Photolithography(also called: Lithography) is an important step in manufacturing process of semiconductor device, the steps of using exposure and development describe the geometric structure in the photoresist layer, then etching the photomask pattern on the transfer to the substrate. The substrate here includes not only the silicon wafer, can also be other the metal layer, the dielectric layer, such as glass, SOS sapphire.
There may be as many as 50 steps of lithography in the process of producing complex integrated circuits, and the number of lithography required to produce the film is less.The photosensitive material in photolithography called photoresist lithography, mainly divided into positive photoresist and negative photoresist.Positive photoresist the part of illuminated will be retained after development, while the negative photoresist was sensitive part will be reserved after developing. Not only sensitive to the photoresist as specified light, also need to keep stable after the metal etching process.Generally,different photosensitive photoresist have different properties.
In etching, a liquid ("wet") or plasma ("dry") chemical agent removes the uppermost layer of the substrate in the areas that are not protected by photoresist. In semiconductor fabrication, dry etching techniques are generally used, as they can be made anisotropic, in order to avoid significant undercutting of the photoresist pattern. This is essential when the width of the features to be defined is similar to or less than the thickness of the material being etched (i.e. when the aspect ratio approaches unity). Wet etch processes are generally isotropic in nature, which is often indispensable for microelectromechanical systems, where suspended structures must be "released" from the underlying layer.
The development of low-defectivity anisotropic dry-etch process has enabled the ever-smaller features defined photolithographically in the resist to be transferred to the substrate material.
Removal of photoresist
After etching or ion implantation, it is no longer necessary to use photoresist as a protective layer and it can be removed
This usually requires a liquid "resist stripper", which chemically alters the resist so that it no longer adheres to the substrate. Alternatively, photoresist may be removed by a plasma containing oxygen, which oxidizes it. This process is called ashing, and resembles dry etching. Use of 1-Methyl-2-pyrrolidone (NMP) solvent for photoresist is another method used to remove an image. When the resist has been dissolved, the solvent can be removed by heating to 80 °C without leaving any residue.
The method of removing is classified as follows:
1.Organic solvent removal: Removal of photoresist by organic solvent
2.Inorganic solvents: By the use of some inorganic solvents, the organic carbon in the organic matter of the photoresist is oxidized to carbon dioxide, and then removed
3.Dry removal of photoresist: stripping photoresist by plasma
In addition to the main process, some auxiliary process also often used, such as large area uniform corrosion to reduce the thickness of the substrate, or the process of remove the edge uneven. Generally,in the production of semiconductor chips or other elements, a substrate needs to be repeated many times.
Merits and demerits
The advantage of photolithography is that it can accurately control the formation of the shape, size, In addition, it can produce a profile on the whole wafer surface.However, the main demerit is that it must be used on plane, the rough surface of its effect will be worse. It also has a very high requirement of substrate cleaning conditions.