Plasma processing with enhanced charge neutralization and process control

Plasma processing with enhanced charge neutralization and process control is disclosed. In accordance with one exemplary embodiment, the plasma processing may be achieved as a method of plasma processing a substrate. The method may comprise providing the substrate proximate a plasma source; applying to the plasma source a first RF power level during a first period and a second RF power level during a second period, the first and second RF power levels being greater than zero RF power level, wherein the second RF power level is greater than the first RF power level; generating with the plasma source a first plasma during the first period and a second plasma during the second period; and applying to the substrate a first bias voltage during the first period and a second bias voltage during the second period, wherein the first voltage has more negative potential than the second voltage.

BACKGROUND

Plasma processing has been widely used in the semiconductor and other industries for many decades. Plasma processing is used for tasks such as cleaning, etching, milling, and deposition. In many plasma processing systems, charge tends to accumulate on the substrate being processed. This charge build-up can result in the development of a relatively high potential voltage on the substrate that can cause plasma processing non-uniformities, arcing, and substrate damage. For example, charge build-up in plasma etching systems can result in non-uniform etch depths and pitting or damage to the surface of the substrate which can reduce process yield. In addition, charge build-up in deposition system can result in non-uniform deposition and damage to the deposited film layer.

More recently, plasma processing has been used for doping. Plasma doping is sometimes referred to as PLAD or plasma immersion ion implantation (PIII). Plasma doping systems have been developed to meet the doping requirements of some modern electronic and optical devices. Plasma doping is fundamentally different from conventional beam-line ion implantation systems that accelerate ions with an electric field and then filter the ions according to their mass-to-charge ratio to select the desired ions for implantation. In contrast, plasma doping systems immerse the target in a plasma containing dopant ions and bias the target with a series of negative voltage pulses. The electric field within the plasma sheath accelerates ions toward the target thereby implanting the ions into the surface of the target.

Plasma doping systems for the semiconductor industry generally require a very high degree of process control. Conventional beam-line ion implantation systems that are widely used in the semiconductor industry have excellent process control and also excellent run-to-run uniformity. Conventional beam-line ion implantation systems provide highly uniform doping across the entire surface of state-of-the-art semiconductor substrates.

In general, the process control of plasma doping systems is not as good as conventional beam-line ion implantation systems. In many plasma doping systems, charge tends to accumulate on the substrate being plasma doped. This charge build-up can result in the development of a relatively high potential voltage on the substrate that can cause unacceptable doping non-uniformities and arcing, which can result in device damage.

SUMMARY

Plasma processing with enhanced charge neutralization and process control is disclosed. In accordance with one exemplary embodiment, the plasma processing may be achieved as a method of plasma processing a substrate. The method may comprise providing the substrate proximate a plasma source; applying to the plasma source a first RE power level during a first period and a second RF power level during a second period, the first and second RF power levels being greater than zero RE power level, wherein the second RF power level is greater than the first RF power level; generating with the plasma source a first plasma during the first period and a second plasma during the second period; and applying to the substrate a first bias voltage during the first period and a second bias voltage during the second period, wherein the first voltage has more negative potential than the second voltage.

In accordance with other aspects of this particular exemplary embodiment, the method may further comprise striking the plasma to generate the first plasma during the first period.

In accordance with further aspects of this particular exemplary embodiment, the method may further comprise etching the substrate during at least one of the first and second periods.

In accordance with additional aspects of this particular exemplary embodiment, the method may further comprise depositing a material during at east one of the first and second periods.

In accordance with further aspects of this particular exemplary embodiment, at least one of the first and the second power levels may be substantially constant during respective ones of the first and the second periods.

In accordance with other aspects of this particular exemplary embodiment, the method may further comprise directing ions from in the first plasma toward the substrate during the first period; and directing electrons from the second plasma toward the substrate during the second period.

In accordance with further aspects of this particular exemplary embodiment, the plasma source may comprise at least one of a planar coil RF antenna and a helical coil RF antenna; and a RF power supply electrically coupled to at least one of the planar coil RF antenna and the helical coil RF antenna.

In accordance with additional aspects of this particular exemplary embodiment, the plasma source may comprise at least one of a planar coil RF antenna and a helical coil RF antenna; and a RF power supply electrically coupled to one of the planar coil RF antenna and the helical coil RF antenna, the other one of the planar coil RF antenna and the helical coil RF antenna being a parasitic antenna.

In accordance with other aspects of this particular exemplary embodiment, the plasma source may comprise a planar coil RF antenna and a helical coil RF antenna; and a RF power supply electrically coupled to the planar coil RF antenna and the helical coil RF antenna.

In accordance with additional aspects of this particular exemplary embodiment, the method may further comprise applying the first RF power level to the one of the planar coil RF antenna and the helical coil RF antenna of the plasma source during the first period; and applying the second RF power level to the other one of the planar coil RF antenna and the helical coil RF antenna of the plasma source during the second period.

In accordance with further aspects of this particular exemplary embodiment, the method may further comprise applying to the plasma source a third RF power level during a third period, wherein the third RF power level is less than the first RF power level; and applying to the substrate the second bias voltage during the third period.

In accordance with other aspects of this particular exemplary embodiment, the second period may immediately follow the first period.

In accordance with additional aspects of this particular exemplary embodiment, the first bias voltage may be negative bias voltage and the second bias voltage may be ground bias voltage.

In accordance with further aspects of this particular exemplary embodiment, the first bias voltage may be negative bias voltage and the second bias voltage may be positive bias voltage.

In accordance with another exemplary embodiment, the plasma processing may be achieved as a method of plasma processing a substrate. The method may comprise applying to a plasma source a first power level during a first period and generating a first plasma containing first ions; applying to the plasma source a second power level during a second period and generating a second plasma containing second ions, the second power level being greater than the first power level; directing, during the first period, the first ions from the first plasma toward the substrate and accumulating charges in the substrate; and decreasing, during the second period, the charge accumulated in the substrate.

