Electroplating Chemical Leveler

Abstract

Presented herein is a method of processing a device, comprising providing an electroplating bath having a leveler, the leveler having a total nitrogen-to-total carbon (TN/TOC) ratio of about 15% or less, bringing a substrate into contact with the electroplating bath, the substrate having a recess formed therein and electroplating the substrate to create a feature substantially free of voids in the substrate recess. Electroplating the substrate is performed for a time period about as long as an electrical response peak of the leveler, and optionally for at least 30 seconds. The leveler may optionally have at least one ingredient free of nitrogen and having a leveling functionality. One ingredient may be a benzene ring free of nitrogen. The leveler TN/TOC ratio is between about 3% and about 15%.

Description

This application claims the benefit of U.S. Provisional Application Ser. No. 61/778,290, filed on Mar. 12, 2013, entitled “Electroplating Chemical Leveler,” which application is hereby incorporated herein by reference.

BACKGROUND

As modern integrated circuits shrink in size, the associated features shrink in size as well. As transistor shrink, features such as through vias and other electroplated elements shrink in size as well. In many instances, various layers of circuit on chips, dies, in packages, on PCBs and other substrates are interconnected between various layers by way of vias. Typically, the vias are connected to traces or other conductive structures to route electrical signals through dielectric layers. One way of forming a conductive via in a via opening is to form the opening and then plate a conductive metal in the inside of the opening. In some instances, copper, gold, aluminum or other material are plated in the via openings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1-4 are cross-sectional views of a plating process for a via opening;

FIG. 5 is a diagram illustrating electrical response time of a leveler according to an embodiment;

FIG. 6. is a diagram illustrating concentration dependent polarization of a leveler according to an embodiment; and

FIG. 7 is a flow diagram illustrating a method of plating using a leveler according to an embodiment.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the illustrative embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the embodiments of the disclosure, and do not limit the scope of the disclosure.

The present disclosure describes embodiments with regard to a specific context, namely controlling plating of recessed features. The embodiments of the disclosure may also be applied, however, to a variety of semiconductor devices, plating scenarios or other electrochemical deposition techniques. Hereinafter, various embodiments will be explained in detail with reference to the accompanying drawings.

Plating of semiconductor device surface features is commonly achieved using an electroplating technique, where an ionic metal in solution is deposited on a substrate. In some instances, a metal may be deposited directly on a semiconductor substrate, or on a seed layer. An electric current passed through the substrate and into the solution causes migration of the metal in the solution to the surface of the substrate. Copper is a commonly used metal for electroplating due to its cost, well understood plating properties, and adhesion to many different substrates. Other metals used in the electroplating process may include gold, aluminum, tungsten, cobalt, nickel, chromium, silver, compounds or alloys of the same, or other conductive materials.

In order to control the rate of plating, surface properties and other variables related to electroplating, various additives may be added to the electroplating solution. Generally, the electroplating solution may be a polar liquid, such as water, with a metal source, and, in some cases, an acid. A uniform deposition of plated material is required for the development of complex conductor structures, especially for the high aspect ratio through hole plating. In electroplating smaller diameter holes and thicker panels, plating distribution becomes an important consideration. Material insufficiently deposited, for example, at the walls of the through holes, or voids in pated structures result in rejection the device.

FIG. 1 illustrates an initial plating step according to an embodiment. A substrate 102 may have a recess 104 formed therein, and the recess 104 may have sidewalls 112 and a bottom surface 114. The substrate 102 may be exposed to an electroplating solution 116 having a leveler to form a plated layer 110. While the recess 104 of FIG. 1 is illustrated as extending partially through the substrate 102, the embodiments presented herein may be applied to a recess or opening extending through the substrate to create a via opening or the like. Additionally, while the sidewalls 112 of the recess 104 are illustrated as being substantially linear and perpendicular to the top surface of the substrate, the sidewalls may be at an angle greater or less than 90 degrees to the substrate 102 surface, and may be curved, in linear sections, or otherwise non-linear.

Throwing power of an electroplating solution is a factor in the reliability of the electroplating process. Throwing power is the ability of an electroplating solution to deposit a uniform thickness of plating across a wafer or board when the electric field varies in different regions of the electroplating bath. In general, plating processes that provide better leveling of the deposit across irregularities on the substrate surface and inside the through holes tend to worsen the throwing power of the electroplating bath.

