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Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping
MNE-00002; No of Pages 4 Micro and Nano Engineering xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Micro and Nano Engineering journal homepage: www.elsevier.com/locate/tsf
Research paper
Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping Adeline Lallart a,b,c,d,⁎, Philippe Garnier a, Elise Lorenceau b, Alain Cartellier c, Elisabeth Charlaix b a
STMicroelectronics, 850 Rue Jean Monnet, 38926 Crolles, Cedex, France Univ. Grenoble Alpes, CNRS, LIPhy, F-38000 Grenoble, France Université Grenoble Alpes, CNRS, Grenoble INP, LEGI, F-38000 Grenoble, France 1 d CEA-LETI, MINATEC Campus, 17 Rue des Martyrs, 38054 Grenoble, France b c
a r t i c l e
i n f o
Article history: Received 31 May 2018 Received in revised form 27 August 2018 Accepted 30 September 2018 Available online xxxx Keywords: Polymer Particle removal efficiency Contamination Particle size Aging
a b s t r a c t The removal of nanometric particles constitutes one of the main challenges for the Integrated Circuits manufacturing. A solution based on the polymer coating and removal without substrate consumption is described and its performances are evaluated. In our experiments, 60 nm SiO2 particles and 40–200 nm Si3N4 particles are used to contaminate Si wafers. Two polymer removal methods are compared, one is purely based on a chemical action while in the other one a chemical and a physical actions are coupled. We demonstrate that a physical action is required to remove particles. The process shows high Particle Removal Efficiency (PRE) up to 87% independently of the particle size and nature. The PRE stays at a constant value, around 85%, for 3 decades of aging time, but the particle removal is not uniform on all the wafer. © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction In the last decades, it has been observed that the number of transistors in integrated circuits has doubled every two years. This law, known as Moore's Law, places enormous constraints on manufacturing processes. For example, it requires us to invent new manufacturing processes but also to ensure more and more critical control over the quality of the surrounding material. Indeed, parasitic particles, emitted from air or manufacturing equipment can deteriorate the performance of integrated microcircuits leading to short circuit or micro masking issues. With the drastic miniaturization of microcircuits, these critical sizes get smaller and smaller. Thus, to continue to achieve high yield various solutions are used to remove particles from microelectronic chips. A common method to remove particles is based on the use of SC1 (NH4OH:H2O2:H2O) chemistry. When put into contact with the microelectronic chip, this chemistry will etch the substrate and/or the particle, leading to a detachment of the particle from the surface and its removal thanks to favorable pH conditions. However, by chemically etching the surfaces of the microelectronic circuit, this method degrades the resolution of the patterns, thus limiting the miniaturization of the circuits [1]. Other methods have been proposed such as acoustic methods [2], spray impact [3] or cryogenic method [4]. However, all of ⁎ Corresponding author. E-mail address: [email protected] (A. Lallart). 1 Institute of Engineering Univ. Grenoble Alpes
them present some limitations, either due to the restricted range of particle that can be cleaned off or due to pattern damages. A new promising method using polymer layer, which has been barely investigated so far, circumvent these limitations. It consists in two steps: first a thin polymer layer, which embeds the particles within it, is spin-coated on the surface. The polymer layer - and the embedded particles - is then removed from the surface either by siphoning it if it is liquid or by peeling it if it is solid [5–7]. In this work, we propose a new way to remove this polymer by chemical dissolution either in a wet bench or by spray and spin drying the chemistry assisted by wafer rotation. In the following, we describe our experimental protocol and discuss whether applying a physical action during the chemical removal process via the impact of spray droplets and centrifugal forces enhance the efficiency of the whole process or not. 2. Material and methods The different steps of our experimental protocol are described on Fig. 1a). We first artificially contaminate 300 mm bare chemically oxidized silicon wafers by spin-drying. To do so, we used deionized water solution containing either monodisperse SiO2 particles with a size of 61.7 nm + −1.6 nm or polydisperse Si3N4 particles ranging from 40 nm up to 200 nm. The initial particles count on the wafer is comprised between 40,000 and 60,000. After this contamination step, a polymer layer with a thickness of 1.8 μm is spin coated on the wafer. Then, this layer is baked during 90 s at 130 °C, which is in the order of
https://doi.org/10.1016/j.mne.2018.09.001 2590-0072/© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Please cite this article as: A. Lallart, et al., Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping, (2018), https://doi.org/10.1016/j.mne.2018.09.001
2
A. Lallart et al. / Micro and Nano Engineering xxx (xxxx) xxx–xxx
Fig. 1. a) Process and time description b) Sketch of the geometry of the spray batch equipment in top view.
