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AN LTCC HYBRID PRESSURE TRANSDUCER FOR HIGH TEMPERATURE
Jolymar González-Esteves (Mechanical Engineering)
University of Puerto Rico, Mayaguez Campus
NSF Summer Undergraduate Fellowship in Sensor Technologies (SUNFEST)
Advisors: Dr. Jorge Santiago–Avilés and Patricio Espinoza
Low Temperature Co-fired Ceramic (LTCC) and thick film
technology, with their mechanical, electrical, and thermal properties make
them an appropriate choice for the development of a pressure transducer. This
research designs a pressure transducer using LTCC and thick film technology.
We show the relation between the size of the diaphragm and resistance to
pressure. We determine the best position of the piezo-resistors, which are
accommodated forming a Wheatstone bridge. Two of the piezo-resistors
measure the deflection on the tangential axis, and the other two measure the
deflection on the radial axis. By using the Wheatstone bridge we can obtain
more accuracy in the output of the transducer.
This paper describes the process of making a pressure transducer with LTCC tape and
thick film technology. The great qualities of this technology show a great promise to develop
a transducer. This paper presents the circuit for compensation and the characterization of the
piezo-resistors used in the design. The simulation of the membrane is another important part
of this research. The paper provides all the steps needed to construct a pressure transducer
Low Temperature Co-fired Ceramic (LTCC) has high density and reliability and a
low manufacturing process cost. LTCC has a uniform structure composed 45% alumina
(Al 2O3), 40% glass, and 15% organic material. Among the properties that make LTCC useful
are high strength, reliability, low Thermal Expansion Coefficient (TCE), refire stability, and
an environmentally safe organic binder and solvent system.
LTCC is very effective in the
microelectronics area because of its thermal, mechanical, and electrical properties.
For this project we use DuPont 951-AT of 114 mm thickness. During the green state
this material is very soft, flexible, and easily dissolved (see Table 1.1). Figures 1.1 and 1.2
depict the firing process. The temperature raised 350°C at a rate of 10°C/min. At 350°C the
organic material is dissolved. The next step is to raise the temperature to 850°C at the same
rate. If this temperature is maintained for 2 minutes, the process is as semi-sintered. If,47
however, the temperature is maintained for 30 minutes, it is called a fully-sintered process.
The difference between these processes is in the arrangement of the particles of glass. In the
semi-sintered process the particles of glass surrounding the molecules of alumina; in the
fully-fired process the particles of glass covers the molecules of alumina. It shrinks 12% in
the x-y plane and 15% in the z plane.
Once fired, the material is very strong (see Table 1.2). It is i deal for use of hybrid
circuits with thick film technology.
Figure 1.1: Fully–Fired Process Figure 1.2: Semi-Fired Process
Table 1.1: Unfired properties
951-AT 114 mm ± 7% (4.5 mils)
12.27% ± 0.3%
15% ± 0.5%
Tensile Strength 1.7 MPa
Young’s Modulus 152 GPAS
Typical properties are based on laboratory test using recommended processing.
Eight- layer laminated structure with no metalization, using recommended processing.48
Table1.2: Typical fired properties
Dielectric constant, k (@ 10MHz)
Dissipation factor (@ 10 MHz)
Insulation resistance (@100 V DC)
Breakdown voltage (@V/25)
Thermal expansion (25°C-300°C)
Refire at 850°C surface
Conforms to setter
Via Diameter Resolution
Maximum Layer Count
100 mm / 100 mm
> 80 layers
1.2 PRESSURE TRANSDUCER
The main parts of a pressure transducer are the diaphragm (the sensitive element), the
electrical device, and the contacts. The diaphragm is where pressure is applied. It has to be
strong enough to resist pressure, and elastic to avoid a fracture when the applied pressure.
The diaphragm suffers a strain or deflection, causing a change in the sensitive element. At
the top of the diaphragm there is an electrical device, which coverts the strain into an
electrical signal, and makes possible measure the applied pressure. The most commonly used
pressure transducers are capacitive and resistive.
1.2.1 CAPACITIVE PRESSURE TRANSDUCER
Capacitive pressure transducers convert a change in the position of the
electroconductive plates forming a capacitor, and a change in the properties of a dielectric
between the plates into an electrical signal. Figure 1.3 shows an example of a capacitive
Figure 1.3: Single Capacitor pressure-sensitive element
The electroconductive diaphragm (1) affected by the pressure (P) moves towards a
stationary electrode (2) positioned side along the diaphragm. The diaphragm and electrode
form a capacitor sensitive to pressure.
