Abstract:
An apparatus and method for processing workpieces, which include a chamber having a coil for inductively coupling RF energy through a dielectric window into the chamber to energize a plasma, and a shield positioned between a sputtering target and the dielectric window to reduce or eliminate deposition of sputtered material onto a portion of the dielectric window. In the illustrated embodiment, the window shield is spaced from the dielectric window to define a gap and has at least one opening, which permit RF energy to be coupled through the gap and through the window shield opening to the interior of the chamber. As a consequence, the coil may be positioned exterior to the chamber to simplify construction and operation of the chamber.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to apparatus and methods for sputtering ionized material from a solid target, or cathode, onto a workpiece with the aid of an inductively coupled plasma. 
     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 is ionized by a plasma and the resulting ions are attracted to the wafer. 
     The wafer is mounted on a support and is biased to a DC potential selected to attract the sputtered, ionized material. Typically, the sputtered material is composed of positive ions and the workpiece is negatively biased. 
     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 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 primary winding of a transformer. The plasma acts as a single turn secondary winding of the transformer. 
     In a number of deposition chambers such as a physical vapor deposition chamber, the chamber walls are often formed of a conductive metal such as stainless steel. Because of the conductivity of the chamber walls, it is often necessary to place the RF coils or electrodes within the chamber itself because the conducting chamber walls would block or substantially attenuate the electromagnetic energy radiating from the antenna. As a result, the coil may be directly exposed to the deposition flux and energetic plasma particles. This is a potential source of contamination of the film deposited on the wafer, and therefore may be undesirable in some applications. To protect the coils, shields can be made from nonconducting materials, such as ceramics. However, many deposition processes involve deposition of conductive materials such as aluminum on the electronic device being fabricated. Because the conductive material will coat the ceramic shield, it will soon become conducting, thus again substantially attenuating penetration of electromagnetic radiation into the plasma. 
     U.S. Pat. No. 5,346,578 describes a system in which a plasma is created for the performance of various types of wafer processing operations, including etching and chemical vapor deposition in a hemispherical quartz vessel surrounded by a similarly shaped exterior induction coil. RF energy is transmitted from the coil through the dome into the chamber to energize the plasma. In the operation of the apparatus discussed in this patent, a reactive gas is introduced into the treatment chamber in order to be ionized by the plasma, the resulting ions being directed to a wafer under the influence of a suitable electric field. It is believed that the apparatus described in this reference is not suitable for the performance of conductive material sputtering processes because sputtered material tends to coat all interior surfaces of a chamber. Hence, the quartz dome would soon become relatively opaque to the RF energy from the coil. 
     Published European Patent Application 0607797 describes a device for generating a plasma in order to perform low pressure chemical vapor deposition or reactive ion etching operations. The system includes a processing chamber having, at its top, a planar spiral coil producing an electromagnetic field which will be coupled with a plasma within the processing chamber, the coil itself being isolated from the interior of the chamber by a flat dielectric window. The window is associated with a conductive shield which is positioned between the window and the coil. The purpose of the shield is to prevent dielectric material from being sputtered from the window. This publication suggests that the window and coil may, alternatively, be domed or hemispherical. 
     The material which is to be ionized in order to be deposited on a wafer or to perform etching is introduced into the chamber in the form of a process gas. As in the case of the apparatus described in U.S. Pat. No. 5,346,578, supra, the surface of the dielectric window which communicates with the interior of the chamber is prone to being coated with deposition material. Therefore, it is believed that this chamber is likewise not well suited to sputtering processes. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an inductively coupled plasma processing apparatus which avoids drawbacks possessed by prior art apparatus of this type. 
     A more specific object of the invention is to produce a more uniform plasma in a processing chamber which contains a metal sputtering target. 
     Another object of the invention is to protect a dielectric window forming part of the boundary of the processing chamber against deposition of sputtered material. 
     A further object of the invention is to provide an improved sputtering apparatus having an external coil for inductively coupling energy into the plasma in the processing chamber. 
     The above and other objects and advantages are achieved, according to the present invention, by an apparatus and method for processing workpieces, which include a chamber having a coil for inductively coupling RF energy through a dielectric window into the chamber to energize a plasma, and a shield positioned between a sputtering target and the dielectric window to reduce or eliminate deposition of sputtered material onto a portion of the dielectric window. In the illustrated embodiment, the window shield is spaced from the dielectric window to define a gap and has at least one opening, which permit RF energy to be coupled through the gap and through the window shield opening to the interior of the chamber. As a consequence, the coil may be positioned exterior to the chamber to simplify construction and operation of the chamber. 
     In another aspect of the invention, the formation of a plasma having more uniform characteristics is facilitated. For example, by placing the plasma energizing coil outside the chamber, the coil may be readily shaped and positioned to achieve a desired plasma distribution without concern for the interaction between the coil structure and the flow of sputtered material in the chamber. Still further, multiple coils can be used while avoiding the expense and complexity of connecting multiple coils to external RF sources through the walls of the chamber. 
