Epitaxial Silicon Wafers Research Articles

Optimization of bulk microdefect performance in epitaxial silicon wafer

Bulk microdefects (BMDs) in epitaxial silicon wafers are pivotal for advanced node integrated circuits, offering enhanced mechanical strength and metal gettering capabilities. This study introduces two heat treatment procedures to optimize BMD density and radial uniformity in lightly doped wafers, addressing the challenges associated with large-diameter crystal growth and the thermal budget constraints of advanced node processes. We demonstrate that Rapid Thermal Annealing (RTA) at temperatures exceeding 1175 °C significantly improves BMD uniformity. A post-RTA stabilization step, when performed prior to epitaxial growth, enhances BMD density by retaining nuclei generated by RTA. Conversely, when the RTA with stabilization step follows epitaxial growth, a higher BMD density and larger BMD size are achieved. Light Scattering Tomography (LST) analysis confirms the optimal conditions for these treatments. The findings contribute to the semiconductor industry by optimizing wafer properties for high-performance ICs.

Epitaxial silicon transition zone measurements by spreading resistance profiling and Fourier transform infrared reflectometry

AbstractSilicon epitaxy is an essential building block in the manufacturing of complementary metal‐oxide semiconductor (CMOS) devices. Accurate determination of epitaxial layer thickness is indispensable for a uniform and reproducible process. In this paper, we compare thickness values of the transition zone (TZ) in silicon epitaxial wafers obtained by two of Semilab's production‐compatible electrical and optical characterization techniques: Fourier‐transform infrared (FTIR) reflectometry and spreading resistance profiling (SRP). We demonstrate a high correlation between TZ thicknesses obtained from the optical modeling of FTIR reflectance spectra and SRP profiles. The dependence of TZ thickness change on the high‐temperature annealing steps is also examined. FTIR reflectometry thus offers a quick, contactless alternative for obtaining structural parameters of an epitaxial layer, and these values can be well matched to those given by SRP.

Hydrogen Termination Effect on SiO2/Si Interface State Density in CH3O-Molecular-Ion-Implanted Silicon Epitaxial Wafer for CMOS Image Sensors

The reduction in SiO2/Si interface state density (Dit) at the SiO2/Si interface region is important to improve the performance of complementary metal-oxide semiconductor (CMOS) image sensors. The CH3O-ion-implanted region stores hydrogen and releases the stored hydrogen during the subsequent heat treatment. This study demonstrates that a CH3O-ion-implanted epitaxial silicon wafer can reduce the Dit and Pb0 center density in SiO2/Si interface regions, as analyzed by quasi-static capacitance–voltage and electron spin resonance measurements, respectively. Both Dit and Pb0 center density in the CH3O-implanted wafer decreased with increasing heat treatment temperature. Moreover, the activation energy is estimated to be 1.57 eV for the hydrogen termination reactions induced by the CH3O-ion-implanted wafer. The activation energy is close to those of hydrogen molecules and Si dangling bonds at the SiO2/Si interface. This result means that Dit can be reduced by hydrogen from inside the silicon wafer, regardless of the heat treatment atmosphere. It has unique characteristics not found in conventional silicon wafers. The termination effect of the CH3O-molecular-ion-implanted epitaxial silicon wafers can contribute to the high electrical performance of CMOS image sensors.

Preparation of thin-film SOI wafer by low-dose ion implantation

Silicon-on-insulator (SOI) devices have many advantages, such as high speed, low energy consumption, radiation-hard, and high integration. In this paper, the separation by implanted oxygen process under low-dose implantation conditions is studied by the two-step implantation method combined with the internal thermal oxidation process. The effects of different types of silicon wafers and different implantation doses on SOI surface defects, top Si thickness, buried oxide (BOX) layer thickness, BOX layer breakdown voltage, and top Si defect density were investigated. Ultra-thin SOI wafers are prepared by epitaxial silicon wafers and control the first implantation dose. The number of surface defects of SOI materials is less than 100 counts, the breakdown voltage of the BOX layer is about 7.8 MV/cm, and the top Si dislocation density is about 8 × 103 cm−2.

