Etching a substrate: processes – Forming or treating electrical conductor article
Reexamination Certificate
1999-11-30
2001-12-18
Gulakowski, Randy (Department: 1746)
Etching a substrate: processes
Forming or treating electrical conductor article
C216S083000, C216S099000, C333S262000
Reexamination Certificate
active
06331257
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Technical Field of the Invention
The present invention relates generally to switches. More particularly, it relates to the design and fabrication of microfabricated electro-mechanical switches.
(b) Description of Related Art
In communications applications, switches are often designed with semiconductor elements such as transistors or pin diodes. At microwave frequencies, however, these devices suffer from several shortcomings. Pin diodes and transistors typically have an insertion loss greater than 1 dB, which is the loss across the switch when the switch is closed. Transistors operating at microwave frequencies tend to have an isolation value of under 20 dB. This allows a signal to ‘bleed’ across the switch even when the switch is open. Pin diodes and transistors have a limited frequency response and typically only respond to frequencies under 20 GHz. In addition, the insertion losses and isolation values for these switches varies depending on the frequency of the signal passing through the switches. These characteristics make semiconductor transistors and pin diodes a poor choice for switches in microwave applications.
U.S. Pat. No. 5,121,089 issued Jun. 9, 1992 to Larson, and assigned to the assignee of the present invention, discloses a new class of microwave switch—the micro-electro-mechanical (MEM) switch. The MEM switch has a very low insertion loss (less than 0.2 dB at 45 GHz) and a high isolation when open (greater than 30 dB). In addition, the switch has a large frequency response and a large bandwidth compared to semiconductor transistors and pin diodes. These characteristics give the MEM switch the potential to replace traditional narrow-bandwidth PIN diodes and transistor switches in microwave circuits.
The Larson MEM switch utilizes an armature design. One end of a metal armature is affixed to an output line, and the other end of the armature rests above an input line. The armature is electrically isolated from the input line when the switch is in an open position. When a voltage is applied to an electrode below the armature, the armature is pulled downward and contacts the input line. This creates a conducting path between the input line and the output line through the metal armature.
Rockwell International has also developed a MEM switch based on an armature design. The Rockwell switch uses a combination of insulating structural layers and metals as the armature, which increases the mechanical durability of the MEM switch, but the control of the mechanical characteristics, such as internal stress and elastic modulus of the insulating layer, is limited by stoichiometric control of silicon dioxide films. The Rockwell switch uses multiple contact points with flat metal contacts that are likely to have time-varying contact characteristics. In addition, the Rockwell switch is fabricated using an organic polyimide as a sacrificial armature support layer. This leaves organic residue on the switch surfaces after fabrication, which are difficult to remove and adversely affect switch performance and reliability.
Texas Instruments has developed a MEM switch based on a diaphragm configuration. The switch comprises of a flexible membrane supported between two posts. When a voltage is applied between the membrane and an electrode beneath the membrane, the membrane is drawn closer to the electrode by an electrostatic force. The closer together the membrane and electrode, the higher the capacitance between the two. High frequency signals are able to transmit through high capacitances and as such these switches do not need to make actual metallic contact in their “closed” position. High applied voltages are needed, however, to deform the membrane, and isolation characteristics at low frequency are very poor because of the inherent coupling capacitances of the switch structure. In addition, the Texas Instruments switch is highly dependent on membrane stress and on the fabrication process itself, such that the switch is susceptible to material creep and fatigue. Accordingly, there is a need for a MEM switch with a resilient structure and reliable mechanical and electrical contact characteristics.
SUMMARY OF THE INVENTION
The present invention relates to a method of design and fabrication of a micro-electro-mechanical switch. Two different fundamental switch structures are discussed and three different fabrication sequences are presented, for a total of six switch designs covered in this work. Every switch design has a number of important features in common. Each switch is designed with a bi-layer or tri-layer armature to give the switch superior mechanical qualities. In addition, the switches have conducting dimples with defined contact areas to improve contact characteristics. The switch is fabricated using improved materials and processes that prevent the armature from sticking to the substrate during fabrication and also ensure superior mechanical qualities and uniform contact properties of the switch.
One embodiment of the invention is a micro-electro-mechanical switch comprising an input line, an output line, and an armature. The input line and the output line are located on top of a substrate. The armature is made of at least one structural layer, a conducting transmission line on top of, below, or between the structural layers, and a suspended armature bias electrode similarly placed. One end of the structural layer is connected to the substrate, and a substrate bias electrode is located on top of the substrate below the suspended armature bias electrode on the armature.
A first end of the conducting transmission line is connected to the output line, and a second end rests above the input line when the switch is in an open position and contacts the input line when the switch is in a closed position. The conducting transmission line also contains a conducting dimple such that the distance between the conducting dimple and the input line is less than the distance between the conducting transmission line and the input line so that the conducting dimple contacts the input line when the switch is in the closed position. The structural layer may be formed below, above, or both above and below the conducting line. The input line, output line, armature bias pad, substrate bias pad, and substrate bias electrode are comprised of a stack of films referred to as the first metal layer which is comprised of a film of gold on top of a 1000 angstrom film of platinum on top of a 250 angstrom film of titanium. The armature bias electrode, suspended transmission line, and contact dimples are made of a film stack referred to as the second metal layer, which is identical to the first metal layer without the platinum film.
The present invention may also be embodied in a process for making a micro-electro-mechanical switch. The process comprises a first step of depositing a first metal layer onto a substrate to form an input line, an output line, substrate bias electrodes, substrate bias pads, and armature bias pads. A support layer, also known as a sacrificial layer, is deposited on top of the first metal layer and the substrate, and a beam structural layer is deposited on top of the sacrificial layer. The beam structural layer forms an armature with one end of the beam structural layer affixed to the substrate near the output line. The process further comprises the steps of removing a portion of the structural layer and a portion of the support layer to create a dimple mold. A conducting dimple is formed in the dimple mold when the conducting transmission line and suspended armature bias electrodes are fabricated by depositing a second metal layer, such that the suspended armature bias electrode is electrically connected to the armature bias pad. A second structural layer may or may not be deposited on top of the second metal layer for stress matching and thermal stability of the switch. Finally, the sacrificial layer is removed from beneath the armature to release the armature and allow the switch to open and close.
The materials and fabrication techniques use
Brown Julia
Foschaar James
Hsu Tsung-Yuan
Hyman Daniel J.
Loo Robert Y.
Ahmed Shamim
Duraiswamy Vijayalakshmi D.
Gulakowski Randy
Hughes Electronics Corporation
Sales Michael W.
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