Abstract:
The present invention is related to methods and apparatus that allow a chalcogenide glass such as germanium selenide (Ge x Se 1-x ) to be doped with a metal such as silver, copper, or zinc without utilizing an ultraviolet (UV) photodoping step to dope the chalcogenide glass with the metal. The chalcogenide glass doped with the metal can be used to store data in a memory device. Advantageously, the systems and methods co-sputter the metal and the chalcogenide glass and allow for relatively precise and efficient control of a constituent ratio between the doping metal and the chalcogenide glass. Further advantageously, the systems and methods enable the doping of the chalcogenide glass with a relatively high degree of uniformity over the depth of the formed layer of chalcogenide glass and the metal. Also, the systems and methods allow a metal concentration to be varied in a controlled manner along the thin film depth.

Description:
RELATED APPLICATION 
       [0001]    This application is a divisional of application Ser. No. 11/710,517, filed Feb. 26, 2007, which is a divisional of application Ser. No. 10/878,060, filed Jun. 29, 2004, now U.S. Pat. No. 7,202,104, issued Nov. 25, 2004, which is a divisional of application Ser. No. 10/164,429, filed Jun. 6, 2002, now U.S. Pat. No. 6,890,780, issued Dec. 11, 2003. This application is related to the disclosure of U.S. application Ser. No. 10/164,646 of Li et al., filed Jun. 6, 2002, now U.S. Pat. No. 6,825,135, entitled ELIMINATION OF DENDRITE FORMATION DURING METAL/CHALCOGENIDE GLASS DEPOSITION. The entirety of each related application is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to memory technology. In particular, the present invention relates to the fabrication of metal-doped chalcogenides. 
         [0004]    2. Description of the Related Art 
         [0005]    Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied. 
         [0006]    In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data for long periods of time when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), and the like. 
         [0007]    U.S. Pat. No. 6,084,796 to Kozicki, et al., entitled “Programmable metallization cell structure and method of making same,” discloses another type of nonvolatile memory device known as a programmable conductor memory cell or a programmable metallization cell (PMC). U.S. Pat. No. 6,084,796 is herein incorporated by reference in its entirety. Such memory cells can be integrated into a memory device, which has been referred to as a programmable conductor random access memory (PCRAM). A chalcogenide glass element is doped with metal, preferably silver (Ag). Application of an electric field with a first polarity causes a conductive pathway to grow along the sidewalls or in the sidewalls of the glass element, whereas an electric field of the opposite polarity dissolves the conductive pathway back into the glass element. If the conductive pathway extends between electrodes at opposite ends of the glass element, the resulting short or relatively low resistance can represent a logic state, e.g., a “1” state for the memory cell, whereas the unshorted, relatively high resistance state can represent another logic state, e.g., a “0” state. Additional applications for a programmable metallization cell include use as a variable programmable resistance and a variable programmable capacitance. 
         [0008]    One conventional technique for producing the programmable conductor memory cell applies silver (Ag) photodoping to a chalcogenide glass such as germanium selenide (Ge 3 Se 7 ). The silver (Ag) photodoping process deposits silver (Ag) over germanium selenide (Ge 3 Se 7 ) and exposes the underlying substrate assembly to a relatively intense source of ultraviolet (UV) radiation for an extended period of time, such as 15 minutes. Disadvantageously, the photodoping process is relatively time-consuming and can slow semiconductor fabrication rates. The photodoping process can decrease the overall process rate especially when it is repetitively applied, such as in the fabrication of a multiple layer stack. Further disadvantageously, the extended exposure to intense UV radiation can induce the glass to convert from an amorphous material to a crystallized material, which thereby results in reduced yields. 
         [0009]    Another disadvantage to producing memory cells with silver (Ag) photodoping of glasses is that relatively precise control of the amount of silver (Ag) that is photodiffused into the glass is necessary. A sufficient amount of silver (Ag) must be incorporated into the glass backbone and yet, the glass must not crystallize. If too much silver (Ag) is photodiffused into the glass, the glass crystallizes. If too little silver (Ag) were to be photodiffused into the glass, the memory cell would not switch properly. 