In accordance with other aspects of this particular exemplary embodiment, the method may further comprise striking the plasma to generate the first plasma during the first period while applying to the plasma source the first power level.

In accordance with further aspects of this particular exemplary embodiment, the the method may further comprise etching the substrate during the first period.

In accordance with additional aspects of this particular exemplary embodiment, the method may further comprise depositing a material during the first period.

In accordance with further aspects of this particular exemplary embodiment, the method may further comprise implanting ions from the plasma into the substrate during the first period.

DETAILED DESCRIPTION

Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all retelling to the same embodiment.

The present teachings will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein. For example, it should be understood that the methods for neutralizing charge in a plasma processing system according to the present invention can be used with any type of plasma source.

Many plasma processing systems operate in a pulsed mode of operation where a series of pulses is applied to the plasma source to generate a pulsed plasma. Also, a series of pulses can be applied to the substrate being plasma processed during the on-periods of the plasma source pulses, which biases the substrate to attract ions for implantation, etching, or deposition. In the pulsed mode of operation, charge tends to accumulate on the substrate being plasma processed during the on-period of the plasma source pulses. When the duty cycle of the plasma source pulses is relatively low (i.e. less than about 25% and sometimes less than 2% depending upon process parameters), the charge tends to be efficiently neutralized by electrons in the plasma and there are only minimal charging effects.

However, there is currently a need to perform plasma processing in a pulsed mode of operation with relatively high duty cycles (i.e. duty cycles above about 2%). Such higher duty cycles are necessary to achieve the desired throughputs and to maintain etching rates, deposition rates, and doping levels that are required for some modern devices. For example, it is desirable to perform poly gate doping and counter doping of some state-of-the art devices by plasma doping with a duty cycle greater than 2%. In addition, it is desirable to perform many plasma etching and deposition processes at duty cycles greater than 2% to increase process throughput to acceptable levels.

As the duty cycle is increased above about 2%, there is a relatively short period of time where the charge on the substrate being plasma processed can be neutralized during the pulse-off period of the plasma source. Consequently, charge accumulation or charge build up can occur on the substrate being plasma processed, which results in the development of a relatively high potential voltage on the substrate being plasma processed that can cause plasma processing non-uniformities, arcing, and substrate damage. For example, substrates containing thin gate dielectrics can be easily damaged by excess charge build up.

The present invention relates to methods and apparatus for neutralizing charge during plasma processing. The method and apparatus of the present invention allow plasma processing to be performed at higher duty cycles by reducing the probability of damage caused by charging effects. In particular, a plasma processing apparatus according to the present invention includes a RF power supply that varies the RF power applied to the plasma source to at least partially neutralize charge accumulation during plasma processing. In addition, the bias voltage to the substrate being plasma processed can be varied to at least partially neutralize charge accumulation. Furthermore, in some embodiment of the invention, the RF power pulses applied to the plasma source and the bias voltage applied to the substrate are synchronized in time and the relative timing of the RF power pulses applied to the plasma source and the bias voltage applied to the substrate being plasma processed is varied to at least partially neutralize charge accumulation on the substrate and/or to achieve certain process goals.

More specifically, in various embodiments, single or multiple RF power supplies are used to independently power the plasma source and bias the substrate being plasma processed so as to at least partially neutralize charge during plasma processing. Also, in various embodiments, the RF power applied to the plasma source and the bias voltage applied to the substrate during plasma processing are applied at relative times to at least partially neutralize charge during plasma processing.

In addition to neutralizing charge, the method and apparatus of the present invention can precisely control at least one of the power to the RF source and the bias applied to the substrate during periods where the plasma processing is terminated (i.e. pulse-off period) in order to achieve certain process goals. For example, the method and apparatus of the present invention can precisely control at least one of the power to the RF source and the bias voltage applied to the substrate during the pulse-off period in order to allow chemical reactions to occur on the surface of the substrate. Such a capability can improve throughput and provide more process control in some etching and deposition processes.

In addition, the method and apparatus of the present invention for plasma doping can precisely control at least one of the power to the RF source and the bias voltage applied to the substrate during the pulse-off period in order to improve the retained dose while plasma doping. The resulting improvement in retained dose will reduce the implant time and thus, will increase plasma doping throughput. In addition to neutralizing charge, the method and apparatus of the present invention can precisely control at least one of the power to the RF source and the bias applied to the substrate during periods where the plasma doping is terminated in order to achieve knock-on type ion implant mechanisms that achieve improved sidewall plasma doping profiles and retrograde doping profiles as describe herein.

FIG. 1Aillustrates one embodiment of a plasma processing system100with charge neutralization according to the present invention. It should be understood that this is only one of many possible designs of apparatus that can perform plasma processing, such as ion implantation, deposition, and etching, with charge neutralization according to the present invention. In particular, it should be understood that there are many possible plasma sources that can be used with the plasma processing system of the present invention. The plasma source shown inFIG. 1includes both a planar and a helical RF coil. Other embodiments include a single planar or a helical RF coil. Still other embodiments include capacitively coupled plasma sources or electron cyclotron resonance plasma sources. One skilled in the art will appreciate that there are many types of equivalent plasma sources.

The plasma processing system100includes an inductively coupled plasma source101having both a planar and a helical RF coil and a conductive top section. A similar RF inductively coupled plasma source is described in U.S. patent application Ser. No. 10/905,172, filed on Dec. 20, 2004, entitled “RF Plasma Source with Conductive Top Section,” which is assigned to the present assignee. The entire specification of U.S. patent application Ser. No. 10/905,172 is incorporated herein by reference. The plasma source101shown in the plasma processing system100is well suited for plasma doping and other precise plasma processing applications that require highly uniform processing because it can provide a very uniform ion flux. In addition, the plasma source101is useful for high power plasma processing because it efficiently dissipates heat generated by secondary electron emissions.

