Issue



Dicing MEMS


07/01/2007







HANDLING WITH CARE

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BY RAMON J. ALBALAK, Ph.D., ADT
Micro-electromechanical systems (MEMS) include a variety of miniaturized intelligent mechanical systems such as accelerometers, flow sensors, motion mirrors, and radio-frequency (RF) devices. MEMS are manufactured by a combination of well-established methods for IC fabrication, together with various micromachining processes to selectively etch away desired portions of the silicon wafer upon which the device is based, or to create additional layers of material, thus forming a three-dimensional functional structure. The final component often contains minute and extremely delicate structures such as cantilevers, bridges, hinges, gears, membranes, and other sensitive features that necessitate special handling and care.


Figure 1. A non-standard cooling block specifically for dicing MEMS.
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Several basic differences make dicing MEMS significantly more challenging than dicing typical ICs or other microelectronic components. MEMS may contain membranes, high-aspect-ratio topography, and/or other pressure-sensitive components that cannot withstand the impact of water encountered during dicing and the subsequent cleaning cycle. These components require a protective mechanism to shield them from the constant flow of liquid. In addition, MEMS often have moving parts that are super-sensitive to contamination, and in which the presence of tiny debris particles may hinder or even halt movement altogether. Specific types of MEMS (e.g. electrostatic actuators) are especially sensitive to electrostatic discharge (ESD) phenomena and may fail upon spontaneous ESD.

Current Dicing Approaches

Several approaches for dicing MEMS may be used to overcome challenges created by their fragility and sensitivity to contamination and ESD. One method is to permanently cap the MEMS, thus creating a physical barrier between the delicate micromechanical parts and the harsh dicing environment. This prevents contamination of the MEMS device by debris, and protects it from the impact of the cooling water flow during dicing and rinse water in the cleaning step that follows. It also reduces the hazards of ESD by eliminating direct contact between sensitive components and the surroundings. Various materials - such as silicon and glass - are used for capping, and several sealing methods are available to attach the cap. After dicing, the cap remains as a permanent part of the MEMS. A variation of this is a temporary protective sacrificial layer that covers the MEMS device during the actual dicing step, and is later removed or washed away. In most cases, this protective layer is a polymeric film that may be removed by either dry or wet techniques. Both these approaches necessitate additional manufacturing steps and processes to place the permanent cap or to form and remove the temporary protective film, and add to the overall cost of manufacturing the device.

Innovative Approaches

More recent developments in dicing processes have resulted in an array of innovative methods and specialized equipment to overcome the difficulties and obstacles presented by the unique sensitivities of MEMS. The preferred approach taken is one in which the dicing equipment and processes - and not the MEMS device itself - are modified to solve these problems.


Figure 2. Ionizing elements in an automatic dicing saw. A. Stationary unit between dicer and cleaner. B. Mobile unit on MHS.
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One of the main factors governing the impact of cooling water on the substrate during dicing is the geometry of the cooling block used. Advanced cooling blocks have been developed specifically for use with MEMS devices, including custom-made models tailored to meet the needs of specific customer applications. Figure 1 shows one example, which exhibits a cooling block including a rear-end nozzle.

Another modification to the cooling system geared towards reducing physical damage to sensitive and delicate features is dicing in a cooling bath - the substrate is completely submerged in the cooling medium during the dicing process. This method essentially reduces the impact of the coolant to zero.

In addition to the negative impact of the streams and sprays of cooling water during dicing, sensitive MEMS devices may also undergo physical damage during the subsequent cleaning cycle, which is most often performed on a system integrated into the dicing saw. More advanced types of fully automatic dicing saws can be equipped with an “atomizing” nozzle configuration, in which the cleaning
insing water is delivered in the form of minute droplets rather than in a continuous stream. The atomized flow of water has improved cleaning properties, thus reducing the probability of MEMS failure due to contamination and debris particles.

