1. Silicon Etch Process Options for Micro- and Nanotechnology using Inductively Coupled Plasmas References [1] See http://www.physicsweb.org/article/world/13/8/9http://www.physicsweb.org/article/world/13/8/9 [2] ‘Successful Fabrication of 2D Photonic Crystals’, Notomi et al, Research Activities in NTT Basic Research Laboratories, Vol.11, August 2001. [3] Finite-difference time-domain method for design and analysis of microcavity coupled submicron-width silicon waveguides, Aroua et al, The European Physical Journal Applied Physics, Vol 17, Issue 3, 215-221, March 2002 [4] Highly selective etch process for silicon-on-insulator nano-devices, T.Wahlbrink, M.C.Lemme et al, Microelectronic Engineering 78-79 (2005) 212 –217. [5] High resolution ICP etching of 30nm lines and spaces in Tungsten and Silicon, A.L.Goodyear, D Olynick et al. JVST. B 18 (6), Nov/Dec 2000 [6] Fabrication of silicon nanopillars using self-organized gold-chromium mask, V.Ovchinnikov et al Material Science and Engineering B69-70 (2000) 459-463 [7] I.Rangelow, “Critical tasks in high aspect ratio silicon dry etching for microelectromechanical systems,” J. Vac. Sci. Technol. A 21(4), Jul/Aug 2003. C.C.Welch a, A.L.Goodyear a, T.Wahlbrink b and M.Lemme b 2) Profile control Figure 3 shows a fine dimension doped or undoped polycrystalline silicon etch stopping on very thin oxide. Figures 4 and 5 show that the profile can be controlled from vertical to a tapered angle if desired (here for photonic crystal holes 0.2µm wide x 0.4µm deep. SiO 2 mask intact). 2. Experimental Details Silicon etch process development has been carried out in commercially available inductively coupled plasma (ICP) etch equipment from Oxford Instruments. Figure 1 is a schematic diagram of the Plasmalab System 100 ICP180 tool suitable for substrates up to 100mm diameter. The Plasmalab System 100 ICP380 tool is similar but has a larger ICP tube and is suitable for substrates up to 200mm diameter. Small samples pieces may be mounted on silicon carrier wafers with thermally conductive glue (note a smaller open load system, the Plasmalab 80plus ICP65 is available if only small samples are to be etched). The Plasmalab ICP Systems have control of substrate temperature from -150°C to +400°C. This very wide range permits great flexibility in processing. Wafers are loaded into the chamber via a load lock and either mechanically or electrostatically clamped to the temperature-controlled lower electrode. Helium pressure is applied to the back of the wafers to provide good thermal conductance between the electrode and the wafer. The process gases are admitted to the chamber and controlled to a pressure usually in the range 1mT to 100mT. Then RF power at 13.56MHz is applied to the ICP coil (up to 3000Watts) to generate a high-density etching plasma. 13.56MHz power is also applied to the lower electrode (0 to 600Watts) for independent control of substrate DC bias. Unused feed gas and volatile etch products are pumped away by a turbo molecular pump (with speed typically 1300l/sec) backed by a wet or dry pump (typically 40-60m 3 /hr). Silicon etching results were assessed by standard methods: thin film thicknesses by ellipsometry or Nanospec, step heights by profilometer and profiles by scanning electron microscopy (SEM). 3. Choice of processes Table 1 summarizes the process performance of the three process strategies as used for micro- and nano- scale etching of silicon. The processes are now further described and discussed mentioning advantages and disadvantages of each. A. Hydrogen Bromide (HBr) based silicon-on-insulator (SOI) process This process is useful when very high selectivity over a silicon dioxide insulator is required, such as for nano- scale MOSFETS [4] where the gate oxide is extremely thin. It is also a good choice for other SOI applications where the selectivity demands are not so high because the process is free from notching effects at the silicon- insulator interface. Such applications include 2d-photonic crystals [2] where the addition of an insulator layer such as silicon dioxide provides vertical confinement of light in the silicon (SiO 2 having a lower refractive index than Si). The HBr-based process has other benefits of smooth controllable profiles, high aspect ratio capability and is very clean. Disadvantages are the need for the corrosive gas HBr, the relatively low etch rate and the depth limitation (the profile becomes difficult to control much beyond 1µm depth). Generally oxide masking is preferred as this yields very high selectivity (so that etching back at the top of the silicon sidewall is unlikely), but a vertical resist mask may also be acceptable. The processing temperature is around room temperature. 