Controlled Ultraviolet (UV) laser ablation is a viable method of micro-machining micro-electro-mechanical systems (MEMS) and micro-optical-electro-mechanical systems (MOEMS). It enables users to capitalize on the strengths of UV laser processing in order to meet the ever-evolving challenges and promise of advancing technology.
MEMS applications demand extremely precise and tight tolerances, high repeatability, and cost-effective processing. Consequently, the speed, accuracy, and simplicity of UV laser micro-machining continue to encourage its emergence as an alternative to other methods. For example, it can be used in applications, such as die separation, where wet processing simply cannot be utilized. Lack of vibration also makes it advantageous in comparison to mechanical cutting.
UV Laser Ablation
UV lasers are very high-powered lasers that can run from hundreds of Hz up to kHz repetition rates (see figure 1). This gives the user precise depth control based on the absorption of the material, and most materials absorb strongly in the UV wavelength range. Additionally, the shorter the wavelength, the finer the resolution one can achieve. This ability to focus to smaller spot sizes is a key factor in MEMS micro-machining capability. UV laser wavelengths can be lithographically projected onto material with very high resolution. Even with the use of simple lenses to shape and direct the beam, micron resolution is easily achieved.
The method of materials removal with UV lasers is unique and direct function of the laser’s characteristic form and energy type. Known as laser ablation, sub-micron layers of materials are removed when small volumes of a material absorb high peak power laser energy. Basically the short wavelength of deep UV light allows each laser pulse to etch a fine sub-micron layer of material (typically <0.1 μm thick). The high peak power of a UV laser light pulse, when absorbed into this tiny volume, results in strong electronic bond breaking in the material. The resulting molecular fragments expand in a plasma plume that carries any thermal energy away from the work piece. As a result, there is little or no damage to the material surrounding the feature just produced.
Depth is obtained by repeatedly pulsing the laser; depth control is achieved through overall dosage control. Controlling the laser exposure allow 3-D features, such as those of the ink jet nozzle pictured in figure 2, to be created in a single step. Additionally, lasers produce a limited taper angle that eliminates the problem of undercutting associated with wet processing.
Features & Materials
MEMS micro-machining involves the creation of complex features such as holes, cones, channels, sample chambers, and other consistently sized, microscopic attributes. These features may also have specific essential characteristics such as sharp-definition, smooth walls, or optically clear surfaces. Short-wavelength (157-248 nm) excimer and UV diode-pumped solid state (DPSS) lasers have exhibited the ability to execute complex features with large-area, ganged processing capabilities, and characteristically smooth cuts. This is particularly true when processing difficult materials such as borosilicate glass, quartz, fused silica, and sapphire.
In laser micro-machining, materials will absorb laser energy differently; the greater the absorption of the material, the easier it is to machine it cleanly and consistently. Virtually all of the semiconductor materials — silicon (Si), gallium arsenide (GaAs), gallium nitride (GaN), sapphire, glass, the full range of ceramics, and polymers (used in microfluidics) — are readily etched by UV lasers. Many of these materials cannot, however, tolerate longer wavelengths without cracking, melting, or shattering. Other materials will exhibit rough holes and edges that do not meet the strict requirements of the application. This occurs, in part, due to the laser’s low resolution. MEMS technology based on difficult materials can clearly benefit from the many strengths of UV laser processing.
This article was written by Jeffrey P. Sercel, president of JPSA Laser, Hollis, NH. The author may be contacted at jpsercel@ jpsalaser.com or (603) 595-7048. Learn more about JPSA online at www.jpsalaser.com .