Abstract:
Systems and methods are disclosed for face target sputtering to fabricate semiconductors by providing one or more materials with differential coefficients of expansion in the FTS chamber; and generating a controlled pressure and size with the one or more materials during sintering.

Description:
BACKGROUND 
     FTS (Facing Target Sputtering) method is a semiconductor fabrication technique that provides high density plasma, high deposition rate at low working gas pressure to form high quality thin film. In a facing target type of sputtering apparatus, at least a pair of target planes are arranged to face each other in a vacuum vessel, and magnetic fields are generated perpendicularly to the target planes for confining plasma in the space between the facing target planes. The substrate is arranged so as to be positioned at the side of the space so that films are produced on the substrate by sputtering. 
     As discussed in U.S. Pat. No. 6,156,172, a typical FTS apparatus includes a vacuum vessel for defining therein a confined vacuum chamber, an air exhausting unit having a vacuum pump system to cause a vacuum via an outlet, and a gas supplying unit for introducing sputtering gas into the vacuum vessel. A pair of target portions are arranged in the vacuum vessel in such a manner that a pair of rectangular shape cathode targets face each other so as to define a predetermined space therebetween. 
     Another FTS apparatus discussed in the &#39;172 patent confines sputtering plasma in a box type of plasma space using a pair permanent magnets so as to face N and S-pole generate magnetic flux circulating perpendicularly the outside space of the first facing targets which defines facing target mode in combination with electric fields perpendicular to target planes in plasma space. The pair of magnets generate a conventional magnetron mode with a closed magnetic flux from the pole of magnets in the vicinity of the outside area of the pair of target planes in addition to the facing target mode. The cathodes of all the targets are arranged so as to recoil and confine the electrons into the plasma space by the aid of both the facing target mode and the magnetron mode. 
     To improve the deposition speed of the equipment, the &#39;172 patent discloses an FTS apparatus which includes: an arrangement for defining box-type plasma units supplied therein with sputtering gas mounted on outside wall-plates of a closed vacuum vessel; at least a pair of targets arranged to be spaced apart from and face one another within the box-type plasma unit, with each of the targets having a sputtering surface thereof; a framework for holding five planes of the targets or a pair of facing targets and three plate-like members providing the box-type plasma unit so as to define a predetermined space apart from the pair of facing targets and the plate-like members, which framework is capable of being removably mounted on the outside walls of the vacuum vessel with vacuum seals; a holder for the target having conduits for a coolant; an electric power source for the targets to cause sputtering from the surfaces of the targets; permanent magnets arranged around each of the pair of targets for generating at least a perpendicular magnetic field extending in a direction perpendicular to the sputtering surfaces of the facing targets; devices for containing the permanent magnets with target holders, removably mounted on the framework; and a substrate holder at a position adjacent the outlet space of the sputtering plasma unit in the vacuum vessel. 
     On a parallel note, manufacturing complex metal oxide targets is a complex process involving multiple sintering, grinding and annealing steps. These steps are difficult even with simple geometries like parallelopipedal plates and strips, but become much more problematic with curved geometries and cylindrical targets. The current process requires a specialized press which costs about $50 k for each shape. Since many different shapes are typically necessary to optimize the magnetron design, and since the optimum shape is typically curved to minimize electrical fields and maximize throughput and cooling flow, the cost of making large magnetrons is often prohibitive, thus leaving the designer with sub-optimal shapes. 
     SUMMARY 
     Systems and methods are disclosed for face target sputtering to fabricate semiconductors by providing one or more materials with differential coefficients of expansion in the FTS chamber; and generating a controlled pressure and size with the one or more materials during sintering. 
     In one embodiment, the system uses differential coefficient of expansion of materials to achieve the necessary controlled pressure and size during the sintering step. By changing the size of the inner pressure ring with temperature while the outer casing is kept at constant shape (a low expansion alloy), a large force can be exerted on the sintered material. This force and temperature compact the material and create a solid out of the sinter powder. 
     In another embodiment, the FTS has an air-tight chamber in which an inert gas is admittable and exhaustible; a first cylindrical target plate; inner and outer cylindrical magnets respectively disposed adjacent to the cylindrical target plate such that magnet poles of different polarities face each other across said plasma region thereby to establish a magnetic field covering the target plate; and a substrate holder adapted to hold a substrate on which an alloyed thin film is to be deposited. 
     Advantages of the above system may include one or more of the following. The system allows multiple shapes of a complex-metal oxide magnetron to be used at a low cost. For example, in one embodiment, the system provides approximately 10× lower cost than conventional systems. 
     The above configuration provides symmetry and scalability. While conventional FTS systems is constrained in size because the magnetic field and process pressure change depending on the distance between the plates, the above circular system can be expanded since the distance between the two circular target plates can be kept constant while both of their diameters are increased. For example, while a conventional FTS system could uniformly cover only a one-inch area with a four-inch target plate separation, the circular system can cover a 12-inch area with the same four-inch target plate separation. Such increased coverage increases the deposition rate to increase productivity and thus lowers operating cost. The compact and simplified configuration also increases reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       In order that the manner in which the above-recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated, in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1A  shows one embodiment of an apparatus for fabricating semiconductor. 
         FIG. 1B  shows magnet arrangement in  FIG. 1A  to provide a symmetrical source. 
         FIG. 1C  shows the system of  FIG. 1B  with a plurality of moving magnets. 
         FIG. 1D  shows the system of  FIG. 1B  with an oxygen trap. 
         FIG. 1E  shows a cross-type facing magnetron. 
         FIG. 1F  shows an exemplary embodiment of a target material. 
         FIG. 2  is an exemplary electron distribution chart. 
         FIG. 3  shows another embodiment of a FTS unit. 
         FIGS. 4-7  show exemplary embodiments of a systems using one or more materials with differential coefficients of expansion in the FTS unit to generate controlled pressure and size with the one or more materials during sintering. 
     
