Background

Compact Discs (CD) and Digital Versatile Discs (DVD) are popular data storage now. The information unit of CD/DVD is 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 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 ideal tool for pit geometry analysis owing to high resolution, high measurement rapidity and possibility of non-destructive measurements of large samples (Fig.1). The using of the SPM under manufacturing of the stamper for CD/DVD is perspective method of quality monitoring. SPMs help to check the stamper quality and to decrease 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 pressing of a polycarbonate. During manufacturing the several parts of stamper surface are controlled and quality of the stamper is appreciated. Also selective control of discs is executed.

AZoM - Metals, Ceramics, Polymer and Composites - 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 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 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 special designed SMENA base for large sample (Fig. 2). This device can provide the same noise level as Solver P7LS provide. Such design doesn't include vacuum holder and motorized positioning stage. Automatic approach is available on requirement. 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.

AZoM - Metals, Ceramics, Polymer and Composites -The NT-MDT SPM Solver P7LS.

Figure 2. The NT-MDT SPM Solver P7LS.

The user has to provide 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 only 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 carbon needle with high aspect ratio. Focused Ion Beam (FEB) grows the carbon needle on the end of tip. "Whisker type" cantilever are suitable for more precision 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 flat surface (also suitable for pits).With the help of these tools it is possible to determine 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.

AZoM - Metals, Ceramics, Polymer and Composites - Main view of the menu "Grain analysis"

Figure 3. 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 flat area (Fig. 4a). If such flat surface is not observed (Fig. 4b), then an error during reading can be made. The depth of the pits is 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 resolution of a fraction of nanometer.

b) Slope of the pit (bump) side.

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

d) Track pitch and track stability.

e) Ratio of pit volume to volume of single pit. This is important technology characteristic of CD/DVD.

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

AZoM - Metals, Ceramics, Polymer and Composites -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 check quality of the CD/DVD surface by revealing surface defects. Two scratch and knoll are seen on Fig. 5.

AZoM - Metals, Ceramics, Polymer and Composites -SPM image of defective surface of the CD disc.

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

The knowledge of the either pit or bump parameters measured at different places of the sample allows effective controlling of the CD/DVD manufacture. Fig. 6 shows 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 approximation of bumps by ellipse. The different parameters were calculated for the bump array. Some of them show in Table1. These defects influence the quality of data reading.

AZoM - Metals, Ceramics, Polymer and Composites -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.

AZoM - Metals, Ceramics, Polymer and Composites -Section of a stamper bump.

Figure 7. Section of a stamper bump.
Other Information and Parameters Determined

Also information about 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.