Abstract:
Multiple microelectromechanical systems (MEMS) on a substrate are capped with a cover using a layer that may function as a bonding agent, separation layer, and hermetic seal. A substrate has a first side with multiple MEMS devices. A cover is formed with through-holes for vias, and with standoff posts for layer registration and separation. An adhesive sheet is patterned with cutouts for the MEMS devices, vias, and standoff posts. The adhesive sheet is tacked to the cover, then placed on the MEMS substrate and heated to bond the layers. The via holes may be metalized with leads for circuit board connection. The MEMS units may be diced from the substrate after sealing, thus protecting them from contaminants.

Description:
BACKGROUND 
   The present invention relates to sealing and packaging of microelectromechanical fabricated devices on a device substrate before dicing. 
   Microelectromechanical systems (MEMS) are miniaturized devices, such as microswitches that may range in size from about 1 micron to about 1 mm. These devices generally require a controlled environment to operate for a long period of time. Introduction of contaminants such as moisture, particulates or oxygen into the gas surrounding the device can cause sticking, contamination, or interference of the metal contacts, leading to device failure. MEMS devices are often manufactured in an array on a silicon wafer, and then separated or diced into individual units. Such separation is called “singulating”. Dicing produces random particles that can contaminate and disable some of the microdevices. 
   There are known wafer-level packaging processes that may use bonding techniques such as a silicon/gold eutectic bond or gold/gold thermal compression bonding. However, these bonding materials produce an electrically conductive bond, which may introduce constraints as to how the wafers can be bonded and how the device layout may be configured. Another known bonding material is glass frit, or powdered glass. However, this must be screen-printed onto one of the wafers, which can cause contamination of MEMS devices, it limits the process capability of through-wafer cavities and has the potential for outgassing over the operational lifetime of the device. 
   BRIEF DESCRIPTION OF THE INVENTION 
   An aspect of the invention resides in forming a cover, and bonding it to a substrate with multiple MEMS devices via a patterned adhesive sheet to seal the MEMS devices before dicing them into individually packaged microdevices. 
   Another aspect of the invention resides in packaging MEMS devices using an adhesive sheet without additional adhesives to reduce the outgassing potential and at a low enough temperature to avoid damage to MEMS actuators. 
   Another aspect of the invention resides in reducing MEMS production cost and increasing device yield and reliability by wafer-level capping under controlled atmospheric conditions prior to dicing. 
   Another aspect of the invention resides in reducing MEMS package size by bonding a MEMS device cover with an efficient hermetic seal that requires minimal material for sealing. 
   Another aspect of the invention resides in reducing MEMS height requirements on a circuit board by forming short MEMS device leads that require negligible space between the MEMS package and circuit board contacts. 
   Another aspect of the invention resides in minimizing radio frequency reflections and interconnection loss and improving thermal efficiency by minimizing lead length and angles to the MEMS internal pads. 
   Another aspect of the invention resides in providing electrical isolation between the cover and the device substrate. 
   Another aspect of the invention resides in bonding of dissimilar materials including semiconductors, insulators and metals. 
   Another aspect of the invention resides in the increased process flexibility of placing the vias through the cover and enabling a solder filled via. 
   Another aspect of this invention resides in the ability of this solder filled via to be mounted to a circuit board using standard die attachment techniques. 
   Another aspect of this invention resides in placing the solder in close proximity to the MEMS device to improve the thermal and electrical performance. 
   Another aspect of the invention resides in providing a standoff that prevents the adhesive sheet from undesirable encroachment. 
   Another aspect of the invention resides in providing a method of improvement for placing the cover a predetermined distance above the MEMS device. This method can be used to add electrical and mechanical functionality to the device cap. 
   Another aspect of the invention is to use the standoffs for making an electrical connection and/or thermal connection to the device. 
   Another aspect of the invention is improving the registration of the adhesive sheet to the cover through the use of mechanical interlock between the standoff and the adhesive sheet. 
   Another aspect of this invention is the scalability of the adhesive sheet and the whole process from small wafers to full flat panel size sized substrates. 
   Another aspect of this invention is the ability of the adhesive sheet to create a hermetic seal over large topology on the substrate. 
   Another aspect of the invention is the reduced stress build up due to thermal strain on the MEMS device due to the elastic modulus of the adhesive sheet being lower than the substrate. 
