Deposition of alkali metal alloys in atomic vapor cells

Abstract

A method for integrating an alkali metal into a vapor cell may include forming an aperture in each substrate of a semiconductor wafer, the wafer comprising a plurality of substrates, each substrate having a first surface and a second surface opposite the first surface and an aperture through the substrate, depositing, within each aperture, a metal including an alkali metal alloy including an alkali metal and one of Gold (Au), Silver (Ag), Copper (Cu), Palladium (Pd), Platinum (Pt) or Aluminum (Al) as an alloying metal, sealing the aperture of each substrate to form a sealed chamber containing the alkali metal alloy, singulating the semiconductor wafer to separate the substrates as a plurality of individual vapor cell devices, and heating the alkali metal alloy to release the alkali metal within the sealed chamber.

Description

[0001] 127242-10502

[0002] 10 / 23 / 2025

[0003] 1

[0004] DEPOSITION OF ALKALI METAL ALLOYS IN ATOMIC VAPOR CELLS

[0005] PRIORITY

[0006] This patent application claims priority from Provisional United States Patent Application number 63 / 711,720, filed October 25, 2024, entitled METHOD FOR DEPOSITING AND INTEGRATING ALKALI METAL ALLOYS IN MEMS VAPOR CELLS and naming Douglas Sparks as the inventor, the disclosure of which are incorporated herein in their entirety, by reference.

[0007] FIELD

[0008] Illustrative embodiments of the invention generally relate to vapor cell deposition and, more particularly, various embodiments of the invention relate to deposition methods for alkali metal alloys in atomic vapor cells.

[0009] BACKGROUND

[0010] Vapor cells are known in the art and have been used for various applications including atomic clocks, quantum sensors, and magnetometers. These devices typically consist of a chamber with transparent windows and an alkali metal vapor. Vapor cells can be fabricated using different approaches, including MEMS (Micro-Electro-Mechanical Systems) techniques for semiconductor-based cells and glass fabrication methods for glass-type cells.

[0011] Current methods for introducing alkali metals like Rubidium (Rb), Cesium (Cs), Sodium (Na), Francium (Fr), Lithium (Li), and Potassium (K) into vapor cells involve manual techniques such as syringe injection of liquid alkali metals, use of metal azides, or alkali-doped Non- Evaporable Getter (NEG) materials. In the NEG approach, materials like Aluminum (Al) and Zirconium (Zr) alloys serve primarily as carriers for the alkali metal, typically in the form of a metal molybdate compound. When heated, the NEG material sorbs oxygen released during the alkali metal activation process, creating a favorable environment for the alkali metal vapor.

[0012] However, these methods are not compatible with high- volume Micro-Electro-Mechanical Systems (MEMS) wafer manufacturing and can introduce unwanted byproducts or particles that degrade cell performance and reliability. The low melting point of Rb (40°C) has prevented its integration into traditional MEMS fabrication processes. Pure alkali metals are also highly reactive in air, which introduces safety considerations when handling or processing pure alkali metals.

[0013] SUMMARY OF VARIOUS EMBODIMENTS

[0014] Illustrative embodiments enable the use of standard metal deposition and patterning techniques known in the field of MEMS wafer fabrication, as well as a safer method of processing the metals. These deposition methods may include sputtering with an alloy target, co-sputtering and co-evaporation. Such methods allow for the integration of Rubidium sources directly into the 127242-10502

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[0016] 2 wafer manufacturing process, dramatically lowering production costs and improving manufacturability of warm vapor quantum sensors.

[0017] In accordance with one embodiment of the invention, a method for integrating an alkali metal into a vapor cell may include forming an aperture in each substrate of a semiconductor wafer, the wafer including a plurality of substrates, each substrate having a first surface and a second surface opposite the first surface and an aperture through the substrate, depositing, within each aperture, a metal including an alkali metal alloy including an alkali metal and one of Gold (Au), Silver (Ag), Copper (Cu), Palladium (Pd), Platinum (Pt) or Aluminum (Al) as an alloying metal, sealing the aperture of each substrate to form a sealed chamber containing the alkali metal alloy, singulating the semiconductor wafer to separate the substrates as a plurality of individual vapor cell devices, and heating the alkali metal alloy to release the alkali metal within the sealed chamber.

[0018] In accordance with other embodiments, the method includes heating the alkali metal alloy with a heating element integrated into the substrates to heat and release the alkali metal to fill the chamber in gaseous form.

[0019] In accordance with other embodiments, the method includes heating the alkali metal alloy with a laser from a location external to the chamber to heat and release the alkali metal to fill the chamber in gaseous form.

[0020] In accordance with other embodiments, depositing the metal includes sputtering the alkali metal alloy to the plurality of substrates.

[0021] In accordance with other embodiments, the method includes forming a cavity in each substrate, the cavity being in fluid communication with the aperture, where depositing the metal includes depositing the metal in the cavity.

[0022] In accordance with other embodiments, depositing the metal includes depositing the metal-by one of evaporation, co-evaporation, sputtering, or co-sputtering.

[0023] In accordance with other embodiments, method for integrating an alkali metal into a vapor cell may include forming a chamber in each substrate of a semiconductor wafer including a plurality of substrates, each substrate having a first surface and a second surface opposite the first surface and an aperture through the substrate, the chamber including the aperture, placing, within the chamber, a metal including an alkali metal alloy-based pellet, the pellet including an alloy of an alkali metal and one of Gold (Au), Silver (Ag), Copper (Cu), Palladium (Pd), Platinum (Pt) or Aluminum (Al) as an alloying metal, sealing the aperture of each substrate to form a sealed chamber containing the pellet, singulating the semiconductor wafer to separate the substrates as a plurality of individual vapor cell devices, and heating the alkali metal alloy to release the alkali metal within the sealed chamber.

