Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24D—TOOLS FOR GRINDING, BUFFING OR SHARPENING
  • B24D13/00—Wheels having flexibly-acting working parts, e.g. buffing wheels; Mountings therefor
  • B24D13/14—Wheels having flexibly-acting working parts, e.g. buffing wheels; Mountings therefor acting by the front face
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B37/00—Lapping machines or devices; Accessories
  • B24B37/11—Lapping tools
  • B24B37/20—Lapping pads for working plane surfaces
  • B24B37/24—Lapping pads for working plane surfaces characterised by the composition or properties of the pad materials
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
  • C08G18/00—Polymeric products of isocyanates or isothiocyanates
  • C08G18/06—Polymeric products of isocyanates or isothiocyanates with compounds having active hydrogen
  • C08G18/08—Processes
  • C08G18/10—Prepolymer processes involving reaction of isocyanates or isothiocyanates with compounds having active hydrogen in a first reaction step

Definitions

• the present invention generally relates to polishing pads, and, in particular to chemical-mechanical polishing (CMP) with a slurry. Moreover, the present invention relates to methods for increasing the CMP removal rate of polishing pads, improving the planarization of work pieces by polishing pads, and improving the General Planarization Efficiency of polishing pads.
• CMP chemical-mechanical polishing
• CMP is a process step in the semiconductor fabrication sequence that has generally become an integral part of the manufacture of semiconductor wafers.
• CMP is used in a variety of applications in the semiconductor fabrication sequence.
• CMP is used in applications referred to as "oxide” or "ILD/PMD", “STI”, “copper”, “barrier”, “poly” and “tungsten". These terms generally indicate the type of material that is being removed.
• the CMP process preferably is configured to expediently remove material and planarize the surface, while leaving it defect free and contamination free.
• Each of these applications may use a different slurry, and therefore the removal mechanism is generally different for each application. For that reason, the optimal condition of each of the applications tends to be different as well.
• a silicon substrate is placed in direct contact with a moving polishing pad.
• a wafer carrier applies pressure against the backside of the substrate, usually while simultaneously rotating the pad.
• a slurry is made available, and is generally carried between the wafer and the pad by the motion of the pad.
• the composition of the slurry may vary from one CMP application to another.
• slurries that are designed to remove insulating materials consist of water, an abrasive and an alkali formulation designed to "hydrolyze" the insulating material.
• Copper slurries on the other hand, generally comprise a combination of: water, an abrasive, an oxidizing agent, a complexing agent, and a chemical to passivize the surface.
• a typical slurry often has very low removal rate on a material it was not designed to remove.
• the CMP polishing pad preferably performs a plurality of engineering functions.
• the pad may desirably be configured to polish at a high removal rate, planarize short ( ⁇ 100 micrometer) distances, planarize long (>100 micrometer) distances up to a certain Planarization Length (see below) determined by the quality of the silicon substrate, not planarize beyond that length, transport slurry, maintain the same friction with the wafer for wafers polished sequentially and without interruptions for hundreds of wafers, clean the wafer surface, not scratch the wafer surface, be replaceable in minimal time, and/or other functions.
• long range planarization is controlled by the bulk properties of the pad and short range planarization is controlled by the surface properties of the pad.
• a concept useful in describing long range planarization is the Planarization Length (L), defined as a lateral dimension characteristic of the pad's ability to planarize. Intrinsic to this concept is Preston's equation, which maintains that when polishing, the removal rate is proportional to force. There are significant deviations to this relationship, but it holds generally.
• a feature that is to be planarized may be modeled as an upraised element.
• a polishing pad will try to planarize this feature, and will succeed in doing so when the pressure exerted by the pad at the top of the feature exceeds the pressure exerted adjacent to the feature.
• the removal rate at the top of the feature will exceed the removal rate adjacent to the feature and over time the feature will decrease in height with polishing.
• Planarization length is the distance from the feature that the pressure has increased to 1/e of the pressure infinitely far from the feature (e is In(IO)).
• Short range planarization has no analog to L. Since the short range planarization is affected by the surface properties, it can vary dramatically using the same polishing pad by varying the amount, of diamond conditioning received by the pad, an action that affects the pad's surface roughness. However, generally the pad surface is conditioned for the purpose of maintaining the removal rate. Therefore with regard to short range planarization, the engineering constraint placed on the pad is rather results-oriented. If written, it would read something like, "...when consistently using a standard conditioning process the pad should exhibit adequate removal rate as well as high short range planarization.". Because of this, the short range planarization requirement generally comes from empirical results. For the copper application, these results would typically be expressed in dishing, which describes the amount of material removed from a narrow copper line. For oxide applications, the results would typically be expressed by the planarization performance of a test structure designed to measure short range planarization.
