NIST Lays Groundwork for Vacuum Measurement Standards

A quantum-based vacuum gauge system developed by researchers at NIST (National Institute of Standards and Technology) passed what scientists considered to be its first step to become a primary standard. Measurement tests showed the system to be intrinsically accurate without the need for calibration.

The advancement is of particular importance to semiconductor fabricators that manufacture chips layer-by-layer in vacuum chambers operating at or below one hundred-billionth the pressure of air at sea level. Fabricators must maintain stringent control of this working environment to ensure product quality.

NIST scientist Stephen Eckel behind a pCAVS unit (silver-colored cube left of center) that is connected to a vacuum chamber (cylinder at right). Courtesy of C. Suplee/NIST.

In opposition to current modalities based on the sensing of electrical currents in rarefied gas molecules ionized by an electron source, the NIST system gauges the quantity of gas molecules remaining in the vacuum chamber by measuring their effect on a microscopic cluster of trapped lithium ions cooled to just above absolute zero and illuminated by laser light. The method is advantageous over electrical modalities as it does not require recalibration — the interaction dynamics between lithium atoms and hydrogen molecules can be calculated exactly from first principles.

The portable cold-atom vacuum standard (pCAVS) — 1.3 L in volume excluding the laser system — can be readily attached to commercial vacuum chambers; a narrow channel connects the chamber interior to the pCAVS core.

In experiments where the scientists connected two pCAVS units to the same chamber, both produced the same measurements within their very small uncertainties.

The devices provided accurate measurements of pressures as low as 40 billionths of 1 Pa, within 2.6% — approximately the same pressure surrounding the International Space Station. Atmospheric pressure at sea level is around 100,000 Pa.

In the pCAVS sensor core, vaporized ultracold lithium atoms are dispensed from a source and then immobilized in a chip-scale magneto-optical trap (MOT) designed and fabricated at NIST. Atoms entering the trap are slowed at the intersection of four laser beams: one input laser beam and three others reflected from a specially designed grating chip. The laser photons are tuned to the right energy level to damp the atoms’ motion.

To confine the atoms in the proper place, the MOT employs a spherical magnetic field produced by a surrounding array of six permanent Nd magnets. The field strength is zero at the center and increases with distance outward. Atoms in higher-field areas are more susceptible to laser photons and are therefore pushed inward. Once the lithium atoms are loaded into the MOT, the lasers are turned off and a small fraction of the atoms (about 10,000) are trapped by the magnetic field.

After waiting a specified period of time, the lasers are turned back on, causing the atoms to fluoresce where they are then counted using a camera. The camera measures luminance — the more atoms in the trap, the more light there is. Each time a trapped lithium atom is struck by one of the few molecules moving around in the vacuum, the collision kicks the atom out of the magnetic trap. The faster the rate of ejection, the more molecules there are in the vacuum chamber.

To mitigate the high cost related to the number of lasers needed to cool and detect the atoms, both pCAVS units receive light from the same laser through a fiber optic switch and take measurements alternately. The scheme allows as many as four units to be connected to the same laser source. In applications requiring multiple sensors, like those at accelerator facilities or semiconductor manufacturing lines, such multiplexing can lower the per-unit cost.

The next step in developing pCAVS is to validate its theoretical underpinning. To translate the loss rate of cold atoms from the magnetic trap into a pressure, quantum scattering calculations are required.

“These calculations are rather complicated,” said Eite Tiesinga, who leads the theoretical effort, “but we believe that their calculations are good to a few percent.”

To test the theory, the team intends to build a special vacuum chamber capable of generating a known pressure — a dynamic expansion standard — and attach a pCAVS to measure that pressure. If the pCAVS and the dynamic expansion standard agree, it will establish evidence that the theory is correct.

The research was published in AVS Quantum Science (www.doi.org/10.1116/5.0095011).