Sub-surface Characterization of Solids

NFMM has been successfully used to detect the topography of metallic features coated by a conductive layer and insulating layer. Feature identification has been carried out by detecting changes in the output signals of NFMM (mag(S11), phase(S11), voltage) as the NFMM scans over a homogenous,

uniform and known film containing the buried features; any detected change in the output reading represents a change in the buried material. In [27], a commercial hybrid AFM-NFMM was used to image

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buried Al defects in a 120 nm thick Ni film deposited onto a Si substrate. The probe operated in contact mode and scanned over the testing sample using several operating frequencies ranging from 1 to 6 GHz in order to image features at different depths from 0 nm to 125 nm. Phase images shown in Figure 2-8, reveal that the Al patterns are resolved for an operating frequency of 1.878 GHz.

Figure 2-8 Testing sample and measured phase of reflection coefficient image at different operating frequencies from [27] © 2011.

Gold traces buried in an 800 nm thick SiO2 layer have been imaged before and after 1000 thermal

cycles also using a commercial AFM-NFMM operating in contact mode at 7 GHz [29]. Two limitations of the AFM-based NFMM are that the vertical travel of the probe is dictated by the z-travel of the AFM head (which can be only a few micrometers) and the depth of study is limited to only about hundreds of nanometers since the probes use cantilevers whose tip size is about 100 nm. Attempts to solve these limitations include the use of non-AFM probes with larger tip size. In [28], a coaxial resonator-based NFMM operating in non-contact mode at a frequency of 763 MHz is used to image MMIC spiral inductors, pads, and traces embedded at 0.8m and 8.6m below the surface using probe tips of radius 10

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operate the commercial AFM-NFMM in transmission mode (TM-NFMM) rather than in reflection mode [45]. For this purpose, an SMA-type-probe acts as a radiating antenna and an AFM cantilever-type tip collects the signal that propagates from the SMA probe through the sample as illustrated in Figure 2-9. Images of surface topography and S21 parameters showed that the TM-NFMM is able to distinguish

different doping levels along the cross section of a bulk silicon substrate. Disadvantages of this approach include the limitation of samples to be studied and the complex sample positioning setup.

Figure 2-9 Experimental setup of the TM-NFMM and measured topography and S21 parameters images of

the testing sample (bulk silicon with varying doping profile along the cross-section) from [45] © 2014.

Subsurface dielectric features have also been imaged at 60 GHz using a microstrip line resonator- based NFMM [64]. The imaged feature is a 200 m wide resin ring pattern buried under 30 m AZ4562 resin. The microwave probe consisted of a gold microstrip line deposited on an alumina substrate. The probe was tapered to 7 m and the scanning was performed at a standoff distance of 5 m. Magnitude and phase-shift images of reflection coefficient showed that the probe was able to resolve the buried ring.

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Imaging of subsurface features immersed in a lossy liquid was presented in [65], where a Split- Ring Resonator (SRR) resonating at 1.2 GHz with dimensions of 20 mm x 20 mm is excited by a rectangular loop. This probe was used for detection of an Aluminum cube (3.2 mm x 3.2 mm x 3.2 mm) immersed at different depths (1 mm – 4 mm) in 1% NaCl solution. In this work, it was demonstrated that the SRR increased the evanescent fields around the probe, compared to the single rectangular loop, and that the increase in the fields improves the sensitivity of the probe to detect the aluminum box. Measurements consisted of 1-D scan profiles of phase change of the probe versus Y-distance showing the presence of the Aluminum box when it was immersed from 1 mm to 4 mm.

Before proceeding further with the background review of cavity perturbation theory and the modeling of the tip-sample interaction, a summary of the capabilities, advantages and limitations of several probes used in the NFMM discussed in this section is summarized in Table 2-3. Six criteria are selected for this comparison including vacuum requirements, approximated scan size, materials that can be characterized, probe size, depth of study, and Q. The designs with the highest Q are those that use cavity and DR-based probes; higher Q improves the sensitivity of the NFMM. The main disadvantage of these probes is that they are bulky compared to the other designs which complicates the integration with distance following feedback systems. On the contrary, coaxial-transmission line and planar-based probes are compact but their Q is not high. STM and AFM-based NFMM designs allow imaging of electromagnetic properties with very high resolution since the probe tip size is on the order of nanometers. This advantage may turn into a limitation if subsurface sensing at depths larger than a few hundreds of nanometers is desired. Additionally, these designs are intended to be used to study samples with very smooth surface and particularly for the AFM-based design, the scan area is limited to about 50

m x 50 m. Finally, important limitations of the STM-based NFMM design are that it requires vacuum to achieve the very high resolution images and the samples should be conductive.

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