Chemical Analysis of Semiconductor Wafer Fabs Environment and Quality Control

• Environment-induced wafer contamination
• Analysis of ultratrace metals in process chemicals and reagents by ICP-MS
• The role of ion chromatography (IC) in chemical analysis of semiconductors
• Analysis of additives and suppressors in copper plating baths by liquid chromatography (LC)

Monitoring the wafer manufacturing environment includes the control and analysis of all media that the silicon wafers are exposed to that can adversely affect their manufacturing yield. These include:

• Deionized (DI) water that is used as a rinsing agent throughout the fabrication of an integrated circuit
• Chemical reagents, acids, bases, and solvents that are used to clean wafer surfaces or etch materials on the wafer surfaces
• Inert and reactive gases that are used during ion implantation and deposition process steps
• Air in clean room and mini-environments throughout the entire manufacturing process

To minimize environment-induced wafer contamination, these gaseous and liquid media have to be analyzed for particles, impurities, and specific contaminants including organics, trace metals, acids, and bases. Depending on the phase of the medium (gas, liquid, solid) and the nature of the contaminants to be controlled, different chromatography and spectrometry analytical techniques have to be used. Table 1 lists many of these technologies, while Table 2 provides an overview of the analytical techniques for semiconductor contaminant analysis.

Cold plasma is not recommended for the analysis of all elements however, (for example it gives reduced sensitivities for high ionization potential elements such as Ge) and therefore a combination of hot and cold plasma conditions must be used in routine analysis. Thermo Scientific quadrupole (Q) ICP-MS systems deliver high productivity with reliable hot and cold plasma operation and fast switching between the modes in one analytical run.

The latest addition to the Thermo Scientific ICP-MS portfolio, triple quadrupole (TQ) ICP-MS is designed to deliver improved interference removal in challenging matrices such as silica matrix and sulfuric acid. The powerful combination of triple quadrupole and cold plasma operation enables ultratrace analyte quantification at sub ppt concentrations in process chemicals and on wafer surfaces for reliable control of elemental impurities in wafer production.

Instrument type	Target applications	Instrument model	Process monitoring	QA/QC	R&D
HR-ICP-MS	UHPC, UPW, VPD	ELEMENT 2 HR-ICP-MS
TQ-ICP-MS	UHPC, UPW, VPD	iCAP TQs ICP-MS
SQ-ICP-MS	UHPC, UPW, VPD	iCAP RQplus ICP-MS

Ionic contamination is a big concern in semiconductor manufacturing processes and in finished devices because small amounts of contamination (parts-per-billion (ppb) to parts-per-million (ppm) concentrations) can cause corrosion, erosion, electromigration, and shorting in devices, on wafers or in final individual electronic components. Ion chromatography (IC) is an efficient analytical technique that can quickly determine both trace and major constituents of a wide variety of process contaminants in the semiconductor industry. This technique provides a convenient means of quantifying common inorganic anions and cations, certain organic additives, transition metals, polyvalent ions, and organic chelating agents. With online IC process monitoring, these components can be determined quickly at low parts-per-trillion levels for immediate remediation steps. IC testing in the semiconductor includes the applications below.

Determination of inorganic anions in ultrapure water

Deionized (DI) water is used throughout the fabrication of an integrated circuit, which may be exposed to DI water hundreds of times during its manufacture. Because contaminants in DI water can distort normal dopant profiles, create inversion layers or cause shorts, or other circuit malfunctions, it is extremely important that semiconductor pure water be maintained at its highest attainable quality. Ion chromatography is used to ensure that the water in semiconductor fabrication facilities is indeed of highest quality and is the only analysis technique recommended by Semiconductor Equipment and Materials International (SEMI) for inorganic anions.

Determination of trace anions in organic solvents

Anion contamination introduced during manufacturing processes can ruin semiconductors and computer components. Trace contaminants in solvents used during manufacturing can cause short circuits, defects in deposition, and corrosion. Component defects such as these reduce yields, which increases manufacturing costs and waste. As device geometries shrink, even lower levels of contamination become problematic. The computer and semiconductor industries need analytical methods to determine trace anions in solvents to help them identify contamination at different stages of manufacturing, so they can take action to prevent future contamination.

Determination of trace cations in concentrated acids

Determination of cations in concentrated acids is important because some cations can combine with anionic contaminants to form insoluble compounds that can cause plating and etching defects in electronics manufacturing processes.

Determination of trace transition metals in reagent grade acids, bases, salts, and organic solvents

The SEMI (Semiconductor Equipment and Materials International) specifications for maximum permitted levels of transition metals in concentrated acids used in fabrication of semiconductor devices is in the range of 0.1 to 1.0 part per billion (ppb). Labor-intensive manual pre-concentration methods are usually required prior to analytical measurements. The chelation ion chromatographic technique (chelation IC) provides the sample pre-concentration step and direct determination of trace transition metals by ion chromatographic separation and post-column derivatization prior to detection.

Contaminants and additives in plating baths

Plating baths for copper electrochemical deposition (ECD) need to be of well controlled composition and free of contaminants to ensure a metal deposition with the correct characteristics. Copper thickness, crystal orientation and grain size need to be optimized to ensure the correct conductance and to minimize degradation due to electromigration. Both ion chromatography and liquid chromatography can determine concentrations of additives, bath components, and bath contaminants and additives to assure the quality of the plating deposition.

Copper plating baths are used to plate copper into submicron features on semiconductor wafers. Copper plating chemistries are designed to provide a fast, efficient fill on the wafer. Maintaining the concentration of the additives at certain levels is critical to the performance of the bath. Plating technology for semiconductor applications requires rigid bath control and disciplined methodology.

Establishing correlations between what is found in the plated film and bath chemistry control parameters is fundamental in producing interconnects that are consistent and reliable. To establish these correlations, it is important to have a clear understanding of the chemical composition of the bath, including suppressor and accelerator bath components, which help moderate the deposition rate of the copper fill since too much or too little of these components in the bath can result in "fill failures".

Cyclic Voltammetric Stripping (CVS) provides indirect bath measurements that provides an incomplete picture as it only measures the combined effect of the additives and by-products on the plating quality.

High performance liquid chromatography (HPLC) and ion chromatography (IC) are analytical techniques which provide important information on the concentration, chemical balance and trend measurement of major constituents such as additives, brighteners, boosters, stabilizers, carriers, levelers, inhibitors, accelerators, transition metals, metal complexes and contaminants in the plating bath. This information provides for improved device quality, reduced scrap rate and reduced costs of bath maintenance. HPLC also has the ability to quantify related by-products found in copper plating baths as well.