Impact of the precursor chemistry and process conditions on the cell-to-cell variability in 1T-1R based HfO2 RRAM devices

The Resistive RAM (RRAM) technology is currently in a level of maturity that calls for its integration into CMOS compatible memory arrays. This CMOS integration requires a perfect understanding of the cells performance and reliability in relation to the deposition processes used for their manufacturing. In this paper, the impact of the precursor chemistries and process conditions on the performance of HfO2 based memristive cells is studied. An extensive characterization of HfO2 based 1T1R cells, a comparison of the cell-to-cell variability, and reliability study is performed. The cells’ behaviors during forming, set, and reset operations are monitored in order to relate their features to conductive filament properties and process-induced variability of the switching parameters. The modeling of the high resistance state (HRS) is performed by applying the Quantum-Point Contact model to assess the link between the deposition condition and the precursor chemistry with the resulting physical cells characteristics.

In this paper, the switching behavior of cells during the forming, set and reset procedures is monitored by an incremental pulse and verify algorithm 18,19 . In order to analyze the peculiarity of the switching behavior activation and the process-induced inter-cell variability, 100 cells per process variation have been considered. To evaluate the endurance properties, 100 switching cycles have been performed to analyze the impact on the switching voltages on the RR. Modeling of the HRS obtained has been performed through the Quantum-Point Contact (QPC) model [20][21][22][23] to link the technology process characteristics 24 with cells performance and reliability.

