Method of copper plating small diameter hole

Abstract

A method of copper plating a small diameter hole which uses a copper sulfate plating solution containing copper sulfate, sulfuric acid, chlorine ions, a sulfur compound, and a surfactant to copper plate the inside of a small diameter hole of an object being plated having a small diameter hole by the PPR method, comprising performing reverse electrolysis by a range of current density of 0.1 to 1 A/dm.sup.2 to peel off a sulfur compound near the opening of the small diameter hole in the sulfur compound adsorbed to the object being plated so as to keep the polarization resistance in the small diameter hole at the time of regular electrolysis lower than that near the opening of the small diameter hole and form a copper plating film of a uniform thickness inside the small diameter hole. Since a high precision, large capacity pulse power supply is not required, the capital costs can be reduced and the inside of the small diameter hole can be plated well.

Description

TECHNICAL FIELD

[0001] the present invention relates to a method of copper plating a small diameter hole.

BACKGROUND ART

[0002] Along with the higher densities of interconnects of electronic components, the through holes and blind vias of circuit boards have become smaller in diameter and reliable formation of plating films in them has become difficult. That is, with the ordinary plating methods, the plating films at the center parts of the through holes or the bottoms of the vias become extremely thin, so there is a problem of reliability. Further, if making the films at the insides thicker by plating for a long time, the cost increases, the openings become blocked, or other new inconveniences arise.

[0003] To solve these problems, the PPR (Periodic Pulse Reverse) plating method for reversing the electrolysis polarity periodically (for example, Japanese Unexamined Patent Pulication (Kokai) No. 2000-68651) and a method using special agitation have been proposed.

[0004] With the conventional PPR plating method, however, pulses of a millisecond (ms) unit are used, so there are the following problems:

[0005] That is, a high performance, expensive pulse power supply able to continuously switch polarities at a high speed is required.

[0006] Further, it is necessary to synchronize two power supplies for plating patterns on the front and back of the substrate, but since the pulses are high in speed, synchronization is extremely difficult.

[0007] Further, the reverse electrolysis current density required is about 1 to 5 times the regular electrolysis current density, so a large capacity power supply is required.

[0008] Further, since the pulses are high in speed (high frequency), it is necessary to lay the interconnects considering the loss due to the inductance of the interconnects.

[0009] Further, in the case of rack plating where the object being plated (substrate) is suspended in a plating solution, the effect of the pulse does not reach the center of the substrate, making this impractical.

[0010] Further, there are various other issues such as the fact that setting the various types of plating conditions is not easy.

[0011] Further, in the case of using a special agitation apparatus, there are the issues that it is difficult to uniformly agitate the solution for all parts and the cost rises.

DISCLOSURE OF THE INVENTION

[0012] Therefore, an object of the present invention is to provide a method of copper plating a small diameter hole not requiring a high precision, large capacity pulse power supply and therefore achieving a reduction in the capital cost and enabling the inside of the small diameter hole to be plated well.

[0013] To achieve the above object, the method of copper plating a small diameter hole according to the present invention is a method of copper plating a small diameter hole which uses a copper sulfate plating solution containing copper sulfate, sulfuric acid, chlorine ions, a sulfur compound, and a surfactant to copper plate the inside of a small diameter hole of an object being plated having a small diameter hole by the PPR method, characterized by performing reverse electrolysis by a range of current density of 0.1 to 1 A/dm.sup.2 to peel off a sulfur compound near the opening of the small diameter hole in the sulfur compound adsorbed to the object being plated so as to keep the polarization resistance in the small diameter hole at the time of regular electrolysis lower than that near the opening of the small diameter hole and form a copper plating film of a uniform thickness inside the small diameter hole.

[0014] The method further preferably comprises, at the time of said reverse electrolysis, performing two-stage reverse electrolysis consisting of performing a first half of reverse electrolysis by a high current density and performing a second half of reverse electrolysis by a current density lower than the first half.

