Nanonics Systems are the Only SPMs that Allow for On-line Integration with Standard MicroRaman Geometries

Transcription

1 Nanonics Systems are the Only SPMs that Allow for On-line Integration with Standard MicroRaman Geometries 2002 Photonics Circle of Excellence Award PLC Ltd, England, a premier provider of Raman microspectral imaging systems, has partnered with Nanonics Imaging Ltd to integrate, for the first time, the worlds of scanned probe microscopy (SPM) with Raman chemical imaging. Seen in this slide is the Nanonics SPM System placed on the microscope stage of the microraman. This combination has been awarded the 2003 Photonics Circle of Excellence Award. All other advertised commercial systems, that indicate a combination of AFM and Raman (see next slide), can either run an AFM scan or a Raman scan but no system other than the Nanonics combination provides simultaneous and on-line data from both modalities. This is an enormous advantage that resolves critical problems in Raman such as intensity comparisons in Raman images and provides for new avenues of improved resolution as will be described in this presentation. 1

2 What New Possibilities Does On-line AFM & Raman Permit? The new possibilities in such simultaneous AFM and Raman imaging result from the fact that in an AFM, as seen in the diagram above, there is a control unit that adjusts the sample piezo scanner (PZT in the diagram) in Z. This is accomplished by reflecting a laser off the AFM cantilever onto a position-sensitive detector. The AFM scanner is then adjusted in Z for alterations in sample topography so that the signal on the position sensitive detector is returned to the original value. Such an adjustment in Z for topographic variations rigidly maintains the sample position at the same distance from the lens for every pixel in the AFM scan. Also it should be realized that an AFM scanner moves with much higher resolution in x and y than conventional Raman scanning systems and this also further improves the performance of the microraman system. 2

3 The Advantages of On line & Simultaneous AFM and Raman Raman intensities can be effectively compared with AFM based autofocus Significant resolution improvements are achieved even without near-field techniques Other on-line advantages Surface enhanced techniques can be transparently applied Other advanced near-field techniques can be readily applied These advantages will be explained in the following slides. The first two advantages will be the topic of the next few slides 3

4 In All Raman Measurements Surface Topography Leads to Artifactual Intensity Comparisons The problem can simply be seen in this diagram where the extent of illumination of the surface of the sample is related to the surface topography. The same problem arises even if the sample is completely flat and as all samples cannot be mounted without some angle relative to the lens. 4

5 On Line AFM Pixel by Pixel Z Adjustment Leads to Effective Raman Intensity Comparison Nano Metric Z Feedback Quadrant Detector AFM with transparent nonobstructing AFM probes Sample topography changes, the z position of the sample is adjusted to keep the signal on the quadrant detector constant Every pixel rigidly maintained to a nanometer relative to the lens When an AFM probe is on-line pixel by pixel there feedback of an AFM alters the Z position of the surface of the sample sitting on a piezoelectric device or PZT to the same position no matter what the surface topography is. Thus the surface of the position of the surface of the sample is rigidly controlled relative to the lens to a single nanometer. 5

6 On Line AFM Pixel by Pixel Z Adjustment Leads to The First True Raman Intensity Imaging 6

7 Diamond Film Raman Intensity With & Without AFM Feedback With AFM Feedback Rigorous Z Feedback is Essential for Comparing Raman Intensities of Less than Flat Surfaces Without AFM Feedback Intensity of a Raman band in this case the vibrational mode of diamond at 1334 cm -1 in a diamond film is entirely different with and without AFM feedback

8 Collage of Raman Intensity at 1333 cm -1 with AFM Topography The colors represent the intensity of the Raman band while the topography is the background. 8

9 Collage of Raman Intensity & AFM Topography of 1333 cm -1 and 1525 cm -1 AFM Topography & Raman 1525 cm -1 Intensity AFM Topography & Raman 1334 cm -1 intensity Blue Arrows Indicate Two of Many Differences of Raman Intensity in the carbon 1525 cm - 1 band and diamond 1334 cm -1 bands and with AFM Topography Such Raman intensity differences in a Raman image could not be compared without on-line AFM z adjustment. Nor would there be any point by point correlation with topography. 9