The presence of debris may also be controlled by reducing ESD that attracts minute particles. Electrostatic charges and the resulting electric fields increase particle deposition due to the Coulomb forces of electrostatic attraction (ESA), and particle attraction is directly proportional to the electric field. When selecting a saw for dicing MEMS it is important to verify that all its components (i.e. basic configuration and any integrated handling and cleaning system) are compliant with the SEMI E78-1102 standard regarding electrostatic compatibility.

Uncontrolled discharge of electrostatic charges can cause localized scorching, and lead to the malfunction of sensitive electronic components often found in MEMS devices. Several factors can contribute to electrostatic charge build-up both during the dicing process and during various stages in which the MEMS-bearing frame is transferred between different stations within the machine. Most dicing operations are conducted using de-ionized (DI) water as the coolant, due to its purity and lack of dissolved ions. However, this lack of ions results in low conductivity of the water, and its inability to conduct electrostatic charges away from the substrate being diced. One solution to ESD phenomena related to the high resistance of DI water is to incorporate a re-ionizing unit, which increases conductivity by adding CO2. The CO2 forms bicarbonate ions that are harmless, yet sufficient to increase conductivity to the desired level.

Dicing saws meant for dicing MEMS should be equipped with ionizing elements that accompany all movement within the machine. Figure 2a shows a schematic of the area between the dicer and cleaning station, emphasizing a stationary ionizer located between the two. Figure 2b shows a second mobile unit connected to the material handling system (MHS). The combination of both units reduces ESD to acceptable levels in full compliance with SEMI E78-1102 standard.

Advanced, fully automatic dicing saws can be equipped with several additional features used for dicing MEMS, such as the built-in ability of the saw to accurately measure the height of the substrate being diced. This feature - termed “height-on-part” - is especially useful when performing partial cuts not all the way through substrates of which the precise height is not previously known, for example when part of the height of the substance is composed of an adhesive layer of undetermined thickness.

The delicacy of some MEMS components and their strict cut-quality requirements often brings up a need for the perfectly balanced blade and flange combination. Recent saw software and hardware advances have led to the development of an in-situ vibration sensor that checks the level of vibrations during dicing, and exhibits an alarm when the level is higher than a predetermined threshold. This system may also be used for dynamic balancing of the blade/flange prior to dicing.


Figure 3. Vibration sensor mounted on the cooling block.
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Some dicing saws also have the capability of performing a unique type of dicing action commonly referred to as a chop- or plunge-cut. In this operation, the blade does not cut the substrate from edge to edge, but rather enters it at some point away from the edge and dices a predetermined distance before being raised out of the substrate. The cut may be partial or transcend the full thickness of the substrate. The general concept for a partial cut is schematically demonstrated in Figure 4. The chop- or plunge-cut may be used, for example, to cut a rectangular section out of a wafer.


Figure 4. Chop- or plunge-cut concept.
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While dicing MEMS (especially when performing partial cuts), it is often of great importance to maintain the square-shaped edge of the dicing blade. During dicing, the blade wears down, resulting in a curved edge. In extreme cases, the shape of the edge is a semi-circle. Specific saws may be equipped with an optional on-line dressing station to maintain blade edge shape. The dressing station option is fully integrated into the saw software package, and all dressing process parameters may be pre-defined along with the frequency at which the blade is dressed. The dressing station may be used for dressing of new blades; periodic cleaning of accumulated buildup, which causes blade overload; and blade reshaping.

Conclusion

Dicing MEMS is a task accompanied by several challenges that stem from the sensitivity of these devices to mechanical pressure, contamination, and ESD. However, there is an array of innovative process- and equipment-based solutions to answer these challenges without the need for modifications of the MEMS device itself in the form of hermetic caps and protective layers.

RAMON J. ALBALAK, Ph.D, engineering manager, may be contacted at Advanced Dicing Technologies, Advanced Technology Center, Haifa, Israel 31905; 972/4-8545222; E-mail: ralbalak@adt-co.com.