Abstract:
A method and apparatus for plasma cleaning a workpiece (W) in a plasma-cleaning chamber ( 20 ) having an interior region ( 30 ). The method comprises the steps of first, loading the workpiece into the plasma cleaning chamber interior region. The next step is pumping down the plasma cleaning chamber interior region down to a pre-determined pressure, with hydrogen as the ambient gas. The next step is forming from the hydrogen gas a plasma ( 36 ) having an ion density in the range between 10 10  and 10 13  cm −3  and an ion energy lower than 30 eV The last step is exposing the workpiece to the plasma for a predetermined time. The apparatus of the present invention preferably includes first and second vacuum processing chambers ( 20  and  120 ), wherein the first chamber performs the plasma cleaning of the workpiece according to the method of the invention, and the second chamber performs an additional process step, e.g., depositing a metal.

Description:
This is a continuation of International Application No. PCT/US01/13002, which was filed on Apr. 23, 2001, and also claims benefit of U.S. application Ser. No. 60/199,354, which was filed Apr. 25, 2000, the contents of both of which are incorporated herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the preparation of workpieces, and in particular relates to the cleaning of workpiece surfaces using a hydrogen-based plasma. 
     A fundamental step in the manufacturing of semiconductor devices, such as integrated circuits (ICs), is the process of forming electrical interconnections, or “contacts.” The formation of a low resistance contact involves the steps of providing a semiconductor workpiece, such as a silicon wafer, cleaning the surface of the workpiece, selectively depositing a metal, such as titanium, on the surface, and thermally annealing the metal. Where the metal is titanium, the annealing process causes the formation of titanium silicide, which consumes some of the underlying silicon. 
     Unfortunately, as minimum circuit dimensions decrease, the use of metals for forming the electrical contacts becomes problematic. This is primarily because the resistivity of the metal-silicon (e.g., titanium silicide) contact increases dramatically when the size of contact (i.e., the “line width”) is one micron or less. Compounding the problem, as line widths diminish below one micron, device junction depths decrease to just a few hundred angstroms. Since the formation of silicides consumes some of the underlying silicon, a reduction in the junction depth to a few hundred angstroms means that the integrity of the junction is at risk. 
     Use of the metal cobalt has been proposed as a solution to the above-described problems associated with titanium-based contacts, and is used in sub 0.25 micron manufacturing processes. However, the use of cobalt in forming contacts introduces additional problems. For example, cobalt does not react with silicon oxides or any of the other likely surface contaminants, such as water and C-F polymers. Consequently, the surface of the wafer prior to cobalt deposition must be far cleaner than what is necessary for other metal-silicon contacts, such as titanium silicide. 
     There are two techniques currently used in semiconductor manufacturing to clean workpiece surfaces prior to forming contacts using cobalt. One method is to clean the wafer in a variety of chemical solutions, including a final step of cleaning the wafer with a hydrofluoric (HF) dip. Though this approach is effective for many cleaning processes (especially those involving 0.5 micron technology and higher), HF is not as sufficiently reliable for sub 0.25 micron technology. Furthermore, this chemical poses significant health risks to operators and technicians. Moreover, workpieces to be processed must be transported from the HF dip tank to the deposition reactor. In this transportation step the workpieces are exposed to air, which oxidizes the exposed surface, thereby degrading device performance and reducing process tolerances. 
     The second workpiece cleaning surface method used prior to cobalt deposition involves sputtering the workpiece surface with argon ions. To be effective, the energy of the ions must be reasonably high. Unfortunately, use of such high-energy ions is problematic. For example, sputtering at such high energies can result in argon being incorporated into the silicon. Such ions can result in the generation of crystal defects as deep as several hundred angstroms. Other problems include erosion of the silicon itself, re-deposition of the sputtered materials, and the penetration of surface contaminants into the silicon. 
