Abstract: A wafer level package for an optical microelectromechanical system (MEMS), and method of forming the same are provided. Generally, the package includes a fused silica window overlying the optical MEMS, and a spacer interposed between the window and a surface of the substrate on which the optical MEMS is formed, the spacer laterally surrounding the optical MEMS. A first preform between an attachment-surface of the substrate and a first attachment-surface of the spacer, and a second preform between a second attachment-surface of the spacer and an attachment-surface of the window form a hermetically sealed cavity enclosing the optical MEMS. The spacer is a nickel-cobalt alloy having a coefficient of thermal expansion between that of the substrate and window, and has a cross-section to enable movement between the window and substrate due to differences in thermal expansion without compromising the hermetically sealed cavity.