Modulated Optical Reflectance Research Articles

Fiber-based modulated optical reflectance configuration allowing for offset pump and probe beams.

A new fiber-based modulated optical reflectance configuration is developed in this work. The technique maintains the fiber-based heating laser (pump) and detection laser (probe) in close proximity at a fixed separation distance in a ceramic ferrule. The pump beam periodically heats the sample inducing thermal waves into the sample. The probe beam measures the temperature response at a known distance from the pump beam over a range of heating modulation frequencies. The thermal diffusivity of the sample may be calculated from the phase response between the input heat flux and the temperature response of a sample having a reflective surface. The unique measurement configuration is ideal for in situ measurements and has many advantages for laboratory-based systems. The design and development of the system are reported along with theoretical justification for the experimental design. The thermal diffusivities of Ge and SiC are measured and found to be within 10% of reported literature values. The diffusivity for SiO2 is measured with a relative difference of approximately 100% from the literature value when the ferrule is in contact with the sample. An additional measurement was made on the SiO2 sample with the ferrule not in contact resulting in a difference of less than 2% from the literature value. The difference in the SiO2 measurement when the ferrule is in contact with the sample is likely due to a parallel heat transfer path through the dual-fiber ferrule assembly.

Modulated optical reflectance method for analysis of magnetoelectric nanomaterials

A recently developed technique of modulated optical reflectance and its applicability to structural and defectoscopic analysis with high spatial resolution of the ferromagnetic LSMO materials is reported. The variation of the optical reflectance is measured within the active area in the laser focus on the surface of the thin film subjected to photothermal modulation. The measurement described is indicative of the magnetoelectric properties of the sample films and is found to be proportional to the thermal variations of the conductance and free carrier density.

Optical fiber hydrogen sensor based on photothermal reflectance detection technique

This article proposes an optical fiber hydrogen (H2) sensor based on photothermal reflectance [hereinafter modulated optical reflectance (MOR)] technique. Our H2 sensor is based on a technique that detects the changes of MOR signals in palladium film, which is widely known to absorb H2 gas. The sensor element is a palladium film deposited on a 2.5-mm-diameter FC-ferrule made from zirconium to realize the optical fiber sensor. Our recently developed "laptop" MOR instrument assembled with optical fiber components is applied to this technique. Thus, an extremely compact photothermal H2 gas sensor system can be constructed. We certified that our technique has hypersensitive less than 1% with a concentration of H2 gas and also demonstrated that the response time is approximately 5 seconds when the sensor head is filled with H2 gas.

Impact of band gap narrowing and surface recombination on photoelectrothermal modulated optical reflectance power curves

Photomodulated optical reflectance is a well established technique for surface and near surface characterizations. In this work, the nonlinear behavior of the differential reflectance as a function of the pump irradiance (104–106W∕cm2) is studied on uniformly and nonuniformly (p-n∕p+-p junctions) doped silicon structures, with a particular emphasis on the impact of band gap narrowing (BGN) and of surface recombination velocities (SRVs). We show that the BGN induced by the presence of excess carriers substantially influences the excess carrier profile. We also explain the unexpected shape of power curves on lowly doped substrate by a time-dependent variation of the SRVs during illumination.

Nonlinear study of photoelectrothermal modulated optical reflectance for active dopant profile extraction

Carrier illumination (CI) is a photoelectrothermal modulated optical reflectance (PMOR) technique for the one dimensional active doping profile characterization of ultrashallow junctions. The specificity of CI as a PMOR technique is to exploit the probe differential reflectance nonlinearity as a function of the pump laser irradiance (104–106W∕cm2). The probe differential reflectance as function of the pump power is called a power curve, and its interpretation provides information on the underlying active doping profile. In a previous work [F. Dortu et al., J. Vac. Sci. Technol. 24, 375 (2006)], the independent extraction of the active doping concentration (N) and the metallurgical junction (Xj) of a chemical vapor deposited boxlike profile was based on two features of the power curve, namely, the inflexion power and the signal at end of range power. However, this method suffers from the difficulty to extract accurately the second derivative and has a limited extraction range (Xj=20–40nm, N=1019–1020∕cm3). In the present work, we present a method making use of the power curve’s first derivative at low and high illumination powers. This method, in principle, allows a much broader extraction range (Xj=10–70nm, N=1018–1020∕cm3) provided that the signal time dependence due to the native silicon oxide charging under intense illumination is taken into account properly. The present work is supported by a two-layer diffusionless nonlinear analytical model, which provides the basic insights of the method, and three dimensional axisymmetric numerical simulations in the framework of the drift-diffusion equations. A procedure to remove the time dependent charging effect is also presented.

