RF _amp; MICROPROCESSOR CIRCUITS FOR MEMS SENSOR PACKAGING

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RF _amp; MICROPROCESSOR CIRCUITS FOR MEMS SENSOR PACKAGING Powered By Docstoc
					       RF & MICROPROCESSOR CIRCUITS FOR MEMS SENSOR
                        PACKAGING
                                              Thomas Jorgensen
                                                 Zensys A/S
                                                  Denmark
                                              www.zen-sys.com
                                              tjo@zen-sys.com
                    Co-Author                                                     Co-Author
                  E. V. Thomsen                                                 Jens Branebjerg
      Department of Micro and Nanotechnology                                        DELTA
      Technical University of Denmark (DTU)                                        Denmark
                     Denmark                                                     www.delta.dk
                  evt@mic.dtu.dk                                                 jab@delta.dk


ABSTRACT
In the resent years wireless Home Automation
products like lightning, thermostats blinds/drapes
and appliance control, energy management, access
control and building automation has reached the
market. With the combination of highly integrated
RF and micro-processor platform and micro
electromechanical systems (MEMS) the wireless
sensors and actuators of tomorrow will be ultra
cheap and small and open new yet un-thought
markets.
These ultra small sensors silently and wirelessly
collect data, for instance monitoring light,
temperature and many other factors. The complex
integration of MEMS, processing circuitry, RF
communication circuitry and energy source results
in a unique and self-contained system that not only      Figure 1 The Z-Wave Module is dedicated for Home
detects its environment but also has the capability      Automation and contains a complete HW and SW platform
of actively change it.                                   including micro-processor, memory, different HW interfaces,
The high integration of the sensors requires single      crystal, RF circuitry and SW protocol. The Module is 11x16mm
                                                         (0.43x0.63”) suitable for most applications. The future requires
chip implementation of the MEMS sensors and the          even smaller and higher integration level.
RF and micro-processor circuitry. With the use of
3-D assembly and MEMS new standards are set for           and appliance control, energy management, access
the product assembly and new materials.                   control and building automation.
This paper will cover some design considerations of       Low cost "ready-to-use" RF platforms (hardware
a low cost wireless Home Environment (HE) sensor          and software) already exist, which in an easy way
including RF and micro-processor circuitry, MEMS          can be integrated in to consumer products without
sensors, the interconnections and packaging. The          having extensive RF and network protocol
low cost is crucial if such small sensors should          knowledge.
penetrate the marked.                                     The next generation of Home Automation products
                                                          must have ultra low cost and be ultra small using
Key words: Wireless, Home Automation, MEMS,               the    latest    technologies    when    designing
Sensor                                                    microprocessor, RF transceiver etc. Additionally
                                                          small MEMS (Micro Electro-Mechanical System)
INTRODUCTION                                              sensors are used opening new and yet un-thought
In the resent years there has been a high interest for    markets. With the use of pervasive computing a
wireless control and monitoring of residential            system with wireless sensors and actuators are able
products like lightning, thermostats, blinds/drapes       to measure and change different parameters keeping
                                                          the environment in balance.



