Post-etch Residue Removal Research Articles

A Cleaning Method for Post-Etch Ruthenium Residue Removal Using UV and Liquid Chemical

As semiconductor devices continue to scale, it is important to evaluate alternative metals on narrower wiring or via structures. Ruthenium (Ru) is one promising candidate because of its lower resistivity compared to the conventional metals such as copper (Cu), cobalt (Co) and tungsten (W) on narrow space. To prevent leakage problems between metal layers caused by residues on the bottom and sidewalls after the metal patterning process, a cleaning process for Ru metal lines is necessary. Although the industry standard using Ammonia Peroxide Mixture (APM) is effective for removing residues, it was ineffective for Ru semi-damascene stack. Therefore, a new cleaning method involving UV treatment followed by APM, which was tested on metal pitch 18 nm patterned structure, was developed. This method showed promising results and is expected to be used in manufacturing of future semiconductor devices.

Wet Cleaning of Molybdenum for Nano Interconnects

Copper (Cu) has been used as the metal for interconnects for many years. Along with the continued scaling of semiconductor devices, especially beyond 10 nm nodes, alternative metals such as ruthenium (Ru) and molybdenum (Mo) have emerged as interesting candidates to replace Cu [1] at certain levels of the interconnect.Despite having higher bulk resistivities with respect to that of Cu, these alternative barrierless metals have lower resistivity at dimensions below ~12 nm. For instance, the resistivity of Mo was found to be lower than that of TaN/Cu/TaN stack of a total thickness below 8 nm [2]. In addition, besides W which is considered as a prime candidate as a first-generation buried power rail (BPR) metal [3,4], Mo has also been introduced as a potential candidate to achieve even lower BPR resistance.For both dual damascene and semi-damascene structures, typically, a wet cleaning step needs to be carried out prior to the next processing step, aiming at a complete removal of the residues generated during the dielectric patterning (dual damascene) or the metal patterning (semi-damascene). Therefore, the compatibility and etching behavior of the metals in chemical solutions must be well understood. In this work, the effect of various chemical solutions on the removal of post-etch residues and the etch characteristics of Mo is investigated.For the cleaning study, we first compared the etching rate on blanket Mo of two commodity chemicals (diluted HF of 0.05% and SC1) and formulated mixtures. The etching characteristics of the as-deposited and 420 °C-annealed Mo were similar for most chemical solutions. 0.05% HF and semi-aqueous alkaline mixtures were found to be highly compatible with Mo, while SC1 mixture significantly etched Mo. The high etching rate of Mo can be explained by the presence of H2O2 in the SC1 mixture, even at low concentration. The surface chemistry of the as-deposited Mo was compared with the surface after different treatments. Figure 1(a) shows the normalized XPS Mo 3d core-level spectra recorded on blanket Mo films after deposition, after Cl2-based plasma treatment and with subsequent wet cleaning. The narrow peak recorded at ~228.0 and 231.1 eV can be assigned to the Mo 3d5/2 and 3d3/2 components, respectively, of the metallic Mo. While the shoulder and broader peak recorded at higher binding energies, ~232.6 and 235.4 eV, are attributed to Mo oxides (3d5/2 and 3d3/2 components, respectively). It is clear that the surface became less oxidized after the plasma treatment. While the clean using 0.05% HF for 1 min did not show any change in terms of surface oxidation, the semi-aqueous mixture (FOTOPUR R-2403 formulation from BASF) clearly dissolved most of the Mo oxides present at the Mo surface. Since the XPS measurement was carried out ex-situ, this result could also be further explained by the dissolution of the surface Mo oxides combined with a better surface passivation provided by the formulated chemical vs. HF solution. In addition, these results also indicate that Mo surface is readily oxidized [5], and it needs to be protected after a dry etching or wet cleaning process. Figure 1(b) displays the atomic ratio of O 1s/Metallic Mo (measured from the Mo 3d level) for the surface described above, giving additional evidence of the low oxygen content of the Mo surface after being cleaned in the formulated chemical solution FOTOPUR R-2403.The assessment of post-etch residue removal was done on a semi-damascene structure (metal pitch = 32 nm), where Mo lines were formed by a non-optimized Cl2-based plasma etch. Figure 2 shows the TEM images obtained after the plasma etch, with the residues formed in between the lines (Figure 2(a)), and with subsequent cleaning using the semi-aqueous formulation FOTOPUR R-2403 from BASF (Figure 2(b)), showing efficient removal of the residues and good compatibility behavior with the top hard mask and the metal lines. The latter thus confirmed the etching rate results on blanket Mo films.Quantification of surface roughness and a study of the oxide growth on Mo surface will also be presented and discussed.

