Metal fusion bonding – Process – Critical work component – temperature – or pressure
Reexamination Certificate
2002-06-25
2004-12-21
La Villa, Michael (Department: 1775)
Metal fusion bonding
Process
Critical work component, temperature, or pressure
C228S262100, C156S272200, C156S273900, C264S045100, C427S375000, C427S376200, C427S397700
Reexamination Certificate
active
06832716
ABSTRACT:
This invention relates to bonded products, to methods of fabrication therefor and to bonding materials for use in such methods.
There are a number of ways in which bonded products, comprising components that have been bonded together, can be fabricated. For example the components may be bonded by welding, soldering, or by the use of an adhesive. Welding involves melting the components so that they bond together. Alternatively one of the components may comprise a bonding material such as an adhesive or a solder. For example if one of the components is a solder, then this component may be melted to form a bond between the other components. Welding technology has a particularly important role to play in the field of silicon microfabrication, which is now an established manufacturing technique for producing micromechanical devices. The technique provides for batch-processing miniaturised silicon devices of great diversity, for example micropumps, accelerometers, pressure sensors and microactuators. Many micromechanical devices comprise several micromachined components, each component being formed from bulk crystalline silicon. Assembly of such devices often involves joining parts of silicon wafers, comprising bulk crystalline silicon, together in a spatially precise, clean-room compatible manner. Silicon wafer bonding technology is therefore an important aspect of device manufacture, for example to ensure that assembled and packaged devices maintain operational reliability.
Many microfabricated devices incorporate electronic circuits, for example circuits to perform in-situ signal processing or provide drive signals for operating the devices. For many applications, the circuits have to be protected from an environment in which the devices are to be used. Such protection is conventionally achieved by encapsulating the devices in respective packages which are sealed by forming a package hermetic seal under vacuum conditions
There are presently two dominant conventional bonding processes for bonding silicon-based components together, namely “direct bonding” and “anodic bonding”. In direct bonding two or more components, comprising bulk crystalline silicon, are assembled so that the surfaces to be bonded are in contact with each other. Heat is then applied to the assemble components so that the associated surfaces form a bond. For many applications temperatures approaching 1000° C. are required before the bond can be formed. In contrast, anodic bonding is often employed to form bonds between silicon and silica components. It involves mating a polished surface of a silicon component to that of a silica component to be joined together and then applying a high electric field across an interface formed where the surfaces mate, thereby mutually polarising the surfaces to form an electrostatic bond at the interface. During anodic bonding, heating the components enhances bonding strength achievable therebetween.
Both of these conventional bonding processes described above suffer a disadvantage that the components need to be heated in their entirety for direct bonding and high electric field strengths are required for anodic bonding. In many situations, electronic circuits are not capable of withstanding annealing temperatures used in direct bonding and high electric field strengths applied in anodic bonding; aluminium interconnections cannot withstand temperatures in excess of 450° C. for example, whereas high electric fields can damage or ionise silicon nitride or silicon dioxide dielectric layers for example. Moreover, bonding strengths provided by direct bonding and anodic bonding are insufficient in certain device applications where high reliability is paramount, for example for micromachined accelerometers which are to be subjected to peak acceleration forces in excess of 25000 g.
A demanding application for encapsulated microfabricated micromachined devices is in biological environments where there are, for example, corrosive biological body fluids. Providing protection from such fluids is particularly important for safety-critical applications where device failure cannot be tolerated, for example in amicrofabricated pace maker arranged to provide heart stimulation. A conventional approach for protecting electronic circuits for use in biological systems is to encapsulate them within welded titanium boxes, titanium being a biocompatible material which biological systems accept by forming a layer of cells thereonto which thereby avoids biological rejection problems. This conventional approach was developed in the 1960's and 1970's where, even in that era, hermetic seals were of a sufficiently high quality to realise a remarkably low failure rate; J Buffet in an article in Medical Progress in Technology 1975 Vol. 3 page 133 reported a failure rate of 13 out of 5800 implanted pacemakers encapsulated within welded titanium enclosures over a three year period.
Although adoption of welded titanium enclosures has been acceptable to health care industries generally, the enclosures tend to be bulky which excludes their use in situations where miniaturisation is of prime importance, for example for incorporation into an human inner ear region to stimulate nerve endings therein. In a publication Advanced Materials 7, 1995 pp. 1033, it is disclosed that silicon is potentially usable, instead of titanium, for enclosures for use in biological systems. Bonded silicon microfabricated micromachined components can thereby not only form devices suitable for use in biological environments but also provide their own encapsulation. However, especially in safety critical applications, seals provided between bonded silicon components must be extremely reliable. Conventional bonding techniques, for example direct bonding and anodic bonding, are often insufficiently reliable for safety critical applications. There is therefore a need for a more reliable bonding technique for bonding together semiconductor components.
Silicon welding has been previously investigated during the 1960's and 1970's and is reported in an article by H Foll and D G Ast in the proceedings of the Ninth International Conference Electron Microscopy, 1978, pp. 428-429. It was quickly abandoned as a reliable process for bonding silicon components because:
(a) welding of silicon components requires them to be heated to an elevated temperature, namely bulk crystalline silicon has a melting point temperature of 1414° C. which means that silicon components to be bonded by conventional silicon welding have to be heated to this temperature; such a high melting point is incompatible with other microcircuit parts, for example aluminium metallisation in an integrated circuit cannot withstand temperatures in excess of 450° C.;
(b) silicon is a brittle material and exhibits a high thermal budget for making it fuse during welding; this greatly increases likelihood of fracture from thermally induced stresses.
It has been reported, by Goldstein in Appl. Phys. A62, p 33-7 (1996), that nanocrystals of silicon, comprising porous silicon, melt at lower temperatures than bulk crystalline silicon. Melting temperatures as low as 200° C., for 4 nm diameter nanocrystals of silicon have been reported, which compares with melting temperatures of 1414 for bulk crystalline silicon. Porous silicon may be fabricated by the chemical dissolution of bulk crystalline silicon as described by L T Canham in Appl. Phys. Lett. Vol 57, p1046 (1990). Provided the pores are sufficiently closely spaced, nanocrystalline silicon can be formed by this technique.
The following items of prior art are relevant to this invention: U.S. Pat. No. 5,628,848, WO 9606700, EP 0461 481 A2, GB 2337255, and GB 2317885. U.S. Pat. No. 5,628,848 relates to the formation of multilayer structures that are sintered together to form a strong bond between the layers. The starting materials for the layers are in the form of powders. WO 9606700 relates to the fabrication of nanoscale particles. The invention also relates to the use of nanoscale particles to join components together. EP 046
Canham Leigh T
Reeves Christopher L
La Villa Michael
Nixon & Vanderhye P.C.
PSIMEDICA Limited
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