Yasuaki Terui

Fabrication technologies for dual 4-kbit stacked SRAM

The process technologies for realizing a 3-D LSI are reported with emphasizing the high quality laser recrystallization and the thermally stable interconnects. A homogeneous recrystallization of the silicon island array was achieved all over a wafer by the dual laser beam recrystallization method (DLB), in which the 2-dimensional energy distribution of the laser beam was precisely controlled. Thermally stable interconnects in the 1st layer were realized by W wiring and stoichiometry controlled W/WSi/Si contact structure. By these key technologies, the 8-kbit CMOS SRAM with two-active layers has been fabricated. It was confirmed that these technologies were available to fabricate the 3-D LSI.

Multilayer CMOS device fabricated on laser recrystallized silicon islands

Multilayer CMOS devices were fabricated by a laser recrystallization technology. The single crystalline silicon islands embedded in an insulator on the top of MOS IC were studied. To minimize the thermal influence on a lower IC during the fabrication process, a CVD-SiO2was used as a retaining wall of silicon islands instead of a LOCOS. In addition, a planarized heat sink (PHS) polysilicon layer was employed to eliminate the impediment to a grain growth at the steps due to a lower IC. The single crystalline silicon islands were successfully obtained. The field effect mobility of NMOS transistors fabricated on single-crystallied silicon islands was calculated as about 400 cm2/V.sec, 490cm2/V.sec. The 10 bit CMOS shift register fabricated just on a lower IC was successfully operated at VDD=6.5 V, fCLK= 1 MHz. The 4 bit CMOS shift register, stacking PMOS on NMOS was also successfully operated.

Narrow-Band KrF Excimer Laser -- Tunable And Wavelength Stabilized

A tunable and wavelength stabilized KrF excimer laser has been developed. Two Fabry-Perot etalons of 2.4 THz and 170 GHz FSR are placed inside of the laser cavity to reduce the lasing bandwidth to 0.003 nm (3 pm). The wavelength was found to be tunable over a range from 248.1 nm to 248.6 nm. Within this tuning range, a stable wavelength setting to ±0.5 pm was achieved with a feedback system composed of a wavelength detector and a controlling mechanism. This wavelength-stabilized tunable KrF excimer laser will find its applications in optical microlithography, plasma diagnostics and photochemical reactions.

Thermally stable W/Silicide/Si contact

A thermal stability of the W/WSi<inf>x</inf>/Si, W/TiSi<inf>x</inf>/ Si, W/TiN/TiS<inf>2</inf>/Si contact structures has been investigated under annealing conditions up to 900°C. Thermal reactions in the W/WSi<inf>x</inf>/Si and W/ TiSi<inf>x</inf>/Si contact structures were strongly affected by a metal-silicon ratio, X, and a material of metal silicide. The silicide layers with precisely controlled X values were formed by the novel co-sputter method. The thermally stable W/WSi<inf>2</inf>/Si contact structure was obtained, while the W/TiSi<inf>2</inf>/ Si contact structure changed into a WSi<inf>2</inf>/TiSi<inf>2</inf>/Si contact structure after annealing. At the TiSi<inf>2</inf>/Si interface, a segregation of oxygen did not occur after 900°C annealing. A thermally stable and low resistance W/Silicide/Si contact structure up to the 900°C process was successfully realized by the W/TiN/TiSi<inf>2</inf>/Si contact structure. The TiN was formed by a rapid thermal nitridation of the TiSi<inf>2</inf>layer. The contact resistivity of the value of 5 × 10^-6Ω.cm²was obtained even after 800°C annealing.

Characterization of W/WSI x /Si Contact Structures under Thermal Annealing for Some X Values

Thermal stability of W/WSi x /Si (X=1.4, 2.1, 2.5) contact structures has been characterized at annealing temperature up to 900°C. Thermal reaction in these contact structures was strongly affected by the X values of deposited WSi x films. The X values were presicely controlled by a novel co-sputter deposition apparatus. After annealing at 900°C, in the samples of X=1.4 and 2.1, the W films were stably maintained. On the other hand, in the sample of X=2.5, the structure changed into the WSi 2. 2 /Si resulting in the disappearance of the W film. The silicidation of the W film in the sample of X=2.5 was likely due to a diffusion of Si into the W film through the precipitated region of the excess Si in the tungsten silicide film. The W/WSi 2.2 /Si structure can provide the electrically stable W-Si contact at annealing temperature up to 800°C.