MICROELECTROMECHANICAL SYSTEM COMPONENT OR A MICROFLUIDIC COMPONENT COMPRISING A FREE-HANGING OR FREE-STANDING MICROCHANNEL

Abstract

The invention relates to a microelectromechanical system (MEMS) component or microfluidic component comprising a free-hanging or free-standing microchannel (1), as well as methods for manufacturing such a microchannel, as well as a flow sensor, e.g. a thermal flow sensor or a Coriolis flow sensor, pressure sensor or multi-parameter sensor, valve, pump or microheater, comprising such a microelectromechanical system component or microfluidic component. The MEMS component allows to increase the flow range and/or decrease the pressure drop of for instance a micro Coriolis mass flow meter by increasing the channel diameter, while maintaining its advantages.

Claims

1-18. (canceled)

19. A microelectromechanical system component or microfluidic component comprising a free-hanging or free-standing microchannel, wherein the microchannel has a substantially circular cross-section, wherein the diameter of the microchannel is at least 10 times the thickness of the microchannel wall and the diameter of the microchannel is at least 20 μm.

20. The microelectromechanical system component or microfluidic component according to claim 19, wherein the diameter of the microchannel is selected from at least 50 μm, at least 100 μm, at least 200 μm, at least 500 μm, and at least 1000 μm.

21. The microelectromechanical system component or microfluidic component according to claim 19, wherein the thickness of the microchannel wall is smaller than 10 μm.

22. A flow sensor comprising the microelectromechanical system component or microfluidic component according to claim 19.

23. The flow sensor according to claim 22, wherein the flow sensor is at least one of a thermal flow sensor, a Coriolis flow sensor, a pressure sensor, a density sensor, a viscosity sensor, a multi-parameter sensor, a valve, a pump and a microheater.

24. A method of manufacturing a free-hanging or free-standing microchannel using an electroplating method comprising the steps of: providing a conductive or non-conductive electroplating wire; when the electroplating wire is non-conductive: forming a conductive coating on the electroplating wire to form a cathode; electroplating a channel wall on the electroplating wire or the conductive coating; and removing the electroplating wire.

25. The method according to claim 24, wherein the microchannel has a substantially circular cross-section, wherein the diameter of the microchannel is at least 10 times the thickness of the microchannel wall and the diameter of the microchannel is at least 20 μm.

26. The method according to claim 24, wherein the electroplating wire is made of acrylonitrile-butadiene-styrene (ABS) or copper.

27. The method according to claim 24, wherein the conductive coating comprises silver.

28. The method according to claim 24, wherein the microchannel wall comprises nickel or copper and the electroplating step is carried out in a nickel or copper electroplating cell using an aqueous nickel or copper solution.

29. The method according to claim 24, wherein the microchannel is incorporated in a microelectromechanical system (MEMS) component or a microfluidic system.

30. The method according to claim 24, wherein the electroplating wire is pre-shaped to be a part of a MEMS system or a microfluidic system before the step of electroplating.

31. The method according to claim 30, wherein the pre-shaped electroplating wire is attached to a MEMS system or a microfluidic system before the step of electroplating.

32. A free-hanging or free-standing microchannel manufactured according to the method of claim 24.

33. A microfluidic system or a MEMS system, comprising the free-hanging or free-standing microchannel of claim 32.

34. A flow sensor, comprising the free-hanging or free-standing microchannel according to claim 32.

35. The flow sensor according to claim 34, wherein the flow sensor is at least one of a thermal flow sensor, a Coriolis flow sensor, a pressure sensor, a density sensor, a viscosity sensor, a multi-parameter sensor, a valve, a pump and a microheater.

36. A microfluidic density sensor comprising: the free-hanging or free-standing microchannel of claim 32; actuating means for vibrating the free-hanging or free-standing microchannel at its resonance frequency; and readout means for measuring and comparing the resonance frequency.

