Apparatus and process for producing high density axially extending plasmas

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FIELD OF THE INVENTION

The present invention relates to a process and apparatus for processing a microelectronic substrate, and more particularly to a process and apparatus for producing a high density axially extended plasma for processing a microelectronic substrate.

BACKGROUND OF THE INVENTION

Plasma generation is useful in a variety of microelectronic fabrication processes, including etching, resist stripping, passivation, deposition, and the like. There are a variety of plasma techniques known for processing microelectronic devices. Most known applications for using plasma can be significantly enhanced if the density of the plasma can be increased and maintained at low pressures, particularly with increasing miniaturization of features of microelectronic devices.

For example, plasma etching is a technique known in the art for patterning a substrate surface to form microelectronic devices and their interconnections. Ideally, substrates are etched primarily in a direction orthogonal to the surface thereof, i.e., in a direction perpendicular to the surface of the substrate. As a material is etched, walls are formed in the material, generally referred to as sidewalls. Ideally etching continues in a substantially vertical direction to the substrate and not laterally into the thus created sidewalls. Such vertical etching with minimal or no lateral etching into the sidewalls is referred to as anisotropic etching.

Plasmas pattern substrates by using directed ions to achieve anisotropic etching. To provide ions with sufficient directionality requires plasma operation at low pressures, typically about 1-20 mTorr. This prevents scattering of the ions by collisions with the gas molecules. Further, it is advantageous to operate plasma processes at high rates for commercial viability. This necessitates the use of high density plasmas.

Currently, several techniques can be used to generate high density plasmas at low pressures. For example, plasma sources such as electron cyclotron (ECR), helicon or MORI, and inductively coupled (RFI/TCP/ICP) sources are becoming increasingly important for plasma processing applications due to their ability to operate at low pressure and high plasma density. ECR processes couple a microwave energy source with a magnetic field to create electron cyclotron resonance in the electrons of a gas to generate a plasma of the gas. This, however, can require high outputs of energy to maintain the plasma. That is, ECR sources typically operate at 2.45 GHz and require an 875 Gauss magnetic field with its attendant costs for coils, power supplies, cooling, and power.

Other techniques generate a plasma by generating helicon or whistler waves in a gas. The helicon waves can be formed by coupling a magnetic field with RF energy, using complicated, large volume antenna structures. This in turn requires complex and large scale reactor design. For example, helicon sources typically use antenna structures external to a cylindrical plasma column to set up either an m=0 or m=.+-.1 mode with a well defined parallel wavelength. This geometry usually necessitates a separate source and downstream processing chamber. Helicon sources typically operate at magnetic fields of 100-400 Gauss because the plasma density, magnetic field, and wavenumber are linked by the helicon dispersion relation.

Still other techniques use inductively coupled plasma sources, which include a flat coil as the coupling element to generate the plasma. Compact reactors can be constructed from inductively coupled plasma generators. However, the plasma is generated near the window and typically has a planar, or "pancake" shape. This can result in small degree of selectivity in etching operations, because of the small chemical reaction surface area of the chamber. RFI/TCP sources with flat spiral coils couple power primarily inductively to the plasma with the RF power deposited primarily within half a skin depth (approximately 0.5-4 cm) of the window, while ECR and helicon sources are wave supported and deposit their power in the plasma bulk.

U.S. Pat. No. 4,810,935 to Boswell discloses a method and apparatus for producing large volume, uniform, high density magnetoplasmas for treating (e.g., etching) microelectronic substrates. The high density plasma is generated using whistler or helicon waves. The plasma is created in a cylinder. A coil is wrapped about the cylinder to create a magnetic field in the cylinder. An antenna is located alongside the cylinder. RF energy, the source of power used to establish the plasma, is coupled to the cylinder by the antenna.

