Abstract:
An apparatus for sputtering material onto a workpiece, composed of: a chamber; a first target disposed in the chamber for sputtering material onto the workpiece; a holder for holding the workpiece in the chamber; a plasma generation area between the target and the holder; a coil for inductively coupling energy into the plasma generation area for generating and sustaining a plasma in the plasma generation area; and a second target disposed in the chamber below the first target and above the coil for sputtering material onto the workpiece.

Description:
FIELD OF THE INVENTION 
     The present invention relates to plasma generators, and more particularly, to methods and devices for generating a plasma to sputter deposit a layer of material onto a substrate during the fabrication of semiconductor devices. 
     BACKGROUND OF THE INVENTION 
     A number of semiconductor device fabrication procedures include processes in which a material is sputtered from a target onto a workpiece such as a semiconductor wafer. Material is sputtered from the target, which is appropriately biased, by the impact of ions created in the vicinity of the target. A certain proportion of the sputtered material may be ionized by a plasma such that the resulting ions can be attracted to the wafer. The wafer is mounted on a support and is usually biased to a potential selected to attract the sputtered, ionized material. Typically, the sputtered material is composed of positive ions and the workpiece is negatively biased. 
     Sputtered material has a tendency to travel in straight line paths from the target to the substrate at angles which are oblique to the surface of the substrate. As a consequence, high aspect ratio (depth to width) features such as trenches and holes on a substrate surface may not be completely filled during deposition because deposition material may build up near the top edges of the high aspect ratio feature and close off a void or cavity. For example, as illustrated in FIG. 1 a,  sputtered material  20  may build up near the upper edges of the high aspect ratio trenches  22  located between features  26  on a substrate  28 . As the sputtered material  20  accumulates, a void or cavity  24  may become closed off within each trench  22 , as illustrated in FIG. 1 b.  To inhibit the formation of such cavities, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate and positioning appropriate vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 10% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term “dense plasma” is intended to refer to one that has a high electron and ion density. 
     There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer. 
     In many high density plasma applications, it is preferable for the chamber to be operated at a relatively high pressure so that the frequency of collisions between the plasma ions and the deposition material atoms is increased to thereby increase the residence time that the sputtered material remains in the high density plasma zone. However, scattering of the deposition atoms is likewise increased. This scattering of the deposition atoms typically causes the thickness of the deposition layer on the substrate to be thicker on that portion of the substrate aligned with the center of the target and thinner in the outlying regions. It has been found that the deposition layer can be made more uniform by reducing the distance between the target and the substrate which reduces the effect of the plasma scattering. 
     On the other hand, in order to increase the plasma density to increase the ionization of the sputtered atoms, it has been found desirable to increase the distance between the target and the substrate. The coil which is used to couple energy into the plasma typically encircles the space between the target and the substrate. If the target is positioned too closely to the substrate, the ionization of the sputtered material can be adversely affected. Thus, in order to accommodate the coil which is coupling RF energy into the plasma, it has often been found necessary to space the target from the substrate a certain minimum distance even though such a minimum spacing can have an adverse effect on the uniformity of the deposition. 
     In addition, certain chambers include a coil which is fabricated from a material which can sputtered. In such chambers, the coil acts as an additional target for sputtering. This feature allows the deposition profile to be modified and may help to improve the uniformity of deposition material on the substrate. 
     It has also been proposed to utilize more than one target. For example, U.S. Pat. No. 5,178,739 describes a chamber having both an end target and a cylindrical target to increase the deposition rate and improve uniformity. However, it is believed that the ionization rate of the sputtered material may not be sufficiently high for many applications. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method and apparatus for generating a plasma within a chamber and for sputter depositing a layer which obviate, for practical purposes, the above-mentioned limitations. 
     These and other objects and advantages are achieved by a plasma generating apparatus having a first target in which, in accordance with one aspect of the invention, a second independent target is positioned between the first target and an RF coil which inductively couples RF energy into a plasma, so that both targets sputter material onto a workpiece to improve the uniformity of the deposition of sputtered material onto the workpiece. In addition, because the second target is positioned above the coil, the ionization rate of the sputtered material from the second target is increased which can reduce the formation of unwanted voids in the deposition layer deposited into vias, channels and other openings. 
     In one illustrated embodiment, the second target has a ring-like structure disposed about the circumference of the plasma generation area. The second target is positioned below a generally planar first target but above the coil that inductively couples electromagnetic energy into the plasma in the plasma generation area to increase the density of the plasma. Therefore, material sputtered from both targets will pass through a substantial portion of the high density plasma before reaching the workpiece. Consequently, it is believed that the ionization rate of sputtered material from both targets will be increased. 
