Patent Application
20060017010
The present invention concerns ion implanters and more particularly an ion implanter having a scanning magnet for use in performing serial implants of a workpiece.
Axcelis Technologies, assignee of the present invention, designs and sells products for treatment of workpieces such as silicon wafers during integrated circuit fabrication. Ion implanters create an ion beam that modifies the physical properties of workpieces such as silicon wafers that are placed into the ion beam. This process can be used, for example, to dope the silicon from which the untreated wafer is made to change the properties of the semiconductor material. Controlled use of masking with resist materials prior to ion implantation as well as layering of different dopant patterns within the wafer produce an integrated circuit for use in one of a myriad of applications.
An ion implantation chamber of an ion beam implanter is maintained at reduced pressure. Subsequent to acceleration along a beam line, the ions in the beam enter the implantation chamber and strike the wafer. In order to position the wafer within the ion implantation chamber, they are moved by a robot into a load lock from a cassette or storage device that is located at high pressure.
One prior art patent relating to an ion implanter is U.S. Pat. No. 5,481,116 to Glavish et al. This patent concerns a magnetic system for uniformly scanning an ion beam. The system has a magnet structure having poles with associated scanning coils and respective pole faces that define a gap through which the ion beam passes. A magnetic field set up by the magnet structure controllably deflects ions that make up the beam.
The present invention concerns an ion beam implanter for implanting a workpiece such as a semiconductor wafer. The ion beam implanter includes an ion beam source for generating an ion beam moving along a path of travel and that can be scanned back and forth away from a beam centerline. A workpiece support positions a wafer in an implantation chamber so that the ions that make up the beam strike the workpiece.
One embodiment of an ion beam implanter that utilizes the invention includes an ion beam source for generating an ion beam moving along a beam line and structure that defines an implantation chamber having an evacuated interior region wherein a workpiece is positioned to intersect the ion beam for ion implantation of an implantation surface of the workpiece by the ion beam. Upstream from the implantation chamber the implanter includes a scanning magnet including a core material comprising an amorphous metal material. An electronic conductor, typically magnet windings sets up a magnetic field for scanning the ions in the ion beam from side to side.
An important aspect of the invention is use of a metallic glass for use as core material for a scanning magnet. This material exhibits sufficient magnetic permeability with low core loss at high scanning frequency to permit scanning from side to side of the beam at relatively high frequencies. These high frequencies are advantageous because the implant uniformity is improved if the scanning frequency is increased. As the workpiece moves within the implantation chamber, the magnet causes the beam to scan back and forth in an orthogonal direction. A high wafer scan frequency means the workpiece has a chance to move only a small amount during a side to side scan of the beam and this “painting” of a band across the workpiece without appreciable wafer movement improves implant uniformity. Higher scan frequencies also permit higher implant throughput (number of wafers per hour) and therefore greater implanter productivity.
These and other features of the exemplary embodiment of the invention are described in detail in conjunction with the accompanying drawings.
Turning to the drawings,
The ion source 12 includes a plasma chamber defining an interior region into which source materials are injected. The source materials may include an ionizable gas or vaporized source material. Ions generated within the plasma chamber are extracted from the chamber by ion beam extraction assembly 28 which includes a number of metallic electrodes for creating an ion accelerating electric field.
Positioned along the beam path 16 is an analyzing magnet 30 which bends the ion beam 14 and directs it through a beam shutter 32. Subsequent to the beam shutter 32, the beam 14 passes through a quadrupole lens system 36 that focuses the beam 14. The beam then passes through a deflection magnet 40 which is controlled by the controller 41. The controller 41 provides an alternating current signal to the conductive windings of the magnet 40 which in turn caused the ion beam 14 to repetitively deflect or scan from side to side at a frequency of several hundred Hertz. In one disclosed embodiment, scanning frequencies of from 200 to 300 Hertz are used. This deflection or side to side scanning generates a thin, fan shaped ribbon ion beam 14a.
Ions within the fan shaped ribbon beam follow diverging paths after they leave the magnet 40. The ions enter a parallelizing magnet 42 wherein the ions that make up the beam 14a are again bent by varying amounts so that they exit the parallelizing magnet 42 moving along generally parallel beam paths. The ions then enter an energy filter 44 that deflects the ions downward (y-direction in
The ribbon ion beam 14a that exits the parallelizing magnet 42 is an ion beam with a cross-section essentially forming a very narrow rectangle that is, a beam that extends in one direction, e.g., has a vertical extent that is limited (e.g. approx ½ inch) and has an extent in the orthogonal direction that widens outwardly due to the scanning or deflecting caused to the magnet 40 to completely cover a diameter of a workpiece such as a silicon wafer.
