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Advanced Electrical Characterization of Charge Trapping in MOS Transistors

4.3 Physical Characterization Methods

Aside from the electrical methods of defect characterization discussed in the previous parts of this chapter, there is a number of experimental methods which give insights to the physical, structural and stochiometric properties of the gate stack and thus allow to draw conclusions as to the nature of the defects present in a device. While these methods were not accessible to the author using the measurement equipment at the institute for microelectronics, for the sake of completeness and their relevance to the topic of this work the most popular of these methods and their use in defect characterization are briefly presented in this section.

4.3.1 Electron Paramagnetic Resonance (EPR)


Figure 4.22: Principle of electron paramagnetic resonance (EPR) spectroscopy. (a) A sample is placed in a homogeneous magnetic field and irradiated by microwave radiation. (b) The magnetic field causes the energy levels of unpaired electrons to split due to the Zeeman effect. When the split caused by the magnetic field coincides with the microwave energy, the absorption peaks.

ESR or EPR spectroscopy was pioneered by Zavoisky and Bleaney in 1944. It is sensitive to unpaired electrons present in a sample. In the context of MOS reliability, EPR was used to show the presence of P(math image) centers at the Si/SiO\( _{\mathrm {2}} \)interface [33, 110]. For this technique, a sample is placed inside a homogeneous magnetic field. The magnetic field can be generated using a Helmholtz coil as shown in Figure 4.22a. The coil is supplied with a DC current which is sweeped during the measurement. This magnetic field causes the magnetic moments of unpaired electrons to align with or against the field due to the Zeeman effect. The two alignment configurations have different energies depending on the strength of the external field, as shown in Figure 4.22b. In addition, the sample is irradiated with microwaves, and the microwave absorption is measured. When the energy of the incident microwaves is close to the split between the low and high energy states, resonance occurs and the microwave absorption of the sample peaks, see Figure 4.22c. The condition for this is given by

(4.13) \{begin}{`} h \nu = g\sub {e} \mu \sub {B} B_0. \{end}{`}

Here, \( g\sub {e} \) is the electron’s g-factor and \( \mu \sub {B} \) the Bohr magneton.

4.3.2 X-Ray Photoelectron Spectroscopy (XPS)

The X-ray photoelectron spectroscopy (XPS) method, developed by Siegbahn et al. in the 1950s can be used to study the surface chemistry of a material. In the context of MOSFETs, it is used to study the structure of oxides and interfaces [111].


Figure 4.23: Principle of XPS. (a) X-ray photons are directed at the target. Upon collision they remove an electron from an atom. The electron moves to the sample surface and is emitted with an kinetic energy \( E\sub {k} \). The binding energy of the electron can then be calculated from the detected energy and the work functions of the sample and the detector. The probed depth can be influenced by the angle of the detector. (b) The resulting peaks in the observed binding energies can be linked to the targeted species and their concentrations. Graph from [112]

XPS is based on the photoelectric effect. The sample is irradiated with X-rays. Upon collision between an X-ray photon and the sample, electrons are ejected from inner orbitals of the irradiated species. Part of them then escape the material and emit from the surface. The difference between the energy of the X-ray photon, the kinetic energy of the detected electron, and the work function of the material and detector gives the binding energy of the electron. From the amount of detected electrons at a specific binding energy the irradiated atomic species can be calculated. By adjusting the angle of the detector, the probed depth can be controlled, see Figure 4.23 for an illustration.

4.3.3 Secondary Ion Mass Spectroscopy (SIMS)

Secondary ion mass spectroscopy (SIMS) is a technique which allows to characterize the surface composition of a sample by sputtering it with ions.


Figure 4.24: Principle of SIMS. (a) The sample is sputtered with ions. Upon collision they remove ions from the sample. These secondary ions are then analyzed in a mass spectrometer. (b) Exemplary results: the composition of the removed material over time is seen in the output of the mass spectrometer. Measurement results from [113]

Historically, emission of atoms and ions from a surface targeted with ions was first observed by J. J. Thomson in 1910 [114]. In 1949, the first prototype instrument was built by Herzog and Viehböck [115].

During SIMS measurements the secondary ions removed from the surface of the sample are measured. For this, the sample is targeted with ions which physically remove ions from the surface. Part of these secondary ions will be neutral, while others will be positively or negatively charged. The charged particles are then analyzed in a mass spectrometer to find the concentrations of atomic species on the sample surface. Due to the progressing removal of the sample surface during measurement, the method allows to profile the sample in depth. As a drawback of this method, the sample is destroyed during measurement. See Figure 4.24 for an illustration.

4.3.4 Neutron Activation Analysis (NAA)

Neutron activation analysis can be used to determine the concentration of elements within a sample. First experiments were performed by George de Hevesy and Hilde Levi in 1935 where they tested radiation induced in lanthanides after being irradiated by neutrons, and found that they can detect even small amounts of dysprosium in other lanthanides [116].

For the experiment, the sample is targeted with neutrons, which causes atoms in the sample to become radioactive isotopes. This induced radioactivity can then be measured. Elements can be distinguished by the type and energy of radiation they emit. The measured spectrum is a mixture of emissions by the contained elements. To characterize the individual components, two methods are possible [116]:

  • • Destructive/ Radiochemical Neutron Activation Analysis: The sample is chemically separated and the individual fractions are analyzed for radiation.

  • • Non-destructive/ Instrumental Neutron Activation Analysis: The intact sample is measured a number of times with intervals in-between to measure decay rates.