Abstract:
An apparatus for treating a substrate with a cryogenic impingement fluid includes a protective enclosure defining an internal cavity, a cryogenic fluid applicator positioned within the internal cavity and a snow generation system connected to the cryogenic fluid applicator. The snow generation system includes a condensing subsystem and a diluent or propellant gas subsystem. Each subsystem is connectable to a common gas source. The condensing subsystem includes a condenser for condensing liquid carbon dioxide into solid carbon dioxide particles, or dry ice snow. The condenser includes at least two segments of differing diameter connected to one another. Liquid carbon dioxide is introduced into the smaller diameter first segment and upon entering the larger diameter second segment, solidifies into dry ice particles. The dry ice particles, along with diluent or propellant gas produced from the diluent subsystem, are delivered to the cryogenic fluid applicator via a coaxial delivery tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
   This application claims the benefit U.S. Provisional Patent Application No. 60/635,400 entitled MEHTOD AND APPARATUS FOR SELECTIVELY TREATING AND INSPECTING A SUBSTRATE filed on 13 Dec. 2004 which is hereby incorporated herein by reference. 

   BACKGROUND OF INVENTION 
   The present invention generally relates to the field of environmental control for performing cryogenic spray cleaning processes. More specifically, the present invention is directed at cleaning or treating miniature electromechanical device surfaces with cryogenic impingement sprays. 
   Conventional precision cleaning processes using cryogenic particle impingement sprays, such as solid phase carbon dioxide, require control of the atmosphere containing a treated substrate to prevent the deposition of moisture, particles or other such contaminants onto surfaces during and following cleaning treatments. Environmental control is required because of localized atmospheric perturbations created by the low temperatures and high velocities which are characteristic of these impingement cleaning sprays. 
   For example, snow particles having a surface temperature of −100 F and traveling through the space between a spray nozzle and a substrate are continuously sublimating in transit and upon impact with the substrate. This rapidly lowers local ambient atmospheric temperature causing contaminants contained therein to condense or “rain-out” of the local atmosphere and onto treated substrate surfaces during or following spray treatments. Moreover, by way of the Bernoulli effect, the cleaning spray stream exhibits lower internal pressure than the surrounding atmosphere which creates venturi currents adjacent to the flow of the stream. These venturi currents cause the local atmosphere surrounding the stream to collapse into the spray stream above the substrate, thus entraining and delivering a mixture of cleaning spray and atmospheric constituents to the substrate. Finally, static charge build-up and accumulation are common to cryogenic sprays due to dielectric and triboelectric characteristics. This presents problems including, for example, potential device damage from electrostatic overstress or electrostatic discharge, and attraction of atmospheric contaminants to treated substrates via electrostatic attractive forces. 
   Micro-environmental control technology is well established and many techniques have been developed over the years to isolate either a process, a substrate or a worker. The purpose of isolation generally includes protecting workers from toxic chemicals, protecting clean rooms from particles, or protecting delicate processes and substrates from the outside environment. 
   There are many examples of techniques to control thermal and electrostatic effects during cryogenic impingement sprays using secondary heated or ionized jets or sprays above the substrate surface and delivered either independently or as a component of the cryogenic spray have been used commercially. For example, U.S. Pat. No. 5,409,418 issued to Krone-Schmidt et al. and U.S. Pat. No. 5,354,384 issued to Sneed et al. suggest direct heated or ionized gas impingement techniques and apparatus for heating, purging and deionizing substrate surfaces. The &#39;384 patent suggests the use of a heated gas, such as filtered nitrogen, to provide a pre-heat cycle to a portion of a substrate prior to snow spray cleaning and a post-heat cycle to the substrate following the snow cleaning. This approach relies on “banking heat” into the substrate portion prior to cryogenic spray cleaning by delivering a heated gas stream to a portion of substrate to prevent moisture deposition and adding heat from a heated gas following cryogenic spray treatment. The &#39;384 patent is primarily useful for removing high molecular weight materials such as waxes and adhesive residues having weakened cohesive energy from surfaces by partially melting or softening them prior to spray treatment. However, the approach of the &#39;384 patent does not work well for most substrate treatment applications because many materials being cleaned, or at least portions thereof, have low thermal conductivity, low mass or because highly thermal conductive materials rapidly lose heat to the sublimating snow during impact. This tends to create localized cold spots on even a mostly hot bulk substrate. Examples of such substrates include ceramics, glasses, silicon and other semi-conductor materials, as well as most polymers. Additionally, many electromechanical devices being cleaned are relatively small, providing no appreciable mass for storing heat. Such examples include photodiodes, fiber optic connectors, optical fibers, end-faces, sensors, dies, and CCD&#39;s, among many others. 