U.S. Pat. No. 4,990,229 to Campbell et al. discloses another apparatus for forming a high density plasma for deposition and etching applications. Again, whistler or helicon waves are used to generate the plasma. The apparatus includes a cylindrical plasma generator chamber. A pair of coils produce an axial magnetic field within chamber. An antenna is mounted on the chamber, to launch RF waves at low frequency along the magnetic field. The plasma is transported by the magnetic field to a separate processing chamber.

U.S. Pat. No. 5,225,740 to Ohkawa discloses a method and apparatus for producing high density plasma using helicon or whistler mode excitation. The high density plasma is produced in a long cylindrical cavity imbedded in a high magnetic field, generated by a coil wound around the plasma chamber. In one embodiment, electromagnetic radiation is coupled axially into the cylindrical cavity using an adjustable resonant cavity to excite a whistler wave in the cylindrical cavity and hence in the plasma. In another embodiment, electromagnetic radiation is coupled radially into the cylindrical cavity using a slow wave structure to excite the whistler wave in the plasma. In both embodiments, the plasma is generated without using electrodes.

U.S. Pat. No. 5,146,137 to Gesche et al. discloses a device for the generation of plasma by means of circularly polarized high frequency waves, such as whistler and helicon waves. The apparatus includes a plasma chamber having an upper cylinder and a lower cylinder, an antenna, and a coil, both of which are placed about the upper cylinder. The coil creates a magnetic field. The antenna generates waves which are coupled through the magnetic field in the plasma in the helicon state. Four electrodes generate an electromagnetic field. First and second voltages, which are phase-sifted by 90.degree., are applied to opposing pairs of electrodes to develop the helicon wave.

U.S. Pat. No. 4,948,458 to Ogle discloses an inductively coupled plasma apparatus. The apparatus includes a generally air tight interior chamber within which the plasma is generated. To induce the desired plasma, an electrically conductive coil is disposed adjacent to the exterior of the enclosure. The coil is substantially planar, including a single conductive element formed into a planar spiral or a series of concentric rings. By inducing a radio frequency current within the coil, a magnetic field is produced which will induce a generally circular flow of electrons within a planar region parallel to the plane of the coil.

Despite these and other plasma processing techniques and apparatus, there exists a need for a plasma process and apparatus which can be used to produce and maintain high density plasma under low pressure conditions. Further, it would be desirable to provide such plasmas without requiring high outputs of energy to maintain the plasma or requiring complicated, large volume antenna structures, and thus complex and large scale reactor design. In addition, it would be desirable to provide an apparatus which includes a single chamber in which the plasma can be generated and the microelectronic substrate processed, and which includes a large chamber wall surface area, and thus a large chemical reaction surface area, to thereby increase the degree of selectivity in etching operations at low pressures.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an apparatus and process for producing a high density plasma.

It is another object of the present invention to provide an apparatus and process for producing a high density plasma for processing samples, such as microelectronic substrates.

It is another object of the present invention to provide an apparatus and process for producing a high density plasma for processing semiconductor substrates without requiring high outputs of energy to maintain the plasma.

It is yet another object of the present invention to provide an apparatus and process for producing a high density plasma for processing microelectronic substrates without requiring complicated, large volume antenna structures, and thus complex and large scale reactor design.

It is yet another object of the present invention to provide an apparatus and process for producing a high density plasma for processing microelectronic substrates which includes a large chamber wall surface area, and thus a large chemical reaction surface area, to thereby increase the degree of selectivity in etching operations at low pressures.

These and other objects are provided according to the present invention by an apparatus and process for producing high density plasmas within a single processing chamber by the interaction of an electrically conductive substantially planar antenna located outside of the processing chamber, and a magnetic field generating means, also located outside of the processing chamber. Radio frequency (RF) current is made to flow through the antenna by supplying a RF voltage from an RF power supply or generator. The substantially planar antenna can be, for example, a planar coil arranged as a spiral or as a series of concentric rings. Alternatively, the planar antenna can be a planar coil arranged as a double spiral.

