A new micromachining method to fabricate
wafer scale, atomically smooth nano-membranes is
described. The delicate membrane is supported on a robust
silicon microsieve fabricated by plasma etching. The
supporting sieve is micromachined independently of the
nano-membrane, which is later fusion bonded to it. The
transferred thin-film membrane can be dense, porous or
perforated according to the application desired. One of the
main application areas for such membranes is in fluidics,
where the small thickness and high strength of the supported
nano-membranes is a big advantage. The novel method
described enables to easily up-scale and interface micro or
nano-membranes to the macro-worl

Berenschot, J.W.

Elwenspoek, M.C.

Jansen, H.V.

Unnikrishnan, S.

English

A new micromachining method to fabricate wafer scale, atomically smooth nano-membranes is described. The delicate membrane is supported on a robust silicon microsieve fabricated by plasma etching. The supporting sieve is micromachined independently of the nano-membrane, which is later fusion bonded to it. The transferred thin-film membrane can be dense, porous or perforated according to the application desired. One of the main application areas for such membranes is in fluidics, where the small thickness and high strength of the supported nano-membranes is a big advantage. The novel method described enables to easily up-scale and interface micro or nano-membranes to the macro-worl

Jansen, Henricus V.

Berenschot, Johan W.

Elwenspoek, Michael Curt

University of Twente Research Information

WAFER SCALE NANOMEMBRANE SUPPORTED ON A SILICON MICROSIEVE  Sandeep Unnikrishnan1, Henri Jansen, Erwin Berenschot and Miko Elwenspoek University of Twente, MESA+ Research Institute, Electrical Engineering Department,  PO Box 217, 7500 AE Enschede, The Netherlands 1 E-mail: s.unnikrishnan@ewi.utwente.nlAbstract ⎯ A new micromachining method to fabricate wafer scale, atomically smooth nano-membranes is described. The delicate membrane is supported on a robust silicon microsieve fabricated by plasma etching. The supporting sieve is micromachined independently of the nano-membrane, which is later fusion bonded to it. The transferred thin-film membrane can be dense, porous or perforated according to the application desired. One of the main application areas for such membranes is in fluidics, where the small thickness and high strength of the supported nano-membranes is a big advantage. The novel method described enables to easily up-scale and interface micro or nano-membranes to the macro-world.  Key Words: Nano-membrane, silicon microsieve, plasma etching, fusion bonding, thin film transfer I INTRODUCTION Micro and nano-membranes find a lot of applications, especially in the field of filtration and selective gas permeation. Typically desired characteristics of membranes for fluidic applications are high flux, good strength and inertness under harsh environments. Higher fluxes require thinner membranes, which in turn need a strong porous support underneath. This is always a tradeoff between the fluid flux and membrane strength since such a support can cause huge pressure drops. The continuously growing interest in nano-membranes and the various problems encountered have stimulated the search for novel membrane fabrication techniques. One of the promising technologies is microfabrication technology, which enables precise control over the membrane properties like thickness, stiffness, porosity, permeability etc. and at the same time offering control of the properties of the support structure. The limitation of using micromachining for thin membranes fabrication is the difficulty to up-scale it in order to make this technology commercially viable for large scale applications.  Prior research has reported fabrication of silicon nitride microsieve as a fluidic filter [1] or as a support for gas permeation [2]. This method, although a break-through in membrane technology, has certain limitations. Since it was based on wet-etching of the silicon support, it is slow and crystal plane dependent and additionally could affect the membrane’s quality since the wet-etching is done after the membrane is applied. Moreover, the effective surface area of the membrane is limited not only by the silicon nitride microsieve support, but also due to the silicon macro-support beneath.   