2.2 Operation Principle

An optical lithography tool generally consists of an illuminator, a photomask, an optical system, and the photoresist spinned on top of the wafer. These basic system components are assembled as shown in Figure 2.3. The operation principle is also demonstrated in Figure 2.3. It is based on the ability of the resist to store an image of the pattern to be printed. The mask already carrying this pattern is flooded with light and the optical system images all parts of the mask simultaneously onto the resist. Exactly, this parallelism is the crucial advantage of optical lithography as it guarantees the high wafer throughput. The light intensity on top of the resist surface is commonly called aerial image. The resist itself is a photosensitive material that changes its chemical composition during light exposure. The pattern is thereby stored in form of a latent bulk image within the resist. During immersion in a developer solution the exposed parts of the resist remain or dissolve depending on its polarity. After the completion of the lithography process a more or less exact replica of the mask pattern is left on the wafer surface.

  

Figure 2.3: An optical lithography tool generally consists of an illuminator, a photomask, an optical system, and the photoresist spinned on top of the wafer. The lithography process is based on the ability of the photoresist to store a replica of the photomask that is used for subsequent processing steps, e.g., etching, deposition or implantation.

A more realistic and detailed sketch of a modern projection printing system is shown in Figure 2.4. The illuminator consists of a light source, an aperture, and a condenser lens. The functionality of each component is described in the next section. The photomask can be a binary chrome mask or a phase-shifting mask. Details can be found in Section 2.4. The optical system in a modern photolithography tool is a reduction projection printing system, although proximity printing becomes interesting again in combination with X-rays. Both methods are discussed in Section 2.5. The processing of the photoresist for shorter wavelengths has become a considerable problem. Some aspects are presented in Section 2.6.

  

Figure 2.4: A schematic modern optical projection printing system. Resolution enhancement is achieved by decreasing the wavelength from UV down to EUV, introducing an aperture below the light source to provide off-axis illumination, using phase-shifting masks, or implementing in-lens filters inside the projection lens. Advanced resist systems are required to resolve the image within the resist.