Abstract:
The present invention is a phase change device with a heater and a selector (e.g., diode) separated by a phase-change alloy. The present invention will find applicability in electronic memory devices.

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. 61/278,052 which was filed on Oct. 2, 2009. 
     
    
     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 phase-change memory (PCM). 
         [0007]    2. Description of Related Art 
         [0008]    Electric resistance of a phase change device varies upon a phase transition of a phase-change alloy (PCA) inside the device between two or more states during the device programming. Phase change device can be read and programmed very quickly and do not require power to maintain their state. Therefore, phase change devices are very useful devices for storing data (e.g., as a computer memory device) and for and configurable electronics. A phase change device constructed from PCA and from a selector (e.g., a transistor or a diode or a Ovshinsky threshold switch or a thyristor or a semiconductor controlled rectifier or a field emitter). Phase change memory (PCM) from plurality of the phase change devices has many of the advantages of both volatile memories such as dynamic random access memories and non-volatile memories such as Flash. 
         [0009]    The electric programming pulses produce current that heats up PCA to or above crystallization temperature about 200 deg. C for the low resistance state and to or above melting temperature about 650 deg. C for the high resistive state due to the Joule effect. 
         [0010]    One of the device electrodes (usually bottom) is made from a material with poor electrical and thermal conductivities in order to create high temperatures in PCA. 
         [0011]    This electrode (so called heater) is deposited between a PCA and a selector as shown in  FIG. 1 . During programming the heater reach high temperatures that exceed PCA melting temperature. This high temperature negatively affects properties of selectors that degraded with phase change device cycling. 
         [0012]    The heater is located between selector and PCA as shown in  FIG. 1  according to prior art. 
         [0013]    In order to reduce this degradation D. H. Kang et. al. proposed to use heat dissipation layer in the paper “Novel Heat Dissipating Cell Scheme for Improving a Reset Distribution in a 512M Phase-change Random Access Memory (PRAM)” published in Symposium on VLSI Technology Digest of Technical Papers, 2007; p. 96-97. Few technological steps are required to manufacture such devices, hence their cost increases. 
         [0014]    What is needed in the art is a phase change device with high endurance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows a generic phase-change device known in prior art. 
           [0016]      FIG. 2  shows a generic device according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The present invention explores a new way to construct a phase-change memory (PCM) that increases its endurance. 
         [0018]      FIG. 1  illustrates a generic device  100  known in prior art. A cell  140  and a selector  120  are located between bitline  210  and wordline  150 . A heater  130  located between the cell  140  and the selector  120  provides possibility of the phase-change cell  140  programming into one of the logic states due to Joule heating propagated from the heater  130  to the cell  140 . Because the heater  130  is located in close proximity to the selector  120  high temperatures negatively affect the properties of selector  120 , for example change p−n junctions in the selector  120 . As the result endurance of the device  100  is low that prevents it usage in high-reliability memory applications. 
         [0019]      FIG. 2  illustrates a generic device  200 , according to an embodiment of the invention. An active cell  230  and a selector  220  are located between a first electrode (so called bitline)  210  and a second electrode (so called wordline)  250 . A heater  240  located between the cell  140  and the wordline  250  provides possibility of the cell  230  programming into one of the logic states due to Joule heating propagated from the heater  240  to the cell  230 . Because the heater  240  is located in far away from the selector  120  and due to small thermal conductivity of the cell  230  the selector  220  does not expose to high temperatures during the cell  230  programming. As the result endurance of the device  200  is high. 
         [0020]    In some embodiments at least one electrical conductor is located between the cell  230  and the selector  220 , or between the bitline  210  and the selector  220 , or between the wordline  250  and the heater  240 . 
         [0000]    In some embodiments at least one electrical insulator is used to reduce contact area between the heater  240  and the cell  230 , or to separate one device  200  from another device  200  in a circuit, or to reduce contact area between the cell  230  and the selector  220 . 
         [0021]    Devices  200  can be used in non-volatile and electrical memory or in configurable electronics (e.g. in programmable gate arrays). 
         [0022]    The bitline  210  and the wordline  250  can be made from a good electrical and thermal conductor, e.g. Al or Cu. 
         [0000]    The memory cell  230  consists of at least one phase change alloy with a small thermal conductivity, e.g. Ge-Sb-Te or In-Sb-Te.
 
The heater  240  consists of at least one material with moderate electrical and thermal conductivity, e.g. TaSiN or TiSiAl or TiN.
 
An electrical insulator can be made from SiO2 or Si3N4.
 
         [0023]    The device  200  or/and apparatus containing at least one of the devices  200  can be produced by methods well known in semiconductor manufacturing. 
       Conclusion 
       [0024]    The main advantage of this invention is high endurance of phase change devices due to insulation of a selector to exposure to high temperatures during a cell programming. 
         [0025]    To summarize, various embodiments of a phase-change devices have been described. In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 
         [0026]    Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying main claims.