Abstract:
An apparatus for packaging of microelectronic devices, including an integral window. The microelectronic device can be a semiconductor chip, a CCD chip, a CMOS chip, a VCSEL chip, a laser diode, a MEMS device, or a IMEMS device. The package can include a cofired ceramic frame or body. The package can have an internal stepped structure made of one or more plates, with apertures, which are patterned with metallized conductive circuit traces. The microelectronic device can be flip-chip bonded on the plate to these traces, and oriented so that the light-sensitive side is optically accessible through the window. A cover lid can be attached to the opposite side of the package. The result is a compact, low-profile package, having an integral window that can be hermetically-sealed. The package body can be formed by low-temperature cofired ceramic (LTCC) or high-temperature cofired ceramic (HTCC) multilayer processes with the window being simultaneously joined (e.g. cofired) to the package body during LTCC or HTCC processing. Multiple chips can be located within a single package. The cover lid can include a window. The apparatus is particularly suited for packaging of MEMS devices, since the number of handling steps is greatly reduced, thereby reducing the potential for contamination.

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 
     This application is related to a co-pending application “Method of Fabricating a Microelectronic Device Package with an Integral Window”, by Kenneth A. Peterson and Robert D. Watson. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of microelectronics, and more specifically to packaging of microelectronic devices in a package having an integral window. 
     Many different types of microelectronic devices require a window to provide optical access and protection from the environment. Examples of optically-interactive semiconductor devices include charge-coupled devices (CCD), photo-sensitive cells (photocells), solid-state imaging devices, and UV-light sensitive Erasable Programmable Read-Only Memory (EPROM) chips. All of these devices use microelectronic devices that are sensitive to light over a range of wavelengths, including ultraviolet, infrared, and visible. Other types of semiconductor photonic devices emit light, such as laser diodes and Vertical Cavity Surface-Emitting Laser (VCSELS), which also need to pass light through a protective window. 
     Microelectromechanical systems (MEMS) and Integrated MEMS (IMEMS) devices (e.g. MEMS devices combined with Integrated Circuit (IC) devices) can also require a window for optical access. Examples of MEMS devices include airbag accelerometers, microengines, microlocks, optical switches, tiltable mirrors, miniature gyroscopes, sensors, and actuators. All of these MEMS devices use active mechanical and/or optical elements. Some examples of active MEMS structures include gears, hinges, levers, slides, tilting mirrors, and optical sensors. These active structures must be free to move or rotate. Optical access through a window is required for MEMS devices that have mirrors and optical elements. Optical access to non-optically active MEMS devices can also be required for inspection, observation, and performance characterization of the moving elements. 
     Additionally, radiation detectors which detect alpha, beta, and gamma radiation, use “windows” of varying thickness and materials to either transmit, block, or filter these energetic particles. These devices also have a need for windows that transmit or filter radiation to and from the device, while at the same time providing physical and environmental protection. 
     The word “transparent” is broadly defined herein to include transmission of radiation (e.g. photons and energetic particles) covering a wide range of wavelengths and energies, not just UV, IR, and visible light. Likewise, the word “window” is broadly defined herein to include materials other than optically transparent glass, ceramic, or plastic, such as thin sheets of metal, which can transmit energetic particles (e.g. alpha, beta, gamma, and light or heavy ions). 
     There is a continuing need in the semiconductor fabrication industry to reduce costs and improve reliability by reducing the number of fabrication steps, while increasing the density of components. One approach is to shrink the size of packaging. Another is to combine as many steps into one by integrating operations. A good example is the use of cofired multilayer ceramic packages. Unfortunately, adding windows to these packages typically increases the complexity and costs. 
     Hermetically sealed packages are used to satisfy more demanding environmental requirements, such as for military and space applications. The schematic shown in FIG. 1 illustrates a conventional ceramic package for a MEMS device, a CCD chip, or other optically active microelectronic device. The device or chip is die-attached face-up to a ceramic package and then wirebonded to interconnect inside of the package. Metallized circuit traces carry the electrical signal through the ceramic package to electrical leads mounted outside. A glass window is attached as the last step with a frit glass or solder seal. Examples of conventional ceramic packages include Ceramic Dual In-Line Package (CERDIP), EPROM and Ceramic Flatpack designs. 
     Although stronger, ceramic packages are typically heavier, bulkier, and more expensive to fabricate than plastic molded packages. Problems with using wirebonding include the fragility of very thin wires; wire sweep detachment and breakage during transfer molding; additional space required to accommodate the arched wire shape and toolpath motion of the wirebond toolhead; and the constraint that the window (or cover lid) be attached after the wirebonding step. Also, attachment of the window as the last step can limit the temperature of bonding the window to the package. 
     FIG. 2 illustrates schematically a conventional molded plastic (e.g. encapsulated) microelectronic package. The chip is attached to a lead frame, and a polymer dam prevents the plastic encapsulant from flowing onto the light-sensitive area of the chip during plastic transfer molding. The window is generally attached with a polymer adhesive. Problems with this approach include the use of fragile wirebonded interconnections; and plastic encapsulation, which does not provide hermetic sealing against moisture intrusion. 
