Methods of directing magnetic fields in a plasma source, and associated systems

A plasma source includes a plasma vessel that includes a dielectric material that encloses a cavity of a toroidal shape. The toroidal shape defines a toroidal axis therethrough. The vessel forms input and output connections, each of the input and output connections being in fluid communication with the cavity. One or more metal plates are disposed adjacent to the plasma vessel for cooling the plasma vessel. A magnetic core is disposed along the toroidal axis such that respective first and second ends of the magnetic core extend beyond axially opposed sides of the plasma vessel. First and second induction coils are wound about the respective first and second ends of the magnetic core. A plasma is generated in the cavity when an input gas is supplied through the input connection and an oscillating electrical current is supplied to the first and second induction coils.

TECHNICAL FIELD

The present disclosure is directed to technology for plasma generation, as may be used, for example, in semiconductor wafer processing. In particular, systems and methods of directing magnetic fields in a plasma source are disclosed.

BACKGROUND

Plasma processing is commonly used in processing of semiconductors and other products. Plasmas can be used for a variety of processing operations including deposition, etching, oxidation, removal of organic material, and other treatments. Generating a plasma involves exposing an input gas to energy input in the form of high electric and/or magnetic fields. At least some of the molecules of the input gas are excited by the fields and gain energy, in some cases becoming ionized.

Processing involves exposing the product to ions or reactive species generated in the plasma, and/or gas molecules that were present in the plasma but were not excited (any combination of plasma-generated ions, reactive species, and/or gas molecules that were not excited by the plasma will be referred to as “plasma products” herein). Generally speaking, there are two modes of plasma processing. In in-situ processing, item(s) being processed are in the location where the plasma is generated. In remote plasma processing, a plasma is generated in a first location, and the plasma products are brought to a second location, where they contact the item(s) being processed. Gas flows, vacuum pumping, electric fields and/or magnetic fields may be utilized to direct the plasma products to the product being processed. Remote treatment is sometimes preferable because parameters that affect the plasma can be controlled at the first location, but at the second location containing the product, the high electric or magnetic fields used to generate the plasma, and/or high velocity ions produced in the plasma, can be reduced or eliminated to avoid damage to certain types of product. Some plasma processing systems provide both in-situ and remote plasma processing capabilities.

Two known methods of generating a plasma are capacitive coupling and inductive coupling. In a capacitively coupled plasma, a high frequency (usually radio frequency, or RF) electric field is applied directly to the input gas to generate the plasma. In an inductively coupled plasma, a magnetic field is provided within a chamber containing the input gas. The magnetic field is often generated by a high power RF signal being transmitted into a coil, so that the magnetic field is generated within the coil, and generates transverse electrical currents within the gas according to the right-hand rule. Some capacitive coupling is often utilized to initiate the plasma by inducing a Townsend avalanche in the input gas, generating charge carriers for the transverse electrical currents. In some cases the coil is wrapped around the location where the plasma is generated; in others coil is wrapped around a magnetic (e.g., ferrite) core to enhance and/or direct the magnetic field to another location where the plasma is generated.

SUMMARY

In an embodiment, a plasma source is disclosed. The plasma source includes a plasma vessel that includes a dielectric material that encloses a cavity of a toroidal shape. The toroidal shape defines a toroidal axis therethrough. The vessel forms input and output connections, each of the input and output connections being in fluid communication with the cavity. One or more metal plates are disposed adjacent to the plasma vessel for cooling the plasma vessel. A magnetic core is disposed along the toroidal axis such that respective first and second ends of the magnetic core extend beyond axially opposed sides of the plasma vessel. First and second induction coils are wound about the respective first and second ends of the magnetic core. A plasma is generated in the cavity when an input gas is supplied through the input connection and an oscillating electrical current is supplied to the first and second induction coils.

In an embodiment, a method of generating a plasma includes supplying an input gas to an input connection of a plasma vessel. The plasma vessel includes a dielectric material that encloses a cavity of a toroidal shape. The toroidal shape defines a toroidal axis therethrough. A magnetic core is disposed along the toroidal axis. The input connection is in fluid communication with the cavity. The method further includes supplying an oscillating electrical current to first and second induction coils that are wound about the magnetic core, such that respective first and second ends of the magnetic core extend beyond axially opposed sides of the plasma vessel, and the oscillating electrical current ignites a plasma within the input gas. The method further includes cooling the plasma vessel by utilizing one or more metal plates disposed adjacent to the plasma vessel.

