Wafer Curvature Measurements Research Articles

Effect of Growth Temperature on Strain during Growth and Crack Suppression in AlGaN Templates on Sapphire Substrates for Deep Ultraviolet Light‐Emitting Diodes

The crack formations in AlGaN templates for deep ultraviolet (DUV) light‐emitting diodes (LEDs) are investigated and successfully suppressed. The strain values in AlGaN thick layers on sapphire substrates by in situ wafer curvature measurements and ex situ X‐ray diffraction measurements are evaluated. It is found that the tensile strain during the AlGaN thick layer growth comes from a thermal expansion difference between the AlGaN thick layer and the sapphire substrate as the temperature changes from the AlN nucleation layer growth to the AlGaN thick layer growth. When the temperature change is 100 °C and less, the tensile strain of 0.1% and less is observed during the AlGaN thick layer growth, resulting in no crack formations in the AlGaN thick layer. Furthermore, a DUV LED layer structure grown on such a crack‐free AlGaN template shows no crack formations. Thus, to suppress crack formation in templates fabricated for DUV LEDs, their growth temperature must be optimized by considering thermal expansions caused by the changes in the growth temperature from the nucleation layer to the thick layer.

A self-referenced interferometer for in situ cryogenic wafer curvature measurements.

A self-referenced interferometer to measure time-varying curvature in mechanically unstable environments is needed in many applications. One application that demands this measurement technique with fast data acquisition, 2D sensitivity, and insensitivity to vibration is the measurement of thermal strain in thin films in operational environments. The diverging beam interferometer described here demonstrates an angular sensitivity to the local curvature using interferograms captured by a CMOS camera. Two-dimensional Fourier analysis is used to extract curvature changes. The interferometer demonstrates an experimental sensitivity to curvature changes on the order of 10-4 m-1 and is used to measure thermal stresses in a cryogenic environment of a polycrystalline titanium nitride thin film on a silicon wafer that exhibits anisotropic curvature.

Utilizing Island Growth in Superlattice Buffers for the Realization of Thick GaN‐on‐Si(111) PIN‐Structures for Power Electronics

In this study, the effect of implementing island growth in an AlN/Al0.1Ga0.9N superlattice on the structural properties of vertical GaN‐on‐Si(111) PIN‐structures is investigated. It is demonstrated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) that islands are formed on‐top of V‐pits present in the AlN nucleation layer and that the island coalescence height can be controlled by the growth temperature. Defect selective etching analyses confirm a noteworthy reduction in the threading dislocation density (TDD), which diminishes from 1.2 × 109 cm−2 ± 7.5 × 107 cm−2 to 8.5 × 108 cm−2 ± 7.3 ×107 cm−2 as the island coalescence height increases from ≈160 nm to ≈450 nm, achieved by increasing the growth temperature. Cross‐sectional transmission electron microscopy (TEM) shows that island growth is particularly favorable for the reduction of a‐type dislocations. As a consequence of the significant reduction of a‐type dislocations in the buffer, stress relaxation during the GaN film growth is reduced, which is supported by in situ wafer curvature measurements and high‐resolution X‐ray diffraction (XRD). Owing to the optimized island growth conditions, thick and crack‐free GaN layers on Si(111) substrates are obtained with an absolute wafer bow of <50 μm.

The effect of microstructure and film composition on the mechanical properties of linear antenna CVD diamond thin films

This study reports the impact of film microstructure and composition on the Young’s modulus and residual stress in nanocrystalline diamond (NCD) thin films (≈250nm thick) grown on silicon substrates using a linear antenna microwave plasma-enhanced chemical vapor deposition (CVD) system. Combining laser acoustic wave spectroscopy to determine the elastic properties with simple wafer curvature measurements, a straightforward method to determine the intrinsic stress in NCD films is presented. Two deposition parameters are varied: (1) the substrate temperature from 400°C to 900°C, and (2) the [P]/[C] ratio from 0ppm to 8090ppm in the H2/CH4/CO2/PH3 diamond CVD plasma. The introduction of PH3 induces a transition in the morphology of the diamond film, shifting from NCD with larger grains to ultra-NCD with a smaller grain size, concurrently resulting in a decrease in Young’s modulus. Results show that the highest Young’s modulus of (1130±50)GPa for the undoped NCD deposited at 800°C is comparable to single crystal diamond, indicating that NCD with excellent mechanical properties is achievable with our process for thin diamond films. Based on the film stress results, we propose the origins of tensile intrinsic stress in the diamond films. In NCD, the tensile intrinsic stress is attributed to larger grain size, while in ultra-NCD films the tensile intrinsic stress is due to grain boundaries and impurities.