• CMP copper CMP
• the copper is typically deposited through the use of electro-deposition, and is typically deposited on a surface in which submicron channels in the underlying oxide layer have been created.
• the copper is deposited in the channels as well as on top of the areas between channels (the "field”).
• the copper deposited in the field between channels is often called the “overburden.”
• a “barrier layer” is provided on the oxide surface before depositing the copper since the copper would otherwise diffuse into the oxide layer and potentially reach the sensitive silicon substrate below, destroying the transistors. Accordingly, the barrier dads the copper channels, preventing diffusion of the copper into the oxide.
• Typical materials for the barrier layer are tantalum (Ta) and tantalum nitride (TaN).
• Ta tantalum
• TaN tantalum nitride
• the objective of the CMP process is to remove substantially all of the copper from the field area and leave the copper in the trenches at the level of the oxide.
• the surface of the copper in the channels is preferably flat and substantially without oxidation, contamination or corrosion on its surface. These conditions are also desirable for other larger areas of copper known as "bond pads," which allow probes to make contact with the conductor, and are typically 100um by 50-75 ⁇ m in dimension.
• the process is usually performed in three steps, involving three rotating platens normally found on commercially available CMP tools. The steps include: 1) removal of the bulk copper; 2) removal of the barrier layer; and 3) buffing of the final surface to remove substantially all residue and to passivate the copper surface. After these three steps, the wafer may be cleaned and dried prior to further processing. Often, the step of removal of the copper is divided into two steps. In the first step, half or more of the copper film is removed leaving the surface substantially planar. In the second step, the remainder of the copper is removed, leaving only barrier material.
• a typical way to carry out process Sequence #1 could be to use the following: Platen 1 ) An IC1000 pad (such as supplied by Rohm and Haas) and a copper slurry such as Cabot 5001 ; Platen 2) An IC1000 pad and a barrier slurry such as Hitachi T-605; and Platen 3) A Politex pad (such as supplied by Rohm and Haas) and water or water and a passivize chemical such as BTA.
• a typical way to carry out process Sequence #2 could be to use the following: Platen 1) An IC1000 pad and a copper slurry such as Cabot 5001; Platen 2) An IC1000 pad and a copper slurry such as Cabot 5001; Platen 3) A Politex pad and a barrier slurry such as Hitachi T-605, followed by a rinse with water or water and BTA for passivation.
• Step 3 in the last case could also be performed with an IC1000 pad, but is much more typically performed with a Politex pad.
• the disadvantage of using the Politex for this purpose is that the pad is basically too soft for the application.
• a typical Politex pad is composed of several layers. The bottom layer is a polyurethane impregnated polyester felt. The next layer is a very porous urethane, and the top layer consists of vertical urethane structures having tapered pores which are wider on the bottom and narrower on top.
• the softness of the pad is advantageous in delivering a very nice final surface (buff), both clean and scratch-free
• the net result of using this pad for the barrier removal step is that the resulting planarity of the wafer is poor.
• the pad As a barrier remover, the pad is exposed initially to both copper and tantalum simultaneously and later when the barrier is cleared, to oxide, copper and tantalum simultaneously. A pad with poor planarizing properties will do a poor job of uniformly removing these materials.
• This nonplanarity can result in bridging (a conductive material remaining where it is supposed to be removed resulting in undesired electrical contact), dishing/erosion (material being removed by the process that should remain in place causing an increase in resistivity if it is copper or shorts if it is oxide), or undesirable surface features that encourage bridging.
• the blown polyurethane material is chemically inert, and is not known to disintegrate in the presence of any of the chemicals it is normally exposed to during CMP.
• Sequence #1 apparently satisfies all requirements; however, it is very slow. It has a much slower throughput (“tpt”), and thus the polishing equipment is poorly utilized.
• tpt the throughput
• the tpt of a tool is a function of its slowest step.
• the slowest step is typically the slowest of the platen steps.
• Sequence #1 involves bulk removal of the copper at the first platen, that platen is likely the tpt limiter. It is not uncommon for that first step to take 3 or 4 minutes, resulting in a maximum tpt of the tool of 15-20 wafers per hour (“wph"). A higher tpt is generally desired.
