GB2533294A

GB2533294A - Micro-hotplates

Info

Publication number: GB2533294A
Application number: GB1422253.3A
Authority: GB
Inventors: Zeeshan Ali Syed; Govett Matthew; Jonathan Stacey Simon
Original assignee: Cambridge CMOS Sensors Ltd
Current assignee: Cambridge CMOS Sensors Ltd
Priority date: 2014-12-15
Filing date: 2014-12-15
Publication date: 2016-06-22
Anticipated expiration: 2034-12-15
Also published as: GB2533294B

Abstract

A micro-hotplate device, particularly for a resistive gas sensor, includes a dielectric membrane 4,5 formed on a semiconductor substrate 1 comprising a back-etched portion, a heater 2 formed within or over of the dielectric membrane, and an electrode layer 7 formed over the dielectric membrane. The electrode layer 7 includes a material comprising titanium nitride (TiN). A gas sensitive material (17, fig. 13) may be formed over the electrodes 7, e.g. aluminium oxide or a polymer. Using TiN as an electrode material enables the device to be fabricated in a CMOS processing facility. The electrodes may be square, circular, interdigitated, concentric rings or spiral.

Description

M icro-hotplates

Field of the Invention

This invention relates to a micro-hotplate device, particularly but not exclusively, for use as a resistive gas sensor.

Background of the Invention

It is well known to fabricate micro-hotplates on a silicon substrate for use in resistive gas sensors. Such structures include a micro-heater embedded within a thin dielectric membrane, typically including silicon dioxide and/or silicon nitride. The membrane thermally isolates the heater from the substrate resulting in low power consumption.

Such structures can be used for resistive gas sensors, by having electrodes on top, onto which a gas sensitive material is deposited to measure the change in resistance in presence of gas. For example, U. Dibbern et. al, "A substrate for thin-film gas sensors in microelectronic technology," Sensors and Actuators B, 1990 describes the design of a micro-hotplate using NiFe alloy as a heater material, in an oxy-nitride membrane. The device has electrodes on top for use in gas sensing. Similarly M. Stankova et. al, "Detection of SO2 and H2S in CO2 stream by means of WO3 -based micro-hotplate sensors" Sensors and Actuators B, 2004 describes micro-hotplates based on a polysilicon heater within an oxy-nitride membrane. M. Baroncini et. al., "Thermal characterization of a microheater for micromachined gas sensors," describes a gas sensor with a micro-heater made from platinum.

Similarly, many such reports can be found in literature using micro-hotplate devices for gas sensors. Reference to some of these are given in I.Simon et.al, "Micromachined metal oxide gas sensors: opportunities to improve sensor performance," Sensors and Actuators B (2001), and S.Z. Ali Et. Al, "Tungsten-Based 501 Microhotplates for Smart Gas Sensors" Journal of MEMS 2008. Designs include CMOS based, as well as nonCMOS compatible devices. The heater materials used in these designs include platinum, tungsten, polysilicon, single crystal silicon as well as MOSFET heaters.

Different membrane materials are used such as silicon dioxide, silicon nitride or porous silicon.

It has been demonstrated that the micro-hotplates include electrodes to measure the resistance of the sensing material. The fabrication of such electrodes that are exposed on top of the membrane has also been demonstrated. Examples of relevant literatures describing micro-hotplates with electrodes are as follows: M. Afridi Et. al, "A monolithic CMOS Microhotplate-Based Gas Sensor System," IEEE Sensors Journal; M. Graf et. al. "CMOS microhotplate sensor system for operating temperature up to 500°C" Sensors and Actuators B 2005; J.F. Creemer et. al, "Microhotplates with TiN heater," Sensors and Actuators A, 148, pp. 416-421, 2008; and P. Parthangal et. al. "Direct synthesis of tin oxide nanotubes on microhotplates using carbon nanotubes as templates," J. Mater. Res, Vol 26, pp. 430-436, 2011.

Summary

In accordance with one aspect of the present invention there is provided a micro-hotplate comprising: a dielectric membrane formed on a semiconductor substrate comprising a back-etched portion; a heater formed within or over the dielectric membrane; and an electrode layer formed over the dielectric membrane, wherein the electrode layer comprises a material comprising titanium nitride (TiN).

In accordance with a further aspect of the present invention there is provided a method of manufacturing a micro-hotplate, the method comprising: forming a dielectric membrane on a semiconductor substrate comprising a back-etched portion; forming a heater within or over the dielectric membrane; and forming an electrode layer over the dielectric membrane, wherein the electrode layer comprises a material comprising titanium nitride (TiN).

Further aspects and preferred features of the invention are set out in the accompanying claims.