10 Polyethylene Films 10

11 With an On-line AFM and Z Adjustment NanoIndentation Can Be Correlated with Material Properties It is now possible to investigate a nanoindentation, measure its topography and correlate on-line the spectral intensity alterations of, for example, silicon strain with topographical position in a nanoindentation The intensity of the Raman band of silicon as a function of height can be compared because of the on-line correlation Only The Nanonics & Fully Integrated Hardware and Software Solution Provides For Such New Worlds of Fully Integrated Spectral Imaging and NanoIndentation 11

12 The Advantages of On-line & Simultaneous AFM and Raman Raman intensities can be effectively compared Significant resolution improvements are achieved even on rough samples Other on-line advantages Surface enhanced techniques can be transparently applied Other advanced near-field techniques can be readily applied 12

13 The Problem with Light Out-of-focus Diffraction Light has two problems when focused by a lens. One is out-of-focus light and the other is diffraction. 13

14 The Confocal Solution A Standard Microscope Illumination Dimension ~0.5 µ XY Resolution ~0.5 µ Z Resolution ~1.6 µ A Confocal Microscope Illumination Dimension ~0.5 µ XY Resolution ~0.25 µ Z Resolution ~0.7 µ Confocal Microscopes Reduce Outof-focus Light by Placing a Confocal Aperture Before the Detector without Changing the Focal Spot Just by adding an aperture (confocal pinhole) we reduce the out of focus light and this reduces the in-focus point spread function of the microscope which is a critical parameter in optical resolution. Such an aperture or confocal pinhole only effects the detection and does not alter the point of illumination. Therefore, with the same dimension for the point of illumination the x, y and z resolution increase by a half. 14

15 With AFM Z Adjustment The Resolution is Increased Sinc The Out-of-focus PSF Above the Sample Does Not Contrib Out-of-focus PSF Produces In-focus PSF Optical Microscope Illumination of a Point Out-of-focus PSF As a result of z adjustment the region above the sample surface does not contribute to the image and if the sample surface is in focus the region above the sample simply does not contribute out-of-focus light and so this essentially eliminates the effect of the out-of-focus PSF above the sample. As a result the resolution with z adjustment is considerably improved over a conventional confocal microraman. This is seen on the next slide. 15

16 Nonetheless There is a Serious Problem In All Confocal Microscopes Including MicroRaman Lens Lens Sample Region of Signal Sample Region of Signal Surface Topography Contributes to Out-of-focus Effects on Resolution 16

17 On-Line Pixel by Pixel Z Adjustment Corrects This Problem in All Confocal Microscopes Including Confocal MicroRaman & Presents A New Way to Reduce the Point Spread Function Lens Lens Sample Region of Signal Sample Region of Signal Lens For Comparison Sample Region of Signal 17

18 With AFM Z Adjustment The Resolution is Increased Sinc The Out-of-focus PSF Above the Sample Does Not Contrib Out-of-focus PSF No Out-of-focus PSF Above the Focal Point Produces In-focus PSF Optical Microscope Illumination of a Point Out-of-focus PSF This Improves the In-focus PSF & Resolution by a Factor of at Least 2 Over Conventional Confocal Imaging As a result of z adjustment the region above the sample surface does not contribute to the image and if the sample surface is in focus the region above the sample simply does not contribute out-of-focus light and so this essentially eliminates the effect of the out-of-focus PSF above the sample. As a result the resolution with z adjustment is considerably improved over a conventional confocal microraman. This is seen on the next slide. 18

19 This Reduction in the Point Spread Function Results in Increased Resolution AFM and Raman Line Profile TiCN Sample: AFM & Raman Line Scan Profile counts Region 1 Region 2 Region microns Height um Raman topography Distance Between 2 AFM and Raman Points = 85nm Region 1: Large AFM change no Raman intensity change because of rigid control of Z Region 2: Small AFM change and material change as indicated by Raman on a scale of 170 nm (two points of 85 nm each). This resolution is obtained by rigid Z control and concomitant reduction in the point spread function Region 3: Large Raman change with uncorrelated topography change A comparison of an AFM and Raman line scan are overlaid point by point. In Region 1 the AFM is changing by ~0.4 m while the Raman is rigidly constant (see blue arrow in Region 1) since the material composition is not changing. In Region 3 the AFM is essentially constant while the Raman which represents the material composition is changing quite drastically. In region 2 the AFM shows a topographic feature which is changing over 170 nm where each point is 85 nm while the Raman is changing over the spatial extent. The Raman alteration is following the change in the AFM topography. 19

