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Applications and Design of Thin Film Capacitors C =
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Applications and Design of
Thin Film Capacitors C
apacitors are an essen-
tial component in any circuit
requiring decoupling, filter-
ing, tuning, or general charge
storage functions. A wide
array of capacitor styles and
technologies presently exists
to cover the broad range of
requirements in todays elec-
tronics market. The type of
capacitor chosen for a particu-
lar use depends upon a num-
ber of variables, including fre-
quency of operation,
Q-factor, breakdown voltage,
and environmental conditions.
MIC Technology offers high-
performance, thin film capaci-
tors integrated on our
PIMIC
TM
substrates as an
answer to circuit designers
concerns about capacitor loss,
size, and assembly difficulty.
These devices are ideal for
MMIC chip decoupling, DC
blocking, and lumped-element
filtering, providing lower loss
than traditional ceramic chip
capacitors. This application
note discusses the design rules
and advantages of using thin
film capacitors in high-density
circuit designs.
Designing With
Capacitors
As a fundamental building block in
electronic circuits, capacitors perform
many useful functions in analog and
digital circuits and come in numerous
shapes, sizes, and materials. At
extremely low frequencies, large val-
ued capacitors are often used to elimi-
nate power supply noise or control
ringing effects in digital circuits. At
microwave frequencies, capacitors are
at ease providing isolation between
DC and RF signals or controlling RF
leakage from high-gain circuits.
Across the entire frequency spectrum,
capacitors are useful in a myriad of
filter topologies for controlling and
shaping RF signals.
Clearly, no single capacitor style
addresses all of these needs due to a
variety of performance parameters
that vary as a function of capacitor
construction. The information provided
in this guide first defines the parame-
ters useful in judging the performance
of thin film capacitors followed by a
detailed review of layout rules and
simulation considerations.
Step 1: Choosing a
Capacitor
MIC Technologys thin film capacitors
are constructed using a Metal-Insula-
tor-Metal approach with either a sili-
con-nitride or polyimide dielectric and
gold electrodes. Connections between
the capacitor and the adjacent circuit-
ry are made using air-bridges, con-
struction of which is detailed later in
this guide. This approach to capacitor
construction is in contrast to other
available technologies including dis-
crete chip capacitors manufactured
using single-layer or multi-layered,
high dielectric ceramics and attached
to base circuitry using epoxy, solder,
and wire bonds. The choice of capaci-
tor style is generally dictated by any
number of the parameters defined
below:
Capacitance Density, Value, and
Tolerance
The capacitance of a parallel plate
structure comes about through the
separation of opposing charges by a
dielectric material. This charge-stor-
age capability is a function of the area
and separation of the plates as well as
the dielectric constant of the separat-
ing material as defined by the follow-
ing equation:
C =
o
A/D
Where:
= Relative Dielectric
Constant of Insulator
o
= 8.854e-14 F/cm
A = Area of Electrodes (cm
2
)
D = Distance between
Electrodes (cm)
Table 1: Capacitance Density, Value, & Tolerance for Thin Film Dielectrics
Dielectric
Dielectric
Dielectric
Capacitance
Available
Material
Constant
Thickness
Density
Values
Tolerance*
Silicon-Nitride
7.8
2200
0.2pF/mil
2
10-500pF
10% (>50pF)
Angstroms
(310pF/mm
2
)
20% (<50pF)
Polyimide
3.0
3 �m
.0055pF/mil
2
0.5-50pF
20%
(8.55pF/mm
2
)
*Special tolerances available Computing the capacitance for a unit
square of dielectric material yields the
capacitance density, a convenient
term which provides a simple basis for
computing capacitor sizes. In qualita-
tive terms, the higher dielectric con-
stant of silicon-nitride combined with
its relatively thin deposition thickness
result in a significantly higher capaci-
tance density than polyimide. This
result leads to the realization that
practical capacitor values for poly-
imide capacitors will be limited to low
values because of rapidly increasing
element sizes. For silicon-nitride
capacitors, practical values range
from 10pF to 500pF resulting in con-
venient capacitor sizes smaller than
their discrete counterparts. Polyimide
capacitors with their lower capaci-
tance density are readily fabricated in
the 0.5pF to 50pF range. Capacitance
tolerance also differs for each of these
materials as a function of capacitor
value due to both lithography toler-
ances and dielectric deposition meth-
ods. For silicon-nitride, tolerances of
�10% are readily achievable for
capacitor values over 50pF, while
below 50pF, standard tolerances are
�20%. In polyimide, all capacitor val-
ues meet a �20% tolerance value.
