poly-Si Etching Research Articles

Silicon etching in NF3/O2 remote microwave plasmas

The etching of silicon in remote microwave discharges fed with NF3/O2 has been investigated. In situ ellipsometry and x-ray photoelectron spectroscopy (XPS) were used to monitor surface effects, while mass spectrometry was used to monitor the gas phase dynamics. Varying the microwave power from 600 to 1400 W has little effect, due to the near complete dissociation of the NF3, even at lower powers. For discharges containing pure NF3, the poly-Si etch rate increases linearly with NF3 flow. When a low proportion of O2 (O2/NF3=0.1) is added to the discharge, the etch rate increases quickly to its maximum of ∼700 nm/min. With further O2 addition, this etch rate decreases below that observed for pure NF3 processing. The fluorine concentration in the processing region decreases for all O2 additions by a dilution effect. For pure NF3 discharges, XPS measurements reveal 1–2 nm thick, highly fluorinated reaction layers with a gradual loss of fluorine content as the NF3 flow is increased. Specimens processed with both NF3 and O2 show much less surface fluorination that decreases with increasing O2 content in the feed gas. At the etch rate maximum, the observed N (1s) signal is also maximized. The reaction layer thickness increases with added O2 and continues to more than 10 nm at O2:NF3 ratios greater than unity. We discuss the enhanced reactivity of the modified Si surface and compare our results with the role of admixed N2 into the CF4/O2 system. We also injected NO directly into the effluent of NF3 and CF4/O2 discharges. For fluorine rich discharges, NO removes the modified surface layer on Si and provides for an enhanced etch rate. In the oxygen rich regime, NO injection can increase both the etch rate and the reaction layer thickness. We will present a mechanism for the enhanced etching of Si in the presence of fluorine, oxygen and the NO molecule.

Characteristics of self bias voltage and poly-Si etching in pulsed helicon wave plasma

Characteristics of the plasma and the poly-Si etching in a pulse modulated helicon wave plasma were studied. The self bias voltage induced on the wafer by r.f. bias of 300 kHz was directly measured with a Pt wafer by high voltage probe. Even though the r.f. bias frequency of 300 kHz was less than the ion plasma frequency, it was observed that the self bias voltage was induced, which contradicts the conventional thoughts. In the off period of the source power in the pulse mode, the weak plasma was sustained only by the r.f. bias power. This weak plasma in the pulse mode induced higher self bias voltage compared with the continuous mode because the self bias voltage decreased as the plasma density increased. The pulse modulated helicon wave plasma with low frequency r.f. bias effectively eliminated the local side etching but enhanced the resist erosion due to higher self bias voltage. The Si etch rate and the selectivity of Si to photo-resist were enhanced by adjusting the duty ratio.

Degree of Cl2 dissociation and etching characteristics in high-density plasmas

At the same input power (1000 W), inductive coupled plasma (ICP) and ultrahigh-frequency (UHF) plasma sources produced electron densities of 1×1011 cm−3 at 3.5 mTorr, yet the UHF plasma was much less dissociated (30%) than the ICP (70%). This can be attributed to differences in the electron energy distribution functions between the ICP and the UHF plasma, especially at low pressure. Under these conditions, Al etching profiles and doped poly-Si-etching rates were investigated to determine how the degree of dissociation influences etching reactions. UHF plasmas produced completely anisotropic Al and poly-Si etching independent of the dopant used when Cl2 was feed gas, whereas under the same conditions, the ICP produced isotropic Al etching profiles and the poly-Si-etching rate was strongly dependent on the dopant used under the same conditions. This implies that the degree of dissociation strongly influences the surface reaction in a high-density Cl2 plasma.

