Patent Publication Number: US-2019173005-A1

Title: Method for fabricating an oxram memory location for limiting dispersions

Description:
The invention relates to RRAM memories, and in particular the methods for fabricating such memories. 
     In order to overcome the limits in terms of miniaturization, power consumption and complexity of fabricating floating-gate non-volatile memory technologies, the semiconductor industry is developing various alternative technologies. Among the alternative non-volatile memory technologies in the process of being developed, RRAM memories have a certain technical advantage. 
     RRAM memories are based on the reversible formation and rupture of a conductive filament: a dielectric material, which is normally insulating, may be forced to be conductive through a filament or a conduction path after application of a sufficiently high voltage. Once the filament is formed, it may be reset or programmed by an appropriately applied voltage. 
     In the particular case of OxRAM memories, the conductive filament is produced from oxygen vacancies in an insulating metal-oxide-based material. OxRAM memories benefit from a very good thermal stability, in theory making it possible to keep the information in a reliable manner for several years at high temperature, with a lifetime having a very large number of programming/deprogramming and/or read cycles. 
     A standard oxide used as insulating material in a memory is HfO 2 . On the macroscopic scale, the zone of the oxide in which the filament is formed has either an insulating state or a conductive state in the programmed state, or a semiconductor state in the reset state. 
       FIG. 1  is a schematic cross-sectional view of an example of an OxRAM memory location  8 . The OxRAM memory location  8  comprises a metallic top electrode  81 , a metallic bottom electrode  82 , and a dielectric  83  inserted between the top electrode  81  and the bottom electrode  82 . 
     The top electrode  81  is here advantageously broken down into a conductive upper element  811 , and a lower element  812  for recovery of oxygen vacancies. The upper element  811  is typically made of TiN, the lower element  812  being for example made of Ti. The lower element  812  typically has a thickness of between 3 and 15 nm. 
     The dielectric  83  is made selectively conductive by the application of an appropriate potential difference between the top electrode  81  and the bottom electrode  82  by a control circuit  90 . The dielectric  83  is intended to allow oxygen vacancies to migrate from this layer  83  to the electrode  81  or  82  in order to form the conductive filament  84  in the programmed state, and vice versa. In the programmed state, a conductive element  84  is formed in the dielectric  83 . This programmed state is retained even in the absence of power supply to the memory location  8 . 
     By applying another potential difference by means of the control circuit  90 , it is possible to deprogram the memory location  8  and rupture the filament  84 . The dielectric  83  has then a high resistance state, HRS. 
     By subsequently applying a read potential difference between the electrodes  81  and  82 , the control circuit  90  can measure the current passing through the memory location  8  in order to determine whether this is in the programmed or reset state. 
     In practice, the dielectric  83  comprises multiple domains having different compositions in the zone of the filament  84 . Thus, in the figure, zones  841  to  845  have been schematically illustrated that have various substoichiometric HfOx phases in the HfO 2 . 
     It has been observed that these different zones  841  to  845  have different bandgap values for semiconductor oxide phases (0&lt;BG&lt;2).  FIG. 2  shows the bandgap energy of various substoichiometric HfOx phases in the HfO 2 : there are two ranges of substoichiometric phases with a metallic behaviour and a semiconductor behaviour. For the semiconductor phases, small stoichiometry variations induce variations of the bandgap.  FIG. 3  shows the thermodynamic stability of the substoichiometric phases: for x&lt;1.5, the entire range of the semiconductor phases are stable and may be formed during the change of memory state from the LRS to the HRS state. Such variations of the bandgap BG induce large dispersions in the electrical behaviour of the various memory locations, and also in the electrical behaviour of one and the same memory location for various programming/resetting cycles. Consequently, the HRS state has a large dispersion of the electrical properties which is a particularly limiting point for the industrialization of the OxRAM technology. 
     The invention aims to resolve one or more of these drawbacks. The invention thus relates to a method for fabricating an OxRAM memory location, as defined in the appended Claim  1 . 
     The invention also relates to the variants of the dependent claims. A person skilled in the art will understand that each of the features of the variants of the dependent claims or of the description may be combined independently with the features of an independent claim without however constituting an intermediate generalization. 
     The invention also relates to an OxRAM memory location, as defined in the appended claims. 