     The use of hydrogen plasma has been proposed as a method for cleaning surfaces. Since the chemical byproducts of hydrogen plasma are essentially gaseous, the cleaning process should be very effective. However, when the use of hydrogen plasma to clean wafers was studied, numerous problems emerged. For example, when using a parallel plate reactive ion etch (RIE) system, severe silicon erosion and the diffusion of hydrogen into the silicon resulted from the high ion energy generated in the source plasma. When a microwave-excited downstream plasma was used, the removal rate of the native oxide and other contaminants was significantly reduced due to the low energy of the hydrogen radicals and the reduction of radical concentration during the transportation from the source to the wafer surface. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to the preparation of workpieces, and in particular relates to the cleaning of workpiece surfaces using a hydrogen-based plasma. 
     A first aspect of the invention is a method of plasma cleaning a workpiece in a plasma-cleaning chamber having an interior region. The method comprises the steps of first, loading the workpiece into the plasma cleaning chamber interior region. The next step is pumping the plasma cleaning chamber interior region down to a pre-determined pressure, with hydrogen as the ambient gas. The next step is forming from the hydrogen gas a plasma having an ion density in the range of 10 10  to 10 13  cm −3  and preferably greater than 10 12  cm −3 , and an ion energy lower than 30 eV and preferably in the range from 10 to 15 eV. The last step is exposing the workpiece to the plasma for a predetermined time. 
     A second aspect of the invention is the method as described above, further including the steps, after the wafer is cleaned, of transferring the workpiece from the plasma cleaning chamber to a processing chamber, and then performing a process step to the workpiece. This process step may be, for example, depositing a metal. 
     A third aspect of the invention is an integrated workpiece processing apparatus for plasma cleaning a workpiece and then processing the workpiece. The apparatus comprises a first vacuum processing chamber adapted to plasma clean a workpiece with a plasma having a high ion density, low ion energy and low plasma potential. The first vacuum processing chamber includes a workpiece support fitted therein. The apparatus also includes a second vacuum processing chamber adapted to perform a process selected from the group consisting of CVD, PVD, sputtering, and etching of a workpiece. The second processing chamber is also fitted with a workpiece support. Further included in the apparatus is a vacuum transfer chamber connecting the first and second chambers. The transfer chamber is sized so that a workpiece may pass between the chambers. The purpose of the transfer chamber is to prevent the workpiece from being exposed to contaminants (i.e., oxygen or water vapor, etc.) after it has been cleaned in the first vacuum processing chamber. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a schematic diagram of the plasma-cleaning system of the present invention, shown as used in combination with a process chamber used to process the workpiece after it is cleaned in the plasma-cleaning chamber; and 
     FIG. 2 is a close-up perspective view of the plasma-cleaning chamber of the system shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to the preparation of workpieces, and in particular relates to the cleaning of workpiece surfaces using a hydrogen-based plasma. The present invention is particularly useful in preparing a workpiece in which low-resistivity metal silicide contacts are to be formed in the process of fabricating semiconductor devices in a silicon workpiece (wafer). 
     With reference now to FIG. 1, there is shown a plasma-cleaning system  12  in the form of an ESRF plasma reactor (though other reactors, such as a neutral-loop plasma (NLP) reactor, an ultra-high frequency (UHF) plasma reactor, and the like could also be used). System  12  comprises a plasma-cleaning chamber  20  as a vacuum processing chamber adapted to perform plasma cleaning of a workpiece W, such as a silicon wafer. Workpiece W has an upper surface WS. Chamber  20  has sidewalls  22 , an upper wall  24  and a lower wall  26  that enclose an interior region  30  capable of supporting a hydrogen plasma  36 . The latter has a high ion density and low ion energy, arising from a low voltage (potential). These plasma properties minimize ion penetration into workpiece surface WS and are a key aspect of the present invention. Chamber  20  includes within region  30  a workpiece support  40  arranged adjacent lower wall  26  for supporting workpiece W while the workpiece is processed in chamber  20 . The workpiece support  40  preferably includes a lifting member (not shown) for vertical translation in order to position workpiece support  40  for workpiece exchange as well as position the workpiece at an optimal position relative to the plasma for process. The optimal position may be one that achieves an acceptable rate for the cleaning process and spatial uniformity of the cleaning process. 
