poly-Si Etching Research Articles

Low-damage etching of poly-Si and SiO2 via a low-energy electron beam in inductively coupled CF4 plasma

Abstract Electron-assisted etching of poly-Si and SiO2 is performed via a grid system in inductively coupled CF4 plasma. The feasibility of electron-assisted etching is discussed with a focus on the low-surface damage of the etching. The etch rate increases with electron beam energy, which indicates that the electrons assist the surface etching process. To verify this, etching of poly-Si and SiO2 is performed in several plasma conditions, which leads to differences in etch rate that depend on the presence or absence of radicals and electron beams. Poly-Si and SiO2 are not etched without radicals of CF4 plasma, but they are etched when such radicals are present. When the electron beam and radicals exist simultaneously, the etch rate increases more dramatically than in the case of a CF4 plasma without an electron beam, demonstrating that the electron beam assists the etching process. Optical emission spectroscopy is employed to verify the F radical does not affect the etch rate increase. The surface roughness is measured after electron-assisted etching and compared with the surface roughness after ion-assisted etching.

Unveiling polycrystalline silicon channel dissolution mechanism in wet etching process of 3D NAND fabrication

3D NAND is one of the most promising storage devices due to its high storage density and low energy consumption. Selective wet etching of Si3N4 by hot-concentrated phosphoric acid (PA) solution is a key process for 3D NAND fabrication. However, the channel material, polycrystalline silicon (poly-Si), is unexpectedly dissolved due to its inevitable exposure to the PA etching solution, which causes device failure and thus limits the stacking density of 3D NAND. Herein, to avoid the detrimental dissolution of poly-Si, we unveil the mechanism of poly-Si dissolution in PA solution from kinetic and thermodynamic perspectives. We find that the etch process shows a two-stage reaction, in which H2O plays a key role in lowering the activation barrier for poly-Si etching. Further thermodynamic analysis demonstrates that poly-Si prefers to be oxidized by H2O to suboxide rather than to SiO2. Combined with surface chemistry characterization, we propose the dissolution mechanism as a two-stage process involving the dissolution of suboxide interface and bulk poly-Si, where the nucleophilic substitution of Si–H with Si–OH on bulk poly-Si surface determines the overall etch rate. This study advances the 3D NAND development by establishing a theoretical foundation for identifying and implementing efficacious strategies to impede detrimental dissolution.

Wet chemical poly-Si(n) wrap-around removal for TOPCon solar cells

This work gives an overview on different technological solutions for polysilicon removal in industrial tunnel oxide passivated contact (i-TOPCon) n-type silicon solar cell fabrication. The removal of parasitically deposited poly-Si layers on the front and the edges is a mandatory requirement for a low reverse bias junction leakage current (Irev). A polysilicon removal study on wafer level shows (i) that the efficient removal of the surface oxide layer before poly-Si etching is crucial to achieve short etching times and (ii) that an additive in the KOH solution enhances etching selectivity between poly-Si and silicon oxide surfaces. To evaluate the impact on device level, TOPCon cells have been fabricated in parallel using in-situ n-doped PECVD and LPCVD layers, as well as ex-situ LPCVD poly-Si layers, with another variation of poly-Si removal processes, namely wet chemical inline, wet chemical batch cluster and atmospheric dry etching (ADE). Our results show that a two-minute inline polysilicon removal in hot KOH meets the Irev qualification in case of as deposited in-situ doped layers, whereas for ex-situ doped layers a batch process with a five-minute KOH etching time is needed. LPCVD poly-Si cells show efficiencies above 23%, PECVD poly-Si cells with a batch cluster poly-Si removal process up to 23.4%.

