Precision Cleaning
January 1998
Vol. VI, No.1

Megasonics Help 'Stream' Line Sensitive Substrate Cleaning

By Mark Beck and Richard Vennerbeck

Semiconductor wafer inspection equipment detects and records images of particles to be be cleaned.

As manufacturers grapple with the challenge of meeting increasingly stringent cleaning requirements while reducing overall production costs, advances in acoustic wave cleaning technology are garnering increased attention. Resulting from a better understanding of high-frequency acoustic streaming and controlled cavitation, megasonics is now recognized as an effective, cost-conscious cleaning method by a growing number of manufacturers in the integrated circuit, hard drive, raw silicon, masks, and flat panel display industries.

Megasonics uses the piezoelectric effect at high frequencies between 700 and 1000 kHz to remove submicron particles from substrates (see Figure 1).

Cleaning is accomplished by exciting a ceramic piezoelectric crystal with a high-frequency AC voltage, causing the ceramic material to change dimension, or vibrate. These vibrations are transmitted by the ceramic transducer to produce megasonic waves in the cleaning fluid.

Figure 1: Particle size vs. frequency for megasonic and ultrasonic cleaning.

Megasonic cleaning safely removes particles through controlled acoustic cavitation, megasonic pulsing, and acoustic streaming. It has proven effective in removing 0.15-micron particles from silicon wafers and other cavitation-sensitive products without causing substrate damage.

Controlled Cavitation

Acoustic cavitation is generally regarded as the principle mechanism of particle removal in acoustic cleaning. In an acoustic field, a bubble or cavity in the liquid can be created when the liquid pressure momentarily drops below the vapor pressure as a result of pressure oscillation. There are four methods of producing cavitation (see Table I).

Table I: Methods of producing cavitation.

The pressure oscillations which acoustic cavitation cause bubbles to contract and expand. Gas from the liquid diffuses into the bubble upon expansion, and leaves the bubble during contraction. When the bubble reaches a size that can no longer be sustained by its surface tension, the bubble will collapse, and the intensity of this collapse on a substrateâs surface is related to the type of acoustic cavitation produced.

There are two types of acoustic cavitation: transient and stable (or controlled). Transient cavities exist for a few cycles, and are followed by a rapid and violent collapse, or implosion, that produces very high local temperatures. Ultrasonic cleaning frequencies, typically between 20 and 350 kHz, transform low-energy/density sound waves into high-energy/density collapsing bubbles, producing transient acoustic cavitation. Transient acoustic cavitation can cause damaging surface erosion in more sensitive substrates.

Megasonic frequencies, 700 to 1000 kHz, produces stable acoustic cavitation bubbles have less time to grow and are smaller, resulting in a less vigorous collapse than in transient cavitation. And the implosion associated with these smaller, gas-filled bubbles is less likely to produce surface damage. Thus, megasonic cavitation is better suited for sensitive substrate surfaces.

Ultrasonics simultaneously cleans all sides of a submerged part, while megasonics cleans only the surfaces of the part facing the acoustic stream formed by the piezoelectric crystal (see Figure 2).

Figure 2: An illustration of the megasonic cleaning process.

Pulsed-Wave Megasonics

In an acoustic cleaning tank working in continuous mode, sound waves are reflected from substrate surfaces, tank walls, and the free surface of the liquid medium. The pressure amplitude, or megasonic power, required to achieve controlled cavitation and acoustic streaming depends on pulse width, dissolved gas content in the cleaning fluid, and power input.

Through research, the threshold pressure needed to initiate cavitation has been found to be a strong function of the pulse width and the duty cycle of the power input into the transducer. The increase of cavitation threshold pressure with a decrease in pulse width is believed to be related to the time needed for a bubble to grow by rectified. With short pulses, bubbles may not have enough time to grow transient cavities.

Megasonics cleaning, therefore, is optimized by pulsing the input power, thus providing effective particle removal and enhanced control over cavitation.

A Stream-Lined Process

Acoustic streaming is the time-independent fluid motion generated by a sound field. This motion, caused by the loss of acoustic momentum by attenuation or absorption of a sound beam, enhances particle dissolution and transports detached particles away from surfaces, decreasing particle redeposition.

Cleaning activity depends not only on the local sound intensity at the substrate surface, but also on the bulk motion of the fluid, which caries removed particles away from substrates. In a closed tank, bulk motion is produced by acoustic streaming. Stable cavitation bubbles also influence the bulk flow through buoyancy forces and microscopic flow through acoustic streaming.

Fluid velocity is a function of the fluid produced by acoustic waves, and the velocity of acoustic streaming. Pressure is also divided into two parts: the acoustic waves and the hydraulic pressure caused by acoustic streaming.

The high-frequency acoustic waves used in megasonic cleaning may either slide, roll, or lift a particle from its initial position on a substrate, depending on the size and shape of the particle from its initial position on a substrate, depending on the size and shape of the particle, as well as the nature of the hydrodynamic force being applied. The combination of megasonic-controlled cavitation and acoustic streaming enables typical substrate exposure times of 1 to 30 minutes.

Ongoing Optimizations

The mechanical effect of both ultrasonics and megasonics can be helpful in speeding particle dissolution and in displacing particles. However, there clearly are application for which megasonic cleaning would be favored.

The impact of ultrasonics and megasonics on substrate surfaces and particle removal provide the basis for identifying the best applications for each process. Table II presents the relative strengths of each.

Table II: Strengths of megasonics and ultrasonics.

Typically applied without any chemicals, megasonic cleaning can help drastically reduce chemical vapor evaporation and the load on air exhaust and replacement systems. Likewise, megasonics reduce costs associated with acquisition and disposal of toxic substances, and optimize the use of cleaning fluids.

Through continuing megasonics research and development, manufacturers can look forward to the removal of contaminants 10 times smaller than bacteria -- down to 0.1 micron: increasingly diluted cleaning mixtures in megasonics baths to improve particle removal; and a continued reduction in the need for toxic solutions.

About the authors

Mark Beck is the president and CEO of ProSys (Campbell, CA), which he co-founded in 1987, and has previously worked for National Semiconductor, Semiconductor Technologies, and Cypress Semiconductor. He received a BS in mechanical engineering and manufacturing management from the University of California, Berkeley and is the co-inventor of tow patents for megasonic cleaning equipment.

Richard B. Vennerbeck is vice president of sales, marketing, and customer support for ProSys, having worked for Veeco Instruments, National Semiconductor, Focus Semiconductor Systems, Lam Research, and Silicon Systems