the glass transition temperature (Tg) of the polymer. The time lapse between the contamination step and the spin coating of the polymer layer, noted τpc, has been varied between 11 h and 19 days. To remove the polymer layer, two methods have been used. First, the wafer is immersed in different tanks containing the chemistry. A tank containing Sulfuric Peroxide Mixture SPM (H2SO4:H2O2) is first used to dissolve the polymer layer associate to a second immersion in a SC1 tank. The second method uses a spray batch equipment to dispense the chemistries. In this case, the contaminated wafers are put on rotation at 80 RPM around a central spray dispenser, which is located 25 cm away from the center of the wafer as shown in Fig. 1b). The SPM and the SC1 chemistries are successively dispensed by a central spray horizontally flowing droplets obtained from the co-flow of nitrogen at a flow rate of 4 L/min and liquid chemistry at 1 L/min. Then, the wafers are finally water rinsed and dried at a rotation of 300 RPM. The time between the polymer coating and the polymer removal (τpr) is between 6 h and 42 days, leading to a queue time (τaging) between the contamination and the polymer removal, corresponding to the addition of the times τpc and τpr, between 17 h and 57 days. The particles on the wafer are counted
after the initial contamination (pre particle counts) and after the complete polymer removal (post particle counts) thanks to a laser diffraction spectrometer (Surfscan SP3 KLA Tencor) with a defect detection ranging between 30 nm up to 2 μm. The efficiency of the method is quantified using the particle removal efficiency (PRE), defined as follow: PRE ð%Þ ¼
initial particles amount−final particles amount 100 initial particles amount
ð1Þ
3. Results To quantitatively compare the methods used to remove the polymer layer, we map the particle distribution on the wafer along with the particle removal efficiency as defined in Eq. (1), in Fig. 2 right after the initial contamination (a), after the wet bench removal step (b) or after the spray batch removal step (d). To quantify the influence of the spray impact on the process, we also provide these two items when the spray
Fig. 2. Particles mapping and PRE after the different processes a) after the initial contamination b) after a polymer coating and the wet bench process c) after spray batch process d) after polymer coating and the spray batch process.
Please cite this article as: A. Lallart, et al., Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping, (2018), https://doi.org/10.1016/j.mne.2018.09.001
A. Lallart et al. / Micro and Nano Engineering xxx (xxxx) xxx–xxx
batch removal step has been carried out without any polymer coating (c). 3.1. Comparison of the removal efficiency using the wet bench and the spray batch process When a chemical action is solely used, like in the wet bench method, no particles are removed. Thus, both the physical action of the rotation and the spray is mandatory in order to withdraw particles from the surface. 3.2. Influence of the polymer presence on the particle removal efficiency using the spray batch equipment We also observe that the presence of the polymer layer is mandatory to obtain high value of PRE. Indeed, when using the spray batch equipment on contaminated wafers without the polymer layer coating (Fig. 2c)), no particles are removed whereas with the polymer layer (Fig. 2d)) a removal efficiency up to 87% is achieved. This ensures that physical action applied by the impact of the spray droplet associated with the fast rotation of the wafer does not participate by itself in the cleaning process. 3.3. Impact of the polymer removal method on the particle removal efficiency When comparing Fig. 2b) and d), we observe the following features: first, when the polymer is chemically removed using the wet bench equipment, so without any physical action, no particles are removed from the surface. Then, when the polymer is removed thanks to a spray batch method which combines chemical and physical actions (via the spray dispense and the wafer rotation) we obtain that 87% of the particles initially present on the surface are withdrawn. Another observation is that the particle removal is not uniform. Denser zones are observed near the central spray dispenser and along straight lines. To understand the origin of these straight lines, we determine their intersection point from images such as the one of Fig. 2d) and find that they intersect at the position of the central spray dispenser which is also the rotation center of the wafer. This suggests that the rotation of the wafer contributes to the particle removal through centrifugal forces. 3.4. Performances of the process, aging time and contamination influence on the particle removal efficiency In the classical cleaning methods, the process performances can vary with different parameters such as the aging time between the contamination and the cleaning process [8,9] or like the contamination nature [8]. Thus, to gain quantitative comparison between the different processes, we measure the PRE for various aging time, in Fig. 3. As can be seen, the particle removal efficiency is constant around 85%, for 3
