System for heating a vapor cell

A vapor cell includes an interrogation cell in a substrate, the interrogation cell having an entrance window and an exit window, and a first transparent thin-film heater in thermal communication with the entrance window. The transparent thin-film heater has a first layer in communication with a first pole contact at a proximal end of the heater and a layer coupler contact at a distal end, a second layer in communication with a second pole contact at the proximal end, and the second layer electrically coupled to the layer coupler contact at the distal end. An insulating layer is sandwiched between the first and second layers. The insulating layer has an opening at the distal end to admit the layer coupler contact and to insulate the remainder of the second layer from the first layer. The first and second pole contacts are available to complete an electric circuit at the proximal end, with magnetic fields for each of the first and second layers oriented in opposing directions when a current is applied through the circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electric heaters used in microsystems, systems, and more particularly to chip-scale heaters used for vapor cell interrogation systems.

2. Description of the Related Art

Advances in microelectromechanical systems (MEMS) have enabled a variety of miniaturized and chip-scale atomic devices used in, for example, gyroscopes, magnetometers and chip-scale atomic clocks (CSAC). With reduced system dimensions come many advantages, including lower operating power and reduced manufacturing cost for the finished device. Of primary importance in many of these MEMS applications, is an atomic vapor cell for use as a frequency-defining element, rather than traditional quartz-crystal resonators, for improved frequency stability.

As is typical for atomic vapor cells during their manufacture, the vapor cell is charged with a sample material that later produces an interrogation gas during heating and subsequent operation. Common sample material examples for atomic vapor cells include rubidium (Rb) and cesium (Cs). The vapor cell is permanently sealed after charging, often using anodic bonding between a silicon substrate containing an interrogation cell enclosing the sample material and a transparent window through which the gas is interrogated after heating. Heaters are typically used to maintain suitable vapor pressure of the sample material in the vapor cell and can be positioned adjacent the gas interrogation cavity of the vapor cell to heat the enclosed sample material. Because the solid form of sample materials such as rubidium and cesium tend to migrate and condense at the coldest portions of the vapor cell, window heaters may be placed directly on the entrance and/or exit windows of the vapor cell to create a suitable thermal profile for reduction of solid sample material buildup over the aperture portion of such windows. Typical window heaters may consist of wire heaters spaced adjacent the aperture portion of the windows or transparent window heaters that may or may not cover the aperture, itself.

SUMMARY OF THE INVENTION

In one embodiment, a vapor cell system is disclosed that includes an interrogation cell in a substrate, the interrogation cell having an entrance window and an exit window and a first multi-layer transparent thin-film heater in thermal communication with the entrance window. To facilitate description of the system, the transparent thin-film heater is described as having proximal and distal ends. A first layer of the heater is in communication with a first pole contact at the proximal end, and a layer coupler contact at the distal end. A second layer of the heater is in communication with a second pole contact at the proximal end, the second layer electrically coupled to the layer coupler contact at the distal end, and an insulating layer is sandwiched between the first and second layers. The insulating layer has an opening at the distal end to admit the layer coupler contact and to insulate the remainder of the second layer from the first layer. The first and second pole contacts are available to complete an electric circuit at the proximal end, with electric currents (and hence magnetic fields) for each of the first and second layers oriented in opposing directions when a current is applied through the circuit.

A heater method is also disclosed that includes driving a current through folded and directionally-opposite current paths in the transparent thin-film heater and heating an entrance window of a vapor cell with heat generated from the multi-layer thin-film heater so that the folded and opposing current paths reduce the magnetic field from what would otherwise exist in a vapor cell heater without the folded and stacked configuration of the multi-layer thin-film heater.

DETAILED DESCRIPTION OF THE INVENTION

In many vapor cell applications, such as CSAC, the device operation requires a stable magnetic field. Field perturbations caused by the time-varying currents in resistive heaters can degrade device performance. A stacked, multi-layer thin-film heater is disclosed for use in combination with a vapor cell to reduce unwanted magnetic fields associated with prior art thin-film heaters and to facilitate migration of sample material condensation away from the optical aperture. In one embodiment, the heater has a plurality of stacked thin-film layers in serial communication to wrap respective current flows during operation to reduce its external magnetic field.

In addition to the issues with thermal profiles, magnetic fields created by the heaters are another concern.

