Shallow Trench Isolation Chemical Mechanical Planarization Research Articles

Release of Redox Additives Via Megasonic Energy for High Selectivity/Low-Stress STI CMP

The advancements in a wide array of semiconductor manufacturing processing have been pivotal for the extension of Moore’s law the Chemical Mechanical Planarization (CMP) process has become an area of focus due to the stringent surface topography requirements. CMP utilizes complex slurry formulations containing abrasive nanoparticles and additives to reduce substrate defectivity and achieve angstrom-level uniformity. One area that has gained significant attention is Shallow Trench Isolation (STI), where the modulation and control of the surface oxidation state of the ceria, specifically the Ce3+ state, is critical to TEOS removal. In the Ce3+ state, defect states (i.e., oxygen vacancies (Ovacs)) on the surface are highly electron deficient (i.e., electrophilic) and allow the nucleophilic substrate to complex. By modulating the electrophilic properties of the particle, the kinetics of surface complexations required for effective TEOS removal can be controlled. This work focuses on developing stimuli-responsive capsules that can be added to STI slurry formulations to promote the controlled release of redox-active additives that can be used to drive the oxidation state of the CeO2 nanoparticle (Ce3+/Ce4+). By encapsulating the additive in a polymeric system, the reactive properties of the slurry are limited until the additive is released. In this work, megasonic action was employed to disrupt the capsules releasing the additives on demand to facilitate the modulation of the CeO2 surface redox states. Results have shown that the addition of a controlled-release capsule with an additive resulted in the increase of Ovacs at the CeO2 surface. This led to an increase from 1800 Å/min to 3450 Å/min using a constant 2.0 W/cm2 continuous flow megasonic flux. Furthermore, the overall defectivity decreased significantly with respect to both particle contamination and surface scratching. Lastly, the nature of the polymeric capsule has been shown to enhance selectivity between TEOS and silicon nitride and preliminarily appears to be megasonic flux dependent.

Restricted hydrolysis reaction of Si3N4 via nonionic polymer adsorption in advanced shallow trench isolation chemical mechanical planarization

The adoption of ultra-fine nanoparticles as an abrasive has garnered notable attention in shallow trench isolation (STI) chemical mechanical planarization (CMP) due to its potential in scratch reduction. However, the poor removal rate, attributed to the physicochemical interaction between abrasive and conventional organic passivation agent, limits the introduction of ultra-fine nanoparticles. Here, a nonionic polymer polyvinyl alcohol (PVA), without particle interaction, is proposed as a novel passivation agent to restrict the hydrolysis reaction of Si3N4 film in the advanced STI CMP process. The selective adsorption of PVA onto the Si3N4 film lowers its removal rate by suppressing the hydrolysis reaction via hydrogen bonding while the removal rate of SiO2 film remains constant. Moreover, the uniformity of SiO2 removal is improved due to the enhanced fluidity of slurry, resulting from increased wettability between pad and slurry. As a result, the removal selectivity between both films has been improved to 113.4:1 and the in-wafer-uniformity (IWU) of SiO2 film is supplemented by 96.8 % simultaneously.

Role of Molecular Structure on Modulating the Interfacial Dynamics for Shallow Trench Isolation (STI) Chemical Mechanical Planarization (CMP) Applications

As feature sizes continue to shrink well beyond the 7 nm node, understanding the delicate balance present in the chemical mechanical planarization (CMP) process is of utmost importance. In order to achieve high through-put and defect-free CMP processes it is critical to develop predictive analytical techniques that directly correlate to macroscopic STI CMP performance metrics (i.e. oxide/nitride removal, defectivity, and dishing/erosion). This work employed a suite of techniques to monitor the CeO2 nanoparticle interfacial redox processes in the presence of structurally diverse rate modulating additives. Specifically, utilizing a UV–vis spectroscopic technique, the Ce3+/Ce4+ ratio in the presence of different slurry additives was monitored and proved to directly correlate to removal rate performance (i.e. an increase in Ce3+/Ce4+ ratio shows an increase in rate). This finding coupled with the rate of dissolved O2 evacuation and a modified QCM technique determined the mode of interaction/adsorption which validates that the mechanism of oxide removal does not strictly depend on redox capacity, but also depends on the dynamic O2 equilibrium at the CeO2 nanoparticle surface. It was determined that the modulation of oxide removal was directly related to the distribution of interactions (i.e. steric vs redox) and was highly dependent on the slurry additive functionality.

