Tatsumi Mizutani

Radiation Damage in SiO2/Si Induced by VUV Photons

Quantitative measurements of radiation damage in SiO2/Si systems induced by VUV photons generated in various microwave plasmas are performed. The SiO2/Si system is irradiated by monochromatic VUV photons, and its flat-band voltage shift (ΔVFB) is measured by the C-V method. The number of effective positive charges generated in the SiO2 layer by a single VUV photon (Rf) is evaluated from the ΔVFB and the total VUV dose. A VUV photon is found to generate effective positive charges in SiO2 when its energy (Ep) is larger than the SiO2 band gap energy (Eg=8.8 eV). The value of Rf is on the order of 10-3~10-2 and increases in proportional to the energy difference between Ep and Eg . A model is proposed to explain these phenomena. The model states that a VUV photon with energy Ep(>Eg) generates an electron-hole pair in the SiO2 and the holes that are not recombined are trapped in the energy state near the SiO2/Si interface.

Sputtering yield and radiation damage by neutral beam bombardment

To clarify the difference between ion beam etching and neutral beam etching, a low‐energy (100–1000 eV) Ne0 neutral beam obtained by charge exchange reaction has been used to bombard Cu, Si, and SiO2 surfaces. The sputtering yields of Cu and Si by Ne0 have been found to be the same as those by Ne+. The sputtering yield of SiO2 by Ne0 is slightly smaller than that by Ne+. It is believed that electronic sputtering mechanisms play only a small role in the sputtering of these materials at these low ion energies. C–V measurements show that the amount of radiation damage caused by Ne0 neutral beam bombardment of the SiO2/Si structure is significantly less than that by a Ne+ ion beam. When the Ne+ dosage increases, the flat‐band voltage shift increases and the thin SiO2 films on Si eventually break down. In Ne0 neutral bombardment, the flat‐band voltage shift saturates with increasing dosage and no breakdown of SiO2 occurs. It is assumed that, in neutral beam bombardment, the surface potential of SiO2 is limited...

Development of neutral‐beam‐assisted etcher

Advanced neutral‐beam‐assisted etching machines have been developed for very low‐damage SiO2 etching. The etching reactions are enhanced by a neutral beam and neutral radicals. This enhancement effect was first demonstrated by a prototype etcher equipped with independent sources for the neutral beam and neutral radicals. Next, a simple co‐axial type etcher was developed. Only one co‐axial discharge tube generates two cylindrical plasmas: the inner one is a neutral beam source; the outer one is a neutral radical source. The typical SiO2 etch rate was 60 nm/min. To further increase the etch rate, the beam and etching characteristics were studied in detail. We found that the neutral radical density had to be increased. To achieve a high etch rate, a tandem type etcher was designed and developed. It generates two plasmas side by side in only one discharge tube. This etcher can supply high‐density uniform neutral radicals from one of the tandem plasmas very close to the specimen. This new etcher is expected to...

Neutral-Beam-Assisted Etching System for Low-Damage SiO2 Etching of 8-Inch Wafers.

A novel neutral-beam-assisted etching apparatus has been developed for large-diameter etching. Neutral radicals from halide molecules and energetic neutral beams are irradiated on the specimen surface. The etching reaction of these neutral radicals is enhanced by neutral-beam bombardment. The neutral-beam source and the neutral-radical source are produced from a magneto-microwave plasma source. These plasmas are arranged in tandem in front of the specimen. This arrangement enables highly uniform supply of neutral radicals over a large area. This etching system was applied to SiO2 etching using Ar and CHF3 mixed gas. The SiO2 etch rate was 76 nm/min, and the etch rate uniformity was within ±7.4% for an 8-inch-diameter specimen and ±3.4% for a 6-inch-diameter specimen. In addition, highly anisotropic etching shapes of submicron patterns were obtained.

Compositional and structural modifications of amorphous SiO2 by low-energy ion and neutral beam irradiation

Abstract Compositional and structural modifications induced by irradiating amorphous SiO 2 with low-energy (300–500 eV) ion and neutral beams of inert gases have been studied. The ion beams were extracted from microwave excited plasmas of inert gases (Ne, Ar and Kr), and the neutral beams were generated by subsequent charge-exchange reactions. The SiO 2 layers modified with these beams have been characterized by Auger electron spectroscopy, Rutherford backscattering spectroscopy, reflection high-energy electron diffraction and transmission electron microscopy. It is shown that neutral beam irradiation does not cause preferential sputtering of oxygen from SiO 2 , whereas ion beam with the same energy causes significant preferential sputtering. For neutral beam irradiation, densification and crystallization of SiO 2 were observed: nanometer sized α-cristobalite or α-quartz crystals were formed when amorphous SiO 2 was irradiated with more than 10 17 neutrals/cm 2 and the densified SiO 2 layer extended several nanometers beneath these crystals. The reasons for these densification and crystallization phenomena have been discussed in terms of the high temperatures and pressures in local spots formed as a result of bombardment by energetic neutral atoms. For ion beam irradiations, these densification and crystallization phenomena have not been observed.