Modulated optical reflectance measurements on La2∕3Sr1∕3MnO3 thin films

The modulated optical reflectance (MOR) measurement technique was applied to colossal magnetoresistive materials, in particular, La2∕3Sr1∕3MnO3 (LSMO) thin films. The contactless measurement scheme is prospective for many applications spanning from materials’ characterization to devices such as reading heads for magnetically recorded media. A contrasted room-temperature surface scan of a 100-μm-wide and 400-μm-long bridge patterned into LSMO film provided preliminary information about the film homogeneity. Then the temperature was varied between 240 and 400K, i.e., through the ferromagnetic to paramagnetic transition. A clear relation between the MOR signal measured as a function of the temperature and the relative derivative of the resistivity up to the Curie temperature was observed. This relationship is fundamental for the MOR technique and its mechanism was explored in the particular case of LSMO. Analysis in the framework of the Drude model showed that, within certain conditions, the measured MOR signal changes are correlated to the changes in the charge-carrier concentration.

Effects of Residual Surface Resistance on the Microwave Properties of YBCO Thin Films

The performance of a planar microwave device fabricated from a YBCO thin film is strongly dependent on the quality and uniformity of the film. In this paper we present a full set of results of the microwave surface impedance at 8 GHz of three films deposited on MgO substrates and patterned into coplanar resonators. We use the Modulated Optical Reflectance (MOR) technique as a complementary, noncontacting, room temperature method of assessing the uniformity of the films. The three films studied have highly uniform MOR signals across the whole of their surfaces, indicative of thin films of very good electrical quality. However, although the measured Q factors of all three resonators are high, they vary significantly from 15000 to 30000 at 15 K, a result that is uncorrelated with the observed uniformity of the MOR signals. We interpret this variability as being due to residual surface resistance associated with microstructural defects that are impossible to detect using MOR. On subtracting the residual resistance we find that the intrinsic, linear microwave properties (i.e. quasiparticle conductivity /spl sigma//sub 1/ and penetration depth /spl lambda/) are very similar, consistent with the uniformity of the MOR signals.

Application of modulated optical reflectance method for high-temperature superconducting film characterization

The modulated optical reflectance (MOR) method is applied for nondestructive characterization of high-temperature superconducting (HTS) films and thin-film devices. An optimized optical scheme is developed employing He–Ne laser and diode laser of 780nm wavelength, and via a photodetector aligned to a lock-in amplifier, which is able to monitor the reflectance and technological inhomogeneity in the focal spot on the sample film. Microwave coplanar resonators patterned from YBa2Cu3O7−x (YBCO) films of 100nm thickness have been examined employing the MOR scheme. Unmodulated reflectance signals and MOR are compared between YBCO film and the substrate, MOR scans provide better indication of the structural differences due to the employed thermal modulation. A characteristic feature of YBCO films with embedded oxide crystals, referred to as assemblies of imperfections, could be detected. Automation of the scanning process and computerization of data acquisition are being developed.

Microwave and modulated optical reflectance studies of YBCO thin films

Planar HTS microwave devices require high quality, homogeneous samples. In this paper, sensitive measurements of the microwave surface impedance of YBCO thin films using coplanar resonators are collated with modulated optical reflectance (MOR) measurements. MOR provides a powerful noncontact, nondestructive and high resolution means of probing local variations in the quality of thin films at room temperature, and consequently has great potential for diagnostic testing of HTS films prior to microwave device patterning. Microwave and MOR inter-comparisons of four YBCO films patterned into 5.2 and 8 GHz coplanar resonators are presented. Superior global microwave response in the superconducting state, such as low surface impedance and low levels of nonlinearity at enhanced powers, correlate with the magnitude and spatial homogeneity of the room temperature MOR signals. The presence of defects in films is investigated using both techniques. Both large scale single defects and film inhomogeneity can be detected using MOR; however, the spatial resolution of the technique is not sufficient to detect single weak links on a sub-micron scale, whose presence can result in severely degraded microwave resonator performance.

