Welcome, I'm Nan Jokerst, And this is our in-depth video about photo lithography. The term photo lithography is used to describe a process in which we use light to print or pattern the surface of a substrate or a metal or dielectric on the substrate. This type of patterning is used a great deal when we make devices, interconnections and structures. For example, the metal interconnect structures on an integrated circuit are very complex and often use over 10 different layers of patterned metal. There are two types of lithography or patterning that we'll discuss in this course. Photo lithography and electron beam lithography. In this video, we will talk about photo lithography, which uses light to transfer a pattern to a substrate. First, we identify what pattern we want to make on our substrate. For example, we might want to pattern a layer of metal that has been deposited onto a silicon substrate as shown here. If there are devices such as transistors in the silicon, then we might want to pattern the metal interconnect to connect the transistors in a certain pattern to make a circuit. A key component in Photolithoraphy Is the photo mask or mask that has the pattern we want to transfer to the wafer. A photo mask is a transparent plate, usually glass or quartz. It has a thin metal pattern. The mask is transparent to light everywhere, but where the metal lies on the mask. This metal will block the light that patterns the substrate. And so, the metal is the pattern that we transfer to the substrate. How do we make a photo mask? Well, photo masks are designed by you the user on a computer, usually using a computer aided design or CAD drawing program. Then, we send the computer file to a commercial photo mask vendor, and the mask is fabricated by the supplier and shipped to us within a few days. Here's an example of a photo mask. Photo mask sizes vary depending upon the size of your substrate. On this photo mask, we can see that the thin metal forms a very complex pattern. Some regions are transparent while other regions have the metal film that make them opaque. To transfer the pattern from our mask onto the wafer, we use a thin polymer film that is sensitive to light. When the polymer is exposed to light through our mask, the polymer is patterned by the light. This light sensitive polymer is called Photo resist. The particular type of photolithography that we will discuss, is called contact photolithography. Contact photolithography is typically used to pattern shapes that are as large as a few centimeters in size, down to about 1 micro meter or we say 1 micron. To start the process, the wafer is first coated with a thin layer of this polymer photo resist using a process known as spin coating. Spin coating is accomplished by depositing a few milliliters of liquid Polymer onto the substrate and spinning the substrate at high speeds. Usually, 3,000 revolutions per minute is common and we spin for 30 seconds to one minute. This spinning process causes the liquid polymer to spread evenly over the wafer forming a uniform thin photo resist film. This is important for high yield of our transferred pattern across the entire substrate because the uniformity of that photo resist thickness across the entire substrate, will ensure that our pattern is transferred accurately across the entire wafer. The wave front in photo resist coating are then heated on a hot plate for one minute to a temperature of about 100 degree Celsius. Note however, that there are many types of photo resist and it's important to follow the instructions for your particular photo resist. Now let's take a closer look at the substrate coated with photo resist. Here's a close up cross-sectional view of the substrate and photo resist. The photo resist thickness may range from a fraction of a micron to 10 or 20 microns or more depending upon the desired process, while a substrate thickness is usually several hundred microns. To transfer our pattern from the mask to the photo resist, we use a tool called a Mask Aligner, to position the mask at the right place with respect to the substrate and to illuminate the photo resist with ultraviolet light, also called UV light. These two steps are referred to as alignment and UV exposure. Here's a photo of a Mask Aligner. The mask aligner is very important, because it allows us to position many pattern layers of material with each layer aligned to each other as they're patterned and we build up a multi layer structure. Now that our substrate is coated with photo resist, we bring the mask into contact with the photo resist after it's aligned using the mask aligner. We then illuminate the mask from above with ultraviolet light. The UV light passes through the transparent glass portions of the photo mask and the areas of the photo mask containing metal will block the UV light. So we're essentially casting a shadow onto the photo resist where the shape of this shadow is defined by that pattern on our photo mask. The portions of the photo resist that are exposed to the UV light will undergo a chemical change that's indicated here with dotted white lines. Remember, photo resist is just sensitive to light. So this light induced chemical change, creates a pattern in the photo resist. Now we remove the mask and the substrate from the mask aligner. We have what's called an exposed substrate. That is, the substrate and photo resist have undergone that UV light exposure and the photo resist is chemically altered in our desired pattern. The next step involves submerging the exposed substrate into a chemical bath known as developer. The developer dissolves the photo resist that was exposed to UV light, but it does not dissolve the photo resist that was not exposed to UV light. So this process is called the develop step and usually takes about one minute. We then remove the substrate from the developer, rinse the substrate with deionized water, and blow it dry with nitrogen gas. Our substrate needs to be really clean for high yield. Deionized water is specifically filtered high purity water used in many chemical processes. Using DI water eliminates trace minerals in water that would act as contaminants to our sensitive chemical processes. Drying with pressurized nitrogen gas ensures that no water remains on our substrate surface. Now we have a photo resist pattern on our substrate. The thin photo resist film contains the same pattern that was on our photo mask. But photo resist is typically only a temporary layer. The patterned photo resist is not permanent, but rather we're going to use it to perform some other processing steps. These steps combined with photolithography are those that are used to make permanent patterned features on our substrate. Let's look at an example of how we can use photo resist patterning to make a permanent pattern on a silicon substrate. An example of such a process is as follows: we start with our silicon substrate as shown here. Then, we vacuum deposit a layer of gold as shown here. Then we perform the photolithography steps just described. So then, we have a pattern of photo resist on top of our gold. We then place this sample in a liquid acid that etches the gold, but does not harm the photo resist and only the exposed gold is etched. Then, we remove the remaining photo resist leaving only the gold pattern on the silicon as shown here. I hope you enjoyed this lesson. Thank you for joining me today.
