Literature for Scanning Probe Microscopy

Transkript

1 Literature for Scanning Probe Microscopy E. Meyer, H. J. Hug, R. Bennewitz Scanning Probe Microscopy The Lab on a Tip Springer Verlag, 2003 (D45 XWO2221+7_d) D. Sarid Scanning Force Microscopy Oxford University Press, 1993 (D33 UFF1760_d) S. Sadewasser, T. Glatzel (Eds.) Kelvin Probe Force Microscopy, 2012 (D33 UIY )

2 Mechanical Probes and their Interaction with Matter Sonde Wechselwirkungsprodukt miniaturisierte mechanische Spitzen Wechselwirkung mit der Probe Partikel z.b. Elektronen Photonen z.b. Licht Wellen z.b. akusti. Wellen Allgem. Größen z.b. Kräfte

3 Principle of Scanning Probe Microscopy Signalverarbeitung Positionierung der Sonde z x y rasternde Sonde h Wechselwirkung DUT

4 Examples of Scanning Probe Techniques Rastertunnelmikroskopie Rasterkraftmikroskopie Tunnelstrom Kraft Licht optische Nahfeldmikroskopie Rasterkapazitätsmikroskopie thermische Rastermikroskopie akustische Nahfeldmikroskopie Kapazität Wärme F Schall

5 Principle of Scanning Force Microscopy Messung der Spitzenauslenkung Positionierung der Messspitze z z x x y y Messspitze (isolierend, leitend, magnetisch) F Kraft DUT

6 Cantilever Sensor Träger Mechanische Schwingung Spitze

7 Frequency Behaviour of a Cantilever 20 a. u. 15 s i m u l a t i o n m e a s u r e m e n t k H z 20 Oszillations Frequenz Resonance Frequency: khz - MHz Limits in RF-Applications

8 Oscillation Modes of a Cantilever 1. transversale Eigenfrequenz: f 1 =76,0738 khz 2. transversale Eigenfrequenz: f 2 =476,534 khz [w.e.] [w.e.] (a) (b) 3. transversale Eigenfrequenz: f 3 =1,33437 MHz 4. transversale Eigenfrequenz: f 4 =2,61504 MHz [w.e.] [w.e.] (c) (d)

9 Phase [deg] Amplitude [nm] Oscillation Modes of a Cantilever ,1 0,01 1E-3 1E-4 1E FEM analytisch Anregungsfrequenz [khz]

10 Force-Distance Curve F rep ~ h -13 F res repulsive abstoßend F att distance h Abstand h Abstand h distance h Abstand h distance h anziehend attractive ~ h -7

11 Working Distances

12 Forces F [w.e.] v. d. Waal Kraft elektr. Kraft magn. Kraft 0 0 z [w.e.]

13 Plate Capacitor Model U S U S L a s e r + D e t e k t o r F e d e r D h U S U U P M h A, e r E r F E U y U P x A e F = ( U - U ) 2 E 2 h 2 P S Messung von Spannungen über Hebelarmauslenkung Einsatz heterodyner Mischtechnik

14 Field Simulation electrical potential [V] Electric field diestibution for even mode

15 Electrostatic force vs. tip distance total electrostatic force electrostatic onto the cantilever electrostatic onto the tip apex electrostatic onto the tip cone

16 Set-up Piezo electric scanner y z Control Unit x Cantilever Tip PC Sample y z x Sample stage Sample driver

17 Oscillation Detection Scheme

18 Contact Mode z h Probe h=const. z=f(x) Messspitzenposition x - Verbiegung des Hebelarms - die Höheninformation ergibt sich aus der z-bewegung des Trägerkörpers - Proben mit großen Höhenunterschieden - harte Proben (z. B. Halbleiteroberflächen)

19 Noncontact Mode z h=const. z=f(x) Probe Messspitzenposition x - Verbiegung des Hebelarms, Veränderung der Amplitude, Phase und Frequenz - die Höheninformation ergibt sich aus der z-bewegung des Trägerkörpers - Proben mit großen Höhenunterschieden - weiche Proben (z. B. Polymere, Nanopartikel)

