Si Wafer Surface Research Articles

Argon Ion Beam Current Dependence of Si-Si Surface Activated Bonding.

In order to optimize the process parameters of Si-Si wafer direct bonding at room temperature, Si-Si surface activated bonding (SAB) was performed, and the effect of the argon ion beam current for surface activation treatment on the Si-Si bonding quality was investigated. For the surface activation under the argon ion beam irradiation for 300 s, a smaller ion beam current (10~30 mA) helped to realize a lower percentage of area covered by voids and higher bonding strength. Especially with the surface activation under 30 mA, the bonded Si-Si specimen obtained the highest bonding quality, and its percentage of area covered by voids and bonding strength reached <0.2% and >7.62 MPa, respectively. The transmission electron microscopy analyses indicate that there exists an ultrathin amorphous Si interlayer at the Si-Si bonding interface induced by argon ion beam irradiation to Si wafer surfaces, and its thickness increases as the argon ion beam current rises. The investigation results can be used to optimize the SAB process and promote the applications of SAB in the field of semiconductor devices.

Micro-crack defects detection of semiconductor Si-wafers based on Barker code laser infrared thermography

Micro-crack defects are easy to form in the semiconductor Si-wafer surface during the production process, which finally effect the quality of silicon based microelectronic products. In order to ensure the quality and performance of products, it is important and necessary to carry out nondestructive testing (NDT) for Si-wafers. This paper experimentally investigates the performance of the Barker code laser infrared thermography (BCLIT) technique for the semiconductor Si-wafer defect detection, using an optical infrared thermography set-up in transmission method. The thermal wave signal response of the laser to the Barker code modulation waveform is collected by an infrared camera, which was stored as image sequences. Then the detectability was studied according to processed results via the Total Harmonic Distortion (THD) algorithm from obtained image sequence. In addition, linear frequency modulation thermal wave imaging (LFMTWI) technique was used to compare and analyze the performance of BCLIT. Experiments performed on the Si-wafer under the low power density condition indicate that the higher of the excitation power and the wider of the crack width, the better the detection effect. Furthermore, the performance of BCLIT is better than the traditional LFMTWI with all cases in evaluation index result of signal-to-noise ratio (SNR). The research results show that the BCLIT technique significantly improve SNR and defect detectability, and it can be used for the semiconductor Si-wafers micro-crack defects detection even under low power density excitation.

Study of Cryogenic Unmasked Etching of "Black Silicon" with Ar Gas Additives.

The influence of Ar gas additives on ≪black silicon≫ formation is shown in this work. The way to achieve the conical shape of Si texture using low Ar dilution is demonstrated. Also, a possibility of silicon nanowire width reduction keeping a high density of array is shown. No damage to the Si structure caused by Ar plasma was detected. The introduction of Ar into the plasma also does not affect electrical properties. The lifetime value after cryogenic etching with 5 sccm Ar flow remains at the same level of 0.7 ms. The resulting black silicon has a low total reflectance of 1 ± 0.5% in the range of 450–1000 nm in the overall 100 mm Si wafer surface.

Effects of High-Pressure Hydrogen Gas Exposure on the Residual Stress Fields and Cracks around Vickers Indentations

Effects of high-pressure hydrogen gas exposure on the residual stress fields and cracks around the Vickers indentations introduced into {100} silicon wafers were investigated. Square chip test pieces (10 × 10 × 0.6 mm3) were cut out from a mirror-finished (001) silicon wafer that was sliced from a large crystal (diameter 300 mm, resistivity 15 Ωcm) grown using the Czochralski (CZ) method. Vickers indentations were introduced on the (001) Si wafer surface. Loads of indentations were 0.98 N and 1.96 N, and the holding time was 30 s. Upon indentation, not only was a large internal stress field formed around the indent, but small cracks were also initiated along the {110} planes from the four corners of the indent. A laser Raman microscope was used to measure the residual stress distribution before and after the hydrogen gas exposure, with four hydrogen gas pressures (from 10 to 100 MPa) applied at 270 °C for 24 h. Crack growth occurred after the hydrogen gas exposure. The crack growth ratio increased up to a hydrogen gas pressure of 35 MPa, with increasing gas pressure. However, for the hydrogen pressures above 35 MPa, the crack growth ratio was saturated, and the upper limit of the crack growth ratio was approximately 9%. Furthermore, it was found that the hydrogen gas exposure caused the compressive residual stress around the indentation to decrease. The results suggest that hydrogen may reduce the residual stress field around the crack, which is related to one of the causes of hydrogen embrittlement.

