Jae-Ho Min

Redeposition of etch products on sidewalls during SiO2 etching in a fluorocarbon plasma. I. Effect of particle emission from the bottom surface in a CF4 plasma

The effect of etch-product redeposition on sidewall properties during the etching of step-shaped SiO2 patterns in a CF4 plasma was examined using a Faraday cage located in a transformer coupled plasma etcher. Sidewall properties were observed for two cases: with and without particles emitted from the bottom surface in normal contact with the sidewall. Particles sputtered from the bottom surface were redeposited on the sidewall, which contributes to the formation of a passivation layer on the surface of the latter. The passivation layer consisted of silicon oxide, SixOy, and fluorocarbon, CxFy, the latter comprising the major species. Ar plasma experiments confirmed that CxFy or a fluorocarbon polymer must be present on the sidewall in order for the SixOy species to be deposited on the surface. The redeposited particles, which were largely F-deficient fluorocarbon species, as evidenced by x-ray photoelectron spectroscopy analyses, functioned as precursors for fluorocarbon polymerization, resulting in a rou...

Oblique-Directional Plasma Etching of Si Using a Faraday Cage

A method for controlling the direction of Si etch profiles was developed using a Faraday cage system and a conventional gas-chopping process. The gas-chopping process consisted of sequentially alternating etching and deposition steps using SF 6 and C 4 F 8 plasmas, respectively. The single-directional oblique etching was achieved using a Faraday cage having a top horizontal grid plane, inside of which a tilted substrate holder was placed. The double-directional etching was also obtained using a cage having two slanted grid planes. The double-directional etch profiles were either symmetrically or asymmetrically oblique according to the angle of the sample holder. During the oblique-directional etching, the shadowing effect by the bombardment of tilt-angled ions was found to affect etch profiles. Due to the shadowing effect, the top width of the pattern and the etch depth decreased as the ion-incident angle increased, and a fluorocarbon film accumulated in the shadow area. By regulating the process conditions to allow a higher mask selectivity, an oblique Si etch profile with a high aspect ratio (etch depth = 5700 nm and aspect ratio = 9.5) was obtained.

Angular dependence of etch rates in the etching of poly-Si and fluorocarbon polymer using SF6, C4F8, and O2 plasmas

The dependences of etch rates on the angle of ions incident on the substrate surface in four plasma/substrate systems that constitute the advanced Bosch process were investigated using a Faraday cage designed for the accurate control of the ion-incident angle. The four systems, established by combining discharge gases and substrates, were a SF6/poly-Si, a SF6/fluorocarbon polymer, an O2/fluorocarbon polymer, and a C4F8/Si. In the case of SF6/poly-Si, the normalized etch rates (NERs), defined as the etch rates normalized by the rate on the horizontal surface, were higher at all angles than values predicted from the cosine of the ion-incident angle. This characteristic curve shape was independent of changes in process variables including the source power and bias voltage. Contrary to the earlier case, the NERs for the O2/polymer decreased and eventually reached much lower values than the cosine values at angles between 30° and 70° when the source power was increased and the bias voltage was decreased. On the other hand, the NERs for the SF6/polymer showed a weak dependence on the process variables. In the case of C4F8/Si, which is used in the Bosch process for depositing a fluorocarbon layer on the substrate surface, the deposition rate varied with the ion incident angle, showing an S-shaped curve. These characteristic deposition rate curves, which were highly dependent on the process conditions, could be divided into four distinct regions: a Si sputtering region, an ion-suppressed polymer deposition region, an ion-enhanced polymer deposition region, and an ion-free polymer deposition region. Based on the earlier characteristic angular dependences of the etch (or deposition) rates in the individual systems, ideal process conditions for obtaining an anisotropic etch profile in the advanced Bosch process are proposed.The dependences of etch rates on the angle of ions incident on the substrate surface in four plasma/substrate systems that constitute the advanced Bosch process were investigated using a Faraday cage designed for the accurate control of the ion-incident angle. The four systems, established by combining discharge gases and substrates, were a SF6/poly-Si, a SF6/fluorocarbon polymer, an O2/fluorocarbon polymer, and a C4F8/Si. In the case of SF6/poly-Si, the normalized etch rates (NERs), defined as the etch rates normalized by the rate on the horizontal surface, were higher at all angles than values predicted from the cosine of the ion-incident angle. This characteristic curve shape was independent of changes in process variables including the source power and bias voltage. Contrary to the earlier case, the NERs for the O2/polymer decreased and eventually reached much lower values than the cosine values at angles between 30° and 70° when the source power was increased and the bias voltage was decreased. On th...

Angular dependence of Si3N4 etch rates and the etch selectivity of SiO2 to Si3N4 at different bias voltages in a high-density C4F8 plasma

The dependence of Si3N4 etch rates and the etch selectivity of SiO2 to Si3N4 on ion-incident angles was studied for different bias voltages in a high-density C4F8 plasma. A Faraday cage and specially designed substrate holders were used to accurately control the angles of incident ions on the substrate surface. The normalized etch yield (NEY), defined as the etch yield obtained at a given ion-incident angle normalized to that obtained on a horizontal surface, was unaffected by the bias voltage in Si3N4 etching, but it increased with the bias voltage in SiO2 etching in the range of −100to−300V. The NEY changed showing a maximum with an increase in the ion-incident angle in the etching of both substrates. In the Si3N4 etching, a maximum NEY of 1.7 was obtained at 70° in the above bias voltage range. However, an increase in the NEY at high ion-incident angles was smaller for SiO2 than for Si3N4 and, consequently, the etch selectivity of SiO2 to Si3N4 decreased with an increase in the ion-incident angle. The ...

