Nanoscale Infrared Spectroscopy, Technique and Applications

Abstract

Conventional IR microscopy is widely used in a range of research but is limited from applications where feature sizes are approximately 10 microns and lower. This is due to the diffraction limit of the mid IR illumination which limits the resolution to roughly three times the wavelength.1 Atomic force microscopy (AFM) has been very successful in visualizing the structure of samples at the nanoscale. Researchers have long recognized that combining the nanoscale resolution of the AFM with the chemical identification of IR spectroscopy would provide a powerful analytical technique to chemically characterize regions of the sample. Several AFM probe-based techniques have been developed in an attempt to surpass the diffraction limit of conventional IR measurements. They include both traditional near field scanning optical microscopy (NSOM) as well as aperatureless near field scanning microscopy (ANSOM). The ANSOM technique has demonstrated impressive lateral resolution 2 but has not produced rich spectra that can be used to characterize a broad range of vibrational resonances. Additionally the ANSOM technique suffers from a limited signal to noise restricting analysis to specific sample classes. Other techniques developed to extend the resolution of IR spectroscopy are based on measuring the local temperature rise from spectral absorption through the use of AFM cantilevers integrated with conventional Fourier Transform Infrared (FTIR) spectrometers.3 These approaches allow broader spectrum measurement than near-field approaches, but the spatial resolution is typically limited due to thermal diffusion to the scale of many microns. We have developed and utilized a novel lab-based instrument that combines AFM and infrared spectroscopy to enable chemical characterization of samples at sub-micron scales. The instrument makes use of a technique called photothermal induced resonance (PTIR).4 PTIR is accomplished by using a traditional AFM probe to sense the rapid local expansion of a sample that absorbs light from a pulsed IR source. As shown in figure 1, the sample is illuminated with a pulsed, tunable IR light source in a configuration similar to the attenuated total reflection (ATR) technique used in conventional IR spectroscopy. When the IR illumination is at a wavenumber which is an absorption band of the sample, it creates a rapid thermal expansion wave that excites resonant oscillations of the AFM cantilever. By measuring the amplitude of the cantilever vibrations as a function of the IR source wavelength, a local IR absorption spectrum can be created. The IR source is continuously tunable from 3600 to 1200 cm -1 . This covers a major portion of the mid-IR including important CH, NH and CO bands, as well as carbonyl and amide I/II bands. We have used this novel technique to image and chemically identify a variety of polymer samples. Figure 2 shows measurements performed on a blend of polycarbonate (PC) and acrylonitrile butadiene styrene (ABS) typically used as an impact modified polymer. The two components could be i...

Nanoscale Chemical-Mechanical Characterization of Nanoelectronic Low-kDielectric/Cu Interconnects

“…For AFM-IR characterization, the polished sample was placed onto the sample holder of a modified nanoIR instrument. 14,22 This allowed a broadly tunable nanosecond optical parameter oscillator (OPO) IR laser to directly impinge onto the sample with a typical pulse length of 10 ns and a 1 kHz repetition rate. The IR laser was oriented with respect to the sample plane at a polar angle of 30 • and an approach angle of 60 • from the front of the AFM probe such that the Arrow AFM probe with a force constant of 0.07-0.4 N/m (NanoandMore, Switzerland) or Access-C AFM probe with a force constant between 0.06-0.9 N/m (AppNano, Mountain View, CA) could co-localize above the sample simultaneously (Fig.…”

“…53 For AFM-IR imaging, a spatial resolution of 25-100 nm has been previously demonstrated using these conditions. 13,14 In comparison, the spatial resolution for a typical transmission FTIR measurement over a similar wavenumber range is 3-10 microns.…”

“…9 However, the development of nanoscale chemical structure metrologies has been hindered by the difficulty of scaling popular micron-scale techniques such as infrared (IR) absorption spectroscopy [10][11][12] into the nanometer regime due to diffraction limits defined by the Rayleigh equation. 13,14 To overcome these limitations, a number of approaches combining optical spectroscopy with scanned probe microscopy (SPM) have been proposed and demonstrated. [13][14][15][16][17][18][19][20] Of these, the atomic force microscope (AFM) IR technique, which is based on photothermally induced resonance (PTIR), 21,22 has recently garnered significant interest due to a demonstrated spatial resolution of 25-100 nm, and a close correlation to micron-scale Fourier transform infrared (FTIR) spectroscopy.…”

“…13,14 To overcome these limitations, a number of approaches combining optical spectroscopy with scanned probe microscopy (SPM) have been proposed and demonstrated. [13][14][15][16][17][18][19][20] Of these, the atomic force microscope (AFM) IR technique, which is based on photothermally induced resonance (PTIR), 21,22 has recently garnered significant interest due to a demonstrated spatial resolution of 25-100 nm, and a close correlation to micron-scale Fourier transform infrared (FTIR) spectroscopy. 13,14 This combination has resulted in the utilization of AFM-IR in a number of biological, [23][24][25] polymeric, [26][27][28] and semiconductor [29][30][31] applications.…”