Abstract:
A method of generating a film during a chemical vapor deposition process is disclosed. One embodiment includes generating a first electrical pulse having a first pulse amplitude; using the first electrical pulse to generate a first density of radicalized species; disassociating a feedstock gas using the radicalized species in the first density of radicalized species, thereby creating a first deposition material; depositing the first deposition material on a substrate; generating a second electrical pulse having a second pulse amplitude, wherein the second pulse amplitude is different from the first pulse width; using the second electrical pulse to generate a second density of radicalized species; disassociating a feedstock gas using the radicalized species in the second density of radicalized species, thereby creating a second deposition material; and depositing the second plurality of deposition materials on the first deposition material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to power supplies, systems, and methods for chemical vapor deposition. 
     BACKGROUND OF THE INVENTION 
     Chemical vapor deposition (CVD) is a process whereby a film is deposited on a substrate by reacting chemicals together in the gaseous or vapor phase to form a film. The gases or vapors utilized for CVD are gases or compounds that contain the element to be deposited and that may be induced to react with a substrate or other gas(es) to deposit a film. The CVD reaction may be thermally activated, plasma induced, plasma enhanced or activated by light in photon induced systems. 
     CVD is used extensively in the semiconductor industry to build up wafers. CVD can also be used for coating larger substrates such as glass and polycarbonate sheets. Plasma enhanced CVD (PECVD), for example, is one of the more promising technologies for creating large photovoltaic sheets, LCD screens, and polycarbonate windows for automobiles. 
       FIG. 1  illustrates a cut away of a typical PECVD system  100  for large-scale deposition processes—currently up to 2.5 meters wide. This system includes a vacuum chamber  105  of which only two walls are illustrated. The vacuum chamber  105  houses a linear discharge tube  110 . The linear discharge tube  110  is formed of an inner conductor  115  that is configured to carry a microwave signal, or other signals, into the vacuum chamber  105 . This microwave power radiates outward from the linear discharge antenna  115  and ignites the surrounding support gas  120  that is introduced through the support gas tube  120 . This ignited gas is a plasma and is generally adjacent to the linear discharge tube  110 . Radicals generated by the plasma and electromagnetic radiation disassociate the feedstock gas(es)  130  introduced through the feedstock gas tube  125  thereby breaking up the feedstock gas to form new molecules. Certain molecules formed during the dissociation process are deposited on the substrate  135 . The other molecules formed by the dissociation process are waste and are removed through an exhaust port (not shown)—although these molecules tend to occasionally deposit themselves on the substrate. This dissociation process is extremely sensitive to the amount of power used to generate the plasma. 
     To coat large substrate surface areas rapidly, a substrate carrier (not shown) moves the substrate  135  through the vacuum chamber  105  at a steady rate, although the substrate  135  could be statically coated in some embodiments. As the substrate  135  moves through the vacuum chamber  105 , the dissociation should continue at a steady rate and target molecules from the disassociated feed gas theoretically deposit on the substrate, thereby forming a uniform film on the substrate. But due to a variety of real-world factors, the films formed by this process are not always uniform. 
     Nonconductive and conductive films deposited utilizing plasma enhanced chemical vapor sources have been achieved with many types of power sources and system configurations. Most of these sources utilize microwaves, radio frequency (RF), high frequency (HF), or very high frequency (VHF) energy to generate the excited plasma species. 
     Those of skill in the art know that for a given process condition and system configuration of PECVD, it is the average power introduced into the plasma discharge that is a major contributing factor to the density of radicalized plasma species produced. These radicalized plasma species are responsible for disassociating the feedstock gas. For typical PECVD processes, the necessary density of produced radicalized species from the plasma must be greater than that required to fully convert all organic materials. Factors such as consumption in the film deposition processes, plasma decomposition processes of the precursor materials, recombination losses, and pumping losses should be taken into consideration. 
     Depending upon the power type, configuration and materials utilized, the required power level for producing the necessary density of radicalized plasma species can unduly heat the substrate beyond its physical limits, and possibly render the films and substrate unusable. This primarily occurs in polymer material substrates due to the low melting point of the material. 