Hardmask engineering by mask encapsulation for enabling next generation reactive ion etch scaling

Miniaturization and scaling of semiconductor devices require development of innovative techniques to sustain advancements. A promising trend is the migration from 2D to 3D device architectures that necessitate fabrication of high-aspect-ratio narrow features through layers of different materials. Reactive ion etching can achieve this but poses unique challenges due to the requirement of etch chemistries capable of etching dissimilar materials with varying etch rates while maintaining high productivity. The choice of hardmask is also crucial, as it plays a critical role in determining efficacy of the etch process and the final shape of the feature being etched. To address these challenges, we introduce a new concept of hardmask engineering that involves a bilayer hardmask scheme consisting of a patterned conventional hardmask encapsulated with a thin layer of etch-resistant ruthenium (Ru) layer. Experimental results for etching multilayer stacks consisting of alternating pairs of SiO2 and Mo show that this engineered hardmask results in improved hardmask remaining and etch profile with smaller critical dimensions (CDs). Technology computer-aided design simulations with the Ru encapsulation layer on conventional carbon hardmask demonstrate increased poly-Si etch depth with reduced bow CD. This concept can be extended to any semiconductor nanofabrication step involving high-aspect-ratio etching where precise control of CDs is essential in the vertical direction.

Electron beam-induced etching of SiO2, Si3N4, and poly-Si assisted by CF4/O2 remote plasma

Electron-stimulated etching of surfaces functionalized by remote plasma is a flexible and novel approach for material removal. In comparison with plasma dry etching, which uses the ion-neutral synergistic effect to control material etching, electron beam-induced etching (EBIE) uses an electron-neutral synergistic effect. This approach appears promising for the reduction of plasma-induced damage (PID), including atomic displacement and lateral straggling, along with the potential for greater control and lateral resolution. One challenge for EBIE is the limited selection of chemical precursor molecules that can be used to produce functionalized materials suitable for etching under electron beam irradiation. In this work, we studied a new experimental approach that utilizes a remote plasma source to functionalize substrate surfaces in conjunction with electron beam irradiation by an electron flood gun. Etching rates (ERs) of SiO2, Si3N4, and poly-Si are reported in a broad survey of processing conditions. The parametric dependence of the ER of these Si-based materials on the operating parameters of the flood gun and the remote plasma source is evaluated. We also identified the processing parameters that enable the realization of material selective removal, i.e., the etching selectivity of Si3N4 over SiO2 and poly-Si over SiO2. Additionally, surface characterization of etched materials is used to clarify the effects of the co-introduction of particle fluxes from the remote plasma and flood gun sources on surface chemistry.

Impact of firing temperature on fire-through metal contacts to P-doped (n) and B-doped (p) poly-Si

Screen-printed contacts to doped polysilicon (poly-Si) layers with high-temperature fire-through pastes is one of the key enablers for widespread industrial acceptance of passivating contact solar cells. In recent years, rapid progress has been made in the development of pastes for contacting doped poly-Si layers. However, there are still gaps in understanding the behavior of fire-through contacts, especially regarding damage to poly-Si layers. In this work we present a comprehensive study of metal contacts formed with fire-through pastes to both phosphorus-doped ( n + ) poly-Si and boron-doped ( p + ) poly-Si layers. The contacts are fired at different temperatures and characterized by evaluating specific contact resistivity and recombination current density under the metal contacts. The metal crystallites formed at the interface between metal fingers and poly-Si is quantitatively analyzed to determine their area fraction. It is observed that the area fraction of the crystallites tends to saturate at higher temperatures for n + poly-Si, however such behavior is not observed for p + poly-Si within the range of firing temperatures relevant for industrial solar cells. Metal crystallites appear to agglomerate at higher temperatures for contacts to both n + and p + poly-Si layers which explains increased recombination due to aggressive etching of poly-Si. For Ag contacts to n + poly-Si best results are achieved when the metal crystallites covered 50% of the surface area at the optimum firing temperature. For Ag–Al contacts to p + poly-Si the crystallites covered 30% of the area, but resulted in higher recombination under the contacts. n-type bifacial solar cells are fabricated with n + poly-Si passivating contacts on the rear side and fired at different temperatures. The solar cell results followed the trends shown by the contacts and efficiency reached maximum for an optimized firing temperature. However, the change in efficiency was dominated by the front side contact characteristics. The results obtained on solar cells validate the observations about contact formation and metal recombination. Etch pits on p + poly-Si observed under SEM at different peak firing temperatures. Different splits represent the peak firing temperatures. A) 710 B) 730 C) 750 D) 770 E) 790. • In depth analysis of fired-through contacts formed to 150 nm doped poly-Si layers at different temperatures. • Contact resistivity improved with increasing temperature, attributed to increased number of metal crystallites. • Excellent contact resistivity ≈0.5 mΩ/cm 2 and ≈4 mΩ/cm 2 achieved for Ag and Ag–Al fired through contacts respectively. • The J 0,metal increased with increasing temperature due to increasingly aggressive etching of the poly-Si layer. • An ideal metal paste should result in optimum number of crystallites that do not extensively etch the poly-Si layer.

