# Real-time buffer gas pressure tuning in a micro-machined vapor cell

S. Dyer  Department of Physics, SUPA, University of Strathclyde, Glasgow G4
0NG, United Kingdom  A. McWilliam  Department of Physics, SUPA, University of
Strathclyde, Glasgow G4 0NG, United Kingdom  D. Hunter  Department of Physics,
SUPA, University of Strathclyde, Glasgow G4 0NG, United Kingdom  S. Ingleby
Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, United
Kingdom  D. P. Burt  Kelvin Nanotechnology, University of Glasgow, Glasgow G12
8LS, United Kingdom  O. Sharp  Kelvin Nanotechnology, University of Glasgow,
Glasgow G12 8LS, United Kingdom  F. Mirando  Kelvin Nanotechnology, University
of Glasgow, Glasgow G12 8LS, United Kingdom  P. F. Griffin  Department of
Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, United Kingdom  E.
Riis  Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG,
United Kingdom  J. P. McGilligan  Department of Physics, SUPA, University of
Strathclyde, Glasgow G4 0NG, United Kingdom<EMAIL_ADDRESS>
###### Abstract

We demonstrate a controllable depletion of the nitrogen buffer gas pressure in
a micro-machined cesium (Cs) vapor cell from the dynamic heating of an alkali
dispenser pill. When the alkali source is laser activated, the gettering
compounds within the alkali pill dispenser reduce the nitrogen (N2) content
from the vapor for fine-tuning of the alkali to buffer gas pressure ratio.
Additionally, we decrease the buffer gas pressure below 100 mTorr to evaluate
the presence of other potential broadening mechanisms. Real-time control of
the gas pressure ratio in the vapor cell will have notable benefits for
refining atomic sensor performance and provide a routine to achieve various
target pressures across a wafer bonded with a uniform back-filled buffer gas
pressure.

††preprint: AIP/123-QED

Precision spectroscopy of atomic ensembles remains at the forefront of quantum
technologies and state-of-the-art metrological instruments. A prominent
platform used to this end is the interrogation of atoms in the vapor phase,
which provides a relatively simple apparatus for probing and controlling the
atomic state. The implementation of vapor phase atomic platforms range from
investigations of fundamental physics Schäpers  _et al._ (2000); Offer  _et
al._ (2018), to applied research in magnetometry Budker and Romalis (2007),
electrometry Sedlacek  _et al._ (2012), and terahertz sensing Downes  _et al._
(2020) among other applications Kitching (2018); Shuker  _et al._ (2008).

In recent years, the miniaturization of atom-optic technology has advanced the
state of field-deployable sensors for practical use in real-world applications
Kitching (2018); McGilligan  _et al._ (2022). At the heart of many of these
instruments is a micro-electro-mechanical-systems (MEMS) alkali vapor cell,
that provides a stage for atom-light interactions Liew  _et al._ (2004). The
MEMS cells typically consist of an etched or mechanically cut silicon cavity
sandwiched between two anodically bonded glass wafers. The design versatility
and mass-producible fabrication processes have resulted in MEMS cells being
widely implemented in compact clocks Knappe  _et al._ (2004); Batori  _et al._
(2022); Carlé  _et al._ (2023); Gusching  _et al._ (2023),
magnetometersSchwindt  _et al._ (2007); Hunter, Dyer, and Riis (2022); Ingleby
_et al._ (2022), and wavelength references Hummon  _et al._ (2018); Hafiz  _et
al._ (2016) as well as recently transferring to cold-atom technology Dyer  _et
al._ (2022a); McGilligan  _et al._ (2020); Bregazzi  _et al._ (2021).

