Direct Correlation of Raman and SPM Imaging

By placing a Nanonics AFM NSOM microscope with its free optical axis, onto a standard Raman microscope, for the first time it is possible to integrate the separate worlds of Raman and AFM.

The integration of the Nanonics MultiView system with the Renishaw RM Series Raman Microscope marks the beginning of a new era in high-resolution Raman spectroscopy.  

Although Raman scattering is currently undergoing a renaissance, it thus far has remained separate and removed from the proliferation of insights that Scanned Probe Microscopies (SPM) can give. In general, investigating a sample with scanned probe microscopies requires removing the sample from the micro-Raman spectrometer. This means that the exact region that was being interrogated by Raman cannot be found again for the chosen SPM imaging technique. Direct correlation of Raman scattering with an SPM technique has been a dream.

Today, with the release of the joint Nanonics and Renishaw MicroRaman system, this dream has become a reality.

Transparent Correlation of Raman and SPM Imaging

The Nanonics MultiView 1000™ system can be directly integrated into the Renishaw RM Series Raman Microscope. These microscopes employ the upright microscope configuration, and the Nanonics MultiView 1000™ is readily placed on the sample stage of such a microscope (see picture). All other systems combining AFM and Raman perform seperately either an AFM scan or a Raman scan. No system other than the Nanonics-Renishaw combination provides simultaneous and on-line data from both modalities. This enormous advantage resolves critical problems in Raman such as intensity comparisons in Raman images and provides for new avenues of improved resolution as described below

AFM without Optical Obstruction

The Nanonics patented cantilevered optical fibers are held between the microscope lens and the sample without obstructing any aspect of the Far-field optics. The tip in these fibers is exposed and illuminated by the lens of the microscope, allowing the user to view the exact region where the SPM and Raman information is being collected. 

Parallel Imaging 

With the combined system, one can now record, in parallel with Raman, a wide variety of scanned probe imaging modalities. For example, while the Si Raman peak of a microcircuit is being monitored to detect stress in the silicon, the Raman spectroscopist can simultaneously measure the circuit's micro-topography with AFM, and its NSOM reflectivity or its electrical properties, such as the dopant concentration. In addition, Nanonics provides software that can display all these images at once for direct and simultaneous comparison and analysis.

Parallel imaging of a Silicon Semiconductor Left: 9 x 7 micron AFM image Right: Raman intesity of the same region at 520nm/cm

High Resolution Raman Mapping with Z-feedback Conventional Raman Mapping 

There is a serious drawback to Raman Spectroscopy when studying non-smooth surfaces. As with all lens-based microscopy techniques, Raman suffers from the problem of out-of–focus light. When a sample is scanned conventionally under the illuminating beam of a Raman microscope, the uneven sample surface will scan in and out of the focal plane. As a result the resolution of the Raman mapping is limited by the large area of the unfocussed beam on the sample. In addition, the point spread function is significantly broader where there are contributions from the out-of-focus light.

As a result the Raman spectra of non-flat surfaces can be very misleading, and tend to misrepresent the true information that could be gained by using Raman.

Raman Mapping with Z-Control

The problem of out-of focus light can be solved by using a Z-feedback mechanism. With this feedback in place the surface of the sample can be kept in the focal plane throughout the scan.   All the Nanonics MultiView AFM platforms have completely free optical axis. This makes them the ideal add on to any Raman system to provide the Z-control necessary for true high resolution Raman mapping.

Examples of the Power of Integrated Raman/AFM

The difference between Raman mapping with and without Z-control can be seen clearly in the examples below. Here the vibrational mode of diamond is represented:

The Vibrational Mode of Diamond at 1334cm^-1

Without Z control

With Z control

The pair of images on the left shows the same area mapped with and without Z-control. The advantage of Z-control is made apparent by the differences between the two.

The image on the right is a collage of AFM topography and Raman intensity of the same sample at two different wavelengths. Note the differences in the intensity of the two images. For example the bright spots at the top of the image at 1334cm-1 which are absent from the image at 1525cm-1.
No other Raman system has sufficient control of Z position to pick out these differences.

Nanoindentation Correlated with Material Properties

To illustrate this combination of the worlds of AFM and Raman spectroscopy, actual data has been obtained on the Si stress problem mentioned above. A 14 x 14 micron AFM height image of a nanoindentation in Si is shown here (figure a) with a line scan (figure b) through a region of this AFM image.

The points on the AFM cross section are points at which Raman microscope spectra were collected. As a result of the nanoindentation, it can be seen that the silicon has been displaced. The question is whether or not these regions correspond to different phases of the silicon that can be correlated with the AFM measurements. Only Raman microprobe spectroscopy can give this information. The Raman spectra were obtained at the same time as the topography was being measured.

Local Stress of MEMs Devices

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 exceeding megapascals since the area of a probe tip is nanometric NanoRaman technology is the ideal for super-resolution silicon stress measurements in floating structures such as combs and forks.
 
The on-line AFM 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 (see below). No other AFM is capable of such a combination.

Raman Shift as a Function of Local Stress Location

Intermittent Contact Mode in Liquids

In addition, the joint Nanonics/Renishaw Raman and SPM system can operate in intermittent contact mode even in liquids. Thus, the whole world of NSOM/SPM imaging of biological materials in physiological media can now be directly correlated with Raman spectra.