Abstract:
Methods of decreasing the dose per pulse implanted into a workpiece disposed in a process chamber are disclosed. According to one embodiment, the plasma is generated by a RF power supply. This RF power supply may have two different modes, a first, referred to as continuous wave mode, where the RF power supply is continuously outputting a voltage. This mode allows creation of the plasma within the process chamber. During the second mode, referred to as pulsed plasma mode, the RF power supply outputs two different power levels. The platen bias voltage may be a more negative value when the lower RF power level is being applied. This pulsed (or multi-setpoint) plasma also assists in reducing dopant deposition on the wafer during the time when CW plasma is on but the bias voltage pulse is in the off-state. In a further embodiment, a delay is introduced between the transition to the pulsed plasma mode and the initiation of the implanting process. In yet another embodiment the plasma is generated at a location in the chamber more judicious to reducing the dose impinging on the wafer, thereby increasing the process time to allow adequate control of the process.

Description:
This application claims priority of U.S. Provisional Patent Application Ser. No. 61/783,789, filed Mar. 14, 2013, the disclosure of which is incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to methods for reducing the ion dose implanted into a workpiece in a PLAD system. 
     BACKGROUND 
     Semiconductor workpieces are processed within process chambers. One such chamber is known as a plasma deposition chamber, which is part of a PLAD system. In operation, one or more dopant gasses are fed into the process chamber. These gasses are energized into a plasma through the use of radio frequency (RF) or other forms of energy, such as by utilizing one or more RF antennas or coils. A workpiece is disposed on a platen within the process chamber. This platen may be in electrical communication with a power supply, which can apply a bias voltage to the platen. When the platen is negatively biased, the positively charged species, or ions, from within the plasma accelerate toward the workpiece, thereby implanting the dopant species in the workpiece. At times when the plasma is on but the bias voltage to the platen is in the off state, there may be conditions conducive to dopant deposition on the wafer surface, instead of dopant implantation into the wafer. 
     Implants performed using PLAD systems typically utilize high concentrations of charged species in the plasma and therefore, perform relatively high dose implants. For example, the dopant concentration implanted in the workpiece using a PLAD system may be between 1E16 and 1E17 ions per square centimeter. This implant may be performed in a relatively short amount of time, such as between 30 seconds and a few minutes. This can be achieved because the concentration of ions within the plasma is typically much greater than that found in an ion beam generated in an ion beam line system. 
     PLAD systems are also effective for conformal doping applications. These include applications where dopant is to be implanted in all exposed surfaces of a three-dimensional structure. Examples of these structures include raised structures, such as fin type structures, and recessed structures, such as trenches. Unlike beam line systems, PLAD systems are effective at implanting ions into both the vertical surfaces and the horizontal surfaces of the workpiece. 
     Recently, a new set of applications, such as CMOS image sensors (CIS) shallow trench isolation (STI) and channel doping for threshold voltage control, has arisen, which require conformal doping at ion concentration levels much less than those typically associated with PLAD systems, such as 1E13. 
     A PLAD system, using present operating parameters, would implant this concentration of charged ions in a workpiece in a very short period of time, such as about 0.5 seconds. This period may be too short to allow adequate process control and guarantee wafer-to-wafer repeatability. Additionally, given the short duration of the implant, the species concentration in the workpiece may not be uniform. 
     Therefore, it would be beneficial if there were a method of achieving low dose doping and particularly, low dose, conformal doping, of a workpiece using a PLAD system. 
     SUMMARY 
     Methods of decreasing the dose per pulse (DPP) implanted into a workpiece disposed in a process chamber are disclosed. According to one embodiment, the plasma is generated by a RF power supply. This RF power supply may have two different modes, a first, referred to as continuous wave mode, where the RF power supply is continuously outputting a RF power level. This mode is typically used for creation of the plasma within the process chamber. During the second mode, referred to as pulsed plasma mode, the RF power supply outputs two different power levels. The platen bias voltage may be a more negative value when the lower RF power level is being applied. This pulsed (or multi-setpoint) plasma also assists in reducing dopant deposition on the wafer during the time when CW plasma is on but the bias voltage pulse is in the off-state. 
