Abstract:
Chalcogenide containing semiconductor devices may be formed with a gradient film between a chalcogenide film and another film. The gradient film may have its chalcogenide concentration decrease as it extends away from the chalcogenide film, while the concentration of the other film material increases across the thickness of the gradient film moving away from the chalcogenide film.

Description:
BACKGROUND 
       [0001]    This relates generally to chalcogenide containing semiconductors including phase change memories and ovonic threshold switches. 
         [0002]    Chalcogenide containing semiconductors include a chalcogenide layer that interfaces with other layers. Traditionally, the adhesion between chalcogenide and other adjacent layers is problematic. Chalcogenides adhere well to pure metals, but pure metals result in high chalcogenide film contamination, causing unwanted doping and chalcogenide material failures. Thus, metal composites, such as metal nitrides, are generally utilized in adjacent layers. 
         [0003]    A phase change memory includes chalcogenide that changes between more amorphous and less amorphous (i.e. more crystalline) phases to store information. An ovonic threshold switch may be used as a select device in a phase change memory. The chalcogenide in the switch generally remains in an amorphous phase. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is an enlarged, simplified cross-sectional view of a chalcogenide containing semiconductor device in accordance with one embodiment; 
           [0005]      FIG. 2  is a graph of composition versus cross-section across the device shown in  FIG. 1  in accordance with one embodiment; and 
           [0006]      FIG. 3  is an enlarged, simplified cross-sectional view of still another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    Referring to  FIG. 1 , a chalcogenide containing semiconductor device  10  may include a first compositional layer  12 , a second compositional layer  16 , and an intermediate gradient film made up of a mixture of the compositions of the films  12  and  16 . At least one of the films  12  or  16  includes a chalcogenide. The chalcogenide, for example, may be germanium, antimony, tellurium (GST) and the device  10  may be a phase change memory. As another example, the chalcogenide may be a chalcogenide generally utilized in connection with ovonic threshold switches, which normally remains in the amorphous phase. Thus, the chalcogenide film may be the film  12 , which is on top of another film  16 , or the chalcogenide film may be below another film, in which case it would be represented by the film  16 . 
         [0008]    For example, when the chalcogenide film  16  is below another film, the other film  12  may be an electrode, such as a metal nitride film. Similarly, when the chalcogenide film  12  is above another film  16 , again, the other film  16  may be an electrode or heating layer that provides Joule heating of the chalcogenide to change its phase. 
         [0009]    In addition, the chalcogenide film  12  or  16  may interface with insulator films  12  or  16 , such as silicon dioxide or silicon nitride. These insulator films may be above or below the chalcogenide film in some instances. In other cases, the chalcogenide film  12  or  16  may interface with a pure metal interfacial film, such as a titanium film, to improve adhesion between the chalcogenide film and other films, such as metal nitrides, insulators, and substrates, etc. 
         [0010]    As indicated in  FIG. 2 , the adhesion between the films  12  and  16  may be improved by providing an intervening gradient film  14  whose composition varies from entirely composition B, adjacent the composition B film  16 , to entirely composition A at the interface between the film  14  and the composition A film  12 . In one embodiment, the transition in atomic composition may be linear with fifty atomic percent at each at the exact center of the gradient film. Other non-linear gradients may be utilized as well. In addition, stepwise gradients, curved gradients, and gradients that are not centered or regular across the film  14  may be utilized, so that the fifty percent atomic composition point may be non-centered in the gradient film  14 , to mention a few examples. 
         [0011]    In order to form the structure  10 , shown in  FIG. 1 , a chalcogenide bulk film may be deposited by standard argon sputtering, for example. Then a deposition step may create the gradient film  14 . For example, there may be an introduction and increase of nitrogen gas content in the sputter chamber up to a typical value for reactive sputtering of metal to form metal nitride, such as titanium nitride, as the film  12 . Then there is a decrease in chalcogenide sputtering power down to zero as the gradient film  14  grows. At the same time, there is an increase of titanium sputtering power from zero to the typical value for titanium nitride deposition, as well as an adjustment of argon composition from the chalcogenide only layer to the titanium nitride only layer. This creates a film  14  with a continuous gradient of composition from chalcogenide to titanium nitride. Then, conventional titanium nitride sputtering may proceed. 
         [0012]    Similarly, a chalcogenide may be deposited on a dielectric film  16 , such as silicon dioxide or silicon nitride. For example, an incoming wafer may have mainly silicon oxide and silicon nitride exposed in a titanium silicon nitride heater material. Prior to chalcogenide bulk deposition, a thin film  14  with a gradient is deposited, including physical vapor deposition sputtering of silicon. The silicon may ensure good adhesion on all exposed materials, including silicon dioxide, silicon nitride, and titanium silicon nitride. After a very few nanometers of deposition of the gradient film  14 , the silicon sputtering source may be reduced down to zero to stop the silicon deposition. In the meantime, the chalcogenide sputtering power switched on and increased up to its typical value for chalcogenide sputtering. Thereafter, only chalcogenide is deposited to form the film  12 . Then a standard capping layer is performed or, again, with a gradient, as described above. 
