SLURRIES OF ABRASIVE INORGANIC OXIDE PARTICLES AND METHOD FOR POLISHING COPPER CONTAINING SURFACES

Abstract: Oxidizing agents are added to slurries of inorganic oxides which have been heated, e.g., autoclaved, to produce abrasive slurries which impart relatively equal polishing rates for conductive metal and insulating layers used to make semiconductor chips. A relatively flexible abrasive slurry in terms of its abrasivity is also provided by this slurry, thereby permitting the modification of a copper polishing slurry's abrasivity when a new insulating material is used to make a chip. When using this method, an increase in particle abrasivity of this slurry can be correlated with a decrease in particle surface area as determined by N2 adsorption (BET method) and abrasivity can be increased (or decreased) by heating the slurry to produce particles with a surface area determined to have the abrasivity desired. The method is particularly suitable for preparing silica-based abrasive slurries and the slurries prepared are particularly useful for polishing chips made with copper conductive circuits and silica-based insulating layers.

Description: SLURRIES OF ABRASIVE INORGANIC OXIDE PARTICLES AND METHOD FOR POLISHING COPPER CONTAINING SURFACES RELATED APPLICATION This application is a continuation-in-part of Serial No. 09/422,384, filed October 21 , 1999 which is a continuation-in-part of provisional application 60/ 105, 141 , ied October 21 , 1998, the contents of which are incorporated by reference. BACKGROUND OF THE INVENTION The field of this invention relates to slurries of abrasive inorganic oxide particles. In particular, it relates to slurries of inorganic oxide particles used to planaπze or polish electronic chips, especially chips containing conductive metal circuits and silica-based insulating layers Copper is increasingly being used as a conductive layer, and tetraethoxysilane (TEOS) dielectric is widely used as an insulating layei in such circuits. The process employing these abrasive slurries is known as a chemical/mechanical planaπzation (or polishing) process, also known as "CMP". Mechanical polishing is imparted by the abrasivity of the inorganic oxide particles in the sluπy and chemical additives included in the slurry impart an additional effect of facilitating dissolution and removal of the surface being polished. Electronic chips are polished or planaπzed because conductive and/or insulating layers are applied in excess during a series of steps needed to create the final circuit of the chip. The damascene process for making electronic chips is an example of when such polishing is used. Briefly, the damascene process applies excess copper onto an insulating layei containing channels that correlate with a desired circuit. Copper tills those channels as well as covers the insulating layei This excess on the insulating layei , or the so-called "overburden," has to be removed by polishing It is desired to polish the deposited copper layer in such a manner that the overburden is completely lemoved before the next layei of material is applied The additional layers are typically applied by photolithography and the undei lying layei s need to be sufficiently planaπzed to maximize the sharpness of focus in the subsequent photolithography steps The sluπy also must piovide uniform polishing across the wafei without undue scratching or pitting of the polished substrate In conjunction with meeting this requirement, it is also fuither desirable to maximize polish rates in order to maximize the pioductivity of high-cost polishing equipment The conductive layei must also be lemoved dunng the damascene piocess with minimal ' dishing (see Figuies 1 A, IB and 1 C) Dishing can occui during the layer deposition process (Figure 1 A) oi as the polishing piocess leaches the insulating layer and is caused by the conductive layer being removed at a faster rate than the adjacent insulating layei (Figuie IB) For example, slumes used in CMP piocesses typically comprise fine sized, l e , submicron, lnoiganic abrasive paiticles In paiticular, micionized amorphous silica particles have proven utility in CMP slurries based on their good colloidal stability and unifoim polishing with minimal sciatching However, these abrasives geneially do not yield equal polishing lates when employed in acidic, oxidizing sluiπes for polishing chips containing copper Specifically, slurries prepared with these abiasives impart significantly lowei polish rates foi a silica-containing insulating layer compaied to a coppei - containing conducting layer As a lesult, coppei is lemoved moie quickly, and significant (and undesirable ) dishing can occui unless the polishing piocess is stopped at exactly the moment that the polishing process exposes the insulating layei By contrast, a slimy with equal polish rates foi copper and insulating layer will lesult in a planar suiface even when polishing is continued briefly after the dielectnc layei is exposed (Figure 1C) Figuies 1 A, I B and 1 C also illustrate the use of a baiπei layei A baπiei layei is a protective layei applied to the suiface of the conductive layei in oidei to limit diffusion of metal, e g , copper, into the dielectric layer during chip processing The presence of a barrier may also play a part in dishing if the barrier polishes at a significantly slowei rate than the coppei It alone may even play a pait in cieating dishing As a result of the different lemoval lates of the above materials, CMP piocesses using known abrasive slurries generally will not impai t uniform polishing across the wafei Therefore, it is generally desirable, e g , in a damascene process, to employ a fust abrasive slurry to remove most of the copper overbuiden These fu st slumes are piimanly effective because they contain aggressive chemical additives, e g , glycine and hydiogen peroxide mixtuies, which accelerate the removal of coppei Theie aie some issues as to when polishing with this fu st sluny should be stopped Suffice it to say many opei atoi s stop the piocess when the