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Fundamental Concepts for The Functionality of Thin Film Thermistors
Temperature Measurement Accuracy Due to NTC Resistive Properties
The operation of NTC thermistors is governed by the principles of semiconductors. At elevated temperatures, metal oxide thermistors, such as those containing manganese, cobalt, nickel, or oxygen, display reduced electrical resistance. There are certain temperature ranges where their resistance decreases as depicted by a curve. This characteristic allows them to detect temperature changes of less than 0.1 degrees Celsius. NTC thermistors outperform standard temperature sensors by being able to detect small changes in temperature without the need for electronic signal processing. The high resistance thermistors are resistant to electrical and thermal shock which is a result of the stable crystal structures formed from the high temperature (1000-1400 degrees Celsius) processing of the thermistors. Thin film thermistors can withstand cycling of increased temperature and decreased temperature without drifting significantly. With a combination of longevity, superior thermal shock resistance, and fast response times, thin film NTC thermistors are ideal for use in sensitive medical applications and automotive applications which require high reliability and varying environmental conditions.
Why Thin-Film Architecture Improves TCR Stability and Response Time
When considering ceramic or wire wound options, the thin film approach has undeniable benefits. Using the technique called sputtering, manufacturers apply the Mn-Co-Ni-O layer to a thickness of just 50 to 250 angstroms. This results in enormous improvements in both uniformity and the reduction of the issues related to the grain boundaries of the individual particles. This results in the Temperature Coefficient of Resistance being far more stable and in the region of 0.5% variation over the typical operating conditions. The films are also highly responsive, with a typical response time of < 100 ms, caused by the thin film layers having such a small thermal mass. The addition of insulating, flexible, polyimide materials allows these devices to perform in applications that are subject to constant mechanical vibration or rapid thermal cycling. This is the reliability that industries demand for difficult factory environments or in unpredictable conditions in automotive applications.
Thin Film Thermistor Manufacturing: Deposition and Patterning Procedures
Optimal Adhesion: Substrate Selection and Surface Preparation
When considering materials, one is most likely to consider alumina and sapphire substrates as these are thermally stable, provide sufficient electrical insulation, and are compatible with metal oxide films. Before any deposition occurs, surface preparation (in the case of alumina substrates) is of absolute importance. The surface is prepared by an ultrasonic cleaning followed by an oxygen plasma etch, a procedure which achieves a roughness of less than 5 nanometers. This is important because one of the many factors influencing the adhesion is the surface roughness, and the smoother the surface the better the adhesion, and it has been shown that the surface preparation procedure described above can reduce the occurrence of delamination of the prepared surface by 70% during the thermal cycling, a factor of great importance in the thermal cycling.
Sputtering as the Preferred Method for Mn–Co–Ni–O Thin Film Thermistor Deposition
Reactive magnetron sputtering is the most reliable method to synthesize Mn-Co-Ni-O thin films while achieving the requested stoichiometry. This method involves the precise control of the sputtering chamber atmosphere with mixtures of argon and oxygen gases, allowing the thin film to maintain a stoichiometry accuracy of approximately 1.5%, while simultaneously achieving deposition rates of approximately 0.2 um/min. When researchers optimize the distance between the target material and the surface of the substrate, they record a decrease in the average number of defects and a notable increase in the consistent temperature coefficient of resistance (TCR) of the film. Comparatively, the films produced via thermal evaporation methods are significantly less dense, and have a decreased adhesion to the substrate. In fact, independent testing has shown thin films created via sputtering to have up to 40% greater material density, which is a key indicator for addressing defects and encourages the use of sputtering for dense applications across numerous fields.
Precision Patterning of Thin Film Thermistors Using Photolithography and Etching
High-precision photolithography allows the manufacturing of micrometer-scale electrodes and sensing structures, with features down to 10 microns of detail. Once this level of resolution is achieved, we perform spin coating of a photoresist, followed by photolithographic exposure through chromium masks and photoresist development. The subsequent step is wet etching using ferric chloride solution to remove all thermistor material that is unmasked. This process achieves a dimensional accuracy of ± 0.8 microns. Most of the time, this level of accuracy is required because sensor resistance levels can be affected by even the smallest variations in dense sensor arrays. The quality of the sensor pattern determines the quality of the sensor and how the sensor responds to thermal changes and the variability of sensor response during operation.
Electrode Integration and Interfacial Engineering for Long-Term Reliability
Ni–Cr and Pt Electrodes: Preventing Diffusion and Ensuring Ohmic Contact Stability
Proper attention to how electrodes are integrated plays a crucial role in mitigating interfacial degradation, one of the primary causes of long term drift problems. For example, nickel chromium alloys serve as effective diffusion barriers because they impede the diffusion of cations from the electrodes into the undesirable regions of the thermistor layer. In addition, platinum electrodes are advantageous because of their low resistance and the stability of their contacts, even after numerous thermal cycling. An engineering approach to enhance interfacial adhesion and minimize undesirable reactions is the application of atomic scale treatments such as controlled oxidation and ion beam surface clearing. Engineering interfacial layers has been shown to reduce contact potential drift to under 0.5 ohm after 10,000 thermal cycles and reduce interfacial stress by 40% relative to the previous metallization. In the end, all of this will improve the measurement accuracy from the very beginning until the device completely wears out.
Processing of Thin Film Thermistors with Quality Assurance & Performance Evaluation Quality Assurance Quality Control methods now in use for thin film thermistors, have proven immensely and repeatably successful in meeting extreme levels of reliability and accuracy. We conduct thermal cycling up to 125 degrees and down to 40 degrees for a more than 1000 cycles to evaluate structural integrity and resistance. For long-term drift tests, accelerated aging requires sample exposure of 85 degrees and 85 percent humidity for 1000 + hours to guarantee drift remains under 1 percent. For electrical testing, we conduct a complete mapping of TCR values, and testing of Electro Magnetic Interference to ensure +/- 0.1 degrees accuracy is maintained over time. Each of our production processes has strict statistical process control to monitor film thickness variation of 5 nanometers and alignment of electrodes using automated optical alignment functions. Our real time thermal imaging during laser trimming captures microscopic (in the micron range), and our burn-in tests eliminate early-failure components. All of the above testing and monitoring ensures our Thermistors achieve operational service life of 100,000 hours in extreme performance demand conditions without failure.
FAQ
What is the main advantage of NTC thermistors?
Thermal NTC thermistors are rather simple devices, but their advantages are remarkable. The main advantage is that NTC thermistors exhibit a considerable degree of stability over long periods of time, and can be compensated for with precision of 0.1 degree Celsius.
What is the main difference between thin film thermistors and others?
Thin-film thermistors are made with a very thin layer of Mn-Co-Ni-O, and so provide a much better uniformity, a faster time response and generally a better replacement for ceramic or wire-wound materials.
What is the impact of substrate preparation in the fabrication process of thermistors?
Correct substrate preparation improves the adhesion of the metal oxides to the substrate, thereby reducing the likelihood of test-related layer separations by about 70%. Smooth layers are more able to resist the test-related separations.
What effect do Ni-Cr and Pt electrodes have on thermistors?
Ni-Cr and Pt electrodes are referred to inside the thermistors because Ni-Cr provides a barrier to diffusion and Pt provides a stable, low resistance contact. It is a combination of the two that reduces drift over time and improves contact stability under repeated cycles.
