Abstract:
A low-cost micro-electro-mechanical system (MEMS) has a mass-produced carrier fabricated as a pre-molded leadframe so that the space of the leadframe center is filled with compound and a two-tier recess is created in the center. The first tier is filled by an inset with a first perforation and a second perforation. An integrated circuit chip with an opening and a membrane at the end of the opening, operable as a pressure sensor, microphone, speaker, etc, is assembled on the inset so that the opening is aligned with the first perforation. The chip is protected by a cover transected by a vent aligned with the second inset perforation. An air channel can then reach from the ambient through the vent and the second perforation to the second tier recess, which connects to the first perforation and the chip opening to the membrane.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related in general to the field of semiconductor devices and processes, and more specifically to the structure and fabrication method of Micro-Electro-Mechanical systems (MEMS) in leadframe-based pre-molded packages with acoustic channel suitable for microphones, speakers, and pressure gauges. 
       DESCRIPTION OF RELATED ART 
       [0002]    The wide variety of products collectively called Micro-Electro-Mechanical Systems (MEMS) are small, low weight devices on the micrometer scale, which may have mechanically moving parts, including transducers and actuators, and may have parts sensitive to thermal, acoustic, or optical energy. Because of the moving and sensitive parts, MEMS have a need for physical and atmospheric protection. Consequently, MEMS typically are placed on a substrate and surrounded by a housing or package, which shields the MEMS against ambient and electrical disturbances, and against stress. 
         [0003]    A MEMS integrates mechanical elements, sensors, actuators, and electronics on a common substrate. MEMS manufacturing aims at using batch fabrication techniques similar to those used for microelectronics devices. MEMS can thus benefit from mass production and minimized material consumption to lower the manufacturing cost, while trying to exploit the well-controlled integrated circuit technology, although today the production of MEMS is still far from the level of maturity that manufacturing and packaging of microelectronics have reached. 
         [0004]    MEMS may be mechanical sensors, both pressure sensors such as microphone membranes, and inertial sensors such as accelerometers, either of which may be coupled with the integrated electronic circuit of the chip. Mechanical sensors react to and measure pressure, force, torque, flow displacement, velocity, acceleration, level, position, tilt, and acoustic wavelength and amplitude. Among the general requirements for pressure sensors are long term stability, small temperature sensitivity, low pressure and temperature hysteresis, resistance to corrosive ambients, and often hermeticity. Two specific bulk micromachining processes employed in MEMS sensor production to create in bulk semiconductor crystals the movable elements and the cavities for their movements are anisotropic wet etching and deep reactive ion etching (DRIE). 
         [0005]    The semiconductor MEMS pressure sensors and microphones are assembled on insulating substrates, which include multi-level metallizations patterned for horizontal and vertical connections. As an example, the substrate may have 4 metallization levels. The assembly typically involves adhesive chip attachment and metal wire bonding to the terminals. In many products, the terminals are positioned similar to the compact Small Outline No-Lead (SON) or Quad Flat No-Lead (QFN) type semiconductor devices, since they show the compact outline of chip-size packages. The substrates also include the air channels needed to expose the membranes to the ambient pressure and acoustic signals to be monitored by the MEMS. For protection and robustness reasons, the chip, wires, and portions of the substrate are packaged in a metal can or molded in a plastic encapsulation. As an example, the dimensions of a MEMS cuboid analog-output microphone with 4 terminals for mobile phones may be about 4.8 mm by 3.8 mm by 1.25 mm, or even as small as 2 mm by 2 mm by 1.25 mm (resulting in a 4 mm 2  footprint). 
       SUMMARY OF THE INVENTION 
       [0006]    Applicants recognized that the fast growing popularity of MEMS with an internal cavity and their wide diversification are strongly dependent on a fabrication cost lower than today. As an example, low manufacturing cost and small size are dominant requirements in order to integrate pressure sensors, microphones, accelerometers and other applications, where a movable member is needed to convert an external analog input into an electrical output, into applications of portable and consumer electronic devices in automotive, medical, and aerospace industries. 