    
    
     
       Other features and advantages of the invention will become apparent from the description which is given below, by way of indication and non-limiting, with reference to the appended drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of a memory location according to the prior art, provided with a programming/read circuit; 
         FIG. 2  is a diagram illustrating the bandgap energy of various substoichiometric phases of a filament zone made in HfO 2 ; 
         FIG. 3  illustrates differences in thermodynamic stability of the various substoichiometric HfOx phases in HfO 2 ; 
         FIG. 4  is a schematic cross-sectional view of a step of a fabrication method according to an example of a first embodiment of the invention; 
         FIG. 5  is a diagram illustrating the bandgap energy of various substoichiometric phases of a filament zone made in an insulator produced according to the invention; 
         FIG. 6  is a diagram illustrating the bandgap energy of various substoichiometric phases, as a function of the proportion of titanium included; 
         FIG. 7  is a comparative diagram of the thermodynamic stability of a substoichiometric alloy of Hf 1-y Ti y O 1.5  relative to Hf 1-y Ti y O 2 , as a function of the value of y; 
         FIG. 8  is a diagram illustrating the proportion of titanium in a filament zone as a function of the depth, for various implantation energy values; 
         FIG. 9  is a cross-sectional view of a step of a fabrication method according to a variant of the first embodiment of the invention; 
         FIG. 10  is a schematic cross-sectional view of an example of the structure of a filament zone obtained by means of the method illustrated in  FIG. 9 ; 
         FIG. 11  is a cross-sectional view of a step of a fabrication method according to an example of a second embodiment of the invention; 
         FIG. 12  is a diagram illustrating the concentration of various components in an Hf 1-y Ti y O 2  layer, as a function of the implantation energy used for the ion implantation; 
         FIG. 13  is a diagram illustrating the titanium concentration as a function of the depth in a memory location, for various implantation energies. 
     
    
    
     The invention proposes a method for fabricating an OxRAM memory location. In this method, it is proposed to implant Ti in a lower HfO 2  layer. This implantation is carried out by an ion implantation of a material chosen from Xe, Kr or Ar, in an upper layer, that includes more than 50% by weight of Ti. The ion implantation in this upper layer induces therein a collision with a recoil effect, leading to an implantation of Ti in the lower HfO 2  layer. 
       FIG. 4  is a schematic cross-sectional view of a stack  1  during a step of a method for fabricating an OxRAM memory location, according to an example of a first embodiment of the invention. 
     The stack  1  here comprises the superposition of a layer  100 , of a layer  101 , of a layer  102  and of a layer  103 . The layer  100  is for example a layer intended to form a bottom electrode of the memory location in the process of being fabricated. The layer  100  may for example be made of any appropriate conductive material, for example made of TiN in this example. The layer  103  is for example a layer intended to form a top electrode of the memory location in the process of being fabricated. The layer  103  is therefore in electrical contact with one face of the layer  101 , by means of the layer  102 . The layer  103  advantageously includes Ti. The layer  103  is for example made from the same material as the layer  100  and is for example formed of TiN in this example. 
     The layer  101  is positioned on the layer  100 . The layer  101  is made of HfO 2 . The layer  102  is positioned on the layer  101 . The layer  102  includes a material having Ti at more than 30% by mole fraction, preferably more than 50% by mole fraction. The layer  102  is here made of Ti. The layer  103  is positioned on the layer  102 . For a layer  102  made of Ti, the layer  103  advantageously makes it possible to prevent the oxidation thereof. A layer  103  made of TiN makes it possible, on the one hand, to prevent the oxidation of the layer  102  made of Ti and, on the other hand, to form an electrode for the memory location in the process of being fabricated, and furthermore to provide Ti for an implantation by collision with recoil effect in the layer  101 . The layer  100  is in electrical contact with one face of the layer  101 . 
     The method implements here a step of ion implantation of a material  2 , from among Xe, Kr or Ar. The ion implantation is carried out here in the layers  103  and  102 . Owing to the ion implantation in the layers  102  and  103 , the collision with recoil effect on Ti of the layers  102  and  103  induces an implantation of this Ti in the layer  101 . It is thus possible to form filament creation zones in the layer  101 , with a composition of Hf 1-y Ti y O 2  type. 
     The diagram from  FIG. 5  shows the bandgap energy of various substoichiometric Hf 1-y Ti y O x  phases in the Hf 1-y Ti y O 2  of the layer  101  obtained, with a value y of 0.2. This diagram shows that the presence of these multiple substoichiometric phases with a Ti alloy makes it possible to homogenize the bandgap energies of these multiple substoichiometric phases, for x≤1.5. A single phase corresponds to a reset state, for x=1.75. Consequently, memory locations using such a dielectric have greatly reduced behaviour dispersions. Thus, the discrimination between a programmed state and a reset state may be made relatively easily. Thus, the potential differences for switching between the programmed and reset states of the memory location may be substantially reduced, without impairing the reading of the memory location. 
