Thin-film Multijunction Thermal Converters Research Articles

Improvements in the Low-Frequency Characteristic and Sensitivity of a Thin-Film Multijunction Thermal Converter in Vacuum

We have improved the ac-dc transfer differences and enhanced the sensitivity in our thin-film multijunction thermal converters (TFMJTCs) in vacuum. The ac-dc transfer differences of the TFMJTCs are less than 1 $\mu $ V/V down to 1 Hz and 10 $\mu $ V/V down to 0.2 Hz owing to the increased thermal time constant. The measured output voltage exceeds 120 mV at $10^{\mathrm {-4}}$ Pa owing to the reduction of heat conduction to the air. The sensitivity of the device, defined as the ratio of the output voltage to the input power, is 2.4 V/W, which is approximately three times larger than that measured at ambient pressure. Numerical simulations of the heat transfer of our TFMJTC explain the experimental results well.

Fabrication of Thin Film Multijunction Thermal Converters With Improved Long-Term Stability

To increase the long-term stability of thin-film multijunction thermal converters (TFMJTCs) developed by the National Metrology Institute of Japan and Nikkohm Corporation Ltd., we prevented their degradation by improving the design and fabrication process. Judging from the Arrhenius model experiments, we successfully increased the lifetime of a renewed TFMJTC to more than 20 years by improving the deposition pattern of a Bi-Sb-Cu-based thin-film thermopile. We also evaluated the thermoelectric effect of the renewed TFMJTCs by the fast-reversed dc method. The low thermoelectric effect of less than 0.1 μV/V has been realized, which is comparable with that of high-quality multijunction thermal converters previously produced by other national metrology institutes.

交流電圧標準を導く薄膜型サーマルコンバータの開発

Thermal voltage converters have been widely used in the electric standard field as a major method to derive AC voltage standards from DC voltage standards. However, there are only a limited number of organizations that are capable of fabricating thermal voltage converters. Furthermore, establishing AC-DC voltage standards has been constrained by the lack of high-quality thermal converters. High-quality thermal converters are used only by national metrology institutes (NMIs) because the conventional thermal converters are too fragile for many calibration laboratories. The National Institute of Advanced Industrial Science and Technology (AIST) has developed new thin-film multi-junction thermal converters(TFMJTCs) to realize a reliable high-performance thermal converter. Development of a durable TFMJTC with a heater on an aluminum nitride (AlN) substrate is expected to make significant contribution to Japanese calibration laboratories.

Improved Stability of Thin‐Film Multijunction Thermal Converters

SUMMARYWe have demonstrated the long‐term stability and environmental durability of a high‐performance thin‐film multijunction thermal converter (TMJTC) with an improved thin‐film thermopile design. We fabricated a new TMJTC with an improved deposition pattern for a Bi‐Sb‐Cu‐based thin‐film thermopile in order to avoid interface delamination between the Bi and the Cu layers. Our TMJTC exhibited good frequency properties, with an ac–dc difference of less than 5 μV/V in the range from 10 Hz to 100 kHz, at a root‐mean‐square (rms) voltage of 10 V. The long‐term stability of the TMJTC has been significantly increased owing to the performance of the developed thin‐film thermopile, as compared with that of previously fabricated TMJTCs, exhibiting an ac–dc difference of less than 2 μV/V after 18 months at room temperature. We also evaluated the temperature, relative humidity, and pressure characteristics of the TMJTC in accordance with the International Electrotechnical Commission (IEC) standards for electronic devices. The measurement results showed that the dependence of the ac–dc transfer difference on the temperature, humidity, and pressure was less than 1 μV/V, which is comparable to the results of thermal converters, constituting the primary standard.

Development of Thin-Film Multijunction Thermal Current Converters With Increased Rated Current

Thin-film multijunction thermal converters (MJTCs) with a heater fabricated on an AlN chip have been developed for ac–dc current transfer standards. The rated current of the MJTC can be increased due to the good thermal conductivity of the AlN substrate, which acted as a heat sink drawing away heat from the heater. The AlN chip was mounted on a polyimide foil with thermocouples of the MJTC produced separately. In the case of an MJTC with a heater resistance of 5 $\Omega$ , which has a bifilar heater pattern to reduce the thermoelectric effects, the nominal current is 200 mA. The observed current level dependence of the ac–dc current transfer differences from 50 to 200 mA for the 5- $\Omega$ MJTC was within 1 $\mu\hbox{V/V}$ at frequency from 1 to 100 kHz. Compared with a conventional thermal converter, the wide range of available input current and small current level dependence of the new MJTCs may reduce uncertainties of ac–dc current transfer standards.