Z-Wave is a trademark owned by Zensys A/S
HOME AUTOMATION                                          By distributing several sensors in each room the
In the Home Automation world the wireless                above parameters can be measured and adjusted in
sensor/actuator products primary objective is to         different areas of the room.
bestow quality of life to the user. All the nodes will   The temperature sensors assure equal temperature
become an important part of our lives, but in ways       in all areas of the room by sending commands to the
that we don’t really notice. A pervasive system is       “local” radiators/floor heating system to adjust the
unobtrusively embedded in the home environment.          temperature (for example the temperature at the
Pervasive computing already exist today including        window is lower than in the middle of the room).
automatic wireless control and monitoring of             The temperature sensors can likewise detect air
lightning, thermostats, movement sensors, air-           drafts by measuring temperature differences
conditioning etc. using “ready-to-use” RF platforms      between the sensors.
[1] containing both hardware (HW) and software           The light sensors assure the wanted light level
(SW) see illustration on figure 1. With the addition     setting in the whole room and the humidity sensors
of newer, smaller, smarter and cheaper                   assure correct humidity in the rooms enabling
sensors/actuators the home gets more and more            optimal comfort and well being. The many gas
intelligent. It does not only mean saving money on       sensors can fast detect smoke from a starting fire
electricity bills, gas etc. due to the energy            assuring fast reaction and decrease the likelihood of
management but does also improve the home                a catastrophic fire.
environment in many ways and thereby improves
quality of life in the home.                             Sensor Platform
The HW platform contains the microprocessor,             The HE Sensor Platform must contain the following
memory, RF transceiver circuitry and the sensor (or      four blocks:
actuator). The SW protocol assures that the
products in a standardized way communicates with             1.   Micro-controller Platform
each other.                                                  2.   RF Transceiver
In order to have a true mass-market penetration the          3.   RF Front-end
overall wireless sensor platform cost must be very           4.   MEMS Sensors
low and easy to install/use. The right tradeoffs             5.   SW Protocol
between technology and cost must therefore be
taken.
                                                         PLATFORM CONSIDERATIONS
HOME ENVIRONMENT SENSOR                                  The HE sensor requires a highly reliable RF
The next generation of sensors will use highly           platform using leading edge technologies ranging
integrated micro-processor, memory, RF and               from chip design to final product design including
MEMS platforms. MEMS is the integration of               selection of wafer technology, layout, assembly
mechanical elements, sensors and actuators on a          methodology, production test etc.
silicon    substrate   through      micro-fabrication    In order to have a true mass market the following
technology. The MEMS are fabricated using IC             prioritized considerations must be taken into
compatible “micro-machining” processes that              account when designing the above five platform
selective etch away parts in the silicon wafer or add    blocks:
new structural layers to form mechanical and                  • Cost
electromechanical devices.                                    • Power Consumption
These highly integrated sensors opens new markets             • Reliability
like home environment control, which help you to
                                                              • Size
feel comfortable at home resulting in a better well
being and health. Some of the parameters, which          The cost must be ultra low as many sensors will be
could be measured by the Home Environment (HE)           located in each room. The power consumption must
sensor, are:                                             also be low in order to get battery lifetime of 10
     • Temperature                                       years or even more. The sensor platform (micro
                                                         controller, RF and MEMS) reliability must be high
     • Light intensity
                                                         in the environments the sensor is operating to assure
     • Humidity
                                                         correct and reliable measurements. Last but not
     • Concentration of different gasses (gas,           least the HE sensor must be small as it ideally is
         fire)                                           “hidden” and cannot been seen by the user.




