Abstract:
A sequence of temperature control in electroless plating for microelectronic processing is disclosed in this invention. This sequence improves the uniformity of the deposit, increases the lifetime of the plating bath and is cost effective. The plating bath is heated to a temperature, which is lower than the minimum deposition temperature, in an apparatus outside the plating chamber. Then the solution is introduced into the plating chamber without the occurrence of the deposition. After the chamber is filled, the solution is heated up to the desired deposition temperature. The deposition is initialized. After the deposition, the solution is returned back to the original tank.

Description:
BACKGROUND OF THE INVENTION 
     Electroless plating refers to the autocatalytic or chemical reduction of aqueous metal ions to metal atoms on a substrate without application of an electrical current. Electroless plating processes and compositions are found in a wide variety of commercial practices and are used for plating a substantial number of metals and alloys onto various substrates. Examples of materials commonly plated through this process can include copper, nickel, gold, cobalt, tin-lead alloys, etc. The substrate surface can be any surface that is either catalytically active itself or can be activated by a catalyst. Possible substrates common in the past include, for example, metal, diamond, and a variety of polymers. Plating processes can be either selective, i.e., only a portion of the substrate surface is catalytically activated to control precisely where metal deposition will occur, or alternatively can be used to coat an entire substrate surface. 
     Electroless plating has been widely used in the microelectronics industry for deposition of layers on semiconductor wafers. For example, electroless plating has been used in the past to form adhesion, barrier and capping layers on substrates. For the purposes of this disclosure, a barrier layer is defined as a layer formed on at least a portion of a substrate surface which can prevent contact between the materials located on either side of the barrier layer. For example, a barrier layer can prevent oxidation or otherwise render passive the material covered by the barrier layer, or alternatively can prevent the material contained in a layer located on one side of the barrier layer from diffusing into a layer located on the other side of the barrier layer. For instance, in the microelectronics industry, Co(W)P and NiP are examples of two barrier layers used in the past for the prevention of copper ion diffusion into the substrate as well as for copper passivation. 
     Electroless plating processes known in the past generally include heating a bath solution to a certain deposition temperature, which generally corresponds to at least the minimum deposition temperature (i.e. the minimum temperature where deposition from that bath to that substrate can occur). After heating, the bath solution is pumped into a plating chamber. In the plating chamber, a substrate having an activated surface is present and the electroless plating begins at or near the time the hot solution contacts the substrate. 
     The plating process itself includes an induction period followed by a steady-state deposition period. The induction period is the time necessary to reach the mixed potential at which the steady-state metal deposition occurs. The deposition is usually designed to occur in a certain pH and temperature range. In a certain range, the deposition rate is proportional to the bath temperature. As such, most electroless plating processes heat the bath to the highest deposition temperature possible to take advantage of the higher deposition rate and increase process throughput. Bath temperature is one of the most important factors affecting the deposition rate of the layer. In addition to the deposition rate, however, bath temperature can also affect the uniformity and composition of the deposit and hence its properties. As such, temperature control of the electroless plating bath continues to be very important in these processes. 
     In addition to high throughput, uniformity of the deposit formed on the substrate is greatly desired. In the past, the bath solution has been introduced into the plating chamber via a rotating showerhead which has a slit opening or holes. Because the temperature of the bath is high, the induction period is short, and the deposition can start essentially upon contact of the solution with the substrate surface. This spray method can significantly affect the uniformity of the deposit formed on the substrate, however, due to, among other factors, the flow pattern of the bath solution as it is fed into the chamber. In addition, uniformity of the deposit can be affected by distribution of temperature on the wafer surface itself as the deposit is formed. 
     Prior art methods of electroless plating have proven problematic in many aspects. For example, a suitable showerhead design has proven very difficult for obtaining high quality products. Variation in head opening shapes and sizes, head rotation speed, and flow rate can cause different flow patterns across the substrate surface and affect deposition uniformity. For instance, one region of the substrate can have greater exposure to the high temperature bath solution and subsequently can have more material deposited at those regions. The design of the showerhead has thus been very important in an attempt to obtain even distribution of the solution. 
