Laser-Induced Carbonization of Polyimide Films for Physically Unclonable Anticounterfeiting Applications

Laser-induced carbonization (LIC) on polymers with aromatic and imide repetitive units, particularly Kapton polyimide (PI), has revolutionized the production of carbon-based devices and sensors. This includes supercapacitors, triboelectric nanogenerators, actuators, heater devices, as well as gas, humidity, strain, and biological sensors. Furthermore, PI demonstrates remarkable mechanical and thermal stability, positioning it as a compelling choice for a myriad of flexible electronic devices.<br/>In contrast to traditional pyrolysis, where polymer materials undergo thermal decomposition in an inert atmosphere, the LIC technique offers a notable advantage in swiftly achieving patterned carbon. This is crucial for electrode fabrication and the creation of individual, isolated devices. Pulsed lasers emit high-intensity pulses with short durations, affecting only a small portion of the sample with limited penetration depth, thereby avoiding damage to the surrounding material. Various lasers, such as carbon dioxide (CO₂, 10.6 µm), infrared (IR, 1064 nm), ultraviolet (UV, 248 nm), and blue (456 nm), are commonly employed for carbonizing PI materials in diverse applications. Significantly, the carbonization mechanism varies with different wavelengths and other parameters such as laser power, speed, frequency, hatch distance, and pyrolysis environment. This is attributed to distinct absorptions by both PI and carbon. Although the exact mechanisms are not universally agreed upon, both photochemical and photothermal processes have been proposed to elucidate this phenomenon.<br/>On the other hand, as the global population grows rapidly, there is an increasing demand for a larger quantity of goods to meet the needs and comforts of individuals. However, this heightened demand has inadvertently given rise to counterfeit markets. To combat this issue, the implementation of anti-counterfeiting tags on products has emerged as a practical solution. Various traditional anti-counterfeiting tags like quick-response codes, radio-frequency identifications, graphical elements, holograms, and watermarks have been utilized for consumer goods. Unfortunately, these tags are susceptible to cloning attacks and are primarily employed as "identifiers" rather than security tags for authentication. In response to the cloning issue, physically unclonable (PU) tags have been introduced. These tags, generated through stochastic processes, possess unique patterns of randomly distributed physical features that are extremely challenging to duplicate. Additionally, it is crucial for the tags to be low-cost and manufactured using easy, reconfigurable, and ultrafast processes. One effective technique for creating a unique and visually appealing PU tag is the LIC method. Notably, the creation of carbon content without the need for external matter eliminates the material cost drawback associated with tag production.<br/>In this study, we demonstrate the fabrication of a unique and visually appealing PU tag based on a facile, low-cost, rapid, and scalable LIC approach. The LIC dot shapes, sizes, and density can be adjusted based on the input design and laser processing parameters, allowing the tags to be reconfigured according to the manufacturer's requirements. A PI sample with LIC spots of varying sizes distributed randomly in an array format is considered a tag. The LIC spot patterns, formed under the same laser beam energy incidence, are attributed to the heterogeneity of aromatic compounds in the PI film. The spot images are classified into three-level bits based on their sizes and brightness, serving as representations of an identity. The digitalized information encoded by these bits can be stored and utilized for authentication purposes. With this innovative approach, unique spot patterns can be directly engraved onto a flexible printed circuit board, providing a highly secure and customizable solution.