Scanning Probe Microscopy for Topographical and Statistical Analysis

Compact Discs (CD) and Digital Versatile Discs (DVD) are popular data storage now. The information unit of CD/DVD is a so-called pit. CD and DVD are made by stamping of a polycarbonate. Nickel stampers are usually used as a stamp that contains bumps. These bumps form pits. The quality of the group of discs depends on the quality of a one stamper, i.e. preliminary control of stamper is necessary.

SPM for Pit Analysis of CDs and DVDs

Magnetic properties of nickel stamper and its large size (140mm) make difficulties for electron microscopy control. There are also devices for surface analysis based on electric principles, but such measurements cannot visualize pit geometry. SPM is an ideal tool for pit geometry analysis owing to high resolution, high measurement rapidity, and the possibility of non-destructive measurements of large samples (Fig.1). The use of the SPM under the manufacturing of the stamper for CD/DVD is the perspective method of quality monitoring. SPMs help to check the stamper quality and to decrease the risk of defect appearances. It is possible to study topography changes of the stamper under various external influences, for example, observation of the deformation owing to polishing or blow. It is possible also to observe topography changes owing to heating during the pressing of a polycarbonate. During manufacturing, the several parts of stamper surface are controlled and the quality of the stamper is appreciated. Also, selective control of discs is executed.

Single pit of CD measured by SPM.

Figure 1. Single pit of CD measured by SPM.

Observing the Magnetization Reversal Process

The experiments in an external magnetic field enable us to observe the magnetization reversal processes. The design of the NT-MDT SPMs allows applying the various external magnetic fields during magnetic force microscopy (MFM) measurements. The present results were obtained with SOLVER P47 equipped with an electromagnet, which can produce magnetic fields up to 500Oe. The studied sample was cobalt polycrystalline patterned film (40nm thick) deposited on high-oriented pyrolytic graphite in the form of the micron size rectangles (Fig. 1).

Equipment and Methods

As appears from the above possibility of the measurements of the large sample is the main demand of CD/DVD industry to SPM. There are three basic configurations of NT-MDT devices that meet this demand and provide effective analyzing of CD/DVD:

1.      SPM Solver P7LS (Fig. 2). The motorized positioning stage, the vacuum holder for samples with the size up to 300 mm in diameter, optical viewing system, and automatic approach make this device most convenient for CD/DVD industry.

2.      SPM Stand Alone SMENA in combination with a special designed SMENA base for large samples (Fig. 2). This device can provide the same noise level as Solver P7LS provide. Such a design doesn't include a vacuum holder and motorized positioning stage. The automatic approach is available on requirement. The positioning of the sample is carried out by hand.

3.      SPM Stand Alone SMENA with special long leg, which allows CD/DVD placing between them.

The NT-MDT SPM Solver P7LS.

Figure 2. The NT-MDT SPM Solver P7LS.

The user has to provide the hard fastening of the sample under tip in this case.

Modes of Operation for Topography Measurements

There are two main modes for topography measurements: contact and semi-contact. Semi-contact mode uses cantilever that oscillates at its resonance frequency. As a result, the tip and sample are in contact with only a small part of the oscillation period. This leads to an appreciable reduction in the destructive action of the friction and capillary forces. Therefore semi-contact mode is more suitable for soft materials such as polycarbonate.

Whisker Type Cantilevers

"Whisker type" cantilevers being manufactured by NT-MDT provides more accurate measurements than standard silicon cantilevers. This type of cantilever consists of a standard silicon cantilever augmented by a carbon needle with a high aspect ratio. Focused Ion Beam (FEB) grows the carbon needle on the end of the tip. "Whisker type" cantilever is suitable for more precise measurements of abrupt steps. Moreover, carbon is a hydrophobic material, therefore the adsorption layer of water is absent on the carbon tip. This also increases the accuracy of the measurements.