       FIG. 6  is a diagram illustrating the bandgap energy of various substoichiometric phases, as a function of the proportion of titanium included in Hf 1-y Ti y O 2  and Hf 1-y Ti y O 1.5 . This diagram shows that the Hf 1-y Ti y O 2  phase retains a high bandgap energy up to a value of y=0.5, which guarantees that good insulation properties will be retained for the Hf 1-y Ti y O 2  phase and that the formation of semiconductor phases will be allowed. It is also observed that the Hf 1-y Ti y O x  phases for x=1 or x=1.5 retain a relatively low and stable bandgap energy irrespective of the value of y. 
       FIG. 7  is a comparative diagram of the thermodynamic stability of a substoichiometric alloy of Hf 1-y Ti y O 1.5  relative to Hf 1-y Ti y O 2 , as a function of the value of y. It is observed that the Hf 1-y Ti y O 2  is more stable than the Hf 1-y Ti y O 1.5 , up to a value of y of 0.3 approximately. Use will thus advantageously be made of a value y≤0.3 in order to generate less Hf 1-y Ti y O and Hf 1-y Ti y O 1.5 , which are more stable and formed in a larger proportion beyond this value. 
     It is deduced from these diagrams that a value of y between 0.05 and 0.3 proves advantageous, for retaining both a reduction of the dispersions, an increase of the thermodynamic stability, and good insulating properties of most of the layer  101 . Preferably, the value of y is between 0.1 and 0.25. 
     The ion implantation of Xe, Kr or Ar is carried out with an inclination of between 5° and 30° relative to the normal to the layer  102 , and preferably between 7° and 25°. Such an inclination for the ion implantation makes it possible to lower the proportion of Ti in the depth of the layer  101 . 
       FIG. 8  is a diagram illustrating the proportion of Ti in the layer  101  as a function of the depth in this layer  101 , for various implantation energy values. The ion implantation is here carried out with an inclination of 7°, and an Xe ion implantation dose of 10 15  cm −2 . In this example, the layer  100  was made of TiN, the layer  101  had a thickness of 10 nm, the layer  102  had a thickness of 5 nm of Ti, and the layer  103  had a thickness of 5 nm of TiN. The diagram illustrates that the concentration of Ti decreases greatly in the depth of the layer  101 , which makes it possible to delimit the zone of the layer  101  that includes a significant concentration of Ti. 
     Furthermore, it was observed that a significant concentration of Ti was implanted in the layer  101 , starting from an implantation energy at least equal to 10 keV. The implantation energy is advantageously at most 40 keV. 
     The implantation in the layer  101  by collision with recoil effect with Xe, Kr or Ar makes it possible to obtain relatively high concentrations of the implanted material, starting from a relatively reduced ion implantation dose, in particular with respect to a process of direct ion implantation in the layer  101 . The duration of the ion implantation may thus be relatively reduced, which shortens the duration of the memory location fabrication method. 
     Advantageously, the ion implantation is carried out with Xe, which proves to be electrically inert and sufficiently heavy to maximize the recoil effect, which is advantageous in order not to disrupt the electrical properties of the memory location by the parasitic implantation thereof in the layer  101 . Advantageously, the Xe ion implantation dose is between 2×10 14  cm −2  and 4×10 15  cm −2 , preferably between 5×10 14  cm −2  and 2×10 15  cm −2 . 
     Advantageously, the layer  103  has a thickness of between 3 and 12 nm, preferably equal to 5 nm. Preferably, the layer  102  has a thickness between 3 and 12 nm, preferably equal to 5 nm. Preferably, the sum of the thicknesses of the layers  102  and  103  is between 6 and 15 nm. Preferably, the layer  101  has a thickness of between 3 and 20 nm, preferably equal to 10 nm. 
     Advantageously, the ion implantation is carried out through an opening  14  of a hardmask  104 , as illustrated in the cross-sectional view of  FIG. 9 , of a variant of the first embodiment. 
       FIG. 10  is a schematic cross-sectional view of an example of the structure of the layer  101 , obtained by means of the variant illustrated in  FIG. 9 . The use of the opening  14  of the mask, combined with the decrease in the concentration of Ti in the thickness of the layer  101 , makes it possible to obtain an implantation of Ti having a cross section or zone  71  of substantially triangular shape. Such cross section  71  of triangular shape makes it possible to limit the volume of the filament zone that may potentially have defects. 
     The cross section  71  of the layer  101  has a given proportion of Ti. The cross section  71  is surrounded here by a cross section or zone  72  of the layer  101  having a much lower proportion of Ti, or even zero Ti. In particular, the cross section  72  may have a proportion of Ti at least two times lower than that of the cross section  71 . 
     The zone  72  forms here a separation between the zone  71  and the interface between the layers  100  and  101 . Advantageously, the thickness of this separation is at least equal to 1 nm, and preferably at most equal to 2 nm (in order to guarantee a conduction across the memory location in the LRS state with a low potential difference). The presence of such a separation makes it possible to reduce the dimension of the zone  71  that may have defects, and makes it possible to obtain a point effect for the filament form, with a very precise location of the filament formed and therefore a limitation of switching dispersions. 