     With reference to FIG. 2, in a preferred embodiment, chamber  20  of plasma-cleaning system  12  includes an inductive coil  50  wrapped around chamber sidewalls  22  so as to surround interior region  30 . Inductive coil  50  may be a helical resonator (i.e. a quarter-wave or half-wave resonator), wherein one coil end  50 E (shown in FIG. 1) is grounded, and the opposite coil end is open. Coil  50  is electrically connected to a chamber RF power supply  60  through a match network MN 1 . For a helical resonator, match network MN 1  will be connected to a tap location generally near grounded end  50 E. The latter is used to maximize RF power transfer to plasma  36 . 
     Between inductive coil  50  and chamber walls  22  is a grounded electrostatic shield  62  (also referred to as an E-shield or Faraday shield) comprising an electrically grounded, conductive sheet with slots  62 S each having a bottom  62 B and a top  62 T. Slots  62 S are aligned parallel with the axis of revolution A of chamber  20  and are typically equally spaced. Slots  62 S may have a width, for example, of between 2 mm-6 mm. The total area covered by all slots  62 S should preferably fall into the range of 0.2 to 5% of the total area of shield  62 . E-shield  62  minimizes capacitive coupling between coils  50  and plasma  36  by limiting the area of slots  62 S through which the electromagnetic field from the coils can couple to the plasma. 
     With reference again to FIG. 1, system  12  also includes a workpiece support RF power supply  70  electronically connected to workpiece support  40  through a match network MN 2 , to supply an electrical bias to the workpiece. 
     Plasma-cleaning system  12  further includes gas supply system  80  in pneumatic communication with plasma-cleaning chamber  20  via a gas conduit  82 . Gas supply system  80  includes a source of hydrogen gas  86  used to create hydrogen plasma  36 . Preferably only hydrogen gas is used for the cleaning process. However, other gases, in particular inert gases, may be used as a dilution gas. For example, helium may be used as a dilution gas. However, due to its high ionization energy, it can raise the “tail” of the electron energy distribution function, which can be a disadvantage to the process. Moreover, argon could be used as the dilution gas. However, due to its mass, it has the disadvantage of increasing the ion bombardment of the sensitive contact surface. Gas supply system  80  also regulates the flow of hydrogen gas to chamber interior region  30 . Gas supply system  80  also connects appropriate gasses to a second processing chamber, as discussed below. 
     Plasma cleaning system  12  also includes a vacuum pump system  90  pneumatically connected to chamber  20  for evacuating interior region  30  to at least approximately 1-100 mTorr. Further included in plasma cleaning system  12  is a workpiece handling and robotic system  94  that transports workpieces W to and from workpiece support  40 . 
     RF power supplies  60  and  70 , gas supply system  80 , vacuum pump system  90  and workpiece handling and robotic system  94  are all electronically connected to and controlled by a main control system  100 . 
     In a preferred embodiment, main control system  100  is a computer having a memory unit MU having both random-access memory (RAM) and read-only memory (ROM), a central processing unit CPU (e.g., PENTIUM™ processor from Intel Corporation), and a hard disk HD, all electronically connected. Hard disk HD serves as a secondary computer-readable storage medium, and may be, for example, a hard disk drive for storing information corresponding to instructions for control system  184  to carry out the present invention, as described below. Control system  100  also preferably includes a disk drive DD, electronically connected to hard disk HD, memory unit MU and central processing unit CPU, wherein the disk drive is capable of accepting and reading (and even writing to) a computer-readable medium CRM, such as a floppy disk or compact disk (CD), on which is stored information corresponding to instructions for control system  100  to carry out the present invention. It is also preferable that main control system  100  have data acquisition and control capability. A preferred control system  100  is a computer, such as a DELL PRECISION WORKSTATION  610 ™, available from Dell Corporation, Dallas, Tex. 
