Abstract:
The invention provides water enriched with ozone by generating ozone from oxygen using short-wavelength ultraviolet light and pumping the generated ozone under pressure through a 0.1-micron filter into a sealed housing of deionized water. The filter is fabricated of a material such as polytetrafluoroethylene which does not react with water and ozone. The filter apertures are sufficiently small to prevent the formation of gas bubbles in the outlet fluid. The highly-purified outlet fluid is usable immediately in semiconductor wafer cleaning.

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/344,867, filed Jun. 28, 1999 now U.S. Pat. No. 6,314,974. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to semiconductor wafer cleaning, and more specifically to processes and apparatus for dissolving ozone in deionized rinse water for cleaning and passivating a semiconductor wafer. 
     Once semiconductor wafers have been cleaned, it is important to provide a thin native oxide layer on the wafers as soon as possible in order to prevent contamination of the wafer during its fabrication. The process of forming this layer is termed surface passivation. 
     Native oxide readily forms on bare silicon wafer surfaces with or without ozone. When it is formed slowly or in an uncontrolled manner it will tend to incorporate high levels of SiO x  particles or other contaminants. In prior art techniques, surface passivation is provided by hydrogen peroxide. It has been found, however, that hydrogen peroxide often contains undesirable metallic contaminants, reducing wafer yield or requiring additional cleaning steps. 
     Further prior art techniques expose the bare silicon wafer surface to high levels of ozone, forming a quick and clean native oxide on the surface. Such a native oxide layer can be provided by subjecting the wafers to a bath of ozone-rich water. In order to achieve a rapid silicon surface conversion to a native oxide, the ratio of ozone to water should be more than 7 parts per million. 
     Current techniques for ozonating water are, however, inadequate. Ozone quickly leaves the water bath and so the wafers do not receive the desired native oxide layer. Furthermore, the existing techniques for generating the ozone and ozonating the water are expensive, complex, and power-hungry. 
     SUMMARY 
     The invention provides a simple, inexpensive, ozone-generating and ozone-capturing apparatus and method for semiconductor wafer cleaning, which require less power than existing methods. The ozone generation apparatus and method uses ultraviolet light at a range of wavelengths concentrated at 185 nm to convert gaseous oxygen (O 2 ) to gaseous ozone (O 3 ). Oxygen gas is pumped into an optically-opaque chamber containing a short-UV high-intensity lamp. The lamp&#39;s ultraviolet light ionizes the molecular oxygen, converting it to ozone. The ozone is then pumped out of the chamber. The ozone-capturing apparatus and method uses a reverse polytetrafluroethylene (Teflon) filter. The filter is immersed in a housing of water. Ozone is pumped into the filter under pressure with a check valve to prevent the back flow of ozone. The receiving housing is filled with water and is likewise sealed. The ozone under pressure is forced out of the Teflon filter and into the surrounding water. The ozonated water is withdrawn from the base of the housing and is passed to a wafer ozone bath for applying the ozonated water to one or more semiconductor wafers. The ozonated water quickly produces on the wafers a thin layer of virtually contaminant free native oxide. This layer of native oxide aids in protecting the wafers from further contaminants during the further wafer processing. Note that native oxide is self-limiting in its growth, with the final thickness (usually&lt;50 Å) dependent on the ambient temperature and pressure under which it is formed. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of the ozonator, including both the ozone-generating apparatus and the ozone-capturing apparatus. 
     FIG. 1 a  is a diagram of the ozone-capturing apparatus alone. 
     FIG. 2 is a diagram of the ozone-generating chamber showing a helical-bulb ultraviolet light source. 
     FIG. 3 is a diagram of the ozone-generating chamber showing a helical-bulb ultraviolet light source, with a baffle to control the direction of gas flow through the chamber. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 1 a  show the invention&#39;s apparatus for generating ozone and dissolving the generated ozone into water. The invention comprises an ozone generator  200  and an ozonator  100 , taking in gaseous oxygen and deionized water, and producing ozone dissolved in the deionized water. Refer first to FIG. 1. A source of oxygen (not shown) delivers oxygen gas via an inlet  201  into an ozone generator  200 , which converts the oxygen to gaseous ozone using ultraviolet light. The ozone is then delivered under pressure to pressure to the ozonator  100 , where it disperses and dissolves into deionized water. The pressure on the ozone gas dissolves the ozone in the deionized water. The ozonated deionized water leaves the ozonator to be used in semiconductor wafer cleaning. 
     The invention generates ozone using ultraviolet source illumination, employing ultraviolet-sensitive photosensors and circuits to drive an indicator to show that the ozone generator is in fact on and working. By contrast to an ordinary power-on indicator, the invention&#39;s circuit physically looks at the ultraviolet source independently to make sure it is alive. If the generator is running and the lamp fails, the indicator shows this fact immediately. 