More specifically, the plasma processing system100includes a plasma chamber102that contains a process gas supplied by an external gas source104. The external gas source104, which is coupled to the plasma chamber102through a proportional valve106, supplies the process gas to the chamber102. In some embodiments, a gas baffle is used to disperse the gas into the plasma source101. A pressure gauge108measures the pressure inside the chamber102. An exhaust port110in the chamber102is coupled to a vacuum pump112that evacuates the chamber102. An exhaust valve114controls the exhaust conductance through the exhaust port110.

A gas pressure controller116is electrically connected to the proportional valve106, the pressure gauge108, and the exhaust valve114. The gas pressure controller116maintains the desired pressure in the plasma chamber102by controlling the exhaust conductance and the process gas flow rate in a feedback loop that is responsive to the pressure gauge108. The exhaust conductance is controlled with the exhaust valve114. The process gas flow rate is controlled with the proportional valve106.

In some embodiments, a ratio control of trace gas species is provided to the process gas by a mass flow meter that is coupled in-line with the process gas that provides the primary dopant species. Also, in some embodiments, a separate gas injection means is used for in-situ conditioning species. Furthermore, in some embodiments, a multi-port gas injection means is used to provide gases that cause neutral chemistry effects that result in across substrate variations.

The chamber102has a chamber top118including a first section120formed of a dielectric material that extends in a generally horizontal direction. A second section122of the chamber top118is formed of a dielectric material that extends a height from the first section120in a generally vertical direction. The first and second sections120,122are sometimes referred to herein generally as the dielectric window. It should be understood that there are numerous variations of the chamber top118. For example, the first section120can be formed of a dielectric material that extends in a generally curved direction so that the first and second sections120,122are not orthogonal as described in U.S. patent application Ser. No. 10/905,172, which is incorporated herein by reference. In other embodiments, the chamber top118includes only a planer surface.

The shape and dimensions of the first and the second sections120,122can be selected to achieve a certain performance. For example, one skilled in the art will understand that the dimensions of the first and the second sections120,122of the chamber top118can be chosen to improve the uniformity of plasmas. In one embodiment, a ratio of the height of the second section122in the vertical direction to the length across the second section122in the horizontal direction is adjusted to achieve a more uniform plasma. For example, in one particular embodiment, the ratio of the height of the second section122in the vertical direction to the length across the second section122in the horizontal direction is in the range of 1.5 to 5.5.

The dielectric materials in the first and second sections120,122provide a medium for transferring the RF power from the RF antenna to a plasma inside the chamber102. In one embodiment, the dielectric material used to form the first and second sections120,122is a high purity ceramic material that is chemically resistant to the process gases and that has good thermal properties. For example, in some embodiments, the dielectric material is 99.6% Al2O3or AlN. In other embodiments, the dielectric material is Yittria and YAG.

A lid124of the chamber top118is formed of a conductive material that extends a length across the second section122in the horizontal direction. In many embodiments, the conductivity of the material used to form the lid124is high enough to dissipate the heat load and to minimize charging effects that results from secondary electron emission. Typically, the conductive material used to form the lid124is chemically resistant to the process gases. In some embodiments, the conductive material is aluminum or silicon.

The lid124can be coupled to the second section122with a halogen-resistant O-ring made of fluoro-carbon polymer, such as an O-ring formed of Chemrz and/or Kalrex materials. The lid124is typically mounted to the second section122in a manner that minimizes compression on the second section122, but that provides enough compression to seal the lid124to the second section. In some operating modes, the lid124is RF and DC grounded as shown inFIG. 1.

In some embodiments, the chamber102includes a liner125that is positioned to prevent or greatly reduce metal contamination by providing line-of-site shielding of the inside of the plasma chamber102from metal sputtered by ions in the plasma striking the inside metal walls of the plasma chamber102. Such liners are described in U.S. patent application Ser. No. 11/623,739, filed Jan. 16, 2007, entitled “Plasma Source with Liner for Reducing Metal Contamination,” which is assigned to the present assignee. The entire specification of U.S. patent application Ser. No. 11/623,739 is incorporated herein by reference.

In various embodiments, the liner is a one-piece or unitary plasma chamber liner, or a segmented plasma chamber liner. In many embodiments, the plasma chamber liner125is formed of a metal base material, such as aluminum. In these embodiments, at least the inner surface125′ of the plasma chamber liner125includes a hard coating material that prevents sputtering of the plasma chamber liner base material.

Some plasma processes, such as plasma doping processes, generate a considerable amount of non-uniformly distributed heat on the inner surfaces of the plasma source101because of secondary electron emissions. In some embodiments, the plasma chamber liner125is a temperature controlled plasma chamber liner125. In addition, in some embodiments, the lid124comprises a cooling system that regulates the temperature of the lid124and surrounding area in order to dissipate the heat load generated during processing. The cooling system can be a fluid cooling system that includes cooling passages in the lid124that circulate a liquid coolant from a coolant source.

A RF antenna is positioned proximate to at least one of the first section120and the second section122of the chamber top118. The plasma source101inFIG. 1illustrates two separate RF antennas that are electrically isolated from one another. However, in other embodiments, the two separate RF antennas are electrically connected. In the embodiment shown inFIG. 1, a planar coil RF antenna126(sometimes called a planar antenna or a horizontal antenna) having a plurality of turns is positioned adjacent to the first section120of the chamber top118. In addition, a helical coil RF antenna128(sometimes called a helical antenna or a vertical antenna) having a plurality of turns surrounds the second section122of the chamber top118.

In some embodiments, at least one of the planar coil RF antenna126and the helical coil RF antenna128is terminated with a capacitor129that reduces the effective antenna coil voltage. The term “effective antenna coil voltage” is defined herein to mean the voltage drop across the RF antennas126,128. In other words, the effective coil voltage is the voltage “seen by the ions” or equivalently the voltage experienced by the ions in the plasma.