   Most significantly, directing a heating spray, or any secondary fluid for that matter, directly at or incident to the substrate surface during and/or following cryogenic cleaning spray treatments causes the entrainment, delivery and deposition of atmospheric contaminants as discussed above. This necessitates housing the cryogenic spray applicator, substrate and secondary gas jets in large, bulky and complex environmental enclosures employing HEPA filtration and dry inert atmospheres, such as included in U.S. Pat. No. 5,315,793, issued to Peterson et al. 
   In the &#39;418 patent, an apparatus is taught for surrounding the impinging cryogenic spray stream with an ionized inert gas. It is proposed that by surrounding a stream of solid-gas carbon dioxide with a circular stream of ionized gas and applying the two components to the substrate simultaneously controls or eliminates electrostatic discharge at the surface during impingement. However, as also suggested by the &#39;384 patent, the &#39;418 patent suggests secondary stream that entrains, delivers and deposits atmospheric contaminants upon the substrate surfaces being treated. Moreover, contact of the ionizing gas with the stream prior to contact with the surface rapidly eliminates ion concentration and is ineffective in controlling electrostatic dishcarge. Still moreover, using the ionizing spray of the &#39;418 patent independent of the snow spray and which is directed at an angle incident to the surface will further re-contaminate the substrate unless, as taught in the &#39;793 patent, the entire operation is performed in a controlled HEPA filtered chamber. 
   As devices become smaller and their complexity increases, it is clearly desirable to have a improved processing technique, including a method and apparatus, that enables the use of environmentally safe cleaning agents to remove unwanted organic films and particles. It is desirable to have a technique which prevents additional particles and residues from being deposited on critical surfaces during application of said impingement cleaning sprays. The complete environmental control technique should include all of the basic environmental controls of thermal control, ionization control, and providing a dry and particle free cleaning atmosphere, but not negatively impacting the performance of the impinging cleaning spray. Moreover it would be highly desirable to have a cleaning capability integrated with the aforementioned controlled environment which provides a compact in-line or bench-top critical cleaning solution for manufacturing operations. 
   BRIEF SUMMARY OF INVENTION 
   The apparatus of the present invention includes a protective enclosure within which is positioned a cryogenic fluid applicator for treating and inspecting a substrate placed therein. The protective enclosure is partially open to the atmosphere and includes a filtered air circulation system and ionization mechanism to provide for a partially-pressurized, heated and ionized re-circulated atmosphere within the protective enclosure to prevent contamination of the substrate. The re-circulated atmosphere flows at a controlled velocity in a manner consistent with the geometry of the cavity and substrate being treated so as not to produce undue turbulence and erratic flow lines within the cavity. The substrate may be held within the cavity by means of a vacuum fixture, operator hands or other suitable fixture. Alternatively, the substrate may be inserted within the partial enclosure, treated and removed using an external robot or conveyed through each side using an automated track. 
   The present invention further includes a snow generation system connected to the cryogenic fluid applicator. The snow generation system includes a stepped capillary condenser having at least two connected segments of tubing with differing diameters to provide increased Joule-Thompson cooling in the conversion of liquid carbon dioxide to solid carbon dioxide, which reduces clogging and sputtering, improves jetting, and allows for greater spray temperature control. Moreover, the stepped capillary condenser produces coarser particles than a single step capillary. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of the present invention. 
       FIG. 2  is a side-view of the present invention taken along lines A—A in  FIG. 1 . 
       FIG. 3  is an illustrated perspective view of a carbon dioxide snow treatment apparatus of the present invention. 
       FIG. 4  is a partial cross sectional view of the carbon dioxide snow treatment apparatus of  FIG. 3 . 
       FIG. 5  is an illustrated perspective view of an alternative embodiment of a snow treatment apparatus of the present invention. 
       FIG. 6  is a partial cross sectional view of the alternative embodiment of a snow treatment apparatus of  FIG. 5 . 
       FIG. 7  is a perspective view of the present invention illustrating the incorporation of a conveyor belt. 
