Mitsuru Okigawa

On-Wafer Monitoring of Vacuum-Ultraviolet Radiation Damage in High-Density Plasma Processes

Vacuum-ultraviolet (VUV) radiation damage was investigated in inductively coupled Ar, He, and O2 plasmas by measuring the hole currents generated in SiO2 film. The hole currents strongly depended on the irradiated plasma VUV wavelength and photon flux (electron density). When the electron density was increased, larger hole currents were observed in the SiO2 film. A VUV wavelength of 58.4 nm in the He plasma and of 130.5 nm in the O2 plasma generated more holes in the SiO2 film than that of 104.8 and 106.6 nm in the Ar plasma did. That is believed to be due not only to the photon energy but also to the dependence of VUV transmittance in SiO2 films on the VUV wavelength. We found that pulse-time-modulated plasma is very effective in reducing the number of holes generated in SiO2 film by plasma VUV irradiation. Compared to the result obtained in the cw plasma, the VUV spectrum intensities and hole currents in the pulse-time-modulated plasma decreased drastically when the electron density was maintained.

Plasma-Radiation-Induced Interface States in Metal-Nitride-Oxide-Silicon Structure of Charge-Coupled Device Image Sensor and Their Reduction Using Pulse-Time-Modulated Plasma

We found that ultraviolet (UV) light from helium discharge plasma and a halogen lamp clearly induce SiO2-Si interface states in a metal-silicon-nitride-oxide-silicon (MNOS) structure. A dark current originating in the interface states of charge-coupled-device (CCD) image sensors also increases by this UV irradiation. Pulse-time-modulated (TM) plasma suppresses the interface states, resulting in the CCD dark current, by decreasing the UV light. On the other hand, results of Capacitance-Voltage (CV) measurement did not show the difference between UV irradiation and no irradiation. This indicates that fixed charges in the SiO2 cannot be generated by the UV lights. Using optical filters, we revealed that a photon energy of 3.90 eV (318 nm) to 4.96 eV (250 nm) causes an increase in the interface states.

Effects of Thermal Annealing for Restoration of UV Irradiation Damage during Plasma Etching Processes

During plasma etching processes, UV and vacuum ultraviolet (VUV) photon irradiation generates defects and causes an increase in interface state density between SiO2 and Si. To understand the effects of pulse-time-modulated plasma on reducing damage, defects in a SiO2 film (E center) after etching processes were measured using the electron spin resonance (ESR) method. We found that the density of the E center was markedly reduced using C4F8/O2 pulse-time-modulated (TM) plasma etching, as compared with continuous wave (CW) plasma etching. This corresponds to the changes in interface state density using the charge pumping method. Conversely, in both cases, these E centers were almost eliminated by H2/N2 annealing at 400 °C. Interface state density, however, remained constant even by annealing in both cases, whereas, interface state density when using TM plasma was much smaller than that when using CW plasma. This result suggests that H2/N2 annealing is not sufficient to restore the interface state density between SiO2 and Si. To reduce the increase in interface state density, UV irradiation damage must first be suppressed during plasma etching. Consequently, a combination of TM plasma etching and thermal annealing is very effective for reducing UV irradiation damage to achieve high reliability in metal–insulator–silicon (MIS) devices.