Electroless Plating Method

Abstract

An electroless plating method which can prevent stoppage of a plating reaction and a decrease in a plating rate is disclosed. This method includes: circulating a plating solution through a plating bath while heating the plating solution; immersing the substrate in the plating solution in the plating bath; forming a first electroless plating film on the substrate while circulating the plating solution at a first flow rate during a period from when the substrate is immersed in the plating solution until a predetermined time elapses; and forming a second electroless plating film on the first electroless plating film while circulating the plating solution at a second flow rate that is lower than the first flow rate after the predetermined time has elapsed.

Description

CROSS REFERENCE TO RELATED APPLICATION

[0001] This document claims priority to Japanese Patent Application Number 2014-056713 filed Mar. 19, 2014, the entire contents of which are hereby incorporated by reference.

BACKGROUND

[0002] Electroless plating is a technique to deposit a plating film onto a substrate, such as a wafer, by chemically reducing metal ions in a plating solution without passing an electric current through the plating solution. In this electroless plating, when the substrate is immersed in the plating solution, a static reduction reaction, such as a substitution reaction or an autocatalytic reaction, occurs to deposit a plating film onto a surface of the substrate.

[0003] Electroplating is a technique to deposit a plating film onto a substrate by applying a voltage between the substrate and an anode. In this electroplating, a voltage of not lower than a threshold value is applied to initiate a plating reaction, and a plating rate is controlled by controlling the voltage applied. In electroless plating, on the other hand, the initiation of plating reaction or the plating rate depends on a temperature and a concentration of a plating solution. Therefore, it is important to control the temperature and the concentration of the plating solution. Generally, the temperature of the plating solution is set at a temperature higher than room temperature in order to initiate the plating reaction promptly after a substrate is immersed in the plating solution and in order to maintain a high plating rate. If the temperature of the plating solution significantly drops in the course of plating, there will be stoppage of the plating reaction or a large decrease in the plating rate. In that case, an oxide film can be formed on the surface of the substrate or bubbles of a reaction gas can adhere to the surface of the substrate, possibly impeding contact of the plating solution with the substrate surface. Consequently, the plating reaction may become unstable or may stop even if the temperature of the plating solution is raised again.

[0004] Electroless plating can be carried out in the following manner. First, a substrate is pretreated by immersing the substrate in a pretreatment solution held in a pretreatment bath (a pretreatment process). In general, this pretreatment process is a process of applying catalytic nuclei, which serve for deposition of a plating film, to the surface of the substrate, or a process of coating the substrate surface with an antioxidant film. The substrate is then immersed in a cleaning liquid held in a cleaning bath to remove the pretreatment solution adhering to the substrate (a cleaning process). This cleaning process is a process of removing the pretreatment solution from the substrate to thereby terminate a reaction in the pretreatment process. After completion of the cleaning process, the substrate is immersed in a plating solution stored in a plating bath to initiate plating of the substrate.

[0005] The cleaning liquid held in the cleaning bath is approximately at room temperature. Therefore, the substrate that is immersed in the cleaning liquid becomes approximately at room temperature. On the other hand, as described above, the temperature of the plating solution is set at a temperature higher than room temperature. Accordingly, when the substrate at room temperature is immersed in the plating solution, the temperature of the plating solution drops, resulting in a decrease in the plating rate.

SUMMARY OF THE INVENTION

[0006] Embodiments, which will be described below, provide an electroless plating method which can prevent stoppage of a plating reaction and can prevent a decrease in the plating rate. The embodiments relate to an electroless plating method for plating a surface of a substrate such as a wafer.

[0007] In an embodiment, there is provided an electroless plating method for plating a substrate, comprising: circulating a plating solution through a plating bath while heating the plating solution; immersing the substrate in the plating solution in the plating bath; forming a first electroless plating film on the substrate while circulating the plating solution at a first flow rate during a period from when the substrate is immersed in the plating solution until a predetermined time elapses; and forming a second electroless plating film on the first electroless plating film while circulating the plating solution at a second flow rate that is lower than the first flow rate after the predetermined time has elapsed.

[0008] In an embodiment, the substrate has an underlying metal and a dielectric film that covers the underlying metal, the dielectric film has an opening through which the underlying metal is exposed, and the first electroless plating film is formed on an exposed surface of the underlying metal.

[0009] In an embodiment, the first electroless plating film is formed in the opening of the dielectric film.

[0010] In an embodiment, the predetermined time is in a range of 30 seconds to 10 minutes.

[0011] In an embodiment, the predetermined time is not more than one-tenth of a time for forming the second electroless plating film while circulating the plating solution at the second flow rate.

[0012] In an embodiment, a flow velocity of the plating solution moving on the substrate when the plating solution is circulating at the first flow rate is in a range of 50 cm/sec to 500 cm/sec, and a flow velocity of the plating solution moving on the substrate when the plating solution is circulating at the second flow rate is in a range of 0.05 cm/sec to 200 cm/sec.

[0013] In an embodiment, a flow velocity of the plating solution moving on the substrate when the plating solution is circulating at the first flow rate is at least three times a flow velocity of the plating solution moving on the substrate when the plating solution is circulating at the second flow rate.