Z-Wave is a trademark owned by Zensys A/S
MICROCONTROLLER PLATFORM                        Z-Wave Single Chip
The Micro processor platform must contain                             System
                                                                                                 Pwr-
                                                                                               Reset /
                                                                                                  On
                                                   Flash Memory                    Timer
an instruction effective processor (CPU),                              Clock          3
                                                                                               Brown-
                                                                                                 out
different HW interfaces, memory and a
wake-up time. An instruction/power                Application SW
                                                                      Timer        8051        10-Bit
effective processor assures fast processing         Protocol SW         2           SFR         ADC
time and low power consumption. To avoid
additional chips/circuitry the micro-                                                                    RF Transceiver
controller must contain as many interfaces
(like ADC’s etc.) as possible to keep the
                                                  SRAM      SRAM                     8051
cost down. The memory should be as small          2kbyte    128 bytes                CPU
as possible and still have room for the
                                                                         Standard 8051: UART0, Timer
protocol and the application SW in order to                 Power                 0, Timer 1
                                                   Triac
keep cost and power consumption as low as           Ctrl
                                                            Saving
                                                              Ctrl
possible.
In order to have optimum battery lifetime
                                                  Intrp.      SPI
power management is of high importance              Ctrl      Ctrl                        I/O Interfaces
and requires careful design if the power
circuitry of the chips/MEMS. The power
management must be an integrated part of        Figure 2 Single Chip solution containing CPU, memory for
the protocol, which means that the micro        protocol and application SW, timers, different HW interfaces
                                                and RF transceiver. The high integration assures low cost and
controller assures that only the circuitry that high reliability.
needs to be powered is on and the remaining
circuitry is powered down. The HE sensor
                                                          868MHZ – 870MHz) keeping power consumption
does not need to make measurements continuously
                                                          low.
but perhaps only every 2-5 minutes depending of
                                                          Frequency Shift Keying (FSK) for RF modulation is
the wanted response time. Therefore the whole
                                                          effective for this application as it has a low level of
sensor must be powered down between
                                                          complexity and has high power efficiency keeping
measurements.
                                                          the cost and power consumption low.
RF TRANSCEIVER
                                                              Microcontroller & RF Transceiver Integr. Level
In the resent years RF circuits like VCO tank and
                                                              Integration of micro processor, memory, different
loop filter has been integrated on-chip and thereby
                                                              HW interfaces, and the RF transceiver into one
reduced the number of external components
                                                              single chip not only reduces the overall RF module
dramatically (typically 12-20 passive components)
                                                              cost but does also reduce the die area and improves
to only very few or even no external passive
                                                              the overall performance. A block diagram of a
components. Integrating the passive components on
                                                              single chip solution is shown in figure 2.
die level does require additional die area, but
                                                              The highly integrated single chip contains all the
enables design of high-Q (quality factor)
                                                              circuitries, which are required to control/monitor
components and reduces the interconnection
                                                              most Home Automation products and HE sensor
distances and thereby makes it more reliable and
                                                              products and have these wirelessly communicate
immune to external noise.
                                                              with each other [1].
In order to achieve long battery lifetime the wireless
devices have to transmit with as low power as
                                                              MEMS RF Switch
possible. The RF transceiver must therefore have a
                                                              As the RF transceiver has both a transmitter and a
high receiver sensitivity. This is achieved by an
                                                              receiver a RF switch has to switch between the
effective RF receiver chip design and a high RF
                                                              transmitter and the receiver when transmitting and
transceiver/RF front-end integration.
                                                              receiving data respectively. This could be done
The frequency modulation methodology used must
                                                              using a MEMS RF switch.
also be selected with power consumption and die
                                                              The MEMS RF switch structure can be
area usage in mind. As a general rule, the use of
                                                              implemented as shown in Figure 3. The MEMS RF
high frequencies leads to a higher power
                                                              switch contains the micro mechanical switch and
consumption. Therefore the chips can with great
                                                              some test structure, which is made on either a high
advantage use the sub-Giga hertz license free ISM
                                                              resistive silicon wafer or a glass substrate
frequency band (US: 902MHz-928MHz, EU:
                                                              (Integrated Passive Device - IPD).




