Takashi Kouzaki

Interface microstructure of titanium thin‐film/silicon single‐crystal substrate correlated with electrical barrier heights

The titanium (Ti)/single‐crystal silicon (Si) interface has been examined by cross‐section high‐resolution transmission electron microscopy (HRTEM) combined for the first time with 2‐nm‐diam probe, energy‐dispersive spectrometry. HRTEM shows that thin Ti‐Si alloy formation always occurs at the interfaces, even in the as‐deposited state. The thickness of the reacted alloy depends on the crystallinity of the Si surface, but does not depend on impurities or doping level. Crystallization of the Ti‐Si alloy depends on the annealing temperature; it remains in the amorphous phase after annealing at temperatures lower than 430 °C, and the C49 TiSi2 crystal phase was observed as the first crystalline phase after annealing at 460–625 °C. The composition of the Ti‐Si alloy at the Si interface is close to TiSi2, and it remains amorphous with variable composition across the alloy. It seems that the electrical barrier height is determined by the degree of crystallinity of TiSi2 at the Si interface. The barrier height f...

Dependence of thermal stability of the titanium silicide/silicon structure on impurities

The effect of impurity on the thermal stability of titanium silicide (TiSi2)/single‐crystal silicon (Si) structures has been studied. It is found that nitrogen and oxygen in the TiSi2 film significantly influence the morphological changes of a TiSi2/Si structure during high‐temperature annealing at 1100 °C for 2–20 s. Nitrogen impurity improves the thermal stability of the TiSi2/Si structure, whereas oxygen degrades it.

Quantitative study of nitrogen doping effect on cyclability of Ge-Sb-Te phase-change optical disks

By nitrogen doping into a Ge-Sb-Te phase change optical disks recording layer, we were able to significantly increase its cyclability. For example, our PD attained, at the maximum, 800,000 overwrite cycles through accurate control of nitrogen concentration. We quantified the nitrogen concentration of recording layer using secondary ion mass spectrometry (SIMS) and determined, from the viewpoint of cyclability, signal amplitude and other parameters, the optimum concentration to be around 2 - 3 at.%. From analyses by thermal desorption mass spectrometry (TDMS) and X-ray diffraction (XD) using powder, we found: (1) nitrogen atoms are mainly bound with Ge to create an amorphous phase of Ge-N; (2) as long as the nitrogen concentration remains around 5 at.%, those Ge, Sb and Te atoms which are not bound with nitrogen form NaCl type crystals. We obtained the following model by combining the results of the above analysis. Nitrogen-doped Ge-Sb-Te recording layer is composed of Ge-Sb-Te grains intermingled with a small quantity of amorphous Ge-N, which exists in the form of a thin film penetrating the grain boundary of Ge-Sb-Te. The Ge-N composing this high-melting-point material layer appears to suppress any micro-material-flow that may occur during overwrite.

Structure and electrical properties of interfaces between silicon films and n+ silicon crystals

We have studied the morphology and electrical characteristics of the interfaces between silicon‐implanted, low‐pressure chemical‐vapor‐deposited (LPCVD) silicon films with an n+ single‐crystal silicon substrate. Using high‐resolution transmission electron microscopy, it is shown that the silicon implant causes a ‘‘balling up’’ of the native oxide layer at the interface and epitaxial growth occurs in the LPCVD silicon film even after rapid thermal annealing at only 940  °C for 30 s. This morphological change results in a realization of a low‐ohmic‐resistivity LPCVD silicon/ n+ single‐crystal silicon contact even at a sub‐half‐μm size, although the unimplanted contact becomes nonohmic. The leakage current for the implanted contact is as low as that for the unimplanted one in shallow junctions.

Morphology of the silicon implanted interface of a polysilicon/single crystal silicon structure

Abstract Employing high resolution TEM, it was shown that the morphological change of polysilicon/single crystal silicon (poly-Si/Si) interface induced by silicon implantation and subsequent annealing at 900° C for 30 min greatly depended on implantation dose. After annealing, the interfacial native oxide layer implanted with 3 × 10 15 cm −2 Si was broken up into a discontinuous layer, which still remained at the interface. For the interface implanted with 1 × 10 16 cm −2 Si, however, “oxide balls” of 10–20 A diameter were formed and the distribution of the oxide balls was broadened (about 100 A width). SIMS analysis revealed that for an interface implanted with 1 × 10 16 cm −2 Si, the distribution of oxygen atoms near the interface broadened by the implantation and that it slightly narrowed after annealing. These morphological changes influenced the contact resistance of poly-Si/Si interface, especially in a sub-half-μm region.

Rapid Thermal Annealed Undoped LPCVD Si/Single Crystal Si Substrate Structures with an Implant of Si Near Interfaces

The breaking up of a native oxide layer of a LPCVD amorphous Si/single crystal n + Si substrate interface by a rapid-thermal annealing was studied from the point of view of oxygen movement and morphological change. Oxygen atoms began to move at 1025 °C. After annealing at 1115 °C for 30sec, the quantity of oxygen atoms near the interface decreased dramatically and a silicon implant near the interface could enhance the decrease. More detailed observation was carried out by cross-section high-resolution transmission electron microscopy. After annealing at 940 °C for 30sec, the native oxide layer was continuous. On the qther hand, with a silicon implant near the interface, it changed into small oxide balls and an epitaxial growth occurred in the LPCVD layer with twins caused by these oxide balls. After annealing at 1115°C for 30sec, even without the silicon implant, a complete epitaxial growth occurred but it seemed that some SiOx particles dissolved into a single crystal Si layer near the interface.

Highly reliable high-temperature aluminum sputter metallization

A highly reliable high-temperature Al-Si-Cu sputter metallization, employing a Ti underlayer to prevent Si from precipitating has been developed, and complete filling of 0.15 micrometers diameter vias with aspect ratio of 4.5 has been achieved. Degree of filling and via chain resistance were improved by increasing the Ti underlayer thickness. This is probably because of improvement in wettability of Al on via sidewall, which is caused by uniform interfacial reaction between Ti underlayers and Al-Si-Cu films. Transmission electron microscopy (TEM) combined with micro energy dispersive spectrometry (EDS) analysis revealed that reacted ball-like precipitates exist at the interface between the first metal and the second metal lines in the filled via, and that the precipitates particles are Al-Ti-Si compounds. No Si precipitation was observed in areas away from or near to the particles. Also, it was found that Al films in the vias consist of one or two single crystalline <111> textured normal to a substrate. The electrical resistance for the 0.3 micrometers sputter filled via was 0.71 (Omega) , which is about one order of magnitude lower than that for a non-filled (conventional) via. The electromigration (EM) resistance of 0.3 micrometers filled vias was found to be four orders of magnitude greater than that for the 0.3 micrometers conventional vias. Furthermore, we confirmed that the EM resistance for the 0.3 micrometers filled via is comparable to the 0.9 micrometers conventional via. Superior EM and stress-induced migration (SM) resistance for the lines have been confirmed.

Roughness Control and Electrical Properties of SiO2/Si(001) Interfaces

Conventional wet cleaning and ultra-high vacuum heated cleaning prior to thermal oxidation were compared with respect to their effects on extremely thin-SiO2/Si(001) interface roughness. Atomically flat silicon-oxide interfaces were obtained for the latter. As for the former, interface roughening seen at the initial oxidation stage seems to affect inversion electron mobility degradation.