US20040074517A1

US20040074517A1 - Surfactants for chemical mechanical polishing

Info

Publication number: US20040074517A1
Application number: US10/277,854
Authority: US
Inventors: Vincent Korthuis; Mona Eissa; Gregory Shinn
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 2002-10-22
Filing date: 2002-10-22
Publication date: 2004-04-22

Abstract

Methods and compositions are disclosed for chemical mechanical polishing (CMP) of semiconductor substrates, post-CMP storage of semiconductor substrates and post-CMP cleaning of semiconductor substrates. The methods and compositions feature the use of surfactants and, in some cases, passivation agents. The methods and compositions are particularly suited to polishing, storing and cleaning semiconductor substrates comprising hydrophobic surfaces.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to the preparation of integrated circuits on semiconductor substrates such as silicon wafers and more particularly to improved methods for chemical-mechanical polishing (CMP), post-CMP storage prior to cleaning, and post-CMP cleaning.

2. Description of Related Art

The semiconductor technology central to the modern integrated circuit has been developing for over a century. In the late nineteenth century, the special properties of the semiconductor selenium were first observed and recognized. The field of semiconductor physics advanced rapidly and the first transistor was proposed in the 1930s. However, not until the late 1940s was a functional point contact transistor constructed. The integrated circuit, which employs a plurality of circuit elements in a monolithic semiconductor substrate rather than using discrete components, was first developed in the late 1950s by Jack Kilby at Texas Instruments, Inc.

Since the late 1950s, integrated circuit technology has evolved rapidly and has revolutionized virtually every industry and capacity in which integrated circuits are used. Today's integrated circuits frequently employ hundreds of thousands or even millions of transistors and highly complex, multi-layered architectures. The proliferation of electronics in general, and integrated circuits in particular, has resulted in large part from the ability to increase circuit functionality while simultaneously reducing device cost and size. An important catalyst for these improvements has been advances in semiconductor processing technologies, the various techniques used to construct circuit elements-e.g., transistors, resistors and capacitors-on the semiconductor substrate, as well as the necessary conducting interconnects between individual circuit elements. Improved materials, equipment and processes have allowed increasingly complex circuits having improved speed, reduced power requirements and a smaller footprint.

Integrated circuits are typically constructed at the surface of a crystalline silicon wafer, although other semiconductors such as gallium arsenide and germanium are also used. The individual circuit elements are formed in and on the wafer surface. The electrical conduction between appropriate circuit elements, and electrical isolation between other circuit elements, is then established using alternating layers of appropriately patterned conductors and insulators. The circuit elements and their interconnections are formed using a series of processing steps including photolithography, thin film deposition, selective etching and ion implantation, as well as various cleaning processes.

Increasingly complex integrated circuits utilize an increasing number of circuit elements, which in turn requires both more electrical conduction paths between circuit elements and a greater number of conductor-insulator layers to achieve these paths. This has proved problematic for several reasons. First, longer interconnect paths means increasing resistance and capacitance, which not only decreases circuit speed by increasing RC-delay times but also increases resistive power loss. Second, an increasing number of layers makes successive layer-to-layer alignment more difficult. This latter problem is compounded by layers that lack global and local planarity. Historically, the techniques available to improve layer planarity in the semiconductor industry have been quite limited.

Until recently, aluminum was the interconnect conductor of choice in integrated circuit processing. Techniques for depositing thin aluminum films are well established and, because aluminum trichloride is somewhat volatile, aluminum can be etched effectively in chlorine plasmas to form patterned aluminum films following appropriate photolithography steps. At the same time, aluminum interconnects have several undesirable properties. First, aluminum is not a particularly good conductor: its resistivity is considerably higher than many other metals. Second, aluminum is particularly susceptible to electromigration, the physical movement of a conductor due to electron flow. Electromigration at grain boundaries results in conductor discontinuities and reduced circuit reliability.

The semiconductor industry is transitioning from aluminum to copper as the electrical conductor of choice for establishing interconnections between circuit elements. Copper has a significantly higher conductivity than aluminum and is inherently more resistant to electromigration. Although these properties of copper have been known for a long time, the absence of acceptable methods for selectively etching or otherwise removing copper have limited its use: unlike aluminum, copper is not amenable to plasma etch. Thus, a key limitation in moving to copper metallization is the ability to etch or otherwise remove copper at the wafer surface.

The shift to copper metallization is being driven by the development of chemical mechanical polishing (CMP), a relatively new technique in semiconductor processing. CMP not only provides a method for copper removal and for forming patterned copper films but also addresses the increased need for local and global planarity in complex integrated circuit architectures.