     In another aspect of the present invention, the second target is positioned to shield the outer edge periphery of the first target, as explained in greater detail below, so that sputtering of material from the outer edge periphery of the first target is substantially reduced. Such an arrangement is believed to increase deposition uniformity also. 
     In another illustrated embodiment, the second target is modified to function as a second antenna coil which, like the first coil, inductively couples electromagnetic energy into the plasma. One end of the second target-coil is coupled to a RF generator through an amplifier and impedance matching network while the other end is coupled to the system ground through a blocking capacitor. The currents through (or voltages applied to) the first and second coils, may have a predetermined phase difference, preferably between ¼π to 1 ¾π Radians. Under appropriate settings, this phase difference in the electromagnetic fields generated by the two antenna coils can launch a helicon wave in the plasma. Such helicon waves can be absorbed more uniformly throughout the plasma, such that a plasma excited using a helicon wave can be excited to higher densities. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     Embodiments of the invention are described with reference to the accompanying drawings which, for illustrative purposes, are schematic and not drawn to scale. 
     FIGS. 1 a  and  1   b  illustrate the formation of voids during a sputter deposition process according to prior art methods. 
     FIG. 2 is a perspective, partial cross-sectional view of a deposition chamber in accordance with one embodiment of the present invention. 
     FIG. 3 is a schematic diagram of the electrical interconnections to the chamber of FIG.  2 . 
     FIG. 4 is a schematic partial cross-sectional view of the chamber of FIG. 2 shown installed in a vacuum chamber. 
     FIG. 5 is a chart depicting the respective deposition profiles for material deposited from the first and second targets of the chamber of FIG.  1 . 
     FIG. 6 is a schematic diagram of the electrical interconnections to a deposition chamber in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIGS. 2 and 3, a deposition in accordance with a first embodiment of the present invention comprises a substantially cylindrical plasma chamber  100  (shown schematically in FIG.  3 ). The plasma chamber  100  of this embodiment may utilize a multi-turn or single turn coil  104  which is carried internally by a shield  106  (FIG.  2 ). The shield  106  protects the interior walls  102  of the vacuum chamber  100  from the material being deposited within the interior of the chamber. 
     Radio frequency (RF) energy from an RF generator  108  (FIG. 3) is radiated from the coil  104  into the interior of the chamber  100 , which energizes a plasma within the chamber  100 . An ion flux strikes a first target  110  positioned at the top of the chamber  102 . The first target  110  is preferably negatively biased by a DC power source  111  to attract an ion flux. The plasma ions eject material from the first target  110  onto a substrate  112  which may be a wafer or other workpiece which is supported by a pedestal support  114  at the bottom of the chamber  100 . A first magnet assembly  116 , preferably a magnetron which includes one or more fixed or rotating magnets  117 , is provided above the first target  110  and produces magnetic fields which sweep over the face of the first target  110  to promote a desired erosion pattern (for example, uniform) on the first target  110 . 
     In accordance with one aspect of embodiments of the present invention, a second target  500  is positioned below the first target  110  but above the coil  104  to sputter material onto the substrate  112  to supplement the material which is being sputtered from the first target  110  onto the workpiece. The second target  500 , which has a ring-like structure in the illustrated embodiment, is disposed about the circumference of the plasma generation area, as explained in greater detail below. 
     The second target  500  may be disposed above the coil  104  and may be biased by a DC or RF power source  400  (FIG.  3 ). An RF power source is preferentially used instead of a DC power source because DC biasing can, in some applications, be too disruptive to the plasma if applied to the second target  500 . It is believed that such a disruptive effect on the plasma can be ameliorated by using the RF power source  400  to induce a DC bias on the second target  500 . As a consequence, the ionization of material sputtered from the first target  110  may be maintained, while, at the same time, the ionization of material sputtered from the second target  500  may also be effected. As a result, the layer deposited onto the substrate  112  includes ionized material from both the second target  500  and the first target  110 , which can substantially improve the uniformity of the resultant layer and minimize the formation of unwanted cavities in the deposition layer. 
     The RF generator  108  is preferably coupled to the coil  104  through an amplifier and impedance matching network  118 . The other end of the coil  104  is coupled to ground, preferably through a blocking capacitor  120  which may be a variable capacitor. The ionized deposition material is attracted to the substrate  112  and forms a layer thereon. The pedestal  114  may be negatively biased by an AC (or DC or RF) source  121  to externally bias the substrate  112 . Alternatively, external biasing of the substrate  112  may optionally be eliminated. 
