Low cost wafer level process for packaging MEMS three dimensional devices

Abstract

An apparatus and method for wafer-level hermetic packaging of MicroElectroMechanical Systems (MEMS) devices of different shapes and form factors is presented in this disclosure. The method is based on bonding a glass cap wafer with fabricated micro-glassblown bubble-shaped structures to the substrate glass/Si wafer. Metal traces fabricated on the substrate wafer serve to transfer signals from the sealed cavity of the bubble to the outside world. Furthermore, the method provides for chip-level packaging of MEMS three dimensional structures. The packaging method utilizes a micro glass-blowing process to create bubbleshaped glass lids. This new type of lids is used for vacuum packaging of three dimensional MEMS devices, using a standard commercially available type of package.

Claims

1. A method for wafer-level vacuum packaging of a MEMS three dimensional device comprising: glassblowing a spherical glass structure on a wafer-level in a cap wafer, using sealing rings for coupling between the structure and metal traces and/or substrate wafer; disposing metal traces on the substrate wafer; disposing a three dimensional MEMS device on the substrate wafer using flip-chip bonding with interconnects to the MEMS device; disposing the cap wafer over the MEMS device on the substrate wafer; and bonding the cap wafer to the substrate wafer to form a cap-handle wafer stack.

2. The method of claim 1 where glassblowing a spherical glass structure on a wafer-level, using sealing rings for coupling between the structure and metal traces and/or substrate wafer comprises: selectively defining a cavity in a handle wafer having a first surface and an opposing second surface; bonding a glass cap wafer onto the handle wafer on the first surface to form a cap-handle wafer stack; forming metallization on the handle wafer on the second surface; heating the cap-handle wafer stack as to allow pressure buildup within the cavity causing plastic deformation of the cap wafer; and removing a layer of the handle wafer disposed beneath the deformation of the cap wafer to open the cavity.

3. The method of claim 1 where bonding the cap wafer to the substrate wafer comprises hermetically sealing the cap-handle wafer stack to the substrate wafer.

4. The method of claim 2 where removing a layer of the handle wafer disposed beneath the deformation of the cap wafer to open the cavity comprises removing the handle wafer disposed beneath the deformation of the cap wafer by wet chemical etching the handle wafer using a metal etch mask.

5. The method of claim 2 where removing the handle wafer to open the cavity comprises removing the handle wafer by anisotropic dry etching the handle wafer.

6. The method of claim 2 where removing a layer of the handle wafer disposed beneath the deformation of the cap wafer to open the cavity comprises removing the handle wafer disposed beneath the deformation of the cap wafer by lapping the cap-handle wafer stack on the second opposing surface disposed beneath the deformation of the cap wafer.

7. The method of claim 2, where forming metallization on the handle wafer on the second surface is performed before bonding the glass cap wafer onto the handle wafer on the first surface to form the cap-handle wafer stack.

8. The method of claim 2, where forming metallization on the handle wafer on the second surface is performed after glassblowing the spherical glass structure on a wafer-level, using sealing rings for coupling between the structure and metal traces and/or substrate wafer.

9. The method of claim 1 where disposing the spherical glass structure as a cap wafer over the MEMS device on the substrate wafer comprises disposing the MEMS device on the substrate wafer by a flip-chip bonding process.

10. The method of claim 2, where removing the handle wafer to open the cavity further comprises removing the handle wafer and then disposing a thin-film deposition layer of getter material.

11. The method of claim 2 where bonding the cap wafer to the substrate wafer comprises producing a hermetic seal by bonding the cap-handle wafer stack to the substrate wafer in vacuum.

12. The method of claim 1 where disposing metal traces on the substrate wafer comprises selectively disposing a first metal layer, selectively disposing an insulating layer on the first metal layer, and selectively disposing a second metal layer on the insulating layer.

13. The method of claim 10, where disposing the thin-film deposition layer of getter material comprises disposing the thin-film deposition layer of getter material on the cap wafer before bonding the cap wafer onto the handle wafer on the first surface to form the cap-handle wafer stack and where a hermetic seal is formed thereafter.

14. A method for fabrication of lids for a chip-level hermetic packaging of MEMS device of varying shape and form factor, comprising: providing a handle wafer; creating cavities into a handle wafer; bonding a cap wafer onto the handle wafer to form a cap-handle wafer stack; forming a metal hard mask on the handle wafer; heating the cap-handle wafer stack in a furnace as to allow pressure buildup within the cavity causing plastic deformation of the cap wafer; and removing portions of the handle wafer defined by the metal hard mask to open the cavity.

