Planarization Of Copper Research Articles

CMP: Four Decades of Innovation and a Promising Future

The invention of CMP by IBM in 1983 represents an inflection point in the evolution of semiconductors. It was a bold and risky idea to mechanically apply abrasives to the surface of a wafer, but this new process for restoring a wafer to a planar state was the key to continued areal scaling and increased device complexity. Initially applied to a simple application for filling trenches with insulator, the potential of this technique was evident. Through understanding of material properties and subsequent manipulation of slurry chemistry, CMP was applied to enable the more challenging and technically critical processes for Shallow Trench Isolation and Tungsten contacts. The late 1990’s saw the introduction of copper interconnects. IBM was first to market this technology which continues to support conductors down to lines widths on the order of 10 nanometers! The CMP process required for copper interconnects demanded a thorough understanding of the materials present and potential surface phenomena for exploitation. For this need, a new and complex mechanism for copper planarization was invented. Equally challenging was the need for a subsequent non-selective barrier removal step. For this a unique, self-balancing approach was also invented, borrowing some elements from previous STI technology. Device architecture has progressed from planar geometry to FINFETs to Gate All Around, or Nanosheet technology. This evolution has required increasingly more CMP process steps with a concurrent demand for increased process control. Gate height control for Nanosheeets is currently paramount for performance, affected by many CMP steps in the process flow. A typical requirement is less than 1nm added variability per CMP step! Looking forward, CMP continues to be tapped to enable technology evolution. Polish steps are becoming part of the patterning process to enable pitch splitting for upcoming nodes. A VTFET integration approach can enable further area scaling but presents integration challenges; CMP is being asked to enable “third color” schemes for the protection of NFET vs PFET devices which is necessary at various points in the in the VTFET process flow. Heterogeneous Integration is demanding nanometer control of topography across large contact pads and large lateral distances. Finally, extreme scaling is creating fundamental challenges for metal polish at the contact and interconnect levels. CMP tool architecture currently does not provide for control of ambient conditions at the wafer surface during wet transport and post CMP cleaning. Subsequently, unwanted metal dissolution is observed. New tool designs will be needed to control wafer surface conditions, and particularly oxygen exposure through all steps within the CMP tool. This talk is dedicated to the special session in honor of Professor S.V. Babu

Molecular Interactions in Copper Chemical Mechanical Planarization: A Phenomenological Study

In this study, the molecular interactions occurring during Chemical Mechanical planarization of Copper in Ferric nitrate–Alumina–Benzotriazole system is investigated. This system is characterized by sudden and dramatic transitions in removal rates when experimental parameters such as downforce and concentrations of the slurry components (Benzotriazole, abrasive, oxidizer) are varied. This behavior is interpreted in terms of the three surface kinetic processes viz. Cu dissolution, film formation and film removal by abrasion, that determine the overall removal rate. A phenomenological model incorporating Frumkin isotherm for the adsorption of Benzotriazole molecules and Langmuir isotherm for the adsorption of surfactants/ions is proposed. Mathematical equations relating the rates of film formation, film removal and metal dissolution to the overall removal rates are derived. The model is validated by comparing the model predictions with experimental data. The non-linear equations describing the sudden transitions are shown to exhibit a cusp catastrophe when the interaction parameter exceeds a certain value.

Urea as a complexing agent for selective removal of Ta and Cu in sodium carbonate based alumina chemical–mechanical planarization slurry

This work reports urea as a promising complexing agent in sodium carbonate-based alumina slurry for chemical?mechanical planarization (CMP) of tantalum and copper. Ta and Cu were polished using Na2CO3 (1 wt. %) with alumina (2 wt. %) in the presence and absence of urea. The effect of slurry pH, urea concentration, applied downward pressure and platen rotational speed were deliberated and the outcomes conveyed. Prior to the addition of urea, the Ta removal rate (RR) was observed to increase with pH from acidic to alkaline, having a maximum RR at pH 11. However, Cu RR decreases with increasing pH with minimum RR at pH 11. With the addition of urea in the slurry, a Cu to Ta removal rate selectivity of nearly 1:1 was encountered at pH 11. The addition of urea simultaneous boosts the Ta RR and suppresses Cu RR at pH 11, as it adsorbs on the metal surface. Potentiodynamic polarization was conducted to determine the corrosion current (Icorr) and the corrosion potential (Ecorr). Electrochemical impedance spectroscopy of both metals was carried out in the proposed formulation and the obtained outcomes are elaborated.