[0024] In accordance with other embodiments, the method may include heating the pellet with a laser from a location external to the chamber to heat and release the alkali metal to fill the chamber in gaseous form. 127242-10502

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[0026] 3

[0027] In accordance with other embodiments, a method for integrating an alkali metal into a vapor cell may include forming a chamber in a glass structure, placing, within the chamber, a metal including an alkali metal alloy-based pellet, the pellet including an alloy of an alkali metal and one of Gold (Au), Silver (Ag), Copper (Cu), Palladium (Pd), Platinum (Pt) or Aluminum (Al) as an alloying metal, sealing the chamber of the glass structure to form a sealed chamber containing the pellet, and heating the pellet.with a laser from a location external to the chamber to heat and release the alkali metal to fill the chamber in gaseous form.

[0028] In accordance with other embodiments, a method of fabricating a vapor cell may include bonding a first transparent plate to a first surface of a semiconductor wafer, the wafer having a plurality of substrates having the first surface and a second surface opposite the first surface, etching the wafer to form an aperture through each substrate, each aperture forming a first opening in the first surface and extending through the wafer and forming a second opening in the second surface, depositing, on the wafer, a layered stack of an alkali metal and another metal at each aperture location, bonding a second transparent plate to the second surface and covering the apertures, each aperture, the first transparent plate, and the second transparent plate forming a sealed chamber in each substrate, such that the layered stack is sealed within each chamber, and singulating the semiconductor wafer to separate the substrates to form a plurality of individual vapor cell devices.

[0029] In accordance with other embodiments, an alkali metal source for a vapor cell may include an alloy of an alkali metal and Gold (Au), the alloy having a melting point above 100°C, the alloy configured to release the alkali metal in gaseous form upon heating, and the alloy in a form of one of a sputtered coating, a pellet, or a layered stack.

[0030] In accordance with other embodiments, the alloy may include one of AuRb, Au2Rb, or AusRb.

[0031] In accordance with other embodiments, the alkali metal may include one of Rubidium (Rb), Lithium (Li), Sodium (Na), Potassium (K), Francium (Fr), or Cesium (Cs).

[0032] In accordance with other embodiments, the alloy may have a melting point of at least 400°C.

[0033] In accordance with other embodiments, a vapor cell may include a semiconductor substrate including a first surface and a second surface opposite the first surface and an aperture through the substrate, a metal deposited within the aperture, the metal including an alkali metal alloy of an alkali metal and one of Gold (Au), Silver (Ag), Copper (Cu), Palladium (Pd), Platinum (Pt) or Aluminum (Al) as an alloying metal, and first and second transparent plates that cover each end of the aperture to form a sealed chamber containing the alkali metal alloy.

[0034] In accordance with other embodiments, the metal is in a gaseous form produced from the metal in a solid form.

[0035] In accordance with other embodiments, the metal may include an alkali metal alloy-based 127242-10502

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[0037] 4

[0038] In accordance with other embodiments, the vapor cell may include one or more heating elements disposed within the substrate, configured to heat the alkali metal alloy to heat and release the alkali metal to fill the sealed chamber in gaseous form.

[0039] In accordance with other embodiments, the alkali metal is heated to release the alkali metal to fill the chamber in gaseous form in response to a laser projecting a beam through one or more of the first or second transparent plates.

[0040] In accordance with other embodiments, the aperture is in fluid communication with a cavity in the substrate and the metal is disposed within the cavity.

[0041] In accordance with other embodiments, a vapor cell may include a sealed glass structure including a chamber and an alkali metal alloy-based pellet, disposed within the chamber. The pellet may include an alloy of an alkali metal and one of Gold (Au), Silver (Ag), Copper (Cu), Palladium (Pd), Platinum (Pt) or Aluminum (Al) as an alloying metal The pellet is configured to release the alkali metal to fill the chamber in gaseous form in response to a laser beam from a location external to the chamber heats the pellet.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Those skilled in the art should more fully appreciate advantages of various embodiments of the invention from the following “Description of Illustrative Embodiments,” discussed with reference to the drawings summarized immediately below.

[0044] Figure 1A schematically shows a side view of a MEMS vapor cell in accordance with illustrative embodiments of the invention.

[0045] Figure IB schematically shows a top view of a MEMS vapor cell in accordance with illustrative embodiments of the invention.

[0046] Figure 2 schematically shows a phase diagram for Gold-Rubidium alloys in accordance with an embodiments of the invention.

[0047] Figure 3A schematically shows a semiconductor substrate in accordance with embodiments of the invention.

[0048] Figure 3B schematically shows an anodically bonded Pyrex bottom layer in accordance with embodiments of the invention.

[0049] Figure 3C schematically shows a photolithographically etched chamber in accordance with embodiments of the invention.

[0050] Figure 3D schematically shows alkali metal and buffer gas added to the chamber in accordance with embodiments of the invention.

[0051] Figure 3E schematically shows an anodically bonded Pyrex top layer in accordance with embodiments of the invention.

[0052] Figure 4 schematically shows assembly steps for a sputtered vapor cell in accordance with a first embodiment of the invention. 127242-10502

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[0054] 5

[0055] Figure 5A schematically shows a MEMS vapor cell heated by a laser in accordance with a first embodiment of the invention.

[0056] Figure 5B schematically shows a MEMS vapor cell heated by a heater strip in accordance with a first embodiment of the invention.

[0057] Figure 6 schematically shows assembly steps for a vapor cell using an AuRb pellet in accordance with a second embodiment of the invention.

[0058] Figure 7 schematically shows a MEMS vapor cell heated by a laser in accordance with a second embodiment of the invention.

[0059] Figure 8 schematically shows a MEMS vapor cell with getter and bond pads in accordance with embodiments of the invention.

[0060] Figure 9 schematically shows a MEMS vapor cell including a sealed chamber in accordance with embodiments of the invention.

[0061] Figure 10 schematically shows a flowchart of a method for fabricating a MEMS vapor cell in accordance with embodiments of the invention.