20 A Flat Germanium Quantum Dots Sample from Padua A Raman image formed by plotting the ratio of the peak at 408 cm -1 to the valley at 420 cm -1 Because of the on-line AFM there is a reduction in the point spread function (PSF) with no contribution of out-of-focus light from above the sample surface. In addition, the stage allows for subpixel movements and this together with the reduction in the PSF results in resolutions of <100 nm, which is half the width of the feature shown in this line scan. 20

21 A Quantum Dots Sample 115 nm Full Width; 53 nm Half width 21

22 Transistor Device 22

23 Mapping of Different Raman Bands Over the Device in Previous Slide Intensity, coun Distance, nm 520 cm cm-1 295cm cm-1 intensity, coun distance, nm Intensity, coun Raman line scan at 493cm-1 493cm-1 295cm cm Distance (nm) First AFM image in report. AFM image shown in this presentation. Higher up line of the two. 23

24 Mapping of Different Raman Bands Over the Device Pattern in Previous Slide Raman line scan at 493cm Intensity, coun nm Distance (nm) Distance (nm) Intensity (coun 24

25 Transistor Device AFM Where is the silicon or the strained silicon in this AFM structure? 25

26 AFM With Simultaneous Raman Chemical Mapping AFM Image Raman Image Mapped At 520 cm -1 Raman Image Mapped At 503 cm -1 26

27 AFM With Simultaneous Raman Chemical Mapping AFM Image Raman Image Mapped At 520 cm -1 Raman Image Mapped At 503 cm -1 27

28 AFM With Simultaneous Raman Chemical Mapping AFM Image Raman Image Mapped At 520 cm -1 Raman Image Mapped At 503 cm -1 28

29 The Advantages of On-line & Simultaneous AFM and Raman Raman intensities can be compared for the first time Significant resolution improvements are achieved even without near-field techniques Other on-line advantages Surface enhanced techniques can be transparently applied Other advanced near-field techniques can be readily applied These advantages will be explained in the following slides. The first two advantages will be the topic of the next few slides 29

30 Systems High Resolution Local Silicon Stress in MEMs Bent & Broken Floating Structures Monochromator Lens R A M A N AFM Probe Raman spectroscopy is a most important technique for measuring silicon strain. On-line AFM can impose finely controlled and well defined strain on silicon with pressures P=F/A) that can exceed megapascals since the area of a probe (A) tip is nanometric NanoRaman technology is the ideal for super-resolution silicon stress measurements in floating structures such as combs and forks 30

31 Systems Silicon Strain 10 microns The on-line AFM (above) allows for defined forces to be imposed on a MEMs cantilever while the on-line Raman measures the shift in the silicon vibrational frequency & silicon strain at the cross. No other AFM is capable of such a combination Note the nonobscuring nature of the glass AFM probe in this on-line CCD image through the Raman microscope 31

32 Systems Silicon Strain Raman spectrum as a function of the local stress Raman shift ( cm 1) pressure(relative) Series1 Series2 Series 1 local pressure increases imposed by the online AFM probe on the MEMs cantilever (x axis) and Raman shift due to silicon strain (y axis) Series 2-local pressure decreases as the probe pressure is reduced on the MEMs cantilever 32

33 The Advantages of On line & Simultaneous AFM and Raman Raman intensities can be compared for the first time Significant resolution improvements are achieved even without near-field techniques Other on-line advantages Surface enhanced techniques can be transparently applied Other advanced near-field techniques can be readily applied These advantages will be explained in the following slides. The first two advantages will be the topic of the next few slides 33