   Another aspect of the device is the shock absorption properties of the adhesive sheet. A MEMS device is sensitive to shock, with a time constant near the resonant frequency of the device that is about 200 KHz or 5 microseconds. A shock absorber may act as a low-pass filter, reducing this component of shock with a relatively short time constant. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
     The terms “top” and “bottom” are not used here because parts of the assembly are processed partly in one orientation, and partly in another. Instead, the terms “first side” and “second side” are used, such that all of the first sides eventually face the same direction in the finished MEMS unit. For conceptual clarity, the sectional views show standoff posts  34  in the section planes. However, standoff posts can be placed in any vacant area of the pattern, as shown in surface views such as  FIGS. 11-13 . 
       FIG. 1  is a first side view of a substrate with multiple MEMS devices. 
       FIG. 2  is a partial sectional view taken along line  2 - 2  of  FIG. 1 . 
       FIG. 3  is a partial sectional view of a cover fabrication in progress, the cover coated with a mask material. 
       FIG. 4  shows the mask material patterned by removing parts of it to produce an etching mask on each side. 
       FIG. 5  shows a first etching process that selectively reduces both sides of the cover where not masked. 
       FIG. 6  shows a working layer deposited on a second side of the cover to facilitate a second etching process from the first side. 
       FIG. 7  shows a second etching process that forms via holes through the cover to the working layer. 
       FIG. 8  shows the patterned cover after stripping of the working layer and the mask material. 
       FIG. 9  shows a first side of an adhesive sheet that is cut to match the MEMS substrate and the cover, and is patterned with cutouts for the MEMS devices, vias, and standoff posts. 
       FIG. 10  shows a first side view of a MEMS unit area on the MEMS substrate  20 . 
       FIG. 11  shows a first side view of a unit area of the cover corresponding to the area of  FIG. 10 . 
       FIG. 12  shows a first side view of a unit area of the adhesive sheet corresponding to the area of  FIG. 10 . 
       FIG. 13  shows a first side view of the MEMS device of  FIG. 10  sealed with the cover and adhesive sheet of  FIGS. 11-12 . 
       FIG. 14  is a partial sectional view of the cover bonded to the MEMS substrate via the intermediate adhesive sheet. 
       FIG. 15  is a partial sectional view showing a metal lining the vias. 
       FIG. 16  shows the vias filled with solder, providing solder leads. 
       FIG. 17  shows a singulated MEMS device soldered to a circuit board. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present inventors have innovatively recognized that certain properties of an adhesive sheet embodying aspects of the present invention are conducive to an improved MEMS package and fabrication process, as will be described in greater detail below. In one example embodiment, the adhesive sheet can be a thermoplastic material. Some thermosets like epoxy or epoxy-based adhesives outgas during the cross-linking process. This gas can enter a MEMS device cavity, limiting control over the cavity environment. A thermoplastic does not need to cure, because there is no cross-linking involved with the bonding process. A thermoplastic also does not necessarily need an adhesive layer on either of the bonding surfaces. Some epoxies and other adhesives need such additional adhesive layer(s) to ensure proper bonding. The adhesive sheet is also an electrical insulator that enables passing electrical traces through the bond line without having to passivate them. The adhesive sheet can be formed without the need of a backing layer, enabling the present fabrication process. The adhesive sheet has a flow temperature high enough to allow post-processing such as solder filling of a via without debonding the cover and a flow temperature low enough to avoid damage to the MEMS devices. The adhesive sheet can be patterned by etching and/or ablation. One possible material for this adhesive sheet is liquid crystal polymer (LCP). 
   The drawings show exemplary structure for packaging MEMS units using wafer-level capping.  FIG. 1  illustrates an example MEMS substrate  20  as may be sealed in accordance with aspects of the present invention. MEMS substrate  20  may be formed as currently known or to be developed in the art. It has a first side  20 A with multiple MEMS units  22 , each of which has an electromechanical actuator  24  and electrical contact pads  26 A-C within a unit area  20 U. It has a second side  20 B.  FIG. 2  shows a schematic partial sectional view of two such MEMS units  22 . 
