Abstract:
A cell in one example comprises an alkali metal and a coating of parylene on an interior surface of the cell. In one implementation, the alkali metal may be an optically pumped gaseous phase of an alkali metal. The parylene coating minimizes interaction of the excited state of the alkali metal, increases lifetime of the excited state, and minimizes interaction of nuclear spin states with the cell walls.

Description:
BACKGROUND 
     This application relates generally to nuclear magnetic resonance (NMR) gyroscopes and atomic clocks, and in particular to fabrication, coating, and sealing of alkali vapor cells suitable for both applications. 
     Atomic clocks and NMR gyroscopes (gyros) utilize generally glass cells containing alkali metal, alkali metal vapor and various other gases. The alkali metal is optically pumped to an excited state. For the optimum operation of either the clock or gyro, interaction of the alkali vapor with the walls must be minimized. One way is to use a buffer gas to reduce the number of collisions with the walls. The other way is to minimize the interaction with the walls when a collision does occur. Various coatings have been utilized to minimize interaction. For atomic clocks, paraffin has been used. For NMR gyros, certain hydrides have been used. 
     Thus, a need exists for a cell having a high transmissivity at wavelengths of interest while minimizing undesired cell wall interactions. 
     SUMMARY 
     The invention in one implementation encompasses a cell. The cell comprises an alkali metal and a coating of parylene on an interior surface of the cell. 
     Another implementation of the invention encompasses a method. The method comprises the steps of forming a cell from an optically transmissive material having an opening therethrough, attaching top and bottom covers to the optically transmissive material, forming an opening in one of the top or bottom covers to provide a fill hole for the cell, depositing a coating of parylene on an interior surface of the cell, and placing an alkali metal within the cell. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which: 
         FIG. 1  is a top plan view of a cell of the prior art. 
         FIG. 2  is a side elevational view of the cell of  FIG. 1 . 
         FIG. 3  depicts a cell arrangement suitable for an atomic clock implementation. 
         FIG. 4  illustrates a cell configuration for an NMR gyro. 
         FIG. 5  is a top plan view of a cell in accordance with one embodiment of the present invention. 
         FIG. 6  is a side elevational view of the cell of  FIG. 5 . 
         FIG. 7  is a top plan view of a cell illustrating heater placement. 
         FIG. 8  is a side elevational view of a cell illustrating creation of a cold spot. 
     
    
    
