Patent Application
20200150069
The disclosure relates to gas sensors, particularly but not exclusively, to micro-machined metal oxide gas sensors.
Metal oxide (MOX) gas sensors are generally based on the deposition of a metal oxide film onto sensing electrodes defined on or within a suitable substrate. Micro-machined MOX gas sensors typically include a membrane, a heater element within the membrane, and interdigitated electrodes in direct contact to the metal oxide (MOX) layer. A typical MOX gas sensor is shown in
It has been demonstrated to use upside-down membranes for metal oxide gas sensors in EP 1 936 364 A1. This is shown in
U.S. Pat. No. 9,506,885 B2 describes a very similar concept of an upside-down membrane, with the MOX layer deposited on the back side of the membrane and the device being operated in a flip-chip configuration. This document also demonstrates electronic circuitry included in the silicon substrate outside the membrane, on the front side (as shown in
It has been demonstrated in state-of-the-art to use gold or platinum electrodes. This extra deposition of gold and platinum increases process complexity and adds cost to the production of a device. Furthermore, neither gold nor platinum is CMOS compatible (due to possible formations of deep traps in the bandgap of the semiconductors) and issues with process integration of such layers post CMOS are challenging from a cost and supply chain perspective.
This disclosure generally relates to micro-hotplates incorporating a membrane, a heater element within the membrane and electrodes in direct contact to a metal oxide (MOX) layer. A recess is formed within the membrane, below one of the surfaces of the membranes and the MOX layer is confined within the recess and making electrical and physical contact to the interdigitated electrodes. The MOX is deposited within the etched recess portion of the membrane and placed above/below the heater, but without making physical connection to the heater. To control the depth of the recess, an etch stop, such as a silicon nitride layer may be used. This silicon nitride layer may also separate the heater from the metal oxide (MOX) and may additionally act as a stress relief layer within the membrane.
Compared to the state-of-the-art gas sensors, the sensing device disclosed may have the following features:
The MOX layer may be fully embedded in the membrane so that the MOX layer is fully confined within the etched recess. The MOX layer surface is therefore below or largely below the surface of the membrane. This allows better confinement, which could improve both the reproducibility and reliability of the device. Confining the MOX layer reduces the spreading of the MOX layer on the surface of the membrane. In state-of-the-art devices this spreading causes reliability and reproducibility issues and also increases heat loss and, thus, power consumption.
Advantageously, this disclosure offers smaller dimensions of the MOX layer and thus smaller dimensions of the micro-hotplate. The confinement of the MOX layer in a cavity further results in
Compared to state-of-the-art sensing devices, the gas sensing device disclosed will have the following advantages:
According to one aspect of the present disclosure, there is provided a gas sensing device comprising; a substrate comprising an etched cavity portion and a substrate portion; a dielectric layer disposed on the substrate, wherein the dielectric layer comprises a dielectric membrane, wherein the dielectric membrane is adjacent to the etched cavity portion of the substrate, and wherein the dielectric membrane comprises an etched recess portion; a heater located within the dielectric layer; a material for sensing a gas, wherein the material for sensing a gas is located within the etched recess portion of the dielectric membrane.
The gas sensing device may further comprise a plurality of electrodes configured to drive the current through the gas sensing material and/or measure the resistance of the gas sensing material (i.e. MOX).
The interdigitated electrodes may comprise two connections for the current to flow through the MOX layer and two other connections for resistance measurement, in order to remove the undesirable effects of the contact resistance. Such contact resistance can introduce yield losses and non-uniformity from device to device or batch to batch. The contact layer can also drift in time. Furthermore, the contact resistance is not affected by the chemical reaction and therefore the sensitivity and selectivity to a particular gas may be lowered or worsened.
Advantageously, this 4-way measurement results in removing the contact resistance and thus increasing the sensitivity, the selectivity, and enhancing the reliability as the contact resistance can deteriorate in time. The heater may comprise a CMOS compatible material. Optionally, the CMOS material may be any of polysilicon, platinum, titanium, tungsten, or a combination of these. The heater may be formed underneath (or below) the polysilicon electrodes.
The gas sensing device may further comprise a plurality of first etch stop layers located on a top surface of the dielectric membrane, each first etch stop layer being located above a plurality of electrodes. These etch stop layers may define the recess laterally and may locate and confine the MOX layer above the heater. The etch stop layers may have a higher resistance to the etchant used in comparison to the rest of the dielectrics in the membrane layers. This allows a recess to be etched within the membrane.
