Megasonics has been used considered and used for many years to meet many of the cleaning challenges, but it has been shown to cause damage to nanoscale device structures such as polysilicon lines.
By Ahmed Busnaina, Northwestern University
Cavitation threshold, defined as the minimum pressure amplitude to induce cavitation, has been identified more than 40 years ago. A lower cavitation threshold indicates that cavitation (micro bubble implosion) occurs more readily. At megasonic frequencies, the cavitation threshold is very high which indicates that it’s unlikely to have cavitation at high (megasonic) frequencies. However, many have clearly shown that cavitation damage does occur at megasonic frequencies using commercial megasonic equipment.
If cavitation is not supposed to happen but does, why? The answer is that cavitation does not occur at megasonic frequencies (400kHz and higher) as shown by many over the last few decades. Cavitation is instead caused by secondary frequencies as low as 40 KHz that exist in megasonic tanks with sufficiently high power to generate ultrasonic cavitation responsible for damage.
Cavitation is a result of bubble implosion that occurs at high pressure amplitudes typically at low frequency (lower than 100 kHz). Frequency and amplitude (power) measurements show that traditional megasonic transducers generate many frequencies as low as 40 kHz at high amplitude (power). For example, a commercial megasonic tank operating at 700 kHz frequency shows siginifcant pressure amplitude (close the pressure at the 700 kHz) at 100kHz or less on top of the transducer or next to it. However, when a narrow band transducer is used where for a narrow bandwidth transducer that operates at 600 kHz frequency does show any large pressure amplitudes at lower frequencies. Narrow band transducer is a term used to indicate that large amplitude at lower frequencies (at or smaller than 100 kHz) are minimized or eliminated. Minimization of the large amplitude at the low frequencies shows that damage does not occur even at high power once the low ultrasonic frequencies (with high amplitudes) are eliminated or minimized.
FIGURE 1 shows SEM images of 120nm (A and C) and 350nm (B and D) lines after cleaning with 100% power for 5 minutes. While a conventional single wafer megasonic tank (3A and 3C) damages the structures the narrow bandwidth transducer (3B and 3D) preserves the patterns. The investigated area is 1mm2. This shows that damage in a conventional megasonic tank is the result of these low frequencies and that by eliminating these low frequencies (with high amplitude) damage can be eliminated without sacrificing effective cleaning.
Transducers in conventional megasonic tanks giving rise to high pressure amplitude at low frequencies is the main culprit and the cause of observed damage. Measured frequencies in conventional megasonic tanks and in a narrow bandwidth megasonic tank and show that in typical megasonic (operating at high frequencies) high amplitude low frequencies exist. Narrow bandwidth transducers reduce do not have high pressure amplitudes at low frequencies. This leads to the reduction or elimination of pattern damage which occur in conventional megasonic tanks. Therefore, if tanks are made to only generate high amplitude at high frequencies only, the issue of damage could be resolved. •