Part 3: Vapour Phase Etching: A User’s Guide

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.

vapour phase etching

Figure 1. Cantilever structures are common in MEMS devices, and stiction between layers (left) can result when wet etching. Vapour phase etching results in stiction-free MEMS devices (right).

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. Be sure to check out our new white paper with a deeper dive on these topics and other factors in the MEMS process equation

See Part 1: Getting a Handle on MEMS Process Variables 

See Part 2: Material Issues in MEMS Etching Processes 

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Part 2: Material Issues in MEMS Etching Processes

MEMS device fabrication requires a broad range of reactions and chemistries to create the structures and functions of these complicated electro-mechanical systems. MEMS etching processes in particular, merit attention for material compatibilities because, of course, the function of etching is to remove material. You need to make sure that you remove what you want to remove without removing anything else.

 

Oxide and Nitride Selectivity for MEMS Etching Processes

Silicon dioxide and silicon nitride layers are very common in MEMS devices, and the removal of sacrificial oxide layers by etching is a cornerstone of MEMS processing. So, it is critical that anoxide etching process does not detrimentally affect any silicon nitride layers that are present for passivation or other purposes. This desired material selectivity can be enhanced with the use of a catalyst. Oxide etches faster in the presence of a specific catalyst, but silicon nitride does not. This approach etches oxide faster with minimal impact on any nitride layers that are exposed to MEMS etching processes.

Even when only oxide is present, it is possible to create a selective etching process by taking advantage of the difference between oxide that is thermally grown and oxide that is deposited with plasma-enhanced chemical vapor deposition (PECVD). Specifically, thermal oxide has a higher density than PECVD oxide, so it is harder to etch. This allows for differential etching using thermal oxide as an etch stop layer when deposited oxide is the sacrificial layer in a MEMS structure. With this control over the PECVD material set and the processes to etch them selectively, it is possible to create MEMS structures with just oxide layers. This can simplify some of the processing, while avoiding the challenges of having a greater mix of materials.

 

The Whole Gamut

The vapour HF etching system produced by memsstar is highly compatible with a wide range of materials, with the list expanding as we continue to work with our customers and support ongoing research in the field.

Table 1 shows a summary of the most common materials known to work as either a sacrificial layer or a functional layer, along with the compatibility of memsstar’s tools.

In the oxide section, only doped oxides such as phospho-silicate glass (PSG) and borophosphosilicate glass (BPSG) are incompatible with memsstar’s HF etching systems. For structural layers, all of the identified materials are compatible, although PECVD silicon nitride needs extra attention to ensure compatibility. (Contact memsstar for expert advice.) For other functional layers, only tantalum and photoresist layers are incompatible, and titanium nitride may or may not be compatible, depending on the TiN deposition process.

MEMS etching processes

Table 1. Compatibility of HF vapour-phase MEMS etching processes using a variety of materials. (*TiN deposition methods differ, depending on the system used, as described by Kumar et al. [1].)

By understanding the fundamentals of these material issues involved in MEMS processing, it is possible to develop a more efficient route to setting up processes for a range of different devices. We describe the range of commonly utilised materials and their compatibility with the HF process, which can inform the material choices of future designs and can reduce development times. Often there is flexibility in choosing materials for interconnect and other features, so understanding how they interact with HF MEMS etching processes can help you optimise these material choices.

Bottom Line:  Material compatibility issues are critical for fabricating MEMS devices, especially with such a wide variety of materials and processes involved. Be sure to check out our new white paper with a deeper dive on these topics and other factors in MEMS fabrication.

See Part 3: Vapour Phase Etching: A User’s Guide 

[1] ‘Failure Mechanisms of TiN Thin Film Diffusion Barriers’, Kumar, N. Purrezaei, K., Lee, B.,  Douglas, E.C., Thin Solid Films, vol 164, 1988, pp. 417-428.

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