Control of Cl 2 plasma by electron-beam-excited plasma and poly-Si etching

An electron-beam-excited plasma (EBEP) apparatus is one of the plasma sources to produce high density plasma (high ion-current) at low pressure. Its attractive advantage is that plasma parameters can simply be controlled by beem-gun parameters: accelarating voltage (V A) and discharge current (I D). Therefore, it is relatively easy to adjust the plasma parameters to suitable condition in process. As one of the applications, poly-Si etching of floating wafer is attempted in a pure CI 2 plasma by means of reactive ion etching. From the experimental results, it is found that etched pattern tends to change from isotropy to anisotropy owing to increase of sheath potential as V A increases, and that etching rate may be controllable because ion current density increases with I D

Comparison of two-dimensional and three-dimensional models for profile simulation of poly-Si etching of finite length trenches

The development of two-dimensional (2D) profile simulators for fabrication of microelectronics features has significantly progressed during the past few years and now enables modeling of etch profile evolution for many different plasma processing conditions. Increasingly complex devices which have three-dimensional (3D) (that is, asymmetric) structures are now being designed. These structures require improved dimensionality in profile simulators to capture their most important features. Under many conditions, such as circular via etching, two-dimensional profile simulators can be used to address 3D structures. A legitimate issue is to what degree these 2D approaches can indeed be applied to truly 3D structures. In this article, we present results from a 3D profile simulator for the purpose of comparing profiles for innately 3D features to results obtained from a 2D profile simulation. It has been found that profiles obtained from the 3D simulators exhibit greater sidewall sloping in three-plane corners than predicted by the 2D simulator. The implication to process design is that a greater degree of overetching will be required to clear these corners than predicted by the 2D simulators. Asymmetries as well as the angular spread of the ion flux distribution are examined to determine their role in 3D profile evolution.

Role of oxygen in ion-enhanced etching of poly-Si and WSix with chlorine

Ion-enhanced etch yields of poly-Si and WSi1.66 with beams of molecular and atomic chlorine and oxygen are reported as a function of neutral-to-ion-flux ratios. The ions used are 500 and 1000 eV Ar+ and 500 eV O2+/O+. Measurements were made in a vacuum chamber with chlorine and oxygen molecules/atoms effusing from an external discharge source as a beam and impacting the selected surface. The etch rates were measured using quartz-crystal microbalances onto which poly-Si and WSi1.66 were deposited. In the ion-flux limited regime, atomic chlorine in comparison to molecular chlorine enhances the etch yield of poly-Si by a factor of 4 and of WSi1.66 by a factor of 9. Therefore, the etch yield selectivity of WSi1.66 over poly-Si, which is 0.7 with molecular chlorine, increased to 1.5 with atomic chlorine in the presence of energetic Ar+ ions. The effect of neutral O/O2 on both molecular and atomic chlorine/Ar+ etching of poly-Si and WSi1.66 is negligible. However, with 500 eV oxygen ions, the etch yields of poly-Si with both Cl and Cl2 are greatly reduced in comparison to those with 500 eV Ar+. The etch yields of WSi1.66 are also lower with oxygen ions, but the reduction is smaller than for the poly-Si case. Therefore, the etch yield selectivity of WSi1.66 over poly-Si increases to 10 with Cl/oxygen ions.

Abrupt reduction in poly-Si etch rate in HBr/O2 plasma

The effect of oxygen on polycrystalline-Si (poly-Si) and SiO2 etching in hydrogen bromide (HBr) reactive ion etching plasmas has been studied by measuring etch rates and using x-ray photoelectron spectroscopy (XPS) to study compositional changes in the surface layer. The etch rate of the poly-Si increases dramatically from 30 to 235 nm/min as the O2 concentration increases from 0% to 25%, whereas the SiO2 etch rate gradually decreases from 3 to 1 nm/min. Above 30% O2 in HBr, the poly-Si etch rate abruptly decreases by a factor of 16 compared with that at 25%. From XPS analysis, it is found that the abrupt decrease of the poly-Si etch rate at O2 concentrations of more than 30% is closely related with the composition and thickness of an SiBrxOy layer formed during the HBr/O2 plasma exposure. The SiBrxOy layer has a composition of nearly SiO2. Br ions cannot permeate the SiBrxOy layer formed in plasmas containing 30% O2 in HBr (or greater), and therefore, the poly-Si etch terminates.

Proximity Effects Correction for Advanced Optical Lithography Processes

This paper describes the performance of various optical proximity correction (OPC) software packages in terms of correction accuracy and effectiveness. Although parts of these software packages have been made commercially available couple of years ago, their performance keeps improving by the new features and capabilities added continuously in time by the software vendors. In this study we address mainly the lithographic performance of the OPC techniques implemented in the software packages, including many of the new features which are still in a development phase. Corrections using one of these software packages were carried out for an advanced sub-0.3 µm i-line process after poly-Si etching. Corrections for an advanced 0.2 µm deep-UV process were carried out using two other OPC software packages, representing two different OPC techniques. Exposures were carried out on state-of-the-art i-line and deep-UV steppers equipped with high numerical aperture lenses and illuminators allowing the user to vary the coherence factor by keyboard control. Optimized optical settings towards reducing optical proximity effects have been used. Results on the effectiveness of reducing linewidth variation and line-end shortening for various pitches using these OPC techniques are reported.

