Andy Singh

Application of synchrotron radiation to TXRF analysis of metal contamination on silicon wafer surfaces

Synchrotron Radiation based Total Reflection X-ray Fluorescence (TXRF) has been shown to meet the critical needs of the semiconductor industry for the analysis of transition metal impurities on silicon wafer surfaces. The current best detection limit achieved at the Stanford Synchrotron Radiation Laboratory (SSRL) for Ni is 8 x 10 7 atoms/cm 2 which is a factor of 50 better than what can be achieved using laboratory based sources. SSRL has established a TXRF facility which meets the cleanliness and stability requirements of the semiconductor industry. This has enabled both industrial and academic researchers to address industrially relevant problems. In addition research is being carried out for the analysis of light elements such as Al and Na.

Zirconia-germanium interface photoemission spectroscopy using synchrotron radiation

An ultrathin zirconia gate dielectric had been successfully incorporated into germanium metal-oxide-semiconductor (MOS) devices demonstrating very high-permittivity gate stacks with no apparent interfacial layer. In this study, synchrotron-radiation photoemission spectroscopy has been applied on the same gate stack to identify and quantify the presence of any interfacial germanium suboxide layer. By taking progressive core-level spectra during the layer-by-layer removal of the zirconia film, an oxidized germanium layer with submonolayer thickness was found, possibly arising from an interfacial Zr–O–Ge bonding configuration. In addition, the offsets in the valence-band spectra were also monitored and the energy-band diagram of the zirconia–germanium heterostructure was constructed. Compared to high-κ gate stacks on Si, the thinner interfacial layer and larger conduction-band offset in high-κ gate stacks on Ge suggest better scalability towards an ultimately higher MOS gate capacitance.

Aluminum impurities in silicon: Investigation of x-ray Raman scattering in total reflection x-ray fluorescence spectroscopy

Total reflection x-ray fluorescence using synchrotron radiation from the Stanford Synchrotron Radiation Laboratory has been used to study Al impurities on Si wafer surfaces. For primary excitation energies below the Si K absorption edge an inelastic resonance scattering due to resonant x-ray Raman scattering is observed. This scattering dominates the background behavior of the Al K fluorescence line, and consequently limits the achievable sensitivity for detection of Al surface contaminants. The energy and angle dependence of the resonant x-ray Raman scattering has been investigated to determine the experimental conditions for which the highest sensitivity for Al can be achieved. We find that for a precise determination of the achievable sensitivity, the specific shape of the continuous Raman background has to be taken into account. Our calculations demonstrate a minimum detection limit for Al of 6×109 atoms/cm2 for a 10 000 s count time.

Detection and characterization of trace element contamination on silicon wafers

Increasing the speed and complexity of semiconductor integrated circuits requires advanced processes that put extreme constraints on the level of metal contamination allowed on the surfaces of silicon wafers. Such contamination degrades the performance of the ultrathin SiO2 gate dielectrics that form the heart of the individual transistors. Ultimately, reliability and yield are reduced to levels that must be improved before new processes can be put into production. It should be noted that much of this metal contamination occurs during the wet chemical etching and rinsing steps required for the manufacture of integrated circuits and industry is actively developing new processes that have already brought the metal contamination to levels beyond the measurement capabilities of conventional analytical techniques. The measurement of these extremely low contamination levels has required the use of synchrotron radiation total reflection x‐ray fluorescence (SR‐TXRF) where sensitivities 100 times better than conve...

X-ray Absorption Spectroscopy on Copper Trace Impurities on Silicon Wafers

Trace metal contamination during wet cleaning processes on silicon wafer surfaces is a detrimental effect that impairs device performance and yield. Determining the chemical state of deposited impurities helps in understanding how silicon surfaces interact with chemical species in cleaning solutions. However, since impurity concentrations of interest to the semiconductor industry are so low, conventional techniques such as x-ray photoelectron spectroscopy cannot be applied. Nonetheless, chemical information on trace levels of contaminants can be determined with x-ray absorption near edge spectroscopy (XANES) in a grazing incidence geometry. In this study, silicon samples were dipped in ultra pure water (UPW) and 2% hydrofluoric (HF) solutions with copper concentrations of 5 and 1000 ppb, respectively. These samples were then analyzed using XANES in fluorescence yield mode to determine the oxidation state of deposited copper contaminants. It was found that copper impurities on the silicon surface from HF solution were metal in character while copper impurities deposited from the spiked UPW solution were deposited as an oxide. These results show that XANES can provide information on the chemical state of trace impurities even at surface concentrations below a few thousandths of a monolayer.

