Abstract:
A method for manufacturing a nanotube non-volatile memory cell is proposed. The method includes the steps of: forming a source electrode and a drain electrode, forming a nanotube implementing a conduction channel between the source electrode and the drain electrode, forming an insulated floating gate for storing electric charges by passivating conductive nanoparticles with passivation molecules and arranging a disposition of passivated conductive nanoparticles on the nanotube, the conductive nanoparticles being adapted to store the electric charges and being insulated by the passivation molecules from the nanotube, and forming a control gate coupled with the channel.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority from European patent application No. EP05108235.2, filed Sep. 8, 2005, which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    An embodiment of the present invention relates to the nanotechnology field. More specifically, the invention relates to nanotube non-volatile memory cells. 
       BACKGROUND 
       [0003]    Devices based on non-volatile memory cells are commonly used in several applications when the data stored therein needs to be preserved even when a power supply is off. 
         [0004]    A largely employed solution for implementing those memory cells is based on floating-gate MOSFETs, which are typically integrated in a silicon substrate for obtaining an EPROM or an EEPROM (such as a flash memory). Typically, a floating-gate MOSFET has two gates stacked over a channel region between a source region and a drain region. The gate (referred to as floating gate) interposed between the channel region and the other gate is electrically insulated from the channel region and the other gate (for example, the floating gate is surrounded with silicon dioxide). The other gate (referred to as control gate) is electrically connected as in a common MOSFET. 
         [0005]    Nowadays, the floating-gate MOSFETs for memory devices typically have to satisfy the existing demand for more and more compact circuits, that continually steers the microelectronics industry into submicron regions. 
         [0006]    In particular, for allowing a more aggressive scaling of the floating gate oxide, recently some innovations for memory devices have been proposed in which gold nanoparticles are exploited as storage elements in traditional silicon devices. In detail, the floating gate is made by a layer of gold nanoparticles passivated with organic molecules over a first layer of insulating material (such as silicon dioxide) of only few nanometers. In this way, considering the length of the passivation molecules, a distance between a silicon surface and the gold nanoparticles is of only tens of nanometers. A second layer of insulating material (such as an organic insulator) overlaps the passivated nanoparticles. 
         [0007]    In order to manufacture high-density memory devices, the control-gate, source, and drain sizes should also be reduced. However, this may impair the electric characteristics of the floating-gate MOSFET. For example, a large electric resistance can be induced or a punch-through phenomenon (in which a depletion zone of the source region contacts a depletion zone of the drain region) can occur. 
         [0008]    Recently, memory cells implemented by exploiting carbon nanotubes have been studied. Carbon nanotubes are carbon allotropes having a structure that can be described as a sheet of graphite, rolled so as to form a cylinder and ended by two hemispheres. The carbon atoms in the nanotube are bound together so as to form hexagons. The nanotube has a nano-sized diameter and a length of the order of hundreds of nanometers; the nanotube, depending on a twist along its axis, can also feature a semiconducting electric behavior (i.e., a non-ohmic behavior affected by a control voltage). 
         [0009]    An example of carbon nanotube memory cell is disclosed in EP1341184, which is incorporated by reference. In this case, a source electrode and a drain electrode are formed on a substrate, and a carbon nanotube is interposed between the source electrode and the drain electrode to implement a channel. A memory element is located over the carbon nanotube, and a gate electrode is formed in contact with the memory element. The memory element includes a first insulating film in contact with the carbon nanotube, a charge storage film deposited on the first insulating film, and a second insulating film formed on the charge storage film (contacting the gate electrode). Particularly, the first and the second insulating films are formed of silicon oxide, and the charge storage film is formed of silicon or silicon nitride. Alternatively, the charge storage film includes a porous film (for example, of aluminum oxide) having a plurality of nanodots filled with a charge storage material (such as silicon or silicon nitride). 
