MANUFACTURING THE SAME

RELATED PATENT APPLICATION

This patent application incorporates by reference, in its entirety, and claims priority to U.S. Provisional Patent Application No. 61/881 ,628, filed September 24, 2013.

TECHNICAL FIELD

The general technical field relates to micro-electro-mechanical systems (MEMS), and more particularly, to a MEMS device including a guard ring surrounding an electrode, and to a method of manufacturing such a MEMS device.

BACKGROUND

Micro-electro-mechanical systems (MEMS) are an enabling technology. Broadly described, MEMS devices are integrated circuits (ICs) containing tiny mechanical, optical, magnetic, electrical, chemical, biological, or other, transducers or actuators. MEMS devices are manufactured using high-volume silicon wafer fabrication techniques developed over the last fifty years for the microelectronics industry. Their resulting small size and low cost make them attractive for use in an increasing number of applications in consumer, automotive, medical, aerospace, defense, green energy, industrial, and other markets.

In many applications, MEMS devices use capacitive sensing as a transduction method. For example, a conventional MEMS pressure sensor can include a fixed electrode and a flexible membrane electrode that moves in response to variations in external pressure. Monitoring the capacitance between the two electrodes can provide a direct measurement of the pressure variations. MEMS motion sensors also generally rely on capacitive measurements. Existing capacitive-based MEMS devices can involve the detection of capacitance variations of the order of femtofarads (fF) or even attofarads (aF). The measurement of such small capacitance variations can become quite sensitive to parasitic capacitance effects between the sensing electrodes and their surroundings.

Accordingly, a need exists in the art for the development of MEMS devices provided with electrodes exhibiting improved performance against parasitic and stray capacitance, current leakage, and other undesired electrical coupling effects that tend to degrade device sensitivity and reliability.

SUMMARY

In accordance with an aspect of the invention, there is provided a method of manufacturing a micro-electro-mechanical system (MEMS) device. The method includes the steps:

a) providing a top cap wafer having opposed first and second sides and a thickness extending therebetween, and a MEMS wafer including a MEMS structure, the MEMS wafer having opposed top and bottom sides, each of the top cap wafer and the MEMS wafer being made of an electrically conductive semiconductor material;

b) forming an electrode structure from the first side into the top cap wafer, the electrode structure including an electrode and a guard ring laterally surrounding and electrically insulated from the electrode, the electrode and the guard ring each extending through the entire thickness of the top cap wafer; and

c) bonding the first side of the top cap wafer to the top side of the MEMS wafer such that an electrical connection is established between the electrode and the MEMS structure.

In some embodiments, step b) includes the substep of: - forming an inner insulation channel extending through the entire thickness of the top cap wafer, the inner insulation channel laterally bordering a first region of the top cap wafer corresponding to the electrode; and

- forming an outer insulation channel extending through the entire thickness of the top cap wafer and spaced from the inner insulation channel, the inner and outer vertical insulation channels together laterally bordering a second region of the top cap wafer corresponding to the guard ring.

In some embodiments, step b) includes the substeps of:

- forming a closed-loop trench in the top cap wafer, the trench having inner and outer sidewalls, the trench extending from the first side partially into the top cap wafer and laterally bordering a volume of the top cap wafer corresponding to the electrode to be formed;

- lining the inner and outer sidewalls of the trench with an electrically insulating material to form an insulator-lined trench;

- depositing an electrically conductive material into the insulator-lined trench, the deposited electrically conductive material corresponding to the guard ring to be formed;

- removing top cap wafer material from the second side of the top cap wafer to expose the electrically conducting material, such that the electrode and the guard ring are formed which are electrically insulated from each other by the electrically insulating material lining the inner sidewalk

In some embodiments, the electrically conductive material includes one of a metal and polysilicon.

In some embodiments, step b) includes the substeps of: - forming spaced-apart inner and outer closed-loop trenches each extending from the first side partially into the top cap wafer, the inner trench laterally bordering a first volume of the top cap wafer corresponding to the electrode to be formed, the inner and outer trenches together laterally bordering a second volume of the top cap wafer corresponding to the guard ring to be formed;

- depositing an electrically insulating material into the inner and outer trenches; and

- removing top cap wafer material from the second side of the top cap wafer to expose the electrically insulating material of the inner and outer trenches, such that the electrode and the guard ring are formed which are electrically insulated from each other by the electrically insulating material deposited into the inner trench.

In some embodiments, the electrically insulating material includes silicon dioxide.

In some embodiments, removing top cap wafer material from the second side of the top cap wafer includes at least one of grinding, polishing and etching.

In some embodiments, the method includes, after step b), the substeps of:

- forming a cap insulating layer on the second side of the top cap wafer;

- partially removing the cap insulating layer to expose at least a portion of the guard ring; and

- forming a first electrical connection on the portion of the guard ring, the first electrical connection being in electrical contact with the guard ring.

In some embodiments, the method includes, after step b), the substeps of:

- partially removing the cap insulating layer to expose at least a portion of the electrode; and - forming a second electrical connection on the portion of the electrode, the second electrical connection being in electrical contact with the electrode.

In some embodiments, the method includes the step of

d) bonding a bottom cap wafer to the bottom side of the MEMS wafer.

In some embodiments, the electrically conductive semiconductor material includes a silicon-based semiconductor. In some embodiments, step a) includes forming the MEMS structure as a pendulous proof mass, and wherein the electrical connection is a capacitive electrical connection.

In accordance with another aspect of the invention, there is provided a micro-electro- mechanical system (MEMS) device. The MEMS device includes:

- a MEMS wafer including a MEMS structure, the MEMS wafer having opposed top and bottom sides;

- a top cap wafer having opposed first and second sides and a thickness extending therebetween, the first side being bonded to the top side of the MEMS wafer, the MEMS wafer and the top cap wafer being made of an electrically conductive semiconductor material;

- an electrode structure formed into the top cap wafer, the electrode structure including an electrode establishing an electrical connection with the MEMS structure and a guard ring laterally surrounding and electrically insulated from the electrode, the electrode and the guard ring each extending through the entire thickness of the top cap wafer.

