Sputtering apparatus

A magnetron assembly for a rotary target cathode comprises a rigid support structure, a magnet bar structure movably attached to the rigid support structure, and at least one actuation mechanism coupled to the rigid support structure and configured to change a distance of the magnet bar structure from a surface of a rotatable target cylinder. The magnetron assembly also includes a position indicating mechanism operative to measure a position of the magnet bar structure relative to the surface of the rotatable target cylinder. A communications device is configured to receive command signals from outside of the magnetron assembly and transmit information signals to outside of the magnetron assembly.

BACKGROUND

Magnetron sputtering of rotating targets is well known and is used extensively for producing a wide variety of thin films on a wide variety of substrates. In the most basic form of rotating-target magnetron sputtering, the material to be sputtered is either formed in the shape of a tube or is adhered to the outer surface of a support tube made of a rigid material. A magnetron assembly is disposed within the tube and supplies magnetic flux, which permeates the target such that there is adequate magnetic flux at the outer surface of the target. The magnetic field produced by the magnetron assembly is designed in a way such that it retains electrons emitted from the target so as to increase the probability that they will have ionizing collisions with the working gas, hence enhancing the efficiency of the sputtering process.

It is becoming increasingly important to compensate for target erosion effects because it is desirable to increase target thickness and operate sputter processes under more sensitive process conditions. The desire for thicker targets is largely driven by fabrication costs of ceramic targets, but is also desirable in order to have a greater inventory of usable material inside the sputter coater in order to run longer coating campaigns. The need to run processes in more sensitive process conditions is driven by the desire to get higher deposition rates, in reactive mode sputtering, and/or to finely control film chemistry.

Fabrication cost for targets of some materials, in particular ceramic transparent conductive oxide (TCO) materials, are relatively high in comparison to the cost of the raw materials. To improve the economy of these targets, it is desirable to increase the thickness of the target material. In this way, the target will have significantly more usable material while adding only minimally to the overall cost of the target, as the fabrication cost does not change significantly. The only significant cost increase is due to the additional raw material used. In addition, thicker targets have the added benefit of allowing longer production campaigns between target changes.

Increasing the target thickness too much, however, can result in inadequate magnetic flux at the target surface when using standard magnetron assemblies. Magnetron designs with higher magnetic flux have recently been introduced to provide the higher magnetic flux required for the thicker targets.

In the case of reactive magnetron sputtering, metallic targets are sputtered in an atmosphere that contains reactive gas such as oxygen or nitrogen. The sputtered material reacts with the reactive gas in order to form a film comprised of compounds of the target material and the reactive gas. The reactive gas also reacts with the target surface, thereby forming reacted compounds on the target surface. The surface compounds greatly reduce the ablation rate. In order to improve the sputtering efficiency, the amount of reactive gas may be carefully controlled so as to minimize the target surface reactions while still achieving the desired film chemistry. In some cases, the processes need to be controlled such that the chemistry of the film is sub-stoichiometric.

This fine control over the process gas makes the process sensitive to small perturbations. The industry has seen considerable technological advances in power delivery and process gas control that have minimized many of the process perturbations. Nevertheless, little has been done to minimize variations in the magnetic confinement of the plasma. As the target erodes, the working surface gets closer to the magnetic assembly and the magnetic field becomes stronger. This changes the confinement of the plasma, altering the dynamics of the sputtering process. This presents a challenge in maintaining long-term stability of the process.

The typical magnetron assembly for rotating cathodes comprises three substantially parallel rows of magnets attached to a yoke of magnetically conductive material, such as steel, that helps complete the magnetic circuit. The direction of magnetization of the magnets is radial with respect to the major axis of the sputtering target. The center row of magnets has the opposite polarity of the two outer rows of magnets.

Magnetic flux of the inner and outer rows of magnets is linked through the magnetically conductive yoke, on one side of the magnets. On the other side of the magnets, opposite the yoke, the magnetic flux is not contained in a magnetically conductive material. Hence, the magnetic flux permeates substantially unimpeded through the target, which is substantially non-magnetic. Thus, two arc-shaped magnetic fields are provided at and proximate to the working surface of the target. These fields retain the electrons and cause them to drift in a direction perpendicular to the magnetic field lines, which is parallel to the rows of magnets. This is known as the E×B drift. In an ordinary arrangement, this drift path is also parallel to the major axis of the target.