     With continuing reference to FIG. 1, in a preferred embodiment of the present invention, plasma-cleaning system  12  is in operable communication with a workpiece processing system  112  for further processing workpiece W after it is cleaned in the plasma-cleaning system. Workpiece processing system  112  includes a process chamber  120  as a second vacuum processing chamber adapted to perform a desired process, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), etching, or sputtering, on workpiece W. Chamber  120  has sidewalls  122 , an upper wall  124  and a lower wall  126  that enclose an interior region  130 . Chamber  120  includes within region  130  a workpiece support  140  arranged adjacent lower wall  126  for supporting workpiece W while the workpiece is processed. Workpiece support  140  includes a heater  142  and heater power supply  144  electrically connected to the heater. Workpiece support  140  is preferably adjustable so that workpiece W can be positioned in different locations within interior region  130 . 
     Workpiece processing system  112  further includes a second gas supply system  180  pneumatically connected to process chamber  120  via gas conduit  182 . Alternatively, a gas supply system  80  could also be used to supply gas to chamber  120 . Also included is a vacuum pump system  190  pneumatically connected to chamber  120  and capable of producing a vacuum of at least approximately 1-100 mTorr. Alternatively, vacuum pump system  90  could also be connected to chamber  120  and used to control the pressure in interior region  30 . 
     Heater power supply  144 , gas supply system  180 , and vacuum pump system  190  are also electronically connected to and controlled by control system  100 . 
     In the present preferred embodiment, systems  12  and  112  are interconnected via a vacuum transfer (or “load lock”) chamber  150  through respective chamber sidewalls  22  and  122 . In this preferred embodiment, workpiece handling and robotics system  94  also transports workpiece W between chambers  20  and  120 , and also removes the workpiece from workpiece support  140  after processing in chamber  120 . This allows for transfer of workpiece W from chamber  20  to chamber  120  while in a contamination-free environment. 
     Also, control system  100  is shown in electronic communication with various systems  160   a,    160   b,  . . .  160   n  which may be conventional individual power sources for individual segments of a segmented electrode (not shown). 
     In the above-described preferred embodiment, the combination of plasma-cleaning system  12  and workpiece processing system  112 , along with the accompanying elements, constitute an integrated workpiece processing apparatus. All of the individual components and systems of systems  12  and  112  can be constituted by known, commercially available components And systems or can be constructed on the basis of knowledge already possessed by those skilled in the art. 
     Method of Operation 
     With continuing reference to FIG. 1, control system  100  causes workpiece handling and robotics system  94  to load a workpiece onto workpiece support  40  in plasma-cleaning chamber  20 . Control system  100  then sends an electronic signal to vacuum system  90  to evacuate interior region  30  of plasma-cleaning chamber  20 . Subsequently, control system  100  signals gas supply system  80  to introduce a controlled flow of a gas composed solely or predominantly of hydrogen, into interior region  30  of chamber  20  while sustaining the desired pressure inside the chamber. When chamber interior region  30  reaches a desired pressure, control system  100  signals RF power supply  60  to energize the gas to form plasma  36  in interior region  30 . 
     Preferably the gas used for the cleaning process is composed only of hydrogen. However, other gases, in particular inert gases, may be used as dilution gas, provided that the gas composition does not introduce any unwanted chemical species in the cleaning process. For example, helium may be used as a dilution gas, although, due to its high ionization energy, it can raise the “tail” of the electron energy distribution function and this can be a disadvantage to the process. Moreover, argon could be used as the dilution gas, although, due to its mass, it has the disadvantage of increasing the ion bombardment of the sensitive contact surface. 
     As mentioned above, plasma  36  is formed so as to have a high ion density (e.g., from 10 10  to 10 13  ion/cm 3 , preferably of order 10 12  ion/cm 3 ), and low ion energy (e.g., less than 30 eV, preferably of order of 10 to 15 eV depending upon the RF bias power). This type of plasma has been found by the present inventors to be well-suited for cleaning substrates that have significant topography e.g., contacts, of contaminants such as oxides, organic residues, etching polymers, heavy metal atoms and water molecules, etc. 