    
    
     DESCRIPTION 
     Referring now to the drawings in greater detail, there is illustrated therein structure diagrams for a semiconductor processing system and logic flow diagrams for processes a system will utilize to deposit a memory device at low temperature, as will be more readily understood from a study of the diagrams. 
       FIG. 1A  shows one embodiment of a reactor  10 . The reactor  10  includes a metal chamber  14  that is electrically grounded. A wafer or substrate  22  to be sputter coated is supported on a pedestal electrode  24  in opposition to the target  16 . An electrical bias source  26  is connected to the pedestal electrode  24 . Preferably, the bias source  26  is an RF bias source coupled to the pedestal electrode  24  through an isolation capacitor. Such bias source produces a negative DC self-bias VB on the pedestal electrode  24  on the order of tens of volts. A working gas such as argon is supplied from a gas source  28  through a mass flow controller  30  and thence through a gas inlet  32  into the chamber. A vacuum pump system  34  pumps the chamber through a pumping port  36 . 
     The FTS unit is positioned to face the wafer  22  and has a plurality of magnets  102 ,  104 ,  106 , and  108  which are part of two facing magnetrons. A first target  110  is positioned between magnets  102  and  104 , while a second target  120  is positioned between magnets  106  and  108 . The first and second targets  110  and  120  define an electron confining region  130 . 
     The two facing magnetrons are elongated resulting in a rectangular configuration. The rectangular configuration is bent into a doughnut shape by uniting the two ends. Thus the system has two bands of facing magnetrons, one inside the other, as shown in  FIG. 1B . By adding magnets of opposite polarity behind the outer and inner target bands, a barrel shaped magnetic field is developed. Thus, on a local scale, the magnetic field is identical to the conventional FTS configuration. The pressure and electric field are identical as well. 
     A power supply  140  is connected to the magnets  102 - 108  and targets  110 - 120  so that positive charges are attracted to the second target  120 . During operation, particles are sputtered onto a substrate  150  which, in one embodiment where the targets  110  and  120  are laterally positioned, is vertically positioned relative to the lateral targets  110  and  120 . The substrate  150  is arranged to be perpendicular to the planes of the targets  110  and  120 . A substrate holder  152  supports the substrate  150 . 
     The targets  110  and  120  are positioned in the reactor  10  to define the plasma confining region  130  therebetween. Magnetic fields are then generated to cover vertically the outside of the space between facing target planes by the arrangement of magnets installed in touch with the backside planes of facing targets  110  and  120 . The facing targets  110  and  120  are used a cathode, and the shield plates are used as an anode, and the cathode/anode are connected to output terminals of the direct current (DC) power supply  140 . The vacuum vessel and the shield plates are also connected to the anode. 
     Under pressure, sputtering plasma is formed in the space  130  between the facing targets  110  and  120  while power from the power source is applied. Since magnetic fields are generated around the peripheral area extending in a direction perpendicular to the surfaces of facing targets  110  and  120 , highly energized electrons sputtered from surfaces of the facing targets  110  and  120  are confined in the space between facing targets  110  and  120  to cause increased ionized gases by collision in the space  130 . The ionization rate of the sputtering gases corresponds to the deposition rate of thin films on the substrate  22 , then, high rate deposition is realized due to the confinement of electrons in the space  130  between the facing targets. The substrate  22  is arranged so as to be isolated from the plasma space between the facing targets  110  and  120 . 
     Film deposition on the substrate  22  is processed at a low temperature range due to a very small number of impingement of plasma from the plasma space and small amount of thermal radiation from the target planes. A typical facing target type of sputtering method has superior properties of depositing ferromagnetic materials at high rate deposition and low substrate temperature in comparison with a magnetron sputtering method. When sufficient target voltage VT is applied, plasma is excited from the argon. The chamber enclosure is grounded. The RF power supply  26  to the chuck or pedestal  24  causes an effective DC ‘back-bias’ between the wafer and the chamber. This bias is negative, so it repels the low-velocity electrons. 
