The surface contamination of wafers, especially by particle contaminants, has been one of the major problems in the semiconductor industry since its inception. The yield on fully processed silicon wafers is inversely related to the defect density of the wafers. One way to decrease defect density is to use efficient cleaning techniques that remove particle contaminants efficiently. Small particles are especially difficult to remove from wafers because of the strong electrostatic forces between the particles and the substrate. It is therefore imperative to find an effective way to remove particles from wafers with efficiency and without damage to the wafers.

Modern wafer manufacturing facilities use stringent contamination control protocols, including the use of clean-room suits, latex gloves, and highly purified ventilation systems. In combination with these protocols, modern manufacturing facilities use various methods of cleaning wafers, often involving pressurized water jet scrubs, rotating wafer scrubbers, wet chemical baths and rinses, and similar systems. These processes, however, are prone to damaging the wafer. In addition, the chemical processes have inherent dangers associated with the use of chemicals, such as sulfuric acid, ammonium hydroxide, and isopropylalcohol.¹

Ultrasonic cleaning involves a variety of complex mechanisms, including cavitation, mechanical vibration, etc., depending on whether liquids are used in the cleaning process or not. A typical ultrasonic source is a plane surface that oscillates at a single frequency, producing a longitudinal wave. Vibrational energy transmitted subsequently propagates through the fluid.²

In this article, we present an efficient method of cleaning bare silicon wafers with the aid of ultrasonic energy from a low-cost transducer. This work is based on a patent (U.S. Patent # 6,766,813) that describes a method of cleaning a wafer.¹The method used in this article can be incorporated into a modified vacuum chuck containing an acoustic wave emitter. Additionally, a stream of cleaning liquid can be directed towards the wafer at a shallow incidence angle.¹However, the present study focuses only on the use of acoustic energy to clean a wafer.

SEMICONDUCTOR WAFER
The semiconductor industry uses various types and sizes of wafers. Depending on the type of process technology, wafer sizes can range from 100–300mm, and both p-type and n-type wafers are common in the industry. In this investigation, experiments were performed using a 100-mm diameter, p-type silicon waferwith a thickness of 540 µm.

CONTAMINANT PARTICLES
Various types and sizes of particles can contaminate semiconductor wafers during the fabrication process. Silicon and silicon dioxide are two commonly found contaminants on semiconductor wafers and, hence, were used in our experiments.Also, it should be noted that particles of nanometer size can reduce the yield of smaller geometry circuits and are to be avoided and removed from wafers if possible. Particles of such small size were not attainable for this study. However, silicon and silicon dioxide particles at the micrometer level were obtained using a U.S. Customary sieve shaker (Gilson Company Inc. SS-12R) and crushed silicon and silicon dioxide. Silicon particle sizes obtained for use in this experiment were: 53–90 µm, 90–125 µm, and 125–300 µm. Silicon dioxide particle sizes obtained for use in this experiment were: 38–45 µm, 45–53 µm, and90–150 µm.