Abstract:
A method of forming a diode structure for a phase-change data storage array, having multiple thin film layers adapted to form a plurality of data storage cell diodes is disclosed. The method includes depositing a first diode layer of CuInSe material on a substrate and depositing a second diode layer of phase-change material on the first diode layer.

Description:
[0001]     This application is a divisional of copending U.S. patent application Ser. No. 10/654,189, filed Sep. 3, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to ultra-high density data storage devices. More particularly, the present invention involves ultra-high density data storage devices having diode memory cells using materials containing copper, indium, and selenium (CIS), and methods of fabricating the diode memory cells.  
       BACKGROUND OF THE INVENTION  
       [0003]     Electronic devices, such as palm computers, digital cameras and cellular telephones, are becoming more compact and miniature, even as they incorporate more sophisticated data processing and storage circuitry. Moreover, types of digital communication other than text are becoming much more common, such as video, audio and graphics, requiring massive amounts of data to convey the complex information inherent therein. These developments have created an enormous demand for new storage technologies that are capable of handling more complex data at a lower cost and in a much more compact package. Efforts are now underway to enable the storage of data on a scale of ten nanometers (100 angstroms) up to hundreds of nanometers, referred to herein as “ultra-high density data storage.” 
         [0004]     One method of storing data at ultra-high densities involves utilizing a directed energy beam. As used herein a “directed energy beam” means a beam of particles, such as electrons, or a beam of photons or other electromagnetic energy, to heat the medium so that it changes states. As used herein, “state” is defined broadly to include any type of physical change of a material, whether from one form to another, such as crystalline to amorphous, or from one structure or phase to another, such as different crystalline structures. As used herein, the term “phase change” means a change between different states in a material.  
         [0005]     A state change may be accomplished by changing a material from crystalline to amorphous, or the reverse, by the application of an electron or light beam. To change from the amorphous to crystalline state, beam power density is increased so as to locally heat the medium to a crystallization temperature. The beam is left on long enough to allow the medium to anneal into its crystalline state. To change from crystalline to amorphous state, the beam power density is increased to a level high enough to locally melt the medium and then rapidly decreased so as to allow the medium to cool before it can reanneal. To read from the storage medium, a lower-energy beam is directed to the storage area to cause activity, such as current flow representative of the state of the storage area.  
         [0006]     To effectively sense contrasts in states or phases of phase-change materials, a diode may be formed having a junction for sensing carrier flow in response to an electron or light beam focused on a data storage memory cell in the phase-change layer. Such diode junctions are utilized for carrier detection in photovoltaic devices, in which light beams impact the diode, and in cathodovoltaic devices, in which electron beams are directed to the diode. Photovoltaic devices include phototransistor devices and photodiode devices. Cathodovoltaic devices include cathodotransistor devices and cathododiode devices. In addition, diode junctions may be utilized for carrier flow detection in photoluminescent and cathodoluminescent devices. Reference is made to copending patent application Ser. No. 10/286,010 [HP 20020-6667] filed on Oct. 31, 2002 for a further description of the structure and function of diode junctions in these devices.  
         [0007]     Diode junction layers need to be composed of materials having electrical properties suitable for generating a desired carrier flow across the diode junction. As used herein, the term “carrier flow” refers to either electron current or the flow of holes, depending on whether the materials are n-type or p-type. As used herein, the term “materials” includes all kinds and types of compounds, alloys and other combinations of elements. Various types of junctions may be formed in the context of the above data storage devices, such as heterojunctions, homojunctions, and Schottky junctions, in order to achieve the desired detection results.  
       SUMMARY OF THE INVENTION  
       [0008]     One embodiment comprises a method of forming a diode structure for a phase-change data storage array, having multiple thin film layers adapted to form a plurality of data storage cell diodes, wherein the method comprises depositing a first diode layer of CIS material on a substrate; and depositing a second diode layer of phase-change material on the first diode layer.  
