Patent Application
20050269688
Proper packaging of optoelectronic and other micro components is important to ensure the integrity of signals to and from the micro components and often determines the overall cost of the assembly. Microelectromechanical systems (MEMS) technology, which is capable of micromachining silicon wafers or other materials with high precision, has become a promising candidate for packaging various types of micro components.
MEMS technology is able to machine hundreds of packaging components on a single silicon wafer to yield high throughput during manufacture and low-cost components. MEMS technology also offers a broad variety of etching processes, both for glass and silicon.
Examples of MEMS devices include electrostatic switches, pressure sensors, acceleration sensors and microfluids.
The present disclosure relates to packages for MEMS devices. The package includes a semiconductor cap structure and a lid substrate, which define an encapsulated area within which the MEMS device is located. Feed-through metallization hermetically seals micro-vias in the semiconductor cap structure and extends through the semiconductor cap structure to provide interconnections coupled electrically to the MEMS device and to an exterior of the semiconductor cap structure.
The invention may be used with various types of MEMS devices.
In various implementations, one or more of the following features may be present. For example, the MEMS device may include a switch having end contacts. The feed-through metallization may provide electrical interconnections coupled, respectively, to the end contacts. Signal line and ground line interconnections may be coupled electrically to a signal strip of a transmission line on an exterior of the semiconductor cap structure. Additional interconnections may be coupled to the MEMS device to enable the switch to be activated between open and closed states by applying electrical signals to conductive pads on an exterior of the package. Thus, the MEMS device may include a first state in which the end contacts for the MEMS device are not in electrical contact with one another, and a second state in which the end contacts are in electrical contact with one another to cause a short circuit so as to block a signal on the transmission line. The transmission line to which a switching function of the MEMS device is to be applied can be routed along the exterior of the package or along the board on which the package is mounted.
The MEMS device may be located, within the encapsulated area, either on the semiconductor cap structure or the lid substrate. In some cases, some parts of the MEMS device may be located on the lid substrate and other parts may be located on the semiconductor cap structure. For example, the MEMS device may include a switch having contact pads, at least one of which is located on the lid substrate. In some implementations, one or more contact pads may be located on the semiconductor cap structure.
The MEMS device may include an actuation pad wherein, during operation, electrical signals are provided from an exterior of the package to the actuation pad via feed-through metallization that hermetically seals a micro-via extending through the semiconductor cap structure. The actuation pad may be located on the semiconductor cap structure or on the lid substrate. Some implementations may include actuation pads on both the semiconductor cap structure and the lid substrate.
The package may include one or more conductive bumps between the semiconductor cap structure and the lid substrate to provide electrical interconnection for at least one of an input signal to the MEMS device, an output signal from the MEMS device or an actuation signal for the MEMS device.
In some implementations, the semiconductor cap structure may include an etch resistant layer between semiconductor layers, and the lid substrate may include a glass wafer.
Various implementations may include one or more of the following advantages. Electrical micro-vias can enable the use of surface mounted technologies (SMT), balanced RF impedance matching between a printed circuit board and the packaged MEMS device can be achieved as well. Additional cavity space may be provided within the package for device headroom. The electrical micro-vias may allow the device to be used with high power and at high frequencies. The hermetic sealing can provide particle-free encapsulation before dicing.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention may be apparent from the description, the drawings, and the claims.
The present disclosure relates to MEMS devices hermetically sealed in a package. The discussion below describes a package for an electrostatic switch which may be implemented, for example, by a metal cantilever, a contact pad under the non-anchored end of the cantilever, and an actuation pad under the main part of the cantilever. However, other electrostatic switches, as well as other MEMS devices, may be packaged using the techniques described here.
As shown in
The lid substrate 24, which may be, for example, a glass wafer, is processed to form a MEMS switch 40. Examples of the MEMS switch include electromechanical actuators comprising cantilevers, bridges or membranes.
A transmission line 30 (e.g., a co-planar or micro-strip line) is provided on the backside of the device with pads 32 (e.g., solder bumps) for attachment to the main platform (e.g., printed circuit board or flex cables). The package may be surface-mounted, for example, to a printed circuit board 33, with the lid substrate 24 serving as a lid for the package.
Formation of the semiconductor cap structure 22 may include micro-machining to provide micro-vias on one side opposite a larger recess 28 on the front or the back of the cap structure. Feed-through metallization through some of the micro-vias in the cap 22 and along the sides of the recess 28 provides electrical interconnections 34, 36 for the signal and ground lines of the transmission line 30. The interconnections 34, 36 are coupled electrically, respectively, to end-contacts 42, 44 of the MEMS switch. The signal line interconnection(s) 34 and the ground line interconnection(s) 36 may be positioned at different locations along the z-axis, with the signal line interconnection(s) 34 coupled electrically to the signal strip of the transmission line 30. The ground line interconnection(s) 36 may be coupled electrically to the ground strip(s) of the transmission line. The end-contacts 42, 44 of the switch 40 may be routed to the electrical interconnections 34, 36 by a rigid ohmic contact (i.e., a solder bump).
