In-situ Ar Plasma Cleaning of Samples Prior to Surface Analysis

Transcription

1 In-situ Ar Plasma Cleaning of Samples Prior to Surface Analysis GE Global Research Vincent S. Smentkowski, Cameron Moore and Hong Piao 04GRC955, October 04 Public (Class ) Technical Information Series

2 Copyright 04. General Electric Company. All rights reserved. ii

3 GE Global Research Technical Report Abstract Page Title In-situ Ar Plasma Cleaning of Samples Prior to Surface Analysis Author(s) Vincent Smentkowski SSO Hong Piao Cameron Moore -- Component Nanostructures & Surfaces Lab, Niskayuna Report Number 04GRC955 Date October 04 Number of Pages Class Public (Class ) Key Words: Surface cleaning, surface contaminants, ToF-SIMS, XPS, plasma, plasma cleaning Abstract: The surface of as received samples is often contaminated with adsorbed layers of hydrocarbons. These surface contaminants can attenuate or mask underlying species of interest. In-situ ion beam sputtering is often used to remove the outer layer of a sample surface and thus remove contaminants; however this erosion process is inherently destructive and can alter the surface of interest. Moreover there are many materials that cannot be cleaned using monoatomic ion beam sputtering as the material(s) may decompose and deposit a layer of fragments onto the outer surface of the material to be analyzed. A non line-of-sight protocol which is able to clean large (mm or greater) areas is desired. We recently demonstrated that ambient air plasma processing can be used to clean the outer surface of samples, however ambient air plasma treatment can result in oxidation of the material. In this presentation we report our first attempts at in-situ plasma cleaning of samples using Ar prior to XPS and ToF-SIMS analysis. We compare Ar plasma cleaning with air plasma cleaning, and report key findings. We also describe instrument modifications that were required in order to allow Ar processing of samples. Manuscript received Oct 7, 04 iii

4 The surface of as received samples is often contaminated with adsorbed layers of hydrocarbons. These surface contaminants can attenuate or mask underlying species of interest, inhibiting or compromising accurate analysis. In-situ ion beam sputtering is often used to remove the outer layer of a sample surface and thus remove contaminants, however this erosion process is inherently destructive and can alter the surface of interest. Moreover there are also many materials that can not be cleaned using monoatomic ion beam sputtering as the material(s) may decompose and deposit a layer of fragments onto the outer surface of the material to be analyzed. Recently gas cluster ion beams (GCIB) have been developed,, which allows for depth profile analysis of organic layers with minimal degradation (and references therein). GCIBs have also been used for low damage surface cleaning 4,5,6. A non line-of-sight protocol which is able to clean large (mm or greater) areas is desired. We recently demonstrated that ambient air plasma processing can be used to clean the outer surface of samples 7, however ambient air plasma treatment can result in oxidation of the material. In this presentation we report our first attempts at in-situ plasma cleaning of samples using Ar prior to XPS and ToF-SIMS analysis. We compare Ar plasma cleaning with air plasma cleaning, and report key findings. In order to allow Ar processing of samples, modifications to both the X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) instruments were required. Namely, Ar gas tanks needed to be attached to the gas inlet on the Evactron plasma cleaners and the pumps on the load lock needed to be modified such that the pumps did not operate at full capacity and deplete the Ar gas reservoir in a few seconds. The in-situ XPS and ToF-SIMS studies were complimented with measurements of the macroscopic rate of contamination removal in order to verify that a given process gas is a viable option. The method consisted of coating quartz crystals with stable hydrocarbons in solid form, and using these in a commercial quartz crystal microbalance (QCM) system installed in a test chamber. The sample was located 5 mm from the plasma. This allowed for a direct comparison air, Ar, and Ar/H (5%) as process gases. We observed that the different gases did not couple the RF power in the Evactron identically, so these removal rate data were compensated by normalization to delivered power. Two conclusions are evident: (i) that Ar has an efficacy ranging from 0 to 40% that of air, and (ii) the addition of 5% hydrogen had negligible effect. The attached poster shows typical experimental results and has a discussion of the results. Over the next few months we plan to obtain tanks of He, Ne, and Xe such that we can evaluate, for the first time, if the surface contamination rate varies with noble plasma gas composition.

5 I. Yamada, J. Matsuo, N. Toyoda and A. Kirkpatrick, Mater. Sci. Eng., R 4, (00). S. Ninomiya, K. Ichiki, H. Yamada, Y. Nakata, T. Seki, T. Aoki and J. Matsuo, Rapid Commun. Mass Spectrom.,, 64 (009). V. S. Smentkowski, G. Zorn, A. Misner, G. Parthasarathy, A. Couture, E. Tallarek, and B. Hagenhoff, J. Vac. Sci. Technol A.,, 060 (0). 4 M. Akizuki, M. Harada, Y. Miyai, A. Doi, T. Yamaguchi, J. Matsuo, G.H. Takoka, C.E. Ascheron, and I. Yamada, Surface Review and Letters, 0 (), 89(996). 5 I. Yamada, J. Matsuo, Z. Insepov, D. Takeuchi, M. Akizuki, and N. Toyoda, J. Vac. Sci. Technol A 4() 780 (996). 6 I. Yamada, J. Matsuo, and N. Toyoda, Nuclear Instruments and Methods in Physics Research B, 06, 80 (00). 7 V.S. Smentkowski, C.A. Moore, J. Vac. Sci. Technol. A. (0) 06F05.