     To reduce the amount of heat loading of the substrate, a method of high power pulsing into the plasma, with off times in between the pulsing, can be used. This method allows the plasma during the short high energy pulses to reach saturation of the radicalized species required for the film deposition process and loss to occur, while reducing the instantaneous and continuous heating of the substrate through the reduction of other forms of electromagnetic radiation. 
     Film property requirements are achieved by setting the process conditions for deposition, including the power levels, pulsing frequency and duty cycle of the source. To achieve required film properties the structure and structural content of the deposited film must be controlled. The film properties can be controlled by varying the radical species content, (among other important process parameters), and as stated in the above, the radical density is controlled primarily by the average and peak power levels into the plasma discharge. 
     To achieve several important film properties, and promote adhesion to some types of substrates, the films organic content must be finely controlled, or possibly the contents must be in the form of a gradient across the entire film thickness. Current technology, which enables control of only certain process parameters, cannot achieve this fine control. For example, current technology consists of changing the deposition conditions, usually manually or by multiple sources and chambers with differing process conditions, creating steps in the film stack up to achieve a gradient type stack. Primarily the precursor vapor content, system pressure, and or power level at one or more times is used to develop a stack of layers. These methods are crude at best and do not enable fine control. Accordingly, a new system and method are needed to address this and other problems with the existing technology. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention that are shown in the drawings are summarized below. These and other embodiments are more fully described in the Detailed Description section. It is to be understood, however, that there is no intention to limit the invention to the forms described in this Summary of the Invention or in the Detailed Description. One skilled in the art can recognize that there are numerous modifications, equivalents and alternative constructions that fall within the spirit and scope of the invention as expressed in the claims. 
     One embodiment includes generating a first electrical pulse having a first pulse amplitude; using the first electrical pulse to generate a first density of radicalized species; disassociating a feedstock gas using the radicalized species in the first density of radicalized species, thereby creating a first deposition material; depositing the first deposition material on a substrate; generating a second electrical pulse having a second pulse amplitude, wherein the second pulse amplitude is different from the first pulse amplitude; using the second electrical pulse to generate a second density of radicalized species; disassociating a feedstock gas using the radicalized species in the second density of radicalized species, thereby creating a second deposition material; and depositing the second plurality of deposition materials on the first deposition material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various objects and advantages and a more complete understanding of the present invention are apparent and more readily appreciated by reference to the following Detailed Description and to the appended claims when taken in conjunction with the accompanying Drawing wherein: 
         FIG. 1  illustrates one type of PECVD system; 
         FIG. 2   a  illustrates a power supply for a PECVD system in accordance with one embodiment of the present invention; 
         FIG. 2   b  is an alternative depiction of a power supply for a PECVD system in accordance with one embodiment of the present invention; 
         FIG. 3  illustrates one example of a pulse-width modulated power signal; 
         FIG. 4  illustrates one example of a pulse-amplitude modulated power signal; 
         FIG. 5  illustrates one example of a frequency-modulated power signal; 
         FIG. 6   a  illustrates one example of a gradient film formed using pulse-width modulation; 
         FIG. 6   b  illustrates one example of a multi-layer gradient film formed using pulse-width modulation; 
         FIG. 7   a  illustrates one example of a gradient film formed using amplitude-width modulation; 
         FIG. 7   b  illustrates one example of a multi-layer gradient film formed using amplitude-width modulation; and 
         FIGS. 8   a - 8   d  illustrate the development of a multi-layer gradient film over time using a pulse-width modulated power signal. 
     
    
    
     DETAILED DESCRIPTION 
     In some PECVD processes the typical radical lifetime (time for the loss of and consumption of the radical species) is long enough so that there can be an off time of the plasma during which the radical density remaining is gradually consumed by the deposition of the film and loss mechanisms. Therefore, by controlling the total radical density during these on and off times of the plasma the chemical makeup of the film can be altered, as well as the over all layer properties of the film. 
     By modulating the power level into the plasma, the on time of the plasma and the timing between the power pulses, the user can make films that were not achievable before in PECVD. The layers could be a single gradient layer or a multiple stack of hundreds to thousands of micro layers with varying properties between each layer. Both processes can create a unique film. 