[0062] Figure 11 schematically shows a MEMS vapor cell based on a glass structure and an alkali metal alloy pellet in accordance with embodiments of the invention.

[0063] Figure 12 schematically shows a flowchart of a method for fabricating an AuRb pellet in accordance with a second embodiment of the invention.

[0064] Figure 13 schematically shows a MEMs vapor cell for an atomic clock application in accordance with embodiments of the invention.

[0065] DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0066] In illustrative embodiments, a chip-scale MEMS vapor cell is provided. A EMs vapor cell may be fabricated using integrated circuit fabrication techniques known from the field of micro -electromechanical systems ("MEMS"). The present application utilizes Gold- Alkali metal alloys as stable carriers for alkali atoms. While the following description focuses on Gold- Rubidium (Au-Rb) alloys as an exemplary implementation, the principles and methods apply to other alkali metals including Cesium (Cs), Potassium (K), Sodium (Na), and Lithium (Li), with appropriate adjustments to the alloying ratios and activation parameters.

[0067] A significant drawback of NEG materials is their tendency to also sorb nitrogen (N2), which creates challenges when attempting to optimize buffer gas mixtures. Standard NEG compositions contain zirconium with additions of aluminum, vanadium, iron, or titanium, and these materials exhibit varying sorption rates for different gases. While they effectively sorb reactive gases like O2, H2O, CO, and CO2, they also sorb N2 at rates that can significantly alter buffer gas pressure and composition over time. This often necessitates switching to alternative buffer gases such as Neon (Ne) rather than using the preferred N2 mixtures, adding complexity and cost constraints to vapor cell design. Despite these limitations, NEG-based approaches have been used in specialized 127242-10502

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[0069] 6 atomic physics and sensor technology experiments. However, these methods suffer from significant drawbacks that prevent high- volume manufacturing, as described herein.

[0070] Current methods are incompatible with MEMS wafer fabrication. Current manual deposition methods cannot be integrated into standard semiconductor manufacturing processes.

[0071] Existing methods introduce contamination and gas composition issues. Existing methods introduce unwanted byproducts or gases that degrade cell performance and reliability. For example, Rb azide methods release nitrogen (N2) during the decomposition process, making precise control of buffer gas composition extremely challenging. This nitrogen release, coupled with the previously mentioned tendency of NEG materials to sorb N2, creates significant obstacles for maintaining consistent and optimal buffer gas mixtures. These gas composition issues directly impact device performance, reliability, and vapor cell lifetime.

[0072] Existing methods introduce safety hazards. Pure alkali metals are highly reactive and flammable in air, introducing significant safety considerations during handling and processing. Current methods that use pipetted liquid alkali metals require complex glove box setups or fully enclosed systems with controlled nitrogen (or other inert gas) environments to prevent dangerous reactions. These specialized handling requirements necessitate expensive safety equipment, dedicated facilities, and extensive operator training, which significantly increases production costs and limits manufacturing scalability.

[0073] Existing methods have temperature limitations. The extremely low melting points of pure alkali metals (e.g., 39.3°C for Rubidium, 28.5°C for Cesium) create significant manufacturing challenges for both MEMS and glass-type vapor cells. In MEMS fabrication, these low melting points are incompatible with standard semiconductor processing steps that often require temperatures well above 100°C, including photoresist baking, anodic bonding, and thin film deposition. For glass-type cells, the low melting point necessitates complex cooling systems during filling operations and restricts subsequent processing options. These temperature constraints force manufacturers to use specialized low-temperature processes with reduced yields or to adopt complex multi-step approaches where the alkali metal is introduced only after all high-temperature steps are completed.

[0074] Existing methods have production throughput limitations. While some progress has been made toward wafer-level processing using azide deposition methods, these approaches still face significant manufacturing challenges. The time required to deposit azide solutions, allow them to dry completely, and then move to sealing processes creates production bottlenecks and extends manufacturing cycle times. These processing delays, combined with the previously mentioned gas control issues, limit the industrial scalability of such methods. The extended process time and complexity result in higher operational costs compared to standard semiconductor manufacturing processes that could utilize more integration-friendly materials.

[0075] Figure 1A schematically shows a side view of a MEMS vapor cell 100 in accordance with illustrative embodiments of the invention. The MEMS vapor cell 100 is based on a semiconductor 127242-10502

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[0077] 7 substrate 104 that is processed and assembled to form a complete MEMS vapor cell 100. The substrate 104 includes an aperture 120 through a thickness of the substrate 104. A Gold- Rubidium (Au-Rb alloy) is deposited within a cavity formed in the substrate 104. The cavity is fluidly coupled to the aperture 120 by a micro channel 116. Top 108A and bottom 108b transparent lids are affixed to top and bottom sides of the substrate 104, respectively, and cover the aperture 120 and the cavity. This creates a sealed chamber 124. In use, the AuRb alloy 112 is heated and releases Rubidium (Rb) atoms within the sealed chamber 124.

[0078] Figure IB schematically shows a top view of a MEMS vapor cell in accordance with illustrative embodiments of the invention. The top view corresponds with the side view shown in Fig. 1A. The cavity includes the AuRb alloy 112, which is coupled to the aperture 120 by the micro channel 116. A top transparent layer 108A covers the sealed chamber 124.

[0079] Figure 2 schematically shows a phase diagram for Gold-Rubidium alloys 200 in accordance with embodiments of the invention. The phase diagram 200 illustrates variation of weight percent Rubidium 208A or atomic percent Rubidium 208B vs. temperature 204. The graph illustrates three Gold-Rubidium alloys: AuRb, Au2Rb, and Au Rb.