37. A method of manufacturing a free-hanging microchannel, comprising the steps of: (a) providing a substrate of a first material; (b) depositing a trench-etch protective layer on at least one of a front side and a back side of the substrate; (c) etching a trench in the first material through the trench-etch protective layer; (d) creating a channel-etch protective layer in the trench, covering a bottom wall and side walls of the trench; (e) etching the first material of the substrate through the channel-etch protective layer and the bottom wall of the trench to form a channel outline centering around the location of the etched-away bottom wall of the trench; (f) removing the trench-etch and channel-etch protective layers from the substrate; (g) depositing a wall-forming layer on the substrate, wherein the wall-forming layer forms a microchannel wall on the channel outline and closes the trench; (h) etching the substrate to remove the first material surrounding the microchannel wall, such that the microchannel becomes free-hanging.

38. The method according to claim 37, wherein the microchannel has a substantially circular cross-section, wherein the diameter of the microchannel is at least 10 times the thickness of the microchannel wall and the diameter of the microchannel is at least 20 μm.

39. The microelectromechanical system component or microfluidic component according to claim 19, wherein the free-hanging or free-standing microchannel is manufactured using a sol-gel method comprising the steps of: forming a mould with a circular channel; flushing the circular channel with a flushing fluid to create an oxidized inner channel wall; cleaning and drying the oxidized inner channel wall; flushing the circular channel with a sol-gel solution; initiating a gelation reaction until a desired microchannel wall thickness is achieved; flushing out the remaining sol-gel solution; and removing the mould material from around the microchannel wall.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0128] The invention will next be explained by means of the accompanying drawings and description of the figures.

[0129] In FIGS. 1a-2g different stages of manufacturing a free-hanging microchannel according to the first method are shown, wherein FIGS. 1a-1g show the different stages using a cross-section of the substrate along the length of the channel and FIGS. 2a-2g show the stages using a cross-section of the substrate perpendicular to the channel:

[0130] FIGS. 1a and 2a schematically show the creation of a first thermal SiO.sub.2 layer as an inlet-etch protective layer, and the etching of an inlet;

[0131] FIGS. 1b and 2b schematically show the creation of an LPCVD SiO.sub.2 layer as a trench-etch protective layer and the etching of three trenches;

[0132] FIGS. 1c and 2c schematically show the creation of a PECVD SiO.sub.2 layer as a further trench-etch protective layer in the three trenches, and the creation of a second thermal SiO.sub.2 layer as a channel-etch protective layer;

[0133] FIGS. 1d and 2d schematically show the etching of the substrate through the bottom walls of the three trenches to form a channel outline;

[0134] FIGS. 1e and 2e schematically show the deposition of a wall-forming layer on the substrate;

[0135] FIGS. 1f and 2f schematically show the deposition of a metal layer on top of the wall-forming layer above a channel;

[0136] FIGS. 1g and 2g schematically show etching the substrate to remove the first material surrounding the channel walls, such that the channels become free-hanging.

[0137] FIG. 3 furthermore schematically shows a layer stack in the channel wall;

[0138] FIG. 4 schematically shows a free-hanging channel next to another partly released free-hanging channel; and

[0139] FIG. 5 schematically shows metal electrodes on a free-hanging channel.

[0140] In FIGS. 6a-6f different stages of manufacturing a free-standing microchannel according to the second method are shown:

[0141] FIG. 6a schematically shows an example of a mould with a circular channel;

[0142] FIG. 6b schematically shows flushing the circular channel with Piranha solution;

[0143] FIG. 6c schematically shows the formation of hydroxyl groups on the channel inner wall;

[0144] FIG. 6d schematically shows flushing the circular channel with a sol-gel solution;

[0145] FIG. 6e schematically shows the creation of a silicon dioxide layer on the inner circular wall; and

[0146] FIG. 6f schematically shows the resulting free-standing channel.

[0147] In FIGS. 7a-7d different stages of manufacturing a free-standing microchannel according to the third method are shown:

[0148] FIG. 7a schematically shows an electroplating wire;

[0149] FIG. 7b schematically shows a conductive layer being applied to the electroplating wire to form a cathode;

[0150] FIG. 7c schematically shows a copper or nickel layer being applied onto the conductive layer; and

[0151] FIG. 7d schematically shows a free-standing (copper or nickel) channel.