Each first etch stop layer may be laterally spaced from one another, or may form a ring or a closed shape at the surface. This helps define the recess laterally within the dielectric membrane, to avoid the MOX layer spreading towards the edge of the membrane. The etch stop layers may also help define the cavity within the substrate.
The etched recess portion may be defined or confined laterally by the plurality of first etch stop layers.
The gas sensing device may further comprise at least one second etch stop layer located within the dielectric layer, each second etch stop layer being located under a plurality of electrodes. The second etch stop layer may define vertically the depth of the gas sensing material and preferably may isolate the MOX layer from the heater.
The at least one second etch stop layer below the electrodes may extend laterally through the dielectric membrane area. The etch stop layer may provide stress relief, and therefore an etch stop layer extending laterally through the membrane can strengthen the membrane. The second etch stop layer may extend through an entire width of the dielectric layer, providing stress relief and strengthening the dielectric layer.
The material used for the etch stop layers may be for example silicon nitride, although other materials that are resistant to the etchant may be used. A preferred way to form this etch stop layer is LPCVD (Low Pressure Chemical Vapour Deposition).
The device may comprise a flip-chip configuration. Preferably the device may be packaged in a flip-chip configuration where the interdigitated electrodes can be made of a CMOS layer such as highly doped polysilicon or silicide or a lower CMOS metal layer such as Tungsten or Titanium. Alternatively the interdigitated electrodes could be made of a noble metal such as Pt and Au.
In prior-art devices, extra deposition of gold and platinum comes with significant yield reduction, as well as the increased process complexity. In the flip-chip configuration, the preferable use of polysilicon or silicides as interdigitated electrodes results in major simplification of the process, cost reduction, and higher yield.
The flip-chip configuration also allows wafer level placement of a permeable membrane on top of the silicon substrate to protect the metal oxide layer. This also reduces the effect of humidity and poisoning. The flip-chip configuration also enables connections of an electronic circuit (ASIC) via solder balls, solder bumps, or copper pillars. The device may be placed above an application specific integration circuit (ASIC) using a flip-chip technique, and may further comprise solder ball or bumps. Advantageously, this reduces the form factor and eliminates the bond wires. This configuration may also allow for wafer level or chip level packaging.
An ASIC containing drive, read out, transducing, and processing circuitry may be attached to the sensor either by solder balls (or bumps) in the flip-chip configuration or by using Through Silicon Vias in the standard configuration. The ASIC may contain temperature and/or humidity sensors.
The etched recess portion of the dielectric membrane may be formed on a front side of the dielectric layer. The front side of the dielectric layer may be defined as the side of the dielectric layer on the opposite side to the substrate. This allows an etched recess in a non flip-chip configuration. This embodiment is easier to manufacture however reduces the choice of materials that can be used for the electrodes. The etched recess portion of the dielectric membrane may be formed over the etched cavity portion of the substrate.
The etched recess portion of the dielectric membrane may be formed on a back side of the dielectric layer. The back side of the dielectric layer may be defined as the side of the dielectric layer which is the same side as the substrate. This allows an etched recess in a flip-chip configuration. The etched recess portion of the dielectric membrane may be formed within the etched cavity portion of the substrate.
The etched recess portion may be formed by wet etching of part of the dielectric membrane using at least one etch stop layer to prevent further lateral or vertical etching.
Alternatively, the etched recess portion may be formed by dry or a combination of dry and wet etching of part of the dielectric membrane using at least one etch stop layer to prevent further lateral or vertical etching.
The etched recess portion may be formed at the same time (in the same step) as the membrane, during the cavity etching by using an addition wet etching of the dielectric membrane. The wet etching may not use any of the etch stop layers or the electrodes. This method to form the recess portion has the advantage that the etched cavity (to define the membrane) and the etched recess portion (where the gas sensing layer is to be confined) are formed within the same step.
The etched recess portion may extend underneath the plurality of electrodes. This allows more contact between the gas sensing material and the electrodes, and can improve sensitivity of the device.
The etched recess portion may not directly contact the heater to avoid the electrical connection of the material for sensing a gas (MOX material) with the heater.
The material for sensing a gas may extend underneath the plurality of electrodes. In other words, a substantial part of the gas sensing material (MOX) may be placed below the electrodes. This allows the electrodes to be closer to the surface of the gas sensing material. This increases sensitivity of the device. The feature of embedding the gas sensing material, partly or entirely, in the membrane has the advantage of the electrodes closer to the surface of the gas sensing material and greater control of the size, the form, and placement within the membrane of the gas sensing material.
