WO2019054872A1 - Micro machined fuel gas combustion unit - Google Patents

Micro machined fuel gas combustion unit Download PDF

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Publication number
WO2019054872A1
WO2019054872A1 PCT/NL2018/050609 NL2018050609W WO2019054872A1 WO 2019054872 A1 WO2019054872 A1 WO 2019054872A1 NL 2018050609 W NL2018050609 W NL 2018050609W WO 2019054872 A1 WO2019054872 A1 WO 2019054872A1
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WO
WIPO (PCT)
Prior art keywords
combustion
micro machined
unit
fuel gas
heater
Prior art date
Application number
PCT/NL2018/050609
Other languages
French (fr)
Inventor
Yiyuan ZHAO
Henk-Willem VELTKAMP
Yaxiang ZENG
Joost Conrad Lötters
Remco John Wiegerink
Original Assignee
Berkin B.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to NL2019559A priority Critical patent/NL2019559B1/en
Priority to NL2019559 priority
Application filed by Berkin B.V. filed Critical Berkin B.V.
Publication of WO2019054872A1 publication Critical patent/WO2019054872A1/en

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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/22Fuels, explosives
    • G01N33/225Gaseous fuels, e.g. natural gas
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N25/00Investigating or analyzing materials by the use of thermal means
    • G01N25/20Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity
    • G01N25/22Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity on combustion or catalytic oxidation, e.g. of components of gas mixtures
    • G01N25/28Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity on combustion or catalytic oxidation, e.g. of components of gas mixtures the rise in temperature of the gases resulting from combustion being measured directly
    • G01N25/30Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity on combustion or catalytic oxidation, e.g. of components of gas mixtures the rise in temperature of the gases resulting from combustion being measured directly using electric temperature-responsive elements
    • G01N25/32Investigating or analyzing materials by the use of thermal means by investigating the development of heat, i.e. calorimetry, e.g. by measuring specific heat, by measuring thermal conductivity on combustion or catalytic oxidation, e.g. of components of gas mixtures the rise in temperature of the gases resulting from combustion being measured directly using electric temperature-responsive elements using thermoelectric elements

Abstract

The invention relates to a micro machined fuel gas combustion unit, comprising a substrate with at least one inlet for supplying fuel and oxygen-containing gas; a micro machined combustion tube connected to said inlet and arranged for chemically reacting the fuel to be measured with the oxygen containing gas in a combustion reaction therein; and at least one an outlet connected to said micro machined combustion tube for discharging waste gases produced in the combustion reaction. According to the invention said micro machined combustion tube is substantially thermally isolated to prevent heat loss to the surrounding, and the micro machined combustion tube has four walls defining a substantially rectangular cross section, wherein at least two opposing walls of the four walls of said combustion tube are provided with a reinforcing supporting structure.

Description

Title: Micro machined fuel gas combustion unit
Description The invention relates to a micro machined fuel gas combustion unit, comprising at least one inlet for supplying fuel and oxygen-containing gas; a micro machined combustion tube connected to said inlet and arranged for chemically reacting the fuel to be measured with the oxygen containing gas in a combustion reaction therein; and at least an outlet connected to said micro machined combustion tube for discharging waste gases produced in the combustion reaction.
Such a unit is known, for example, from US2016195482 where it is incorporated in a device for determining the combustion value of a fuel. With the device of the prior it is possible to supply the fuel to be measured to the device in a continuous manner, mix said fuel with the oxygen-containing gas being supplied and subsequently combust the fuel (whether or not continuously), and thus determine the combustion value of the fuel.
The Wobbe Index (Wl) or Wobbe number is an indicator of the interchangeability of fuel gases such as natural gas, liquefied petroleum gas (LPG), and town gas and is frequently defined in the specifications of gas supply and transport utilities. An integrated Wobbe Index (Wl) meter is desired in industries such as the central heating systems and fuel gas supplies in many countries. Miniaturized on-chip fuel gas combustion and local temperature sensing facilitate the measurement of the Wl and determine the exchangeability of different fuel gases.
One of the challenges in micro machined combustion units is to establish and maintain the microscale combustion process in the microchannel. Microflames may suffer from flame extinction, for example.
It is therefore an object of the invention to provide an improved micro machined fuel gas combustion unit, in particular with improvements with respect to flame extinction.
To this end, the invention provides a micro machined fuel gas combustion unit of the aforementioned kind, comprising a substrate with at least one inlet, a micro machined combustion tube, and at least one outlet. The micro machined combustion tube is substantially thermally isolated to prevent heat loss to the surrounding. This may be obtained by providing a thermally insulating layer about the combustion tube. In one embodiment, the thermally insulating layer is a fluid layer, such as a gas (air). In a preferred embodiment, the combustion tube is a free hanging mechanical structure, with which the thermally insulating layer is already established. The micro machined combustion unit according to the invention has four walls defining a substantially rectangular cross section, wherein at least two opposing walls of the four walls of said combustion tube are provided with a reinforcing supporting structure.
