US 11,180,844 B2

5

metal vapor cells 200. The process can include introducing
a buffer gas in cell cavity 216, wherein the alkali metal is
inert to the buffer gas.

In the process for making alkali metal vapor cell 200 with

alkali metal vapor cell apparatus 250, disposing depositions

assembly 228 on sealing wafer 220 in fluid communication
with transmission apertures 224 of sealing wafer 220 such
that sealing wafer 220 is interposed between preform wafer
210 and deposition assembly 228 includes mechanical
clamping of deposition assembly 228 to sealing wafer 220
with a spring or screw-based clamp designed and adjusted to
press the two components together; chemical adhesion of
deposition assembly 228 to sealing wafer 220 with appro-
priate glue or other adhesive; or wafer-bonding of deposition
assembly 228 to sealing wafer 220 by anodic bonding,
fusion bonding, or eutectic bonding.

In the process for making alkali metal vapor cell 200,
disposing alkali metal precursor 234 in deposition assembly
228 includes delivering a liquid solution that includes alkali
metal precursor 234 into deposition assembly 228; and
heating the liquid to evaporate the solvent leaving the
precursors in deposition assembly 228 in solid form. Heat-
ing can occur by placing the deposition assembly 228 on a
heated surface. Optionally, the liquid composition can be
dried by exposing the liquid composition to a dry gaseous
environment. Upon drying the liquid composition, alkali
metal precursor 234 is stable at room temperature.

In the process for making alkali metal vapor cell 200,
disposing sealing wafer 220 on preform wafer 210 includes

introducing sealing wafer 220 and preform wafer 210 into an_ 3

evacuated bonding chamber 299; translating 220 with
respect to 210 such that 220 comes into contact with 210;
and lateral aligning wafers.

In the process for making alkali metal vapor cell 200,

aligning transmission apertures 224 of sealing wafer 220 3

with cell cavities 216 includes depositing alignment marks
on bonding sealing wafer 220 and cavity layer 214, the
alignment marks being visible from outside bonding cham-
ber 299 when alkali metal vapor cell apparatus 250 is inside
bonding chamber 299. Alignment of alignment marks is
with respect to each other before reaction of chemical
precursors 234 to align transmission apertures 224 to cell
cavities 216. Accordingly, after alignment, transmission
apertures 224 of sealing wafer 220 are in fluid communica-
tion with cell cavities 216 of preform wafer 210.

In the process for making alkali metal vapor cell 200,
subjecting alkali metal precursor 234 in deposition assembly
228 to a reaction stimulus includes heating the alkali metal
precursors to elevated temperature after insertion of depo-
sition assembly 228 and preform wafer 210 into bonding
chamber 299 and evacuation of this chamber of gas. In an
embodiment, subjecting alkali metal precursor 234 in depo-
sition assembly 228 to the reaction stimulus includes heating
alkali metal precursor 234. Here, electrically resistive heat-
ers in the platen of the bonding machine that hold deposition
assembly 228 are activated by passing an electrical current
through them, which results in them getting hot and heat
transferred to deposition assembly 228 and alkali metal
precursor 234. In an embodiment, subjecting alkali metal
precursor 234 in deposition assembly 228 to the reaction
stimulus includes irradiating alkali metal precursor 234 with
ultraviolet radiation. Here, ultraviolet light, e.g., from a
discharge lamp or laser, is transmitted into deposition
assembly 228 through windows on deposition assembly 228
transparent to ultraviolet light.

In the process for making alkali metal vapor cell 200,
producing, from alkali metal precursor 234, alkali metal

40

45

6:

6

vapor in deposition assembly 228 in response to subjecting
alkali metal precursor 234 to the reaction stimulus includes
a chemical reaction that occurs between alkali metal pre-
cursors, one of which is a compound containing an alkali
atom. Under the reaction stimulus, the alkali metal precur-
sors react to form pure alkali metal and a reaction product
not subsequently used. The pure alkali metal evaporates to
fill deposition assembly 228 with an alkali metal vapor.

In the process for making alkali metal vapor cell 200,
communicating, simultaneously in parallel, the alkali metal
vapor from deposition assembly 228 to cell cavities 216
through transmission apertures 224 includes heating of the
deposition assembly to form a vapor of sufficient pressure
and effusing the alkali vapor through the transmission aper-
tures

In the process for making alkali metal vapor cell 200,
receiving, in cell cavities 216, alkali metal vapor from
transmission apertures 224 includes applying a temperature
difference between deposition assembly 228 and preform
wafer 210 and condensing the alkali vapor as a solid or
liquid alkali metal condensate 236 onto the surface of the
cell cavities 216.

In the process for making alkali metal vapor cell 200,
moving sealing wafer 220 laterally with respect to preform

s wafer 210 such that cover layer 222 encapsulates alkali

metal condensate 236 in cell cavities 216 includes moving
cover layer 222 with respect to preform wafer 210 perpen-
dicular to the wafer surfaces (e.g., vertically) to separate the
wafers; lateral translation of cover layer 222 with respect to
preform wafer 210 using mechanical manipulators; and
translating, e.g., in a vertical direction, cover layer 222 with
respect to preform wafer 210 such that the wafers are once
again in contact,

In the process for making alkali metal vapor cell 200,
bonding sealing wafer 220 to preform wafer 210 to make
individually sealed alkali metal vapor cells 200 in preform
wafer 210 can include anodic bonding, thermocompression
bonding, fusion bonding, eutectic bonding, or a combination
thereof. In an embodiment, bonding is anodic bonding that
includes applying an elevated voltage across a conductive
wafer and a glass or ceramic wafer placed in surface contact
at elevated temperature such that impurity ions in the wafers
are transported within the wafers resulting in electrostatic
forces and an oxidation process at the interface that bonds
the wafers together. In an embodiment, bonding is fusion
bonding that includes placing two silicon wafers in contact
after appropriate processing of the wafer surfaces; heating to
elevated temperature to form the bond; subsequent anneal-
ing at elevated temperature. In an embodiment, bonding is
eutectic bonding that includes depositing metallic films on
the wafer surfaces; aligning of the wafers such that the
surfaces are in contact and the metallic films are in contact;
and heating to an elevated temperature to allow the metallic
films to diffuse into one another. As a result of bonding

5 sealing wafer 220 to preform wafer 210, alkali metal vapor

cells 200 are individually sealed.

The process can include removing deposition assembly
228 from sealing wafer 220 by abrasive machining, laser
cutting or chemical etching. In an embodiment, the process
further includes dicing, after bonding sealing wafer 220 to
preform wafer 210, preform wafer 210 to provide a select
number of individually sealed alkali metal vapor cells 200.
Dicing includes abrasive machining or laser cutting of the
parts of the wafer between the cells 200 without disrupting
the seal formed by the bonding.

