Secondary Ion Mass Spectrometry (SIMS) is a highly sensitive analytical technique used for depth profiling and trace element analysis in semiconductors. Accurate quantification in SIMS relies on well-characterized standards, which are essential for converting raw ion counts into meaningful concentration values. The development and use of SIMS standards, including implanted standards and certified reference materials (CRMs), have been critical in ensuring reliable and reproducible measurements in semiconductor research and manufacturing.
The earliest SIMS quantification methods relied on relative sensitivity factors (RSFs), which correlate ion yields of analytes to those of a known matrix element. However, RSFs are matrix-dependent and can vary significantly between different materials and instrument conditions. To address this, implanted standards became a cornerstone of SIMS quantification. These standards are created by ion implantation, where a known dose of the analyte is introduced into a well-characterized substrate. The implantation energy and dose are carefully controlled, allowing the creation of a reference material with a known depth distribution.
Implanted standards must be validated using complementary techniques such as Rutherford Backscattering Spectrometry (RBS) or neutron activation analysis (NAA) to confirm the implanted dose. For example, boron-implanted silicon standards are widely used in semiconductor analysis, with implantation doses typically ranging from 1e12 to 1e16 atoms/cm². The accuracy of these standards depends on precise dose measurement, uniformity of implantation, and stability of the reference material over time.
Certified reference materials (CRMs) provide another layer of reliability in SIMS quantification. These materials are produced under stringent conditions and are accompanied by documentation detailing their composition, homogeneity, and measurement uncertainty. CRMs for SIMS are often developed by national metrology institutes or industry consortia. For instance, the National Institute of Standards and Technology (NIST) has developed SRM 2135, a silicon wafer with implanted arsenic, for use in SIMS calibration.
The choice between implanted standards and CRMs depends on the specific application. Implanted standards are highly customizable, allowing researchers to tailor the dopant species, concentration, and depth profile to match their analytical needs. However, they require careful preparation and validation. CRMs, on the other hand, offer traceability to international standards but may not cover all possible dopant-matrix combinations needed in semiconductor research.
One challenge in SIMS quantification is matrix effects, where the ion yield of an element varies depending on the host material. To mitigate this, matrix-matched standards are essential. For example, quantifying oxygen in gallium arsenide (GaAs) requires a GaAs standard rather than a silicon-based one. The development of matrix-matched standards has been a significant focus in SIMS research, particularly for compound semiconductors and emerging materials like gallium nitride (GaN) and silicon carbide (SiC).
Another consideration is the detection limits and dynamic range of SIMS measurements. Implanted standards must cover the full range of concentrations relevant to the analysis. For ultra-trace analysis, standards with very low doses (below 1e12 atoms/cm²) are necessary, while high-concentration standards (above 1e20 atoms/cm³) are needed for heavily doped materials. The linearity of the SIMS response must also be verified across this range to ensure accurate quantification.
Recent advancements in SIMS standards include the use of delta-doped layers, where a very thin, high-concentration layer of the analyte is embedded in the matrix. These structures provide a sharp reference peak for depth calibration and sensitivity factor determination. Delta-doped standards are particularly useful for high-resolution depth profiling in advanced semiconductor devices, such as FinFETs and heterostructures.
The semiconductor industry’s shift toward new materials, such as 2D semiconductors and high-mobility oxides, has driven the need for specialized SIMS standards. For example, quantifying impurities in transition metal dichalcogenides (TMDCs) like MoS₂ requires standards that account for their layered structure and anisotropic sputtering behavior. Similarly, organic semiconductors present unique challenges due to their sensitivity to ion beam damage, necessitating low-dose standards and specialized analytical protocols.
Interlaboratory comparisons and round-robin studies have played a crucial role in validating SIMS standards and measurement protocols. These studies involve multiple laboratories analyzing the same sample to assess reproducibility and identify potential biases. For instance, the International SEMATECH Manufacturing Initiative (ISMI) has conducted cross-lab studies on boron and phosphorus quantification in silicon, leading to improved standardization practices.
In summary, the development and use of SIMS standards have evolved significantly to meet the demands of semiconductor analysis. Implanted standards and CRMs provide the foundation for accurate quantification, while matrix-matched and delta-doped standards address the challenges of emerging materials. Continued advancements in standard preparation, validation, and interlaboratory collaboration will be essential as semiconductor technologies push toward smaller feature sizes and novel material systems.