34 Enhanced NSOM Raman Shen and coworkers Nanonics provides probes with nanoparticles of gold and silver at the probe tip that is exposed to the optical axis of the Raman microscope. This allows for developments in the area of AFM controlled surface enhanced Raman spectroscopy. In this slide is shown the enhancement of the silicon signal with the tip in contact (Figure 2) while a image of grating with this technique of enhancement is shown in Figure 4. 34

35 Photonics Solutions The Multiview 2000 The First Sample and Probe Scanning SPM MultiView 2000 is important since the tip can be scanned relative to a Gaussian focused laser beam which has polarizations along the tip in the side lobes 35

36 NSOM Raman Enhancement on Opaque Sample Lens Carbon Nanotubes Silicon MEMs 10 micron The geometry of these surface enhancement near-field optical measurements is a reflection geometry as shown in the upper right of this slide. The top images are with the probe in contact (purple) and the probe out of contact (blue) for the carbon nanotubes and with the probe in contact (purple) and out of contact (light blue) for the MEMs device. Today, Nanonics can produce reliable probes for near-field surface enahcements with gold or silver nanoparticle probes. This is seen in the upper right image in which the gold nanoparticle is shown diagrammatically concentrating the blue light and producing red backscattered light. Nanonics produces the only systems in the world with a free optical axis from above the sample and this allows for such backscattering Raman measurements on opaque samples. 36

37 A Summary of On-line AFM & Raman Advantages Raman intensities can be effectively compared Significant resolution improvements are achieved with AFM autofocus Other on-line advantages such as imposed pressure on MEMs or other external pertubations while monitoring alterations in material properties Surface enhanced techniques can be transparently applied Other advanced near-field techniques can be readily applied 37

38 Scattering NSOM Raman TM Can Only Be Performed with an On-line AFM Lens Lens Only the complete integration that and Nanonics Provides allows for Evanescent Field NSOM Raman TM The Nanonics platform has a completely free area below the scanner that allows for evanescent field excitation No other SPM system allows this The Evanescent Field NSOM Raman TM or Scattering Raman TM Protocol: A laser beam is coupled into a prism to generate an evanescent field on the surface A sample on a glass slide is placed in contact with the prism. The evanescent field illuminates the sample, but no Raman signal is detected by the lens, because most of this evanescent field is non-propagating However, if an AFM probe with a nanoparticle is brought into contact with the sample, the light from the evanescent field is scattered away from the surface and into the lens of the microscope. This light includes Rayleigh light (the wavelength of the illumination) and Raman light which is separated by the Raman System. Furthermore, the nanoparticle could enhance the Raman signal considerably if the particle is made of gold or silver. The Raman spectrum, from each point in the sample, corresponding to the size of the nanoparticle is detected in the scattered light. This is one unique approach to super resolution Raman mapping that only the fully integrated Nanonics/ system provides. 38

39 Shadow NSOM Raman TM Can Only Be Performed with an On-line AFM Lens Near-field Optics: Subwavelength Illumination to Nanometric Shadowing Nature Biotechnology 21, 1380 (2003) Lens Raman Spectroscopy with CCD detection is ideal for difference spectroscopy Nanonics has produced AFM sensors with an opaque nanoparticle exposed to the optical axis Shadow NSOM Uses the Best Attributes of the miroraman with the Nanometric Positioning Ability of an On line AFM Patented Shadow NSOM Raman TM Uses the Best Attributes of microraman with On-line AFM. To Obtain a Shadow NSOM Raman TM : An opaque nanoparticle probe is brought in contact with a point on the sample with subnanometric AFM control A CCD spectrum is stored for this point The probe is retracted without sample movement. Another CCD spectrum is recorded and stored for the same point The Difference Spectrum is what was shadowed by the probe tip This is only possible with the /Nanonics integration The Tip & Sample Scanning Module, The NSOM/SPM 2000 TM, allows for tip retraction without sample movement, which is especially suitable for Shadow NSOM TM 39

40 A Summary of On-line AFM & Raman Advantages Raman intensities can be effectively compared Significant resolution improvements are achieved with AFM autofocus Other on-line advantages such as imposed pressure on MEMs or other external pertubations while monitoring alterations in material properties Surface enhanced techniques can be transparently applied Other advanced near-field techniques can be readily applied 40