Method for forming micromachined structure

The invention provides a method of fabricating a micromachined structure, and in particular to a method of forming a micro-electro-mechanical system (MEMS) structure. A thin silicon cantilevered or suspended structure used to make micromachined structures is first formed from a SOI wafer or a bulk silicon wafer, followed by formation of the micromachined structures by semiconductor manufacturing techniques.

BACKGROUND

The present invention relates to a method of forming a micromachined structure, and in particular to a method of forming a micro-electro-mechanical system (MEMS) structure.

In recent years, micromachined devices, such as micro-mirror devices, microswitches, microactuators and the like, having small movable structures, have been gradually developed in many applications. Particularly, in the case of forming micromachined devices using technologies for semiconductor integrated circuits, such as those which include photolithographic processing, micromachined structures can be reproduced accurately. Thereby, the micromachined structures can be arrayed easily on a substrate, produced at low cost, and respond quicker than structures produced by prior techniques because of their reduced size.

Generally, movable micromachined structures can be actuated by electrostatic force, magnetic force or Van der Waals' force, etc., depending on choice of shapes and compositions. The micromachined structures can be realized by bulk micromachining or surface micromachining process using semiconductor IC manufacturing techniques of mass production at low cost and miniature size.

While micromachined structures are formed from a processing wafer or layer, the processing wafer or layer should be thin enough for the micromachined structures to respond quickly. However, it is noted that such thin processing wafers or layers are usually floppy and fragile.

SUMMARY

Accordingly, the invention provides a method of forming a micromachined structure, and in particular a method of forming a micro-electro-mechanical system (MEMS) structure.

The invention provides a method for forming a micromachined structure, comprising: providing a substrate, such as a wafer, having an intermediate layer interposed between a first layer and a second layer, patterning a free surface on the second layer, adhering the patterned free surface of the second layer to a first handle substrate, such as a first handle wafer, via a first adhesive layer, removing the first layer, removing the intermediate layer to expose a surface of the second layer, releasing the first adhesive layer and the first handle substrate from the patterned free surface of the second layer, bonding the patterned free surface of the second layer to a substrate, such as a wafer or a glass substrate, with integrate circuit devices, so as to form the micromachined structure thereon.

The invention also provides a method for forming a micromachined structure, comprising: providing a processing substrate, such as a wafer, providing a handle substrate, such as a handle wafer, adhering the processing substrate to the handle substrate via an adhesive layer, thinning the processing substrate, patterning a free surface on the processing substrate, bonding the patterned free surface of the processing substrate to a supporting substrate, such as a wafer or a glass substrate, with integrate circuit devices, releasing the adhesive layer and the handle substrate from the processing substrate, so as to form the micromachined structure thereon.

DESCRIPTION

The invention discloses a method of forming a micromachined structure, and in particular to a method of forming a micro-electro-mechanical system (MEMS) structure.FIGS. 1A˜1Fdemonstrate a method known to the inventor of forming a thin suspended structure which can be used to make micromachined structures. This is not prior art for the purposes of determining the patentability of the invention. This merely shows a problem found by the inventor.

As shown inFIG. 1A, a silicon-on-insulator (SOI) wafer100having an insulator layer104interposed between a first silicon layer102and a second silicon layer106is first provided. The second silicon layer106has a free surface108and a substantially co-planar surface110abutting the insulator layer104. The insulator layer104may comprise an oxide layer, such as silicon oxide.

Referring toFIG. 1B, the free surface108of the second silicon layer106is patterned by photolithography and etching techniques known to those skilled in the art, such that recesses112are formed in the patterned free surface108′ of the second silicon layer106′ of the SOI wafer100′.

InFIG. 1C, a substrate120, such as a wafer or glass substrate with integrated circuit devices114or the like therein or thereon, is provided. The patterned free surface108′ of the silicon layer106′ is then bonded to the substrate120, using an appropriate method such as surface-active bonding (SAB), anodic bonding, adhesives, heat bonding, or any other suitable means.