6 In-situ Ar Plasma Cleaning of Samples Prior to Surface Analysis Vincent S. Smentkowski, Hong Piao, C.A. Moore General Electric Global Research Center, Research Circle, Niskayuna, NY 09, USA XEI Scientific, Inc., 755 E. Bayshore Rd., Suite 7,Redwood City, CA 9406, USA Time (s) Background In an industrial setting many of the samples analyzed, as received, by X-ray Photoelectron Spectroscopy (XPS) and/or Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) reveal a hydrocarbon signature that results from processing, handling, and/or ambient exposure. With the high surface specificity of XPS and especially ToF-SIMS the contamination (hydrocarbon, and/or silicones) signal can attenuate and/or mask the signals of species of interest. To counteract this issue, an in-situ ion beam is often used to remove the outer layer of a sample surface via sputtering and thus remove contaminants. However this erosion process is inherently destructive and can alter the surface of interest and/or change the topography/microstructure of the surface. Moreover there are also many materials that cannot be cleaned with this method as the material(s) may decompose and deposit a layer of fragments onto the surface of the material to be analyzed. Thus an alternative to sputter cleaning is sought. In 0, we demonstrated that ambient air based plasmas can be used to remove both hydrocarbons and polydimethylsiloxane (PDMS) from the surface of materials prior to XPS and ToF-SIMS analysis. Unfortunately, ambient air plasma processing can result in oxidation of the material to be analyzed and for some analysis is not desired. In this presentation we report our first attempts at in-situ plasma cleaning of samples using Ar gas prior to XPS and ToF- SIMS analysis. We compare Ar plasma cleaning with air plasma cleaning, and report key findings. Samples: Si wafer with an intrinsic oxide and Cu foil Experimental: An XEI Scientific Evactron 5 de-contaminator plasma cleaner was mounted onto the top flange of the load lock of both an Ion-Tof TOF-SIMS IV instrument, and a Kratos AXIS Ultra DLD XPS instrument. Ambient air processing was performed using the air in the lab and the load lock pumps running in the normal configuration (but at a higher than normal load). Noble gas processing using Ar gas was performed by connecting tanks of research grade Ar gas to the gas inlet using swage lock fittings. Throttle valves, and a second set of pumps, were used to pump the load lock on both the XPS and ToF-SIMS instruments such that the Ar gas did not deplete in the gas line during plasma processing. Each specimen was plasma treated for 0 minutes. Samples were analyzed as received, transferred into the load lock for in-situ cleaning without ambient air exposure, then transferred back to the analysis region without air exposure for post plasma cleaning analysis. Results: Measuring Removal Rate: It is useful to measure the macroscopic rate of contamination removal to verify that a given process gas is a viable option. The method consisted of coating quartz crystals with stable hydrocarbons in solid form, and using these in a commercial quartz crystal microbalance system installed in a test chamber 5 mm from the plasma. This allowed for a direct comparison air, Ar, and Ar/H (5%) as process gases. We observed that the different gases did not couple the RF power in the Evactron identically, so these removal rate data were compensated by normalization to delivered power. These results are presented in the adjoining figure where two conclusions are evident: (i) that Ar has an efficacy ranging from 0 to 40% that of air, and (ii) the addition of 5% hydrogen had negligible effect. Removal Rate (a.u./wa ) 5 4 Hydrocarbon Removal Rate vs Pressure (Normalized to delivered RF power) Air Ar/H Argon Pressure (mtorr) Results: ToF-SIMS Si Sample: The ToF-SIMS peak areas were measured, and ratios to 0 Si were determined. Table lists the peak ratios for select species. Table, Si C CH CH CH CH O NH4 Na F As Received After Ar Plasma As Received After Air Plasma The plots below show the change in C (left) and hydrocarbon species (right) following plasma treatment Results: XPS Si Sample: The table below shows the changes of composition measured by XPS C F Na O Si As Received < After Ar Plasma.0.4 < After Air Plasma < The spectral overlay plot shows the change in oxidation state Sip Si wafer 0 mins Ar plasma 0 mins air plasma Si-O Si(0) Binding Energy (ev) C -- Ar C - Air The oxide feature remained after Ar plasma cleaning. Ambient air plasma cleaning results in the highest degree of surface oxidation Intensity (counts) Cu Sample: The table below shows the changes of composition measured by XPS C Cl Cu N O As Received After Ar Plasma After Air Plasma The spectral overlay plot shows the change in oxidation state of Cu Cup CH -- Ar CH -- Ar CH - Ar CH - Air CH - Air "CH air" 0 0 It is of interest to note that certain species (such as F) increase following plasma treatment. F SiO ToF-SIMS depth S Cl profile measurements 0 reveal that the intensity of these species C increase after the surface contaminants are removed Cu foil 0 mins Ar plasma 0 mins air plasma Binding Energy (ev) CuO Cu(OH) Cu O CuO Cu Sample Table, Cu C CH CH CH5 CH CH5 CH7 O NH4 Na As Received After Ar Plasma As Received After Air Plasma Cu LMM Binding Energy (ev) Ar plasma cleaning significantly reduced the amount of surface Cu(OH) and CuO species but increased the amount of Cu O. Air plasma cleaning removed the surface Cu(OH) species. Summary and Conclusions: Both ToF-SIMS and XPS show that Ar plasma s can be used to remove hydrocarbon species from the surface of samples. Plasma processing using ambient air results in oxidation of the sample oxidation is reduced when Ar is used as the processing gas Removal of the outer layer of hydrocarbon surface contaminants often exposes other elements (such as N, Na, F)); ToF-SIMS and XPS depth profiling shows that these elements are indeed present in the near surface region of the specimen and these species do not originate from plasma cleaning References: V.S. Smentkowski, C.A. Moore, J. Vac. Sci. Technol. A. (0) 06F05 and therein