Engineering Surface Texture of Pads for Improving CMP Performance of Sub-10 nm Nodes

Shallow trench isolation (STI) chemical mechanical planarization (CMP) will continue to be a critical step in the device fabrication of future technology nodes. A CMP process needs the right combination of pad, slurry, and pad conditioner to be able to deliver the best performance. Engineered pad surface along with usage of non-Prestonian ceria slurries can be used to solve key STI CMP challenges. Herein, it is demonstrated that relative to a reference pad, reducing and optimizing pad surface roughness, can help improve material removal rate by 26%–95% at different wafer pressures, lower the dishing by 34%–51% for feature size of 50–200 μm, thereby leading to an improved planarization performance. This work illustrates that the surface texture of a pad can be used to achieve a stable dishing range over different feature scales for various over polish times. Also, optimizing the pad surface roughness minimizes slurry consumption by 33% and enables polishing at a lower downforce. Further, it is demonstrated that relative to a reference pad, defectivity can be improved up to 85% for the pad without impacting the planarization performance by reducing the pad material hardness and polishing at an optimized pad surface roughness.

Probing Oxide-Nitride Selectivity As a Function of CeO2 Valence State Relevant to STI CMP

Electrical isolation within a desired circuit requires shallow trench isolation (STI) to separate active regions with tetraethyl orthosilicate (TEOS) filled trenches. STI chemical mechanical planarization (CMP) is used to facilitate bulk oxide removal by selectively targeting TEOS overburden with colloidal suspensions (slurries) containing ceria (CeO2) nanoparticles. A critical factor in maintaining high removal rates is the presence of CeO2 surface oxygen vacancies (Ovac); commonly associated with unoxidized Ce3+ particles which when filled, change the oxidation state to a less reactive Ce4+ form. The goal of this work is to utilize modified slurries at various Ce3+/Ce4+ ratios and determine the influence of oxidation state on oxide/nitride removal rates. Characterization of the CeO2 particles was determined through UV/Visible and fluorescence spectroscopy along with a novel UV/Visible spectroscopic flow cell technique. The modified slurries were then used to polish both TEOS and Si3N4 wafers to probe selectivity with respect to various mechanical parameters. Preliminary results show that greater concentrations of Ce3+ valence states, in slurries containing a novel reducing agent, result in higher oxide removal rates than the unmodified slurry. Future work will focus on characterization of the post polish effluent to determine the influence of polishing on certain particle properties, such as oxidation state, zeta potential, and size.

Controlling Germanium CMP Selectivity through Slurry Mediation by Surface Active Agents

New developments and device performance requirements in microelectronics industry add to the challenges in chemical mechanical planarization (CMP) process. One of the recently introduced materials to semiconductor manufacturing is germanium which enables improved device performance through better channel mobility in shallow trench isolation (STI) applications for advanced circuits. This paper focuses on controlling germanium/silica selectivity for advanced STI CMP applications through slurry modification by surface active agents. Surface adsorption characteristics of cationic and anionic surfactants on germanium and silica wafers are analyzed in order to control selectivity as well as the defectivity performance of the CMP applications. The effects of surfactant charge and concentration (up to self-assembly) are studied in terms of slurry stability, material removal rates and surface defectivity. Surface charge manipulation by the surfactant adsorption on the germanium surface is presented as the main criteria on the selection of the proper surfactant/oxidizer systems for CMP. The outlined correlations are systematically presented to highlight slurry modification criteria for the desired selectivity results. Consequently, the paper evaluates the slurry selectivity control and improvement criteria for the new materials introduced to microelectronics applications with CMP requirement by evaluating the germanium silica system as a model application.