In some embodiments, the process includes introducing a
buffer gas in cell cavity 216, wherein the alkali metal is


U.S. Patent Nov. 23, 2021 Sheet 12 of 15 US 11,180,844 B2

(A)

(B)
238 216 238 216 238 216
i) i) i) ‘ 214
\ \ \ \ \ \
aT ava ava ‘
i i |

An AA
i i

218 218 218 218 218 218 218

Figure 12


US 11,180,844 B2

13

deposition cavities integrated onto the wafer containing the
cell cavities or integrated in a second deposition wafer that
can be fabricated to match the cell cavity pattern on the
preform wafer. Advantageously, processes herein include
physical translation, under vacuum, of one wafer with
respect to a second wafer for deposition of the alkali metal
into the cell before translation and provide a clean and
contiguous bonding surface over the openings in the cell
cavities after translation.

The articles and processes herein are illustrated further by
the following Examples, which are non-limiting.

EXAMPLE

Compact, alkali metal vapor cells produced on a wafer
preform.

FIG. 15 shows a wafer of alkali vapor cells fabricated
using the processes described herein. Shown are deposition
cavities 216 and cell cavities 238 with alkali metal conden-
sate 236 visible in cell cavities 238 as whitish material. This
water of cells was fabricated with as described herein with
no buffer gas intentionally introduced into the cavity. Suc-
cessful bonding is indicated by darker regions between cell
cavities and deposition cavities. The image was acquired
through transparent glass sealing wafer 220 and onto
opaque, black silicon cavity layer 214. The cell cavities and
deposition cavities were fabricated using lithographic pat-
terning and deep reactive ion etching of silicon.

While one or more embodiments have been shown and

described, modifications and substitutions may be made 3

thereto without departing from the spirit and scope of the
invention. Accordingly, it is to be understood that the present
invention has been described by way of illustrations and not
limitation. Embodiments herein can be used independently
or can be combined.

All ranges disclosed herein are inclusive of the endpoints,
and the endpoints are independently combinable with each
other. The ranges are continuous and thus contain every
value and subset thereof in the range. Unless otherwise
stated or contextually inapplicable, all percentages,
when expressing a quantity, are weight percentages. The
suffix “(s)” as used herein is intended to include both the
singular and the plural of the term that it modifies, thereby
including at least one of that term (e.g., the colorant(s)
includes at least one colorants). “Optional” or “optionally”
means that the subsequently described event or circumstance
can or cannot occur, and that the description includes
instances where the event occurs and instances where it does
not. As used herein, “combination” is inclusive of blends,
mixtures, alloys, reaction products, and the like.

As used herein, “a combination thereof” refers to a
combination comprising at least one of the named constitu-
ents, components, compounds, or elements, optionally
together with one or more of the same class of constituents,
components, compounds, or elements.

All references are incorporated herein by reference.

The use of the terms “a” and “an” and “the” and similar
referents in the context of describing the invention (espe-
cially in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by con-
text. “Or” means “and/or.” It should further be noted that the
terms “first,” “second,” “primary,” “secondary,” and the like
herein do not denote any order, quantity, or importance, but
rather are used to distinguish one element from another. The
modifier “about” used in connection with a quantity is
inclusive of the stated value and has the meaning dictated by

S

2

40

50

3!

3

14

the context (e.g., it includes the degree of error associated
with measurement of the particular quantity). The conjunc-
tion “or” is used to link objects of a list or alternatives and
is not disjunctive; rather the elements can be used separately
or can be combined together under appropriate circum-
stances.

What is claimed is:

1. A process for making a plurality of alkali metal vapor
cells, the process comprising:

providing a preform wafer comprising:

a basal member;

a cavity layer disposed on the basal member; and

a plurality of cell cavities disposed in the cavity layer
and bounded by a cavity wall of the cavity layer;

providing a sealing wafer comprising:

a cover layer; and

a plurality of transmission apertures disposed in the
cover layer and bounded by a cover wall of the cover
layer;

disposing a deposition assembly on the sealing wafer in

fluid communication with the transmission apertures of
the sealing wafer such that the sealing wafer is inter-
posed between the preform wafer and the deposition
assembly;

disposing an alkali metal precursor in the deposition

assembly;
disposing the sealing wafer on the preform wafer;
aligning the transmission apertures of the sealing wafer
with the cell cavities such that the transmission aper-
tures of the sealing wafer are in fluid communication
with the cell cavities of the preform wafer;

subjecting the alkali metal precursor in the deposition
assembly to a reaction stimulus;

producing, from the alkali metal precursor, alkali metal

vapor in the deposition assembly in response to sub-
jecting the alkali metal precursor to the reaction stimu-
lus;

communicating, simultaneously in parallel, the alkali

metal vapor from the deposition assembly to the cell
cavities through the transmission apertures;

receiving, in the cell cavities, the alkali metal vapor from

the transmission apertures;

producing, from the alkali metal vapor, an alkali metal

condensate in the cell cavity;
moving the sealing wafer laterally with respect to the
preform wafer such that the cover layer encapsulates
the alkali metal condensate in the cell cavities; and

bonding the sealing wafer to the preform wafer to make
individually sealed alkali metal vapor cells in the
preform wafer.

2. The process of claim 1, further comprising removing
the deposition assembly, from the sealing wafer after pro-
ducing, from the alkali metal precursor, alkali metal vapor in
the deposition assembly.

3. The process of claim 1, further comprising dicing, after
bonding the sealing wafer to the preform wafer, the preform
wafer to provide a select number of individually sealed
alkali metal vapor cells.

4. The process of claim 1, further comprising introducing
a buffer gas in the cell cavity, the alkali metal being inert to
the buffer gas.

5. The process of claim 1, wherein subjecting the alkali
metal precursor in the deposition assembly to the reaction
stimulus comprises heating the alkali metal precursor.



U.S. Patent Nov. 23, 2021 Sheet 3 of 15 US 11,180,844 B2

(A)

(B)

(co) C 224 f oad ” 254
T
} A —___} _ } a fp 252
222
on | } L | | P20
ade 226 ode 226 { 222 os

224

Figure 3


US 11,180,844 B2

7

disposed in buffer gas. Introducing the buffer gas can include
filling bonding chamber interior 298 of bonding chamber
299 in which alkali metal vapor cell apparatus 250 is
enclosed with the buffer gas after the alkali metal precursors
are reacted, but before bonding; and performing bonding in
the presence of the buffer gas to seal the gas inside the cell
cavity 216.

