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MOEMS device technology and packaging for PCB design


The micro-optoelectronic mechanical system (MOEMS) is an emerging technology that has become one of the most popular technologies in the world. MOEMS is a micro-electro-mechanical system (MEMS) that uses a photonic system, which contains micro-mechanical optical modulators, micro-mechanical optical switches, ICs and other components, and uses the miniaturization, multiplicity, and microelectronics of MEMS technology to achieve Seamless integration of optical devices and electrical devices. Simply put, MOEMS is the further integration of system-level chips. Compared with large-scale opto-mechanical devices, PCB design MOEMS devices are smaller, lighter, faster (with higher resonance frequency), and can be produced in batches. Compared with the waveguide method, this free space method has the advantages of lower coupling loss and smaller crosstalk. The changes in photonics and information technology have directly promoted the development of MOEMS. Figure 1 shows the relationship between microelectronics, micromechanics, optoelectronics, fiber optics, MEMS and MOEMS. Nowadays, information technology is developing rapidly and constantly updated, and by 2010, the speed of light opening can reach Tb/s. Increasing data rates and higher-performance new-generation equipment requirements have driven the demand for MOEMS and optical interconnects, and the application of PCB design MOEMS devices in the field of optoelectronics continues to grow.

PCB design MOEMS devices and technology PCB design MOEMS devices are divided into interference, diffraction, transmission, and reflection types according to their physical working principles (see Table 1), and most of them use reflection type devices. MOEMS has achieved significant development in the past few years. In recent years, due to the increase in demand for high-speed communication and data transmission, the research and development of MOEMS technology and its devices have been greatly stimulated. The required low loss, low EMV sensitivity, and low crosstalk high data rate reflected light PCB design MOEMS devices have been developed.

Nowadays, in addition to simple devices such as variable optical attenuators (VOA), MOEMS technology can also be used to make tunable vertical cavity surface emitting lasers (VCSEL), optical modulators, tunable wavelength selective photodetectors and other optical devices. Active components and filters, optical switches, programmable wavelength optical add/drop multiplexers (OADM) and other optical passive components and large-scale optical cross-connects (OXC).

In information technology, one of the keys to optical applications is commercialized light sources. In addition to monolithic light sources (such as thermal radiation sources, LEDs, LDs, and VCSELs), MOEMS light sources with active devices are particularly concerned. For example, in a tunable VCSEL, the emission wavelength of the resonator can be changed by changing the length of the resonator through micromechanics, thereby realizing high-performance WDM technology. At present, a support cantilever tuning method and a movable structure with a support arm have been developed.

A MOEMS optical switch with a movable mirror and a mirror array was also developed for assembling OXC, paralleling, and on/off switch arrays. Figure 2 shows a free-space MOEMS fiber optic switch, which has a pair of U-shaped cantilever actuators for lateral movement of the fiber. Compared with the traditional waveguide switch, its advantages are lower coupling loss and smaller crosstalk.

An optical filter with a wide range of continuously adjustable is a very important device in a variable DWDM network, and MOEMS F_P filters using various material systems have been developed. Due to the mechanical flexibility of the tunable diaphragm and effective optical cavity length, the wavelength tunable range of these devices is only 70nm. Japan's OpNext company has developed a MOEMS F_P filter with a record tunable width. The filter is based on multiple InP/air gap MOEMS technology. The vertical structure is composed of 6 layers of suspended InP diaphragms. The film is a circular structure and is supported by three or four suspension frames. Rectangular support table connection. Its continuous tunable F_P filter has a very wide stop band, covering the second and third optical communication windows (1 250 ~ 1800 nm), its wavelength tuning width is greater than 112 nm, and the actuation voltage is as low as 5V.

 MOEMS design and production technology Most MOEMS production technology is directly evolved from the IC industry and its manufacturing standards. Therefore, body and surface micro-machining and high-yield micro-machining (HARM) technology are used in MOEMS. But there are other challenges such as die size, material uniformity, three-dimensional technology, surface topography and final processing, unevenness and temperature sensitivity.