The RF energy from the antenna excites the gaseous medium within the chamber to form a helicon or whistler wave within the gas. This generates the high density plasma. The magnetic field generating means generates a magnetic field within the processing chamber, the magnetic field being perpendicular to the plane of the antenna. This magnetic field causes elongation or axial extension of the high density plasma, i.e., the helicon wave is propagated along the magnetic field lines.

Thus the interaction of the planar antenna and the magnetic field results in the formation, maintenance, and propagation of a helicon wave sustained plasma within the processing chamber. This plasma can be used in a variety of processing applications, such as deposition, etching, and the like.

The present invention provides advantages over prior techniques without the corresponding problems associated therewith. The end launch configuration of the present invention differs from previous helicon sources in that the parallel wavelength is not defined by the antenna. The RF power absorption and the plasma generation are remote from the reactor walls, only a modest magnetic field is required (which can be reduced to zero near the substrate), and the configuration is compact. Further, the reactor configuration allows the aspect ratio (height/diameter) to be increased as compared to a purely inductively coupled reactor without sacrificing plasma density at the substrate. This can be advantageous in improving processing conditions with regard to pumping speed, reaction product exhaust, control of the wall chemistry, and the like.

In addition, by applying a magnetic field perpendicular to the planar antenna, the helicon wave plasma can be elongated or axially extended within the processing chamber. As a result, a larger processing chamber can be used. This in turn provides a greater amount of chamber wall surface area for plasma chemical reactions to take place, thus increasing the selectivity of the particular process.

In contrast, ECR tools require large scale magnets to generate the necessary large magnetic field, in the range of 875 Gauss, and thus require high outputs of energy, high operating costs, etc. Prior tools used to generate helicon plasmas have used complex antenna structures thought necessary to generate the desired helicon wave configuration. Inductively coupled plasma tools produce flat plasmas, with limited selectivity and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings which form a portion of the original disclosure of the invention:

FIG. 1 is a side cross-sectional view of a plasma processing apparatus in accordance with the present invention;

FIG. 2 is a perspective view of one embodiment of a planar antenna electrically coupled to a RF source;

FIG. 3 is a perspective view of another embodiment of a planar antenna;

FIG. 4 is a top plan view of yet another embodiment of a planar antenna;

FIGS. 5a, 5b, and 5c are graphs illustrating radial profiles of B.sub..theta. -probe voltage (peak-to-peak) versus: (a) coil current for p=7.5 mTorr and z=3.9 cm; (b) axial distance z from a dielectric window in the apparatus of the invention for p=7.5 mTorr and I=50 A; and (c) pressure for z=6.4 cm and I=50 A, respectively;

FIGS. 6a and 6b are graphs illustrating axial scans of the B.sub..theta. -probe voltage (peak-to-peak) versus pressure with I=25 A and I=75 A, respectively;

FIGS. 7a and 7b are graphs illustrating (a) the phase versus distance plot, and (b) the imaginary parallel wavenumber (k.sub..parallel.i) versus distance and pressure, each for the data of FIG. 6b, respectively;

FIGS. 8a and 8b are graphs illustrating axial scans of B.sub..theta. -probe voltage (peak-to-peak) versus coil current with (a) pressure=1 mTorr and (b) 25 mTorr, respectively;

FIGS. 9a and 9b are graphs illustrating the imaginary wavenumbers k.sub.i (cm.sup.-1) for the data illustrated in FIGS. 8a and 8b, respectively, plotted versus magnetic field, along with theoretical value (dashed line) and the measured real wavenumber k.sub.r (solid line); and

FIGS. 10a and 10b are graphs illustrating density profiles versus (a) field and (b) pressure magnetic, respectively.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein. Rather, this embodiment is provided so that the disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. For purposes of clarity the scale has been exaggerated.