(Invited) Towards Metrology to Enable Standardizing CMP Consumables

In the semiconductor industry, there is a need to establish fundamental, mechanism-based, correlation(s) between process conditions, consumables (e.g., pads and slurries), and observed process performance in Chemical-Mechanical Polishing (CMP) [1-5]. Such understanding can lead to the creation of industry-wide standards that can help reduced cost of ownership. Along these lines, the Chemical Mechanical Planarization Consumables (CMP-C) Task Force under Liquid Chemicals Global Technical Committee within the SEMI Standards Program has been actively developing a guide (SEMI C100) for reporting CMP pad hardness [6], and pad density.In this paper, we revisit our decades old, but still relevant, quest for suitable mechanism-based metrology for CMP-pad. While we focus on polyurethane-based CMP pads, we will touch on alternative pad materials. [10-11]. Using both static and dynamic the dynamic mechanical analysis (DMA) [7-8] and the FTIR [9] we monitored changes in pad polymer properties under various chemical and mechanical stresses. Pad sample were analyzed: prior to use (fresh); after a 24-hr soak in silica-containing oxide slurry (basic); and after oxide polishing (used). Upon comparison it was observed that a characteristic transition feature due to water is present at sub-ambient temperatures in both the slurry soaked and used pads. Exposure of as-received pads to basic oxide slurry results in a broad, high temperature transition thought to be the result of chemical-induced disruption of macrostructural units. Polishing (load-enhanced chemical exposure) introduces further changes to the polymer network represented by an apparent degradation of the pad polymer.References1) P. B. Zantye, et al, 'Chemical mechanical planarization for microelectronics applications, Materials Science and Engineering: R: Reports, 2004, 45, 3–6, doi: 10.1016/j.mser.2004.06.002.2) Zantye, Parshuram B., "Processing, Reliability and Integration Issues In Chemical Mechanical Planarization" (2005). Graduate Theses and Dissertations, https://scholarcommons.usf.edu/etd/9283) S. Deshpande et al, “The Electrochemical Society, find out more Chemical Mechanical Planarization of Copper: Role of Oxidants and Inhibitors”, 2004 J. Electrochem. Soc. 151 G7884) Y. S. Obeng et al., "Impact of CMP consumables on copper metallization reliability," in IEEE Transactions on Semiconductor Manufacturing, vol. 18, no. 4, pp. 688-694, Nov. 2005, doi: 10.1109/TSM.2005.858457.5) Obeng, Yaw S., et al. "Characterization of ‘In-Process’ Degradation of Polyurethane CMP Pads." Chemical Mechanical Planarization V 2002 (2002): 1.6) SEMI C100 - Guide for Reporting Chemical Mechanical Planarization (CMP) Polishing Pads Hardness Used, published by SEMI Standards (https://store-us.semi.org/products/c10000-semi-c100-guide-for-reporting-chemical-mechanical-planarization-cmp-polishing-pads-hardness-used-in-semiconductor-manufacturing, accessed December 15, 2020)7) Li et al, “Dynamic Mechanical Analysis (DMA) of CMP pad materials”, MRS Proceedings, January 2000, Cambridge University Press, DOI: 10.1557/proc-613-e7.3.18) H. Lu et al, "Applicability of dynamic mechanical analysis for CMP polyurethane pad studies” Materials Characterization, 2002, 49, 2, 177-186, DOI: 10.1016/S1044-5803(03)00004-4.9) H Lu, et al., "Quantitative analysis of physical and chemical changes in CMP polyurethane pad surfaces, Materials Characterization", 2002,49, 1, 35-44, DOI: 10.1016/S1044-5803(02)00285-1.10) S. Deshpande, et al. “Surface-modified polymeric pads for enhanced performance during chemical mechanical planarization”, Thin Solid Films, 2005, 483, 1–2, 261-269, DOI: 10.1016/j.tsf.2004.12.063.11) Zantye, P., et al., “Metrology of Psiloquest's Application Specific Pads (ASP) for CMP Applications”. MRS Proceedings, (2004). 816, K5.6. doi:10.1557/PROC-816-K5.6

