提高SEM / FIB的吞吐量

Vacuum-based processes display lower performance in the presence of adventitious hydrocarbons that are volatilized from various sources, such as solvents and oils, as well as chamber and sample surfaces¹。For instance, in electron microscopy, the presence of hydrocarbons (HC) causes unwanted effects such as image blur, “black square” and “black frame” formation during prolonged beam exposure times²。These and associated issues have directed the need for RF-driven plasmas to decontaminate vacuum chambers via production of excited-state species (usually oxygen radicals) that mildly remove contamination. It is crucial to note that these systems produce electrically neutral cleaning species that flow from the plasma into the chamber so the decontamination is realized by mild chemical etching.

The oxygen radicals formed in the plasma oxidize carbon compounds, producing CO₂，co和h₂O, which are evacuated from the instrument. Quantum chemistry rules regarding energy loss state that these oxygen atoms do not react with diatomic molecules in two body collisions but require a third body to kinetically remove excess energy. Oxygen radicals also react on solid surfaces such as metals where they can react or recombine with hydrocarbons. Several studies performed at XEI Scientific using pre-contaminated quartz crystal microbalances (QCM) to measure cleaning rates have demonstrated that Evactron cleaning is highly effective at removing hydrocarbons^{3, 4}。Faster decontamination rates were noted in FIBs and SEMs fitted with turbo molecular pumps.

The present generation of Evactron plasma cleaners includes models such as the EP illustrated in Figure 1 and the new E50 in Figure 2. The EP and E50 models are engineered for high efficiency cleaning with lesser cost by simplification of both software and hardware. The compact design of the model EP makes it a versatile solution for small sample preparation chambers and SEM/FIB loadlocks.

Figure 1.The Evactron Model EP De-contaminator system (a) includes a desktop controller and the KF clamp together plasma radical source. (b) The small size PRS fits on a FESEM column with numerous analytical accessories.

Figure 2.The Evactron Model E50 De-contaminator system (a) includes a desktop controller and the newly designed external hollow cathode plasma radical source for 50 Watt operation. (b) The Evactron E50 PRS mounted on a SEM port has a clean, compact footprint.

Technology

Invented in 1999, the Evactron plasma cleaner is based on RF hollow cathode excitation (RFHCE) plasma which is uniquely available from XEI Scientific. Other plasma cleaners use other excitation techniques such as inductively coupled plasma (ICP) that use more power and produce more heat for similar cleaning rates. RFHCE forms more neutral radicals for chemical etching and fewer energetic ions that can result in sputter damage to the surfaces being cleaned

The original Evactron plasma source used an internal plasma electrode that was built to be used on chambers at pressures between 1 Torr and 200 m Torr attained by roughing pumps. Lower pressures were avoided because of the then common oil diffusion pumps used to acquire high vacuum (down to 10^-7Torr）。对新SEM的涡轮分子泵转换使得可以在10中涵盖血浆清洁到高真空^-4托罗范围。在这些低压下，输入气体流量和泵速度之间存在平衡，在10中提供压力^-2to 10^-4Torr. In the newEvactron E50 model,gas input down to 6 sccm will develop a plasma with a chamber pressure in the 10^-4T range, but with a loss of cleaning rate because of lower radical production (Figure 3). In this pressure range, there is a trade-off between low pressures for low flow and higher cleaning rates from more input gas flow.

Figure 3.At low-pressure cleaning rates begin to drop as fewer oxygen molecules are available for radical production. Higher RF power increases the cleaning rate and less flow lowers it.

Materials and Methods

为了证明Evactron Turbo等离子体清洁对泵浦时间和HC污染去除的影响，采用两种型号的Evactron等离子体清洁剂进行了研究：

1. A Model Evactron EP Plasma De-Contaminator was positioned on a large, extremely oil-contaminated 50 L vacuum chamber fitted with a 450 L/second turbo molecular pump, MKS 972B dual range pressure gauge, 14 CFM scroll pump, and a residual gas analyzer (RGA). All pump down curves are from atmosphere where the chamber is vented for 10 minutes to accept configuration changes. A baseline pump down curve from atmosphere was taken and RGA data taken when the measured pressure was 1E-7 Torr. To contaminate, a two-inch nipple was contaminated with two drops of pump oil and was installed on the chamber. The nipple was wrapped in bake tape and foil, and then it was heated to 120 °C while being pumped on with only the roughing pump. This bake process lasted for ~ 18 hours. Pump down curves and RGA data were captured before and after plasma cleaning.

Figure 4.50升测试室用于测量原始腔室的泵送时间，以在重污染，污染和等离子体清洁后的污染物。

RGA spectra of contamination (Figures 7-9) and pump down curves (Figure 11) were collected before and after plasma cleaning so as to show the effects of Evactron Turbo Plasma cleaning on pump downtime and hydrocarbon removal. The chamber is said to be in pristine condition when the HC peaks on the RGA spectrum are less than a partial pressure of 2 × 1^-10Torr.