[0014] In an embodiment, the electroless plating method further comprises cleaning the substrate by immersing the substrate in a cleaning liquid while maintaining the cleaning liquid within a predetermined temperature range, wherein immersing the substrate in the plating solution in the plating bath comprises immersing the cleaned substrate in the plating solution in the plating bath, and the predetermined temperature range is from 30.degree. C. to a temperature higher by 10.degree. C. than a temperature of the plating solution.

[0015] In an embodiment, the electroless plating method further comprises deaerating the cleaning liquid.

[0016] In an embodiment, the electroless plating method further comprises supplying an inert gas into the cleaning liquid.

[0017] In an embodiment, there is provided an electroless plating method for plating a substrate, comprising: supplying a heated plating solution to the substrate at a first flow rate to form a first electroless plating film on the substrate; and after a predetermined time has elapsed since supply of the plating solution is started, supplying the heated plating solution to the substrate at a second flow rate that is lower than the first flow rate to form a second electroless plating film on the first electroless plating film.

[0018] By supplying the heated plating solution to the plating bath or the substrate at the first flow rate that is higher than the second flow rate, a plating solution on the substrate can be quickly replaced with the heated plating solution. This operation can prevent a decrease in the temperature of the plating solution in contact with the substrate, thereby preventing stoppage of a plating reaction and a decrease in a plating rate. Moreover, by switching the flow rate of the plating solution from the first flow rate to the second flow rate, a shape of a film can be prevented from becoming non-uniform due to the flow of the plating solution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a schematic view of an electroless plating apparatus according to an embodiment;

[0020] FIG, 2 is a schematic view showing a plating-solution heating device provided in a plating bath;

[0021] FIG. 3 is a graph illustrating timing of switching a flow rate of a plating solution;

[0022] FIG. 4A is a diagram showing a plating film as formed when a flow velocity of a plating solution is low;

[0023] FIG. 4B is a diagram showing a plating film as formed when the flow velocity of the plating solution is too high;

[0024] FIG. 5 is a diagram showing an electroless plating film as formed according to the embodiment;

[0025] FIG. 6 is a schematic view of a modified example of the plating apparatus shown in FIG. 1;

[0026] FIG. 7 is a schematic view of the plating apparatus further including flow control valves;

[0027] FIG. 8 is a schematic view of another modified example of the plating apparatus shown in FIG. 1;

[0028] FIG. 9A is a schematic view of a one-by-one face-down type plating apparatus that is designed to plate substrates one by one;

[0029] FIG. 9B is a schematic view of a face-up type plating apparatus;

[0030] FIG. 10 is a schematic view of an electroless plating apparatus according to another embodiment;

[0031] FIG. 11 is a schematic view showing a heater provided in a cleaning bath;

[0032] FIG. 12 is a schematic view of a modified example of the plating apparatus shown in FIG. 10;

[0033] FIG. 13 is a schematic view of another modified example of the plating apparatus shown in FIG. 10;

[0034] FIG. 14 is a schematic view of a plating apparatus including an inert-gas supply unit;

[0035] FIG. 15 is a schematic view of yet another modified example of the plating apparatus shown in FIG. 10;

[0036] FIG. 16 is a schematic view of the plating apparatus further including a room-temperature bath for storing an unheated cleaning liquid therein; and

[0037] FIG. 17 is a schematic view of the plating apparatus, which corresponds to the modified example shown in FIG. 13, further including a room-temperature bath.

DESCRIPTION OF EMBODIMENTS

[0038] Embodiments will now be described with reference to the drawings. The same reference numerals are used in FIGS. 1 through 17 to refer to the same or corresponding elements, components, etc., and duplicate descriptions thereof will be omitted. FIG. 1 is a schematic view of an electroless plating apparatus according to an embodiment. The electroless plating apparatus of this embodiment is a batch processing-type plating apparatus capable of processing multiple substrates at a time. However, the electroless plating apparatus may be a one-by-one processing-type plating apparatus which processes substrates in a one-by-one manner. The electroless plating apparatus will be hereinafter simply referred to as plating apparatus.

[0039] As shown in FIG. 1, the plating apparatus of this embodiment includes a plating bath 1 for storing a plating solution therein, a plating-solution heating device 3 for heating the plating solution, and a circulation unit 2 for creating a flow of the plating solution in the plating bath 1. Multiple substrates W are disposed in a vertical position in the plating bath 1 and arranged parallel to each other.

[0040] The circulation unit 2 is configured to be capable of switching a flow rate of the plating solution, flowing into the plating bath 1, between a first flow rate and a second flow rate that is lower than the first flow rate. More specifically, the circulation unit 2 includes a plating-solution circulation line 5 coupled to the plating bath 1, a pump 6 for circulating the plating solution through the plating bath 1 and the plating-solution circulation line 5, and a pump controller 7 for switching a rotational speed of the pump 6 between a first rotational speed to achieve the first flow rate and a second rotational speed to achieve the second flow rate.

[0041] One end of the plating-solution circulation line 5 is coupled to an upper portion of the plating bath 1, and the other end of the plating-solution circulation line 5 is coupled to a bottom of the plating bath 1. The plating solution is delivered from the upper portion of the plating bath 1 through the plating-solution circulation line 5 to the bottom of the plating bath 1. The plating solution flows from the plating-solution circulation line 5 into the bottom of the plating bath 1 to form an upward flow in the plating bath 1. The upward flow of the plating solution moves on surfaces of the substrates W. Thus, the plating solution, which has been heated by the plating-solution heating device 3, flows into the plating bath 1, and the plating solution in the plating bath 1 is agitated gently, whereby the plating solution at a uniform temperature spreads throughout the plating bath 1.