         [0009]     Other aspects and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the accompanying drawings, illustrates by way of example the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a partial side view of an exemplary data storage system;  
         [0011]      FIG. 2  is a plan view of the storage system shown in  FIG. 1 ;  
         [0012]      FIG. 3  is a schematic view of a portion of the storage system shown in  FIG. 1 ;  
         [0013]      FIG. 4  is a schematic side view of the diode structure according to one embodiment of the present invention;  
         [0014]      FIG. 5  is a flow chart describing a method for fabricating the diode structure shown in  FIG. 4 ;  
         [0015]      FIG. 6  is a bar graph showing EBIC diode characteristics according to four diode embodiments of the present invention; and  
         [0016]      FIG. 7  is a graph showing the current-voltage characteristics of a diode according to one of the embodiments of  FIG. 6 . 
     
    
       [0017]     The same numerals in the Figures are assigned to similar elements in all the figures. Embodiments of the invention are discussed below with reference to the Figures. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the invention extends beyond these limited embodiments.  
       DETAILED DESCRIPTION  
       [0018]     Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.  
         [0019]      FIG. 1  shows an exemplary ultra-high density data storage system  100  having electron emitters  102  and  104  and a phase-change data storage medium  106 . Electron emitter  102  and  104  are mounted above the storage medium  106 , that has a number of storage areas, such as  108 , that are impacted by electron beams from the emitters. Micromovers, based on micro-electromechanical systems (MEMS) technology, cause relative movement between the phase-change medium  106  and the electron emitters  102 . Micromover  110  is connected to the storage medium  106  and moves it relative to the emitters  102 ,  104 , so that each emitter can impact a number of different data storage areas.  
         [0020]      FIG. 2  is a top view of the cross section A-A in  FIG. 1 , showing the micro-mover  110  fabricated to scan the medium  106  in the X and Y directions. The storage medium  106  is supported two sets of thin-walled micro-fabricated flexible beams or springs  112  and  114  which flex to allow the medium  106  to move in the X direction with respect to a supporting frame  122 . A second set of springs  116  and  118  are connected between the supporting frame  122  and the outer casing  120  and flex to allow the medium  106  and frame  122  to move in the Y direction with respect to the casing  120 . The field emitters scan over the medium, or the medium scans over the field emitters in the X-Y directions by electrostatic, electromagnetic or piezoelectric means known in the art.  
         [0021]      FIG. 3  shows an exemplary diode structure  140  comprising the phase-change layer  106  described above and a second diode layer  142  forming a diode junction  144  at the interface of the two layers. Typically, phase-change layer  106  and second diode layer  142  have different electrical characteristics, to encourage the movement of carriers across the junction. For example, phase change layer  106  may be n-type and second diode layer  142  may be p-type. Doping may be used to change or enhance the electrical characteristics of each layer. An external circuit  146  is connected to the diode to impress a voltage across the junction so as to create a bias, either in a reverse direction or in a forward direction, depending on the needs of the circuit. An output  148  is generated by circuit  146  and is representative of the magnitude of carrier flow across junction  144 .  
         [0022]     Reading or detecting is carried out by directing a narrow beam of electrons  105  onto the surface of phase change layer  106  at data storage area  108 . The incident electrons excite electron-hole pairs  107  near the surface of the storage area  108 . The diode  140  is reversed-biased by external circuit  146  so that the minority carriers that are generated by the incident electrons drift toward the diode junction  144 . Minority carriers that do not recombine with majority carriers before reaching the junction are swept across the junction, causing a current to flow in the external biasing circuit  146 .  