Alkali metal precursor 234 can be disposed on chamber
wall 232 of deposition assembly 228 that opposed cell cavity
216 in cavity layer 214 of preform wafer 210. In an
embodiment, with reference to FIG. 9, FIG. 10, FIG. 11,
FIG. 12, FIG. 13, FIG. 14, and FIG. 15, cavity layer 214
includes deposition cavity 238 bounded by cavity wall 218
and disposed among cell cavity 216. Here, a process for
making a plurality of alkali metal vapor cells 200 in alkali
metal vapor cell apparatus 250 includes providing preform
wafer 210. Preform wafer 210 includes basal member 212;
cavity layer 214 disposed on basal member 212; a plurality
of cell cavities 216 disposed in cavity layer 214 and bounded
by cavity wall 218 of cavity layer 214; and a plurality of
deposition cavities 238 disposed in cavity layer 214 and
bounded by cavity wall 218 of cavity layer 214, such that
each deposition cavity 238 is adjacent to a cell cavity 216.
The process also includes disposing alkali metal precursor
234 in deposition cavity 238 of preform wafer 210; and
providing sealing wafer 220. Sealing wafer 220 includes
cover layer 222 that can be a blank. The process also
includes disposing sealing wafer 220 on preform wafer 210
so that deposition surface 240 of sealing wafer 220 faces and

is in fluid communication with deposition cavities 238 of 3

preform wafer 210, such that deposition cavities 238 are
individually isolated from fluid communication with other
deposition cavities 238 and cell cavities 216; subjecting
alkali metal precursor 234 in deposition cavities 238 to a

reaction stimulus; producing, from alkali metal precursor 3

234, alkali metal vapor in deposition cavities 238 in
response to subjecting alkali metal precursor 234 to the
reaction stimulus; communicating, simultaneously in paral-
lel, the alkali metal vapor from deposition cavity 238 to
deposition area 242 on deposition surface 240 of sealing
wafer 220 that covers deposition cavity 238, such that each
deposition area 242 is local to an individual deposition
cavity 238 and physically separate from other deposition
cavities 238 and cell cavities 216; optionally contacting
deposition area 242 with the alkali metal vapor from depo-
sition cavity 238; producing, from alkali metal vapor, alkali
metal condensate 236 on deposition area 242; separating
sealing wafer 220 from preform wafer 210; moving sealing
wafer 220 laterally with respect to preform wafer 210 and
aligning deposition areas 242 of sealing wafer 220 with cell
cavities 216 of preform wafer 210 such that individual
deposition areas 242 of sealing wafer 220 are in fluid
communication with an individual cell cavity 216 of pre-
form wafer 210 in an aligned position and contacting sealing
wafer 220 with preform wafer 210 in the aligned position to
encapsulate alkali metal condensate 236 in individual depo-
sition area 242 in individual cell cavities 216; and bonding
sealing wafer 220 to preform wafer 210 to make individually
sealed alkali metal vapor cells 200 in preform wafer 210.

In the process for making alkali metal vapor cell 200,
disposing alkali metal precursor 234 in deposition cavity
238 of preform wafer 210 includes mixing precursor mate-
rials in solid form or dissolved into a liquid and the depo-
sition of mixed precursor materials into deposition cavity
238 by manual or automated injection.

In the process for making alkali metal vapor cell 200,
disposing sealing wafer 220 on preform wafer 210 so that

0

40

&

0

3

6:

8

deposition surface 240 of sealing wafer 220 faces and is in
fluid communication with deposition cavities 238 of preform
wafer 210 includes manual or automated motion of the
sealing wafer 220 with respect to the preform wafer 210
within the bonding chamber 299.

In the process for making alkali metal vapor cell 200,
subjecting alkali metal precursor 234 in deposition cavities
238 to a reaction stimulus includes heating the alkali metal
precursors to elevated temperature after insertion of sealing
wafer 220 and preform wafer 210 into the bonding chamber
299 and evacuation of this chamber of gas. In an embodi-
ment, subjecting alkali metal precursor 234 in deposition
assembly 228 to the reaction stimulus includes heating alkali
metal precursor 234. Here, electrically resistive heaters in
the platen of the bonding machine that hold the deposition
assembly 228 are activated by passing an electrical current
through them, which results in them getting hot and the heat
being transferred to the deposition assembly 228 and alkali
metal precursor 234. In an embodiment, subjecting alkali
metal precursor 234 in deposition assembly 228 to the
reaction stimulus includes irradiating alkali metal precursor
234 with ultraviolet radiation. Here, ultraviolet light, e.g.,
from a discharge lamp or laser, can be transmitted into
deposition assembly 228 through windows on deposition
assembly 228 that are transparent to ultraviolet light.

In the process for making alkali metal vapor cell 200,
producing, from alkali metal precursor 234, alkali metal
vapor in deposition cavities 238 in response to subjecting
alkali metal precursor 234 to the reaction stimulus includes
a chemical reaction that occurs between two or more alkali
metal precursors, one of which is a compound containing an
alkali atom. Under the reaction stimulus, the alkali metal
precursors react to form pure alkali metal and additional
reaction products not subsequently used. The pure alkali
metal then evaporates to fill the deposition cavity 238 with
an alkali metal vapor.

In the process for making alkali metal vapor cell 200,
communicating, simultaneously in parallel, the alkali metal
vapor from deposition cavity 238 to deposition area 242 on
deposition surface 240 of sealing wafer 220 that covers
deposition cavity 238 includes heating of the deposition
assembly to form a vapor of sufficient pressure and the
effusion of this alkali vapor toward deposition surface 240.

In the process for making alkali metal vapor cell 200,
producing, from alkali metal vapor, alkali metal condensate
236 on deposition area 242 includes forming a temperature
differential between preform wafer 210 and sealing wafer
220 such that the alkali metal condenses in solid or liquid
form on the deposition area 242.

In the process for making alkali metal vapor cell 200,
separating sealing wafer 220 from preform wafer 210
includes the manual or automated motion of the sealing
wafer vertically with respect to the preform water inside the
vacuum chamber without exposing deposition surface 240 to

5 oxidizing gas such as oxygen or water vapor.

In the process for making alkali metal vapor cell 200,
moving sealing wafer 220 laterally with respect to preform
wafer 210 and aligning deposition areas 242 of sealing wafer
220 with cell cavities 216 of preform wafer 210 includes the
manual or automated motion of sealing wafer 220 with
respect to preform wafer 210 within the vacuum chamber
using mechanical actuators; alignment of sealing wafer 220
with respect to preform wafer 210 using alignment marks
deposited onto, or etched into, the wafers using lithographic
processes; and verification relative alignment of the wafers
with an imaging systems. As a result, individual deposition
areas 242 of sealing wafer 220 are in fluid communication


US 11,180,844 B2

11

cavities 216 can be lithographically defined or easily etched,
e.g., in parallel using the same process sequence, and can be
silicon, glass, or a metal. Moreover, cell cavities 216 can
also be abrasively or laser machined. In an embodiment,
cavity layer 214 includes a circular polished wafer of silicon,
100 mm in diameter and 1 mm thick, into which cell cavities
216 have been defined using lithographic patterning and
subsequently etched using wet chemical etching or deep
reactive ion etching. A diameter of cell cavity 216 can be
from 10 1m to 10 mm, specifically from 100 ym to 1 mm,
and more specifically from 0.5 mm to 1 mm. An axial length
of cell cavity 216 can be from 10 um to 10 mm, specifically
from 100 jum to 1 mm, and more specifically from 0.5 mm
to I mm. An aspect ratio of cell cavity 216, given by a length
of the cavity wall 218 to a diameter of the call cavity 216,
can be from 0.001 to 1000, specifically from 0.1 to 10 and
more specifically from 0.9 to 1.1. A volume of cell cavity
216 can be from 1000 um* to 1000 mm*, specifically from
10°? mm? to 10 mm, and more specifically from 0.1 mm?
to 10 mm*