     Referring to the embodiment illustrated in FIG. 4, the first target  110 , which is generally disk-shaped, is supported by a first insulative ring assembly  300  which engages the first target  110 . A side  131  of the first target  110  is sealably stacked on top of a surface  133  of the first insulative ring assembly  300 . An O-ring seal (not shown) may be provided between the side  131  and the surface  133  to provide a vacuum tight assembly from the vacuum chamber. The first insulative ring assembly  300  is in turn supported by the second target  500 . A surface  135  of the first insulative ring assembly  300  is sealably stacked on top of a surface  501  of the second target  500 . An O-ring sealing surface may also be also provided between the surface  135  and the surface  501 . The first target  110  is negatively biased and is preferably insulated from the second target  500  which is biased by a different source, for example, the RF power source  400 . Therefore, the first insulative ring assembly  300  which may be made of a variety of insulative materials including ceramic spaces the first target  110  from the second target  500  to facilitate separately biasing the two targets. 
     The second target  500  is supported by a second insulative ring assembly  310  which engages both the second target  500  and the vacuum chamber wall  102 . Thus the second target  500  may act as an extension of the vacuum chamber wall  102 . A surface  503  of the second target  500  is sealably stacked on top of a surface  505  of the second insulative ring assembly  310 , and a surface  507  of the second insulative ring assembly  310  is also sealably stacked on top of a surface  509  of the vacuum chamber wall  102 . O-ring seals may be provided between the surfaces  503  and  505 , and between the surfaces  507  and  509 . The second insulative ring  310  is supported by the vacuum chamber wall  102  which is preferably at ground. The second insulative ring  310 , which also may be made of a variety of insulative materials including ceramics, spaces the second target  500  from the chamber  100  so that the second target may be appropriately biased by the RF power source  400 . A backing plate  520  may be attached to the outer surface of the second target  500 . 
     As seen in FIG. 4, the second target  500  may be shaped to shield the periphery or the outer edges of the first target  110 , and provide a narrow strip of space between the outer edge surface  139  of the first target  110  and an upper edge surface  141  of the second target  500 . By shielding the outer edge surfaces of the first target  110 , a dark space  700 , which has a reduced or preferably zero plasma density, may be formed in the narrow strip of space between the first target surface  139  and the second target surface  141  such that sputtering of material from the first target outer edges may be reduced. 
     As illustrated in FIG. 4, a sputtering face surface  143  of the second target  500  may be slanted downwardly away from the center of the chamber  100 , thereby forming an angle  530  greater than ninety degrees with respect to the bottom sputtering surface  145  of the first target  110 . The formation of an angle greater than ninety degrees between the side  145  and the side  143  can reduce sputtering of material from the first target  110  onto the second target  500  because much of the sputtered material travels parallel to the vertical axis of the chamber  100  or at relatively small oblique angles relative to the vertical axis. If the angle were less than ninety degrees, more sputtered material from the first target  110  could contact the second target  500  and deposit on the second target  500  while traveling parallel to the vertical axis of the plasma chamber  100  towards the workpiece. Material sputtered from the second target  500  onto the first target  110  is also minimized by having an angle greater than ninety degrees. However, the above arrangement may not completely prevent the targets from depositing material on each other since some of the sputtered material travels at an oblique angle with respect to the vertical axis of the plasma chamber  100 . 
     Although the second target  500  is illustrated as having a generally convex exposed sputtering face comprising the surfaces  141  and  143 , it should be appreciated that the second target  500  may have an exposed sputtering face having other shaped surfaces including flat and concave surfaces. 
     Certain embodiments may utilize a second magnet assembly, preferably a rotating magnet assembly  550  adjacent to the second target  500 , which rotates around the second target  500  and produces magnetic fields that sweep across the face of the second target  500  to promote uniform erosion of the second target  500 . The second rotating magnet assembly  550  may have an annular form and include a plurality of pole pairs. In the illustrated embodiment, the poles of each pair are spaced from one another in the vertical direction, i.e., parallel to the center axis of chamber  100 . The pole pairs are spaced from adjacent pole pairs circumferentially around the magnet assembly  550 . Magnet assembly  550  can be mounted and operated to rotate continuously in a single direction about the vertical center axis of chamber  100 . Magnet assembly  550  can also be mounted and operated to oscillate back and forth with an amplitude that allows the path of travel of one magnet pole pair to overlap that of each adjacent pole pair. The magnetic field created by the second rotating magnet assembly  550  traps electrons, within the vicinity of the second target  500 , that ionize the chamber gas to produce ions which in turn impact the surface of the second target  500  to sputter material from the second target. 
