Patent Application
20090144970
At least one embodiment of the present invention pertains to a Micro-Electro-Mechanical System (MEMS), and more particularly, to fabrication of MEMS parts on a substrate and integration of the MEMS parts onto an application platform.
Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common substrate, such as a silicon substrate, through microfabrication technology. While the electronics are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), the micromechanical components are fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
MEMS technology is based on a number of tools and methodologies, which are used to form small structures with dimensions in the micrometer scale (one millionth of a meter). Significant parts of the technology have been adopted from integrated circuit (IC) technology. For instance, similar to ICs, MEMS structures are, in general, realized in thin films of materials and patterned with photolithographic methods. Moreover, similar to ICs, MEMS structures are, in general, fabricated on a wafer by a sequence of deposition, lithography and etching.
With the increasing complexity of MEMS structures, the fabrication process of a MEMS device also becomes increasingly complex. Conventionally, a MEMS structure comprising a large number of MEMS parts with multiple vertical layers deep (e.g., a MEMS probe card) is built on a single substrate, using a sequence of deposition steps across an entire wafer. A concern with the conventional methodology is that a defect or contamination occurring in any deposition step and in any individual MEMS part may cause the entire wafer to fail. Thus, there is a need to improve the conventional fabrication process in order to increase the yield of MEMS devices and reduce the cycle time and costs.
One or more embodiments of the present invention are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:
In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.
A method for fabricating a Micro-Electro-Mechanical System (MEMS) part and an apparatus comprising the MEMS part are described. In one embodiment, an array of MEMS parts is fabricated on a substrate. The MEMS parts are individually detached (“picked”) from the substrate, and then attached (“placed”) to an application platform in an unpackaged state using die attachment techniques. This “pick-and-place” technique greatly increases the flexibility with respect to how the MEMS parts are fabricated and used. For example, the array of MEMS parts may be detached from the substrate concurrently, or one or more parts at a time. Each of the MEMS parts may be attached to the same or different application platforms. Further, the MEMS parts attached to the same application platform may be fabricated on the substrate in a first arrangement and then attached to the application platform in a second arrangement, where the first arrangement and the second arrangement may have different spacing between the MEMS parts, different orientations of the MEM parts, or a combination of both.
The term “MEMS part” herein refers to a sub-structure (e.g., a mechanical part, an optical part, an electrical part, or the like) of a micro-machine, a micro-machining processed structure, or a MEMS processed structure. Typically, a MEMS part has dimensions ranging from 10×10×10 μm to 5000×5000×5000 μm. Examples of a MEMS part include a probe in an array of probes, which can be arranged on an application platform to form a probe card. A probe card uses the probes to establish an electrical path between an electronic test system and a wafer for testing and validation of the wafer. Other examples of a MEMS part include an optical laser module, optical lenses, micro-gears, micro-resistors, micro-capacitors, micro-inductors, micro-diaphragms, micro-relays, micro-springs, waveguides, micro-grooves, and the like.
One feature of the technique described herein is that the MEMS parts on the application platform, which is used in the final application, are fabricated on a substrate different and separate from the application platform. The term “substrate” herein refers to the substrate used only in the fabrication process without involvement in the operations of the MEMS parts and the final MEMS device incorporating the MEMS parts. Examples of a substrate for fabricating a MEMS part include, but are not limited to, ceramics, glasses, metal plates, plastic plates, and semiconductor wafers. A non-silicon substrate, compared to a Si-based substrate, offers a larger number of standard sizes and is available as a thicker and non-circular standard substrate. Further, a non-silicon substrate is inert to most chemicals used during fabrication processes. Most substrates, including Si-based substrate, can be processed with the MEMS parts thereon. Processed materials on the substrates can be later removed or dissolved without damaging the substrates. Therefore, the substrate for fabricating MEMS parts, as described herein, is a “reusable substrate,” unless otherwise indicated. A reusable substrate can be reused for a next batch of MEMS part fabrication after the MEMS parts are detached therefrom and residual substances are removed.
The term “application platform” herein refers to a platform (e.g., a substrate) used in operation as part of an operational MEMS device (e.g., a probe card, a laser module, etc.). An application platform may comprise, but is not limited to, semiconductor, glass, ceramics, low-temperature co-fired ceramics (LTCC), high-temperature co-fired ceramics (HTCC), metal, dielectric materials, organic materials, or any combinations of the above, that are suitable for the attachment of a MEMS part and for the final application purposes. An application platform has components fabricated thereon for specific application purposes. The components include, but are not limited to, electrical connection, electrical contact, electrical isolation, electrical grounding, integrated circuit (IC) module, application specific IC (ASIC) module, dielectric patterning, conducting opening definition, mechanical support, mechanical protection, thermal conduction, electrostatic discharge (ESD) protection, confinement for parts, and wire bonding pads. One or more MEMS parts are to be attached to this application platform to complete the integration of a MEMS device. It is understood that an application platform may include one or more MEMS parts fabricated on one or more reusable substrates. The MEMS parts attached to an application platform may be of different orientations, shapes, sizes and materials, and may have different functions.
According to embodiments of the present invention, MEMS parts are fabricated on a substrate different from the substrate used for the final application. Thus, yield of the individual MEMS parts does not directly affect the yield of the final product that integrates one or more of the MEMS parts. A selection process of acceptable MEMS parts may be performed before the MEMS parts are attached to the application platform. Defective MEMS parts may be discarded before the attachment process or left on the reusable substrate.