In some embodiments, the electrode structure includes: - an inner insulation channel extending through the entire thickness of the top cap wafer, the inner insulation channel laterally bordering a first region of the top cap wafer corresponding to the electrode; and

- an outer insulation channel extending through the entire thickness of the top cap wafer and formed into the top cap wafer and spaced from the inner insulation channel, the inner and outer vertical insulation channels together laterally bordering a second region of the top cap wafer corresponding to the guard ring. In some embodiments, the inner and outer insulation channels are each made of an electrically insulating material including silicon dioxide.

In some embodiments, the MEMS device includes:

- a cap insulating layer formed on the second side of the top cap wafer and defining a guard ring contact opening over at least a portion of the guard ring; and

- a first electrical connection providing electrical contact to the guard ring through the guard ring contact opening. In some embodiments, the cap insulating layer further defines an electrode contact opening over at least a portion of the electrode, the MEMS device including a second electrical connection providing electrical contact to the electrode through the electrode contact opening. In some embodiments, the electrically conductive semiconductor material includes a silicon-based semiconductor. In some embodiments, the guard ring is made of an electrically conductive material including one of metal, silicon and polysilicon.

In some embodiments, the MEMS device includes a bottom cap wafer bonded to the bottom side of the MEMS wafer.

In some embodiments, the MEMS structure includes a pendulous proof mass, and wherein the electrical connection is a capacitive electrical connection. Other features and advantages of the embodiments of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs 1 A and 1 B illustrate two configurations of a measurement setup in which a coaxial cable including a central conductor surrounded by a shield is connected between a capacitor whose capacitance is to be measured and a measurement amplifier. In Fig 1A the shield is grounded and the central conductor is unguarded, while in Fig 1 B the shield is held at the same potential as the central conductor and acts as a guard surrounding the central conductor.

Fig 2A is a schematic perspective view of a MEMS device, in accordance with an exemplary embodiment. Fig 2B is a schematic cross-sectional view of the MEMS device of Fig 2A. Fig 2C is another schematic cross-sectional view of the MEMS device of Fig 2A, illustrating sources of parasitic capacitances that may affect the sensitivity and reliability of measurements obtained by the MEM device.

Figs 3A to 3G schematically illustrate steps of a method for manufacturing a MEMS device, in accordance with an exemplary embodiment. Figs 4A to 4H schematically illustrate steps of a method for manufacturing a MEMS device, in accordance with another exemplary embodiment. Fig 5 is an enlarged view of a portion of Fig 3F, illustrating a guard ring bordered by inner and outer trenches filled with an electrically insulating material.

Fig 6 is an enlarged view of a portion of Fig 4G, illustrating a guard ring bordered by electrically insulating material lined on the inner and outer sidewalls of a trench.

Fig 7 is a schematic cross-sectional view of a MEMS device, in accordance with another exemplary embodiment, wherein a guard ring is provided around a conductive electrical lead. It should be noted that the appended drawings illustrate only exemplary embodiments of the invention, and are therefore not to be construed as limiting of its scope, for the invention may admit to other equally effective embodiments.

DETAILED DESCRIPTION

In the following description, similar features in the drawings have been given similar reference numerals, and, in order to not unduly encumber the figures, some elements may not be indicated on some figures if they were already identified in preceding figures. It should also be understood that the elements of the drawings are not necessarily depicted to scale, since emphasis is placed upon clearly illustrating the elements and structures of the present embodiments.

The present description generally relates to a MEMS device provided with an electrode guard ring. The present description also generally relates to a method of manufacturing a MEMS device provided with an electrode guard ring. Throughout the present description, the term "MEMS device" is meant to encompass any suitable device such as, but not limited to, motion sensors, pressure sensors, magnetometers, actuators, micro-fluidic devices, micro-optic devices, and any other MEMS device where it may be relevant to apply the techniques and principles described herein. Among the different types of MEMS devices, motion sensors have seen their use grow steadily over the past decades. MEMS motion sensors are used to sense changes in the state of motion of an object, including changes in position, velocity, acceleration or orientation, and encompass devices such as accelerometers, gyroscopes, vibrometers and inclinometers. It is to be noted that although an embodiment the MEMS device described below is implemented as a motion sensor, this embodiment is presented only as a non-limiting example of applying the techniques and principles described herein to a MEMS device. Broadly described, the embodiments of the MEMS device include a MEMS wafer having opposed top and bottom sides, a top cap wafer having opposed first and second sides and a thickness extending therebetween, and an electrode structure. The first side of the top cap wafer is bonded to the top side of the MEMS wafer. Both the MEMS and top cap wafers are made of an electrically conductive semiconductor material. The MEMS wafer is provided with a MEMS structure which can include or be embodied by any sensing and/or control element or combination of sensing and/or control elements such as, but not limited to, membranes, diaphragms, proof masses, comb sensors, actuators, transducers, micro-valves, micro-pumps, and the like. The electrode structure is formed into the top cap wafer and includes an electrode establishing an electrical connection with the MEMS structure, as well as a guard ring laterally surrounding and electrically insulated from the electrode. Each of the electrode and guard ring extends through the entire thickness of the top cap wafer. As used herein, the term "guard ring" refers to an electrically conducting element of the MEMS device, which is electrically insulated from and laterally surrounds the electrode of the MEMS device. The guard ring forms a closed electrical loop within the top cap wafer. Likewise, the term "electrode" is intended to refer to an electrically conducting element of the MEMS device that is used to establish an electrical connection with the MEMS structure of the MEMS device in view of transmitting signals (e.g., electrical signals such as charges, voltages or currents) to and/or from the MEMS structure. Depending on a given implementation of the MEMS device, the electrode can be capacitive (e.g., a drive or sense capacitor), conductive, (e.g., a resistive electrical element, an inductive electrical element or a lead), or a combination thereof. Accordingly, the electrical connection established between the electrode and the MEMS structure can involve a resistive electrical connection, a capacitive electrical connection, an inductive electrical connection, or a combination thereof.

