Abstract:
A cavitation cleaning system and method for using the same to remove particulate contamination from a substrate including providing at least one substrate immersed in a cleaning solution said cleaning solution contained in a cleaning solution container. The container further includes means for producing gaseous cavitation bubbles of ultrasound energy, said gaseous cavitation bubbles arranged to contact at least a portion of the at least one substrate; applying ultrasound energy to create gaseous cavitation bubbles to contact the substrate to remove adhering residual particles in a substrate surface cleaning process; and, recirculating the cleaning solution through a particulate filtering means.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention generally relates to semiconductor wafer manufacturing and more particularly to methods for cleaning semiconductor wafers to remove particulate contamination.  
         BACKGROUND OF THE INVENTION  
         [0002]    In creating a multiple layer (level) semiconductor device on a semiconductor wafer, each layer making up the device may be subjected to one or more deposition processes, for example using chemical vapor deposition (CVD) or physical vapor deposition (PVD), and usually including one or more dry etching processes. A critical condition in semiconductor manufacturing is the absence of particulate on the wafer processing surface, since microscopic particles may interfere with and adversely affect subsequent processing steps leading to device degradation and ultimately semiconductor wafer rejection.  
           [0003]    While the wafer cleaning process has been always been a critical step in the semiconductor wafer manufacturing process, ultraclean wafers are becoming even more critical to device integrity. For example, as semiconductor feature sizes decrease, the detrimental affect of particulate contamination increases, requiring removal of ever smaller particles. For example, particles as small as 5 nm may be unacceptable in many semiconductor manufacturing processes. Further, as the number of device layers increase, for example to 5 to 8 layers, there is a corresponding increase in the number of cleaning steps and the potential for device degradation caused by particulate contamination. To adequately meet requirements for ultraclean wafers in ULSI and VLSI the wafer surface must be essentially free of contaminating particles.  
           [0004]    Another factor in modern processing technology that increases the incidence of particle contamination is the deposition of carbon doped oxides as IMD layers to achieve dielectric constants of less than about 3.0. The IMD layers are typically deposited by a plasma enhanced CVD (PECVD), low pressure CVD (LPCVD) or high density plasma CVD (HDP-CVD). In these processes, a degree of sputtering occurs as the layer of material is deposited causing a higher degree of particulate contamination as the deposition time increases. In addition, PVD processes are typically used to deposit films of metal, for example barrier/adhesion layers within anisotropically etched features or for metal filling an anisotropically etched feature. PVD processes tend to coat the inner surfaces of the processing chamber with a metal film, flaking off to contaminate a wafer process surface as the metal film increases in thickness and are subjected to cyclic thermal stresses. Other processes that frequently resulting particulate contamination include plasma etching processes where a photoresist layer is etched away during an ashing process. Over time, the buildup of ashing residue within a plasma etching chamber increases the probability that a semiconductor wafer will become contaminated by particulates.  
           [0005]    Particulate contamination may cause ‘killer defects’ resulting in integrated circuit opens or shorts by occluding a portion of a circuit or providing a shorting path between two conductive lines of a circuit.  
           [0006]    Common processes in use for cleaning wafers include cleaning solutions based on hydrogen peroxide. At high pH values (basic) organic contamination and oxidizable particles, are removed by an oxidation process. At low pH (acidic) metal contamination is desorbed from the wafer surface by forming a soluble complex.  
           [0007]    Typically, to reduce processing times and increase throughput, in prior at processes, ex-situ cleaning processes are performed following particle generating processes such as plasma etching or PECVD film deposition. For example, common particle removal mechanisms which may be exploited, depending on the particle and how it adheres to the surface, include dissolution, oxidizing degradation and dissolution, physical removal by etching, and electrical repulsion between a particle and the wafer surface.  
           [0008]    Standard wafer cleaning processes have included mechanical scrubbing and ultrasonic, for example megasonic agitation of the wafer surface in a cleaning solution or in deionized water to effectuate particulate removal. A shortcoming of mechanical scrubbers includes is a demonstrated difficulty in removing particles smaller than about 300 nm. In addition, mechanical scrubbers may produce and unacceptable level of scratching in soft materials. In using a megasonic source of agitation, megasonic transducers operating in a frequency range near about 1 MHz are typically attached to the side or bottom portion of a cleaning tank filled with cleaning solution with the surfaces of the process wafers arranged parallel to the direction of traveling or standing megasonic waves induced at the side of the cleaning tank. The transducer is typically rectangular shaped and integrally attached to the cleaning tank to achieve megasonic cleaning action. A shortcoming of prior art megasonic cleaning processes is the relatively low level of cavitation action produced by megasonic transducer arrangements of the prior art. Under certain conditions the ultrasonic energy also creates cavitation bubbles within the liquid where the sound pressure exceeds the liquid vapor pressure. When the cavitation bubbles collapse, energy is released causing turbulent flow which can dislodge particles adhering to the wafer surface. Typical megasonic transducers of the prior art have not sufficiently coupled ultrasonic energy into the cleaning solution to create a sufficiently high density of cavitation bubbles to achieve primarily cavitation collapse induced particulate cleaning. As a result prior art processes have not been fully successful in removing smaller particles from wafer process surfaces, particularly those smaller than about 0.3 microns.  