The technique described herein may be useful for a variety of products that include one or more MEMS parts. Illustratively, the final products may include a laser module in which a laser source (a MEMS part) is integrated and aligned with one or more lenses (which may also be MEMS parts). In this scenario, the substrate to which the laser module is bonded is the application platform. The attachment process of the laser module can be performed using die attachment techniques, e.g., the technique of bonding a die to a substrate commonly used in semiconductor industry. The application platform may have been patterned to form electrical connections among the components thereon and to a system external to the application platform. In another scenario, the final products may include a probe card including a plurality of MEMS probes. The location of the probes on the probe card may be customized. Fabricating the probes on the probe card substrate (i.e., the application platform) generally involves a sequence of processing steps. Conventionally, a defect developed during any processing step in any of the probes may render the entire probe card unusable. Using the techniques described herein, the probes may be fabricated on a separate substrate, and only the good probes are selected and attached to the probe card substrate. The probe card substrate may be patterned to electrically connect each of the probes to an external printed circuit board for transmitting probe signals.
Referring to
Island 23 provides support to one or more adjacent MEMS part 25 (only one is shown in
After the formation of MEMS part 25, further processing operations (not shown) may be performed to pattern the MEMS part 25 into a final structure.
In
In
In an alternative embodiment, instead of etching sacrificial layer 22, a Si-based substrate may replace reusable substrate 21 described above and selectively etched to a depth enough to free MEMS part 25. In this alternative embodiment, MEMS part 25 and the island 23 may be formed directly on top of the Si-based substrate. After formation, the Si-based substrate is then etched away from underneath MEMS part 25 such that there is no direct contact between MEMS part 25 and the substrate. A substantial amount of the Si-based substrate underneath island 23 remains to keep island 23 and MEMS part 25 in place. Thereafter, MEMS part 25 can be detached from island 23 in the same manner as described in the embodiment of
One notable feature of process 200 is that MEMS part 25 is detached from reusable substrate 21 in a highly controlled manner by the use of island 23 as an anchoring structure. Island 23 ensures that MEMS part 25 stays at a fixed location after the removal of sacrificial layer 22 from underneath MEMS part 25. When hundreds or thousands of MEMS parts are fabricated concurrently on a reusable substrate, the use of islands minimizes the possibility that these MEMS parts can be randomly scattered all over the reusable substrate after sacrificial layer 22 is removed. Scattered MEMS parts would be difficult to pick up and susceptible to damage.
In one embodiment, a MEMS part may be manually picked up by a tool (e.g., a pair of tweezers) suitable for a specific MEMS part. In another embodiment, a MEMS part may be picked up by a machine, such as a die attach machine with a custom-made pick-up arm to pick up a MEMS part from a target location on the substrate. A die attach machine is commonly used for picking up a die from as well as bonding the die to a substrate with high precision.
In one embodiment, the adhesion of the MEMS part 25 to application platform 41 may be accomplished by a die attachment technique, such as applying a bonding material 42 between the MEMS part 25 and application platform 41 (or a component on the application platform). To improve the adhesion, the MEMS part 25 and application platform 41 can be patterned with cavities and protrusions. The type of bonding material 42 includes, but is not limited to, epoxy, glue, paste, cement, silicone, conductive adhesive, eutectic metal, and any combination of the above. Some bonding material 42, e.g., eutectic metal and solder, may be in the form of a template or a coupon. Bonding material 42 can be applied to, or formed on, application platform 41 and/or the bonding surface of MEMS part 25 manually, or by a machine or equipment. Bonding material 42 may be applied or formed by electrical forming, thin film deposition, spin patterning, spray patterning, laminating, chemical forming, soldering, thermal compression, chemical join, thermal laminating, dispensing, mechanical locking or any combination of the above.
After the formation of bonding material 42, MEMS part 25 may be attached to application platform 41 by a machine with custom-made parts using any of the die attachment techniques described above. It is to be noted that the die attachment techniques described herein are applied to a MEMS part and an application platform, instead of a die and a PCB. Unlike a die residing in a package, a MEMS part is not packaged and may have any three-dimensional shape.
Typically, a MEMS part (e.g., MEMS part 25) has dimensions ranging from 10×10×10 μm to 5000×5000×5000 μm. A machine with custom-made parts is able to pick up, orient, align and attach the MEMS part to a specific location of an application platform with desired precision and orientation. Examples of such a machine include a die-attach machine, a pick-and-place machine, and a flip-chip machine, all of which are commercially available and can be operated manually, semi-automatically, or automatically. Typically, the pick-up head or arm of the machine can be custom made to pick up a MEMS part of a specific size and shape. Generally, a MEMS part with the dimensions described above may be picked up by vacuum, mechanical grabbing, mechanical locking, magnetic force, or any suitable methods. The machine that performs the attachment may be the same machine or a different machine that detaches the MEMS part from the reusable substrate.
Moreover, the adhesion of the MEMS part to the application platform may be enforced by local heating and/or pressure. The adhesion may be further enforced by local chemical application, or special local environments (such as forming gas or flux). Depending on the machine type and the degree of automation, the time it takes to attach a MEMS part can be as short as seconds. More than one MEMS part can be attached at a time. For example, a die-attach machine may be configured to pick up one or more MEMS parts and attach them to the specific locations on the application platform.
In some embodiments, after the attachment process, a MEMS part can be detached from its application platform for repair or replacement purposes. In an embodiment where a solder is used to join the MEMS part to the application platform, an attached MEMS part may be detached from the application platform by applying local heating above the solder melting point to the joining spot. An attached MEMS part may be detached from the application platform manually or by a machine. The machine may be the same machine as the one performing the attachment, or a different machine.
After the attachment of the MEMS part and subsequent integration process (e.g., integration with a PCB as described at block 150 of
Thus, a method and apparatus for fabricating MEMS parts on a reusable piece have been described. It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.
Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense.