20 Oscillation during Noncontact Mode Dh ms ( ) Dh ms ( r (h 2 ),h 2 ) Dh ms ( r (h 3 ),h 3 ) H TP ( ) Dh ms ( ) im Abstand h h 1 Dh ms ( ) im Abstand h = h 2 Dh ms ( ) im Abstand h = h 3 Dh ms ( r (h 3 ),h 2 ) Dh ms ( r (h 2 ),h 3 ) r (h 3 ) r (h 2 ) r (h 1 ) h 1 h 2 h3 x 1 x 2 x 3 x ohne Dämpfung mit Dämpfung

21 5000 nm 5000 nm Electrical Force Microscopy (EFM) y y line 1 line 2 line 3 x 5000 nm x 5000 nm U U

22 EFM Topography 7 5 n m 0 n m 2000 n m 2000 nm 7 5 n m l i n e w i d t h 7 5 n m l i n e s p a c i n g 0 n m Spannungskontrast y hohes Signal Voltage contrast, Frequency 13 khz niedriges Signal 375 nm

23 Principle of RF-EFM u p(f p) probe signal u p Ampl. -f p f p f u (f ) m m DUT signal u m Ampl. 2 0 a. u s i m u l a t i o n m e a s u r e m e n t amplitude spectrum of force -f m f m Ampl. low pass behaviour of cantilever f k H z 2 0 Oszillations Frequenz -2f m -2f p -(f p + f m ) - f D D f = f m - f p 2f p 2f m f p + f m f

24 RF-EFM Ausgangsleitung TWA 0.5 Widerstand Luftbrücken Bogen Ausgang Eingang T4 T3 T2 T1 MESFET : untersuchte Testgebiete

25 RF-EFM 100µm 100µm high [a.u.] 50µm 50µm 0µm 0µm 50µm 100µm 0µm 0µm 50µm 100µm low 7,5 GHz 20 GHz

26 Elektron. Energie Kelvin Probe Force Microscopy (KPFM) Spitze Gap Probe eu cpd W vac W F e A A S Material A mit e A, A B e B Material B mit e B, B e A A e Oberflächenladungen B e B e A A UKelvin U B CPD e B U 1 e CPD A B

27 Kelvin Probe Force Microscopy Kraft auf die Spitze: F el x ~ U x 2 U(x) U ext. F el (x) + U DC U AC U F x U x U x U U sin t el, ω CPD drop DC x ~ UCPD x Udrop x UDC U DC wird so geregelt, dass F el, = 0: U Kelvin U DC x U x U x drop CPD AC Ortsaufgelöste Informationen aus U Kelvin : i) U ext. = 0 V: Informationen über das Material ( Mat qu CPD + Spitze ) ii) U ext. 0 V: iii) Fall ii) Fall i): zusätzlich Informationen über den Spannungsabfall quantitative Aussagen über den Spannungsabfall (Subtraktionsmethode)

28 KPFM on Heterostructures 10 nm AlGaN layer STEM-Aufnahme Kelvin-Spannung

29 KPFM on Heterostructures Schichtstruktur der LED p-kontakt p: GaP Fenster p: InAlP i: InAlP AlGaInP MQW n: InAlP n: DBR n:gaas Substrat n-kontakt transp. Kunststoff LED 2.0 U Kelvin p-kontakt [ V ] V V 1.5 V 1.0 V 0.5 V 0.0 V -0.5 V -1.0 V -1.5 V U ext. = Spitzenposition [ µm ] Linescans Gehäuse

30 KPFM on Heterostructures p-kontakt Kelvin Spannung U Kelvin [ V ] Substrate 2 1 n-contact Spitzenposition [ µm ] 1.84V 1.5 V 1.0 V 0.5 V 0.0 V -0.5 V -1.0 V -1.5 V -2.0 V Spitzenposition [ µm ] = V Diode Die U ext. = 0V-Kurve gibt Informationen über U CPD für die verschiedenen Materialien Die U ext. 0V-Kurven geben Information über U CPD und U drop