Thermal Shrinkage Behavior of CH3O-Multielement-Molecular-Ion-Implantation-Induced Dislocation Loops Studied by Real-Time Transmission Electron Microscopy Observation

We investigated the thermal behavior of dislocation loops formed in a CH3O-multielement-molecular-ion-implanted epitaxial silicon (Si) wafer by real-time cross-sectional TEM observation with in situ heating. We found that the CH3O-ion-implantation-induced faulted Frank dislocation loops (FDLs) shrink at a low rate at the beginning of heat treatment (1st stage), and then the shrinkage rate rapidly increased (2nd stage), resulting in the dissolution of the defects. The activation energies for the shrinkage of FDLs in the 1st and 2nd stages (E D-1 and E D-2) were found to be 2.94 ± 0.31 and 4.95 ± 0.25 eV, respectively. The shrinkage behavior in the 1st stage is the desorption of C and O atoms that segregated along the edge of an FDL because of the interaction between the CH3O-ion-implantation-induced FDL and the segregated impurities. On the other hand, the 2nd stage corresponds to the desorption of Si atoms from FDLs and its migration. Compared to our previous study on the shrinkage behavior of CH4N-ion-implantation-induced FDLs (J. Electrochem. Soc. 169, 047521 (2022)), E D-2 of the CH3O-ion-implantation-induced FDLs is almost the same as that of the CH4N-ion-implantation-induced FDLs, while the values of E D-1 are quite different. The difference between the E D-1 values of CH3O- and CH4N-ion-implantation-induced FDLs is suggested to be the difference of the kind of segregated impurities. Our experimental results suggest that thermal stability of the dislocation loop is determined by the kind of segregated impurities around the dislocation loop.

Reduction of White Spot Defects in CMOS Image Sensors Fabricated Using Epitaxial Silicon Wafer with Proximity Gettering Sinks by CH2P Molecular Ion Implantation.

Using a new implantation technique with multielement molecular ions consisting of carbon, hydrogen, and phosphorus, namely, CH2P molecular ions, we developed an epitaxial silicon wafer with proximity gettering sinks under the epitaxial silicon layer to improve the gettering capability for metallic impurities. A complementary metal-oxide-semiconductor (CMOS) image sensor fabricated with this novel epitaxial silicon wafer has a markedly reduced number of white spot defects, as determined by dark current spectroscopy (DCS). In addition, the amount of nickel impurities gettered in the CH2P-molecular-ion-implanted region of this CMOS image sensor is higher than that gettered in the C3H5-molecular-ion-implanted region; and this implanted region is formed by high-density black pointed defects and deactivated phosphorus after epitaxial growth. From the obtained results, the CH2P-molecular-ion-implanted region has two types of complexes acting as gettering sinks. One includes carbon-related complexes such as aggregated C-I, and the other includes phosphorus-related complexes such as P4-V. These complexes have a high binding energy to metallic impurities. Therefore, CH2P-molecular-ion-implanted epitaxial silicon wafers have a high gettering capability for metallic impurities and contribute to improving the device performance of CMOS image sensors. (This manuscript is an extension from a paper presented at the 6th IEEE Electron Devices Technology & Manufacturing Conference (EDTM 2022)).

In Situ Transmission Electron Microscopy Study of Shrinkage Kinetics of CH4N-Molecular-Ion-Implantation-Induced Extended Defects

