The process of photolithography uses photo light (UV light ~375nm wavelength) to pattern thin films of specialized organic materials (also known as photoresist or resist). Photoresist is a light sensitive material which changes in its chemical structure when exposed to UV light. Chemically modified photoresist is then removed in a chemical solution (also known as developer) to form a pattern of photoresist. Typically, a physical mask (also known as photomask) is used to control the exposed area of photoresist that sees a UV light.
Step-by-Step illustration of Photo Lithography Process:
STEP 1: Start with Si wafer (substrate) with Resist on top
STEP 2: Purchase (or Fabricate) mask (Cr patterened on Glass)
STEP 3: Flip mask and place on top of Resist coated Si. Typically, there is no separation between the two. Expose UV (~375nm) light above the glass mask (not shown in picture)
STEP 4: Part of the resist exposed to UV light is chemically modified (shown in darker color)
STEP 5(a): Substrate is kept in chemical solution (developer) (not shown). If chemically modified resist is removed after development, the process is called Positive lithography.
STEP 5(b): Substrate is kept in chemical solution (developer) (not shown). If chemically modified resist stays after development, the process is called Negative lithography.
Alternate to mask based Photo Lithography Process:
UV laser (375nm) is another alternative to mask based UV lithography. Here, the UV laser is rastered on the resist coated substrate. UV laser motion (or areas exposed) is controlled by a design drawn in AutoCAD. Laser based exposure is often a slower photolithography process ( ~ hours) in comparison to mask based exposure (~ min). However, the UV laser provides the repeatability and feature capability down to 0.6 micro. Most applications of UV laser lithography include designing a one time photomask (Resist/Cr/Glass) or writing features with size below 1 micro. IMSE offers the Heidelberg UV Laser Writer to accomplish this process. Click the link to learn more and start the training process.