A Novel Technique for the Simultaneous Collection of Reflection and Transmission Data from Thin Films in the Extreme Ultraviolet

Studies of thin films in the Extreme Ultraviolet (EUV) are difficult given that most materials readily absorb photons of these energies. By depositing a thin film of the material of interest on a silicon photodiode, transmission measurements can be made throughout the EUV. If the measurements are made in a range of low absorption, the extinction coefficient, k, can be found with relative ease. However, if the material’s absorption is considerable, reflection measurements are needed to supplement the transmission data in order to find the optical constants n and k. The technique developed allows for reflection and transmission measurements to be taken simultaneously, which combined, account for all of the measurable photons from the original beam: (those which cannot be counted are photons absorbed into the thin film material). Also, the technique presented allows for data to be collected from practically all angles of incidence. This technique has been applied to a thin film of scandium oxide (d=65 nm), with measurements taken over wavelengths from 2.5-25 nm, and at angles of incidence 12 degrees from grazing to normal. Introduction Since most materials have yet to be definitively studied in the Extreme Ultraviolet (EUV), experiments which minimize the number of unknowns present in the model describing the data are preferred. For studying thin films in the EUV, transmission measurements are an excellent first step in developing an optical understanding of a material. Subsequent experiments build upon the information found from the transmission experiments. Thin film samples are prepared by depositing thin films directly on the face of photodiodes. This process greatly facilitates the collection of transmission data, as all photons transmitted through the film are guaranteed to be incident upon the detector face. The transmission data can then be used to determine the extinction coefficient k (which is the imaginary portion of the complex index of refraction, N = n + ik ) of the deposited film. Successive reflection measurements of the same sample, can be used to calculate the refractive index using the newly determined extinction coefficient. A schematic illustrating the use of a photodiode with a thin film on its detection surface is offered in Figure 1. The optical properties of thin films in the EUV are sensitive to a variety of parameters. Among these are: film thickness, presence of impurities, interface quality, film density, and grain size and orientation. Each of the listed items are nanoscopic in scale, such that even subtle variations in of one of the variables will effect the optical performance of the sample. Also, since it is difficult to ensure the uniformity of a thin film with regard to any of these physical parameters, the correlation between the reflection and transmission data may not be optimal if the Figure 1 The detection face of a photodiode is coated with a thin film to be studied. In doing so, the signal reported by the photodiode corresponds to photons that have traveled through the thin film. With the use of a second, auxiliary detector, reflection measurements of the thin film can be made. At the bottom of the figure, distribution of the photons is illustrated—the energy from the incident will be present in the reflected and transmitted beams, or lost as photons are absorbed by the thin film (suggested by the dashed line representing the transmission beam). Tranmission Measurement Incident energy can go to only three places Reflection Measurement Au xi lia ry de te ct or Bare