Typical examples of Micro Hole Drilling in Ceramics, Plastic and Metals.
Machining dimensions are proportional to approximately twice the laser wavelength. The shorter the laser wavelength, the smaller the feature size is realizable. By using short wavelength lasers, with short pulse duration, machining dimension as small as 1 micron is realizable.
Methods of Laser Micromachining:
Laser micromachining is performed by two different methods: Direct Write or Mask Projection.
Direct Write has a number of advantages:
Two direct write methods are commonly used: The fixed beam approach involves moving the part on an X-Y motion stage; alternatively the laser beam can be moved very quickly by a pair of galvonometers, programmable spinning mirrors to direct the beam in the X and Y axis within a defined field.
- i) Maskless: There is no mask pattern. Just point and shoot the laser beam.
- ii) Ease of programming: The drawing is converted by a CAD/CAM program to the machine code, used to drive the motion controller of the laser system. The laser beam is focused to a small spot size (typically 10-25 microns) and the beam traces the pattern to be cut. If the laser is drilling a hole larger than the spot size, a method called trepanning is used where the beam is directed in a series of concentric circles to “fill” the larger hole. Often, to accelerate processing times, the beam is directed by galvanometer or scanning mirrors to direct the beam to a specific location.
Mask Projection has a number of advantages:
- i) Complex pattern: Any complex pattern such as an “8” or “S” can be produced flawlessly without any stitching or scalloping issues (caused by overlapping a circular laser spot) because the entire pattern is machined at one time.
- ii) Edge quality: When laser micromachining blind channels, the excimer laser can image a long thin rectangular image with perfectly straight line edges. When drilling holes, perfectly circular holes with “ink jet-type” precision is achieved.
- iii) Throughput: A large process area can be laser micromachined at one time, maximizing process throughput and lowering costs.
- iv) The business case for using excimer lasers in a mask projection method usually comes down to process throughput. Having UV laser sources up to 100W in average power, as opposed to 2.5W to 10W with DPSS, allows excimer lasers to be a cost-effective manufacturing solution.
- v) This is especially the case at 193nm laser wavelength that is not currently attainable by DPSS lasers at reasonable average powers (ie; 30W). The 193nm laser wavelength is used for laser micromachining specific materials such as pebax, nylon, glass and bioabsorbable materials. To automate the mask projection technique, programmable mask changers are deployed where multiple mask pattern are mounted on a high speed linear mask stage, permitting different mask patterns to be shuttled in on-the-fly.
DPSS (Diode-pumped Solid State Lasers) 1064nm, 532nm, 355nm, 266nm wave length), with Picosecond and Femtosecond pulse parameters. Diode-pumped solid-state lasers are solid-state lasers that are pumped by a series of diode bars. There has been significant development over the years, propelled by the microelectronics industry (micro vias in cell phones and other hand-held portable devices) to reach the ultraviolet spectrum at 355nm and 266nm. The fundamental laser wavelength is 1.06 microns and non-linear crystals are used to double (532nm), triple (355nm) and quadruple (266nm) the laser wavelength with the penalty of lower average power and higher pulse-pulse variation.
DPSS Pulsed Laser
Picosecond and Femtosecond pulsed lasers are solid state lasers that produce a pulse train at high repetition rates. The laser consists of an oscillator, regenerative amplifier, amplifier pump laser and stretcher/compressor unit. In some ways, this technology is viewed as the “holy grail” because the extremely short pulse duration removes material as a multiphoton ablation process, ideal for any material type with little or no heat affected zone. The technology is becoming more industrialized, packaged in a single unit, with average power of a few watts and repetition rates up to 5 khz.
DPSS lasers operate at high repetition rates (50+ khz), suited for direct write applications such as laser cutting of plastics and thin metal foils or laser drilling of non-repeatable hole patterns in polymers and ceramics. These lasers operate in the fundamental wavelength (1.06 microns) but can be doubled (532nm), tripled (355nm) or quadrupled (266nm) to handle a variety of materials and machining patterns.
The use of non-linear harmonic modules such as tripler (355nm) and quadrupler (266nm) allow Diode-pumped solid state lasers (DPSS) to reach the ultraviolet spectrum, opening up new laser micromachining applications.
