Method and apparatus to reduce the leakage rate of a hermetic cavity
A chip scale vapor cell and millimeter wave atomic clock apparatus are disclosed. The chip scale vapor cell includes a first substrate and a second substrate bonded to the first substrate with a bonding material. A primary hermetic cavity includes a first bottom wall and first sidewalls formed in the first substrate and a first top wall formed by the lower surface of the second substrate. A secondary hermetic cavity includes a second bottom wall and second sidewalls formed in the first substrate and a second top wall formed by the lower surface of the second substrate. The secondary hermetic cavity is separate from the primary hermetic cavity and surrounds the perimeter of the primary hermetic cavity. A gas, which can be a dipolar molecular gas, is sealed in the primary hermetic cavity and the secondary hermetic cavity at a given initial pressure.
1. A chip scale vapor cell comprising:
a first substrate;
a second substrate bonded to the first substrate with a bonding material, the second substrate comprising an upper surface and a lower surface;
a primary hermetic cavity comprising: a first bottom wall and first sidewalls formed in the first substrate; and a first top wall formed by the lower surface of the second substrate;
a secondary hermetic cavity comprising a second bottom wall and second sidewalls formed in the first substrate, and a second top wall formed by the lower surface of the second substrate, the secondary hermetic cavity being separate from the primary hermetic cavity, and the secondary hermetic cavity comprising: a first region surrounding a perimeter of the primary hermetic cavity; and a second region lying under the primary hermetic cavity, the first and second regions being conjoined; and
a gas sealed in the primary hermetic cavity and the secondary hermetic cavity at an initial pressure, the gas comprising a dipolar molecule.
2. The chip scale vapor cell of claim 1 , wherein the initial pressure is less than 1.0 mbar.
3. The chip scale vapor cell of claim 1 , wherein the initial pressure is approximately 0.1 mBar or less.
4. The chip scale vapor cell of claim 1 , wherein the gas comprises water vapor.
5. The chip scale vapor cell of claim 1 , wherein a volume of the secondary hermetic cavity is equal to or greater than one tenth the volume of the primary hermetic cavity.
6. The chip scale vapor cell of claim 1 , wherein a volume of the secondary hermetic cavity is equal to or greater than a volume of the primary hermetic cavity.
7. The chip scale vapor cell of claim 1 , wherein the secondary hermetic cavity comprises a gettering agent to absorb the dipolar molecule.
8. A chip scale vapor cell comprising:
a first substrate;
a second substrate bonded to the first substrate with a bonding material, the second substrate comprising an upper surface and a lower surface;
a primary hermetic cavity comprising a first bottom wall and first sidewalls formed in the first substrate and a first top wall formed by the lower surface of the second substrate;
a secondary hermetic cavity comprising a second bottom wall and second sidewalls formed in the first substrate, a second top wall formed by the lower surface of the second substrate, the secondary hermetic cavity being separate from the primary hermetic cavity and surrounding a perimeter of the primary hermetic cavity; and
a gas sealed in the primary hermetic cavity and the secondary hermetic cavity at an initial pressure, the gas comprising a dipolar molecule;
the first bottom wall, first sidewalls and first top wall comprising a conductive surface, and the first top wall comprising: a first non-conductive aperture on the lower surface of the second substrate that provides an electromagnetic field entrance to the primary hermetic cavity; and a second non-conductive aperture on the lower surface of the second substrate; and spaced from the first non-conductive aperture, that provides an electromagnetic field exit from the primary hermetic cavity.
9. The chip scale vapor cell of claim 8 , further comprising: a first conductive coupling structure formed on the upper surface of the second substrate proximate the first non-conductive aperture; and a second conductive coupling structure formed on the upper surface of the second substrate proximate the second non-conductive aperture.
10. A millimeter wave atomic clock apparatus comprising:
a vapor cell, comprising:
a first substrate,
a second substrate bonded to the first substrate with a bonding material, the second substrate comprising an upper surface and a lower surface,
a primary hermetic cavity comprising a first bottom wall and first sidewalls formed in the first substrate and a first top wall formed by the lower surface of the second substrate,
a secondary hermetic cavity comprising a second bottom wall and second sidewalls formed in the first substrate, a second top wall formed by the lower surface of the second substrate, the secondary hermetic cavity being separate from the primary hermetic cavity, the secondary hermetic cavity surrounding a perimeter of the primary hermetic cavity and
a dipolar molecular gas sealed in the primary hermetic cavity and the secondary hermetic cavity at a given initial pressure; and
a transceiver circuit mounted to the second substrate and electrically coupled to provide an alternating electrical output signal at a first end of the primary hermetic cavity and to receive an alternating electrical input signal at a second end of the primary hermetic cavity, the transceiver circuit coupled to selectively adjust a frequency of the alternating electrical output signal and to provide a reference clock signal.
11. The millimeter wave atomic clock apparatus of claim 10 , wherein the first bottom wall, the first sidewalls and the first top wall comprise a conductive surface and the first top wall further comprises a first non-conductive aperture for providing an electromagnetic field entrance to the primary hermetic cavity and a second non-conductive aperture spaced from the first non-conductive aperture for providing the electromagnetic field exit from the primary hermetic cavity.
12. The millimeter wave atomic clock apparatus of claim 11 , wherein the vapor cell further comprises:
a first conductive coupling structure formed on the upper surface of the second substrate proximate the first non-conductive aperture and coupled to receive the alternating electrical output signal from the transceiver circuit,
a second conductive coupling structure formed on the upper surface of the second substrate proximate the second non-conductive aperture and coupled to provide the alternating electrical input signal to the transceiver circuit.
13. The millimeter wave atomic clock apparatus of claim 12 , wherein the initial pressure is approximately 0.1 mBar or less.
14. The millimeter wave atomic clock apparatus of claim 13 , wherein the dipolar molecular gas comprises water vapor.
15. The millimeter wave atomic clock apparatus of claim 14 , wherein the secondary hermetic cavity comprises a first region that surrounds the perimeter of the primary hermetic cavity and a second region that lies under the primary hermetic cavity, the first and second regions being conjoined.
16. The millimeter wave atomic clock apparatus of claim 14 , wherein the secondary hermetic cavity comprises a plurality of secondary hermetic cavities, a first secondary hermetic cavity of the plurality of secondary hermetic cavities surrounding the perimeter of the primary hermetic cavity and each successive secondary hermetic cavity of the plurality of secondary hermetic cavities surrounding an outside perimeter of a previous secondary hermetic cavity.