In an embodiment, a plasma source includes a plasma vessel including a dielectric material that encloses a cavity of a toroidal shape. The toroidal shape defines a toroidal axis therethrough. The vessel forms input and output connections, each of the input and output connections being in fluid communication with the cavity. Two metal plates are disposed adjacent to axially opposed sides of the plasma vessel such that the plasma vessel is between the metal plates. The two metal plates form channels therein for a cooling fluid, typically a liquid, for cooling the plasma vessel. A magnetic core, including a single piece of magnetic material with a channel therein for the cooling fluid, is disposed along the toroidal axis such that respective first and second ends of the magnetic core extend beyond axially opposed sides of the plasma vessel. First and second induction coils are wound about the respective first and second ends of the magnetic core in opposing directions, such that magnetic fields induced by the first and second induction coils in the magnetic core are in opposite directions along the toroidal axis. The plasma source further includes a Faraday shield having components that encircle the plasma vessel in an axial direction, the components being electrically connected with the two metal plates. A plasma is generated in the cavity when an input gas is supplied through the input connection and an oscillating electrical current is supplied to the first and second induction coils.

DETAILED DESCRIPTION

The present disclosure may be understood by reference to the following detailed description taken in conjunction with the drawings briefly described below. It is noted that, for purposes of illustrative clarity, certain elements in the drawings may not be drawn to scale. Specific instances of an item may be referred to by use of a numeral in parentheses (e.g., induction coils250(1),250(2)) while numerals without parentheses refer to any such item (e.g., induction coils250).

FIG. 1schematically illustrates major elements of a plasma processing system100, according to an embodiment. Plasma processing system100is a remote processing system that includes a remote plasma system (RPS)130that receives one or more source gases110and RF energy from an RF generator120. Plasma and/or plasma products generated in RPS130are directed to a processing chamber140. Plasma processing system100is illustrated as a semiconductor processing system that exposes a wafer50, placed on or held by a fixture150, to the plasma or plasma products. A vacuum pump160removes the plasma, plasma products, and/or other byproducts of processing from chamber140. It is understood by those skilled in the art that plasma processing system100is a generalized system illustration. Other embodiments of a plasma processing system may include features not shown inFIG. 1. Such additional features may include but are not limited to sensors, control electronics, product handling systems, multiples of any of the components shown, different schemes for connecting gases and vacuum among the system components, in-situ plasma generation capability within chamber140, and the like.

FIGS. 2A, 2B and 2Cschematically illustrate components of RPS130,FIG. 1, according to an embodiment.FIG. 2Ais a side elevational view of certain components, whileFIGS. 2B and 2Care angled elevational views of only some of the components shown inFIG. 2A, with slight variations. It is understood that RPS130may have many other components than those shown; the items shown inFIGS. 2A, 2B and 2Care selected for clarity of illustration.

RPS130is an inductively coupled plasma generator. RPS130includes a plasma vessel200that receives input gas in an input connection220and provides plasma products at an output connection230. Plasma vessel200encloses a cavity202of a toroidal shape that defines an axis203therethrough. Input connection220and output connection230are in fluid communication with cavity202. It is understood that the locations of input connection220and output connection230are arbitrary and may vary among embodiments. Plasma vessel200may be made of a dielectric material, for example aluminum nitride, that has desirable properties such as high thermal conductivity, an ability to withstand high temperature, and compatibility with the desired input gases and their resulting plasma products. Those skilled in the art will note that dielectric materials are generally not used for plasma vessels of inductively coupled plasma systems due to thermal constraints. In embodiments, the present disclosure overcomes these constraints by use of appropriate materials (e.g., aluminum nitride), maximizing power coupling into the plasma itself (thus minimizing waste heat generation) and efficient cooling.

Input energy to RPS130is provided by application of an oscillating electrical current at RF inputs260(1) and260(2) that connect with induction coils250(1) and250(2) respectively. Coils250(1) and250(2) wrap around a magnetic (e.g., ferrite) core240that passes through plasma vessel200, as shown. Axis R inFIG. 2Aand in other drawings denotes a radial direction relative to axis203, about which plasma vessel200is symmetric except for input and output connections220,230.FIGS. 3A-3D and 4A-4Bprovide further schematic illustration of the directions and effects of magnetic fields generated by coils such as250(1),250(2).

