Patent Publication Number: US-2010123502-A1

Title: System for providing a substantially uniform potential profile

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     This application claims priority to provisional patent application Ser. No. 61/134,385, filed Jul. 9, 2008, and provisional patent application Ser. No. 61/209,788, filed Mar. 11, 2009, both of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION  
     The present invention relates to system for providing a substantially uniform potential profile. In particular, the present invention relates to a system for providing a substantially uniform potential profile that can be used with plasma. 
     BACKGROUND OF THE INVENTION 
     Semiconductor materials are utilized in many different applications. Thus, there is a continued need to fabricate semiconductor material quickly and at reduced cost. The fabrication of semiconductor materials often includes a deposition step and an etching step. Deposition encompasses any process that grows, coats, or otherwise transfers material onto another substance, such as a wafer, and etching includes any process that removes a portion of the transferred material from the other substance. Deposition can be accomplished by use of plasma in chemical vapor deposition, and etching can be completed by plasma asking. Thus, plasma can be used in the processes of deposition and etching. 
     Plasma is any gas in which a significant percentage of the atoms or molecules are ionized. In deposition, plasma can be used in chemical vapor deposition (CVD) which is a chemical process wherein a substrate or a wafer is exposed to one or more volatile precursors, which react or decompose on a surface of the substrate to produce the desired deposit. CVD processes involving plasma include microwave-assisted plasma CVD, plasma-enhanced CVD, and remote plasma-enhanced CVD. In CVD processes involving plasma, a thin film is deposited on a surface as a portion of the plasma changes phase to a solid on the surface. 
     The plasma is generally created by a radiofrequency (RF) signal or a direct current discharge between two electrodes. When plasma is created by an RF signal, the RF signal is typically around 13 MHz, however a very high frequency (VHF) RF signal provides a faster deposition process and thus faster manufacturing of semiconductor materials. Unfortunately, a VHF RF signal creates standing waves when the signal is applied to the relatively large electrodes required for photovoltaic cells and large flat panel displays. Standing waves produce non-uniform deposition rates and poor crystalline qualities for plasma-enhanced CVD for depositing amorphous and micro-crystalline silicon. 
     Thus, there is a need in the art for a system that uses VHF RF signals to manufacture semiconductor material that minimizes the effects of standing waves. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a system for providing a substantially uniform potential profile. 
     An exemplary embodiment of the invention provides a system for providing at least two output signals to produce a substantially uniform potential profile. The system includes a signal generator adapted to emit a frequency at least about 30 megahertz, a splitter in communication with the signal generator, and a signal manipulator in communication with the splitter. The splitter is adapted to split the signal of the signal generator into the two output signals, and the signal manipulator is adapted to manipulate a phase, a gain, or an impedance of the two output signals. The signal manipulator manipulates the two output signals so that the two output signals produce the substantially uniform potential profile. 
     Another exemplary embodiment of the invention provides a system for providing at least two output signals to produce a substantially uniform potential profile. The system includes a phase adjuster, signal generators in communication with the phase adjuster, and an impedance matcher to substantially match an input impedance of a load in communication with the system. The signal generators are adapted to emit a signal with a frequency at least about 30 megahertz with a phase controlled by the phase adjuster. The phase adjuster manipulates the two output signals so that the at least two output signals produce the substantially uniform potential profile. 
     Yet another exemplary embodiment of the invention provides a system for providing at least two signals to produce a substantially uniform potential profile. The system includes a first signal generator adapted to emit a first signal with a first phase shift, a second signal generator adapted to emit a second signal with a second phase shift, and a controller in communication with the first signal generator and the second signal generator. The second signal generator is in communication with the first signal generator. The controller is adapted to incrementally change the first phase shift and the second phase shift at a predetermined time increment. At least one of the first phase shift and the second phase shift is adjusted to produce the substantially uniform potential profile. 
