Patent Application
20120024694
This invention relates to sputtering systems and, in particular, to a magnetron for use in a sputtering system.
Sputtering systems are widely used in the semiconductor manufacturing industry for depositing materials on semiconductor wafers. Sputtering is sometimes referred to as physical vapor deposition, or PVD. In a sputtering operation, thin films comprising materials such as Al, Au, Cu, Ta are deposited in a vacuum on silicon wafers or other substrates.
The present assignee has obtained a U.S. Pat. No. 7,479,210 on a sputtering tool, which is shown in the prior art
Prior art
Since the present invention is a new magnetron assembly, shown in
The top cover of the sputtering system 12 has been removed. A robotic arm (not shown) in a wafer transport module inserts and removes wafers 41 via the access port 14. Typical wafer sizes are 6, 8, and 12 inches, and the system is customized for the particular workpieces for processing.
In one embodiment, the system 12 simultaneously processes three or more wafers 41 (preferably five or six) using three or more sputtering targets 43.
A pallet 36 rotates to align a wafer 41 below an appropriate target 43. Each target 43 may be a different material for forming successive thin films of different materials on a wafer 41. The wafers 41 are supported on wafer support areas 32. A wafer support area 32 is an indented area in pallet 36 sized to accommodate the particular wafers being processed.
Four pins (not shown) below pallet 36 are raised, using pin bellows 39, to extend through four holes in the wafer support area 32 to temporarily lift the wafer 41 during insertion of the wafer 41 into the chamber and removal of the wafer 41 from the chamber.
The pallet 36 is mounted on a rotatable table 40. Pallet 36 and table 40 may be formed of aluminum. Pallet 36 may be continuously rotated at any speed or may be temporarily stopped to control the deposition of a sputtered material from a target 43 overlying a wafer.
A chamber shield 35 prevents contaminants from accumulating on the vacuum chamber wall.
The entire back surface of each wafer is thus in electrical and thermal contact with pallet 36, which is in turn in electrical and thermal contact with table 40.
The temperature of the wafers is controlled by flowing a coolant 44 (
An external cooling source 50 cools the coolant (e.g., water) and recycles the coolant back to table 40. Flexible tubing 51 from the cooling source 50 attaches to a rotatable coupler 52 for providing a sealed coupling between the rotating copper tubes 46 (input and output) and the stationary tubing 51 to/from the cooling source 50.
An RF and DC bias source 54 is electrically coupled to the copper tube 46 by the rotatable coupling 52 to energize table 40 and thus energize pallet 36 and the wafers for the sputtering process. In another embodiment, table 40 is grounded, floated, or biased with only a DC voltage source.
When the chamber is evacuated and back filled with a certain amount of Ar gas at a certain pressure (for example, 20 milli-torr) and the gas is energized with a DC source, an RF source, or a combination of the two sources, an electromagnetic field is coupled inside the chamber to excite a sustained high density plasma near the target surface. The plasma confined near the target surface contains positive ions (such as Ar+) and free electrons. The ions in the plasma strike the target surface and sputter material off the target. The wafers receive the sputtered material to form a deposited layer on the surface of the wafers. In one instance, up to twenty kilowatts of DC power can be provided on each target. In such a case, each target can deposit approximately 1 micron of metal per minute on an underlying work piece.
The chamber wall is typically electrically grounded in processing operations.
A bias voltage on the wafers can drive a flux of an electrically charged species (Ar+ and/or atomic vapor sputtered off the target) to the wafers. The flux can modify the properties (for example, density) of the sputtered material to the wafers.
Generating a plasma for sputtering and the various biasing schemes are well known, and any of the known techniques may be implemented with the described sputtering system.
In a preferred embodiment, the chamber gas is provided by a distribution channel at the bottom of the chamber, rather than from the top, which reduces particle contamination during the sputtering process and allows optimization of the magnetron assembly.
A seal 57 provides a seal around shaft 49 in order to maintain a low pressure in the chamber.
A cross-contamination shield 96 helps confine sputtered material to an area under the target.
The sputtering system 12 uses a magnetron assembly, outside the vacuum, to further control the bombardment of the target by the plasma. A magnet 60 is located behind each target 43 so that the plasma is confined to the target area. The resulting magnetic field forms a closed-loop annular path acting as an electron trap that reshapes the trajectories of the secondary electrons ejected from target into a cycloidal path, greatly increasing the probability of ionization of the sputtering gas within the confinement zone. Inert gases, specifically argon, are usually employed as the sputtering gas because they tend not to react with the target material or combine with any process gases and because they produce higher sputtering and deposition rates due to their high molecular weight. Positively charged argon ions from the plasma are accelerated toward the negatively biased target and impact the target, resulting in material being sputtered from the target surface.