Generation and extinction characteristics of negative ions in pulse-time-modulated electron cyclotron resonance chlorine plasma

The time dependence of plasma parameters in a pulse-time-modulated electron cyclotron resonance plasma of and Ar source gases is measured. It is found that plasma produces a large quantity of negative ions during afterglow and decay times of electron density, electron temperature and sheath potential of plasma are much smaller than those of Ar plasma. The negative ions in plasma stay for a long time in the afterglow and they quickly become extinct when the microwave power is turned on. These characteristics suggest that the pulse modulation of plasma produces a large amount of negative ions and changes the flow of charged particles through the sheath region to the substrate surface, which enables us to improve highly anisotropic and charge-free poly-Si etching.

WSi2/Polysilicon Gate Etching Using TiN Hard Mask in Conjunction with Photoresist

We found that the selectivity between TiN and poly-Si film was high enough in Cl2/O2 plasma to use TiN as an etching mask for poly-Si gate etching. We also discovered that the oxidation of the TiN surface is the reason why TiN serves as an etching mask in poly-Si etching using Cl2/O2 plasma. By using a TiN hard mask in conjunction with a thin photoresist, we were able to reduce electron shading damage. The low aspect ratio of the space of a line-and-space (L&S) pattern is the major reason for the reduction in electron shading damage in this TiN mask process. A portion of the TiN surface on which no charge up occurs is exposed to the plasma during the overetch step because photoresist is eroded by ion bombardment. This is an another reason for the reduced electron shading damage.

Mechanisms for Microscopic Nonuniformity in Low-Pressure, High-Density Plasma Etching of Poly-Si in Cl2 and Cl2/O2 Mixtures

Etch anisotropy and microscopic uniformity have been investigated in low-pressure, high-density plasma etching environments. Polycrystalline Si films masked with a photoresist pattern of lines and spaces were etched in electron cyclotron resonance (ECR) Cl2 and Cl2/O2 plasmas with additional rf biasing. Experiments were performed by varying the gas pressure, substrate temperature, and percentage of O2 added. Moreover, the profile evolution during etching was simulated taking into account the transport of neutral and ionic species in microstructures and the following surface reactions: adsorption of neutral reactants, ion-stimulated desorption of reaction products, surface oxidation, and redeposition of etch products. The etched profiles obtained in ECR Cl2 and Cl2/O2 plasmas were compared with simulated results, and interpreted in terms of the deposition of etch products and the surface oxidation that occur competitively during etching. Etch products desorbed from the surface in microstructures have a sticking coefficient S p\\lesssim0.1, while etch products arriving from the plasma have a much larger coefficient S p\\gtrsim0.5. It was also shown that the competitive surface oxidation by incoming oxygen atoms plays a more important role in achieving aspect-ratio-independent etching than the deposition of etch products: the increased deposition of etch products resulted in enhanced sidewall tapering with little change in vertical etch rate; the surface oxidation resulted in inverse reactive-ion-etching lag without significant change in sidewall tapering.

Effects of etch products and surface oxidation on profile evolution during electron cyclotron resonance plasma etching of poly-Si

Evolution of etched profiles has been numerically investigated during electron cyclotron resonance (ECR) Cl2/O2 plasma etching of polycrystalline Si. The calculations included the processes of chemically enhanced ion etching and passivation layer formation, which are important in such low-pressure, high-density plasma etching environments. The sidewall passivation was modeled by taking account of the redeposition of etch products on sidewalls of etched features. In addition, the effect of surface oxidation was also included in the model. Etched profiles were then simulated to examine effects of the neutral-to-ion flux ratio onto the substrate, sticking coefficient of etch products, and additional incident fluxes of etch products and oxygen atoms. Numerical results indicated that in typical ECR plasma etching environments, where the gas pressure is P0≲1 mTorr and the ion current density is Ji∼10 mA/cm2 onto the substrate, the chlorinated surface coverage α is microscopically nonuniform on sidewalls and bottom surfaces of etched features: α≲0.3 at the bottom and α≊1 on the sidewalls. This microscopic nonuniformity in coverage was found to lead to inversely tapered etched profiles without any sidewall passivation. These profiles changed to vertical and further tapered, when simultaneous redeposition of etch products were taken into account with their sticking coefficients Sp≳0.1. Furthermore, it was shown that in the presence of oxidation, the sidewall surfaces are easily oxidized, and the lateral etching is effectively reduced during overetch.