     The efficiency of the facing magnetron deposition can be further increased by incorporating a secondary additional magnetron excitation system ( 238 ) with a separate power supply  237  that increases the number of positive ions that are then accelerated into the wafer surface by the back bias. 
       FIG. 1B  shows in more detail the magnetron structure that allows the configuration to be symmetrical and results in a small and uniform source. In this embodiment, an elongated central or middle magnet  302  is encircled by an inner ring magnet  304 A. An inner target ring  306 A encircles the ring magnet  304 A and faces a second target ring  306 B. The second target ring  306 B is in turn encircled by an outer ring magnet  304 B. The arrangement forms a “doughnut.” 
     Although  FIG. 1  shows a single doughnut, a plurality of doughnuts (one doughnut inside another doughnut) can be used. Thus, for a double doughnut, four magnets and four circular target plates are used. For a triple doughnut, six magnets and six circular target plates can be used.  FIG. 1C  shows the system of  FIG. 1B  with a plurality of moving magnets  308 A- 308 B. 
     During operation, a parallel magnetic field having a portion parallel to the surface of the cylindrical target rings  306 A-B effect generation of a magnetron-mode electromagnetic field in the vicinity of the surface over the entire periphery of each of the facing targets. Also, a magnetic field extending between the facing targets  306 A-B causes facing-mode electromagnetic fields within the space between the facing targets  306 A-B. As a result, high-density plasma is generated over the entire surface of each of the targets  306 A-B using a small and uniform source. 
       FIG. 1D  shows the system of  FIG. 1B  with an oxygen ion trap  310  in place of the target  306 A. The oxygen ion trap  310  traps oxygen in a three-dimensional quadrupole electric field generated basically by combining an RF electric field and a DC electric field. The ion trap device is constructed by cylindrical and disc electrodes in which an ion trapping space is created around the center of the space surrounded by the electrodes. In these constructions, the electrodes are composed of a ring electrode, and two end cap electrodes placed at both ends of the ring electrodes, wherein the RF voltage is normally applied to the ring electrode. In either electrode construction, the mass to charge ratio (m/e) of an ion determines whether the ion is trapped in the trapping space in a stable manner, or whether its movement becomes unstable and it collides with the electrodes, or it is ejected from an opening of the electrodes. 
       FIG. 1E  shows a cross-type facing magnetron. In this embodiment, a plurality of square FTS source sub-chambers  360 A,  360 B,  360 C, and  360 D are positioned adjacent each other and share walls and magnets  362  and  364 . The arrangement of  FIG. 1E  allows a four fold pattern which will increase coverage on target material  366 . For example, a conventional square FTS apparatus with 4″ target plate separation can cover about 2″ of a 4″ wafer. The shared wall arrangement of  FIG. 1E  can cover a larger area such as 12″, for example, while increasing uniformity. Each sub-chamber yields a cosine distribution, which is additive since the chambers are in close proximity. By optimizing the exact positions, a smooth distribution will result. 
       FIG. 1F  shows an exemplary embodiment of a target material. The target material is typically a CMO ceramic which is difficult to shape into a circular shape. Therefore the embodiment of  FIG. 1F  has small rectangular CMO plates or strips  384  which will interlock to approximate a circular shape. The plates or strips  384  have backing plates  390  which is circular in shape. 
       FIG. 2  illustrates an exemplary electron distribution for The method of  FIG. 1A . The electron distribution follows a standard Maxwellian curve. Low energy electrons have two characteristics: they are numerous and they tend to have non-elastic collisions with the deposited atoms, resulting in amorphization during deposition. High-energy electrons come through the back-biased shield, but they effectively “bounce” off the atoms without significant energy transfer—these electrons do not affect the way bonds are formed. This is especially true because high energy electrons spend very little time in the vicinity of the atoms, while the low energy electrons spend more time next to the atoms and can interfere with bond formation. 