By providing a reinforcing supporting structure in two opposing side walls, it becomes possible to provide a larger span between these side walls, and with this the cross sectional area of the tube may be increased without adversely affecting the mechanical strength of the tube. With the increased cross sectional area, large internal volumes of the combustion tube compared to the outer surface area of the tube are obtained, which leads to a relatively low surface-to-volume ratio. This is beneficial, as will be explained below.
It was found that in microchannels in the prior art, where the characteristic diameter is smaller than the critical quenching diameter, the huge surface-to-volume ratio induces quenching of the flame. Due to the relatively large surface-to-volume ratios in micro/meso-channels, gas burning in a small channel suffers from flames extinction that is induced by thermal quenching and radical quenching.
It was found that to overcome the radical quenching, channel inner wall materials should be chemically inert to avoid radical adsorption and further recombination to cause radical extinction.
It was furthermore found that to overcome thermal quenching, thermal loss to the environment should be minimized to ensure to be smaller than the heat generated from combustion. With the reinforcing supporting structure according to the invention, it is possible to provide a large channel with a relatively big cross-sectional area that is necessary to maintain a continuous flame propagation to obtain the adiabatic flame temperature. This way, the micro flames stability is improved and the object of the invention is achieved.
The reinforcing supporting structure in the two opposing side walls may comprise trenches that are formed in said substrate, and that are filled with a material that is different from the material of said substrate. This way the filled trenches act like pillars increasing the mechanical strength of the combustion tube.
Further embodiments of the invention will be explained below. The substrate may be an Silicon on Insulator (SOI) substrate or similar substrate. The substrate comprises a device layer, a buried layer, such as a BOX layer, and a handle layer. In an embodiment, the Silicon On Insulator substrate comprises a device layer having a thickness of approximately 50 pm, a BOX layer having a thickness of approximately 200 nm and a handle layer having a thickness of approximately 400 pm. The micro machined combustion tube is in this embodiment at least partly provided in the device layer. A wall of the combustion tube may be bound or formed by at least part of the buried layer, in particular the BOX layer. The reinforcing supporting structure according to the invention is at least partially made from a material that is different from the device layer material to create a mechanical stable channel.
The reinforcing supporting structure may comprise a pillar element. The pillar element may extend over the entire height of the side wall. The reinforcing supporting structure may comprise or consist of a polysilicon material. Alternatively, monocrystalline silicon material can also be used.
The micro machined combustion tube may be a free standing tube for providing thermal isolation. The tube may be provided in an opening of a substrate, and may be connected with two ends to said substrate. This way, the tube forms a beam with two fixed supports with respect to said substrate. Of course, more connections to the substrate may be provided, for providing improved mechanical strength, whilst maintaining thermal isolation.
The connection of the combustion tube to the substrate may be made by means of flexure structure elements that are designed to both suspend the floating channels for mechanical strength, and deform flexibly to relax the induced thermal stress during combustion.
In an embodiment, at least one wall connecting said opposing walls comprises a membrane structure. This membrane structure connecting said opposing walls, which have said reinforcing supporting structure, functions a span between said opposing side walls. These membranes may be several millimeters long and wide and can be connected to the reinforcing supporting structure for better mechanical strength. The membrane structure may comprise a silicon rich silicon nitride (SiRN) membrane, in particular a low-stress SiRN membrane, although other materials are conceivable as well. In an embodiment, the thickness of the membrane may be in the range of 0.1 pm to 10.0 pm. The thickness may be different for different membranes. In a particular embodiment, one of the membranes has a thickness of approximately 1 .6 pm and the other of approximately 3.2 pm.
In an embodiment, at least one of the four walls of the micro machined combustion tube comprises a heater element. The heater element is arranged for heating up at least a wall of the combustion tube to provide excess enthalpy.
In an embodiment, each of said two opposing walls comprise at least one heater element. This provides for an easy way to provide excess enthalpy to the fuel gas in the combustion tube.
In an embodiment, at least one wall connecting said two opposing walls comprises said heater element.
Said heater element may be a silicon and/or platinum heater. The heater (or heaters) may be embedded in the side walls of silicon-rich silicon nitride (SiRN). Additional platinum heaters and temperature sensors may be deposited on top of the structures for accurate heat management.
In an embodiment, the reinforcing supporting structure is configured to function as a mixing device.
In an embodiment, the reinforcing supporting structure is configured to function as a heater element.
