustrate one embodiment of a control system that may be used to vary the resistance values of the resistive elements  140  thereby controlling the current flow through the conductive segments  130  and, optionally, the conductive fingers  235 . Generally stated, the control system comprises a power supply circuit  400  to supply power for the control system, an electromagnetic communications link  120  for communicating with the stator control system  110 , a processor circuit  410  for executing the programmed operations of the control system, the resistive elements  140  for controlling the current flow through the individual segments  130  and, optionally, fingers  235 , and a resistive element interface  415  providing an interface between the processor  410  and the resistive elements  140 .  
         [0034]    The power supply circuit  400  preferably uses batteries  420  as its power source. The negative side of the battery supply is referenced to the brush contact (ground). Three 3V lithium coin cells are used to provide 9V to the input of a LT1521 5 VDC regulator  425 . This ensures 3.5 volts of compliance. The op-amp U 3  and corresponding circuitry monitors the output of the 5 VDC regulator LT1521 and provides an interrupt to the 87251 processor U 17  when the batteries require replacement.  
         [0035]    The processor U 17  is preferably an 87251 microcontroller and controls communication with the control system. One of the communications links is the electromagnetic radiation link  120  which is preferably implemented as an infra-red communications link that provides a communications interface with a corresponding infra-red communications link in the stator control system  115 .  
         [0036]    When the rotor assembly  80  is in a “home position” with respect to the stator assembly  85 , the processor U 17  may receive data over the link  120  from the stator control system  110 . The data transmitted to the control system over the link  120  of the disclosed system includes sixteen/twenty, 8-bit channel data (see below). The processor U 17  controls the return of an ack/checksum and an additional battery status byte to the stator control system  110 . The data received by the control system is stored by the processor U 17  in battery backed RAM.  
         [0037]    Once the data is verified, the processor U 17  controls the resistive element interface  415  to select the proper resistance value for each of the resistive elements  140 . In the illustrated embodiment, the resistive elements  140  can be divided into individual resistive channels  1 - 20  respectively associated with each of the conductive segments  130  and, optionally, each of the conductive fingers  235 . Since the current thief assembly  95  of the illustrated embodiment uses sixteen segments  130  and there are four conductive fingers  235  that are used, either sixteen or twenty resistive channels may be employed.  
         [0038]    As shown with respect to the exemplary resistive channel  1 , each resistive channel  140  is comprised of a plurality of fixed resistors that may be selectively connected in parallel with one another to alter the effective resistance value of the channel. Eight fixed resistors are used in each channel of the disclosed system.  
         [0039]    Each channel is respectively associated with an octal latch, shown here as U 1  for channel  1 . The output of each data bit of the octal latch U 1  is connected to drive a respective MOSFET Q 1 A-Q 4 B that has its source connected to a respective fixed resistor of the channel.  
         [0040]    The processor U 17  uses its Port  2  as a data bus to communicate resistor selection data to the octal latches of the resistive element interface  415 . Ports  1  and  0  of the processor U 17  provides the requisite clock and strobe signals to the latches. After the requisite data has been communicated to the octal latches, the processor U 17  preferably enters a sleep mode from which it awakes only during a reset of the system or when the stator control system  110  transmits further information through the infra-red link.  
         [0041]    Based on the data communicated to each of the octal latches, various selected ones of the MOSFETs for the respective channel are driven to effectively connect corresponding fixed resistors in parallel with one another and effectively in series with the respective segment  130  or finger  235 . The resistance values of the fixed resistors for a given channel are preferably weighted to provide a wide range of total resistance values for the channel while also allowing the resistance values to be controlled with in relatively fine resistance value steps.  
         [0042]    The foregoing control system is preferably mounted for co-rotation with the rotor assembly  80 . Preferably, the control system is mounted in the hub  210  in a location in which it is not exposed to the electroplating solution  105 .  
         [0043]    One embodiment of the stator control system  110  is shown in FIGS.  15 - 16 . The stator control system  110  includes an 87251 processor  440  that contains the programming for the stator control system operation. The primary function of the stator control system  110  is to receive programming information from the main control system  65  over an RS-485 half duplex multi-drop communications link  430 . The programming information of the disclosed embodiment includes the sixteen/twenty, eight bit values used to drive the MOSFETs of the resistive element interface  415 . Data transmitted from the stator control system  110  to the main control system  65  includes: an ack/checksum OK and an additional byte containing a product detection bit, a meniscus sense bit, and a rotor control system battery status bit.  
         [0044]    Communications between the current control system  90  and the stator control system  110  should be kept to a minimum to conserve battery power in the rotor control system. Due to the gain limitations of the micro-power characteristics of the integrated circuits used in the current control system  90 , the baud rate used for the communications should be maintained between 600 baud and 1.2K baud. The static RAM of the rotor control system is non-volatile. As such, the channel resistance programming values are stored so long as there is power in the batteries. Communications between the stator control system  10  and the current control system  90  need only take place when the batteries are replaced or when different plating characteristics are necessary.  