Improved Stability of a Thin-Film Multijunction Thermal Converter

Improved Stability of a Thin-Film Multijunction Thermal Converter

Development of Thin-Film Multijunction Thermal Converters With a Novel Structure

A new type of thin-film multijunction thermal converter (MJTC), which can be fabricated using a simple process, has been developed for use as high-frequency ac-dc transfer standards. The design of this MJTC reduces the coupling between a heater structure and thermocouples at high frequencies, and enables its characteristics to be calculable from the shape of the thermal converter (TC). The influence of the electromagnetic coupling between the heater and the thermocouples can be reduced to less than 20 μV/V at 100 MHz by improving the structure of the thermocouples. The nominal input current is increased to 50 mA by sputtering a heater on an aluminum nitride (AlN) substrate featuring good thermal conductivity, enabling direct comparison measurements between TCs with input resistances of 50 Ω and 1 k Ω. The frequency characteristics of our MJTCs are in a good agreement with the calculated results in the frequency range between 1 and 100 MHz. This paper describes the unique structure of the new MJTC and its frequency characteristics in the frequency range of 1-100 MHz.

New Thin-Film Multijunction Thermal Converter Based on P+ Polysilicon/Al Thermocouples

This paper presents the design, fabrication and preliminary results for a new thin-film multijunction thermal converter based on P+ Polysilicon/Al thermocouples and tantalum nitride heater. The experiment results show that the output voltage and ac–dc voltage transfer differences of the FMJTC increases with frequency in both contacting and non-contacting temperature sensing schema. It is help to understand the physics of transfer differences in the PMJTC.

Design and Fabrication of MJTCs on Quartz Substrates at NIST

Wet and dry etching is employed in the fabrication of new planar thin-film multijunction thermal converters (MJTCs) on quartz membranes and crystalline quartz chips at the National Institute of Standards and Technology (NIST). The use of crystalline quartz as a material for the membrane and chip improves the performance of the MJTC in the frequency range of 100 kHz-100 MHz. Simulations of the ac-dc voltage transfer difference for a heater resistance of 400 Omega in the frequency range of 1-100 MHz show a reduction in the ac-dc transfer difference of more than one order of magnitude, in comparison with the MJTCs fabricated on silicon chips. The devices that have been fabricated, although not optimized for 100 MHz, have been shown to have reasonable performance for their 20-V maximum input.

New Thin-Film Multijunction Thermal Converter Design for Improved High-Frequency Performance

New thin-film planar multifunction thermal converters (PMJTC) were developed to improve the high-frequency ac-dc transfer differences. The heater resistor and thermocouples of these PMJTCs were produced on different substrates: an AIN chip for the heaters and polyimide film for the thermocouples, using simple fabrication processes. The thermocouples were moved from the conventional high-potential heater position, and placed close to the ground electrode of the input circuit. This new configuration upgrades the performance of ac-dc transfer differences above 10 kHz by improving both the electromagnetic coupling between the heater and thermocouples and the output resistance of thermocouples. Using a high thermal conductivity AIN substrate, through which heat is conducted from the heater to the hot junctions of the thermocouples, almost the same sensitivity as that of PMJTCs on silicon is obtained.

Explanation for the AC–DC Voltage Transfer Differences in Thin-Film Multijunction Thermal Converters on Silicon Chips at High Frequencies

In this paper, an explanation for the ac-dc voltage transfer differences at frequencies greater than 100 kHz of the standard thin-film or planar multijunction thermal converter (PMJTC) developed at the Physikalisch-Technische Bundesanstalt and fabricated on a silicon chip has been found. The simulation indicates that the capacitance of the bond pads of the heater is the reason for an increase of the ac-dc voltage transfer difference of PMJTCs with higher heater resistance to negative values. Bond pads of smaller size and larger distance are shown to improve the frequency response

1-A and-120 mA Thin-Film Multijunction Thermal Converters

We report on the development of thin-film multijunction thermal converters for the measurement of AC currents up to 10 A without relying on parallel current shunts. The materials and designs chosen for these devices have been optimized to provide high accuracy over a wide range of input levels and frequencies. Preliminary measurements on a 10-A chip made with a copper-gold heater indicate that its ac-dc difference is nearly zero from 10 Hz to 100 kHz, and that the AC-DC difference is independent of input current level.

Evanohm R 합금 히터를 사용한 크로멜-콘스탄탄 다중접합 열전변환기의 제작 및 특성

A thin-film multijunction thermal converter was fabricated through the process using 6 inch silicon wafer semiconductor process and bulk micromachining. Evanohm R alloy and chromel-constantan were used as a heater and thermocouple materials, respectively. The temperature coefficient of resistance of Evanohm R heater was about 75.12 ppm/ and the voltage sensitivity of the thermal converter indicated about 5.75 mV/mW in air. The transfer differences, measured by FRDC-DC method in the frequency range from 20 Hz to 10 kHz, showed the value under about 1.36 ppm, 0.83 ppm for the film thickness of 500, 200 nm, respectively. And in case of a 200 nm-thick thermal converter, the AC-DC transfer differences seems to be stabilized below the value of 1 ppm in the frequency range from 1 kHz to 500 kHz.