Mechanism of highly selective etching of SiCN by using NF3/Ar-based plasma

As part of the self-aligned processes to fabricate a 3D device, highly selective SiCN etching using NF3/Ar-based gas plasma generated by microwave electron-cyclotron resonance was investigated. The etching rate of SiCN etched by NF3/Ar plasma was higher than that of various other materials, namely, SiO2, Si3N4, poly-Si, TiN, and Al2O3. Extremely highly selective etchings of SiCN with regard to various materials are possible by forming protective layers on nonetched materials by adding gases to the NF3/Ar plasma. The effects of adding gases to the NF3/Ar plasma on various other materials were studied by analysis using optical emission spectroscopy and x-ray photoelectron spectroscopy (XPS). The three key findings of these analyses are summarized as follows. First, highly selective etching of SiCN to poly-Si was achieved by adding oxygen to the NF3/Ar etching plasma. This etching was made possible because poly-Si etching was inhibited by forming a 1.0-nm-thick oxidized layer to protect the poly-Si surface from the etching reaction with fluorine radicals. Second, highly selective etching of SiCN to SiO2 and Si3N4 was achieved by using NF3/Ar-based plasma with added SiCl4. In this etching, silicon-containing deposited layers were formed on the SiO2 and Si3N4 surfaces. The deposited layers protected the surfaces from being etched by reacting with fluorine radicals. Third, highly selective etching over TiN was achieved by using hydrogen-added plasma. The XPS results show that a thin protective layer containing TiNxFy and ammonium fluoride was formed on the TiN surface. The protective layer formed on the TiN surface effectively protects the TiN from being etched by fluorine radicals.

Reaction Kinetics of Poly-Si Etching in TMAH Solution

Tetramethylammonium hydroxide (TMAH) is a metal-free strong alkaline solution which can etch poly-Si. The concentration of dissolved gas as well as the concentration of TMAH affects etching rate of poly-Si. The detailed kinetics of poly-Si etching in TMAH solution is investigated in this study. The effect of water and TMAH concentration on the etching kinetics of poly-Si was investigated by using various concentrations of TMAH solution. It is found that H2O in TMAH solution plays an important role in etching poly-Si. Presence of dissolved CO2 and O2 in TMAH solution tends to inhibit etching of poly-Si. The concentration of dissolved CO2 and O2 in TMAH were reduced by Ar bubbling, thereby the poly-Si etching rate increased.

Reactive ion etched, self-aligned, selective area poly-Si/SiO2 passivated contacts

Front/back poly-Si/SiO2 contact devices suffer from low short-circuit current density, Jsc, due to parasitic optical absorption in the front poly-Si layer. Thin poly-Si (~20 nm) allows for high Jsc but is not compatible with screen-printed fire-through contacts. We therefore study the effects of post-deposition etching of a thick poly-Si (200 nm) front layer by reactive ion etching (RIE) using the metal grid lines as a self-aligned mask. We show that passivation is maintained in the device during RIE and that Jsc is increased by a gain in the blue quantum efficiency response. However, our specific etching parameters cause non-uniform etching of the poly-Si leading to premature loss of passivation without optimal gain in Jsc. Etched, unpassivated layers can be re-passivated with a H-containing dielectric layer leading to a gain in Jsc, open circuit voltage,Voc, Fill-Factor, FF, and efficiency.

