Praktikum I, Autumn Semester 2008/09 Experiment 13/14; 03.12.2008 Softlithography and Atomic Force Microscopy Authors: Claudio Zihlmann (zclaudio@student.ethz.ch) and Philippe Knüsel (pknuesel@student.ethz.ch) Assistance: Samuel Hertig and Wei Hu
1. Abstract The purpose of the experiment was to stamp a pattern on a gold-coated silicon-sample through softlithography. Three different experiments have been made in order to see the differences in the pattern. In the second part, the prepared samples have been analyzed with atomic force microscopy. Unfortunately, we were not able to get AFM-pictures of our samples. By analyzing a picture of another group, we could see the properties of the pattern and explain them in spite of our failure. On the top of the pattern, the surface was less rough than in the environment. 2. Introduction a. Softlithography Lithography can be divided in two different parts: Photolithography and softlithography. The main difference between these two methods is that Photolithography can be used only for flat substrates while softlithography can also stamp rough surfaces. In this experiment the focus is on softlithography. Fig. 1: (a) production of a PDMS stamp out of a master, (b) stamping alkanethiol molecules on a substrate [1] Softlithography describes a stamping process to create structures in the micro- and nanometer range. Highly defined patterns can be placed on the surface of a material. To create a predefined pattern a stamp is needed. This stamp is made out of polydimethylsiloxane (PDMS) which is an elastic polymer. This PDMS is put on a previously 1
created master structure with the desirable pattern. As soon as the PDMS is solid it can be removed and is ready to use (Fig. 1 a). Before using the stamp it is inked with alkanethiol solution which molecules will be later on the substrate and build the pattern. Afterwards the stamp is put on the substrate and the molecules of the alkanethiol can be set on the surface of the substrate where a direct contact exists between the substrate and the stamp. After removing the stamp the defined pattern out of alkanethiol molecules is on the substrate with non-coated areas between the molecules (Fig. 1 b). b. Atomic Force Microscopy The atomic force microscopy is a method that is used to make picture of surfaces. The whole system consists of a piezoscanner, on which the sample is fixed. The sample can be moved in x-, y- and z-direction through a voltage, that makes the piezoscanner extend in these directions. As the voltage can exactly be set, that methods permits a placement of the sample. A sharp tip is fixed on a cantilever. Fig. 2: Working principle of the AFM. The Laser deflection is measured with a photodiode. The sample is moved by a piezoscanner. [1] The tip scans the surface line by line, measuring the displacement (orthogonal to the sample-surface). With that data, the system calculates the properties of the surface and is able to plot a map of the surface or a 3- dimensional model. The displacement of the tip is caused by two different types of forces between the atoms. There is the attractive van-der-waals-force in large ranges and a repulsive force in short ranges, due to the overlapping of the electron orbitals. Referring to the Pauli exclusion principle, this force is called the Pauli repulsion. So for every distance between the atoms, the resulting force is the difference between both the attractive and the R 0 Fig. 3: The attractive van-der-waals-force decreases proportional to R -6, the Pauli-repulsion to R -12. At R 0, the Potential is minimal. [1] 2
repulsive force. This is known as Lennard-Jones-Potential (Fig. 3). As the AFM does not work with visible light of electricity to measure the surface, these properties of the sample do not matter. In order to measure the displacement of the cantilever and the tip, a laser and a photodiode are used. By measuring the deflection of the laser on the cantilever, the computer can calculate the orthogonal displacement. There are two different modes that can be applied with AFM, depending on the sample that has to be analyzed. Contact mode There are different types of contact mode. The surface can be scanned with constant force. Hereby, the tip does not move vertically and the force on it is constant. This is controlled by checking the deflection. With that feedback, the system sets the voltage for the piezoscanner for every point so that the height changes. This permits to keep the force constant. With the voltage, the height can be calculated. Instead of keeping the force constant and measure the change of height, another mode keeps the height constant. The whole principle is contrary to the mode of constant force, thus the force on the tip will be changed. The consequential displacement of the cantilever is measured with the laser deflection, which permits to calculate the height of the surface point. The third possibility in contact mode is the lateral force microscopy. When sliding on the surface lateral force induce a torque on the cantilever. The resulting horizontal displacement of the laser-beam can be measured with the photodiode. Because of the lateral force s weakness and being influenced by other parameters like the roughness, the resolution of LFM is not as good as the one of the force modes. Non-contact mode In non-contact mode, the tip does not touch the surface but is about 10 100 nm above it. It is oscillated at (or