Patent Application
20250178887
The current application claims priority to European Patent Application No. 23214335.4, filed on Dec. 5, 2023, the entire contents of which are hereby incorporated by reference.
This disclosure relates to electronic devices and more particularly to Microelectromechanical components. The present disclosure further concerns eutectic bonding of microelectromechanical components with a metal standoff.
Microelectromechanical systems (MEMS) are the technology of microscopic devices which combine mechanical and electrical features. MEMS devices, also called MEMS elements, can have either simple or complex structures with various moving parts. They include devices such as gyroscopes, acceleration sensors, magnetometers, and pressure sensors. MEMS devices may be fabricated from a silicon wafer by microfabrication techniques.
Electronic chips, also called dies, are prepared by manufacturing various electronic structures on a substrate and cutting the substrate into small, chip-size pieces. These chips may be MEMS devices. Several MEMS elements may be built on the same die. This allows significant cost and area reduction. Once the MEMS elements are built, they may need to be sealed in a particular environment depending on their function.
Eutectic bonding, also known as eutectic soldering, is a well-established semiconductor bonding technology that involves bonding wafers together by high pressure using a seal formed of two or more metal films which transform into a eutectic alloy at a specific temperature (eutectic temperature). Due to their easy preparation techniques and good wettability, eutectic alloys form excellent seals in electronic devices. Eutectic bonding is widely used in MEMS technology for the hermetic sealing of MEMS components. During the bonding process, at the eutectic temperature, the eutectic alloy is in liquid phase. Because bonding process involves pressing wafers together at high pressure, in some circumstances, it may be challenging to accurately control the gap height between the bonded wafers, and thus the MEMS element cavity height, due to the softness of the liquid eutectic alloy. The use of standoffs formed of a semiconducting material to control the gap height between eutectically bonded wafers is a known solution. For example, document U.S. Pat. No. 10,308,507B2 discloses a gap control structure in MEMS devices wherein the gap control structure comprises a standoff formed of a semiconducting material. However, it may be challenging to reduce the MEMS element size when such gap control structures are included in MEMS devices.
In view for the foregoing, a component and a method for manufacturing the same are provided to allow MEMS vertical gap height control in eutectically bonded microelectromechanical components. Examples provided in this disclosure describe ways to implement the solution. The improvement is achieved by features of the microelectromechanical component and a manufacturing method.
In an exemplary aspect, a microelectromechanical component is provided that includes a cap wafer having a top surface and a bottom surface, the top surface defining a horizontal xy-plane and a vertical z-direction that is perpendicular to the xy-plane; a structure wafer that has a top surface and a bottom surface, the structure wafer being bonded to the cap wafer so that the cap wafer and the structure wafer are aligned with each other in the z-direction; an electrical connection in a gap-control region, the electrical connection comprising a metal layer that extends in the z-direction from the bottom surface of the cap wafer to the top surface of the structure wafer; and one or more metal standoffs at the bottom surface of the cap wafer in the gap-control region. In this aspect, the one or more metal standoffs are adjacent to the electrical connection and extend in the z-direction from the bottom surface of the cap wafer to the top surface of the structure wafer. Moreover, the one or more metal standoffs comprise a metal having a compressive strength that is greater than a compressive strength of the metal layer in the electrical connection.
In another exemplary aspect, a method is provided for manufacturing a microelectromechanical component with a metal standoff, the microelectromechanical component including a cap wafer having a bottom surface and a top surface that defines a horizontal xy-plane and a vertical z-direction that is perpendicular to the xy-plane, a cap wafer gap-control region in the cap wafer, and a structure wafer having a top surface and a bottom surface, the structure wafer comprising a structure wafer gap-control region. In this aspect, the method includes forming one or more metal standoffs at the bottom surface of the cap wafer in the cap wafer gap-control region; forming a metal layer on the bottom surface of the cap wafer in the cap wafer gap-control region so that the metal layer is adjacent to the one or more metal standoffs; placing the cap wafer on top of the structure wafer so that the cap wafer gap-control region is aligned with the structure wafer gap-control region in the z-direction; and bonding the cap wafer to the structure wafer so that the metal layer extends in the z-direction from the bottom surface of the cap wafer to the top surface of the structure wafer and the one or more metal standoffs extend in the z-direction from the bottom surface of the cap wafer to the top surface of the structure wafer. Moreover, a compressive strength of the metal forming the one or more metal standoffs is greater than a compressive strength of the metal layer in the electrical connection.
The exemplary aspects of the disclosure are based on the idea of including at least one metal standoff inside the MEMS element. The metal standoff provides an accurate control of the MEMS gap height during the eutectic bonding of the component and enables element size reduction. This provides new improvements of gap control and robustness of the eutectic bond in the microelectromechanical component.