DETAILED DESCRIPTION

First Method

[0152] As will be shown with respect to FIGS. 1a-5, and as discussed in the foregoing, the Applicant has come up with a first advantageous fabrication process to fabricate relatively large, free-hanging of free-standing microchannels 1, preferably integrated with electrodes 23 for actuation and read-out of a micro Coriolis mass flow sensor. The process improves on the previously presented SCT. The resulting microchannels 1 have a circular or near-circular (such as partially hexagonal) cross-section with a diameter of for instance up to 180 μm and a channel-wall 7 thickness for instance up to 10 μm without the use of wafer bonding. The channels 1 can be released from the bulk to allow the vibrational movement needed for a Coriolis mass flow meter (MFM). Integrated metal electrodes 23 can be used for actuation and read-out of the sensor.

[0153] However, as discussed in the foregoing, many applications, like liquid chromatography or “Lab-on-a-Chip” use higher flowrates while requiring the low volumes of microfluidic flowmeters. To be able to increase the flow-range and/or decrease the pressure drop of the micro Coriolis MFM the channel-diameter needs to be increased, while maintaining its advantages (fast, accurate, low volume, small form factor). However, inherent to the SCT process, the channels have a flat top and a maximum channel-wall thickness. Both of these limit the maximum size of the free-hanging channel and increase the sensitivity to pressure when the channel size increases.

[0154] The fabrication process shown in FIGS. 1a-5 implements two main changes to the SCT. By isotropically etching the channel 1 from the bottom 16 of a trench 15 instead of at the surface of the wafer 2, the “flat top” inherent to SCT can be avoided. Furthermore, by preferably using a stack of multiple materials as the channel wall 7, the wall-thickness can be increased beyond what the intrinsic stress of the silicon-rich silicon nitride (SiRN) used in SCT allows.

Fabrication Outline

[0155] In FIGS. 1a-5 a short overview of the first fabrication process is shown.

[0156] As shown in FIGS. 1a and 2a, the fabrication preferably may start with wet thermal oxidation of a e.g. 525 μm thick, highly doped silicon wafer 2 as the substrate 2. This thermal SiO.sub.2 (t-SiO.sub.2) layer 4 functions as an inlet-etch protective layer 4 and may be used as a hard-mask during the inlet 10 etch from the back side 12 of the wafer 2.

[0157] As shown in FIGS. 1b and 2b, after the inlet-etch, the inlet-etch protective layer 4 may be stripped and a new SiO.sub.2 layer 5 (as the trench-etch protective layer 5) is deposited using low pressure chemical vapour deposition (LPCVD) of tetraethyl orthosilicate (TEOS, Si(OC.sub.2H.sub.5).sub.4). This trench-etch protective layer 5 is patterned with rectangular holes, such as three rectangular holes, of for instance 10 μm wide and 50 μm long which may form a longitudinal outline of the channels 1. The SiO.sub.2 layer 5 is then used as a hard-mask to etch the trenches 15 of for instance 110 μm deep through which the channels 1 will be etched.

[0158] As shown in FIGS. 1c and 2c, this SiO.sub.2 layer 5 may also be deposited inside the inlet 10 to protect the inlet 10 during the channel etch. Next, a thermal oxidation step may be carried out, which forms a protective t-SiO.sub.2 layer 18 as the channel-etch protective layer 18 on the silicon in the trench 15. This channel-etch protective layer 18 will protect the trench side-walls 17 during the channel etch. Using e.g. plasma enhanced chemical vapour deposition (PECVD), a thick SiO.sub.2 layer 6 as a further trench-etch protective layer 6 may be deposited on the front side 11 of the wafer 2.

[0159] The PECVD process is preferably not conformal, which means that virtually no SiO.sub.2 will be deposited inside the trenches 15, while a relatively thick layer is deposited on the surface of the wafer 2. A directional plasma etch may be used to remove the t-SiO.sub.2 at the bottom 16 of the trenches 15. Since the etch-rate of this process decreases heavily inside the trenches 15, the PECVD SiO.sub.2 layer 6 at the surface (front side 11) needs to be much thicker than the t-SiO.sub.2 layer at the bottom 16.

