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1、十二、二次离子质谱十二、二次离子质谱（Secondary Ion Mass Spectrometryfor Surface Analysis）近代分析实验原理（Introduction of modern analytical methods）12surface chemical analysis techniqueAdvantages (Compared with AES and XPS):l Detection of all the chemical elements in the periodic table, including hydrogen which cannot be det
2、ected by the AES or XPS;l Detection of elements in concentrations as low as 106, while AES or XPS detection limits are concentration levels of 0.1 atom%;l Limitation of the detection to the top one or two atomic layers of a solid surface (1 nm);lDistinguish between different isotopes of elements.Sec
3、ondary Ion Mass Spectrometry (SIMS)SIMS examines the mass of ions, instead of energy of electrons, escaped from a solid surface to obtain information on surface chemistry.The term secondary ion is used to distinguish primary ion that is the energy source for knocking out ions from a solid surface.dy
4、namic SIMSstatic SIMSfor depth profiling of chemistryfor surface chemical examinationQualitativethe complicated nature of secondary ion yield from a solid surface.destructive typeNondestructive type31. Basic PrinciplesSecondary particle generation by an energetic primary particle.lElectronslneutral
5、species of atoms or moleculeslionsmajority and not useful in SIMSCarrychemical informationmore than a simply one-to-one knock-out41.1 Secondary Ion GenerationThe sputtering process in SIMS: (a) direct collision sputtering; (b) collision cascade; and (c) thermal sputtering.extremely fast1015 to 1014
6、seconds1014 to 1012 seconds1013 to 1010 secondsOnly a small portion of secondary particles ( 1% of total secondary particles) are ionized and become the secondary ions that are analyzed in SIMS.Slow collision sputteringA sputtered particle faces competition between ionization and neutralization proc
7、esses when it escapes a sample surface.5secondary ion currentprimary ion fluxthe sputter yieldthe ionization probabilitythe probability for positive ionsthe fractional concentration of species m in the surface layertransmission of the detection systemVariation of positive ion yield among chemical el
8、ements under bombardment of 13.5 keV O1. (o) indicates a pure element, () indicates a compound.Under the same experimental conditions6Under the same experimental conditions:The yield of elemental secondary ions can vary by several orders of magnitude across the periodic table; Also, the secondary io
9、n yield for a particular element dramatically varies with chemical state.the positive Al ion yield in its oxide state is 100 times higher than in its pure state.7lFirst,the point at which the secondary particles escape from the surface is not the point of the initial impact by the primary ion. Inste
10、ad, there is a surface damage zone that includes the points of primary particle impact and secondary particle escape.lSecond, the cascade collision results in generation of secondary ions with much lower energy than that of primary ions.lThird,there is significant variation in secondary ion yield wi
11、th chemical elements and chemical states of the surface, which makes quantitative analysis difficult.The important features of the sputter ionization process81.2 Dynamic and Static SIMSHigh flux of primary ion bombardmentremove many layers of atoms in the surface region and provide elemental distrib
12、utions in a depth profilelow flux of primary ionsStatic SIMS. Primary ion bombardment results in surface damage and emission of ions and neutral atoms. The damaged area is quantified by the damage cross-section (). In static SIMS the total primary ion dose should be limited so that only small portio
13、n of the surface is damaged. a, surface layer; b, bulk; c, damaged zone; d, depth of damage zone; r, radius of damage zone.the density of surface atoms as about 1015 atoms cm2less than 10% of the surface atoms or molecules are bombarded.9the primary ion flux (Ip)damage cross-section ()1 A = 6.21018
