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Structure and method for releasing stressy metal filmsRelated Patent Categories: Stock Material Or Miscellaneous Articles, Structurally Defined Web Or Sheet (e.g., Overall Dimension, Etc.), Including Components Having Same Physical Characteristic In Differing DegreeStructure and method for releasing stressy metal films description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080095996, Structure and method for releasing stressy metal films. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This is a divisional of U.S. Application. No. 11/300,872 filed Dec. 15, 2005 by the same inventors and claims priority therefrom. This divisional application is being filed in response to a restriction requirement in that prior application. BACKGROUND [0002] Spring structures in MicroElectroMechanical systems (MEMS) have become increasingly important in a wide variety of applications. These applications include electronic packaging, test and measurement probing, integrated high quality factor inductors, electrical interconnects, fluid distribution and printing applications. [0003] Traditional methods of forming small spring structures have disadvantages. Often the fabrication process used to produce these springs produce sharp angled bends or "kinks" in the spring at or near the point where the spring lifts up from the substrate such as is shown in FIG. 6. When the angled bend is oriented in a direction that opposes the spring flex the angled bend serves as a weak point that is prone to failure with repeated use. [0004] Alternate fabrication techniques exist to avoid such bends. However, these alternative techniques use precisely controlled timing of etch rates. Etch rates depend on many parameters such as etchant concentrations, the exact composition and thickness of the layer being etched, and the spring geometry. Consistently reproducing the multiple parameters is difficult in a commercial production environment. [0005] Thus a method for forming spring structures that does not rely on timing to control etching and results in spring structures that do not form angled bends that are susceptible to breakage is needed. SUMMARY [0006] A method of forming a suspended structure is described. The method converts a region of a release layer to form an anchor of the suspended structure. A release layer is first deposited over a substrate. At least one region of the release layer is treated to alter the chemical structure such that an anchor region of the release layer is resistant to a selective etchant and a release region of the release layer is etchable by the selective etchant. The selective etchant is then used to perform an etch of the release layer such that the release region releases a release portion of an overlying layer, the release portion of the overlying layer to form a suspended structure, an anchor portion of the overlying layer remains attached to the anchor region. The method described is useful for forming various structures, but has particular application in the fabrication of spring structures. BRIEF DESCRIPTION OF THE DRAWINGS [0007] FIGS. 1-3 provide a schematic side view of an example lithography operation for forming a stressed metal spring. [0008] FIG. 4 shows a top view and FIG. 5 a side view of a spring fabrication method where the release layer is patterned prior to deposition of the spring material thereby allowing a portion of the spring material to adhere directly to an underlying substrate. [0009] FIG. 6 shows a side view of a spring fabricated using a release layer patterned prior to deposition of the spring material. [0010] FIG. 7 shows a top view and FIG. 8 a side cross sectional view of a spring material deposited over a continuous release layer where an anchor region of the continuous release layer has been altered to resist etching. [0011] FIG. 9 is a flow chart that describes an example procedure of forming a suspended structure such as a spring by oxidizing a portion of a continuous release layer. [0012] FIG. 10 shows an example side cross sectional view of a spring fabricated by converting an anchor region of a release layer. [0013] FIG. 11 shows a side cross sectional view of a spring material deposited over a continuous release layer where the anchor region of the continuous release layer expands when it is altered to resist etching. [0014] FIG. 12 shows a side cross sectional view of a spring fabricated using the method of FIG. 9 when an expansion of the release layer anchor region occurs. [0015] FIG. 13 shows a top view of an example membrane structure. [0016] FIG. 14 shows a side cross sectional view of the membrane structure of FIG. 13. DETAILED DESCRIPTION [0017] A planar self terminating release process for fabricating small suspended structures is described. The technique converts selected areas of a release layer to an etch resistant material. An overlying material is deposited over the release layer, the overlying material includes an anchor portion deposited over the converted etch resistant material. After etching the release layer, the etch resistant material serves as an anchor region that fixes an anchor portion of the overlying material to an underlying substrate. The remaining portion of the overlying material has been undercut and thus remains suspended. The described technique is particularly useful for forming spring structures. [0018] Micro-spring structures are typically formed by depositing a spring material over a release layer. As used herein, "spring material" is broadly defined as any material upon which a stress may be induced to cause the material to curl out of a plane. Typically, after curling out of the plane, the spring material is flexible and can apply a force approximately proportional to displacement over a small distance. Example spring materials include MoCr alloys sputtered at different ambient pressures to induce a stress gradient within the thickness of the material, Ni electroplated using different chemistries to produce internal stress differences between the different layers, layers of bi-morph materials that have different internal stresses, and layers of bi-morph materials that have different thermal expansion coefficients. One particularly useful spring material is a "stressed metal". As used herein, "stressed metal" is defined as a spring structure with an internal stress gradient. Stressed metals are typically formed by depositing multiple sublayers, each sublayer deposited at a different temperature or pressure such that differing atomic packing densities in each sublayer result in an the internal stress gradient. A detailed description of forming a stressed metal spring is provided in U.S. Pat. No. 6,528,350 entitled "Method for Fabricating a Metal Plated Spring Structure" by David Fork and U.S. Pat. No. 5,613,861 entitled "Photolithographically Patterned Spring Contact" by Smith et al. which is hereby incorporated by reference. [0019] FIGS. 1-3 provide a schematic side view of an example lithography operation for forming a stressed metal spring. In FIG. 1, a release layer 104 and a seed layer 108 are deposited over a substrate 100. Release layer 104 is selected to be a material that can be easily etched to "release" a spring material that is subsequently deposited over the release layer. In one embodiment, release layer 104 is a sputtered titanium (Ti) layer. Continue reading about Structure and method for releasing stressy metal films... 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