     In one embodiment, a method of performing low dose implantation of a workpiece in a process chamber is disclosed. This method comprises applying continuous wave (CW) RF power to a plasma coil and creating a plasma in the process chamber; maintaining the CW RF power applied to the plasma coil for a first predetermined period; transitioning the CW RF power to a pulsed RF power after the first predetermined period to reduce a concentration of ions in the plasma, wherein at least a first RF power level and a second lower RF power level, are repeatedly generated, causing the plasma to vary in ion concentration as a function of time; maintaining the pulsed RF power applied to the plasma coil for a second predetermined period; and implanting ions into the workpiece after the second predetermined period by applying a negative bias voltage to the workpiece while maintaining the pulsed RF power applied to the plasma coil. In a further embodiment, the bias voltage associated with the workpiece may be pulsed with a low magnitude bias voltage during the second predetermined period. In another embodiment, the bias voltage associated with the workpiece may not be pulsed until the implant begins. 
     In another embodiment, a method of performing low dose implantation of a workpiece in a process chamber is disclosed. The method comprises creating a plasma in the process chamber; operating a RF power supply in a pulsed plasma mode after the plasma is created to reduce a concentration of ions in the plasma, wherein, in the pulsed plasma mode, the RF power supply repeatedly generates at least a first RF power level, and a second lower RF power level, causing the plasma to vary in ion concentration as a function of time; waiting a time period; and implanting ions into the workpiece after the time period by applying a first negative bias voltage to the workpiece when the second lower RF power is generated and applying a second bias voltage, less negative than the first negative bias voltage, to the workpiece when the first RF power is generated. 
     In a third embodiment, a method of performing low dose implantation of a workpiece in a process chamber is disclosed. The process chamber comprises a top cover with coils disposed thereon. The method comprises creating a plasma in the process chamber by applying a RF power to the coils; waiting a first time period for conditions within the process chamber to stabilize after creating the plasma; applying, after the first time period, in an alternating manner, a first RF power to the coils and a second RF power, less than the first RF power, to the coils, causing the plasma to vary in ion concentration as a function of time; waiting a second time period for conditions within the process chamber to stabilize after the applying; and implanting ions into the workpiece after the second time period by pulsing a negative bias voltage to the workpiece, wherein a temporal relationship exists between the bias voltage pulsed to the workpiece and the RF power applied to the coils so that a more negative bias voltage is applied to the workpiece when the second RF power is applied to the coils. In a further embodiment, the top cover comprises a top horizontal surface, vertical walls and horizontal walls, where the coils are disposed on the vertical walls and the horizontal walls. The method further comprises applying RF power only to the coils disposed on the vertical walls. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  shows a PLAD implant system that may be used with one embodiment; 
         FIG. 2  is a timing diagram using pulsed plasma mode; 
         FIG. 3  is the timing diagram of  FIG. 2  showing additional chamber parameters; 
         FIG. 4  is a timing diagram showing delayed implantation with pulsed plasma mode according to one embodiment; 
         FIG. 5  is a timing diagram showing delayed implantation with pulsed plasma mode according to another embodiment; 
         FIG. 6  illustrates a flow chart showing the delayed implant process according to one embodiment; and 
         FIG. 7  shows a configuration of RF coils that may be used with any of the embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, the time required for typical PLAD implants is sufficiently long so as to allow adequate process control and excellent wafer-to-wafer repeatability. In contrast, low dose implants would require less time. Therefore, in order to operate the PLAD tool in low-dose mode that meets the requisite wafer-to-wafer repeatability and process control, the time required to perform the low dose implant may need to be increased. 