     In yet another aspect of the invention, the formation of a more uniform sputter deposition material flux may be facilitated. For example, by placing the plasma energizing coil or coils outside the chamber, the sputtering target or targets may be more readily shaped and positioned to achieve a desired deposition flux without concern for the interaction between any internal coil structure and the target. In various illustrative embodiments, the target has an annular ring structure which may improve the uniformity of deposition onto the workpiece. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a simplified, elevational view, partly in cross section, of a first embodiment of apparatus according to the invention. 
     FIG. 2 is a plan view of one embodiment of the window shield of FIG.  1 . 
     FIG. 3 is a partial cross-sectional view of a portion of the window and window shield of FIG. 1; 
     FIGS. 4 and 5 are views similar to those of FIG. 1 showing further embodiments of apparatus according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a simplified illustration of processing apparatus in which an inductively coupled plasma ionizes material sputtered from a target for delivery to a workpiece. Components of such apparatus which are conventional in this art are not illustrated so that novel components according to the invention can be viewed and comprehended more easily. Such conventional components include conduits for delivering a gas which is used to form the plasma, components for supporting the target and its associated magnet assembly and components for appropriately displacing the workpiece support. 
     The processing apparatus includes a vacuum chamber  2  having at its top an opening which is closed by a dielectric window  4 . As explained in greater detail below, in accordance with one aspect of the invention, positioned in front of the window  4  is a sputter shield  5  which permits RF energy to be transmitted from a source exterior to the sputtering chamber  2 , through the window  4  and inductively coupled with the plasma in the interior of the chamber, notwithstanding the deposition of sputtered conductive material onto the inner surface of the window  4 . 
     The vacuum chamber  2  also has a second metal shield (not shown separately) that is coextensive with all of the chamber walls and normally electrically grounded, as shown. Within chamber  2  there is installed a workpiece support  6  providing a support surface. A workpiece  7  that may be constituted by one or a plurality of semiconductor wafers or panels is mounted on the support surface provided by workpiece support  6 . 
     Within chamber  2  there is mounted, in any suitable manner known in the art for conventional sputtering targets, a ring-shaped metal sputtering target  8 . Target  8  may be a solid body of an appropriate sputtering metal and may be formed to have an internal helical passage  10  through which a cooling fluid can be caused to flow. 
     Target  8  is surrounded by a permanent magnet assembly  12  which generates a magnetic flux configured to facilitate the creation of ions adjacent to the target  8 . These ions strike one or more external surfaces of target  8  in order to dislodge, or sputter, atoms or clusters of atoms from target  8 . Magnet assembly  12  may be mounted to be stationary or to rotate about a vertical center axis of chamber  2  which is concentric with target  8 . 
     Support  6  and target  8  may be appropriately biased, in accordance with conventional practice in the art, to suitable, typically negative, potentials by AC and DC voltage sources  14  and  16 , respectively. Although illustrated as contained wholly within the chamber  2 , it is contemplated that the target  8  and assembly  12  may be mounted in the walls of the chamber  2  so that the outer portion of the target  8  and the assembly  12  are on the exterior of the chamber  2 . 
     The dielectric window  4  is made, for example, of quartz and having a domed shape. This shape may be, for example, in the form of a segment of a sphere. However, other pressure resistant shapes are also possible. The window  4  may be formed of other nonconductive materials such as ceramic. 
     Above window  4  there are mounted two conductive coils  22  and  24  which are wound to conform to the domed shape of window  4 . Coils  22  and  24  are spiral wound with respect to a vertical center axis (not shown) that may be coaxial with the vertical center axis of chamber  2 , with coil  22  being enclosed, or surrounded, by coil  24 . One end of each of coils  22  and  24  is connected to ground via a respective DC isolation capacitor  26 ,  28 , while the other end of each of coils  22  and  24  is connected to a suitable, adjustable RF power source  30 ,  32 . Each RF power source  30 ,  32  may be of any suitable conventional type. Coils  22  and  24  are enclosed in an RF shielding can  34 . 
     The surface of dielectric window  4  which faces the interior of chamber  2  is covered by the metal shield  5 , in the form of a dome-shaped sheet or plate. Window shield  5  is preferably made of an electrically conductive material and is coextensive with window  4  in order to cover the entire surface of window  4  which faces the interior of chamber  2 . Window shield  5  may be conductively connected to the lower wall shield of chamber  2  and grounded thereby. In addition, window shield  5  may be of the same material as the wall shield of chamber  2 . 
     As shown more clearly in FIG. 2, window shield  5  is provided with a plurality of radially extending slots  38  which are distributed around the circumference of shield  5 . Preferably, slots  38  extend from the outer edge of shield  5  substantially to the center thereof. Thus, slots  38  divide shield  5  into a plurality of segments  36  which are substantially electrically insulated from one another. If necessary for support purposes, adjacent segments of shield  5  may be connected together by coupling members made of electrical insulating material. Slots  38  minimize the appearance of eddy currents in window shield  5 , while allowing for efficient coupling of RF energy from the coils to the interior of the processing chamber. 