In an embodiment, the reinforcing supporting structure is configured to function both as a mixing device as well as a heater element. Thus, because of the construction of the reinforcing supporting structure as a refilled trench, the reinforcing support structure/pillar may simultaneously - and advantageously - function as a heater element as well as a mixing device, emphasizing the highly multifunctional nature of the reinforcing supporting structure.
The method according to the invention allows for fabricating mechanically stable, thermally isolated microfluidic channels with silicon heaters embedded in the sidewalls, using trench-assisted surface channel technology (TASCT).
Sidewall heating results in highly uniform heating while allowing high heating powers because of the relatively large cross-sectional area (20 pm x 50 pm) of the silicon heaters. In demonstrator devices a maximum temperature of 400°C was reached at a heating power of 1 .4 W, limited by mechanical stress. The method allows a wide range in channel widths and heater thicknesses. The latter allows variation of the power dissipation and thus the temperature profile along the length of the channel.
In most fabrication technologies for suspended microchannels, heating is only possible using heaters on top of the channels, resulting in temperature gradients within the cross-section of the channel. The method according to the invention allows to incorporate resistive heaters inside the sidewalls of the channels, enabling heating from two sides which results in a more uniform temperature profile.
Furthermore, the relatively large cross-sectional area of the heaters allows large heating powers. Important applications are high-temperature physical parameter sensing and (bio)microreactors. In most applications, flow rates up to 1 g h-1 (± 0.3 ml_ s-1 ) are desired.
Within the method the final shape of the microchannels is independent of the actual channel etch. The outline is defined by using refilled trenches as etch stops. The final channel cross-section is square or rectangular, with a height defined by the used SOI wafer and a width defined by the design. Besides the possibility of sidewall heating, the process also allows in-channel structures like strengthening pillars or mixing-enhancers. Thus, the reinforcing support structures or pillars may simultaneously function as heater(s) AN D mixer(s), which, in practice, has many advantages.
In an exemplary embodiment, straight, 8,500 pm long, channels with sidewall heaters and resistive Pt temperature sensors can be fabricated in a p-type SOI wafer (1 e-3-1 e-2 Qcm) via the proposed method. The fabrication may comprise three stages:
1 ) Microfluidic channels are fabricated by Bosch etching 50 pm deep trenches in the device layer (DL) and refilling them with a multi-layer system, Bosch etching of inlets in the handle layer (HL), reactive ion etching (RI E) of a slit pattern in the hard mask on the DL, isotropically etching the channels with XeF2, and as final step the formation of the inner wall of the microfluidic structure by low-pressure chemical vapor deposition (LCPVD) of SiRN.
2) Sensor structures and the interfacing of the sidewall heaters are fabricated by first etching openings to the sidewall heaters via RI E, directly followed by sputtering of Ta and Pt and patterning this via ion beam etching. Then, a capping layer of SiNx is deposited via plasma-enhanced chemical vapor deposition and patterned with RI E.
3) Microchannels are suspended as final step via a multi-step approach in which the silicon in the device and handle layers is etched away by XeF2.
For illustrative purposes, in another exemplary embodiment, the method comprises five stages:
Etch stop and channel outline
A highly-doped SOI substrate with a device layer of 50 pm, a BOX layer of 200 nm, and a handle layer of 450 pm will be oxidized via wet thermal oxidation at 1 150 °C in a suitable furnace. This S1O2 layer will serve as a hard mask during the trench etching. For that, it will be patterned with 3 pm wide trenches via conventional l-line photo-lithography and S1O2 reactive ion etching (RI E) in a suitable plasma etcher. The high aspect ratio trenches of 3 pm wide will be etched completely down to the BOX layer with a notching-free Bosch process with a low frequency (LF) end-step using a suitable deep reactive ion etching (DRIE) plasma system.
Then, a layer of 2 pm parylene-C will be deposited conformally via chemical vapour deposition (CVD) in a suitable system.
This layer will serve as BOX layer protection during subsequent hard mask stripping. The surface parylene-C will be etched back using an O2 plasma in a barrel etcher. Here, we will take advantage of the fact that etching on the surface has a higher rate than etching inside the trenches, i.e. the etch rate is limited by the aspect ratio of the trench.
Subsequently, the Si02 hard mask will be stripped in buffered HF (7: 1 N H F: H F) and the remaining parylene-C will be stripped away in a piranha solution (3: 1 H2S04: H202) at °90 C.
The trenches will be refilled with low-stress (50MPa) SiRN via low pressure chemical vapour deposition (LPCVD) in a suitable furnace with a Sih C /N h /lN flow. These refilled trenches will act as etch stops during the channel etch later on. Channel and chamber etch
First, a layer of Cr will be sputtered with a sputter machine on top of the low stress SiRN layer. A slit pattern will be patterned in between two adjacent trenches, which are forming the channel side walls, via 1-line photo-lithography and etched with RI E in a suitable etcher. These patterned Cr and SiRN layers will be used as etch mask during the isotropic channel etch, like in the conventional SCT process. The Cr will act as an etch mask during SiRN and Si etch, preventing the increase of the slit width in the SiRN .
