Brent A. Sperling

Time-resolved surface infrared spectroscopy during atomic layer deposition

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.

Quantum cascade laser-based measurement of metal alkylamide density during atomic layer deposition.

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.

In Situ Time-Resolved Attenuated Total Reflectance Infrared Spectroscopy for Probing Metal–Organic Framework Thin Film Growth

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.

Measurements of Metal Alkylamide Density During Atomic Layer Deposition Using a Mid-Infrared Light-Emitting Diode (LED) Source

A nondispersive infrared (NDIR) gas analyzer that utilizes a mid-infrared light emitting diode (LED) source was demonstrated for monitoring the metal alkylamide compound tetrakis(dimethylamido) titanium (TDMAT), Ti[N(CH3)2]4. This NDIR gas analyzer was based on direct absorption measurement of TDMAT vapor in the C–H stretching spectral region, a spectral region accessed using a LED with a nominal emission center wavelength of 3.65 μm. The sensitivity of this technique to TDMAT was determined by comparing the absorbance measured using this technique to the TDMAT density as determined using in situ Fourier transform IR (FT-IR) spectroscopy. Fourier transform IR spectroscopy was employed because this technique could be used to (1) quantify TDMAT density in the presence of a carrier gas (the presence of which precludes the use of a capacitance manometer to establish TDMAT density) and (2) distinguish between TDMAT and other gasphase species containing IR-active C–H stretching modes (allowing separation of the signal from the LED-based optical system into fractions due to TDMAT and other species, when necessary). During TDMAT-only delivery, i.e., in the absence of co-reactants and deposition products, TDMAT minimum detectable molecular densities as low as ≈∼4 × 1012 cm−3 were demonstrated, with short measurement times and appropriate signal averaging. Reactions involving TDMAT often result in the evolution of the reaction product dimethylamine (DMA), both as a thermal decomposition product in a TDMAT ampoule and as a deposition reaction product in the deposition chamber. Hence, the presence of DMA represents a significant potential interference for this technique, and therefore, the sensitivity of this technique to DMA was also determined by measuring DMA absorbance as a function of pressure. The ratio of the TDMAT sensitivity to the DMA sensitivity was determined to be ≈∼6.0. To further examine the selectivity of this technique, measurements were also performed during atomic layer deposition (ALD) of titanium dioxide using TDMAT and water. During ALD, potential interferences were expected from the evolution of DMA due to deposition reactions and the deposition on the windows of species containing IR-active C–H stretching modes. It was found that the interfering effects of the evolution of DMA and deposition of species on the windows corresponded to a maximum of only ≈∼6% of the total observed TDMAT density. However, this level of interference likely is relatively low compared to a typical chemical vapor deposition process in which co-reactants are introduced into the chamber at the same time.

In situ infrared spectroscopy during La2O3 atomic layer deposition using La(iPrCp)3 and H2O

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.

Evaluation of Silicon Wafer-Based Internal Reflection Elements for Use with in Situ Fourier Transform Infrared (FT-IR) Spectroscopy

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.

Nondispersive Infrared Gas Analyzer for Vapor Density Measurements of a Carbonyl-Containing Organometallic Cobalt Precursor:

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.

Modified RCA clean transfer of graphene and all-carbon electronic devices fabrication

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.