Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With window means
Reexamination Certificate
2003-04-23
2004-10-26
Thai, Luan (Department: 2827)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With window means
C257S778000, C257S682000, C257S693000, C257S698000, C257S704000, C257S737000, C257S738000, C257S779000, C257S780000
Reexamination Certificate
active
06809413
ABSTRACT:
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'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's), side-brazed dual-in-line packages (DIP'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)
Peterson Kenneth A.
Watson Robert D.
Sandia Corporation
Thai Luan
Watson Robert D.
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