de Boer, Meint J.

Lötters, Joost Conrad

Veltkamp, Henk-Willem

Wiegerink, Remco J.

Yu, Qihui

English

This paper presents a new fabrication method to realize a channel underneath the surface of a single silicon substrate for micro Coriolis flow sensing application, as well as other multiparameter systems, such as thermal flow sensors. It allows to fabricate channels with a nearly perfect circular cross section without bonding of two wafers. With this technology, problems such as leakage, deformation of channels due to pressure and temperature that affect the performance of micro Coriolis flow sensor can be reduced significantly. Moreover, it leaves only very little topography on the surface of substrate, hence integration of actuation and readout systems by metal deposition can still be followed easily afterwards

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2. Actuators Poster or OralFABRICATION METHOD OF MICROFLUIDIC CHANNELS WITH CIRCULAR CROSSSECTION FOR MICRO-CORIOLIS MASS FLOW SENSORQ. Yu1, H. Veltkamp1, M.J. de Boer1, R.J. Wiegerink1 and J.C. Lötters1,21 MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands2 Bronkhorst High-Tech BV, Ruurlo, The NetherlandsABSTRACTThis paper presents a new fabrication methodto realize a channel underneath the surface of asingle silicon substrate for micro Coriolis flow sensingapplication, as well as other multiparameter systems,such as thermal flow sensors. It allows to fabricatechannels with a nearly perfect circular cross sectionwithout bonding of two wafers. With this technology,problems such as leakage, deformation of channelsdue to pressure and temperature that affect theperformance of micro Coriolis flow sensor can bereduced significantly. Moreover, it leaves only very littletopography on the surface of substrate, hence integrationof actuation and readout systems by metal depositioncan still be followed easily afterwards.KEYWORDSCircular cross section, Buried channel technology,Wet etch, HNA, Flow sensors, MicrofluidicsINTRODUCTIONThe concept of this new method for fabricatingmicrofluidic channels for micro Coriolis flow sensingapplication is based on the combination of BuriedChannel Technology (BCT) [1, 2] and silicon isotropicwet etching, which is known as HNA etching [3].With this combination, it allows to fabricate a channelburied in Si substrate. With a starting point of etchingat the bottom of a trench instead of at the substratesurface, a channel with a circular cross section can berealized by isotropic etching of Si. Compared to theSurface Channel Technology (SCT), which was usedas a standard technology for the fabrication of Coriolismass flow meter and other microfluidic devices [4, 5, 6],this new method has the potential to achieve much largermicro structures.Buried Channel TechnologyBuried Channel Technology, or so called BCT,derived from the well-known SCREAM process, wasinvented in 1999 at MESA+ Institute to fabricate microstructures for fluidic applications in silicon substrate[1, 2, 7]. It allows to fabricate channels buried in thesilicon substrate. The main steps of this process aretrench etching, coating, bottom coating removal andetching of channel. Figure 1 shows the cross sectionof channel fabricated by BCT. By adjusting the widthand depth of trenches, different sizes of channels canbe realized in one substrate with only one step oflithography, which gives the possibility to integrate multimicrofluidic devices with one lithographic mask.Figure 1: SEM image of cross sections of channels fabricatedby BCT [1].Si Isotropic Wet EtchingSilicon isotropic wet etching, also known as HNAetching, is a chemical etching of silicon by a mixtureof hydrofluoric acid (H), nitric acid (N) and aceticacid (A), while some researchers like to use waterinstead of acetic acid. It is proceeded by a sequentialoxidation-followed-by-dissolution process [3, 8]. Thecomplete reaction can be described as:Si + 2 HNO3 + 6 HF −−→ H2SiF6 + 2 HNO2 + 2 H2OThe factor that limits the etch rate is highlydependent on the composition of mixture. When theconcentration of nitric acid is low and hydrofluoricacid is high, the etch rate of process is limited bythe oxidation step. When in opposite situation, thelimit is then the dissolution of SiO2. B. Schwartzet al investigated and explained in detail the etchingmechanism behind this process in [3]. Figure 2 shows2019The 4th Conference on MicroFluidic Handling Systems, 2–4 October 2019, Enschede, The Netherlands103a semi-circular cross section of etched silicon substratethrough a 10µm-wide line.Figure 2: SEM image of cross sections of channels etched inHNA solution (69%HNO3:50%HF:H2O=2:1:2) for 60minwith SiRN mask.Figure 3: Cross section view of fabrication steps for creatingburied channels. (a): Bare silicon wafer. (b): Silicon waferwith 500nm SiO2. (c): Patterning of line openings. (d):Trench etching. (e): Stripping of SiO2. (f): SiRN coating. (g):Trench bottom coating removal. (h): Channel etching.Figure 4: SEM image of bottom coating removed trench(3µm wide) from early test to optimize the recipe of process.Figure 5: SEM image of cross section of a channel after65min etching in HNA (69%HNO3:50%HF:H2O=2:1:2).FABRICATIONAn overview of fabrication process can be seen inFigure 3. The steps are