bai nei layer is fu st exposed Aftei polishing with the aggiessive slurry, a second sluny which does not contain glycine is employed to finish the polishing on a l inei level to ensuie copper is completely removed from eveiy location on the chip outside of the conducting Either of the two slumes above, however, tend to cause the dishing effect desci ibed above At least some dishing occui s when the polishing using the fu st slun y is earned out until the bai nei layei is leached and that dishing lemains or is even amplified aftei the second polishing step In addition, new insulating and barπei materials aie f iequently being developed These new materials will typically have different pi operties and as a lesult will show different polishing l ates Accoidingly, when these matenals aie introduced, the operatoi of the piocess will eithei need to adjust the abrasivity of the existing polishing slui ry oi completely leplace the existing sluiry system with another having the appiopnate abi asivity It would be moie desn able to adjust the existing sluiry than to find a leplacement slui ry Howevei it has been found that modifying conventional slurries foi polishing conductive suifaces, e g , copper, has not lesulted in the selectivity desned Alumina slurries, and fumed silica slurries, have been used in the past to polish copper surfaces. U.S. Patent 5,527,423 to Neville, et al. discloses examples of such slurries. The '423 patent to Neville et al. discloses CMP slurries comprising fumed silicas or fumed alumina particles dispersed in a stable aqueous medium. Neville also mentions that precipitated alumina can be used Neville et al. disclose that the particles have a surface area ranging fiom about 40 m7g to about 430m7g, an aggregate size distribution less than about 1 0 micron and a mean aggregate diameter less than about 0.4 microns. This patent also discusses references that teach the addition of agents, such as hydrogen peroxide, or alkaline materials to CMP slurries Other patents that disclose CMP slurries containing hydrogen peroxide and/or othei acidic or alkaline additives include U S Patent 5,700,838 to Fellei , et al., U S Patent 5,769,689 to Cossaboon, et al., U.S. Patent 5,800,577 to Kidd and U.S. Patent 3,527,028 to Oswald. In general, slurries such as these are based on the concept of selecting an inorganic oxide particle and either relying on the particles' inherent abrasive properties for polishing or by including additional additives to the slurry in order to adjust the abrasive and/or polishing eifects imparted by the slurry. U S Patent 4,304,575 to Payne discloses the prepaiation of aqueous silica sols for use as abrasive matenals in mechanically polishing semiconductor wafers. Payne ' s method for preparing the sol comprises heating an initial alkaline aqueous silica sol containing a mixtuie of relatively smaller particles and relatively larger particles. It is stated by Payne that the smaller particles dissolve and redeposit on iarger particles thereby producing an aqueous silica sol in which the majonty of the silica particles have a size significantly largei than the laiger silica pai tides in the staiting mixed sol Payne's materials are prepaied from sols having aveiage particle size less than 100 millimicrons and piefeiably having final paiticle size of about 180 millimicrons. A similar disclosure is set forth in U.S. Patent 4,356, 107 also to Payne. It is still desirable to find abrasive slurries which provide relatively equal selectivity among copper and the other various layers used to make electronic chips. It is also desirous to devise methods of making abrasive slurries in such a way that the abrasiveness of the particles can be easily adjusted to meet the polishing requirements at hand without having to resort to modifying the chemical makeup of the slurry or a new starting material for the abrasive particle. SUMMARY OF THE INVENTION In this invention, oxidizing agents are combined with slurries of fine, porous, inorganic oxide particles which have been prepared by heating the particles, e.g., in an autoclave, to modify and/or increase the particles' abrasivity. These slurries preferably have a median particle size in the range of 0.1 to about 0.5 microns, and substantially all of the particle size distribution is below one micron. Slurries produced by this heating process (without oxidizing agent) have abrasive properties such that an alkaline slurry (e.g., at pH 10.8) consisting of water and the inorganic oxide particles removes silica at a rate of at least 120 mm/ minute at 200 psi.rpm. This measurement was made at a solids content of 12.6% by weight, at a pH of about 10.8 and with a Strasbaugh 6CA polisher with a SUBA 500 pad at a two minute polish time. The oxidizing agents added to these slurries include those known in the art, e.g. hydrogen peroxide. It has also been found that when oxidizing agent is added to slurries of inorganic oxide particles prepared in this fashion, and preferably the pH of the slurry is adjusted appropriately, the resulting slurry polishes copper at a rate which is relatively equal to its rate of polishing conventional insulating and barrier layers. As mentioned above, autoclaving slurries of the above-mentioned porous particles imparts an increased abrasiveness to the particles. This is reflected in increased removal rates of silica substrate at standard polishing conditions This increase in paiticle abiasivity strongly conelates with a deciease in particle surface aiea as determined by N 2 adsorption (BET method) Such a coπelation can be used for pioviding methods of simply modifying the abrasivity of the slurry It is thought that this increase in particle abrasiveness and associated decrease in particle surface area is attributable to silica Uansport during the autodaving piocess whereby silica is preferentially dissolved from sharply convex surfaces within the porous paiticle and redeposited at sharply concave sui faces at the junction of silica subunits (ultimate particles) that make up the poious paiticle This