The size of magnets 60 depends on the size of the wafers, which determines the size of the targets. In one embodiment, a magnet 60 is about 10.7 inches (27 cm) long and about 3 inches (7.6 cm) wide at its widest part. An eight inch wafer may use a target that is from 10-13 inches long in the radial direction. A twelve inch wafer may use a target that is from 13-18 inches long in the radial direction.
Since the plasma makes the targets hot, a coolant channel is provided in each target support plate 59 through which a coolant flows. The highest heat is generated under the magnet 60. Adequate and uniform cooling becomes a problem for high density plasmas.
The structure of
Although the sputtering system 12 is very good, there is a practical limit on the ionization power that is supplied to maintain a high sputtering rate. The scanning magnet 60 covers approximately one-half the target at any instant, and the ionization power must be sufficient to create a high density plasma in the area of each target being influenced by the relatively large area magnetic field for a desired high sputtering rate. Such high power requires a large amount of cooling of the target backing plate 59 via the coolant channel in the plate 59. Uneven cooling of the target results in nonuniform sputtering and erosion.
Additionally, the large magnet causes some nonuniform erosion/sputtering of the target due to some target areas being subjected to different average magnetic fluxes over time.
What is needed is an improved sputtering system 12, using mostly its existing components, that achieves a higher sputtering rate with the same ionization power and cooling of the target, and which provides more uniform erosion of the target.
In a preferred embodiment, each of the large scanning magnets in
In one embodiment, the same central scanning actuator of
The smaller magnet exhibits about the same magnetic flux density (gauss) as the larger magnet of
The relative cycle periods of the linear motor and scanning actuator are set so that, over a suitable time period, the magnet scans over substantially the entire surface of the target. Such suitable cycles depend on the particular shapes of the target and magnet. In one embodiment, the linear motor varies its speed depending on the position of the magnet along the screw gear, such as to slow the linear movement of the magnet near the wide end of the target, while the magnet scans through arcs at a constant rate, to achieve uniform coverage of the target by the magnet. In other words, the linear movement of the magnet is varied so that the magnet overlies all portions of the target about equal times during a complete cycle. In one embodiment, the magnet moving along the linear path actually stops (dwells) at one or more positions along the path, such as at the widest portion of the target, while the magnet continuously scans through arcs back and forth across the target. Such dwells times at various positions are for the purpose of achieving a desired erosion over the entire surface of the target.
In one embodiment, the target is 15-17 inches long, requiring the arm to be a similar length.
The new magnetron may be used in conjunction with the system of
Elements with the same numbers in the various figures are the same.
A scanning actuator 70, located at the center of the top wall of the chamber, rotates through an arc about axis 72 and then reverses. The actuator 70 may the same as actuator 66 in
A scanning controller 73 controls the actuator 70 to rotate through a predetermined angle (less than 120 degrees) and then reverse direction.
Attached to the actuator 70 and extending radially is an arm 74. The arm 74 comprises a support beam with a slot. Within the slot is a screw gear 78. A motor 80 at the base of the arm 74 is connected to the screw gear 78 for turning the screw gear 78 in one direction through a predetermined angular rotation and then turning the screw gear 78 in the opposite direction through a predetermined angular rotation. The motor 80 may be a servo motor or a stepper motor.
A linear movement controller 82 controls the motor 80 speed and direction. Since the speed can be variable during a single cycle, the controller 82 is programmable.
A permanent magnet 84 has a substantially triangular shape, being formed of three straight edges and rounded corners. The magnet 84 comprises a ferrous backing plate populated with a pattern of relatively small magnets affixed to the plate. The magnet 84 will be described in more detail later with respect to
The view of
Affixed to the back of the magnet's backing plate is a standard screw gear threaded sleeve that receives the screw gear 78 so that rotation of the screw gear 78 causes the magnet 84 to move linearly with respect to the arm 74 while remaining in a single plane.
