ION BEAM TECHNIQUES. Ion beam characterization techniques are illustrated in Fig

Transcription

1 ION BEAM TECHNIQUES Ion beam characterization techniques are illustrated in Fig

2 ION BEAM TECHNIQUES Incident ions are absorbed, emitted, scattered, or reflected leading to light, electron or X-ray emission. Aside from characterization, ion beams are also used for ion implantation. We discuss two main ion beam material characterization methods 1. Rutherford backscattering spectrometry (RBS) 2. Secondary ion mass spectrometry (SIMS) 2

3 2. Secondary Ion Mass Spectrometry (SIMS) Secondary ion mass spectrometry, also known as ion microprobe and ion microscope, is one of the most powerful and versatile analytical techniques for semiconductor characterization. It was developed independently by Castaing and Slodzian at the University of Paris and by Herzog and collaborators at the GCA Corp. in USA in the early 1960s. One of the major purposes: moon rock analysis. The technique is element specific and is capable of detecting all elements as well as isotopes and molecular species. Lateral resolution is typically 100 μm, but can be as small as 0.5 μm with depth resolution of 5 to 10 nm. ions RBS The depth resolution is on the order of 10 ~ 20 nm for film thicknesses 200 nm. Beam diameters are commonly around 1 to 2 mm but microbeam backscattering with beam diameters as small as 1 μm is possible. 3

4 Hydrogen Isotopes # of neutrons zero one two Atomic mass 1.0 u 2.0 u 3.0 u 4

5 Working mechanism A primary ion beam impinges on the sample and atoms from the sample are sputtered or ejected from the sample. Most of the ejected atoms are neutral and cannot be detected by conventional SIMS, but some are positively or negatively charged. This fraction was estimated as about 1 % of the total. RBS: around 10 6 of the number of incident ions backscattered (elastically). The mass/charge ratio of the ions is analyzed, detected as a mass spectrum, as a count, or displayed on a fluorescent screen. The detection of the mass/charge ratio can be problematic, since various complex molecules form during the sputtering process between the sputtered ions and light elements like H, C, O, and N typically found in SIMS vacuum systems. The mass spectrometer only recognizes the total mass/charge ratio and can mistake one ion for another. 5

6 The basis of SIMS, is the destructive removal of material from the sample by sputtering and the analysis of the ejected material by a mass analyzer. Static mode Image mode Dynamic mode A%2F%2Fwww.geos.ed.ac.uk%2Ffacilities%2Fionprobe%2FSIMS4.pdf&ei=tn21UIuwKI7KmAX9tIC4Aw&u sg=afqjcngbr8dj9lk-szr1tblpk7fw17g2yg 6

7 Primary ion sources Three basic types of ion guns are employed. 1. Ions of gaseous elements: for instance noble gases (Ar +, Xe + ), oxygen (O -, O 2+ ), or even ionized molecules such as SF 5+ (generated from SF 6 ) or C 60+. (by duoplasmatrons or electron ionization) This type of ion gun is easy to operate and generates roughly focused but high current ion beams. => An electron ionization ion source is typically used to ionize noble gas atoms such as He, Ne or Ar. Oxygen primary ions are often used to investigate electropositive elements due to an increase of the generation probability of positive secondary ions. 7

8 Electron ionization SIMS Apparatus: ion gun (1) (Ions of gaseous elements) (Ar) (Ar + ) a cathode: e - into a vacuum chamber. Ar is introduced (very small quantities). Ar + e - Ar + + 2e - forming a plasma. The plasma is then accelerated through a series of highly charged grids, and becomes a high speed ion beam. 8

9 Primary ion sources 2. The surface ionization source, generates Cs + primary ions. Cesium atoms vaporize through a porous tungsten plug and are ionized during evaporation. Depending on the gun design, fine focus or high current can be obtained. while cesium primary ions are often used when electronegative elements are being investigated. 9

10 Cesium surface ionization source (2) Cs is heated to approximately 900 C => Cs vapor is formed. Cs vapor moves to an enclosed region between the cathode, which is cooled, and the ionizer, which is heated. Some of Cs condenses onto the cool surface of the cathode, some of Cs comes in contact with the ionizer surface (immediately ionized). Cs + leaves the ionizer is accelerated toward and focused onto the cathode, sputtering material from the cathode at impact. Some of the sputtered material gains an electron in passing through Cs coating on the surface of the cathode, and forms the negatively charged beam. This negative beam is accelerated out of the source. cool electropositive heated the University of Notre Dame 10

