Part II: Thin Film Characterization

Transcription

1 Part II: Thin Film Characterization General details of thin film characterization instruments 1. Introduction to Thin Film Characterization Techniques 2. Structural characterization: SEM, TEM, AFM, STM 3. Chemical characterization: AES, SIMS, RBS, XRD, XPS, XRF 4. Thickness characterization: Optical techniques, stylus profilometry, etc. Size: Cost: - varies from a portable desktop interferrometer to the 50-ft long accerlerator and beam line of a Rutherford backscattering (RBS) facility - ranges from the modest cost of test instruments required to measure electrical resistance of films to the approximate $1,000,000 price tag of a commercial SIMS spectrometer. Operating environment: - varies from the ambient in the measurement of film thickness to the torr vacuum required for the measurement of film surface composition. Referenced Textbooks and Articles 1. M. Ohring, The Materials Science of Thin Films, Academic Press, San Diego (1992) 2. J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C. Fiori and E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis, Plenum, New York (1981) 3. W.A. Pliskin and S.J. Zanin, in Handbook of Thin Films Technology, eds. L.I. Maissel and R. Glang, McGraw-Hill, New York (1970) Sophistication: - At one extreme is the manual scotch-tape film peel test for adhesion, and at the other is an assortment of electron microscopes and surface analytical equipment where operation and data gathering, analysis, and display are essentially computer-controlled.

2 Film Properties Structural Characterization 1. Scanning Electron Microscopy (SEM) Common required film properties - Film morphology and structure - Film compositions - Film thickness Other properties related to specific applications - hardness, adhesion, stress, electrical conductivity, reflectivity, refractive index, magnetic domains, electrical polarization, etc. Specifications Figure 1. A picture of an ultra-high resolution SEM. (From Hitachi High-Technologies Canada, Inc.) Image resolution 0.5 guaranteed (magnification = 600,000x ) 1.8 1kV guaranteed (magnification = 150,000x ) (secondary electron image with test specimen) Magnification: 60x - 2,000,000x Accelerating voltage: kv Electron gun: cold field emission source

3 Wehnelt Cylinder Figure 3. Wehnelt cylinder. Figure 2. A schematic of the electron and x-ray optics of a combined SEM-EPMA (Electron Probe Microanalysis).

4 Important Features of SEM 1. Electron gun 1.1 Tungsten hairpin electron gun - size: ~100 µm wire - tip radius: ~100 µm - thermionic emission: resistive heating (i f ) is required. - source size: ~ µm - usage: reliable, properties are well understood, relatively inexpensive, not so high brightness (for low magnification applications), stable high currents (for X-ray microanalysis) - At a temperature of 2700 K and a brightness of 10 5 A/cm 2 sr, a typical lifetime is about h in a reasonably good vacuum (10-5 torr). 1.2 Lanthanum hexaboride (LaB 6 ) electron gun - most common thermionic electron source - high brightness (5-10 times > tungsten) - typical operating temperature: 1800 K lower operating temperature increases the lifetime of the filament. - requires better vacuum condition (better than 10-7 torr) LaB 6 is extremely chemically reactive when hot and it readily forms compounds with elements in the gas phase that poisons the cathode and reduce its emission. The volatile oxides of La and B which form in the presence of water vapor or oxygen may cause a significant loss of material. - size of single crystal block: ~100 µm - tip radius: 1-10 µm - operation: activation period (slow heating of filament current) is required to evaporate contaminants on the surface of LaB 6. -usage: LaB 6 is more expensive (10 times > tungsten) has longer filament life.

5 1.3 Field emission electron guns - resistive heating of filament (to eject electrons) not required - electrons emitted (tunneled) by high applied electric field (> 10 7 V/cm) - made of wire of single crystal tungsten fashioned into a sharp point and spot-welded to a tungsten hairpin. - tip radius: < 100 nm - Types of field emitter (a) cold field emitter source size is small that little demagnification of the beam is necessary to form a small spot on the order of 1-2 nm. energy spread is low which improves performance for low-voltage operation. require vacuum better than torr need to heat tip (from time to time) to about 2500 K to clean off adsorbed gas atoms. The process is called "flashing". (b) thermal field emitter heating a <100> single crystal tungsten field emitter to 1800 K in a strong electric field, building up and sharpening up the tip. continuous heating of the tip at K to prevent gas molecules from adsorbing on tip's surface may be operated in 10-9 torr vacuum condition. (c) Schottky emitter operated at 1800 K uses a ZrO coating on a <100> tungsten facet at the tip to reduce the work function and flat emitting area provides good emission stability. need greater demagnification to produce 1-2 nm spot. (a) (b) (c) Figure 4. SEM micrographs of (a) tungsten hairpin filament, (b) LaB 6 filament and (c) field emission electron gun.

