High density plasma deposition and etching apparatus

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plasma deposition or etching method and various apparatus for depositing a thin film onto a substrate or for removal (etching) of a film from a substrate. The present invention includes the use of a new and significantly better high density plasma deposition and etching apparatus, a significantly improved magnetic means for the plasma source region and operation with a specified range of processes and gases. Applications of the present invention include the removal by etching of a layer from a surface, the removal by sputtering of a layer from a surface, or the deposition of a layer onto a surface.

2. Related Art

Etching

Plasma etching involves using chemically active atoms or energetic ions to remove material from a substrate. It is a key technology in the fabrication of semiconductor integrated circuits. However, before the advent of microwave plasmas utilizing electron cyclotron resonance (ECR), it was becoming difficult for conventional plasma etching techniques to satisfy the requirements dictated by the increase in device packing density. Specifically, the requirement for fine pattern etching without undercutting (anisotropic etching) and the requirements for low damage and high selectivity could hardly be satisfied at the same time.

Deposition

Plasma Enhanced Chemical Vapor Deposition (PECVD) is a widely used technique to deposit materials on substrates in a host of applications. In normal Chemical Vapor Deposition (CVD) the chemical reaction is driven by the temperature of the substrate and for most reactions this temperature is high (>800.degree. C.). The high substrate temperature needed precludes use of this method in a number of applications particularly in microelectronics, displays and optical coatings. The role of the plasma is to dissociate and activate the chemical gas so that the substrate temperature can be reduced. The rate of dissociation, activation and ionization is proportional to the density of the plasma. It is therefore of importance to make the plasma as dense as possible.

Sputtering

Sputtering is also a widely used method for depositing materials onto substrates for a wide variety of applications such as the production of hard or decorative coatings and the coating of glass. In general, a plasma is produced at the sputter target material and the sputter target is biased to a negative voltage of around 700 V. Plasma ions (generally argon) impact the surface and sputter the material which then transports as neutral atoms to a substrate. Reactive gases can be introduced to chemically react with the sputtered atoms at the host substrate in a process called reactive sputter deposition. Rate is often important and it is therefore important to make the plasma as dense as possible. Ionization of reactive gases is also important and is helped by having plasma in the vicinity of the substrate material. Sputtering is also done by ions accelerated in an ion or plasma gun and then made to bombard the sputter target. In this case, a bias voltage on the target is not necessary. For sputtering insulating materials, RF voltage bias can applied to the sputter target.

Existing Methods

There are presently two widely used methods for plasma deposition and etching, the parallel plate reactor and the ECR plasma deposition system. There are also methods based on the use of RF to produce plasma including ordinary induction techniques and techniques based on whistler waves.

Parallel Plate Reactor (Diode)

The RF diode has been widely used for both deposition and etching. It is described in detail in the book by Chapman ("Glow Discharge Processes" John Wiley & Sons 1980). It uses RF at 13.56 MHz capacitively coupled to one electrode while the other electrode is grounded. The pressure in the system is typically 1 mtorr to 1 torr and the plasma density is typically 10.sup.10 electrons per cm.sup.3. The rate at which both deposition or etching occurs is dependent on the density of the plasma and the density (pressure) of the reactive gas used to etch, or, in CVD processes, to deposit.

In etching, the high pressure needed to sustain the discharge causes collisions between the ions and the background gas. This causes the paths of the etching ions or atoms to be randomized or non-directional, leading to undercutting of the mask. This is referred to as an isotropic etch. It is desirable to have the etch atoms or ions be directional so that straight anisotropic etches can be achieved. At the high pressure used in RF diode discharges, it is necessary for the ions to have high energy (up to 1 KeV) to achieve an anisotropic etch. However, the high energy of the ions can cause damage to the substrate, film materials or photoresist.

The plasma is sustained by secondary electrons that are emitted by ions impacting the cathode. These electrons are accelerated by the voltage drop across the sheath which is typically 400-1000 V. These fast electrons can bombard the substrate causing it to have a high voltage sheath drop. This high voltage can accelerate the ions leading to damage of the substrate or film material. The presence of high energy electrons leading to high voltage sheath drops is undesirable.

Electron Cyclotron Resonance Plasmas

The advent of using microwaves at 2.45 GHz and a magnetic field of 875 Gauss to utilize electron cyclotron resonance allowed the generation of high density plasmas at low pressure. The advantages of this technique for plasma etching are described by Suzuki in U.S. Pat. No. 4,101,411 and in an article entitled "Microwave Plasma Etching" published in Vacuum, Vol. 34, No. 10/11, 1984. Due to a low gas pressure (0.04-0.4 Pa) and high plasma density (1.7-7.times.10.sup.11 electrons/c.sup.3) anisotropic etch with high etch rates is achievable.