3
decades of aging time. Moreover, we also varied the time between the contamination and the polymer coating (τpc) and the time between the polymer coating and the polymer removal (τpr) and didn't observe any variation. We also report in Fig. 4, PRE for different nature and size of the particles. Here also, the PRE exhibits a plateau at 85% + −5%. 4. Discussion To understand these results, we recall that when a solid particle is deposited over a solid surface in the presence of humidity, the adhesion force between the particle and the solid surface, which is of capillary origin, varies as a function of time τaging as written below [10]: F adhesion τaging ¼ γd
τ 1 ln aging P sat τ0 ln Pv
ð2Þ
where γ is water/air surface tension, d is a distance taking into account the geometrical characteristics of the contact and is proportional to the particle radius, Psat is the saturated water pressure, Pv the water vapor pressure and τ0 order of time needed to condense one liquid layer. This time dependence of the adhesion force can be explained considering either that the contact area of the particles with the surface increases over time due to the plastic deformation of the beads or the formation of silica bridges between the surface and the particle due to water action. Eq. (2), which has been used successfully in several different experimental configurations, suggests that the longer the time between contamination and cleaning, the more firmly attached the particles and the less efficient the cleaning process. In addition, the adhesion force of eq. (2) depends on the particle: the higher the adhesion energy (via γ), the greater the adhesion force. Fig. 3 suggests, however, that PRE is neither dependent of aging time nor of the particle nature despite the fact that the adhesion energy on SiO2 of Si3N4 particles is higher than the one of SiO2 particles. To understand these two surprising features, we assume that the particles are bounded to the polymer film due to the large surface they exhibit at their contact. Thus, when the film detaches from the surface, the particles follow it and also detaches from the surface. This mechanism explains why the PRE we measure is neither influenced by the aging time nor by the particle size or nature. To understand under which conditions the film can detach from the surface, we use the theoretical framework used to describe rupture and delamination of thin films deposited over a solid surface. When a thin film (such as our polymer film) deposited over a solid substrate is put into tension, the general principle is that the crack propagates if the elastic energy released during the propagation exceeds the crack energy. However, in that case, the film fracture and delaminates in a limited volume defined by Lh2 where L is the length of the fracture and h the polymer film (in the order of 2 μm) [11]. This would only induce delamination in a very
Fig. 3. Influence of the aging time on the particle removal efficiency.
Please cite this article as: A. Lallart, et al., Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping, (2018), https://doi.org/10.1016/j.mne.2018.09.001
4
A. Lallart et al. / Micro and Nano Engineering xxx (xxxx) xxx–xxx
This method is still under study and need improvement. Indeed, new chemical solutions for polymer removal are required for its compatibility with all kind of substrates. The polymer and the chemical solution used for its removal need to be environmental friendly and finally the efficiency of this method should be demonstrated at an industrial scale. Conflict of Interest I confirm that there are no known conflicts of interest associated with this publication Acknowledgement The experiments were performed in the frame of the joint development program with STMicroelectronics, “Laboratoire Interdisciplinaire de Physique (LIPhy)”, “Laboratoire des Ecoulements Géophysique et Industriels (LEGI)” and the “CEA LETI”. Fig. 4. Influence of the particle nature and size on the removal efficiency.
