A spectroscopic and topographic investigation of abrasive nanoparticle retention in polyurethane chemical mechanical planarization (CMP) pads has been carried out as a function of ex situ pad conditioning parameters for Cu CMP. X-ray photoelectron spectroscopy (XPS) and scanning electron microscope energy dispersive spectroscopy (SEM-EDS) were utilized to quantitatively characterize the presence of alumina nanoabrasive in the pad following Cu CMP. As expected, conditioning did not modify the overall pad morphology but did increase local asperity roughness with increasing conditioner downforce. As expected, the magnitude of the conditioner downforce reduced the overall average alumina nanoabrasive retention, albeit weakly, as determined via XPS. However, local EDS analysis revealed that increased downforce does significantly reduce nanoabrasive retention at pad asperities. In contrast, increased downforce increased the amount of nanoabrasive present in the polyurethane pad surface pores. Moreover, overall conditioning time was shown to be a more dominant parameter compared to conditioner downforce regarding removal of abrasive particles from both pad asperities and pores.

In this study, incremental loading and interferometry analysis were performed on Rohm and Haas IC1000 plain pad surfaces to investigate the effects of diamond size and conditioning force on pad topography. The Rohm and Haas IC1000 plain pad was conditioned by Mitsubishi Materials Corporation 100-grit and 325-grit diamond discs under 3.6 and 8.0 lb. Two pad samples were taken for each disc and each conditioning force test. For each pad sample, a custom-made incremental loading device (ILD) was used to measure pad surface mechanical response under different loads. When loads were applied to the pad sample surface, the pad surface displacement was measured. A decay length was extracted from the log plot of the pressure ratio vs. the change in pad surface displacement. The decay length generally increased with the diamond size and conditioning force. Both dry and wet pad samples were analyzed by the incremental loading device. Results showed that wet pad samples had higher decay lengths than dry pad samples. Separately, optical interferometry was used to probe dry pad sample surfaces without contact and produce surface height probability density functions (PDFs). For all pad samples, the pad surface height PDFs exhibited an exponential tail and a decay length was extracted, which represented the distance over which the pad surface height probability dropped by a factor of 1/e. The decay lengths extracted from the optical interferometry analysis were consistent with those extracted from the incremental loading experiments, indicating that the optically measurable pad surface characteristic had a practical interpretation in terms of the pad surface mechanical response.

Planarization efficiency is a key parameter to evaluate the process effectiveness of CMP. When the Cu thickness was removed beyond the step height, under certain conditions, the recessed trenches become protruded, resulting in reverse topography. This reverse topography during bulk Cu removal can only be attributed to the low down force polishing, electrical driving Cu removal and Cu passivation mechanism in ECMP process. In this paper, the concept of reverse topography is introduced and the removal selectivity difference on the trench area and field area during bulk Cu removal is discussed. In ECMP, a “real” 100% planarization efficiency can be achieved when the Cu thickness removed is less than the trench step height. This new understanding in planarization efficiency sheds lights on ways to improve CMP productivity

It is becoming increasingly clear that understanding the small scale polishing mechanisms operating during CMP requires knowledge of the nature of the pad-wafer contact. Dual Emission Laser Induced Fluorescence (DELIF) can be used to study the fluid layer profile between the polishing pad and the wafer during CMP. Interactions between the polishing pad surface and the wafer can then be deduced from the fluid layer profile. Previous investigations of pad-wafer interactions using DELIF include in-situ measurements of average fluid layer thickness and asperity layer compressibility, surface roughness measurements and polishing pad rebound into etched wells. In this paper, DELIF is used to determine pad-wafer contact, the point at the fluid film thickness goes to zero. We present a technique and some preliminary data for instantaneous measurement of in-situ pad-wafer contact using DELIF. The imaging area is 1.30×1.74 mm with a resolution of 2.5 μm/pixel. At this magnification, some regions imaged contain contact, whereas others do not. For the contact regions discussed in this paper, contact percentage varies from 0.07% to 0.27% on a Cabot Microelectronics D100 polishing pad. The asperity contact area increases with applied load, which was varied from 0.28 to 3.1 psi.

