Huai-Yu Cheng

Wet etching of Ge2Sb2Te5 films and switching properties of resultant phase change memory cells

Wet-etching of amorphous Ge2Sb2Te5 films was studied by ICP and XPS spectrometries. It is thought that wet-etching arises from chemical etching that starts with bond breakages, oxidation of each element and subsequent dissolution of the resultant oxides. The Ge element debonds more easily from the Ge–Sb–Te matrix than the Te element, but Ge oxide is more stable than Te oxide. The Te element debonds more sluggishly than Ge, although Te oxide is quite unstable. As a result, Ge is the first leached element that dominates the etching process. Sb is the most difficult element to leach in Ge2Sb2Te5 thin films. Cells of phase-change random access memory (PRAM) were successfully manufactured using the wet-etching process, and studies of the switching properties revealed a low threshold voltage of 0.60 ± 0.15 V.

Characteristics of Ga–Sb–Te Films for Phase-Change Memory

Novel materials based on the Ga-Sb-Te system are proposed as a medium for phase-change memory (PCM). Studied compositions were located along the pseudo-binary tie-line GaSb-Sb₈Te₂, in which the atomic ratio Sb/Te of GaSbTe films is higher by incorporating GaSb into Sb₈Te₂, indicating a possible higher crystallization speed hereof. The GaSbTe films were explored by systematic evaluations of crystallinity, crystallization, and melting temperatures (T_x, T_m), resistivity at crystalline and amorphous states (R_c, R_a), R_a/R_c ratio and temperature-dependent electrical resistivity for PCM. The Ga₃ Sb₈Te₁ composition, exhibiting single endothermic peak, lower T_m, higher T_x/T_m and R_a/R_c ratios, suitable T_x, and electrical resistivity at crystalline state, is highly potential for PCM to reduce the reset current

Wet-etching characteristics of Ge/sub 2/Sb/sub 2/Te/sub 5/ thin films for phase-change memory

Etching of phase-change memory thin films is essential in the processing for the manufacture of devices. The Ge/sub 2/Sb/sub 2/Te/sub 5/ thin film as a typical material for such purposes can be control-etched by an aqueous solution of 20% nitric acids (HNO/sub 3/). It was found that the Ge/sub 2/Sb/sub 2/Te/sub 5/ films in amorphous state could be etched more uniformly than that in crystalline state. The etch rate can be well controlled to be 4.6 nm/s using such a solution, resulting in macroscopic and microscopic uniformity on amorphous films. It is therefore suggested that the crystallization annealing of Ge/sub 2/Sb/sub 2/Te/sub 5/ thin films should be done after a wet etching process in the manufacture of phase-change random access memories.

Thermal Stability and Electrical Resistivity of SiTaN x Heating Layer for Phase-Change Memories

Highly resistive SiTaN, amorphous films were studied focusing on their use as heating layers for phase-change random-access memory (PCRAM). The measured electrical resistivity of SiTaN x films between 0.069 and 1.21 Ω cm fulfills the requirements of a suitable heating layer suggested by simulations and is determined by nitrogen content which is tunable up to 52.8%. The SiTaN x films with higher nitrogen contents showed excellent thermal stability with amorphous structure sustained until at least 900°C. However, these samples exhibited a high degree of temperature-dependent resistivity and showed a negative temperature coefficient of resistance (TCR) at the temperature range 400-500°C. The negative TCR increases with increasing N content and Si/(Si + Ta) ratio. The binding energy of SiTaN x films changes with nitrogen content and Si/(Si + Ta) ratio, conforming with the performance of electrical resistivity. By considering electrical resistivity at the range 0.1-0.15 Ω cm and low TCR, SiTaN x compositions, 33.5-34.5 Si, 15.0-16.0 Ta, and 48-50 N in atom %, are recommended optimal for PCRAM heating layers. The single cells with a SiTaN x heating layer underwent preliminary testing.

Highly electrical resistive SiTiNx heating layers and diffusion barriers for PCRAM

Highly electrical resistive amorphous SiTiNx films were explored as both heating layers and diffusion barriers for the cells of phase-change random-access memory (PCRAM). The measured electrical resistivity of SiTiNx films, determined by the nitrogen content x tunable up to 38.68%, is between 0.039–0.69 Ω cm which fulfils the requirements of a suitable heating layer suggested by simulations. SiTiNx films with a medium range of nitrogen content showed excellent thermal stability, very smooth surface and amorphous structure sustaining until at least 800 °C, thus suitable for application in PCRAM. However, SiTiNx films exhibit a high degree of temperature dependent resistivity, and show a different temperature coefficient of resistance with different N contents at the temperature range 400–500 °C. The chemical binding characteristics of SiTiNx films with such higher electrical resistivity were also investigated. The binding energy of Ti 2p spectra in SiTiNx films changes with nitrogen content and shifts to higher energies confirming their susceptibility to oxidation. Besides, interface analysis showed that these films perform excellently as a diffusion barrier between a W bottom electrode and the Ge2Sb2Te5 phase-change layer after evaluation annealing at 600 °C for 30 min.

Tungsten Added Sb

Sb80Te20 has the merit of high crystallization speed yet with low crystallization temperature (Tx ~132degC), and hence, not suitable for use as a medium of phase change random access memory (PCRAM). We proposed to add refractory metals to solve this problem. It was found that W-added Sb80Te20 show increased Tx up to 233degC with increasing W. More important, the melting temperature of W-Sb-Te materials, 536-539degC irrespective of W content, is more than 80degC lower than that of Ge2Sb2Te5. They show a two to four orders of magnitude lowering in resistance during phase change from an amorphous to a crystalline state. With these promising properties, the composition Sb80Te17W 3 is recommended as a potential candidate for PCRAM

_{80}

Structural changes at high temperature of amorphous TaS1 2 C x films deposited on Si(100) were evaluated. Increased carbon content remarkably raises crystallization temperature; thus, TaSi 2 C x films (x > 16 atom %) sustain amorphous phase at 800°C for at least 30 min. A preliminary evaluation of such films as a diffusion barrier of Cu metallization in a sandwich scheme Si(100) / TaSi 2 C x (20 nm)/Cu showed the stability of 750°C (x = 19 atom %) or 800°C (x = 22 atom %) for at least 5 min without a sharp increase in sheet resistance nor the formation of Cu 3 Si. Because Ta, Si, and C are compatible with integrated-circuit processing, these films are readily applicable as diffusion barriers in Cu metallization.

Te

A Bi-doped ZnO layer was prepared by means of electrodeposition. The lattice structure of the deposits examined by grazing-incidence X-ray diffraction showed a ZnO wurtzite structure without impurity phases [metallic Zn, Bi, or Zn(OH) 2 ]. It was found that the surface morphology, Bi content, and electrical property of the deposits were strongly dependent on deposition conditions, such as bath composition, applied potential, and even the counter electrode used (Pt-coated Ti gauze or Zn sheet). The Bi content in the deposits is 0.09-1.58 atom % Bi as determined by inductively coupled plasma atomic emission spectroscopy. The sheet resistance of the deposit with Bi content 1.58 atom % exhibited a 4 order of magnitude improvement compared to that of undoped ZnO. By X-ray photoelectron spectroscopy it was found that Bi in ZnO matrix is in a mixed state of Bi° (26%) and Bi 3+ (74%), which contributes to the defect chemistry for better electric conduction. The 1.58 atom% Bi-doped ZnO layer exhibited p-type conduction with carrier concentration and mobility equal to 1.18 X 10 17 cm 3 and 32.7 cm 2 V -1 s -1 , respectively.