FACULTY OF ENGINEERING
LAB SHEET
ENT3026
DIAGNOSTIC TECHNOLOGY
TRIMESTER 2 2012/2013
DT1 –STRUCTURAL CHARACTERIZATIONS USING ATOMIC FORCE MICROSCOPY (AFM)
*Note: On-the-spot evaluation will be carried out during or at the end of the experiment. Students are advised to read through this lab sheet before doing experiment. Your performance, teamwork effort, and learning attitude will count towards the marks.
STRUCTURAL CHARACTERIZATIONS USING
ATOMIC FORCE MICROSCOPY (AFM)
Part A:AFM – Procedures
1.0. Objective
Atomic Force Microscopy has been one of the most effective techniques for surface characterization of thin film and low dimension structures. This experiment is designed to familiarize the student to the practical aspects in the operation principles of atomic force microscope and its applications in structural characterization of surface structures.
2.0. Equipment Required
- Atomic Force Microscope
3.0. Components Required
- Standard Calibration Sample (Square grid patterns of 3m, 5m and 10m)
4.0. Introductions
4.1. What is Atomic Force Microscope (AFM)?
An AFM is a mechanical imaging instrument that measures the three dimensional topography as well as physical properties of a surface with a sharpened probe. The primary uses for the AFM imaging are:
- Visualization
The AFM measures three dimensional images of surfaces and is very helpful for visualizing surface topography.
- Spatial Metrology
Nanometer sized dimensions of surface featuresare measurable with the AFM.
- Physical Property Maps
With different imaging modes it is possible to measure surface physical property maps.
4.2. Basic Operation of Atomic Force Microscope
Typically, when we think of microscopes, we think of optical or electron microscopes. Such microscopes create a magnified image of an object by focusing electromagnetic radiation, such as photons or electrons, on its surface. Optical and electron microscopes can easily generate two dimensional magnified images of an object’s surface, with a magnification as great as 1000X for an optical microscope, and as large as 100,000X for an electron microscope. Although these are powerful tools, the images obtained are typically in the plane horizontal to the surface of the object. Such microscopes do not readily supply the vertical dimensions of an object’s surface, the height and depth of the surface features.
Unlike traditional microscopes, the AFM does not rely on electromagnetic radiation, such as photon or electron beams, to create an image. An AFM is a mechanical imaging instrument that measures the three dimensional topography as well as physical properties of a surface with a sharpened probe. The sharpened probe is positioned close enough to the surface such that it can interact with the force fields associated with the surface. Then the probe is scanned across the surface such that the forces between the probe remain constant. An image of the surface is then reconstructed by monitoring the precise motion of the probe as it is scanned over the surface. Typically the probe is scanned in a raster-like pattern.
The force sensor in an atomic force microscope is typically constructed from a light lever. In the light lever, the output from a laser is focused on the backside of a cantilever and reflected into a photo detector with two sections. The output of each of the photo-detector sections is compared in a differential amplifier. When the probe at the end of the cantilever interacts with the surface, the cantilever bends, and the light path changes causing the amount of light in the two photo-detector sections to change. Thus the electronic output of the light lever force sensor is proportional to the force between the probe and sample.
Figure 1: Basic operation of AFM.
Although the AFM is capable of extreme magnification, it is not a large instrument. An AFM that is capable of resolving features as small as a few nanometers can be easily installed in a laboratory on a desk top. The greatest deterrent to high magnification with the AFM is often environmental vibrations that cause the probe to have unwanted vibrations.
4.3. Basic Components of Atomic Force Microscope
- Scan Head
Figure 2: AFM scan head.
Controller
Figure 3: AFM controller.
- Stage
An AFM stage is where the sample is placed when measured.
(a)
(b)
Figure 4: (a) Standard stage, and (b) Motorized high precision xy stage.
Computer and Software
Software in the computer is used for acquiring and displaying AFM images. Also, software for processing and analyzing AFM images typically resides in the computer.
Figure 5: Nanosurf easyScan2 software.