References limited volume. However, Marthelot et al. [12] recently highlighted a novel fracture mechanism of thin films where delamination and propagation occur simultaneously. In that case, the mechanism is active below the standard critical tensile load for channel cracks and selects a robust interaction length scale on the order of 30 times the film, which spontaneously replicates and propagates. In our case, the centrifugal force and the additional stress to which the polymer layer is subjected due to solvent addition, would be sufficient to induce this spontaneous delamination. The inhomogeneity in PRE observed in 2d) also lies in this picture. The polymer located far from the point around which the polymer rotates will be subjected to a higher tension due to centrifugal forces and thus more prone to crack and delaminate. 5. Conclusion Miniaturization in the microelectronic field lead to new challenges concerning particles removal. The cleaning method based on the use of a polymer coating and removal open new trails with high removal efficiency (87%) whatever the aging time, particle nature or size. This method, which combines a physical and a chemical action to remove nanoparticles is thus very efficient, yet the removal is still non-uniform.
[1] F. Tardif, A. Danel, O. Raccurt, Understanding of wet and alternative particle removal processes in microelectronics: theoretical capabilities and limitations, J. Telecommun. Inf. Technol. 1 (2005) 8. [2] W. Kim, T.H. Kim, J. Choi, H.-Y. Kim, Mechanism of particle removal by megasonic waves, Appl. Phys. Lett. 94 (2009) 081908. [3] J. T. Snow, K. Miya, M. Sato, M. Kato, and T. Tanaka, “Advances in Particle Removal without Damage,” 2015. [4] N. Narayanswami, R. Analytics, K.C. Optomec, S. Carolina, Evaluation of Particle Removal Efficiency in Wafer cleaning Processes, Semicond. Int. Mag. (January) (2000). [5] P. Lin, S. Pi, H. Jiang, Q. Xia, Mold cleaning with polydimethylsiloxane ( PDMS ) for nanoimprint lithography, Nanotechnology 24 (32) (2013). [6] H.F. Okorn-Schmidt, et al., Particle cleaning Technologies to Meet Advanced Semiconductor device Process Requirements, ECS J. Solid State Sci. Technol. 3 (1) (2013) N3069–N3080. [7] T. W. Walker and C. W. Frank, “Enhanced particle removal using viscoelastic fluids,” no. May 2015, 2013. [8] C.-C. Chiang, B. Wu, S. Raghavan, Particle Deposition and Removal of Relevance to Wet Processing in Semiconductor Manufacturing, Part. Sci. Technol. 33 (5) (2015) 150706133355006. [9] J. Tang, A.A. Busnaina, The effect of Time and Humidity on Particle Adhesion and Removal, J. Adhes. 74 (2000) 411–419. [10] J. Bocquet, L. Charlaix, E. Ciliberto, C. Crassous, Moisture-induced ageing in granular media and the kinetics of capillary condensation, Nature 396 (1998) 735–737. [11] J. Marthelot, Rupture et délamination de films minces, Université Pierre et Marie Curie - Paris VI, 2014. [12] J. Marthelot, B. Roman, J. Bico, J. Teisseire, D. Dalmas, F. Melo, Self-replicating cracks: a collaborative fracture mode in thin films, Phys. Rev. Lett. 113 (8) (2014) 22–26.