redeposition should thus strengthen the porous silica paiticle and inciease its abrasivity The elevated temperatures associated with autodaving seive to accelerate this dissolution-iedeposition process by increasing silica solubility A similar process takes place in alkaline aqueous suspensions of silica pai tides held at loom temperature or temperatuies up to ambient pressure boiling (~ 100°C), but much longei times would be lequned Therefore abiasivity of the pai tides in this invention can be modified ovei a wide range of pioperties by modifying the heating conditions used to leposition the inorganic oxide within the particles' poie structuie Accordingly, the abiasivity of the pai tides can be adjusted as new insulating matenals are being combined with copper conductive layers BRIEF DESCRIPTION OF THE DRAWINGS Figuies 1A, I B and 1 C lllustiate the dishing effect cieated by conventional abiasive slurries, as well as illustrate the equal selectivity of the abrasive pioperties foi copper (Cu), silica (SιO 2 ) and baiπer (e g , TaN) possessed by the invention, e g , equal selectivity Figuie 2 is a graph which lllustiates that inci easing the seventy of heating conditions according to this invention deci eases the suiface aiea of the sluiπed pai tides as well as increases the abiasiveness of those pai tides Results for Al through A3 are those for slurries prepared according to the invention. The polishing rates from those slurries are compared to the polishing rates from a prior art chemical mechanical polishing slurry containing fumed silica (Rodel ILD 1300) having a BET surface area of 105 m 2 /g. The polishing rates reported aie illustrated by the rate at which silica dielectric material is removed at rates in nanometers per minute at various pressures (psi) and angular velocity rates (rpm) imparted by the polishing equipment. The pressure (P) referenced is the pressure between the polishing pad and the wafer. The velocity (V) referenced is the angular velocity at which the polishing pad is rotated during polishing Figure 3 is a graph indicating that the abrasiveness of a sluny, as reflected in material removal rate, is idated to the paiticle suiface area (SA) of the slurry at a constant P*V The surface area is plotted as an inverse of the actual BET surface area measured. DETAILED DESCRIPTION The initial steps in preparing the slurries of this invention comprise forming a slurry of inorganic oxide particles and then milling and separating particles from the slurry under conditions and in a manner sufficient to create a dispersion comprising particles having a particle size distribution suitable foi chemical mechanical polishing, e g., polishing copper conductive layei s and silica insulating layers. ( 1 ) Parent Inorganic Oxide Particles Inoiganic oxides suitable for piepaπng the sluny include piecipitated inorganic oxides and inoiganic oxide gels It is preferable that the inorganic oxide is soluble. Slightly soluble inorganic oxides can be used as well if the heating steps described later below are appropriately adjusted to alter the abrasivity of the selected inorganic oxide at the pH conditions needed to solubihze that inorganic oxide The initial inorganic oxide slurries are referred to herein as "parent inorganic oxides," "parent particles" or "parent dispersions". Amorphous silica gels are particularly suitable parent inorganic oxides. The dispersion can also be prepared from mixed inorganic oxides including SιO 2 • Al O 3 , MgO • SιO 2 • AI 2 O 3 . Mixed inorganic oxides are prepared by conventional blending or cogelling procedures. In embodiments comprising gels, the dispersions are derived from porous inorganic oxide gels such as, but not limited to, gels comprising SiO 2 , Al 2 O 3 , AlPO , MgO, TιO 2 , and ZrO 2 . The gels can be hydrogels, aerogels, or xerogels. A hydrogel is also known as an aquagel which is formed in water and as a result its pores are filled with water. A xerogel is a hydrogel with the water removed. An aerogel is a type of xerogel from which the liquid has been removed in such a way as to minimize any collapse or change in the gel's structure as the water is removed. Silica gels commercially available as Syloid ® grade gels, e.g., grades 74, 221 , 234, 244, VV300, and Genesis I M silica gels are suitable parent inorganic oxides. Methods of preparing inorganic oxide gels are well known in the art. For example, a silica gel is prepared by mixing an aqueous solution of an alkali metal silicate (e.g., sodium silicate) with a strong acid such as nitric 01 sultuπc acid, the mixing being done under suitable conditions of agitation to form a clear silica sol which sets into a hydrogel, i.e., macrogel, in less than about one-half hour. The resulting gel is then washed. The concentration of inorganic oxide, i.e., S1O2, formed in the hydrogel is usually in the range of about 10 to about 50, preferably between about 20 and about 35, and most preferably between about 30 and about 35 weight percent, with the pH of that gel being from about 1 to about 9, preferably 1 to about 4 A wide range of mixing temperatures can be employed, this lange being typically fi om about 20 to about 50°C. The newly formed hydrogels are washed simply by immersion in a continuously moving stream of water which leaches out the undesirable salts, leaving about 99.5 weight percent or more pure inorganic oxide behind. The porosity of preferred parent silica gels can vary and is affected by the pH, temperature, and duration of the water used to wash the gel. Silica gel washed at 65-90°C at pH's of 8-9 for 15-36 houi s will usually have surface areas (SA) of 250-400 and form aerogels with pore volumes (PV) of 1 .4 to 1 7 cc/gm. Silica gel washed at pH's of 3-5 at 50-65°C for 15-25 hours will have SA's of 700-850 and form aerogels with PV's of 0.6- 1 3. These measurements are generated by N 2 porosity analysis. Methods for preparing other inorganic oxide gels such as alumina and mixed inorganic oxide gels such as silica/alumina cogels are also well known in the art. Methods for