P-type poly-Si/SiO contact by aluminium-induced crystallization of amorphous silicon

In this work, we explored aluminium-induced crystallization (AIC) of amorphous Si (a-Si) as a low-temperature alternative to make p-type poly-Si/SiOx contacts for Si solar cells. In this approach, a stack of Al and a-Si deposited on a SiOx-passivated Si wafer surface is annealed at temperatures below 577 °C (the eutectic temperature of Al–Si mixture), which induces the exchange of Al and Si layer positions in the stack and the simultaneous crystallization of a-Si to Al-doped p-type poly-Si, potentially leading to the formation of an Al/poly-Si/SiOx passivating contact. The Al-doped AIC poly-Si has low carrier concentration, and thus to compensate for this, we used highly B-doped a-Si for AIC and could achieve a tenfold increase in the carrier concentration. We found that the typical thickness of 1.5 nm of the passivating SiOx layer is too low for this AIC-based approach, as Al reduces SiOx during the process and degrades its passivation quality. By starting with a thicker SiOx layer, we could demonstrate that AIC poly-Si/SiOx contacts with low contact recombination is achievable. However, the contact characteristic remains non-ohmic. This can be improved by extended annealing after AIC, during which Al diffuses into the Si substrate. The Al diffusion, however, damages the SiOx layer and in turn severely increases contact recombination. We conclude that with AIC poly-Si/SiOx contacts the process window to achieve low contact resistivity and low contact recombination seems to be very narrow, if existing at all.

Density Functional Theory Study on Dependence of Stability of Fe, Cu, and Ni Atoms on Surface Orientation of Si Crystal

The deep levels related to metal impurities degrade the yield of the complementary metal oxide semiconductor (CMOS) image sensors due to the increase in white pixels. Regarding the gettering technology, the stability of major contaminated metals, such as Fe, Ni, and Cu atoms near Si (100) wafer surface was clarified. However, recent high-performance devices including the latest CMOS image sensors involve complex surface structures including (110) and (111) surfaces. In this study, density functional theory calculations were performed to investigate the dependence of the stability of Fe, Ni, and Cu atoms on Si surface orientation. Our key findings are as follows. (1) The stability of Fe, Ni, and Cu atoms is in the order of (110) > (111) > (100) Si surface. (2) The stability of each metal is in the order of Cu > Fe > Ni regardless of surface orientation. (3) Fe, Ni, and Cu are unstable in the ultra-thin SiO2 film formed on the surface. These results suggest that controlling metal contamination, e.g., by using a chemical solution that removes the oxide film on the (110) and (111) surface, is very important in the manufacturing process for CMOS image sensors. In addition, thermal oxidation should be performed immediately when the Si surface is exposed.

Theoretical study of stress impact on formation enthalpy and thermal equilibrium concentration of impurities and dopants in Si single crystal

In Si-based VLSI manufacturing processes, complicated plane stress is applied near the (001) surface of Si wafers as device feature sizes further shrink. For this technical trend, the control of impurities including contamination metals and light elements as well as dopants becomes more and more important to improve device performance. In this study, density functional study calculations were performed to evaluate the dependence of the formation energy (Ef), volume change (ΔVol), and formation enthalpy (Hf) of impurities and dopants on isotropic or plane stress up to 2 GPa. Thirteen metal atoms (Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Hf, Ta, and W), which are concerned in LSI processes, 4 light elements (H, C, N, and O), which exist normally in CZ-Si wafers, and 3 dopants (B, P, and As), which are generally used for Si, were considered. It was found that the dominant factor in the change of Hf was the ΔVol. On the basis of calculated results, we classified impurities and dopants according to the stress that increases the thermal equilibrium concentration (Ceq) in Si crystal. Furthermore, the stress dependence of the Hf of impurities and dopants in Si was formulated. The results obtained in the present work should be a useful database for controlling the behavior of impurities and dopants in stressed Si crystal.

Simulation of Nitrogen Indiffusion and Its Impact on Silicon Wafer Strength

Herein, the concentration profiles of nitrogen after nitriding rapid thermal annealing (RTA) of Si wafer surfaces are simulated. A model describing the fundamental partial differential equations for diffusion and interaction of nitrogen, vacancies, interstitials, and nitrogen vacancy (NV) complexes is used for this thermal process and the equations are solved simultaneously. In addition, thermal stress tests via intentionally induced stress due to the temperature gradient in an RTA process are conducted. By these wafer strength tests, which apply a controlled stress level, the relative slip robustness of wafers with nitrogen‐indiffused RTA and polished wafers are compared quantitatively.

