Si Etch Rate Research Articles

Silicon Anisotropic Etching in Ternary Solution Composed of TΜΑΗ+Triton+NH2ΟΗ

Wet chemical etching is extensively used in the fabrication of microelectromechanical systems (MEMS) owing to its several benefits such as low cost, easy handling, and bulk production [1, 2]. TMAH and KOH are most widely employed etchants for silicon anisotropic etching. In these two etchants, TMAH is preferred when complementary metal oxide semiconductor (CMOS) compatibility is a major concern and the oxide layer is used as mask material [2, 3]. Etching behavior of TMAH can be modified by the incorporation of different kinds of additives such surfactants, alcohol, etc. [2-4]. Significant amount of research has been done to modify the etching characteristics by adding some additives, however very less is reported on TMAH-based ternary solutions. Present research reports the anisotropic etching characteristics of silicon in various concentrations of TMAH (5 wt%, 15 wt%, and 25 wt%) solutions containing 10% NH2OH and Triton-X-100 ranging from ppb to ppm level. The etch rate, undercutting at convex corners, and etched surface morphology are investigated in different compositions of ternary solutions (TMAH+NH2OH+Triton). Optical microscope, scanning electron microscope (SEM), and three-dimensional (3D) measuring laser microscope are employed for the characterization of etched surface morphology and to measure the etch depth and undercutting at convex corners. Figure 1 presents the etch rate of Si{100} and the undercutting rate (U <110> = l <110> /d, l <110>: undercutting length along <110> direction, d: etch depth) at convex corners. High etch rate and undercutting are achieved in NH2OH-added 15 wt% TMAH, while significantly less undercutting with reasonably good etch rate of Si{100} (Er = 18 µm/hr) is obtained in 25 wt% TMAH+10%NH2OH+1ppm Triton. The high undercutting is desirable when freestanding structures are fabricated, while it is unwanted effect when microstructures with convex corners have to be fabricated such as mesa structure. The present research is very useful for engineering applications for the fabrication of microstructures using CMOS-compatible silicon bulk micromachining for MEMS-based devices.

A new single-component low-cost emitter etch-back process for silicon wafer solar cells

A way of achieving lightly doped emitter is a combination of a heavy emitter diffusion and emitter etch back, which has an added advantage of phosphorous diffusion gettering. However, this chemical emitter etch-back process must fulfil some critical requirements, e.g. cost-effectiveness, near-conformal Si etching even after deep emitter etch back, controlled Si etch rate, post-etch clean Si surface and lowest safety issues in chemical handling and drainage. In this work, we report a new low-cost (less than 1 US Cents/wafer), single-chemical, non-acidic, high-throughput emitter etch-back process for tube-diffused emitters for crystalline Si wafers. This process uses only sodium hypochlorite solution at 80 °C as the Si etchant. This process is versatile with its applications on phosphorous and boron tube-diffused monocrystalline Si and phosphorous tube-diffused multicrystalline Si wafers. The preparation, usage and drainage of this highly diluted solution are easy and safe. The Si etching process leads to excellent spatial uniformity over large-area Si wafers (243 cm2). With deep etch back resulting in a change of sheet resistance by ~60 Ω/sq, the standard deviation value changes by only 2.7%. High surface conformity in the etch-back surface is evident from reflectance studies. Quasi-steady-state photoconductance and photoluminescence imaging are used to demonstrate improved electrical parameters of the etch-back wafers.

Concurrent effects of wafer temperature and oxygen fraction on cryogenic silicon etching with SF6/O2 plasmas

Cryogenic plasma etching is a promising technique for high‐control wafer development with limited plasma induced damage. Cryogenic wafer temperatures effectively reduce surface damage during etching, but the fundamental mechanism is not well understood. In this study, the influences of wafer temperature, gas mixture and substrate bias on the (cryogenic) etch rates of Si with SF6/O2 inductively coupled plasmas are experimentally and computationally investigated. The etch rates are measured in situ with double‐point reflectometry and a hybrid computational Monte Carlo – fluid model is applied to calculate plasma properties. This work allows the reader to obtain a better insight in the effects of wafer temperature on the etch rate and to find operating conditions for successful anisotropic (cryo)etching.