[0160] As shown in FIGS. 1d and 2d, the channels 1 may then be etched in the bulk of the wafer 2, through the trenches 15, by, for instance, an isotropic gas-phase XeF2 etch. The etch process is generally isotropic, so it results in a round channel 1 centering around the location of the etched-away bottom wall 16 of the etch-trenches 15.

[0161] As shown in FIGS. 1e and 2e, all SiO.sub.2 may be removed using a wet HF etch and the channel wall 7 may be deposited. The first layer preferably is a t-SiO.sub.2 layer, followed by LPCVD of silicon-rich silicon nitride (SiRN) layer, a polycrystalline silicon (polySi) layer and a final SiRN layer (as more clearly shown in FIG. 3).

[0162] As shown in FIGS. 1f and 2f, the thickness of the total stack can be e.g. up to 10 μm (depending on the etch-trench width) and may be conformally deposited, meaning it is deposited inside the channel outlines 19 and on the front side 11 and back side 12 of the wafer 2. It may close the trenches 15 through which the channel 1 is etched, resulting in an enclosed, leak-free channel 1. The only wetted material inside the channels 1 is silicon-rich silicon nitride (SiRN). After the channel 1 is closed, a gold layer 8 with chromium adhesion layer may be deposited and patterned to form the electrodes for actuation and read-out of e.g. a Coriolis sensor.

[0163] As shown in FIGS. 1g and 2g, finally, the layer-stack on top of the wafer 2 may be etched using a directional plasma etch to reach the bulk silicon next to the channel 1. Using a semi-isotropic SF.sub.6 plasma etch and an isotropic gas-phase XeF.sub.2 etch, the silicon around the channels 1 may be removed to allow for free movement of the channels 1.

Fabrication Results

[0164] At the start of the etch, the silicon loading is very small and plenty of etchant can reach the silicon, while the reaction products can be removed. This results in a completely isotropic etch. However, when the channel diameter increases, the silicon load also increases while the trenches 15 through which the etchant and reaction products have to diffuse does not increase. As a result, the XeF.sub.2 concentration inside the channels will decrease, changing the etch kinetics, eventually resulting in a less desirable hexagonal cross-section.

Channel Wall

[0165] Previously, the channel wall 7 was made using a single silicon-rich silicon nitride layer (SiRN). Due to intrinsic stress in the layer available, the layer can only be 2 μm thick. This means that, when the channel 1 diameter increases, the strength and stiffness of the channel wall 7 may not increase with it. As a result, the mechanical behaviour of the free-hanging channel 1 will increasingly depend on external influences like applied pressure and temperature. To create free-hanging channels 1 with a thicker channel wall 7, preferably a layer stack is used where every layer has a function: The SiO.sub.2 protects the channel wall 7 during the XeF.sub.2 release etch.

[0166] SiO.sub.2 has a much better selectivity compared to SiRN which allows to release the complete channel 1.

[0167] The SiRN at the inside of the channel wall 7 results in a chemically inert fluid path. Due to intrinsic stress, the maximum thickness of these SiRN layers is approximately 2 μm and can thus not be used for the whole channel wall 7. It is also deposited at the outside of the channel wall 7 to make sure the stress in the channel wall 7 is mostly balanced.

[0168] The polySi can be deposited e.g. up to 10 μm while its stress can be tuned by annealing. As a result, this layer can be used to reach a specific wall thickness without influencing the stress inside the channel wall 7.

[0169] FIG. 3 shows the top of an etch-trench which has been closed with a wall-forming layer 9 comprising such a layer stack. From bottom to top, FIG. 3 shows the t-SiO.sub.2 and SiRN layers 25, 26 of 1 μm thick, the 5 μm poly-silicon layer 27, the second SiRN layer 28 of 1 μm and finally a photoresist layer 3 on top.

