Characterization of pure boron depositions integrated in silicon diodes for nanometer-deep junction applications (2010)

Doping technologies for formation of ultrashallow and highly-doped p+ junctions are continuously demanded to face the challenges in front-end processing that have emerged due to the aggressive downscaling of vertical dimensions for future semiconductor devices. As an alternative to implantations, current solutions are based on in-situ boron (B) doping during Si/SiGe chemical vapor deposition (CVD) by using diborane (B2H6) as the dopant gas. In this context, a few studies have demonstrated p+-like doping behavior of n-type (100)-oriented Si surfaces after exposure solely to B2H6 in an oxygen-free atmosphere without any extra addition of silane-based sources. As illustrated in Chapter 1, this doping process relies on the thermal decomposition of the source gas, so that the available boron atoms may stick to the surface, chemically react with silicon atoms, and diffuse into the substrate. Contrary to other doping impurities, by appropriately varying the source gas parameters and the exposure time, the reaction kinetics can also cause the boron density at the silicon surface to significantly increase beyond the solid solubility in Si at the given processing temperature. Thus, a boron layer can be formed. However, this property has not been explored so far with respect to reliable integration in Si-based device technologies, since boron segregation has been commonly addressed as a drawback of this doping method. This thesis presents the characterization of nanometer-thick B-layers formed during exposure to diborane in a commercially available CVD system at either atmospheric or reduced pressures down to 500C by using B2H6 at high concentrations. The process, as described in Chapter 2, substantially differs from previous approaches both with respect to the low temperature used and the gas exposure conditions. The former is generally very attractive for versatile use of a doping technology, while the excessive B adsorption intentionally promoted on the Si surface, i.e. the deposition of a B-layer, is demonstrated to offer unprecedented advantages for the formation of ultrashallow and low leakage pn-junctions. Analytical techniques, such as transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS), in conjunction with an extensive electrical characterization are applied to investigate the material and electrical properties of the B-layers as a function of the deposition conditions. The experimental results are also validated by process and device simulations. The formation of B-layers is slower the lower the temperature and the diborane partial pressure, and mainly controlled by the exposure time at high gas flow rates that provide good conditions for segregation of boron atoms on the Si surface. While gas parameters can determine the transition from surface Si doping to B-deposition, the temperature mainly influences the final composition of the deposited film that can vary from amorphous boron (a-B) to a boron-silicon compound, i.e. boron-silicide, (BxSiy), for temperatures increasing from 500C to 800C. The deposition exhibits high selectivity to Si, isotropy, and uniformity for any surface topography and patterning. The time dependence of the B-layer growth is quite linear and a similar grading coefficient is observed for the boron surface density. The chemical reactivity of boron with HNO3-based acid solutions can be used for the removal of the layer. Furthermore, the growing B-film will act as a source for boron thermal diffusion during the CVD process itself, and the crystalline Si substrate is p-doped up to the B solid solubility. The as-diffused active boron density is also shown to be quantitatively controlled by the exposure time. Moreover, both the relatively low deposition temperature and the absence of any defect formation, which could cause enhanced-diffusion effects, ensure junction depths lower than 10 nm even after prolonged depositions. In Chapter 3, the properties of the deposited B-layers are further explored with respect to formation of high-quality, ultrashallow junctions in p+n diode configurations. Ohmic contacts, diodes, and pnp bipolar structures are fabricated and characterized under different B2H6 exposure conditions. As B-deposition is commenced, the Fermi level of the exposed Si surface is rapidly shifted towards the valence band, as one would expect for electrically active p-type doping. This is beneficial for formation of very low-ohmic contacts on p-type surfaces, while pn diodes are formed on n-type Si substrates. In the latter case, the near-ideal saturation current can be tuned from high Schottky-like values to low deep-pn-junction-like values by increasing the deposited B-layer thickness by just a few nm. The integration of B-deposited emitters in pnp structures has shown that the presence of a distinct a-B layer, which occurs for min-long exposures, is an effective way to suppress the electron minority