For example, in an embodiment, an electroplating solution for plating copper may have copper sulfate (CuSO₄) and sulphuric acid (H₂SO₄). The sulphuric acid disassociates the copper ions from the copper sulfate, allowing the copper sulfate to migrate to the substrate and form copper plate. However, it has been discovered that, with nanoscale features, the rate of copper plating may be so rapid that the surface finish of the copper is not acceptable. Additionally, when plating recesses in a substrate, and in vias or high aspect ratio recesses in particular, the rate of copper plating may be so rapid that the copper plate closes off over the top of a recess or opening before the interior of the recess has completely filled with solidly plated material, resulting in a void or cavity in the metal via. Such voids may be particularly undesirable because the lack of conductive material introduced by the void creates a greater electrical resistance, possibly bringing the feature out of tolerance.

In order to provide a smoother plated surface and reduce the errors introduced in small features by plating, many electroplating solutions also include additives such as brighteners, levelers, and suppressors. Organic compounds are added to an electroplating bath and act as levelers and brighteners, increasing uniformity of metal deposition on different regions of the PCB including through holes and recesses. Additionally, salts such as chlorides, may also be included in an electroplating bath to as a brightener and to increase the deposition of plating materials Organic compound such polyethylene glycol (PEG) or, alternatively, polyalkylene glycol (PAG) function as suppressors, while organic sulfides such as Bis(3-sulfopropyl)-disodium-sulfonate (C₆H₁₂Na₂O₆S₄) (SPS) work as accelerators.

A leveler frequently has ingredients with nitrogen functional group and is added to the bath at a relatively low concentration. Traditional leveling involves the diffusion or migration of strongly current suppressing species to corners or edges of macroscopic objects which otherwise plate more rapidly than desired due to electric field and solution mass transfer effects. In the case of plating recesses or opening, the function of a leveler is to stop the excessive growth rate of copper over a feature to be filled while not otherwise perturbing the process.

However, large nitrogen concentrations found in levelers can cause voids and cavities, particularly in high aspect ratio recesses such as via openings. In particular, nitrogen in levelers tends to interfere with adsorption of suppressors on the plated surface. The levelers displace the suppressor, causing plating to occur too rapidly, and causing the corners of via openings to plate faster than the plating can build up from the bottom, closing off the tops of the openings and creating voids or cavities in the electroplated vias.

In an embodiment, the TN/TOC in the leveler may be between about 3% and about 15%, and in another embodiment, may be about 5%. It has been determined that the reduced TN/TOC results in less interference with the suppressor by the leveler and a longer electrical response peak in the electroplating solution 116. In such an embodiment, the electrical response peak time of the electroplating solution 116 may be greater than thirty seconds, permitting recesses in the electroplating target to fill from the bottom 114 before plating at the corners 106 of a recess 104 closes the recess 104.

A leveler may have one or more organic compounds acting as a leveler, and reducing the amount of a nitrogen containing compound may permit reduction of the overall TN/TOC ratio. For example, in some levelers, a compound having multiple nitrogen groups may be replaced with a compound having fewer nitrogen groups, or with a compound being free of nitrogen. For example, benzotriazole (C₆H₅N₃) (BTA), Janus Green B, thiourea (SC(NH₂)₂, polyvinylpyrrolidone (C₆H₉NO)_n (PVP) and polyacrylamide (C₃H₅NO)_n components may be completely or partially replaced with non-nitrogen hydrocarbons or hydrocarbons having a greater carbon-to-nitrogen ratio, lowering the overall TN/TOC ratio. In an embodiment, benzene (C₆H₆) or a similar benzene ring or cyclic hydrocarbon may be used as a constituent to reduce the overall leveler TN/TOC ratio. In such an embodiment, toluene, ethylbenzene, xylene compounds, phenol or the like may be used to replace a nitrogen containing leveler ingredient. The leveler may, in an embodiment, have a constituent component that provides leveling capabilities or functionality and is free of nitrogen.

