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Sensor Business Digest

September 2004

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Highly Innovative Technology Poised to Significantly Impact the Thermistor Market
In the dynamic sensor industry, a company can never be totally complacent, even if they are a long-standing provider of sensors based on conventional, widely - accepted technology. There are profound opportunities for a new sensor technology, which provides genuine, significant performance and cost benefits, and can open up new applications, to encroach upon and eventually displace existing technologies.

In the very significant market for NTC (negative temperature coefficient) thermistors, which are thermally-sensitive resistors whose resistance decreases as the temperature rises, there is clear potential for a readily - manufacturable sensing technology that would further extend the market opportunities for NTC thermistors by providing a wider temperature range, higher sensitivity, greater interchangeability, and a smaller form factor at an attractive price.

NTC thermistors comprise a very significant part of the overall thermistor market. The remainder of the thermistor market is accounted for by PTC (positive temperature coefficient) thermistors, whose resistance level increases sharply as the temperature rises over a limited range.

NTC thermistors are used in a plethora of applications, notably temperature sensing and control, as well as such applications as temperature compensation, inrush current limiting, circuit or equipment protection, surge protection, RF or microwave power measurement, sequential switching, voltage regulation or limiting, etc.

Key markets/applications for NTC thermistors include automotive (e.g., intake air temperature, coolant temperature, HVAC/climate control, heated seats, etc.); medical (e.g., fever thermometers, thermal dilution heart catheters, blood analyzers, kidney dialysis, incubators, etc.); telecommunications/office equipment (including transistor temperature stabilization, battery packs, copy machines, fax machines, printers, switching power supplies, etc.); consumer products (e.g., air conditioners, thermostats, oven temperature control, refrigeration, water heaters, washing machines, dish washers, pool and spa control, coffee makers, microwave ovens, toasters, hair dryers, ice makers, fire detection equipment); and industrial/HVAC (e.g., food processing, oven temperature control, food storage and transport, laminating equipment, hot glue dispensing equipment, thermoplastic mold equipment, soldering irons, photographic processing, HVAC temperatures probes and boiler probes, etc.).

Moreover, NTC thermistors are employed in a wide array of other applications, such as, for example, fluid and gas flow measurement, thermal conductivity gas detection, gas chromatographs, liquid level detection, laboratory analysis and research equipment, and aerospace/military applications.

For the past 50 + years NTC thermistors have been made from ceramic semiconductor oxides, such as manganese, nickel, cobalt, iron, copper, titanium, and chromium. In the traditional, well-entrenched process for fabricating NTC thermistors, a mixture of metal oxide powders is combined with a binder into the appropriate geometry and sintered at high temperature to form various shapes, such as beads, rods, disks, chips, washers, etc. The ceramic NTC thermistor element generally has an upper temperature limit of about 300 degrees C.

Ceramic NTC thermistors can provide greater sensitivity to temperature change than RTDs (resistance temperature detectors), thermocouples, or semiconductor temperature sensors based upon p-n-junctions. They are also inexpensive relative to these temperature sensors. However, ceramic NTC thermistors change their resistance in a humid environment; and they typically have a non-linear resistance versus temperature characteristic over their temperature range, thereby requiring a linearization circuit or limiting their use over a smaller temperature range to minimize non-linearity.

To make use of ceramic thermistors over a wide temperature range with standard simple electronics, its resistance-temperature relationship should be predetermined over a working temperature range and then be stored in a memory of a look up table of a data processing system. While such a data processing approach provides high accuracy over a wide temperature range, it requires relatively expensive data processing hardware and software.

Unfortunately, there is no standard resistance versus temperature R (T) characteristics established for ceramic NTC thermistors. Each ceramic thermistor supplier provides their own resistance-temperature look-up tables based on the type of employed oxides and in-house production technology. Due to the severe degree of fragmentation of the ceramic thermistor industry, the development of an integrated digitizer has not been feasible; it would force IC designers to provide variety of R (T) curves and still limit compatibility of the ICs to few thermistor production lines.

AdSem, Inc., a small start-up in Palo Alto, CA, has developed and offers the first silicon and germanium high-temperature NTC thermistors that can significantly eclipse the performance of conventional ceramic NTC thermistors. AdSem's NTC semiconductor thermistors, based on proprietary, patent-pending technology, offer compelling benefits, compared to conventional NTCs, such as: a wider temperature range; higher thermo sensitivity; a very high degree of interchangeability; high stability; ease of manufacturing and ability to be more cost-effectively produced; smaller form factor; resilience to negative environmental conditions (e.g., moisture/humidity); versatility of device configuration; and compatibility with semiconductor and microelectronic packaging.

Moreover, employment of semiconductor Si and Ge as a thermistor material creates the opportunity for development of the new generation of multi-functional sensing devices on the same chip (for example, pressure and temperature sensors, acceleration and temperature, etc.)

The fundamental property of both Ge and Si high temperature thermistors is the universality of their resistance-temperature dependence for any thermistor size and any thermistor configuration. In addition, R (T) dependence for Si and Ge is described precisely by the analytical exponential function. The combination of these two properties allows the development of standard inexpensive integrated electronic digitizer (front end electronics, converting R directly into temperature data) and simplifies greatly the application of Si and Ge termistors. The universal nature of R (T) characteristics of AdSem's semiconductor thermistors makes development of such ICs a straightforward procedure and potentially a highly successful venture that will provide end users with a plug-and-play, off the shelf solution for accurate and yet cost effective temperature sensing.