5.0. Standard Operating Procedures for easyScan2 AFM system
The basic steps involved in operating an Atomic Force Microscope are as follow:
(1)Prepare Sample
(2)Place Sample on Stage
(3)Probe Approach
(4)Scan Sample
(5)Zoom on Feature
(6)Tip Retract
(1) Prepare Sample
The easyScan2 AFM can be used to examine any material with a surface roughness that does not exceed the height range of the scanning tip (14 μm). Nevertheless the choice and preparation of the surface can influence the surface tipinteraction. Examples of influencing factors are excess moisture, dust, grease etc. Because of this some of the samples need special preparation to clean their surface. Generally, clean as little as possible. If the surface is dusty, try to measure on a clean area between the dust. It is possible to blow coarse particles away with dry, oil free air, but small particles generally stick so well to the surface that they cannot be removed. Note that bottled pressurized air is generally dry, but pressurized air from an in-house supply is generally not. In this case an oil filter should be installed. Blowing dust away by breath is not advisable, because it is not dry. When the surface is contaminated with grease, oil or something else, the surface should be cleaned with a solvent. Suitable solvents are distilled or deionized water, alcohol or acetone, depending on the contaminant. The solvent should be very pure, in order to prevent the collection of impurities on the surface when the solvent evaporates. The sample should be cleaned several times to remove dissolved and redeposited contaminants if it is very contaminated. Delicate samples can be cleaned in an ultrasound bath.
(2) Place Sample on Stage
The sample is placed on the stage by using a suitable tweezer. If vacuum suction is used, the sample must cover the opening completely to prevent air leakage. The vacuum pump is then turn on to provide suction to hold the sample at its place.
(3) Probe Approach
Turn on the easyScan2 Controller and start the easyScan2 Software on the control computer. The main program window appears, and all status lights are turned off. Now a Message ‘Controller Startup in progress’ is displayed on the computer screen, and the module lights are turned on one after the other. When initialization is completed, a Message ‘Starting System’ is shortly displayed on the computer screen and the Probe Status light, Scan Head status light of the detected scan head and modules will light up. To start measuring, the tip must come within a fraction of a nanometer of the sample without touching it with too much force. To achieve this, a very careful and sensitive approach of the cantilever is required. This delicate operation is carried out in two steps: Manual coarse approach and the automatic final approach.
- Manual Coarse Approach
Lower the scan head manually until the cantilever is between 1 to 2 mm from the sample. This is achieved by referring to the right magnifying lens, where the cantilever must be lowered to obtain a reflective image of itself on the sample.
- Automatic Final Approach
Click the positioning icon on the software, and select ‘approach’. The software will perform automatic approach to bring the cantilever down to the set point value. A message ‘Approach done’ appears when the approach has been completed.
Figure 6: Software automatic approach panel.
Figure 7: View of cantilever after approach: left: side view, right: top view.
(4) Scan Sample
The instrument was set to automatically start measuring after the automatic approach. Two representations of the ongoing measurement are drawn in the Imaging panel. One representation is a color coded height image, called a Color map, the other is a plot of height as a function of X* position called a Line graph. Watch the displays for a while until about a quarter of the measurement has been measured. With the current setting, the software automatically adjusts the contrast of the Color map, and height range of the Line graph to the data that have been measured.
Figure 8: Imaging window.
(5) Zoom on Feature
After the measurement has been completed, activate the color map graph by clicking on it. Click in the imaging toolbar. The mouse pointer becomes pen shaped when moving over the color map and the Tool Results panel is zooming in on an overview measurement displayed. Select an interesting region by drawing a square with the mouse pointer. Click on one corner of the region using the left mouse button, and keep the button pressed. Drag the mouse to the other corner of the region, and then release it. The size and the position of the square are shown in the Tool Result panel. Release the mouse button when the square’s size covers approximately one period of the grid. Confirm the selection by double clicking the color map graph using the left mouse button. Now the selection is enlarged to the whole display size. You can abort the zoom function by clicking again. The microscope will now start measuring a single grid period.
Figure 9: Zooming in on an overview measurement.
(6) Tip Retract
After completing the scanning, the tip is retracted from the sample by clicking on the ‘Retract’ button on the positioning window. A soft hissing sound is heard as the motor retracts the tip away from the sample. Perform manual retracting and remove the sample from the stage.
