Patent Publication Number: US-6217735-B1

Title: Electroplating bath with megasonic transducer

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to wet process chemistry treatment of substrates, e.g., semiconductor wafers and the like, and is more particularly directed to a technique for plating a flat workpiece in a manner that is efficient and also minimizes surface defects. The invention also concerns a technique that facilitates robotic handling of the articles. 
     Electroplating plays a significant role in the production of many rather sophisticated technology products, and has recently begun to be used for metallization of semiconductor devices. Recently there has been interest in using plating techniques to form copper conductors on silicon to increase the power or speed of the semiconductor devices. 
     A number of techniques for electro-depositing or coating on an article face been described in the patent literature. 
     A recent technique that employs a laminar flow sparger or injection nozzle within the plating bath is described in my recent U.S. Pat. No. 5,597,460, granted Jan. 28, 1997. The means described there achieve an even, laminar flow across the face of the substrate during the plating operation. A backwash technique carries the sludge and particulate impurities away from the article to be plated, and produces a flat plated article of high tolerance, such as a high-density compact disc master or semiconductor wafer. The techniques in that patent improve the flow regime for the plating solution within the tank or cell, as the flow regime is regarded as being crucial for successful operation. Flow regime is affected by such factors as tank design, fluid movement within the process vessel, distribution of fluid within the vessel and at the zone of introduction of the solution into the vessel, and the uniformity of flow of the fluid as it is contacts and flows across the substrate in the plating cell. 
     In the plating cell as described in said U.S. Pat. No. 5,597,460, a plating bath contains the electrolyte or plating solution, in which the substrate to be plated is submerged in the solution. A sparger or equivalent injection means introduces the solution into the plating bath and forms a laminar flow of the electrolyte or plating solution across the surface of the substrate to be plated. A circulation system draws off the solution from the anode chamber, together with any entrained particles, and to feeds the solution through a microfilter so that all the particles of microscopic size or greater are removed from the plating solution. Then the filtered solution is returned to the sparger and is re-introduced into the plating cell. 
     The flow regime as described in said U.S. Pat. No. 5,597,460 is further improved by the geometry of the well that forms the tank for the plating bath. The well has a cylindrical wall that is coaxial with the axis of the substrate. This arrangement was intended to avoid corners and dead spaces in the plating cell, where either the rotation of the substrate or the flowing movement of the plating solution might otherwise create turbulences. 
     An increased evenness in plating is achieved by the technique of my U.S. Pat. No. 5,634,564 in which a rotary blade or wiper is positioned in the plating bath. 
     Electroless plating is favored in many applications, and especially in those where there is no electrically conductive layer that could serve as a cathode. Accordingly, electroless plating is now seen as an economical alternative to sputtering or vacuum deposition. This is especially true for metals that are difficult to deposit using sputtering or plasma techniques. One advantageous approach to electroless plating is disclosed in my earlier U.S. Pat. No. 5,865,894. In that arrangement, a megasonic transducer adjacent the floor of the plating cell applies megasonic energy at a frequency of about 0.2 to 5 MHz to the solution. The frequency can be above 1 MHz, and in some cases above 5 MHz. The megasonic waves distribute the solution evenly on the substrate, and also break up any bubbles or concentrations that may lead to defects in the plated surface. 
     Megasonic plating technique can improve the process for electroplating silicon wafers, and an example of this technique, in which the flow regime is further improved by imposing a rotary motion, as described in my U.S. Pat. No. 5,865,894. The megasonic transducer and the rotary blade can be incorporated together in a plating cell, as described and illustrated in my pending U.S. patent application Ser. No. 08/954,239, filed Oct. 20, 1997 now U.S. Pat. No. 5,904,827. 
     The techniques described in my U.S. Pat. No. 5,932,027 permit mounting the substrate and lowering the substrate into the plating cell to be automated or robotized. Automation and robotization of the insertion, removal, and transport of the workpiece from one process cell to another make it possible to conduct the entire multiplestep plating operation in a clean or super-clean environment. In the technique of that application, the carrier for the substrate is disposed on a sealable door for the plating cell. The door opens to a loading position, which is preferably the horizontal position, and closes to a position which preferably holds the substrate vertically in the plating chamber. The door sealably seats onto an opening in a side wall of the cell. For electroplating use, a cathode ring may be disposed at the periphery of the door opening for making electrical contact with the substrate when the door is closed. This arrangement can lend itself to robotization of the plating process, but nevertheless requires the transfer of the substrate from a transfer holder to a platen associated with the plating cell. Moreover, mechanical and fluid handling considerations must be addressed because of the need to move the substrate between horizontal and vertical orientations. 