The present invention provides a fully CMOS-compatible or CMOS-based micro-hotplate design based on a closed dielectric membrane structure with titanium nitride (TiN) as the material for the resistive sensing electrodes. Here the closed dielectric membrane refers to a dielectric membrane in which the dielectric membrane is released by back etching of the underlying semiconductor substrate. The membrane is not connected to the substrate using any bridge structure (like in suspended membranes), but is supported along its entire perimeter by the substrate. The closed membrane structure is advantageous because it can be fabricated in the CMOS process using relatively less processing steps. Furthermore, the advantage of the closed-form back-etched membrane is that it is generally mechanically stable as the dielectric layers of the membrane are fully supported by the substrate. TiN is used in many CMOS foundries for various processes, and as an adhesion layer between metal and dielectric. Therefore it is possible to manufacture the CMOS-based micro-hotplate and the electrodes within the same CMOS process. Additionally TiN can make good contact with gas sensing material, and is resistant to corrosion. TiN is therefore an improved material for manufacturing the gas sensing electrodes.

Most of the prior-art documents on micro-hotplate based gas sensors disclose gold or platinum electrodes. However both gold and platinum are not compatible with CMOS processes or CMOS processing steps. As a result the micro-hotplates have to be either fabricated in a non-CMOS process, or the electrodes have to be deposited as a postCMOS process. Post-CMOS deposition has its own challenges in that the electrodes have to be deposited before back etching of the substrate, in which case the equipment used for the back etching will be contaminated. Alternatively, the deposition has to be done after back etching, in which case it is very difficult to process wafers with membranes, as most equipment is designed to handle wafers without membranes. For example, many wafer handling systems use a vacuum, which can damage the membranes on wafers.

In addition most high volume fabrication facilities are reluctant to work with gold or platinum as it needs to be kept well away from their CMOS fabrication facilities and needs separate handling, infrastructure and protocol.

The present invention overcomes these disadvantages. The micro-hotplate device may be used for gas sensing, where the micro-hotplate includes a micro-heater embedded within or on top of a dielectric membrane that is supported on a semiconductor substrate, with the membrane having sensing electrodes exposed at the top made from titanium nitride (TiN). This is a CMOS compatible material and therefore the electrodes may be manufactured within the same CMOS processing steps for manufacturing the dielectric membrane. The use of TiN as the material of the electrodes of the CMOS-based microhotplate having back-etched substrate is therefore advantageous. The present invention can also use tungsten as the electrode material. Tungsten is also a fully CMOS compatible material.

The device is preferably fabricated using CMOS-based or CMOS-usable materials.

Here the terms "CMOS-based" material or "CMOS-usable" material refer to the materials which are compatible in the state-of-art CMOS processing steps or CMOS process. In this case the heater may be a resistive heater made from CMOS materials such as tungsten, aluminium, titanium, polysilicon, molybdenum or single crystal silicon. The heater may also be a MOSFET heater to allow easier drive control. The dielectric membrane itself may include layers of silicon dioxide and/or silicon nitride as well as silicon on glass. The starting wafer may be either bulk silicon, or a silicon on insulator (S01) wafer. The membrane may be formed by back etching the supporting semiconductor substrate. The membrane cavity may either have near vertical sidewalls (formed by the used of Deep Reactive Ion Etching (DRIE)), or may have sloping sidewalls (formed by the used of anisotropic or crystalographic etching methods such as potassium hydroxide (KOH) or TetraMethyl Ammonium Hydroxide (TMAH)). The use of DRIE allows circular membranes to be made more easily.

Alternatively the device may also be fabricated with some or all non-CMOS materials.

For example the heater may be fabricated from platinum, or a supporting semiconductor substrate other than silicon may be used.

The heater in the device may be circular, rectangular, or any other shape. It may be patterned in a meander shape, ring shape or multiringed shape. It can have electrical connections through either two connections, or by four connections to accurately monitor the resistance. In the case of a resistive heater, it may be formed on any of the available conductive layers in the process. The heater may have an additional material layer above or below it, such as titanium or titanium nitride to improve the adhesion of the material. Additionally the device may have more than one heater either made within the same layer or with different layers or materials. The heater may also be formed on top of the membrane and in such an arrangement the heater is the same plane as the TiN electrodes. The heater in this case may include titanium nitride or any other metal.

The dielectric membrane itself may be circular, rectangular, or rectangular shaped with vias and/or rounded corners to reduce the stresses in the corners, but other shapes with or without vias are possible as well. The top passivation of the device may be either silicon dioxide or silicon nitride. To improve the mechanical stability of the membrane, it may also have other structures embedded within the membrane. For example beams of metal or single crystal silicon may be embedded, or a metal or single crystal silicon layer covering the whole area of the membrane, or spin on glass for planarization and/or stress relief.

The device may further include a temperature sensor formed or embedded within the dielectric membrane. This may be a resistive temperature sensor made from a metal, or polysilicon, or single crystal silicon, or can be a diode. When using a CMOS process, the diode may be made with the same CMOS layer used in the CMOS process to make transistors such as n-channel and p-channel FETs. When the semiconductor structure includes a MOSFET structure manufactured using the CMOS processing steps, the cathode of the diode may therefore be formed by using the source of the drain layers of n-channel MOSFET while the anode of the diode can be formed by using the same source or drain layers of a p-channel MOSFET.