Effects of O2 addition on in-plasma photo-assisted etching of Si with chlorine

Addition of oxygen was used to control the in-plasma photo-assisted etching (PAE) of p-type Si(100) and poly-Si in a high density, inductively coupled, Faraday-shielded, Ar/Cl2 (225/25 SCCM), 60 mTorr plasma. After etching, samples were transferred under vacuum to an UHV x-ray photoelectron spectroscopy chamber for surface analysis. Samples etched under PAE conditions (ion energies below the ion-assisted etching, IAE, threshold) had a thicker surface oxide and lower [Cl] surface concentration, when compared to samples etched under IAE conditions (ion energies above the IAE threshold). PAE was found not to be affected by 0.1 or 0.25 SCCM O2 addition, while etching stopped with more than 0.5 SCCM O2 addition. IAE with RF power on the sample stage, resulting in −65 V self-bias, was not affected by up to 2 SCCM of oxygen addition but decreased rapidly when more than 5 SCCM O2 was added to the plasma. These results imply that PAE may be completely suppressed, while IAE occurs unobstructed. The implications of these findings are discussed in view of applications involving continuous wave and pulsed-plasma processes.

Utilizing photosensitive polymers to evaluate UV radiation exposures in different plasma chamber configurations

Monitoring vacuum ultraviolet (UV/VUV) emission in plasma systems is challenging as it requires specialized diagnostic systems or sensors to be compatible with these devices. This study addresses different reactor configurations and plasma chemistries with various levels of UV emission and their effects on a known set of polymers. First, the effect of He plasma treatment of organic underlayer (NFC-1400) and e-beam resist (hydrogen silsesquioxane) on line-space patterning of polysilicon at sub-100 nm pitch was investigated. By applying He plasma exposure, both before and after patterning of the optical underlayer, a significant improvement in line edge roughness (LER) from 2.5 to 1.4 nm was observed. To understand the plasma treatment mechanisms, polymers were exposed to synchrotron VUV light at 62 nm (or 20.0 eV), which coincides with the He VUV emission range (50–60 nm), followed by etching of poly-Si, and were found to have similar LER results. The refraction index measurements for both the optical underlayer and the e-beam resist revealed the key finding that the polymers absorption maxima corresponded exactly to the wavelength of ∼60 nm. Second, the impact of inductively coupled and microwave plasma configurations on etch rates and chemical properties of photoresists were investigated. Identifying specific photon-induced modifications on polymers can help detect UV/VUV emission in the plasma and decouple ion and photon effects on materials. Poly(methyl methacrylate) and poly(4-vinylphenol)-based photoresists were exposed to argon (Ar) and nitrogen (N2) plasmas. Surface and elemental analyses confirmed that plasma effects on chemical modifications, surface roughness, and etch rate were significantly higher for poly(methyl methacrylate) compared to poly(4-vinylphenol)-based photoresists. Detailed elemental and molecular structure analyses of these polymers showed relatively higher damage to both species caused from the inductively coupled plasma, which is ultimately correlated with a higher UV/VUV emission.

Electron beam injection from a hollow cathode plasma into a downstream reactive environment: Characterization of secondary plasma production and Si3N4 and Si etching

