Microsystems assembly and interconnection
TWI Bulletin, July/August 1997
Microsystems can generally be defined as microstructures and their associated intelligence/ signal processing as electronics. Microstructures are small mechanical components, such as transducers, sensors, actuators or motors and are machined from materials such as silicon. Their features are measured in microns. Development and use of microsystems is seen as an emerging technology which will expand rapidly over the next 20 years.
Fig.1 Surface machined accelerometer (Note gaps between fingers in the order of 5µm) Courtesy of Robert Bosch
Currently, sensors are the main commercial application for this technology, with their primary market being the automotive industry for components such as fuel injection pressure sensors and accelerometers for airbag systems. Microstructures are machined in multiple arrays on silicon wafers, using a combination of etching and plating/deposition technology. They range from simple diaphragms to complex free standing comb structures (Fig.1). In future, more complex structures/intelligent systems are predicted; these may be miniature motors/pumps/actuators and their electronic control circuits for applications such as 'smart pills' (implants that match drug dosage to body requirements) and micro-robotic operating machines, to replace catheters, thus obviating the need for long umbilical control leads.
TWI's long involvement with the transducer and sensors industry, combined with experience drawn from participation in the Europractice Microsystems Technology project, (part of the European Union's Europractice Programme to promote the uptake of microelectronics technology), confirms the view that assembly and interconnection technology are significant factors in microsystem viability. Choice of packaging design/materials and the assembly route can fundamentally affect the performance and service reliability of individual micromachined devices and the cost of the complete system.
Fig.2 Scanning catheter (concept drawing)
The prime requirements are to protect the microsystem from its operating environment, and/or the operating environment from the microsystem, ensure reliable interconnections both within and to or from the microsystem, without detriment to its function, and all at minimum cost. Figure 2 illustrates the potential complexity of such packaging systems.
Fig.3 Basic assembly of a microsystem
The majority of current microsystem assembly and interconnection technologies are based on those developed for the microelectronics industry. Furthermore, there are no standards or preferred options, the choice is usually a compromise between availability, performance and cost. Figure 3 shows a typical microsystem assembly.
Major elements of the assembly operation are:
- microstructure handling
- microstructure bonding
- substrate design and mounting
- device protection/package sealing
Selection of joining processes / materials has a major influence on stress levels induced in the microstructure and reworkability of the product, which may be important for prototyping. It also dictates the temperature to which the device will be exposed during the bonding process and limits the temperature of subsequent manufacturing processes. The choice includes anodic bonding, diffusion and eutectic bonding (eg Au/Si), soldering and adhesives. Selection will depend on the packaging materials, assembly process and service requirements. Anodic bonding is generally limited to the direct joining of a silicon device to a glass support substrate. Diffusion, eutectic bonding and soldering usually require the joint interfaces to be metallised prior to assembly. Adhesives (thermoset and thermoplastic) can accommodate a wide range of surfaces to which they will adhere, but cannot provide the level of joint hermeticity which some microsystems require.
Whilst the goal of microsystems is to incorporate the microstructure and its associated control/telemetry electronics into a single semiconductor die, most current systems use multiple dies. The mounting technique used for such dies is dictated by the design requirements (eg size, weight, cost) of the final product, but is often based on conventional electronic packaging, substrate materials and attachment techniques. Options for substrates range from simple, low cost PCBs to complex active silicon substrates. The choice will influence the product's capacity to meet the required level of interconnection in the space available, dissipate (or transmit) heat and withstand harsh environments.
Use of a single monolithic die containing both microstructure and electronic processing components minimises space, weight, signal transmission losses and assembly costs. However, it significantly increases the complexity of the device manufacturing route and reduces process yields at all stages of manufacture. This approach has yet to gain widespread acceptance.
Mounting a substrate to its package poses similar problems to large area die bonding ie greater stress levels in the joint. This can often be overcome by using compliant attach materials matching the TCE (thermal coefficient of expansion) of the component materials. The attach materials are those available for die bonding, but the assembly process will probably have to be completed at a lower temperature to avoid damaging previously completed bonds.
Fig.4 Flip-chip die interconnection by solder bumping
The electrical connections from microstructure to its associated control/signal processing chip or chips and then to the microsystems Input/Output (I/O) pins can be by established wire bonding techniques which allow a high level of production flexibility. The flip-chip technique (Fig.4) is a high density interconnect process using solder, thermocompression bonding or conductive adhesives. This method also has potential for mounting the microstructure directly onto its control device. The choice of interconnect technology depends on the application requirements (eg size, cost, interconnect density, package type) and availability of dies.
The microstructure and its associated electronics need protection from the operating environment to ensure long life. This is typically achieved by sealing in a hermetic container - an approach owing its technology to conventional electronic packaging. Hermetic packages are usually metal (eg Ni or Au plated Ni-Fe-Co alloy) or ceramic. Lids are brazed, soldered or welded in place in a controlled environment. Interconnection to the microsystem is normally achieved through glass-to-metal seals or plated tracks/filled vias. These packaging techniques not only ensure good protection from the local environment, but also allow inclusion of a vacuum or gas/fluid inside the cavity. In some cases this may be necessary for device operation.
Fig.5 Wafer level protection of individual microstructure
There is growing interest in locally protecting the sensitive microstructures as an alternative to mounting them in expensive hermetic packages. This can be achieved after micromachining, whilst the array of structures is still in the wafer, using a machined or flat glass plate placed over the silicon wafer. The two are anodically (electrostatically) bonded together (Fig.5). The individual microstructures, or microsystems, are then cut from the wafer and can be mounted in a polymer package or resin encapsulated.
With a few notable exceptions, manufacturing volumes of microsystems are still relatively low compared to those of microelectronics. Therefore the techniques used for handling dies containing microstructures, both between foundry and customer, and during the assembly operation, have not been considered a problem; manual or semi-automatic handling, with tweezers or soft tip vacuum pick-ups has sufficed. However, as volumes and device complexity increase, new automated handling techniques will need to be developed to prevent damage to the microstructures, unless wafer-level protection is applied.
Fig.6 Micro pressure sensor (concept drawing)
Microsystems, just as any other system, require transmission media for signals going into and out of them. For instance: a simple pressure sensor (Fig.6) requires the input pressure to be transmitted fluidically (via an incompressible fluid) onto its piezo-resistive diaphragm, from which the output is transmitted as an electrical signal. Major signal transmission media are:-
- electrical (eg wires)
- optical (eg optical fibres)
- fluidic - gas and liquid (eg pipes/tubes)
- mechanical (eg levers/push rods)
It is therefore necessary to ensure that media interconnection techniques, suitable for microstructure applications, are available. Electrical interconnection is well understood and established - techniques drawn from the microelectronic field meet known microsystems needs. Optical and fluidic interconnection technologies for microsystems are still in their infancy and rely upon the applications of 'scaled-down' versions of existing larger technologies. Development of more reliable and/or cost effective techniques is likely to occur only if product volumes increase significantly.
The question of mechanical interconnection eg interchangeable tools for micro actuators, remains to be addressed. As with optical and fluidic interconnection, economics seem likely to dictate the direction of this work.
Other considerations in selecting assembly and interconnection technologies may be the provision of contact with the microstructure, protection of other parts of the microsystem (necessary for pressure and chemical sensor devices), or the systems' ability to be free from dirt traps and/or be sterilised (necessary for medical products).
Successful exploitation of any microsystem product will depend heavily upon the correct selection of appropriate packaging interconnection/assembly technology. This should be addressed at the outset of product design, and developed in-parallel with it to ensure a fully integrated, reliable, product.
