Analysis of Angstrom-Level Lubricant Thicknesses on Hard Disk Surfaces with FT-IR Spectroscopy

Manufacturing processes in the semiconductor and electronics industry have advanced such that depositing very thin layers of lubricant on hard disk drives is now routine. Such thin layers of lubricants often have thicknesses at the angstrom level, pushing the limits of Fourier transform infrared spectroscopic sensitivity, which is the primary coating thickness measurement tool. Presented here is a method that demonstrates the ability to measure 11-Å layers, with potential for sub-1-Å determinations.

The electronics and semiconductor industries continue to demand more advanced and sensitive sample analysis techniques to keep up with the continuing miniaturization trends in their products.

The demand to measure and characterize smaller and thinner samples is becoming increasingly common these days because of technological advances. Hard disk manufacturers are developing new processes whereby extremely thin layers of lubricant can be coated onto hard disk surfaces. This poses a challenge as the lubricant thickness approaches sub-1-Å levels.

Such thin layers are not straightforward to analyze with traditional Fourier transform infrared (FT-IR) spectroscopy. Findings in this article focus mainly on providing a solution to characterizing angstrom-level thin layers of lubricant on hard disk samples.

Experimental

An Agilent Cary 660 FT-IR spectrometer coupled with a PIKE Technologies Advanced Grazing Angle (AGA) accessory was used to analyze three hard disk samples for lubricant thickness measurements. The AGA accessory was mounted onto the main sample compartment of the spectrometer. The three hard disk samples (HD1, HD2, and HD3) have highly reflective, smooth surfaces with lubricant thicknesses of 12.5 Å, 14 Å, and 11 Å, respectively.

Sample collection conditions are as follows: The detector was a linearized narrow-band MCT detector. The spectral resolution was 16 cm^-1. There were 64 co-added scans.

Agilent Resolution Pro software was used for quantitative calibration.

Results and Discussion

The hard disk samples HD1, HD2, and HD3 were analyzed in grazing angle reflectance mode using the grazing angle accessory. This accessory is designed to optimize grazing angle measurements at 80°. Figure 1 shows the optical design (1) of the accessory, in which the beam from the spectrometer is focused onto a circular pin mirror. The reflected beam from this mirror strikes the sample at an 80° angle of incidence. The area of the sample that is being measured is equivalent to the size of the circular pin mirror, in this case, a 0.5-in. diameter spot size.

Figure 1: Optical design of the grazing angle accessory.

Measurements were taken with and without a polarizer and the respective results were compared. For each sample, four different surface locations were analyzed, with quadruplicate measurements at each of the four locations. The sample spectrum was ratioed against a background spectrum obtained with a gold mirror. An average FT-IR spectrum was obtained from the 16 spectra collected from each of the three hard disk samples. The subsequent three FT-IR spectra were used to study and deduce the difference in lubricant thickness between the three hard disk samples. Figure 2 shows the three overlaid spectra obtained without the polarizer.

Figure 2: IR Absorption band at 1290 cm-1 for HD1, HD2, and HD3 samples, without polarizer.

The absorbance intensities of the three samples were quantified at the IR absorbance peak of 1290 cm^-1. This peak typically represents the characteristic fluorocarbon used in the coating of surfaces of such hard disk drives.

Quantitative Calibration

A calibration can be based on a user-defined peak area or peak height as shown in Figure 3. The left and right baseline points of the peak can be altered by simply clicking and dragging the points dynamically.

Figure 3: Quantitative analysis: peak height of sample HD1.

The quantitative calibration was prepared from the different thicknesses of the hard disk samples of known thickness. A data point in the calibration curve can be included or excluded from the calibration. The quantitation software selects the data points for plotting the calibration curve, as well as the samples' respective thicknesses.

Figure 4 gives a representation of how the calibration shows excellent linearity and a correlation coefficient of 1.000.

Figure 4: Linear calibration plot of fluorocarbon peak height at 1290 cm-1.

As shown in Table I, the peak height absorbances correspond accordingly to the known sample thickness, that is, HD1, HD2, and HD3 — 12.50 Å, 14.00 Å, and 11.00 Å, respectively.

Table I: Corresponding absorbance intensities of samples HD1, HD2, and HD3

Use of Polarizer

A polarizer was used to get maximum sensitivity and increase the signal. P-polarized light will enhance the surface species measured, making features in the spectrum more prominent. The use of a polarizer with a liquid-nitrogen-cooled MCT detector is an ideal way to achieve optimum results. The results shown in Figure 5 verify this approach, whereby the peak intensities of HD1, HD2, and HD3 are enhanced by a factor of two.

Figure 5: Increased absorbance intensities with use of p-polarizer.

Conclusion

The quantitative calibration curve confirms excellent linearity and sensitivity of the data collected from the different lubricant thickness samples HD1, HD2, and HD3. The use of a polarizer (p-polarized light) also intensified the specific peak of interest by a factor of ~2, facilitating the possibility of sub-1-Å measurements.

The recent trend and popularity in the semiconductor and electronics industries to measure monolayer thin lubricants continues to increase. Lubricant monolayers can decrease to sub-1-Å levels, posing a significant challenge to FT-IR analysis. An FT-IR spectrometer combined with a grazing angle specular reflectance accessory and linearized high sensitivity MCT detector is suitable to measure ultrathin layers of lubricants on hard disk surfaces.

References

(1) PIKE Technologies Advanced Grazing Angle Accessory User Manual.

(2) S. Boyd and J. Kirkwood, "Quantitative Analysis using ATR-FTIR Spectroscopy," Application Note SI-1374.

Heng Soo Chin and Wong Chee Ping are with Agilent Technologies in Yishun, Singapore. Dr. Mustafa Kansiz is with Agilent Technologies in Mulgrave, Victoria, Australia. Direct correspondence to: mustafa.kansiz@agilent.com