Also, in some embodiments, at least one of the planar coil RF antenna126and the helical coil RF antenna128includes a dielectric layer134that has a relatively low dielectric constant compared to the dielectric constant of the Al2O3dielectric window material. The relatively low dielectric constant dielectric layer134effectively forms a capacitive voltage divider that also reduces the effective antenna coil voltage. In addition, in some embodiments, at least one of the planar coil RF antenna126and the helical coil RF antenna128includes a Faraday shield136that also reduces the effective antenna coil voltage.

A RF source130, such as a RF power supply, is electrically connected to at least one of the planar coil RF antenna126and helical coil RF antenna128. In many embodiments, the RF source130is coupled to the RF antennas126,128by an impedance matching network132that matches the output impedance of the RF source130to the impedance of the RF antennas126,128in order to maximize the power transferred from the RF source130to the RF antennas126,128. Dashed lines from the output of the impedance matching network132to the planar coil RF antenna126and the helical coil RF antenna128are shown to indicate that electrical connections can be made from the output of the impedance matching network132to either or both of the planar coil RF antenna126and the helical coil RF antenna128.

In some embodiments, at least one of the planar coil RF antenna126and the helical coil RF antenna128is formed such that it can be liquid cooled. Cooling at least one of the planar coil RF antenna126and the helical coil RF antenna128will reduce temperature gradients caused by the RF power propagating in the RF antennas126,128.

In some embodiments, the plasma source101includes a plasma igniter138. Numerous types of plasma igniters can be used with the plasma source101. In one embodiment, the plasma igniter138includes a reservoir140of strike gas, which is a highly-ionizable gas, such as argon (Ar), which assists in igniting the plasma. The reservoir140is coupled to the plasma chamber102with a high conductance gas connection. A burst valve142isolates the reservoir140from the process chamber102. In another embodiment, a strike gas source is plumbed directly to the burst valve142using a low conductance gas connection. In some embodiments, a portion of the reservoir140is separated by a limited conductance orifice or metering valve that provides a steady flow rate of strike gas after the initial high-flow-rate burst.

A platen144is positioned in the process chamber102a height below the top section118of the plasma source101. The platen144holds a substrate146for plasma processing. In many embodiments, the substrate146is electrically connected to the platen144. In the embodiment shown inFIG. 1, the platen144is parallel to the plasma source101. However, in one embodiment of the present invention, the platen144is tilted with respect to the plasma source101to achieve various process goals.

A platen144is used to support a substrate146or other workpieces for processing. In some embodiments, the platen144is mechanically coupled to a movable stage that translates, scans, or oscillates the substrate146in at least one direction. In one embodiment, the movable stage is a dither generator or an oscillator that dithers or oscillates the substrate146. The translation, dithering, and/or oscillation motions can reduce or eliminate shadowing effects and can improve the uniformity of the ion beam flux impacting the surface of the substrate146.

A bias voltage power supply148is electrically connected to the platen144. The bias voltage power supply148is used to bias the platen144and the substrate146so that ions in the plasma are extracted from the plasma and impact the substrate146. In various embodiments, the ions can be dopant ions for plasma doping or inert or reactive ions for etching and deposition. In various embodiments, the bias voltage power supply148is a DC power supply, a pulsed power supply, or a RE power supply. In one embodiment of the plasma processing apparatus according the present invention, the bias voltage power supply148has an output waveform that is independent of the output waveform of the RF source130that powers at least one of the planar coil RF antenna126and helical coil RF antenna128. In another embodiment of the plasma processing apparatus according the present invention, the bias voltage power supply148has an output waveform that is synchronized to the output waveform of the RF source130that powers at least one of the planar coil RF antenna126and helical coil RF antenna128. The bias voltage power supply148and the RF source130can physically be the same power supply with two different outputs or can be separate power supplies.

A controller152is used to control the RF power supply130and the bias voltage power supply148to generate a plasma and to bias the substrate146so as to at least partially neutralize charge accumulation during plasma processing according to the present invention. The controller152can be part of the power supplies130,148or can be a separate controller that is electrically connected to control inputs of the power supplies130,148. The controller152controls the RF power supply130so that pulses are applied to either or both of the planar coil RF antenna126and the helical coil RF antenna128with at least two different amplitudes. Also, the controller152controls the RF power supply130and the bias voltage power supply148so that the pulses are applied to at least one of the planar coil RF antenna126and the helical coil RF antenna128, and also to the substrate146at relative times that at least partially neutralize charge accumulation during plasma processing according to the present invention.

One skilled in the art will appreciate that there are many different possible variations of the plasma source101that can be used with the features of the present invention. See, for example, the descriptions of the plasma sources in U.S. patent application Ser. No. 10/908,009, filed Apr. 25, 2005, entitled “Tilted Plasma Doping.” Also see the descriptions of the plasma sources in U.S. patent application Ser. No. 11/163,303, filed Oct. 13, 2005, entitled “Conformal Doping Apparatus and Method.” Also see the descriptions of the plasma sources in U.S. patent application Ser. No. 11/163,307, filed Oct. 13, 2005, entitled “Conformal Doping Apparatus and Method.” In addition, see the descriptions of the plasma sources in U.S. patent application Ser. No. 11/566,418, filed Dec. 4, 2006, entitled “Plasma Doping with Electronically Controllable implant Angle.” The entire specification of U.S. patent application Ser. Nos. 10/908,009, 11/163,303, 11/163,307 and 11/566,418 are herein incorporated by reference.

In operation, the controller152instructs the RF source130to generate RF currents that propagate in at least one of the RF antennas126and128. That is, at least one of the planar coil RF antenna126and the helical coil RF antenna128is an active antenna. The term “active antenna” is herein defined as an antenna that is driven directly by a power supply. In many embodiments of the plasma processing apparatus of the present invention, the RF source130operates in a pulsed mode. However, the RF source130can also operate in the continuous mode.