11 3. The liquid metal ion source (LMIS) Operates with metals or metallic alloys, which are liquid at room temperature or slightly above. The liquid metal covers a tungsten tip and emits ions under influence of an intense electric field. LMIS: a field ion emission source Generate highly bright positive ion beams from neutral atoms or molecules. Taylor cone's tip sharper the electric field becomes stronger. LMIS: particularly used in ion implantation or in focused ion beam (FIB). W 11

12 A gallium source (able to operate with elemental gallium), recently developed sources for gold, indium and bismuth use alloys which lower their melting points. Ga: T m : ~ 30 C, liquid at rt. Low vapor pressure: good for high vacuum The LMIG provides a tightly focused ion beam (< 50 nm) with moderate intensity and is additionally able to generate short pulsed ion beams. It is therefore commonly used in static SIMS. 12

13 The secondary ion yield is significantly lower than the total yield, but can be influenced by the type of primary ion. Electronegative oxygen (O 2+ ) enhances species for electropositive elements (e.g., B and Al in Si) which produce predominantly positive secondary ions. O 2+ easily strip off electrons from the sputtered atoms. Electronegative elements (e.g., P, As and Sb in Si) have higher yields when sputtered with electropositive ions like cesium (Cs + ). Cs easily to donate an electron to the sputtered atoms. The secondary ion yield for the elements varies over five to six orders of magnitude. 13

14 SIMS: sputtering process Primary ions Sputtered particles: Positive/negative or neutral Vacuum tens of nm Cascade mixing (later) 14

15 Sputtering is a process in which incident ions lose their energy mainly by momentum transfer as they come to rest within the solid. (displace atoms within the sample.) The primary ion loses its energy in the process and comes to rest tens of nm below the sample surface. Sputtering takes place when atoms near the surface receive sufficient energy from the incident ion to be ejected from the sample. The escape depth of the sputtered atoms is generally a few monolayers for primary energies of 10 to 20 kev typically used in SIMS. Ion bombardment leads not only to sputtering, but also to ion implantation and lattice damage. The sputtering yield is the average number of atoms sputtered per incident primary ion; it depends on the sample or target material, its crystallographic orientation, and the nature, energy, and 15 incidence angle of the primary ions.

16 The yield for SIMS measurements with Cs +, O 2+, O, and Ar + ions of 1 to 20 kev energy ranges from 1 to 20 (sputtered atoms per ion). Sputtering yield (Y) = mean number of emitted atoms/incident particle 1. mass, energy of incident ion beam 2. properties (structure, composition, etc.) of target 3. incident angle Sputtering yield of Si as a function of energy of Ar + Why? Energy Prof. F. Ernst Case Western Reserve University 16

17 The yield usually increases with the ion energies. However, sometimes, at very high energies => the yield decreases because of the increasing penetration depth; thus, increasing energy loss below the surface => not all the bombarded atoms are able to reach the surface to escape. 17

18 Sputtering yield of Si as a function of different ion mass What is important, however, is not the total yield, but the yield of ionized ejected atoms or the secondary ion yield, because only ions can be detected. 18

19 RBS vs. SIMS Different primarily in the energy range of ion beam RBS: high energy ions (MeV) A small fraction (10 6 ) : elastic collisions, backscattered SIMS: low energy ions (kev) 1 % charged ions Prof. F. Ernst Case Western Reserve University 19

20 Energy loss of energetic ions Several semi-empirical stopping power formulas have been developed. The most popular: the Ziegler, Biersack and Littmark model. Prof. Kai Nordlund Both charged and uncharged particles lose energy while passing through matter. The stopping power depends on the type and energy of the radiation and on the properties of the material it passes. J. Lindhard, M. Scharff, and H. E. Shi tt. Range concepts and heavy ion ranges. Mat. Fys. Medd. Dan. Vid. Selsk., 33(14):1,