Generally, the extent of the ribbon ion beam 14a is sufficient to, when scanned, implant an entire surface of the workpiece 24. Assume the workpiece 24 has a horizontal dimension of 300 mm. (or a diameter of 300 mm.). The magnet 40 will deflect the beam such that a horizontal extent of the ribbon ion beam 14a, upon striking the implantation surface of the workpiece 24 within the implantation chamber 22, will be at least 300 mm.
A workpiece support structure 50 both supports and moves the workpiece 24 (up and down in the y direction) with respect to the ribbon ion beam 14 during implantation such that an entire implantation surface of the workpiece 24 is uniformly implanted with ions. Since the implantation chamber interior region is evacuated, workpieces must enter and exit the chamber through a loadlock 60. A robotic arm 62 mounted within the implantation chamber 22 automatically moves wafer workpieces to and from the loadlock 60. A workpiece 24 is shown in a horizontal position within the load lock 60 in
In a typical implantation operation, undoped workpieces (typically semiconductor wafers) are retrieved from one of a number of cassettes 70-73 by one of two robots 80, 82 which move a workpiece 24 to an orienter 84, where the workpiece 24 is rotated to a particular orientation. A robot arm retrieves the oriented workpiece 24 and moves it into the load lock 60. The load lock closes and is pumped down to a desired vacuum, and then opens into the implantation chamber 22. The robotic arm 62 grasps the workpiece 24, brings it within the implantation chamber 22 and places it on an electrostatic clamp or chuck of the workpiece support structure 50. The electrostatic clamp is energized to hold the workpiece 24 in place during implantation. Suitable electrostatic clamps are disclosed in U.S. Pat. No. 5,436,790, issued to Blake et al. on Jul. 25, 1995 and U.S. Pat. No. 5,444,597, issued to Blake et al. on Aug. 22, 1995, both of which are assigned to the assignee of the present invention. Both the '790 and '597 patents are incorporated herein in their respective entireties by reference.
After ion beam processing of the workpiece 24, the workpiece support structure 50 returns the workpiece 24 to a horizontal position and the electrostatic clamp is de-energized to release the workpiece. The arm 62 grasps the workpiece 24 after such ion beam treatment and moves it from the support 50 back into the load lock 60. In accordance with an alternate design the load lock has a top and a bottom region that are independently evacuated and pressurized and in this alternate embodiment a second robotic arm (not shown) at the implantation station 20 grasps the implanted workpiece 24 and moves it from the implantation chamber 22 back to the load lock 60. From the load lock 60, a robotic arm of one of the robots moves the implanted workpiece 24 back to one of the cassettes 70-73 and most typically to the cassette from which it was initially withdrawn.
Scanning Magnet 40
The conductors 120, 122 extend in a direction that parallels the direction of ion movement as ions enter the magnet 40. Portions of the conductors are positioned on either side of a centerline through the magnet 40. See
As seen in
The two formed sections 130a, 130a′ are arranged in the magnet with the longer prong of the “U” to the outer side of the magnet center passageway, as shown if
Each of the ten sections when in their respective location within the magnet form the overall core of the magnet. This core comprises two side segments 131, 134 and a center segment 132 having a surface 135 which bounds the beam passageway through the magnet. In one exemplary embodiment of the invention, a surface 135 of the core has a width between the two side segments 131, 134 (including the width of the channels C that accommodate the windings) of approximately ten (10) inches. The two side segments 131, 134 extend upwardly in the ‘y’ direction above the generally planar surface 135 of the center segment 132 and in one embodiment the distance from the plane 135 to an exposed face of the side segments 131, 134 is about three (3) inches.
Each of the core sections 130a-130e and 130a′-130e′ is made up of many individual magnet laminations which are thin generally planar sheets or ribbons that are wound about a mandrel 202 to form the magnet sections (130a for example). The exposed planar surface of the center segment 132 of the overall core is made up of a combination of the cut ends of the smaller prongs of each of the ten “U” shaped core sections. As shown in
The laminations or sheets are constructed from an alloy of amorphous metal material, commonly referred to in the art as metallic glass. These amorphous metal alloys differ from conventional metals used, such as grain-oriented Silicon steel, in that they have a non-crystalline structure and possess unique physical and magnetic properties. Amorphous-metal alloys differ from their crystalline counterparts in that they consist of atoms arranged in near random configurations devoid of order. The amorphous metal alloy material is ferromagnetic, i.e., has a magnetic permeability much greater than 1. The amorphous metal alloy material is typically formed from metals comprising cobalt, iron, and nickel. More particularly one suitable amorphous metal material is chosen from an alloy of cobalt, iron, and nickel with the concentrations of the metals chosen to reduce the cost of producing the sheets while maintaining sufficiently high magnetic flux saturation density, i.e., greater than 1.5 Tesla. An important property of the metallic glass is that it exhibits low core loss at high frequency, typically more than ten times lower than the core loss of Silicon (transformer) steel. The low core loss reduces the power consumption of the scanning magnet 40 as well as cooling requirements and, therefore, operating temperature.
