Understanding the Role of Sonochemical and Sono-electrochemical Parameters in Semiconductor Cleaning

Abstract

Over the years, megasonic energy has been widely used in the semiconductor industry for effective particle removal from surfaces after chemical mechanical planarization (CMP) processes. As a sound wave propagates through a liquid medium, it generates two effects, namely, acoustic streaming and acoustic cavitation. Acoustic streaming refers to time independent motion of liquid due to viscous attenuation, while cavitation arises from the bubble activity generated due to the difference in the pressure field of the propagating wave. Cavitation can be classified into two categories, (1) stable and (2) transient cavitation. When a bubble undergoes continuous oscillations over repeated cycles it is known to exhibit stable cavitation, while a sudden collapse is referred to as transient cavitation. Due to the rapid implosion of the transient cavity, drastic conditions of temperature (5,000-10,000 K) and pressure (hundreds of bars) are generated within and surrounding the bubble. If this phenomenon occurs close to the substrate, it causes damage to the sub-micron features. In this study, emphasis has been laid on understanding acoustic cavitation as it is critical to achieving high cleaning efficiency without any feature damage. The research work described in this dissertation has been divided into three sections. In the first part of the dissertation, the development of a novel sono-electrochemical technique for removal of sub-micron (300 nm) silica particles from conductive surfaces (Ta) has been discussed. The technique employs megasonic field at low pulse time and duty cycle in conjunction with an applied electrical field for achieving superior particle removal efficiency (PRE). In order to demonstrate the effectiveness of the sono-electrochemical technique, cleaning studies were conducted using 300 nm silica particles both in the presence and absence of an applied electrical field in air and argon saturated solutions. In the presence of the megasonic field (0.5 W/cm², 10% duty cycle, 5ms pulse time) alone, about 55% PRE was observed in Ar saturated DI water, while in the presence of the sono-electrochemical field (-1.5V vs Ag/AgCl (sat. KCl)), about 80% PRE was measured. The enhancement in particle removal efficiency was attributed to oscillating hydrogen bubbles formed from water reduction in close vicinity of the tantalum surface, that grow to a resonant size under suitable acoustic conditions and likely cause removal of particles. Interestingly, increasing the applied potential to -2V (vs Ag/AgCl (sat. KCl)) enhanced the particle removal efficiency to about 100%. Investigations were also performed in solutions containing 10 mM potassium chloride (KCl). The results revealed that even at low applied potentials of -1.5V, almost complete particle removal was achieved. This improvement in PRE was attributed to a combined effect of microstreaming and electro-acoustic forces. The results revealed that almost complete removal of particles could be achieved at low power density and duty cycle when a sound field at 1 MHz is used in conjunction with electrochemistry. The second study focuses on the effect of acoustic frequency and transducer power density for the development of a damage-free megasonic cleaning process. Here, an effort was made to characterize cavitation activity at acoustic frequencies of 1, 2 and 3 MHz by means of electrochemical, acoustic emission and fluorescence spectroscopy techniques. Studies conducted with a microelectrode using ferricyanide as an electroactive species showed that at 1 MHz and 2 W/cm², current peaks with a rise and fall time of about 30-50 ms and 80-120 ms were observed, respectively, which were indicative of transient cavitation behavior. Interestingly at higher frequencies (3 MHz), symmetric and oscillatory behavior in the current was observed. The rise and fall times were about 3 orders of magnitude lower at about 50 µs. This oscillatory behavior in the current at 3 MHz was attributed to the presence of stable cavities. Furthermore, hydrophone studies supported the microelectrode studies as they showed a reduction of about two orders of magnitude in the intensity of transient cavitation as frequency was increased from 1 to 3 MHz. Hydroxyl radical (OH*) capture measurements using terephthalate dosimetry corroborated the above results as they illustrated an order of magnitude decrease in OH* generation rate at 3 MHz compared to 1 MHz. These studies suggest that the use of higher megasonic frequencies may be more suitable for damage-free and effective cleaning of patterned surfaces in the semiconductor industry. In the last part of the dissertation, we investigate the effect of solution parameters on cavitation characteristics using a bicarbonate based alkaline chemical cleaning formulation that has been previously demonstrated to be beneficial in achieving effective megasonic cleaning and low damage. The results of this study revealed that in the presence of ammonia (NH₃) or carbonate/bicarbonate ions at concentrations greater than 75 mM or 200 mM respectively, the measured rate of generation of hydroxyl radicals at 1 MHz and 2 W/cm² was significantly reduced. The lower rate of OH· was attributed to scavenging of radicals in these solutions and additionally due to reduced transient cavitation in ammonia solutions. Hydroxyl radical measurements at higher power density of 8 W/cm² showed that carbonate ions were better scavengers of hydroxyl radicals than bicarbonate ions. The study on the effect of bulk solution temperature illustrated that the rate of generation of OH· increased with increase in temperature from 10 to 30 °C suggesting enhanced transient cavitation at higher temperatures (in the investigated range). The use of optimum concentration of ammonia or carbonates ions in cleaning formulation and bulk solution temperature would likely provide desired cleaning with minimum damage.