VARIOUS PACKAGING CHALLENGES AND SOLUTIONS
6.1: Introduction
Micro electro mechanical systems (MEMS) is an exciting field which integrates concepts of physics, electrical engineering, mechanical engineering, biomedical, electronics, chemical and material science to create unique low cost and miniature devices for a variety of applications. Over the past few years RF-MEMS fabricated using semiconductor, micro-fabrication technology have gained significant interest for wireless communication applications owing to their small size, integration capability and superior performance. For instance, using RF-MEMS switches, RF circuits such as variable capacitors, tunable filters, phase shifters and signal routers have been demonstrated.
RF-MEMS packaging is a promising field with diverse methods being explored to balance the technological and cost constraints. The packaging cost and expenditure involved in assembling the final product must be acceptable. Additionally, it must protect the MEMS during the product life and not degrade its RF performance and should have low parasitics like resistance, capacitance, or inductance. Integration of RF-MEMS is another aspect that lowers the number of components needed for an application.
MEMS packages must have the ability to meet at least one or more of the following criteria:
· Isolate non-sensing areas from the sensing areas, often in harsh, corrosive, or mechanically demanding environments;
· Not impede mechanical action, such as tilting, twisting, rotating, sliding, or vibrating;
· Allow the transfer of fluids from one region to another.
Due to many applications, device structures, and needs for MEMS, many packaging techniques have to be developed to meet the needs of the applications. Packaging of MEMS is different than packaging of ICs because MEMS need direct interaction with their environment. So, MEMS cannot be physically isolated from the environment, it needs selective access too. This creates a problem for the package as it has to provide simultaneously both the protection from and interaction with the environment. Unlike IC die packaging, MEMS dice need to interface with the environment for sensing, interconnection, and actuation.
MEMS packaging is application specific and the package allows the physical interface of the MEMS device to the environment. Harsh environments may create different challenges for the packaging of MEMS. In addition to challenges related to the environment of the MEMS chip and interfacing it with the environment, challenges also exist inside the MEMS package with the die handling, die attach, interfacial stress, and outgassing.
Here, RF-MEMS packaging challenges are described with some solutions and suggestions. The motivation for overcoming the challenges is the low cost and the ubiquitous applications of MEMS. MEMS packaging is by now far behind the capabilities of MEMS designers, and it is the purpose of this chapter to allocate the challenges of MEMS packaging and create knowledge and a curiosity of MEMS packaging in the packaging society.
6.2: RF-MEMS Product Integration
An RF-MEMS product is made of three main cost components that are the MEMS device, the control ASIC and the package. RF function is provided with the MEMS device such as an ohmic switch, capacitive switch, varactor, resonator, tunable FBAR, or multi-element circuits. The MEMS devices are incorporated with the ASIC on the same wafer by an integrated approach or by a hybrid approach.
In the integrated approach, the MEMS and ASIC process can be monolithically integrated by three basic scenarios: MEMS First, MEMS Last, or embedded MEMS. In contrast, for the hybrid approach, the MEMS device is fabricated using a MEMS specific process. Most RF-MEMS devices, such as a switch, varactor, or resonator, are active elements that respond mechanically to some electrical input signal.
Passive components (like capacitors, resistors, inductors, etc.) are integrated with active RF electronics in GaAs, SiGe, SOS, or other non- CMOS process. With appropriate modifications to the materials and other considerations, low contact resistance switches may be integrated with a CMOS process, or the matter can be prevented or delayed by subsequent hybrid integration approach.
6.3: Challenges in integration of RF-MEMS:
Obviously, the goal of IC packaging is to provide physical support and electrical interface for the chip(s), supply signal, power, and ground interconnections, and allow heat dissipation; to isolate the chip physically from its environment. But MEMS devices have to interface with their environment. Another issue is the media compatibility of the MEMS package. MEMS devices may need to operate in diverse environments like under automobile hoods, intense vibrations, in salt water, strong acids or other chemicals.