In an embodiment, the substrate 102 may be placed in an electroplating solution 116, with a leveler at, in an embodiment, a 5% TN/TOC ratio. Without a leveler and suppressor, the plating layer corners 108 tend to plate or grow most rapidly due to the relatively large surface area in relation to plated layer 110 surface volume. However, with the leveler having a reduced TN/TOC between about 15% and about 3%, the suppressor function tends to dominate, slowing the plating process. A voltage may be applied to the substrate and an anode in the electroplating solution 116, and a plated layer 110 is formed over the exposed surfaces of the substrate 102. The recess 104 may be plated with copper or another conductive metal.

In one embodiment, the recess 104 may have a width of about 30 nm to about 60 nm, and a depth of at least about 120 nm to about 200 nm, with an aspect ratio of about 4 to about 7. In an embodiment, electroplating a recess 104 with an aspect ratio of 5 may fill the recess 104 to form a via or feature generally free of voids due to the bottom-up plating abilities of the reduced nitrogen leveler.

FIG. 2 illustrates an early state of plating in an embodiment. Using a reduced nitrogen leveler permits the suppressor to prevent the plating layer corners 108 from closing off the recess 104. In an embodiment, the plating layer 110 forms a bottom portion 110A without voids on the bottom surface 114 of the recess 104. The plating layer 110 also tends to grow on the sidewalls of the recess 104 at a more uniform rate, the plating layer 110 on the sidewalls 112 being generally free of a plating layer corner 108 that is substantially thicker than the plating layer on the sidewalls 112. In an embodiment where the recess 104 is a through via opening, or otherwise extends through the substrate 102, the substantially uniform growth of the sidewall 112 plating layer 110 will fill the void from the sides, rather than the bottom surface 114 of a recess 104 since no bottom surface 114 exists. Uniform sidewall 112 plating layer 110 growth on the sidewalls avoids voids or cavities in a filled via structures in such an embodiment.

FIG. 3 illustrates an intermediate stage of plating according to an embodiment. The plating layer bottom portion 110A continues to grow while the plating layer corners 108 maintain a separation 302 permitting plating solution to escape the unplated portion of the recess 104.

FIG. 4 illustrates a plated recess according to an embodiment. A plating layer 110 will form a substantially solid and substantially void-free via 402 in the recess 104 by plating from the bottom or sides without closing out the recess 104 by the plating layer corners 108 (FIG. 3). In an embodiment, the plating layer 110 will have a substantially even or level top surface 404.

FIG. 5 illustrates the electrical response time of a leveler having different TN/TOC ratios. A leveler having about a 5% TN/TOC ratio exhibits an electrical response 502 that initially drops upon plating initiation, and then peaks around the 35 second mark 508, resulting in an electrical response peak greater than 30 seconds. A leveler having about a 28% TN/TOC ratio exhibits an electrical response 504 with an electrical response peak around the 10 second mark 506. In order to adequately plate the substrate 102, the substrate 102 may be plated for a period having a duration about as long as the electrical response peak time of the leveler. For example, a substrate 102 may be plated in a leveler having about a 5% TN/TOC ratio for about 30 second or more, since the electrical response peak occurs around 30 seconds.

A reduced leveler nitrogen content further provides greater control over the plating process and plating rate. FIG. 6 illustrates leveler polarization as a function of leveler concentration. The polarization curves of a leveler with a 28% TN/TOC ratio at 2 seconds 604 and 4.5 seconds 602 show roughly 300% greater deviation from an average 610 at different leveler concentrations (shown in ml/L concentration) than the deviation from the average 612 of polarization curves for a leveler with a 5% TN/TOC ratio at 2 seconds 606 and 4.5 seconds 608. Thus, a leveler with a reduced nitrogen content, such as about 5% TN/TOC will tend to have more predictable polarization levels at different leveler concentrations.

FIG. 7 is a flow diagram illustrating a method 700 of plating using a leveler according to an embodiment. A substrate 102 may be provided in block 702. The substrate 102 may have one or more recesses 104. The substrate 102 may be applied, or submersed, in an electroplating bath having a leveler with reduced TN/TOC ratio. The reduced TN/TOC ratio of the leveler causes the electroplating solution 116 to plate at a relatively slow rate. The substrate 102 may be plated for at least about 30 seconds in block 706.