[0042] Although not shown in the drawings, the plating bath 1 may be composed of a plating-solution storage bath where the substrates W are to be immersed in the plating solution, and an overflow bath located adjacent to the plating-solution storage bath. In that case, one end of the plating-solution circulation line 5 is coupled to the overflow bath, while the other end of the plating-solution circulation line 5 is coupled to the bottom of the plating-solution storage bath. The plating solution overflows the plating-solution storage bath into the overflow bath. The plating solution that has flowed into the overflow bath is returned to the plating-solution storage bath through the plating-solution circulation line 5.

[0043] The plating-solution heating device 3 is mounted to the plating-solution circulation line 5 and is configured to heat the plating solution that is circulating through the plating bath 1 and the plating-solution circulation line 5. As shown in FIG. 2, the plating-solution heating device 3 may be provided in the plating bath 1. Such a construction can also heat the plating solution in the plating bath 1. The plating-solution circulation line 5 is also provided with a filter 9 for removing unwanted materials from the plating solution, and a flow meter 8 for measuring the flow rate of the plating solution. The flow meter 8 and the filter 9 are located downstream of the pump 6, while the plating-solution heating device 3 is located upstream of the pump 6. When the pump 6 is in operation, the plating solution circulates through the plating bath 1, the plating-solution heating device 3, and the plating-solution circulation line 5. A temperature measuring device 60 is installed in the plating bath 1 in order to measure the temperature of the plating solution in the plating bath 1. The temperature measuring device 60 is coupled to the plating-solution heating device 3, and the plating-solution heating device 3 is on-off controlled so as to keep the temperature of the plating solution within a predetermined management range.

[0044] The pump controller 7 is coupled to the pump 6. The pump 6 is configured to change the flow rate of the plating solution flowing into the plating bath 1 in accordance with a command from the pump controller 7. The rotational speed of the pump 6 is controlled by the pump controller 7. This pump controller 7 is configured to switch the rotational speed of the pump 6 between a first rotational speed to achieve the first flow rate and a second rotational speed to achieve the second flow rate.

[0045] FIG. 3 is a graph illustrating timing of switching the flow rate of the plating solution. In FIG. 3, vertical axis represents the flow rate of the plating solution flowing into the plating bath 1, i.e. the flow rate of the plating solution circulating through the plating-solution circulation line 5, and horizontal axis represents time. When the substrates W begin to be immersed in the plating solution, the pump 6 receives a command from the pump controller 7 to rotate at the first rotational speed. Accordingly, the plating solution is supplied to the plating bath 1 at the first flow rate. After a predetermined time has elapsed since the substrates W were immersed in the plating solution, the pump 6 receives a command from the pump controller 7 to rotate at the second rotational speed. Accordingly, the plating solution is supplied to the plating bath 1 at the second flow rate. Although the predetermined time may vary depending on various plating conditions, the predetermined time is preferably set in a range of 30 seconds to 10 minutes (e.g., 5 minutes). The total plating time is, for example, 30 minutes to 90 minutes.

[0046] A flow velocity [m/min] of the plating solution moving on the substrates W can be approximately calculated by dividing the flow rate [m.sup.3/min] of the plating solution, which is measured by the flow meter 8 (see FIG. 1), by a horizontal cross-sectional area [m.sup.2] of the plating bath 1.

[0047] Plating of a substrate W may be carried out in the following manner. The following description illustrates a case where aluminum is used as an underlying metal of the substrate W, and a nickel plating solution is used as the plating solution. First, the substrate W is cleaned with an aqueous solution of nitric acid (an acid cleaning process). After the acid cleaning process, the substrate W is cleaned with pure water. Thereafter, aluminum oxide formed on the surface of the substrate W is removed with a zincate solution, so that the surface of the substrate W is coated with zinc (a zincate treatment). This zinc that is formed on the surface of the substrate W functions as an antioxidant film. The acid cleaning process and the zincate treatment are referred to as pretreatment process.

[0048] Next, the substrate W is transported by a not-shown transport mechanism to the cleaning bath 10 shown in FIG. 1, and is immersed in a cleaning liquid in the cleaning bath 10. The cleaning bath 10 is a storage bath that stores the cleaning liquid therein for cleaning the substrate W before plating of the substrate W. This process of cleaning the substrate W with the cleaning liquid is referred to as a cleaning process. The substrate W is then transported by the transport mechanism to the plating bath 1, and is immersed in the plating solution held in the plating bath 1.

[0049] In general, a temperature of the cleaning liquid in the cleaning bath 10 is approximately equal to room temperature. Therefore, the substrate W, when immersed in the cleaning liquid in the cleaning bath 10, becomes approximately at room temperature. On the other hand, the temperature of the plating solution is higher than room temperature. Accordingly, when the substrate .W at room temperature is immersed in the plating solution, the temperature of the plating solution drops. In order to prevent this, the circulation unit 2 supplies the plating solution to the plating bath 1 at the first flow rate during a period from when the substrate W is immersed in the plating solution until a predetermined time elapses. Because the plating solution flows into the plating bath 1 at the relatively high flow rate, the plating solution around the substrate W and the entirety of the plating solution in the plating bath 1 can be quickly replaced with the heated plating solution. This operation can prevent a decrease in the temperature of the plating solution that is in contact with the substrate W, thereby preventing stoppage of a plating reaction and a decrease in the plating rate. When the plating solution is supplied to the plating bath 1 at the first flow rate, the first flow velocity of the plating solution moving on the substrate W is in a range of for example, 50 to 500 [cm/sec].