Wet Cleaning of Molybdenum for Nano Interconnects

The effect of various chemical solutions on the etch characteristics of molybdenum and on the removal of post-etch residues formed between Mo lines was investigated. In contrast to diluted SC1 mixtures, it was found that diluted HF solution (0.05%) and formulated semi-aqueous alkaline mixtures are highly compatible with Mo. The high etching rate measured for SC1 is attributed to the presence of the oxidizer in the mixture. Post-etch residue removal assessment performed on a semi-damascene structure with metal pitch of 32 nm showed that cleaning using diluted HF did not completely removal the residues. On the contrary, a clean using FOTOPUR R-2403, a semi-aqueous formulated mixture, resulted in efficient removal of the residues. The effect of oxygen in the plasma gas mixture and subsequent wet cleaning on the thickness of Mooxides was also performed on blanket Mo

Formation and Wet Removal of Organic and Titanium-Containing Residues on Patterned TiN/Porous Low-k Structure

The formation of the residues after patterning of a patterned metal hard mask/dielectric stack was characterized using surface-sensitive methods (attenuated total reflection-Fourier transform infrared spectroscopy and angle-resolved X-ray photoelectron spectroscopy) and scanning electron microscopy. The effect of a reducing plasma post-etch treatment (PET), aging, and moisture on the evolution of the residues was investigated. Both polymer-based (CFx) and metal-containing (TiFx) residues were identified on the exposed surface of the stack. Without the PET, the XPS data collected at different take-off angles suggested that the metal-containing residues were covered by a thin layer of polymer-based residues. The metal-containing residues were found to continue to evolve as a function of aging time if the layer of polymer-based residues was removed by the PET, making the metal hard mask in direct contact with cleanroom air. Exposing the MHM/dielectric stack to a 100% humidity environment did not induce further growing of the TiFx residues. Despite the PET only had limited removal of the TiFx-based residues, it was efficient in removing the polymer-based CFx residues, which in turn led to a better cleaning efficiency after the subsequent wet clean. The results in this study also confirmed that diluted HF dissolved TiFx-type residues and had no CFx removal capability. Although only two chemicals are shown as examples for post-etch residue removal, the type of test structure and methodology shown in this study can be applied for investigation and optimization of any other chemical mixture candidates for cleaning purpose.

Rapid Recovery Process of Plasma Damaged Porous Low-k Dielectrics by Wet Surface Modifying Treatment

A rapid repair process of plasma damaged SiCOH in combination with post-etch residue removal has been developed. The carbon depletion layer caused by plasma dry etching was repaired by subsequent surface modifying SAM treatment, which resulted in replenishment of carbon not only on the surface but also a few nm toward the bulk. This repairing technique provides a high-quality hydrophobic surface under conditions of low temperature and short process time. In addition, the SAM layer can be expected to act as an adhesion promotor with metal materials.

Characterization of Etch Residues Generated on Damascene Structures

For patterned TiN/silicon oxide/low-k dielectric stack, fluorinated etch residues were detected on the TiN surface, the dielectric sidewall and bottom, regardless of the low-k material used in the stack. XPS results showed that they consisted of polymer-based (CFx) residues deposited on trench sidewall and bottom, and metal-based (TiFx) residues mainly deposited on top surface. In terms of post-etch residue removal, the efficiency of various wet clean solutions can be clearly distinguished for CFx, and TiFx using the same patterned porous low-k stack. These results also demonstrate that the removal of both TiFx and CFx residues generated during the plasma is possible in one step with optimized chemical and process.

Development of a Cu and W Compatible PERR Clean in BEOL Advanced Interconnect Patterning

Low-k integration for 22 nm ½ pitch technology requires continuous development of compatible cleaning processes. In this paper the use of acidic (pH:0.5-3) and alkaline (pH:8-13.5) aqueous based chemistries are demonstrated for the removal of post-etch residues after low-k etch and the aim to remove the TiN HM. These chemistries are also compatible with W and Cu, respectively. At the same time, no decrease in CD was observed, which means that the exposed low-k material was not attacked by the cleaning processes used.

