Column

Transitions in Bonding Technologies : from a Golden Mask to MEMS
Osamu Ohashi, Professor, Graduate School of Science and Technology
Niigata University
photolithography. The method of bonding is anodic bonding,which employs electrostatic attraction in the glass interface
that accompanies the migration of sodium ions. This method
can be used at the relatively low temperature of about 300;.
In addition to microreactors, devices having
micromechanical structures (MEMS) include various
accelerometers and optical switches. Assembled of
micromechanical parts formed in silicon using
photolithography, these devices have practical applications in
combination with processing technologies from multiple fields.
I believe that complex micromechanical process andassembly technologies represented in devices having
micromechanical structures should be one of Japan's focuses in
the future.

Throughout the history of humankind, technology has
progressed slowly and, at times, rapidly. One technology thathas advanced with the others is welding/bonding technology.
The history of bonding began with a golden mask that wasdiscovered in the tomb of the Egyptian king Tutankhamen. The
goldwork of the mask had been assembled around 1300 BCE
using thermocompression. Bonding in Japan began with therepair of a bronze bell. Then in 747, the Daibutsu, or Great
Buddha, of Nara was cast in block units and subsequently
assembled using a brazing-like technique. Around 900, atechnique for manufacturing Japanese swords was established.
When guns arrived in Tanegashima in 1543, material processing
techniques advanced so rapidly that guns could bemanufactured domestically soon thereafter. These bonding andprocessing techniques were performed by combusting charcoal
as a heat source, heating the materials in a reducing gas toprevent oxidation, and joining gold and iron alloys throughforge welding and brazing with no melting.
When iron came to be produced in blast furnaces after theIndustrial Revolution, iron and steel materials were bonded
mechanically by forming holes in two iron plates and inserting
rivets to join them together. This riveted construction can beseen in the Eiffel Tower, constructed in 1889, and theKomagata Bridge in Asakusa, Japan, shown in Photo 1. The heatsource evolved from charcoal to electricity, then arcs, electronbeams, and laser beams, as new heat sources were developed.
These heat sources have been applied to welding/bonding
techniques to develop a new fusion welding for melting theprocess points.
Recently, materials are gaining higher functionality; parts
are being made smaller; and there is increasing demand for
precision bonding and methods for joining dissimilar materials.
There are numerous cases in which assembly is not possible
with the conventional melt welding due to metallurgicalproblems. Thus, technologies have been sought and developedfor joining materials in their solid phase without including amolten phase in order to decrease these metallurgical problems.
Fig. 2 shows a sample wound coil for use in a cell phone.
The entire coil is smaller than a grain of rice. The coil is
produced from a urethane-coated copper wire having a
diameter of 30 µm, and this insulated copper wire is bonded tothe silver electrode of a terminal by thermal compression.
Fig. 3 shows a sample microreactor having heat exchange
circuits above and below the reactor circuit. A stainless steel
leaf having a path no greater than 0.5 mm, is bonded using
thermocompression (diffusion bonding) under a high vacuum
so that the material does not deform.
The thermocompression in these bonding techniques was
the same used to assemble Tutankhamen's golden mask, but
remarkable advancements have been made in techniques forcontrolling the bonding process.
Various accelerometers have been manufactured in recentyears. Many of these have been incorporated in automobiles
and have helped improve riding comfort and safety.
Accelerometers are parts formed by joining and assemblingsilicon and glass that have been micromachined according toFig. 1 Rivet structure for the Komagata Bridge in Asakusa

Fig. 2 Wound coil for a cell phone (manufactured by Taiyo Yuden)Fig. 3 Layered heat exchanger cut to show the internal
structure (manufactured by Yachida)6