Abstract:
A method is disclosed for pre-release plastic packaging of MEMS and IMEMS devices. The method can include encapsulating the MEMS device in a transfer molded plastic package. Next, a perforation can be made in the package to provide access to the MEMS elements. The non-ablative material removal process can include wet etching, dry etching, mechanical machining, water jet cutting, and ultrasonic machining, or any combination thereof. Finally, the MEMS elements can be released by using either a wet etching or dry plasma etching process. The MEMS elements can be protected with a parylene protective coating. After releasing the MEMS elements, an anti-stiction coating can be applied. The perforating step can be applied to both sides of the device or package. A cover lid can be attached to the face of the package after releasing any MEMS elements. The cover lid can include a window for providing optical access. The method can be applied to any plastic packaged microelectronic device that requires access to the environment, including chemical, pressure, or temperature-sensitive microsensors; CCD chips, photocells, laser diodes, VCSEL&#39;s, and UV-EPROMS. The present method places the high-risk packaging steps ahead of the release of the fragile portions of the device. It also provides protection for the die in shipment between the molding house and the house that will release the MEMS elements and subsequently treat the surfaces.

Description:
FEDERALLY SPONSORED RESEARCH 
     The United States Government has rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     None Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to the field of microelectronics and more specifically to plastic packaging of microelectromechanical systems (MEMS) and integrated microelectromechanical systems (IMEMS) devices. 
     Examples of MEMS and IMEMS devices include airbag accelerometers, microengines, optical switches, gyroscopic devices, sensors, and actuators. For current commercially packaged MEMS and IMEMS components, the steps of packaging and testing can account for at least 70% of the cost. Also, the current low-yields of MEMS packaging are a “show-stopper” for the eventual success of MEMS. Conventional electronic packaging methods, although expensive, are not presently adequate to carry these designs to useful applications with acceptable yields and reliability. 
     Current packaging methods first release the MEMS elements at the wafer scale, followed next by probe testing. Unfortunately, probed good MEMS are often lost in significant quantity due to damage during subsequent packaging steps. These subsequent steps include die separation, die attach to the package, wirebonding (or other interconnection methods), and sealing with hermetic or dust protection lids. Electrostatic effects, moisture, and rough handling can damage the fragile MEMS elements or wirebonds. There is a need to ruggedize the MEMS elements and wirebonds against damage during each of these packaging steps. Herein, the word “wafer” can include silicon; gallium arsinide (GaAs); or quartz wafers or substrates (e.g. for MEMS structures). 
     At some stage in the fabrication process the MEMS elements must be released (e.g. made functional) by etching away a sacrificial, protective layer of silicon dioxide or silicate glass that surrounds the MEMS elements. Typically, this is done at the wafer scale. A typical wet release procedure includes acid etching in hydrofluoric or hydrochloric acid, followed by rinsing and drying. Alternatively, dry plasma etching with chemically active ions, such as oxygen, chlorine, or fluorine ions, can be used. It is critical that the release process does not damage other features on the MEMS or IMEMS device, such as metal interconnects. Dry etching processes are generally less damaging to the fragile MEMS elements than wet processes, but can take more time to complete. A desirable goal is to postpone the release step until the last possible moment. 
     Many different types of microelectronic devices require an opening in the protective package that exposes a sensitive or active area to the surrounding environment. For example, MEMS elements (e.g. gears, hinges, levers, slides, mirrors, optical sensors, chemical sensors, etc.) must not be encapsulated in plastic because these free-standing structures must be able to move, rotate, etc. Also, MEMS packages can require optical access through a window to permit viewing and inspection for calibration and performance characterization of operating MEMS elements. 
     Likewise, microsensors that have chemically sensitive, pressure-sensitive, or temperature-sensitive areas (sometimes combined with IC&#39;s, CMOS or Bipolar chips, etc.) must be freely exposed to the environment through an opening or openings in the plastic package. Finally, optically active microelectronic devices can require optical access through an opening in the plastic package (the package may, or may not, have a window attached across the opening). Examples of optically active devices include charge coupled devices (CCD), photocells, laser diodes, vertical cavity surface emitting lasers (VCSEL&#39;s), and UV erasable programmable read-only memory chips (UV-EPROM&#39;s). While some of these devices emit light, and while others receive light; both are considered to be “optically active”. 
     Therefore, some method of creating an opening in the plastic package is needed for these types of microelectronic devices. 
     Butler, et al. disclose a method of creating an opening in a plastic package using laser ablation to vaporize and cut a window through a 60 micron thick protective dielectric layer (e.g. 2 layers of Kapton film bonded with thermoplastic adhesive), thereby exposing the unreleased MEMS device or microsensor to the environment. See J. T. Butler, V. M. Bright, and J. H. Comtois, “Multichip Module Packaging of Microelectromechanical Systems”,  Sensors and Actuators  A 70 (1998) 15-22. In the same reference, Butler, et al. disclose using a two-step process where the majority of the protective layer is removed by a high-power laser, followed by removing the remaining thin layer by using a low-power laser, or by using a plasma etching process. Then, after creating the opening, a conventional release etch can be performed to release the MEMS elements. 