In this paper we illustrate a new micromachining process, which counters the above mentioned drawbacks by using a thin-film transfer technique to assemble the nano-membrane onto the silicon support, based on fusion bonding and RIE etching [3, 4]. The support is a silicon microsieve with well defined micropores giving high strength and high flux. In this way, strong and atomically smooth membranes with large usable surface area can be fabricated (figure 1).     Figure 1. Cross-sectional representation of wafer scale nano-membrane supported on the silicon microsieve Depending on the application, the membrane could be dense, porous or perforated and of different materials. In this paper, we show an example of fabricating a wafer scale dense nano-membrane using this technique. II FABRICATION The fabrication process is a single mask process. Dual side plasma etching is the key technique used for micromachining the silicon sieve. Figure 2 illustrates the outline of the fabrication process. The process begins with the dry oxidation of a silicon wafer to grow 200nm of silicon dioxide (fig.2-a). After photolithography to define the microsieve pattern, the resist pattern is transferred into the oxide layer using buffered-HF (fig.2-b). Silicon wafers Nano-membrane Silicon microsieve support   a)   b)    c)   d)   e)            f)    g)   Figure 2. 2D Fabrication process outline for nano-membrane development The pattern used has hexagonally packed holes of 10µm diameter and a periodicity of 20µm, distributed over a circular area of Ø8cm. Subsequently, the pattern is etched 90µm deep into the silicon wafer by DRIE etching using a time multiplexed Bosch recipe (fig.2-c). After the resist is stripped off the wafer, it is dry oxidized for 10 minutes at 1100°C to burn off the fluorocarbon residue deposited during the plasma etching step, rendering also 50nm of silicon dioxide (fig.2-d). Then, the oxide on the back side is removed and the wafer is back-etched with a SF6 plasma based isotropic etch recipe. This back-etch process is time controlled, whereby the process has to be stopped when the oxidized micropores are reached from the backside. Subsequently, this 90µm thick microsieve membrane is dipped in 50% HF to strip the silicon oxide (fig.2-e). This wafer is then fusion bonded to a silicon wafer with a thin-film on it, in this case 200nm thermal oxide (fig.2-f). Prior to the direct fusion bonding, the microsieve wafer and the thin-film wafer are thoroughly cleaned in Piranha (96% H2SO4 + 31% H2O2 = 3:1 mixture at 100°C) for 30 minutes to ensure a hydrophilic bondable surface. The thin-film wafer is then plasma back-etched until the 200nm oxide (fig.2-g), resulting in Ø8cm oxide membrane supported on a silicon microsieve of 18% porosity.  III EXPERIMENTAL The bond interface was characterized using an IR-imaging. The transferred thin-film was checked using SEM to confirm that it is not buckled and has no cracks. The quality of the silicon dioxide thin-film on top of the supporting silicon microsieve was tested within a helium gas flow set-up (figure 3). The test samples were 3mm in diameter and were sealed onto glass tubes. The trans-membrane helium pressure was varied (up to 5 bars), with a time duration of 1 hour between increments.       Figure 3. Gas flow test-setup Furthermore, just the microsieve without the thin-film was tested with compressed air-flow through it for determining the flux and pressure drop.  IV RESULTS AND DISCUSSIONS IV.1 FABRICATION  Figure 4-b shows the 90µm deep highly anisotropic micropores in the silicon wafer.  (a)      (b)   Figure 4. Micropores of Ø10μm etched 90μm deep into silicon wafer a) the top view b) the cross section  The thickness of the microsieve membrane is defined by the depth of these micropores. The maximum pore depth achieved by tuning the anisotropic Bosch recipe, was around 150μm for Grow 200nm oxide on Silicon wafer Photolithography and oxide etching with Microsieve pattern Plasma etching of 90µm deep micropores Dry oxidation at 1100°C  Silicon back-etch and oxide strip Direct fusion bonding and annealing at 1100°C for 2 hours Silicon back-etch to selectively stop on the 200nm oxide membrane Pressure sensor Flow-control Helium source Sample holder holes of Ø5μm. The porosity of the microsieve can be increased without affecting its strength, which can be compensated by increasing its thickness. Figure 5 shows the SEM picture of the wafer scale nano-membrane on top of the microsieve.              (a)            (b)  (c)  Figure 5.  a) Ø8 cm silicon microsieve supporting a 200nm SiO2 membrane b) SEM picture showing the thin-film membrane bonded over the microsieve c) cross-section of (a) The isotropic wafer back-etch process in the Adixen-SE plasma etcher exhibited non-uniformity due to accumulation of unused etch-radicals near the non-etchable wafer clamp ring of the plasma equipment (figure 6-a). The problem due to this non-uniform etching is that the microsieve support membrane finally becomes thinner at the edge than at the center of the wafer, which makes it very fragile to handle.           (a)         (b) Figure 6. a) Problem of non-uniform back-etching b) A sacrificial silicon ring is used as a solution  As a solution to this problem a 1cm wide laser-cut silicon sacrificial ring was used, which consumes the extra etch-radicals, thus eliminating the non-uniformity (figure 6-b). The ring was immobilized onto the wafer using fomblin oil. Depending on its thickness, the ring can be reused multiple times. The usage of such a sacrificial ring mask also offers an additional advantage that the microsieve wafer after etching has a residual non-etched ring around it, which acts as a strong handle to it. Another solution to this clamp-ring induced non-uniformity problem could be the usage of electrostatic wafer clamping instead of mechanical clamping. Using such a non-mechanical clamp, the thin-film silicon wafer can be completely etched away (without a residual ring) after fusion bonding.  Although the back-etch recipe used here has a high selectivity to silicon dioxide, still the oxide stop layer inside the micropores (fig2-d) needs to be adequately thick (at least 15nm) to withstand the sputtering by the ions propelled by the plasma potential. On breakage of this oxide layer, the etch radicals can get into the micropores and start attacking them, especially since the cooling would be no longer effective. Figure 7-b shows the effects of this attack on a microsieve with Ø5μm holes, in comparison with a successful case (fig7-a).                  (a)    (b)                                        Figure 7. Bottom side view of the plasma back-etched Ø5μm microsieve a) still with the protective oxide layer b) widened micropores due to broken protective oxide layer  The fusion bonding of the thin-film is done only after the micropores are back-etched, so that there would not be any problem due to gas entrapment during pre-bonding or annealing of the bond at 1100°C.   The initial oxide layer under the resist serves the purpose of a temporary layer for protecting the bondable surface against resputtered deposition of impurities from the plasma etching equipment. The other function of this oxide layer is to avoid non-uniformity on the bondable surface that could form during thermal oxidation of the micropores [5]. Shown in figure 8 is the AFM image of a 23nm protruding micropore edge measured after stripping a 300nm oxide thermally grown over the microsieve. These protrusions act as roughness on SiO2 membrane Silicon microsieve  Clamp ring Flow of etch radicals Excess etch radicals Etch-profile Clamp ring Sacrificial Silicon ring Etch-profile Flow of etch radicalsthe bondable surface of the wafer. By using a flat oxide layer on top of the wafer during oxidation, this undesirable effect could be avoided.     Figure 8. AFM image of a micropore with 23nm protruding edge, measured after stripping a 300nm oxide.  The thin-film transfer offers also the big advantage of modular fabrication, in which the silicon microsieves can be fabricated as independent modules and later different kinds of thin-films can be selectively transferred onto it. Nano-membranes of other materials like silicon nitride or polysilicon could also be formed by this technique, provided they are fusion bondable to silicon. Metallic nano-membranes can also be formed using this method by depositing a thin-film of the desired metal over the silicon dioxide nano-membrane as a sacrificial layer and then by etching the latter away.  