Statistical Treatment of Data

The acquired results need statistical treatment. NT-MDT software contains a menu of tools "Grain analysis" (Fig. 3), which was developed for statistically treating the particles lying on a flat surface (also suitable for pits). With the help of these tools it is possible to determine the geometrical size of pits, approximate pits by different figures, such as rectangular, ellipse, and rectangular with rounded sides (imitation of CD/DVD pits). On the basis of such approximation the direction of axes, the angles between them, and other parameters are determined.

Main view of the menu "Grain analysis"

Figure 3. The main view of the menu "Grain analysis".

Parameters Determining CD/DVD Stamper Quality

CD/DVD parameters for checking The main test characteristics, which determine either CD/DVD or stamper quality, are:

a)     Pit (bump) form and pit (bump) size. For example, pits (bumps) must have a flat area (Fig. 4a). If such a flat surface is not observed (Fig. 4b), then an error during reading can be made. The depth of the pits is an important parameter owing to its influence on the amplitude of signal during reading. The SPM Solver P7LS is able to measure the pit height with a resolution of a fraction of a nanometer.

b)     Slope of the pit (bump) side.

c)     Roughness of pit (bump) surface, which influences reflection of the laser beam.

d)     Track pitch and track stability.

e)     The ratio of pit volume to the volume of the single pit. This is an important technology characteristic of CD/DVD.

f)       A number of pits per area unit, i.e. data density.

AFM images of the surface of the nickel stampers made by different technologies. Images obtained by semi-contact mode, SOLVER P7LS.

Figure 4. AFM images of the surface of the nickel stampers made by different technologies. Images obtained by semi-contact mode, SOLVER P7LS.

NT-MDT software can calculate all these parameters for the investigated area.

Surface Quality and Surface Defects

Also, SPM checks the quality of the CD/DVD surface by revealing surface defects. Two scratch and knoll are seen in Fig. 5.

SPM image of defective surface of the CD disc

Figure 5. SPM image of the defective surface of the CD disc.

Stamper Topography

The knowledge of either pit or bump parameters measured at different places of the sample allows effective control of the CD/DVD manufacture. Fig. 6 shows the topography of a stamper. After measurements of the topography the menu "grain analysis" was applied to obtained topography data. Black lines on Fig.7 mark contour of bumps at preset levels; red lines are an approximation of bumps by an ellipse. The different parameters were calculated for the bump array. Some of them show in Table1. These defects influence the quality of data reading.

Surface topography of a CD/DVD stamper.

Figure 6. Surface topography of a CD/DVD stamper.

Table 1. Analysis of surface features of the CD/DVD stamper.

 

Grain 10

Grain 6

Grain 9

Z-Lev

116.70nm

116.70nm

116.70nm

Dvolume

291.08nm

361.17nm

415.57nm

Dsquare

756.80nm

1056.55nm

1270.31nm

Length

883.93nm

1581.00nm

2153.94nm

Width

825.00nm

899.00nm

953.89nm

Xpos

6046.41nm

4112.79nm

3468.24nm

Ypos

4404.36nm

2117.78nm

4112.79nm

Orient

85.28°

85.28°

85.28°

Error

13.20%

19.20%

22.30%

Where: Z-Lev is preset level of section (Fig. 7); Dvolume (Dv) - effective size of bump above level ZLev (3√V); Dsquare(Ds) - effective size of bump at level Z = 116.7nm (√S); Length - bump length; Width - bump width; Xpos,Ypos - coordinates of the bump center; Orient - angle of bump orientation; Error - error of approximation of real bump by ellipse.

Section of a stamper bump

Figure 7. Section of a stamper bump.

Other Information and Parameters Determined

Also, information about the slope of the bump sides (Fig. 7) and other parameters is available. Table 2 presents ratios of areas (αS) and volumes (αV) for different bumps.

Table 2. The ratio of areas and volumes for different bumps.

αv10-6

1.24

αS10-6

1.397

αv10-9

1.42

αS10-9

1.68

αv9-6

1.145

αS9-6

1.202

The determined parameters can be used for the analysis of bump geometry, revealing the technical defects and other applications.

 

This information has been sourced, reviewed and adapted from materials provided by NT-MDT Spectrum Instruments.

For more information on this source, please visit NT-MDT Spectrum Instruments.

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