As the scanning actuator 70 scans at a constant rate, the motor 80 moves the magnet 84 at either a constant speed or a varying speed up and down the length of the target 86 so that ultimately the magnet 84 covers substantially the entire area of the triangular target 86 after one or more cycles of the repetitive angular and linear movement of the magnet. In one embodiment, the actuator 70 completes a back and forth scan in between 1-3 seconds, and the motor 80 completes and up and down movement of the magnet between 1-5 seconds. The cycle times may be greater or less than these periods, depending on the sizes of the target and magnet.
Due to the triangular shape of the target 86, to achieve uniform coverage of the target 86 by the magnet 84, the linear motor 80 varies its speed depending on the position of the magnet 84 along the screw gear 78. For example, the linear movement of the magnet 84 is slowed near the wide end of the target, and may even be dwelled, while the magnet scans through arcs at a constant rate. In other words, the linear movement of the magnet may be varied so that the magnet overlies all portions of the target about equal times during a complete cycle. Additionally, due to edge effects, the plasma may not be uniformly created across the surface of the target 86, and the linear speed of the magnet 84 may be controlled to vary to cause substantially uniform erosion of the target even though the magnet 84 does not overlie all portions of the target 86 for equal times during a complete cycle. For example, the speed of the magnet 84 may need to be slowed or dwelled at the narrow end of the target 86. The variation in speed may be programmed into the controller 82 based upon empirical data after long periods of testing and examining the erosion of the target 86.
In one embodiment, the variation in linear speed may not repeat for each linear scan of the magnet 84 in order to achieve full coverage of the target.
In one embodiment, the magnet moving along the linear path actually stops (dwells) at one or more positions along the screw gear, such as at the widest portion of the target 86, while the magnet continuously scans through arcs back and forth across the target. Such dwells times at various positions are for the purpose of achieving a desired erosion (e.g., uniform erosion) over the entire surface of the target. Without such dwelling, certain areas of the target may be overlapped by the magnet more than others due to the multi-direction scanning of the magnet, or some areas may be not covered due to the interaction of the multi-direction scanning. In one embodiment, the magnet dwells at a position along the screw gear for up to five seconds by stopping the linear motor 80. Such control by the linear motor 80 is programmed into the motor's controller and may be based on computer simulation and actual testing results. When a dwell time is used, a period for the motor 80 to move the magnet up and down the arm 74 may exceed 20 seconds.
In one embodiment, the magnet 84 has a shape generally corresponding to the shape of the target but smaller in all dimensions. The magnet will typically be between one-quarter to one-half the size of the target. Since the magnet 84 is smaller than a magnet having the same length as the target, it must be scanned along the length of the target to fully cover the target over time. The smaller magnet will create a higher power density, compared to a full-length bigger magnet, for the same input power into the system because all the power is concentrated in a smaller footprint. This increases the ion concentration at the target, which increases the deposition rate.
In
The individual magnets 106 along the edge of magnet 84 are smaller that the inner magnets so that the magnetic field extends close to the edge of the magnet. The span of a magnetic field can be approximated by the distance between the centers of the two opposite poles. Hence, the diameters of the outer magnets 106 are made small (e.g., 0.5-1 cm). The inner rings of magnets 106 may be larger. In the example, the magnets 106 may be rectangular or circular.
The magnetron assembly of
The described sputtering system allows for all three targets to concurrently sputter the same or different materials on the wafers during a batch process. This increases throughput and allows the sputtering of alloys or layers on the wafers without breaking a vacuum. To select an alloy composition, one target may be one material, and the other two targets may be a second or third material. For depositing stacked layers of distinct materials, then only one material may be deposited at a time (e.g., one target energized at a time or multiple targets of the same material energized at a time). For depositing mixed layers (e.g. alloys of distinct materials), then all targets may be energized at the same time, assuming the targets are of different materials.
More targets and wafers than shown in the examples may be employed in the system. For example, there may be eight targets. The number of such targets is limited only by the ability to build increasingly narrow magnets, which deliver a suitable magnetic flux on the target surface.
Conventional aspects of the system that have not been described in detail would be well known to those skilled in the art. U.S. Pat. No. 6,630,201 and U.S. Patent Application Publication 2002/0160125 A1 are incorporated herein by reference for certain conventional aspects primarily related to creating a plasma and supplying gas to a process chamber.
Although the system has been described with respect to forming a metal film on semiconductor wafers, the system may deposit any material, including dielectrics, and may process any workpiece such as LCD panels and other flat panel displays. In one embodiment, the system is used to deposit materials on multiple thin film transistor arrays for LCD panels.
Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.
