MEMS Pressure Sensor Built Using BEOL Metal Layers of Solid-State Semiconductor Processes

Abstract

A MEMS pressure sensor is provided having a membrane made of one of a plurality of metal layers. A lid is disposed over the membrane and connected to a plurality of cavity walls at a distal end of the membrane. The lid includes an array of holes disposed over an area of ​​the lid. A fixed metal electrode is disposed under the lid.

Description

[Technical field]

[0001] This application claims priority to U.S. Provisional Application No. 63 / 231,503, filed August 10, 2021, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] An integrated circuit is a semiconductor device that includes a substrate of semiconductor material onto which a series of layers are deposited using photolithographic techniques. These layers are doped and polarized to create electrical elements (e.g. resistors, capacitors, or impedances) or electronic elements (e.g. diodes or transistors). Other layers are subsequently deposited to form the structure of interconnect layers required for electrical connections.

[0003] Microelectromechanical or Microelectromechanical Systems (MEMS) are miniature electromechanical devices fabricated using layer deposition techniques based on photolithography techniques. MEMS provide cavities or hollow spaces in their interior that can be filled with liquids or gases. Conventional integrated circuits are completely solid devices, i.e. without any hollows. Hollows can be defined as cavities larger than hollow at the atomic or subatomic scale. The interior of a MEMS can contain moving elements. The moving elements can be connected by one of their ends to the rest of the MEMS structure or can be completely loose (i.e. not physically attached to the surroundings) in an at least partially closed housing (to prevent loose parts from "escaping" the MEMS). A chip can contain a MEMS device and an integrated circuit (IC), the IC being able to control the MEMS.

[0004] One application of MEMS is pressure sensors. To build a pressure sensor, in principle, it is enough to have a membrane with a constant pressure on one side and a membrane on the other side that samples the air or gas at the pressure that needs to be measured. This membrane will then bend or deform depending on the pressure difference between the two sides, but if there is a fixed electrode on one side that does not move, the capacitance between the fixed electrode and the movable electrode will change, and by measuring this change in capacitance, the change in pressure can be known.

[0005] While existing designs for building pressure sensors are easy to implement, such state-of-the-art pressure sensors developed using CMOS processes suffer from a number of significant drawbacks that severely limit the performance, reliability and yield of the sensors, rendering them nearly useless. Summary of the Invention [Problem to be solved by the invention]

[0006] Another problem with existing pressure sensors developed using CMOS processes is that when new materials are added to create the seal, such as aluminum sputtering, PI evaporation, or other materials, such implementations significantly increase the membrane thickness, thereby significantly reducing the sensitivity of the sensor.

[0007] Yet another problem is that the reference pressure will be the pressure in this chamber that is generated underneath the membrane once it is sealed. This is a problem because in CMOS, like most if not all semiconductor processes, there is known to be outgassing. This means that there will be gas molecules that will enter the reference cavity and eventually the cavity will not be completely sealed. Thus the pressure can change slightly over time depending on the pressure difference between the cavity and the outside air. Thus the reference pressure in the cavity will drift over time and this effect is accelerated with temperature.

[0008] Moreover, another potential problem is that the membrane is completely exposed. The sensor device is not resistant to harsh conditions such as dust and water. Dust, water, and other contaminants falling on the membrane can cause sensitivity changes and drift, eventually permanently damaging the device. Reliability is insufficient for some applications. This can be problematic in applications where dirty atmospheres are present, such as e-cigarettes and tire pressure monitoring systems (TPMS). Also, as with existing capacitive sensors, it is desirable to simplify the detection mechanism to reduce costs and improve reliability. [Means for solving the problem]

[0009] According to one aspect of the subject matter described in this disclosure, a MEMS pressure sensor is provided. The MEMS sensor includes a membrane made of one of a plurality of metal layers. A lid is disposed over the membrane and connected to a plurality of cavity walls at a distal end of the membrane. The lid includes an array of holes disposed over an area of ​​the lid. A fixed metal electrode is disposed under the lid.

[0010] According to another aspect of the subject matter described in this disclosure, a MEMS pressure sensor is provided. The MEMS pressure sensor includes a membrane made from one of a plurality of metal layers. The membrane includes a plurality of holes. An electrode is located below the membrane and made from another of the plurality of metal layers, and has the same shape as the membrane or at least extends over an area of ​​the membrane where the plurality of holes are located.

[0011] According to another aspect of the subject matter described in this disclosure, a MEMS pressure sensor is provided. The MEMS pressure sensor includes a metal layer made from one of a plurality of metal layers. A pedestal is located below the metal layer and made from another set of the plurality of metal layers. The pedestal reduces a capacitance gap at a center of the metal layer.

[0012] According to another aspect of the subject matter described in this disclosure, a MEMS pressure sensor is provided. The MEMS pressure sensor includes a metal layer made from one of a plurality of metal layers. A pedestal is located below the metal layer and made from another set of the plurality of metal layers. The other set of metal layers includes a plurality of wing-shaped electrodes.

[0013] According to another aspect of the subject matter described in this disclosure, a MEMS pressure sensor is provided. The MEMS pressure sensor includes a metal layer made from one of a plurality of metal layers. A piston is located below the metal layer and made from another set of the plurality of metal layers. When pressure is applied to the metal layer, the piston captures a displacement of the metal layer.

[0014] According to another aspect of the subject matter described in this disclosure, a MEMS pressure sensor is provided. The MEMS pressure sensor includes a metal layer made from one of a plurality of metal layers. A pedestal is located below the metal layer and made from another set of the plurality of metal layers. The metal layer includes a membrane integrally connected to a wing portion of one of the plurality of wing-like metal layers.

[0015] Additional features and advantages of the present disclosure are described in, and will be apparent from, the detailed description of the present disclosure.

[0016] The present disclosure is illustrated by way of example, and not by way of limitation, in the accompanying drawings, in which like reference numerals are used to refer to like elements, and it is emphasized that various features may not be drawn to scale and that dimensions of various features may be arbitrarily increased or reduced for clarity of discussion. [Brief description of the drawings]

