Agata Sakic

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.

Optical stability investigation of high-performance silicon-based VUV photodiodes

Silicon p+n photodiodes fabricated by a pure boron deposition technology are evaluated for detection in the Vacuum Ultra-Violet (VUV) spectral range from 115 nm to 215 nm wavelengths, where the attenuation length in silicon is only a few nanometers. It is demonstrated that the junctions formed by the B-layer process are so ultrashallow that the photodiodes are able to provide a very good sensitivity, in the order of 0.1 A/W, in the whole VUV range. Moreover, the demonstrated stability also appears to be very good if isolating layers on the diode surface are avoided.

Silicon photodiodes for high-efficiency low-energy electron detection

Solid-state electron detectors have been fabricated using a p+n silicon photodiode where the p+ region is created by a chemical-vapor deposition (CVD) surface doping from diborane B 2 H 6 . The as-obtained nm-deep p-type layer is resistant to conventional metal etchants, which allows elimination of both entrance contacts and protection layers from the photosensitive surface. This approach lowers the dead layer energy loss, while keeping near theoretical efficiency at high electron energies. The photodiodes have outstanding performance in terms of electron signal gain at low energies achieving 60% and 74% of the theoretical gain value at 500 eV and 1 keV, respectively. The ideal I–V characteristics and the small over-the-wafer spread of the dark current indicate a defect-free p+n junction, as well as a reliable and reproducible process.

Series resistance optimization of high-sensitivity si-based VUV photodiodes

Recently, silicon ultrashallow p+ n photodiodes, fabricated by a pure boron deposition technology (B-layer diodes), were evaluated for detection in the Vacuum Ultra-Violet (VUV) spectral range from 115 nm to 215 nm wavelengths, where the attenuation length in silicon is only a few nanometers. Superior sensitivity in the order of 0.1 A/W in the whole VUV spectral range was reported [1]. Next to the sensitivity, another important parameter of any photodetector is the response time, which is directly related to its series resistance. In this work a study of the relation between the sensitivity and the series resistance of the B-diodes is presented, supported by simulation results and optical/electrical experimental results. Moreover, practical methods for designing a high sensitivity VUV photodiodes while keeping a relatively low series resistance, are proposed. The experimental results demonstrate that by modifying the diode structure, the series resistance can be effectively reduced. At the same time, the B-layer diodes still maintain a high VUV sensitivity.

Arsenic-doped high-resistivity-silicon epitaxial layers for integrating low-capacitance diodes

An arsenic doping technique for depositing up to 40-μm-thick high-resistivity layers is presented for fabricating diodes with low RC constants that can be integrated in closely-packed configurations. The doping of the as-grown epi-layers is controlled down to 5 × 1011 cm−3, a value that is solely limited by the cleanness of the epitaxial reactor chamber. To ensure such a low doping concentration, first an As-doped Si seed layer is grown with a concentration of 1016 to 1017 cm−3, after which the dopant gas arsine is turned off and a thick lightly-doped epi-layer is deposited. The final doping in the thick epi-layer relies on the segregation and incorporation of As from the seed layer, and it also depends on the final thickness of the layer, and the exact growth cycles. The obtained epi-layers exhibit a low density of stacking faults, an over-the-wafer doping uniformity of 3.6%, and a lifetime of generated carriers of more than 2.5 ms. Furthermore, the implementation of a segmented photodiode electron detector is demonstrated, featuring a 30 pF capacitance and a 90 Ω series resistance for a 7.6 mm2 anode area.

PureB low-energy electron detectors with closely-packed photodiodes integrated on locally-thinned high-resistivity silicon

A Pure Boron (PureB) photodiode technology is developed for low-energy (down to 200 eV) electron detectors, and implemented in a versatile production-ripe process for highspeed detectors for Scanning Electron Microscopy (SEM). It is here investigated with respect to transfer from the existing low-resistivity-silicon (LRS) process to a locally-thinned high-resistivity-silicon (HRS) process. In this way a lower capacitance, i.e., a wider low-doped region, can be achieved. A trade-off must be made to meet the demands of optimized SEM imaging: the larger lateral depletion of each photodiode can be in conflict with the requirement to have the detector divided into arrays of several electrically-separated closely-packed photodiodes. Here a 37% reduction in total capacitance is achieved, with junction capacitance of <; 1 pF/mm2, while detection efficiency, series resistance, dark current, and packing density are kept within specifications.

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.

Epitaxial growth of large-area p + n diodes at 400 ºC by Aluminum-Induced Crystallization

Aluminum-Induced Crystallization is applied on crystalline Si substrates. By this method a physical-vapor-deposited amorphous Si layer is successfully transformed into a monocrystalline solid-phase epitaxy (SPE) p-doped layer at an anneal temperature of 400°C. The as-grown epitaxial layer takes on the orientation and the lattice constant of the substrate. It is shown that a complete coverage over large areas is possible if the c-Si interface is free of nucleation centers. This can be achieved by the proper oxide-patterning and/or chemical treatments of the substrate surface before deposition of the Al mediator layer. High-quality p+n diodes have been fabricated with areas up to 1×1 cm2, having ideality factors down to 1.02 and low leakage currents in the 2-3 nA/cm2 range. The full coverage by p+ SPE-Si is confirmed by material analysis.