WO2001019935A1

WO2001019935A1 - Slurry for forming insoluble silicate during chemical-mechanical polishing

Info

Publication number: WO2001019935A1
Application number: PCT/US2000/025222
Authority: WO
Inventors: Craig D. Lack; Peter A. Burke; Qiuliang Luo; Vikas Sachan; Vilas N. Koinkar
Original assignee: Rodel Holdings, Inc.
Priority date: 1999-09-15
Filing date: 2000-09-15
Publication date: 2001-03-22
Also published as: JP2003509855A

Abstract

An aqueous slurry for chemical-mechanical polishing of silica substrates comprising: water, submicron abrasive particles, and a silica protecting agent having a solubility in water of less than 0.01 g/100 mL. The silica protecting agent is selected from alumina and zirconia.

Description

SLURRY FOR FORMING INSOLUBLE SILICATE DURING CHEMICAL-

MECHANICAL POLISHING

The invention relates to a slurry for rninimizing or lowering oxide removal from silica surfaces during chemical-mechanical polishing, CMP, especially for pohshing substrates containing smaller and higher density features.

US Patent 5,476,909 discloses a known aqueous slurry for CMP polishing of a semiconductor substrate having a composite of sihca and a metal comprising, comprising: water and submicron abrasive particles. The slurry further has a compound which suppresses the rate of removal of sihca. One problem prevalent throughout the pohshing industry is the unwanted removal of sihca from the substrate during pohshing, known as, erosion. The features on sihca substrates are decreasing in size and the density of features on sihca substrates is increasing. Thus, it is becoming more and more important to protect the sihca surface in order to πώiimize inconsistencies in the polished substrate. It is thus desirable to provide a pohshing slurry that protects the sihca surface while still providing acceptable metal removal rates.

According to the invention, an aqueous slurry is provided which is useful for the chemical-mechanical pohshing of sihca substrates, comprising: water, submicron abrasive particles, and a sihca protecting agent. According to the invention, a method is provided for chemical-mechanical polishing of sihca substrates with a pohshing slurry comprising: water, submicron abrasive particles, and a sihca protecting agent.

Embodiments of the invention will now be described by way of example.

It has been found that the oxide removal rate decreases when an agent is present that is capable of forming an insoluble or nearly insoluble silicate. The surface, therefore, of a sihca substrate can be protected by the presence of a species that forms an insoluble or nearly insoluble silicate with or on the sihca surface. It is presumed that such species remains in contact or in close proximity to the oxide surface in order to protect it. Thus, the presence of a sihca protecting agent in a slurry for CMP has been found to reduce the oxide removal rate.

A sihca protecting agent, as used herein, can be a submicron abrasive particle, an oxidizing agent, an additional additive, or a combination of two or more of these components. A sihca protecting agent is an agent capable of forming a sihcate which is other than a water soluble sihcate. A water soluble sihcate, as defined herein, is a sihcate whose solubility in cold water (g/100 mL) is greater than 0.01. Preferably, the sihca protecting agent is capable of forming a sihcate having a solubility less than 0.01 g/lOOmL. More preferably, the sihca protecting agent is capable of forming a sihcate having a solubility less than 0.001 g/lOOmL. Even more preferably, the sihca protecting agent is capable of forming a sihcate having a solubility less than 0.0001 g/lOOmL. Still more preferably, the sihca protecting agent is capable of forming a sihcate which is insoluble in water. Another way of describing a sihca protecting agent, as used herein, is an agent which forms a sihcate upon contact with sihca, wherein the formed sihcate is capable of remaining on or near the sihca surface thereby protecting it from pohshing, erosion, or both. Contact of the sihca protecting agent and sihca meaning the contact which occurs throughout the entire pohshing process.

An aqueous non-ferric slurry is provided which is useful for the chemical- mechanical pohshing of silica substrates, comprising: water, submicron abrasive particles, and a sihca protecting agent.

A non-ferric method is provided for reducing sihca erosion during chemical- mechanical polishing of sihca substrates using a pohshing slurry comprising: water, submicron abrasive particles, and a sihca protecting agent.

A sihca protecting agent, as used herein, can be a submicron abrasive particle, an oxidizing agent, an additional additive, or a combination of two or more of these components. A sihca protecting agent is an agent capable of forrning a sihcate which is other than a water soluble sihcate.

