Quantification of Contamination Using Quartz Thickness Monitors

Abstract

Hydrocarbon (HC) contamination is a persistent problem for users of electron microscopes, often leading to image distortion and interference with nanoprobing [1][2][3][4]. Quantifying the amount of contamination present by easy, low cost methods has been a challenging problem. Contamination levels in Electron Microscopes are usually determined by measuring electron beam deposition of HC on a clean surface and imaging the resulting HC mound or black square [5]. If the specimen can be tilted the height of the deposited mound can be measured. The problem with this technique is that in a relatively clean instrument, it may take 20 to 200 minutes to accumulate a significant deposit for measurement.As an alternative, a quartz monitor could be mounted on the stage for a new method of quantifying contamination. Quartz crystal thin film thickness monitors are a standard technique for measuring the vacuum deposition of thin films and are available from many manufacturers [6]. Commonly used for measuring sputter coating, they can be adapted to measure contamination in electron microscopes and the rate of contamination removal by plasma cleaning. Liquid cooling of the monitor is not needed as it is for Sputter coating processes, but an RF feedthrough port is needed for the monitor. The electron beam is scanned on the monitor and the thickness build rate of HC is available almost immediately.The Evactron® De-Contaminator (D-C) has been available for contamination removal in SEMs since 1999 [7]. The Evactron D-C uses low power radio frequency (RF) generator plasma in order to produce oxygen radicals which clean an electron microscope (EM). The oxygen radicals chemically react with the HCs to form volatile oxidation products such as H 2 O, CO and CO 2 . These volatile compounds can be pumped out of the EM chamber. The efficiency of the Evactron process has been demonstrated qualitatively. A light HC deposit can be removed from a mirrored surface in minutes, but achieving a reproducible deposit and removal data is difficult.Efforts have been made to provide quantitative tests of the efficiency of the Evactron cleaning process using a quartz crystal thickness monitor. A picture of the quartz crystal thin film thickness monitor is shown in Figure 1. The thickness monitor (McVac -MCM 160) was placed inside the vacuum chamber ~1 cm directly in front of a port fitted with an 11 cm long piece of steel vacuum tube containing 5 to 7 drops of oil. The end of the vacuum tube was capped with an adapter flange fitted with a needle valve that was used to bleed air through the tube. The chamber pressure was adjusted to 0.15 mbar with the needle valve. The tube containing the oil was heated for 5-7 minutes and then allowed to cool for ca. 5 minutes. Visual inspection normally revealed a thin film of oil on the thickness meter, on the end cap, and lining the opening into vacuum chamber. This method produced more contamination than would normally be seen in an EM; however, enough oil was placed on the thickness monitor so that the rate of oil re...

Comparing the Effects of Different Gas Mixtures and Vacuum Chamber Geometries on the Evactron® Cleaning Process

“…3 with the QCTM on the vacuum port side and the Evactron D-C cleaning hydrocarbons. The loss rates are higher than expected, especially when compared to loss rates seen in more open chambers [2]. This suggests that the geometry used is funneling the radicals into the areas near the QCTM.…”

“…The standard explanation of how the Evactron D-C device works is that oxygen radicals are created in the plasma and flow downstream, ashing carbon contamination. Using quartz crystal thickness meters (QCTMs), the effectiveness of the Evactron process has been quantified as a function of cleaning parameters such as chamber pressure during cleaning, RF power, and distance from the plasma source [2]. QCTM measurements can now be extended in order to consider the effect of different gas mixtures and chamber geometries on cleaning.…”

20th Anniversary of Evactron® Plasma Cleaners for SEMs and FIBs

“…XEI developed the use of quartz crystal monitors to measure hydrocarbon removal rates in 2007 [4]. This direct testing method was used in research to improve cleaning rates.…”

Visualizing Evactron® Turbo Plasma™ Cleaning in nanoflight® Movies

“…Previously hydrocarbon removal has been monitored by means of quartz crystal microbalances [2] and residual gas analysis [3]. For the first time the process of plasma cleaning has been recorded as it happens, not simply before and after images.…”