Abstract:
A phase change device includes a native oxide grown on the surface of a first phase change alloy layer. The native oxide is punched through during the first electrical pulse applied between the device electrodes. An aperture created in the native oxide limit a region of localized heating during the device programming. A method for the phase change device fabrication includes a native oxide formation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of, and incorporates herein by reference in its entirety, U.S. Provisional Patent Application No. 61/215,452 which was filed on May 6, 2010. 
     
    
     REFERENCE TO A SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
       [0002]    Not Applicable. 
       REFERENCE REGARDING FEDERAL SPONSORSHIP 
       [0003]    Not Applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0004]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0005]    1. Field of the Invention 
         [0006]    The invention relates to phase-change devices and, in particular, to their architecture and processes for manufacturing. 
         [0007]    2. Description of Related Art 
         [0008]    The electric resistance of a phase change device varies in wide range under programming pulses. Resistance of phase-change devices can be read and programmed very quickly and do not require power to maintain their value. Therefore, phase change devices are very useful for non-volatile memories. 
         [0009]    Other electrical properties of a phase change device (such as threshold switching voltage or capacitance) can be also altered by programming current pulses and values of these properties do not significantly change after programming. Therefore, phase change devices are very useful for reconfigurable electronics. 
         [0010]    The high programming current is the main problem of phase change devices. It is possible to decrease the programming current by
       a) Reduction of an active device volume;   b) Diminish heat losses during a device programming;   c) Selection of a robust phase change alloy with small thermal conductivity and low melting temperature.       
 
         [0014]    Several patents and publications address the problem of high programming current. The only closest prior art documents are described here. 
         [0015]    Breakdown device with insulator layer between two phase-change alloy layers is proposed in US Patent Application 20070200202 “Phase change memory structure having an electrically formed constriction” by Nowak and Lu. This device has small programming current but has pure yield (because phase change alloy sometimes not fill the electrically formed aperture) and relatively small endurance and programming current stability (because strong thermal mismatch of insulator and phase change alloy). 
         [0016]    Double phase change alloy layer device is proposed in US Patent Application 20080186762 “Phase-change memory element”. This device has small programming current that is very sensitive to ill-controlled slope of pore between two phase change alloy layers and, as the result, different devices have different programming currents, hence it is difficult to create an apparatus that consist of several such devices. 
         [0017]    The phase change devices should have small cost and good performance for all applications of these devices. 
         [0018]    What is needed in the art is a phase change device with low-energy programming, high endurance, stability and retention and a simple method for such devices manufacturing. 
       SUMMARY OF THE INVENTION 
       [0019]    Broadly speaking, the embodiments of the present invention fill industry needs by providing robust and low-energy consuming phase change devices methods for their manufacturing. 
         [0020]    New constructions of phase change devices are described in some embodiments of the present invention. A phase change device has native oxide that serves as breakdown layer between two phase change alloy(s) layers in one or more embodiments of the present invention. Methods of the phase change device with native oxide manufacturing are described in some embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” embodiment in this disclosure are not necessarily to the same embodiment, and they mean at least one. 
           [0022]      FIG. 1A  shows a generic phase change device with native oxide before the first pulse application. 
           [0023]      FIG. 1B  shows a generic phase change device with native oxide after the first pulse application. 
           [0024]      FIG. 1C  shows a generic phase change device with native oxide after a programming pulse application. 
           [0025]      FIG. 2  illustrates a process of a phase change device fabrication flow chart. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Several exemplary embodiments of the invention will now be described in details with reference to the accompanying drawings. 
         [0027]    For the sake of simplicity only simplest phase change device and process of its fabrication are described in details below, although one or more embodiments of the invention are applicable for other types of phase change device and manufacturing processes. 
         [0028]    A phase change device  100  ( FIG. 1A ) has a first electrode  110 , a first phase change alloy layer  120  formed on at least a portion of an upper surface of the first electrode  110 , a native oxide  130  of the first phase change alloy  120  formed on an upper surface of the first phase change alloy  120 , a second phase change alloy layer  140  formed on an upper surface of the native oxide  130 , and a second electrode  150  formed on at least a portion of an upper surface of the at least second phase change alloy layer  140 . 
         [0029]    The electrodes  110  and  120  can be made from metals, doped or degenerate semiconductors, superconductors. Electrodes  110  and  120  can be made from the same material or from the different materials, e.g., from TiSiN or carbon. 
         [0030]    The layers  120  and  140  can be made from the same or from different phase change alloys based on a chalcogene such as Te or Se or pnictide such as Sb or As, e.g. from Ge—Sb—Te and from In—Sb—Te. 
         [0031]    At least one of the first  110  and second  150  electrodes has electrical conductivity equal or large than an electrical conductivity of at least one of the first  120  and second  140  phase change alloys. At least one of the first  110  and second  150  electrodes has thermal conductivity equal to or larger than a thermal conductivity of at least one of the first  120  and second  140  phase change alloys. 