         [0010]    Another disadvantage to the photodoping process is that the ultraviolet light is attenuated by the silver film as the ultraviolet light penetrates through the silver film. Such attenuation varies exponentially with the thickness of the film. In one example, with 300 nanometers (nm) wavelength ultraviolet radiation, the intensity of the ultraviolet radiation decreases to only about 10% of its initial intensity after penetrating through 650 angstroms (Å) of silver film. This attenuation renders photodoping to be impractical with relatively thick films, and requires relatively precise control of the thicknesses of the silver (Ag) and chalcogenide glass films. In order to form a thick film with a UV photodoping process, the UV photodoping process is disadvantageously applied repetitively to relatively thin films of silver (Ag). In addition, the varying attenuation of the ultraviolet light continues as the silver (Ag) dopes the chalcogenide glass. Further disadvantageously, this attenuation in intensity of the ultraviolet light as the ultraviolet light penetrates material results in a non-uniform depth profile of the doped silver (Ag) in the chalcogenide glass. 
       SUMMARY OF THE INVENTION 
       [0011]    Embodiments of the present invention include systems and methods that overcome the disadvantages of the prior art. The systems and methods described herein allow a chalcogenide glass, such as germanium selenide (Ge x Se 1-x ), to be doped with a metal such as silver (Ag), copper (Cu), and zinc (Zn), without utilizing an ultraviolet (UV) photodoping step to dope the chalcogenide glass with the metal. Other examples of chalcogenide glasses that can be used include germanium sulfide (Ge x S 1-x ) and arsenic selenide (As x Se 1-x ). Advantageously, embodiments of the invention co-sputter the metal and the chalcogenide glass and allow for relatively precise and efficient control of a constituent ratio between the doping metal and the chalcogenide glass. Further advantageously, the systems and methods enable the doping of the chalcogenide glass with a relatively high degree of depth-profile uniformity. Also, the systems and methods allow a metal concentration to be varied in a controlled manner along the thin film depth. 
         [0012]    One embodiment according to the present invention is a nonvolatile memory cell including a first electrode, a second electrode, and a memory cell body disposed between the first electrode and the second electrode. The memory cell body includes a layer of germanium selenide (Ge x Se 1-x ) that is uniformly doped over depth with a metal such as silver (Ag), copper (Cu), or zinc (Zn). 
         [0013]    Another embodiment according to the present invention is a deposition system adapted to fabricate a nonvolatile memory cell body in a substrate assembly. The deposition system includes a deposition chamber, a first target, and a second target. The deposition chamber is adapted to hold the substrate assembly. The deposition system is further configured to sputter metal from the first target and to sputter germanium selenide (Ge x Se 1-x ) from the second target at the same time to co-deposit the metal and the germanium selenide (Ge x Se 1-x ). In one arrangement, the deposition system sputters silver (Ag) from the first target. In another arrangement, the deposition system sputters copper (Cu) or zinc (Zn) from the first target. The deposition system can further include a control configured to control the deposition rate of the metal and the deposition rate of the germanium selenide such that the nonvolatile memory cell body is deposited at a selected ratio between the metal and the germanium selenide in the cell body. 
         [0014]    Another embodiment according to the present invention is a process of fabricating a nonvolatile memory structure in a substrate assembly. The process includes forming a bottom electrode, co-sputtering metal and germanium selenide (Ge x Se 1-x ), and forming a top electrode. In other embodiments, a metal selenide and germanium; selenium and a mixture of a metal and germanium; or a metal, germanium, and selenium are co-sputtered. 
         [0015]    Another embodiment according to the present invention is a process of forming a layer in a substrate assembly. The layer is capable of supporting the growth conductive pathways in the presence of an electric field. The process includes providing elemental silver (Ag) in a first sputtering target, providing germanium selenide (Ge x Se 1-x ) in a second sputtering target, selecting a first sputtering rate for silver (Ag), selecting a second sputtering rate for germanium selenide (Ge x Se 1-x ), sputtering the silver (Ag), and sputtering the germanium selenide (Ge x Se 1-x ) at the same time as sputtering the silver to produce the layer. 
         [0016]    Advantageously, the co-sputter deposition of silver (Ag) and germanium selenide (Ge x Se 1-x ) allows the silver (Ag) to dope the sputtered germanium selenide (Ge x Se 1-x ) in the layer with a relatively uniform depth profile. In one arrangement, the first sputtering rate is determined by selecting a first sputtering power for silver (Ag), and the second sputtering rate is determined by selecting a second sputtering power for germanium selenide (Ge x Se 1-x ). The process preferably further includes selecting a ratio between the silver (Ag) and the germanium selenide in the layer, using the ratio to determine the first sputtering rate, and using the ratio to determine the second sputtering rate. 
