Physical vapor deposition method using backside gas cooling of workpieces

Abstract

A circular PVD chamber has a plurality of sputtering targets mounted on a top wall of the chamber. A pallet in the chamber is coupled to a motor for rotating the pallet about its center axis. The pallet has a diameter less than the diameter of the circular chamber. The pallet is also shiftable in an XY direction to move the center of the pallet beneath any of the targets so all areas of a workpiece supported by the pallet can be positioned directly below any one of the targets. A scanning magnet is in back of each target and is moved, via a programmed controller, to only be above portions of the workpiece so that no sputtered material is wasted. For depositing a material onto small workpieces, a cooling backside gas volume is created between the pallet and the underside of sticky tape supporting the workpieces.

Claims

1. A method of operation of a physical vapor deposition device, the device having a chamber configured to create a low pressure environment in the chamber while sputtering materials on a workpiece, the method comprising: providing a workpiece support platform in the chamber, the support platform having openings for a backside gas; mounting a workpiece on a sticky tape, wherein a ring provides a frame for the sticky tape; providing one or more magnets on the support platform for attracting the ring to the one or more magnets; positioning the ring over the one or more magnets such that the ring is attracted to the one or more magnets; providing a ridge on the support platform that creates a gas seal with respect to the sticky tape as the ring is attracted to the one or more magnets on the support platform, so that an area between the tape and the support platform forms a sealed backside gas volume; and introducing a backside gas into the backside gas volume for cooling the workpiece supported by the tape.

2. The method of claim 1 wherein the sticky tape has at least one opening that exposes a back surface of the workpiece to the backside gas volume.

3. The method of claim 1 wherein the workpiece is one of a plurality of workpieces supported on a top surface of the sticky tape for batch processing of the workpieces.

4. The method of claim 1 wherein the one or more magnets are permanent magnets.

5. The method of claim 1 wherein the ring is formed of a high magnetic permeability material.

6. The method of claim 1 wherein the ring is formed of steel.

7. The method of claim 1 further comprising depositing a metal shield over the workpiece.

8. The method of claim 1 further comprising depositing a backside metal over the workpiece.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a side view of the sputtering system in accordance with one embodiment of the invention.

(2) FIG. 2 is a bisected perspective view of the system of FIG. 1.

(3) FIG. 3 is a bisected side view of the system of FIG. 1.

(4) FIG. 4 is a perspective view of an XY stage which moves the rotating pallet in a controlled XY direction.

(5) FIG. 5 is a schematic cross-sectional view of the pallet, showing the liquid coolant channel, the backside coolant gas channel, the pallet motor, the XY mount for the motor and pallet, the RF/biasing source, and the programmed controller (a processor).

(6) FIG. 6 is a top down view of the magnetron over the wedge-shaped targets above the vacuum chamber.

(7) FIG. 7 illustrates how the magnet over the target moves linearly as well as in an arcing motion to cover all areas of the target and to prevent sputtering over areas where no workpiece is directly underneath the target.

(8) FIG. 8 illustrates how the magnet is composed of many smaller magnets to create a substantially uniform magnetic field behind the target.

(9) FIG. 9 illustrates how a large rectangular panel (a workpiece on the pallet in the vacuum chamber) is rotated by the pallet and shifted in an XY plane below the targets and how the magnets for the targets can be controlled to only pass over portions of the target directly above the panel.

(10) FIG. 10 illustrates how the rectangular panel is rotated and shifted in an XY plane within the vacuum chamber to obtain full sputtering coverage of the panel.

(11) FIG. 11 illustrates the pallet on which is mounted four steel rings supporting a sticky tape, with packaged integrated circuit chips mounted on the tape for receiving a sputtered metal shielding layer.

(12) FIG. 12 is a schematic cross-section of one of the steel rings being mounted over the pallet, showing a backside cooling gas channel in the pallet, a circular ridge, and magnets on the pallet.

(13) FIG. 13 illustrates how the magnets on the pallet pull down the steel ring to create a backside gas seal around the periphery of the sticky tape for cooling the packages with a backside gas.

(14) Elements with the same numbers in the various figures are the same.

DETAILED DESCRIPTION

(15) The present assignee has obtained a U.S. Pat. No. 7,479,210, and has a published application US 2012/0024694, describing a sputtering tool, and the present invention is an improvement over those tools. U.S. Pat. No. 7,479,210 and US 2012/0024694 are incorporated herein by reference. Accordingly, only aspects of the improved tool that are relevant to the present invention are described herein in detail. Other features of the tool may be obtained by reviewing the assignee's above-identified publications.