[0015] Further, the method performs the regular electrolysis in several tens to several hundreds of seconds and performs the reverse electrolysis in several seconds to several tens of seconds.

[0016] Further, the method preferably performs the regular electrolysis in a range of current density of 1 to 2 A/dm.sup.2.

[0017] It is preferable to use a low electrical resistance, high copper concentration copper sulfate plating solution set to a sulfuric acid concentration of 150 to 250 g/l and a concentration of copper sulfate of 130 to 200 g/l. Further, a copper sulfate plating solution set to a sulfuric acid concentration of around 200 g/l and a concentration of copper sulfate of around 150 g/l is preferable in terms of safety.

[0018] Further, it is possible to bury the inside of the small diameter hole by copper plating.

BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 is a graph of the relationship between the sulfuric acid concentration and the resistance of a plating solution.

[0020] FIG. 2 is a graph of the relationship between the sulfuric acid concentration and the saturation copper sulfate concentration.

[0021] FIG. 3 is a graph of the relationship between the reverse electrolysis potential and the magnitude of the polarization resistance at the time of regular electrolysis.

[0022] FIG. 4 is a schematic view of a current waveform in an embodiment of the present invention.

[0023] FIG. 5 is a sectional photograph of a through hole of Example 1.

[0024] FIG. 6 is a sectional photograph of a through hole of Example 2.

[0025] FIG. 7 is a sectional photograph of a through hole of Example 3.

[0026] FIG. 8 is a sectional photograph of a through hole of Example 4.

[0027] FIG. 9 is a sectional photograph of a through hole of Comparative Example 1.

[0028] FIG. 10 is a sectional photograph of a through hole of Comparative Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] Next, a preferred embodiment of the present invention will be explained in detail based on the attached drawings.

[0030] First, the copper sulfate plating solution will be explained.

[0031] The copper sulfate plating solution is comprised of copper sulfate as a copper source, sulfuric acid for adjusting the conductivity, chlorine ions (chloride) and a surfactant as suppressants, and a sulfur compound functioning as a plating accelerator.

[0032] To improve the throwing power of the plating in the small diameter hole, it is preferable to make the solution high in copper concentration. Further, to reduce the electrical resistance of the plating solution, the greater the amount of the sulfuric acid, the better. However, if the amount of the sulfuric acid becomes greater, the copper sulfate becomes hard to dissolve. If in excess, the copper sulfate will end up precipitating. Therefore, a balance between the two is required.

[0033] FIG. 1 is a graph of the relationship between the sulfuric acid concentration and copper sulfate concentration and the resistance of a plating solution and compares the case of an electrical resistance of 5% sulfuric acid as "1". Further, FIG. 2 is a graph of the relationship between the sulfuric acid concentration and the saturation copper sulfate concentration.

[0034] As clear from FIG. 1, with a sulfuric acid concentration of 150 g/l or more, the electrical resistance is low and substantially stable. Therefore, to obtain a plating solution with a low electrical resistance, it is preferable to make the sulfuric acid concentration 150 g/l or more. Further, to make a plating with a high copper concentration, it is preferable to make the sulfuric acid concentration 250 g/l or less.

[0035] Further, the region below the line of FIG. 2 is the region where the solution can be used as a copper plating solution. In the range of the above sulfuric acid concentration (150 to 250 g/l), dissolution is possible in the range of copper sulfate concentration of about 130 to 200 g/l.

[0036] In the above range, to obtain a plating solution able to be used stably while maintaining a high copper concentration, it is optimal to adjust the sulfuric acid concentration to around 200 g/l and the concentration of copper sulfate to around 150 g/l.

[0037] Note that the above "around" means.+-.5%.

[0038] As a chlorine ion source, hydrochloric acid, sodium chloride, potassium chloride, ammonium chloride, etc. may be mentioned. These may be used alone or in combination. The amount added is, as chlorine ions, one in the range of 10 to 200 mg/l, but around 35 mg/l is preferable.

