FIELD OF INVENTION
The present invention relates to the fabrication and packaging of optical microelectro-electromechanical devices (MEMS or MOEMS), carrying tiltable mirrors integrated on substrates; and improved MEMS structures and devices provided thereby, being more particularly directed to the precise alignment or tiling of such devices or dies on single packaging transparent optical substrates and the like, without restriction on the size of, or the layout upon, the substrate, and with ready adaptability for large scaling.
The present invention, as above stated, generally relates to the packaging of electronic integrated circuits, and more specifically to the packaging of MEMS devices with optical components, such as tiltable or orientable mirrors; being primarily concerned with the means by which optical and electrical inputs and outputs are made, utilizing packaging substrates, and how such packaging can monolithically employ general-purpose optical components.
Recent attention, however, has been paid to making multichip modules (MCM), systems-on-chip (SOC) and microscale optomechanical devices for a variety of applications. The MCMs are devoted to miniaturization of electronic systems into one packaged module where many hybrid technologies may be employed; while the SOC has focused on the integration of many electronic functions, (analog, digital and RF, etc.), monolithically onto a single VLSI die. Optoelectronic devices are beginning to follow along the MCM route where many optical components, such as lenses, beams splitters, lasers, detectors, and fiber optics and the like, are integrated onto MCM-like carrier substrates.
The present invention falls under the particular purview of the so-called flip-chip (FC) bonded multichip modules (MCM) that employ substrates that act not only as a mechanical attachment and electrical wiring point, but also as an optical interface. Typical MCMs incorporate an insulating or non-conductive substrate resembling a printed circuit board (PCB) where metalization is placed for the creation of interconnection circuitry. The substrates have regions where VLSI dies are attached, face-up to the substrate; and, following attachment to the substrate, are then wirebonded to complete the electrical connection. An example of such structures is disclosed in U.S. Pat. No. 6,147,876,creating a special substrate for die potting. Other forms of substrates, interconnection methodologies, materials, and architectures have also been proposed for face-up VLSI MCMs.
More recently, the previously mentioned flip-chip bonding of dies to substrates and of dies-to-dies for face-to-face solder attachments have also been proposed as in, for example, U.S. Pat. No. 6,150,724, illustrating die-on die/die-on-wafer flip-chip bonding. In such cases, solder is used to attach, align and electrically connect the VLSI to another die or a sub-wafer package. By utilizing surface tension while the solder is in its liquid state, the floating die placed face down onto the target substrate is drawn laterally until minimal misalignment between the target substrate and the die is achieved. Such techniques are shown, as a further example, in U.S. Pat. No. 6,151,173, employing solder microballs to achieve 1 micron alignment. In this case, the solder microballs are utilized to control the solder coating thickness, which plays an important role in alignment accuracy. In the field of MEMS or MOEMS, the use of such flip-chip bonding has been employed to mix differently processed die substrates in order to achieve hybrid integration of MEMS components for an optomechanical device, such as an optical scanner of Xerox Corporation, employing flip chip process for MEMS applications in silicon optical bench integration. In addition, flip-chip bonding has also been used for the self-alignment of optical fiber arrays to substrates that have waveguide components monolithically integrated, as described at http://www.rereth.ethz.ch/phys/quantenelectornik/melchior/pj.17.html. In this case, the surface tension of the solder bond draws the fiber arrays into alignment relative to the substrate.
In much of the prior art, the attach substrate has been opaque or not at all considered for optical interconnection functions or its optical properties. Recently, however, some consideration has been given to the use of the attach substrates as an optical path. VLSI dies with detectors or transmitters, for example, have been bonded to an optical substrate that provides an optical path for interconnection, as illustrated in U.S. Pat. No. 6,097,857, describing optical and electrical interconnections using such a substrate with integrated holograms, wherein VLSI chips are flip-chip bonded to the optical substrate. As another illustration, transmission through a VLSI substrate has also been considered for optical interconnection as in U.S. Pat. No. 6,052,498.
Up until the present invention, however, it does not appear that the prior art has taken into full account the problem of integrating many MEMS dies with high alignment accuracy onto an optically transmissive substrate that provides not only electrical connectivity but also simultaneously provides means to integrate passive or active optical components (as later discussed). MCM and flip-chip approaches heretofore only covered the many die-to-single substrate attachments. One of the purposes of this invention, on the other hand, is to create a substrate that provides both electrical and optical interconnection to optical MEMS-integrated circuits and components requiring critical alignment, say as low as +/−1 micron. The present state-of-the-art of VLSI processing, unfortunately, does not pragmatically provide a mechanism for creating die sizes beyond 20 mm on a side without defects. Sizes exceeding 20 mm on a side, indeed, require stitching of stepper repeated masks—a process that encompasses more defects per area that often result in defective mirrors or electronics, increasing the risk of unacceptable dies and producing wafers with very poor yield.
As a result, very large arrays of MEMS devices, sizes exceeding 40 mm on a side, have not heretofore been possible with the stitched stepper mask approach, for example, on a single silicon die, particularly where the die is approaching the wafer sizes. In addition, the scalability of the MEMS devices, typically on the order of 1 square mm in area, to thousands of devices, is currently seriously limited. To combat these problems, known good—die approaches have been the industry standard; that is, VLSI dies and correspondingly MEMS dies, are tested, and only known good parts are selected out for packaging. To effectively use known good dies in the creation of a larger optical MEMS array, however, a precise alignment of MEMS die-to-MEMS die is necessary in order to maintain beam integrity, requiring a high-accuracy tiling approach. In addition, while lens arrays can be used over the MEMS array to reduce the overall size of the MEMS and increase the amount of real estate available for integrated electronics and interconnections, a critically tight alignment of the lens arrays to the center of the MEMS mirror is required to avoid misfocusing of the optical beams.