Dry etching of polysilicon with high selectivity using a chlorine-based plasma in an ECR reactor

Halogen-bearing gases have proved to be useful plasma discharge etchants for the fabrication of sub-micron poly-Si gate structures. In this paper, investigations of chlorine-based plasmas generated by an electron cyclotron resonance (ECR) reactor for poly-Si etching is studied. The influences of added oxygen, microwave power and RF power on etching characteristics are discussed. In addition, the etching mechanisms of the underlying oxide are developed. Finally, the Cl 2/HBr/O 2 mixed system is examined. The combined plasma exhibits the features of high selectivity, high anisotropy and high etching rate.

Negative ion measurements and etching in a pulsed-power inductively coupled plasma in chlorine

Anomalous side wall etching, called `notching' in gate poly-Si etching, is suppressed in a pulsed-power chlorine inductively coupled plasma (ICP). To understand the mechanism, comprehensive time-resolved measurements were performed on such key parameters as chlorine negative ion density, electron density, electron temperature and plasma potential. Comparison of these data with argon afterglows reveals a rapid electron cooling and a remarkable electron density drop which are caused by electron dissociative attachment forming abundant negative ions. The measurements of RF bias and plasma potential suggest a new mechanism of notch-free etching. Namely, the substrate potential in the positive RF phase instantaneously exceeds the plasma potential in the afterglow by a considerable amount, . Then electrons are accelerated through a sheath and neutralize positive charges on the gate oxide layer. Finally, the poly-Si etching process utilizing abundant is examined, focusing on the bias-frequency-dependence.

New ultrahigh-frequency plasma discharge for overcoming the limitations of etching processes

Electron, ion, and neutral temperatures are measured in an ultrahigh-frequency (UHF) plasma by Langmuir probe and Doppler-shifted laser-induced fluorescence. The electron, ion, and neutral temperatures are found to be 1.5–2.0 eV (Ar plasma), 0.066 eV for Ar+, and 0.036 eV for Ne, respectively, and are lower than those reported for electron cyclotron resonance and helicon wave plasmas. The low temperatures cause lower dissociations of CHF3 gas even in the plasma production region of the UHF plasma source. The plasma is expected to dramatically improve the selectivity of SiO2 to the underlying Si. Additionally, the plasma can be used to accomplish a notch-free poly-Si etching profile and microloading-free Si trench etching with a high etching rate and high anisotropy with a narrow space pattern of less than 0.3 μm. It is suggested that the charge accumulation with the narrow space pattern is eliminated because of the low electron temperature in the UHF plasma.

SiO2 Etching in C4F8/O2 Electron Cyclotron Resonance Plasma

We report the application of C4F8/O2 electron cyclotron resonance (ECR) plasma to SiO2 etching. The effect on SiO2 etching characteristics of O2 addition to C4F8 ECR plasma is investigated. In C4F8 ECR plasma (no O2 addition), strong reactive ion etching (RIE) lag effect of SiO2 and poly-Si etching is observed. However, with O2 addition, the RIE lag effect of SiO2 etching decreases markedly whereas the RIE lag effect of poly-Si etching decreases gradually. Consequently, SiO2 etching selectivity to poly-Si increases. The results of mass spectrometric measurement using two orifices indicate that surface reaction probability of fluorocarbon molecules is affected by O2 addition.

