Case study: In-line metrology provides hands-free MEMS manufacturing
By Shari Farrens, SUSS MicroTec
More MEMS companies are going fabless every day and pushing production and process development to contract manufacturers and foundries. The commercialization of MEMS products, especially more consumer-oriented products such as portable displays, accelerometers for game units and gyros for image stabilization in digital cameras, are significantly changing manufacturing methods and tool sets.
According to Yole Development, there are roughly 24 foundries and contract manufacturers fabricating more than 85 percent of all MEMS devices worldwide. These companies have a revenue base just shy of $300 million today and will approach $500 million by the end of the decade. With rapid product ramps and decreasing margins on consumer products, it is essential to go to automated manufacturing techniques. This is especially true for assembly techniques such as wafer bonding.
Wafer bonding is a first-level packaging step in which the functional MEMS device is capped or sealed just prior to end-of-line packaging and testing. Historically the moving components are fabricated via sacrificial or surface micromachining methods in a starting substrate. The cap wafer is a second substrate - glass or silicon - that is placed on top of the finished devices to seal the small mechanical components from the outside world.
To complete the device fabrication the two wafers must be aligned and then permanently bonded to one another. In order to maintain margins and increase die density, devices have shrunk and alignment accuracy has become increasingly important. Maintaining alignment accuracy and monitoring the registry is an important and critical component of yield in MEMS manufacturing.
There are several key nodes in production flow where alignment accuracy can be measured and monitored. Process control monitoring starts with the very first set of alignment keys that are placed on the surface with lithography masks. By the time the device is ready for bonding, as many as ten or more mask layers may have been used, and it is important to understand whether there has been any deviation - run out - during these process steps that will prevent die to die alignment across the full wafer surface. Typical values for run out errors fall into the 0.1 to 0.5 micrometer range.
When the wafers go into the bond aligner they are aligned relative to one another with less than 0.1 micrometer offset. The associated mechanical motion and clamping of the substrates into position during the alignment step can lead to shifting. In a production environment it is imperative that post clamp accuracy be monitored and recorded automatically.
This requires that the automated aligner software have a high degree of flexibility to simultaneously determine the overlay accuracy of the two alignment keys and provide the option for switching of viewing mode to examine the wafers after they are clamped in position. All good bond alignment systems allow for this examination and the ability to back up and repeat the process until the required specifications are met.
The final step is transfer of the aligned pair to a thermal compression bonder for permanent bond formation. The thermal expansion during annealing must be symmetric and is controlled by independent upper and lower heaters with a temperature uniformity of less than or equal to one percent. It is recommended to use an air column approach for the applied force during the annealing cycle so that no shear forces are applied to the stack and induce additional shifting.
Once bonded, the wafer pair would normally be placed in an exit cassette for human assessment by off-line metrology. In the new generation of production bond clusters it is now possible to return bonded pairs to the aligner immediately after annealing for in-line metrology. The software automatically checks the alignment and records any deviations from the recipe setpoints for allowed limits.
The software is so advanced that wafers that are out of specification can trigger calls to the production manager and/or cessation of the production run. This level of automation is a first for MEMS manufacturing and enables hands-off operation of production wafer bonders with forecasted results. This revolutionary in situ metrology capability prevents lost lots because any process excursion is immediately identified and dealt with.
It now appears as though the MEMS industry will once again follow some of the IC trends: lowering manufacturing costs through higher volumes; standardizing processing; and automated foundry fabrication. The ability to produce MEMS products with less human interference will indeed lead to lower cost manufacturing and higher yields, but only if safeguards can be put in place. Using in situ metrology to check bonding is an essential step along that path.
Shari Farrens is chief scientist, wafer bonder division at SUSS MicroTec (www.suss.com) in Waterbury Center, Vt.