Micro-Electro-Mechanical Systems (MEMS) device fabrication presents unique challenges for process engineers and their choice of equipment. This is largely due to the three-dimensional (3D) nature and electro-mechanical functioning of MEMS products. In fact, with these additional challenges, a fundamentally different etching process – vapour phase etching – is the best match for these technical requirements.
Better Process Control
Isotropic etching steps used in conventional semiconductor device fabrication are typically accomplished with a liquid-based process, also known as wet etching. This is the straightforward method of removing material from wafers in a short time by immersing them in a liquid etchant solution bath. During the process, the etchant is consumed and the solution is replenished at a pace dictated by the rate of etchant usage and evaporation from the baths. A common wet etching process uses hydrofluoric acid (HF) to remove silicon dioxide (SiO2). As an etchant, HF is strong enough to remove SiO2 at the rate of 1µm/min. This makes it an efficient process for cleaning or when bulk layers are moved and precise control is not needed.
Vapour phase etching (VPE) is an alternative approach where the etching is accomplished by an etchant in the gas phase. This is a much more precise process because the vapour is easier to control with gas flow controllers and pressure controllers in the equipment. The liquid in a wet etching process cannot be managed as well with process variables, so VPE has advantages when very precise control is needed, as in MEMS devices.
Drier Process for Reduced Stiction
For MEMS processing, vapour phase etching provides a significant advantage by preventing stiction. A wet etchant typically requires several rinsing steps to ensure that the solution is removed from the devices, followed by a drying process. Although sophisticated, clean, rinse, and dry processes, such as critical point drying, have been developed and implemented over the years, it is still impossible to totally remove residue and contaminants that are left behind when the liquid is removed. This imperfect surface treatment resulting from wet etching contributes to stiction that can prevent MEMS devices from functioning properly.
At a microscopic scale, stiction is the permanent adhesion of a material to another due various forces such as Coulombic attractions, van der Waals forces, electrostatic charges, localised hydrogen bonding, or other surface phenomena. If these forces are great enough, then the freestanding component of the MEMS device will adhere to the device substrate (Figure 1). Vapour-phase etching methods have become the only certain way of preventing stiction.
Enabling More Material Options
The nature of VPE makes it more compatible with a broader set of materials. For example, a VPE process with HF can be used when aluminium is present, whereas HF wet etching is not compatible with aluminium. Liquid HF is actually HF acid when used in an etch bath, however anhydrous HF (aHF) is used in the vapour phase. The different phase of the HF can affect how it interacts with the surfaces that are exposed to it. For many materials, that doesn’t make a difference, but for aluminium it does. Liquid HF etching (i.e., with HF acid) is too corrosive for aluminium, so VPE is the right choice when there are Al bonds pads or functional electrical layers as part of the device.
Conversely, conventional polymer photoresist can be used as a masking layer with HF wet etching, but it isn’t a good match for vapour phase etching. The vapour phase HF molecule is more mobile and therefore able to diffuse through the pores in polymers like photoresist. This allows it to react with layers beneath the photoresist, which is not a desired outcome. Process knowledge like this is critical in optimizing MEMS processing.
Bottom Line: Vapour phase etching is a fundamentally different process from conventional wet etching, and its advantages in process control, eliminating stiction, and greater material compatibility make it a critical part of the MEMS process toolbox.