A material etching system was developed by combining beam electron injection from a direct current hollow cathode (HC) electron source with the downstream reactive environment of a remote CF4/O2 low temperature plasma. The energy of the injected beam electrons is controlled using an acceleration electrode biased positively relative to the HC argon discharge. For an acceleration voltage greater than the ionization potential of Ar, the extracted primary electrons can produce a secondary plasma in the process chamber. The authors characterized the properties of the secondary plasma by performing Langmuir probe measurements of the electron energy probability function (EEPF) 2.5 cm below the extraction ring. The data indicate the existence of two major groups of electrons, including electrons with a primary beam electron energy that varies as the acceleration voltage is varied along with low energy electrons produced by ionization of the Ar gas atoms in the process chamber by the injected beam electrons. When combining the HC Ar beam electron with a remote CF4/O2 electron cyclotron wave resonance plasma, the EEPF of both the low energy plasma electron and beam electron components decreases. Additionally, the authors studied surface etching of Si3N4 and polycrystalline Si (poly-Si) thin films as a function of process parameters, including the acceleration voltage (0–70 V), discharge current of the HC discharge (1–2 A), pressure (2–100 mTorr), source to substrate distance (2.5–5 cm), and feed gas composition (with or without CF4/O2). The direction of the incident beam electrons was perpendicular to the surface. Si3N4 and polycrystalline silicon etching are seen and indicate an electron-neutral synergy effect. Little to no remote plasma spontaneous etching was observed for the conditions used in this study, and the etching is confined to the substrate area irradiated by the injected beam electrons. The electron etched Si3N4 surface etching rate profile distribution is confined within a ∼30 mm diameter circle, which is slightly broader than the area for which poly-Si etching is seen, and coincides closely with the spatial profile of beam electrons as determined by the Langmuir probe measurements. The magnitude of the poly-Si etching rate is by a factor of two times smaller than the Si3N4 etching rate. The authors discuss possible explanations of the data and the role of surface charging.

A Unique Approach to Generate Self-Aligned T-Gate Transistors in Counter-Doped Poly-Si With High Etching Selectivity and Isotropy

A unique approach for fabricating poly-Si thin-film transistors (TFTs) with self-aligned T-shaped gate (T-gate) structure is reported. A counter-doped poly-Si process comprises an in-situ doped ${n}^{+}$ poly-Si deposition followed by a subsequentshallow implantation of BF $_{{2}}^{+}$ . Both high etching isotropy in ${n}^{+}$ poly-Si and high etching selectivity between ${n}^{+}$ poly-Si and B-doped poly-Si in a Cl2-based plasma process are the key enablers for the fabrication of our T-gate structures. Thanks to good control in the shape and deformation of our T-gate structure, sidewall air-gap spacers in combination with self-aligned Ni silicided gate and source/drain were established. High-performance sub-micron poly-Si TFTs are evidenced by superior transfer characteristics measured on TFTs with effective gate length of $0.15~\mu \text{m}$ . The unique T-gate structure provides an effective way for possible production of poly-Si radio-frequency TFTs viable for emerging new applications.

(Invited) Wet Etching inside Advanced High Aspect Ratio Structures: Impact of Dissolved Oxygen

Semiconductor technology is currently facing the challenge of processing structures with nanometer scale openings and high aspect ratios (HAR) in order to enable the manufacture of 3D nano-devices like FINFETs, 3D-NAND cells and DRAM capacitors. Simultaneously, process control has been pushed towards sub-nanometer dimensions as 1nm variation starts to impact device performance. These requirements pose a growing challenge to wet processing inside nanometer scale openings in HAR structures of advanced 3D devices. Top to bottom etch uniformity with nanometer etch control has become a paramount necessity. High selectivity silicon etch is a critical process for 3D-NAND. Typical silicon wet etch processes have very high selectivity towards silicon dioxide. Even a monolayer of silicon dioxide can affect the silicon etch rate. Dissolved oxygen (DO) has become a key aspect during silicon etching. The dissolved oxygen can react with the exposed silicon creating silicon dioxide slowing the silicon etch. In our paper, we specifically looked at the etching of poly Si by tetramethylammonium hydroxide (TMAH) and how the etch rate is affected by DO in the solution. Figure 1a shows experimental results of etching a blanket poly Si wafer depending on the oxygen in the environment: normal air or pure nitrogen. Even though the TMAH is dispensed with a low amount of DO, as it flows across the wafer it can absorb or desorb oxygen to/from the atmosphere and accordingly change the center to edge etch rate considerably. In order to test if the hypothesis was correct, we modeled the DO concentration in the TMAH solution and predicted the DO profile inside HAR structures. Figure 1c shows the plan for modeling the DO in order to predict the etch rate variability: a liquid flow model predicts the temperature and DO across the wafer then this model feeds into an additional model for the DO concentration inside the HAR feature. These models then report the etch rate as a function of wafer position and depth in feature, so long as the etch rate as a function of temperature and DO is known. Table 1 shows the equations for the wafer surface liquid model. Equation 1 calculates the temperature and volumetric flow rate across the wafer and equation 2 uses the results of equation 1 to calculate the DO concentration. Table 2 shows the chemical reactions and rate equations for modeling the DO concentration inside a HAR feature, where the DO was consumed as it diffuses down into the feature and reacts with the poly Si. The rate of reaction 1b is assumed to be much faster than 1a, so the important rate equations are 1a, 2, and 3 with their associated rate constants. Figure 1b shows the results of the wafer surface liquid model in predicting the DO concentration and resulting etch rate across the wafer. The process environment O2 was varied in which the atmosphere is either 0.1% O2, standard 20% O2, or a carefully controlled 6% O2 chosen to maintain a constant DO in the TMAH across the wafer, even with temperature variation. However as we will show in the full paper, center to edge etch rate uniformity is insufficient for good process control, as DO consumption within HAR features causes very different etch rates between the top and bottom of the HAR structure. We will then show that this difficulty can be solved by using a much more powerful oxidizer than DO such as H2O2, ensuring a uniform etch rate across the HAR. Figure 1

