Abstract:
An electromechanical device having a size no larger than about 10 microns utilizing a working fluid in the high pressure liquid or supercritical fluid state. A process of preparing the electromechanical device involves the introduction of the liquid or supercritical fluid therein which permits the retention of the working fluid in the liquid or supercritical state after introduction.

Description:
BACKGROUND OF THE DISCLOSURE  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is directed to an electromechanical (MEMS) device which utilizes a working fluid and a process of making the same. More specifically, the present invention is directed to a MEMS device which utilizes a working fluid having a size no greater than about 10 microns wherein the working fluid is a high pressure liquid or a supercritical fluid and a process of making a MEMS device which involves introducing a high pressure liquid or a supercritical fluid therein.  
           [0003]    2. Background of the Prior Art  
           [0004]    The development of MEMS devices has significantly advanced in recent years. This development corresponds to the extensive growth in the use of integrated circuits involving semiconductor devices. Although the development of MEMS devices has rapidly developed in recent years, advances in MEMS devices requiring the utilization of a working fluid has been slower. This is because of problems associated with the inability of the working fluid to traverse through openings provided in the MEMS device.  
           [0005]    The aforementioned problems have become more pronounced with the development of MEMS of ever decreasing size. Obviously, as newly developed integrated circuits become smaller and smaller, MEMS devices, employed in applications involving integrated circuits, have been required to correspondingly decrease in size. Although the development of MEMS devices of smaller and smaller size has continued apace, as evidenced by such developments as those embodied in U.S. Pat. Nos. 6,164,933 and 6,227,809, which describe micropumps, and U.S. Pat. No. 5,323,999, directed to a micro-sized gas valve, a major deterrent to this development is the constraint provided by the inability of working fluids to flow in micron-sized and even nanometer-sized devices. This is because, as those skilled in the art are aware, of the inability of working fluids to penetrate into such tiny-sized spaces. This, in turn, is the result of the relatively high surface tension of most working fluids. That is, the higher the surface tension of a fluid, the more difficult it is for that fluid to traverse through a very small sized opening.  
           [0006]    The technical literature has addressed this problem in the development of MEMS devices. Burger et al., 14 th  IEEE Inter. Conf. Micro Electro Mechanical Systems, 418-421 (January, 2001) describes a cryogenic micromachined cooler suitable for cooling from ambient temperature to 169° K. and below. The working fluid in this MEMS cooler device is ethylene which is present as a liquid and a gas. The MEMS cooler, however, is attached to a source of ethylene and the system is required to be sealed off in order to maintain specific thermodynamic conditions necessary to retain ethylene under conditions required for cryogenic operation.  
           [0007]    Although this cryogenic MEMS machine represents an improvement in MEMS heat exchange technology, it does not provide the requisite mobility, requiring as it does the presence at all times of a source of fresh working fluid, necessary to extend the utility of MEMS devices requiring a working fluid to very small sized devices.  
           [0008]    It is thus apparent that there is a significant need in the art for a new MEMS device which utilizes a working fluid, which need not be tethered to a source of the working fluid, having a low enough surface tension so that it can be used in the ever smaller sizes required of newly developed MEMS devices.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    A new MEMS device requiring the use of a working fluid and a method of producing the same has now been developed which is characterized by the use of a working fluid having very low surface tension such that the MEMS device may be as small as nanometer-sized. The MEMS device provided with a working fluid, although capable of flowing through all openings provided in the MEMS device, is also characterized by the self contained nature of the working fluid. That is, the MEMS device is unattached to any working fluid source, representing as it does a true closed loop system, wherein the working fluid provides the same operability associated with MEMS devices of the prior art which require an appended working fluid source.  
           [0010]    In accordance with the present invention a microsized MEMS device which utilizes a working fluid is provided. The working fluid is a high pressure liquid or a supercritical fluid. The MEMS device is provided with a connecting device which not only acts to permit introduction of the working fluid under thermodynamic conditions consistent with the maintenance of the fluid in the liquid or supercritical state but which maintains the fluid under those conditions even after removal of those thermodynamic conditions.  
           [0011]    In further accordance with the present invention a process of providing a MEMS device having a size no greater than about 10 microns utilizing a working fluid is provided. In this process a high pressure liquid or a supercritical fluid is introduced into the micronsized MEMS device utilizing a working fluid under thermodynamic conditions consistent with the maintenance of the working fluid in the liquid or supercritical state. High pressure liquid or supercritical fluid is introduced into the MEMS device until the pressure of the liquid or supercritical fluid in the MEMS device reaches the pressure of the liquid or supercritical fluid source. Thereupon, a device provided in the MEMS device closes and seals the working fluid in the MEMS device from the source, trapping the working fluid therein. The thermodynamic conditions are thereupon changed to ambient. However, the working fluid in the MEMS device remains in the liquid or supercritical state. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The present invention may be better understood by reference to the accompanying drawings of which:  
         [0013]    [0013]FIG. 1 is a schematic representation of an apparatus suitable for introducing a high pressure liquid or a supercritical fluid into a MEMS device; and  
         [0014]    [0014]FIG. 2 is a schematic representation of an aspect of a MEMS device illustrating means for introducing a liquid or a supercritical fluid therein. 
