WO2002084700A1

WO2002084700A1 - Inductively coupled plasma control with external magnetic material

Info

Publication number: WO2002084700A1
Application number: PCT/US2002/010547
Authority: WO
Inventors: Takumasa Nishida; Shu Nakajima; Neil Benjamin; Andrew Perry
Original assignee: Lam Research Corporation
Priority date: 2001-04-12
Filing date: 2002-04-02
Publication date: 2002-10-24

Abstract

A plasma reactor for processing a substrate in a plasma has a dielectric window disposed in a wall of a chamber. An RF antenna coil (402) is disposed outside of the chamber and adjacent to the window. A magnetic core (420) is disposed outside of the chamber and adjacent to the coil. The magnetic core comprises a material having a magnetic dipole moment in a range of about 10ν to about 1000ν. The magnetic core comprises a ferrite material that surrounds at least a portion of the coil. The position of the magnetic core relative to the dielectric window can be adjusted.

Description

S P E C I F I C A T I O N

TITLE OF INVENTION

INDUCTIVELY COUPLED PLASMA CONTROL WITH EXTERNAL

MAGNETIC MATERIAL

FIELD OF THE INVENTION The present invention relates to a radio frequency (RF) inductor for a plasma processor. More particularly, the present invention relates to a plasma inductor having a magnetic material disposed adjacent to a coil.

BACKGROUND OF THE INVENTION RF plasma sources are widely used in plasma processing technology for large scale manufacturing of semiconductor chips (etching, deposition, ion implantation, sputtering), large panel displays, and ion sources. Inductively coupled plasma (ICP) sources as well as microwave plasma (MWP) sources have been more popular in a new generation of plasma reactors due to their ability to maintain a high-density plasma at a very low gas pressure, and due to their capability to separate the process of plasma generation from that of ion acceleration.

FIG. 1 illustrates the magnetic field lines produced by a conventional RF powered antenna coil 102. The current through the coil produces a magnetic field that is strong and uniform at point A but relatively weak at point B. One side of the coil (above or below the coil) acts as a north pole and the other side as a south pole. It is commonly known that a current-carrying coil acts as a magnetic dipole.

FIG. 2 illustrates the magnetic field lines produced by a conventional RF powered antenna coil coupled with a gas. This typical ICP source comprises a flat helix inductor coil 202 and a metallic discharge chamber 204 (filed with an operating gas) having a dielectric window 206 (typically quartz) which separates a discharge volume of the chamber 204 from the RF antenna coil 202 thereby maintaining a plasma 208 within chamber 204. The direction of the electric filed induced by the coil 202 is shown by the pair of arrows in FIG. 2. The base 210 of the chamber 204 supports a workpiece 212 such as a substrate being processed by the plasma. The coil 202 excites the plasma 208 by heating electrons in the plasma region near the vacuum side of the dielectric window 206 by oscillating inductive fields produced by the coil 202 and coupled through the dielectric window 206. Inductive currents which heat the plasma electrons are derived from the RF magnetic fields produced by RF currents in the coil 202. The spatial distribution of the magnetic field is a function of the sum of the fields produced by each of the turns of the coil 202. The field produced by each of the turns is a function of the magnitude of RF current in each turn. For a typical spiral design such as coil 202, the RF currents in the coil 202 are distributed to produce a ring-shaped region 208 where power is absorbed by the plasma 208. Magnetic field lines are illustrated in FIG. 2 coupling with the plasma 208. On the opposite side of the coil 202, the magnetic field lines retains their free-space shape as illustrated in FIG. 1. The ring-shaped plasma forming region 208 abuts the vacuum side of the dielectric window 206. At low pressures, in the 1.0 to lOmTorr range, diffusion of the plasma from the ring shaped region produces a plasma density peak in a central portion of the chamber, along a chamber center line away from the window. At intermediate pressure ranges, in the 10 to lOOmTorr range, gas phase collision of electrons, ions and neutrons in the plasma prevent substantial diffusion of the plasma charged particles outside of region 208. As a result, there is a relatively high plasma flux in a ring like region 212 about the workpiece 212 but relatively low plasma fluxes in the center and peripheral regions of the workpiece 212.

