Ion mass separation using RF extraction

An apparatus which has the capability of filtering unwanted species from an extracted ion beam without the use of a mass analyzer magnet is disclosed. The apparatus includes an ion source having chamber walls that are biased by an RF voltage. The use of RF extraction causes ions to exit the ion source at different energies, where the energy of each ion species is related to its mass. The extracted ion beam can then be filtered using only electrostatic energy filters to eliminate the unwanted species. The electrostatic energy filter may act as a high pass filter, allowing ions having an energy above a certain threshold to reach the workpiece. Alternatively, the electrostatic energy filter may act as a low pass filter, allowing ions having an energy below a certain threshold to reach the workpiece. In another embodiment, the electrostatic energy filter operates as a bandpass filter.

FIELD

Embodiments relate to an apparatus for performing mass separation, and more particularly, an ion source having RF extraction and an electrostatic energy filter disposed downstream from the ion source.

BACKGROUND

Ions are used in a plurality of semiconductor processes, such as implantation, amorphization, deposition and etching processes. These ions may be created within an ion source chamber and extracted through an extraction aperture in the ion source chamber.

There are several different types of ion implantation systems. A first type is referred to as a beam-line system. In a beam-line system, ions are extracted from an ion source, pass through a mass analyzer to select particular ions based on their mass to charge ratio, and are made into a parallel ribbon ion beam. Beam-line systems may also include deceleration stages, and other components to make the resulting ribbon ion beam more uniform.

A second type of ion implantation system is referred to as a plasma immersion ion implantation system. In these systems, the workpiece is disposed within the plasma chamber. Ions from the plasma are attracted toward the workpiece by negatively biasing the workpiece relative to the plasma.

A third type of ion implantation system uses a plasma chamber to create a plasma. Ions from that plasma are extracted through an extraction aperture and accelerated directly toward a workpiece, which is disposed outside the plasma chamber.

Each type of ion implantation has advantages and shortcomings. For example, the third type of system is relatively low cost and generates ion beams having high current, but lacks the capability to filter out unwanted species. It would be beneficial if there were an apparatus that retains the advantages of this third type of system, but was also able to filter unwanted species from the extracted ribbon ion beam.

SUMMARY

An apparatus which has the capability of filtering unwanted species from an extracted ion beam without the use of a mass analyzer magnet is disclosed. The apparatus includes an ion source having chamber walls that are biased by an RF voltage. Disposed outside the extraction aperture of the ion source are extraction optics, which may be grounded or DC biased. The use of RF extraction causes ions to exit the ion source at different energies, where the energy of each ion species is related to its mass. The extracted ion beam can then be filtered using only electrostatic energy filters to eliminate the unwanted species. The electrostatic energy filter may act as a high pass filter, allowing ions having an energy above a certain threshold to reach the workpiece. Alternatively, the electrostatic energy filter may act as a low pass filter, allowing ions having an energy below a certain threshold to reach the workpiece. In another embodiment, the electrostatic energy filter operates as a bandpass filter.

According to one embodiment, an apparatus for extracting an ion beam is disclosed. The apparatus comprises an ion source having a plurality of chamber walls defining an ion source chamber, wherein one of the chamber walls comprises an extraction plate having an extraction aperture, wherein the extraction plate is biased using an RF voltage; extraction optics, disposed outside the ion source chamber, to extract an ion beam from the ion source chamber through the extraction aperture; and an electrostatic energy filter disposed downstream from the extraction optics to selectively allow certain ions from the ion beam to reach a workpiece. In certain embodiments, the extraction optics are DC biased. In certain embodiments, the electrostatic energy filter uses only electric fields to manipulate the ion beam. In some embodiments, the electrostatic energy filter functions as a high pass filter, passing ions having an energy greater than a first predetermined value. In other embodiments, the electrostatic energy filter functions as a low pass filter, passing ions having an energy less than a second predetermined value. In yet other embodiments, the electrostatic energy filter functions as a band pass filter, passing ions having an energy between a first predetermined value and a second predetermined value. In certain embodiments, the apparatus comprises a second electrostatic energy filter disposed between the electrostatic energy filter and the workpiece.