         [0007]    Applicants saw that to build MEMS on substrates with patterned multi-level metallizations is not only a high cost approach, but also limits the choice of materials and configuration available to the MEMS component. Furthermore, the complexity of multi-metal level substrate fabrication makes these MEMS inflexible to rapidly changing customer requirements or market opportunities. 
         [0008]    Applicants solved the problem of mass-producing low cost MEMS with a membrane suitable for pressure and sound sensing, microphones, and speakers, by assembling the chip with the membrane on a carrier including a pre-molded leadframe. The molding process, which embeds the leads of the leadframe, also forms a two-tier recess. An inset (made of plastic, copper, silicon, or other suitable material) placed in the first tier is attached to the step between the tiers and confines the second tier to the function of an internal air channel. The inset, which provides the area for assembling the chip with the membrane, has a perforation for connecting the air channel to the external ambient, and another perforation for connecting the air channel to the membrane. Since the size and the outline of the inset as well as the number and location of the perforations can be customized, the inset adds low-cost design variability to the MEMS fabrication. In addition, the inset may be made of inexpensive amorphous silicon, which has the same coefficient of thermal expansion (CTE) as the single-crystalline silicon of the chip and thus supports the effort to minimize CTE mismatches between the package materials. The assembly process does not employ lasers and thus supports cost reduction. 
         [0009]    Applicants further discovered that a leadframe of the QFN/SON type allows greater system level integration with other components such as package-on-package MEMS, thus increasing the electrical product efficiency. 
         [0010]    Embodiments of this invention include the usage of air pressure, electrostatic force, gravity, gas composition etc., to deflect a membrane or a beam for building pressure sensors, microphones, accelerometers, relays, thermopiles, and other applications where a movable member is needed to convert an external input into an electrical output. 
         [0011]    Exemplary MEMS of the pressure sensor family, operating on capacitive changes caused by a movable membrane, may offer 20 to 30% lower fabrication cost, when the chip with the membrane is assembled on a pre-molded leadframe with lid, instead of being fabricated in conventional manner with a multi-level substrate. 
         [0012]    According to one embodiment of the present invention, a MEMS is provided. The MEMS comprising: a carrier having a height and a surface, the carrier embedding at its periphery a plurality of metal leads; a recess in the carrier, remote from the periphery, the first tier extending from the surface to a stair towards the second tier; an inset attached to the stair and having a first perforation and a second perforation; a integrated circuit chip attached to the inset, the chip having an opening extending through the chip capped by a foil, the opening in communication with the first perforation of the inset; and a cover attached to the surface of the carrier enclosing the chip and having a vent in communication with the second perforation of the inset. 
         [0013]    According to another embodiment of the present invention, a MEMS is provided. The MEMS comprising: a carrier having a height and a surface, the carrier embedding at its periphery a plurality of metal leads; a recess in the carrier, remote from the periphery, extending from the surface to a depth less than the carrier height; an inset attached to the stair and having a first perforation and a second perforation. 
         [0014]    According to another embodiment of the present invention, a method for fabricating a MEMS is provided. The method comprising the steps of: providing a leadframe having a plurality of leads surrounding a central open space, the leads made from a metal sheet having a height; filling the open space with a polymeric compound, thereby embedding the leads and forming a carrier having the height and a surface; forming a recess in the open space; attaching an inset having a first perforation and a second perforation; attaching an integrated circuit chip having an opening extending through the chip, the opening capped by a foil at the circuit side and unobstructed at the opposite side, onto the inset so that the opening is in communication with the first perforation; and attaching a cover onto the surface of the carrier to enclose the chip. 
         [0015]    According to another embodiment of the present invention, a method for fabricating a MEMS is provided. The method comprising the steps of: providing a leadframe having a plurality of leads surrounding a central open space, the leads made from a metal sheet having a height; filling the open space with a polymeric compound, thereby embedding the leads and forming a carrier having the height and a surface; forming a recess in the open space; and attaching an inset having a first perforation and a second perforation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a cutaway of a schematic MEMS with an acoustic air channel in a leadframe-based pre-molded package according to the invention. 
           [0017]      FIG. 2  A is a top view of a portion of an exemplary pre-molded leadframe strip used for fabricating a plurality of MEMS according to the invention. 
