Abstract:
A critical point drying apparatus for sample preparation in electron microscopy and semiconductor wafer production includes a computer system to automate the operational modes in drying the specimen. These operational modes controlled by the computer system are: cooling, in which a drying chamber is cooled; starting, in which the specimen chamber is filled with a transitional fluid; purging, in which the transitional fluid purges an intermediary fluid from the drying chamber and the purged intermediary fluid is collected by a collector condenser; heating, in which the drying chamber is heated to elevate the transitional fluid to its critical point temperature and pressure; and bleeding, in which the drying chamber is depressurized to atmospheric pressure at a very slow rate until the drying chamber is completely vented, which signals the end of the drying operation. The computer system interfaces with a remote client terminal to update the status of the operation of the critical point drying apparatus.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 09/658,185 filed Sep. 8, 2000, now U.S. Pat. No. 6,493,694 the disclosure of which is incorporated herein by reference. application Ser. No. 09/658,185 was filed under 35 U.S.C. §111(a) claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing dates of Provisional Application No. 60/206,726 filed on May 28, 2000 pursuant to 35 U.S.C. §111(b). The U.S. application Ser. No. 09/658,185 is incorporated herein by reference for all it discloses. 
    
    
     BACKGROUND OF THE PRESENT INVENTION 
     1. Technical Field of the Present Invention 
     This present invention relates to improvements in critical point dryers for sample preparation for electron microscopy and semiconductor wafer manufacturing and especially to a computer controlled critical point drying apparatus. The present invention is embodied in a method for remotely viewing a critical point drying apparatus using a computer system, a computer system for implementing the method of remotely viewing the critical point drying apparatus, and a computer program product bearing software instructions that implement the method of remotely viewing the critical point drying apparatus. 
     2. Description of the Related Art 
     The following references provide useful background information on the indicated topics, all of which relate to the present invention, and are incorporated herein by reference: 
     U.S. Pat. No. 4055,904 issued to Home on Nov. 1, 1977 describes an automatic method of operating the purge and bleed modes for a critical point dryer. 
     U.S. Pat. No. 4,104,808 issued to Home et al. on Aug. 8, 1978 describes a critical point dryer wherein the purge and bleed modes are controlled semi-automatically. 
     There will now be provided a discussion of various topics to provide a proper foundation for understanding the present invention. 
     In order to examine biological specimens under a scanning electron microscope, the biological specimens must be completely dried and coated with a thin conductive layer. It is important that the drying process be accomplished without disturbing the microstructure of the biological specimen to be examined. Depending upon the biological specimen&#39;s structure, three techniques are available for drying the biological specimen. The first method is air drying by evaporation of the cellular water. While suitable for bacteria or other rigid structures, this method is detrimental to the structures of many biological specimens. The surface tension forces, which turn a grape into a raisin during the drying process, cause sufficient distortion in the cell structure of many biological specimens thereby rendering them useless. The second method is sublimation or freeze-drying. This method is useful only for very small specimens. Additionally, unless the lengthy technique is followed precisely, structural damage from thermal expansion or ice crystal formation often results. The third method utilized is the phase transitional or critical point drying which produces consistently reproducible results without the drawbacks of the preceding two methods. 
     Along with being used to prepare specimens for the scanning electron microscope, critical point drying may also be used in the production of MEMS (Micro-Electro-Mechanical Systems) devices. The critical point drying process helps for a sticktion free release of microstructers in the MEMS device. 
     In critical point drying, a dehydrating fluid such as ethanol or acetone gradually replaces the water contained in a specimen. This maintains the three-dimensional hydrated morphology of the structure under study. However, if the ethanol or acetone evaporates, surface tension forces would cause structural damage and destroy the specimen&#39;s usefulness. 
     Critical point drying devices for sample preparation in electron microscopy are known in the art. The prior art critical point dryers utilize the technique of substituting a transitional fluid for the dehydrating fluid in the cell structure and then removing the transitional fluid. A critical point dryer heats and pressurizes the biological specimen until above the critical pressure and critical temperature. The critical temperature is defined as the temperature above which a gas cannot be liquefied by pressure alone. The critical pressure is the pressure that results when a substance exists as a gas and a liquid in equilibrium at the critical temperature. The critical point of a liquid is when its temperature and pressure are at or above the critical temperature and pressure and the densities of the liquid phase and vapor phase are identical. This critical point is characterized by an absence of phase boundaries that normally exist between a liquid and its vapor at temperatures and pressures below the critical point. This absence of a phase boundary eliminates the boundary forces that exist when changing a liquid to a gas. These boundary forces often cause the destruction of the extremely delicate specimens when changing its internal liquid to a gas below the critical point. Therefore, the solution which is applied in a critical point drying process is to remove the internal liquid from the biological specimen above its critical pressure and temperature to eliminate the boundary force destruction that would otherwise result. 
     Although all fluids have a characteristic critical point which should allow direct removal without the use of dehydrating or transitional fluids, the critical point temperature and pressure of water is 374.2° C. and 218 atmospheres. Achieving these temperatures and pressures would cause severe damage to most biological specimens and therefore a fluid having a lower critical temperature and pressure is normally substituted. Commonly, a dehydrating fluid is used that is miscible with water (e.g., ethanol or acetone) as an intermediate stage between the specimen containing water and a specimen containing transitional fluid. 
     Typically, and in the prior art dryers, the transitional fluid commonly used is carbon dioxide (CO 2 ) because it is easy to use, more economical, less noxious and provides consistently better results than other transitional fluids. The critical temperature and pressure of carbon dioxide is 31° C. and 1,072 psi, respectively, thus reducing the potential for destruction of the specimen structure. 
     The known instruments and apparatuses for critical point drying of biological specimens include, of course, a drying chamber that is connected a supply of the transitional fluid with various regulating valves, temperature gauges and a means for heating the chamber. A skilled technician must carefully control the application, heating, pressurizing and removal of the transitional fluid, thus requiring not only time but also constant attention. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention has been made in view of the above circumstances and is intended to overcome the above problems and limitations of the prior art. 
     Additional aspects and advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from the description, or may be learned by practice of the present invention. The aspects and advantages of the present invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     According to a first aspect of the invention, a critical point drying chamber for drying specimens comprising a chamber and at least one heater is provided. The apparatus further comprises a first valve assembly that supplies a cooling fluid to the chamber. The apparatus further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the chamber. The apparatus further comprises a third valve assembly that allows an intermediary fluid to be purged from the chamber. The apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the chamber. The apparatus further comprises a condenser collector that collects intermediary fluid exiting the third valve assembly. 
     According to a second aspect of the invention, a critical point drying apparatus for drying specimens comprises a cylindrical drying chamber having concave end portions and at least one heater. The apparatus further comprises a first valve assembly that supplies a cooling fluid to the drying chamber. The apparatus further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the drying chamber. The apparatus further comprises a third valve assembly that allows an intermediary fluid to be purged from the drying chamber. The apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the drying chamber. 
     According to a third aspect of the invention, a critical point drying apparatus for drying specimens comprises a drying chamber having at least one heater. The apparatus further comprises a first valve assembly that supplies a cooling fluid to the drying chamber. The apparatus further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the drying chamber. The apparatus further comprises a third valve assembly that allows an intermediary fluid to be purged from the drying chamber. The apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the drying chamber. The apparatus further comprises a computer system that operates the first, second, third and fourth valve assemblies and that activates the at least one heater to heat the transitional fluid above the critical point temperature and to pressurize the transitional fluid above the critical point pressure. 
