Presented at SPIE'99
A Meso-Scale Electro-magnetically Actuated Normally Closed Valve
Realized on LTCC Tapes
Mario Gongora-Rubio a; Luis Sola-Laguna b; Michael Smith b and Jorge J. Santiago-Aviles c
a
Instituto de Pesquisas Tecnologicas (IPT), São Paulo, Brazil
b
DuPont Photopolymer & Electronic Materials, Research Triangle Park, USA,
c
Department of Electrical Engineering, University of Pennsylvania, Philadelphia, USA,
ABSTRACT
Sensors and actuators with promising characteristics in aggressive environments and high temperatures have been
developed using low temperature co-fired ceramic tape technology. We would like to report our work on an electromagnetically actuated normally closed valve.
This is a hybrid device which utilizes a purely LTCC tape electro-magnet and fluid flow manifold, combined with an
anisotropically etched silicon rectangular planar spring, and a high energy product SmCo mini-permanent magnet. Device
dimensions are in the meso (intermediate) range with the smallest features (fluid conduit in the manifold) of 400 µm and the
largest (the actuating coil) of 15 mm. All parts of the electromagnet and the fluid flow channels were machined from
DuPont 951 series, alumina based LTCC tapes utilizing either a numerically controlled milling machine, a puncher or an
isotropic etching technique involving the glassy binder of a partially sintered LTCC tape.
Two versions of this device have been fabricated. The first one, a hybrid, and an all ceramic (LTCC) valve. The hybrid
device, currently under evaluation, consists of five layers of planar spiral coils, connected as to preserve the magnetic field
direction. The total coil resistance is high (120 Ohms) and thermal considerations limits the current to 150 mA. Using a 900
Gauss SmCo magnet (1mm diameter) we obtained 200 micrometers deflection of the rectangular planar spring with no
hydraulic load. The best results so far are with the hybrid valve consisting of a silicon 30 micro-meter thick rectangular
planar spring with a polysiloxane sealing element.
Keywords: LTCC Technology, Electromagnetic Actuators, Microvalves, , Meso-EMS, Hybrid Microelectronics.
1. INTRODUCTION
For certain applications the small size characteristics of MEMS structures can be traded off for low cost production and high
manufacturing yields. Low Temperature Co-fired Ceramic tape materials used in Multi-Layer packages offers the potential
of emulating a great deal of silicon sensor technology at the meso scale level. The goal of our research is to access MicroSystem Technology -MST- the hybrid way, using LTCC (Low Temperature Co-fired Ceramics), Thick Films and Silicon
Technologies.
A substantive MST problem being addressed today is fluid handling for miniaturized chemical analytical systems. For
larger MST-3D applications, that is for meso-structures with minimum feature size in the range from 10 to several hundred
µm, it would be desirable to have a material compatible with hybrid micro-electronics, with suitable thermal, mechanical
and electrical properties, easy to fabricate and inexpensive to process. Such a material is the Green Ceramic Tape multilayer
system [1], with important features such as the possibility of fabricating three-dimensional structures using multiple layers.
In this work we would like to emphasize electromagnetic actuators for meso-systems exploring Green Ceramic Tapes.
MEMS/MST developments strongly influence industrial growth. Expanding the horizon of new materials and processes for
sophisticated or currently nonexistent applications. Such as, Microsystems for chemical analysis, e.g. F.I.A (Fluid Injection
Analysis); for drug delivery; for environmental data acquisition; Inertial Microsystems for disabled assistance, Hybrid
Microsystems for automotive applications, automata stabilization and others..
______________________________________
Correspondence: Email: gongoram@ipt.br or Santiago@ee.upenn.edu
2. HYBRID LTCC TECHNOLOGY
LTCC was conceived as a technology that could have the advantages of both Thick Film and HTCC technologies [11], as
shown in Figure 1. It attained a technological versatility by its excellent properties for doing packaging and MCM
applications, rendering good conductors, low associated capacitances, simpler processes and high layer count.