Low-frequency AC-DC voltage transfer standards with new high-sensitivity and low-power-coefficient thin-film multijunction thermal converters

New planar multijunction thermal converters (PMJTCs) with low-power coefficient and high dynamic range using CuNi44/BiSbTe thermocouples have been manufactured. In series connection with carefully evaluated range resistors for voltages up to 1000 V they result in AC-DC voltage transfer standards with known AC-DC voltage transfer difference at low frequencies, for which the voltage step-up procedure is not necessary any more.

High-frequency thin-film multijunction thermal converter on a quartz crystal chip

The design of a new planar or thin-film multijunction thermal converter (PMJTC) on a quartz membrane and a quartz crystal chip results in a calculable standard for AC-DC voltage transfer for frequencies from 100 kHz to 100 MHz with improved uncertainties. This is due to smaller dielectric losses and higher insulation resistance in the quartz membrane and the quartz chip. The window with the membrane is anisotropically etched into the quartz crystal chip. For the first measurements, the temperature of the heater was radiometrically sensed with a thin-film infrared sensor and later with an array of 100 Cu-CuNi thermocouples. At frequencies higher than 100 kHz, the AC-DC voltage transfer differences are one order smaller than with the standard PMJTC with the Si/sub 3/N/sub 4//SiO/sub 2//Si/sub 3/N/sub 4/ sandwich membrane on a silicon chip.

New thin-film multijunction thermal converters with negligible low frequency AC-DC transfer differences

The AC-DC transfer differences of thin-film thermal converters at low frequencies are caused by nonlinearities in the heat transport mechanisms and in the thermal-to-electric conversion process. The resulting power coefficient of the sensitivity has been compensated by adding another nonlinear temperature sensitive component to the thermal-to-electric conversion process. This is a thin-film Ni resistor mounted under the thermocouples and connected to the output of the thermocouples. The Ni resistor together with the internal resistance of the thermocouples forms a resistive divider for the output voltage. The positive temperature coefficient of the Ni resistor in the divider compensates for the negative power coefficient of the sensitivity. As a result, converters used at a rated voltage of 1.5 V or a rated current of 7 mA have an AC-DC transfer difference below 0.3 /spl mu/V/V and 0.3 /spl mu/A/A respectively with a standard uncertainty of 0.3 /spl mu/V/V and 0.3 /spl mu/A/A respectively at a frequency of 10 Hz.

A new fabrication process for planar thin-film multijunction thermal converters

Advanced thin-film processing and packaging technologies are employed in the fabrication of new planar thin-film multijunction thermal converters (MJTCs). The processing, packaging, and design features build on experience gained from prior NIST demonstrations of thin-film converters and are optimized for improved sensitivity, bandwidth, manufacturability, and reliability.

Development of thin-film multijunction thermal converters at PTB/IPHT

The thin-film multijunction thermal converter (PMJTC) developed in cooperation between Physikalisch-Technische Bundesanstalt (PTB) and Institut fur Physikalische Hochtechnologie e.V. (IPHT) is today's most sensitive and accurate standard for the precise measurement of electrical AC quantities in the frequency range of 10 Hz-1 MHz. Thin-film technology and micromechanics in silicon were essential for this success. The thin-film heater and bismuth/antimony thermocouples with high Seebeck effect deposited on a thin membrane of low heat conductance result in the attractively high sensitivity of the PMJTC which allows voltage measurements down to 100 mV to be performed. The statistics of the mass production of the PMJTCs show that PMJTCs built into a housing with an N-connector at the input can be reproducebly manufactured with an AC-DC voltage transfer difference smaller than 0.1 /spl mu/V/V at 1 kHz, 8 /spl mu/V/V up to 100 kHz, and below 40 /spl mu/V/V up to 1 MHz for a heater resistance of 90 /spl Omega/. A compensation circuit has been added on the chip which results in low-frequency PMJTCs (LF-PMJTCs) with AC-DC transfer differences below 0.3 /spl mu/V/V at 10 Hz.

The relationship between microsystem technology and metrology

Microsystem technology (MST) has enabled silicon sensors to evolve from simple transduction elements to microsystems (micro-instruments) that include readout circuits, self-test, and auto-zeroing facilities. This paper discusses the impact of MST in the instrumentation and measurement (I&M) field. In metrology, in particular, the development of electrical reference standards by using microtechnology has opened a wide variety of potential applications, such as the Josephson junction array (DC voltage reference) and thin-film multijunction thermal converters (AC voltage and AC current reference). It is shown that MST has even more to offer to the I&M field. Two devices that have highly benefited from MST. thermal and capacitive RMS-to-DC converters are discussed in historical perspective., Subsequently, a recently developed microdevice, the pull-in voltage reference, which may have a huge impact in I&M applications, is outlined. Finally, it is demonstrated that recent developments in electrical and nonelectrical metrology system concepts offer special opportunities for on-chip cointegrated silicon microsystem realizations.

Low-frequency characteristics of thin-film multijunction thermal voltage converters

Low-frequency errors of thin-film multijunction thermal voltage converters are estimated using a simple model based on easily measured parameters. The model predictions are verified by measuring the converter's frequency characteristic using a digitally synthesized source.