Abstract:
A nozzle and method of providing CO 2  for cleaning includes providing a CO 2  flow; phase transferring the CO 2  flow into gaseous CO 2  and CO 2  pellets; interrupting the CO 2  flow with a screen member; retaining the CO 2  pellets of a select larger size upstream of the screen member; permitting the CO 2  pellets of a select smaller size to pass through the screen member for cleaning.

Description:
BACKGROUND OF THE INVENTION 
     The present inventive embodiments relate to nozzles for removing particle impurities from surfaces and structures during for example wafer processing without damaging the structures themselves. 
     The use of carbon dioxide (CO 2 ) spray to remove particles sized from micrometers to as small as nanometers from surfaces has emerged in cleaning technology as an acceptable replacement for Freon cleaning. Effective delivery of CO 2  will remove the particulate contamination and organic films (light oils, silicone lubricants, etc.) by momentum transfer between the CO 2  snow and contaminant. 
     CO 2  snow is used to clean for example optical components. In general, the CO 2  jet needs to be controlled in its combination of solid snow called “pellets” of CO 2  (i.e. dry ice) and gaseous CO 2 . The CO 2  snow properties such as size, velocity, density and flux can be controlled by the design of the nozzle employed, as well as by other characteristics of the CO 2  at the CO 2  source such as pressure and temperature. Carbon dioxide snow cleaning is dry, nonabrasive, chemical-free and residue-free, thus making this cleaning procedure attractive for many critical cleaning applications. 
     In a conventional nozzle, the nucleation, growth and compacting of CO 2  snow takes place after expansion through an orifice separating inlet liquid CO 2  (at approximately 800 psi) from the expanding gas phase in the downstream cavity of the nozzle (so called barrel). The CO 2  plume contains snow pellets with sizes that can exceed 50 micrometers (μm) in diameter, and yet still have velocities comparable to smaller size snow pellets. These larger pellets (whether they are CO 2  or a contaminant initiating from the CO 2  source tank) travel with a momentum sufficiently large to damage fine structures of the workpiece being cleaned or processed, if the number and momentum of the particles additively exceeds the device damage threshold value. 
     CO 2  molecules can coalesce or agglomerate onto a CO 2  assembly. This agglomeration of the CO 2  molecules occurs during passage through the orifice or directly after, where the liquid CO 2  converts to the CO 2  gas phase in the downstream barrel-like tube. Additionally, contaminants may build up or accumulate on the wall of the barrel along with the CO 2  agglomerates. “Adders” result from contaminants on the CO 2  source which accumulate from agglomeration in the nozzle and then are deposited on a surface of the workpiece or wafer being cleaned. The adders are thus transported in the CO 2  stream from the nozzle onto the object or surface to be cleaned. Such adders when discharged from known nozzles can cause contamination by adhering to the very surface that the nozzle is being employed to clean and can possibly damage the surface as well. Mitigating the agglomeration at the barrel interior surface would correspondingly reduce if not eliminate the adders and problems associated therewith. 
     To overcome this deficiency, it is known to purify a CO 2  reservoir to less than parts per billion (“ppb”) if at all possible, and to clean and degrease an interior surface of the nozzle of lubricants which were used during machining and drilling by electropolishing, extrusion techniques, etc., to construct the nozzle. Of course, some residue, such as a film layer, may remain from nozzle fabrication disposed at an inner surface of the nozzle, which residue is reduced to an extent by baking-out the nozzle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present embodiments, reference may be had to the description of the embodiments which follow taken in connection with the accompanying drawings, of which: 
         FIG. 1A  shows a cross-sectional plan view of a nozzle embodiment; 
         FIG. 1B  shows another cross-sectional plan view of another nozzle embodiment; 
         FIG. 1C  shows still another cross-sectional plan view of a nozzle embodiment; 
         FIGS. 2A-2C  show screen or sieve elements which can be used with the nozzle embodiments of  FIGS. 1A-1C ; 
         FIG. 3A  shows a cross-sectional plan view of another nozzle embodiment; 
         FIG. 3B  shows a cross-sectional plan view of another nozzle embodiment; 
         FIG. 4  shows a cross-sectional plan view of another nozzle embodiment for reducing electrical charge of the CO 2 ; 
         FIG. 5  shows a cross-section view of another nozzle embodiment; 
         FIGS. 6A-6D  show cross-sectional plan views of other nozzle embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present nozzle embodiments of the  FIGS. 1-6  reduce the amount of contaminants in the CO 2  plume or stream used for cleaning, by trapping the contaminants and/or reducing the size of the contaminants, and also controlling the size, velocity and flux of the CO 2  pellets in the plume or stream. 