Shallow Trench Isolation Chemical Mechanical Planarization: A Review

Electrical isolation of the billion or so active components in each integrated device is achieved using shallow trench isolation (STI) which requires chemical mechanical planarization (CMP) involving silicon dioxide removal at a high rate and stopping on an underlying silicon nitride film. Several colloidal slurries with various additives can yield the desired high rate selectivity between the oxide and nitride films during CMP while maintaining an acceptably low nitride rate. Here, many of such high selectivity STI CMP slurries described in the literature are reviewed along with the characteristics of the colloidal dispersions like the abrasives, additives, the interactions between them and with the films being planarized and the associated pH range in which the high selectivity is observed. The mechanisms proposed to explain the high reactivity of ceria with oxide, the role of additives in suppressing the nitride removal rate and resulting high selectivity are discussed. Reduction of a multitude of defects in post-CMP processed STI structures still remains an important challenge, especially as the feature sizes continue to shrink.

Control of adhesion force between ceria particles and polishing pad in shallow trench isolation chemical mechanical planarization.

The adhesion force between ceria and polyurethane (PU) pad was controlled to remove the step height from cell region to peripheral region during Shallow Trench Isolation Chemical Mechanical Planarization (STI-CMP) for NAND flash. Picolinic acid was found to be adsorbed on ceria particles at pH 4.5 following a Langmuir isotherm with the maximum adsorbed amount of 0.36 mg/m2. The ceria suspension with full surface coverage of picolinic acid showed a threefold increase in the number of adhered ceria particles on the PU pad over non-coated ceria particles. It was shown that the coverage percent of picolinic acid on ceria corresponds well with the amount percent of adsorbed ceria on PU pad. The change in adsorbed particles was directly reflected in the CMP polishing process where significant improvements were achieved. Particularly, convex areas on the chip experienced higher friction force from the attached abrasives on the PU pad than concave areas. As a result, the convex areas have increased removal rate of step height compared to the ceria suspension without picolinic acid. The changing profiles of convex areas are reported during the step height reduction as a function of polishing time.

The Effect of Amino Acids on Step Height Reduction in STI CMP

Shallow trench isolation (STI) process has been a crucial technology to isolate the transistors in ultra large scale integration (ULSI) device [1]. In STI process, step height of dielectric material is inevitably formed after gap-filling process due to the different pattern density. Chemical mechanical planarization (CMP) process has been employed to remove the step height. Remaining step height would cause defects such as dishing and erosion, therefore the reduction of step height can be considered as one of the critical issues in STI-CMP process.Several groups have studied about the step height reduction using ceria due to its high removal rate of SiO2 [2]. Lim et al. reported that the effect of particle size on step height reduction. They showed that the large particle size could improve the step height reduction by its high removal rate. Recently, the needs of small particle have been increased because the defect control has been more important as the design rule is getting narrow. Therefore, it is inevitable to develop new methodology to enhance the step height reduction. Here, we used different amino acid to modify the surface of ceria and enhance the interaction between the ceria slurry and SiO2.We investigated the effect of ceria slurries prepared with L-serine, proline, glutamine and amino butyric on step height reduction. We calculated the interaction forces between ceria with different amino acids and SiO2 film by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The result shows that ceria slurry with L-serine or proline has higher attractive interaction forces with SiO2 than the slurry with glutamine or amino butyric acid (Fig. 1).The results of SiO2 pattern as different pattern density and line width show that the ceria slurry with L-serine or proline produces higher step height reduction than the slurry with glutamine or amino butyric (Fig. 2). As a result, we found that the high interaction forces between ceria slurry and SiO2 contribute to high step height reduction and it is stronger in case of the slurry with L-serine or proline than the slurry with glutamine and amino butyric acid.

