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Ion beam deceleration properties of a newly developed low-energy ion beam implantation system were studied. The objective of this system was to produce general purpose low-energy (5 to 15 keV) implantations with high current beam of hundreds of μA level, providing the most wide implantation area possible and allowing continuously magnetic scanning of the beam over the sample(s). This paper describes the developed system installed in the high-current ion implanter at the Laboratory of Accelerators and Radiation Technologies of the Nuclear and Technological Cam-pus, Sacavém, Portugal (CTN).

Low energy ion implantation became very important nowadays, either on microelectronics technology, thin film studies or plasma-wall studies in fusion reactors. Traditional ion deceleration uses a system that includes an electrostatic Einzel lens to focus and a target disk to decelerate the ion beam [

To produce ion implantations at low energies it is necessary to decelerate the beam, keeping it focused. Positive beam deceleration is performed by applying a positive voltage to the target (V_{A}), in order to create an axial electric field in the opposite direction of propagation of the ion beam (E). This is the method (

The difficulty in the deceleration process is the creation of a uniform axial field in order to decelerate the beam without defocus. The most common method is to use Einzel lenses [

In order to correct the dispersion of the ion beam allowing magnetic scanning to run continuously it was necessary to study the behaviour of a positive ion beam in two aspects: in front of a charged target and in the region inside a charged ring.

This study, using simple electromagnetism theory, showed that a positive ion beam deflection of length y_{i} due to a uniform electric field with opposite direction of the beam can be described by [

where

being Ec_{o} the initial beam energy and α the incident angle and E_{x} the electric field on the x direction, given by

where V_{a} is the voltage applied to the target, C_{a} the self-capacitance and R_{a} the target radius.

As the field is non-uniform, the final deflection y_{f} is given by the sum of elementary regions of uniform fields as in

being n the number of considered uniform regions.

On the other hand, the study of a charged ring showed that the electric field in the y direction inside the ring, that forms the single electrode lens, can be given by

where

being V the voltage applied to the ring, l the ring length, R the ring radius, r the electrode radius and k_{1}, k_{2} and k_{3} geometrical constants of the system.

Thus the beam deflection due to a charged ring is given by

where d is the distance between the target and the lens.

Thus, using Equations (4) and (7) was possible to determine the voltage to apply to the electrostatic lens to compensate for the deflection of the ion beam caused by the charged target. The theoretical approach was also useful to determine the best overall geometry of the system.

To confirm these results, the simulation software Simion 3D 8.0 was used to simulate the performance of the electrostatic lens, enabling to verify the best target and lens dimensions and gap in between to subsequently build the system [

In

In

Through this figure is also possible to verify that the deflection increases as the target is biased at higher potentials, as expected i.e. as final energy becomes smaller, the beam becomes defocused.

For subsequent simulations a lens was introduced. As shown in

_{L}) for which the most significant trajectories are better compensated for various values of the target potential (V_{A}), using the same geometry in both cases.

V_{A} [kV] | V_{L} [kV] Theoretical | V_{L} [kV] Simulated |
---|---|---|

5 | 14 | 12.5 |

10 | 16.5 | 15.5 |

14 | 18.5 | 17 |

The equation for the deflection caused by the electric field (7), is in general terms in agreement with the simulations, as long it is applied to flat samples to avoid changes in the geometry of the target and consequently in the electric field.

In

In this circuit, two high voltage (20 kV) power supplies are used, one to bias the lens (V_{L}) and the other to bias the target (V_{A}). Resistors R_{1} and R_{2} are used to improve voltage stability, while the resistor R_{3} is intended to measure the beam current [

Three sets of tests were performed in order to proof homogeneity of implantations using the studied system. Each test consists of an implantation of four samples of silicon with an Argon beam with final energy of 5 keV. The implantation area was 10 × 10 cm^{2} and beam current of 100 μA (

Only the third test, with deceleration and lens, shows homogeneity. Notice that, again as predicted by simulation, the fluence is a bit higher for the outmost samples due to beam over-deflection. With this experimental result it was observed that there is a greater homogeneity in the samples 9 to 12 with the largest difference between fluences of the order of 14% while in samples 1 to 4 and 5 to 8 are about 42% and 38% respectively.

Through this test, and taking into account the theory the simulation and the results, it is possible to confirm that the maximum limit that each sample can be within the edge of the target is about 6 cm (sweeping amplitude of 16 cm ). This can be verified in ^{2} with acceptable homogeneity.

Using the developed system was possible to perform implantations at 5 keV energy continuously scanning of the ion beam over an area of 10 × 10 cm^{2} with accuracy for a general purpose low-energy implantation system. In addition it was possible to compare the differences between ion implantation based on energy and beam focus.

In this paper the results obtained for implantations at low energies (below 15 keV) were presented. These results were achieved based on the system developed with an electrostatic lens of a single electrode, allowing performing implantations at low energies with a focused beam using the magnetic beam scanning over a large area, gaining versatility for research involving ion implantation.

The system developed and applied to the CTN implanter can be used in other implanters, for general purpose low energy implantations where a very high homogeneity and/or very high fluence control is not an issue.

J.Lopes,J.Rocha, (2015) Simple General Purpose Ion Beam Deceleration System Using a Single Electrode Lens. World Journal of Engineering and Technology,03,127-133. doi: 10.4236/wjet.2015.33014