Effect of Sheet Electron-Beam Irradiation (SEBI) on Water Wettability of Mirror-Finished Si Wafer
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ABSTRACT The influence of sheet electron beam irradiation ( SEBI ) on the water-wettability of the (100 ) plane on the Si wafer etched by hydrofluoric acid ( HF ) is investigated. The wettability energy is estimated from the contact angle of water. The SEBI treatment decreases the contact angle and increases the interfacial energy. Aging decreases the change in the interfacial energy. A rate process is inferred for the interfacial energy changes on the Si wafer.
INTRODUCTION Wafer bonding is useful to device processing [ 1 ]. The wafer bonding ability is determined by the coverage of adsorbed water molecules on the wafer surface [ 2 ]. If we can change the condition of the silicon wafer surface, the adsorption of the waiter molecule can be controlled. The authors have found that sheet electron beam irradiation ( SEBI ) homogeneously changes the water-wettability of silicon wafers. So, SEBI homogeneously activates the surface in a short time without the need of other treatments. Therefore, we study and discuss the effect of sheet electron beam irradiation ( SEBI ) on the interfacial energy of mirror-polished Si wafers versus water.
EXPERIMENTAL PROCEDURE Mirror-polished silicon wafers were etched by hydrofluoric acid. Sheet electron beam irradiation was homogeneously performed by an electrocurtain processor ( Type : CB1175/15/180L, Energy Science Inc. 8 Gill street, Woburn, MA 01801, USA., Iwasaki Electric Group Company) [ Refs. 3 &4 ]. The sheet electron beam was generated by a tungsten filament in vacuum. The acceleration potential and the irradiation current were 175 kV and 4 mA, respectively. Since the most important feature is the sheet electron beam, homogeneous treatments with the electron beam were performed under a protective nitrogen gas at atmospheric pressure in 365 Mat. Res. Soc. Symp. Proc. Vol. 477 01997 Materials Research Society
the apparatus. The concentration of oxygen was below 400 ppm. The SEBI treatment was not continuously performed. In order to control the temperature of the sample surface, the irradiation time was kept constant at 0.25 s as the sample was transported by a conveyer. The conveyer speed was 10 m/min. The temperature was below 323 K just after the irradiation. The repeated
exposures were used to increase the dose of irradiation. The amount of irradiation was proportional to the irradiation time ( ti ) by the following equation. (1)
Dose=0.36(MGy/s) X ti(s)
The silicon wafers etched by the hydrofluoric acid and then irradiated by SEBI were aged under air atmosphere at 373 K. The wettability is evaluated by the interfacial energy between silicon wafer and water. It is measured by the contact angle of distilled water on the ( 100 ) plane of mirror-polished silicon wafers etched by hydrofluoric acid ( HF ). The wettability was evaluated by measuring the contact angle 0 ina drop of water. The 0 value is measured by taking a photograph of the drop of distilled water on the ( 100 ) plane of the silicon wafer ( see Fig. 1). If the wettability is high, 0 has a low value. The mass
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