Contrast enhancement of modulated optical reflectance microscopy of semiconductor devices

Photothermal microscopy based on combined optical and electrical excitation has been applied to insulating lines prepared by focused ion beam implentation on oxygen implanted mesas. The double excitation technique permits to image electrical properties of the implanted structures which are complementary to simple defect tracing when a single excitation is applied. In addition, the contrast of imaging of implanted structures can be increased considerably. Apart from combining modulated optical excitation with electrical direct current voltages, double modulation configurations can improve the contrast further, by detecting either on a higher harmonic of the single excitation frequency or on the sum or difference of two different excitation frequencies. The influence of different parameters characterizing the experimental configuration gives information on the physical processes of the signal generation, indicating that the observed contrast enhancement is mainly of thermal origin related to the photoinduced currents.

Modulated optical reflectance characterization of high temperature superconducting thin film microwave devices

The modulated optical reflectance (MOR) technique is shown to provide a room temperature, noncontact, nondestructive and high spatial resolution means of assessing high temperature superconducting (HTS) thin film quality. Room temperature MOR characterizations of a number of 8 GHz planar HTS resonators indicating a range of property variations and local degradations in HTS film performance are shown to be consistent with results obtained at low temperatures by the electron beam induced voltage contrast technique. The microwave performances of some of the resonators are found to exhibit nonlinear characteristics that can be explained by HTS defects revealed by the MOR technique.

Plasma etching and ion implantation on silicon, characterized by laser-modulated optical reflectance

Plasma etching and ion implantation lead to changes of the electronic and thermal properties of semiconductor materials, which have been measured with the help of the modulated optical reflectance. Imaging based on modulated optical reflectance applied to control etching and ion implantation in the production of semiconductor devices is more efficient at higher modulation frequencies where both thermal waves and charge carrier waves contribute to the measured signal.

Electron-beam-induced voltage contrast and modulated optical reflectance studies of YBCO thin film resonators

Electron-beam-induced voltage contrast (EBIV) and modulated optical reflectivity (MOR) have been used to study the electrical properties of superconducting microwave resonators. The three resonators studied were patterned (YBCO) films on MgO substrates. The YBCO films consisted of a wide central track of 8 mm length separated from the ground plane by . A qualitative correlation was observed between the EBIV and MOR signals observed throughout the central track of the resonator. The EBIV results were taken close to the superconducting transition while the MOR data were obtained at room temperature. Both techniques are dependent on the local electrical properties which in turn are governed to a large extent by the local oxygen content within the superconductor. The correlation between the MOR technique and the more established EBIV technique has shown MOR to be a useful inspection tool for superconducting devices, particularly as MOR is a contactless room-temperature technique.

Nonlinear photothermal modulated optical reflectance and photocurrent phenomena in crystalline semiconductors: II. Experimental

Experimental photothermal modulated optical reflectance (PMOR) and photocurrent (PC) data from silicon and germanium samples have been obtained at high laser pump beam intensities and correlated with a theoretical model of nonlinear carrier recombination. In principle, the PMOR signal consists of both thermal and Drude (free-carrier) components; nonlinear recombination influences it through its direct effect on the modulated carrier density, and its indirect effect on the modulated sample temperature. All of the five samples discussed in this work showed evidence of nonlinear carrier recombination in their PC and PMOR behaviour. Although few of the PMOR data were indicative of a dominant Drude component, the effect of nonlinear recombination upon the carrier density was quite apparent in the PC data which were obtained to correlate with the PMOR data. The experimental results clearly indicate that the PMOR effect is sensitive to nonlinear recombination, albeit not to as large a degree as the transverse photocurrent.

Nonlinear photothermal modulated optical reflectance and photocurrent phenomena in crystalline semiconductors: I. Theoretical

The effect of nonlinear carrier recombination on the photothermal modulated optical reflectance (PMOR) signal of a crystalline semiconductor has been theoretically modelled. The PMOR signal consists of both thermal and Drude (free-carrier) components, and nonlinear recombination influences it through its direct effect on the modulated carrier density, and its indirect effect on the modulated sample temperature. Quadratic recombination was shown to lead to a sublinear intensity dependence of the photocurrent amplitude at elevated optical excitation intensities; cubic recombination leads to a supralinear intensity dependence of the PMOR amplitude accompanied by a progressive phase lag dependence on pump laser-beam intensity. The effect of nonlinear carrier and heat-flux gradients was also considered and found to be negligible with PMOR detection.