Thus, according to an embodiment, a method of forming a device may comprise bringing a substrate into contact with an electroplating solution having a leveler with a TN/TOC ratio of less than about 15% and electroplating the substrate in the electroplating solution for a time period about as long as an electrical response peak of the leveler, optionally for at least 30 seconds. The leveler TN/TOC ratio may be between about 3% and about 15%, and may optionally be about 5%. The leveler may further optionally have at least one constituent ingredient free of nitrogen and having a leveling functionality. The substrate may have a recess electroplating the substrate may comprise forming a via in the recess, the via substantially free of voids. The substrate may be electroplated with copper.

According to another embodiment, a method of processing a device may comprise providing an electroplating bath having a leveler, the leveler having a TN/TOC ratio of about 15% or less, bringing a substrate into contact with the electroplating bath, the substrate having a recess formed therein and electroplating the substrate to create a feature substantially free of voids in the substrate recess. Electroplating the substrate may be performed for a time period about as long as an electrical response peak of the leveler, and optionally for at least 30 seconds. The leveler may optionally have at least one ingredient free of nitrogen and having a leveling functionality. One ingredient may be a benzene ring free of nitrogen. The leveler may have a TN/TOC between about 3% and about 15%, and may optionally have a TN/TOC of about 5%.

A leveler according to an embodiment may comprise a hydrocarbon free of nitrogen and having a leveling functionality, wherein the electroplating leveler has an electrical response peak of at least about 30 seconds, and wherein the electroplating leveler has a total nitrogen-to-total carbon (TN/TOC) ratio of about 15% or less. The hydrocarbon of the leveler may be a benzene ring free of nitrogen, optionally benzene.

Although embodiments of the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Claims

1. A method of forming a device, comprising:

bringing a substrate into contact with an electroplating solution, the electroplating solution having a leveler with a total nitrogen-to-total carbon (TN/TOC) ratio of less than about 15%; and

electroplating the substrate in the electroplating solution for a time period having a duration about as long as an electrical response peak time of the leveler.

2. The method of claim 1, wherein the electroplating the substrate comprises electroplating the substrate for at least about 30 seconds.

3. The method of claim 1, wherein the leveler TN/TOC ratio is between about 3% and about 15%.

4. The method of claim 1, wherein the leveler TN/TOC ratio is about 5%.

5. The method of claim 1, wherein the substrate has a recess and wherein the electroplating the substrate comprises electroplating the recess of the substrate and forming a via in the recess, the via substantially free of voids.

6. The method of claim 1, wherein the electroplating the substrate comprises electroplating the substrate with copper.

7. The method of claim 1, wherein the bringing a substrate into contact with an electroplating solution comprises providing a leveler having at least one constituent ingredient free of nitrogen, the at least one constituent ingredient further having a leveling functionality.

8. A method of processing a device, comprising:

providing an electroplating bath having a leveler, the leveler having a total nitrogen-to-total carbon (TN/TOC) ratio of about 15% or less;

bringing a substrate into contact with the electroplating bath, the substrate having a recess formed therein; and

electroplating the substrate in the electroplating solution, the electroplating the substrate comprising forming an electroplated feature in the recess, the feature substantially free of voids.

9. The method of claim 8, wherein the electroplating the substrate is performed for a time period having a duration about as long as an electrical response peak time of the leveler.

10. The method of claim 8, wherein the electroplating the substrate is performed for at least about 30 seconds.

11. The method of claim 8, wherein the leveler has at least one ingredient free of nitrogen, the at least one ingredient further having a leveling functionality.

12. The method of claim 8, wherein the leveler has at least one ingredient that is a benzene ring free of nitrogen.

13. The method of claim 8, wherein the leveler comprises at least benzene.

14. The method of claim 8, wherein the leveler TN/TOC ratio is between about 3% and about 15%.

15. The method of claim 8, wherein the leveler TN/TOC ratio is about 5%.

16. An electroplating leveler, comprising:

a hydrocarbon free of nitrogen and having a leveling functionality;

wherein the electroplating leveler has an electrical response peak of at least about 30 second, and wherein the electroplating leveler has a total nitrogen-to-total carbon (TN/TOC) ratio of about 15% or less.

17. The electroplating leveler of claim 16, wherein the hydrocarbon is a benzene ring free of nitrogen.

18. The electroplating leveler of claim 16, wherein the hydrocarbon is benzene.

19. The electroplating leveler of claim 16, wherein the leveler TN/TOC ratio is between about 3% and about 15%.

20. The electroplating leveler of claim 16, wherein the leveler TN/TOC ratio is about 5%.