     A problem with using an ablative process (e.g. laser ablation) to create an opening in a plastic package is potential damage to MEMS elements (or other sensitive surfaces) caused by overheating, and fracture or warping due to thermal stresses. Accurate control is required to prevent accidentally cutting through the protective layer and damaging the sensitive area below. Another potential problem is unwanted deposition of laser-ablated debris on to adjoining surfaces. Finally, the rate that material can be removed by laser ablation or dry plasma etching is generally much slower than that which can be achieved by non-ablative methods, including acid (e.g. wet) etching, mechanical machining, water jet cutting, vacuum or thermal processing methods. 
     In U.S. Pat. No. 5,897,338, Kaldenberg teaches a method for encapsulating an integrated semi-conductor circuit (die) comprising the following steps: a) mounting the semi-conductor circuit onto the surface of a lead frame, b) attaching connecting wires between the contact surfaces of the semi-conductor circuit and selected parts of the lead frame (bonding operation), c) by means of a mould producing a plastic housing which at least encapsulates the semi-conductor circuit, the supporting surface, the bonding wires and part of the lead frame, wherein d) the mould comprises an inwards extending section of which the end surface in the closed situation of the mould extends parallel to the free upper side of the integrated semi-conductor circuit at short distance thereof, and e) before closing the mould a layer of heat resistant deformable material in the form of a ring or a continuous layer is brought in between the upper side of the integrated semi-conductor circuit and the end surface of the inwards extending part, which layer not or hardly adheres to the plastic housing. The combination of the inwards extending part and the ring or layer of deformable material serve together to completely exclude any encapsulant from touching the surface of the IC, and serve to exclude any encapsulant from flowing into the volume directly above the designated surface area of the IC. 
     Kaldenberg does not discuss in the &#39;338 patent any application of his method to packaging of MEMS or IMEMS devices. However, if Kaldenberg&#39;s method of the &#39;338 patent were to be applied to packaging of MEMS or IMEMS device, potential problems could include: damaging the MEMS elements if the inwards extending section is pushed down too hard; sticking of the deformable material to the MEMS surface; and sticking of the encapsulant to the inwards extending section. 
     The present invention differs from Kaldenberg&#39;s &#39;338 patent in at least two ways. Firstly, in the present invention, the opening in the plastic package is created by physically removing (e.g. perforating) the encapsulant in a volume located above the sensitive area (e.g. “designated area of the IC”). Kaldenberg, on the other hand, never allows any of the flowable encapsulant to fill-in this volume because his patent teaches the step of inserting an excluding member through a hole in the top of the transfer mold frame prior to encapsulating. 
     Secondly, in the present invention, encapsulant is allowed to flow across the face of the designated area of the IC. Kaldenberg, on the other hand, teaches the use of a deformable ring or layer placed in contact with the surface of the IC to specifically prevent the flow of encapsulant across the face of the designated area. If Kaldenberg&#39;s &#39;338 method were to be used without using the deformable ring or layer, then some encapsulant would flow across the face of the designated surface area of the IC; thereby defeating the purpose of Kaldenberg&#39;s invention, namely, to provide complete access to the IC after molding, without the need to perform any additional material removal steps. Kaldenberg does not teach or suggest in the &#39;338 patent any method, or means for, removing encapsulant material which may have flowed across the face of the IC. Kaldenberg uses the deformable ring or layer to prevent such a flow from occurring, thereby obviating the need for removing encapsulant after the molding step. 
     In the present invention, a perforation is created in an electrically insulating material above the sensitive area. A commercial device currently exists for this purpose. Conventionally called a “decapsulator”, or “jet etcher”, such a device removes plastic material by directing a stream of heated acid etchant perpendicular to the surface being etched. A gasket can be used to confine the area being etched. Conventional etching solutions comprise fuming nitric acid, fuming sulfuric acid, or mixtures of the two acids. The etching solutions can be heated to 250° C., and +/−1% temperature control is provided. An example of a decapsulator device is the “D Cap-Delta” dual acid system sold by B&amp;G International, Santa Cruz, Calif., 95060. 
     Commercially available decapsulators are used for creating an opening in plastic packages to expose an integrated circuit (IC) contained therein. The etching solutions described above preferentially dissolve common plastic compounds, including epoxy resin, without attacking the underlying integrated circuit. Reasons for decapsulating the package include inspection, testing, failure analysis, and repair. Visual inspection can confirm the integrity of wirebond connections. Probe pins can be placed onto the exposed surface pads to test the operation of the IC (e.g. probe test). It is also desirable to test IC&#39;s while the package is still mounted to the printed circuit board. Failure analysis also requires a method of opening up selected portions of a microelectronic device potted in plastic. Finally, damaged wirebond connections can be repaired, once the broken connection is exposed. 
     Commercially available decapsulators are not presently used to create an opening in plastic packages or covers for the purpose of exposing released (or unreleased) MEMS devices. They are also not presently used for the purpose of “activating” microsensors, e.g. by exposing the active sensor area freely to the surrounding environment. Decapsulators are also not presently used to create an opening for providing optical access to optically active areas on integrated circuits (e.g. CCD chips and UV-EPROMS). 