The slurries of the present invention may contain an oxidizing agent (which may or may not be a sihca protecting agent). Useful oxidizing agents include any water soluble composition capable of receiving an electron from the metal atoms at the surface of the substrate during the polishing operation. By receiving electrons from the metal surface of the substrate, the oxidizing agent can transform metal atoms at the substrate surface into water soluble anions. In this way, the oxidizing agent promotes a type of dissolving of the metal into the slurry's aqueous medium. Useful oxidizing agents include acids, salts, peroxides and the like. Ordinary skill and experimentation may be necessary in selecting an oxidizing agent, depending upon the pohshing system and substrate chosen. Preferred oxidizing agents would generally include: nitrates, sulfates (including persulfates), iodates (including periodates), hydrogen peroxide and/or acid derivatives thereof. Preferred particles of the present invention are readily dispersible in an aqueous medium. The particles preferably have a surface area ranging from about 40, 60, 80, 100, 150, 200 m² /g to about 250, 300, 350, 400, 430 m² /g, and an aggregate size distribution less than about 1.0 micron, a mean aggregate diameter less than about 0.4 micron. The particles of the present invention are metal oxides selected from sihca, alumina, ceria, zirconia and/or derivatives thereof and optionaUy can further include second metal oxide. The slurries of the present invention can be stable, but are more preferably meta-stable.

Slurries of the present invention may also comprise complexing agents which include compounds having at least two acid moieties present in the structure which can affect complexation to the target metal, such as, titanium. Acid moieties are defined as those functional groups having a dissociable proton. These include, but are not limited to, carboxyl, carboxylate, hydroxyl, sulfonic and phosphonic groups. Carboxylate and hydroxyl groups are preferred, as these are present in the widest variety of effective species. Particularly effective are structures which possess two or more carboxylate groups with hydroxyl groups in an alpha position, such as straight chain mono- and di- carboxyhc acids and salts including, for example, mahc acid and malates, tartaric acid and tartarates and gluconic acid and gluconates. Also effective are tri- and polycarboxyhc acids and salts with secondary or tertiary hydroxyl groups in an alpha position relative to a carboxyhc group such as citric acid and citrates. Also effective are compounds containing a benzene ring such as ortho, di- and polyhydroxybenzoic acids and acid salts, phthahc acid and acid salts, pyrocatecol, pyrogallol, galhc acid and gallates and tannic acid and tannates. The most preferred complexing agents of the present invention will tend to complex with metal anions, forming a 5 or 6 member ring, whereby the metal atom forms a portion of the ring. Sihcates which are insoluble or have a solubility (g per lOOmL) less than 0.01 include aluminum sihcate, bismuth sihcate, cadmium silicate, calcium sihcate, cobalt sihcate, iron sihcate, lead sihcate, lithium sihcate, magnesium sihcate, manganese sihcate, hydrogen sihcate (silicic acid), strontium sihcate, thorium sihcate, zinc sihcate, or zirconium sihcate. Thus, a sihca protecting agent would be an agent capable of forming one of these sihcates upon contact with the sihca surface during the pohshing process. Oxidizing agents in compositions of the present invention may be comprised of nitrates, iodates, perchlorates, sulfates, peroxides, or any other commonly known oxidizing agent. Counter-ions such as sodium, hthium, calcium, potassium, ammonium, and magnesium can be used. Generally oxidizing agents are used in slurries for CMP at about 1, 2, 3, 4, 5, 6, 7, 8, 9, to 10% by weight. Preferably, the oxidizing agent is present at about 2, 3, 4, 5, 6, to 7% by weight. Preferably, the oxidizing agent is other than hydrogen peroxide and a ferric oxidizer (e.g., ferric nitrate, ferric sulfate, ferric chloride, and ammonium ferrate).

Oxidizing agents can be classified into two types: (1) agents which can form insoluble or nearly insoluble sihcates, and (2) agents which are incapable or nearly incapable of forming insoluble or nearly insoluble sihcates. Type (1) oxidizing agents include iodic acid, hthium iodate, calcium iodate, silver iodate, ammonium iodate, and lead iodate. Type (2) oxidizing agents include potassium iodate, hydrogen peroxide and ferric nitrate..

Potassium iodate is a preferred oxidizing agent when alumina or zirconia are used as submicron abrasive particles. More preferably, potassium iodate is present at about 2, 3, or 4% by weight. Even more preferably potassium iodate is present at about 3% by weight.

Iodic acid is a preferred oxidizing agent when a type (2) particle [type (2) particles are discussed later in the present specification] is present, especially titania. It has been found that the use of iodic acid provides a slurry which sufficiently oxidizes the metal surface being pohshed while providing a decreased oxide removal rate when compared with potassium iodate. Preferably, the iodic acid is present in the slurry in an amount greater than 0% by weight and less than about 1.8% by weight. More preferably, the iodic acid is present at about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, or 1.7% by weight. Even more preferably, the iodic acid is present at about 1.5% by weight.