In some embodiments, one of the planar coil antenna126and the helical coil antenna128is a parasitic antenna. The term “parasitic antenna” is defined herein to mean an antenna that is in electromagnetic communication with an active antenna, but that is not directly connected to a power supply. In other words, a parasitic antenna is not directly excited by a power supply, but rather is excited by an active antenna in close proximity, which in the apparatus shown inFIG. 1Ais one of the planar coil antenna126and the helical coil antenna128powered by the RF source130. In some embodiments of the invention, one end of the parasitic antenna is electrically connected to ground potential in order to provide antenna tuning capabilities. In this embodiment, the parasitic antenna includes a coil adjuster150that is used to change the effective number of turns in the parasitic antenna coil. Numerous different types of coil adjusters, such as a metal short, can be used.

The RF currents in the RF antennas126,128then induce RF currents into the chamber102. The RF currents in the chamber102excite and ionize the process gas so as to generate a plasma in the chamber102. The plasma chamber liner125shields metal sputtered by ions in the plasma from reaching the substrate146.

The controller152also instructs the bias voltage power supply148to bias the substrate146with negative voltage pulses that attract ions in the plasma towards the substrate146. During the negative voltage pulses, the electric field within the plasma sheath accelerates ions toward the substrate146for plasma processing. For example, the electric field within the plasma sheath can accelerate ions toward the substrate146to implant the ions into the surface of the substrate146, to etch the surface of the substrate146, to produce a chemical reaction on the surface of the substrate146for either etching or deposition, or to grow a thin film on the surface of the substrate146. In some embodiments, a grid is used to extract ions in the plasma towards the substrate146in order to increase the energy of the ions.

When the RF source130and the bias voltage power supply148are operated in the pulse mode under some processing conditions, such as with relatively high duty cycles, charge can accumulate on the substrate146. Charge accumulation on the substrate146can result in the development of a relatively high potential voltage on the substrate146being plasma processed that can cause processing non-uniformities, arcing, and device damage. Charge accumulation on the substrate can be greatly reduced by generating multi-level RF waveforms with the RE source130and biasing the substrate146according to the present invention. In addition, certain process goals, such as process rates and process profiles, can be achieved by generating multi-level RF waveforms with the RF source130and biasing the substrate146according to the present invention.

FIG. 1Billustrates another embodiment of a plasma processing system170with charge neutralization according to the present invention. The plasma processing system170is a capacitive RF discharge system. Capacitive RF discharge plasma processing systems are well known in the industry. The plasma processing system170includes a process chamber172having a process gas inlet174that receives a feed gas from a mass flow controller which flow through the plasma discharge area. The process chamber172also includes an exhaust port175that is coupled to a vacuum pump that removes effluent gases. Typically a throttle valve is positioned in the exhaust port175that is coupled to a vacuum pump to control the pressure in the chamber172. Typically operating pressures are in the 10-1000 mT range.

The plasma processing system170includes two planar electrodes, which are often called parallel plate electrodes176. The parallel plate electrodes176are driven by an RF source178. The parallel plate electrodes176are separated by a gap that is in the range of 2-10 cm. A blocking capacitor180is electrically connected between the output of the RF source178and the parallel plate electrode176. The blocking capacitor180is used to remove DC and low frequency signals from the drive signal. The RF drive signal is typically in the 100-1000V range. The parallel plate electrodes176are typically driven by 13.56 MHz signal, but other frequencies are also suitable.

In conventional capacitive RF discharge plasma processing systems, the substrate is positioned directly on the bottom parallel plate. However, the plasma processing system170includes an insulator182that is positioned between the bottom plate and the substrate184. The insulator182allows the substrate184to be biased independently of the parallel plate electrodes176which are driven by the RF source178. A separate substrate bias voltage power supply186is used to bias the substrate184. An output of the substrate bias voltage power supply186is electrically connected to the substrate184that is positioned in the insulator182.

A controller188is used to control the RF power supply186and the bias voltage power supply186to generate a plasma and to bias the substrate184so as to at least partially neutralize charge accumulation during plasma processing according to the present invention. The controller188can be part of the power supplies178,186or can be a separate controller that is electrically connected to control inputs of the power supplies178,186. The controller188controls the RF power supply178so that multi-level RF pulses are applied to the parallel plate electrode176with at least two different amplitudes. Also, the controller188controls the RF power supply178and the bias voltage power supply186so that the RF pulses are applied to the parallel plate electrodes176at relative times that at least partially neutralize charge accumulation during plasma processing according to the present invention.

The operation of the plasma processing system170is similar to the operation of the plasma processing system100. The controller188instructs the RF source178to generate RF currents that propagate to the parallel plate electrodes176to generate a plasma between the parallel plates from the feed gas. The controller188also instructs the bias voltage power supply186to bias the substrate184with negative voltage pulses that attract ions in the plasma towards the substrate184. During the negative voltage pulses, the electric field within the plasma sheath accelerates ions toward the substrate184for plasma processing. For example, the electric field within the plasma sheath can accelerate ions toward the substrate184to implant the ions into the surface of the substrate184, to etch the surface of the substrate184, to produce a chemical reaction on the surface of the substrate184for either etching or deposition, or to grow a thin film on the surface of the substrate184.

When the RF source178and the bias voltage power supply186are operated under some processing conditions, charge can accumulate on the substrate184. Charge accumulation on the substrate184can result in the development of a relatively high potential voltage on the substrate184being plasma processed that can cause processing non-uniformities, arcing, and device damage. Charge accumulation on the substrate184can be greatly reduced by generating multi-level RF waveforms with the RF source178and biasing the substrate184according to the present invention. In addition, certain process goals, such as process rates and process profiles, can be achieved by generating multi-level RF waveforms with the RF source178and biasing the substrate184according to the present invention.

The methods and apparatus of the present invention can be applied to numerous other types of plasma processing systems. For example, the methods and apparatus of the present invention can be applied to ECR plasma processing systems, helicon plasma processing systems, and helicon resonator plasma processing systems. In each of these systems, the RF source generates a multi-amplitude pulsed RF waveform that has at least two RF power levels. Also, in many embodiments, the substrate is biased by a bias voltage power supply that generates a bias voltage waveform that can be synchronized to the RF waveform driving the plasma source with a controller.