         [0017]    Another embodiment according to the present invention is a process that controls a constituent ratio during production of a memory cell body. The ratio is controlled by selecting a first deposition rate of a metal such as silver (Ag), copper (Cu), or zinc (Zn) selecting a second deposition rate of germanium selenide (Ge x Se 1-x ), controlling the first deposition rate by selecting a first sputtering power used to deposit the metal, and controlling the second deposition rate by selecting a second sputtering power used to deposit the germanium selenide (Ge x Se 1-x ). 
         [0018]    Another embodiment according to the present invention is a process to configure a deposition system used to fabricate a memory cell body for a nonvolatile memory cell. The process includes receiving an indication of a desired constituent ratio, and calculating a deposition rate for a metal and a deposition rate for germanium selenide (Ge x Se 1-x ) that provides the desired ratio. The calculated deposition rate for the metal is further related to a sputter power for a metal target, and the calculated deposition rate for germanium selenide (Ge x Se 1-x ) is related to a sputter power for a germanium selenide (Ge x Se 1-x ) target. The process configures the deposition system to sputter the metal from the metal target at the calculated sputter power, and configures the deposition system to sputter germanium selenide (Ge x Se 1-x ) from the germanium selenide (Ge x Se 1-x ) target with the calculated sputter power. The metal can be silver (Ag), copper (Cu), or zinc (Zn). In another embodiment, the process includes storing a configuration of the deposition chamber, measuring the deposition rate for the metal versus sputter power, measuring the deposition rate for germanium selenide (Ge x Se 1-x ) versus sputter power, and storing the measured information such that it can be later retrieved by the process to configure the deposition system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0019]    These and other features of the invention will now be described with reference to the drawings summarized below. These drawings and the associated description are provided to illustrate preferred embodiments of the invention and are not intended to limit the scope of the invention. 
           [0020]      FIG. 1  schematically illustrates a co-sputter deposition system according to an embodiment of the present invention. 
           [0021]      FIG. 2  is a schematic cross section of a memory cell with a memory cell body formed by co-sputtering a metal and germanium selenide (Ge x Se 1-x ) glass. 
           [0022]      FIG. 3  is a flowchart that generally illustrates a process of co-sputtering metal and germanium selenide (Ge x Se 1-x ) glass. 
           [0023]      FIG. 4  is a flowchart that generally illustrates a process of configuring a deposition system to co-sputter metal and germanium selenide (Ge x Se 1-x ) glass. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Although this invention will be described in terms of certain preferred embodiments, other embodiments that are apparent to those of ordinary skill in the art, including embodiments which do not provide all of the benefits and features set forth herein, are also within the scope of this invention. Accordingly, the scope of the present invention is defined only by reference to the appended claims. 
         [0025]    Embodiments of the present invention allow a chalcogenide glass such as germanium selenide (Ge x Se 1-x ) to be doped with a metal such as silver (Ag), copper (Cu), or zinc (Zn) without the performance of an ultraviolet (UV) photodoping step. Other examples of chalcogenide glasses that can be used include germanium sulfide (Ge x S 1-x ) and arsenic selenide (As x Se 1-x ). The value of x can vary in a wide range. Although the value of x can theoretically range from 0 to 1, the fabrication of a programmable conductor random access memory (PCRAM) should maintain the value of x such that the underlying combination of chalcogenide glass doped with the metal remains in an amorphous state. It will be understood by one of ordinary skill in the art that the value of x can depend on the amount of the metal that dopes the chalcogenide glass. The selection of a value of x will be described later in connection with  FIG. 2 . 
         [0026]    Advantageously, embodiments of the invention co-sputter the metal and the chalcogenide glass. This provides a relatively precise and efficient control of a constituent ratio between the doping metal and the chalcogenide glass. Further advantageously, the doping of the chalcogenide glass with the metal can be produced with a relatively high degree of depth profile uniformity. It will be understood by one of ordinary skill in the art that there are at least two types of uniformity with respect to a doping profile. A first type, a lateral doping profile, varies depending on the deposition system. For example, variations in the projected light intensity of an ultraviolet source across the surface of the deposited film can produce lateral variations in the doping profile. By contrast, the attenuation of the ultraviolet light as the ultraviolet light penetrates through the metal and the chalcogenide glass gives rise to variations in depth profile uniformity. 