Z-Wave is a trademark owned by Zensys A/S
                                                         of such wireless sensor nodes continues to decrease
                                                         and as their prevalence increases, effective power
                                                         supplies become a larger problem.
                                                         Batteries has a fairly stable voltage, electronic
                                                         devices can often be run directly from the battery
                                                         without any intervening power electronics. Non-
                                                         chargeable batteries are called primary power
                                                         sources where as rechargeable batteries are called
                                                         secondary power source. Rechargeable batteries
                                                         therefore, in the context of wireless sensor
                                                         networks, require another primary power source to
 Figure 3 MEMS and RF Chip Interconnection               charge them. In most cases it would be cost
                                                         prohibitive to manually recharge the batteries in
The micromechanical switch is an electro-statically      each device. In the future an energy-scavenging
actuated device containing a metaltipped                 source on the node itself, such as a solar cell, could
polysilicon actuator and a control electrode made of     be used to recharge the battery. One item to
polysilicon. The metallized tip connects wires on        consider when using rechargeable batteries is that
the RF counter-chip.                                     electronics to control the charging profile must be
The actuator has an internal stress gradient, which      used increasing the overall cost.
curls the actuator away from the device. The metal       One type of batteries, which may become an ideal
tip shorts two pads on the counter RF chip.              battery for wireless sensors is the thick film battery
Applying a voltage to the polysilicon electrodes         [3] as is undergoing a large development progress.
underneath pulls back the actuators and opens the        This type of battery allows printing of layers with
two pads. The actuators themselves are grounded.         specific thickness. This flexibility enables the
The electrical current does not flow along the           realization of electrochemical cells with different
actuator, because a layer of silicon nitride isolates    capacities.
the contact metal tip from the actuator arm.             The ultra-thin battery (see Figure 4) consists of a
As the number of masks on the IPD substrate adds         silver oxide cathode and a zinc anode, whereby
to cost the number of masks should be kept on a          these active layers in an well defined way can be
minimum.                                                 applied on the current collectors by thick film
The IPD is mounted onto the RF device using flip         technology.
chip bonding to keep the size and thereby the cost
of the IPD down. Compared to wire bonding the
flip chip solder balling (or stud bumping) process is
more expensive today. The cost has to be reduced in
order for the process to be used in these types of
low cost HE sensors.
The cost of RF Switch IPD as a standalone IPD is
relatively high due to the manufacturing, testing and
mounting process. The RF Switch IPD should
therefore with great advantage also contain other
MEMS circuitries (sensors etc.) as long they uses
                                                           Figure 4 Structure of a thick film battery
the same processes.

ENERGY SOURCE                                            Some sensors are producible in thick film
The scaling down in size and cost of CMOS                technology and the sensor and battery may be
electronics has far outpaced the scaling of energy       produced on the same substrate using the same
density in batteries, which are by far the most          process and thereby decrease manufacturing/testing
prevalent power source currently used. Therefore,        cost.
the power supply is usually the largest and most         The future energy source vision is to have a cubic
expensive component of the emerging wireless             millimeter battery providing enough power to
sensor nodes being proposed and designed. This has       perform simple tasks each second for 10 years and
in the resent years led to a large research in battery   maybe even square millimeter solar cells as
technology based around the vision of ubiquitous         secondary power source providing enough power to
networks of wireless sensors. As the size and cost       have a sensor running forever.




Z-Wave is a trademark owned by Zensys A/S
MEMS Sensor
The primary requirements of the sensors must be
determined when selecting the sensors for any
sensor-based system. These are selectivity,
sensitivity and stability.
For Home Automation application especially long-
term stability is critical, because recalibration of the
sensors is normally not an option and stability of 10
or more years are required. The requirement on
selectivity and sensitivity is less demanding.
MEMS is known for its long term stability and
therefore a preferred sensor technology in safety
critical application like airbag sensors, tire pressure
sensors and gyros in cars.
When taking the requirements of low cost, low
power consumption and small size into account
MEMS are favorable for Home Automation and HE
applications.