Probing the Mechanisms of Metal CMP Using Tribo-Electroanalytical Measurements: Results for a Copper/Malonate System

The experiments reported in this work explore certain fundamental mechanistic aspects of assessing slurry formulations for chemical mechanical planarization (CMP), an important processing step of integrated circuit (IC) fabrication. We use a model system involving abrasive-free planarization of copper (wiring material in ICs) with a pH-varied (∼6–10) CMP slurry containing malonic acid (MA, a complexer) and sodium percarbonate (an oxidizer). The analytical protocols necessary to probe such a CMP system are illustrated by combining linear sweep voltammetry (LSV), open circuit potential (OCP) transients, chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS), all operated under tribological controls of CMP. LSV helps to establish the CMP enabling reaction mechanisms. The information obtained from transient data of OCP and CA is incorporated in a phenomenological model of tribo-corrosion to further corroborate the CMP mechanism. EIS provides further verification and more details of the CMP reactions. Cu removal rates increase with increasing concentrations (0.0–0.10 M) of MA, and in agreement with a proposed CMP mechanism, exhibit a correlation with the rates of tribo-corrosion. The results illustrate a quantitative diagnostic framework for studying CMP mechanisms in the tribo-electroanalytical approach.

Correlating Shear Force and Coefficient of Friction to Platen Motor Current in Copper, Cobalt, and Shallow Trench Isolation Chemical Mechanical Planarization at Steady-State Conditions

In a follow up from our earlier work that focused on highlighting the relationship among platen motor current (PMC), shear force (SF), and coefficient of friction (COF) in non-steady-state cases, we examined whether PMC was a reliable indicator that could be used in place of SF and COF data at steady-state conditions. For the 12 cases studied, we examined 60 distinct steady-state steps from their associated Stribeck+ curves. Data averaging, coupled with a trend matching algorithm, showed that for large time spans at steady-state, PMC was a reasonably good indicator of both SF and COF, as 68.5% of the time, PMC and SF trends matched (ranges between 62% to 86%) for all 60 cases. Regarding PMC and COF, we observed trend matching for 68.8% of the time (ranges between 62% and 86%) for all 60 cases. However, correlations between SF and PMC, and also between COF and PMC were found to be very poor (in most cases, non-existent) since at small timescales PMC was not sensitive enough to capture the instantaneous stick-slip occurrences and other important tribological and fluid dynamics phenomena present in CMP.

Correlating Shear Force and Coefficient of Friction to Platen Motor Current in Copper, Cobalt, and Shallow Trench Isolation Chemical Mechanical Planarization at Highly Non-Steady-State Conditions

In this study we determined the relationships that exist between the coefficient of friction (COF) and platen motor current (PMC), and also between shear force and PMC at highly non-steady-state conditions during chemical mechanical planarization (CMP). For the 12 cases studied (8 for copper, and 4 for dielectric CMP including one as a cobalt buff step), we found that real-time PMC data closely mimicked shear force data as evident from high average values of correlation coefficient and coefficient of determination for all 12 cases (0.913 and 0.835, respectively). As for COF vs. PMC, average correlation coefficient and coefficient of determination for Cases A through D were 0.949 and 0.900, respectively. However, for the remaining 8 cases, correlation coefficients ranged from zero to 0.425, with coefficients of determination ranging from zero to 0.180. The reason behind the strong correlations seen in the first 4 cases, and the lack of any correlation seen in the remaining 8 cases, were explained based on the particulars of the dominant tribological mechanism for each case.