     Other objects, advantages and salient features of the invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a normalized potential profile at about 13.56 MHz for a center-fed 100 cm×100 cm source; 
         FIG. 2  is a normalized potential profile at about 81.36 MHz for a center-fed 100 cm×100 cm source; 
         FIG. 3  is a schematic of a system for providing a substantially uniform potential profile according to an embodiment of the invention; 
         FIG. 4  is a normalized potential profile at about 81.36 MHz for a multi-fed 100 cm×100 cm source; 
         FIG. 5  is a schematic of a system for providing a substantially uniform potential profile according to another embodiment of the invention; 
         FIG. 6  is a schematic of a system for providing a substantially uniform potential profile according to yet another embodiment of the invention; 
         FIG. 7  is a schematic of signal generators according to a fourth embodiment of the invention; 
         FIG. 8  is a schematic of the output of two signal generators according to a fifth embodiment of the invention; 
         FIG. 9  is a chart of the phase relationship of signals of the signal generators shown in  FIG. 8 ; 
         FIG. 10  is a schematic of the output of four signal generators according to a sixth embodiment of the invention; 
         FIG. 11  is a chart of the phase relationship of signals of the signal generators shown in  FIG. 10 ; 
         FIG. 12  is a schematic of the output of five signal generators according to a seventh embodiment of the invention; 
         FIG. 13  is a chart of the phase relationship of signals of the signal generators shown in  FIG. 12 ; and 
         FIG. 14  is a schematic of a system with a controller according to an eight embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-14 , the invention relates to providing a substantially uniform potential profile. Such a substantially uniform potential profile can be applied to a plasma that, for example, is used in the manufacture of semiconductor material. The invention allows the use of very high frequency (VHF) plasma. VHF plasma can be used for semiconductor etching and deposition processes. Using VHF plasma provides deposition rates for materials, such as, amorphous silicon (a:Si), nanocrystalline silicon (nc-Si), and microcrystalline silicon (uc-Si) that are approximately seven to eight times greater than deposition rates using plasma at about 13.56 Mhz. However, VHF plasma develops a non-uniformity over a large surface area, and the non-uniformity of VHF plasma limits its use in semiconductor material fabrication. The invention can minimize the non-uniformity of VHF plasma. 
     Referring to  FIG. 1 , a normalized potential profile at about 13.56 MHz for a center-fed 100 cm×100 cm source is shown. Along the z-axis of the figure, the potential value of the profile has been normalized to zero, and the area that the potential profile covers is plotted along the x and y axes. As shown in the figure, the potential profile at about 13.56 MHz is relatively uniform. That is, the potential profile has a potential value very close to zero (as indicated on the z-axis) over the entire area defined by the x and y axes. However, as the frequency increases, the potential profile becomes less uniform. 
     Referring to  FIG. 2 , a normalized potential profile at about 81.36 MHz for a center-fed 100 cm×100 cm source is shown. Similar to  FIG. 1 , the potential value normalized to zero is plotted along the z-axis, and the area that the potential profile covers is plotted along the x and y axes. Comparing  FIG. 2  and  FIG. 1 , as the frequency enters the very high frequency range, approximately 30 Mhz to approximately 300 MHz, the potential profile becomes increasingly non-uniform. As shown in the figure, the potential profile has a shape with a prominent protuberance in its approximate center. Following the potential profile from the edge of the area to the approximate center of the area covered by the profile (located at approximately 50 on the x axis and at approximately 50 on the y axis), the normalized potential value rises from approximately zero to approximately 1.0 to 1.5. Thus, to utilize VHF plasma, the non-uniformity of the applied potential must be minimized. 
     Referring to  FIG. 3 , a schematic of a system  100  according to one embodiment of the invention is shown. The system  100  includes at least a signal generator  102 , a splitter  104  in communication with the signal generator  102 , and a signal manipulator  106  in communication with the splitter  104 . 
     The signal generator  102  provides a repeating or non-repeating signal for the system  100 . The signal generator  102  can be an electronic signal generator in either the digital or analog domain, a function generator, an arbitrary waveform generator, a tone signal generator, an audio signal generator, a video signal generator, a radiofrequency signal generator, a combination of the aforementioned, or some other component that provides a signal. In the embodiment shown in  FIG. 3 , the signal generator  102  is a VHF signal generator that provides a VHF radiofrequency (RF) signal for use in the system  100 . Although only a single signal generator  102  is shown, there can be more than one signal generator  102 . The single signal generator  102  shown is exemplary only and not meant to be limiting. The optimal number of signal generators  102  may be more than the one shown. The exact number of signal generators  102  depends on, for example, the configuration of the system  100  or the requirements of any components in communication with the system  100 . 