15. The method of claim 14 where creating cavities into a handle wafer comprises microglassblowing a bubble in the cap wafer.

16. The method of claim 14 where bonding a cap wafer onto the handle wafer to form a cap-handle stack comprises anodically bonding a borosilicate glass wafer to a silicon wafer.

17. The method of claim 14 where heating the cap-handle wafer stack as to allow pressure buildup within the cavity causing plastic deformation of the cap wafer comprises microglassblowing a bubble in the cap wafer with a diameter of 10 mm or greater at at least 134 Torr pressure within the furnace.

18. The method of claim 17 where microglassblowing a bubble in the cap wafer comprising microglassblowing a bubble with a height of 6.8 mm or less so that removing the handle wafer to open the cavity does not break the bubble.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1a and 1b are diagrams of prior art MEMS packaging approaches.

(2) FIGS. 2a and 2b are photographs of prior art three dimensional MEMS devices.

(3) FIG. 3 is a side cross-sectional view of a hermetically sealed three dimensional MEMS device.

(4) FIGS. 4a-4f are side cross-sectional diagrams illustrating the bubble-shaped glass structures fabrication process steps: FIG. 4a shows the step of laying down a photoresist mask. FIG. 4b illustrates a Si DRIE etching step to define a cavity. FIG. 4c illustrates a step of glass wafer anodic bonding. FIG. 4d illustrates a step of metal deposition and patterning. FIG. 4e shows a step of glass-blowing and FIG. 4f a step of Si wet etching.

(5) FIG. 5 is a side cross-sectional diagram of the geometry of the blown structure.

(6) FIGS. 6a-6d are side cross-sectional diagrams illustrating the fabrication process for the substrate wafer.

(7) FIG. 7 is an exploded diagram of the wafer-level packaging of three dimensional MEMS devices.

(8) FIG. 8 is a graph of the time and temperature/current to anodically bond 100 m thin borosilicate glass wafer to the Si wafer.

(9) FIG. 9 is a photograph of 100 m thin borosilicate glass wafers anodically bonded to the Si wafer.

(10) FIG. 10 is a photograph of a gold hard mask on the Si side of the glass-to-Si wafer stack.

(11) FIG. 11 is a photograph of glass bubbles blown at pressure in the range of 134 Torr-262 Torr. The height of the bubble exceeds 10 mm. 50 mm.sup.3 three dimensional MEMS TIMU for the scale.

(12) FIG. 12 is a photograph of a 50 mm.sup.3 three dimensional MEMS IMU packaged, using the LCC standard package and the bubble-shaped glass lid.

(13) The disclosure and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the embodiments defined in the claims. It is expressly understood that the embodiments as defined by the claims may be broader than the illustrated embodiments described below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(14) As a demonstration of using the packaging method of the illustrated embodiments of the invention, a 50 mm.sup.3 three dimensional MEMS TIMU device as shown in FIG. 2a was packaged, using the LCC standard package and a bubble shaped glass lid. Recent developments in the MEMS processing enable building complex-shaped three dimensional MEMS structures, including those using polymers to create three dimensional elements. One example of a three dimensional MEMS device is a folded mems inertial measurement unit (IMU), as seen in FIG. 2a. Folded IMU approach is based on wafer-level fabrication of planar structures with in-situ fabricated single-axis inertial sensors. These structures are then folded in a three dimensional shape, using co-fabricated polyimide flexible hinges. Another example is a chip-scale combinatorial atomic navigator, as seen in FIG. 2b. The device is assembled into a three dimensional configuration with Helmholtz coils and Rb vapor cell inside a three dimensional structure. Optical components and inertial sensors are integrated on the side walls of the folded pyramid.

(15) A large variety of shapes and form factors of MEMS devices makes it difficult to use standard wafer-level packaging techniques and standard packages. For example, common wafer-level packaging techniques are not suitable for packaging of MEMS three dimensional devices, such as the folded MEMS TIMU. Wafer-level thin-film encapsulation approach is not compatible with three dimensional MEMS structures due to a limited thickness of a sacrificial layer, which defines the height of the microcavity. Packaging approaches based on hermetic wafer-to-wafer bonding techniques successfully address the most of the packaging needs of the flat MEMS devices. However, in order to use these approaches for packaging of three dimensional MEMS structures, sophisticated fabrication steps are required, such as etching deep cavities in the handle Si/glass wafers (up to 10 mm deep). In order to use a standard package for packaging three dimensional MEMS structures, significant modification in package geometry is needed. This includes the increased depth of the package cavity (up to 10 mm deep). Using a customized package can result in increased cost of the final product.