     As previously mentioned, in order to accommodate the coil  104  to facilitate ionization of the plasma, it has been found beneficial to space the target  110  from the surface of the workpiece  112 . However, this increased spacing between the target and the workpiece can adversely impact the uniformity of the material being deposited from the target. As indicated at  250  in FIG. 5, such nonuniformity typically exhibits itself as a thickening of the deposited material toward the center of the workpiece with a consequent thinning of the deposited material toward the edges of the workpiece. 
     In accordance with one feature of embodiments of the present invention, this nonuniformity can be compensated by sputtering deposition material not only from the first sputter target  110  above the workpiece but also from the second target  500  disposed about the circumference of the plasma generation area and below the first target  110 . Because the edges of the workpiece  112  are closer to the second target  500  than the center of the workpiece  112 , it is believed that a greater proportion of the material sputtered from the second target  500  may be deposited near the edges of the workpiece  112  than at the center, as indicated at  252  in FIG.  5 . This is the reverse of the deposition pattern of sputtered material from the first target  110 . By appropriately adjusting the ratio of the RF power level of the bias applied to the second target  500  to the DC power level of the bias applied to the first target  110 , it is believed that the deposition level of the material being sputtered from the second target  500  can be selected in such a manner as to compensate for nonuniformity of the deposition profile of the material from the first target  110  such that the overall deposition profile of the layer from both sources of sputtered material as indicated by the deposition profile  254  in FIG. 5 can be substantially more uniform than that which has often been obtained from the first target  110  alone. 
     As set forth in copending application Ser. No. 08/680,335, filed Jul. 10, 1996 (Attorney Docket No. 1390-CIP/PVD/DV) entitled “Coils For Generating A Plasma And For Sputtering” by Jaim Nulman, Sergio Edelstein, Mani Subramani, Zheng Xu, Howard Grunes, Avi Tepman, John Forster, assigned to the assignee of the present application, the coil for inductively coupling RF energy into the plasma may be adapted to become yet another target to sputter material onto the workpiece. In the aforementioned copending application Ser. No. 08/680,335, the ratio of the RF power applied to the coil in comparison to the DC power applied to the target affected the ratio of amount of material sputtered from the coil in comparison to the target. Similarly, it is presently believed that the amount of sputtering which originates from second target  500  as compared to the sputtering which originates from the first target  110  is also a function of the RF power applied to the second target  500  relative to the DC power applied to the first target  110 . 
     By adjusting the ratio of the RF power to the DC power applied, the relative amounts of material sputtered from the first target  110  and the second target  500  may be varied so as to achieve improved uniformity. Thus, a particular ratio of the second target RF power to the first target DC power may achieve a smaller degree of non-uniformity of the layer of material deposited from both the coil and the target. It is believed that as the RF power to the second target  500  is increased relative to the DC power applied to the first target  110 , the deposited layer tends to be more edge thick. Conversely, it is also believed that by decreasing the ratio of the RF power to the second target  500  relative to the DC power applied to the first target  110 , the center of the deposited layer tends to grow increasingly thick relative to the edges as represented by the increasingly positive percentage of non-uniformity. Thus, by adjusting the ratio of the RF power to the second target  500  relative to the DC power biasing the first target  110 , the material being sputtered from the second target  500  can be increased or decreased as appropriate to compensate for non-uniformity of the material being deposited from the first target  110  to achieve a more uniform deposited layer comprising material from both the target and the coil. 
     Although the second target  500  is illustrated as having a continuous generally ring-shape, it should be appreciated that the second target  500  may comprise other shapes including separate, spaced segments. 
     In another embodiment as illustrated in FIG. 6, the second target  500  functions as a second antenna coil which inductively couples electromagnetic energy into the plasma. A vertical slit is provided through the annular-shaped second target  500  to form two separate ends  502  and  504 . One end  502  of the second target is coupled to the RF generator  400  through a second amplifier and impedance matching network  450  while the other end  504  is coupled to the system ground through a blocking capacitor  410 . 
     The coil  104  functions as a first antenna coil for inductively coupling electromagnetic energy into the plasma. The second target  500  likewise functions as a second antenna coil to inductively couple electromagnetic energy into the plasma. 
     Imposed upon the plasma is a substantially uniform, axially oriented magnetic field (as represented by magnetic line of force  190 ). The magnetic field may be generated by, for example, Helmholtz coils coaxial with the chamber axis. The RF energy radiated by the coil  104  into the interior of the plasma chamber  100  is preferably phase shifted by a predetermined amount from the RF energy radiated by the second target  500  such that a helicon wave is launched and maintained in the plasma chamber  100 . The phase of the RF energy can be shifted by using a phase shift regulating network, preferably to effect a phase shift between ¼π to 1 ¾π Radians. 