Attenuated total reflection FT-IR spectroscopy coupled with a hydrogen-deuterium exchange technique is employed to investigate the surface chemistry of two polishing pads, a thermoplastic polyurethane (TPU) pad and a commercial cast urethane (PU) pad. A time/temperature-dependent reduction of nitrogen-hydrogen stretching modes (νN-H) was observed when the pads were immersed in deuterium oxide (D2O). Compared to TPU pads, the PU pads showed an ambiguous correlation between dependence of the band intensity changes and D2O-immersion time/temperature. For both types of pads, a sharp decrease in νN-H band intensity and an unexpected rebound in νN-H band intensity were observed within 24 hrs after immersing the pads in D2O. However, a comparable rebound in νN-H band intensity was not observed for conditioned TPU pad samples. This phenomenon is proposed to be due to a rearrangement of polar groups at the pad surface when the pad is immersed in water, which may influence pad performance on the asperity level.

Although dielectric polishing is primarily mechanical in nature, surface chemical effects can be tailored to enhance material removal, selectivity and planarity. The use of ceria abrasives in oxide polishing is particularly attractive because of favorable polishing characteristics that are generally not obtainable using conventional fumed or colloidal silica abrasives. Unfortunately, a characteristic of ceria abrasive is an enhanced tendency toward defectivity in comparison with conventional silica. Entirely novel composite structures comprising 300 nm polymer particles coated by ceria (specific surface area of the powder 61 m2/g), are achieved by either adding silane coupling agents (Composite A) or tuning the pH in order to form electrostatic attractive interactions between the core and the shell (Composite B). The polymer core shows mechanical properties that are highly tunable by variation of its synthesis parameters, while the major advantage of the ceria coating is an enhanced chemical action of the abrasive particles, commonly referred to as the chemical tooth model. In this study we report the evolution of RR, haze, and defects such as particles and scratches in CMP experiments on high-density plasma (HDP) silicon oxide using four abrasive types (14 nm primary size ceria particles, 300 nm polymer particles, composites A and B) at pH 3 and 10. Interestingly, the two types of composite exhibit different RR. This is attributed to differences in morphology and surface composition. Composite B presents more similarities with the ceria, as confirmed by particle / silica surface adhesion forces in different pH solutions, measured by a colloidal AFM technique.

Examining CMP at any scale, one finds coupled contact mechanics and fluid mechanics. Increasingly sophisticated experimental and computational techniques have revealed aspects of solid-solid interaction and slurry flow at the wafer and groove scale and, more recently, at the texture scale. Successful prediction of CMP performance hinges on identifying universal physics that span these scales. In this paper we first review results of novel asperity-scale experiments that characterize the pad texture both as a solid topography subject to contact deformation and as an equivalent porous medium for slurry flow. These measures reveal that much of the texture volume is inactive as flow space, a feature confirmed quantitatively by computational modeling of flow across a conditioned CMP pad surface built from 3-D microscopy images. For hydrodynamics, the findings establish active fluid volume per unit area as the property that bridges from asperity scale to wafer scale. We then derive a fundamental basis for CMP removal rate prediction based on contact and hydrodynamics, using a Sommerfeld number defined across the groove and texture length scales. The resulting equation, containing a single unknown proportionality constant, demonstrates that the often used product of downforce and table speed tracks removal rate only when the hydrodynamic state affords adequate pad-wafer contact. Departures from the Preston equation attributed in other models to chemically-limited regimes of CMP are explained in the present treatment as changes in hydrodynamic film thickness and contact area—a fact confirmed by direct measurement. Removal rate predictions are discussed for ILD, STI, and copper processes using both conventional and non-Prestonian slurries, including variations in downforce, table speed, temperature, pad properties, and groove design. Finally, the influence of regional pad-wafer hydrodynamics is illustrated by applying the contact-hydrodynamics equation to grooves specially configured to vary the slurry film thickness from wafer center to edge. Local removal rates are well predicted using locally defined values of the groove-texture Sommerfeld number, confirming the generality of the contact-hydrodynamic description at least from wafer to asperity scale. Findings are further discussed in the context of next-generation pad architectures—not only to achieve more effective pad-wafer contact and slurry delivery, but also to favorably decouple contact and fluid mechanics in CMP pad design.

