Environment-resistant module, micropackage and methods of manufacturing same

An environment-resistant module which provides both thermal and vibration isolation for a packaged micromachined or MEMS device is disclosed. A microplatform and a support structure for the microplatform provide the thermal and vibration isolation. The package is both hermetic and vacuum compatible and provides vertical feedthroughs for signal transfer. A micromachined or MEMS device transfer method is also disclosed that can handle a wide variety of individual micromachined or MEMS dies or wafers, in either a hybrid or integrated fashion. The module simultaneously provides both thermal and vibration isolation for the MEMS device using the microplatform and the support structure which may be fabricated from a thin glass wafer that is patterned to create crab-leg shaped suspension tethers or beams.

TECHNICAL FIELD

This invention relates to an environment-resistant modules, micropackages and methods of manufacturing same.

BACKGROUND

SUMMARY

An object of at least one embodiment of the present invention is to provide an environment-resistant module, a micropackage and methods of manufacturing same.

In carrying out the above object and other objects of the present invention, an environment-resistant module including a packaged micromachined or MEMS device is provided. The module includes a micromachined or MEMS device including at least one bonding site and a package having an inner surface which forms a cavity and an outer surface which communicates with the environment. The module further includes a microplatform or isolation platform located within the cavity. The microplatform includes at least one bonding site. The device is coupled to the microplatform at their respective bonding sites. The module still further includes a flexible, thermally isolating support structure to support the microplatform and the device within the cavity. The microplatform and support structure provide both thermal and vibration isolation of the device. The module further includes a path of electrically conductive material formed on the microplatform and on the support structure.

The package may include a substrate and a capsule connected to the substrate at a bonding area to at least partially form the cavity.

The package may completely encase the microplatform and the device to allow hermetic or vacuum encapsulation of the microplatform and the device.

The module may include at least one feedthrough through the package to electrically connect the conductive material to the environment.

The at least one feedthrough may include a vertical or a horizontal feedthrough.

The vertical feedthrough may extend through the substrate or the capsule.

The support structure and the microplatform may be defined by a layer of a wafer such as a glass wafer.

The substrate may include a wafer such as a semiconductor wafer.

The support structure may include a plurality of isolation suspension beams or tethers.

The module may further include a heater and a temperature sensor formed on the microplatform.

The module may still further include at least one shock absorption layer formed inside the package.

The module may further include at least one anti-radiation shield formed inside the package.

The module may still further include a getter layer formed inside the package.

At least a portion of the package may be optically transparent.

At least a portion of the package may be open to the environment.

Further in carrying out the above object and other objects of the present invention, a micropackage is provided. The micropackage includes a semiconductor wafer and an insulating layer or film bonded to the wafer. The insulating layer or film has a hole which extends completely therethrough. Electrically conductive material is formed in the hole and is electrically connected to an isolated portion of the wafer. The conductive material and the isolated portion of the wafer form a substantially vertical feedthrough for signal transfer through the package.

The wafer may at least partially form a substrate or a capsule of the package.

Still further in carrying out the above object and other objects of the present invention, a method of making a module is provided. The method includes providing a micromachined or MEMS device including at least one bonding site. The method further includes providing a substrate and providing a microplatform including at least one bonding site. The method still further includes providing a flexible support structure to support the microplatform above the substrate, aligning the respective bonding sites and bonding the microplatform to the device at the respective bonding sites.

The support structure may be flexible and the step of bonding may include flexing the support structure above the substrate. The substrate prevents flexing of the support structure beyond a predetermined amount.

At least one embodiment of a generic, wafer-level environment-resistant microinstruments (i.e., micromachined or MEMS devices) package and related processes are provided. This unique technique provides thermal and mechanical isolation from the environment, which may deteriorate the device performance. It also can package and handle a wide variety of individual MEMS chips or wafers in either a hybrid or integrated fashion, using a new and novel MEMS die transfer/assembly technique. The microinstrument is batch integrated/transferred onto a microplatform and suspended over a substrate wafer by the thermal and mechanical isolation suspensions. The microinstrument is then encapsulated in a cavity to provide vacuum or hermetic sealing. Micro-heaters and temperature sensors can be integrated to maintain the microinstruments at a fixed temperature (oven-control). The electrical signal leads can be defined vertically or laterally.

