September 06, 2017

In our everyday lives, pressure can mean a looming work deadline, a final exam, or bases loaded in the bottom of the ninth. In scientific terms, pressure is a physical quantity that's important for many things, including car tires, blood circulation and breathing, semiconductor manufacturing of high-speed computer chips, and the weather.

Accurately measuring pressure for important applications currently requires a large device in a lab. But the National Institute of Standards and Technology (NIST) is aiming to move a promising new laser-based pressure-measurement technique it invented into the larger world, potentially reducing the costs for everything from airplane flights to chip manufacturing. NIST and MKS Instruments, Inc., of Andover, Massachusetts, have signed a collaborative agreement to make the technique more portable and transportable so that it could be developed into a commercial product.

Better measurements of pressure will have many applications, said NIST physicist Jay Hendricks, the leader of the project. For example, the Federal Aviation Administration worked with industry to safely reduce the vertical distance between nearby airplanes to 1,000 feet, and further reductions are possible. Since the air pressure surrounding an aircraft changes with altitude, a more precise pressure sensor could help make this happen, Hendricks said.

“In the future, this could lead to savings in fuel costs,” he said. With reduced vertical separation, flight controllers could safely arrange planes more densely, enabling both fuel savings and more frequent on-time landings.

In manufacturing semiconductor chips, such as those in smartphones, Hendricks said, engineers must adjust the pressure of the gas environments in which the chips are made. Conventional pressure sensors, known as capacitance diaphragm gauges, are precise, but their readings must be kept within tight values, requiring engineers to adjust the sensors on a regular basis. More stable and accurate measurements of pressure could make semiconductor yields more reliable and less prone to defects, reducing costs for manufacturers and consumers.

In defense applications, aircraft such as the Apache helicopter must fly low to survey terrain. Such aircraft rely on pressure sensors to ensure they are flying safely. But “on any one day a sensor might be throwing a fit,” Hendricks said, and the airplane could be grounded until the problem is found. “If you could make a portable, handheld, NIST-traceable standard that you can use in the field, you could reduce downtime,” he said.

Standard pressure-measurement devices have limitations that limit performance in these applications. So, NIST set out to make a state-of-the-art sensor that could improve performance while providing fundamental measurements of pressure.

To measure the pascal, the SI unit of pressure, the scientific community has traditionally relied on a tall, bulky device known as a mercury manometer. Elemental mercury, which is a hazardous neurotoxin, adjusts its height in response to changes in pressure. “We’ve been using mercury manometers for almost 375 years, and they’ve served us well,” said Hendricks. “They were state of the art in their day. But about five years ago, we realized that there’s got to be a better way to do this.”

The result is called the FLOC—the fixed-length optical cavity. It’s a rectangular slab of translucent material, known as ultra-low expansion (ULE) glass, that you can hold in your hand. It contains two thin tubes or “cavities” through which laser light can travel. Besides being 20 times smaller in size and mercury-free, the FLOC’s higher resolution also enables it to measure pressure changes that are 36 times smaller than the traditional mercury standard. In principle, the device can measure any gas at low pressure, such as the gas in a semiconductor fabrication facility, to high pressures, such as on the sea floor. Presently, different kinds of pressure sensors are required to cover the same range.

To measure pressure with the FLOC, researchers fill one of the cavities with the gas to be measured while the other cavity remains empty. Laser light shines through both cavities, but travels more slowly in the cavity with gas. The density of the gas alters the wavelength of the light in the cavity in a way that depends on the gas’s pressure. In response to this change, the researchers adjust the frequency of the laser so that the light once again resonates in the cavity. By measuring the difference between frequencies in the light exiting the empty and gas-filled cavities, the pressure is determined. Since the laser light’s interactions with the gas can be calculated from first principles of quantum mechanics, the researchers are making a quantum-based measurement of the pascal.

The FLOC has been successful in the laboratory, but it’s currently too bulky for most commercial uses. The entire apparatus fits on a large table, with precisely positioned lasers and other optics equipment. Under a Cooperative Research and Development Agreement (CRADA) signed by NIST and MKS, the two organizations will work to make smaller prototypes. This will include using fiber optics instead of more conventional laser optics, for example. The goal, Hendricks said, is to reduce the size and cost of the system until it can fit in a suitcase and the cost is competitive with existing technologies—making it suitable for aircraft, weather stations, chip factories and uses not yet imagined.

"MKS is excited to join NIST in further developing this technology,” said Phil Sullivan, CTO of MKS's Pressure and Vacuum Measurement Solutions business. “MKS Instruments brings 50 years of pressure measurement experience, and additionally MKS is very well suited to the optical developments needed for this project." 

On the scientific side, Hendricks and his colleagues are developing an advanced version for the laboratory known as a variable length optical cavity—the VLOC—that will help make precise measurements of the gases used in the cavities, typically helium and nitrogen, and will further increase the accuracy of the technology.

For Hendricks, the main focus remains clear. “How do you get this technology outside the walls of NIST?” he said. “This partnership with industry is a great way to make this happen.”