preparing such gels are disclosed in U.S. Patent 4,226,743, the contents of which aie incorporated by leference. Fumed inorganic oxides such as silicas and aluminas can also be chosen as the parent inorganic oxide. The production of fumed silicas and aluminas is a well-documented process and involves the hydrolysis of suitable feedstock vapor, such as silicon tetrachloride or aluminum chloride, in a flame of hydrogen and oxygen Once an inorganic oxide is selected for the parent dispersion, a dispersing medium tor the slurry of the selected inoi ganic oxide is chosen The slurry can be prepared using residual watei in inorganic oxide gels which have been drained, but not yet dried, and to which additional water is added. In another embodiment, dried inorganic oxides, e.g., xei ogels, are dispersed in water. In general, the parent dispersion should be in a state that can be wet milled. The size of the parent particles only needs to be sufficient such that the mill being used can produce a dispersion having the desired particle size distribution. In most embodiments, the parent dispersion has a median paiticle size approximately in the range of 10 to 40 microns. In embodiments prepared from a drained inorganic oxide gel, the drained gel may first be broken up into gel chunks and premilled to produce a dispersion of particles in the range of 10 to 40 microns. (2) Milling and Centπfuging The parent dispersion is then milled. The milling is conducted "wet", l e., in liquid media chosen as the dispersing medium. The general milling conditions can vary depending on the feed material, residence time, impeller speeds, and milling media particle size. Suitable conditions and residence times are described in the Examples These conditions can be varied to obtain the particular particle size distnbution, typically below one micron. The techniques for selecting and modifying these conditions aie known to those skilled in the art The milling equipment used to mill the parent inorganic oxide particles should be of the type capable of seveiely milling materials thiough mechanical action. Such mills are commercially available, with hammer and sand mills being particularly suitable for this purpose. Hammer mills impart the necessary mechanical action through high speed metal blades, and sand mills impart the action through rapidly churning media such as zirconia or sand beads. Impact mills can also be used. Both impact mills and hammer mills reduce particle size by impact of the inorganic oxide with metal blades The milled slurry is then centntuged to separate the dispersion into a supernatant phase, which comprises the particles of the final product, and a settled phase, which comprises laiger particles which usually are removed to prepare the final abrasive slurry. The supernatant phase is removed from the settled phase, e g , by decanting, with the supernatant being further processed according to the invention Conventional centrifuges can be used for this phase separation A commeicially available centrifuge suitable tor this invention is identified in the Examples below In some instances, it may be preferable to centrifuge the supernatant two, thiee or more times to furthei remove large particles remaining after the initial centrifuge The particles of the slurry recovered from the milling and centπfuging are porous. Silica gel slurries recovered from these steps typically have pore volumes similar to that of the parent inorganic oxide. The porosity of particles recovered from milling and centπfuging of other parent inorganic oxides depends on the inorganic oxide and how it is made. For example, slurries prepared from parent precipitated and fumed inorganic oxides have pore volumes less than that of the parent inorganic oxide (3) Heating the Slurry The centπfuged slurry then is thermally treated under conditions sufficient to altei and adjust the distribution of inorganic oxide within the pore structure of the particles, thereby altenng the hardness or abrasiveness of the particles As indicated earlier, it is believed that in heating conditions such as those in an autoclave, inorganic oxide, e g., silica, preferentially dissolves from sharply convex surfaces, i.e., those found around the edges (rims) of pores, and redeposits at sharply concave surfaces, such as those at the juncture of ultimate particles which form the pores of the inorganic oxide particles. It is believed that repositioning inorganic oxide to these junctures strengthens the particle structure and as a result creates a harder and moie abrasive particle. Treating the centπfuged slurry in an autoclave is one method of thermal treatment that can be used to make the inventive sluny. By "autoclave" it is meant a pressure reactor which allows for heating of the slurry above the ambient pressure boiling point of the slurry's solution phase For aqueous slurries, this temperature is about 100°C. The pH of the slurry is adjusted before it is placed in the autoclave and depends on the inorganic oxide selected for the slurry. The pH is adjusted so as to optimize the solubility of the inorganic oxide, thereby decreasing the residence time in the autoclave. However, the pH should not be such that the amount of inorganic oxide solubihzed results in unwanted agglomeration and piecipitation of secondary inorganic oxide particles when the slurry is cooled to ambient temperature. For example, slurries of silica can be adjusted to a pH of 8-10 prior to thermal treatment and the final pH depends on the substrate which will be planaπzed by the final slurry. The autoclave conditions used depend on the desned hardness and the type of inorganic oxide selected for the slurry It has been found that the more severe the autoclave conditions used, e g , higher temperature and/or longer lesidence time in the autoclave, the haider the particles become, thereby increasing the abrasiveness of the particles For water based slurries, the temperature employed for the autoclave should at least be 100°C When preparing silica-based abiasive slurries foi polishing