     According to a fourth aspect of the invention, a critical point drying apparatus for drying specimens, comprising a drying chamber having concave end portions and at least one heater is provided. The apparatus further comprises a first valve assembly that supplies a cooling fluid to the drying chamber wall. The apparatus further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the interior of the drying chamber. The apparatus further comprises a third valve assembly that allows an intermediary fluid to be purged from the interior of the drying chamber. The apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the interior of the drying chamber at a predetermined rate. The apparatus further comprises a computer system that operates the first, second, third and fourth valve assemblies and that activates the at least one heater to heat the transitional fluid above the critical point temperature and to pressurize the transitional fluid above the critical point pressure. 
     According to a fifth aspect of the invention, a critical point dryer apparatus comprising a drying chamber and a computer system wherein the computer system is adapted to controlling the drying chamber during a critical point drying process is provided. The computer system comprises a processor and a memory including software instructions adapted to enable the computer system. The software instructions enable the computer system to cool the drying chamber to a first chamber temperature. The software instructions further enable the computer system to fill the drying chamber with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber at the first chamber temperature such that the transitional fluid completely displaces an intermediary fluid within a first time period. The software instructions further enable the computer system to activate at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching critical point equilibrium. The software instructions further enable the computer system to maintain the transitional fluid at the critical point equilibrium for a second time period. The software instructions further enable the computer system to bleeding the transitional fluid from the drying chamber while maintaining the drying chamber at the second chamber temperature and allowing the drying chamber pressure to drop. 
     According to a sixth aspect of the invention, a computer program product for enabling a computer system to control the drying chamber during a critical point drying process is provided. The computer program product comprises software instructions for enabling the computer system to perform predetermined operations, and a computer readable medium bearing the software instructions. The predetermined operations comprise cooling the drying chamber to a first chamber temperature. The predetermined operations comprise filling the drying chamber with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber at the first chamber temperature such that the transitional fluid completely displaces the intermediary fluid within a first time period. The predetermined operations comprise activating at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching critical point equilibrium. The predetermined operations comprise maintaining the transitional fluid at the critical point equilibrium for a second time period. The predetermined operations comprise bleeding the transitional fluid from the drying chamber while maintaining the drying chamber at the second chamber temperature. 
     According to a seventh aspect of the invention, an article of manufacture, which comprises a computer readable medium having stored therein a computer program to control a drying chamber during a critical point drying process, is provided. The article of manufacture comprises a first code segment which, when executed on a computer, cools the drying chamber to a first chamber temperature. The article of manufacture comprises a second code segment which, when executed on a computer, fills the drying chamber with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber at the first chamber temperature such that the transitional fluid completely displaces the intermediary fluid within a first time period. The article of manufacture comprises a third code segment which, when executed on a computer, activates at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching a critical point equilibrium. The article of manufacture comprises a fourth code segment which, when executed on a computer, maintains the transitional fluid at the critical point equilibrium for a second time period. The article of manufacture comprises a fifth code segment which, when executed on a computer, bleeds the transitional fluid from the drying chamber while maintaining the drying chamber at the second chamber temperature. 
     An eighth aspect of the invention provides a critical point drying apparatus comprising a drying chamber with at least one heater and a first valve assembly that supplies a cooling fluid to the drying chamber. The critical point drying apparatus further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the drying chamber and a third valve assembly that allows an intermediary fluid to be purged from the drying chamber. The critical point drying apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the drying chamber. The critical point drying apparatus further comprises a computer system that operates the first, second, third and fourth valve assemblies and that activates the heater to heat the transitional fluid above the critical point temperature and to pressurize the transitional fluid above the critical point pressure. The computer system is connected to a network and the computer system outputs mode update messages and elapsed time messages. 
     A ninth aspect of the invention provides a critical point drying system, comprising a drying chamber having at least one heater and a first valve assembly that supplies a cooling fluid to the drying chamber. The critical point drying system further comprises a second valve assembly that supplies a transitional fluid having a critical point temperature and critical point pressure to the drying chamber and a third valve assembly that allows an intermediary fluid to be purged from the drying chamber. The critical point drying apparatus further comprises a fourth valve assembly that bleeds the transitional fluid from the drying chamber. The critical point drying apparatus further comprises a computer system that operates the first, second, third and fourth valve assemblies and that activates the heater to heat the transitional fluid above the critical point temperature and to pressurize the transitional fluid above the critical point pressure. The critical point drying apparatus also comprises a server comprised of a plurality of client terminals, wherein the computer system is connected to the server through a network and the computer system outputs mode update messages and elapsed time messages to at least one of the plurality of client terminals. 
     A tenth aspect of the invention provides a critical point dryer apparatus that comprises a drying chamber and a computer system wherein the computer system is adapted to controlling the drying chamber during a critical point drying process. The computer system includes a processor and a memory including software instructions for a series of drying chamber modes. The software instructions are adapted to enable the computer system to cool the drying chamber to a first chamber temperature and to fill the drying chamber with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber at the first chamber temperature such that the transitional fluid completely displaces an intermediary fluid within a first time period. The software instructions are further adapted to activate at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching a critical point equilibrium, and to maintain the transitional fluid at the critical point equilibrium for a second time period. The software instructions are further adapted to bleed the transitional fluid from the drying chamber while maintaining the drying chamber at a second chamber temperature and allowing the drying chamber pressure to drop. The software instructions are further adapted to enable the computer system to output a mode update message each time the computer system cycles through a new drying chamber mode. 
     An eleventh aspect of the invention provides a computer program product for enabling a computer system to control a drying chamber during a critical point drying process. The computer program product includes software instructions for enabling the computer system to perform predetermined operations for a series of drying chamber modes, and a computer readable medium bearing the software instructions. The predetermined operations include cooling the drying chamber to a first chamber temperature and filling the drying chamber with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber at the first chamber temperature such that the transitional fluid completely displaces the intermediary fluid within a first time period. The predetermined operations further comprise activating at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching a critical point equilibrium, and maintaining the transitional fluid at the critical point equilibrium for a second time period. The predetermined operations further comprise bleeding the transitional fluid from the drying chamber while maintaining the drying chamber at the second chamber temperature, and outputting a mode update message each time the computer system cycles through a new drying chamber mode. 
     A twelfth aspect of the invention provides an article of manufacture, which comprises a computer readable medium having stored therein a computer program having a plurality of drying chamber modes for controlling a drying chamber during a critical point drying process. The computer program comprises a first code segment which, when executed on a computer, cools the drying chamber to a first chamber temperature, and a second code segment which, when executed on a computer, fills the drying chamber with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber at the first chamber temperature such that the transitional fluid completely displaces the intermediary fluid within a first time period. The computer program further comprises a third code segment which, when executed on a computer, activates at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching a critical point equilibrium, and a fourth code segment which, when executed on a computer, maintains the transitional fluid at the critical point equilibrium for a second time period. The computer program further comprises a fifth code segment which, when executed on a computer, bleeds the transitional fluid from the drying chamber while maintaining the drying chamber at a second chamber temperature, and a sixth code segment, which, when executed on a computer, outputs a mode update message each time the computer cycles through a new drying chamber mode. 