[0080] An alloy of an alkali metal is less susceptible to oxidation than an alkali metal alone, and so use of such an alloy is safer than uses of an alkali metal alone and also simplifies vapor cell fabrication at least in that using such an alloy does not require special handling that would be required by a pure alkali metal. Due to their high reactivity, alkali metals must be handled in an inert atmosphere to prevent unwanted reactions with oxygen or water. Recognizing the difficulty of working with Rubidium alone at room temperatures, the present application utilizes various forms of Rb alloys, including forms of Gold-Rubidium (AuRb). AuRb alloys may include AuRb, Au Rb, and Au2Rb. AuRb alloys serve as stable carriers for the Rb atom until activation is needed. Rb may be released from the alloy through controlled heating, such as with a laser or thin-film heating elements, as discussed herein. Phase diagram analysis guides synthesis of AuRb intermetallic compounds with engineered thermal properties.

[0081] Due to their high reactivity, alkali metals must be handled in an inert atmosphere to prevent unwanted reactions with oxygen or water. Rb and Gold are combined together in the desired ratio to form an alloy. Precise control of the alloy composition is crucial as the properties of the alloy can vary significantly depending on the ratio of the alkali metals, as determined from the phase diagram.

[0082] The diagram illustrates a change in alloy melting point as a function of the type of AuRb alloy. As shown in the diagram, a 50%-50% composition of Au and Rb results in a melting point of 498 °C. This is much easier to work with than pure Rb, which has a melting point of 39.3 °C (102.7 °F). Similar phase relationships exist for other Gold-Alkali metal compositions, with each exhibiting significantly higher melting points than their pure alkali metal counterparts. This higher melting point provides several critical advantages: 127242-10502

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[0084] 8

[0085] 1 . The alloy remains solid at room temperature and during standard processing steps

[0086] 2. The alloy is less reactive with oxygen and water than pure alkali metals

[0087] 3. The alloy can be handled using standard equipment without specialized inert environments

[0088] 4. The alloy can be deposited using standard metal deposition techniques

[0089] 5. The alloy does not release unwanted gases during activation, unlike azide-based methods

[0090] 6. The Gold component can serve as a condensation site for the alkali metal when not in use

[0091] The Gold-alkali metal alloys serve as stable carriers for alkali atoms until activation is needed. The alkali metal can be released from the alloy through controlled heating, such as with a laser or thin-film heating elements, as discussed herein.

[0092] Phase diagram analysis guides synthesis of Gold-Alkali metal intermetallic compounds with engineered thermal properties. Engineered AuRb alloys create thermally stable compounds that enable industrial-scale vapor cell fabrication. Precise control of the alloy composition is crucial as the properties of the alloy can vary significantly depending on the ratio of the alkali metals (determined from the phase diagram 200 or another phase diagram corresponding to different alkali metal alloy combinations).

[0093] Figure 3 A schematically shows a semiconductor substrate 104 in accordance with embodiments of the invention. The substrate 104 may be typically a silicon chip cut from a larger wafer after processing steps have been completed. In most embodiments, the chip may have a rectangular footprint.

[0094] Figure 3B schematically shows an anodically bonded Pyrex bottom layer in accordance with embodiments of the invention. In one embodiment, a thin transparent window 108B such as Pyrex is anodically bonded to the bottom surface of the semiconductor substrate 104. The transparent window 108B allows a laser beam to pass through the window and release the Rubidium atoms, as will be described later.

[0095] Figure 3C schematically shows a photolithographically etched chamber in accordance with embodiments of the invention. A photolithographic process may be used to create a void through the thickness of the semiconductor substrate 104. The dimensions and aspect ratio of the etched chamber may be circular or rectangular, depending on the target sensor requirements. In some embodiments, a 1mm by 1mm chamber is etched and is generally centered on the semiconductor substrate 104.

[0096] Figure 3D schematically shows alkali metal and buffer gas added to the chamber in accordance with embodiments of the invention. Once the bottom transparent layer 108B is in place, the void or chamber is filled with an alkali metal alloy such as AuRb, Au2Rb, or AusRb, to name but a few examples. In some embodiment, after depositing or applying the alkali metal to the chamber, the chamber may be filled with a buffer gas, in some cases a mixture of gases such as Argon (Ar) and Nitrogen (N2) or Argon (Ar) and Neon (Ne). 127242-10502

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[0098] 9

[0099] Figure 3E schematically shows an anodically bonded Pyrex top layer 108 A in accordance with embodiments of the invention. Other silicon to glass bonding methods may be used, such as glass frit or thermocompressive bonding. In one embodiment, a thin transparent window 108A such as anodically bonded Pyrex is bonded to the top surface of the semiconductor substrate 104. The transparent window 108 (108 A or 108B) allows a laser beam to pass through the window and release Rubidium atoms through the heating the action of the alloy, as will be described later. Once this layer 108 A is in place, a MEMS vapor cell including the semiconductor substrate 104, top 108 A and bottom 108B transparent layers and a sealed chamber 124 is ready for activation. In other embodiments, the transparent windows 108A, 108B may be silicon carbide, sapphire, or other material transparent to the target wavelength light source used in the application.

[0100] Figure 4 schematically shows assembly steps for a sputtered vapor cell 400 in accordance with a first embodiment of the invention. The first embodiment uses a commercial sputtering process to deposit an alkali metal within the MEMS vapor cell. The sputtering process for MEMS vapor cells 100 has many advantages over the conventional MEMS vapor cell assembly processes. It enables wafer-level parallel processing, allows more automation with associated production cost savings and higher throughput per time period, has lower per-unit costs, and a lower risk of vapor cell contamination. The Gold (Au) surface also acts as a condensation site for Rb, which assists in keeping Rb from condensing on the transparent glass, which might occlude the optical pathway. No Nitrogen (N2) is released, which might otherwise affect inert buffer gas concentrations, if present.

[0101] At step A, a silicon semiconductor wafer is obtained that is typically used in a standard MEMS fabrication process. Other base wafers may be used depending on the final application requirements. The wafer is divided using a masking process into a number of equal-sized chips or substrates at step B that become the silicon substrate 104.