After bonding, the first silicon layer102of the SOI wafer100′ located away from the substrate120is thinned by grinding or polishing, as illustrated inFIG. 1D. Since the second silicon layer106′ must be thin enough to form a micromachined structure, cracks may occur therefrom due to external mechanical forces applied during grinding or polishing.

Etching process is then performed to remove the remaining grinded or polished first silicon layer102′, using the insulator layer104as an etch stop layer, as illustrated inFIG. 1E. Etching may be accomplished by suitable etching techniques known to those in the art, such as reactive ion etching (RIE).

Next, the insulator layer104is removed, leaving the second silicon layer106′ with recesses112bonded to the substrate120, as shown inFIG. 1F. In this case, the insulator layer104, an oxide layer, may be removed by Buffered Oxide Etch Solution (BOE) etching. The second silicon layer106′, a thin silicon cantilevered or suspended structure, can make micromachined structures using technologies for semiconductor integrated circuits, such as photolithographic and etching process. Depending on choices, the thin silicon cantilevered or suspended structure acts as a beam member, mirror element or the like for the micromachined devices that may be actuated by the integrated circuit devices114or the like.

In the above method, the micromachined structures are fabricated from a SOI wafer, combining the bonding process with subsequent grinding or polishing, such that the thin silicon cantilevered or suspended structure may crack at wafer edge due to external mechanical forces applied by the grinding or polishing process, causing a large amount of yield loss.

Accordingly, the invention provides a method of forming a micromachined structure, and in particular to a method of forming a micro-electro-mechanical system (MEMS) structure. A processing substrate, such as a SOI wafer, a bulk silicon wafer or stacked layers, having a thin suspended structure for the micromachined structure is thinned by grinding or polishing before the wafer having the thin suspended structure is bonded to a substrate, such as a wafer or glass substrate, with integrated circuit devices.

FIGS. 2A˜2Hare a series of schematic cross-sections of a method of forming a thin suspended structure from a SOI wafer according to a preferred embodiment of the invention, whereby the problem of cracking is avoided.

As shown inFIG. 2A, a silicon-on-insulator (SOI) wafer200having an insulator layer204interposed between a first silicon layer202and a second silicon layer206is first provided. The second silicon layer206has a free surface208and a substantially co-planar surface210abutting the insulator layer204. The insulator layer204may comprise an oxide layer, such as silicon oxide.

Referring toFIG. 2B, the free surface208of the second silicon layer206is patterned by photolithography and etching techniques known to those skilled in the art, such that recesses212are formed in the patterned free surface208′ of the second silicon layer206′ of the SOI wafer200′. The second silicon layer206′ with recesses212acts as a beam member, mirror element or the like of micromachined devices, which should be thin sufficiently.

InFIG. 2C, a first handle wafer220, such as a glass wafer, is provided. The first handle wafer220is then adhered to the patterned free surface208′ of the second silicon layer206′ of the SOI wafer200′ via a first adhesive layer222such as hot-melt glue. Furthermore, an optional protective layer224may be disposed between the patterned free surface208′ of the second silicon layer206′ and the first adhesive layer222to protect the second silicon layer206′ from damage by external mechanical forces. The optional protective layer224may comprise a photoresist layer.

The first adhesive layer222can also provide protection from damage caused by external mechanical forces, such that the protective layer224is not necessary.

After the first handle wafer220adheres to the patterned free surface208′ of the second silicon layer206′, the first silicon layer202of the SOI wafer200′ located away from the first handle wafer220is thinned by grinding or polishing, as illustrated inFIG. 2D. The first adhesive layer222and the optional protective layer224can protect the thin patterned second silicon layer206′ used to make a micromachined structure away from cracking during the grinding or polishing process.

Etching process is then performed to remove the remaining ground or polished first silicon layer202′, using the insulator layer204as an etch stop layer, as illustrated inFIG. 2E. Etching may be accomplished by suitable etching techniques known to those in the art, such as reactive ion etching (RIE).