    
    
     The above and other aspects and advantages of the present invention will become apparent from the following detailed description and with reference to the accompanying drawing figures. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification illustrate embodiments of the present invention and, together with the written description, serve to explain the aspects, advantages and principles of the present invention. In the drawings, 
     FIG. 1 illustrates a perspective view of a first embodiment of the critical point drying apparatus; 
     FIG. 2 illustrates a perspective view of a second embodiment of the critical point drying apparatus; 
     FIG. 3 is a top view of the drying chamber illustrating the location of the purge outlets, the fill inlets, the cool inlet and the cool outlet; 
     FIG. 4 is a top view of the chamber cover; 
     FIG. 5 illustrates a cross-sectional view along lines IV—IV of the chamber cover, the cover viewing port and the cover viewing window; 
     FIG. 6 illustrates a cross-sectional view of the drying chamber along lines I—I showing the fill inlets, the purge outlets, the chamber viewing port and the chamber viewing window; 
     FIG. 7 illustrates a cross-sectional view of the drying chamber along lines II—II showing the temperature sensor, the heat sensor, the cool sensor and the heater; 
     FIG. 8 illustrates a cross-sectional view of the drying chamber along lines III—III showing the cool inlet and the cool outlet; 
     FIG. 9 illustrates the various valves and connection lines for the routing of cooling fluid and transitional fluid through the critical point drying apparatus; 
     FIG. 10 illustrates the various valves and connection lines for the routing of cooling fluid and transitional fluid through a critical point drying apparatus that is connected to a closed loop refrigeration system; 
     FIG. 11 illustrates a side view of the collector condenser; 
     FIG. 12 illustrates the data flow paths between the computer system and the various valves and sensors; 
     FIG. 13A illustrates a view of the spacer ring; 
     FIG. 13B illustrates a side view of the wafer holder; 
     FIG. 13C illustrates a side view of the chamber insert for the drying chamber; 
     FIG. 14 illustrates the process steps executed by the computer system when controlling and remotely viewing a critical point drying process in the drying chamber; 
     FIGS. 15A-15F illustrate the process steps, in more detail, executed by the computer system when controlling and remotely viewing a critical point drying process within the drying chamber; 
     FIG. 16 is a table showing the valve openings and closings for each drying mode controlled by the computer system; 
     FIG. 17 illustrates an exemplary network for remotely displaying mode status and elapsed time for a critical point drying process. 
     FIG. 18 illustrates an embodiment of a remote display for a client terminal that enables remotely viewing a critical point drying process according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Prior to describing the aspects of the present invention, some details concerning certain terms of art will be provided to facilitate the reader&#39;s understanding of the present invention and to set forth the meaning of various terms. 
     As used herein, the term “computer system” encompasses the widest possible meaning and includes, but is not limited to, microprocessors, standalone processors, networked processors, mainframe processors, and processors in a client/server relationship. The term “computer system” is to be understood to include at least a memory and a processor. In general, the memory will store, at one time or another, at least portions of executable program code, and the processor will execute one or more of the instructions included in that executable program code. 
     As used herein, the term “embedded computer system” includes, but is not limited to, an embedded central processor and memory bearing object code instructions. Examples of embedded computer systems include, but are not limited to, personal digital assistants, cellular phones and digital cameras. In general, any device or appliance that uses a central processor, no matter how primitive, to control its functions can be labeled has having an embedded computer system. The embedded central processor will execute one or more of the object code instructions that are stored on the memory. The embedded computer system can include cache memory, input/output devices and other peripherals. 
     It will be appreciated that the term “predetermined operations” and the term “computer system software” mean substantially the same thing for the purposes of this description. It is not necessary to the practice of this present invention that the memory and the processor be physically located in the same place. That is to say, it is foreseen that the processor and the memory might be in different physical pieces of equipment or even in geographically distinct locations. 
     As used herein, one of skill in the art will appreciate that “media” or “computer-readable media” may include a diskette, a tape, a compact disc, an integrated circuit, a cartridge, a remote transmission via a communications circuit, or any other similar medium useable by computers. For example, to distribute computer system software, the supplier might provide a diskette or might transmit the instructions for performing predetermined operations in some form via satellite transmission, via a direct telephone link, or via the Internet. 
     Although computer system software might be “written on” a diskette, “stored in” an integrated circuit, or “carried over” a communications circuit, it will be appreciated that, for the purposes of this discussion, the computer usable medium will be referred to as “bearing” the instructions for performing predetermined operations. Thus, the term “bearing” is intended to encompass the above and all equivalent ways in which instructions for performing predetermined operations are associated with a computer usable medium. 
     Therefore, for the sake of simplicity, the term “program product” is hereafter used to refer to a computer useable medium, as defined above, which bears instructions for performing predetermined operations in any form. 
     A detailed description of the exemplary embodiments of the present invention will now be given referring to the accompanying drawings wherein like reference numerals refer to similar parts in the several views. 
     The general operation of a critical point dryer will be provided to facilitate the reader&#39;s understanding of the present invention. This description of the general operation of a critical point dryer is by no means limiting on the operation of the present invention. 
     The first step in a critical point drying operation is to cool the drying chamber to a temperature that will condense the transitional fluid to be added later. A cooling fluid flows around the drying chamber and evaporates in a heat exchange relationship with the drying chamber. Preferably, the cooling fluid is liquid carbon dioxide. If a closed loop refrigeration system is used to cool the drying chamber, Freon™ or its equivalent can also be used. Next, the treated specimen is placed in the drying chamber along with an amount of the dehydrating fluid, such as ethanol, methanol or acetone. The specimen has previously been dehydrated with the dehydrating fluid. After the treated specimen has been placed in the drying chamber and the cover secured, a transitional fluid fills the drying chamber. After the drying chamber is filled, the dehydrating fluid is purged from the treated specimen in the drying chamber. Preferably, the transitional fluid is liquid carbon dioxide. The purging of the dehydrating fluid is controlled over a predetermined time period. In an aspect of the invention, the critical point drying apparatus cycles through another filling of the drying chamber with the transitional fluid to ensure that the transitional fluid completely fills the drying chamber. The drying chamber is then heated to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching critical point equilibrium. Once critical point equilibrium is reached, the equilibrium is maintained for a certain length of time. After the equilibrium time period has expired, the drying chamber temperature is maintained while the drying chamber pressure is reduced or bled-off very slowly. When the drying chamber pressure drops below a predetermined threshold, the drying chamber is then vented to release any residual pressure that may hinder removal of the chamber cover and the chamber heat is shut off. 
     Referring to FIG. 1, a perspective view showing the external configuration of an embodiment of the critical point drying apparatus  1  is shown. The housing  24  encloses the internal valves, wiring, piping, switches, relays and computer system components that make up the critical point drying apparatus  1 . A power switch  22  applies electrical power to the dryer through a fuse. The operation indicator lights indicate the individual operation that is being undertaken in the drying chamber  40 . Preferably, the operation indicators are light emitting diodes (LED). The critical point drying apparatus  1  has the following operation indicators: cool LED  16 , fill LED  17 , purge LED  18 , heat LED  19 , bleed LED  20  and vent LED  21 . The temperature gauge  23  and the pressure gauge  69  provide visible indicators of the present conditions within the drying chamber  40 . Transitional fluid and cooling fluid enter the critical point drying apparatus  1  through the inlet port  52 . Exhausted cooling fluid exits the critical point drying apparatus  1  through the cool exit port  51 . Purged dehydrating fluid and exhausted transitional fluid exits the critical point drying apparatus  1  through purge port  50 . 
     Referring to FIG. 2, a perspective view showing the external configuration of another embodiment of the critical point drying apparatus  1 ′ is shown. The critical point drying apparatus  1 ′ is shown with a cool supply port  71 . The cool supply port  71  is connected to a closed loop refrigeration system. Cooling fluid circulates through the critical point drying apparatus  1 ′ by entering the cool supply port  71  and exiting through the cool exit port  51 . Transitional fluid enters the critical point drying apparatus  1 ′ through the inlet port  52 . Purged dehydrating fluid and exhausted transitional fluid exits the critical point drying apparatus  1 ′ through purge port  50 . 
     Referring to FIG. 3, a top view of the drying chamber  40  is illustrated. The placement of the various inlets and outlets shown in FIG. 3 is by no means limiting and is shown for illustration purposes only. The crosssectional view along lines I—I includes the fill inlets and the purge ports and is shown in more detail in FIG.  6 . The cross-sectional view along lines II—II includes the heater and is shown in more detail in FIG.  7 . The cross-sectional view along lines III—III includes the cool inlet and the cool outlet and is shown in more detail in FIG.  8 . As shown in FIG. 3, the fill inlets  43 - 44  have an angled portion  39  that is angled relative to the drying chamber wall  47  such that when the transitional fluid enters the drying chamber  40 , it will flow into the drying chamber  40  in a swirling fashion. The swirling of the transitional fluid as it enters the drying chamber  40  allows for an even and thorough purge of the intermediary fluid from the drying chamber  40 . 
