Secondary ion mass spectrometry, also known under the acronym SIMS, is a widespread technique for the analysis of surfaces and small volumes. It is an extremely powerful technique, capable of very high sensitivity, high mass resolution and high depth resolution. It can be used to determine the elemental, molecular and isotopic composition of a sample. SIMS uses a focused beam of ions (primary ions) to sputter a material and produce a localized ion emission characteristic of the material itself (secondary ions). Typical ion beams used in SIMS are reactive primary ion beams (Cs+, O2+, O−) enhancing the emission of negative and positive secondary ions, respectively, and cluster ion beams (Arn+, C60+, Bin+, Aun+). While smaller Bi and Au clusters are mainly used for imaging applications, C60 and large Ar clusters have been demonstrated to be of highest interest for organic depth profiling. The secondary ions emitted from the specimen are analyzed by mass spectrometry.
A secondary ion mass spectrometry device generally includes at least one device for producing and focusing primary ions and a device for collecting and measuring the secondary ions. The device for measuring the secondary ions generally includes an extraction system for secondary ions, transfer optics, a mass spectrometer and a detection system. Many different arrangements of the above elements exist and many different types of spectrometer (e.g. magnetic sector, time of flight, quadrupole, ion trap . . . ) may be used to perform SIMS. These are well known in the state of the art.
As the sensitivity of SIMS analysis is determined in part by the collection and transmission of secondary ions through the complete device, efficient extraction of the secondary ions is extremely important for all types of spectrometer.
The extraction fields required for collection of secondary ions can have a number of deleterious effects on the primary ion beam. The beam may be deflected changing both the position and angle of impact. And aberrations may be introduced, increasing the spot size of the primary beam decreasing the achievable lateral resolution. This last consideration is particularly important for imaging SIMS. One method of minimizing the aberrations introduced by the extraction field is to ensure that the primary and secondary ions beam are coaxial in the vicinity of the sample. One example of a SIMS instrument that uses this configuration is the Cameca NanoSIMS 50. However the precise arrangement used by the NanoSIMS imposes the limitation that the primary and secondary ions must be of opposite polarity. Thus negative primary ions must be used for the analysis of positive secondary ions and vice versa.
Prior art patent document published WO2014/108376 A1 discloses a mass spectrometer device for separating ions in accordance with their mass-to-charge ratio. The mass spectrometer device successively comprises an ion source, an electrostatic sector, a magnetic shunt, a magnetic sector and detection means. The magnetic sector achieves separation of ions originating from the source of ions according to their mass-to-charge ratios. The electrostatic sector comprises spherical electrodes which define between them a deflection gap. The electrostatic sector is used in retarding mode in order to reduce the energy of the ion beam entering the electrostatic sector. The combination of magnetic sector an electrostatic sector is used to provide an achromatic focusing of the secondary ions.
Prior art document of SCHUELER B: “Microscope imaging by time-of-flight secondary ion mass spectroscopy”, MICROSCOPY, MICROANALYSIS, MICROSTRUCTURE, LES ULIS, FR, vol. 3, no. 2/3, 1 Apr. 1992 (1992-04-01), pages 119-138, XP002559196, ISSN: 1154-2799, DOI: 10.1051/MMM:0199200302-3011900; discloses a charged particle beam deflecting system providing a stigmatic image in the detection plane. The system includes several pairs of spherical sectors, each pair forming a spherical deflecting gap for a secondary beam. The system is also fitted with Herzog shunts and Matsuda plates. Such devices typically provide a potential closer to an ideal l/r potential in the deflecting gap. In this case, the devices are used to produce slightly toroidal field to specifically compensate for time of flight errors introduced in the accelerating region of the spectrometer.