     The splitter  104  is in communication with the signal generator  102  and transforms the output of the signal generator  102  into two or more signals based on the signal from the signal generator  102 . The two or more signals then become outputs from the splitter  104 . The splitter  104  can be an analog or digital filter, a hybrid coil, a bridge transformer, a combination of the aforementioned, or some other component that can transform an input signal into two or more output signals. Although only a single splitter  104  is shown, the single splitter  104  shown is exemplary only and not meant to be limiting. The optimal number of splitter  104  may be more than the single one shown. The exact number of signal generators  102  depends on, for example, the configuration of the system  100 , the number of signal generators  102 , or the requirements of any components in communication with the system  100 . 
     The signal manipulator  106  is in communication with the splitter  104  and manipulates the signal from the splitter  104 . The signal manipulator  106  can be a phase adjuster, a gain adjuster, an impedance matcher, a frequency manipulator, a combination of the aforementioned, or some other component that can manipulate a signal. The signal manipulator  106  can also include a signal transformer that can transform a signal of one kind into a signal of another kind. For example, the signal transformer can transform an audio signal, video signal, an optic signal, or some other signal into a radiofrequency signal that can be used by the system  100 . Furthermore, the signal manipulator  106  can include components to transmit the signal from the splitter  104  to a component in communication with the system  100 . The signal manipulator  106  can include one or more wires, a wireless transmitter, a wireless receiver, one or more coaxial cables, a microstrip, combinations of the aforementioned, or some other component or components able to transmit or communicate a signal. Similar to the other components of the system  100 , the number of signal manipulators  106  shown is exemplary only and not meant to be limiting. The optimal number of signal manipulators  106  may be more or less than the twelve signal manipulators  106  shown. The exact number of signal manipulators  106  depends on, for example, the configuration of the system  100 , the number of splitters  104 , or the requirements of any components in communication with the system  100 . 
     In the embodiment shown in  FIG. 3 , the system  100  is in communication with a plasma source with multiple RF inputs. Also, the depicted system  100  includes a single VHF RF generator as the signal generator  102 . The VHF RF generator provides RF power to the system  100  shown, and the splitter  104  transforms the RF power into multiple RF outputs at the output of the splitter  104 , In the system  100  shown, the splitter  104  provides twelve RF outputs, and the RF outputs are transmitted to the signal manipulator  106 . The signal manipulator  106  of the depicted embodiment includes a combined phase and gain adjuster  108 , an impedance matcher  110 , and other components, such as coaxial cables or microstrips, to communicate the signal from the splitter  104  to the plasma source with multiple RF inputs. The phase and gain adjuster  108  has phase/gain adjustment circuitry, and the impedance matcher  110  includes impedance matching circuitry to transform the input impedance of the plasma chamber for maximum power transfer. 
     Referring to  FIG. 4 , a normalized potential profile at about 81.36 MHz for a multi-fed 100 cm×100 cm source is shown. The system  100  shown in  FIG. 3  adjusts the phase and amplitude of the RF signals from the splitter  102  such that the potential profile on the plasma source is substantially uniform as shown in  FIG. 4 . The normalized potential profile shown is for a multi-feed plasma source with phase and amplitude adjustment, such as the one shown in  FIG. 3 . 
     Referring to  FIG. 5 , a schematic of a system  200  according to another embodiment of the invention is shown. Similar to system  100 , the system  200  includes at least a signal generator  202 , a splitter  204  in communication with the signal generator  202 , and a signal manipulator  206  in communication with the splitter  204 . However, the system  200  has more than one signal generator  202 , and a splitter  204  in communication with each signal generator  202 . 
     The several signal generators  202  can be operated at different output power levels. The signal generators  202  may have a single common input or output. Alternatively, one of the signal generators  202  may act as a source for a master signal that is transmitted to the other signal generators  202  so that the other signal generators  202  can operate at substantially the same signal. With the differences noted above, the signal generators  202  are otherwise substantially similar to the signal generator  102 . Thus, a further detailed description of the signal generators  202  is omitted. 
     The splitters  204  and the signal manipulators  206  are substantially similar to the splitter  104  and signal manipulator  106 , respectively, of system  100 . Thus, detailed descriptions of the splitters  204  and the signal manipulator  206  are omitted. 