silicon-containing layeis, the slurry can be heated at 120- 180°C foi 20-30 hours In geneial, silica embodiments become unstable at tempeiatuies highei than 200°C and should be avoided if surfactants cannot be added to the desired abrasive slurry to leduce the instability. Likewise, heating the inorganic oxide to temperatures below 100°C require longer heating times to affect redeposition of the inorganic oxide. As indicated earlier, the abrasiveness of the particles increases and the BET surface area measured for the particles is i educed as heating severity increases As mentioned eai liei , it is believed that the suiface aiea leduction is caused when inoiganic oxide dissolves and lepositions to the junctures between ultimate particles The data in the Examples below show that poie volume and surface aiea are reduced after autodaving, and it is believed that the repositioning occurs at the expense of poie volume and the surface area associated with the pores lost Particles having BET surface aieas less than 120 m 2 /g and prefeiably less than 60 m 2 /g can be prepared according to this invention The pore volume of these particles is typically in the lange of 0 2 to 0 6 cc/g, as measuied by nitiogen porosimetiy at 0 967 P/Po Accordingly, a method foi imparting a desned abiasivity foi a selected inorganic oxide sluiry can be earned out by fust identifying an abiasivity oi abrasivities as determined by a polishing ιate(s) of a substrate, e g , a silica substrate to be used with a copper conductive surface. BET surface area for those particles is also determined. Then, once an abrasivity or polishing rate has been selected for a substrate to be worked upon, one can reproduce a suitable slurry by preparing a slurry of porous inorganic oxide particles having a measurable BET surface area and then heating the slurry to obtain the particle BET surface area which was identified and associated with the desired abrasivity. As indicated, the surface area referred to herein is that measured using conventional N 2 BET surface area techniques. To measure the surface area (and pore volume) for these slurries, the pH is adjusted to minimize surface area reduction that can occur during drying. The slurries also have to be dried to make these measurements and are dried using conventional techniques, e.g., heating the slurries to about 90 to about 130°C for periods long enough to dry the slurry to a powder. The examples below show that the abrasivity of silica slurries, as measured by silicon dielectric removal rates, can be varied along a relatively wide range of hardnesses. The examples below show that silica removal rates of at least 150, at least 200 and at least 250 mm per minute can be obtained. This method is an advantage when a manufacturer is faced with polishing a variety of materials and each of the materials require a different abrasive material and/or polishing rate. With Applicant's invention, the slurries used to polish these materials can be prepared from one material, e.g., silica, without having to add other essential abrasives. Accordingly, once the slurry has been adjusted to a suitable pH, the slurry of the invention can consist essentially of dispersing medium and the inorganic oxide particles of the invention. (4) The Final Abrasive Slurry As indicated earlier, substantially all of the particle size distribution for the final abrasive slurry should be less than one micron. The data below indicates that the particle size distribution of the slurry after heating is substantially the same as the distribution of the slurry after milling. Preferred embodiments have a median particle size less than 0.5 microns and in the range of 0. 1 to about 0.3 microns. The particle size distribution is measured using conventional light scattering instrumentation and methods. The sizes reported in the Examples were determined by a LA900 laser scattering particle size analyzer from Horiba Instruments, Inc. The solids content of the dispersion varies and depends on the solids content of the feed particle dispersion. The solids content of the dispersion is generally in the range of 1 -30% by weight and all other ranges encompassed therein. A solids content in the range of 10 to 20% by weight is particularly suitable when using silica gel as slurries for polishing silica insulating dielectric layers. In general, the dispersion's solids content and the dispersion's viscosity should be such that the dispersion easily flows between the wafer to be polished and the polishing pad used to polish the wafer. The pH of the slurry is dependent upon the inorganic oxide selected and the substrate to be planarized by the slurry. Silica slurries of this invention are particularly suitable for polishing copper when used with silica dielectric insulating layers. Silica dielectric layers prepared from tetraethoxysilane (TEOS) are illustrative. Slurries for polishing copper usually are adjusted to a pH in the range of 4-6 but they can be operable over a more general range of 3- 6.5. The pH can be adjusted using standard pH modifiers such as nitric acid to lower the pH of a basic abrasive particle dispersion prepared from silica. As indicated earlier, stable silica dispersions typically have a pH of about 9- 10. The pH can be modified to the desired pH as the final slurry is made or it can be modified just prior to being used. For a slurry which is modified to the appropriate pH at a time well before the slurry is used, stabilizing agents can be added to maintain the slurry's stability. The oxidizing agent used in the invention is preferably an inorganic or organic per-compound. A per-compound as defined by Haw ley's Condensed Chemical Dictionary is a compound containing at least one peroxy group (-O-O-) or a compound containing an element in its highest oxidation state. Examples of compounds containing at least one peroxy group include, but are not limited to, hydrogen peroxide, urea hydrogen peroxide, dipersulfates (S 2 O 8 =), peracetic acide, percarbonates, organic peroxides such as benzoyl peroxide, and