Today, CMP is an essential step in the manufacture of almost every modern integrated circuit. According to the 1997 National Technology Roadmap for Semiconductors, the typical logic device in 2004 will include seven inner-layer dielectric (ILD) CMP steps, seven metal CMP steps and one shallow trench isolation (STI) CMP step. Put simply, CMP is quickly becoming a central aspect of semiconductor processing in the formation of integrated circuits.

CMP is a method of fabricating planar structures by the selective removal of topography-generating features. The CMP process involves the controlled removal of material on the wafer surface through the combined chemical and mechanical action of a slurry of abrasive particles and a polishing pad on the semiconductor wafer. The slurries used in CMP are best classified by the types of films they are intended to planarize. In semiconductor manufacturing, CMP processes are most commonly used for films comprised of silicon oxide, tungsten, copper, tantalum and titanium. CMP of copper films, for example, often employs slurries based with a complexing agent which offer high copper ion solubility.

In addition to polishing of metallization layers, CMP processing generally also involves barrier layer and dielectric layer polishing. The dielectric layer is frequently comprised of an oxide material such as silica that forms the electrically insulating layer between conducting metal layers. A barrier layer is a layer disposed between two layers that prevent one layer from contaminating the other layer and vice versa. Copper metallization schemes often employ barrier metals such as tantalum or tantalum-rich alloys between the copper and dielectric layers to minimize cross-contamination between those layers. A integrated CMP processing technique should allow the polishing and planarization of alternating layers such as those described e.g., a layer comprising copper on a layer comprising tantalum or Ta alloy, on a layer comprising oxide.

Following the CMP process, wafers are typically subjected to a post-CMP cleaning process to remove particulate and molecular contaminants before continuing the construction of the integrated circuit. For wafers processed in batches rather than individually, storage techniques are used following the CMP process and prior to the post-CMP cleaning process. Storing the wafers frequently consists of placing them in a cassette filled with an appropriate liquid such as water.

For a variety of reasons, currently available CMP techniques are less than optimal. First, the CMP process involves the use of small, abrasive particles that can prove difficult to remove from the wafer surface. Although the slurry particles serve a valuable role during CMP, they constitute particulate defects following the CMP process. Consequently, techniques for improving the removal efficiency of slurry particles are desirable. In addition, molecular contaminants can be introduced during the CMP process that are not always effectively removed during post-CMP cleaning. For wafers processed in lots, the wafer storage process can introduce additional problems. It has been noted that wafers removed from storage solutions can evidence streaking wherein contaminants appear preconcentrated in certain areas on the wafer surface. Furthermore, exposed copper surfaces are susceptible to corrosion, resulting in undesirable etching during the post-CMP storage and cleaning processes.

BRIEF SUMMARY OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention include improved methods and compositions for chemical mechanical polishing (CMP) of semiconductor substrates, storage of semiconductor substrates following CMP prior to cleaning processes (“post-CMP storage”) and subsequent cleaning processes (“post-CMP cleaning”).

One preferred embodiment of the present invention involves a method for CMP that utilizes a surfactant during the CMP process, preferably when a hydrophobic surface is exposed. Optionally, the method includes the use of a passivation agent during the CMP process.

Another preferred embodiment of the present invention involves a method for storing semiconductor substrates that utilizes a surfactant in a post-CMP storage liquid. Optionally, a passivation agent is also present in the post-CMP storage liquid. Yet another preferred embodiment of the present invention involves a composition of a post-CMP storage liquid comprising water, a surfactant and a passivation agent.