When a two step pohshing process is being used, the preferred amount of iodic acid present depends on the step. In the first step, the iodic acid is preferably present in an amount greater than 0% by weight and less than 1.8% by weight. More preferably in the first step, the iodic acid is present at about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, or 1.7% by weight. Even more preferably, the iodic acid is present at about 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, or 1.7% by weight. Still more preferably, the iodic acid is present at about 1.5% by weight. In the second step of the two step pohshing process, the iodic acid is present in the slurry in an amount less than 1.8% by weight. Preferably, in the second step the iodic acid is present in an amount less than 1% by weight. More preferably, in the second step the iodic acid is present in an amount less than 0.5% by weight. Even more preferably, in the second step the iodic acid is present in an amount less than 0.1% by weight. The submicron abrasive particles in the compositions of the present invention may be comprised of any of the oxides used for chemical-mechanical pohshing such as, alumina, sihca, ceria, titania, and zirconia. These particles can be classified into two types: (1) particles capable of forming insoluble or nearly insoluble sihcates (e.g., alumina and zirconia) and (2) particles incapable or nearly incapable of forming an insoluble or nearly insoluble sihcate. When a type 1 particle is used, then no other sihca protecting agent is required. A second or third sihca protecting agent can be used in combination with a type 1 particle. When a type 2 particle is used, then a sihca protecting agent is required. The sihca protecting agent can selected from an oxidizing agent, another additive, or a type 1 particle (i.e., a combination of type 1 and type 2 particle). Generally the total amount of abrasive particles used in slurries of the present invention is about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15% by weight. When the submicron abrasive particles are fumed sihca, then they are preferably present at about 4, 5, 6, 7, 8, 9, or 10% by weight, even more preferably about 7% by weight. When the submicron abrasive particles are colloidal sihca, then they are preferably present at about 4, 5, 6, 7, 8, 9, or 10% by weight, even more preferably about 8.5% by weight. Preferably, the submicron abrasive particles of the present invention are absent of an organic solubility coating (e.g., a phthalate compound coated thereon).

Alumina particles have been found to form aluminum sihcate. Alurriinum sihcate is an amphoteric species which associates with the sihca surface. Thus, the aluminum sihcate, once formed, tends to stay on the sihca surface and protect it. Consequently, alumina is a preferred abrasive particle. Preferably, the alumina is present at about 1, 2, 3, 4, 5, 6, to 7% by weight, even more preferably 2, 2.5, 3, 3.5, 4, to 4.5% by weight, still more preferably about 3% by weight. AJiirnina is an abrasive particle that is available in many different forms (e.g., alpha- alumina, gamma-alirmina, delta-alumina, and amorphous (non-crystalline) alumina). Preferably, α and γ alumina particles are present when alumina particles are used. When both types of alumina are present, the γ alumina is preferably present in about 0.5, 1, 2, 3, 4, to 5 weight percent, more preferably from about 1, 2, 3, to 4 weight percent and even more preferably from about 2, 2.25, 2.5, 2.75, to 3 weight percent. A particularly preferred concentration of γ alumina is about 2.4% by weight. When both types of alumina are present, the alumina is preferably present in about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, to 5 weight percent, more preferably from about 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, to 1 weight percent and even more preferably from about 0.4, 0.5, 0.6, 0.7, to 0.8 weight percent. A particularly preferred concentration of α alumina is about 0.6% by weight.

For slurries containing titania and alumina particles, the amount of titania is preferably present in an amount of about 0.05, 0.1, 0.25, 0.5, 0.75, 1, 1.5, to 2 weight percent, more preferably between about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, to 1 weight percent, and even more preferably from about 0.2, 0.3, 0.4, to 0.5 weight percent, and still more preferably about 0.3% by weight.

When titania and alumina particles are both present, then preferably from about 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, to 20 weight percent of the particles present in the composition are titania, from about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, to 20 weight percent of the particles present in the composition are α alumina, and from about 60, 65, 70, 75, 80, 85, 90, 95, to 98 weight percent of the abrasive submicron particle present in the composition is γ alumina. More preferably, about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, to 15 wt.% of the abrasive submicron particle present in the composition are titania, about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, to 15 wt.% of the abrasive submicron particle present in the composition are alumina, and about 70, 75, 80, 85, to 90 wt.% of the abrasive submicron particle present in the composition are γ alumina. Even more preferably, about 10% by weight of the abrasive submicron particles present in the composition are titania, about 10% by weight of the abrasive submicron particles present in the composition are α alumina, and about 80% by weight of the abrasive submicron particles present in the composition are γ alumina. It has been found that a more uniform particle size provides greater erosion protection. Thus, it is preferred that substantially ah of the alumina particles are all less than about 5μ. Substantially, as used in this instance is intended to mean at least 90, 91, 92, 93, 94, 95, 96, 97, 98, 99. 99.5, 99.9% of the alumina particles are less than about 5μ. Alumina is often prepared via milling. The milling process, unfortunately, results in milling medium being present with the alumina particles. The milling medium can be seen under a transition electron microscope to be fractured chunks. Alumina particles, in contrast, appear to be more rounded under the transition electron microscope. Milling medium, due to its fractured shape, can cause undesirable scratching, gouging, scoring, etc... during CMP. It is therefore preferable to remove the milling medium from the alumina particles prior to use in CMP slurries. Preferably, the alumina used in the present invention has been passed at least once through a 5μ filter. It is preferred that the majority of the slurry is passed through the filter 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 times. Majority, as used here, is preferably 95, 96, 97, 98, 99, 99.5, or 99.9 wt.% of the alumina particles. It has been found that such filtering increases the copper removal as compared with unfiltered alumina.

