H. Liebl

Secondary−ion mass spectrometry and its use in depth profiling

During the past decade, since commercial instruments have become available, secondary−ion mass spectrometry has been increasingly applied as a very sensitive technique for surface characterization. Depth−concentration profiling has emerged as one of the most important applications, owing to the high−depth resolution obtainable. The intrinsic depth resolution of some 10 A is degraded by uneven erosion, sample mixing owing to the atomic collisions, and at very low concentration, profiling by the sample consumption required. Quantitative analyses are more difficult than with other techniques, but are feasible. The utilization of chemical signal enhancement by oxygen flooding or oxygen bombardment is very helpful.

Design of a combined ion and electron microprobe apparatus

Abstract A new instrument for surface research has been developed which combines an ion microprobe and an electron microprobe. Beams of ions of electrons with energies up to 20 keV are run through the same demagnifying lens systems and brought to a focus of about one micron of less diameter on the sample. Both beams can be used separately or, by virtue of a novel lens design, simultaneously. Primary particles scattered on the surface and secondary particles released from the surface can be analyzed by a combined energy and mass spectrometer with high solid angle acceptance. Scanning is provided for either one of the two primary beams. and scanning micrographs can be obtained either with analyzed particles or with unanalyzed particles. In order to facilitate investigation of surface phenomena, the whole apparatus is laid out in ultra high vacuum technology and can be baked at 250°C.

Cluster induced secondary electron emission

Abstract By means of a new ion-electron converter having a spherically shaped stainless-steel conversion electrode, the electron yields of various atomic ions, molecular ions and cluster ions were investigated for a fixed ion energy of 15 keV. The pulse height distributions obtained can be approximately described by Poisson distributions for atomic as well as for molecular ions. In most cases larger electron yields were observed for oxide ions and cluster ions than for atomic ions of the same velocity.

An Electronic Aperture for In-Depth Analysis of Solids with an Ion Microprobe

Abstract An electronic aperture for in-depth concentration measurements with an ion microprobe is described. Measurements on polycrystalline metal samples were performed to test the ability of the instrument and its applicability in sputtering yield investigations. On an amorphous target the depth-resolution was found to be satisfactory but still amenable to improvement by a proper choice of primary ion energy and mass.

Beam Optics in Secondary Ion Mass Spectrometry

Abstract Secondary ion mass spectrometry (SIMS), a technique for characterizing solids and surfaces, is distinguished by high sensitivity and its applicability to detect all elements. It can be used for surface research, in-depth concentration profiling, isotopic work, and the identification of compounds. Combined with imaging or microprobe techniques, these applications can be made with high spatial resolution. The main fields of application are surface physics, materials science, metallurgy, semiconductor physics and technology, geochemistry and biochemistry. Following a general discussion of the method and its intrinsic limitations, the ion-optical possibilities of achieving high lateral resolution are discussed, viz., ion emission microscopy and ion microprobe techniques. Since SIMS is a destructive method of solids analysis, there exists a definite correlation between microanalysis (small volumes) and trace analysis (small concentrations), which depends on the overall transmission of the secondary ions from the sample through the mass spectrometer to the detector. In the design of ion probe microanalyzers, therefore, particular attention must be paid to optimize the secondary ion transmission. In ion microprobe instruments, this requirement must be reconciled with that for a primary beam having a very small diameter but still a reasonable current. The current which can be focused into a small spot on the sample surface depends on the brightness and energy spread of the ion gun and on the image aberrations of the focusing optics, especially the last lens. These relations are discussed with examples. Finally, an objective lens design is shown which combines the requirement of finely focusing the primary beam with that of optimally collecting the secondary ions and forming them into a beam with low emittance.

Study of an iodine discharge in a duoplasmatron

Abstract An iodine discharge in a duoplasmatron hollow cathode ion source allows the extraction of negative iodine species for possible application in SIMS. The electronegative character of iodine produces efficient negative-ion formation with a simple resultant mass spectrum. Iodine vapor from a reservoir containing solid iodine must be delivered to the discharge in a controlled and reproducible manner. Discharge stability is enhanced by the use of a stabilizing gas. Argon is particularly advantageous in that it contributes no additional negative ions which would complicate the primary beam in a SIMS application. The intensity and stability of the iodine discharge depends upon several parameters, including total gas pressure, the iodine-argon pressure ratio, the discharge current and the duoplasmatron magnet field strength. Stainless steel cathodes have been used at discharge currents up to 100 mA.

Experimental and Statistical Investigations on the Measurement of Very Low Ion Currents in Mass Spectrometers

Ion-electron converters with low background current have been applied for the investigation of residual gas mass spectra. From the abundance distribution of the secondary electrons, measured with scintillation and surface barrier detectors, counting losses can be determined for different ion masses. Ion currents as low as some 10−22 A are measurable. The corresponding error limits are considered by a statistical analysis including the background current. Criteria are given to distinguish a mass peak from the background within a definite error probability.

Combined electrostatic objective and emission lenses for microcharacterization of surfaces

Abstract It is shown how the matrix-transfer method can be used to treat electrostatic particle-optical assemblies for finding solutions where a collimated beam of charged primary particles is focused on a flat sample surface, while at the same time charged secondary particles emitted with low energy from the surface as a consequence of the primary particle bombardment are guided backward through the assembly to form a well collimated beam suitable for further handling. Examples of solutions are presented for primary and secondary particles of equal and opposite charge polarity.

Optimum Sample Utilization in Secondary Ion Mass Spectrometry

Abstract Secondary ion mass spectrometry (SIMS) is capable of detecting all elements, most of them with high sensitivity. Since the sample volume is eroded during SIMS analysis, particular attention must be paid to the optimum utilization of the consumed sample material. Post-ionization of the sputtered neutrals by different methods has had only limited success; there are basic problems in post-ionizing the neutrals and then effectively collecting the ions for mass spectrometric analysis. The better alternative is to maximize the ionized fraction of the sputtered particles by a suitable choice of the sputtering conditions. The next step is then effective collection and transmission of the secondary ions through the mass analyzer. Various types of mass analyzer are suitable for SIMS: quadrupoe mass filters, sector field mass spectrometers, dc accelerators, time-of-flight mass spectrometers, and sector field mass spectrographs. The choice depends on the analytical requirements such as trace sensitivity, spatial resolution, mass-resolving power, number of elements of interest and duration of analysis. For optimum performance regarding these parameters a large double-focusing mass spectrograph with on-line simulataneous multi-element detection is required.

Stigmatic sector-field energy analyzer without second-order angular image aberrations

Abstract An electrostatic sector field consisting of a toroidal condenser is described which images stigmatically like a spherical condenser but has a larger energy dispersion. Moreover, both the radial and axial image aberrations of second order can be made to vanish simultaneously, which is not possible with a spherical condenser.