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Atomic layer deposition (ALD) of Al2O3 using trimethylaluminum and H2O is known to proceed through sequential surface reactions that leave the surface alternately terminated with AlCH3 and OH groups. Using in situ reflection–absorption infrared spectroscopy in a flow reactor, we monitor the consumption of AlCH3 groups by brief pulses of H2O at temperatures between 105 and 300 °C. At low temperatures, the surface reactions occur by what appears to be two stages that can be fit to a biexponential decay. Rate laws based on species of AlCH3 groups that also predict a biexponential decay are found to depend on unrealistic activation energies for their constituent reactions when applied to the data. However, a model in which the effective activation energy changes linearly with AlCH3 coverage does adequately fit the data. This model produces the apparent biexponential decay at low temperatures, and it confirms prior suggestions of a coverage dependence in the rate constant. The decrease in the effective activat...

We show that commercially sourced n-channel silicon field-effect transistors (nFETs) operating above their threshold voltage with closed loop feedback to maintain a constant channel current allow a pH readout resolution of (7.2±0.3)×10(−3) at a bandwidth of 10 Hz, or ≈3-fold better than the open loop operation commonly employed by integrated ion-sensitive field-effect transistors (ISFETs). We leveraged the improved nFET performance to measure the change in solution pH arising from the activity of a pathological form of the kinase Cdk5, an enzyme implicated in Alzheimer’s disease, and showed quantitative agreement with previous measurements. The improved pH resolution was realized while the devices were operated in a remote sensing configuration with the pH sensing element off-chip and connected electrically to the FET gate terminal. We compared these results with those measured by using a custom-built dual-gate 2D field-effect transistor (dg2DFET) fabricated with 2D semi-conducting MoS(2) channels and a signal amplification of 8. Under identical solution conditions the nFET performance approached the dg2DFETs pH resolution of (3.9±0.7)×10(−3). Finally, using the nFETs, we demonstrated the effectiveness of a custom polypeptide, p5, as a therapeutic agent in restoring the function of Cdk5. We expect that the straight-forward modifications to commercially sourced nFETs demonstrated here will lower the barrier to widespread adoption of these remote-gate devices and enable sensitive bioanalytical measurements for high throughput screening in drug discovery and precision medicine applications.

The electron energy band alignment of the monolayer MoSe2/oxide/Si system is characterized by internal photoemission spectroscopy, where the oxide is Al2O3 or SiO2. Raman and photoluminescence spectroscopic measurements confirm the high quality of monolayer MoSe2 exfoliated with gold film as medium. At the oxide flat-band condition, the band offset from the monolayer MoSe2 valence band maximum to the Al2O3 and SiO2 conduction band minimum are measured to be (4.10 ± 0.05) eV and (4.80 ± 0.05) eV, respectively. By referencing the recently reported band gap value of 2.18 eV for monolayer MoSe2, we obtain the electron affinity of monolayer MoSe2 to be (3.8 ± 0.1) eV on Al2O3/Si and (3.5 ± 0.1) eV on SiO2/Si. It is believed that the results from this study will help accelerate the design of electronic and optoelectronic devices that employ this class of two-dimensional materials.

Silicon wafer-based internal reflection elements (IREs) present many practical advantages over the prisms conventionally used for attenuated total reflection (ATR) spectroscopy in the infrared. We examine two methods of using minimally prepared IREs that have appeared in the literature, edge-coupled (EC) and prism-coupled (PC), in conjunction with a liquid flow cell. Polarization measurements show that radiation entering the PC-IRE becomes depolarized due to stress-induced birefringence, and transmission through the edge of the EC-IRE also affects the polarization state. Quantification of the noise and a calibration using a series of sodium acetate solutions show the sensitivity of the PC-IRE outweighs the lower noise obtainable with the EC-IRE.