     In addition, the high temperature of the bath itself has often been disadvantageous in these processes. For example, loss of water from the bath due to evaporation can cause a change of concentration of the components and consequent change in deposition rate. In order to avoid this, the composition of the bath solution must be closely monitored and water must be frequently replenished. Additionally, reducing agents used in baths will often experience accelerated decomposition at high temperature, thus the lifetime of the bath solution can be shortened due to the high bath temperature. Moreover, in the processes of the past, a large volume of solution is usually held at high temperature and recirculated through the system for each subsequent plating operation. Such systems can demand high energy inputs and create high operating expenses. 
     Prior art processes also often require a long preparation time prior to deposition. Usually the tank size for the bath solution is 10 gallons or larger for industrial use. Such a tank size requires a long period of time for heating the solution from room temperature to deposition temperature. In addition, after the process is finished and the heater is shut down, the solution will often be circulated for a long period of time until cooled sufficiently to avoid excess bath decomposition. 
     As such, there is a need for an improved electroless plating process which can provide high quality, uniform deposition on a substrate at a high level of throughput, as well as increase bath lifetime and decrease energy requirements of the system. 
     SUMMARY OF THE INVENTION 
     In general, the present invention is directed to an electroless plating method which includes a temperature control sequence for the bath solution which is used in the plating process. More specifically, in one embodiment the present invention comprises conveying an electroless plating bath solution from a holding tank to a plating chamber, wherein the electroless plating bath solution is at a temperature less than but relatively close to the minimum deposition temperature of the bath solution. Once in the plating chamber, the bath solution can be heated to the deposition temperature and electroless deposition can take place. 
     Any suitable electroless plating bath solution can be used in the process of the present invention. In general, an electroless plating bath solution can comprise one or more sources of metal ion, a reducing agent, and a complexing agent. For instance, in one embodiment the source of metal ions can be cobalt sulfate and sodium tungstate. In an alternative embodiment, nickel chloride and nickel sulfate can be the metal ion sources. One example of a possible reducing agent can be sodium hypophosphate. One example of a possible complexing agent in the electroless plating bath solution can be sodium citrate. After filling the holding tank, at least a portion of the bath solution can then be removed from the holding tank, optionally preheated and moved into a plating chamber while at a temperature less than the minimum deposition temperature of the bath. 
     Once in the plating chamber, where the solution contacts at least a portion of a substrate, the bath solution can be further heated to a deposition temperature, at which time electroless deposition can occur and the desired deposit can form on the substrate. In one embodiment, the deposition temperature can be between about 60° C. and about 90° C. More specifically, the deposition temperature can be between about 70° C. and about 75° C. At lower deposition temperatures, preheating of the bath solution may not be necessary. If desired, the bath solution can be returned to the holding tank after the electroless deposition and the solution can be cooled and recycled. 
     When preheating of the solution is desired, the bath solution can be preheated to a temperature such that the temperature of the solution filling the plating chamber can be less than the minimum deposition temperature of the bath solution. For example, the bath solution can be preheated to a temperature less than about 10° C. below the minimum deposition temperature of the bath solution. More specifically, the bath solution can be preheated to a temperature between about 5° C. and about 10° C. below the minimum deposition temperature of the bath solution. In one embodiment, the bath solution can be preheated to a temperature of between about 50° C. and about 55° C. prior to charging it to the plating chamber. 
     In one embodiment, the entire holding tank of bath solution can be preheated in the holding tank itself prior to transferring the solution to the plating chamber. Alternative, only a portion of the bath solution can be removed from the larger holding tank and then preheated prior to charging the plating chamber with the portion of the bath solution. For instance, less than about 25% of the total volume of bath solution can be removed from the holding tank and preheated and particularly less than about 15% of the total amount of bath solution can be removed from the holding tank and preheated. In one embodiment, less than about 10% of the total amount of the bath solution can be removed from the holding tank and preheated. 
     The bath solution can be preheated prior to entering the plating chamber by any suitable method. For instance, the bath solution can be preheated in a separate pre-heat tank or alternatively in a heated line as it flows from the holding tank to the plating chamber. 