[0017] [Figure 1A] 1A-1E are schematic diagrams of a process for forming a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 1B]1A-1E are schematic diagrams of a process for forming a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 1C] 1A-1E are schematic diagrams of a process for forming a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 1D] 1A-1E are schematic diagrams of a process for forming a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 1E] 1A-1E are schematic diagrams of a process for forming a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Diagram 2] FIG. 2 is a schematic diagram of a cross-sectional view of the MEMS pressure sensor of FIGS. 1A-1D, according to some embodiments. [Diagram 3] FIG. 3 is a SEM image of the MEMS pressure sensor of FIGS. 1A-1D, according to some embodiments. [Figure 4A] 4A-4D are schematic diagrams of a process for forming a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 4B] 4A-4D are schematic diagrams of a process for forming a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 4C] 4A-4D are schematic diagrams of a process for forming a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 4D] 4A-4D are schematic diagrams of a process for forming a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Diagram 5]FIG. 5 is a schematic diagram of a cross-sectional view of the MEMS pressure sensor of FIGS. 4A-4D, according to some embodiments. [Figure 6] FIG. 6 is a schematic diagram of a cross-sectional view of a MEMS pressure sensor according to some embodiments. [Figure 7] FIG. 7 is a schematic diagram of an exploded view of a MEMS pressure sensor (silicon dioxide, passivation, and substrate not shown) according to some embodiments. [Figure 8] FIG. 8 is a schematic diagram of a cross-sectional view of the MEMS pressure sensor of FIG. 7 according to some embodiments. [Figure 9] FIG. 9 is a schematic diagram of a cross-sectional view of a MEMS pressure sensor having an array of holes around the periphery of the membrane, according to some embodiments. [Figure 10A] 10A-10B are schematic diagrams of an exploded view and a side view of a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 10B] 10A-10B are schematic diagrams of an exploded view and a side view of a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 11A] 11A-11B are schematic diagrams of an exploded view and a side view of metal layers of a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 11B] 11A-11B are schematic diagrams of an exploded view and a side view of metal layers of a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 11C] FIG. 11C is a schematic diagram of a cross-sectional view of the pressure sensor shown in FIGS. 11A-11B, according to some embodiments. [Figure 12A] 12A-12B are schematic diagrams of an exploded view and a side view of metal layers of a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). [Figure 12B]12A-12B are schematic diagrams of an exploded view and a side view of metal layers of a MEMS pressure sensor according to some embodiments (silicon dioxide, passivation, and substrate not shown). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The figures and descriptions provided herein are simplified to show aspects relevant for a clear understanding of the devices, systems, and methods described herein, and for clarity, other aspects that may be found in typical similar devices, systems, and methods may be omitted. Those skilled in the art will recognize that other elements and / or operations are desirable and / or necessary to implement the apparatus, systems, and methods described herein. However, because such elements and operations are well known in the art and do not facilitate a better understanding of the present disclosure, descriptions of such elements and operations may not be provided herein. However, the present disclosure is deemed to essentially include all such elements, variations, and modifications to the described aspects that are known to those skilled in the art.

[0019] The terms used herein are for the purpose of describing particular exemplary embodiments only and are not intended to be limiting. For example, as used herein, the singular forms "a," "an," and "the" may be intended to include the plural unless the context clearly dictates otherwise. The terms "comprise," "include," "comprise," "comprise," and "have" are inclusive and thus specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The method steps, processes, and operations described herein should not be construed as necessarily requiring the execution in the particular order discussed or illustrated, unless specifically identified as an order of execution. It should also be understood that additional or alternative steps may be employed.

[0020] Terms such as first, second, third, etc. may be used herein to describe various elements, components, regions, layers, and / or sections, but these elements, components, regions, layers, and / or sections should not be limited by these terms. These terms may be used only to distinguish one element, component, region, layer, or section from another element, component, region, layer, or section. That is, terms such as "first," "second," and other numerical terms, when used herein, do not imply any order or sequence unless clearly indicated by the context.

[0021] Aspects of the present disclosure are directed to a MEMS pressure sensor and its formation. The pressure sensor includes a lid, a membrane, and a fixed electrode. Additionally, the sensor may include a membrane having the same shape as the vertical wall at the edge of the lid, which may be joined to close a cavity. The cavity may include a reference pressure. The top metal layer of the sensor may be the top metal layer of the process or may be in another layer. In some embodiments, it may have two arrays of holes.

[0022] The pressure sensor includes one internal array of holes across the lid to etch silicon dioxide in a cavity made by the lid, membrane, and surrounding walls, and an outer array of holes to etch the volume around this cavity. In this way, the membrane can be created using a metal layer that does not have to be the top metal layer of the process. In some embodiments, the metal layer does not have any holes, so the metal layer does not have to be sealed.

[0023] The pressure sensor is constructed using backend-of-the-line (BEOL) metal layers of a solid-state semiconductor process.

[0024] For this purpose, various solutions have been proposed, for example using a combination of plasma and / or wet etching with HF and other chemistries, but these processes, especially those involving wet etching, are difficult to implement in mass production with high yields.

[0025] A simple post-processing approach proposed previously is to use a single vapor HF (vHF) maskless post-processing step. vHF etches away silicon oxide present between metal layers in the BEOL, leaving all metal behind. This was proposed by Baorab. Due to its simplicity, it is the lowest cost CMOS post-processing approach. Moreover, it can be implemented in the same CMOS foundry or in the packaging or assembly house.

[0026] In this approach, the MEMS device is built using metal layers (usually Al or AlCu and W), but other metal layers such as Cu can also be used. With proper design, it is possible to confine the oxide inside the metal case. Other materials can be used, but they must be in the CMOS BEOL. Most approaches to date use special packaging, such as laminate packages like LGA, to protect the MEMS. This increases cost and size, minimizing or eliminating the size and cost advantages that can be gained by building MEMS using CMOS processes.

[0027] Baolab proposed to use a top metal layer to protect the MEMS while providing small holes to allow the vHF to penetrate into the MEMS cavity. A second set of post-processing steps consisting of Al sputtering and patterning is then applied to properly seal the MEMS device, which typically adds 10% cost to the CMOS process. This simplifies the packaging requirements, eliminating the need to use laminates or other special packaging. Instead, standard packaging techniques such as QFN can be used. This reduces the cost and size of the final IC.

[0028] In addition to the top metal layer, a bottom metal layer was used to complete the metal cavity in which the MEMS device will be located. This was done to limit the etching of vHF towards the bottom considering that in most CMOS processes there is doped silicon oxide under the bottom metal layer, M1. Doped silicon oxide reacts very aggressively to vHF, causing the etch rate to increase rapidly and leaving a very messy residue that is difficult to remove. This makes the design portable to most CMOS processes, as it would otherwise only be applicable to specialized processes that do not have doped silicon oxide under the BEOL's lowest metal layer.

[0029] Baolab's solution, like other solutions that use BEOL materials to implement MEMS devices, surrounds the MEMS device with metal walls, defining a MEMS cavity in the ASIC die. The electronics are then placed around it. The implementation of these metal walls is made with a stack of metal layers (usually made of aluminum) and vias (usually tungsten). However, for lower CMOS nodes, i.e. below 0.18um processes, the material can be different, mainly copper. As a rule, this is not a straight vertical wall, since DRC rules require the metal layer to extend beyond the ends of the vias. However, some exceptions to this can be made if there is an interest in increasing the side area exposed to the wall, such as in the case of in-plane volume sensors. This becomes the DRV that the fab must accept.

[0030] With Baolab's solution, in principle, a vertical metal wall would connect the top and bottom metal planes, electrically shorting all MEMS cavities. Usually, this is not of concern, or at least it does not happen everywhere in the cavities. To solve this problem, Baolab used vertical interleaved anchor structures. These structures allow the vHF to move up and across the silicon oxide layer until it is depleted, leaving behind silicon oxide that is not etched. In this way, a mechanically consistent wall is obtained without electrically shorting the top and bottom metal plates.

[0031] One reason this is particularly effective is that the silicon oxide layers that are typically deposited between metal layers in the BEOL of a CMOS process are composed of two different sublayers, each with a different oxide density. Thus, one of these layers etches slower with vHF than the other. Thus, etching the silicon oxide vertically using vHF becomes more difficult (i.e., takes longer) than etching horizontally. The etch then propagates faster along one of the silicon oxide sublayers. Using these anchor structures forces the vHF to etch across all the slow etch rate sublayers without propagating quickly through the faster etch rate sublayers. These interleaved anchors can also be used to add columns or pillars at various locations on the MEMS to increase the consistency of the top metal surface. This is particularly important in terms of supporting later encapsulation, typically with Al sputtering, to prevent bending of the top metal surface that would ultimately destroy or disable the MEMS device.