One of the most advanced dense-plasma sources based on an inductive RF discharge (ICP) is shown in FIG. 3. This typical ICP source comprises a flat helix inductor coil 302 and a metallic discharge chamber 304 (filed with an operating gas) having a dielectric window 306 which separates a discharge volume of the chamber 304 from the inductor coil 302 thereby maintaining a plasma 308 within the chamber 304. The direction of the electric field induced by the coil 302 is shown as a pair of arrows in FIG. 3. The base 310 of the chamber 14 supports a substrate being processed 312 or an ion extracting arrangement for creating an ion beam. An external RF power source 314, connected to coil 302 via an impedance matching device 316, maintains the RF current in coil 302. Coil 302 is magnetically shielded from both the RF source 314 and the matching device 316 with a conductive enclosure 318 typically made of aluminum or copper. However, the side effects of magnetically shielding coil 302 within the electrically conductive enclosure 318 are mirror-currents. The oscillating magnetic fields generate a current within the conductive enclosure 318. Small arrows shown in FIG. 3 around the enclosure 318 illustrate the induced mirror-currents. On the opposite side of coil 302 facing the chamber 304, the RF current induces an azimuthal RF electric field which maintains an azimuthal RF discharge current producing a plasma 308. The matching device 316 is an essential part of the ICP inductor. It performs two important functions. The first one is to match the 50 Ohm conventional output resistance of the RF power source 314 with the coil 302 impedance (depending on plasma parameters) for efficient power transfer to ICP inductor. The second one is to tune the coil 302 circuit to a resonance with an operating frequency, thereby, to enhance resonantly the RF current in the coil 302.

For a typical ICP inductor driven at a standard industrial application frequency of 13.56 MHZ, and with the RF power transferred into the plasma being around 1 kW, the inner volume of the discharge chamber 1 is a few liters, and the operating gas pressure is in the range 1-100 mTorr, the resonant RF current of the coil is a few tens of Amperes, and the RF voltage across the inductor coil 4 is a few kV. Under these conditions, the RF power loss in the matching device, connectors, and the inductor coil itself (due to its final resistance) is comparable to that transferred to the plasma. Moreover, due to the coil 102 and the conductive enclosure 318 proximity, an RF current is induced along the chamber wall. This effect results in an additional power loss due to chamber heating. Therefore, the power transfer efficiency to the plasma is significantly less than 1, since significant power is dissipated in the ICP source hardware of all practically realized devices.

The ICP coil 302 acts as an inductively coupled plasma source and emits an RF magnetic field to both upper (grounded coil housing 318) space and lower (plasma chamber 304) space. The later contributes to plasma generation, and the former does not. In fact, the coupling of the RF magnetic field to the grounded housing 318 causes a loss of power in the form of heat and current. In addition, because of the presence of the RF magnetic field in the upper space, the ICP coil housing 318 needs to have a minimum clearance thereby increasing the dimensions of the plasma processor. Accordingly, a need exists for an apparatus and a method for steering magnetic flux back toward a coil for increasing the magnetic field in and coupling to a plasma.

BRIEF DESCRIPTION OF THE INVENTION A plasma reactor for processing a substrate in a plasma has a dielectric window disposed in a wall of a chamber. An RF antenna coil is disposed outside of the chamber and adjacent to the window. A magnetic core is disposed outside of the chamber and adjacent to the coil. The magnetic core comprises a material having a magnetic dipole moment in a range of about lOμ to about lOOOμ. The magnetic core comprises a ferrite material that surrounds at least a portion of the coil. The position of the magnetic core relative to the dielectric window can be adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present invention and, together with the detailed description, serve to explain the principles and implementations of the invention. In the drawings: FIG. 1 is a schematic representation illustrating magnetic field lines around a RF powered inductive coil in accordance with the prior art;

FIG. 2 is a schematic representation illustrating magnetic field lines around a RF powered inductive coil coupled with a plasma in a discharge chamber in accordance with the prior art;

FIG. 3 is a schematic representation showing a plasma inductor with a discharge chamber in accordance with the prior art;

FIG. 4 is a schematic representation showing a plasma inductor coupled with a- magnetic material above a discharge chamber in accordance with a specific embodiment of the present invention;

FIG. 5 is a perspective view of a magnetic material in accordance with a specific embodiment of the present invention; FIG. 6 is a schematic top view of a magnetic material in accordance with an alternative embodiment of the present invention;

FIG. 7 is a schematic side cross-sectional view of a magnetic material in accordance with an alternative embodiment of the present invention; and

FIG. 8 is a schematic side cross-sectional view of a part of the magnetic material in accordance with an alternative embodiment of the present invention.