A nondispersive infrared (NDIR) gas analyzer was demonstrated for measuring the vapor-phase density of the carbonyl-containing organometallic cobalt precurso μ2-η2-(tBu-acetylene) dicobalthexacarbonyl (CCTBA). This sensor was based on direct absorption by CCTBA vapor in the C≡O stretching spectral region and utilized a stable, broadband IR filament source, an optical chopper to modulate the source, a bandpass filter for wavelength isolation, and an InSb detector. The optical system was calibrated by selecting a calibration factor to convert CCTBA absorbance to a partial pressure that, when used to calculate CCTBA flow rate and CCTBA mass removed from the ampoule, resulted in an optically determined mass that was nominally equal to a gravimetrically-determined mass. In situ Fourier transform infrared (FT-IR) spectroscopy was performed simultaneously with the NDIR gas analyzer measurements under selected conditions in order to characterize potential spectroscopic interferences. Interference due to CO evolution from CCTBA was found to be small under the flow conditions employed here. A CCTBA minimum detectable molecular density as low as ≈3 × 1013 cm−3 was calculated (with no signal averaging and for a sampling rate of 200 Hz). While this NDIR gas analyzer was specifically tested for CCTBA, it is suitable for characterizing the vapor delivery of a range of carbonyl-containing precursors.

A nondispersive infrared gas analyzer was demonstrated for investigating metal alkylamide precursor delivery for microelectronics vapor deposition processes. The nondispersive infrared analyzer was designed to simultaneously measure the partial pressure of pentakis(dimethylamido) tantalum, a metal precursor employed in high volume manufacturing vapor deposition processes to deposit tantalum nitride, and dimethylamine, the primary decomposition product of pentakis(dimethylamido) tantalum at typical delivery conditions for these applications. This sensor was based on direct absorption of pentakis(dimethylamido) tantalum and dimethylamine in the fingerprint spectral region. The nondispersive infrared analyzer optical response was calibrated by measuring absorbance as a function of dimethylamine and pentakis(dimethylamido) tantalum density. The difference between the mass of material removed from the ampoule during flow tests as measured gravimetrically and as determined optically, by calculating flow rates from the nondispersive infrared analyzer measurements, was only ≈2 %. The minimum detectable molecular densities for pentakis(dimethylamido) tantalum and dimethylamine were ≈2 × 1013 cm−3 and ≈5 × 1014 cm−3, respectively (with no signal averaging and for a sampling rate of 200 Hz), and the corresponding partial pressures were ≈0.1 Pa and ≈2 Pa for pentakis(dimethylamido) tantalum and dimethylamine, respectively (for an optical flow cell temperature of 93 ℃). Pentakis(dimethylamido) tantalum could be detected at all conditions of this investigation and likely the majority of conditions relevant to high volume manufacturing tantalum nitride deposition. Dimethylamine was not detected at all conditions in this study, because of a lower nondispersive infrared analyzer sensitivity to dimethylamine compared to pentakis(dimethylamido) tantalum and because conditions of this study were selected to minimize DMA production. While this nondispersive infrared gas analyzer was specifically developed for pentakis(dimethylamido) tantalum and dimethylamine, it is suitable for characterizing the vapor delivery of other metal alkylamide precursors and the corresponding amine decomposition products, although in the case of some metal alkylamides a different bandpass filter would be required.

Metal-mediated exfoliation has been demonstrated as a promising approach for obtaining large-area flakes of two-dimensional (2D) materials to fabricate prototypical nanoelectronics. However, several processing challenges related to organic contamination at the interface of a 2D material and gate oxide must be overcome to realize robust devices with high yields. Here, we demonstrate an optimized process to realize high-performance field-effect transistor (FET) arrays from large-area (∼5000 μm2), monolayer MoS2 with a yield of 85%. A central element of this process is an exposed material forming gas anneal (EM-FGA) that results in uniform FET performance metrics (i.e., field-effect mobilities, threshold voltages, and contact performance). Complementary analytical measurements show that the EM-FGA process reduces deleterious channel doping effects by decreasing organic contamination while also reducing the prevalence of insulating molybdenum oxide, effectively improving the MoS2-gate oxide interface. The uniform FET performance metrics and high device yield achieved by applying the EM-FGA technique on large-area 2D material flakes will help advance the fabrication of complex 2D nanoelectronic devices and demonstrate the need for improved engineering of the 2D material-gate oxide interface.