         [0013]    Thus, in some embodiments, as indicated in  FIG. 3 , there may be multiple gradient films utilized. For example, where the composition B film  26  is chalcogenide, it may have the first gradient film  24  above it and a second gradient film  28  below it. Depending on the composition A film  22  and the composition C film  30 , the first and second gradient films may be different films. For example, the composition A film  22  may be titanium nitride, so that the first gradient film  24  has a gradient between chalcogenide and titanium nitride, while the composition C film  30  may be silicon or silicon dioxide, silicon nitride, or titanium silicon nitride and may include a gradient film in which the concentrations of those materials decrease across the gradient film and a concentration of chalcogenide increases across the gradient film. 
         [0014]    In some embodiments, the gradient film is quite thin, less than five nanometers, and, in some embodiments, it is less than ten Angstroms. 
         [0015]    Programming to alter the state or phase of the material may be accomplished by applying voltage potentials to the address lines, thereby generating a voltage potential across a memory element including a phase change material. When the voltage potential is greater than the threshold voltages of any select device and memory element, then an electrical current may flow through the phase change material in response to the applied voltage potentials, and may result in heating of the phase change material. 
         [0016]    This heating may alter the memory state or phase of the material, in one embodiment. Altering the phase or state of the material may alter the electrical characteristic of memory material, e.g., the resistance or threshold voltage of the material may be altered by altering the phase of the memory material. Memory material may also be referred to as a programmable resistance material. 
         [0017]    In the “reset” state, memory material may be in an amorphous or semi-amorphous state and in the “set” state, memory material may be in a crystalline or semi-crystalline state. The resistance of memory material in the amorphous or semi-amorphous state may be greater than the resistance of memory material in the crystalline or semi-crystalline state. It is to be appreciated that the association of reset and set with amorphous and crystalline states, respectively, is a convention and that at least an opposite convention may be adopted. 
         [0018]    Using electrical current, memory material may be heated to a relatively higher temperature to melt and then quenched to vitrify and “reset” memory material in an amorphous state (e.g., program memory material to a logic “0” value). Heating the volume of memory material to a relatively lower crystallization temperature may crystallize or devitrify memory material and “set” memory material (e.g., program memory material to a logic “1” value). Various resistances of memory material may be achieved to store information by varying the amount of current flow and duration through the volume of memory material. 
         [0019]    The ovonic threshold switch is either on or off depending on the amount of voltage potential applied across the switch and, more particularly, whether the current through the switch exceeds its threshold current or voltage, which then triggers the device into an on state. The off state may be substantially electrically non-conductive and the on state may be a substantially conductive state with less resistance than the off state. 
         [0020]    In the on state, the voltage across the switch, in one embodiment, is equal to its holding voltage V hold +IR on , where R on  is the dynamic resistance from the extrapolated X axis intercept V hold . For example, an ovonic threshold switch may have a threshold voltage V th  and, if a voltage potential less than the threshold voltage of the switch is applied across the switch, then the switch may remain off or in a relatively high resistance state so that little or no electrical current passes. 
         [0021]    Alternatively, if a voltage potential greater than the threshold voltage of the select device is applied across the device, then the device may turn on, i.e., operate in a relatively low resistance state so that significant electrical current passes through the switch. In other words, one or more series connected switches may be in a substantially electrically non-conductive state at less than a predetermined voltage, e.g., the threshold voltage as applied across a switch. The switch may be in a substantially conductive state if greater than a predetermined voltage is applied across the switch. 
         [0022]    In one embodiment, each switch may comprise a switch material that is a chalcogenide alloy. The switch material may be a material in a substantial amorphous state positioned between two electrodes that may be repeatedly and reversibly switched between a higher resistance off state that is generally greater than about 1 megaOhms and a relatively lower resistance on state that is generally less than about 1000 Ohms in series with the holding voltage by the application of electrical current or potential. 
         [0023]    Each switch is a two-terminal device that has an IV curve similar to that of a phase change memory element that is in an amorphous state. However, unlike a phase change memory element, the ovonic threshold switch does not change phase. That is, the switching material of the ovonic threshold switch is not a phase programmable material and, as a result, the switch may not be a memory device capable of storing information. For example, the switching material may remain permanently amorphous and the IV characteristics may remain the same throughout the operating life. 
         [0024]    In the low voltage, low electric field mode, where the voltage applied across the switch is less than the threshold voltage V th , the switch may be off or non-conducting and exhibits a relatively high resistance. The switch may remain in the off state until a sufficient voltage, namely, the threshold voltage, is applied or a sufficient current is applied, namely, the threshold current, that switches the device to a conductive relatively low resistance on state. After a voltage potential of greater than about the threshold voltage is applied across the device, the voltage potential across the device may drop or snapback to a holding voltage V hold . Snapback may refer to the voltage difference between the threshold voltage and the holding voltage of the switch. 
         [0025]    In the on state, the voltage potential across the switch may remain close to the holding voltage as current passing through the switch is increased. The switch may remain on until the current through the switch drops below a holding current. Below this value, the switch may turn off and return to a relatively high resistance, non-conductive off state, until the threshold voltage and current are again exceeded. 
         [0026]    References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application. 
         [0027]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.