The attractiveness of Picosecond or Femtosecond lasers is the ultra short pulse duration that offers the possibility of machining any material with minimal recast material and no heat affected zone.
DPSS laser systems have a very short pulse duration that can be six orders of magnitude shorter than excimer lasers.
Excimer lasers operating in the ultra-violet wavelength (193nm, 248nm, 308nm) with average power approaching 100 Watts, are ideal for processing of plastics, glass, ceramics and thin metals with tolerances approaching 1 micron.
Eximer lasers utilize the mask projection technique that is akin to laser lithography, the same leading edge technology that manufactures next generation microcomputer chips. The excimer laser illuminates a mask (that is not in contact with the part) that contains a pattern such as a circle, rectangle or any complex shape. This pattern is imaged by downstream optics to produce an identical yet significantly smaller pattern.
Most often, the mask is fabricated inexpensively by chemically etching stainless steel masks whose pattern is typically 150 microns (0.006″) or greater. Since the mask projection technique optically reduces or “demagnifies” the mask pattern by an integral amount (typically 5 to 30 times), simple low cost methods can be used to fabricate the mask. In some specific applications, with dimensional requirements less than 5 microns, glass mask (metal deposited on quartz) are used.
An excimer laser is gas powered laser, automatically filled with a specific gas mix and run for a period of time before the existing gas fill is evacuated and replenished with new gas. The excimer laser achieves the highest average power (up to 100 Watts) in the ultraviolet spectrum, with laser wavelengths ranging from 157nm, 193nm, 248nm, 308nm and 351nm.
Excimer lasers became industrial workhorses when the semiconductor industry gravitated from UV lamp sources to excimer lasers to produce next generation computer chips. Today’s lithographic tools, scanner and steppers, deploy excimer lasers. In the case of laser micromachining applications, excimer lasers are utilized as lithographic tools in the range of 1 micron features (or higher).
DPSS and Excimer lasers ablate or vaporize the material, minimizing debris or heat affected zone. This photo chemical ablation process works by the laser breaking the molecular bonds within the material and ablated material is ejected upward and away from the material surface at supersonic speed. DPSS laser have picosecond or femtosecond pulse parameters. These temporal pulse parameters are so short that there isn’t time to generate heat, minimizing any thermal effects such as melting. This is especially the case as the wavelength gets shorter, e.g., 532nm, 352nm, 266nm, 193nm.
The DPSS laser source operates at high repetition rates with an etch rate of approximately 0.1 to 0.5 microns per pulse. The laser can drill through or blind holes. By counting the number of pulses, the depth of machining can be accurately controlled. As an example, these lasers are ideal for selective removal of a top coating or layer without damaging the underlying layer removing material precisely, like peeling an onion layer-by-layer.
Photo below illustrates the significance of pulse parameters Å¡ Picosecond vs Nanosecond
CO2 lasers are gas powered lasers with a sealed laser tube (no gas filling required) that have been the mainstay of laser machining for decades. These lasers operate in the infrared spectrum (10.6 microns) with average powers in the kilowatts with high repetition rates. In the field of laser micromachining, the applications are limited because the smallest achievable spot size is 50-75 microns diameter. Still, the laser is applicable for certain “through hole” applications where high throughput and low operating costs are required.
Short pulsed CO2 lasers operating at 10.6 micron wavelength. The CO2 (Carbon Dioxide) lasers operate at average power of 150W with repetition rates of 10khz. The CO2 laser has the highest processing speed, two orders of magnitude faster than excimer lasers and one order of magnitude faster than DPSS lasers.
Of course, there are consequences to this processing speed. They include thermal effects such as potential melting or cracking, limited machining dimensions (0.003″) and tolerances (0.0015″) and not suitable for blind hole drilling or precise selective material removal.
CO2 lasers are the industrial workhorse of laser machining utilizing a spot size of .003″ or .004′. Machining tolerances approach one-half the laser spot size. CO2 laser machining is a thermal process, liquefying the material to allow coaxial gas to push the machined material out the back side. Unlike a UV laser (DPSS or Excimer at 355nm or 266nm), the CO2 laser vibrates the molecular bonds of the material, generating heat that is used to liquefy the material. Similar to DPSS lasers, CO2 lasers deploy a direct write approach where the focused beam traces the pattern to be machined.