Patent Application
20190172696
The invention relates to a method of performing Secondary Ion Mass Spectrometry (SIMS), comprising:
Secondary Ion Mass Spectrometry (SIMS) is a technique used to analyze the composition of solid surfaces and thin films by sputtering a surface of a specimen with a (focused) primary ion beam and collecting and analyzing ejected/ablated secondary ions. The mass/charge ratios of these secondary ions are measured with a mass analyzer/mass sorter (such as a sector field mass spectrometer, time-of-flight mass analyzer, quadrupole mass analyzer, etc.) to determine the elemental, isotopic, or molecular composition of the surface, typically to a depth of ca. 1 to 2 nm. SIMS can be performed in a dedicated/standalone SIMS apparatus, but it can also be performed in situ in a Charged Particle Microscope that has been provided with a SIMS module/mass analyzer.
Charged particle microscopy is a well-known and increasingly important technique for imaging microscopic objects, particularly in the form of electron microscopy. Historically, the basic genus of electron microscope has undergone evolution into a number of well-known apparatus species, such as the Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), and Scanning Transmission Electron Microscope (STEM), and also into various sub-species, such as so-called “dual-beam” apparatus (e.g. a FIB-SEM), which additionally employ a “machining” Focused Ion Beam (FIB), allowing supportive activities such as ion-beam milling or Ion-Beam-Induced Deposition (IBID), for example. More specifically:
As an alternative to the use of electrons as irradiating beam, charged particle microscopy can also be performed using other species of charged particle. In this respect, the phrase “charged particle” should be broadly interpreted as encompassing electrons, positive ions (e.g. Ga or He ions), negative ions (e.g. Oxygen), protons and positrons, for instance.
It should be noted that, in addition to imaging and performing (localized) surface modification (e.g. milling, etching, deposition, etc.), a charged particle microscope may also have other functionalities, such as performing spectroscopy, examining diffractograms, studying ion channeling/ion backscattering (Rutherford Backscattering Spectrometry), etc.
In all cases, a Charged Particle Microscope (CPM) will comprise at least the following components:
An issue with SIMS is that its efficiency/sensitivity is dependent upon the ionization yield of the associated milling/ablation process, i.e. the number of (charged) ablated ions as a fraction of the total number of (neutral or charged) ablated particles. This ionization yield is generally quite low, with a typical value of the order of about 0.01—meaning that only ca. 1% of the ablated/sputtered particles appear as ions. Since only charged ablated particles can be analyzed/counted by a mass analyzer, one can understand why the ionization yield is a major limiting factor in the overall performance of a SIMS system.
It is an object of the invention to address this issue. More specifically, it is an object of the invention to provide a SIMS technique/apparatus with an augmented ionization yield compared to the prior art.
These and other objects are achieved in a method as set forth in the opening paragraph above, characterized by:
The basic idea behind the invention is to increase the ionization yield at the ablation region (the ion-irradiated region of the specimen) by provision of an electron-acceptor gas species at/proximal said region. Fluorine has the highest electronegativity of the entire periodic table, and perfluoroalkanes are completely saturated with fluorine (i.e. all hydrogen atoms have been replaced by fluorine), thus making them very strong electron acceptors. The inventor has observed that a local supply of perfluoroalkane molecules close to the ion beam impact point on the specimen will induce the release of fluorine atoms and produce free radicals and excited F (fluorine) atoms that are eager to steal an electron from an ablated neutral particle, thereby turning it into a charged ion and thus increasing the ionization yield. Moreover, the inventor has observed that F-containing molecules do not tend to deposit onto an ion-irradiated specimen—most likely due to the high release of F, which tends to enhance milling at the expense of deposition; although the presence of a perfluoroalkane gas can tend to decrease the milling speed to some extent (because released carbon has to be milled away), ion-induced deposition has not been observed. In addition, the inventor has observed that the contribution of surface diffusion effects obviates the need for a high-density gas cloud in front of the beam impact point, thus avoiding any significant deterioration in resolution.
As provided by the present invention, an enhanced ionization degree/yield during ion milling dramatically improves analysis sensitivity, and is thus a step toward higher-resolution SIMS. This benefit is achieved independently of the detector or primary ion beam species used. This contrasts, for example, with certain known prior-art approaches that attempt to use specific primary ion beam species—such as oxygen or cesium—to achieve an improved ionization yield, as opposed to the use of a catalytic gas.
In an embodiment of the invention, the employed catalytic gas comprises CnF2n+2, with an alkane length n selected to lie in a range 5-15, more preferentially 8-12. Such an embodiment is advantageous in that this sub-family of molecules has been found to have a sticking coefficient that is a good compromise between not too low (so that the residence time on the specimen is sufficient to allow satisfactory reaction with the ions) and not too high (so that the layers do not pile up and become difficult to pump out). However, the skilled artisan will realize that other molecules/molecular groups can alternatively be chosen, depending on the particulars of a given analytic situation.