As described in greater detail below, in some embodiments, the provision of a guard ring around an electrode of the MEMS device can improve the sensitivity of the electrode by reducing parasitic and stray capacitance, current leakage and other adverse electrical coupling effects between the electrode and its surroundings. It should be noted that the term "guard ring" is not limited to circular or oval ring-shaped structures in a strict geometric sense, but may admit a variety of shapes, including rectangular, polygonal, and other more complex shapes.

Those skilled in the art will recognize that using a "guard" to reduce parasitic capacitance and other adverse electrical coupling effects in low signal measurements has been known in the field of electronics. For example, guarded cables are commonly employed to eliminate or at least mitigate leakage currents and stray capacitance in electrical leads. Figs 1 A and 1 B illustrate two configurations of a charging-rate measurement setup in which a coaxial cable 100 including a central conductor 102 surrounded by a shield 104 is connected between a capacitor whose capacitance C _x is to be determined and a measurement amplifier 106. In Fig 1A, the shield 104 is grounded and the central conductor 102 is unguarded. The shield and the low side of C _x are at the same ground potential, so that measuring the current / charges not only the capacitor of capacitance Cx, but also the capacitor of capacitance C _c established between the central conductor 102 and the grounded shield 104 of the coaxial cable 100. Additionally, leakage current can flow between the central conductor and the shield since it is at a different potential. Meanwhile, in Fig 1 B, the shield 104 is connected to the output terminal of the measurement amplifier 106 (or a separate voltage supply at the same potential) so that the central conductor 102 and the shield 104 are held at the same potential. Thus no leakage current can flow between the conductor and the shield and no charge is generated across the capacitor formed by the conductor and shield since the capacitor electrodes are at the same potential. In this configuration, it can be said that the central conductor 102 is "guarded" by the shield 104, which contributes to eliminating or at least reducing parasitic cable capacitance C _c and leakage current, and enables a more direct measurement of the unknown capacitor Cx.

Embodiment of a MEMS device

In accordance with an aspect, there is provided a MEMS device including an electrode guard ring.

Referring to Figs 2A and 2B, there are shown a schematic perspective view and a schematic cross-sectional view, respectively, of a MEMS device 10 according to an embodiment. In this exemplary embodiment, the MEMS device 10 consists of a multi- wafer stack implemented as a MEMS motion sensor. Other types of MEMS devices can be used in other embodiments, as mentioned above. The MEMS device 10 includes a top cap wafer 12 having opposed first and second sides 121 , 122 and a thickness 123 extending therebetween; a MEMS wafer 16 having opposed top and bottom sides 161 , 162; and, optionally, a bottom cap wafer 14 having opposed first and second sides 141 , 142 and a thickness 143 extending therebetween.

It will be understood that throughout the present description, and unless stated otherwise, positional descriptors such as "top" and "bottom" should be taken in the context of the figures and should not be considered as being limitative. In particular, the terms "top" and "bottom" are used to facilitate reading of the description, and those skilled in the art of MEMS will readily recognize that, when in use, MEMS devices can be placed in different orientations such that the "top" and "bottom" cap wafers and the "top" and "bottom" sides of the MEMS wafer may be positioned upside down in certain configurations.

The MEMS wafer 16 can consist of a standard wafer, a silicon-on-insulator (SOI) wafer, or of multiple wafers. For example, in the embodiment of Figs 2A and 2B, the MEMS wafer 16 is an SOI wafer including a device layer 20, a handle layer 22 and an insulating layer 24 sandwiched between the device layer 20 and the handle layer 22. The device layer 20 and the handle layer 22 of the MEMS wafer 16, as well as the top and bottom cap wafers 12, 14 can be made of an electrically conductive semiconductor material, for example a silicon-based semiconductor.

The MEMS wafer 16 is provided with a MEMS structure 17 which, in the illustrated embodiment, includes a pendulous proof mass 171 coupled to a peripheral region of the MEMS wafer 16 via flexible springs 27. As used herein, the term "proof mass" refers broadly to any predetermined inertial mass used in a MEMS motion sensor, such as an accelerometer or a gyroscope, whose displacement serves as a reference for the motion to be measured or monitored. The flexible springs 27 join the proof mass 171 to the peripheral region of the MEMS wafer 16 for providing a restoring force to the proof mass 171. This restoring force enables the proof mass 171 to move relative to the peripheral region of the MEMS wafer 16 in response to a motion experienced by the MEMS device 10. This relative displacement of the proof mass 171 may be sensed by electrodes or other suitable transducing means.

Referring still to Figs 2A and 2B, when the top cap wafer 12, the MEMS wafer 16 and the bottom cap wafer 14 are assembled to form the MEMS device 10, the first side 121 of the top cap wafer 12 is bonded to the top side 161 of the MEMS wafer 16, and the first side 141 of the bottom cap wafer 14 is bonded to the bottom side 162 of the MEMS wafer 16. The top cap wafer 12, the MEMS wafer 16 and the bottom cap wafer 14 are typically bonded with a conductive bond. In this particular embodiment, the top cap wafer 12 is bonded to and in electrical contact with the device layer 20, while the bottom cap wafer 14 is bonded to and in electrical contact with the handle layer 22. The insulating layer 24, which typically consists of buried oxide, insulates the top half of the MEMS device 10 from the bottom half. SOI conducting shunts (not shown) can be provided that extend through the insulating layer 24 to electrically connect the device layer 20 and handle layer 22, in specific desired places.