Cell cavity 216 of cavity layer 214 terminates at basal
member 212. Basal member 212 can be a polished wafer to
prevent the release of alkali vapor outside the cell cavity 214
and prevent the contamination of the cell by gases from the
outside and can be glass, silicon, and the like. Moreover,
basal member 212 may have thin oxide coatings applied to
it, e.g., to prevent diffusion of alkali vapor into the glass and
control reflections of light from the glass surface. In an
embodiment, basal member 212 is a round polished boro-

silicate or aluminosilicate glass wafer 100 mm in diameter 3

and 1 mm thick. It is contemplated that basal member 212
can include a material with low permeability to helium such
as aluminosilicate glass, a material with a thermal expansion
with temperature close to that of sealing platen 220. A

coeflicient of thermal expansion of basal member 212 can be 3

from 1x10~/K to 5x10°°K, specifically from 2.5x10°/K to
2.7x10-°/K. A cross-sectional shape of cell cavity 216 can
be, e.g., round, square, rectangular, and the like. A volume
of cell cavity 216 can be from 1000 um? to 1000 mm’,
specifically from 10~? mm? to 10mm°, and more specifically
from 0.1 mm* to 10 mm®. Once sealed, the gas inside cell
cavity 216 can have a pressure at room temperature from 0
kPa to 500 kPa, specifically from 30 kPa to 100 kPa, and
more specifically from 50 kPa to 60 kPa. It is contemplated
that basal member 212 and the cavity layer can be a
monolithic component or separate components. For the
monolithic component, instead of etching through holes in
the Si wafer and bonding glass to one side, blind holes are
etched into the wafer such that the wafer includes the basal
member and the cavity layer.

With reference to, FIG. 10, FIG. 11, FIG, 12, FIG. 13,
FIG. 14, and FIG. 15, alkali metal vapor cell apparatus 250
includes preform wafer 210 having cavity layer 214 dis-
posed on basal member 212; and sealing wafer 220 disposed
on cavity layer 214. Cavity layer 214 includes deposition
cavity 238 bounded by cavity wall 218 that are arranged
proximate and adjacent to a neighboring cell cavity 216. It
is contemplated that, in a row of deposition cavities 238 and
cell cavities 216, deposition cavities 238 and cell cavities
216 can be alternatingly disposed cavity layer 214 as shown
in FIG. 11. Other arrangements are contemplated such as a
row of exclusively deposition cavities 238 arranged adjacent
to a row of exclusively cell cavities 216 so that alkali metal
condensate 236 can be transferred on sealing wafer 220
between deposition cavity 238 and cell cavity 216. Lateral
translation of sealing wafer 220 relative to preform wafer
210 includes linear translation and rotary translation. A

S

20

2

40

50

12

number of deposition cavities 238 and a number of cell
cavities 216 can be selected based on a density of alkali
metal vapor cell 200 on preform wafer 210 desired. More-
over, as shown in FIG. 13, sealing wafer 220 can include a
blank with exterior surface 258 and deposition surface 240
arranged on opposite faces of sealing wafer 220. Deposition
surface 240 receives alkali metal condensate 236 from alkali
metal precursor 234 disposed in deposition cavity 238.

Alkali metal precursor 234 can include an alkali halide, an
azide, a chromate, a molibdate, or a combination thereof,
and can be combined with a reducing agent such as barium
azide, calcium, zirconium or aluminum. Exemplary alkali
halides include RbCl, CsCl, KCI, RbBr, CsBr, RbBr, and the
like. Exemplary alkali azides include RbN,, CsN,, KN,. and
the like. It is contemplated that alkali metal precursor 234
can include RbN,, CsN3, KN, that produce alkali metal in
response to be subjected to ultraviolet light. Alkali halides
can be reacted with BaN, or Ca to produce alkali metal. In
an embodiment, alkali metal precursor 234 includes BaN,
and an alkali chloride, which react to form BaCl, N,, and
alkali metal in response to heating. The alkali metal and
alkali metal condensate 236 independently can include
potassium, rubidium, cesium, or a combination thereof.

Alkali metal precursor 234 can be disposed in a liquid
composition for delivery to deposition assembly 228 or
deposition cavity 238. In addition to alkali metal precursor
234, the liquid composition can include a solvent for mixing
of constituents of alkali metal precursor 234 and communi-
cating alkali metal precursors 234 into deposition cavity
238. Exemplary solvents include water, alcohol, and the like.

Buffer gas can be included in alkali metal vapor cell 200.
The buffer gas is inert to the alkali metal, and exemplary
buffer gases include N,, Ar, Ne, CH,, and the like. The
buffer gas can have a partial pressure at room temperature
from 0 kPa to 500 kPa, specifically from 30 kPa to 100 kPa,
and more specifically from 50 kPa to 60 kPa.

Alkali metal vapor cell 200, alkali metal vapor cell
apparatus 250, and processes disclosed herein have numer-
ous beneficial uses, including as an alkali metal vapor source
in atomic clocks, atomic magnetometers, gyroscopes based
on nuclear magnetic resonance, wavelength references,
quantum information storage devices, voltage references,
current references, temperature sensors and other sensors
and instruments. Advantageously, alkali metal vapor cell
200, alkali metal vapor cell apparatus 250, and processes
disclosed herein overcome technical deficiencies of conven-
tional articles or processes, e.g., alkali vapor cells made
using glass-blowing techniques, microfabricated alkali
vapor cells made using alkali precursors that remain in the
cell after bonding and activation, and the like.

Moreover, alkali metal vapor cell 200, alkali metal vapor
cell apparatus 250, and processes herein have numerous
advantageous properties. In an aspect, large numbers of
alkali vapor cells can be fabricated on a single set of wafers

5 using a single process that reduces fabrication cost and

cell-to-cell variability as compared with conventional pro-
cesses. Very small alkali vapor cells can be fabricated by
adjusting the dimensions of the lithographic process used to
fabricate transmission apertures 224, cell cavities 216, and
deposition cavities 238. Vapor cells with very few impurities
can be fabricated since alkali metal precursors remain in a
physical location separate from the alkali vapor cell after
reaction. Moreover, vapor cells with many buffer gas com-
positions over a large range of pressures, or no buffer gas at

5 all, can be fabricated.

Alkali metal vapor cell 200, alkali metal vapor cell
apparatus 250, and processed herein unexpectedly uses


US 11,180,844 B2

1
PROCESS FOR MAKING ALKALI METAL
VAPOR CELLS.

CROSS REFERENCE TO RELATED
APPLICATIONS

The application claims priority to U.S. Provisional Patent
Application Ser. No. 62/692,970 filed Jul. 2, 2018, the
disclosure of which is incorporated herein by reference in its
entirety.

STATEMENT REGARDING FEDERALLY
SPONSORED RESEARCH

This invention was made with United States Government
support from the National Institute of Standards and Tech-
nology (NIST), an agency of the United States Department
of Commerce. The Government has certain rights in the
invention. Licensing inquiries may be directed to the Tech-
nology Partnerships Office, NIST, Gaithersburg, Md.,
20899; voice (301) 301-975-2573; email tpo@nist.gov.