FIG. 1 illustrates a side cross sectional view of a plasma processing apparatus or tool in accordance with the present invention, designated generally as 10. The plasma processing tool includes a processing chamber 12, having first and second opposing ends 50 and 52, which defines a generally air-tight interior chamber within which the plasma is generated and within which a substrate to be processed is treated. The chamber is preferably circularly symmetric about an axis 54. One or more support surfaces or chucks 14 are provided within the interior of the process chamber 12 for supporting a substrate 16 to be treated, such as a semiconductor wafer or other microelectronic device. The chuck may be connected to a radio frequency source 17 which provides a biasing potential to the chuck. In addition, techniques known in the art can be used to control the substrate temperature during processing.

The processing chamber 12 includes in at least one wall thereof a dielectric window or shield 18. The dielectric window 18 provides a vacuum seal for the processing chamber 12, and allows penetration therethrough of electric and magnetic fields generated as described in more detail below. The dielectric window 18 is preferably formed of quartz, although other dielectric materials known in the art can be used. The thickness of the dielectric window 18 is not critical, and is selected to be sufficient to withstand the differential pressure created by a vacuum within the processing chamber 12.

The processing chamber also includes a gas inlet port 20 for introduction of a process gas and an outlet port 22 for exhaustion or removal of the gas and reaction products. The gas can comprise a single component or a mixture of gas components, and is selected according to the type of etching or surface treatment of the substrate that is required. For example, in processing silicon dioxide (SiO.sub.2) substrates, a hydrocarbon gas, or a halogenated hydrocarbon gas, such as CH.sub.4, CHF.sub.3, and the like, can be used.

Techniques and apparatus of supplying a process gas within the interior of the process chamber 12 are well known in the art. The location of inlet port 20 is not critical and gas can be introduced at any point which provides for the even distribution of the gas within the processing chamber. For example, as illustrated in FIG. 1, to enhance the uniformity of gas distribution, a distribution ring 24 as known in the art can be used. Typically such a gas distribution ring will include an annular member and a series of ports distributed substantially equally around the ring and extending from the annular member to the open center of the ring. This arrangement is such that a uniform flow of gas is directed toward the chuck 14. When present such a ring is advantageously located opposite from the substrate chuck 14 and adjacent the dielectric window 18.

A plasma is established within chamber 12 by establishing a magnetic field within processing chamber 12 and coupling radio frequency energy into the gas within chamber 12 using an RF antenna as described below. The RF energy from the antenna excites the gaseous medium within the chamber 12 to form a helicon wave within the plasma. This helicon wave is then propagated along the magnetic field lines. This generates an elongated high density plasma, i.e., a plasma which is axially extended along axis 54. As used herein, the term "high density plasma" refers to a plasma having a density above about 1.times.10.sup.11 cm.sup.-3.

To induce the desired plasma, an electrically conductive, substantially planar antenna 26 is provided outside the processing chamber 12 adjacent the dielectric window 18. Antenna 26 can comprise a single conductive element formed into a substantially planar spiral as illustrated in FIG. 2. Alternatively, antenna 26 can comprise a single conductive element formed into a series of concentric rings, as illustrated in FIG. 3. In FIG. 3, planar antenna comprises a series of concentric loops 28, where each succeeding loop is connected by a short transverse member 30. In yet another embodiment of the invention, antenna 26 can comprise a substantially planar double coil antenna, as illustrated in FIG. 4. Other antenna configurations can be used in accordance with the present invention, for example a solid disc-like planar electrode. With a solid disc-like planar electrode, the dielectric window should be very thin, i.e., less than about 1 millimeter, or be replaced by a dielectric feed through behind the solid electrode.

The specific configuration of the antenna 26 is selected based upon the particular desired plasma wave configuration, which in turn can be based upon the particular processing required, i.e., deposition, etching, etc., substrate material, and the like. For example, the antenna configurations of FIGS. 2 and 3 are particularly useful in launching a m=0 mode helicon wave configuration. Alternatively, the antenna configuration of FIG. 4 is particularly useful for launching a m=.+-.1 mode helicon wave in the plasma generated in processing chamber 12.