       FIG. 1  shows one embodiment of a PLAD implant system that may be used for low dose implants. The PLAD implant system  100  includes a plurality of walls  110 , which define a process chamber  120 . An RF energy source  130  is disposed outside the process chamber  120 , preferably in close proximity to, or in contact with, one or more of the walls  110  of the process chamber  100 . This RF energy source  130  may comprise one or more coils  133 , which as energized by an RF power supply  135 . In some embodiments, a matching circuit  137  is disposed between the RF power supply  135  and the coils  133 . While  FIG. 1  shows the coils arranged on the upper surface of the process chamber  100 , the disclosure is not limited to this embodiment. For example, coils may be disposed on one or more vertical walls  110 , if desired. In other embodiments, the upper surface of the process chamber may comprise both horizontal and vertical surfaces and the coils  133  may be disposed on any or all of these surfaces. 
     One or more dopant gasses enter the process chamber  120  via one or more gas inlets  140 . Various types of dopant may be used. For example, in some embodiments, the dopant gasses may include BF 3  and argon. In other embodiments, PH 3  and hydrogen may be used. Other dopant gasses may also be used and the disclosure is not limited to any particular dopant gasses or mixtures thereof. 
     Although not shown, a vacuum pump and valves, such as pendulum or throttle valves, may be in communication with the process chamber  120  to insure that the pressure within the process chamber  120  stays within a desired range. Similarly, a pressure sensor may be included to monitor the pressure within the chamber  120 , and other peripheral hardware to ensure smooth operations of the tool. 
     Energization of the coils  133  creates a plasma  150  within the process chamber  120 . A workpiece  160  is also disposed in the process chamber  120  and is located on a platen  170 . This platen  170  is in electrical communication with a bias power supply  175 , which applies a bias voltage to the platen  170  as determined by controller  180 . 
     A controller  180  may be used to control the actions within the PLAD system  100 . This controller  180  may include a processing unit, in communication with a memory device. The memory device may comprise instructions, which, when executed by the processing unit, allow the controller  180  to perform the actions described herein. The controller  180  may be in communication with a variety of sensors, such as, for example, pressure sensors, temperature sensors, and voltage detectors, to monitor the activity of the PLAD system  100 . In addition, the controller  180  controls the operation of the various power supplies, including, for example, the RF power supply  135  and the bias power supply  175 . 
     In normal operation, the RF power supply  135  may provide a continuous oscillating output, such that the gas in the process chamber  120  is continuously energized. This mode of operation may be referred to as continuous wave (CW) mode. In CW mode, the RF power supply  135  may output a power of 250 W or more to power the coils  133 . In one embodiment, the RF power supply  135  generates 500 W of power. 
     To reduce the concentration of ions in the plasma  150 , the RF power supply  135  may be pulsed so as to affect the density of the plasma  150 . In other words, the RF power supply  135  may have a variable output, which in turn affects the density and intensity of the plasma  150  that is generated. This second mode of operation may be referred to as pulsed or multi-setpoint plasma mode. In this pulsed mode, the RF power supply  135  generates at least two different outputs, a higher RF power level, similar to that generated during CW mode, and a lower RF power level. The duty cycle of the RF power supply  135  may vary, and may be, for example, 30% at the higher power level and 70% at the lower RF power level. However, other duty cycles are also possible and the disclosure is not limited to any particular duty cycle. 