     As best seen in FIG. 3, window shield  5  is spaced a small distance from window  4  by a gap  40 . As a result, even if portions of the inner surface  41  of window  4  which are aligned with slots  38  become coated with conductive deposition material as represented by a coating  42 , those portions  43  of window inner surface  41  behind the window shield segments  36  will remain substantially uncoated as shown in FIG.  3 . Consequently, paths for flow of magnetic field energy from coils  22 ,  24  into chamber  2  will exist in gap  40  between window  4  and window shield  36  and in slots  38 , as represented by path  44 . Although some stray deposition material may deposit on the window inner surface portions  43 , it is believed that the deposition will remain sufficiently sparse to permit substantial RF coupling through the window portions  43 . 
     In the illustrated embodiment, the gap  40  between the window  4  and the window shield  5  is approximately 1-2 mm in which the length of the segments  36  have a width which varies from near zero at the center of the window shield  5  to a width of approximately 12 cm (five inches) at the perimeter. The gap  40  is preferably selected to be small enough to minimize the intrusion of deposition material behind the window shield segments  36  and hence minimize the coating of deposition material onto the window inner surface portions  43  behind the segments  36 . On the other hand, the gap  40  should be sufficiently large to allow a large number of layers of successive coatings  42  of conductive deposition material to be built up on the window inner surface during successive wafer treatments without the stack of coatings  42  being sufficiently thick to bridge the gap  40  to the window shield  5  and hence block the paths  44  through the gaps  40 . 
     Once the number of coatings has reached a level in which the gaps  40  may begin to close, the window  4  and window shield  5  may be cleaned in situ or alternatively, the window  4  may be replaced with a clean window. In some applications, partial closing of the gaps  40  may be tolerated without adversely affecting the plasma density level and hence the ionization rate of the deposition material. 
     According to preferred embodiments of the invention, as illustrated in the drawing, coils  22  and  24  are independently powered, coaxially mounted solenoid coils. Outer coil  24  preferably has an outer diameter substantially equal to the diameter of dielectric window  4 , while inner coil  22  preferably has an outer diameter in a range of ¼ to ½ of the diameter of dielectric window  4 . By adjusting the relation between the power supplied to the two coils, nonuniformities in the plasma created within chamber  2  can be reduced or eliminated. In particular, more homogeneous plasma densities throughout the processing region can be established and maintained. 
     In further accordance with preferred embodiments of the invention, coils  22  and  24  have a uniform coil pitch, i.e. the spacing between adjacent turns of each coil, and preferably also between the outer turn of coil  22  and the inner turn of coil  24 , will be substantially identical. However, the winding pitch can also be made nonuniform in order to improve plasma uniformity in a given installation. 
     In the disclosed embodiments, coils  22  and  24  may be tubular, ribbon shaped, etc., and may contain a channel for circulation of cooling water. Because the coils  22  and  24  are entirely external to the pressure chamber  2 , coupling the coils to suitable RF and coolant sources is readily facilitated because such couplings need not pass through the pressure chamber walls. 
     In the embodiment shown in FIG. 1, magnet assembly  12  surrounds target  8 . The magnetic field produced by magnet assembly  12  serves to enhance ionization within the portion of the plasma field adjacent target  8  and promotes increased deposition rates and uniform target etching according to principles already known in the art. 
     The second embodiment shown in FIG. 4 is identical to the embodiment of FIG. 1 except for the structural arrangement of the target and its associated magnet assembly. Specifically, in the embodiment shown in FIG. 4 magnet assembly  52  is positioned above target  48 , i.e. between target  48  and coils  22 ,  24 . Here again, magnet assembly  52  may be either stationary or rotatable about the vertical center axis of chamber  2 . In the embodiment of FIG. 4, the magnetic field produced by magnet assembly  52  will be oriented at right angles to the magnetic field produced by magnet assembly  12  of the embodiment shown in FIG.  1 . 
     Target  48  has a ring, or annular, shape, as does target  8  of FIG. 1, and both targets  8  and  48  have an inner diameter which is not smaller than the diameter of the workpiece support surface of support  6 . However, target  48  is shaped to be compatible with the orientation of the magnetic field produced by magnet assembly  52 . 
     Yet another embodiment shown in FIG. 5 differs from that of FIG. 4 solely with respect to the dimensions of the target and its associated magnet assembly. In the embodiment shown in FIG. 5, target  58  and magnet assembly  62  are dimensioned and positioned to be located in a vertical projection of the space between the outer turn of coil  22  and the inner turn of coil  24 . However, there may be some horizontal overlap between the inner and outer diameters of target  58  and magnet assembly  62 , on the one hand, and the outer diameter of coil  22  and the inner diameter of coil  24 , on the other hand. 
     According to one feature of the present invention, it is believed that the combination of an annular, or ring-shaped, target as a source of sputtering material and a domed spiral coil for producing an inductively coupled plasma which ionizes metal sputtered from the target, can serve to produce a medium to high density inductively coupled plasma having a more homogeneous plasma density and ionized metal flux than previous ionized sputtering arrangements. In addition, the provision of a slotted metal shield covering the dielectric window can prevent a deposited metal film from covering the entire window while allowing sufficient RF energy to be coupled through the window and shield into the processing region enclosed by chamber  2 . 
     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.