An in situ XRF system for composition mapping of thin film IR sensors

Abstract

Ge 1-x Sn x thin films are capable of detecting mid- to long IR wavelengths. However, due to the size difference between Ge and Sn atoms, homogeneous alloys are difficult to manufacture in thicknesses greater than a couple hundred nanometers. To address the difficulty in manufacturing these IR detecting alloys, we developed an in situ-capable x-ray fluorescence (XRF) system to characterize and monitor thin film composition and concentration information as a function of position during thin film growth. The inclusion of a focusing polycapillary optic enabled the x-ray source and detector distances from the sample to be large enough for placement outside the deposition chamber. RF sputtered, metastable Ge 1-x Sn x thin films (∼ 4 μm, ∼ 8 pm) were compositionally mapped with the XRF system. Since Sn phase separation destroys the IR capabilities of the Ge 1-x Sn x sensor, it is imperative to prevent this during deposition. Additionally, to produce an IR sensing array, more gradual changes in composition are desired. With our system, both Sn phase separation and more gradual changes in alloyed Sn concentrations were observed in 1000 s. However, the signal-to-noise ratio was such that 100 s would have been sufficient, meaning this system could be used in situ to characterize the alloy composition during deposition. A spatial resolution of ± 25 μm was obtained by oversampling with the focusing optic (100 μm spot size). From these measurements, the minimum detectable limits were on the order of nanograms using the Sn-L α signal, which corresponds to picograms from a Sn-K α signal. Such low levels are usually only possible with a rotating anode source 10 3 times more powerful than the low power sealed tube source used in this experiment. Additional ultra thin samples (< 100 nm) made by ion implantation were also analyzed with this XRF system. Sn was ion implanted into single crystal Ge, resulting in a sample representative of early stage thin film growth. In 300 s, a detectable signal was obtained, indicating the viability of this system for in situ, thin film, composition monitoring and characterization of uniformly alloyed metastable semiconductors for IR sensors.