3-ways to clean optical samples for better deep UV performance measurements

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3-ways to clean optical samples for better deep UV performance measurements Dave Riley, and Erik Schoeffel* of McPherson, 7-A Stuart Road, Chelmsford MA 1824 USA Correspondence to erik.

The vacuum environment contributes to measureable changes in optics.

1 3-ways to clean optical samples for better deep UV performance measurements Dave Riley, and Erik Schoeffel* of McPherson, 7-A Stuart Road, Chelmsford MA 1824 USA Correspondence to By definition a vacuum is devoid of matter. When we measure optical samples in a vacuum, they change while we watch. The vacuum environment contributes to measureable changes in optics. Our spectrophotometer is capable of better precision and can measure changes due to vacuum, the material loss in performance, in real time. These experiments explore the implementation and results of various forms of sample preparation and cleaning. These results are useful to metrology laboratories as well as manufacturers of analytical instruments, semiconductor process control, material scientists and other researchers. These findings indicate that it may be time for National Institute of Standards to revisit the possibility of reproducible standards for deep UV 1 to 3 nanometer region. Vacuum ultraviolet spectrophotometer for optical characterization The deep and vacuum ultraviolet is a challenging area to work in. Many materials absorb very strongly. The wavelength region below 35nm has very few, if any, standard reference material (1). One reason is the energetic ultraviolet light polymerizes many residual materials while measurements are attempted. There is also the sensitivity of samples to contamination. Contamination by monolayer s of water, oxygen, or oils have reportedly been responsible for transmission losses of 15% at 157nm (2). While the specific details of the contamination or loss of performance are difficult to ascertain, it is relatively easy to measure loss of transmission over time in a sensitive vacuum spectrophotometer. 1

2 Loss of transmission over fifteen minutes; spectrophotometer stability (red trace), sample (brown trace) The McPherson VUVAS Spectrophotometer (3) was used for these tests. It delivers absolute measurements at wavelengths from 115 nanometers up to 2 nanometers. A single detector is used to collect reference and sample data. Instrument performance can be qualified in situ and without extra accessories. The following cleaning techniques and measurement data inform and perhaps guide adoption of tools and procedures for better optical measurements in the deep ultraviolet. Samples used in these experiments are commercially available (ISP Optics Corp.) magnesium fluoride windows 25 mm diameter and 2 mm thick. Magnesium fluoride was used for the tests because it transmits ultraviolet light well, especially wavelength less than 1 nanometers. High reflective mirrors with aluminum coatings over coated with magnesium fluoride were used in the mirror cleaning tests. Samples were handled with care, by their edges and as little as possible. The samples were exposed to our labs atmosphere as little as possible. We added orientation marks to maintain consistent mounting in sample holders. Initially we benchmarked the samples by measuring them in as shipped condition, before contamination or cleaning. Only after did cleaning attempts begin. Samples received various cleaning treatments, including: 1) Methanol (HPLC grade) flush and subsequent wipe with Kimwipes (Kimtech Science Brand) 2) Ultraviolet ozone (non-contact) scrub for 6 seconds under Mercury grid lamp 3) Plasma cleaning in microwave excited low pressure argon gas environment Samples were wiped with Methanol and Kimwipes before UV-ozone and plasma cleaning. This per the crystal grower s instruction to remove ambient / atmospheric contamination accumulated during handling. As a final test, samples were intentionally contaminated with a well pressed finger. The sample s transmission was measured then the part thoroughly cleaned and measured again. Results are compared to as shipped and those previously cleaned to determine effectiveness of the cleaning and measurement process. 2