[0039] The sulfur compound is not particularly limited, but sodium 3-mercapto-1-propane sulfonate or sodium 2-mercaptoethane sulfonate, bis-(3-sulfopropyl)-disulfide disodium, or another sulfur compound may be preferably used alone or in combination.

[0040] The amounts added of these sulfur compounds are effectively slight amounts of addition of around 1 mg/l.

[0041] The surfactant is also not particularly limited, but polyethylene glycol, polypropylene glycol, or another surfactant may be used alone or in combination.

[0042] The amount added of the surfactant used may be in the range of around several mg/l to 10 g/l.

[0043] The surfactant is present together with the chlorine ions, so increases the polarization resistance at the cathode.

[0044] On the other hand, the sulfur compound reduces the polarization resistance at the cathode and acts as an accelerator.

[0045] When the copper sulfate plating solution contains a surfactant, chlorine ions, and a sulfur compound, the polarization resistance at the cathode surface depends on the balance of the amounts of adsorption of these additives. In particular, the sulfur compound is adsorbed at the surface of the object being plated and has a strong effect of reducing the polarization resistance at the adsorbing surface. Therefore, suppression of the amount of adsorption of the sulfur compound leads to control of the polarization resistance.

[0046] In the present invention, at the time of regular electrolysis, the polarization resistance at the surface of the object being plated (circuit board etc.) or the opening side of the small diameter hole is controlled to be high and the polarization resistance inside the small diameter hole to be low so as to form overall a plating film with a uniform thickness.

[0047] Therefore, reverse electrolysis is performed to peel off the sulfur compound adsorbed at the front surface of the object being plated or near the opening of the small diameter hole. By this, at the time of regular electrolysis, a difference is given to the polarization resistance as explained above.

[0048] The electrical resistance of a plating system is defined as the sum of the polarization resistance and the electrical resistance of the plating solution.

[0049] Therefore, if making the electrical resistance of the plating solution sufficiently small with respect to the polarization resistance, the electrical resistance of the plating system and therefore the current inversely proportional to the same will greatly depend on the magnitude of the polarization resistance. As explained above, the sulfuric acid concentration in the copper plating solution is made high and the electrical resistance of the plating solution is kept low so as to facilitate control of the polarization resistance.

[0050] The polarization resistance Rc was defined as:

Rc=.vertline.V.sub.SCE-V.sub.0.vertline./I (V.sub.0 is an equilibrium potential)

[0051] and the polarization resistance was measured from the potential and the current.

[0052] FIG. 3 is a graph obtained by measuring the magnitudes of the polarization resistances when performing reverse electrolysis on a circuit board by various potentials for 10 sec, then performing regular electrolysis. Note that the polarization resistance shows the polarization resistance at the front surface of the object being plated (circuit board). The polarization resistance inside a small diameter hole is naturally lower than the polarization resistance at the surface.

[0053] As shown in FIG. 3, in the range of an electrolysis potential at the time of reverse electrolysis of 0.10 to 0.16V (vs SCE), the polarization resistance at the time of regular electrolysis changes. It is therefore learned that it is possible to control the polarization resistance at the time of regular electrolysis by the electrolysis potential at the time of reverse electrolysis. The higher the electrolysis potential, the higher the polarization resistance. That is, the higher the electrode potential, the more peeling of the sulfur compound occurs and the higher the polarization resistance at the time of regular electrolysis.

[0054] Further, it deserves special mention that when setting the time for the reverse electrolysis to a long time of 10 sec, a range of potential enabling control of the polarization resistance at the time of regular electrolysis was obtained. This suggests that long period PPR plating can be performed.

[0055] The experiments showed that the time for reverse electrolysis is sufficiently one in the range of 1 sec to several tens of sec.

[0056] Note that if controlling the reverse electrolysis by the potential, the current density might run wild, so control by the current density is preferable. The current density for dealing with the above potential is 0.1A to 1/dm.sup.2.

