Hiroyuki Kumano

The Fundamental Threshold Level—a New Parameter for Predicting Cavitation Erosion Resistance

In order to predict and avoid cavitation erosion, it is necessary to know the resistance of materials to cavitation impacts. It is proposed that only cavitation impacts with energy larger than a certain threshold level affect the cavitation erosion of materials. Weak cavitation impacts have small impact energy, which cannot produce elastic or plastic deformation on materials. Cavitation erosion does not take place in this situation. On the other hand, strong cavitation impacts have large impact energy, which generates plastic deformation and/or causes damage with mass loss. It is proposed that only cavitation impacts that are larger than a certain threshold level affect the cavitation erosion of materials. In this paper, the existence of such a threshold level is revealed experimentally. The fundamental threshold level of the material is a new parameter suitable for the prediction of cavitation erosion rate. The erosion rates for several materials were determined by using a cavitating jet erosion test, which forms a new ASTM standard, ASTM G 134. The cavitation impacts induced by the cavitating jet were measured by means of a specially made PVDF transducer and the energy of the cavitation impacts were then calculated. The fundamental threshold levels for several materials were obtained from the relationship between the erosion rate and the energy of the cavitation impacts. A method to predict cavitation erosion quantitatively from the fundamental threshold level of the material and the measurement of cavitation impacts is proposed.

Evaluation of the effectiveness of back-side damage gettering in silicon introduced by a cavitating jet

Photocapacitance measurements have been performed to evaluate the electrical effectiveness of gettering by back-side damage, introduced by a cavitating jet into silicon wafers. The silicon wafers, which had their back sides damaged previously in localized areas, were intentionally contaminated and subsequently thermally treated to diffuse the contamination through the wafer. The density of deep levels varied between the areas with back-side damage and those without. The results obtained on back-side damaged areas were closer to those on the original starting material. These results confirm that the back-side damage introduced by a cavitating jet can function as gettering sites.

Evaluation of the Damage for Gettering in Silicon Wafer Introduced by a Cavitating Jet

Damage for gettering can be introduced by a high speed submerged water jet with cavitation, i.e., a cavitating jet, into a silicon wafer. Gettering is an important technique for removing unwanted impurities from the surface of the silicon wafer that is active device region. Metal contaminations diffuse in bulk of the wafer and adhere to the defects through the thermal treatments in semiconductor processes. If the damage was intentionally introduced into the silicon wafer, these contaminations can be gathered within the intended region. Consequently, the surface of the wafer can be kept free from impurities. The method presented here utilizes cavitation impacts to introduce the damage for gettering. By using cavitation impacts, the damage can be introduced without the use of particles which form sources of contamination during farther wafer handling, as in shot blasting that is a popular technique. The cavitation impacts caused by a cavitating jet were used since the intensity of cavitation impacts can easily be controlled by controlling hydraulic parameters. The gettering effect of the damage introduced by the cavitating jet was already confirmed, but the detail information of the introduced damage has not been obtained. After applying thermal treatment to the wafer treated by the cavitating jet, there are Oxidation-induced stacking faults (OSF) which would be gettering sites. However, the source of OSF is not yet confirmed. In this paper, the damage on the silicon wafer surface introduced by the cavitating jet for gettering is observed to evaluate the source of OSF.

Gettering of Cu in Silicon Wafer by Using Cavitation Impacts

A novel gettering method using cavitation impacts is presented. Gettering is very important technique for IC manufacturing. Silicon wafers used as a substrate for semiconductors are often exposed to contamination during the device processes. If crystal defects are intentionally introduced into back-side of silicon wafer, metal impurities such as Cu and Fe in the wafer are trapped in the defects and gather into the region during heat cycles. As a result, the zone near the surface of the wafer that is used as active device region is kept off unwanted impurities. This technique is called gettering. In this paper, to introduce backside damage, which is one of the gettering techniques, cavitation impacts are utilized. Cavitation bubbles produce high-pressure impacts upon collapsing. The suitable damage can be introduced by controlling the intensity of cavitation impacts. The high speed submerged water jet with cavitation, i.e., a cavitating jet, was used to cause cavitation impacts. The cavitating jet can introduce backside damage without the use of particles, as in shot blasting that is popular technique. In order to confirm the gettering effectiveness of the damage introduced by a cavitating jet, an experimental study was carried out. The silicon wafer treated by the cavitating jet was intentionally contaminated with solution of Cu(NO3)2. The wafer was then thermally treated. The surface was observed after etching that makes defects on the surface observable. On the surface of the wafer having no gettering effectiveness, defects which were induced by contamination are observed. If the wafer has gettering effectiveness, defects are not observed on the surface. Gettering effectiveness of the damage introduced by the cavitating jet was shown.