A key step in the fabrication of the MEMS cell is the deposition technique
used for the introduction of alkali atoms into the vapor cell. While the
purity of the atomic species within the cell is critical to decrease unwanted
collisional shifts and broadening, the ratio of the alkali and buffer gas
plays an essential role in the performance of the atomic sensor Hasegawa  _et
al._ (2011). A number of options have been demonstrated as mass-producible
solutions to vapor deposition, such as alkali-azidesDyer  _et al._ (2022b);
Liew, Moreland, and Gerginov (2007), chloridesBopp, Maurice, and Kitching
(2020); Knappe  _et al._ (2005), graphite reservoirs Kang  _et al._ (2017) and
chromate based micro-pill dispensers Douahi  _et al._ (2007a); Hasegawa  _et
al._ (2011). For each of these methods, the inclusion of a substantial
additional buffer gas pressure is typically achieved by back-filling the
vacuum bonder with the desired mixture of gases prior to cell encapsulation
Kitching (2018). However, this approach can result in cell-to-cell variations
across the wafer, as has been recently shown in the case of alkali-azide
deposition Dyer  _et al._ (2022b). While deviations from the target pressure
can be addressed by changing the operational temperature of the cell Kroemer
_et al._ (2015); Hasegawa  _et al._ (2011), this approach would be
unfavourable for integrated optics packages were the cell and laser require
stabilization at a common value Vicarini  _et al._ (2019). Importantly, the
method of back-filling a buffer gas pressure during bonding is applied across
an entire wafer, such that all cells derived from the same wafer are
restricted to a single target buffer gas pressure and composition, with
additional cell complexity required for individual cell tunability Maurice
_et al._ (2022).

In this Letter, we demonstrate the real-time fine tuning of the nitrogen
buffer gas pressure within a micro-machined cesium vapor cell. A thick silicon
wafer is bonded to borosilicate glass within a nitrogen environment, with an
alkali pill deposited within the silicon frame. When laser activated, the pill
source provides a saturated cesium vapor pressure within each cell. We show
that the initial back-filled nitrogen pressure can be controllably decrease by
laser heating the non-evaporable getter compound within the dispensing pill.
In this work we show by tracking the broadening and pressure shift in the sub-
Doppler features of $D_{1}$ Cs spectroscopy during pill heating, that the
nitrogen content can be fine-tuned over a pressure range of 150 Torr to below
100 mTorr. The stability of the target pressure within the cell is shown to
remain stable over a prolonged period of months after the heating of the pill
has stopped. Finally, the zero-intensity linewidth is measured for a cell with
a greatly diminished buffer gas pressure. This approach could prove beneficial
to the fine-tuning of buffer gas cells in a plethora of atomic sensors, while
also enabling fabrication routines that realize many individual target buffer
gas pressures derived from the constituent cells of a single wafer.

Figure 1: (a)-(c): Wafer bonding procedure. Wafers are heated to
300$\,{}^{\circ}$C and chamber is pumped down to a vacuum pressure of
$P=10^{-5}\,$Torr. Nitrogen is back-filled to the target pressure at this
temperature. The wafer is then diced into constituent vapor cells and measured
at $T_{cell}=60\,^{\circ}$C. (d): Illustration of the measurement apparatus.
The Cs vapor cell is heated on a hotplate while absorption of the 894.6 nm
probe is monitored. Activation is performed with a 1070 nm laser aligned to an
alkali pill dispenser within the micro-machined vapor cell.

A simplified schematic of the vapor cell bonding and measurement procedure is
shown in Fig. 1. A 6 mm thick silicon wafer is water-jet cut to the desired
cavity geometry, with two chambers connected via a meandering channel to avoid
glass darkening from pill activation in the spectroscopy region of the cell
Dyer  _et al._ (2022c). The processed silicon wafer is first pre-bonded to a
lower borosilicate glass (BSG) wafer. The pill dispensers are then deposited
within the silicon frame. The final bond is carried out within a vacuum
apparatus, with the process illustrated in Fig. 1 (a). The pre-formed wafer
stack and upper BSG wafer are held on independent surface electrodes while the
vacuum is pumped down to the order of 10${}^{-5}\,$Torr. Following vacuum
evacuation, nitrogen is back-filled within the chamber to a pressure of 305
Torr while the wafers are heated to 300$\,{}^{\circ}$C, shown in Fig. 1 (b). A
high voltage is applied across the wafers to anodically bond a hermetic seal
for vacuum encapsulation. The wafers are then removed from the chamber and
mechanically diced into constituent vapor cells, shown in Fig. 1 (c). An
individual cell is then placed within a spectroscopy set-up and heated to an
operational temperature of 60$\,{}^{\circ}$C on a hotplate, illustrated in
Fig. 1 (d).

An 894.6 nm distributed Bragg reflector (DBR) laser is used to resolve
spectroscopy of the $D_{1}$ transitions from the $F=3$ and $F=4$ ground-states
of Cs. The light is fibre coupled and split into 3 arms. The first arm is
coupled into a Fabry-Pérot cavity to calibrate the frequency scan from the
laser. The second arm is aligned through a 7-cm-long Cs vapor cell to
calibrate the frequency scan with respect to the Cs $D_{1}$ spectrum. The
third arm is aligned through the main chamber of the MEMS cell onto a
photodiode to record absorption spectra for each buffer gas pressure during
the tuning process. The probe beam has a beam waist of 700$\,\mu$m (1/e2
radius) and average intensity of 75$\,\mu$W/cm2.