Yet another preferred embodiment of the present invention involves a method for post-CMP cleaning of semiconductor substrates that uses a surfactant, preferably in a liquid and with the application of a mechanical force. Yet another preferred embodiment of the present invention involves a composition of a post-CMP cleaning liquid comprising water, a surfactant and a passivation agent.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the present invention, reference will now be made to the accompanying drawings, wherein: [0022]
FIG. 1 depicts a wafer in a CMP apparatus comprising a platen and a wafer carrier; [0023]
FIG. 2 depicts the chemical structure of benzotriazole; and [0024]
FIGS. [0025] 3A-3D depicts the cross-section of a wafer during various stages of a multi-step CMP process.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the manufacture of integrated circuits, wafer surface planarity and quality are of extreme importance. In order to achieve the degree of planarity required to produce ultra-high density integrated circuits, CMP processes are being employed. In general, CMP involves pressing a semiconductor wafer against a moving polishing surface that is wetted with a chemically reactive, abrasive slurry. Conventional slurries are either acidic or basic, and generally contain either fumed or colloidal alumina, silica, zirconium oxide, magnesium oxide, or cerium oxide abrasive particles. The polishing surface usually is a planar pad made of a relatively soft, porous material such as polyurethane. [0026]
The term “semiconductor substrate” as used herein refers to a substrate comprised of a semiconductor material upon which an integrated circuit is being, will be or has been constructed. Typically, the substrate is in the form of a thin, circular wafer. A variety of semiconductor materials are used in semiconductor processing such as silicon and gallium arsenide. Furthermore, other semiconductor materials exist and may be amenable to the methods and compositions disclosed herein. Therefore, without limiting the scope of the present invention, the preferred embodiments of the present invention involve a semiconductor substrate comprised of silicon. More preferably, the semiconductor substrate is a single-crystal silicon wafer. Furthermore, the term “semiconductor substrate” also encompasses a substrate comprised of a semiconductor upon which other materials have been deposited. [0027]
The term “surface material” as used herein refers to a material located at the surface of a semiconductor substrate. Because integrated circuits processing involves the deposition of patterned thin films on the semiconductor substrate, there will frequently be two or more surface materials on a semiconductor substrate. Consequently, references herein to “surface material” are not intended to imply a single surface material and instead refer to one or more particular surface materials. [0028]
The semiconductor processing industry has generally determined that barrier materials facilitate the construction of high-yield, high-reliability copper interconnects. As used herein, the term “barrier layer” is a layer disposed between two layers that prevent one layer from contaminating the other layer and vice versa. Barrier layers may be deposited by any manner over the surface of the semiconductor substrate and may be any other region that functions to protect one layer from an otherwise adjacent layer or vice versa. Barrier layers generally used in the semiconductor industry include, for example, tantalum nitride, titanium nitride, titanium/tungsten, tungsten, tantalum, composites thereof, and the like. Preferably, barrier layers are comprised of tantalum nitride when copper metallization is used. The barrier layer may be of any appropriate thickness. Preferably, the barrier layer is a few tens of angstroms to several hundred angstroms in thickness. [0029]
During the course of integrated circuit processing-e.g., during CMP processing both copper surfaces and barrier metal surfaces may be exposed. Differences in the galvanic potentials of the exposed metals can give rise to a type of corrosion in which the more easily oxidized metal is etched from the wafer surface. This phenomenon is frequently referred to as galvanic corrosion. Because of its galvanic potential, copper is particularly susceptible to galvanic corrosion. [0030]
Some of the preferred embodiments of the present invention apply to processes involving hydrophobic surfaces exposed on semiconductor substrates. Hydrophobic surfaces are typically non-polar surfaces that do not interact favorably with polar solvents such as water. Because hydrophobic surfaces are not wet effectively by these polar solvents, hydrophobic surfaces are cleaned less effectively than are hydrophilic surfaces when polar solvents are used. Furthermore, hydrophobic surfaces exposed to polar solvents are susceptible to beading, meaning that the polar solvent forms beads on the surface rather than wetting the surface. As the bead evaporates, the residual contaminants become preconcentrated in small spots on the hydrophobic surface, resulting in localized stains or spots with high contaminant levels. [0031]
Hydrophobic surfaces are frequently generated during the processing of integrated circuits on semiconductor substrates. For example, some low-k dielectrics films such as organosilica glasses (OSGs) are hydrophobic. Similarly, some passivation agents applied to otherwise hydrophilic surfaces result in hydrophobic surface. For example, a copper surface passivated with benzotriazole results in a hydrophobic surface. Hydrophobic surfaces may be characterized based upon their contact angle with an appropriate liquid material. Without limiting the scope of the present invention, a hydrophobic material or surface as used herein refers to a material or surface having a contact angle with water of greater than five degrees. [0032]
Various dielectric materials are used during semiconductor processing. Commonly, such dielectric materials are comprised of a silica-containing material. As used herein, the term “silica” encompasses pure silica, any type of modified silica such as OSGs or doped silica and other materials comprising silica. [0033]