Zinc-hyperdoped silicon nanocrystalline layers prepared via nanosecond laser melting for broad light absorption

Current techniques for hyperdoping of Si wafer surface by femtosecond laser or ion implantation have encountered several problems for device fabrication, such as severe lattice defects, shallow doping depth, poor electrical activity of impurities, difficult collection of charge carriers, or weak compatibility with present complementary metal–oxidesemiconductor processes. To provide a new way to solve these problems, in this study, Zn-hyperdoped Si layers were prepared on polycrystalline Si substrates by vacuum evaporation and nanosecond (ns) laser melting under different conditions of deposition and laser treatment. As a result, the hyperdoped Si layers of up to micron-level thickness with a broad light absorption (above 90% from the ultraviolet to the visible range and up to 54% for the near-infrared spectrum) and good electrical transport properties (electron mobility of up to 88.5 cm2⋅V−1s−1 and sheet resistance of ~ 100 Ω⋅square−1) were obtained. These hyperdoped Si layers can be flexibly superimposed on the existing Si optoelectronic devices: Such tandem structure avoids the damage of substrate and provides both the layers and substrate properties. In addition, the variation in the microstructural characteristics of the Zn-hyperdoped Si layers consisting of surface and cross-section structures, Zn impurity distribution and concentration, and crystallinity were carefully studied to clarify the relationships between their preparation conditions, microstructures, and physical properties. The results show that a matching between the thickness of Si-Zn deposited layer and the ns-laser irradiation fluence is crucial to achieve a full melting process and to obtain excellent Zn-hyperdoped Si layers. Their near-infrared absorption and electrical transport properties are determined by the combined effects of surface antireflection, Zn-hyperdoping, and nanocrystallization.

Etching characteristics of si wafer thinning in HF/H2O binary solution for microelectronic and nanopackaging applications

The wet etching of silicon surfaces by the use of acidic or fluoride solutions is of both technological and fundamental significance, which is essentially to be applied to produce a reliable silicon chip at desired thickness for microelectronic packaging. In this work, we have investigated the wet etching effect on thickness dissipation, weight loss, etching rate, surface morphology and crystalline nature of Si wafer immersed in the 48 % HF/water solution. The etch rate was ascertained from the variation of weight loss and depth etched against time. The results show that the thickness reduction and weight loss of silicon increases as the etching time increases. Roughened surface is observed on the etched Si wafer surface under the high-resolution optical microscope. From the XRD analysis, it shows that the crystalline peak intensity of silicon becomes weaker after etching, implying the reduced light scattering from the formed amorphous structure surface on the Si substrate. After all, this finding can be valuably referred to produce a reliable Si thin wafer, which is crucial in thinner microelectronic devices fabrication and nanopackaging, and in turn reduces environmental pollution and energy consumption for future sustainability.

Selective rounding for pyramid peaks and valleys improves the performance of SHJ solar cells

AbstractThe first step in the fabrication of SHJ (silicon heterojunction) solar cells is forming random pyramids on the Si wafer surfaces. The pyramid profile would affect the following amorphous silicon deposition. Two CP (chemical polishing) solutions consist of HNO3 + HF and O3 + HF, respectively, are used to modify the pyramid profile. It is found that these two solutions have selective rounding effects on the pyramid profiles. The first one rounds the pyramid valleys and the second one rounds the pyramid peaks. The combination of the two CP methods improves the passivation quality of amorphous silicon and promotes the conversion efficiency of SHJ cells.

Dry Phosphorus silicate glass etching and surface conditioning and cleaning for multi-crystalline silicon solar cell processing

As an alternative to the wet chemical etching method, dry chemical etching processes for Phosphorus silicate glass [PSG} layer rel11ova] using Tri?uormethane/Sulfur Hexafluoride (CHF3/ SF6) gas mixture in commercial silicon-nitride plasma enhanced chemical vapour deposition (SiN-PECVD) system is applied. The dependence of the solar cell performance on the etching temperature is investigated and optimized. It is found that the SiN-PECVD system temperature variation has a signi?cant impact on the whole solar cell characteristics. A dry plasma cleaning treatment of the Si wafer surface after the PSG removal step is also investigated and developed. The cleaning step is used to remove the polymer ?lm which is formed during the PSG etching using both oxygen and hydrogen gases. By applying an additional cleaning step, the polymer ?lm deposited on the silicon wafer surface after PSG etching is eliminated. The effect of different plasma cleaning conditions on solar cell performance is investigated. After optimization of the plasma operating conditions, the performance of the solar cell is improved and the overall gain in ef?ciency of 0.6 % absolute is yielded compared to a cell without any further cleaning step. On the other hand, the best solar cell characteristics can reach values close to that achieved by the conventional wet chemical etching processes demonstrating the effectiveness of the additional O2/H2 post cleaning treatment.

