Background
The resistance strain sensor is a device capable of converting external strain change into resistance change, and has wide application in the field of measurement and measurement. On the one hand, the most of the measurements of physical quantities such as force, torque, speed and acceleration can be converted into indirect measurements of strain, so that the strain sensor becomes the core of most measuring instruments on the market at present. On the other hand, the device has simple structure, small size, stable and reliable performance and convenient use. However, the resistance strain sensor is easily affected by the ambient temperature, and generally, in order to reduce the influence of the temperature on the resistance, a bridge circuit and the like are often required to be built for temperature compensation, for example, chinese patent publication No. CN106768217B discloses a compensation method for the resistance strain sensor, which calculates the length of the bridge arm resistance with the aid of a computer by setting a label on the strain sensor, but this results in a complicated structure and difficult solution, and is not convenient for miniaturization and integration of the sensor. As for the preparation of the self-compensation composite film strain gauge proposed in chinese patent publication No. CN105755438B, the temperature self-compensation is realized by forming the sensitive layer of the sensor from two materials of TaN and PdCr with opposite resistance temperature coefficients, but the method adopts a multilayer structure, so that the preparation process is complicated, and the strain gauge can only be prepared on a hard substrate.
In addition, in the fields of biomedical instruments, wearable devices and the like, higher requirements are provided for the flexibility and the stretchability of the strain sensor. Particularly in the field of wearable devices, simultaneous measurement of the temperature of the environment is sometimes required in addition to the measurement of strain. In order to meet the requirement of flexibility of the strain sensor, people mainly rely on the improvement of a sensitive layer material of the strain sensor, for example, chinese patent publication No. CN101598529 discloses a strain sensor prepared by using an elastic fabric doped with conductive particles and an elastomer matrix as a sensitive material, and the strain sensor can measure a strain value of 50% at most. However, the processing technology of the patent is complex, conductive particles in the fabric need to be cleaned and solidified, miniaturization of the sensor is difficult to achieve, meanwhile, the flexible strain sensor does not achieve a temperature compensation function, and the environmental temperature can have a large influence on strain measurement.
Therefore, in order to meet the requirements of flexible strain measurement in the fields of biomedical instruments, wearable devices and the like, provide real-time temperature compensation for the strain sensor and reduce environmental errors during strain measurement, the invention provides a flexible strain sensor chip with a temperature compensation element and a preparation method thereof.
Disclosure of Invention
In view of the defects in the prior art, the present invention aims to provide a flexible strain sensor chip with a temperature compensation element and a preparation method thereof.
According to the invention, a flexible strain sensor chip with a temperature compensation element is provided, comprising: the device comprises a flexible substrate 1, a resistance strain sensor 2, an insulating isolation layer 3 and a resistance temperature sensor 4;
the resistance strain sensor 2 is arranged on the upper surface of the flexible substrate 1;
the insulating isolation layer 3 is arranged on the upper surface of the resistance strain sensor 2, and meanwhile, lead electrodes of the resistance strain sensor 2 are exposed;
the resistance temperature sensor 4 is disposed on the upper surface of the insulating isolation layer 3.
Preferably, the resistive strain sensor 2 is patterned by using a mask sputtering method, a mask etching or a lift-off process.
Preferably, the resistance temperature sensor 4 adopts a magnetron sputtering method to deposit a film, and utilizes a mask sputtering method or lift-off process to realize patterning.
Preferably, the flexible substrate 1 is an organic polymer material prepared by spin coating.
Preferably, a sensor protection layer 5 is also included;
the sensor protection layer 5 covers the upper surface of the resistance temperature sensor 4 while exposing the lead electrodes of the resistance temperature sensor 4.
Preferably, the material adopted by the resistance strain sensor 2 comprises one of copper-nickel alloy, camar or nickel-chromium alloy, and the material adopted by the resistance temperature sensor 4 comprises platinum and gold.