Results/Discussion
In order to evaluate the impact of the HfO 2 deposition parameters on the switching characteristics of memristive devices, a halide (HA) Hf precursor was used at two different deposition temperatures (150 °C and 300 °C) and compared with a metalorganic (MO) Hf precursor used at the same deposition temperatures. The physical analysis as X-Ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) of the HfO 2 films were performed after finalizing the complete CMOS process flow. It should be mentioned, that the HfO 2 films were subsequently covered by a stacked Ti/TiN layer and finally annealed at 400 °C in N 2 /H 2 ambient for 30 min.
As shown in Fig. 1(b), X-Ray diffraction studies were performed to study the microstructure of the HfO 2 films. According to XRD, the HfO 2 deposited by the use of the metalorganic precursor (MO) at 150 °C and 300 °C were grown in the amorphous state. The amorphous microstructure of thin HfO 2 films, grown by the use of metalorganic precursor is consistent with reported ALD studies, the as-deposited amorphous films starts to crystallize after post-annealing at about 500 °C 25,26 . In contrast to the metalorganic precursor based deposition, the HfO 2 films deposited at 300 °C by the use of the halide precursor are polycrystalline in the monoclinic phase. At the deposition temperature of 150 °C, the HfO 2 film was grown in the amorphous state, which remains stable at post-annealing temperatures of 400 °C 27 . The crystallinity of the HfO 2 films was further examined microscopically by Transmission electron microscopy and the results are shown in Fig. 2. The HfO 2 films, grown by the use of the MO precursor are amorphous ( Fig. 2(a,b)), while the HfO 2 film, deposited using the HA precursor at 300 °C is grown in a polycrystalline structure. However, the obtained TEM results confirm the previous X-Ray studies.
The carbon content, oxygen concentration and Hf/O ratio in the HfO 2 films are analyzed by depth profiling of the memristive stack by using X-ray photoelectron spectroscopy. As illustrated in Fig. 1(a), the carbon content in the HfO 2 films is strongly affected by the precursor. Using the halide precursor strongly reduces the carbon content in the HfO 2 to less than 1%, while the films deposited by the metalorganic precursor contain 7-9% carbon residuals caused the molecular structure of the used metalorganic precursor 25 . It has to be added, that the small carbon peak at the interface between HfO 2 and the Ti x O y N z film is caused by the vacuum break between the HfO 2 ALD and the growth of the Titanium layer by PVD (Plasma Vapor Deposition). In addition, the oxygen content is found decreasing with increasing amount of carbon atoms, as listed in Table 1. The different process conditions were labelled as A, B, C and D as summarized in Table 1.
During the CMOS fabrication flow of the RRAM array, several annealing steps at 400 °C for 30 min were applied. These steps activate the scavenging properties of the Ti layer, resulting in the oxygen content reduction in the HfO 2 layer 28 . Due to the layer asymmetry of the resistive MIM device, the distribution of the oxygen vacancies in the HfO 2 layer is not uniform. There is a strong gradient from the top interface (i.e., with Ti) to the bottom interface (i.e., with TiN) of the HfO 2 film. Hence the sensitivity of the XPS depth profile setup is not sufficient to provide the exact oxygen vacancies content but sufficient to provide the oxygen concentration. In case of carbon, the concentration can be assumed as homogenous since the carbon atoms are incorporated through the deposition process and not during the annealing step.
The schematic and cross-sectional TEM images of the integrated RRAM cell including the metal lines, the MIM materials and the Tungsten-based via connections are shown in Fig. 3. In order to study the impact of the HfO 2 deposition condition on the switching behavior systematically, the pristine currents of the memristive devices at 1 Volt are investigated primarily. As shown in Fig. 4, the currents are strongly affected by the deposition conditions as well as the gettering activities of the Ti layers. In case of the amorphous microstructures of HfO 2 : with increasing carbon content, the current is also increasing. The large pristine current fluctuations between different cells of the poly-crystalline HfO 2 film (process A) are caused by the high affinity of the grain boundaries to charged oxygen vacancies causing leakage paths in some of the cells 29,30 .
Afterwards, the memristive cells were formed by the so-called form and verify algorithm. The cumulative distributions of the forming voltages are shown in Fig. 4(a). The films deposited by the process conditions C and D are the ones where the pristine currents (see Fig. 3) are the largest caused by the high carbon content and the low oxygen concentration in the HfO 2 layers. Lower electric potentials are needed to form these memristive cells.
Process B features a very low carbon concentration in the amorphous dielectric layer, resulting in a slightly difficult forming operation (i.e., larger voltage requested as shown in Fig. 5(a)), but with a higher degree of uniformity as shown by its pristine current. The films deposited by process A exhibit a very low carbon concentration, although the poly-crystalline structure of the dielectrics makes it less reliable and controllable in terms of forming, as shown in Fig. 5(a). All processes with amorphous microstructures are providing a clear trend: Larger carbon concentration increase the pristine currents, causing a decrease of the median forming voltage, a decrease of the median read current after forming/set, and an increase of the median read current after reset. The decrease of the LRS currents with raising carbon content, as illustrated in Fig. 5(b) could be related to the higher probability of having carbon atoms next to the narrowest part of the filament. Such atoms could create preferential conductive paths, repelling the oxygen vacancies from moving into that region and limiting the conductive filament growth 22 .  In order to identify the endurance characteristics of the memristive devices, 100 to 1000 cycles using the incremental pulse and verify algorithm were performed. The average HRS and LRS resistances and their standard deviation were evaluated by cycling 20 cells per deposition process variation. As illustrated in Fig. 6, the HRS and LRS states of process B remain stable during the endurance cycling, mainly due to the low content of carbon as well as the amorphous microstructure of the HfO 2 film. With increasing carbon content (processes C and D) or introducing grain boundaries (process A) the stability and controllability of the HRS states is reduced due to an increase of the leakage current, causing a reduction of the resistance ratio.  The resistance ratio (RR), V SET , and V RES average values and normalized variances (i.e., the ratio between variance and the average value) as function of cycling are reported in Fig. 7. The Halide based precursor processes (A, B) reach the thresholds of the set and reset algorithms at lower V SET /V RES values with respect to the metalorganic precursor based processes (C, D). As shown in Fig. 7(a), process A demonstrates the largest RR at the beginning of the cycling stress, but also a fast reduction during the endurance test. A better stability of the RR during cycling is observed for the amorphous HfO 2 films. Moreover, the endurance performance seems to be related to the carbon content: film B, corresponding to the amorphous HfO 2 layer with the lowest carbon content, demonstrates the highest RR with the lowest normalized variance (see in Fig. 7(b)) after 100 cycles with an excellent stability during cycling. There is a clear trend: With increasing carbon content, RR is reduced.