The preferred embodiments of the present invention employ one or more surfactants. Surfactants are typically molecules having both a polar moiety and a non-polar moiety that assist in the wetting of hydrophobic surfaces by polar solvents. Surfactants may be characterized as ionic or nonionic. Ionic surfactants may be further characterized as cationic or anionic. As will be evident to a person of ordinary skill, a variety of surfactants may be suitable in the embodiments of the present invention. Therefore, without limiting the scope of the invention, the preferred embodiments of the present invention employ anionic surfactants. More preferably, the anionic surfactant is Coppeready® Post Clean Surfactant (Ashland Chemical). Depending upon its characteristics, the surfactant employed in the present invention may be employed neat or as a component in a liquid. The preferred embodiments of the present invention employ the surfactant as a component of an aqueous liquid used during a CMP process, a post-CMP storage process, or a post-CMP cleaning process. [0034]
Some of the preferred embodiments of the present invention employ a passivation agent to coat an exposed surface material. Numerous passivation agents may prove adequate in the present invention. Therefore, without limiting the scope of the present invention, suitable passivation agents include benzotriazole (BTA), hydroquinone, pyrogallol, gallic acid and combinations thereof. The passivation agent may be provided neat or in an appropriate liquid. Preferably, the passivation agent is BTA alone or in an aqueous liquid and the exposed surface material comprises a copper surface exposed following a CMP process. [0035]
CMP Processes [0036]
FIG. 1 depicts single-[0037] platen CMP apparatus 10 for polishing semiconductor wafer 20. Single-platen CMP apparatus 10 comprises wafer carrier 30 and platen 40 supporting polishing pad 50. Preferably, single-platen CMP apparatus 10 includes a controller that allows a variable down force to be applied to wafer carrier 30, allows wafer carrier 30 and platen 40 to be rotated at variable and independent rates, and allows slurry and/or other materials to be applied between the surface of semiconductor wafer 20 and polishing pad 50. During operation, a preselected down force is preferably applied to wafer carrier 30 to achieve a desired polish pressure. Also during operation, wafer carrier 30 is preferably rotated at a desired rate while platen 40 is preferably rotated in an opposing direction at a desired rate. Preferably, a slurry having a pH between about 3 and about 11 and comprised of slurry particles having an average diameter of between about 20 and about 200 nanometers (nm) is present during polishing. More preferably, the slurry particles are comprised of alumina. The combined action of the down force of wafer carrier 30, the rotation of wafer carrier 30 and platen 40 and polishing pad 50, and the chemical and mechanical effects of the slurry combine to polish the surface of semiconductor wafer 20.
According to the preferred embodiments of the present invention, the CMP apparatus is a multi-platen CMP apparatus. A multi-platen CMP apparatus comprises a plurality of individual platens and their associated wafer carriers and polishing pads. Multi-platen CMP apparatuses allow for the processing of multiple wafers in parallel or in series. For example, a three-platen CMP apparatus could be used to process three wafers in parallel at three times the throughput of a single-platen CMP apparatus. Such an arrangement would typically use the same slurry, down force and rotation rates on each of the three platens. Alternatively, where multiple CMP processes must be performed on each wafer, a multi-platen apparatus may be used to process wafers in series. For example, a three-platen CMP apparatus could be used to process three wafers in series wherein the wafer on the first platen is subjected to a first CMP process, the wafer on the second platen is subjected to a second CMP process, and the wafer on the third platen is subjected to a third CMP process. Such an arrangement would frequently use different slurries, down forces and/or rotation rates on each of the three platens depending on the nature of the individual CMP processes. The preferred embodiments of the present invention employ a three-platen CMP process. [0038]
The duration of each CMP process may be determined by any suitable method. For example, the duration of each CMP process may be calculated by reference to the removal rate and the layer thickness. Alternatively, the duration of each CMP process may be determined using any suitable endpoint detection technique. For example, endpoint detection may involve Eddy current where the film thickness is monitored or an optical measurement in which an energy source impinges upon the wafer and the reflectivity of the wafer is measured. As the surface layer is removed from the wafer over time to expose the underlying layer, Eddy current or reflectivity of the wafer will change measurably. Upon the detection of this change in Eddy current or surface reflectivity, the polishing process can be terminated. The preferred embodiments of the present invention employ monitoring film thickness on first platen and stopping prior to exposing the barrier. The second platen uses a low down force polishing process where the barrier is exposed and monitoring of the reflectivity of the wafer signals endpoint, after which a timed polish to remove residual copper is employed. The third platen removes the barrier material off the surface of the dielectric by a timed polish. [0039]
To accomplish planarization, CMP must etch and polish high film areas at a substantially faster rate than low areas. Copper CMP involves the creation of a thin passivating layer that is susceptible to abrasion in high areas but that is resistant to etching in low areas. The passivating layer may either be a native oxide film—e.g., Cu[0040] ₂O—or a normative film formed from the addition of a separate reagent such as benzotriazole (BTA), as depicted in FIG. 2. Heterocyclic-nitrogen-based compounds such as BTA form organometallic complexes at the copper surface. The ability of the nitrogen atom in these chemicals to bond at the copper surface facilitates the formation of a hydrophobic passivating film.