As illustrated in FIGS. 1, 2, and 3, the antenna includes a center tap 32 and an outer tap 34. Radio frequency (RF) current is made to flow through antenna 26 by applying an RF voltage between center tap 32 and outer tap 34. The outer tap 34 is shown as a circuit ground in FIG. 2. Alternatively, tap 34 can be the driven side and tap 32 can be the ground. In addition, each of taps 32 and 34 can be capacitively isolated from ground so as to adjust the capacitive coupling of antenna 26.

The RF voltage is applied from an RF power supply or generator 36. The RF generator will typically operate at a frequency from about 0.5 MHz to about 50 Mhz, and preferably at about 13.65 MHz. The RF generator can be any of the types of RF generators known in the art for the operation of microelectronic substrate processing equipment.

As best illustrated in FIG. 2, the RF voltage passes from generator 36 through a cable 38 to a matching box or circuit 40, and the output is coupled to the planar antenna 26, which in turn resonantly couples the RF power into the gaseous medium within the chamber 12. Matching box 40 typically includes two variable capacitors 42 and 44. The capacitors are provided to adjust the circuit resonance frequency with the frequency output of the RF generator. Impedance matching maximizes the efficiency of power transfer to the planar antenna 26, and thus coupled into the plasma, and minimizes the power that is reflected back along the cable to the RF power supply.

The skilled artisan will appreciate that other circuit designs can be used for resonantly tuning the operation of the planar antenna 26 and for matching the impedance of the antenna with the RF generator.

Inducing a radio frequency current within the antenna 26 creates a RF or oscillating magnetic field. The magnetic field penetrates the dielectric window of chamber 12 in the form of a helicon wave. The helicon wave induces plasma formation within the gaseous medium within chamber 12 by energizing or exciting the electrons of the gas.

The antenna 26 is adapted to couple the radio frequency power into the gas in processing chamber 12 to thereby generate the plasma. The plane of the antenna 26 is oriented substantially parallel to the dielectric window 18 and the chuck 14 within processing chamber 12. The planar antenna can be placed close to the dielectric window, preferably from about 0.5 mm to about 2 mm from the window, and can be touching the dielectric window. The antenna 26 advantageously can be up to about 5 centimeters or greater from chuck 14. The skilled artisan will recognize that these exact distances can vary depending upon the particular application.

The planar antenna 26 will generally be circular, although ellipsoidal patterns, such as that illustrated in FIG. 4, and other deviations from true circularity can be used. The antenna is substantially planar, although deviations from true planarity can also be tolerated. Adjustments to the profile of the antenna may be made to modify the shape of the electric field which is capacitively coupled to the plasma.

The diameter of the antenna 26 will generally correspond to the size of the plasma to be generated. Advantageously, the diameter of the antenna is from about 10 centimeters to about 30 centimeters. The diameter of the antenna can be greater or smaller depending upon for example the size and number of substrates to be treated. For example, to treat a single 8 inch diameter substrate, the diameter of the antenna 26 can range from about 6 centimeters to about 10 centimeters.

The number of turns of antenna 26 is dependent upon other factors, such as the diameter of the antenna, the number of substrates to be treated, whether the antenna is a single or double coil, the desired inductance, and the like. For example, using a single coil antenna as illustrated in FIG. 2 to treat a single substrate, the antenna can include about 2 to about 4 turns, although this number can be greater or smaller.

The planar antenna 26 is constructed of any electrically conductive metals, typically copper. In addition, antenna 26 can be hollow to allow for water or air cooling. The antenna can have a current carrying capacity from about 5 to 30 amps.

