To meet demands for smaller size and higher performance regarding high-frequency or radiofrequency (RF) component parts of cellular phones and the like, researches and developments of electric equipment using MEMS technologies are being vigorously carried out. In MEMS devices intended for radiofrequency uses, an RF switch, a variable capacitance or the like employs a movable portion that has a cantilever structure or a both-ends-supported beam (bridge shape) structure that is formed from a metal material structure of low resistance. The movable portion is displaced by piezoelectric drive, electrostatic drive, etc. For performing a desired function, the position of the movable portion should be stably controlled.
An MEMS switch is a mechanical switch that has a static or stationary electrode and a movable electrode facing each other and that performs on-off actions by driving or actuating the movable electrode to contact to or separate from the static electrode. The mechanical switch allows reduction in parasitic capacitance, and is low in loss, high in insulation and less in distortion in signal waveform, in comparison with switches that use semiconductor elements.
In a radiofrequency (RF) circuit, an MEMS capacitance is connected in series to or loaded on a RF line so as to define the frequency characteristic or adjust the distributed constant of the RF line. By using a variable capacitance, it is possible to change the resonance frequency or change the distributed constant. Generally, a variable-capacitance element has such a structure that a fixed electrode and a movable electrode face each other and the capacitance between the electrodes is changed by displacing the movable electrode.
Such a movable electrode is formed of a flexible metal structure that is formed by, for example, plating. Electrolytic plating requires an electricity feeding layer. For example, a stack of an adherence layer that provides adherence with a base and a seed layer made of the same material as a plate layer is formed by sputtering or the like. The adherence layer is formed of, for example, a metal layer of Ti, Cr, Mo, etc. The plate layer is formed, for example, of a highly electroconductive metal such as copper (Cu), gold (Au), etc.
In order to provide for a free space below the flexible metal structure, a method including forming a sacrificial film in a free space, forming a metal structure on top of the sacrificial film, and then removing the sacrificial film is employed. The sacrificial film that is used in this method may be, for example, a metal film of copper, aluminum, etc., an inorganic dielectric film of silicon oxide, silicon nitride, aluminum oxide, etc., or an organic dielectric film of a photosensitive resin, etc.
For example, in order to form a cantilever type movable electrode on a ceramics substrate, a pedestal portion is first formed by processes of forming an adherence layer/seed layer by sputtering or the like, forming a structure that defines a plating region by using a resist pattern or the like, forming a pedestal metal layer by electrolytic plating process, removal of a structure such as the resist pattern or the like, removal of the adherence layer/seed layer that is unnecessary, etc., and a sacrificial film that fills a free space is formed through formation of an adherence layer/seed layer by sputtering or the like, formation of a structure that defines a plating region through the use of a resist pattern or the like, formation of a sacrificial metal layer by an electrolytic plating process, removal of the structure of the resist pattern or the like, etc., and a cantilever structure is formed through formation of an adherence layer/seed layer by sputtering or the like, formation of a structure that defines a plating region through the use of a resist pattern or the like, formation of a movable beam portion metal layer by an electrolytic plating process, removal of the structure of the resist pattern or the like, etc. Thereafter, the sacrificial film and the unnecessary adherence layer/seed layer are removed, so that the cantilever type movable electrode is formed.
The cantilever structure, which is mainly formed from a good conductor such as gold or the like, includes an adherence layer and a seed layer as a base. A metal layer formed by sputtering and a metal layer formed by plating are different from each other in purity and the like and exhibit different physical properties even if the two layers are of the same metal. A stack of metal layers having different physical properties is a stack of metal layers whose thermal expansivities are different.
An electric equipment is subjected to a reflow step at about 260° C. or a temperature impact test at −20° C. to +80° C. Due to the different thermal expansivities, stress occurs between the stacked metal layers, and warpage or strain occurs. For example, a distal end of the cantilever structure warps and becomes displaced upward. In some cases, the distal end of the cantilever structure is displaced upward by 10 μm or more. As a result, the electric equipment fails to operate at a predetermined operating voltage. A cause of such warpage is considered to be that the structure is constructed of a stacked layer structure of layers of different metal materials. For example, the adherence layer and the plated layer are formed of different metal layers.
For example, the adherence layer may have a higher resistivity than the plated layer. If a contact surface of a switch is covered with an adherence layer, high resistance results. In order to reduce such resistance, it has been proposed to remove the adherence layer from the contact surface of a switch (e.g., Japanese Patent Application Publication No. 2007-196303 (JP 2007-196303 A) and Japanese Patent Application Publication No. 2009-252672 (JP 2009-252672 A)).