[0050] After the predetermined time has elapsed since the substrate W was immersed in the plating solution, the circulation unit 2 supplies the plating solution to the plating bath 1 at the second flow rate that is lower than the first flow rate. After a predetermined plating time has elapsed, the substrate W is raised from the plating solution, so that plating of the substrate W is terminated. When the plating solution is supplied to the plating bath 1 at the second flow rate, the second flow velocity of the plating solution moving on the substrate W is in a range of, for example, 0.05 to 200 [cm/sec].

[0051] In particular, it is desirable that the flow velocity of the plating solution moving on the substrate W when the plating solution is circulating at the first flow rate be at least three times the flow velocity of the plating solution moving on the substrate W when the plating solution is circulating at the second flow rate.

[0052] In electroless plating, the flow velocity of the plating solution in contact with the substrate W is preferably low. FIG. 4A is a diagram showing a plating film as formed on the substrate W when the flow velocity of the plating solution is low, and FIG. 4B is a diagram showing a plating film as formed on the substrate W when the flow velocity of the plating solution is too high. As shown in FIGS. 4A and 4B, the substrate W has underlying metals (metal pads) 19. A dielectric film 20, which may be composed of a photoresist or silicon nitride (SiN), is formed so as to cover the underlying metals 19. The dielectric film 20 has openings 21 through which the underlying metals 19 are exposed. Exposed surfaces of the underlying metals 19 are formed in these openings 21. Electroless plating films 70 are formed on the exposed surfaces of the underlying metals 19.

[0053] As shown in FIG. 4A, when the flow velocity of the plating solution is low, the plating films 70 are deposited isotropically and the plating films 70 are formed in a normal shape. In contrast, when the flow velocity of the plating solution is too high, an additive, which suppresses deposition of the plating films 70, concentrates on a part of each plating film 70, thereby causing the plating film 70 to have a non-uniform shape, called "chipped", as shown in FIG. 4B. Moreover, a turbulent flow of the plating solution may be produced around an edge of the substrate W, resulting in deformation of the plating films 70 or non-uniformity of thickness of the plating films 70.

[0054] The second flow rate to be selected is such that the flow of the plating solution does not lead to such non-uniform shape of the plating films 70 while the temperature of the plating solution on the surface of the substrate W in the plating bath 1 can be kept uniform. If the flow rate of the plating solution flowing into the plating bath 1 is too low, the temperature of the plating solution may differ e.g., between an upper region and a lower region in the plating bath 1, resulting in non-uniform thickness of the plating films 70 over the substrate surface.

[0055] If the substrate W is immersed in the plating solution held in the plating bath 1 when the plating solution is circulating at the second flow rate, the temperature of the plating solution may decrease and the plating reaction may not be initiated. Even if the temperature of the plating solution gradually increases and the plating reaction starts, there may be a variation in the temperature of the plating solution in the plating bath 1, which may cause non-uniform thickness of the plating films 70 over the surface of the substrate W.

[0056] In this embodiment, therefore, the plating solution is circulated at the first flow rate at least during the period from when the substrate W is immersed in the plating solution in the plating bath 1 (in this embodiment from a time when a lower end of the substrate W is brought into contact with the plating solution) until the predetermined time elapses. The circulation of the plating solution at the first flow rate may be started before the substrate W is immersed in the plating solution held in the plating bath 1. Even if the temperature of the plating solution temporarily drops as a result of the immersion of the substrate W, the plating solution in the plating bath 1 is replaced with the high-temperature plating solution in a short time, because the plating solution is circulating at the relatively high flow rate. Therefore, the plating reaction starts promptly after the substrate W is immersed in the plating solution in the plating bath 1. Further, plating films 70 having a uniform thickness can be formed. Because a circulation time of the plating solution at the first flow rate is short relative to a total plating time, the high flow rate of the plating solution at an initial stage of plating has a relatively small influence on a final shape of the plating films 70.

[0057] FIG. 5 is a diagram showing an electroless plating film as formed according to this embodiment. First, electroless plating films (first electroless plating films) 71 are formed on the exposed surfaces of the underlying metals 19, while the plating solution is flowing on a substrate W at the first flow velocity. The electroless plating films 71 are formed in the openings 21. Therefore, in spite of the high flow velocity, the flow of the plating solution hardly affects the shape of the plating film. Furthermore, because plating with the first flow velocity is performed only for a short initial period in a total plating time, the high flow velocity has a little influence on the final shape of the plating film. After the formation of the first electroless plating films 71, electroless plating films (second electroless plating films) 72 are formed on the first electroless plating films 71, respectively, while the plating solution is flowing on the substrate W at the second flow velocity. The second electroless plating films 72 may project upwardly from the surface of the dielectric film 20 as shown in FIG. 5, or may be formed only within the openings 21 of the dielectric film 20.

[0058] It is desirable that the time for forming the first electroless plating film while circulating the plating solution at the first flow rate be not more than one-tenth ( 1/10) of the time for forming the second electroless plating film while circulating the plating solution at the second flow rate.

