Photolithography

While investigating our Electronics Lab, it became clear that we have equipment and capabilities beyond the average electronics lab.  I was amazed to find that for any invention prototype in need of an electronics board, we have four different techniques, which can be utilized alone or in combination to create one or many: direct cutting by using a routing machine, pulse platting (similar to electroplating), vacuum deposition metallization, and photolithography. As a result of this “exposure”, I became interested in the photolighography process, and how it was developed.

Photolithography combines photography and lithography to generate an image and can be utilized for a variety of applications. In 1796, Alois Senefelder, a German actor and playwright, invented lithography as an inexpensive method for publishing his theatrical works.  It was the first new printmaking technique to emerge in nearly 300 years (the first originated from Johannes Gutenberg sometime around 1439, after whom one of the IV Lab’s buildings is named).

Lithography is a method of printmaking using a stone, metal, or polyester plate with a completely smooth surface. Lithography works because of the mutual repulsion of oil and water. The image is drawn on the surface of the print plate with an oil-based medium (hydrophobic) such as a wax crayon, while the negative image remains water-retaining (hydrophilic). When the plate is introduced to a compatible printing ink and water mixture, the ink will adhere to the positive image and the water will clean the negative image.

"A Lithographic Press". Pen lithograph, c1820, drawn and published by Rudolf Ackermann, leading publisher and printseller in Regency London.

Today, lithography is used to produce posters, maps, books, newspapers, and packaging.  Just about any mass-produced item with print and graphics takes advantage of the lithographic process.  In addition to high-volume printing, lithography is also used by artists to create fine art print additions. Learn more about the lithographic process in this informative video by the Frist Center displayed in conjunction with a Chuck Close exhibition.

Photography creates images by recording light or other electromagnetic radiation, either chemically or through a light sensitive material. In 1826, Joseph Niepce produced the world’s first photograph on a polished pewter plate. The light-sensitive material he used was a thin coating of bitumen, a natural petroleum tar.  After a long exposure (8+ hours) the bitumen was sufficiently hardened where exposed to light and the unhardened part was removed with a solvent.  This created a positive image with the light regions represented by hardened bitumen and the dark regions by bare pewter.

"View from the Window at Le Gras" Niepce's earliest surviving photograph, circa 1826.

Photolithography shares some fundamental principles with photography, specifically through the use of light sensitive materials. Photolithography was developed from the asphaltum process, which uses a grainy lithographic stone coated in bitumen, which is exposed to UV light to generate an image on the surface. The coated stone is then inked, and printed to produce a very course half-tone image.

Alphonse Poitevin experimented with the asphaltum process and eventually substituted photosensitive dichromated albumen for the bitumen, which he was able to wash in water to produce a planographic, or smooth, surface for printing.  This produced a cleaner and more detailed image. Poitevin’s 1855 invention is the basis for the photolithographic transfer process we use today.

Just as Poitevin’s process used light to generate an image, photolithography today uses light to transfer a pattern from a photomask to a light-sensitive polymer, or photoresist, which hardens when exposed to ultra-violet (UV) light.

For example if you wanted to transfer a pattern onto a copper plate, you would first create a photomask, or opaque material with the desired pattern as a transparent, allowing light to shine through the pattern.  Next, you would cover the surface of the copper plate with a photoresist material and place the photomask above it to control light exposure.  Now, expose the photomask and photoresist to UV light (the intensity and length of exposure is determined by the type of photoresist). After exposing the photomask and photoresist surface to UV light, the copper plate is left with areas of hardened and unhardened photoresist. A solvent is then used to dissolve the unhardened photoresist revealing the transferred pattern.  Once the surface is exposed, a series of chemical treatments that either engraves the exposed pattern into the surface, or a new material is deposited into the pattern. This is a fairly basic description of photolithography; for a more detailed visual description of the photolithography process check out lithography expert Chris A. Mack’s website.

Photolithography is used in a variety of industries including: offset printing, screen printing, electronic board, circuit board, semiconductor, and even special products such as LCD displays or ink jet printers.  It is used in micro fabrication because it can produce extremely small patterns down to tens of nanometers in size, and provides exact control over the shape and size of the pattern or image and is very cost effective.

In our Electronic’s Lab we utilize photolithography to create unique micro patterns for invention prototypes.  Most recently, we took advantage of the photolithography process as one step in the development of our Metamaterials Surface Antenna Technology.

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3 Comments

  1. Steve
    Posted November 29, 2012 at 2:31 pm | Permalink

    Ever think about dichromatic Photolithography?

    • Katie Miller
      Posted December 7, 2012 at 8:20 am | Permalink

      Dichromatic photolithography is yet another interesting process. To my knowledge we have not utilized this technique in our lab, but if a prototype would benefit from the process, I am sure we would take advantage of it.

  2. Luna
    Posted January 8, 2013 at 8:58 am | Permalink

    Woow, you can’t believe how helpful this was to my project. THANK YOU

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