The device may also have a heat spreading plate made of metal or polysilicon, or in the case of Silicon on Insulator substrates, the device may have a single crystal silicon plate. The role of the spreading plate is to spread more uniformly the heat and thus improve the uniformity of the temperature in the heater area.

The electrodes, made from titanium nitride, can be of any shape, for example square or circular. They may be interdigitated or just two electrodes side by side or any other shape that allows resistance or capacitance measurement of a sensing material deposited on top of them. They may also be multiringed with the rings interdigitated. The titanium nitride layer used to form the electrodes may also have an additional layer or layers below it to improve the adhesion to the membrane. The titanium nitride layer may also have tracks that electrically connect the electrodes to the bond pads. These tracks may be covered by a passivation material such as silicon dioxide and/or silicon nitride to protect it, while leaving the electrodes exposed. The passivation may extend to the very edge of the electrodes, creating a small "well" that makes it easier to deposit the sensing material within the electrode area. Alternatively the passivation layer may cover most of the electrode area while leaving some parts of each electrode exposed.

To use the device as a gas sensor, a gas sensitive material may be deposited on the titanium nitride electrodes. The sensitive material may be a metal oxide, a polymer, or a nanomaterial such as graphene or carbon nanotubes, or a combination of these. These may also be doped or mixed with other materials to improve performance. In case a metal oxide is used it could be any metal oxide such as tin oxide, tungsten oxide, zinc oxide, chromium oxide or any combination of metal oxides. The device can also be used as a humidity sensor. In this case the material deposited on top of the TiN electrodes may be sensitive to humidity, for example a metal oxide such as aluminium oxide, or a conductive polymer. The sensing material can be any material sensitive to gas or humidity and is not limited by the examples given. The sensing material may be deposited by any method, for example screen printing, sputtering, chemical vapour deposition (CVD), ink-jet, drop coating, flame spray pyrolysis or atomic layer deposition (A LD).

Arrays of such micro-hotplate may also be fabricated on a single chip. The micro-hotplates in the array may be either identical or different. They may have a different shape, different heater material, different gas sensitive material, and even a different electrode material. However, at least one of the micro-hotplates may have titanium nitride as the electrode material.

The TiN electrodes may be plated with another material onto which the gas sensing material is then deposited. For example, an electroless plating process may be used to plate a metal (for example gold or platinum) onto the TiN electrodes. The TiN layer may provide the patterning of the electrodes and the tracks to the bond pads. The TiN layer may operate as a seed layer for the electroless deposition process. In this case the plated metal may be used to form the contact with the sensing material. This could be used for example in the case where the gas sensing material may react with the titanium nitride electrodes at room temperature or at high temperatures.

The dielectric membrane may have one or more small holes. The holes perform the function that they prevent the membrane cavity from having an air tight seal -which prevents stresses due to air expansion in the cavity during device use.

The device may integrate analogue or digital CMOS circuitry on the same chip ( the resistive gas sensor). This may be achieved because of the use of the CMOS technology to fabricate the micro-hotplate. The circuitry may be a drive circuit for the heater, including a simple current supply, for example using a current mirror circuit, or a more complex circuit to allow heater drive through a constant voltage, constant current or constant power circuit, or a pulse width modulation (PWM) drive. There may also be circuitry to measure the temperature sensor in the membrane, and additionally using it to have a feedback loop to allow a constant temperature control of the heater. Circuitry may also be implemented to drive the heater or the sensing material using a bidirectional current or voltage.

Other circuitry may be read-out circuitry for the temperature sensor, or the sensing layer. This may include an amplifier, a filter, as well as an analogue to digital converter. Digital circuits may also be integrated to allow digital processing of the signal. Furthermore, an off-membrane temperature sensor, based on a thermodiode, a resistive temperature sensor, or Vptat or Iptat circuits may also be integrated.