     High precision electroplating in the past has required either rotation of the substrate or rotation of a wiper to induce the removal of hydrogen bubbles from the surface to obtain uniform plating free of defects. For this reason the substrate had to be positioned either in a vertical orientation or in a diagonal or slant orientation, rather than horizontal, so that the bubbles would not collect on the surface. There has not been an effective technique for plating wafers or other substrates in a horizontal, circuit-side-down, orientation. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an electroplating arrangement which reliably and evenly plates metallic material onto a wafer or other substrate, and which avoids the drawbacks of the prior art. 
     It is another object of this invention to provide an electroplating arrangement in which the substrate is maintained in a horizontal position, and which can be adapted easily to an automated wafer handling mechanism. 
     According to one aspect of the present invention, a planar face of a substrate is plated with a metal layer. The plating is carried out with the substrate held horizontal, and with the planar face, i.e., the circuit side of the wafer of other substrate, oriented downwards. The associated electroplating bath employs an elongated, horizontally extending tray that has an open top. An elongated, horizontally extending megasonic transducer is situated at the base of the tray, and an anode extends horizontally above the transducer. A sparger arrangement supplies a flow of a process fluid, such as an electrolyte, into the tray, and the megasonic transducer creates a transverse ridge of the electrolyte or other wet process fluid that projects upwards from the tray. This ridge can contact a substrate passing over the tray. The method of plating involves applying megasonic energy to the transducer to create the transverse ridge of said electrolyte. A plating current is applied between the anode and the substrate. The substrate is oriented horizontally and face down so it is in position to contact the transverse ridge of electrolyte, and then either the substrate holder or the tray is moved in the direction across the ridge to effect relative motion as between substrate and tray so that the ridge sweeps across the face of the substrate. A rinser may be positioned alongside the electrolyte tray for rinsing the substrate after the plating operation. 
     The anode may be in the form of a transverse metal rod or similar conductive member that extends parallel to the transducer and above it within said electrolyte. Alternatively, the anode may be incorporated as a lens of the transducer, and may take the form of a stainless steel surface affixed onto a top of the transducer. The holder for the wafer or other substrate may be a heated or unheated chuck. The megasonic energy also heats the substrate where the ridge contacts it, and in some cases may partially or fully heat-treat the metallization. 
     Preferably, the sparger arrangement creates a non-turbulent flow of the electrolyte that emanates from each side of the megasonic transducer. In a preferred mode, the sparger arrangement includes first and second rows of openings, one row disposed along one side of the transducer, and one row disposed along the other side. 
     The above and many other objects, features, and advantages of this invention will become more fully appreciated from the ensuing detailed description of a preferred embodiment, which is to be considered in conjunction with the accompanying Drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a plan of an electroplating bath arrangement according to one preferred embodiment of this invention. 
     FIG. 2 is a sectional of this embodiment taken at line  2 — 2  of FIG.  1 . 
     FIG. 2A is an enlargement of a portion of FIG.  2 . 
     FIG. 3 is a sectional elevation for explaining the operation of this invention. 
     FIG. 4 is a sectional end view of a second embodiment. 