     In one embodiment, a negative bias voltage is applied to the platen  170  by the bias power supply  175  when the lower RF power is applied to the coils  133  (e.g. during a first period). Because the plasma is being energized at a lower power level, this causes a smaller number of ions to be implanted during the bias voltage pulse, as compared to CW mode. Thereafter, during a second period, a more positive bias voltage, such as −100V or greater, is applied to the platen  170  by the bias power supply  175 , while the higher RF power level is supplied to the coils  133 . The term “more positive bias voltage” refers to any voltage; positive, negative or ground; which is greater than the first bias voltage. Thus, if the first bias voltage is, for example, −500V, the second more positive bias voltage may be any negative voltage that is more positive than −500V, such as, for example, −100V. The second more positive bias voltage may also be ground or any positive bias voltage. Stated differently, the first bias voltage is more negative than the second bias voltage. The higher RF power level serves to create more ions and electrons. In one embodiment, the implant occurs during the lower RF power and more negative bias voltage (e.g. the first period). During the application of higher RF power to the coils  133  and the more positive bias voltage to the platen  170  (e.g. the second period), a plasma with a greater concentration of ions and electrons is generated. The workpiece  160  may remain positively charged after the implant and this charge may attract electrons from the plasma, which serves to neutralize the workpiece  160 . Of course, this is only one embodiment, and the relationship between the RF power levels and the bias voltages may be different than that described above. For example, the higher RF power level may overlap with the more negative bias voltage in some embodiments. In other embodiments, the higher RF power level may correspond to the assertion of the more negative bias voltage. Any desired temporal relationship between the RF power supply  135  and the bias power supply  175  may be possible. Furthermore, any combination of negative or positive bias voltages and RF power levels can be utilized. Therefore, the use of the term “pulsed plasma mode” is intended to represent any configuration where the RF power supply  135  is not driven at a constant power level, thereby causing the plasma to vary in ion concentration as a function of time. Further, in some embodiments, pulsed plasma mode may also indicate a temporal relationship between the RF power level and the bias voltage. 
     In one embodiment, the RF power supply  135  is transitioned to pulsed plasma mode after the first pulse of the bias power supply  175 . In other words, the RF power supply  135  is maintained in continuous wave mode until it is time to implant the workpiece  160 . The indication that an implant is about to start may be, for example, the bias voltage pulse created by the bias power supply  175 . Once the implant begins, the RF power supply  135  may only be at the higher RF power level during the times when the bias voltage is at the more positive bias voltage. It has been found that modulating the output of the RF power supply  135  relative to the pulsed DC bias  175  may increase the implant time. 
       FIG. 2  represents a timing diagram that shows one embodiment of this process. Waveform  250  represents the output of RF power supply  135 , while waveform  260  represents the bias voltage, which is the output of the bias power supply  175 . First, as described above, and shown at time  200 , the RF power supply  135  is turned on and is in CW mode. This results in a plasma  150  being generated in the process chamber  120 . The bias voltage  260  is pulsed to a negative voltage at time  210 , which causes the controller  180  to direct the RF power supply  135  to transition to pulsed plasma mode. This causes a reduction in the RF power level, as shown at time  210 . The negative voltage of this bias pulse is sufficient to attract positive ions from the plasma  150  toward the workpiece  160 . In some embodiments, this bias pulse may be a negative voltage in the hundreds to thousands of volts, such as but not limited to −1000V. When the pulse ends, at time  220 , the controller  180  directs the RF power supply  135  to return to the first RF power level. When the next negative bias pulse occurs at time  230 , the RF power supply  135  transitions back to the lower RF power level. This cycle repeats until a sufficient number of bias pulses have been generated to achieve the desired ion concentration in the workpiece  160 . While  FIG. 2  (as well as  FIGS. 3-5 ) show the lower RF power level corresponding to the more negative bias voltage, other embodiments are possible. The temporal relationship between these two voltages may be varied as required by the application. 
     The actions described in the timing diagram shown in  FIG. 2  may be coordinated by the controller  180 . 
     In some embodiments, the process of  FIG. 2  may not produce the desired wafer-to-wafer repeatability.  FIG. 3  shows the timing diagram as  FIG. 2 , but includes other parameters, such as chamber pressure  270  and the behavior of matching circuit  137 . Note that at time  200 , when the RF power supply  135  transitions to continuous wave mode, the pressure  270  within the process chamber  120  varies, as the PLAD system  100  adapts to the change in the internal environment. Simultaneously, the creation of the plasma causes significant fluctuation in, among other parameters, the chamber pressure, which causes the matching circuit  137  to react. Waveform  280  is used to illustrate the changes in matching circuit  137 . The actual shape of the waveform  280  is not important, rather it is intended to illustrate the transient behavior of the matching circuit  137  when the RF power supply  135  changes operating modes. If the time duration between time  200  and time  210  is sufficiently long, the pressure  270  will stabilize, as will the behavior  280  of the matching circuit  137 . However, the transition to pulsed plasma mode at time  210  causes another response in pressure  270  and the behavior  280  of matching circuit  137 . This response may not stabilize quickly, especially for very short time low-dose applications before the pressure and matching circuit  137  have stabilized. Note that while  FIG. 3  shows the pressure  270  and behavior  280  of the matching circuit  137  stabilizing after three bias pulses (i.e. by time  240 ), this is only illustrative. In operation, the pressure  270  and the behavior  280  of the matching circuit  137  may take much longer or shorter to stabilize. During the time prior to time  240 , if any implants are performed, the desired dose per pulse (DPP) may not be implanted. Furthermore, there may be unwanted variation in the DPP for successive bias voltage pulses. 