         [0032]    The phase change alloy  120  (or  140 ) has low viscosity above it glass transition temperature and can easily fill the aperture forming during the first electrical pulse that breaks the native oxide  130 . In some embodiment the viscosity of the alloy  120  (or  140 ) is below 5 Poise at the melting temperature. 
         [0033]    The native oxide  130  selected from the group consisting germanium oxide, silicon oxide, tellurium oxide, antimony oxide, indium oxide, gallium oxide has thickness below 20 nm, preferably below 3 nm. 
         [0034]    The native oxide  130  has the thermal expansion coefficient close (or the same as) to the thermal expansion coefficient of phase change alloy  120  (or  140 ). The native oxide  130  has a small thermal conductivity close (or the same as) to the thermal expansion coefficient of phase change alloy  120  (or  140 ). The native oxide  130  has a small thermal boundary resistivity with phase change alloy  120  (or  140 ). 
         [0035]    The breakdown voltage for native oxide  130  is smaller than 20V, preferably smaller than 1V. The breakdown current for native oxide  130  is smaller than 1 mA, preferably smaller than 10 uA. The duration of pulse that break the native oxide  130  is shorter than 1 ms, preferably shorter than 10 ns. 
         [0036]    The native oxide  130  blocks electrical current flow between the electrodes  110  and  150  until a breakdown pulse is applied to the electrodes. The breakdown pulse opens an aperture  160  in the native oxide  130  as shown in  FIG. 1B . The aperture  160  size is smaller than 50 nm, preferably smaller than 5 nm. 
         [0037]    Electrical programming of the device  100  by a programming circuit coupled with the phase change device  100  brings a part  170  of at least one of the first and second phase change alloys  120  or/and  140  to a new state. The part  170  shown in  FIG. 1C  is located mostly within the aperture  160  in the native oxide  130 . As the result of the programming a parameter of the device  100  is changed due to alteration of the part  170  of the alloy  120  or/and  140 . The parameter is selected from the group consisting electrical resistance, impedance, capacitance, threshold switching voltage, optical reflectivity. A new value of the parameter can be read by at least one of interface devices coupled with the phase change device  100 . 
         [0038]    Phase change devices compromise at least K electrodes (K&gt;2), at least L phase change alloy layers (L&gt;2), at least M native oxides of phase change alloys formed on the layers&#39; surfaces (1&lt;M≦L), and at least two of phase change alloy layers are electrically connected with at least two electrodes in some embodiments of this invention. 
         [0039]    A flowchart for the device  100  manufacturing is shown in  FIG. 2 . The manufacturing includes standard steps of a semiconductor device process such as a first electrode  110  formation, a deposition of a first phase change alloy layer  120  on at least a portion of an upper surface of the first electrode  110 , a formation a native oxide  130  at an upper surface of the first alloy  120 , a deposition of a second phase change alloy layer  140  on at least a portion of the native oxide  130 , and a formation of second electrode  150  on at least a portion of an upper surface of the at least second phase change alloy layer  140 . 
         [0040]    In order to create the native oxide  130  the chamber for a phase change alloy deposition is filled with oxygen or oxygen-contained gases that contact the upper surface of the first alloy  120  at temperatures between 20 deg. C. and 900 deg. C. in some embodiments. 
         [0041]    Electron or ion beam creates a weak spot in native oxide  130  before the second layer  140  deposition during the device fabrication in some embodiments. 
         [0042]    The formation method for at least one of the first  110  and second  150  electrodes selected from the group consisting thermal evaporation, spin-on a liquid, vacuum sputtering, chemical vapor deposition, atomic layer deposition, electrolyze. The deposition method for at least one of the first  120  and second  140  phase change alloys selected from the group consisting thermal evaporation, spin-on a liquid, vacuum sputtering, chemical vapor deposition, atomic layer deposition, electrolyze, sol-gel deposition. The deposition material for the first and second phase change alloys can be the same or different, and selected from the group consisting a chalcogenide (e.g. tellurium), a pnictide (e.g. antimony), germanium, silicon, indium, gallium. 
         [0043]    At least one of the first and second phase change layers  120  or/and  140  is compromising more than one alloy in some embodiments. The group consisting germanium oxide, silicon oxide, tellurium oxide, antimony oxide, indium oxide, gallium oxide can be formed on the surface of the layer  120  during the native oxide  130  growth in some embodiments. 
         [0044]    The electrodes  110  and  150  are formed from the group consisting metals, doped semiconductors, superconductors in some embodiments. The materials for electrodes  120  and  140  can be the same or different in some embodiments. Because the programming part  170  does not contact electrodes  120  and  140 , the requirements to these electrodes are not so tight as the requirements for electrodes of phase change devices known in prior art. 
         [0045]    The main advantage of some embodiments of this invention is the phase change devices with low programming current that can be manufactured in simple process with high yield. Proposed in some embodiments of this invention devices have high stability of the programming current during device functioning, high endurance and good performance. One skilled in the art can easily produce the phase change devices according to their architecture and manufacturing methods described in embodiments of this invention. 
         [0046]    The foregoing description of an example of the preferred embodiment of the invention and the variations thereon have been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description.