     Referring to FIG. 4, a top view of the chamber cover  75  is illustrated. Cross-section IV—IV includes the mounting stud holes  76  and is shown in more detail in FIG.  5 . The chamber cover  75  has a cover viewing port  80  for observing the interior of the drying chamber  40 . Preferably, the mounting stud holes  76  are evenly arranged around the perimeter of the chamber cover  75 . 
     Referring to FIG. 5, the cross-section of the chamber cover  75  along lines IV—IV is illustrated in more detail. This is the cross-section along lines IV—IV referred to by FIG.  4 . The chamber cover  75  secured by securing knobs (not shown) and is provided with a cover viewing window  78  for viewing the specimen during the operation of the critical point drying apparatus  1 . 
     The cover viewing window  78  is mounted on the axial center of the chamber cover  75 . Preferably, the cover viewing window  78  comprises quartz or an equivalent material. The operator can view the interior of the drying chamber  40  through the cover viewing port  80  in the chamber cover  75  and monitor the progress of the critical point drying sequence. The cover viewing window  78  is mounted with a precision machined viewing window gasket  77  that is pressure fit into the cover viewing port  80  in the chamber cover  75 . Preferably, the viewing window gasket  77  comprises Teflon™ or an equivalent material. The viewing window gasket  77  holds the cover viewing window  78  in place, thereby providing a seal that withstands high pressure and will not be damaged by the intermediary fluids. 
     In an aspect of the invention, the chamber cover  75  has a concave surface  79  on its underside that has a concave pitch. The concave pitch acts to evenly displace the internal pressure of the drying chamber  40  exerted on the chamber cover  75 . The concave surface  79  allows the critical point drying apparatus  1  to accommodate large specimens without warping the chamber cover  75 . For example, wafers used in integrated circuit manufacturing can be upwards of twelve inches in diameter. A conventional chamber cover would likely warp and be difficult to remove from the drying chamber. 
     Referring to FIG. 6, the cross-section of the drying chamber  40  along lines I—I will be described in more detail. The drying chamber  40  is also provided with fill inlets  43 - 44  and purge outlets  45 - 46  that allow the transitional fluid to fill and flow through the chamber. As the size (i.e. diameter) of the drying chamber  40  increases, the fill inlets  43 - 44  and the purge outlets  45 - 46  serve to increase the efficiency of the critical point drying apparatus  1 . The fill inlet  43 - 44  has an angled portion  39  that is angled relative to the drying chamber wall  47  such that when the transitional fluid enters the drying chamber  40 , it will flow into the drying chamber  40  in a swirling fashion. The swirling of the transitional fluid as it enters the drying chamber  40  allows for an even and thorough purge of the intermediary fluid from the drying chamber  40 . A small chamber may require only a single fill inlet  43  and a single purge outlet  45  to adequately flow the transitional fluid through the drying chamber  40 . However, as drying chamber size increases, a plurality of fill inlets  43 - 44 , are used and all have an angled portion  39  that is arranged at an angle relative to the drying chamber wall  47 , which efficiently circulates the incoming transitional fluid. An added benefit of multiple fill inlets is that the drying chamber  40  fills at a much faster rate, thereby inducing less disturbance to the specimen situated in the drying chamber  40 . Similarly, a large drying chamber has a plurality of purge outlets  45 - 46  situated at the lowest point of the drying chamber  40  that aids in the collection and purging of the intermediary fluid. 
     The drying chamber  40  has a chamber viewing window  114  mounted in the axial center of the drying chamber  40 . Preferably, the chamber viewing window  114  comprises quartz or an equivalent material. The chamber viewing window  114  is lighted from below and the operator can view the interior of the drying chamber  40  through the cover viewing port  80  in the chamber cover  75  and monitor the progress of the critical point drying sequence. The chamber viewing window  114  is mounted with a precision machined viewing window gasket  115  that is pressure fit into a chamber viewing port  116  in the bottom of the drying chamber  40 . Preferably, the viewing window gasket  115  comprises Teflon™ or an equivalent material. The viewing window gasket  115  holds the chamber viewing window  114  in place, thereby providing a seal that withstands high pressure and will not be damaged by the intermediary fluids. 
     Mounting studs  110  extend upward through the chamber cover  75  and in conjunction with internally threaded securing knobs (not shown), fixably and sealably mount the chamber cover  75  to the drying chamber  40 . By drilling through the drying chamber wall  47  into the mounting studs  110 , stainless steel mounting pins  111  can be inserted into the mounting studs  110  to prevent any movement. 
     The bottom of the drying chamber  40  has a chamber concave surface  48  with a concave pitch. The concave pitch acts to each active to evenly displace the internal pressure of the drying chamber  40 . The chamber concave surface  48  allows the intermediary fluid to collect and exit through the purge ports  45 - 46  that are located at the lowest point of the drying chamber. The concavity thus assists with dispersing the internal pressure and with the complete purging of the intermediary fluid through the bottom of the drying chamber  40 . The chamber concave surface  48  allows the critical point drying apparatus  1  to accommodate large specimens without warping in the chamber cover  75 . 
     A cover gasket  113  provides a seal between the chamber cover  75  and the top portion of the drying chamber  40 . A seal groove  112  is formed in the top portion of the drying chamber  40  that directly opposes the chamber cover  75 . The cover gasket  113  is set into the seal groove  112 , and projects slightly above the top surface of the drying chamber  40 , thereby ensuring a tight seal when the chamber cover  75  is secured. Due to the pressure in the drying chamber  40 , the cover gasket  113  must be made from a material that is inert to the fluids used in the drying chamber  40  and must be able to withstand the chamber pressure without deformation. Preferably, the cover gasket  113  comprises Teflon™ or an equivalent material. 
     Referring to FIG. 7, the cross-section of the drying chamber  40  along lines II—II is illustrated in more detail. The drying chamber  40  also has at least one heater  32  mounted in the drying chamber wall  47  to heat the transitional fluid above the critical point. Preferably, the heater  40  is a wire-wound resistance heater that is controlled by the computer system  99 . 
     A temperature sensor (not shown) is mounted in the wall of the drying chamber  40  is connected to a temperature gauge  73  to provide an indication of the drying chamber temperature. In addition, three other temperature sensors are mounted on the drying chamber wall  47 . A heat sensor  31  (normally closed) opens when the temperature in the drying chamber  40  reaches a predetermined level and the opening of the heat sensor  31  is monitored by the computer system  99 . Preferably, the heat sensor  31  is a thermostatic sensor and opens when the drying chamber temperature exceeds 42° C. A cool sensor  32  (normally closed) opens when the temperature in the drying chamber  40  drops past a predetermined level and the opening of the cool sensor  32  is monitored by the computer system  99 . Preferably, the cool sensor  32  is a thermostatic sensor and opens when the drying chamber temperature is less than 5° C. Finally, the safety sensor  30  ensures that the heater  33  does not raise the drying chamber temperature past a predetermined safety level. If the drying chamber temperature exceeds the predetermined safety level, all power to the heater  33  is cut off. Preferably, the safety sensor  32  is a thermostatic sensor and opens when the drying chamber temperature exceeds 50° C. 
     Referring to FIG. 8, the cross-section of the drying chamber  40  along lines III—III is illustrated in more detail. A cooling fluid is circulated to cool the drying chamber wall  47  by passing into the cool inlet  41  through the walls of the drying chamber and the connecting line  49  and out through the cool outlet  42 . Preferably, liquid carbon dioxide is used as a cooling fluid. The cooling fluid cools the drying chamber  40  by adiabatic cooling, which is turned on and off automatically via thermostatic controls. The fittings and tubing through which the cooling fluid flows is preferably stainless steel. In order to protect the integrity of the specimen, all external and internal surfaces of the critical point drying apparatus  1  are both chemically and moisture resistant, and all internal surfaces are inert to the intermediary and transitional fluids, such as liquid carbon dioxide and ultra-pure alcohol. In addition, all internal and external surfaces are grounded to guard against static discharge that is harmful to semiconductor wafers. 
