GDMS Applied to Materials Analysis
How GDMS is used in Tosoh SMD's thin film program.
Dr. Nick Hill,,Tosoh, Grove City, Ohio -- Semiconductor International, 6/1/1998
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| Tosoh SMD uses two of VG Elemental's GDMS instruments in its laboratory. |
Glow discharge mass spectrometry
(GDMS) is a high-performance, quality control tool for solid sample
analysis and is widely accepted in the semiconductor industry as the
method of choice for performing trace analysis of high-purity metals.
For example, Tosoh SMD, a supplier of thin film materials, utilizes two
of VG Elemental's GDMS instruments in its laboratory. Used as a primary
control tool in Tosoh SMD's quality control and R&D departments, GDMS
enables the measurement of a wide range of elements at detection limits
of better than 10 parts per billion (ppb).
In addition to the measurement of trace metal impurities of incoming materials, Tosoh also utilizes GDMS to evaluate materials that have been altered in-house by melting or powder processing. This ensures that after alteration, the materials still meet Tosoh SMD's requirements. At Tosoh, GDMS is used exclusively for any solid metallic sample that is four nines or better purity. Some of its non-solid and nonconducting samples are sent to to contract labs for analysis.
The GDMS technique
The GDMS technique involves preparing a sample that is sputtered in an argon glow discharge. Sputtered neutrals are then ionized in the plasma, extracted from the source and accelerated down the flight path of a Nier-Johnson reverse geometry double-focusing mass spectrometer. In this process, ions are resolved according to their mass-to-charge ratio and detected on a combination Faraday cup/Daly knob detector. This allows the matrix elements and trace impurities to be detected in the same experiment. Samples are generally prepared as 2-3 mm diameter pins about 20 mm long, utilizing analytical results in dilution factors of between 50-200 times.
Evaluation of commercial surface cleaners
When Tosoh sought to eliminate CFCs from its now CFC-free manufacturing process, it was necessary to evaluate CFC cleaning vs. alternative methods. Again, this was accomplished using GDMS.
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| Fig. 1. GDMS was used to measure carbon, sodium, sulfur, chlorine and iron contamination left on various samples after cleaning. |
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| Fig. 2. Depth profiling was used to evaluate an aluminum-silicon-copper interconnect. |
For this work, witness samples were prepared from a variety of common target materials all having the same surface finish as sputtering targets. These samples were then soaked in the same fluids used in Tosoh SMD's machining operations. To establish a baseline, a representative number of samples were cleaned in Tosoh's cleaning systems. Surface contamination was measured using the flat cell GDMS technique. Six elements that were used in Tosoh SMD's manufacturing facility and that most concerned the company's customers were chosen: carbon, sodium, silicon, sulfur, chlorine and iron.
Figure 1 reflects data compiled during this evaluation project. It shows carbon on an aluminum surface obtained using the flat sample cell. The initial carbon signal is from surface contamination. The signal then decays to a steady state that represents carbon in the bulk material. In this case, elemental signals are displayed in relative terms, as Tosoh SMD did not have good calibration for carbon in aluminum using the flat cell. Further, the goal of the experiment was to compare the relative cleanliness of the two techniques. Ultimately, a CFC-free cleaning process was chosen.
Metal film analysis depth profile
Depth profiling is another application in which GDMS was used as an R&D tool. An aluminum-silicon-copper (Al-1% Si-0.5% Cu) film was deposited onto a silicon wafer. Then the content of the aluminum, silicon and copper was measured through the depth of the film to determine how they varied (Fig. 2). The depth profile of the sputtered film indicates the variation of the silicon and copper through the film depth. By run nine, the film has been sputtered through to the silicon substrate.
Tosoh SMD has used GDMS as a means of analyzing high-purity materials. Material specifications for the semiconductor industry are changing dramatically. As the need for higher purities and lower limits of detection continue to expand, glow discharge mass spectrometry will continue to meet industry demands.