     
    
     DETAILED DESCRIPTION  
       [0015]    A MEMS device which requires the utilization of a working fluid is depicted by reference numeral  16 . During introduction of the working fluid MEMS device  16  is disposed in a filling zone  14  of a processing chamber  12 . Therein a high pressure liquid or supercritical fluid from a source  30  is introduced into device  16 . To ensure that the liquid or supercritical fluid remains in the liquid or supercritical state during introduction, thermodynamic conditions in processing chamber  12  are maintained under conditions which insure retention of the fluid in the liquid or supercritical state. Those thermodynamic conditions are a function of the physical characteristics of the working fluid. For example, when the working fluid is carbon dioxide, neon, nitrogen, argon, xenon, sulfur hexafluoride or propane, processing chamber  12  is maintained at a pressure in the range of between about 1,000 psi and about 8,000 psi. More preferably, the pressure within the processing chamber  12  is in the range of between about 2,000 psi and about 5,000 psi. At this pressure an additional fluid, ammonia, may be utilized. Still more preferably, the pressure within processing chamber  12  is about 3,000 psi. It is at this pressure that the most preferred working fluid, carbon dioxide, is most usefully employed. The temperature within processing chamber  12  is maintained in a range of between about 32° C. and about 100° C. Preferably, the temperature within processing chamber  12  is maintained in a range of between about 50° C. and about 80° C. Still more preferably, the temperature within processing chamber  12  is in the range of about 70° C.  
         [0016]    Since it is critical that the aforementioned thermodynamic conditions be maintained during the filling of the working fluid into the MEMS device  16 , processing chamber  12  may be controlled by a heat controller  32  which has the capability of monitoring the temperature therein by means of a thermocouple  26 . The measured temperature is adjusted by heat jacket  18 , controlled by controller  32 , in accordance with temperature control means well known in the art.  
         [0017]    As stated above, a high pressure liquid or supercritical fluid is introduced into MEMS device  16 , disposed in filling zone  14  of processing chamber  12 . This fluid, introduced into MEMS device  16 , is provided by a liquid or supercritical fluid source  30 . As shown in FIG. 1, the liquid or supercritical fluid source  30  may be prepressurized by a pump  28 , disposed downstream of the source of the liquid or supercritical fluid  30 . The high pressure liquid or supercritical fluid is conveyed into filling zone  14  of processing chamber  12  by means of a connecting means  36  provided as part of MEMS device  16  as discussed below.  
         [0018]    Turning now to the MEMS device  16 , that device is disposed in processing chamber  12 , which, as indicated above, is maintained under conditions which are suitable for the maintenance of the working fluid in the liquid or supercritical state. The MEMS device  16  includes a plurality of conduits  37  into which a liquid or supercritical fluid is introduced. The liquid or supercritical fluid is introduced through a connecting means  36  provided on the device  16 . The connecting means  36  operates on the principle of a check valve. Indeed, a check valve suitable for introducing a liquid or supercritical fluid into a MEMS device is described in copending application Ser. No. ______ (Atty. Docket: YOR920010084US1 (14266)), incorporated herein by reference.  
         [0019]    It is emphasized that check valve designs other than those set forth in copending application, Ser. No. (Atty. Docket: YOR920010084US1 (14266)), as the connecting means  36  component of MEMS device  16 , wherein the check valve principle, underlying the embodiments detailed therein, may be utilized.  
         [0020]    The introduction of liquid or a supercritical fluid into MEMS device  16  in processing chamber  12  is completed when the pressure of the high pressure liquid or supercritical fluid in MEMS device  16  is equal to the pressure of the source  30 . At this point the MEMS working fluid, the liquid or supercritical fluid, is fully charged into MEMS device  16 . Thereupon, in accordance with the operation of connecting means  36 , as discussed in copending application Ser. No. ______ (Atty. Docket: YOR920010084US1 (14266)), the conduit between the source of liquid or supercritical fluid and the MEMS device  16  is closed by the closing of a plug in connecting means  36  trapping the working fluid therein. Thus, the working fluid is held in MEMS device  16  at the pressure of its introduction. Therefore, the next step, the removal of thermodynamic conditions consistent with the maintenance of the working fluid in the high pressure liquid or supercritical fluid state, does not change the state of the working fluid in MEMS device  16  insofar as that fluid is trapped therein under the pressure at which it was introduced therein. Stated differently, the replacement of the thermodynamic conditions consistent with the maintenance of high pressure liquid or supercritical fluid conditions in processing chamber  12  with those of ambient does not change the pressure of the liquid or supercritical working fluid in MEMS device  16 . Hence, the working fluid remains a liquid or supercritical fluid.  
         [0021]    Examples of MEMS devices requiring a working fluid, within the contemplation of the present invention, include a heat exchanger, a closed loop pumping apparatus, a closed loop hydraulic system and the like.  
         [0022]    The MEMS device, as suggested previously, is no larger than micron-sized. That is, the size of the MEMS device is no larger than about 10 microns. More preferably, the maximum size of the MEMS device is no larger than about 1 micron.  
         [0023]    The above embodiments are given to illustrate the scope and spirit of the present invention. These embodiments will make apparent, to those skilled in the art, other embodiments and examples. These other embodiments and examples are within the contemplation of the present invention. Therefore, the present invention should be limited only by the appended claims.