         [0023]     Writing onto diode  140  is accomplished by increasing the power density of the electron beam  105  enough to locally alter some property of the diode at storage area  108 , such as collection efficiency of minority carriers. In  FIG. 4 , a schematic side view of a diode configuration  200  in accordance with one embodiment of the present invention is shown. A phase-change layer  202  is composed of indium and selenium in an indium selenide (InSe) compound which can be reversibly changed from an amorphous to a crystalline state by heating and cooling the InSe compound at suitable rates. This state change is useful for data storage, since the amorphous state is less conductive that the crystalline state. The InSe compound may be any InSe-type material. As used herein the terms “InSe compound,” “InSe-type material” and “InSe layer” mean any compound of InSe, In 2 Se 3  or a material in the group having the formula In x Se 1-x .  
         [0024]     When the InSe layer  202  is changed from the crystalline state to the amorphous state at the storage area  212 , the electrical properties of the InSe layer significantly change. Accordingly, the number of carriers swept across the diode junction will be significantly different in the amorphous state than when InSe is in a crystalline state.  
         [0025]     In  FIG. 4 , the second diode layer  204  is disposed below the phase-change layer  202  to form a diode junction  206 . Since polycrystalline InSe-type materials are naturally n-type, the second diode layer  204  may be a p-type semiconductor layer or capable of being p-doped. The second diode layer  204  may be made of a material of copper indium selenide (CuInSe), also known as CIS. The terms “CIS,” “CIS compound” or “CIS material” refer to a material having any ratio of copper, indium, and selenium, such as CuInSe 2 , including but not limited to CIS doped with gallium (CIGS). The term “CIGS” refers to CuInSe doped with gallium to form various stoichiometric materials of copper indium gallium selenide including but not limited to (Cu(In, Ga)Se 2 ) and Cu(In 1-x Ga x )Se 2 .  
         [0026]     CIS materials are desirable for memory diode materials for a number of reasons. CIS compounds are similar to InSe-type materials, differing in the additional Cu element and a change in crystalline structure. Although the structures of these two compounds are different, the diode interface between layers of the two compounds is effective. Also, CIS can be readily doped with a p-type dopant which is required to work with InSe. Further, the CIS layer can be formed in the same vacuum system as an InSe layer, enabling the formation of a clean high quality interface between the two layers. This diode structure also reduces processing costs, since the time required for an additional fabrication stage is eliminated. In addition, CIS and CIGS materials tend to have relatively high melting temperatures, compared to InSe-type materials, so heat applied to the phase-change layer should not affect the CIS or CIGS layer. Finally, CIS materials tend to have good diode junction characteristics.  
         [0027]     The diode configuration  200  shown in  FIG. 4  may be used as a detection device in various data storage detection devices, such as photovoltaic devices, cathodovoltaic devices, photoluminescent devices and cathodoluminescent devices, all described above.  
         [0028]      FIG. 5  discloses a method of fabrication  300  of the diode structure shown in  FIG. 4 . Fabrication of the diode structure  200  starts at step  310  with the selection of a substrate layer  210  from silicon or various silicon oxide materials. In the present embodiment, substrate  210  is preferably of a material that can be doped with sodium, which promotes the growth of the subsequent layers. Such material includes, but is not limited to, silicon, silicon oxide, soda lime glass, and other semiconductor materials. At step  312 , an electrical potential field layer, or back contact,  208  is fabricated on the surface of substrate  210 . Field layer  208  is fabricated from molybdenum (Mo) using a sputtering technique, known to those skilled in the art, to a thickness of 0.5-2.0 microns, with 1 micron being used in this embodiment. Other suitable conductive materials are indium, titanium nitride, platinum, gold, zinc oxide, indium tin oxide and/or nickel.  
         [0029]     Next, at step  314 , second diode layer  204  is fabricated on field layer  208 . Other suitable phase-change materials may be used. In this embodiment, diode layer  204  is preferably fabricated with a CIS compound and is doped with a p-type dopant. Second diode layer  204  has a thickness range of about 1 to 6 microns with 3.5 microns being used in this embodiment, and is applied by elemental evaporation in a vacuum. Other methods of fabrication can include sputtering or electrodeposition using an electrolyte of K 2 SO 4 , CuSO 4 , In 2 S(SO 4 ) 3 , and/or SeO 2 . As previously mentioned, second diode layer  204  may also be doped with gallium to form CuInGaSe 2  or other CIGS compounds.  
