Reduced formation of asperities in contact micro-structures

Details Reduced formation of asperities in contact micro-structures Reduced formation of asperities in contact micro-structures

2002-06-28

2004-11-02

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S231000, C359S248000

Reexamination Certificate

active

06813059

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to micro-structures which are configured to move. More particularly, the present invention is related to micro-structures which are configured to move and contact.
BACKGROUND OF THE INVENTION
A number of micro-machines utilize movable cantilevers, ribbon structures or other similar micro-structures. Typically, these micro-structures are extremely thin; on the order of hundreds or thousands of Angstroms, and are formed through release etch processes.
Optical MEM (micro-electro-mechanical) devices are used to modulate one or more wavelengths of light. Optical MEM devices can have applications in display, print and electrical device technologies. Examples of an optical MEM device which utilize suspended micro-ribbon structures to modulate light are disclosed in the U.S. Pat. Nos. 5,311,360, 5,841,579 and 5,808,797, all issued to Bloom et al., the contents of which are hereby incorporated by reference.
Briefly, an optical MEM device described in the above referenced patents has one or more sets of movable ribbons that comprise a support layer and a reflective top-layer. The support layer is preferably a silicon nitride layer and the reflective top-layer is preferably an aluminum layer. The ribbon structures are typically secured to a substrate through opposite ends of the ribbon, whereby center portions of the ribbons, referred to herein as the active portions, move up and down to modulate an incident light source.
The ribbons are configured to move by applying a bias voltage across the ribbons and the substrate. In order to effectively modulate light, the distance that the ribbons are moved must be controllable and reproducible. In one construction, an optical MEM device has ribbons that are suspended at a fixed distance from the substrate and the ribbons are selectively moved to contact the substrate. Unfortunately, this results in weld spots or surface artifacts generally referred to herein as asperities. Asperities which develop over time can greatly effect the ability of the ribbons to move in a controllable and/or reproducible manner and can cause the ribbons to stick to the substrate. There are many MEM devices, including cantilever devices and oscillator devices, where movable structures which contact the substrate, or other micro-structure, is advantageous if the formation asperities and sticking of the contacting parts can be avoided or reduced.
SUMMARY OF THE INVENTION
A device, in accordance with the embodiments of the invention, comprises one or more micro-structures suspended over a substrate. The micro-structures can be, but are not limited to, cantilevers, ribbons and combs structures configured to move relative to the substrate and contact a portion of the substrate. Preferably, the micro-structures are ribbons having lengths in a range of about 50 to about 500 microns and widths in a range of about 4.0 to about 40 microns and are configured to modulate light having one or more wavelengths in a range of about 300 to about 3000 nanometers.
In accordance with the embodiments of the invention, the substrate has a metal-insulator-metal construction comprising a lower metal layer and an upper metal layer with an insulator layer sandwiched therebetween. Preferably, the upper metal layer and the insulator layer are patterned with vias to expose portions of the lower metal layer and to provide contact regions for complimentary contact regions on the ribbons. In a preferred embodiment, the substrate has a metal-insulator-metal construction comprising titanium nitride metal layers and a silicon oxide insulator layer.
The ribbons have at least one metal under layer. The metal under layer comprises contact regions and non-contact regions. In accordance the with embodiments of the invention, the ribbons also have a metal-insulator-metal construction. For example, the ribbons have an under layer of titanium nitride, a top layer of aluminum and a silicon nitride insulator layer sandwiched therebetween.
In operation, a bias voltage is applied across selected micro-structures, or ribbons, and the upper metal layer of the metal-insulator-metal construction on the substrate. The lower metal layer is maintained at a reduced potential relative to the applied bias voltage and is preferably maintained at a zero, or near to zero, potential relative to the applied bias voltage. The bias voltage between the selected micro-structures and the upper metal layer of the metal-insulator-metal construction on the substrate urges the selected micro-structures to move towards the substrate and to contact the substrate. The micro-structures and substrate make contact through the contact regions in the lower metal layer of the metal-insulator-metal construction of the substrate and contact regions on the under layer of the micro-structures.
Contact regions of the micro-structures preferably protrude, such that the contact regions of the micro-structures insert, or fit, into the vias patterned through the upper metal layer and the insulator layer of the metal-insulator-metal construction of the substrate. Because the potential difference between the contact regions of the micro-structures and the contact regions of the substrate are minimized, or reduce, the formation of asperities and sticking of contacting parts is also minimized or reduced.
In accordance with the method of the invention, a micro-device is made by forming a substrate structure comprising a metal-insulator-metal construction. In accordance with the embodiments of the invention, a lower metal layer of titanium nitride is deposited to a thickness in range of about 200 to about 2000 Angstroms. Over the lower metal layer, a insulator layer of silicon oxide and an upper metal layer of titanium nitride are deposited to a thickness in a range of about 200 to about 2000 Angstrom. The insulator layer and the upper metal layer of the metal-insulator-metal construction are patterned with contact vias. The metal-insulator-metal construction can be patterned with the contact vias using selective deposition processes, etch process or a combination thereof.
Over the substrate, micro-structures, or ribbons, are preferably formed by first depositing a layer of poly-silicon and etching the layer of poly-silicon to form support regions for coupling a device layer to the substrate. The device layer is then formed over the patterned poly-silicon layer and is cut into ribbons with the appropriate dimension using any suitable process, such as a reactive ion etch process. After the device layer is cut into ribbons, the underlying poly-silicon is etched using any suitable processes, such as a xenon diflouride etch process to release the ribbons.


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