     Because of the helicon wave, the energy distribution of the plasma may be more uniform and the density of the plasma may be increased. A plasma excited using a helicon wave can be excited to densities in the range of 10 11  to 10 13  electrons cm −3 . The higher plasma density is beneficial to ionize a higher proportion of material sputtered from the targets. As a result, the sputtered material can be more responsive to electric fields adjacent to the wafer  112 , which enhances the perpendicularity of the metal flux to the wafer  112 . Consequently, fine features may be coated more uniformly, and high aspect ratio holes and trenches may be filled with little or no void formation. Such electric fields may be induced by electrically biasing the wafer and/or pedestal negatively with respect to the plasma with an RF supply  121  (FIG. 3) to impose an HF RF signal (e.g., 13.6 MHz) to the pedestal through a matching network. The above arrangement also allows for operation of the plasma chamber  100  at a pressure below 10 mT. 
     In another embodiment, the coil  104  can also be adapted to sputter material from the coil  104  onto the workpiece to supplement the material being sputtered from the targets  101  and  500  onto the workpiece as set forth in greater detail in the aforementioned copending application Ser. No. 08/680,335. The coil  104  can be made of the same type of material as the targets  110  and  500  or the coil  104  can be made from a different type of material. For example, the coil  104  and the targets  110  and  500  can be made of titanium or aluminum. If it is desired to deposit a mixture or combination of materials, the first and second targets and the coil  104  can be formed from the same mixture of materials or alternatively from different materials such that the materials combine or mix when deposited on the substrate. In addition, the RF power applied to the coil  104  can be set independently of the biases applied to the targets  110  and  500  for optimization of the plasma density for ionization. 
     The coil  104  may be carried on the shield  106  by a plurality of coil standoffs which electrically insulate the coil  104  from the supporting shield  106 . The insulating coil standoffs have an internal labyrinth structure which permits repeated deposition of conductive materials from the target  110  onto the coil standoffs while preventing the formation of a complete conducting path of deposited material from the coil  104  to the shield  106  which could short the coil  104  to the shield  106  (which is typically at ground). 
     RF power may be applied to the coil  104  by feedthroughs which are supported by insulating feedthrough standoffs. The feedthrough standoffs, like the coil support standoffs, permit repeated deposition of conductive material from the target onto the feedthrough standoff without the formation of a conducting path which could short the coil  104  to the shield  106 . Thus, the coil feedthrough standoff has an internal labyrinth structure somewhat similar to that of the coil standoff to prevent the formation of a short between the coil  104  and the wall  140  of the shield. 
     The chamber shield  106  may be generally bowl-shaped and include a generally cylindrically shaped, vertically oriented wall  140  to which the standoffs are attached to insulatively support the coil  104 . The shield may further have a generally annular-shaped floor wall  142  which surrounds the chuck or pedestal  114  which supports the workpiece  112  which has an 8″ diameter in the illustrated embodiment. A clamp ring may be used to clamp the wafer to the chuck  114  and cover the gap between the floor wall of the shield  106  and the chuck  114 . The chamber shield  106  is grounded to the system ground. 
     The appropriate RF generators and matching circuits are components well known to those skilled in the art. For example, an RF generator such as the ENI Genesis series which has the capability to “frequency hunt” for the best frequency match with the matching circuit and antenna is suitable. The frequency of the generator for generating the RF power to the coil  104  is preferably 2 MHz but it is anticipated that the range can vary from, for example, 1 MHz to 100 MHz. An RF power setting of 4.5 kW is preferred but a range of 1.5-5 kW is believed to be satisfactory. In some applications, energy may also be transferred to the plasma by applying AC or DC power to coils and other energy transfer members. A DC power setting for biasing the target  110  of 3 kW is preferred but a range of 2-5 kW and a pedestal bias voltage of −30 volts DC is believed to be satisfactory for many applications. 
     A variety of precursor gases may be utilized to generate the plasma including, for example, Ar 1  H 2  ,or reactive gases such as NF 3  and CF 4 . Various precursor gas pressures are suitable including, for example, pressures of 0.1-50 mTorr. For ionized PVD, an Ar gas or an Ar/N 2  gas mixture pressure between 10 and 100 mTorr is preferred for best ionization of sputtered material. 
     It will, of course, be understood that modifications of the present invention, in its various aspects, will be apparent to those skilled in the art, some being apparent only after study others being matters of routine mechanical and electronic design. Other embodiments are also possible, their specific designs depending upon the particular application. As such, the scope of the invention should not be limited by the particular embodiments herein described but should be defined only by the appended claims and equivalents thereof.