The Si inside the microfluidic channels and chamber structures will be etched away through the slits with a suitable etching device. After the channel and chamber etch, the Cr layer will be stripped away in wet Cr etchant.
Channel wall formation and closure
After etching away the Si, the inner channel and chamber walls will be formed via another LPCVD run of low-stress SiRN, which will be conformally grown to a thickness slightly more than half the slit width (total layer thickness: ±1 .5 times the slit width). This way, a full closure of all the slits will be ensured, thus completely closing the channel. The use of LPCVD to close the channels is the same as in the conventional SCT process.
Electrical connects and metal deposition In order to create electrical contacts to the Si side wall heater structures, the two SiRN layers will be patterned via l-line photo-lithography and RI E in a suitable plasma etcher.
Then, the metallic layers (Pt and an adhesion layer), which serve as both the interfacing between the Si side wall heaters and the macro world, and resistive heaters and temperature sensors will be sputtered.
The adhesion between Pt and the substrate can only withstand elevated temperatures of above 500 °C when a proper adhesion layer is used. From previous work it is learned that Ti will not survive elevated temperatures and causes delamination, hole formation and agglomeration of the Pt. Therefore, an adhesion layer of Ta will be used, which is known to withstand higher temperatures. First, a thin 5 nm Ta layer will be sputtered in a sputter device, directly followed by a 400 nm Pt layer. The metallic layers will be patterned via ion beam etching in an suitable etcher.
Channel release
As a final step, the channels and chambers will be released in order to create a suspended system, which is thermally isolated from the bulk Si. This release will be done in two steps.
First, a directional etch will be performed with the Bosch process, after which an isotropic etch with will be used to remove the remaining Si in all directions, creating cavities of sufficient size.
Both steps will be performed in a suitable (same) etcher. The hole etched with the Bosch process will reduce the etch time of the isotropic etch, and therefore limits the exposure time of SiRN to SF6.
According to an aspect, a device for determining the combustion value of a fuel is provided, the device comprising:
- a micro machined fuel gas combustion unit according to the invention and as described above;
- a measurement unit for measuring at least a measure of the amount of energy released by the combustion reaction; and
- a control unit connected to said measurement unit and arranged for determining the combustion value of the fuel based on the measured amount of energy released by the combustion reaction.
With the combustion unit according to the invention, such a device becomes more reliable as thermal and radical quenching is reduced.
The combustion unit may be provided a chip, in particular a system chip with a silicon substrate provided on a carrier. In this embodiment the device comprises a system chip that is provided with said combustion unit connected to the fuel inlet and the gas inlet, which combustion unit is provided with a combustion chamber for chemically reacting the fuel to be measured with the oxygen containing gas in a combustion reaction therein; a gas outlet connected to the combustion chamber for discharging waste gases produced in the combustion reaction; as well as
means for measuring at least a measure of the amount of energy released by the combustion.
To determine a measure of the combustion value of the fuel to be measured, it is preferred to measure a temperature increase resulting from the combustion. In one embodiment this can be realised in that the device is provided with at least one temperature measuring element which may be disposed on or near the combustion unit. The means for measuring at least a measure of the amount of energy released upon combustion thus comprise a temperature measuring element. In one embodiment, the temperature measuring element is disposed at a distance from the combustion chamber, such that the temperature measuring element will be exposed to smaller temperature increases. The temperature measuring element may be an integrated platinum resistance sensor.
In an embodiment, the device comprises at least one flow measurement unit provided upstream of the micro machined combustion unit. The device according to the invention preferably comprises a flow measurement unit for determining the density and/or the flow rate of the fuel. A suitable flow measurement unit is a flow measurement unit of the Coriolis type, for example, whose construction and operation are known per se to those skilled in the art, as follows from EP 1 719 983, for example. Using such a flow measurement unit, the flow rate (mass flow and/or volume flow) of the fuel to be measured can be determined in a relatively inexpensive and reliable manner. Said determination can take place just before the fuel reaches the combustion chamber, which will further increase the precision of the device. Additionally, the flow measurement unit of the Coriolis type is suitable for determining the density of the fuel to be measured while the fuel to be measured is being supplied. Thus it becomes possible to measure the density also momentarily, so that variations in said density can be taken into account in the determination of the combustion value, if desired, which further increases the precision. The use of the aforesaid flow measurement unit, for example of the Coriolis type, makes it possible to realise a relatively compact flow measurement unit, so that the entire device can be relatively small. Another advantageous aspect is the fact that such a sensor is relatively inexpensive, since comparable technologies may be used to incorporate these units into a single substrate / chip. The flow measurement unit of the Coriolis type thus makes it possible to realise a compact and manageable construction, and in addition to that it is relatively inexpensive.