Plasma vessel200is flanked by one or more cooling plates, for example cooling plates210(1),210(2) and/or210(3) that have high thermal conductivity. In embodiments, cooling plates210may be made of copper or alloys thereof. A gap242between magnetic core240and cooling plates210, and/or optional apertures212within cooling plates210, advantageously allow magnetic fields induced by induction coils250into the vicinity of plasma vessel200, as discussed below. Geometries of apertures212that are depicted inFIG. 2Bare representative only, and only two such apertures are depicted, for clarity of illustration.

Apertures212may be advantageously provided in cooling plates210as shown inFIGS. 2B and 2C. Apertures212advantageously allow the magnetic fields generated by coils250to extend into plasma vessel200, and may help to disrupt parasitic electrical currents within cooling plates210such that more of the total energy radiated by induction coils250couples to plasma within plasma vessel200. Apertures212that are radial slots are especially advantageous in terms of disrupting such parasitic electrical currents.FIG. 2Billustrates an embodiment with cooling plate210(2) that defines apertures212(1) as elliptical shapes with edges that are within an outer circumference of cooling plate210(2).FIG. 2Cillustrates an embodiment with cooling plate210(3) that defines a single aperture212(2) as a radial slot with edges that intersect outer and inner edges of an outer circumference of cooling plate210(2). Providing at least one slot such as aperture212(2) that completely interrupts azimuthal current paths about toroidal axis203is advantageous in terms of coupling RF energy from induction coils250(FIG. 2A) into plasma vessel200. The illustration of single aperture212(2) is exemplary only; in embodiments, more than one aperture212(2) may be provided, and/or apertures similar to aperture212(1) and aperture212(2) may both be present.

Power dissipated as heat within RPS130is removed by a cooling system, for example a liquid cooling system. In particular, cooling plates210(1) and210(2) are in thermal contact with respective sides of plasma vessel200and may contain tubes or channels within which a cooling fluid flows. The cooling fluid is typically a liquid. In embodiments, one or more cooling fluids may flow serially or in parallel through any or all of induction coils250, magnetic core240and/or other components of RPS130. In addition to cooling plates210(1) and210(2), additional cooling features may be provided, for example about the front, rear, top or bottom of plasma vessel200. When the additional cooling features are formed of metal or other material that impedes or otherwise affects magnetic fields, design and placement of such features is done with consideration for their effect on magnetic fields around and in plasma vessel200, as now described.

FIGS. 3A-3Dschematically illustrate exemplary magnetic field configurations that are achievable with certain combinations of magnetic core elements and windings carrying electrical currents, according to embodiments. InFIGS. 3A-3D, 4 and 5, directions of current and magnetic fields are shown with>< symbols.

FIG. 3Ashows a simple, straight magnetic core (e.g., ferrite) element310and an induction coil320that wraps about element310. When current passes through induction coil320in the direction noted, a magnetic field330develops and is shaped by the presence of element310. Because all magnetic fields form closed loops, when magnetic field330reaches ends of element310it extends out into a space surrounding element310to complete each loop, as shown. Those skilled in the art will understand that magnetic field330also extends into and out of the plane of the drawing, radially symmetric about element310. It is also understood that induction coil320could be broken into two or more induction coil segments wrapped about element310and connected in series, as long as all of the segments wrap in the same direction about element310.

InFIG. 3B, a magnetic core element340forms a closed shape such that a magnetic field360generated by an induction coil350loops through element340without extending into space around element340. The configuration shown inFIG. 3Bis utilized in certain plasma generators wherein a plasma chamber (not shown) lies within element340such that magnetic field360induces an electrical current within the plasma chamber.

InFIG. 3C, an induction coil370wraps around magnetic core element340differently than coil350; in particular, induction coil360wraps around opposing sides of element340so as to generate magnetic fields380(1),380(2) that oppose one another within side elements345(1),345(2) of the closed shape formed by element340. In this configuration, magnetic fields380(1),380(2) are forced to exit elements345(1),345(2) to complete their respective loops.

InFIG. 3D, straight magnetic element310(as inFIG. 3A) has an induction coil390wrapped around it such that a winding direction of coil390changes from one side of magnetic element310to the other. Thus, when current passes through induction coil390as indicated, opposing magnetic fields395(1) and395(2) develop. In this configuration, similar to the case shown inFIG. 3C, magnetic fields395(1),395(2) are forced to exit elements310to complete their respective loops. Those skilled in the art will understand that magnetic fields395(1),395(2) also extend into and out of the plane of the drawing, radially symmetric about element310.