In-Situ Microscale Compression Testing of Li-Si Micropillars

Silicon is considered as an ideal anode material for high-capacity Li-ion batteries (LIB) thanks to its high theoretical capacity, but the commercial viability is severely hampered by considerable volume expansion of Si upon lithiation that can lead to mechanical failure (e.g., cracks and delamination). Hence, understanding the mechanical behaviors of the Li-Si system is of high interest, but the experimental studies have been limited to thin film electrodes by means of wafer curvature measurements or nanoindentation. In this work, we for the first time present in-situ microscale compression testing of Li-Si micropillars and extract the key mechanical properties (e.g., Young’s modulus, yield strength, and fracture energy) from the resulting stress-strain curves. Prior to testing, Si micropillars of various sizes/shapes and oriented at different crystal directions were lithiated/delithiated at room temperature. We then performed the microscale compression testing on these lithiated Si micropillars using a flat-head indenter in environmental scanning electron microscope (ESEM) while obtaining live video images for data-image correlation. The results suggest that the measured elastic moduli of the Li-Si system agree with the values obtained from the standard nanoindentation experiments but with much smaller variance. The precise nature of these experiments allows us to observe the mechanical properties dependent on the crystal orientation of Si and lithiation rate. In addition, various failure behaviors of Li-Si under compressive stresses will be presented, which is difficult to be obtained through the existing experimental techniques.

Mechanical and optical properties of amorphous silicon nitride-based films prepared by electron cyclotron resonance plasma-enhanced chemical vapor deposition

Silicon nitride (SiNx) based films have been recognized as essential dielectric films in the microelectronics and optoelectronics industry due to their desirable properties, such as high electrical insulation, excellent thermal stability, and compatibility with integrated circuit fabrication processes. They are also a potential candidate for fabricating wavelength-selective reflective coatings and surface passivation layers in building-integrated photovoltaics technologies. SiNx-based films are one of the popular choices for antireflective coatings in photovoltaics as well. Recently, SiNx and oxynitride (SiOyNx) based thin film optical filters have been explored to provide distinct color rendering to solar-charged electrical vehicles. Since solar cells have a lifespan of many years and the coating surfaces are substantial, it is essential to produce films with controlled optical and mechanical properties and maintain mechanical integrity against corrosion and wear. This study aims to design a deposition process and optimize minimal parameters for stable plasma conditions during multilayer deposition of SiNx and SiOyNx films using an electron cyclotron resonance plasma-enhanced chemical vapor deposition reactor with SiH4/N2/O2/Ar precursor mixtures at 120 °C. The primary goal was to investigate the influence of gas flow adjustments on the optical and mechanical properties, specifically targeting the refractive index and mechanical properties of the films. We measured the refractive index and the absorption of the films using variable angle spectroscopic ellipsometry. Then, we evaluated the mechanical residual stress ex situ using the wafer curvature measurement method. We have determined the elastic modulus and the hardness of the films using nanoindentation. The experimental results have shown a significant dependence of the optical and mechanical properties on the deposition parameters. To investigate the factors contributing to the intrinsic mechanical stress and to better understand SiNx film degradation mechanisms, we have studied the effect of postdeposition thermal cycling on the properties of the films. Several thermal-cycling experiments from room temperature to 400 °C were performed on different SiNx films, and the results showed an irreversible variation of the mechanical stress toward the tensile stresses caused by delamination of the films, while the refractive index remained unchanged.

