MOEMS packaging technology

Optical alignment: 

In order to obtain a reliable and low-loss system. Alignment of optical devices in MOEMS is the most important. At present, MOEMS has two methods: passive alignment and active alignment. Passive alignment is usually achieved at one time during the fabrication process. Manufacturing errors or temperature variations can reduce the accuracy of the alignment. These errors can be compensated by the active alignment system. Active alignment is more complicated, but active alignment helps to reduce system tolerances and achieve real-time alignment of optical devices. Photo-alignment for multimode applications can use passive waveguide structures such as Si V-grooves. A well-established method of assembling a MOEMS module is the use of a passively aligned photo-subsystem assembly based on Si-optical step/Si micromachined technology. It can also be used for passive alignment of single-mode optical fibers with hybrid integrated optical or electrical components, depending primarily on the accuracy of the V-grooves. This packaging technology has been developed to wafer-level self-aligned Si substrates. In order to prevent the fiber from moving, InP waveguides are used instead of the manual operation of the fiber. Due to the inaccuracy of the MOEMS technology itself, active alignment must also be used for most single-mode devices such as OXC.

In the field of free space optical interconnects and optical storage, integrated micro-optical subsystems with special requirements are modeled and standardized. In order to achieve alignment requirements, the degree of freedom of positioning must be minimized and prefabricated modules with positioning devices have been developed. In order to be able to freely combine different standard components, it is critical to establish mechanical and optical standards. A typical self-assembled MOEMS optical switch has taken a big step towards a high degree of integration.

Housing: 

The geometric interface requirements of MOEMS are similar to planar integration. In planar free space integration, light propagation occurs at an off-axis angle within the substrate, and all light functions are accomplished at the surface of the substrate. Therefore its interface is also located on the surface of the substrate. Therefore, such as the traditional IC package can not be packaged. The chip is generally placed in a closed enclosure to prevent sensitive optics from being affected by outside light, but a light path must be left and a light guide cover or window needs to be designed inside the enclosure. Nowadays, MOEMS has many commercialized packaging technologies. The widely used packaging methods include three common types of ceramics, plastics and metals. Because ceramics are safe, reliable, stable, strong, and will not bend and deform, MOEMS mostly uses ceramic cavity housings. The ceramic shell is often made up of a base or a stem to which one or more dies are attached by an adhesive or solder, the cover being transparent glass. To ensure good sealing performance. For example, LCC snap array ceramic cavity housings using snap technology are smaller and less expensive than leaded packages, and wire bonding and flip bonding are suitable for electrical interconnections.

Wiring and Electrical Interconnections: 

All MOEMS packages must provide optical and electrical interconnections. Wire soldering is a conventional technique for electrically connecting a die and a package. With flip chip (FC) technology, solder balls can be placed throughout the chip area, providing higher density I/O connections. However, since the heating process of the molten solder can damage the chip and produce a different axis, it cannot be used for opto-mechanical assembly. An effective solution is to determine the electrical contact paths (including the conductivity through the substrate) from the surface of the MOEMS to the outer surface of the package, make the vias of these via deep RIE etching techniques, and apply the isolation and conductive layers.

In addition, there is an incompatibility between the circuit, the conventional process of metal wiring, and the anisotropic deep etching process in the fabrication of Si MOEMS. In the process of Si anisotropic deep etching for making micro-mechanical structures, the finished circuit and metal wirings are easily corroded and damaged. The general solution is: using Au as the protective film for the circuit and wiring; after the electrode lead hole is concentrated and diffused, Al is evaporated on the glass cover as a wire bond, and then pressed together. Both of these methods increase the difficulty of the process and limit the integration and miniaturization of Si MOEMS. For this reason, a method of using SiO2/Cr as a protective film has also been developed. The process is simple, the cost is low, and the compatibility between the processes is realized. Optical Interconnections: The key to optical interconnects for PCB design MOEMS devices is to reduce alignment losses. In a precise V-groove, the glass fiber is fixed with a very stable adhesive, and the die must be aligned by passive or active adjustment.

In addition to the development and design of PCB design MOEMS devices, attention should also be paid to MOEMS assembly technology on PCBs. In the optical interconnection of optoelectronics and MOEMS, the concern for backplanes and printed boards (PCBs) is growing. However, PCB assembly has no rules to follow. The basic principle is to use devices, packages, and assemblies as a system that interacts with each other. The impact of MOEMS on PCB assembly is currently being studied and PCB assembly processes and standards need to be developed.

A good solution is to use polymer-wave conductive photo-circuit boards, that is, PCB carriers combined with light structures. For optical links, an additional optical layer with a thermal boss waveguide structure is chosen. The additional optical layer includes a lower cladding layer, a core layer, and an upper cladding layer, and is formed into a thin sheet by a standard laminating technique of a PCB manufacturing process, and finally becomes an electro-optical circuit board (EOCB). FIG. 5 shows the assembly of the EOCB, which includes an electro/optical carrier, a photovoltaic device, and a driver. VCSEL and PIN optoelectronic devices can be coupled directly to the waveguide. This layer of light is placed in the middle of the flat plate housing in order to protect the light structure with a high thermal load during welding. EOCBs are then made by standard lamination.

By direct butt coupling, the coupling between the optoelectronic device and the waveguide can be achieved. The connection process also solves the problem of precise alignment of the optoelectronic device in the thin layer with the optical multimode structure and minimizes the axis offset between the device and the waveguide axis. In addition, direct docking coupling also limits crosstalk between adjacent channels due to the reduced beam broadening effect. The entire butt-coupled optoelectronic device device for EOCB is shown in FIG. At present, EOCB test card system with light emitters, drivers and plug-ins has been developed.

The promising HDI MCM packaging process In addition, the HDI MCM packaging process suitable for MEMS is a promising method. This is also a new application of MEMS technology into optoelectronic multi-chip modules (OE-MCM). Since the HDI MCM packaging process in a common substrate has the ability to support multiple types of die, it is well suited for MOEMS packaging. HDIMCM provides flexibility for MOEMS integration and packaging, so there is no need to change MEMS or electronics fabrication processes. After using the standardized HDI process to complete the window required for packaging the MOEMS chip, a large-area laser cutting technique can be used to cut the chip to be connected to the MOEMS. Open the window necessary to physically access the MEMS die. However, one of the disadvantages of the MCM or panel level is that passive optical structures (such as beam splitters or beam combiners) cannot be implemented in the optical fiber, and only the splicing method can be used. Therefore, MOEMS cannot be assembled using standard SMD processes and other methods of increasing costs must be used.

Development prospects MOEMS is an emerging technology that provides light weight, miniaturization, and low-cost optical components for telecommunication and data communications applications, enabling the moveable structure with monolithic integration of micro-optical components and becoming the 21st century electronics One of the representative technologies in the field.

MOEMS is receiving great attention from research institutes and industry. Sandia National Laboratory, University of Colorado and other research institutes in the United States have successively developed valuable PCB design MOEMS devices, and set off an upsurge in the development of MOEMS optical switches and other optoelectronic devices. Currently MOEMS has begun commercialization. For example, the commercialized MOEMS optical system has been used for the most advanced digital projectors and has begun trial operation in digital cinema.

MOEMS is a new type of packaged device. Its components and packaging are special applications, so it is different from the standard microelectronics method. The MOEMS has the largest proportion of its packaging costs. MOEMS packaging not only ensures the expected performance of the product, but also makes the device reliable and competitive in the market. MOEMS will have a place in this emerging technology field and will face a series of issues such as product manufacturing repeatability, packaging and process flow standardization, core device reliability and longevity. That is not only to develop device technology, but also to develop packaging technology.