Low volatility precursors are widely utilized in chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes. Compared to gases and high volatility liquid precursors, delivery of low volatility liquid and solid precursors can be problematic, with solid precursors being particularly so. To investigate some of these delivery issues, the performance of a vapor draw vessel was characterized for the delivery of pentakis(dimethylamido) tantalum (PDMAT), a low-volatility solid precursor at preferable delivery temperatures, for reduced-pressure cyclical CVD and ALD processes. Vessel characterization involved determining (1) a source efficiency as a function of process conditions and (2) the degree of PDMAT decomposition as a function of temperature and vessel idle time. The PDMAT partial pressure, flow rate, and mass per injection used to determine the source efficiency were determined from measurements obtained using a custom-designed non-dispersive infrared gas analyzer. For a series of injections after an idle/purge sufficiently long to saturate the vessel head space, the source efficiency decreased from a maximum slightly less than unity for the first injection until a consistent value was reached that was approximately one half to one third of the maximum value. A comparable trend was observed for mass delivered per injection. For the conditions used in this investigation, the source efficiency decreased when the injection time was increased to longer than 1 s, when pressure was decreased, and when the carrier gas flow rate was increased. Although the corresponding mass per injection increased with these changes, the increase in mass was less than that predicted had the carrier gas been saturated. The source efficiency did not depend strongly on temperature and only moderately on vessel idle durations (4–16 s). The degree of PDMAT decomposition was evaluated by measuring the partial pressure of dimethylamine (the primary PDMAT decomposition product under the conditions of this investigation) using the same gas analyzer. For a given idle time, the amount of dimethylamine delivered more than doubled as vessel temperature was increased from 68 to 78 °C.

Thin film vapor deposition processes, e.g., chemical vapor deposition, are widely used in high-volume manufacturing of electronic and optoelectronic devices. Ensuring desired film properties and maximizing process yields require control of the chemical precursor flux to the deposition surface. However, achieving the desired control can be difficult due to numerous factors, including delivery system design, ampoule configuration, and precursor properties. This report describes an apparatus designed to investigate such factors. The apparatus simulates a single precursor delivery line, e.g., in a chemical vapor deposition tool, with flow control, pressure monitoring, and a precursor-containing ampoule. It also incorporates an optical flow cell downstream of the ampoule to permit optical measurements of precursor density in the gas stream. From such measurements, the precursor flow rate can be determined, and, for selected conditions, the precursor partial pressure in the headspace can be estimated. These capabilities permit this apparatus to be used for investigating a variety of factors that affect delivery processes. The methods of determining the pressure to (1) calculate the precursor flow rate and (2) estimate the headspace pressure are discussed, as are some of the errors associated with these methods. While this apparatus can be used under a variety of conditions and configurations relevant to deposition processes, the emphasis here is on low-volatility precursors that are delivered at total pressures less than about 13 kPa downstream of the ampoule. An important goal of this work is to provide data that could facilitate both deposition process optimization and ampoule design refinement.

The design and operation of a simple, optically-accessible modular reactor for probing thermal thin film deposition processes, such as atomic layer deposition processes (ALD) and chemical vapor deposition (CVD), is described. This reactor has a nominal footprint of 225 cm(2) and a mass of approximately 6.6 kg, making it small enough to conveniently function as a modular component of an optical train. The design is simple, making fabrication straightforward and relatively inexpensive. Reactor operation is characterized using two infrared absorption measurements to determine exhaust times for tetrakis(dimethylamino)titanium and water, proto-typical ALD precursors, in a pressure and flow regime commonly used for ALD.

In situ chemical measurements of solution/surface reactions during metal-organic framework (MOF) thin film growth can provide valuable information about the mechanistic and kinetic aspects of key reaction steps, and allow control over crystal quality and material properties. Here, we report a new approach to study the growth of MOF thin films in a flow cell using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Real-time spectra recorded during continuous flow synthesis were used to investigate the mechanism and kinetics that govern the formation of (Zn, Cu) hydroxy double salts (HDSs) from ZnO thin films and the subsequent conversion of HDS to HKUST-1. We found that both reactions follow pseudo-first order kinetics. Real-time measurements also revealed that the limited mass transport of reactants may lead to partial conversion of ZnO to HDS and therefore leaves an interfacial ZnO layer beneath the HDS film providing strong adhesion of the HKUST-1 coating to the substrate. This in situ flow-cell ATR-FTIR method is generalizable for studying the dynamic processes of MOF thin film growth, and could be used for other solid/liquid reaction systems involving thin films.

