Patent Document

FIELD OF THE INVENTION 
     The present invention is directed toward computer memory, and more particularly to resistive non-volatile memory, such as a phase change memory device. 
     BACKGROUND OF THE INVENTION 
     There are two major groups in computer memory: non-volatile memory and volatile memory. Constant input of energy in order to retain information is not necessary in non-volatile memory but is required in the volatile memory. Examples of non-volatile memory devices are Read Only Memory (ROM), Flash Electrical Erasable Read Only Memory, Ferroelectric Random Access Memory, Magnetic Random Access Memory (MRAM), and Phase Change Memory (PCM). Examples of volatile memory devices include Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM). The present invention is directed to resistive non-volatile memories, such as phase change memory. In phase change memory, information is stored in materials that can be manipulated into different phases. Each of these phases exhibit different electrical properties which can be used for storing information. The amorphous and crystalline phases are typically two phases used for bit storage (1&#39;s and 0&#39;s) since they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase. 
     Glass chalcogenides are a group of materials commonly utilized as phase change material. This group of materials contain a chalcogen (Periodic Table Group 16/VIA) and a more electropositive element. Selenium (Se) and tellurium (Te) are the two most common semiconductors in the group used to produce a glass chalcogenide when creating a phase change memory cell. An example of this would be Ge2Sb2Te5 (GST), SbTe, and In2Se3. However, some phase change materials do not utilize chalcogen, such as GeSb. Thus, a variety of materials can be used in a phase change material cell as long as they can retain separate amorphous and crystalline states. 
     One problem in PCM and CMOS integration is the drive current required to create a phase change in the phase change material. Small MOSFET devices necessary to provide high density of bits per unit area do not provide enough current to switch the resistive state of phase change materials. A closely packed diode array can provide enough drive current for a resistive phase change in a memory cell but a significant amount of current crosses over to adjacent memory cells (“cross-talk”) in high-density storage devices. Another possibility is the use of a bipolar junction transistor (BJT). However, the integration of CMOS and BJT arrays has proven difficult. Thus, it is desirable to find a method and structure capable of providing the drive current necessary for resistive phase change in a memory cell, minimizing cross-talk between memory cells, and easily integrated into a CMOS circuit. 
     BRIEF SUMMARY OF THE INVENTION 
     One exemplary aspect of the present invention is a memory cell. The memory cell being comprised of, at least in part, a substrate, an insulating region, a base layer, an emitter layer, a plug, a phase change element, and a top electrode. The substrate can be, but is not limited to, bare silicon substrate or silicon substrate with a layer of insulating material deposited on the top surface of the silicon substrate. The insulating region includes insulating material formed above the substrate. The base layer includes a first semiconductor material above the substrate and contained within the insulating region. The emitter layer includes a second semiconductor material above the base layer and contained within the insulating region. Furthermore, the second semiconductor material is substantially polycrystalline silicon. The plug includes conductive material above the emitter layer and contained within the insulating region. The phase change element includes phase change material above the plug and contained within the insulating region. The top electrode layer includes conductive material above the phase change element and contained within the insulating region. 
     Another exemplary aspect of the present invention is a memory array comprising one or more memory cells, at least one of the memory cells comprising a substrate, an insulating region, a base layer, an emitter layer, a plug, a phase change element, and a top electrode. The substrate is comprised of silicon. The insulating region includes insulating material above the substrate. The base layer includes silicon above the substrate and contained within the insulating region, the base layer forming a word-line. The emitter layer includes a semiconductor material above the base layer and contained within the insulating region. Furthermore, the semiconductor material is substantially polycrystalline silicon. The plug includes conductive material above the emitter layer and contained within the insulating region. The phase change element includes phase change material above the plug and contained within the insulating region. The top electrode layer includes conductive material above the phase change element and contained within the insulating region, the top electrode forming a bit-line. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  illustrates a starting substrate. 
         FIG. 2  illustrates shallow trench isolation processes and base layer deposition. 
         FIG. 3  illustrates gate oxide layer deposition. 
         FIG. 4  illustrates polycrystalline silicon layer deposition. 
         FIG. 5  illustrates a patterned etch. 