     FIG. 4A is an enlargement of a portion of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     A megasonic plating arrangement  10  according to an embodiment of this invention is illustrated in FIGS. 1,  2  and  2 A. Although not shown here, the arrangement  10  can be disposed within a cabinet, with a clean air system and automatic wafer handling equipment so that the plating, and possibly other process steps, are carried out automatically for a number of wafers or other workpieces in sequence. The system also includes fluid treatment and replenishment equipment, which is not shown here but would be understood by those skilled in this art. As shown in the plan view of FIG. 1, an elongated tray or basin  12  is preferably formed of a durable non-reactive, non-conductive material such as polypropylene. The tray is elongated in one horizontal direction, and has an open top. The tray has an elongated central portion  14  whose bounds are defined by a generally rectangular weir  16 . As shown in FIG. 2, the weir  16  has a sawtooth upper edge  18  that serves as a spillway for fluid exiting the central portion  14 . A trough  20  surrounds the weir  16  and receives any liquid that spills over the weir. A megasonic transducer  22  is situated in the base of the tray at the central portion  14 , and here is shown elongated in the long direction of the tray. A stainless steel rod  24 , which serves as anode, is positioned to extend through one wall of the weir  16  and across the central portion, above and centered over the transducer  22 . A pair of spargers  26  are formed on the floor of the tray and are situated one opposite sides of the transducer  22 . Each sparger  26  is in the form of a semicylindrical tubular member  28 , with a row of openings  30  through the floor of the tray  12  to introduce a fluid flow into the central portion  14  of the tray. The spargers create a laminar, i.e., non-turbulent flow of the electrolyte out over the top of the megasonic transducer  22 . The openings  50  can be holes, pores or foramina. Instead of a row of openings  30 , the spargers  26  can be provided with a slit or a series of slits. There can be more than one row on a side. Still other configurations are possible within the scope of this invention. Here the holes  30  are shown drilled through at a slant in the direction towards the mid-line of the tray  12  and of the transducer  22 . Also shown here is a drain tube  32  for conducting away fluid that enters the trough  20 . While not shown here, the drain tube connects with a return conduit to pump, filtration, and fluid conditioning equipment, and the conditioned fluid is eventually supplied again to the spargers  26 ,  26 . 
     As shown in FIG. 2A, there can be a cover  40  beneath the tray  12  to protect the megasonic transducer. This is removable to service the transducer. As also shown here, the transducer  22  fits against a generally rectangular opening  42  in the floor of the tray, so that the transducer is in direct contact with the fluid. The opening  42  serves as a megasonic lens. 
     The plating operation of this invention is illustrated in FIG. 3, in which the parts and elements described earlier are identified with the same reference numbers. In this embodiment, the megasonic transducer  22  directs its energy upwards into the fluid in the tray  12  directly over the opening or lens  42 . This pushes the fluid upwards, in effect creating a fluid wall or ridge  44 . The frequency is selected within the megasonic range (nominally 500 KHz to 5 MHz) as is the strength of the applied megasonic signal, so as to achieve an optimal ridge  44 . This ridge extends for the length of the opening  42 , and rises above the top of the weir  16 . A chuck or platen  46  is situated for horizontal motion, and is disposed to hold a semiconductor wafer  50 , or other substrate to be plated, in a horizontal, face-down orientation, i.e., with the plated face  52  of the wafer  50  facing downwards towards the tray  12 . The chuck  46  is moved in the horizontal direction perpendicular to the long direction of the tray  12  and transducer  22 , i.e., across the ridge or wall  44 . This draws the wafer  50  through the plating fluid above the rod  24 . A plating current supply (not shown) has its positive side connected to the rod  24  and its negative side connected to the face  52  of the wafer. The plating supply may have a nominal plating voltage, e.g., 12 volts. 
     The horizontal positioning of the chuck and wafer, and the motion of the chuck relative to the tray, causes a relative horizontal motion so that the ridge or wall  44  sweeps across the face  52  of the wafer  50 . When this happens, the electroplating fluid contacts the wafer  50  only along a horizontal line of contact, and the electrolyte does not flow to other areas of the wafer face  52 . The electroplating is carried out non-turbulently and only along a well defined zone of contact, resulting in extreme evenness and a high level of repeatability from one wafer to the next. 
     The electroplating operation can require one pass or more than one pass, as required by the materials and chemistry involved. 
     Another embodiment of this invention is illustrated in FIGS. 4 and 4A, in which elements that correspond to the same elements in FIGS. 1 and 2 are identified with the same reference numbers, but raised by  100 . In this case, the tray  112  and spargers  126  are identical with those in the previous embodiment, with the weir  116  and the trough  120  serving the same purpose as previously. However, in place of the transverse rod  24 , in this embodiment the anode is formed of a conductive surface  124  formed on the top surface of the megasonic transducer  122 . As shown in FIG. 4A, on the top of the transducer  122  there is a dielectric or insulating layer  123 , with a stainless-steel or similar non-reactive conductive layer  124  affixed on top of the dielectric layer  123 . 
     While the invention has been described with reference to a preferred embodiment, it should be recognized that the invention is not limited to that precise embodiment, or to the variations herein described. Rather, many modifications and variations would present themselves to persons skilled in the art without departing from the scope and spirit of the invention, as defined in the appended claims.