Abstract:
An apparatus for cleaning a substrate includes a source of pressurized carrier gas and a body of cleaning agent in liquid form. A first conduit directs the pressurized carrier gas from the carrier gas source to the body of cleaning agent. A second conduit carries a flow of the carrier gas away from the body of the cleaning agent. The carrier gas flow carried by the second conduit includes cleaning agent in vapor form acquired from the body of cleaning agent. A nozzle is coupled to the second conduit to cause droplets of the cleaning agent to impinge upon a first face of the substrate to be cleaned.

Description:
BACKGROUND 
     1. Technical Field 
     This invention relates to semiconductor device manufacturing, and more particularly to the post chemical mechanical polishing (CMP) cleaning of semiconductor substrates. 
     2. Background Information 
     During the manufacture of semiconductor devices, after particular manufacturing steps, it is desired or required to remove contaminant particles. If not removed, such particles may cause defects in the device being manufactured or otherwise interfere with the manufacturing process. 
     For example, integrated circuits are typically formed on silicon wafers by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. After a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. The substrate may be periodically planarized via a process such a chemical mechanical polishing. This method typically includes the mounting of the substrate on a carrier or polishing head and the placing of the exposed surface of the substrate against a rotating polishing pad. A polishing slurry, which may include chemically-reactive agents and/or abrasive particles, may be introduced to facilitate the polishing. The polishing leaves the surfaces of the substrate contaminated with polishing byproducts typically including silica, alumina, or other abusive particles from the slurry as well as a variety of other particles. Other residues from the polishing include the slurry itself, and often rubber or lubricant residue left by the carrier on the unpolished surface of the substrate. 
     A variety of methods and apparatus have been used or proposed for substrate cleaning after CMP. Broadly characterized, these include immersive and spray techniques. One immersive technique involves placing the substrates in an alkaline solution of ammonium hydroxide, water and hydrogen peroxide, and subjecting the solution to ultrasonic agitation to remove contaminants. The substrates may then be rinsed and dried. 
     Prominent among spray techniques are a variety of cryogenic techniques. Such techniques require the use of a high pressure gas and frequently may include the introduction of a cleaning agent. The cleaning agent is frozen by the expansion of the gas through a nozzle and is thus impinged upon the surface of the substrate as a spray of frozen particles. Cryogenic methods typically make intensive use of the gas which may prove expensive. 
     Somewhat intermediate of the immersion and spray methods are methods which involve directing a stream of liquid onto a substrate surface being cleaned. A liquid cleaning agent is sprayed from a high pressure nozzle, with an associated high kinetic energy, for dislodging small particles from the surface. Such methods may make intensive use of the cleaning agent, with a relatively large droplet or jet size as compared with the size of the particles being removed. Thus, in addition to the high cost of the high volume of cleaning agent, expensive high pressure pumps may be required and the cleaning agent may need to be filtered to avoid damage to the substrate. Accordingly, it is desirable to provide a substrate cleaning system and method which is efficient in its use of consumable products such as cleaning agent and does not present high equipment costs. 
     SUMMARY 
     In one aspect, the invention is directed to a method for cleaning a substrate. A substrate is provided having first and second generally flat faces. A source of pressurized carrier gas is provided. A source of cleaning agent in liquid form is provided. A flow of the carrier gas is directed along a flow path from the source upstream to the substrate downstream. The cleaning agent is introduced to the flow of carrier gas at least at a first location along the flow path so as to form a vapor of the cleaning agent. The vapor is condensed to form droplets of cleaning agent in the flow at a second location along the flow path, downstream of the first location. The flow of carrier gas containing the droplets is caused to impinge on at least the first face of the wafer so as to clean the first face of debris. Implementations of the inventive method may include one or more of the following. The introduction of the cleaning agent may include bubbling the carrier gas through a body of cleaning agent in liquid form. The body of cleaning agent may be heated to a temperature above an ambient temperature. The vapor may be condensed by externally cooling the flow of carrier gas. The flow of carrier gas containing the droplets may be caused to impinge on both the first and second faces of the substrate and on a substrate perimeter. 