Additionally, the outer rows of magnets are slightly longer that the inner row of magnets, and additional magnets, of the same polarity as the outer rows, are placed at the ends of the assembly between the two outer rows creating the so-called “turn-around” areas of the drift path. This has the effect of connecting the two drift paths, hence forming one continuous ovular “racetrack” drift path. This optimizes the retention of the electrons and therefore optimizes the efficiency of the sputtering process.

As the target erodes, the working surface comes closer to the magnet assembly, and the intensity of the magnetic field, at the working surface, increases in a non-linear fashion. For finely controlled processes it very desirable to modify the magnetic field, as the target erodes, so as to minimize variability of the process, thereby making the process easier to control over the course of the target life.

The need for changing the magnetic field as the target erodes is well known, and has been accomplished in the case of planar sputtering cathodes. The need for an adjustable magnetron for rotating cathodes has gone unsatisfied, however, because the geometry and mechanical structure of the cathodes make the task especially challenging.

SUMMARY

A magnetron assembly for a rotary target cathode comprises a rigid support structure, a magnet bar structure movably attached to the rigid support structure, and at least one actuation mechanism coupled to the rigid support structure and configured to change a distance of the magnet bar structure from a surface of a rotatable target cylinder. The magnetron assembly also includes a position indicating mechanism operative to measure a position of the magnet bar structure relative to the surface of the rotatable target cylinder. A communications device is configured to receive command signals from outside of the magnetron assembly and transmit information signals to outside of the magnetron assembly.

DETAILED DESCRIPTION

An apparatus and technique for rotating-cathode magnetron sputtering is provided that deals with the variation in magnetic intensity at the sputtering surface as a target erodes, which results in changing process conditions. By magnetically compensating for target erosion effects, the present approach improves process stability.

In some embodiments, adjustments in the position of a magnetron assembly can be made by pneumatic or hydraulic pressure that pushes against a spring loaded mechanical structure. The spring can push the magnetron assembly either towards the nearest or towards the furthest distance from the target's working surface while the pneumatic or hydraulic pressure pushes against the spring, in the opposite direction. The applied pressure will determine the position of the assembly. In such embodiments, the pressure-carrying line can be disposed within a central water tube, on which the magnetron assembly is generally mounted. This location is beneficial because the water tube remains static. Thus, no seals are required on moving parts and reliability is optimized.

In one embodiment, energy is provided to a pneumatic actuator through a pressurized gas cylinder disposed within the target assembly. The high-pressure gas cylinder can be a commercially available carbon dioxide (CO2) cartridge, for example.

In other embodiments, the motion for making adjustments can be provided by a cable, which also can be disposed within the water tube. In one example, the cable can be rotary, such as in a speedometer of a car. In another example, the cable can be push/pull, such as in a hand-brake cable on a bicycle.

Depending on the specific design of the cathode being used, some embodiments of the present approach can provide motion directly by way of a rotary or linear shaft that is disposed within the water tube along the axis of the target assembly. The shaft passes from air to water through an air-to-water seal, such as a rotary seal. In the case of linear motion, the motion can be transmitted via a bellows, which provides complete air-to-water isolation and transfers motion through compression or expansion of the bellows.

In some cathode designs, one end of the target assembly is attached to an end-block through which all functions pass, and the other end of the target is capped. This type of cathode is referred to as a single-ended cathode. The capped end may or may not be supported by a bearing. In this type of cathode design, the above mentioned bellows can be part of the end-cap design.

Another method of providing a mechanical feed-through is magnetically. A magnetic assembly inside the target structure can magnetically couple to an assembly outside the target. The motion of the external assembly will translate, via the magnetic link, through a solid wall. Such an arrangement can most easily be implemented as part of the end-cap of a single-ended cathode design.

In other embodiments, adjustments can be driven by taking advantage of the motion of the target rotation relative to the static magnetron/water-tube assembly. This can be done by providing a mechanical structure, such as gears, to harness the rotary motion of the target to drive the actuators used to make the adjustment. In another embodiment, adjustments can be driven by harnessing the water flow through the cathode, as by a turbine or water-wheel mechanism, such as when the water flows through the central water bar. In such cases as the target rotation or water flow driven embodiments, it is necessary to provide a structure for engaging and dis-engaging the mechanism from outside the coater, such as a switching mechanism, which can use any of the mechanical feed-through methods previously discussed.

