Patent Description:
Ignition of plasma in an inductively coupled plasma (ICP) source or a transformer coupled plasma (TCP) source typically involves application of a high-voltage pulse within the plasma vessel to create an initial breakdown of the gas into ionized state that then generates the plasma. The time duration and magnitude of the high voltage pulse are typically fixed at some predetermined values designed to achieve plasma breakdown at the preferred operating conditions. Conventional ignition schemes are designed to deliver the highest practical pulse voltage to break down the gas and create a plasma. In some cases, this approach can apply unnecessary stress on the electronics and also applies unnecessarily high electric fields across the biased plasma blocks.

In conventional plasma sources, a higher ignition voltage is chosen to account for unfavorable gas conditions which might inhibit the ignition. These unfavorable conditions can include, for example, high gas pressure, low gas flow, or poisoning, i.e., the presence of contaminants in the gas, or a lack of sufficient electron density to create an initial plasma breakdown. The high ignition voltage results in a high electric field that in turn increases the probability of creating an avalanche breakdown of the gas where a few electrons in the high-field region gain sufficient energy to ionize the gas molecules, thereby releasing more electrons in the high-field region, leading to the avalanche breakdown of the gas. In such conventional systems, with this voltage adjustment implemented, even in favorable gas conditions, the high-voltage is applied, which can wear the electronics more quickly, and can increase the chances of arcing inside the plasma vessel. The high-voltage ignition pulse train can lead to arcing and degradation, which in turn generates particulate defects that could reduce the yield of high performance chips in advanced semiconductor processing applications.

<CIT> discloses methods, systems, and computer program products for measuring and controlling parameters of a plasma generator. A current in a primary winding of a transformer that generates a plasma is measured. A voltage across a secondary winding of the transformer is measured. Based on the current of the primary winding and the voltage across the secondary winding, a parameter of the plasma is determined. The parameters includes a resistance value associated with the plasma, a power value associated with the plasma, or both.

According to a first aspect of the invention, a method of determining the health of a plasma system by monitoring an ignition process for igniting a plasma within a plasma confining volume is provided, in accordance with claim <NUM>.

According to a second aspect of the invention, an apparatus for determining the health of a plasma system by monitoring an ignition process for igniting a plasma within a plasma confining volume is provided, in accordance with claim <NUM>.

Further embodiments of the invention are defined in the dependent claims.

The present disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the present disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings.

The present disclosure is directed to an approach to ignition of plasma in a plasma source. In exemplary embodiments as described herein, the plasma source is a toroidal plasma source. It will be understood that the present disclosure is also applicable to other plasma source configurations. The plasma source of the present disclosure is of the type described in <CIT> and <CIT>.

<FIG> includes a schematic functional block diagram of a toroidal plasma source <NUM> for producing activated gases, to which the present disclosure is directed. Source <NUM> includes a power transformer <NUM> which couples electromagnetic energy into a plasma <NUM>. The power transformer <NUM> includes a high-permeability magnetic core <NUM>, a primary coil <NUM>, and a plasma chamber <NUM>. The plasma chamber <NUM> allows the plasma <NUM> to form a secondary coil of the transformer <NUM>. The power transformer <NUM> can include additional magnetic cores and conductor coils (not shown) which form additional primary or secondary coils.

The plasma chamber <NUM> can be formed from a metallic material such as aluminum or a refractory metal, a coated metal such as anodized aluminum, or can be formed from a dielectric material such as quartz, sapphire, pressed alumina, or other dielectrics. One or more sides of the plasma chamber <NUM> can be exposed to a process chamber <NUM> to allow reactive particles (radicals) generated by the plasma <NUM> to be in direct contact with a material or surfaces to be processed (not shown). Alternatively, the plasma chamber <NUM> can be located at a distance from the process chamber <NUM>, allowing activated radicals to flow to the process chamber <NUM> while charged particles recombine during the gas transport. A sample holder <NUM> can be positioned in the process chamber <NUM> to support the material to be processed. The material to be processed can be biased relative to the potential of the plasma. Alternatively the reactive particles generated in the plasma source may be used to react with the materials comprising the process chamber to remove any undesired deposits in the process chamber. The reactive particles may also be utilized to generate other molecules through reactions with other gases introduced downstream of the plasma source (in the plasma chamber). In other instances, the reactive particles generated in the plasma source may be used to react with and remove undesired deposits in other components besides the processing chamber, such as gas delivery lines, pump forelines, valves, etc., that may accumulate unwanted deposits over time.

Plasma source <NUM> also comprises a RF power supply <NUM>. In some embodiments, the switching power supply <NUM> includes a voltage supply <NUM> directly coupled to a switching circuit <NUM> containing a switching semiconductor device <NUM>. The voltage supply <NUM> can be a line voltage supply or a bus voltage supply. The switching semiconductor device <NUM> can be a set of switching transistors. The switching circuit <NUM> can be a solid state switching circuit. An output <NUM> of the circuit <NUM> can be directly coupled to the primary winding <NUM> of the transformer <NUM>.

The toroidal plasma source <NUM> can include a means for accelerating the free charges that provides an initial breakdown event that ignites a plasma in the plasma chamber <NUM>. The initial ionization event can be a short, high-voltage pulse, which is applied to the plasma chamber <NUM>. The pulse can have a voltage of approximately <NUM>-<NUM>,<NUM> volts and can be approximately <NUM> microsecond to <NUM> milliseconds in duration. A continuous RF voltage of <NUM>-<NUM>,<NUM> volts can also be used to produce the initial ionization event, and the voltage is disconnected after gas breaks down. A gas with low ionization potential such as argon, Neon or Xenon may be introduced into the plasma chamber <NUM> to reduce the voltage required to ignite the plasma. Ultraviolet radiation may also be used to aid the creation of free charges in the plasma chamber <NUM> that provide the initial ionization event that ignited the plasma in the plasma chamber <NUM>.