MEMS Technology                                            Figure 5 Pressure sensor, humidity sensor, CO2
The MEMS technology is very similar to the                 sensor [7].
semiconductor technology as many of the materials
and processes are the same. These processes are
known for their extreme high yield, and long term          Generally, most MEMS sensors are not directly
stability and low cost in mass production. The best        compatible with conventional CMOS processing
know example of this is the extreme purity and             and integration to a complete monolithic system
microscopic homogeny of monocrystalline silicon,           will not always be possible. As an example, many
which is the basis of microelectronics and also            MEMS devices are made using bulk silicon
gives the basic properties of semiconductor and            micromachining where KOH is used for anisotropic
MEMS sensors.                                              etching of silicon – a process not directly
Depending on the transducer principle used in              compatible with CMOS processing. This suggests a
MEMS sensors different properties can be                   hybrid multifunctional micro system where all
optimised. When low power is required sensors              sensors are made on one chip and the sensor
using capacitive transducer principles are                 electronics and the required microprocessor and RF
favourable especially at low frequencies. Today            transceiver on another chip. Using advanced
MEMS components are available that can be used             packaging schemes these chips are assembled into a
in HE sensors. In Fig 5 shows 2 sensors based on           system. Here the challenge is to provide a low cost
capacitive readout and one CO2 sensor based on an          packaging that allows both tight integration and an
optical measurement principle.                             open window to the sensors that requires direct
                                                           contact to the surroundings.
Integration of several sensors on one chip                 To realize multi sensor systems a shift from current
When striving towards miniaturised sensor systems          mainstream MEMS sensor research focus is
several new challenges for the MEMS technology             necessary. In MEMS research the focus is on
appear. As the dimensions of the sensor system             selectivity and sensitivity of a single sensor whereas
goes down the integration level of the sensors must        for sensor systems the important point is to have a
increase. For the tightest integration several sensors     high stability of the complete system.
must be fabricated on the same sensor chip.                As an example of a recent multi-sensor chip multi-
                                                           sensor is the chip developed for fisheries research
                                                           [6] shown in figure 6. This sensor can determine
                                                           pressure, temperature, light intensity and the
                                                           conductivity of the water and an O-ring is used for
                                                           sealing the remaining circuitry from the water as
                                                           outlined on Figure 7. This kind of sensor can be
                                                           used in a miniaturised data storage tag that is
                                                           mounted on fish. After tagging the fish is released




Z-Wave is a trademark owned by Zensys A/S
to the sea and later, when fishermen return the tag,    Productions and test of MEMS sensors
the stored data can be retrieved. Such tags are used    It is commonly accepted that between 40-60% of
in today’s fisheries research. The sensor is made       the productions cost of MEMS is in the packaging
using bulk micromachining of silicon where the          and test. In order for a disruptive technology, like
pressure sensor is made using KOH anisotropic           MEMS, to reach its ultimate potential,
etching. Implantation of Boron forms the resistors      understanding the cost components followed by
used for the piezoresistive pressure sensor, for the    applying an aggressive cost reduction program is
pn junction used to measure the light intensity and     the only way a MEMS will succeed.
for the thermistor based temperature sensor.            The productions cost of MEMS based sensor
                                                        systems are mainly determined by:

                                                             •   Chip area on the MEMS sensor chip
                                                             •   Complexity in automatic assembly and
                                                                 packaging
                                                             •   Testing time

                                                        Most of the chip area of MEMS sensor is used for
                                                        interconnection pads and for sealing the sensor area
                                                        exposed to the environment from the electronics on
                                                        the chip. Integration of all sensors on one MEMS
                                                        chip will therefore minimize the total area and
                                                        thereby the cost.
                                                        The costs of assembly and packaging are mainly
                                                        given by the number of chips in the system, the
                                                        number of critical sealing processes and the ease of
                                                        automations. When integrating all sensors on one
                                                        chip all elements of cost are minimized. Further
                                                        ease of assembly and packaging is very much given
                                                        by optimizing the MEMS chip layout for automatic
Figure 6 Picture of a 4X6 mm2 multi-sensor chip         production.
for fisheries research. The dark paths are titanium     Test and calibration of sensors are often a time
silicide conductors and the light areas are gold used   consuming process because the sensors must be
for bonding-pads and electrodes. The front-side is      exposed to different sensor stimuli like
protected by a Si3N4 film.                              temperatures, gas and humidity concentrations and
                                                        light intensities. The test and calibration are
                                                        normally done on the final product. At this point the
                                                        product cost is high and the size largest. Therefore
                                                        cost of sensors rejected in the test is high and
                                                        handling and stabilizations time during testing is
                                                        long.
                                                        To reduce the cost of MEMS sensors test and
                                                        calibration, a system has been developed for
                                                        automatic test and calibration on chip or wafer
                                                        level. In this system a wafer with many hundred or
                                                        thousands of MEMS sensor chips is exposed to the
                                                        same sensor stimuli and automatic measured by and
                                                        automatic probing station [5]. In the test system
                                                        shown in figure 8 the probing station is placed in a
Figure 7 Chip design layout. The different sensors      tank with controlled gas content and pressure.
for Temperature (T), Pressure (D), Electrical           Further temperature, light and other sensor stimuli
conductivity (C) and Light intensity (L), are centred   can be controlled as well and exposed to the MEMS
with respect to the interface O-ring shown as the       sensor chips. This test method minimizes the cost of
grey circle. The contact areas (A) are placed on the    rejects and reduces the testing time dramatically.
dry ends of the chip.                                   The measured sensor data is stored and used to
                                                        program the calibration values into the electronics