Generally, photolithography technology is widely used to make structural patterns. In addition, maskless photolithography can also be used to make conventional patterns. For example, it is used on the surface of photosensitive materials such as polymers. In order to obtain a low refractive index surface, a two-dimensional pattern can also be produced, which can replace the traditional multilayer anti-reflective coating and can be used in MOEMS to improve its performance. The materials used and their deposition techniques are similar to standard IC processes, such as Si thermal oxidation, LPCVD, PECVD, sputtering, electroplating, etc., and different types of wet etching and dry etching techniques can also be used. For example, SiV-shaped grooves can be made accurately by wet anisotropic etching, and they are widely used for the alignment and packaging of optical fibers and optoelectronic devices. Micro-mirrors can be fabricated by wet reactive ion etching (DRIE) and surface micro-machining. A non-planar structure with a large longitudinal-to-mode ratio can also be obtained by using fine honing technology.

At present, the most used method is the micromechanical silicon wafer planar technology with chip bumps, which makes standard and low-cost IC assembly methods possible. In order to protect the chip, the surface of the chip can be sealed by a gel coating, and the recessed in-flow soldering method (IRS) can be used as a method to improve the wafer-level packaging. Some new MOEMS products are particularly sensitive to temperature. Devices with leads are generally welded by hand, while surface mount devices are welded by laser.

Successful technologies such as analog feedback loop (FEA), process optimization and secondary design have been adopted in MOEMS. In addition to mechanical, thermal, and electrical simulations, optical simulation (BPM) and performance appraisal were also introduced. In addition, due to high optical alignment requirements, in order to achieve complete optical device packaging and interconnection requirements, packaging technology has been introduced in the design simulation. Figure 3 shows the MOEMS design simulation and technical process procedures.

 MOEMS packaging technology In addition to the research and development of practical PCB design MOEMS devices, the main challenge at present is to assemble and package reliable devices in a dedicated package. Although many devices have been developed, there are few devices that can work reliably in the market. One of the reasons is the difficulty in packaging and the difficulty in realizing reliable and low-cost optical links. Especially as PCB design MOEMS devices enter the application field, the main problem is optical alignment and packaging. In addition, the actual loss of PCB design MOEMS devices also depends on the packaging technology.

Different from the standard packaging method, MOEMS components and packaging are special applications. Because each PCB design MOEMS device is non-standard development, and different applications have different packaging requirements, the MOEMS manufacturing technology is mainly packaging technology, and the packaging cost is in MOEMS It accounts for the largest proportion in the system, which is 75%-95% of the total cost of the system. Therefore, some developers say: Packaging is a process rather than a science.

Generally, MOEMS packaging is divided into three levels: chip level, device level, and system level. Among them, chip-level packaging includes chip passivation, isolation and welding, providing power paths, signal conversion and interconnection leads, and passivation protection and isolation of sensing elements and actuators; device-level packaging includes signal measurement and conversion, leads Bonding and component soldering; system-in-package includes packaging, production, assembly and testing. Package of 2×2 optical switch using glass fiber and ball lens. This high-performance, low-yield, mass-produced MOEMS optical switch can meet the requirements of all-optical networks for devices.


  MOEMS packaging requirements

MOEMS packaging requirements are: resistance to mechanical and thermal shock, vibration and chemical resistance, and long life. Including wafer and wafer adhesion thickness, wafer cutting, die fixing chip placement process, thermal control, stress isolation, hermetic packaging, inspection and adjustment.

Chip and chip adhesion thickness: The chip adhesion is generally quite thick (above 1mm), but nowadays the standard IC packaging market is developing in multiple dimensions, which poses a major challenge to packaging technology, because certain traditional assembly equipment cannot be used. There are no standardized tools.

Wafer cutting: The wafer cutting process is the biggest problem. Using viscose carrier tape to manually operate, water flow and vibration can destroy the tiny surface micromechanical structure. In addition, cutting before the sacrificial layer is etched increases the cost. Since MOEMS first-level packaging does not have to contact the surrounding environment, this problem can be solved. Thermal control: Because thermal fluctuations can cause unstable performance, and different CTE materials can cause light to be out of axis, thermal control is required in the chip and the package. A radiator such as a thermal regulator can be used for cooling to maintain a constant temperature. Chip mounting uses solder or epoxy filled silver material with high thermal conductivity.