     For this purpose, the CO 2  reservoir is purified to less than ppb and the connecting tube to the nozzle is cleaned by Acetone rinse, Isopropyl Alcohol (IPA) rinse, followed by a 24 hour bake-out at nearly 300° C. in a nitrogen (N 2 ) purged environment. The nozzle is initially degreased from lubricants used in machining and drilling by electropolishing, and extrusion techniques. The subsequent precision cleaning procedure includes hot ultra pure de-Ionized (DI) water rinse, acetone bath with ultrasonic, IPA bath with ultrasonic, followed by hot DI water ultrasonic bath. Even with such elaborate and careful cleaning methods some residue will remain as thin film layers disposed on an inner wall of the nozzle. Additional and final cleaning steps are required to mitigate these residues, a twenty-four hour bake-out of the nozzle at nearly 300° C. in a N 2  purge environment is carried out first, and subsequently ultra-high purity CO 2  is run through the nozzle (purged) for a sufficient period of time, after which the plume or exhaust of CO 2  from the nozzle evaluated by counting the “adders” on a clean silicon wafer before and after exposure to the CO 2 . The total adders from the nozzle will approach zero as the remaining residual contamination is reduced. The nozzle is determined to be qualified for a cleaning application when the total adders present is zero for a predetermined size of adders. 
     The present nozzle embodiments control the CO 2  particle size and velocity distribution as well as the flux of CO 2  (molecules or agglomerated particles) emitted from the nozzle to allow the CO 2  plume or CO 2  molecular beam to clean the contaminated workpiece without damaging the structures on the workpiece. The nozzle traps contaminates existing along a pathway of the CO 2  stream from being carried by the CO 2  plume to the workpiece. 
     The present embodiments prevent the nucleated CO 2  snow particles and contaminants, having larger than a specific diameter, from reaching the workpiece. This is accomplished by disposing a screen or sieve for preventing the large CO 2  particles from being emitted from the nozzle to the workpiece. The screen can be of different constructions as discussed below and disposed downstream of a nozzle orifice at select locations in a barrel of the nozzle. The size of the openings in the screen determines the maximum size of snow particles (and contaminants) that can escape through the screen. All dimensions recited below are by way of example only. 
     Referring to  FIGS. 1A-1C , there is shown a nozzle  10  which includes a housing  12  formed by a sidewall  14 . The sidewall  14  is of metallic construction. The housing  12  includes a reservoir portion  16  and a barrel portion  18 . The reservoir and barrel portions  16 ,  18 , respectively, may be formed as an integral unit or alternatively the barrel portion  18  may be press fit, soldered or welded to the reservoir portion  16 . 
     The reservoir portion  16  of the housing  12  includes a chamber or reservoir  20  in which liquid CO 2  is provided. The barrel portion  18  includes a passageway  22 , which extends to an outlet  23  of the barrel  18 . An orifice or port  21  of the reservoir portion  16  interconnects the reservoir  20  with the passageway  22 . The reservoir  20  is connected to a source (not shown) of liquid CO 2 . 
     The barrel portion may have a length “L” of up to as much as 1-1.5″ (2.54-4.0 cm). An internal diameter “D” of the passageway  22  may range from 0.05-0.5 inches (1 mm-13 mm). 
