Abstract:
The present invention concerns an apparatus comprising a fixture and a sputtering device. The fixture may be configured to position a semiconductor wafer in a plasma. The sputtering device may be configured to sputter metal atoms onto a surface of the wafer in a direction perpendicular to the surface.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and/or architecture for implementing semiconductors generally and, more particularly, to a method and/or architecture for salicidation of cobalt on a semiconductor wafer. 
     BACKGROUND OF THE INVENTION 
     The conventional method of forming cobalt salicide (i.e., self aligned silicide) on a surface of a silicon semiconductor wafer is via the following steps: (i) depositing cobalt on the surface of the wafer using a conventional (i.e., non-directional) DC sputtering system, (ii) depositing either a titanium or titanium/titanium nitride cap over the cobalt layer using the conventional DC sputtering system to getter up oxygen, (iii) reacting the cobalt with the underlying active silicon regions using a first stage rapid thermal process in a temperature range from 400° C. to 550° C., (iv) chemically stripping the cobalt that was not over silicon and thus not reacted, (v) converting the cobalt silicon compound formed in the first rapid thermal process (i.e., step (iii)) to a fully reacted and converted cobalt salicide using a second rapid thermal process at a temperature range from 700° C. to 1000° C. The conventional process has many steps and each step (i) costs money, (ii) adds to the cycle time, and (iii) can create scrap when not performed properly. 
     It would be desirable to have a method and/or architecture for salicidation of a cobalt layer on a semiconductor wafer that (i) implements fewer steps than the conventional method, (ii) reduces costs, and/or (iii) reduces cycle time. 
     SUMMARY OF THE INVENTION 
     The present invention concerns an apparatus comprising a fixture and a sputtering device. The fixture may be configured to position a semiconductor wafer in a plasma. The sputtering device may be configured to sputter metal atoms onto a surface of the wafer in a direction perpendicular to the surface. 
     The objects, features and advantages of the present invention include providing a method and/or architecture for forming a cobalt salicide on a surface of a semiconductor wafer that may: (i) eliminate a step in the salicidation process, (ii) reduce production cost, (iii) reduce process cycle time, (iv) minimize wafer damage, (v) provide a uniform distribution of metal ions in the wafer, and/or (vi) increase the accuracy of cobalt distribution within the wafer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which: 
     FIG. 1 is a block diagram of a preferred embodiment of the present invention; 
     FIG. 2 is a block diagram illustrating an operation of the present invention; 
     FIG. 3 is a block diagram illustrating an alternate operation of the present invention; and 
     FIG. 4 is a block diagram illustrating another alternate operation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a block diagram illustrating a system  100  is shown in accordance with a preferred embodiment of the present invention. The system  100  may be implemented as a directionalized sputtering system. The system  100  may comprise a target  102 , one or more RF coils  104  (e.g., RF coils  104   a  and  104   b ), one or more shields  106  (e.g., shields  106   a  and  106   b ), a plasma field  108 , a wafer  112 , and a pedestal  114 . 
     The target  102  may be a source of atoms  110 . In one example, the target  102  (and the atoms  110 ) may comprise cobalt. In another example the target  102  may be implemented as Hf, Mo, Ni, Pd, Pt, Ta, W, Zr, Cr, etc. In yet another example (e.g., when forming a cap over a cobalt layer), the atoms  110  may comprise titanium (Ti), or titanium/titanium nitride (Ti/TiN). However, any appropriate metal, material or metal compounds may be implemented accordingly to meet the design criteria of a particular application. The system  100  may be configured to first implement the target  102  using cobalt and then implement another target  102   a  using an alternative metal, material or compound (e.g., titanium, titanium/nitride) to form one or more layers on the same wafer  112 . The alternative metal, material or compound may form a cap over the cobalt. 