         [0045]    The stator control system  110  includes an on-board watchdog timer which is software enabled/disable. The watchdog timer is enabled after power-on reset and register initialization. One of the on-board timers also provides a timer for controller operation and I/O debounce routines.  
         [0046]    The stator control system  110  also includes a meniscus sense circuit  450  as shown on FIG. 16. Just prior to product plating, a start signal at PP 8  from the processor  440  enables relay K 1 . In response, the signal at PP 10  output from the meniscus sense circuit  450  is provided to the processor  440  when the product contacts the plating solution. This latching signal causes the control system to stop downward motion and retract, for example, 0.050 in to provide the meniscus pull described above. Mechanisms for lowering and raising the semiconductor wafer  55  may be constructed in effectively the same manner as such mechanisms are implemented on the Equinox® semiconductor processing machine available from Semitool, Inc., of Kalispell, Mont.  
         [0047]    The stator control system  10  also provides a wafer sensor interface  455  at J 2 . The external product sensor (not illustrated) may be, for example, an open collector optical sensor such as one available from Sunx.  
         [0048]    On initialization of the control system  110 , the processor  440  preferably stores $FF to all of the ports. The following table lists the port assignments for the processor.  
                           TABLE 1                                   PORT   FUNCTIONALITY                           P0[0..7]   NOT USED           P1.0 (PP8)   MENISCUS SENSE START/STOP           P1.1 (PP9)   MENISCUS SENSE RESET           P1.2 (PP10)   MENISCUS SENSE SIGNAL           P1.3 (PP11)   WAFER/PRODUCT SENSE           P1.4 (PP12)   NOT USED           P1.5 (PP13)   NOT USED           P1.6 (PP14)   RS-485 TRANSMITTER ENABLE           P1.7 (PP15)   RS-485/OPTICAL LINK SELECT           P2 [0 . . . 7]   NOT USED           P3.0 (RxD)   RECEIVER DATA           P3.1 (TxD)   TRANSMITTER DATA           P3.2 (PP24) THROUGH   NOT USED           P3.7 (PP29)                      
 
         [0049]    A further embodiment of the current thief  95  and corresponding rotor assembly  80  is set forth in FIG. 17. In the illustrated embodiment, the segments  130  are preferably formed from stainless steel and are secured to a polymer base  475  that, in turn, is secured to the hub  210 . Each of the segments  130  projects beyond the inner parameter of the base  475  toward the wafer support area, shown generally at  480 .  
         [0050]    In the illustrated embodiment, each finger  235  is associated with a corresponding insulating anvil support  485 . As such, the wafer  55  is gripped between the end of conductive fingers  235  and the respective anvil supports  485  to secure the wafer for rotation of the rotor assembly  80  during the electroplating process.  
         [0051]    The circuits for the current control system  90  are disposed on, for example, printed circuit board  500 . Electrical connection between each of the segments  130  and the corresponding resistive element  140  on board  500  is facilitated through the use of a plurality of stand-offs  490 . Each stand-off  490  extends from a respective connection to one of the resistive elements  140  on the printed circuit board  500  through the base  475  and into electrical engagement with a respective one of the conductive segments  130 . The stand-offs  490  also function to secure the board  500 , hub  210 , and base  475  to one another.  
         [0052]    The entire assembly  510  may be disposed for rotation or pivoting about a horizontal axis. In a first position shown in FIG. 18, the wafer is faced downward toward the plating solution for processing. In a second position, the entire assembly is inverter to expose the wafer to manipulation by, for example, mechanical arms or the like. To assist in removal of the wafer from the processing area  480 , the assembly  510  is provided with a plurality of pneumatically actuated lifter mechanisms  515 . When actuated, the lifter mechanisms  515  lift the wafer to a level beyond the current thief assembly  95  to allow placement of the wafer into and removal of the wafer from the assembly  510 .  
         [0053]    [0053]FIG. 18 illustrates the rotor assembly  80  in its home position with respect to the stator assembly  85 . In this position, the IR transmit links  115  and  120  are aligned for communication.  
         [0054]    Other embodiments of the control system of FIGS.  9 - 14  are also suitable for use with the current thief assembly  95 . For example, the control system may be implemented without a processor, instead allowing the processor of the stator control system  110  to shift the resistor selection data bit-by-bit through shift registers of the current control system  90 . In such instances, further IR links may be used to communicate shift register timing signals to the system  90  to allow the stator control system  110  to control the shifting operations. Such timing signals are specific to the particular manner in which the current control system is designed and are not particularly pertinent here.  
         [0055]    Numerous modifications may be made to the foregoing system without departing from the basic teachings thereof. Although the present invention has been described in substantial detail with reference to one or more specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the scope and spirit of the invention as set forth in the appended claims.