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Nanofabrcation Series: Photolithography

Have you ever wondered how your computer or phone can store so much information into such a compact space? As we discussed in our previous blog post, scientists and engineers use a process known as nanofabrication to create the billions of tiny transistors that make up the ones and zeros that translate to computations and displayed data in our electronic devices.

Modern day microchips and microprocessors found within your electronic device are usually made using nanolithography—a molecular printing and nanofabrication technique that allows the modification of material surfaces. These structures can have the lateral dimension ranging from the size of 10’s of atoms to 100’s of nanometers (nm). For reference, the width of a single human hair is about 100,000 nm signifying the value of lithography in nanofabrication to manufacture compact devices.

To give you some idea of the nanoscale, 10 hydrogen atoms laid side by side measure a nanometer across, a strand of DNA is 2.5 nm in wide, while a red blood cell is about 7,000 nm thick, and a human hair is approximately from 100,000 nm wide. [Image credit: Conte, Raffaele. "When size matters: the importance of nanoscale for pharmaceutically active substances"]

The word “nanolithography” comes from the combination of three Greek words: “nano” (small), “lithos” (stone), and “grapho” (to write), to produce the literal translation of “small writing on stone.” Its literal meaning is consistent with the utilization of the technique—to modify the surfaces of materials at a nanoscale.

Lithography was invented by a Bavarian playwright, Alois Senefelder, in 1798 who unintentionally discovered that he was able to reproduce his scripts using greasy crayons on limestone to print multiple copies. In 1826, Joseph Niepce, a French scientist, produced the world’s first photograph, however it wasn’t until the mid-1950s, that these fundamental principles were applied in such a way to create patterned surfaces with the intention to make small electronic devices. The movement to shrink transistors from millimeters to microns and eventually to nanometer scale, was derived from the need to increase computing power with integrated circuits, following Gordon Moore’s Law or prediction that that the number of transistors per silicon chip doubles every two years (see previous blog post for more details).

If you visit a nanofabrication facility, you will find all photolithography rooms colored in seemly orange or yellow lighting. The photoresists used in the process are sensitive to UV light, so in these unique spaces, all light sources have UV filters as to not prematurely expose the polymers with ambient light. Think of it equivalently as the old dark rooms used to develop film! [Image credit: University College London Faculty of Mathematical & Physical Sciences]

Processing nano-structures using photolithography is quite similar to developing photographic films. The material surface or substrate is uniformly coated with a thin UV sensitive polymer, known as a photoresist. The coated substrate is subsequently cured. The photoresist is then covered with a photomask which acts as a stencil with opaque and transparent sections to outline the desired structures. Next, the open areas are uniformly exposed to UV light, leading them to chemically change their structural and chemical integrity. The now exposed photoresist is then soaked in a developer solution. Depending on the type of photoresist selected, the exposed photoresist areas will dissolve away in the developer, while the unexposed areas remain (known as a positive photoresist) or in the case of a negative photoresist, the exposed photoresist areas will remain, while the unexposed areas dissolve away. In both cases, the resulting substrate is now patterned with areas covered and uncovered with photoresist.

A 2D cross-section of the step-by-step process in which a positive photoresist on a substrate is coated, cured, masked, exposed, and developed. In the pictures above, the grey color represents the substrate, red represents the unexposed photoresist, black represents the masked sections created by the photomask, purple represents the exposed areas of the photoresists, and light blue represents the developer solution.

Using the photoresist as a mask to only effect the exposed areas, material is subsequently either added to surface via thin-film deposition or removed via an etching process. In the method described here, the features on the surface are a direct one-to-one translation in size (no reduction). Restricted by the diffraction of light, this technique is typically limited to approximately 500 nm using UV light. Modern photolithography is done using a technique known as projection photolithography. In this method, prior to reaching the photoresist, the light passing through the photomask passes through a series of lenses, shrinking the image and focusing it onto the substrate. In this way, along with using shorter wavelength light, modern photolithographic systems can produce features below 10 nm.

A photolithographically patterned silicon wafer after exposure and development. The dully rainbow colors in the bulk/un-patterned areas of the wafer are a result of interference generated by the semitransparent nature of the remaining photoresist. In the patterned areas, the more brilliant colors are a result of diffraction caused by the patterned surface, similar what you would find on the patterned backside of a CD or DVD. [Image credit: Jasmine Sabio]

There are several other more specialized types of lithography techniques designed to serve many other uses. Despite being expensive and time-consuming, electron beam lithography (EBL) can generate structures that can be as small as 10 nm. This utilizes tightly focused beams of electrons instead of photons (light) that disperse and expose a pattern in a polymer which can be used to create high-resolution structures. New multi-photon lithography techniques allow for the patterning of polymers in three dimensions, opening an entirely new space of applications. Scanning probe lithography (SPL) utilizes a nanoscale tip that is capable of removing or adding material in a serial method as it moves along a surface. These are just a few examples of the many emerging ways to pattern nanoscale features on surfaces.

Nanolithography is just one of the nanofabrication techniques that has revolutionized science and engineering. With varying significant uses across almost every scientific and technological application space, this indispensable technique continues to advance, creating the next generation lithography techniques with more variety of uses and capabilities. In the continuation of this nanofabrication blog series, we will cover the additional complimentary processes needed to fully realize nano-structured devices.

About the Author: Alara Tuncer is a master’s student in NYU GSAS studying Biology. Aspiring to become a science writer, she is very interested in simplifying and translating complex scientific knowledge, theories and practices to a general audience to shape social and political conversations regarding science.

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