     Flip-chip mounting of semiconductor chips is a commonly used alternative to wirebonding. In flip-chip mounting the chip is mounted face-down and then reflow soldered using small solder balls or “bumps” to a substrate having a matching pattern of circuit traces (such as a printed wiring board). All of the solder joints are made simultaneously. Excess spreading of the molten solder ball is prevented by the use of specially-designed bonding pads. Flip-chip mounting has been successfully used in fabricating Multi-Chip Modules (MCM&#39;s), Chip-on-Board, Silicon-on-Silicon, and Ball Grid Array packaging designs. 
     Flip-chip mounting has many benefits over traditional wirebonding, including increased packaging density, lower lead inductance, shorter circuit traces, thinner package height, no thin wires to break, and simultaneous mechanical die-attach and electrical circuit interconnection. Another advantage is that the chips are naturally self-aligning due to surface tension when using molten solder balls. It is also possible to replace the metallic solder bumps with bumps, or dollops, of an electrically-conductive polymer or epoxy (e.g. silver-filled epoxy). Flip-chip mounting avoids potential problems associated with ultrasonic bonding techniques that can impart stressful vibrations to a fragile (e.g. released) MEMS structure. 
     Despite the well-known advantages of flip-chip mounting, this technique has not been widely practiced for packaging of MEMS devices, Integrated MEMS (IMEMS), or CCD chips because attaching the chip face-down to a solid, opaque substrate prevents optical access to the optically-active, light-sensitive surface. 
     The cost of fabricating ceramic packages can be reduced by using cofired ceramic multilayer packages. Multilayer packages are presently used in many product categories, including leadless chip carriers, pin-grid arrays (PGA&#39;s), side-brazed dual-in-line packages (DIP&#39;s), flatpacks, and leaded chip carriers. Depending on the application, 5-40 layers of dielectric layers can be used, each having printed signal traces, ground planes, and power planes. Each signal layer can be connected to adjacent layers above and below by conductive vias passing through the dielectric layers. 
     Electrically conducting metallized traces, thick-film resistors, and solder-filled vias or Z-interconnects are conventionally made by thick-film metallization techniques, including screen-printing. Multiple layers are printed, vias-created, stacked, collated, and registered. The layers are then joined together (e.g. laminated) by a process of burnout, followed by firing at elevated temperatures. Burnout at 350-600 C. first removes the organic binders and plasticizers from the substrate layers and conductor/resistor pastes. After burnout, these parts are fired at much higher temperatures, which sinters and densifies the glass-ceramic substrate to form a dense and rigid insulating structure. Glass-forming constituents in the layers can flow and fill-in voids, corners, etc. 
     Two different cofired ceramic systems are conventionally used, depending on the choice of materials: high-temperature cofired ceramic (HTCC), and low-temperature cofired ceramic (LTCC). HTCC systems typically use alumina substrates; are printed with molybdenum-manganese or tungsten conducting traces; and are fired at high temperatures, from 1300 C. to 1800 C. LTCC systems use a wide variety of glass-ceramic substrates; are printed with Au, Ag, or Cu metallizations; and are fired at lower temperatures, from 600 C. to 1300 C. After firing, the semiconductor die is attached to the fired HTCC (or LTCC) body; followed by wirebonding. Finally, the package is lidded and sealed by attaching a metallic, ceramic, or glass cover lid with a braze, a frit glass, or a solder seal, depending on the hierarchy of thermal processing and on performance specifications. 
     Use of cofired multilayer ceramic structures for semiconductor packages advantageously permits a wide choice of geometrical designs and processing conditions, as compared to previous use of bulk ceramic pieces (which typically had to be cut and ground from solid blocks or bars). Ceramic packages with high-temperature seals are generally stronger and have improved hermeticity, compared to plastic encapsulated packages. It is well known to those skilled in the art that damaging moisture can penetrate polymer-based seals over time. Also, metallized conductive traces are more durable than freestanding wire bond segments, especially when the traces are embedded and protected within a layer of insulating material. 
     In summary, conventional methods and designs for packaging of light-sensitive microelectronic devices attach the window (or cover lid containing a window) after completing the steps of die attachment and wirebonding of the chip or MEMS device to the package. Many processing steps are used, which can expose the fragile MEMS structures to particulate contamination and mechanical damage during packaging. 
     What is needed is a packaging process that minimizes the number of times that a MEMS device is handled and exposed to temperature cycles and different environments, which can possibly lead to contamination of the device. This can be accomplished by performing as many of the package fabrication steps as possible before mounting the MEMS device. What is needed, then, is a packaging process that attaches the window to the package before mounting the chip to the package. It is also desired that the window be attached to the package body at a high temperature to provide a strong, hermetic bond between the window and the body. What also is needed is a method where the MEMS device faces away from the cover lid, so that contamination is reduced when the cover lid is attached last. 
     Electrical interconnections from the chip to the package are needed that are stronger and less fragile than conventional wirebonds. What also is needed is a package having a high degree of strength and hermeticity. In some cases, it is also desired to stack back-to-back multiple chips, of different types (e.g. CMOS, MEMS, etc.) inside of a single, windowed-package. 
     Use of the phrase “MEMS devices” is broadly defined herein to include “IMEMS” devices, unless specifically stated otherwise. The word “plastic” is broadly defined herein to include any type of flowable, dielectric composition, including polymer compounds and spin-on glass-polymer compositions. The phrases “released MEMS structures”, “released MEMS elements”, and “active MEMS elements” and “active MEMS structures” are used interchangeably to refer to a device having freely-movable structural elements, such as gears, pivots, hinges, sliders, tilting mirrors; and also to exposed active elements such as chemical sensors, flexible membranes, and beams with thin-film strain gauges, which are used in accelerometers and pressure sensors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form part of the specification, illustrate various examples of the present invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 shows a schematic cross-section view of a conventional ceramic microelectronic package, where the window or cover lid is attached last, after the microelectronic device has been joined (face-up) to the base and wirebonded. 