(16) It is therefore desirable to develop a new hermetic packaging method for three dimensional MEMS devices, such as for folded MEMS TIMUs. Our approach utilizes a microglass-blowing process for wafer-level packaging of three dimensional MEMS devices, based on bonding a glass cap wafer with fabricated bubble-shaped glass structures to the handle glass/Si wafer. Metal traces fabricated on the handle wafer serve to transfer signals from the sealed cavity of the bubble to the outside world. The illustrated embodiments of the present invention also provide a method for chip-level packaging of MEMS three dimensional devices, using bubble-shaped glass lids and standard commercially available type of packages.

(17) Consider first wafer-level vacuum packaging of MEMS three dimensional devices. We introduce a new method for wafer-level vacuum packaging of MEMS three dimensional devices. Our approach is based on using a micro glass-blowing technique to fabricate the glass bubble-shaped structures on wafer level. Three dimensional MEMS devices 10 are hermetically sealed by bonding a glass cap wafer 12 with bubble-shaped structures to the handle glass/Si wafer, as seen in FIG. 3. The packaging process involves four steps: a. bubble-shaped glass structures 12 are fabricated on wafer-level, using glassblowing process with sealing rings 18 between structure 12 and metal traces 14 or wafer 16 below; b. metal traces 14 are deposited on the substrate wafer 16; c. three dimensional MEMS devices 10 are attached to the substrate wafer 16, using flip-chip bonding with interconnects 20 to device 10; d. cap wafer with bubble-shaped structures 12 is bonded to the substrate wafer 16.

(18) Wafer-level fabrication process for the bubble-shaped glass structures 12 starts with patterning a 1 mm thick silicon wafer 32 with a layer of photoresist 22, as illustrated in FIG. 4a. 700 m deep cavities 24 are then etched using deep reactive ion Etching (DRIE), as seen in FIG. 4b. The photoresist 22 is removed with acetone and a 100 m thin borosilicate glass wafer 26 is anodically bonded to the silicon wafer 32, as seen in FIG. 4c. This is followed by deposition and pattering a metal hard mask 28 for the subsequent Si wet etching process, as seen in FIG. 4d. Gold, chromium or titanium can be used as masking material. Next, the bonded wafers are placed inside a furnace at 350 Torr pressure and at an 850 C. temperature (above the softening point of the glass). Under the high temperature, the pressure of the trapped air inside the cavities 24 increases and the glass deforms into spherical shapes 12, as seen in FIG. 4e. The samples are then quickly removed from the furnace and cooled down to the room temperature. Once glass-blowing step is complete a 300 m layer of the silicon handle wafer 32 is removed, using wet etching process and the metal etch mask 28 defined at the previous step, as seen in FIG. 4f. Ethylenediamine pyrocatechol (EDP)-based etchant, or KOH-based etchant can be used for this purpose. This is followed by deposition of 5000 thick gold layer 30 on top of the 500 thick chromium adhesion layer 28. Metal is then patterned to define a seal ring 18 for subsequent wafer bonding.

(19) An analytical model developed in the art was used to predict the shape of the bubble 12. Glass-blowing takes place inside the furnace at 350 Torr pressure and 850 C temperature. The predicted geometry of the bubbles 12 for different diameters of the etched cavities 24 are summarized in Table 1. FIG. 5 shows a cavity 24 diameter of d.sub.0, a height hg of the bubble 12, a substrate wafer 16 thickness of he and ring seal 18 thickness b.sub.0.

(20) TABLE-US-00001 TABLE 1 Predicted height of the blown structures. Diameter of a cavity, Height of a blown d0, mm bubble, hg, mm 8.6 6.2 9.6 6.5 10.6 6.8 11.6 7.0

(21) A process flow diagram for the substrate wafer 16 is shown in FIG. 6. First, a 500 adhesion layer of chrome and a 5000 thick gold layer, collectively denoted by reference numeral 34, is deposited on the glass wafer, as seen in FIG. 6a. Metal layers 34 are then patterned in order to define the interconnection traces features, as seen in FIG. 6b. Metal traces serve to transfer signals from the sealed cavity of the bubble 12 to the outside world. A silicon nitride layer 36 is than deposited to provide electrical insulation, as seen in FIG. 6c. Next, we deposit another 500 adhesion layer of chrome and 5000 layer of gold, collectively denoted by reference numeral 38 and pattern these layers in order to define the seal ring features 18, as seen in FIG. 6d.