[0059] Besides nickel, examples of the metal of a plating film as formed by electroless plating according to this embodiment may include cobalt, copper, gold, and an alloy thereof.

[0060] FIG. 6 is a schematic view of a modified example of the plating apparatus shown in FIG. 1. Those constructional features of this plating apparatus, which will not be described below, are the same as those of the above-described plating apparatus shown in FIG. 1. As shown in FIG. 6, the circulation unit 2 includes a first pump 11 and a second pump 12 for circulating a plating solution through the plating bath 1 and the plating-solution circulation line 5, and a first valve 15 and a second valve 16 attached to the plating-solution circulation line 5. Part of the plating-solution circulation line 5 is constituted by a first delivery line 13 and a second delivery line 14 that extend parallel to each other. The entirety of the plating-solution circulation line 5 may be constituted by the first delivery line 13 and the second delivery line 14. The first pump 11 and the first valve 15 are mounted to the first delivery line 13. The second pump 12 and the second valve 16 are mounted to the second delivery line 14. The first valve 15 and the second valve 16 are located downstream of the first pump 11 and the second pump 12, respectively.

[0061] A pump controller 7 is coupled to the first pump 11, the second pump 12, the first valve 15, and the second valve 16, and controls the operations of the first pump 11, the second pump 12, the first valve 15, and the second valve 16. The first valve 15 and the second valve 16 are configured to open and close fluid passages of the first delivery line 13 and the second delivery line 14, respectively, upon receiving commands from the pump controller 7.

[0062] The first pump 11 is a high-speed pump for delivering the plating solution at a high flow rate, while the second pump 12 is a low-speed pump for delivering the plating solution at a low flow rate. Thus, the first pump 11 is configured to deliver the plating solution through the first delivery line 13 at a predetermined first flow rate, while the second pump 12 is configured to deliver the plating solution through the second delivery line 14 at a predetermined second flow rate that is lower than the predetermined first flow rate. The first flow rate is such a flow rate as to allow the plating solution in the plating bath 1 to flow on a substrate W at the first flow velocity when the second valve 16 is closed, while the second flow rate is such a flow rate as to allow the plating solution in the plating bath 1 to flow on the substrate W at the second flow velocity when the first valve 15 is closed.

[0063] The plating-solution heating device 3 is attached to the plating-solution circulation line 5. When the first pump 11 and/or the second pump 12 is in operation, the plating solution circulates through the plating bath 1, the plating-solution heating device 3, and the plating-solution circulation line 5.

[0064] The pump controller 7 closes the second valve 16 and opens the first valve 15, and causes the first pump 11 to operate during the period from when the substrate W is immersed in the plating solution until a predetermined time elapses. The plating solution is delivered by the first pump 11 through the first delivery line 13 at the first flow rate. As a result, a flow of the plating solution, moving on the substrate W at the first flow velocity, is created in the plating bath 1. After the predetermined time has elapsed, the pump controller 7 closes the first valve 15, opens the second valve 16, stops the operation of the first pump 11, and causes the second pump 12 to operate. The plating solution is delivered by the second pump 12 through the second delivery line 14 at the second flow rate. As a result, a flow of the plating solution, moving on the substrate W at the second flow velocity, is created in the plating bath 1.

[0065] FIG. 7 is a schematic view of the plating apparatus further including flow control valves 17, 18. Those constructional features of this plating apparatus, which will not be described below, are the same as those of the above-described plating apparatus shown in FIG. 6. As shown in FIG. 7, a first flow control valve 17 may be attached to the first delivery line 13, and a second flow control valve 18 may be attached to the second delivery line 14. The first flow control valve 17 is configured to regulate the flow rate of the plating solution flowing in the first delivery line 13, and the second flow control valve 18 is configured to regulate the flow rate of the plating solution flowing in the second delivery line 14. The operations of the first flow control valve 17 and the second flow control valve 18 are controlled by the pump controller 7.

[0066] FIG. 8 is a schematic view of another modified example of the plating apparatus shown in FIG. 1. Those constructional features of this plating apparatus, which will not be described below, are the same as those of the above-described plating apparatus shown in FIG. 1. As shown in FIG. 8, part of the plating-solution circulation line 5 is constituted by a delivery line 27 and a return line 23 that extend parallel to each other. The circulation unit 2 includes a pump 25 attached to the delivery line 27, and a flow control valve 24 for regulating a flow rate of the plating solution flowing backward through the return line 23. The flow control valve 24 is attached to the return line 23.

[0067] The flow control valve 24 is coupled to the pump controller 7, so that the operation of the flow control valve 24 is controlled by the pump controller 7. After the operation of the pump 25 is started, most of the plating solution is returned to the plating bath 1, while part of the plating solution flows into the return line 23, as shown by the arrows in FIG. 8. The flow control valve 24 is configured to change the flow rate of the plating solution flowing backward through the return line 23, thereby switching the flow velocity of the plating solution, flowing on a substrate W, between the first flow velocity and the second flow velocity.

[0068] The flow rate of the plating solution, returned to the plating bath 1, increases when the flow control valve 24 decreases the flow rate of the plating solution flowing through the return line 23. As a result, a flow of the plating solution, moving on the substrate W at the first flow velocity that is higher than the second flow velocity, is created in the plating bath 1. The flow rate of the plating solution, returned to the plating bath 1, decreases when the flow control valve 24 increases the flow rate of the plating solution flowing through the return line 23. As a result, a flow of the plating solution, moving on the substrate W at the second flow velocity that is lower than the first flow velocity, is created in the plating bath 1.