Effect of NH2OH on Etching Characteristics of Si{100} in KOH Solution

Potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) are most extensively used etchants for wet anisotropic etching process. Amongst these two etchants, KOH is a low cost etchant and provides high etch rate anisotropy between Si{111} and Si{100} planes. Increasing the etch rate is an important research problem for both academic and industrial applications. In this research, we study the effect of hydroxylamine (NH2OH) in 20 wt% KOH on the etching characteristics of Si{100}. This type of wafer orientation (i.e. Si{100}) is selected owing to its popularity in fabricating planar semiconductor devices (e.g. diode, transistor, complementary metal oxide semiconductor (CMOS), etc.) and MEMS components (e.g. cantilever, diaphragm, etc.). A systematic parametric analysis of various concentrations of hydroxylamine (from 0 to 20% in step of 5%) added 20 wt% KOH is carried out and its effect on the etching characteristics is discussed. We have found that the etch rate of Si{100} is increased threefold when 15% NH2OH is added to 20 wt% KOH solution. Additionally, this concentration also exhibits improved etching characteristics including undercutting rate, etch selectivity, and surface morphology. We have obtained improved etching characteristics when NH2OH is added to 20 wt% KOH. Due to an improvement in etching characteristics, present research is very useful to pave the path toward application in microfabrication industries as well as academic research laboratories.

The high-resolution nanostructuring of Si wafer surface with 10 nm scale using a combined ion bombarding technique and chemical reaction

We describe a highly efficient technique for nanostructuring silicon (Si) wafer surfaces with high-resolution (< 15 nm) and high aspect ratio (20) structures without any deposition processes. Our strategy is based on advanced secondary sputtering lithography (SSL), which combines physical and chemical plasma etching during an ion bombardment process. Compared with general SSL techniques using Ar gas only, the reactive radicals assisted the SSL and promoted the Si etching rate to simultaneously deposit the etched Si materials onto the side surface of a pre-patterned polymer. In addition, various three-dimensional Si nanostructure shapes could be developed simply by controlling the pre-patterned polymer, thereby providing a simple and versatile approach to customizing this technique.

Insights into the mechanism of in-plasma photo-assisted etching using optical emission spectroscopy

Various mechanisms have been proposed to explain photo-assisted etching (PAE) of Si, including photogenerated carrier-mediated etching, photon-stimulated desorption, and photon-induced damage (breaking of Si–Si bonds) caused mainly by vacuum ultraviolet photons irradiating the substrate. In this study, the authors use optical emission spectroscopy to gain an insight into possible in-plasma PAE mechanisms. Emissions from Cl, Si, SiCl, and Ar were recorded as a function of power while etching p-Si in Cl2/Ar Faraday-shielded inductively coupled plasmas at a pressure of 60 mTorr with no substrate bias. Under these conditions, ion-assisted etching was negligible and PAE was dominant. The Si:Ar optical emission intensity ratio, ISi/IAr (proportional to the etching rate of Si), increased substantially with power. Accounting for the contribution to this signal from the dissociation of SiClx (x = 1–4) etch products, the residual increase in the emission indicated that the PAE rate also increased with power. Time resolved emissions were also recorded in a pulsed plasma where power was modulated between 500 and 300 W. ISi/IAr was found to modulate with the instantaneous power. This rules out the photon-induced damage mechanism since, if this mechanism was dominant, the ISi/IAr signal would not be modulated.

Highly Selective Wet Silicon Etch Chemistry and Process for Advanced Semiconductor Packaging

Abstract A wet silicon etch chemistry and a process using the chemistry as a simple and cost-effective alternative to the polish/plasma etch silicon thinning process are presented in this paper. The new etch chemistry improves the Si etch rate over traditional etchants such as tetramethylammonium hydroxide (TMAH). The chemistry has very high silicon etch selectivity (ratio of Si etch rate to another film etch rate) over SiO2 and over copper films, with etch selectivity greater than 8000:1 and 2500:1 respectively. Chemical compatibility with typical packaging materials such as polybenzoxazole (PBO) and polyimide (PI) are also discussed. Additionally, real production TSV wafers have been successfully processed on an advanced single wafer etch tool using the chemistry. High uniformity and smooth surface finishing have been achieved.

Etching characteristics of SiC, SiO2, and Si in CF4/CH2F2/N2/Ar inductively coupled plasma: Effect of CF4/CH2F2 mixing ratio

This study investigated the etching characteristics and mechanisms of SiC, Si, and SiO2 in CF4/CH2F2/N2/Ar inductively-coupled plasmas. The investigation showed that a change in the CF4/CH2F2 mixing ratio at fixed N2 and Ar fractions in a feed gas causes a decrease in the etching rates of SiC and Si, but results in an almost constant SiO2 etching rate. Plasma chemistry was analyzed using Langmuir probe diagnostics and optical emission spectroscopy. The good agreement between the behaviors of both the SiC and the Si etching rates with a change in F atom density suggested a neutral-flux-limited etching regime for these materials. On the contrary, the SiO2 etching process appeared in the transitional regime of the ion-assisted chemical reaction and was influenced by both neutral and ion fluxes.