     What is desired is a method that provides a low-cost, high-yield, and high-capacity commercial packaging technique that releases the MEMS elements after covering or encapsulating the MEMS or IMEMS devices, delicate wirebonds, other electrical interconnections, and die attachments in a hardened plastic material. What is also desired is to minimize the number of processing steps that are performed after releasing the MEMS elements, in order to minimize the risk of damaging the fragile MEMS elements. It is also desired to use a material removal process that can create an opening in a plastic protective layer, without damaging the underlying sensitive area underneath (e.g. IC&#39;s, MEMS elements, or sensor elements). 
     The present invention solves these problems by firstly creating an opening in the plastic protective layer by using a non-ablative material removal process, which exposes the sensitive area to the surrounding environment; and, secondly, releasing any MEMS elements by wet or dry etching the unreleased MEMS device with access provided by the opening created in the plastic cover. 
     Throughout this application, use of the word “MEMS” is intended to also include “IMEMS” devices, unless specifically stated otherwise. Likewise, the word “plastic” is intended to also broadly include any type of flowable dielectric composition. 
     The present method places the high-risk packaging steps ahead of the release of the fragile portions of the device. It also provides protection for the die in shipment between the molding house and the house that will release the MEMS elements and subsequently treat the surfaces. This packaging scheme is very inexpensive compared to placing a device in a discrete package. It permits one to obtain large numbers of functional devices for performance evaluation and debugging of designs. It lets the devices go through any rough handling or environments associated with packaging while still protected in encapsulating plastic. Adhesion problems, such as during die attach, are minimized because surfaces have not yet been treated with proprietary materials. This method can be used for inserting MEMS/IMEMS devices into existing commercial IC plastic packaging lines with minimal impact, thereby minimizing costs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form part of the specification, illustrate various embodiments of the present invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1A shows a schematic cross-section view of a first example, according to the present invention, of a microelectronic device having a sensitive area substantially covered by an electrically insulating material. 
     FIG. 1B shows a schematic cross-section view of a first example, according to the present invention, of a microelectronic device having a sensitive area after perforating the insulating material, thereby exposing the sensitive area. 
     FIG. 2A shows a schematic cross-section view of a second example, according to the present invention, of an unreleased MEMS device that has been die-attached to a paddle, wirebonded to a lead frame, and encapsulated in a molded plastic package. 
     FIG. 2B shows a schematic cross-section view of a second example, according to the present invention, of an unreleased MEMS device, held upside down, where the plastic package can be perforated with an acid etching solution, confined by an external gasket. 
     FIG. 2C shows a schematic cross-section view of a second example, according to the present invention, of an unreleased MEMS device, held upside down, where the MEMS elements are being released by dissolving the sacrificial glass layer with an acid etching solution, confined by an internal gasket. 
     FIG. 2D shows a schematic cross-section view of a second example, according to the present invention, of a released MEMS device that is protected in a sealed cavity by an attached window that provides optical access to the MEMS device. 
     FIG. 3A shows a schematic cross-section view of a third example, according to the present invention, of a wafer with multiple integrated circuit modules, each having a sensitive area. 
     FIG. 3B shows a schematic cross-section view of a third example, according to the present invention, after covering the top of the wafer, including the sensitive areas, with an electrically insulating coating. 
     FIG. 3C shows a schematic cross-section view of a third example, according to the present invention, where the insulating coating can be perforated by two different methods, mechanical milling, and acid etching, thereby exposing the sensitive areas to the surrounding environment. 
     FIG. 4A shows a schematic cross-section view of a fourth example, according to the present invention, of an unreleased MEMS device attached to a lead frame paddle, and wirebonded to the lead frame. 
     FIG. 4B shows a schematic cross-section view of a fourth example, according to the present invention, of an unreleased MEMS device attached to a lead frame paddle, and wirebonded to the lead frame, placed inside a two-piece mold assembly, where the upper part of the mold assembly has an integral, inwardly extending protrusion that does not touch the top of the MEMS device. 
     FIG. 4C shows a schematic cross-section view of a fourth example, according to the present invention, of an unreleased MEMS device attached to a lead frame paddle, and wirebonded to the lead frame, placed inside a two-piece mold assembly, after being transfer molded and encapsulated with a plastic encapsulant. 
     FIG. 4D shows a schematic cross-section view of a fourth example, according to the present invention, of an unreleased MEMS device attached to a lead frame paddle, and wirebonded to the lead frame, placed inside an alternative two-piece mold assembly, where the upper part of the mold assembly has an attached separate mold insert member that that does not touch the top of the MEMS device. 
     FIG. 4E shows a schematic cross-section view of a fourth example, according to the present invention, of an unreleased MEMS device attached to a lead frame paddle, and wirebonded to the lead frame, being removed from the two-piece mold assembly, after the plastic encapsulant has cured and hardened, showing a locally thinner region of plastic directly above the unreleased MEMS elements. 