[0032] The main problem with these anchor walls is that although they keep the top and bottom metal surfaces electrically disconnected, providing mechanical robustness, the electrical capacitance between them is very large, because inside the sandwiched anchor structure there are large surfaces placed close to each other, one connected to the top plate and the other to the bottom plate, and to make matters worse, a significant portion of this surface is filled with silicon oxide.

[0033] Another problem related to the previous one is that there is a critical trade-off between the parasitic capacitance between the top and bottom metal plates and the production yield and reliability. To minimize this parasitic capacitance, one can minimize the length of the anchor structure to reduce the number of fingers and / or their height and / or increase the etching time. In this way, if one wants to minimize this parasitic capacitance, one gets a small anchor structure that leaves a minimal amount of silicon oxide inside it after vHF etching. However, this is a very weak structure and prone to easy mechanical failure due to mechanical shock and vibration or simply when sealing or packaging the device. It also leads to low yields. This is because even a slight overetching can completely remove the silicon oxide in the anchor structure, causing collapse of the top and bottom, making the device completely unusable. In production, this requirement of a critical vHF etch must be avoided, as it always leads to a loss of yield. The reason is that the etch rate and the silicon oxide etched in the MEMS cavity depends not only on the vHF machine and the recipe applied, but also on the CMOS process. One can tightly control the vHF machine and its recipe, but not the CMOS process. The tolerance of the CMOS process is usually around 30%.

[0034] Besides the fact that all metal layers may be required to implement a MEMS device (hence the need for special packaging processes, the need for specific CMOS processes that do not use doped silicon oxide under the bottom metal plate, and the need for large parasitic capacitances), two major issues with all solutions that use CMOSBEOL materials to implement MEMS are yield and reliability. These issues become even more important when using the Baolab approach, which uses top and bottom metal planes. But without them, the process becomes more complex and expensive, losing not only the cost benefits, but also the mass production, time to market, and even performance benefits.

[0035] One of the major issues found when using BEOL metals in CMOS processes to implement MEMS devices is vertical stress gradients, which are minimized with bespoke MEMS manufacturing processes. However, in CMOS, these metal lines are not intended to implement mechanical structures, but only electrical connections surrounded by silicon oxide in solid-state ICs, so residual stresses are of less concern and are usually large. In addition to large residual stresses, large vertical stress gradients are usually found. As a result, the metal bends or curls, usually upwards, but sometimes downwards in some layers, especially in the upper layers. This bending becomes a major concern when using top and bottom metal planes, because then the available vertical clearance spacing above and below the device is minimized and can easily be touched. If a MEMS device touches a top or bottom metal surface, it becomes unusable. This leads to very low yields and reliability.

[0036] One possibility to alleviate this problem a little is to increase this vertical gap distance and reduce the number of metal layers used in the MEMS device itself. However, this leads to a larger gap, which reduces the relative capacitance change of a given sensor for the same displacement, thus reducing the performance in the out-of-plane direction. It also forces the proof masses to be smaller in the case of inertial sensors, which would not allow the use of all available metal layers, further reducing performance. Also, reducing the number of metal layers used to build the moving parts of the device, such as the proof masses in the case of inertial sensors, would further increase the curvature, as will be explained later. Therefore, it is necessary to minimize the curvature height of the MEMS device. This is defined as the maximum vertical displacement in the out-of-plane direction of the metal layers along the entire MEMS device or its specific elements.

[0037] One of the known solutions to this problem is to stack two or more metal layers. In this way, we increase the radius of curvature of the resulting metal structure and reduce the overall height of curvature. However, although this is a good solution when designing some parts of MEMS devices, such as the proof mass of an inertial sensor (where we want to make the size of other parts, such as the springs, as large as possible to improve the sensitivity of the sensor), it results in a very large stiffness and a significant decrease in sensitivity. In fact, as soon as we increase the thickness, we get a very stiff spring, since the stiffness is inversely proportional to the cube of the length and the thickness. This means that the sensitivity of the sensor is very low and the driving voltage of the actuator is large. Moreover, the stacking of many layers is limited by the number of metal layers in the process, so if we need to change or use a CMOS process with more metal layers in the BEOL, its costs will increase rapidly.

[0038] In summary, we need to reuse the BEOL materials present in the standard CMOS process, use vHF to etch away some of the silicon oxide in the MEMS cavity, and find a suitable design to mount the MEMS devices, which can later be packaged in WLCSP, with very high yield, reliability, and performance.

[0039] Yet another problem with using a vHF etch post-processing step after CMOS is that the SiN passivation layer that is deposited and patterned on top of the CMOS wafer is partially etched away by the vHF. This means that unless a very short vHF etch step is performed, most or all of the SiN passivation layer is etched away. This leaves troublesome residues in the wafer and exposes the entire wafer, including the ASIC areas where the silicon oxide should not be etched away.

[0040] A known solution to prevent this is to increase the silicon content of the passivation, usually measured by the refractive index or RI of the layer. Although not technically complicated, this requires process adjustments that are very difficult for large mainstream foundries to accept. Ultimately, this requirement means that fully standard CMOS processes cannot be used, thus losing some of the benefits of low cost, fast time to market, and high volume manufacturing capabilities.

[0041] In various aspects, the systems, devices, and methods disclosed herein result from or use a vHF etch to etch away portions of silicon oxide in the BEOL of a CMOS process to release material present in the BEOL that comprises a MEMS device (e.g., as described in U.S. Pat. No. 11,312,617, the entire contents of which are incorporated herein by reference).

[0042] Some embodiments may include a method of fabricating a MEMS device using materials from the back-end of a CMOS process, the method including applying a vHF post-treatment and post-backing to form a MEMS device to form: a membrane made from one of a plurality of metal layers; a lid disposed over the membrane and connected to a plurality of cavity walls at a distal end of the membrane, the lid including an array of holes disposed over a region of the lid; and a fixed metal electrode disposed under the lid. Some embodiments may include a method of fabricating a MEMS device using materials from the back-end of a CMOS process, the method including applying a vHF post-treatment and post-backing to form a MEMS device to form: a membrane made from one of a plurality of metal layers, the membrane including a plurality of holes; and an electrode disposed under the membrane and made from another metal layer of the plurality of metal layers, the electrode having the same shape as the membrane or at least extending over a region of the membrane where the plurality of holes are located.

[0043] All inventive concepts described throughout this application apply in principle to CMOS, but may also be applied to the BEOL of other solid-state semiconductor processes such as BiCMOS, GaAs, SiGe, GaN, SOI, etc.

[0044] 1A-1E are schematic diagrams of a process for forming a MEMS pressure sensor 100 according to some embodiments (silicon dioxide, passivation, and substrate are not shown). The pressure sensor 100 is constructed using the top four metal layers of the back-end of a CMOS process. In particular, a 0.18 μm node with a total of six metal layers is used. This means that the bottom two metal layers (M1 and M2) are not used. Thus, the metal layers M1 and M2 can be used to implement an application specific integrated circuit (ASIC) underneath. The pressure sensor 100 includes a circular metal layer M4 as shown in FIG. 1A. The metal layer M4 is configured to act as a bottom metal plane and an electrode. As shown in FIG. 1B, a circular metal layer M5 is disposed on the metal layer M4. The metal layer M5 includes a membrane 102 disposed in its central region. Furthermore, the metal layer M5 has several holes forming an outer array of holes 104 around the periphery of the membrane 102.