Analysis of stress in sputter-deposited films using a kinetic model for Cu, Ni, Co, Cr, Mo, W

Many published studies have quantified film stress evolution for different processing conditions and deposition methods. Here, data from multiple wafer curvature measurements in the literature (for evaporated and sputter-deposited Cu, Ni, Co, Cr, Mo and W) are analyzed in terms of a kinetic model to develop a comprehensive picture of the processes that control film stress. The model includes the effects of film growth kinetics, grain growth and incoming particle energy. Non-linear least squares fitting of the data to this model enables the determination of kinetic parameters that control the stress for each material. The fitting for each material is done in a way that optimizes the parameters simultaneously for all the measurements, both sputtered and evaporated. Parameters that depend only on the material are constrained to have a common value among all the data sets for that material. The validity of the resulting parameters is evaluated by comparing with values estimated from the underlying physical mechanisms.

Strain engineering in III-V photonic components through structuration of SiNx films

We describe work to quantify the effects of structured dielectric thin films, such as SiNx, at the surface of III-V semiconductors, in terms of strain engineering with applications to photonic components such as waveguides and lasers. We show that the strain in the semiconductor can be engineered by controlling the stress in the dielectric thin film by tuning its deposition process. In the first part of this study, we describe how we can control the amount of this built-in mechanical stress, in the case of SiNx, over a large range, from highly tensile (300 MPa) to highly compressive (−800 MPa), using two different kinds of plasma-enhanced chemical vapor deposition reactors: a standard capacitively coupled reactor with radiofrequency excitation and an electron cyclotron resonance reactor with microwave excitation. We focused on characterizing and understanding these thin films' optical and chemical bonding properties through spectroscopic ellipsometry and Fourier transform infrared spectroscopy. We have also studied their mechanical properties experimentally using the wafer curvature measurement technique, microstructure fabrication, and nanoindentation measurements. In the second part, we show accurate measurements of the strain distribution induced within GaAs wafers when such thin films are structured in the shape of elongated stripes of variable width, using standard optical lithography and plasma etching. For this, we map the anisotropic deformation, measuring the degree of polarization of the spectrally integrated photoluminescence (PL) generated within GaAs by excitation with a red laser. PL from the bulk cubic semiconductors such as GaAs and InP is unpolarized, whereas anisotropic strain produces some degree of polarization. These maps were measured either from the semiconductor surface or from cleaved cross sections. They provide a detailed and complete image of the crystal deformation in the vicinity of the structured stressor film. Finally, we have performed some finite element simulations trying to reproduce the experimental maps. This investigation covering the different steps, including control of the built-in stress within the SiNx thin films, mapping of the anisotropic deformation field generated within the semiconductor beneath the structured films, and numerical simulation of these effects, allows us to propose a set of recipes that can be employed for strain engineering of III-V photonic components. Our simulation scheme is helpful for the design of the photonic components, e.g., to predict the local changes in the refractive index due to the photoelastic effect.

Kinetics and mechanisms of stress relaxation in sputtered silver thin films

The post-growth stress relaxation in thin polycrystalline Ag films, deposited by direct-current magnetron sputtering under different growth conditions, was explored through wafer curvature measurements. It exhibits exponential behaviour with three distinct characteristic time components τ. The slowest one, namely τth≃200 s, is ascribed to thermal stress inherent to the deposition method. In fact, the performed temperature measurements match perfectly with an exponential stress variation with heating or cooling, as predicted by a thermal exchange model detailed in this work. Based on a comparison between different deposition conditions (continuous/interrupted sputter deposition and evaporation), the case of Mo deposition and stress relaxation modelling, the remaining components are assigned to the out-diffusion of atoms from grain boundaries (τgb≃3 s) and to changes in the grain surface shape induced by grain boundary grooving (τsurf≃20 s). The relaxation amplitude of the first mechanism varies linearly with the steady-state stress at the end of growth in agreement with theoretical expectations. Yet, that of the second one does not. However, clues point to a kinetic limitation of atomic diffusion mechanism along grain boundaries. This study provides proofs of the simultaneous occurrence of several mechanisms of stress relaxation in thin metallic films.