Figure 2: (a): Measured spectroscopy of the Cs $D_{1}$ line for N2 pressures
of 150 Torr (black), 100 Torr (red) and 100 mTorr (dashed blue). (b): Time
sequence of the inferred nitrogen pressure as it is decreased with vertical
black lines indicating when the 1070 nm laser is on. (c): Ratio of the
measured linewidth broadening and frequency shift as a function of time shown
in red with the expected value in black.

Since the Cs in the MEMS cell is derived from the pill source, there is no
initial signal until the pill source has been activated. A fibre laser at 1070
nm is used for pill heating, and focused to fill the surface area of the
$\approx$1 mm diameter pill. The Cs source is an alkali-pill composed of Cs-
chromate mixed with a non-evaporable getter (NEG) compound Douahi  _et al._
(2007a, b). Recent work has shown that the heating of NEGs can be used for the
passive pumping of vacuum cells by removing gases capable of diffusing into
the bulk and undergoing irreversible chemical reactions with the Al and Zr
mixture of the NEG McGilligan  _et al._ (2022); Boudot  _et al._ (2020);
Burrow  _et al._ (2021); Little  _et al._ (2021); Scherer, Fenner, and Hensley
(2012). Among the getterable elements, the nitrogen content within a vapor
cell has been shown to be getterable Lucivero  _et al._ (2022); Noor, Asadian,
and Shkel (2020). To initiate an alkali density, the pill was heated by the
laser for 40 s with 3 W to release the Cs. After an hour of heating the cell
at 100$\,\degree$C to encourage alkali diffusion through the meandering
channel Dyer  _et al._ (2022c), a saturated Cs vapor was measured with the
broadening and shift indicating a N2 pressure of $\approx$150 Torr at
60$\,{}^{\circ}$C. The measured initial pressure is lower than the expected
pressure of 177 Torr, inferred from the ideal gas law. This discrepancy could
be attributed to N2 consumption into the pill during the first heating pulse.

Following the release of Cs, the pill was re-heated with 3 W bursts of varied
duration. This level of laser power was selected as it provided a temporal
response from the pill that enabled fine control of the buffer gas pressure.
Spectra taken for cells tuned between 150 Torr and $\leq$100 mTorr are shown
in Fig. 2 (a). During the gettering process, the N2 pressure is inferred from
both the linewidth and collisional frequency shift, which are extracted from
fitting the spectrum with Voigt profiles. The impact of N2 pressure on the
$D_{1}$ line of Cs has been previously characterisedAndalkar and Warrington
(2002), with coefficients for the broadening and shift determined as 19.8
MHz/Torr and -8.4 MHz/Torr respectively for a cell at 21$\,{}^{\circ}$C. The
inferred N2 pressure during the gettering process is shown in Fig. 2 (b),
where the pressure is shown with red data points after each subsequent
activation cycle, indicated by vertical black lines. During the pressure
depletion process, the ratio of the broadening and shift was found to be
$\Gamma_{BG}/\Delta_{BG}\simeq$-2.24$\pm$0.02, in good agreement with the
expected value for Cs $D_{1}$ in the presence of N2 Andalkar and Warrington
(2002). The tracking of this ratio is provided in Fig. 2 (c), where the
consistently measured ratio expected for a N2 dominated background pressure
indicates an insignificant presence of other gas species within the cell.

Following the initial demonstration of controllably decreasing the N2 pressure
with the alkali dispensing pill, the stability of the target pressure was
investigated to elucidate if the NEG compound continues to remove N2 after the
laser heating has been removed. For this study, two cells were tuned to
P${}_{N_{2}}\approx 100\,$Torr at 60$\,{}^{\circ}$C while monitoring the
evolution of the inner pressure environment for a period beyond 50 days. The
cells were maintained at different temperatures between the subsequent
measurement windows to determine if a higher cell operational temperature
would impact the pressure stability, potentially occurring from an increased
N2 diffusion rate into the getter material at higher temperatures. Following
the heating periods, the cell pressure was then measured again at an
operational temperature of 60$\,{}^{\circ}$C. The results of this study are
shown in Fig. 3 in blue squares and red circles for maintained temperatures of
60∘C and 120∘C respectively. Over the course of the measurement, the N2
pressure remained constant around 102 Torr for the measurement temperature,
indicating that the gettering compound does not significantly remove content
from the vapor in this cell operational temperature range. A linear fit to the
cell pressure as a function of time in the cells maintained at 60∘C and 120∘C
provide drifts of $\approx\,-0.3\,$mTorr/day and $\approx\,3\,$mTorr/day
respectively, consistent with no change of buffer gas pressure.