     FIG. 2 shows a schematic cross-section view of a conventional plastic molded microelectronic package, where the microelectronic device, lead frame, and window are encapsulated in a plastic body by a transfer molding process. 
     FIG. 3A shows a schematic cross-section view of a first example of a microelectronic package according to the present invention, with the package having an integral window attached to a ceramic body including an first (lower) plate, a second (upper) plate, and an attached cover lid. 
     FIG. 3B shows a schematic cross-section view of the second example of a microelectronic package according to the present invention, with the package having an integral window cofired with a cofired multilayered assembly of twelve individual layers, and an attached cover lid. 
     FIG. 4A shows a schematic cross-section view of a third example of a microelectronic package according to the present invention that is similar to the second example of FIG. 3B, but with a cofired window substantially filling up the aperture through the first plate. 
     FIG. 4B shows a schematic cross-section view of a fourth example of a microelectronic package according to the present invention that is similar to the second example of FIG. 3B, but with a cofired window mounted to a recessed lip located inside of the first plate, recessed from the second surface of the first plate. 
     FIG. 4C shows a schematic cross-section view of a fifth example of a microelectronic package according to the present invention that is similar to the second example of FIG. 3B, but with a window mounted flush to the bottom surface of the first plate. 
     FIG. 5 shows a schematic cross-section view of a sixth example of a microelectronic package according to the present invention, with the package having an integral window cofired to a cofired multilayered assembly including an first (bottom) plate, a second (middle) plate, a third (top) plate, and an attached cover lid, for packaging a pair of stacked chips, including a MEMS chip flip-chip mounted to the first plate, and a second chip attached to the backside of the MEMS chip, wirebonded to the second plate. 
     FIG. 6A shows a schematic cross-section view of a seventh example of a microelectronic package according to the present invention that is similar to the first example of FIG. 3A, but with the cover plate removed, and also having a second package, substantially identical to the first example of FIG. 3A (also without a cover plate), where the second package has been inverted and joined to the first package, thereby forming a substantially symmetric package. 
     FIG. 6B shows a schematic cross-section view of an eighth example of a microelectronic package according to the present invention that is similar to the first example of FIG. 3A, but with the cover plate removed, and also having a second package, substantially identical to the first example of FIG. 3A (also without a cover plate), where the second package has been stacked above the first package and joined to the first package, thereby forming a stacked, double-package. 
     FIG. 6C shows a schematic cross-section view of a ninth example of a microelectronic package according to the present invention that is similar to the sixth example of FIG. 5, but with the cover plate removed, and also having a second package, substantially identical to the first example of FIG. 5 (also without a cover plate), where the second package has been inverted and joined to the first package, thereby forming a substantially symmetric package. 
     FIG. 6D shows a schematic cross-section view of a tenth example of a microelectronic package according to the present invention that is similar to the first example of FIG. 5, but with the cover plate removed, and also having a second package, substantially identical to the first example of FIG. 5 (also without a cover plate), where the second package has been stacked above the first package and joined to the first package, thereby forming a stacked, double-package. 
     FIG. 7 shows a schematic top view along line  1 — 1  of FIG. 3A of a sixteenth example of a microelectronic package for housing at least one microelectronic device according to the present invention, illustrating examples of the electrically conducting metallized traces located on the upper surface of the first plate, including interconnect bumps, interior bond pads, exterior bond pads, and a conductive via. 
     FIG. 8 shows a schematic top view of a seventeenth example of a microelectronic package for housing at least one microelectronic device according to the present invention, wherein the package can be a multi-chip module (MCM), including multiple integral windows and multiple microelectronic devices in a two-dimensional array. 
     FIG. 9 shows a schematic side view of a eighteenth example of a microelectronic package for housing at least one microelectronic device according to the present invention, wherein the window further comprises a lens for optically transforming light passing through the window. 
     FIG. 10A shows a schematic side view of an example of a microelectronic package for housing a microelectronic device, according to the present invention. 
     FIG. 10B shows a schematic side view of an example of a microelectronic package for housing a microelectronic device, according to the present invention. 
     FIG. 10C shows a schematic side view of an example of a microelectronic package for housing a microelectronic device, according to the present invention. 
     FIG. 11 shows a schematic side view of an example of a microelectronic package for housing a microelectronic device, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a package for housing at least one microelectronic device, comprising a hollow assembly of stacked, electrically insulating plates and an integral window. 
     It should be noted that the examples of the present invention shown in the figures are sometimes illustrated with the window facing down, which is the preferred orientation during flip-chip bonding. However, those skilled in the art will understand that the completed package can be oriented for use with the window facing upwards. It should also be noted that all of the figures show only a single microelectronic device, illustrated as a microelectronic device or pair of chips. It is intended that the method and apparatus of the present invention should be understood by those skilled in the art as applying equally to a plurality of chips or devices packaged in a one-dimensional or a two-dimensional array, as in a multi-chip module (MCM), including multiple windowed-compartments, and including having a window on either side of the package. 