[0045] As shown in FIG. 1C, a via structure 106 is disposed around the membrane 102. The via structure V5 includes a double-wall structure made of two via rings 114A and 114B that define the internal cavity of the pressure sensor 100 and secure the ends of the membrane 102, as shown in FIG. 1E. Furthermore, the via structure V5 can be used to trap oxides inside for pressure measurement. As shown in FIG. 1D, a circular metal layer M6 is disposed on the metal layer M5 and serves as a lid for the pressure sensor 100. The metal layer M6 includes an inner array of holes 108 and an outer array of holes 110. The holes of the inner hole array 108 are arranged to provide access to the membrane 102. Also, the holes of the outer arrays of holes 104 and 110 are aligned to provide access to the metal layer M4. In some embodiments, the holes of the inner hole array have a distance of at least 20 μm from each hole.

[0046] In some embodiments, metal layers M4, M5, and M6 have a non-circular shape. Metal layers M4, M5, and M6 are constructed using BEOL metal layers of a solid-state semiconductor process.

[0047] The pressure sensor 100 is built using the top four metal layers of the back-end of the CMOS process. In particular, a 0.18 μm node with a total of six metal layers is used. This means that the bottom two metal layers (M1 and M2) are unused and can be used to implement the ASIC circuitry underneath. The bottom metal layer is M4, which has a diameter of 100 μm. This is the bottom fixed electrode of the capacitance. The metal layer M5 above metal layer M4 is of the same size. The holes in the outer array of holes 110 are 0.8 μm in size and are located in an annular region of radius 14 μm to 18.5 μm in metal layer M5.

[0048] The purpose of these holes is to allow a vapor hydrofluoric acid (vHF) etching process to be performed completely under the 25 μm diameter membrane 102 during post-processing of the wafer. Any oxide between metal layers M5 and M4 on membrane 102 is completely etched away. For this reason, the holes must be located on the outside of membrane 102. It is also preferable that the holes do not extend too far, as this would increase etching around the perimeter of membrane 102 and reduce its mechanical integrity.

[0049] Metal layer M6 has the same shape as metal layer M5. The holes of the inner array of holes 108 are arranged in a circular area on metal layer M6 with a diameter of 25 μm. This is to etch all the silicon oxide on the membrane 102. Via structure V5 is a via layer between metal layers M5 and M6. Via rings 114A and 114B may have a standard width like via structure V5, typically 0.36 μm. In this case, via rings 114A and 114B are configured to trap oxide inside, building a more robust composite wall of oxide + tungsten in the horizontal direction. This wall is located outside the membrane 102 but inside the outer array of holes 104 of metal layer M5.

[0050] In some embodiments, a single via ring is used to form the wall. In some embodiments, multiple via rings are used to form the wall. This may be applicable in some applications where a very wide temperature range is required. In such situations, due to the mismatch in thermal coefficients between aluminum, tungsten, and silicon oxide, a metal only wall is used and only one via ring is required.

[0051] The membrane 102 is free of any silicon oxide on both the top and bottom and can be secured at its edges by the via structures V5 and walls made using silicon dioxide. The remaining part of the metal layer M5 outside the membrane 102 does not function as a membrane. This is for two reasons: 1.) there is no pressure difference between the top and bottom surfaces; and 2.) most of the remaining part of the metal layer M5 is embedded in silicon dioxide. Thus, the distal part of the metal layer M5 is firmly attached to the surrounding silicon dioxide of the BEOL, thus providing firm support for the membrane 102. Note that the vHF post-treatment runs out before reaching the edge of the metal layer M5.

[0052] Electrical connections to the ASIC for sensing capacitance are made at any point between metal layer M5 or M6 and metal layer M4. In some embodiments, capacitance sensing is made using a conventional transconductance amplifier circuit. In some embodiments, capacitance sensing is made using MEMS to implement an RC circuit that is periodically charged. The output is sent to a comparator to count the time it takes to charge a capacitor to a particular value. In some embodiments, capacitance sensing is achieved using MEMS capacitance to implement a ring oscillator that generates a frequency that may depend on the capacitance of the MEMS pressure sensor. This can be used to drive a counter, and after a period of time, a count number is obtained depending on the capacitance value of the pressure sensor. In some embodiments, capacitance sensing is made using other circuits that can be used to measure capacitance.

[0053] 2 is a schematic diagram of a cross-sectional view of the MEMS pressure sensor 100, according to some embodiments. In particular, FIG. 2 shows a silicon dioxide layer 202 that has been processed using a vHF etching process. Additionally, passivation layers 204A and 204B associated with the CMOS processing of the pressure sensor 100 are disposed over the silicon dioxide layer 202 to protect and shield the silicon dioxide layer 202 from corrosion. A seal 206 may be disposed over the inner array of holes 108.

[0054] One reason for using the seal 206 is to protect the membrane 102 from being exposed at all. Any dirt or contaminants would have to pass through the outer array of holes 110, which is already difficult due to their small size, and then travel and reach the surface of the metal layer M4. If the inner array of holes 108 on the membrane 102 was left open and the holes around it were sealed, some dirt or contaminants could pass through the metal holes on top of the inner array of holes 108. This would directly reach the upper surface of the membrane 102. Therefore, this approach of sealing the inner array of holes 108 and leaving the holes in the outer area adds an additional level of protection. Another reason for using the seal 206 is that it maintains the pressure reference within the membrane 102, and all the inner surfaces of the membrane 102 can be made of metal (aluminum and tungsten). This prevents or makes very difficult outgassing effects that cause this reference pressure to drift over time.

[0055] In some embodiments, the seal 206 is formed using laser bleeding or similar techniques to melt metal locally so that the holes of the inner array of holes 108 are sealed. In some embodiments, the seal 206 is formed using sealing. In some embodiments, standard semiconductor and lithography techniques can be used to deposit and pattern the sealant over the holes of the inner array of holes 108. In some embodiments, the seal 206 is formed using sputtering of aluminum. In some embodiments, the seal 206 is formed using a standard PI seal such as a wafer level chip scale package (WLCSP) process. In some embodiments, the seal 206 is formed using silicon nitride residue of the passivation layer 204 to seal the holes of the inner array of holes 108. In some embodiments, the seal 206 is formed by depositing bumps using standard bumping techniques in WLCSP.

[0056] The ends of metal layer M4 extend and are anchored firmly into silicon dioxide layer 202 to form capacitive anchors 210A and 210B, and the ends of metal layer M5 extend and are anchored firmly into silicon dioxide layer 202 to form capacitive anchors 212A and 212B. Capacitive anchors 214A and 214B are defined by the ends of metal layer M4 that extend and are embedded firmly into silicon dioxide layer 202. Capacitive anchors 210, 212, and 214 are configured to provide mechanical support to metal layers M4, M5, and M6 and to provide the capacitance necessary to measure pressure at membrane 102.

[0057] Pressure sensor 100 includes a cavity 216 for trapping gas for measuring pressure. Cavity 216 provides cavity walls 218A and 218B formed using via structure V5. Cavity walls 218A and 218B are configured to cooperate with seal 206 to minimize outgassing and increase pressure measurement accuracy.

[0058] FIG. 3 is an SEM of a MEMS pressure sensor 100, according to some embodiments. In particular, FIG. 3 shows residue 302 resulting from etching the SiN passivation layer. This is an undesirable effect due to the long etch time associated with the vHF etch process required to completely etch away all oxide underneath the membrane 102, which cannot have holes. The residue 302 even blocks the etch holes in the inner array of holes 108 and the outer array of holes 110. One way to address this issue is to use a less aggressive approach to the vHF etch process. Another is to adjust the etch time. Another option is to increase the silicon content of the passivation layer to increase the RI, since this increases the resistance of the SiN being etched.