     FIG. 4F shows a schematic cross-section view of a fourth example, according to the present invention, of an unreleased MEMS device attached to a lead frame paddle, and wirebonded to the lead frame, encapsulated in a plastic package, where the thinner region of plastic above the unreleased MEMS elements can be removed by a first acid etching process. 
     FIG. 4G shows a schematic cross-section view of a fourth example, according to the present invention, of an unreleased MEMS device attached to a lead frame paddle, and wirebonded to the lead frame, encapsulated in a plastic package, where the perforation has been completed, and the sacrificial layer surrounding the MEMS elements can be removed by a second acid etching process. 
     FIG. 4H shows a schematic cross-section view of a fourth example, according to the present invention, of a released MEMS device attached to a lead frame paddle, and wirebonded to the lead frame, encapsulated in a plastic package, where an open cavity has been created above the released MEMS elements, and where a window has been attached to a recessed lip in the plastic package, for providing optical access to the MEMS device. 
     FIG. 5A shows a schematic cross-section view of. a fifth example, according to the present invention, of a microelectronic device having an sensitive area covered by a temporary protective material, wherein the temporary protective material is substantially encased in an electrically insulating material. 
     FIG. 5B shows a schematic cross-section view of a fifth example, according to the present invention, of a microelectronic device having an sensitive area covered by a temporary protective material, wherein the electrically insulating material has been perforated to provide access to the temporary protective material. 
     FIG. 5C shows a schematic cross-section view of a fifth example, according to the present invention, of a microelectronic device having an sensitive area covered by a temporary protective material, wherein the temporary protective material has been removed, thereby exposing the sensitive area to the surrounding environment. 
     FIG. 6A shows a schematic cross-section view of a sixth example, according to the present invention, of a plurality of IC&#39;s or MEMS devices disposed on a wafer. 
     FIG. 6B shows a schematic cross-section view of a sixth example, according to the present invention, of a plurality of IC&#39;s or MEMS devices disposed on a wafer, each having an sensitive area covered by a temporary protective material. 
     FIG. 6C shows a schematic cross-section view. of a sixth example, according to the present invention, wherein the wafer is being cut into multiple individual device dies. 
     FIG. 6D shows a schematic cross-section view of a sixth example, according to the present invention, of individual IC&#39;s or MEMS device dies, each having an sensitive area covered by a temporary protective material. 
     FIG. 6E shows a schematic cross-section view of a sixth example, according to the present invention, of a microelectronic device die having an sensitive area covered by a temporary protective material, where the die is attached to a lead frame paddle, and wirebonded to the lead frame. 
     FIG. 6F shows a schematic cross-section view of a sixth example, according to the present invention, of a microelectronic device die attached to a lead frame paddle, and wirebonded to the lead frame, placed inside a two-piece transfer mold assembly. 
     FIG. 6G shows a schematic cross-section view of a sixth example, according to the present invention, of a microelectronic device attached to a lead frame paddle, and wirebonded to the lead frame, after being encapsulated in plastic, and after being perforated to provide access to the temporary protective material. 
     FIG. 6H shows a schematic cross-section view of a sixth example, according to the present invention, of a microelectronic device attached to a lead frame paddle, and wirebonded to the lead frame, encapsulated in plastic, after the temporary protective material has been removed, also with a window attached across the perforation. 
     FIG. 7A shows a schematic cross-section view of a seventh example, according to the present invention, of a microelectronic device die attached and wirebonded to a package, and encased in plastic. 
     FIG. 7B shows a schematic cross-section view of a seventh example, according to the present invention, of a microelectronic device die attached and wirebonded to a package, and encased in plastic; after being perforated to provide access to the temporary protective material. 
     FIG. 7C shows a schematic cross-section view of a seventh example, according to the present invention, of a microelectronic device die attached and wirebonded to a package, and encased in plastic; after the temporary protective material has been removed, thereby proving free access to the sensitive area through the perforation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a method for activating at least one sensitive area of a microelectronic device, comprising the steps of providing a microelectronic device having at least one sensitive area substantially covered by an electrically insulating material; and perforating the insulating material in a location above the sensitive area by using a non-ablative material removal process; whereby the sensitive area becomes exposed to the surrounding environment. 
     In this context, the word “activating” comprises at least three meanings. Firstly, “activating” can mean creating an opening in the insulating material to freely expose an active sensing area of the microelectronic device to the surrounding environment. Examples of an active sensing area include a chemical sensing area, a pressure sensing area, and a temperature sensing area, or a combination thereof. Secondly, “activating” can mean providing an opening in the insulating material to allow optical access to optically active areas, (e.g. CCD chips, photocells, laser diodes, VCSEL&#39;s, and UV-EPROM&#39;s). Thirdly, “activating” can mean providing an opening in the insulating material to allow access for performing a step of releasing any unreleased MEMS elements in the microelectronic device. 
     FIG. 1A shows a schematic cross-section view of a first example of a microelectronic device  10  having a sensitive area  12  substantially covered by an electrically insulating material  14 , according to the present invention. 