Please cite this article as: A. Lallart, et al., Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping, (2018), https://doi.org/10.1016/j.mne.2018.09.001
Contents lists available at ScienceDirect
Micro and Nano Engineering journal homepage: www.elsevier.com/locate/tsf
Research paper
Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping Adeline Lallart a,b,c,d,⁎, Philippe Garnier a, Elise Lorenceau b, Alain Cartellier c, Elisabeth Charlaix b a
STMicroelectronics, 850 Rue Jean Monnet, 38926 Crolles, Cedex, France Univ. Grenoble Alpes, CNRS, LIPhy, F-38000 Grenoble, France Université Grenoble Alpes, CNRS, Grenoble INP, LEGI, F-38000 Grenoble, France 1 d CEA-LETI, MINATEC Campus, 17 Rue des Martyrs, 38054 Grenoble, France b c
a r t i c l e
i n f o
Article history: Received 31 May 2018 Received in revised form 27 August 2018 Accepted 30 September 2018 Available online xxxx Keywords: Polymer Particle removal efficiency Contamination Particle size Aging
a b s t r a c t The removal of nanometric particles constitutes one of the main challenges for the Integrated Circuits manufacturing. A solution based on the polymer coating and removal without substrate consumption is described and its performances are evaluated. In our experiments, 60 nm SiO2 particles and 40–200 nm Si3N4 particles are used to contaminate Si wafers. Two polymer removal methods are compared, one is purely based on a chemical action while in the other one a chemical and a physical actions are coupled. We demonstrate that a physical action is required to remove particles. The process shows high Particle Removal Efficiency (PRE) up to 87% independently of the particle size and nature. The PRE stays at a constant value, around 85%, for 3 decades of aging time, but the particle removal is not uniform on all the wafer. © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction In the last decades, it has been observed that the number of transistors in integrated circuits has doubled every two years. This law, known as Moore's Law, places enormous constraints on manufacturing processes. For example, it requires us to invent new manufacturing processes but also to ensure more and more critical control over the quality of the surrounding material. Indeed, parasitic particles, emitted from air or manufacturing equipment can deteriorate the performance of integrated microcircuits leading to short circuit or micro masking issues. With the drastic miniaturization of microcircuits, these critical sizes get smaller and smaller. Thus, to continue to achieve high yield various solutions are used to remove particles from microelectronic chips. A common method to remove particles is based on the use of SC1 (NH4OH:H2O2:H2O) chemistry. When put into contact with the microelectronic chip, this chemistry will etch the substrate and/or the particle, leading to a detachment of the particle from the surface and its removal thanks to favorable pH conditions. However, by chemically etching the surfaces of the microelectronic circuit, this method degrades the resolution of the patterns, thus limiting the miniaturization of the circuits [1]. Other methods have been proposed such as acoustic methods [2], spray impact [3] or cryogenic method [4]. However, all of ⁎ Corresponding author. E-mail address: [email protected] (A. Lallart). 1 Institute of Engineering Univ. Grenoble Alpes
them present some limitations, either due to the restricted range of particle that can be cleaned off or due to pattern damages. A new promising method using polymer layer, which has been barely investigated so far, circumvent these limitations. It consists in two steps: first a thin polymer layer, which embeds the particles within it, is spin-coated on the surface. The polymer layer - and the embedded particles - is then removed from the surface either by siphoning it if it is liquid or by peeling it if it is solid [5–7]. In this work, we propose a new way to remove this polymer by chemical dissolution either in a wet bench or by spray and spin drying the chemistry assisted by wafer rotation. In the following, we describe our experimental protocol and discuss whether applying a physical action during the chemical removal process via the impact of spray droplets and centrifugal forces enhance the efficiency of the whole process or not. 2. Material and methods The different steps of our experimental protocol are described on Fig. 1a). We first artificially contaminate 300 mm bare chemically oxidized silicon wafers by spin-drying. To do so, we used deionized water solution containing either monodisperse SiO2 particles with a size of 61.7 nm + −1.6 nm or polydisperse Si3N4 particles ranging from 40 nm up to 200 nm. The initial particles count on the wafer is comprised between 40,000 and 60,000. After this contamination step, a polymer layer with a thickness of 1.8 μm is spin coated on the wafer. Then, this layer is baked during 90 s at 130 °C, which is in the order of
https://doi.org/10.1016/j.mne.2018.09.001 2590-0072/© 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Please cite this article as: A. Lallart, et al., Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping, (2018), https://doi.org/10.1016/j.mne.2018.09.001
2
A. Lallart et al. / Micro and Nano Engineering xxx (xxxx) xxx–xxx
Fig. 1. a) Process and time description b) Sketch of the geometry of the spray batch equipment in top view.
