Metal fusion bonding – Process – Applying or distributing fused filler
Reexamination Certificate
2000-11-30
2002-09-24
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Applying or distributing fused filler
C228S179100, C385S088000, C257S678000, C438S106000, C438S118000
Reexamination Certificate
active
06454160
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a method for hermetically encapsulating microsystems in situ. At least one microsystem mounted on a substrate is encapsulated under a metal capsule made in situ. “Mounted” means either placing the microsystem, made beforehand, on the substrate, or making the microsystem in situ on the substrate. Preferably, several microsystems of micrometric dimensions are manufactured together on the same substrate. The encapsulation enclosing the microsystem must be sealed hermetically and leave said microsystem free of movement inside the capsule.
“Microsystems” means three-dimensional structures, i.e. microoptoelectromechanical devices (MOEMS) or microelectromechanical devices (MEMS) such as reed contactors, accelerometers, micromotors, sensors of micrometric size, which need to be left free to move after encapsulation. The construction of said microsystems can be made on an insulating substrate or on a substrate comprising integrated circuits which have been made beforehand. In this latter case, it is possible to use the metal contact pads of the integrated circuit to begin depositing the metal layers which will form part of the microsystem and to allow it to be electrically connected to said circuit.
Swiss Patent No. 688213, by the same Applicant, discloses a reed contactor or contactor with strips of micrometric size and the manufacturing method thereof. The contactor comprises metal strips at a distance from each other in the rest state which are made by electrolytic means in several steps and are attached to a base plane. The strips are formed of an iron and nickel alloy, deposited by an electrolytic method. This alloy has the property of being ferromagnetic so that the strips are able to be put in contact with each other when a magnetic field passing through them creates a force of attraction therebetween. This contactor is encapsulated under a hollow cover which is fixed for example using an epoxy adhesive material onto the base plane. The latter may be a glass substrate or an insulating layer obtained by oxidising the surface of a silicon substrate. The cover is formed of a glass plate in which cavities are formed by chemical etching. This plate allows each contactor to be enclosed in each of the etched cavities. The plate may be bonded onto the base plane or soldered by an eutectic or anodic solder. In a final operation, the multitude of contactors thereby made and sealed are separated by cutting or dicing operation.
In this type of embodiment, it is necessary to machine the glass plate separately from the substrate on which the contactors are manufactured. This constitutes a drawback. Moreover, the plate has to be bonded precisely onto the base plane using an epoxy adhesive material. The sealing is not hermetic over the long term, since the epoxy resin absorbs water and degasses substances capable of disturbing the operation of the contactor. In other embodiments, an heat treatment for encapsulating the contactor can be destructive.
In Swiss Patent No. 688213 it will also be noted that during contact resistance measurements between the metal strips, prior to encapsulation of the contactors, the contact resistance average of all the contactors made on a same substrate was centred around 10 ohms. After said encapsulation, this contact resistance average was measured rising to 10 to 60 ohms.
European Patent No. 0 302 165 discloses a sheet of tin which is formed by stamping to act as the metal dome for an integrated circuit. This stamped sheet is then bonded onto a base plate where the integrated circuit is placed so as to close said circuit under the dome. The whole assembly is subsequently coated with a layer of polyethylene. The adhesive material, as explained above, can cause contamination of the microsystem. Consequently it does not allow hermetic encapsulation to be guaranteed. It is also not possible to design the dome in situ by stamping. Moreover, making these stamped sheets, which have to be individually placed on each microsystem, complicates the encapsulation of several microsystems mounted on a same substrate.
In the field of combined micromechanical and electronic devices, the use of sacrificial layers is already known. One can cite the case in which one wishes for example to make a metal bridge between an integrated circuit and a sensor. On the other hand in the case of making an hermetic metal encapsulation for microsystems, the use of sacrificial layers is not known.
U.S. Pat. No. 5,798,283 discloses a method for manufacturing at least one microelectronicmechanical device with an electronic circuit. A cavity is etched in the substrate for example made of silicon in order to house therein the micromechanical device. The latter is constructed using different layers of polysilicon in order to obtain elements able to be free of movement. The device has to be protected using layers of silicon oxide or nitride so that the subsequent steps for making the integrated circuit can be performed. This protection of the micromechanical device is necessary to protect it against dopant diffusion temperature (boron, phosphorous for example) which can be higher than 700° C. Such temperature can partly destroy the elements of said micromechanical device designed with certain metals with a low melting point. Such protecting layers also allow to avoid doping said elements if polysilicon is involved to be avoided.
Once the integrated circuit operations are finished, two openings arranged in a protective layer disposed above layers of SiO2 or Si3N4 allow said layers of SiO2 or Si3N4 to be partly removed by chemical etching. That allows thus to release the micromechanical device and to leave it free of movement. During such removal, precautions must be taken to avoid too great a lateral etching, because the integrated circuit is constructed beside the micromechanical device.
Instead of making two openings in the protective layer, it might have been envisaged to use only one layer of porous polysilicon in order to remove the layers of SiO2 or Si3N4 by chemical etching, in particular using fluorohydric acid, through the polysilicon, and then to rinse with deionised water.
Several drawbacks of said method from this document can be cited. Firstly, the encapsulation is made using non-metallic layers. Moreover, a cavity has to be arranged beforehand in the substrate to house therein the microsystem by etching techniques similar to those used in the microelectronic field. The microsystem has also to be protected while the corresponding integrated circuit is being made with layers which can withstand high temperatures. Consequently, there is no question of depositing metal layers in particular by electrolytic means on said micromechanical device to create an hermetic metal encapsulation.
European Patent No. 0 435 530 discloses an electronic system hermetically sealed by metal layers one of which is deposited by electrolytic means. The electronic system is an association of different integrated circuits, with high density interconnection (HDI). These circuits are housed and bonded using polymers in a cavity micro-machined in a glass or ceramic substrate. A first metal layer, in particular made of chromium or titanium, is sputtered onto a dielectric layer which overhangs the interconnections made for the different circuits. This first layer allows to coat the entire structure and to come into contact with the surface of the substrate. Subsequently, a second metal layer is deposited by electrolytic means above the first layer in order to create a thicker protective layer against various contaminating elements able to disturb the circuits.
European Patent No. 0 435 530 provides no teaching for making an encapsulation for microsystems, such as reed type contactors. One drawback is that the polymers used to bond the circuits, produce gases, i.e. degas. That thus creates defects which will be noticeable as regards the proper operation of the contactor. Moreover, it is to be noted that creating a metal capsule via a sacrificial metal layer remove
Asulab S.A.
Dunn Tom
Edmondson L.
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