Understanding residual stress in thin films: Analyzing wafer curvature measurements for Ag, Cu, Ni, Fe, Ti, and Cr with a kinetic model

An analytical model for the evolution of residual stress in polycrystalline thin films is used to analyze numerous previously reported wafer curvature measurements obtained for a variety of materials and processing conditions. The model, which has been described in previous publications, considers stress-generating mechanisms that occur at the grain boundary as it forms between adjacent grains and stress due to the subsurface grain growth in layers that have already been deposited. Current work extends the model to include different types of microstructural evolutions. A set of parameters for each dataset is obtained by non-linear least square fitting. Model parameters that are not expected to depend on the processing conditions are constrained to have a common value when fitting the multiple datasets for each material. The dependence of the fitting parameters on the material and process conditions is evaluated and compared with the physical mechanisms implemented in the model.

Stress Engineering of Dielectric Films on Semiconductor Substrates

Dielectric thin films deposited by plasma enhanced chemical vapor deposition (PECVD) have been extensively studied over the last decades due to their interesting optical and electrical properties besides their many applications in microelectronic and optoelectronic devices. Recently published studies have shown the impact of the mechanical properties of amorphous dielectric films on semiconductor substrates [1]. In strain engineering, stressed films are used to control on demand the physical properties of semiconductors at the surface such as bandgap energy, dielectric constant, and refractive index. We aim to study in this work how to control the distribution of the strain field beneath a dielectric film and how the changing of the residual stress affects the physical properties of the dielectric film itself. We deposited hydrogenated amorphous silicon nitride a-SiN:H films on Si, InP, and GaAs substrates using a capacitively coupled plasma reactor CCP-PECVD with a radiofrequency (RF) power at 13.56 MHz.The a-SiN:H films were deposited at 280 °C, with a thickness of approximatively 500 nm, using a SiH4/NH3/N2/Ar precursor mixture. The RF power injected into the plasma allows a tunable residual stress and a wide range of built-in stress, from tensile (+ 300 MPa) to compressive (– 400 MPa). To evaluate the residual stress in our deposited thin films, we used the standard method of wafer curvature measurements. The thickness and the refractive index were characterized by variable angle spectroscopic ellipsometry (VASE). The determination of Young’s modulus and hardness of the a-SiN:H films was performed by nanoindentation. We noticed that the adjustment of the residual stress leads to the modification of the film in terms of optical and mechanical properties. In order to investigate the deformation induced in the semiconductor, an understanding of the semiconductor mechanical behavior on a microscopic scale is required. Thus, we performed a detailed investigation of the effect of strain on the degree of polarization (DOP) of the photoluminescence signal on direct bandgap substrates [2]. After examining the DOP profiles beneath the film, it is interesting to note that the anisotropic deformation extends to significant depths (~ 8 µm), as illustrated in figure 1, while the horizontal distribution of the stress can propagate beyond the edge of the sample by a few microns (see figure 2). The confinement of light in some photonic devices such as photoelastic planar waveguides can be achieved by a photo-elastic effect in semiconductor using stressed dielectric films [3].[1] S. Gérard et al., “Photoluminescence mapping of the strain induced in InP and GaAs substrates by SiNx stripes etched from thin films grown under controlled mechanical stress,” Thin Solid Films, vol. 706, p. 138079, Jul. 2020, doi: 10.1016/j.tsf.2020.138079.[2] D. T. Cassidy, C. K. Hall, O. Rehioui, and L. Bechou, “Strain estimation in III-V materials by analysis of the degree of polarization of luminescence,” Microelectron. Reliab., vol. 50, no. 4, pp. 462–466, Apr. 2010, doi: 10.1016/j.microrel.2009.11.003.[3] P. A. Kirkby, P. R. Selway, and L. D. Westbrook, “Photoelastic waveguides and their effect on stripe-geometry GaAs/Ga 1-xAlxAs lasers,” J. Appl. Phys., vol. 50, no. 7, pp. 4567–4579, Jul. 1979, doi: 10.1063/1.326563. Figure 1