     Referring to FIG. 9, a schematic diagram showing the flow of fluids through the drying chamber  40  and the various control valves is illustrated. The transitional fluid is also utilized to cool the drying chamber  40  through the cooling circuit shown. The transitional fluid, preferably liquid carbon dioxide, is provided at the inlet port  52  and then flows through a filter assembly  53  to the 3-way tee  54 . The filter assembly  53  removes any particulate matter from the transitional fluid prior to entering the various valves of the critical point drying apparatus  1 . Preferably, the filter assembly  53  is a stainless steel filter that removes particulate up to 0.5 microns in diameter in order to protect the specimen in the drying chamber  40 , as well as the critical point drying apparatus valves. Preferably, all tubing, fittings, valves, etc. are stainless steel. In addition, a rupture disc that will burst and evacuate the system in case of an undesirable rise in pressure protects the entire pressure system (not shown). 
     The transitional fluid is then piped to a pair of computer system controlled solenoid operated valves: the fill valve  56  and the cool valve  55 . The fill valve  56  and cool valve  55  also comprise metering valves to control the flow rate of fluids through the valves. 
     When the computer system  99  energizes the cool valve  55  solenoid, the cool valve  55  supplies the transitional fluid through the metering valve to the drying chamber  40  at the cool inlet  41 . The metering valve regulates the flow of transitional fluid through the wall of the drying chamber. When the cool valve  55  is energized, the transitional fluid flows out from the cool valve  55  and through the connection line to the drying chamber  40 , wherein the transitional fluid is evaporated and ducted throughout the wall of the drying chamber  40 . The flow is from the cool inlet  41  through the wall of the drying chamber  40  to the cool outlet  42 . The warmed vaporized cooling fluid is ducted out of the critical point drying apparatus  1  at the cool port  51 . If a closed loop refrigeration system is used, the cooling fluid is cycled back to a refrigeration unit. The closed loop system may use a refrigerant other than liquid carbon dioxide, such as Freon™. 
     When the computer system  99  energizes the fill valve  56 , transitional fluid flows into the fill inlets  43 - 44  to fill or purge the drying chamber  40 . When the drying chamber  40  is being filled with the transitional fluid, the computer system  99  energizes the fill valve  56 , and transitional fluid flows from the fill valve  56 , through a check valve  57 , a 4-way tee  58  and into the drying chamber  40  through the fill inlets  43 - 44 . The in-line check valve  57  protects the fill valve from any backflow to the drying chamber. 
     A connection line from the 4-way tee  58  is connected to a high-pressure sensor  67 , a low-pressure sensor  68  and a pressure gauge  69 . The low-pressure sensor  68  opens when the chamber pressure drops below a predetermined low pressure point, and the opening of the low-pressure sensor  68  is monitored by the computer system  99 . An acceptable range for the predetermined low pressure point is from 100 to 600 psi. Preferably, the predetermined low pressure point is 400 psi. 
     The high-pressure sensor  67  opens when the chamber pressure exceeds a predetermined high pressure point, and the opening of the high-pressure sensor  67  is monitored by the computer system  99 . An acceptable range for the predetermined high pressure point is from 1175 and 1600 psi. Preferably, the predetermined high pressure point is 1200 psi. 
     A pressure relief valve (not shown) is also connected to the pressure sensors. The pressure relief valve will release the pressure from the critical point drying apparatus  1  when the drying chamber pressure exceeds a predetermined limit. The pressure relief valve is heated and the heating of the pressure relief valve is thermostatically controlled independent of the computer system  99 . An acceptable range for the pressure relief valve is from 1200 to 1600 psi. Preferably, the pressure relief valve opens at 1250 psi. 
     A rupture disc  72  is also connected to the pressure sensors. The rupture disc  72  acts as an additional safety feature and is set to burst when the drying chamber pressure goes above a predetermined limit. An acceptable range for the rupture disc  72  is from 1900 to 3000 psi. Preferably, the rupture disc  72  ruptures at 2100 psi. 
     When the intermediary fluid is to be purged from the drying chamber  40 , the computer system  99  commands the purge valve  63  to open. The purge valve  63  is a solenoid valve. The intermediary fluid is forced from the drying chamber  40  through the purge outlets  45 - 46  in the drying concave surface  48  and into the connection line through a filter assembly  61 , which is connected to the purge valve  63 . The purge valve  63  is heated to prevent the purge valve  63  from freezing when the transitional fluid or the intermediary fluid passes through the purge valve  63 . The heating of the purge  63  valve is thermostatically controlled independent of the computer system  99 . The purge valve  63  also includes a metering valve to control the flow rate at which the transitional fluid or intermediary fluid is purged. Attached to the exiting end of the purge metering valve is a check valve  65  to prevent fluid backflow through the purge valve  63  into the drying chamber  40 . The purged fluid exits the critical point drying apparatus  1  through the purge outlet  50 . Throughout the entire purging process, the computer system  99  monitors the drying chamber temperature and keeps the drying chamber  40  below a predetermined temperature, preferably 5° C. or less. 
     The cycle time for executing a purge of the intermediary fluid from the drying chamber  40  is controlled by the computer system  99 . Preferably, the purge time is adjusted by a purge timing control  25  that is located on the housing  24  of the critical point drying apparatus  1 . After the purge cycle for the purging of the intermediary fluid is complete, the computer system  99  closes the purge valve  63  and allows the fill valve  56  to continue filling the drying chamber  40  with transitional fluid. This ensures the transitional fluid fills the drying chamber  40  completely. The computer system  99  then advances the drying chamber  40  into the heating cycle. 
     The computer system  99  activates the heater  32  to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching critical point equilibrium. Preferably, the heater  32  raises the drying chamber temperature to at least 31° C. or greater, which, in turn, causes the temperature and pressure of the transitional fluid to reach its critical point temperature and pressure. 
     After the computer system  99  has determined that the specimen has been at the critical point equilibrium for a sufficient amount of time, the computer system  99  commands the bleed valve  62  to open, thereby allowing the transitional fluid to exhaust out of the drying chamber  40  and exit the critical point drying apparatus  1  through the purge outlet  50 . The bleed valve  62  is a solenoid valve. When the transitional fluid is exhausted, it flows from the drying chamber  40  into the bleed valve  62  and then into the check valve  64 . The check valve  64  prevents backflow from backing through the bleed valve  62  into the drying chamber  40 . The bleed valve  62  also comprises a metering valve to control the bleed rate. Preferably, the metering valve allows the system pressure to decrease at a rate of 100 psi/minute. This bleed rate prevents the transitional fluid from recondensing. In addition, the bleed valve  62  is thermostatically heated to prevent the bleed valve  62  from freezing as the transitional fluid flows through it. During the bleed process, the computer system  99  maintains the drying chamber temperature at 31° C. or above. This temperature level prevents recondensation on the specimen. When the drying chamber pressure is reduced to 400 psi, the computer system  99  turns off the heater  32  and switches from bleed to vent mode. The computer system  99  commands the bleed valve  62  to close and the purge valve  63  to open. This returns the drying chamber to atmospheric pressure quicker. 
     Referring to FIG. 10, in another aspect of the invention, the drying chamber  40  can be cooled using a closed loop refrigeration system. The closed loop system is coupled to the drying chamber  40  and allows the drying chamber  40  to be cooled without blasting liquid carbon dioxide through the drying chamber  40 . This feature of the present invention provides for savings on the amount of carbon dioxide that is used during a specimen drying sequence. In addition, the closed loop system may use a refrigerant other than liquid carbon dioxide, such as Freon™. Cooling fluid enters the critical point drying apparatus  1  through cool supply port  71  and is piped to the cool valve  55 . A common supply for cooling fluid and transitional fluid is not used. 