[0170] In this case, the trench was only 10 μm wide, and it was completely closed during deposition of the polySi layer 27. As a result, the final SiRN layer 28 is only to be found on top of the wafer 2 and not at the inside of the channel wall 7.

Silicon Isotropic Wet Etching

[0171] Silicon isotropic wet etching, also known as HNA etching, is a chemical etching of silicon by a mixture of hydrofluoric acid (H), nitric acid (N) and acetic acid (A), while some researchers like to use water instead of acetic acid. It is proceeded by a sequential oxidation-followed-by-dissolution process. The complete reaction can be described as:

Si+2HNO.sub.3+6HF->H.sub.2SiF.sub.6+2NO.sub.2+2H.sub.2O

[0172] The factor that limits the etch rate is highly dependent on the composition of the mixture. When the concentration of nitric acid is low and hydrofluoric acid is high, the etch rate of process is limited by the oxidation step. When in the opposite situation, the limit is then the dissolution of SiO.sub.2.

[0173] With a sufficient supply of etchant, together with an ultrasonic bath to accelerate diffusion of etchant and etching product, the maximum diameter of etched channels 1 will be limited only by the depth of trench 15 and thickness of SiRN mask. From the earlier test, SiRN is a good material to serve as a mask for HNA solution, with an etch rate of approximately 23 nm per minute. However, a thicker layer of SiRN can stand longer during etching, as a trade-off, it also becomes more difficult to remove the layer at the bottom 16 of trenches 15. Thus an alternate material can also be considered to serve as mask for HNA solution.

Channel Release

[0174] Releasing the channels 1 “from the bulk” is done in three steps. First, the channel wall 7 layer stack 9, which is also deposited on the top/front side 11 of the wafer may be patterned with release windows next to the channels 1 using a directional plasma etch. To prevent the exposed polySi layer 27 from etching during the bulk etch, a new photoresist layer 3 is applied with smaller release windows. This photoresist requires excellent planarization properties to be able to fully cover the 10 μm step and also allow for accurate enough patterning for comb-fingers used for capacitive read-out. Next, e.g. 200 μm deep trenches may be etched through the release windows using a directional plasma etch. To remove the silicon underneath the channel 1, an isotropic XeF.sub.2 etch is done through these trenches.

[0175] To keep the silicon loading during this etch low (and thus the etch-rate relatively high), parallel channels 1 were made around the free-hanging channel 1 to act as an etch-stop. This is shown in FIG. 4. The “pillars” above the channel 1 show a periodic pattern which coincides with the e.g. 50 μm long etch-trenches with e.g. 10 μm spacing between. The channel wall 7 shows a periodic pattern with the same periodic pattern.

Actuation and Read-Out

[0176] FIG. 5 shows the metal electrodes 23 on a free-hanging channel 1, i.e. FIG. 5 shows some metal electrodes 23 on the free-hanging channel 1 at the top for actuation. Below that, the closed channel-etch-trenches 21 are still visible. In the middle of FIG. 5, comb fingers 22 for capacitive read-out are shown. One side of the comb structure 22 is attached to the free-hanging channel 1, the other side to the bulk of the chip. Since the comb fingers 22 significantly reduce the etch-rate of the release etch, extra release holes 24 are etched above and below the comb fingers 22.

[0177] Thus, the Applicant has developed a fabrication process for the fabrication of relatively large, round free-hanging microchannels 1. The channels 1 can have a diameter up e.g. to 180 μm with a channel wall 7 thickness of e.g. up to 10 μm. Integrated metal electrodes 23 can be used for actuation and read-out of e.g. a microfluidic sensor, such as a high flow micro Coriolis mass flow sensor.

Second Method

[0178] A second method that may result in (almost perfectly) circular channels 1 with various diameters and channel wall 7 thicknesses comprises a so-called “sol-gel” method.

[0179] FIGS. 6a-6f show a process outline of how this method can be used to realize channels 1 with a channel wall 7 of e.g. silicon dioxide. A sol-gel mixture may be created by e.g. mixing TEOS, MTES, ethanol and water in a 1:1:1:1 volumetric ratio, adding HCl to adjust the pH to 4.