     FIG. 1B shows a schematic cross-section view of a first example according to the present invention, similar to FIG. 1A, wherein an opening  16  has been made in material  14  by perforating material  14  in a region located above sensitive area  12 . Opening  16  exposes the sensitive area  12  to the surrounding environment, thereby activating the sensitive area. 
     The method of perforating the electrically insulating material  14  in FIG. 1B can include any non-ablative material removal process. The non-ablative material removal process can include mechanically machining (e.g. drilling, milling, and grinding), water jet cutting, ultrasonically machining with an abrasive fluid, wet etching, dry etching cutting, vacuum or thermal processing methods. 
     FIG. 2A shows a schematic cross-section view of a second example, according to the present invention, of an unreleased MEMS device  10  that has been die-attached to a paddle  18 , wirebonded  22  to a lead frame  20 , and encapsulated in a plastic package  14  by transfer molding. Device  10  can include unreleased MEMS elements  24 , meaning that elements  24  are surrounded by a sacrificial protective layer  26 , which can be made of silicon dioxide, silicate glass, or vapor-deposited polymer (e.g. parylene). Wirebonds  22  can be attached to bond pads (not shown) on the surface of device  10 . These bond pads have been exposed by cutting vias  28  through protective layer  26  with a laser, or other tool, as is well-known in the art. In this example, sensitive area  12  can include MEMS elements  24 . 
     FIG. 2B shows a schematic cross-section view of a second example, according to the present invention, that is similar to FIG. 2A, where the plastic package  14  can be perforated by directing a stream of a first acid etching solution  32  in a direction substantially perpendicular to the surface of package  14 . An external gasket  30  can be used to confine the spray of acid  32 . It is important to limit the etching process to only the zone of perforation  16  so that other members (e.g. wirebonds  22 ) aren&#39;t unintentionally set free by removing plastic from adjacent regions. Package  14  is held upside down so that fluid and waste debris fall down and away from sensitive area  12 , thereby reducing the potential for contamination. 
     Solutions used for etching plastic (e.g. epoxy resin) conventionally comprise fuming nitric acid, fuming sulfuric acid, or mixtures of the two acids. Preferred etching fluids include heated fuming nitric acid at about 80-100 C for etching NOVOLAC type epoxies, and heated fuming sulfuric acid at about 230-260 C, or non-fuming sulfuric acid at about 230-260 C for etching glob-top polymer compositions. Polymer compositions can be etched by other solvents, as well. Also, if the encapsulant material is water-soluble, then water can be used as the etching fluid. 
     A commercial device currently exists for the purpose of creating an opening  16  in a plastic package  14 . Conventionally called a “decapsulator”, or “jet etcher”, such a device removes plastic material by directing a stream of heated acid etchant perpendicular to the surface being etched. A gasket can be used to confine the area being etched. Conventional etching solutions comprise fuming nitric acid, fuming sulfuric acid, or mixtures of the two acids. The etching solutions can be heated to 250 C, and +/−1% temperature control is provided. An example of a decapsulator device is the “D Cap-Delta” dual acid system sold by B&amp;G International, Santa Cruz, Calif., 95060. Other decapsulating devices are also described in U.S. Pat. No. 5,855,727 to Martin, et al., and in U.S. Pat. No. 5,932,061 to Lam. 
     Nitric acid and sulfuric acid are both attractive etching solutions because they typically do not attack or damage the underlying integrated circuit. Importantly, they also do not attack (e.g. etch) the sacrificial layer of silicon dioxide or silicate glass which typically surrounds the MEMS elements. This is particularly avantageous because the wet etching process can be allowed to proceed at a high rate to 100% perforation of the plastic encapsulant  14 , without concern for accidentally damaging the underlying silicon dioxide or silicate glass sacrificial layer (unlike ablative processes, such as laser ablation). 
     Alternatively, plastic package  14  can be perforated by a dry process (e.g. a plasma etching process, a reactive ion etching process, or an ion milling process). Each of these dry etching processes can use a chemically active ion (e.g. oxygen, chlorine, and fluorine). Plasma etching processes are well-known to those skilled in the art. 
     In another embodiment of the present invention, the non-ablative step of perforating plastic package  14  can be performed in more than one stage. For example, using a two-stage process, the majority of the thickness of the plastic layer  14  can be first removed by a fast removal process. This can comprise using a wet acid etching solution with a high velocity jet stream. Or, it can comprise mechanical machining with a high-speed milling tool. Then, in the second stage, a much slower process can be used to remove the remaining thin layer of plastic  14 . Examples of slower processes can include wet etching with a low-velocity stream, or with a stream of a dilute concentration of acid product. Dry plasma etching can also be used, which is typically a slower process. The primary reason for switching to a much slower process at the final stage of perforation is to remove the last amount of plastic using a gentle process. This minimizes the possibility of accidentally damaging the sensitive area underneath. 