In the following, the disclosure will be described in greater detail by exemplary embodiments with reference to the accompanying drawings, in which:
Figure la illustrates an example of the sectional view of a microelectromechanical component comprising a metal standoff;
The disclosure describes a microelectromechanical component comprising a cap wafer and a structure wafer. The cap wafer has a top surface and a bottom surface, and the top surface of the cap wafer defines a horizontal xy-plane and a vertical z-direction which is perpendicular to the xy-plane. The structure wafer has a top surface and a bottom surface. The cap wafer and the structure wafer are bonded to each other so that the cap wafer and the structure wafer are aligned with each other in the z-direction. The microelectromechanical component further comprises an electrical connection and a gap-control region, and the electrical connection is in the gap-control region. The electrical connection comprises a metal layer. The metal layer extends in the z-direction from the bottom surface of the cap wafer to the top surface of the structure wafer. The microelectromechanical component further comprises one or more metal standoffs, and the one or more metal standoffs are located at the bottom surface of the cap wafer in the gap-control region. The one or more metal standoffs are adjacent to the electrical connection and extend in the z-direction from the bottom surface of the cap wafer to the top surface of the structure wafer. The compressive strength of the metal forming the one or more metal standoffs is greater than the compressive strength of the metal layer in the electrical connection.
Any direction or plane which is parallel to the xy-plane defined by the cap wafer can be called horizontal. The direction which is perpendicular to the xy-plane can be called vertical direction. Expressions such as “top”, “bottom”, “above”, “below”, “up” and “down” refer in this disclosure to differences in the vertical z-coordinate. These expressions do not imply anything about how the device should be oriented with respect to the earth's gravitational field when the component is in use or when it is being manufactured.
Figure la illustrates an example of the sectional view of a microelectromechanical component comprising a metal standoff 104. In this example, the cap wafer 100 and the structure wafer 101 are bonded to each other via a seal 105 so that the cap wafer and the structure wafer are aligned with each other in the z-direction. The seal 105 may be a eutectic seal which is formed by two or more metals or by a combination of one or more metals and a metalloid which transform into a eutectic alloy at a specific temperature. The microelectromechanical component further comprises a gap-control region 102 and an electrical connection located in the gap-control region 102. The electrical connection may connect the cap wafer to the structure wafer. This connection may for example connect electrical signals and/or electrical potentials between the micromechanical structure and system electronics. In this example, the electrical connection comprises a metal layer 103. The metal layer 103 is attached to the bottom surface of the cap wafer 100 and extends in the z-direction from the bottom surface of the cap wafer 100 to the top surface of the structure wafer 101. The microelectromechanical component further comprises a metal standoff 104. The metal standoff 104 is attached to the bottom surface of the cap wafer 100 in the gap-control region 102. The metal standoff 104 is adjacent to the metal layer 103 and extends in the z-direction from the bottom surface of the cap wafer 100 to the top surface the structure wafer 101. In this example, the metal standoff 104 is in contact with the metal layer 103. Alternatively, the metal standoff may be beside the metal layer. This option may apply to any embodiment in this disclosure. The surface area of the metal standoff in the xy-plane may be of the same order of magnitude or larger than the surface of metal layer 103 in the xy-plane.
In any embodiment presented in this disclosure, the one or more metal standoffs may comprise Ti, or the one or more metal standoffs may comprise W. For example, the metal standoff 104 may be made of one or more metals that include but are not limited to Ti, W, Cr, V, or hard metal alloys, or hard metalloid. The metal layer 103 in the electrical connection may be made of one or more metals that include but are not limited to Al, Au, Ag, Cu, Sn, or soft metal alloys. These options may apply to any embodiment in this disclosure. The compressive strength of the one or more metals forming the metal standoff 104 is greater than the compressive strength of the one or more metals forming the metal layer 103. In other words, the one or more metals composing the metal layer 103 are softer than the one or more metals composing the metal standoff 104. These options may apply to any embodiment in this disclosure. The metal standoff may be formed by a variety of deposition methods such as sputtering, chemical vapor deposition, molecular beam epitaxy, electron beam physical vapor evaporation, atomic layer deposition, or laser metal deposition. The metal layer may be formed by a variety of deposition methods such as sputtering, chemical vapor deposition, molecular beam epitaxy, electron beam physical vapor evaporation, atomic layer deposition, or laser metal deposition. These options may apply to any embodiment in this disclosure. While the metal layer may serve as an ohmic contact in the MEMS, due to its hardness, the metal standoff provides an accurate control of the MEMS gap height during the eutectic bonding of the component, mechanical stress reduction of the electrical contact, as well as the possibility for element size reduction.
The microelectromechanical component may comprise a plurality of metal standoffs. The metal standoffs may be substantially evenly distributed around the metal layer.
The cap wafer may be a semiconducting wafer, or an insulating wafer such as a glass layer, or a wafer comprising semiconducting parts and insulating parts, or a wafer comprising metal parts and insulating parts, or integrated circuitry (IC). The insulating parts may be made of glass or other insulating materials such as silicon dioxide, silicon nitride or aluminium oxide. These options may apply to any embodiment in this disclosure.