FIG. 1illustrates one embodiment of a vapor cell101that uses as its foundation a substrate102, preferably silicon crystal. An interrogation cell104having a generally circular cross section and inner wall(s)105is formed extending through opposite sides of the substrate102. The interrogation cell104is in vapor communication with a reservoir cell106, preferably through a trench108. The reservoir cell106receives a sample material to charge the vapor cell for later gas interrogation, in accordance with one embodiment described, below. The reservoir cell106also provides a place for sample material, preferably rubidium (Rb) or cesium (Cs), that is not in vapor phase to condense on the coolest part of the vapor cell, outside an optical aperture110of the interrogation cell104, and provides a place outside of the optical aperture for any non-volatile Rb oxides and hydroxides residual from cell filling. The reservoir cell106extends partially or fully into the substrate102and, although illustrated as having a generally triangular cross section, may be formed into other shapes to better accept the sample material. For example, the reservoir cell106may be formed into a rectangular or circular cross section in order to facilitate introduction of the sample material.

An exit window, preferably a transparent window112, is coupled to the substrate102on a side opposite from the reservoir cell106. The transparent window112is preferably formed from borosilicate glass, although other materials may be used to both seal the interrogation chamber104and to provide suitable transparency for later electromagnetic (EM) interrogation of the vapor cell101. If formed of borosilicate glass, such coupling is preferably accomplished by anodic bonding, with the transparent window112covering the interrogation chamber104on one side of the substrate. Other bonding techniques may be used to bond the window to the substrate102, however, such as through the use of glass frit, metal to metal thermal compression, solder or other bonding materials. A transparent entrance window116, preferably borosilicate glass, is coupled to the substrate102on a side opposite from the transparent exit window112, such as by anodic bonding, to vapor seal the reservoir cell106and interrogation cell104from the external environment.

A stacked, multi-layer thin-film heater114is in thermal communication with the transparent entrance window116at the optical aperture110of the interrogation cell114through a transparent heater substrate118. Preferably, the heater114heats the entrance window116uniformly. In an alternative embodiment, the heater114is configured to heat the optical aperture110annularly, such as if the heater was formed with annular, rather than, solid rectangular, stacked thin-film layers. Similarly, a second multi-layer, thin-film heater120is in thermal communication with the transparent exit window112at an exit optical aperture (not illustrated) of the interrogation cell114through a second transparent heater substrate122. Each of the transparent heater substrates (116,122) are preferably composed of borosilicate glass, although other suitably transparent and heat-resistant materials may be used. The thin-film heater114does not cover the reservoir cell116to facilitate migration of sample material condensation away from the optical aperture110.

In one vapor cell designed for use in a chip-scale atomic clock (CSAC) device and using a 2 mm silicon wafer thickness, the interrogation cell diameter is preferably 2 mm and the various other elements of the vapor cell have the approximate thicknesses and widths listed in Table 1.

FIGS. 2 and 3are assembled and exploded perspective views, respectively, of the vertically stacked and multi-layer thin-film heater used on the vapor cell illustrated inFIG. 1. Preferably, the heater114is formed of multiple thin-film zinc-oxide (ZnO) or Indium Tin Oxide (ITO) layers electrically coupled in serial fashion, each layer substantially separated by an insulator, on the transparent heater substrate118. More particularly, a first pole pad302is coupled to a first thin-film layer304through a first pole distribution strip306at a proximal end204of the heater114. At a distal end206of the heater114, a coupler contact308is coupled to the first thin-film layer304and extends through a slot or other opening310established in an insulating layer312disposed on the first thin-film layer304. A second layer314is seated on the insulating layer312and is electrically coupled to the coupler contact308, with the remainder of second layer314insulated from the first thin-film layer304by the insulation layer312sandwiched between them. A second pole pad316is coupled to the second layer314through a second pole distribution strip319. The first and second pole distribution strips (306,319) extend along proximal edges of their respective layers to promote more uniform current distribution, and hence temperature, through their respective thin-film layers in view of the relative location of the coupler contact (308). The pole pads (302,316), pole distribution strips (306,319) and coupler contact (308) are preferably formed of metal such as gold (Au), but may be formed with any suitable metal or other conductor. The insulator is a suitable dielectric, such as Silicon Dioxide (SiO2). In an alternative embodiment, the insulator is aluminum oxide or other suitably transparent material. Through the appropriate selection of heater first and second layer (304,314) thicknesses, widths and lengths, appropriate temperature uniformity and cell heating is provided to the entrance aperture110illustrated in FIG. A. The illustrated heater114may be utilized on either or both sides of the vapor cell101to facilitate migration of sample material condensation away from optical apertures of the vapor cell101.