           [0018]      FIG. 2B  shows a cross section along line  2 B- 2 B of the pre-molded leadframe strip of  FIG. 2A . 
           [0019]      FIG. 2C  shows a cross section along line  2 C- 2 C of the pre-molded leadframe strip of  FIG. 2A . 
           [0020]      FIG. 3A  is a phantom X-ray top view of a singulated MEMS fabricated according to the invention with a pre-molded leadframe and acoustic air channel. 
           [0021]      FIG. 3B  shows a cross section along line  3 B- 3 B of the singulated MEMS of  FIG. 2A , displaying a cross section of the inset resting on the molded ledge. 
           [0022]      FIG. 3C  shows a cross section along line  3 C- 3 C of the singulated MEMS of  FIG. 3A , displaying another cross section of the inset resting on the molded ledge. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]      FIG. 1  illustrates a cutaway of an exemplary embodiment  100  from a family of MEMS, which is based on the usage of air pressure, electrostatic force, gravity, gas composition etc., to deflect a membrane or a beam for building pressure sensors, microphones, speakers, accelerometers, relays, thermopiles, and other applications where a movable member is needed to convert an external input into an electrical output. 
         [0024]    As  FIG. 1  shows, an integrated circuit chip is  101  is attached by adhesive film  102  onto a carrier  110 . This carrier includes a polymeric or other compound  111  embedding metallic leads  112 , and an inset  120 . The exemplary MEMS has a square-shaped outline of about 2 by 2 mm side length. Other embodiments may have greater or smaller side lengths, or they may have a rectangular outline. As  FIG. 1  indicates, there are three leads on two opposite sides of body  110 ; other embodiments may have more leads or fewer leads; the leads may be distributed symmetrically or clustered; or not all leads may be positioned at the body outline. 
         [0025]    The material of portion  111  of carrier  110  may be a polymerized molding compound based on an epoxy and filled with inorganic fillers such as silicon dioxide or silicon carbide, or it may be any other suitable plastic compound. The leads  112  may be copper, a copper alloy, aluminum, or any other metal or alloy used in the semiconductor industry as the basis for leadframes. The surface of the leads  112  may have a metallurgical surface configuration suitable for wire bonding (thin layer of silver or gold) or soldering (layer of tin, thin layers of nickel and palladium or gold). The carrier portion made of polymeric compound  111  has a height  111   a,  and leads  112  have a height  112   a.  In  FIG. 1 , height  111   a  is the same as height  112   a  due to the starting leadframe and subsequent molding process used for fabricating body  110  (see below). Consequently, the surface of carrier  110  has the same flat plane for the leads and the plastic portion; in  FIG. 1 , the surface is designated  110   a.  In the exemplary embodiment of  FIG. 1 , height  112   a  of the leads is between about 0.35 and 0.40 mm, and height  111   a  of the polymeric compound is also between about 0.35 and 0.40 mm. 
         [0026]      FIG. 1  further indicates that, towards the center of carrier  110  and extending from surface  110   a,  a recess is formed into the polymeric compound  111 . The recess is structured in two tiers  114  and  115 . In the exemplary embodiment of  FIG. 1 , the two tiers are shown to have the same depth; in other embodiments, the depths may be different. The tiers of  FIG. 1  have a depth between about 0.12 and 0.13 mm. The sum of the two tiers has a total depth less than the height  111   a  of the plastic carrier portion. Between tier  114  and tier  115  is a stair  116 , which projects from the first tier  114  to the second tier  115 . In the exemplary embodiment of  FIG. 1 , the stair stretches around the periphery of the recess; in other embodiments there may be a plurality of discrete stairs.  FIG. 1  indicates that as a result of the recess, the bottom area of second tier  115  forms a channel  117  through a major portion of carrier  110 ; this channel  117  is a portion of the air channel of the MEMS (see detail below). 