         [0030]     At step  316 , after application of the second diode layer  204 , the first diode layer  202  is fabricated on layer  208 . In the present embodiment, first diode layer  202  is fabricated with In 2 Se 3 , being a natural n-type material. First layer  202  has a thickness range of 50-200 nm, with 100 nm being used in this embodiment. First layer  202  may be applied by elemental evaporation in vacuum, which may be the same vacuum used to apply the second layer  204 . In an alternative embodiment, fabrication of first layer  202  can be performed with electrodeposition using an electrolyte.  
         [0031]     In an embodiment in which layer  204  is a CIGS material, layer  204  may be grown using a three-stage co-evaporation process. First, an (InGa) 2 Se 3  layer is deposited at 400° C., followed by evaporation of Cu and Se at 550° C. to make the film slightly Cu-rich. The composition is restored to slightly Cu-poor by the addition of (InGa) 2 Se 3  again. Next, the InSe layer is thermally evaporated from elemental sources onto a heated surface. InSe is a natural n-type material, so no doping is required. Further, Cu can diffuse from the second diode layer of CuInSe into the InSe top layer to enhance the n-type doping.  
         [0032]     At step  318 , electric grid contacts  214  are fabricated on the surface of layer  202  because the first layer of InSe  202  has a low electrical conductivity. Contacts  214  are made of a conductive material the same as or similar to field layer  208 . For example, grid contacts  214  can be made of Mo at a thickness ranging from 0.5 to 5 microns, with 1 micron used in this embodiment. Alternately, the grids may be made of other suitable materials, such as Au or In. The material may be sputter-deposited or made by any other suitable fabrication techniques known to those skilled in the art. Grid contacts  214  and field layer  208  may be the connection points for applying a voltage source circuit to the diode  200  during a read operation.  
         [0033]     Testing has been performed on sample diodes as embodied in the present invention. The tests have included measuring the I-V characteristics of the diodes, performing quantum efficiency (optical), and performing electron beam induced current (EBIC) measurements, which simulate a “read” mode for data storage.  
         [0034]      FIG. 6  contains bar graphs showing EBIC gain as a function of emitter beam voltage for diodes having In 2 Se 3  as the top layer with In or Au contacts and Si or CIGS as a bottom layer. The EBIC gain is diode current output divided by electron beam current. In the first two samples, In 2 Se 3  was applied after the substrate was exposed to air. The second two samples (in situ) were fabricated by depositing both layers in a vacuum without exposure to air during the process.  
         [0035]     As shown in the  FIG. 6  graph, the EBIC variations are not linear with variations in emitter beam energy, because more energetic electron beams result in both more electron-hole pairs and deeper penetration. Although 5 kV beams may be used, beams of 1 kV or less may be utilized in most applications. Beam current may vary from 0.5 nA to as high as 200 nA, with about 20 nA being used in some applications. As seen from the graph, EBIC gain is considerably better with CIGS rather than Si as a bottom diode layer. Also, the gain is substantially higher if the diode is fabricated in a vacuum.  
         [0036]      FIG. 7  shows the current-voltage characteristics for three samples of the first diode listed in  FIG. 6 . As shown in  FIG. 7 , the applied voltage causes essentially zero current until it reaches about 200 mV, after which there is a sharp increase in current. The open circuit voltage is 5 mV and the short circuit current density is about 2 to 3 mA cm 2 .  
         [0037]     It is to be understood that the above-referenced arrangements are illustrative of the application for the principles of the present invention. Numerous modifications and alternative arrangements can be devised without departing from the spirit and scope of the present invention while the present invention has been shown in the drawings and described above in connection with the exemplary embodiments(s) of the invention. It will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth in the claims.