Abstract:
Atomic layer deposition (ALD) can be used to form a dielectric layer of zirconium oxide for use in a variety of electronic devices. Forming the dielectric layer includes depositing zirconium oxide using atomic layer deposition. A method of atomic layer deposition to produce a metal-rich metal oxide comprises the steps of providing a silicon substrate in a reaction chamber, pulsing a zirconium precursor for a predetermined time to deposit a first layer, and oxidizing the first layer with water vapor to produce the metal-rich metal oxide. The metal-rich metal oxide has superior properties for non-volatile resistive-switching memories.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to resistive-switching material for non-volatile memory devices, and particularly related to layer formation techniques such as atomic layer deposition of zirconium oxides. 
       BACKGROUND OF THE INVENTION 
       [0002]    Nonvolatile memory elements are used in systems in which persistent storage is required. For example, digital cameras use nonvolatile memory cards to store images and digital music players use nonvolatile memory to store audio data. Nonvolatile memory is also used to persistently store data in computer environments. 
         [0003]    Nonvolatile memory is often formed using electrically-erasable programmable read only memory (EEPROM) technology. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals. 
         [0004]    As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory elements with increasingly smaller dimensions. However, as device dimensions shrink, scaling issues are posing challenges for traditional nonvolatile memory technology. This has led to the investigation of alternative nonvolatile memory technologies, including resistive switching nonvolatile memory. 
         [0005]    Resistive switching nonvolatile memory is formed using memory elements that have two or more stable states with different resistances. Bistable memory has two stable states. A bistable memory element can be placed in a high resistance state or a low resistance state by application of suitable voltages or currents. Voltage pulses are typically used to switch the memory element from one resistance state to the other. Nondestructive read operations can be performed to ascertain the value of a data bit that is stored in a memory cell. 
         [0006]    Resistive switching based on transition metal oxide switching elements formed of metal oxide (MO) films has been demonstrated. Although metal oxide (MO) films such as these exhibit bistability, the resistance of these films and/or the ratio of the high-to-low resistance states is (are) often insufficient to be of use within a practical nonvolatile memory device. For instance, the resistance states of the metal oxide film should preferably be significant as compared to that of the system (e.g., the memory device and associated circuitry) so that any change in the resistance state change is perceptible. Since the variation in the difference in the resistive states is related to the resistance of the resistive switching layer, it is often hard to use a low resistance metal oxide film to form a reliable nonvolatile memory device. For example, in a nonvolatile memory that has conductive lines formed of a relatively high resistance metal such as tungsten, the resistance of the conductive lines may overwhelm the resistance of the metal oxide resistive switching element if its resistance was not sufficiently high. This may make it difficult or impossible to sense the state of the bistable metal oxide resistive switching element. Similar issues can arise from integration of the resistive switching memory element with current steering elements, such as diodes and/or resistors. The resistance of the resistive switching memory element (at least in its high resistance state) is preferably significant compared to the resistance of the current steering elements, so that the unvarying resistance of the current steering element does not dominate the resistance of the switching memory element, and thus reduce the measurable difference between the “on” and “off” states of the formed memory device (i.e., logic states of the device). However, since the power that can be delivered to a circuit containing a series of resistive switching memory elements and current steering elements is typically limited in most conventional nonvolatile memory devices (e.g., CMOS driven devices), it is desirable to form each of the resistive switching memory elements and current steering elements in the circuit so that the voltage drop across each of these elements is small, and thus resistance of the series connected elements does not cause the current to decrease to an undesirable level due to the fixed applied voltage (e.g., ˜2-5 volts). 