Development of a Cu and W Compatible PERR Clean in BEOL Advanced Interconnect Patterning

In back-end of line (BEOL), the fluorinated polymer deposited on dielectric sidewalls during patterning [1,2] must be removed prior to the subsequent processing steps to achieve good adhesion and coverage of materials (metals) deposited in the etched features. Recent results clearly showed the presence of a highly fluorinated layer deposited on the trench sidewalls during the plasma etch using a fluorocarbon based plasma [3]. The challenges related to the wet cleaning of such structures is the efficient removal of post-etch residues (PERR) by showing high selectivity and compatibility towards a variety of new materials exposed to the wet clean chemistry [4]. This study focuses on the use of acidic (pH:0.5-3) and alkaline (pH:8-13.5) aqueous based chemistries that are compatible with W and Cu, respectively. A schematic overview of a BEOL interconnect structure used for the N10 node (22 nm ½ pitch) is shown in Figure 1. The low-k material and hardmask that are used in this work/stack structure are respectively an OSG type of low-k material with a target value of k=2.4 (~20% open porosity) and TiN. Compatibility tests of different alkaline and acidic chemistries have been performed and proven to be compatible with the OSG 2.4 low-k dielectric material. The alkaline chemistries were compatible with Cu, while the acidic chemistries were compatible with W metal. The etch rate behavior of TiN was also checked. Spectroscopic ellipsometry, capacitance measurements (to measure k-value) and sheet resistance measurements were performed for this study. The test vehicle used to check the cleaning efficiency is a 45 nm ½ pitch BEOL trench short loop structure (Figure 2). A layer of low-k dielectric (80 nm) was first deposited on a 300 mm Si wafer, followed by the deposition of a 30 nm oxide HM and TiN layer (~25 nm), and a BARC/193-nm photoresist layer. The TiN layer was first etched using a HBr/Cl2 plasma, followed by the Oxide HM and OSG etch using a C4F8/CF4-based plasma (TEL Tactras). All dies are fully covered with the same type of 45 nm trench structures. A large choice of techniques was selected to characterize the cleaning efficiency of this test vehicle.One of the techniques used to investigate the cleaning of the CFx residue and TiN removal efficiency, is X-ray photoelectron spectroscopy (XPS). From figure 3, it is clear that the alkaline mixture removes almost completely the TiN within 2 min and at the same time the fluorine-containing residues have almost disappeared. The acidic formulated mixture removed TiFx (684.5 eV) completely while the CFx (688.3 eV) residues were only partially removed. A comparison of these acidic and alkaline formulated mixtures with citric acid and a TMAH:H2O2 mixture respectively, results in various cleaning efficiency but none of the commodity chemistries achieved complete removal of CFx and TiFx. The result obtained on a real device structure of 22.5 nm ½ pitch confirms the good performance obtained on the 45 nm ½ pitch BEOL structure. The SEM images shown in Figure 4 give an evidence that both Cu (M1 level) and the porous dielectrics are fully compatible with the chemistry. Electrical results using this alkaline chemistry, show a very good yield for CD lines, MF structure, and via chains will also be presented and discussed. [1] Y. Furukawa et al., Microelectron. Eng., 70, 267 (2003). [2] Q. T. Le et al., J. Electrochem. Soc. 159, H208 (2012). [3] T. Mukherjee et al., ECS Solid-St. Lett. 2, N11-N14 (2013). [4] E.Kesters et al.,Solid State Phenomena, 219, 201 (2015). Figure 1

Integration of Wet Cleaning in 45 Nm Pitch BEOL Processing

A partial-trench via first with self-aligned double patterning (SADP) approach is used to realize a 45 nm pitch using a dual damascene compatible hardmask. This back-end of line (BEOL) integration approach is used to investigate the ability to integrate advanced low-k materials and more specifically to assess their patterning performance. Throughout this SADP integration approach, different specific cleaning steps are required. A first cleaning process is needed after spacer etch and amorphous-carbon (a-C) strip to remove the spacer etch residues that are formed. Further in the integration flow, a TiN clean is needed together with a post-etch residue removal (PERR) after low-k etch.

UV-induced modification of fluorocarbon for improvement of subsequent wet removal

The effect of UV irradiation was studied for its potential application for post-etch residue removal in the back-end of line. A model fluorocarbon polymer (CFx) deposited by plasma polymerization of CF4/CH2F2 was used as test vehicle. This work mainly focused on investigating the effect of various UV exposure parameters including atmosphere, pressure, temperature, UV power density and dose on the magnitude of change of the polymer properties which in turn affected its cleanability by wet solution. A partial removal of CFx (x=1–3), CFCF2, and CO occurred during UV irradiation in oxygen. Increasing the UV dose and oxygen pressure resulted in a more dramatic removal of the polymer. In terms of material removal in the subsequent wet clean, experimental results also showed that the UV dose and the atmosphere in which the UV pre-treatment is carried out represent the most critical parameters. The presence of a minimum concentration of reactive species such as oxygen radicals during UV treatment was necessary to allow an efficient modification that made polymer fragments more hydrophilic and led to the removal of the polymer in the subsequent wet clean. Other parameters such as the power density of the UV source and temperature appear to play a minor role in polymer removal.