The material for sensing a gas may not extend above an upper surface of the dielectric membrane. In other words, the material for sensing a gas may be completely confined below an upper surface of the dielectric membrane.
The material for sensing a gas may be embedded in the dielectric membrane. In other words, the material for sensing a gas may not extend beyond a surface of the dielectric membrane and may be entirely embedded in the dielectric membrane.
The material for sensing a gas may not be formed underneath the plurality of electrodes. In other words, the material for sensing a gas may not be formed directly underneath or below the electrodes, and may only be formed above the electrodes and extending downwards in the gap between the electrodes. This makes the fabrication process simpler and provides mechanical support to the electrodes.
The electrodes may comprise a CMOS compatible material, optionally this may be polysilicon or silicides.
The electrodes may comprise a first polysilicon layer, and the heater may comprise a second polysilicon layer.
The electrodes may comprise a first polysilicon layer and a second polysilicon layer.
The electrodes may comprise a first pair of electrodes comprising interdigitated electrodes, and a second pair of electrodes interleaving between the first pair of electrodes. The first pair of electrodes may be configured such that a current bias is applied across them, and the second pair of electrodes may be configured to measure a voltage between them.
Preferably, this 4-way electrode configuration may comprise two sets of interdigitated electrodes, and two electrodes interleaving between the fingers of the two interdigitated electrodes. A current bias may be applied across the two interdigitated electrodes, and the voltage may be measured across the two interleaving electrodes. Using polysilicon in this arrangement is preferable as polysilicon patterning techniques allow good resolution of the fingers and can pack the electrodes in a small space.
Alternatively, other electrode configurations may also be used, including having four rectangular parallel electrodes, where current bias is applied to the outer most electrodes, while the voltage is measured on the inner two electrodes. Several other electrode designs are also possible.
CMOS technologies offer the polysilicon width (which normally defines the length of the MOS gate of CMOS transistors) as the smallest dimension controllable in the manufacturing process. Therefore polysilicon electrodes with widths of sub-micrometre dimensions can be manufactured. The distance between adjacent fingers of the electrodes may also be of sub-micrometres. This aspect ratio results in a much denser structure of electrodes, which further lowers the resistance of the gas sensing layer. This is particularly useful in situations where the resistances of the MOX layers are very high (MΩ range) and the high aspect ratio allows them to be reduced to below 1 MΩ.
A width of at least some of the electrodes may be of sub-micrometre dimension. The length of the electrodes may be substantially larger than the width of the, such that the polysilicon electrodes have a high aspect ratio.
According to a further embodiment of the disclosure, there is provided a sensor array based on a gas sensor as described above. This may be built and integrated within the same die to improve sensitivity, calibration, and selectivity to different gases. Different MOX layers may be deposited on each of the individual sensors, and each of the different sensors may be operated at different temperatures or with a different drive to improve selectivity and sensitivity.
According to a further aspect of the present disclosure, there is provided a gas sensor array assembly comprising: an array of a plurality of gas sensing devices as described above, wherein the plurality of devices are formed on the same chip. The sensing devices may have the same gas sensing materials or different gas sensing materials, and may be operated at different temperatures of driving conditions. This allows a gas sensor array assembly which is selectively sensitive to different gases or provides a dual or differential output.
According to a further aspect of the present disclosure, there is provided a method of manufacturing a gas sensing device according to any preceding claim, the method comprising: forming a substrate; forming a dielectric layer disposed on the substrate; forming a heater within the dielectric layer; forming an etched cavity portion within the substrate; forming an etched recess portion within the dielectric layer; and forming a material for sensing a gas within the etched recess portion.
The method may further comprise forming one or more electrodes coupled with the material for sensing a gas. The electrodes may be formed in a CMOS compatible process.
The term “CMOS compatible process” covers the processing steps used within a CMOS process as well as covers certain processing steps performed separately from the CMOS process, but utilizing processing tools usable in the CMOS processing steps.
Complementary metal-oxide-semiconductor (CMOS) technology is used to fabricate integrated circuits. The CMOS term refers to the silicon technology for making integrated circuits. CMOS processes ensure very high accuracy of processing identical transistors (up to billions), high volume manufacturing, very low cost and high reproducibility at different levels (wafer level, wafer to wafer, and lot to lot). CMOS comes with high standards in quality and reliability.