(16) FIGS. 1-3 illustrate the inventive sputtering system 12 in accordance with one embodiment of the invention. The sputtering system 12 may be used for workpieces such as semiconductor wafers, display panels, mechanical parts, and other workpieces requiring the deposition of thin films. Examples of thin films include Al, Cu, Ta, Au, Ti, Ag, Sn, NiV, Cr, TaNx, Hf, Zr, W, TiW, TiNx, AlNx, AlOx, HfOx, ZrOx, TiOx, magnetic films, and various alloys of these materials.

(17) In FIG. 1, the top cover 14 covers the magnetron and other components above the vacuum chamber 16. A rotating pallet and various targets reside within the vacuum chamber 16.

(18) In one embodiment, the system 12 can process any size workpiece that can fit on the rotating pallet 18 (FIGS. 2 and 3), such as a large display panel. Alternatively, the system 12 can simultaneously process multiple standard semiconductor wafers (e.g., five or more) using multiple sputtering targets (e.g., 3-6).

(19) The pallet 18 rotates to move a workpiece directly below an appropriate wedge-shaped target. Each target may be a different material for forming successive thin films of different materials on a workpiece, or the targets may be the same material. The targets are directly below an associated target backing plate 20 (FIG. 2).

(20) The pallet 18 has a central shaft 22 (FIG. 3) that is rotated by a motor 24, such as a stepper motor or a servo motor. The pallet 18 may be formed of aluminum. The pallet 18 may be continuously rotated at any speed or may be temporarily stopped to control the deposition of a sputtered material from a target overlying a workpiece.

(21) The servo or stepper motor 24 is mounted on a mounting area 26 of an XY stage 28. FIG. 4 illustrates the XY stage 28 in greater detail. The XY stage 28 has a stationary top plate 30 mounted to the bottom surface of the vacuum chamber 16. A middle plate 32 moves in an X direction relative to the top plate 30 using an X linear motor 34. The linear motor includes a small rotating motor with a cam that engages a long screw so that the screw is moved in a linear direction with respect to the motor. The motor may be attached to the top plate 30, and the screw is rotatably mounted on the middle plate 32. Tracks keep the middle plate 32 slidably attached to the top plate 30. Similarly, the bottom plate 36 is slidably attached to the middle plate 32 using tracks and a Y linear motor 38 that is identical to the X linear motor 34.

(22) A large center opening 42 in the XY stage 28 accommodates the pallet shaft 22 and allows some XY movement of the shaft 22 within the opening 42. As shown by the arrows 44 in FIG. 3, the XY stage 28 can move the rotatable pallet 18 within the vacuum chamber 16 in an XY plane. The amount of movement can be any amount, depending on the size of the system. In one embodiment, the movement may be greater than 4 inches.

(23) A bellows 46 (FIG. 3) extends through the hole 42 in the XY stage 28 and surrounds the shaft 22 to maintain a vacuum seal around the shaft 22.

(24) The magnetron assembly 50 in the top (non-vacuum) compartment of the system 12 moves a magnet around each of the target backing plates 20. The magnets attract the ions in the sputtering gas to the targets to accelerate and direct the ions. The ions then knock out atoms of the target for sputtering the target material on the workpiece substantially directly below the magnet. The aluminum pallet 18 has a potential that attracts the sputtered material to the workpiece. More detail regarding the magnetron assembly 50 is presented later.

(25) FIG. 5 is a schematic cross-sectional view of an area of the pallet 18, with various features exaggerated for ease of illustration. The pallet motor 24 is shown rotating the shaft 22, and the motor 24 is mounted on the XY stage mounting area 26.

(26) The pallet 18 has liquid channels or tubing 51 that receive a liquid coolant from a recirculating cooling source 52. The coolant flows through input and output lines 54/55 in the shaft 22 and within the pallet 18. A coupler 58 provides a rotating seal for the input/output lines 54/55.

(27) The coupler 58 is shown also coupling an optional RF source 60 to the aluminum pallet 18 for generating a plasma and for attracting ions to the workpiece.