UV VIS Studies of Ce(III)/Ce(IV) Redox Reactions to Understand Abrasive and Work Surface Interactions during STI CMP

In the current fabrication processes for integrated chips (IC), shallow trench isolation (STI) scheme is used to isolate and insulate transistors. Chemical mechanical planarization (CMP) is a key step in STI integration scheme. High selectivity slurries (HSS) which yield high silicon dioxide to silicon nitride polish rate are often used, and this is achieved by controlling the solid loading and the chemical concentration of additives [1]. Ceria based slurries with suitable additives are known to provide high selectivity [2, 3, 5]. Ceria is known to interact chemically with the silicon dioxide surface during CMP [2,3] and it is proposed that Ce(III) might play a role in the chemical interaction [4], but the exact nature of the species interacting with oxide are not known.Addition of hydrogen peroxide (H2O2) to ceria based slurries is reported to suppress the oxide and nitride removal rates [4]. However, when other abrasives are employed, hydrogen peroxide does not modify the removal rate significantly [4]. Ce (III) and Ce (IV) ions are known to exhibit UV VIS absorption peak at different wavelength regions.In this work a study on the kinetics of Ce (III) and Ce (IV) ions in the presence of H2O2is undertaken using UV-Visible Spectroscopy. Ce (III) chloride and Ce (IV) sulfate are employed for the initial study and the effect of hydrogen peroxide on changing the equilibrium composition is analyzed. The absorption is also monitored as a function of pH and time, to understand the dynamics of the system. Further, ceria abrasives are employed in the study and the nature of the species dissolving into the aqueous medium in presence of and absence of hydrogen peroxide, at different pH values are analyzed. The results show that the equilibrium between Ce (III) and Ce (IV) is shifted by the hydrogen peroxide which may help explain the suppression of oxide and nitride CMP rate by the addition of peroxide to ceria based slurries.

Effect of La Doping of Ceria Abrasives on STI CMP Selectivity

Shallow trench isolation (STI) is used to isolate the transistors in an integrated chip and chemical mechanical planarization (CMP) is a key step in the STI integration scheme [1]. Ideally a high silicon dioxide removal rate and a low silicon nitride removal rate (i.e. high selectivity) is needed. High selectivity can be achieved when amino acids such as L-proline are used as additives along with ceria abrasives [2-3] but they do not work well with other abrasives [4]. Even when ceria is used as abrasive high selectivity is not observed in some cases [5].In the present study, ceria is synthesized using sol gel and liquid phase synthesis (LPS). Polishing experiments are done on different ceria abrasives with same additives. To study the effect of impurity on polishing, lanthanum is added to ceria during the synthesis stage. The polishing results show that ceria without any impurity give high selectivity with L-proline while ceria with lanthanum as impurity does not give high selectivity.To compare the results of polishing, commercial ceria with high and low purity obtained from sigma aldrich (ceria-SA) are considered for polishing experiments. Polishing with commercial ceria show similar results obtained with synthesized ceria. The ceria particles both commercial and synthesized were characterized by x-ray diffraction, energy dispersive spectra and scanning electron microscopy techniques. Polishing experiments were also conducted with milling the commercial ceria to evaluate if the size of ceria abrasive plays a role in modifying the selectivity.

Abrasive and additive interactions in high selectivity STI CMP slurries

Ceria based slurries with additives are widely used in shallow trench isolation (STI) chemical mechanical planarization (CMP) process to obtain high selective removal of silicon dioxide over silicon nitride. In this study ceria from different sources were used as abrasives and l-proline and l-glutamic acid were used as additives, with a focus on identifying the interactions between abrasives and additives and their effect of selectivity. Ceria particles were characterized using X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) while the additive abrasive interactions were evaluated by UV–Visible spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. While slurries with l-proline yielded high selectivity only with certain type of ceria, slurries with l-glutamic acid were found to be less sensitive to the ceria source than those with l-proline, and yielded high selectivity regardless of the source of ceria used. The purity of the abrasive and its crystal structure appear to play a significant role in determining the selectivity.