In some embodiments, when bonded together, the top cap wafer 12, the MEMS wafer 16 and the bottom cap wafer 14 together define a cavity 31 for receiving and housing the MEMS structure 17, as depicted in the cross-sectional view of Fig 2B. In some embodiments, at least part of the cavity 31 can be defined by a top recess 31 1 formed by removing top cap wafer material from a central region of the first side 121 of the top cap wafer 12 prior to bonding the first side 121 of the top cap wafer 12 to the MEMS wafer 16. In other embodiments, at least part of the cavity 31 can alternatively or additionally be defined by a bottom recess 312 formed by removing bottom cap wafer material from a central region of the first side 141 of the bottom cap wafer 14 prior to bonding the first side 141 of the bottom cap wafer 14 to the MEMS wafer 16. In such a case, those skilled in the art will recognize that the top cap wafer 12 and the MEMS wafer 16 are bonded to each other at their respective peripheral regions, and similarly for the bottom cap wafer 14 and the MEMS wafer 16. As mentioned above, the MEMS device 10 of Figs 2A and 2B is a motion sensor capable of sensing the motion of a pendulous proof mass 171 to determine any of a number of parameters (e.g., acceleration, velocity, angular rate or orientation) indicative of a state of motion of an object or structure to which the MEMS device 10 is associated. In the exemplary embodiment of Figs 2A and 2B, the motion of the proof mass is measured using capacitive sensing techniques.

For this purpose, the MEMS device 10 can be provided with a plurality of electrodes including top and bottom electrodes 13, 15 provided in the top and bottom cap wafers 12, 14, respectively, and forming capacitors with the proof mass 171 . Depending of the particular implementation of the MEMS device 10, some of the top and bottom electrodes 13, 15 can be operated a driving electrodes and be connectable to driving means, while other ones of the top and bottom electrodes 13, 15 can be operated as sensing electrodes and be connectable to sensing means. The top and bottom electrodes 13, 15 may alternatively be reconfigurably connectable to driving and sensing means, for switching between drive and sense modes. As used herein, the terms "driving means" and "sensing means" refer broadly to any electronic circuitry configured to deliver electrical signals to and receive electrical signals from the electrodes in order to drive and sense a response from the MEMS structure (e.g., a proof mass) of the MEMS device, respectively.

For example, in the embodiment of Figs 2A and 2B, the plurality of electrodes includes five top electrodes 13 (one central driving electrodes and four peripheral sensing electrodes) and five bottom electrodes 15 (one central driving electrodes and four peripheral sensing electrodes), which extend through the entire thicknesses 123, 143 of the top and bottom cap wafers 12, 14, respectively. Of course, the number, size, shape, position, arrangement and type (e.g., driving or sensing) of electrodes can vary depending on the application in which the MEMS device is to be used. It is to be noted that the architecture of the embodiment of the MEMS device 10 illustrated in Figs 2A and 2B can allow for the placement of electrodes and electrical leads above, below, and/or around the MEMS structure 17 (e.g., the proof mass 171 ) for measuring signals from the MEMS structure 17 (e.g., acceleration and/or angular rate). This architecture can be referred to as a "three-dimensional through-chip-via" (3DTCV) architecture and can allow routing all the signals to one side of the chip where they can be accessed for signal processing. For example, the signals detected by the top and bottom electrodes 13, 15 can be routed to electrical contacts connected to the top surface of the MEMS device 10, to which a CMOS chip can be connected for controlling and/or measuring the electrical signals to/from the MEMS device 10. In particular, electrodes 13, but also leads, bond pads, guard rings and other structures, can be defined in the top cap wafer 12 by insulating channels. The insulating channels can be made using a through-silicon-via (TSV)-like fabrication process in which the vias are completely filled with an insulator, or by an insulator and conductor, as the insulating channels are used essentially to for isolation purposes, and not to carry electrical signals vertically through MEMS device 10.

Referring still to Figs 2A and 2B, the capacitance of each capacitor formed between the proof mass 171 and the top or bottom electrodes 13, 15 can be monitored to determine the motion of the proof mass 171 in response to external forces. In some implementations of the MEMS device 10, the top and bottom electrodes 13 and 15 are used to measure very low electrical signals (e.g., charges, voltages or currents) associated with correspondingly small capacitance variations. For example, in a conventional MEMS motion sensor, the value of the "undetected" capacitance can be of the order of 1 picofarad (pF), and the capacitance variations corresponding to proof mass motions of interest can be of the order of fF or even aF. The measurement of such small capacitance variations can become quite sensitive to parasitic capacitance and other deleterious electrical coupling effects between the sensing electrodes and the surrounding in the MEMS device. For example, turning briefly to Fig 2C, there are shown some parasitic capacitors 18 that may appear between the top electrodes 13 and other elements of the MEMS device 10 and that may affect the reliability of the measurement of the motion of the proof mass 171 (or, more generally, a response of interest of the MEMS structure 17).

Referring back to Figs 2A and 2B, in order to eliminate or at least reduce the impact of parasitic and stray capacitance, current leakage, and other adverse electrical coupling effects, the MEMS device 10 according to embodiments of the invention includes one or more guard rings 54. Each guard ring 54 forms a closed-loop electrical circuit that laterally surrounds and is electrically insulated from a corresponding electrode 13, 15 formed in the top and bottom cap wafers 12, 14. As used herein, the terms "lateral" and "laterally" refers to directions that lie in a plane perpendicular to the thickness of the top or the bottom cap wafer, that is, in plane perpendicular to the wafer stacking direction of the MEMS device. Accordingly, each guard ring 54 forms a two-dimensional continuous or closed-loop conducting path or channel around its associated electrode 13, 15. Additionally, each guard ring 54 and its associated electrode 13, 15 extend through the entire thickness 123, 143 of the top or bottom cap wafer 12, 14 and define what is referred to as an "electrode structure" 19.