     FIG. 3A shows a schematic cross-section view of a first example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, comprising a hollow assembly  10  of stacked, electrically insulating plates. The assembly  10  of FIG. 3A has an interior interconnect location  12  disposed on an interior surface of hollow assembly  10 , and an exterior interconnect location  14  disposed on an exterior surface of assembly  10 . Assembly  10  further comprises a first plate  16 . Plate  16  has a first surface  20 , an opposing second surface  18 , and a first aperture  22  through plate  16 . Plate  16  also has an electrically conductive metallized trace  24  disposed on surface  18 , for conducting an electrical signal between interior interconnect location  12  and exterior interconnect location  14 . Plate  16  further comprises a first window  26  bonded to plate  16 , for providing optical access to a microelectronic device  100  that could be disposed within assembly  10 . 
     In FIG. 3A, assembly  10  further comprises a second plate  30 , which has a third surface  34 , an opposing fourth surface  32 , and a second aperture  36  through plate  30  for providing physical access to insert device  100  into package  8 . Surface  18  of plate  16  is bonded to the surface  34  of plate  30  to form assembly  10 . At least one lateral dimension of aperture  36  is slightly larger than the corresponding lateral dimension of aperture  22 . Aperture  22  is substantially aligned with aperture  36 . The lateral dimensions of aperture  36  are slightly larger than the lateral dimensions of chip or device  100 , so that chip or device  100  can fit inside of aperture  36 . 
     In FIG. 3A, window  26  is attached flush to plate  16 . The attachment can comprise a first seal  38 . Other mounting arrangements will be disclosed later. The shape of aperture  22  and aperture  36  can be polygonal (e.g. square or rectangular) or circular. Aperture  22  can have a different shape than aperture  36 . The horizontal surfaces of device  100 , plate  16 , plate  18 , and window  26  all can be substantially coplaner. Microelectronic device  100  can comprise a microelectronic device  100 . 
     In FIG. 3A, microelectronic device  100  can be flip-chip mounted (e.g. flipped facedown, with optically active area  109  of chip or device  100  facing towards window  26 ) to surface  18  of plate  16 . The method of flip-chip mounting is well-known to those skilled in the art. Surface  18  can comprise a bond pad  44  electrically connected to metallized trace  24  at interior interconnect location  12 . Microelectronic device  100  can include interconnect bumps pre-attached to chip or device  100 . Alternatively, surface  18  can comprise an interconnect bump  46 , connected either to metallized trace  24  or to bond pad  44  at interior interconnect location  12 . Interconnect bump  46  can comprise an electrically conductive material (e.g. gold, gold alloy, aluminum, solder, and silver-filled epoxy) for electrically connecting chip or device  100  to metallized trace  24  or bond pad  44 . Alternatively, bump  46  can comprise a non-conducting, adhesive material (e.g. epoxy resin, polyimide, silicone, or urethane) for providing mechanical attachment of chip or device  100  to surface  18 . 
     In FIG. 3A, package  8  can include a bond pad  28  attached to assembly  10  at exterior interconnect location  14 . Bond pad  28  can be electrically connected to metallized trace  24 . Package  8  can also include an electrical lead  40  attached to assembly  10  at exterior interconnect location  14 . Lead  40  can be electrically connected to metallized trace  24 . Optionally, lead  40  can be attached to bond pad  28 . Assembly  10  can also comprise an electrically conductive via  54 , which can be in electrical communication with metallized trace  24 . Via  54  can be oriented perpendicular to surface  18 , and can be disposed from surface  18  to surface  16 . Via  54  can be made electrically conducting by filling hole  54  with solder or other flowable, electrically conducting material. 
     In FIG. 3A, assembly  10  can include a cover lid  42  attached to surface  32  of plate  30 . Attachment of cover lid  42  can complete the packaging of semiconductor chip or device  100  inside of a sealed package  8 . Cover lid  42  can include a second window (not shown in FIG.  3 A), for providing optical access through aperture  36 . Optionally, the ambient air inside of sealed package  8  can be substantially removed before attaching cover lid  42 , and replaced with at least one gas other than air. This other gas can include an inert gas (e.g. argon, nitrogen, or helium). Helium can be easily detected by a conventional helium leak detector, thereby providing information on the hermetic quality of the joints and seals in package  8 . The level of humidity can also be adjusted prior to sealing package  8  with cover lid  42 . 
     In FIG. 3A, plate  16  is attached to plate  18 . This attachment can comprise a second seal  48  disposed in-between surface  18  and surface  34 . Seal  48  can have an annular shape. Likewise, the attachment between cover lid  42  and plate  30  can comprise a third seal  50 . Seal  50  can also have an annular shape. The bonding material used for either seals  38 ,  48  or  50  can comprise a hermetic sealant (e.g. a braze alloy, a frit glass compound, a glass-ceramic composite, a glass-polymer compound, a ceramic-polymer compound, or a solder alloy) or an adhesive material (e.g. an epoxy resin, a polyimide adhesive, a silicone adhesive, or a urethane adhesive). Selection of a particular material for seal  38 ,  48  or  50  should take into consideration the hierarchy of thermal processing for the entire packaging process. Here, “thermal hierarchy” means that the highest temperature processes (e.g. sintering, joining, etc.) are performed first, followed by progressively lower temperature processes, with the lowest temperature process being performed last in the sequence of fabrication steps. 