Measurements and modeling of residual stress in sputtered TiN and ZrN: Dependence on growth rate and pressure

In situ wafer curvature measurements have been conducted on sputter-deposited nitride films (TiN, ZrN) to investigate the effect of growth rate and pressure on stress. For each growth rate, the stress becomes more compressive at lower pressure. At high pressure the stress shows a weak dependence on growth rate, while at low pressure, it becomes more compressive at higher growth rates. The results are interpreted in terms of a kinetic model that includes the effects of growth kinetics and energetic particle bombardment. The overall agreement suggests that the model can reproduce the dependence on processing conditions in multiple nitride systems.

Dislocation plasticity and detwinning under thermal stresses in nanotwinned Ag thin films

Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic “signature” in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic – ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the “signature” thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars.

Determination of Stresses in Incrementally Deposited Films From Wafer-Curvature Measurements

Abstract We report closed-form formulas to calculate the incremental-deposition stress, the elastic relaxation stress, and the residual stress in a finite-thickness film from a wafer-curvature measurement. The calculation shows how the incremental deposition of a new stressed layer to the film affects the amount of the film/wafer curvature and the stress state of the previously deposited layers. The formulas allow the incremental-deposition stress and the elastic relaxation to be correctly calculated from the slope of the measured curvature versus thickness for arbitrary thicknesses and biaxial moduli of the film and the substrate. Subtraction of the cumulative elastic relaxation from the incremental-deposition stress history results in the residual stress left in the film after the whole deposition process. The validities of the formulas are confirmed by curvature measurements of electrodeposited Ni films on substrates with different thicknesses.

Stress Bearing Mechanism of Reduced Graphene Oxide in Silicon-Based Composite Anodes for Lithium Ion Batteries.

Despite the significant research that has been carried out to improve cycling performance of lithium ion batteries (LIB) with silicon (Si) based composite electrodes, limited studies have been performed on these materials to evaluate the effects of internal microstructural changes and stress evolution on the electrochemical performance. Here, combined ex situ and in situ investigations on the accommodation of volume expansion in Si-based nanocomposite electrodes are reported. This work emphasizes the importance of conductive agents in light of the poor electronic conductivity of Si. A detailed comparison between commonly used carbon black (CB) and reduced graphene oxide (rGO) shows that these materials have substantial effects on microstructural evolution and internal stress in Si based composite electrodes that are employed in lithium ion cells. This study provides the first monitoring of stress evolution in Si-rGO based composite electrode during electrochemical cycling using in situ wafer curvature measurements. The prepared Si-rGO based electrode exhibits almost 10 times lower stress generation and consequently higher cycling performance in electrochemical cells. The resulting 3D networked structure not only acts as an electronic conduit to the encapsulated active materials but also serves as a mini-electrochemical reaction chamber which hinders the formation of the solid electrolyte interphase (SEI) and limits the pulverization of active material and the evolution of severe stress during cycling. Moreover, investigations of the microstructural changes and internal charge transfer resistance in the electrodes after cycling provide further evidence that rGO produces superior structures for energy storage.

Fracture properties of thin film TiN at elevated temperatures

We provide an experimental and theoretical description of the high temperature fracture behaviour of TiN thin films. For this, we employ molecular dynamics and density functional theory, to show that the surface energies drop insignificantly between 0 and 1000 K. We utilise these results to predict a slight decrease of the fracture toughness over the aforementioned temperature range.For the experimental perspective, we use unbalanced DC reactive magnetron sputtering to synthesise a TiN film, on which we perform in situ high temperature microcantilever bending tests. Upon increasing the testing temperature from room temperature to 773 K our results present a slight, irreversible decrease of KIC, once the deposition temperature of the film (~653 K) is exceeded.Based on our theoretical groundwork, as well as complementary data produced by X-ray diffraction, nanoindentation, transmission electron microscopy, and wafer curvature measurements, we identify growth defect recovery as the main reason behind the decrease of KIC. We observe no change in the deformation and/or fracture mechanism of TiN across the experimentally investigated temperature range. Using an analytical model based on continuum mechanics, we estimate the influence of macro residual stresses on the temperature-dependent fracture toughness of TiN attached to a Si (100) wafer.