FIG. 4is a cross-sectional, schematic illustration of a magnetic configuration of an RPS400that may be understood according to the principles explored inFIGS. 3A-3D, according to an embodiment. It is understood that RPS400may have many other components than those shown; the components shown inFIG. 4are selected for clarity of illustration. A dashed line6-6′ indicates a section that corresponds to magnetic field modeling data shown inFIG. 6.

RPS400includes a toroidal plasma vessel405flanked by cooling plates410(1) and410(2). A magnetic (e.g., ferrite) core440extends through the center of plasma vessel405, as shown. Induction coils450(1) and450(2) are connected as shown, and wrap about ends of magnetic core440. In operation, an RF source (not shown) drives induction coils450(1) and450(2) (or, alternatively, coils450(1) and450(2) may not be connected as shown, but may be driven in parallel such that they are driven in phase with one another). Accordingly, induction coils450(1),450(2) and magnetic core440set up a magnetic field460, as shown. Magnetic field460does not pass through cooling plates410, and thus extends beyond cooling plates410and plasma vessel405, as shown. Those skilled in the art will understand that magnetic field460also extends into and out of the plane of the drawing, radially symmetric about magnetic core440. Although stray fields may exist within plasma vessel405, overall magnetic field density is low within plasma vessel405due to the arrangement of magnetic core440through plasma vessel405, and cooling plates410flanking plasma vessel405in the axial direction.

FIG. 5is a cross-sectional, schematic illustration of a magnetic configuration of an RPS500that may also be understood according to the principles explored inFIGS. 3A-3D, according to an embodiment. It is understood that RPS500may have many other components than those shown; the components shown inFIG. 5are selected for clarity of illustration. A dashed line7-7′ indicates a section that corresponds to magnetic field modeling data shown inFIG. 7.

RPS500includes a toroidal plasma vessel505flanked by cooling plates510(1) and510(2). A magnetic (e.g., ferrite) core540extends through the center of plasma vessel505, as shown. Induction coils550(1) and550(2) are connected as shown, and wrap about ends of magnetic core540. Note that induction coils550(1) and550(2) are connected differently as compared with induction coils450(1) and450(2),FIG. 4; the arrangement shown inFIG. 5provides a very different result from that shown inFIG. 4. In operation, an RF source (not shown) drives induction coils550(1) and550(2). Accordingly, induction coils550(1),550(2) and magnetic core540set up magnetic fields560(1) and560(2), as shown. Magnetic fields560(1) and560(2) do not pass through cooling plates510, but can pass through plasma vessel505. Thus, magnetic fields560(1) and560(2) set up similar to magnetic fields395(1) and395(2),FIG. 3D. Those skilled in the art will understand that magnetic fields560(1) and560(2) also extend into and out of the plane of the drawing, radially symmetric about magnetic core540.

The arrangement shown inFIG. 5provides a significant radial magnetic field in plasma vessel505, and thus provides significantly better RF power coupling into plasma vessel505than the arrangement shown inFIG. 4provides for plasma vessel405. As understood by those skilled in the art, higher RF power coupling is desirable because it increases plasma products produced per power input. This effect can be utilized to improve throughput of a processing tool that receives the plasma products, to reduce the power that must be supplied in order to generate the same plasma products, and/or to reduce the overall size and power consumption of RPS500for a given amount of plasma products needed for processing. Yet, it may be considered counterintuitive to provide opposing magnetic fields within a single magnetic core, as shown inFIGS. 3D and 5, because magnetic cores are usually used to confine and direct a magnetic field in a single direction.

In RPS500, even though multiple axial magnetic field directions are implemented, use of a single magnetic core540may be advantageous, as compared to use of multiple magnetic core pieces, for a variety of reasons. Multiple magnetic core pieces would have to be aligned precisely and mechanically stabilized, because opposing fields560(1),560(2) would generate repulsive forces on mechanically separate pieces of magnetic material. Less than perfect alignment or mechanical instability, due to multiple magnetic core pieces, may degrade uniformity of plasma generation within plasma vessel505. Non-uniform plasma generation may, in turn, lead to undesirable effects such as reduced generation of plasma products and/or non-uniform spatial distribution of the plasma products as they are directed from an RPS system to a processing location (see, e.g.,FIG. 1). Also, a single magnetic core540allows easy fabrication of a center channel to pass coolant through, but separate magnetic core pieces with cooling channels would be much more difficult to fabricate and might not result in uniform cooling (e.g., for separate magnetic core pieces, a channel would have to double back within each of the pieces, possibly leading to asymmetry and potential for uneven cooling).