Suzuki, in U.S. Pat. No. 4,101,411, describes a plasma etching apparatus using ECR. Natsuo, in U.S. Pat. No. 4,401,054 describes a plasma deposition apparatus utilizing ECR. In U.S. Pat. No. 4,876,983 there is described a plasma etching apparatus to improve uniformity and have the specimen close to the source chamber.

While this technique is desirable over the parallel plate reactor in many respects, it has several limitations. The magnetic field needed is very high (1-2 kGauss) which means that heavy, power consuming electromagnets must be used. The maximum density is limited by either cut-off in certain configurations or by refraction in other configurations to the value of 1.times.10.sup.12 electrons/cm.sup.3 in the source. The expense of the power supply and necessary hardware to generate and transmit the microwaves is high. The uniformity (or width of the plasma profile) is not very good.

RF Helicon Whistler Wave Plasmas

The first use of helicon type whistler waves to generate dense plasmas was described in 1970 by Boswell in the journal, Physics Letters, Vol. 33A, pp 457-458 (1970) which showed an antenna configuration used by Ovchinnikov. This type of antenna excites an m=1 mode. The frequency of excitation was 8 MHz. The density profile of the 10 cm plasma was found to be quite peaked, particularly at the higher magnetic field strengths needed for high densities. In Boswell, U.S. Pat. No. 4,810,935, two mathematical relationships are required to be satisfied. These equations are in fact overly restrictive and not applicable to the approach outlined by Campbell, Conn and Shoji in U.S. Pat. Nos. 4,990,229 and 5,122,251.

In these publications the mechanism for efficient coupling of the RF energy to the plasma could not be explained. Chen, in an Australian National university report, explained the mechanism as Landau damping.

Chen, in a paper presented in August 1988 and published in the journal, Plasma Physics and Controlled Fusion, Vol. 33, 1991, describes a system using whistler waves to generate dense plasmas for use in advanced accelerators. The type of antenna used in this arrangement was similar to that used by Boswell in that it excited the m=1 mode and was a type known as the Nagoya Type III antenna. This type of antenna is explained in a paper by Watari (1978). The frequency of excitation was 30 MHz.

Campbell, Conn and Shoji, in U.S. Pat. Nos. 4,990,229 and 5,122,251 describe new and highly efficient antenna means designed to excite the m=0 and the m=1 modes, and to control the wave number of the excited wave. This is important in obtaining the maximum plasma density, in generating the broadest spatial plasma density profile in the source and process chamber regions, and in providing control over the electron temperature in the plasma.

Efficiency of plasma production by low frequency whistler waves depends on the coupling of RF energy into the plasma. As discussed by Campbell et al. in U.S. Pat. No. 4,990,229, an important mechanism for damping of the RF energy is Landau damping. The phase velocity of the whistler wave is given by .omega./k.sub.z, where k.sub.z is given by the dispersion relation and depends on the plasma density and vacuum magnetic field strength. Ideally, the phase velocity of the wave should be near the maximum of the ionization potential of the gas we wish to ionize. From the dispersion relation for the m=0 mode, the higher the value of k.sub.z, the higher the density. However, the phase velocity of the wave is .omega./k.sub.z and so increasing k.sub.z decreases the energy of the electrons that are accelerated by the wave. If the k.sub.z is too high then the energy of the electrons may fall below the ionization potential.

Also, Campbell, Conn and Shoji in the above-mentioned patents use a magnetic bucket means in conjunction with the plasma generator to provide a uniform plasma density over large circular or rectangular areas. They use one or multiple plasma generators in conjunction with cylindrical or rectangular magnetic buckets to provide a uniform density over a large area for the coating or etching of substrates such as are needed for IC or flat panel display processing. They use expansion of the magnetic field to allow deposition or etching over a large area.

Other RF Induction Sources

Other existing methods use RF circuit resonances to generate plasma. These methods are less efficient than those using low frequency whistler waves, and do not generate high density plasmas. Ogle, in U.S. Pat. No. 4,948,458 describes an RF means to produce planar plasma in a low pressure process gas using an external planar spiral coil (or series of concentric rings) and connected to a second loop which is positioned to allow for effective coupling of the circuit and for loading of the circuit at the frequency of operation. Steinberg et al., in U.S. Pat. No. 4,368,092, describes a plasma generating system employing a helical inductive resonator for producing the plasma external to an etching chamber. The plasma is non-uniform and passes through a tube before utilization. U.S. Pat. No. 4,421,898, describes an inductively-coupled plasma generating apparatus, where a transformer having a magnetic core induces electron circulation in an insulating tube carrying a process gas. Gas ionization is non-uniform, and exposure to the wafer occurs downstream. U.S. Pat. No. 4,626,312, describes a conventional parallel plate plasma etcher where the wafer is situated on a lower electrode and a plasma is generated by applying radiofrequency energy across the lower electrode and a parallel upper electrode. U.S. Pat. Nos. 4,668,338 and 4,668,365, describe magnetically-enhanced plasma processes for reactive ion etching and chemical vapor deposition, respectively. Flamm et al. in U.S. Pat. No. 4,918,031 describes an L-C circuit referred to as a helical resonator which consists of an inner helically shaped copper coil surrounding a quartz tube and attached at one end to a cylindrical copper shield. The opposite end of the inner coil is unterminated. No external magnetic field is employed in these approaches and all generate plasmas at low pressure in the 1-10 mtorr range but at moderate density on the order of 10 cm.sup.3 in the quartz source tube or just below a planar spiral coil and without a high degree of spatial uniformity. No externally generated magnetic field is employed in these RF plasma generators.

