Microscopic Batteries for Remote    
Sensors and Other Microcircuits    
Wafer with 1200 Ni/Zn Cells
The increasing miniaturization of electronics has created a continuing application of integrated circuit in our lives.  An important developing area for integrated circuit is remote sensors.  An ever-expanding variety of sensors is available, to help monitor such things as temperature, humidity, motion, pressure, radiation, the presence of certain chemicals.  Sen-sors detect physical conditions from their environment, and convert the information into an interpretable form, usually an electrical signal.  Most sensors require electrical energy to operate, and this is usually provided through wires to and from a host.  
It is often desirable to have inexpensive sensors that do not need to be physically wired to either an energy source for operation or a host for transmitting information from the sensor.  Such remote sensors would greatly simplify many complex situations where monitoring and sensing are needed.  For example, battleships of the future will require as many as a million sensors located throughout the ship.  Obviously, it is not desirable to hard-wire all sensors.  Also, the increasing need to monitor the health of our environment mandates more simplified and powerful sensor concepts than are presently available.
The difficulty in developing remote sensors has been the energy source. A viable remote sensor would be able to store enough energy to operate for a suitable length of time; alternately, the sensor would be able to collect energy from the environment (such as from solar or other radiative energy, motion, or heat) to operate the sensor.  Batteries are contem-plated for sensors that are intended to be autonomous.  However, the strategy of using bat-teries that are presently available has been wholly unsatisfactory, as the battery is usually the largest contributor to both the overall weight and volume of the sensor device. One reason for this is the lack of availability of miniature batteries. The smallest commercial batteries, such as the button cells used in watches, calculators and hearing aids, are huge when com-pared with the rest of the sensor device.  A second reason is the need for energy supply at (relatively) high power levels.  High power is often needed instantaneously for actuating sensors or wireless communication.  Commercially available batteries are typically designed to maximize the amount of energy they store, rather than to release their stored energy at high power levels. Consequently, batteries must often be overly large to supply needed power to a circuit.  A further limitation of small commercial batteries is that few are secon-dary batteries (i.e., rechargeable), a requirement for many sensor applications.
Microscopic Batteries for Remote Sensors
In response to the energy storage and needs of MEMS (“MicroElectroMechanical Systems,” an important class of sensors), scientists from Bipolar Technologies have devel-oped a family of microscopic batteries which can be used with remote sensors and other microcircuits.  Batteries as thin as a human hair can be made, using the same low-cost/high volume fabrication processes used to make MEMS and other integrated circuits.  Both re-chargeable and nonrechargeable batteries are made.  They can be formed into almost any shape or size, with varying voltage, capacity, and power.
Our microscopic batteries can be implemented in a variety of different ways.  First, they can be made separately from an existing device, and then connected externally.  Alternately, batteries can be made as an integral element of the MEMS (or other microcircuit).  In other words, the batteries can be built as part of an integrated circuit.  In this manner, the same high volume/low cost processes used to make other integrated circuit devices can be used to make devices which have batteries built in.  An integrated circuit could contain a single cell, or an array of cells, which can be connected in varying series/parallel combinations to achieve different operating voltages or capacities.
Ni/Zn Cells in FabricationMicroscopic batteries can also be used to reduce overall power consumption of some integrated circuits.  They can be placed throughout a circuit, for energy storage and supply for given moving parts.  Power is supplied to the circuit at low, stored temporarily in small cells, and then released upon need at high power.  In this manner, the energy effi-ciency of many devices can be improved significantly.
Applications
Microscopic Remote Sensor Concept
Remote sensors are perhaps the biggest immediate application for integrable micro-scopic batteries. The sensor element operates and collects a measurement signal, using en-ergy supplied from the battery; the processed signal is then sent from the transmitter, also using the energy from the battery.  The energy scavenger (such as miniature solar collectors) replenishes the energy removed from the battery. 
The entire device operates at average power levels of approximately 0.01 mW.  Higher power is required during signal transmission, which lasts a few ms at levels of several mW.  Numerous examples of “microsensors” (i.e.  < 1 cm2, operating at ≤ 1 mW).  Miniature signal transmitters (R/F, infrared, optical) are also available, which operate at relatively low power, and have transmission ranges of 10 – 100 m.
Remote Autonomous Sensor ConceptThe availability of the three component technologies for a remote sensor (sensor element, signal transmitter, and energy source) can now enable a new class of microscopic sensors.  This new class of sensors will operate at average power levels of the order of 10 µW, and will be under 1 cm2 in area.  This sensor technology is enabled by our micro-scopic, batteries.  A miniature hybrid power supply, containing a battery and energy harvester (such as a photovoltaic array), sufficient to power such a sensor, would be < 1 cm2.  Sensors can operate for an extended time without energy supplied from the scavenger, such as when a satellite is on the dark portion of its orbit, during the nighttime in a battlefield, or when the lights are off in a battleship.  The battery also can operate the sensor device at the high power levels that are periodically needed (during signal transmission, for example).  Typical energy scavengers cannot.  Also, the high power density (W/cm2) achieved with our batteries is not possible with other microscopic battery options using a solid electrolyte. 
Flexible Sensing Surfaces (Space)
Another application of microscopic batteries is with flexible sensing surfaces.  Many space system designs would greatly benefit from the availability of sensing arrays that can be conformably mounted to a non-flat satellite or vehicle surface. Such surfaces require autonomous power sources, perhaps powered through integrated solar cell panels. The sensor, circuitry, microscopic battery, and solar cells can all be mounted on a flexible membrane surface using conventional integrated circuit mounting techniques such as die attach/wire bonding or flip chip.  Autonomous power sources are necessary in order to re-duce power consumption and improve reliability.
Flexible Sensing Surface SchematicMicroscopic batteries can be formed on a flexible substrate, or can be fabricated as part of the flexible membrane substrate on which integrated circuits and sensors are mounted.  This substrate can also be used to mount other system components. The figure below illustrates a microscopic battery fabricated on a polymeric membrane, can also serve as the carrier for devices, integrated circuits, solar cells for recharging the microscopic bat-teries, and the interconnect to electrically connect the components. A protective coating is deposited to protect all components.  This membrane can be used as part of a smart sensing surface.
Other Applications
Several applications for our microscopic batteries have been shown here.  Obviously, many others are possible.  It is our hope that microscopic batteries will be widely used, to make even greater improvements in MEMS and other IC systems.
Microscopic Battery Technology
We have to this point focused on two different types of well-known battery chemistries.  Others are under development.  Both types have good rechargeability, and can be made into many different sizes and shapes.
The first type uses the nickel-zinc chemistry.  The cell consists of a nickel oxide positive electrode, a potassium hydroxide/water electrolyte, and a zinc negative electrode.  After the various cell components are fabricated, the cell is filled with electrolyte and then sealed under a polymer film.  The nickel/zinc cell offers the advantage of very high power operation.  It has an operating voltage of 1.5 - 1.6 V per cell. 
The other battery type, still under development, is based on the lithium-ion chemis-try.  The cell employs a metal oxide positive electrode, a solid polymer electrolyte, and a carbon anode.  The lithium ion battery operates at lower power than the nickel/zinc cell, but it holds nearly four times the energy (per volume). It has a higher cell voltage (4 V).  Fur-ther, the all-solid state construction makes it even easier to fabricate cell in a wide variety of curved.  The choice of battery type will depend on the application.  The attached data sheets demonstrate the relative capabilities of the two battery types, in addressing a sample applica-tion requiring 22 V operation.

