Description of Instrumentation and Procedures - Advances in Biochemical Engineering Biotechnology - page 11

technique is surface micromachining, in which a sacrificial layer is selectively
etched from below an etch-resistant thin film. Static and dynamic micromecha-
nical structures have been fabricated by applying these processes in combina-
tion with low-pressure chemical vapour deposition on polysilicon. Such
systems, i.e. that have the electronic properties of semiconductors, are compara-
ble with thermistors, where there is a change in resistance as a function of tem-
perature, or a thermopile based on the Seebeck effect or the p -n junction for the
diode and transistor. In addition, integrated thermistors and thermopiles can be
designed by doping boron into polysilicon, in order to achieve a temperature
dependent change in resistance or to form a thermocouple in the presence of
aluminium or gold.
An additional advantage of the integrated circuit technology is the ability to
integrate the various components, such as the transducer, reactor, valve, pump
etc., within the electronic system, forming refined flow-analysis systems on
silicon wafers. Several approaches, such as electrostatic, electromagnetic, piezo-
electric, thermopneumatic and thermoelectric can be employed for force trans-
duction in the microvalves, these are also applicable to micropumps. Based on
these approaches, two versions of micropumps have been developed. These are
connected in parallel; the first pump (dual pump) is activated with periodic two-
phase voltage, while the second pump (the buffer pump) is driven by two piezo-
electric actuators. Microsensors of two kinds are described below: a thermopile
based- and a thermistor based microbiosensor.

2.3.1
Thermopile-Based Microbiosensor

The thermopile-based microbiosensor (Fig. 5) is fabricated on a quartz chip. Its
functioning is based on the Seebeck effect: V is the voltage

output of one thermocouple; n stands for the number of thermocouples,

D V=n a ab D T, where D

T is
the temperature difference between the hot junction and the cold junction, and
a ab is the relative Seebeck coefficient, which is dependent on the composition of
the material and on the working temperature. For small temperature ranges, the
Seebeck coefficient

D

a ab can be considered to be constant. Thus, the voltage
output of the thermocouple is proportional to the temperature difference,

T
between the hot and the cold junctions. A thermopile was constructed by
connecting a number of thermocouples in series. The thermopile has a much
larger voltage output than a single thermocouple for the same temperature
difference, since the output from the thermopile is equal to the sum of the out-
puts from each thermocouple. When the cold junction is maintained at a con-
stant temperature and the hot junction is placed proximally to the exothermic
enzyme reaction, the detection of the output voltage from the thermopile is
directly related to the substrate concentration.
The integrated thermopile (1.6

D

¥ 10 mm) was manufactured by the following

method. A quartz chip (25.2 0.6 mm) was used as a substrate instead of

a silicon wafer, in order to reduce the heat conductivity of the chip. A 0.5

¥ 14.8 ¥

m
thick layer of polysilicon was deposited using LPCVD (low-pressure chemical
vapour deposition) onto the quartz substrate. The layer was boron-doped using

m