all done with standard recipein the cleanroom of MESA+. First, silicon substrateis cleaned and oxidized to obtain a layer of 1µm SiO2(Figure 3b), serving as hard mask for Si etching in latersteps. Lines with width of 10µm are etched throughthis layer (Figure 3c) and then followed by BOSCHprocess to produce trenches (Figure 3d). After that,SiO2 layer is stripped by HF (Figure 3e) and coatingof trenches is done by LPCVD 1µm low stress siliconrich nitride (SiRN) (Figure 3f). To remove the coating atthe bottom of trenches, a PECVD SiO2 layer is used asmask to protect SiRN on substrate surface. Due to thefact that trenches are of high aspect ratio and PECVD isa non-conformal deposition, bottom of the trench is notcovered by this SiO2 layer. Thus by running a plasmaetching of SiRN, coating at bottom of trenches can beremoved while substrate surface and sidewall of trenches2019The 4th Conference on MicroFluidic Handling Systems, 2–4 October 2019, Enschede, The Netherlands104stay. Figure 4 shows the bottom of trench after removingthe coating layer. After successfully removing thebottom coating of trenches, the substrate is then etchedin HNA solution (69%HNO3:50%HF:H2O=2:1:2) for65mins. The cross section is shown in Figure 5.DISCUSSIONCompare the result shown in Figure 5 to Figure 2,the etch rate of silicon decreases significantly, this is dueto the fact that diffusion of etching product and etchantthrough the deep trench is more difficult than throughline openings on the substrate surface. A magneticstirrer or ultrasonic bath can be the solution to achievea higher etch rate. Together with a sufficient supply ofetchant, the maximum diameter of etched channels willbe limited only by the depth of trench and thicknessof SiRN mask. From the earlier test, SiRN is a goodmaterial serves as mask for HNA solution, with an etchrate of approximately 2∼3nm per minute. A thickerlayer of SiRN can stand longer during etching, however,as a trade-off it also becomes more difficult to removethe layer at the bottom of trenches. Thus an alternatematerial can be considered to serve as mask for HNAsolution for the future.CONCLUSION & OUTLOOKA brief introduction of how to fabricate channelsburied in a silicon substrate with a nearly perfect circularcross section is presented. The result is preliminary butshows a potential for producing extra-large channels ofdiameters up to few hundreds µm with this technology.Compared to channels fabricated by standard SCT, themain advantages of this new method are: 1) Reduction oftemperature and pressure dependency for micro Corilisflow sensing application due to the nearly circular crosssection, 2) A wider flow range can be covered withdifferent sizes of channels. Furthermore, the cost offabrication can be reduced as well since channels areetched by wet chemicals instead of plasma. Next stepof the work will be focused on optimizing fabricationsteps to realize an extra-large channel, then followed bysealing and releasing to achieve a free suspended tubefor micro Coriolis flow sensing application.REFERENCES[1] Meint J. de Boer, R. Willem Tjerkstra, J.W.(Erwin) Berenschot, Henri V. Jansen, G.J.Burger, J.G.E (Han) Gardeniers, Miko Elwenspoekand Albert van den Berg “Micromachining ofBuried Micro Channels in Silicon”, Journal ofMicroelectromechanical Systems Volume: 9, Issue:1, March, 2000, pp. 94-103.[2] Rint Willem Tjerkstra, “ISOTROPICETCHING OF SILICON IN FLUORIDECONTAINING SOLUTIONS AS A TOOL FORMICROMACHINING” University of Twente,Enschede, 1999.[3] B. Schwartz and H. Robbins, “Chemical Etching ofSilicon IV . Etching Technology”, J. Electrochem.Soc. 1976 volume 123, issue 12, 1903-1909.[4] Jarno Groenesteijn, Meint J. de Boer1, JoostC. Lötters, Remco J. Wiegerink, “A versatiletechnology platform for microfluidic handlingsystems, part I: fabrication and functionalization”,Microfluid Nanofluid (2017) 21:127.[5] Jarno Groenesteijn, Meint J. de Boer1, JoostC. Lötters, Remco J. Wiegerink, “A versatiletechnology platform for microfluidic handlingsystems, part II: fabrication and functionalization”,Microfluid Nanofluid (2017) 21:126.[6] Jarno Groenesteijn, “Microfluidic platform forCoriolis-based sensor and actuator systems”University of Twente, Enschede, 2016.[7] R.W. Tjerkstra, M. De Boer, E. Berenschot,J.G.E. Gardeniers, A. van den Berg and M.Elwenspoek, “ETCHING TECHNOLOGY FORMICROCHANNELS”, Proceedings IEEE TheTenth Annual International Workshop on MicroElectro Mechanical Systems. An Investigation ofMicro Structures, Sensors, Actuators, Machines andRobots (Jan 1997), Nagoya, Japan[8] Wing Cheong Hui, “How to prevent a runawaychemical reaction in the isotropic etching of siliconwith HF/HNO3/CH3COOH or HNA solution”, Proc.SPIE 5276, Device and Process Technologies forMEMS, Microelectronics, and Photonics III, (2April 2004).CONTACT* Q. Yu, q.yu-3@utwente.nl2019The 4th Conference on MicroFluidic Handling Systems, 2–4 October 2019, Enschede, The Netherlands105

Fabrication method for microfluidic channels with circular cross section for micro-Coriolis mass flow sensor

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Fabrication method for microfluidic channels with circular cross section for micro-Coriolis mass flow sensor

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