Micro-relay contact structure for RF applications

Details Micro-relay contact structure for RF applications Micro-relay contact structure for RF applications

2000-11-09

2003-07-01

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Long line elements and components

Switch

C333S105000, C200S181000, C335S078000

Reexamination Certificate

active

06587021

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to electrical and electronic circuits and components and more particularly to micro-electromechanical (MEM) relays for radio frequency (RF) applications.
BACKGROUND OF THE INVENTION
A MEM relay is an electrical micro-relay operated by an electrostatic charge, magnetic, piezoelectric or other actuation mechanism and manufactured using micro-electromechanical fabrication techniques. A MEM relay uses an electrically activated structure to mechanically close a set of electrical contacts. A MEM relay can be used to control RF signal flow in a wide range of electronic applications including telecommunications applications.
Current efforts in designing RF micro-relays or RF switches using MEM fabrication techniques concentrate on the actuator design and on the closed circuit RF characteristics of the RF signal path. The open circuit isolation of the signal path is partially determined by the physical separation of the contact structure, and is an uncontrolled parameter in most MEM microrelay structures. Additionally in some applications, conventional MEM relay designs do not provide sufficient isolation between the actuation mechanism and the signal contact structure of the MEM relay operating at radio frequencies. A problem occurs when large RF voltages on the signal contact structure activate or self-bias the electrostatic actuator or other high impedance actuator and causing the micro-relay to become uncontrollable. Also, large control signals on the actuator structure can couple onto the signal contact structure and as a result, disrupt or interfere with the flow of very weak signal currents.
The electrical isolation between the signal path and the actuation control path in a MEM relay distinguishes the MEM relay from a MEM switch. The term “RF Micro-relay” is normally used to designate a
4
terminal device with two terminals used for an actuation function and two terminals used to control the flow of the RF signal in an external circuit. The term “RF MEM switch” is so used interchangeably with RF micro-relay. However, the RF MEM switch function may also include a condition where the actuation process and the signal control process have one or more common elements, such as a common ground. The “RF MEM switch” could then be a two terminal device or a three terminal device as well as a four terminal device. The term “micro-relay” will be applied to the 4 terminal device, and the contact structure within the micro-relay used to control the RF signal flow in an external circuit will be referred to as contacts.
Conventional MEM fabrication technology tends to limit the type of contact metals and shapes that can be supported. The contacts fabricated in the conventional manner tend to have lifetimes in the millions of cycles or less. One of the problems encountered is that microscale contacts on MEM devices tend to have very small regions of contact surface (for example 5 micrometers by 5 micrometers). The portion of the total contact surface that is able to carry electrical current is limited by the microscopic surface roughness and the difficulty in achieving planar alignment of the two surfaces making mechanical and electrical contact. Furthermore, most conventional MEM switches or relays have only one contact set. The contacts that would seem to have hundreds or thousands of square micrometers of contact surface available are actually multiple small point contacts with a much smaller equivalent contact surface area. The high current densities in these small effective contact regions create microwelds and surface melting, resulting in impaired or failed contacts. Such metallic contacts tend to have relatively short operational lifetimes, usually in the millions of cycles.
A MEM contact structure can be fabricated using either surface micromachilling or bulk micromachining techniques including deep reactive ion etch (DRIE). Surface micromachining builds a MEM structure on the surface of a substrate by the proper combinations of depositing and etching MEM fabrication materials. The deposition and etching is usually based on a pattern needed to selectively obtain the desired end mechanical structure. State of the art surface micromachining requires a wet etching process and uses liquids in various stages of the fabrication and the releasing process. In the MEM manufacturing process, moving structures are created by depositing the desired material in a mold composed of sacrificial MEM fabrication material which defines the shape of the end movable structure. The sacrificial material is etched away as the final step in manufacturing, and this releases the movable portion of the MEM structure. Bulk micromachining builds the MEM structure within the substrate material but exposed at the substrate surface. The etching process can cut away portions of the substrate surface and body to form the MEM structure. The etching process can also undercut the structure. Undercutting the structure allows lateral motion in the full 2-dimensional surface plane of the substrate. The actual motion available depends on the design of the movable parts. Bulk micromachining also uses deposition and etching processes. Some methods of bulk micromachining also use wet processes. DRIE is a fully dry process of bulk micromachining. The use of liquids has been found to result in difficult cleaning requirements, contamination of the MEM device, and a problem of MEM operation known as “stiction”, a combination of stickiness and friction. Dry MEM fabrication processes are believed to be free of the stiction problem. DRIE creates high-aspect ratio, 3-dimensional structures in silicon, with thicknesses ranging from microns to hundreds of microns. DRIE allows micro-machined structures to be combined readily with CMOS electronics and devices constructed using traditional bulk and surface micro-machining techniques.
SUMMARY OF THE INVENTION
In view of the above problems and limitations of existing MEM relays and in accordance with the present invention, it would, therefore, be desirable to have a MEM relay having one or more of the following characteristics: a multi-point contact system using a self-aligning structure, improved and controllable open circuit isolation characteristics, electrostatic shielding between the actuator system and the RF signal switching system and electrostatic shielding between the relay signal contacts. It is also desirable to have a MEM relay which can be fabricated using a dry fabrication process such as DRIE or other dry bulk micro-machining techniques.
In accordance with an aspect of the present invention, the MEM relay includes a housing, a first signal contact in the housing, a second signal contact in the housing, a grounded electrostatic actuator shield in the housing forming a signal contact region and an actuator region and an aperture formed in the housing to connect the signal contact region and the actuator region. The MEM relay also includes an actuator, with an open and closed position connected to an actuator insulator that passes through the aperture and is connected to a movable shorting contact. The shorting contact can electrically connect the first signal contact to the second signal contact thereby completing the relay circuit. With such an arrangement the MEM relay has an electrostatic (Faraday) shield between the actuator system and the RF signal switching system that improves isolation between the signal contact structures and the actuator mechanism. The shield contributes to the open circuit isolation of the signal path. The electrostatic shield also prevents large RF voltages on the signal contact structure from activating or self-biasing an electrostatic actuator or other high impedance actuator and causing the micro-relay to become uncontrollable. The electrostatic shield will also prevent large control signals on the actuator structure from coupling onto the signal contact structure and disrupting or interfering with the flow of very weak signal currents. The electrostatic shield in alternate configurations can

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