Patent Application
20070205096
The present invention relates to an apparatus used for processing a semiconductor substrate.
FTS (Facing Target Sputtering) method is a semiconductor fabrication technique that provides high density plasma, high deposition rate at low working gas pressure to form high quality thin film. In a facing target type of sputtering apparatus, at least a pair of target planes are arranged to face each other in a vacuum vessel, and magnetic fields are generated perpendicularly to the target planes for confining plasma in the space between the facing target planes. The substrate is arranged so as to be positioned at the side of the space so that films are produced on the substrate by sputtering.
As discussed in U.S. Pat. No. 6,156,172, a typical FTS apparatus includes a vacuum vessel for defining therein a confined vacuum chamber, an air exhausting unit having a vacuum pump system to cause a vacuum via an outlet, and a gas supplying unit for introducing sputtering gas into the vacuum vessel. A pair of target portions are arranged in the vacuum vessel in such a manner that a pair of rectangular shape cathode targets face each other so as to define a predetermined space therebetween.
Another FTS apparatus discussed in the '172 patent confines sputtering plasma in a box type of plasma space using a pair permanent magnets so as to face N and S-pole generate magnetic flux circulating perpendicularly the outside space of the first facing targets which defines facing target mode in combination with electric fields perpendicular to target planes in plasma space. The pair of magnets generate a conventional magnetron mode with a closed magnetic flux from the pole of magnets in the vicinity of the outside area of the pair of target planes in addition to the facing target mode. The cathodes of all the targets are arranged so as to recoil and confine the electrons into the plasma space by the aid of both the facing target mode and the magnetron mode.
To improve the deposition speed of the equipment, the '172 patent discloses an FTS apparatus which includes: an arrangement for defining box-type plasma units supplied therein with sputtering gas mounted on outside wall-plates of a closed vacuum vessel; at least a pair of targets arranged to be spaced apart from and face one another within the box-type plasma unit, with each of the targets having a sputtering surface thereof; a framework for holding five planes of the targets or a pair of facing targets and three plate-like members providing the box-type plasma unit so as to define a predetermined space apart from the pair of facing targets and the plate-like members, which framework is capable of being removably mounted on the outside walls of the vacuum vessel with vacuum seals; a holder for the target having conduits for a coolant; an electric power source for the targets to cause sputtering from the surfaces of the targets; permanent magnets arranged around each of the pair of targets for generating at least a perpendicular magnetic field extending in a direction perpendicular to the sputtering surfaces of the facing targets; devices for containing the permanent magnets with target holders, removably mounted on the framework; and a substrate holder at a position adjacent the outlet space of the sputtering plasma unit in the vacuum vessel.
In sputtering, ionized atoms bombards a surface. The most common form of sputtering process is sputter deposition where the ionized atoms by bombarding a target, eject neutral atoms from the target material to deposit on a substrate placed in a suitable location to intercept the ejected atoms. The other form of sputtering process is sputter etch where the ionized atoms bombard a substrate to form a pattern on the substrate.
The ionized atoms are typically large atomic weight gases, such as argon and xenon, created in a plasma and then accelerated into the target cathode by an electric field. In some cases, reactive gases instead of inert gas of argon or xenon are used, resulting in the deposition of a compound of the target material and the reactive gas species. A dc diode sputtering system uses a high dc voltage, typically between 200 to 800 V, to create a plasma discharge, comprising a target material as a cathode electrode, and a substrate as an anode electrode where deposition occurs. The dc diode sputtering systems are characterized by high voltages and low currents, and thus susceptible to charging and arcing. Further, dc diodes are inadequate for dielectric deposition due to the rapid build-up of positive charges on the surface of the insulator and the difficult of secondary electron emission from the cathode.
Replacing the dc power supply with an rf (radio frequency) power supply, typically at a frequency of 13.56 MHz, results in an rf sputtering system with reduced charging and arcing problems. The rf voltage also results in changes to the electron and ion motions, producing better energy coupling and higher plasma densities.
Further improvement to the sputtering system to improve the deposition rate is the introduction of a magnetron system, consisting of an external magnetic field superimposed upon the electric field. Magnetron sputter devices are characterized by crossed electric and magnetic fields, with the electric field perpendicular and the magnetic field parallel to the target cathode. The magnetic field confines the glow discharge plasma, and traps the electrons moving under the influence of the electric field, resulting in an increase in the gas atom-electron collision probability, the ionization rate of argon atoms, the frequency of argon ions striking the target, and thus leading to a higher sputter deposition rate. The basic arrangement of the magnetic field is to create a closed loop travel of the electrons which establishes the region of preferred sputtering.