di-t-butyl peroxide, mixtures thereof and either as is or in the form of their respective acid, salts, and adducts. Other suitable oxidizing agents include periodic acid, periodiate salts, perbromic acid, perbromate salts, perchloric acid, perchloric salts, perboric acid, perborate salts, permanganates, permanganate salts, and chromate salts. Monopersulfates (SO*-,=) are also suitable oxidizing agents and include compounds which include the oxidizing SO<-= group as shown below. O // χ O-O-S-O-X 2 // O where X, and X 2 are each individually H, (Si(R') λ , NH 4 , N(R") 4 and alkali earth metals such as Li, Na, and K; where R' is an alkyl group, preferably having from 1 to 10 carbon atoms, and wherein R" is H, alkyl group, aryl group, or mixtures thereof including, for example, wherein N(R") 4 is NMe , NBu , NMeBu 3 , NHEt 3 and so forth. Suitable classes of monopersulfates include combinations of KHSOs, KHSO and K 2 SO . Another suitable monopersulfate oxidizing agent is ammonium persulfate. Other oxidizing agents include nitric acid and derivatives of nitric acid; salts of transition metals such as potassium ferricyanide; and organic oxidizing agents such as nitrobenzene. The oxidizing agent may be present in the overall chemical mechanical polishing slurry in an amount ranging from about 0.1 to about 20.0 weight percent. It is preferred that the agent is present in the slurry in an amount ranging from about 0.2 to about 10.0 weight percent. As indicated earlier, the dispersions of the invention are preferably designed for polishing conductive metal surfaces on semiconductor wafers wherein these surfaces are part of integrated circuits constructed with dielectric insulating and/or barrier layers. As discussed eai her, these layei s comprise materials which polish at different rates, thereby ci eating the potential for dishing as illustrated in Figure 1 The slui nes of this invention ai e particularly suitable for polishing, but can be used on other conductive metals such as tungsten They are especially suitable in a damascene process in which semiconductors are constructed from copper and a silica based dielectric layer, e.g , that prepared from TEOS, and a barrier layer such as that comprising tantalum (Ta) oi tantalum nitride (TaN) The sluny of the invention can be used as the sole slurry for removing copper ovei burden, oi as the fu st or second slumes mentioned eai liei with lespect to the discussion of the damascene process In general the slurries of this invention generally impart relatively equal removal rates for copper, TEOS and any Ta oi TaN present, with the polish rates of each (when polished using the invention) being at least seventy (70%) of the other, i.e , the removal rate of removing TEOS and TaN do not differ by more than 30% from the copper removal rate Indeed, the examples below show selectivity rates for a coppei , TEOS and TaN in which the rate of one is at least 80% of the othei , I e., the rates do not diffei by more than 20%. A variety of additives, such as surfactants, polymeric stabilizers or othei surface active dispersing agents, can be added to the inventive dispei sion to stabilize it against settling, flocculation and decomposition of the oxidizing component Examples of suitable sui factants aie disclosed in Kii k-Othmei , Encyclopedia of Chemical Technology, 3ιd Edition, Vol 22 (John Wiley & Sons, 1983), Sislet & Wood, Encyclopedia of Surface Active Agents (Chemical Publishing Co , Inc , 1964) and available manufacturing literature, including for example McCutcheon's Emulsifiers & Detergents, North American and International Edition (McCutcheon Division, The MC Publishing Co, 1991 ), Ash, The Condensed Encyclopedia of Surfactants (Chemical Publishing Co , Inc , 1989), Ash, What Every Chemical Technologist Wants to Know About . . Emul.s ers and Wetting Agents, Volume I (Chemical Publishing Co , Inc. 1988), Tadros, Surfactants (Academic Press, 1984); Napper, Polymeric Stabilization of Colloidal Dispersion (Academic Press, 1983); and Rosen, Surfactants & Interfacial Phenomena, 2nd edition (John Wiley & Sons, 1989), all of which are incorporated herein by reference. A surfactant consisting of a copolymer of polydimethyl siloxane and polyoxyalkylene ether is suitable. Such stabilizers are used in amounts ranging from 0.001 % to about 0.2% by weight. The slurnes of this invention can be used with conventional polishing equipment and pads. The examples below illustrate the perf ormance of this invention using a Strasbaugh 6CA polisher unit using a SUBA 500 pad or a SUBA 1 C 1400 pad These examples, howevei , are merely illustrative of certain embodiments of the invention and are not intended to any way limit the scope of this invention as recited in the claims appended hereto ILLUSTRATIVE EXAMPLES Examples 1 -3 show the flexibility in adjusting abrasiveness of the inventive dispersions with respect to removal rates of insulating layers. Example I Prepai ation of Base Silica Gel Sluny Appi oximately 30 gallons of an aqueous suspension oi an mtei mediate density (ID) hydrous gel were prepared The term "ID gel" is used to refer to hydrogel which is washed in a pH range of 5- 10 after it has been initially formed and as a result has a density which is slightly less than gels prepared from hydrogels which are washed under more acidic conditions These lattei gels are lefeired to as regulai density (RD) gels A slurry was prepared by dispersing the ED hydrogel, milling it in an ACM mill and partially drying the hydiogel to piepare a hydious silica gel having a 55% by weight total volatiles content The hydrous gel slurry was then milled further in a NETZSCH media mill ( 12 liters, 1.2 mm zirconium silicate media) at a rate of 1 gallon per minute. This milled slurry was then centπfuged using a Dorr-Oliver disc-nozzle type centrifuge (9 3 inch disc diameter) at about 9000 rpm's (correlates to about 10,000 G's). The resulting slurry was designated as Base Silica Slurry A. Base Silica Slurry A was measured to have 90% of the particles at or below 0 4 