What is Clean?

by Megan Harberts

Have you ever heard of a cleanroom before? You probably think that is what your parents want your room to be, but what does it mean for a laboratory to be in a cleanroom?  Have you ever seen a picture of someone wearing a white suit in a yellow room with a bunch of equipment?


Nanosystems Laboratory (NSL) cleanroom at The Ohio State University

That is one type of cleanroom used for scientific research, as well as manufacturing of circuits and solar cells and I am a graduate student at OSU who does research in one of these rooms.

So what does it mean for the room to be clean and why do you have to wear a special suit?  Cleanrooms provide an environment where there are very few particles in the air. More and more technology is being done at the nanoscale, this means a lot of the components are on the order of nanometers.  There are 109 nanometers in one meter.  Typical dust particles are on the order of microns, there are 106 microns in one meter.  Therefore, a single dust particle landing on a device can ruin it by bridging connections or introducing unwanted impurities.

How can you keep a room free of dust particles? Well, where do the particles come from?  Some are from dirt and other things outside, others are from dead skin cells and fibers. One way to keep from adding new particles to the room is to wear a suit to keep particles from coming off your body and to keep things out of the room like fabric and paper which tend to shed fibers.

Before the 1960’s, the best practice was to wear a suit, and keep the room sealed as best as they could. Like you do when you clean your room, they tried to remove particles that got into the room by vacuuming, using a special vacuum that sucks up and traps small particles with a filter to prevent them from getting out of the vacuum, but when particles got into the room they still caused problems.  This all changed when physicist Willis Whitfield invented the laminar flow cleanroom, which revolutionized the way cleanroom processes worked.  The idea is to constantly cycle filtered air into the room.  Air coming into the room is passed through HEPA (high-efficiency particulate air) filters in the ceiling and then is passed out through vents on or near the floor so that gravity can help pull out any particles in the air.  50 years later this is still the standard for cleanrooms for research, manufacturing, and even in hospitals.

A picture of the airflow in a laminar flow cleanroom coming in through filters.

A picture of the airflow in a laminar flow cleanroom coming in through filters.

This is the basis for all cleanrooms, but they come in different classifications.  These classifications are based on the number of particles that are 0.5 microns or larger measured with an instrument called a particle counter. They range from class 1 to class 100,000.

For rooms that are the cleanest, class 1, the users have to wear a full suit and a respirator so that they don’t introduce particles from inside their bodies.  I work in a class 10,000 cleanroom, meaning we expect there to be no more than 10,000 particles in a square meter, so I only have wear a hair net, gown, booties, and gloves.

This me in my cleanroom outfit.

This is me in my cleanroom outfit.

Other ways we keep the dust level down include sticky mats that we step on when we enter the lab and vacuuming every week.  The doors to our lab are not sealed so our lab is kept at positive pressure so air always flows out of the room around the doors and the only air that flows in passes through filters.

Sticky mat at the entrance to the cleanroom.

Sticky mat at the entrance to the cleanroom.

As I mentioned earlier, a lot of cleanrooms have yellow light and I always wondered why before I started working in one.  It turns out that the yellow lights and window covering are not necessary for keeping the room clean, but they are necessary for doing photolithography which is common process done in cleanrooms. As I said before, a lot of new devices have components on the scale of nanometers and one of the ways to make small circuits and devices is with photolithography.

Photolithography is similar to how pictures are developed. During device processing, a sample is covered with a light sensitive material called photoresist, which does not react to yellow light. The photoresist covered sample is then exposed to ultra violet light under a patterned mask.  Only parts of the photoresist on the sample will be removed when it is placed in a developer solution. The photoresist that remains protects the sample to allow somebody to now either remove material from the exposed parts, called etching, or cover them with an additional material like a metal for contact pads. Later the photoresist can be removed and you can end up with something like my sample with a specific pattern.  In my research, we have to have small contacts on the sample because the materials are can be hard to process on a larger scale and the tools we have for measuring do not leave much room.

Here is a one of my samples which is about 1 cm long on each side with a very small circuit on it. The mask covered the clear parts during the processing.

Here is a one of my samples, which is about 1 cm long on each side, with a very small circuit on it. The mask covered the clear parts during the processing.

Photolithography is just one of many types of processes that are done in cleanrooms.  While it can be cumbersome to have to put on all those extra layers every time you go to work, doing work in a dust free environment ensures that very small devices can be made consistently, which is important for both research and manufacturing. This tells you a little bit about what clean means for my research, but the question, “What is Clean?” can mean very different things if you work in a biology lab or even when it comes to your room.


About Megan Harberts

picture of me

I am one of the founders of “A Day in the Life” and a current graduate student at Ohio State University working on my PhD in condensed matter physics.  Outside of school and working on the blog, I play water polo for the OSU club team. Leave a comment or find me on Twitter @meganharberts if you want to know more!