SURFACE DAMAGE EFFECTS IN ULTRASONIC CLEANING OF SILICON WAFERS

The cleaning of Si wafers due to kHz-frequency ultrasonic treatment in a watercontaining bath is studied. The cleaning stages observed with varying treatment time are discussed. It is found that, during the first 60-90 min, organic hydrocarbon contaminants can be effectively removed from the wafer surface. This is evidenced by the disappearance of organic-related absorption peaks and remarkable shortening of the photovoltage decay transients. At longer times, the subsurface crystalline quality is degraded and the wafer performance gradually deteriorates. The decay curves become double-exponential profiles, developing fast initial decays and longer ones at greater instants, indicative of a subsurface trap generation at this treatment stage. This is accompanied by the broadening of the X-ray rocking curves. The likely origin of the presented effects is discussed. Introduction Cleaning of surfaces in silicon wafers has become one of the most critical operations in device processing technologies. The complete removal of contaminations and the passivation of rechargeable states on the surfaces are very important issues for improving energy conversion efficiency of Si solar cells. Ultrasonic treatment (UST) is a promising tool to clean Si wafers with the benefit of reducing hazardous waste [1, 2]. Since the force imparted by acoustic cavitation and streaming is non-selective, not only contaminants but also wafers can be attacked. It is assumed that a damage threshold for UST energy exists, although the physical mechanism behind the ultrasonic-induced damage is not completely understood. Here, we study an interplay of the surface cleaning and damage effects of Si wafers subjected to a kHz-frequency UST, which is performed in distilled water.