BRIEF DESCRIPTION

Disclosed is a process for making a plurality of alkali
metal vapor cells in an alkali metal vapor cell fabrication
apparatus, the process comprising: providing a preform
wafer comprising: a basal member; a cavity layer disposed
on the basal member; and a plurality of cell cavities disposed

in the cavity layer and bounded by a cavity wall of the cavity 3

layer; providing a sealing wafer comprising: a cover layer;
and a plurality of transmission apertures disposed in the
cover layer and bounded by a cover wall of the cover layer;
disposing a deposition assembly on the sealing wafer in fluid

communication with the transmission apertures of the seal- 3

ing wafer such that the sealing wafer is interposed between
the preform wafer and the deposition assembly; disposing an
alkali metal precursor in the deposition assembly; disposing
the sealing wafer on the preform water; aligning the trans-
mission apertures of the sealing wafer with the cell cavities
such that the transmission apertures of the sealing wafer are
in fluid communication with the cell cavities of the preform
wafer; subjecting the alkali metal precursor in the deposition
assembly to a reaction stimulus; producing, from the alkali
metal precursor, alkali metal vapor in the deposition assem-
bly in response to subjecting the alkali metal precursor to the
reaction stimulus; communicating, simultaneously in paral-
lel, the alkali metal vapor from the deposition assembly to
the cell cavities through the transmission apertures; receiv-
ing, in the cell cavities, the alkali metal vapor from the
transmission apertures; producing, from the alkali metal
vapor, an alkali metal condensate in the cell cavity; moving
the sealing wafer laterally with respect to the preform wafer
such that the cover layer encapsulates the alkali metal
condensate in the cell cavities; and bonding the sealing
wafer to the preform wafer to make individually sealed
alkali metal vapor cells in the preform wafer.

Disclosed is a process for making a plurality of alkali
metal vapor cells in an alkali metal vapor cell apparatus, the
process comprising: providing the preform wafer compris-
ing: a basal member; a cavity layer disposed on the basal
member; a plurality of cell cavities disposed in the cavity
layer and bounded by a cavity wall of the cavity layer; and
a plurality of deposition cavities disposed in the cavity layer
and bounded by the cavity wall of the cavity layer, such that
each deposition cavity is adjacent to a cell cavity; disposing
an alkali metal precursor in the deposition cavity of the

2

40

&

3

2

preform wafer; providing a sealing wafer comprising a cover
layer that comprises a blank; disposing the sealing wafer on
the preform wafer so that a deposition surface of the sealing
wafer faces and is in fluid communication with the deposi-
tion cavities of the preform wafer, such that the deposition
cavities are individually isolated from fluid communication
with other deposition cavities and cell cavities; subjecting
the alkali metal precursor in the deposition cavities to a
reaction stimulus; producing, from the alkali metal precur-
sor, alkali metal vapor in the deposition cavities in response
to subjecting the alkali metal precursor to the reaction
stimulus; communicating, simultaneously in parallel, the
alkali metal vapor from the deposition cavity to a deposition

s area on the deposition surface of the sealing wafer that

covers the deposition cavity, such that each deposition area
is local to an individual deposition cavity and physically
separate from other deposition cavities and cell cavities;
contacting the deposition area with the alkali metal vapor
from the deposition cavity: producing, from the alkali metal
vapor, an alkali metal condensate on the deposition area;
separating the sealing wafer from the preform wafer; mov-
ing the sealing wafer laterally with respect to the preform
wafer and aligning the deposition areas of the sealing wafer

‘5 with the cell cavities of the preform wafer such that the

individual deposition areas of the sealing wafer are in fluid
communication with an individual cell cavity of the preform
wafer in an aligned position and contacting the sealing wafer
with the preform water in the aligned position to encapsulate
the alkali metal condensate in individual deposition areas in
individual cell cavities; and bonding the sealing wafer to the
preform wafer to make individually sealed alkali metal
vapor cells in the preform wafer.

BRIEF DESCRIPTION OF THE DRAWINGS.

The following description should not be considered lim-
iting in any way. With reference to the accompanying
drawings, like elements are numbered alike

FIG. 1 shows steps in a process for making a plurality of
alkali metal vapor cells from an alkali metal vapor cell
apparatus;

FIG. 2 shows a plan view of an alkali metal vapor cell

5 apparatus in panel A and a cross-section along line A-A in

panel B;

FIG. 3 shows, in panel A, a plan view of a deposition
assembly disposed on a sealing wafer of the alkali metal
vapor cell apparatus shown in FIG. 2, a cover surface of a
sealing wafer in panel B, and in panel C a cross-section
along line A-A shown in panel B for the deposition assembly
disposed on the sealing wafer;

FIG. 4 shows (in panel A) a plan view of the deposition
assembly shown in FIG. 3, a bottom view of the deposition

5 assembly in panel B, and in panel C a cross-section along

line A-A shown in panel B for the deposition assembly;

FIG. 5 shows (in panel A) a plan view of a sealing wafer
shown in FIG. 2 and, in panel B, a cross-section along line
A-A shown in panel B;

FIG. 6 shows, in panel A, a plan view of a preform wafer
that includes cavity layer 214 on a basal member for the
alkali metal vapor cell apparatus shown in FIG. 2 and, in
panel B, a cross-section along line A-A shown in panel A;

FIG. 7 shows, in panel A, a plan view of a basal member

5 of the preform wafer for the alkali metal vapor cell apparatus

shown in FIG. 2 and, in panel B, a cross-section along line
A-A shown in panel A;


U.S. Patent Nov. 23, 2021 Sheet 8 of 15 US 11,180,844 B2

(A) 214 216

(B)

214

Figure 8


U.S. Patent Nov. 23, 2021 Sheet 5 of 15 US 11,180,844 B2

(A) 220 224

(B) 220
224 224
224
222 222 222
226) 226
222 Seo a
( il 256 226 226 256

Figure 5


US 11,180,844 B2

Sheet 4 of 15

Nov. 23, 2021

U.S. Patent

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Figure 4


US 11,180,844 B2

3

FIG. 8 shows, in panel A, a plan view of a cavity layer of
the preform wafer for the alkali metal vapor cell apparatus
shown in FIG. 2 and, in panel B, a cross-section along line
A-A shown in panel A;

FIG. 9 shows steps in a process for making a plurality of 5
alkali metal vapor cells from an alkali metal vapor cell
apparatus;

FIG. 10 shows a plan view of an alkali metal vapor cell
apparatus in panel A and a cross-section along line A-A in
panel B; 1

FIG. 11 shows, in panel A, a plan view of a cavity layer
having deposition cavities and cell cavities disposed on a
basal member for the alkali metal vapor cell apparatus
shown in FIG. 10, and panel C shows a cross-section along
line A-A shown in panel A; 1

FIG. 12 shows, in panel A, a plan view of the cavity layer
shown in FIG. 11, and panel B shows a cross-section along
line A-A shown in panel A;

FIG. 13 shows, in panel A, a perspective view of a sealing
wafer shown in FIG. 10 prior to forming an alkali metal
condensate thereon; panel B shows a plan view of the
sealing wafer, and panel C shows a cross-section along line
A-A shown in panel B;

FIG. 14 shows, in panel A, a plan view of the sealing
wafer shown in FIG. 13 after forming an alkali metal 2:
condensate thereon, and panel B shows a cross-section along
line A-A shown in panel A; and

FIG. 15 shows a plurality of alkali metal vapor cells in an
alkali metal vapor cell apparatus.