In some embodiments, the high-voltage pulse is applied to an electrode <NUM> positioned in the plasma chamber <NUM>. It is noted that the electrode does not necessarily need to be inside the plasma chamber. Rather, the electrode need only couple the energy (high voltage) into the gas inside the plasma chamber. For example, the energy may be coupled into the gas through a window made of a material such as sapphire. In some embodiments, the short-duration, high-voltage pulse is applied to an electrode <NUM> that is capacitively coupled to the plasma chamber <NUM> through a dielectric. In other embodiments, the plasma chamber <NUM> is exposed to high-energy radiation emitted from a source such as an ultraviolet light source or laser <NUM> that is optically coupled to the plasma chamber <NUM>. The radiation causes the initial ionization event that ignites the plasma. In yet another embodiment, the high-voltage pulse may be applied to an electrically isolated portion of the plasma chamber where the electrically biased region is isolated from the adjacent grounded regions through a gap or dielectric insulation separating the two.

The toroidal low-field plasma source <NUM> can also include a circuit <NUM> for measuring electrical parameters of the primary winding <NUM>. Electrical parameters of the primary winding <NUM> include the current driving the primary winding <NUM>, the voltage across the primary winding <NUM>, the bus or line voltage supply generated by the voltage supply <NUM>, the impedance of the primary winding <NUM>, the average power in the primary winding <NUM>, and the peak power in the primary winding <NUM>.

In addition, the plasma source <NUM> can include a means for measuring relevant electrical parameters of the plasma <NUM>. Relevant electrical parameters of the plasma <NUM> include the plasma current, loop voltage, impedance, and/or power. For example, the source <NUM> can include a current probe <NUM> positioned around the plasma chamber <NUM> to measure the plasma current flowing in secondary of the transformer <NUM>. The plasma source <NUM> can also include an optical detector <NUM> for measuring the optical emission from the plasma <NUM>. In addition, the plasma source <NUM> can include a power control circuit <NUM> that accepts data from one or more of the current probe <NUM>, the power detector <NUM>, and the circuitry <NUM>, and then adjusts the power in the plasma by adjusting the current in the primary winding <NUM>.

In operation, a gas is flowed into the plasma chamber <NUM> until a pressure substantially between <NUM> millitorr and <NUM> torr is reached. The gas can comprise a noble gas, a reactive gas or a mixture of at least one noble gas and at least one reactive gas. The circuit <NUM> containing switching semiconductor devices supplies a current to the primary winding <NUM> that induces a potential inside the plasma chamber <NUM>. The magnitude of the induced potential depends on the magnetic field produced by the core according to Faraday's law of induction and the frequency at which the switching semiconductor devices operate. An ionization cascade event that forms the plasma can be initiated in the chamber. The ionization event can be the application of a voltage pulse to the electrode <NUM> in the chamber <NUM> or to the electrode <NUM> that is capacitively coupled to the plasma chamber <NUM>. Alternatively, the ionization event can be exposing the chamber <NUM> to high-energy radiation.

<FIG> includes a schematic functional block diagram of a system <NUM> including a plasma source <NUM> of the type illustrated in <FIG>, which includes a toroidal plasma tube or block <NUM>, also referred to herein as a plasma confining region <NUM>, in which a plasma is ignited, according to the present disclosure. A total potential drop around the entire block loop <NUM> is referred to herein as VLoop. VLoop is a function of the voltage across the primary coil. In a typical toroidal plasma source, the loop voltage may be allowed to vary with the gas operating pressure, as the impedance of the plasma will change in accordance with different operating conditions. The loop potential may also be changed as desired by changing the plasma current setpoint for a given gas, flow and pressure condition. Plasma confining region <NUM> includes one or more, e.g., four, dielectric breaks 134A, 134B, 134C, 134D, around the plasma loop. To ignite the plasma in the loop, the ignition voltage, e.g., pulse or series of pulses, referred to herein as VSpark, is applied across one or more of the dielectric breaks 134A, 134B, 134C, 134D. In a successful plasma ignition, sometime after application of the ignition voltage, breakdown occurs, and the plasma ignites and fills the entire loop <NUM>.

<FIG> also illustrates the ignition system used to generate the VSpark ignition voltage, according to exemplary embodiments. The ignition system includes an inverter circuit, i.e., switching power supply <NUM>, which generates the initial waveform used to generate VSpark. Alternatively, Vspark may be generated from a different circuit at a different frequency. The signal generated by inverter circuit <NUM> is applied to resonant circuit <NUM>, which includes inductors L1 and L2 and capacitor C1. Resonant circuit <NUM> is tuned to have a resonant frequency at the desired plasma excitation voltage VSpark switching frequency. As shown in <FIG>, capacitor C1 is in parallel with the plasma load. The signal of circuit <NUM> is applied to the primary winding <NUM> of a transformer <NUM>. It should be noted that this particular resonant circuit is described as an illustrative exemplary circuit. Other types and configurations of tunable resonant circuits can be used.