         [0027]    As  FIG. 1  shows, inset  120  is resting on stair  116  and may be attached to stair  116  by an adhesive film such as a B-stage chip attach compound; examples include epoxy- and polyimide-based formulations. The inset has a size (area and outline) and a thickness to fill the first tier recess  114  and thus acts as a cover for the second tier  115  and a carrier for attaching the semiconductor chip. Inset  120  may be made of polycrystalline silicon to offer the same coefficient of thermal expansion (CTE) as chip  101 ; alternatively, inset  120  may be made of plastic or a metal such as a copper alloy, aluminum, or nickel, or other suitable compounds.  FIG. 1  further indicates that inset  120  has a first perforation  121  on one end of the inset, and a second perforation  122  on the opposite end of the inset. Both perforations open to the channel  117 . 
         [0028]    The integrated circuit of chip  101  is located on the chip surface  101   a  in  FIG. 1 . Surface  101   a  also has the chip terminals  103  for connections  130  to contacts or leads of the carrier  110 . The chip surface opposite surface  101   a  is, at least partially, attached to inset  120  by adhesive film  102 . This film may be a B-stage chip attach compound, for instance an epoxy-based formulation; alternatively, adhesive film  102  may be a polyimide layer. As  FIG. 1  illustrates, chip  101  has an opening  104  extending through the height of the chip. Opening  104  may be shaped as a cylinder, a tuncated cone, or any other suitable stereometrical form. At the circuit side, the opening  104  is capped by a movable part  105 . In the embodiment of  FIG. 1 , the movable part  105  stretches across the opening  104  and is anchored in the chip in x- and y-directions along the circumference of the opening. In other embodiments, the movable part extends only partially across the opening  104 . 
         [0029]    Electrical connections  130  may be gold wires as used in conventional semiconductor ball bonding, with a wire diameter of about 25 μm. Wires  130  connect chip terminals  103  to the metal leads  112  embedded in carrier  110 . 
         [0030]    Movable part  105  is may be made of metal such as copper, nickel, or aluminum, in some embodiments with a surface made of palladium or gold. For many embodiments, movable part  103  has an area between about 0.04 and 0.25 mm 2  and a thickness between about 5 and 25 μm. In this thickness range, movable part  105  can act as a membrane, which is a foil flexible in the z-direction, movable normal to the surface  101   a  and thus also normal to body surface  110   a  in the space of the chip opening  104 . As a membrane, part  105  is sensitive to external pressure changes arriving from z-direction through opening  104 , bending the membrane inward and outward of opening  104 . 
         [0031]    As  FIG. 1  shows, a cover  140  is attached to the surface  110   a  of the carrier and portions of inset  120 . Cover  140  may be made of metal such aluminum, or it may be plastic such as a molded part. Cover  140  is shaped like a can to encapsulate chip  101  and electrical connections  130 , and has a vent  141 , which extends through the height of cover  140 . Vent  141  is aligned with the second perforation  122  of inset  120 . This alignment allows a continuous spatial connection of vent  141  through channel  117  to the movable part  105  at the end of chip opening  104 . 
         [0032]      FIGS. 2A ,  2 B, and  2 C illustrate an example of a portion of a carrier generally designated  200 , including a leadframe strip embedded in pre-molded compound, used for fabricating the MEMS in  FIG. 1 .  FIG. 2A  shows a top view of a portion of the exemplary original metallic leadframe strip surrounded by molding compound. The exemplary leadframe resembles the leadframes used in conventional semiconductor technology for QFN—(Quad Flat No-Lead) and SON-type (Small Outline No-Lead) devices. Included in  FIG. 2A  are a line  2 B- 2 B and a line  2 C- 2 C.  FIG. 2B  is a cutaway view of the carrier along line  2 B- 2 B, and  FIG. 2C  is a cutaway view of the carrier along line  2 C- 2 C. 
         [0033]    The metal in the portion of the leadframe strip in  FIG. 2A  is shaded. For many MEMS embodiments, the leadframe strip is made from a sheet metal of copper or a copper alloy; in other MEMS, the leadframe strip may be made of a leadframe strip of aluminum or an aluminum alloy. For facilitating wire attachment in the bonding step, one leadframe surface may have plated spots of thin layers of silver or gold; for facilitating solder attachment in the assembly step, the opposite leadframe surface (not shown in  FIG. 2A ) may have a plated layer of tin, or plated thin layers of nickel and of a noble metal such as palladium or gold. The leads have a height (see  FIG. 2C ), which reflects the thickness of the metal sheet. 