It will be understood that by providing a guard ring 54 that extends through the entire thickness of a cap wafer 12, 14 and that border the entire periphery of its associated electrode 13, 15, the guard ring 54 can laterally guard the sidewalls of the electrode 13, 15 from the rest of the cap wafer 12, 14 through the entire thickness of the electrode 13, 15 and not just at the surface. In particular, the provision of a guard ring 54 around an electrode 13, 15 of the MEMS device 10 can shield or protect that electrode 13, 15 from parasitic capacitance, leakage current and other unwanted electrical coupling effects that could otherwise degrade the integrity of the electrode signals (e.g., electrical signals such as charges, voltages or currents) transmitted from and/or to the MEMS structure 17.

In the embodiment of Figs 2A and 2B, a guard ring 54 is provided around each of the four sensing electrodes 13 in the top cap wafer 12 and around each of the four sensing electrodes 15 in the bottom cap wafer 14. The guard ring 54 is preferably made of an electrically conductive material 32 including one of metal (e.g., copper), silicon and polysilicon. Of course, the material, number, size, shape and position of the guard rings and the type of electrodes (e.g., driving and sensing) to which the guard ring paired can be varied in other embodiments. For example, in some embodiments, electrodes of only one of the top and bottom cap wafers may be provided with guard rings without departing from the scope of the present invention.

Referring still to Figs 2A and 2B, in some embodiments, an electrode structure 19 formed in the top cap wafer 12 can include an inner insulation channel 21 and an outer insulation channel 23 laterally spaced from the inner insulation channel 21 . Each of the inner and outer insulation channels 21 , 23 extends through the entire thickness 123 of the top cap wafer 12 and is made of an electrically insulating material 30, preferably including silicon dioxide (S1O2). The inner insulation channel 21 laterally borders a first region of the top cap wafer 12 that corresponds to the electrode 13 of the electrode structure 19. Meanwhile, the inner and outer insulations 21 , 23 together laterally border a second region of the top cap wafer 12 corresponding to the guard ring 54 of the electrode structure 19. Depending on the particular implementation of the MEMS device, the inner and outer insulation channels 21 , 23 can have various configurations and structures, and be fabricated according to different techniques and processes. For example, in some embodiments, the inner and outer insulation channels 21 , 23 can be embodied by spaced-apart inner and outer closed-loop trenches 28a, 28b formed into the top cap wafer 12 and filled with electrically insulating material 30 (see, e.g., Fig 3E). In other embodiments, such as in Figs 2A and 2B (see also Fig 4F), a single closed-loop trench 28 having inner and outer sidewalls 31 a, 31 b can be formed in the top cap wafer 12, and the inner and outer insulating channels 21 , 23 can be formed respectively by lining the inner and outer sidewalls 31 a, 31 b of the trench 28 with an electrically insulating material 30. An electrically conductive material 32 (e.g., a metal such as copper, silicon, polysilicon or any suitable conductor) may then be deposited into the insulator-lined trench to form the guard ring 54. In this second configuration, the use of a single trench 28 in which are formed the guard ring 54 and both the inner and outer insulation channels 21 , 23 can allow for the area occupied by the guard ring 54 to be reduced, and thus the area occupied by the electrode 13 to be increased.

More regarding the various fabrication techniques and structural features of the MEMS device, and in particular the guard ring, will be discussed further below.

Referring to Fig 2B, in some embodiments, the MEMS device 10 can include a cap insulating layer 40 formed on the second side 122 of the top cap wafer 12 and defining a guard ring contact opening 55 over at least a portion of each guard ring 54. The MEMS device 10 can also include a first electrical connection or contact 42 providing an electrical connection to each guard ring 54 through the guard ring contact opening 55. The cap insulating layer 40 can further define an electrode contact opening 57 over at least a portion of an electrode 13 in the top cap wafer 12, so that a second electrical connection or contact 43 can provide an electrical connection to the electrode 13 through the electrode contact opening 57. The first and second electrical connections 42, 43 may be formed by depositing a metallic layer 41 , in some scenarios with an additional sticking or barrier layer, on the cap insulating layer 40 and by patterning the metallic layer 41 to form the first and second electrical connections 42, 43. A passivating layer 45 may also be applied over the first and second electrical connections 42, 43. In some embodiments, the passivating layer 45 can extend over substantially the entire surface of the second side 122 of the top cap wafer 10. Finally, openings may be formed in the passivating layer 45 to expose at least partially the first and second electrical connections 42, 43.

It will be understood that by connecting the first connection 42 to an electrical source (which can be the same or a different electrical source than the electrical source to which the second electrical connection 43), the guard ring 54 can be held at the same or at a similar electrical potential as its associated electrode 13, thus contributing to reducing parasitic capacitance and other detrimental electrical coupling effects between the electrode 13 and its environment. It is to be noted that the cap insulating layer is not depicted in Fig 2A in order to show the electrodes 13 and the guard rings 54 formed in the top cap wafer 12. It should be noted that a guard ring can be used not only to guard a capacitive electrode, as in Figs 2A to 2C, but also a conductive electrode or lead. For example, referring to Fig 7, in some embodiments, in addition to the guard rings 54 provided around the capacitive electrodes 13 and 15, an additional guard ring 54 may be provided that surrounds a MEMS electrical lead 47 (e.g., a feedthrough or a conductive wafer plug) connecting a MEMS electrical connection 44 to the MEMS structure 17 via a spring 27. The arrow 58 indicates the conducting path followed by the MEMS lead 47 between the MEMS electrical connection 44 and the MEMS structure d. When the guard ring electrical connection 42 is held at the same potential as the MEMS connection 44, then no or almost no leakage current can flow from the MEMS lead to the cap.