         [0007]    As nonvolatile memory device sizes shrink, it is important to reduce the required currents and voltages that are necessary to reliably set, reset and/or determine the desired “on” and “off” states of the device to minimize overall power consumption of the memory chip as well as resistive heating of the device and cross-talk between adjacent devices. Moreover, as nonvolatile memory device sizes shrink it becomes increasing necessary to assure that the “set” and “reset” currents used to change the state of the memory element are not too large so as to require higher voltage transistors for chip control circuitry, as well to minimize damage to or alter the electrical or physical properties of the one or more layers found in the formed memory device. A large current flowing through the current carrying lines in a memory array can also undesirably alter or disturb the memory state of other interconnected devices or possibly damage portions of the adjacently connected devices, due to an appreciable amount of “cross-talk” created between them due to resistive heat transfer. Therefore, there is a need to limit and/or minimize the required current used to sense and program the logic states of each of the interconnected devices in an effort to reduce chip overall power consumption as well as improve device longevity and reduce the chance that cross-talk between adjacently connected devices, which can alter a nonvolatile memory&#39;s device state. Therefore, it is desirable to form a nonvolatile memory device that has low programming currents when switching the device between the “on” and “off” states. 
       SUMMARY OF THE DESCRIPTION 
       [0008]    The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
         [0009]    A novel resistive-switching material for a semiconductor device and method for atomic layer deposition to manufacture the resistive-switching material are disclosed. The resistive-switching material is a metal-rich metal oxide, such as zirconium oxide. Accordingly, a semiconductor device comprises a first electrode, such as a poly-silicon substrate, a metal oxide film may then be deposited onto the first electrode by atomic layer deposition (ALD). A zirconium precursor may be pulsed into an ALD reactor, followed by purging with an inert gas to remove the excess precursor from the processing chamber. Then an oxidizer, may be pulsed into the chamber to oxidize the metal precursor. The resistive switching material is formed as a metal rich oxide (MO x ) where x is less than 2. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings are not to scale and the relative dimensions of various elements in the drawings are depicted schematically and not necessarily to scale. 
           [0011]    The techniques of the present invention can readily be understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  illustrates an array of resistive switching memory elements in accordance with an embodiment of the invention. 
           [0013]      FIG. 2A  is a logarithmic plot of current (I) versus voltage (V) for a memory element according to some embodiments of the present invention. 
           [0014]      FIG. 2B  is a logarithmic plot of current (I) versus voltage (V) plot for a memory element that illustrates a resistance state change according to some embodiments of the present invention. 
           [0015]      FIG. 3  illustrates a flowchart describing a process for forming a memory element according to some embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A detailed description of one or more embodiments is provided below along with accompanying figures. The detailed description is provided in connection with such embodiments, but is not limited to any particular example. The scope is limited only by the claims and numerous alternatives, modifications, and equivalents are encompassed. Numerous specific details are set forth in the following description in order to provide a thorough understanding. These details are provided for the purpose of example and the described techniques may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
         [0017]    According to some embodiments, resistive-switching memory elements can be formed that use resistive-switching metal oxide layers. According to some embodiments, a metal-insulator-metal (MIM) memory element can be formed from two electrodes and one or more layers of metal oxide materials disposed there-between. In some embodiments, metal oxide layers can be formed using atomic layer deposition (ALD). The ALD process parameters and materials can be optimized to reduce the forming voltage and increase the site yield of memory elements described herein. For example, the purge duration, the substrate temperature of the deposition process, and metal materials can be optimized. 
         [0018]      FIG. 1  illustrates a memory array  100  of resistive-switching memory elements  102 . Memory array  100  may be part of a memory device or other integrated circuit. Read and write circuitry may be connected to memory elements  102  using signal lines  104  and orthogonal signal lines  106 . Signal lines such as signal lines  104  and signal lines  106  are sometimes referred to as word lines and bit lines and are used to read and write data into the elements  102  of array  100 . Individual memory elements  102  or groups of memory elements  102  can be addressed using appropriate sets of signal lines  104  and  106 . Memory element  102  may be formed from one or more layers  108  of materials. In addition, the memory arrays shown can be stacked in a vertical fashion to make multi-layer 3-D memory arrays. 
         [0019]    Any suitable read and write circuitry and array layout scheme may be used to construct a nonvolatile memory device from resistive-switching memory elements such as element  102 . For example, horizontal and vertical lines  104  and  106  may be connected directly to the terminals of resistive-switching memory elements  102 . This is merely illustrative. 