     Window  26  can comprise an optically transparent material (e.g. a borosilicate glass, a quartz glass (i.e. fused silica), a low-iron, a leaded glass, a tempered glass, a low thermal-expansion glass, or a transparent ceramic, such as sapphire). Alternatively, a transparent plastic or polymer-based material can be used (e.g. PMMA). Some plastics are transparent in the UV spectrum. Silicon can be used for windows that are transparent in the IR spectrum. Preferably, the window&#39;s coefficient of thermal expansion (CTE) is about equal to the CTE of plate  16 . Alternatively, the mismatch in CTE between window  26  and plate  16  can be chosen avantageously so that window  26  is placed in compression. Window  26  can optionally comprise optical quality properties (e.g. purity, flatness, and smoothness). 
     Window  26  can comprise means for filtering selected wavelengths of light. Coloring dyes, or other elements, can be added to the glass or plastic formulations to form windows that can filter light, as is well-known to the art. Anti-reflection coatings can be applied to the surface or surfaces of window  26  to reduce reflection and/or increase transmission. Also, surface treatments (e.g. thin-film coatings or controlled surface roughness) can be applied to the periphery of window  26  in order to improve the wettability of molten solders and brazes, and to improve the adhesion of window  26  to plate  16 . The same surface treatments can also be applied to the mating surfaces of other pairs of surfaces to be joined, including plates  16  and  30 , and cover lid  42 . Window  26  can also be made of a metal or metal alloy, for use in packaging of a microelectronic device used for detecting energetic particles. 
     In FIG. 3A, assembly  10  includes plates comprising an electrically insulating material (e.g. a ceramic, a polymer, a plastic, a glass, a glass-ceramic composite, a glass-polymer composite, a resin material, a fiber-reinforced composite, a glass-coated metal, or a printed wiring board composition) well-known to the art. The ceramic material can comprise alumina, beryllium oxide, silicon nitride, aluminum nitride, titanium nitride, titanium carbide, or silicon carbide. Fabrication of ceramic parts can be performed by processes well-known to the art (e.g. slip casting, machining in the green state, cold-isostatic pressing (CIP) followed by hot-isostatic pressing (HIP) or sintering, and uniaxially hot/cold pressing, or rapid forging). Fabrication of plastic and polymer parts can be performed by processes well-known to the art (e.g. transfer molding, injection molding, and machining of printed wiring board (PWB) sheets). 
     For severe environments, ceramic packages are generally stronger and more hermetic than plastic encapsulated packages. The preferred construction of a microelectronic package with an integral window can use cofired ceramic multilayers. The multiple, stacked ceramic layers are formed by casting a blend of ceramic and glass powders, organic binders, plasticizers, and solvents into sheets or tapes. The organic components provide strength and flexibility to the green (unfired) sheets during substrate personalization and fabrication. Burnout at a relatively low temperature (e.g. 350-600 C.) removes the organic binders and plasticizers from the substrate layers and conductor/resistor pastes. After burnout, these parts are fired at much higher temperatures, which sinters and densities the glass-ceramic substrate to form a dense, rigid, insulating structure. Glass-forming constituents in the layers can flow and avantageously fill-in voids, corners, etc. 
     Two different cofired ceramic systems are conventionally used, depending on the choice of materials: high-temperature cofired ceramic (HTCC), and low-temperature cofired ceramic (LTCC). If the ratio of ceramic to glass is high (9/1, or greater), the green substrate layer can only be sintered (e.g. densified) at high firing temperatures (e.g. 1300 to 1800 C.). Consequently, the thick-film pastes (e.g. to form metallized trace  24 ) that are typically cofired with the substrate also have to withstand these high temperatures, such as tungsten, or alloys of molybdenum and manganese. The dielectric consists of glass fillers in a ceramic matrix. This system is referred to as HTCC. Alternatively, the dielectric can be a ceramic-filled glass matrix, which can be sintered at much lower firing temperatures (e.g. 600 C. to 1300 C.). Thick-film metallization can comprise high-conductivity metals, such as gold, silver, copper, silver-palladium, and platinum-gold. This system is referred to as LTCC. 
     If hermetic packaging is not required, then polymer-based materials can be used. Multilayer printed wiring board (PWB) materials can be used for constructing assembly  10 . In this system, metallized trace  24  is fabricated by using an etched-foil process, well-known to those skilled in the art. Similar to cofired ceramic multilayers, the multiple layers of PWB composition are stacked and laminated under pressure and temperature in a single bonding step (e.g. co-bonded) to form a multilayered assembly  10 . 
     FIG. 3B shows a schematic cross-section view of a second example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, comprising a hollow assembly  10  of stacked, electrically insulating plates comprising multiple layers of ceramic tape stacked and laminated under simultaneous pressure and temperature (e.g. cofiring) to form a multi-layered cofired ceramic assembly  10 . Such a construction technique readily accommodates the preferred stepped interior-surface profile, as required to hold window  26  and chip or device  100 , since the individual layers are easily punched-out or cut (e.g. via a laser, water-jet, or mechanical press) into shapes of varying sizes that can be stacked and cofired to form multi-layered cofired ceramic assembly  10 . For example, FIG. 3B shows an arrangement for integrating window  26  into first plate  16  comprising an encased joint geometry  39  (where the edges of window  26  are embedded inside plate  16 ). If a bulk ceramic plate were used, it would be very difficult to manufacture such a reentrant, recessed feature for housing window  26  therein. However, by using a laminated, multilayered construction, this is relatively easy to do. 