     Refer to FIG.  2 . In the inventive ozone generator  200 , oxygen gas is pumped into an inlet  201  at one end of a stainless-steel-walled chamber  203  in which is mounted lengthwise a helical ultraviolet lamp  205  radiating short-wavelength (principally 185 nm) ultraviolet light. The ultraviolet lamp  205  is long, with quartz walls. The lamp  205  is mounted in a closed, stainless-steel chamber because any but the briefest exposure to short-wavelength ultraviolet radiation can burn human skin and eyes. The oxygen under pressure passes the length of the lamp to outlet  207 . The ultraviolet light in the short wavelengths ionizes the oxygen, converting the molecular oxygen to molecular ozone. The converted ozone exits the chamber  203  via outlet  207  at the end opposite the inlet  201 . The rate of pumping of oxygen into the chamber  203  is such as to cause the gas exiting the chamber  203  at the outlet  207  to be saturated ozone. An ultraviolet-sensitive photosensor  202  is used to drive an indicator to show that the ozone generator is in fact on and working. By contrast to an ordinary power-on indicator in the same circuit as the device being powered, the invention&#39;s circuit physically looks at the ultraviolet source independently to make sure it is alive. If the generator is running and the lamp stops producing ultraviolet light, the indicator shows this fact immediately even if the lamp is still drawing current. 
     Refer to FIG. 1 a.  The ozonator  100  includes a mixing container  102  and a sealing cover  109 . An ozone inlet line  104  passes through the seal cover  109 . The inlet line  104  has a one way gas check valve  101  for preventing reverse flow of ozone. The inlet line  104  is coupled to the dispersion filter  103 . The filter  103  is located entirely withing the mixing container  102  and is surrounded by deionized water. The filter  103  has a central cylindrical chamber  107  that receives the ozone from the ozone inlet line  104 . The filter  103  has a membrane that has pores of suitable size to permit passage of ozone from the chamber  107  into mixing region  108  of the mixing container  102 . The membrane pores are sized to permit to prevent passage of water molecules into chamber  107 . In the preferred embodiment the filter membrane is a 0.1 micron, polytetrafluoroethylene (Teflon) filter. The pore size may be greater to less than 0.1 microns so long as the pores are large enough to permit the passage of ozone and small enough to restrict the passage of water. The outlet of the mixing container has an orifice  120  for creating a back pressure in the mixing container  108 . The orifice  120  may be fixed or variable in size. A variable orifice can be formed by a flow control valve that is either manually or automatically adjusted to provide a desired back pressure in mixing container  102 . The ozonator dissolves the ozone gas in deionized water (DIH 2 O). 
     In operation, the filter  103  serves as a boundary surface between an inner volume  107  and an outer volume  108  in container  102 . Ozone is introduced under pressure into inner volume  107  through inlet line  104  having an in-line check valve  101 . Highly deionized water is introduced from inlet line  106  into outer volume  108  in container  102 . Filter  103  allows the pressurized ozone inside the inner volume  107  to dissolve into the water in outer volume  108 , thereby ozonating the water. The ozonated water is then discharged through discharge outlet  105 , and pumped into a container holding semiconductor wafers requiring surface passivation. An orifice  120  is located at the outlet of the ozonator and is in fluid communication with the discharge line  105 . The orifice is restricted in cross-section relative to the inlets  104  and  106 . This restriction maintains pressure in container  102  at a level which will insure that the concentration of dissolved ozone in the outlet ozonated water is at least 7 parts per million. The purity of the invention&#39;s output enables the wafer surfaces to be rapidly oxidized, leaving on each wafer a thin oxide layer with virtually no metallic or organic contamination. In an alternate embodiment, there orifice is eliminated and the discharge line  105  provides the necessary back pressure by having a diameter smaller than the diameter of the water inlet line  106 . 
     Since the ozone dissolves into the water in a sealed container  102 , there is little or no free ozone mixed with the discharge fluid. The ozonating operation is normally carried out at a temperature of about 20 degrees C.±2 degrees C. If desired, the temperature of the intake water can be controlled to vary the amount of ozone in the discharge. Lower temperatures will result in more ozone dissolving in the water. In the preferred embodiment, the discharge fluid from orifice  130  is at least 7 parts per million of ozone. 
     Refer to FIG. 3, which shows an alternate embodiment of the ozone generator chamber. In this embodiment, the chamber  203  is fitted with a helical stainless steel baffle  206  that causes the gases moving through the chamber to move more nearly tangentially parallel to the helical contour of the lamp  205 . The inner diameter of the baffle  206  is sufficiently large to permit removal and replacement of the lamp  205  along its longitudinal axis whenever necessary. 
     Existing ozone generation methods for semiconductor wafer ozonators cost well above $50,000, and generate ozone by electrical arcing between plates. The invention&#39;s method, by contrast, is a clean way of generating ozone which, in combination with the invention&#39;s ozonator, is inexpensive both to purchase and to operate. 
     The above examples are not intended to limit the spirit and scope of the invention. Those skilled in the art understand that further additions, modifications and changes may be made to the invention without department from the appended claims.