         [0027]      FIG. 1  schematically illustrates a co-sputter deposition system  100  according to an embodiment of the present invention. The illustrated co-sputter deposition system  100  includes a first inlet  102  adapted to introduce an inert gas, such as argon (Ar). A second inlet  104  allows a vacuum pump to evacuate an interior of the co-sputter deposition system  100  to a relatively low pressure. 
         [0028]    A first target  110  provides a source of chalcogenide glass, preferably germanium selenide (Ge x Se 1-x ). The first target  110  is coupled to a first target electrode  112 , which in turn is coupled to a first power supply  114 . In one arrangement, the first power supply  114  is configured to pulse direct current (DC) to sputter material from the first target  110 . In one arrangement, the first target  110  is germanium selenide (Ge x Se 1-x ), e.g., Ge 3 Se 7 . 
         [0029]    A second target  120  provides the source of the metal that dopes the germanium selenide (Ge x Se 1-x ). The metal can be silver (Ag), copper (Cu), and zinc (Zn), which will advantageously diffuse relatively quickly into the chalcogenide element. The second target  120  is coupled to a second target electrode  122 , which in turn is coupled to a second power supply  124 . In one arrangement, the second power supply  124  is configured to apply direct current (DC) to sputter material from the second target  120 . 
         [0030]    The co-sputter deposition system  100  sputters chalcogenide glass from the first target  110  and simultaneously sputters the metal from the second target  120  to a substrate  130  to produce a layer  140  of chalcogenide glass doped with the metal. In the illustrated co-sputter deposition system  100 , the substrate  130  rests on an electrode  106 , which is at ground potential. The relative removal rates and thus, deposition rates, of material from the first target  110  and the second target  120  approximately determine the doping profile of the layer  140 . 
         [0031]      FIG. 2  illustrates one embodiment according to the present invention of a memory cell  200  with an active layer formed by co-sputtering metal and a chalcogenide glass. In one embodiment, the metal is silver (Ag). In other embodiments, the metal is copper (Cu) or zinc (Zn). In one embodiment, the chalcogenide glass is germanium selenide (Ge x Se 1-x ), e.g., Ge 3 Se 7 . The illustrated memory cell  200  includes a first electrode  202 , a memory cell body  204 , an insulator  208 , and a second electrode  210 . 
         [0032]    The first electrode  202  is formed on a substrate assembly. The substrate assembly can correspond to a variety of materials including plastic and silicon. Preferably, the first electrode  202  is part of an elongated conductor in a crosspoint array so that the memory cell  200  can be programmed and read. The first electrode  202  can be made from a variety of materials and from combinations of materials such as tungsten (W), nickel (Ni), silver (Ag), and titanium (Ti). 
         [0033]    The memory cell body  204  is formed on the first electrode  202 . In the illustrated embodiment, the memory cell body  204  is a co-sputtered layer of silver (Ag) and germanium selenide (Ge x Se 1-x ). In another embodiment, the memory cell body  203  is a co-sputtered layer of copper (Cu) and germanium selenide (Ge x Se 1-x ) or a co-sputtered layer of zinc (Zn) and germanium selenide (Ge x Se 1-x ). A variety of combinations of metal and chalcogenide glass elements can be used to form the memory cell body  204 . In another embodiment, the metal and chalcogenide glass elements are co-sputtered from three separate targets, e.g., a silver target, a germanium target, and a selenium target. 
         [0034]    The memory cell body  204  of the memory cell  200  should be formed such that the metal-doped chalcogenide glass in the memory cell body  204  is in an amorphous state. The skilled practitioner will appreciate that where the chalcogenide glass is germanium selenide (Ge x Se 1-x ), the state of the metal-doped chalcogenide glass, i.e., whether it is amorphous or crystalline, depends on both the value of x and the amount of metal that dopes the chalcogenide glass. 
         [0035]    A phase diagram can be used to select a value for x and to select the amount of metal that is to dope the chalcogenide glass such that the chalcogenide glass remains amorphous. Such a phase diagram can be found in a reference from Mitkova, et al., entitled “Dual Chemical Role of Ag as an Additive in Chalcogenide Glasses,”  Physical Review Letters,  Vol. 86, no. 19, (Nov. 8, 1999), pp. 3848-3851, (“Mitkova”) which is attached hereto as Appendix 1 and which is hereby incorporated herein by reference in its entirety.  FIG. 1  of Mitkova illustrates two glass-forming or amorphous regions for germanium selenide (Ge x Se 1-x ) doped with silver (Ag). In one example, where x is 30, i.e., 0.30, so that the germanium selenide glass is Ge 30 Se 70 , the amount of silver (Ag) used to dope the germanium selenide should fall within about 0 to 18% or within about 23% to 32% by atomic percentage versus the amount of selenide (Se). 