FIG. 2Aillustrates a prior art waveform200generated by the RF source130having a single amplitude that can cause charge accumulation on the substrate146(FIG. 1) under some conditions. The waveform200is at ground potential until the plasma is generated with a pulse having a power level PRF202. The power level PRF202is chosen to be suitable for plasma doping and many plasma etching and plasma deposition processes. The pulse terminates after the pulse period TP204and then returns to ground potential. The waveform then periodically repeats.

FIG. 2Billustrates a prior art waveform250generated by the bias voltage supply148that applies a negative voltage252to the substrate146(FIG. 1) during plasma processing to attract ions in the plasma. The negative voltage252is applied during the period T1254when the waveform200generated by the RF source130has a power equal to the power level PRF202. The negative voltage252attracts ions in the plasma to the substrate146for plasma processing. The waveform200is at ground potential during the period T2256when the plasma processing is terminated. At relatively high duty cycles (i.e. greater than about 25% and in some cases greater than about 2%), charge tends to accumulate on the substrate146during the pulse period T1254when the waveform250generated by the RF source130has a power equal to the power level PRF202.

The methods and apparatus of the present invention allow plasma processing, such as plasma doping, plasma etching, and plasma deposition, to be performed at higher duty cycles by reducing the probability of damage caused by charging effects. There are numerous methods according to the present invention to power the plasma source101and to bias the substrate146being processed to at least partially neutralize charge accumulation on the substrate146.

FIG. 3Aillustrates a RF power waveform300generated by the RF source130(FIG. 1) according to the present invention that has multiple amplitudes for at least partially neutralizing charge accumulation on the substrate146(FIG. 1). The waveform300is pulsed and has a first302and a second power level304, which are indicated in the figure as PRF1and PRF2, respectively. However, it should be understood that waveforms with more than two amplitudes can be used in the methods of the present invention to at least partially neutralize charge accumulation on the substrate146. It should also be understood that the waveforms may or may not have discrete amplitudes. For example, the waveforms can be continuously changing. That is, in some embodiments, the waveforms can ramp with positive or negative slopes. Also, the waveforms can ramp in a linear or in a non-linear rate.

The first power level PRF1302is chosen to provide enough RF power to at least partially neutralize charge accumulation on the substrate146when the substrate146is not biased for plasma processing. The second power level PRF2304is chosen to be suitable for plasma processing, such as plasma doping, plasma etching, and plasma deposition. In various embodiments, the waveform300generated by the RF source130including the first and second power levels PRF1302, PRF2304is applied to one or both of the planar coil RF antenna126and the helical coil RF antenna128(seeFIG. 1). In one specific embodiment, the waveform300generated by the RF source130is applied to one of the planar coil RF antenna126and the helical coil RF antenna128when it is at the first power level PRF1302and is applied to the other of the planar coil RF antenna126and the helical coil RF antenna128when it is at the second power levels PRF2304. In another specific embodiment, the waveform300generated by the RF source130is applied to one of the planar RF antenna126and the helical coil RF antenna128when it has a first frequency and is applied to the other of the planar coil RF antenna126and the helical coil RF antenna128when it has a second frequency that is different from the first frequency as described in connection withFIGS. 5A-5C.

The waveform300shown inFIG. 3Aindicates that the first power level PRF1302is greater than the second power level PRF2304. However, in other embodiments, the first power level PRF1302is less than the second power level PRF2304. Also, in some embodiments, the waveform300includes a third power level that is zero or some relatively low power level when the substrate146is not biased for plasma processing as described in connection withFIG. 6.

The waveform300also indicates a first pulse period TP1306corresponding to the time period were the waveform300has a power equal to the first power level PRF1302and a second pulse period TP2308corresponding to the time period were the waveform has a power equal to the second power level PRF2304. The total multi-amplitude pulse period for the waveform300TTotal310is the combination of the first pulse period TP1306and the second pulse period TP2308. For example, in one embodiment, the first and second pulse periods TP1306, TP2308are both in the range of 30-500 μs and the total pulse period TTotal310is in the range of 60 μs-1 ms. In other embodiments, the total pulse period TTotal310can be on order of 1 ms or greater.

FIG. 3Aindicates that the frequency of the waveform300during the first pulse period TP1306is the same as the frequency of the waveform300during the second pulse period TP2308. However, it should be understood that in various embodiments, the frequency of the waveform300during the first pulse period TP1306can be different from the frequency of the waveform300during the second pulse period TP2308as described in connection withFIGS. 5A-5C. In addition, the frequency of the waveform300can be changed within at least one of the first and the second pulse periods TP1,306, TP2,308.

Thus, in some embodiments, the waveform300includes both multiple frequencies and multiple amplitudes that are chosen to at least partially neutralize charge accumulation during plasma processing. In addition, in some embodiments, the waveform300includes both multiple frequencies and multiple amplitudes that are chosen to improve certain process parameters, such as the retained dose for plasma doping. Furthermore, in some embodiments, the waveform300includes both multiple frequencies and multiple amplitudes that are chosen to assist in achieving certain process goals. For example, the waveform300can include both multiple frequencies and multiple amplitudes to improve process control and to increase process rates.

Also, the waveform300can include both multiple frequencies and multiple amplitudes to achieve knock-on ion implants to form retrograde doping profiles. Also, the waveform300can include both multiple frequencies and multiple amplitudes to achieve certain etching profiles and etching process goals, such as achieving high aspect-ratio etching profiles. In addition, the waveform300can include both multiple frequencies and multiple amplitudes to achieve certain deposition profiles and process goals, such as depositing material into high aspect-ratio structures, depositing conformal or near conformal coating, and filling gaps in trenches and other device structures.

