V. Mohammadi

Robust UV/VUV/EUV PureB Photodiode Detector Technology With High CMOS Compatibility

This paper gives an assessment of old and new data relevant to the optical and electrical performance of PureB photodiodes for application in the wavelength range 2 nm to 400 nm. The PureB layer, fabricated by depositing pure boron on Si, forms the anode region of devices that function as p+n junction diodes. The results show that the high sensitivity and high stability of the PureB diodes is related to the integrity of the interface with the Si. When measures are taken to retain a complete PureB coverage, thermal processing steps with minute long exposure to temperatures up to 900 °C do not compromise the robustness and a lower-than-ideal but still high responsivity is maintained. Besides the thermal processing considerations, other aspects that impact the integration of PureB in CMOS are reviewed.

VUV/Low-Energy Electron Si Photodiodes With Postmetal 400

Pure boron (PureB) chemical-vapor deposition performed at 400°C is applied as a postmetalization process module to fabricate near-ideal p+n photodiodes with nm-thin PureB-only beam-entrance windows. The photodiodes have near-theoretical sensitivity and high stability for optical characterization performed with either UV light down to a wavelength of 270 nm or low-energy electrons down to 200 eV.

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In this paper, an analytical model is established to describe the deposition kinetics and the deposition chamber characteristics that determine the deposition rates of pure boron (PureB-) layers grown by chemical-vapor deposition (CVD) from diborane (B2H6) as gas source on a non-rotating silicon wafer. The model takes into consideration the diffusion mechanism of the diborane species through the stationary boundary layer over the wafer, the gas phase processes and the related surface reactions by applying the actual parabolic gas velocity and temperature gradient profiles in the reactor. These are calculated theoretically and also simulated with fluent software. The influence of an axial and lateral diffusion of diborane species and the validity of the model for laminar flow in experimental CVD processes are also treated. This model is based on a wide range of input parameters, such as initial diborane partial pressure, total gas flow, axial position on the wafer, deposition temperature, activation energy of PureB deposition from diborane, surface H-coverage, and reactor dimensions. By only adjusting these reactor/process parameters, the model was successfully transferred from the ASM Epsilon One to the Epsilon 2000 reactor which has totally different reactor conditions. The models predictive capabilities have been verified by experiments performed at 700 °C in these two different ASM CVD reactors.

PureB Deposition

Surface reaction mechanisms are investigated to determine the activation energies of pure boron (PureB) layer deposition at temperatures from 350 °C to 850 °C when using chemical-vapor deposition from diborane in a commercial Si/SiGe epitaxial reactor with either hydrogen or nitrogen as carrier gas. Three distinguishable regions are identified to be related to the dominance of specific chemical reaction mechanisms. Activation energies in H2 are found to be 28 kcal/mol below 400 °C and 6.5 kcal/mol from 400 °C to 700 °C. In N2, the value decreases to 2.1 kcal/mol for all temperatures below 700 °C. The rate of hydrogen desorption is decisive for this behavior.

An analytical kinetic model for chemical-vapor deposition of pureB layers from diborane

Resistance measurement structures are designed for monitoring thickness variations in nanometer-thin pure-boron (PureB) layers deposited on Si for (photo-)diode applications where angstrom-level variations have an impact on performance. In millimeter-large windows a fine resolution is achieved with metal-contact arrays patterned directly on the PureB. For micron-sized windows Kelvin structures provide a sensitive solution.

Temperature dependence of chemical-vapor deposition of pure boron layers from diborane

Integrated resistors are fabricated by using pure boron (PureB) depositions to create a p-type conductive layer on n-type silicon. Sheet resistance values in the 100 kΩ/□ range are reliably and reproducibly realized. The resistors made in this material are linear and display low temperature coefficients of a few hundred ppm/°C and good tolerances.

Thickness evaluation of deposited pureb layers in micro-/millimeter-sized windows to Si

In industrial applications, particularly in vacuum ultraviolet applications and low-energy electron detection systems, a periodic surface cleaning of the used photon/electron detectors is required to prevent the buildup of carbon contaminating layers [1-3]. Such applications can be found in synchrotron measurements, space payload equipment, next-generation extreme-ultraviolet (EUV) lithography and high-resolution Scanning Electron Microscopes (SEMs). One effective way to remove the carbon contamination is to use aggressive gasses, such as hydrogen radicals (H*) and oxygen plasma [1-3]. In previous publications we have reported the excellent optical and electrical performance of silicon-based PureB photodiodes produced by high-temperature (HT, 700°C) pure boron chemical vapor deposition (CVD) [12-16]. Also the stability of these HT PureB photodiodes under hydrogen radicals cleaning and oxygen plasma cleaning is reported [4, 5]. Recently, a low-temperature (LT, 400°C) PureB CVD process has been introduced, which is fully CMOS compatible [17]. In this work, a review study is presented of the effect of detrimental environment, particularly related to H* and oxygen plasma cleaning, on the performance of the both HT and LT PureB-diodes.

High-ohmic resistors using nanometer-thin pure-boron chemical-vapour-deposited layers

The pattern dependency of pure-boron (PureB) layer chemicalvapor depositions (CVD) is studied with respect to the correlation between the deposition rate and features like loading effects, deposition parameters and deposition window sizes. It is shown experimentally that the oxide coverage ratio and the size of windows to the Si on patterned wafers are the main parameters affecting the deposition rate. This is correlated to the gas depletion of the reactant species in the stationary/low-velocity boundary layer over the wafer. An estimation of the radius of gas depletion for Si openings and/or diffusion length of diborane in this study yields lengths in the order of centimeters, which is related to the boundary layer thickness. The deposition parameters; pressure and flow rates are optimized to minimize the pattern dependency of the PureB deposition rates.

Stability characterization of high-performance PureB Si-photodiodes under aggressive cleaning treatments in industrial applications

In this paper, optimization of the process flow for PureB detectors is investigated. Diffusion barrier layers between a boron layer and the aluminum interconnect can be used to enhance the performance and visual appearance of radiation detectors. Few nanometers-thin Zirconium Nitride (ZrN) layer deposited by reactive sputtering in a mixture of Ar/N2, is identified as a reliable diffusion barrier with better fabrication process compatibility than others. The barrier properties of this layer have been tested for different boron layers deposited at low and high temperatures with extensive optical microscopy analyses, electron beam induced current, SEM, and electrical measurements. This study demonstrated that spiking behavior of pure Al on Si can be prevented by the thin ZrN layer thus improving the performance of the radiation detectors fabricated using boron layer.

Pattern Dependency of Pure-Boron-Layer Chemical-Vapor Depositions

The interest in nanostructures of silicon and its dopants has significantly increased. We report the creation of an ultimately-shallow junction at the surface of n-type silicon with excellent electrical and optical characteristics made by depositing an atomically thin boron layer at a relatively low temperature where no doping of silicon is expected. The presented experimental results and simulations of the ab initio quantum mechanics molecular dynamics prove that the structure of this new type of junction differs from all other known rectifying junctions at this time. An analysis of the junction formation has led to the conclusion that the chemical interaction between the surface atoms of crystalline silicon and the first atomic layer of the as-deposited amorphous boron is the dominant factor leading to the formation of a depletion zone in the crystalline silicon which originates from the surface. The simulation results show a very strong electric field across the c-Si/a-B interface systems where the charge transfer occurs mainly from the interface Si atoms to the neighboring B atoms. This electric field appears to be responsible for the creation of a depletion zone in the n-silicon resulting in a rectifying junction-formation between the n-silicon and the atomically thin boron layer.