1. Field of the Invention
This application relates to new and novel methods for cleaning both surfaces and liquids. More particularly, the present invention concerns methods of removing small particulate matter from surfaces, and from fluids in which such particles may be suspended.
2. Description of Related Technology
Cleaning techniques are as varied as the contaminates and media to be cleaned. Precision cleaning has taken on great importance in recent years due to the necessity in certain fields, such as computer disks and integrated circuit manufacture, where the removal of microscopic particulate matter is essential to assure the basic efficacy of the device or manufacturing process. For example, in the manufacture of silicon wafers as used in the integrated circuit industry, a rejection rate of 20 to 30 percent can be caused solely by the presence of particulate matter on the surface of the wafers.
Various methods have been employed in the field of precision cleaning, including ultrasonics, megasonics, wiping, brush scrubbing, low pressure surfactant spraying, high pressure jet spraying, etching and centrifugal spraying. Each of these methods will now be briefly discussed.
In ultrasonic cleaning, a part is immersed in a suitable liquid medium and sonicated or agitated at a high frequency (18 to 120 kilohertz). This usually lasts for several minutes, and then the part is rinsed and dried. Cavitation occurs when microscopic bubbles form in the liquid medium and then violently collapse or implode, mechanically scouring the part to be cleaned and displacing and loosening the contaminants. There are many advantages to ultrasonic cleaning. It is fast, effective and safe to use. It requires less heat than other cleaning methods, and when used properly it can vigorously clean delicate parts without harming surface finishes. Also, there is no need to dismantle assemblies. Disadvantages of ultrasonic cleaning include its complexity and the generation of noise. In fact, ultrasonic cleaning makes so much noise at frequencies lower than 20 kilohertz that a 40 kilohertz frequency is recommended even though it is less efficient.
Megasonic cleaning is another cleaning method which consists of basically the same steps as ultrasonic cleaning: immersion, agitation or sonication, rinsing and drying. The major difference between the two is that while ultrasonic frequencies range from 18 to 120 kilohertz, megasonic frequencies are in the range of 0.8 to 1 megahertz with input power densities ranging from 5 to 10 watts per centimeter. Whereas the cleaning action in ultrasonic cleaning comes from cavitation, the cleaning action in megasonic cleaning comes from high pressure waves pushing and tugging at contaminants lodged on a part's surface. There are many advantages to megasonic cleaning. It causes almost-no scratches, breakage or chipping since substrates are not transferred or subjected to any mechanical stress. It is three to four times more productive than scrubbing or chemical cleaning at an equal or lower investment cost and produces superior wafer cleanliness. Megasonic cleaning consumes only about 1/8 the amount of chemical solvents when compared to conventional chemical cleaning, and megasonic cleaners have low maintenance and are simpler to automate. Megasonic cleaners are also able to use more chemically active cleaning solutions such as hydrogen peroxide and amonium hydroxide.
Megasonic cleaning also has some disadvantages. For example, the solvent system must be adapted to the particular contaminant-substrate bonding and the transducer matrix is not a commercial item. Cleaning solutions such as strong hydrofluoric acid cannot be used, and a substrate container must be designed to minimize obstruction to the megasonic beam.
Although it may not be as efficient as sonic cleaning, wiping is another successful cleaning method. It is an inefficient but effective method of particle removal that is commonly used to clean optical surfaces. Besides the amount of time it takes, a major drawback to wiping is that particles can be deposited from the tissue or the solvent being used. Wiping is also unable to reach irregular surface geometries and the results depend on the wiper's skill and attention to detail.
A related method of cleaning is brush scrubbing. In the brush scrubbing method, the brush never actually touches the surface being cleaned due to the hydrophilic nature of the brush. There is always the film of the scrubbing solution between the brush and the surface. The hydrophilic brush will only remove contaminants from hydrophobic surfaces. Surfaces that are hydrophilic are more difficult to clean because suspended contaminants can precipitate onto them. There are several factors which contribute to making brush scrubbing ineffective. First, the aqueous neutral detergent solutions used with scrubbers can leave behind thin nylon films. When used with chemically active cleaning solutions, rapid corrosion can occur. If this is true, chemical cleaning is then also required, and this inevitably introduces more particles. Another problem can arise when brushes become infested with dirt particles and debris from surface breakage or chipping. When this happens, the brushes themselves can become sources of contamination and scratches. Finally, scrubbers are sequential in operation and can only clean one part, and often only one side, at a time.