Synthesis of Silica-Coated Ceria Particles for STI-CMP in a Single Step by Flame Spray Pyrolysis with an Emulsion

Silica-coated ceria particles for STI-CMP (Shallow Trench Isolation Chemical Mechanical Planarization) were prepared in a single step by using a modified flame spray pyrolysis with an emulsion precursor solution. The prepared particles were characterized by using TEM and zeta potential measurement. The isoelectric points (IEPs) of the silica-coated ceria particles are decreased with increasing the amount of silica coating and approach to two, which is IEP of silica. This result macroscopically confirms that silica is reasonably coated on the ceria surface. TEM analysis also supports that the silica layer is indeed formed on ceria particle and the thickness is around 5 nm. The silica-coated ceria suspension was found to have lower viscosity compared with the bare ceria suspension. Therefore, dispersibility of CMP suspension is expected to be enhanced at the expense of decreased polishing rate. However, it is decreased by only 6.5% compared with the polishing rate of bare ceria when 5 wt % of silica is coated on ceria.

The Synergetic Role of Pores and Grooves of the Pad on the Scratch Formation during STI CMP

The polishing pad plays a vital role in achieving the desired removal rates and level of surface planarity during the chemical mechanical planarization (CMP) process. Generally the pad containing both pores and grooves is used for the shallow trench isolation (STI) CMP process. After polishing the wafer, many scratches would be produced on the surface, especially chatter mark scratches on the oxide surface. In this work, the role of pores and grooves of the pad on scratch formation was studied with STI-patterned wafers with three types of pads: pad with only pores, pad with only grooves, and pad with both grooves and pores. The pad with only grooves produced more irregular-shaped scratches when compared to the pad with only pores, but the length of the scratch was smaller. A regular chatter mark type scratch would be formed only when the pad contained both pores and grooves. The most likely occurring phenomenon might be the stick-slip motion during the process. The removal rates with three different pads were compared and they were inversely correlated to each other. The presence of pores would be essential to decrease the scratch number. However the scratch number could be greatly reduced with the combination of both pores and grooves. Overall, the number of scratches would be greater with the absence of pores, and the scratch number and intensity substantially reduced with the combination of both pores and grooves.

Increase in the Adsorption Density of Anionic Molecules on Ceria for Defect-Free STI CMP

We have investigated the adsorption density of anionic molecules on a ceria surface and its effect on suppressing defects on the films during shallow trench isolation (STI) chemical mechanical planarization (CMP). Intermolecular vacancies in poly(methyl methacrylate) (PMMA) caused by ionic repulsive forces between its carboxyl groups provide adsorption sites on the ceria surface for 3-hydroxypentanedioic acid-3-carboxylic acid (hydrogen citrate), which has a molecular length of 8.39 Å. This allows for an increase in adsorption density. The coadsorption of PMMA and hydrogen citrate on the ceria surface leads to an increase in negative charge at the ceria particle–water interface and provides repulsive forces between the ceria particle and the surface. Ceria slurry prepared with the coadsorption of PMMA and hydrogen citrate shows a suppression of remaining particles (90.9%) and microscratches (100%) on the film, resulting from a decrease in the adhesion force of ceria particles on the film. This effect was analyzed by force measurement using atomic force microscopy.

(111) Faceted Ceria and Its Influence on STI CMP for Memory Devices Below 50 nm

We investigated the effects of (111) faceted ceria nanoparticles on the removal rate of oxide films in shallow trench isolation (STI) chemical mechanical planarization (CMP). The X-ray diffraction peak ratio of (111) to (200) for the (111) faceted ceria particles synthesized by a flash-creation method was 4.08, while the ratio for polyhedral ceria synthesized by a solid-state displacement reaction was 2.65. The highest surface density of atoms in the (111) plane led to an increase in removal rates. Faceted ceria slurry yielded a higher oxide removal rate and greater selectivity than the polyhedral ceria slurry in STI CMP evaluation. .