[0059] When a voltage is applied to the two electrodes (the bottom fixed electrode through metal layer M4 and the membrane through metal layers M5 or M6), the change in capacitance is measured with an impedance analyzer and the stiffness and resonant frequency are calculated by deriving the sensitivity to pressure (change in capacitance vs. change in pressure). Surprisingly, the membrane is relatively softer than expected. This softness can be attributed to the longer etching time required to etch the silicon dioxide underneath the long membrane without holes compared to other MEMS sensors such as inertial sensors. Longer etching times result in more etching of the ARC / TiN sublayer of the metal layer, decreasing the overall stiffness of the membrane. This is favorable as it leads to a more sensitive pressure sensor and can be tuned using longer or shorter etching times in the vHF etching process. After a certain time, once all the ARC / TiN sublayers are gone, the softness and sensitivity of the pressure sensor will not improve any more.

[0060] 4A-4D are schematic diagrams of a process for forming a MEMS pressure sensor 400 according to some embodiments (silicon dioxide, passivation, and substrate not shown). The pressure sensor 400 includes a circular metal layer M4, as shown in FIG. 4A. The metal layer M4 is configured to function as a bottom fixed electrode with a diameter of 100 μm. A circular metal layer M5 is disposed on top of the metal layer M4, as shown in FIG. 4B. The metal layer M5 is configured to act as a membrane for the pressure sensor 300. A dual ring via 402 having a diameter of 25 μm is disposed on the metal layer M5, as shown in FIG. 4C. A circular metal layer M6 is disposed on top of the dual ring via 402, as shown in FIG. 4D. In this case, the metal layer M6 is configured as a lid for the pressure sensor 400. Additionally, the metal layer M6 includes an inner array of holes 404 and an outer array of holes 406.

[0061] In some embodiments, metal layers M4, M5, and M6 have a non-circular shape. Metal layers M4, M5, and M6 are constructed using BEOL metal layers of a solid-state semiconductor process.

[0062] The pressure sensor 400 is constructed such that the membrane / metal layer M5 does not extend to the silicon dioxide layer. In this case, the metal layer M5 includes the capacitive anchor because the vertical walls of the cavity constructed with the double ring via 406 at the end of the metal layer M5 are sufficient to anchor the membrane M5 at its end. This is explained in more detail in FIG. 5.

[0063] In some embodiments, metal layer M6 is not used and is an optional feature.

[0064] 5 is a schematic diagram of a cross-sectional view of a MEMS pressure sensor 400, according to some embodiments. In particular, FIG. 5 shows a silicon dioxide layer 502 that has been processed using a vHF etching process. Additionally, passivation layers 504A and 504B are disposed over the silicon dioxide layer 502 to protect and shield the silicon dioxide layer 502 from corrosion. A seal 506 may be disposed over the holes of the inner array of holes 108. Note that the seal 506 is formed using the same approach as described herein for the seal 206.

[0065] Pressure sensor 400 includes a cavity 516 for trapping gas to measure pressure. Cavity 516 includes cavity walls 518A and 518B formed using dual ring via V5. Cavity walls 518A and 518B are configured to cooperate with seal 506 to minimize outgassing and increase pressure measurement accuracy. Additionally, each cavity wall 518A and 518B includes a region 520 filled with oxide from silicon dioxide layer 502 that was not etched by the vHF etch process.

[0066] The ends of metal layer M4 extend and are anchored into silicon dioxide layer 502 to form capacitive anchors 510A and 510B. The ends of metal layer M6 extend and are anchored into silicon dioxide layer 502 to form capacitive anchors 512A and 512B. Capacitive anchors 510 and 512 provide mechanical support to metal layers M4 and M6 and are configured to provide the capacitance necessary to measure pressure at membrane / metal layer M5. The ends of the cavity walls 518A and 518B provide sufficient mechanical support to anchor the membrane / metal layer M5.

[0067] FIG. 6 is a schematic diagram of a cross-sectional view of a MEMS pressure sensor 600, according to some embodiments. Pressure sensor 600 is similar to pressure sensor 400. The main difference is the addition of larger cavity walls 602A and 602B to create a larger cavity 604 and a shorter length of metal layer M1 of pressure sensor 600. The larger cavity walls 602A and 602B may be formed using an additional dual ring via, such as vertically stacked dual ring via 406. Pressure sensor 600 reduces parasitics between metal layer M1 with an extension of metal layer M6 by reducing the size of metal layer M1, but it does not need to be the same size as metal layer M6. The shorter length of metal layer M1 and larger cavity walls 602 protect the IMD oxide under metal layer M1, since no vHF etching process is required to reach metal layer M1 for processing. It is preferable to protect the IMD oxide under metal layer M1 from the vHF process. This is because the oxide is usually doped and will react badly and ruin the wafer.

[0068] FIG. 7 is a schematic diagram of an exploded view of a MEMS pressure sensor 700 according to some embodiments (silicon dioxide, passivation, and substrate are not shown). The pressure sensor 700 includes a circular metal layer M1, which is a bottom metal surface that supports all the structures of the pressure sensor 700 and prevents the vHF process from reaching the IMD oxide below the metal layer M1. The diameter of the metal layer M1 may be 90 μm. A ring via V1 is disposed on the metal layer M1 with a diameter of 25 μm. A metal layer M2 is disposed on the ring via V1. The metal layer M2 includes a ring via 702 with a circular metal surface 704 set within the ring via 702. The diameter of the ring via 702 is 42 μm, and the diameter of the circular metal surface 704 is 25 μm. A via structure V2 is located on the metal layer M2. The via structure V2 includes a first via ring 706 and a second via ring 708 is disposed within the first via ring 706. The first ring via 706 has a diameter of 42 μm and the second ring via 708 has a diameter of 25 μm.

[0069] Metal layer M3 is disposed on via structure V2. Metal layer M3 includes a ring via 710 having a circular metal surface 712 located within the ring via 710. The ring via 710 may have a diameter of 42 μm and the circular metal surface 712 may have a diameter of 25 μm. Via structure V3 is disposed on metal layer M3. Via structure V2 includes a first via ring 714 and a second via ring 716 located within the first via ring 714. The first ring via 714 may have a diameter of 42 μm and the second ring via 716 may have a diameter of 25 μm.

[0070] Metal layer M4 is disposed over via structure V3. Metal layer M4 includes a ring via 718 having a circular metal surface 720 located within ring via 718. Ring via 718 may have a diameter of 42 μm and circular metal surface 720 may have a diameter of 25 μm. Via structure V4 is a ring via 722 located over metal layer M4. Ring via 722 may have a diameter of 42 μm.

[0071] Metal layer M5 is disposed on top of via structure V4. Metal layer M4 includes ring via 724 and membrane 726 of pressure sensor 700, which is a circular metal surface located within ring via 718. The diameter of ring via 724 is 42 μm and the diameter of membrane 726 is 25 μm. Via structure V5 is disposed on top of metal layer M5. First ring via 714 may have a diameter of 42 μm. Via structure V5 includes a first via ring 728 having a double ring via 730 disposed within first via ring 728. Double ring via 730 is configured to define the vertical walls of the membrane cavity and traps silicon oxide therein after the vHF etching process. This is shown in more detail in FIG. 8.

