Dosimetry for lithographic applications

Abstract

Because of the demand for miniaturization of devices, lithographic tools are becoming more expensive and sophisticated with future emphasis being placed on x rays, e beams, and ultraviolets in the range of 2200–3200 Å. It is increasingly important to be able to easily monitor exposure dose and uniformity from tool to tool in an industrial environment. This paper investigates the use of commercially available dosimeter films for lithography applications. These films consist of radiachromic dye derivatives in various polymers. Upon exposure to ultraviolet light (<3400 Å) and other ionizing radiation, these films undergo photoionization, and change from a colorless to a deeply colored state as a direct function of the radiation exposure received. Therefore, the optical density is a means of measuring the amount of incident radiation. For e-beam dosimetry, a measurable optical density change is observed between 0.1 and 20 μC/cm2 with saturation occurring at ∠25 μC/cm2. Maximum penetration range of the 25-kV beam was determined to be 20–25 μ. For x-ray application, the film is similar to e-beam, with a linear response up to 100 mJ/cm2 using the Al–Kα line. For mid UV at 3130 Å and deep UV at 2200 Å, the dosimeter is very sensitive with a 5% change in transmission occurring for every 1 mJ/cm2 dose variation.