Potential application areas using this technique are:

3. High performance pressure sensor or microphone packaging.

5. High performance IR sensor packaging.

DETAILED DESCRIPTION

Existing micromachined or MEMS device packages have not been able to provide advanced isolation functionalities, for example, from temperature and vibration in efficient ways. Technical issues are addressed herein for making advanced isolation possible as well as making the developed technology as generic as possible so it can be applied to various applications without significant change.

A generic vacuum package that can suit a number of different devices and applications, and that can provide isolation from environmental disturbances such as temperature and vibration will be of value for many MEMS devices [1].

A new environmentally isolated package design, a generic transfer approach for the integration of monolithic and hybrid MEMS into the package, and new vertical feedthroughs for signal transfer are provided herein.

Package Design

FIGS. 1aand 1bare schematic views of one embodiment of the package. The package typically has three major components and in at least one embodiment includes: (i) a supporting substrate such as a silicon wafer or ceramic or metal substrate that may incorporate signal feedthroughs; (ii) a thin glass wafer or substrate which provides thermal and mechanical isolation using isolation suspensions made from the glass; and (iii) a cap silicon wafer or capsule for final vacuum/hermetic encapsulation if needed. In one embodiment, the MEMS die is flipped over and attached onto a glass microplatform, which is, in turn, supported by isolation suspensions over a shallow recess formed in the supporting silicon wafer substrate. Interconnect lines are formed on the glass suspension beams and transfer electrical signals between pads on the glass microplatform and vertical feedthroughs through the bottom silicon wafer. The attached MEMS die is oven-controlled by a heater and temperature sensor integrated on the microplatform and is thereby maintained at a fixed temperature. Vibration isolation is provided by the suspensions made of the glass.

The isolation suspensions should be stiff enough to mechanically support the platform and withstand shock/vibration, but long and flexible enough to provide thermal and vibration isolation. Both of these requirements are achieved using glass as the support and thermal isolation material. Glass has a relatively high Young's modulus and a low thermal conductivity. A thin (100 μm) glass wafer may be used to form these suspensions. The thin wafer is easy to etch and pattern using a batch-level wet etching process. Shock absorption layers [2], an anti-radiation shield for higher thermal isolation, and a getter layer for the high vacuum environment may also be formed inside the package [3].

The MEMS device is fabricated on a separate substrate, and transferred onto a microplatform that is an integral part of a second wafer or substrate. The MEMS device may be one or more of several devices such as, for example, a temperature sensor, a heater, a quartz device, a gyroscope, an accelerometer, a biomedical device, a microfluidic device and/or a magnetometer. More than one MEMS devices may be included in the same package and/or on the same microplatform. For example, multiple temperature sensors and/or heaters may be used on one microplatform for additional precision and accuracy when regulating temperature or to achieve a more uniform temperature distribution across the entire unit. The MEMS device may also be another microplatform.

An oven circuit acting as a circuit component (individual die, dies or boards) may also be included with the MEMS device that control the temperature of the microplatform utilizing a temperature sensor and heater that are either integrated on the platform, have direct contact with the platform, or are separated from the platform. Alternatively, or in combination, the MEMS device may include or be in communication with a device circuit acting as a circuit component that communicates with the MEMS device to enable the device to function properly.

Packaging around the MEMS device is described below. However, it should be understood that, as will be described, such packaging is optional. For example, the device can be bonded to the microplatform while not in a cavity of a package; the device may be bare and not vacuum sealed, exposing the device to some surrounding environment.

If packaging is included, the transferred device may be vacuum or hermetically sealed on top by a cap wafer or capsule. The electrical signal leads are defined vertically on the support substrate. The vertical feedthroughs can be formed on the cap wafer or capsule, and lateral feedthroughs are also possible.

The packages shown inFIGS. 1aand 1bcan provide isolation from two different sources: mechanical and thermal input. Mechanical isolation is provided through two elements: isolation suspensions that damp out the low-level and higher frequency vibration signals, and shock stops that limit the range of travel of the transferred device during high g shocks. Thermal isolation is also provided by these isolation suspensions, which are designed and fabricated to have very high thermal resistance. A control method can be executed to keep temperature constant using a heater and a temperature sensor integrated on the isolation platform or on the MEMS dies. Since the devices are highly thermal-isolated, the power consumption for the constant temperature control is very low.

