Abstract:
An improved method of doping a workpiece is disclosed. In this method, a film comprising the species to be implanted is introduced to the surface of a planar or three-dimensional workpiece. This film can be grown using CVD, a bath or other means. The workpiece with the film is then subjected to ion bombardment to help drive the dopant into the workpiece. This ion bombardment is performed at elevated temperatures to reduce crystal damage and create a more abrupt doped region.

Description:
[0001]    This application claims priority of U.S. Provisional Ser. No. 61/413,816 filed Nov. 15, 2010, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    This invention relates to doping a workpiece and, more particularly, to doping a workpiece using ion bombardment at an elevated temperature. 
       BACKGROUND 
       [0003]    Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. 
         [0004]    The scaling of planar bulk silicon complementary metal oxide semiconductor (CMOS) devices has limitations. For example, at sub-22 nm nodes, the integrated circuit (IC) industry is transitioning to fully depleted (FD) planar or three-dimensional device structures. Doping of these structures using a beam-line ion implanter causes damage to the crystal structure of the silicon, which may not be completely annealed. Active areas in these devices that are damaged or amorphized by ion implantation fail to have the silicon crystal structure restored. This results in degradation of the electrical characteristics in the device. Planar or conformal doping using plasma doping (PLAD) may still introduce some crystal damage. Furthermore, PLAD may produce an oxide or other undesirable films on the workpiece. 
         [0005]    Use of a dopant-containing deposited layer also has been attempted. The dopant “drive-in” was performed in a later thermal anneal step. While crystal damage is mostly avoided, a high thermal budget is required and less abrupt dopant profiles are made. This ultimately degrades device performance. Accordingly, there is a need in the art for an improved method of doping, and, more particularly, doping a workpiece using ion bombardment at an elevated temperature. 
       SUMMARY 
       [0006]    An improved method of doping a workpiece is disclosed. In this method, a film comprising the species to be implanted is introduced to the surface of a planar or three-dimensional workpiece. This film can be grown using CVD, ALD, MLD, plasma, plasma assisted deposition, a bath or other means. The workpiece with the film is then subjected to ion bombardment to help drive the dopant into the workpiece. This ion bombardment is performed at elevated temperatures to reduce crystal damage and create a more abrupt doped region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0008]      FIG. 1  is a cross-sectional view of a plasma doping system; 
           [0009]      FIGS. 2A-C  are cross-sectional side views of one embodiment of the process disclosed herein; 
           [0010]      FIGS. 3A-B  show different implant angles; and 
           [0011]      FIG. 4  shows a representative FinFET that may be created using the process disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The embodiments of this process are described herein in connection with a plasma doping system. However, these embodiments can be used with other systems and processes involved in semiconductor manufacturing or other systems that use implantation. For example, in an alternate embodiment, a beam-line ion implanter, an ion implanter that modifies a plasma sheath, or flood implanter is used. Thus, the invention is not limited to the specific embodiments described below. 
         [0013]    Turning to  FIG. 1 , the plasma doping system  100  includes a process chamber  102  defining an enclosed volume  103 . The process chamber  102  or workpiece  105  may be cooled or heated by a temperature regulation system, such as within a load lock or using a pre-chiller. A platen  104  may be positioned in the process chamber  102  to support a workpiece  105 . The platen  104  also may be cooled or heated by a temperature regulation system. Thus, the plasma doping system  100  may incorporate hot or cold implantation of ions in some embodiments. In one instance, the workpiece  105  may be a semiconductor wafer having a disk shape, such as, in one embodiment, a 300 mm diameter silicon wafer. However, the workpiece  105  is not limited to a silicon wafer and may be a flat panel, solar cell, or other workpiece. The workpiece  105  may be clamped to a flat surface of the platen  104  by electrostatic or mechanical forces. In one embodiment, the platen  104  may include conductive pins for making connection to the workpiece  105 . 
         [0014]    The plasma doping system  100  further includes a source  101  configured to generate a plasma  106  from an implant gas within the process chamber  102 . The source  101  may be an RF source or other sources known to those skilled in the art. In one instance, an RF source with at least one coil around the process chamber  102  is used. This RF source may be coupled to a matching network and RF generator. 
         [0015]    The platen  104  may be biased. This bias may be provided by a DC or RF power supply. The plasma doping system  100  may further include a shield ring, a Faraday sensor, or other components. In some embodiments, the plasma doping system  100  is part of a cluster tool, or operatively-linked process chambers  102  within a single plasma doping system  100 . Thus, numerous process chambers  102  may be linked in vacuum. 
