Nitrogen Investigation by SIMS in Two Wide Band-Gap Semiconductors - Diamond and Silicon Carbide

At a Glance

Metadata	Details
Publication Date	2022-05-31
Journal	Materials science forum
Authors	Marie Amandine Pinault-Thaury, François Jomard
Institutions	Université Paris-Saclay
Citations	4
Analysis	Full AI Review Included

Executive Summary

This analysis details the optimization of Secondary Ion Mass Spectrometry (SIMS) for determining the low-level nitrogen (N) detection limit (DL) in wide band-gap semiconductors, Diamond and Silicon Carbide (SiC), crucial for power electronics applications.

Core Achievement: Established a reliable, non-time-consuming SIMS protocol using standard diamond conditions combined with High Mass Resolution (HMR) settings for N detection in both Diamond and SiC.
Target Ion: Nitrogen is detected via the molecular ion ¹²C¹⁴N^- (mass M ~ 26 amu), requiring HMR (M/ΔM ~ 7,500) to separate it from the matrix interference ¹³C₂^-.
SiC Detection Limit: Achieved a Nitrogen [DL] of 5x10¹⁵ at/cm³ in SiC, which is sufficiently low for monitoring critical n-type doping levels (typically 10¹⁴ to 10¹⁵ at/cm³).
Diamond Detection Limit: Determined a Nitrogen [DL] of 2x10¹⁷ at/cm³ in diamond. This higher value is attributed primarily to the difficulty in obtaining lightly doped diamond standards for accurate calibration.
Method Validation: The raster size method was successfully employed to isolate the constant background signal (DL) from the material signal, confirming the viability of the technique for deep profiling (up to 8 µm in SiC) of multi-layer structures.
Sensitivity Ratio: The detection limit for nitrogen in SiC is approximately 40 times lower (more sensitive) than in diamond under these optimized conditions.

Technical Specifications

Parameter	Value	Unit	Context
SIMS Instrument	IMS7f-CAMECA	N/A	Dynamic Magnetic Sector SIMS
Primary Ion Species	Cs⁺	N/A	Used for detecting electro-negative elements
Primary Ion Energy	10	keV	Standard Cs⁺ beam setting
Sample Bias	-5000	V	Used for negative secondary ion detection
Interaction Energy	15	keV	Effective energy of primary ions
Incidence Angle	23	°	Relative to sample normal
Target Molecular Ion	¹²C¹⁴N^-	amu	Detected mass M ~ 26
Required Mass Resolution	~7,500 to 8,000	M/ΔM	High Mass Resolution (HMR) setting
Analysis Vacuum Limit	~5x10^-10	mbar	Ultra-high vacuum required to limit atmospheric background
N Detection Limit ([DL]) in SiC	~5x10¹⁵	at/cm³	Achieved DL for lightly doped SiC
N Detection Limit ([DL]) in Diamond	~2x10¹⁷	at/cm³	Achieved DL for diamond
RSF (Diamond)	3.30x10¹⁸	at/cm³	Relative Sensitivity Factor (RSF) for N
RSF (SiC)	1.34x10¹⁸	at/cm³	Relative Sensitivity Factor (RSF) for N
Routine Raster Size	150 x 150	µm²	Standard analysis area (Diamond)
Routine Sputtering Rate	0.25 to 0.4	nm/s	Standard rate in diamond matrix

Key Methodologies

The SIMS analysis utilized standard diamond conditions combined with the Raster Size Method and High Mass Resolution (HMR) to accurately determine the nitrogen detection limit (DL).

Standard SIMS Setup:
- The analysis employed a Cs⁺ primary ion beam (10 keV) and detected negative secondary ions (Cs⁺/M^- configuration).
- The sample was biased at -5000 V, resulting in a 15 keV interaction energy and a 23° incidence angle.
- Primary ion intensity was kept low (40 to 60 nA) to maintain a reasonable sputtering rate (0.25 to 0.4 nm/s).
High Mass Resolution (HMR) Tuning:
- Nitrogen was detected using the molecular ion ¹²C¹⁴N^- (26.003074 amu).
- HMR was required to separate ¹²C¹⁴N^- from the mass interference ¹³C₂^- (26.00671 amu).
- Entrance and exit slits of the spectrometer were slightly closed to achieve a mass resolution (M/ΔM) of approximately 7,500 to 8,000.
Detection Limit (DL) Determination via Raster Size Method:
- The primary ion intensity (I_p) was held constant (e.g., 40 nA for SiC).
- The raster size (sputtered area) was reduced step-by-step (e.g., 250x250 µm² down to 100x100 µm²).
- Reducing the raster size increases the primary ion density (J_p) and the signal intensity (I(¹²C¹⁴N^-)) coming from the material.
- The measured intensity I(¹²C¹⁴N^-) was plotted as a function of J_p.
- The y-intercept of the resulting linear fit represents the constant background signal (DL in counts per second, cps), which is independent of the material sputtering rate.
Quantification:
- The DL (cps) was converted to concentration ([DL] in at/cm³) using the formula: [DL] = DL * RSF / I_matrix.
- RSF (Relative Sensitivity Factor) was determined using implanted (diamond) or bulk (SiC) standards with known N content.
- The matrix intensity (I_matrix) was monitored using the ¹³C₂^- ion signal.

Commercial Applications

The ability to accurately measure and profile low-level nitrogen contamination and doping is critical for the manufacturing and quality control of advanced semiconductor devices based on SiC and Diamond.

Industry/Application	Relevance to Nitrogen Control
High Power Electronics (SiC)	Nitrogen is the primary n-type dopant in SiC. Precise control of N concentration (especially in the 10¹⁴ to 10¹⁵ at/cm³ range) is necessary to optimize device performance, particularly breakdown voltage.
High Frequency/RF Devices	SiC and Diamond offer superior properties (high electron mobility, high thermal conductivity) for next-generation RF components, requiring strict control over trace impurities like N that affect carrier concentration.
Diamond Growth Optimization	Nitrogen is often intentionally introduced during diamond synthesis to increase the growth rate. Low-level SIMS monitoring ensures that N content is minimized in the final electronic grade material to avoid deleterious effects.
Semiconductor Quality Assurance	SIMS depth profiling validates the thickness and purity of homoepitaxial layers and multi-layer structures (e.g., delta-doped or buried layers) by detecting differences in N concentration between the layer and the substrate.
Material Research & Development	Provides quantitative data (RSF values) necessary for understanding nitrogen incorporation mechanisms during synthesis processes (e.g., CVD) in both SiC and diamond.

View Original Abstract

Diamond and Silicon Carbide (SiC) are promising wide band-gap semiconductors for power electronics, SiC being more mature especially in term of large wafer size (200 mm). Nitrogen impurities are often used in both materials for different purpose: increase the diamond growth rate or induce n-type conductivity in SiC. The determination of the nitrogen content by secondary ion mass spectrometry (SIMS) is a difficult task mainly because nitrogen is an atmospheric element for which direct monitoring of N ± ions give no or a weak signal. With our standard diamond SIMS conditions, we investigate 12 C 14 N - secondary ions under cesium primary ions by applying high mass resolution settings. Nitrogen depth-profiling of diamond and SiC (multi-) layers is then possible over several micrometer thick over reasonable time analysis duration. In a simple way and without notably modifying our usual analysis process, we found a nitrogen detection limit of 2x10 17 at/cm 3 in diamond and 5x10 15 at/cm 3 in SiC.

Tech Support

Original Source

DOI: https://doi.org/10.4028/p-684nsi

References

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