Analysis of Si Wet Etching Effect on Wafer Edge

We investigated the effect of Si wet etching on the vertical step at wafer edge. We found that the concave-convex shape appeared at the wafer edge after Si etching by the Atomic Force Microscopy analysis. From the liquid simulation and the detailed evaluation of Si etching rate, we revealed that the concave-convex shape was formed by the distribution of the fluid velocity at the wafer edge.

Chemical downstream etching of Ge, Si, and SiNx films

This work reports on selective isotropic dry etching of chemically vapor deposited Ge thin film, release layers using a Shibaura chemical downstream etcher with NF3 and Ar based plasma chemistry. Relative etch rates between Ge, Si, and SiNx are described with etch rate reductions achieved by adjusting plasma chemistry with O2. Formation of oxides reducing etch rates was measured for both Ge and Si, but nitrides or oxy-nitrides created using direct injection of NO into the process chamber were measured to increase Si and SiNx etch rates while retarding Ge etching. Observation of preferential etching of Ge in the presence of Si and SiNx is also observed with lateral etch rates reaching 19.2 μm/min for the Ge layers. Results presented here demonstrate the use of Ge as a microelectromechanical systems device dry release layer in the presence of Si and SiNx making it a highly advantageous technology, especially for optical devices.

Elucidating the effects of gas flow rate on an SF6 inductively coupled plasma and on the silicon etch rate, by a combined experimental and theoretical investigation

Experiments show that the etch rate of Si with SF6 inductively coupled plasma (ICP) is significantly influenced by the absolute gas flow rate in the range of 50–600 sccm, with a maximum at around 200 sccm. Therefore, we numerically investigate the effects of the gas flow rate on the bulk plasma properties and on the etch rate, to obtain more insight in the underlying reasons of this effect. A hybrid Monte Carlo—fluid model is applied to simulate an SF6 ICP. It is found that the etch rate is influenced by two simultaneous effects: (i) the residence time of the gas and (ii) the temperature profile of the plasma in the ICP volume, resulting indeed in a maximum etch rate at 200 sccm.

Effect of molecular structure of alcohols on wet anisotropic etching of silicon

The study concerns anisotropic etching of silicon (100) and (110) planes in potassium hydroxide aqueous solutions saturated with different monohydric alcohols (C3H7OH, C4H9OH and C5H11OH). The main goal of the research is to systematically examine the influence of length and branching of alcohol molecules alkyl chains on etching process. The etch rate ratio R(100)/R(110)>2 and (110) planes having zigzag pattern are obtained as a result of etching in KOH solutions with all considered alcohols. Additionally, it is demonstrated that convex corners of spatial structures etched in aforementioned solutions are probably formed by exact or vicinal {221} planes. It is shown that for alcohols whose molecules have more than 3 carbon atoms the Si (100) surface is covered with pyramidal hillocks after etching. Moreover, the etch rate of (110) plane decreases as the alcohol molecules alkyl chain becomes longer and more branched. The attempt to explain the impact of molecular structure of different alcohols on the Si etch rates is made, based on results of surface tension and contact angle measurements.

Influence of temperature on the formation of SiF2 molecules

Chemical etching of silicon in SF6 plasma is considered. Dependences of Si etching rate on pressure of reactive species are measured at two different temperatures. Atomic layer etching of silicon with F atoms is analyzed using theoretical results obtained by fitting the experimental data. It is found that at high pressure, the formation of SiF2 molecules is the etching-rate limiting process. As a result, dependences of Si etching rate on pressure of reactive species have pronounced maxima. The increase in temperature during Si etching in SF6 plasma shifts maximum etching rate to lower pressure. This phenomenon is caused by an increased chemisorption of F atoms on the surface.

Efficiency of silane gas generation in high-rate silicon etching by narrow-gap microwave hydrogen plasma

The silicon (Si) etching characteristics and the related efficiency of the etched Si to generate SiH4 gas in narrow-gap high-pressure microwave H2 plasma have been investigated. It was found that cooling of the Si sample is effective to obtain a high etching rate even under high pressure conditions, and the excess temperature increase of both the gas and Si sample can be suppressed even at an input power density of more than 250 W cm−3, probably because of the narrow plasma gap. The local etching depth monotonically increased with increasing H2 pressure and input plasma power, whereas the etching weight decreased with increasing H2 pressure. By simultaneously increasing the H2 pressure and input power, a maximum Si etching rate of 38 μm min−1 was achieved. This is considered to be related to the high H density generated in the narrow-gap microwave plasma at relatively low temperatures. The energy efficiency of Si etching and the utilization efficiency of the etched Si and H2 gas for SiH4 formation are discussed. Lower input power is favorable for high energy efficiency of Si etching. The Si utilization efficiency, which is defined as the molar ratio of generated SiH4 to etched Si, increases with decreasing average gas residence time in the plasma, whereas H2 utilization efficiency is independent of the gas residence time.