     Referring to  FIG. 6 , a schematic of a system  300  according to yet another embodiment of the invention is shown. The system  300  includes at least a signal generator  302  that is in communication with a phase adjuster  308 , and an impedance matcher  310  that is in communication with the signal generator  302 . When compared to the system  100 , the phase adjuster  308  is upstream of the signal generator  302 . 
     The system  300  shown in  FIG. 6  is in communication with a plasma source with multiple RF inputs. Also, the depicted system  300  includes more than one VHF RF generator as the signal generator  302 . The several VHF RF generators provide RF power to the system  300  shown. Each of the VHF RF generator is able to operate at different power levels and at different phase relationships when compared to the other VHF RF generators in system  300 . Each output from the VHF RF generators is then transmitted to the plasma chamber through RF connections, such as coaxial cables and microstrips, and an impedance matcher  310  that includes impedance matching circuitry, to transform the input impedance of the plasma chamber for maximum power transfer. 
     Referring to  FIG. 7 , a schematic of signal generators  402 ,  404 ,  406 , and  408  is shown. In the embodiment shown, the signal generators  402  are VHF RF signal generators, but the invention is not limited to VHF RF signal generators. One of the signal generators  402  provides output phase information to the other signal generators  404 ,  406 , and  408 . Thus, signal generator  402  can be designated the master, and the other signal generators  404 ,  406 , and  408  can be designated slave. The phase information can be machine code, low level RF (also known as common exciter oscillator or CEX), a combination of the two, or some other component or signal that transmits phase information between signal generators  402 ,  404 ,  406 , and  408 . The master signal generator  402  can coordinate the phase control of the other signal generators  404 ,  406 , and  408  so that their respective outputs have substantially the same or different phases. Although four signal generators  402 ,  404 ,  406 , and  408  are shown, the number of signal generators  402 ,  404 ,  406 , and  408  shown is exemplary only and not meant to be limiting. The optimal number of signal generators  402 ,  404 ,  406 , and  408  may be more or less than the four shown. The exact number of signal generators  402 ,  404 ,  406 , and  408  depends on, for example, the configuration of the system  100 , the number of outputs required, or the requirements of any components that receive the signal from the signal generators  402 ,  404 ,  406 , and  408 . Furthermore, a single signal generator with multiple outputs may be used instead of the signal generators  402 ,  404 ,  406 , and  408 . 
     In the embodiment shown, the master signal generator  402  is substantially similar to the slave signal generators  404 ,  406 , and  408 . However, the master signal generator  402  can adjust, for example, the phase shift from approximately 0° to approximately 360°, the incremental change in the phase from approximately 0.01° to approximately 360°, and the time period between incremental changes in the phase from approximately 1 microsecond to approximately 100 minutes. The slave signal generators  404 ,  406 , and  408  substantially follow the master signal generator  402 . Each of the slave signal generators  404 ,  406 , and  408  can have their own independent control loop and power measurement. 
     Referring to  FIG. 8 , a schematic of a system  500  with two signal generators that provide signals  502  and  504 . The figure shows an example of a two RF output system  500  with two signal generators, such as a master and a slave, that provide signals  502  and  504 , respectively. The signals  502  and  504  can be transmitted to, for example, electrodes used for plasma-enhanced chemical vapor deposition (CVD). The system  500  can include two discrete signal generators or a single signal generator with two outputs. The signals  502  and  504  begin with different phases but are incremented by substantially the same amount at substantially the same time. In particular, one signal  502  starts at about 0°, and the other signal starts at about 360°. Then, after approximately 0.2 seconds of time has elapsed, each signal  502 ,  504  increments by about +2°. Thus, as shown in  FIG. 9 , at time 0, one signal  502  is at 0°, and the other signal  504  is at 180°. At time 0.2 seconds, one signal  502  is at 2°, and the other signal  504  is at 182°. At time 0.4 seconds, signal  502  increases to 4°, and signal  504  increases to 184°. Thus, each time that 0.2 seconds elapses, each signal  502 ,  504  increases by 2°. Therefore, at time 1.0 second, signal  502  has increased to 10°, and signal  504  has increased to 190°. 