FIG. 3Billustrates a bias voltage waveform350generated by the bias voltage supply148(FIG. 1) according to the present invention that applies a negative voltage352to the substrate146during plasma processing to attract ions. The bias voltage waveform350is synchronized with the RF power waveform300. However, it should be understood that the pulses in the bias voltage waveform350are not necessarily aligned with the pulses in the RF power waveform300. The negative voltage352is applied during the second pulse period TP2308when the waveform350generated by the RF source130has a power equal to the second power level PRF2304. The waveform350is at wound potential during the first pulse period TP1306when the plasma processing is terminated and the waveform300has a power equal to the first power level PRF1302.

Applying a waveform to the plasma source101(FIG. 1) with two different power levels where the first power level PRF1302is applied by the RF source130during the period TP1306when the waveform350generated by the bias voltage supply148(FIG. 1) is at ground potential will assist in neutralizing charge accumulated on the substrate146(FIG. 1). Electrons in the corresponding plasma will neutralize at least some of the charge accumulated on the substrate146.

FIG. 3Cillustrates a waveform360generated by the bias voltage supply148(FIG. 1) according to the present invention that applies a negative voltage362to the substrate146during plasma processing to attract ions and that applies a positive voltage364to the substrate146after plasma processing is terminated to assist in neutralizing charge on the substrate146. The negative voltage362is applied during the second pulse period TP2308when the waveform300generated by the RF source130has a power equal to the second power level PRF2304. The waveform360is at a positive potential364during the first pulse period TP1306when the waveform300generated by the RF source130has a power equal to the first power level PRF1302.

Applying a waveform to the plasma source101(FIG. 1) with two different power levels where the first power level PRF1302is applied by the RF source130(FIG. 1) during the first period TP1306when the waveform360generated by the bias voltage supply148(FIG. 1) is at a positive potential364will assist in neutralizing charge accumulated on the substrate146(FIG. 1). Electrons in the corresponding plasma will neutralize at least some the charge accumulated on the substrate146. In addition, the positive voltage364applied the substrate146will also neutralize at least some of the charge accumulated on the substrate146.

FIGS. 4A-Cillustrate a RF power waveform400generated by the RF source130(FIG. 1) and bias voltage waveforms402,404generated by the bias voltage supply148(FIG. 1) according to the present invention that are similar to the waveforms300,350, and360described in connection withFIGS. 3A-3C, but that are displaced in time relative to the waveforms300,350, and360so as to perform plasma process with both the first and the second power levels PRF1302, PRF2304. In this embodiment, the RF power waveform400and the bias voltage waveforms402,404are synchronized, but the pulses in the RF power waveform400are not aligned with the pulses in the bias voltage waveforms402,404.

Changing the power generated by the RF source130during plasma processing allows the user to more precisely control the amount of charge that is accumulating on the surface of the substrate146during plasma processing to achieve certain process goals and effects. For example, increasing the power near the end of the second pulse period TP2308will enhance the neutralization of charge accumulated on the substrate146.

FIGS. 5A-Cillustrates a RF power waveform500generated by the RF source130(FIG. 1) with a variable frequency and corresponding bias voltage waveforms502,504generated by the bias voltage supply148(FIG. 1) according to another embodiment of the present invention. The waveform500is similar to the waveforms300,400described in connection withFIGS. 3 and 4. However, the RF powers in the first and second pulse periods TP1306, TP2308are the same and the frequencies in the first and second pulse periods TP1306, TP2308are different. Changing the frequency of the waveform500changes the ion/electron density and, therefore, changes the charge neutralization efficiency.

Thus, in one embodiment, the frequency of the waveform500in the first pulse period TP1306is different from the frequency of the waveform500in the second pulse period TP2308and these frequencies are chosen to at least partially neutralize charge accumulation during plasma processing. The waveforms502,504are similar to the waveforms350and360that were described in connection withFIG. 3. In other embodiments, the waveforms502,504are displaced in time relative to the waveform500, similar to the displacement of waveforms402,404that were described in connection withFIG. 4.

In addition, in one aspect of the present invention, parameters, such as the multiple power levels generated by the RF source130, the frequency of the waveform500in the first and second pulse periods TP1306, TP2308, and the relative timing of the waveform500with respect to the waveforms generated by the bias voltage supply148(FIG. 1), are chosen to achieve certain process goals. For example, generating multiple power levels with the RF source130where one power level is generated by the RF source130when the bias voltage is at ground potential allows the user to use less power during plasma processing and/or to reduce process times because some plasma processing will occur when the bias voltage is at ground potential.

Also, in one embodiment of the present invention, at least one of the multiple power levels generated by the RF source130(FIG. 1), the frequency of the waveform500in at least one of the first and second pulse periods TP1306, TP2308, and the relative timing of the waveform500with respect to the waveforms generated by the bias voltage supply148(FIG. 1) are chosen to improve the retained dose on the substrate146(FIG. 1) when performing plasma doping. For example, using less power during plasma processing will result in less deposition and, therefore, a higher retained dose in the substrate. The operating pressure, gas flow rates, type of dilution gas, and plasma source power can also be selected to further improve the retained dose with this method.

Also, in another embodiment of the present invention, at least one of the multiple power levels generated by the RF source130(FIG. 1), the frequency of the waveform500in at least one of the first and second pulse periods TP1306, TP2308, and the relative timing of the waveform500with respect to the waveforms generated by the bias voltage supply148are chosen to improve sidewall coverage during plasma processing. The term “improve sidewall coverage” is referred to herein as increasing the ratio of the deposition rate of material on the sidewall to the deposition rate of material on the surface of the surface of the substrate perpendicular to the ion flux. Achieving better sidewall coverage is important for many applications, such as conformal doping and conformal deposition applications. For example, many three-dimensional and other state-of-the-art devices required conformal doping and conformal deposition.

