The final parameters for the optimized process can be seen in Table4.3.
Table 4.3
The optimized parameters for etching a sub-wavelength blazed grating.
Parameter Optimal value
During the optimization process, we observed that the gas flow and RF power parameters have a significant effect both on the etching depth and the shape of the structure. The shape of the structure shifted significantly when the gas composition was changed in the process. We found that when the ratio between the SF6 and O2 gases were set to be around 57 % and 43 % of total gas volume, the result was close to the desired structure. The gas flow parameters were set for these gases at 20 sccm and 15 sccm respectively. These were found to have the best pattern preservation while also providing a good selectivity. After finding decent values for the gas flow, the effects of adjusting the RF power were studied. The tests were conducted by having the same gas flow. RF power was set at first to 200 W. The power was then gradually lowered until 160 W was reached, where we achieved the best blazed
grating structure in this work. With these two parameters set we managed to achieve the best possible shape and surface relief in this study.
Etching time was studied and we concluded that the time of one minute is ad-equate for this process. With this etching time, some resist may remain on top of the structure, but this can be etched away by using oxygen plasma and should not affect the resulting shape of the structure. The optimal etching time could be determined more accurately, but the effect of other variables during the RIE process would make it a tedious and time-consuming process. Due to the unforeseen cir-cumstances caused by the global pandemic COVID-19, the university and thus the cleanroom environment was shut from students during this work which prevented us from testing the etching time further.
Chapter V
Conclusions
The optimization of an RIE etching process for a sub-wavelength blazed structure was achieved during this study. These structures were etched into a silicon substrate using an unknown resist material as a mask. The etching process was optimized in parts. We studied the effects of gas composition, RF power and etching time to find the best process for transferring the grating structure from the resist into the substrate. Of these, gas composition and RF power were the most important ones for controlling the shape of the structure. The methods used during this study were also discussed.
The shape of the structure was found to be the best when the gas composition was set to 57 % of SF6 gas and 43 % of O2 gas. This was also found to etch away the whole mask layer from the top of the structure. These values were set as a baseline before optimizing the RF power parameter. Different values of RF power were tested between 150 W and 220 W. We found that the best shape of the structure was achieved with 160 W of RF power. These tests were performed with a chamber pressure of 10 mTorr and without helium back cooling.
With the surface shape, the other parameter we were interested in during this study was the selectivity of the process. This was found to be around 0.72 for the two runs with this optimized recipe. This was not near to the target value of 1, but the shape of the structure was good with the optimized process. It might be possible to achieve selectivity of 1 with an extensive redesign of this process but due to the limited amount of time available for this study, this could not be done here.
The shape of the structure was good, and by modifying the original resist layer, one could achieve the desired depth of structure into the silicon with our recipe. The
limitation of our process is that it etches the mask layer away too early, which leads to the process starting to etch the surface uniformly. This starts to flatten out the structure. To alleviate this problem, one could decrease the amount of oxygen in the process. This would reduce the etch rate of the resist layer, and thus, it would theoretically let us etch for longer periods. It would also increase the anisotropicity of the etching process, which could help with the selectivity. However, to still preserve the shape one would need to tune the other parameters in this process.
Optimizing an RIE process is an endless task. The parameters can be changed to practically unlimited combinations. Furthermore, the reproducibility and stability of these RIE processes tend to change with the machinery used. Therefore, one should take these optimized parameters more as a guideline for a similar process, and then further tune the parameters to achieve their desired structure with their equipment. Nonetheless, the optimization process described in this work is a good way to optimize these processes
References
[1] J. M. Hollas, Modern spectroscopy, 2nd ed. ed. (Wiley, Chichester, 2004).
[2] E. Hecht, Principles of Lithography, Vol. 3, (SPIE Press, 2010).
[3] C. Palmer, “Diffraction Grating Handbook,” Newport Corporation (2005).
[4] T. Buß, C. L. C. Smith, M. Brøkner Christiansen, R. Marie, and A. Kristensen,
“Sub-wavelength surface gratings for light redirection in transparent substrates,”
Applied Physics Letters 101 (2012).
[5] X. Yadong, F. Yangyang, and C. Huanyang, “Steering light by a sub-wavelength metallic grating from transformation optics,”Scientific Reports 5 (2015).
[6] H. Subbaraman, X. Xu, J. Covey, and R. T. Chen, “Efficient light coupling into in-plane semiconductor nanomembrane photonic devices utilizing a sub-wavelength grating coupler,”Optics Express20, 20659–20665 (2012).
[7] J. Chovan, A. Kuzma, and F. Uherek, “Design and simulation of apodizet SOI fiber to chip coupler by sub-wavelength structure,”International Conferense on Transparent Optical Networks 1–4 (2013).
