Dicing MEMS (1063251)
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Dicing MEMSDr. Ramon J. AlbalakAdvanced Dicing Technologies,Haifa, IsraelH e a d q u a r t e r s – Advanced Dicing Technologies Ltd.www.adtdicing.comAdvanced Technology Center, Haifa 31905, Israel, Tel.: 972-48545222, Fax: 972-4-8550001IntroductionMicro-Electro-Mechanical Systems (MEMS) include a large variety of miniaturizedintelligent mechanical systems such as accelerometers, flow sensors, motion mirrors andradio-frequency devices. MEMS are manufactured by the combination of wellestablished methods for the fabrication of integrated circuits (ICs) together with variousmicromachining processes to selectively etch away desired portions of the silicon waferupon which the device is based or to create additional layers of material, thus forming athree-dimensional functional structure. The final component may contain minute andextremely delicate structures such as cantilevers, bridges, hinges, gears, membranes andother sensitive features that necessitate special handling and care.Several basic differences make the dicing of MEMS significantly more challenging thanthat of typical ICs or other microelectronic components.
MEMS may containmembranes, high aspect-ratio topography and other pressure-sensitive components thatcannot withstand the impact of water encountered during dicing and the subsequentcleaning cycle, and raise the need for a protective mechanism to shield them from theconstant flow of liquid. In addition, MEMS often have moving parts that are supersensitive to contamination, and in which the presence of tiny debris particles may hinderor even halt movement altogether. Specific types of MEMS (e.g. electrostatic actuators)are especially sensitive to ESD phenomena and may fail upon spontaneous electrostaticdischarges.H e a d q u a r t e r s – Advanced Dicing Technologies Ltd.www.adtdicing.comAdvanced Technology Center, Haifa 31905, Israel, Tel.: 972-48545222, Fax: 972-4-8550001Current Dicing ApproachesThere are several approaches for dicing MEMS that may be employed in order toovercome the challenges created by their fragility and sensitivity to contamination andESD.
One approach is to permanently cap the MEMS, thus creating a physical barrierbetween the micromechanical parts and the environment. This method both prevents thecontamination of the MEMS device by debris and protects it from the impact of the flowof cooling water during dicing and rinse water in the cleaning step that follows. It alsoreduces the hazards of ESD by eliminating direct contact of sensitive components withthe surroundings. Various materials – such as silicon and glass – are used for capping,and several sealing method may be utilized in order to attach the cap.
After dicing, thecap remains as a permanent part of the MEMS. A variation of the capping method is theuse of a temporary protective sacrificial layer that covers the MEMS device during theactual dicing step, and is later removed or washed away. In most cases, this protectivelayer is a polymeric film that may be removed by either dry or wet techniques.The methods outlined above necessitate additional manufacturing steps and processes inorder 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 by ADTAdvanced Dicing Technologies (ADT) has developed an array of innovative methods andspecialized equipment to overcome the difficulties and obstacles presented by the uniquesensitivities of MEMS to the traditional dicing and subsequent cleaning processes.
Theapproach taken by ADT is one in which the dicing equipment and process – and not theMEMS device itself - are modified in order to solve these problems.H e a d q u a r t e r s – Advanced Dicing Technologies Ltd.www.adtdicing.comAdvanced Technology Center, Haifa 31905, Israel, Tel.: 972-48545222, Fax: 972-4-8550001One of the main factors that govern the impact of cooling water on the substrate duringdicing is the geometry of the cooling block used. ADT has developed cooling blocksspecifically for use with MEMS devices, including custom-made cooling blocks tailoredto meet the needs of specific customer applications.
An example of such a cooling blockis shown in Figure 1 which exhibits a cooling block including a rear-end nozzle.Figure 1: A non-standard cooling block developed by Advanced Dicing Technologiesspecifically for dicing MEMS.An additional modification to the cooling system geared towards reducing physicaldamage to sensitive and delicate features is dicing in a cooling bath in which the substrateis completely submerged in the cooling medium during the dicing process.
This methodessentially reduces the impact of the coolant to zero.In addition to the negative impact of the streams and sprays of cooling water duringdicing, sensitive MEMS devices may also undergo physical damage during thesubsequent cleaning cycle, which is most often performed on a cleaning systemintegrated into the dicing saw itself. The 7200 family of fully automatic dicing sawsrecently developed by ADT, can be equipped with a unique ‘atomizing’ nozzleconfiguration, in which the cleaning/rinsing water is delivered in the form of minuteH e a d q u a r t e r s – Advanced Dicing Technologies Ltd.www.adtdicing.comAdvanced Technology Center, Haifa 31905, Israel, Tel.: 972-48545222, Fax: 972-4-8550001droplets rather than in the form of a continuous stream.
