Using ATR to Study Coated SI Wafers

ATR spectroscopy is a surface-sensitive method of infrared sampling, which is commonly used in the analysis of thin films and monolayers on surfaces. Areas that require this approach include the semiconductor, electronic and optical industries. To examine coatings on semiconductor substrates like Ge and Si, as well as metallic substrates such as gold and aluminum, multiple approaches have been undertaken.

These methods, for reflective substrates, include grazing angle specular reflectance and grazing angle Ge-ATR (the grazing angle specular reflectance is a non-contact method and the latter requires contact between the coated surface and the ATR crystal). The methods which are frequently used for transmissive substrates include the following: making ATR crystals from coated wafers, holding a coated wafer to an ATR crystal under above-critical angle conditions and transmission.

Three methods are compared in this work, all of which examine double-side polished coated wafers: transmission, grazing angle Ge-ATR spectroscopy, and an ATR technique where the infrared is coupled into the wafer. This comparison will be expanded upon in the form of an overview listing the benefits and drawbacks of the various methods used for the infrared analysis of thin coatings on single-side polished wafers and coatings on metals.

Experimental

A commercial FT-IR spectrometer was used to collect all spectra at 8 cm^-1 resolution and a DTGS detector was used to undertake 64 scans. Three Si wafers were examined to investigate each technique.

The following list elaborates on the three samples: one double-side polished (DSP), 0.770-mm thick single-side coated wafer with an unknown SAM; one DSP, 0.50 mm-thick single side coated wafer with a toluene residue atop 1000 Å thick Al coating (EMF); as well as one single-side polished (SSP) 0.770 mm-thick wafer with an unknown SAM. By using a pipette to disperse toluene (Alfa Aesar, CAS# 108- 88-3) on one side of the wafer and then letting it evaporate before collecting data, the toluene residue was obtained. For the toluene coated sample, the data was then baseline corrected.

Figure 1. Each technique’s accessories: Four-Pass Transmission (left), WalfIRTM (middle), VariGATRTM (right).

Figure 2. Each sample’s transmission spectra, where Sample 1 (red) is a DSP single-side coated wafer with an unknown SAM coatings, and sample 2 (blue) is a DSP single side coated wafer with a toluene residue atop a 1000 Å thick Al coating.

A four-pass transmission accessory was used to collect sample spectra for the transmission method. This possesses an incident angle of 75° and a built-in polarizer, therefore minimizing interference fringes. By dividing the absorbance by a factor of 4, data was adjusted from four pass to single-pass transmission.

To obtain the grazing angle ATR measurements, the VariGATR™, a variable grazing-angle accessory with a mounted germanium crystal, was used at a 65° incident angle. To perform this technique, it requires contact between the Ge crystal and the coated surface of the Si wafer. In this case, the contact was achieved by using the built-in slip clutch to compress each sample against the crystal.

The WafIR™ technique was used for the ATR technique, in which the infrared radiation is coupled into the wafer. This is a multiple reflection accessory which works by using the Si wafer as the ATR crystal, coupling the light into and out of the wafer using two 45° prisms. It provides 33 reflections from the coated surface for a 0.770 mm-thick silicon wafer and 51 reflections of the coated surface of a 0.50 mm-thick Si wafer.

The design of the WafIR is such that it is created to limit the contact area on the sample and to apply contact to the coating outside the sampled area. The 112 in-oz (0.79 N-m) slip-clutch is used to apply force.

Results and Discussion

The spectra of all the DSP single side coated wafer samples measured with the four-pass transmission accessory are displayed in Figure 2. As a result of the C-H stretches, each of the samples has peaked at 2922 cm^-1 and 2852 cm^-1. Weak band intensities are shown by the DSP single-side coated (non-metallic coating) wafer sample for transmission.

In Figure 3, the spectra of sample 1 measured with Ge-ATR and internal-wafer ATR are compared. For both methods from the C-H stretches, there are peaks at 2922 and 2852 cm^-1 for both methods. It is significant to note the relatively strong intensity of the C-H stretches with the internal wafer ATR technique. These band intensities are shown by the internal-wafer ATR spectrum to be about 10 times greater than Ge-ATR. This sensitivity has been caused by infrared energy coupled into the wafer, which results in multiple bounces within the wafer and greater interaction of the evanescent wave with the sample.

Figure 3: This figure shows Internal-wafer ATR (red) vs Ge-ATR (blue) spectra of a DSP single-side coated wafer with an unknown SAM coating.

Figure 4: Internal-wafer ATR (red) vs Ge-ATR (blue) spectra of a DSP single side coated wafer with a toluene residue atop a 1000 Å thick Al coating is displayed here.

Figure 5: Figure 5 shows the transmission spectra for a SSP single-side coated wafer (non-metallic coating), where the polished side up is red and blue is the polished side down.

Figure 4 demonstrates the spectra of toluene residue on a single side 1000 Å thick aluminum-coated DSP Si wafer measured using Ge-ATR and internal-wafer ATR. For comparison, Figure 2 displays the transmission spectrum for this sample. For the toluene residue, the transmission is more sensitive than internal-wafer ATR with peak intensities which approximate to double that of the internal-wafer ATR.

However, the sensitivity displayed by Ge-ATR is clearly more than internal-wafer ATR, undergoing peak intensities, which are about twenty times greater than the latter. Ge-ATR has a greater sensitivity overall when all three methods are compared concurrently, which demonstrates that the best method depends on the sample type.

The grazing angle Ge-ATR is ideal for use with DSP single-side coated wafers with a metallic coating. Unlike the other samples, which are all DSP single-side coated wafers, an SSP single-side coated wafer with a non-metallic coating was examined for further investigation. For comparison, using each method, a spectrum was measured of the wafer placed polished side up and polished side down.

It was difficult to determine the sample spectra for transmission (Figure 5) because the baseline produced was sloping, which was likely attributed to scattering. This issue of scattering causes immense difficulties in analyzing such a sample using internal-wafer ATR (Figure 6). The result is spectra with a lot of noise, regardless of wafer orientation.

Figure 7, however, portraying the Ge-ATR method, shows that the polished side down spectrum displays absorption in the 800-900 cm^-1 spectral region. For the polished side up measurement, no absorption is observed. This is due to the poor contact between the ATR crystal and the unpolished surface of the wafer.

Conclusion

In conclusion, it has been determined that the effectiveness of the internal wafer ATR and the Ge-ATR method is dependent on the size of the wafer and its surface finish, as well as its material. The band intensities produced by the internal wafer ATR method are much stronger by comparison than the grazing angle Ge-ATR technique and the transmission measurements for the DSP single-side coated wafer with non-metallic coating.

In contrast, however, the most sensitive was Ge-ATR in examining the DSP single-side coated Si wafer with an optically thick 1000 Å metallic coating, in addition to the SSP single-side coated Si wafer. The weakest method overall for coated Si wafers was the transmission.

Figure 6: This figure shows the internal-wafer ATR spectra of an SSP single-side coated wafer (non-metallic coating), where polished side up is red and polished side down is blue.

Figure 7: Figure 7 displays the Ge-ATR spectra of an SSP single-side coated wafer (non-metallic coating), where polished side up is red and polished side down is blue.

To see if the internal-ATR method sufficiently retains polarization to allow the study of the orientation of species on the wafer surface (and to compare its sensitivity to that of the grazing angle Ge-ATR), further work is in progress. More work is necessary for the form of study done on a metallic coated SSP single side coated wafer, which in theory is predicted to have similar results to an SSP nonmetallic single side coated wafer.

Table 1. Comparison of the techniques.

This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.

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