First version of the ion gun

On the basis of SIMION calculations, the first version of the source was projected in the AutoCAD14 system and built. Gold foil was installed as the sputter target. Gold was chosen due to its monomisotopic nature, large atomic mass, and great efficiency of negative ionization under cesium bombardment.

During operation, a DC current of 6-7 A was passed through the tungsten wire for heating filament. It was emitting an electron current of 20 mA. It was emitting the current of electrons at the value about 20 mA. With an accelaration voltage between wire and bombarded ionizer of 900 V, the power delivered by electrons to ionizer was about 18 W. In addition, tungsten filament itself consumed power about 20 W.

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Fig. 6.8. Primary ions spectra.

All insulators were made from commercially available alumina oxide ceramic tubes. Mass-spectra of produced gold ions in dependence on the Wien filter plates voltage is shown on the fig. 6.8.

Primary currents for monomer of gold at the value 50 nA were achieved at the current of Cs+ ions of 65 Aµ . The intensity of gold dimer and gold trimer were at the order of magnitude lower.

It is interesting to note of presence in the spectra heterogeneous clusters containing gold and two cesium atoms. Such clusters may be of interest for surface analysis by SIMS technique. Bombarding by this this kind of particles allows using advantages of polyatomic ion sputtering simultaneously with delivering chemically reactive atoms of cesium and, therefore, increasing the efficiency of negative ion production.

One of the drawbacks of this construction was related to failure of sputter target insulators. This was the most critical part. During operation they must sustain high

voltage in the presence of corrosive cesium vapor. It was noticed that after few days of operation leak currents in the range of few µA were appearing slowly leading to the arcing. Some modifications were made in the cathode-lens assembly to solve the problem. Special shields were installed close to ceramic insulators from cesium vaporization, the length of the insulators was increased from 22 mm to 35 mm, but these efforts only prolonged the „life“ time of insulators. Another disadvantage was that for cleaning these insulators a dismounting of the whole ion gun was required.

From the SIMION simulations in fig. 6.5 it is seen that the diameter of the cesium beam is only two times smaller than the size of the target. Simulations show that in this design it was not possible to reduce the cesium beam spot size due to inhomogeneous field of the extractor where the reflection occurs. The real size of the cesium beam is nearly the same as it was predicted by SIMION. The size of the cesium beam was determined from sputter erosion of the target.

To increase the current density from the ion source it is desirable to reduce the size of the cesium ion beam on the sputter target. Which could be achieved only by implementation of new ion optical design.

The procedure of working with the ion source is beginning from applying all voltages to the source electrodes (sputter target, lenses, Wien Filter plates, deflection plates, etc). After that the current source for the tungsten wire is switched on at its minimum value. Increasing the current through the wire, the electron current for ionizer heating must appear. Warming up of ionizer is performed in two to three steps during 10 minutes until working parameters of heater are achieved (usually 20 mA of electron current).

For alignment of the source, the second slit at the exit of the Wien filter can be used. This slit is constructively isolated from body of the source and therefore the current onto it can be measured. After ion current on the slit is detected at some value of the voltages on the first lens and Wien filter plates, the next step is the optimization of the ion current on the sample. Optimization on the absolute value of the ion current on the sample is a rather complex procedure which can include iterative alignment of all ion optics together. This procedure must begin with alignment of the potential on the first lens together with voltage on the Wien filter. After that the second lens with deflection plates must be optimized. When the ion current is adjusted to its maximum value, current measured on the second slit must be less than the one on the sample. At this point playing with potentials on the Wien filter plates must show two peaks of the ion current of monomers on the slit. The local minimum in the middle corresponds to the maximum value of ion current on the sample. An additional alignment may be performed by the precise positioning of permanent magnets of the Wien filter in direction perpendicular to the axis of the source. Optimal position corresponding to the maximal value of ion current and highest resolution may be found when magnets are slightly displaced from middle. This may be explained in the way that inhomogeneous magnetic field may additionally focus the ion beam.

Another critical part of the design – is the alignment of the Cs ions source directly opposite to the hole in the sputter target plate.

During operation the ion source may by degassing, especially after reloading new cesium salt. Degassing is usually slowly increasing during operation and it is related to the warming chamber of the source. After one hour operation temperature of the ion source body closely to Cs source may be 60oC and pressure may increase up to _2×10−7 mbar.