Method of manufacturing a MEMS device

In accordance with another aspect, there is provided a method of manufacturing a MEMS device, the MEMS device including an electrode guard ring. The method for manufacturing the MEMS device will be described with reference to the diagrams of Figs 3A to 3G and Figs 4A to 4H, which schematically illustrate steps of a first and a second exemplary embodiment, respectively. It will be understood, however, that there is no intent to limit the invention to these two embodiments, for the method may admit to other equally effective embodiments. It will also be understood that the manufacturing method can, by way of example, be performed to fabricate a MEMS device like that described above with reference to Figs 2A to 2C, or any other suitable MEMS device provided with one or more electrode guard rings. First embodiment of the manufacturing method

Referring to Figs 3A to 3G, there are schematically illustrated fabrication steps of a first exemplary embodiment of the method for manufacturing a MEMS device.

Referring to Fig 3A, the method first includes a step of providing a top cap wafer 12 and a MEMS wafer 16. The top cap wafer 12 has opposed first and second sides 121 , 122 and a thickness 123 extending therebetween, and the MEMS wafer 16 has opposed top and bottom sides 161 , 162. The top cap wafer 12 and the MEMS wafer 16 are each made of an electrically conductive semiconductor material 30 such as, for example a silicon-based semiconductor.

The MEMS wafer 16 can be provided as a standard wafer, an SOI wafer, or as multiple wafers. In the present embodiment, the MEMS wafer 16 is an SOI wafer including a device layer 20, a handle layer 22 and an insulating layer 24 sandwiched between the device layer 20 and the handle layer 22, as described above. The MEMS wafer 16 includes a MEMS structure 17 which, as also described above, can include or be embodied by any sensing element or combination of sensing elements such as, but not limited to, membranes, diaphragms, proof masses, comb sensors, actuators, transducers, micro-valves, micro-pumps, and the like. In the present embodiment, the MEMS device is a MEMS motion sensor so that the step of providing the MEMS wafer 16 can include forming the MEMS structure 17 as a pendulous proof mass 171 coupled to a peripheral region of the MEMS wafer 16 via flexible springs 27. The flexible springs 27 provide a restoring force to the proof mass 171 which enables the proof mass 171 to move relative to the peripheral region of the MEMS wafer 16 in response to external forces.

In some embodiments, the step of providing the top cap wafer 12 can include a preliminary step of forming a recess 31 1 by removing top cap wafer material from a central region of the first side 121 of the top cap wafer 12. The recess 31 1 may eventually form part of a cavity whose role is to house the MEMS structure once the top cap wafer 12 is bonded to the MEMS wafer 16, as described below.

The method next includes a step of forming one or more electrode structures from the first side into the top cap wafer, wherein each electrode structure include an electrode and a guard ring laterally surrounding and electrically insulated from the electrode. As will be described below, in some embodiments, the step of forming one such electrode structure may be conceptually described as including two substeps. The first substep can involve forming an inner insulation channel extending through the entire thickness of the top cap wafer, wherein the inner insulation channel laterally borders a first region of the top cap wafer corresponding to the electrode to be formed. The second substep can involve forming an outer insulation channel also extending through the entire thickness of the top cap wafer and spaced from the inner insulation channel, such that the inner and outer vertical insulation channels together lateral borders a second region of the top cap wafer corresponding to the guard ring to be formed.

Depending on the particular implementation of the method, forming the inner and outer insulation channels to define the electrode and the guard ring can involve different fabrication techniques and structural features, as will be appreciated by comparing the present embodiment of Figs 3A to 3G and the second exemplary embodiment described below with reference to Figs 4A to 4H.

Referring now to Figs 3B and 3C, the step of forming an electrode structure can include a first substep of forming spaced-apart inner and outer closed-loop trenches 28a, 28b each extending from the first side 121 partially into the top cap wafer 12. In particular, Fig 3B is a schematic plan view of the top cap wafer 12 illustrating the two-dimensional closed-loop paths defined by the inner and outer trenches 28a, 28b. It can be seen that the inner trench 28a laterally borders a first volume 29a of the top cap wafer 12, which corresponds to the electrode to be formed, and that the inner and outer trenches 28a, 28b together laterally border a second volume 29b of the top cap wafer 12, which corresponds to the guard ring to be formed. The trenches 28a, 28b may be produced using a selective etching process. Selective etching processes for producing trenches in a wafer body are well known in the art and need not be described in further detail herein. Typically, a deep silicon reactive ion etch (DRIE) is used to etch high aspect ratio features with vertical sidewalls into silicon, but other etching processes may be used.

Referring to Fig 3D, the step of forming an electrode structure can include a second substep of depositing an electrically insulating material 30 into the inner and outer trenches 28a, 28b. The electrically insulating material 30 may include silicon dioxide (Si0 ₂ ) or any other suitable material. Those skilled in the art will appreciate that numerous deposition processes including thermal oxidation, low pressure chemical vapor deposition (LPCVD), or plasma enhanced chemical vapor deposition (PECVD) may be used to fill the inner and outer trenches 28a, 28b with the electrically insulating material 30. It will also be understood that in other embodiments, the substep of depositing an electrically insulating material into the inner and outer trenches may include forming an insulation layer one the sidewalls of each trench, followed by filling each trench with an electrically conductive material. Turning now to Fig 3E, the step of forming an electrode structure 19 can include a third substep of removing top cap wafer material from the second side 122 of the top cap wafer 12 to expose the electrically insulating material 30 of the inner and outer trenches 28a, 28b. As known in the art, the step of removing top cap wafer material from the second side 122 of the top cap wafer 12 can include at least one of grinding, polishing and etching. As a result, the electrode 13 and the guard ring 54 are formed and are electrically insulated from each other by the electrically insulating material 30 deposited into the inner trench 28a. In particular, the electrode 13 and the guard ring 54 each extends through the entire thickness 123 of the top cap wafer 12, and both the electrode 13 and the guard ring 54 are made of the same electrically conductive semiconductor material as the top cap wafer 12. Also, it will be understood that the exposed inner and outer trenches 28a, 28b filled with electrically insulating material 30 correspond respectively to the inner and outer insulating channels introduced above.