         [0034]    In the example of  FIG. 2A , leadframe portion includes two square-shaped units of a side length of approximately 2 mm, outlined and connected by rails, or tie bars,  201 . Attached to the rails, each unit includes a plurality of leads  112 ; in the example of  FIG. 2A , there are 6 leads grouped in 2 sets of 3 leads positioned symmetrically on opposite rails. The units, however, do not include the conventional pad for attaching the semiconductor chip, which, in typical leadframes, is located remote from the rails (or periphery). Instead, the central portion of the units remains open space. In the example, leads  112  have a width  202  of about 0.25 mm at the attachment to the rails, a length  203  of about 0.5 mm, and a pitch  204  center-to-center of about 0.5 mm. 
         [0035]    The top view of the carrier  200  in  FIG. 2A  displays the area of the stair  116 , which separates the first tier and the second tier of the recess in the molded portion of carrier  200 .  FIG. 2A  indicates the edge line of first tier  114  and the edge line of the second tier  115 . The bottom area of the second tier forms the channel  117  of the finished device. 
         [0036]    The cross section of the carrier  110  in  FIG. 2B  along line  2 B- 2 B illustrates the height  111   a  of the plastic portion  111  of the carrier and the outline of the two-tier recess in the plastic portion. Since the leadframe may be embedded in the encapsulation compound by a molding process, height  111   a  of the plastic carrier portion is the same as the leadframe height (see also  FIG. 2C ). The depth  216  of the recess, extending from the surface  110   a  of the carrier to the bottom of channel  117  formed by the second tier, is smaller than the height  111   a.  The first portion of depth  216  is the first tier  114  and the second portion is the second tier  115 . The embodiment of  FIG. 2B  shows the tiers to contribute equal amounts to the total depth  216 ; in other embodiments, they contribute unequal amounts. The recess volume created by the first tier will be filled by inset  122  (see  FIGS. 3B and 3C ). Since first tier  114  has a larger width than the second tier  115  (about 15% in the embodiment of  FIG. 2B ), the stair  116  is formed between them. Stair  116  provides the support for inset  122  as displayed in  FIG. 1 . 
         [0037]    In the leadframe used for the embodiment in  FIG. 2B , the tie bars  201  are shown to have a smaller height than the height  111   a  of the carrier, which, as stated, is equal to the thickness of the original leadframe. This feature facilitates the operation of trimming the leadframe strip and singulating the discrete units from the strip after completing the assembly and packaging processes. The feature is created by half-etching the metal of the leadframe strip along the tie bars. 
         [0038]    The cross section of the carrier  110  in  FIG. 2C  along line  2 C- 2 C illustrates the height  112   a  of the metallic lead portions  112  of the carrier together with the height  111   a  of the plastic portion  111 . As stated above, height  112   a  is the same as height  111   a.  In other embodiments with different assembly needs for the semiconductor chip, the heights may be different. The recess displayed in the cross section of  FIG. 2C  features a wider area for stair  116  than the recess displayed in the cross section of  FIG. 2B , indicating a non-uniform width of step  116 . In other embodiments, the width of stair  116  may be uniform. In addition, the cross section of  FIG. 2C  displays shorter geometries of the first and second tiers and the channel  117  compared to the geometries of the tiers and the channel displayed in the cross section of  FIG. 2B , indicating the elongated shape of the recess, the tiers, and the channel. 
         [0039]      FIGS. 3A ,  3 B, and  3 C illustrate a top view and two cutaways along the lines  3 B- 3 B and  3 C- 3 C of the exemplary embodiment shown in perspective view in  FIGS. 1 and 4 .  FIG. 3A  is an X-ray top view of the embodiment generally designated  100  after singulating the leadframe strip into discrete units and attaching the cover for protecting the chip together with its membrane and bond wire connections. In the embodiment  100 , the outline of cover  140  determines the outline of the completed MEMS, about 2 by 2 mm. Reaching the top surface of the cover is vent  141 , which has in this exemplary MEMS a circular outline; in other embodiments, the vent may have different outlines. 