Also, in another embodiment of the present invention, waveforms are generated by the RF source130(FIG. 1) with certain multiple power levels, multiple frequencies, and relative timings with respect to the waveforms generated by the bias voltage supply148(FIG. 1) so as to create knock-on ion implants for plasma doping. The term “knock-on ion implant” is defined herein as a recoil ion implant where an ion is implanted through the surface layers of the substrate146to drive the dopant material into the substrate146.

The ions used for the knock-on ion implant can be an inert ion species, such as He, Ne, Ar, Kr and Xe, which can be formed from an inert feed gas. In some embodiments, the mass of the knock-on ions is chosen to be similar to a mass of the desired dopant ions. The RF source130(FIG. 1) generates a RF power that is sufficient to direct the knock-on ions toward the substrate146(FIG. 1) with enough energy to physically knock the deposited dopant material into both the planar and non-planar features of the substrate146(FIG. 1) upon impact. Also, the operating parameters, such as chamber pressure, gas flow rate, plasma source power, gas dilution, and duty cycle of pulsed bias supply, can be chosen to enhance knock-on ion implants.

Knock-on ion implant can be used to form retrograde doping profiles. The waveforms are generated by the RF source130(FIG. 1) with certain multiple power levels, multiple frequencies, and relative timings with respect to the waveforms generated by the bias voltage supply148so as to create a retrograde profile, such as a retrograde doping profile or a retrograde deposited film profile. The term “retrograde profile” is defined herein as a profile where the peak concentration of the profile is below the surface of the substrate. See, for example, U.S. patent application Ser. No. 12/044,619, entitled “A Method of Forming a Retrograde Material Profile Using Ion Implantation, which is assigned the present assignee. The entire specification of U.S. patent application Ser. No. 12/044,619 is incorporated herein by reference.

For plasma doping, it is sometimes desirable to form retrograde ion implant dopant profiles because it is difficult to precisely control the depth of ion implanted layers for many reasons. For example, during plasma doping, there could be some unintentional etching of the surface of the substrate caused by physical sputtering and chemical etching. In addition, there could be some unintentional deposition on the surface of the substrate. Furthermore, there can be a significant ion implant energy distribution due to many factors, such as the presence of multiple ion species, collisions between ions, non uniformities in the plasma sheath, presence of secondary electron emissions, displacements currents formed due to parasitic impedances, and the application of non-ideal bias pulses.

In addition, it is sometimes desirable to form retrograde ion implant dopant profiles because surface-peak dopant profiles are very sensitive to post deposition or post implant processes since most of the maximum peak concentration of deposited or implanted material is located at or near the surface of the substrate. In particular, the photo-resist strip process typically performed after implantation will remove a significant amount of dopant material near the surface.

In other embodiments, the waveforms are generated by the RF source130with certain multiple power levels, multiple frequencies, and relative timings with respect to the waveforms generated by the bias voltage supply148so as to achieve certain process goals or process profiles, such as etching profiles. For example, the multiple power levels, multiple frequencies, and relative timings with respect to the waveforms generated by the bias voltage supply148can be chosen to achieve high aspect-ratio etching profiles or certain types of deposition profiles.

One skilled in the art will appreciate that waveforms generated by the RF source130(FIG. 1) according to the present invention can have both multiple amplitudes and multiple frequencies and can have various relative timings with respect to the waveforms generated by the bias voltage supply148(FIG. 1). In fact, there are an almost infinite number of possible waveforms with multiple power levels and multiple frequencies that can be generated by the RF source130(FIG. 1) and relative timing with respect to the waveforms generated by the bias voltage supply148(FIG. 1) that will at least partially neutralize charge and/or achieve the process goals described herein.

FIG. 6illustrates measured multi-set-point RF power and control signal waveforms600according to one embodiment of the present invention. The waveforms600include RF power and control signal waveforms as a function of time beginning at time t0. The waveforms600show an ion implantation period602, a charge neutralization period604, and a power off period606.

Referring toFIGS. 1 and 6, at time t0, the controller152(FIG. 1) generates an implant pulse608that instructs the bias voltage power supply148(FIG. 1) to bias the substrate146(FIG. 1) with a negative voltage pulse that attracts ions in the plasma towards the substrate146. The rise time of the implant pulse602is about 30 microseconds. Also, at time t0the controller152generates a RF pulse control signal that initiates a RF power waveform610having a first power level. In the ion implantation period602, the controller152generates a first RF pulse control signal612that causes RF currents to flow in at least one of the RF antennas126and128(FIG. 1) thereby striking a plasma. The rise time of the first RF pulse control signal612is about 30 microseconds.

The charge neutralization period604begins when the first RF pulse control signal612and the implant pulse signal608both return to zero. The fall time of the first RF pulse control signal and the implant pulse control signal is about 20 microseconds. In the charge neutralization period604, the controller152generates a second RF pulse control signal614that ramps the RF power waveform610to a second power level. In many embodiments, the second power level is greater than the first power level as shown inFIG. 6. However, in other embodiments, the second power level can be any power level including a power level that is lower than the first power level. The rise time of the second RF pulse control signal is also about 30 microseconds. In the charge neutralization period604, at least some of the charge on the substrate146is efficiently neutralized by electrons in the plasma. This partial or complete charge neutralization reduces undesirable charging effects on the substrate146.

The power off period606begins when the second RF pulse control signal614returns to zero. The fall time of the second RF pulse control signal614is about 20 microseconds. In the power off period606, the RF power is extinguished, which terminates the plasma. The methods of plasma processing with enhanced charge neutralization according to the present invention can be employed with many different multi-set-point RF power and control signal waveforms600.

It should be understood that the methods for charge neutralization according to the present invention can be used with numerous other types of plasma processing apparatus. For example, the methods for charge neutralization can be used with plasma processing apparatus that have inductively coupled plasma (ICP) sources, helicon resonator plasma sources, microwave plasma sources, ECR plasma source, and capacitive coupled plasma sources. In fact, any type of plasma source that can be operated in a pulsed mode can be used to perform the methods of the present invention.

EQUIVALENTS

While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art, may be made therein without departing from the spirit and scope of the invention.