Thick Film
Disadvantages
- Multiple
printing
steps
- Multiple
firings
- Thickness control
of dielectric
- Limited layer
count
LTCC
HTCC
Advantages
Disadvantages
Advantages
- High conductivity - High print
- Low Conductivity
Resolution of
metals
Metals (W,Mo)
condutors
(Au,Ag)
- Single firing
- Complex Process
- Low Q
- Good dielectric
Dielectrics
- No printed
thickness
resistors
control
- Printed
- Low surface
resistors
- High Capital
roughness
- Low processing - Unlimited
investment
temperature
Layer count
Si TCE Match
Figure 1 Comparison of hybrid ceramic technologies
Green ceramic tape technology has been used in the last ten years for high reliability applications in military, avionics and
automotive areas, as well as in MCM's for communications and computer applications. Main reasons for using LTCC green
ceramic techniques as a MST technology are:
• Simplicity of tape machining in the green state with feature size of 10µm to 20mm;
• Mass production methods can be readily applied;
• Thermo-physical properties can be promptly modified, e.g. thermal conductivity of single layers;
• Other technologies can be integrated, using its hybrid nature;
• Tapes of different compositions can be formulated to obtain desired layer properties, e.g. magnetic permeability;
• Layer count can be high;
• Possibility of auto-packed devices fabrication
• Fabrication techniques are relatively simple, inexpensive and environmentally benign.
Green Tapes are easily fabricated while still in the green, they are soft, pliable, and easily dissolved and abraded. Once the
material is fired and fully sintered, it becomes tough and highly rigid. LTCC ceramics have uniform grain structures. The
primary material is alumina Al2O3. A glass frit is incorporated as part of the binder to lower processing temperatures, insure
non-permeability of the final material, and material compatibility with thick film technology. An organic vehicle is part of
the initial tape to provide the desired rheology. The tapes are cast onto a substrate (usually Mylar™) by doctor blade to
different thickness in the range of 100 to 400 µm.
The co-fired ceramics are sometimes referred to as Green Tapes or "GT” (a trademark of DuPont Photopolymer &
Electronic Materials).
One of the most important aspects of the GT technology is that it provides a very convenient medium for fabricating threedimensional fluidic structures using multiple layers. In the green state, the layers are conventionally processed to form the
features needed for the overall function of the 3D structure such as vias, cavities, channels, and internal electrical elements
such as capacitors, resistors, and interconnections. Complete processing sequence for green ceramic tapes is depicted in
Figure 2.
Individual layers are then stacked, registered, and laminated to yield the desired structure. In the conventional processing
the registration holes and the vias are usually punched. Subsequently, heat and pressure are applied to the stack to complete
the lamination process using pressures of 3000 PSI, at 90°C.
Slit & Blank
Pre-Condition
Layer 1
Layer 2
Layer n
Structure
Formation
Structure
Formation
Structure
Formation
Via Fill
Via Fill
Via Fill
Film deposition
Film Deposition
Lamination
Firing
Test
Figure 2 Green ceramic tape processing
The laminates are then ready for sintering, which is done in an air furnace 875 °C. during the sintering process the tapes
shrink in all dimensions. DuPont 951 GT™ tape shrinks 12 % in the x, y plane and 15 % along the z-axis and can be
compensated for in the design process.
3. HYBRID MESO-SCALE VALVE
Sensors and actuators with promising characteristics in aggressive environments have been developed using low
temperature co-fired ceramic tape technology. Our group has developed gas flow sensors[4], eddy current proximity
sensors[6] and other devices [5]. In the present paper we report our work on an electro-magnetically actuated normally
closed valve [7].
Microvalves are necessary to execute fluid control functions in micro-fluidic applications, some advantages of
miniaturization of this devices are:
•
•
•
•
•
Small sizes;
Short response time;
Low power consumption;
Low inactive volume;
Good dynamic characteristics.
Main problems with silicon micro-machined valves are: Manipulation of biological fluids having cells or bacteria with
dimensions in the hundreds of microns; Moving parts using for microvalves can block or clog the devices.
Several implementations of micro-valves using silicon technologies appear in the literature [8] but so few using hybrid
techniques. In this work is presented a non-moving parts hybrid electromagnetic meso-scale valve, fabricated using LTCC,
thick film and silicon technologies.