           [0009]    Another shortcoming of prior art cleaning processes, for example megasonic cleaning processes, is the tendency for the removed particles to reattach to the wafer surface. For example, following treatment of a large number of wafers in megasonic cleaners of the prior art, the cleaning solution must frequently be replaced to avoid particulate recontamination of process wafers.  
           [0010]    There is therefore a need in the semiconductor wafer processing art to develop an ultrasonic cleaning system and method driven primarily by cavitation whereby particulate contamination is effectively removed from a process wafer surface while avoiding particulate recontamination.  
           [0011]    It is therefore an object of the invention to provide an ultrasonic cleaning system and method driven primarily by cavitation whereby particulate contamination is effectively removed from a process wafer surface while avoiding particulate recontamination, in addition to overcoming other shortcomings and deficiencies of the prior art.  
         SUMMARY OF THE INVENTION  
         [0012]    To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a cavitation cleaning system and method for using the same to remove particulate contamination from a substrate.  
           [0013]    In a first embodiment the method for using the cavitation cleaning system includes providing at least one substrate immersed in a cleaning solution said cleaning solution contained in a cleaning solution container said cleaning solution container further including means for producing gaseous cavitation bubbles by application of ultrasound energy said gaseous cavitation bubbles arranged to contact at least a portion of the at least one substrate; applying ultrasound energy to create gaseous cavitation bubbles to contact the at least one substrate to remove adhering residual particles in a substrate surface cleaning process; and, recirculating the cleaning solution through a particulate removing filtering means during the substrate surface cleaning process.  
           [0014]    These and other embodiments, aspects and features of the invention will be better understood from a detailed description of the preferred embodiments of the invention which are further described below in conjunction with the accompanying Figures.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIGS. 1A-1B are representative cross sectional side views of portions of exemplary cavitation cleaning systems including several embodiments of the present invention.  
         [0016]    [0016]FIGS. 2A and 2B are schematic representations of a portion of an exemplary cavitation cleaning system for use in the cavitation cleaning process according to an embodiment of the present invention.  
         [0017]    [0017]FIG. 3 is a process flow diagram including the cavitation cleaning process using the cavitation cleaning system according to embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Referring to FIG. 1A, in an exemplary first embodiment, an ultrasonic cavitation system is provided for producing cavitation bubbles for assisting in removal of particulate contamination adhering to semiconductor process wafers. A cavitation cleaning module  12  enclosed by a housing e.g.,  12 A is provided having a cavitation power system (not shown), including at least one vibration generator  14  powered by transducers, for example disposed in an upper portion of the cleaning module  12 . The vibration generator  14 , includes cavitation bubble producers (e.g., cavitation rods as shown in FIG. 2A) mounted in vibrational producing relationship with the vibration generator  14  for coupling ultrasonic energy into the cavitation bubble producers to produce cavitation bubbles. Preferably, the cavitation bubbles produced by the cavitation bubble producers are formed to flow in a direction parallel to a wafer process surface to contact the wafer process surfaces within the cleaning module  12 .  
         [0019]    For example, the cleaning module  12  is preferably configured to accept a cassette holding a plurality of wafers e.g.,  16 A the wafer surfaces are preferably arranged below the cavitation bubble producers to contact the cavitation bubbles flowing according to gravitational influence and/or induced fluid flow toward and parallel to the wafer surfaces as indicated conceptually by directional arrows, e.g.,  18 . Disposed in a lower portion of the cleaning module  12 , is a particulate filtering means  20 A, arranged below the wafers. The particulate filtering means for example, preferably includes of a series of flow passageways as is known the art including repeatedly changing flow path directions such that upon a change of flow direction, a particle in solution is captured along the flow passageway wall the wall including ribbing or raised areas for capturing and retaining the particles. While many suitable particulate filters are commercially available it is important that the pressure drop across the filter be minimized to allow a continuous flow of cleaning solution to be provided to the cleaning module  12  to fill the cleaning module to a level to at least cover cavitation bubble producers attached to the at least one vibration generator  14  disposed in an upper portion of the cleaning module  12 . Preferably, the cleaning solution is recirculated from a cleaning solution reservoir  22  following filtering of the cleaning solution to remove particulates from the cleaning solution.  