Silicon etching using only Oxygen at high temperature: An alternative approach to Si micro-machining on 150 mm Si wafers

Using a combination of low-pressure oxygen and high temperatures, isotropic and anisotropic silicon (Si) etch rates can be controlled up to ten micron per minute. By varying the process conditions, we show that the vertical-to-lateral etch rate ratio can be controlled from 1:1 isotropic etch to 1.8:1 anisotropic. This simple Si etching technique combines the main respective advantages of both wet and dry Si etching techniques such as fast Si etch rate, stiction-free, and high etch rate uniformity across a wafer. In addition, this alternative O2-based Si etching technique has additional advantages not commonly associated with dry etchants such as avoiding the use of halogens and has no toxic by-products, which improves safety and simplifies waste disposal. Furthermore, this process also exhibits very high selectivity (>1000:1) with conventional hard masks such as silicon carbide, silicon dioxide and silicon nitride, enabling deep Si etching. In these initial studies, etch rates as high as 9.2 μm/min could be achieved at 1150 °C. Empirical estimation for the calculation of the etch rate as a function of the feature size and oxygen flow rate are presented and used as proof of concepts.

Silicon etching characteristics for tetramethylammonium hydroxide‐based solution with additives

In this reported work, the anisotropic etching properties of single-crystal silicon with tetramethylammonium hydroxide (TMAH) solution are examined in detail. The variations in the Si etching rate and surface morphology at different etching temperatures and TMAH concentrations are evaluated. The effect of isopropyl alcohol (IPA) additive is also examined. As the TMAH concentration (10–25 wt%) decreased, the etching rate increased from 10 to 70 μm/h at temperatures between 70 and 90°C. On the other hand, the etched surface roughness became degraded as the hillock density and corner undercut ratio increased. The additive of IPA affected the morphology of the etched surface to be flat. In addition, the IPA additive improved the etched surface significantly by decreasing the hillock density on the surface.

Application of Si and SiO2 Etching Mechanisms in CF4/C4F8/Ar Inductively Coupled Plasmas for Nanoscale Patterns.

An investigation of the etching characteristics and mechanism for both Si and SiO2 in CF4/C4F8/Ar inductively coupled plasmas under a constant gas pressure (4 mTorr), total gas flow rate (40 sccm), input power (800 W), and bias power (150 W) was performed. It was found that the variations in the CF4/C4F8 mixing ratio in the range of 0-50% at a constant Ar fraction of 50% resulted in slightly non-monotonic Si and SiO2 etching rates in CF4-rich plasmas and greatly decreasing etching rates in C4F8-rich plasmas. The zero-dimensional plasma model, Langmuir probe diagnostics, and optical emission spectroscopy provided information regarding the formation-decay kinetics for the plasma active species, along with their densities and fluxes. The model-based analysis of the etching kinetics indicated that the non-monotonic etching rates were caused not by the similar behavior of the fluorine atom density but rather by the opposite changes of the fluorine atom flux and ion energy flux. It was also determined that the great decrease in both the Si and SiO2 etching rates during the transition from the CF4/Ar to C4F8/Ar gas system was due to the deposition of the fluorocarbon polymer film.

Catalytic Activity of Ru for Metal-Assisted Etching of Si

Metal-assisted etching of Si is an electroless method that can produce porous Si by immersing metal-modified Si in a HF solution without electric bias. We reported that the etching rate of noble metal-modified Si in a simple HF solution including oxygen under dark conditions depended on the catalytic activity of metal for oxygen reduction. Only palladium exhibits high activity for the etching under dissolved oxygen-free and dark conditions. In this study, the catalytic activity of Ru for the etching of Si has been investigated. Ru nanoparticles are electrodeposited on Si substrates. Electroless displacement deposition using RuCl3 solution including HF can form Ru nanoparticles not on mirror-polished n-Si substrate but on roughed one. The etching rate of Ru-particle-deposited Si is similar to that of Rh case which is changed with dissolved oxygen concentration. On the roughed Si substrates, the Ru nanoparticles assist the etching even under oxygen-free and dark conditions.