     FIG. 2C shows a schematic cross-section view of a second example, according to the present invention, that is similar to FIG. 2B, where the MEMS elements  24  are being released by dissolving the sacrificial glass layer  26  with a second acid etching solution  34 , confined by an optional internal gasket  31 . This step is also conventionally called a release step, because after the MEMS elements  24  are “released”, they are then free to move, rotate, tilt, etc. Releasing by wet etching can include using an acid solution comprising hydrochloric acid, hydrofluoric acid, or any combination thereof. After wet etching, MEMS elements  24  can be dried by methods that can reduce unwanted stiction, including sublimation and supercritical drying, as is well-known to those skilled in the art. 
     Alternatively, MEMS elements  24  can be released by a dry process, such as plasma etching (in the manner as described above). Dry plasma etching can also be used to release MEMS elements that can be protected by a sacrificial coating of a water-insoluble, vacuum-deposited carbon-based, parylene-type organic polymer coating or silicon nitride coating. Parylene is a generic name for thermoplastic polymers based on poly-para-xylyene monomers, and have low dielectric constants, low water affinity, low film stress, high electrical resistivity, are inert to organic solvents, including water, and may be conformally deposited from the vapor state without solvents or high temperature cures. In this embodiment, a parylene coating can replace the conventional sacrificial layer of silicon dioxide or silicate glass. 
     After releasing the MEMS elements  24  in FIG. 2C, optional coatings can be applied to the released MEMS elements  24  in order to reduce friction, improve performance, and increase the lifetime of these moving components. One advantage of waiting until this stage in the fabrication process to apply anti-stiction coatings is that there is no concern with possible contamination of the backside of the MEMS device die, or the bonding pads, with anti-stiction coatings (since the MEMS die have already been attached to the paddle  18  and encapsulated in plastic). This is not the case when anti-stiction coatings are applied at the wafer scale (after wafer-scale release). The subsequent removal of these unwanted coatings on the backside of the wafer can damage or contaminate the released MEMS elements. Likewise, application of coatings to the wafer backside that are designed to promote or enable the die attachment process may similarly damage the released MEMS elements by adsorption of harmful materials. 
     Those skilled in the art will understand that the release step illustrated in FIG. 2C would not need to be performed if the sensitive area  12  of device  10  did not have any unreleased MEMS elements  24 . 
     FIG. 2D shows a schematic cross-section view of a second example, according to the present invention, that is similar to FIG. 2C, of a released MEMS device  24  that can be protected in a sealed cavity  25  by an attached window  36  that provides optical access  40  to the MEMS device. Window  36  can be attached to plastic package  14  by polymer seal  38 . Alternatively, an opaque cover lid (not shown) can be attached in place of window  36 , to provide dust and environmental protection to released MEMS device  24 . A window  36  or cover lid would not be used if open-access to the surrounding environment is required, e.g., for a chemically-sensitive or pressure-sensitive microsensor. The other structures (e.g. wirebonds  22 ) remain safely encapsulated inside of plastic molded package  14 . 
     FIG. 3A shows a schematic cross-section view of a third example, according to the present invention, of a wafer  42  with multiple integrated circuit modules  44 , each having a sensitive area  12 . Sensitive area  12  can be an active sensing area, an optically-active area, or can comprise MEMS elements in a MEMS or IMEMS device. Active sensing area can be a chemical sensing area, a pressure sensing area, or a temperature sensing area, or a combination thereof. Optically-active areas can be CCD chips, photocells, laser diodes, VCSEL&#39;s, and UV-EPROM&#39;s, or a combination thereof. CMOS or Bipolar integrated circuits can be combined with sensitive area  12  to form an integrated microelectronic device including a sensitive area  12 . 
     FIG. 3B shows a schematic cross-section view of a third example, according to the present invention, that is similar to FIG. 3A, after covering the top of wafer  42 , including the sensitive area  12 , with an electrically insulating coating  14  (e.g. glob-top polymer, epoxy resin, parylene, silicon dioxide). Alternatively, the entire silicon wafer  42  could be encapsulated in a plastic body (not shown). 
     FIG. 3C shows a schematic cross-section view of a third example, according to the present invention, that is similar to FIG. 3B, where the insulating coating  14  can be perforated by two different methods, mechanical milling  46 , and acid etching  32 , thereby exposing the sensitive areas to the surrounding environment through perforation  16 . 
     FIG. 4A shows a schematic cross-section view of a fourth example, according to the present invention, of an unreleased MEMS device  10  attached to a paddle  18  of a lead frame  20 , and wirebonded  22  to the lead frame  20 . Device  10  can include unreleased MEMS elements  24  that can be surrounded by a sacrificial protective layer  26 , which can be made of silicon dioxide, silicate glass, or vapor-deposited polymer (e.g. parylene). Wirebonds  22  can be attached to bond pads (not shown) on the surface of device  10 . These bond pads have been exposed by cutting vias  28  through protective layer  26  with a laser or other tool, as is well-known in the art. In this example, sensitive area  12  can include MEMS elements  24 . 