An additional advantage of a flow measurement unit of the Coriolis type is that it is very suitable for measuring the flow rate and/or the density both of a gaseous fuel and of a liquid fuel, or even of combinations thereof. Thus it is possible to determine the combustion value both of gaseous and of liquid fuels.
As indicated before, it is advantageous when the control unit is arranged for determining the Wobbe index of the fuel.
In an embodiment, the measurement unit comprises an oxygen sensor. The oxygen sensor may in one embodiment be provided downstream of the combustion chamber for determining a measure of the amount of residual oxygen in the combustion gas. Alternatively or additionally, catalytic detection of non-combusted components may take place. Such an embodiment can be of relatively simple and compact construction.
The invention will next be explained by means of the accompanying drawings and description of the figures. In figures 1 -9 different stages of fabricating a micro machined channel from a substrate are shown, and figures 10-15 show how a micro machined fuel gas combustion unit according to the invention may be made. In particular it is shown:
Fig. 1 - a schematic cross sectional view of a substrate, such as a Silicon on Insulator wafer, wherein a hard mask is formed using wet thermal oxidation;
Fig. 2 - Patterning of the hard mask via RI E, and trench formation via
BOSCH DRI E etching;
Fig. 3 - BOX protection with parylene-C via CVD;
Fig. 4 - Parylene-C etching with O2 plasma and hard mask stripping in BHF;
Fig. 5 - Parylene-C stripping in piranha, dry thermal oxidation of SI to create isolation layer, and trench filling with polycrystalline Si via LPCVD;
Fig. 6 - Patterning of isotropic etch mask with RI E;
Fig. 7 - Isotropic etching of Si to create channels;
Fig. 8 - Deposition of low-stress SiRN via LPCVD;
Fig. 9 - Release Etching of channels with isotropic etching; Fig. 10 - A schematic cross sectional view of a substrate, such as a Silicon on Insulator wafer, with a hard mask, that is used for forming a combustion unit according to the invention;
Fig. 1 1 - Formation of a plurality of desired trenches;
Fig. 12 - Filling of the plurality of trenches;
Fig. 13 - Etching of the inlet hole and channels;
Fig. 14 - Sealing the channel and sputter metal on top, etching the outlet from the combustor chamber ceiling membrane;
Fig. 15 - Etching side and bottom cavities to fully release the channels from the substrate.
Figures 1 - 9 schematically show an embodiment of a method of fabricating a micro machined channel, using Trench-Assisted Surface Channel Technology (TASCT), which may be used in fabricating a micro machined combustion unit according to the invention.
In general, the method comprises the steps of:
- Providing a substrate 1 1 of a first material (Fig. 1 );
- Forming at least two trenches 21 , 22 in said substrate 1 1 by removing at least part of said substrate 1 1 (Fig. 2);
- Forming at least two filled trenches 31 , 32 by providing a second material different from said first material and filling said at least two trenches 21 , 22 with at least said second material (Fig. 5);
- Forming an elongated cavity 51 in between said filled trenches 31 , 32 by removing part of said substrate 1 1 extending between said filled trenches 31 , 32 (Fig. 7); and
- Forming an enclosed channel 5 by providing a layer of material 61 in said cavity 51 and enclosing said cavity 51 (Fig. 8).
As shown in Fig. 9, said enclosed channel 5 may be partially released from said substrate by defining an exterior of said channel 5 by removing at least part of said substrate 1 1 . By removing part of said substrate for defining an exterior of said channel, the fabrication of free-hanging, mechanical stable and thermally isolated channels may be realized.
The process will now be described in more detail.
Fig. 1 shows that an SOI wafer 1 1 (having for example a 50 pm device layer 13, a 200 nm BOX layer 14, and a 400 pm handle layer 15) is oxidized via wet thermal oxidation (for instance at 1 150 °C) for creating a hard mask 12. This S1O2 layer 12 is patterned with 3 pm wide trenches to create the actual mask 12.
Fig. 2 shows that the high aspect ratio trenches 21 , 22, which may for example be 3 pm wide, are etched completely down to the BOX layer 14 with a Bosch process using a DRI E plasma system.
Then, referring to Fig. 3, the trenches 21 , 22 are filled with a polymer 71 , in particular parylene-C, which in the embodiment shown is deposited as a conformal 2 pm thick layer via CVD. The chosen thickness is, in this case, more than half the trench 21 , 22 width, ensuring full filling of the trenches 21 , 22.
Fig. 4 shows that the surface parylene-C is etched back using an 02 plasma in a barrel etcher. Advantageously, etching on the surface has a higher rate than etching inside the trenches, which means that the S1O2 hard mask 12 is stripped in BHF (7: 1 NhUFi H F) whilst protecting the BOX layer 14. The remaining polymer 71 is stripped away in piranha solution (3: 1 H2S04: H202), for instance at 90 °C.
