Photolithography is the word applied to the process of using photographic or optical means to produce a photoresist mask for a printed circuit board. Photolithography as it applies to circuit boards involves two steps. The first part is creating the "artwork" that is to be used for the second part, which is "masking" with the photoresist.
The ultimate goal is to produce a circuit board that is the proper size to allow the components to fit properly. To this end, photography may be used to guarantee the final artwork is essentially the exact size required, by enlarging or reducing the master. The master may be produced in several ways:
The goal of any of these methods is to produce "camera ready" artwork. This is artwork that has had everything done to it to make it ready to be converted into a photographic mask for making the resist patterns on the photoresist coated printed circuit board.
There are options as to how the photomask is used to create the resist mask, as well. Each of these processes may have a positive or negative version.
Coated circuit boards are available from suppliers, or you may get a spray photoresist (from the same suppliers). The type you use is determined by two things: the type of final artwork you have and the type of etch process you intend to use. If you are going to use a photographic negative to create a positive (cover the areas to be preserved), you would use a negative photoresist. If you are going to use a photographic positive to create a positive mask, you would use a positive photoresist. These are the most common cases.
If you generate your artwork by computer, and make the output actual size, you will almost always use the positive method. If you generate your artwork at 2x or 4x, and have it reduced, you will almost always use the negative method, since the result of the photographic reduction is a negative image. You may also pay a bit more and get positive contacts made from the negative, if you want to use the positive process. It is a matter of preference. The positive method is more widely used.
Negative photoresist is a coating that is hardened by exposure to UV light. A negative image of the circuit board is placed in contact with the photoresist by clamping in a frame made of glass. A UV light, such as a mercury vapor or bug zapper bulb, is placed at a distance of several feet, and the frame is exposed for several minutes. The circuit board is developed in a negative resist developer and rinsed. A cotton ball may be used to very lightly brush off the partially dissolved resist while holding the circuit board in distilled water. The circuit board is then etched in your chosen etchant.
Negative photoresist cost less overall than positive photoresist, but gives about 1/4 the maximum resolution of the positive photoresist. Negative photoresist is still the most common for dry films, and the least for pre-sensitized PC boards.
Positive photoresist is a coating that is softened by exposure to UV light. A positive image of the circuit board is placed in contact with the photoresist by clamping in a frame made of glass. A UV light, such as a mercury vapor or bug zapper bulb, is placed at a distance of several feet, and the frame is exposed for several minutes. The circuit board is developed in a positive resist developer and rinsed. A cotton ball may be used to very lightly brush off the partially dissolved resist while holding the circuit board in distilled water. The circuit board is then etched in your chosen etchant.
Positive photoresist costs more overall than negative photoresist, and gives about 4 times the resolution. This can be an important benefit if you need to make a very tight circuit board and the rest of your process is up to the task.
Resist films are sheet material applied to the surface of a cleaned circuit board, usually by heat and pressure from a specialized laminator. Once applied, they are not too different from a negative photoresist coating. They tend to have a more even thickness across the surface of the circuit board, are generally thinner, and have better resolution than liquid photoresists. They are available from several sources, and can be applied using water and a squeegee, then dried with a hair dryer or oven. Puretch dry film photoresist is a film used for any etching, including circuit boards. eBay sources for dry film photoresist. The circuit board is is developed in a very weak soda ash solution.
There are several sources of presensitized PCB blanks, with either positive photoresist or negative photoresist already on the copper. These are generally dry film coated, because it is easier in a production environment than liquid flood or photoresist spray. MG Chemicals seems to be most abundant in my area.