The present invention relates generally to microfabrication methods. More particularly, the invention relates to methods for forming and positioning moldable permanent magnets on electromagnetically actuated microfabricated components, such as electrical switches. Micromachining is a recent technology for fabricating micromechanical moving structures. In general, semiconductor batch fabrication techniques are employed to achieve what is in effect three-dimensional machining of single-crystal and polycrystalline silicon and silicon dielectrics, producing such structures as micromotors, microsensors and switches. Thus, except for selective deposition and removal of materials on a substrate, conventional assembly operations are not involved.
Electromagnetically actuated micromechanical components include permanent magnets attached to a microfabricated moving structure. A separate electromagnet, appropriately energized, produces a controllable magnetic field which cooperates with the permanent magnet to cause the microfabricated element to move.
One example of an electromagnetically actuated microfabricated component is disclosed in an article by B. Wagner and W. Benecke, "Microfabricated Actuator with Moving Permanent Magnet" IEEE MEMS '91, Nara, Japan (1991). Another example is disclosed in commonly-assigned Ghezzo et al. U.S. patent application Ser. No. 08/000,172, filed Jan. 4, 1993, entitled "Micromechanical Moving Structures Including Multiple Contact Switching System, and Micromachining Methods Therefor."
A temporary structure, known alternatively as a release layer or as a sacrificial layer, is critical for micromachining because it allows moving parts to be formed by self-registered casting methods, with subsequent selective etching to remove the sacrificial layer. Since micromachining originated from the technology of silicon integrated circuit processing, low-temperature SiO.sub.2 is commonly employed as a sacrificial layer material. However, metals may also be employed for the sacrificial layer. For example, the use of a copper sacrificial layer is disclosed in the above-identified application Ser. No. 08/000,172.
The fabrication of such structures also requires that magnets somehow be attached to or formed on a movable element. In general, two approaches have previously been employed.
The first approach has been to attach magnets using pick-and-place equipment. This approach, however, has several disadvantages. In particular, the overall process is relatively time-consuming, in that suitable magnets must be purchased, and the magnets must be cut to a specific size, which is difficult with tiny elements. The magnets are then carefully placed and secured in position employing a suitable adhesive. Thus, while rare earth NdFeB magnets are effectively used in miniature components as small as 1 mm, they are difficult to machine, and in general are costly to produce.
The second approach has been to deposit magnetic material employing RF sputtering techniques. It has been observed, however, that crack-free rare earth transition metal films cannot be deposited with thicknesses greater than about 10 .mu.m. Such is reported for example in F. Cadieu, "Permanent Magnet Thin Films: A Review of Film Synthesis and Properties" Physics of Thin Films, M. Francombe and J. Vossen, Editors, Volume 16, Academic Press Inc., 1992. In addition, to obtain the desired physical properties, the films need to be heated to temperatures greater than 400.degree. C. While such may be possible in the case of silicon micromachining in general, such temperatures cannot be used in the presence of materials employed in high density interconnect (HDI) technology.
More particularly, what is known as high density interconnect (HDI) technology for multi-chip module packaging, is disclosed for example in Eichelberger et al. U.S. Pat. No. 4,783,695. Very briefly, in systems employing this high density interconnect structure, various components, such as semiconductor integrated circuit chips, are placed within cavities formed in a ceramic substrate. A multi-layer overcoat structure is then built up to electrically interconnect the components into an actual functioning system. To begin the multi-layer overcoat structure, a polyimide dielectric film, such as Kapton polyimide (available from E. I. DuPont de Nemours & Company, Wilmington, Del.), about 0.5 to 3 mils (12.7 to 76 microns) thick, is laminated across the top of the chips, other components and the substrate, employing Ultem.RTM. polyetherimide resin (available from General Electric Company, Pittsfield, Mass.) or another thermoplastic as an adhesive. The actual as-placed locations of the various components and contact pads thereon are determined by optical sighting, and via holes that are adaptively laser drilled in the Kapton film and adhesive layers in alignment with the contact pads on the electronic components. Exemplary laser drilling techniques are disclosed in Eichelberger et al. Pat. Nos. 4,714,516 and 4,894,115; and in Loughran et al. Pat. No. 4,764,485, each of which is incorporated by reference. Such HDI vias are typically on the order of one to two mils (25 to 50 microns) in diameter. A metallization layer is deposited over the Kapton film layer and extends into the via holes to make electrical contact to chip contact pads. This metallization layer may be patterned to form individual conductors during the deposition process, or it may be deposited as a continuous layer and then patterned using photoresist and etching. The photoresist is preferably exposed using a laser which is scanned relative to the substrate to provide an accurately aligned conductor pattern upon completion of the process. Exemplary techniques for patterning the metallization layer are disclosed in Wojnarowski et al. U.S. Pat. Nos. 4,780,177 and 4,842,677; and in Eichelberger et al. Pat. No. 4,835,704 which discloses an "Adaptive Lithography System to Provide High Density Interconnect" each of which is incorporated by reference. Any misposition of the individual electronic components and their contact pads is compensated for by an adaptive laser lithography system as disclosed in U.S. Pat. No. 4,835,704. Additional dielectric and metallization layers are provided as required in order to make all of the desired electrical connections among the chips.
As is described in detail hereinbelow, one aspect of the present invention is the fabrication of molded permanent magnets on micromechanical structures employing a modified HDI technology, including use of a polymer such as a polyimide (e.g. Kapton) as a mold material.