     The RF coils  104  may be implemented as a pair of coils (e.g., the coils  104   a  and  104   b ) positioned on either side of the plasma field  108 . The RF coils  104  may be configured to generate an electromagnetic field across the plasma  108 . In one example, the RF coils  104   a  and  104   b  may be positioned on opposing sides of the plasma  108 . However, any appropriate number of RF coils  104  and appropriate positioning may be implemented accordingly to meet the design criteria of a particular application. The shields  106  may be implemented as a heat resistant material that will not interfere with the electromagnetic field generated via the RF coils  104 . The shields  106  may be positioned to separate the plasma field  108  from the RF coils  104 . The shields  106  may be configured to protect the RF coils  104  from heat generated in the plasma  108 . 
     Examples of sputtering deposition systems that implement RF coils include the Applied Material “Ion Metal Plasma” system and the Novellus “Hollow Cathode Magnetron” system. However, any appropriate sputtering system may be implemented accordingly to meet the design criteria of a particular application. The electromagnetic field generated via the RF coils  104  may be configured to control ionization of the atoms  110  in the plasma  108 . The electromagnetic field generated via the coils  104  may positively ionize the atoms  110  (e.g., ionized atoms  110   a ). The electromagnetic field generated via the coils  104  may be configured to control the location that ionized atoms  110   a  are deposited on a surface of the wafer  112  that is exposed to the plasma  108 . 
     The electromagnetic field generated via the coils  104  may suspend the ionized atoms  110   a  in the plasma field  108 . The length of time that the ionized atoms  110   a  are suspended in the plasma field  108  may control the ratio of ionized atoms  110   a  to non-ionized atoms  110 . The atoms  110  may randomly sputter off the target  102 . The atoms  110  may disperse upon entering the plasma field  108  due to the random sputtering from the target  102 . The electromagnetic field generated via the RF coils  104  may be configured to align the ionized atoms  110   a  to form concentrated directional (e.g., perpendicular) deposition of the ionized atoms  110   a  on the surface of the wafer  112  exposed to the plasma  108 . 
     The plasma field  108  may comprise a stream of gas. The plasma field  108  may comprise an inert gas and/or an inert gas mixture (not shown). In one example, the inert gas may be implemented as argon. However, any appropriate inert gas and/or gasses may be implemented accordingly to meet the design criteria of a particular application. The plasma field  108  may generate heat that may be sufficient to ionize some of the gas atoms that comprise the plasma  108 . The ionized gas atoms may impact the target  102  and cause the atoms  110  to be sputtered off the target  102 . The atoms  110  may migrate from the target  102  and into the plasma field  108 . 
     The wafer  112  may be implemented in silicon. In other examples, the wafer  112  may be implemented in gallium arsenide, germanium, gallium nitride, aluminum phosphide, diamond, Si 1−x Ge x , and/or AlxGa 1−x As, where 0≦x≦1. However, any appropriate semiconductor wafer material may be implemented accordingly to meet the design criteria of a particular application. The pedestal  114  may be implemented as an electrically conductive material. 
     The wafer  112  may be positioned on the pedestal  114  in line with and below the plasma field  108 . In one example, a DC current (not shown) may be coupled to the pedestal  114 . The DC current may be configured to generate a negative electrical potential across the wafer  112 . The positively charged ions  110   a  may be accelerated out of the plasma field  108  and toward the wafer  112  in response to the negative electrical potential across the wafer  112 . 
     In another example, the velocity of ionized atoms  110   a  may be controlled in response to vapor pressure of the plasma  108  in the system  100 . Increased vapor pressure of the plasma  108  generally increases the density of the plasma  108 . The increased density of the plasma  108  may increase the number of collisions of the ionized atoms  110   a  and the plasma  108 . The increased density of the plasma  108  may reduce the velocity of the ionized atoms  110   a  moving towards the wafer  112 . The increased velocity of the ionized atoms  110   a  in response to (i) the electromagnetic field generated via the coils  104  and/or (ii) the electrical potential across the wafer  112  may forcefully drive the ionized atoms  110   a  into the wafer  112 . The velocity of the ionized atoms  110   a  is generally a function of the DC potential applied across the wafer  112  and/or the vapor pressure in the system  100 . 