   

   DETAILED DESCRIPTION 
   An apparatus to selectively treat and inspect a substrate is generally indicated  10  in  FIGS. 1 and 2 . The apparatus  10  includes a protective enclosure  12  which defines a mini-environment or cavity  14  for providing an instantaneous curtain or sheath of re-circulated and controlled atmosphere when treating or inspecting substrates  16  positioned therein. The protective enclosure  12  includes a ceiling,  18  walls  20 , base  22  and removable electrostatic-discharge dissipative side panels  24 , all of which provide a partial enclosure about the substrate  16  during processing and thus forming the cavity  14  therein. Each side panel  24  includes an upper aperture  26  containing a pane of transparent material  28  to allow further lighting within the cavity  14 . The protective enclosure  12  is designed to have a portion open to the ambient atmosphere for insertion of the substrate  16  to be treated. The enclosure  12  may be constructed of any variety of materials including, but no limited to, metals, ceramics, glasses and conductive or electrostatic-discharge dissipative polymers, and combinations thereof. While it is preferable that the protective enclosure  12  include a substantially box-style configuration, it should be noted that the protective enclosure  12  may be formed of any geometrical shape in order to accommodate the substrate  16  to be treated. The substrate  16  may be held within the cavity  14  by means of a vacuum fixture (not shown), operator hands or other suitable fixture. Alternatively, the substrate  16  may be inserted, articulated, cleaned and removed using an external robot or conveyed through each side using an automated track, as will be discussed in greater detail. 
   A re-circulated atmosphere  30 , which may be ionized, flows at a controlled velocity in a manner consistent with the geometry of the protective enclosure  12  and substrate  16  being treated so as not to produce undue turbulence and erratic flow lines within the cavity  14 . Thus the airflow may be circular, rectangular or any other shape as desired to form the appropriate flow patterns within the open cell cavity  14 . Still moreover, the protective enclosure  12  may be designed to be interchangeable to accommodate any number of substrates  16  and substrate geometries, such as reel-to-reel substrates (not shown). The internal cavity  14  is further bounded above and below, respectively, by a regenerated heated clean air outlet plenum  34  positioned within the ceiling  18  and a return air plenum  36  positioned within the base  22  for capturing contaminated air. A regenerated and heated atmosphere  30  is derived by re-circulating air from the perforated return air plenum  36 . The regenerated atmosphere  30  is fed through an integrated heater-blower motor  38  and through a filter cartridge  40 . The filter cartridge  40  is preferably an ultra low penetration air (ULPA) filter, however, other suitable filters known in the art are well within the scope of the present invention. The regenerated atmosphere  30  flows in a circular motion from the outlet plenum  34 , through the cleaning cavity  14 , and down through the return plenum  36 . Alternatively, various baffles or diffusers (not shown) may be affixed to the outlet plenum  34  to re-direct or diffuse clean air flow over the substrate  16 . The apparatus  10  of the present invention further includes an internal point ionizer  42  positioned within cleaning cavity  14  to provide DC, AC or photon ionization  44  to the clean air flow  30 . The ionizer  42  is powered by an ionization power supply  46  connected via a power cable  48  to the ionizer  42 . The regenerated atmosphere  30  re-circulates between the space comprising above cavity ceiling, along cavity walls, and downward through the return plenum  36  in the base  22  of the protective enclosure  12  resulting in the substrate  16  being contained between the ceiling  18 , walls  20 , and base  22 , protected from ambient atmosphere in a sheath of clean dry ionized atmosphere. 
   To treat the substrate  16 , a carbon dioxide spray treatment nozzle  50  is positioned within the enclosure  12  by means of a bracket  52 . The spray treatment nozzle  50  is preferably positioned such that an emitted spray  54  is directed at a suitable angle and distance from the exemplary substrate  16  to perform the snow treatment operations. The spray treatment nozzle  50  is preferably a co-axial nozzle as taught by the present inventor and fully disclosed in U.S. Pat. No. 5,725,154, which is hereby incorporated herein by reference. More preferably, the spray treatment nozzle is a tri-axial type delivering apparatus as taught by the present inventor and fully disclosed in U.S. Provisional Application No. 60/726,466, which is also hereby incorporated herein by reference. It should be noted, though, that any type of nozzle capable of emitting carbon dioxide, in either solid or plasma phases, is well within the scope of the present invention. 
   A proximity sensor  56  is also positioned within the cavity to detect the presence of the substrate  16  to automatically start or stop the heater-blower motor  38  and ionizer  42 . Also connected to the apparatus  10  are a supply of clean-dry-air or CDA  58 , a supply of carbon dioxide liquid or gas  60  and a source of electrical power  62 . An electronic actuator, such as a footswitch  64 , is connected to the apparatus  10  using a suitable electronic control cable  66 . 
