MEMS - Semiconductor Process Integration Monolithic Manufacturing Technology(The Search for a New MEMS Sensing Principle)

Susumu Sugiyama, Professor, COE Promotion Organization, Ritsumeikan University
Toshiyuki Toriyama, Professor, Department of Micromachine Systems Engineering,
College of Science and Engineering, Ritsumeikan University

Yoshimasa Isono, Professor, Department of Micromachine Systems Engineering,
College of Science and Engineering, Ritsumeikan University
Forming components comprising semiconductor ICs andsensors, actuators, or other MEMS devices in an integrated
manner will make it possible to create more compact, more
highly functional and more reliable devices. The need for
improvement is particularly keen in areas such as automobilecomponents and medical sensors, in which advancement is amust.
In the case of MEMS semiconductor integration, thetechnical complexity increases along with the level ofintegration, due to the complexity of conducting the designusing different processes, the duplication of manufacturing
equipment and so on. Moreover, whereas the size of thetransistors making up the IC is in the sub-µm domain, the size ofthe elements making up the sensor are in the µm domain. As aresult, a size of several mm is needed to accommodate the entire
sensor, and this increases the size of the chip with which the IC
is integrated. In this way, the complexity of the technologies
used in the manufacturing process and the resulting lowered
yield and increased chip size reduce the yield constant, making
it impossible to maintain profitability in cost terms.
The purpose of this development project was to resolve thisproblem by developing a manufacturing technology for creating
MEMS and semiconductors in an integrated manner, thereby
providing higher added value and more advanced functions. The
ultimate goal was to increase the international competitivenessof Japan’s unique leading-edge technologies.
In particular, we aimed to create the advantages that would
be obtained from higher added value and more advanced
functions. We focused on the piezoelectric element that is one of
the basic sensor elements, working to determine its properties
at the sub-µm level and to find a sensing principle that would
lead to higher sensitivity.

Fig. 1 Stand-alone Si nanowire manufactured using Scanning Probe Nano Lithography (SPNL)

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OutputFig. 2 Structure of Nanoelectromechanical system (NEMS) resonant device

Determining the piezoresistance effect of nanoscale silicon
The piezoresistant elements used as sensing elements inmechanical sensors are simple in terms of structure anddetection path. For this reason, they have been widely used inpressure sensors and acceleration sensors and so on. As shownin Fig. 3, when the width of the silicon piezoresistant element isreduced to the sub-µm level or below, the coefficient ofpiezoresistance, which affects sensor sensitivity, increases
greatly. We focused on this phenomenon in our efforts toincrease sensor sensitivity. First, we reduced the width of thesilicon piezoresistant element to 100 nm or below andconducted a theoretical analysis of piezoresistance based on the
low-dimensional band structure. Based on these analyticalpredictions, we plan to manufacture a silicon piezoresistantelement and determine the actual coefficient of piezoresistance.
It is thought that, if the prediction is correct, sensor sensitivity
will be increased anywhere from several times to nearly anorder of magnitude greater.

Achieving a nanomechanical structure and determining
nanoelastic properties
It is not yet clear whether the same mechanical properties
are maintained or different mechanical properties are exhibitedwhen a mechanical sensor mechanism is reduced to the sub-µm
scale or smaller. The purpose of this study was to measure thisexperimentally in the hope that the results would serve as
useful data for the design of highly integrated MEMS. Fig. 1
shows an autonomous silicon nanomechanical structure
manufactured by means of Scanning Probe Nano Lithography(SPNL). Fig. 2 shows the structure of a nanoelectromechanicalsystem (NEMS) resonant device. From these resonanceproperties, the mechanical properties were measured and theirapplication in sensors was studied. The planned width of thenanowire and the gap between electrodes is less than 100 nm.

Fig. 3 Relationship between Si piezoresistance effect and section area5