NIST microfabricates atomic vapor cells for Chip-Scale Atomic Clocks.

A team of researchers from NIST have met a major milestone in the Defense Advanced Research Projects Agency's Chip Scale Atomic Clock (CSAC) program by successfully microfabricating a cesium (Cs) vapor cell with a volume of less than 10 m[m.sup.3] using silicon micromachining techniques. Additionally, the researchers have developed vapor cell filling techniques and measured the effect of vapor cell buffer gas pressure on the intrinsic line width of Cs vapor atomic transitions. The cells developed by the team have volumes just under 10 m[m.sup.3], with spectral line widths less than 1 kHz for a 9.2 GHz Cs hyperfine absorption line. Other cells fabricated at NIST using the same process, and with volumes as small as 1 m[m.sup.3], have line widths only slightly larger.

Miniaturized atomic clocks (volume less than 1 c[m.sup.3] have military and civilian applications, including synchronization of encryption keys and communication networks and to help prevent jamming of global positioning systems.

Novel excitation techniques have been investigated by NIST, including coherent population trapping (CPT) resonances in which two optical fields are applied to a Cs vapor cell. The two fields are obtained by modulating the drive current for a vertical-cavity surface-emitting laser, thus generating laser sidebands separated by about 9.2 GHz. This eliminates the need for a resonating cavity and permits the cell size to be reduced to below the wavelength of the microwave radiation. At a critical frequency difference between the two laser frequencies, a dark-line or CPT resonance occurs, signified by a decrease in the Cs vapor optical absorption. This resonance then is used to lock the modulation frequency of the laser. The advantage of using the CPT method over conventional microwave excitation is that the clock can be made much smaller and simpler. Eventually, the goal of the NIST part of the CSAC Program is to develop a clock physics package (including laser, laser optics, vapor cell, vapor cell heater, photodetector, and magnetic shield) based on the CPT scheme with volume less than 3 m[m.sup.3].

Silicon micromachining methods currently being developed at NIST can be used to make vapor cells with dimensions well below 1 m[m.sup.3]. In addition to a high level of miniaturization, other advantages include lower cost, higher reproducibility, and integration with control electronics and sensors.

The team has developed two methods for filling and sealing the micromachined cells: chemical reaction between barium azide and cesium chloride in an ultrahigh vacuum system followed by anodic bonding of the silicon chip to a glass window in a nitrogen buffer gas ambient, and direct injection of liquid Cs in an anaerobic chamber followed by anodic bonding to a glass window in a nitrogen buffer gas ambient. In principle, both processes are scalable to wafer-level production.

CONTACT: John Moreland, [phone]; [e-mail].