     The layer formed on the substrate through the process of the present invention can be any type of layer. For instance, the layer can cover the entire surface of the substrate or alternatively can cover only a portion of the substrate, as in a pattern. The layer can also be any desired thickness. Specifically the layer can be less than about 200 Å thick. More specifically, the layer can be between about 50 Å and about 100 Å thick. 
     In one embodiment, the substrate which can have the electroless plating layer deposited upon it can be a semiconductor wafer. For example, the deposit on the wafer can be a barrier layer which can act as a passivation layer for a copper layer already applied to the wafer. 
     Other features and aspects of the present invention are discussed in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the appended figures in which: 
         FIG. 1  illustrates one embodiment of the flow sequence for the electroless plating bath solution of the present invention; 
         FIG. 2A  illustrates one embodiment of the plating chamber of the present invention when the chamber is closed; 
         FIG. 2B  illustrates the plating chamber of  FIG. 2A  when open, as when a substrate is being loaded into the chamber for processing; 
         FIG. 3  illustrates another embodiment of the flow sequence for the electroless plating bath solution of the present invention; 
         FIG. 4A  illustrates another embodiment of the plating chamber of the present invention when the chamber is closed; and 
         FIG. 4B  illustrates the plating chamber of  FIG. 4A  when open, as when a substrate is being loaded into the chamber. 
       Repeat use of references characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention which broader aspects are embodied in the exemplary construction. 
     Electroless plating provides a method of forming a layer on at least a portion of a substrate surface without the application of an electrical current. The present invention provides an electroless plating method with improved uniformity of deposition while decreasing the energy requirements of the system and increasing the life of the electroless plating bath solution. 
     In general, the method of the present invention comprises introducing a temperature control sequence to an electroless plating bath solution in any electroless deposition process which takes place at higher than ambient temperature. More specifically, the present invention comprises introducing the electroless plating bath solution to the plating chamber at a temperature below the minimum deposition temperature, and then, only after the plating chamber has been filled, heating the bath solution in the plating chamber to a deposition temperature at which electroless deposition can occur and the deposit can form on the substrate. If desired, the process of the present invention can also include preheating the bath solution prior to charging the plating chamber. For example, the bath solution can be heated to a temperature slightly below the minimum deposition temperature prior to charging the solution to the plating chamber. 
     In one embodiment of the present invention, rather than charging all of the bath solution from the holding tank to the plating chamber, only a portion of the bath solution can be taken from the larger holding tank and charged to a small plating chamber. In this embodiment, the smaller portion of the bath solution can be heated to a temperature somewhat less than the minimum deposition temperature of the solution prior to filling the plating chamber. 
     The plating chamber can already contain the substrate to be processed when the bath solution is introduced. After the bath solution has been introduced into the plating chamber, the solution is then heated to the deposition temperature which is at least equal to the minimum deposition temperature of the solution and may be somewhat higher. The solution is held at or near the deposition temperature for a suitable amount of time such that electroless deposition occurs and a layer is formed on the substrate surface where desired. After the desired deposition time, the bath solution can be removed from the plating chamber and returned to the holding tank. 
     The process of the present invention can provide many improvements over heated electroless plating processes known in the past. For example, uniformity of deposition problems found in past processes due to filling the plating chamber with a bath solution which is already at or above the deposition temperature can be eliminated. Additionally, in some embodiments of the present invention, only a small portion of the total bath solution is heated at any one time, so the energy requirements of the system can be low. Moreover, by heating the bath solution only during the deposition process and immediately cooling the solution again by circulating it back into the larger, cooler holding tank, the effective life of the bath solution can be extended since thermal decomposition of the bath components can be minimized. Other advantages of the present invention are numerous and will become apparent throughout the remainder of this disclosure. 
     The methods of the present invention are suitable for use with any desired electroless plating process which can be used to deposit a layer onto a substrate at an elevated temperature. For instance, the present invention is suitable for either acid or alkaline electroless plating bath solutions. In general, electroless plating solutions include one or more sources of metal ions, a reducing agent, a complexing agent, and/or any other desired constituents, such as, for example, stabilizers, pH adjusters, salts, etc. which can be used to obtain the desired plating characteristics of the bath. Such solutions have been known and utilized in the past in many different industries, including, for example, the microelectronics industry. 