We present an investigation on the carbon scrolls produced from chemical vapor deposition (CVD) grown graphene. A wet, polymer-mediated process is used to transfer CVD grown graphene from copper foil to a Si/SiO2 or metal substrate. Upon the removal of polymer and drying, tearing of the graphene occurs and carbon scrolls are produced from the ruptured graphene. We demonstrate that the optical and electronic properties of these scrolls are comparable to those from exfoliated graphene. Optical microscopy and scanning electron microscopy show that the isolated scrolls exhibit a similar morphology to those formed from exfoliated graphene. Raman spectroscopy has been used to investigate the conformational change from graphene to scrolls. The scrolls have a more pronounced Raman D peak than graphene and display a large variation in the integrated intensity of Raman G and G′ peaks at different scroll locations. Raman spectroscopy and transmission electron microscopy confirm that the graphene layers in the scrolls are non-AB stacked. The scroll field-effect-transistor (FET) device exhibits an ambipolar behavior, resembling the graphene FET device. This work demonstrates the possibility of fabricating carbon scrolls using CVD grown graphene on various substrates and paves the way for advancing their applications.

We have developed a portable photoacoustic spectrometer that offers routine, precise and accurate measurements of the molar concentration of atmospheric carbon. The temperature-controlled spectrometer continuously samples dried atmospheric air and employs an intensity-modulated distributed feedback laser and fiber amplifier operating near 1.57 µm. For measurements of carbon dioxide in air, we demonstrate a measurement precision (60-s averaging time) of 0.15 µmol mol−1 and achieve a standard uncertainty of 0.8 µmol mol−1 by calibrating the analyzer response in terms of certified gas mixtures. We also investigate how water vapor affects the photoacoustic signal by promoting collisional relaxation of the carbon dioxide.

Delivery of low-volatility precursors is a continuing challenge for chemical vapor deposition and atomic layer deposition processes used for microelectronics manufacturing. To aid in addressing this problem, we have recently developed an inline measurement capable of monitoring precursor delivery. Motivated by a desire to better understand the origins of what is now observable, this study uses computational fluid dynamics and a relatively simple model to simulate the delivery of pentakis(dimethylamido)tantalum (PDMAT) from a commercial vapor draw ampoule. Parameters used in the model are obtained by fitting the performance of the ampoule to a limited dataset of PDMAT delivery rates obtained experimentally using a non-dispersive infrared sensor. The model shows good agreement with a much larger experimental dataset over a range of conditions in both pulsed and continuously flowing operation. The combined approach of experiment and simulation provides a means to understand the phenomena occurring during precursor delivery both quantitatively and qualitatively.

A monolayer of diruthenium molecules was self-assembled onto the silicon oxide surface in a semiconductor capacitor structure with a “click” reaction for nonvolatile memory applications. The attachment of the active molecular monolayer was verified by x-ray photoelectron spectroscopy. The prototypical capacitor memory devices in this work employed a metal/oxide/molecule/oxide/Si structure. With the intrinsic redox-active charge-storage properties of diruthenium molecules, these capacitor memory devices exhibited fast Program and Erase speed, excellent endurance performance with negligible degradation of the memory window after 105 program/erase cycles, and very good 10-year memory retention. These experimental results indicate that the redox-active ruthenium molecular memory is very promising for use in nonvolatile memory applications.

The performance of a bubbler to deliver the low-volatility, liquid cobalt precursor μ2-η2-(tBu-acetylene) dicobalthexacarbonyl (CCTBA) for reduced-pressure chemical vapor deposition and atomic layer deposition processes was characterized. A relatively large process window was investigated by varying carrier gas flow rate, system pressure, and bubbler temperature. For this range of conditions, the CCTBA partial pressure was measured using a custom-designed nondispersive infrared gas analyzer, and the CCTBA flow rates were derived from these partial pressure measurements. The dependence of CCTBA flow rate on these process parameters was modeled to obtain a deeper understanding of the factors influencing bubbler performance. Good agreement between measured and modeled CCTBA flow rates was obtained using a model in which the pressure drop between the bubbler head space and the pressure measurement location was included and in which a constant CCTBA partial pressure in the bubbler head space for a given bubbler temperature was assumed. The dependence of CCTBA head space partial pressure on temperature was parameterized in the form of the August equation, which is commonly used to describe the temperature-dependence of vapor pressure. While this report was focused specifically on CCTBA, the results of this study indicate that this method for estimating the precursor delivery rate from a bubbler should be applicable to other low-volatility, liquid precursors.The performance of a bubbler to deliver the low-volatility, liquid cobalt precursor μ2-η2-(tBu-acetylene) dicobalthexacarbonyl (CCTBA) for reduced-pressure chemical vapor deposition and atomic layer deposition processes was characterized. A relatively large process window was investigated by varying carrier gas flow rate, system pressure, and bubbler temperature. For this range of conditions, the CCTBA partial pressure was measured using a custom-designed nondispersive infrared gas analyzer, and the CCTBA flow rates were derived from these partial pressure measurements. The dependence of CCTBA flow rate on these process parameters was modeled to obtain a deeper understanding of the factors influencing bubbler performance. Good agreement between measured and modeled CCTBA flow rates was obtained using a model in which the pressure drop between the bubbler head space and the pressure measurement location was included and in which a constant CCTBA partial pressure in the bubbler head space for a given bubble...