(28) In addition to cooling the pallet 18, a backside coolant gas source 62 is also provided to supply a backside gas, such as argon or other inert gas, through openings 64 in the pallet 18. A gas channel 66 in the shaft 22 supplies the gas to a diffuser 68 in the pallet 18 for distributing the gas to the openings 64. The coupler 58 may also provide a gas seal to allow the shaft 22 to rotate while the backside gas is being supplied. A gas exit channel for maintaining a desired gas pressure is outside of the cross-section. Optionally, the gas may also exit into the chamber without a return path. The use of the backside gas to cool a workpiece is discussed later. The backside gas feature is not always used.

(29) A controller 70 comprises a programmed processing system and automatically controls the XY stage 28, motor 24, RF source 60, coolants, and target magnets pursuant to a pre-programmed routine to deposit one or more sputtered layers on the workpiece.

(30) The RF source 60 (FIG. 5) is electrically coupled to the aluminum pallet 18 by the rotatable coupling 58 to create a plasma. In another embodiment, the pallet 18 is grounded, floated, or biased with only a DC voltage source.

(31) When the chamber 16 is evacuated and 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 workpieces receive the sputtered material to form a deposited layer on the surface of the workpieces. 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. A typical RPM of the pallet 18 during the deposition process is 5-30 RPM. The pallet 18 may be rotating during deposition or stopped.

(32) The chamber wall is typically electrically grounded during sputtering operations.

(33) A bias voltage on the workpieces can drive a flux of an electrically charged species (Ar+ and/or atomic vapor sputtered off the target) to the workpieces. The flux can modify the properties (for example, density) of the sputtered material to the wafers.

(34) 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.

(35) FIG. 6 is a top down schematic illustration of the target backing plates 20 and the magnetron assembly 50. Targets are mounted to the top ceiling of the vacuum chamber directly under the target backing plates 20. The targets have the same outline as the plates 20. A contamination shield 71 (a vertical wall) extends any length between targets for preventing cross-contamination. Some possible positions for the magnet 72 during motion are shown in dashed outline.

(36) FIG. 7 illustrates a single scanning magnet 72 over a target 74. The view is looking up from the target 74 though the target backing plate. In one embodiment, the delta-shaped magnet 72 is about 10.7 inches (27 cm) long and about 3 inches (7.6 cm) wide at its widest part. A motor 76 (also shown in FIG. 3) moves all magnets 72 back and forth in an arc path 78 over the target backing plate 20 and target 74. The motor 76 may be any type of scanning actuator, such as a servo/stepper. The rate of scan can be any rate. The motor 76 scans the magnet 72 to the left and right edges of the target for each scan of the magnet 72. When the magnet 72 is over a portion of the target 74, that portion will be primarily bombarded by ions in the plasma and deposit sputtered material on the underlying workpiece. The magnet 72 is also moved linearly along any portion of the length of the target 74 by a linear motor. The linear motor includes a motor 80 that rotates a screw 82 within a track 84. The back of the magnet 72 has a threaded coupling (like a nut) that is moved linearly by the rotating screw 82 in the direction of the arrow 86. The linear motor can be any type of linear actuator.

(37) A scanning controller 88 controls the simultaneous arcing scan of the magnets 72, and a linear movement controller 90 independently, and individually, controls the linear movement of an associated magnet 72. Each magnet, associated with a different target, may be controlled differently, depending on the requirements of the sputtering for that target. The controllers 88 and 90 may be part of the controller 70 of FIG. 5.

(38) More specifically, the function of the magnet 72 is as follows. The magnet 72 confines the plasma 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.

(39) The scanning controller 88 oscillates all the magnets 72 back and forth in unison over their associated targets at an oscillating period of between 0.5-10 seconds. The magnets 72 are oscillated so that the magnetic fields are not always at the same position relative to the target. By distributing the magnetic fields evenly over the target, target erosion is uniform.

(40) FIG. 8 illustrates the magnet 72 in more detail. The magnet 72 has a triangular or delta shape with rounded corners. In one embodiment, the thickness of magnet 72 is between 0.5-1 inch thick (12-31 mm). In the example of FIG. 8, there are three rings (nested patterns) of individual magnets 94, where adjacent rings have opposite poles so that a magnetic field spans across one ring to the next. Some magnetic field lines 96 are shown. Since there are three rings of magnets, there are two racetracks of field lines. These magnetic fields pass through the target backing plate 20 (FIG. 6) and intersect the target attached to the underside of the target backing plate 20. The plasma density at the target (and thus the erosion rate) is greatest at the highest magnetic field intensity. The sizes, shapes, and distribution of the individual magnets 94 are selected to create a uniform erosion of the target. The magnets 94 are mounted to a magnetic backing plate 98, also known as a shunt plate, which may be formed of a ferrous material. The shape and magnetic properties of the shunt plate 98 may be altered to optimize the performance of the magnet 72. The magnet 72 may instead be an electromagnet.