         [0010]    A memory cell based on a vertical nanotube is instead disclosed in EP1420414, which is incorporated by reference. In detail, the memory cell includes a source region formed into a substrate; a nanotube array is composed of a plurality of nanotube columns vertically grown on the substrate (so that an end of the nanotube array is in contact with the source region). A memory element is formed around the nanotube array, and a control gate is formed around the memory element. A drain region is in contact with the other end of the nanotube array. In this solution, the memory element includes a first insulation layer formed around the nanotube array, an electron-storing layer formed around the first insulation layer, and a second insulation layer formed around the electron-storing layer (contacting the gate electrode). Particularly, the first and the second insulation layers are silicon oxide layers, and the electron-storing layer is a silicon layer, a silicon nitride layer, or a porous layer (filled with silicon or silicon nitride). 
         [0011]    However, in the solutions known in the art, the memory element has a conventional three-layers structure. This structure may suffer from an inherent limit in its minimum thickness (for example, of the order of tens of nanometers). 
         [0012]    In addition, the corresponding manufacturing process of the memory cells based on the nanotubes may be relatively complex and expensive. This may hinder the widespread diffusion of such a technology. 
       SUMMARY 
       [0013]    According to an embodiment of the present invention, the idea of exploiting passivated conductive nanoparticles for forming the floating gate is suggested. 
         [0014]    Particularly, an embodiment of the invention proposes a method for manufacturing a nanotube non-volatile memory cell. The method starts with the step of forming a source electrode and a drain electrode. The method continues by forming a nanotube, which implements a conduction channel between the source electrode and the drain electrode. An insulated floating gate (for storing electric charges) is then formed. This result is achieved by passivating conductive nanoparticles with passivation molecules, and then arranging a disposition of passivated conductive nanoparticles on the nanotube. The conductive nanoparticles are adapted to store the electric charges, and are insulated by the passivation molecules from the nanotube. The method ends with the step of forming a control gate coupled with the channel. 
         [0015]    In an embodiment of the invention, a carbon nanotube is used. 
         [0016]    The conductive nanoparticles may be made of gold. 
         [0017]    In an embodiment of the invention, the passivated nanoparticles are adsorpted on the nanotube. 
         [0018]    As a further enhancement, the passivated nanoparticles, the nanotube or both of them may be functionalized for facilitating the adsorption. 
         [0019]    Suggested examples of passivation molecules are also proposed (such as hydrocarburic or aromatic thiols). 
         [0020]    In an embodiment, the adsorption is achieved by dipping the nanotube into a solution with the passivated nanoparticles. 
         [0021]    A way to further improve the solution is by forming the control gate with a layer of polymer electrolyte. 
         [0022]    Examples of the polymer electrolyte are also proposed. 
         [0023]    A further embodiment of the present invention provides a corresponding nanotube non-volatile memory cell. 
         [0024]    A still further embodiment of the present invention provides a non-volatile memory device including those memory cells. 
         [0025]    Features and advantages of at least one embodiment of the invention will be best understood by reference to the following detailed description, given purely by way of a non-restrictive indication, to be read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  shows a cross-sectional view of a non-volatile nanotube memory cell according to a first embodiment of the present invention; 
           [0027]      FIGS. 2A-2D  are cross-sectional views of the memory cell of  FIG. 1  at various stages of a manufacturing process according to an embodiment of the present invention; and 
           [0028]      FIG. 3  shows a cross-sectional view of a non-volatile nanotube memory cell according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    With reference to the drawings, and in particular to  FIG. 1 , a non-volatile nanotube memory cell  100  is shown according to an embodiment of the invention. The memory cell  100  is formed on a substrate  105 . The substrate  105  may be of a material resistant to relatively high temperatures and inert with respect to chemical elements or compounds exploited during a manufacturing process of the memory cell  100 . For example, the substrate  105  is a silicon substrate, but also other materials such as alumina, zirconia, silica, titania, aluminum nitride, silicon nitride, boron nitride, metallic silicates, titanates, niobates, glass ceramics and quartz are suitable. 
         [0030]    The memory cell  100  further includes a source electrode  110  and a drain electrode  115  formed over an upper surface  120  of the substrate  105 . The source and drain electrodes  110  and  115  are separated from each other and made, for example, of gold; the source and drain electrodes  110  and  115  have, for example, a height ranging between 30 nm and 60 nm (for example 30 nm). 