A first secondary winding <NUM> of transformer <NUM> is applied to a second circuit <NUM>, which delivers the plasma ignition voltage, VSpark (typically higher than the primary loop voltage). To initiate ignition of plasma, the output voltage from circuit <NUM> is applied to one or more of the biased blocks <NUM>, <NUM>, while the adjacent blocks <NUM>, <NUM> are grounded. Blocks <NUM>, <NUM>, <NUM>, <NUM> are electrically conductive blocks, made of a, for example, aluminum, or other such conductive material. Dielectric breaks 134A, 134B, 134C, 134D can be formed as shown between blocks, for example, by coating conductive blocks <NUM>, <NUM>, <NUM>, <NUM> with a dielectric material, such as, for example, aluminum oxide, or other similar insulating dielectric material. In the particular exemplary illustration of <FIG>, the output signal from circuit <NUM>, i.e., VSpark, is applied to biased blocks <NUM> and <NUM>. This creates an electric field across dielectric breaks 134A and 134C due to the voltage difference between biased block <NUM> and grounded blocks <NUM> and <NUM>, respectively. Similarly, an electric field is created across dielectric breaks 134B and 134D due to the voltage difference between biased block <NUM> and grounded blocks <NUM> and <NUM>, respectively. These electric fields initiate ignition of the plasma in the plasma confining volume within the interior of blocks <NUM>, <NUM>, <NUM>, <NUM>. The plasma loop acts as the second secondary winding <NUM> of transformer <NUM> after the plasma loop formation is complete. It should be noted that, even though <FIG> shows second secondary winding <NUM> of transformer <NUM> adjacent to the transformer core, it is actually the plasma itself formed within the plasma confining volume that serves as second secondary winding <NUM> of transformer <NUM>. Thus, <FIG> schematically illustrates the block ignition according to the present disclosure, showing the two secondary loops that enable VSpark and VLoop to be driven off the same primary coil <NUM> from transformer <NUM>.

According to the exemplary embodiments, the voltage during ignition and the resulting plasma breakdown is affected by the switching frequency of the power supply in the resonant switching power supply circuit, including inverter circuit <NUM>, tank circuits <NUM> and <NUM> and transformer <NUM>, where the voltage around the primary coils of transformer <NUM> is controlled by the operating frequency. <FIG> is a graph illustrating a relationship between the switching frequency and the resulting ignition voltage VSpark, according to exemplary embodiments. It should be noted that the values for VSpark and Switching Frequency are exemplary only. Referring to <FIG> and <FIG>, the voltage applied for ignition VSpark as well as the plasma loop voltage VLoop are multiples of the primary coil voltage in accordance with the turns ratio of the coils on the respective secondary coils <NUM>, <NUM> of transformer <NUM>. As the operating frequency of the inverter is changed, e.g., via a gate driver, towards the natural resonant frequency of the circuit (=<NUM>/sqrt(LC)), the voltage across primary coil <NUM> increases toward a maximum, thus correspondingly increasing the voltages on the secondary coils <NUM>, <NUM> that control the spark and loop voltages VSpark and VLoop (proportional to TurnsRatio* Primary Coil Voltage). As the voltage on the plasma loop and the ignition circuits increases, there is a higher probability of creating a breakdown of the gas creating a plasma. The voltage required to generate the plasma for a given gas flow and pressure is thus indicative of the recombination losses of electrons in the plasma block and hence can serve as a valuable diagnostic for measuring the surface condition of the plasma blocks.

In some exemplary embodiments, VSpark can be tuned by removing capacitors in the ignition circuit to change resonant frequency of the ignition circuit. In one embodiment, a variable VSpark relay could be used in the ignition circuit to remove capacitance and change resonant frequency. In another method, adjustable capacitors similar to those used in RF matching networks could be used to accomplish the same result.

Hence according to the present disclosure, various adjustments and modification can be made to parameters that affect the ignition of plasma. These adjustments and modifications can be made based on the conditions of the particular ignition scenario, such as, for example, a cold-start situation or a poison, i.e., contamination situation. These adjustments can include, for example, one or more of changing the switching frequency of the power supply, i.e., inverter circuit; changing the pulse width of the power supply, i.e., inverter circuit; changing the resonant frequency of the plasma breakdown electronics through modifying the capacitance of the circuit and/or modifying the properties of the inductor used to couple to the plasma; and switching in the different voltage levels via alternative circuits.

<FIG> includes a graph showing the positive peak voltage used for igniting the plasma. The levels represent the peak (positive) value of the spark voltage applied for certain time periods according to exemplary embodiments. It is important to note that the spark voltage may be applied as square alternating pulses switching from positive to negative peak values switching at the RF frequency. This train of pulses may be applied for a given time to enable plasma ignition within that time period. Specifically, <FIG> illustrates the timing in seconds of a series of VSpark voltage pulse trains, referred to as Normalized VSpark, showing only the positive half of the Voltage pulse envelope. As illustrated in <FIG>, as the ignition sequence progresses, in some exemplary embodiments, the voltage across the dielectric break <NUM> increases. In the particular exemplary sequence illustrated in <FIG>, in successive pairs of plasma breakdown pulse trains, the voltage across the plasma loop is increased. However, during each pulse train, the peak voltage level is constant. After three successive pairs of pulse trains, the voltage assumes a constant level for subsequent pulse trains.

In some exemplary embodiments, the VSpark signal follows a sinusoidal waveform having an amplitude which swings between the peak positive value illustrated by the pulse train envelope in <FIG> to the peak negative value. The repetition rate of the sinusoidal pulses in each pulse train can be approximately <NUM> to <NUM> and typically between <NUM> and <NUM>. As noted above, <FIG> illustrates only the positive peak voltage of the sinusoidal pulses over a given time period. As illustrated, the voltage value of the positive peak voltage is changed/controlled between consecutive pulse trains.

