A quartz crystal microbalance for investigating the angular distribution of particles sputtered from realistic mineral samples

Abstract

Planets and other objects in the solar system are exposed to a stream of ions originating from the sun, the solar wind. It has sufficient energy to alter the surface of airless bodies like Mercury via sputtering processes and may create a tenuous exosphere of ejected particles surrounding these objects. Knowing the material parameters relevant for sputtering processes is therefore crucial to understand planetary surfaces. Based on previous efforts with a quartz crystal micro balance as catcher, a more flexible system for evaluating sputtering yields of realistic mineral samples was developed. This allows for direct comparison between glassy thin films and mineral pellet samples with varying properties like grain size or porosity.

Introduction

Irradiating thin films of material deposited onto Quartz Crystal Microbalances (QCM) is a common technique for determining ion sputtering yields. It allows precise measurements of the samples mass change during irradiation with ion beams [1, 2, 3]. When Pulsed Laser Deposition (PLD) is used for the transfer of sample material onto a quartz, the original sample stoichiometry can often be reproduced [3]. Due to material constraints of the quartz crystals, the temperature during deposition may not exceed 846 K [4], resulting in glassy sample films. 
It is however desired to irradiate mineral analogue samples, as the number and angular distribution of sputtered particles might be different for crystals and glassy films and depend on the grain size. One way of approaching this is by weighing a bulk sample before and after irradiation [5]. This technique lacks the high precision of QCMs, which allow determining changes of sample mass equal to less than a monolayer of material [1]. An alternative approach is the detection of ejecta, for example with a pressure gauge, or a QCM as catcher [6, 7]. 
At TU Wien, a technique using a QCM for catching sputtered material (C-QCM) was developed earlier, showing the ability to determine sputtering yields from bulk samples with such a method [7]. However, this previous setup was limited in its geometry, with the C-QCM placed on a linear manipulator. In this setup, determining the angular distribution of sputtered material required changing distance and tilt angle between sample and C-QCM.

New setup for angular resolved measurements

In the new setup, the sample is mounted on a rotatable holder, which allows for an angular resolved investigation of sputtering yields (angle α in Figure 1). The C-QCM is mounted on a second sample holder and the angle α_C between both can be varied independently from α. Sample and C-QCM are placed coaxial, which guarantees a fixed distance and a variable tilt angle. This is of advantage, as the distance dependence of the C-QCM signal follows an inverse-square law and can therefore significantly increase the measurement uncertainty.

Figure 1: Principle of the Measurement using a QCM as a catcher. The distance can be kept constant while the angles sample/ions α and sample/C-QCM α_C are varied.

For the irradiation experiments, ions with solar wind velocity of 440 km s^-1 [8] are generated in an Electron Cyclotron Resonance ion source. They are then guided towards the sample holder using electromagnets and electrostatic lenses [9].
To avoid charging up of the insulating mineral pellets, the setup is also equipped with an electron flood gun. It delivers electrons with an energy of about 30 eV and is pointed towards the irradiated area, neutralizing the positive charge otherwise accumulating on the sample.
The sample holder is equipped with an ohmic heater and a K-type thermocouple. Combined with a Quadrupole Mass Spectrometer, not only irradiations at different sample temperatures, but also Thermal Desorption Spectroscopy (TDS) measurements after irradiations are possible. These can give important information about the implantation and diffusion of solar wind ions in minerals.
A QCM thin film sample and a pellet sample can be installed into the experimental chamber at the same time. Both mounts are placed in the same plane which is on the axis of rotation. By moving between both samples with the help of a linear feedthrough, C-QCM signals can be compared directly as the geometry stays the same. This has the additional benefit, that sputtering yields of the thin film sample can be directly evaluated and used for calibration.
This new setup allows investigation of sputtering yield and angular distribution for a wide variety of realistic samples. Effects of grain size, surface roughness and porosity on sputtering can be studied and compared with simulations [10] to get a more complete picture on sputtering processes on Mercury's surface.

References

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