FIG. 4is a cross-section view of the embodiment illustrated inFIG. 2illustrating magnetic fields generated by individual thin-film layers of the heater, that are each configured to reduce the heater's resultant external magnetic field during operation. When a current source402is connected between first and second pole pads (302,316), current (I) flows from the first pole pad302, through the first thin-film layer304and to the coupler contact308, with the first layer304generating a magnetic field B1. From the coupler contact308, the current flows through the second thin-film layer314to the second pole pad316, with the second layer314producing a magnetic field B2. Because the current I is configured to wrap in directionally-opposite directions, magnetic fields B1and B2generally oppose one another. Each positionally adjacent vertically stacked thin-film layer induces a directionally-opposite magnetic field, thereby resulting in a greatly reduced total magnetic field outside of the heater114than would otherwise exist without the wrapping configuration. In an alternative embodiment, additional wrapped current paths may be provided, with the sum of the magnetic fields preferably opposing one another to reduce the total summed magnetic field outside of the heater.

In one heater designed for operation at 1-10 V. for use with a rubidium-charged vapor cell as illustrated inFIG. 1, the dimensions and operating parameters of the multi-layer heater are as shown in Table 2.

FIGS. 5-10are top plan views of alternative embodiments of a multi-layer thin-film heater configured with adjacent vertically stacked thin-film layers to induce directionally-opposite magnetic fields in response to a current. Similar to the embodiment illustrated inFIGS. 2 and 3, first and second pole pads (500,502) are formed on a substrate504. The first pole pad500is electrically connected to a layer coupler contact506through a first thin-film layer508that either serpentines around (SeeFIGS. 5,8and10) or circumscribes (FIGS. 6,7and9) a perimeter of the heater. A second thin-film layer510is electrically coupled to the layer coupler contact506, and follows back over the path of the first layer508, with the remainder of second thin-film layer510insulated from the first thin-film layer508by an insulation layer512sandwiched between them. The second thin-film layer510is electrically connected to the second pole pad502, preferably through a hole514etched in the insulator512. The pole pads (500,502) and coupler contact (506) are preferably formed of metal such as gold (Au), but may be formed with any suitable metal or other conductor. The insulator is a suitable dielectric, such as silicon dioxide (SiO2). In an alternative embodiment, the insulator is aluminum oxide or other suitably transparent material. Through the appropriate selection of heater first and second layer (500,502) thicknesses, widths and lengths, appropriate temperature uniformity and cell heating may be provided to an entrance aperture such as those illustrated inFIGS. 1-3. For example,FIG. 5may have ITO layer thicknesses of 510 Å resulting in 3.6K ohm resistance.FIGS. 6,7,8may have thicknesses of 200 Å, 510 Å and 250 Å, respectively, resulting in 13.8K, 4.2K and 17K ohm resistance, respectively.FIGS. 9 and 10may have thicknesses of 250 Å and 200 Å, respectively resulting in 9.7K and 25K ohm resistance, respectively.

The vapor cell illustrated inFIG. 1may be formed and assembled in a variety of different processing steps.FIG. 11illustrates one embodiment of multiple vapor cells with associated heaters assembled on a single wafer1102prior to dicing into individual vapor cells. An array1104of vapor cells are formed in the wafer1102, preferably on an exit window1106, and an entrance window1108is bonded to the wafer after the vapor cells are charged with a sample material (not shown). Each vapor cell1110in the array of vapor cells1104preferably has an interrogation cell-reservoir cell pair1112in vapor communication with each other through a trench1114or other pathway. In an alternative embodiment, the vapor cell does not have a reservoir cell, but rather the interrogation cell itself is charged with a sample material. Preferably, heaters1116are formed separately from the vapor cells1110on a heater substrate1118. If heaters are provided on the exit window1106, a separate heater substrate1120would be provided. After the vapor cells are charged and sealed with their respective transparent entrance and exit windows (1108,1106), the heater substrate1118having the heaters1116is aligned with the array of vapor cells1104and bonded over the vapor cell assembly, such as by anodic bonding or adhesive bonding, to complete assembly of the vapor cells prior to dicing along dicing lines1120. Alternatively, the heaters may be diced and be individually assembled onto the vapor cells.

While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of this invention.