[0072] Metal layer M6 is disposed over via structure V5. Metal layer M6 includes a circular metal surface 732 configured as a lid for pressure sensor 700. Metal layer M6 further includes an inner array of holes 734 and an outer array of holes 736. The holes in the inner array of holes 734 are positioned to provide access to membrane 726, and the holes in the outer array of holes 736 are aligned to provide access to metal layer M1. This is shown in more detail in FIG.

[0073] In some embodiments, the metal layers M1-M6 have a non-circular shape.The metal layers M1-M6 are constructed using BEOL metal layers of a solid-state semiconductor process.

[0074] In some embodiments, metal layer M6 is not used and is an optional feature.

[0075] FIG. 8 is a schematic diagram of a cross-sectional view of a pressure MEMS sensor 700, according to some embodiments. In particular, FIG. 8 shows a silicon dioxide layer 802, similar to the silicon dioxide layer 202 described in FIG. 2, that has been processed using the vHF etching process described herein. Additionally, passivation layers 804A and 804B are disposed on the silicon dioxide layer 802 to protect and shield the silicon dioxide layer 802 from corrosion. A seal 806 may be disposed on the inner array of holes 734. Note that the seal 806 is formed using the same approach as described herein for the seal 206. In some embodiments, the seal 806 is not used and is an optional component.

[0076] Pressure sensor 700 includes a cavity 816 for trapping gas to measure pressure. Cavity 816 includes cavity walls 818A and 818B formed using via structure V5. Cavity walls 818A and 818B are configured to cooperate with seal 806 to minimize outgassing and increase the accuracy of pressure measurements. Additionally, each cavity wall 818A and 818B includes an area 821 filled with oxide from silicon oxide layer 802 that was not etched by the vHF etch process.

[0077] The ends of metal layer M6 extend and are firmly embedded in silicon dioxide layer 802 to form capacitive anchors 810A and 810B. The ends of metal layer M1 extend and are firmly embedded in silicon dioxide layer 502 to form capacitive anchors 812A and 812B. Capacitive anchors 810 and 812 are configured to provide mechanical support for metal layers M6 and M1 and the capacitance required to measure membrane pressure 726. The ends of cavity walls 818A and 818B provide sufficient mechanical support to anchor membrane 726.

[0078] The pedestal 820 is formed using via rings V1, 708, and 716 in conjunction with metal layer M1 and metal planes 704, 712, and 720. In particular, the pedestal 820 is configured to support the membrane 726 and lid (metal layer M6) and add distance between the membrane and the fixed electrode below. The pressure sensor 700 uses a different anchor type than the pressure sensors 100 and 400. The pedestal 820 can result in a more compact design, but the etching can still determine the horizontal size below the passivation layer 802. Nevertheless, even with the correct dimensions and process parameters, parasitic capacitance can be reduced. The pedestal 820 and the vHF etch need to be tuned to trade between parasitic capacitance and yield. Additionally, the pedestal 820 includes some areas 824 filled with oxide from the silicon oxide layer 802 that were not etched by the vHF etch process.

[0079] Vertical capacitive anchors 822A and 822B are formed using via rings 702, 706, 710, 714, 718, 722, 724, and 728 and metal planes 704, 712, 720, and 726. Vertical capacitive anchors 822A and 822B are configured to protect and prevent the sidewalls of silicon dioxide layer 802 to which they are adjacent from being etched by the vHF etch process. Additionally, vertical capacitive anchors 822A and 822B provide additional support to metal layer M6, which is configured to be the lid of pressure sensor 700.

[0080] FIG. 9 is a schematic diagram of a cross-sectional view of a MEMS pressure sensor 900 having an array of holes 902 around a periphery of a membrane 904, according to some embodiments. In particular, FIG. 9 shows a silicon dioxide layer 906, similar to the silicon dioxide layer 202 described in FIG. 2, that has been processed using the vHF etching process described herein. Additionally, passivation layers 908A and 908B are disposed on the silicon dioxide layer 906 to protect and shield the silicon dioxide layer 904 from corrosion. Seals 910A and 910B may be disposed on the array of holes 902. Note that the seals 910A and 910B are formed using the same approach as described herein for the seal 206. In some embodiments, the seals 910A and 910B are not used and are optional components.

[0081] In some embodiments, seals 910 and 910B are formed using vapor deposition or by laser bleed or other techniques.

[0082] In some embodiments, the array of holes is disposed throughout the metal layer M6 that forms the membrane 904.

[0083] In some embodiments, the array of holes is located at the periphery of the metal layer M6 or within a predetermined radial distance from the center of the metal layer M6 (as shown in FIG. 9). This allows for a longer etching time, which etches away the TiN sublayer of the metal layer, exposing only the Al layer, reducing the stiffness of the layer and increasing its sensitivity to pressure 900. In some embodiments, annealing, such as at 300° C. for 1 hour, further reduces the stiffness of the metal layer.

[0084] The pressure sensor 900 includes metal layers M5 and M6. The metal layer M5 is configured as a bottom electrode. The ends of the metal layer M5 are embedded and secured in the silicon dioxide layer 906 to form capacitive anchors 912A and 912B. The ends of the metal layer M6 are embedded and secured in the silicon dioxide layer 906 to form capacitive anchors 914A and 914B. A portion of the metal layer M6 is used as the membrane 904 of the pressure sensor 900. The configuration of the pressure sensor 900 eliminates the need to seal the membrane 904 because the membrane 904 is hole-free. Additionally, the holes of the array of holes 902 are formed on the metal layer M6 and around the membrane 914.

[0085] 10A-10B are schematic diagrams of an exploded view and a side view of a MEMS pressure sensor 1000 without showing silicon dioxide, passivation, and substrate, according to some embodiments. The pressure sensor 1000 includes a circular metal layer M1 that serves as a bottom electrode with a diameter of 100 μm, as shown in FIG. 10A. A ring via V1 is disposed on the metal layer M1. A circular metal layer M2 is disposed on the ring via V1. A ring via V2 is disposed on the metal layer M1. A circular metal layer M3 is disposed on the ring via V2. A ring via V3 is disposed on the metal layer M1.

[0086] Metal layer M4 is disposed on ring via V3. Metal layer M5 is located on metal layer M4. Metal layer M5 is further configured to include membrane 1004 of pressure sensor 1000. Metal layer M5 further includes an array of holes 1002 having a number of holes on a top surface of metal layer M5. The top layer of pressure sensor 1000 is preferably configured to be membrane 1004 while minimizing stiffness.

[0087] In some embodiments, the metal layers M1-M5 of the pressure sensor 1000 have a non-circular shape. The metal layers M1-M6 are constructed using BEOL metal layers of a solid-state semiconductor process.

[0088] When pressure is applied, the membrane 1004 deflects at its center location with minimal deflection around the edges, which means that the outer regions of the membrane 1004 have minimal contribution to the capacitance change. However, these outer regions of the membrane 1004 may contribute to the initial capacitance.

[0089] The pressure sensor 1000 forces a more significant contribution to the initial capacitance at the center of the membrane 1004 where the displacement or deflection is greatest. A way to do this is to place the metal layer M1 at several levels below the membrane 1004, rather than directly below it. As shown in FIG. 10B, a cylindrical shaped pedestal 1006 is formed by stacking the metal layers M1-M4 and the ring vias V1-V3. Note that the pedestal 1006 is configured to reduce the capacitance gap at the center of the membrane 1004 and increase the capacitance gap at the periphery. The pedestal 1006 may be located at the center of the membrane 1004 and attached to the metal layer M1.