         [0036]    In the illustrated embodiment, the insulator  208  surrounds the memory cell body  204 . The insulator  208  insulates the memory cell body  204  from other memory cells and also prevents the undesired diffusion of metal atoms and ions. The insulator  208  can be formed from a variety of materials such as silicon nitride (Si 3 N 4 ). 
         [0037]    The second electrode  210  is formed on the memory cell body  204  and on the insulator  208 . In one embodiment, the second electrode  210  also forms part of a line, preferably perpendicular to a lower line as part of a crosspoint array. The second electrode  210  can be formed from a variety of materials such as copper (Cu), zinc (Zn), silver (Ag), and the like. An electric potential applied between the first electrode  202  and the second electrode  210  generates an electric field in the memory cell body  204 , which in turn causes conductive pathways in the memory cell body  204  to grow or shrink in response to the applied electric field. 
         [0038]      FIG. 3  illustrates a process  300  of co-sputtering metal and germanium selenide (Ge x Se 1-x ) glass. The process provides  310  a metal target from which metal is to sputtered onto a substrate assembly. The metal can be silver (Ag), copper (Cu), or zinc (Zn). The process proceeds to provide  320  a germanium selenide (Ge x Se 1-x ) target from which germanium selenide (Ge x Se 1-x ) is to sputtered onto the substrate assembly. In one embodiment, the germanium selenide (Ge x Se 1-x ) target is a germanium selenide (Ge 30 Se 70 ) target. 
         [0039]    The process proceeds to select  330  a deposition rate for the metal. In one embodiment, the process selects a relatively constant deposition rate for the metal. In another embodiment, the process selects a variable deposition rate for the metal that can be used to vary a doping profile of the metal in the resulting metal-doped germanium selenide (Ge x Se 1-x ) layer. The deposition rate for the metal is approximately related to the removal rate of material from the metal target. In turn, the removal rate of the material from the metal target is approximately related to the sputter power applied to the metal target. This allows sputter power to control the deposition rate for the metal. It will be understood by one of ordinary skill in the art, however, that the deposition rate versus sputter power varies according to the configuration of the deposition system and the material that is sputtered. 
         [0040]    The process selects  340  a deposition rate for germanium selenide (Ge x Se 1-x ). In one embodiment, the deposition rate for germanium selenide (Ge x Se 1-x ) is relatively constant. In another embodiment, the deposition rate for germanium selenide (Ge x Se 1-x ) can vary and can be used to vary the doping profile of the metal in the metal-doped germanium selenide (Ge x Se 1-x ) layer. The deposition rate for the germanium selenide (Ge x Se 1-x ) is approximately related to the removal rate of material from the germanium selenide (Ge x Se 1-x ) target and, in turn, approximately related to the sputter power applied to the germanium selenide (Ge x Se 1-x ) target. This allows the process to select  340  the deposition rate by a selection of sputter power. 
         [0041]    The relative deposition rates between the metal and the germanium selenide (Ge x Se 1-x ) determine the amount of doping of the metal to the germanium selenide. For example, where a silver (Ag) deposition rate is about 17.8% of the total film deposition, the resulting film is doped at about 32 atomic percent of silver (Ag). In another example, where the silver (Ag) deposition rate is about 9% to about 56% of the total film deposition, the resulting film is doped at about 18.3% to about 69.6% silver (Ag) by atomic percentage. 
         [0042]    The process sputters  350  the metal and the germanium selenide (Ge x Se 1-x ) from their respective targets. In one embodiment, the process sputters  350  metal in accordance with a direct current (DC) sputter process, and the process sputters  350  germanium selenide (Ge x Se 1-x ) in accordance with a pulse DC sputter process. In a pulse DC sputter process, a positive voltage is periodically applied for a short period of time to the target to reduce or eliminate charge build up in the target. It will be understood by one of ordinary skill in the art that the sputter power used to generate a particular deposition rate will vary depending on the configuration of the deposition system. For the purposes of illustration only, one embodiment of the invention uses 30 Watts (W) of DC sputter to sputter silver (Ag) and sputters germanium selenide (Ge 30 Se 70 ) with 575 W of pulse DC sputter to produce a doped film with about 32% silver (Ag) by atomic weight. 
