Banishing Impurities for Successful Semiconductors: How to Achieve Consistent Part-Per-Quadrillion Trace Metal Detection

Kyoko Kobayashi

SPECTROSCOPY: As an application scientist, walk us through the various steps you take to prepare an inductively ICP-MS for ultratrace impurity analyses.

KOBAYASHI: Throughout my career as an application scientist working for PerkinElmer in Japan, I developed an in-depth expertise developing inductively coupled plasma mass spectrometry (ICP-MS) methods for ultratrace impurity measurements in various chemicals used in the semiconductor and electronics industry. These chemicals are diverse and span from acids, to bases, to a wide selection of organic solvents. In some cases, these chemicals are proprietary mixtures that many companies use to protect their unique manufacturing processes and gain competitive advantages.

Before I tackle any project, I must inquire about the sample type. Is it aqueous, an organic solvent, or a blend? If the sample is an aqueous solution, then my next question is, what kind of acid or base is it? If the sample is an organic solvent or a mixture, water solubility and boiling point are key parameters in defining next steps. From this information, the type of sample introduction system, such as torch, injector, spray chamber and nebulizer, can be determined.

Once the sample introduction system is configured, the next steps are to decide on instrument conditions and subsequent method development, which I will address later in this interview. What makes my day-to-day job easier is PerkinElmer’s semiconductor and fine chemicals recommended operating conditions, a compilation of methods that our customers can leverage to decide the appropriate sample introduction system and select from predefined methods for common chemicals.

SPECTROSCOPY: What is important to consider for ultratrace chemical samples analysis by ICP-MS?

KOBAYASHI: The most important consideration for ultratrace analysis is contamination and contamination management. Most customers I assist have their instruments operating in a cleanroom. Contamination management in clean- rooms involves strict protocols for personnel hygiene, proper attire, controlled airflow, high-efficiency particulate air filtration systems, and careful handling of materials and equipment—all aimed at minimizing the introduction of air- borne particles that can contaminate the chemical being analyzed. In the world of ICP-MS, this is referred to as the background equivalent concentration (BEC), the best indicator to the cleanliness of the analysis and the effectiveness of the spectral interference removal approach that is being used. That said, higher BECs result in higher limits of detection (LODs). Therefore, lower BECs are always desired for trace metal analysis.

SPECTROSCOPY: How do you optimize methodologies for your customers?

KOBAYASHI: When I start method development, sample matrix information is very important. For example, if I am developing a method for hydrochloric acid (HCl), I need to pay attention to chlorine (Cl)-based interferences. If I am running an organic solvent or a blend of organic solvents, I need to pay attention to carbon (C)- and probably phosphorus (P)- and sulphur (S)-based interferences. For example, if the sample is a photoresist mixture, it will contain high levels of carbon and, most likely, phosphorus and sulphur. The same thought process applies to analyzing silicon wafers: I will need to pay attention to silica (Si)-based interferences.

Depending on the matrix, abundant elements tend to form polyatomic interferences that will interfere on other elements, affecting the BEC and LOD for that specific element. For in- stance, if I am analyzing HCl, the ³⁵Cl ¹⁶O⁺ polyatomic molecule will interfere on ⁵¹V⁺. Therefore, I need to manage this polyatomic interferent using the Universal Cell Technology (UCT) that the PerkinElmer NexION® series of ICP-MS instruments is equipped with. In this specific case, I will use the UCT operating in Dynamic Reaction Cell (DRC) mode where ammonia (NH₃) as a reactive gas is introduced. Ammonia reacts with the ³⁵Cl ¹⁶O⁺ polyatomic ion inhibiting it from interfering on ⁵¹V⁺. In another instance, the introduced re- active gas reacts with the element of interest and the newly formed cluster is measured at a higher mass. This is referred to as mass shift. In the same HCl method, improved BEC and DL for P and S is achieved when we mass shift these elements with O₂ as a reactive gas. P and S will now be quantified as ³¹P¹⁶O⁺ and ³²S¹⁶O⁺ at masses 47 and 48, respectively.

SPECTROSCOPY: Why and when do you use cold vs warm vs hot plasma?

KOBAYASHI : The use of plasma modes has its merits, improving the BECs and DLs of a multitude of elements based on their ionization potential and sample matrix. In general, ionization potential increases from left to right across a period in the periodic table and decreases from top to bottom within a group.

From the ionization potential point of view, alkali metals (group 1) and alkaline earth metals (group 2) which have low ionization potentials preer cold plasma, while the halogens (group 17) and the chalcogens (group 16) with high ionization potentials are best analyzed in hot plasma. I suggest using warm plasma for the best BECs and DLs of group 1 and 2 in samples with elevated matrix. The more matrix the analyzed sample has, the more energy is needed to achieve optimal ionization. In addition, cold plasma suppresses the ionization of some elements such as Na, K, Cr and Fe from the cones. Summarizing, cold plasma is used for elements with low ionization potential in samples with light matrix, such as ultrapure water and pure chemicals like H₂O₂, HNO₃ and HCl, while hot plasma is preferred for samples with high matrix, such as organic blends and silica-based samples, and for elements which have high ionization potential.

SPECTROSCOPY: What other parameters do you work with when developing a method?

KOBAYASHI: When the sample volume is limited, I choose the optimal nebulizer that gives the appropriate sample flow rate. However, we need to pay attention to this lower sample flow rate, which produces a hotter plasma, resulting in higher BECs for some analytes. When sample analysis time is important for the operating cost or the sample volume, I approach the method development from both angles: sample flow rate and analysis time. Analysis time is optimized by minimizing gas switching and gas flow changes.

SPECTROSCOPY: What do you enjoy most about your work and why?

KOBAYASHI: The most exciting part of my job is having the opportunity to work with the latest ICP-MS technology. Working with various customers from a variety of industries and countries is also very exciting. While I am regarded as an ICP-MS expert, I am constantly learning from my interactions with all the customers I assist.