         FIG. 6  illustrates spacer formation. 
         FIG. 7  illustrates protective nitride formation. 
         FIG. 8  illustrates sacrificial layer formation. 
         FIG. 9  illustrates trench formation. 
         FIG. 10  illustrates emitter layer formation. 
         FIG. 11  illustrates recessing of the emitter layer. 
         FIG. 12  illustrates sacrificial layer removal. 
         FIG. 13  illustrates protective nitride removal. 
         FIG. 14  illustrates silicidation. 
         FIGS. 15A  and B illustrate an insulating layer deposition and trench formation. 
         FIGS. 16A  and B illustrate liner and plug formation. 
         FIGS. 17A  and B illustrate integration of the memory elements and metallization. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is described with reference to embodiments of the invention. Throughout the description of the invention reference is made to  FIGS. 1-17 . When referring to the figures, like structures and elements shown throughout are indicated with like reference numerals. 
     As described in detail below, an aspect of the present invention is a phase change memory (PCM) cell structure with a PNP bipolar junction transistor (BJT) access device. The BJT access device provides the drive current to operate the PCM device with minimal crosstalk between adjacent individual memory cells. Furthermore, memory cells of the present invention may be advantageously formed together with CMOS devices, such as MOSFETs, on the same substrate. Thus, embodiments of the present invention can utilize both the low crosstalk advantages of the BJT memory cell design and the CMOS power consumption advantages of peripheral circuitry on the same integrated circuit. 
       FIG. 1  illustrates an exemplary embodiment of a P-doped silicon substrate  102 . As discussed in detail below, the silicon substrate  102  acts as a collector in the PNP BJT access device. In an alternate embodiment, a blanket P+ implant is performed on the silicon substrate  102 . It is noted that although a PNP BJT structure is described herein, those skilled in the art will understand the claimed invention may also be achieved using a NPN BJT structure with drive polarities and dopant species reversed. Thus, the substrate  102  may alternatively be doped with N material. 
       FIG. 2  illustrates a base layer  204  is formed on the substrate  102  by epitaxial growth or ion-implantation. A shallow trench isolation (STI) process is performed. Those skilled in the art will recognize various etching techniques such as lithographic masks with directional reactive ion etch (RIE) can be utilized to perform the STI process. The trenches are filled with a first insulating layer  202  comprised of an insulating (dielectric) material such as silicon dioxide (SiO2). The base layer  204  will act as the base in the PNP BJT access devices. The base layer  204  may be comprised of a semiconductor material with negative charge carriers, such as N-doped silicon or N+ silicon. The base layer  204  might incorporate Germanium as well. 
     Turning to  FIG. 3 , a CMOS gate oxide layer  302  is deposited above the insulating layer and the base layer. While the CMOS gate oxide layer  302  is deposited on the entire surface of the insulating layer and the base layer the CMOS gate oxide layer  302  will not be a part of the memory cells. The CMOS gate oxide layer  302  will only be a part of the CMOS devices in the integrated circuit. 
     In  FIG. 4 , a CMOS polycrystalline silicon layer  402  is deposited above the CMOS gate oxide layer  302 . Like the CMOS gate oxide layer  302 , the CMOS polycrystalline silicon layer  402  illustrated in  FIG. 4  will not be a part of the memory cells. In one embodiment of the invention, the CMOS polycrystalline silicon layer  402  is comprised of doped polycrystalline silicon. Those skilled in the art will recognize that the doping of the CMOS polycrystalline silicon layer  402  depends on the type of CMOS being created in the integrated circuit. 
     Now turning to  FIG. 5 , a patterned etch on the CMOS polycrystalline silicon layer  402  and the CMOS gate oxide layer  302  is carried out. Those skilled in the art will recognize that the patterned etch may be performed with lithographic masks and RIE to etch the CMOS polycrystalline silicon layer  402  and the CMOS gate oxide layer  302 . Other techniques known in the art, such as using a hydrogen fluoride (HF) etch, may be used during this patterning step. 