S

DETAILED DESCRIPTION

A detailed description of one or more embodiments is
presented herein by way of exemplification and not limita-
tion. 3

It has been discovered that a process for making alkali
metal vapor cells provides millimeter-scale and sub-milli-
meter alkali metal vapor cells in large numbers at a wafer
scale that include a vapor of alkali atoms. Advantageously,
in the process, a glass lid wafer as a sealing wafer is
translated laterally with respect to a preform wafer after
depositing an alkali metal condensate 236 and before bond-
ing the sealing wafer to the preform wafer. Holes in the
sealing wafer that are connected to the deposition assembly,
ora second array of holes in the cavity layer, and filled with 45
a chemical precursor, provide a source of alkali atoms to be
integrated as part of one of the wafers and eliminates a
separate alkali atom source, which can be complicated,
contaminant-infused, or inefficient to implement. Benefi-
cially, processes for making alkali metal vapor cells herein
provide alkali metal vapor cells in large numbers in parallel
at a wafer-scale, rather than producing the alkali metal vapor
cells individually in serial. As used herein, “wafer-scale”
refers to making a plurality of alkali metal vapor cells on a
single wafer simultaneously in parallel, wherein a number of 5
individual alkali metal vapor cells produced on the single
water can be, e.g., 1000 vapor cells on a single 4-inch wafer.
Unexpectedly, the processes produce very clean alkali metal
vapor cells that can include a selected buffer gas or an
absence of buffer gas.

Conventional fabrication of cells based on deposition of
alkali metal are complicated and can involve two moving
elements so that conventional processing is difficult to
implement in conventional bonding tools that have one
moving element. Processes herein overcome this technical 6:
problem. Moreover, the alkali metal vapor cells provided
herein are sealed cells containing a vapor of alkali metal

40

g

60

4

such as potassium, rubidium or cesium and can be used in
atomic clocks, magnetometers, and other instruments. The
processes herein considerably lower fabrication costs than
conventional methods of fabricating vapor cells.

The processes herein further overcome technical prob-
lems making alkali metal vapor cells with micromachined
silicon and glass and solves the problem of how to dispose
alkali metal atoms into a cell and how to seal the cell so that
the cells contain highly pure alkali metal atoms even though
alkali metal is highly reactive and difficult to handle. Ben-
eficially, the processes herein overcome undesirable reaction
products inside the cells. These undesirable reaction prod-
ucts can cause difficulties in using the cells by, e.g., obscur-
ing optical access through cell windows or causing changes
in the cell environment after bonding. However, absence of
undesirable reaction products is achieved in the processes
herein.

In an embodiment, with reference to FIG. 1, FIG. 2, FIG.
3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, and FIG. 8, a process for
making a plurality of alkali metal vapor cells 200 in preform
wafer 210 includes: providing preform wafer 210. Preform
wafer 210 includes basal member 212; cavity layer 214
disposed on basal member 212; and a plurality of cell
cavities 216 disposed in cavity layer 214 and bounded by
cavity wall 218 of cavity layer 214. The process also
includes providing sealing wafer 220. Sealing wafer 220
includes cover layer 222 and a plurality of transmission
apertures 224 disposed in cover layer 222 and bounded by
cover wall 226 of cover layer 222. The process also includes
disposing deposition assembly 228 on sealing wafer 220 in
fluid communication with transmission apertures 224 of
sealing wafer 220 such that sealing wafer 220 is interposed
between preform wafer 210 and deposition assembly 228;
disposing alkali metal precursor 234 in deposition assembly
228; disposing sealing wafer 220 on preform wafer 210;
aligning transmission apertures 224 of sealing wafer 220
with cell cavities 216 such that transmission apertures 224 of
sealing wafer 220 are in fluid communication with cell
cavities 216 of preform wafer 210; subjecting alkali metal
precursor 234 in deposition assembly 228 to a reaction
stimulus; producing, from alkali metal precursor 234, alkali
metal vapor in deposition assembly 228 in response to
subjecting alkali metal precursor 234 to the reaction stimu-
lus; communicating, simultaneously in parallel, the alkali
metal vapor from deposition assembly 228 to cell cavities
216 through transmission apertures 224; receiving, in cell
cavities 216, alkali metal vapor from transmission apertures
224; optionally producing, from the alkali metal vapor,
alkali metal condensate 236 in cell cavity 216; moving
sealing wafer 220 laterally with respect to preform wafer
210 such that cover layer 222 encapsulates alkali metal
condensate 236 in cell cavities 216; and bonding sealing
wafer 220 to preform wafer 210 to make individually sealed
alkali metal vapor cells 200 in preform wafer 210. As used
herein, “sealed” and variants thereof (e.g., seal, sealing, and
the like) refer to a fluid impermeable barrier the prevents
communication of fluids into or out of alkali metal vapor cell
200.

After making the alkali metal vapor cells 200, the process
can include removing deposition assembly 228 from sealing
wafer 220 after producing, from alkali metal precursor 234,
alkali metal vapor in deposition assembly 228. In an
embodiment, the process further includes dicing, after bond-
ing sealing wafer 220 to preform wafer 210, preform wafer
210 to provide a select number of individually sealed alkali


U.S. Patent Nov. 23, 2021 Sheet 7 of 15 US 11,180,844 B2

(A)

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Figure 7


US 11,180,844 B2

15

6. The process of claim 1, wherein subjecting the alkali
metal precursor in the deposition assembly to the reaction
stimulus comprises irradiating the alkali metal precursor
with ultraviolet radiation.

7. The process of claim 1, wherein disposing the alkali
metal precursor in the deposition assembly comprises:

delivering a liquid composition comprising the alkali

metal precursor into the deposition assembly; and
heating the liquid composition to evaporate a solvent of
the liquid composition.

8. The process of claim 1, wherein bonding the sealing
wafer to the preform wafer comprises anodic bonding,
fusion bonding, eutectic bonding, or a combination thereof;

the cavity layer comprises a silicon wafer;

the basal member and the cover layer independently

comprise glass; and

a volume of the cell cavity is from 100 um? to 10 mm*

9. The process of claim 1, wherein the alkali eta precursor
comprises an alkali halide, an azide, or a combination
thereof.

10. The process of claim 1, wherein the alkali metal
precursor and the alkali metal condensate independently
comprise potassium, rubidium, cesium, or a combination
comprising a plurality of the foregoing alkali metals.