     In particular, plasma  36  cleans workpiece W in the manner described as follows. As mentioned above, a key aspect of the present invention is the low plasma potential resulting from the use of E-shield  62 . This means that if no bias is applied to workpiece W from workpiece RF power supply  70 , the energy of the ions and the electrons in plasma  36  reaching the workpiece will be very low. Thus, in the case of no external applied bias from workpiece RF power supply  70 , there is significantly reduced ionic bombardment of workpiece surface WS. Adjusting the bias applied to workpiece support  40  increases the kinetic energy of ions arriving at substrate surface WS. In addition, varying the amount of RF power from RF power supply  60  alters the ion density in plasma  36 , while maintaining the energy of the ions at substantially the same level (typically on the order of 5 to 30 eV depending upon the RF bias power). This allows for control over the anisotropic etch characteristics of plasma  36 . Thus, plasma  36  can be adjusted to have the right balance of “etching strength” (i.e., ion energy)—enough to clean workpiece surface WS without significantly etching into the surface—combined with the proper directionality (i.e., anisotropic etch, in a direction perpendicular to the workpiece surface). 
     The chemical and mechanical processes involved in the removal of contaminant material from the sensitive surfaces at the bottom of high aspect ratio contacts formed in workpiece W can be categorized as a plasma enhanced etch. In a preferred embodiment of the present invention, a hydrogen chemistry is employed to volatize a thin layer of contaminants subject to a shower of H +  and H 2   +  ions. The primary independent process parameters available for adjusting the process include the gas specie(s), gas flow rate, chamber gas pressure, RF source power, and RF bias power (or workpiece holder peak-to-peak voltage). The above independent process parameters are then adjusted to provide an ion density and ion energy in the ranges provided above, wherein they are fine tuned to optimize the process. A preferred range of operating parameters for the cleaning process are 10 to 1000 sccm of hydrogen gas, preferably about 200 sccm, 1 to 500 mTorr chamber pressure, preferably 10 to 100 mTorr, 1 to 5 kW RF source power, preferably 3 kW at 50 mTorr, and 0 to 20 Volts (peak-to-peak RF bias on the workpiece holder (or chuck). Typically, the RF frequency for both the inductive coil and the chuck bias would be, for example, 13.56 MHz. 
     Upon terminating the cleaning process, control system  100  sends an electronic signal to wafer handling and robotics system  94 , which then acts in response to the signal to remove move workpiece W from workpiece support  40 . At this point, workpiece W can be transported through vacuum transportation chamber  150  to wafer support  140  in processing chamber  120  via wafer handling and robotics system  94 . In a preferred embodiment of the present invention, workpiece W is a semiconductor wafer having a patterned surface (e.g., contact areas) that have been cleaned in plasma-cleaning chamber  20 . Further, processing chamber  120  is preferably capable of depositing a metal layer to form low-resistance metal-silicide electrical connections (e.g., contacts) in the process of fabricating a semiconductor device. Any operation to be performed in process chamber  112  will be performed in a manner already known to those skilled in the art. 
     Although the above-described plasma-cleaning system  12  has been described in connection with an ESRF plasma reactor, it will be understood that alternate systems, including a neutral loop plasma (a Faraday shield in the form of a coil partially inside the reactor), or an ultra high frequency plasma, or an inductively coupled plasma (ICP) system capable of forming a high-density, low-potential plasma are also suitable for practicing the process of the present invention. 
     In fact, the many features and advantages of the present invention are apparent from the detailed specification and thus, it is intended by the appended claims to cover all such features and advantages of the described process which follow in the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those of ordinary skill in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described. Moreover, the process and apparatus of the present invention, like related apparatus and processes used in the semiconductor arts tend to be complex in nature and are often best practiced by empirically determining the appropriate values of the operating parameters, or by conducting computer simulations to arrive at best design for a given application. Accordingly, all suitable modifications and equivalents should be considered as falling within the spirit and scope of the invention.