The thermal stability of end-of-range (EOR) defects formed in a CH4N-molecular-ion-implanted epitaxial silicon (Si) wafer was studied by transmission electron microscopy (TEM) observation. By plan-view TEM observation, we found that the density and size of the CH4N-ion-implantation-induced EOR defects negligibly changed upon heat treatment at temperatures below 1000 °C, whereas the EOR defect density was drastically reduced by heating at 1100 °C. This result suggests that almost all CH4N-ion-implantation-induced EOR defects were sufficiently thermally stable to maintain their size at temperatures below 1000 °C, and that above 1100 °C, most of the EOR defects lost their stability, shrank and finally dissolved. Additionally, by in situ cross-sectional TEM observation during heat treatment, we found a large difference in the shrinkage rates of the EOR defects between at the beginning of heat treatment and the last minute of just before defect disappearance. We found that the EOR defects began to gradually shrank at the beginning of heat treatment (1st stage), and then the shrinkage rate rapidly increased (2nd stage), finally resulting in the dissolution of the defects. The activation energies for the shrinkage of EOR defects in the 1st and 2nd stages (E D-1 and E D-2) were found to be 7.55 ± 1.03 and 4.57 ± 0.32 eV, respectively. The shrinkage behavior in the 1st stage is likely to be due to the thermally activated desorption of C and N species that segregated along the edge of an EOR defect. On the other hand, from the E D-2 value, the shrinkage behavior in the 2nd stage is deduced to be due to the desorption of interstitial Si atoms. These findings suggest that this two-stage shrinkage behavior is peculiar to the EOR defects formed in the CH4N-ion-implanted epitaxial Si wafer, and that the interaction between the EOR defect and the impurities segregated at the edge of the defect affects the thermal robustness of the molecular-ion-implantation-induced EOR defects.

Silicon Wafer Gettering Design for Advanced CMOS Image Sensors Using Hydrocarbon Molecular Ion Implantation: A Review

We have developed silicon epitaxial wafers with high gettering capability using hydrocarbon molecular ion implantation for advanced Complementary Metal-Oxide-Semiconductor (CMOS) image sensors. These wafers have three unique silicon wafer characteristics for improvement of CMOS device electrical parameter such as high metallic impurity gettering, oxygen out-diffusion barrier effects from Czochralski silicon (CZ) substrate and hydrogen passivation effect for interface state defect at Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . We demonstrate that double epitaxial growth silicon wafers have an extremely high gettering capability during CMOS device fabrication process. We also found that gettering capability strongly dependence on oxygen impurity amount in hydrocarbon molecular ion implantation projection range. We believe that this novel silicon wafer can drastically contribute to the improvement of CMOS image sensor device performance such as white spot defect and dark current.

Internal and External Gettering of Iron Contamination in Power Technologies

Substrate gettering and the contribution to the contamination reduction of backside layers are evaluated after high‐temperature annealing, following intentional iron contamination at the backside in silicon epitaxial wafers typical of power technologies. Iron is detected at the frontside by deep‐level transient spectroscopy within a depth corresponding to the actual devices. Herein, contamination occurring at the beginning of the semiconductor process flow is simulated and the role of the different gettering mechanisms is isolated. Substrate boron in epitaxial p over p+ wafers is effective in more than halving iron contamination at the front compared with a p‐only substrate, especially at high contamination doses. In the absence of a specific thermal cycle for oxygen precipitation and growth in the bulk, long thermal treatment at 1200 °C induces a significant precipitate growth even in the beginning of the process, greatly contributing to iron gettering in the bulk. However, this effect is found to strongly depend on the silicon condition after crystal growth. No significant contribution to iron gettering from a backside polycrystalline silicon layer is found after high‐temperature annealing, whereas a backoxide acts as diffusion barrier, effectively screening the substrate from contamination only for short thermal treatments or annealing temperature below 1000 °C.

Reduction of Dark Current in CMOS Image Sensor Pixels Using Hydrocarbon-Molecular-Ion-Implanted Double Epitaxial Si Wafers.