Generally, a permanent magnet structure is employed to generate the magnetic field, but electromagnetic devices can also be used for this purpose. The magnetic field can also be moved by a motor drive to average out the intrinsic nonuniformity, and to spread the erosion pattern on the target face. The ion bombardment of the target also impart a significant amount of thermal energy to the target, thus the heat generated during sputtering must be adequately removed to ensure optimum performance of a magnetron system. By applying a back bias to the substrate, the deposited film can have certain amount of control of ion bombardment of the growth films. This is called bias sputtering, resulting in densifying and planarizing the depositing film.
Another improvement to sputter technology is to ionize the sputtered atoms to enhance the directionality of a deposition, a technique sometimes called IPVD (ionized physical vapor deposition). An IPVD system is based on a magnetron sputtering, with the addition of an inductively coupled rf plasma located between the cathode and the substrate for in-flight ionization of the sputtered atoms. Thus there are two somewhat separate plasmas within the process chamber with the same background gas. One is a conventional dc or rf plasma of the magnetron, located close to the cathode target, so that ions can strike the cathode target to cause sputter emission of the target atoms. A second plasma, generated by the inductive coil, is presented between the target cathode and the substrate. Thus a high fraction of the sputtered atoms emitted from the target are ionized by electron bombardment when they pass through this plasma. The ionized sputtered atoms drift within the plasma, and can either accelerated to the substrate due to the plasma potential (typically +10 V) and the substrate potential (0 to −50 V), or accelerated toward the target cathode by the magnetron voltage (typically −400 V), sputtering more atoms from the cathode.
In a first aspect, a wafer magnetron includes a support for supporting a wafer; a generator to provide an electric field substantially perpendicular to the wafer surface; and a magnet system for generating a magnetic field, a portion of the magnetic field lines being parallel to the wafer surface.
In a second aspect, a wafer magnetron processing chamber includes a vacuum process chamber; a plasma generator to generate a plasma within the process chamber; a support for supporting a wafer, the wafer positioned within the process chamber; a generator to provide an electric field substantially perpendicular to the wafer surface; and a magnet to generate a magnetic field having a portion of the magnetic field lines parallel to the wafer surface.
In a third aspect, a method for processing a semiconductor wafer includes trapping electrons in the vicinity region of the wafer surface by a wafer magnetron effect, wherein the trapped electrons ionize the atoms and ions before reaching the wafer surface; and increasing an ionization rate for atoms and ions bombarding the wafer surface.
The system ionizes the neutral atoms by employing a magnetron effect at the wafer substrate rather than just at the target. By establishing a magnetic field in the vicinity of the substrate to trap electrons, the neutral atoms traveling to the substrate can be ionized by collisions with the electron cloud. The electrons in this magnetic confinement can have sufficient energy to ionize incoming atoms and not being annihilated by the ions. A high proportion of the incoming atoms can be excited by this electron cloud and then accelerated by a back bias field onto the wafer surface. The high number of mobile ions at the surface can provide enough surface mobility to significantly decrease the required deposition temperature, possibly even down to room temperature.
The electron cloud can be self-generated by secondary electron emission, or by electron capturing from nearby plasma environment. The electrons can be ejected from the substrate by the energetic bombardment of the incoming atoms or ions, and they become captured in the electron cloud above the wafer by the present invention wafer-vicinity magnetron. The electron cloud can also be generated from ionizing radiation, microwaves or electron bombardment, with electron bombardment is preferable because it can be contained in the specific area of interest. The electron population can be generated or maintained by a field emission tip array or an electron gun at the wafer-vicinity magnetron boundary. By adjusting the electron flow and geometry, the ions can even be partially neutralized before hitting the wafer surface, giving some additional control. By shaping the magnetic field dynamically, the imperfections caused by the non-uniform deposition pattern of standard magnetrons can further be corrected.
In one embodiment, a system generates a wafer-vicinity magnetron effect, providing a cross electric field and a magnetic field in the vicinity of the wafer. The presence of the wafer-vicinity magnetron creates a magnetic confinement of the electrons in the region near the wafer surface, thus trapping an electron cloud and providing a means for ionizing the atoms to strike the wafer.
The wafer-vicinity magnetron is independent of the conventional magnetron technology, thus it can be used in a magnetron sputtering system where there would be two magnetron units, one for the cathode target and one for the wafer substrate. The present invention wafer-vicinity magnetron can also be used in systems not employing conventional magnetron technology, such as non-magnetron dc or rf diode sputter systems, plasma-enhanced chemical vapor deposition systems, reactive ion etching systems, ion beam system, or even thermal deposition or processing system.