Integration of Fe3O4 nanospheres and micropyramidal textured silicon wafer with improved photoelectrochemical performance

Silicon (Si)-based composites have attracted extensive attention for photoelectrochemical (PEC) application. Herein, micropyramidal textured Si wafer was constructed by wet chemical etching method. Hydrothermally synthesized Fe3O4 nanospheres were further deposited on the micropyramids by a simple dip-coating approach. The integrated Fe3O4 nanospheres and Si micropyramids (denoted as Fe3O4@SiMPs) revealed 20 times of higher PEC efficiency than planar Si wafer, without obvious photocurrent decay and crystalline structure change during chronoamperometry test. The greatly enhanced PEC performance of Fe3O4@SiMPs is largely attributed to the seamless integration of Si micropyramids and Fe3O4 nanospheres. The micropyramidal textured structure can enhance light absorption by reducing the surface reflectance and enhancing the light trapping effect. The micropyramids and Fe3O4 nanospheres on the surface of planar Si wafer offer more specific surface area for the contact of electrode with electrolyte. The Fe3O4 nanospheres not only protect the micropyramids from corrosion, but also accelerate the PEC kinetics by promoting charge transfer between the electrode and the electrolyte. This study can inspire the optimal design of Si wafer-based nanocomposites as efficient PEC catalysts for overall water splitting.

Influence of surface structure on the performance of mono-like Si PERC solar cell

Mono-like Si wafers were textured by both alkaline solution and metal-assisted chemical etching (MACE) process. The influences of surface textures on the diffusion process, surface passivation, electrical property and performance of solar cells were investigated. The results show that the pyramid-like structures obtained by alkaline texturing facilitates a higher surface doping concentration, larger junction depth after diffusion and better passivation effect, compared with the honeycomb-like structures generated by MACE texturing. In the same wafer textured by alkaline solution or MACE, the mono-crystalline region exhibits a higher quantum efficiency than the multi-crystalline region. By combining the MACE and alkaline texturing processes, the honeycomb- and pyramid-like structures were generated on the surface of mono-like Si wafer, resulting in a conversion efficiency of 21.39% that is higher than that of the individually MACE or alkaline-textured solar cells by 0.4% and 0.02%, respectively.

3D impurity profiles of doped/intrinsic amorphous-silicon layers composing textured silicon heterojunction solar cells detected by atom probe tomography

Laser-assisted atom probe tomography was used to identify the impurity distribution in Si heterojunction (SHJ) solar cells composed of thin doped/intrinsic amorphous Si layers on the textured surface of a crystalline Si wafer. A site-specific lift-out technique involving a focused ion beam enabled the selection of a ∼2 × 2 μm2 area on an arbitrary pyramidal surface. The distributions of B, P and C in the amorphous Si layers introduced by p-type (trimethyl-borane or diborane) or n-type (phosphine) dopant gases were investigated. Standard guidelines for the assessment of the H content in amorphous Si of SHJ solar cells were provided.

Different Cr Contents on the Microstructure and Tribomechanical Properties of Multi-Layered Diamond-Like Carbon Films Prepared by Unbalanced Magnetron Sputtering

To investigate the effects of Cr doping on the microstructure and tribological properties of multi-layered diamond-like carbon (DLC) films, Cr-DLC films with Cr contents ranging from 0 to 22 at.% were deposited on 0Cr19Ni10 stainless steel and Si wafer surfaces by unbalanced magnetron sputtering. The thickness of the transition layer is 180 nm and the Cr-DLC film ranging from 1.3 to 1.5 µm. The microstructure, mechanical and tribological properties of the films were systematically investigated at room temperature in atmospheric environment. Results show that increasing the Cr target current can effectively improve the incident particle density and reduce surface roughness. As a strong carbon metal, Cr has significant influence on the structure and properties of the film for bonding with carbon atoms during deposition. High-energy Cr particles are favorable for films to increase the sp3 hybrid bond ratio and reduce internal stress. The high-hardness sp3 hybrid phase, Cr carbide, oxide dispersion can effectively improve the mechanical properties of the film, but overdoping is detrimental for mechanical properties of the film. The internal stress shows a continuous trend of decreasing with the increasing Cr content and it reduced up to 80% compared with pure DLC films. The films prepared with Cr sputtering current of 0.4 A demonstrate the best tribological properties, and the friction coefficient and wear rate are 0.15 and 2.9 × 10−7 mm3/Nm, respectively.

