Patent Application
20120142136
Micro-electro-mechanical systems (MEMS) devices are typically formed with semiconductor fabrication techniques to create small mechanical structures on a surface of a substrate such as a silicon wafer. In the production of MEMS devices such as gyroscopes or accelerometers, such semiconductor fabrication techniques are often used to create a number of moving structures that can be used to sense displacement in response to rotation or acceleration of the device about an input or rate axis.
High performance MEMS gyroscopes must be packaged in a vacuum, while MEMS accelerometers need to be packaged in gas. Both atmospheres of vacuum and gas must be stable over time, which means that all seals must be hermetic. A hermetic seal around the MEMS device also protects it from dust and damage during fabrication, post-fabrication handling, and operation. Since packaging of MEMS devices is often done one at a time, or in relatively small batches, the packaging process tends to be expensive.
Wafer level packaging (WLP) reduces costs by doing the packaging in batch fabrication as well as often allowing a simpler, cheaper final package to be used. For these reasons, a large number of approaches to WLP have been devised. Nevertheless, few approaches have been commercially successful because the requirements are stringent and difficult to achieve.
Most WLP designs provide a hermetically-sealed cover over the MEMS device. In high-performance devices that utilize MEMS gyroscopes and accelerometers, not only is a hermetically-sealed cover required, but also the cover needs to be a precisely-defined distance from the device for use as an “upper sense plate.” This spacing requirement makes WLP even more difficult since it severely limits the choices available for bonding the cover to the device. The easiest bonding technologies (e.g., adhesives, solders, frits, etc.) introduce large, uncontrolled gaps between the cover and the device and are, therefore, not suitable.
A process for packaging micro-electro-mechanical systems (MEMS) devices comprises providing a lower cover wafer having a first surface and an opposing second surface, providing an upper cover wafer having a first surface and an opposing second surface, providing a semiconductor wafer including a plurality of MEMS devices on a substrate layer, bonding the semiconductor wafer to the first surface of the lower cover wafer, and bonding the second surface of the upper cover wafer to the semiconductor wafer. The first surface of the lower cover wafer and the second surface of the upper cover wafer define a plurality of hermetically sealed cavity sections when bonded to the semiconductor wafer such that each of the MEMS devices is located inside one of the sealed cavity sections. A plurality of holes are formed that extend from the first surface of the upper cover wafer to the second surface of the upper cover wafer after the upper cover wafer is bonded to the semiconductor wafer. A metal lead layer is then deposited in each of the holes to provide an electrical connection with the MEMS devices.
Features of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings. Understanding that the drawings depict only typical embodiments and are not therefore to be considered limiting in scope, the invention will be described with additional specificity and detail through the use of the accompanying drawings, in which:
In the following detailed description, embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be utilized without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense.
A wafer level packaging process for micro-electro-mechanical systems (MEMS) devices is provided. The packaging process is compatible with MEMS inertial sensors such as accelerometers and gyroscopes. The packaging process allows for lower cost, higher reliability, and smaller sensors. The present approach to wafer level packaging of MEMS devices is an improvement over conventional methods.
For example, in one conventional approach, a silicon wafer containing MEMS structures is anodically bonded to a lower glass substrate. The MEMS structures are contained in an epitaxial layer of the silicon wafer, and the remainder of the wafer is removed by silicon etching. Prior to etching, holes are formed in the lower glass substrate. The holes open on to conductive silicon pads that maintain a hermetic seal around the hole and also provide a path for electrical connection from the back of the lower glass substrate to the MEMS device, which will eventually be inside a sealed cavity. Following the hole formation, the silicon substrate is removed as described above, and metal is deposited on the back of the lower glass substrate and in the holes. Finally, an upper glass wafer is anodically bonded to the silicon, sealing the device in a cavity containing gas (for an accelerometer) or vacuum (for a gyroscope).
In the conventional approach described above, the holes are made in the lower glass substrate prior to upper glass bonding because an electrical contact to all silicon features is required during anodic bonding of the upper glass substrate. This approach, however, makes the wafers fragile as the holes significantly weaken the lower glass substrate. Since the silicon MEMS device, after removal of its substrate, is quite thin, the device adds little or no structural support to make up for the holes in the lower glass substrate. Therefore, the glass is fragile and subject to breaking during subsequent fabrication steps, reducing wafer yield.
In the present approach to wafer level packaging of MEMS devices, the holes are not formed in the glass wafer when it is thin and fragile. In the present technique, the holes are only formed at the end of the process when the bonded wafers are double in thickness and, therefore, more rigid and robust. As the bonded wafers provide vastly improved rigidity, and there are so few handling steps left in the process, the bonded wafers can accommodate the larger number of holes needed for MEMS gyroscopes.
A semiconductor wafer 120 such as a silicon wafer is provided that includes one or more MEMS devices on a substrate layer. The MEMS devices include various microstructures contained in an epitaxial layer on the substrate layer of semiconductor wafer 120. The MEMS devices can include one or more MEMS inertial sensors, such as one or more gyroscopes and/or one or more accelerometers, which have been prefabricated in semiconductor wafer 120 by standard techniques.
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When the metal runners are present on lower cover wafer 110, the metal runners electrically short all metal and silicon features together. If the metal runners are not present, any flexible structure on the silicon wafer 120 would be pulled up during bonding by the electrostatic force, and be bonded to the upper substrate 130. After bonding the metal runners can be cut using laser trimming through lower cover wafer 110. The bonding step in
As illustrated in
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In an alternative approach, the foregoing process steps are carried out as described above for
The lower and upper cover plates 210, 230 are processed with appropriate mechanism support and trough structures prior to bonding with mechanism substrate 222 such that cover plates 210, 230 cooperate with MEMS device mechanism 220 when assembled together. The mechanism support and trough structures define a plurality of sealed cavity sections 234 when cover plates 210, 230 are bonded with mechanism substrate 222. The MEMS device mechanism 220 is located inside of sealed cavity sections 234. The cover plates 210, 230 can be formed in respective glass wafers of a type having a thermal expansion coefficient substantially matched to that of silicon. Examples such glass wafers include Corning Pyrex, Schott Borofloat 33, or Hoya SD2.
A plurality of holes 240 extend through upper cover plate 230 to a plurality of respective connecting portions 224 of mechanism substrate 222. The holes 240 are tapered inwardly from an external surface 236 of cover plate 230 to connecting portions 224. The connecting portions 224 are sized to completely seal off holes 240 from cavity sections 234. A plurality of metal runners 242, such as gold traces, are located in lower cover plate 210. The metal runners 242 may be partially submerged within shallow troughs formed on an inner surface of lower cover plate 210 prior to bonding with mechanism substrate 222.
A plurality of metal lead layers 250, such as gold traces, extend into each of holes 240 and provide electrical connection to MEMS device mechanism 220 through connecting portions 224. A plurality of bond pads 254 are located adjacent to each of holes 240 on an external surface 236 of upper cover plate 230. The bond pads 254 are electrically coupled to metal lead layers 250 and provide for routing signals into and out of sealed MEMS device 200.
A plurality of holes 320 each extend through upper glass plate 304 to a plurality of connecting portions 324. The holes 320 taper inwardly such that their diameters are larger at the top of glass plate 304 and smaller where holes 320 meet connecting portions 324. A plurality of metal lead layers 322 extend into each of holes 320 and provide electrical connections to device mechanism 310 through connecting portions 324. A plurality of bond pads 328 are located adjacent to holes 320 on an external surface of glass plate 304 and are within support frame 314. This allows all electrical connections to device mechanism 310 to be made from inside of support frame 314.
The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