Polycrystalline-silicon thin-film transistor fabricated with Cu gate controlled morphology by plating mode

A planarized copper gate thin-film transistor (TFT) using metal-induced laterally-crystallized polycrystalline‑silicon (poly-Si) was fabricated and characterized in this study. The planarized copper gate was able to structurally alleviate the drawbacks of copper and enhance stability to adapt in TFTs. Moreover, the surface of copper was systematically investigated by an electroplating process involving leveling additives such as thiourea and chloride in acidic sulphate-plating baths. These additives can improve surface morphology and ensure a smoother surface of the copper gate, which influence the electrical property of poly-Si TFT while reducing the surface roughness scattering effect. As the gate surface morphology was enhanced, the device exhibited superior electrical characteristics in field-effect mobility, on/off current ratio, and subthreshold slope.

Improvements in Stribeck Curves for Copper and Tungsten Chemical Mechanical Planarization on Soft Pads

Understanding the tribology of polishing processes using an improved Stribeck curve (i.e. referred as Stribeck+ curve) elucidates the essential factors governing the mechanical aspects of chemical mechanical planarization. This study investigated tribological and thermal aspects of copper and tungsten blanket wafer polishing on the Politex soft pad with a multitude of gradual yet continuous changes in polishing pressure and sliding velocity. This work also attempted to understand the frictional and thermal transients associated with such dynamic processes. Results showed that polishing with the Politex pad produced stable COF values that indicated a “boundary lubrication” regime throughout. In contrast, copper polishing on hard pads produced tremendous spread of data clusters having a trend in COF that started with “boundary lubrication” and then transitioned to the “mixed lubrication” mode. Regarding temperature effects, the plots between average pad surface temperature and the product of COF, velocity and downward pressure showed a linear relationship between the two parameters for all copper and tungsten polishing used in conjunction with Politex and hard pads.

A Ring Type Residue Formation in Post Cu CMP Cleaning

A chemical mechanical planarization (CMP) of copper (Cu) damascene structure is prone to defect formation due to variety of chemical agents in slurry and chemical complex formation. In post CMP cleaning process, which should remove such defects, a mechanical scrubbing of wafer by porous and elastic brushes are eligibly used. Although, chemical effects are the main mechanism of defect detachment and dissolution during cleaning process, mechanical effects also play key role by transportation of chemical agent and discharge of reacted species. The most relevant defect caused by mechanical effect is ring shaped scratch due to exceeding contact pressure. In this study, however, we focused on ring shaped residue after post Cu CMP cleaning which is not caused by direct solid contact. To investigate root cause, analysis are carried out such as chemical composition of residue, surface charge map on a wafer, kinematic analysis on brush and wafer motion, and fluid dynamic simulation on chemical motion. In conclusion, it is revealed that both chemical and mechanical effects are interacted to form ring shaped residue. Elimination of defects by chemical effect is suppressed by mechanical motion of brush and chemical fluid. Figure 1

Inhibition Mechanism of Benzotriazole in Copper Chemical Mechanical Planarization

During the process of chemical mechanical planarization (CMP) of copper, benzotriazole (BTA) is the most commonly used inhibitor in the slurry. Though the corrosion inhibition mechanism has been studied widely, the mechanism of BTA layer on copper surface in CMP slurries should be further investigated. In this paper, the adsorption mechanisms of BTA were studied by static corrosion tests. Besides, the surface composition was measured by XPS. Combining with CMP experiments, the material removal mechanism of copper CMP depending on pH values was investigated. It was found that the formation of passive film, consisting of Cu-BTA complex, adsorption of BTA and copper oxides, played a dominant role under acidic conditions. While the surface film composed of adsorption layer of BTA and copper oxides under alkaline conditions. The inhibition mechanism of BTA varied with pH values, resulted in corresponding changes of material removal rate and coefficients of friction.