Characteristics of Nickel/Zinc Microscopic Batteries
Essentially any size (> 0.0001 cm2) is possible.  One is shown here.
It should also be noted that these characteristics are based on stand-alone batteries, as shown in the photograph below, and more optimum arrangements are possible for a given application.

2 mm X 1mm Cells
Property Single Cell
 Sixteen Cell Array
Capacity (µA•hr) 18.9
 18.9
Peak Current (µA) ~2000
 ~2000
Discharge Power (1 µA; µW) 1.5
 24
Discharge Time (1 µA; hr) 25
 25
Recharge Time (min) 1-15
 1-15
Operating Voltage (V) 1.50-1.65
 1.50-1.65
Length (mm) 1.68
 7.02
Width (mm) 1.09
 4.66
Thickness (µm) 120
 120
Ni/Zn Cell PhotoNI/Zn Cell Schematic











Characteristics of Lithium/Ion Microscopic Batteries

2 mm X 2 mm Cells
Property
 Single Cell
 Six Cell Array
Capacity (µA•hr) 56
 56
Peak Current (µA) 800
 800
Discharge Power (1 µA; µW) 4
 24
Discharge Time (1 µA; hr)
56
 56
Recharge Time (min) 2-30
 2-30
Operating Voltage (V) 3.3-4.0
 20-34
Length (mm) 2
 5
Width (mm) 2
 7
Thickness (µm) 200
 200
Li-Ion Cell Schematic








1 cm X 1 cm Cells
Property
 Single Cell
 Six Cell Array
Capacity A•hr) 1380
 1380
Peak Current (µA) 20000
 20000
Discharge Power (1 µA; µW) 4
 24
Discharge Time (1 µA; hr)
1380
 1380
Recharge Time (min) 5-15
 5-15
Operating Voltage (V) 3.3-4.0
 20-34
Length (mm) 10
 21
Width (mm) 10
 32
Thickness (µm)
 200
 200