According to another embodiment, an apparatus for extracting an ion beam is disclosed. The apparatus comprises an ion source having a plurality of chamber walls defining an ion source chamber, wherein one of the chamber walls comprises an extraction plate having an extraction aperture, wherein the extraction plate is biased using an RF voltage; extraction optics, disposed outside the ion source chamber, to extract an ion beam from the ion source chamber through the extraction aperture; and an electrostatic energy filter disposed downstream from the extraction optics, wherein the electrostatic energy filter comprises at least one electrode comprising an aperture therethrough, to selectively allow ions from the ion beam having a certain energy to pass through the aperture and to reach a workpiece. In certain embodiments, the electrostatic energy filter comprises a first electrode biased at a first positive voltage, such that ions having an energy less than the first positive voltage are repelled by the first electrode, so that ions having an energy greater than the first positive voltage pass through the aperture. In a further embodiment, the apparatus further comprises a second electrode biased at a negative voltage disposed between the first electrode and the workpiece to accelerate ions passing through the aperture of the first electrode. In certain embodiments, the electrostatic energy filter comprises an entry electrode, an exit electrode and at least one central electrode, each electrode comprising an upper plate and a lower plate, defining an aperture therebetween, where the upper plate and lower plate are independently biased, and wherein apertures of the electrodes are not linearly aligned. In certain embodiments, the electrostatic energy filter comprises a plurality of central electrodes. In certain embodiments, the electrostatic energy filter comprises an entry electrode, an exit electrode and at least one central electrode, each electrode comprising two independently biased spaced apart conductive rods, defining an aperture therebetween, wherein apertures of each electrode are not linearly aligned.

According to another embodiment, an apparatus for extracting an ion beam is disclosed. The apparatus comprises an ion source configured to extract an ion beam, wherein each species of ions in the ion beam has a unique ion energy distribution function; and an electrostatic energy filter, disposed downstream from the ion source, to selectively pass certain species of ions toward a workpiece, based on the unique ion energy distribution function. In certain embodiments, the electrostatic energy filter comprises a resolving aperture, such that only ions having a desired energy pass through the resolving aperture and are directed toward the workpiece. In some embodiments, the apparatus comprises a second electrostatic energy filter, disposed between the electrostatic energy filter and the workpiece, to selectively pass certain species of ions toward the workpiece, based on the unique ion energy distribution function.

DETAILED DESCRIPTION

FIG. 1shows a first embodiment of an apparatus that may be used to separate ions according to mass using only electrostatic energy filters. The apparatus includes an ion source100. The ion source100comprises a plurality of chamber walls111defining an ion source chamber110. An RF antenna120may be disposed against a dielectric window113. This dielectric window113may comprise part or all of one of the chamber walls111. The RF antenna120may comprise an electrically conductive material, such as copper. An RF power supply130is in electrical communication with the RF antenna120. The RF power supply130may supply an RF voltage to the RF antenna120. The power supplied by the RF power supply130may be between 0.1 and 10 kW and may be any suitable frequency, such as between 1 and 15 MHz. Further, the power supplied by the RF power supply130may be pulsed.

While the figures show the RF antenna120disposed against a dielectric window outside the ion source chamber110, other embodiments are also possible. For example, the plasma may be generated in a different manner, such as by a Bernas ion source, a capacitively coupled plasma (CCP) source, an indirectly heated cathode (IHC or another plasma source). The manner in which the plasma is generated is not limited by this disclosure.

In certain embodiments, the chamber walls111may be electrically conductive, and may be constructed of metal. In certain embodiments, these chamber walls111may be electrically biased by bias power supply140. The bias voltage applied to the chamber walls111establishes the potential of the plasma within the ion source chamber110. The difference between the electrical potential of the plasma and the electrical potential of the ground electrode180may help determine the energy that the extracted ions possess.

One chamber wall, referred to as the extraction plate112, includes an extraction aperture115. The extraction aperture115may be an opening through which the ions generated in the ion source chamber110are extracted and directed toward a workpiece10. The extraction aperture115may be any suitable shape. In certain embodiments, the extraction aperture115may be oval or rectangular shaped, having one dimension, referred to as the length, which may be much larger than the second dimension, referred to as the height. In certain embodiments, the length of the extraction aperture115may be as large as two meters or more. As described above, in certain embodiments, all of the chamber walls111and the extraction plate112are electrically conductive. In other embodiments, only the extraction plate112is electrically conductive and in communication with the bias power supply140. The remaining chamber walls111may be made of a dielectric material. The bias power supply140may bias the chamber walls111and the extraction plate112at a RF voltage of between 0.5 kV and 10 kV, and a frequency of between 0.1 and 50 MHz.