Referring to Fig 3F, once the electrode 13 and guard ring 54 have been formed, the method can include successive substeps of (i) forming a cap insulating layer 40, typically thermally or chemical vapor deposited silicon dioxide, on the second side 122 of the top cap wafer 12; (ii) partially removing the cap insulating layer 40, for example by selective etching, to expose at least a portion of the guard ring 54 and at least a portion of the electrode 13; and (iii) forming first and second electrical connections 42, 43 on the exposed portions of the guard ring 54 and the electrode 13, respectively. The first and second electrical connections 42, 43 may be formed by depositing a metallic layer 41 (in some cases including an underlying sticking or barrier layer) on the cap insulating layer 40 and by patterning the metallic layer 41 to form the first and second electrical connections 42, 43. The first and second electrical connections 42, 43 are formed so that they establish an electrical contact with the guard ring 54 and the electrode 13, respectively. A passivating layer 45 may also be applied over the first and second electrical connections 42, 43. As mentioned above, the passivating layer 45 can extend over substantially the entire surface of the second side 122 of the top cap wafer 10. Finally, openings may be formed in the passivating layer 45 to expose at least partially the first and second electrical connections 42, 43.

As mentioned above, it will be understood that by connecting the first connection 42 to an electrical source (which can be the same or a different electrical source than the electrical source to which the second electrical connection 43), the guard ring 54 can be held at the same or at a similar electrical potential as its associated electrode 13, thus contributing to reducing parasitic capacitance and other detrimental electrical coupling effects between the electrode 13 and its environment.

Finally, in Fig 3G, the method includes a step of bonding the first side 121 of the top cap wafer 12 to the top side 161 of the MEMS wafer 16 to form the MEMS device 10 in a manner such that an electrical connection is established between the electrode 13 and the MEMS structure 17. In the illustrated embodiment, the electrical connection between the electrode 13 and the MEMS structure 17 is a capacitive electrical connection, but a resistive or inductive electrical connection may be envisioned without departing from the scope of the present invention.

Bonding the first side 121 of the top cap wafer 12 to the top side 161 of the MEMS wafer 16 can made with a conductive bond. For example, fusion bonding can be used but other alternatives can be considered, such as using a conducting material. Bonding can be made for example using gold thermocompression bonding, or gold- silicon eutectic bonding.

Optionally, the method may also include a step of bonding a bottom cap wafer 14 to the bottom side of the MEMS wafer 16. As mentioned above, the bottom cap wafer 14 may, but need not, be provided with electrodes 15 surrounded by guard rings 54. In such embodiments, the number, size, shape and configuration of the electrodes 15 and guard rings 54 formed in the bottom cap wafer 14 may, but need not, be identical to the number, size, shape and configuration of the electrodes 13 and guard rings 54 formed in the top cap wafer 12.

In some embodiments, the method steps illustrated in Fig 3G may be performed before the steps performed in Figs 3E and 3F, without departing from the scope of the present invention. Those skilled in the art will recognize that, in some embodiments, it may be desirable or necessary that electrodes, guard rings and other structures formed into a cap wafer of a MEMS device satisfy certain criteria or requirements in terms of size. In particular, the minimum width of the inner and outer insulating channels bordering a guard ring and its associated electrode is generally controlled by the resolution of the etching techniques used to defined trenches formed in the cap wafers. As the size of MEMS devices is reduced, so is the available surface area to that can be used to form electrodes. In turn, a smaller electrode is associated with lower capacitance and reduced sensitivity. Furthermore, in embodiments of the invention, the maximum area that can be occupied by electrodes is further reduced by the presence of guard rings.

Referring to Fig 5, there is provided an enlarged view of a portion of Fig 3F, illustrating a guard ring 54 bordered by inner and outer trenches 28a, 28b filled with an electrically insulating material 30. In some embodiments, due to limitations of existing microfabrication processes, each of the inner and outer trenches 28a, 28b is typically between 5 and 50 micrometers ( m) wide. The guard ring 54 generally has to be at least as wide as the trenches 28a, 28b. In the embodiment of Figs 3A to 3G, it is seen that the electrode 13 of an electrode structure 19 is surrounded by twice the width of each of the guard ring 54, the inner trench 28a and the outer trench 28b, which can amount to a total width of 2x150=300 m per electrode. Accordingly, when the size of MEMS devices is reduced, the area around electrodes occupied by guard rings and insulation channels may eventually become an appreciable fraction of the area occupied by the electrodes themselves. It may therefore be desirable, in some implementations, to decrease the area of the cap wafers devoted to guard rings and, correspondingly, to increase the area occupied by active electrodes.

Second embodiment of the manufacturing method

Referring to Figs 4A to 4H, there are schematically illustrated fabrication steps of a second exemplary embodiment of the method for manufacturing a MEMS device. As will be described below, this second exemplary embodiment can provides a reduction of the area occupied by guard rings, with a resulting increase in electrode area. It is to be noted that the second exemplary embodiment illustrated in Figs 4A to 4H share common steps with the first exemplary embodiment described above and illustrated in Figs 3A to 3G. Accordingly, the description of these common steps and of any features or variants thereof that were provided above will not be repeated in detail hereinbelow.