The impact of hydrocarbon-molecular (C3H6)-ion implantation in an epitaxial layer, which has low oxygen concentration, on the dark characteristics of complementary metal-oxide-semiconductor (CMOS) image sensor pixels was investigated by dark current spectroscopy. It was demonstrated that white spot defects of CMOS image sensor pixels when using a double epitaxial silicon wafer with C3H6-ion implanted in the first epitaxial layer were 40% lower than that when using an epitaxial silicon wafer with C3H6-ion implanted in the Czochralski-grown silicon substrate. This considerable reduction in white spot defects on the C3H6-ion-implanted double epitaxial silicon wafer may be due to the high gettering capability for metallic contamination during the device fabrication process and the suppression effects of oxygen diffusion into the device active layer. In addition, the defects with low internal oxygen concentration were observed in the C3H6-ion-implanted region of the double epitaxial silicon wafer after the device fabrication process. We found that the formation of defects with low internal oxygen concentration is a phenomenon specific to the C3H6-ion-implanted double epitaxial wafer. This finding suggests that the oxygen concentration in the defects being low is a factor in the high gettering capability for metallic impurities, and those defects are considered to directly contribute to the reduction in white spot defects in CMOS image sensor pixels.

Hydrogen passivation for reduction of SiO2/Si interface state density using hydrocarbon-molecular-ion-implanted silicon wafers

The reduction in the density of SiO2/Si interface state (Dit) in the isolation region and transfer transistor gate oxide is necessary to improve the performance of complementary metal-oxide semiconductor (CMOS) image sensors. In this study, we demonstrated that a hydrocarbon-molecular-ion-implanted epitaxial silicon wafer can reduce the Dit and Pb0 center density in SiO2/Si interface regions analyzed by quasi-static capacitance–voltage and electron spin resonance measurements, respectively. The Dit and Pb0 center density of wafers without hydrocarbon molecular ions increased after annealing at 700 °C. On the other hand, the Dit and Pb0 center density of wafers implanted with hydrocarbon molecular ions decreased after annealing at 700 °C. We also estimated the activation energy to be 1.67 eV for the hydrogen termination reactions with hydrogen molecules and Si dangling bonds at the SiO2/Si interface. The termination effects of the hydrocarbon-molecular-ion-implanted epitaxial silicon wafers can contribute to the high electrical performance of CMOS image sensors.

Fundamental Characteristics of Cyanide‐Related Multielement Molecular Ion‐Implanted Epitaxial Si Wafers for High‐Performance CMOS Image Sensors

Fundamental characteristics such as metal‐gettering capability and defect morphology of a cyanide‐related multielement molecular (CH4N) ion‐implanted epitaxial silicon (Si) wafer are investigated. It is found that the CH4N ion‐implanted epitaxial Si wafer has a higher gettering capability for transition metallic impurities than a hydrocarbon molecular (C3H5) ion‐implanted epitaxial Si wafer. This higher metal‐gettering capability of the CH4N ion‐implanted epitaxial Si wafer may be due to the formation of stacking faults as well as carbon (C) agglomeration defects in the CH4N ion‐implanted region during the epitaxial Si layer growth process. The formation of stacking faults may be a specific phenomenon in the case of the CH4N ion implantation. In addition, the CH4N ion‐implanted region can trap high concentrations of hydrogen (H) and nitrogen (N) atoms during the epitaxial Si growth. This fact suggests that the distribution of N atoms is strongly associated with the defect morphology and C distribution in the CH4N ion‐implanted region. The CH4N ion‐implanted epitaxial Si wafer with these characteristics has the potential to improve the performance of complementary metal‐oxide‐semiconductor (CMOS) image sensors.

Molecular and Atomic Hydrogen Diffusion Behavior by Reaction Kinetic Analysis in Projection Range of Hydrocarbon Molecular Ion for CMOS Image Sensors

In this study, two types of hydrogen diffusion behavior in the projection range of a hydrocarbon molecular ion after high‐temperature heat treatment for the passivation of interface state defects at SiO2/Si interface of the CMOS image sensor is presented. The hydrogen peak concentration in the hydrocarbon ion projection range is observed by secondary ion mass spectrometry analysis after silicon epitaxial growth and the subsequent high‐temperature heat treatment. Moreover, the hydrogen peak concentration strongly depends on heat treatment time after epitaxial growth and the subsequent heat treatment. Two dissociation activation energies by reaction kinetic analysis using the results of the time dependence of hydrogen diffusion behavior are also determined. Assuming two dissociation reactions, the activation energy from the projection range of a hydrocarbon molecular ion is derived. The results of reaction kinetic analysis show that the dissociation activation energies are 0.79 eV for molecular hydrogen and 0.42 eV for atomic hydrogen. The dissociation activation energy of 0.79 eV indicates molecular hydrogen diffusion activation energy, whereas that of 0.42 eV indicates atomic hydrogen diffusion from the projection range of a hydrocarbon molecular ion. Therefore, it is believed that the molecular and atomic hydrogen diffusion behavior of the hydrocarbon molecular ion implanted silicon epitaxial wafers can contribute to the effective reduction in the density of interface state defects at the SiO2/Si interface.