         [0020]    If desired, other electrical devices may be associated (i.e., be one or more of the layers  108 ) with each memory element  102 . These devices, which are sometimes referred to as current steering elements, may include, for example, diodes, p-i-n diodes, silicon diodes, silicon p-i-n diodes, transistors, etc. Current steering elements may be connected in series in any suitable locations in memory element  102 . 
         [0021]    During a read operation, the state of a memory element  102  can be sensed by applying a sensing voltage (i.e., a “read” voltage) to an appropriate set of signal lines  104  and  106 . Depending on its history, a memory element that is addressed in this way may be in either a high resistance state or a low resistance state. The resistance of the memory element therefore determines what digital data is being stored by the memory element. If the memory element has a high resistance, for example, the memory element may be said to contain a logic zero (i.e., a “0” bit). If, on the other hand, the memory element has a low resistance, the memory element may be said to contain a logic one (i.e., a “1” bit). During a write operation, the state of a memory element can be changed by application of suitable write signals to an appropriate set of signal lines  104  and  106 . 
         [0022]      FIG. 2A  is a logarithmic plot of current (I) versus voltage (V)  200  for a memory element  102 .  FIG. 2A  illustrates the set and reset operations to change the contents of the memory element  102 . Initially, memory element  102  may be in a high resistance state (“HRS”, e.g., storing a logic zero). In this state, the current versus voltage characteristic of memory element  102  is represented by solid line HRS  202 . The high resistance state of memory element  102  can be sensed by read and write circuitry using signal lines  104  and  106 . For example, read and write circuitry may apply a read voltage V. READ  to memory element  102  and can sense the resulting “off” current I OFF  that flows through memory element  102 . When it is desired to store a logic one in memory element  102 , memory element  102  can be placed into its low-resistance state. This may be accomplished by using read and write circuitry to apply a set voltage V SET  across signal lines  104  and  106 . Applying V SET  to memory element  102  causes memory element  102  to switch to its low resistance state, as indicated by dashed line  206 . In this region, the memory element  102  is changed so that, following removal of the set voltage V SET , memory element  102  is characterized by low resistance curve LRS  204 . As is described further below, the change in the resistive state of memory element  102  may be because of the filling of traps (i.e., a may be “trap-mediated”) in a metal oxide material. 
         [0023]    The low resistance state of memory element  102  can be sensed using read and write circuitry. When a read voltage V READ  is applied to resistive switching memory element  102 , read and write circuitry will sense the relatively high “on” current value I ON , indicating that memory element  102  is in its low resistance state. When it is desired to store a logic zero in memory element  102 , the memory element can once again be placed in its high resistance state by applying a reset voltage V RESET  to memory element  102 . When read and write circuitry applies V RESET  to memory element  102 , memory element  102  enters its high resistance state HRS, as indicated by dashed line  208 . When the reset voltage V RESET  is removed from memory element  102 , memory element  102  will once again be characterized by high resistance line HRS  204 . Voltage pulses can be used in the programming of the memory element  102 . 
         [0024]    A forming voltage V FORM  is a voltage applied to the memory element  102  to ready the memory element  102  for use. Some memory elements described herein may need a forming event that includes the application of a voltage greater than or equal to the set voltage or reset voltage. Once the memory element  102  initially switches, the set and reset voltages can be used to change the resistance state of the memory element  102 . A lower forming voltage may be desirable (e.g. less than 5 volts) to reduce the likelihood of damage to the memory element. As is described herein, certain process techniques can be used to form memory elements that have lower forming voltages. 
         [0025]    The bistable resistance of resistive-switching memory element  102  makes memory element  102  suitable for storing digital data. Because no changes take place in the stored data in the absence of application of the voltages V SET  and V RESET , memory formed from elements such as element  102  is nonvolatile. As can be appreciated, it is desirable for memory element  102  to have a large difference between off current and on current (i.e., a high I ON /I OFF  ratio), which causes the on and off states of the memory element to be more discrete and easily detectable. 