         [0016]    During operation, the source  101  is configured to generate the plasma  106  within the process chamber  102 . In one embodiment, the source  101  is an RF source that resonates RF currents in at least one RF antenna to produce an oscillating magnetic field. The oscillating magnetic field induces RF currents in the process chamber  102 . The RF currents in the process chamber  102  excite and ionize the implant gas to generate the plasma  106 . The bias provided to the platen  104  and, hence, the workpiece  105  will accelerate ions from the plasma  106  toward the workpiece  105  during bias pulse on periods. The frequency of the pulsed platen signal and/or the duty cycle of the pulses may be selected to provide a desired dose rate. The amplitude of the pulsed platen signal may be selected to provide a desired energy. With all other parameters being equal, a greater energy will result in a greater implanted depth. 
         [0017]      FIGS. 2A-C  are cross-sectional side views of one embodiment of the process disclosed herein. In  FIG. 2A , the workpiece  105  is illustrated. The workpiece  105  has a plurality of trenches  200 . While this particular embodiment of the workpiece  105  is illustrated, other workpieces with different three-dimensional structures are possible. For example, a FinFET or other vertical device also may be used. Furthermore, while a three-dimensional surface is illustrated in  FIG. 2A , a planar surface or a selected portion of a workpiece also may be doped using the embodiments disclosed herein. For instance, if selective deposition or selective doping is used, only part of a workpiece may be processed in this manner. Thus, the embodiments disclosed herein may be applied to numerous different workpiece types and shapes known to those skilled in the art. 
         [0018]    In  FIG. 2B , a film  201  is deposited on the workpiece  105 . This film may be applied using chemical vapor deposition (CVD), may be applied using a bath, or may be deposited in a PLAD system such as that illustrated in  FIG. 1 . Of course, the film  201  may be applied using other methods known to those skilled in the art, such as atomic layer deposition (ALD) or molecular layer deposition (MLD). In some embodiments, the thickness of the film is such that it is thick enough to provide adequate numbers of dopant atoms that will be later driven into the workpiece. The film must also be thin enough to not fill the trenches  200  between the 3 dimensional structures. In some embodiments, the film is less than ten monolayers thick, although other thicknesses may be used, based on the geometry of the workpiece, such as the depth and width of the trenches  200 . 
         [0019]    In one particular embodiment, a plasma tool that modifies a plasma sheath is used for selective deposition. In other embodiments, other mechanisms may be used to allow for selective deposition. For example, a photoresist or hard mask may be used over portions of the workpiece to prevent the formation of film on that portion of the workpiece. 
         [0020]    The film  201  covers a surface of the trenches  200  in the workpiece  105 . This film  201  may contain any n-type or p-type dopant, such as but not limited to boron, arsenic, or phosphorus. 
         [0021]    In  FIG. 2C , ions  202  bombard the workpiece  105  and the film  201 . In one instance the bombardment ions  202  are argon, though other noble gases, other inert species, or other species may be used. For example, non-doping species such as silicon, germanium, neon, or carbon may also be used. In another instance, a dopant is used for the bombardment ions  202 , which may at least partially dope the workpiece  105 . The implant energy used may vary, depending on the mass of the bombardment ions, the mass of the dopant ions, the angle of implant, and other parameters. For example, an implant energy of between 1 and 20 keV may be used for lighter bombardment ions, such as silicon, carbon and neon, while an implant energy of between 3 and 30 keV may be used for heavier bombardment ions, such as argon and germanium. 
         [0022]    The implant angle is determined, at least in part, by the ratio of the width of a trench  200  to its depth. For example, as seen in  FIG. 3A , if the ratio of trench width to depth is high (indicating a shallow, wide trench), the maximum implant angle θ 1  may be greater than the maximum implant angle θ 2  where the ratio of the trench wide to its depth is low (indicating that it is a deep, narrow trench), as shown in  FIG. 3B . Greater implant angle may require lower implant energies than smaller implant angles. In this example, implant angle is defined as the deviation from the normal, or perpendicular direction, relative to the plane of the workpiece  105 . 