Referring to Fig 4A, the method first includes a step of providing a top cap wafer 12 and a MEMS wafer 16. The top cap wafer 12 has opposed first and second sides 121 , 122 and a thickness 123 extending therebetween, and the MEMS wafer 16 has opposed top and bottom sides 161 , 162. The top cap wafer 12 and the MEMS wafer 16 are each made of an electrically conductive semiconductor material 30 such as, for example a silicon-based semiconductor. As in Fig 3A, the MEMS wafer 16 may be an SOI wafer including a device layer 20, a handle layer 22 and an insulating layer 24 sandwiched between the device layer 20 and the handle layer 22. The MEMS wafer 16 includes a MEMS structure 17 which can include or be embodied by any sensing element or combination of sensing elements such as, but not limited to, membranes, diaphragms, proof masses, comb sensors, actuators, transducers, micro-valves, micro-pumps, and the like. In the present embodiment, the MEMS device is a MEMS motion sensor including a pendulous proof mass 171

The method next includes a step of forming one or more electrode structures from the first side into the top cap wafer, wherein each electrode structure include an electrode and a guard ring laterally surrounding and electrically insulated from the electrode. Referring of Figs 4B and 4C, the step of forming an electrode structure can include a first substep of forming a single closed-loop trench 28 having inner and outer sidewalls 31 a, 31 b, rather than forming a pair of spaced-apart inner and outer electrodes as in the embodiment of Figs 3A to 3G. The trench 28 extends from the first side 121 partially into the top cap wafer 12, and may be defined using a suitable etching process. In particular, Fig 4B is a schematic plan view of the top cap wafer 12 illustrating the two-dimensional closed-loop path defined by the single trench 28. It can be seen that the trench 28 laterally borders a volume 29 of the top cap wafer 12 corresponding to the electrode 13 to be formed. Referring to Fig 4D, the step of forming an electrode structure can include a second substep of lining the inner and outer sidewalls 31 a, 31 b of the trench 28 with an electrically insulating material 30 to form an insulator-lined trench. The electrically insulating material 30 may include silicon dioxide (S1O2) or any other suitable material. Various deposition processes may be used to line the inner and outer sidewalls 31 a, 31 b of the trench 28 with the electrically insulating material 30.

Referring to Fig 4E, the step of forming an electrode structure can include a third substep of depositing an electrically conductive material 32 into the insulator-lined trench, for example polysilicon or a metal such as copper. The deposited electrically conductive material 32 corresponds to the guard ring to be formed.

Referring to Fig 4F, the step of forming an electrode structure can include a fourth substep of removing top cap wafer material from the second side 122 of the top cap wafer 12 to expose the electrically conducting material 32, which can include at least one of grinding, polishing and etching. As a result, the electrode 13 and the guard ring 54 are formed and are electrically insulated from each other by the electrically insulating material 30 lining the inner sidewall 31 a. As in Fig 3E, it is seen that the electrode 13 and the guard ring 54 each extends through the entire thickness 123 of the top cap wafer 12. Also, it will be understood that in Fig 4F, the inner and outer sidewalls 31 a, 31 b lined with electrically insulating material 30 correspond respectively to the inner and outer insulating channels introduced above. Referring to Fig 4G, once the electrode 13 and guard ring 54 have been formed, the method can include successive substeps of (i) forming a cap insulating layer 40, typically thermally or chemical vapor deposited silicon dioxide, on the second side 122 of the top cap wafer 12; (ii) partially removing the cap insulating layer 40, for example by selective etching, to expose at least a portion of the guard ring 54 and at least a portion of the electrode 13; and (iii) forming first and second electrical connections 42, 43 on the exposed portions of the guard ring 54 and the electrode 13, respectively. As mentioned above, the first and second electrical connections 42, 43 may be formed by depositing a metallic layer 41 (in some cases including an underlying sticking or barrier layer) on the cap insulating layer 40 and by patterning the metallic layer 41 to form the first and second electrical connections 42, 43. The first and second electrical connections 42, 43 are formed so that they establish an electrical contact with the guard ring 54 and the electrode 13, respectively. A passivating layer 45 may also be applied over the first and second electrical connections 42, 43, and be etched to form openings exposing at least partially the first and second electrical connections 42, 43.

Finally, in Fig 4H, the method includes a step of bonding the first side 121 of the top cap wafer 12 to the top side 161 of the MEMS wafer 16 to form the MEMS device 10 in a manner such that an electrical connection is established between the electrode 13 and the MEMS structure 17. Optionally, a bottom cap wafer 14 may also be bonded to the bottom side of the MEMS wafer 16, which may or may not electrode guard rings 54. In some embodiments, the method steps illustrated in Fig 4H may be performed before the steps performed in Figs 4F and 4G, without departing from the scope of the present invention.

Referring to Fig 6, there is provided an enlarged view of a portion of Fig 4G, illustrating a guard ring 54 defined by electrically conducting material 32 filing a trench 28 whose inner and outer sidewalls 31 a, 31 b were previously lined with an electrically insulating material 30. As mentioned above, in some embodiments, due to limitations of existing microfabrication processes, the minimum achievable width of the trench 28 typically ranges from 5 to 50 m, with the guard ring 54 being necessarily at least slightly narrower than the trench 28. Accordingly, in the embodiment of Figs 4A to 4H, an electrode 13 in an electrode structure 19 is surrounded only by twice the width of a trench 28, which can amount to a total width of 2x50=100 m per electrode. It will thus be appreciated that the electrodes 13 can advantageously occupy a larger fraction of the top cap wafer 12 in the embodiment of Figs 4A to 4H than in the embodiment of Figs 3A to 3G described above.

Of course, numerous modifications could be made to the embodiments described above without departing from the scope of the present invention.