21 Electronic stopping refers to the slowing down of a projectile ion due to the inelastic collisions between bound electrons in the medium and the ion moving through it. The collisions may result both in excitations of bound electrons of the medium, and in excitations of the electron cloud of the ion as well. Number of collisions an ion experiences with electrons is large. In the beginning at high energies, the ion slows down mainly by electronic stopping. When the ion has slowed down sufficiently, the collisions with nuclei become more and more probable, finally dominating the slowing down. Nuclear stopping power refers to the interaction of the ion with the nuclei in the target. Nuclear stopping increases when the mass of the ion increases. Nuclear stopping is larger than electronic stopping at low energy. However, for very light ions slowing down in heavy materials, the nuclear stopping is weaker than the electronic at all energies. 21

22 At extremely high ion energies, one also has to consider radiative stopping power which is due to the emission of bremsstrahlung in the electric fields of the particles in the material traversed. Important only for electrons. 22

23 Preferential sputtering issue (1) Selective or preferential sputtering can occur in multi-component or polycrystalline targets when the components have different sputtering yields. The component with lowest yield becomes enriched at the surface while that with the highest yield becomes depleted. However, once equilibrium is reached, the sputtered material leaving the surface has the same composition as the bulk material and preferential sputtering is not a problem in SIMS analysis. as time progresses, a steady state is reached and eventually produces sputtering yields representing the bulk concentration. Rich sputtered species to allow direct measurement of composition. 23

24 Matrix effect (2) SIMS has not only a wide variation in secondary ion yield among different elements, it also shows strong variations in the secondary ion yield from the same element in different samples or matrices - the matrix effect (2). For example, the secondary ion yield for oxidized surfaces is higher than for bare surfaces by as much as A striking example is a SIMS profile of B or P implanted into oxidized Si obtained by sputtering through an oxidized Si wafer. The yield of Si in SiO 2 is about 100 times higher than the yield of Si from the Si substrate. A plot of yield versus sputtering time shows a sharp drop when the sample is sputtered through the SiO 2 -Si interface. Si Chemical bonding environment. SiO 2 (B or P) Si 24

25 Cascade mixing (3) For SIMS, the most important type of atomic mixing is cascade mixing, resulting from primary ions striking sample atoms and displacing them from their lattice positions, leading to homogenization of all atoms within the depth affected by the collision cascade. Dopant atoms originally present at a given depth in the sample will distribute throughout this mixing depth as sputtering proceeds and the dopant profile will give a deeper distribution than the true distribution. 25

26 Incident primary ion Cascade mixing θ Secondary ions Transient depth Cascade mixing Steady state depth staff.science.nus.edu.sg/~pc4250/2006/lectures/1.%20sims.ppt It is important that the primary ion penetration depth be kept to a minimum for shallow dopant profiling. 26

27 Side-wall effect (4) Another instrumentation effect that complicates SIMS analysis is the edge or wall effect. To obtain good depth resolution, it is important that only the signal from the flat, bottom portion of the sputtered crater be analyzed. Atoms are also ejected from the crater bottom as well as from the sidewalls during sputtering. But the sidewalls of an ion-implanted sample, especially near the top surface, contain a much higher doping density than the crater bottom. If secondary ions from the edge of the crater are included in the analytical signal, the depth profile will have poor depth resolution. 27

28 How to avoid side-wall effect 1. the detectors are only open, when the beam reaches a central region of the sputter-crater. With this method it is possible to avoid edge effects and to get only a signal of a defined sputter-depth. 2. Using electronic gating of the secondary ion yield signal or a lens system, it is possible to detect only those ions from the central part of the crater. SIMS crater 28

29 A high vacuum is very important for SIMS (10 6 torr). This is needed to ensure that secondary ions do not collide with background gases on their way to the detector. It also prevents surface contamination by adsorption of background gas particles during measurement. This is particularly important for low mass species like hydrogen. Standards The usual approach is one of using standards with composition and matrices identical or similar to the unknown. Ion implanted standards are very convenient and also very accurate. The implant dose of an ion-implanted standard can be controlled to an accuracy of 5% or better. When such a standard is measured, one calibrates the SIMS system by integrating the secondary ion yield signal over the entire profile. Calibrated standards are, therefore, very important for accurate 29 SIMS measurements.

