Carbon dioxide exists as a low-density gas at standard temperature and pressure conditions and possesses phase boundaries with a triple point (Solid-Liquid-Gas co-exist in equilibrium like a glass of ice cubes and water) and a critical point (Liquid-Gas have identical molar volumes). Through pressure or temperature modification, carbon dioxide can be compressed into a dense gas state. The term ‘Dense Phase Carbon Dioxide’ is used herein to describe all phases of carbon dioxide: liquid state, supercritical state, dense gas state, and solid-state. These states have densities that are within the range of liquid-like or near-liquid substances.
Compressing carbon dioxide at a temperature below its critical temperature (C.T.) liquefies the gas at approximately 70 atm. Cooling liquid-state or gas-state carbon dioxide to its freezing point causes a phase transition into solid-state carbon dioxide. Compressing carbon dioxide at or above its critical temperature and critical pressure (C.P.) also increases its density to a liquid-like state, however there is a significant difference between compression below and above the critical point.
Compressing carbon dioxide above its critical point does not effect a phase change. In fact, carbon dioxide at a temperature at or above 305 K (88 F) cannot be liquefied at any pressure, yet the density for the gas may be liquid-like. At the critical point the density is approximately 0.47 g/ml. At or above this point carbon dioxide is termed a supercritical fluid (SCF). Supercritical carbon dioxide can be compressed to a range of liquid-like densities, yet it will retain the diffusivity of a gas. Continued compression of supercritical carbon dioxide causes continued increase in density, approaching that of its liquid phase.
Solid-state carbon dioxide is useful for removing particulates and trace organic residues from surfaces, using a process typically called Snow Cleaning or Snow Departiculation. Similar to liquid and supercritical fluid cleaning agents, solid-state carbon dioxide's cleaning power can also be described in both physical and chemical terms. The process of snow departiculation can be described as a kinetic energy transfer process called “Linear Momentum Transfer” in accordance with the following vector quantity:
P=MV, where
P—Linear Momentum of Solid Carbon Dioxide Particle or Surface Particle
M—Mass of Solid Carbon Dioxide Particle or Surface Particle
V—Velocity of Solid Carbon Dioxide Particle or Surface Particle
A stream of solid carbon dioxide particles having significant mass and velocity impact a stationary surface particle causing the surface particle with a given mass to accelerate away from the surface to a given velocity in accordance with the following equation:Vsp=(Mcp/Msp)Vcp, where
Vsp—Velocity of Surface Particle
Mcp—Mass of Carbon Dioxide Particle
Msp—Mass of Surface Particle
Vcp—Velocity of Carbon Dioxide Particle
The physical energy transferred during a snow departiculation process is usually sufficient to overcome strong electrostatic and intermolecular adhesive forces, commonly referred to as Van der Waal's forces, that hold small particles to the surface.
The mechanism for the removal of trace organic films using snow is not fully understood, but has been postulated to be a combination of momentum transfer and a phase change of minute solid carbon dioxide particles from solid-state to liquid-state (compression) and subsequent solutioning of trace surface residues. According to the phase diagram for carbon dioxide, a minimum impact compression of approximately 6 atm (88 psi) at 195 K is required to produce a liquid interphase. Energy transformations are possible other than the formation of a liquid phase, including particle fragmentation or shearing, gas phase transition (sublimation), and temperature rise in the solid (thermal energy) at impact.
Solid carbon dioxide is being used in a number of commercial product cleaning applications to remove trace organic and inorganic residues and particulates. Liquid carbon dioxide is rapidly expanded (joule-thompson expansion) through an orifice of a valve to form a mixture of subcooled gas state and solid state carbon dioxide—referred to as “snow” or “dry ice”.
Solid carbon dioxide is applied in conventional applicators according to two types of applicators, described as Type I and Type II snow cleaning applicators as follows:
Type I Snow Applicator: Liquid carbon dioxide, stored in a high pressure bottle, is expanded from 850 psi at 298 K through a suitable nozzle into gas state (the propellant) and solid state carbon dioxide (the cleaning agent) and directed at a substrate. Conventional Type I applicators are commonly used in precision cleaning applications at close proximity to a substrate and have relatively simple operation and low-cost designs.