     In one embodiment, electroless plating solutions can be used in the process of the present invention which deposit layers, such as adhesion, barrier and capping layers on semiconductor wafers at an elevated temperature. For example, an electroless plating solution can be formed which can be used to deposit a metal phosphide barrier layer on a substrate, such as a semiconductor wafer, at an elevated temperature. For instance, a metal phosphide barrier layer can be plated on top of a previously formed copper layer on a substrate to prevent oxidation of the copper. Alternatively, a metal phosphide barrier layer can be plated on a substrate to prevent ions from one layer, such as a subsequently formed layer, from diffusing down into layers or the substrate material itself below the barrier layer. 
     One embodiment of an electroless plating bath solution suitable for use in the process of the present invention is a solution which can be used to deposit a cobalt-tungsten-phosphorous alloy barrier layer on a semiconductor wafer. One possible electroless plating bath solution for such an embodiment can include, for example, between about 0.03M and about 0.1M cobalt sulfate, between about 0.01M and about 0.1M sodium tungstate, between about 0.1M and about 0.5M reducing agent such as sodium hypophosphate, between about 0.1M and about 1M complexing agent such as sodium citrate, and any other desired additives such as, for example, a combination of boric acid (H 3 BO 3 ), potassium hydroxide (KOH), and a surfactant such as RE610 available from the Rhodia Corporation. For example, the electroless plating bath solution can contain between about 0.1M and about 1.0M boric acid and between about 0.01 g/l and about 0.05 g/l surfactant. In one embodiment, the bath solution can be formed with a pH of between about 9 and about 9.5. 
     In an alternative embodiment, an electroless plating bath solution can be formed which can be used in the process of the present invention to deposit a nickel phosphide layer on a substrate. In this embodiment, the plating bath can be similar to the solution discussed above, but can be formed with nickel chloride (NiCl 2 ) and nickel sulfate (NiSO 4 ) as the metal ion sources. 
     In accordance with the present invention, once formed, the electroless plating bath solution can be charged to the plating chamber at a temperature which is less than the minimum deposition temperature of the solution. In general, the electroless plating solution can enter the plating chamber at a temperature which is no more than about 10° C. less than the desired deposition temperature. After filling the plating chamber, the bath solution can then be heated to the deposition temperature such that electroless deposition occurs and a deposit is formed on the substrate. The bath solution can then be removed from the plating chamber, and the process can be repeated. 
     The bath solution can be charged to the plating chamber at ambient temperature. In an alternative embodiment, however, the bath solution can be preheated. In general, whether the bath solution is preheated depends mostly on the desired deposition temperature. For example, if the deposition temperature is within 10° C. of the ambient temperature, no preheating may be required. Otherwise, for most applications, the electroless plating bath solution is preheated prior to being charged to the plating chamber. 
     One embodiment of a possible flow sequence for an electroless plating bath solution utilized in the present invention is illustrated in FIG.  1 . In this embodiment, the process of the present invention includes charging a holding tank  100  with a bath solution. The amount of solution charged to the holding tank  100  can be any desired amount and can depend upon individual process conditions. For example, in one embodiment, about ten gallons of a bath solution can be formed and charged to holding tank  100 , although other embodiments of the invention can include holding tanks which can accommodate smaller or larger volumes of the bath solution. If desired, holding tank  100  can be sealed to prevent contamination of the solution such as through a nitrogen purge (not shown). Holding tank  100  can also include, if desired, an agitator  108 , to keep the bath well mixed in the tank. In this particular embodiment, the bath solution in the holding tank  100  can generally be at ambient temperature or can even be cooled somewhat so as to prolong the life of the bath solution. 
     In the embodiment illustrated in  FIG. 1 , rather than using the entire bath contents in the plating chamber, a portion of the bath solution can be removed from the holding tank  100  and conveyed to a plating chamber  120 . The portion of the bath solution removed from holding tank  100  can, in general, be less than about 25% of the total bath. Specifically, the portion of the bath solution removed from the holding tank can be less than about 15% of the total bath. More specifically, less than about 10% of the total bath can be removed from the holding tank  100 . 