Elimination of blind zone in nanoparticle removal on silicon wafers using a double-beam laser shockwave cleaning process

Laser shockwave cleaning (LSC) has been gaining increasing attention due to its advantages in decontaminating various micron- and nano-scale contaminated particles on silicon (Si) wafer surfaces. However, the particles in a blind zone right under the laser-induced plasma, the origin of the shockwave, cannot be removed efficiently, thereby impeding the conventional LSC process in industrial wafer cleaning applications. Here, we demonstrate a simple but intriguing method of eliminating the blind zone in nanoparticle removal on Si wafers by employing a double-beam laser shockwave cleaning process (DLSC). The size of the uncleaned blind zone in DLSC is reduced by 98% compared to the traditional single-beam LSC (SLSC) process with a laser pulse energy of 150 mJ and a gap distance between the laser focus and the wafer of 1 mm. The time-resolved, laser-induced plasma evolution reveals that the steady single-laser-beam-induced plasma splits into multiple plasma kernels distributed around the focus point due to the disruption from the second laser beam. The formation of multiple plasma kernels makes it possible to expose the particles in the blind zone to the cleaning-effective zone of the shockwave from a nearby passing plasma. The angle between the two laser beams plays a critical role in eliminating the blind zone. The comparison of the particle removal force between DLSC and SLSC suggests that the horizontal drag force from a nearby passing plasma acts on the particles in the blind zone and facilitates their removal.

Sub-surface modifications in silicon with ultra-short pulsed lasers above 2 µm

Nonlinear optical phenomena in silicon such as self-focusing and multi-photon absorption are strongly dependent on the wavelength, energy, and duration of the exciting pulse, especially for wavelengths &gt; 2 µ m . We investigate the sub-surface modification of silicon using ultra-short pulsed lasers at wavelengths in the range of 1950–2400 nm, at a pulse duration between 2 and 10 ps and pulse energy varying from 1 µJ to 1 mJ. We perform numerical simulations and experiments using fiber-based lasers built in-house that operate in this wavelength range for the surface and sub-surface processing of Si-wafers. The results are compared to the literature data at 1550 nm. Due to a dip in the nonlinear absorption spectrum and a peak in the spectrum of the third-order nonlinearity, the wavelengths between 2000 and 2200 nm prove to be more favorable for creating sub-surface modifications in silicon. This is the case even though those wavelengths do not allow as tight focusing as those at 1550 nm. This is compensated for by an increased self-focusing due to the nonlinear Kerr-effect around 2100 nm at high light intensities, characteristic for ultra-short pulses.

Inverted Si:PbS Colloidal Quantum Dot Heterojunction-Based Infrared Photodetector.

Silicon and PbS colloidal quantum dot heterojunction photodetectors combine the advantages of the Si device and PbS CQDs, presenting a promising strategy for infrared light detecting. However, the construction of a high-quality CQDs:Si heterojunction remains a challenge. In this work, we introduce an inverted structure photodetector based on n-type Si and p-type PbS CQDs. Compared with the existing normal structure photodetector with p-type Si and n-type PbS CQDs, it has a lower energy band offset that provides more efficient charge extraction for the device. With the help of Si wafer surface passivation and the Si doping density optimization, the device delivers a high detectivity of 1.47 × 1011 Jones at 1540 nm without working bias, achieving the best performance in Si/PbS photodetectors in this region now. This work provides a new strategy to fabricate low-cost high-performance PbS CQDs photodetectors compatible with silicon arrays.

Stability, geometry and electronic properties of BHn (n = 0 to 3) radicals on the Si{0 0 1}3 × 1:H surface from first-principles

A new generation of radiation detectors relies on the crystalline Si and amorphous B (c-Si/a-B) junctions that are prepared through chemical vapor deposition of diborane (B2H6) on Si at low temperature (~400 °C). The Si wafer surface is dominated by the Si{0 0 1}3 × 1 domains that consist of two different Si species at low temperature. Here we investigate the geometry, stability and electronic properties of the hydrogen passivated Si{0 0 1}3 × 1 surfaces with deposited BHn (n = 0 to 3) radicals using parameter-free first-principles approaches. Ab initio molecular dynamics simulations using the density functional theory (DFT) including van der Waals interaction reveal that in the initial stage the BH3 molecules/radicals deposit on the Si(–H), forming (–Si)BH4 radicals which then decompose into (–Si)BH2 with release of H2 molecules. Structural optimizations provide strong local relaxation and reconstructions at the deposited Si surface. Electronic structure calculations reveal the formation of various defect states in the forbidden gap. This indicates limitations of the presently used rigid electron-counting and band-filling models. The attained information enhances our understanding of the initial stage of the PureB process and the electric properties of the products.