Film Structure and Quality Requirements for Nanometer-Thick Film Pressure Sensors
2020-08-22
The multilayer film of the nanofilm pressure sensor is fabricated using a vacuum ion-beam sputtering process. Because the thickness of its core sensitive film is less than 100 nm, it is referred to as a nanofilm. Pressure sensors manufactured using this process are called nanofilm pressure sensors.
1. Thin-film structure
The typical thin-film structure of a nanometer-thick film pressure sensor is shown in Figure 1. In the figure, the thin-film substrate is a circular, flat, metallic elastomer; the first layer of the film is a dielectric insulating film that serves as an isolation layer, typically made of Al2O3, SiO2, or a multilayer structure thereof. The second layer is a metal-sensitive film that responds to strain; four strain resistors forming a Wheatstone bridge are fabricated using photolithographic techniques and constitute the core of the sensing element. The third layer is a passivation protective dielectric film, which isolates the strain resistors from the ambient air, preventing corrosion caused by oxidation, moisture, and sulfides, thereby ensuring the stability and reliability of the strain resistors. Common materials for this layer include Al2O3 and SiO2. The fourth layer is a gold-plated window film used for full-wire lead-out; it contacts the strain resistor film to enable electrical signal extraction. After the thin-film fabrication and photolithographic processes, the finished pressure-sensing elastomer is obtained, as shown in Figure 2.
Figure 1: Typical Film Structure of a Nanofilm Pressure Sensor
Figure 2: Elastomer after photolithography of the nanofilm pressure sensor
II. Film Quality Requirements
Depending on the different functions performed by the four-layer film, the quality requirements for the film also vary. They are described below:
1. Requirements for dielectric insulating films
① Insulation Strength Requirements: For finished sensors, the insulation resistance between the strain gauge and the sensor housing must be at least 104 MΩ. The withstand voltage has increased from an initial 50 V, through a mid-term level of 100 V, to the current standard of 250 V; meanwhile, the insulation resistance has risen from a few MΩ in the early stages, through tens of MΩ in the mid-term, to over 5,000 MΩ today. This progress is closely linked to advancements in thin-film processing technology.
② The dielectric insulating film exhibits strong adhesion and high bonding strength to the surface of the metallic elastomer. It must possess a certain degree of elasticity, meaning that under the maximum usable microstrain (e.g., 2500 με), the dielectric film should not crack or fracture. Moreover, within the specified measurement range, the film must remain functional after undergoing more than 106 cycles of bending deformation without failure, and it must also retain its elasticity when subjected to temperatures up to 350℃.
③ The thermal expansion coefficients of the dielectric insulating film and the metallic elastomer are essentially identical, so no internal stress is generated due to the difference between them, thereby preventing instability in the sensor’s output.
④ The sensor thus fabricated should exhibit minimal creep. In addition to rigorous heat treatment of the metallic elastomer, the dielectric insulating film deposited on it must have high adhesion, be dense, and free from defects such as pinholes and voids. If creep occurs, it will lead to increased zero-point shift errors and degrade the sensor’s linearity.
⑤ The dielectric insulating film contains few impurities and is free of adsorbed gases. As a result, it can prevent the degradation of insulation performance, increased leakage current, and even insulation failure caused by impurity migration and gas release during use.
In summary, the dielectric insulating film layer should possess the following characteristics: a high resistivity; a high breakdown voltage; excellent insulation performance; good adhesion to stainless steel; effective transmission of elastic deformation; and high thermal stability.
2. Requirements for strain-sensitive metal films
① Strain-sensitive metal films typically consist of two or more elements. It is required that the compositional makeup of the film after deposition be identical to that of the bulk material, thereby ensuring that its temperature coefficient and sensitivity remain unchanged.
② After heat treatment, the residual stress in the strain-sensitive metal thin film should be minimized as much as possible. The presence of residual stress can lead to zero-point drift errors. It is desirable that the thermal expansion coefficients of the deposited strain-sensitive metal thin film and the dielectric insulating film be as closely matched as possible. Moreover, while ensuring a stable and continuous film with an average thickness, the ideal film thickness should be controlled as precisely as possible. If the film is too thick, lattice defects within the film will increase, leading to greater distortion and higher internal stress; additionally, the resistance value will become unstable. On the other hand, if the film is too thin, it will be discontinuous and cannot be stabilized through heat treatment. At the same time, considering the need for high resistance, the bridge resistance should be maintained within the range of (4–6) kΩ. A higher resistance results in lower power consumption, thereby reducing the instability of the resistance caused by self-heating.
③ The resistance of the fabricated strain gauges remains stable over a wide temperature range: for instance, if the sensor’s stability is 0.1% FS, the change in resistance should be less than 0.05%. For applications requiring long-term operational stability, the resistance stability requirement is even stricter—less than 0.001%. Achieving such high resistance stability is quite challenging and necessitates the use of certain compositional modification techniques to minimize resistance variations as much as possible.
Thanks to Sono-Mate’s advanced sensitive material technology, the sensor’s zero-point temperature drift can be as low as 0.0002% FS/℃ without any compensation—100 times lower than that of conventional products.
In summary, the strain-sensitive metallic thin-film layer should possess the following characteristics: high sensitivity of resistivity to strain; minimal temperature dependence in strain detection; a high resistivity coefficient; low temperature influence on the resistivity coefficient; and high chemical and structural stability.
3. Requirements for Passivation Protective Coating Films
In addition to the thermal expansion coefficient and adhesion requirements, the thickness of the passivation protective film must not be too great; otherwise, it may easily crack and get damaged.
In summary, the passivation protective dielectric film layer should possess the following characteristics: high electrical insulation; high moisture resistance; good adhesion; high thermal stability; high resistance to ion penetration; and low material stress.
4. Window gold-plated film
The lead-frame window is very small, requiring the gold-plated film on the window to adhere well. During ball bonding, the gold film must not detach from the sensitive film layer nor damage the insulating layer. Gold is a heavy metal that easily diffuses and penetrates, potentially causing insulation failure.
In summary, the gold-plated thin film layer in the lead window should possess the following characteristics: good electrical connection with the strain layer; low resistivity; compatibility with subsequent processing steps; high thermal stability; excellent adhesion; and a resistance temperature coefficient that matches that of the strain layer.
To summarize the requirements for film quality in the aforementioned nanofilm pressure sensors, there are two main aspects: mechanical properties and electrical properties. The mechanical properties include: adhesion of the film, internal stress, thermal stress, good elasticity after deformation under applied force, and a low coefficient of thermal expansion. As for the electrical properties, they primarily involve excellent insulation performance of the dielectric film, high breakdown strength, low leakage current, moderate sheet resistance of the metal film, and good stability of the resistance value. Both the mechanical and electrical properties of the film are strongly dependent on the film-processing equipment and the associated process technology. Even with the same processing equipment, differences in film quality can be significant due to variations in deposition process parameters—this is because the film-forming process is highly complex and sensitive to the conditions under which it takes place.
Sensor Nobleman has achieved breakthroughs in key technologies for nanofilm pressure sensors and has independently developed a series of nanofilm pressure sensor products. These products are manufactured by integrating sensitive materials with elastomers at the atomic level, using semiconductor-like fabrication processes. They boast advantages such as high precision, a wide operating temperature range, high stability, and high reliability. As a result, they are widely used in fields including aerospace, weapons systems, petrochemicals, metallurgy, rubber processing, nuclear industry, transportation, diesel power systems, and the Internet of Things.
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