The invention provides a preparation method of a flexible strain sensor chip with a temperature compensation element, which is characterized by comprising the following steps:
firstly, spin-coating a layer of PDMS on a hard substrate by adopting a spin-coating method and semi-curing;
secondly, spin-coating a layer of flexible substrate 1 on the upper surface of the semi-cured PDMS formed in the first step by adopting a spin-coating method;
thirdly, sputtering a layer of alloy sensitive material on the upper surface of the flexible substrate 1 formed in the second step by adopting a magnetron sputtering method to obtain a resistance strain sensor 2;
fourthly, covering a layer of insulating isolation layer 4 on the resistance strain sensor 2 formed in the third step by using a spin coating method, and simultaneously exposing the lead electrode by using an etching method;
and fifthly, depositing a film by adopting a magnetron sputtering method, realizing patterning by utilizing a mask sputtering method or a lift-off method, and sputtering a layer of Ti/Pt on the insulation isolation layer 2 prepared in the fourth step to obtain the resistance temperature sensor 4, wherein Ti is used as an adhesive layer.
Preferably, the method further comprises the following steps:
sixthly, covering a sensor protective layer 5 on the upper surface of the resistance temperature sensor 4 in the fifth step by using a spin coating method, and exposing lead electrodes of the resistance temperature sensor 4 and the resistance strain sensor 2 by using an etching method;
and seventhly, separating the flexible substrate 1 from the PDMS to obtain the strain sensor chip.
Preferably, in the fourth step: the alloy sensitive material is one of Cr/NiCr, Cr/CuNi and Cr/Karma alloy, and the Cr is used as an adhesive layer and has the thickness of 10 nm-30 nm; the thickness of the sensitive material NiCr or CuNi or the Carma alloy is between 200 and 900 nm.
Preferably, in the fifth step: ti is used as an adhesive layer, and the thickness is 10 nm-30 nm; the thickness of the sensitive material Pt is between 200 and 900 nm.
Compared with the prior art, the invention has the following beneficial effects:
the invention can realize the simultaneous measurement of temperature and strain and provide real-time temperature compensation for strain measurement, and has the characteristics of flexibility, simple structure, low cost and high measurement precision.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that it would be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit of the invention. All falling within the scope of the present invention.
Fig. 1 and 2 are schematic structural diagrams of a flexible strain sensor chip with a temperature compensation element, where fig. 1 is a cross-sectional view of the chip, and fig. 2 is a top view of the chip.
In fig. 2, a flexible strain sensor chip with a temperature compensation element comprises: flexible substrate 1, resistance strain sensor 2, insulating isolation layer 3, resistance temperature sensor 4, sensor protective layer 5, wherein:
the resistance strain sensor 2, the insulation isolation layer 3, the resistance temperature sensor 4 and the sensor protection layer 5 are all arranged on the flexible substrate 1; wherein: the resistance strain sensor 2 is prepared above the flexible substrate 1; the resistance temperature sensor 4 is prepared above the insulating isolation layer 3; the resistance temperature sensor 4 and the insulating isolation layer 3 are arranged above the resistance strain sensor 2, and lead electrodes of the resistance strain sensor 2 are exposed at the same time; the sensor protection layer 5 covers the upper surface of the resistance temperature sensor 4, and simultaneously exposes the lead electrode of the resistance temperature sensor 4;
when the sensor chip deforms under the action of unknown physical quantity, the resistance strain sensor 2 outputs resistance under the action of the unknown physical quantity according to resistance strain effect, and strain value under the action of corresponding physical quantity is obtained according to the relationship between the resistance and strain of the sensor; meanwhile, the resistance temperature sensor 4 outputs corresponding ambient temperature, and the temperature can also compensate the strain value measured by the resistance strain sensor 2 so as to eliminate the resistance temperature effect. The flexible substrate can enable the sensor chip to be conveniently covered on the surfaces of various tested objects, and the stretchable characteristic of the flexible substrate can also be used in the wearable field.