[0057] Further, in the case of a current density larger than 0.5 A/dm.sup.2, the surface of the object being plated becomes rough, so it is preferable to control the process by a current density in the range of 0.1 to 0.5 A/dm.sup.2. However, in the case of an object being plated where roughness would not be that much of a problem, control by the above current density would also be possible.

[0058] In this way, it is possible to control the extent of peeling of the sulfur compound by the current density at the time of reverse electrolysis and as a result it is possible to control the polarization resistance at the time of regular electrolysis.

[0059] The magnitude of the polarization resistance cannot be measured inside a small diameter hole of an object being plated, but since reverse electrolysis has almost no effect inside a small diameter hole, it may be considered that there is almost no peeling of the sulfur compound inside the small diameter hole other than near the opening. Therefore, the polarization resistance at the time of regular electrolysis inside a small diameter hole is maintained low as it is, current flows into the small diameter hole, and the throwing power is improved even inside the small diameter hole. The setting of the current density at the time of reverse electrolysis, the reverse electrolysis time, the current density at the time of regular electrolysis, and the setting of the electrolysis time may be performed, while measuring the thickness, in accordance with the object being plated.

[0060] FIG. 4 schematically shows the current waveform of PPR plating in this embodiment.

[0061] The optimal current density at the time of reverse electrolysis is 0.1 to 0.5 A/dm.sup.2, while the optimal electrolysis time is around 1 to 10 seconds.

[0062] Further, experiments have shown that if performing two-stage reverse electrolysis at the time of reverse electrolysis consisting of performing the first half reverse electrolysis by a high current density and performing the second half reverse electrolysis by a current density lower than the first half, a better effect is obtained.

[0063] The improvement in the effects by performing the reverse electrolysis in two stages is believed to be due to the following reason: That is, in the first half (first stage) reverse electrolysis, the peeling action at the outside of a through hole is strong and the peeling action at the inside of the through hole is weak. In the second half (second stage) reverse electrolysis, the potential for the peeling is extremely weak, so the outside of the through hole is just slightly peeled, while the inside is not subject to almost any peeling action and rather adsorption of the sulfur compound proceeds.

[0064] Therefore, in the first stage reverse electrolysis, the surface compound at the outside of the through hole is reliably peeled, while in the second stage reverse electrolysis, the sulfur compound is adsorbed at the inside of the through hole while the peeled state of the outside of the through hole is maintained due to the weak peeling action. As a result, compared with performing the reverse electrolysis in one stage, the difference in adsorption and concentration of the sulfur compound at the outside and inside of the through hole becomes greater and a larger difference in polarization resistance occurs.

[0065] Further, with only one stage of reverse electrolysis, depending on the shape of the through hole, sometimes the inside of the through hole will end up being overly peeled and a sufficient difference in polarization resistance between the outside and inside of the through hole will not be able to be obtained. In such a case as well, it is possible to reliably obtain a difference in polarization resistance by performing a second stage of weak reverse electrolysis.

[0066] The current density at the time of regular electrolysis should be around 1.5 A/dm.sup.2 (not particularly limited to this. May be decided viewing the throwing power of the plating) and the electrolysis time around 50 to 200 sec.

EXAMPLES

[0067] The copper sulfate plating solution used was one of the following composition in all cases:

1 Copper sulfate 5-hydrate 150 g/l Sulfuric acid 200 g/l Polyethylene glycol 4000 3 g/l SPS 1 mg/l Chlorine ions 35 mg/l Note that "SPS"means bis-(3-sulfopropyl)-disulfide disodium.

Example 1

[0068] A circuit board of a thickness of 0.8 mm having through holes of opening diameters of 0.1 mm was plated by the PPR method under the following conditions:

[0069] Regular electrolysis: Current density: 1.5 A/dm.sup.2, electrolysis time: 120 sec

[0070] Reverse electrolysis: Current density: 0.5 A/dm.sup.2, electrolysis time: 10 sec

[0071] Plating time: 76 min

[0072] As a result, the thickness ratio: (thickness of center part of through hole/thickness of surface of substrate).times.100 was 91.3%.

