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Nanoscale Chemical and Morphological Characterization of Ultrathin Atomic Layer Deposition Films

Atomic-layer-deposition (ALD) of various oxides can be utilized for diverse applications in the semiconductor industry, as well as lesser known applications such as drug passivation.  The self-limiting nature of the adsorption ensures atomic layer-by-layer deposition of a highly uniform and conformal coating with thickness control at the atomic level.  The nature and quality of the ALD film will depend on the precursor and post-annealing processes.  Currently, there is not an adequate analytical tool that can analyze the surface chemistry of such ultra-thin films.  In this note, PiF-IR spectra on ultra-thin films of Al2O3, ZrO2 and TiO2 are presented to show how IR PiFM can be used to characterize ALD grown films.  

Figure 1 shows the PiF-IR spectra of ALD grown Al2Ofilms of differing thickness, ranging from 1 to 50 nm.  The bulk FTIR from literature [1] is shown together with the PiF-IR spectra.  Interestingly, PiF-IR peaks are sharper and better defined compared to the bulk FTIR.  For all film thicknesses, most of the signals are observed between ~ 750 cm−1 to 910 cm−1.  For thicker films (> 5 nm), there is a slow increase in the signal in the 500 cm−1 to 750 cm−1 range with increasing film thickness.  According to literature, the peaks in the region of 500 – 750 cm−1 are assigned to AlVI, whereas the shoulder at 750 and the line at 890 cm−1 are assigned to AlIV[1]; AlIV and AlVI refer to 4-fold and 6-fold coordination for aluminum in tetrahedral and octahedral configuration, respectively.  The peak at around 870 cm−1 is assigned to the bending vibrations of Al-O bond whereas the band at around 670 cm−1 is assigned to Al-O-Al bond.  Based on these band assignments, the two films with 1 and 2.5 nm thickness may consist mostly of the tetrahedral coordination.  The films thicker than 5 nm seem to gradually include octahedral coordination also.  If we integrate the PiF signal, we can see that the total signal grew with thickness up to 10 nm and then decreased for the 25 and 50 nm thick film. This kind of behavior has been observed with PiFM measurements of polymer films. It arises due to the decrease in the magnitude of the tip-enhanced field as the tip moves away from the dielectric substrate [2]. The spectrum for the 5 nm thick film looks quite distinct from the other spectra, with new peaks that are not seen in other thicknesses. The prominent peak at around 1260 cm−1 is usually associated with Si-CH3, which may indicate that the silicon substrate may have been contaminated with organic contaminants before the ALD process.

Figure 1. PiF-IR spectra acquired on ALD grown films of various thicknesses. The bulk FTIR from reference 1 also presented for comparison.  

Figure 2 shows the PiF-IR spectra for 2.5 nm thick films of Al2O3, ZrO2, and TiO2, all grown via ALD. The y-scale has been adjusted for the three films to make the comparison easier.  Even for 2.5 nm thick films, the difference between the three types of oxides is quite clear.  The availability of a tunable laser that can extend down to below 600 cm−1 allows IR-PiFM to identify these oxides unequivocally. Note that the spectrum for ZrO2 shows peaks above 900 cm−1 that are like those observed for the 5 nm thick Al2O3 in Figure 1, most likely due to the contaminated substrate as well. 

Figure 2. PiF-IR spectra acquired on ALD grown films (2.5 nm thick) of different materials. 

Another advantage of IR PiFM is that the distribution of the ultrathin ALD film can be mapped with nanoscale spatial resolution.  Figure 3 presents the IR PiFM analysis of ALD grown halfnium nitride (HfN) on a grating structure that has a self-assembled monolayer (SAM) of organic molecules (1-octadecanethiol) that are masking the metal structures.  The topography of the structure (bottom left image) reveals that HfN growth was not as well defined as designed.  The PiFM image at 1471 cm-1 maps the SAM molecule via its C-H bending mode, and it shows that the SAM coverage has many voids, exposing the metal to HfN growth.  The image at 971 cm−1 maps the HfN film, and it shows that HfN has grown on all regions without SAM coverage, including the metal lines with SAM voids, creating the observed jumbled topography; the region in the yellow ellipse shows that HfN (red image) occupies the voids on SAM (the voids appear dark against the green SAM molecules). The region in the white ellipse tells a different story: the voids on SAM are not covered by HfN film, suggesting that the voids developed as a result of HfN growth process so that neither SAM nor HfN occupy those locations. 

Figure 3. Alternating lines of Cu and SiO2 with a half pitch of 50nm, have undergone SAM deposition to selectively coat the Cu lines followed by HfN ALD growth process.  PiFM images at 1471 cm−1 and 971 cm−1 highlight the SAM and HfN molecules, respectively. The bottom left image shows the jumbled topography due to the HfN that invaded the Cu lines as a result of incomplete coverage by SAM. The bottom right image shows the two PiFM images combined.  Sample Credit: IMEC    

As shown above, IR PiFM can be used to analyze ultrathin films grown by ALD technique for its local chemical states and spatial distribution with unprecedented spatial resolution and sensitivity.  

References:

[1]      R. R. Toledo et al., Nova Scientia, 10, 83 (2018)

[2]      J. Jahng et al., Anal. Chem. 90, 11054 (2018)

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