Photolithography

Almost all top-down manufacturing involves one or more photolithography fabricationsteps, so we give a brief outline of this technique here. From a lightsource light is directed through a mask carrying the circuit design down onto the substrate wafer covered with a photo-sensitive film, denoted the photoresist. Depending on the local photo-exposure defined by the photolithographic mask the photoresist can be partly removed by a chemical developer leaving well-defined parts of the substrate wafer exposed to etching or metal deposition. The substrate wafer is typically a very pure silicon disk with a thickness around 500 μm and diameter of 100 mm (for historic reasons denoted a 4 inch wafer). Wafers of different purities are purchased at various manufacturers. The photolithographic mask contains (part of) the design of the microsystem that is to be fabricated. This design is created using computer-aided design (CAD) software. Once completed the computer file containing the design is sent to a company producing the mask. At the company the design is transferred to a glass plate covered with a thin but non-transparent layer of chromium. The transfer process is normally based on either the relatively cheap and fast laser writing with a resolution of approximately 1.5 μm and a delivery time of around two weeks, or the expensive and rather slow electron beam writing with a resolution of 0.2 μm and a delivery time of several months. The photo exposure is typically performed using the 356 nm UV line from a mercury lamp, but to achieve the line widths of sub 100 nm mentioned in Sec. 1.1 an extreme UV source or even an X-ray source is needed. To achieve the best resolution must minimize note only the wavelength λ of the exposure light, but also the distance d between the photolithographic mask and the photoresist-covered substrate wafer, and the thickness t of the photoresist layer. The minimum line width wmin is given by the approximate expression

wmin = 3 /2 ( λ(d + t). ) ^½

If d = 0 nm the mask is touching the photo-resist. This situation, denoted contact printing,improves the resolution but wears down the mask. If d > 0 nm, a case denoted proximity printing, the resolution is pourer but the mask may last longer. It is difficult to obtain wmin < 2 μm using standard UV photolithography. The photoresist is a typically a melted and thus fluid polymer that is put on the substrate wafer, which then is rotated at more than 1000 rounds per minute to ensure an even and thin layer of resist spreading on the wafer. The photoresists carry exotic names like SU-8, PMMA, AZ4562 and Kodak 747. The solubility of the resists is proportional to the square of the molecular weight of the polymer. The photo-processes in a polymer photoresist will either cut the polymer chains in small pieces (chain scission) and thus lower the molecular weight, or they will induces cross-linking between the polymer chains and thus increase the molecular weight. The first type of resists is denoted the positive tone photoresists, they will be removed where they have been exposed to light. The second type is denoted the negative tone photoresists, they will remain where they have been exposed to light.