Determination of copper nanoparticle size distributions with total reflection X-ray fluorescence spectroscopy

Total reflection X-ray fluorescence (TXRF) analysis is extensively used by the semiconductor industry for measuring trace metal contamination on silicon surfaces. In addition to determining the quantity of impurities on a surface, TXRF can reveal information about the vertical distribution of contaminants by measuring the fluorescence signal as a function of the angle of incidence. In this study, two samples were intentionally contaminated with copper in non-deoxygenated and deoxygenated ultrapure water (UPW) resulting in impurity profiles that were either atomically dispersed in a thin film or particle-like, respectively. The concentration profile of the samples immersed into deoxygenated UPW was calculated using a theoretical concentration profile representative of particles, yielding a mean particle height of 16.1 nm. However, the resulting theoretical profile suggested that a distribution of particle heights exists on the surface. The fit of the angular distribution data was further refined by minimizing the residual error of a least-squares fit employing a model with a Gaussian distribution of particle heights about the mean height. The presence of a height distribution was also confirmed with atomic force microscopy measurements.

The Nucleation and Growth of Cu Nanoclusters on Silicon Surfaces

Due to the recent adoption of copper interconnect technology by the semiconductor industry, there has been great interest in understanding the kinetics and mechanisms of copper metal deposition on silicon wafer surfaces in ultra pure water (UPW) solutions. To study the kinetics of the copper deposition mechanism on silicon surfaces, silicon [100] samples were immersed in non‐deoxygenated and deoxygenated UPW solutions contaminated with a copper concentration of 100 ppb with dipping times ranging from 5 to 300 seconds and then measured using total reflection x‐ray fluorescence (TXRF) at the Stanford Synchrotron Radiation Laboratory (SSRL). By measuring the Cu fluorescence signal as function of angle of incidence of the incoming x‐rays, it was possible to ascertain whether the deposited copper was atomically dispersed or particle‐like in nature. It was established that in non‐deoxygenated UPW, the copper is incorporated atomically into the silicon surface oxide as a copper oxide, while in deoxygenated UPW, ...

Surface trace element characterization of synthetic single crystal Al2O3 at the SSRL

Detailed surface trace impurity element analyses (Ti, Cr, Fe, Co, Ni, Cu, and Zn) have been performed on synthetic sapphire grown by the HEM™ technique. Both bulk and surface measurements, yielding information on the spatial distribution of the elements are needed as part of our effort to obtain correlations between optical absorption at 1064 nm and trace element concentrations. Transition metal elements (e.g. Ti, Cr, and Fe) are known to produce absorption in sapphire and therefore were the focus of our study. We report results from our use of total reflection x‐ray fluorescence (TXRF) to determine trace transition elements on the surface of synthetic sapphire. The measurements show concentrations for Cr at 109 atoms/cm2 to Fe at 1012 atoms/cm2 and are consistent with bulk trace element studies done using instrumental neutron activation analysis (INAA).

Investigation of Na impurities on Si wafer surfaces using TXRF

Synchrotron Radiation from the Stanford Synchrotron Radiation Laboratory (SSRL) has been used as an excitation source for Total Reflection X-ray Fluorescence Analysis (TXRF) of Na impurities on Si wafer surfaces. A wafer intentionally contaminated by a droplet containing 1.4×1014 atoms/cm2 of sodium and a wafer uniformly contaminated with 4.4×1012 atoms/cm2 of Na were investigated. The minimum detection limit for this element has been found to be 1.1×1011 atoms/cm2 for the blanket sample and 3×1011 atoms/cm2 for the droplet sample. Theoretical considerations show that the detection limit for Na can be further improved by at least a factor of 2 by exploiting the tunability of synchrotron radiation to even lower excitation energies.