       FIG. 2   a  illustrates a system constructed in accordance with one embodiment of the present invention. This system includes a DC source  140  that is controllable by the pulse control  145 . The terms “DC source” and “DC power supply” refer to any type of power supply, including those that use a linear amplifier, a non-linear amplifier, or no amplifier. The DC source  145  powers the magnetron  150 , which generates the microwaves, or other energy waves, that drive the inner conductor within the linear discharge tube (not shown). The pulse control  145  can contour the shape of the DC pulses and adjust the set points for pulse properties such as duty cycle, frequency, and amplitude. The process of contouring the shape of the DC pulses is described in the commonly owned, entitled “SYSTEM AND METHOD FOR POWER FUNCTION RAMPING OF MICROWAVE LINEAR DISCHARGE SOURCES,” which is incorporated herein by reference. 
     The pulse control  145  is also configured to modulate the DC pulses, or other energy signal, driving the magnetron  150  during the operation of the PECVD device. In some embodiments, the pulse control  145  can be configured to only modulate the signal driving the magnetron  150 . In either embodiment, however, by modulating the DC pulses, the power level into the plasma can also be modulated, thereby enabling the user to control radical density and make films that were not achievable before in PECVD. This system can be used to form variable, single gradient layers or a multiple stack of hundreds to thousands of micro layers with varying properties between each other. 
       FIG. 2   b  illustrates an alternate embodiment of a power supply. This embodiment includes an arbitrary waveform generator  141 , an amplifier  142 , a pulse control  145 , a magnetron  150 , and a plasma source antenna  152 . In operation, the arbitrary waveform generator  141  generates a waveform and corresponding voltage that can be in virtually any form. Next, the amplifier  142  amplifies the voltage from the arbitrary waveform generator to a usable amount. In the case of a microwave generator (e.g., the magnetron  150 ) the signal could be amplified from +/ — 5 VDC to 5,000 VDC. Next, the high voltage signature is applied to the magnetron  150 , which is a high frequency generator. The magnetron  150  generates a power output carrier (at 2.45 GHZ in this case) that has its amplitude and or frequency varied based upon the originally generated voltage signature. Finally, the output from the magnetron is applied to the source  152  to generate a power modulated plasma. 
     Signal modulation can be applied by the pulse control  145  to the arbitrary waveform generator  141 . Signal modulation is a well-known process in many fields—the most well known being FM (frequency modulated) and AM (amplitude modulated) radio. But modulation has not been used before to control film properties and create layers during PECVD. Many forms of modulation exist that could be applied to a waveform power level, duty cycle or frequency, but only a few are described below. Those of skill in the art will recognize other methods. Note that modulation is different from simply increasing or decreasing the power or duty cycle of a power signal into a source. 
       FIG. 3  illustrates pulse-width modulation, which varies the width of pulse widths over time. With pulse-width modulation, the value of a sample of data is represented by the length of a pulse. 
       FIG. 4  illustrates pulse-amplitude modulation, which is a form of signal modulation in which the message information is encoded in the amplitude of a series of signal pulses. In the case of plasma sources the voltage, current or power level can be amplitude modulated by whatever percentage desired. 
       FIG. 5  illustrates frequency modulation (FM), which is the encoding of information in either analog or digital form into a carrier wave by variation of its instantaneous frequency in accordance with an input signal. 
     Referring now to  FIGS. 6 and 7 , they show two examples of multi-layer films that could be produced with two differing forms of modulation, pulse-width and pulse-amplitude modulation. Both of these figures illustrate the film layers deposited on the substrate and the corresponding modulated power signal that is used to generate the plasma. Notice that the power signal is modulated during the deposition process, which differs from establishing and leaving initial set points that are static during the deposition process. 
     Referring first to  FIG. 6   a,  it illustrates a variable film  157  produced by pulse-width modulation. In this embodiment, the cycle between short pulse widths and long pulse widths is relatively long. This long cycle produces a variable-gradient coating on the substrate that varies through its thickness from a flexible, organo-silicon film located next to the substrate to a rigid, dense SiO2 or SiOxNy film. The film produced by this process becomes harder and more rigid as it extends out from the substrate. 
     A benefit is realized with this single, variable gradient layer because the flexible, softer portion of the film bonds better to the substrate than would the dense, rigid portion. Thus, the pulse width modulation allows a film to be created that bonds well with the substrate but also has a hardened outer portion that resists scratches and that has good barrier properties. This type of film could not be efficiently created without a modulated power signal. 
