William A. Kimes

In Situ Gas Phase Diagnostics for Hafnium Oxide Atomic Layer Deposition

Atomic layer deposition (ALD) is an important method for depositing the nanometer-scale, conformal high κ dielectric layers required for many nanoelectronics applications. In situ monitoring of ALD processes has the potential to yield insights that will enable efficiencies in film growth, in the development of deposition recipes, and in the design and qualification of reactors. This report will describe the status of a project to develop in situ diagnostics for hafnium oxide ALD processes. The focus is on an examination of the utility of Fourier transform infrared spectroscopy and diode laser spectroscopy for optimizing deposition conditions, rather than simply monitoring precursor delivery. Measurements were performed in a single-wafer, warmwall, horizontal-flow reactor during hafnium oxide ALD involving tetrakis(ethylmethylamino) hafnium and water. Measurements were performed near the wafer surface under a range of deposition conditions in an effort to correlate gas phase measurements with surface processes.

Time-resolved Fourier transform infrared spectroscopy of the gas phase during atomic layer deposition

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.

Thin-Film Resistance Thermometers on Silicon Wafers

We have fabricated Pt thin-film resistors directly sputtered on silicon substrates to evaluate their use as resistance thermal detectors (RTDs). This technique was chosen to achieve more accurate temperature measurements of large silicon wafers during semiconductor processing. High-purity (0.999 968 mass fraction) platinum was sputter deposited on silicon test coupons using titanium and zirconium bond coats. These test coupons were annealed, and four-point resistance specimens were prepared for thermal evaluation. Their response was compared with calibrated platinum–palladium thermocouples in a tube furnace. We evaluated the effects of furnace atmosphere, thin-film thickness, bond coats, annealing temperature and peak thermal excursion of the Pt thin films. Secondary ion mass spectrometry (SIMS) was performed to evaluate the effect of impurities on the thermal resistance coefficient, α. We present typical resistance versus temperature curves, hysteresis plots versus temperature and an analysis of the causes of uncertainties in the measurement of seven test coupons. We conclude that sputtered thin-film platinum resistors on silicon wafers can yield temperature measurements with uncertainties of less than 1 °C, k = 1 up to 600 °C. This is comparable to or better than commercially available techniques.

Effects of wafer emissivity on rapid thermal processing temperature measurement

Lightpipe radiation thermometers (LPRTs) are widely used to measure wafer temperatures in rapid thermal processing (RTP) tools. Using blackbody-calibrated LPRTs to infer the wafer temperature, it is necessary to build a model to predict the effective emissivity accounting for the wafer and chamber radiative properties as well as geometrical features of the chamber. The uncertainty associated with model-corrected temperatures can be investigated using test wafers instrumented with thin-film thermocouples (TFTCs) on which the LPRT target spot has been coated with films of different emissivity. A model of the wafer-chamber arrangement was used to investigate the effects of Pt (/spl epsiv//sub s/=0.25) and Au (/spl epsiv//sub s/=0.05) spots on the temperature distribution of the test wafers with the emissivity of 0.65 and 0.84. The effects of the shield reflectivity and the cool lightpipe (LP) tip on the wafer temperature were evaluated. A radiance analysis method was developed and a comparison of model-based predictions with experimental observations was made on a 200 mm wafer in the NIST RTP test bed. The temperature rises caused by the low-emissivity spot were predicted and the cooling effect of the LP tip was determined. The results of the study are important for developing the model-corrected temperature measurement and uncertainty estimates using LPRT in semiconductor thermal processes.

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.

Characterization of metal oxide nanofilm morphologies and composition by terahertz transmission spectroscopy

An all-optical terahertz absorption technique for nondestructive characterization of nanometer-scale metal oxide thin films grown on silicon substrates is described. Example measurements of laser-deposited TiO2 and atomic layer-deposited films of HfO2 are presented to demonstrate applicability to pure Y2O3, Al2O3, and VOx and mixed combinatorial films as a function of deposition conditions and thickness. This technique is also found to be sensitive to HfO2 phonon modes in films with a nominal thickness of 5 nm.

Time-resolved surface infrared spectroscopy during atomic layer deposition of TiO2 using tetrakis(dimethylamido)titanium and water

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).

Emissivity compensated pyrometry for specular silicon surfaces on the NIST RTP test bed

Since pyrometric thermometry is a noncontact method, it has great promise as a technique for monitoring silicon wafers during rapid thermal processing (RTP). Absolute values of surface emissivity are required when making pyrometric temperature measurements. One approach to obtaining these values is the use of emissivity compensated pyrometry, where a reflectometer is integrated into the pyrometer to allow real-time emissivity measurement. While this technique has been successfully applied to metal organic chemical vapor deposition (MOCVD) of compound semiconductors, it has not been applied to RTP. Although such measurements require that the surface be a specular reflector, they promise real-time traceable temperature measurements that are independent of the nature of the wafer. Here we discuss measurement of wafer temperature for polished wafers and an initial attempt to measure a patterned wafer during heating inside the RTP test bed at the National Institute of Standards and Technology

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.

Determining the thermal response time of temperature sensors embedded in semiconductor wafers

We present a non-contact method for the determination of the thermal response time of temperature sensors embedded in wafers. In this method, a flash lamp illuminates a spot on the wafer in periodic pulses; the spot is on the opposite side from the sensor under test. The thermal time constant of the sensor is then obtained from measurement of its temporal response, together with a theoretical model of heat flows both into the sensor and laterally within the wafer. Experimental data on both platinum resistance thermometers (PRTs) and on thermocouples embedded in silicon wafers show good agreement with the heat transfer models. Values of the thermal response time for a wide range of experimental parameters agree to within standard deviations of 8% (PRTs) and 20% (thermocouples), demonstrating the self-consistency of our results. The method is directly applicable to determining the thermal properties of sensors used in instrumented silicon wafers. We anticipate that the method will have use in development of new sensor attachment methods, in verifying the proper attachment of sensors during production, and in confirming that the thermal attachment has not degraded with age or thermal cycling. To simplify the application of the method, we have produced a table of calculated relevant quantities to be used in relating the measured signal to the thermal response time.