     After removal from the holding tank  100 , the bath solution can be pre-heated to a temperature such that the bath solution can be charged to the plating chamber  120  at a temperature which is only slightly less than the minimum deposition temperature of the solution. For example, in one embodiment the bath solution can be charged to the plating chamber  120  at a temperature less than about 10° C. below the minimum deposition temperature of the solution. More specifically, the bath solution can be charged to the plating chamber  120  at a temperature between about 5° C. and about 10° C. below the minimum deposition temperature of the bath solution. 
     In one embodiment, as illustrated in  FIG. 1 , a portion of the bath solution can be removed from tank  100  via line  105  to a smaller pre-heat tank  110 . Pre-heat tank  110 , if desired, can include an agitator  104  and can be sealed, such as through a nitrogen purge, to avoid contamination. The bath solution can be pre-heated in pre-heat tank  110  by any suitable method. For example, in one embodiment, pre-heat tank  110  can include a heating plate which can be immersed into the solution and then used to heat the solution. Alternatively, the solution can be heated from the bottom of the tank by use of a heated tank base. Any suitable heating method can be utilized to pre-heat the bath solution. After pre-heating, the bath solution can be moved from pre-heat tank  110  to plating chamber  120  through line  102 . 
     In an alternative embodiment of the present invention, such as that illustrated in  FIG. 3 , rather than pre-heating the bath solution in pre-heating tank  110 , the bath solution can be pre-heated in a heated line  107 . In this embodiment, heated line  107  can be heated by any suitable method known in the art and can carry the bath solution from holding tank  100  to plating chamber  120 . As the solution moves through heated line  107  it can be heated to the desired pre-heat temperature by the time it reaches plating chamber  120 . 
     It should be understood that though  FIGS. 1 and 3  illustrate embodiments of the invention wherein the plating chamber is smaller than the holding tank and only a portion of the entire contents of the bath solution is moved from the holding tank to the plating chamber at one time, the methods discussed in these embodiments are equally applicable to those embodiments wherein the entire contents of the holding tank are moved to the plating chamber at one time for processing one or more wafers in-a plating chamber roughly equivalent in size to the holding tank. 
     In yet another alternative embodiment of the present invention, the bath solution can be charged to the plating chamber with no intermediate preheating of the solution at all, such as when it is shown to be more cost effective to heat the bath solution to the deposition temperature in only one step in the plating chamber rather than in two separate heating steps. For example, in one embodiment of the present invention, the minimum deposition temperature of the bath solution can be quite low, such as, for example, between about 5° C. and about 10° C. above the ambient temperature. In such an embodiment, it may be preferable to not pre-heat the bath solution, since it is already close to the minimum deposition temperature. In this case, the bath solution can flow directly from the holding tank  100  to the plating chamber  120  with no intermediate pre-heating. 
     Once the bath solution has been pre-heated, as desired, it can be transferred to the plating chamber  120 . One possible embodiment of plating chamber  120  is illustrated in FIG.  2 A. In general, plating chamber  120  can be about the same size as pre-heating tank  110 . Plating chamber  120  includes a substrate holder  206 , held on an arm  204  for holding a substrate  210 , such as a semiconductor wafer. The plating chamber  120  can also include a gas inlet  220 , which can allow an inert gas such as nitrogen or argon to enter the plating chamber when sealed to help protect the wafer  210  from possible contamination. Although illustrated with only a single substrate  210 , substrate holder  206  can also be designed to hold multiple substrates. 
     The substrate  210  will generally be placed into the plating chamber  120  prior to filling the plating chamber with the bath solution.  FIG. 2B  illustrates one method of loading the substrate  210  into the plating chamber  120 . In this embodiment, arm  204  can retract with a piston action, lowering substrate holder  206  below the base level of chamber  120  such that substrate  210  can be loaded onto substrate holder  206 . Once loaded, arm  204  can move up into the closed position, sealing substrate holder  206  against the sides of the chamber  120  and closing the chamber. The chamber can then be purged of any possible contaminants prior to filling with the bath solution via line  102 . 
