Dopant SIMS depth profiling

In ion implantation the gaseous dopant is ionized and accelerated into the wafer. SIMS has been used extensively to obtain dopant concentration depth profiles because it is the most sensitive of the surface analytical techniques and because sputtering is an intrinsic part of the dynamic SIMS process (40). AES, combined with ion sputtering, has also been used to obtain depth profiles for high dose implants (41). 

In the semiconductor industry, SIMS has been particularly useful for the depth profiling of dopants that are present in silicon in very low concentrations. As an example, a SIMS depth profile for boron implanted into silicon is shown in Figure 27. One of the significant features is that we can detect about 101S boron atoms/cm3 in a silicon matrix of 5 x 1022 atoms/cm3. This illustrates an ability to detect 20 ppb. Also, the method spans 5 orders of magnitude in boron concentration. No other technique can span such a large range accurately. 

In this review the various modes of SIMS and examples of their applications are discussed. SIMS depth profiles are widely used to study dopant profiles and Intermetallic diffusion. The extreme surface sensitivity and low concentration detection limits of SIMS make It useful for Investigation of substrate and metallization cleaning processes. SIMS elemental Imaging Is also used In contamination studies. The ability of SIMS to provide Isotopic Information has allowed elegant mechanistic studies. The Identification and determination of the relative abundance of various molecular or elemental species by SIMS Is applicable to the development characterization and understanding of microelectronic processing. The capability of SIMS In the area of quantitative analyses Is also discussed. 

Indispensable and continue to be Important In the development of models for range statistics In Ion Implantation. SIMS depth profiles are also used to monitor and develop an understanding of the diffusion of dopants during laser and thermal annealing processes. Metallization and thin films have also been Investigated by SIMS. In addition SIMS depth profiles are useful for failure analysis and problem solving. 

The out diffusion of dopants from highly doped wafer areas Into the epitaxial layer during epitaxial deposition or prebake Is called autodoping. SIMS depth profiles during autodoping showed a pile up of As at the SI substrate surface. Spreading resistance measurements showed that the source of the autodoping to be electrically Inactive As (41). 

FIGURE 41.8 SIMS depth profile of Mg-implanted GaN. The implanted Mg is used to quantify this p-type dopant in GaN. 

The most common application of dynamic SIMS is depth profiling elemental dopants and contaminants in materials at trace levels in areas as small as 10 pm in diameter. SIMS provides little or no chemical or molecular information because of the violent sputtering process. SIMS provides a measurement of the elemental impurity as a function of depth with detection limits in the ppm—ppt range. Quantification requires the use of standards and is complicated by changes in the chemistry of the sample in surface and interface regions (matrix efiects). Therefore, SIMS is almost never used to quantitadvely analyze materials for which standards have not been carefiilly prepared. The depth resoludon of SIMS is typically between 20 A and 300 A, and depends upon the analytical conditions and the sample type. SIMS is also used to measure bulk impurities (no depth resoludon) in a variety of materials with detection limits in the ppb-ppt range. 

The use of SIMS to obtain quantitative information has traditionally been recognized as a very difficult task complicated by the fact that internal standards and sensitivity factors cannot be applied universally. One approach, which has met with some success in dynamic SIMS, is the construction of a depth profile of implanted dopant (of known total dopant dosage) and then to relate the integrated area under the depth profile curve to the total concentration of dopant. This allows determination of concentration at any... 

SIMS is used for quantitative depth profile determinations of trace elements in solids. These traces can be impurities or deliberately added elements, such as dopants in semiconductors. Accurate depth prohles require uniform bombardment of the analyzed area and the sputter rate in the material must be determined. The sputter rate is usually determined by physical measurement of the crater depth for multilayered materials, each layer may have a unique sputter rate that must be determined. Depth prohle standards are required. Government standards agencies like NIST have such standard reference materials available for a limited number of applications. For example, SRM depth profile standards of phosphorus in silicon, boron in silicon, and arsenic in silicon are available from NIST for calibration of SIMS instmments. P, As, and B are common dopants in the semiconductor industry and their accurate determination is critical to semiconductor manufacture and quality control. 

SIMS (Magnetic sector) Secondary Ion Mass Spectrometry Dopant and impurity depth profiling, suri e and microanalysis Secondary ions 10 -10 atoms/cm (ppb-ppm) 50-300A 1.0 pm (imaging) >30 pm (depth profiling)... 

With its analysis speed, LA-ICP-MS lends itself for quick checks of film consistency (e.g., for metal interconnects or hard disk platters) [96]. The fact that the ablation volume per laser shot can be rather large in comparison with pulsed ions can be utilized to measure the total dopant dose of ion implantation. Here the distribution of the dopant is not depth profiled step by step as with SIMS, but the laser parameters are chosen... 

Ion implants, particularly for semiconductor materials, are widely used as standards in SIMS. Implants can be made with accurate and controllable dopant concentrations. The implant concentration is known in atom/cm. Depth profiling the implant to obtain the integrated ion signal of the implanted dopant and accurately measuring the depth allows for the conversion of atom/cm to atom/cm. This assumes a constant sputtering rate through the depth profile, SIMS ion intensities linear with concentration, and an accurate depth measurement of the analysis crater [28]. 

Loesing R (2001) Development of high resolution depth profiling of ultrashallow dopant implants with SIMS. Dissertation, North Carolina State University, http //repository.lib.ncsu. edu/ir/bitstream/1840.16/5181/1/etd.pdf... 

Dynamic SIMS has long been an established technique for the detection of low concentration species in bulk samples (e.g. dopant levels in semiconductors). While the sample is being sputtered (see Sec. 2.3), a quadrupole mass analyzer detects the resulting fragments in real time, resulting in a depth profile of the desired species in the sample or, with calibration, an indication of the concentration of the desired species in the bulk of the sample. The method is fast and simple, but a quadrupole mass analyzer can only tune into one particular mass-to-charge ratio at a time, or... 

SIMS Dopant and impurity depth profiling, surface microanalysis Secondary ions... 

SIMS is a very surface-sensitive technique because the emitted particles originate from the uppermost one or two monolayers. The dimensions of the collision cascade are rather small and the particles are emitted within an area of a few nanometers diameter. Hence, SIMS can be used for microanalysis with very high lateral resolution (50 nm to 1 pm), provided such finely focused primary ion beams can be formed. Furthermore, SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the solid in a controlled manner to obtain information on the in-depth distribution of elements.109 This dynamic SIMS mode is widely applied to analyze thin films, layer structures, and dopant profiles. To receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (<1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is used widely for the characterization of molecular surfaces (see Figure 3.10). 

This Datareview discusses the redistribution of typical dopant atoms in GaN during the implant activation anneal. Secondary ion mass spectroscopy (SIMS) spectra of the impurity profiles (impurity concentration versus depth into the sample) before (as-implanted) and after annealing are presented. SIMS analysis is the primary method of characterising impurity distributions in semiconductors [2], This information can be used to roughly estimate a diffusivity, D , of the dopant at the temperatures studied by invoking the relationship... 

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