The flexible substrate, the insulating isolation layer and the sensor protection layer are polyimide films formed by adopting a spin coating process and performing high-temperature curing, and polyimide formed by adopting the spin coating process and performing high-temperature curing has good insulativity and high corrosion resistance.
The resistance strain sensor adopts a magnetron sputtering method to deposit a film, and utilizes a mask sputtering method or lift-off process to realize patterning, and the material is copper-nickel alloy. The resistance temperature sensor adopts a magnetron sputtering method to deposit a film, utilizes a mask sputtering method or lift-off process to realize patterning, and adopts platinum as a sensitive material.
As shown in fig. 2, the resistance strain sensor 2 is located below the resistance temperature sensor 4, and an insulating layer 3 is prepared in the middle so that there is no contact between the resistance strain sensor 2 and the resistance temperature sensor 4.
In a preferred embodiment, the strain resistance sensor 2 and the resistance temperature sensor 4 are made of metal by a mask sputtering method, and the resistance temperature sensor 4 is made of platinum with good stability.
Further, the shapes of the strain resistance sensor 2 and the resistance temperature sensor 4 are set by the shape of a mask, and may be an arc shape, a long strip shape, or other shapes. The bending part of the strain resistance sensor 2 adopts a short circuit design, so that the transverse effect of the strain resistance sensor is reduced.
In a preferred embodiment, the flexible substrate 1, the insulating isolation layer 3 and the sensor protection layer 5 are formed by spin coating and high temperature curing, the flexible substrate 1 is used for supporting the resistance strain sensor 2 and the resistance temperature sensor 4, the insulating isolation layer 3 is used for electrically isolating the resistance strain sensor 2 from the resistance temperature sensor 4, and the sensor protection layer 5 is used for protecting the entire sensor from external influences.
Based on the structure, the flexible integrated temperature strain sensor chip structure can be prepared by adopting the following preparation method, and specifically comprises the following steps:
firstly, spin-coating a layer of PDMS on a hard substrate by adopting a spin-coating method and semi-curing;
secondly, spin-coating a layer of polyimide with the thickness of 10-20 microns on the upper surface of the semi-cured PDMS formed in the first step by adopting a spin-coating method, and curing at 300 ℃ after spin-coating to obtain a flexible substrate 1;
and thirdly, spinning and coating a layer of photoresist of 8-13 um on the flexible substrate 1 prepared in the second step, and exposing and developing to obtain a pattern of the resistance strain sensor 2.
Fourthly, the flexible substrate 1 patterned in the third step is placed into a magnetron sputtering machine, a high-purity Cr target material and a high-purity Karman alloy target material are used, and the background is vacuumized to 10 DEG-3~10-4And Pa, introducing Ar gas and adjusting the working pressure to 0.3-0.9 Pa. The sputtering power is adjusted to be 150-200 w, 15-60 s of Cr is sputtered to obtain a Cr bonding layer with the thickness of about 10-30 nm, then the sputtering power is adjusted to be 150-200 w, 200-600 s of the Karma alloy is sputtered, and the thickness of the obtained Karma alloy film is about 200-900 nm.
And fifthly, washing the sputtered structure with acetone to remove the photoresist and the redundant metal on the photoresist, thereby obtaining the resistance strain sensor 2. Then, deionized water is used for cleaning and drying.
Sixthly, covering a layer of polyimide with the thickness of 10-20 um on the resistance strain sensor 2 formed in the fifth step by using a spin coating method again, semi-curing, and etching out a lead electrode of the resistance temperature sensor 2 by using a NaOH solution with the concentration of 3% -8%;
and seventhly, coating a layer of photoresist of 8-13 um on the structure obtained in the sixth step in a spinning mode, and obtaining the graph of the resistance temperature sensor 4 through exposure and development.
Eighthly, placing the structure subjected to the patterning in the seventh step into a magnetron sputtering machine, and vacuumizing the background to 10 ℃ by using a high-purity Ti target material and a high-purity Pt target material-3~10-4And Pa, introducing Ar gas and adjusting the working pressure to 0.3-0.9 Pa. The sputtering power is adjusted to be 150-200 w, Ti with the thickness of about 10 nm-30 nm is obtained by sputtering for 15-60 s, then the sputtering power is adjusted to be 150-200 w, Pt with the thickness of 200-600 s is sputtered, and the thickness of the obtained Pt film is about 200-900 nm.
