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Application Note Semion™ Ion Energy and Flux Analyser

Multi-grid Retarding Field Energy Analysers (RFEA) are widely used to measure the energy distribution functions of ions arriving at a surface. The IEDF is important in understanding the role of ions in processes such as etch and deposition. Deviations in ion energy can lead to charging effects on a substrate, causing substrate damage. Variations in ion flux can affect etch uniformity.

A key element in the design of the Semion RFEA probe is the ability to measure the ion energy distributions impacting a dc, pulsed, or radio-frequency biased electrode in a plasma discharge. Semion is compact so that the need for differential pumping is avoided. The RFEA probe is designed to sit on the electrode surface, in place of the substrate, and the signal cables are fed out through the reactor side port. This alleviates the need for modifications to the rf electrode - as is normally the case for analyzers built into biased electrodes.

In this application note, a test is performed with a plasma which is maintained by a inductively coupled antenna at the top electrode (not shown) and a wide-band amplifier supplies an rf bias power to the lower electrode through a blocking capacitor to achieve capacitive coupling. This note demonstrates the ability of the Semion system to measure ion energies in the range 0-150eV for various rf bias voltage levels, discharge pressures, rf bias frequencies (in the range 500 kHz to 60 MHz) and rf bias waveforms – both sinusoidal and dual-frequency.

A schematic of the experimental setup used for RFEA measurements is given in Figure 1.

Figure 1: Schematic of the Semion RFEA Probe attached to a biased electrode of an RF plasma chamber

The Semion RFEA probe is placed on the surface of the electrode (Figure.2). Tests were conducted using Argon gas in a pressure range of 1-50 mTorr. The plasma was sustained by the inductive source at a constant power of 300W with no Faraday shield present.

Figure 2: RFEA Probe mounted on electrode

The RFEA probe consists of a number of entrance orifices facing the plasma and each allows a sample of the ions arriving through the sheath into the RFEA probe for analysis. The RFEA probe has a cylindrical shape, with plasma facing front face of at least 60 mm diameter. The overall height of the RFEA probe is less than 5mm. The analyzer is electrically connected to the electrode and floats at the same potential. Figure 2, shows the RFEA Probe mounted in a 300 mm disc (‘dummy' wafer), with its front face flush with that of the disc, to be used in an industrial scale reactor in place of a semiconductor wafer thus allowing a measurement of the IED impacting the wafer surface.

The RFEA probe is a multi-grid system. The first grid, G1, is electrically connected to the back side of the plasma facing orifices, to reduce the sampling area open to the plasma, minimizing disturbance to the sheath electric field. A second grid, G2, is biased negatively with respect to the potential of G1, to repel plasma electrons that enter the analyzer. A third grid, G3, is used to discriminate ions with different energies. At each bias step in a voltage sweep applied to the discriminator, only ions with sufficient energy to overcome the potential barrier can pass G3 and reach the collector plate. The collector plate is biased slightly negative to draw any ions that pass G3 towards it but does not accelerate these ions unnecessarily to cause significant secondary electron emission. An alternative grid arrangement can be used to suppress secondary electrons emitted from the collector although this is not usually necessary.

For each bias step of the sweep voltage applied to the discriminator the corresponding ion current to the collector is recorded. Importantly, G2, G3 and C are designed to float at the electrode/analyzer rf potential, when rf biased, to accurately measure the IED. The rf potential is capacitively coupled from the electrode/analyzer surface (both are in electrical contact) to each grid and collector through the capacitance between them and the analyzer body.

Low pass filters with very high input impedance at the frequencies of interest are placed between each grid and collector and the RFEA electronics. The filters ensure that the grids/collector maintain the electrode rf potential. These high input impedance filters also prevent loading of the rf electrode. Critically, these filters also have high attenuation at the output preventing any rf voltage drop across the Semion Control Unit electronics. The filters are designed to cover the range 1kHz to 100MHz and include most of the common frequencies used in low temperature rf plasma processes.

Cables are taken from the RFEA and brought to a vacuum feed-through with a re-entrant ceramic tube in which the filters are mounted. The cables are sealed in a flexible ceramic shield and are resistant to discharge temperatures and to reactive species present in the process.

Figure 3: RFEA sensor mounted into a standard 70mm holder (cable not shown)

The requirements on the filter input impedance are that a) it is large relative to that of the rf electrode, on which it is mounted, so as not to alter its impedance to ground and b) it is large with respect to the impedance associated with the capacitance between the RFEA internal grids and its outer casing. This means that there is no significant loading of the electrode impedance and that there is a negligible difference between electrode rf bias and grid rf bias.

The Semion data acquisition unit provides the required bias voltages for all the grids and measures the small currents drawn by the collector. Data is transferred conveniently from the scanner to a PC for analysis across a universal serial bus (USB) connection. As the RFEA is floating at the dc self bias of the rf driven electrode on which it is mounted (as would a wafer under processing conditions) the bias voltages applied to the grids and collector need to be referenced to this potential to obtain the actual ion energies.

The Semion RFEA is unique in that it uses the technology developed to float RF Langmuir probes in order to isolate the RFEA from electrical ground. The analyser is built into a dummy wafer and placed on the chuck or wafer holder. Probe holder sizes from 70mm to 300mm are available. The RFEA sensor does not disturb the RF bias and the measured Ion Flux and Ion Energy are similar to that seen by a wafer.

Figure 4: A Bi-modal distribution of ion energy with changing bias voltage in a 300W Argon inductively coupled plasma with 13.56MHz RF bias on the substrate.

Figure 5: A Bi-modal distribution of ion energy with changing bias frequency in a 300W Argon inductively coupled plasma with 13.56MHz RF bias on the substrate.

Figure 6: A multi-modal distribution of ion energy with a dual frequency bias in a 300W Argon inductively coupled plasma with 2 and 30 MHz RF bias on the substrate.