     In another aspect, the invention is directed to an apparatus for cleaning a substrate. The apparatus includes a source of pressurized carrier gas and a body of cleaning agent in liquid form. A first conduit directs the carrier gas from the source to the body of cleaning agent. A second conduit carries the flow of the carrier gas away from the body of cleaning agent. The flow carried by the second conduit includes cleaning agent in vapor form acquired from the body of cleaning agent. A nozzle is coupled to the second conduit for causing flow of carrier gas containing droplets of the cleaning agent to impinge at least a first face of the substrate. 
     Implementations of the inventive apparatus may include one or more of the following. The apparatus may include a heater for heating the body of cleaning agent to a temperature above an ambient temperature. The apparatus may include a cooling unit cooling the flow of carrier gas containing cleaning agent vapor. The first conduit may extend into the body of cleaning agent. The first conduit may terminate in a sparger, the sparger emitting bubbles of the carrier gas into the cleaning agent. The body of cleaning agent may be contained within a vessel and the second conduit may extend from a headspace within the vessel, the headspace containing carrier gas and cleaning agent in vapor form. The carrier gas may be compressed air or compressed nitrogen. The carrier gas may be introduced to the body of cleaning agent at a pressure of approximately 80 psi. The cleaning agent may comprise a solution of approximately 5% NH 4 OH in deionized water. Impingement of the droplets on the first face of the substrate may act so as to remove residue from the chemical mechanical polishing operation from the first face of the substrate. 
     According to a further aspect, the invention provides an apparatus for cleaning byproducts of chemical mechanical polishing from a face of a substrate. The apparatus includes an inlet, a mixing unit and a nozzle. The inlet receives the flow of carrier gas. The mixing unit introduces a cleaning liquid into the flow of carrier gas at a location wherein the pressure of the carrier gas is less than 100 psi above an ambient pressure. The nozzle directs the flow of carrier gas to impinge the face of the substrate with droplets of the cleaning liquid. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic view of a wafer cleaning system according to the present invention. 
     FIG. 2 is a partial schematic cross-sectional view of a nozzle of a wafer cleaning system according to the present invention. 
     FIG. 3 is a partial schematic cross-sectional view of a condensing section of a wafer cleaning system according to the present invention. 
    
    
     LIKE REFERENCE NUMBERS AND DESIGNATIONS IN THE VARIOUS DRAWINGS INDICATE LIKE ELEMENTS. 
     DETAILED DESCRIPTION 
     FIG. 1 shows a wafer cleaning system  110 . The system provides for the mixing of a liquid cleaning agent  12 A with a carrier gas  14  for purposes of cleaning a wafer  16 . 111  an exemplary embodiment the liquid cleaning agent  12 A consists of an approximate 5% solution of ammonium hydroxide (NH 4 OH) in deionized water, drawn from a source which may take the form of first storage tank  18 . The carrier gas  14  may be drawn from a source which may take the form of second storage tank  20 . In an exemplary embodiment, the carrier gas is compressed nitrogen. An alternate source of carrier gas is ambient air which may be compressed to a desired pressure by means of a compressor  22 . 
     The cleaning agent  12 A may be drawn from the first tank storage  18  through a cleaning agent supply line or conduit  24  to a mixing tank  26 . This may be accomplished by gravity feed, by a pump  28  in-line in the cleaning agent supply line  24 , or by other means. A gas supply line or conduit  30  leads from the second storage tank  20  via a valve  32  and/or leads from the compressor  22 . The gas supply line  30  extends into the mixing tank  26  and terminates at a foraminous sparger  34  located within a body  12 B of liquid cleaning agent in the mixing tank  26 . The gas supply line  30  thus forms a first portion of a flow path  38  of carrier gas from the source to the nozzles  66  (FIG.  2 ). The sparger  34  permits a diffusion of the flow of carrier gas into the body  12 B of cleaning agent. The carrier gas is introduced to the body  12 B of cleaning agent at an approximate pressure of 80 psi above the ambient atmospheric pressure. The carrier gas forms bubbles  40  which become saturated with vapors of the cleaning agent. The bubbles  40  rise and burst through the surface  42  of the body  12 B of cleaning agent and thus fill a headspace  44  of the mixing tank  26  with carrier gas saturated with cleaning agent. 
