MEMS fabrication is developed from IC fabrication. Many techniques and materials used in IC fabrication are reused in MEMS fabrication for the advantages of low cost, high reliability and performance. However, MEMS fabrication is still different from IC fabrication at some aspects.
1.Unconventional Materials - MEMS fabrication involves more variety of materials. Besides the conventional materials used in IC fabrication, MEMS fabrication also use other materials. MEMS can also be made from quartz, ceramics, and polymide etc.
2. Lack of Standard Processes - IC fabrications have converged to certain standard processes, which can be used to implement all kinds of circuit functions, while MEMS fabrication is much more customized and diversified among different applications and different foundries. For example, pressure sensors and inkjet printing nozzles are fabricated by bulk micromachining, and airbag accelerometers and micromirror projection arrays are fabricated by surface micromachining. There is currently no library of design rules available for MEMS
3.Feature Size - Feature size of MEMS fabrication is normally larger than IC fabrication. When IC fabrication feature size has shrink to 25nm, the smallest features of MEMS devices are still in 0.5um to 1um range. This translates to cheaper mask cost than IC fabrication.
4.Mechanical Properties - MEMS fabrication cares about the mechanical properties (such as residue stress, density, young’s modulus etc.) much more than IC fabrication. Because the purpose of MEMS fabrication is to make micromachines, we more care about their mechanical properties, especially for material forming the structures. The properties of interest include Young’s modulus, yield strength, density, residual stress and stress gradients, and long term stability of these properties. The fabrication process configuration could greatly affect these properties. For example, in a process flow using polysilicon as structure materials, the deposition temperature and annealing process of polysilicon can significantly change its density, Young’s modulus, residue stress, and stress gradient, and thus optimization of process settings to control the mechanical properties of structural materials are crucial in MEMS fabrication.
5.Unique Unit Processes - MEMS fabrication shares many of unit processes from IC fabrication, however, it also has some unique requirements. MEMS fabrication use the same photolithography, wet and dry etch, oxidation, diffusion, LPCVD, and sputter deposition as IC fabrication. But some unit processes, such as plating, molding, and wafer bonding, are more common in MEMS than in mainstream IC fabrication. MEMS fabrication requires much deeper etch, and thicker deposition of materials (high aspect ratio) for better mechanical performance (such as high sensitivity and better signal to noise ratio). DIRE are conceived for MEMS fabrication to make deep trench, or through holes (up to 0.5mm). For some MEMS process flows, deposition of thick (up to 20-30um) polysilicon layer are crucial, while in IC fabrication, the polysilicon deposition is normally less than 1um thick. For MEMS process using SOI wafer as the starting material, the top silicon layer are normally 10-30um thick, but for SOI electronics, the top silicon thickness is much less than 0.1um.
6. Release Process - MEMS fabrication needs release process. For MEMS devices with movable parts, a unit process is required to release the movable parts from the substrate. The sacrificial layers are removed by wet etching or dry etching in the release process. During the process of wet etching release, the liquid surface tension of etchant could drag the movable parts to the substrate and cause stiction issues. CPD (Critical Point Drying) wet etch is required for MEMS release to avoid stiction issues, or dry etch can be used to avoid stiction.
7.Stiction - MEMS fabrication needs to consider stiction problem, and perhaps special layer to prevent stiction, Stiction is caused by strong adhesion forces between MEMS movable structures and the substrate. Once contact is made, the magnitude of these forces is sufficient to deform and attract these structures to each other or to the substrate, resulting in device failure. This type of failure is one of the dominant sources of yield loss in MEMS fabrication. The basic approaches in MEMS fabrication to prevent stiction include increasing surface roughness and/or lowering solid surface energy by coating the structure surface with low surface energy materials (for example, some kind of monolayer) .
8. Doping - For many MEMS applications, the doping of materials doesn’t require very accurate control and the doping level are normally high since the purpose of doping is only to make the material conductive. This could cause compatibility problem with IC fabrication process when two processes are integrated on the same die or share some equipments.
9. Wafer Bonding - MEMS fabrication usually need wafer bonding to form protective caps/cavities, or wafer level packaging, or implement the integration of ASIC and MEMS transducers. Also some very thick layers or heavy mass can be implemented by wafer bonding. For example, three wafers are bonded together to form bottom, up, and movable capacitor plates for high performance accelerometer devices.
10. Less Layers - The number of layers/masks in MEMS fabrication is usually less than IC fabrication. Some MEMS processes can be much simpler than IC fabrication. This is partially due to that the interconnection need in MEMS device is less than IC.
11. Front/Back Side Processing - Some MEMS devices need to be processed on both front side and back side while IC fabrication is focused on one side of the wafer. For example, Pressure sensor need to etch the backside to form cavity and membrane and doping on the front side to form the piezoresistors on the membrane, and the front-backside process alignment is required.
12. MEMS Package Stress - Thermal and mechanical properties of package are important factors to MEMS fabrication. The package stress can cause deflection and stress MEMS structures and thus change the device behavior while IC is less affected by mechanical stress. There are differences in MEMS packaging, whether wafer- or die-level.
13. Several orders of magnitude difference in wafer volume
14. Volume Production - Most MEMS applications are first time through the process – cannot jump into volume production right after the first prototype wafer run
15. Qualification of Process After process and design lockdown, to transition to volume production, the process must be qualified by Statistical Quality Control & Statistical Process Control, (SQC/SPC) implementation.