     The presence of the large positively biased shield affects the plasma, particularly close to the pedestal electrode  24 . As a result, the DC self-bias developed on the pedestal  24 , particularly by an RF bias source, may be more positive than for the conventional large grounded shield, that is, less negative since the DC self-bias is negative in typical applications. It is believed that the change in DC self-bias arises from the fact that the positively biased shield drains electrons from the plasma, thereby causing the plasma and hence the pedestal electrode to become more positive. 
       FIG. 3  shows another embodiment of an FTS system. In this embodiment, a wafer  200  is positioned in a chamber  210 . The wafer  200  is moved into the chamber  210  using a robot arm  220 . The robot arm  220  places the wafer  200  on a wafer chuck  230 . The wafer chuck  230  is moved by a chuck motor  240 . One or more chuck heaters  250  heats the wafer  200  during processing. 
     Additionally, the wafer  200  is positioned between the heater  250  and a magnetron  260 . The magnetron  260  serves as highly efficient sources of microwave energy. In one embodiment, microwave magnetrons employ a constant magnetic field to produce a rotating electron space charge. The space charge interacts with a plurality of microwave resonant cavities to generate microwave radiation. One electrical node  270  is provided to a back-bias generator such as the generator  26  of  FIG. 1 . 
     In the system of  FIG. 3 , two target plates are respectively connected and disposed onto two target holders which are fixed to both inner ends of the chamber  210  so as to make the target plates face each other. A pair of permanent magnets are accommodated in the target holders so as to create a magnetic field therebetween substantially perpendicular to the surface of the target plates. The wafer  200  is disposed closely to the magnetic field (which will define a plasma region) so as to preferably face it. The electrons emitted from the both target plates by applying the voltage are confined between the target plates because of the magnetic field to promote the ionization of the inert gas so as to form a plasma region. The positive ions of the inert gas existing in the plasma region are accelerated toward the target plates. The bombardment of the target plates by the accelerated particles of the inert gas and ions thereof causes atoms of the material forming the plates to be emitted. The wafer  200  on which the thin film is to be disposed is placed around the plasma region, so that the bombardment of these high energy particles and ions against the thin film plane is avoided because of effective confinement of the plasma region by the magnetic field. The back-bias RF power supply causes an effective DC ‘back-bias’ between the wafer  200  and the chamber  210 . This bias is negative, so it repels the low-velocity electrons. By also moving the magnetron or chuck vertically with motor  260  such that the distance between them is changed during deposition, the uniformity of the magnetic field can further be increased. 
     The manufacturing of complex metal oxide targets involves multiple sintering, grinding and annealing steps. In several applications it is desirable to make a cylindrical target or curved geometries. To provide such non-planar geometries, one embodiment uses differential coefficient of expansion of materials to achieve a controlled pressure and size during the sintering operation. 
       FIG. 4  shows one exemplary block  300  that houses a cylinder  304 . Block  300  can be made from a controlled expansion alloy such as Invar (Fe64/Ni36), among others. The cylinder  304  can be made from a ceramic sinter material, for example. The size of the cylinder  304  can be varied with temperature, while the outer casing such as the block  300  is kept at constant shape (a low expansion alloy), a large force can be exerted on the sintered material. This force and temperature compact the material and create a solid from the sinter powder. 
     In one exemplary configuration, the diameter of the cylinder  304  will change about 18*900×10 e-6 or 2%. For a 12″ diameter target, the change in diameter will be about 6 mm. Since the desired cylinder thickness is about 3 mm, the starting thickness can be 6 mm on both sides and this will be reduced to the desired 3 mm after the sintering. 
     The sintering temperature can be precisely adjusted to give the best material properties while also giving the correct thickness the linear expansion of a heated solid or liquid can be measured by a quantity α, the coefficient of linear expansion as follows: 
     