carrier injection from the n-substrate. This results in an effective Gummel number 60 times higher than that of the diffused emitter only. The doping efficiency is also demonstrated to be superior to that in conventional B-doped Si epitaxy and comparable to B+ / BF2+ ion implants. Although for increasing thickness the series resistance through this high-resistive layer will eventually dominate the I-V behavior, processing conditions can be found where exceptionally low values of both series resistance and saturation current can be achieved. However, the high-ohmic property can be used as a means of fabricating very compact, small area, and non-linear resistors. The compatibility of the doping technique with standard Si device manufacturing is also proven. Also for the use of hard-mask materials other than SiO2 the selectivity of the B-layers to deposit only in the contact openings to the Si has been demonstrated. Furthermore, due to the excellent isotropic coverage, the B-deposition can be applied to non-planar device schemes such as trenches and recessed-contact technologies. Although the boron chemical concentration significantly exceeds the solid solubility at the silicon surface, for as-deposited B-layers the active dopants of the c-Si substrate is found to be essentially limited by substitutional incorporation at the deposition temperature. However, in Chapter 4 the B-layer is demonstrated to act as a well-controlled source of dopant for solid-phase diffusion during any subsequent in-situ or ex-situ high-temperature annealing step. The presence of a sufficiently thick B-layer offers, due to its thermal stability, the additional advantage of being able to minimize boron evaporation. Thus, a higher dopant activation can be obtained with a good control of the resulting junction depth. Furthermore, the use of B2H6 exposure and thermal anneal as post-processing gives more insights into the influence of processing parameters on the boron adsorption mechanism. In particular, at very low temperatures, hydrogen termination of the silicon surface is assumed to influence the sticking of boron atoms. In this respect, the carrier gas can play a significant role. Finally, thermal anneals are also applied to increase the doping efficiency of B-layers that act as emitters in pnp bipolar transistors. The unique properties of the boron CVD deposition are exploited with great advantages in p+n diodes fabricated at 700C for the integration of two distinct device technologies: varactor diodes for adaptive functions in radio frequency (RF) applications and photodiodes for detection of ultraviolet (UV) radiation. In the former case, B-deposition offers a defect-free, low-temperature process module within the silicon-on-glass (SOG) substrate-transfer technology to form one-sided p+n-junctions that can preserve hyper-abrupt arsenic profiles needed for highly-linear tunable varactors. On the other hand, the fundamental advantages of the B-layers for use as a novel p+ front-layer in UV detectors are given by the extremely ultrashallow and highly-doped junction that is instrumental in the collection of the photogenerated carriers. Since both devices operate in reverse biasing mode, in Chapter 5 the electrical performance of the B-layers is investigated when reverse voltages are applied up to the expected diode breakdown limit. Device simulations demonstrate that the high electric field induced by the nm-deep p+ junction at the anode contact edges is responsible for band-to-band tunneling current. Thus, the reverse I-V characteristics of as-deposited B-doped varactors would suffer from high leakage current and premature breakdown. Diffused p+ guard-rings are proposed as a solution to reduce the electric field crowding at the contact rim. In addition, n+ channel-stop implants are used to prevent the depletion region from extending excessively under the MOS structure formed by the anode metallization on the SiO2 isolation layer and approach regions with reduced generation-recombination lifetimes. Then, the silicon-on-glass varactor technology is illustrated along with the electrical characterization results. A noteworthy improvement of the reverse I-V performance is shown for varactor implementations with either uniform or hyper-abrupt 1/x^2 As profiles when BF2+ and P+ implants are included in the process to form guard-rings and channel-stop regions, respectively. The devices have unprecedented low reverse current and an increased operating voltage range close to the theoretical breakdown limit. At the same time, the desired capacitance-voltage relationship is still preserved by the compatibility of the thermal processing steps with the As profiles. Chapter 6 describes the integration of B-layers in a silicon-based planar p+n photodiode technology for radiation detection in the complete UV spectral range down to soft X-ray wavelengths. The B2H6 exposure conditions for the formation of the sensitive surface are specifically optimized to minimize any possible quantum efficiency loss d