The wafer-vicinity magnetron provides a magnetic field and an electric field in the vicinity of the wafer substrate and the presence of electrons to be trapped in the electromagnetic field. The magnetic field can be generated from a permanent magnet system or from an electromagnetic device. The electric field can be generated from a wafer back bias system, or by an externally applied electric field. The electrons can be externally supplied or internally present in the process chamber, for example due to a plasma condition or due to the bombardment of the substrate to liberate secondary electrons.
The wafer-vicinity magnetron can be directed toward the sputter deposition, to provide ionized sputtered atoms having high energy to reach the wafer substrate. One preferred embodiment of the present invention wafer-vicinity magnetron is to provide low temperature deposition by sputtered ions having enough energy to provide the surface mobility needed for either a surface reaction or a crystalline arrangement.
The wafer-vicinity magnetron effect could enable crystalline deposition virtually independent of wafer temperature, and possibly even at room temperature. Currently, most prior art systems use temperature to provide enough surface mobility or with a back bias potential where the ions can be accelerated into the wafer, lowering the required surface temperature. However, the back bias potential technique is not really effective since most of the atoms reaching the wafer are neutral, and thus not affected by the back bias field. The present invention wafer-vicinity magnetron provides a method to ionize the atoms in the vicinity of the wafer, and together with a back bias field, accelerating the ions into the wafer to provide surface mobility and thus reducing wafer temperature. The atom ionization is accomplished by a wafer-vicinity magnetron trap with various ways to generate the trapped electrons.
Various techniques can be used to ionize atoms within a given volume. Ionizing radiation, microwaves or electron bombardment can all be used to create a plasma. Of these methods, electron bombardment is preferable because it can be contained in the specific area of interest. The electrons are ejected from the substrate by the energetic depositions and they end up in the electron cloud above the wafer. The electrons in this cloud have sufficient energy to ionize incoming atoms and not being annihilated by the ions. A magnetic field is used to contain the electron charge. The electrons are then used to ionize and excite a very high proportion of the incoming ions, which are then accelerated by the back bias field onto the wafer surface. The high number of mobile ions at the surface can significantly decrease the required deposition temperature, down to room temperature.
Though the sputtering assembly shown is either a planar magnetron sputtering assembly or a hollow cathode magnetron sputtering assembly, any sputtering assembly can be used in the present invention, for example, a conical sputtering assembly, a cylindrical magnetron sputtering assembly, a s-gun sputtering assembly, a facing target system sputtering assembly.
The magnetic field generated by the wafer-vicinity magnetron is preferably spread so the magnetic field covers a larger wafer area, thus providing a more uniform deposition. This can be accomplished by spreading the magnetic poles, by superimposing the magnetic fields of several magnets to shape the magnetic field, or by a motor drive system to move the magnet system. If the magnetic field parallel to the wafer surface is increased, the sputter atom ionization rate also increases, but it would be saturated. Thus, there is a limit to the useful intensity of the magnetic field, depending on electric field, particle velocity, chamber pressure, plasma pressure, and other physical parameters of the sputtering system.
The power supply on the target cathode may be a dc voltage or a rf voltage. In dc sputtering, the power supply applied to the target is typically between −200 to −800 V, with the other electrode connected to the wafer substrate or to the chamber ground. The power supply on the wafer back bias may be a dc voltage or an rf voltage, with the power supply applied to the target is typically between 10 to −100 V.
RF frequency is typically a rf frequency of 13.56 MHz for use with a LC matching network, or a rf frequency of 1.9 MHz for use with a variable frequency-matching network. At the rf frequency, the electrons response more readily than the ions due to their higher mobility, and thus this creates a net negative electric field bias on the target or the wafer.
The background gas is typically at a pressure from 0.5 mTorr to about 100 mTorr to provide the initiation and sustaining the plasma. The gas flow is typically about 10-100 sccm.
Electrons emitted due to ion bombardment are accelerated and collide with gas atoms. At low pressures or high energy, electrons travels far and generated ions can be easily lost. Thus, magnetron system can trap the electrons, primary for the target magnetron, and secondary for the wafer magnetron to improve ionization efficiencies. A parallel magnetic field to the cathode or wafer surface can confine electrons to the vicinity of the cathode or wafer and therefore can further ionize the atoms. The ExB electron drift currents makes a closed loop, and thus being trapped near the cathode or wafer surface. The wafer magnetron effect is created by an array of magnets to produce a magnetic field normal to the electric field at the wafer surface.