[0180] Referring to FIGS. 6a-6c, the mixture may be placed in an oven for 12 hours. A circular mould 29 is made preferably in PDMS using the ESCARGOT method. As shown in FIG. 6b, the mould 29 may then be flushed with Piranha solution 30 for e.g. 35-70 s for having hydroxyl groups 31 to form covalent bonds, and may subsequently be rinsed with DI water and dried by nitrogen flow.

[0181] As shown in FIG. 6d, next, the sol-gel mixture 32 may be slowly flushed through the channel 37 e.g. using a syringe pump. To initiate the gelation reaction, the channel 37 is preferably placed on a hot plate at 55° C. This temperature may be limited by two factors: ethanol evaporation that leads to a non-uniform channel and the glass transition temperature of PDMS, when PDMS is used. When the desired coating thickness is reached, air flow may be used to flush out the remaining solution.

[0182] FIG. 6e shows the resulting silicon dioxide layer 33 inside the circular channel 37 with a diameter of e.g. 600 μm. A free-standing channel 1 can then be realized by selectively dissolving the PDMS of the mould 29 around the silicon dioxide layer 33. The thickness of the silicon dioxide layer 33 can be controlled e.g. by the time and temperature of the layer-forming process, and the rate of the syringe pump. However, it should be noted that the creation of the free-standing channel 1 can be challenging, because PDMS does not easily dissolve in the Piranha solution and, as the skilled person will understand, care should be taken that the silicon dioxide layer 33 does not also dissolve.

Third Method

[0183] A third method that can be used is electroplating. Plating can be performed on wires 34 (with a circular cross-section) of for instance Acrylonitrile-Butadiene-Styrene (ABS), which can afterwards be dissolved by e.g. acetone. FIGS. 7a-7d show possible process steps of this method. The copper electroplating cell may comprise a copper anode, the ABS wire 34 as cathode, an aqueous copper solution and a DC power supply (not shown). However, nickel or other metals can in principle also be used.

[0184] To produce an electrically conductive cathode the FDM-ABS wire 34 may be coated by silver conductive paint 35 and dried at room temperature. A volume of 1 litre aqueous copper solution preferably contains 0.1 kg copper salt, 0.11 litre 96% sulphuric acid, 0.15 ml 50% HCl, 10 ml HL11 starter and 0.25 ml HL13 grain refiner. DI water may be added to reach the required volume. The electroplating may be done e.g. at room temperature with a current density of 3 A/dm.sup.2 for 1 h. After electroplating, the substrate is rinsed with DI water to prevent oxidation of the electroplated copper 36. Next, the wire 34 may be immersed in e.g. a beaker of acetone for 12 hours to completely dissolve the ABS wire 34 inside. Instead of ABS, PA or PVA polymer could also be used which can be dissolved in hot water. FIG. 7d shows a schematic depiction of the resulting free-standing channel 1.

[0185] This plating method can be used to predictably manufacture microchannels with a very high degree of circularity, e.g., the channels 1 produced by the plating method have a high degree of rotational symmetry. Flat parts can be prevented by selecting a highly circular wire 34.

[0186] The method was repeatedly tested using ABS wires 34 with a diameter of 120 to 1000 μm. Table 1 shows examples of measured wall thicknesses for several different runs of nickel electroplating for every size of ABS wire 34. The thinnest wall thickness achieved was approximately 8 μm. The thickest walls were 60 μm, and thicker walls can be achieved if the electroplating step is permitted to continue even longer.