14、charged particles per second1013 cm2Ip = 1 Acm2 (= 6.21012 ions cm2)t 1 s1013 ions cm2, for static SIMSfor a 10-min durationIp 2.7 nAcm2102. InstrumentationSIMS instrumentation.mass analyzer systemultra-high vacuumtrajectories of ions remain undisturbed during SIMS surface analysis.112.1 Primary Ion
15、 Systemion sourceprimary ions with a certain kinetic energyion filterpurifies the primary ions and rejects unwanted ions in the primary ion beamdeflectormakes the focused ion beam raster on the sample surface in two orthogonal directionsAr+, Xe+, O2+, Ga+, Cs+, Bi+ ionsElectron bombardment source.(1
16、05 Am2 per solid angle)2.1.1.1 electron bombardment sources2.1.1 Ion sources12Duoplasmatron ion source. The magnetic field marked B intensifies the plasma by confining electrons close to the axis.up to 107 Am2 per solid angleLiquid metal ion source. The primary ion beam is extracted from the Taylor
17、cone of liquid metal.(1010 Am2 per steradian solid angle)high spatial resolution2.1.1.4 Surface ionization sources (Cs+)2.1.1.2 Plasma ion sources2.1.1.3 Liquid metal ion sourcesThe O2+ ion beam is commonly used because it can increase positive secondary ion yield.Ga+ (29.8 C)as small as 10 nmunifor
18、m kinetic energy andreasonable spatial resolutionBi (271.3 C)132.1.2 Wien FilterThe Wien filter for ion mass selection.Under the same electric field for accelerating the primary ions, the time for two ions to reach the sample will be different if their mass is different. Time accuracy is extremely i
19、mportant in the most widely used form of static SIMS, time-of-flight SIMS.The primary ion beam likely contains ions with differentmass such as isotopic ions and impurity ions.same kinetic energyMass-dependent velocity142.2 Mass Analysis SystemDetect: ratios of mass to electric chargemagnetic sector
20、analyzer,quadrupole analyzer,time-of-flight analyzer2.2.1 Magnetic Sector Analyzerextraction potentialmagnetic field strengthmass-to-charge ratiosuitable for dynamic SIMSmass resolutionby sequential selectionin which a few specific elemental ions are measured.扇形磁场分析器四极杆分析器飞行时间分析器152.2.2 Quadrupole M
21、ass AnalyzerA quadrupole analyzer. The oscillations of ions are generated by combined DC and AC electric fields using four cylindrical rods: (a) travel path of secondary ions in the analyzer; and (b) electrode arrangement of the analyzer.To obtain a whole mz1 spectrum, the analyzer increases the vol
22、tages but keeps the ratio of DC:AC voltages constant.162.2.3 Time-of-Flight AnalyzerTime-of-flight mass analyzer. The secondary ion beam is reflected by a mirror to correct the flight time of ions with identical mz1.the most widely used analyzer in static SIMSsame kinetic energyMass-dependent veloci
23、typulsed primary ions 10 ns17popular for static SIMShigh resolution, high transmission and high sensitivityThe major shortcoming of the ToF analyzer is its use of pulse primary ions. The ratio of primary beam on- to off-time is only about 104. Thus, it is not efficient for analysis such as depth pro
24、filing of chemical elements.183. Surface Structure Analysisthe static SIMS technique(ToF SIMS)Time-of-flight SIMS instrumentation.3.1 Experimental AspectsPrimary IonsFlood Gunsolves the problem of surface charging of insulating samplesSample Handlingavoid chemical contamination andsurface roughening
25、The positioning of the sampleHigher energy may not be favorable because it can cause excessive fragmentation and make SIMS spectra difficult to analyze.stand-alone instrumenthigh-mass ionsincrease the yield of secondary ions193.2 Spectrum InterpretationPositive ion ToF SIMS spectrum of polystyrene.
26、聚苯乙烯mass is in atomic mass units20Negative ion ToF SIMS spectrum of polystyrene.the spectra can be either that of positive ions or negative ions.21Negative ToF spectrum of NaNO3.SIMS spectra appear more complicated than those of AES and XPS because there are more peaks than the basic ion components.