The ion bombardment to the target cathode also provides significant thermal energy to the target, and thus target magnetron system must include a cooling assembly since magnetron sputtering system produces very large ion currents, causing a very intense and localized heating of the target. In contrast, this ion current heating is a desired feature of the present invention wafer-vicinity magnetron since the high energy of the ions is beneficial to the thin film deposition, to provide the thermal energy needed for either the deposition reaction or the surface mobility needed for low temperature crystalline formation.
The wafer magnetron assembly utilizes magnetic field in proximity to the wafer to produce electron traps near the wafer, thereby increasing the ionization rate of ions and atoms reaching the wafer. The electric field, created by the wafer back bias or wafer to ground configuration, is substantially perpendicular to the wafer surface. The magnetic field from the magnetron usually starts and returns to the wafer surface to form a closed arch. By shaping the magnetic configuration into a circular or racetrack shape, the electrons will follow a closed loop drift path above the surface of the wafer, providing the ionization by colliding with the atoms present near the wafer surface. The required magnetic flux density is generally greater than about 200 gauss. To improve uniformity across the wafer, the drift path is relatively translated across the surface of the wafer. This can be achieved by either moving the wafer or preferably, by moving the magnet by means of a motor drive, the so-called rotating-magnet magnetron system.
A rf back bias can develop large electron currents due to the high electron mobility, and thus if a capacitor is placed in series, it allows a negative bias to accumulate, typically half the peak-to-peak rf voltage. This bias creates an electric field for the wafer substrate, trapping the secondary electron emission from the wafer with the wafer-vicinity magnetron assembly.
Also, by splitting the rf power between the cathode and the substrate, the substrate acts as a cathode and thus is bombarded by ions from the plasma. By applying a wafer magnetron assembly, the electron emission from the ion bombardment can be trapped, to further ionize the incoming atoms or ions.
The electron population can further be maintained by an electron gun such as a hollow cathode electron source, a filament electron beam, or a field emission tip array at the magnetron boundary. By adjusting the electron flow and geometry, the ions can even be partially neutralized before hitting the wafer surface, giving some additional control.
Further control to correct the imperfections caused by the non-uniform deposition pattern of standard magnetrons can by imposed by dynamically shaping the magnetic field, for example, by using electromagnetic devices.
The systems and methods ionize the neutral atoms by employing a magnetron effect at the wafer substrate. By establishing a magnetic field in the vicinity of the substrate to trap electrons, the neutral atoms traveling to the substrate can be ionized by collisions with the electron cloud. The electrons in this magnetic confinement can have sufficient energy to ionize incoming atoms and not being annihilated by the ions. A high proportion of the incoming atoms can be excited by this electron cloud and then accelerated by a back bias field onto the wafer surface. The high number of mobile ions at the surface can provide enough surface mobility to significantly decrease the required deposition temperature, possibly even down to room temperature.
Additionally, the wafer 200 is positioned between the heater 250 and a magnetron 260. The magnetron 260 serves as highly efficient sources of microwave energy. In one embodiment, microwave magnetrons employ a constant magnetic field to produce a rotating electron space charge. The space charge interacts with a plurality of microwave resonant cavities to generate microwave radiation. One electrical node 270 is provided to a back-bias generator.
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It is to be understood that various terms employed in the description herein are interchangeable. Accordingly, the above description of the invention is illustrative and not limiting. Further modifications will be apparent to one of ordinary skill in the art in light of this disclosure.
The invention has been described in terms of specific examples which are illustrative only and are not to be construed as limiting. The invention may be implemented in digital electronic circuitry or in computer hardware, firmware, software, or in combinations of them.
Apparatus of the invention for controlling the fabrication equipment may be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a computer processor; and method steps of the invention may be performed by a computer processor executing a program to perform functions of the invention by operating on input data and generating output. Suitable, processors include, by way of example, both general and special purpose microprocessors. Storage devices suitable for tangibly embodying computer program instructions include all forms of non-volatile memory including, but not limited to: semiconductor memory devices such as EPROM, EEPROM, and flash devices; magnetic disks (fixed, floppy, and removable); other magnetic media such as tape; optical media such as CD-ROM disks; and magneto-optic devices. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs) or suitably programmed field programmable gate arrays (FPGAs).
While the preferred forms of the invention have been shown in the drawings and described herein, the invention should not be construed as limited to the specific forms shown and described since variations of the preferred forms will be apparent to those skilled in the art. Thus the scope of the invention is defined by the following claims and their equivalents.