Brief Description of the Preferred Embodiments

Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 illustrates a schematic cross-section of a micro-hotplate including an SOI starting wafer with titanium nitride electrodes for resistive gas sensing; Figure 2 illustrates a schematic cross-section of an alternative micro-hotplate including a bulk semiconductor starting wafer with titanium nitride electrodes for resistive gas sensing; Figure 3 illustrates a schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes where there is an additional layer below the heater to improve adhesion of the heater material layer; Figure 4 illustrates a schematic cross-section of an alternative micro-hotplate where the titanium nitride layer above the dielectric membrane has an additional layer below it to improve the adhesion of the layer; Figure 5 illustrates a schematic cross-section of an alternative micro-hotplate where the membrane cavity has sloping sidewalls formed by the used of anisotropic back etching; Figure 6 illustrates a schematic cross-section of an alternative micro-hotplate including a diode temperature sensor underneath the heater; Figure 7 illustrates a schematic cross-section of an alternative micro-hotplate including heat spreading plates above and the below the heater; Figure 8 shows a top view of a square micro-hotplate with Titanium Nitride electrodes; Figure 9 shows a top view of a circular micro-hotplate with Titanium Nitride electrodes; Figure 10 shows a top view of some possible designs of electrodes for measuring the resistance of the sensing material on resistive gas sensors; Figure 11 shows a top view of a CMOS chip with a micro-hotplate based gas sensors and interface circuitry on the same chip; Figure 12 shows a top view of a 2x2 array of micro-hotplates; Figure 13 shows a schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes and a gas sensitive material deposited on the electrodes; Figure 14 shows a schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes and a material plated on the titanium nitride material; Figure 15 shows a schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes with a passivation layer on top of the electrode tracks which extends to the electrode region; Figure 16 shows a schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes and a passivation on top of the electrode tracks, and a sensing material on top of the electrodes; Figure 17 shows the schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes and a passivation on top of the titanium nitride tracks which also covers the much of the electrode area while leaving parts/portions of the electrodes exposed; Figure 18 shows the schematic cross-section of an alternative micro-hotplate with both the electrodes and the heater made in the same layer of titanium nitride, and Figure 19 shows the top view of a micro-hotplate with titanium nitride electrodes, and a ring heater also made in the same titanium nitride layer.

Detailed Description of the Preferred Embodiments

Figure 1 shows a schematic cross-section of a micro-hotplate made in an Sal process with titanium nitride electrodes for resistive gas sensing. The device includes a silicon substrate 1 and a membrane including buried oxide 4, dielectric layers 5 (comprising silicon dioxide and/or silicon nitride) and a passivation layer 6 which is supported by the substrate 1. A resistive heater 2 is embedded within the dielectric membrane, and tracks or interconnects 3 are used to connect the heater to the pads. On top of the passivation there are electrodes 7 formed of titanium nitride which can be used to make contact to a sensing material, and titanium nitride tracks/interconnect 8 which connects the electrodes to the bond pads on the chip. The etching of the substrate 1 is performed by deep reactive ion etching (DRIE) to achieve near vertical sidewalls of the cavity.

Figure 2 shows a schematic cross-section of an alternative micro-hotplate where the starting substrate 1 is a bulk semiconductor substrate. In this case there is no buried oxide layer. The other features are the same as those shown in Figure 1 and thus carry the same reference numerals.

Figure 3 shows a schematic cross-section of an alternative micro-hotplate where the heater 2 has an adhesion and/or diffusion barrier 9 underneath the heater material 2.

Figure 4 shows a schematic cross-section of an alternative micro-hotplate where the titanium nitride layer 7 that is used for the sensing electrodes has an adhesion and/or diffusion barrier layer 10 underneath.

Figure 5 shows a schematic cross-section of an alternative micro-hotplate where the membrane cavity or the back-etched portion has sloping sidewalls. These are typically caused when anisotropic or crystalographic back etching methods, such as potassium hydroxide (KOH) or tetra methyl ammonium hydroxide (TMAH) are used.

Figure 6 shows a schematic cross-section of an alternative micro-hotplate where there is a diode 11 underneath the heater 2, but embedded or formed within the membrane.

Figure 7 shows a schematic cross-section of an alternative micro-hotplate device with heat spreading plates 13 above the heater 2. These spreading plates 13 may be made of any material such as single crystal silicon, polysilicon, titanium nitride, or a CMOS metal such as tungsten, aluminium, titanium or molybdenum, or a non-CMOS metal such as gold or platinum.

Figure 8 shows a top view of a rectangular micro-hotplate with titanium nitride electrodes 7. In this case a square meander heater 2 is embedded within the square or rectangular membrane 14. Interdigitated electrodes of titanium nitride 7 are on top of the membrane.

Figure 9 shows a top view of a circular micro-hotplate with a circular heater 2 and circular interdigitated titanium nitride electrodes 7.

Figure 10 shows some different shapes for electrodes used for resistive gas sensing.

Figure 10(a) shows a circular interdigitated shape; Figure 10(b) shows interdigitated concentric rings; and Figure 10(c) shows electrodes in a spiral shape. Many other different shapes are possible. In particular, the electrodes can be rectangular in shape, or can be just two electrodes side by side. These figures show only a few possible examples, and to one versed in the art many different shapes are possible.

Figure 11 shows a top view of a chip 15 with a micro-hotplate device 14 and circuitry 16 on the same chip. This is made possible in the case that a CMOS process is used to fabricate the device, allowing both the sensor device and circuitry to be on the same chip. The use of titanium nitride as an electrode material is compatible with CMOS processing.

Figure 12 shows a top view of a 2x2 array of circular micro-hotplates. In this case these are shown with each micro-hotplate having an identical configuration. However, they may be different, having different shapes, sizes, heater or electrode materials, or different sensing materials on each micro-hotplates. Different array sizes and configurations are possible.

Figure 13 shows a schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes 7 and having a gas sensitive material 17 deposited on the electrodes 7.

