Micro-Nano Laser Machining
Typical examples of Micro Hole Drilling in Ceramics, Plastic and Metals.

Laser Micromachining: Definition Laser micromachining is defined as laser cutting, drilling, etching, stripping, skiving materials such as plastics, glass, ceramic and thin metals with dimensions from 1 micron (0.00004”) to 1mm (0.040"). Recommended maximum machining thickness is 1mm. Typically, laser drilling holes are tapered where the entrance diameter is larger than the exit diameter. The typical half angle side wall taper is 3 to 5 degrees, which is material dependent.

Laser Wavelengths:

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.

Laser Sources:

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.

1 | 2 | 3