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Critical dimension (CD) shrink and patterning of contact features via plasma etching were studied for typical resistive random access memory (RRAM) stacks. These consist of SiO2 and Si3N4 (total thickness of 65 80 nm) with NiO or pure Ni at the bottom. First, the contact patterning of RRAM stacks was investigated for 90 nm contacts. Thus, a standard high power contact etch recipe was shown to give rise to resist strip challenges due to the incorporation of sputtered Ni in the resist film. Therefore, a low-sputter-yield contact etch recipe based on a CF4/H2/Ar gas chemistry was introduced. The ion sputter efficiency of the recipe was estimated from a blanket SiO2 sputter-yield experiment in Ar plasma for the same recipe settings: this yielded values close to the Ni sputter-yield threshold. Second, plasma-assisted CD shrink was studied in combination with the newly developed patterning scheme to get the contact CD well below the initial 90-nm litho print size. It was shown that a low contact etch power regime could also provide a larger window for contact CD shrink using a C2H4-based chemistry: e.g. the demonstrated CD shrink from 90 nm down to sub-40 nm was shown to be extremely challenging in the case of a high power regime due to polymer instability enhanced with the resulting thickness increase. Perhaps, the relaxation of the polymer film stress, which was measured to be in the range of 1200-1500 MPa, is more easily triggered at higher power settings, which leads to polymer blistering. Finally, the optimization of the plasma-assisted CD shrink step in combination with the low-sputter-yield contact etch recipe was demonstrated to be able to provide CDs as small as 27 nm. The demonstrated approach shows that plasma-assisted CD shrink can provide a robust test vehicle for research programs that require the patterning of small features in the sub-40-nm CD range.
We have characterized the in-film stress and the mechanical constants of a growth-dominated phase-change SbTe-alloy, a material beneficially used in the line-cell phase change memory architecture. The influence of the thickness of the films (5-120 nm) and of the substrate used for its deposition (Si, SiO2 and SiC) was studied. The characterizations were carried out on both amorphous and crystalline films. The crystallization temperature was determined by the resistance changes as a function of the temperature. The mechanical characteristics of the films were measured by the wafer curvature method. We observed that the mechanical behavior of these films was strongly dependent on their thicknesses and on the substrate material. For the thinnest amorphous films, the in-film stress was highly compressive (with the largest compressive stress for films on SiC), while it tended to be very low (fully relaxed films) for films thicker than 20 nm. The amorphous films furthermore did not reveal any stress relaxation in time-dependent stress measurements. Therefore, it was not possible to quantify the viscosity of the SbTe-alloy. This result maybe related to a lower defect state in the very thin films used, or to a blocking of the defects by enhanced reactivity of these thin SbTe-alloy films with their surroundings, both resulting in the absence of relaxation in the amorphous state. Finally, the coefficients of thermal expansion (CTE) of the amorphous and crystalline SbTe-alloys were similar, 1–3×10−6 K−1. These values were comparable to the CTE's of the substrates, clearly indicating that for thin films the substrates dominate their thermal expansion behavior.
The microstructural and electrical characterizations of RuxTa1-x alloys obtained from Ru-Ta laminates are presented. The films were deposited on SiO2 and HfO2 and capped with TiN to avoid oxidation of the top surface. The alloys were attained by post-anneal thermal treatments in the range of 500-1000 °C in Ar atmosphere. Co-sputtered RuxTa1-x alloys were used as references. In particular, Ru0.4Ta0.6 phase could be obtained when the Ru-Ta laminate was annealed at 1000 °C. The alloying reaction is limited either by the tantalum nitride or oxide formation being the source for Nitrogen the TiN capping used on top of the stack and the Oxygen either the dielectric films or the one stuffing the films after exposure to the atmosphere.
Independent of the Ta content a mid gap work function was obtained. Measured WF's in laminate-obtained alloys and alloys themselves differ from other literature data, where a more n-type like WF are measured, and indicating process dependence. In the present study mid-gap or rather p-type work functions were found, 4.5 eV < WF < 4.9 eV.
The impact of material crystallization characteristics on the switching behavior of phase change memory cells has been investigated using finite element simulation. Both a conventional vertical cell and a horizontal line cell have been analyzed, using the widely used Ge2Sb2Te5 (GST) which is a nucleation dominated material for the vertical cell, and Ag5.5In6.5Sb59Te29 (AIST) which is a growth dominated material for the horizontal cell. Nucleation and growth models were implemented for both materials. Both RESET and SET program cycles were simulated. From these simulations, it was shown that the crystallization models gave realistic results for switching voltages, currents and switching times for the two different cell types. It is found that for GST, both nucleation (at lower voltages) and growth (at higher voltages) can play an important role in the crystallization. However, for AIST, crystal growth from non-amorphized crystal regions dominated over nucleation for all program conditions. The high growth rate of AIST moreover is shown to allow much shorter SET times in the line cell compared to that of GST in the vertical cell.
The electrical and material characterization of Ti(C)N deposited by metal organic chemical vapor deposition (MOCVD) technique, as metal gate electrode for advanced CMOS technology is investigated. The effects of the plasma treatment, post anneal treatment and the thickness variation of the Ti(C)N film on the flat band voltage (VFB) and effective work function (WF) of the Poly-Si/Ti(C)N/SiO2 Poly-Si/Ti(C)N/SiO2 gate stack s are reported. We found that both the in-situ plasma treatment and post anneal treatment help in reducing the carbon content (organic) in the film making it more metallic compared to the as-deposited films. However, the post anneal treatment was found to be a better option for getting rid of hydrocarbons as compared to plasma treatment from the gate dielectric integrity point of view. The thickness variation of post annealed Ti(C)N film ranged from 2.5 nm to 10 nm lead to WF shift of upto ~350 mV for both Poly-Si/Ti(C)N/SiO2 and Poly-Si/Ti(C)N/HfO2 gate stacks.