In situ infrared (IR) spectroscopy has proven to be an extremely valuable tool for understanding various aspects of chemical vapor deposition (CVD) and atomic layer deposition (ALD). Solid film deposits, gaseous precursors and by-products, and growth surfaces are all of interest in these processes, and IR spectroscopy is capable of providing useful data for each. Qualitative measurements are sometimes sufficient, but quantitative spectroscopy is often desired for work involving process monitoring or process development. Relating absorbancetoconcentrationscanbeproblematicforsurfacesandsolid deposits due to interactions, dielectric screening, etc., but measurements of the gas phase are straightforward if absorptivities are known. Efforts to obtain accurate IR reference spectra of CVD and ALD precursors, however, have been limited. 1 Commercial quantitative spectral libraries, if they include any precursors at all, typically contain only the most common ones (e.g., SiH4 and WF6). This is despite the use of IR spectroscopy for process monitoring in manufacturing 2 and for process development in the laboratory. 3,4 There is a need, therefore, for carefully obtained spectra of the less common precursors and for a general method of obtaining them. This work presents a static measurement for determining the IR absorptivities of low vapor pressure organometallic compounds used as precursors for CVD and ALD. We use Fourier transform IR (FTIR) spectroscopy to this end. Since the focus of this work is related to CVD and ALD processes where the compounds are typically heated to increase their vapor pressure, we obtain reference spectra at elevated temperatures. Heated samples are known to cause photometric errors in FT-IR spectroscopy, so we use a setup that is demonstrated to provide correct spectra. The apparatus and method for emission correction are discussed fully in the Experimental section. Quantitatively accurate IR spectra of tetrakis(dimethylamido)titanium (TDMAT), Ti[N(CH3)2]4 ,a re measured in the temperature range of (352 to 476) K. TDMAT is an alkylamido organometallic compound, a class of molecules that has found widespread use as CVD and ALD precursors in the semiconductor industry. TDMAT, in particular, is commonly used for the CVD of TiN and is under investigation for the ALD of TiO2. Qualitative IR spectra of TDMAT have been reported a number of times in the literature, 5‐11 which offers an opportunity to compare the results to previous studies and to elucidate the discrepancies. The temperature-dependent IR absorptivity is presented in the Results section, and a comparison to prior results can be found in the ∗

This work presents a novel method for obtaining surface infrared spectra with sub-second time resolution during atomic layer deposition (ALD). Using a rapid-scan Fourier transform infrared (FT-IR) spectrometer, we obtain a series of synchronized interferograms (120 ms) during multiple ALD cycles to observe the dynamics of an average ALD cycle. We use a buried metal layer (BML) substrate to enhance absorption by the surface species. The surface selection rules of the BML allow us to determine the contribution from the substrate surface as opposed to that from gas-phase molecules and species adsorbed at the windows. In addition, we use simulation to examine the origins of increased reflectivity associated with phonon absorption by the oxide layers. The simulations are also used to determine the decay in enhancement by the buried metal layer substrate as the oxide layer grows during the experiment. These calculations are used to estimate the optimal number of ALD cycles for our experimental method.

We present the results of a thorough study of wet chemical methods for transferring chemical vapor deposition grown graphene from the metal growth substrate to a device-compatible substrate. On the basis of these results, we have developed a "modified RCA clean" transfer method that has much better control of both contamination and crack formation and does not degrade the quality of the transferred graphene. Using this transfer method, high device yields, up to 97%, with a narrow device performance metrics distribution were achieved. This demonstration addresses an important step toward large-scale graphene-based electronic device applications.