Table 6.1: Current packaging parameters, challenges, and solutions
PackagingParameters / Challenges / Solutions to these challenges and parameters
Release and
Stiction / Stiction of devices / Freeze drying, supercritical CO, drying, roughening of contact surfaces, non-stick coatings
Dicing / Contamination risks / Release dice after dicing, flush chip surface to remove contaminants
Die handling / Device failure, top die face is
very sensitive to contact / Fixtures that hold MEMS dice by sides rather than top face
Stress / Performance degradation and
resonant frequency shifts / Low modulus die attach, annealing, compatible CTE match-ups
Outgassing / Stiction, corrosion / Low outgassing epoxies, cyanate esters, low modulus solders, new die attach materials, removal of outgassing vapors
Testing / Possible device failure after many assembly steps / Post release, pre-package, and/or post package testing
The package, during operation, must be able to withstand the environment(s). Another challenge in MEMS packaging is the effect on reliability that packaging parameters induce. The package is part of the complete system and should be designed as the MEMS chip is designed, according to the specific application. The chip, package, and environment all must function together and all must be well-matched.
The materials and design considerations and limitations remain the main issue. The main scientific challenge is material characteristics. The material properties vary with their use, processing, and the heat treatments given to the materials. Not all the materials used, react the same to these parameters, so compromises must be made. Some materials may be hard to obtain with R&D production. Low quantities of materials are used, and suppliers are hesitant to trade small quantities or build up novel few products.
However, the material properties generally get better at the microscale due to a decrease in the number of defects in the materials. The defect density remains the same as in macroscale devices. Packaging of MEMS dice is application specific and hence, desired process steps could vary significantly. Thus, it is important to classify MEMS dice from the packaging requirements and develop the packaging standards and concerned knowledge.
Table 6.1 summarizes the current packaging parameters, challenges, and solutions that can be exercised to minimize or eradicate the different challenges of packaging. These solutions are incomplete, but are methods that have been exercised and are guidelines for further research and expansion of RF-MEMS technology. The packaging parameters and challenges are briefly described below.
6.3.1: Release and Stiction
An important step in MEMS packaging is the release of the MEMS dice. The polysilicon features are bounded by silicon dioxide that saves the features and stops them from damaging and contaminating. This oxide must be etched away for the liberation of the structures. This is done with an HF etch, which is choosy between SiO2 and silicon. It is done just in a simple and economical way in wafer with a batch process, but this causes grim impurity risks and harm during the dicing of the released wafer.
The most difficult time is after dicing. Each chip must be released independently instead of the full wafer in a single time. The MEMS parameters are guarded right through the potentially fatal dicing phase. In addition, there is also a risk of stiction during and after the release. Stiction occurs from the capillary action of the evaporated rinse solution in the crevices between elements like cantilevers and the substrate. This stiction may cause the MEMS devices of no use after all the assets have by now been spent in them. Figure 6.1 shows an example of the stiction. The capillary forces have pulled down the beam-type structure.
Stiction taking place after release is a key issue to be dealt with. It should be avoided also. Some effective approaches are freeze drying and supercritical CO2 drying. These methods get rid of the liquid surface tension from the drying process avoiding stiction from happening. This, however, does not stop stiction throughout the lifespan of the device. For preventing stiction all the way through the lifetime of the device, the surface can be roughened to minimize contact area or non-stick coatings can be employed to the device surfaces.
Figure 6.1: Beam-type structure being pulled down by the capillary forces
Stiction can also be decreased during the process of designing by employing dimples in locations of the device that are prone to the stiction. These small protrusions on the bottom on a structure can very much diminish the contact area between the MEMS device structure and the substrate. A cantilever beam with dimples on the bottom surface is illustrated in the figure 6.2.
Figure 6.2: An illustration of dimples used for avoiding stiction.