An E50 model Evactron De-Contaminator was mounted on a 22 L vacuum chamber to measure cleaning rates as a function of distance from the plasma source for the data charted in Figure 10. For the E50 system, cleaning rates were measured by means of QCMs placed 25 cm and 0 cm from the mounting nipple with the plasma radical source operating at 20-70 W.

Results and Discussion

对于Evactron等离子体清洁技术，五分钟或更小的短清洁时间是足以消除所有烃污染物，如在图像对之前和之后所示（图5和6）所示。

Figure 5.Before cleaning.

Figure 6.After 5 minutes of cleaning.

Such results are established by the comparison of RGA spectra in Figure 7, a pristine chamber before plasma cleaning, Figure 8 after contamination, and Figure 9, after 10 minutes of plasma cleaning. This rapid cleaning allows vacuum chambers to be kept pristine, and SEMs/FIBs to have more uptime for imaging and analysis.

Figure 7.Pristine chamber before contamination shows all HC peaks are absent from RGA scan, leaving residual water and atmosphere.

Figure 8.RGA扫描用碳氢化合物污染腔室。Characteristic hydrocarbon peak series are present. To assure a pristine chamber, six 5 minute plasma cleans at 20 watts using an Evactron EP were executed with a 2 minute wait time between cleans.

Figure 9.The RGA scan after 10 minutes shows that the cleaning byproducts including atmosphere and water are still being detected but all signs of the hydrocarbon contamination are gone.

泵停机时间的长度基于FIB和SEM中的HC污染水平而不同。因此，泵停机可以用作真空系统清洁度的标志。图10显示了在正常的SEM / FIB工作距离下实现了数百埃/分钟的有效HC净化速率。图11中的数据说明了Evactron等离子体清洁器可以显着减少泵浦泵的泵和SEM以及HC污染，从而提高样品处理吞吐量，而不会牺牲分析的质量。

Figure 10.的Evactron E50更高的权力和low pressures offers good cleaning rates throughout larger vacuum chambers.

Figure 11.等离子体清洁室达到了3e的真空^-6Torr in 20 minutes pump down whereas the contaminated chamber did not reach this level after 44 minutes.

Remote plasma cleaning can be done on FIBs and SEMs at pressures below 75 mTorr (10 Pa) during direct pumping with a turbo molecular pump. Earlier studies⁵揭示低室压力升高了清洁速率和观察到清洁的距离。低压通过增加平均自由路径并通过三体碰撞降低氧自由基的重组率来提高清洁效率。该结果与低压下的新空心阴极设计一起允许大型仪器真空室的更有效的清洁策略。

低压下的点火还允许在清洁短时间（1-5分钟）后缩短等离子体，导致涡轮泵快速返回基础压力以去除反应产物，然后重新启动等离子体再次执行清洁周期。虽然在基础压力下，任何看不见的烃都可以在腔室内进行脱气并重新分配，因为没有自由路径分子流动。可重新分配的产品可以在以下等离子体周期中消除。循环等离子体清洁被证明是在高真空下使用这种新的等离子体清洁技术来获得原始腔室的高效方法。典型的涡轮分子泵输入流量为20 sccm提供良好的清洁，并不会过热泵。

Conclusions

As the need for superior quality data and higher throughput of samples in Scanning Electron Microscopes (SEMs) and Focused Ion Beam (FIB) systems increases, so does the requirement to shorten pump downtimes between loading samples. Industry demands SEM/FIB systems to be working round-the-clock and preferably maintained in pristine condition with uncompromised image quality. Frequent venting of the FIBs or SEMs to load samples adds contamination and moisture into the vacuum chamber, resulting in much longer pump downtimes and lower efficiency. Evactron cleaning removes this contamination effortlessly. Samples can be imaged more speedily in a clean environment, and if contamination is detected, it can be swiftly removed, the SEM pumped back to operating pressure quickly and, with only minutes of cleaning delay, imaging and analysis are restarted. The result is increased productivity.

The new Evactron^®涡轮等离子™De-Contaminators去除碳氢化合物(HC) contamination from FIBs, SEMs, and other analytical tools using a gentle, down-stream plasma afterglow process. At turbopump pressures, Evactron cleaning becomes faster and spreads all through the chamber. This is because of longer mean-free-paths that cause less recombination of oxygen radicals in the required three-body collisions and reduced scattering to chamber walls. In the majority of cases, short plasma cleaning cycles are adequate to remove contamination and considerably reduce pump downtime, allowing for high throughput for sample processing and analysis.

TheEvactron E series of plasma cleanersoffer quick, effective, and powerful cleaning over a broad range of pressures, enabling superior quality, artifact-free images, and better efficiency of sample analysis. The ‘Fastest Way to Pristine’ is a slogan that translates into the fastest productivity for your laboratory.

参考和进一步阅读

1. 沙利文,n . et al。(2002)。Microsc. Microanal.8(2), 720.
2. Joubert, L.-M. (2013).Microsc。Anal.27(4), 15.
3. Gleason, M.M. et al., (2007).Microsc. Microanal.13(2), 1734.
4. Morgan, C.G. et al., (2007).Microsc. Microanal.13（2），1736。
5. Vane, R. et al., (2016)Microsc. Microanal.8(2), 720.

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