The atomized flow of water hasimproved cleaning properties, thus reducing the probability of MEMS failure due tocontamination and debris particles.The presence of debris may also be controlled by reducing electrostatic discharges thatattract minute particles. Electrostatic charges and the resulting electric fields increaseparticle deposition due to the Coulomb forces (electrostatic attraction = ESA), andparticle attraction is directly proportional to the electric field.
ADT’s 7200 series ofautomatic dicing saws (basic configuration with integrated handling and cleaning system)has been certified as being in compliance with theSEMI E78-1102 standard regarding electrostaticcompatibility (Figure 2).Figure 2: Certification of compliance of Advanced Dicing Technologies 7200 series ofautomatic dicing saws with the SEMI E78-1102 standard.In addition to particle attraction, the uncontrolled discharge of electrostatic charges cancause both physical damage in the form of localized scorching and lead to malfunction ofsensitive electronic components often found in MEMS devices. Several factors cancontribute to electrostatic charges build-up both during the dicing process and during thevarious stages in which the MEMS-bearing frame is transferred between differentstations within the machine.
Most dicing operations are conducted using deionized (DI)water as the coolant. DI water is used for its purity and lack of dissolved ions. However,this lack of ions results in low conductivity of the water, and its inability to conductelectrostatic charges away from the substrate being diced. One solution to the ESDH e a d q u a r t e r s – Advanced Dicing Technologies Ltd.www.adtdicing.comAdvanced Technology Center, Haifa 31905, Israel, Tel.: 972-48545222, Fax: 972-4-8550001phenomena related to the high resistance of DI water is to incorporate a reionizing unit,which increases conductivity by the addition of carbon dioxide gas. The carbon dioxideforms bicarbonate ions that are harmless, yet sufficient to increase conductivity to thedesired level. As stated, electrostatic charges may originate from the movement of thesubstrate between the different stations in the saw.
ADT’s 7200 series of dicing saws areequipped with ionizing elements that accompany all movement within the machine.Figure 3a shows a schematic of the area between the dicer and cleaning stationemphasizing a stationary ionizer located between the two. Figure 3b shows a secondmobile unit connected to the material handling system (MHS). The combination of bothunits reduce ESD to acceptable levels in full compliance with SEMI E78-1102 standard.abFigure 3: Ionizing elements in the ADT 7200series automatic dicing saws. (a) Stationaryunit between dicer and cleaner; (b) Mobileunit on MHS.H e a d q u a r t e r s – Advanced Dicing Technologies Ltd.www.adtdicing.comAdvanced Technology Center, Haifa 31905, Israel, Tel.: 972-48545222, Fax: 972-4-8550001The 7200 family of fully automatic dicing saws has several additional features which areutilized in the dicing of MEMS.
One such feature is the built-in ability of the saw toaccurately measure the height of the substrate being diced. This feature termed “heighton part” is especially useful when performing partial cuts (i.e. not all the way through)into substrates of which the height is not precisely know beforehand, for example whenpart of the height of the substance is composed of an adhesive layer of undeterminedthickness. An additional ability of the 7200 dicing saws commonly utilized in dicingMEMS with a beveled blade is the machines capability to determine the correct dicingdepth according to the width of the top of the kerf.
By measuring the top opening of thekerf the saw continuously calculates the correct dicing depth and compensates for bladewear. Accordingly, this feature is termed “z-compensation.The 7200 family of dicing saws also has the capability of performing a unique type ofdicing action commonly referred to as a “chop cut” or a “plunge cut”. In this operationthe blade does not cut the substrate from edge to edge, but rather enters it at some pointaway from the edge and dices a predetermined distance before being raised out of thesubstrate. The cut may be partial or transcend the full thickness of the substrate. Thegeneral concept is schematically demonstrated in Figure 4 for a partial cut.
The chop- orplunge-cut may be used, for example, to cut a rectangular section out of a wafer.H e a d q u a r t e r s – Advanced Dicing Technologies Ltd.www.adtdicing.comAdvanced Technology Center, Haifa 31905, Israel, Tel.: 972-48545222, Fax: 972-4-8550001(a)(b)(c)(d)Figure 4: Chop- or plunge-cut concept.While dicing MEMS (especially when performing partial cuts), it if often of greatimportance to maintain the square-shaped edge of the dicing blade. During dicing, theblade wears down resulting in a curved edge. In extreme cases the shape of the edge is asemi-circle.
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