Electrical signals for switching the actuator 40 may be routed from the backside of the package to an actuation pad 41 by feed-through metallization through additional micro-vias formed in the cap and metallization lines routed from the cap 22 to the actuator pad on the substrate 24. The interconnection for the actuator signal line is indicated by the dashed lines in the drawings.
In
The actuator 40 may be switched between two states. One state (i.e., the “open” state) of the switch occurs when the front edge of the first end-contact 42 is not in contact with the second end-contact 44 of the switch. The second state (i.e., the “closed” state) occurs when the first end-contact 42 is in contact with the second end-contact 44. In the closed state, the electrical contact between the two end-contacts 42, 44 of the actuator 40 causes a short-circuit between the signal and ground strips on the transmission line 30, thus blocking the signal on the transmission line 30.
Hermetic encapsulation of the MEMS device may be realized, for example, by (i) the feed-through metallization sealing the micro-vias and (ii) the solder or other sealing ring between the cap 22 and the lid substrate 24.
Although only one MEMS device is shown in the drawings, other embodiments may include two or more MEMS devices in a single package. The package may be used with a wide range of frequencies.
Formation of the Semiconductor Cap Structure and Feed-Through Metallization:
The cap 22 may comprise, for example, a semiconductor material such as silicon, so that the recess 28 can be formed by known etching processes. Various techniques may be used to form the recess 28 and the micro-vias for the feed-through metallization. One such technique uses a multilayer structure that includes a substantially etch-resistant layer sandwiched between first and second semiconductor layers. The first and second semiconductor layers may include, for example, silicon, and the etch-resistant layer may include, for example, silicon nitride, silicon oxy-nitride or silicon dioxide. The through-holes (i.e., micro-vias) may be formed using a double-sided etching process in which the first and second layers are etched until the etch-resistant layer is exposed to define the locations of the through-holes. The semiconductor layer in which the larger recess 28 is to be formed may be etched over an area that corresponds to the positions of all or a large number of the through-holes. The through-holes then may be completed by removing part of the etch-resistant layer.
The through-holes may be hermetically sealed, for example, using an electro-plated feed-through metallization process as the base for the through-hole connections.
Further details of a suitable double-sided etching and feed-through metallization process are disclosed in U.S. Pat. No. 6,818,464, which is assigned to the assignee of the present application. The disclosure of that patent is incorporated herein by reference.
Other Implementations
Various modifications may be made to the embodiment of
In some implementations, one or more of the contact pads may be either on the cap structure 22, on the lid substrate 24 or on both the cap structure and the lid substrate. In the latter case, the switch may function as a router with a single input and two outputs.
In the embodiment of
In other embodiments, both end-points of the RF signal may be located on the cap structure 22, inside the encapsulated space. An example of such a design is illustrated in
The foregoing designs can be simplified by thinning down the back-side of the cap structure 22.
Various techniques may be used for the thinning process, including mechanical grinding or polishing techniques. Further details of such techniques are disclosed in U.S. application Ser. No. 11/082,507, filed on Mar. 17, 2005 and assigned to the assignee of the present disclosure.
In some implementations, the sealing technology of the package may be based on the use of a metal sealing ring (e.g., solder). The sealing ring also may be used to route the electrical “ground” interconnection to the cantilever of the switch 40.
The cap structure 22 and the lid substrate 24 may be bonded, for example, by direct wafer-to-wafer bonding techniques (e.g., anodic bonding) in which a metal interface is not required for the bonding and the encapsulation. An example of such a design is illustrated in
In applications such as electrostatic switches in which the cantilever needs to be grounded, a metal contact between the rigid part of the cantilever and the cap can be formed by two metal stand-offs being pressed against each other during the bonding of the two wafers (see, e.g., bump 38A in
In the foregoing implementations, at least part of the switch (e.g., the cantilever) is located on the glass wafer 24 that serves as the lid of the package. However, in other implementations, all parts of the switch may be integrated into the cap structure 22 in which the electrical feed-through connections are provided, as indicated by
Other implementations are within the scope of the claims.
This disclosure claims the benefit of U.S. Provisional Application Nos. 60/576,765, filed on Jun. 3, 2004, and 60/662,644, filed on Mar. 17, 2005.
| Number | Date | Country | |
|---|---|---|---|
| 60576765 | Jun 2004 | US | |
| 60662644 | Mar 2005 | US |