the glass transition temperature (Tg) of the polymer. The time lapse between the contamination step and the spin coating of the polymer layer, noted τpc, has been varied between 11 h and 19 days. To remove the polymer layer, two methods have been used. First, the wafer is immersed in different tanks containing the chemistry. A tank containing Sulfuric Peroxide Mixture SPM (H2SO4:H2O2) is first used to dissolve the polymer layer associate to a second immersion in a SC1 tank. The second method uses a spray batch equipment to dispense the chemistries. In this case, the contaminated wafers are put on rotation at 80 RPM around a central spray dispenser, which is located 25 cm away from the center of the wafer as shown in Fig. 1b). The SPM and the SC1 chemistries are successively dispensed by a central spray horizontally flowing droplets obtained from the co-flow of nitrogen at a flow rate of 4 L/min and liquid chemistry at 1 L/min. Then, the wafers are finally water rinsed and dried at a rotation of 300 RPM. The time between the polymer coating and the polymer removal (τpr) is between 6 h and 42 days, leading to a queue time (τaging) between the contamination and the polymer removal, corresponding to the addition of the times τpc and τpr, between 17 h and 57 days. The particles on the wafer are counted
after the initial contamination (pre particle counts) and after the complete polymer removal (post particle counts) thanks to a laser diffraction spectrometer (Surfscan SP3 KLA Tencor) with a defect detection ranging between 30 nm up to 2 μm. The efficiency of the method is quantified using the particle removal efficiency (PRE), defined as follow: PRE ð%Þ ¼
initial particles amount−final particles amount 100 initial particles amount
ð1Þ
3. Results To quantitatively compare the methods used to remove the polymer layer, we map the particle distribution on the wafer along with the particle removal efficiency as defined in Eq. (1), in Fig. 2 right after the initial contamination (a), after the wet bench removal step (b) or after the spray batch removal step (d). To quantify the influence of the spray impact on the process, we also provide these two items when the spray
Fig. 2. Particles mapping and PRE after the different processes a) after the initial contamination b) after a polymer coating and the wet bench process c) after spray batch process d) after polymer coating and the spray batch process.
Please cite this article as: A. Lallart, et al., Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping, (2018), https://doi.org/10.1016/j.mne.2018.09.001
A. Lallart et al. / Micro and Nano Engineering xxx (xxxx) xxx–xxx
batch removal step has been carried out without any polymer coating (c). 3.1. Comparison of the removal efficiency using the wet bench and the spray batch process When a chemical action is solely used, like in the wet bench method, no particles are removed. Thus, both the physical action of the rotation and the spray is mandatory in order to withdraw particles from the surface. 3.2. Influence of the polymer presence on the particle removal efficiency using the spray batch equipment We also observe that the presence of the polymer layer is mandatory to obtain high value of PRE. Indeed, when using the spray batch equipment on contaminated wafers without the polymer layer coating (Fig. 2c)), no particles are removed whereas with the polymer layer (Fig. 2d)) a removal efficiency up to 87% is achieved. This ensures that physical action applied by the impact of the spray droplet associated with the fast rotation of the wafer does not participate by itself in the cleaning process. 3.3. Impact of the polymer removal method on the particle removal efficiency When comparing Fig. 2b) and d), we observe the following features: first, when the polymer is chemically removed using the wet bench equipment, so without any physical action, no particles are removed from the surface. Then, when the polymer is removed thanks to a spray batch method which combines chemical and physical actions (via the spray dispense and the wafer rotation) we obtain that 87% of the particles initially present on the surface are withdrawn. Another observation is that the particle removal is not uniform. Denser zones are observed near the central spray dispenser and along straight lines. To understand the origin of these straight lines, we determine their intersection point from images such as the one of Fig. 2d) and find that they intersect at the position of the central spray dispenser which is also the rotation center of the wafer. This suggests that the rotation of the wafer contributes to the particle removal through centrifugal forces. 3.4. Performances of the process, aging time and contamination influence on the particle removal efficiency In the classical cleaning methods, the process performances can vary with different parameters such as the aging time between the contamination and the cleaning process [8,9] or like the contamination nature [8]. Thus, to gain quantitative comparison between the different processes, we measure the PRE for various aging time, in Fig. 3. As can be seen, the particle removal efficiency is constant around 85%, for 3
3