         [0026]    In its most basic form, the layers  108  of the memory element  102  include two electrodes (each having one or more materials and/or layers) and one or more layers of one or more metal oxides disposed in between. The memory element  102  generally has a metal-insulator-metal (MIM) capacitor structure, although other structures such as metal-insulator-insulator-metal (MIIM) and metal-insulator-insulator-insulator-metal (MIIIM) can be used. 
         [0027]    Without being bound by theory, in some embodiments the memory element  102  may use a switching mechanism that is mediated in the bulk of the metal oxide. Generally, defects are formed in, already exist in the deposited metal oxide, and existing defects can be enhanced by additional processes. Defects may take the form of variances in charge in the structure of the metal oxide. For example, some charge carriers may be absent from the structure (i.e., vacancies) or additional charge carriers may be present (i.e., interstitials). Therefore, by applying a voltage to the memory element  102 , the defects, such as traps, can either be filled or emptied to alter the resistivity of a metal oxide and resistive-switching memory elements can be formed using these principles. 
         [0028]    The metal oxides may be of any phase, including crystalline and amorphous. The deposited metal oxides can have vacancy defects such as different level of oxygen vacancy defects created by depositing metal rich zirconium oxide film, vacancies (missing atoms), and interstitials (extra atoms). Amorphous-phase metal oxides may have increased resistivity, which in some embodiments can lower the operational currents of the device to reduce potential damage to the memory element  102 . 
         [0029]      FIG. 2B  is a logarithmic plot of current (I) versus voltage (V)  220  for a memory element  102  that demonstrates a resistance state change. The plot  220  shows a voltage ramp applied to the memory element  102  along the x-axis and the resulting current along a y-axis. The line  222  represents the response of an Ohmic material when the ramped voltage is applied. An Ohmic response is undesirable, since there is no discrete voltage at which the set or reset occurs. 
         [0030]    Generally, a more abrupt graph like graph  224  is desired. The graph  224  begins with an Ohmic response  224   a , and then curves sharply upward  224   b . The graph  224  may represent a set operation, where the memory element  102  switches from the HRS  202  to the LRS  204 . 
         [0031]    Without being bound by theory, non-metallic percolation paths are formed during a set operation and broken during a reset operation. For example, during a set operation, the memory element  102  switches to a low resistance state. The percolation paths that are formed by filling traps increase the conductivity of the metal oxide, thereby reducing (i.e., changing) the resistivity. The voltage represented by  224 b is the set voltage. At the set voltage, the traps are filled and there is a large jump in current as the resistivity of the metal oxide decreases. 
         [0032]    The set voltage shown here is very discrete (i.e., vertical), which is desirable to ensure the switching of the memory element occurs at a repeatable voltage. Additionally, a high ratio of on current to off current (i.e., a high I ON /I OFF  ratio), for example 10 or greater, is desirable because it indicates a large difference in the resistivity of the metal oxide when in the HRS and LRS, making the state of the memory element easier to determine. Finally, it is desirable to have low set, reset, and forming voltages in order to avoid damage to the memory elements and to be compatible with complementary device elements such as diodes and/or transistors in series with the memory element  102 . 
         [0033]    The resistive-switching memory elements generally have a metal-insulator-metal (MIM) structure in which resistive-switching insulating layers are surrounded by two conductive electrodes. The resistive-switching layer includes a metal oxide (such as HfO x , ZrO x , AlO x , TiO x , TaO x , etc.) layer or a film stack consisting of a metal oxide film as the host switching material with another metal oxide as the coupling layer. 
         [0034]    Without being bound to theory, a number of models have been proposed for the driving mechanism in resistive-switching involving an interface-type conducting path, such as electrochemical migration of oxygen vacancies and trapping of charge carriers (hole or electron). The switching characteristics of the resistive-switching memory elements can be tailored by controlling the defects within the metal oxides. However, using conventional PVD deposition technique, it is difficult to vary metal oxide film composition and control defect concentrations, particularly for very thin films less than 50 A. ALD is a deposition technique that produces high-quality films with precise thickness control and good conformality. By varying the precursor and oxidizer concentration, different film compositions of metal oxides can be achieved. Ozone is a typical oxidant used in the formation of metal oxide materials. If the metal oxide materials can be formed metal-rich instead of stoichiometric composition, the switching layer will have a deficit of oxygen, which manifests as oxygen vacancy defects. 