3.1
Electromagnetic actuation
Electromagnetic techniques are suitable to hybrid meso-systems, because: can generate large forces; can produce large
displacements; has good performance with temperature, adequate velocity response and is a robust and non expensive
technique [9], [3]. Forces of magnetic origin can be generated by the interaction of a magnetic field intensity H with an
electrical current I or a Magnetization M. Lorenz force for a wire with current I is, see [2]:
dF = µo ⋅ I ⋅ ds × H
For a vertical actuator the force generated by the interaction of a magnetic field intensity Hz created by a current I in a coil
and a permanent Magnet with magnetization Mz is:
dF = M z
d
H dV
dz z
So the force generated in this scheme depends on the Hz rate of change. So if the magnet is placed in a flexible spring, the
applied force will be according to[10]:
Fz = Mz ∫
d
H z dV
dz
Hz for given geometry can be calculated integrating the Biot-Savart law. As a result of this force the spring generate a
displacement proportional to the force divided by the equivalent spring constant k.
∆z = Fz ⋅ k −1
In figure 3 is displayed the block diagram of an electromagnetic actuator, comprising a magnet a coil, to create interaction
forces generated by a magnetic field intensity and a magnetization, and a flexible spring to obtain the desired displacement.
∆z
dFz
Magnetization
(Mz)
Resultant
Force in
Z direction
Elastic
Element
(Diaphragm)
Magnetic Field
Intensity
Hz produced
by Coil
Figure 3 Electromagnetic actuator block diagram
It is clear that this electromagnetic actuator approach can be readily used to implement the proposed meso-scale valve.
4. HYBRID MESO-SCALE VALVE IMPLEMENTATION
Using some of LTCC technology possibilities it was implemented a hybrid meso-scale valve. This device has a multilayer
coil, a fluidic system and a flexible diaphragm with a bonded magnet, associated to a media interface, as shown in figure 4.
Figure 4 Conception of a hybrid meso-scale valve.
3.1
Fluidic Subsystem
Fluidic subsystem can be implemented with three LTCC tapes as presented in figure 5. Top layer inlets carry work fluid to a
channel located in the intermediate layer and to the meso-scale valve chamber third layer is channel base.
Figure 5 Meso-scale valve fluidic system
Silicon flexible diaphragm is attached to top LTCC layer using polysiloxane deposited with a suitable dispenser, this was
done in order to obtain the correct distance between center coil and magnet. A Polysiloxane Valve seat is also deposited
onto the top layer, with a controlled dispenser, to prevent valve leak.
In figure 6 its possible to visualize Volumetric Flow Vs. input pressure for fabricated fluidic subsystem with fluid conduits
of 400 x200 µm.
Volumetric Flow for open Microvalve
1,80
Volumetric
Flow in
cm3/min
1,60
1,40
1,20
1,00
0,80
0,60
0,40
0,20
0,00
0
2
4
6
8
10
12
14
16
18
20
Pressure in kPascal
Figure 6 Volumetric Flow for open Meso-scale valve.
3.2 Flexible Diaphragm
Flexible diaphragm associated with a rare earth magnet allow electromagnetic actuation, it was implemented using silicon
technology for a square spiral spring that is covered with an polisiloxane film. In figure 7 it is depicted a process for flexible
diaphragm fabrication.
Si p++
Thickness
Definition by
Diffusion
Aluminum
mask
deposition
Definition of
d
Spring
geometry
Silicon
Si p++
Silício
Al
PhotoResist
Si p++
Silício
Definition
of Area
and RTV
dispensing
RTV
Si p++
Silício
Silicon
PhotoResist
Al
Si p++
Silicon
Plasma etching
Silicon
Plasma
etching
Cleaning &
Si Nitride
deposition
Al
Si p++
Silicon
Backside Si
anisotropic
Etching and
diaphragm
release
d
RTV
Si p++
Silicon
Cleaning &
magnet
bonding
Anisotropic etching
Si Nitride
Mask
Magnet
Silicon
Si p++
RTV
Figure 7 Flexible diaphragm fabrication process
Flexible diaphragms were fabricated using the described process as well as using LTCC ceramics as shown in digital
photographs depicted in figure 8. Flexible diaphragms were simulated using F.E.M techniques giving results with good
agreement with experimental measures.