It is preferable that the alumina particles be substantially free of the rnilling medium. SubstantiaUy free is intended to mean that 95, 96, 97, 98, 99, 99.5, or 99.9wt.% of the submicron particles are alumina. Preferably, about 95 wt.% of the submicron particles are alumina, more preferably, 96wt.%, even more preferably 97 wt.%, stiU more preferably 98wt.%, further preferably 99wt.%, even further preferably 99.5wt.%, and stiU further preferably 99.9wt.%. The preferred complexing agents of the present invention are a class of compounds, having at least two acid moieties present in the structure which can effect complexation to the target metal anion. Preferably, the pKa of the first acid species is not SubstantiaUy larger than the pH of the pohshing solution. "Substantially" is intended to mean about 1 unit (pKa or pH). Compounds which act as complexing agents or chelating agents for SiO₂ are described in great detaU in U.S. 5,391 ,258 and U.S. 5,476,606, the contents of which are herein incorporated by reference.

Acid species are defined as those functional groups having a dissociable proton. These include, but are not limited to, carboxyl, carboxylate, hydroxyl, sulfonic and phosphonic groups. Carboxylate and hydroxyl groups are preferred as these are present in the widest variety of effective species.

Particularly effective complexing agents of the present invention have a structure which possess one or more carboxylate groups with hydroxyl groups in an alpha position, such as straight chain mono- and di-carboxyhc acids and salts including, for example, mahc acid and malates, tartaric acid and tartrates and gluconic acid and gluconates. Also effective complexing agents are tri- and polycarboxyhc acids and salts with secondary or tertiary hydroxyl groups in an alpha position relative to a carboxyhc group such as citric acid and citrates. Also effective complexing agents are compounds containing a benzene ring such as ortho di- and polyhydroxybenzoic acids and acid salts, phthahc acid and acid salts, pyrocatecol, pyrogaUol, gallic acid and gaUates and tannic acid and tannates. The reason for the effectiveness of these compounds is believed to he in the extensive electron delocalization observed in the structures. This delocalization leads to a high degree of stability for the conjugate base in solution, as evidenced by the low pKa values: Tartaric acid: pKai =3.02 Citric acid: pKaι=3.1

Phthahc acid: pKaι=2.95 The pKa limitations set forth in the present invention are due to the requirement that the free anion or conjugate base must be present in reasonable concentration for the complexing effect to occur. At pH«pKa httle free anion is present. At pH=pKa, the acid is 50% dissociated. At pH»pKa, essentiaUy aU of the acid is present as the anion. Thus the dissociation constant must be chosen to reflect the range of pH values normaUy encountered in pohshing. IdeaUy, the pH of the pohshing composition should be equal to or greater than a value equal to the pKai of the additive used for sihca rate suppression. If the pKai of the additive is SubstantiaUy greater than the composition pH, insufficient free metal anion is produced in solution and the advantageous complexing effect is inhibited. Thus additives such as tartaric, citric and phthahc acid (pKai < 3.1) should be effective over a pH range corresponding to the normal pH range encountered in pohshing metals (pH.about.4-11) and would be preferred. In contrast, addition of pyrocatechol (pKaι«10) would only be useful at very high solution pH and would have a more restricted utility. The complexing agents in accordance with the present invention are preferably used in concentrations of from about 1, 2, 3, 4, 5, 6, 7, 8, 9, to 10 weight percent, more preferably about 2, 3, 4, 5, 6, to 7 wt.%. Preferably, the complexing agent is mahc acid, tartaric acid, gluconic acid, glycohc acid, citric acid, phthahc acid, pyrocatecol, pyrogaUol, gallic acid, or tannic acid. More preferably, the complexing agent is citric acid. Another more preferred complexing agent is glycohc acid. Preferably, citric acid is present in a concentration of about 1, 1.5, 2, 2.5, to 3 wt.% and more preferably about 2.0 wt. %. Preferably, glycohc acid is present in a concentration of about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, to 5 wt.% and more preferably about 3.0 wt. %. Complexing agents may be used in the compositions of this invention individuaUy or in combinations of two or more. Preferred complexing agents of the present invention wiU tend to complex with metal anions, forming a 5 or 6 member ring, whereby the metal atom forms a portion of the ring.

Features being used on substrates today are often around 5μ. However, new technologies are aUowing the size of features to decrease to about 0.18μ. Such newer, smaUer features, wiU require more sophisticated and specialized slurries. The present method of pohshing is preferably performed on a substrate with features of about 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 to 0.5μ. More preferably, the present method is performed on a substrate with features of less than 0.4μ, even more preferably, less than 0.3μ, stiU more preferably, less than 0.2μ, and still further preferably about 0.18μ. Improvements in wafer manufacturing has led to an increase in feature density. Just as smaUer features wiU require more specialized slurries, so wiU higher density features. The present method of pohshing is performed on a substrate containing tungsten features with a density of greater than 15%, more preferably greater than 20%, and even more preferably greater than 25%. The present method of pohshing is preferably performed on a substrate containing copper features with densities up to 60%, more preferably up to 90%, and even more preferably up to 95%.