<FIG> includes a logical flow diagram illustrating the logical flow of the square ignition sequence of <FIG>, according to some exemplary embodiments. Referring to <FIG>, a plasma ignition command is generated and transmitted in step S202. Next, a peak voltage level VSpark is chosen for the first pulse train in step S204. The voltages for ignition pulses can be stored in a look-up table, based on various parameters related to the particular plasma generation application. Next, the ignition pulse train is generated in step S206. During this step, inverter circuit <NUM> is enabled and a pulse timer is started. Next, a check is performed to determine whether plasma has ignited and is being generated in step S208. If so, flow continues to the plasma-on mode in step S210, via Yes branch S209. If not, a check is performed to determine whether the prescribed pulse duration time has been reached in step S212, via No branch S213. If the pulse duration has not been reached, then plasma is checked for in step S208 along No branch S211. This loop checking for plasma until the pulse time duration expires continues via No branches S211 and S213 until the prescribed pulse duration is reached, and the pulse number is incremented in step S214, via Yes branch S215. Next, a check is performed to determine whether the maximum number of pulses has been reached, i.e., whether the last pulse of the VSpark sequence has completed, in step S216. If not, then flow returns along No branch S217 to step S204, at which a voltage for the next pulse in the VSpark waveform is selected. On the other hand, if it is determined in step S216 that the last pulse has been reached, then flow moves to step S218 along Yes branch S219, wherein it is concluded that an ignition failure has occurred.

<FIG> includes a graph of a waveform for ignition of plasma, having triangular or sloped ignition pulse trains, according to exemplary embodiments. Specifically, <FIG> illustrates the timing in seconds of a series of VSpark voltage pulse trains, referred to as Normalized VSpark. The diagram of <FIG> is completely analogous to that of <FIG>, with the difference between the two pulse train envelopes is that the pulse train envelope of <FIG> is a "square" ignition pulse train envelope, and the pulse train envelope of <FIG> is a "triangular" ignition pulse train envelope. As illustrated in <FIG>, as the ignition sequence progresses, in some exemplary embodiments, the voltage across the dielectric break <NUM> increases. In the particular exemplary sequence illustrated in <FIG>, in successive plasma breakdown pulses the voltage across the plasma loop is increased. Furthermore, in contrast to the square ignition sequence illustrated in <FIG> and <FIG>, in the triangular ignition sequence of <FIG>, even during each pulse train, the voltage level increases for successive pulses. In the particular exemplary embodiment illustrated in <FIG>, after six successive pulse trains, the voltage assumes a constant level for subsequent pulse trains. As illustrated in <FIG>, the peak voltage during a pulse train need not be constant. For example, during a pulse train, the peak voltage can vary in a step-wise fashion over time within the pulse train.

<FIG> includes a logical flow diagram illustrating the logical flow of the triangular or sloped ignition sequence of <FIG>, according to some exemplary embodiments. Referring to <FIG>, a plasma ignition command is generated and transmitted in step S222. Next, a voltage level VSpark is chosen for the first pulse in step S224. The voltages for the first ignition pulse can be a preset value, based on various parameters related to the particular plasma generation application. Subsequent pulses can have a starting voltage level equal to the end voltage level of the previous pulse. Next, the ignition pulse is generated in step S226. During this step, inverter circuit <NUM> is enabled and a pulse timer is started. Next, a check is performed to determine whether plasma has ignited and is being generated in step S228. If so, flow continues to the plasma-on mode in step S230, via Yes branch S229. If not, a check is performed to determine whether the prescribed pulse duration time has been reached in step S232, via No branch S233. If the pulse duration has not been reached, then flow moves along No branch S231 to step S240, where the voltage level of the pulse is increased. Again, a check is made for plasma in step S228. This loop checking for plasma and increasing the pulse voltage until the pulse time duration expires continues via No branches S231 and S233 until the prescribed pulse duration is reached, and the pulse number is incremented in step S234, via Yes branch S235. Next, a check is performed to determine whether the maximum number of pulses has been reached, i.e., whether the last pulse of the VSpark sequence has completed, in step S236. If not, then flow returns along No branch S237 to step S224, at which a voltage for the next pulse in the VSpark waveform is selected. On the other hand, if it is determined in step S236 that the last pulse has been reached, then flow moves to step S238 along Yes branch S239, wherein it is concluded that an ignition failure has occurred.

<FIG> includes a schematic diagram of a toroidal plasma tube, plasma block, or plasma confining region <NUM>, as described above in detail, with connections for applying ignition voltage VSpark and monitoring voltage in plasma block <NUM>, according to some exemplary embodiments. Referring to <FIG>, the monitored voltage in plasma block <NUM> is identified herein by the variable VSpark_Readback. The voltage VSpark_Readback is sensed across plasma block <NUM> as shown, or in any of many other possible locations on plasma block <NUM>. Ignition voltage VSpark, referenced to ground, is shown applied across plasma block <NUM>. Ground reference is shown at the body of plasma block <NUM>.

<FIG> is a waveform diagram over time of the programmed VSpark ignition pulse train envelope waveform, VSpark_Program, identified as <NUM>, according to some exemplary embodiments. The VSpark_Program pulse train envelope waveform <NUM> is analogous to the pulse train envelope waveforms illustrated in <FIG> and <FIG>. As shown in <FIG>, each pulse train of VSpark_Program <NUM> includes an initial VSpark _Program voltage ramp <NUM> of relatively short time duration. The time duration of each pulse train is indicated by a pulse train duration arrow <NUM> along the time axis.