     In FIG. 3B, assembly  10  comprises twelve individual layers of ceramic tape stacked and laminated to form a monolithic, unitized body having an integral window  26 . The part of assembly  10  grouped as plate  16 ′ comprises six individual layers (e.g. sheets) of glass-ceramic tape (e.g. layers  61 ,  62 ,  63 ,  64 ,  65 , and  66 ). Likewise, the part of assembly  10  grouped as plate  30 ′ comprises six additional individual layers (e.g. layers  67 ,  68 ,  69 ,  70 ,  71 , and  72 ). Each layer can be individually personalized with the appropriate inside and outside dimensions. Metallized trace  24  can be deposited on the upper surface of layer  66  (corresponding to surface  18  of FIG. 3A) prior to stacking of the individual layers. Window  26  can be inserted into the stack of layers after the surrounding layers  61 ,  62 ,  63 , and  64  have been stacked and registered. The remaining eight layers (e.g.  65 - 72 ) can be stacked and registered after window  26  has been inserted. Then, the entire stack of twelve layers (e.g.  61 - 72 ) can be clamped and fired at the appropriate temperature and pressure for the required time to form a unitized, monolithic body including an integral window  26 . 
     In FIG. 3B, it is not necessary to join plate  16 ′ to plate  30 ′ with a separate seal  48  because this joint is made simultaneously with all of the other layers during the cofiring or co-bonding process. 
     Those skilled in the art will understand that other thicknesses for plates  16 ′ and  30 ′ can be formed by laminating a different number of layers of the cofired ceramic multilayered material (or co-bonded PWB material). For example, the third example shown in FIG. 4A of the present invention illustrates an example where plate  16 ′ comprises a fewer number of layers (e.g. two layers:  63  and  64 ). In this case, aperture  22  is substantially filled up by window  26 . In this case, window  26  can be fabricated integrally with plate  16 ′ by casting molten glass, or by molding a liquid polymer, directly into aperture  22 . 
     In the example shown in FIG. 4A, the size of aperture  22  (and, hence, window  26 ) is much smaller than the size of chip or device  100 . It is not required that the size of window  26  be similar to the size of aperture  22 . Also, the example of FIG. 4A shows that the centerline of aperture  22  does not align with the centerline of aperture  36 , e.g. aperture  22  is offset from aperture  36 . It is not required that aperture  22  be aligned with aperture  36 . However, aperture  22  can be substantially aligned with aperture  36 . Those skilled in the art will understand that more than one small aperture  22  can be included in plate  16 ′, for providing multiple locations for providing optical access to chip or device  100 . 
     FIG. 4B shows a schematic cross-section view of a fourth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, that is similar to the second example of FIG. 3B, but with window  26  attached to recessed lip  58  formed inside of plate  16 ′, wherein the lip can be recessed away from second surface  20  of first plate  16 ′. Recessed lip  58  can be easily formed by using a cofired multilayered construction, as described previously. FIG. 4B also illustrates that plates  16 ′ and  30 ′ can extend laterally an unlimited distance beyond the immediate material surrounding apertures  22  and  36 . 
     Alternatively, the width of plates  16 ′ and  30 ′ can be limited to extending only a short distance beyond the apertures  22  and  36 , as illustrated in FIG.  4 A. In this example, plates  16 ′ and  30 ′ can be considered to be a frame for a package that might be housing a single device or chip. 
     FIG. 4C shows a schematic cross-section view of a fifth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention that is similar to the second example of FIG. 3B, but with window  26  attached flush to second surface  20  of first plate  16 ′. Window  26  can be attached to plate  16 ′ with seal  38 . Seal  38  can comprise a hermetic sealant material or an adhesive material, as described previously. Alternatively, window  26  can be cofired integrally with plates  16 ′ and  30 ′. 
     FIG. 5 shows a schematic cross-section view of a sixth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, that is similar to the first example of FIG. 3A; wherein assembly  10  further comprises a second electrically conductive metallized trace  82  disposed on third surface  34  of plate  30 ; and a third plate  80  bonded to third surface  34 , wherein plate  80  includes a third aperture  84  through plate  80 ; and further wherein at least one lateral dimension of aperture  84  is slightly larger than the corresponding lateral dimension of aperture  36 ; and wherein aperture  84  is substantially aligned with aperture  36 . Assembly  10  can further comprise a second bond pad  86  or second electrical lead  88  attached to metallized trace  82 . Assembly  10  can further comprise a second solder-filled via  90 , vertically disposed inside plate  30 . Those skilled in the art will understand that additional plates having apertures and metallized traces can be stacked on top of previous plates, to construct as many levels as is needed. 
     FIG. 6A shows a schematic cross-section view of a seventh example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention; further comprising a second package  9  that is substantially identical to the first example of package  8  in FIG. 3A, wherein second package  9  can be inverted and bonded with seal  60  to package  8  to form a sealed, symmetric package capable of housing at least two microelectronic devices. In this example, second package  9  serves the function of cover lid  42  (e.g. to cover and seal package  8 ). 