Substrates which can be polished using slurries of the present invention are comprised of sihca and a layer of at least one metal selected from aluminum, copper, and tungsten. Often times a barrier layer or film is used between the aluminum, copper or tungsten and the sihca. The barrier layer is preferably at least one layer comprised of titanium, titanium nitride, tantalum, and tantalum nitride. Alternatively, two different barrier layers can be used, preferably titantium/titanium nitride or tantalum/tantalum nitride. A preferred substrate is one wherein a copper layer is separated from the sihca substrate via a tantalum layer. Another preferred substrate is one wherein a copper layer is separated from the sihca substrate via tantalum and tantalum nitride layers. Another preferred substrate is one wherein a tungsten layer is separated from the sihca substrate via a titanium layer. Another preferred substrate is one wherein a tungsten layer is separated from the sihca substrate via titanium and titanium nitride layers. The slurries in accordance with the present invention can be coUoidaUy stable or meta-stable. When a slurry is agitated into a uniform dispersion, then placed at rest, a stable slurry wiU tend to stay uniformly dispersed. Perhaps a very thin line of decantant might form at the very top of the slurry after several days or so, but fundamentaUy the particles generaUy remain weU dispersed throughout at least 90% of the slurry, even after being at rest for more than two weeks.

On the other hand, metastable slurries of the present invention wiU immediately start to faU out of suspension when at rest. TypicaUy within a few hours (of being at rest), a large line of decantant wiU tend to form at the top of the slurry. Within 48 hours (of being at rest), as much as 80% or more of the slurry particles wiU tend to be located in the bottom two thirds of the slurry, and after being at rest for more than two weeks, the slurries of the present invention wiU generaUy have over 80% of the slurry particles located in the bottom half of the slurry.

The meta-stable slurries of the present invention are not unstable, but rather (unlike an unstable slurry), the particles wiU agglomerate and faU out of suspension when the slurry is at rest, but then, wiU immediately de-agglomerate and redisperse with simple agitation. In comparison, an unstable slurry wiU NOT readUy de-agglomerate and re- disperse with simple agitation, because unstable slurries wiU form stage 2 agglomerates (stage 1 and stage 2 agglomeration is further defined below). Agglomerates have generaUy been considered undesirable for pohshing. However, agglomeration occurs in two stages, and Applicant has discovered that only stage 2 agglomeration causes the predominant undesirable effects upon chemical mechanical pohshing performance. The metastable slurries of the present invention wiU generaUy not form stage 2 agglomerates, but rather wiU SubstantiaUy only form stage 1 agglomerates. Unlike stage 2 agglomerates, stage 1 agglomerates wiU readUy de-agglomerate with simple agitation (e.g., vigorous shaking of the slurry for about 5 seconds or less).

Stage 1 agglomeration involves agglomerated particles held together primarUy by van der Waal forces. Stage 2 agglomeration can occur after stage 1 agglomeration, wherein the particles then fuse together over time, causing the particles to be primarily held together not by van der Waal forces, but rather covalent (or similar-type high energy) bonding between the particles. The slurries of the present invention comprise an appropriate amount of ionic species and/or other adjuvants which diminish or otherwise prevent stage 2 agglomeration. The ionic species used in the present invention are adjusted to dirriinish, inhibit or otherwise disrupt any charge layer around each particle in the slurry. For example, the anionic species in the aqueous medium wiU interact with, dirriinish or otherwise disrupt any positively charged layer around any particle, and the cationic species in the aqueous medium wiU interact with, dirriinish or otherwise disrupt any negatively charged layer around any particle.

This disruption of any charge layer around each particle SubstantiaUy removes or diminishes electrostatic repulsion between particles. Such dirninished electrostatic repulsion de-stabilizes the slurry and enables the particles to move sufficiently close to one another to induce a van der Waals bond between the particles, thereby creating stage 1 agglomerates. Stage 1 agglomeration may also involve hydrogen bonding between particles. A critical feature of the present invention is the absence of a force sufficient to repel and overcome the van der Waals forces between the particles, and therefore the slurries of the present invention wUl (when at rest) readUy form stage 1 agglomerates and (partiaUy or whoUy) faU out of suspension. During agglomeration, particles are able to move sufficiently close to one another to induce van der Waals bonds, and these bonds bias the particles together. Wh e the particle are biased together by van der Waals forces, a second stage of agglomeration can then occur. This second stage involves bridging between the particles. Bridging occurs due to the equilibrium reactions between the particle surface and the aqueous medium surrounding the particles. The surface of the particle wiU tend to dissolve into the aqueous medium, then precipitate onto the particle(s). When the precipitate bridges between two particles, thereby covalently bonding the particles together, this becomes stage 2 agglomeration. For example, although alpha alumina is generaUy inert (i.e., tends to resist dissolving) in an aqueous medium, conventional alpha alumina has about 1 weight percent (or more) of gamma alumina. The gamma alumina is far less inert in an aqueous medium and wiU typicaUy (reversibly) dissolve, creating AlO₂ ^" in a basic medium and Al⁺³ in an acidic medium. In either case, the reaction is reversible and the ions which dissolve from the particle wiU re-deposit back onto the particle(s).