31 Kelvin Probe Force Microscopy

32 Kelvin Probe Force Microscopy Detector Laser Amplitude signal Topography Control unit V(x) V DC (V Kelvin ) V AC Lock-In-Amplifier + Kelvin Controler ω Ref

33 Kelvin-Spannung [V] Kelvinspannung [V] KPFM on Nanowires 368 nm Au 0.00 nm nominell n Topographie nominell p V p n Kelvinspannung Uni Duisburg-Essen, HLT Kelvin-Spannung -0, , , , , Ort [µm] V Nachweis einer Dotierstoffänderung

34 Kelvin voltage (V) Voltage drop (V) KPFM on Nanowires biased V = 1 V unbiased contact isolator substrate contact V ext Tip position (µm)

35 KPFM on Graphene U ext = 0 V V U ext = +1.0 V 1.10 V U ext = -1.0 V V V -

36 Voltage drop (V) KPFM on Graphene 1.09 V V Conductivity: 3.7x10 5 S/m Sheet Resistance: 2.7 kw/sq Contact Resistivity: 6.3x10-7 Wcm U ext = +2.0 V +1.5 V +1.0 V +0.5 V -0.5 V -1.0 V -1.5 V Position (nm)

37 Probes for Current Measurements cantilever ferromagnetic coating probetip M cantilever magnetic dipole N S m magnetic spatial resolution < 25 nm

38 Principle of Current Measurements

39 Principle of Current Measurements H DUT, tangential a m p l i t u d e H D U T, n o r m a l magnitude x S DUT phase X H DUT, normal x

40 Current Measurements Topographie Amplitude Phase 930nm µm 0nm 20µm 20µm 20µm 20µm µm 0µm 0µm 0µm 20µm 20µm 0µm 20µm 0µm 20µm

41 Current Measurements Line 2 Line 3 Line 4 Line 5 i T 35 µm Area a 2i T 2i T 2i T 45 µm Line 1 i L1 i L2 0 µm 0 µm 0 µm 0 µm Line scan 1 Line scan 2 Line scan 3 Line scan 4 i L3 i L4

42 Current Measurements 90 µm 2 i T Line scan b i L1 i L2 Line scan b 0 µm 0 µm 90 µm 0µm 4175µm 0 max Topography Amplitude [u.a.] Phase

43 Measuring signal S [a.u.] Current Measurements 8 Evaluation area 6 Measuring signal S 1 from Linescan 1 for i L1 Measuring signal S 2 from Linescan 2 for i L2 Measuring signal S 3 from Linescan 3 for i L3 Measuring signal S 4 from Linescan 4 for i L Position above the line 1 [µm]

44 current in line 1 [a.u.] 10 8 Current Measurements measured current expected current i L4 6 i L3 current in linescan 4 4 i L2 current in linescan 3 2 i current in L1 linescan 2 current in linescan node 2. node 3. node Position above line 1 4. node

45 Conductive Atomic Force Microscopy (c-afm) Topography- Controller A I V DC - conductivity variations - Low to medium conducting material to semiconducting material - pa - µa sensitivity

46 Conductive Atomic Force Microscopy (c-afm) GaN LEDs mostly grown epitaxially on sapphire or SiC substrates Large mismatch of the lattice constants Large mismatch of the thermal expansion 100 nm High density of threading dislocations Formation of pits with six {10-11}-oriented sidewalls V-defects

47 Conductive Atomic Force Microscopy (c-afm) T = 950 C C-AFM KPFM topography V=-6.5V n-doped n-gan MQW p-gan d = nm nm µav sapphire nm µa V nm T = 960 C 3 p-doped p-gan MQW p-gan sapphire d = 25 nm nm µav µa nmv 2 V=-6.0V V=-6.5V V=-7.0V 500 nm

48 Tunneling Atomic Force Microscopy (TUNA) fa pa current amplifier - similar to c-afm - higher sensitivity - ultra-low currents - fa pa - uniformity or surface roughness of thin dielectrics