Several techniques for creating a ribbon for fabricating a core are known. One known construction technique is known as planar flow casting. In this variation of chill-block melt spinning, molten metal is forced through a slotted nozzle in close proximity (≈0.5 mm) to the surface of a moving substrate. A melt puddle is formed which is simultaneously contacting the nozzle and the substrate and is thereby constrained to form a stable, rectangular shape. While the flow of molten metal through the nozzle is controlled by pressure, it is also dependent on a gap or spacing between the nozzle and the substrate. Using planar flow casting, amorphous metal ribbon widths up to 300 mm have been realized, and widths up to 210 mm are commercially available. Once the ribbon or individual sheet is formed (such as the sheets used to fabricate the core sections 130a, 130b etc) it is wound about a supporting mandrel. A binder is included with the amorphous metal material and can be either a silicate or a glass. After winding the ribbon forms a coiled spiral that is held together with a suitable adhesive such as epoxy. One suitable amorphous metal alloy material for use in creating the core sheets is commercially available from Metglas having a place of business at Jimmy W. Jordan 440 Allied Drive, Conway, S.C. 29526 and sold under product designation 2605SA1. This product provides extremely low core loss (less than 0.2 W/kg at 60 Hz, 1.4 Tesla) or 30% of the core loss of grade M-2 electrical steel (core loss at 50 Hz is approximately 80% of 60 Hz values) and high permeability (maximum D.C. permeability (μ)-annealed-600,000; cast-45,000). A data sheet describing the properties of this product is commercially available from Metglas and is incorporated herein by reference. The details of amorphous metals and the process of creating a ribbon of material is disclosed in, “Amorphous Metals in Electric-Power Distribution Applications,” Nicholas DeCristofaro, MRS Bulletin, Volume 23, Number 5 (1988) P. 50-56, and is hereby incorporated by reference in its entirety.
The ions that make up the beam 114 that enters the magnet entrance 124 are shaped upstream by the quadrupole focusing structure. There are always ions, however, that will deviate from the normal path and some of these ions impact upon structure of the magnet 40. To avoid damage to the structure of the center portion 132 of the magnet the magnet includes top and bottom entrance shields 140,142 constructed from steel. The shields are constructed from planar steel laminations which are bound together by a suitable adhesive that reduces contamination in the region of the beam line.
The two halves of the magnet yoke (all ten core sections in the exemplary embodiment) are supported by structure above and below the beam line that includes mounting flanges 150, 152 that support the yoke and saddle coils. The saddle coils are constructed from hollow electrically conductive conduits through which a coolant such as water is routed during operation of the magnet. Prior to assembly, the conduits are electrically insulated with thin coatings of enamel or epoxy. The assembled saddle coil is held together by a vacuum compatible epoxy glue, typically cured in vacuum. Extending downwardly from the top flange 150 and upwardly from the bottom flange 152 are end plates 154, 155, 156, 157. These end plates are metal and define passageways through which suitable coolant such as water is also routed. As seen in
In operation control electronics coupled to bus bars 170 energize the saddle coils to create an alternating magnetic field that deflects the ions entering the magnet by a varying amount that depends on the instantaneous field strength when the ion enters the magnet. The B field has a vector component in generally the positive y direction with one polarity of coil energization and a vector component in generally the negative y direction with the second polarity electrical energization. This alternating field polarity in the positive and negative ‘y’ direction, as seen in the figures, produces a side to side beam scan in the x-z plane, since the larger the field magnitude, the greater the force on the ion, hence the smaller the bend radius of the ion inside the scanning magnet, since charged particles in magnetic fields follow circular trajectories, and therefore the greater the deflection. A triangular wave energization of the saddle coils produces a constant beam scan velocity transverse to the direction of travel of the unscanned beam. In the case of the scanning magnet, the scanning field or magnet current has to be accurately controlled to control the beam scan angle. In practice, the waveform is modulated to change scan speed and the time-averaged ion flux across the workpiece to obtain high dose uniformity of the implant.
While the present invention has been described with a degree of particularity, it is the intent that the invention includes all modifications and alterations from the disclosed design falling with the spirit or scope of the appended claims.
