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MFM Metrology and Analysis of Hard Disks

Introduction

Three hard disk samples (#1, #11, and #12) were submitted for magnetic force microscope (MFM) analysis, which is one function of the Nano-R2™ AFM systems. A PNI hard disk (4 GB) full of data was used as a reference. This work is to show the machine capability to image topography and map magnetic field of samples. Examples of measurement results are shown and discussed below.

Experiment

All sample measurement was carried out using the Close-Contact Mode of Nano-R2™ AFM equipped with a light lever scanner. The Zhgt and Zdem channels are open to obtain topographic and MFM images respectively. We used MFMR probes that were coated by 40 nm thick cobalt alloy. The probes were magnetized by a strong magnet prior to measurement.

The Nano-R2™ system was calibrated in the X, Y, and Z axes before measurement. We utilized the standard samples to calibrate Zheight channel, and resulting system’ accuracy was ± 1 %. The resonant frequency (65 KHz) of probes was tested to comply with the manufacturers’ datasheet. We minimized the drive amplitudes to enhance the measurement stability in the Close-Contact Mode. Raw scanning data were processed using the NanoRule+™ (V 2.5.04) software. Different scanning rates, resolutions and angles were used to achieve optimal results.

Results and Discussion

Topography and MFM

We scanned at least 6 different localities of the #1 hard disk (inside three marks). The sample was clean and uniform. Figure 1A is a typical topographic image (20 × 20 μm) acquired by the Close-Contact Mode. The surface was characterized by polishing lines. The MFM image (Figure 1B) of the same area was acquired by adjusting the setpoint of Z-piezo travel. This allows the magnetic tip to fly about 60-100 nm over the surface of hard disk. The stray field is sensed this way. As shown, information stored in bits expresses “high” or “low” level of signal in MFM because of different orientations of local magnetic domains in hard disk.

Figure 1: #1 Hard Disk (A) Topography; (B) MFM image acquired by raising the
magnetic tip ~ 80 nm above the surface.

We scanned 3 different localities of the #11 hard disk (inside the mark), and found that the surface topography is rougher than the #1 hard disk, presumably because of the degausser process. The polishing lines were similar to the #1 hard disk, but micron-sized particles appeared on the surface (Figure 2A). Figure 2B shows the MFM image of the same area. The regular bits in Figure 1B were never found while raising the magnetic tip. Instead, the magnetic residues (arrow indicated) appeared on the whole surface.

Figure 2: #11 Hard disk (A) Topography; (B) MFM image acquired by raising the magnetic tip ~80 nm
above the surface. The bit microstructures (Figure 1B) were never found on this sample surface.

Measurement and Analysis

NanoRule+™ is a powerful tool to process and quantitatively analyze acquired images. We used NanoRule+™ to measure 3D data of the samples. Example results of 3D image and data analysis are shown in Figures 3 and 4. Figure 3 is a high-resolution scan of the #1 hard disk. Each individual bit and spacing between two adjacent bits can be clearly seen.

Figure 4 illustrates the line analysis for bits found on surface of the #1 hard disk. The 9.96 × 9.96 μm MFM image was on different spot from Figure 3. The pair 1 represents the linewidth of the bit as 0.247 μm; the pair 2 represents the last bit as 72.789 mv above the surface, which can be used to analyze status of magnetic dipoles. The bit executed a repelling force

Figure 3: 3D MFM image of #1 hard disk
(9.96 × 9.96 μm).

and pushed the tip upward during scanning because of magnetic coupling between tip and the sample; this results in the bit appearing higher than the surface. The pair 3 represents the bit-to-bit spacing of 362 nm. Another line analysis shows a bit is about 1.45 μm long (not shown). The measurement method was similar to Figure 4.

In order to verify the degaussering process, a PNI reference hard disk (4GB) full of data was analyzed by MFM. Figure 5 shows a typical MFM image. Similar to the #1 hard disk, the regular bit tracks were found uniformly on the disk surface.

Figure 4: Example of analysis of bit height, width and bit-to-bit spacing on #1 hard disk. The
pair 1, 2 and 3 represent linewidth of 247 nm, height of 72.789 mv above the surface, and
spacing of 362 nm, respectively. Bit length is about 1.45 microns.

Figure 5: MFM image of the PNI reference hard disk (4 GB).

Conclusion

This measurement shows example results of topography and MFM images on reference hard disks. The Close-Contact Mode and cobaltcoated probes were used for all cases. Quantitative analysis of bit 3-dimension in MFM images was performed by using NanoRule+™. We did not take the average of the data.

Pacific Nanotechnology’s Nano-R2™ AFM is shown fully capable for study and verification of the degaussering process and results on the storage media.