at least near to) its own resonance. The oscillation-amplitude, the phase and the frequency are modified by acting forces between the tip and the surface. This permits to get information about the surface of the sample. Due to its measuring the weaker long-range attractive forces, the non-contact mode usually results in a lower resolution. Another problem in non-contact mode is that it is hardly possible to define the x-ycoordinates while oscillating the tip. In order to resolve this problem, another type of noncontact mode has been developed: The dynamic non-contact mode, also called tapping mode or intermittent contact mode. During tapping mode, the tip touches the surface in every circle before being detached by a restoring force provided by the cantilever. This allows to get a higher resolution than in non-contact mode. As the non-contact mode have a 3
minimal contact with the surface, they are often preferred for soft samples (e.g. biological materials) in spite of their lower resolution. As mentioned before, the tip has to be sharp. After having described the working principle of the AFM, the reason for the tip s being is obvious. A broken tip would result in wrong maps of the surface (Fig. 4). Fig. 4: Influence of a broken tip in the AFM. Instead of imaging the surfacetopography, the system would plot the tip geometry. [1] 3. Materials and Methods a. Softlithography With the alkanethiol softlithography method three differently structured gold surfaces have been prepared. The difference in the preparation was just in the last step. The PDMS stamp was already prepared and we removed the absorbed dust by ultrasonication in an ethanol bath during ten minutes. After drying with the nitrogen jet we inked the PDMS stamp with dodecanthiol (0.005 mm) solution and put it on the gold-coated silicon sample for five minutes. Then we removed the stamp and repeated this procedure for each of our three gold-coated silicon samples. Experiment A For the experiment A we just washed the sample with ethanol and dried it afterwards with the nitrogen jet. Experiment B In experiment B we put one drop of mercaptoundecanoic acid (0.01 mm) on the sample to backfill the non-coated areas. Then we swung it in ethanol and dried it afterwards with the nitrogen jet. Experiment C For the experiment C we prepared a solution consisting of 200 mg sodium cyanid and 100 mg potassium hexacyanoferrate in 20 ml sodium hydroxide (1 M). Then we swung the 4
sample in this solution for 10 seconds. Afterwards we washed the sample first with distilled water and then we ethanol. Finally we dried it with the nitrogen jet. Fig. 5: The different preparation of the surfaces of the gold-coated silicon samples [1] b. Atomic Force Microscopy In our experiment, we used an atomic force microscope that was manufactured by Asylum Research. We used the tip model MSCT-AUNM and the mode MFP 3D. By checking the pattern on the video-screen, we tried to scan areas with a single element of the pattern. 4. Results On the samples of the experiment C, a pattern could be recognized by viewing it with a simple light microscope. The other two samples have not been analyzed with a light microscope, because the pattern would not have been observable. In the second part of the experiment, the samples should have been analyzed by atomic force microscopy. Due to its better quality (checked in the first part with the light microscope), the sample C of the other group (Simon Bachmann and Louis Schär) was examinated. The pattern could be seen on the screen with a higher range. Unfortunately, as soon as it was scanned with the AFM, it was impossible to see any pattern. Different settings were tried, without result. This took a long time, so that only one sample could be analyzed with the AFM. In order to describe what should have been seen on the screen, our assistant (Wei Hu) sent us some pictures of other groups. 5
Fig. 6: Another group s example. On the left side, there is the picture of one element of the pattern. On the right side, the cross-section of the pattern is shown. 5. Discussion On figure 1, the pattern can be seen very well. The red line shows where the height has been measured for the graph on the right side. It has a side-length of approximately 10 µm. The height is of 15 nm. There is something on the picture that looks like a shadow. This could be the result of a little movement during the stamping of the sample. The red line on the right part of figure 1 shows that the roughness is different on the top of the pattern than on the environment. The pattern is less rough on the top than in the environment as the jumps of the line are not as high between approximately 12 µm to 30 µm. This is caused by the acid that attacks the non-coated material. This acid does not attack regularly, so some parts are more attacked than others. The reasons why the AFM-experiment did not work in our case is not clear. It was very difficult to zoom in to get a higher resolution. The pattern could be seen on the video-screen, but we were not able to zoom exactly on the pattern. 6
6. References Fig. 1 : taken from [1] Fig. 2: taken from [1] Fig. 3: taken from [1] Fig. 4: taken from [1] Fig. 5: taken from [1] Fig. 6: Picture of the AFM-experiment of another group, received from Wei Hu. [1] Laboratory course instruction Praktikum I, Teil Werkstoffe, HS 08/09: Versuch 13 & 14: Softlithographie und AFM, Materials Science BSc, ETH Zürich. 7