Method for sealing and backside releasing of microelectromechanical systems

Disclosed are methods for fabricating encapsulated microelectromechanical systems (MEMS) devices. A MEMS device fabricated on a CMOS wafer is encapsulated using an etch resistant thin film layer prior to the release of the MEMS device. Once CMOS processing is completed, the wafer is etched to release the MEMS device. If the MEMS is fabricated on a silicon-on-insulator (SOI) wafer, the buried oxide of the SOI wafer acts as an etch stop for the etching. A sacrificial layer(s) is accessed and removed from the back side of the wafer, while the front side of the wafer is protected by a masking layer. The MEMS device is released without having any detrimental effects on CMOS components. If desired, the wafer can be mounted on another substrate to provide hermetic or semi-hermetic sealing of the device.

BACKGROUND

The present invention relates generally to microelectromechanical systems processing and fabrication methods, and more particularly, to methods for sealing (i.e., packaging) and backside releasing of microelectromechanical systems (MEMS).

Reliable sealing/packaging of movable micromechanical devices is a very critical and challenging step in commercial development of such devices in industrial environments. The challenging aspect is having a hermetically sealed cap on top of the MEMS device that completely isolates it from the surrounding environment while maintaining the movability for the critical mechanical parts of the structure. In other words, the cap should not be in touch with any of the movable sections of the micromechanical system. Furthermore, for most micromechanical devices, operation in an inert or stable environment (sometimes vacuum) is a necessity or helps maximize the performance. On the other hand, the ability to integrate micromechanical sensors and actuators with active electronic circuitry is of great interest and is a key step in achieving higher levels of performance and integration in microelectronics.

Many high-performance MEMS devices use silicon dioxide as a sacrificial layer during the fabrication process. The sacrificial dioxide must be removed at the end of the fabrication process to release the device and render it movable and/or functional. The removal of the silicon dioxide is typically carried out in a hydrofluoric acid and de-ionized water (HF/H20) solution, which could also attack and damage passivation and interconnect layer(s) of a CMOS wafer. Therefore, sealing and release techniques that alleviate this problem are of great interest.

Various references discuss MEMS devices and processing methods for producing such devices. These include U.S. Pat. No. 7,023,065 of F. Ayazi et al., issued Apr. 4, 2006, U.S. Pat. No. 6,841,861 of Fredrick T. Brady issued January, 2005, U.S. Pat. No. 6,743,656 of Orcutt, et al., issued June 2004, U.S. Pat. No. 6,469,909 of Simmons, issued October 2002, a paper by S. Pourkamali and F. Ayazi, entitled “High frequency capacitive micromechanical resonators with reduced motional resistance using the HARPSS technology,” proceedings, 5 Silicon RF topical meeting 2004, pp. 147-150, and a paper by S. Pourkamali, Z. Hao and F. Ayazi, entitled “VHF single crystal silicon side supported disk resonators—Part II: implementation and characterization” Journal of Micro Electro Mechanical Systems, Vol. 13, Issue 6, December 2004, pp. 1054-1062. U.S. Pat. No. 5,963,788 of Barron et al. issued Oct. 5, 1999 discloses a method to integrate MEMS devices with CMOS circuits. However, none of these references disclose or suggest a method for encapsulating the MEMS portion of a CMOS wafer, and releasing the wafer for long period of time in HF without damaging the CMOS portion.

It would be desirable to have methods for sealing and backside releasing of microelectromechanical systems with the possibility of providing a vacuum environment to the microelectromechanical device. It would also be desirable to have methods that are suitable for sealing and releasing MEMS integrated with CMOS or microelectronics circuits on a common substrate. It would be desirable to have microelectromechanical devices that are fabricated using the methods.