As noted above, the desired plasma is generated by the apparatus of the invention by coupling the RF power from antenna 26 into the gaseous medium of chamber 12 with a magnetic field substantially perpendicular to the plane of the antenna 26. In the apparatus of the invention, the magnetic field is generated by magnetic field generating means 46 positioned outside of the processing chamber 12. Magnetic field generating means 46 can be any of the types of devices known in the art for generating a magnetic field in plasma processing applications, for example, a coil about the exterior of processing chamber 12, a set of permanent magnets, and the like. The axes of planar antenna 26, magnetic coil 46 and chuck 14 are generally parallel to one another and preferentially colinear with the axis 54 of chamber 12.

The strength of the magnetic field produced by magnetic field generating means can vary. The process of the invention is effective with magnetic fields as low as 5 Gauss. Stated differently, magnetic field generating means 46 can be used to generate an axial magnetic field having a strength of about 5 Gauss or greater in processing chamber 12. For example, the axial magnetic field can have a strength of about 5 Gauss for the standard RF frequency of 13.56 Mhz.

When the helicon wave plasma is induced by antenna 26, the magnetic field elongates or axially extends the plasma in the chamber along axis 54. Stated differently, when no magnetic field is present, planar antenna 26 induces a plasma having a first thickness 56 as designated in FIG. 1. The magnetic field generated by magnetic field generating means 45 elongates or axially extends the plasma production region along axis 54 to produce a plasma having a second thickness, designated generally as 58 in FIG. 1, which is greater than the first thickness 56. As a result, a larger chamber can be used, which in turn provides a greater amount of chamber wall surface area for plasma chemical reactions to take place.

This is advantageous for the reasons described below. Plasma processes, such as ECR, helicon, inductively coupled plasma, and the like, can provide high density plasmas at low pressures. However, other problems arise with the use of such plasmas, for example, with etching of silicon dioxide. The oxide should be selectively etched with respect to the silicon so that silicon is not removed in the process. Accordingly, the rate of oxide etching advantageously is about 20 to 30 times that for silicon. Etching of oxide typically uses a fluorocarbon chemistry, such as CHF.sub.3. The plasma will dissociate the CHF.sub.3 or other input gas and produce a variety of fluorocarbon species, along with F and H. To achieve the desired selectivity and oxide etch rate, it is necessary to form certain fluorocarbon species in the plasma (such as CF.sub.3, although the exact identity of such species is not known) and to minimize the F concentration.

Prior etching processes run at high pressures have achieved this by gas phase reactions in the volume of the plasma to "scavenge" F, typically by reacting F with H to form HF. This is believed to also lead to the formation of desired fluorocarbon species to assist with selectivity.

At low pressures, however, the gas phase reactions are much slower. This is believed to be due to the fact that at low pressures, the gas densities are lower, so that the reactions are much less frequent. As a result, the gas phase reactions are believed to be ineffectual at low pressures in reducing the F concentration and producing the desired fluorocarbon for selectivity.

It is believed possible to remove F by reactions occurring on the walls of a plasma reaction or processing chamber which contains the substrate. These reactions are believed to be increasingly more effective as the area of the chamber walls is increased. Therefore, at low pressures, as necessitated by processing of increasingly smaller devices, it is desirable to have a larger reactor surface area for controlling the plasma chemistry. In the present invention, because the walls of the reaction chamber can be extended, a greater surface area is provided on which the gas in the plasma can catalyze to form the desired chemical species for processing. This is advantageous for increasing selectivity in processing. In addition more wall space is available for pump ducts to exhaust the reaction products.

The operating pressure is dependent upon the particular process being performed. The range of pressures in which the plasma can be generated is broad, ranging from about 1 mTorr to about 1 Torr. Processes and apparatus for maintaining a desired pressure within the interior of the processing chamber are known in the art.

Quantitative Description

A description of the present invention for a particular set of parameters (rf frequency, pressure, magnetic field) will now be provided. A much wider range of rf frequency, pressure and magnetic field parameters can also be used in accordance with the present invention.

In the absence of magnetic fields, RF fields are expected to penetrate on the order of a skin depth into the plasma, .delta..about.c/.omega..sub.pe