The method of the present invention can be conveniently performed in either a standalone SIMS apparatus or a SIMS module in a CPM, for example. Because the invention is independent of the primary ion beam species used, one can conveniently make use of the Ga or Xe ion beam that is commonly employed in ion-beam microscopy. An example of such in situ application in a CPM is given in more detail in
The invention will now be elucidated in more detail on the basis of an exemplary embodiment and the accompanying schematic drawing, in which:
The column 1 (in the present case) comprises an electron source 9 (such as a Schottky gun, for example) and an illuminator 2. This illuminator 2 comprises (inter alia) lenses 11, 13 to focus the electron beam 3 onto the specimen 6, and a deflection unit 15 (to perform beam steering/scanning of the beam 3). The microscope M further comprises a controller/computer processing apparatus 25 for controlling inter alia the deflection unit 15, lenses 11, 13 and detectors 19, 21, and displaying information gathered from the detectors 19, 21 on a display unit 27.
The detectors 19, 21 are chosen from a variety of possible detector types that can be used to examine different types of “stimulated” radiation emanating from the specimen 6 in response to irradiation by the (impinging) beam 3. In the apparatus depicted here, the following (non-limiting) detector choices have been made:
By scanning the beam 3 over the specimen 6, stimulated radiation—comprising, for example, X-rays, infrared/visible/ultraviolet light, secondary electrons (SEs) and/or backscattered electrons (BSEs)—emanates from the specimen 6. Since such stimulated radiation is position-sensitive (due to said scanning motion), the information obtained from the detectors 19, 21 will also be position-dependent. This fact allows (for instance) the signal from detector 21 to be used to produce a BSE image of (part of) the specimen 6, which image is basically a map of said signal as a function of scan-path position on the specimen 6.
The signals from the detectors 19, 21 pass along control lines (buses) 25′, are processed by the controller 25, and displayed on display unit 27. Such processing may include operations such as combining, integrating, subtracting, false colouring, edge enhancing, and other processing known to the skilled artisan. In addition, automated recognition processes (e.g. as used for particle analysis) may be included in such processing.
In addition to the electron column 1 described above, the microscope M also comprises an ion-optical column 31. This comprises an ion source 39 and an illuminator 32, and these produce/direct an ion beam 33 along an ion-optical axis 33′. To facilitate easy access to specimen 6 on holder 7, the ion axis 33′ is canted relative to the electron axis 3′. As alluded to hereabove, such an ion (FIB) column 31 can, for example, be used to perform processing/machining operations on the specimen 6, such as incising, milling, etching, depositing, etc. Alternatively, the ion column 31 can be used to produce imagery of the specimen 6. It should be noted that ion column 31 may be capable of generating various different species of ion at will, e.g. if ion source 39 is embodied as a so-called NAIS source; accordingly, references to ion beam 33 should not necessarily been seen as specifying a particular species in that beam at any given time—in other words, the beam 33 might comprise ion species A for operation A (such as milling) and ion species B for operation B (such as implanting), where species A and B can be selected from a variety of possible options.
Also illustrated is a Gas Injection System (GIS) 41, which can be used to effect localized injection of gases, such as etching or precursor gases, etc., for the purposes of performing gas-assisted etching (IBIE) or deposition (IBID), for instance. Such gases can be stored/buffered in a reservoir 41′, and can be administered through a narrow nozzle 41″, so as to emerge in the vicinity of the intersection of axes 3′ and 33′, for example.
It should be noted that many refinements and alternatives of such a set-up will be known to the skilled artisan, such as the use of a controlled environment within (a relatively large volume of) the microscope M, e.g. maintaining a background pressure of several mbar (as used in an Environmental SEM or low-pressure SEM).
In the context of the present invention, the microscope M is further provided with a mass analyzer module 43. If ion beam 33 is directed onto a region of specimen 6, it will cause localized ablation of specimen material—some of which will be ionized and some (most) of which will be neutral. Ionized constituents of the ablated specimen material can be captured by mass analyzer module 43, which will sort and count them according to mass/charge ratio—thus giving qualitative/quantitative information regarding the specimen's (localized) constitution. So as to improve the ionization yield of this specimen ablation process, GIS 41 is used by the preset invention to administer a catalytic gas comprising a component selected from the group comprising perfluoroalkanes and their isomers. As set forth above, the presence of such a fluorine-rich catalytic gas in the vicinity of (the intersection of ion axis 33′ with) specimen 6 causes more secondary ions to be produced, via a fluorine-based “electron-grabbing” process.
In a specific, non-limiting example, the following parameters can be used:
| Number | Date | Country | Kind |
|---|---|---|---|
| 17199158.1 | Oct 2017 | EP | regional |