[0090] It is preferable not to use an additional top metal layer, such as M6, to minimize the thickness of the pressure sensor 1000. In some embodiments, the membrane includes a hole that is not located in the center or at an edge of the membrane 1004.

[0091] In some embodiments, pressure sensor 1000 includes additional vias to form pedestal 1006. In some embodiments, membrane 1004 is mounted on other metal layers M1-M4. In some embodiments, membrane 1004 is mounted on the top metal layer if metal layer M5 is not the top metal layer.

[0092] 11A-11B are schematic diagrams of the metal layer development and side view of a MEMS pressure sensor 1100 with a full leverage membrane according to some embodiments (silicon dioxide, passivation, and substrate not shown). The pressure sensor 1100 includes a circular metal layer M1, which serves as the bottom electrode, as shown in FIG. 11A. A metal layer M2 is located on the metal layer M1. The metal layer M2 is configured as a movable electrode. A via layer V2 is located on the metal layer M2. The via layer V2 has a similar design to the metal layer M2 and is movable. A metal layer M3 is located on the via layer V2. The metal layer M3 has a similar structure to the metal layer M2 and is movable. The via layer V3 includes a number of vias 1102 arranged in a circle on the metal layer M3. A metal layer M4 is located on the via layer V3. The metal layer M4 consists of several bridges or springs 1104 arranged on the vias 1102.

[0093] The via layer V4 is located on the metal layer M4. The via layer V4 has a first portion 1106 located on the bridge or spring 1104. A second portion 1108 of the via layer V4 is located on the remaining portion of the metal layer M4. The metal layer M5 is located on the via layer V4. The metal layer M5 includes a first portion 1110 located on the first portion 1106 of the via layer V4. A second portion 1112 of the metal layer M5 is located on the remaining portion of the metal layer M4. The via layer V5 is a ring via located on the first portion 1110 of the metal layer M5. The metal layer M6 is located on the via layer V5. The metal layer M6 is configured to include the membrane 1114 of the pressure sensor 1100.

[0094] Metal layers M1 and M4-M6 are configured to have a circular shape and radial dimensions, as shown in FIG. 11B. In some embodiments, metal layers M1 and M4-M6 are configured to have a shape other than circular. Metal layers M2 and M3 have precise radial dimensions, but are relatively smaller than metal layers M1 and M4-M6. Via layer V2 has a circular shape and is similar in size to metal layers M2 and M3 in the case of a via layer. Via layer V4 is circular and radially larger than via layer V2, but smaller than metal layers M1 and M4-M6.

[0095] In some embodiments, the metal layers M1-M5 of the pressure sensor 1000 have a non-circular shape. The metal layers M1-M6 are constructed using BEOL metal layers of a solid-state semiconductor process.

[0096] 11C shows a cross-sectional view of a pressure sensor 1100, according to some embodiments. In this case, a passivation layer 1122 is disposed on a silicon dioxide layer 1120. In the embodiment shown in FIG. 1, as shown in FIG. 11C of the pressure sensor 1100, an array of holes 1120 may be disposed on the membrane 1114. It is noted that the pressure sensor 1100 utilizes the membrane 1114 to generate or sense ultrasonic pressure. In some embodiments, the membrane 1114 may not include an array of holes.

[0097] Additionally, the pressure sensor 1100 includes an internal piston 1116 at its center to capture the displacement of the membrane 1114 and transmit it towards the inside of the pressure sensor 1100. A bridge or spring 1104 is attached to the central piston 1122 on one side and to the fixed support 1112 on the other side, such that a vertical displacement of the piston 1122 causes the bridge or spring 1104 to tilt. Metal layers M2 and M3 and via layer V2 are tilted parallel to the bridge or spring 1104, including metal layer M1. The internal electrodes 1120A and 1120B are implemented using stacked metal layers M2 and M3, as they would bend too much if only one metal layer was used.

[0098] In some embodiments, the metal layers M4 and M5 include a hole in the middle to allow the displacement of a central piston 1116 (which is made of the same metal layers M4 and M5 plus via layers V4 and V5 that connect the metal layer M5 to the membrane 1114 made of metal layer M5). This hole in the center of the pressure sensor 1100 is filled with a straight line that coincides with a bridge at the level of the metal layer M4.

[0099] In some embodiments, bridges or springs 1104 made of metal layer M4 tilt when the central piston is displaced vertically. Each is attached to a section of an internal electrode constructed of metal layers M2 and M3 and via layer V3. Thus, when bridges or springs 1104 tilt, internal electrodes 1120A and 1120B also tilt because they move and tilt parallel to the bridges or springs 1104 to which they are attached.

[0100] In some embodiments, if metal layer M6 is not the top metal layer, membrane 1104 is mounted on the top metal layer.

[0101] A similar approach for implementing pressure sensor 1100 is described in US Pat. No. 11,312,617, the entire contents of which are incorporated herein by reference.

[0102] 12A-12B are schematic diagrams of an exploded view and a side view of metal layers of a MEMS pressure sensor 1200, according to some embodiments. As shown in FIG. 12A, the pressure sensor 1200 includes a circular metal layer M1, a via layer V1, a metal layer M2, a via layer V2, a metal layer M3, a metal layer M4, a via layer V4, and a circular metal layer M5. The metal layers M2, M3, and the via layer V2 are each configured to be wing-shaped and similar in size. Also, as shown in FIG. 12B, the metal layers M2, M3, and the via layers V1, V2 are stacked to form a pedestal 1206 located on the metal layer M1. In this case, the pedestal 1206 is located away from the center of the pressure sensor 1200. The metal layer M4 includes a wing-like portion 1202 and a membrane 1204 integrally connected to the wing-like portion 1202. The pedestal 1206 is located below the wing-like portion 1202, where the maximum displacement occurs. Via layer V4 is a via 1208 disposed on the distal end of membrane 1204. Metal layer M5 is disposed on via layer V4.

[0103] The pressure sensor 1200 does not include a piston as in the pressure sensor 1100. The wings 1202 can start from an optimal anchor point with maximum tilt of the membrane 1204 when a pressure difference is applied to the distal end of the membrane 1204. Additionally, the size of the wings 1202 can be increased and / or pedestals 1206 can be added to the ends of the wings 1202, resulting in maximum displacement. This enhances the effect of the edges of the wings 1202 on the total capacitance, thus maximizing the sensitivity of the pressure sensor 1200.

[0104] In some embodiments, an ASIC is provided that operates as a tire pressure monitoring system (TPMS), the ASIC including a MEMS pressure sensor as described herein, the pressure sensor being monolithically integrated into the BEOL of the ASIC.

[0105] In some embodiments, an ASIC is provided that operates as an e-cigarette sensor. The ASIC includes a MEMS pressure sensor as described herein. The pressure sensor is monolithically integrated in the BEOL of the ASIC. In an e-cigarette, the pressure sensor is used to detect the inhalation of air from a user. This is done using a differential pressure sensor. When a user inhales and inhales, the pressure drops faster at the front of the differential pressure sensor than at the back reference. As a result, the pressure difference is detected and at a certain threshold, a signal is triggered that turns on the filament of the e-cigarette.