A new weakly alkaline slurry for copper planarization at a reduced down pressure

This study reports a new weakly alkaline slurry for copper chemical mechanical planarization (CMP), it can achieve a high planarization efficiency at a reduced down pressure of 1.0 psi. The slurry is studied through the polish rate, planarization, copper surface roughness and stability. The copper polishing experiment result shows that the polish rate can reach 10032 Å/min. From the multi-layers copper CMP test, a good result is obtained, that is a big step height (10870 Å) that can be eliminated in just 35 s, and the copper root mean square surface roughness (sq) is very low (< 1 nm). Apart from this, compared with the alkaline slurry researched before, it has a good progress on stability of copper polishing rate, stable for 12 h at least. All the results presented here are relevant for further developments in the area of copper CMP.

Effects of chemical mechanical planarization slurry additives on the agglomeration of alumina nanoparticles II: Aggregation rate analysis

The aggregation rate and mechanism of 150nm alumina particles in 1mM KNO3 with various additives used in chemical mechanical planarization of copper were investigated. The pH of each suspension was ∼8 such that the aggregation rate was slow enough to be measured and analyzed over ∼120min. In general, an initial exponential growth was observed for most suspensions indicating reaction-limited aggregation. After aggregate sizes increase to >500nm, the rate followed a power law suggesting diffusion-limited aggregation. Stability ratios and fractal dimension numbers were also calculated to further elucidate the aggregation mechanism.

Effects of CMP slurry additives on the agglomeration of alumina nanoparticles 1: General aggregation rate behavior

The aggregation behavior for 150nm alumina particles suspended in 1mM KNO3 solutions with various additives used in chemical mechanical planarization of copper was investigated. Three behaviors were observed: no aggregation, reversible aggregation where large agglomerates formed almost instantaneously, and steady aggregation where particle sizes grew over the duration of the measurement. In general steady aggregation occurred at high pH for all suspensions, while no aggregation occurred at acidic pH, except with suspensions with sodium dodecyl sulfate, where reversible aggregation was observed. No aggregation was observed at near neutral pH for all suspensions. Zeta potential and isoelectric points for each suspension were also measured.

Leakage Current Behavior of the Bottom Gate Poly Thin-Film Transistor Fabricated by Copper Damascene Process

Bottom-gated polycrystalline-silicon (poly-Si) thin-film transistors (TFT's) with a planarized copper (Cu) gate for large-area displays have been fabricated and characterized. The 500 nm depth of trenchs are filled up with 400 nm, 500 nm, 600 nm thickness of Cu using the damascene process of VLSI technology, poly-Si TFT's with 100 nm thick gate insulator are fabricated on the Cu gate. As the Cu gate's thickness becomes thinner, the anomalous leakage current of poly-Si TFT's is reduced significantly both before and after electrical stressing. The results simulated by 3D electric field simulator demonstrate that the structure of planarized gate in bottom-gate TFT can effectively reduce the electric field causing the field emission between the gate and the drain.

5-methyl-1H-benzotriazole as potential corrosion inhibitor for electrochemical-mechanical planarization of copper

According to the electrochemical analysis, the corrosion inhibition efficiency of 5-methyl-1H-benzotriazole (m-BTA) is higher than that of benzotrizaole (BTA). The inhibition capability of the m-BTA passive film formed in hydroxyethylidenediphosphonic acid (HEDP) electrolyte containing both m-BTA and chloride ions is superior to that formed in m-BTA-alone electrolyte, even at a high anodic potential. The results of electrical impedance spectroscopy, nano-scratch experiments and energy dispersive analysis of X-ray (EDAX) indicate that the enhancement of m-BTA inhibition capability may be due to the increasing thickness of passive film. Furthermore, X-ray photoelectron spectrometry (XPS) analysis indicates that the increase in passive film thickness can be attributed to the incorporation of Cl− into the m-BTA passive film and the formation of [Cu(I)Cl(m-BTA)]n polymer film on Cu surface. Therefore, the introduction of Cl− into m-BTA-containing HEDP electrolyte is effective to enhance the passivation capability of m-BTA passive film, thus extending the operating potential window.