[0102] At step C, a bottom transparent layer (e.g. borosilicate glass) 108B is bonded to a bottom surface of the semiconductor substrate 104. The bottom glass 108B may serve as an etch stop in subsequent etching steps.

[0103] At step D a cavity is etched in the semiconductor substrate 104. Photolithography and etching processes are used to create cavities and apertures through the semiconductor wafer at each device location. The etching may include formation of vapor channels connecting various chambers within each device. At step E, an alkali metal coating 112 (AuRb shown) is sputtered into one or more surfaces of the cavity.

[0104] At step F, etching (for example Deep Reactive Ion Etching - DRIE) is used to form an aperture 120 through a thickness of the semiconductor substrate 104. The aperture 120 may be any shape, although a generally round aperture 120 is illustrated herein. Finally, at step G a top transparent layer (e.g. borosilicate glass) 108 A is bonded to a top surface of the semiconductor substrate 104 while a buffer gas mixture 404 is within the sealed chamber 124. Buffer gas 404 is introduced in a controlled environment (such as within a specialized bonder). The bonding may 127242-10502

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[0106] 10 be accomplished through anodic bonding, thermocompression bonding, or glass frit sealing depending on temperature constraints and hermeticity requirements. The buffer gas 404 may include an inert gas such as Argon (Ar) mixed with Nitrogen (N2) or Neon (Ne) depending on the final application requirements. Following step G, the completed wafer assembly is then singulated (diced) to separate the individual vapor cell devices.

[0107] Once the MEMs vapor cell is fully fabricated, a laser beam projected through the vapor cell inner surface will impinge on the AuRb alloy target and release Rb atoms within the sealed chamber 124. The sputtering process for MEMS vapor cells has many advantages over conventional MEMS vapor cell assembly processes, including it enables wafer-level parallel processing, allows more automation with associated production cost savings, provides higher throughput per time period, results in lower per-unit costs, reduces the risk of vapor cell contamination, the Gold surface acts as a condensation site for Rb, and no nitrogen gas (N2) is released, which avoids disrupting carefully controlled buffer gas concentrations.

[0108] Figure 5A schematically shows a MEMS vapor cell heated by a laser 500 in accordance with a first embodiment of the invention. In the first embodiment, the alkali metal is sputtered onto the cavity of the semiconductor substrate 104. A laser (e.g., a Neodymium-doped Yttrium Aluminum-Gamet (YAG) laser 504 provides laser activation of Rb atoms into the vapor cell 508. This provides a one-time release of Rubidium within the sealed chamber 124. The YAG laser 504 rasters across the AuRb alloy to heat and release the Rubidium atoms. This method provides a one-time release of Rubidium within the sealed chamber.

[0109] Figure 5B schematically shows a MEMS vapor cell heated by a heater strip in accordance with a first embodiment of the invention. Fig 5B provides a top view of a MEMS vapor cell. The semiconductor substrate 104 includes a heater strip 512 surrounding the AuRb alloy 112. The heater strip 512 uses resistive heating to heat the AuRb alloy and release Rb in gaseous form. The semiconductor substrate 104 may include bond pads 812, as shown in Fig. 8, to apply a voltage to the heater strip 512 that heats the AuRb alloy 112 to release the Rb atoms into the vapor cell 516. This resistive heater strip 512 may be used to heat the AuRb alloy to release additional Rb and control Rb vapor density within the sealed chamber 124 over the vapor cell lifetime. In this embodiment, Gold may also serve as a local condensation site for Rubidium when not in use in order to keep Rb from condensing on the transparent windows, which may block the optical pathway and impact the functionality of the vapor cell.

[0110] Figure 6 schematically shows assembly steps for a vapor cell using an AuRb pellet 600 in accordance with a second embodiment of the invention. The second embodiment forms the alkali metal into a “pellet” form 612 and deposits the “pellet” 612 within the MEMS vapor cell 600. At step A, a silicon substrate wafer compatible with MEMS fabrication methods is obtained. Other base wafers may be used depending on the final application requirements. The wafer is divided using a masking process into a number of chips at step B that become the silicon substrate 104. At step C, a bottom transparent layer (e.g. borosilicate glass) 108B is bonded to a bottom surface of 127242-10502

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[0112] 11 the semiconductor substrate 104. At step D a pair of holes are etched in the semiconductor substrate 104 using deep reactive ion etching (DRIE) or an equivalent method. A bridge between the holes is at a reduced thickness to allow fluid communication between the holes after sealing.

[0113] At step E the AuRb pellet 612 and optional buffer gas 404 is placed in the chamber. The pellet 612 is formed as shown and the pellet formation process is described in Fig. 11. Finally, at step F a top transparent layer (e.g. borosilicate glass) 108A is bonded to a top surface of the semiconductor substrate 104 while a buffer gas mixture 404 is within the sealed chamber 124. In some embodiments (depending on application and purpose) the sealed chamber 124 is partially to completely evacuated or pressurized.

[0114] Figure 7 schematically shows a MEMS vapor cell heated by a laser 700 in accordance with a second embodiment of the invention. A YAG laser 504 releases Rb atoms through heating the AuRb alloy pellet 612 into the vapor cell 508. The pellet-based alkali metal alloy approach is particularly advantageous for manufacturers transitioning from traditional glass vapor cell fabrication to higher-volume production methods, as it provides a reliable alkali metal source that doesn't require specialized handling equipment or introduce unwanted gases.

[0115] Figure 8 schematically shows a MEMS vapor cell with getter and bond pads 800 in accordance with embodiments of the invention. The illustrated embodiment utilizes a two-cavity embodiment having getter 808 interspersed with AuRb 112 and a pair of bond pads 812. The getter 808 may be a thin film material, such as Titanium (Ti), Zirconium (Zr), Cobalt (Co), Iron (Fe), Nickel (Ni), Magnesium (Mg), and alloys of those elements, to name but a few examples. The getter may be required in some applications to maintain a target vacuum or assist in maintaining a precise buffer gas mixture and pressure in the sealed vapor cell.