In accordance with the present invention, these and other problems of tiling many MEMS have now been successfully addressed by using a custom-fabricated optically (for example, visible or near-infrared band) transmissive substrate. This substrate may have monolithically integrated optical components, such as lenses, diffractive gratings, optical absorbers, and transmission filters, and the like; and its MEMS chips are flip-chip-bonded onto support pillars or posts that act as the electrical and mechanical connections and also provide the mechanism for self-alignment. Instead of creating MCMs with standard VLSI dies and an optical substrate, or MCMs on non-optical substrates, the technique of the present invention rather builds an optical MCM (OMCM) with MEMS devices. This invention allows for physically integrated means to set the optical path, as for a lens which focuses light onto the MEMS mirror. By using lenses to optically address the MEMS arrays, smaller mirrors are then possible, enabling greater area for monolithic electronics integration. By using a self-aligned flip-chip approach in this manner, moreover, the MEMS are accurately aligned to these passive optical components. The invention, furthermore, does eliminate the need for attaching monolithic lens arrays to the MEMS device following the packaging process; but it does require careful handling of the released MEMS dies during the bonding process, and careful control of the solder bonding process to assure 100% yield of the bonded dies, as later more fully explained.
OBJECTS OF INVENTION
A primary object of the invention, accordingly, is to provide new and improved critically aligned optical MEMS dies particularly suitable for large packaged substrate arrays, and that shall not be subject to the above-described prior art limitations and difficulties; but that, to the contrary, enable large packaged array constructions through the integrated packaging of MEMS devices with optical components, such as lenses, wherein electrical and optical inputs and outputs are integrally provided upon an optical substrate monolithically embodying such optical components.
A further object is to provide such novel structures and devices wherein an integrated physical optical path is provided for the lenses which focus light onto the MEMS mirrors and optically address the MEMS array, thereby enabling the use of small mirrors and providing greater area for monolithic electronics integration in the substrate.
An additional object is to provide a new and improved method of manufacturing such novel devices.
Other and further objects will be explained hereinafter and are more particularly delineated in the appended claims.
In summary, the invention embraces an assembled array of optically and electrically interacting optical MEMS dies physically and electrically integrally attached upon a light-transmissive substrate carrying a pattern of printed electrical circuit interconnections, whether transmissive, opaque or a combination of both properties, for operating the dies, the light-transmissive substrate being integrated monolithically with optical components (passive and/or active) to provide accurate and fixed optical alignment of the MEMS and the optical components interacting therewith.
In its fabrication aspects, the invention provides a method for enabling the precision assembly of optical MEMS arrays upon a single substrate without substantial restriction on the size or layout of the substrate, that comprises, custom-forming a plurality of MEMS dies each carrying electrical signal-controllable mirrors; and forming a light-transmissive substrate of desired size to accommodate the plurality of MEMS dies, while monolithically integrating into the light-transmissive substrate, optical components useful for light-path interfacing with the MEMS dies. Integral printed electrical circuit interconnections are provided on the substrate for operation of the mirrors of the dies and the dies are physically and electrically integrally attached along and upon the single optically transmissive substrate, and with electrical connection to the printed circuit, thereby to provide also for the accurate and fixed optical alignment of the MEMS dies and the optical components optically interacting therewith, and enabling the focusing of light onto the MEMS mirrors along fixed optical paths for optically addressing the array without requiring adjustments.
A scalable approach is thus provided for packaging the optical microelectro-electromechanical system devices onto the light-transmitting, (preferably optically transparent) printed wiring or circuit substrate. This approach allows for the creation of custom-defined optical paths, by a lens array, antireflecting and/or absorbent surfaces, optical grating surfaces, wavelength specific filters, etc. The substrate contains photolithographically defined metalization or conductors that represent the desired electrical printed circuit interconnections of the many MEMS dies, once integrally bonded to the substrate. These printed circuit conductors are defined with well-known high-resolution lithography tools allowing for inter-MEMS die conductor placement accuracy of less than 1 micron. The mounting substrate accepts separately custom-manufactured MEMS dies that are bonded physically and simultaneously electrically connected to the substrate using the before-mentioned present state-of-the-art flip-chip solder attach tools. Solder reflow techniques are preferably employed precisely to align MEMS dies to the transparent optical substrate. This precise alignment process maintains tight alignment of the MEMS devices relative to the substrate, and, indeed, provides for MEMS die alignment as low as+/−1 micron, and similar tight alignment for the optical components monolithically integrated into the packaging substrate. The overall device package of the invention thus allows for the precise placement and tiling of many MEMS dies onto a single substrate without restriction on the size or layout of the substrate.
Preferred and best mode designs, techniques and configurations are hereinafter more fully explained.
The OMCM approach of the invention, moreover, allows for the integration not only of passive optical elements, but interacting active optical elements, as well. Passive optical elements include, for example, the before-described lenses, diffractive optical elements, arbitrary phase elements, optical masks, mirrors, polarizers, etc. In addition to such and other passive elements, active optical components can well be integrated onto or into either the MEMS substrate or the OMCM substrate. The active optical components may be lasers, photodetectors, modulators, switches, and filters, as examples. MEMS devices are typically integrated on silicon substrates that readily allow for the cointegration of MEMS and photodetectors onto a single MEMS substrate, as illustrated in FIG. 9. In this figure, a photodetector 14 is integrated into the MEMS substrate either laterally placed with respect to the MEMS device M or as part of the MEMS device. In this case, the OMCM substrate acts as a masked window 8. An alternative to this configuration is where the photodetector is integrated onto or within the OMCM substrate, as illustrated in FIG. 10. In this case, the OMCM substrate may act as a window for the photodetector, or it positions the photodetector relative to the MEMS device M to collect light reflected from the MEMS device.