In further embodiments, adjustments can be made by internal motors such as servos or stepper motors contained within the target assembly, which can be either submergible or contained inside a water-tight housing. These motors can apply torque directly to an adjustment screw, or there can be intermediate mechanics. The intermediate mechanics can be worm gears, bevel gears, or rack and pinions, for example, which change the direction of motion. Such mechanics can also serve as gear reductions in order to adjust the relative torque between the output torque of the motor and the desired torque to be applied to the adjustment screw.

In an alternative approach, piezo-electric motors can be utilized in the actuation mechanism. The piezo-electric motor provides a linear motion. This motion can be translated to rotary motion, to drive an adjustment screw, by applying tangential force to a gear that is affixed to the adjustment screw. Other linear motion options include electric solenoids, and pneumatic or hydraulic cylinders.

While power for internal motors can be provided by wires that are run through the water bar assembly, difficulties can arise in shielding these wires from the electrical power applied to the cathode, and because of the extra connection required when assembling the target. Alternately, power can be routed in by brush contacts, especially on a capped end of a single-ended cathode.

Another method of driving internal motors is by providing battery packs within the target cavity. In this approach, power can be switched on and off by any of the mechanisms previously discussed.

In general, pneumatic embodiments of the actuator require less power than motorized embodiments.

In another embodiment, an electronic internal control module can be located within the target assembly. Command and feedback communications between the operator and the internal control module can be done by a variety of methods that do not require significant alteration of the cathode.

One method for remote communicating with the internal control module is by a power-line overlay signal. In this case, the communications signal is transmitted through the same conductance path as the power applied to the target. However, communications frequencies must be chosen to be very different from any power frequency of the sputtering power supply. Additionally, it may be necessary to send redundant signals to compensate for electrical noise that is occasionally generated by the sputtering process. This method of communication has the advantages of being easily implemented in most rotary cathode designs with virtually no modifications to the cathode structure and requires no special feed-through.

Alternate methods of communicating with the control module include transmitting signals through a window in the cathode or target assembly. The most convenient place for such a window is as the center of the end cap of the single-ended type cathode. Exemplary types of signals that can be sent through this window include radio, Wi-Fi, Bluetooth, optical, magnetic induction, or the like. Digital optical communications have the advantage of being immune to interference from electromagnetic noise produced by the sputtering process, but the communication path needs reasonable shielding from light. Radio and Wi-Fi signals need shielding from electro-magnetic noise. Magnetic induction communication involve two inductive coils in close proximity, where a first coil is activated by an electric current and a second coil acts as a pick-up coil that generates a voltage signal in response to the magnetic field produced by the first coil. All of these methods can provide two-way communications. A variation of the magnetic induction method is to replace one of the coils with a Hall sensor, but this limits communications to one-way.

Another alternative for remote communication is by use of a pair of ultrasonic transceivers. Ultrasonic communication has the advantage in that there is more versatility in where the transceivers are mounted, since they do not require line-of-sight or any special window through which to transmit. Additionally, ultrasonic transceivers do not suffer from any electro-magnetic noise, optical noise, or optical impedance. The advantages of ultrasonic communication make it easier to retrofit cathodes produced by a variety of manufacturers.

A method of sensing the position of the magnet assembly, relative the target working surface, is also provided. In one approach, direct measurement is performed using an analog or digital linear motion indicator. If the motion is driven by servos or stepper motors, a feedback signal is available from these motors. An alternate method for sensing position is to measure gas pressure inside the pneumatic elements. Another method is to have a magnet and a Hall probe mounted in the apparatus such that they move relative to one another as adjustments are made. The Hall probe will have a different voltage output depending on its distance from the magnet.

The various techniques disclosed herein can be used to position the entire magnet assembly as a single unit, or to position multiple points along the length of the magnet assembly, independently, so as to make it possible to also adjust uniformity of the process.

FIGS. 1-3 and 6illustrate various views of a magnetron assembly100for a rotatable target cylinder according to one embodiment. In general, magnetron assembly100includes a rigid support structure102such as a coolant tube, a magnet bar structure104movably attached to support structure102, and one or more actuation mechanisms108coupled to support structure102. The actuation mechanisms108are configured to change a distance of magnet bar structure104from a surface of the rotatable target cylinder.

