Yasushi Ishikawa

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.

Surface reactions during etching of organic low-k films by plasmas of N2 and H2

Surface reactions during etching of organic low-k film by N2 and H2 plasmas were studied through observations of the surface resident species using in situ infrared spectroscopy and in vacuo electron-spin-resonance techniques. We observed surface modifications by the formation of CN and NH bonds after exposure to plasmas generated from N2 and H2. The number of carbon dangling bonds were greater in processes where H2 was present. The passivation of carbon dangling bonds leads to CH3, NH3, and CN functionalities, which are the precursors for etching products that are desorbed, which includes volatile forms such as HCN and C2N2.

Ultraviolet-induced damage in fluorocarbon plasma and its reduction by pulse-time-modulated plasma in charge coupled device image sensor wafer processes

Highly sensitive charge coupled device (CCD) image sensors present a number of serious problems, such as increased dark current and interface states induced by plasma etching processes. In particular, irradiation with ultraviolet (UV) photons (200 to 310 nm) generates this damage. UV absorption at the Si∕SiO2 interface may contribute to increasing the density of the interface states. To solve this problem we investigated optimum fluorocarbon gas chemistries and the effect of pulse-time-modulated (TM) plasma. We found that selecting appropriate gas chemistries and using TM plasma drastically reduced the dark current in CCDs.

Reduction of ultraviolet-radiation damage in SiO2 using pulse-time-modulated plasma and its application to charge coupled 44 device image sensor processes

We found that vacuum-ultra/violet (VUV) light emitted by plasmas causes dark current in charge coupled device (CCD) image sensors. When a CCD was irradiated with inductively coupled plasmas using helium (He), argon (Ar) or oxygen (O2) gas, the He plasma caused higher plasma-induced dark current in the CCD than the Ar or O2 plasmas. To investigate the influence of VUV radiation in He plasma, the plasma-induced electric current in SiO2 was measured using two types of on-wafer monitoring devices to separate the effects of He ions and He VUV radiation. One monitoring device has an aluminum filter that only allows He VUV 58.4 nm to pass through, and the other has no filter. We performed time-resolved-measurement experiments using the two types of devices in pulse-time-modulated He plasma (He-TM plasma). With the filter device, the decay of the VUV intensity after plasma off corresponded completely to the decay of the plasma-induced current curve. On the other hand, in the no-filter device, decay curve of the p...

Reduction of plasma-induced damage in SiO2 films during pulse-time-modulated plasma irradiation

Use of a pulse-time-modulated (TM) plasma is an effective way to reduce vacuum ultra-violet (VUV) radiation damage in SiO2 films because such a plasma can reduce the quantity of high-energy electrons while maintaining the electron density during the off time of the TM plasma. To understand the effects of VUV radiation, we measured the VUV-induced current in SiO2 films using a simple on-wafer monitoring technique. We found that the plasma-induced-current in the SiO2 films strongly depended on the VUV photon energy. Under the same conditions, the density of E′ centers in SiO2 films also depended on the photon energy. That is, the plasma-induced currents in the SiO2 films detected by on-wafer monitoring corresponded to the density of E′ centers (dangling bonds) in the films. By using a TM plasma, the plasma-induced current and the E′ center density in SiO2 films can be reduced during the plasma-off time. Consequently, use of a TM plasma can eliminate VUV radiation damage. These results also confirm that our ...

Charging and Coulomb staircase effects in silicon nanodisk structures fabricated by defect-free Cl neutral beam etching process

A defect-free nanometer-scale silicon disk (nanodisk) on thin SiO2 film was precisely fabricated by using Cl neutral beam etching of a 3.5–4-nm-thick polycrystalline silicon on 1.4–3-nm-thick underlying SiO2 with a 7-nm-diameter ferritin iron core mask. Kelvin force microscope observations revealed that nanodisks could maintain injected positive and negative charges. Additionally, Coulomb staircases were observed by I-V measurement of a nanodisk at a temperature of 25K. These results indicate that the nanodisk fabricated in this research had a precise quantum effect structure and attained the single electron property. This process has great potential in the development of future quantum effect devices.

In vacuo measurements of dangling bonds created during Ar-diluted fluorocarbon plasma etching of silicon dioxide films

Dangling bond creation processes during fluorocarbon plasma etching of silicon dioxide (SiO2) films were studied using an in vacuo electron spin resonance technique. In a range of about 10nm underneath the interface of the SiO2 films with an amorphous fluorinated carbon film that was top-covered, a Si dangling bond in the films (E′ center, g value 2.0003) was located. Density of the E′ center was sustained during etching processes created by the illumination of vacuum ultraviolet emissions, higher photon energy than the bandgap of SiO2. The etching mechanism in this system is discussed taking into account the experimental results.

Prediction of ultraviolet-induced damage during plasma processes in dielectric films using on-wafer monitoring techniques

We measured electron-hole pairs generated in dielectric film using our developed on-wafer monitoring technique to detect electrical currents in the film during the plasma etching processes. The electron-hole pairs were generated by plasma induced ultraviolet (UV) photons, and the number of electron-hole pairs depends on the UV wavelength. In SiO2 film, UV light, which has a wavelength of less than 140nm, generates electron-hole pairs, because the band gap energy of the film is 8.8eV. On the other hand, in Si3N4 film, which has a band gap energy level of 5.0eV, UV light below 250nm induces the electron-hole pairs. Additionally, we evaluated the fluorocarbon gas plasma process that induces UV radiation damage using multilayer sensors that consisted of both SiO2 and Si3N4 stacked films. In these cases, electron-hole pair generation depended on the dielectric film structure. There were more electron-hole pairs generated in the SiO2 deposited on the Si3N4 film than in the Si3N4 deposited on the SiO2 film. As a...

Damage-free surface treatment of carbon nanotubes and self-assembled monolayer devices using a neutral beam process for fusing top–down and bottom–up processes

Plasma etching processes have been used for the past 30 years to shrink the pattern size of integrated devices. However, the inherent problems of plasma processes, such as ultraviolet photon radiation damage, limit the effectiveness of etching and surface treatments of nanoscale devices. To overcome these problems, we developed a neutral beam surface treatment process. The process uses neutral beams and a defect-free surface process to fabricate carbon nanotubes and self-assemble mono-layer devices. We found that neutral beams can be used to produce atomically defect-free surfaces in carbon nanotubes and organic molecules. This technique has potential for fabricating nanodevices.