Automatic measurements using scanning probe microscopy (SPM) involves measurements of the programed areas with SPM methods, automatic adjustment of scanning parameters, and automatic data analysis. Following are the application areas of the automated SPM measurements:
- Quality control in industry, for instance, control of the DVD/CD disk surface
- SPM modifications (nanolithography) on the macroscopic areas
- SPM measurements of macroscopic areas by moving the scanning area over a sample surface with the aid of the positioning system. Consequently, areas that have a size of several centimeters or millimeters can be measured using SPM (maximum scan range in commercial SPMs is restricted to ~100 µm)
- Combinatorial Material Research (high-throughput characterization of archives of samples with distinctive chemical composition or of samples prepared at distinctive conditions)
The Solver LS SPM
The modified Solver LS SPM equipped with special software is a vital tool for automated characterization and modification of surfaces at all fundamental SPM modes. The sample holder of the Solver LS for holding four standard 4 inch silicon wafers is illustrated in Figure 1. Each wafer can hold a large amount of samples deposited, for instance, by ink-jet printing.
Figure 1. Modified SOLVER LS for automated measurements (left). Positioning platform for four 4-inch silicon wafers (right).
The automated measurements menu for 25 points is illustrated in Figure 2. The coordinates of each point are saved in the program. An optical image of the current position (with a resolution of up to 1.5 µm) is automatically captured by the software, which then carries out the SPM measurement, moves the sample to the next saved position, and so on. To acquire a specific parameter or statistics for all the measured areas, all the saved data can be automatically processed using the software.
Figure 2. Menu of the automated SPM.
Results reported below were acquired by the team of Professor U.S. Schubert (Eindhoven University of Technology, The Netherlands, http://www.schubert-group.com) in partnership with the Dutch Polymer Institute. These experiments were performed using the modified Solver LS for automatic measurements.
Automatic Measurements of Photo Embossed Polymer Gratings
Periodically elevated relief structures are formed by selective irradiation through a mask of a sample consisting of a photo initiator, monomers, and a pre-polymer (Figure 3). In the case of applications in display technology, one aim is to realize the highest possible relief structures. Several sample preparation conditions — such as the temperature in the development stage, the intensity of light, the period of the applied mask, composition, and the initial film thickness — govern the formation of the elevated structures.
Figure 3. AFM picture of the structure with 20 microns pitch (left); schematic presentation of the complete sample (right).
Investigation of Optimal Processing Conditions
For the investigation of the optimal processing conditions, a combinatorial setup, in which two parameters were modified simultaneously on a large substrate, was selected. The resultant sample includes four rows of polymer gratings that have different pitches (5, 10, 20 and 40 µm). There are 11 areas in each row, which were prepared under different conditions (for example, a temperature gradient or light intensity at the time of development) (Figure 3, right). The sample’s total size is 25 x 102 mm. Automatic SPM analysis can be performed to find out the proper conditions of the sample preparation, for example, at which it is possible to achieve the maximum aspect ratio of the polymer grating. Figure 4 depicts the relationship between grating height and light intensity (for the sample obtained by using intensity gradient mask). The sample includes 44 areas: 11 values of energy dose and four pitches were used for preparing the polymer grating. Automatic measurements have been carried out in tapping mode. The results of the investigation are four dependences for each grating period that indicate modifications of height with increase in energy dose. This information enables determining the conditions optimal for preparing the sample.
Figure 4. Results of automatic measurements of the library of samples. Dependence of height of the polymer grating on light intensity (energy dose) for four pitches of grating: 5, 10, 20, and 40 microns.
Analysis of Macroscopic Areas
The large areas can be measured using SPM only by moving the SPM head over the sample surface with the positioning system. Automatic SPM has been used to analyze the thickness of the spin-coated polymer film over a distance of 3.5 cm. As shown in Figure 5, a knife was used to scratch the spin-coated film deposited on silicon. Figure 6 illustrates the measurement of the thickness of the film in 19 positions along the scratch. The coordinates of those positions are saved in the software before scanning.
Figure 5. Optical images of the scratch (scratch is indicated by red arrows).
The film thickness is calculated as the distance between maximums depending upon the number of pixels on z-coordinates of pixels. The analysis of film thickness (Figure 6, right) indicates that the middle part of the film is fairly uniform, while an area measuring 7 mm near the edge of the film has variable thickness, suggesting that the material has moved outside at the time of spin-coating.
Figure 6. SPM image of the scratch (left), determination of the film thickness as distance between peaks on statistics (center), final result: dependence of film thickness on distance along the scratch (right).
Automated Nanolithography on Macroarea
Electrochemical oxidization of a monolayer of octadecyl trichlorosilane (OTS) deposited on a silicon wafer can be performed using conductive SPM tips. Under normal conditions, a thin water layer permanently exists on the surface. Thus, decomposition products enable the terminal –CH3 groups of the OTS layer to be changed to –COOH upon the application of voltage to the tip. The dimensions of the smallest modified area can be equal to that of the tip size (it is also dependent on the applied voltage, humidity, and so on). The oxidation result can be viewed on lateral force image in contact mode. Macroscopic lithographic patterns with minimal detail in the nanometer scale are formed by translation of the lithographic pattern over a large area by moving the positioning stage. The lateral force distribution for an oxidized OTS film is illustrated in Figure 7.
Figure 7. Lateral force microscopy of oxidized areas: pattern unit (left), 9 units (right).
The unit of this pattern (Figure 7, left) has been translated over a large area by moving the positioning stage. The right image in Figure 7 illustrates only a portion of the modified area that is larger than the scan size. A total of 100 units (10 x 10) measuring 0.2 x 0.2 mm have been made within 2 hours.
Note: A full list of references can be obtained by referring to the original document.
This information has been sourced, reviewed and adapted from materials provided by NT-MDT Spectrum Instruments.
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