27、223.3 Element IdentificationlThe great majority of metals give exclusively positive ions in static SIMS conditions. lThe most electropositive elements, such as alkali and alkali earth metals, give intense positive ion peaks. lThe most electronegative elements, such as O, F, Cl, Br and I give intense
28、 negative ion peaks. lHydrogen gives a high yield of both H+ and H ions. The CH peak is commonly more intense than the C peak.lN is not usually detectable by an elemental peak.lP is usually detected from peaks of PO2 and PO3.sub-monomer, n-mer and oligomerA spectrum for polymer: mass valuesthe monom
29、er massa small number of monomer units or monomer units plus fragmentsat high mz1, in which peaks are generated by a process called cationization.单体identifying polymer moleculesbasic chemical informationmore information on the macromolecular structure.低聚物complicated23Cationization is the laying down
30、 of a thin layer of organic substance or polymer on a silver substrate. ToF SIMS is able to detect the peaks of organic molecules (M) attached to Ag as Mx+Ag+. The peaks of cationization can reach mz1 levels at a scale of 103.Positive ToF SIMS spectrum of a polymeric substance that has a repeat unit
31、 of C10H20.estimating the average molecular weight distribution of polymeric materials24254. SIMS Imagingin a scanning mode,similar to EDS and AESSIMS images of ceramic coating on titanium obtained with scanning ToF SIMS. Ga+ primary ion beam generates the positive ion images of Ca, Ti, O and OH. Ch
32、emical gradients of O and OH in the coating layer (left-hand sides of images) are revealed.a cross sectionthe calcium phosphateCoatingTi substrateThe commonly used diameters of ion beams for scanning images are about 200 nm to 1 m.磷酸钙26Image Qualityhigh spatial resolution with high signal-to-noise r
33、atiosSmaller pixel sizeNot sufficient secondary ionsincrease the ionization probabilityprimary ions with heavy masssurface topographic featurespossible artifactsignals from secondary ions will vary even there is not chemical variation1.2.Particularly, for insulating samplessample should as flat as p
34、ossibleWe may also check whether there is a topographic effect by comparing SIMS images for several types of secondary ions.275. SIMS Depth Profiling5.1 Generation of Depth Profilesmagnetic sector analyzer(high mass resolutionGood transmission)SIMS depth profile of the GaAsSiAl2O3 system.10m10 m to
35、500 m500 mbeam energy 120 keVbeam density 1Acm2An O2+ ion beam is commonly used for positive ions profiles. Cesium ions, Cs+, are widely used for generating high negative ionyields of the elements that exhibit low positive ion yields.the detection limit28Definition of depth resolution and its effect
36、 on depth profiling.5.2 Optimization of Depth Profilingdetection sensitivitydetection limitdepth resolutionthe ability to detect a given element in a samplethe lowest atom concentration of a given element that can be detected in a samplethe accuracy of depth measurement for elemental concentrationsa
37、 depth range (z or t) in which the concentration (or secondary ion intensity) varies from 84.1315.87% at a depth profile with a step-like chemical gradient.295.2.1 Primary Beam EnergyHigher beam energyatoms to move deeper into the sample while causing some atoms to movetoward the surface.lower beam
38、energy is desirable5.2.2 Incident Angle of Primary Beamaffects sputtering yield, ionization probability and depth resolution.Primary O2+ beam energy effects on depth profile of As doped in SiO2 substrate.mobilize atoms in a sampleThe roughening by a beam normal to the surface is more severe than by
39、a beam with small incident angle.305.2.3 Analysis AreaSchematic illustrating the importance of selecting an analysis area smaller than the crater bottom.31the secondary neutral mass spectroscopy (SNMS)32P-type behavior of Sb doped ZnOSb doped p-type ZnO/undoped n-type ZnO/ZnO/MgO/ c-sapphire33substrate/MgO (2)/Pt (2)/Co60Fe20B20 (0.8)/MgO (2)/ SiO2 (t)SIMS depth profiles of oxygen for the films without (t = 0 nm, green solid line) and with (t = 2 nm, orange dashed line) the SiO2 capping layer. Expected layers with the etching time are denoted at the bottom of the graph.