     In an alternative embodiment of the plating chamber, as shown in  FIGS. 4A and 4B , the substrate  210  can have a diameter slightly greater than the diameter of the chamber  120 . In this embodiment, the substrate can be loaded onto substrate holder  206  when the chamber is open, as shown in FIG.  4 B and similar to the embodiment shown in FIG.  2 B. When arm  204  moves up into the closed chamber position in this embodiment, the substrate  210  itself can contact the sides of chamber  120  and form a seal with the chamber walls, thereby closing the chamber, as shown in FIG.  4 A. Such an embodiment may be utilized to minimize contact between the back side of the wafer and the plating bath solution. 
     Substrate  210  can be any desired substrate and can have a surface of any suitable material. For example, substrate  210  can include a metal, diamond, or polymer material on the surface of the substrate which can be coated by the process of the present invention. In one embodiment, substrate  210  can be a silicon based substrate such as those used in forming semiconductor devices. Additionally, the substrate can be previously coated with one or more materials prior to being subjected to the process of the present invention. 
     The surface of the substrate can be either naturally catalytically active or can be activated by a catalyst in order that the metal ions in the bath solution can deposit on the substrate surface at the deposition temperature as desired. In one embodiment, palladium can be used as the surface catalyst, though other suitable catalysts are well known in the art and could also be used. In addition, the electroless plating process can be either selective, i.e., only a portion of the substrate surface is catalytically activated to control precisely where metal deposition will occur, or alternatively can be used to coat the entire surface of the substrate. 
     The amount of the bath solution which can be charged to the plating chamber can depend on the size of the plating chamber, which, in turn, can depend on the size and number of substrates to be processed at a single time. In one embodiment, the plating chamber can be large, and the contents of the entire holding tank  100  can be charged to the plating chamber  120 . Alternatively, the plating chamber can be quite small, and only a portion of the bath solution can be removed from the holding tank and charged to the plating chamber. For example, in one embodiment, a 200 mm wafer can be processed in a plating chamber similar to that illustrated in  FIG. 2A  using a volume of bath solution of about 1.5 liters. In another embodiment, a 300 mm wafer can be processed in a larger plating chamber using a volume of bath solution of about 3.5 liters. As the pre-heated bath solution enters plating chamber  120  through line  102 , it can fill plating chamber  120  to a point such that the top surface of wafer  210  is submerged and is somewhat below the surface of the bath solution. 
     The bath solution can be charged to the plating chamber by any suitable means. For example, line  102  can simply empty into plating chamber  120  with no specific pressure or flow fittings required. No deposition will occur as the plating chamber is being filled in the present invention since the bath solution is not at the minimum deposition temperature when it is charged to the plating chamber and first contacts the substrate. Therefore, the flow pattern of the bath solution over the substrate as the chamber is filled can have little or no effect on the subsequent electroless deposition and a more uniform deposition can be attained by the methods of the present invention. 
     After the bath solution has filled the plating chamber  120 , the bath solution can be heated to the deposition temperature. For example, in one embodiment, heating plate  208  can be lowered into the bath solution to heat the solution. While the bath solution is being heated, heating plate  208  can also be rotated in the solution such that the solution is kept well mixed and heat distribution through the solution can be improved. In general, the depth of the bath solution over the substrate should be enough that the heating plate  208  can contact and heat the solution without coming into contact with the substrate  210 . 
     The solution can be heated in the plating chamber such as by heating plate  208  to a deposition temperature which is at least equal to the minimum deposition temperature of the bath solution. If desired, however, the solution can be heated to a temperature somewhat higher than the minimum deposition temperature, in order to increase the deposition rate of the process. In any event, the deposition temperature of the solution should not exceed temperatures which could cause the bath solution to become unstable and begin to decompose. 