Role of polysilicon in poly-Si/SiO x passivating contacts for high-efficiency silicon solar cells.

In this study, we focused on understanding the roles of a polysilicon (poly-Si) layer in poly-Si/SiOx/c-Si passivating contacts. Passivating contact formation conditions were varied by changing the doping method, annealing temperature and time, polysilicon layer thickness, and polysilicon doping concentration. Our observations indicated that the roles of polysilicon are contact, in-diffusion barrier action, field effect, gettering, and light absorption. Based on the observations, a iVOC of 741 mV was obtained. Finally, to increase JSC with high VOC, the polysilicon was etched after hydrogenation to reduce light absorption with high passivation quality. iVOC was not affected by etching; moreover, by etching the polysilicon from 300 nm to 60 nm, the cell efficiency increased from 20.48% to 20.59% with increasing JSC, constant VOC, and fill factor.

Influence of phosphorus doping on the chemical etching rate of polycrystalline silicon films

The chemical etching of intrinsic and n-type poly-Si with phosphorus concentration 5×1018cm−3 in Cl2 environment is investigated. It is found that phosphorus doping increases formation and desorption of SiCl2 molecules. The activation energy of Si+Cl2→SiCl2 reaction for intrinsic poly-Si is equal to 1.73±0.24eV, and it decreases to 1.51±0.16eV when n-type poly-Si films are used. Meanwhile, the desorption activation energy of SiCl2 molecules for intrinsic poly-Si is equal to 1.86±0.16eV, and it decreases to 1.54±0.21eV when n-type poly-Si films are used. The etching-rate limiting process is determined using the calculated mean times of elementary processes.

Introducing pinhole magnification by selective etching: application to poly-Si on ultra-thin silicon oxide films

Carrier selective junctions formed by polycrystalline silicon (poly-Si) on ultra-thin silicon oxide films are currently in the spotlight of silicon photovoltaics. We develop a simple method using selective etching and conventional optical microscopy to determine the pinhole density in interfacial oxide films of poly-Si on oxide (POLO)-junctions with excellent electrical properties. We characterize the selective etching of poly-Si versus ultra-thin silicon oxide. We use test structures with deliberately patterned openings and 3 nm thin oxide films to check the feasibility of magnification by undercutting the interfacial oxide. With the successful proof of our concept we introduce a new method to access the density of nanometer-size pinholes in POLO-junctions with excellent passivation properties.