Part B: AFM – Experimental Work
- Place the sample with square grid pattern with actual size of 5m on the AFM stage. Follow the AFM standard operating procedure. Select the appropriate data filter to get the clear square grid pattern on the imaging window.
2. Adjust rotation angle to ensure the image is horizontally and vertically aligned as shown in Figure 10 below. Take note that there will be more than four square grid patterns on your first measured/scanned overviewAFM image. Note down the scanning area (dimension of x- and y- axis). Save your AFM image as bitmap file into your thumb drive.
Figure 10: AFM image with four square grid patterns.
3. Zoom in the feature to fit for four square grid patterns on the image monitor. An example of AFM image with four square grid patterns is shown above in Figure 10. Save your AFM images with four square grid patternsas bitmap files into your thumb drive.
4.From the print out of 5m square grid pattern (overview AFM image), draw lines on selected edges of square grid pattern as shown in Figure 11. Using a ruler, measure the horizontal (dx) and vertical (dy) distances.
Figure 11: AFM image with four square grid patterns.
5.Calculate the x- and y-axis magnification of Mxand My for the square grid pattern of 5m, respectively using the following formula:
x-axis magnificationy-axis magnification
Mx = My =
where
D = measured distance (provide the distance in m)
d = actual distance (5m +5m)
Part C: AFM – Sample Evaluations
You are given two metallic copper(Cu) samples, Sample A & B, fabricated with low and high substrate temperatures, respectively.
- Apply the AFM measurement procedures in order to characterize the structural properties of the Cu samples, including 2-D and 3-D sample morphology, RMS roughness, and lateral feature size.
- Analyze the AFM measurement results.
- Evaluate and comparethesamples structural properties from the analysis on the AFM measurements.
- Identify the sample which exhibits higher crystallinity/lower crystallinity, justify your answers based on your analysis on the AFM measurements of the Cu samples.
Part D: AFM – System Design
- Design a nano-patterning system based on the above AFM measurement setup and procedure. The system should include all necessary constituents (including a few equipments not utilized for AFM measurements but needed for patterning structures) in order to perform a nanostructure patterning on silicon oxide (SiO2).
- Please sketch and identify/label the nano-patterning system, explain the functionality of each constituent, and explain the patterning principle of the perceived nano-patterning system mainly based on the AFM measurement setup.
- What are the advantages and disadvantages of AFM in comparison to SEM (scanning electron microscopy) in terms of surface morphology/topography inspection?
- Based on the AFM measurements, comment on the potential applications of AFM in the area of nano-science.
Marking Scheme
Lab (10%) / Assessment Components / DetailsHands-On & Efforts (2%) / The hands-on capability of the students and their efforts during the lab session will be assessed.
Lab Report
(6%) / Each student will have to submit his/her lab final report to the subject lecturerwithin 10 days of performing the lab experiment. The report should coverthe followings:
- Introductions, which includebackground information on AFM, and general summary of the lab experiment work.
- Experimental, which include the AFM experimental work, and measurement procedures.
- Results and Discussions, which include the AFM measurement results and analysis, with neat diagram/graph/image of measurement results and recorded data, and the discussion on the measurement results and the system design.
- Conclusions, which include a conclusion on the sample structural characterizations using AFM and AFM sample measurement results.
- List of References, which include all the technical references cited throughout the entire lab report.
and/or conference proceedings (e.g.
Format of references:The references to scientific journals and text books should follow following standard format:
Examples:
[1] Chan KY, Bunte E, Stiebig H, Knipp D. “Influence of low temperature thermal annealing on the performance of microcrystalline silicon thin-film transistors”. Journal of Applied Physics, 2007, vol. 101, p. 074503.
[2] Hodges DA, Jackson HG. “Analysis and design of digital integrated circuits”. New York, McGraw-Hill Book Company, 1983, p. 76.
Reports must be typed and single-spaced, and adopt a 12-point font for normal texts in the report.
Any student found for plagiarism in their reports will have the assessment marks for this component (6%) forfeited.
On the Spot Evaluation
(2%) / The students will be evaluated on the spot based on the AFM experiments and the observations on the sample structural characterizations.
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