     The zone  71  may be formed by using an ion implantation with an angle of between 20° and 25° relative to the normal to the layer  101 . Such an angle makes it possible to promote the point effect. 
     The diagram from  FIG. 12  illustrates the influence of the implantation energy of the ion implantation on the concentration of various components in the layer  101 . The blackened area corresponds to the concentration of Ti, the dotted area corresponds to the concentration of Xe, and the broken line corresponds to the concentration of N. It can generally be observed that the respective concentrations of N and of Xe remain relatively low irrespective of the implantation energy, and typically below 1%. It can in particular be deduced therefrom that the superposition of the layers  102  and  103  makes it possible to limit the amount of Xe implanted in the layer  101 . It can also be deduced therefrom that the presence of the layer  103  does not induce an excessive concentration of N in the layer  101 . A limited concentration of N in the layer  101  following the ion implantation makes it possible to avoid disrupting the switching of the memory location. It can also be observed that a high implantation energy made it possible to implant a relatively large amount of Ti in the layer  101 . A memory location characteristic of a fabrication method according to the invention has a layer  101  having a concentration of Xe typically of between 0.1% and 0.9%. 
     According to a variant, it is possible to carry out a stepwise annealing, in a range extending from 300° C. to 450° C.: this makes it possible to make the Ti diffuse into the layer  101  and to optimize the implantation profile thereof. Such an annealing is compatible with the thermal budgets used in the metallization layers. 
       FIG. 11  is a cross-sectional view of a stack  1  during a step of a method for fabricating an OxRAM memory location, according to an example of a second embodiment of the invention. 
     The stack  1  comprises here the superposition of a layer  100  and of a layer  101 , that may have the dimensions and compositions described in detail in the first embodiment. The layer  100  is here also in electrical contact with one face of the layer  101 . 
     The stack  1  further comprises a TiO 2  layer  105 . The layer  105  is positioned on the layer  101 . The layer  105  has a thickness of between 3 and 12 nm, preferably equal to 5 nm. In this embodiment, the TiO 2  layer  105  may be obtained by previously carrying out the deposition of Ti on the layer  101 , then by letting this layer of Ti oxidize in order to form the TiO 2  layer  105 . 
     The method here also carries out a step of ion implantation of a material  2 , from among Xe, Kr or Ar. The ion implantation is carried out here in the layer  105 . Owing to the ion implantation in the layer  105 , the collision with recoil effect on Ti of the layer  105  induces an implantation of this Ti in the layer  101 . It is thus possible to form filament creation zones in the layer  101 , with a composition of Hf 1-y Ti y O 2  type. The ion implantation may be carried out with the same inclination, dose or implantation energy parameters. 
     The ion implantation is carried out with a layer  105  that is bare and directly in contact with the layer  101 . Thus, for a given implantation energy, the proportion of Ti that can be implanted in the layer  101  by collision with recoil effect may be increased. Furthermore, owing to the density of oxygen in the layer  105 , the implantation by collision with rebound effect promotes the formation of the Hf 1-y Ti y O 2  phase in the layer  101 . It is thus possible to better control the switching between the programmed state and the reset state of the memory location. 
     The fabrication method may comprise a subsequent step of depositing a layer of TiN on the layer  105 , with a view to forming another electrode of the memory location. 
     The diagram from  FIG. 13  represents the concentration of Ti as a function of the depth, in a stack  1 , for various Xe implantation energies. The depth D=0 corresponds to the interface between a layer  102  as described previously and a layer  101  having a thickness of 10 nm. The solid line curve corresponds to an Xe implantation energy of 6 keV, the dotted line curve corresponds to an Xe implantation energy of 15 keV, the broken line curve corresponds to an Xe implantation energy of 22 keV, another broken line curve corresponds to an Xe implantation energy of 28 keV, and the dot-and-dash line curve corresponds to the plot of the Hf. 
     It can be observed, on the one hand, that the decrease in the proportion of Ti in the layer  101  decreases relatively linearly, irrespective of the implantation energy. Such an implantation can therefore quite easily enable a Ti implantation profile having a cross section or zone  71  of substantially triangular shape to be obtained, as illustrated in  FIG. 10 . Furthermore, it can be observed that the implantation energy makes it possible to influence quite precisely the concentration of Ti at the interface between the layers  101  and  102 , as a function of the Xe implantation energy. It is furthermore observed that the implantation energy makes it possible to control quite precisely the Ti implantation depth, and therefore to control quite precisely the dimension of a thickness of a zone  72  between the cross section  71  and the interface between the layers  100  and  101 .