Metal alkylamide compounds, such as tetrakis(ethylmethylamido) hafnium (TEMAH), represent a technologically important class of metalorganic precursors for the deposition of metal oxides and metal nitrides via atomic layer deposition (ALD) or chemical vapor deposition. The development of in situ diagnostics for processes involving these compounds could be beneficial in, e.g., developing deposition recipes and validating equipment-scale simulations. This report describes the performance of the combination of two techniques for the simultaneous, rapid measurement of the three major gas phase species during hafnium oxide thermal ALD using TEMAH and water: TEMAH, water, and methylethyl amine (MEA), the only major reaction by-product. For measurement of TEMAH and MEA, direct absorption methods based on a broadband infrared source with different mid-IR bandpass filters and utilizing amplitude modulation and synchronous detection were developed. For the measurement of water, wavelength modulation spectroscopy utilizing a near-IR distributed feedback diode laser was used. Despite the relatively simple reactor geometry employed here (a flow tube), differences were easily observed in the time-dependent species distributions in 300 mL/min of a helium carrier gas and in 1000 mL/min of a nitrogen carrier gas. The degree of TEMAH entrainment was lower in 300 mL/min of helium compared to that in 1000 mL/min of nitrogen. The capability to obtain detailed time-dependent species concentrations during ALD could potentially allow for the selection of carrier gas composition and flow rates that would minimize parasitic wall reactions. However, when nitrogen was employed at the higher flow rates, various flow effects were observed that, if detrimental to a deposition process, would effectively limit the upper range of useful flow rates.

In this work, a Fourier transform infrared spectroscopy-based method is developed to measure the gas-phase dynamics occurring during atomic layer deposition. This new technique is demonstrated during the deposition of hafnium oxide using tetrakis(ethylmethylamido)hafnium and water vapor. The repeatability of the deposition process is utilized to signal average across multiple cycles. This approach required synchronizing the precursor injection pulses with the moving mirror of the spectrometer. The system as implemented in this work achieves spectra with a time resolution of ≈150 ms, but better resolution can be easily obtained. Using this technique, the authors are able to optically measure transients in the molecular number densities of the precursors and product that are the effects of mass transport and surface reactions.

Tetrakis(ethylmethylamido)hafnium and water are commonly used precursors for atomic layer deposition of HfO 2 . Using reflection absorption infrared spectroscopy with a buried-metal-layer substrate, we probe surface species present during typical deposition conditions. We observe evidence for thermal decomposition of alkylamido ligands at 320 °C. Additionally, we find that complete saturation of the SiO 2 substrate occurs in the first cycle at ≈100 °C whereas incomplete coverage is apparent even after many cycles at higher temperatures. The use of this technique as an in situ diagnostic useful for process optimization is demonstrated.

Infrared spectra of surface species have been obtained during atomic layer deposition using tris(isopropylcyclopentadienyl)lanthanum, La(iPrCp)3, and water as precursors at 160 °C and 350 °C. Gas-phase spectra of La(iPrCp)3are obtained for comparison. At low temperature, ligand exchange is seen to occur, and carbonate formation is found. With extended purging, the organic ligands are found to be stable on the surface, and carbonates are not formed. These observations indicate that carbonate formation is occurring during exposure to the precursors. At high temperature, the La precursor is observed to decompose leaving an opaque deposit containing relatively little hydrogen.

In situ spectroscopic ellipsometry was used to analyze hafnium diboride thin films deposited by chemical vapor deposition from the single-source precursor Hf(BH4)4. By modeling the film optical constants with a Drude-Lorentz model, the film thickness, surface roughness, and electrical resistivity were measured in situ. The calculated resistivity for amorphous films deposited at low temperature ranged from 340to760μΩcm. These values are within 25% of those measured ex situ with a four-point probe, indicating the validity of the optical model. By modeling the real-time data in terms of film thickness and surface roughness, the film nucleation and growth morphology were determined as a function of substrate type, substrate temperature, and precursor pressure. The data show that at low precursor pressures (∼10−6Torr) and at low substrate temperatures (

We investigate the evolution of surface morphology during hot-wire chemical vapor deposition of amorphous silicon films onto rough substrates. Using in situ spectroscopic ellipsometry, we find that the surface smoothens as the film grows. However, postdeposition atomic force microscopy reveals that the roughness is actually increasing linearly. We resolve this discrepancy by examining the power spectrum densities of the atomic force images, which indicate that the growth surface experiences both short-range smoothening and global roughening. The ellipsometry data are consistent with the short-range atomic force microscopy data, but they exclude information about the long-range components of roughness. The slope of the power spectrum density indicates surface diffusion is the dominant smoothening mechanism; the linear increase in roughness is consistent with columnar growth caused by self-shadowing.