<FIG> includes a logical flow diagram illustrating the logical flow of voltage monitoring during plasma ignition illustrated in the timing curve of <FIG>, according to some exemplary embodiments. Referring to <FIG>, an ignition command is sent in step S260. Next, VSpark _Program is determined, such as from a look-up table, and the ignition voltage VSpark, based on VSpark _Program, is applied to plasma block <NUM> to initiate the ignition procedure, in step S262. Also, VSpark_Readback is sensed in step S272. Next, VSpark_Program VP is compared to VSpark_Readback VR in step S264. If VP < VR, then a system error is declared in step S266. If VP = VR, then it is concluded in step S268 that no capacitive plasma breakdown has occurred, which can be due to a "cold start" in which insufficient free electrons are present in the plasma block <NUM> for ignition, or a high level of contamination is present in plasma block <NUM>, i.e., a "poison" condition exists. If VP > VR, then it is concluded in step S270 that capacitive plasma breakdown has occurred to some degree and that a moderate poison condition exists in plasma block <NUM>.

<FIG> includes a schematic diagram of toroidal plasma tube, plasma block, or plasma confining region <NUM>, as described above in detail, with connections for applying ignition voltage VSpark and, therefore, ignition current ISpark, according to some exemplary embodiments. Ignition voltage VSpark, referenced to ground, is shown applied across plasma block <NUM>. Ground reference is shown at the body of plasma block <NUM>. Referring to <FIG>, the current ISpark_Readback is sensed by a sensor or sensing circuit <NUM>, which includes a transformer <NUM> having a turns ratio of N:<NUM>. A sense voltage Vs can be sensed across sense resistance Rs, the sense current Is passing through Rs resulting in sense voltage Vs. <MAT> and <MAT> therefore, <MAT>.

<FIG> includes a logical flow diagram illustrating the logical flow of current monitoring during plasma ignition illustrated in the timing curve of <FIG>, according to some exemplary embodiments. Referring to <FIG>, <FIG> and <FIG>, an ignition command is sent in step S280. Next, the intended ignition current ISpark _Program is determined, such as from a look-up table, and the ignition voltage VSpark, based on ISpark _Program, is applied to plasma block <NUM> to initiate the ignition procedure, in step S282. Also, ISpark_Readback is sensed in step S292. Next, ISpark _Program IP is compared to ISpark_Readback IR in step S284. If IP > IR, then a system error is declared in step S286. If IP=IR, then it is concluded in step S288 that no capacitive plasma breakdown has occurred, which can be due to a "cold start" in which insufficient free electrons are present in the plasma block <NUM> for ignition, or a high level of contamination is present in plasma block <NUM>, i.e., a "poison" condition exists. If IP < IR, then it is concluded in step S290 that capacitive plasma breakdown has occurred to some degree and that a moderate poison condition exists in plasma block <NUM>.

<FIG> include waveform diagrams over time of the programmed VSpark ignition waveform and resulting sensed VSpark_Readback signal, under different ignition conditions, according to some exemplary embodiments. In the three illustrated scenarios of <FIG>, contamination level, in these specific example, using N<NUM> gas as a contaminant, is varied to produce different ignition results. That is, the poison level is varied to illustrate different types of ignition and ignition failure. All of the scenarios of <FIG> are implemented at a pressure of <NUM> Torr at an Argon flow rate of <NUM> standard liters per minute (slm). In <FIG>, N<NUM> gas is introduced as a contaminant at a flow rate of <NUM> standard cubic centimeters per minute (sccm). In <FIG>, N<NUM> gas is introduced as a contaminant at a flow rate of <NUM> sccm. In <FIG>, no contamination is introduced.

Referring to <FIG>, with the highest of the three contamination levels, no breakdown occurs, such that ignition fails. In the poor ignition conditions of <FIG>, high voltage is applied for a large amount of time, and there is no creation of plasma, or localized breakdown. Thus, the voltage applied on the midblocks and on the plasma loop stays at the high value determined by the resonant circuit of the ignition circuit.

Referring to <FIG>, with a somewhat lower level of contamination, only localized breakdown occurs. That is, full loop breakdown does not occur. In this case, the poison (contaminant) level is preventing electrons from accelerating around the entire plasma loop, meaning that ignition of the plasma loop fails. In the case of <FIG>, in the absence of contamination, after high voltage is applied to the plasma block, a localized breakdown in the high field regions near the biased blocks is created. The localized breakdown acts as a resistive path to ground, thus decreasing the ignition voltage as the charge on the capacitor bleeds down thru the resistor (as illustrated in <FIG>). Once the plasma loop is completed, it shunts the secondary loop on the transformer, due to the very low resistive impedance in a plasma, thus effectively dropping the voltage that can be held on the primary coils. Hence, VSpark and VLoop decrease substantially, and localized breakdown occurs. The localized breakdown cascades around the entire plasma loop, resulting in complete loop breakdown and complete, successful plasma ignition.

<FIG> and <FIG> includes two waveform diagrams over time of the programmed VSpark ignition waveform VSpark _Program, the resulting sensed VSpark_Readback signal, and the VLoop signal, under a cold start ignition scenario and a normal ignition scenario, respectively, according to some exemplary embodiments. Referring to <FIG>, in the first ignition attempt, longer total ignition time, i.e., longer time to VLoop breakdown, is caused by longer time to initial breakdown due to cold start conditions, i.e., a shortage of free electrons in the plasma block. In the first ignition attempt, the time to ignition, i.e., total VLoop breakdown, was approximately <NUM>. That is, the time to initial breakdown, when VSpark_Readback = VSpark _Program, was approximately <NUM>, followed by immediate loop breakdown thereafter, i.e., VSpark_Readback << VSpark_Program. In the second (normal) ignition, illustrated in <FIG>, both initial and loop breakdowns occurred almost immediately.

