Monday, July 29, 2013

Photolithography in the Clean Room

At Boston University's Photonics Center (PHO) there is a "Cleanroom" also known as the Optoelectronic Processing Facility (OPF).  OPF is a multi-user cleanroom, or a Shared Facility at BU, meaning that all of the equipment inside this lab is available for any faculty/staff member or student to use as long as they are trained or have the assistance of the laboratory manager, Mr. Paul Mak.  The cleanroom gets it's name because it's "so clean." The room has less particles in the air than any ordinary laboratory room, so if a researcher is examining a super small device/machine in the cleanroom, then there would be a less likely hood of a particle of dust in the air contaminating the device.  There are actually two cleanrooms at BU, one is the Class 1,000 Cleanroom and the other is
Helen Fawcett Outside
the Class 100 cleanroom. Which cleanroom do you think is "cleaner"? You'd be right, if you said that the Class 100 cleanroom is cleaner because the Class 100 cleanroom has less particles in the air that the Class 1,000 cleanroom. In the picture above and on the right, you will see (from Left to Right) Ben Fawcett (high school student), Jessica Leach (RET), and myself dressed up in funny-looking "bunny suits."  We need to wear these outfits in order to enter the cleanroom so that we do not bring in any unwanted particles from our hair, skin, clothes, shoes, or hands; we're also wearing safety goggles to protect our eyes from anything harmful inside the lab.  To the left, you'll see Helen Fawcett (Manager of Operations and Technical Programs and Primary Investigator of RET at BU) is standing outside the cleanroom.  The doors are sealed off and need to remain shut in order to maintain the Class 100 cleanroom.

Inside Class 100 cleanroom
  I was lucky enough to spend a day in the cleanroom using a process of Photolithography.  Using photolithography, I was able to create/print a design onto a clear transparency, which served as the mask which allows only certain areas to allow light to shine through.  After the mask was printed, we used the clean room to deposit and etch away the design onto a silicon wafer. In the photo to the right, you will see that I am placing a silicon wafer (looks like a small CD disk with out a hole in the center) onto the spinner.  I needed to use special tweezers to pick up and hold the silicon wafer on it's edge so that I didn't contaminate/dirty the silicon wafer with a fingerprint before we even started!

Miss Lagas' Mask
Pouring Photoresist
  To the left, you will see Paul Mak (laboratory manager) monitoring me as I poured the photoresist onto the silicon wafer, which is on the spinner; the spinner will spin the silicon wafer and as it does this, it forces the photoresist to spread out into a thin and even layer on the silicon wafer.  The photoresist kind of works like film in an old 35mm camera (before the ages of digital photography). The film inside the 35mm camera is exposed to light and the image that is being taken, with some areas having more or less light come through to the film.  During
Deposited Gold
photolithography, the photoresist is exposed to a certain amount of UV light and whatever part of the photoresist that is exposed to the UV light will get washed away. After photoresist is poured onto the silicon wafer then the wafer is placed under UV light, but the mask (the clear transparency with my design) is placed between the UV and the wafer so that only the blank areas of the design are exposed to the UV light.  Once the UV light exposure is complete, then gold (yes gold!) is deposited onto the silicon wafer and then the silicon wafer looks like a sheet of gold, until the wafer is placed in an acetone wash, which you can watch below!  Look for the finished product on Miss Lagas' desk when we're back at school!


  1. Dear Miss,
    You have a great article, Can I use it in my blog. Thanks in advance

    1. This comment has been removed by the author.

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