DETAILED DESCRIPTION

Disclosed are microelectromechanical devices and packaging methods that encapsulate a MEMS portion of a CMOS wafer using an etch resistant (HF-resistant) thin film layer (such as silicon nitride, polysilicon, metal, or polymers) prior to releasing of the MEMS device (in HF solution, for example). Once the CMOS circuit and MEMS device processing is completed, the wafer is wet or dry etched from the back side beneath the MEMS device portion. If the MEMS device is fabricated on a silicon-on-insulator (SOI) wafer, for example, the buried oxide of the SOI wafer acts as an etch stop for backside etching. The front side of the wafer is protected by a masking layer, and the sacrificial layer(s) of the MEMS device is then accessed and removed from the back side of the wafer. The MEMS device is therefore released without having any detrimental effect on CMOS circuit. If desired, after releasing the wafer can be mounted on or hermetically bonded to another wafer (carrier wafer or package substrate) to provide hermetic or semi-hermetic sealing of the MEMS device. The hermetically-bonded carrier wafer or package substrate can have getter thin-films deposited on the areas exposed to the MEMS portion to ensure long-term vacuum.

The disclosed method is a generic packaging and/or sealing and release method that can be used with other types of sacrificial layers besides silicon dioxide, including germanium, and aluminum, for example. The method is also not limited to CMOS wafers and can be applied to many types of MEMS devices on various substrates, including silicon-carbide-on-insulator substrate (with or without circuitry), for example.

Referring to the drawing figures,FIGS. 1-5illustrate an exemplary processing method10for producing a MEMS device20on a CMOS silicon-on-insulator (SOI) wafer11(such as a silicon-on-oxide wafer11) that is used to encapsulate and release the MEMS device20from the CMOS SOI wafer11. The CMOS SOI wafer11comprises a lower silicon handle layer12, an insulating layer13such as a silicon dioxide layer13(1-3 micrometers in thickness), and an upper device layer14that can be a few micrometers to a few hundreds of micrometers in thickness. It should be mentioned that the use of SOI wafers is not necessary to render the sealing and backside release technique disclosed in this application operational. Regular silicon wafers can be used equally well with this technique. The advantage of the SOI wafer is in providing electrical isolation between the body of the MEMS devices and the substrate, as well as providing an etch stop during the backside release of the MEMS device, which in turn enables accurate control of the thickness of the MEMS device and protects it from the silicon etchant. Silicon carbide, silicon carbide-on-insulator, (ultra) nano-crystalline diamond, gallium arsenide wafers can be used equally well in this method.

As is shown inFIG. 1, a representative MEMS device20, such as a capacitive micromechanical resonator, a gyroscope or an accelerometer, for example, is fabricated in the CMOS wafer11. The MEMS device could be fabricated prior to, between, or after the CMOS fabrication steps. The MEMS device can be fabricated using the well-known HARPSS process as outlined in U.S. Pat. No. 7,023,065 of Ayazi et al. The MEMS device20is fabricated with a movable or vibratable silicon structure21surrounded by a removable sacrificial layer22, such as silicon dioxide, for example. The MEMS device20is sealed using an etch resistant layer23, such as a low pressure chemical vacuum deposited (LPCVD) hydrofluoric acid (HF) resistant layer23, which may be silicon nitride and/or polysilicon, for example.

As is shown inFIG. 2, a conventional CMOS circuit24is fabricated that is typically electrically connected to the MEMS device20and other components fabricated on the CMOS SOI wafer11using lithographically-defined interconnects (not shown). The MEMS device20and CMOS circuit24are covered by a passivation layer25that protects the devices20,24. Although not necessary, peripheral trenches can be etched around the MEMS device(s) through the upper device and oxide layers of the SOI wafer, and filled with the HF-resistant layer23to block lateral undercut of the buried oxide of the SOI wafer during the backside release. As will be discussed below, the MEMS device20needs to be released, i.e., the sacrificial layer surrounding all or portion of the MEMS device needs to be removed to render the device movable so that it can properly function. This is achieved by removing the sacrificial layer22, typically using a HF/H20 solution. However, the CMOS portion of the wafer11should not be exposed to the HF/H20 solution as it can also attack and remove the passivation and/or the metallization layers25and damage the CMOS circuit24.

