5.6.1. Techniques

Today, we’ll explore bulk micromachining. This technique focuses on selectively removing material from a silicon wafer to form structures like membranes and diaphragms. Can anyone share what etching is?

Isn't etching the process where you remove layers of material by using chemicals or other methods?

Exactly! We use wet etching with solutions like KOH, which selectively dissolve silicon. It leads to angled sidewalls due to its anisotropic nature. Can anyone explain what anisotropic means?

It means that the etching happens more in one direction than another, right?

Great! Now, dry etching like RIE gives us better precision and vertical walls. Why do you think this matters?

It’s important for creating very detailed designs in MEMS devices, like accelerometers.

Correct! So, remember the key points: bulk micromachining uses etching to create structures, and the choice between wet and dry etching affects precision and wall characteristics.

Now, let’s look at surface micromachining. Unlike bulk micromachining, which etches into the silicon, this technique builds structures layer by layer on the surface. Can anyone think of an advantage of this approach?

It allows for more complex designs, right?

Exactly! By using materials like polysilicon for the structural layer and silicon dioxide for sacrificial layers, we can integrate electronic components easily. What’s a typical application you can think of?

Maybe micro gears and RF MEMS switches?

Yes! Complex microstructures like gears benefit greatly from surface micromachining. Remember that the sacrificial layers must be removed after creating the movable parts; this is key!

Next, we cover High-Aspect-Ratio Micromachining (HARMS). Why do you think we would create structures with high aspect ratios?

For applications that need tall and narrow geometries, like in biomedical implants.

Exactly! Techniques like the LIGA process and DRIE are fundamental here. Can anyone explain what the LIGA process involves?

It combines deep X-ray lithography with electroplating, right?

Correct! It produces very fine and high structures, which are crucial for specialized devices like microturbines or microfluidic channels. Remember this technique for its critical applications!

Let’s dive into wafer bonding. This technique is vital for creating multi-layered MEMS devices. What methods can you recall from our readings?

There’s anodic bonding, which bonds silicon to glass with heat and an electric field.

Good! And what about fusion bonding?

That one reconnects silicon to silicon at a very high surface flatness!

Yes! Remember, wafer bonding methods ensure we can build complex structures and seal spaces like fluidic channels safely. This integration is crucial, especially in pressure sensors and microfluidic systems.

Lastly, we’ll explore soft lithography techniques which use polymers for fabricating flexible MEMS. What processes can you remember?

I think there’s replica molding where you cast PDMS onto a mold?

Exactly! And why do you think PDMS is commonly used?

It’s flexible and bio-compatible, perfect for medical applications!

Correct! Soft lithography is instrumental for devices like lab-on-chip systems and wearable sensors. Keep in mind the benefits of flexibility and lower costs in MEMS fabrication when using this technique.