SUMMARY OF THE INVENTION

The present invention utilizes low frequency RF whistler waves to generate plasmas of high density for use in plasma etching, deposition, and sputtering equipment. In conjunction with a source tube into which a gas is injected and along the central axis of which a magnetic field is established, a single loop antenna is disposed in a plane transverse to the central axis. The angle of the antenna plane is 90.degree. if it is desired to excite only M=0 mode, or at less than 90.degree. if it is desired to excite components in both M=0 and M=1 mode. The gas is a noble or reactive gas at a pressure of 0.1 mtorr to 200 mtorr. The magnetic field strength is in the range of 10 to 1000 gauss and the antenna is driven with RF energy of 100 W to 5 KW at a frequency range of 2 MHz to 50 MHz. With the antenna placed along the tube source at a sufficient distance along the axis from the gas injection end, the other end defining an open egress zone leading to a process chamber, the single loop antenna surprisingly provides highly efficient wave coupling to establish a high density and high current plasma.

In accordance with other features of the invention, the plasma generated by this plasma source is supplied to a process chamber including a magnetic bucket system for holding the plasma away from the process chamber walls. The arrangement provides, in combination, a uniform plasma density over a large circular area, so that a large substrate may be etched or otherwise processed. Another feature is that a magnetic cusp zone may be established, at the material surface being processed, to homogenize and make more uniform the plasma at that location. An aspect of this is that the magnetic cusp position relative to the substrate may be time modulated to enhance uniformity and reduce sensitivity to substrate location.

Further, the magnetic field may be expanded to allow deposition or etching over a large area and current flows may be equalized by serial driving of antennas in systems having more than one antenna. Other features reside in configurations which employ one or more multiple geometrical areas for coating or etching of square or rectangular substrates, or a linear juxtaposition for coating or etching large substrates.

The invention provides a module with a highly efficient magnetic means of transporting plasma from a plasma generator means to a substrate located on a cooled substrate holder located in a substrate process chamber and in which the processing of the substrate is highly uniform and the substrate process module is compact.

The invention provides a gas distribution means in the top of the process chamber as an integral part of the process chamber structure in order to attain highly efficient plasma operation and highly uniform processing of the substrate while permitting the process module to be reduced in height.

The invention attains highly efficient plasma operation in a compact substrate process module which can attain excellent characteristics for the etching of IC wafers as represented by high etch rate, high uniformity, high selectivity, high anisotropy, and low damage.

The invention achieves high density and highly uniform plasma operation at low pressure from 0.3 mtorr to 5 mtorr for etching an IC substrate and from 1 mtorr to 30 mtorr for deposition of films on to substrates.

The invention provides a substrate processing system capable of operating with a wide variety of gases and combinations of gases, including highly reactive and corrosive gases.

The invention provides such a substrate processing system capable of etching or depositing films listed in Table 1 and Table 2 using gases fed into the plasma generator region, the process chamber region, into one region or another, or into both regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting the principle of operation and RF current flow in an antenna constructed according to the invention as shown in U.S. Pat. No. 4,990,229.

FIGS. 2A, 2B and 2C are schematic views of antennas constructed according to the principles of the invention.

FIG. 3 is a schematic diagram depicting the principle of operation and RF current flow in a plasma source constructed according to the invention as shown in U.S. Pat. No. 5,122,251.

FIGS. 4A and 4B illustrate in schematic form two basic configurations of a plasma deposition or etching apparatus accordance with this invention.

FIG. 5A is a schematic diagram of a second example of a system in accordance with the present invention in which the plasma source region is connected to a magnetic bucket region where uniformity requirements are important.

FIG. 5B is a plan view of the arrangement of FIG. 5A, taken along the line 3A--3A in FIG. 5A.

FIG. 6A is a perspective view of a third example of a system in the present invention for deposition or etching over a large rectangular area where uniformity is important.

FIG. 6B is a plan view of the arrangement of FIG. 6A, taken along the line 4A--4A in FIG. 6A.