Micromachining and Material Processing with Amplified Picosecond Lasers

June 1, 2012

Short pulsed lasers are presently an important tool for manufacturing a variety of devices from smart phones to photovoltaics. When wielded correctly, these lasers can remove material with far greater precision than mechanical tools or more conventional laser processing using CW or pulsed Qswitched lasers. New developments in the design of picosecond laser amplifiers have resulted in improved durability, maintainability, and lower costs.

Figure 1. Comparison of laser ablation mechanism between picosecond and long pulsed conventional lasers.

The ability to ablate material while minimizing heat is important in numerous military and defense applications. These include microstructuring the surfaces of metal parts for low-friction turbines, as well as machining volatile and even explosive materials.

In the world of laser fabrication, nanosecond and Q-Switched lasers have limitations because they cannot efficiently cut transparent material and often suffer from bad edge cut quality and surface roughness. With far shorter pulses, picosecond lasers are capable of better cut quality and can produce structures that conventional lasers cannot. They are considered universal tools in microfabrication as they are less sensitive to the properties of the materials and can even cut transparent material. When wielded properly, picosecond lasers can remove material with extreme precision due to an ablation mechanism with negligible thermal and mechanical stress. This ablation mechanism is precise enough to selectively remove extremely thin layers (10-100nm) without disturbing the underlying material. Picosecond laser micromachining now enables new methods for manufacturers to produce structures comparable to traditional semiconductor lithographic masks, but without the masks or dangerous chemicals, and in essentially any material that is desired.

Cold Ablation/Multiphoton Ionization

Figure 2: SiC Drilling (top left) heat deposited during laser drilling causes significant damage to surrounding material. (top right) Picosecond pulsed lasers result in the ability to produce clean, sharp cuts without burrs or recast. (bottom) Trenches cut into ceramic show how picosecond ablation can produce smooth surfaces due to the reduced heat affected zone.

The mechanism with which conventional industrial lasers, having nanosecond or longer pulses, cut material is through heat deposition, which results in melting and burning of the material. Mechanical tools cause mechanical stress, they cannot achieve high precision, and will wear out. Both tools are limited in their applications in many industries because they cause micro cracks on silicon wafers, which leads to device failure.

Picosecond lasers have much shorter pulses than conventional lasers and, thus, pack all their energy into a shorter time interval. When such intense (>J/cm2) bursts of energy collide with the surface of any material, the strong electromagnetic field of the focused laser pulse rips the electrons out of their atoms and the exposed material becomes ionized. The ensuing plasma is very thin and results in material removal with extremely precise edges. Picosecond lasers offer a solution to the demand for high precision and selective material removal for all materials. Picosecond lasers minimize the heat affected zone and the resulting cuts are clean and precise — important advantages in today’s precision manufacturing environments. Some important examples are discussed below.

Glass

Microscopic features in glass, sapphire, and other transparent materials are easily achieved with picosecond micromachining. Material removal rates in glass can vary from ~0.1 to >1 mm3/min*W for optimized processes. The microfluidic mixer structures of Figure 3 are milled from one side. The fluid can travel from one 20-um diameter entrance hole, along the branches and into the mixing chamber. Burrs are not allowed in micrfluidic microstructures as they cause fluid flow obstruction.

In this design, a second glass slide is required to seal the structure. In such designs, the second glass slide with a different structure can be stacked on top of the first slide to create a complex, 3- dimensional, multilayer circuit or mixing chamber. Multilayer mixing chambers with complex stacking structures can be easily and inexpensively fabricated using picosecond lasers and cost-effective micrometer precision stage control. Since picosecond lasers are universal cutting tools, with minimal adjustment to the laser pulse energy, the same design can be used to fabricate devices using other materials such as sapphire and Si.

Steel

Numerous applications exist, particularly in the manufacturing of medical devices, in which stainless steel and other tough alloys must be micromachined without defects such as burrs. With picosecond lasers, material removal rates can be achieved ranging from 0.05 to 0.2mm³/min*W. Shown in Figure 4 is a micro gear produced for use in conjunction with a microfluidic mixer for secondary stage fluid mixing. Such tiny devices can be integrated into existing MEMS actuators. To ensure the smooth operation of these sensitive devices, the edge quality must be burr-free. Difficulty in fabrication of such pieces include handling and removal of debris from the cutting process. In this case, the only post-production procedure employed is the use of cleaning tissue to remove debris generated from cutting. Existing microgears made from silicon often suffer from debris formation due to wear and tear over time. Stainless steel gears promise to be significantly more durable and can minimize or eliminate debris formation from wear and tear.

Silicon

Picosecond lasers have found a number of applications in solar cells and other silicon-based applications. The sun-facing surface of a solar cell can be textured to absorb as much light as possible in order to achieve greatest efficiency. Since a single pulse can modify the silicon surface, the modified surface can be used as a nucleation zone for chemical etching in traditional silicon processing methods. Picosecond pulses can also dig inverted pyramidal features on silicon.

During solar cell manufacturing, layers of conductive material are deposited on top of each other. To avoid electrically shorting the solar cell to itself, a thin scribe line is needed to electrically isolate the solar cell. While nanosecond lasers can perform this task, there is a very high risk for nanosecond lasers to form microcracks within the thin solar cell substrate. Sometimes the crack will not appear for years, but even that cannot be tolerated. Microcracks can completely change the electronic structure within the silicon crystal and may cause unwanted electrical insulation.

Crystalline solar cells are generally pseudo-squares. A silicon wafer is diced on 4 edges and the off-cuts of the solar grade pure silicon are recycled back into the melt pool. Traditionally, dicing is done using a diamond saw. Diamond saws wear out over time and both nanosecond lasers and diamond saws can cause undesired micro cracking, which leads to losses in solar cell efficiency and shorter cell lifetimes. Picosecond lasers can eliminate these microcracks while dicing at m/sec speeds.

Plastics

Figure 6. An Array of 10-um diameter holes on thin polymer film (left). AFM image on the right shows selectivity of picosecond ablation. A 200-nm conformal coating has been removed using a single pulse exposing a smooth copper surface.

Picosecond lasers also have important applications in the processing of polymers and other soft and thermally sensitive materials and coatings. Excimer lasers are used to make <50 um holes in very thin polyamide (PA) film, useful for printers and drug administration. Figure 6 shows an array of 10 um holes in PA film. These small holes can be used to deposit a very small amount of material on surfaces and also printing stensils. Here we have 10um exit holes with 100um spacing. Nanosecond lasers cannot achieve such high quality due to burrs and melting effects.

Of particular importance in manufacturing of thin film devices is the ability to remove insulating layers to expose microscopic circuitry for testing or repair. Transparent coatings such as ITO or conformal coatings pose an insurmountable problem for nanosecond lasers that either transmit through the coatings, damaging the layers underneath, or deposit heat that degrades underlying structures. Correct optimization of picosecond material removal can result in clean and complete layer removal, as shown in Figure 6.

The need for picosecond material processing is expected to grow by faster than 40% over the next few years and has been adopted as standard manufacturing practice by many well known firms. Attodyne currently produces commercially available picosecond laser systems for low to medium throughout applications such as repair of thin film transistors (TFT, used in flat panel displays such as TVs and laptops), repair of printed circuit boards and masks (used for the production of microchips), and also dicing, scribing and interconnecting wafers (e.g. for microchip and solar panel manufacturing).

This article was written by Darren Kraemer, President, and Dr. Michael Cowan, CEO, Attodyne Inc. (Toronto, Ontario, Canada). For more information, Click Here .