When van der Waal forces bias two particles together, this re-depositing (of the dissolved alumina back onto the particles) can cause bridging between the two particles. Indeed, by dissolving and re-forming, the two particles tend to slowly fuse together into a single rigid mass. Over time, the agglomerates wiU be so rigidly fused together that a hard dense sediment (of stage 2 agglomerates) is formed. Stage 2 agglomerates generaUy cannot be effectively broken down into their original particles, except by the apphcation of high energy, e.g., mUling or high shear mixing.

The invention recognizes that only this bridging (stage 2 agglomeration) is harmful to the pohshing performance of a metal slurry. Apphcant has further discovered that if such bridging is inhibited or whoUy prevented, dramaticaUy improved pohshing performance can occur, even if the particles undergo stage 1 agglomerating (i.e., agglomeration SubstantiaUy free of bridging) due to van der Waal forces between particles. This is preferably done by adjusting the slurry chemistry to obtain the desired state, e.g., an optimal ionic strength.

Without bridging, agglomerated particles wiU readUy de-agglomerate with rninimal agitation. Indeed, van der Waal forces are extremely weak, arguably the weakest forces which can exist between two separate bodies of matter. Without bridging, these van der Waal forces (and any hydrogen bonding between the particles) are easUy overcome, and any agglomeration is not detrimental to pohshing. Agglomeration without bridging wiU generaUy cause a slurry to form a fluffy "cloud" or layer toward the bottom of a slurry container, once left undisturbed for a period of time. With only minimal agitation, the cloud of agglomerates readUy break apart and re-disperse in the medium. TypicaUy, vigorous shaking of the container for less than a minute (more preferably less than 30 seconds, yet more preferably less than 15 second and yet more preferably in less than 5 seconds) wUl de-agglomerate the slurries of the present invention and cause the particles to uniformly disperse within the aqueous medium.

A further critical feature of the present invention is the inhibition or prevention of stage 2 agglomeration, after stage 1 agglomeration. This is accomplished by the incorporation of appropriate ionic species or other adjuvants which inhibit the fusing of stage 1 agglomerated particles into stage 2 agglomerated particles.

In a preferred embodiment, stage 2 agglomeration is inhibited by coating particles with a surfactant or polyelectrolyte prior to incorporating the particles into a slurry system. Alternatively, the surfactant or polyelectrolyte can be incorporated onto the particles after the particles are incorporated into the slurry system. The surfactant and/or polyelectrolyte wiU tend to remain in close proximity to the slurry particles, thereby stericaUy hindering the particles from coming sufficiently close to one another to enable bridging or stage 2 agglomeration. It has been surprisingly discovered that stage 1 agglomeration can occur even in the presence of surfactant or polyelectrolyte at the surface of the particles, and that the presence of the surfactant or polyelectrolyte wUl keep the particles sufficiently apart to inhibit or prevent stage 2 agglomeration.

Stage 2 agglomeration can also be inhibited by the use of complexing agents which inhibit deposition or sedimentation from the aqueous medium onto the agglomerated particles. Useful complexing agents include appropriate chelating compounds, ordinary skiU and experimentation may be necessary in choosing appropriate chelating agents, depending upon the type of potential sedimentation or deposition for any particular slurry system. GeneraUy speaking, water soluble, polar organic compounds having one or more (preferably two or more) Lewis acid moieties can be advantageous as complexing agents in accordance with the present invention. Preferred complexing agents include multifunctional acid or acid-hydroxide, water soluble organic compounds, such as, citric acid.

Stage 2 agglomeration can also be inhibited by modifying the solubility of "potential bridging" materials in the slurry (material in the slurry which is capable of deposition or sedimentation). Possible modifications may include pH modification, temperature modification, ionic strength modification and the like. Ordinary skUl and experimentation may be necessary to determine the appropriate modification, depending upon the particular slurry system selected. Sonification is a method that can be used to determine whether agglomerates are stage 1 agglomerates (stage 1 agglomerates are agglomerates which are held together primarily only by Van der Waal forces, e.g., no bridging) or stage 2 agglomerates (stage 2 agglomerates are agglomerates which are held together by Van der Waal forces and also by bridging). GeneraUy speaking conventional, low energy sonification will break up stage 1 agglomerates but not stage 2 agglomerates. Any agglomeration of the present invention (due to the slurry being at rest for a period of time, e.g., 2 hours or more) is principaUy stage 1 agglomeration. Hence, the agglomerated particles of the present invention are de- agglomerated by sonification. De-agglomeration can be measured by taking a particle size distribution before and after sonification. After sonification, the size distribution should shift, thereby showing smaUer particles. Thereafter, the slurry (when at rest) wiU tend to once again (stage 1) agglomerate.