• the method of improving a removal rate of a pad comprises the steps of: producing a body of a pad of which at least the top layer is polyurethane; introducing into the body an additive which decreases the ER of the pad so as to increase a chemical-mechanical planarization removal rate; and using as the additive a substance which at least contains starch.
• said using step includes introducing substantially one pound of starch in a 25 pound mix to reduce the elastic rebound by approximately eight percentage points.
• introduction of at least a substance containing starch into the polyurethane body of the pad significantly decreases the elastic rebound and as a result the chemical-mechanical planarization removal rate of the pad is increased.
• one feature of the present invention includes a method of copper CMP for copper film deposited on a surface of a semiconductor wafer.
• the method comprises the steps of removing a bulk copper to a barrier; substantially removing the barrier; and subsequently buffing, and wherein said barrier removing and buffing includes using a polyurethane polishing pad having at least one layer fabricated from a mix composed of a prepolymer with an isocyanate concentration of between about 6.5% and about 11.0% or from a monomer and an addition of isocyanate to achieve substantially the same molal concentration, in which a shore D hardness of the layer is less than about 35%.
• An exemplary embodiment of the present invention includes a polishing pad for removal of a barrier and subsequent buffing in CMP of a copper film deposited on a surface of a semiconductor wafer.
• the polishing pad is fabricated from a mix composed of the polymer with an isocyanate concentration of between about 6.5% and about 11.0% or from a monomer and an addition of isocyanate to achieve substantially the same molal concentration, and has a shore D hardness less than about 35%.
• Fig. 1 illustrates a planarization of urethane polishing pads as a function of NCO concentration produced in accordance with the present invention
• Fig. 2 is a plot illustrating a relationship between a hardness of a pad and a number of scratches.
• a method for improving the removal rate of a polishing pad.
• removal rates may be affected by the composition of the slurry used with the pad, several pad-related factors affect removal rates as well.
• grooves can improve removal rates by, for example, delivering the slurry to the wafer-pad interface.
• Various patterns, pitches, widths and depths of these grooves may be also affect removal rates.
• Removal rates are also related to the friction which exists between the wafer and the pad in the presence of the slurry. This friction differs for different pads and can be affected by factors such as the size of pores in the pad, the density of the pad, the material composition of the pad, and/or the like.
• the pads used for CMP are anelastic.
• An anelastic material exhibits both elastic and inelastic properties.
• a measure of the amount of inelasticity of a material is the Elastic Rebound, or the Elastic Recovery (ER).
• ER Elastic Rebound
• the fractional amount of recovery after a set period of time is the ER.
• polyurethane pads and oxide wafers exhibit CMP removal rates that have been found to be strongly negatively correlated with ER.
• a method of improving the removal rate of a polishing pad includes constructing the pad to have a lower ER and thus a higher removal rate. It has been found that the ER of a pad may be lowered by adding starch to the pad components during the mixing of the pad ingredients (as described more fully below).
• Starch is a known urethane additive, for example to increase biodegradability (e.g. pat# 6,228,969), improve flame retardance and to enhance foaming, (e.g. pat# 4,374,208).
• a CMP pad may comprise, in addition to other components, a starch.
• a CMP pad may comprise: 20 lbs ADIPRENE L325 Liquid Polyether Urethane from Crompton Uniroyal, 4.7 lbs MOCA (4,4-methylene-bis-chloroaniline), 0.5 Ib L-6100 silicone surfactant from GE Silicones, and 1 Ib starch.
• the method comprises the steps of mixing, for about 5 minutes, the starch (or a starch-containing additive) together with the L-325 Liquid Polymer Urethane and with the L-6100 silicone surfactant.
• the method may further comprise the step of mixing (for about one additional minute) an accelerant, such as MOCA, with the mixed starch/L-325/L-6100.
• the method may further include the steps of pouring the mix into a mold, curing it at room temperature for 15 minutes, placing it into an oven at a temperature of approximately 250 0 F for approximately 8 hours, cooling the resultant cake and slicing it with a skiving-type blade.
• the slices are then fashioned into polishing pads through additional steps such as grooving, applying adhesive and/or applying a subpad.
• the Elastic Rebound of the pads is reduced by a full eight absolute percentage points, from about 94% to about 86%, with an r-square value of approximately 66%.
• the ER may be reduced from approximately 94% to approximately 90%.
