Si and GaAs surface oxidation with Atomic Force Microscopy (AFM) at various ambient humidity Kyongseon Lim Oxidation of Si and GaAs surface were performed using a negatively biased AFM tip. From previous papers, it was known that there is a relationship between applied voltage, height of oxide and depth of the etching on the surface after oxide removal. In this experiment, previous experiments were revived and furthermore any relationship between the ambient humidity and height of oxide was sought. From the papers read previously (1,2), it was learned that when a semiconductor surface is oxidized in tapping mode on AFM using a negatively charged tip, there is a correlation between the height of the oxide that is formed and the width of the etch on the surface after removing the oxide. These previous experiments, oxidation was done in the atmosphere with humidity of approximately 70% (2). In this experiment, ambient humidity is another variable along with the voltage. Three sets of data on each of semiconductor surfaces, Si and GaAs, are taken varying ambient humidity from 70% to 90 % and 60% to 80% respectively. Voltage applied ranges from -18 volts to -32 volts for Si surface and from -6 volts to -24 volts for GaAs surface, for GaAs being more easily oxidized. The sample was first thoroughly cleaned with acetone and isopropyl alcohol. Next the sample is dipped into HF for less than 15 seconds to remove the naturally formed oxide on the surface and rinsed with running DI water for 5 minutes. When the sample is prepared, a set of lines varying applied voltages with intervals of 1 volt are drawn by the AFM using the Digital Instrument Nanoman program. (See Appendix for the.jpg images of the patterns) After taking the image of oxides, the sample was dipped into HF again to remove the formed oxide by nanolithography, and then a new image of depth on the surface where the oxide used to be present was taken. The humidity was kept at 70% for easy comparison with the previous experimental results. The positive correlation between the height of the oxide and the depth of the etching was found again with revival of the experiment. Magnitudes of depth and height on GaAs surface are larger than that of Si surface. The apparent trend is that the height of the oxide is increased as applied voltage is increased; therefore the depth of the etching is increased as well. Quantitative evaluation of the trend was not included.
height, depth(nm) height,depth(nm) 1.8 height depth 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2-16 -18-20 -22-24 -26-28 -30-32 -34 voltage(v) Fig 1. Relationship between height of oxide and depth on Si surface 12 height depth 10 8 6 4 2 0-4 -6-8 -10-12 -14-16 -18-20 -22-24 -26 Voltage(V) Fig 2. Relationship between height of oxide and depth on GaAs surface The second variable was ambient humidity. Ambient humidity for Si sample is varied from 70% to 90%. The actual range for ambient humidity of 70% is 68 ~ 73%, that of 80% is 78 ~ 83%, and that of 90% is 88 ~ 90%. Ambient humidity for GaAs sample is varied from 60% to 80%, and the actual range for the humidity of 60% is from 58 ~ 62%, that of 70% is from 71 ~ 74%, and that of 80% is 79 ~ 83%. This
crude control of ambient humidity has caused an error in taking a stable data trend. It was expected that the width and/or height will increase as the humidity increases. If the density of ambient water molecules near the tip and the sample interface is higher, more O anions will be produced and that results in production of more oxide (1). However a definite correlation could not be found for either. From the experiment one can find that both height and width of the oxide on the Si sample were the greatest around ambient humidity of 80%, and these on the GaAs sample were greatest around ambient humidity of 70%. The height of the oxide is relatively less affected by ambient humidity than the width. The initial expectation was not proven to be right or wrong. Discussion of possible sources of error will be followed by the graphs.
Height(nm) height(nm) 1.8 70% 80% 90% 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2-18 -20-22 -24-26 -28-30 -32-34 voltage(v) Fig 3. Height of oxide as a function of bias voltage at various humidity. (Si) 10 60% 70% 80% 9 8 7 6 5 4 3 2 1 0-4 -6-8 -10-12 -14-16 -18-20 -22-24 -26 Voltage(V) Fig 4. Height of oxide as a function of bias voltage at various humidity (GaAs) The x-axis shows the control variable which is the bias voltage on the tip of the AFM. The y-axis shows the height of the oxide formed and the unit is in nm. The data was taken three times using the same procedure but varying ambient humidity. Oxide formed on Si sample is one tenth of magnitude smaller than that on GaAs sample.
width(um) width(um) 0.14 0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 70% 80% 90% -18-20 -22-24 -26-28 -30-32 -34 voltage(v) Fig 5. Width of oxide as a function of bias voltage at various humidity (Si) 0.26 60% 70% 80% 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06-4 -6-8 -10-12 -14-16 -18-20 -22-24 -26 Voltage(V) Fig 6. Width of oxide as a function of bias voltage at various humidity (GaAs) The y-axis on the above graphs shows the width of the oxide formed and the unit is in μm. Once again, the graphs show that the width of oxide formed on Si sample is also smaller than that on GaAs sample. The positive correlation between voltage and the width is less evident than the height in all humidity ranges. From the result, one can find that there is the optimal humidity that the
oxidation occurs at best. High humidity did not necessarily cause the size of the oxide to be larger. It shows that when there is enough O anions in the environment, the sample no longer oxidizes more than it needs to; oxidation seems to reach its saturation at some point. After it has reached its saturation point, spare water molecules do not assist in favor of more oxidation but hinder oxidation. One possible explanation for this is that too many water molecules are busy bumping into each other, it actually interrupts neighboring water molecules from making bonds with the semiconductor atoms on the surface. Possible sources of error are from the procedures in carrying out the experiment and there was an effort to minimize error by effective data analysis. To reduce the actual time for data acquisition, once humidity reached at maximum, the next experiment was taken starting at the maximum humidity and data was taken while the humidity is being lowered by the controller. This procedure might have caused the access water molecules to be present in the atmosphere but was not close enough to be read by the humidity reader. There were many general obstacles in taking data, for the preparation of the sample was not done in the clean room. Taking a good image was especially hard because even the smallest dirt could have been attached to the tip or the sample surface and that ruins the image or disrupts actual etching on the surface. Even right after cleaning the sample, moving through each step introduces new dirt to be included in the image as you can see from the fourth picture in Appendix as an example. The images were not fine enough to have exact measurements. Even after filtering out noise and flattening may not have eliminated all bumpiness of the surface, therefore it resulted in crude measurements of data. Nevertheless Gaussian function of the profile of the oxide was modified to reduce this kind of error. A new slope term has been added to the usual Gaussian function to minimize the error due to the inclined and/or uneven sample surface. The data from this experiment indicates that there is a most favorable range of ambient humidity at which the oxidation occurs. The optimal humidity is around 70-80%.
Appendix 1. Oxides on Si surface at 70% ambient humidity 2. Etching on Si surface at 70% ambient humidity 3. Oxide on GaAs surface at 70% ambient humidity 4. Etching on GaAs surface at 70% ambient humidity
Reference (1) Snow, Jerrigan, and Campbell, Appl. Phys. Lett. 76, 1782 (2000) (2) Bo, Rohkinson, Tsui, and Sturm, Appl. Phys. Lett. 81, 3263 (2002) Acknowledgement I thank Professor Rohkinson for providing me an opportunity to take a part in his lab to obtain advanced laboratory credits. It was fun setting up the lab and watching others set up the lab. I learned a lot from it and wish to work with the group again next year. I have enjoyed getting to know the cutting edge technology which I have never had a chance to work on. I put importance that I had my hands on experience that prepared me for further study in this field. I also thank him for letting me turn in the paper during the summer even after the deadline but I have to apologize for delay. I also apologize that I broke many AFM tips during the experiment. I also appreciate continual support and love from my parents.