     DETAILED DESCRIPTION 
     Cells made using MEMS (micro-electromechanical systems) technology have been developed for chip-scale atomic clocks.  FIGS. 1 and 2  illustrate the basic principle. The cell  100  uses a standard pyrex-silicon-pyrex wafer sandwich, in which relatively transparent pyrex wafers  101  are bonded to an open silicon structure  202  to form a cell. Generally, the wafers  101  are bonded to the silicon  202  by anodic bonding. A mirrored surface  102  may or may not be included. Naturally, the mirrored surface  102  would be included in a double-pass arrangement where the detector would be on the same side of the cell as the light source  201 .  FIG. 3  depicts a cell arrangement suitable for an atomic clock, in which the detector  301  is oppositely disposed from the light source  201 . In  FIGS. 1-3 , the light source  201  is a VCSEL (Vertical Channel Surface Emitting Laser). Much better performance can be obtained for a gyro implementation by utilizing the arrangement shown in  FIG. 4 , which exhibits light transparency (high transmissivity) in the near infrared (IR) on two orthogonal axes. 
     Both atomic clocks and NMR gyros use optically pumped alkali atoms. In a clock, the hyperfine splitting of the ground state gives the time-stable frequency required in a precise clock. In a gyro, the spin moment of the Zeeman levels are transferred via collision to the nuclei of noble gas atoms. The subsequent precession of these moments about an applied magnetic field is observed by their effect on the alkali atoms and detected as modulation of a light beam. By comparing the precession frequencies of two noble gas systems, desired rotation effects can be extracted. The basic operation of a gyro system is described in detail in U.S. Pat. No. 4,157,495, the disclosure of which is fully incorporated by reference thereto as though fully set forth herein. 
     Since both atomic clocks and NMR gyros employ alkali metals optically pumped from their ground state, a competitive advantage could be obtained by designing an instrument that contains both clock and gyro functionality. It would require adding a number of features to the clock, including the need for a transverse interrogation beam to obtain gyro precession signals, more uniform and directed magnetic field, and perhaps separate light sources for gyro signals. Virtually all small inertial navigation systems currently envisioned need both gyro and clock functions. However, although each individual instrument would be more complicated, the sum would be both smaller and less costly than having dedicated instruments for each function. 
       FIGS. 5 and 6  illustrate the design and fabrication processes and sequence for NMR gyro cells. The cells  500  are filled at the wafer level, and then sealed at the wafer level under an atmosphere of the required gases (like Xe and Kr or selected isotopes thereof). First, a cell opening  601  is cut into the pyrex wafer  603 . Top and bottom silicon covers  602  are anodically bonded to the pyrex wafer  603 , one at a time, with one of the top or bottom covers  602  having a fill hole  501  provided therein. The interior of the cell  601  is then coated with parylene. 
     Parylene is an extremely inert polymer film. It is deposited in vacuum and is completely conformal. It has a wide working temperature range from −200 to +200 deg C. The films can be optically transparent as required for either atomic clock or NMR gyro applications. It performs as a coating like paraffin, but is much more uniformly deposited and has a much larger working temperature range. The monomer deposits as a film inside the cell with only a small fill hole ( 501  in  FIG. 5 ). Cells are then sealed by melting a film of parylene over the fill hole. 
     It is anticipated that parylene will manifest spin lifetime enhancing properties similar to paraffin. It is known that an NMR gyro utilizes two kinds of spins. One is the spin of orbital electrons of the alkali metal. This state is obtained by optical pumping. Both clock and gyro use this state. This state normally has a lifetime of the order of a millisecond. Paraffin has been shown to increase this lifetime substantially, perhaps up to one second. Since parylene is also a hydrocarbon, it should be similarly effective, and certainly performs better than bare glass cell walls. 
     The other kind of spin is the nuclear spin. Xe129 and Xe131, both of which occur in natural Xenon, can be caused to have preferred spin direction by the interaction with optically pumped alkali. With large cells and certain coatings, Xenon spin can have a lifetime well over 100 seconds. While paraffin may not have been studied extensively with respect to nuclear spin lifetime, there is reason to believe that both paraffin and parylene may be effective for nuclear spins as well. While nuclear spin lifetime is of primary importance, an improvement in the electron (or atomic) spins would be beneficial in both gyro and clock applications. 
     Filling and sealing is done in a controlled atmosphere, such as in a glove box. The inside of the glove box is heated to a temperature above the melting point of the alkali metal. Rubidium, for example, has a melting temperature of about 38.5 degrees C. An aliquot of liquid metal is then pipetted into each cell via the fill hole. Of course, to ensure that the liquid metal pipettes properly, the cell, the pipette, and the source of alkali metal should all be maintained at or above the alkali metal&#39;s melting temperature. The atmosphere in the glove box contains other gaseous components as required for the operation of the gyro or clock. Then a film of parylene on a stretch frame is placed over the array of cells in the wafer and brought into contact with the cells. A circular melt zone is then created around the fill hole. This could be automated with a step and repeat table in the glove box. To create the circular melt zone, a tiny shaped, temperature-controlled “iron” could be used, or a controlled power laser, such as a CO 2  laser could be programmed to scan a circular path to melt the parylene. 
     Obviously, the sequence could be modified to dice before filling and sealing and do filling and sealing at the chip level if precautions are taken not to contaminate the cells during dicing. The parylene can be removed from the top and bottom of the wafer by plasma etch. 
     It may be worth noting that the fill hole  501  may be located other than the center of the chip and the relative height and width of the cell may be various to optimize NMR gyro performance. Further, the interior of the cell may not be square, but could be round or any other shape that would optimize NMR gyro performance. 
     Since it is necessary to control the temperature of the cell  500  very roughly at 100 degrees C., a heater ( 701 , in  FIG. 7 ) may be added to the cell  500 . This can be done by standard lithography and deposition (e-beam, sputtering) methods. In the gyro assembly, the cell will be suspended in a vacuum to minimize gyro power. By judicious placement of the heaters  701 , a gradient can be established within the cell  500  such that the areas that must be transparent are the hottest, so that in operation, excess alkali metal does not deposit in these areas. 
       FIG. 8  illustrates how a cold spot can be insured by thinning a portion of the cover  602  to decrease lateral heat flow, and placing or depositing a high emissivity coating  801  at the center of the thinned area  802 . The heater would be placed on the opposite silicon surface. Note that the sealing area and cold spot could be combined. 
     The steps or operations described herein are just exemplary. There may be many variations to these steps or operations without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted, or modified. 
     Although exemplary implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.