AN ALTERNATIVE TO WET PROCESSING
BY JEFFREY P. SERCEL
Ultraviolet (UV) laser micromachining of microelectromechanical systems (MEMS) and micro-optoelectromechanical systems (MOEMS) is emerging as a viable, and often preferable, alternative to wet processing and other methods, thanks to its speed, accuracy and simplicity. It can be used in applications such as die separation, where wet processing cannot be used, and is superior to mechanical cutting because of its lack of vibration. One of the key concerns of potential users of this technique is the type of byproducts or debris created by UV laser micromachining, its nature, effect and removal procedures. While the amount and type of debris created by UV laser micromachining (photo ablation) varies with the material being machined, there are methods of removal and remediation that are effective and support the successful application of UV lasers to MEMS manufacturing.
MEMS and MOEMS are the integration of mechanical and optical elements, sensors, actuators and electronics on a common substrate (predominantly silicon) through microfabrication technology. While the electronics are fabricated using IC process sequences, the micromechanical components are fabricated using compatible micromachining processes that selectively remove parts of a wafer to form the mechanical and electromechanical devices. Chemical etching is one commonly used method. Others include ion beam and plasma etching, molding devices, mechanical sawing, etc. Increasingly, however, UV lasers are proving to be a better choice for MEMS micromachining processes.
 Figure 1. UV laser cutting MEMS. |
The emerging demands of MEMS micromachining require extremely precise, tight tolerances, high repeatability and cost-effective processing. Short-wavelength (157 to 248 nm) excimer and UV diode-pumped solid-state (DPSS) lasers are ideal for such applications — particularly with regard to processing difficult materials such as borosilicate glass, quartz, fused silica and sapphire — exhibiting the ability to execute complex features with large-area and ganged processing capabilities and characteristic smooth cuts in such applications such as precise drilling of microscopic apertures.
MEMS micromachining may require complex features, holes, cones, channels and sample chambers of microscopic size, of uniform and consistent size, with certain essential characteristics. These may include sharply-defined features, smooth walls, optically clear surfaces, and they must be produced with high repeatability and at production speeds sufficient to make their production economically feasible. Chemical etching, compared to laser micromachining, requires more process steps, poses environmental concerns because of chemistries used, and has materials compatibility issues.
 Figure 2. Various micronozzles control beam shape and intensity. |
In laser micromachining, different materials absorb laser energy differently; the greater the absorption of the material, the easier it is to machine it cleanly and consistently. Many materials can be effectively micromachined with longer-wavelength lasers (e.g., Nd:YAG); however many materials such as certain types of glass, sapphire, etc., cannot tolerate longer wavelengths without cracking, melting or shattering. Other materials will exhibit rough holes (caused in part by low resolution) and edges that do not meet the strict requirements of the application. Difficult materials, such as quartz and fused silica, can be effectively processed using short wavelength (157-nm) UV lasers. Due to the high absorptivity of short-wavelength UV by the material, micromachining is crisp, precise and repeatable. UV lasers are high-average-power sources that operate at a variety of user-selectable wavelengths from 157 to 351 nm. This allows processes to be optimized based on absorption. Submicron layers of materials can be removed with each laser pulse. Short 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.
Photo Ablation
The method of materials removal with UV lasers is unique and a direct function of the laser's characteristic form and energy type. Known as photo ablation, this occurs when small volumes of materials absorb high peak power laser energy. When matter is exposed to focused UV laser light pulses, the energy of the pulse is absorbed in a thin layer of materials, typically <0.1-µm- thick, due to the short wavelength of deep UV light. The high peak power of an UV laser light pulse, when absorbed into this tiny volume, results in strong electronic bond breaking in the material. The molecular fragments that result 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 produced. Each laser pulse etches a fine submicron layer of material. The ejecting material carries the heat away with it. Depth is obtained by pulsing the laser repeatedly; depth control is achieved through overall dosage control.
With many materials, particularly softer materials such as polymers, the byproducts of photo ablation are nearly vaporized and carried off in the plasma plume, with only a small fraction of the ablated material (such as carbon) remaining to redeposit on the workpiece. This is especially true of light micromachining operations. With drilling and complex patterns where typically a greater volume of material is removed, and if the material is particularly hard (such as fused silica and sapphire), there will be a greater volume of residue remaining. It is much finer and decreased in volume than, for example, the rough byproducts of diamond saw cutting and mechanical scribing.
Due to the small size of their moving parts and close tolerances between them, MEMS are particularly sensitive to any type of particulate contamination that could result from a micromachining process other than chemical etching. While the ejected submicron fragment plume resulting from laser photo ablation is often of minimal concern, it is prudent to remove the material so that it will not affect the operation and reliability of the MEMS. Removal of the dust is sometimes problematic. Some MEMS cannot see a wet process, so rinsing or water removal is not possible. Vacuum and purge removal is effective when carefully combined with an airbrush or airknife, but must be balanced so as not to merely blow the dust onto other MEMS or into areas where it cannot be removed and can create reliability issues.
Protective Film Technique
For MEMS applications where the use of water can be tolerated, a water-soluble film protective technique has been developed that is applied to the device before micromachining begins. This film traps the small remnant debris and allows it to be removed with the film after micromachining is finished. Methods are currently being investigated by JPSA to effectively apply this protective cover in a mass production environment. Difficulties with the process include the small size of the MEMS parts, as well as their large numbers.