         [0031]    The source and drain electrodes  110  and  115  are connected by a semiconductive carbon nanotube  125  formed over the surface  120  of the substrate  105  (with the nanotube  125  having a longitudinal axis parallel to the surface  120 ). The nanotube  125  implements a channel, which permits electric charges (or carriers) to move between the source electrode  110  and the drain electrode  115 . The nanotube  125  has a diameter ranging between 0.8 nm and 3 nm (for example 2-3 nm), and a length ranging between 100 nm and 20 μm (for example 1 μm). 
         [0032]    An upper surface of the nanotube  125 , opposite to a surface thereof in contact with the substrate  105 , is covered with passivated conductive nanoparticles (i.e., nanoparticles coated so as to reduce their reactivity). The passivated nanoparticles are arranged over the nanotube  125  in such a way to form a disposition  130  (like a layer); the layer  130  of passivated nanoparticles implements a floating gate (for trapping the electric charges from the nanotube  125 ), which defines the memory element of the cell  100 . The molecules exploited for passivating the nanoparticles act as an insulating shield around them (of a thickness, for example, of about 2-3 nm), in such a way as to insulate the nanoparticles electrically from the nanotube  125 . The layer  130  of passivated nanoparticles has a very small size, with a thickness ranging between 2 nm and 7 nm (for example 5 nm). 
         [0033]    For example, the nanoparticles  130  are made of a transition metal, such as gold, and functionalized with long chain hydrocarburic or aromatic thiols, in such a way to facilitate their anchoring to the upper surface of the nanotube  125 . For this purpose, the hydrocarburic or aromatic thiols (exploited for the passivation) are ended, for example, with pending aryl, pyrenyl, antracenyl, fenantrenyl, or fullerenic groups (see, for example, “Chemical Physics Letters 364 (2002) 290-296”, “Pure Appl. Chem., Vol. 74, No. 9, pp. 1731-1738, 2002”, Nano Letters 2002 Vol. 2, No. 1 29-35”, “Langmuir 2001, 17, 6393-6395”, “Langmuir 2002, 18, 1840-1848”, “Langmuir 1999, 15, 5329-5332”, and “Chem. Commun. 2002, 2560-2561”, all of which are incorporated by reference). These groups are able to interact with the upper surface of the nanotube  125  by Van der Waals forces, n-stackings and the like, so as to permit a physical adsorption into the upper surface of the nanotube  125  without altering its semiconductive properties (i.e., due to the adhesion of an extremely thin layer of passivated nanoparticles  130 ). 
         [0034]    In an embodiment, a control gate  135  is formed over the layer  130  of passivated nanoparticles. The control gate  135  is an electrochemical gate with an aqueous electrolyte solution exploited as dielectrics (see, for example, “Nano Letters 2004 Vol. 4, No. 5 927-931” or “Nano Letters 2004 Vol. 4, No. 4 623-627”, all of which are incorporated by reference). Particularly, a polymer electrolyte is used as gate medium in which an aqueous solution, such as LiClO 4 , 3H 2 O, is dissolved. For example, the polymer electrolyte is poly(ethylene oxide) (PEO, average weight M n  550 or 1000), poly(ethylenimine) (PEI, average weight M n  800), or poly(acrylic acid) (PAA, average weight M n  1240). Weight ratios of polymer to LiClO 4  solution, for example, are 2.3:1, 2.5:1 and 1.6:1 for PEO, PEI and PAA, respectively. The control gate  135  has a height ranging between 30 nm and 60 nm (for example 30 nm). A gate electrode  150  is formed over the control gate  135  (for example, by means of a silver wire). 
         [0035]    The main steps of an exemplary process for the manufacturing of the above-described memory cell are explained hereinbelow with reference to  FIGS. 2A-2D  according to an embodiment of the invention. 
         [0036]    Considering  FIG. 2A , at a first step the upper surface  120  of the silicon substrate  105  is covered by a thin film  205  (e.g., less than 2 nm, for example 1 nm) of a catalyst used for the formation of the nanotube. The catalyst is a metal, such as cobalt, nickel or iron. The catalyst film  205  is grown, for example, by physical procedures, such as evaporation, sputtering and laser ablation. Also chemical procedures can be exploited in which the substrate  105  is coated with, for example, a solution containing a metal catalyst. 