Water is often employed as the oxygen source in metal oxide atomic layer deposition (ALD) processes. It has been reported that variations in the amount of water delivered during metal oxide ALD can impact the oxide film properties. Hence, one contribution to optimizing metal oxide ALD processes would be to identify methods to better control water dose. The development of rapid, quantitative techniques for in situ water vapor measurements during ALD processes would be beneficial to achieve this goal. In this report, the performance of an in situ tunable diode laser absorption spectroscopy (TDLAS) scheme for performing rapid, quantitative water partial pressure measurements in a representative quarter-inch ALD delivery line is described. This implementation of TDLAS, which utilizes a near-infrared distributed-feedback diode laser and wavelength modulation spectroscopy, provides measurements of water partial pressure on a timescale comparable to or shorter than the timescale of the gas dynamics in typical AL...

An in situ gas-phase diagnostic for the metal alkylamide compound tetrakis(ethylmethylamido) hafnium (TEMAH), Hf[N(C2H5)(CH3)]4, was demonstrated. This diagnostic is based on direct absorption measurement of TEMAH vapor using an external cavity quantum cascade laser emitting at 979 cm−1, coinciding with the most intense TEMAH absorption in the mid-infrared spectral region, and employing 50 kHz amplitude modulation with synchronous detection. Measurements were performed in a single-pass configuration in a research-grade atomic layer deposition (ALD) chamber. To examine the detection limit of this technique for use as a TEMAH delivery monitor, this technique was demonstrated in the absence of any other deposition reactants or products, and to examine the selectivity of this technique in the presence of deposition products that potentially interfere with detection of TEMAH vapor, it was demonstrated during ALD of hafnium oxide using TEMAH and water. This technique successfully detected TEMAH at molecular densities present during simulated industrial ALD conditions. During hafnium oxide ALD using TEMAH and water, absorbance from gas-phase reaction products did not interfere with TEMAH measurements while absorption by reaction products deposited on the optical windows did interfere, although interfering absorption by deposited reaction products corresponded to only ≈4% of the total derived TEMAH density. With short measurement times and appropriate signal averaging, estimated TEMAH minimum detectable densities as low as ≈2 × 1012 molecules/cm3 could be obtained. While this technique was demonstrated specifically for TEMAH delivery and hafnium oxide ALD using TEMAH and water, it should be readily applicable to other metal alkylamide compounds and associated metal oxide and nitride deposition chemistries, assuming similar metal alkylamide molar absorptivity and molecular density in the measurement chamber.

Graphene is regarded as a promising material that could be the basis for future generations of low-power, faster, and smaller electronics [1,2]. Currently, chemical vapor deposition (CVD) growth method is the only way that can produce large area monolayer graphene up to tens of inches with high quality [3,4], which makes it the most promising graphene producing method for large scale device applications. The first step necessary in fabricating devices from CVD-grown graphene, is to transfer the graphene from the metal growth substrate onto a device-compatible substrate (typically an insulator). It is crucial to device performance, yield, and uniformity that the quality of the graphene is not degraded during this transfer process.

Using Rayleigh-Rice scattering theory we have studied the influence of surface morphology on the optical response of self-affine surfaces. We have established a mathematical relationship between the surface roughness ($d$) as determined by spectroscopic ellipsometry (SE) using the effective medium approximation (EMA) and the parameters controlling the morphology of the surface: root-mean-square roughness ($w$), correlation length ($\ensuremath{\xi}$), and roughness (Hurst) exponent (\ensuremath{\alpha}). These three parameters affect the roughness value measured by ellipsometry. However, when the correlation length is smaller than the wavelength, the dependence is contained in a single parameter $w\ensuremath{\delta}$ that is proportional to the product of the surface roughness and the local slope $\ensuremath{\delta}=w/\phantom{w{\ensuremath{\xi}}^{\ensuremath{\alpha}}}\phantom{\rule{0.0pt}{0ex}}{\ensuremath{\xi}}^{\ensuremath{\alpha}}$. The fact that the local slope of a surface increases only very slowly during growth explains the linear dependence experimentally found between $w$ as measured by scanning-probe microscopy and the vertical roughness determined by the effective medium approach.