Particle contamination characterization in a helicon plasma etching tool

There is much current interest regarding the formation, transport, charging, and behavior of particulate contamination in high density plasma tools, as these tools are generally regarded as the future of plasma processing for the semiconductor industry due to the need to obtain greater anisotropy and faster process rates concurrently with reduced surface damage. Because of the low pressures in which these tools typically operate (<5 mTorr), the likelihood of homogeneous nucleation processes leading to particle contamination problems is low. Similarly, the effect of ion drag is also expected to be greater, possibly leading to reduced particle trapping effects. Yet, few laser light scatter studies have been performed in high density plasma tools. This study is the first in situ characterization of particle contamination in a plasma etching tool using a helicon source. It was performed during normal process conditions for poly-Si etching and also under intentionally altered process conditions designed to produce particles. The effect of a bipolar, electrostatic chuck on wafer particle deposition was also investigated. Results showed that under normal process conditions, few particles were deposited onto the wafer; those that were observed were attributed to thermal stress effects resulting in flaking of deposition films, probably on the quartz bell jar of the source. Results also suggest that the electrostatic chuck increased particle deposition when the clamping voltage was applied. No trapping was observed over the wafer or near the source. However, some trapped particles were observed below the wafer platform. These particles showed unusual motion, but probably have minimal effect on wafer contamination. The issues pertaining to particle contamination formation and transport in this high density plasma tool are discussed.

プロセス雰囲気と表面制御 ＥＣＲプラズマによるＰｏｌｙ‐Ｓｉエッチングにおける反応生成物の診断と形状制御

Plasma-surface interactions for poly-Si gate etching have been studied in Cl2 and Cl2/O2 electron cyclotron resonance (ECR) plasmas, through in situ diagnostics of reaction products during etching and numerical simulation for the etched profile evolution. The in situ observation of product species was conducted both in the gas phase and on the substrate surface, by employing Fourier transform infrared (FTIR) absorption spectroscopy. For both Cl2 and Cl2/O2 plasmas, silicon tetrachloride SiCl4 was the only IR-absorbing etch product species detected in the gas phase, while unsaturated SiClx (x= 1-3) as well as SiCl4 were observed on the surface; moreover, a broad absorption feature due to Si-O vibrations or silicon oxides was found to occur both in the gas phase and on the surface, implying competitive chlorination and oxidation of the surface, and also competitive etching and surface inhibitor deposition. Based on these observations, the surface chemistry was modeled for Cl2 and Cl2/O2 plasma etching of poly-Si, including ion-assisted reaction processes, physical sputtering or desorption of adsorbates by energetic ion bombardment, and passivation layer formation consisting of surface oxidation and inhibitor deposition. Moreover, numerical calculations were performed for the profile evolution during etching, by employing the surface chemistry model proposed and taking into account a variety of neutral transport mechanisms in microstructures that are known from the literature. The etched profiles numerically and experimentally obtained were then compared to unravel competitive mechanisms responsible for the anisotropy and microscopic uniformity in poly-Si gate etching in low-pressure, high-density chlorine-containing plasmas.

Role of Oxygen in Poly-Si Etching by Cl2/O2 Plasmas

The increase in poly-Si etch rate at low O2 concentration (1-5%) in Cl2/O2 plasmas was investigated using optical emission spectroscopy (OES) and X-ray photoelectron spectroscopy (XPS). The OES spectrum indicates that the Cl (837.5 nm) intensities increase at low O2 concentration during the poly-Si etching but the dissociation of Cl2 is not promoted by the addition of O2 to Cl2. SiCl (287 nm) intensities decrease in the range of O2 concentration where poly-Si etch rate increases. The result of XPS shows that by increasing O2 concentration from 0% to 3%, the amount of Cl and O adsorbed onto poly-Si surface decreases and increases, respectively. In Cl2/O2 plasmas, a cause of the poly-Si etch rate increase may be that oxygen atoms adsorb more easily on the poly-Si surface than chlorine atoms, and this weakens the Si-Si back bonds.

New Ultra-High-Frequency Plasma Source for Large-Scale Etching Processes

A low temperature, uniform, high-density plasma is produced by an ultra-high-frequency (UHF) discharge using a new spokewise antenna. The plasma is uniform within ±5% over a diameter of 30 cm. The plasma density, 1×1011 cm-3, for low electron temperatures of 1.5-2.0 eV, is almost proportional to the UHF power even at a low UHF power. No magnetic field is needed to maintain a high-density plasma. Consequently, the plasma source is fairly simple and lightweight. The plasma source can accomplish a notch-free poly-Si etching profile with a high etching rate at a narrow space pattern of less than 0.3 µ m.