     In one embodiment, shown in  FIG. 4 , the initiation of pulsed plasma mode is decoupled from the bias pulses. In other words, the system starts as described before, with the RF power supply  135  entering continuous wave mode at time  400 . At some later time, such as time  410 , the controller  180  determines that the chamber conditions, such as pressure  270  and the behavior  280  of the matching circuit  137  are stable. At this time, or some time thereafter, the controller  180  instructs the RF power supply  135  to begin pulsed plasma mode. Note that this occurs without the pulsing of the bias voltage  260 . Thus, in this case, the pulsed plasma mode refers only to the time varying nature of the RF power level. Again, the pressure  270  and behavior  280  of matching circuit  137  respond to this change. The width and frequency of the pulses generated by the RF power supply  135  during the period between time  410  and time  420  may be predetermined, such as based on expected bias voltage pulse width and frequency. In other embodiments, other values may be used. At some later time, such as time  420 , the controller  180  determines that the pressure  270  and behavior  280  of the matching circuit  137  are again stable. At this time or some time thereafter, the controller  180  instructs the bias power supply  175  to begin generating pulses. It is this action that initiates the implantation process. As was described previously, the RF power supply  135  may be coordinated such that its high RF power level is active during the times when the bias voltage  260  is at the more positive bias voltage, although other embodiments are possible. By delaying the start of the implantation process until time  420 , the stability of the conditions within process chamber  120  may be improved. This may result in more controllable DPP and wafer-to-wafer repeatability, especially for short time low-dose applications. 
     In some embodiments, the controller  180  may be configured such that the bias pulses  260  necessarily trigger the initiation of pulsed plasma mode. In other words, the controller  180  cannot instruct the RF power supply  135  to enter pulsed plasma mode until the first bias pulse occurs.  FIG. 5  shows a timing diagram of an embodiment intended to improve DPP and wafer-to-wafer repeatability in this scenario. In this embodiment, the system starts as described before, with the RF power supply  135  entering continuous wave mode at time  500 . At some later time, such as time  510 , the controller  180  determines that the chamber conditions, such as pressure  270  and the behavior  280  of the matching circuit  137  are stable. At this time, or some time thereafter, the controller  180  instructs the bias power supply  175  to begin creating pulses. However, unlike  FIG. 3 , these pulses have a much smaller magnitude, such as, for example, −1V to −100V. This first smaller magnitude voltage may be sufficient to initiate the pulsed plasma mode, as seen in  FIG. 5 . However, the small magnitude of the first bias voltage insures that few, if any, ions are actually implanted in the workpiece  160  during the time between time  510  and time  520 . The bias power supply  175  continues pulsing the bias voltage  260  with this first low magnitude voltage, until the controller  180  determines that the conditions within the process chamber  120  are stable. At time  520 , when the controller  180  makes this determination, the magnitude of the bias voltage pulse is increased, thereby allowing implantation of the workpiece  160  to take place. This second high magnitude voltage may be, for example, any negative voltage, up to −20 kV. In some embodiments, the high magnitude voltage may be, for example, between −500V and −2000V, such as −1000V. As was described previously, the RF power supply  135  may be coordinated such that the lower RF power level is supplied during the times when the bias voltage  260  is being pulsed, or may have some other relationship. Note that while  FIG. 5  shows the bias voltage  260  switching directly from the first low magnitude voltage to a second high magnitude voltage, other embodiments are possible. For example, the controller  180  may cause the bias power supply  175  to transition between these two values by increasing the magnitude of successive pulse voltages until the second high magnitude bias voltage is reached. For example, the bias voltage  260  can be modulated from the low magnitude voltage in order to trigger pulsed plasma mode, to the high magnitude voltage that is used in implant, in a ramped manner with any ramp rate. 