In one preferred embodiment, the present invention is directed to methods of using surfactants in CMP processes. CMP processes can be characterized, in part, based on both the surface material being removed and the underlying layer. The identities of both the surface and underlying layer are important in defining the CMP process because ideally CMP processes offer high etch selectivities for the surface layer material relative to the underlying layer material. In other words, CMP processes desirably have much higher etch rates for the surface layer material than for the underlying layer material so that a given CMP step essentially terminates once the underlying layer material is reached. Clearly, because CMP process strategies should be formulated with an eye towards not only the surface layer material but also the underlying layer material, a large number of CMP processes exist. The methods of the present invention are applicable to a wide variety of CMP applications. Therefore, without limiting the scope of the present invention, the preferred embodiments disclosed herein include a three-platen CMP process for polishing a common design structure in modern integrated circuits: namely a threelevel stack having a surface material comprising copper on a barrier material comprising tantalum on an insulator material comprising silica. [0041]
FIGS. 3A, 3B, [0042] 3C and 3D depict a cross-section of a semiconductor substrate during various stages of a multi-step CMP process. FIG. 3A depicts the surface topography of semiconductor substrate 100 prior to CMP comprising surface layer 110, barrier layer 120, and dielectric layer 130. Preferably, surface layer 110 comprises copper, barrier layer 120 comprises tantalum and dielectric layer 130 comprises an organosilica glass (OSG). Such architectures can be formed on semiconductor substrates through a series of processing steps. For example, dielectric layer 130 can be formed as a unpatterned layer by blanket deposition on semiconductor substrate 100. Dielectric layer 130 may be, for example, silica formed by the decomposition of tetraethylorthosilicate (TEOS) on semiconductor substrate 100 or by a spin on glass process. Preferably, dielectric layer 130 is a low-k dielectric such as an organosilica glass (OSG). Dielectric layer 130 may then be patterned and etched to form contact holes exposing selected areas of semiconductor substrate 100 using standard photolithographic techniques. Following deposition and patterning of dielectric layer 130, barrier layer 120 may then be conformally deposited over dielectric layer 130, including in the contact holes to contact the layer underneath dielectric layer 130 on semiconductor substrate 100. The material deposited to form barrier layer 120 generally depends on the material comprising surface layer 110. For example, when surface layer 110 comprises tungsten (W), barrier layer 120 typically comprises titanium (Ti)—e.g., Ti, titanium nitride (TiN), or a Ti/TiN stack. However, when surface layer 110 comprises copper (Cu), barrier layer 120 typically comprises tantalum (Ta) e.g., Ta, tantalum nitride (TaN) or a Ta/TaN stack. According to one preferred embodiment, surface layer 110 comprises copper and barrier layer 120 comprises Ta or TaN or Ta/TaN bilayer.
FIGS. 3B, 3C and [0043] 3D depict a cross-sectional view of the surface of semiconductor substrate 100 of FIG. 3A at various stages of the process of the preferred embodiments of the present invention. Silicon wafers having a surface topography of copper on a tantalum-rich barrier on an organosilica glass (OSG) as depicted in FIG. 3A were subjected to a three-platen CMP process for removal of copper and barrier metal. FIG. 3B depicts a cross-section of semiconductor substrate 100 of FIG. 3A following CMP on the first platen in which a bulk copper removal is performed. FIG. 3C depicts a cross-section of semiconductor substrate 100 of FIG. 3B following CMP on the second platen in which a residual copper removal is performed. FIG. 3D depicts a cross-section of semiconductor substrate 100 of FIG. 3C following CMP on the third platen in which barrier layer 120 was removed.
As depicted in FIG. 3B, the first CMP process of the three-step CMP process comprises the bulk removal of [0044] surface layer 110 on the first platen. In the first CMP process, semiconductor substrate 100 is loaded on a first wafer carrier associated with the first platen in a three-platen CMP apparatus. A controller initially causes a high pressure water spray and a surfactant to be sprayed on a polishing pad located on the platen to clean the surface of the pad prior to polish. The controller then causes a dispenser to dispense a first slurry to the surface of the polishing pad. Preferably, the first slurry is dispensed at a flow rate between about 50 and about 500 milliliters per minute (ml/min). More preferably, the slurry is dispensed at a flowrate of about 200 ml/min for 200 mm wafers. The first slurry is any slurry suitable for polishing a surface layer comprising copper on a barrier layer comprising tantalum. Preferably, the first slurry has a selectivity ratio of greater than ten-to-one for copper to tantalum. Typical slurries appropriate for a copper CMP process include an alumina abrasive having an average particle size of between 20 and 200 nm, an oxidizer and a corrosion inhibitor. Preferably, the oxidizer is hydrogen peroxide (H₂O₂) and the corrosion inhibitor is benzotriazole.
During the first CMP step, the controller also causes the first wafer carrier to rotate. Preferably, the first wafer carrier rotates at a rate of between about 18 and about 150 rotations per minute (rpm). At the same time, the controller also causes the first platen to orbit relative to the first wafer carrier. Preferably, the first platen orbits at a rate of between about 20 and about 150 rpm. The controller also causes a polishing pressure to be exerted between the platen and the wafer carrier. Preferably, the polishing pressure is between about 1 and about 6 pounds per square inch (psi). [0045]
The combined chemical etching effect of the slurry and the mechanical abrasion effect of the pressure and rotation cause the removal of surface material from [0046] semiconductor substrate 100. Because the first CMP polishing process is used to remove the bulk of surface layer 110 without polishing barrier layer 120, the first CMP step may be optimized for a high removal rate. Preferably, the removal rate is between about 1,000 and about 10,000 A/min. The first CMP process continues until the bulk of surface layer 110 is removed and stops slightly before barrier layer 120 is exposed. Consequently, as illustrated in FIG. 3B, a relatively thin surface layer 110 remains above barrier layer 120.