Y2O3 nanosheets as slurry abrasives for chemical-mechanical planarization of copper

Abstract Continued reduction in feature dimension in integrated circuits demands high degree of flatness after chemical mechanical polishing. Here we report using new yttrium oxide (Y2O3) nanosheets as slurry abrasives for chemical-mechanical planarization (CMP) of copper. Results showed that the global planarization was improved by 30% using a slurry containing Y2O3 nanosheets in comparison with a standard industrial slurry. During CMP, the two-dimensional square shaped Y2O3 nanosheet is believed to induce the low friction, the better rheological performance, and the laminar flow leading to the decrease in the within-wafer-non-uniformity, surface roughness, as well as dishing. The application of the two-dimensional nanosheets as abrasive in CMP would increase the manufacturing yield of integrated circuits.

Electrochemical Mechanical Polishing of Copper with High Permittivity Abrasives

Electro-Chemical Mechanical Polishing (ECMP) is a promising low-pressure copper planarization method for the interconnection containing low-k material. ECMP utilizes anodic dissolution of copper to raise the material removal rate. However, the surface roughness is usually not acceptable in ECMP because the selective removal ability of electrochemical dissolution is poor. Different permittivity abrasives have different influence on surface quality in ECMP. High permittivity abrasives can enhance selective removal ability to get better polishing results through aggrandizing the potential drop in electrode/electrolyte interface. In this article, three abrasives with different permittivity are compared using polishing experiments. The results show that rutile TiO2, permittivity of which is larger than anatase TiO2 and SiO2, gets the best surface quality among the three abrasives. The composition of electrolyte with rutile TiO2 was also optimized to improve polishing performance. The chosen electrolyte contains 0.5 mol/L glycine, 0.1 mol/L TSA, 1 wt% rutile TiO2. The material removal rate got by using this electrolyte is 0.06 mg/min and the surface roughness is Ra8.9 nm.

Factors Influencing Hydroxyl Radical Formation in a Photo-Induced Confined Etching System

In this paper, we studied the formation of free OH on a TiO2nanotube array electrode in a photo-induced confined etching system. We used fluorescence spectroscopy, transient photocurrent response, electrochemical impedance spectroscopy(EIS), and Mott- Schottky analysis to investigate the influence of several key factors, including the applied potential, the illumination time, and the pH value. The highest efficiency for the photoelectrocatalytic formation of free OH on the TiO2nanotube array electrode was achieved at an applied potential of 1.0 V(vs a saturated calomel electrode(SCE)); the photoelectrocatalytic generation and consumption of free OH quickly approached a steady state in this system, as the confined etching layer formed by OH remained stable during illumination. This may allow good control of the etching precision during continuous etching processes. The highest efficiency for the photoelectrocatalytic formation of free OH on the TiO2nanotube array electrode was observed at pH 10. The results have an important significance for regulating and optimizing photo-induced confined etching system, which can be used to improve the etching speed or the leveling precision during the planarization of copper.

Effects of Alkaline Nano-SiO&lt;sub&gt;2&lt;/sub&gt; Abrasive on Planarization of 300mm Copper Patterned Wafer

Silica abrasive plays an important role in chemical mechanical planarization (CMP) of copper. In this paper, effect of different silica abrasive concentrations on copper removal rate and planarization performance of copper was investigated. The results show that the copper removal rate was increased as the concentration of silica abrasive increase. However, excessive abrasive will lead to a decreased copper removal rate. The initial step height values of the multilayer copper wafers were all about 2500Å, and after being polished for 30s, the remaining values of step height of slurry A, B, C and D were 717 Å, 906 Å, 1222 Å and 1493 Å. It indicates that alkaline copper slurries with different abrasive concentrations all had a good planarization performance on copper patterned wafer CMP. As the abrasive concentration increased, the planarization capability was enhanced.