Saturday, July 25, 2009

IMEC Update 7/25/09

I know I've neglected to discuss the research I'm doing over here in Belgium. Part of the reason is that my research groups on both sides of the pond require there own confidentiality. With respect to the Gracias lab, the work we've done with the nanocubes has not yet been published. As a result, I'm not at the liberty to provide details about their fabrication.

On the IMEC side, all employees have to sign a confidentiality waiver about not providing internal information to 3rd parties. You see these at almost every company, so it's not surprising that IMEC had their own.

Anyway, in terms of my day to day operations, I'm currently in the fabrication stage of the nanocubes. They're structurally similar to the microcontainers that we fabricate in the Gracias lab and their fabrication is based on lithography.

First, we need define a pattern for our structures. What shape/size do we want them to be? Here at IMEC, I'm currently working with structures that are approximately 500 nm, which in terms of relative size is about 200 times smaller than the width of a human hair.

In order to define a pattern, we first need to spin coat some type of resist material onto a silicon wafer. We then use photolithography or electron beam lithography to selectively remove the exposed or non-exposed portion of the film depending on whether the resist material is positive or negative. In the semiconductor industry, the go-to resist material is polymethyl methacrylate (PMMA). After exposing the film, it needs to be developed in solution such that certain areas of the film are selectively removed. After we define our pattern, we can then move onto the next step.

Material Deposition
After we have our pattern, we need to deposit some sort of metal onto a silicon wafer via thermal evaporation, electron beam evaporation, or sputter coating. In these steps, metal atoms are removed from a source and then deposited onto a sample. In terms of thermal evaporation, the metal source is heated and the solid evaporates, allowing metal atoms to rise and coat the wafer that's placed above the source. To learn more about these techniques, please consult wikipedia. There's no sense in me discussing these in depth here when there are outside sources available.

Once we have our material deposited, we need to go through a lift-off process. After completing the above steps, we have a silicon wafer coated with a resist and a metal. Depending on our pattern, some sections of the wafer consist of silicon/resist/metal and other sections are simply silicon/metal. In the lift-off process, we dissolve the resist and what remains is our pattern which is now composed of just a metal.

After repeating these processes several times, we can fabricate structures composed of different shapes and compositions.

As I mentioned earlier, for the past couple weeks I've been involved with fabricating our nanocubes. I've mostly been in the clean room making samples using the techniques I described above. All throughout the process I'm checking the condition of the samples after each step both with an optical microscope and a scanning electron microscope (SEM).

One of my highlights so far while working here at IMEC is my personal operation of the SEM. These instruments cost hundreds of thousands of dollars and it's been exciting to operate these on my own. Undergrads at Hopkins aren't trained on the Materials Science SEM, so I've never been able to operate the instrument back at school.

One of my other highlights while here at IMEC has been working with hydrogen fluoride (HF). It's a very dangerous chemical such that when it comes into contact with water it forms hydrofluoric acid, which is incredibly corrosive. It interferes with nerve function and if it contacts your skin, you probably wouldn't even notice the burn. If you've ever taken a chemical safety class you've probably seen the effects of hydrofluoric acid. If not, you can google it if you're interested. However, be warned that some images may be disturbing.

Granted, the HF I've used was buffered, so it wasn't pure HF. The risks however still apply. My first use of HF was a very memorable event and I think I'll forever associate HF with IMEC and Belgium. I admit that might have sounded vaguely nerdy, but we're all nerds at Hopkins, right?

Anyway, that's what I've been up to at IMEC the last couple weeks. Fabrication and imaging. Once I have final structures I'll be taking optical measurements of my samples. In the next week or two I should hopefully be starting that up.

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