Jin-Kwan Lee

Analysis of deposition phenomena and residual stress in plasma spray coatings

Abstract An analytical model was developed to describe the deposition process. Assuming the plane solid-liquid interface to move with the solidification rate calculated by this deposition model, the thermal history was also calculated by a finite difference method. The solidification rate changes periodically, causing a temperature fluctuation in the coating layer and substrate during deposition. As the liquid remains discontinuosly at the coating surface, interlamellar layers, such as oxide or glass layers, form discontinuosly. These phenomena can be confirmed in the Ni17Cr4Si3B deposit. Using the stress model proposed in this study, the stress distribution within the ZrO 2 coating and iron substrate was derived from the thermal history by a finite element method. The shape of the predicted residual stress distribution was in good agreement with that measured by X-ray diffraction in the PSZ coating. The differences between the theoretical predictions and experimental results are discussed.

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 ...

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...

Cyclic Deposition/Etching Process to Etch a Bowing-Free SiO2 Contact Hole

High aspect ratio SiO 2 contact holes were etched using a cyclic process, which consisted of alternating etching and deposition steps using C 4 F 6 /CH 2 F 2 /O 2 /Ar and C 4 F 6 /CH 2 F 2 /Ar plasmas, respectively, to minimize the sidewall bowing of the hole. A CF x -film deposition step was added to recover the eroded slope of the mask and to deposit an additional passivation film on the sidewall. The introduction of the deposition step also improved the etch selectivity of SiO 2 toward the amorphous carbon layer mask and the open ratio of the contact hole, which was defined as the ratio of the hole diameter at the bottom to that at the top of the hole. By regulating the durations of the deposition and etching steps in the cyclic process, the degree of bowing, which was 9 nm when using a continuous-etching process, was reduced to 1 nm during the etching of a 2.3 μm deep SiO 2 contact hole with a diameter of 118 nm.

Angular dependences of SiO2 etch rates in C4F6/O2/Ar and C4F6/CH2F2/O2/Ar plasmas

The angular dependences of SiO2 etch rates for C4F6/O2/Ar and C4F6/CH2F2/O2/Ar plasmas were investigated using a Faraday cage system. In the absence of CH2F2, the steady-state fluorocarbon film that formed on the SiO2 surface was thin enough (<10 A) for ions to penetrate through the film. The normalized etch yield (NEY) curve in this case showed a maximum value of 1.74 at an ion-incident angle of 70°, illustrating that physical sputtering was a major contributor to the SiO2 etching. The addition of CH2F2 to C4F6/O2/Ar plasmas produced thicker and more etch-resistant fluorocarbon films, leading to a decrease in the ion energy transfer depth through the steady-state films. This implies that physical sputtering was suppressed when CH2F2 was present in the plasma, demonstrated by a decrease in the maximum NEY and the ion-incident angle at the maximum NEY.