DETAILED DESCRIPTION Embodiments of the present invention are described herein in the context of an inductively coupled plasma control with external magnetic material. Those of ordinary skill in the art will realize that the following detailed description of the present invention is illustrative only and is not intended to be in any way limiting. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to implementations of the present invention as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts.

In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It will, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer's specific goals, such as compliance with application- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.

In accordance with the present invention, FIG. 4 illustrates a plasma processor 400 that can be used for processing a semiconductor, dielectric or metal substrate. The plasma processor 400 comprises an inductor coil 402, such as a flat helix coil, and a chamber 404 filed with an operating gas such as bichloride. However, any other type of inductor may be suitable so long as flux lines are generated. Any other type of coils such as a helical cylindrical coil may be used. An electro-magnetic transparent window 406, such as a quartz window, separates the operating gas in the vacuum chamber 404 from the RF antenna coil 402 thereby maintaining a plasma 408 within the chamber 404. The base of the chamber 404 contains a plasma processed substrate 410, such as a semiconductor wafer, mounted on an electrically conductive support 412. An external RF power source 414, connected to the coil 402, via a matching device 416, maintains the RF current in the coil 402. Coil 402 is electro-magnetically shielded from both the RF power source 414 and the RF matching device 416 with a conductive enclosure 418.

A magnetic material 420 is disposed between the coil 402 and both RF power source 414 and the matching device 416 such that the magnetic material 420 is adjacent to the coil 402 and opposite to the vacuum chamber 404. A gap exists on either side of the magnetic material 420 between the coil 402 and both the matching device 416 and the RF source 414. The magnetic material 420 preferably surrounds at least a portion of the coil 402.

The magnetic material 420 preferably has a relatively high magnetic dipole moment in a range of about lOμ to about lOOOμ. An example of a high magnetic dipole moment material may be ferrite, powdered iron, or any other magnetic shielding material. The magnetic material 420 may also have about the same diameter of the coil 402 and may also include a cavity 422 in its center small enough for allowing the matching device 416 to be connected with the coil 402. The magnetic material 420 is non-conductive.

The magnetic material 420 steers magnetic flux lines as illustrated in FIG. 4 away from the enclosure 418. Because of the high magnetic permeability of the magnetic material 420, magnetic flux lines easily flow through the magnetic material 420. The steering of the magnetic flux lines above the coil 402 causes the magnetic field lines to concentrate below the coil 402. Although the total flux generated by coil 402 remains the same, the density of the flux lines coupling with the plasma in chamber 408 has increased. As illustrated by comparing FIG. 3 to FIG. 4, without the use of magnetic material 420 above coil 402, there are a few flux lines represented near the plasma 308. FIG. 4 illustrates the higher density of flux lines coupling with the plasma 408. The increase in density of flux lines induces more plasma rendering the induction process more efficient. It also provides a mechanism for forming a more uniform plasma in chamber 404.

A more uniform plasma density distribution is obtained by disposing the magnetic material 420 above the coil 402 to steer the magnetic field produced by coil 402. The upward magnetic field is returned back to the coil 402 so that the loss of electro-magnetic energy above the coil 402 is minimized. The increment of magnetic field increases the coupling efficient to plasma resulting in an optimized flux profile.

Because the magnetic material 420 serves to redirect the upward magnetic fields, the inductive coil housing can be made more compact by reducing the clearance to the housing 418 above the coil 402.

FIG. 5 illustrates one embodiment of the magnetic material 420 in the shape of a "pot core". A magnetic material 500 may include a cavity 502 in its center. The cavity 502 may preferably have a radius of about the same radius of any coil currently used in a plasma processing chamber. However, the radius of the magnetic material 500 is not limited to being the same as the radius of the coil. The magnetic material 500 may have a thickness of about the same of any coil used in a plasma processing chamber. The bottom 504 of magnetic material 500 may have a planar surface while the top 506 may have a recess 508. The recess 508 extends over the surface of the top 506 from the edge of the cavity 502 to an outer radius near to the outer edge of magnetic core 500. The recess 508 may form an arch over the surface of top 506 extending from the edge of cavity 502 to the outer radius. The magnetic core 500 is disposed adjacent to the inductor coil 402 in FIG. 4 such that the top 506 is facing the coil 402.