         [0040]    Further indicated in the X-ray view are, by dashed outlines, the semiconductor chip  101 , which, in this exemplary MEMS, is shaped as a square with about 1 mm sides, and the foil  105  capping the opening through the chip; the inset  120 , which fills the first tier of the recess and has the first perforation  121  and the second perforation  122  on opposite ends; the second tier  115  of the recess, which provides the channel  117  between the first perforation and foil, and the second perforation and vent; and the leads  112  embedded in the carrier compound. 
         [0041]    The cross section of the exemplary discrete MEMS in  FIG. 3B  (along offset line  3 B- 3 B in  FIG. 3A ) illustrates the common height  112   a  of the metallic leads  112  of the carrier and the height  111   a  of the plastic portion  111  of the carrier. The common height is a consequence of the pre-molding process of the leadframe to embed the leads and create a carrier. The recess displayed in the cross section of  FIG. 3B  features the formation of the air channel  117  from the second tier recess  115 , after the inset  120  has been attached to stair  116  of the pre-molded carrier by using, for example, a B-stage polymeric adhesive formulation designated  301 . As mentioned above, the depth of the first tier in the exemplary MEMS is between about 0.12 and 0.13 mm, and the inset fills the first tier in height and area. Consequently, the thickness of inset  120 , together with the thickness of the adhesive  301 , is in the same range. 
         [0042]    In  FIG. 3B , the semiconductor chip  101  is attached partially onto inset  120  and partially onto plastic compound  111  by using, for instance, a B-stage polymeric adhesive formulation designated  302 . The height  101   b  of chip  101  is between about 0.3 and 0.4 mm. Consequently, a height  142  of cover  140  between about 0.7 and 0.8 mm is sufficient to accommodate the arch heights of the wire ball bonds as well as the z-axis movements of foil  105 . The total height  303  of the packaged MEMS falls thus into the range from approximately 1.0 to 1.3 mm. In  FIG. 3   b , the opening  104  of chip  101  is schematically shown to continue to foil  105  by one or more narrow portions  104   a,  such as through-silicon vias (TSVs); however, this feature is dependent on the function of the MEMS (pressure sensor, microphone, speaker, etc.). Consequently, in some devices, opening  104  may continue to foil  105  without constriction; in other devices, there may be a plurality of constrictions or TSVs. 
         [0043]    The cutaway of  FIG. 3C  displays the layout of the complete air channel in the exemplary MEMS embodiment. The portion formed by polymeric compound  111  of carrier  110  includes channel  117 ; this channel connects the ambient inlet through the vent  141  of cover  140  with the movable part (foil, membrane)  105  at the end of opening  104  in chip  101 . The air channel is facilitated by inset  120 , which couples the channel  117  to chip opening  104  and foil  105  by the first perforation of the inset, and to the ambient and vent  141  by the second perforation  122  of the inset. 
         [0044]    Another embodiment of the invention is a method for fabricating a MEMS. In the first step of the process flow, a leadframe strip is provided, which has a plurality of units, wherein each unit having leads surrounding a central open space. As stated above, the units do not exhibit pads for attaching a chip. The units are held together by tie bars or rails. The rails and the leads are stamped or etched from a metal sheet with a height. For thinning, the rails are then half-etched to reduce the original metal thickness by about 50%. For many embodiments, the sheet material is copper; the copper surfaces may have treatments such as oxidation or roughening for enhancing adhesion to molding compounds. On one surface, the leads may have spot-plated silver for facilitating wire bonding, and on the opposite surface, the leads may have plated metal (such as nickel and palladium or gold) for facilitating solder attachment. 
         [0045]    In the next process step, the open space of the units is filled with a polymeric compound, thereby embedding the leads and forming a plate-like carrier having the height of the leads, and a common surface with the leads. For many embodiments, a low-cost technique for filling the open spaces is the transfer molding technique: The strip is loaded in a steel mold with a cavity of the length of the strip and a height of the leadframe thickness. An epoxy-based encapsulation compound with a high percentage of inorganic fillers is pressed into the cavity to embed the leadframe strip. As a result, the strip is formed into a robust plate-like carrier, which resembles the carrier of many QFN and SON semiconductor devices. 