<FIG> and <FIG> include two waveform diagrams over time of the programmed VSpark ignition waveform VSpark _Program, the resulting sensed VSpark_Readback signal, and the VLoop_Sendback signal, under poison (contamination) ignition conditions, in the case of a long-duration ignition scenario and a failed ignition scenario, respectively, according to some exemplary embodiments. Referring to <FIG> and <FIG>, during <NUM> ignition attempts under poisoned ignition conditions, VSpark_Readback and VLoop_Sendback were recorded. After localized breakdown, VSpark_Readback drops in amplitude, and VLoop_Sendback still continues to ramp to full voltage (since the plasma loop is not completely closed, the secondary loop representing plasma in a transformer is open and can sustain the higher loop Voltage). This Vloop_Sendback collapses (reduces to a lower amplitude) as the plasma loop is formed and becomes proportional to the Plasma Current X Plasma Impedance. Fluctuations in VSpark_Readback, as indicated by the dip in the signal seen in <FIG>, can be concluded to indicate arcing or an unstable plasma loop in the plasma block, contributing to the failed ignition. It is noted that, after <NUM>,<NUM> ignitions, the inlet plasma block was severely damaged by the plasma. It is believed that the total time duration when there is initial breakdown but incomplete plasma loop formation, as shown in <FIG> and <FIG>, is related with plasma block life, since the ions formed in the vicinity of the biased plasma blocks can be accelerated back into the plasma block surfaces and cause damage to the protective dielectric coatings thus lowering the useful life of the blocks, and, in accordance with the present disclosure, can be used as a trigger for block replacement and/or process interlock.

According to exemplary embodiments, by starting at a lower ignition voltage, the system is able to ignite under favorable gas conditions at a lower voltage, which results in reduced electric fields within the plasma block (E =V/thickness), thus reducing the possibility of a catastrophic breakdown of the dielectric in the high field region, which could result in ablation of the coating causing particles.

The approach of the present disclosure provides the most benign ignition possible. Also, according to the disclosure, process and RPS health monitoring through changes in ignition voltage can be carried out over longer-term operation. Block coating integrity, i.e., dielectric strength, and changes therein may be determined from this voltage. According to the disclosure, the breakdown voltage can also serve as a quality control metric for the block and different feed gases.

When high voltage is applied to the blocks for igniting the plasma, there can be a parasitic arcing event if the coating is compromised in any location and cannot hold the electrostatic field. This can occur when the local dielectric breakdown strength is weaker than the applied electric field and can result in an arc to other grounded regions in the vicinity of the biased electrode. This may cause coating to erode in this area due to arcing damage induced during the electrical breakdown. Such erosion and damage to the block coatings can be minimized by the smart ignition scheme provided herein, where the voltage can be ramped to enable plasma strike at the lowest possible potential and via smart learning where the previously learned potential for a given process condition can be applied to strike plasma for processing the next substrate.

The present disclosure also provides for diagnostic monitoring for plasma ignition conditions. According to exemplary embodiments, the voltage needed for ignition also provides a means for health check of the system and can be used to monitor drifts in the process conditions over longer periods of time. This can also be a means to protect substrates from being processed incorrectly due to inadvertently drifting plasma or block conditions, and can further serve as a means to monitor plasma block health in case there is erosion of the block due to the exposure of the block/coating surfaces to highly reactive plasma environments, especially with halogen , O or H containing gases, or due to deposition of other species on the block surfaces, either due to back streaming of condensable precursors from the process chambers or due to the gases being used in the remote plasma sources, e.g., C or S or Al, etc. containing precursors. Alternatively, this can involve chemical modification of the plasma facing surfaces of the remote plasma source. This can cause changes in the time or voltage needed for ignition. Such changes can provide useful diagnostics for a system using these remote plasma sources for integrating a trigger for other process or maintenance activities to reset the plasma conditions to within the control limits. These diagnostics can be used for "On-wafer" processing applications where a minor change in radical output could cause significant yield loss to the end user. Having a diagnostic that can effectively be used to monitor radical recombination in a plasma source is useful in such applications that merit a high degree of process control.

The present disclosure also provides a "smart," i.e., self-learning, ignition scheme for plasma sources. The self-learning ignition scheme for plasma source according to the disclosure can be used for minimizing the plasma ignition time when processing multiple substrates with the same processes. The approach involves recording the plasma strike conditions for different stages in a process recipe sequence from the first few substrates (learning substrates) and then using the "learned" values for processing the remaining substrates. This assures process repeatability and minimizes/prevents scrap events that can be very costly to the end users. Typical semiconductor device substrates are worth tens of thousands of dollars, which is typically worth more than the cost of a remote plasma unit itself. The smart (self-learning) approach of the present disclosure also minimizes erosion of the blocks by minimizing the time the block coating is exposed to high fields as well as minimizes the magnitude of the field that is required to ignite the plasma, thus enabling a superior defect performance, e.g., minimizing the generation of defects, that is important for any on-wafer processes where the device yields are directly correlated to defects in the films generated during process.

Thus, according to the plasma ignition approach of the present disclosure, a VSpark_Ratio signal is generated from the ratio of VSpark_Readback/VSpark_Program in the plasma block to determine and report back if plasma ignition is occurring in cold start or poison conditions, or if ignition was normal. This guides any remediation techniques applied to the situation.

Also, according to the disclosure, a timer is used with VSpark _Ratio = VSpark_Readback/VSpark_Program to give an approximation of block health. In general, the longer the time with localized breakdown and no loop breakdown, the more damage results to plasma blocks.

Also, according to the disclosure, plasma chamber conditions can be determined during ignition using the time duration to ignition and/or the value of VSpark_Program needed for successful ignition.