Glass tethers can be fabricated from a thin glass wafer, or from a thick glass wafer that is mechanically thinned, or from a thick deposited glass layer, or from a thick glass/oxide layer that is deposited on a semiconductor wafer using a number of different techniques.

Fabrication

FIG. 2jis an exploded view of one embodiment of the package andFIGS. 2a-2iillustrate a corresponding process flow. This process can be divided into three major parts: (i) preparation of the supporting substrate, (ii) MEMS die transfer, (iii) final encapsulation and vertical feedthroughs formation.

Supporting Substrate Fabrication

Referring toFIG. 2a, a shallow recess is formed in a standard thickness, highly-doped supporting silicon wafer using DRIE (Deep Reactive Ion Etching), and a metal layer for shock absorption layer as well as anti-radiation shield is deposited inside the recess. Gold is typically used because it is soft and highly reflective [2].

Referring toFIG. 2b, the silicon wafer is then anodically bonded to a 100 μm-thick glass wafer as shown inFIG. 2b. Other techniques are contemplated that bond or otherwise adhere the silicon wafer (substrate) to the glass wafer (that later forms the microplatform, support structure, and outer regions of material). For example, the glass wafer may be mounted to the silicon wafer utilizing conductive or nonconductive paste, soldering, reflow flip chip bonding, bump bonding, anodic bonding, and/or eutectic bonding. These techniques can be used to bond any part of the microplatform to the substrate. For instance, the microplatform can be mounted to the silicon wafer (substrate) via conductive paste such that electrical signals can be sent to and from the microplatform through the paste. These exemplary bonding techniques can also be used to bond the glass wafer (that later forms the microplatform, support structure, and outer regions of material) to the package (that is discussed below).

Referring toFIGS. 2cand 2d, via holes for the vertical feedthroughs are first wet etched into the glass wafer using a 49% HF solution, and then filled with a metal film as further shown inFIGS. 3a-3d. The via holes are filled by the metal layer. Good conformal coverage is shown. The measured contact resistance between the contact metal and the highly doped silicon substrate shows ohmic characteristics with less than 2 ohm resistance. By this via and feedthrough structure, the footprint of the package can be reduced. A lateral feedthrough instead of the vertical feedthrough can also be applied in this package.

Referring toFIG. 2e, metal interconnection lines between the vias and the bonding pads on the platform are then defined. Finally, the isolation platform and suspensions are patterned by wet etching the glass using a 49% HF solution as shown inFIG. 2f. This defines a microplatform for the MEMS device, support structure in the form of suspensions that support the microplatform, and an outer region of material about the support structure that supports the support structure.

In summary, first, a bottom recess is formed on the support bottom wafer, and the shock absorption and radiation shield layer is deposited. It is then bonded with another glass wafer, out of which isolation suspension will be formed. For the vertical feedthrough interconnection, via holes are made by etching the glass wafer. The electrical interconnection lines are defined and then the isolation suspensions are patterned.

MEMS Transfer Process

Before transferring the MEMS dies, suitable metal layers may be deposited on the MEMS die using a shadow mask process. The shadow mask may be made of patterned SU8 film on a silicon wafer with several holes, each of which corresponds to each of the bonding pads on the MEMS die.

Referring toFIG. 2g, MEMS dies are then flipped over and bonded to the pads on the glass isolation platform using transient liquid phase (TLP) bonding [4]. This process may be referred to as reflow flip chip bonding. The reflow flip chip process sequence and results are shown inFIGS. 7a-7f. The alignment of the MEMS dies to the substrate is done using a micromachined guide wafer ofFIG. 7a. It has through-wafer holes where each of MEMS dies is placed. This transfer/bonding process is a batch process and could support any size and shape die. The glass platform is supported using the flexible glass suspensions (FIG. 7b). It is flexible enough so when the die is being bonded to the platform it bends and touches down on the bottom silicon wafer (i.e.,FIG. 7e), but it springs back up due to the high stiffness of the suspensions (i.e.,FIG. 7f). The guide wafer is removed after die attach and bonding. All transfer/bonding process is done using standard wafer bonding equipment including a standard wafer bonding tool (FIGS. 7dand 7e).