IV.2 CHARACTERISATION  The helium leak test confirmed that the silicon oxide membrane was still dense and crack-free after its transfer onto the silicon microsieve. Although high temperature tests were not yet performed, theoretically, the membrane stack can withstand temperatures up to the softening temperature of the nano-membrane material (~1000°C in this case) or that of silicon. The flow measurements using just the silicon microsieve showed a pressure drop of 20·10-3 bar for an air flux of 9.8·107 l/bar.m2.hr through it. Since the flow is still in the incompressible and laminar regime, using Poiseuille's pipe flow formula a flux of 12.5·107 l/bar.m2.hr was calculated (assuming constant pore diameter). This deviation is probably due to the entrance effects, which have been neglected. The microsieve was strong enough to withstand at least 7 bars trans-membrane pressure (the maximum pressure available with our test equipment). The sieves could withstand even a rough pulsating flow test of ~0.5Hz pulses. Such pulses might be necessary to clean perforated membranes, which could get clogged during fluid filtration applications. All these tests proved the robustness and low flow resistance of the silicon microsieve support. The chemical inertness of the membrane stack was found to be good after dip-testing for ca. 30 minutes in aggressive solutions like hot concentrated HNO3 (69% at 95°C) or Piranha (96% H2SO4 + 31% H2O2 at 100°C). For even higher inertness, the support and/or the membrane can be coated with another inert layer.  V CONCLUSION We have demonstrated a new micromachining process to fabricate a stress free, large surface area nano-membrane supported on a robust silicon microsieve. Dual side plasma etching of the silicon wafer was exhibited as a successful method to fabricate wafer scale microsieves with a single mask process. Thin-film transfer via fusion bonding was used to assemble the nano-membrane atop the microsieve support. The strong and inert silicon microsieve with its well defined pores was capable of letting high gas fluxes through them without a large pressure drop, thus being an ideal solution for up-scaling microfabricated membranes to the macro-world. The membranes made by this new technology can be applied in many filtration and selective gas permeation applications. REFERENCES [1] S.Kuiper, C.J.M.van Rijn, W.Nijdam, M.C.Elwenspoek, Development and applications of very high flux microfiltration membranes, J. Membr. Sci. 150 1–8, 1998 [2] H.D.Tong, F.C.Gielens, J.G.E.Gardeniers, H.V.Jansen, J.W.Berenschot, M.J.de Boer, J.H.de Boer, C.J.M.van Rijn, and M.C.Elwenspoek, Microsieve supporting Pd-Ag alloy membrane and application to hydrogen seperation, J. MEMS, Vol. 14, No.1, 2005 [3] E.H.Klaassen, K.Petersen, J.M.Noworolski, J.Logan, N.I.Maluf, J.Brown, C.Storment, W.McCulley, G.T.A.Kovacs, Silicon fusion bonding and deep reactive ion etching: a new technology for microstructures, Sensors and Actuators A, 52, 132-139, 1996 [4] K.Midtbo, A.Ronnekleiv, D.T.Wang, Capacitive micromachined ultrasonic transducers fabricated by advanced wafer bonding and RIE etch, XX Eurosensors conference, September 2006  [5] P.N.Minh and T.Ono, Nonuniform silicon oxidation and application for the fabrication of aperture for near-field scanning optical microscopy, Appl. Phys. Lett.,Vol.75, No.26, 1999 

Wafer scale nano-membranes supported on a silicon microsieve

NARCIS 

D.T.Wang, Capacitive micromachined ultrasonic transducers fabricated by advanced wafer bonding and

Development and applications of very high flux microfiltration membranes,

J.G.E.Gardeniers, H.V.Jansen, J.W.Berenschot, M.J.de Boer, J.H.de Boer, C.J.M.van Rijn, and M.C.Elwenspoek, Microsieve supporting Pd-Ag alloy membrane and application to hydrogen seperation,

N.I.Maluf, J.Brown, C.Storment, W.McCulley, G.T.A.Kovacs, Silicon fusion bonding and deep reactive ion etching: a new technology for microstructures,

Nonuniform silicon oxidation and application for the fabrication of aperture for near-field scanning optical microscopy,

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Wafer scale nano-membranes supported on a silicon microsieve

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