         [0020]    For example, multiple cleaning solution outlets e.g.,  24 A may be provided in the lower portion of the cleaning module  12  to provide cleaning solution recirculation outlet pathways after at least partially passing through particulate filtering means  20 . For example a plurality of cleaning solution outlets e.g.,  24 A, communicate with a common fluidic flow pathway e.g.,  24 B to capture the cleaning solution and return it to the cleaning solution reservoir  22 . The cleaning solution is in turn provided to the cleaning module  12  from cleaning solution reservoir  22 , for example by fluidic pumping means  25  for pumping cleaning solution along fluidic flow pathway  24 C to a recirculation inlet e.g.,  24 D provided in an upper portion of the cleaning module  12 , for example, preferably above a process wafer level, to maintain a cleaning fluid level to cover the cavitation bubble producers. optionally, the cleaning solution reservoir includes a heat exchange unit e.g.,  22 B for heating or cooling the recirculating cleaning solution to maintain a cleaning solution temperature. Optionally, another particulate filtering means (not shown) may be included in the cleaning solution reservoir  22  for removing particles in the cleaning solution similar to particle filtering means  20 A. In addition, preferably, a controller (not shown) is in communication with temperature sensor units disposed in both the cleaning module  12  and the cleaning solution reservoir  22  as well as in responsive communication with the heat exchange unit  2 B and fluidic pumping means  25  for automated control of a cleaning solution temperature and recirculation flow rate to maintain a predetermined cleaning solution level at a predetermined temperature in cleaning module  12 .  
         [0021]    Referring to FIG. 1B is shown another embodiment showing an alternative side view of the cleaning module  12  and solution reservoir  22  with the wafer e.g.,  16 A having a major surface arranged perpendicular to the view direction. Two vibration generators  14 A and  14 B are shown in an upper portion of the cleaning module  12  including respectively attached cavitation bubble producers  14 D and  14 C disposed over wafer e.g.,  16 A. In an exemplary embodiment, one or more cleaning solution flow pathways e.g.,  26 A,  26 B, are provided in a sidewall of the cleaning module  12 , preferably above the particle filtering means  20  to provide for unobstructed flow. Fluid pumping means e.g.,  28 A and  28 B, for example conventional fluidic pumps, are provided in respective cleaning solution flow pathways e.g.,  26 A, and  26 B to provide for accelerated drainage of the cleaning solution from cleaning module  12  following termination of cavitation. For example, it has been found that is frequently advantageous to quickly drain the cleaning fluid from the cleaning module following a cavitation cleaning process to prevent reattachment of particles remaining in the cleaning solution to the process wafer surface following termination of cavitation. For example, in operation, following a cavitation cleaning process, cavitation power supply is terminated to the at least one vibration generator  14 , followed by accelerated removal of the cleaning solution from cleaning module  12  by aid of pumping means e.g.,  28 A and  28 B, and respective solution flow pathways e.g.,  26 A, and  26 B, to cleaning solution reservoir  22 , for example at flow rates of about 1 to about 10 gallons per minute. For example, preferably, the cleaning solution is removed before it can diffuse or migrate a distance from about a center portion between wafer surfaces to a wafer surface. Particles remaining in the cleaning solution may be removed from the cleaning solution in a subsequent particulate filtering process, for example, by passing the cleaning solution through particulate filtering means included in the cleaning module  12  and/or the cleaning solution reservoir  22 .  
         [0022]    Referring to FIG. 2A is shown a cross sectional side view of an exemplary cavitation bubble producer  30 , for example a cavitation rod preferably formed of quartz having a hollowed cylindrical portion forming a longitudinally extending length portion e.g.,  32 A, preferably closed at a distal end  32 B, and a cylindrical base portion e.g.,  34 A opposite the distal end  32 B, attached to the longitudinally extending length portion  32 A, for example, by means of a tapered transition portion  34 B tapered down from a larger radius of the base portion  34 A to the smaller radius of the axially extending length portion  32 A. Optionally attached to base portion  34 A is an inlet  32 C for supplying a flow of gas, for example, preferably compressed air or nitrogen to pass along the axially extending length portion  32  at a predetermined flow rate and pressure for producing cavitation bubbles. Formed over at least a portion of the axially extending length portion  32 A are apertures, e.g.,  36 A, formed to penetrate the wall of the hollowed cylinder forming the longitudinally extending length portion  32 A to communicate with a gas flow within the longitudinally extending length portion  32 A. Preferably, the apertures penetrating the axially extending length portion  32 A are formed at a predetermined radial spacing and predetermined axial (longitudinal) spacing along at least a portion of the longitudinally extending length portion  32 A. For example, in an exemplary embodiment, the apertures, e.g.,  36 A, are from about 0.2 microns to about 0.5 microns in diameter. For example, the apertures, e.g.,  36 A, include a plurality of apertures having a spacing between adjacently disposed apertures of from about  2  aperture diameters to about  10  aperture diameters measured from an aperture opening edge.  