A mechanistic product development approach will be presented that utilized selected thermodynamic and kinetic data for development of a Cu post-CMP cleaner for use after polishing with either high or low pH slurry. A Pourbaix diagram offers the thermodynamically stable Cu species vs. pH and open circuit potential (OCP) for an aqueous cleaner. Pourbaix diagrams do not provide any information on the kinetics for dissolution of water soluble species. Cu oxide dissolution rates were measured using an electrochemical cell. Deposition and removal rates for benzotriazole (BTA) and 1,2,4-triazole (TAZ) were measured using an electrochemical cell with a quartz crystal microbalance (QCM). Passivator film deposition was investigated to determine the film type and thickness resulting from a typical CMP process. BTA and TAZ films deposited near neutral pH were thicker than those deposited at high pH. TAZ films were significantly thinner (∼20Å) than BTA films deposited under similar conditions, approaching a self assembled monolayer (SAM). The BTA removal rate for a citrate solution with a pH of 4 was 20Å/min., an order of magnitude lower than that for TMAH-containing solutions. The TAZ removal rate for an oxalate solution with a pH of 4 was 240Å/min., which was twice the removal rate for BTA deposited under similar conditions. An appropriate TAZ solution was found to displace BTA on Cu and leave a hydrophilic, thin (∼20Å) passivating layer of TAZ. Other Cu passivators were also examined but only TAZ protected all Cu crystallographic orientations observed. BTA, 5-aminotetrazole (5-ATA) and 4-carboxybenzotriazole (CBTA) did not protect some Cu crystal orientations, leading to etching of entire grains and increased surface roughness. BTA and CBTA yielded hydrophobic Cu surfaces, leading to surface wetting issues and the potential for watermarks. A new alkaline Cu post-CMP cleaner was developed to remove residues resulting from slurries, regardless of their pH, which utilizes TAZ to displace BTA and protect Cu, significantly lower organic defects, reduce surface roughness, provide a hydrophilic Cu surface, and significantly extend the staging time allowed between the clean process and subsequent wafer processing.

A new particle sizing and counting method based on the coupling of flow field-flow fractionation (FFF) with dual-sensor, single particle optical sensing (SPOS) detection is reported. The integration of FFF and SPOS systems was accomplished by means of a dilution interface that preserved the resolution of FFF-separated particles. Analysis of a model mixture of polystyrene latex standards of different diameters established that the FFF-SPOS system can resolve particles into discrete peaks for subsequent particle counting. Application of this method for the analysis of a colloidal silica standard demonstrated its use for materials commonly employed as CMP abrasives. Further development and refinement of the technique will enable compositional and structural analyses of heterogeneous large particle populations constituting commercial CMP slurries.

A volume-pixel, or “voxel”-based wear model was developed in order to predict feature-scale wear in chemical-mechanical polishing (CMP), and was compared to the measured evolution of lithographically-patterned features during full-scale CMP tests. In order to conduct this study, a lithographic technique was used in order to pattern a set of raised square features into Cu-coated silicon wafers. A two-dimensional (2D) contact profilometer was used to measure the topography of an isolated feature on the wafer both before polishing and at various intervals throughout the polishing process. For wear modeling, the voxel modeling framework involved the conversion of each of the pre-polished feature topographies into a square matrix of cuboids, which allowed for contact mechanics and wear modeling to take place between the interacting cuboids of the sample and a simulated polishing pad. After the predicted wear of the Cu feature was then compared to experiment.

The purpose of this study was to characterize KOH based electrolytes and effects of additives on electro-chemical mechanical planarization. The electrochemical mechanical polisher was made to measure the potentiodynamic curve and removal rate of Cu. The potentiodynamic curves were measured in static and dynamic states in investigated electrolytes using a potentiostat. Cu disk of 2 inch was used as a working electrode and Pt electroplated platen was used as a counter electrode. KOH was used as the electrolyte. H2O2 and citric acid were used as additives for the ECMP of Cu. In static and dynamic potentiodynamic measurements, the corrosion potential decreased and corrosion current increased as a function of KOH concentration. In dynamic state, different potentiodynamic curve was obtained when compared to the static state. The current density did not decrease in passivation region by mechanical polishing effect. The static etch and removal rate were measured as function of KOH concentration and applied voltage. In ECMP system, polishing was performed at 30 rpm and 1 psi. The removal rate was about 60 nm/min at 0.3 V when 5 wt% KOH was used. Also, the effect of additive was investigated in KOH based electrolyte on removal rates. As a result, The removal rate was increased to 350 nm/min when 5wt% KOH, 5vol% H2O2, 0.3 M citric acid were used.