The stage 1 agglomerates of the present invention are stable, and stable is intended to mean that the stage 1 agglomerates will resist stage 2 agglomeration for a period of at least 3 months. Preferably, less than 15 percent (by volume) of the stage 1 agglomerates wiU become stage 2 agglomerates when at rest for 3 months, more preferably less than 10 percent, yet more preferably less than 5 percent, yet more preferably less than 2 percent and yet more preferably less than 1 percent of the stage 1 agglomerates will become stage 2 agglomerates when at rest for 3 months.

The invention recognizes that particles capable of providing stage 1 agglomeration (without also causing stage 2 agglomeration) provide a superior metal pohshing slurry relative to slurries having a force sufficient to repel and overcome the van der Waals forces between the particles, e.g., do not agglomerate. Hence, Applicants have found that agglomeration is not the problem, but rather bridging (e.g., the formation of hard, dense sediment) is what harms slurry performance. Not only can agglomerating slurries function weU as a metal polishing slurry, but indeed, a slurry system which enables particle agglomeration without bridging has been found to surprisingly provide improved polishing performance, particularly in the pohshing of metal layers as part of the manufacture of semiconductor devices. Stage 1 particle agglomerates generaUy have diminished electrostatic layer(s) and such particles tend to provide improved polishing by better interacting with the surface chemistry of a polishing substrate. Furthermore, the ionic species which inhibit or destroy electrostatic layers (around the particles) are preferably selected to provide other polishing advantages. For example, an ionic species can be used to buffer pH, provide a complexing agent to other ions in suspension (inhibit re-deposition), and/or provide selectivity (certain ionic species may protect portions of a surface, so that other portions of the surface wiU exhibit a higher removal rate).

Ionic species such as acids, bases, salts, complexing agents, surfactants, electrolytes, and the like are aU weU known, and indeed, such ionic species are, broadly speaking, known for chemical mechanical pohshing. However, it has been surprisingly discovered that when an appropriate level of ionic species is introduced into a pohshing slurry (e.g., where the aqueous medium has a sufficient ionic strength), pohshing performance of the slurry can be improved and unwanted particle bridging can be SubstantiaUy inhibited. The ionic strength can be adjusted by the use of agents such as acids, bases, and salts. Examples of such ionic strength adjusting agents include ammonium hydroxide, ammonium chloride, ammonium bromide, ammonium acetate, ammonium sulfate, ammonium nitrate, ammonium dihydrogenphosphate, ammonium hydrogenphosphate, ammonium benzoate, ammonium carbamate, ammonium carbonate, ammonium iodate, ammonium glycolate, ammonium citrate, iodic acid, glycohc acid, and citric acid. A preferred ionic strength adjusting agent is ammonium chloride. Preferably, the ionic strength adjusting agent is present in the slurry in about 0.01, 0.05, 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, to 5.0% by weight, more preferably about 2% by weight. In an embodiment, the slurry's total ion concentration is greater than 0.001 molar, more preferably greater than 0.01 molar, yet more preferably greater than 0.05 molar , yet more preferably greater than 0.1 molar, yet more preferably greater than about 0.2 molar and yet more preferably greater than about 0.5 molar. In a preferred embodiment, the total ion concentration is also less than 2 molar, more preferably less than 1 molar. In a preferred embodiment, the metal oxide particles also have a maximum zeta potential greater than about plus or minus 0.10 millivolts in an aqueous medium having an ion concentration of less than 0.001 molar.

The slurries of the present invention are particularly weU suited for pohshing operations having high polishing surface speeds. For example, many newer polishing machines are polishing at increasingly higher revolutions per minute, and the slurries of the present invention are particularly weU suited for such high speed polishing (e.g., rotary polishing speeds greater than 100 rpm, greater than 150 rpm and/or greater than 200 rpm). The slurries of the present invention are also weU suited for pohshing dielectrics

(silica), including low k dielectrics, such as porous sihca, or organic low k dielectrics, such as fluoro polymers or copolymers.

In an embodiment, any stage 1 agglomerate transported onto the polishing interface or region wiU de-agglomerate or otherwise wear, rather than scratch or otherwise cause defects on the surface being polished.

In another embodiment of the present invention, the ionic strength of the slurry is adjusted after the polishing operation, thereby restoring (or increasing) the electrostatic layers around each particle. This in turn wUl generaUy cause the particles to be more easUy cleaned or otherwise removed from the polished surface. The present invention is particularly advantageous for fumed particles, since firmed particles generaUy have more potential sites for stage 2 agglomeration.