The thermal conductivities of thin films of amorphous Si (a-Si) deposited by hot-wire chemical vapor deposition (HWCVD) are measured by time-domain thermoreflectance (TDTR). Amorphous Si samples prepared at the National Renewable Energy Laboratory (NREL) show an anomalous enhancement in thermal conductivity compared to other forms of a-Si and compared to the prediction of the model of the minimum thermal conductivity. The thermal conductivity of the NREL HWCVD a-Si samples also decreases with increasing frequency of the temperature fields used in the experiment. This frequency dependence of the thermal conductivity is nearly identical to the results of our previous studies of crystalline semiconductor alloys; a comparison of the frequency dependence to a phonon-scattering model suggests that Rayleigh-type scattering controls the mean-free path of $\ensuremath{\sim}5\text{ }\text{meV}$ phonons in this material. Amorphous Si films prepared at University of Illinois (U. Illinois) do not show an enhanced thermal conductivity even though Raman vibrational spectra of the U. Illinois and NREL samples are nearly identical. Thus, the thermal conductivity of a-Si depends on details of the microstructure that are not revealed by vibrational spectroscopy and measurements by TDTR provide a convenient method of identifying novel microstructures in amorphous materials.

Atomic layer deposition of titanium dioxide using tetrakis(dimethylamido)titanium (TDMAT) and water vapor is studied by reflection-absorption infrared spectroscopy (RAIRS) with a time resolution of 120 ms. At 190 °C and 240 °C, a decrease in the absorption from adsorbed TDMAT is observed without any evidence of an adsorbed product. Ex situ measurements indicate that this behavior is not associated with an increase in the impurity concentration or a dramatic change in the growth rate. A desorbing decomposition product is consistent with these observations. RAIRS also indicates that dehydroxylation of the growth surface occurs only among one type of surface hydroxyl groups. Molecular water is observed to remain on the surface and participates in reactions even at a relatively high temperature (110 °C) and with long purge times (30 s).

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

We use postdeposition atomic force microscopy and in situ spectroscopic ellipsometry to analyze the roughening of hydrogenated amorphous silicon films deposited by hot-wire chemical vapor deposition at 150°C. From the atomic force microscopy data, the root-mean-squared roughness w increases with deposition time t as w∝tβ with β=0.37±0.02, and the correlation length ξ increases as ξ∝t1∕z with 1∕z=0.31±0.02. From the height-difference correlation function, we obtain a roughness exponent α=0.87±0.04 and a root-mean-squared local slope δ, which increases as δ∝tκ with κ=0.17±0.03. These measurements are indicative of anomalous growth, which we attribute to geometric shadowing. However, the roughening behavior we observe using atomic force microscopy is not reproduced in the spectroscopic ellipsometry data. This contradicts previous reports which found a linear relationship between the thickness of the optical roughness layer and the root-mean-squared roughness. We discuss the discrepancy between the two techni...

We use fluctuation electron microscopy to determine changes in the medium-range structural order of un-hydrogenated amorphous silicon thin films after they are exposed to atomic hydrogen at a substrate temperature of 230 °C. The films are deposited by magnetron sputtering at either 230 or 350 °C substrate temperature to obtain starting states with small or large initial medium-range order, respectively. The in-diffusion of atomic hydrogen causes the medium-range order to decrease for the small initial order but to increase for the large initial order. We suggest that this behavior can be understood in terms of classical nucleation theory: The ordered regions of small diameter are energetically unstable and can lower their energy by evolving towards a continuous random network, whereas the ordered regions of large diameter are energetically stable and can lower their energy by coarsening towards the nanocrystalline state.

A recent article suggested that “paracrystalline” silicon could be detected in very thin hydrogenated amorphous silicon films by using spectroscopic ellipsometry. We show that the important features of the ellipsometry data can be reproduced using a simple optical model of amorphous Si:H, with no assumptions about unusual structures or thickness-dependent optical properties.