GB2386747A

GB2386747A - Fullerene ion gun

Info

Publication number: GB2386747A
Application number: GB0126838A
Authority: GB
Inventors: Rowland Hill; Paul William Miles Blenkinsopp
Original assignee: Ionoptika Ltd
Current assignee: Ionoptika Ltd
Priority date: 2001-11-08
Filing date: 2001-11-08
Publication date: 2003-09-24
Also published as: GB0126838D0

Abstract

A fullerene ion gun capable of producing a beam of C60 ions, which is pulsed, mass filtered and has sufficient intensity for use as a probe in static time-of-flight secondary ion mass spectrometry (S-TOF SIMS). C60 powder is held in a cylindrical reservoir 2, which is heated by a heater 4 in order to vaporize the powder. C60 vapour issues from a nozzle 5 into an ionization chamber enclosed by a grid 6. A circular filament 7 releases electrons which are accelerated by the grid 6 into the centre of the chamber, where fullerene ions are formed by electron bombardment of the C60 vapor. Ions are extracted by an electrode 8 and formed into a probe by electrostatic lens 9 and 12, which may be scanned over a sample by scanning electrodes 13. The gun also has components for mass filtering 14, pulsing the beam on and off 10, and includes a bend 11 for rejection of neutrals from the ion beam.

Description

FULLERENE ION GUN

This invention relates to an ion source and ion-optical column which will generate a focused beam of C60 ions and ions of other ffillerene type molecules. Such an ion beam is suitable for a range of surface analytical techniques.

Ion guns are widely used in analysis and microfabrication techniques. They take a range of forms depending on the ion species which is required and the required spot shape of the ion beam. In an ion gun, ions are created by one of a range of ionising processes in an ion source and there are several different types of source that are conventionally used. Whatever the source, it is usually placed in close proximity to an extraction electrode to create a spray of ions from the source. These ions proceed along an optical column which contains electrostatic lenses, defectors and other ion-

optical devices to bring the ion beam to a focus at a particular point. Usually, this point lies on the surface of a sample to be analysed or on the surface of a work piece to be etched or to undergo a deposition process. The whole is contained inside a vacuum system. The extraction electrode and lenses are usually cylindrically symmetrical. Such ion beams are used, for instance, in secondary ion mass spectrometry (SIMS) in which the focused ion beam is directed at a sample and causes secondary ions to be ejected from the sample. These secondary ions are collected into a mass analyser and a mass spectrum of a small area ofthe surface is produced. By scanning the primary ion beam over an area of the sample, typically using deflector plates to steer the beam in a line-and- flyback raster, an image ofthe distribution of a particular ion species can

be generated. With some types of ion source, the ion beam has a very small spot diameter and the spatial resolution of the ion image can be less then 0.1 micron.

However, for many SIMS applications, a spot diameter of 1 micron is suitable.

When SIMS techniques are applied in organic chemistry, it is highly desirable that large intact molecules (or at least large fragments) are released from the sample surface under the impact of the primary ion species, to facilitate identification of the molecules in the mass spectrum. Atomic or light molecular ion beams conventionally used in the technique cause a very high degree of fragmentation of surface molecules. A C60 ion beam, the ions having mass 720 A.M.U., has the potential to reduce this fragmentation through its softer impact and thereby to enhance yields of large molecules from the surface.

A fullerene ion source suitable for SIMS applications must have sufficient intensity that the beam impinging on the sample can have a spot size in the range of I to 10 microns, whilst having sufficient ion current to produce an adequate yield of secondary ions from the surface.

One type of ion source is the electron bombardment source. Such a source is used for generation of ion beams from the atoms or molecules of a gas, usually a noble gas or oxygen. The gas is leaked into a vacuum chamber at low pressure. Some of the gas is lost directly to the vacuum system, but some enters an ionisation chamber where it is bombarded by low energy electrons to produce ions of the gas. Because the gas is leaked into the vacuum chamber from outside, the point of entry is often quite remote from the ionisation chamber for reasons of electrical isolation. The density of the

electron flux inside the ionisation chamber is limited by the rate of escape of electrons and their capture by surfaces at positive voltage.

According to the present invention, an electron bombardment ion source can be so constructed such that the gas is supplied through a nozzle directly into the centre of the ionization chamber. The gas is fullerene vapour produced by the heating of a C60 powder reservoir situated near to the ionisation chamber. Furthermore, repeller electrodes are used to force electrons into orbits which repeatedly pass through the centre of the ionisation chamber. By these means, the source brightness becomes sufficient to allow the production of an ion beam with a sufficiently small spot size and sufficiently high current to be useful in SIMS applications. i''"4'"; In the present invention, ions are extracted from the source by an extraction electrode and are transported through the optical column to form a beam which is suited to imaging Time of Flight SIMS. For this application, the beam must be pulsed, mass filtered and scanned, and it must be free from neutral atoms.

A specific embodiment of the invention is shown in figure 1. The C60 powder ( 1) is contained in a cylindrical reservoir (2) which is itself held in a copper surround (3).

This assembly is heated by a heater (4) to vaporise the C60 powder. C60 vapour issues from the nozzle (5) into the centre of the ionisation chamber which is enclosed by the grid (6). Outside the grid is a circular filament (7) This releases electrons which are then accelerated by the voltage on the grid into the centre of the chamber. The reservoir and grid are at high voltage, typically 20kV, in order that an ion beam can be produced with sufficient energy to have a small focus on a grounded sample. As

fullerene ions are formed by interaction with the electrons, an extraction electrode (8), at a potential of several hundred volts relative to the grid, attracts the ions for transportation into the optical column. The ions initially pass through an electrostatic lens (9) which forms a field image of the source in between a pair of deflector plates

(10) which can be used to pulse the beam on and off. In order to reject neutrals from the beam, it is necessary that the beam path should bend, and in this embodiment, a 1 bend (11) is built into the column at the position ofthe deflector plates. The beam continues along the column and is focused onto the sample by a second lens (12) and is scanned over the sample by a set of scanning electrodes (13). The optical column includes a means of mass filtering the beam (14). The whole assembly is contained in a vacuum housing (15). Electrical connections to the various components enter the vacuum housing via vacuum feedthroughs (these connections not shown).

Claims

1) A fullerene ion gun in which: a) fullerene vapour is introduced directly into the ionisation chamber of an electron bombardment source through a nozzle, b) a fullerene ion beam is produced which is focused on a sample and scanned over an area of the sample c) a bend in the beam path acts to reject neutrals from the beam, and d) a means of mass filtering is used to select one fullerene from the mixture in the beam.

2) A fullerene ion gun as in claim (1) in which the mass filter is a Wien filter.

3) A fullerene ion gun as in claim (1) in which the mass filter is a dual purser system. --I 4) A fullerene ion gun as in claimant (1) to (3) in which the scanning is done by double deflection electrodes before the final lens.

5) A fullerene ion gun as in claims (1) to (4) in which the double deflection electrodes are successive and ratioed in length to form a compact assembly and to give a square, or near square, raster.

6) A fullerene ion gun as in claims (1) to (5) which includes an aperture selection mechanism. 7) A fullerene ion gun as in claims (1) to (6) which includes electrodes for alignment and for shaping of the beam.