     FIG. 4B shows a schematic cross-section view of a fourth example, according to the present invention, that is similar to FIG. 4A, where device  10  has been placed inside of a two-piece mold assembly  47 , wherein assembly  47  includes a lower part  50  and an upper part  48 , wherein the upper part  48  has an integral, inwardly extending protrusion  54  that does not touch the top surface of the MEMS device  10 . This creates a positive gap  52  in-between the top surface of the MEMS device  10  and the bottom surface of protrusion  54  for plastic encapsulant  14  to flow into. One reason for using protrusion  54  is to reduce the thickness of encapsulant that has to be removed during the perforation step, thereby reducing the time and cost required to complete that step. 
     FIG. 4C shows a schematic cross-section view of a fourth example, according to the present invention, that is similar to FIG. 4B, after being transfer molded in a two-piece mold assembly  47 , and encapsulated with a flowable plastic encapsulant  14 . Many possible “Plastic” compounds can be used, including: epoxy, resin, plastics, glob top polymers, gels, silicones, rubber, thermosetting plastics, thermoplastic plastics, two-part epoxies, UV curable epoxies, fast curing epoxies, slow curing epoxies, water-soluble compounds, water-insoluble compounds, NOVOLAC epoxies, anhydride epoxy, polyimide epoxy, polyphenylene sulfide polymers, polyetherimide polymers, polyethersulfone polymers, liquid crystal polymers, urethanes, polyesters, transparent, opaque, and hardenable resins. One purpose of the encapsulating step is to provide structural support for the electrical leads, and to ruggedize the microelectronic device against breakage, contamination, abuse, electrostatic effects, moisture, light, and associated handling stresses. 
     FIG. 4D shows a schematic cross-section view of a fourth example, according to the present invention, that is similar to FIG. 4A, where the mold assembly  47  can have a separate mold insert member  56  attached to the inside surface of the upper part  48 . The lower surface of mold insert member  56  does not touch the top surface of the MEMS device  10 , thereby creating a positive gap  52  for plastic encapsulant  14  to flow into. 
     FIG. 4E shows a schematic cross-section view of a fourth example, according to the present invention, that is similar to FIG. 4D, after the plastic encapsulant has cured and hardened, wherein a thin layer of plastic  58  has flowed and filled-in gap  52 . Removal of the upper mold part  48  reveals a region of partial perforation  16 ′ created by inwardly extending integral protrusion  54 . 
     FIG. 4F shows a schematic cross-section view of a fourth example, according to the present invention, that is similar to FIG. 4E, where the thin layer of plastic  58  can be removed by using a first acid etching process  32  (as described earlier), accessed through partial perforation  16 ′. 
     FIG. 4G shows a schematic cross-section view of a fourth example, according to the present invention, that is similar to FIG. 4F, where perforation  16  has been completely opened, and the sacrificial layer  26  surrounding the MEMS elements  24  can be removed (e.g. released) by using a second acid etching process  34  (as described earlier). 
     FIG. 4H shows a schematic cross-section view of a fourth example, according to the present invention, that is similar to FIG. 4G, where an open cavity has been created above the released MEMS elements  24 , and where a window  36  has been attached to a recessed lip  60  in the plastic package, for providing optical access  40  to the MEMS device. Recessed lip  60  can be formed by machining package  14  after the plastic has cured and hardened. Alternatively, lip  60  can be formed during transfer molding by using an appropriately shaped ledge (not shown) located on protrusion  54 , or located on mold insert member  56 . 
     FIG. 5A shows a schematic cross-section view of a fifth example, according to the present invention, of a microelectronic device  10  having an sensitive area  12  covered by a temporary protective material  15 , wherein the temporary protective material is encased in an electrically insulating material  14 . Temporary protective material  15  is not the same material as insulating material  14 . 
     The temporary protective material can include a parylene polymer. The parylene coating can be poly-para-xylylene, poly-para-xylylene which has been modified by the substitution of a chlorine atom for one of the aromatic hydrogens, or poly-para-xylylene which has been modified by the substitution of the chlorine atom for two of the aromatic hydrogens. The method can include blending the parylene polymer with a reactive material to form a copolymer coating. The reactive material can include a monomer containing silicon, carbon, or fluorine, or a combination thereof. The temporary protective material can include silicon nitride, metal (e.g. aluminum or tungsten), a vapor deposited organic material, cynoacrylate, a carbon film, a self-assembled monolayered material, perfluoropolyether, hexamethyidisilazane, or perfluorodecanoic carboxylic acid, silicon dioxide, silicate glass, or combinations thereof. 
     FIG. 5A also shows an alternative method comprising the steps of providing a microelectronic device  10  having a sensitive area  12 ; covering at least the sensitive area  12  in a temporary protective material  15 ; substantially encasing the temporary protective material with an electrically insulating material  14  that is different than the temporary protective material  15 ; and substantially hardening the electrically insulating material  14  before perforating the electrically insulating material. 
     The encasing step in FIG. 5A can comprise encapsulating the device in the electrically insulating material  14 . The encasing step can comprise using a process selected from pouring, casting, spin-on coating, and glob top overmolding. The temporary protective material  15  can comprise a flowable elastomeric material selected from rubber, silicone, and polyurethane. The step of removing the temporary protective material  15  can comprise picking the material out by hand or by a robot manipulator. The temporary protective material  15  can comprise a dissolvable material that is dissolvable with a solvent that also does not substantially dissolve the electrically insulating material. The temporary protective material  15  can comprise a low-melting point material (e.g. wax, solder), which can be melted and flowed out of opening  13 . 
