One of the most widely used methods for creating nanoscale circuit components is Photolithography. In the 1820's a French scientist by the name of Nicephore Niepce developed the first photoresist, a component fundamental to photolithography. A photoresist is a substance that undergoes a chemical reaction when it is exposed to light. Niepce's photoresist was a material known as Bitumen of Judea, a kind of naturally occurring asphalt. A sheet of stone, metal, or glass was coated with a thin layer of this bitumen, which became less soluble where it was exposed to light. Areas that were unexposed could then be removed using a solvent, and the resultant exposed areas of the sheet were etched using a chemical bath. After the remaining photoresist was removed, the sheet could then be used as a printing plate.
Photolithography today is in many ways similar to the original process invented by Niepce. In general, modern photolithography involves a procedure with six steps:
In photolithography, a wafer is a thin slice of semi conductive material, such as crystalline silicon, that forms the base for the photolithographic process. Before the wafer can be used it must first undergo several steps of cleaning and preparation to remove organic contaminants and prepare it chemically for application of a photoresist.
Photoresist can be applied to the wafer in several ways, most of which make use of a device know as a "spin coater". A spin coater is composed of a small drum with a rotating disk inside. The wafer is placed on the disk, where suction from a vacuum holds it in place, and the wafer is rotated at a predetermined speed. In most laboratory applications, resist is added before rotation by applying a few drops of resist to the center of the wafer using a pipette. The spin coater is then activated, and the photoresist is dispersed across the wafer using a two step process. In the first step the wafer is rotated at a speed of about 500 rpm for about 30 seconds. This evenly distributes the resist across the entire surface of the wafer. At this point the spin coater accelerates, and the rotation speed and spin time of the wafer, combined with the viscosity of the photoresist, determine the final thickness of the resist coating. Photoresist coatings are generally 1-6 microns thick, though thicker coatings are sometimes used in commercial applications.
After application of the photoresist, the wafer undergoes a post-apply bake(also termed a "softbake" or "prebake") to stabilize the photoresist prior to exposure. This step also increases adhesion between the resist and the wafer, and helps prevent contamination of the sample by airborne particles. The prebake process is often performed using a hotplate. Though temperature and bake time differ slightly depending on the photoresist used, typical prebake conditions require a temperature between 90 and 100 degrees Celsius for a period of 60-90 seconds. A common method used to test if the postapply bake is complete is performed by touching the edge of the sample with tweezers. If the surface of the wafer is tacky or if a gentle touch by the tweezers leaves a mark on the surface of the sample, the bake process is not yet complete.
Once the prebake is complete, the wafer is ready for alignment and exposure. In most forms of photolithography, the design to be created on the resist coated wafer is dictated using a photomask. A photomask generally consists of a transparent glass sheet on which is printed the pattern to be copied to the wafer surface. Darkened areas of the photomask prevent penetration of light to the resist below, while transparent areas of the mask allow the resist beneath to react chemically. In the alignment stage of photolithography, the photomask is carefully aligned with the resist coated wafer. In some commercial applications the mask is elevated a few nanometers above the surface of the mask so as to prevent long term damage to the mask by chemicals in the resist, but in most laboratory settings the mask is placed in direct contact with the wafer so as to achieve the minimum feature size in the resist pattern.
