W.B. de Boer

On the optimization of SiGe-base bipolar transistors

Extensive computer simulations of NPN SiGe-base bipolar transistors were performed to examine the effect of the Ge profile in the electrical characteristics. It is shown that extra charge storage in the emitter-base (E-B) junction, caused by the Ge profile, affects the device performance considerably. In addition, it is shown that an abrupt Ge profile in the middle of the base region is optimal for a given critical layer thickness of approximately 600A.

High-Efficiency Silicon Photodiode Detector for Sub-keV Electron Microscopy

A silicon photodiode detector is presented for use in scanning electron microscopy (SEM). Enhanced imaging capabilities are achieved for sub-keV electron energy values by employing a pure boron (PureB) layer photodiode technology to deposit nanometer-thin photosensitive anodes. As a result, imaging using backscattered electrons is demonstrated for 50-eV electron landing energy values. The detector is built up of several closely packed photodiodes, and to obtain high scanning speed, each photodiode is engineered with low series resistance and low capacitance values. The low capacitance (<; 3 pF/mm2) is facilitated by thick, almost intrinsically-doped epitaxial layers grown to achieve the necessarily wide depletion regions. For the low series resistance, diode metallization has been patterned into a conductive grid directly on top of the nanometer-thin PureB-layer front-entrance window. Finally, a through-wafer aperture in the middle of the detector is micromachined for flexible positioning in the SEM system.

Versatile silicon photodiode detector technology for scanning electron microscopy with high-efficiency sub-5 keV electron detection

A new silicon electron detector technology for Scanning Electron Microscopy, based on ultrashallow p+n boron-layer photodiodes, features nm-thin anodes enabling low-energy electron detection with record-high sensitivity down to 200 eV. Designs with segmented, closely-packed photodiodes and through-wafer apertures allow flexible configurations for optimal material and/or topographical contrasts. A high scanning speed is obtained by growing a well-controlled, lightly-doped, tens-of-microns-thick epi-layer for low capacitance, and by patterning a conductive grid directly on the photosensitive surface for low series resistance.

Pure-boron chemical-vapor-deposited layers: A new material for silicon device processing

This paper places focus on the special properties of pure boron chemical-vapor deposition (CVD) thin-film layers that, in several device applications, have recently been shown to augment the potentials of silicon device integration. Besides forming a reliable an efficient dopant source for both ultrashallow and deep p+n junctions, the deposited amorphous boron (α-B) layer itself, even for sub-nm thicknesses, is instrumental in suppressing minority electron injection from the n-region into the p+ contact. Therefore, even for nm-shallow junctions where the current levels mainly will approach high Schottky-like values, the diodes exhibit saturation current levels that can become as low as that of conventional deep junctions. Moreover, the α-B layer has chemical etch properties that make it particularly suitable for integration as the front-entrance window in photodiodes for detecting nm-low-penetration-depth radiation and charged particles.

Deep p + junctions formed by drive-in from pure boron depositions

This paper presents a new method of supplying the high doses of boron needed for creating several micron deep p+n junctions. Chemical vapor deposition (CVD), in a Si/SiGe epitaxial reactor, of nanometer-thick pure boron layers is used to fabricate 5 μm deep p+n junctions. The 10 min B deposition is combined with a 195 min drive-in at 1100°C to give a resulting sheet resistance of 3.1 Ω/sq. For as-deposited B-layers in windows through an silicon dioxide isolation to the Si substrate, reactions of the Si with oxide at the perimeter of the deposited windows will be enhanced by the presence of the B-layer during the high-temperature drive-in. Detrimental effects such as lateral contact window widening, small surface defects and/or large spikes formation, are avoided by capping the surface of the windows with either thermal oxide in a selective process or a low-pressure CVD (LPCVD) oxide during the drive-in. A good electrical quality of the oxide capping layer was achieved. The surface morphology was investigated by atomic force and scanning electron microscopy (AFM/SEM) analysis and found to depend on the overall method of fabrication.

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

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.

Extremely ultrashallow junctions for a high-linearity silicon-on-glass RF varactor-diode technology

A review is given of the junction technology used to realize and preserve special high-doped, abrupt doping profiles for silicon-on-glass high-frequency, high-linearity varactors. Three advanced doping techniques are used: (i) reduced-pressure chemical- vapor deposition (CVD) Si-epitaxy with sophisticated control of arsenic doping during deposition, (ii) pure boron atmospheric-pressure CVD for formation of an extremely ultrashallow p+n diode, and (iii) excimer laser annealing of implanted arsenic to form ultrashallow n+ regions.

Applications of PureB and PureGaB ultrashallow junction technologies

A review is given of present and potential applications of pure dopant deposition of boron and gallium integrated as the p+-region in p+n ultrashallow junctions. Pure B (PureB) layers have been applied in several large area Si diode applications where nm-shallow junctions are required: high-linearity, high-quality varactor diodes for RF adaptive circuits and photodiode detectors for low-penetration-depth beams such as extreme/ vacuum/deep-ultraviolet (EUV, VUV, DUV) light and low-energy electrons. The integration of these types of detectors in CMOS is discussed along with some points that may make the pure dopant depositions attractive for source/drain fabrication in advanced pMOS transistors. Pure Ga capped with pure B (PureGaB) layers have been demonstrated as the p+-region in p+n Ge-on-Si diodes that are sensitive to infrared wavelengths (> 1 μm) both in avalanche and Geiger mode.