Effect of temperature on deposition layer formation in HBr/N2/fluorocarbon-based plasma

The effects of wafer temperature on etching rate and surface composition were investigated to clarify the surface reaction mechanism under HBr/N2/fluorocarbon-based gas plasma for developing a process for three-dimensional NAND flash devices. The etching rates of both polycrystalline silicon (poly-Si) and SiO2 were found to increase at a wafer temperature of 20 °C as compared with those at 60 °C. Comparing the gas combination of fluorocarbon/N2 and HBr/N2 mixtures, the temperature dependence of SiO2 etching rates was considered to relevant to the sticking probability of fluorocarbon polymers. To determine the cause of the temperature dependence of the poly-Si etching rate, surface composition was evaluated by thermal-desorption-spectroscopy and laser-sputtered-neutral-mass-spectrometry analyses. Ammonium bromide was confirmed in the deposition film at a wafer temperature of 20 °C. The observed increase in poly-Si etching rate at lower temperatures was possibly caused by increased amounts of nitrogen, hydrogen, and bromine fixed to the surface with the formation of ammonium bromide.

Introduction of a High Selectivity Etching Process with Advanced SiNx Etch Gas in the Fabrication of FinFET Structures

The FinFETs [1-3] is one of the candidates of further high performance device compared to the conventional planar MOSFETs. It is important to form the channel as high and thin structure for the performance of FinFETs. Such a high aspect channel is superior to the gate bias controllability to the channel and current capability per unit area. High selectivity etch technologies are required in the fabrication of high aspect fin channel. Those technologies can also realize a fabrication of fin as a fine structure, which leads to the possibility of a newly technology such as a applying of different orientation in Si fin surface [4]. In this work, we fabricated FinFET structure with high selectivity etching using newly developed SiNx etch gas SSY525 (a prototype developed by Zeon Corporation) and we found that SSY525 is one of the promising material for the fabrication of FinFETs. We evaluated the etching characteristics of SSY525. The evaluations were carried out with a microwave-excited plasma etching equipment [5]. The microwave power, the RF power of bottom electrode, the chamber pressure and the flow rate of SSY525 were 1000 W, 90W, 5.3 Pa, and 15 sccm, respectively. Figure 1 (a) shows the etch rates of SiNx and the selectivity to Si and SiO2 as a function of the O2 flow rate. The etching characteristics of CH3F, which is conventionally used in SiNx etching is also plotted in Fig. 1 (b) as a comparison. The flow rate of CH3F was 15 sccm. The higher etching selectivity of SiNx to Si and SiO2 was obtained with SSY525 compared with CH3F. In the case of SSY525, the highest selectivity was obtained with O2 flow rate of 0 sccm, and then the selectivities were 62 to the Si and over 100 to the SiO2, respectively. Contrary to the SSY525, in the CH3F, the highest selectivity was obtained with O2 flow rate of 8 sccm, and then the selectivities were 8 to the Si and 9 to the SiO2, respectively. In the case of CH3F, the etching of poly-Si and SiO2 are suppressed by introducing a small amount of O2. Meanwhile in the case of SSY525, those etching are suppressed by depositing a film on Poly-Si and SiO2, therefore SSY525 realized a high selectivity etching [6]. Furthermore, in these conditions, the etch rates in the both gases were almost the same; the etching rate of SSY525 was 57 nm/min and of CH3F was 55 nm/min, respectively. Next, we carried out the shapes evaluation after the etching with SSY525 and CH3F. We used a SOI wafers, which was patterned line and space (L/S=30 nm/100nm, height=75 nm) with Si represent a fin structure and deposited 40 nm SiNx film over the L/S pattern for evaluation. The SiNx film on the Si fin was etched under the 40% over-etching condition with SSY525 and CH3F, respectively. The just etching means here, the time to etch the Si fin height and SiNx film thickness. The flow rates of etchant gases were 15 sccm, and the O2 flow rates were set at the etch selectivity condition maximum, that is, 0 sccm in SSY525 and 8 sccm in CH3F, respectively. For both etchant conditions, the microwave power, the RF power of bottom electrode, and the chamber pressure were 1000 W, 90W, and 5.3 Pa, respectively. Then, these Si fin after etching were evaluated the shapes. The results were shown in Fig. 2. The SiNx film between the Si fins was completely removed with both gases. Though, with the CH3F, there were losses of the top of Si fin and bottom oxide during the over-etching, with SSY525, there were almost no less of those parts. Since SSY525 has higher selectivity than CH3F, therefore the top of Si fin and bottom oxide almost maintained the pristine structures. Finally, we fabricated FinFET structure introducing SSY525 in the spacer etch process, and these indicate that the fabrication of the fine shape FinFET is possible by using SSY525 (Fig. 3). Acknowledgment This research has been carried out at fluctuation free facility of New Industry Creation Hatchery Center, Tohoku University. Reference D. Hisamoto, et al., IEDM Tech. Dig. 1032, 1998. D. Hisamoto, et al., IEEE Trans. Elec. Devices, 47, 2320, 2000. X. Huang, et al., IEDM Tech. Dig., 67, 1999. W. Cheng, et al., Microelectron. Eng., 84, 2105, 2007. T. Ohmi, et al., J. Phys. D: Appl. Phys. 39, R1 2006. Y. Nakao, et al., ECS Trans. 61, 29, 2014. Figure 1

