Instrument Introduction

Services are what you'd expect: electricity, ultra-high purity argon, water cooling, high purity nitrogen (for the spectrometer) and compressed air (for pneumatic valves).

Pulsed RF GDOES is the result of cross collaboration of HORIBA Scientific with the plasma coating research community. This is the most advanced analytical GD technique available now.

There was absolutely no reason to limit the use of analytical GD instruments to the steel industry. Systems are now used both in material science labs at universities and in industries where they contribute to the development of new materials with coatings at the nano-scale upward or help to monitor photovoltaic devices manufacturing, i.e., to understand the origin of corrosion on painted car bodies, to assess the composition of precious metals, to control hard disks or LED manufacturing, etc.

With “Pulsed RF GDOES,” thin and thick layers of conductive or isolating materials are readily analyzed. The HORIBA Pulsed RF GDOES instruments are named GD Profiler 2 and GD Profiler HR.

In a glow discharge, a low pressure gas is flushed in the chamber. When the RF power is switched, an electrical plasma is set up. The sample is sputtered by ions of Ar layer by layer, and neutral species enter in the plasma. These atoms removed from the material and entering the plasma are excited through collisions with electrons or metastable carrier gas atoms. The characteristic spectrum emitted by these excited atoms is measured by the spectrometer.

The process is a dynamic one: new species constantly enter the plasma as the sputtering continues, therefore the spectrum constantly changes as new layers are sputtered.

Ar is the most common gas employed in GD. It is not costly and the low flow required for GD operation (about 0.2 l/min during analysis) will make a 200 bars cylinder bottle last for months. As no UHV is used in GD, the technique entirely relies on the cleanliness of the source and, to a great extent, on the Ar purity.

Ar can excite nearly all elements except F. To measure F (or monitor Ar in the sample) Ne has to be used.

The Glow Discharge source is responsible of the creation of the plasma. The plasma is located inside the anode. The plasma assures the sputtering of the sample and the excitation of the sputtered species. It is crucial to get flat craters for depth profile.

It is also important to generate the maximum light for sensitivity. With Pulsed RF, a stable plasma can be obtained on thin/thick films, conductive or non-conductive, with minimal thermal effects.

Plasma is the fourth state of matter. Plasma is an ionized gas electrically neutral (containing same number of negative and positive species – respectively electrons and positive ions in a positive plasma).

Plasmas can be characterized by their ionization efficiency σ = n/(n N) where n is the number of charged particles (n~ne~np) and N is the number of neutral particles.

Glow Discharge Plasmas are weakly ionized (σ <10^-4). They are generated at room temperature by applying an electric field to a low pressure gas  nd are called “cold plasmas”. The electric field accelerates the electrons to sufficient energies to ionize, through collisions, the gas molecules.

A GD plasma requires a low pressure gas atmosphere and the application of an electric field between two electrodes.

The glow is not spatially uniform.

Glow Discharge plasmas are very interesting, simultaneously being a source of light, a source of charged particles and a source of active species.

In its most simple configuration, a “GD plasma” consists of two parallel plane electrodes immersed in a cell with a low pressure gas and electrically connected to a generator. As the voltage increases over a threshold value, the cell begins to “glow”; a plasma is created and a current flows through the electrodes. Ions form a layer of positive charge close to the cathode where the applied potential is mainly redistributed; the “sheath”. In  he steady state, the plasma is sustained by the secondary electron emission process.

The typical GD curve V/I is as follows. The exact values depend on the cathode material, the type of gas, the gas pressure, etc. The EF region – named abnormal Glow Discharge – corresponds to a functioning point where the entire cathode surface facing the plasma is covered by the plasma and where an increase of V corresponds to an increase in A. This will be the functioning region for our analytical GD.

At first look there is little in common between the GD source where the plasma is confined in a small tube, typically 4 mm in diameter, and the large plasma chambers used for material deposition.

But the physical principles of the two plasmas are the same with only a scaling factor. This has helped us benefit from the tremendous knowledge acquired over the years in plasma deposition to bring GD to a new standard: the Pulsed RF GDOES instrument.

In addition, these large plasma chambers are often equipped with our spectrometers to observe the plasma changes during the process. Customers who prepare new coatings with such low pressure plasmas (HPIMS, magnetron sputtering, PVD, etc. – acronyms will differ depending on the actual technology applied) often use our Pulsed RF GDOES instruments to characterize their coatings, reinforcing their complementarity.

The analytical GD source features a geometry which has not changed much since Grimm's original concept. Schematics of a typical design and a photo are shown below. The anode (grounded) is normally a circular tube of 4 mm in diameter. Facing the anode, and maintained at proper distance of the anode by a spacer (here the white coloured ceramic), the sample is the powered electrode. The sample seals the GD source by simple application onto an o-ring.

A primary vacuum is assured within the anode and in the interstitial space between sample and ceramic. A continuous flow of gas (often Ar) is applied and regulated at the typical low pressure of GD plasmas: a dynamic process is therefore present.

This geometrical configuration restricts the plasma in the anodic tube.