The capacitive pressure transducer has some disadvantages. There is a small change in
the capacitance as a response to the mechanical signal. Accuracy is affected by wetting parts,
capillary effects, some build up on electrodes, and an unpredictable change on the
of the medium (for example forming gas fraction bubbles, fumes on the
surface of liquid, and so on).
The construction and the assembly must be precise. In light of
those disadvantages and the available resources, we chose to make a resistive pressure
1.2.2 RESISTIVE PRESSURE TRANSDUCERS
Resistive pressure transducers are the most common in the market. When pressure is
applied to the diaphragm, it deflects proportional to the change in resistance. The change in
resistance is measured by a strain gage. Because these pressure transducers are very sensitive
to the deflection, their measurements are very precise.
The resistive transducer has a very simple construction, most commonly of silicon.
Silicon is a high-precision, high-strength, and high-reliability material. It is very functional
where miniaturized precision mechanical devices must be fabricated in large quantities.
High-temperature treatment, bulk imperfections, and depositions of different films on the
surface of the single-crystal silicon cause a concentrate stress and cleavage on the material.
Silicon is best used at low temperatures because materials deposited on the surfaces have
variations in TCE and develop a nonuniform deformation under heating. For those reasons
we used LTCC tape to elaborate the pressure transducer.
LTCC does not change its properties with the change in temperature and ideal for it
can be sintered below 1000°C. It is very resistant to pressure and ideal for use with thick film
technology. LTCC is piezoelectric, so it often is employed to convert mechanical signals into
The permittivity of an insulating, or dielectric, material is commonly symbolized by the Greek letter epsilon, e; the permittivity of a vacuum, or free space, is
symbolized e0; and their ratio e/e0, called the dielectric constant, is symbolized by the Greek letter kappa, k.50
electrical signals. It is very practical for micro-technology and is low cost compared to
1.3 DESCRIPTION OF THE PRESSURE TRANSDUCER
The membrane is the sensitive part of the transducer. Pressure on the membrane
deflects it; the deflection is proportional to the pressure applied. The membrane receives
more stress at the edge (see Figure 1.4). Therefore, the membrane should not be square, or it
can fracture easily. A circular shape is recommended. On the top of the membrane we put the
Figure 1.4: Stressed part of a transducer.
1.3.2 STRAIN GAGES
Strain Gages use the change in resistance of an electrical conductor as a response to
the measured deformation. In our transducer we utilize bonded strain gages to measure the
strain or stresses on the element of construction. The bonded strain gages have to be strong
enough to withstand tensile and compressive stress. They must also have a high gage factor.
The gage factor shows how sensitive is the material. Bonding strain gages have a low
temperature coefficient of resistance and high resistivity , these qualities make it very
sensitive. The change in the resistance is given by:
DR = Kxex +Kyey (1)
Where: R = resistance
DR = change in resistance
Kx and Ky = axial and transversal gage factor, respectively;
ex and ey = strains in the direction of x and y axes, respectively.
There areon the market several kinds of strain gages. Each one has different material,
purposes and application.51
Our strain gage consists in four piezo-resistors, forming a wheatstone bridge (see
Figure 1.5). The Wheatstone bridge has four arms, all predominantly resistive.
Figure 1.5: Wheatstone bridge
If a resistor has a resistivity r, length l, width w, and thickness t (see Figure 1.6), its
R=r l (3)
Figure 1.6: Piezo-resistor
The sensitivity of the deflection is given by the gage factor (FG). Equation shows the
variation of the resistance due to some deflections.
FG = dR/R (4)
Where: e is the applied pressure
R is the resistance
dR is the change in resistance
If we differentiate the equation (3) and apply the Poisson ratio (v)*** we obtain:
dR = dr + (1+2v )e (5)
Exfoliation is a recommended technique for obtaining the diaphragm. In the
exfoliation we use Hydrofluoric acid, which is known to etch glass. When a semi-sintered
tape is submerged vertically in the Hydrofluoric acid during a period of time. The tape is
separate in three layers. Figure 2.1 depicts the separation of the layers. We use is of 114mm.
of thick tape, the thinnest available DuPont 951. When we put the layer on the hydrofluoric
acid solution, the solution dissolves the layer.