[8] R. Halir, A. Ortega-Mo˜nux, J. H. Schmid, C. Alonso-Ramos, J. Lapointe, D. Xu, J. G. Wang¨uemert-P`erez, I. Molina-Fern´andez, and S. Janz, “Recent Advances in Silicon Waveguide Devices Using Sub-Wavelength Gratings,”IEEE Journal of Selected Topics in Quantum Electronics20, 279–291 (2014).
[9] K. Young, Q. Wen, S. Hanany, H. Imada, J. Koch, T. Matsumura, O. Suttmann, and V. Sch¨utz, “Broadband millimeter-wave anti-reflection coatings on silicon using pyramidal sub-wavelength structures,” Journal of Applied Physics 121 (2017).
[10] H.-M. Xia, K.-H. Song, R.-R. Wang, and L.-L. Peng, “Study on a new sub-wavelength grating for antireflective structure of solar cell,” in ISAPE2012 (2012), pp. 778–781.
[11] G. Oehrlein, “Reactive-ion Etching,”Physics Today 39, 26–33 (1986).
[12] M. J. Madou, Fundamentals of microfabrication: the science of miniturization, Vol. 5, (Pearson, 2017).
[13] Z. Cui, Nanofabrication: Principles, Capabilities and Limits, Vol. 2, (Springer, 2017).
[14] S. Franssila, Introduction to microfabrication, 2nd ed. ed. (John Wiley & Sons, Chichester, West Sussex, England, 2010).
[15] K.-S. Lee, M.-H. Ha, J. Kim, and J.-W. Jeong, “Damage-free reactive ion etch for high-efficiency large-area multi-crystalline silicon solar cells,”Solar Energy Materials and Solar Cells 95, 66–68 (2011).
[16] H. J. Levinson, Principles of Lithography, Vol. 3, (SPIE Press, 2010).
[17] C. A. Mack, Fundamental principles of optical lithography: the science of mi-crofabrication (Wiley, Chichester, West Sussex, England, 2007).
[18] C. Constancias, S. Landis, S. Manakli, L. Martin, L. Pain, and D. Rio, “Pho-tolithography,” inLithography (John Wiley and Sons, 2013), pp. 1-38.
[19] B. Yu, L. Chang, S. Ahmed, H. Wang, S. Bell, C.-Y. Yang, C. Tabery, C. Ho, Q. Xiang, T.-J. King, J. Bokor, C. Hu, M.-R. Lin, and D. Kyser, “FinFET Scaling to 10 nm Gate Length,”Diges. International Electron Devices Meeting 251–254 (2002).
[20] I. Vartiainen, Polarization control and coherence-polarization mixing by sub-wavelength gratings, PhD thesis (University of Eastern Finland, 2011).
[21] C. Constancias, S. Landis, S. Manakli, L. Martin, L. Pain, and D. Rio, “Electron Beam Lithography,” in Lithography (John Wiley and Sons, 2013), pp. 101-182.
[22] L. Lind, “Simplified schematic of an EBL system,” (2020), Figure.
[23] J. Orloff, M. Utlaut, and L. Swanson, “Physics of liquid metal ion sources,” in High Resolution Focused Ion Beams (Springer, 2003).
[24] J. Taniguchi, H. Ito, J. Mizuno, and T. Saito, Nanoimprint Technology: Nan-otransfer for Thermoplastic and Photocurable Polymers (Wiley, 2013).
[25] J. M. Stauffer, Y. Oppliger, P. Regnault, L.Baraldi, and M. T. Gale, “Electron beam writing of continuous resist profiles for optical applications,”Journal of Vacuum Science and Technology B 10, 2526 (1992).
[26] A. Schleunitz and H. Schift, “Fabrication of 3D patterns with vertical and sloped sidewalls by grayscale electron-beam lithography and thermal annealing,”
Microelectronic Engineering 88, 2736–2739 (2011).
[27] S. Zhang, J. Shao, J. Liu, C. Xu, Y. Ma, and Y. Chen, “Multistep Aztec profiles by grayscale electron beam lithography for angle-resolved microspectrometer applications,”Journal of Vacuum Science and Technology B 32 (2014).
[28] Y.-R. Luo, Comprehensive Handbook of Chemical Bond Energies, Vol. 1, (CRC Press, 2007).
[29] M. Muttalib, R. Chen, S. Pearce, and M. Charlton, “Optimization of reactive-ion etching (RIE) parameters for fabricatreactive-ion of tantalum pentoxide (Ta 2 O 5 ) waveguide using Taguchi method,” (2017).
[30] X. Wang, H. Qi, and Y. Zhang, “Reactive ion etching optimization method based on orthogonal experiment,” in Proceedings of 2013 IEEE 11th Interna-tional Conference on Electronic Measurement and Instruments, ICEMI 2013, Vol. 2 (2013), pp. 700–703.
[31] LEO, “SEM LEO 1550 Gemini specification sheet,” (1997).