UV laser micromachining is typically accomplished using short-pulse and short-wavelength DPSS lasers with shorter pulse widths than are typically available. All UV laser wavelengths can be used, including 157-, 193- and 248-nm.
This method has a number of advantages over wet processing. There are no chemical materials involved, and fewer process steps. Material is removed in a one-step process, faster than plasma etching. Laser micromachining allows 3-D features to be created in a single step by controlling the laser exposure. Additionally, lasers produce a limited taper angle that eliminates the problem of undercutting associated with wet processing.
In UV laser processing, the shorter the UV wavelength used, the better the material will absorb energy. This allows the operator to strip very fine and controlled layers of material off with each laser pulse. The UV laser is a very high-powered laser that can run of hundreds of Hz, up to kHz repetition rates. 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. The ability to focus to smaller spot sizes is a key factor in MEMS micromachining capability.
 Figure 3. Cross section SEM of a tiny inkjet nozzle. |
Virtually all of the semiconductor materials — silicon, GaAs, gallium nitride, sapphire, glass, the full range of ceramics, polymers (used in microfluidics) — are readily etched by UV lasers. When substrate material is photo ablated by a UV laser pulse, the material, whether vaporized or reduced to a submicron-sized particulate, does not necessarily go away. If it is a finely divided powder, for example (particularly with semiconductor materials or ceramics), it may be redeposited on the surface of the substrate.
What's Left?
With a longer pulsed laser, the materials removed at the beginning of the pulse and at the tail end of the pulse tend to be absorbed by the plasma plume that is formed. Superheating scatters the material. With shorter laser pulses, there is less scattering of the debris field. That material is ejected out in the ablation process, supersonically, and carries away the heat of the laser pulse and ablation process with it. This allows the micromaching process to operate virtually heat-free, so that there is little or no thermal effect on surrounding material or microstructures.
The size of the debris particles depends on the material that is being ablated. With polymers, the composition of the debris can go down to the molecular level, consisting of cracked polymers, various compounds, and gases. With solids such as silicon dioxide, silicon, or ceramics, the debris cloud can consist of constituent metals and oxides, very fine submicron particles. These types of particles are a concern for redeposition. The range of the debris field can range from 10s to 100s of microns, depending on how much material is being removed. Generally, adjacent devices are within that range, particularly when conducting a dicing operation where one is actually creating the device itself.
Debris Removal Methods
Removal of the debris created by the laser micromachining process can involve a variety of methods and techniques. One method (where wet processing or contact with water or liquids is allowed) is the use of a liquid assist with the laser, where the waste materials are actually carried or rinsed away with the liquid. With UV laser lasers, a liquid stream or a fine mist aimed near the target area will carry away the debris, but has the disadvantage of requiring an increase in the number of laser pulses needed in order to etch the part. The use of CO2 'snow' to collect debris particles is another method.
A unique approach is to use a 'sacrificial' soluble coating or layer that the laser will burn through to etch the part; the ejected debris collects or settles out on this coating, which is then washed away after processing, taking the debris with it. There are a wide variety of coatings, but of course they must be compatible with the end result that the customer is trying to achieve.
Still another method is to process in a vacuum, where the mean free path for ejected plasma is increased. This allows the debris material easier egress from the ablation area (fewer air or gas molecules to collide with). But processing in a vacuum is complicated and impractical for most facilities and production situations with technical and equipment capitalization issues.
Where liquid assist is not or cannot be used, the most common debris removal method is to use a jet of purge gas in conjunction with a vacuum nozzle, whereby the vacuum nozzle creates a venturi effect to draw out matter from the debris field. The purge gas is introduced into the ablation zone, with the vacuum creating a low-pressure zone that serves as an exhaust that literally scoops the debris up off the surface and carries it away through the vacuum nozzle. Many different types of purge gas can be used. However, in UV laser processing, it is more commonplace to use an inert gas such as nitrogen, argon or helium rather than a reactive gas such as air or oxygen. Helium offers the advantage of a very high ionization potential, resulting in a very long mean free path for debris ejection due to the small, light nature of helium atoms and helium's plasma suppression qualities. A high-pressure, aggressive flow of helium, therefore, reduces the amount of debris generated to the surface of the immediate area adjacent to an ablation. The disadvantage to this flow is that it can negatively affect the laser beam in high-resolution imaging. The flow must be strictly controlled or beam distortion can result. Helium increases the etch rate in UV laser ablation, possibly due to a reduction in the volume of plasma in the ejection plume, since plasma will absorb the energy of the laser beam.
The larger the cut that is being ablated, the more debris is produced. Material removal must be managed so that it does not interfere with the ablation process, or leave residue behind. The removal process must match the rate at which debris is being generated.
Conclusion
UV laser photo ablation is a viable method offering many advantages over other techniques for micromachining MEMS. The process creates debris that, depending on the material being removed and its volume, may require removal to prevent redeposition on the substrate or on adjacent parts. A number of removal technologies have proven effective, offering the user the ability to capitalize on UV laser processing's strengths to meet the ever-evolving challenges and promise of advancing MEMS technology.
JEFFREY P. SERCEL, president, may be contacted at JPSA Laser, 17D Clinton Drive, Hollis, NH 03049; (603) 595-7048; e-mail: [email protected].