This experiment proved that the thickness of the layer has to be taken into account in
the exfoliation process. For this reason we decided to use a fully-fired layer for the
Figure2.1: Exfoliation Process
2.2 SIZE OF THE TRANSDUCERS
To make the body of the transducers we laminate eight layers of the green tape and
make a perforation of .625 inch in diameter, and then fire with a fully-fired process. Once it
is fired, we paste another layer on top of it but without perforation with QQ550 (a glaze used
to paste ceramics). The part of this layer that is on the top of the perforation will be the
diaphragm. Then it is fired, but this time by raising the temperature to 550°C at a rate of
10°C/min. Once is in 550°C stays there for 45 min. The last step is to decrease the
temperature to room temperature (see figure 2.2).
The next step in the construction of this transducer is the printing of the piezo-
resistors and the conductors. For the piezo-resistors we use a “strain gage paste.” This paste
is not in the market, because is an experimental paste of the DuPont series. This paste is
compatible with the conductors’ paste. It has a low thermal coefficient of resistance,
reliability, and low noise.53
Figure 2.2: Firing process for QQ550
For thick film resistors the resistivity r in function of temperature is given by:
r = ÖT e
(T0 0/T) ^(1/4)
Where:T0 is the typical temperature (it depends on the structure of the material);
T is the actual temperature
We print the piezo-resistors to eliminate organic material of the paste. We fired in a
hot plate at 550°C during ten minutes. The conductors are printed on the ends of the piezo-
resistors. To print the contacts we use 6134 paste of the DuPont series. The main component
of the paste is AgPd. The printing process is the same that we employed to print the piezo-
resistors. Finally we fire transducer in a fully-fired process. This will be transducer A. We
make a second transducer using same process, but changing the diameter of the diaphragm.
In this occasion the diameter of the diaphragm is 0.3 inch. This will be transducer B
When we apply the same pressure to the transducers, both of them break
Transducer A broke completely at the edge of the diaphragm. Transducer B cracked at
the edge of the diaphragm (see Figure 2.2). This experiment proves that a transducer with
a smaller diaphragm, can resist more pressure; with a bigger diaphragm resist less
pressure. Both transducers receive more stress at edge of the diaphragm.54
Transducer A Transducer B
Figure 2.2: Experimental Transducers
2.3 DESIGN OF THE TRANSDUCER
Using the previous results and the background of pressure transducer, LTCC and
thick film technology, we design a pressure transducer. Our pressure transducer consists in16
circular diaphragms of 0.25 inch of diameter. Each diaphragm has a Wheatstone bridge with
four piezo-resistors. Two piezo-resistors will measure the tangential strain; the other two will
measure the radial strain. The strain is given by:
st = 3w [ (m + 3) x˛ - (m + 1) ] (7)
sr = 3w [ (3m + 1) x˛ - (m + 1) ] (8)
Each piezo- resistor is 3/64 in and 1/64 in wide. At the ends of the piezo-resistors are
the bonding pads, which are 1/64 inch length and 1/32 inch wide. With these dimensions we
can approximate the offset to zero. Figure 2.3 depicts the design of the transducer
Figure 2.3 Design of the transducer55
2.4 SCREEN PRINTING
We cannot do the print of the piezo-resistors and contacts by hand, because of their
sizes. To make a precise print of the piezo-resistors and contacts we used a screen-printing
machine. In this machine we put a screen of the Wheatstone bridge with the desired
dimensions. We place the paste on the screen and the machine applies very precisely paste on
top of the diaphragm.
The next step in this research is to construct the proposed transducer. Prove the
functioning of the transducer is also indispensable step to complete the research. We expect
that the transducer have a good response time and low noise. We expect to compare our
pressure transducer with a silicon pressure transducer. The low cost and the mechanical,
electrical and thermal properties of LTCC, show a great promise for microelectronics
Thanks to the National Science Foundation for its support of the REU - SUNFEST
program for giving me the opportunity of make this research. Thanks to my advisor, Dr.
Jorge Santiago-Aviles, Patricio Espinoza, Vladimir Dominko and Mario Gongora for helping
me in my research and for advising me. Also I would also like to thank Seu Wah Low
(Mary), Juan F. Ortiz, and Jason Gillman, for helping with research.
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and Company.Accesed July 15,1999.
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4. Smiddeltoek, S.A. Audet. Silicon Sensors and Actuators
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&sctn=10> Accessed August 5, 1999.