ITO Details
Direct Write Laser Patterning, Dry Etching and Structuring of TCO Coatings on Glass & Plastic Displays at Laserod
The following is based on a paper by Rodney Waters, originally published in LIA Handbook of Laser Materials Processing, Laser Institute of America, Magnolia Publishing, Inc., Orlando FL, 2001, p.637. It was a distinct honor to have been invited to contribute the section on Laser Patterning. We offer this as proof of our pioneer world leader status in laser patterning, etching, scribing and structuring of Transparent Conductive Oxide (TCO) Display Coatings. TCO coating examples include Indium Tin Oxide (ITO), carbon nanotube (CNT), chrome, gold and all others without exception.
Principles of Laser Patterning by Solid State Lasers
- Direct write maskless pattern generation
- Dry process laser patterning and etching, single step
- Up to 1.6m SCAN AREA and up to 5m/sec WRITING RATE
TCO Films on Flat Panel Displays form circuit patterns after the film has been patterned.
We have been laser patterning ITO on glass and PET since 1985. For the Fluke Corp. we designed and built a Q-switched green YAG laser for patterning, etching, structuring, and even scribing of touch panels. However, until recently we have mainly worked on R&D applications as laser patterning was too slow and thus too expensive to compete against traditional etching. Recent advances in laser and motion technology have now allowed us to develop systems increasing our laser ablation rate ten fold or more. Laser etching TCO is an upcoming and beneficial alternative to traditional methods.
The conventional method of patterning transparent conductive materials is a photographic process involving photoresist spinning, exposing, developing, wet chemical etching, stripping and baking. The exposure step may require a photomask, produced also by multi-step wet chemistry. Conventional patterning consists of acid etching photoresist to remove TCO not protected by the mask.
In contrast, the laser method is a dry process performed in a clean environment. All the photolithography process steps are replaced by a single laser ablation operation. Another difference is the solid state laser is a serial direct writing process whereas photolithography is a parallel, write it all at once, process. Worthy of note is that an excimer gas laser with projection metal mask is also parallel but not maskless. Direct laser patterning without a mask eliminates strippers, etching acid and resist waste discharge. Finally, compared to excimer lasers, the solid state type, commonly known as YAG, is quite compact and enjoys more than 80 percent lower cost of operation.
Advantages of the dry etch laser technique are:
- No chemical pollution
- No undercut
- No swelling
- Low cost
- Fast prototyping, hours instead of many weeks
- Uniform and repeatable
We can laser pattern both TCO coated glass and plastic displays at three solid state laser wavelengths: 1064, 532 and 355nm. Plastic substrates are usually “PET” which encourage roll-to-roll (R-to-R) web processing (also called reel-to-reel) to facilitate handling and prevent scratches which cause opens. For low density laser lines, we are faster, better, cheaper.
Laser Patterning Flexible Plastic Displays
Laser patterning of flexible displays comprising ITO film on PET substrates holds promise for several reasons:
- No laser burning of the PET for improved transparency of the flexible display
- R-to-R flexible display processing is possible.
- Sheet resistivity can be any value and always results in excellent isolation.
- Laser writing rates of up to 5 meters/second are feasible over large or small areas.
- Our Class 100 clean room assures high levels of laser patterning quality.
The laser process works best for low densities of laser lines. Laser patterning is optimum for line removal. For large area removal we depend on high speed scanning. Laser lines are spaces in the coating. Naturally, the lower the density of laser lines (spaces), the faster the laser patterning, and thus the more attractive is laser processing. Finally, our single step laser cutting process yields a benefit of minimizing scratches on delicate flexible PET displays. The less handling, the fewer the scratches and the better the yield.
It is worthy to repeat that laser patterning is maskless. Bypassing the photomask step saves time and money. Laser pattern generation and structuring is therefore useful for prototyping, quick turnaround, and high volume production of low laser line density displays. Finally, laser lithography (dry etching) can be highly price competitive with photolithography.
How the laser produces patterns
Patterning involves generating grid lines, fanouts, fiducial marks, borders and alignment aids. A dxf download from an Autocad file is usually the input to the laser, patterned displays are the output.
Basically, laser ablating of TCO is drilling a series of overlapping blind holes in the thin coating. Drilling proceeds at a rate of one blind hole per laser pulse. Holes are then overlapped at high speed.
When the laser beam is passed through a lens to focus to a spot in the range of 0.010mm to 0.125mm on the coating, a single laser pulse vaporizes the coating. The beam stops at the substrate, whether glass or plastic, and does not burn it. Overlapping these spots by 50-90 percent produces a space in the TCO coating.
The relationship is:
Writing Speed = Laser Spot Size x Laser Rep Rate x (1-Spot Overlap)
M1-DLP for accuracy
M2-DLP for speed
M3-DLP for large parts
Laser Patterning Equipment – Laserod’s Basic Product Line:
We offer three basic patterning models: M1-DLP, M2-DLP and M3-DLP. Each of these comes with either IR fiber laser or UV diode laser so that’s 6 models. Throw in a green laser to make 9 models. M1 is a fixed beam with XY stage moving part, M2 is a galvo moving beam and M3 is a gantry with beam moving in the X-axis (side to side) with the part moving in the Y-axis, (in and out). Available also are custom systems featuring multiple beam splitting to increase throughput.
Our patterning lasers are all solid-state type, not a gas like excimer or CO2. They are frequency tripled (355nm) diode pumped (UV diode for short) or diode pumped seed oscillator with fiber amplifier (IR fiber for short). Our laser systems feature computer controlled motion driven by dc servo or linear motors, in two-axis translation stages with single or dual beam delivery optics to double throughput. An alternate choice of motion is a fast low-mass galvanometer (usually called just a galvo) , fiducial registration, high magnification inspection built-in and integral fume removal. There’s more. Please call us.
Conclusion
Laser patterning via dry etching shows promise as an economical method for patterning a variety of TCO coated glass and plastic displays. It is dry etch, non-contact, non-chemical, environmentally “green,” single step, direct write, maskless, ablative process ideal for both quick turnaround and high throughput for certain products. Laser processing is repeatable, reliable, low operating cost, low consumable cost and easy to automate. Furthermore, laser pattern generation is low maintenance. IR fiber lasers are more reliable than UV diode lasers and five times faster, but UV has the advantage of smaller feature size. Finally, laser patterning cleanly to high electrical isolation is of supreme importance.
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