     Referring to FIG. 11, a condenser collector  130  allows the intermediary fluid (i.e., the dehydrating fluid) to be purged from the drying chamber  40  in a safer and more controlled manner. The condenser collector  130  is connected to the purge port  50 . Preferably, methyl alcohol or ethyl alcohol is used as intermediary fluid for the specimen drying process, although these fluids are relatively toxic. When the intermediary fluid is purged from the drying chamber  40  under pressure, the intermediary fluid often exits the drying chamber  40  in a frozen state. The condenser collector  130  transforms the intermediary fluid back into a liquid state so that it may be drained from the condenser collector  130  easily. Preferably, the condenser collector  130  uses a thermostatically controlled heated reservoir  131  with a concave bottom portion  133 . The drain line  134  the condenser collector  130  is located on the low point of the concave bottom portion  133  and a drain valve  135  regulates the exit flow of the material. 
     The outgoing intermediary fluid, both in its gaseous state and frozen state, enters through a reservoir inlet  137  on the upper portion of the reservoir  131 . The gas is allowed to exit through the gas vent  136  of the reservoir  131 , while the frozen portions of the intermediary fluid remain in the reservoir  131 . The heater  132  then warms the reservoir  131  and the frozen intermediary fluid returns to a liquid state and is then drained from the reservoir  131 . Preferably, the condenser collector  130  can be constructed from stainless steel, brass or polyvinyl chloride. The reservoir  131  of the condenser collector  130  is insulated to prevent condensation from developing on the surfaces of the reservoir  131 . 
     Referring to FIG. 12, the computer system  99  with software that is adapted to perform critical point drying techniques when coupled to a critical point drying apparatus  1  will now be described in summary fashion. As described above, the computer system  99  includes a processor for executing software instructions adapted to enable the computer to control a critical point drying chamber and its associated valve and heaters. The computer system  99  includes a memory  101  that stores the software instructions adapted to enable the computer system  99  to control the drying chamber  40  and associated valves. The computer system  99  also comprises an I/O port  102  that allows the computer system  99  to be re-programmed and to upload/download data. The I/O port  102  also provides the computer system  99  with interfacing capability to a network, such as a Local Area Network (LAN) or a Wide Area Network (WAN). The I/O port  102  also supports Internet and Intranet connections as well. The I/O port  102  allows current status data to be sent from the computer system  99  to remote client terminals for the purpose of remotely monitoring an on-going critical point drying process. As shown in FIG. 12, the computer system  99  receives inputs from the temperature and pressure sensors through the data lines  103 . Commands to open or close the cool valve  55 , the fill valve  56 , the bleed valve  62  and the purge valve  63  are sent over the data lines  103 . Finally, the computer system  99  receives commands from the indicator switches on the housing  24  that indicate the various modes, and the computer system  99  also lights the appropriate indicator switch to inform the operator which mode is currently being executed. 
     Referring to FIGS. 13A-13C, when the critical point drying apparatus  1  will be used to process semiconductor wafers, the drying chamber must be outfitted with a mechanism to hold the semiconductor wafer in a suspended position so that the drying process can be achieved. The critical point drying apparatus  1  that are designed to process semiconductor wafers are outfitted with a wafer holder  124 , a spacer ring  120  and a chamber insert  121 . Preferably, these components come in different sizes to accommodate different wafer sizes, and are made out of Teflon™ or an equivalent material. 
     The spacer ring  120  is placed in the bottom of the appropriate wafer holder  124 , then a semiconductor wafer can be placed on top, and another spacer ring  120  can then be put in place so that another semiconductor wafer can be added. This method of suspended stacking allows multiple wafers to be successfully processed during one run. 
     Since semiconductor wafers come in several different sizes, there are several different size wafer holders  124 . When a smaller wafer holder  124  is needed to process a semiconductor wafer, a chamber insert  121  is used to hold the wafer holder  124  in place and reduce the amount of transitional fluid used. For example, if the drying chamber  40  has a six and one half inch internal diameter and the operator wishes to process a four inch wafer, then a chamber insert  121  is put into place (having a six inch outer diameter with cavity  123  that has a four inch internal diameter). The wafer then is placed in the four-inch wafer holder  124  that is placed inside the chamber insert  121 . 
     The wafer holder  124  is comprised of a body  125  and a handle  128 . The body  125  has a plurality of fluid holes  127  that allow the transitional fluid to reach the semiconductor wafers stacked inside the body  125 . 
     Referring to FIG. 14, as described above, these software instructions can be resident on the microprocessor  100  or stored on a separate memory  101 . At Step S 1000 , the computer system  99  executes software instructions to cool the drying chamber  40  to a first chamber temperature. When the first chamber temperature has been reached, the computer system  99  will output a mode update message to a remote terminal indicating that the cool cycle has started. At S 1100 , the computer system  99  executes software instructions to fill the drying chamber  40  with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber  40  at the first chamber temperature, and then the computer system  99  executes software instructions to purge the intermediary fluid from the drying chamber  40 . At the completion of both the fill and purge cycles, the computer system  99  outputs mode update messages to a remote terminal indicating that these cycles have started. At S 1200 , the computer system  99  executes software instructions to fill to ensure the drying chamber  40  is completely filled. When the drying chamber  40  has been filled, the computer system  99  outputs a mode update message to a remote terminal indicating that the second fill cycle has started. At S 1300 , the computer system  99  executes software instructions to activate a heater  32  to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching critical point equilibrium. When the critical point equilibrium has been reached, the computer system  99  outputs a mode update message to a remote terminal indicating that the critical point equilibrium has been reached. At S 1400 , the computer system  99  executes software instructions to maintain the transitional fluid at the critical point equilibrium for a second time period. At the expiration of the second time period, the computer system  99  outputs a mode update message to a remote terminal indicating that the equilibrium time period has expired. At S 1500 , the computer system  99  executes software instructions to bleed the transitional fluid from the drying chamber  40  while maintaining the drying chamber  40  at the second chamber temperature and allowing the drying chamber pressure to drop. After the transitional fluid has been bled from the drying chamber  40 , the computer system outputs a mode update message to a remote terminal indicating that the transitions fluid has been bled from the drying chamber  40 . At S 1600 , the computer system  99  executes software instructions to vent the transitional fluid from the drying chamber  40 , and the computer system  99  outputs a mode update message that the drying chamber  40  is now venting transitional fluid. 
     The software instructions adapted to enable the computer system  99  to control the drying chamber  40  and the associated valve assemblies will now be described in greater detail. 
     If commanded to by the operator, the computer system  99  executes diagnostic routines to determine if the critical point drying apparatus  1  is in working order. These diagnostics are described in greater detail below. 
     Next, the software instructions executed by the computer system  99  cool the drying chamber  40  to a predetermined value. As described above, at S 1010 , the computer system  99  commands the cool valve  55  to open, thereby allowing the cooling fluid to flow from the inlet port  52 , through the cool valve  55  and into the drying chamber walls, thereby cooling the drying chamber  40  to the desired temperature level. As described above, by commanding the cool valve  55  to open, a cooling fluid flows around the drying chamber and evaporates in a heat exchange relationship with the drying chamber  40 . The computer system outputs a mode update message to a remote terminal (if connected) that the cool mode in the drying cycle has started. At S 1020 - 1040 , a determination is made whether the drying chamber  40  has reached the desired temperature. If not, the cool valve remains open. 