   An inspection device  68 , including for example a stereo microscope or CCD camera and monitor, is removably affixed to a front panel  70  by means of a mounting bracket  72  to be in visual communication with the spray applicator  50  and substrate  16 . Alternatively, the inspection device  68  can be situated using a separate stand (not shown). To aid in the inspection, a light source  78  is connected to the inspection device  68  using a ring light  80 . To prevent an operator  84  from introducing human contaminants such as skin or hair into the micro-environment during cleaning and inspection operations, a transparent sneeze guard  86  is included. The operator may be grounded via a wrist strap  88  and grounding element (now shown) through a suitable ground connection plug  90  which provides electrostatic discharge protection for the substrate  16  being treated by the operator  84 . Alternatively, the grounding element (not shown) may be connected directly to the exemplary substrate  16  being treated and inspected. For further grounding of the apparatus  10 , a common grounding bus is provided internally which is connected to a suitable ground  94 . 
   In operation, the operator  84  positions the substrate  16  within the cleaning cavity  14 . Upon so doing, the proximity sensor  56  activates to turn on the heater-blower motor  38  and ionizer  42 . The operator  84  then depresses the footswitch  64  to activate a snow generation system  320  or  340 , whereby high-velocity snow particles travel from the system via delivery conduit  32  and emit from spray applicator to be directed at the substrate  16  for treatment. Preferably, the snow treatment system  320  or  340  is that as taught by the present inventor and fully disclosed in U.S. application Ser. No. 11/301,442 entitled CARBON DIOXIDE SNOW APPARATUS, filed concurrently with the present application and claiming priority from U.S. Provisional Application No. 60/635,230, both of which are hereby incorporated herein by reference. 
   The carbon dioxide snow treatment system  320  is generally indicated at  320  in  FIG. 3 . A dense fluid  330 , preferably liquid carbon dioxide, enters the capillary condenser  326  whereupon passing therethrough, or in conjunction with the applicator  322 , is condensed and solid carbon dioxide snow  332  exits the mixing spray nozzle along with the propellant gas  328  or any uncondensed carbon dioxide. Referring to  FIG. 4 , the capillary condenser  326  includes a capillary tube  334  covered by suitable insulation  336 , such as such as for example, 0.318 cm (0.125 inch) of self-adhering polyurethane insulation foam tape as supplied by Armstrong World Industries, Inc. of Lancaster, Pa., which is wrapped about the capillary tube  34  in a helical fashion with 50% overlap. The capillary tube  334  includes segmented capillaries  338  that have step-wise increasing diameters, indicated by d 1 , d 2 , d 3  and d 4 , respectively, which increase in a feed-wise direction, indicated by arrow A. Thus, d 1 &lt;d 2 &lt;d 3 &lt;d 4 . It should be noted, though, that capillary tube  334  of  FIG. 4  is for illustrative purposes only, and that the capillary tube  334  of the present invention need only include at least two segments  338 , and it is well within the scope of the present invention to provide a capillary tube  334  with three or more segments  38  as well, depending upon the particular application. The capillary  334  is preferably constructed of a PolyEtherEtherKetone (PEEK) polymer. However, other suitable tubular materials are well within the scope of the present invention including, but not limited to, Teflon® or other clean and flexible materials. As stated, the capillary condenser tube  334  includes at least two segments  338 , with each segment  338  preferably having a length ranging from 0.3 m (1 foot) to 7.32 m (24 feet) and inside diameters ranging from 0.127 mm (0.005 inches) to 3.175 mm (0.125 inches). Such tubing should be able to withstand propellant gas pressures ranging up to about 7 MPa (1000 psi) and temperatures ranging between 203 K and 473 K. The interconnections  339  between the segments may be Swagelok or finger-tight compression fittings. 