       
         
           
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     α=Coefficient of linear expansion (SI: 1/° C.) 
     ΔL=Change in length (SI: m) 
     ΔT=Change in Temperature (SI: ° C.) 
     This coefficient is defined in such a way that it measures the percentage change in the length per degree temperature change as shown in  FIG. 5 . Exemplary coefficients of thermal expansion for various materials are shown below: 
                                               Coefficients of Thermal Expansion at 20° C.                    Linear Coefficient   Volumetric Coeff.           Substance   α (1/° C.)   β = 3α (1/° C.)                       Aluminum   24 × 10 −6     72 × 10 −6             Brass   19 × 10 −6     57 × 10 −6             Copper   17 × 10 −6     51 × 10 −6             Glass (ordinary)    9 × 10 −6     27 × 10 −6             Glass (Pyrex)    3 × 10 −6      9 × 10 −6             Iron/Steel   12 × 10 −6     36 × 10 −6             Lead   29 × 10 −6     87 × 10 −6                          
Thermal Expansion of Volume:
 
             β   ≡       Δ   ⁢           ⁢     V   /     V   o           Δ   ⁢           ⁢   T                   {           β   =     3   ⁢   α             (     for   ⁢           ⁢   Solids     )                 Δ   ⁢           ⁢   V     =     β   ⁢           ⁢     V   o     ⁢   Δ   ⁢           ⁢   T                             V   =       V   o     ⁡     (     1   +     βΔ   ⁢           ⁢   T       )                                 
Thermal Expansion of Area:
 
     
       
         
           
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       FIG. 7  shows another embodiment with a curved core  400  (such as a steel core) surrounded by precision cylinders  410  and  430  with differential thermal expansion coefficients. A sintered material  420  is positioned between the cylinders  410  and  430 . This embodiment provides plates  410  and  430  with different materials having different thermal expansion constants, the same relative thickness change can be obtained regardless of diameter. An advantage of this embodiment is that the hardware can be easily reused for different sizes of the target. 
     The method can be used to make shapes other than cylinders as well. As such, the shapes most optimized for a particular magnetron target can be manufactured. In yet another embodiment, a compaction of the material from the top can be used to achieve a uniform sintered compound. As shown in  FIG. 8 , a bottom plate  500  remains fixed in position. Above the bottom plate  500 , an outer plate  502  and an inner plate  504  flanks the sintered material. A top pressure plate  506  is provided to adjust the final position  510  of the sintered material. 
     It is to be understood that various terms employed in the description herein are interchangeable. Accordingly, the above description of the invention is illustrative and not limiting. Further modifications will be apparent to one of ordinary skill in the art in light of this disclosure. 
     The invention has been described in terms of specific examples which are illustrative only and are not to be construed as limiting. The invention may be implemented in digital electronic circuitry or in computer hardware, firmware, software, or in combinations of them. 
     Apparatus of the invention for controlling the fabrication equipment may be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a computer processor; and method steps of the invention may be performed by a computer processor executing a program to perform functions of the invention by operating on input data and generating output. Suitable processors include, by way of example, both general and special purpose microprocessors. Storage devices suitable for tangibly embodying computer program instructions include all forms of non-volatile memory including, but not limited to: semiconductor memory devices such as EPROM, EEPROM, and flash devices; magnetic disks (fixed, floppy, and removable); other magnetic media such as tape; optical media such as CD-ROM disks; and magneto-optic devices. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs) or suitably programmed field programmable gate arrays (FPGAs). 
     While the preferred forms of the invention have been shown in the drawings and described herein, the invention should not be construed as limited to the specific forms shown and described since variations of the preferred forms will be apparent to those skilled in the art. Thus the scope of the invention is defined by the following claims and their equivalents.