Stress isolation: The mechanical or thermal stress in the PCB design MOEMS device is related to its working principle. It is believed that functional problems and stress problems caused by mismatch loss can reduce reliability and performance, and are often caused by the slow shrinkage of the adhesive or epoxy that connects the silicon chip to the package.

Hermetic packaging: Hermetic packaging is often used to increase the long-term reliability of the device. Generally, it is evacuated or filled with inert gas to prevent moisture, water vapor and pollution from entering the shell or corroding the environment. Metal, ceramic, silicon or millimeter-thick glass must be used to make air-tight tube shells, and air-tight connections must be ensured when electrical and optical interconnections are made.

 Inspection and adjustment: Due to small deviations in the manufacturing process, PCB design MOEMS devices must be inspected to meet the required technical indicators. One is to use laser trimming resistors or laser ablation methods, and the other is to use electronic compensation methods.

         MOEMS packaging technology

MOEMS packaging technology can be divided into the main aspects of die fixation, housing, wiring and optical interconnection. In MOEMS, commercial devices require practical MOEMS mixed reliable and safe shielded packaging. Due to the non-contact and non-intrusive nature of optics, PCB design of MOEMS device packaging is much easier than MEMS device packaging, and MEMS design can be used, but excellent and reliable optical alignment is required.

Optical alignment: In order to obtain a reliable and low-loss system. The alignment of optical devices is the most important in MOEMS. At present, MOEMS has two methods: passive alignment and active alignment. Passive alignment is usually achieved once during the manufacturing process. Manufacturing errors or temperature changes can reduce the accuracy of alignment. These errors can be compensated by an active alignment system. Active alignment is more complicated, but active alignment helps reduce system tolerances and can achieve real-time alignment of optical devices. Optical alignment for multi-mode applications can use passive guided wave structures like Si V-grooves. A mature method for assembling MOEMS modules is to use passive alignment photonics assembly based on Si optical step/Si micromechanical technology. It can also be used for passive alignment of single-mode fiber and hybrid integrated optical or electrical components, mainly depending on the accuracy of the V-groove. This packaging technology has been developed to wafer-level self-aligned Si substrates. In order to prevent the optical fiber from moving, the InP waveguide is used to replace the manual operation of the optical fiber. Due to the insufficient accuracy of the MOEMS technology itself, active alignment must be used for most single-mode devices like OXC.

In the field of free-space optical interconnection and optical storage, the integrated micro-photonic system with special requirements is simulated and standardized. In order to meet the alignment requirements, the degree of freedom of positioning must be minimized, and prefabricated modules with positioning devices have been developed. In order to freely combine different standard components, the key is to establish mechanical and optical standards. The typical self-assembled MOEMS optical switch has taken a big step towards high integration.

Shell: The geometric interface requirements of MOEMS are similar to those of planar integration. In the planar free space integration, since the light propagates in the substrate at an off-axis angle, and all optical functions are completed on the surface of the substrate. Therefore its interface is also located on the surface of the substrate. Therefore, it cannot be packaged with a traditional IC package. Generally, the chip is placed in a closed shell to prevent sensitive optical devices from being affected by external light, but a light channel must be reserved, and a light guide cover or window needs to be designed in the shell. Nowadays, MOEMS has many commercial packaging technologies, and the widely used packaging methods include ceramic, plastic and metal three common types. Because ceramics are safe, reliable, stable, strong, and will not bend or deform, most MOEMS uses ceramic cavity shells. The ceramic shell is usually composed of a base or a tube socket connected with one or more die through adhesive or solder, and the cover is made of transparent glass. To ensure good sealing performance. For example, the LCC snap array ceramic cavity shell using snap technology is smaller and lower in cost than a leaded tube shell, and wire pressure welding and reverse welding are suitable for electrical interconnection.

Wiring and electrical interconnection: All MOEMS packages must provide optical and electrical interconnection. Wire welding is a traditional technique for electrically connecting the die and the case. Using flip chip (FC) technology can arrange solder balls in the entire chip area and provide higher density I/O connections. However, since the heating process of melting the solder can damage the chip and produce the phenomenon of different axes, it cannot be used for opto-mechanical assembly. An effective solution is to determine the electrical contact channels from the surface of the MOEMS to the outer surface of the package (including the conductivity through the substrate), make the through holes of these channels by deep RIE etching technology, and coat the isolation layer and the conductive layer.