Unlike brush scrubbing, low pressure surfactant spraying relies on chemical means to remove particles. Its success depends on the effectiveness of the detergent that it sprays. The pressure of the jet itself, which can vary from 5 psi to 80 psi, is not nearly enough by itself to remove particles. Therefore, the compatability of the detergent with the contaminant and the surface is crucial. Detergents are surfactants which function by reducing the surface tension of water. They remove soils through emulsification and by concentrating at water interfaces. At high concentrations, detergent solutions form micelles. Micelles occur when the detergent carbon chain forms a low polarity region that is stabilized by having the polar ends in contact with the water. The cleansing action occurs because the lowered water surface tension allows the detergent to penetrate and the micelles to dissolve greases and oils by taking them into the carbon regions.
High pressure jet cleaning operates under very different principles than surfactant spraying. It works when the shear force it exerts is greater than the adhesion force holding a particle to a surface. The method consists of a high velocity jet of liquid sweeping across a surface at pressures of 100 to 4,000 psi. The main advantage of high pressure jet scrubbing is that it is able to remove microscopic debris from difficult surface geometries such as depressions and circuit corners.
Etching is one of the most common cleaning methods used. It is a chemical cleaning method that consists of dissolving unwanted substances on a surface and is not as severe to the surface as a mechanical means of surface treatment. Etching is closely related to acid cleaning and is one of the most important procedures in microelectronic device fabrication. Etching is performed as a sequential purification process in which oxidation and dissolution of residual or organic impurities in certain metal contaminants occurs in a mixture of water, hydrogen peroxide and ammonia peroxide. A solution of hydrogen peroxide and hydrochloric acid is used to remove heavy metals and to prevent displacement replating from solution by forming soluble complexes with the resulting ions. These solutions are chosen because they are completely volatile. They are also less hazardous than other possible cleaning mixtures and present no disposal problems. This method works because at a high pH, hydrogen peroxide solutions are effective at removing organic contaminants by oxidation and at a low pH they are effective at desorbing metal contaminants by complexing. This technique is particularly useful in cleaning silicon device wafers, quartz tubes, and parts used in semiconductor processing.
Centrifugal spray cleaning is an effective cleaning alternative to chemical immersion processes. It is commonly used for cleaning wafers. The wafers are enclosed in a sealed chamber purged with nitrogen. As the wafers spin, they are subjected to a series of continuous fine sprays of reagent solutions, including a hot aqueous solution of hydrogen peroxide and ammonium hydroxide, an aqueous solution of hydrochloric acid and hydrogen peroxide, and high purity water. Recontamination is prevented by arranging the sprays so that each wafer is continuously exposed to fresh solutions. Wafers are sprayed at 2,500 psi. After the wafers have been sprayed, they are dried with nitrogen. Centrifugal spray cleaning relies on centrifugal force, shear force and solvency.
A commercial apparatus for cleaning semiconductor wafers is disclosed in U.S. Pat. No. 4,186,032, issued to Ham. In this device, super-heated steam is passed over the surface of the wafer, the condensate being permitted to form and drip off the wafer and drying being accomplished by heating a chamber to a temperature in excess of 100.degree. C. Note that this apparatus is limited to cleaning loose foreign surface matter and chemical impurities.
The following chart presents the current state of the art of methods for surface particle removal:
______________________________________ CLEANING GENERAL MECH- LOWER LIMIT CHARAC- METHOD ANISM PARTICLE SIZE TERISTICS ______________________________________ ultrasonic cavitation 25 microns frequency range of 18 to 100 kilohertz; may damage surface megasonic high pressure 0.2 microns frequency range waves eight-tenths to one megahertz; 15 minutes duration wiping shear 5 microns time-consuming brush mechanical 0.5 microns does not work scrubbing and sheer well on hydro- philic surface low mechanical 0.2 microns pressure range pressure and of 5 to 80 spray detergency pounds per inch; relatively time-consuming high shear 0.2 microns pressure range pressure of 100 to 4,000 spray pounds per square inch; relatively slow; can damage surface ______________________________________
Each of the cleaning methods heretofore discussed has required either the use of an active mechanical component, the use of special chemical solutions, or labor intensive human participation in order to achieve a satisfactory level of effectiveness. Furthermore, each cleaning method discussed is limited to a particular class of media to be cleaned, and, given the stringent requirements of certain industries, the absolute cleaning effectiveness of each method is not yet at a uniform, commercially acceptable level.