     FIG. 6B shows a schematic cross-section view of an eighth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention; further comprising a second package  9  that is substantially identical to the first example of package  8  in FIG. 3A, wherein second package  9  can be stacked and bonded with seal  60  to package  8  to form a stacked double-package capable of housing at least two microelectronic devices. 
     FIG. 6C shows a schematic cross-section view of a ninth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention; further comprising a second package  9  that is substantially identical to the sixth example of package  8  in FIG. 5, wherein second package  9  can be inverted and bonded with seal  60  to package  8  to form a sealed, symmetric package capable of housing at least four microelectronic devices. In this example, second package  9  serves the function of cover lid  42  (e.g. to cover and seal package  8 ). 
     FIG. 6D shows a schematic cross-section view of a tenth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention; further comprising a second package  9  that is substantially identical to the sixth example of package  8  in FIG. 5, wherein second package  9  can be stacked and bonded with seal  60  to package  8  to form a stacked double-package capable of housing at least four microelectronic devices. 
     In an eleventh example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, that is similar to the first example of FIG. 3A; package  8  further comprises a microelectronic device  100  mounted within assembly  10 . Chip or device  100  can be attached to surface  18 . Chip or device  100  can be flip-chip mounted via interconnect bump  46  to metallized trace  24 . Chip or device  100  can comprise a light-sensitive chip or device (e.g. CCD chip, photocell, laser diode, optical-MEMS, or optical-IMEMS device). Light-sensitive chip or device  100  can be mounted with a light-sensitive side  109  facing towards window  26 . An optional seal  52  can be made between chip or device  100  and first surface  18  of plate  16 , after flip-chip bonding has been performed. Seal  52  can have an annular shape. Seal  52  can provide protection from particulate contamination of the optically active face of chip or device  100  (e.g. active MEMS structures), as well as a second layer of environmental protection in addition to third seal  50 . 
     In a twelfth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, that is similar to the sixth example of FIG. 5; package  8  further comprises a pair of microelectronic devices,  100  and  102 , mounted within assembly  10 . Chip or device  100  can be attached to surface  18 . Chip or device  100  can be flip-chip mounted via interconnect bump  46  to metallized trace  24 . Second chip or device or device  102  can be bonded to the backside of chip or device  100  with bond  104 . Methods for bonding chips or devices back-to-back include anodic bonding, gold-silicon eutectic bonding, brazing, soldering, and polymer-adhesive attachment. Assembly  10  can further comprise a wirebonded electrical lead  106 , electrically attached to metallized trace  82  and to chip or device  102 . Chip or device  102  can include a second light-sensitive side  110  mounted face-up, e.g. facing towards cover lid  42 . Although not illustrated, cover lid  42  can be attached to assembly  10  using a recessed lip similar to the recessed lip  58  shown in FIG.  4 B. Cover lid  42  can be made of a transparent material. Cover lid  42  can also comprise a cofired ceramic multilayered material, which includes a cofired integral window. 
     In a thirteenth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, that is similar to the sixth example of FIG. 5; package  8  further comprises a pair of microelectronic devices,  100  and  102 , mounted within assembly  10 . Second chip or device  102  can be flip-chip mounted to metallized trace  82  disposed on surface  32 . 
     In a fourteenth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, that is similar to the ninth example of FIG. 6A; package  8  further comprises a pair of microelectronic devices,  100  and  102 , mounted within assembly  10 . In this example, cover lid  42  includes a second window  108  for providing optical access to light-sensitive side  110  of chip or device  102 . 
     Optional exterior electrical connections  112  can easily be made on the exterior surface of assembly  10 , to provide means for conducting electrical signals between chip or device  100  and chip or device  102 , as needed. 
     In a fifteenth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, that is similar to the ninth example of FIG. 6C; package  8  further comprises a first pair of chips or devices, joined to each other back-to-back, and mounted to a first package  8 , and a second pair of chips or devices, joined to each other back-to-back, and mounted to a second package  9 , wherein the second package  9  is inverted and bonded to the first package  8  (as in FIG.  6 C). In this example, a combination of flip-chip and wirebonded interconnects can be used for interconnecting the chips or devices to the four different levels of metallized circuit traces. Also, each of the four chips or devices can comprise optically-active elements, including MEMS structures, thereby providing the possibility of passing an optical signal through both apertures by direct transmission, or by conversion of optical signals to electrical, and back to optical via the optically-active chips or devices. This can be accomplished, in part, by using exterior connections in-between the four different levels of traces  24 . 
     FIG. 7 shows a schematic top view along line  1 — 1  of FIG. 3A of a sixteenth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention. Multiple metallized traces can fan out from a smaller pitch to a larger pitch on the periphery of plate  16 . Seals  48  and  52  can have the shape of an annular ring. 
     FIG. 8 shows a schematic top view of a seventeenth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, wherein package  8  can be a multi-chip module (MCM) having a two-dimension array of microelectronic devices. In this example, package  8  includes three compartments having an integral window  26 . These windows can be LTCC or HTCC cofired simultaneously along with the rest of the package. Additional microelectronic devices  116  and microelectronic components  118  (e.g. capacitors, resistors, IC&#39;s) can be surface mounted to package  8  by conventional techniques, including flip-chip bonding and wirebonding. Cofired windows  26  and/or cover lids  42  can be placed on either side, or both, of the MCM package  8 . Multiple light-sensitive chips or devices can be mounted inside of the multiple windowed compartments. 