Ions are accelerated towards the cathode and have enough energy to sputter the cathode/sample material. Sputtered species enter the plasma and are excited by collisions. De-excitation of excited species creates light, i.e., photon characteristics of the sample material. As sample material is continuously sputtered, the measured light reflects the temporal evolution of the sputtered species.

The special geometrical configuration of the analytical source – notably the double differential pumping with 2 pumps accounts for many of the specificities and crucial properties of the instrument.

The "anode" is the mainly positive electrode. The copper tube is the anode. It is electrically grounded.

In GDOES, the sample is the cathode (“mainly” negative electrode in most part of the RF cycles). It is connected to the generator. This is why the sample chamber is closed during analysis.

The standard diameter is 4 mm corresponding to optimum compromise between crater shape and quantity of light collected. 2 mm anodes are also available.

2 mm is the practical minimum size, because below that size, the quantity of collected light is very small. However, 1 mm anodes do exist and have been used in some experiments.

Fig 73: 1 mm spots done on a ceramic (the ruler below confirms the spot size)

Of course larger anodes are available and used in some applications where more light collection is needed. 7 mm is standard but 6 mm, 8 mm and 10 mm have also been designed for some customers.

The application on precious metals shown at the GD day with outstanding detection limits was done with a 8 mm anode.

The 10 mm anode was, for instance, used to measure He with Ne gas – a poster showing the result is available – He is difficult to excite, so collecting more light was useful.

Anodes are usually round, as this is easier to manufacture; there is no other reason for this design. Other shapes (for anodes and ceramics) have been made for special applications, as shown here.

The Anode-to-Sample Gap is the distance between the front surface of the anode and the front surface of white ceramic.

The anode-to-sample gap is a crucial parameter for reproducible GDOES data. Increasing the anode-to-sample gap increases the source impedance. This change in source impendance then affects the relationship between current and voltage in the plasma. An anode-to-sample gap between 0.1 mm and 0.2 mm is generally recommended. Small gaps are usually beneficial for improving depth resolution; larger gaps allow running the discharge for longer times without creating a short circuit between the anode and sample.

The geometry of the GD source assures that only the sample will be sputtered away. This is only true if the tip of the anode ends into the dark space. The distance between anode and sample is therefore crucial, in the range of 150 μm and should be carefully monitored. As this distance increases, the crater shape will be affected and the electric parameters of the discharge will also change.

The double pumping assures that deep craters even down to 150-200 μm can be obtained by evacuating away the sputtered particles. However, as the photo above illustrates, a certain re-deposition of material on the edges of the crater usually occur. When the crater is too deep, the re-deposition peak will be so close to the anode that discharge will stop.

Pulsed operation is a way to minimize the sample heat while keeping the instantaneous power high (and having more light). The pulsed RF mode has been studied extensively by HORIBA Scientific, in cooperation with plasma researchers.

A proprietary patent allows us to automatically match even in pulsed mode.

Pulsed RF is the most advanced source available for analytical GD. DC and RF power supplies have previously been used for powering the analytical GD plasmas.

Historically DC sources came first, as materials under investigation were conductive metals exclusively, and they remain much cheaper to build. RF sources have a much wider range of applications as they can be used for metals, non-metals and hybrid configurations. They are recognized to also be superior for ultra thin layers notably. RF sources permit Plasma Cleaning on the samples prior to analysis.

RF sources finally offer the advantage to be able to use within the same calibration, metals and non-metals which is beneficial even when conductive layers are to be measured, as it offers a way to find samples to calibrate “difficult” elements or ranges. (As an example, a carbide can be used as high point for C, an alumina layer used for calibrating O, a glass sample for Ca or Na, various polymers for H, etc.)

The RF source operates at 13.56 MHz which is the frequency used for plasma deposition systems. Over the course of an RF cycle, the voltage becomes alternatively positive and negative, preventing the accumulation of charges. The two electrodes become alternatively anode and cathode over a cycle (each 74 ns!).

But in our analytical GD, the two electrodes are not plan and parallel.

One electrode (as seen from the plasma) is actually the zone of the sample facing the anode (and not the entire sample), where the other one (as seen from the plasma) is the inner wall of the anode. The two electrodes are therefore asymmetric. The current (current density × surface area) having to be identical on both electrodes, the current density is higher on the small electrode (the sample) and therefore the electric field is higher (meaning that only the sample is being sputtered).

The potential automatically moves to negative values; a “DC bias voltage” builds itself at the sample surface and enables operation of the source on non-conductors as well as conductors.

Pulsed RF is the most advanced source available for analytical GD. DC and RF power supplies have previously been used for powering the analytical GD plasmas.

RF plasmas provide softer sputtering than DC ones, but they are more energetic due to the additional energy supplied by the oscillation mechanism of the RF power.

Depending on the source, various parameters can be controlled or monitored – real power, voltage, pressure, current, pulsing frequency, duty cycle, etc. They have a direct effect in particular on the sputtering rate and on the crater shape that can vary from concave to flat to convex.