Mechano-Electro-Chemical Insights into the Danger of Using Precious Metal Electrodes for Oxygen Surface Exchange Measurements

This talk will explore the effect intentionally added Pt impurities have on the oxygen surface exchange coefficient of praseodymium doped ceria, as determined from wafer curvature measurements.

In situ wafer curvature measurement and strain control of AlInN/GaN distributed Bragg reflectors

We measured in situ wafer curvature evolutions of 25-pair AlInN/GaN DBRs on sapphire substrates and found that strains in the DBRs were gradually increased in a compressive direction as the epitaxial growth progressed. The increase of the strain was originated in the AlInN layers of the DBRs, suggesting a possibility that the InN mole fraction was increased with 0.03%–0.05%/pair. In order to compensate the increase of the strain in the AlInN layers, we grew the DBR by gradually increasing AlInN growth temperatures with 1°C/5 pairs, resulting in narrower satellite peaks of an X-ray diffraction pattern.

Dislocation Plasticity and Detwinning Under Thermal Stresses in Nanotwinned Ag Thin Films

Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic signature in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic - ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the signature thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars.

(Invited) Surface Contaminant Effects on the Properties of Pr-Doped Ceria Determined Via Wafer Curvature and X-Ray Diffraction Measurements

Praseodymium doped ceria (PCO) is a model mixed ionic electronic conductor (MIEC) solid oxide fuel cell (SOFC) cathode material. Under SOFC operating conditions, PCO undergoes the reaction (1): ½ O2 + 2Pr’ Ce + VӦ = 2PrCe + Oo x The kinetics of this reaction can be represented by the chemical oxygen surface exchange coefficient (kchem). It has been reported that surface contaminants can affect the kinetics of this reaction, either boosting or deteriorating the oxygen exchange performance of MIECs (2, 3). Hence, in this work, the effect of surface contaminants on the Pr0.1Ce0.9O2-δ (10PCO) thin film properties were investigated. First, YSZ (yttria doped zirconia) or MgO (magnesium oxide) single crystal substrates were coated with (100)-oriented >21 nm average grain sized 10PCO thin films containing various surface contaminants via Pulsed Laser Deposition. The stress-temperature behavior of these samples was then measured using a dual substrate wafer curvature measurement technique utilizing a multi-beam optical stress sensor (MOSS) to determine the 280-700oC in-plane 10PCO thermal-chemical expansion coefficient, Young’s modulus, and oxygen nonstoichiometry, as described in Ref. (4). The curvature relaxation of these samples reacting to abrupt changes in the surrounding oxygen partial pressure from synthetic air (20% O2-80% Ar) and 10 times diluted synthetic air (2% O2-98% Ar), and vice-versa, were measured to determine the high temperature 10PCO kchem. Finally, the 500-700oC films strains in the 10PCO|YSZ samples were analyzed by measuring the 10PCO (200) peak position via high temperature X-Ray diffraction (HTXRD) and compared to the film stresses measured via MOSS to determine the Young’s modulus and thermal-chemical expansion coefficient (assuming no difference between the in-plane and out-of-plane values). The close agreement of the data in Figures 1a and 1b show that the in-plane and out-of-plane bulk properties measured here were identical to each other, within the measurement errors, and similar to some of the previous values reported for bulk 10PCO. Further, no difference in bulk properties were observed for films with various surface contaminants. However, as shown in Figure 2, Si surface impurities significantly reduced kchem while noble metal surface impurities likely increased it. These results 1) will be important for evaluating the real-world tolerance of 10PCO in real-world SOFCs, 2) highlight the benefit of using electrode-free kchem measurement techniques, and 3) help explain the large kchem variation seen in the literature for identical materials at identical conditions.