     Referring to  FIGS. 2A-2C , there is shown a plurality of screens or sieves  24 A,  24 B,  24 C (collectively “ 24 A- 24 C”), each of which is constructed for being disposed in the passageway  22  of the barrel portion  18  of the nozzle  10 , as shown in  FIGS. 1A-1C  for example. The screens  24 A- 24 C will have a diameter sufficient to permit them to be disposed to extend across or span the passageway  22  and impede the flow of CO 2  through the passageway  22 . 
     The screens  24 A- 24 C are constructed with different perforations. For example, the screen  24 A includes elongated perforations  26  arranged as slots (also called multi-slots). The screen  24 A can be disposed in the passageway  22  such that the perforations  26  extend vertically or perhaps horizontally, or in any angle therebetween, as needed. The screen  24 A is usually used when the flux of the CO 2  particles from a particular nozzle is required to be of a given size range with no larger particles. Thus, the screen is equivalent to a cutoff filter for the larger particles that would damage a delicate device, while permitting the particles capable of efficient, non-detrimental cleaning to pass through the screen. The dimensions of the elongated slots or perforations  26  can be uniform or can vary as shown in  FIG. 2A . The dimensions of the slots  26  can range from a width of 1 μm-10 μm, and the length can be from 1-5 mm. The screens  24 A- 24 C permit CO 2  pellets of a select or pre-determined size only to be able to pass through the particular screen. 
     The screen  24 B includes a plurality of circular perforations or holes  28  extending therethrough (called a multi-channel plate). Any number of perforations  28  can be formed in the screen  24 B. Each one of the perforations  28  can have the same diameter, which can range from 0.5 μm-10 μm, or the perforations  28  can have different diameters. The arrangement of the perforations  28  resemble a multi-channel plate. The length of each perforation  28 , which resembles a tube extending through the screen  24 B, may be from 0.5 mm to 3 mm. The perforations  28  or tubes act as a collimator for the CO 2  stream (or molecular beam), and in effect replaces or obviates the need for the barrel portion  18 . See also  FIG. 3B  discussed further below. 
     The screen  24 C is manufactured of a plurality of crossed-hatched metallic wires to provide a wire mesh having a multiplicity of apertures  30  therein. The wire mesh can be arranged in any manner of crosshatching in order to provide perforations  30  of common or varying sizes to ensnare CO 2  particles of certain sizes from passing through the passageway  22  of the barrel portion  18 . 
     Referring still to  FIGS. 1A-1C  and  2 A- 2 C, it can be seen that at least one of the screens  24 A- 24 C is selected to be disposed in the passageway  22  of the nozzle  10 . Depending upon the type of cleaning application and the component to be cleaned, the screen  24 A- 24 C is selected based upon the position in which the screen will be disposed in the passageway  22 . For example, different screens provide different size and velocity distribution of the CO2 snow. The position of the screen in the barrel affects the size and velocity distribution, as well as the degree of collimation of the beam leaving the barrel for the workpiece. 
     The orifice  21  has an inlet at the reservoir portion  16  with a diameter of as much as 3-5 mm, transitioning to an outlet at the barrel portion  18  where it enters the passageway  22  having a diameter of 1.0 mm to 10 mm for interconnecting the reservoir  20  with the passageway  22 . The liquid CO 2  in the reservoir  20  passing through the orifice  21  defuses to create combined CO 2  gaseous and solid phase CO 2  at the passageway  22  of the barrel portion  18 . The size distribution of the solid CO 2 , however, is broad. The screens  24 A- 24 C will catch or ensnare the large CO 2  particles and large contaminants that may cause damage to the substrate to be cleaned. 
     As shown in  FIGS. 1A-1C , the screen can be disposed at different locations along the interior of passageway  22  of the barrel  18 . The position of the screen in the passageway  22  affects particle size emitted from the outlet  23 , i.e. the screen inserted closer to the outlet  23  allows less agglomeration downstream of the screen and therefore tighter (narrower) size distribution. However, the CO 2  stream will become less parallel (collimated or straight) if the screen is closer to the outlet  23 . As a result, the CO 2  beam spreads out or expands after leaving the barrel  18  and therefore covers a larger area on the workpiece, although with less intensity. This arrangement is suitable for cleaning finer structures on the workpiece. On the other hand, positioning the screen closer to the orifice  21  allows more collimation of the beam, but with larger agglomeration of CO 2 , an arrangement more suitable for removal of larger contaminates on the workpiece. 