(41) FIG. 9 is identical to FIG. 6 except that the outline of a relative large panel 100 (a workpiece) is shown mounted on the rotating pallet.

(42) FIG. 10 shows the panel 100 mounted on the pallet 18. The inside wall 102 of the vacuum chamber 16 is also shown. Note that the diameter of the pallet 18 is significantly smaller than the diameter of the chamber 16 to allow the pallet 18 to be moved in an XY plane within the chamber 16.

(43) In the prior art sputtering chambers having a rotating pallet, if a large workpiece, such as the panel 100, were mounted on the pallet, the middle area of the workpiece could not be positioned under a target, since the targets do not extend over the center of the chamber. However, in the present invention, the center 104 of the pallet 18 is movable so as to be under any of the targets by shifting the XY stage 28 (FIGS. 3 and 4). Therefore, by controlling the rotation of the pallet 18 in conjunction with the XY stage 28, all portions of the panel 100 may be positioned under any target for receiving a sputtered layer of the target material. The arrows 106 and 108 illustrate the rotation of the pallet 18 and the XY movement of the pallet 18. In one embodiment, the diameter of the pallet 18 is at least 4 inches less than the diameter of the chamber 16 to allow the center of the pallet 18 to be shifted up to plus/minus 2 inches in order to position the panel 100 under a target.

(44) Referring back to FIG. 9, it can be seen that the corners of the rectangular panel 100 extend closer to the wall 102 of the chamber 16 than the flat sides of the panel 100. When the corner of the panel 100 is below the outer end of the target (having the same outline as the target backing plate 20), the magnet 72 needs to be positioned, via the scanning and linear motors 76/80, above the outer end of the target to promote sputtering onto the corner area of the panel 100. However, when the side of the panel 100 is below a target, the side may only be below the inner one-third or less of the target. Thus, sputtering from the outer end of the target will waste target material (and generate debris) when no portion of the panel 100 is below the outer edge. Therefore, in accordance with one embodiment of the invention, the linear movement controller 90 (FIG. 3) is programmed to control the magnet 72 movement so that the magnet 72 is substantially moved only over portions of the panel 100 (or other workpiece). The dimensions of the panel 100 are programmed into the controller 70 (FIG. 5) or 90 (FIG. 7) and the position of the panel 100 at any time is determined by the position of the pallet 18.

(45) In one application, the magnet 72 is continuously scanned in an arc via the motor 76 and controller 88 (FIG. 7) during the entire sputtering process. In another application, the controllers 88/90 of FIG. 7 are programmed to control the magnet 72 to stop scanning or slow scanning, via the combination of the scanning/linear motors 76/80, in order to minimize the waste of target material by reducing/preventing sputtering when there is no workpiece under areas of the target.

(46) If all the targets are the same material and the pallet is rotating the rectangular panel 109 under the targets, the scanning/linear motors for one magnet will control their associated magnet in one way to ensure the magnet is only over a portion of the panel 100 and the scanning/linear motors for another magnet may control their associate magnet in a different way to ensure all magnets are only above a portion of the panel 100 so no sputtering material is wasted.

(47) Although the panel 100 in FIG. 9 is shown having a diagonal dimension approximately equal to the diameter of the pallet 18, the panel 100 can be any size. If the workpiece is small, such as a silicon wafer, the wafers may be positioned around the periphery of the pallet 18 so XY shifting of the pallet 18 is not needed to position the wafers directly under each target.

(48) The XY shifting of the pallet 18 and the control of the magnets 72 to limit the waste of target material are also applicable where multiple wafers are distributed over the pallet 18, where there are spaces between the wafers, and where the wafers cover the center area of the pallet 18. The positioning of the magnets 72 and the shifting of the pallet 18 are controlled to substantially limit sputtering material to only the wafer surfaces and not the pallet 18 surface.

(49) Another use of the sputtering system 12 is to sputter a copper film or other metal film over a batch of packaged integrated circuit chips, or over some other structures, to act as a shield to mitigate electromagnetic interference (EMI). Such packaged ICs would not have good thermal contact to the pallet 18 so could not be cooled by cooling the pallet 18 (using the liquid coolant). One technique to support an array of packaged ICs is shown in FIGS. 11-13.

