Silicon dioxide atomic layer deposition at low temperature for PDMS microlenses coating

Short Communication Silicon dioxide atomic layer deposition at low temperature for PDMS microlenses coating Florival M. Cunha a , Jo˜ ao R. Freitas a , Sara Pimenta a,b,* , Manuel F. Silva a,b , Jos´ e H. Correia a,b a CMEMS-UMinho, University of Minho, 4800-058, Guimar˜ aes, Portugal b LABBELS-Associate Laboratory, Braga/Guimar˜ aes, Portugal ARTICLE INFO Keywords: Polymeric microlenses Surface roughness Optical efficiency ABSTRACT The optical performance of a microlens is strongly affected by its surface roughness, which depends on its fabrication process. High-surface roughness leads to scattering issues, decreasing the optical efficiency of a microlens. In this work, a polydimethylsiloxane microlens was coated with a very thin-film of silicon dioxide (30 nm), deposited by plasma-enhanced atomic layer deposition at a low temperature (50 ◦C). The main goal was to passivate the polymeric microlens and reduce its surface roughness. Atomic force microscopy was performed before and after the silicon dioxide coating, confirming the surface roughness reduction by a factor of 3.2. The microlens was observed by scanning electron microscopy after silicon dioxide coating. Surface elemental composition analysis of the silicon dioxide thin-film was also performed through X-ray photoelectron spectroscopy, confirming the formation of silicon dioxide and a stoichiometric ratio of silicon to oxygen (Si:O) close to 27:58. The refractive index of the deposited film was measured by ellipsometry, obtaining a value of 1.4617 at 470 nm. 1. Introduction The main challenge of using dielectric coatings in optical microdevices is the achievement of low-absorption and low-scattering losses. Additionally, the coating films must be deposited with precise thickness and uniformity [1]. Atomic layer deposition (ALD) is a high-quality thin-film deposition technique with unique features, including excellent film thickness control, high uniformity, and conformality, even on complex or structured surfaces [2–7]. ALD is a special type of chemical vapor deposition (CVD), where gaseous precursors are sequentially and independently exposed to the substrate. The precursor pulses are separated by purging cycles. Due to this cycle-based process, the reaction does not happen in the gas phase but is only surface-limited. Using ALD, the resulting film thickness is precisely controlled (at a sub-nanometer scale) by the number of ALD cycles [1,8,9]. Due to the high conformity of the ALD, the roughness of the substrate can decrease because the deposition fills all the holes in the substrate and turns the surface less rough [10]. ALD deposition of dielectric thin-films at low-temperature substrate (<250 ◦C) typically results in amorphous films with low-surface roughness, and thus low-scattering losses can be expected [1,11]. Silicon dioxide (SiO 2 ) is the most widely used dielectric thin-film in the semiconductor industry for photonics and electronic applications due to its excellent insulating properties and can be easily deposited by ALD at low-temperature depositions. This is especially interesting considering polymers as substrate materials, making it possible to coat a substrate with an ultrathin oxide film with good electrical properties [12–14]. Ultra-thin SiO 2 films have also been reported as good components for the nanotechnology field, such as dielectric materials in microelectronic devices, anticorrosion/passivation films, etc. [15–17]. Other oxides (e.g., titanium dioxide (TiO 2 ) and alumina (Al 2 O 3 )) are reported in the literature as good coatings to increase the organic solvent resistance of a polymer (polydimethylsiloxane (PDMS)) or increase its hydrophilicity [18,19]. The surface roughness of a microlens is an important parameter for evaluating its optical performance and it is mainly affected by the fabrication process. High-surface roughness leads to scattering issues, which decreases the optical efficiency of the microlens [20]. In this work, a PDMS microlens, previously microfabricated with the * Corresponding author. CMEMS-UMinho, University of Minho, 4800-058, Guimar˜ aes, Portugal. E-mail address: [email protected] (S. Pimenta). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2024.116167 Received 29 April 2024; Received in revised form 26 August 2024; Accepted 24 September 2024 Optical Materials 157 (2024) 116167 Available online 24 September 2024 0925-3467/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/bync-nd/4.0/ ). process reported at [21], was coated with a SiO 2 thin-film (30 nm thick), deposited at a low temperature (50 ◦C) by plasma-enhanced atomic layer deposition (PEALD), using a SENTECH Instruments GmbH system. Therefore, the main goal of this work was to perform the passivation of a thermally sensitive PDMS microlens by PEALD technique and to reduce its surface roughness, achieving the reduction of optical scattering losses. Furthermore, the characterization techniques used during this study, specifically atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and ellipsometry prove the PDMS microlens optimization. 2. Materials and methods As previously referred, the SiO 2 PEALD was performed using a SENTECH Instruments GmbH system. This ALD system integrates an ellipsometer for real-time film thickness monitoring. The Bis(diethylamino)silane (BDEAS) precursor was selected with the O 2 -plasma as a co-reactant. BDEAS is a usual precursor for SiO 2 ALD due to its chemical stability [15]. The substrate (PDMS microlens) and the reactor were heated up to 50 ◦C, the precursor BDEAS up to 60 ◦C, and the precursor’s lines up to 125 ◦C. The O 2 -plasma was excited at 13.6 MHz by radio frequency (RF) power of 200 W and a flow rate of 200 sccm. The reactor vacuum pressure was throttle-controlled at 20 Pa. Table 1 shows the recipe conditions used. AFM was used to measure the surface roughness of the PDMS microlens before and after the SiO 2 PEALD coating. The AFM measurements were performed with a BRUKER system (Dimension Icon) with a tip model RTESPA-150 in contact mode. SEM measurements were also performed after the SiO 2 PEALD coating, using a NanoSEM system (FEI Nova 200) by normal secondary electron (SE) mode, with an acceleration voltage of 10 kV, and with a Gold/Palladium (Au/Pd) antistatic coating for high conductivity. XPS measurements were performed to obtain a chemical composition analysis of the SiO 2 thin-film, using a ThermoFisher Scientific system (SCALAB250Xi) with a monochromated X-ray source Al K α (hv = 1486.68 eV) radiation, operated at 220 W, 14.6 kV, spot size 650 μ m. The XPS spectra were collected at pass energies 100 eV and 40 eV for survey spectra and individual elements respectively (N 1s, Al 2p, Si 2p, O 1s, and C 1s scan). The energy step for individual elements was 0.1 eV. The XPS spectra were peak-fitted using Avantage data processing software. For peak fitting the Shirley-type background subtraction was used. All the XPS peaks are to be referenced to adventitious carbon C 1s, C–C peak at 284.8 eV. Quantification has been done using sensitivity factors provided by the Avantage library. Charge neutralization was achieved with both low-energy electron and argon ion flood guns (<0.1 eV, 120 μ A, and 70 μ A current, respectively) during XPS measurements. Finally, ellipsometry of the SiO 2 thin-film was also performed to access the refractive index of the thin-film, using a J.A. Woollam Co. system (alphaSE Ellipsometer) with wavelengths from 381 nm to 893 nm, at different measurement angles (65◦, 70◦, and 75◦) and applying the Sellmeier model. The general form of the model is given by (1) [22]: n(λ)2= ε ∞+AUV λ2 λ2−λ2 UV +AIR λ2 λ2−λ2 IR (1) where nis the refractive index of the material at a given value of λ,A UV and A IR are the amplitude of the ultraviolet (UV) and infrared (IR) poles, respectively, λ UV and λ IR are the positions of the UV and IR poles, and ε ∞ is the low-frequency offset. In the software of the ellipsometer, the Sellmeier model was used to fit the pole parameters and the lowfrequency offset, extracting the SiO 2 refractive index and thickness. The mean square error (MSE) of the fitting process was 0.947. 3. Results and discussion The SiO 2 PEALD thin-film was monitored using the ALD system in situ ellipsometer. Fig. 1 shows the growth parameters of the SiO 2 PEALD thin-film at 50 ◦C. The recipe starts with a stabilization period of approximately 10 min before the first cycle to ensure the maintenance of all the deposition parameters (pressure, temperature, gas flow rate). A growth per cycle (GPC) of 1.32 Å/cycle was measured by the ellipsometer. A more detailed view of the first three cycles is shown in Fig. 1 (b). The exposition time for the BDEAS and the O 2 -plasma is indicated in the shaded regions. A thickness gain was observed during the BDEAS exposure, while a decrease in thickness occurred during the O 2 -plasma exposure. This decrease is attributed to the replacement of diethylamine groups ((CH 3 CH 2 ) 2 N) on the surface with oxygen atoms during the O 2 - Table 1 PEALD recipe for the SiO 2 thin-film deposition. Number of ALD cycles BDEAS O 2plasma Pulse time (ms) Purge time (s) Pulse time (ms) Purge time (s) 227 180 5 5000 1 Fig. 1. Thickness measured by the ALD system in situ ellipsometer for SiO 2 PEALD thin-film at 50 ◦C: (a) growth over many cycles; (b) detailed growth over the first three cycles. F.M. Cunha et al. Optical Materials 157 (2024) 116167 2 plasma exposure, thereby reducing the amount of material attached to the surface. AFM measurements were performed in the PDMS microlens before and after the SiO 2 coating. Fig. 2 shows the AFM surface topography of the PDMS microlens before and after the SiO 2 coating. Considering an area of 1 ×1 μ m 2 on the top of the microlens, it was measured an average surface roughness (R a ) of 1.6 nm before the SiO 2 coating, and 0.5 nm after the SiO 2 coating. Thus, it was confirmed that the surface roughness was reduced by a factor of 3.2, which brings less scattering issues and improves the optical efficiency of the PDMS microlens. As an example, an average surface roughness of 3.7 nm is reported in the literature for a PDMS microlens and considering the same surface area on the top of the microlens of 1 ×1 μ m 2 [23]. Therefore, the PEALD coating could be an alternative method for reducing the surface roughness and improving polymeric-microlens optical efficiency. Fig. 3 (a) shows a top view SEM image of the array of PDMS microlenses coated with the SiO 2 PEALD thin-film, highlighting the analyzed PDMS microlens in this work. Fig. 3 (b) shows a cross-section SEM image of a PDMS microlens coated with a SiO 2 PEALD thin-film. XPS is a highly sensitive technique for surface elemental composition analysis of very thin-films (below 40 nm thick). Fig. 4 shows the XPS of the SiO 2 PEALD thin-film. The stoichiometric ratio of silicon to oxygen (Si:O) was found to be close to 27:58, by calculating the XPS peak areas of the Si 2p and O 1s. This value is very close to the expected value of 1:2. The appearance of Si 2p at ≈103 eV confirmed the formation of SiO 2 [24]. Fig. 5 shows the refractive index as a function of wavelength for the SiO 2 PEALD thin-film, obtained by ellipsometry (applying the Sellmeier model). The refractive index of the SiO 2 at 470 nm is 1.4617, which is very close to the PDMS refractive index, 1.4014 also at 470 nm [25]. This small refractive index difference doesn’t have a significant impact on the SiO 2 coated PDMS microlens refractive properties, which is pretended in this work. To obtain the SiO 2 refractive index, 30 nm of SiO 2 was grown on a small bare Si wafer next to the microlens sample. Ellipsometry measurements were performed in situ during its growth process and ex situ on a different ellipsometer to confirm its thickness. Fig. 2. AFM surface topography at the top of the PDMS microlens (area of 1 ×1 μ m 2 ): (a) without coating; (b) with the PEALD thin-film SiO 2 coating. R a : average surface roughness. Fig. 3. SEM image of PDMS microlenses with the SiO 2 PEALD coating: (a) top view; (b) cross-section view. F.M. Cunha et al. Optical Materials 157 (2024) 116167 3 4. Conclusions In this work, a PDMS microlens was coated with a very thin-film of SiO 2 deposited by PEALD at a low temperature (50 ◦C). The main goal was to passivate a thermally sensitive PDMS microlenses and to reduce its surface roughness. AFM measurements performed before and after the coating confirmed the surface roughness reduction by a factor of 3.2. Also, the XPS results confirmed the formation of a SiO 2 thin-film by PEALD. Finally, ellipsometry characterization confirmed that the SiO 2 PEALD thin-film refractive index is very close to the PDMS refractive index at the same wavelength, as required. PEALD coating is an interesting approach to passivate optical structures (even polymeric) to increase their efficiency and reduce scattering issues. CRediT authorship contribution statement Florival M. Cunha: Writing –original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Conceptualization. Jo˜ ao R. Freitas: Writing –review &editing, Validation, Methodology, Investigation, Formal analysis, Conceptualization. Sara Pimenta: Writing –original draft, Validation, Methodology, Formal analysis, Conceptualization. Manuel F. Silva: Writing –review &editing, Validation, Methodology, Conceptualization. Jos´ e H. Correia: Writing – review &editing, Supervision, Resources, Conceptualization. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgments This work was supported by the CMEMS-UMinho Strategic Project (UIDB/04436/2020 and UIDP/04436/2020) and the MPhotonBiopsy (PTDC/FIS-OTI/1259/2020, http://doi.org/10.54499/PTDC/FIS-OTI/ 1259/2020). Florival M. Cunha and Jo˜ ao R. Freitas thank FCT (Fundaç˜ aoparaaCiˆ enciaeaTecnologia)forthePh.D.grants(2023.00594. BD, https://doi.org/10.54499/2023.00594.BD and 2020.07708.BD, https://doi.org/10.54499/2020.07708.BD, respectively). 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