Figure 3: Inferred buffer gas pressure from the shift $\Delta_{BG}$ for a cell
kept at 60 $\degree$C (blue squares) and another at 120 $\degree$C (red
circles).

Finally, to evaluate the presence of additional broadening mechanisms, a vapor
cell was exposed to multiple laser pulses in an attempt to remove all N2
background gas. The previous apparatus was modified to allow for Doppler-free
counter-propagating pump-probe spectroscopy of the $F=4\rightarrow
F^{\prime}=3$ transition, as shown in Fig. 4 (a). The linewidth of the sub-
Doppler resonance is monitored for various pump beam intensities to
extrapolate back to a zero-intensity linewidth. During this measurement the
pump and probe beams had a waist of 0.9 mm (1/e2 radius) at the vapor cell and
the probe beam intensity is held constant at 0.1 mW/cm2, to provide a
sufficient signal-to-noise ratio for fitting. A typical Lorentzian fit to the
sub-Doppler feature is shown in Fig. 4 (b). The power broadened linewidth is
expected to follow the form $\Gamma=\Gamma_{0}\sqrt{1+I/I_{sat}}$, where
$\Gamma_{0}=$ $2\pi\times 4.5$ MHz and $I_{sat}$=2.5 mW/cm2 Steck (2001). Here
a zero-intensity linewidth of $\Gamma=$8.4$\pm 0.5\,$MHz was obtained as
demonstrated in Fig. 4 (c). To eliminate contributions from systematic
broadening mechanisms such as residual Doppler broadening, laser linewidth and
transit-time broadening the measurement was repeated with a glass-blown vapor
cell which yielded a zero-intensity linewidth of $\Gamma=$7.0$\pm 0.4\,$MHz.
By comparing the zero-intensity linewidths a residual broadening of
$\approx$1.4$\pm$0.6 MHz, is obtained for the depleted MEMS vapor cell. If we
attribute this broadening to remaining nitrogen within the cell, this would
place an upper bound on the N2 pressure of 100 mTorr at 21∘C. In the future we
will investigate alternative spectroscopic techniques such as two-photon
spectroscopy Newman  _et al._ (2021); Georgiades, Polzik, and Kimble (1994) to
better estimate the residual pressure within the cell.

In conclusion, we have demonstrated a simple approach to buffer gas pressure
tuning in micro-machined vapor cells. This approach greatly simplifies current
methods for in-situ tuning of the cell pressure ratio with a minimal footprint
on the cell dimensions. The demonstrated tunability and expected longevity
from this technique could prove advantageous to the fabrication of buffer gas
based atomic sensors, such as clocks and optically pumped magnetometers, where
such a method could be used for real-time fine-tuning of the sensor
sensitivity. Additionally, this approach could greatly reduce manufacturing
costs by enabling an array of cell target pressures to be extracted from a
single wafer with a back-filled buffer gas pressure.

Figure 4: (a): Sub-Doppler spectroscopy of the $F=4\rightarrow F^{\prime}=3$
transition (unaveraged) with pump intensity 1.2 mW/cm2 and vapor cell at room
temperature. (b): Spectra (black circles) and Lorentzian fit (red line) of the
sub-Doppler resonance extracted from (a). (c): The sub-Doppler linewidth as a
function of pump intensity for the MEMS cell (red circles) and glass-blown
cell (blue squares) with fit lines of the form
$\Gamma=\Gamma_{0}\sqrt{1+I/I_{sat}}$ shown in solid red and dashed blue lines
respectively.

###### Acknowledgements.

The authors would like to thank Dr. Jonathan Pritchard for his work on the
optical absorption spectroscopy setup and Dr. Terry Dyer for his assistance
and training with wafer dicing. Additionally, we would like to thank Dr. Aidan
Arnold for useful conversations. The authors acknowledge funding from Defence
Security and Technology Laboratory. The authors would like to acknowledge
support from the INMAQS collaboration (EP/W026929/1). J. P. M gratefully
acknowledges funding from a Royal Academy of Engineering Research Fellowship.

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