1. Andrew Jacquier Brigham Young University
Silicon Wafer Cleaning for EUV Reflectance Measurements by Cold, High-Pressure CO2 Jet
Andrew Jacquier
Brigham Young University
Andrew Jacquier
March 19, 2005
Spring Research Conference
College of Physical and Mathematical Sciences, BYU

2. Why EUV Optics? EUV Lithography Soft X-Ray Microscopes EUV Astronomy
I work with the EUV thin film optics group. We are dedicated to researching Extreme Ultraviolet light and the ways of effectively reflecting it.
As mentioned before, the extreme ultraviolet has potential applications in lithography, microscopy and astronomy. The EUV will be an important tool if we can learn to reflect it well. Accurately measured optical constants are vital to these applications. The optical constants of materials in the EUV may be calculated by measuring its reflectance as a function of angle and fitting that reflectance to the correct values.

3. Hydrocarbons and “Stuff”
Thin films and silicon wafers naturally build up a layer of contaminants.
The contaminants interfere with measurements of the optical properties of the mirror.
Thin films and silicon wager naturally build up a layer of contaminants – the air around us is full of them and there is no avoiding it completely. Only in a sealed, clean environment is this not a problem.
Because of the small wavelength, EUV light can be affected even by a relatively small amount of hydrocarbon contaminants. This frustrates our ability to accurately measure the optical properties of the mirror. The wavelength of EUV light is on the order of 3 nanometers, so even a 10 angstrom layer of hydrocarbons is significant.
This diagram shows a mirror made of a silicon substrate, a native oxide layer, a thin layer of metal and a layer of hydrocarbon contaminants.

4. Hydrocarbon Buildups Lower Reflectance
This is a computer simulation of how a layer of hydrocarbons change the reflectance of a silicon substrate. Note, for example, the reflectance at twenty degrees incidence. The reflectance is just less than 50% with no hydrocarbons but as we increase them, this decreases; it is at 40% reflectance with 30 Angstroms of hydrocarbons. As we go from 0 degrees to 30 degrees incidence, the reflectance as a function of angle is significantly different at 30 Angstroms of hydrocarbons than it is at 0. This change in reflectance leads to incorrect values for the optical constants.
Reduced Reflectance with Hydrocarbon Thickness. Theoretical change in reflectance vs. grazing angle and organic thickness. (at λ=40.0 nm)

5. Cleaning Methods
Many different cleaning methods have been used to clean silicon wafers for thin film deposition.
Opticlean®
Problem: 20 A Residue
Oxygen Plasma Etch
Problem: Not Local and Can Oxidize Non-Protected Surfaces
High Intensity UV Light in Air
Problem: Oxidizes Metals, even gold
Cold, High-Pressure CO2 Jet
We are interested in methods that would clean off the hydrocarbon residue without damaging any other thin films on the mirror. The cleaning methods we are searching for are inexpensive, do not require a full clean-room setup and do not require much expertise or skill. We are familiar with four different cleaning techniques. The first is a peel-coat product called Opticlean. It is applied like fingernail polish and removed by peeling the dry product off of the substrate. This process removes large particles, but leaves a hydrocarbon residue of approximately 20 Angstroms. We have also used the oxygen plasma etch, but this process is expensive, a clean room is necessary and it can oxidize surfaces. High Intensity UV light in air appears promising but also tends to oxidize materials heavily, even metals such as gold. Finally there is the Cold, High-Pressure CO2 Jet.

6. Cold, Pressurized CO2
We used a CO2 Snow Cleaning Solenoid Unit pressurized CO2 gun by Applied Surface Technologies.
The unit uses freezing, pressurized CO2 to blow particles and contaminates off of a surface.
We tested this unit on
silicon wafers with a
native oxide layer.
This method uses a solenoid unit which looks kind of like a gun – it is connected to a tank of pressurized CO2. By allowing the CO2 to expand freely while shooting it out of the gun, it cools down rapidly into a snow. When directed at the silicon wafer, the CO2 snow blows particles and contaminants off of the surface. We tested this process to see if it could clean hydrocarbon residue as well as large-scale particulates.
The stream of CO2 snow was pretty intense. Whether we held the sample in the air with tweezers or kept it on the counter with the snow jet blowing down on it, the sample was almost always in danger of shattering. I remember a few samples shattering and therefore being unusable.

7. Testing Procedure
We tested the CO2 gun cleaning system by using a spectroscopic ellipsometer to measure the thickness of hydrocarbon layers before and after cleaning.
We can accurately detect changes in average thickness up to .1 Å
In all of our trials, we used a spectroscopic ellipsometer to measure the thickness of the thin film layers. This works by shining polarized light onto the sample and then analyzing the polarization of the reflected light. Knowing that the wafer was silicon, and knowing the ways that silicon reflects light, we could then determine the thickness of each thin layer on top of the silicon substrate.
By measuring the thickness of the hydrocarbon layer before and after the CO2 cleaning was performed, we were able to see how the CO2 changed the layer and thus if it was successful in cleaning or not.

