Hydrocarbon Contamination Cleaning fgor Secondary Ion Mass Spectrometry (SIMS) Samples

In recent years, there has been a growing interest in analyzing dopants and contaminants in thin layers in near surface regions, as device structures are becoming smaller and smaller.

Secondary Ion Mass Spectrometry (SIMS) is suitable for studying the elementary composition of a surface, as well as the near surface region of samples with sensitivity down to parts per billion. In order to achieve maximum device performance, atmospheric contaminants with low atomic weight such as and oxygen (O), carbon (C), and hydrogen (H) should have low concentration levels.

SIMS Analysis

In SIMS analysis, contamination of hydrocarbon on surfaces to be examined can produce false results. In addition, hydrocarbons are abundantly present in ambient atmosphere and tend to deposit on the surface of samples when exposed to air.

In SIMS analysis, the surface’s atmospheric contaminants can be pushed within the sample by the primary ion beam and appear as contamination in the sample - this produces distorted or spurious profiles. It is important for operators to know whether they are determining the surface’s composition or just viewing an artifact caused by contamination.

Challenges Involved in Surface Composition Analysis

When Scanning Electron Microscopes (SEMs) are operated in the electron landing energy mode or low beam voltage to image the surfaces down to the nanometer scale, contamination of hydrocarbon in the SEM chamber networks with the electron beam and produces artifacts in the SEM image.

Such artifacts will produce poor image quality and can prevent quantitative use of SEM images.

Solution

One way to overcome this problem is to install the ibss GV10x DS Asher onto the microscopes. The GV10x operates by utilizing inductive coupled RF plasma to produce radicals. These radicals in turn enter the SEM chamber and chemically etch the hydrocarbon contamination.

Since the GV10x is capable of operating at low SEM chamber pressure and high RF power, cleaning takes place instantly and effectively.

When the GV10x DS Asher is periodically operated, hydrocarbons are removed quickly from the SEM chamber and enables users to achieve better images, particularly at low beam voltage and high magnification.

Another important aspect is whether there is a way to transfer the cleaning solution from the SEM to SIMS.

GV10x DS Asher

Evans Analytical Group (EAG) has a separate SIMS group that uses sophisticated systems at its facilities located in New Jersey and California. The group provides SIMS services to help customers understand impurity levels, dopant profiles, and composition of their samples.

They tested the ibss GV10x DS Asher to determine whether its cleaning capabilities can also be utilized for the SIMS operation. First, they utilized air and pure hydrogen and then positioned the GV10x onto a port on the loadlock to their SIMS system, as illustrated in Figure 1.

GV10x DS Asher Source (white box in middle of picture) installed on the loadlock chamber of the SIMS analysis system.

Figure 1. GV10x DS Asher Source (white box in middle of picture) installed on the loadlock chamber of the SIMS analysis system.

Before the SIMS analysis, the sample was cleaned with the GV10x. Using air, the group was able to observe the removal of carbon at a high rate from surfaces by contrasting SIMS results before and after cleaning. However, during this process, they observed the growth of a thin oxynitride layer subsequent to cleaning.

To prevent the growth of the oxynitride layer, EAG analysts cleaned the surface with hydrogen gas, instead of cleaning it with air. First, a SIMS analysis was carried out on a silicon sample with shallow Carbon implant.

SIMS analysis of the surface of a silicon sample revealing relative concentrations of silicon, carbon and oxygen before and after cleaning with the GV10x DS Asher.

Figure 2. SIMS analysis of the surface of a silicon sample revealing relative concentrations of silicon, carbon and oxygen before and after cleaning with the GV10x DS Asher.

As shown in Figure 2, a considerable amount of carbon was observed on the surface prior to the ashing process. As the sample depth increases the silicon concentration increases quickly.

The relative oxygen concentration differs between 10^21-10^22 atoms/cc, while there is a noticeable decrease in the concentration of carbon from 10^21 to 10^19 atoms/cc at the surface post cleaning.

Next, the sample was cleaned with the GV10x DS Asher by using pure hydrogen gas. The GV10x was operated for a period of 15 minutes with the loadlock chamber pressure at 50mTorr and the RF power set to 50W.

The results obtained were remarkable. The concentration of oxygen increased only marginally, while the concentration of carbon at the surface reduced more than an order of magnitude. The increased concentration of oxygen is because of the remaining oxygen in the H2 gas and loadlock chamber. The surface cleaning slightly increases the concentration of hydrogen entrenched into the sample.

Conclusion

EAG’s SIMS group is aiming to use the GV10x DS Asher with hydrogen to clean the samples prior to examination.

Using the GV10x with hydrogen gas provides a suitable solution to carry out sensitive analysis of surfaces using SIMS without artifacts produced by contamination.

This information has been sourced, reviewed and adapted from materials provided by IBSS Group

For more information on this source, please visit IBSS Group.

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