The structure wafer may be a semiconductor device layer which has been attached to a support layer (not illustrated). The device layer may be a layer of silicon. The device layer and the support layer may for example be parts of a silicon-on-insulator (SOI) substrate where MEMS elements can be formed by patterning the top silicon layer (the device layer). These options may apply to any embodiment in this disclosure.
The electrical connection may further comprise an electrically conductive via. The electrically conductive via may extend along the z-direction through the cap wafer, and the metal layer may extend along the bottom surface of the cap wafer so that it meets the electrically conductive via. The electrically conductive via may extend through a portion of the cap wafer thickness. Alternatively, the electrically conductive via may extend through the whole cap wafer thickness.
The cap wafer may comprise a semiconducting part and an insulating part, wherein the semiconducting part is located on top of the insulating part so that the bottom surface of the insulating part forms the bottom surface of the cap wafer. Alternatively, the cap wafer may comprise a conductive part, such as a metal part, and an insulating part, wherein the metal part is located on top of the insulating part so that the bottom surface of the insulating part forms the bottom surface of the cap wafer.
The top surface of the structure wafer may comprise a protrusion wherein the protrusion is located in the gap-control region. The metal layer may extend from the bottom surface of the cap wafer to the protrusion, and the one or more metal standoffs may extend in the z-direction from the bottom surface of the cap wafer to the protrusion. The metal layer and the protrusion, which acts as an anchor, may serve as a good ohmic press-on contact inside the element. The protrusion may have different shapes such as a truncated pyramid shape, or a pillar-like shape. The pillar like-shape may be, but is not limited to, a cylinder, a hexagonal prism, or a cuboid. These options may apply to any embodiment in this disclosure.
The method comprises: (1) forming one or more metal standoffs at the bottom surface of the cap wafer in the cap wafer gap-control region, (2) forming a metal layer on the bottom surface of the cap wafer in the cap wafer gap-control region so that the metal layer is adjacent to the one or more metal standoffs, (3) placing the cap wafer on top of the structure wafer so that the cap wafer gap-control region is aligned with the structure wafer gap-control region in the z-direction, (4) bonding the cap wafer and the structure wafer together so that the metal layer extends in the z-direction from the bottom surface of the cap wafer to the top surface of the structure wafer and the one or more metal standoffs extend in the z-direction from the bottom surface of the cap wafer to the top surface of the structure wafer. The compressive strength of the metal forming the one or more metal standoffs is greater than the compressive strength of the metal layer in the electrical connection. The components described above may include any structural feature which is included in the following presentation of manufacturing method.
The structure wafer may be a semiconductor device layer which has been attached to a support layer. The device layer may be a layer of silicon. The device layer and the support layer may for example be parts of a silicon-on-insulator (SOI) substrate where MEMS elements can be formed by patterning the top silicon layer (the device layer).
The cap wafer may be a semiconducting wafer, or an insulating wafer such as a glass layer, or a wafer comprising semiconducting parts and insulating parts, or a wafer comprising metal parts and insulating parts or integrated circuitry (IC). The insulating parts may be made of glass or other insulating materials such as silicon dioxide, silicon nitride or aluminium oxide.
The compressive strength of the one or more metals forming the metal standoff 404 is greater than the compressive strength of the one or more metals forming the metal layer 413. In other words, the metal layer is more malleable than the metal standoff. The metal standoff provides an accurate control of the MEMS gap height during the eutectic bonding of the component.
The method may comprise forming a protrusion in the structure wafer gap-control region before placing the cap wafer on top of the structure wafer so that the top surface of the structure wafer comprises the protrusion.
The method for manufacturing a microelectromechanical component may comprise the step of forming an electrically conductive via in the cap wafer gap-control region before forming the one or more metal standoffs at the bottom surface of the cap wafer. The electrically conductive via may extend along the z-direction through the cap wafer and the metal layer may meet the bottom of the electrically conductive via. The electrically conductive via may extend through a portion of the cap wafer thickness. Alternatively, the electrically conductive via may extend through the whole cap wafer thickness.
In general, it is noted that the exemplary embodiments described above are intended to facilitate the understanding of the present invention and are not intended to limit the interpretation of the present invention. The present invention may be modified and/or improved without departing from the spirit and scope thereof, and equivalents thereof are also included in the present invention. That is, exemplary embodiments obtained by those skilled in the art applying design change as appropriate on the embodiments are also included in the scope of the present invention as long as the obtained embodiments have the features of the present invention. For example, each of the elements included in each of the embodiments, and arrangement, materials, conditions, shapes, sizes, and the like thereof are not limited to those exemplified above and may be modified as appropriate. It is to be understood that the exemplary embodiments are merely illustrative, partial substitutions or combinations of the configurations described in the different embodiments are possible to be made, and configurations obtained by such substitutions or combinations are also included in the scope of the present invention as long as they have the features of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 23214335.4 | Dec 2023 | EP | regional |