         [0035]    Increasing the temperature of the precursor source increases the partial pressure of the precursor, which can result in a more metal-rich switching layer by increasing the concentration of the metal precursor in the chamber and in the film. In some embodiments of the present invention, water vapor or isopropyl alcohol is chosen as the oxidant source. These oxidants are not as aggressive as ozone and allow the metal to oxygen ratio to be controlled in the film. The partial pressure of the oxidant can be lowered by reducing the source temperature which reduces its vapor pressure. For example, water vapor has a vapor pressure of about 17.5 torr at 20C and about 4.9 torr at 1C. The partial pressure can also be reduced by diluting the oxidant with an inert gas such as argon. By using water vapor as the oxygen source (which is less reactive than other oxygen sources such as ozone) and reducing the vapor pressure for water vapor, the ALD process window can be extended. Since water vapor is less reactive than ozone, the use of water vapor reduces the formation of SiO x  between the silicon substrate and the metal oxide during the process. For example, the SiO x  layer formed between the substrate and the metal oxide may be as thin as about 10 A when water vapor is used as the oxidant. The formation of SiO x  is an undesirable effect of the ALD process. A thicker SiO x  layer results in a higher forming voltage of the resulting resistive-switching element. 
         [0036]      FIG. 3  is a flowchart describing a process  300  for forming a memory element. The process  300  is a general description of techniques used to form the memory elements described above. The process  300  describes techniques for forming a memory element  102  generally including two electrodes and at least one layer of metal oxide disposed there-between. Although certain processing techniques and specifications are described, it is understood that various other techniques and modifications of the techniques described herein may also be used. 
         [0037]    In operation  302 , a first electrode is formed. The first electrode can be formed on a substrate, for example, a silicon substrate that may include one or more layers already formed thereon. The first electrode may be formed over a signal line such as a bit line or a word line. The first electrode can be poly-silicon, a silicide, titanium nitride, tungsten, or other appropriate material. In some embodiments, a titanium nitride electrode is formed using PVD or other suitable process. These descriptions are given as examples; the actual process conditions can vary greatly depending on the material to be deposited, the tool used to deposit the material, and the desired speed of deposition. Other processing techniques, such as ALD, PLD (pulsed laser deposition), CVD (chemical vapor deposition), evaporation, etc. can also be used to deposit the first electrode. In some embodiments, the first electrode is in contact with one of the signal lines  104  or  106 . The first electrode may have any thickness, for example between about 10 A and about 2000 A. 
         [0038]    In operation  304 , the substrate is heated, for example by heating a pedestal on which the substrate is mounted in an ALD chamber. In this way, a pedestal temperature is maintained for an ALD deposition process so that the metal oxide layer(s) can be deposited. Typically, the substrate is heated to about 250 C. 
         [0039]    In operation  306 , at least one metal oxide layer is formed using ALD. The metal oxide layer can be a resistive-switching metal oxide layer, such as a high bandgap (i.e. greater than 4 eV) material including zirconium oxide, hafnium oxide, aluminum oxide, yttrium oxide, tantalum oxide, etc. ALD is a multi-step process used to deposit the materials. The operation  306  is a single ALD cycle. An ALD-deposited layer typically includes multiple cycles to deposit a layer of a desired thickness, so the process  306  is repeated until the desired layer thickness has been deposited. The metal oxide layer can include single metal oxide (i.e. zirconium oxide, hafnium oxide, aluminum oxide, etc.), can use co-injection or nanolaminates to deposit a binary metal oxide, or can use multiple layers of metal oxides. For example, a metal oxide “base” layer can be deposited over a metal oxide “diffusion barrier” layer. In other embodiments, three or more different layers of materials can be deposited. 