     For gentler cleaning with the CO 2 , the screen, for example screen  24 A, can be disposed proximate the outlet  23  at an end of the barrel  18  of the nozzle  10  as shown in  FIG. 3A . A collar  32  is mounted at the outlet  23  of the nozzle  10  for supporting the screen  24 A proximate the outlet. The collar  32  can be permanently affixed to the outlet  23  of the barrel portion  18  or alternatively, releaseably engagable to the outlet of the barrel portion  18 . The arrangement of the collar  32  with respect to the outlet  23  and in order to accommodate the screen  24 A at the outlet, provides for an increased cross-sectional area at the outlet resulting in the CO 2  plume no longer being collimated (parallel or straight). As a result, the force per unit area of the CO 2  stream upon the structures at the workpiece is substantially reduced. This type of nozzle arrangement is useful for cleaning workpieces with fine and high aspect ratio structures thereon. Each one of the screens  24 A,  24 B,  24 C may include the collar  32 , so that the screens can be selected and changed out by manipulating the collar  32  with respect to the barrel  18  so as not to compromise the cleanliness of the passageway  22 . 
     The screens  24 A- 24 C of the embodiments trap the large CO 2  particles so that same sublime at an upstream side of the screen. Eventually, the CO 2  particles become small enough to pass through the screen with acceptably-sized CO 2  particles and also with a reduced flux, for effective non-damaging cleaning of the workpiece. 
     Referring to  FIG. 3B , one of the screens  24 A- 24 C is selected, by way of example only, to be mounted at the outlet of the orifice  21 , thereby obviating the need for the barrel portion  18  and the passageway  22 . The screen  24 B can be permanently mounted to the reservoir portion  16  or removably mountable to the portion  16 . Use of the screen  24 B for example mounted to the reservoir portion  16  brings the perforations  28  of the screen into registration with the orifice  21  outlet, wherein the CO 2  stream emerging downstream from the screen  24 B remains collimated with CO 2  particles not exceeding a particular size. 
     The CO 2  snow particles may also carry electrostatic charges that could adversely affect the cleaning ability of the nozzle  10 . The charges are created by tribo-action, i.e. friction between two species (also called tribo-charging) such as between the CO 2  particles and a wall of the passageway  22 .  FIG. 4  shows a nozzle assembly that will mitigate any charges present. The screen  24 A- 24 C and downstream thereof are electrically isolated from the rest of the assembly and can be grounded or elevated to different potentials for charge transfer purposes to or from the CO 2  snow. As shown in  FIG. 4 , an electrical insulator ring  34  is mounted to the barrel portion  18  of the nozzle  10 . The ring  34  is mounted proximate the outlet  23  of the barrel portion  18 . A voltage source  36  is connected or wired to the screen  24 A or the collar  32  for the screen. The mitigation occurs when a CO 2  particle passes through for example the screen  24 A. A negatively charged CO 2  particle passing through the screen  24 A will be stripped of electric charge creating a neutral CO 2  particle downstream of the screen  24 A. The ring  34  and the voltage source  36  can be used as well with the other embodiments herein described to mitigate electrostatic charges. If the embodiment does not call for using one of the screens  24 A- 24 C, then the voltage source  36  can be connected to the nozzle or barrel itself. 