Effect of composition of post etch residues (PER) on their removal in choline chloride–malonic acid deep eutectic solvent (DES) system

In back end of line (BEOL) processing of copper/dielectric stacks, the composition of the post etch residues (PER) affects their removal by wet chemical formulations. For the removal of PER composed mainly of copper oxides and fluorides, deep eutectic solvent (DES) systems composed of choline chloride (CC) and urea (U) or choline chloride (CC) and malonic acid (MA) have been shown to be very effective. This paper will discuss the applicability of CC/MA system with and without the addition of glacial acetic acid (HAc) for the removal of residues that exhibit both inorganic and organic character. Different types of residues were created by CF4/O2 plasma etching of deep UV (DUV) photoresist (PR) films on Cu. By varying the time of etching, post-etch residues of varying inorganic and organic composition were generated and characterized using XPS. The effectiveness of CC/MA and HAc/CC/MA systems in removing the prepared post etch residues was characterized by SEM, XPS and single frequency impedance measurements. The results indicate that HAc/CC/MA system is very effective in removing different types of residues.

Removing W Polymer Residue from BEOL Structures Using DSP<sup>+</sup> (Dilute Sulfuric-Peroxide-HF) Mixture – A Case Study

As the feature size of semiconductor device shrinks continuously, various high-K metals for 3-D structures have been applied to improve the device performance, such as high speed and low power consumption. Metal gate fabrication requires the removal of metal and polymer residues after etching process without causing any undesired etching and corrosion of metals. The conventional sulfuric-peroxide mixture (SPM) has many disadvantages like the corrosion of metals, environmental issues etc., DSP+(dilute sulfuric-peroxide-HF mixture) chemical is currently used for the removal of post etch residues on device surface, to replace the conventional SPM cleaning [. Due to the increased usage of metal gate in devices in recent times, the application of DSP+chemicals for cleaning processes also increases [.

Wet Removal of Post-Etch Residues by a Combination of UV Irradiation and a SC1 Process

In back-end of line processing (BEOL), the polymer deposited on the dielectric sidewalls during the etch process must be removed prior to subsequent processing steps to achieve high adhesion and good coverage of materials deposited in the etched features [1, . Typically, this is done by a combination of a short plasma treatment and a diluted wet clean, or by wet cleans alone. On the one hand, for porous dielectric stacks, a mild plasma treatment that preserves the integrity of the low-k dielectrics would not be sufficient to effectively remove this residue. With regard to wet clean, diluted aqueous solutions (e.g. HF-based) are not efficient for polymer removal without etching the underlying dielectric to lift off the polymer, leading to unacceptable critical dimension (CD) loss. In addition, analytical techniques available for direct characterization of sidewall residues are limited. For a fast screening of potential chemistries capable of dissolving/removing polymer residues generated during the low-k etch, a model fluoropolymer was deposited on a blanket, checkerboard low-k substrate. The present study mainly focused on the characterization of model polymer after deposition (as-deposited) and after immersion in aqueous and solvent-based cleaning solutions. The polymer removal efficiency was influenced/ improved by UV treatments prior to wet clean processes. In the second part of the study, selected UV treatment conditions and cleaning solutions were applied to low-k patterned structures using Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) to characterize the dielectric sidewall before and after UV modification and the subsequent cleaning process.

Effect of Water Addition to Choline Chloride–Urea Deep Eutectic Solvent (DES) on the Removal of Post-Etch Resid ues Formed on Copper

Feasibility of dissolution of post-etch residues (PERs) formed on copper in formulations containing a mixture of urea (U)-choline chloride (CC) deep eutectic solvent (DES) and water (W) has been investigated. PER films were formed on copper surface by spin coating deep ultraviolet photoresist film, followed by CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma etching. The residue removal process was characterized using X-ray photoelectron spectroscopy and scanning electron microscopy techniques. Effective removal of PER was obtained in water-DES solutions containing as high as 90% water in the temperature range of 20-40 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. Additionally, the etch rates of copper and siloxane-based low-k dielectric material in water-DES solutions were found to be lower than that in deaerated 250:1 [H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O: hydrofluoric acid (HF) (49%) volume ratio] dilute HF solutions.

