Adhesive bonding and miscellaneous chemical manufacture – Delaminating processes adapted for specified product – Delaminating in preparation for post processing recycling step
Patent
1992-03-20
1993-11-16
Powell, William A.
Adhesive bonding and miscellaneous chemical manufacture
Delaminating processes adapted for specified product
Delaminating in preparation for post processing recycling step
29622, 156653, 156656, 156657, 1566591, 156662, 437182, H01L 21306, B44C 122, C03C 1500
Patent
active
052620000
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a micromechanical switch, and to a method of making such a switch. In particular, the invention is concerned with the fabrication of micromechanical beams, bridges and torsion elements for use in optical switches and modulators.
Silicon based micromechanical switches, which incorporate micromachined deflecting beams, bridges or torsion elements (switch elements) are known. Typically, this type of device is formed by etching a switch element from monocrystalline silicon, the etching process being such as to form a cavity or well beneath the switch element. Electrodes are then added (or formed in the monocrystalline silicon) for controlling the switch element. When making mirror switches or modulators, the surface of the switch element is provided with a coating of reflective metal. Such switches offer significant advantages compared with conventional switches, these advantages arising from their small size, fast response and negligible aging effects. Moreover, they can be manufactured by techniques that are compatible with standard integrated circuit (IC) processing methods, and so offer the potential of batch processing and of integration with associated electronic circuitry.
For some optical switching applications, it is advantageous to use larger switch elements than those which can be made by known micromachining techniques (typically 30-75 .mu.m). Unfortunately, fabrication problems arise as the size of the switch element is increased. Thus, if the switch element has a dimension greater than 300 .mu.m, a very high degree of etching selectively is required. This is because the region to be protected from etching is coated with an etch-resistant masking layer as thin as 0.1 to 0.2 .mu.m, and the material beneath the switch element (which is to be undercut) must be completely eroded. This calls for a long over-etch, which needs a selectivity of more than 3000:1 between the undercut region and the etch mask. To give some idea of this constraint, Si.sub.3 N.sub.4 is usually used as a protective mask against the usual oxide etchant (buffered HF), but the Sio.sub.2 : Si.sub.3 N.sub.4 etch ratio is only 50:1. Similarly, where an anisotropic Si etchant is used to etch {100} planes quickly, whilst "etch-stopping" on {111} planes, selectivity is typically 50:1 for ethylene diamine pyrocatechol (EDP) and water and between 100: 1 and 300:1 for KOH.
Moreover, it is important that the edges of the cavity formed beneath the switch element are well defined during the undercut etching process. Thus, if an undercut of several hundred .mu.m is to be formed (which is necessary for large area switch elements), a lateral etch stop layer is required around the cavity edges to prevent the cavity being enlarged by several hundred .mu.m per side. Since some enlargement is tolerable, a selectivity of greater than 100:1 is adequate here. However, even this degree of etch selectivity is difficult to achieve.
Another problem that can arise with the manufacture of mirror switches/modulators is that the metal reflectivity can be affected by the etching process. In order to overcome this problem, the metal may be passivated (i.e. coated with non-etching material) prior to the deep undercut etch step. This passivating layer must be formed by a low temperature process to avoid thermal damage to the metal film and stress damage to the switch element. Unfortunately, low temperature passivating layers have a poor etch resistance, so it is difficult to prevent the metal reflectivity being affected.
When the switch element of such a pattern is controlled electronically, very high voltages may be required in which case it is important to obtain good isolation/breakdown strength between the control electrodes. If the switch element is a torsion element, this is very difficult to achieve monolithically. Furthermore, it is difficult to prevent the torsion bars of such a torsion element from deflecting under the forces used to control the element.
The aim of the invention is to provide a method of making micromechanical s
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patent: 4624741 (1986-11-01), Daniele
patent: 4674180 (1987-06-01), Zavtacky et al.
patent: 4783237 (1988-11-01), Aine et al.
patent: 4805038 (1989-02-01), SeLigson
patent: 4849070 (1989-07-01), Bly et al.
International Electron Devices Meeting 1986, Los Angeles pp. 184-187; S. Sugiyama: "Micro-diaphragm pressure sensor".
McLaughlin Judith C.
Welbourn Anthony D.
British Telecommunications public limited company
Powell William A.
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