Mechanism of Radiation Damage in SiO2/Si Induced by vuv Photons

The generation yield (Rf) of effective positive charges at an SiO2/Si interface, induced by a vacuum ultraviolet (vuv) photon, is measured as a function of the substrate temperature by a low-temperature microwave plasma etching apparatus. The Rf decreases monotonously with decreasing temperatures from 300 K down to 120 K. The model is verified by analyzing the experimental results according to the effective positive charge generation model previously proposed. From this analysis, it is proposed that an ultra low-damage plasma process becomes possible by eliminating the holes generated by the vuv photons from the SiO2 film keeping the specimens at low temperatures.

Radiation damage in SiO2/Si induced by low‐energy electrons via plasmon excitation

We have found definite evidence which proves that radiation damage in SiO2/Si induced by low‐energy electrons is caused by plasmon excitation in SiO2. The SiO2/Si sample was irradiated by accelerated thermoelectrons, and the flat‐band voltage shift, ΔVFB, of the sample was measured by the C‐V method. The effective charge generation yield, Rf, in SiO2/Si was evaluated from the ΔVFB and the electron dose. The effective positive charges were measured in p‐type SiO2/Si, and the effective negative charges were measured in n‐type SiO2/Si. This is because interface states behave like positive charges in p‐type SiO2/Si and negative charges in n‐type SiO2/Si. The Rf in p‐type SiO2/Si oscillated as a function of the incident electron energy of 5–150 eV with several clear peaks. These peaks correspond to the quantum energy of a bulk plasmon or a surface plasmon. Both plasmons decay into electron‐hole pairs. The holes that escape from recombination with the electrons are trapped at the SiO2/Si interface and cause eff...

Neutral-Beam-Assisted Etching of SiO2?A Charge-Free Etching Process?

Highly directional and low-damage dry etching of SiO2 is realized by neutral-beam-assisted etching reactions. In this etching, low-energy neutral beams and neutral radicals are simultaneously supplied to the specimen, and the etching reactions are enhanced by the neutral beam bombardment. Low-energy (500 eV or less) Ar° neutral beams are generated from Ar+ ion beams by charge exchange reactions. Neutral radicals are generated in a microwave plasma of CHF3. The specimen surface is subjected only to these electrically neutral particles by eliminating charged particles with the retarding grids. The neutral-beam-assisted etching enables deep submicron pattern delineation without any serious radiation damage such as dielectric breakdown. It is a promising substitute for the current plasma etching as a future low-damage etching process.

Generation of low‐energy neutral beams and radiation damage of SiO2/Si by neutral bombardment

In order to apply to neutral beam etching, several hundred eV neutral beams have been generated by charge exchange reaction using a magneto‐microwave plasma ion source. The charge exchange cross sections for Ar+ and Kr+ are unexpectedly small, whereas the cross section for Ne+ of 300 eV is 1.7×10−15 cm2 in good agreement with published results. The small cross sections for Ar+ and Kr+ in the present experiments are explained in terms of a nonresonant charge exchange scheme. The capacitance–voltage (C–V) measurements show that the radiation damage to SiO2/Si by Ne0 neutral beam bombardment is much less than those by Ne+ ion beam and by vacuum ultraviolet photons. Consequently, the neutral beam process is a promising method for charge‐free and low‐damage surface processes.

Lower plasma‐induced damage in SiO2/Si at lower temperatures

We found that the radiation damage induced in a SiO2/Si system during plasma processing depends strongly on the specimen temperature. The surfaces of the SiO2 have been exposed to a microwave plasma at different temperatures and the resultant damage has been evaluated by capacitance‐voltage (C‐V) measurements. The flatband voltage shift (ΔVFB) for the specimen exposed to the plasma at 126 K has been found to be only 1/3 of that at 300 K. In case of vacuum ultraviolet photon irradiation through a thin Al film, the ΔVFB for the irradiation at 126 K has been only 1/5 of that at 300 K. It is believed that this lower plasma‐induced damage at the lower temperature is due to the small mobility of hole in SiO2 at lower temperatures. Plasma etching at low temperature has the advantage of low damage generation in the SiO2/Si structures.