Post-etch residue removal using choline chloride–malonic acid deep eutectic solvent (DES)

Eutectic mixture of choline chloride (CC) and malonic acid (MA) in a molar ratio of 1:1 has been evaluated as a potential chemical system for the removal of residues produced by CF4/O2 plasma etching of copper coated with DUV photoresist. Immersion cleaning was performed in the liquid at the eutectic composition at 40 and 70 °C. Residue removal rate was screened using scanning electron microscopy and verified using X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy measurements. The results presented in this paper show that choline chloride–malonic acid eutectic is effective in removing post-etch residues and has the potential to function as a back end of line cleaning formulation.

Wafer Surface Charging Model for Single-Wafer Wet-Spin Processes

Wet chemical processes in integrated circuit (IC) manufacturing are used in many applications, e.g., post-etch residue removal and pre-deposition surface treatment. While advanced single-wafer wet spin tools are part of the critical tool-set for advanced IC fabrication, non-optimized tool hardware and/or process may induce different types of wafer surface charging issues. In this paper, a physical model to fundamentally explain surface charging induced by a single-wafer wet spin tool is described. The model is based on the advection of surface charges from wafer-center to wafer-edge resulting from the shear flow of the liquid. The charge distribution in the diffuse layer adjacent the wafer surface is calculated by solving the coupled Poisson's and current continuity equations. As often practiced in the industry in characterizing this type of wafer surface charging, a thermally grown silicon dioxide surface is used as the model surface and de-ionized water as the liquid medium. Good agreement is obtained between experimental and calculated surface charging potentials for radial positions extending from wafer-center to approximately 130 mm on standard 300 mm diameter wafers. The observed charging potential trends with respect to radial position, wet process time, and wafer spin speed are well explained by the current model.

Prevention of AlCu Line Galvanic Corrosion after Fluoride Containing Stripper Cleaning: A Case Study

In this paper, the status of galvanic corrosion on AlCu lines during the AlCu post etch and ashing residue removal process is studied with a fluoride containing stripper in details, including DIW rinse condition and dry process. The experimental data show that galvanic corrosion on AlCu lines is not observed in the chemical cleaning process, but inclined to be caused by the rinse and dry processes in this case. Different scenarios in the rinse process, such as overflow or quick dump rinse (QDR), the times of quick dump rinse cycles, CO2 bubbled DIW and metal protection solution rinse, are studied. At the same time, two kinds of dry processes are also evaluated. The results show the optimized rinse and dry processes can eliminate the galvanic corrosion on AlCu lines without the change of wafer manufacture prevenient process and cleaning solution.

Use of Urea-Choline Chloride Eutectic Solvent for Back End of Line Cleaning Applications

A liquid mixture formed from two benign chemicals, urea and choline chloride, in the molar ratio of 2:1 has been evaluated for post etch residue (PER) removal from copper surface. Residue films were prepared by etching DUV photoresist films coated on copper in CF4/O2 plasma. Film removal was evaluated under immersion cleaning conditions using X-ray photoelectron spectroscopy and scanning electron microscopy and verified using electrochemical impedance spectroscopy measurements. The residue film was effectively removed in the temperature range of 40–70°C. The results of this study show that choline chloride and urea based DES system has the potential to function as a back end of line cleaning formulation.

Using a Novel Solvent-based Technology to Preserve Low-k Material when Removing Gap Fill Material

As the technology nodes shrink, device makers are running into barriers with traditional post etch process flows. In the back-end-of-line (BEOL), the porous low-k materials can be damaged by the etch chemistries, the dry ash plasmas and the subsequent cleaning processes. Replacing the dry ash and typical wet cleaning processes with a one step selective solvent-based removal process can work to protect the etched low-k. The solvent-based blend must remove the reactive ion etch (RIE) residues and gap fill material used in the patterning step while preserving the low-κ from further damage. This paper will focus on the removal of films and post etch residues in under 2 minutes with a novel solvent formulation.

Screening and Evaluation of Different Wet Cleaning Solutions for Post Etch Residue Removal in BEOL Applications

Two types of wet cleaning solutions were screened for post etch residue removal in back end of line applications: aqueous acid chemistries and organic solvents. Their compatibility with plasma damaged low-k materials and their polymer removal efficiency were checked on blanket low-k layers and single damascene layers, respectively. Aqueous acid chemistries under study showed either to be compatible with plasma damaged low k material but had poor polymer removal efficiency, or showed good polymer removal but also removal of the plasma damaged low-k. A correlation between the damaged low-k and the amount of etching additive in aqueous acid chemistry could be established. Initial tests with organic solvents demonstrated that they were compatible with the damage low-k layer. Concerning polymer removal, some partial dissolution of polymers was observed for the first time. Furthermore, the cleaning performance of organic solvents, and hence the electrical performance, can be improved by applying megasonic power during the clean process.