In addition, there is incompatibility between the conventional process of circuit and metal wiring and the anisotropic deep etching process in the production of Si MOEMS. In the process of Si anisotropic deep etching of the micromechanical structure, the completed circuit and metal wiring are susceptible to corrosion and damage. The general solutions are: use Au as a protective film for circuits and wiring; after densely spreading the electrode lead holes, evaporate Al on the glass cover as the lead solder joints, and then press them together. But these two methods both increase the process difficulty and limit the integration and miniaturization of Si MOEMS. For this reason, a method of using SiO2/Cr as a protective film was developed. The process is simple, the cost is low, and the compatibility between processes is realized. Optical interconnection: The key to PCB design for optical interconnection of MOEMS devices is to reduce alignment loss. Use a very stable adhesive to fix the glass fiber in a precise V-groove, and align the die with passive or active adjustment.

In addition to the development and design of PCB design MOEMS devices, attention should also be paid to the assembly technology of MOEMS on the PCB. In the optical interconnection of optoelectronics and MOEMS, attention to backplanes and printed circuit boards (PCBs) is growing. But PCB has no rules to follow in assembly. The basic principle is to treat devices, packaging, and assembly 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 a polymer wave conductive optical circuit board, that is, to combine the PCB carrier and the optical structure. For optical links, an additional optical layer with a thermal boss waveguide structure is selected. The additional optical layer includes a lower cladding layer, a core layer, and an upper cladding layer, and is made into a thin sheet by the standard laminating technology of the PCB manufacturing process, and finally becomes an electro-optical circuit board (EOCB). Figure 5 shows the assembly of the EOCB, which includes electrical/optical carriers, optoelectronic devices and drivers. Such as VCSEL and PIN optoelectronic devices can be directly coupled with the waveguide. The optical layer is placed in the middle of the flat tube shell to protect the optical structure with high thermal load during welding. Then EOCB is made through standard laminates.

Through direct butt coupling, the coupling between the optoelectronic device and the waveguide can be realized. The connection process also solves the problem of precise alignment between the optoelectronic device and the optical multimode structure in the thin layer, and minimizes the axis offset between the device and the waveguide axis. In addition, since the effect of beam broadening is reduced, cross-talk between adjacent channels is also restricted through direct butt coupling. The entire optoelectronic device device for butt coupling of EOCB is shown in FIG. 6. At present, an EOCB test plug-in board system with optical transmitters, drivers and plug-ins has been developed.

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

Development prospects MOEMS is an emerging technology. It provides light, miniaturized and low-cost optical devices for telecommunications and data communication applications, and realizes a movable structure with monolithic integration of micro-optic components. It has become a 21st century electronics One of the representative technologies in the field.

MOEMS is receiving great attention from research units and industry. The Sandia National Laboratory, the University of Colorado and other research institutions have successively developed valuable PCB design MOEMS devices, and set off an upsurge in the development of MOEMS optical switches and other optoelectronic devices in the world. At present, MOEMS has begun to be commercialized. For example, the commercial MOEMS optical system has been used in the most advanced digital projectors and has begun trial operation in digital cinemas.

The MOEMS market is promising. It is said that the value of optical switches that entered the market in 2003 amounted to 440 million to 10 billion U.S. dollars. In 2003, the market share of MOEMS was 8% of the total MEMS market. Table 2 shows the types and shares of the MOEMS application market.

As a new type of packaged device, MOEMS has components and packages for special applications, so it is different from standard microelectronics methods. Its packaging cost accounts for the largest proportion in MOEMS. The MOEMS package must not only ensure the expected performance of the product, but also make the device performance reliable and competitive in the market. If MOEMS wants to occupy a place in this emerging technology field, it will face a series of issues such as the repeatability of product manufacturing, the standardization of packaging and process flow, and the reliability and life of core devices. That is not only to develop device technology, but also to develop packaging technology. Although the packaging of MOEMS is difficult, it is developing very fast, and there are many commercial packaging technologies. This means that there is no shortage of solutions, and lack of how to apply it to MOEMS production. MOEMS and its device technology have a bright future in the field of information technology and optoelectronics in the future.