11. A process for making a plurality of alkali metal vapor
cells, the process comprising:

providing a preform wafer comprising:

a basal member;
a cavity layer disposed on the basal member;

a plurality of cell cavities disposed in the cavity layer 3

and bounded by a cavity wall of the cavity layer; and
a plurality of deposition cavities disposed in the cavity
layer and bounded by the cavity wall of the cavity
layer, such that each deposition cavity is adjacent to
a cell cavity;
disposing an alkali metal precursor in the deposition
cavity of the preform wafer;
providing a sealing wafer comprising a cover layer that
comprises a blank:
disposing the sealing wafer on the preform wafer so that
a deposition surface of the sealing wafer faces and is in
fluid communication with the deposition cavities of the
preform wafer, such that the deposition cavities are
individually isolated from fluid communication with
other deposition cavities and cell cavities;
subjecting the alkali metal precursor in the deposition
cavities to a reaction stimulus;
producing, from the alkali metal precursor, alkali metal
vapor in the deposition cavities in response to subject-
ing the alkali metal precursor to the reaction stimulus;
communicating, simultaneously in parallel, the alkali
metal vapor from the deposition cavity to a deposition
area on the deposition surface of the sealing wafer that
covers the deposition cavity, such that each deposition
area is local to an individual deposition cavity and
physically, separate from other deposition cavities and
cell cavities;

20

2

45

16

contacting the deposition area with the alkali metal vapor
from the deposition cavity;

producing, from the alkali metal vapor, an alkali metal

condensate on the deposition area;

separating the sealing wafer from the preform wafer;

moving the sealing wafer laterally with respect to the

preform wafer and aligning the deposition areas of the
sealing wafer with the cell cavities of the preform wafer
such that the individual deposition areas of the sealing
wafer are in fluid communication with an individual
cell cavity of the preform wafer in an aligned position
and contacting the sealing wafer with the preform
wafer in the aligned position to encapsulate the alkali
metal condensate in individual deposition area in indi-
vidual cell cavities; and

bonding the sealing wafer to the preform wafer to make

individually sealed alkali metal vapor cells in the
preform wafer.

12. The process of claim 11, further comprising dicing,
after bonding the sealing wafer to the preform wafer, the
preform wafer to provide a select number of make individu-
ally, sealed alkali metal vapor cells.

13. The process of claim 11, further comprising introduc-

Sing a buffer gas in the cell cavity, the alkali metal being inert

to the buffer gas.

14. The process of claim 11, wherein subjecting the alkali
metal precursor in the deposition cavity to the reaction
stimulus comprises heating the alkali metal precursor.

15. The process of claim 11, wherein subjecting the alkali
metal precursor in the deposition cavity to the reaction
stimulus comprises irradiating the alkali metal precursor
with ultraviolet radiation.

16. The process of claim 1, wherein disposing the alkali
metal precursor in the deposition cavity comprises:

delivering a liquid composition comprising the alkali

metal precursor into the deposition cavity; and
heating the liquid composition to evaporate a solvent of
the liquid composition.

17. The process of claim 1, wherein:

bonding the sealing wafer to the preform wafer comprises

anodic bonding, fusion bonding, eutectic bonding, or a
combination thereof;

the cavity layer comprises a silicon wafer;

the basal member and the cover layer independently

comprise glass; and

a volume of the cell cavity is from 100 jum? to 10 mm?

18. The process of claim 1, wherein the alkali metal
precursor comprises an alkali halide, an azide, or a combi-
nation thereof.

19. The process of claim 1, wherein the alkali precursor
metal and the alkali metal condensate independently com-
prise potassium, rubidium, cesium, or a combination com-
prising a plurality of the foregoing alkali metals.

+ ke ee


a2 United States Patent

US011180844B2

(10) Patent No.: US 11,180,844 B2

Kitching (45) Date of Patent: Nov. 23, 2021
(54) PROCESS FOR MAKING ALKALI METAL (56) References Cited
VAPOR CELLS
U.S. PATENT DOCUMENTS
(71) Applicant: Government of the United States of ,
5,192,921 A 3/1993. Chantry et al
America, as represented by the Ses A Steen aa \
Secretary of Commerce, Gaithersburg, 5827105 (Continued) sal
MD (US)
(72) Inventor: John Kitching, Boulder, CO (US) FOREIGN PATENT DOCUMENTS
EP 1895372 A2 3/2008,
(73) Assignee: GOVERNMENT OF THE UNITED
SEP RSeM LED itn OTHER PUBLICATIONS
SECRETARY OF COMMERCE, Knappe, S. et al., “Atomic Vapor Cells for Miniature Frequency
Gaithersburg, MD (US) References”, IEEE International Frequency Control Symposium
0) Not subject to any diseleimes the ermorthig tnaPDA Exhibition Joinly with the 17th Puopean Frequency and
lotice: subject to any disclaimer, the term of this Time Forum, 2003, p. 31-32
patent is extended or adjusted under 35 (Continued)
U.S.C. 154(b) by 352 days.
Primary Examiner — Brian K Talbot
ry
(21) Appl. No.: 16/357,650 (74) Attorney, Agent, or Firm — Office of Chief Counsel
(22) Filed: Mar 19,2019 for National Institute of Standards and Technology
7 J :
(65) Prior Publication Data 6) ABSTRACT
Making alkali metal vapor cells includes: providing a pre-
US 2020/0002802 Al Jan. 2, 2020 form wafer that includes cell cavities in a cavity layer;
providing a sealing wafer having a cover layer and trans-
Related U.S. Application Data mission apertures; disposing a deposition assembly on the
(60) Provisional application No. 62/692,970, filed on Jul, Sealing wafer: disposing an alkali metal precursor in the
2, 2018. deposition assembly; disposing the sealing wafer on the
, preform water; aligning the transmission apertures with the
G1) Int Ch cell cavities; subjecting the alkali metal precursor to a
C23C 14/04 (2006.01) reaction stimulus; producing alkali metal vapor in the depo-
C23C 14728 (2006.01) sition assembly; communicating the alkali metal vapor to the
(Continued) cell cavities; receiving, in the cell cavities, the alkali metal
(2) Us.cl vapor from the transmission apertures; producing an alkali
‘8. Cl. metal condensate in the cell cavity; moving the sealing
CPC 8G Os BROAN 1/0008 wafer such that the cover layer encapsulates the alkali metal
(2013.01); B23K 1/20 (2013.01); condensate in the cell cavities; and bonding the sealing
(Continued) wafer to the preform wafer to make individually sealed
(58) Field of Classification Search alkali metal vapor cells in the preform wafer.
USPC... . 427/248.1

See application file for complete search history.

19 Claims, 15 Drawing Sheets



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US 11,180,844 B2

9

with an individual cell cavity 216 of preform wafer 210 in
an aligned position and in the aligned position encapsulate
alkali metal condensate 236 in individual deposition area
242 in individual cell cavities 216.

In the process for making alkali metal vapor cell 200,
bonding sealing wafer 220 to preform wafer 210 to make
individually sealed alkali metal vapor cells 200 in preform
wafer 210 includes applying an elevated voltage across a
conductive wafer and a glass or ceramic wafer placed in
surface contact at elevated temperature such that impurity
ions in the wafers are transported within the wafers resulting
in electrostatic forces and an oxidation process at the inter-
face that bonds the wafers together. In an embodiment,
bonding is fusion bonding that includes placing two silicon
wafers in contact after appropriate processing of the wafer
surfaces; and heating to elevated temperature to form the
bond; subsequent annealing at elevated temperature. In an
embodiment, bonding is eutectic bonding that includes the
deposition of metallic films on the wafer surfaces; aligning
the wafers such that the surfaces are in contact and the
metallic films are in contact; and heating to elevated tem-
perature to allow the metallic films to diffuse into one
another.