Delay-Time Effect on the Transistor Performance after Reactive Ion Etching

Since recessed channel array transistor (RCAT) was adopted from 100 nm feature size, gate-induced-drain-leakage (GIDL) has been the major cause of refresh fail. [1] The process of inner-gate-recess was adapted and GIDL in RCAT structure has been remarkably reduced by lowering electric field at overlap region. Fig.1 (a) and (b) show the VSEM image of inner-gate-recessed-RCAT and the electric field simulation, respectively. The process of recessing gate-poly was executed at both cell array transistor such as RCAT and planar transistor in logical circuits. In case of planar transistor, there was no need to recess poly-Si except SiO2 layer. During the etching of poly-Si in the cell array, the thin oxide layer in the planar area should be preserved to prevent Si-pitting. HBr/O2 plasma has been used to improve the selectivity of poly-Si to gate oxide in gate-stack etching. HBr/O2 plasma produced polymer residues such as SiBr and SiBrOx layer on the underlying gate oxide and polymer residues should be removed. [2, 3] Fig.2 shows the variation of stand-by current of DRAM and propagation time of inverter (tPD) with wafer number in a lot investigated. Wafer number indicates the order in processing. As the number of wafer increased, stand-by current increased and tPD decreased. Fig.3 shows the saturation current of PMOS and the saturation current also decreased with increasing wafer number. Structural analysis was evaluated, and the difference between the first wafer and the last wafer was found. There was thicker SiO2 layer on the gate stack in the first wafer. Especially, to make the gate recessed profile of RCAT, the over-etch time increased and the exposure time in HBr/O2 plasma also increased. It induced to increase polymer residues and unexpected SiO2 offset spacer. As the HF removal was delayed for measuring critical dimension (CD) of gate stack and thickness of remain substrate, deposited SiBr among the polymer residues reacted with O2 in the air and additional SiO2 was grown. [4] The first wafer in the lot investigated had much more delay time than the last wafer. Therefore thicker SiO2 in the first wafer was grown than that of the last wafer, and the SiO2 in the first wafer was insufficiently removed by HF clean. SiO2 offset made under-lap from gate to source/drain and induced increasing resistance and decreasing saturation current. [5, 6] Fig.4 (a) and (b) show the TEM profiles and explain the cause of saturation current difference obtained from the same lot. Immediate HF cleaning after gate etching was introduced and CD was measured with increasing time by the scanning electron microscope (SEM). Fig. 5 shows the CD variation monitored as a function of delay time. While SiO2 layer was steadily grown with increasing delay time without HF cleaning, the SiO2 and residual SiBr layer on the sidewall of gate stack were easily removed and additional SiO2was not grown in case of immediate HF clean. As the integration process is more complicated and delicate, small variation brings the big failure. Delay time after reactive ion etching should be reduced, and immediate cleaning should be appropriately considered.