Figure 14 shows a schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes 7, but with an additional material 18 plated on the titanium nitride electrode layer 7 which will be used to form a contact to a gas sensing material.

Figure 15 shows a schematic cross-section of an alternative micro-hotplate, where the titanium nitride layer 8 is covered with a passivation 19, so that the interconnect tracks/interconnects 8 between the electrodes and the bond pads are protected while the electrode area is left un-passivated. This protects the interconnects 8 and also makes it easier to confine the sensing material deposition within the electrode area. The passivation will typically be silicon nitride or silicon dioxide, or a combination of these two, but other materials may also be used.

Figure 16 shows a schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes 7 where a passivation 19 is used, and a sensing material 17 is deposited on the electrodes and in the "well" formed by the passivation.

Figure 17 shows a schematic cross-section of an alternative micro-hotplate with titanium nitride electrodes 7, but with an alternate passivation design, where the passivation 19 covers most of the electrode area while still leaving some parts of each electrode exposed.

Figure 18 shows the schematic cross-section of an alternative micro-hotplate with the titanium nitride electrodes 7 and the heater 2 both in the same layer, with the heater also made from titanium nitride Figure 19 shows the top view of a micro-hotplate with both the heater 2 and electrodes 7 made from the same titanium nitride layer. The heater in this case is a rectangular ring heater surrounding the electrodes. However, other shapes and configurations are also possible, including having a heater interweaved with the electrodes. There can also be an additional passivation on this layer which covers the heater but not the electrodes.

It will be appreciated that all the figures described above have some common features and they carry the same reference numbers in the figures. For example, the passivation layer 6, dielectric layers 5, heater 2, heater interconnect 3, electrode layer 7 and electrode layer interconnect 8 are shown in most of the figures described above.

The skilled person will understand that in the preceding description and appended claims, positional terms such as 'above', 'below', 'front', 'back', 'vertical', 'underneath' etc. are made with reference to conceptual illustrations of a semiconductor device, such as those showing standard cross-sectional perspectives and those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to a semiconductor device when in an orientation as shown in the accompanying drawings.

Furthermore, the invention can be defined using the following clauses: 1. A micro-hotplate comprising: a dielectric membrane formed on a semiconductor substrate comprising a back-etched portion; a heater formed within or over the dielectric membrane; and an electrode layer formed over the dielectric membrane, wherein the electrode layer comprises a material comprising titanium nitride (TiN).

2. A micro-hotplate according to clause 1, wherein the dielectric membrane is supported along its entire perimeter by the semiconductor substrate.

3. A micro-hotplate according to clause 1 or 2, wherein the dielectric membrane is formed using an etching technique for back-etching the substrate, the etching technique being selected from a group comprising deep reactive ion etching (DRIE), anisotropic or crystallographic wet etching, potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH).

A micro-hotplate according to clause 1, 2, or 3, wherein the dielectric membrane comprises: a membrane cavity comprising vertical side walls or sloping side walls; one or more dielectric layers comprising silicon dioxide and/or silicon nitride one or more spin on glass layers; and a first passivation layer over the one or more dielectric layers.

5. A micro-hotplate according to clause 4, wherein the electrode layer is formed over the first passivation layer of the dielectric membrane.

6. A micro-hotplate according to clause 4 or 5, further comprising an adhesion or diffusion barrier layer between the first passivation layer and the electrode layer.

A micro-hotplate according to any preceding clause, wherein the electrode layer comprises a plurality of patterned structures.

8. A micro-hotplate according to clause 7, wherein the patterned structures have one or more shapes selected from a group comprising: (1) square and/or circular (2) interdigitated electrodes (3) electrodes arranged in shapes of concentric rings (4) spiral shaped electrodes, and (5) only two electrodes next to one another.

9. A micro-hotplate according to any preceding clause, further comprising a first interconnect layer on the dielectric membrane, the first interconnect layer comprising a material comprising TiN.

10. A micro-hotplate according to clause 5, wherein the first interconnect layer laterally connects the electrode layer to a bond pad.

11. A micro-hotplate according to clause 9 or 10, further comprising a second passivation layer formed on the first interconnect layer only so that the electrode layer is exposed.

12. A micro-hotplate according to clause 9 or 10, further comprising a second passivation layer formed on the first interconnect layer and on a portion of the electrode layer so that a remaining portion of the electrode layer is exposed.

13. A micro-hotplate according to any one of clauses 4 to 12, wherein the first and second passivation layers comprise a material comprising silicon dioxide and/or silicon nitride.

14. A micro-hotplate according to any preceding clause, further comprising a sensing layer formed over the electrode layer.

15. A micro-hotplate according to clause 14, further comprising a metal layer between the sensing layer and the electrode layer.

16. A micro-hotplate according to clause 15, wherein the metal layer is formed using an electroless plating or electroplating technique.

17. A micro-hotplate according to clause 16, wherein the electrode layer is a seed layer in the electroless plating or electroplating technique.