The magnetic material 402 may be combined with an optimized RF antenna coil design to improve the plasma uniformity in chamber 404. However, even with a simple or conventional ICP coil design, the plasma uniformity can be adjusted by changing the shape and position of the magnetic material 402. FIG. 6 is a top view illustrating an alternative embodiment of a magnetic material 600 used in combination with the inductor coil 402. The plasma density distribution can be further adjusted by moving the individual elements 602 of the magnetic material 600 to optimize the flux return. The position of each individual element 602 may be adjusted manually or with the use of a motor. Such adjustment may be performed before, during, or after the processing. FIG. 6 illustrates a magnetic material 600 divided into four equal pie segments 602. Any other number of segments may be used to further tune the effects of the magnetic material 600.

FIG. 7 illustrates the basic component shapes of magnetic material 700. A top 702 provides a lateral path for the flux lines to travel. An outer crown 704 provides an incoming or outgoing flux line path. An annus 706 is disposed in the center of magnetic material 700 for providing an outgoing or incoming flux line path. It should be noted that the outer crown 704 may be extended further into the plasma chamber for steering the flux lines. The position of each individual component may further be adjusted azimuthally or vertically (in-situ real-time) to optimize the flux return and therefore the plasma density distribution in the plasma chamber.

The top 702 may be further broken down into several movable elements 802 as illustrated in FIG. 8. The individual elements 802 may be adjusted to form a dome or any other shape that would optimize the return flux lines. Therefore, changing the position of the magnetic material with respect to the coil may also change the efficiency of the coupling to the plasma.

Other ways to further tune the magnetic material 402 for optimizing the coupling to the plasma may include changing the temperature of the magnetic material 402, i.e. by cooling it, or modifying the DC magnetic field saturation of the magnetic material affecting the coupling to the plasma.

While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.

Claims

CLAIMS What is claimed is:

1. A plasma reactor for processing a substrate in a plasma, said reactor comprising: a chamber; a dielectric window disposed in a wall of said chamber; an RF antenna coil disposed outside of said chamber and adjacent to said window; and a magnetic core disposed outside of said chamber and adjacent to said RF antenna coil.

2. A reactor according to claim 1, wherein said magnetic core further comprises a material having a magnetic dipole moment in a range of about lOμ to about lOOOμ.

3. A reactor according to claim 2, wherein said magnetic core surrounds at least a portion of said RF antenna coil.

4. A reactor according to claim 2, wherein said magnetic core further comprises a ferrite material.

5. A reactor according to claim 2, wherein said RF antenna coil is a flat helix inductor coil.

6. A reactor according to claim 2, further comprising a positioner positioning at least a portion of said magnetic core relative to said dielectric window.

7. A reactor according to claim 2, further comprising a positioner positioning at least a portion of said magnetic core relative to said dielectric window during processing.

8. A reactor according to claim 6, wherein said positioner is manually controlled.

9. A reactor according to claim 6, wherein said positioner is controlled with a motor.

10. A method for processing a substrate in a plasma, said method comprising: placing the substrate in a chamber having a dielectric window; providing a gas to the chamber; coupling an RF energy into the chamber through the dielectric window with an RF antenna disposed adjacent to the window on the outside of the chamber; and controlling the shape of the magnetic field induced by the RF antenna with a magnetic core disposed adjacent to the RF antenna on the outside of the chamber.

11. The method according to claim 9 wherein said controlling further includes moving said magnetic core relative to said dielectric window.

12. The method according to claim 9 wherein said controlling further includes moving said magnetic core relative to said dielectric window during processing.

13. A plasma reactor for processing a substrate in a plasma, said reactor comprising: means for placing the substrate in a chamber having a dielectric window; means for providing a gas to the chamber; means for coupling an RF energy into the chamber through the dielectric window with an RF antenna disposed adjacent to the window on the outside of the chamber; and means for controlling the shape of the magnetic field induced by the RF antenna with a magnetic core disposed adjacent to the RF antenna on the outside of the chamber.

14. The plasma reactor according to claim 12 wherein said plasma reactor further includes means for adjusting the position of said magnetic core relative to said dielectric window during processing.

15. The plasma reactor according to claim 12 wherein said plasma reactor further includes means for moving said magnetic core relative to said dielectric window.