         [0037]    Successively, the catalyst film  205  is processed by exploiting a conventional mask  210  for a lithographic etching. Particularly, the catalyst film  205  is patterned for obtaining catalyst clusters regularly arranged over the upper surface  120  of the silicon substrate  105 . The etching may be a wet etching or a dry etching (e.g., RIE). Other lithographic procedures can be exploited, such as electron-beam lithography and Focused Ion Beam lithography. Furthermore, a lift-off technique can be applied in which a resist is deposited and patterned over the upper surface  120  of the silicon substrate  105 , the metal catalyst is deposited, and then the patterned resist with the overlying metal catalyst is removed. 
         [0038]    As depicted in  FIG. 2B , the nanotube  125  is grown, for example, by means of a Chemical Vapor Deposition (shortly, CVD) process. During the CVD process different hydrocarbons such as benzene, pentane, acetylene, methane and also carbon monoxide are decomposed over the catalyst clusters at temperatures between 500° C. and 1200° C., in presence of a carrier gas, such as H 2 , Ar, and NH 3 . The chamber used for this procedure is a UHV (Ultra High Vacuum) reactor where it is possible to control different parameters (such as gas flow, pressure, temperature) and then the nanotube growth. Alternatively, the nanotube  125  is synthesized by means of a laser ablation of a graphite target or by means of an arc-discharge technique in which two carbon electrodes are exploited. 
         [0039]    Still referring to  FIG. 2B , the (metal) source and drain electrodes  110  and  115  are realized by a process similar to that described above for obtaining the catalyst clusters. Particularly, evaporation, sputtering or chemical deposition techniques can be exploited for the deposition of a metal layer, followed by patterning procedures. 
         [0040]    Considering now  FIG. 2C , the nanotube  125  is decorated with passivated nanoparticles. For this purpose, the nanoparticles, functionalized by long chain hydrocarburic or aromatic thiols with suitable pending groups, are synthesized, for example, by the well-known Brust&#39;s procedure or a slightly modified version thereof. Generally, this procedure is based on gold reduction reaction occurring in a biphasic medium by employing NaBH 4 , as reducing agent, in presence of tetraoctylammonium bromide, as phase transfer agent, and dodecanthiol, as capping molecules. The dodecanthiol molecules are chemisorbed over the gold nanoparticle surface. The dodecanthiol molecules can be replaced, for example, with pyrenethiol molecules by a place-exchange reaction. Alternatively, it is possible to obtain stabilized gold nanoparticles with tetraoctylammonium bromide and then to replace the physically adsorpted surfactant molecules with pyrenethiol molecules. In order to obtain the layer  130  of nanoparticles over the nanotube  125 , the substrate  105  supporting the nanotube  125  is dipped into a functionalized gold nanoparticle solution so as to allow the gold nanoparticles to anchor to the nanotube  125 . 
         [0041]    Referring to  FIG. 2D , the control gate  135  is then obtained by using polymer electrolytes as gate medium. The polymer electrolytes are prepared, for example, by directly dissolving LiClO 4 , 3H 2 O in the desired polymer. Particularly, a rigorous drying of PEO electrolytes and LiClO 4  is performed. PEI electrolytes are injected into a poly-(dimethylsiloxane) fluidic channel placed over the layer  130  of nanoparticles. Alternatively, PAA and LiClO 4  are dissolved in acetone and the memory cell  100  is spin coated; then, PAA and LiClO 4  are dried under vacuum for several hours. However, a combination of additives and different host polymer materials can be exploited; moreover, microfabrication techniques and other methods (such as a layer-by-layer deposition of polymers) are tenable. 
         [0042]    Finally, the gate electrode  150  is obtained in a similar way to that exploited for obtaining the source and drain electrodes  110  and  115 . 
         [0043]    It should be appreciated that the above-described process may be used for manufacturing a plurality of nanotube memory cells at the same time. Moreover, the process steps described above may be incorporated in a process for fabricating further different transistors, and especially for fabricating complex electronic circuits (such as memory devices). 
         [0044]    The electrochemical control gate  135  may limit a leakage current therethrough (e.g., below 500 pA) when relatively high voltages are applied. 