This reflow flip chip transfer technique has several advantages. First, it is generic. Therefore, any kind of MEMS device can be assembled since the dies are transferred after they are fabricated using any given process. Second, the electrical and mechanical connections between the isolation platform and the MEMS die are performed at the same time. Third, it provides flexibility to both the MEMS device and the bonding pad material selection since the materials required for bonding are deposited after the MEMS device fabrication. In addition to various kinds of TLP bonding, other die attachment approaches, such as thermo compression and solder bonding can also be used. A requirement may be that the die attach bond should survive the temperature of the bonding step described hereinbelow to achieve hermetic/vacuum encapsulation.

In summary, the reflow flip chip technique is generic, so that any device with different size, shape and contacts location can be transferred at a time; a precise alignment is possible during the transfer; the electrical connection between the isolation platform and the substrate is possible at the moment of the transfer bonding; and various bonding mechanism such as In—Au TLP, Au—Au thermal compression bonding can be used for the attachment of MEMS device to the platform.

Once the MEMS device is bonded to the microplatform, an optional additional microplatform can be bonded or closely located to the opposite side of the MEMS device (above the device inFIG. 2g) according to any of the bonding techniques described herein.

While flip chip bonding has its benefits, it should be understood that other methods of bonding or otherwise connecting the device to the microplatform are contemplated. For example, the device may be mounted on the microplatform using conductive or nonconductive paste. If the paste is conductive, the MEMS device can be mechanically adhered and electrically connected to the microplatform via the paste. In another example, welding or soldering, preferably (but not necessarily) using conductive metals, can be utilized to mechanically and electrically connect the MEMS device to the microplatform. In other embodiments, the MEMS device can be mounted to the microplatform via bump bonding, anodic bonding, and/or eutectic bonding.

Whether flip chip bonding or other bonding or adhesion methods are utilized, pads may be provided that connect the MEMS device to the microplatform. This is shown inFIGS. 7band 7cin which platform contacts are provided on the microplatform, and device contacts are provided on the MEMS device. These pads provide a special separation between an under surface of the MEMS device and an upper surface of the microplatform. The pads may or may not include a conductive material for transferring electric signals between the MEMS device and the microplatform. The MEMS device can be electrically connected to the pads using conductive paste, soldering, reflow flip chip bonding, bump bonding, eutectic bonding, and/or wirebonding.

Pads on the MEMS device may be located on the opposite side of the MEMS device (above the device inFIG. 2g) and can be connected to the pads on the microplatform using bonding methods such as wirebonding or any other applicable bonding method.

FIGS. 4a-4bshow the isolation platform and the transferred die is shown inFIG. 4cwhich is a SEM image of the fabricated isolation suspension with electrical interconnection. The thin metal interconnections are patterned, and then the isolation suspension is defined. The suspension design provides thermal/mechanical isolation and electrical lead out.

After any of the above-described methods of bonding techniques are utilized to secure the MEMS device to the glass wafer or microplatform, a capsule can enclose the MEMS device, as will be described below. However, it should be understood that the capsule is optional. In other words, the step of encapsulation shown inFIG. 2hand described below can be omitted.

Referring toFIG. 2h, vacuum/hermetic encapsulation is achieved using a cap or capsule bonded to the supporting substrate. This can be done using a variety of bonding techniques, including anodic and Au—Si eutectic bonding.

The capsule can comprise one or more of a plurality of materials. For example, the capsule may be made of metal, ceramic, plastic, silicon, and/or glass. These materials may also make up the underlying substrate.

FIGS. 5a-5dshows the picture of the fabricated packages, and the SEM pictures of the vertical feedthrough, which is formed in the supporting substrate. The vertical feedthroughs are electrically isolated by silicon DRIE. Either wire-bonding or a flip-chip technique can be applied. The samples inFIG. 5aare fabricated using anodic bonding.

Referring toFIG. 2i, after vacuum packaging, the vertical feedthroughs are completed and formed by DRIE etching through the supporting silicon wafer from the backside. This vertical feedthrough technique reduces the footprint of the package, and, unlike lateral feedthroughs, enables the use of flip chip bonding of the die. The vertical feedthrough is very robust in that it shows no problem with wire bonding. The measured contact resistance is <2Ω (this is measured between the metal on the glass substrate to the bottom of the Si feedthrough, which is low enough for most applications).