         [0043]      FIG. 4  illustrates a process  400  of configuring a deposition system to co-sputter metal and germanium selenide (Ge x Se 1-x ) glass. In one embodiment, the metal is silver (Ag), copper (Cu), or zinc (Zn) and the germanium selenide (Ge x Se 1-x ) is germanium selenide (Ge 30 Se 70 ). It will be understood that in other embodiments, a different chalcogenide glass substitutes for the germanium selenide (Ge x Se 1-x ) glass. For example, germanium sulfide (Ge x Se 1-x ) or arsenic selenide (As x Se 1-x ) can also be used. The process selects  410  a desired ratio for the metal to the germanium selenide in the active layer. The ratio can be relatively constant to form a relatively uniformly doped layer of metal-doped chalcogenide glass, or can be variable to allow a metal to dope the chalcogenide glass with a selected doping profile. 
         [0044]    The process proceeds to calculate  420  a deposition rate for the metal and a deposition rate for the germanium selenide to produce the desired doping of the metal in the germanium selenide (Ge x Se 1-x ). A broad variety of methods can be used to calculate  420  the deposition rates. In one embodiment, the process calculates  420  the deposition rates by, for example, referring to a lookup table containing pre-calculated deposition rates for particular doping levels. In another embodiment, the process calculates  420  the deposition rates in real time, and scales calculations as necessary to maintain deposition rates within the capabilities of the applicable deposition system. 
         [0045]    The process proceeds to relate  430  the specified deposition rates to sputter power levels. Where the deposition rates of the various materials sputtered versus sputter power for the configuration of the deposition system is available, the process can retrieve the sputter power to be used by reference to, for example, a database. The sputter power levels for a given configuration are related to the deposition rates and can be used to control the doping profile of the deposited film. In one embodiment, the process collects and maintains in a database, the configuration of the deposition system and data of deposition rates versus sputter power for a collection of materials deposited for later retrieval. 
         [0046]    The process proceeds to con figure 440  a sputtering tool for co-sputtering. In one embodiment, the process configures the tool for DC sputter of the metal target at the specified power level for the desired deposition rate. In one embodiment, the process configures the tool for pulsed DC sputtering of the germanium selenide (Ge x Se 1-x ) target at the specified power level for the desired deposition rate. 
         [0047]    While illustrated primarily in the context of co-sputtering a metal and germanium selenide (Ge x Se 1-x ) to produce a ternary mixture of a metal-doped chalcogenide glass, it will be understood that the co-sputtering techniques described herein to fabricate a memory cell body are applicable to other combinations suitable for forming metal-doped chalcogenide glass elements. 
         [0048]    One combination includes co-sputtering the metal, germanium (Ge), and selenium (Se) from three separate targets. The metal can correspond to a metal that diffuses relatively quickly into the glass, for example, silver (Ag), copper (Cu), and zinc (Zn). Another combination includes co-sputtering a metal selenide, such as Ag y1 Se 1-y1 , Cu y2 Se 1-y2 , or Zn y3 Se 1-y3  with germanium (Ge) from two separate targets. Another combination includes co-sputtering a germanium metal mixture, such as Ge z1 Ag 1-z1 , Cu z2 Ge 1-z2 , or Zn z3 Ge 1-z3 , and selenium (Se) from two separate targets. In the illustrated equations, the values of y1, y2, y3, z1, z2, and z3 should be maintained such that the deposited material is in an amorphous state. Advantageously, these other combinations can provide the metal-doping of a chalcogenide glass with a relatively high degree of depth-profile uniformity and control. 
         [0049]    The chalcogenide glass can also include germanium sulfide (Ge x S 1-x ) or arsenic selenide (As x Se 1-x ). Metal-doped germanium sulfide can be formed by co-sputtering metal and germanium sulfide from two separate targets. Another combination includes sputtering a metal sulfide and germanium from two separate targets. Metal-doped arsenic selenide can likewise be formed by co-sputtering metal and arsenic selenide from two separate targets. In another combination, a metal arsenide and selenium are sputtered from two separate targets. 
         [0050]    Various embodiments of the present invention have been described above. Although this invention has been described with reference to these specific embodiments, the descriptions are intended to be illustrative of the invention and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined in the appended claims.