     After the specimen has been placed in the drying chamber  40  immersed in an intermediary fluid and the chamber cover  75  secured, at S 1110 , the computer system  99  commands the fill valve  56  to open, thereby allowing the transitional fluid to flow into the interior of the drying chamber  40 . This is known as the fill mode and the computer system  99  allows the transitional fluid to flow into the drying chamber  40  for a preset amount of time. The computer system outputs a mode update message to a remote terminal (if connected) that the first fill mode in the drying cycle has started. At the expiration of the preset time period, the computer system  99  commands the fill valve  56  to close. At S 1115  to S 1120 , a determination is made if the preset time period has expired. If not, at S 1120 , the fill valve remains open and the computer system  99  outputs a mode update message to a remote terminal (if connected) of the elapsed time since the first fill mode started. At S 1125  to S 1135 , an additional determination is made whether the drying chamber  40  has remained at the desired temperature. If not, as S 1130 , the cool valve is opened to cool the drying chamber  40  and the computer system  99  outputs a mode update message to a remote terminal (if connected) of the elapsed time since the first fill mode started. Otherwise, as S 1135 , the cool valve is closed to cool the drying chamber  40  and the computer system  99  outputs a mode update message to a remote terminal (if connected) of the elapsed time since the first fill mode started. 
     Next, at S 1140 , the computer system  99  executes the software instructions for purging the intermediary fluid from the drying chamber  40 . This is known as the purge mode, such that the transitional fluid completely fills the drying chamber  40  and purges the intermediary fluid from the treated specimen in the drying chamber  40 . The computer system outputs a mode update message to a remote terminal that the purge mode in the drying cycle has started. As shown by S 1145  to S 1150 , the purging of the dehydrating fluid is controlled over a predetermined time period. At S 1135 , the computer system  99  outputs a mode update message to a remote terminal of the elapsed time since the purge mode started. As shown by S 1155  to S 1165 , while the drying chamber  40  is being filled and purged, the computer system  99  maintains the drying chamber  40  at the predetermined temperature by controlling the cool valve  55 . At S 1160  and  1165 , the computer system  99  outputs a mode update message to a remote terminal of the elapsed time since the purge mode started. Preferably, the predetermined temperature is 0° C. When the computer system  99  has determined that the purging cycle has reached the end of its time period, at S 1270 , the fill valve  56 , the purge valve  63  and the cool valve  55  are closed. In addition, the computer system  99  outputs a mode update message to a remote terminal of the elapsed time since the purge mode started. The drying chamber  40  now should be completely filled by the transitional fluid. 
     Next, to ensure that the drying chamber  40  is completely filled, at S 1210  to S 1270 , the computer system  99  executes software instructions that cycle back into the fill mode following the purging of the intermediary fluid. At S 1210 , the computer system outputs a mode update message to a remote terminal that the second fill mode in the drying cycle has started. The cycling back into the fill mode ensures that the drying chamber  40  is completely filled by the transitional fluid. Therefore, the fill valve  56  is reopened by the computer system  99  to facilitate this task. During this fill, as shown by S 1240  to S 1260 , the computer system  99  monitors the temperature and can activate the cool valve  55  if the drying chamber  40  needs to be cooled. During this time, the computer system  99  outputs the elapsed time since the second fill mode started to a remote terminal (see S 1250  and S 1260 ). Once complete, at S 1270 , all the valves are commanded closed. 
     At S 1310  to S 1340 , after the computer system  99  has completed the second chamber fill, the drying chamber  40  is heated to pressurize the transitional fluid to its critical point as well as raise its temperature to the critical point. At S 1310 , the computer system outputs a mode update message to a remote terminal that the heating mode in the drying cycle has started. The computer system  99  executes software instructions to activate the heater  33  mounted in the wall of the drying chamber. The computer system  99  monitors the temperature to ensure that the chamber temperature does not exceed a preset limit. Preferably, this temperature limit is between 48 and 50° C. 
     The software instructions also comprise instructions that command the computer system  99  to indicate to the operator that the pressure and temperature are above the critical point equilibrium. Preferably, the computer system  99  causes the heat LED  19  to flash thereby indicating that the specimens in the drying chamber  40  have reached the critical point equilibrium. The critical point equilibrium is maintained for a programmed amount of time. At S 1320 , if the heat thermostatic sensor  30  and the high pressure sensor  67  indicate that the critical point equilibrium has not been reached, the software instructions command the computer system  99  to indicate to the operator that there is a problem with the critical point drying sequence. At S 1330 , the computer system  99  will flash all the operation indication LEDs to indicate a problem with the critical point drying sequence. The operator is then allowed to press a switch to return to an earlier stage in the process at which point the computer system  99  will then take over and complete the drying sequence. 
     At S 1410  to S 1420 , the computer system  99  executes software instructions to determine if the specimen has been at critical point equilibrium for a predetermined time period. At S 1410 , the computer system outputs a mode update message to a remote terminal that the critical point equilibrium in the drying cycle has started. At S 1420 , a determination is made whether the equilibrium time period has expired. If not, then, at S 1430 , the computer system  99  outputs the elapsed time since the critical point equilibrium was reached to a remote terminal. 
     At S 1510  to S 1520 , once the programmed amount of time at the critical point equilibrium has expired, the computer system  99  executes software instructions to bleed off the pressure in the drying chamber. At S 1510 , the computer system outputs a mode update message to a remote terminal that the bleed mode in the drying cycle has started. The computer system  99  opens the bleed valve  62  and controls the bleeding of the pressure in the drying chamber  40  down between 100 and 600 psi while maintaining drying chamber temperature above 31° C. or above. Preferably, the low pressure threshold is 400 psi. When the computer system  99  is executing the bleed mode, the computer system  99  controls the bleeding of pressure from the drying chamber  40  in an even fashion. In dropping from 1100 psi to 400 psi, the computer system  99  allows the pressure to vent slowly from the drying chamber  40 . In addition, the computer system  99  commands the heater  33  disposed on the drying chamber  40  to maintain the temperature of the drying chamber  40  at 31° C. or above. 
     At S 1610  to S 1620 , when the drying chamber pressure reaches 400 psi however, the software instructions command the computer system  99  to shut off the heaters and vent the remaining pressurized gas from inside the drying chamber  40  directly out the vent line. At S 1620 , the computer system outputs a mode update message to a remote terminal that the vent mode in the drying cycle has started. The purge valve  63  is opened and the bleed valve  62  is closed. The purge valve  63  will remain open until another run is commenced or the critical point drying apparatus  1  is powered down. 
     In another aspect of the present invention, the predetermined operations of the software instructions comprise instructions for conducting diagnostic testing of several sensors used to control the critical point drying sequence. The software instructions test the high pressure sensor  67  (or critical point pressure sensor), the low pressure sensor  68 , the heat sensor  31  and cool sensor  32 . The computer system  99  executing the software instructions indicates on the display if each of the above-identified sensors is operational. Preferably, the computer system  99  lights an operation indication LED that indicates to the operator that the sensor is working properly. 
     The predetermined operations of the software instructions also comprise diagnostic instructions that support the testing of switches, their corresponding operation indication LED switches and microprocessor interrupt service routine. For example, to test the purge LED switch  18 , pressing the purge LED switch  18  while the critical point drying apparatus  1  is powered up will cause the purge LED switch  18  to light if the computer system  99  is executing the software correctly. In another example, pressing the heat LED switch  19  when powering up the apparatus causes the operation indication LEDs to individually light in sequence, thereby indicating that the microprocessor clock is operating correctly. Preferably, the operation indication LEDs light sequentially in fifteen-second intervals. 
     The predetermined operations of the software instructions also comprise calibration instructions for the cool sensor  32 . The computer system  99  will cool the drying chamber  40  independently of the cool sensor  32 . The fill LED switch  17  will indicate the status of the cool sensor  32  and will allow the operator to determine if the cool sensor  32  is operating correctly or that it needs adjustment. 