High-temperature superconductor thin-film characterization by the modulated optical reflectance technique

Measurements are presented of the modulated optical reflectance (MOR) of laser-ablated thin films of YBa2Cu3O7-x having a range of electrical characteristics. It is found that the room-temperature MOR signal provides a good indication of film quality. There is a strong correlation between MOR signal amplitude and a film's normal- and superconducting-state characteristics. The origins of the MOR signal are discussed and it is suggested that it is directly dependent on the characteristic linear temperature dependence of carrier concentration found in high-temperature superconductors. The potential of MOR for superconducting thin-film device inspection is noted.

Non-destructive depth profiling of silicon ion implantation induced damage in silicon (100) substrates

In this study, the ion implantation of silicon (100) substrates with 28Si + ions is characterised using spectroscopic ellipsometry (SE), modulated optical reflectance (MOR) and Rutherford backscattering spectroscopy (RBS). Samples were implanted with ion energies of 2 MeV, 1 MeV and 200 keV to create a series of deep-level and surface-damaged layers. The implantation dose was varied from 3.4 × 10 14 ions cm −2 to 1.5 × 10 16 ions cm −2. All ion implantations were performed at room temperature. It was observed that the amplitudes and positions of the E 1 and E 2 silicon peaks are sensitive indicators of the degree of crystallinity of the silicon substrate. Monitoring of these parameters may provide a rapid route for water mapping of ion-implantation uniformity. The SE oscillations observed below 2.3 eV reflect the depth of the ion-implanted damage layer. In addition, analysis of the complete spectroscopic data yields damage profiles consistent with both measured and calculated RBS and TRIM ion-implantation moments.

Sensitivity of a modulated optical reflectance probe to process-induced lattice disorder

Process induced lattice disorder in Si(100) wafers via a 28Si + ion beam has been characterised using Rutherford backscattering spectroscopy (RBS) and modulated optical reflectance (MOR). An exact correlation between the measured photoreflectance amplitude response and displaced silicon atom density is obtained. A theoretical calculation of the sensitivity of the technique to lattice disorder is presented.

Three-dimensional theory to study photothermal phenomena of semiconductors. I. Modulated optical reflectance

A thorough theoretical study of the modulated optical reflectance (MOR) response of semiconductor samples has been presented. The three-dimensional plasma and thermal equations are solved analytically, and then numerical calculations are given for the high-frequency MOR signal. The influences of the optical, thermal, and electronic parameters on the MOR signal are discussed in detail. The generation mechanisms of the MOR signal will be clarified. On the other hand, the mathematical analyses also provide a quantitative tool by which one can fit the theoretical curves to experimental data to determine several material parameters, such as the surface recombination velocity, the lifetime of photogenerated carriers, and thermal conductivity of the materials.

Comparison of modern uniformity-mapping techniques

When ion implantation was first introduced for semiconductor processing, implanter specifications of 2% wafer uniformity and 2% wafer-to-wafer repeatability were actually better than the precision of the measurement equipment and techniques available. In the past ten years, the performance of both the ion implanter and its requisite process monitoring equipment has greatly improved. In addition, techniques using modern technology and software have been introduced. Six characterization methods are currently available for monitoring ion implant uniformity: sheet resistance, spreading resistance, pulsed capacitance-voltage ( C- V), modulated-optical reflectance, optical constant monitoring using ellipsometry, and optical densitometry. Sheet resistance, spreading resistance and pulsed C- V are direct electrical measurements of the active dopant atoms in the implanted wafer. The other three techniques monitor dose by measuring the damage induced by the ion beam; modulated-optical reflectance and optical constant monitoring measure the damage in an implanted wafer, whereas optical densitometry measures the damage in a photoresist coating on a glass substrate. This paper will review each of these methods and discuss their relative merits. It will be clear from this presentation that wise process engineers should use more than one of these methods to characterize ion implant equipment as well as monitor the ion implantation process.