Using the material options available at
MIC Technology results in the capaci-
tance densities and values shown in
Table 1.
Loss and Quality Factor
During its operation, a capacitor
charges and discharges stored energy
at a rate determined by the time vary-
ing voltage across its electrodes and
the loading of adjacent circuitry. An
ideal capacitor returns all of this
stored energy to its network, while real
capacitors incur losses during the
charge/discharge cycle. These losses
are typically due to polarization of
dipoles or charge leakage within the
dielectric material. A figure of merit for
capacitor loss most commonly used is
the quality factor or Q factor which is
defined as follows for a simple parallel
plate capacitor:
Q = Quality Factor
= 2 x energy stored per cycle
energy lost per cycle
=
1
Loss Tangent
When a capacitor is fully integrated
into a circuit including interconnec-
tions such as epoxy and wirebonds,
the Q value drops due to the parasitic
resistances and inductances these
connections add. The larger size of
discrete capacitors as compared to
thin film capacitors, also contributes to
wavelength-related resonance prob-
lems in some circuits. With thin film
capacitors, the added parasitics due
to interconnection are an order of
magnitude smaller than those added
with discrete chip capacitors. The
resultant Q value in the actual circuit
is calculated based upon a series
resonant model which incorporates
resistive losses and parasitic induc-
tance as follows:
Q = Quality Factor =
o
L/R
Where:
o
= 1/(L x C)
1/2
A comparison of integrated thin film
capacitor performance is shown in
Table 2 compared to a discrete capac-
itor before and after integration into a
completed circuit design.
Breakdown Voltage
The electric field created across a
capacitor is a function of the voltage
applied to the electrodes and their
separation. At excessively high elec-
tric field values, the dielectric material
can become conductive, resulting in
current flow through the capacitor and
eventual failure. The applied voltage
value where a capacitor begins to
conduct is usually referred to as the
breakdown voltage. Dielectric materi-
als such as silicon-nitride and ceram-
ics typically have dielectric strengths
on the order of one to ten million volts
per centimeter although other factors
such as fabrication approach, material
surface finishes, and dielectric
purity/consistency may play into the
ability to achieve a specific breakdown
value in a given capacitor design.
Proper process control and material
choices, however, allow extremely thin
dielectric materials to tolerate 10s to
100s of volts reliably. Table 3 high-
lights breakdown voltages for MIC
Technologys thin film capacitors.
Temperature Coefficient of
Capacitance
As with resistors, capacitors can also
have variation in value as a function of
temperature with behavior described
by the temperature coefficient of
capacitance or, TCC. This term is
typically used for linear dielectric
materials to compare performance at
differing temperatures to the 25�C
value. Depending on the dielectric
material, these temperature character-
istics can be positive or negative-
going with temperature. Often, with
high-dielectric titanate materials used
in chip capacitors, the temperature
coefficient may have opposite charac-
teristics at low and high temperatures.
Variation in these less stable materials
is typically expressed as % capaci-
tance change versus temperature.
Using silicon-nitride as a dielectric
material has the advantage of provid-
ing a temperature-stable capacitance
characteristic. Table 4 compares sili-
con-nitride to other common capacitor
dielectrics.
Table 2: Quality Factors for Thin Film Capacitors
Loss Integrated
Insertion
Tangent
Dielectric
Discrete
Q Value
Loss
Capacitor Type
@ 5 Ghz
Thickness
Q Value
@ 1 MHz
@ 10 Ghz
Thin Film SiN
.0004
2200 Angstroms
2,500
1000 typ.
.03 dB
Thin Film Polyimide
.01
3 �m
100
75 typ.
0.1 dB
High Q Ceramic
.0001
150-250 �m
10,000
200-500 typ.
0.11 dB
Table 3: Breakdown Voltages for Thin Film Capacitors
D