NiCr thin films

Fig. 7.4.6 a and b Comparison of output span of poly-Si and metal (NiCr) thin-film sensor elements... 

Metal (NiCr) thin film sensor element change of output span related to output span at room temperature... 

The material of the sensing layer strongly influences the characteristics of the pressure sensor and the difficulty of the calibration process. Fig. 7.4.6 shows the change in sensitivity of two different sensing layer materials (poly-Si and thin-film NiCr metal) against temperature. Poly-Si shows a nonlinear dependence on temperature, whereas NiCr is quite linear. Also the variation of the sensitivity is much lower for thin-film NiCr. Both high linearity and low variation of sensitivity contribute to a simple and therefore cost-effective calibration concept for high-pres-... 

In thin-film metallization by evaporation or sputtering of thin metal films onto a ceramic surface (Chapter 28), it has been demonstrated that a sequence of layers of different metals is required for optimum film properties. The first layer is usually a refractory metal such as Ti, Cr, or NiCr this layer provides adhesion to the ceramic. These elements are reactive and bond through redox reactions with the substrate. The second layer acts as a diffusion barrier. The barrier material will usually be a noble metal, preferably Pt or Pd. The top layer will be the metal of choice for the particular application, for example, Au for wire-bonding applications and Ni or Ag-Pd for solderability. 

Photoprint (Sheet resistor material) Thin-film etch (NiCr, Pt, NiP)... 

The properties of deposited films are highly dependent on the processing conditions and substrate properties such as surface chemistry and finish. Typically, a thin adhesion layer, such as Cr, Ti, NiCr, or TaN, is first deposited onto the substrate. This is followed by the high-conductivity metallization layer. Sometimes it is necessary to apply a final barrier layer over the metalhzation to prevent oxidation. 

Thin-fikn circuits typically consist of three layers of material deposited on a substrate. The bottom layer serves two purposes It is the resistor material and it also provides the adhesion to the substrate. The adhesion mechanism of the film to the substrate is an oxide layer that forms at the interface between the two. The bottom layer must therefore be a material that oxidizes readily. The most common types of resistor material are nichrome (NiCr) and tantalum nitride (TaN). Gold and silver, for example, are noble metals and do not adhere well to ceramic surfaces. 

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