18. A micro-hotplate according to any one of clauses 14 to 17, wherein the electrode layer, or the layer deposited thereon, is configured to measure resistance and/or capacitance of the sensing layer.

19. A micro-hotplate according to any one of clauses 14 to 18, wherein the sensing layer comprises a metal oxide material or a combination of metal oxides.

20. A micro-hotplate according to clause 19 wherein the sensing layer comprises a metal oxide selected from a group comprising tin oxide, tungsten oxide, zinc oxide, chromium oxide or the sensing layer comprises a combination of said metal oxides.

A micro-hotplate according to clause 19 or 20, wherein the metal oxide material is pure, doped or catalysed material. 21. 22. 23. 24. 25. 26.

A micro-hotplate according to any one of clauses 14 to 18, wherein the sensing layer comprises a material selected from a group comprising polymers, nanowires, nano-rods, nanotubes, nanoparticles and nano-plates.

A micro-hotplate according to any one of clauses 14 to 18, wherein the sensing layer is a porous layer and/or a nanostructured layer.

A micro-hotplate according to any one of clauses 14 to 23, wherein the sensing layer is a gas sensitive layer.

A micro-hotplate according to any one of clauses 14 to 18, wherein the sensing layer comprises a material comprising aluminium oxide or a polymer.

A micro-hotplate according to clause 25, wherein the sensing layer is a humidity sensing layer.

27. A micro-hotplate according to any one of clauses 14 to 26, wherein the sensing layer is deposited using a technique selected from a group comprising screen printing, sputtering, chemical vapour deposition (CVD) ink-jet, drop coating and flame spray pyrolysis or atomic layer deposition (ALD).

28. A micro-hotplate according to any preceding clause, wherein the heater is a resistive heater comprising a CMOS usable material comprising aluminium, copper, titanium, molybdenum, polysilicon, single crystal silicon, tungsten or titanium nitride.

29. A micro-hotplate according to any one of clauses 1 to 27, wherein the heater is a MOSFET heater.

30. A micro-hotplate according to any one of clauses 1 to 27, wherein the heater comprises platinum.

31. A micro-hotplate according to any preceding clause, further comprising a titanium/titanium nitride layer adjacent the heater to act as an adhesive, or diffusion barrier layer.

32. A micro-hotplate according to any preceding clause, wherein, when the heater is formed over the dielectric membrane, the heater is located on the same plane as the electrode layer.

33. A micro-hotplate according to clause 32, wherein the heater comprises TiN.

34. A micro-hotplate according to any preceding clause, wherein the heater comprises at least two materials formed in at least two different layers.

35. A micro-hotplate according to any preceding clause, wherein the heater is circular, rectangular, meander, ring or multiringed shaped.

36. A micro-hotplate according to any preceding clause, wherein the dielectric membrane has a circular shape.

37. A micro-hotplate according to any one of clauses 1 to 35, wherein the dielectric membrane has a square or rectangular shape with or without rounded corners.

38. A micro-hotplate according to any preceding clause, further comprising a spreading plate located within the dielectric membrane, wherein the spreading plate comprises single crystal silicon, polysilicon or a metal.

39. A micro-hotplate according to any preceding clause, wherein the micro-hotplate is a CMOS based micro-hotplate in which the heater comprises a CMOS interconnect metal, and the dielectric membrane comprises CMOS dielectric layers.

40. A micro-hotplate according to clause 39, wherein the semiconductor substrate is a bulk silicon substrate or an SOI substrate.

41. A micro-hotplate according to any preceding clause, further comprising a temperature sensor located within the dielectric membrane.

42. A micro-hotplate according to clause 41, wherein the temperature sensor is a diode, or a resistive temperature sensor comprising metal, polysilicon or single crystal silicon.

43. A micro-hotplate according to any preceding clause, wherein the dielectric membrane comprises one or more holes.

44. A micro-hotplate according to clause 39, further comprising circuitry integrated on the same chip as the micro-hotplate.

45. An array of micro-hotplates incorporating a micro-hotplate according to any preceding clause, wherein the array of micro-hotplates is arranged on the same chip.

46. An array of micro-hotplates according to clause 45, wherein each micro-hotplate in the array is identical.

47. An array of micro-hotplates according to clause 45, wherein the micro-hotplates in the array are different.

48. An array of micro-hotplates according to clause 47, wherein each micro-hotplate in the array comprises different materials for the heaters, electrode layers and sensing layers, and wherein at least one micro-hotplate in the array comprises TiN as the material of the electrode layer.

49. A method of manufacturing a micro-hotplate, the method comprising: forming a dielectric membrane on a semiconductor substrate comprising a back-etched portion; forming a heater within or over the dielectric membrane; and forming an electrode layer over the dielectric membrane, wherein the electrode layer comprises a material comprising titanium nitride (TiN).

50. A method according to clause 49, wherein forming the dielectric membrane comprises forming the dielectric membrane such that it is supported along its entire perimeter by the semiconductor substrate.

