Excimer laser ablation has proven itself to be a lower-cost method of patterning thin film metal circuits or devices on plastic substrates without using photo-resist. This Laser Direct Pattern Etching (LDPE) technique generates a complete circuit directly onto the substrate by a process referred to as photo-ablation.
There are a number of new applications being developed, which require "low cost" electronic circuits. These applications include disposable electronic products such as RFID tags and biosensors as well as lower cost plastic electronics and flexible displays. A number of companies are involved in the development of organic materials, which exhibit semiconductor properties that can be used to produce various transistor or sensor circuits.
Patterned conductive layers of materials such as copper, gold, silver, palladium and others can be directly patterned using this Excimer process. Typically, patterning of these thin film layers is performed using costly photolithographic methods. Photo-ablation is basically a subtractive process where the thin metal layer is etched directly by a high-energy projected UV image of the circuit from the laser. These metal layers typically need to have a thickness < 300nm in order for this ablation process to occur.
Tamarack Scientific based in California USA, has developed a laser photo-ablation system which directly etches the circuit pattern on the substrate and is an environmentally friendly, one-step dry process, eliminating a number of steps found in conventional lithographic process, including resist coating, soft bake, exposing, developing, hard bake, etching and removal of the resist and cleaning.
In Tamarack's process, ablation patterns are defined by a photo mask and transferred to the thin film metalized substrate using a projection lens. The use of a mask allows generation of complex shapes and provides the ability to change patterns by simply changing the mask. Depending on the fluence required for the process, the photo mask can be either standard semiconductor grade chrome/quartz or special aluminum/quartz. Depending on the size of the circuit the appropriate lens reduction ratio is selected. Several lenses are available with 5X or 2.5X reductions with field sizes from 50 mm to 35 mm respectively. Larger circuits or devices can be provided using Tamarack's 1-to-1 scanning projection technologies.
In a reel-to-reel system where the area of the circuit is reasonably small, a single circuit can in many cases be patterned with one pulse from the laser. Many high-power industrial Excimer lasers can operate at a frequency of 300 hertz. System throughput is a function of the laser energy and the fluence required for the process. Depending on the metal and its thickness, the fluence can range from 75 mj/cm sq. to 300 mj/cm sq.
As an example: for a 100 mj/cm sq. process and a 300 watt laser, a reel-to-reel, film-fed system can produce 250 each 0.6 square inch circuits per second or 15,000 circuits per minute.
Laser Direct Pattern Etching (LDPE) using high-power lasers and project optics can be used to directly etch thin metal layers on plastic substrates forming circuits. With this new method of patterning, thin metal circuits can be produced at very high production rates, significantly reducing the cost of circuit production.
For more attend Printed Electronics USA, 12-15 November 2007.