[0116] A cavity 816 includes an AuRb 112 deposit next to a getter 808. In the illustrated embodiment, the top transparent layer 108 A may be attached to a top surface of the semiconductor substrate 104 by a glass frit seal 804. In addition to a glass frit seal 804, other bonding methods may be used such as anodic or thermal compression.

[0117] A power source may be connected to the bond pads 812 to resistively heat a heater strip 512 and release Rb into the vapor cell 820. In one embodiment, a laser may heat the Rb and activate the getter 808 to capture most of trapped reactive gases. Then, the bond pads 812 and heater strip 512 may be used to periodically release more Rb and reactivate the getter 824 sorption properties.

[0118] Figure 9 schematically shows a MEMS vapor cell including a sealed chamber 900 in accordance with embodiments of the invention. The MEMS vapor cell 900 includes two stacked structures 904 and 908. Each stacked structure 904, 908 may include a heating element layer 912, a dielectric film 916, a surface layer 920 (e.g. aluminum oxide), an adhesion layer 924, and an alkali alloy layer 928.

[0119] In operation, stacked structure 904 is heated (e.g., using the heating element layer 912), and the heat causes the alkali alloy layer 928 to release an alkali metal (e.g., Rubidium, Cesium, 127242-10502

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[0121] 12 etc.) into the sealed chamber. When stacked structure 904 is not heated, the alkali alloy layer 928 may condense the alkali metal back onto or into the alkali alloy layer 928.

[0122] In illustrative embodiments, either stacked structure 904 and / or 908 may be operated as a vapor source, an alkali metal condenser, or alternately as both a vapor source and an alkali metal condenser. In operation, stacked structures 904 and 908 may be used to control the quantity of alkali metal vaper density in a sealed chamber 124.

[0123] In some embodiments, one or both of stacked structures 904 and 908 may passively operate as a condenser to capture alkali metal from the sealed chamber 124 to prevent the alkali metal from undesirably condensing on one or more transparent plates 108 A, 108B. As the vapor cell cools, the Rb (for example) tends to condense and accumulate on the coolest surface available. The objective is to get the Rb to condense on the Au / AuRb vs the transparent plates, thus preventing occlusion of the optical pathway.

[0124] The embodiment of Figure 9 may implement a continuous vapor cell alkali metal (e.g.. Rubidium) recycling in which alkali metal (e.g.. Rubidium) vapor is generated from a hot source and recondensed on a cool Gold ( Au) or a Gold alloy surface.

[0125] For example, condensation of Rubidium can occur on Gold (Au) and possibly on Platinum (Pt), Silver (Ag), Copper (Cu), Palladium (Pd) and other metal surfaces. Such a Rubidium condensation layer may be pure Rubidium coating the Gold, Platinum, Silver, Copper or other metal surface.

[0126] The adhesion layer 924 may include Titanium (Ti), Chromium (Cr), Tungsten (Yv) or Tungsten-Titanium (TiW), to name but a few examples.

[0127] The heating method will heat the thin film Rubidium alloy and a thin film getter. As the Rubidium vapor is released the alloy composition changes at the surface - the condenser metals can perform other functions in the device such as solder bond pads, wire bond pads, electrical traces, temperature sensors such as Pt Resistance Temperature Detectors (RTDs). In one embodiment, electrical traces and RTDs may be passivated by a dielectric.

[0128] Figure 10 schematically shows a flowchart of a method for fabricating a MEMS vapor cell in accordance with embodiments of the invention. Flow begins at block 1004.

[0129] At block 1004, a first transparent plate 108B is bonded to a first surface of a semiconductor substrate 104. Flow proceeds to block 1008.

[0130] At block 1008, an aperture 120 is etched through the semiconductor substrate 104. The aperture 120 is etched through a thickness of the semiconductor substrate 104 such that light energy will pass through the aperture 120 of the packaged MEMS vapor cell 100. In some embodiments, step 1008 also includes forming a cavity 816 in the substrate 104, where the cavity 816 is in fluid communication with the aperture 120. Flow proceeds to block 1012.

[0131] At block 1012, an alkali metal 112 is deposited onto the semiconductor substrate 104. In one embodiment, the alkali metal 112 is deposited using a sputtering process. In another embodiment, the alkali metal is formed into a pellet and the pellet is placed in a cavity 816 formed 127242-10502

[0132] 10 / 23 / 2025

[0133] 13 in the substrate, the cavity 816 having a fluid connection with the aperture 120. In one embodiment, the alkali metal 112 may be a Gold- Rubidium alloy. In some embodiments block 1012 may be repeated to build a layered structure having multiple layers. Flow proceeds to optional block 1016.

[0134] At optional block 1016, a getter 808 is applied to the aperture 120. The getter 808 absorbs unwanted gases and maintains a vacuum within the sealed chamber 124. Flow proceeds to optional block 1020.

[0135] At optional block 1020, the aperture 120 is filled with an inert or buffer gas 404. In one embodiment, the buffer gas 404 fill operation may be performed as part of a lid seal operation. Flow proceeds to block 1024.

[0136] At block 1024, a second transparent plate is bonded to a second surface of the semiconductor substrate 104 to form a sealed chamber 124 in the substrate 104. In some embodiments, the MEMs vapor cell 100 is partially to completely evacuated or pressurized. Flow proceeds to block 1028.

[0137] At block 1028, the alkali metal 11 is heated. Some embodiments heat the alkali metal 112 by heating the alkali metal (e.g., metal alloy) with a heating strip 512 integrated onto or into the substrate 104. Other embodiments heat the alkali metal 112 by heating the metal with a laser 504 external to the substrate 104. Other embodiments heat the alkali metal 112 via radiant heating. For example, glass vapor cells (see Fig. 11) may be healed by an external or laser or flame to release the Rb atoms. Flow ends at block 1028.