     The predetermined operations of the software instructions also comprise instructions to ensure that the buildup of static electricity does not affect the operation indication LED switches. The computer system  99  constantly checks the status of the operation indication LED switches to ensure that they are operating properly. If an operation indication LED has prematurely switched off, the computer system  99  determines at what point of the critical point drying sequence the critical point drying apparatus  1  is in, and then switches on the appropriate operation indication LED. 
     Another aspect of the present invention will now be discussed. The present invention may be embodied on a computer program product for enabling a computer system to perform critical point drying techniques when coupled to a critical point drying apparatus. The software instructions that enable the computer system to perform predetermined operations as required by the present invention are borne on a computer readable medium. The predetermined operations borne on the computer program product comprise software instructions for cooling the drying chamber to a first chamber temperature. Preferably, the first chamber temperature is between 5 and −10° C. The software instructions further enable the computer system to output a mode update message to a remote terminal, thereby indicating that the cool mode of the drying process has started. 
     The predetermined operations borne on the computer program product further comprise software instructions for filling the drying chamber  40  with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber  40  at the first chamber temperature such that the transitional fluid completely displaces the intermediary fluid within a first time period. As noted above, the first chamber temperature is preferably between 5 and −10° C. The software instructions further enable the computer system to output a mode update message to a remote terminal and indicate that the first fill mode of the drying process has started. The software instructions also enable the computer system to output the elapsed time since the first fill mode started. 
     The predetermined operations borne on the computer program product maintains the drying chamber  40  at the first chamber temperature while the intermediary fluid is exhausted from the interior of the drying chamber  40 . The software instructions further enable the computer system to output a mode update message to a remote terminal and indicate that the purge mode of the drying process has started. The software instructions also enable the computer system to output the elapsed time since the purge mode started. 
     When the intermediary fluid is purged, the predetermined operations borne on the computer program product ensure that the drying chamber is completely filled with transitional fluid to ensure a successful drying cycle. This is done by going through a quick fill cycle. The software instructions further enable the computer system to output a mode update message to a remote terminal and indicate that the second fill mode of the drying process has started. The software instructions also enable the computer system to output the elapsed time since the second fill mode started. 
     The predetermined operations borne on the computer program product further comprise software instructions for activating at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching a critical point equilibrium. Preferably, the predetermined operations borne on the computer program product command a heater  32  on the drying chamber  40  to heat the transitional fluid to at least 31° C. or above. The software instructions further enable the computer system to output a mode update message to a remote terminal and indicate that the heat mode of the drying process has started. The predetermined operations borne on the computer program product also notify the operator if the critical point equilibrium was successfully reached. 
     The predetermined operations borne on the computer program product further comprise software instructions for maintaining the transitional fluid at the critical point equilibrium for a second time period. The predetermined operations borne on the computer program product notify the operator that the transitional fluid in the drying chamber is at its critical point equilibrium. The software instructions further enable the computer system to output a mode update message to a remote terminal and indicate that the critical point equilibrium of the drying process was reached. The software instructions also enable the computer system to output the elapsed time since the critical point equilibrium was reached. 
     The predetermined operations borne on the computer program product further comprise software instructions for bleeding the transitional fluid from the drying chamber  40  while maintaining the drying chamber  40  at the second chamber temperature. Preferably, the predetermined operations borne on the computer program product command the at least one heater  32  to maintain a drying chamber temperature of at least 31° C. while the transitional fluid is bled from the drying chamber  40 . The software instructions further enable the computer system to output a mode update message to a remote terminal and indicate that the bleed mode of the drying process has started. The software instructions further enable the computer system to output a mode update message to a remote terminal and indicate that the vent mode of the drying process was reached. 
     Another aspect of the present invention will now be discussed. The present invention may be embodied on an article of manufacture, which comprises a computer readable medium having stored therein a computer program to control a drying chamber during a critical point drying process. The article of manufacture comprises a computer program product that bears a first computer code segment which, when executed on a computer, cools the drying chamber  40  to a first chamber temperature. Preferably, the first chamber temperature is between 5 and −10° C. 
     The article of manufacture further comprises a second computer code segment which, when executed on a computer, fills the drying chamber  40  with a transitional fluid having a critical point temperature and critical point pressure while maintaining the drying chamber  40  at the first chamber temperature such that the transitional fluid completely displaces the intermediary fluid within a first time period. As noted above, the first chamber temperature is preferably between 5 and −10° C. The second computer code segment, when executed on a computer, maintains the drying chamber  40  at the first chamber temperature while the intermediary fluid is exhausted from the interior of the drying chamber. When the intermediary fluid is purged, the second computer code segment, when executed on a computer, ensures that the drying chamber  40  is completely filled with transitional fluid to ensure a successful drying cycle. 
     The article of manufacture also comprises a third computer code segment which, when executed on a computer, activates at least one heater to raise the transitional fluid to its critical point pressure and critical point temperature, thereby reaching a critical point equilibrium. Preferably, the third computer code segment commands a heater  32  on the drying chamber  40  to heat the drying chamber  40  to at least 31° C. or above. The third computer code segment, when executed on a computer, also notifies the operator if the critical point equilibrium was successfully reached. 
     The article of manufacture further comprises a fourth computer code segment which, when executed on a computer, maintains transitional fluid at the critical point equilibrium for a second time period. The fourth computer code segment, when executed on a computer, notifies the operator that the transitional fluid in the drying chamber is at its critical point equilibrium. 
     Finally, the article of manufacture further comprises a fifth computer code segment which, when executed on a computer, bleeds the transitional fluid from the drying chamber  40  while maintaining the drying chamber  40  at the second chamber temperature. Preferably, the fifth computer code segment, when executed on a computer, commands the at least one heater  32  to maintain a drying chamber temperature of at least 31° C. while the transitional fluid is bled from the drying chamber  40 . 
     Referring to FIG. 17, an exemplary network according to the present invention is illustrated. As described earlier, the computer system  99  comprises a microprocessor  100 , a memory  101  and an I/O port  102 . The I/O port  102  provides access to a network for the computer system  99 . The I/O port  102  supports a variety of standard protocols, i.e., FTP, HTTP, TCP/IP, Ethernet, etc. for information transmission. The I/O port  102  is connected to a server  110  and the server  110  handles the mode update messages and the elapsed time messages from the computer system  99 . The server  110  is connected to a plurality of client terminals  111 - 113 . These client terminals are the remote terminals that receive the mode update messages and the elapsed time messages sent from the computer system  99 . 
     Referring to FIG. 18, a status display  120  for the mode update messages and the elapsed time messages sent from the computer system  99  is illustrated. The status display  120  is an exemplary display for a client terminal, and additional information could be added to the status display  120  if desired by the user. The status display  120  comprises a status region  125 . The status region  125  lists all the modes that the critical point drying apparatus  1  will cycle through. The mode that is currently in use will be indicated in some manner, i.e., highlighting, underlining, bolding, flashing, etc. As shown in the status region  125 , certain drying modes also have an elapsed time indicator. In the elapsed time message from the computer system  99 , the length of time allotted to the particular mode is included along with the amount of time that has passed since the mode started. This allows the user, at his client terminal, to see how much time has passed since the mode started, or how much time is left for the mode to finish execution. 
     The status display  120  also has a user name region  130 . An operator inserts his/her name into the new user name region  131 , and clicks on the update button  132 . The name of the new operator will appear in the user name region  130 . In addition, the operator can input his/her contact information into the new contact information region  133 , and clicks on the update button  132 . The contact information of the new operator will appear in the user name region  130 . 
     The foregoing description of the exemplary embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated. 
     Thus, while only certain embodiments of the present invention have been specifically described herein, it will be apparent that numerous modifications may be made thereto without departing from the spirit and scope of the present invention. Further, acronyms are used merely to enhance the readability of the specification and claims. It should be noted that these acronyms are not intended to lessen the generality of the terms used and they should not be construed to restrict the scope of the claims to the embodiments described therein.