Also, according to the disclosure, arc detection can be generated using the noise on the VSpark _Ratio = VSpark_Readback/VSpark_Program signal. As illustrated in <FIG>, the VSpark_Readback signal exhibits erratic behavior during an unsuccessful ignition. Specifically, the value of VSpark_Readback drops from the value of VSpark _Program, but shows low-frequency noise, which is indicative of arcing or unstable plasma formation conditions. Thus, detection of this noise can be used to diagnose occurrence of an arcing or unstable plasma formation condition.

Also, according to the disclosure, it should be noted that all of the elements of the disclosure are equally applicable using a measured current signal that provides VSpark bias to the plasma blocks.

<FIG> includes a schematic logical flow diagram of a plasma ignition procedure, according to some exemplary embodiments. Referring to <FIG>, the ignition sequence starts by starting a timer at t = <NUM>, in step S402. Next, the time t is checked to determine if timeout has been reached, in step S404. In some exemplary embodiments, timeout can be set at <NUM>-<NUM> seconds. If timeout is reached, flow moves along No branch out of step S404 to step S406, where the sequence is aborted, and ignition failure is reported due to ignition window timeout limit exceeded. Flow then moves to step S436 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum VSpark_Readback reached during ignition is recorded. If timeout is not reached, then flow moves along the Yes branch out of step S404 to step S408, where the value of VSpark_Program is set, such as by obtaining the value from a look-up table. Next, in step S410, it is determined whether the value of VSpark_Program is less than a preset maximum VSpark value. If not, then flow moves along the No branch out of step S410 to step S412, where the sequence is aborted, and ignition failure is reported due to VSpark limit exceeded. Flow then moves to step S436 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum VSpark_Readback reached during ignition is recorded. If the maximum VSpark value is not exceeded, then flow moves along the Yes branch out of step S410 to step S414, where the VSpark_Readback signal is measured. The VSpark _Ratio signal = VSpark_Readback/VSpark_Program is computed in step S416. In step S418, if the VSpark-Ratio signal is greater than <NUM>, meaning the VSpark_Readback signal is greater than the VSpark _Program signal, then flow moves along the Yes branch out of step S418 to step S420, where the sequence is aborted, and a system error is reported. Flow then moves to step S436 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum VSpark_Readback reached during ignition is recorded. If the ratio is not greater than <NUM>, then flow moves along the No branch out of step S418 to step S422, where it is determined whether the ratio is less than some first threshold value, referred to herein as Threshold <NUM>, which, in some particular exemplary embodiments, is <NUM>. If not, then flow moves along the No branch out of step S422 to step S424, where it is concluded that no initial breakdown has yet occurred, and flow returns to step S404. If the ratio is less than Threshold <NUM>, then flow moves along the Yes branch out of step S422 to step S426, where it is determined whether the ratio is less than some second threshold value, referred to herein as Threshold <NUM>, which, in some particular exemplary embodiments, is <NUM>. If not, then flow moves along the No branch out of step S426 to step S428, where it is concluded that loop closure has not yet occurred, and, therefore, that a poisoned ignition condition exists. Flow then moves to step S430, where the loop voltage is increased, and then to step S432, where it is determined whether the new loop voltage is below the maximum allowable VLoop value. If not, then flow moves along the No branch out of step S432 to step S438, where an ignition poisoned condition is reported, and a notice is issued requiring purging of the system before a retry of ignition. Flow then moves to step S436 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum VSpark_Readback reached during ignition is recorded. If the new loop voltage is below the maximum allowable VLoop value, then flow moves along the Yes branch out of step S432 back to step S404.

<FIG> includes a schematic logical flow diagram of another plasma ignition procedure, according to some exemplary embodiments. Referring to <FIG>, the ignition sequence starts by starting a timer at t = <NUM>, in step S452. Next, the time t is checked to determine if timeout has been reached, in step S454. In some exemplary embodiments, timeout can be set at <NUM>-<NUM> seconds. If timeout is reached, flow moves along No branch out of step S454 to step S456, where the sequence is aborted, and ignition failure is reported due to ignition window timeout limit exceeded. Flow then moves to step S480 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded. If timeout is not reached, then flow moves along the Yes branch out of step S454 to step S458, where the value of VSpark_Program is set, such as by obtaining the value from a look-up table. Next, in step S460, it is determined whether the value of VSpark_Program is less than a preset maximum VSpark value. If not, then flow moves along the No branch out of step S460 to step S462, where the sequence is aborted, and ignition failure is reported due to VSpark limit exceeded. Flow then moves to step S480 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded. If the maximum VSpark value is not exceeded, then flow moves along the Yes branch out of step S460 to step S464, where the ISpark_Readback and ILoop_Readback signals are measured. In step S466, it is determined whether ISpark_Readback exceeds a preset threshold current value I_Arc_Threshold for arcing. If the threshold is exceeded, then flow moves along the Yes branch out of step S466 to step S468, where the sequence is aborted, and potential arcing is reported. Flow then moves to step S480 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded. If the arcing threshold is not exceeded by ISpark_Readback, then flow moves along the No branch out of step S466 to step S470, where it is determined whether the current value of ISpark_Readback exceeds a first ignition current threshold I_Ignitionthreshold1. If not, then flow moves along the No branch out of step S470 to step S474, where it is concluded that no initial breakdown has yet occurred. Flow then returns back to step S454. If ISpark_Readback exceeds I-Ignitionthreshold1, then flow moves along the Yes branch out of step S470 to step S472, where it is determined whether ISpark_Readback exceeds a second ignition current threshold I_Ignitionthreshold2. If so, then flow moves along the Yes branch out of step S472 to step S478, where it is concluded and reported that a successful ignition has been achieved. Flow then moves to step S480 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded. If ISpark_Readback does not exceed the second ignition current threshold I_Ignitionthreshold2, then flow moves along the No branch out of step S472 to step S476, where it is concluded that no loop closure has occurred, and, therefore, that a poisoned ignition condition exists. Next, flow moves to step S482, where the loop voltage is increased, and then to step S484, where it is determined whether the new loop voltage is below the maximum allowable VLoop value. If not, then flow moves along the No branch out of step S484 to step S486, where an ignition poisoned condition is reported, and a notice is issued requiring purging of the system before a retry of ignition. Flow then moves to step S480 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded. If the new loop voltage is below the maximum allowable VLoop value, then flow moves along the Yes branch out of step S484 back to step S454.