     Once the desired deposition temperature is reached, heating plate  208  can be withdrawn from the bath solution. If desired, heating plate  208  can be rotated while held above the bath solution, to allow bath solution adhering to heating plate  208  to fall back into the bath. At or near the time the deposition temperature is reached by the bath solution, electroless deposition can begin. Because the bath solution is evenly distributed across the substrate surface prior to the onset of electroless deposition, the methods of the present invention can greatly enhance uniformity of the deposit formed on the substrate. 
     A temperature control process can be used to monitor the temperature of the bath solution in the plating chamber so that if the bath temperature drops below some preset minimum the heating plate  208  can again be lowered into the bath solution and the solution can be reheated to the deposition temperature. 
     In one embodiment of the present invention, a metal phosphide barrier layer can be plated onto a substrate using a bath solution such as that discussed above. In such an embodiment, the minimum deposition temperature of the solution can be about 60° C., and the deposition temperature of the bath solution can be anywhere between about 60° C. and about 90° C., at which point the solution can become unstable. While deposition can occur at the minimum deposition temperature, the process can be slow, with an induction period of up to about five minutes prior to reaching steady-state metal deposition. As such, it may be desirable to heat the bath solution to a higher deposition temperature, for example, between about 70° C. and about 75° C. In this temperature range, induction period can be very short, and deposition can begin almost immediately once the bath solution reaches the deposition temperature. 
     The amount of time the substrate can be held in the heated bath solution can depend upon many factors, including, for example, desired film thickness. In general, any desired film thickness can be formed by the methods of the present invention. For instance, a film thickness of less than about 200 Å can be formed by the methods of the present invention. More specifically, a layer of between about 50 Å and about 100 Å thick can be formed by the methods of the present invention. 
     In general, the steady state deposition period of the process of the present invention can be less than about 10 minutes. Specifically, steady state deposition period can be less than about 5 minutes. More specifically, steady state deposition period can be less than about 3 minutes. 
     In one embodiment of the invention, a metal-phosphide layer can be formed on the substrate surface, and the bath solution can be heated to a deposition temperature of between about 70° C. and about 75° C. In such an embodiment, a deposition layer of between about 50 Å and 100 Å can be formed on at least a portion of the substrate surface with a steady state deposition period of less than about 2 minutes. More specifically, in such an embodiment, a metal-phosphide deposition layer of between about 50 Å and 100 Å can be formed with a steady state deposition period of about 1 minute. 
     Referring again to  FIG. 1 , after deposition has occurred, the bath solution can be removed from the plating chamber  120  via line  115  and returned to holding tank  100 . The holding tank will generally be at a temperature lower than the temperature of the bath solution coming from the plating chamber  120 . Therefore, as the bath solution is returned to the holding tank  100  the bath solution temperature will drop. For instance, when only a portion of the total bath solution has been used in the plating chamber  120 , the returning bath solution can mix with the larger volume of cooler solution in the holding tank and quickly drop in temperature. Similarly, if all of the bath solution has been transferred to the plating chamber in the process, the bath solution can drop in temperature when transferred back to the empty holding tank. By maintaining the bath solution at low temperature for as much of the process as possible, high temperature decomposition of bath solution constituents can be avoided. This can prolong the effective lifetime of the bath solution. In addition, by quickly lowering the temperature of the bath solution used in the plating chamber, water evaporation from the system can be minimized, and the need for water replenishment to the bath solution can be greatly decreased and in some embodiments, eliminated altogether. 
     If desired, the process of the present invention can be a continuous process which can rapidly process a number of substrates. For example, in one embodiment, only a portion of the total amount of the bath solution in the holding tank can be charged to the plating chamber. In this embodiment, after the plating chamber is filled with the bath solution, a second portion of bath solution can be removed from the holding tank. If desired, this second portion can be preheated in the time that the first portion of bath solution is being heated to the deposition temperature and the electroless deposition is occurring in the plating chamber. Thus, as soon as the first deposition process is complete and the plating chamber is emptied of bath solution, the processed substrate can be removed from the plating chamber and a second, unprocessed substrate can be inserted. When the second substrate is in place, the second portion of bath solution can fill the plating chamber and the second substrate can be plated. Thus, a fast, continuous electroless plating process can be established. 
     These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims.