Scanning Opto-Acoustic Microscope


The goal of this project is to develop a new type of high resolution all-optical opto-acoustic instrument (SOAM) in which optical methods are used for both the generation and detection of sound. The proposed opto-acoustic microscope is designed to operate in a “planar” wide area or scanning probe modes, to enable novel means for study of a wide range of samples with resolution in the 10-100nm range in structures that are difficult to access by presently available advanced research tools, such as AFM or SEM, etc. Below is an SEM image of one type of samples that can be measured by SOAM, but is not accesible to technique such as AFM due to its high aspect ratio, simply because the AFM tip will not be able to get into the deep trenches of the (as shown) sample.

Figure 1. SEM image of a sample with high aspect ratio.

The SOAM instrument concept is being developed towards a prototype instrument which we here separate to two  configurations for different regimes of application, where the difference lies in primarily their lateral (x-y) spatial resolution: (i) the “planar” instrument where the x-y resolution is limited by the spot size of the incident laser beams (down to ~ 1 μm resolution) with applications to e.g. measurement of nanostructures which contain a (laterally) periodic variation of features; and (ii) the scanning probe instrument, where ultrasound in the frequency range of ~ 10GHz (wavelength <50nm) is focused to a probe spot size of 50 nm and below). In each case, the depth resolution (z) of the instrument is aimed to reach ultimate nm scale ‘sensitivity’.

Our recent technical innovations include new optoacoustic component concepts, such as the incorporation of an optical (Fabry-Perot) reonator cavity to enhance the optoacoustic efficacy of the SOAM probe. Fig. 2 shows a schematic of an Al-mirror pair optical cavity for the “planar” instrument. On a lab bench, our “ultrasound echo ranging” experiments on deep etched silicon trench samples (from Novellus Inc) have shown an initial ability to investigate the quality of periodic trench samples with depth/width ratio at least 10 (e.g. 420 nm/30nm). A sample of experimental data is shown in Fig. 3. Very importantly, advanced simulation techniques have been developed to aid in translation of the experimental data to detailed decoding of deep trench features in terms of small deviations from perfect/ideal structures.

Figure 2. "Planar" cavity enhanced SOAM

Figure 3. Planar SOAM configuration and measurment of depth of narrow trenches.

We have begun the design of a SOAM instrument prototype, where optical, acoustic, and supporting electromechanical components are integrated. An overview schematic of such an instrument is shown in the figure below.

The spacing between the sample and the cavity mount is controlled by two piezo stages combined to give a translational range down to sub-micron. CCD camera is used to assist the locating of the structured sample.