     At  FIG. 6 , nitride spacers  606  are deposited around the pillar structures of the polycrystalline silicon layer  402  and the CMOS gate oxide layer  302 . In addition, rapid thermal oxide (RTO) spacers  608  are disposed around the spacers  606 . Those skilled in the art will recognize that the formation of the RTO spacers  608  may be performed utilizing thermal oxidation techniques. The formation of the nitride spacers  606  is also well known to those skilled in the art. After the nitride spacers  606  are formed, implants  610  are implanted only on the base layer of the CMOS devices  602  using a mask  612  to tune the characteristics of the peripheral CMOS devices. The implants  610  in the CMOS devices  602  are doped while the base layer  204  for the BJT devices is protected by the mask  612 . 
     In  FIG. 7 , the mask is removed and a protective nitride layer  702  is deposited above the top surface of the structure. Thus, the nitride layer covers the implants  610 , the RTO spacer  608 , the insulating layer  202 , and the base layer  204 . The protective nitride layer  702  protects the CMOS devices  602  from etching performed during the formation of the BJT access devices. 
     Turning to  FIG. 8 , a sacrificial insulating layer  802  is deposited above the protective nitride layer  702 . In one embodiment of the invention, the sacrificial insulating layer  802  is comprised of silicon dioxide. 
     In  FIG. 9 , trenches  902  are formed in the areas of the BJT access devices. As illustrated, the trenches  902  are etched through the sacrificial insulating layer  802 , through the protective nitride layer  702 , and through the sidewalls of the RTO spacers  608 . The trenches  902  stop on the base layer  204 . Also illustrated, the nitride spacers  606  and the RTO spacer  608  under the sacrificial insulating layer  802  still remain. The remaining nitride spacers  606  and the RTO spacer  608  are used for the BJT access devices. Those skilled in the art will recognize that a lithographic mask can be formed over the surface of the sacrificial insulating layer  802  and a RIE or wet chemical etching can be performed to etch through the aforementioned layers and structures. 
     In  FIG. 10 , an emitter layer  1002  is formed within and completely filling the trenches  902  (see  FIG. 9 ) and above the base layer  204 . The emitter layer  1002  serves as the emitter in the PNP BJT access device. The emitter layer  1002  may be comprised of a variety of semiconductor materials with positive charge (“holes”). An example of this would be P-doped polycrystalline silicon or P+ polycrystalline silicon. This layer could be formed by, for example, low pressure chemical vapor deposition (LPCVD), rapid thermal chemical vapor deposition (RTCVD), or atomic layer chemical vapor deposition (ALCVD). This region could be doped by, for example, ion implantation or by diffusion during the deposition process. In one embodiment of the invention, a very thin insulator layer comprising of SiO2 may be present between the interface of the emitter layer  1002  and the base layer  204 . The purpose of this very thin insulator layer, if used, is to ensure that the emitter layer  1002  will not turn into single-crystalline form, but will remain in polycrystalline form, during the subsequent fabrication steps. A BJT having a polycrystalline emitter has smaller base current than a BJT having a single-crystalline emitter. Thus, a small base current reduces cross-talk among memory cells. 
     In one embodiment of the present invention, after the emitter layer  1002  has been deposited into the trenches, a chemical-mechanical polish (CMP) is performed so the surface of the sacrificial insulating layer  802  and the surface of the emitter layer  1002  are level and parallel to the surface of the substrate  202 . 
     Referring to  FIG. 11 , the emitter layer  1002  is recessed. The emitter layer  1002  is recessed so that the emitter layer  1002  may be contained within the RTO spacers  608 . In one embodiment of the invention, a timed RIE is employed since there is no etch stop preventing the complete etching of the emitter layer  1002 . 
     Turning to  FIG. 12 , the sacrificial insulating layer  802  (see  FIG. 11 ) is removed. An etch stopping on the nitride spacer and the emitter layer is employed. Wet etches such as dilute hydrofluoric acid (DHF) may be utilized for this step of the process. 
     In  FIG. 13 , the protective nitride layer  702  (see  FIG. 12 ) is removed, leaving the nitride spacer  606 , the CMOS devices  602 , and PNP BJT device  1302  exposed. Those skilled in the art may recognize that the removal of the protective nitride layer can be performed by a timed RIE that does not remove the emitter layer  1002  or base layer  204  (polycrystalline silicon). Alternatively, a timed RIE with a lithographic mask over the emitter layer  1002  and polycrystalline silicon may be used during this step. 