Proc. Precursor T dep (°C) Carbon content Oxygen content Microstructure
As shown in Fig. 7(c,e), the V SET and V RES values increase with raising carbon content in the amorphous films. Considering the variances, illustrated in Fig. 7(d,f), devices B still demonstrate the lowest values and the highest stability during cycling, confirming that the carbon content plays a fundamental role on cells' performance and reliability 21 . This means, when the conductive filaments in the devices B are correctly formed, the subsequent set and reset operations are not impacted by carbon impurities, resulting in reduced dispersion of the switching voltage values. The cycling behavior of the memristive devices C and D is different, the subsequent set/reset operations are more difficult and the required voltages for set/reset switching increase as well as their dispersion. Moreover, the oxygen concentration, which decreases when the carbon content increases, plays a role. The reduced concentration of oxygen vacancies reduces the ion mobility 31 , hence a higher voltage is needed to move the oxygen vacancies in order to recreate and rupture the filament. Since the deposition process B provides the best performance of the memristive devices after 100 cycles, the endurance test has been extended to 1000 cycles: no relevant variation of the parameters has been observed.
In order to correlate the obtained experimental results, obtained by pulsed induced switching, with the quantum mechanical nature of the currents in LRS and HRS, additional DC measurements were performed. To understand the impact of the carbon within the HfO 2 film on the cells' conduction properties after the reset process, the quantum point contact model is applied to the IV characteristics. In this regard, the Quantum Point Contact (QPC) model has been used 16 , allowing the interpreation of the I-V characteristics measured after the reset operation by the following equation: The model parameters are the barrier height Φ which is associated with the bottom of the first quantized level, the curvature parameter α, which is related to the potential barrier curvature, assuming a parabolic longitudinal potential and the symmetry parameter β which represents the fraction of the applied bias that drops at the source side of the conductive filament and defines the constriction symmetry. When β is very close to 1, almost the complete applied bias voltage drops at the source side of the filament, hence the constriction is highly asymmetric. The quantum conductance unit G 0 = 2e 2 /h corresponds to the conductance of a single conduction mode nanowire, where e is the electron charge and h the Planck's constant. The parameter G/G 0 represent the non-ideality of the filaments. This parameter should be 1 in case of an ideal filament structure. The series transistor integrated in the memristive 1T-1R cells (see Fig. 2) has a significantly large area with a proper current driving capability (i.e., W = 1.14 µm, L = 0.24 µm, µ 0 = 1000 cm 2 /Vs, t ox = 5 nm). During the read operation the transistor constantly works in the linear region with a fixed resistance which is negligible compared to one of the 1 R element. Therefore, the transistor is not included in the simulation of the HRS current by the QPC model. The average values and standard deviations of the fitting parameters are illustrated in Fig. 8. The extracted curvature parameter α is quite similar for the amorphous HfO 2 films grown by the process conditions B, C and D, while α is slightly larger in the poly-crystalline film HfO 2, deposited by process A. Larger α could be interpreted as an increase of the width of quantum mechanical barrier. This increasement could be ascribed to the change of the shape of the constriction caused by the microstructure of the HfO 2 film. The width of the barrier and its variability is mainly impacted by the microstructure of the HfO 2 film and is not affected by the carbon content.
Concerning the impact of the carbon content to the height of the quantum mechanical barrier, Φ is decreasing with raising the carbon content, as illustrated in Fig. 8(b). The decreasing barrier height leads to larger HRS currents, as illustrated in Fig. 6(b) and consequently to a reduction of the resistant ratio, as shown in Fig. 7(e).
Within the series of amorphous HfO 2 films, the process variation B leads to the lowest variability of the barrier height Φ, providing the highest cell-to-cell uniformity. The largest variability of the barrier height is evaluated for HfO 2 films deposited by process A, which is mainly caused by the poly-crystalline structure.
The QPC parameter β is defining the position of the constrction point. Independent of the process conditions, the evaluated values for the parameter β are close to 1, as illustrated in Fig. 8(c). Therefore the constriction point is  Fig. 9(a), creating an asymmetric constriction due to the presence of the Ti layer 18 . However, the presence of carbon residuals as well as the microstructure of the HfO 2 layer don't have an impact to the asymmetric position of the constriction point.
In addition to the extracted quantum mechanical parameters, the relationship between α and the potential barrier thickness d can be calculated as 16,32 :  where m * is the electron effective mass in the constriction. The equivalent radius r of the constriction point, corresponding either to a single filament or to multiple conductive filaments in parallel, can be calculated as: where z 0 = 2.404 is the first zero of the Bessel function J 0 16 . The HRS structure of the filament obtained after the reset process is sketched in Fig. 9a).
The average values of d and r as a function of the carbon content are illustrated in Fig. 9(b,c). The average barrier width is decreasing with increasing carbon content, whereas the average radius is increasing.
The impact of carbon residuals on the width of the barrier d and the conductive filament constriction radius r could be explained by additional trap levels inside the HfO 2 band gap formed by carbon ions 21 , generating a reduction of the barrier.
When the carbon content increases, the LRS current decreases, as illustrated in Fig. 5(b). This negative impact could be related to the creation of partially formed filaments involving carbon defects, which prevent the complete growth of the oxygen vacancy based conductive filament growth. To sum up, the origin of the cell-to-cell variability can be enlightened by the use of the QPC model. By applying the QPC model to the DC-switching curves, the variability is affected by he microstructure of the deposited dielectric film as well as its carbon content.