decades of aging time. Moreover, we also varied the time between the contamination and the polymer coating (τpc) and the time between the polymer coating and the polymer removal (τpr) and didn't observe any variation. We also report in Fig. 4, PRE for different nature and size of the particles. Here also, the PRE exhibits a plateau at 85% + −5%. 4. Discussion To understand these results, we recall that when a solid particle is deposited over a solid surface in the presence of humidity, the adhesion force between the particle and the solid surface, which is of capillary origin, varies as a function of time τaging as written below [10]: F adhesion τaging ¼ γd
τ 1 ln aging P sat τ0 ln Pv
ð2Þ
where γ is water/air surface tension, d is a distance taking into account the geometrical characteristics of the contact and is proportional to the particle radius, Psat is the saturated water pressure, Pv the water vapor pressure and τ0 order of time needed to condense one liquid layer. This time dependence of the adhesion force can be explained considering either that the contact area of the particles with the surface increases over time due to the plastic deformation of the beads or the formation of silica bridges between the surface and the particle due to water action. Eq. (2), which has been used successfully in several different experimental configurations, suggests that the longer the time between contamination and cleaning, the more firmly attached the particles and the less efficient the cleaning process. In addition, the adhesion force of eq. (2) depends on the particle: the higher the adhesion energy (via γ), the greater the adhesion force. Fig. 3 suggests, however, that PRE is neither dependent of aging time nor of the particle nature despite the fact that the adhesion energy on SiO2 of Si3N4 particles is higher than the one of SiO2 particles. To understand these two surprising features, we assume that the particles are bounded to the polymer film due to the large surface they exhibit at their contact. Thus, when the film detaches from the surface, the particles follow it and also detaches from the surface. This mechanism explains why the PRE we measure is neither influenced by the aging time nor by the particle size or nature. To understand under which conditions the film can detach from the surface, we use the theoretical framework used to describe rupture and delamination of thin films deposited over a solid surface. When a thin film (such as our polymer film) deposited over a solid substrate is put into tension, the general principle is that the crack propagates if the elastic energy released during the propagation exceeds the crack energy. However, in that case, the film fracture and delaminates in a limited volume defined by Lh2 where L is the length of the fracture and h the polymer film (in the order of 2 μm) [11]. This would only induce delamination in a very
Fig. 3. Influence of the aging time on the particle removal efficiency.
Please cite this article as: A. Lallart, et al., Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping, (2018), https://doi.org/10.1016/j.mne.2018.09.001
4
A. Lallart et al. / Micro and Nano Engineering xxx (xxxx) xxx–xxx
This method is still under study and need improvement. Indeed, new chemical solutions for polymer removal are required for its compatibility with all kind of substrates. The polymer and the chemical solution used for its removal need to be environmental friendly and finally the efficiency of this method should be demonstrated at an industrial scale. Conflict of Interest I confirm that there are no known conflicts of interest associated with this publication Acknowledgement The experiments were performed in the frame of the joint development program with STMicroelectronics, “Laboratoire Interdisciplinaire de Physique (LIPhy)”, “Laboratoire des Ecoulements Géophysique et Industriels (LEGI)” and the “CEA LETI”. Fig. 4. Influence of the particle nature and size on the removal efficiency.
References limited volume. However, Marthelot et al. [12] recently highlighted a novel fracture mechanism of thin films where delamination and propagation occur simultaneously. In that case, the mechanism is active below the standard critical tensile load for channel cracks and selects a robust interaction length scale on the order of 30 times the film, which spontaneously replicates and propagates. In our case, the centrifugal force and the additional stress to which the polymer layer is subjected due to solvent addition, would be sufficient to induce this spontaneous delamination. The inhomogeneity in PRE observed in 2d) also lies in this picture. The polymer located far from the point around which the polymer rotates will be subjected to a higher tension due to centrifugal forces and thus more prone to crack and delaminate. 5. Conclusion Miniaturization in the microelectronic field lead to new challenges concerning particles removal. The cleaning method based on the use of a polymer coating and removal open new trails with high removal efficiency (87%) whatever the aging time, particle nature or size. This method, which combines a physical and a chemical action to remove nanoparticles is thus very efficient, yet the removal is still non-uniform.
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Please cite this article as: A. Lallart, et al., Cleaning surfaces from nanoparticles with polymer film: impact of the polymer stripping, (2018), https://doi.org/10.1016/j.mne.2018.09.001