However, the step of filling the trenches 21 , 22 with a polymer 71 , in particular parylene-C, can be omitted. Therein, the hard mask 12 is removed and it is accepted that the BOX layer 14 in the trenches 21 , 22 is etched.
In the next step, shown in Fig. 5, the trenches are refilled. In an embodiment the trenches are filled with a material comprising at least one non-Silicon layer. In the embodiment shown the trenches are refilled with a multilayer system consisting of a thin S1O2 etch-stop layer 35 grown via dry thermal oxidation and subsequent filling via LPCVD of polycrystalline Si 36. This way, filled trenches 31 , 32 are obtained. Of course, other materials such as Silicon Nitride are conceivable as well. In that case, the Silicon Nitride may already function as an etch-stop 5 layer for some etching techniques, meaning that the additional S1O2 35 layer is not necessary anymore. Thus in effect, the trench may be filled, in an embodiment, with a filling material, which may be a layered material or a single material. The filling material may function as an etch stop layer for specific etching techniques.
Preferably, the polycrystalline Si 36 is then removed from the surface of the wafer 1 1 by means of isotropic silicon etching to prevent problems when etching the cavity 51 .
As shown in Fig. 6 a slit pattern 41 , which will be used as isotropic etch mask, is etched between two adjacent trenches 31 , 32 with RIE. Now referring to Fig. 7, it is shown that the Si of the device layer 13 is etched away through the slits 41 with an isotropic gas phase process, stopping on the S1O2 etch-stop 35. This way the cavity 51 is formed.
After etching away the Si 13 for forming the cavity 51 , the inner channel wall of the channel 5 is formed via LPCVD of low-stress silicon rich silicon nitride 61 , which is conformally grown to a thickness slightly more than half the slit 41 width, ensuring full closure of all slits 41 (see Fig. 8).
As an optional last step, the channels are completely etched free from the top and the bottom, with a suitable etching process, for example an isotropic gas phase etch or semi-isotropic RI E etch.
The TASCT process as described herein may start with a single SOI wafer and may use XeF2 to etch channel 5 through slits 41 arrays and release channel 5 from the bulk substrate 1 1 , due to XeF2 has fast etch rate and high selectivity for silicon over silicon dioxide. Other etching techniques are possible as well. In the depth direction, the Box layer 14 can act as the silicon etch stop through the slits 41 , therefore channels 5 are confined in the device layer 13. In the planar direction, high aspect-ratio trenches 21 , 22 are etched in the device layer 13, all the trench walls 31 , 32 are coated with thin layer of thermal oxide 35 as XeF2 etch stop. Therefore, within the trenches 31 , 32 confined device layer 13, through the slits 41 arrays channels 5 can be etched and result with the designed shape and sizes. Outside the trench confined channels, the handle layer 15 and device layer 13 silicon can be etched by XeF2 until reaching the etch stop, which may be the thermal oxide trench walls 35.
As will be seen, the side walls of the channel 5 formed may be given a heater function. To this end, highly doped device layer silicon 13 is encapsulated/sandwiched within two refilled trenches 31 , and these trenches all have thermal oxide coatings 35 and can provide electrical isolation from the bulk silicon substrate 13. In this way, by heating up the channel from the sidewall directions, thermal loss to the environment can be minimized.
The channel 5 top and bottom surfaces may be made from low-stress SiRN membranes, they can be connected by the pillars and sidewalls made from trenches 31 , 32 to achieve good mechanical strength. Therefore these thin membranes can be several millimeters long or wide.
The top and bottom membrane thickness may be determined by the width of the rectangular slits 41 . Heaters and sensors can be placed on top of the channel ceiling surface to efficiently heat up the channel 5 from the top and sense temperature profile. The bottom membrane may be made very thin and transparent which gives good access to microscopic views of flame location in the channel.
The method described above also allows springs and suspensions structures to be made. In a first embodiment, channel structure or solid silicon in the device layer may be used, as they are defined by trenches, therefore any desired shapes such as serpentine springs can be made. Second choice is using a thin membrane of 500nm thick TEOS, which functions as the slits hard mask, as the spring or suspension by etching all the silicon beneath it by XeF2. In conclusion, with the method according to the invention a lot of freedom in designing the desired shape and size for the flexure and suspension is possible, due to the high selectivity of XeF2 over silicon than silicon dioxide.
The channels obtainable with the method described above may be made with a relatively large cross-sectional area, which is advantageously in terms of fluid dynamics (boundary layer, laminar/turbulent flow, flow development).