Not all silicon technologies are CMOS technologies. Examples of non-CMOS technologies include: lab technologies (as opposed to foundry technologies), screen printing technologies, bio-technologies as for example those employed in making fluidic channels, MEMS technologies, very high voltage vertical power device technologies, technologies that use materials which are not CMOS compatible, such as gold, platinum or radioactive materials.
The method may further comprise forming a plurality of etch stop layers within the dielectric layer. The etch stop layers may control the shape, size and location of the etched recess portion.
Some preferred embodiments of the disclosure will now be disclosed by way of example only and with reference to the accompanying drawings, in which:
Some examples of the device are given in the accompanying figures.
The dielectric membrane region 13 is patterned or etched such there is a recess 50 in the dielectric membrane region 13 for the gas sensing MOX material 7 to be positioned or confined, when deposited from one side (e.g., from the back side) of the dielectric membrane region 13. A silicon nitride layer 8 is formed within the dielectric membrane region 13, below the electrodes 5. A further silicon nitride layer 12 is formed on the top of the dielectric membrane region 13, except at the region where the gas sensing material 7 is to be formed. The silicon nitride layers 8, 12 have a higher resistance to the etchant used to etch the semiconductor substrate 4 and/or the dielectric membrane 13. The silicon nitride layers 8, 12 act as an etch stop, allowing a recess 50 to be etched in the membrane 13. The silicon nitride layers 8, 12 also provide stress relief and define the cavity 6. The gas sensing material 7 is formed within the etched recess 50, on the dielectric membrane region 13.
A gas sensing material 7 is deposited or grown within the substrate cavity 6, in the recess 50 of the dielectric membrane 13. The gas sensing material makes electrical contact to a pair of interdigitated electrodes 5 which are formed within the dielectric layer 3. The electrodes 5 are configured to measure resistance and/or capacitance of the gas sensing material 7. A heater 2 and heater tracks 23 are embedded within the dielectric layer 3, which when powered raises the temperature of the gas sensing MOX layer 7. The heater 2 is formed within the dielectric membrane area 13. In this embodiment, the heater 2 is a micro-heater and can be made from a metal such as Tungsten, Platinum, Gold or Titanium. It will be understood that the etch stop layers 8, 12 may be formed of silicon nitride or may be formed of other materials which have different etch selectivity to the rest of the membrane.
The gas sensor 1 is formed in a flip-chip configuration. The gas sensor can be placed above a circuit (e.g. an application specific integrated circuit (ASIC) or printed circuit board (PCB)), using Solder balls, solder bumps, copper pillars, or stud bumps 9 for connection. The solder balls 9 are typically placed on solderable pads, 10, and can be formed within the CMOS process or post-CMOS at wafer level or chip level on both the IR device and the ASIC.
In one example, the gas sensing material 7 can be a metal oxide such as tin oxide, tungsten oxide, Alumina oxide, zinc oxide, copper oxide, a combination of those metal oxides, or other metal oxides. In further examples, the gas sensing material 7 can be un-doped or doped with elements such as platinum (Pt) or palladium (Pd).
In this embodiment, a substantial part of the gas sensing MOX material, 7 is underneath or below the electrodes 5, which means that the electrodes 5 are closer to the surface of MOX material 7. Thin gas sensing MOX layers have increased sensitivity in comparison with thicker MOX layers. Using traditional inkjet or drop coating techniques on plain surfaces without a cavity, results in relatively thick and uncontrolled sizes of the MOX layer 7. In this gas sensor the effect of embedding, partly or entirely, the MOX within the membrane has the advantage of an equivalent thinner layer (with the electrodes closer to the MOX surface that is exposed to the gas) and more controlled size of MOX layer.
Step 1 (S1): Start with a substrate.
Step 2 (S2): Deposit a dielectric layer with embedded heater. Etch stop layers may also be formed.
Step 3 (S3): Etch substrate to form a cavity.
Step 4 (S4): Etch the dielectric layer. This may be done using the etch stop layers, as etchant will not etch the stop layers or the electrodes.
Step 5 (S5): Deposit metal oxide sensing layer within the recess.
Whilst step 3 and step 4 are separate steps, they may also be carried out in a single step by using a deep reactive ion etch for the cavity etch followed by a wet oxide etch for the recess.
The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘overlap’, ‘under’, ‘lateral’, ‘vertical’, etc. are made with reference to conceptual illustrations of a sensing 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 sensing device when in an orientation as shown in the accompanying drawings.
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.