The process for making alkali metal vapor cell 200 further
can include dicing, after bonding the sealing wafer 220 to
the preform wafer 210, the preform wafer 210 to provide a
select number of make individually sealed alkali metal vapor
cells 200. Dicing includes abrasive machining or laser
cutting of the parts of the wafer between the cells 200
without disrupting the seal formed by the bonding.

‘The process for making alkali metal vapor cell 200 further
can include introducing a buffer gas in the cell cavity 216,
wherein the alkali metal is inert to the buffer gas. Introduc-
ing the buffer gas can include filling bonding chamber

interior 298 of bonding chamber 299 in which preform 3

wafer 210 and sealing wafer 220 are enclosed with such gas
after the alkali metal precursors are reacted, but before
bonding; and bonding in a presence of the buffer gas to seal
the buffer gas inside cell cavity 216.

In the process for making alkali metal vapor cell 200,
subjecting alkali metal precursor 234 in deposition cavity
238 to a reaction stimulus includes heating the alkali metal
precursors to elevated temperature after insertion of sealing
wafer 220 and preform wafer 210 into the bonding chamber
299 and evacuation of this chamber of gas. In an embodi-
ment, subjecting alkali metal precursor 234 in deposition
assembly 228 to the reaction stimulus includes heating alkali
metal precursor 234. Here, electrically resistive heaters in
the platen of the bonding machine that hold the deposition
assembly 228 are activated by passing an electrical current
through them, which results in them getting hot and the heat
being transferred to the deposition assembly 228 and alkali
metal precursor 234. In an embodiment, subjecting alkali
metal precursor 234 in deposition assembly 228 to the
reaction stimulus includes irradiating alkali metal precursor
234 with ultraviolet radiation. Here, ultraviolet light is
generated from a discharge lamp or laser and is transmitted
into the deposition assembly 228 through windows on
deposition assembly 228 transparent to ultraviolet light.

In the process for making alkali metal vapor cell 200,
disposing alkali metal precursor 234 in deposition cavity
238 includes delivering a liquid composition that includes
alkali metal precursor 234 into deposition cavity 238; and
heating the liquid to evaporate a solvent of the composition.
Heating can occur by placing the preform wafer 210 on a
heated surface. Optionally, the liquid composition can be
dried by exposing the liquid composition to a dry gaseous

S

40

&

10

environment. Upon drying the liquid composition, alkali
metal precursor 234 is stable at room temperature.

In the process for making alkali metal vapor cell 200,
bonding sealing wafer 220 to preform wafer 210 to make
individually sealed alkali metal vapor cells 200 in preform
wafer 210 can include anodic bonding, thermocompression
bonding, fusion bonding, eutectic bonding, or a combination
thereof. In an embodiment, bonding is anodic bonding that
includes the application of elevated voltage across a con-
ductive wafer and a glass or ceramic wafer placed in surface
contact at elevated temperature such that impurity ions in the
wafers are transported within the wafers resulting in elec-
trostatic forces that pull the wafers together and an oxidation
process at the interface that bonds the wafers together. In an

5 embodiment, bonding is fusion bonding that includes plac-

ing two silicon wafers in contact after appropriate process-
ing of the wafer surfaces; heating to elevated temperature to
form the bond; subsequent annealing at elevated tempera-
ture. In an embodiment, bonding is eutectic bonding that
includes the deposition of metallic films on the wafer
surfaces; the alignment of the wafers such that the surfaces
are in contact and the metallic films are in contact; the
heating to elevated temperature to allow the metallic films to
diffuse into one another. As a result of bonding sealing wafer

Ss 220 to preform wafer 210, alkali metal vapor cells 200 are

individually sealed.

With reference to, FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 6,
FIG. 7, and FIG. 8, alkali metal vapor cell apparatus 250
includes preform wafer 210 having cavity layer 214 dis-
posed on basal member 212; sealing wafer 220 disposed on
cavity layer 214; and deposition assembly 228 disposed on
sealing wafer 220. Deposition assembly 228 includes exte-
rior surface 254, interior 230 bounded by chamber wall 232,
and precursor surface 252 on which alkali metal precursor
234 can be disposed. Deposition assembly 228 can be made
of various materials including glass, silicon, metals and the
like, including combinations of the aforementioned materi-
als. A cross-sectional shape of deposition assembly 228 can
be, e.g... round, square, rectangular and the like. A volume of
the deposition assembly 228 can be from 0.001 cm? to 100
cm’, specifically from 1 cm? to 5 cm*. Moreover, deposition
assembly 228 can be opaque or partially transparent to light
at a variety of wavelengths to allow the direction of light into
the interior 230 if needed for alkali precursor activation.

Chamber wall 232 of deposition assembly 228 can extend
from precursor surface 252 and terminate in contact with
sealing wafer 220. Sealing wafer 220 can include cover layer
220 in which is disposed transmission apertures 224
bounded by cover wall 226; and cover surface 256 that
contacts cavity layer 214 of preform wafer 210. Sealing
wafer 220 can include a polished or unpolished wafer of in
which transmission apertures 224 are fabricated to direct the
alkali vapor into specific spatial regions defined by trans-
mission apertures 224 while preventing the alkali vapor

5 from being directed into surfaces under cover surface 256

such as the top surface of cavity layer 214 and can be glass,
silicon, or metal. Moreover, sealing wafer 220 can be round,
square, or some other shape with lateral dimensions between
1 cm and 50 cm and thickness between 0.01 mm and 1 em.
In an embodiment, preform wafer 210 includes a round
polished borosilicate or aluminosilicate glass wafer suitable
for anodic bonding to silicon, and can be, e.g., 100 mm in
diameter and 1 mm thick.

Prior to forming alkali metal condensate 236 from alkali

5 metal precursor 234, cavity layer 214 contacts sealing wafer

220. Cavity layer 214 can include a material compatible with
micromachining or additive manufacturing such that cell


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US 11,180,844 B2
Page 2

(51) Int. Ch

B23K 1/20 (2006.01)
B23K 1/00 (2006.01)
€23C 14/18 (2006.01)
(52) U.S. Cl.
CPC €23C 14/18 (2013.01); C23C 14/28

(2013.01); B8IC 2203/031 (2013.01); YIOT
29/49982 (2015.01)

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BL 5/2003 Nathanson et al.
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B2 6/2006 Riley, Jr. et al.
B2 12/2011 Touchberry et al
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8,480,805 B2* 7/2013 Enzenroth

C23 14/243

118/726
8,906,470 B2* 12/2014 Overstolz GO4F 5/14
427/557
9,568,565 B2* 2/2017 Parsa GO4F 5/14
9,639,062 B2 5/2017 Dyer et al.
10,447,286 B2* 10/2019 Park HO3L 7/26
10,509,369 BI* 12/2019 Lemke HO3L 7/26
10,605,840 BI* 3/2020 Amarloo GOIN 21/0303
10,859,981 B1* 12/2020 Ramirez-Serrano .... CO3C 27/06
2005/0007118 Al* 1/2005 Kitching GO4F 5/14
324/464
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