Research On Crack Patterning By Photolithography-Controlled Neutral Plane

Cracks are often considered a material defect,and researchers are always trying to prevent cracks by various methods.However,more and more scholars have recently discovered that cracks are not just defects.They also have many applications.For example,cracks can be applied to patterning of nanostructures,fabrication of nanochannels,fabrication of nanowires,study of interactions between cell and extracellular matrix,fabrication of high-frequency memory and fabrication of flexible sensors.Controlling crack patterning is the key to various applications.However,the existing methods for controlling crack patterning have problems such as the inability to accurately control the position,density and length of cracks,the limitation of metal types,the need to reduce the bonding force between the metal and the substrate,the complexity of the manufacturing process and the short crack lengths.For this reason,this paper proposes a method of crack patterning by photolithography-controlled neutral plane,and analyzes it theoretically.The method to control the cracks patterning process is to make a photoresist pattern on the metal surface through a photolithography technique firstly,and then bend the sample so that the cracks only appear on the metal film without the photoresist.For the mechanism study of this method,we perform a mechanical analysis of the production process.Through the mechanical analysis,we find that when the sample is bent,the neutral plane without photoresist is closer to the substrate than the neutral plane with photoresist,and the metal is farther away from the neutral plane.Therefore,the metal is subjected to greater strain so cracks are more likely to occur.To adjust the manufacturing process guided by the mechanical analysis,we precisely control the position,density and length of cracks.In addition,this method is not limited by the type of materials,and crack patterning can be controlled on gold,silver,and copper films.Using the method proposed in this paper,strain sensors are fabricated and its performance is tested.For examining the performance of sensors,what of the sensor made by the method proposed in this paper and the sensor made by the bending method is tested.It is found that the gauge factor?GF?value of the strain sensor made by the method proposed in this paper is up to17000 within the measuring range of 0-1.5%.It is the highest GF value in the strain sensor of this measuring range.The relative standard deviation between specimens is only 1/10 of that of sensors made by the bending method.In addition,we find that as the crack density decreases the measuring range is smaller,but the gauge factor is the largest in the measuring range.When the crack length decreases,the measuring range and the GF are both lower.We successfully apply the sensors to detect human respiratory signals,human pulse signals,pressure,and temperature detection.In terms of temperature detection,the average temperature coefficient of resistance?TCR?of the sensor is up to 2.69×10^-1/K.Compared with the currently proposed sensors that utilizing metal resistance changes with temperature,the TCR is improved by two orders of magnitude,and could measure remote temperature.