         [0023]    Referring in FIG. 2B in operation, a plurality of cavitation bubble producers (rods) e.g.,  30 A,  30 B,  30 C are mounted with the base portion e.g.,  34 A in vibrational relationship with vibration generator  38  which is arranged having embedded transducers (not shown) connected to power source  40  as is known in the art to cause the vibration generator  38 , and thus the attached cavitation rods e.g.,  30 A,  30 B,  30 C, to vibrate at a predetermined frequency. For example, it will be appreciated that the cavitation rods have a predetermined characteristic vibrational resonant frequency that is dependent on both their dimensions and material, i.e., quartz. It is frequently preferable to operate at or near the characteristic resonant frequency of the cavitation rods to maximize a cavitation bubble density. For example the resonant frequency of the individual rods may vary slightly among one another, making the most effective operating frequency of the vibration generator an average of the various resonant frequencies. While the dimensions of the cavitation rods may be varied to vary an operating resonant frequency, it has been found that operating resonant frequencies of between about 400 kHz and about 2 MHZ are suitably used where a sufficient density of cavitation bubbles are be produced to effectively remove particles including smaller particles, for example, having a diameter smaller than about 0.3 microns without damaging to the process wafer.  
         [0024]    Still referring to FIG. 2B, in operation, the vibration generator  38  is supplied with a gas flow, for example along gas flow pathway  42 A in communication with gas source  44 A. A flow controller, for example a mass flow controller  44 B is disposed between the gas source  44 A and vibration generator  38 . A gas supply manifold  42 B is disposed in vibration generator  38  for individually supplying gas flow to the individual cavitation rods, e.g.,  30 A,  30 B,  30 C. For example, in operation the vibration generator  38  is supplied with a resonant frequency cavitation power from power source  40  for producing a vibrational frequency at or near the resonant frequency of the cavitation rods submerged in cleaning solution to produce cavitation bubbles in the cleaning solution. At about the same time a predetermined gas flow of preferably nitrogen or compressed air is supplied from gas source  44 A to the cavitation rods, at a predetermined gas flow rate, for example at flow rate of about 1 sccm to about 1000 sccm. The cavitation (operating) power is then adjusted to a predetermined operating power level for achieving resonant frequency and producing a predetermined density of cavitation bubbles and/or interactively adjusted to achieve a desired cavitation bubble density.  
         [0025]    In operation, the cavitation rods are arranged to be submerged in a cleaning solution and to extend perpendicular to and above a wafer process surface. When cavitation bubbles are created to exit through the apertures in the cavitation rods into the cleaning solution, the cavitation bubbles are drawn downward by at least one of gravity and induced fluidic flow to contact the process wafer surfaces. The cavitation bubbles preferably contact the process wafer surfaces including at least partially surrounding and encompass contaminating particles thereby removing the contaminating particles into the cleaning solution where they are preferably subsequently removed by particle filtering means.  
         [0026]    The cleaning solution may be any cleaning solution including deionized water or other solutions frequently used for cleaning. For example exemplary cleaning solutions may include at least one of a solution of hydrogen peroxide (H 2 O 2 ) and sulfuric acid (H 2 SO 4 ), a solution of hydrogen peroxide with choline ((CH 3 ) 3  N(CH 2 CH 2 OH)OH), a solution of H 2 O 2  and NH 4 OH and a solution of H 2 O 2  and HCl, and a solution of a carboxylic group containing acid, such as citric acid, and deionized water.  
         [0027]    Referring to FIG. 3 is a process flow diagram including a cavitation bubble cleaning method using the cavitation bubble cleaning system according to several embodiments of the present invention. In process  301 , a process wafer, for example a cassette of process wafers, is immersed in a cleaning solution contained in a cleaning module having a cavitation cleaning system including cavitation rods for forming cavitation bubbles to contact the process wafer surfaces. In process  303 , the cleaning solution is supplied in recirculating relationship to the cleaning module to cover the process wafers including the cavitation rods. In process  305 , a gas flow is supplied to the cavitation rods and a cavitation power is supplied to the vibration generator to vibrate the cavitation rods at a predetermined frequency to form a plurality of cavitation bubbles within the cleaning solution. In process  307 , the cavitation bubbles contact the process wafer surfaces for a period of time to substantially remove particulate contamination. In process  309 , the cavitation power is turned off and the remaining cleaning solution within the cleaning module is pumped out at an accelerated flow rate to avoid particle recontamination of the wafer surfaces.  
         [0028]    The preferred embodiments, aspects, and features of the invention having been described, it will be apparent to those skilled in the art that numerous variations, modifications, and substitutions may be made without departing from the spirit of the invention as disclosed and further claimed below.