According to the invention, the method allows the fabrication of a small sized combustion chamber to burn gas blends on chip and measure adiabatic flame temperature. With additional integration of a micro-Coriolis sensor, it is possible to determine the Wobbe Index of any gas blend. In particular when a free-hanging channel is used, thermal heat loss to the environment may be minimized to ensure to be smaller than the heat generated from combustion. In general, a large channel with bigger cross-sectional area may be formed, as this is advantageous to maintain a continuous flame propagation to obtain the adiabatic flame temperature. To overcome the radical quenching, channel inner wall materials may be made chemically inert to avoid radical adsorption and further recombination to cause radical extinction.
Fabrication of a micro machined combustion unit according to the invention uses process steps as described with reference to Figs. 1 -9, and will next be explained with reference to Figs. 10-15.
Fig. 10 shows the SOI wafer 1 1 1 with device layer 1 13, BOX layer 1 14 and handle layer 1 15.
Fig. 1 1 shows how a number of trenches 121 -127 are formed in the device layer 1 13. To this end, a hard mask (not shown here, compare reference 12 in Figs. 1 and 2) may be provided on top; the trenches 121 -127 may be formed by means of BOSCH DRI E etching; parylene-C may be provided in the trenches via CVD; O2 plasma may be used for parylene-C etching and piranha may be used for parylene-C stripping, as described with respect to Figs. 3-4.
As shown in Fig. 12, the trenches 121 -127 may be filled for creating filled trenches 131 -137. As described before, this process may use a multilayer system consisting of a thin S1O2 etch-stop layer grown via dry thermal oxidation and subsequent filling via LPCVD of polycrystalline Si. This way, filled trenches 131 -137 are obtained. Due to the filling of the trenches, a top multilayer 236 is provided on top of the substrate. This top layer is similar to the layer 35, 36 as shown in Fig. 5. For reasons of clarity it is shown here as a single layer, and is shown separated from the filled trenches, although in fact the trenches 131 -137 and the top multilayer 236 are integrally connected.
Fig. 13 shows how a mask is provided in the top multilayer 236, with which the combustion chamber 151 and an outlet 154 may be produced. To this end, the process as described with reference to Fig. 5, 6 and 7 may be used. Inlet 153 may be made in a similar way.
Fig. 14 shows the creation of (partially) enclosed channels 151 , 154 by filling with a layer of material (61 , see Fig. 8). For reasons of conciseness, this layer of material is indicated with reference sign 336 in Fig. 14; however this layer 336 may be a multilayer, similar to the layers 35, 36, 61 as shown in Fig. 8. Fig. 14 also shows the formation of heaters 201 , 202 and 203 in side walls and top wall of the combustion chamber 151 . For the side walls this may comprise that trenches 133, 135 actually consist of two closely spaced trenches that are refilled. Encapsulated /sandwiched within two refilled trenches 31 is provided a highly doped device layer silicon 13. These trenches all have thermal oxide coatings 35 and can provide electrical isolation from the bulk silicon substrate 13. In this way, by heating up the channel from the sidewall directions, thermal loss to the environment can be minimized.
Fig. 15 shows how the combustion chamber 151 and outlet 154 are released from the surrounding substrate. This way an isolated combustion chamber 151 is obtained.
The process described above with respect to Figs. 10-15 may use a single SOI wafer where channels are realized in the device layer. The fabricated channels 153, 151 , 154 comprise a rectangular cross-section. High aspect ratio trenches of 3 pm wide and 50 pm deep, for example, are used to fabricate these large channels. It will be understood that other ratios and dimensions are conceivable as well. These deep trenches 121 -127 can be refilled with polysilicon to function as pillars to support large membranes. In an embodiment the trenches are filled with a material comprising at least one non-Silicon layer. These refilled trenches 131 -137 may also be the channel 151 sidewalls and define the desired channel shape and width, in particular when the channel sidewall functions as an etching stop layer. The height of the channel side walls may be 50 pm, as defined by the device layer 1 13 thickness of the SOI wafer 1 1 1 . The top and bottom of the channels are formed by low-stress SiRN membranes, which may have a thickness of 3.2 pm and 1 .6 pm, respectively. These thin membranes can be several millimeters long and wide and can be connected by the pillars for better mechanical strength. The structures may be released from the substrate 1 1 1 by selective isotropic etching of the handle layer 1 15 and device layer 1 13 (see Fig. 15). The result is a thermally stable, chemically inert and strong structure with good thermal insulation. Highly doped silicon heaters (201 , 203) are defined by the refilled trenches 133, 134 to heat up the combustor chamber 151 side walls. As stated before, the reinforcing support structures or pillars defined by the refilled trenches may simultaneously function as heater(s) and mixer(s), which, in practice, has many advantages.