[0069] The plating-solution heating device 3 is attached to the plating-solution circulation line 5. When the pump 25 is in operation, the plating solution circulates through the plating bath 1, the plating-solution heating device 3, and the plating-solution circulation line 5. As with the plating apparatus shown in FIG. 2, the plating-solution heating device 3 may be provided in the plating bath 1.

[0070] In the above-described embodiments the flow velocity of the plating solution moving on a substrate W is switched between the first flow velocity and the second flow velocity, while it is also possible to switch the flow velocity of the plating solution between three or more different flow velocities.

[0071] While the embodiments have been described with reference to the plating apparatus in which plating operations for substrates W are performed repeatedly with the plating solution circulating through the plating bath 1, the present invention is also applicable to a plating apparatus of a type that discards a plating solution each time plating of one substrate or one batch of substrates is completed. FIG. 9A is a schematic view of a one-by-one face-down type plating apparatus, and FIG. 9B is a schematic view of a face-up type plating apparatus. The face-down type plating apparatus has a substrate holder 28 that holds a substrate W in a horizontal position with its front surface facing downward. This face-down type plating apparatus is configured to immerse the substrate W in a plating solution held in a plating bath 1 and supply the plating solution upwardly from a bottom of the plating bath 1 while rotating the substrate holder 28 together with the substrate W to thereby plate the substrate W.

[0072] The face-up type plating apparatus has a substrate holder 29 that holds a substrate W in a horizontal position with its front surface facing upwardly. This face-up type plating apparatus is configured to supply a plating solution onto a surface of the substrate W from above while rotating the substrate holder 29 together with the substrate W to thereby plate the substrate W. These types of plating apparatuses can also achieve the same effects as described above by supplying a heated plating solution to the surface of the substrate W at a relatively high flow rate (first flow rate) and, after a predetermined time has elapsed since the supply of the plating solution is started, supplying the plating solution to the surface of the substrate W at a relatively low flow rate (second flow rate).

[0073] FIG. 10 is a schematic view of an electroless plating apparatus according to another embodiment. As shown in FIG. 10, the plating apparatus includes a cleaning bath 10 for storing a cleaning liquid for cleaning a substrate W before plating of the substrate W, and a cleaning-liquid heating device 30 for maintaining the cleaning liquid in the cleaning bath 10 within a predetermined temperature range. The cleaning-liquid heating device 30 includes a heater 33 for heating the cleaning liquid, and a heated-cleaning-liquid supply line 31 for supplying the heated cleaning liquid into the cleaning bath 10. The temperature of the cleaning liquid in the cleaning bath 10 is measured by a temperature measuring device 80. This temperature measuring device 80 is coupled to the heater 33, and the operation of the heater 33 is controlled so as to keep the temperature of the cleaning liquid within the predetermined temperature range.

[0074] One end of the heated-cleaning-liquid supply line 31 is coupled to a lower portion of the cleaning bath 10, while the other end of the heated-cleaning-liquid supply line 31 is coupled to a not-shown cleaning liquid supply source. The heated-cleaning-liquid supply line 31 is provided with an on-off valve 32 for opening and closing a fluid passage of the heated-cleaning-liquid supply line 31, and a heater 33. The plating apparatus further includes an operation controller 46 for controlling operation of supplying the heated cleaning liquid into the cleaning bath 10. This operation controller 46 is coupled to the on-off valve 32, and is configured to control the opening and closing operations of the on-off valve 32.

[0075] As described above, when the substrate W is immersed in the cleaning liquid whose temperature is approximately equal to room temperature, the substrate W becomes approximately at room temperature. On the other hand, the temperature of the plating solution is higher than room temperature. Accordingly, when the substrate W at room temperature is immersed in the plating solution, the temperature of the plating solution drops.

[0076] Thus, in order to make the temperature of the substrate W higher than room temperature, the cleaning-liquid heating device 30 supplies the cleaning liquid that has been heated by the heater 33 to the cleaning bath 10 through the heated-cleaning-liquid supply line 31. When the on-off valve 32 is opened, the heated cleaning liquid is supplied to the cleaning bath 10. The substrate W is immersed in the heated cleaning liquid in the cleaning bath 10; so that the substrate W is cleaned and heated. After the cleaning of the substrate W, the heated substrate W is transported to the plating bath 1, where the substrate W is immersed in the plating solution, so that the substrate W is plated. The cleaning operation in this embodiment can reduce a difference in temperature between the substrate W and the plating solution, thereby preventing a decrease in the temperature of the plating solution. As shown in FIG. 11, the heater 33 may be provided in the cleaning bath 10.

[0077] The temperature range of the cleaning liquid is preferably from 30.degree. C. to a temperature higher by 10.degree. C. than the temperature of the plating solution. For example, when the temperature of the plating solution is 50.degree. C., the temperature of the heated cleaning liquid is not less than 30.degree. C. and nor more than 60.degree. C.

