Nested Hermetic Cavities for Millimeter Wave Atomic Clock Leakage
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Solution Overview
Problem
Current wafer bonding techniques fail to maintain a very low pressure inside hermetic cavities for extended periods, which is crucial for millimeter wave chip scale atomic clocks using dipolar molecular vapors, as the quality factor of the transition degrades quickly with increasing pressure.
Innovation Solution
A primary hermetic cavity surrounded by a secondary hermetic cavity, which acts as a buffer zone, maintaining the same initial internal atmosphere and slowing down leakage rates by absorbing environmental gases before they reach the primary cavity, thus maintaining low pressure within the primary cavity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If current wafer bonding techniques are used to seal the hermetic cavity, then the manufacturing process is simple and cost-effective, but the leakage rate is too high to maintain very low pressure for extended periods
Solution Approach 1:
The hermetic seal is divided into two distinct layers: a first hermetic seal and a second hermetic seal. This segmentation allows each seal to independently contribute to the overall leakage rate, with the second seal providing an additional barrier that significantly reduces the total leakage rate while using standard wafer bonding techniques for both seals.
Solution Approach 2:
The patent applies beforehand cushioning by implementing a second hermetic seal that anticipates and compensates for leakage through the first seal. This pre-established protective layer ensures that even if the first seal degrades over time, the second seal maintains the required pressure integrity for the intended operational lifetime.
2Reliability
If a single hermetic cavity is used, then the device structure is simple, but the leakage rate into the primary cavity is too high
Solution Approach 1:
The patent implements a nested cavity structure where a second hermetic cavity is formed around the perimeter of the first hermetic cavity. The second cavity acts as a buffer zone that surrounds and protects the primary cavity, creating a nested configuration that reduces leakage into the first cavity while maintaining a compact integrated structure.
Solution Approach 2:
The second hermetic cavity serves as an intermediary buffer zone between the external environment and the first hermetic cavity. This intermediate layer absorbs and slows down the leakage process, preventing direct contamination of the primary cavity and significantly extending its pressure maintenance capability.
3Reliability
If the hermetic cavity pressure increases, then the manufacturing and sealing becomes easier, but the quality factor of the molecular transition degrades quickly
Solution Approach 1:
By segmenting the seal into two independent hermetic seals and the cavity into two nested cavities, the system can maintain very low pressure (0.1 mBar) in the first cavity for extended periods. This segmentation enables the use of low pressure conditions required for high transition quality factor while using achievable bonding techniques for both seals.
Solution Approach 2:
The second hermetic seal provides beforehand cushioning against pressure increase in the first cavity. By anticipating potential leakage through the first seal, the second seal maintains the low pressure environment necessary for high transition quality factor over the device's operational lifetime.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration significantly extends the time the primary cavity can maintain low pressure, achieving the required leakage rate of 1.72E-14 atm-cc/s, essential for the stability and longevity of millimeter wave CSACs.
Implementation Method 1
Leakage must first cross the secondary cavity to gain access to the primary cavity, thus slowing the leakage rate into the primary cavity
Implementation Method 2
The secondary cavity contains an initial internal atmosphere that is identical to the internal atmosphere of the primary cavity and provides a buffer zone for leakage from the environment
Data Source
AI summary
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.