Secondary Ion Mass Spectrometry (SIMS) Sample Cleaning

By AZoNano.com Staff Writers

Topics Covered

Introduction
SIMS: A High Sensitivity Surface Analysis Technique
Challenges Involved in Surface Composition Analysis
Solution
Using GV10x DS Asher for SIMS Operation
Conclusion
About IBSS Group

Introduction

Secondary Ion Mass Spectrometry (SIMS) is a technique used in materials science to study the composition of thin films and solid surfaces by sputtering the sample’s surface with a focused primary ion beam. In recent years, there has been an increasing interest in studying dopants and contaminants in thin layers near surface in samples, as the device structures get smaller and smaller.

SIMS can be used for examining the elementary composition of the surface as well as near surface region of samples with sensitivity down to parts per billion. In order to get the maximum performance from the device, atmospheric contaminants with low atomic weight such as and oxygen (O), carbon (C), and hydrogen (H) should have low concentration levels.

SIMS: A High Sensitivity Surface Analysis Technique

In SIMS analysis, contamination of hydrocarbons on the surface of the sample to be assessed may produce incorrect results. Hydrocarbons exist abundantly in ambient atmosphere and often deposit on the surface of samples when exposed to air.

During SIMS analysis, the atmospheric contamination on the surface can be embedded into the sample by means of a primary ion beam; this contamination leads to distorted or spurious profiles. Therefore, it is very 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

In science and technology, an issue in one field may also have been noticed in another field of study. However, a solution to this issue may have already been identified. 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 interacts with the electron beam and produces artifacts in the SEM image. These artifacts will produce poor image quality and can obstruct quantitative use of SEM images.

Solution

One way to address this problem is to install the ibss GV10x DS Asher onto the microscopes. The GV10x uses inductive coupled RF plasma to generate radicals, which in turn enter the SEM chamber and chemically etch the hydrocarbon contamination. Since the GV10x is able to work at low SEM chamber pressure and high RF power, cleaning takes place instantly and effectively.

When the GV10x DS Asher is regularly operated, hydrocarbons are removed quickly from the SEM chamber which enables users to achieve better images, particularly at low beam voltage and high magnification. Yet another important issue is whether there is a way to transfer the cleaning solution from the SEM to SIMS.

Using GV10x DS Asher for SIMS Operation

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

The GV10x DS Asher has been tested by the group to determine whether its cleaning capabilities can also be utilized for the SIMS operation. First, the team utilized air and pure hydrogen and then positioned the GV10x onto a port on the loadlock to their SIMS system, as shown in Figure 1.

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 GV10x was used to clean the sample. Then, they used air 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 also observed the growth of a thin oxynitride layer following cleaning.

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

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 illustrated in Figure 2, 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 considerable reduction in the concentration of carbon from 10^21 to 10^19 atoms/cc at the surface post cleaning.

Next, the GV10x DS Asher was used to clean the sample 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.

Thereafter, the results obtained were remarkable. The oxygen concentration increased only marginally, while the concentration of carbon at the surface reduced more than an order of magnitude. The increased concentration of oxygen is due to the presence of residual 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 continues to use the GV10x DS Asher with hydrogen to clean the samples prior to analysis. The GV10x allows EAG to carry out sensitive analysis of surfaces using SIMS without artifacts produced by contamination.

About IBSS Group

Vince Carlino founded ibss Group, Inc. in 2002, to turn brilliant ideas into affordable tools. In 2007 ibss Group began cooperating with an inventor of a unique, patented plasma source.

ibss Group developed and produced the GV10x, a new paradigm in situ downstream plasma asher.

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

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

Date Added: Oct 24, 2013 | Updated: Oct 25, 2013
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