     FIG. 3  shows a partial sectional view of a cover  28  with first and second sides  28 A,  28 B coated with a mask material  30  such as silicon nitride (Si 3 N 4 ). The cover may be made of silicon as currently known or to be developed in the art of MEMS packaging.  FIG. 4  shows the mask material  30  patterned by removing parts of it to produce an etching mask on each side. This patterning and removal can be done by photolithography and dry etching or other methods as currently known or to be developed. 
     FIG. 5  shows an example anisotropic etching process  32 A,  32 B that may be used to selectively reduce sides  28 A,  28 B of the cover  28  where not masked. This process may be done wet or dry. This can be done with a directed solution based etchant such as potassium hydroxide (KOH). Etching may form standoff posts  34  on the second side  28 B of the cover.  FIG. 6  shows a working layer  36  such as copper deposited on the second side  28 B of the cover  28  to facilitate a second etching process on the first side  28 A. The working layer  36  may be deposited by sputtering or other means as currently known or to be developed. In one example embodiment, a function of the working layer  36  is to protect the first side of the cover from the etchant. Alternately, the standoff posts can be formed on the first side  20 A of the substrate  20 . The standoff posts  34  not only provide registration of the adhesive sheet  42 , but also provide consistent separation between the substrate  20  and the cover  28 . 
     FIG. 7  shows a second example etching process  38  that may be used to form holes  40  through the cover  28  for through-connections, or vias.  FIG. 8  shows the finished patterned cover  28  after stripping off the working layer  36  and the mask material  30 . Stripping may be done by a wet chemical etch that selectively etches the working layer. If the working layer is copper, ferric chloride may be used as the preferred etchant. The cover  28  may now be coated with silicon dioxide  39  (SiO 2 ) by thermal oxidation or plasma enhanced chemical vapor deposition, as a way of electrically isolating the vias formed in the cover from the solder and MEMS device. It will be appreciated that sputtered SiO 2  could also be used. 
     FIG. 9  shows a first side  42 A of an adhesive sheet  42  that is cut, for example by precise laser machining or die punching, depending on accuracy requirements, to match the MEMS substrate  20  and the cover  28 , and is patterned with cutouts  44 ,  46 ,  48  for the MEMS devices, vias, and standoff posts respectively within unit areas  42 U. The adhesive sheet may be about 50 microns thick or in a range of 10-500 microns.  FIG. 10  shows a unit area  20 U of the MEMS substrate  20 .  FIGS. 11 and 12  show corresponding areas  28 U,  42 U of the cover  28  and the LCP sheet  42  respectively. 
     FIG. 14  is a partial sectional view of the cover  28  bonded to the MEMS substrate  20  via the intermediate adhesive sheet  42 . This bonding may be done in the following stages: 1) Place the cover  28  on the adhesive sheet  42  with the standoff posts  34  inserted in the standoff cutouts  48  in the adhesive sheet. This accurately registers the patterns of the cover  28  and the adhesive sheet  42 . 2) Tack the adhesive sheet  42  to the cover  28  by applying a first temperature sufficient to adhere the adhesive to the cover. For example if the adhesive sheet is LCP a first temperature of about 200° C. or in the range from about 100° C. to about 400° C. may be used. 3) Place the cover  28  on the MEMS substrate  20  with the adhesive sheet  42  between them. The cover  28  may be registered with the MEMS substrate  20  using conventional wafer alignment and fixturing techniques. 4) Heat the layers so that the adhesive sheet reaches a second temperature sufficient to bond the MEMS substrate, the adhesive sheet, and the cover together. For example if the adhesive sheet is LCP, a second temperature of about 290° C. or in the range from about 230° C. to about 400° C. for 60 minutes may be used. 
   The standoff posts  34  may be arranged to maintain a constant separation between the MEMS substrate  20  and the cover  28 . The standoff posts  34  may be about 90% as tall as the adhesive sheet thickness, or in a range of 85-98% as tall. Clamping pressure may be used to press the cover and substrate toward each other during bonding to eliminate voids between the adhesive sheet and the cover or substrate. The height of the standoff posts  34  allows the adhesive sheet to flow sufficiently to bond the substrate and cover, and to conform to the edges of the pads  26 A-C, without spreading into the cutouts. The standoff posts  34  allow sufficient pressure to be applied to overcome minor warping of the substrate or cover that may have occurred during fabrication. Thus a wider range of fabrication techniques and more forgiving tolerances of flatness of the cover and substrate can be used, increasing yield, and reducing cost. 
