Levitated particles in a Radio Frequency generated dusty plasma

It’s SCIENTIFIC FACT that crystals can be grown in a radio frequency propagated dusty plasma. These crystals can indeed act as cloud condensation nuclei and/or ice nuclei. Dusty plasmas are a pertinent field of research in the semiconductor business. The main connection here not being so much dusty plasma itself, but the dust particles. Dust particles are a necessary component of what? CLOUDS.

“Scientists in the semiconductor industry, rather than astrophysicists, stumbled onto a significant discovery as they searched for the source of particulate contamination of semiconductor wafers. It had been widely believed that particle contamination of silicon substrates occurred mainly during handling of wafers in air. So attention was focused on improving clean−room standards. Nobody thought to check whether the contamination might be happening inside the plasma processing reactors that are used to deposit and etch thin films on the wafers”
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“Gary Selwyn of IBM made a serendipitous discovery. He was carrying out a routine measurement with laser−induced fluorescence, to determine the concentrations of reactive gases in a plasma.4 When Selwyn shined his laser into the plasma, his attempted measurement of weak optical fluorescence was overwhelmed by scattering of the incident light. The laser light was illuminating clouds of micron−sized particles electrically suspended in the plasma above the wafer (see figure 2).

Selwyn found that particles actually formed and grew in the gas phase (see box 1), aggregating material from gases that were thought to have been exhausted by the vacuum pump. Then, at the fateful moment when the plasma−generating RF power was switched off, the particles fell and contaminated the wafer. Selwyn’s discovery revealed that much of the particle contamination responsible for costly yield losses was happening not just anywhere in the clean rooms, but inside the plasma reactors.

Whereas a dusty plasma is something of enduring interest to an astronomer, it was a vexing problem to be avoided by the semiconductor manufacturer. Nevertheless, the two communities suddenly found common ground. Both needed to understand the charging mechanisms and the forces that transport particles from one place to another in a plasma. But not all the progress was visible; secretiveness is often the rule in the semiconductor industry. Successful solutions are often hidden away as proprietary secrets.

Some solutions are known to involve plasma−chamber designs that exploit various forces on particles to divert them toward the vacuum pump. There have also been changes in the method of coupling RF energy to the plasma. Rather than relying solely on capacitive coupling, manufacturers now commonly also use inductive coupling to power a plasma−processing reactor. As a result, the electric fields are too weak to levitate particles large enough to cause killer defects on etched wafers.

The discovery by the semiconductor industry that RF−powered plasmas can levitate dust particles turned out to be a boon for basic plasma physicists, who study such things as waves and instabilities in ionized gases. Plasma physicists had heard astronomers talk of dusty plasmas in space, and they were eager to study them. The laboratory experimenter, however, has the difficulty that dust particles, unlike plasma ions and electrons, are so massive that they fall rapidly to the bottom of the chamber. Experimenters needed a way to fill a volume of plasma with particles, but gravity seemed sure to thwart them.

Then came Selwyn’s unexpected discovery. Immediately after the appearance of his 1989 paper, plasma experimenters worldwide realized how they could levitate particles in an RF−generated plasma. Soon, other laboratory methods of filling a plasma volume were developed as well….

One can, for example, maintain a dusty plasma by constantly showering particles in from above.”
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Figure 2. “Rings of dust particles encircling silicon wafers in a plasma processing device. In an accidental 1989 discovery, laser light was shone into a plasma used to etch Si wafers so that the expected weak optical fluorescence would monitor concentrations of reactive gas. Instead, the fluorescence was overwhelmed by scattering of the incident light off unanticipated clouds of micron−sized particles electrically suspended in the plasma above the wafers. Although great pains had been taken to minimize dust contamination of the clean room, it was discovered that the particles actually formed and grew in the plasma. When the RF power that generates the plasma is turned off, the particles fall onto the wafer, contaminating it. (Inset) An electron−microscope image of a 20−μm−diameter particle from such a dust cloud.”

“RF−powered plasmas can levitate dust particles”

“maintain a dusty plasma by constantly showering particles in”

“the particles actually formed and grew in the plasma”

Now…

“When the RF power that generates the plasma is turned off, the particles fall onto the wafer, contaminating it.”

People and the planet are the “wafer”. I wonder what kind of health effects this has generated?

http://dusty.physics.uiowa.edu/~goree/papers/PhysicsTodayJuly2004.html#cap2
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