     In  FIG. 5 , a nozzle  70  is shown having a sidewall  72  with an inner surface  74  defining a chamber or passageway  76  extending through the nozzle. The inner surface  74  of the nozzle  70  extends to an outlet  80  of the nozzle  70 , wherein the inner surface  74  provides an outwardly extending concave shape shown generally at  81  of the outlet  80 . The sidewall  72  extends from the outlet  80  as a perforated portion  82  having at least one or alternatively a plurality of holes  84  or apertures formed therein. The holes  84  may be angled with respect to a longitudinal axis  73  of the nozzle  70  as shown in  FIG. 5 . Openings  86  of the holes  84  face or open toward the CO 2  being emitted from the outlet  80 . The holes  84  trap contaminants and agglomerates that are exhausted from the outlet  80  of the nozzle. The shape  81  causes the CO 2  to expand, thereby transitioning to the gas and solid phase such that the larger CO 2  pellets are directed to the openings  86  where they become trapped in the holes  84 . 
     Selecting in combination, as necessary, screens and electric potential to be added to the CO2 nozzles  10 ,  70  provides for being able to control the flux, size and velocity distributions of CO 2  snow particles in the CO 2  plume, and also assists in mitigating impurities ejected from the nozzle. 
     Alternatively, none, any number of or all of the holes  84  may be open to the atmosphere to permit the exhaust of any bubbles or contaminates in the CO 2  beam as it travels from the passageway  76 . This provides for better stability of the CO 2  plume or beam. The holes  84  or slits provide a phase separator, i.e. the slits  84  permit the gas phase to escape the passageway  76  so that solid CO 2  particles for the most part continue down the perforated portion  82 . A screen  24 A for example can also be disposed at the end of the portion  82  using a collar  32  similar to that shown in  FIG. 3A . 
       FIGS. 6A-6D  disclose a plurality of barrel-less nozzle embodiments. 
     In particular, the nozzle embodiments at  FIGS. 6A-6D  include nozzles  50 ,  52 ,  54 ,  56  (collectively “ 50 - 56 ”). The nozzles  50 - 56  each include a sidewall  58  and an orifice  60  through which liquid CO 2  passes to phase into solid and gaseous CO 2 . The size D of the orifice  60  controls the size and flux or amount of CO 2  molecules and pellets that are discharged from the nozzles. The shape of the orifice  60  substantially controls the velocity of the solid CO 2  particles produced and the degree of collimation of the CO 2  particle beam, and also the size and the flux of the CO 2  particles in the beam. The sidewall  58  of each of the nozzles  50 - 56  includes an inner surface  62  defining an interior chamber or passageway  64  for each of the nozzles  50 - 56 . The nozzles  50 - 56  use the structure of the orifice  60  in order to control the CO 2  plume or stream, and the size of the CO 2  particles therein. A thickness of the sidewall  58  for each of the nozzles is represented by “X”, while a diameter of the orifice  60  is represented by “D”. The thickness X can range from 0.5 to 5 mm; while the diameter D can range from 50-500 μm. The orifice  60  can have different shapes for both an inlet of the orifice  65  at the passageway  64 , and an outlet of the orifice  60  at for example surface  65 . 
     Referring to the nozzles  50 ,  52  of  FIGS. 6A and 6B , the nozzles  50 ,  52  function similarly, except that the sidewall  58  of the nozzle  52  permits the nozzle  52  to be positioned closer to the workpiece due to a reduced exterior angle surface  59 , such as being truncated, when a non-vertical nozzle angle is required during cleaning. Both the inlet and outlet, respectively, of the orifice  60  are flat-ended. 
     The nozzle  54  of  FIG. 6C  has an orifice  60  with a concave exhaust surface  65 . As the CO 2  beam exits the orifice  60 , the beam is expanded, collimation is controlled and velocity is increased. 
     The nozzle  56  of  FIG. 6D  has an orifice  60  with a straight cylindrical outlet, but a concave surface inlet  66 . There is some expansion of the CO 2  beam, although it does remain narrower, and velocity of the beam is increased. Generally the velocity of CO 2  from nozzle the  54  is greater than the velocity of CO 2  from the nozzle  56  having the identical diameter D. 
     The embodiments of  FIGS. 6A-6D  mitigate the level of contaminants and agglomerates due to the absence of the barrel for these nozzles. The barrel is considered the bulk area for CO 2  nucleation and growth. 
     It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described and claimed herein. It should be understood that the embodiments described herein are not only in the alternative, but may be combined.