3 RESULTS Fresh from the package Samples were scanned for their initial transmission value. No cleaning was attempted before this scan; samples were measured as received. Samples from the package showed signs of contamination with wavelength dependent losses greatest in the 115 to 1 nanometer region Magnesium fluoride window sample measured as received Wiped with Kimwipes and methanol After the benchmark, samples were wiped with a fresh methanol soaked Kimwipe and then inserted into the sample chamber. Vacuum pump time is less than five minutes. Samples wiped with methanol and Kimwipes show significant gains in transmission value throughout the 115 to 2 nanometer range with greatest gains coming from 115 to 1 nanometers and slightly smaller gains up to 2 nanometers. Note that even the purest laboratory grade methanol will leave molecular trace that will effect transmission Blue trace measured as received, red trace measured after methanol wipe Measurements show repeated wiping does not remove traces of polymerized residual methanol from the sample. This is the best condition obtained for this sample with methanol wiping. Higher percentage transmission was attained only in the initial scan, when the sample had yet to be exposed to vacuum or UV. 3

4 Blue trace measured as received; others show change vs. time (15 minute increments) Transmission value measured every fifteen minutes shows increasing vacuum contamination of samples repeatedly wiped with methanol and Kimwipe. The cleaner a sample is, the faster the vacuum environment and UV exposure will show measureable polymerized contamination. Consistently wiping the sample with methanol can only produce near maximum transmission values. It does provide reproducible transmission values each time the sample is wiped. Additional UV-ozone scrub and/or plasma cleaning further improve measured results. UV-ozone scrubbing Samples are wiped with methanol and Kimwipes before UV-ozone scrubbing. The sample previously used in the methanol wipe experiment was removed from the sample chamber and cleaned in a UV-ozone scrubber for 6 seconds. The result has higher transmission values Black trace measured as received, red methanol wiped, blue UV-ozone scrubbed This indicates the UV-ozone scrubber is able to remove polymerized coatings and residual methanol. UV-ozone scrubbers are extremely effective for cleaning these types of samples and simultaneously clean both sides of UV transparent parts in (typically) less than 6 seconds. 4

5 Freshly UV-ozone scrubbed samples begin to show signs of vacuum contamination in as little as 15 minutes. UV-ozone scrub is the only method used on reflective optics with polymerized vacuum contamination or hydrocarbon molecules left from the coating process Two unique Al+MgF2 coated mirrors before and after UV-ozone scrubbing UV-ozone scrubbing appears to be very reliable and suitable for the most demanding laboratory environments. It is useful for both transmission and reflection optics. Experimentation with scrubbing time shows variations. A specific standard operating procedure is required and should include (at least) time spent in the scrubber, how the sample was prepared, and how it is handled and stored. Microwave-excited plasma cleaning The sample was scanned to check the starting transmission value. It was then cleaned in microwave-excited plasma cleaner for 1 seconds in low pressure argon atmosphere. Results of this cleaning show significant gain in transmission value, somewhat higher than that of UV-ozone scrubbing Black trace measured as received, red plasma cleaned Vacuum contamination can be seen in as little as fifteen minutes after cleaning. Continued cleaning/measurement cycles show good repeatability. 5

6 Black trace measured as received; others show change of plasma cleaned sample vs. time (15 minute increments) Note that plasma cleaning may destroy mirrors and coatings and can produce excessive heat during cleaning. This can damage and even fracture samples especially when there are microscopic cracks or inclusions. Experimenting with time exposed to the plasma is crucial to successfully utilizing this process. The implementation of a well researched and practical standard operating procedure is clearly warranted because of the variables involved. Cleaning heavy contamination Samples were intentionally finger printed on both sides and exposed under vacuum to fifteen minutes of nanometer vacuum ultraviolet light. They were scanned for transmission value with the fingerprint in place. Since the only method that can remove the physical finger print residue is the methanol soaked Kimwipe all samples were wiped with methanol before any other cleaning was attempted. Any residue that remained was removed by either UVozone scrubbing for 6 seconds or plasma cleaning for 1 seconds Black trace measured as received, blue has fingerprint, red methanol wipe cleaned 6