[0078] FIG. 12 is a schematic view of a modified example of the plating apparatus shown in FIG. 10. Those constructional features of this plating apparatus, which will not be described below, are the same as those of the above-described plating apparatus shown in FIG. 10. As shown in FIG. 12, the plating apparatus includes a deaerator 38 for deaerating a cleaning liquid, a cleaning-liquid circulation line 36 coupling cleaning bath 10 to the deaerator 38, a pump 37 for circulating the cleaning liquid between the cleaning bath 10 and the deaerator 38 through the cleaning-liquid circulation line 36, and a filter 39 for removing unwanted materials from the cleaning liquid flowing through the cleaning-liquid circulation line 36. One end of the cleaning-liquid circulation line 36 is coupled to an upper portion of the cleaning bath 10, while the other end of the cleaning-liquid circulation line 36 is coupled to a bottom of the cleaning bath 10.

[0079] When a substrate W is immersed in the cleaning liquid held in the cleaning bath 10, oxygen contained in the cleaning liquid may accelerate oxidization of the underlying metal of the substrate W. The deaerator 38 is provided in order to remove oxygen from the cleaning liquid. Because the cleaning liquid supplied into the cleaning bath 10 is deaerated by the deaerator 38, oxidation of the substrate W can be prevented.

[0080] FIG. 13 is a schematic view of another modified example of the plating apparatus shown in FIG. 10. Those constructional features of this plating apparatus, which will not be described below, are the same as those of the above-described plating apparatus shown in FIG. 10. As shown in FIG. 13, heater 33 may be attached to the cleaning-liquid circulation line 36. In this embodiment, the plating apparatus is not provided with the heated-cleaning-liquid supply line 31 and the on-off valve 32, shown in FIG. 10. The cleaning-liquid circulation line 36 couples the cleaning bath 10 to the heater 33, which is configured to heat a cleaning liquid flowing through the cleaning-liquid circulation line 36. The heater 33 may be provided in the cleaning bath 10. The heater 33 in the plating bath 10 can also heat the plating solution held in the plating bath 10.

[0081] An unheated-cleaning-liquid supply line 42 for supplying an unheated cleaning liquid into the cleaning bath 10 is coupled to the cleaning bath 10. A supply valve 44 is attached to the unheated-cleaning-liquid supply line 42. This supply valve 44 is configured to open and close a fluid passage of the unheated-cleaning-liquid supply line 42. The opening and closing operations of the supply valve 44 are controlled by operation controller 46. The unheated cleaning liquid is a cleaning liquid that is not heated by a heating device, such as a heater.

[0082] As shown in FIG. 14, the plating apparatus may include an inert-gas supply unit 40 for supplying an inert gas, such as nitrogen gas, into cleaning liquid. Those constructional features of this plating apparatus, which will not be described below, are the same as those of the above-described plating apparatus shown in FIG. 10. The inert-gas supply unit 40 includes a diffuser tube 47 disposed at the bottom of the cleaning bath 10, and an inert-gas supply line 48 for supplying the inert gas into the diffuser tube 47. When the inert gas is supplied into the cleaning liquid, bubbles of the inert gas are formed in the cleaning liquid to remove the dissolved oxygen from the cleaning liquid. Accordingly, oxidization of a substrate W can be prevented.

[0083] FIG. 15 is a schematic view of yet another modified example of the plating apparatus shown in FIG. 10. Those constructional features of this plating apparatus, which will not be described below, are the same as those of the above-described plating apparatus shown in FIG. 10. As shown in FIG. 15, the plating apparatus includes unheated-cleaning-liquid supply line 42 for supplying unheated cleaning liquid into the cleaning bath 10, a drain line 41 for draining the unheated cleaning liquid from the cleaning bath 10, and a drain valve 43 attached to the drain line 41. The drain valve 43 is configured to open and close a fluid passage of the drain line 41. Supply valve 44 is attached to the unheated-cleaning-liquid supply line 42. The drain line 41 is coupled to the bottom of the cleaning bath 10.

[0084] The plating apparatus further includes operation controller 46 for controlling the operation of supplying the cleaning liquid into the cleaning bath 10 and the operation of draining the cleaning liquid from the cleaning bath 10. The operation controller 46 is configured to control the opening and closing operations of the on-off valve 32, the drain valve 43, and the supply valve 44.

[0085] Opening and closing operations of the on-off valve 32, the drain valve 43, and the supply valve 44 will now be described. First, the on-off valve 32 and the drain valve 43 are closed and the supply valve 44 is opened to allow the unheated cleaning liquid to be supplied into the cleaning bath 10 through the unheated-cleaning-liquid supply line 42. The supply valve 44 is closed when the cleaning bath 10 is filled with the unheated cleaning liquid. A substrate W is transported by a not-shown transport mechanism to a predetermined position in the cleaning bath 10 and immersed in the unheated cleaning liquid, whereby the substrate W is cleaned.

[0086] After the cleaning of the substrate W, the drain valve 43 is opened to drain the unheated cleaning liquid from the cleaning bath 10. After draining the unheated cleaning liquid, the drain valve 43 is closed and the on-off valve 32 is opened to allow a heated cleaning liquid to be supplied through the heated-cleaning-liquid supply line 31 into the cleaning bath 10. The on-off valve 32 is closed when the cleaning bath 10 is filled with the heated cleaning liquid. The substrate W is immersed in the heated cleaning liquid, whereby the substrate W is cleaned and heated. The heated substrate W is transported by transport mechanism to the plating bath 1. The substrate W is immersed in a plating solution in the plating bath 1 so that plating of the substrate W is started. After a predetermined plating time has elapsed, the substrate W is raised from the plating solution, whereby plating of the substrate W is terminated.

