Plasma and Thin Film Analysis
We strive to achieve a deep understanding of the fundamentals of pulsed laser deposition (PLD) of thin films in order to gain control over the process.
We make use of analytical techniques, such as XRD, SIMS, RBS and ICP-Mass spectrometry, to investigate the structure as well as the stoichiometry of the thin films deposited under different conditions. In addition, we also look at the PLD plasma properties based on in-situ plasma analysis approaches, such as mass spectrometry, emission spectrometry and plasma imaging. Combining the plasma composition with the film composition, we seek to achieve a quantitative monitoring and control of the thin film properties during a PLD process.
Combining quadrupole mass selector combining with kinetic energy selector
In order to investigate the mass and energy distribution of specific species in the plasma in more details, a quadrupole mass spectrometer with kinetic energy analyzer has been applied in our lab. This set up allows the direct measurement of kinetic energy distributions of positive, negative and neutral species with defined mass.
The mass spectroscopy setup makes a better understanding of the composition and energy distribution of the plasma to be possible. The figure below gives information about the detected positive, negative and neutral species in laser induced plasma.
We can also combine the mass spectrometer with the Multichannel Scaler system to measure the arrival time of the species in the plasma with defined mass and kinetic energies.
Mass spectrum of negative ions for a La0.4Ca0.6MnO3 ablation plasma using a 193nm ArF laser at a N2O pressure of 1.5x10-1Pa and a fluence of 1.5J/cm2.
Kinetic energy distributions for LaO+ in vacuum, O2, and N2O.
ArF excimer laser (λ=193nm, ν=5Hz), Φ=1.5J/cm2
Target to detector distance: 4cm
Langmuir probe is one of the most simple and direct approaches to monitor the quality of the laser induced plasma. It works by inserting a metal probe with variable bias voltage into the plasma. The time of the arrival signal of the charge species in plasma can be directly obtained at each bias voltage, as shown in the left figure, which can be used to estimate the speed of the plasma. Besides, through the relationship between the magnitude of applied bias voltage and detected current signal, the temperature of electrons can also be determined.
TOA curve of 355 nm laser induced Ag plasma under different probe bias voltage.
Electron temperature (Te) can be deduced from the retarding region of the semi-logarithmic I-V curve.
Secondary Ion Mass Spectrometry (SIMS)
18O SIMS depth profile of SrTiO3 on SrTi18O3 grown at Ts=750°C, 650°C, and room temperature. The sharp drop of the 18O signal near the SrTiO3 surface for the film grown at TS=750°C could be related to a back-exchange of 16O at room temperature.
18O SIMS depth profile of SrTiO3 on LaAl18O3 grown at Ts=750°C, 650°C, and room temperature.
18O SIMS depth profile of LaAlO3 on SrTi18O3 grown at Ts=750°C, 650°C, and room temperature.
Combining the mass spectrometer with an Ion Gun (Hal IG20) and an Electron Flood Gun (PREVAC) makes it possible for second ion mass spectroscopy (SIMS) profile of both films depth and mapping.
Complementary to mass spectroscopy, we investigate the excited species in the laser induced plasma by emission spectroscopy. The first photo shows that the image of the laser induced plasma is projected onto the entrance slid of the monochrometer via the indicated optical beam path. Through the combination of different delay times, we can record a spatially and time resolved emission spectrum of the plasma. The second image shows a La0.6Ca0.4MnO3 spectrum recorded with the spectrometer set-up.
In addition to the frequency, time and space resolved spectroscopy, we do time, space and frequency resolved imaging. One example is shown in the following animation where the plasma expansion of silver was recorded between the initial impact of the laser beam on the target and 10µsec after with a time resolution of 500nsec for each frame. After approx. 2µsec a rebound of the impinging Ag species from the heater is observed. These rebounded species travel back even as far as the target.
Experimental set-up to record the optical laser-induced emission spectra
La0.6Ca0.4MnO3 spectrum recorded between 550 and 825nm