Wafer-Scale Micro-Knife Sealed Vacuum Cells for Quantum Devices

TL;DR

The paper presents a wafer-scale micro-knife edge plastic-deformation bonding technique to hermetically seal evacuated atomic vapor and atom-beam cells built on selectively etched fused silica. By employing in-situ Al2O3 coatings and Ti micro-knives, diffusion-based bonding at grain boundaries achieves a single robust interface suitable for wafer-scale integration, with leak rates below about $2.8e-10$ mBar·L/s and lifetimes exceeding $1$ year. Activation of Cs pills and getters is performed post-bonding under low-temperature processing constraints, achieving wafer-scale yields above $85\%$. This approach enables chip-scale quantum devices, including atomic clocks and photonics-integrated cold-atom systems, and offers a path toward ultra-high-vacuum, low-dissipation optomechanical platforms on a wafer.

Abstract

Advanced integration technologies greatly enhance the prospects and reliability of practical quantum sensors, atomic clocks, and quantum information technologies. The performance and proliferation of these devices at chip-scale is contingent upon developing low leak and low gas permeation vacuum cells using wafer-scale techniques. Here we demonstrate both evacuated atomic beam cells and atomic vapor cells using plastic deformation micro-knife bonding of selectively etched fused silica wafers. The cells are characterized using saturated absorption spectroscopy and fluorescence measurements. Vapor cells are mechanically robust exhibiting sheer-force strength ($\sim 15$MPa), demonstrate long lifetimes ($> 1$ year), low residual gas pressures $ (\ll 10^{-3} \, \text{mbar}) $, and leak rates below fine-leak testing sensitivity ($\ll 2.8 \times 10^{-10} \frac{\text{mBar} \cdot \text{L}}{\text{s}}$). Micro-knife bonding greatly simplifies the fabrication process for complex chip scale atom-beam devices and atomic vapor cells while identifying a path to future chip-scale cold atom devices, improved chip scale atomic clocks, and fieldable dissipation-dilution-limited optomechanics.

Figures (5)

• Figure 1: (a) A conventional extreme-high-vacuum chamber cuts into a compliant metal gasket and pressure is maintained with a mechanical bolt. (b) A micro-knife evacuated vacuum chamber is realized by plastically deforming and diffusion bonding two metals together. (c) Close up of a fabricated all fused silica atom-beam and simple atomic vapor cells with a dime for scale. (d) All-glass vacuum cells provide a high-degree of optical access enabling background free fluorescence measurements. (e) Background free saturated absorption spectrum of the D$1$ line ($895$nm) from an evacuated Cs vapor cell. Some residual gas causes slight broadening. (f) (Top) Spectra taken of a Cs atom-beam in the source region immediately after activation. (Bottom) Spectra taken from the atom beam in the drift region using side-excitation shows Cs collimation. The dashed line represents a fitted spectrum and shows a beam spectral FWHM of $\sim 115$ MHz. For panels (d-f) cell temperature is between $\sim (80-90) \,$° C. Saturated absorption and fluorescence spectra are measured using different experimental setups.
• Figure 2: (a) Internal cavities and other structures (such as micro-capillary arrays) are fabricated using selective laser etching. A coating consisting of Al$_2$O$_3$ is deposited to reduce He permeation. A thick compliant metal layer is then deposited to form sealing surfaces. Inset showing the vertically stacked micro-capillaries $(50 \, \mu\text{m} \times 75 \, \mu\text{m} \times 2 \, \text{mm})$ patterned in a single step with the cavity to connect the source region and the drift region in an atom-beam device. (b) A capping wafer is fabricated by first depositing Al$_2$O$_3$ and then depositing knives followed by a capping/bonding layer of metal. Inset showing a knife junction in a honeycomb seal. (c) After outgassing, the wafers are brought together and bonded. Inset: SEM image showing a cleaved interface. The knife is bonded into a Cu compliant layer with some damage from cleaving such that the capping wafer has detached from the knife bottom. (d) Diffusion at the grain boundary between the two layers creates a hermetic seal and provides mechanical strength.
• Figure 3: (a) Process interdependency for atomic vapor cell devices including realized bonds, pill activation, and relevant coatings. Shown are typical direct (fusion) bonding conditions, anodic bonding conditions, and metal-metal thermo-compression bonding conditions for flat wafers. Test bonds (not vapor cells) at $40$° C show that ultra-low temperature bonding is possible. (b) Calculated diffusion constant and diffusion length for Cu and Al for bulk and grain boundaries (G.B.). The exact value for this plot may vary depending on values such as grain boundary size.
• Figure 4: (a) Scanning electron microscope (SEM) image of a racetrack knife showing vacuum moats. (b) Picture of patterned honeycomb seals showing unconventional vacuum moats. Note: Images are from different devices and test bonds.
• Figure 5: Shear force testing shows the knife-edge bond strength is linear with increasing temperature. A maximum bond strength of $\sim 15$MPa is observed.