Download 13_chapter 9

CHAPTER
IX
APPLICATION IN AN ASTABLE
MULTIVIBRATOR
9.1.
Introduction
In this space age thin film structures are finding
increasing applications in almost all scientific /indus­
trial advances and fabrications.
With the advances in
the field of solid state lasers and fibre outics, thin
film composites find use as wave guides /'\/.
In the
field of electronics with the present trend towards
microminiaturization in the form of very large scale
integrated circuits, the use of thin films has become
^ery essential to the consummation of such an end-product.
'I'hey find use in thin film transistor circuits or
integrated circuits as insulators, dielectric layers,
resistoyn and interconnections.
Polycrystalline films
of metals and oxides find industrial applications in the
field of optical devices and in electronics.
The pheno­
menal growth of thin film research and development owes
wrcch to the stimulus provided by the early utilitarian
interest in the applications of optical films in mirrors
and interferometers.
Thin films are also found to play
a role in solar energy applications.
The study of titanium oxide is of interest because
of the high value of its dielectric constant /2,3/.
It
is found to have interesting electrical characteristics.
In recent years extensive studies have been made on this
compound and they have been used as antireflection coatings
165
in thin film solar absorbers /A/.
It has also been
reported that titanium oxide coatings can be used as
hyu rogen detectors /f>/.
Titanium nitride films also
possess, interesting electrical, mechanical and optical
characteristics.
This has been exploited as hard
decorative cootirigs on Vatch cases /6/.
The property
that both these films have a high dielectric constant
renders it possible to use them in electronic cireeiits.
In the present study, thin film capacitors, formed
with the oxide and nitride films as dielectrics along
with thin film resistors prepared by vacuum evaporation
have been used in,, cm
(xstable, -
; circuit and its
performance analysed.
In the circuit a '555' integrated
chip has been used as the aetive component.
3,?..
Design of the R and C Components and Circuit layout
9.2.1.
Resistor
Thin film resistor and capacitor have been used
in the
astable multivibrator
(Fig.9.1).
The aspect
ratio i.e., the ratio of the length to the width of the
resistor pattern has been chosen in considration with the
value of the resistor, the available space, the resistivity
of the film and the thickness of the film.
The 80 > 20
nickel-chromium alloy has been used for the fabrication of
resistors since they possess very high stability of
resistance value and low temperature coefficient of
♦J5-.t3.Jg?
4
React
S
?GO
Discharge
threshold
%
Output
555
Control
voltage
trigger
(mu
6
•T\
i
*ig*9.1.
datable multivibrator circuit fabricated using
Uiin fiici components*
c1
166
0
resistance.
A film thickness of 225 A having a specific
resistivity of 225 ^u-fLcm and sheet resistance 100-n\-per
square has "been cnosen in order to obtain stable and
reliable film resistors.
A circuit with discrete resis­
tors shows that the circuit could function effectively
Vhen the resistance is in the order of 12Kwv.
Hence a
mask having dimensions of 0.05 x 6 cm 2 with 120 squares,
has been designed to give a resistance of 12K^e.
9.2.2.
Capacitor
• The basic circuit component for charge storage is a
metal-insulator-metal sandwich structure.
The metal
electrodes were formed by the deposition of aluminium
o
of thickness 1000 A.
Titanium oxide and titanium nitride
films deposited by the ion plating technique, have been
used as the dielectric layers, in 'the fabrication of the
capacitors.
The capacitors, formed with titanium oxide
and titanium nitride as dielectrics and having the dimen­
sions given in table 9.1 are made use of in the ;
a stable .
circuit.
TABLE 9.1.
Thickness !
Material
Method
Area
1
cm
2
Capacitance
pF
TitaniumO
Oxide |
Ion plating
1050
0.04
0.014
TitaniumO
Nitride|
Ion plating
800
0.04
0.012
______________
1H7
9.2.3.
Fabrication of tho astable multivibrator.
A multivibrator
circuit has been designed as in
Fig.9.1 incorporating resistors and capacitors.
The
masks that have been used in the formation of the resis­
tors and capacitors have been fabricated by the photo­
lithographic technique.
Titanium oxide and titanium nitride thin film capa­
citors with aluminium ©lao-trodess have been formed as
described in section 2.6. The nickel chromium wirev&s then
evaporated from resistively heated
form the resistors.
tungsten filament to
The substrate has been maintained
at a temperature of 200°C during the deposition of the
resistor.
The deposition rate, distance between the
source gpd the substrate, degree of vacuum etc.have
been
controlled in order to get resistors of required values.
The 1555* timer chip is the active component in the
cdTcait.
If the circuit is connected as shown in
the figure it will trigger itself and free run as a multi­
vibrator.
The external capacitor charges through R^ and
Rg only.
Thus the duty cycle which can be determined from
R^ and Rg may be precisely set by the ratio of these two
resistors.
In this mode of operation the capacitor charges
and discharges between 1/3 V
and 2/3 V
.
The charge
and discharge times and therefore the frequency are inde­
pendent of the supply voltage.
lb8
The charge time is given by,
(9.1)
t1 = 0.693 (Ra+Rb)c
and the discharge time by
(9.2)
t2 = 0.693 (BgjC
Thus the total period is given by,
(9.3)
T = t., + t2 = 0.693 (Ra+2Rb)G
The frequency of oscillation is then
f = 1
T
9.3.
=
1.44 .
(Ra+2Rb)C
(9.4)
Performance
The performance of the circuit has been studied
by analysing the output waveform with an oscill<fscppe.
The waveforms obtained across the resistor are shown
in Pig.9.2.
The frequency of theqptfcblo has been obtained
experimentally from the oscilloscope measurements.
The
value of the frequency has also been calculated using
the equation 9.4.
Both these values are given in table 9.2.
Table 9,2.
—
Capacitor
Material
Frequency by
Experiment
Frequency by
Calculation
Titanium^
Oxide 5
•
4.7 kHz
4.5 kHz
12.3 kHz
12.0 kHz
Titanium^
NitrideO
Ib9
It has "been observed from the table 9.2 that the
experimentally obtained frequencies are agreeing well
with the calculated values in both the cases.
Hie
performance of the circuit is very much comparable with
the conventional circuits
tors and capacitors.
designed with standard resis­
170
References
1.
A.F. Pereev, A.V. Mikhailov, Cl.A. Muranova,
Sov. J.Opt.Technol., .42, 161 (1976).
2.
Y. Katsuta, A.E. Hill, A.M. Phahle and J.H. Calderwood,
Thin Solid Films, ..13, 53 ( 1973).
3.
M. Takeuchi, T.
Itoh and H. Wagasaka, Thin Solid
films, 51, 83 (1978).
4.
S. Yoshida, Thin Solid Films, 56, 321 (1978).
5.
1.A. Harris, J. Electrochem.Soc., J27, 2657 (1980).
6.
B. Zega, M. Kornmann and J.Amiguet, Thin Solid
Films, 4£, 577 (1977).
Pi-3.9.2.
Output wctfefomo of tho actable multivibrator
fabricated with thin film components*