Tutorial #65
Dr. Donald Gray and Charlotte Frederick
July 2006
Introduction
Twenty-first century device cleaning is a growing challenge for microelectronics packaging. More delicate features, tighter offsets, continuing miniaturization and tenacious residues are pushing older cleaning tools such as In-Spray, Ultrasonic energy and Centrifugal Flow to and beyond their limits. Vacuum Cavitational Streaming (VCS*) is a newly introduced cleaning and drying technology that is non-damaging, cleaning fluid neutral, and processes devices in an oxygen-free environment.
What is Vacuum Cavitational Streaming?
VCS is an “on-surface” cleaning technology that releases mechanical and chemical energies directly onto “targeted” foreign matter, disrupting the surface liquid boundary layer to remove and transport the foreign material into the bulk cleaning fluid.
Like boiling, VCS forms vapor bubbles on nucleation sites. Vapor bubbles only nucleate and grow directly on surface particles and foreign material. Since nucleation and cavitation depend on the presence of contaminate particles, bubble growth does not occur on clean surfaces. Cleaning continues only until the desired level of cleanliness is achieved. Damage to a surface is highly unlikely because of these natural principles of controlled nucleation.
How does Vacuum Cavitational Streaming target-clean?
VCS controls cleaning by manipulating pressure in a sealed chamber containing devices submerged in a liquid. Reducing the total chamber pressure below the vapor pressure of the liquid causes vapor bubbles to form on the solid surfaces. Typical nucleation sites for bubbles include imperfections, crevices, foreign material and contaminates. The bubble size and production rate are similar to that produced in a boiling liquid.
VCS energy transfer mechanisms are controlled by the vacuum level applied to the bulk cleaning fluid. Once vapor bubbles are formed on contaminates, their powerful chemical and mechanical energies are transferred directly to surfaces by “implosion of vapor bubbles” and “detachment of vapor bubbles.” Both energy release mechanisms disrupt the fluid boundary layer, forcing contaminates into the bulk fluid for removal and dissolution.
Figure 1 (67k PDF) illustrates the implosion effects on the boundary layer in VCS cleaning.
Implosion of vapor bubbles releases energy directly on targeted contaminates, disrupting the boundary layer, dislodging the contaminates, and sweeping them into the bulk cleaning fluid where dissolution occurs. This process is modulated by varying pressure until the desired level of cleaning is achieved.
Detachment of vapor bubbles carries the insoluble contaminates away from the surface on the growing vapor bubble surfaces. This mechanism continuously produces a steady stream of carrier vapor bubbles traversing the stagnant boundary layer. The bubbles carry the released contaminates directly into the turbulent cleaning fluid or vapor, where they are removed from the process chamber. Swarms of bubbles sweep the surface, with transfer times measured in milliseconds. Departing bubbles allow fresh cleaning fluid to rush in behind, and the process repeats. A steady wake of fluid behind the rising bubbles sweeps the insoluble contaminates away.
Figures 2 and 3 show the results of VCS cleaning of contaminates and imperfections from a blind via, virtually inaccessible to competing cleaning systems.
Figure 2. Before VCS Cleaning.
The via holds trapped fibers and has a roughened surface.
Figure 3. After VCS cleaning.
The fibers are gone, and the surface imperfections are smoothed.
Summary
VCS cleaning provides consistent performance through the management and control of thermodynamic principles that disrupt and torment the liquid boundary until surfaces are contaminate free. VCS cleaning:
Provides direct energy transfer to surfaces, making difficult to reach areas such as porous surfaces, vias and patterned surfaces no challenge.
Maximizes the cleaning performance of the cleaning fluid by concentrating the cleaning fluid the vapor state as the bubble grows and releases the jetted vapor directly on the targeted contaminate as the bubble implodes.
As the pressure begins to implode the vapor bubble, the vapor phase condenses, the bubble disappears, and high velocity stream of fluid rushes in providing additional cleansing of the surfaces.
Takes place in a vacuum, perfect for cleaning oxide-forming materials such as low-k copper.
Integrates drying with vacuum within a single vacuum chamber, where the environment is controlled and managed to achieve previously unobtainable sub-micron cleaning and drying performance.
*VCS is a trademark of HyperFlo LLC