51. A method according to clause 49 or 50, wherein forming the dielectric membrane comprises using an etching technique to back-etch the semiconductor substrate to form the back-etched portion.

52. A method according to clause 51, wherein the etching technique is selected from a group comprising deep reactive ion etching (DRIE), anisotropic or crystallographic wet etching, potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH).

53. A method according to any one of clauses 49 to 52, wherein forming the dielectric membrane further comprises: forming one or more dielectric layers comprising silicon dioxide and/or silicon nitride; and forming a first passivation layer on the one or more dielectric layers.

54. A method according to clause 53, further comprising forming the electrode layer over the first passivation layer of the dielectric membrane.

55. A method according to clause 54, further comprising forming a metal layer on the electrode layer using an electroless plating or electroplating process.

56. A method according to clause 54 or 55, further comprising forming a sensing material over the electrode layer using a technique selected from a group comprising screen printing, sputtering, chemical vapour deposition (CVD) ink-jet, drop coating, flame spray pyrolysis or atomic layer deposition (ALD).

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS: A micro-hotplate comprising: a dielectric membrane formed on a semiconductor substrate comprising a back-etched portion; a heater formed within or over the dielectric membrane; and an electrode layer formed over the dielectric membrane, wherein the electrode layer comprises a material comprising titanium nitride (TiN).
A micro-hotplate according to claim 1, wherein the dielectric membrane is supported along its entire perimeter by the semiconductor substrate.
3. A micro-hotplate according to claim 1 or 2, wherein the dielectric membrane is formed using an etching technique for back-etching the substrate, the etching technique being selected from a group comprising deep reactive ion etching (DRIE), anisotropic or crystallographic wet etching, potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH).
4. A micro-hotplate according to claim 1, 2, or 3, wherein the dielectric membrane comprises: a membrane cavity comprising vertical side walls or sloping side walls; one or more dielectric layers comprising silicon dioxide and/or silicon nitride; one or more layers of spin on glass, and a first passivation layer over the one or more dielectric layers.
A micro-hotplate according to claim 4, wherein the electrode layer is formed over the first passivation layer of the dielectric membrane.
A micro-hotplate according to claim 4 or 5, further comprising an adhesion or diffusion barrier layer between the first passivation layer and the electrode layer.
A micro-hotplate according to any preceding claim, wherein the electrode layer comprises a plurality of patterned structures.
8. A micro-hotplate according to claim 7, wherein the patterned structures have one or more shapes selected from a group comprising: (1) square and/or circular (2) interdigitated electrodes (3) electrodes arranged in shapes of concentric rings (4) spiral shaped electrodes, and (5) only two electrodes next to one another.
9. A micro-hotplate according to any preceding claim, further comprising a first interconnect layer on the dielectric membrane, the first interconnect layer comprising a material comprising TiN.
10. A micro-hotplate according to claim 5, wherein the first interconnect layer laterally connects the electrode layer to a bond pad.
11. A micro-hotplate according to claim 9 or 10, further comprising a second passivation layer formed on the first interconnect layer only so that the electrode layer is exposed.
12. A micro-hotplate according to claim 9 or 10, further comprising a second passivation layer formed on the first interconnect layer and on a portion of the electrode layer so that a remaining portion of the electrode layer is exposed.
13. A micro-hotplate according to any one of claims 4 to 12, wherein the first and second passivation layers comprise a material comprising silicon dioxide and/or silicon nitride.
14.
15.
16.
17.
18.
19.
20.

A micro-hotplate according to any preceding claim, further comprising a sensing layer formed over the electrode layer.

A micro-hotplate according to claim 14, further comprising a metal layer between the sensing layer and the electrode layer.

A micro-hotplate according to claim 15, wherein the metal layer is formed using an electroless plating or electroplating technique.

A micro-hotplate according to claim 16, wherein the electrode layer is a seed layer in the electroless plating or electroplating technique.

A micro-hotplate according to any one of claims 14 to 17, wherein the electrode layer or the layer deposited thereon is configured to measure resistance and/or capacitance of the sensing layer.

A micro-hotplate according to any one of claims 14 to 18, wherein the sensing layer comprises a metal oxide material or a combination of metal oxides.