     Using the directionalized sputtering of the system  100 , cobalt (e.g., the atoms  110 ) may be reacted and/or mixed with the underlying wafer  112  (e.g., silicon atoms) via one or more of the following phenomena: 
     (i) The cobalt may react with the wafer  112  in response to the elevated temperature of the surface of the wafer  112  generated in response to the incidental energy from the directional (e.g., perpendicular) deposition of the cobalt and/or the subsequent Ti (or Ti/TiN) (described in connection with FIGS.  2  and  4 ). 
     (ii) The high velocity of the cobalt generated via the directionalized (e.g., perpendicular) sputtering of the cobalt may cause the cobalt to be physically driven into the wafer  112  without the surface of the wafer  112  attaining a temperature sufficient to react with the cobalt (described in connection with FIG.  3 ). 
     (iii) The directionalized (e.g., perpendicular) sputtering of a Ti (or Ti/TiN) cap on the wafer  112  may cause the Ti (or Ti/TiN) to impact the underlying cobalt with sufficient velocity to cause the cobalt to be physically driven into the wafer  112  when the temperature of the wafer  112  is not sufficiently high to generate a reaction between the cobalt and the wafer  112  (described in connection with FIG.  4 ). 
     (iv) The perpendicular sputtering of the cobalt or subsequent titanium (or titanium nitride) cap at a system  100  temperature in range of 400° C. to 500° C. (in contrast to the conventional deposition which is conducted at a system temperature of less than 350° C.) may cause the cobalt to react with the wafer  112 . 
     Referring to FIG. 2, a block diagram illustrating an operation of the directional sputtering system  100  is shown. A silicon wafer  112   a  may comprise a salicide layer  116 . When cobalt ions  110   b  impact the silicon wafer  112   a , evolved heat (e.g., thermic energy) may be released. The velocity of the cobalt ions  110   b  and an incidence angle of the cobalt ions  110   b  to the surface of the silicon wafer  112   a  generally determines the amount of incidental thermic energy that will be transferred from the cobalt ions  110   b  to the silicon wafer  112   a . The incidence angle and velocity of the cobalt ions  110   b  may be factors in determining the surface temperature of the silicon wafer  112   a.    
     To provide maximum energy transfer from the cobalt ions  110   b  to the silicon wafer  112   a , the cobalt ions  110   b  generally impact the silicon wafer  112   a  at an angle perpendicular to the surface of the silicon wafer  112   a . The evolved heat may raise the surface temperature of the silicon wafer  112   a . The elevated silicon wafer  112   a  surface temperature may cause the cobalt ions  110   b  to penetrate and mix into (e.g., react with) the silicon wafer  112   a . The reaction of the cobalt and the silicon wafer  112   a  may form the layer of cobalt salicide  116 . The layer of cobalt salicide  116  may be formed on the surface of the silicon wafer  112   a  exposed to the plasma  108 . 
     Referring to FIG. 3, a block diagram illustrating another operation of the system  100  is shown. The system  100  may be configured to provide sufficient kinetic energy to the cobalt ions  110   b  such that the cobalt ions  110   b  are driven into the silicon wafer  112   a.    
     The cobalt may be ionized via the coils  104 . After ionization and alignment (e.g., perpendicular to the surface of the wafer  112 ) via the coils  104 , the cobalt ions  110   b  may gain velocity as the DC potential across the silicon wafer  112   a  causes the cobalt ions  110   b  to migrate out of the plasma field  108 . The acceleration of the cobalt ions  110   b  toward the silicon wafer  112   a  may provide the cobalt ions  110   b  with kinetic energy. The cobalt ions  110   b  may have sufficient kinetic energy to enter the interstitial spaces between the silicon atoms of the silicon wafer  112   a  upon impact. The cobalt ions  110   b  may mix with the silicon wafer  112   a  atoms. The cobalt salicide layer  116  may be formed as cobalt atoms  110   c  become mixed with the silicon atoms. The force driven process of generating the cobalt salicide layer  116  may be accomplished without the silicon wafer  112   a  surface elevated to a temperature sufficient to cause a reaction of the cobalt atoms  110   c  with the silicon wafer  112   a  (e.g., the temperature of the surface of the silicon wafer  112   a  may be less than 250° C.). However, the temperature of the surface of the silicon wafer  112   a  sufficient for a reaction of the cobalt atoms  110   c  with the silicon wafer  112   a  may also depend on factors such as the plasma gas  108  composition, vapor pressure of the plasma  108 , the amount of power supplied to the system  110 , cooling of the pedestal  114 , the size of the chamber that contains the system  100 , etc. The temperature of the surface of the silicon wafer  112   a  sufficient for the reaction of the cobalt atoms  110   c  with the silicon wafer  112   a  may be greater or less than 250° C. (e.g., a range of 100° C. to 700° C.). 