• the CMP removal rate is increased by reducing the elastic rebound of the pad through the addition of starch or a starch like substance to the pad during its formation.
• planarization of the work piece may also be improved.
• planarization test structures which are topographical structures which contain regular periodic arrays arranged in regions of different pitch and width. Due to practical limitations related to metrology, the smallest of these to yield an accurate measurement has a 100 micrometer pitch at 50% density (i.e. alternating high and low structures of equal width) and the largest had a 500 micrometer pitch at 50% density. Planarization capability is measured by a repeating sequential polish/measure action in which the remaining amplitude and the average amount of oxide removed is recorded for each step. The sequence is complete when the structure is planarized below a "noise" level. This sequence yields a curve which can be further reduced to single figure of merit of planarization called the Planarization Efficiency.
• GPE General Planarization Efficiency
• urethane polishing pads are produced by combining a basic polyol resin with an isocyanate ("NCO"). It is to be understood that the whole pad can be produced in this way or only its working layer, in the event when the pad is a multi-layer pad. In a new and special way, the isocyanate is introduced with a concentration of between about 6.5% and about 11.0% into the basic resin.
• polishing pads have been tested, in particular a polishing pad L-100 with isocyanate concentration of 4.0%, a polishing pad L-200 with isocyanate concentration of 7.5% and L-325 with isocyanate concentration of 9.5% from the Adiprene series, and resin 2505 with isocyanate concentration of 11.6 % from the Royalcast series, all from Crompton Uniroyal Chemical.
• urethane polishing pads with isocyanate concentration between about 6.5% and about 11.0% were tested, they achieved the planarization within the range of about 72% to about 95%.
• a breakdown of the data into groups normally expected to affect planarization does not alter the result.
• grooving is known to generally negatively affect the GPE of the pad by structurally weakening the surface.
• the aforementioned planarization range was found to hold both for pads with and without grooving.
• the use of a soft subpad can negatively affect the GPE (by virtue of its effect on the long range planarization). Again in this case the results were unaffected when considering solo pads and pad stacks separately. It is therefore understood that the NCO concentration controls physical properties of the pad which affect both the short and the long range planarization capability.
• the concentration of the isocyanate can be within a range of about 6.5% to about 8.5%. In this case the urethane pad or its working layer is softer and suitable for barrier buff. In accordance with another preferable feature of the present invention, the concentration of the isocyanate can be within a range of about 8.5% to about 11.0%. In this case the urethane pad or its working layer is harder and is suitable for interlayer dielectric.
• the urethane pad or its layer can be composed of polyester polyurethane or a polyether polyurethane.
• the urethane pad or its working layer can contain abrasive particles which may comprise one or more of the following components: silica, alumina, ceria, titania, diamond, and silicon carbide.
• the urethane pad or its working layer can be absent of abrasive particles.
• the urethane pad or its working layer in accordance with the present invention may also include a filler.
• a method for improving CMP of a copper film deposited on a surface of a semiconductor wafer is provided.
• the method may, for example, comprise the steps of removing the bulk copper to a barrier; removing the barrier; and subsequently buffing, wherein said barrier removal and buffing include using a polyurethane polishing pad fabricated from a mix composed of a prepolymer or a monomer and an addition isocyanate to achieve substantially the same molal concentration, in which a shore D hardness is less than about 35%.
• the isocyanate concentration is between about 6.5% and about 8.5%, the smaller value generally leading to a softer polishing pad.
• the present invention also relates to a polyurethane polishing pad for removal of a barrier and subsequent buffing in a chemical mechanical polishing process of a copper film deposited on a surface of a semiconductor wafer, the polishing pad fabricated from a mix composed of prepolymer with an isocyanate concentration of between about 6.5% and about 11.0% or from a monomer and an addition of isocyanate at a concentration to achieve substantially the same molal concentration, with a shore D hardness less than about 35%.
• the pad is made from a polyurethane mix in which the isocyanate concentration is between about 6.5% and about 8.5%, resulting in pads on the softer side of the hardness spectrum.
• the graph indicates the desirable hardness of the pad is approximately under 35 on the Shore D scale.
• the Politex is measured to be 25 on this scale by the same measurements (indicated on graph), and for reference, the IC1000 is measured to be 56 (indicated on graph) and reported to be 55 in information published by Rohm and Haas Corporation.