     By changing the modulation of the power signal, a multilayer gradient coating can be deposited on the substrate.  FIG. 6   b  illustrates this type of substrate and film  160 . In this embodiment, the cycle between short pulse widths and long pulse widths is relatively short, thereby creating multiple layers. These individual layers can also vary from less dense to more dense within a single layer—much as the single gradient layer in  FIG. 6   a  does. 
     In this embodiment, a less-dense, organo-silicon layer is initially deposited on the substrate. This type of layer bonds best with the substrate. The next layer is slightly more dense, and the third layer is an almost pure SiO2 or SiOxNy layer, which is extremely dense and hard. As the pulse width modulates to shorter pulse widths, the next layer is again a less-dense, organo-silicon layer that bonds easily to the dense layer just below. This cycle can repeat hundreds or even thousands of times to create a multilayer, gradient film that is extremely hard, resilient, and with good barrier properties. Further, this film can be produced with a minimal amount of heat and damage to the substrate. 
       FIGS. 7   a  and  7   b  illustrate another series of films similar to those shown in  FIGS. 6   a  and  6   b . These films, however, are created using pulse-amplitude modulation. Again, both a single gradient film  165  or a multilayer gradient film  170  can be created using modulation techniques. Note that this process works for almost any precursor and is not limited to silicon-based compounds. 
     Variable films can be created with other modulation techniques. In fact, there are many modulation technologies that could be implemented to effectively control the radical species density and electromagnetic radiation in relation to time, including, PWM—Pulse Width Modulation, PAM—Pulse Amplitude Modulation, PPM—Pulse Position Modulation, AM—Amplitude modulation, FM—Frequency Modulation, etc. Again, these techniques involve modulating a power signal during film deposition rather than setting an initial power point or duty cycle. 
     Referring now to  FIGS. 8   a  through  8   d,  they show an example of pulse-width modulation and its possible affects on the films properties for SiO2 and or SiOxNy. A sine wave signal is used to drive the pulsing frequency at a fixed peak power level to increase or decrease the short term average power into the plasma. The sine wave shown is the drive signal, and it also indicates power. 
     At the beginning portion (left side) of the  FIG. 8   a , the modulation increases the power level per given time interval by increasing the on-time and decreasing the off-time of the plasma, thus increasing the instantaneous radical density and electromagnetic components of the plasma. This process increases the radical density to the point at which all material was converted and deposited and a new material is the dominate contributor to the growing film stack, SiO2 or SiOxNy.  FIG. 8   b  shows the dense layer formed next to the substrate during this phase. 
     In the center of the drive signal, the on-time is at its lowest and off-time at its highest value. This effect decreases the instantaneous radical density to the point at which all material was consumed and the precursor material again becomes the dominate contributor to the growing film stack.  FIG. 8   c  shows the less-dense, more-organic layer formed during the second phase. This layer is deposited on the first layer. 
     Finally in the last portion of the waveform, the process returns to saturation of the radical density like in the first portion of the waveform. This phase deposits a hardened, dense layer.  FIG. 8   d  shows the dense, third layer deposited on the second layer. Accordingly, the three phases together leaving an inter layer of organic material between two hard, dense layers—thereby introducing flexibility into the entire film stack. 
     These modulation techniques can be used during inline or dynamic deposition processes. By utilizing these modulation techniques with the dynamic deposition process, it is possible to produce alignment layers for applications such as LCD displays, thereby replacing the polymide layers presently being used. 
     In summary, this discovery allows the user to achieve PECVD films not possible in the past, possibly with extended film properties and qualities not possible to date. The higher quality thin films are achieved from the ability to actively control the plasmas radical/electromagnetic radiation densities in continuous way per unit time by contouring the average and or peak power level per time interval. The drive waveform can be any waveform or even an arbitrary function. This technique can also be used to control the localized etching rate when the source and system is configured to do so. 
     In conclusion, the present invention provides, among other things, a system and method for controlling deposition onto substrates. Those skilled in the art can readily recognize that numerous variations and substitutions may be made in the invention, its use and its configuration to achieve substantially the same results as achieved by the embodiments described herein. Accordingly, there is no intention to limit the invention to the disclosed exemplary forms. Many variations, modifications and alternative constructions fall within the scope and spirit of the disclosed invention as expressed in the claims.