1. Introduction Silicon is suitable material for a continually expanding range of micro- and nanoscale opto-and microelectronic devices such as 2d-photonic crystals [1,2], micro-silicon waveguides [3], nano-SOI MOSFETS [4], grating structures [5] and nanopillar structures [6]. However until recently exploitation of such technology has been restricted by the difficulty of fabricating the ever-smaller features and higher aspect ratios demanded. In this work several solutions are presented for the micro- and nanoscale etching of silicon for devices using inductively coupled gas plasmas (ICP). Typically the important features of a silicon etching process for micro- and nanotechnology are as follows: 1) Feature sizes down to 0.1µm or less with aspect ratios at least 2:1 2) Controllable sidewall profile (generally vertical needed) 3) Smooth contamination free sidewalls 4) Sufficiently high selectivity over the mask and if applicable high selectivity over underlying layers. 5) Good uniformity and good reproducibility A choice of three process strategies are described which meet the above needs while offering complementary benefits in tackling the various challenges arising in the micro- and nanoscale etching of silicon. All three process strategies may be performed using the same ICP platform. ProcessHBrRT F- base Cryogenic Depth [µm]0.05µm to 1µm 0.05µm to 10µm 0.2µm to >100µm Feature size [nm] ≥5nm Aspect ratio≥5:1 ≥10:1 Etch rate [nm/min] ≥100≥200≥300 Uniformity<±5% Selectivity Si:oxide >100:1>5:1>10:1 Selectivity Si:resist >1:1>3:1>5:1 Profile 80-92° Sidewall roughness <10nm B. Room temperature fluorinated chemistry silicon etch process This option has the benefit of using a non-corrosive octofluorocyclobutane (C 4 F 8 ) – sulfur hexafluoride (SF 6 ) chemistry at around room temperature. Like the HBr-based process it offers smooth controllable profiles and high aspect ratio capability. It has higher selectivity over resist masks than the HBr process and its depth range is considerably greater. However selectivity over oxide is much lower and there may be a tendency for notching at oxide interfaces if extended over etches are needed to clear extreme topography. Also, using a polymerizing gas (C 4 F 8 ) the process is not perfectly clean and will require periodic plasma cleaning to maintain performance but overall the room temperature F-base process is a good all round process usable for most micro- and nano-scale silicon etching applications. The main control for profile angle is the C 4 F 8 – SF 6 gas ratio as Figure 6 shows. Figures 7 and 8 show the flexibility of the process in its use for a 1µm wide x 5µm deep waveguide and for 50nm wide trenches (300nm deep) respectively. Figure 3: HBr-based etch of 34nm polySi lines and spaces stopping on 3nm gate SiO 2. HSQ masked. a Oxford Instruments Plasma Technology, North End, Yatton, Bristol BS49 4AP, UK Tel +44 1934 837000, Fax +44 1934 837001, e-mail colin.welch@oxinst.co.ukcolin.welch@oxinst.co.uk b Advanced Microelectronic Center Aachen (AMICA), AMO GmbH, Huyskensweg 25, 52074, Aachen, Germany. email lemme@amo.delemme@amo.de Figure 6. Profile angle as a function of % C 4 F 8 in SF 6 Angles 90° a re-entrant profile C. Cryogenic silicon etch process The cryogenic silicon etching offers the best performance of all the options by using sub-minus 100°C processing temperatures in conjunction with a non-corrosive chemistry: sulfur hexafluoride (SF 6 )-oxygen (O 2 ). At these temperatures the etch product (a silicon oxy-fluoride) is marginally volatile and provides sidewall passivation for good profile control [7]. Like the other processes smooth controllable profiles are achieved but the cryogenic process excels in its high selectivity over resist (and oxide) and its very high aspect ratio capability. It can also etch much deeper than the other processes and is extremely clean. Of course the main disadvantage is the need for liquid nitrogen cooling. The primary controls of profile angle are the O 2 :SF 6 gas flow ratio and the temperature. These parameters determine the amount and volatility of the oxy-fluoride sidewall passivation respectively. Low temperature and high O 2 :SF 6 encourages a tapered profile. By balancing the parameters a vertical profile may be readily achieved if that is the target. For instance, Figure 9 shows profile control by means of the O2 flow rate. Figure 10 shows 20:1 aspect ratio silicon waveguides etched with the cryogenic process, while Figure 11 shows 0.1µm trenches etched to 1µm depth. Figure 7. RT F-based etch 1µm wide x 5µm deep Si waveguide Figure 8: RT F-based etch 50nm wide trenches in Si (6:1 aspect ratio). Courtesy of ITRI/MIRL, Taiwan Figure 10 Cryogenic Si etch 0.5µm wide x 10µm deep waveguides Figure 11 Cryogenic Si etch 0.1µm gaps etched 1µm deep. Aspect ratio 10:1 Table 1. Summary of process performance for silicon micro- and nano-scale etching Figure 1: Plasmalab System100 ICP 180 Schematic Figure 4. Vertical HBr-based SOI etch. Figure 5. Tapered HBr-based SOI etch. Figure 9 Cryogenic Si etch Profile control by means of the O 2 flow rate in SF 5 Figure 2: Selectivity of polysilicon over SiO 2 as a function of O 2 flow in 50sccm HBr 1) Selectivity control Oxygen substitution is used to raise selectivity of silicon over SiO 2. Figure 2 shows that extremely high selectivities can be achieved once the O 2 level reaches about 10%.