Conclusion
The impact of the precursor chemistry as well as the affect of the deposition temperature have been studied. In order to evalaute the endurance characteristics of the memristive devices, 100 switching cycles using the incremental pulse and verify algorithm were performed. With increasing carbon content or introducing grain boundaries in the HfO 2 films, the stability and controllability of the HRS states is reduced due to an increase of the leakage current, causing a reduction of the resistance ratio. The grain boundaries of the poly-crystalline HfO 2 films are causing a high cell-to-cell variability during the endurance test.
Amorphous HfO 2 films deposited by using the halide precursor provide the highest inter-cell and intra-cell uniformity. Metalorganic precursors-based processes result in amorphous HfO 2 films as well, although the carbon content is higher. The inter-cell uniformity seems to be affected by the carbon content: HfO 2 films with high carbon content show reduced restistance ratios and an increased variability of the set and reset parameters.
In order to understand the impact of the carbon content in the HfO 2 films on the cells' switching characteristics, the quantum point contact model was applied to the IV curves. The height of the quantum mechanical barrier is decreasing with raising carbon content. The decreasing barrier height leads to larger HRS currents and consequently to a reduction of the resistant ratio. In contrast to the height of the barrier, the width of the barrier and its variability is mainly impacted by the microstructure of the HfO 2 film and is not affected by the carbon content.
In conclusion, HfO 2 based memristive cells manufactured with halide precursors at low deposition temperature provide the most promising results in terms of cell-to-cell variability and switching reliability.

Methods
Preparation and analytical characterization of the HfO 2 based 1 T1R RRAM arrays. The 1T-1R memory cells are constituted by a select nMOS transistor manufactured in BiCMOS technology (width of 1.14 µm and length of 0.24 µm), which also sets the current compliance, whose drain is in series to a MIM stack. The MIM area is equal to 0.4 µm 2 . Metal 1 as well as Metal 2 are metallic layer stacks, consisting of Ti/TiN/Al/TiN/Ti. The MIM integrated on the metal line 2 of the BiCMOS process is composed by 150 nm TiN top and bottom electrode layers deposited by magnetron sputtering, a 7 nm Ti layer, and an 8 nm HfO 2 layer deposited through thermal ALD with the four different processes. A halide (HA) Hf precursor (HfCl 4 ) was used for processes A and B, whereas for processes C and D a metalorganic (MO) Hf precursor was used in combination with H 2 O as oxygen source.
The process flow of the samples used for XRD analytics was slightly modified. The HfO 2 films, deposited by the MO-based precursor were grown on TiN films, which were grown by atomic vapour deposition (AVD). These TiN films were deposited at 400 °C from a pure Ti(NEt 2 ) 4 precursor and NH 3 by AVD. The HfO 2 grown by the HA-based precuror are deposited on PVD TiN. The PVD TiN layers were reactively deposited using a d.c. magnetron sputtering of Ti and nitrogan as reactive gas at room temperature.
The carbon content, oxygen concentration and Hf/O ratio in the MIM stack are analyzed via X-ray photoelectron spectroscopy (XPS) for all processes. The XPS measurements were performed after annealing the MIM stack at 400 °C for 30 minutes. This annealing step activates the scavenging properties of the Ti layer, resulting in the oxygen content reduction in the HfO 2 layer. Due to the layer asymmetry of the resistive MIM device, the distribution of the oxygen vacancies in the HfO 2 is not uniform. There is a strong gradient from the top interface (i.e., with Ti) to the bottom interface (i.e., with TiN) of the HfO 2 film, hence the sensitivity limit of the XPS depth profile is too small to provide the exact oxygen vacancies content but sufficient to provide the oxygen concentration. In case of carbon, the concentration can be assumed as homogenous since the carbon atoms are incorporated through the deposition process and not during the annealing step.

Electrical characterization. The test environment for cells characterization consists in a Keithley 4200-SCS
wafer-level tester. The Forming/Set/Reset operations were performed by using an incremental step pulse (V STEP = 0.1 V) and verify algorithm 18 . A sequence of increasing voltage pulses is applied on the drain of the cell during Forming and Set, with a transistor gate voltage V G = 1.5 V to set the Forming/Set current compliance, whereas the sequence of increasing voltage pulses is applied on the source of the cell during Reset, with a transistor gate voltage V G = 2.8 V which leads to a 120 µA compliance current. All pulses feature duration of 10 µs in order to maximize the switching yield 7 . After every pulse a read-verify operation is performed, where the cell current I read was measured by applying 0.2 V on the drain of the cell with V G = 1.5 V and a read time T read = 10 µs. When the read current reaches I target = 10 µA the Forming and Set operations are stopped, whereas during Reset the operation is stopped when the read current reaches I target = 2 µA. V FORM , V SET and V RES denote the voltages at which the algorithms targets are reached during Forming, Set and Reset operations, respectively. These parameters reflect the operation of the memory when a Set/Reset algorithm is considered, since they guarantee that a sufficiently high read margin is obtained.
In the DC mode used for QPC characterization, V D was raised from 0 to 2 V during the set operation with V G = 1.5 V and V S was increased from 0 to 2 V during the reset procedure with fixed V G = 2.8 V, respectively. A sweep ramp of 1 V/s was used.