49 Theory of TUNA for Gate oxide thickness measurement measured I ~ A eff exp (- Cd ox ) calculated (theory) const. only one fit parameter 2R C A eff = (R C + )² R C + A eff lateral resolution nm

50 current I [pa] Local I-V Curve E-field [MV/cm] exp. data fit charging (~1 C/cm²) breakdown (>10 C/cm²) voltage V gate [V]

51 current [pa] I-V Curve on 4.8 nm Oxide voltage V gate [V]

52 current [pa] I-V Curves on Oxides of Different Thickness d ox (nom) d ox (fit) 3.3 nm 3.3 nm 4.7 nm 4.8 nm 5.3 nm 5.4 nm 7.8 nm 7.9 nm 8.7 nm 8.8 nm 10.0 nm 10.2 nm 15.0 nm 14.9 nm 19.2 nm 19.2 nm voltage V gate [V] note: increasing sensitivity with thinner oxides

53 LOCOS Process LOCOS: Local oxidation of silicon; isolates the active areas of different transistors by a thick field oxide Feldoxid SiN Vogelschnabel Silizium

54 8.5 nm thick EEPROM Tunneling Oxide (2 µm)² topography current image oxide thickness V g =10V 0 50 [nm] [fa] 8 10 [nm]

55 8.5 nm thick EEPROM Tunneling Oxide (2 µm)²

56 Scanning Capacitance Microscopy (SCM) Accumulation e C 0 e ra d Depletion Conductive tip Conductive tip C max Dielectric e e e n-doped + V - C min Dielectric e e e n-doped - V + DC = C max - C min DC ~ 1/Depletion Lenght DC ~ 1/Carrier Concentration

57 Charges, Capacitance CV-Curves of MOS Capacitor Source: MultiProbe, Inc.

58 CV- and dc/dv-curves of MOS Capacitor p-si negative n-si positive low doping high high doping low dc/dv values

59 n-type CV Curve Depletion zone decreases with doping concentration Source: MultiProbe, Inc. The amplitude of C gives information of the doping concentration

60 Phase shift for C/ V for n- and p-type Source: MultiProbe, Inc. Only with phase information a differentiation between n- and p-type doping is possible

61 Capacitance Sensor Source: J.J.Kopanski: Scanning Capacitance Microacopy; in Scanning Probe Microscopy Vol.1

62 Scanning Capacitance Microscopy Source: J. J. Kopanski: Scanning Capacitance Microscopy; in Scanning Probe Microscopy Vol.1

63 Qualitative SCM

64 Refresh Transistor of a DRAM Topography SCM-Signal Source: Institut für Angewandte Physik, Arbeitsgruppe Wiesendanger, Universität Hamburg (30 30)µm 2

65 Measurement pn-junction Position Passing device Topography SCM-Signal pn-junction is implanted inbetween poly-silicon contacts Failing device pn-junction shifted to the left-hand contacts Institut für Angewandte Physik, Universität Hamburg Arbeitsgruppe Wiesendanger

66 Cross Section of a 256 Mbit DRAM Topography SCM

67 Cross Section of a 256 Mbit DRAM Topography SCM

68 Scanning Thermal Microscopy (SThM) Measurement of resistance change Positioning of the probe via AFM z y Resistive tip x Heat flow DUT

69 Set-up for SThM Topographic datas Distance control Detector Piezo Lock-in amplifier Wheatstone 20µm bridge DUT G.B.M. Fiege et al., Microelectron. Reliabil. 38(1998)

70 MESFET during breakdown Example of SThM Gate Drain Source 15µm Topography G.B.M. Fiege et al., Microelectron. Reliabil. 39(1999) Temperature distribution Light emission image

71 Gate-Drain Breakdown of a MOSFET Topography Temperature distribution T +6.2K 0 Source T 0 Drain Gate 20µm G.B.M. Fiege et al., Microelectron. Reliabil. 38(1998)

72 Overview Strom, TUNA Spannung