       FIG. 14  illustrates silicidation of the exposed base layer  204  and the emitter layer  1002 . Those skilled in the art will recognize that a variety of metals, such as nickel (Ni), tungsten (W), or cobalt (Co), may be employed for silicidation. The metal is deposited above the emitter layer  1002  and above the exposed base layer  204 . A rapid thermal anneal (RTA) is performed to form the bonds between the silicon and metal, and a wet metal etch is performed to remove any un-reacted metal. 
     Now turning to  FIGS. 15A and 15B , a second insulating layer  1502  is deposited over the CMOS and PNP BJT devices.  FIG. 15B  is a cross sectional view of  FIG. 15A  along line  15 B- 15 B. The first insulating layer  204  and the second insulating layer  1502  create an insulating region  1514  containing the base layer  204  and the emitter layer  1002 . In addition, several trenches are formed in the insulating layer  1502 . Those skilled in the art will recognize the trenches may be formed utilizing normal RIE processes, as described above. 
     Referring to  FIG. 15A , a set of CMOS polycrystalline silicon trenches  1504 , a set of CMOS base layer trenches  1506 , and a set of emitter layer trenches  1508  are formed. The trenches are formed such that the bottoms of the trenches are the surfaces of the silicide formed over the structures. 
     Referring to  FIG. 15B , a set of base layer trenches  1510  are formed through the second insulating layer  1502  so that the bottom of the trenches are the surface of the silicide formed in base layer  204 . A set of collector contact trenches  1512  are formed through the insulating region  1514  (the second insulating layer  1502  and the first insulating layer  202 ), and into the substrate. 
       FIG. 16A  shows a deposition of a liner and tungsten (W) plug  1602  filling the set of CMOS polycrystalline silicon trenches  1504 , the set of CMOS base layer trenches  1506 , and a set of emitter layer trenches  1508  (see  FIG. 15A ).  FIG. 16B  illustrates a deposition of a liner and tungsten (W) plug  1602  filling the set of base layer trenches  1510  and the set of collector contact trenches  1512  (see  FIG. 15B ). In one embodiment of the invention, the tungsten plug  1602  in the CMOS polycrystalline silicon trenches is coupled to memory decoding and selection logic (not shown). The tungsten plug  1602  in the CMOS base layer trenches is coupled to ground. The tungsten plug  1602  in the set of emitter layer trenches is utilized to form a bit-line for the memory cells. 
       FIGS. 17A and 17B  illustrate integration of the memory element  1710  and metallization. In one embodiment, the memory element  1710  is comprised of phase change elements.  FIG. 17B  is a cross sectional view of  FIG. 17A  along line  17 B- 17 B. Memory cells  1702  and the CMOS device  1716  are connected with metal lines. The phase change element  1710  is formed above the tungsten plug  1602  in the memory cells  1702 . The phase change element  1710  may be formed from a chalcogenide. In one embodiment of the invention, the chalcogenide utilized is, but not limited to, a germanium-antimony-tellurium (GST) chalcogenide glass (Ge2Sb2Te5). Those skilled in the art will recognize methods for phase change element  1710  formation. 
     For the memory cells  1702 , bit-lines  1708  and word-lines  1706  are formed from a conductive metal such as copper (Cu). As illustrated, the bit-lines  1708  in  FIG. 17A  and the word-lines  1706  in  FIG. 17B  are perpendicular to each other. Those skilled in the art will recognize the methods required for bit-line  1708  and word-line  1706  formation. 
     Also formed during integration and metallization are CMOS to CMOS connections  1704  made of a conductive metal such as copper (Cu), tungsten (W) connectors  1712  for the CMOS devices  1716 , and collector contacts  1714 . Those skilled in the art will recognize the processes necessary to form phase change elements  1710 , the integration of memory cells  1702 , and metallization. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. Having thus described the invention of the present application in detail and by reference to embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.

Technology Category: 5