<FIG> includes a schematic logical flow diagram of another plasma ignition procedure, according to some exemplary embodiments. Referring to <FIG>, the ignition sequence starts by starting a timer at t = <NUM>, in step S552. Next, the time t is checked to determine if timeout has been reached, in step S554. In some exemplary embodiments, timeout can be set at <NUM>-<NUM> seconds. If timeout is reached, flow moves along No branch out of step S554 to step S556, where the sequence is aborted, and ignition failure is reported due to ignition window timeout limit exceeded. Next, flow moves to step <NUM>, where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded, and the value of ILoop_Readback is recorded. If timeout is not reached, then flow moves along the Yes branch out of step S554 to step S558, where the value of VSpark_Program is set, such as by obtaining the value from a look-up table. Next, in step S560, it is determined whether the value of VSpark_Program is less than a preset maximum VSpark value. If not, then flow moves along the No branch out of step S560 to step S562, where the sequence is aborted, and ignition failure is reported due to VSpark limit exceeded. Next, flow moves to step <NUM>, where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded, and the value of ILoop_Readback is recorded. If the maximum VSpark value is not exceeded, then flow moves along the Yes branch out of step S560 to step S564, where the ISpark_Readback and ILoop_Readback signals are measured. In step S566, it is determined whether ISpark_Readback exceeds a preset threshold current value I_Arc_Threshold for arcing. If the threshold is exceeded, then flow moves along the Yes branch out of step S566 to step S568, where the sequence is aborted, and potential arcing is reported. Next, flow moves to step <NUM>, where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded, and the value of ILoop_Readback is recorded. If the arcing threshold is not exceeded by ISpark_Readback, then flow moves along the No branch out of step S566 to step S570, where it is determined whether the current value of ISpark_Readback exceeds a first ignition current threshold I_Ignitionthreshold1. If not, then flow moves along the No branch out of step S570 to step S574, where it is concluded that no initial breakdown has yet occurred. Flow then returns back to step S554. If ISpark_Readback exceeds I-Ignitionthreshold1, then flow moves along the Yes branch out of step S570 to step S572, where it is determined whether ILoop_Readback exceeds a loop current threshold I_Loop_threshold1. If so, then flow moves along the Yes branch out of step S572 to step S578, where it is concluded and reported that a successful ignition has been achieved. Flow then moves to step S580 where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded, and the value of ILoop_Readback is recorded. If ILoop_Readback does not exceed the loop current threshold I_Loop_threshold1, then flow moves along the No branch out of step S572 to step S576, where it is concluded that no loop closure has occurred, and, therefore, that a poisoned ignition condition exists. Next, flow moves to step S582, where the loop voltage is increased, and then to step S584, where it is determined whether the new loop voltage is below the maximum allowable VLoop value. If not, then flow moves along the No branch out of step S584 to step S586, where an ignition poisoned condition is reported, and a notice is issued requiring purging of the system before a retry of ignition. Next, flow moves to step <NUM>, where the ignition sequence is declared complete, the time to ignition t is recorded and the maximum ISpark_Readback reached during ignition is recorded, and the value of ILoop_Readback is recorded. If the new loop voltage is below the maximum allowable VLoop value, then flow moves along the Yes branch out of step S584 back to step S554.

As described above, the plasma ignition approach described herein in detail can be used in monitoring health and status of the overall system. <FIG> includes a logical flow diagram illustrating the logical flow of a method of using plasma ignition parameters to monitor health and status of a plasma source, according to some exemplary embodiments. Referring to <FIG>, in step S502, parameters associated with the plasma ignition are read. The parameters include time t, maximum VSpark_Readback, ISpark_Readback, ILoop_Readback. In step <NUM>, the parameters are stored in a memory and/or processing device, such as an internal controller, a semiconductor memory, an external storage device or data comparator, system controller, or any such device. In step S506, the present parameter values are compared to historical values, which can be short-term stored values or longer-term average values. Target values for the parameters are determined from known good conditions. In step S508, possible corrective actions are taken based on the comparisons in step S506. The corrective actions may include one or more of: (i) cleaning and/or replacing plasma block(s), (ii) increasing purge times, (iii) adjusting RPS settings to target same process performance, (iv) replacing RPS, (v) scheduling chamber or RPS preventive maintenance, (vi) checking and/or replacing MFCs/gas feed lines.

Claim 1:
A method of determining the health of a plasma system (<NUM>) by monitoring an ignition process for igniting a plasma (<NUM>) within a plasma confining volume (<NUM>), comprising:
generating an ignition signal with an ignition circuit;
applying the ignition signal between a biased region and a grounded region in the vicinity of the plasma confining volume (<NUM>);
sensing a parameter related to the ignition circuit;
comparing the sensed parameter values to historical values; and
determining at least one corrective action for the plasma system (<NUM>) based on the comparison of the sensed parameter values and historical values, wherein the at least one corrective action comprises providing a purge gas through the plasma confining volume (<NUM>) to reduce an amount of contaminant in the plasma confining volume (<NUM>) before a retry of ignition.