(50) In FIG. 11, steel rings 112, or other rings formed of a high magnetic permeability material, have a sticky tape 114 secured to their underside so the tape 114 is taut across the opening of the rings 112. The rings 112, acting as frames, may be other shapes, such as rectangular. The tape 114 has rectangular openings 116 (FIG. 12) that are customized for the particular packages 115 being shielded. The openings 116 are just large enough for the bottom land grid array (LGA) or ball grid array (BGA) electrodes 118 to be within the openings, and the outer edges of the package create a seal with the top surface of the sticky tape 114, as shown in FIG. 12. It is not desirable to directly adhere the electrodes 118 to the sticky tape for at least the reasons that the sputtered material for the shielding layer may short out the electrodes and that the metal electrodes may be pulled off the packages when they are removed from the tape 114.

(51) In the example shown, there are four identical arrays of packages 115 mounted on the pallet 18 for receiving a metal shielding layer.

(52) In one example, if the packages 115 were not cooled somehow during the sputtering process, the temperature of the packages 115 would reach 220 C., and the packages 115 are only rated to withstand 150 C. Therefore, so means of cooling the packages 115 is required.

(53) Applicant's system cools the packages 115 with a recirculating backside gas, such as argon or other inert gas. As shown in FIG. 5, the shaft 22 of the pallet 18 has a channel 66 for a coolant gas, and the pallet 18 has a manifold 68 and openings 64 for the gas to exit the surface of the pallet 18. As shown in FIG. 12, the pallet 18 also includes openings 119 for a return path of the gas for cooling the gas. The gas exit channel is also along the pallet shaft.

(54) In FIG. 12, the pallet 18 surface includes magnets 120 and a circular ridge 122, assuming the opening of the rings 112 is circular. Other shapes are envisioned.

(55) As shown in FIG. 13, the magnets 120 pull the steel ring 112 downward so that the ridge 122 creates a gas seal with respect to the underside (non-sticky side) of the tape 114 and creates an open backside gas volume. The magnets 120 do not need to contact the entire periphery of the ring 112. The magnets 120 may be permanent magnets or electromagnets.

(56) In one embodiment, the pallet 18 has many functions, and a special surface plate 126 (FIG. 12), having the magnets 120 and ridges 122, is affixed to the top of a flat pallet plate 128, such as with screws. For other uses, a different surface plate may be affixed to the flat pallet plate 128 such as for sputtering on the panel 100 in FIG. 10 or on multiple wafers.

(57) During sputtering of Cu to deposit a shielding layer over the packages 115, the backside gas 129 is recirculated, by the backside coolant gas source 52, within the backside gas volume 130 behind the packages 115 to a predetermined pressure. The heat is removed from the packages 115 and the tape 114 by the recirculating backside gas 129. The temperature of the packages 115 is highly controllable by controlling the flow of the backside gas. Return paths 119 for the gas 129 may be included around the periphery of the backside gas volume 130.

(58) Other workpieces besides packages may be cooled using the basic technique of FIGS. 11-13. High heat dissipation during sputtering is essential for devices such as microprocessors, analog chips, etc. These wafers may be very thin (e.g., 30-200 microns) as a result of a back-grinding process, where the front surface of the wafer is mounted on a sticky tape during the grinding process. The tape 114 is then mounted to the rings 112, and a backside metal (e.g., Ti/Ni/Au) is sputtered over the back surface of the wafers. The wafers are cooled by the backside gas. The wafers may then be singulated while still attached to the sticky tape 114. The invention applies to any workpiece that may not be in good thermal contact with the pallet 18 or needs extra cooling.

(59) The invention pertaining to FIGS. 11-13 is independent of the rotating pallet 18 and the other aspects of the PVD system described with respect to FIGS. 1-10. The support surface for the steel rings 112 may be stationary. The sticky tape 114 can be used in other ways, such as supporting workpieces without openings in the tape 114. The invention will cool the backside of the tape and any workpieces in thermal contact with the tape 114. In one embodiment, the ring 112 material itself may not have a high magnetic permeability, such as the ring 112 being a dielectric, but the ring 112 has areas of high magnetic permeability, such as thin steel plates adhesively affixed to the bottom of the ring 112.

(60) 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.

(61) Although the system has been described with respect to examples of forming a metal film on workpieces, the system may deposit any material, including dielectrics, and may process any workpiece. In one embodiment, the system is used to deposit materials on multiple thin film transistor arrays for LCD panels. The invention is not limited to the specific examples described herein.

(62) 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.