Atomic force microscopy (AFM) was used to examine chemical-mechanical processes on Si (100) surfaces. Places where the underlying silicon was exposed etched in basic solution, producing structures 100 nm or less in size. Etching occurs only in the presence of combined mechanical and chemical effects. By performing AFM in basic solution, the entire etching process could be observed directly. High-force scans were used to remove oxide and initiate etching in selected locations, followed by low-force scans which imaged the etching process. Although roughness initially increased during etching, the final surfaces were smooth. The etching was measured for different applied loads, numbers of scans, concentrations of the etching solution, and time. The oxide layer was extremely sensitive to applied stress, and even very light scanning caused the oxide layer to dissolve more rapidly. Once the oxide layer was removed, chemical etching proceeded with or without AFM scanning, but if AFM scanning was continued additional material was removed, probably by a tribochemical mechanism on pure Si.

The use of density gradient stabilized centrifugal sedimentation (disc centrifugation) for the characterization of abrasive particles in CMP slurries is reported. For slurries prepared with ceria and colloidal silica abrasives, the technique is demonstrated as capable of providing highly repeatable analyses of the abrasive's apparent particle size distribution (PSD). The addition of water soluble organic additives to the slurries is shown to produce large shifts in the apparent PSD relative to the PSD of the pure abrasive particles. Particle-additive interactions driving the shift in apparent PSD could not be interpreted with confidence due to the lack of accurate densities for particle-abrasive complexes formed in the slurry. To address this problem, sucrose density gradients prepared using H2O and D2O were tested as spin fluids of different densities to analyze a colloidal silica standard with a narrow PSD. Preliminary results comparing experimental disc centrifuge data with theoretical simulations of the disc sedimentation pattern suggest that this technique can potentially characterize both the particle diameter and density of abrasive particles in a CMP slurry.

Nanoparticles provide multiple functionalities to the performance of CMP slurries. These include mechanical surface abrasion, mass transport of slurry chemistries between the pad and wafer surface, or increased chemical reactivity of some key additives by in-situ interactions with the particles surfaces. Since most of the inorganic nanosized oxides used as common abrasives in CMP slurries (silica, alumina, ceria) have chemically reactive hydrophilic surface functionalities in a large pH range (2 – 12), we can assume that significant interactions between the inorganic particles surface and some of the slurry additives (organic surfactants, oxidizers, film-forming ligands, removal rates promoters, etc.) could have a meaningful impact on slurry ultimate performances (removal rates, planarity, defectivity, etc.)

In this presentation we will highlight the significance of slurry abrasives as surface modified nanoparticles/chemical carriers, able to directly participate and control the metal oxidation/removal mechanism, removal rates and other polishing characteristics. We will discuss two relevant examples, both involving fumed silica as the carrier particle in two different low pH slurries of variable complexities in terms of design (with/without particles surface modifiers) and performance requirements for tungsten CMP. We will provide a complex variety of analytical evidence (TEM, SEM, FT-IR, GPC, cyclic voltammetry, MS-TOF) in order to support the proposed mechanism of “chemically activated fumed silica”, in its natural (no interactions with organic additives in the slurry) and surface modified form (in-situ interactions with organic additives), as a carrier of selective slurry components with enhanced chemical activity, that ultimately controls the tungsten CMP mechanism and the ability of the slurry to efficiently and predictably remove the oxidized tungsten film formed at the wafer surface.

Using a pad substitute material, we measure the number of active or working diamonds on a conditioner and find that it is generally a small fraction of the total number on the disc. The number of active diamonds also increases with the applied load and varies somewhat with sliding direction. However, even among the active diamonds, most do relatively little cutting. A few diamonds on a disc do most of the deep bulk cutting, a larger fraction skim the higher areas of the pad surface and most of the diamonds on the disc apparently merely help to support the load. While all of the diamonds that make contact, cutting or otherwise, may show evidence some of mechanical wear, wear of the small number of deep bulk cutting diamonds may be responsible for declining cut rates and for surprisingly low observed conditioner lifetimes.

In this work, we propose a wafer level dynamic ECMP model based on time-evolving current density distributions across the wafer. The copper layer on the wafer surface is discretized, and the potential and current density distributions are calculated based on the applied voltage zones and metal film thicknesses across the wafer. The copper removal rate is proportional to the current density, and thus the copper thickness (and conductance) can be calculated as a function of position on the wafer and polish time. Using a time-stepping simulation, the model is able to capture the wafer level non-uniformity and time-dependence of ECMP removal. The model is also able to capture the time-varying voltage zones used in ECMP, and can be used to find optimal voltage zone control schemes to achieve improved wafer-level uniformity.