         [0046]    Concurrent with the step of filling the open space in each unit, a two-tier recess is created in the central area of the space of each unit so that the recess extends from the surface of the carrier to a depth, which is smaller than the height of the carrier. In addition, the recess is formed so that a stair projects from the first to the second tier around the recess periphery. The stair marks the transition from the first to the second tier. 
         [0047]    Insets are then provided as piece parts. An inset is attached to the stair of each unit, preferably using a B-stage epoxy-based adhesive film. The inset is structured to exhibit the contour, area, and thickness of the first tier recess and is thus able to fill the first tier. The inset further has a first perforation at one end of the inset, and a second perforation at the opposite end of the inset. After the inset has been attached to the stair, it acts as a lid to the second tier recess, confining the space of the second tier to act as an air channel. It is advantageous for many embodiments to have the inset made of low-cost polysilicon, since this choice renders the inset&#39;s CTE equal to the chip&#39;s CTE, reducing thermo-mechanical stress. Alternatively, the inset may be made of plastic or a metal such as copper. 
         [0048]    In the next process step, integrated circuit chips are provided, wherein each chip has at least one opening extending through the chip thickness. At the circuit side of the chips, each opening is capped by a foil, which is thin enough to function as the movable part of the MEMS. At the side of the chip opposite to the circuit, the opening is unobstructed. A chip is attached onto the inset of each unit so that the chip opening is aligned with the first perforation of the inset. The attachment process preferably uses a B-stage epoxy-based adhesive film. The terminals of each chip are then connected to the leads embedded in the carrier of each unit; the ball wire bonding technique offers a low-cost method. 
         [0049]    Next, covers are provided, which may be shaped like a can to fit over the area of a unit and have a height sufficient to protect the chip and the wire bonds. The covers further have a vent through the height of the cover. A cover is attached to the surface of each carrier unit so that the vent is aligned with the second perforation of the inset and the cover encapsulates the chip of the unit. 
         [0050]    Finally, the packaged units are singulated from the strip. The low-cost technique is a sawing step along the leadframe rails which cuts through the half-etched rails and the thinner molding compound. Alternatively, the package substrates may be singulated prior to the addition of the insert or prior to the addition of the chip, allowing the manufacture of the package substrates to be independent of the manufacture of the finished devices. 
         [0051]    While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, the invention applies to any material for the MEMS package, including plastics and ceramics, and to any integrated, including silicon, silicon germanium, gallium arsenide, or any other semiconductor or compound material used in manufacturing. 
         [0052]    As another example, the use of a pre-molded leadframe with inset as a carrier with an air channel for the package of a MEMS can be applied to piezoresistive pressure sensors, where the conversion of pressure to an electronically detectable signal relies on the elastic deformation of a membrane, or generally of a structure, that is exposed to the pressure. 
         [0053]    As another example, the use of a pre-molded leadframe with inset as a carrier with an air channel for the package of a MEMS can be applied to resonant pressure sensors, where the resonance frequency depends on the mechanical stress in the vibrating microstructure. 
         [0054]    As another example, recess may be formed having a single tier covered by the inset. For example, the inset could be fit inside the single tier and be spaced apart from the bottom of the tier to form an air channel. The spacing may be controlled by standoffs inerted in the recess, molded into the recess, or formed as part of the inset. In a similar manner, the inset may be positioned on the upper surface of the package substrate and optionally include one or more structured formed in the inset extending into the recess to ensure alignment of the inset. 
         [0055]    In yet another example, the second perforation may be embodied by a gap between an edge of the inset and an edge of the recess. The second perforation is not required to be enclosed by the package cover, although covering the second perforation limits access to the second perforation and the air channel by particulates. The use of a cover also allows both the length and shape of the passage from the second perforation to the environment to be tuned for particular applications. 
         [0056]    It is therefore intended that the appended claims encompass any such modifications or embodiments.