[0138] Figure 11 schematically shows a MEMS vapor cell based on a glass structure and an alkali metal alloy pellet 1100 in accordance with embodiments of the invention. The glass structure 1104 is an alternative to a silicon substrate 104.

[0139] At step A, a glass vapor cell with an open fill sprue 1108 is filled with an inert buffer gas 404. At step B, an AuRb pellet 612 is placed or dropped through the open fill sprue 1108 into the interior of the glass vapor cell 1104. At step C the fill sprue is sealed 1112 by known methods. At step D, the pellet 612 is heated to release Rb atoms 1116. The heating may be performed by a YAG laser 504 as shown or an external flame or other radiant heat source.

[0140] Figure 12 schematically shows a flowchart of a method for fabricating an AuRb pellet in accordance with a second embodiment of the invention. Flow begins at block 1204.

[0141] At block 1204, an alloy of Gold and Rubidium is created (e.g., AuRb, AU2RB, AusRb), by known processes, including layered formation using a “pellet” 612 press using high pressure or other methods. Flow proceeds to block 1208.

[0142] At block 1208, the AuRb alloy is drawn into a wire form. Diameter of the wire may depend on a sufficient vapor density for the chose vapor cell size and volume. Flow proceeds to block 1212.

[0143] At block 1212, the wire AuRb alloy is cut into a pellet form. Similar to step 1208, the length of the pellet may depend on a sufficient vapor density for the chose vapor cell size and volume. Flow ends at block 1212. 127242-10502

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[0145] 14

[0146] Figure 13 schematically shows a MEMs vapor cell for an atomic clock application 1300 in accordance with embodiments of the invention. Vapor cells may be incorporated into atomic clock systems 1300 including chip-scale atomic clocks (CSACs), micro atomic clocks, and precision timing references. Some embodiments may use coherent population trapping (CPT) or other quantum effects to create stable frequency references that are critical for telecommunications, navigation systems, and secure communications infrastructure.

[0147] A container providing a magnetic shield 1304 (mu metal, etc.) protects components within from stray magnetic fields. A laser 1320, typically a vertical cavity surface emitting laser (VCSEL), projects a linearly polarized beam through a series of optical components, then through the vapor cell 100, to a receiving photodiode 1332 within the container 1304. These optical components can include beam expansion, quarter waveplate for changing linearly polarized light to circularly polarized light, and potentially beam splitting, and beam columniation. These optical components are selected based on the final system requirements and implementation desired.

[0148] The photodiode 1332 provides a light intensity signal to a signal processor 1308. The signal processor 1308, in the case of a Coherent Population Trapping (CPT) atomic clock embodiment, processes the detected CPT resonance signal to extract timing information. It compares the detected resonance frequency with the expected atomic transition frequency of the alkali metal (Rb in this example) and generates an error signal. This error signal is used to discipline the quartz reference oscillator 1312, adjusting its frequency to maintain precise synchronization with the atomic resonance. The disciplined output from the quartz oscillator provides a highly stable clock reference signal 1340.

[0149] The frequency multiplier 1316 receives the reference signal from the oscillator 1312 and multiplies it to generate the microwave frequencies necessary for the CPT effect. This multiplied signal drives the laser 1320 (typically a VCSEL), modulating it at the precise frequency difference required to create quantum interference between atomic energy levels in the alkali vapor. This completes the feedback loop, as the laser light passes through the vapor cell, and the resulting CPT resonance is detected by the photodiode, continuing the cycle of measurement and adjustment that maintains the clock's accuracy.

[0150] For the illustrated application, the internal pressure of the sealed chamber 124 may be approximately 300 Torr and the buffer gas 404 may be a mixture of Argon (Ar) and Nitrogen (N2) or Argon (Ar) and Neon (Ne).

[0151] In other embodiments, the vapor cell 100 may be used for quantum sensing applications, such as atomic magnetometers, inertial sensors, or electric field sensors. As atomic magnetometers, vapor cells 100 may be used for precise magnetic field measurements with sensitivities approaching femtoTesla levels, enabling applications in geophysical surveys, medical diagnostics (such as magnetocardiography and magnetoencephalography), and underwater detection systems. As inertial sensors, atom interferometry-based gyroscopes and accelerometers may use these vapor cells to provide high-precision navigation capabilities without relying on 127242-10502

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[0153] 15 external references like GPS. As electric field sensors, Rydberg atom-based sensors may detect RF and microwave signals with high sensitivity and accuracy.

[0154] Various embodiments of the invention have been described in fulfillment of the various objectives of the invention. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the present invention as defined in the following claims.

Claims

20AMENDED CLAIMS received by the International Bureau on 23 March 2026 (23.03.2026)1. A method for integrating an alkali metal into a vapor cell, comprising: forming an aperture in each substrate of a semiconductor wafer, the wafer comprising a plurality of substrates, each substrate having a first surface and a second surface opposite the first surface and an aperture through the substrate; depositing, within each aperture, a metal comprising a pre-formed intermetallic alloy comprising an alkali metal and Gold (Au), the alloy comprising one of AuRb, AioRb, or AusRb, and having a melting point of at least 400° C; sealing the aperture of each substrate to form a sealed chamber containing the alkali metal alloy; singulating the semiconductor wafer to separate the substrates as a plurality of individual vapor cell devices; and heating the pre-formed intermetallic alloy, the alloy serving as a primary alkali vapor source.

2. The method of claim 1, wherein heating the alkali metal alloy comprises heating the alkali metal alloy with a heating element integrated into the substrates to heat and release the alkali metal to fill the chamber in gaseous form.

3. The method of claim 1, wherein heating the alkali metal alloy comprises heating the alkali metal alloy_with a laser from a location external to the chamber to heat and release the alkali metal to fill the chamber in gaseous form.

4. The method of claim 1, wherein depositing the metal comprises sputtering the pre-formed intermetallic alloy from an alloy target prior to sealing5. The method of claim 1, further comprising: forming a cavity in each substrate, the cavity being in fluid communication with the aperture, wherein depositing the metal comprises depositing the metal in the cavity.