         [0040]    In operation  308 , a precursor pulse is applied to a substrate by introducing the first reagent into an ALD chamber. A portion of the precursor adsorbs onto the surface of the substrate. The precursor can be a metal-containing precursor that is used to form a metal oxide. For example, the first reagent can be tetrakis (ethylmethylamino) zirconium (TEMAZ) to deposit zirconium oxide, trimethylaluminum (TMA) to deposit an aluminum oxide layer, or tetrakis (diethylamido) hafnium (TDEAHf), tetrakis(dimethylamido)hafnium (TDMAHf), tetrakis(ethylmethylamido)hafnium (TEMAHf) or hafnium chloride (HfCl 4 ) to deposit a hafnium oxide layer, or tris(methylcyclopentadienyl)-yttrium ((Cp) 3 )Y to deposit yttrium oxide. In other embodiments, lower bandgap (i.e. less than 4 eV) materials, such as titanium oxide, have also been shown to exhibit switching properties. Precursors such as tetraisopropoxide (TTIP) can be used to deposit titanium oxide. 
         [0041]    In accordance to some embodiments of the present invention, ZyALD™ is used as the zirconium precursor. ZyALD™ is used only as an example; those skilled in the art will understand that any suitable ALD precursor may be used. Self-limited growth can be obtained up to the temperature of 350 C. ZyALD™ is available from Air Liquide having its headquarters in Paris, France. The ampoule temperature is set at approximately 90 C. In some embodiments, a pulse time of about 30 seconds was used for the precursor pulse in a carrier gas of Ar. The Ar carrier flow rate may be about 200 sccm. 
         [0042]    In operation  310 , the excess (non-adsorbed portion) of the precursor is purged, for example by purging the ALD chamber to remove excess precursor. In some embodiments, the purge time for the precursor may be about 70 seconds. 
         [0043]    In operation  312 , a reactant gas is applied to the substrate. The reactant gas combines with the adsorbed precursor to form a metal oxide. The reactant gas is an oxidizer, for example ozone, water vapor, isopropyl alcohol, or oxygen. In some embodiments, water vapor is used as the oxidizer. Water vapor is less reactive than other oxidizers such as ozone. Being less reactive, the interfacial SiO x  layer between the Si substrate and the metal oxide is reduced. Typically, the interfacial SiO x  layer between the Si and ZrO x  film is less than 10 A. In some embodiments, the water vapor ampoule temperature is set to about 1 C. The water vapor pulse time is about 10 seconds. 
         [0044]    In operation  314 , the unreacted reactant gas is purged from the chamber. The purge gas may be Ar and may have a flow rate of about 200 sccm. The purge time for the reactant may be about 70 seconds. 
         [0045]    In operation  316 , it is determined whether another ALD deposition cycle is to be performed. A typical ALD cycle may form a layer that is about 0.5 A. To form a 50 A metal oxide layer, about 100 cycles would need to be performed. If in operation  316 , it is determined that the desired number of cycles has been performed or that the desired thickness has been reached, the process  300  continues to operation  318 . If more ALD cycles need to be completed, the process  300  returns to operation  308 . 
         [0046]    Accordingly, the ALD ZrO x  process of some embodiments uses ZyALD™ as Zr precursor and water vapor as the oxidizer. Using water vapor as the oxidizer has positive advantages over other commonly used oxidizers. The different level of oxygen vacancy defects created by depositing metal rich zirconium oxide film improves switching performance such as lower forming voltage and lower switching voltages. The resulting film stoichiometry (i.e. Zr:O atomic ratio) may be adjusted from about 1:2 (i.e. stoichiometric) to about 1:1.7 (i.e. metal rich). Advantageously, the resulting film stoichiometry (i.e. Zr:O atomic ratio) may be adjusted to about 1:1.88. 
         [0047]    In operation  318 , a second electrode is deposited over the metal oxide layer. In some embodiments, the second electrode may be a noble or near-noble metal such as iridium, iridium oxide, platinum, ruthenium, ruthenium oxide, titanium nitride, doped poly-silicon, tungsten or tantalum nitride. The second electrode may be deposited using PVD, ALD, CVD, PLD, evaporation, or other suitable technique. 
         [0048]    Although the foregoing examples have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed examples are illustrative and not restrictive.