Crystalline structure of ceria particles controlled by the oxygen partial pressure and STI CMP performances

The effect of the crystalline structures of nano-sized ceria particles on shallow trench isolation (STI) chemical mechanical planarization (CMP) performance was investigated. The ceria particles were synthesized via a solid-state displacement reaction method, and their crystalline structure was controlled by regulating the oxygen partial pressure at the reaction site on the precursor. The crystalline structures of ceria particles were analyzed by the high-resolution TEM nano-beam diffraction pattern. In a calcination process with a high oxygen concentration, the synthesized ceria particles had a cubic fluorite structure (CeO 2), because of the decarbonation of the cerium precursor. However, a low oxygen concentration results in a hexagonal phase cerium oxide (Ce 2O 3) rather than the cubic phase due to the insufficient oxidation of Ce 3+ to Ce 4+. In the STI CMP evaluation, the ceria slurry prepared with the cubic CeO 2 shows enhanced performances of the oxide-to-nitride removal selectivity.

Constraints on removal of Si3N4 film with conformation-controlled poly(acrylic acid) in shallow-trench isolation chemical–mechanical planarization (STI CMP)

The effect of changes in poly(acrylic acid) (PAA) conformation on removal of Si3N4 film was investigated. PAA was used as a passivation agent by adsorption on an Si3N4 film in shallow-trench isolation chemical–mechanical planarization (STI CMP). Adsorption behavior of PAA on the Si3N4 film and the conformation transition were determined by adsorption isotherms and force measurements using atomic force microscopy (AFM) as a function of ionic strength. AFM results revealed that, as ionic strength increases, the repulsive force between the negatively charged carboxylate groups along the backbone of PAA is reduced due to counterion screening and to the changes of PAA conformation from a stretched to a coiled configuration. At high ionic strength, the coiled conformation of PAA formed a dense passivation layer on the Si3N4 film, which led to suppression of the removal rate of Si3N4 film from 72 to 61 Å/min in the STI CMP process.

Reduction of Large Particles in Ceria Slurry by Aging and Selective Sedimentation and its Effect on Shallow Trench Isolation Chemical Mechanical Planarization

The effect of aging and selective sedimentation of ceria slurry on the slurry characteristics, such as the particle size distribution, the large-particle count, and the dispersion stability, and on the number of defects on the wafer surface during the shallow trench isolation (STI) chemical mechanical planarization (CMP) process was investigated. The aging time and temperature were considered as important variables giving great influence on the slurry characteristics and STI–CMP. By applying aging and selective sedimentation, the number of large particles produced by either strong agglomeration or soft flocculation in ceria slurry was successfully reduced, and the dispersion stability of the ceria particles was also improved. A CMP evaluation demonstrated that the number of defects on the wafer surface was significantly reduced, while maintaining the reasonable oxide removal rate and oxide-nitride selectivity.

Effect of Calcination Process on Synthesis of Ceria Particles, and Its Influence on Shallow Trench Isolation Chemical Mechanical Planarization Performance

The effect of the calcination process for cerium carbonate, a precursor of ceria, on the degree of synthesis and colloidal properties of ceria particles in aqueous media, which greatly influence shallow trench isolation (STI) chemical mechanical planarization (CMP) performance, was investigated. A two-step calcination process, consisting of decarbonation and crystal growth, resulted in a higher degree of synthesis for the same crystal size, a narrower particle size distribution, and better dispersion of the ceria particles, than conventional one-step calcination. These properties enhanced certain aspects of STI CMP performance, leading to a higher oxide removal rate, a better selectivity between oxide and nitride, and fewer defects, including remaining particles.