     Referring to  FIG. 10 , a schematic of the output from four signal generators is shown. The figure shows an example of a four RF output system  600  with four signals  602 ,  604 ,  606 , and  608 . The signals  602 ,  604 ,  606 , and  608  can be from four discrete signal generators or from a single signal generator with four outputs. The signals  602 ,  604 ,  606 , and  608  can be transmitted to, for example, electrodes used for plasma-enhanced CVD. The signals  602 ,  604 ,  606 , and  608  begin with different phases but are incremented by generally the same amount at generally the same time. Specifically, in the embodiment shown, the signals  602 ,  604 ,  606 , and  608  are placed to substantially form a ring, with signal  602  at the top of the ring in the figure. The other depicted signals are arranged clockwise from signal  602 . The signal  602  at the top of the ring and the signal  606  at the bottom of the ring begin at 0°, while the signals  604  and  608  at the sides of the ring begin at 180°. The terms “top,” “bottom,” and “sides” are not meant to be limiting, but rather to describe the positional relationship between the signals  602 ,  604 ,  606 , and  608  with respect to each other. Then, after 0.5 second of time has elapsed, each signal  602 ,  604 ,  606 , and  608  increments by +2°. Thus, as shown in  FIG. 11 , at time 0, signal  602  is at 0°, signal  604  is at 180°, signal  606  is at 0°, and signal  608  is at 180°. At time 0.5 second, signal  602  is at 2°, signal  604  is at 182°, signal  606  is at 2°, and signal  608  is at 182°. At time 1.0 second, signal  602  is at 4°, signal  604  is at 184°, signal  606  is at 4°, and signal  608  is at 184°. Thus, each time that 0.5 second elapses, each signal  602 ,  604 ,  606 , and  608  increases by 2°. Therefore, at time 2.5 seconds, signal  602  is at 10°, signal  604  is at 190°, signal  606  is at 10°, and signal  608  is at 190°. 
     Referring to  FIG. 12 , a schematic of the output from five signal generators according to another embodiment is shown. Unlike system  600  shown in  FIG. 10 , the system  700  includes signal generators that are at different power levels. The signal generators provide signals  702 ,  704 ,  706 ,  708 , and  710 . The signals  702 ,  704 ,  706 ,  708 , and  710  can be from five discrete signal generators or from a single signal generator with five outputs. The signals  702 ,  704 ,  706 ,  708 , and  710  can then be transmitted to, for example, electrodes used for plasma-enhanced CVD. As shown in the figure, the signals  702 ,  704 ,  706 ,  708 , and  710  are arranged in a ring with signal  710  at the center. Signal  702  is at the top of the ring, and the other signals  704 ,  706 , and  708  are arranged clockwise from signal  702 . The signals  702 ,  704 ,  706 ,  708 , and  710  begin with different phases but are incremented by substantially the same amount at substantially the same time. Also, in the depicted embodiment, signals  702 ,  704 ,  706 , and  708  are at 1 kW, while the center signal is at 4 kW. Furthermore, the system  700  can sense the phase of each of the signals  702 ,  704 ,  706 ,  708 , and  710  to provide real time feedback for better precision and repeatability. 
     As shown in  FIG. 13 , at time 0, signal  702  is at 180°, signal  704  is at 182°, signal  706  is at 184°, signal  708  is at 186°, and the center signal  710  is at 0°. After 0.2 seconds of time has elapsed, each signal increments by +2° except for the center signal  710 . Thus, at time 0.2 second, signal  702  is at 182°, signal  704  is at 184°, signal  706  is at  186 °, signal  708  is at 188°, while the center signal  710  remains at 0°. Then, at time 0.4 second, signal  702  is at 184°, signal  704  is at 186°, signal  706  is at 188°, signal  708  is at 190°, while the center signal  710  remains at 0°. The process continues so that at time 1.0 second, signal  702  is at 190°, signal  704  is at 192°, signal  706  is at 194°, signal  708  is at 196°, and the center signal  710  is at 0°. 
     Referring to  FIG. 14 , a schematic of a system  800  with a single controller  810  is shown. In the depicted embodiment, the controller  810  controls all the signal generators  802 ,  804 ,  806 , and  808  of the system  800 . Each signal generator  802 ,  804 ,  806 , and  808  is substantially slaved to the controller  810 . Thus, when the controller  810  is a phase controller as shown in the figure, the controller  810  can send phase information to each signal generator  802 ,  804 ,  806 , and  808  to control the phase relationship between the signal generators  802 ,  804 ,  806 , and  808 . Also, as shown in the figure, the system  800  can include one or more phase detectors  812  that can provide feedback information to the controller  810 . 
     While a particular embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.