Deep etching of silicon with smooth sidewalls by an improved gas-chopping process using a Faraday cage and a high bias voltage

A silicon substrate, masked with oxide lines with a spacing of 1μm, was etched using a gas-chopping process designed to enhance mask selectivity and produce a highly anisotropic etch profile with ripple-free sidewalls. The gas-chopping process employed a high bias voltage of −200V and a Faraday cage in the etching step. The use of a high bias voltage relieved the curvature of sidewall ripples that are generated during etch cycles due to the bombardment of energetic ions on the convex portions of the ripples, thus flattening the sidewall surface. The use of a Faraday cage served to suppress the erosion of the mask, thus leading to a high mask selectivity. This can be attributed to an increase in the intrinsic etch selectivity of the Si substrate to the mask due to an increase in the ratio of neutral to ion fluxes on the substrate, and by the suppression of mask faceting due to a reduction in the etch yields of the oxide mask and a CFx film formed on the surface of the mask.

Dependences of bottom and sidewall etch rates on bias voltage and source power during the etching of poly-Si and fluorocarbon polymer using SF6, C4F8, and O2 plasmas

The dependences of bottom and sidewall etch rates on the bias voltage and source power in four plasma/substrate systems constituting the advanced Bosch process were investigated using a Faraday cage and a step-shaped substrate specially designed for the accurate observation of lateral and vertical etch rates. The four systems, established by combining discharge gases and substrates, were SF6/poly-Si, SF6/fluorocarbon polymer, O2/fluorocarbon polymer, and C4F8/Si. For etch systems using SF6/poly-Si, SF6/polymer, and O2/polymer, the degree of anisotropy showed a higher dependence on the bias voltage than on the source power. As the bias voltage was increased, the degree of anisotropy obtained in SF6/poly-Si decreased while that for the SF6/polymer and O2/polymer increased. The contribution of spontaneous etching by reactive radicals to the etch rates increased in the order of SF6/polymer<O2/polymer<SF6/poly-Si, while that of ion-enhanced chemical etching by ions to the degree of anisotropy increased in the ...

Angular dependence of SiO2 etch rate at various bias voltages in a high density CHF3 plasma

The dependence of the SiO2 etch rate on the angle of ions incident on the substrate surface was studied over a bias voltage range from −20 to −600 V in a high-density CHF3 plasma using a Faraday cage to control the ion incident angle. The effect of the bottom plane on the sidewall etching was also examined. Differences in the characteristics of the etch rate as a function of the ion angle were observed for different bias voltage regions. When the absolute value of the bias voltage was smaller than 200 V, the normalized etch rate (NER) defined as the etch rate normalized by the rate on the horizontal surface, changed following a cosine curve with respect to the ion incident angle, defined as the angle between the ion direction and the normal of the substrate surface. When the magnitude of the bias voltage was larger than 200 V, the NER was deviated to higher values from those given by a cosine curve at ion angles between 30° and 70°, and then drastically decreased at angles higher than 70° until a net depo...

Contribution of bottom-emitted radicals to the deposition of a film on the SiO2 sidewall during CHF3 plasma etching

The contribution of radicals emitted from the bottom of an etched pattern to the deposition of a film on the sidewall in a CHF3 plasma was investigated using a specially designed experimental system. In the experimental system, a target (1), simulating the bottom of an etched pattern, emits various types of radicals after collision with incident ions. Among the radicals and ions that are incident on a target (2), which is located in close proximity to the target (1), to simulate the sidewall of an etched pattern, ions were prevented from arriving at the surface by placing a double grid structure in front of the target (2). The deposition rate and the composition of the film formed on the target (2) were measured for the cases where the target (1) was made of different materials and subject to impingement by ions at different bias voltages. In another set of experiments, a rectangular tube of a predetermined length was located in front of the target (2) for the purpose of estimating the probability of radi...

Effect of sidewall properties on the bottom microtrench during SiO2 etching in a CF4 plasma

Factors that affect microtrench formation during the etching of SiO2 in a CF4 plasma were investigated using an experimental set-up, which comprised a Faraday cage and step-shaped substrates consisting of a SiO2 bottom and slanted sidewalls of different materials. This set-up permitted the etched micropattern to be observed on a magnified scale under conditions that are similar to actual processes. SiO2 and fluorocarbon polymer were used as sidewall surface materials to investigate the effect of sidewall properties on the microtrench. The depth of the microtrench characteristically changed with the sidewall angle, reaching a maximum at 70° under the condition used in this study. This can be explained by the effect of two competitive factors: secondary etching of the bottom by ions reflected from the sidewall, which contributes to the bottom etch rate, and the shadowing effect of the sidewall, which decreases the etch rate. Secondary etching, as estimated from the difference in the bottom etch rates betwee...

Etch Characteristics of Silsesquioxane-based Low Dielectric Constant Material in Fluorocarbon Plasma

The etch characteristics of hydrido-organo-siloxane-polymer (HOSP), a typical silsesquioxane-based low-dielectric material, were compared with those of silicon dioxide in CF4 and CHF3 plasmas. The etch-rate ratios of the two materials are more significantly affected by the types and pressure of plasma gases than by plasma bias voltage, indicating that the relative etch rates are determined largely by the density of radicals than by the energy of ions incident on the substrate surface. The etching of silsesquioxane is accompanied by an increase in the CF2 radical density and a significant decrease in F radical density, indicating that silsesquioxane is etched via the sequential dissociation of Si–CH3 and cage-like Si–O bonds by reaction with F radicals. Based on the findings herein, we propose that the relative amounts of cage-like and network Si–O bonds remaining in silsesquioxane after etching can be controlled by adjusting the parameter, (F radical density)2/(CF2 radical density).