     Optionally, a heater  46  may be provided with a heating element  48  for heating the body  12 B of cleaning agent to a temperature above an ambient temperature. Typical ambient temperatures may run from about 19° C. to about 24° C. in most factory operations. This allows for an increased concentration of cleaning agent vapor in the carrier gas bubbles  40  diffused through the body  12 B of cleaning agent. An exemplary heater  46  may use an electric resistive heating element  48  within the tank  26  or the heat may be applied externally to the tank. Optionally, a fan  49  may be provided to induce current within the body  12 B of cleaning agent to help keep the cleaning agent mixed and at a more even temperature throughout the mixing tank  26 . 
     The flow path continues with a flow  50  of carrier gas containing cleaning agent vapor (hereinafter “gas/vapor”) exiting the headspace  44  through a conduit or cleaning line  52 . The cleaning line  52  may pass through a cooling jacket  54  of a cooling unit  56  which includes a cooling coil  55  (FIG.  3 ). The cleaning line  52  diverges and terminates at cleaning heads  60 A,  60 B and  61 . The upper cleaning head  60 A is provided for cleaning the upper face or surface  74  of the substrate  16  while the lower cleaning head  60 B is provided for cleaning the lower face or surface  76  of the substrate. The perimeter cleaning head  61  is oriented perpendicular to the heads  60 A and  60 B and is directed to clean the perimeter  78  of the substrate  16 . The cleaning line  52  may include flexible or extensible lengths  62  to facilitate motion of the cleaning heads  60 A and  60 B. The sweeping motion of the cleaning heads facilitates the use of a lower flow rate than would be required with fixed heads covering a broader area. The low flow rate enables use of a less expensive, lower capacity system. 
     As is shown in FIG. 2, the underside (the side facing the respective wafer surface  74  and  76 )  64  of each upper and lower cleaning head  60 A and  60 B bears an array of nozzles  66 . Similarly, the inboard side  63  of the cleaning head  61  faces the substrate perimeter  78  and bears a single nozzle  66 . In one embodiment, the flow of gas/vapor passes from the cleaning line  52  (FIG. 1) into the interiors  68  of the cleaning heads  60 A and  60 B and the interior  69  of the cleaning head  61  and through the nozzles  66  to form jets  70 . As each jet  70  expands upon exiting the associated nozzle  66 , the expansion causes condensation of the cleaning agent into droplets  72  within the jet. The droplets from nozzles  60 A,  60 B and  61  are caused to impinge the upper and lower faces or surfaces  74  and  76  and the perimeter  78 , respectively, of substrate  16  so as to assist in the removal of contaminants  75 . 
     As is shown in FIG. 1, in one embodiment, the substrate  16  is rotated approximately about its central axis  100  in a direction  101  while the cleaning heads  60 A and  60 B reciprocate along a linear path  102  extending substantially from the axis  100  to the perimeter  78  of the substrate. This may be done with the substrate perimeter secured between three rollers  79  which rotate the substrate while leaving its upper and lower surfaces  74  and  76  exposed. Alternatively, in situations where only an upper cleaning head  60 A is provided, the lower surface  76  of the substrate  16  may be engaged to a vacuum turntable (not shown). In an exemplary configuration, used with a 200 mm diameter wafer, the cleaning heads  60 A and  60 B are generally disc-shaped having a diameter of approximately 25 mm. There are approximately 3-10 nozzles arrayed across each cleaning head  60 A and  60 B and each nozzle is formed as a right circular bore having a diameter and a length each of approximately 0.1-0.5 mm. The cleaning heads  60 A and  60 B are reciprocated along the path  102  with the nozzles approximately 5-15 mm from the upper and lower surfaces  74  and  76  of the substrate. As wafer size increases, such as for a 300 mm wafer or greater, it may be desired to increase the number of nozzles or to provide multiple cleaning heads for each surface of the wafer. The heads associated with each surface may be independently movable or may move as a unit. The cleaning head  61  may be located a similar distance from the perimeter of the substrate and the nozzle in the cleaning head  61  may be of a similar geometry to the nozzles of the cleaning heads  60 A and  60 B. 
