Semion Single Theory

Theory of Operation: Semion System
Retarding Potential Analyser | Retarding Field Energy Analyser

Introduction

Direct measurement of the ion energy distribution (IED) and total ion flux can be performed with the Semion System, using our advanced retarding field energy analyser (RFEA) technology. The RFEA is constructed from process compatible materials and the sensor’s miniature size allows it to be mounted on the substrate or any other surface inside the reactor. RFEAs have been used for decades to measure IEDs in plasma discharges with limited success. Most designs require mounting on a grounded surface to avoid complications with substrate biasing. Early designs were typically bulky and differential pumping was required for the device to operate even at the low pressures encountered in many plasma processes. The Semion System incorporates a miniature design to avoid the need for differential pumping. Operating pressures of up to 300mTorr can be achieved in Argon discharges. The Semion System also uses high impedance low-pass filters to allow the RFEA to float at the substrate bias potential. The system supports bias frequencies in the range 1kHz to 100MHz and bias potentials up to 3kV peak-to-peak maximum. High temperature cabling connects the RFEA to the external data acquisition unit through a vacuum feedthrough, which is mounted at the reactor wall and enables the sensor to operate to 250°C. Button probe sensing elements are easily replaceable, which is a convenient feature for the user especially when operating in processes using aggressive etching chemistry sets.

In-situ layout of the Semion System in a typical plasma chamber setup

Figure 1: In-situ layout of the Semion Sensor System in a typical plasma chamber setup

Theory of Operation

Figure 2 shows a schematic of the Semion RFEA design. Ions enter the RFEA through an array of sampling apertures exposed to the plasma (only one aperture is depicted for simplicity). A grid G0, covers the internal side of the apertures and reduces the open area ‘seen’ by the plasma to a scale less the Debye length to prevent plasma entering the analyser. A second grid G1, in a plane parallel to G0, is biased with a negative potential relative to G0 to repel any electrons that may enter the device. A third grid G2, is biased with a positive potential sweep, creating a potential barrier for the positive ions. A fourth grid G3, creates a negative potential barrier that prevents electrons from escaping and ensures that they are also collected. A collector plate C, oriented in the same plane as the grids, collects the current of ions which cross the potential barrier set by G2. The data acquisition unit records the ion current at each potential applied to G2 and the graphical user interface displays the resultant current-voltage characteristic. The IED is also displayed - obtained by differentiation of the current-voltage characteristic. The potential configuration is depicted in figure 2.

 

Figure 2: Schematic of the Semion RFEA structure and grid potential configuration

The analyser (including G0, G1, G2, G3 and C), floats at the AC/RF component of the substrate bias potential. This is achieved by means of high impedance low-pass filters. These high impedance filters prevent disturbance of the applied bias signal and provide sufficient attenuation at the output to protect the measurement electronics. The RFEA chassis also floats at the DC bias component of the powered electrode potential. The required DC electric fields between adjacent grids are produced by setting the grid potentials relative to each other (not relative to ground). The Semion feedthrough interface provides a filtered connection to the RFEA chassis to enable a direct measurement of the acceptance angle of a sampling orifice is approximately 45º allowing detection of ions arriving at the surface within this angle. The calculated IED is the energy distribution of the ions perpendicular to the electrode surface.

Typical Results

A typical Semion system installation is shown in figure 3. The RFEA was mounted at the biased substrate holder in a plasma etch reactor. The plasma is sustained by the 2.45GHz source. The substrate holder is biased with 2MHz RF power to control the energy of the ions impacting the substrate. The working gas was pure Argon at a pressure of 10mTorr. IEDs were measured for a range of RF power levels applied to the substrate holder.

Typical Semion Sensor Holder Installation

Figure 3: Semion Sensor Holder Installation

A typical current-voltage characteristic and IED are shown in figure 4. The RF power was set at 50W and the argon gas pressure was 10mTorr. The bi-modal saddle shaped IED structure associated with sinusoidal biasing is clearly visible.

Current-voltage characteristic and IED

Figure 4 (a) Current-voltage characteristic (dashed) and IED measured at 50W and 10mTorr
(b) Ion energy distribution varies as a function of RF power applied to the substrate holder while the pressure is maintained at 10mTorr throughout

 

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Theory of Operation: Semion System | Retarding Potential Analyser | Retarding Field Energy Analyser