The actuation mechanisms108are covered by an actuator housing109. A position indicating mechanism is located in actuator housing109and is operative to measure the position of magnet bar structure104relative to the surface of the rotatable target cylinder. The magnet bar structure104includes an array of substantially parallel rows of magnets110attached to a yoke112, as shown inFIG. 6. The yoke112is comprised of a magnetically conductive material such as steel, which helps to complete a magnetic circuit.

A control housing106partially surrounds support structure102and contains a communications device configured to receive command signals from outside of magnetron assembly100and transmit information signals to outside of magnetron assembly100. The control housing106also encloses an electronic controller in operative communication with actuation mechanisms108. The communications device can be a transceiver that is operatively coupled to the electronic controller. The transceiver can be a radio frequency (RF) transceiver, an optical transceiver, or an ultrasonic transceiver, for example. As shown inFIG. 1, control housing106defines a position feedback connection port114and one or more actuation connection ports116.

The position indicating mechanism can be implemented as a built-in position sensor in each actuation mechanism108. The position sensor can measure the position of magnet bar structure104either by direct sensing or by an indirect metric. For example, the position indicating mechanism can be implemented with a Hall probe and a magnet in an analog sensor. Alternatively, the position indicating mechanism can be implemented with a digital indicator, such as a plunger style digital indicator, which transmits data directly to an operator without additional processing.

In addition, a power source can be provided to energize actuator mechanisms108and the electronic controller. The power source can be fully self-contained within the volume of the magnetron assembly. For example, a power supply such as a battery pack can be located in control housing106.

The actuation mechanisms108can be implemented in various ways. For example, the actuation mechanisms108can include a spring-loaded pneumatic structure or a spring loaded hydraulic structure. Alternatively, the actuation mechanisms108can include a rotary cable or a push/pull cable.

In one embodiment, each actuation mechanism108can include a spring-loaded pneumatic bellows, bladders, or cylinders. In this approach, adjustment points are spring loaded with the pressure in the pneumatic element pushing in the opposite direction. Reserve energy for making adjustments can be stored in a compressed gas supply such as a CO2gas cartridge. Adjustments can be made by adjustment valves that either release compressed gas from the CO2gas cartridge into the pneumatic elements, or release gas from the pneumatic elements into cooling water inside the target.

Further details of one embodiment of actuation mechanisms108with built-in position sensing are depicted inFIGS. 4 and 5. In this embodiment, the actuation mechanisms108include a sensor port120for a Hall probe, a pneumatic actuation port122configured to receive a compressed gas, and a bellows124such as a welded bellows in communication with pneumatic actuation port122. A control shaft130is coupled to bellows124and yolk112of magnet bar structure104. A return spring128is coupled to control shaft130, and a magnet126is located in control shaft130for Hall probe feedback. The Hall probe/magnet in this embodiment is an analog detector for position sensing.FIG. 6illustrates exemplary locations within control housing106for a control board113for the controller, one or more solenoid valves115, transceivers117, a CO2gas cartridge118in fluid communication with the solenoid valves, and a battery119to run the control board, which are used with the actuation mechanisms108ofFIGS. 4 and 5.

As discussed previously, the actuation mechanisms can alternatively be implemented with a motorized structure, such as a servo, a stepper motor, or a piezo-electric motor. Any number of mechanical configurations can be used for the drive motion. One example is a screw jack, which may additionally incorporate right-angle gears or reduction gears. In these embodiments, position sensing of the magnet bar can be carried out through feedback from the motorized structures.

FIGS. 7 and 8illustrate a magnetron assembly140implemented with a motorized structure in a rotatable target cathode170according to one embodiment. In general, magnetron assembly140is disposed inside of a target cylinder172and includes a rigid support structure142, a magnet bar structure144movably attached to support structure142, and a plurality of motorized actuation mechanisms146coupled to support structure142. The motorized actuation mechanisms146include geared stepper motors148, which can have a 100:1 gear reduction, for example. A set of bevel gears150is operatively coupled to stepper motors148. The bevel gears150can have a 4:1 gear reduction, for example. A threaded housing152is mated with each of bevel gears150. A threaded post154is coupled between magnet bar structure144and threaded housing152. An actuator housing155encloses each of motorized actuation mechanisms146.