         [0023]    Returning to  FIG. 2C , the workpiece  105  is kept at an elevated temperature during this bombardment using the bombardment ions  202 . This heating may use lamps, pre-heating, or heating of a platen  104  that the workpiece  105  is disposed on. The bombardment ions  202  knock-in dopants from the film  201  to form a doped region  203 . This doped region  203  is formed on all surfaces of the workpiece  105 , including in or around the trenches  200 . Although the bombardment ions  202  are shown as being implanted perpendicular to the plane of the workpiece  105 , the disclosure is not limited to this embodiment. As shown in  FIGS. 3A-B , the bombardment ions  202  may be implanted at an angle, where that angle may be determined based on the geometry of the trenches  200 . 
         [0024]    The temperature is configured to prevent formation of amorphous layers in the doped region  203  and to initiate “radiation enhanced diffusion.” The temperature may be between 150° C. and 800° C. depending on the dopant in the film  201 , ion bombardment conditions, and geometry of the surface of the workpiece  105 . In other embodiments, the temperature may be between 400° C. and 600° C. The temperature and ion bombardment conditions may be tuned independently to optimize the workpiece  105 . 
         [0025]    For example, a FinFET  300 , is shown in  FIG. 4 . The FinFET  300  includes a drain  301  and source  302 , which are three dimensional structures, also described as fins. A gate  303  is wrapped over the fin so as to create contacts on three sides of the fin. Spacers  304  are typically located on either side of the gate  303  (although only one is shown in  FIG. 4 ). It may be desirable to dope the drain  301  and source  302 . 
         [0026]    To achieve this, a hard mask, such as silicon dioxide or silicon nitride, may be placed on the gate  303  and spacers  304 . A film containing the desired dopant, as described in conjunction with  FIG. 2B , is then applied to the surface of the workpiece, so as to cover the drain  301  and the source  302 . The photoresist inhibits the formation of film on the gate  303  and spacer  304 . After the film has been deposited, the temperature of the workpiece is elevated, and bombardment ions  202  are introduced to knock in the dopant into the workpiece. The use of the combination of elevated temperature and bombardment has several benefits. First, typical thermal diffusion creates a doped region which does not have an abrupt junction. By knocking the dopant into the workpiece, the junction that is produced can be much more abrupt. Further, the use of bombardment also lowers the thermal budget associated with diffusion. 
         [0027]    It may also be desirable to introduce doped regions beneath the spacers  304 . By varying the temperature during the bombardment, it is possible to create desired diffusion patterns beneath the spacers  304 . These regions may not be created without the combination of bombardment and an elevated temperature, as it is the elevated temperature that allows the dopant to diffuse under the spacer  304 . Furthermore, the use of bombardment allows these regions to have more abrupt junctions than would be possible with only thermal diffusion. 
         [0028]    The ion  202  bombardment in one instance is performed in a separate chamber from deposition of the film  201 . In another embodiment, the ion  202  bombardment and film  201  deposition occur in the same chamber, such as in a PLAD system as illustrated in  FIG. 1 . If the same chamber is used, then the two steps may occur without breaking vacuum. 
         [0029]    A supplemental rapid thermal anneal (sRTA), a millisecond anneal (MSA), or a “multi-pass” laser anneal may be eliminated by use of the elevated temperature during bombardment of the ions  202 . This may reduce the complexity and cost associated with the annealing step. However, an anneal may still be performed after the step illustrated in  FIG. 2C . For example, a laser anneal may be used to achieve a high concentration of electrically-active dopant. 
         [0030]    The elevated temperature used during the ion bombardment provides multiple benefits besides improved device performance. First, it provides a dynamic anneal of defects introduced into the workpiece  105  due to the ions  202 . Amorphous layers in the workpiece  105  or doped region  203  may be reduced or prevented. Second, it reduces sensitivity of dopant diffusion. Third, it lowers the temperature required for diffusion of the dopants in the film  201  due to diffusion enhancement by point defects introduced during the knock-in process. Thus, desired diffusion depth may be achieved at a lower processing temperature by using “radiation enhanced diffusion.” This minimized thermal budget may be used for the post-implant anneal between the source drain extension (SDE) and the gate in a transistor structure. This may enable high-k metal gate (MG) stacks for MG first formation schemes and other temperature-sensitive elements of a transistor architecture. Fourth, more abrupt dopant profiles may be created compared to using a pure thermal drive-in process. The ion bombardment enables a more abrupt profile as a diffusion length component related to a thermal diffusion is reduced. 
         [0031]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, any claims should be construed in view of the full breadth and spirit of the present disclosure as described herein.