Example 2

[0073] A circuit board of a thickness of 0.8 mm having through holes of opening diameters of 0.15 mm was plated by the PPR method under the following conditions:

[0074] Regular electrolysis: Current density: 1.5 A/dm.sup.2, electrolysis time: 120 sec

[0075] Reverse electrolysis: Current density: 0.5 A/dm.sup.2, electrolysis time: 10 sec

[0076] Plating time: 76 min

[0077] As a result, the thickness ratio was 101.4%.

Example 3

[0078] A circuit board of a thickness of 0.8 mm having through holes of opening diameters of 0.1 mm was plated by the PPR method under the following conditions:

[0079] Regular electrolysis: Current density: 1.5 A/dm.sup.2, electrolysis time: 120 sec

[0080] Reverse electrolysis 1: Current density: 0.5 A/dm.sup.2, electrolysis time: 5 sec

[0081] Reverse electrolysis 2: Current density: 0.1 A/dm.sup.2, electrolysis time: 5 sec

[0082] Plating time: 76 min

[0083] As a result, the thickness ratio was 109.8%.

Example 4

[0084] A circuit board of a thickness of 0.8 mm having through holes of opening diameters of 0.15 mm was plated by the PPR method under the following conditions:

[0085] Regular electrolysis: Current density: 1.5 A/dm.sup.2, electrolysis time: 120 sec

[0086] Reverse electrolysis 1: Current density: 0.5 A/dm.sup.2, electrolysis time: 5 sec

[0087] Reverse electrolysis 2: Current density: 0.1 A/dm.sup.2, electrolysis time: 5 sec

[0088] Plating time: 76 min

[0089] As a result, the thickness ratio was 110.8%.

Comparative Example 1

[0090] The above copper sulfate plating solution was used to plate by the DC method a circuit board of a thickness of 0.8 mm having through holes of opening diameters of 0.1 mm under the following conditions:

[0091] Current density: 1.35 A/dm.sup.2

[0092] Plating time: 76 min

[0093] As a result, the thickness ratio was 53.9%.

Comparative Example 2

[0094] The above copper sulfate plating solution was used to plate by the DC method a circuit board of a thickness of 0.8 mm having through holes of opening diameters of 0.15 mm under the following conditions:

[0095] Current density: 1.35 A/dm.sup.2

[0096] Plating time: 76 min

[0097] As a result, the thickness ratio was 54.8%.

[0098] FIG. 5 to FIG. 10 are sectional photographs (magnification 75.times.) of through holes. FIGS. 5, 6, 7, and 8 show through holes of Examples 1, 2, 3, and 4, while FIGS. 9 and 10 show through holes of Comparative Examples 1 and 2.

[0099] As explained above, in Examples 1 to 4, the thickness ratio (throwing power) became substantially 100% and it was possible to form a copper plating film of a uniform thickness on the surface and inside the small diameter hole.

[0100] In particular, when performing the reverse electrolysis in two stages, the result is that the plating thickness of the inside of the small diameter hole becomes greater than the plating thickness on the surface. In this case, if extending the plating time, the inside of the small diameter hole can be buried by the plating.

[0101] Further, in the above embodiment, the explanation was given taking as an example the plating of a through hole, but the plating can be similarly performed for plating in a micro blind via.

INDUSTRIAL APPLICABILITY

[0102] As explained above, according to the method of the present invention, it is possible to obtain a long cycle PPR plating method in second units and, since no high precision, large capacity pulse current is required, possible to reduce the capital costs.

[0103] Further, it is possible to easily synchronize the current for plating at the front and back of a substrate and laying of interconnects considering loss due to inductance is no longer required.

[0104] Further, it is possible to form a plating film of a substantially uniform thickness at the surface, opening part, and inside of a small diameter hole. several tens of seconds.