Instead of (or in combination with) the vertical feedthroughs ofFIG. 2i, other feedthroughs may be utilized that enable an electrical connection between the MEMS device and its microplatform with an outside environment that uses the signals sent from the MEMS device. For example, embodiments in which the capsule or the underlying substrate is not utilized may render the use of vertical feedthroughs disadvantageous or even impossible. In such situations, there must still be some form of electrical connection from the MEMS device and the microplatform to the outside environment. Any such connection can still be referred to as a feedthrough. The microplatform may be electrically connected to this feedthrough via conductive paste, soldering, reflow flip chip bonding, bump bonding, eutectic bonding, and/or wirebonding.

Results

A thermal impedance of 3000K/W has been measured for the isolation platform, which corresponds to a power consumption of 43 mW when the platform is oven-controlled at 80° C. and the external environment temperature is −50° C. The thermal isolation can be modified and improved as needed for different applications. The resonant frequency of the platform after a 4.5×4.5×0.5 mm3MEMS die is transferred and attached to the platform has been calculated to be <1 kHz. This resonant frequency can be designed to suit a particular application for vibration isolation. The vacuum and hermeticity of the package is determined by the bonding techniques. For example Au—Si eutectic bonding has been shown to provide sub-10 mTorr vacuum with <2 mTorr variation for almost two years [3].

Conclusion

A new, robust, and generic way of packaging MEMS for isolation against environmental parameters has been developed. Thermal and mechanical isolations are achieved simultaneously using glass isolation suspensions. The packaging technology allows for both wafer-level and die-level packaging of MEMS devices, and can handle a wide variety of MEMS chips. The package is capable of both hermetic and vacuum encapsulation, and provides vertical feedthroughs through the package substrate to save space. This approach is suitable for many different MEMS devices, including high performance gyroscopes, accelerometers, infrared imagers, or any applications requiring low power temperature control, vibration isolation, and hermetic/vacuum packaging for stable operation.

Elements and Variants

The environment-resistant MEMS package can be broken down into key elements.FIGS. 8a-8hshow each element of the developed technology with boxes in phantom lines. Also,FIGS. 8a-8hshow other possibilities that can be derived from the technology that has been developed.

FIG. 8ashows vacuum/hermetic encapsulation by a silicon-insulator (glass)-silicon structure. The bonding method can be various such as anodic bonding, solder bonding, intermetallic bonding, etc.

FIG. 8bshows vertical feedthroughs using vias through the insulating layer (glass) and silicon substrate isolation. Metal is filled in the vias for the electrical interconnection. The feedthroughs can be located in either the support substrate or the cap wafer.

FIG. 8cshows the generic die-to-wafer transfer and assembly method.

FIG. 8dshows thermal and vibration isolation by the isolation suspensions.

FIG. 8eshows 2nd-level vibration isolation by suspensions formed out of a supporting wafer.

FIG. 8fshows packaging (encapsulation) with an outlet for communicating between the packaged device and the environment. This may be for pressure sensor and microphone applications.

FIG. 8gshows packaging (encapsulation) with a transparent window. Image sensor and IR sensor are possible applications.

Any possible combination of above, for example, the encapsulation ofFIG. 8fand the SOG ofFIG. 8h.

FIGS. 9aand 9bshow potential modifications of the isolation platform and suspensions. Instead of using a glass wafer for both platform and suspension material, it is possible to form the isolation platform out of a Si wafer and the isolation suspension out of a dielectric film (possibly thin glass wafer). As shown in the figures, the isolation suspension can be located at the top or bottom of the isolation platform.

FIGS. 9cand 9dandFIGS. 6a-6cshow potential modification of the vertical feedthroughs. The vertical feedthroughs can be formed in the cap wafer or capsule. Dielectric film (possibly thin glass wafer) for electrical isolation between vias can be located either at the top or bottom of the cap wafer as shown inFIGS. 9cand 9d, respectively. The vertical feedthroughs can be fabricated in the cap wafer (top vertical feedthrough) using the same technique used in forming the bottom vertical feedthroughs.FIG. 6ais a schematic perspective view showing the fabricated package with top vertical feedthroughs. In this case, via holes are exposed on the outside of the package.

Although individual dies and transfers are shown, the technology will also work with full wafers containing actual MEMS devices. A device wafer would be bonded, for example, to the platform substrate and then the device wafer would be diced or etched to singulate the individual dies.