- Silicon
- LTCC ceramics
Figure 8 Some flexible diaphragms fabricated
3.3 Actuating coil implementation
The hybrid coil consists of several layers of planar spiral coil layers. We choose a square spiral coil of 1 x 1 cm2 designed
with special geometry in order to have small quantity of interconnecting vias and connected as to preserve the magnetic
field direction. Figure 9 displays actuating coil geometry and layer interconnection of fabricated device.
Even Layer
Odd Layer
Layer 1
Even Layer
Layer 2
Odd Layer
Layer 3
Even Layer
Layer 4
Odd Layer
Layer 5
Even Layer
Base
Figure 9 Actuating coil geometry and layer interconnection
A single layer was designed to have silver conductors of 80 µm lines and 10 µm thickness with 80 µm space between lines,
rendering a 20 turn single layer coil. The total coil resistance is high (120 Ohms), but this is possible to lower this value
using tape machining techniques to obtain 60 µm thickness. Due to coil high resistance, thermal considerations limits the
current to 150 mA.
Typical single layer with twenty silver turns and cross section of central part of coil can be seen in figure 10. Vias of 250
µm were used to ensure layer interconnection
A five and eight layers coil have been fabricated using DuPont 951 Green Tape system following typical LTCC tape
process sequence. Fluidic subsystem was also fabricated at the same time.
10 mm
COIL TURN
COIL VIA
250 µm
Figure 10 Typical single layer and cross section of central part of coil
In figure 11 can be seen experimental measures of magnetic field generated as a function of coil center separation.
11
Field variation in z direction
10
B for 100mA
9
8
7
Magnetic
Field in
Gauss
6
5
4
1,0
1,5
2,0
2,5
Distance from coil center in (mm)
Figure 12 Typical magnetic field generated by coil with 100 mA excitation
5. EXPERIMENTAL WORK
Complete fabricated meso-scale valve is shown in figure 13. This is a hybrid device which utilizes a purely LTCC tape
electro-magnet and fluid flow manifold, combined with an anisotropically etched silicon rectangular planar spring, and a
high-energy product SmCo mini-permanent magnet.
Device dimensions are in the meso (intermediate) range with the smallest features (fluid conduit in the manifold) of 400
micrometers and the largest (the electromagnet, coil) of 12 mm. All parts of the electromagnet and the fluid flow channels
were machined from DuPont 951 series, alumina based LTCC tapes utilizing either a numerically controlled milling
machine, a puncher or an isotropic etching technique involving the glassy binder of a partially sintered LTCC tape.
As shown before multilayer actuating coil and fluidic subsystem are sintered together. Flexible diaphragm is bonded to
LTCC substrate using a dispensed gasket of polysiloxane as well as inlet plastic flanges.
Fluid Out
Fluid In
Magnet
Flexible diaphragm
12 mm
Figure 13 Hybrid meso-scale valve
The hybrid device consists of 5 layers of planar spiral coils. The total coil resistance is high (120 Ohms). Using a 900 Gauss
SmCo magnet (1mm diam) was obtained 200 micrometers deflection of the silicon 30 microns thick rectangular planar
spring with a polysiloxane sealing.
In Figure 14 diaphragm displacement of flexible diaphragm vs. current coil is shown, having distance between magnet and
coil as a parameter.
250
Current 200
(mA)
150
100
Diaphragm displacement Vs coil current
for various magnet-coil distances
2 mm
2.25mm
2.5 mm
50
0
-50
-100
-150
-200
-250
-150
-100
-50
0
50
100
150
Diaphragm displacement (µm)
Figure 14 Flexible diaphragm displacement vs. coil current
CONCLUSIONS
We presented in this work an inexpensive, easy to fabricate hybrid meso-scale valve manufactured in LTCC technology the
same as many IC packaging systems. Fabrication procedures were delineated and some experimental results were presented
This techniques may lead to fluidic meso-systems where the fluidic devices can simultaneously serve as part of the IC
package.
AKNOWLEDGEMENTS
One of us (MGR) would like to thank FAPESP for partial funding of the research. JJ.S-A would like to acknowledge the
support of DARPA grant no. N66001-97-1-8911 and the generosity and support of DuPont Photopolymer and Electronic
Materials, our industrial collaborators.
We would like to acknowledge Dr. Edgar Charry Rodriguez for very useful discussions.
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