microns. A second sample of a similar gel was piepared, except that the hydrous silica gel slurry had a 50% by weight total volatiles content. This hydrous gel slurry was then media milled using the same NETZSCH mill while being fed at 0.2-0 25 gallon per minute This milled sluny was then centnfuged under more severe conditions to yield a finer particle size colloid designated as Base Silica Slurry B. Specifically, this slurry was centnfuged a second time at 90 minutes at around 1 ,500 G's The particle size distribution of Silica Slurry B was measured to have 90% particles at or below 0.2 microns. Silica Slurry A Silica Slurry B Silica Concentration 17% 16% (% solids by weight) Particle Size, μ 10%< .14 .09 50%< .23 .12 90%< .40 .17 N 2 BET Surface Area, m 2 /g 219 232 N 2 PV (.967 P/Po), cc/g .96 .62 Example 2 Autodaving of Submicron Silica Gel Suspensions Three 3 gallon samples of the Base Silica Slurry A and one 3 gallon sample of Base Silica Slurry B were diluted to approximately \ 2.1% solids, pH adjusted to 9.5 (KOH), then sealed in a stainless steel bomb and then aged at the time/temperature conditions given in the table below Particle size, pH, and N 2 porosimetry evaluations of the autoclaved products are also given The slurries weie adjusted to a pH of 6 before drying and conducting the N 2 porosimetry measurements This adjustment minimizes surface area reduction during the drying process necessary to measure the surface aiea, thereby making the measurements more accurate. The samples were dried for these measurements using conventional techniques, e.g., heating the slurry to 105°C until dry. Autodaving results in a significant surface area loss for each of the base silica suspensions, but substantially no change in particle size. Autoclaving of Submicron Silica Gel w •500 ID Hydrous Gel Base Autoclave Condition Size, μ N 2 PV BET SA Sample Hrs fC 10% < Η ) °/<< 0 c /< * < EH '/, solids R ) (m-Ve) Base Silica A - - 14 23 40 16 6 96 2 19 A- 1 30 125 π 24 40 10 8 12 6 55 83 A-2 25 150 15 26 42 10 7 12 8 5 1 59 A-3 28 170 16 27 45 10 8 12 7 27 42 Base Silica B — -- 09 12 17 1 6 1 62 232 B- l 16 120 09 12 17 10 7 12 6 44 1 10 Example 3 Evaluation of Autoclaved Slurries for SιO 2 Polish Rate Prior to polishing rate evaluation, a sample of the Base Silica Slurry A was diluted with DI water to 12.7% solid This is the data reported for Base Silica A in Figure 1 . Then this sample and each of the autoclaved slurries A- 1 through A-3 and B- l were adjusted to a pH range of 10.7- 10.9 with KOH. These samples and a sample of a commercial sluny of fumed silica (ILD 1300 slurry from Rodel) were evaluated for SιO 2 removal rate using 4 inch SιO 2 coated Si wafei s. Polish rate tests weie made using a Strasbaugh 6CA polishei with a SUBA 500 pad employing a two minute polish time The distance between the center of the polishing pad and the center of the wafer was set at five inches. Different polishing conditions (pressure (P), and angulai velocity (V) of the polishing pad) were used These conditions and the results are reported in Figuie 2 showing SιO 2 polish (removal) l ate tor the base silica slurries as a function of polishing seventy (pressure times angular velocity of the polishing pad) Pressuie is presented as pounds pei squai e inch (psi) and angulai velocity is piesented as i evolutions per minutes (rpm) The data show a significant increase in polish rate with increase in autoclave severity. The rates range from approximately 50% of the commercial polish slurry rate for the non-autoclaved silica gel product to approximately twice the rate for the commercial polish slurry rate. Furthermore, a strong correlation between observed polish rate and reciprocal surface area of the autoclaved silica gel slurries is shown in Figure 3. This data indicates that the abrasiveness of inoiganic oxide particles can be adjusted by altering the suiface aiea of the particles using the autoclave and modifying the conditions to obtain a certain suiface aiea and the abrasive properties associated with that particular surface area. Example 4 Preparation of Abrasive Slurries tor Copper Polishing Approximately fifty (50) gallons of a silica xerogel powdei (Grace Syloid 63) was suspended as an aqueous slurry (-20% solids) and adjusted to pH 9.0 with KOH. The slui ry was then media milled and centnfuged resulting in a slurry of approximately 12% solids with a median particle size of 0.2 l μ. Twenty-four gallons of this slurry were then autoclaved tor 27 hours at 150°C. The resulting autoclaved slurry and a second slurry from the above preparation which had not been autoclaved (i.e., the "non-autoclaved slurry" leterred to below) weie adjusted to a pH of 4 with nit c acid Concentrated hydi ogen peroxide was added to the slumes in an amount to yield a final slurry concentration of 10% SιO 2 and 5% H 2 O 2 Analysis of the shin ies is given in the table below. Example 5 Copper, Barrier (TaN), and Insulation (TEOS) Removal Rates Both the autoclaved and non-autoclaved slurries were evaluated foi copper bamer and SιO 2 dielectric (TEOS) lemoval i ates These evaluations weie made using a Stiasbaugh 6CA polishei with a grooved I C 1400 pad at conditions of 6 psi and 40 lpm Foi companson, a commei cially available colloidal silica-based Klebosol 30H25 slurry was also evaluated Chemical and particle size analyses of this sample aie not available Polishing rate data aie given in the following table Values normalized with lespect to the copper removal rate are given in parentheses Polishing Rate (Selectivity) Comparison Removal Rate, nm/min Slurry A Slurry B Commercial Copper Polishing (Autoclaved) (Non-Autoclaved) Slurry-Klebosol 30H25 Copper 111 ( 1 00) 114 ( 1 00) 72 ( 1 00) Barrier (TaN) 126 ( 1 14) 109 (0 96) 43 (0 60) Dιelectnc (TEOS) 103 (0 93) 43 (0 38) 24 (0 33) With the autoclaved sluny both the bainei and the TEOS polish i ates are within 15% of the coppei polish late Both the non autoclaved silica sluny and Klebosol slurry have dielectric polish rates that are less than 40% of the copper polish rate.