30 Static mode Image mode Dynamic mode 30

31 SIMS can give three types of results. 1. Static SIMS (surface analysis) The aim is to obtain sufficient signal to provide a compositional analysis of the surface, without actually removing a significant fraction of a monolayer. This requires the use of very low ion fluxes (around cm -2 ) to ensure that each ion is statistically-likely to impact upon fresh, undamaged surface and that the sputtered secondary ions are representative of the original surface, rather than surface that has already been "damaged" by earlier ion impacts. In this form, the technique is capable of providing information about adsorbed molecular layers or the topmost atomic layer of the solid surface. 31

32 2. Dynamic SIMS (Depth Profiling) The aim of depth profiling is to obtain information on the variation of composition with depth below the initial surface. Such information is obviously particularly useful for the analysis of layered structures such as those produced in the semiconductor industry. Since the SIMS technique itself relies upon the removal of atoms from the surface, it is by its very nature a destructive technique, but also, ideally suited for depth profiling applications. A depth profile of a sample may be obtained simply by recording sequential SIMS spectra as the surface is gradually eroded away by the incident ion beam probe. A plot of the intensity of a given mass signal as a function of time, is a direct reflection of the variation of its abundance/concentration with depth below the surface. 32

33 B in Si 11 B + The time-to-depth conversion is usually made by measuring the sputter crater depth after the analysis is completed. An example of the conversion of yield or intensity versus time to density versus depth profile is given in Fig , showing both 33 the raw SIMS plot and the dopant density profile.

34 One of the main advantages that SIMS offers over other depth profiling techniques (e.g. Auger depth profiling) is its sensitivity to very low (sub-ppm, or ppb) concentrations of elements. This is particularly important in the semiconductor industry where dopants are often present at very low concentrations. The depth resolution achievable (e.g. the ability to discriminate between atoms in adjacent thin layers) is dependent upon a number of factors, including: ions 1. the uniformity of etching by the incident ion beam 2. the absolute depth below the original surface to which etching has already been carried out 3. the nature of the ion beam utilized (i.e. the mass & energy of the ions) as well as effects related to the physics of the sputtering process itself (e.g. ion-impact induced burial). 34

35 With TOF-SIMS instruments the best depth resolution is obtained using two separate beams; one beam is used to progressively etch a crater in the surface of the sample under study, whilst short-pulses of a second beam are used to analyze the floor of the crater. This has the advantage that one can be confident that the analysis is exclusively from the floor of the etch crater and not affected by sputtering from the crater-walls. wall floor SIMS crater 35

36 3. Surface Imaging using SIMS If the aim of the measurement is to obtain compositional images of the surface formed from the secondary ion spectrum with minimum possible damage to the surface, then the main problem is to ensure that sufficient signal is obtained at the desired spatial resolution while minimizing the ion flux incident on any part of the surface. This is most easily achieved by switching from the traditional instrumental approach of using continuous-flux ion guns and quadrupole mass spectrometer detectors, to using pulsed ion sources and time-of-flight (TOF) mass spectrometers. 36

37 Chemical imaging with ToF-SIMS. The primary ion source is rastered across the sample surface and a mass spectrum is collected at each pixel. For every mass range of interest, an intensity plot, which maps the distribution of that signal across the sample, can be generated. The intensity plots can be color coded and overlaid. In this example, an electronic device composed of titanium (m/z 48, green) and barium (m/z 138, blue) is chemically imaged. 37

38 Instrumentation Approaches There are two instrumentation approaches to SIMS: (1) the ion microprobe; (2) the ion microscope. (1) The ion microprobe An ion analog of the electron microprobe. The primary ion beam is focused to a fine spot and rastered over the sample surface. The secondary ions are mass analyzed and the mass spectrometer output signal is displayed on a CRT in synchronism with the primary beam to produce a map of secondary ion intensity across the surface. The spatial resolution is determined by the spot size of the primary ion beam and resolutions lower than 1 μm are possible. The mass spectrometer consists of electrostatic and magnetic sector analyzers in tandem. usually referred to as MS/MS 38

39 Electrostatic Analyzer (an ion beam is focused for energy) Magnetic Analyzer (focus mass angular dispersions) r B r v Force Equations: Force Equations: Electrical force: F e = -q E where E = V d Magnetic force: F m = qvb Centrifugal force: F c = mv2 r v F e = -q E = -q V d = mv2 r = 2KE v KE = qvr v 2d r v Centrifugal force: F c = mv2 r B qvb = mv2 r mv = qbr B B m 2 v 2 = q 2 r B2 B 2 m q = qr B2 B 2 mv 2 = qr 2 B B 2 2KE 39