And ninthly, removing the sputtered photoresist and redundant metal on the sputtered photoresist by using acetone again to obtain the resistance temperature sensor 4. Then, deionized water is used for cleaning and drying.
Tenth step, covering a layer of polyimide with the thickness of 10-20 um on the upper surface of the resistance temperature sensor 4 prepared in the ninth step by using a spin coating method, and etching lead electrodes of the resistance strain sensor 2 and the resistance temperature sensor 4 by using a NaOH solution with the concentration of 3% -8%;
and step eleven, separating the flexible substrate 1 from the PDMS by adopting a mechanical method to obtain the sensor chip.
The invention provides a flexible strain sensor chip with a temperature compensation element, which is realized by utilizing a micro-nano processing technology, has the characteristics of small volume and high response speed, and can simultaneously measure strain and temperature; the flexible substrate has the characteristics of flexibility, stretchability and stable structure.
The polyimide is used as the flexible substrate, the isolation layer and the protective layer, so that the flexible and stretchable characteristic can be provided for the sensor, and the polyimide has good insulating property and high corrosion resistance.
The resistance strain sensor is made of Cr/CuNi or Cr/NiCr or Cr/Cama alloy material, and has high strain sensitivity coefficient. The Cr is used as an adhesive layer to effectively enhance the bonding force between the substrates.
The resistance temperature sensor adopts Ti \ Pt material, has higher resistance temperature coefficient, and the material is stable and corrosion resistant. Ti as an adhesive layer can effectively enhance the bonding force between the substrates.
To verify the implementation effect, fig. 3 is a piezoresistive response curve of a flexible resistance strain sensor according to an embodiment of the present invention, wherein the thickness of the Cr bonding layer is about 30nm, and the thickness of the sensitive film is about 500 nm. As shown in FIG. 4, the film still maintains an electrically conductive state during continuous 5 times of cyclic loading and unloading tests when subjected to unidirectional strain, and the resistance changes in loading and unloading show a good linear relationship, the curves of multiple times of loading and unloading are basically overlapped to indicate that the mechanical hysteresis of the flexible strain sensor is small, and meanwhile, the initial resistance value does not change in an unloaded state to indicate that the zero drift of the flexible strain gauge is small. In the loading and unloading stage of t being 0-200 s, the maximum micro strain borne by the flexible strain sensor is 253 mu epsilon, the maximum resistance change rate delta R/R is 0.000445, and the sensitivity coefficient of the flexible strain sensor is 1.76 according to the sensitivity coefficient calculation formula K being delta R/(R epsilon). According to the calculation and the result of fig. 4, it can be seen that the flexible strain sensor has the characteristics of high response speed, high sensitivity and good linearity, and can perform multiple strain measurements under the condition of electric conduction.
FIG. 4 is a graph showing the resistance of a Pt resistance temperature sensor as a compensation element according to an embodiment of the present invention, wherein the thickness of the Ti adhesion layer is about 30nm, and the thickness of the sensitive film is about 200 nm. As shown in FIG. 4, the resistance of the resistance temperature sensor has good linearity with the temperature change, and the resistance temperature coefficient of the resistance temperature sensor can be obtained to be 248 ppm/DEG C through linear fitting. The current environmental temperature value can be calculated through the resistance temperature coefficient provided by the resistance temperature sensor, so that the temperature compensation of the strain measurement is realized.
In the description of the present application, it is to be understood that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like indicate orientations or positional relationships based on those shown in the drawings, and are only for convenience in describing the present application and simplifying the description, but do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and thus, should not be construed as limiting the present application.
The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes or modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention. The embodiments and features of the embodiments of the present application may be combined with each other arbitrarily without conflict.