Proximity Gettering Design of Hydrocarbon⁻Molecular⁻Ion⁻Implanted Silicon Wafers Using Dark Current Spectroscopy for CMOS Image Sensors.

We developed silicon epitaxial wafers with high gettering capability by using hydrocarbon–molecular–ion implantation. These wafers also have the effect of hydrogen passivation on process-induced defects and a barrier to out-diffusion of oxygen of the Czochralski silicon (CZ) substrate bulk during Complementary metal-oxide-semiconductor (CMOS) device fabrication processes. We evaluated the electrical device performance of CMOS image sensor fabricated on this type of wafer by using dark current spectroscopy. We found fewer white spot defects compared with those of intrinsic gettering (IG) silicon wafers. We believe that these hydrocarbon–molecular–ion–implanted silicon epitaxial wafers will improve the device performance of CMOS image sensors.

Well structure engineering to improve the responsivity of p-on-n SiPM developed at KAIST-NNFC

This paper describes the electrical and optical properties of the p-on-n silicon photomultiplier (SiPM) developed at KAIST-NNFC. In particular, we present a comparison of the new and old versions in terms of several SiPM characteristics to highlight the improvements achieved through internal structure engineering. The new-version sensors were fabricated on a 200 mm n-doped epitaxial silicon wafer with an abrupt p+/n junction structure identical to that used in the old-version sensors using 0.18μm CMOS technology. Based on the previous work, several changes were applied to the improved sensors, such as rapid thermal processing (RTP) conditions and ion implantation conditions for junction formation. In the work, we demonstrate that the reverse current is reduced by more than a factor of 104 with the modified RTP conditions. Furthermore, we show that the breakdown voltage is decreased by nearly 20% and the PDE in the blue-light regime is enhanced by nearly a factor of two as a result of internal structure engineering.

Gettering mechanism in hydrocarbon-molecular-ion-implanted epitaxial silicon wafers revealed by three-dimensional atom imaging

The interaction of iron (Fe) with defects induced by a high hydrocarbon-molecular-ion-implantation dose of 1 × 1016 cm−2 in a Czochralski-grown silicon substrate and an epitaxial growth layer was investigated using secondary ion mass spectroscopy, transmission electron microscopy, and laser-assisted atom probe tomography (L-APT). High-dose hydrocarbon-molecular-ion-implantation formed two types of defects in the projection range: stacking faults and carbon agglomerates. It was demonstrated that the dominant gettering mechanisms of the two types of defects differ. Carbon agglomerates formed by implantation of the epitaxial growth layer exhibited high gettering efficiency for Fe. The L-APT data indicated that the Fe gettering efficiency is strongly affected by the distribution of oxygen atoms in carbon agglomerates. It is suggested that Fe gettering on agglomerates is due to the strong electronic interaction between carbon agglomerates and Fe but suppressed by oxygen atoms in agglomerates.

Comparison on mechanical properties of heavily phosphorus- and arsenic-doped Czochralski silicon wafers

Heavily phosphorus (P)- and arsenic (As)-doped Czochralski silicon (CZ-Si) wafers generally act as the substrates for the epitaxial silicon wafers used to fabricate power and communication devices. The mechanical properties of such two kinds of n-type heavily doped CZ silicon wafers are vital to ensure the quality of epitaxial silicon wafers and the manufacturing yields of devices. In this work, the mechanical properties including the hardness, Young’s modulus, indentation fracture toughness and the resistance to dislocation motion have been comparatively investigated for heavily P- and As-doped CZ-Si wafers. It is found that heavily P-doped CZ-Si possesses somewhat higher hardness, lower Young’s modulus, larger indentation fracture toughness and stronger resistance to dislocation motion than heavily As-doped CZ-Si. The mechanisms underlying this finding have been tentatively elucidated by considering the differences in the doping effects of P and As in silicon.