Following the first CMP process, the wafer is rinsed with Coppeready® Post Clean Surfactant (Ashland) before being transferred to a second wafer carrier. Without limiting the scope of the invention, such rinses serve several important purposes. First, these rinses eliminate contaminants and particles introduced or created during the prior CMP process. Furthermore, the CMP slurries used to polish copper are radically different and may even be chemically incompatible with the CMP slurries used to polish the underlying barrier material. Thus, a rinsing process reduces the deposition of contamination from one polishing pad on another polishing pad. In addition to rinsing the wafer, one or more various adjacent platens (especially the second platen and/or the third platen), may also be rinsed along with the wafer. This rinsing of the wafer, the second platen, and/or the third platen, significantly reduces cross contamination between platens. [0047]
Referring to FIG. 2C, the next step is a second CMP process in which [0048] surface layer 110 remaining above barrier layer 120 is removed. In the second CMP process, semiconductor substrate 100 is loaded on a second wafer carrier associated with a second platen in the conventional three-platen CMP apparatus. During the second CMP process, the down force pressure and rotation rates for the second wafer carrier and platen are reduced relative to the first wafer carrier and first platen to reduce the removal rate. The controller initially causes a high pressure water spray and a surfactant to be sprayed on a polishing pad located on the platen. To clean the surface of the pad prior to the polish. Preferably, the surfactant flow is between 10 and 100 ml/min and the deionized water high pressure spray is 1-10 L per minute. Preferably, the first slurry is again dispensed at a flow rate between about 50 and about 500 milliliters per minute (ml/min). During the second CMP process, the controller causes the second wafer carrier to rotate. Preferably, the second wafer carrier rotates at a rate of between about 20 and about 120 rpm and provides a polishing pressure of about 1 to about 3 psi. At the same time, the controller also causes the second platen to orbit relative to the first wafer carrier. Preferably, the second platen orbits at a rate of between about 20 and about 150 rpm. The second CMP process continues until barrier layer 120 is exposed.
As depicted in FIG. 3C, the CMP process on the second platen removes the remaining copper down to [0049] barrier layer 120. Because of the recessed surface material from surface layer 100 in FIG. 3B, as depicted in FIG. 3C, following the polishing on the second platen, material from both surface layer 110 and barrier layer 120 are exposed. After the optical endpoint signal is received, BTA is then applied while the surface wafer is being polished for a fixed time period. The force applied to the wafer to polish is then reduced and BTA saturates the surface of the pad and wafer.
Referring to FIG. 3D, the next step is a third CMP process in which newly-exposed [0050] barrier layer 120 is removed. In the third CMP process, semiconductor substrate 100 is loaded on a third wafer carrier associated with a third platen in the three-platen CMP apparatus. During the third CMP process, barrier layer 120 is removed to expose dielectric layer 130 underneath. The controller initially causes a high pressure water spray and a surfactant to be sprayed on a polishing pad located on the platen to clean the surface of the pad. Preferably, the surfactant flow is between 10 and 100 ml/min and the deionized water high pressure spray is 1-10 L per minute. The controller then causes a dispenser to dispense a second slurry to the surface of the polishing pad. Preferably, the second slurry is dispensed at a flow rate between about 50 and about 500 milliliters per minute (ml/min). Preferably, the second slurry is any slurry suitable for polishing a barrier layer comprising tantalum tantalum nitride on silica. Preferably, the second slurry has a selectivity ratio of greater than ten-to-one for tantalum to silica.
During the third CMP process, the controller also causes the third wafer carrier to rotate. Preferably, the third wafer carrier rotates at a rate of between about 18 and about 36 rotations per minute (rpm) and provides a polishing pressure of about 3 to about 6 psi. At the same time, the controller also causes the third platen to orbit relative to the first wafer carrier. Preferably, the third platen orbits at a rate of between about 20 and about 150 rpm. The third CMP process continues until the [0051] barrier layer 120 is removed and dielectric layer 130 is exposed, as illustrated in FIG. 3D.
Following the third CMP process, the wafer is rinsed with BTA at flow between 50-200 ml/min while the wafer is in contact with the surface of the pad for a time period between 1 and 30 s. The wafer is then rinsed with a high pressure spray of deionized water and surfactant where the flow of the water is between 1 L/min and 10 L/min and the surfactant flow between 10 ml/min and 100 ml/min before being transferred for storage prior to a post-CMP cleaning process. Post-CMP Storage Processes and Compositions [0052]
Frequently, semiconductor substrates such as silicon wafers are processed using batch techniques in which lots comprising multiple wafers are subjected to identical processing steps in serial fashion. For example, in the case of CMP processing, a lot or batch of wafers may be processed one by one until the CMP process has been completed for all wafers before subjecting the lot to post-CMP cleaning processes. This type of batch processing results in the need to store wafers between processes. [0053]
Although BTA and related chemicals are useful during the CMP processing step, they introduce difficulties during the post-CMP cleaning step. BTA, like organic chemicals in general, is hydrophobic and the passivating layer formed using BTA is hydrophobic. Consequently, the aqueous liquids used to clean the wafer surface following copper CMP processing are not entirely effective at wetting the wafer surface and the result is an undesirably high number of defects following the post-CMP storage and post-CMP cleaning processes. [0054]
The storage liquid described in the preferred embodiments of the present invention may find application in any storage operation employed during semiconductor processing. Therefore, without limiting the scope of the invention, the preferred embodiments of the present invention employ the storage liquid during post-CMP storage. Preferably, the storage solution is employed during post-CMP storage when a hydrophobic surface is exposed on the semiconductor substrate. According to one preferred embodiment, the hydrophobic surface is a low-k dielectric comprised of an OSG. According to another preferred embodiment, the hydrophobic surface is a passivated copper surface. [0055]