         [0045]    Alternatively, the control gate  135  includes a first thin layer (e.g., 15 nm) of a dielectric, such as silicon dioxide, and an overlapping second layer made of a conductive material, such as aluminum. The gate electrode  150  is formed over the control gate  135  and it is made, for example, of titanium carbide. However, this solution may have the drawback that the oxide growth or deposition often needs to be performed at high temperatures, which might be dangerous for the organic molecules coupling the gold nanoparticles with the nanotube  125 . 
         [0046]      FIG. 3  shows a memory cell  300  according to another embodiment of the present invention (the elements corresponding to those depicted in  FIG. 1  are denoted with the same reference numerals and their description is omitted for the sake of simplicity). 
         [0047]    In this case, a back control gate  335  is formed into the substrate  105  (particularly, below the nanotube  125 ), instead of being formed on the top of the memory cell  300 . 
         [0048]    The back control gate  335  is formed, for example, by a first layer of P-doped silicon  345  (the letter P indicates that the doping ions are acceptor) coated, for example, with thermal silicon dioxide  350 . In detail, before forming the catalyst clusters, a selective implant of P type dopants is performed in the substrate  105  and, successively, a layer of silicon dioxide is selectively obtained at the surface  120  of the substrate  105 , for example, by oxidation at high temperature. Alternatively, the back control gate  335  is made of a metal layer, such as of aluminum coated with subnanometer-thick native Al 2 O 3  or tungsten coated with silicon dioxide. 
         [0049]    In the manufacturing process of this embodiment of the memory cell  300 , the step of forming the back control gate  335  is performed before the nanotube growth and the passivated nanoparticles decoration thereof. Accordingly, the control gate formation does not interact or damage the passivating molecules exploited for the floating gate. 
         [0050]    Nevertheless, it should be noted that the memory cells with top control gates such as shown in  FIGS. 1 and 2D  often show better performance than the memory cells with bottom control gates such as shown in  FIG. 3 . 
         [0051]    In both cases, the exploitation of the nanotube decorated with passivated nanoparticles allows shrinking the memory cell sizes (with respect to nanotube memory cells exploiting the conventional three-layers floating gate structure). The memory element including the passivated nanoparticles anchored to the nanotube has a very small thickness of only few nanometers (instead of the typical thickness of tens of nanometers of the three-layers floating gate structures). 
         [0052]    In addition, the proposed manufacturing process is of simple application; the same manufacturing process is also cost and time effective (with respect to the process in which a layer-by-layer technique is applied). 
         [0053]    All of the above may foster the widespread use of the nanotube technology. 
         [0054]    Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many modifications and alterations. Particularly, although the present invention has been described with a certain degree of particularity with reference to embodiments thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible; moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment of the invention may be incorporated in any other embodiment as a general matter of design choice. 
         [0055]    For example, similar considerations apply if the memory cell has a different structure or includes equivalent components (such as it is based on a vertical geometry with respect to the substrate); in any case, it should be appreciated that the numerical examples described above (such as relating to the size of the elements of the memory cell) are merely illustrative and must not be interpreted in a limitative manner. Likewise, the manufacturing process of the memory cell may comprise equivalent steps (exploiting alternative techniques and/or materials); moreover, it is possible to have additional steps, or the steps may be executed in a different order. Alternatively, any other type of nanotube may be used, either of the single-walled (SWNT) or multi-walled (MWNT) type. 
         [0056]    Even though the above-described embodiments have been specifically designed for carbon nanotubes, this not to be interpreted in a limitative manner (with the same technique that may also be applied to nanotubes made of another material, such as inorganic nanotubes). 
         [0057]    Likewise, the use of different conductive nanoparticles is not excluded (for example, made of aluminum or magnesium). 
         [0058]    It should also be appreciated that alternative techniques for forming the layer of passivated nanoparticles are within the scope of the invention. 
         [0059]    In addition, the use of different passivation molecules is contemplated. 
         [0060]    Similar considerations apply if the passivated nanoparticles are adsorpted on the nanotube with a different procedure (for example, by functionalizing the nanotube in alternative or in addition to the passivated nanoparticles). 
         [0061]    Likewise, any other polymer electrolyte may be used for forming the control gate. 
         [0062]    At the end, the proposed memory cell leads itself to be used in any non-volatile memory device, such as a flash of EEPROM, and a system such as a computer system, may incorporate such a non-volatile memory device.