Claims: What is Claimed 1. A slurry comprising (a) dispersing medium, (b) inorganic oxide particles wherein a slurry of these particles has abrasive properties such that a slurry consisting of water and the inorganic particles having a solids content of 12.6% by weight and pH of about 10.8 removes silica at a rate of at least 120 nm/minute at 200 psi-rpm using a Strasbaugh 6CA polisher with a SUBA 500 pad at a two minute polish time, and (c) oxidizing agent. 2. The slurry of claim 1 wherein the inorganic oxide particles have a BET surface area in the range of 40 to 120 m7g and the removal rate of silica is in the range of 150 to 250 nm/minute. 3. The slurry of claim 2 wherein the dispersing medium is water and the inorganic oxide particles comprise silica. 4. The slurry of claim 3 wherein the silica comprises silica gel. 5 The slurry of claim 1 wheiein the silica has a BET surface aiea of 60 m7g or less. 6. The slurry of claim 2 wherein the silica has a pore volume in the range of 0.2 to 0 6 cc/g. 7. The slurry of claim 1 wherein the median particle size of the inorganic oxide particles is in the range of 0 1 to about 0.5 and substantially all of the particle size distribution is below one micron. 8 The slurry of claim 1 wherein the inorganic oxide particles are selected tiom the group consisting of silica gel, fumed silica, precipitated silica, and alumina. 9. The slurry of claim 8 wherein the sluny is prepared by heating silica gel to at least 100°C. 10 The slurry of claim 9 wheiein the slurry is prepared in an autoclave 1 1. The slurry of claim 1 wherein the oxidizing agent comprises hydrogen peroxide. 12. A process for polishing a semiconductor water containing ( 1 ) a conductive metal surface and (2) at least one surface other than the conductive surface, the polishing method comprising (a) contacting the water with a polishing pad, (b) pioviding a slurry comprising oxidizing agent and inorganic oxide particles to an interface between the water and polishing pad, wheiein the inorganic oxide is such that a sluny consisting of the inorganic oxide particles and water at a solids content of 12 6% by weight and pH of about 10.8 removes silica at a rate of at least 120 nm/minute at 200 psi.rpm using a Strasbaugh 6CA polisher with a SUBA 500 pad at a two minute polish time, and (c) removing the other layer (2) from said wafer at a rate which is at least within 30% of the rate at which the conductive metal surface ( 1 ) is removed 3 A process of claim 12 wherein the insulating layer is dielectric and compnses silicon 14 A process of claim 12 wheiein the inorganic oxide particles are selected from the group consisting of silica gel, fumed silica, precipitated silica, and alumina. 15 A process of claim 14 wherein the slurry is prepared by heating a silica gel to at least 100°C. 16 A process of claim 15 wherein the slurry is prepared using an autoclave. 17. A process of claim 12 wherein (c) has a removal rate of at least >100 150 nm/minute. 18. A process of claim 15 wherein the inorganic oxide particles have a BET surface area in the range of 40 to 120 m7g 19. A process of claim 15 wherein the inorganic oxide particles have a BET surface area of 60 m7g oi less. 20 A process of claim 18 wherein the inorganic oxide particles have a poie volume in the range of 0.2 to 0 6 cc/g 21 A process of claim 12 wherein the oxidizing agent is hydrogen peroxide 22 A slurry comprising (a) dispersing medium, (b) inorganic oxide particles having a BET suiface area in the range of 40 to 120 m7g, a pore volume in the range of 0.2 to 0 6 cc/g, and a median particle size in the range of 0 1 to 0 5 microns, and (c) oxidizing agent 23. A slurry of claim 22 wherein the slurry contains 1 -30% solids 24 A slurry of claim 23 wherein at a pH of 4 and a 10% solids content the slurry removes copper at a rate which ditfeis by at most 30% f iom a rate of lemoving tetraethoxysilane undei the same conditions when using a Strasbaugh 6CA polisher with a giooved 1 C 1400 pad at 6 psi and 50 rpm.

Inventors: PRYOR JAMES NEIL, PRYOR, JAMES, NEIL
Applicants: GRACE W R & CO, W. R. GRACE & CO.-CONN.
Application year: 2001
Main classification: C09G1
IPC codes: B24B37, C09G1, C09K3, H01L21