     Referring to FIG. 4, a block diagram  100 ′ illustrating an alternate operation of the system  110  is shown. The system  100 ′ may comprise titanium (or titanium nitride) ions  110   d  (e.g., the target  102   a  may comprise Ti and/or Ti/TiN). The system  100  may be configured to directionally (e.g., perpendicularly) sputter the Ti ions  110   d  over the cobalt atoms  110   c  to form a cap. 
     The layer of cobalt atoms  110   c  may first be deposited on the surface of the silicon wafer  112   a . In one example, the layer of cobalt atoms  110   c  may be deposited on the surface of the silicon wafer  112   a  that is exposed to the plasma  108  using the directionalized sputtering system  100 . However, any appropriate cobalt deposition apparatus and/or process may be implemented accordingly to meet the design criteria of a particular application. 
     When the layer of cobalt atoms  110   c  has been deposited on the silicon wafer  112   a , the titanium (or Ti/TiN) ions  110   d  may be deposited over the layer of cobalt atoms  110   c . The Ti (or Ti/TiN) ions  110   d  may be deposited over the cobalt atoms  110   c  using the system  100  similarly to the deposition of the atoms  110  described in connection with FIGS. 1-3. The titanum ions  110   d  may impact the layer of cobalt atoms  110   c  with sufficient velocity to transfer kinetic energy from the titanium ions  110   d  to the cobalt atoms  110   c . The transfer of kinetic energy to the cobalt atoms  110   c  may cause the cobalt atoms  110   c  located in the cobalt layer to (i) react with the silicon wafer  112   a  and/or (ii) forcefully penetrate the silicon wafer  112   a . The cobalt atoms  110   c  and silicon may form a salicide layer  116  via the processes described in connection with FIGS. 1-3. 
     The present invention may be configured to react and/or mix the cobalt and underlying silicon during the cobalt and/or subsequent titanium or titanium/titanium nitride deposition steps. The present invention may eliminate a first rapid thermal process and reduce salicidation process costs. 
     In one example, the present invention may comprise performing the deposition of the cobalt (or a subsequent titanium or titanium nitride cap) to the silicon wafer  112   a  at a system  100  temperature in a range of 400° C. to 500° C. Conventional cobalt deposition systems operate at temperatures of less than 350° C. The higher operating temperature of the system  100  may provide for a better yield of cobalt salicide due to the elevated surface temperature of the silicon wafer  112   a . The present invention may reduce scrap. 
     The present invention may eliminate the first rapid thermal annealing step in the salicidation process by implementing an architecture and/or method that provides the energy required to react the cobalt and the silicon during the deposition of the cobalt and/or during the deposition of the titanium (or titanium nitride) cap. The energy for the reaction and/or mixing of the ions  110   a  with the wafer  112  may be supplied by the directionalized sputtering system  100 . The energy for the reaction and/or mixing of the cobalt with the silicon may be transferred to the silicon wafer  112   a  as either thermal or kinetic energy. 
     After the directionalized sputtering step described above has been performed, the system  100  may implement the steps of: 
     (i) implementing a chemical strip to remove unreacted cobalt; and 
     (ii) implementing a second rapid thermal process in the range of 700° C. to 1000° C. to fully convert the cobalt silicon compound formed in the directionalized sputtering process to a fully reacted and converted cobalt salicide. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.