Type II Snow Applicator: Liquid carbon dioxide is first expanded into solid carbon dioxide using a suitable “dry ice machine”, packed into dry pellets of uniform size, or shaved into a powder, and then fed into a spray apparatus using compressed air to propel the solid carbon dioxide from a spray nozzle. The air and solid mixture impacts the surface. A Type II applicator is typically used for cleaning large rigid structures because of its more aggressive action at close proximity (i.e., for coating removal) and long-range particle cleaning action (large and hard snow pellets). However, Type II equipment and operational costs are significantly higher than Type I systems.
Type I and Type II applicators include:
1. Fixed position applicators
2. Pistol grip applicators
Conventional applicators are designed to have a single spray pattern, with various interchangeable nozzle designs for different substrates and surface cleaning applications. Type II designs can also vary impact energy through control of compressed air pressure whereas Type I designs cannot. Disadvantages associated with these mechanical designs include:                1. Fixed spray pattern.        2. Non-interchangeability of applicator designs (fixed<−>handheld<−>robotic).        3. Bulky configurations.        4. Uncontrolled tribocharging (electrostatic buildup) of non-metallic substrates such as plastics.        5. Rapid localized substrate cooling and subsequent deposition of contaminating residues.        6. Ineffective deep hole cleaning.        7. Expensive equipment costs.        
Conventional snow cleaning applicators (Type I and II) suffer from the following disadvantages:                1. Impact energy and the amount of snow particles available at the surface decreases as the distance from the expansion valve to the applicator nozzle increases. Type I applicators must have an expansion valve located close to the nozzle and the nozzle must also be very close to the substrate to effectively remove residues.        2. Entrainment of ambient air which often contains moisture, particles, and other contaminating residues which condense onto surfaces which have been supercooled by the snow particle/gas stream.        3. Externally applied environmental control measures such as heated air and particle control hinder the cleaning performance and are applied so generally that localized condensation, particle entrapment, or tribocharging still occur. Conventional applications employ macro-environmental control (clean rooms, infrared heaters etc.) measures. Type I snow applicators cannot be used in relatively uncontrolled environments.        4. The process of expanding liquid carbon dioxide into solid state and subsequent contact of solid state carbon dioxide with surfaces causes a phenomenon called tribocharging, whereas the solid carbon dioxide (primary dielectric) builds electrostatic charge of up to 5 to 15 mJ at 10 KV to 20 KV as it contacts a substrate (secondary dielectric). This type of electrical charge build-up can be extremely damaging to microelectromechanical devices (or can induce latent ESD defects) and will cause a departiculated surface to become an attractor (magnets) of airborne particles following snow cleaning operations. Electrostatic effects can be caused through direct contact of charged solid carbon dioxide particles with the substrate which causes a discharge event or current flow through the surface (direct discharge) or may be caused through electrostatic field exposure and subsequent charging of the surface (induced charging).        5. In many applications, spray cleaning is performed independent of and prior to operations such as microwelding, adhesive bonding and thermal curing and soldering. Moreover, following production operations such as CMP the substrate is wet with aqueous residues and must be dried prior to snow cleaning operations. A method and apparatus is needed to serve as an integrated simultaneous drying, cleaning and production tool.        6. Xenon flashlamp technology is used with solid carbon dioxide (Type II) to remove old paint from aircraft surfaces. This type of technology uses an intense UV radiation (not a laser) burst to pyrolize substrates which produces a large amount of heating radiation as a by-product. The pyrolized paint is swept away from the substrate using a flow of carbon dioxide pellets. This technology is large and bulky and cannot be used to precisely clean small parts commonly found in the semiconductor, electrooptical and electronics markets. A precision coherent photon-based technique is needed to remove small contaminants from intricate assemblies.        7. In some applications, solid phase carbon dioxide chemistry requires physicochemical modification to provide enhanced separation and surface finishing capabilities. To date, no effective technique has been demonstrated to accomplish this requirement.        8. Type II applicators also suffer from being too aggressive (i.e., substrate damage), very noisy, bulky and too costly for most precision substrate cleaning applications.        9. Conventional snow cleaning applicators do not lend themselves to integration with production processes such as stamping, welding, bonding, curing and abrasive surface finishing operations because of the aforementioned problems discussed above.        