[0087] FIG. 16 is a schematic view of the plating apparatus further including a room-temperature bath 50 for storing an unheated cleaning liquid. As shown in FIG. 16, an unheated-cleaning-liquid introduction line 52 for introducing an unheated cleaning liquid into the room-temperature bath 50 is coupled to the room-temperature bath 50. An introduction valve 53 for opening and closing a fluid passage of the unheated-cleaning-liquid introduction line 52 is attached to the unheated-cleaning-liquid introduction line 52. Operation controller 46 is coupled to the introduction valve 53, so that the opening and closing operations of the introduction valve 53 are controlled by the operation controller 46.

[0088] When the introduction valve 53 is opened, the unheated cleaning liquid is introduced through the unheated-cleaning-liquid introduction line 52 into the room-temperature bath 50. A substrate W is immersed in the unheated cleaning liquid held in the room-temperature bath 50, so that the substrate W is cleaned. A heated cleaning liquid is supplied through the heated-cleaning-liquid supply line 31 into the cleaning bath 10 until the cleaning bath 10 is filled with the heated cleaning liquid. The substrate W is transported from the room-temperature bath 50 to the cleaning bath 10, where the substrate W is immersed in the heated cleaning liquid. The substrate W is cleaned and heated by the heated cleaning liquid. The heated substrate W is transported by a transport mechanism to the plating bath 1. The substrate W is immersed in a plating solution in the plating bath 1, so that plating of the substrate W is started. After a predetermined plating time has elapsed, the substrate W is raised from the plating solution, whereby plating of the substrate W is terminated.

[0089] FIG. 17 is a schematic view of the plating apparatus, which corresponds to the modified example shown in FIG. 13, further including the room-temperature bath 50. Also in this plating apparatus, a substrate W is first cleaned with the unheated cleaning liquid in the room-temperature bath 50, and is subsequently cleaned and heated by the heated cleaning liquid held in the cleaning bath 10.

[0090] One of the above-described embodiments of the plating apparatuses may be combined with the other. For example, the cleaning bath 10 shown in FIG. 10 may be employed in the plating apparatus shown in FIG. 1. Further, the cleaning bath 10 and the room-temperature bath 50, shown in FIG. 16, may be employed in the plating apparatus shown in FIG. 1.

[0091] While the present invention has been described with reference to preferred embodiments, it is understood that the present invention is not limited to the embodiments described above, and is capable of various changes and modifications within the scope of the technical concept as expressed herein.

Claims

1. An electroless plating method for plating a substrate, comprising: circulating a plating solution through a plating bath while heating the plating solution; immersing the substrate in the plating solution in the plating bath; forming a first electroless plating film on the substrate while circulating the plating solution at a first flow rate during a period from when the substrate is immersed in the plating solution until a predetermined time elapses; and forming a second electroless plating film on the first electroless plating film while circulating the plating solution at a second flow rate that is lower than the first flow rate after the predetermined time has elapsed.

2. The electroless plating method according to claim 1, wherein: the substrate has an underlying metal and a dielectric film that covers the underlying metal; the dielectric film has an opening through which the underlying metal is exposed; and the first electroless plating film is formed on an exposed surface of the underlying metal.

3. The electroless plating method according to claim 2, wherein the first electroless plating film is formed in the opening of the dielectric fihn.

4. The electroless plating method according to claim 1, wherein the predetermined time is in a range of 30 seconds to 10 minutes.

5. The electroless plating method according to claim 1, wherein the predetermined time is not more than one-tenth of a time for forming the second electroless plating film while circulating the plating solution at the second flow rate.

6. The electroless plating method according to claim 1, wherein a flow velocity of the plating solution moving on the substrate when the plating solution is circulating at the first flow rate is in a range of 50 cm/sec to 500 cm/sec, and a flow velocity of the plating solution moving on the substrate when the plating solution is circulating at the second flow rate is in a range of 0.05 cm/sec to 200 cm/sec.

7. The electroless plating method according to claim 1, wherein a flow velocity of the plating solution moving on the substrate when the plating solution is circulating at the first flow rate is at least three times a flow velocity of the plating solution moving on the substrate when the plating solution is circulating at the second flow rate.

8. The electroless plating method according to claim 1, further comprising: cleaning, the substrate by immersing the substrate in a cleaning liquid while maintaining the cleaning liquid within a predetermined temperature range, wherein immersing the substrate in the plating solution in the plating bath comprises immersing the cleaned substrate in the plating solution in the plating bath, and the predetermined temperature range is from 30.degree. C. to a temperature higher by 10.degree. C. than a temperature of the plating solution.

9. The electroless plating method according to claim 8, further comprising: deaerating the cleaning liquid.

10. The electroless plating method according to claim 8, further comprising: supplying an inert gas into the cleaning liquid.

11. An electroless plating method for plating a substrate, comprising: supplying a heated plating solution to the substrate at a first flow rate to form a first electroless plating film on the substrate; and after a predetermined time has elapsed since supply of the plating solution is started, supplying the heated plating solution to the substrate at a second flow rate that is lower than the first flow rate to form a second electroless plating film on the first electroless plating film.