Development and inspection

The exposed wafers are processed using either dip or spray development. While in the dip etch step an entire batch can be developed in a caustic solution and then rinsed, in spray development each wafer is processed individually. As in resist coating, the wafer sits on a chuck and is continuously sprayed with developer solution while rotating slowly. Once the resist has been fully developed, the wafer is rinsed with sprayed water to stop the development process. Some advantages of spray development over the dip process are as follows:

• the smallest structures can be exposed
• the developer solution is continuously renewed: contamination is prevented
• the amount of developer solution used is considerably lower

During development, certain areas of the resist dissolve, depending on the resist type, so that a patterned wafer remains at the end. Exposure triggers a reaction in the resist in which the sensitizer is converted into an acid. During development, this carboxylic acid is converted into a water-soluble salt with sodium hydroxide (NaOH) according to the following equation:

Since residues of the developer remain on the wafer with potassium or sodium hydroxide solutions, metal-ion-free developers such as TMAH (tetramethylammonium hydroxide) are also used. A further curing step (hard-bake) makes the resist resistant for subsequent processes, such as etching or ion implantation.

The resist structure is now inspected. Under oblique light incidence, a microscope can be used to detect the uniformity of the resist layer, as well as poor focusing and resist accumulation. If the resist lines are too thick or too thin, the resist has to be removed and the process repeated. Likewise, the resist structure must be precisely aligned to the underlying layer, otherwise the coating and exposure processes also have to be repeated. For this purpose, there are various alignment marks for checking the alignment accuracy and the line width:

The width of the resist lines is checked with a microscope: light rays strike the wafer perpendicularly and are not scattered back to the objective at edges. This makes black lines visible as boundaries of the structures, and by measuring their spacing with the aid of the microscope's magnification, the width of the lines can be calculated.