Therefore, as is shown inFIG. 3, a solid (unpatterned) masking layer26is deposited on the front side of the wafer11on top of the passivation layer25. Then, the back side of the CMOS SOI wafer11is etched, using either a dry or wet etching procedure, to remove a portion of the handle layer beneath the MEMS device20, and the silicon oxide layer13acts as an etch stop. This produces a cavity27beneath the MEMS device20.

Then, as is shown inFIG. 4, the MEMS device20is released from the SOI wafer21using hydrofluoric acid (HF) solution, for example. Releasing the MEMS device20removes the sacrificial layer22surrounding the moveable portion of the MEMS device20comprising the vibratable or movable silicon structure21. The masking layer26is removed, as is shown inFIG. 5. This completes fabrication of the exemplary MEMS device20. Note that the MOEMS device20may be fabricated using the techniques outlined in U.S. Pat. No. 7,023,065, the contents of which are incorporated herein by reference.

In addition, and as is shown inFIG. 5, after the masking layer26is removed, the SOI wafer21comprising the MEMS device20and CMOS circuit24may be mounted on or bonded to a second substrate31or wafer31, comprising a carrier wafer31or package substrate31.

FIG. 6illustrates another embodiment of a CMOS SOI wafer11comprising a MEMS device20and a CMOS circuit24fabricated using the method disclosed with reference toFIGS. 1-5. This embodiment of the wafer11includes an intermediate metallic bonding layer32(e.g., gold) interposed between the handle substrate12and the second substrate31or wafer31(carrier wafer31or package substrate31).FIG. 6also illustrates that a getter material/layer33or nanogetter layer33may be disposed in the cavity beneath the MEMS device20. The getter33acts to create a vacuum surrounding the MEMS device20.

FIG. 7illustrates another exemplary embodiment of a MEMS device20fabricated in accordance with the disclosed method. In this embodiment, the MEMS device20is fabricated on the back side of the CMOS wafer11. The CMOS wafer can be an SOI wafer in which the MEMS device is fabricated in the handle layer prior to, between or after fabrication of the CMOS circuit24. In fabricating this embodiment, the optional masking layer26is used on the front side of the wafer11on top of the passivation layer25to protect the CMOS circuit24during release of the backside MEMS device. The MEMS device20can be electrically connected to the front side CMOS circuit24using through wafer electrical via lines or using some external wiring means. In this embodiment, the use of the HF-resistant layer23is not necessary.

FIG. 8illustrates yet another exemplary embodiment of a MEMS device20fabricated in accordance with the disclosed method. This embodiment is substantially the same as is shown inFIG. 7, but the SOI wafer21is mounted17to the second substrate31or wafer31, comprising the carrier wafer31or package substrate31. An optional cavity is formed in the carrier wafer31or package substrate31to create a clearance between the MEMS device and the second substrate31or wafer31. A getter material/layer33or nanogetter layer33may be disposed in the cavity in the carrier wafer31or package substrate31which is used to create a vacuum surrounding the MEMS device20.

FIG. 9illustrates yet another exemplary embodiment of a MEMS device20fabricated in accordance with the disclosed method. This embodiment is substantially the same as is shown inFIG. 8, with the SOI wafer21mounted17to the carrier wafer31or package substrate31. In addition, this embodiment of the wafer11includes an intermediate metallic bonding layer32such as gold interposed between the handle substrate12and the carrier wafer31or package substrate31. A cavity is formed in the carrier wafer31or package substrate31. A getter material/layer33or nanogetter layer33may be disposed in the cavity in the carrier wafer31or package substrate31which is used to create a vacuum surrounding the MEMS device20.

Thus, methods of sealing and backside releasing of microelectromechanical systems (MEMS) devices have been disclosed. It is to be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles discussed above. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.