FIG. 6shows magnetic field modeling data corresponding to line6-6′,FIG. 4. Modeled magnetic field, in arbitrary units, is on the vertical axis while spatial position in the axial direction is on the horizontal axis. First and last regions610and650correspond to leftmost and rightmost portions of line6-6′ respectively, that is, portions that are outside plasma vessel405and both cooling plates410(1),410(2). Regions620and640correspond to cooling plates410(1) and410(2) respectively (e.g., where the magnetic field does not penetrate). Central region630corresponds to plasma vessel405. AsFIG. 6shows, the magnetic field within region630is high at the edges (points A and C) but nonuniform, and dips to a low value at its center (point B). Thus, the highest region of the magnetic field is roughly at the walls of plasma vessel405, while the center of plasma vessel405sees a low magnetic field. In inductively coupled plasmas, asymmetry in magnetic field distribution can disadvantageously lead to unbalanced plasma ionization distribution, and subsequent nonuniformity in plasma product generation and transportation towards a downstream processing location.

FIG. 7shows magnetic field modeling data corresponding to line7-7′,FIG. 5. Modeled magnetic field, in arbitrary units, is on the vertical axis while spatial position in the axial direction is on the horizontal axis. First and last regions710and750correspond to leftmost and rightmost portions of line7-7′ respectively, that is, portions that are outside plasma vessel505and both cooling plates510(1),510(2). Regions720and740correspond to cooling plates510(1) and510(2) respectively (e.g., where the magnetic field does not penetrate). Central region730corresponds to plasma vessel505. AsFIG. 7shows, the magnetic field within plasma vessel505is again relatively high at the edges (points D and F) but much more uniform than the distribution shown inFIG. 6. The dip at its center (point E) is also less pronounced. The field at each point in central regions630and730respectively ofFIGS. 6 and 7is the sum of contributions from each of the outer regions, so the central region730magnetic distributions can be further modified by decreasing the distance between the cooling plates and induction coils. In this manner, it is believed that a central region magnetic distribution can be achieved that is essentially flat, or even higher in the center than at the edges. This is expected to increase production of plasma products in turn, leading to the beneficial results discussed above, and others. For example, magnetic field distribution that is higher in the center than at the edges may enhance ionization near the center region, resulting in a higher radical density toward the center for direct diffusion toward the outside of the plasma vessel. Conversely, a magnetic field distribution that is higher in the edges than at the center may result in radical diffusion toward the center region.

FIG. 8schematically shows components of a Faraday shield assembly800disposed adjacent to a toroidal plasma vessel820, according to an embodiment. Toroidal plasma vessel820is shown in ghost outline only, for clarity of illustration. An axial direction denoted by A is shown, as is a representative radial direction denoted by R, but it is understood that the radial direction is perpendicular in all directions about a toroidal axis of plasma vessel820.

Components810of Faraday shield800encircle the toroidal shape of plasma vessel820in the radial and axial directions to lower electric field differentials in those directions, to reduce capacitive plasma coupling and ion strike energy at corresponding inner walls of plasma vessel820. In embodiments, Faraday shield800does not include azimuthally oriented (e.g., in the direction of curvature of plasma vessel820) components because such components may develop high parasitic currents in response to the magnetic field utilized to excite the plasma within plasma vessel800. Faraday shield800advantageously covers a low fraction (e.g., less than 10% or less than 2%) of surface area of the cylindrical inner and outer surfaces of plasma vessel820, in order to provide maximum magnetic field exposure to plasma vessel820. In embodiments, components810are connected to ground or another fixed potential. Connections among components810and/or connecting components810to the fixed potential are advantageously made outside an inner cavity of plasma vessel820(e.g., where a magnetic core is disposed; seeFIGS. 2A, 2B) to avoid such connections having an azimuthal orientation.

In embodiments, components810are for example metal wires, strips or rods; also, components810may be portions of cooling plates disposed adjacent to plasma vessel820(e.g., cooling plates210,FIGS. 2A and 2B, cooling plates410,FIG. 4or cooling plates510,FIG. 5). These types of components810may also be mixed, for example Faraday shield800may be implemented by connecting wires, strips or rods in the axial direction with cooling plates that form radial portions. Although varying types of metals, and varying shapes, can be utilized to form components810, such components are advantageously arranged symmetrically about plasma vessel820to preserve symmetric and uniform plasma product generation, as discussed above.