     FIG. 9 shows a schematic side view of a eighteenth example of a microelectronic package  8  for housing at least one microelectronic device according to the present invention, wherein window  26  further comprises a lens  96  for optically transforming light passing through the window. Lens  96  can be used for focusing or concentrating light onto a smaller, or specified, area on chip or device  100 . Lens  96  can be formed integrally into window  26 , or can be attached separately to the surface of window  26 , as in lens  98 . More than one lens  96  could be integrated with window  26 , with each lens having different optical properties. Alternatively, a divergent lens  96  can be used to spread the light. 
     Alternatively, the example of FIG. 9 can comprise an array of binary optic lenslets made integral with the window  26 . Binary optics technology is the application of semiconductor manufacturing methods to the fabrication of optics. A lens or lens array is laid out on a computer CAD program and transferred to a photo-mask using an e-beam or other writing process. A series of photo-masks are used, in conjunction with various etch steps, to build up the structures of interest. This fabrication technique can be used to make arrays of lenses with 1 micron features in completely arbitrary patterns. Lenslet arrays are straightforward to make with these methods, and can be extremely high quality with no dead space between elements. The advantage of binary optics is that the optical fabrication is not limited to spheres and simple surfaces. Lenslet arrays can be effectively used to performing optical remapping, such as transforming a round aperture into a square pupil. More details on the utility and methods for fabricating binary optic lenslet arrays can be found in U.S. Pat. No. 5,493,391 to Neal and Michie; as well as U.S. Pat. No. 5,864,381 by Neal and Mansell. Both of these referenced U.S. Patents are commonly assigned to the present assignee, Sandia Corporation of Albuquerque, N. Mex. 
     The present invention can also comprise an electrically-switched optical modulator attached to the package. Alternatively, electrically-switched optical modulator can replace window  26 , such as a lithium niobate window. In the example of a lithium niobate window, application of voltages around 5-6 V can switch the material from being transparent to being opaque, at a frequency of a few billion times per second. Such an active window can be used as a very fast shutter to control the amount of light being transmitted through window  26 . More details about use of lithium niobate as a light modulation device can be found in U.S. Pat. No. 5,745,282 to Negi. 
     FIG. 10A shows a schematic side view of an example of a package  8  for housing a microelectronic device  100 , according to the present invention. Package  8  comprises an electrically insulating plate  16  having a first surface  20 , an opposing second surface  18 , and an aperture  22  disposed through the plate; an electrical conductor  24  disposed on second surface  18 ; and an integral window  26  covering aperture  22 . Window  26  is directly bonded to plate  16  without using a separate adhesion layer (e.g., polymer adhesive) disposed in-between window  26  and plate  16 . Plate  16  can comprise a multilayered material, such as low-temperature cofired ceramic (LTCC) multilayer or printed wiring board (PWB) composition. Window  26  is mounted flush with first surface  20  of plate  16 , on a recessed lip. Microelectronic device  100  is flip-chip mounted to conductor  24 . Conductor  24  can comprise a metallized trace or electrical lead. 
     FIG. 10B shows a schematic side view of an example of a package  8  for housing a microelectronic device  100 , according to the present invention. Package  8  comprises an electrically insulating plate  16  having a first surface  20 , an opposing second surface  18 , and an aperture  22  disposed through the plate; an electrical conductor  24  disposed on second surface  18 ; and an integral window  26  covering aperture  22 . Window  26  is directly bonded to plate  16  without using a separate adhesion layer (e.g., polymer adhesive) disposed in-between window  26  and plate  16 . Plate  16  can comprise a multilayered material, such as low-temperature cofired ceramic (LTCC) multilayer or printed wiring board (PWB) composition. The joint between window  26  and plate  16  comprises an encased joint geometry (as previously defined with reference to FIG.  3 B). Microelectronic device  100  is flip-chip mounted to conductor  24 . Conductor  24  can comprise a metallized trace or electrical lead. 
     FIG. 10C shows a schematic side view of an example of a package  8  for housing a microelectronic device  100 , according to the present invention. Package  8  comprises an electrically insulating plate  16  having a first surface  20 , an opposing second surface  18 , and an aperture  22  disposed through the plate; an electrical conductor  24  disposed on second surface  18 ; and an integral window  26  covering aperture  22 . Window  26  is directly bonded to plate  16  without using a separate adhesion layer (e.g., polymer adhesive) disposed in-between window  26  and plate  16 . Window  26  is mounted to the first surface  20  of plate  16 . Microelectronic device  100  is flip-chip mounted to conductor  24 . Conductor  24  can comprise a metallized trace or electrical lead. 
     FIG. 11 shows a schematic side view of an example of a package  8  for housing a microelectronic device  100 , according to the present invention. Package  8  comprises an electrically insulating plate  16  having a first surface  20 , an opposing second surface  18 , and an aperture  22  disposed through the plate; an electrical conductor  24  disposed on the second surface; and an integral window  26  substantially filling up aperture  22 . Window  26  is directly bonded to plate  16  without using a separate adhesion layer (e.g., polymer adhesive) disposed in-between window  26  and plate  16 . Window  26  can be fabricated by casting molten glass, or by molding a liquid polymer, directly into aperture  22 , thereby substantially filling up the space defined by aperture  22 . Microelectronic device  100  is flip-chip mounted to conductor  24 . Conductor  24  can comprise a metallized trace or electrical lead.