An overview of silicide development for the 65 nm node and beyond is presented. The scaling behavior of Co based and Ni based silicides to sub-100 nm junctions and sub-40 nm gate lengths was investigated. Co and Co-Ni silicides required a high thermal budget to achieve low diode leakage. Even for lower thermal budgets, the sheet resistance of Co and Co-Ni silicides increased at gate lengths below 40 nm. NiSi had low sheet resistance down to 30 nm gate lengths exhibiting a reverse linewidth effect (sheet resistance decreased with decreasing linewidth), achieved lower contact resistivity than CoSi2 and lower diode leakage for similar sheet resistance values. Bridging issues cannot be ignored for NiSi, in particular for thicker Ni films, higher RTP temperatures and in the presence of Ti. Material issues for the application of NiSi were also investigated. Ni2Si was found to grow with diffusion limited kinetics in the 225-300°C range, with an activation energy of 1.5 eV. Results of the kinetic studies were used to design a two-step RTP process that limited the silicide thickness on small features by a low thermal budget first RTP step, reducing the reverse linewidth effect and avoiding excessive silicidation. In the presence of an interfacial oxide, undesired epitaxial NiSi2 pyramidal grains grew directly at temperatures as low as 310°C on p+ Si. Thermal stability of NiSi was also investigated. We found that the initial mechanism of degradation for thin NiSi films was agglomeration, with activation energies of 2.5-3 eV. The surface after agglomeration remained quite flat with alternating NiSi and exposed Si areas, while the interface roughened significantly. Thick films also degraded initially by agglomeration at low temperatures, but by transformation to NiSi2 at higher temperatures. The addition of Pt improved thermal stability of NiSi films against agglomeration. The Ni/Si-Ge reaction was also studied, finding that the addition of Ge reduced the thermal process window and resulted in a slightly higher resistivity.
TiO2 (rutile) single crystal plates with (001) and (110) orientation were immersed in an aqueous solution of Ba(OH)2 0.5 M at temperatures of 100 and 150 °C for 1 and 4 hours in order to grow BaTiO3on them. SEM micrographs of the samples fabricated on the (001) surface of rutile displayed isolated grains with an average height ranging from 200 nm at 100 °C to 700 nm at 150 °C. On the other hand, samples with the (001) orientation exhibited no growth at 100 °C and only a few grains along lines attributed to the polishing process of the substrate at 150 °C. The image of backscattered electrons indicated that barium is concentrated on the grains in all cases. Only the (001) samples exhibited reflections of cubic BaTiO3, as indicated by x-ray dif- fraction, as well as distinct Ba signals under x-ray photoelectron spectrometry. These results agree with the hypothesis of a dissolution-precipitation growth mechanism, in which dissolution is possible for the (001) face, but not for the (110) one, which is the most stable of the low-index faces of this material. Similar treatments were applied to ZrO2:Y2O3 crystals, leading to no film growth.
In the process of developing thin film electro-optical waveguides we investigated the influence of different substrates on the optical and structural properties of epitaxial BaTiO3 thin films. These films are grown by on-axis pulsed laser deposition (PLD) on MgO(100), MgAl2O4(100), SrTiO3(100) and MgO buffered A12O3(1102) substrates. The waveguide losses and the refractive indices were measured with a prism coupling setup. The optical data are correlated to the results of Rutherford backscattering spectrometry/ion channeling (RBS/C). X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). BaTiO3 films on MgO(100) substrates show planar waveguide losses of 3 dB/cm and ridge waveguide losses of 5 dB/cm at a wavelength of 633 nm.
Within our program to develop ferroelectric thin film optical waveguides, we have studied the growth of epitaxial waveguides BaTiO3 on r-plane sapphire substrates with a MgO buffer layer. The films were prepared by pulsed laser deposition (PLD). Their structural properties were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), Rutherford backscattering (RBS) in random and channeling (RBS-c) configuration and atomic force microscopy (AFM). They displayed good crystalline quality, characterized by an RBS-c minimum yield of about 4–6%, a full width at half maximum (FWHM) of the XRD rocking curve measurement of the BaTiO3(200) reflection of 0.32° and a rms roughness of 1.2 nm in a film of ∼ 1.0 μm thickness. The epitaxial relationship was found to be BaTiO3(100) // MgO(100) // A12O3(1102). The refractive index, the birefringence and the optical losses have been measured.
The influence of controlled potential conditions on the structure and phases of BaTiO3 films grown on titanium by the hydrothermal-electrochemical method was studied. The experiments were conducted using a three electrode high pressure electrochemical cell in a 0.2 M Ba(OH)2 electrolyte at 150 °C. The spontaneous initial nucleation linked to pure hydrothermal BaTiO3 formation was inhibited by cathodically protecting the titanium electrode since its immersion in the electrolyte. The application of initial nucleation pulses of varying cathodic potentials affected the grain size of the deposit. The growth of tetragonal BaTiO3 was achieved combining the initial inhibition of the pure hydrothermal growth, followed by nucleation under controlled potentiostatic conditions.
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