40 In the electrostatic analyzer, the ions travel between two parallel plates separated a distance d with a radius of curvature r V. A potential V between the two plates permits only those ions with the proper energy E to be transmitted without striking either plate, where KE is d: a distance between two plates K (1) In the magnetic sector spectrometer, a magnetic field B curves the ion of mass m, charge q, and energy E into a path of radius r B according to Substituting Eq. (1) into (2) gives r V : a radius of curvature V: a potential between the two plates = qb 2 r 2 B qvr 2 v 2d = B 2 r B2 d Vr v (2) 40

41 (2) The ion microscope A direct imaging system, analogous to an optical microscope or a TEM. The primary ion flood beam illuminates the sample and secondary ions are simultaneously collected over the entire imaged area with a resolution on the order of 1 μm. The spatial distribution of the secondary image is preserved through the system using an electrostatic and magnetic sector analyzer in tandem, amplified by a microchannel plate, and displayed on a fluorescent screen. A small aperture may be inserted to select an area for analysis. The lateral resolution of this imaging method is dependent on the beam size, which can be as small as 50 nm. 41

42 Proper mass resolution is essential for unambiguous SIMS analysis. For example, a SIMS mass/charge (m/e) spectrum for high-purity Si obtained with an O 2+ primary ion beam contains 28 Si +, 29 Si +, and 30 Si + isotopes, polyatomic Si 2+ and Si 3+ as well as many molecular species involving oxygen. Oxygen is not from the sample itself, but are due to the oxygen primary beam causing oxygen implantation and subsequent sputtering. The mass resolution can be as high as 40,000, equivalent to resolving two masses differing by only 0.003%. Such high mass resolution is required for detecting ions for which there are interferences. For example, 31 P ( amu) has a very similar mass/charge ratio as 30 Si 1 H ( amu) and 29 Si 1 H 2 ( amu). 54 Fe is similar to the 28 Si 2 dimer. This plethora of signals requires a high resolution spectrometer. e 42

43 Detectors: Quadrupole SIMS (1) Quadrupole SIMS: a quadrupole mass analyzer consists of four parallel rods with an oscillating electric field through which the ions pass. Robust and less expensive than the electrostatic-magnetic sector analyzers, but has lower resolution. Due to lower extraction potentials, it is suitable for analyzing insulating samples, but it cannot distinguish between ions with close mass/charge ratios. DC and RF (180 out of phase) voltages. Only ions with a limited range of m/z reach the transducer. All the others strike the rods and converted to neutral. 43

44 The trajectory in the oscillating electric fields applied to the rods. Each opposing rod pair is connected electrically. A RF + a DC voltages are applied between one pair of rods and the other. 44

45 Electrostatic or magnetic spectrometers depend on serial scanning of an electrostatic or magnetic field, requiring narrow slits for only those ions with the correct mass/charge ratio to be transmitted. This reduces the transmittance of the spectrometer substantially to values as low as 0.001%. A permanent magnet or an electromagnet to cause the beam to travel in a circular path (60, 90, or 180 ) Ions of different m/z can be scanned across the exit slit by varying the field strength of the magnet or the accelerating potential between slits A and B. 45

46 MS/MS in tandem (2) Magnetic sector MS Higher-mass ions are deflected less than lower-mass ions. Scanning the magnet enables ions of different masses to be focused on the monitor slit. => the ions have been separated only by their masses. Electrostatic sector To obtain a spectrum of good resolution (the same m/z) Ions have to be filtered by their kinetic energies. => Their energy distributions corrected for and are focused at the double focusing point on the detector slit. 46

47 Time-of-flight SIMS (TOF-SIMS) (3) slit A SIMS approach without this limitation is time-of-flight SIMS (TOF-SIMS): more ion collection. Separation of ions by mass occurs during the transit of the ions to the detector located at the end of the tube. 47

48 Instead of continuous sputtering by an ion beam, in TOF-SIMS, the incident beam consists of pulsed ions from a liquid Ga + gun with beam diameters as small as 0.3 μm. These pulses typically have a frequency of 10 to 50 khz and a lifetime of 0.25 ms. Ions are sputtered in brief bursts and the time for these ions to travel to the detector is measured. Equating the kinetic and potential energy gives v: the ion velocity V: the potential The transit time t t is simply L/v, where L is the path length from sample to detector, leading to the expression m q = 2V v 2 = 2Vt 2 t L 2 48