According to one preferred embodiment of the present invention, semiconductor substrates such as wafers that have completed one processing step and are awaiting another process step are placed in an aqueous liquid comprising one or more surfactants. Preferably, the concentration of the surfactant is between approximately 0.01 percent by weight and approximately 10 percent by weight. This storage may further comprise one or more passivation agents when the exposed surface includes a material susceptible to degradation such as by galvanic corrosion. Preferably, if the surface includes a copper surface material, the passivation agent is BTA and is present at a concentration of between about 0.002 weight percent and about 1 weight percent. It may also in conjunction with the BTA solution contain a surfactant solution to be at a concentration of between 10 ppm to 500 ppm. [0056]
Post-CMP Cleaning Processes and Compositions [0057]
The cleaning liquids described in the preferred embodiments of the present invention may find application in any cleaning operation employed during semiconductor processing. Therefore, without limiting the scope of the invention, the preferred embodiments of the present invention employ the cleaning liquid during post-CMP cleaning. Preferably, the cleaning liquid is employed during post-CMP cleaning when a hydrophobic surface is exposed on the semiconductor substrate. According to one preferred embodiment, the hydrophobic surface comprises a low-k dielectric comprised of an OSG. According to another preferred embodiment, the hydrophobic surface comprises a passivated copper surface. [0058]
The constituents of the cleaning liquid may be mixed together in any order. Preferably, though, surfactant is added to an aqueous liquid comprising the other components of the cleaning liquid. Ideally, the resulting solution is filtered before use using a 0.1 μm or better filter. The cleaning liquids employed in the preferred embodiments of the present invention are aqueous liquids. Such aqueous liquids may be low pH solutions having a pH less than 7 (the pH of pure water) and comprising one or more acidic components such as hydrochloric acid, acetic acid or citric acid. Alternatively, such aqueous liquids may also be high pH solutions having a pH greater than 7 and comprising one or more basic components such as tetramethylammonium hydroxide. Such aqueous liquids may also be neutral pH solutions having a pH of approximately 7 and comprising either no acidic or basic components or alternatively, comprising both acidic and basic components whose combined effect is to retain a pH of approximately 7. The preferred embodiments of the present invention employ low pH cleaning liquids. [0059]
The cleaning processes of the present invention may employ low pH cleaning liquids, high pH cleaning liquids and/or neutral pH cleaning liquids. Preferably, though, the cleaning processes employed herein comprise sequential cleaning processes in which two or more distinct cleaning liquids are employed. Preferably, one or more of these cleaning liquids comprises a surfactant. More preferably, a cleaning process comprises cleaning a surface material on a semiconductor substrate in a low pH cleaning solution, followed by cleaning in a high pH cleaning solution, followed by an approximately neutral pH cleaning solution, preferably purified water. [0060]
The cleaning processes of the present invention can be performed at any stage of integrated circuit processing. However, the preferred embodiments of the present invention employ post-CMP cleaning processes that follow a CMP process. Preferably, these cleaning processes are performed under conditions that facilitate the removal of both particulate and molecular contaminants on the semiconductor substrate surface. Preferably, the cleaning processes of the present invention include the application of mechanical force to assist in the removal of contaminants from the wafer surface. This type of mechanical force includes any type of mechanical agitation. Therefore, without limiting the scope of the present invention, representative examples of mechanical agitation include buffing, rubbing, brushing or vibrating the surface of the semiconductor substrate. Preferably, the mechanical agitation comprises sonic or ultrasonic energy during cleaning. The term “ultrasonic” as used herein includes megasonic cleaning, which generally comprises not only of high-frequency mechanical vibrations, but also the application of a shearing force caused by directed beams that run parallel to the silicon surface. [0061]
During a cleaning process, the temperature may be maintained so as to facilitate contaminant removal from the surface material on the semiconductor substrate. Preferably, the temperature in the cleaning liquid is maintained between approximately ambient temperature and approximately 50° C. Most preferably, cleaning processes employing low pH cleaning liquids are performed at approximately ambient temperatures and cleaning processes employing high pH and neutral pH cleaning liquids are performed at between 20 and 45° C. [0062]
Following cleaning processes, semiconductor substrates such as wafers may be subjected to an appropriate drying step to remove residual liquids from the substrate surface. The preferred embodiments of the present invention include an isopropyl alcohol (IPA) drying step in which one or more surfactants are present on the surface of the wafer. [0063]
While the preferred embodiments of the present invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. [0064]
Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus the claims are a further description and are an addition to the preferred embodiments of the present invention. Use of the term “optional” with respect to any element of a claim is intended to mean that the subject element is required, or alternatively, is not required. Both alternatives are intended to be within the scope of the claim. The discussion of a reference in the Description of Related Art, if any, is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. The disclosures of all patents, patent applications and publications cited herein are hereby incorporated herein by reference, to the extent that they provide exemplary, procedural or other details supplementary to those set forth herein. [0065]