To provide thermal isolation, relatively large cavities may furthermore be provided underneath and to the sides of the combustion chamber. The cavity underneath may have a height of for instance up to 400 pm, such as 200 - 400 pm, for instance 300 - 400 pm. The cavities to the side may have a width (each) of for instance up to 400 pm, such as 200 - 400 pm, for instance 300 - 400 pm.

Claims

1 . Micro machined fuel gas combustion unit, comprising a substrate with
at least one inlet for supplying fuel and oxygen-containing gas;
- a micro machined combustion tube connected to said inlet and arranged for chemically reacting the fuel to be measured with the oxygen containing gas in a combustion reaction therein, wherein said micro machined combustion tube is substantially thermally isolated by means of a thermally isolating layer to prevent heat loss to the surrounding; and
- at least one outlet connected to said micro machined combustion tube for discharging waste gases produced in the combustion reaction;
wherein the micro machined combustion tube has four walls defining a substantially rectangular cross section forming a channel, wherein at least two opposing walls of the four walls of said combustion tube are provided with a reinforcing supporting structure and wherein the channel inner wall materials are chemically inert.
2. Micro machined fuel gas combustion unit according to claim 1 , wherein said reinforcing supporting structure comprises polycrystalline or monocrystalline silicon.
3. Micro machined fuel gas combustion unit according to claim 1 or 2, wherein at least one wall connecting said opposing walls comprises a membrane structure.
4. Micro machined fuel gas combustion unit according to claim 3, wherein said membrane structure comprises silicon rich silicon nitride.
5. Micro machined fuel gas combustion unit according to claim 1 -4, wherein at least one of the four walls of the micro machined combustion tube comprises a heater element.
6. Micro machined fuel gas combustion unit according to claim 5, wherein each of said two opposing walls comprise a heater element.
7. Micro machined fuel gas combustion unit according to claim 5 or 6, wherein at least one wall connecting said two opposing walls comprises said heater element.
8. Micro machined fuel gas combustion unit according to claim 5, 6 or 7, wherein said heater element is a silicon heater or a platinum heater.
9. Micro machined fuel gas combustion unit according to any one of the preceding claims, wherein the reinforcing supporting structure is configured to function as a mixing device.
10. Micro machined fuel gas combustion unit according to any one of the preceding claims, wherein the reinforcing supporting structure is configured to function as a heater element.
1 1 . Micro machined fuel gas combustion unit according to any one of the preceding claims, wherein the reinforcing supporting structure is configured to function both as a mixing device as well as a heater element.
12. Device for determining the combustion value of a fuel, the device comprising:
- a micro machined fuel gas combustion unit according to any one of the preceding claims;
- a measurement unit for measuring at least a measure of the amount of energy released by the combustion reaction; and
- a control unit connected to said measurement unit and arranged for determining the combustion value of the fuel based on the measured amount of energy released by the combustion reaction.
13. Device according to claim 12, wherein said measurement unit comprises at least a temperature sensor provided on or near the micro machined combustion tube.
14. Device according to claim 12 or 13, wherein the device comprises at least one flow measurement unit provided upstream of the micro machined combustion unit.
15. Device according to claim 14, wherein the flow measurement unit is of the Coriolis type.
16. Device according to ant one of the preceding claims 12-15, wherein the control unit is arranged for determining the Wobbe index of the fuel.
17. Device according to any one of the preceding claims 12-16, wherein the measurement unit comprises an oxygen sensor.
PCT/NL2018/050609 2017-09-15 2018-09-17 Micro machined fuel gas combustion unit WO2019054872A1 (en)

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Citations (4)

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Publication number Priority date Publication date Assignee Title
EP1719983A1 (en) 2005-05-02 2006-11-08 Berkin B.V. Coriolis mass flow meter using contactless excitation and detection
WO2009117218A1 (en) * 2008-03-18 2009-09-24 Solid-State Research Cmos-compatible bulk-micromachining process for single-crystal mems/nems devices
US20110083710A1 (en) * 2005-07-08 2011-04-14 Ying Hsu Energy-Efficient Micro-Combustion System for Power Generation and Fuel Processing
WO2014104889A1 (en) * 2012-12-27 2014-07-03 Berkin B.V. Device and method for determining the combustion value of a fuel

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EP1719983A1 (en) 2005-05-02 2006-11-08 Berkin B.V. Coriolis mass flow meter using contactless excitation and detection
US20110083710A1 (en) * 2005-07-08 2011-04-14 Ying Hsu Energy-Efficient Micro-Combustion System for Power Generation and Fuel Processing
WO2009117218A1 (en) * 2008-03-18 2009-09-24 Solid-State Research Cmos-compatible bulk-micromachining process for single-crystal mems/nems devices
WO2014104889A1 (en) * 2012-12-27 2014-07-03 Berkin B.V. Device and method for determining the combustion value of a fuel
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