AdSem's silicon thermistors, which are highly suited for applications in the -20 to +500 degrees C temperature range in such areas as medical, automotive, telecommunications, industrial or equipment monitoring, etc., have a temperature coefficient of resistance of -7.3%/degrees C @ 25 degrees C; a beta value of 6600 K @ 25 degrees C; and a resistance range of 1-108 Ohm. AdSem can provide the silicon NTC thermistors with interchangeability of +/- 0.1 degrees C @ -20 to +500 degrees C and +/- 0.05 degrees C @ +25 to +200 degrees C. The response time of the silicon NTCs is less than 1 second.

AdSem's germanium NTC thermistors, which are quite valuable in diverse applications where small size, high thermo sensitivity, and a lower resistance value (compared to the silicon thermistors) are key drivers, have a temperature range of -100 to +300 degrees C; a temperature coefficient of resistance of -5.0%/degree C @ 25 degrees C; a beta value of 4700 K @ 25 degrees C; a resistance range of 1 - 108 Ohm; and a rapid response of < 1 second. The interchangeability can be provided down to +/- 0.1 degrees C @ -100 - 0 degrees C, and of 0.05 degrees C @ 0 - +200 degrees C. The minimum dimensions for AdSem's NTC sensors, which are currently available, are of 100 x 100 x 100 um3.

AdSem's thermistors are produced by standard Si /Ge wafer processing techniques, which gives the company an opportunity for using outside vendors - -highly productive microelectronic factories and semiconductor packaging foundries. AdSem exercises a fabless model of thermistor production and is capable of producing millions of units per month. To fully utilize this packaging opportunity, AdSem developed the thermistor designs, including Si and Ge fully lead-free SMT in 0402, 0603 and 0805 packages with flex leads with maximum operating temperature of 150oC. This technology provides very cost-effective high-volume production of high performance thermistors for the one of the most usable temperature ranges. AdSem, which introduced its first high temperature silicon and germanium NTC thermistors at Sensors Expo 2003, also has developed and offers unique germanium cryogenic thermistors for any temperature. They are fabricated by use of novel doping methods of Ge, developed by AdSem`s founders. These methods allow regulation of Ge low-temperature conductivity at low- and ultra-low temperatures and production of inexpensive cryogenic thermistors with exquisite sensitivity at any given low-temperature down to 10 mK. AdSem's super-wide Ge thermistors for the range of 300K - 20 mK and 300K -1K are highly interchangeable thermistors for low- and ultra-low (T <1K) temperatures, and are unique and can also be used for low-temperature nuclear and infrared bolometers. All standard cryogenic thermistors are also available from AdSem but with improved sensitivity and extended operating temperature ranges: 300 -77 K, 300K -20K, and 300 - 4.2K.

Thus, this start-up company not only succeeded in the development and production of the first silicon and germanium high temperature NTC thermsitors, but it is, reportedly, the only one in the world which offers semiconductor thermistors with the best performance for any high- and low- temperature between 20 mK and 500oC.

Thermistor users in some key application sectors such as, for example, industrial, analytical instruments, and medical instruments, have tested AdSem's thermistors and found such sensors to definitely provide a superior level of sensitivity, dynamic range, or interchangeability, compared to ceramic thermistors. Dr. Michael Kozhukh, President and Chief Technical Officer, noted that AdSem semiconductor thermistors are targeting both high-temperature and cryogenic temperature sensor markets and offer both performance and price benefits that will revolutionize temperature-sensing industry. He explained that AdSem's high temperature Si and Ge NTC thermistors can replace conventional ceramic thermistors and, in many cases, platinum RTD, which are used virtually in every existing industrial market, adding that there are numerous untapped, potentially lucrative applications for AdSem's cutting-edge silicon and germanium thermistors that have to be developed. AdSem also is looking producing novel NTC and PTC semiconductor thermistors with extended upper temperature range of 1,000 degrees C.

The high degree of interchangeability of Si/Ge thermistors eliminates the need for manual trimming that is a standard procedure for production of ceramic interchangeable thermistors and allows customers to readily replace one thermsitor with another. The high interchangeability facilitates high-volume applications for thermistors; for example, using thermsitors in all rooms of a home or commercial building. There are also key opportunities for AdSem to offer disposable thermistors for medical applications and temperature measurements in construction structures (such as concrete solidification process, for example).

In addition, AdSem's small, sensitive, and cost-effective silicon and germanium thermistor dies with universal R (T) characteristics and high-volume reproducibility fit perfectly with respect to requirements for thermal management and thermal optimization of Si-in-package (SIP) systems. There has reportedly been a proliferation of 3D/stacked packing technologies, including Si-in-package technology. Such composite ICs are reportedly produced in high volumes. In such SIP technology, a number of silicon dies with active and passive components are in a single package where there is potentially large power dissipation inside the package. Temperature measurement of these complicated systems serves to optimize their temperature and operating performance.

To optimally capitalize on market opportunities, AdSem focuses on providing new products for a range of promising applications. In this vein, around the beginning of this year, the company introduced the following products: 150-micron germanium thermistors for medical applications (for cardiac vascular applications, particularly); rapid-response wireless probes with one or several thermistor sensor elements or surface mount thermistors for measuring temperature distribution; flexible probes with one or several surface mount micro packaged thermistors for measurement of temperature distribution; new surface mount micro-packaged germanium thermistors; and glass-encapsulated silicon thermistors.

The appearance of novel Si and Ge high temperature NTC thermistors will stimulate great changes in the large temperature-sensing industries, as is the case in any industry after development of the revolutionary technology. SBD has pegged global revenues for NTC thermistors used in temperature sensing and control applications at about $416 million in 2003.

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