Effect of additives on the anisotropic etching of silicon by using a TMAH based solution

In this study, the anisotropic etching properties of single crystal silicon were examined using a tetramethyl ammonium hydroxide (TMAH). The variations in the Si etching rate and surface morphology at different etching temperatures and TMAH concentrations were evaluated. The effects of different additives were also examined. As the THAM concentration (10–25 wt. %) decreased, the etching rate increased from 10 μm/h to 70 μm/h at temperatures between 70°C and 90°C. On the other hand, the etched surface roughness became degraded as the hillock density and corner undercut ratio increased. To solve these problems, four additives, pyrazine, ammonium persulfate (AP), ammonium hydrogen sulfate (AHS), and isopropyl alcohol (IPA), were added to the TMAH solution. The experimental results showed that these additives play an important role in increasing the etching rate up to 10–20%. The etched surface was also improved significantly by the decreased hillock density on the surface. The addition of IPA to the TMAH solution showed excellent results in improving the etched surface flatness and the undercutting compensation. On the other hand, one of the characteristics of IPA is the decrease in etching rate with increasing amount of IPA.

Catalytic Activity of Ru for Metal-Assisted Etching of Si

Metal-assisted chemical etching of Silicon (Si) in hydrofluoric acid (HF) solution is an electroless method to produce porous Si (1, 2). Such etching generally uses not only metal-modified Si but also an oxidizing agent as hydrogen peroxide, metal ions and oxygen. We reported that etching rate of platinum (Pt), rhodium (Rh), gold (Au) and silver (Ag)-particle-modified Si depended on catalytic activity of metal for oxygen reduction (3), and palladium (Pd) exhibits high catalytic activity for the etching of Si without an oxidizing agent even under dissolved oxygen-free and dark conditions (4). This etching reaction using Pd is explained by electron injection into the conduction band of Si due to Pd assisted anodic oxidization of Si (Fig. 1). In this study, we have investigated the catalytic activity of ruthenium (Ru), which is in the same platinum group as Pd, for etching of Si in a HF solution. Single-crystalline n-Si wafers (CZ, (100), ca. 10 Ω cm) were used as substrates. Si wafers were immersed in 1 mM (M = mol dm-3) ruthenium chloride (RuCl3) solution including 0.15 M HF at 313 K for electroless displacement metal deposition. Metal deposited Si wafers were immersed in a 7.3 M HF solution at 298 K under dark conditions for 24 h. Argon or oxygen gas bubbling was applied to the HF solution before and during etching to remove or saturate the dissolved oxygen in the solution, respectively. The etching rate of Si was measured by a gravimetric procedure. No metal deposition occurred on mirror-polished Si wafers even by immersing in the deposition solution for even long time of 24 h. On the other hand, on Si wafers roughed with sand paper (#1200), Ru particles were deposited by immersing in the solution for 5 min. The etching rates of such Ru-particle-deposited Si wafers in 7.3 M HF solution were 1.1×10-5 and 2.0×10-4 g cm-2 h-1 under oxygen-free and -saturated conditions, respectively. The etch rate in the oxygen-free case is much higher than 1.2×10-6 g cm-2 h-1 of Pt-particle-modified case and one order lower than 3.4×10-4 g cm-2 h-1 the of Pd case. Figure 2 shows the SEM images of Ru-particle-deposited Si wafers before and after etching with the oxygen free HF solution. Ru particles were localized on scratches before etching (fig. 2a). After etching, a porous layer covered whole surface of Si (Fig. 2b and c). Pores, which have Ru particle at the bottom, were formed. The depth of pores, ca. 0.2 mm, is much shorter than 1 mm of depth calculated using the gravimetric procedure. These results indicate that Ru has much lower activity for electroless displacement deposition than that of other noble metals and high catalytic activity for metal assisted etching of Si similar to Pd. ACKNOWLEDGEMENTS The present work was partly supported by JSPS KAKENHI (26289276). REFERENCES Z. Huang, N. Geyer, P. Werner, J. Boor, U Gösele, Adv. Mater., 23,285 (2011).S. Yae, Y. Kawamoto, H. Tanaka, N. Fukumuro, and H. Matsuda, Electrochem.Commun., 5, 632 (2003).S. Yae, Y. Morii, N. Fukumuro, H. Matsuda, Nanoscale Res. Lett., 7, 352 (2012).S. Yae, M. Tashiro, M. Abe, N. Fukumuro, and H. Matsuda, J. Electrochem. Soc., 157, D90 (2010). Figure 1