     FIGS. 5 and 6  illustrate an alternative carbon dioxide snow treatment apparatus  340  of the present invention including a flexible capillary condenser  342  connected to a divergent/convergent nozzle  344 . The capillary condenser  342  similarly includes a capillary tube  346  having segmented capillaries  348   a ,  348   b ,  348   c  and  348   d  that have step-wise increasing diameters d 1 , d 2 , d 3  and d 4 , respectively, which increase in a feed-wise direction, indicated by arrow B. The capillary  342  is preferably constructed of PEEK polymer. However, other suitable tubular materials are well within the scope of the present invention including, but not limited to, Teflon® or other clean and flexible materials. As stated, the capillary condenser tube  342  includes at least two segments  348 , with each segment  348  preferably having a length ranging from 0.3 m (1 foot) to 7.32 m (24 feet) and inside diameters ranging from 0.127 mm (0.005 inches) to 3.175 mm (0.125 inches). Such tubing should be able to withstand propellant gas pressures ranging up to about 7 MPa (1000 psi) and temperatures ranging between 203 K and 473 K. The interconnections  349  between the segments may be Swagelok or finger-tight compression fittings. The capillary tube  342  is positioned within a propellant gas tube  350 . A heated propellant gas  352  is carried within the flexible propellant delivery tube  350  to the nozzle  344 . The propellant tubing  350  may be constructed of any number of suitable tubular materials including Teflon, Stainless Steel overbraided Teflon®, Polyurethane, Nylon, among other clean and flexible materials having lengths ranging from 0.3 m (1 foot) to 7.3 m (24 feet) or more and inside diameters ranging from about 0.65 cm (0.25 inches) to about 1.3 (0.50 inches). Such tubing  346  should be able to withstand propellant gas pressures ranging between about 0.07 MPa (10 psi) and 1.72 MPa (250 psi) and temperatures ranging between 293 K and 473 K. The exemplary flexible condenser  342  of the alternative embodiment  340  is terminated with the rigid mixing spray nozzle  344  which contains a convergent mixing nozzle portion and a divergent expansion nozzle portion (not shown) as is known in the art. Dense fluid  353 , preferably liquid carbon dioxide, enters the capillary assembly  346  and forms carbon dioxide snow particles as the carbon dioxide progresses through the at least two capillary segments  348 . Upon entering the nozzle  344 , carbon dioxide snow particles discharge from the capillary condenser assembly  346 , mixing with propellant gas  352  discharged from the propellant aerosol tube  350 , thus forming a solid-gas carbon dioxide spray  354 . The carbon dioxide aerosol spray  354  discharges from the nozzle  344  and is selectively directed at a substrate surface (not shown). 
   Being that both embodiments  320  and  340  include similar stepped capillary assemblies  334  and  346 , respectively, reference to one shall include reference to the other and all their like parts, for purposes of convenience, unless stated otherwise. Capillary segments  338  are constructed to have increasing, or stepped, diameters in the direction of flow because it has been discovered that by providing stepped capillaries of increasing diameter, certain performance advantages over single capillary diameters are resulted. For instance, when employing carbon dioxide as the dense fluid, larger and harder snow particles can be generated from a relatively smaller feed supply of carbon dioxide. Also, starting with an internal capillary diameter as little about 0.5 mm (0.020 inches) in the first capillary segment, restricted flow into and down the capillary condenser tube is resulted. It has also been discovered that by manipulating the number of steps and incrementally increasing the capillary step diameters, various ranges of solid phase particle size distribution can be produced. Stepped capillary condensation more efficiently condenses the liquid and vapor to solid through sharp near-isobaric expansion cooling while also producing a more desirable range of impact shear stresses. 
   However, it should be noted that any system for producing carbon dioxide snow is well within the scope of the present invention. The operator  84  can view the treatment process and inspect the substrate  16  either through direct vision or with assistance of the inspection device  68 . 
   A control panel  96  contains all the necessary control valves, pressure regulators, gauges and switches necessary to monitor and control the spray cleaning process. The control panel  96  contains a main power switch  98  which activates the entire system, a spray mode switch  100  which switches spray cleaning operations from continuous spray cleaning mode to stand-by mode or to pulse cleaning mode. The exemplary control panel  96  also contains a carbon dioxide pressure gauge  102  and a CDA or propellant pressure gauge  104 . The control panel  96  contains a pulse cycle switch  106  which varies and controls the spray cleaning pulse rate in sub-second pulse increments from 1 to 10 cycles per second or more. A propellant pressure regulator  108  is included to control the carbon dioxide spray pressure from between 0.07 MPa (10 psi) and 1.72 MPa (250 psi) and a carbon dioxide snow metering valve  110  to control carbon dioxide snow flow from zero to about 45 Kg (100 pounds) per hour or more. Finally, the control panel  96  features a digital temperature controller  112  to control the spray propellant temperature between 20 C and 200 C. 
   Alternatively, and referring to  FIG. 7 , an automatic in-line cleaning conveyor  116  is incorporated. Upon incorporating the in-line cleaning conveyor, side panels  24  include lower apertures  118  that allow the conveyor  116  and substrates  16  to pass therethrough during operation. Also, a process indicator light  120  is included to indicate the operating mode of the cleaning system along with a machine controller (not shown) to coordinate operations between the conveyor  116  and the spray cleaning nozzle  50 . In operation, the conveyor  116  travels through the lower apertures of the side panels  24  and into the cavity  14  of the cleaning system to position each substrate  16  proximate to the spray applicator  50 . The conveyor  116  may proceed continuously through the cleaning cavity  14 , or may pause momentarily at selected intervals to allow the spray applicator  50  to adequately treat each substrate  16 . After treatment, the conveyor  116  carries the treated substrate  16  out of the cavity  14  and to the next stage in the processing, if any. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.