     The electrically insulating material  14  in FIG. 5A can comprise a flowable plastic material (e.g. epoxy resin) that is etchable by nitric acid or fuming nitric acid but is not etchable by sulfuric acid or fuming sulfuric acid. In this example, temporary protective material  15  can comprise a glob top polymer. 
     Alternatively, the electrically insulating material  14  can comprise a flowable plastic material (e.g. glob top polymer) that is etchable by sulfuric acid or fuming sulfuric acid but is not etchable by nitric acid or fuming nitric acid. In this example, temporary protective material  15  can comprise an epoxy resin. 
     FIG. 5B shows a schematic cross-section view of a fifth example, according to the present invention, that is similar to FIG. 5A, wherein the electrically insulating material  14  has been perforated to provide access to the temporary protective material  15  through opening  13 . 
     FIG. 5C shows a schematic cross-section view of a fifth example, according to the present invention, that is similar to FIG. 5B, wherein the temporary protective material  15  has been removed, thereby activating the sensitive area  12  by exposing it to the surrounding environment through perforation  16 . 
     FIG. 6A shows a schematic cross-section view of a sixth example, according to the present invention, of a plurality of IC&#39;s or MEMS devices  44  disposed on wafer  42 , with each of the devices  44  having a sensitive area  12 . 
     FIG. 6B shows a schematic cross-section view of a sixth example, according to the present invention, that is similar to FIG. 6A, wherein each sensitive area  12  is covered by a temporary protective material  15 . 
     FIG. 6C shows a schematic cross-section view of a sixth example, according to the present invention, that is similar to FIG. 6B, wherein wafer  42  is being cut into multiple individual device dies  10  in a manner well-known to those skilled in the art. 
     FIG. 6D shows a schematic cross-section view of a sixth example, according to the present invention, that is similar to FIG. 6C, of four individual IC&#39;s or MEMS device dies  10 , each having an sensitive area  12  covered by a temporary protective material  15 . 
     FIG. 6E shows a schematic cross-section view of a sixth example, according to the present invention, that is similar to FIG. 6D, where the die  10  has been attached to a paddle  18 , and wirebonded to the lead frame  20  with wirebonds  22 . 
     FIG. 6F shows a schematic cross-section view of a sixth example, according to the present invention, that is similar to FIG. 6E, of a microelectronic device  10  placed inside a two-piece transfer mold assembly  47  made of upper frame  48  and lower frame  50 . The top of temporary protective material  15  does not touch the inside surface of upper frame  48 , so that temporary protective material  15  can be substantially encased in insulating material  15  in the next step. 
     FIG. 6G shows a schematic cross-section view of a sixth example, according to the present invention, that is similar to FIG. 6F, after being encapsulated in plastic  14 , and after being perforated to provide access to the temporary protective material  15  through opening  13 . In this example, the width of opening  13  is smaller than the width of temporary protective material  15 . 
     FIG. 6H shows a schematic cross-section view of a sixth example, according to the present invention, that is similar to FIG. 6G, after the temporary protective material  15  has been removed by wet acid etching. Also, a window  36  has been attached across the perforation  16 . In this example, the width of window  36  is smaller than the width of the package  14 . 
     FIG. 7A shows a schematic cross-section view of a seventh example, according to the present invention, of a microelectronic device die  10 , having an sensitive area  12  covered by a temporary protective material  15 , wherein the device die  10  has been attached and wirebonded to package  62 , and then flowable plastic has been poured into package  62 , thereby surrounding and encasing device die  10 , wirebonds  22 , and temporary material  15  in an electrically insulating material  14 . Package  62  can comprise a low-temperature or high-temperature cofired ceramic multilayered body with metallized conductive traces interconnecting wirebonds  22  to external bonding pads (not shown). Alternatively, package  62  can comprise a multilayered polymer printed wiring board material and construction. 
     FIG. 7B shows a schematic cross-section view of a seventh example, according to the present invention, that is similar to FIG. 7A, after material  14  has been perforated to provide access to the temporary protective material  15  through opening  13 . 
     FIG. 7C shows a schematic cross-section view of a seventh example, according to the present invention, that is similar to FIG. 7B, after the temporary protective material  15  has been removed, thereby proving free access to the sensitive area  12  through the perforation  16 . 
     The particular examples discussed above are cited to illustrate particular embodiments of the invention. Other applications and embodiments of the apparatus and method of the present invention will become evident to those skilled in the art. For example, although the figures illustrate only a single MEMS device, the method described herein applies equally well to packaging of multiple MEMS or IMEMS devices. Also, the method of perforating one side of a plastic package can be applied to an opposite side. One example of this can be perforating both sides of a pressure-sensitive area on a microsensor device that is encapsulated in plastic. By perforating both sides, then a pressure differential can be sensed from one side of the device to the other side. The actual scope of the invention is defined by the claims appended hereto.