Disposed outside and proximate the extraction aperture115are extraction optics. In certain embodiments, the extraction optics comprises a ground electrode180. The ground electrode180may be a single electrically conductive component with a ground aperture185disposed therein. Alternatively, the ground electrode180may be comprised of two electrically conductive components that are spaced apart so as to create the ground aperture185between the two components. The ground electrode180may be a metal, such as titanium. The ground electrode180may be electrically connected to ground. Of course, in other embodiments, the ground electrode180may be biased using a separate power supply. The extraction aperture115and the ground aperture185are aligned.

In other embodiments, the extraction optics may be more complex. For example, the extraction optics may include one or more additional electrodes. For example, there may be one or more electrodes that are disposed between the extraction plate112and the ground electrode180. In other embodiments, there may be one or more electrode disposed between the ground electrode180and the electrostatic energy filter200. The configuration of the extraction optics may vary and is not limited by this disclosure.

Located downstream from the ground electrode180is an electrostatic energy filter200. The electrostatic energy filter200, unlike traditional mass analyzers, utilizes only electrical fields to filter the ion beam1based on mass. More particularly, the electrostatic energy filter200uses electrical fields to manipulate the energy of the ion beam1. The energies of the ions that are extracted from the ion source100are related to their mass. Thus, the electrostatic energy filter200has the effect of filtering the ion beam based on the mass of the ions. Since the mass of each species is different, the electrostatic energy filter200has the effect of filtering based on the type of species. Further, as described above, this filtering is performed without the use of magnets or magnetic fields.

In operation, feed gas from a gas storage container150is introduced to the ion source chamber110through a gas inlet151. The RF antenna120is energized by the RF power supply130. This energy excites the feed gas, causing the creation of a plasma. Ions in that plasma are typically positively charged. Because the ground electrode180is more negatively biased than the chamber walls111and the extraction plate112, the ions exit the extraction aperture115in the form of an ion beam1. The ion beam1passes through the extraction aperture115, the ground aperture185, the electrostatic energy filter200and travels toward the workpiece10.

As described above, the bias power supply140supplies an RF voltage to the chamber walls111through a blocking capacitor141. The RF voltage output from the bias power supply140may be in the form: Vbias=V0sin(2πf), where V0is the amplitude of the bias voltage and f is the frequency of the bias voltage.

Once enabled, the ion source will develop an average positive potential, referred to as the self-bias voltage. This self-bias voltage is typically a positive value. Thus, the voltage of the plasma can be expressed as: Vplasma=Vselfbias+V0sin(2πf), where Vselfbiasis the self-bias voltage, V0is the amplitude of the bias voltage and f is the frequency of the bias voltage.

Ions are attracted toward the ground electrode180whenever the electrical potential of the plasma is positive relative to the ground electrode180. If Vselfbiasis greater than or equal to V0, the plasma will always be more positive than the ground electrode180. If Vselfbiasis less than V0, the plasma will be more positive than the ground electrode180whenever sin(2πf) is greater than −Vselfbias/V0. The self-bias voltage may be a function of V0, the size of the ground electrode180, the size of the extraction plate112, and the frequency of the bias voltage.

However, since the ions have a finite mass and inertia, the ions cannot react instantaneously to the changing plasma potential. For example, heavy ions cannot respond to the high frequency changes in the RF voltage. As a result, these heavy ions are all extracted with an energy that is roughly equal to the self-bias voltage. Lighter ions have less inertia and therefore have an ability to react to the changing plasma potential. Thus, lighter ions are extracted at different energies than heavier ions.

Stated differently, different ions have different ion energy distribution functions. For example,FIG. 2shows the ion energy distribution for various species being extracted from an ion source which utilizes an RF voltage having a frequency of 13.56 MHz. Line300shows the ion energy distribution function for a heavy species of ions. Note that nearly all of these ions have the same energy, as the ion energy distribution function of the heavy species is one very narrow peak303. The energy level of these heavy species at this narrow peak303may be very close to the self-bias voltage of the plasma, as described above. Further, the distribution of the heavy species is very narrow, extending between line301and line302.