Claims

What is claimed is:

1. A method for the chemical-mechanical polishing of a surface material on a semiconductor substrate comprising:

a. applying a liquid comprising a surfactant to the surface material; and

b. polishing the surface material to create a newly-exposed surface material.

2. The method of claim 1 wherein the surface material comprises copper.

3. The method of claim 2 wherein the newly-exposed surface material comprises copper.

4. The method of claim 3 further comprising applying a passivation agent to the newly-exposed surface material.

5. The method of claim 4 wherein the passivation agent is selected from the group consisting of benzotriazole, hydroquinone, pyrogallol, gallic acid and combinations thereof.

6. The method of claim 4 wherein the passivation agent comprises benzotriazole.

7. The method of claim 1 wherein the surfactant is an anionic surfactant.

8. The method of claim 1 wherein the surfactant is an cationic surfactant.

9. The method of claim 1 wherein the surfactant is an nonionic surfactant.

10. The method of claim 1 wherein the surfactant is Coppeready® Post Clean Surfactant.

11. The method of claim 1 wherein the concentration of surfactant in the liquid is between about 0.01 and about 10 weight percent.

12. The method of claim 2 wherein the newly-exposed surface material comprises tantalum.

13. The method of claim 1 wherein at least part of the newly-exposed surface material is hydrophobic.

14. The method of claim 1 wherein the newly-exposed surface material comprises a low-k dielectric material.

15. The method of claim 1 wherein the newly-exposed surface material comprises an organosilica glass.

16. A method for the chemical-mechanical polishing of a surface material on a semiconductor substrate comprising:

a. polishing the surface material;

b. applying a liquid comprising a surfactant to a newly-exposed surface material; and

c. polishing the newly-exposed surface material.

17. The method of claim 21 wherein the surface material comprises copper.

18. The method of claim 22 wherein the newly-exposed surface material comprises copper.

19. The method of claim 23 further comprising applying a passivation agent to the newly-exposed surface material.

20. The method of claim 24 wherein the passivation agent is selected from the group consisting of benzotriazole, hydroquinone, pyrogallol, gallic acid and combinations thereof.

21. The method of claim 24 wherein the passivation agent comprises benzotriazole.

22. The method of claim 16 wherein the surfactant is an anionic surfactant.

23. The method of claim 16 wherein the surfactant is an cationic surfactant.

24. The method of claim 16 wherein the surfactant is an nonionic surfactant.

25. The method of claim 16 wherein the surfactant is Coppeready® Post Clean Surfactant.

26. The method of claim 16 wherein the concentration of surfactant in the liquid is between about 0.01 and about 10 weight percent.

27. The method of claim 22 wherein the newly-exposed surface material comprises tantalum.

28. The method of claim 21 wherein at least part of the newly-exposed surface material is hydrophobic.

29. The method of claim 21 wherein the newly-exposed surface material comprises an organosilica glass.