TABLE-US-00001 TABLE 1 Diameter of Thickness of the nickel coating the ABS wire (wall thickness) 120 μm 8 μm, 20 μm, 40 μm 128 μm-130 μm 8 μm, 10 μm, 20 μm, 40 μm, 50 μm, 60 μm 165 μm 8 μm, 10 μm, 20 μm, 40 μm, 50 μm 169 μm 10 μm, 20 μm, 40 μm, 50 μm, 60 μm 178 μm 10 μm, 20 μm, 30 μm, 233 μm 10 μm 20 μm, 30 μm, 43 μm 550 μm 10 μm, 20 μm, 40 μm, 50 μm, 60 μm 600 μm 10 μm, 20 μm, 40 μm, 50 μm, 60 μm 862 μm 10 μm, 15 μm, 20 μm, 30 μm, 40 μm, and 50 μm 866 μm 10 μm, 15 μm, 20 μm, 30 μm, 40 μm, and 50 μm 1000 μm  9 μm, 10 μm, 15 μm, 20 μm, 30 μm, 40 μm, and 50 μm

[0187] Thus the method can be used to manufacture channels with ratio of wall thickness to channel diameter from approximately 1:3 to 1:115. In addition, the channel walls were found to be highly uniform.

[0188] The plating method can be used to manufacture a microfluidic density sensor with low fabrication cost. Such a density sensor could preferably comprise a free-standing U-shaped channel, where the straight leg parts are connected to the straight middle part by rounded edges. In this case, the density sensor tube is made of nickel.

[0189] The channel is made using electroplating. Before immersion in the bath, a 600 μm diameter ABS wire is bent into the U-shape (optionally using a mould), deep coated with silver paint and dried. The anode of the electroplating cell is a 99.99% pure nickel bar, whereas the conductive or non-conductive electroplating wire forms the cathode. The electroplating is allowed to continue until the desired wall thickness of 20 μm was reached. Then the wire was removed from the electroplating bath and placed in an ABS-dissolving liquid consisting of acetone, and optionally heated. The resulting channel has a highly circular cross-section with a diameter of 600 μm and a wall thickness of 20 μm.

[0190] The U-shaped density sensor channel is 12*12 mm long and broad, and may be used in a set-up similar to those mentioned in aforementioned Groenesteijn, et al., 2012. The density sensor was further assembled by placing three external magnets parallel to the length of the channel, attaching the power supply and connecting the inlet to a fluid supply and the outlet to a drain. Preferably, a filter and a degasser are placed before the inlet. Alternating current is applied to the density sensor tube in the presence of an external magnetic field to vibrate the channel by Lorentz force. When the vibrating channel fills with fluid, the mass increases and this increase can be detected through change in (twist mode) resonance frequency. The mode of detection can change depending on the channel shape used. This density sensor has a spring constant of 1.56*10.sup.4 N/m and a of 3.78*10.sup.5 at atmospheric pressure and room temperature. The circular cross-section of the channel results in low dependency on gauge pressure.

LIST OF REFERENCE NUMERALS

[0191] 1. Microchannel [0192] 2. Substrate of first material (silicon) [0193] 3. Photoresist layer [0194] 4. First thermal SiO.sub.2 layer [0195] 5. LPCVD SiO.sub.2 layer [0196] 6. PECVD SiO.sub.2 layer [0197] 7. Channel wall [0198] 8. Metal layer [0199] 9. Wall-forming layer [0200] 10. Inlet [0201] 11. Front side [0202] 12. Back side [0203] 13. Inlet side wall [0204] 14. Inlet top wall [0205] 15. Trench [0206] 16. Trench bottom wall [0207] 17. Trench side wall [0208] 18. Second thermal SiO.sub.2 layer [0209] 19. Channel outline [0210] 20. Continuous flow channel [0211] 21. Closed channel etch-trench [0212] 22. Comb structure with comb fingers [0213] 23. Metal electrodes [0214] 24. Release hole [0215] 25. First wall-forming layer (thermal SiO.sub.2) [0216] 26. Second wall-forming layer (SiRN) [0217] 27. Third wall-forming layer (polySi) [0218] 28. Fourth wall-forming layer (SiRN) [0219] 29. Circular mould [0220] 30. Piranha solution [0221] 31. Hydroxyl group [0222] 32. Sol-gel mixture [0223] 33. Silicon dioxide layer [0224] 34. ABS wire [0225] 35. Silver layer [0226] 36. Copper layer [0227] 37. Circular channel of mould