• GPE General Planarization Efficiency
• a chemical-inert blown polyurethane is selected for the inventive pad. Therefore, in accordance with the present invention the above described polyurethane polishing pad is proposed as well as the above described method of removing barrier and buffing.
• a polyurethane polishing pad may comprise an additive which includes methyl alcohol, or water, or starch of any type including corn starch.
• the pad can have a thickness of between 10 mils and 200 mils, and preferably of 80 mils.
• the pad can be stacked on a subpad or the pad may be a single- layered pad.
• the pad in accordance with the present invention can be grooved or ungrooved, perforated or unperforated.
• the composition begins with either a prepolymer resin with an NCO concentration of between about 6.5% and about 11.0%, or a polyol and an amount of isocyanate required to achieve the same molal concentration.
• the prepolymer is preheated to about 100 0 F, and may be mixed until said temperature is achieved.
• Additives may then be added as desired.
• Exemplary additives include initiators such as water, blowing agents such as Vazo, catalysts such as a tertiary amine, and surfactants such as L-6100, and a silicone surfactant typically used to regulate cell size supplied by Crompton.
• the pad is formed from a mix comprising at least one of a prepolymer in combination with an isocyanate concentration of between about 6.5% and about 11.0% to achieve a molal concentration, and a monomer in combination with sufficient isocyanate to achieve substantially the same molal concentration.
• an accelerant such as MOCA (4,4'-Methylenebis- [o-chloroaniline]
• the pad is formed from a mix comprising at least one of a prepolymer in combination with an isocyanate concentration of between about 6.5% and about 11.0% to achieve a molal concentration, and a monomer in combination with sufficient isocyanate to achieve substantially the same molal concentration.
• Pores may be created either by the incorporation of air during the mixing or by the intentional addition of pore creators, such as microballoons as taught in Reinhardt, U.S. patent #5,578,362.
• the mix is pored into a mold, which may be preheated. While the exothermic reaction is sufficient to bring the temperature of the center of the cake to some 27O 0 F and thus to a complete reaction, surface cooling may prevent the outside of the cake from reacting in a reasonable period of time. Therefore, the cake may be cured in an oven. This is typically performed at a temperature of about 24O 0 F for about 6 hours. The cake is then cooled and sliced to fabricate the polishing pads.
• the cake After curing, the cake is allowed to cool, and once cooled, it can be sliced using, for example, a stationary skiving blade and movable table. Usable slices of the 'cake' are then made into pads by applying adhesive to one face, mounting to a subpad, if desired, grooving the usable surface, finishing with a smoothing process, inspecting, and/or packaging the pad.
• a pad can both planarize and pass the scratch-inducing test, then it can also be used in applications where a buff alone is needed. As such, some pads might have additional applications such as post-oxide buff. Further, one could conceive of other simple non-CMP cleaning applications commonly carried out in a semiconductor fabrication environment.
• the pad planarizes, one could conceive of using it for example, to remove remaining copper to the barrier. In this case, since it is softer than the primary pad it will not have the same long-range planarization. However, if the first copper removal step is done with the hard pad, the copper surface should be well- planarized, making use of a slightly softer pad feasible. Another potential application of this pad is therefore also primary copper CMP.
• the pad may be used on any of a number of substrates, such as a bare silicon wafer, a semiconductor device wafer, a magnetic memory disk, and/or the like.
• the pads can be made by any one of a number of polymer processing methods, such as but not limited to, casting, compression, injection molding, extruding, web-coating, and sintering.
• At least one layer of the pads may be single phase or multiphase, where the second phase could include polymeric microballoons, gases or fluids.
• the second phase could also be an abrasive such as silica, alumina, calcium carbonate, ceria, titania, germanium, diamond, silicon carbide or combinations thereof.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Health & Medical Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Medicinal Chemistry (AREA)
• Polymers & Plastics (AREA)
• Organic Chemistry (AREA)
• Mechanical Treatment Of Semiconductor (AREA)
• Finish Polishing, Edge Sharpening, And Grinding By Specific Grinding Devices (AREA)
• Polishing Bodies And Polishing Tools (AREA)
• Compositions Of Macromolecular Compounds (AREA)

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Citations (6)

• 2005
  • 2005-08-24 WO PCT/US2005/030226 patent/WO2006026343A1/en active Application Filing
  • 2005-08-24 KR KR1020077004576A patent/KR20070057157A/ko not_active Application Discontinuation
  • 2005-08-24 JP JP2007530116A patent/JP2008511181A/ja not_active Withdrawn

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