(22) Once fabrication process is complete, the three dimensional MEMS devices 10 are attached to the substrate wafer 16, using flip-chip bonding and hermetically sealed by bonding a cap wafer with the bubble-shaped structures 12 to the substrate wafer 16. Sealing can be performed in vacuum, using gold eutectic bonding. Thin-film getter layer can be deposited inside the bubbles 12 in order to maintain a high vacuum inside a hermetically sealed cavity.

(23) Consider a new type of lid for chip-level vacuum packaging of MEMS three dimensional devices 10. In a chip-level vacuum packaging method in accordance with the illustrated embodiments of the present invention, a micro glass-blowing technique is utilized to fabricate the glass bubbles 12 on a wafer level. Three dimensional MEMS devices 10 are hermetically sealed, using glass bubble shaped lids 12 and standard commercially available packages. Bubble-shaped glass lids 12 fabrication process is similar to the one used for the wafer-level packaging and has been described above in connection with FIGS. 4a-4f. Packaging of MEMS three dimensional device 10 is performed by gold eutectic bonding in vacuum of the fabricated lids 12 to the standard package, such as an LCC package, DIP package, etc. Following the process flow described in FIGS. 4a-4f, the bubble-shaped glass structures 12 for chip-level vacuum packaging of MEMS three dimensional structures were successfully built. In a first step, 1 mm thick double-sided Si wafers 32 were spin-coated with an AZ P4620 photoresist 22 and the mask for the DRIE etch was photolithographically defined, similar to that shown in FIG. 4a. In the next step, 700 m deep cavities 24 were DRIE etched, like that seen in FIG. 4b. A 100 m thin borosilicate glass wafer 26 was then anodically bonded to the silicon wafer 32, covering all the etched cavities 24, like that shown in FIG. 4c. In the anodic bonding procedure, the wafers 26, 32 were assembled together and heated on a hotplate to about 400 C. The bonding profile is shown in the graph of FIG. 8. When an electric field is applied across the assembly, the current in the circuit starts increasing indicating that the bonding process has started. The current reaches 1.6 mA at a temperature of 324 C. and then starts decreasing indicating that the bonding is complete. FIG. 9 is a photograph of a batch which illustrates the stack of the glass-Si wafers as would be seen at FIG. 4c. A 500 seed layer of Cr followed by 2 m thick layer of gold was deposited and patterned on the Si side of the glass-to-Si wafer stack, resulting in the stack as shown by the photograph of FIG. 10. Next, the bonded wafers were placed inside a furnace at an 850 C. temperature (above the softening point of the glass). The samples were blown at pressure in the range of 134 Torr-262 Torr, FIG. 11. The height of the glass-blown lids exceeded 10 mm for blowing at 134 Torr pressure. The samples blown at lower pressure break during the cooling process. Once the blowing process was complete, the 300 m Si handle wafer 32 was etched in order to open the bubble cavity 12, like that shown in FIG. 4f. Samples with a bubble height less than 6.8 mm were successfully etched, using KOH solution. Samples with larger height bubbles break during KOH etch. A deep reactive ion etch (DIRE) was performed on these samples to open the bubble cavity. A 500 seed layer of Cr followed by 2 m thick layer of gold 38 was then deposited using a shadow mask. Gold pattern defines the seal ring 18. A 50 mm.sup.3 three dimensional MEMS TIMU 10 was then packaged, using the LCC standard package and the bubble shaped glass lid, as shown in the photograph of FIG. 12. The diameter of the bubble exceeds 10 mm.

(24) Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the embodiments. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the embodiments as defined by the following embodiments and its various embodiments.

(25) Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the embodiments as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the embodiments includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. A teaching that two elements are combined in a claimed combination is further to be understood as also allowing for a claimed combination in which the two elements are not combined with each other, but may be used alone or combined in other combinations. The excision of any disclosed element of the embodiments is explicitly contemplated as within the scope of the embodiments.

(26) The words used in this specification to describe the various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.

(27) The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of subcombination.

(28) Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.

(29) The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the embodiments.