We report measurements of single asperity wear on oxidized silicon surface in aqueous KOH using atomic force microscopy (AFM), where the single crystal silicon tip was used both to tribologically load and image the surface. AFM was also operating in the lateral (frictional) force mode (LFM) to investigate the pH dependence of kinetic friction between the tip and the SiO2 surface. We found that the Si tip wear amount strongly depended on the solution pH value and was at a maximum at around pH 10. It was also found that the Si removal volume in moles was approximately equal to that of SiO2 irrespective of the solution pH value. This equality implies that the formation of the Si-O-Si bridge between one Si atom of the tip and one SiO2 molecule of the specimen at the wear interface, followed by the oxidation of the Si surface, finally the bond rupture by the tip movement, the dimeric silica (OH) 3Si-O-Si(OH) 3, including the Si-O-Si bridge is dissolved in the KOH solution. It was also found the frictional force is highly sensitive to the pH values of the solution and peaked at around pH 10. These results indicate that the interfacial reaction would be affected by the frictional force between the Si tip and the SiO2 surface, due to an increased liquid temperature and a compressive stress in Si and SiO2 networks. Strong influence is observed by the pH of the ambient solution confirming the important role of the OH- in the wear mechanism. We present a microscopic removal mechanism which is determined by an interplay of the diffusion of water in Si and SiO2.

Chemical mechanical polishing (CMP) is a process commonly used to planarize or polish thin film surfaces to enable stacking of additional levels to enhance lithographic patterning of wafers. It is used to make surfaces atomically smooth and is also an interim step in integrated circuit (IC) manufacturing. CMP is an example of a tribological regime called Particle-Augmented Mixed Lubrication (PAML) as named by the authors. PAML occurs when two surfaces in relative motion under load are partially separated by an intervening fluid-particle mixture. The load is supported by both asperities and fluid, and the interface is further complicated by the addition of nanoparticles. PAML involves four core components that must be modeled integrally—fluid mechanics, particle dynamics, contact mechanics, and material removal (wear). This work introduces the fundamental tenets of PAML, and describes how it is an effective first principle multi-physics approach to modeling CMP. By inputting the artificial random topographies for the pad and wafer with their actual mechanical properties, the PAML modeling simulation results predict the instantaneous material removal as the wear volume caused by particle-induced wear. These discrete instantaneous material removal events lead to the cumulative wear seen during CMP over a short time. Although only a small fraction of the time of the actual CMP process, tests of 120μs show that the cumulative material removal occurring over the entire simulation is approximately 0.012μm3. This work suggests that a generalized multi-physics modeling simulation of the CMP process is plausible.

Silicon Carbide has a unique combination of properties that include a nearly diamond-like hardness, intrinsic electrical semiconductivity and a high thermal conductivity. This combination of properties has led to it's use in a number of applications including substrates for Light Emitting Diodes (LEDs), power, RF (radio frequency) and other electronic devices in addition to mirror substrates and optical devices as well as stop layers in integrated circuit chip manufacture. In addition, the chemical inertness and high hardness of SiC has historically resulted in low removal rates during chemical mechanical Planarization (CMP). Recent efforts in our labs have led to being able to polish single crystal silicon carbide at removal rates up to 400 nm/hr yielding a root mean squared roughness on the order of a nanometer as determined by AFM and interferometry. The high rates and smoothness obtained are expected to translate to other types of silicon carbide. Fundamental studies by FTIR, streaming potential and ESCA have been done to elucidate the mechanism of silicon carbide polishing.

The hydrophobicity of poly Si is reported to introduce different polishing behavior with careful control of post CMP cleaning process. The purpose of this study was to investigate the effect of poly Si wettability on its CMP behavior. The adhesion force of polymeric particle on the poly Si wafer surfaces was measured in the KOH solution (pH 11) as a function of solution A concentration. Adhesion force decreased and saturated as a function of concentration of solution A. The change of surface wettability affects not only the polishing rates but also the level of contamination on wafer because the interactions between particles and substrates are dependent on the wettability of the surface. Also, hydrophobic poly Si surfaces attracted much more pad particles with water marks than hydrophilic