     In another embodiment, the pulsed (or multi-setpoint) plasma mode may be triggered by the bias pulse at time  210  (See  FIG. 2 ). However, the pulse width of the bias voltage  260  may be significantly different from the width of the pulsed RF power. As an example, the leading edge of the bias voltage pulse at time  210  may trigger the pulse plasma mode in the RF waveform  250 , but the trailing edge of the bias pulse, at time  220 , may not coincide with the end of the low RF power level. In other words, rather than using low magnitude pulses (as in  FIG. 5 ) to trigger the pulsed plasma mode, bias pulses of short time duration are used to trigger pulsed plasma mode. These short time duration bias pulses may be sufficiently short to insure that only a small amount of ions are implanted during each short bias pulse. 
       FIG. 6  illustrates a flowchart showing an embodiment of the delayed implant process. As seen in Box  600 , the controller  180  begins operation by initiating CW mode. In this mode, the plasma  150  is created within the process chamber  120 . In Box  610 , the controller  180  monitors chamber conditions, waiting for them to stabilize. Once this has occurred, the controller  180  initiates pulsed plasma mode, as shown in Box  620 . As described in conjunction with  FIGS. 4 and 5 , this can be accomplished in a number of ways. In one embodiment, the RF power supply  135  transitions to pulsed plasma mode without any activity by the bias power supply  175 . In another embodiment, small magnitude bias pulses are initiated, which cause the transition to pulsed plasma mode. Even though small magnitude bias voltage pulses may be occurring in Box  620 , for purposes of this disclosure, this is not considered to be an implant process. In other words, the number of ions that may be implanted during Box  620  is sufficiently small to not be considered an implant process. In another embodiment, short duration bias pulses are used to initiate pulsed plasma mode. Again, these pulses are sufficiently short so that this is not considered an implant process. The transition to pulsed plasma mode in Box  620  causes chamber conditions to respond. The controller  180  then waits until the chamber conditions are again stable, as shown in Box  630 . Once the process chamber  120  is determined to be stable, the controller initiates the implant process, in Box  640 . This is accomplished by generating bias voltage pulses of sufficient time duration and magnitude to allow ions to implant the workpiece  160 . 
     Delayed implant has been shown to reduce DPP and extend the process time to ensure wafer-to-wafer repeatability. By reducing the concentration of ions in the plasma  150  through the use of pulsed plasma mode, the time required to implant a workpiece to dopant concentrations of 1E13-1E14 is increased to, for example, 8-9 seconds. 
     Additional increases in time can be achieved using the apparatus of  FIG. 7 . In this embodiment, the plasma chamber  700  has a top cover that is comprised of an upper horizontal surface  710 , vertical walls  730  and second horizontal walls  720 . Other components associated with a plasma chamber, such as those shown in  FIG. 1 , may also be part of the plasma chamber  700 . Coils  133  may be disposed on vertical walls  730  and second horizontal walls  720 . When the coils  133  disposed on second horizontal walls  720  are energized, plasma  725  is created in the position indicated in  FIG. 7 . Similarly, when the coils  133  disposed on vertical walls  730  are energized, plasma  735  is created in the position indicated in  FIG. 7 . It has been observed that, in some embodiments, such as those which include BF 3  as a dopant gas, the DPP of a workpiece that is implanted using plasma  735  may be about 30% less than the DPP of a workpiece that is implanted using plasma  725 . In other words, utilization of only the coils  133  disposed on the vertical walls  730  may result in a longer implant time. The use of these coils  133 , in combination with delayed implant, as described above, may sufficiently increase implant time for low dose applications, allowing improved DPP and wafer-to-wafer repeatability. 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.