6 2. Electron lenses Figure 5. Schematic diagram of a directly heated LaB 6 emitter that directly replaces a tungsten filament assembly. - electromagnetic lenses - demagnify the image of the crossover in the electron gun (d 0 ~ µm for a thermionic gun) to the final spot size on the specimen (1 nm - 1 µm) - Types of lenses (a) condenser lenses: demagnify electron-beam crossover diameter (b) objective lenses: final lens in the column for probe formation pinhole lens: - large specimen can be studied and its size is limited by size of speciment chamber below the lens. - large variability working distance allows excellent depth of field at long working distance (and long focal length). - working distance: 5-40 mm - focal length: mm - subject to lens aberration and hence reduction of image resolution. immersion lens: - specimen is immersed in magnetic field - focal lengths: 2-5 mm - only a small specimen can be studied. - lowest aberrations, smallest probe size, highest image resolution. Figure 6. Schematic diagram comparing tip shapes for the Schottky emitter, cold field emitter, and thermal field emitter. (adapted from Tuggle et al., 1985)

7 Figure 7. A rotationally symmetric electron lens where the coil windings are inside the iron shell and the field is produced across the lens gap between polepieces. (adapted from Hall, 1966) Figure 8. Enlarged schematic of polepiece area showing the forces on an electron that cause it to be focused. Figure 9. Two objective lens configurations. (a) Asymmetrical pinhole lens or conical lens, allowing a large speciment to be placed outside the lens. (b) Symmetrical immersion lens, where a small specimen is placed inside the lens.

8 3. Electron-Specimen Interactions 3.1 Elastic scattering - affect the trajectories of the beam electrons inside the specimen without altering the kinetic energy of the electron. - responsible for back-scattered electrons (0 E BSE E 0 with energy distribution peaking in the range E 0 for targets of intermediate and high atomic number.) 3.2 Inelastic scattering - results in a transfer of energy from the beam electrons to the atoms of the specimen. - responsible for generation of secondary electrons (0 E SE 50 ev with most probable energy of 3-5 ev) Auger electrons characteristic and Bremsstrahlung (continuum) X-rays electron-hole pairs in semiconductors and insulators long-wavelength electromagnetic radiation in the visible, ultraviolet and infrared regions of the spectrum (cathodoluminescence) lattice vibrations (phonons) electron oscillations in metals (plasmons) Figure 10. (a) electron and photon signals emanating from tear-shaped interaction volume during electron-beam impingement on specimen surface. (b) Energy spectrum of electrons emitted from specimen surface. (c) Effect of surface topography on electron emission. (From Ref. [1]) Figure 11. Schematic illustration of the origin of two sources of secondary electrons in the sample. Incident beam electrons (B) generate secondary electrons (SE I ) upon entering the sample. Backscattered electrons (BSE) generate secondary electrons (SE II ) while leaving the sample. λ is the mean free path for secondary electrons.

9 4. Detectors 4.1 Everhart-Thornley detector - most commonly used in SEM - used to detect secondary electrons and sometimes backscattered electrons Figure 12. Monte Carlo electron-trajectory simulations of the interaction volume in iron as a function of beam energy: (a) 10 kev, (b) 20 kev, (c) 30 kev Figure 13. Schematic diagram of the Everhart-Thornley detector: B, backscattered-electron trajectories; SE, secondary-electron trajectoreis; F, Faraday cage (bias range -50 V to +250 V); S, scintillator, with thin metallic coating (high bias, +12 kv) supply to the scintillator coating; LG, light guide; PM, photomultiplier.

10 4.2 Dedicated backscattered electron detector - usually placed near near the specimen - Examples: scintillator detector, BSE-to-SE detector, solid-state diode detector 4.3 Electron-Beam-Induced Current (EBIC) - use the specimen as a detector - when the primary electron beam strikes the surface, electron-hole pairs are generated and the resulting current is collected to modulate the intensity of the CRT image. 4.4 X-ray detectors - gas-proportional counter 5. Applications of SEM Figure 14. (a) Schematic diagram of a solid-state BSE detector, showing separation of electron-hole pairs by an applied external potential. (b) Typical installation of a solid-state detector on the polepiece of the objective lens. - topographic contrast (surface imaging) - compositional contrast - elevated-pressure (1-10 torr) microscopy - stereo microscopy

11 Figure 15. Magnification series of a fracture surface of high-purity iron, illustrating the rapid surveying capability of the SEM. The images are recorded at constant objective lens strength and working distance. Note that focus is maintained throughout the series and there is no image rotation. Figure 16. (a) Optical micrograph of the radiolarian Trochodiscus longispinus (b) SEM micrograph of same radiolarian. The greater depth of focus and superior resolving capability are apparent.

12 Figure 17. High-resolution secondary electron images of the surface of the magnetic storage of a computer hard disc. Image recorded at 35 kev on a field-emission SEM with signal collection from an E-T detector. Two magnifications of the same area are shown. Figure 18. The specimen is Raney nickel, which contains multiple phases that produce atomic number contrast and surface defects that produce topographic contrast. (a) specimen is viewed with a positively biased E-T detector, showing predominantly topographic contrast. (b) Four-quadrant solid-state BSE detector gives an atomic number contrast.

13 Figure 19. Atomic number (compositional contrast) observed in an aluminum-nickel alloy (Raney nickel). (a) Backscatteredelectron image derived from a negatively biased Everhart- Thornley detector. (b) Direct specimen-current image of the same region; note the contrast reversal compared to (a).