6.3.2: Wafer Dicing
The dicing of the wafer into the individual dice is another challenge in packaging of the RF-MEMS. The dicing is characteristically finished with a diamond saw of thickness about a few mils. This requires the coolant to flow over the surface of the very sensitive dice along with silicon and diamond particles that are formed during sawing process. These particles combined with the coolant can contaminate the devices and get into the crevices of the features resulting in the failure of the devices. A substitute to wafer dicing is wafer cleaving. Wafer cleaving is frequently done in III-V semiconductor lasers and has applications in MEMS. There is no requirement of any coolant in the wafer cleaving and there is not production of near as many particles as sawing also.
6.3.3: Die Handling
Another area of MEMS system fabrication and assembly is die handling that is at present not gathering the MEMS dice needs. Because of the fragile surface features of MEMS, these chips cannot be displaced by employing vacuum pick-up heads, as in customary IC die assembly. The MEMS dice must be picked up and handled by the edges that will need novel infrastructure for the automatic handling of MEMS. These new edge handling techniques would likely be restricted to MEMS since present vacuum pick-ups will continue to labor for ICs very well.
The MEMS die handling could be performed with the help of fingers or clamps that carefully handle these MEMS dice by their edges, or collets that fit present pick-and-place equipment. For handling the MEMS chips, die handling tools and methods that handle the chips by the edges will become common in intermediate to high volume MEMS packaging houses. In order to handle chips by the edges is not easier than by the top surface on account of very much reduced surface area and increased dexterous requirements of the pick-and-place tool.
Wafer level encapsulation eradicates the need for particular die handling fixtures. Here, a capping wafer is bonded to the top of a device wafer. These wafers can be bonded in a vacuum to create a permanent vacuum inside each device chip. These wafers can be bonded using direct bonding, but the required temperature is about 1000° C. Glass frit or anodic bonding is more regularly used since the processing temperature is between 450 and 500° C. However, the glass frit may cause stress in the die if the glass is not chosen with a CTE close to that of silicon.
6.3.4: Stress
Plenty of stress is created in the films, when polysilicon is deposited. This stress can be annealed out at a temperature of about 1000°C. This is most efficient if the polysilicon is deposited amorphously and then annealed to shape a polycrystalline structure. This creates the lowest stress arrangements with the least defects in polysilicon. The second sources of stress results from the die attach material at the interface between the MEMS die and the package substrate.
Depending upon the die-attach-material and coefficient of thermal expansion (CTE) mismatch between the package and the chip, interfacial stress can take place inside a MEMS package. Reliability is much reduced due to excess stress in the package. This stress can be caused by stress inducing fabrication processes, CTE mismatch in the die attach, lid sealing, or shrinkage. The stress may damage the devices; may misalign the gear teeth, tensile stress may cause the resonant frequency to increase and can also lead to device fracture, and extra compressive stress results in long beam structures to buckle.
The solders like AuSn or AuSi may put too much stress on the structures and cause warp or failure the features as well as the die itself. Stress effects also deteriorate as chip sizes increase. The stress can be minimized by employing smaller modulus die attach materials that deform with the expansion and contraction of the chip and package. These low modulus die attach materials may also allow creep over time. Stress relaxation can be very bad in die attach materials because a change in the stress state will lead to changes in device performance.
6.3.5: Outgassing
The die attach compounds outgas as they cure when epoxies or cyanate esters are used. These moisture and organic vapors redeposit on the features, in crevices, and on bond pads. This results in to stiction of the structure. This also leads to corrosion of the device. Die attach materials with a low Young’s Modulus, like epoxies; also permit the chip to shift during ultrasonic wire bonding, causing low bond strength. Solutions to outgassing challenges are die attach materials with very low outgassing and the removal of outgassing vapors during curing of the die attach.
6.3.6: Testing
Another significant problem is testing of these devices. With almost every step of the assembly potentially lethal to the MEMS chips, the chips can fail at any time during the processing. No one wishes to package an awful chip, it is too costly and too time consuming. However, no one can have enough money to test the chips after every assembly step. The packaging engineer must decide when and if any testing of the devices during packaging would be suitable based on cost and yield figures of the device. A final test of a completely packaged device may signify that in spite of all earlier tests and care taken, the part failed during lid seal that makes all earlier efforts to nil.