A micro-hotplate according to claim 19 wherein the sensing layer comprises a metal oxide material selected from a group comprising tin oxide, tungsten oxide zinc oxide, chromium oxide, or the sensing layer comprises a combination of said metal oxides.
21. A micro-hotplate according to claim 19 or 20, wherein the metal oxide material is pure, doped or catalysed material.
22. A micro-hotplate according to any one of claims 14 to 18, wherein the sensing layer comprises a material selected from a group comprising polymers, nanowires, nano-rods, nanotubes, nanoparticles and nano-plates.
23. A micro-hotplate according to any one of claims 14 to 18, wherein the sensing layer is a porous layer and/or a nanostructured layer.
24. A micro-hotplate according to any one of claims 14 to 23, wherein the sensing layer is a gas sensitive layer.
25. A micro-hotplate according to any one of claims 14 to 18, wherein the sensing layer comprises a material comprising aluminium oxide or polymer.
26. A micro-hotplate according to claim 25, wherein the sensing layer is a humidity sensing layer.
27. A micro-hotplate according to any one of claims 14 to 26, wherein the sensing layer is deposited using a technique selected from a group comprising screen printing, sputtering, chemical vapour deposition (CVD) ink-jet, drop coating, flame spray pyrolysis or atomic layer deposition (ALD).
28. A micro-hotplate according to any preceding claim, wherein the heater is a resistive heater comprising a CMOS usable material comprising aluminium, copper, titanium, molybdenum, polysilicon, single crystal silicon tungsten, or titanium nitride.
29. A micro-hotplate according to any one of claims 1 to 27, wherein the heater is a MOSFET heater.
30. A micro-hotplate according to any one of claims 1 to 27, wherein the heater comprises platinum.
31. A micro-hotplate according to any preceding claim, further comprising a titanium/titanium nitride layer adjacent the heater to act as an adhesive, or diffusion barrier layer.
32. A micro-hotplate according to any preceding claim, wherein, when the heater is formed over the dielectric membrane, the heater is located on the same plane as the electrode layer.
33. A micro-hotplate according to claim 32, wherein the heater comprises TiN.
34. A micro-hotplate according to any preceding claim, wherein the heater comprises at least two materials formed in at least two different layers.
35. A micro-hotplate according to any preceding claim, wherein the heater is circular, rectangular, meander, ring or multiringed shaped.
36. A micro-hotplate according to any preceding claim, wherein the dielectric membrane has a circular shape.
37. A micro-hotplate according to any one of claims 1 to 35, wherein the dielectric membrane has a square or rectangular shape with or without rounded corners.
38. A micro-hotplate according to any preceding claim, further comprising a spreading plate located within the dielectric membrane, wherein the spreading plate comprises single crystal silicon, polysilicon or a metal.
39. A micro-hotplate according to any preceding claim, wherein the micro-hotplate is a CMOS based micro-hotplate in which the heater comprises a CMOS interconnect metal, and the dielectric membrane comprises CMOS dielectric layers.
40. A micro-hotplate according to claim 39, wherein the semiconductor substrate is a bulk silicon substrate or an SOI substrate.
41. A micro-hotplate according to any preceding claim, further comprising a temperature sensor located within the dielectric membrane.
42. A micro-hotplate according to claim 41, wherein the temperature sensor is a diode, or a resistive temperature sensor comprising metal, polysilicon or single crystal silicon.
43. A micro-hotplate according to any preceding claim, wherein the dielectric membrane comprises one or more holes.
44. A micro-hotplate according to claim 39, further comprising circuitry integrated on the same chip as the micro-hotplate.
45. An array of micro-hotplates incorporating a micro-hotplate according to any preceding claim, wherein the array of micro-hotplates is arranged on the same chip.
46. An array of micro-hotplates according to claim 45, wherein each micro-hotplate in the array is identical.
47. An array of micro-hotplates according to claim 45, wherein the micro-hotplates in the array are different.
48. An array of micro-hotplates according to claim 47, wherein each micro-hotplate in the array comprises different materials for the heaters, electrode layers and sensing layers, and wherein at least one micro-hotplate in the array comprises TiN as the material of the electrode layer.
49. A method of manufacturing a micro-hotplate, the method comprising: forming a dielectric membrane on a semiconductor substrate comprising a back-etched portion; forming a heater within or over the dielectric membrane; and forming an electrode layer over the dielectric membrane, wherein the electrode layer comprises a material comprising titanium nitride (TiN).
50. A method according to claim 49, wherein forming the dielectric membrane comprises forming the dielectric membrane such that it is supported along its entire perimeter by the semiconductor substrate.
51. A method according to claim 49 or 50, wherein forming the dielectric membrane comprises using an etching technique to back-etch the semiconductor substrate to form the back-etched portion.
52. A method according to claim 51, wherein the etching technique is selected from a group comprising deep reactive ion etching (DRIE), anisotropic or crystallographic wet etching, potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH).
53. A method according to any one of claims 49 to 52, wherein forming the dielectric membrane further comprises: forming one or more dielectric layers comprising silicon dioxide and/or silicon nitride; and forming a first passivation layer on the one or more dielectric layers.
54. A method according to claim 53, further comprising forming the electrode layer over the first passivation layer of the dielectric membrane.
55. A method according to claim 54, further comprising forming a metal layer on the electrode layer using an electroless plating or electroplating process.
56. A method according to claim 54 or 55, further comprising forming a sensing material over the electrode layer using a technique selected from a group comprising screen printing, sputtering, chemical vapour deposition (CVD) ink-jet, drop coating or flame spray pyrolysis or atomic layer deposition.
57. A micro-hotplate and a method of manufacturing a micro-hotplate, as hereinbefore described with reference to and as illustrated in the accompanying drawings.