6. The method of claim 1, wherein depositing the metal comprises:21 depositing the metal-by one of evaporation, co-evaporation, sputtering, co-sputtering, or piezo-based printing.

7. A method for integrating an alkali metal into a vapor cell, comprising: forming a chamber in each substrate of a semiconductor wafer comprising a plurality of substrates, each substrate having a first surface and a second surface opposite the first surface and an aperture through the substrate, the chamber comprising the aperture; placing, within the chamber, a metal comprising a pre-formed intermetallic alloybased pellet, the pellet comprising one of AuRb, AuiRb, or AusRb, and having a melting point of at least 400°; sealing the aperture of each substrate to form a sealed chamber containing the pellet; singulating the semiconductor wafer to separate the substrates as a plurality of individual vapor cell devices; and heating the alkali metal alloy to release the alkali metal within the sealed chamber.

8. The method of claim 7, wherein heating the alkali metal alloy comprises heating the pellet with a laser from a location external to the chamber to heat and release the alkali metal to fill the chamber in gaseous form.

9. The method of claim 7, wherein in response to the chamber is formed in the substrates of the semiconductor wafer, the method further comprises: forming a cavity in each substrate, the cavity being in fluid communication with the aperture of each substrate.

10. A method for integrating an alkali metal into a vapor cell, comprising: forming a chamber in a glass structure; placing, within the chamber, a metal comprising an alkali metal alloy-based pellet, the pellet comprising an alloy of an alkali metal and one of Gold (Au), Silver (Ag), Copper (Cu), Palladium (Pd), Platinum (Pt) or Aluminum (Al) as an alloying metal; sealing the chamber of the glass structure to form a sealed chamber containing the pellet; and heating the pellet with a laser or other form of radiant heating from a location external to the chamber to heat and release the alkali metal to fill the chamber in gaseous form.22I L A method of fabricating a vapor cell, comprising: bonding a first transparent plate to a first surface of a semiconductor wafer, the wafer having a plurality of substrates having the first surface and a second surface opposite the first surface; etching the wafer to form an aperture through each substrate, each aperture forming a first opening in the first surface and extending through the wafer and forming a second opening in the second surface; depositing, on the wafer, a layered stack of an alkali metal and another metal at each aperture location; bonding a second transparent plate to the second surface and covering the apertures, each aperture, the first transparent plate, and the second transparent plate forming a sealed chamber in each substrate, such that the layered stack is sealed within each chamber; and singulating the semiconductor wafer to separate the substrates to form a plurality of individual vapor cell devices.

12. An alkali metal source for a vapor cell, comprising: an alloy of an alkali metal and Gold (Au), the alloy having a melting point above 100°C, the alloy configured to release the alkali metal in gaseous form upon heating and the alloy in a form of one of a sputtered coating, a pellet, or a layered stack.

13. The alkali metal source of claim 12, wherein the alloy comprises one of AuRb, AmRb, or AusRb.

14. The alkali metal source of claim 12, wherein the alkali metal comprises one of Rubidium (Rb), Lithium (Li), Sodium (Na), Potassium (K), Francium (Fr), or Cesium (Cs).

15. The alkali metal source of claim 12, wherein the alloy has a melting point of at least 400°C.

16. A vapor cell, comprising: a semiconductor substrate comprising a first surface and a second surface opposite the first surface and an aperture through the substrate; a metal deposited within the aperture, the metal comprising a pre-formed intermetallic alloy comprising an alkali metal and Gold (Au), the alloy comprising one of AuRb,23AuiRb, or AusRb, and having a melting point of at least 400° C, wherein the alloy serves as a primary alkali vapor source; and first and second transparent plates that cover each end of the aperture to form a sealed chamber containing the alkali metal alloy.

17. The vapor cell of claim 16, wherein the metal is in a gaseous form produced from the metal in a solid form.

18. The vapor cell of claim 16, wherein the metal comprises an alkali metal alloy-based pellet.

19. The vapor cell of claim 16, further comprising: one or more heating elements disposed within the substrate, configured to heat the alkali metal alloy to heat and release the alkali metal to fill the sealed chamber in gaseous form.

20. The vapor cell of claim 16, wherein the alkali metal is heated to release the alkali metal to fill the chamber in gaseous form in response to a laser projecting a beam through one or more of the first or second transparent plates.

21. The vapor cell of claim 16, wherein the aperture is in fluid communication with a cavity in the substrate and the metal is disposed within the cavity.

22. A vapor cell, comprising: a sealed glass structure comprising a chamber; and a pre-formed intermetallic alloy pellet comprising an alkali metal and Gold (Au), the alloy comprising one of AuRb, AuiRb, or AusRb, and having a melting point of at least 400° C, the pellet configured tojelease the alkali metal to fill the chamber in gaseous form in response to a laser beam or other form of radiant heating from a location external to the chamber heats the pellet.[0001]24[0002]Statement under Article 19(1)[0003]Applicant: Mesa Quantum Systems, Inc.[0004]Int'l Application No: PCT / US2025 / 052188[0005]Int'l Filing Date: 23 October 2025[0006]Title of Invention: DEPOSITION OF ALKALI METAL ALLOYS IN ATOMIC VAPOR CELLS[0007]Atty Docket: 127242-10502[0008]Date Transmitted: March 23, 2026[0009]Original claims 1, 4, 7, 16, and 22 are amended to recite the pre-formed nature of the intermetallic alloy that is deposited into the aperture.[0010]Respectfully submitted,[0011]DATE: March 23, 2026 / Thomas J. Lavan / [0012]Thomas J. Lavan[0013]Agent for Applicant[0014]USPTO Registration No. 58,487[0015]Nutter McClennen & Fish LLP 155 Seaport Boulevard Boston, Massachusetts 02210 Tel: (617) 439-2230 Fax: (617) 310-9230

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