     FIG. 15  shows a lining  50  of a metal such as copper applied in the vias  40  through either a liftoff process or a patterned wet etch process as a primer layer for solder. The lining  50  may also be applied by sputtering.  FIG. 16  shows the vias  40  filled with solder  52  that provide external leads/bumps. The bonded layers  20 ,  28 , and  42  may then be diced or separated along their MEMS unit boundaries  20 U,  28 U,  42 U, producing individual, or singulated, packaged MEMS units  54 .  FIG. 17  shows a singulated MEMS package  54  soldered to contacts  56  on a circuit board  58 . It will be appreciated that the vias do not have to be solder filled. For example, in lieu of solder-filled vias one could perform stud bumping with a suitable material, such as gold. This approach would still keep the direct path from the MEMS device to a next level assembly and provide a flip-chip capability. 
   It will be appreciated that a MEMS packaging technology embodying aspects of the present invention provides advantages over other capping methods such as eutectic bonding or thermal compression bonding. For example, LCP bonds strongly to silicon without additional adhesives, and provides an effective hermetic seal that prevents moisture and oxygen from entering the device cavity. This provides a simple, low-cost bonding process. In addition, wafer-level capping protects the MEMS devices while the atmosphere is controlled, prior to dicing. The capping also enables the devices to be sealed with a controlled environment, such as a specific gas, Nitrogen or Argon, and a desired pressure. This allows greater flexibility for wafer-level testing and dicing, because, once capped, the devices are no longer subject to contamination from those activities. This allows a higher device yield and higher reliability. 
   The above described structural and/or operational relationships produce a covered MEMS device on a substrate. An adhesive sheet is disposed between the substrate and the cover. It has cutouts for the MEMS device. Standoff posts span between the substrate and the cover though further cutouts in the adhesive sheet. The substrate, the adhesive sheet, and the cover are bonded together in registration, and vias are provided to pads on the MEMS device. The MEMS device is sealed and protected from contamination. The atmosphere of the MEMS chamber is clean, and the adhesive does not intrude upon the MEMS device. This is all achieved at a high production yield and low cost. 
   One embodiment of the invention can use LCP as the adhesive sheet. LCP is highly hermetic, allowing reduced seal margins, thus smaller MEMS packages. It exhibits a helium permeability of 2.19*10 −11  cm 3 *atm/s. It has low moisture absorption of 0.1% at 23° C. over 24 hours. It bonds to glass, copper, gold, silicon, LCP, and other materials without additional adhesives at a relatively low transition temperature of about 280° C. The bonding occurs above this glass transition temperature. The amount of temperature over 280° C. will affect the fluidity of the LCP. In contrast, polyimide is much more moisture absorbent (0.4-2.5% at 23° C. over 24 hours), has a higher coefficient of expansion, and higher transition temperature; epoxy seals are less hermetic; glass frit uses higher temperatures, and can contaminate the MEMS with particles; brazed metal is expensive and conductive; glass-to-glass anodic bonding uses a high temperature; gold-to-gold thermo-compression bonding is conductive. 
   After capping, the MEMS package  54  can be soldered directly to a circuit board as in  FIG. 17  using the solder bumps  52  directly under the MEMS package by heating the MEMS package and/or the circuit board to a solder reflow temperature. This requires negligible lead space between the MEMS package and the circuit board contacts, and has no changes of lead direction between the MEMS pads and the circuit board contacts. Thus it reduces radio frequency reflections and interference caused by angles and other discontinuities in the conduction path, and improves thermal performance. The bonding of the adhesive sheet uses a temperature low enough not to interfere with the MEMS device. At higher temperatures the device actuators can bend, altering the device performance or causing device failure. Thus, this technology provides a low cost, hermetically sealed, high-yield, reliable, low-profile package that allows relatively high power interconnects. 
   It is contemplated that a package embodying aspects of the present invention may provide the ability to continue wafer processing after capping. It is further contemplated that if the lid is kept planar, then the package can be made compatible with multi-chip interconnect technology, such as “chips first” multi-chip module (MCM) processes. Also a package embodying aspects of the present invention may be compatible with conventional die handling equipment and infrastructure, e.g. reel packaging, pick and place machines, etc. 
   While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.