The magnetron assembly140also includes an electronic controller156in operative communication with motorized actuation mechanisms146. A communications device such as an ultrasonic transceiver/transducer158is operatively coupled to electronic controller156. Power for stepper motors148and electronic controller156can be provided by a battery pack160. A control housing162encloses electronic controller156and battery pack160.

The target cylinder172is rotatably attached to an end block174as shown inFIG. 7. An ultrasonic transceiver/transducer176is mounted on end block174and is in communication with ultrasonic transceiver/transducer158.

FIG. 9illustrates a sputtering apparatus200according to another embodiment that is configured for two-way optical communications. A rotatable cathode target cylinder210is disposed within a vacuum chamber212having an outer wall215. The target cylinder210is operatively coupled to a motor213mounted on outer wall215outside of vacuum chamber212. A magnetron assembly100, such as described previously with respect toFIGS. 1-3, is located within target cylinder210.

As depicted inFIG. 9, an optical communications box214is located outside of vacuum chamber212on outer wall215. A first fiber optic cable216in atmosphere is optically coupled to a first optical transceiver in optical communications box214. The fiber optic cable216is also coupled to a vacuum coupler218, which provides a feedthrough from atmosphere to vacuum for fiber optic cable216. A second fiber optic cable224is coupled to a second optical transceiver inside control housing106through a cathode water cooling circuit of target cylinder210. A fiber optic window220in an end cap222of target cylinder210allows an optical signal to be transmitted between fiber optic cable216and fiber optic cable224.

FIG. 10is a schematic illustration of a sputtering apparatus300according to another embodiment, which is configured for two-way ultrasonic communications between a magnetron assembly310, located within a rotary cathode assembly320in a vacuum chamber340, and an external controller344outside of vacuum chamber340. The magnetron assembly310includes a magnet bar structure312, and a plurality of motorized actuation mechanisms314mechanically coupled magnet bar structure312. An internal electronic controller316is in operative communication with motorized actuation mechanisms314, such as through motor control cables318that can include two sets of twisted pairs. A battery pack housed with electronic controller316provides power to motorized actuation mechanisms314and electronic controller316.

The rotary cathode assembly320includes a target cylinder322, which can be filled with water, rotatably coupled to an end block324. A first ultrasonic transceiver326is mounted inside of target cylinder322and is in signal communication with electronic controller316, such as through an ultrasonic communication wire328that can include one twisted pair. A second ultrasonic transceiver330is mounted on end block324over an insulator332and is in ultrasonic communication with ultrasonic transceiver326. The external controller344, which can be operated by a user, is in signal communication with ultrasonic transceiver330, such as through an ultrasonic communication wire334that passes through a vacuum coupler336, which provides a feedthrough from atmosphere to vacuum chamber340.

In one implementation, electronic controller316is capable of controlling up to twelve (12) axes of motion for magnet bar structure312, with only one motor of a motorized actuation mechanism314being controlled at any one time. The control theory for electronic controller316can be adapted to move each motor a small amount in a given sequence. Controlling only one motor at a time simplifies the control system and reduces the battery requirements as there is a lower instantaneous power draw. In addition, the control lines can be routed with a communications bus on the magnet bar side of an I-beam support. A water sealed electrical connection can be used between each controlled unit and the communications bus.

In another embodiment, a system for two-way transmission of information between a magnetron assembly contained within a cathode target assembly and outside of a vacuum chamber can also be provided. For example, two-way communications can be carried out by two RF transceivers strategically placed, with one transceiver inside the target assembly and one transceiver outside the target assembly but inside the vacuum chamber. The transceiver inside the target assembly is directly connected to the electronic controller. The transceiver in the vacuum chamber is connected to the outside via an electrical feed-through in the chamber wall. It is necessary to provide a window between transceiver antennas that is transparent to the communication signal. The window can be located as part of the end cap of the single-ended cathode.

EXAMPLE EMBODIMENTS

Example 1 includes a magnetron assembly comprising a rigid support structure; a magnet bar structure movably attached to the rigid support structure; at least one actuation mechanism coupled to the rigid support structure and configured to change a distance of the magnet bar structure from a surface of a rotatable target cylinder; a position indicating mechanism operative to measure a position of the magnet bar structure relative to the surface of the rotatable target cylinder; and a communications device configured to receive command signals from outside of the magnetron assembly and transmit information signals to outside of the magnetron assembly.