7 Black trace measured as received, blue has fingerprint, red UV-ozone scrubbed clean Black trace measured as received, blue has fingerprint, red plasma cleaned Summary Methanol with Kimwipes In the absence of UV-ozone scrubbers and plasma cleaners it can be recommended that all samples be wiped with laboratory grade solvent and Kimwipes or similar. The choice of solvent is subject to change. Determine what is best for the optics and safe for the operators (methanol, isopropyl, et cetera). Careful attention must be paid to the method of wiping that should be repeated by subsequent users. It is relatively easy to acquire and this method appears to be quite repeatable (within two or three percent) and with practice may be better. The methanol wipe cannot provide ultimate transmission values as it leaves some residue that may be subsequently polymerized. UV ozone Scrub UV-ozone scrub is the only method that can be used to clean mirrors, especially those with soft coatings. UV-ozone scrubbing is extremely effective in removing polymerized contaminants from a number of materials including sample types used here. It cannot remove gross or bulk contamination and is ineffective on some materials, such as organic salts contained in human skin oils. (4) Because of this some pre cleaning of the sample must occur and UV-ozone scrubbing plus the methanol wipe makes an important part of the operating procedure. Plasma Cleaning Plasma cleaning is used throughout industry for a number of cleaning purposes. In the case of ultraviolet transparent materials sensitive to contamination, it is extremely effective at quickly removing polymerized contaminants. It is capable of damaging some types of optics so great care and perhaps some experimentation is required to realize the correct 7

8 amount of time needed in plasma. One perfected, plasma cleaning is extremely repeatable, and ultimate peak transmission is possible and expected Black trace measured as received compared to same part cleaned three different ways Nitrogen purge results Each sample was cleaned by three different methods and transmission scanned. Subsequently, each sample was exposed 3 minutes in test chamber, 15 minutes of that time with addition of UV illumination. Transmission scan was done at the conclusion of the 3 minute exposure period and then 3 and 45 minutes later, to track change over time. This test was done in the VUVAS spectrophotometer, identical to that used above, the vacuum pump was removed and dry nitrogen gas flowed through instead. All measured values are higher for scans in nitrogen purge. Even with no UV illumination or exposure, nucleation and/or particle and ion condensates (3) in vacuum result in optical performance losses Plasma cleaned and measured in nitrogen purge (red) and in 5x1E-5 Torr vacuum (blue) 8

9 Plasma cleaned and measured in nitrogen purge at 15, 3 and 6 minutes A salient feature of nitrogen purge systems is the fact that spectrophotometer performance and material measurements remain stable over time. This is of importance to manufacturers of UV transparent materials, researchers working with these materials, and manufacturers interested in coatings for these wavelength regions. These cleaning methods, in combination with a nitrogen purged VUVAS spectrophotometer, would allow NIST to improve precision on radiometric and optical-characterization tests especially in the 1 to 2 nanometer wavelength region. Conclusion Cleaning has allowed us to prepare a sample in a reproducible manner and subsequently measure that samples foreign contamination, polymerized in situ, and measure it. We explored UV illumination and the vacuum environments influence on measurements. This demonstrated a direct connection to the vacuum environment as precipitating cause of changes during sample measurements and/or contamination that may become polymerized, and that UV exposure alone does not contaminate a sample without the presence of the vacuum environment., References 1. John P. Hammond C.Chem MRSC, New Certified Reference Materials for the determination of photometric accuracy in UV spectrophotometry, Spectroscopy Europe 13(4) 1 2. W. Pantely and D. Collier, Taking the UV Challenge, OE Magazine, October 1 3. McPherson specification sheet VUVAS 4. John R. Vig, UV/Ozone cleaning of surfaces, Surface Contamination,

A STUDY OF THE EFFECTIVENESS OF THE REMOVAL OF HYDROCARBON CONTAMINATION BY OXIDATIVE CLEANING INSIDE THE SEM.

A STUDY OF THE EFFECTIVENESS OF THE REMOVAL OF HYDROCARBON CONTAMINATION BY OXIDATIVE CLEANING INSIDE THE SEM. Neal Sullivan, Tung Mai, Scott Bowdoin* and Ronald Vane** A poster paper presented at Microscopy