Next, the insulator layer204may be removed to expose the surface210of the second silicon layer206′, as shown inFIG. 2F. In this case, the insulator layer204, an oxide layer, may be removed by Buffered Oxide Etch Solution (BOE) etching.

A second handle wafer230, such as a glass wafer, adheres to the surface210of the second silicon layer206′ via a second adhesive layer232, such as hot-melt glue, at a temperature higher than the step of adhering the patterned free surface208′ of the second silicon layer206′ to the first handle wafer220via the first adhesive layer222, such that the step of adhering the surface210of the second silicon layer206′ to the second handle wafer230via the second adhesive layer232may be accomplished simultaneously with the step of releasing the first adhesive layer222from the patterned free surface208′ of the second silicon layer206′, as shown inFIG. 2G.

The second handle wafer230may provide support for the floppy and fragile second silicon layer206′ bonding to a wafer240with integrated circuit devices214or the like therein or thereon. The bonding process may comprise an appropriate method such as surface-active bonding (SAB), anodic bonding, adhesives, heat bonding, or any other suitable means. In addition, the second adhesive layer232and the second handle wafer230are removed from the wafer240easily and cleanly by a heat treatment, as shown inFIG. 2H.

The second silicon layer206′ with recesses212, a thin silicon cantilevered or suspended structure, can be used to make micromachined structures using technologies for semiconductor integrated circuits, such as photolithographic and etch process. Depending on choices, the thin silicon cantilevered or suspended structure acts as a beam member, mirror element or the like of the micromachined devices that may be actuated by the integrated circuit devices214or the like.

In view of the extreme fragility of such thin second silicon layer206′, the second silicon layer206′ must be handled with great care before bonding to the substrate240. The embodiment of the present invention inverts the bonding process and grinding or polishing process, such that problems of cracking at wafer edge can be avoided easily.

FIGS. 3A˜3Eare a series of schematic cross-sections of a method of forming a thin suspended structure from a silicon wafer according to another embodiment of the invention, whereby the problem of cracking is avoided.

As shown inFIG. 3A, a processing wafer300, such as a bulk silicon wafer of conventional thickness, is adhered to a handle wafer304via an adhesive layer302. The handle wafer304may comprise a glass wafer. The adhesive layer302may comprise hot-melt glue. The processing wafer300is then thinned by conventional techniques, such as a wet etching process with a suitable etchant, a dry etching process with a reactive gas, or a grinding or polishing process, to a thickness suitable for fabricating micromachined structures, as shown inFIG. 3B.

The thinned processing wafer300′, having a free surface306and a substantially co-planar surface308abutting the adhesive layer302, is supported by the handle wafer304to avoid cracking.

Referring toFIG. 3C, the free surface306of the thinned processing wafer300′ is patterned by photolithography and etching techniques known to those skilled in the art, such that the processing wafer300″ with recesses310and patterned free surface306′ is obtained. The patterned processing wafer300″ acts be as a beam member, mirror element, or the like of micromachined devices, which should be sufficiently thin.

InFIG. 3D, the patterned free surface306′ of the processing wafer300″ is bonded to the supporting wafer312with integrated circuit devices314or the like therein or thereon. The bonding process may comprise an appropriate method such as surface-active bonding (SAB), anodic bonding, adhesives, heat bonding, or any other suitable means. In addition, the adhesive layer302and the handle wafer304are removed easily and cleanly by a heat treatment, such that the surface308of the patterned processing wafer300″ is exposed, as shown inFIG. 3E.

The patterned processing wafer300″, a thin silicon cantilevered or suspended structure, can be used to form micromachined devices using technologies for semiconductor integrated circuits, such as photolithographic and etching process. Depending on choices, the thin silicon cantilevered or suspended structure acts as a beam member, mirror element or the like of the micromachined devices that may be actuated by the integrated circuit devices314or the like.

In view of the extreme fragility of the thin patterned processing wafer300″, the thin patterned processing wafer300″ must be handled with great care before bonding to the supporting wafer312. Accordingly, the embodiment of the invention provides a method of forming a micromachined structure, such that problems of cracking at wafer edge can be avoided easily.