Conventional snow cleaning processes do not have a method for real-time analysis of cleaned surfaces to accept or reject a particular cleaning operation. This is especially advantageous for in-line continuous quality control monitoring of surface cleaning performance.
Conventional ESD Control Methods used with Solid Carbon Dioxide:
Air Ionization—air is ionized using a DC or AC ionizer that is then flushed over an affected surface. The problem with this approach is that flowing air induces contamination through introduction of humidified air and potential particles. Also ionizing air impingement requires flooding the surfaces to be cleaned. This process can subtract from the cleaning energy. Moreover, the charges present within the structure of the cleaning agent are not reduced effectively using this technique.
U.S. Pat. No. 5,409,418 proposes a nozzle-mounted secondary gas ionizer which surrounds the snow stream with oppositely charged ions during impingement. U.S. Pat. No. 5,725,154 proposes neutralizing charges during snow cleaning following each cleaning pulse with a separate propellant gas neutralization pulse.
Most prior art suffers from these typical drawbacks:                Impossible to precisely control charges being delivered to a substrate—each substrate and atmosphere is different.        The portion of a substrate being impacted by the sublimable cleaning agent is not affected by neutralizing ions—only the circumference of the snow spray is affected.        Backside or nearby electrostatic charging due to electric fields is not affected by these techniques—electric fields pervade the materials creating complexly charged surfaces.        
Nuclear Ionization—the substrate is exposed to radioactive particles (alpha). This process is line-of-sight and very short range. Obstructions of the smallest variety will eliminate beneficial ionization using this technique.
Fong '786, referenced herein, uses nuclear ionization to reduce accumulated electrostatic charges contained on solid carbon dioxide stored and mixed within a storage hopper and prior to and during delivery into a high pressure feed line. Fong '786 suffers from all of the drawbacks cited above.
Grounding—the substrate in grounded to earth using a suitable resistor to bleed charges at an acceptable rate. The main problem with this approach is that the electrostatic charge and electrical overstress are not effectively controlled on non-conductive substrates.
Antistatic Chemicals—this approach is the most effective on preventing charge creation by the cleaning agent. However this method tends to, by itself, become a source of chemical contamination within the cleaning process. To date no use of antistats within cryogenic cleaning agents is known
Moreover, in cleaning quartz lenses, as well as many other non-conductive substrates it is difficult to control electrostatic charging of the quartz substrate during sublimable spray cleaning. Flooding the surfaces with ionized air only works prior to and following snow cleaning. During snow cleaning, as much as 2000 volts of electricity of positive and negative charge can be created following the snow-surface tribocharging contact event. Contaminants such as particles tend to move in relationship to thermal and electric field gradients—both of which are present in snow cleaning.
The backside of the quartz is typically opposite in charge (conservation of charge) during snow cleaning, therefore the particles once lifted from a front surface migrate around the substrate within a thin-film of subcooled atmosphere and become attracted to the oppositely charged surface on the backside. Quartz cannot be grounded and commonly used antistatic chemical agents contained in the cryogenic cleaning agent would leave stains during cleaning.
A photoelectric effect has been advantageously employed in different arts for decades. In certain commercial ionization applications, the photoelectric effect is used to produce highly energetic photons from 0.13 to 0.41 nm (9.5 to 3 KeV) to ionize an atmosphere surrounding a substrate during a production process.
As such there is a present need to provide an alternative dense fluid spray cleaning and separation apparatus and process which overcomes the limitations of conventional dense fluid spray technology and provides an environmentally-safe cleaning and finishing alternatives to organic solvents.
As such there is a present need to provide clean and effective electrostatic control method during sublimable cleaning processes. Since electrostatic charging is most prevalent in cryogenic cleaning such as carbon dioxide, argon or liquid nitrogen blasting—a three-dimensional ionization method and device is needed to resolve electrostatic charging effects in complex substrates being cleaned, regardless of composition, shape and size.