Claims

4. (AS ONCE AMENDED HEREIN) A method of copper plating a small diameter hole as set forth in claim 1, characterized by performing the regular electrolysis in a range of current density of 1 to 2 A/dm.sup.2.

5. (AS ONCE AMENDED HEREIN) A method of copper plating a small diameter hole as set forth in claim 1, characterized by using a low electrical resistance, high copper concentration copper sulfate plating solution set to a sulfuric acid concentration of 150 to 250 g/l and a concentration of copper sulfate of 130 to 200 g/l.

6. (AS ONCE AMENDED HEREIN) A method of copper plating a small diameter hole as set forth in claim 1, characterized by burying the inside of the small diameter hole by copper plating.

7. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 2, characterized by performing the regular electrolysis in several tens to several hundreds of seconds and performing the reverse electrolysis in several seconds to several tens of seconds.

8. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 2, characterized by performing the regular electrolysis in a range of current density of 1 to 2 A/dm.sup.2.

9. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 3, characterized by performing the regular electrolysis in a range of current density of 1 to 2 A/dm.sup.2.

10. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 2, characterized by using a low electrical resistance, high copper concentration copper sulfate plating solution set to a sulfuric acid concentration of 150 to 250 g/l and a concentration of copper sulfate of 130 to 200 g/l.

11. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 3, characterized by using a low electrical resistance, high copper concentration copper sulfate plating solution set to a sulfuric acid concentration of 150 to 250 g/l and a concentration of copper sulfate of 130 to 200 g/l.

12. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 4, characterized by using a low electrical resistance, high copper concentration copper sulfate plating solution set to a sulfuric acid concentration of 150 to 250 g/l and a concentration of copper sulfate of 130 to 200 g/l.

13. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 2, characterized by burying the inside of the small diameter hole by copper plating.

14. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 3, characterized by burying the inside of the small diameter hole by copper plating.

15. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 4, characterized by burying the inside of the small diameter hole by copper plating.

16. (AS NEW HEREIN) A method of copper plating a small diameter hole as set forth in claim 5, characterized by burying the inside of the small diameter hole by copper plating. REMARKS The foregoing amendment to the specification is submitted to correct a typographical error. The claim amendments are set forth to delete the multiple dependencies of the original claims. No new matter is presented.

1. A method of copper plating a small diameter hole which uses a copper sulfate plating solution containing copper sulfate, sulfuric acid, chlorine ions, a sulfur compound, and a surfactant to copper plate the inside of a small diameter hole of an object being plated having a small diameter hole by the PPR method, said method of copper plating a small diameter hole characterized by performing reverse electrolysis by a range of current density of 0.1 1 A/dm.sup.2 to peel off a sulfur compound near the opening of the small diameter hole in the sulfur compound adsorbed to the object being plated so as to keep the polarization resistance in the small diameter hole at the time of regular electrolysis lower than that near the opening of the small diameter hole and form a copper plating film of a uniform thickness inside the small diameter hole.

2. A method of copper plating a small diameter hole as set forth in claim 1, characterized by, at the time of said reverse electrolysis, performing two-stage reverse electrolysis consisting of performing the first half reverse electrolysis by a high current density and the second half reverse electrolysis by a current density lower than the first half.

3. A method of copper plating a small diameter hole as set forth in claim 1 or 2, characterized by performing the regular electrolysis in several tens to several hundreds of seconds and performing the reverse electrolysis in several seconds to several tens of seconds.

4. A method of copper plating a small diameter hole as set forth in claim 1, 2, or 3, characterized by performing the regular electrolysis in a range of current density of 1 to 2 A/dm.sup.2.

5. A method of copper plating a small diameter hole as set forth in claim 1, 2, 3, or 4, characterized by using a low electrical resistance, high copper concentration copper sulfate plating solution set to a sulfuric acid concentration of 150 to 250 g/l and a concentration of copper sulfate of 130 to 200 g/l.

6. A method of copper plating a small diameter hole as set forth in any one of claims 1 to 5, characterized by burying the inside of the small diameter hole by copper plating.