BRIEF DESCRIPTION OF THE DRAWINGS
The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
FIGS. 1-5 are side views of various intermediate structures produced by a method according to the present invention;
FIGS. 6A and 6B are side and bottom views of an alternate embodiment of the structure of FIG. 1;
FIG. 7 is a side view of another alternate embodiment of the structure of FIG. 1;
FIG. 8 is a side view of the structure of FIG. 5 that includes components applied according to an alternate embodiment of the present invention;
FIG. 9 is a block diagram of a method according to the present invention; and
FIG. 10 is a block diagram of an alternate method according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a side cross-sectional view of a structure 18 that includes a mechanism wafer 20, a metal layer 22, a metal or polymer layer 24, and a sacrificial (or handle) wafer 26. The mechanism wafer 20 and sacrificial wafer 26 may be standard single side polished silicon wafers. The metal layer 22 is produced by metallizing a bottom face 28 of the mechanism wafer 20, and the metal or polymer layer 24 is produced by metallizing or polymerizing a top face 30 of the sacrificial wafer 26. The faces 28, 30 with attached layers 22, 24 are bonded to each other using various bonding methods; bonding methods include low temperature thermal compression bonding for a metal to metal bond. The mechanism wafer 20 is thinned to a desired thickness using, for example, lapping and polishing.
FIG. 2 shows the structure of FIG. 1 after etching the mechanism wafer 20 to form various components of a MEMS device. The mechanism wafer 20 is masked and etched to the metal layer 22 (which may function as an etch stop), after which the mask is removed. Etching may include, for example, wet chemical etching selective to metal such as etching in tetramethyl-ammonium-hydroxide (TMAH) solution or in hydrazine solution. Etching may further include, for example, plasma etching using flourine or chlorine radicals. Plasma etching may include Deep Reactive Ion Etching (DRIE) to fabricate high aspect ratio structures in silicon. In the latter, the metal layer does not only function as an etch stop layer but also prevents lateral etching of the structure known as “footing” or “notching”.
FIG. 3 shows the structure of FIG. 2 after bonding the mechanism wafer 20 to a patterned silicon device substrate wafer 32. Bonding methods include low temperature, temporary silicon-to-silicon fusion bonding the mechanism wafer 20 to the device substrate wafer 32.
FIG. 4 shows the structure of FIG. 3 after selective etching of the sacrificial wafer 26 to the metal or polymer layer 24. Etching may be accomplished by using wet chemical solutions such as TMAH or hydrazine, or by using plasma etching with flourine or chlorine radicals DRIE. The metal or polymer layer 24 acts as an etch stop. Etching may include a normal silicon etch technique which stops on metal (wet or dry), or alternatively underetching the entire wafer 26 through perforations with a selective metal wet etch.
FIG. 5 shows the structure of FIG. 4 after removal of the metal and polymer layers 22, 24. Removal may be accomplished by metal etching in acidic solutions and by polymer etching using a solvent or in a plasma with oxygen radicals.
FIGS. 6A and 6B show side cross-sectional and top views, respectively, of an embodiment of a perforated sacrificial wafer 34 of the present invention. This embodiment may be substituted for the structure of FIG. 1. The perforated wafer 34 may be perforated by etching holes and trenches using DRIE with flourine radicals. The perforated wafer 34 allows removal of the metal or polymer layer 24 without destruction of the wafer 34. The layer 24 is removed by introducing etchant into the perforations of the wafer 34. The etchant is chosen such that it will not etch the wafer 34, but will etch the layer 24. The wafer 34 is released upon removal of the layer 24, and may be reused.
FIG. 7 shows an alternate embodiment of the structure of FIG. 1. A layer 36 is either metal or polymer, and replaces the metal and metal or polymer layers 22, 24 of FIG. 1.
FIG. 8 shows the structure of FIG. 5 after an optional additional layer 38 has been added to the structure 18. The layer 38 may be a capping wafer or an additional mechanism layer, or both, and may include silicon. Additional layers (not shown) may be attached.
FIG. 9 is a block diagram of a method 40 according to the present invention. At a block 42, a silicon-on-metal (SOM) wafer with an active layer, a sacrificial layer, a metal layer, and a metal or polymer layer is formed. At a block 44, the active layer is patterned and etched to form MEMS components, and the internal metal layer may be used as an etch stop. At a block 46, the patterned SOM wafer is bonded to a cover wafer. At a block 48, the sacrificial layer of the SOM wafer is removed. Finally, at a block 50, the metal layer and metal or polymer layer are selectively removed.
FIG. 10 is a block diagram of an alternate method 52 according to the present invention. At a block 54, a first substrate wafer is provided. At a block 56, a first metallic layer is precipitated on a first surface of the substrate wafer. At a block 58, a sacrificial layer is bonded to the first metallic layer. At a block 60, structures such as beams and trenches are formed in the first substrate wafer. At a block 62, a second substrate wafer (which may be patterned) is bonded to a second surface of the first substrate wafer. At a block 64, the metallic layer is dissolved which releases the sacrificial layer.
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
Patent Application
20080032501