Line320shows the ion energy distribution function for a very light species of ion. Note that the light species are extracted at a much broader range of energy levels, extending from line321to line322. However, there is one large peak323that occurs at an energy much lower than the self-bias voltage, and one smaller peak324at an energy much greater than the self-bias voltage.

Line310shows the ion energy distribution function of a species having a mass between the heavy species and the light species. Like the light species, line310has a dual peak profile, where each of those peaks are closer to the self-bias voltage than the peaks for the light species. Specifically, the peak313is at a higher energy than peak323, while peak314is at a lower energy than peak324. Further, the amplitude of these two peaks313,314are somewhat greater than the amplitudes of the peaks323,324of the light species. This may be because all of the medium mass ions are extracted over a narrower range of energies. The range of energies for the medium mass species is between line311and line312.

Based on this graph, it can be seen that the ion energy distribution is tighter (i.e. span a smaller range of energies) for heavier species. Lighter species have a much wider distribution of energies. Thus, the use of an extraction voltage that includes an RF component results in an ion beam1that has multiple species, where each of those species has a particular ion energy distribution function that is related to its mass.

The fact that the extracted ions in the ion beam1have a unique ion energy distribution function may be exploited to filter unwanted species from the extracted ion beam.

An electrostatic energy filter200may be used to select one or more of these extracted species. The complexity of the electrostatic energy filter200may be varied, based on the desired species. For example, referring toFIG. 2, if it is desired to have an ion beam that comprises only the light species, the electrostatic energy filter200may be used to only allow ions having an energy greater than that represented by line312to pass to the workpiece10. Alternatively, the electrostatic energy filter200may be used to only allow ions having an energy less than that represented by line311to pass to the workpiece10.

In one embodiment, shown inFIG. 3A, the electrostatic energy filter200may comprise a single conductive plate400, having an aperture401, that is biased at a predetermined voltage, such as a voltage that is slightly greater that that represented by line312. All ions that are extracted with an energy level less than this predetermined voltage will be repelled by the conductive plate400, and will not pass through the electrostatic energy filter200. Ions having an energy greater than the predetermined voltage will pass through the aperture401and travel toward the workpiece10.

Thus, in one embodiment, the electrostatic energy filter200may be one conductive plate that are biased at a predetermined voltage to form a high pass filter so as only to pass the lightest species.

Of course, the electrostatic energy filter200may comprise a plurality of conductive plates. For example,FIG. 3Bshows an electrostatic energy filter200having a conductive plate400with an aperture401, that is positively biased at a first predetermined voltage to repel ions having an ion energy less than the first predetermined voltage. A second conductive plate410, having an aperture411, may be disposed downstream from the conductive plate400. This second conductive plate410may be negatively biased so as to accelerate the ions that pass through the aperture401. In this particular embodiment, the apertures401,411may be aligned so that the ions travel in a straight path.

WhileFIGS. 3A and 3Bshow an embodiment where the electrostatic energy filter200behaves as a high pass filter, other configurations are also possible.FIG. 4Ashows one such configuration. In this configuration, the electrostatic energy filter200includes an entry electrode510, an exit electrode520, and one or more central electrodes500disposed between the entry electrode510and the exit electrode520. Each electrode is a conductive material, such as a conductive plate. Further, each electrode comprises an aperture. Each electrode comprises two spaced apart conductive plates, where the space between the two plates forms the aperture. These spaced apart plates may be referred to as the upper plate and the lower plate of the electrode. The upper plate and the lower plate of each electrode may be independently biased at different voltages to cause the ions to be deflected. Further, each central electrode500may be biased independently of every other central electrode500. The apertures of the electrodes are not linearly aligned. In the way, the ion beam is deflected as it passes through the electrostatic energy filter200.

In certain embodiments, disposed downstream from the exit electrode520is a plasma flood gun region580. The plasma flood gun region580comprises a tunnel590into which electrons are targeted. The tunnel590also has an entrance aperture591and an exit aperture592. The entrance aperture591may also function as a resolving aperture, as described in more detail below.

