By Atomica
Wafer bonding plays an integral part in microelectromechanical systems (MEMS). Wafer bonding is literally sticking two different pieces of material together at the wafer level using different techniques. The two wafers might be separated by an insulator like silicon dioxide, or one of the wafers may have a cavity and the other wafer has some structure that you want to protect under atmosphere or vacuum by bonding the wafers together. Atomica's bonding capabilities include these six bond types used relatively regularly, with current programs running each of them, plus temporary wax bonds done regularly: - Low temp metal alloy: Hermeticity Yes, Vacuum High, Temp 200°C, Difficulty Low, Bond Line Width 100 μm, Bond Cost $$$ - Si fusion: Hermeticity Yes, Vacuum High, Temp 1050°C, Difficulty High, Bond Cost $$ - Glass frit: Hermeticity Yes, Vacuum Medium, Temp 420 - 440°C, Difficulty Medium, Bond Line Width 400 μm, Bond Cost $ - Anodic: Hermeticity Yes, Vacuum High, Temp 370°C, Difficulty Medium, Bond Line Width 50+ μm, Bond Cost $$ - Au Au thermo-compression: Hermeticity Yes, Vacuum High, Temp 200 -300°C, Difficulty High, Bond Line Width <100 μm, Bond Cost $$$ - Polymer: Hermeticity No, Vacuum No, Temp 200°C, Difficulty Medium, Bond Line Width 200 μm, Bond Cost $$ A low temp metal alloy bond is a proprietary one we've developed here. Basically, we have two different metal elements that come together. We heat that during the bond process while pressing and they form an alloy that is much stronger and has a much higher melting point than the bond temperature. So, we bonded around 200 C, and then it can withstand up to about 500 C after that without reflowing or moving. The silicon fusion bond usually involves silicon to silicon or more frequently, silicon to silicon dioxide. You have two surfaces that are extremely flat and extremely low roughness, you bring them into contact with each other, and then you anneal them at a high temp. They fuse into one material. The glass frit bond is a paste of sorts that has glass embedded in it as well as some solvents and various other chemistries. You screen-print that onto the wafer (just like you would a screen-printed T-shirt) into the bond line configuration. You bring the wafers together and heat them. That drives out all the solvents and turns it into a much more solid glass from that paste that you originally started with. The anodic bond involves a silicon wafer and a special kind of glass wafer (borofloat) that has a lot of salt ions. You bring the wafers together and apply a very high voltage across those two wafers. You get an ion migration across the interface and the wafers fuse together. It's a very high-strength bond that can be done relatively easily – a very good bond when you can allow for one of the wafers to be glass. A lot of people do gold-gold (Au-Au) thermocompression bonds. One of the challenges is that you have to apply a large amount of force to get that bond to work properly. You also have to know how to prepare the surface properly. We have our own proprietary way of making a gold-gold thermocompression bond work better than just the kind of standard approach. The bond is good but it does add some complexity, which is why that's one of the higher cost bonds (also because it's gold which is a relatively expensive material). With a polymer bond, you're using some kind of polymer, like a glue. You pattern that polymer into the shape of the bond line that you want to use, press the wafers together, and drive out the solvents from that glue to solidify the polymer. Even within some of these bonds, there are different types. For instance, there are many types of polymers. We have expertise in choosing which polymer is the best to use and how you would apply that polymer to the wafer. We also have some proprietary approaches that aren't available anywhere else that allow for some unique benefits. Our proprietary bonds are patented; they are not available anywhere else and provide a unique solution. We have experience developing other bonds. We've actually come up with some bonds that allow for both compliance (with topography) and rigidity (for a very good, strong bond) at the same time for some specific biological applications where that is necessary. Atomica's 8 inch (200mm) Bonding Line. It's one thing to bond wafers at six-inch – we've been doing that for 20 years – but being able to do it at eight-inch. This introduces another level of complexity because you have much larger surface areas and you've got to have more uniformity across the wafer. There aren't that many places in the world that have the expertise, the bonders, the chemical-mechanical polishing (CMP), electroplating, and all the other tools necessary to create good bonds on eight-inch wafers in a production foundry environment.