Kerfless epitaxial silicon wafers with 7 ms carrier lifetimes and a wide lift-off process window

Silicon wafers contribute significantly to the photovoltaic module cost. Kerfless silicon wafers that grow epitaxially on porous silicon (PSI) and are subsequently detached from the growth substrate are a promising lower cost drop-in replacement for standard Czochralski (Cz) wafers. However, a wide technological processing window appears to be a challenge for this process. This holds in particularly for the etching current density of the separation layer that leads to lift-off failures if it is too large or too low. Here we present kerfless PSI wafers of high electronic quality that we fabricate on weakly reorganized porous Si with etch current densities varying in a wide process window from 110 to 150 mA/cm2. We are able to detach all 17 out of 17 epitaxial wafers. All wafers exhibit charge carrier lifetimes in the range of 1.9 to 4.3 ms at an injection level of 1015 cm−3 without additional high-temperature treatment. We find even higher lifetimes in the range of 4.6 to 7.0 ms after applying phosphorous gettering. These results indicate that a weak reorganization of the porous layer can be beneficial for a large lift-off process window while still allowing for high carrier lifetimes.

Design and Performance of a Pinned Photodiode CMOS Image Sensor Using Reverse Substrate Bias.

A new pinned photodiode (PPD) CMOS image sensor with reverse biased p-type substrate has been developed and characterized. The sensor uses traditional PPDs with one additional deep implantation step to suppress the parasitic reverse currents, and can be fully depleted. The first prototypes have been manufactured on an 18 µm thick, 1000 Ω·cm epitaxial silicon wafers using 180 nm PPD image sensor process. Both front-side illuminated (FSI) and back-side illuminated (BSI) devices were manufactured in collaboration with Teledyne e2v. The characterization results from a number of arrays of 10 µm and 5.4 µm PPD pixels, with different shape, the size and the depth of the new implant are in good agreement with device simulations. The new pixels could be reverse-biased without parasitic leakage currents well beyond full depletion, and demonstrate nearly identical optical response to the reference non-modified pixels. The observed excessive charge sharing in some pixel variants is shown to not be a limiting factor in operation. This development promises to realize monolithic PPD CIS with large depleted thickness and correspondingly high quantum efficiency at near-infrared and soft X-ray wavelengths.

Gettering Sinks for Metallic Impurities Formed by Carbon-Cluster Ion Implantation in Epitaxial Silicon Wafers for CMOS Image Sensor

Gettering sinks for metallic impurities formed by carbon-cluster ion implantation in epitaxial silicon wafers have been investigated using technology computer-aided design and atom probe tomography (APT). We found that the defects formed by carbon-cluster ion implantation consist of carbon and interstitial silicon clusters (carbon-interstitial clusters). Vacancy-type clusters are not dominant gettering sinks for metallic impurities in the carbon-cluster ion implanted region. APT data indicated that the distribution of oxygen atoms in the defects differs between Czochralski-grown silicon and epitaxial silicon wafers. The high gettering efficiency observed in carbon-cluster ion implanted epitaxial silicon wafers in comparison with Czochralski-grown silicon wafers is due to the distribution of oxygen atoms in the defects. Defects not containing O atoms provide strong gettering sinks for metallic impurities. These defects are formed by only carbon-interstitial clusters. Oxygen atoms inside the defects modify the amount of carbon-interstitial cluster formation on the defects. It is suggested that the gettering efficiency for metallic impurities in carbon-cluster ion implanted epitaxial silicon wafer is determined by the amount of carbon-interstitial clusters.