49 L v = t t = ml 2 2qV Because all ions entering the tube have the same kinetic energy, their velocities in the tube vary inversely with their masses, with the lighter particles arriving at the detector earlier than the heavier ones. => Assuming the species with the same charges Typical flight time is in the microsecond range for a 1 m tube. 49

50 A major advantage of TOF-SIMS is the absence of narrow slits in the spectrometer increasing the ion collection by %. This allows the incident beam current to be reduced significantly compared to conventional SIMS, which reduces the sputtering rate greatly. In fact, the sputtering rate is so low that it may take an hour to remove a fraction of a monolayer. Such low sputtering rates allow characterization of organic surface layers. Furthermore, since m/q is determined by time of flight, very large and small ion fragments can be detected, much larger than in other SIMS approaches. a TOF-SIMS spectrum of an organic layer contains hundreds of peaks. TOF-SIMS has also proven very sensitive to surface metals. Surface densities as low as 10 8 cm 2 have been detected for Fe, Cr, and Ni on Si. In contrast to conventional RBS with a sensitivity of around cm 2, HIBS can reduce that to the cm 2 range. 50

51 The TOF mass spectrometers are a much more efficient way of acquiring spectral data, and also provide good resolution and sensitivity up to very high masses. Using such instruments, SIMS images with a spatial resolution of better than 50 nm are obtainable. 51

52 Applications I A major source of the limited sensitivity of SIMS is the fact that most of the sputtered material is neutral and cannot be detected. Secondary neutral mass spectrometry (SNMS) or resonance ionization spectroscopy (RIS) Analyze the sputtered neutral atoms. Ions are still required for mass analysis system. Ionize sputtered neutral atoms after they have left the specimen The neutral atoms are ionized by a laser or by an electron gas and then detected. The pulsed laser volatizes and ionizes a small volume of the sample and the ions are analyzed in a time-of-flight mass spectrometer. ACS Nano, 2011, 5 (4), pp

53 Significant sensitivity enhancements over conventional SIMS are achieved. High speed of operation, is applicable to inorganic as well as organic samples and has microbeam capability with a spatial resolution of 1 μm. It is primarily used in failure analysis where chemical differences between contaminated and control samples must be rapidly assessed. Applications II SIMS has found its greatest utility in semiconductor characterization, especially for dopant profiling. Because matrix effects are minor and ion yields can be assumed to be linearly proportional to densities. Furthermore, the substrate sputters very uniformly, at least for Si. 53

54 n-type p-type As B Poly-Si Si Poly-Si Si An example profile in Fig shows that arsenic, boron, and oxygen can be determined in a single measurement. This sample was formed by diffusing As and B from a poly-si layer deposited on the Si substrate. The plot shows the location of the junction (N As = N B ) and the location of the poly-si/substrate interface (oxygen peak). 54

55 ToF SIMS depth profiling Ge HfO 2 HfO 2 Ge Er 2 O 3 (13%) HfO 2 Ge No Er With Er as grown and annealed Microelectronic Engineering 88 (2011) Ge diffusion: decrease when Er is introduced in HfO 2 In HfO 2 : more Ge migrates to the high-k surface than in Er-HfO 2. 55

56 Pros Conclusions High sensitivity: especially for light elements (hydrogen is possible.) High surface sensitivity: important for depth profiling. Information about the chemical surface composition due to ion molecules. Detection of the various isotopes of an element. Cons Destructive method. High selectivity, depending on the element. Secondary ion yield for an element varies with the surrounding elemental composition (matrix dependence). Interference of molecules and isotopes in the mass spectrum Quantitative analysis is quite complicated facilities/sims.htm 56

57 Factors that need to be considered in data analysis are crater wall effects, ion knock-on, atomic mixing, diffusion, preferential sputtering, and surface roughening. Some of these are instrumental and can be alleviated to some extent, but others are intrinsic to the sputtering process. 57

58 Sensitivity Versus Detection Limit Sensitivity alone cannot be related to system performance, since it is only an indication of signal strength. Detection limits are direct indicators of system performance, since both detection limits and performance are functions of the signal-to-noise (S/N) ratio. 58