[0106] There is interest in using MEMS pressure sensors in SMD technology to reduce costs as well as size and soldering errors during assembly. Currently, most e-cigarettes use electret-type pressure sensors. However, MEMS pressure sensor ICs are small. As a result, the narrower air path to reach the back reference of the pressure sensor can be more easily clogged or isolated for a while due to oils and dirt contained inside the e-cigarette. When this happens, the back pressure reference will no longer recognize the actual pressure of the surroundings.

[0107] Then, if the ambient pressure changes due to environmental conditions and / or the user moves to a higher or lower altitude, the front of the pressure sensor will detect it and trigger a threshold to heat the filament when the user is not inhaling. A way to prevent this is to use a non-differential pressure sensor as disclosed in this patent. A circuit (preferably digital) can track the ambient pressure. Preferably, this can be done by doing a moving average of the measured pressure. The moving average may not respond immediately to the measured pressure, allowing for detection of inhalation events. Note that other types of averages may be used. There is no risk of the back reference pressure input port becoming clogged or isolated from the actual pressure, since there is no such port and only the main pressure input port is sufficiently exposed to air to measure the pressure. The MEMS pressure sensor described herein can be used to operate as a differential pressure sensor in an e-cigarette.

[0108] In some embodiments, an integrated circuit operating as a microphone is provided, the integrated circuit including a MEMS pressure sensor as described herein, the pressure sensor being monolithically integrated within the BEOL of the integrated circuit.

[0109] In some embodiments, an integrated circuit operating as an ultrasonic sensor is provided that includes a MEMS pressure sensor as described herein, the pressure sensor being monolithically integrated within the BEOL of the integrated circuit.

[0110] Elements or steps of different described implementations may be combined to form other implementations not specifically described above. Elements or steps may be omitted from the described systems or processes without adversely affecting their operation or the operation of the overall system. Additionally, various separate elements or steps may be combined into one or more individual elements or steps to perform the functions described herein.

[0111] Reference herein to "one implementation" or an "implementation" means that a particular feature, structure, or characteristic described in connection with that implementation is included in at least one implementation of the disclosure. The appearances of the phrases "in one implementation," "in some implementations," "in one example," "in some examples," "in some cases," "in one embodiment," or "in some embodiments" in various places in the specification are not necessarily all referring to the same implementation or embodiment.

[0112] Finally, the above description of the embodiments of the present disclosure has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the present disclosure to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. It is intended that the scope of the present disclosure be limited not by this detailed description, but by the claims of this application. As will be understood by those skilled in the art, the present disclosure can be embodied in other specific forms without departing from its spirit or essential characteristics. Thus, the present disclosure is intended to be illustrative, but not limiting, of the scope of the present disclosure, which is set forth in the claims.

Claims

1. A MEMS pressure sensor comprising: A membrane made of one of a plurality of metal layers; A lid disposed on the membrane and connected to a plurality of cavity walls at the distal end of the membrane, wherein the lid includes an array of holes disposed on a region of the lid; and A fixed metal electrode disposed under the lid.

2. The MEMS pressure sensor according to claim 1, wherein the lid is formed using a metal layer different from the membrane among the plurality of metal layers, and the membrane is formed using a first metal layer under the lid.

3. The MEMS pressure sensor according to claim 1, wherein the membrane and the lid include a circular shape.

4. The MEMS pressure sensor according to claim 3, wherein at least one of the lid and the membrane includes a diameter of about 25 μm, 50 μm, 75 μm, or 100 μm.

5. The MEMS pressure sensor according to claim 1, wherein the metal layer is formed using a post-process metal layer of a solid semiconductor process.

6. The MEMS pressure sensor according to claim 1, wherein the cavity wall is formed using at least one via structure.

7. The MEMS pressure sensor according to claim 6, wherein the at least one via structure includes a double-ring via.

8. The MEMS pressure sensor according to claim 1, wherein the array of holes protects the membrane from contaminants.

9. The MEMS pressure sensor according to claim 1, wherein the lid includes a second array of holes located in a second region on the lid different from the region defined by the array of holes.

10. The MEMS pressure sensor according to claim 9, wherein the second region of the lid is disposed away from the membrane.

11. The MEMS pressure sensor according to claim 9, wherein the membrane includes a third array of holes aligned with the first array of holes of the lid.

12. The MEMS pressure sensor according to claim 1, wherein a set of metal layers among the plurality of metal layers is configured to be a pedestal.

13. The MEMS pressure sensor according to claim 12, wherein the pedestal reduces the capacitance gap at the center of the membrane.

14. The MEMS pressure sensor according to claim 13, wherein the membrane is deflected from its central position when pressure is applied.

15. The MEMS pressure sensor according to claim 12, wherein the set of metal layers is wing-shaped.

16. The MEMS pressure sensor according to claim 13, wherein the membrane is integrally connected to a wing-like portion of one of the plurality of metal layers.

17. The MEMS pressure sensor according to claim 14, wherein the membrane is connected to a lid via a via connected to a distal end of the membrane.

18. A MEMS pressure sensor comprising: a membrane made from one of a plurality of metal layers, the membrane including a plurality of holes; and an electrode disposed under the membrane and made from another metal layer of the plurality of metal layers, the electrode having the same shape as the membrane or at least extending over a region where the plurality of holes of the membrane are located.

19. The MEMS pressure sensor according to claim 18, wherein the metal layer is formed using a post-process metal layer of a solid semiconductor process.

20. The MEMS pressure sensor according to claim 18, wherein the membrane has a distance of at least 20 μm from each of the plurality of holes.

21. The MEMS pressure sensor according to claim 18, further comprising a piston formed to capture displacement of the membrane when pressure is applied to the membrane.

22. The MEMS pressure sensor according to claim 21, wherein a set of the plurality of metal layers is a movable electrode configured to tilt when the piston is displaced.

23. A MEMS pressure sensor comprising: a metal layer made from one of a plurality of metal layers; and a pedestal disposed under the metal layer and made from another set of the plurality of metal layers, the pedestal reducing an electrostatic capacitance gap at the center of the metal layer.

24. The MEMS pressure sensor according to claim 23, wherein the metal layer is deflected from its central position when pressure is applied.

25. The plurality of metal layers includes metal layers M1 to M6, the metal layer M6 is the uppermost metal layer, the metal layer M5 is the second uppermost metal layer, and the pedestal is made of the metal layers M1 to M4 and a plurality of via structures V1 to V3. The MEMS pressure sensor according to claim 24.

26. A MEMS pressure sensor comprising: a metal layer made from one of a plurality of metal layers; and a pedestal disposed under the metal layer and made from another set of the plurality of metal layers, the another set of metal layers including a plurality of wing-like metal layers.

27. The MEMS pressure sensor according to claim 26, wherein the metal layer is connected to another metal layer via a via connected to the distal end of the metal layer. **Claim 28** A MEMS pressure sensor comprising: A metal layer made from one of a plurality of metal layers; and A piston disposed under the metal layer and made from another set of metal layers of the plurality of metal layers, the piston capturing displacement of the metal layer when pressure is applied to the metal layer. **Claim 29** The MEMS pressure sensor according to claim 28, wherein one set of the plurality of metal layers is a movable electrode configured to tilt when the piston is displaced. **Claim 30** A MEMS pressure sensor comprising: A metal layer made from one of a plurality of metal layers; and A pedestal disposed under the metal layer and made from another set of metal layers of the plurality of metal layers, the pedestal including a membrane integrally connected to a wing portion of one of a plurality of wing-shaped metal layers. **Claim 31** The MEMS pressure sensor according to claim 30, wherein the pedestal is located away from the center of the metal layer.

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