     As is shown in FIG. 3, the optional cooling jacket  54  on supply line  52  may be used to cool the flow  50  in the interior  80  of the cleaning line  52 . The cooling causes condensation of droplets  82  of cleaning agent within the cleaning line (i.e., preforming of droplets prior to expansion at the nozzles  66 ). The carrier gas containing these droplets may then pass through the cleaning heads  60 A,  60 B and  61  and their nozzles  66 . The precooling of the flow  50  provided by the cooling jacket  54  and the associated preforming of the droplets allows larger droplets to be formed than would be achieved by expansion at the nozzles alone. An exemplary cooling unit  56  may comprise a conventional phase change refrigeration system or may comprise a conventional Peltier-type unit. In certain embodiments, the cooling may be by an amount of approximately 10 to 20° C., for example from a temperature of 50° C. down to a temperature approximately 30° C. 
     Accordingly, via manipulation of factors including the temperature of the body  12 B of cleaning agent (achieved by the heater  46 ), the cooling of the flow of gas/vapor (achieved by the cooling unit  56 ), and the selection of nozzle size and geometry (which influences the expansion), the quantity and size of cleaning agent droplets  72  impinged upon the wafer may be controlled. In one embodiment, a preferred typical droplet size is about 40 micrometers at the point of impingement with the surface  74 . A preferred range for droplet size is from about 10 to about 100 micrometers or, more particularly, from about 20 to about 50 micrometers. Smaller size droplets may be less effective at cleaning, while larger size droplets may be associated with high consumption of the cleaning agent. 
     Alternatively, if the vapor pressure of the particular cleaning agent is relatively high, it may be desirable to cool the body  12 B of cleaning agent to reduce the amount of cleaning agent introduced to the carrier gas. For example, if the cleaning agent is a particularly high concentration of ammonia in deionized water, cooling may be desired to reduce the vapor pressure. In such a case, element  46  could be formed as a cooling unit with element  48  being formed as a cooling element such as a serpentine tube for phase transition cooling. Alternatively, where the vapor pressure is particularly high, means such as the sparger  34  may not need to be provided to introduce the vapor to the carrier gas. 
     The invention facilitates the use of a relatively low pressure carrier gas which can be much easier and less expensive to supply than a high pressure carrier gas. Accordingly, in one preferred embodiment, the carrier gas may be provided by a house compressed air system as is common in laboratory and industrial settings. Such systems typically provide compressed air at a pressure of approximately 80 pounds per square inch (psi) above the ambient pressure. A house compressed nitrogen supply, which may be derived from a number of sources, may also be used at similar pressures. In the illustrated embodiment using house compressed air or nitrogen at 80 psi, the vast majority of the pressure drop between the 80 psi source pressure and the ambient pressure occurs upon expansion at the nozzle. Thus, the introduction of the cleaning agent to the carrier gas in the mixing tank  26  occurs at substantially the source pressure. The source pressure is preferably greater than 40 psi and less than 100 psi. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, a variety of other methods exist for generating the required aerosol of cleaning agent droplets in the carrier gas. Certain methods which may be suitable are described by William C. Hinds, Aerosol Technolog, 1982, John Wiley &amp; Sons, Inc., pp. 379-395, the disclosure of which is incorporated herein by reference. A variety of cleaning head and nozzle configurations and movements are also possible. 
     Accordingly, other embodiments are within the scope of the following claims.