8. Problems with measurement
Silicon dioxide is basically optically identical to most hydrocarbons as a thin film
We add a thin layer of hydrocarbons to the surface of the mirror and measure the apparent oxide level before and after cleaning.
We can either add hydrocarbons with Opticlean or with DADMAC
We have a problem with this method, however. The optical constants of silicon dioxide (the natural oxide of silicon) and the optical constants of hydrocarbons are very similar. Considering the thickness of the layers, the ellipsometer can not differentiate between a layer of hydrocarbons and silicon dioxide. Instead, all it sees is a single layer of apparent oxide. We needed a way of making sure that we are trying to clean hydrocarbons rather than oxidation.
What we did to solve this problem was measure the apparent oxide thickness first, then add a layer of hydrocarbons, measure the apparent oxide thickness again, clean the mirror, and finally measure the apparent oxide thickness a third time. What this did was give us a layer of “apparent oxide” which we knew in reality to be hydrocarbons. We then measured the change that the CO2 caused in this hydrocarbon layer.
We used two different sources of hydrocarbons. The first was the Opticlean product. This had the benefit of removing the large particles so we could measure just the removal of the residue. It’s drawback, however, was the fact that it took a bit of skill and patience to peel it off the silicon wafer. As an easier alternative, we used DADMAC, a chemical which attaches chains of hydrocarbons to any surface which is immersed in it.

9. Trial 1 Original test of CO2 procedure
Opticlean was used to remove most large-scale contaminants
Two samples showed some significant cleaning of the hydrocarbon layer
Original test performed by William Evans
The first trial was done about a year ago, and was presented at last year’s Spring Research Conference by Bill Evans, a member of our research group. The hydrocarbon added was the residue left after the use of Opticlean. Our results showed two samples with some significant cleaning.

10. Trial 1 Results
The silicon wafer was broken into six different pieces and were exposed to different amounts of time under the CO2 gun. Since the gun is hand held, it is practically impossible to standardize the distance from the gun to the sample or the angle at which we held it. This is most likely the reason why the results are so sporadic. The last sample shows that 120% of the Opticlean was removed, telling us that there was definitely a layer of hydrocarbons on the sample before the Opticlean was used, and that the Opticlean itself did not remove it. This trial showed some initial success, in that there were a few examples of significant cleaning and that none of the samples gained hydrocarbons through the CO2 cleaning.

11. Trials 2 and 3
In the second and third trials, we used DADMAC as the layer of hydrocarbons.
The second trial showed that the CO2 mostly did more damage than it cleaned
Thinking that it was possible that the cold temperature of the CO2 was causing water to condense, we heated sample after spraying with CO2.
Two more trials were conducted to see if we could reproduce the results of the first trial. Both the second and third trials were used with DADMAC as the layer of hydrocarbons. The second trial showed that the CO2 snow added hydrocarbons more often than not. We were puzzled by this and thought that the coldness of the CO2 snow might be affecting our measurements. Perhaps, we thought, water was condensing onto the silicon. We ran the third trial in an effort to try and reduce this water. After measuring the thickness of the apparent oxide after the CO2 snow cleaning, we placed the sample in a hot chemical storage bin in Dr. Linford’s lab (like a refrigerator), then measured the apparent oxide level again.

12. Trial 2 Results (DADMAC & CO2)
Here are the results of the second test, showing that the CO2 added hydrocarbons in two cases, and in one case as much as tripled it. This was disheartening, to say the least. This test might simply be telling us that DADMAC is harder to remove than Opticlean, but most likely there was a source of hydrocarbons making our samples dirty after or during the cleaning process. I think that it is unlikely that an exposure to CO2 snow would cause a sample to triple the hydrocarbon layer. At the same time, there was one sample which was cleaned 28% and another at 52%, so it might still be promising.

13. Trial 3 Results (Heating after CO2)
In the case were we heated the sample after the CO2 process, we again found that there is little chance that the CO2 snow, as we have used it, will clean a residue. It also shows me how we can create better experiments. The first sample shows that after 0 seconds of CO2 snow, the layer was reduced by 30%. This tells me that the layer was probably about 30% lower than we had originally measured it to be. When we conduct experiments in the future we will make sure to take the measurements at different spots on the sample and average them.

14. Results, Conclusions, and Directions for Further Study
In conclusion, the CO2 jet only gives intermittent success because it didn’t always remove residue.
Performance does not seem to depend on time of exposure, perhaps because of the random nature of the process.
Future tests might be warranted, but for now we will try other techniques.
This method did give some examples were the sample was cleaned, but these examples were few and far between. One interesting fact is that the Opticlean was cleaned much more readily than the DADMAC. Other than that, however, in order to isolate the conditions under which we might reasonably expect the sample to be cleaner, we would have to conduct many other tests changing angle of incidence, distance from the gun to the sample, and time of exposure. At this time we will probably not continue with this line of research.
Instead, we are currently excited to be able to conduct further studies in cleaning samples with a combination of Opticlean and ultraviolet light.

15. Acknowledgements I would especially like to thank William Evans
Dr. David Allred
Dr. Linford
The BYU Thin Film Optics Research Group
I would like to conclude my presentation by personally thanking my partner in crime, Bill Evans, as well as Dr. David Allred, Dr. Matthew Linford and the rest of the BYU Thin Film Optics Research Group. Thank you.