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We have harnessed water to suit our needs. We’ve built dams, levees and canals, diverting, containing and channeling water to nurture the growth of farms and cities, generate power and expedite transportation. We process water, filtering, purifying, distilling and bottling it to slake our ever-increasing thirst. Now on the agenda: Creating a “new water” with the power to rid the world of hazardous substances.
Water is liquid between 0ºC (32ºF) and 100ºC (212ºF) at the normal atmospheric pressure of 1 atmosphere (atm). When water reaches a temperature below 0ºC (32ºF), it becomes solid matter — ice — whereas it evaporates into the air when it is heated to 100ºC (212ºF).
On the other hand, “this new water, so-called supercritical water, is a dense, gaseous solvent,” explains Dr. Takeshi Sako, professor in the energy system section in the graduate school of science and technology at Shizuoka University. When water is heated beyond 374ºC (705ºF), then maintained at a pressure above 218 atm, a phenomenon called phase change occurs and the water becomes supercritical water — the fourth phase of matter besides solid, liquid and gas.
Transforming to a supercritical state
Top left: Lighter gas goes up, heavier liquid goes down.
Top right: When heated to just below the critical temperature, liquid and gas start mixing.
Bottom right: Liquid and gas are completely mixed as the temperature and pressure reach the critical point.
Bottom left: The concoction is heated further, transforming it into a supercritical state.
Note: Methanol was used for the purpose of these photographic images. Its critical point is 239ºC (462ºF) and 80 atm.
(Courtesy of Professor Takeshi Sako, Shizuoka University)
Supercritical water has the power of a superman. Why is supercritical water so powerful? Because it’s in a state of high temperature and high density. High temperatures mean molecules move very fast, colliding with hazardous substances with greater force; high density means more molecules packed into a space, increasing the frequency of collisions. In other words, destructive power is maximized and dissolving time is minimized.
Because it can diffuse through solids like a gas and dissolve materials like a liquid, it is ideal for use as a solvent for particularly nasty, hazardous substances like dioxins and PCBs (polychlorinated biphenyls, best known for their use as cooling and insulating fluids).
Dr. Sako says the Japanese government has been using supercritical water to cope with PCBs in the Tokyo area since 2005. “Within the next 10 years, the government plans to use supercritical water to dispose of all PCBs.” Still, a lot remains to be done in order to make supercritical water a commercially viable and sustainable enterprise. Dr. Sako notes that more affordable materials need to be developed to build the reactors that house the supercritical water. More efficient and cost-effective methods of crushing waste materials and feeding them into the high-pressure reactors must also be developed.
Whether we like it or not, water runs into every corner of our lives. It’s a convenience: A tall, cool glass or a refreshing pool’s worth on a hot summer day. It can also be a nuisance, like when your bath overflows or your baseball game gets rained out. At its fiercest, it’s a force to be reckoned with, a deluge capable of flooding streets or even wiping out an entire city.
How we manage water will have a lot to do with how long we will be able to survive as a species. Scientists and laymen alike will tell you this isn’t just a sermon — it’s a fact. Without water, we’re lost. However, as sure as ice floats, humans will find a way to work with water, making sure it will continue the life-giving cycle it began billions of years ago.
Water and semiconductors are closely linked. The manufacture of LSI circuitry involves a series of chemical processes, each of which leaves residue that must be washed away before the next step in the assembly process commences. Only purified water can be used in this application. If impurities are present, they adhere to the semiconductors, interfering with the performance of the electrical circuitry. Today, thanks to intensive research and development, high-quality purified water is readily available.
The numerical aperture (NA) of IC scanners determines the circuitry’s line width, which is an indicator of LSI quality. (The narrower the line width, the better the performance.) The refractive index of the material through which the light passes limits NA. If the light is passing through air that has a refractive index of 1.0, NA can never exceed 1.0. Nikon placed purified water, which has a refractive index of 1.44, between the lens and the wafer, achieving a breakthrough that goes beyond the NA 1.0 barrier.
In 2006, Nikon began shipping an immersion scanner with NA 1.07, the first production scanner in the world that employed purified water and realized a performance beyond NA 1.0. Nikon’s latest model, the NSR-S610C, has achieved NA 1.30 and is now in use at plants to produce LSIs with a line width of less than 45 nm.
Nikon employed purified water and a 193 nm ArF excimer laser for a major breakthrough in ultra-microfabrication. Synthetic silica glass and calcium fluoride are the only two materials through which a 193 nm ArF excimer laser wavelength can pass. As air has a refractive index of 1.0, however, it limits performance, even when these materials are used. Nikon’s breakthrough: Purified water allows us to go beyond the NA 1.0 barrier. Purified water is, so to speak, a third lens material. Once again, Nikon is at the very frontier of the technologies that foster the evolution of the information society.