The slurries of the present invention comprise constituents which not only inhibit or prevent stage 2 agglomeration, but also, are sufficiently benign to the metal surface being polished to have a static metal etch rate of less than 50 Angstroms per minute, more preferably less than 40 Angstroms per minute, yet more preferably less than 30 Angstroms per minute, yet more preferably less than 20 Angstroms per minute and yet more preferably less than 10 Angstroms per minute (up to and including 0 Angstroms per minute). The polishing compositions of the invention can be created before or during the polishing operation. If created during a polishing operation, the polishing fluid can be introduced into a pohshing interface and then some or aU of the particles can be introduced into the polishing interface by means of particle release from a polishing pad. For example, a polishing pad type substrate comprising particles is described in U.S. 5,692,950 to Rutherford, et al. (which is hereby incorporated into this specification by reference), and in the use of such a polishing substrate during polishing, the pohshing substrate wiU release particles, into the polishing interface which also contains a polishing fluid. As the polishing fluid and particles mix (in accordance with the present invention), they become a metastable polishing slurry, whereby the slurry wiU be capable of forming stage 1 agglomerates without substantial formation of stage 2 agglomerates. EXAMPLES The foUowing general polishing procedures were used:

Wafers were pohshed on a Westech 372U pohshing machine (available from IPEC Planar) using a IC1400 K groove primary pohshing pad, a Pohtex Regular Embossed secondary pohshing pad, DF200 carrier film, TBW 100 Grit Diamond conditioner, and 150mm wafer size. The IC1400K pad was mounted to the primary platen and 20 pre condition sweeps with DI. Pohtex regular embossed pad was mounted to the secondary table and preconditioned with the 6" stiff bristle hand brush and DI water hand sprayer, 8 scrapes, and 8 brushes. The conditioning parameters were 7 psi DF, 3 platen sweeps (post with DI Water), 70 rpm platen speed, and 75 rpm disk speed. The foUowing polishing parameters for each phase of polishing are used (depending on the tested substrate): Parameter Phase 1 Phase 2 Phase 3

(Primary Pol

Poly)

Time (seconds) 5 Variable 10

DF (PSI) 3 7 5

Back Press. -3 0 (2 for Ti) 0.5

DF Ramp 5 5 5

Carrier 60 60 60

Table 60 60 60

Slurry Flow 125 125 0

Rinse Off- Off On

EXAMPLE 1

The foUowing slurries were tested on 6" wafers containing Cu, TaN, and SiO₂.

Slurry¹ KI0₃ CA SSA PVP α-AhOs γ-Al₂0₃ PH

Control-a 2.0 1.0 2.0 0.2 0.6 2.4 3.6

Iodic Acid 1.0² 1.0 2.0 0.2 0.6 2.4 3.6

Control-b 2.0 1.0 2.0 0.2 0.6 2.4 3.6

'All numbers are given in weight percentages. Water comprises the remaining weight of the slurries. CA=citric acid. SSA=sulfosalicylic acid. PVP is poly-vinylpyrolidone, MW= 10,000.

²1% HIO3 was used in place of KI0₃.

Observed Metal/Oxide removal rates (A/min)

Slurry* Cu TaN Thermal Oxide

Control-a 3290 88 163

Iodic Acid 2434 90 110

Control-b 3939 91 168 As can be seen, the presence of iodic acid surprisingly decreases the oxide removal rate as compared with a simUar slurry using potassium iodate as the oxidizer.

Claims

Claims:

1. An aqueous slurry for pohshing a semiconductor substrate of sihca and metal composites comprising, water, and submicron abrasive particles, characterised by; a sihca protecting agent forming a sihcate having a solubility in water of less than

O.Olg/lOOmL.

2. A slurry according to Claim 1, wherein the sihca protecting agent is selected from alumina and zirconia.

3. A slurry according to Claim 1 further comprising an oxidizing agent.

4. A slurry according to Claim 1 , wherein the sihca protecting agent forms a sihcate selected from aluminum sUicate, calcium sUicate, cobalt sUicate, lead sihcate, hthium sUicate, manganese sUicate, hydrogen sUicate, zinc sUicate, and zirconium sϋicate.

5. A slurry according to Claim 1, wherein the slurry further comprises a complexing agent.

6. A slurry according to Claim 5, wherein the complexing agent is citric acid.

7. A slurry according to Claim 5, comprising, on a weight basis: about 1, 2, 3, 4, 5, 6, 7, 8, to 9% submicron abrasive particles, about 1, 2, 3, 4, 5, 6, to 7% oxidizing agent, and about 1, 2, 3, 4, 5, 6, to 7% of complexing agent.

8. A slurry according to Claim 5, comprising on a weight basis: about 3% submicron particles, about 3% potassium iodate, about 2% citric acid.

9. A slurry according to Claim 5, wherein the submicron abrasive particles are selected from sihca, ceria, and titania and the oxidizing agent is selected from iodic acid, periodic acid, hthium iodate, calcium iodate, sUver iodate, ammonium iodate, and lead iodate.

10. A method of pohshing a substrate comprising sihca, wherein the substrate is pressed against a pohshing pad, the substrate and the pad are moved relative to each other, and a pohshing composition is apphed to the pad during the pohshing operation, the polishing composition, comprising: water, submicron abrasive particles, and a sihca protecting agent.