Example 2 includes the magnetron assembly of Example 1, wherein the actuation mechanism comprises a spring-loaded pneumatic structure or a spring loaded hydraulic structure.

Example 3 includes the magnetron assembly of Example 1, wherein the actuation mechanism comprises a rotary cable or a push/pull cable.

Example 4 includes the magnetron assembly of Example 1, wherein the actuation mechanism includes a motorized structure comprising a servo, a stepper motor, or a piezo-electric motor.

Example 5 includes the magnetron assembly of any of Examples 1-4, wherein the position indicating mechanism measures the position of the magnet bar structure by direct sensing or by an indirect metric.

Example 6 includes the magnetron assembly of any of Examples 1-4, wherein the position indicating mechanism comprises a Hall probe and a magnet.

Example 7 includes the magnetron assembly of any of Examples 1-4, wherein the position indicating mechanism comprises a digital indicator that transmits data directly to an operator without additional processing.

Example 8 includes the magnetron assembly of Example 4, wherein the position indicating mechanism comprises feedback from direct coupling with the motorized structure.

Example 9 includes the magnetron assembly of any of Examples 1-8, further comprising an electronic controller in operative communication with the actuation mechanism.

Example 10 includes the magnetron assembly of Example 9, further comprising a battery power supply configured to energize the actuation mechanism and the electronic controller.

Example 11 includes the magnetron assembly of any of Examples 9-10, wherein the communications device comprises a transceiver operatively coupled to the electronic controller.

Example 12 includes the magnetron assembly of Example 11, wherein the transceiver comprises a radio frequency transceiver, an optical transceiver, or an ultrasonic transceiver.

Example 13 includes the magnetron assembly of Example 1, wherein the actuation mechanism comprises a sensor port for a Hall probe; a pneumatic actuation port configured to receive a compressed gas; a bellows in communication with the pneumatic actuation port; a control shaft coupled to the bellows and the magnet bar structure; a return spring coupled to the control shaft; and a magnet in the control shaft for Hall probe feedback.

Example 14 includes the magnetron assembly of Example 1, wherein the actuation mechanism comprises a stepper motor; a bevel gear operatively coupled to the stepper motor; a threaded housing mated with the bevel gear; and a threaded post coupled between the magnet bar structure and the threaded housing.

Example 15 includes a rotary cathode assembly for a sputtering apparatus, the rotary cathode assembly comprising a rotatable target cylinder having an inner surface that defines an interior passageway; a coolant tube positioned within the interior passageway of the target cylinder; a magnet bar structure movably attached to the coolant tube within the interior passageway; a plurality of actuation mechanisms coupled to the coolant tube and configured to change a distance of the magnet bar structure from the inner surface of the target cylinder; a plurality of position indicating mechanisms each associated with a respective one of the actuation mechanisms and operative to measure a position of the magnet bar structure relative to the inner surface of the target cylinder; an electronic controller in the target cylinder and in operative communication with the actuation mechanisms; a first transceiver located in the target cylinder and operatively coupled to the electronic controller; and a second transceiver located outside of the target cylinder and in signal communication with the first transceiver; wherein as the target cylinder erodes during operation of the sputtering apparatus, the actuation mechanism adjusts the position of the magnet bar structure with respect to the inner surface of the target cylinder in response to control signals from the electronic controller.

Example 16 includes the rotary cathode assembly of Example 15, wherein the first and second transceivers comprise optical transceivers.

Example 17 includes the rotary cathode assembly of Example 16, further comprising a first fiber optic cable coupled to the first transceiver and in optical communication with a second fiber optic cable coupled to the second transceiver.

Example 18 includes the rotary cathode assembly of Example 17, further comprising an end cap on the target cylinder, the end cap having a window that allows an optical signal to be transmitted between the first fiber optic cable and the second fiber optic cable.

Example 19 includes the rotary cathode assembly of Example 15, wherein the first and second transceivers comprise ultrasonic transceivers.

Example 20 includes the rotary cathode assembly of Example 19, wherein the second transceiver is in signal communication with a user operated external controller.

While a number of embodiments have been described, it will be understood that the described embodiments are to be considered only as illustrative and not restrictive, and that various modifications to the described embodiments may be made without departing from the scope of the invention. The scope of the invention is therefore indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.