As seen inFIG. 4A, the path of the ion beam1is not linear. In fact, the bias applied to each of the upper and lower plates of the central electrodes500creates electric fields that cause the ion beam1to be deflected as it passes through apertures in each of the central electrodes500. If all of the ions had the same energy, the electrical fields in the electrostatic energy filter200would cause all of the ions to follow the path of the ion beam1shown inFIG. 4A. In other words, the electrical fields cause the ions to deflect downward. However, as described above, with respect toFIG. 2, the ions that are extracted from the ion source100have different energies.

FIG. 4Bshows another embodiment of the electrostatic energy filter200. In this embodiment, the conductive plates that make up each electrode are replaced by conductive rods. Thus, the electrostatic energy filter200is still made up of an entry electrode560, an exit electrode570and one or more central electrodes550. However, each of these electrodes is made up of two spaced apart conductive rods, having an aperture therebetween. Each of the spaced apart conductive rods may be independently biased. The operation of this embodiment is the same as that described with respect toFIG. 4A.

The electrical fields in the electrostatic energy filter200cause these ions to follow different paths.FIGS. 5A-5Cshow three different scenarios using the electrostatic energy filter200ofFIG. 4B. For example, ions having the desired energy may follow the path of ion beam1. This is shown inFIG. 5A. The ions pass through the electrostatic energy filter200, and through the entrance aperture591. The ions exit the tunnel590and travel toward the workpiece10.

Ions having greater than the desired energy may not be deflected downward to the same extent. Thus, ions with too much energy may not pass through the entrance aperture591.FIG. 5Bshows the projected path593of ions that have greater energy than desired. Note that the projected path593of these ions is deflected to a lesser extent than those inFIG. 5A. Consequently, the ions are not deflected sufficiently so as to pass through the entrance aperture591. Thus, the entrance aperture591functions as a resolving aperture, permitting only ions with a prescribed energy to pass.

Similarly, ions having less than the desired energy may be deflected to a greater extent than desired. These ions with too little energy may strike the lower part of one of the central electrodes550, or may not pass through the entrance aperture591.FIG. 5Cshows the projected path594of ions that have less energy than desired. In this figure, the ions are pulled back toward one of the central electrodes550.

Thus,FIGS. 5A-5Cshow that only ions of the desired energy pass through the electrostatic energy filter200and reach the workpiece10.

WhileFIGS. 4A-4B and 5A-5Cshow a plasma flood gun region580disposed after the electrostatic energy filter200, there are other embodiments. For example, a resolving aperture600may be disposed after the electrostatic energy filter200, as shown inFIG. 6. This resolving aperture600may have the effect of stopping ions having an energy different than the desired energy level. In other words, the resolving aperture600may perform the same function as the tunnel590in the plasma flood gun region580. WhileFIG. 6shows the embodiment ofFIG. 4Awith a resolving aperture600, the resolving aperture may also be used with the embodiment ofFIG. 4B.

Further, these various embodiments can be combined. For example, the high pass filter ofFIG. 3Amay be used in conjunction with the electrostatic energy filter200ofFIG. 4A or 4B. Further, the electrostatic energy filter200ofFIG. 4A or 4Bmay be cascaded, where each filter is designed to pass a particular band of energies.

In operation, the frequency and amplitude of the bias voltage may be selected to provide optimum ion energy distribution functions and ion beam quality, taking into account subsequent filtering. For example, the frequency may be selected to create a desired energy gap between the peak of interest and the other peaks (seeFIG. 2). In certain embodiments, the bias power supply140may be a variable power supply so that the frequency may be modified based on the desired species. As described above, the spacing between peaks may be dependent on the frequency of the bias voltage.

Further, while this disclosure describes a sinusoidal bias voltage, other embodiments are also possible. For example, the bias voltage may be a sawtooth shape. In certain embodiment, the bias voltage comprises a multifrequency RF signal.

The present apparatus has many advantages. First, by utilizing a modulated bias voltage, it is possible to separate an ion beam by mass without the use of expensive magnets. Further, the size of the system is much reduced by the use of a modulated bias voltage with an electrostatic energy filter. Additionally, in some embodiments, the electrostatic energy filter may simply be a biased conductive plate, further reducing the cost and size of the apparatus. Further, tuning of the electrostatic energy filter may be more straightforward.