Z-Wave is a trademark owned by Zensys A/S
during the very simple final testing of the            testing all sensor parameters (light, temperature,
component.                                             pressure etc.) in the same testing sequence.



                                                       Reference:

                                                       [1] “Low Cost RF Solution for Home Automation”
                                                       Thomas Jorgensen, Zensys A/S, The IMAPS
                                                       Nordic Annual Conference, 2004, Denmark.

                                                       [2] “Power Sources for Wireless Sensor Networks”,
                                                       Roundy, S., Steingart, D., Fréchette, L., Wright, P.
                                                       K., Rabaey, J., Proc. 1st European Workshop on
                                                       Wireless Sensor Networks (EWSN '04), Berlin,
                                                       Germany, Jan.19-21, 2004

                                                       [3] “Preparation and Characterization of Thick-Film
                                                       Ni/MH Battery”, Jing-Shan Do*, Sen-Hao Yu and
                                                       Suh-Fen Cheng, Department of Chemical
                                                       Engineering, Tunghai University, Taichung, Taiwan

Figure 8 MEMS sensors chips are automatically          [4] “Ultrathin Thick-film Batteries for Application
tested using an automatic probing station placed in    in Smartcard”, Dr. Sabine Stolle, Dr. Christel
a gas tank.                                            Kretzschmar. Fraunhofer Institut für Keramische
                                                       Technologien und Sinterwerkstoffe, Dresden
SW PROTOCOL
Not only does the sensor cost have to be low but the   [5] “Functional Testing and Calibration of Sensors
SW protocol also needs to be designed for ease of      on Wafer, Die and Module Level”, Bay, J,
installation and use. The ideal scenario is that the   Branebjerg, J, DELTA Hørsholm Denmark,
SW protocol is invisible and maintenance free to       Sensor&Test 2003, Nürnberg, Germany
end-user.
                                                       [6]” FISH & CHIPS: Single Chip Silicon MEMS
                                                       CTDL Salinity and Light Sensor
CONCLUSION                                             for Use in Fisheries Research”
Wireless Home Automation products are now              A. Hyldgård, O. Hansen & E. V. Thomsen
available on the market. With the introduction of      Paper accepted for the MEMS 2005 conference
MEMS sensors and with the use of highly
integrated RF and micro-processor chips new types      [7] All sensors made by Vaisala
of products like a HE sensor can be a part of the      (www.vaisala.com)
Home Automation system improving the comfort in
the home.
In order to have a true mass-market penetration of
ultra small wireless Home Automation products and
HE sensors the overall cost must be ultra low. This
requires that the whole sensor platform is highly
integrated and the production line testing and
assembly of both MEMS and electronic must
undergo an aggressive cost reduction program in
order to succeed. As mentioned in this paper this
can be done by implementing all sensors on one die
and the RF, micro-processor, memory and sensor
electronics on another limiting the system to two
dies. MEMS sensor chips can be tested in a full
automatic probing station located in a gas tank




Z-Wave is a trademark owned by Zensys A/S

				
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