Vertical and bevel-structured SiC etching techniques incorporating different gas mixture plasmas for various microelectronic applications

This study presents a detailed fabrication method, together with validation, discussion, and analysis, for state-of-the-art silicon carbide (SiC) etching of vertical and bevelled structures by using inductively coupled plasma reactive ion etching (ICP-RIE) for microelectronic applications. Applying different gas mixtures, a maximum bevel angle of 87° (almost vertical), large-angle bevels ranging from 40° to 80°, and small-angel bevels ranging from 7° to 17° were achieved separately using distinct gas mixtures at different ratios. We found that SF6 with additive O2 was effective for vertical etching, with a best etching rate of 3050 Å/min. As for the large-angle bevel structures, BCl3 + N2 gas mixtures show better characteristics, exhibiting a controllable and large etching angle range from 40° to 80° through the adjustment of the mixture ratio. Additionally, a Cl2 + O2 mixture at different ratios is applied to achieve a small-angel bevels ranging from 7° to 17°. A minimum bevel angel of approximately 7° was achieved under the specific volume of 2.4 sccm Cl2 and 3.6 sccm O2. These results can be used to improve performance in various microelectronic applications including MMIC via holes, PIN diodes, Schottky diodes, JFETs’ bevel mesa, and avalanche photodiode fabrication.

A variety of fluorine-, chlorine-, and bromine-based plasma chemistries-including NF 3 , NF 3 /O 2 , SF 6 / O 2 , SF 6 /He, SF 6 /O 2 /Ar, ICl, IBr, Cl 2 /Ar, and BCl 3 /Ar 14, 17-20 -have been studied for SiC etching. Among them, fluorine-based chemistries are the most effective gases when compared to other mixtures in terms of their ease of implementation and etching rate. In general, SiC dry etching is conducted using fluorine radicals as a primary source, followed by additive gases such as O 2 , Ar, N 2 , and H 2 as a secondary source-to control and enhance the etching process. The highest etching rate can be achieved with both SF 6 and NF 3 , because of their rapid dissociation in plasma; however, SF 6 is the preferred feed gas based on cost and safety considerations 20 .
Recently, researchers have put plenty of efforts into developing various microelectronic devices based on the bevel structure. Large-angle bevel structure is commonly used in the application of PIN diodes, Schottky diodes, static induction transistors (SITs), and junction FETs (JFETs). To maintain the high performance and reliability of these devices, a smooth surface and accurate bevel angle should be guaranteed so that the issue of electric field concentration can be efficiently solved 21 . Several SiC diodes, transistors, and switches with high breakdown voltages (greater than 10 kV) have been reported, using a mesa shape with junction termination extensions formed by a notably large-angle bevel structure for SiC etching, which contributes to improve the breakdown voltage by alleviating the electric field crowding at the device edges 22 . As for the small-angle bevel, it plays an important role in the application of avalanche photodiodes (APD). Particular concern has been focus on the mesa structure with bevel sidewalls instead of vertical sidewalls. It can be referred that bevelling the sidewalls suppresses edge breakdown 23 . The effect of bevelled sidewalls is to increase the depletion width at the surface of the device, and therefore improve the breakdown voltage of the APD.
The use of the conventional SF 6 + O 2 gas mixture for bevel etch of the SiC material is limited by the difficulties in controlling both the mixture ratio and the RF power 24 . Accordingly, chlorine-based gas mixtures are carried out and have become notably promising for obtaining a smooth surface on the epitaxial layer. Most importantly, an independently controls of system parameters can be realized 25 .
In this work, the vertical etching of SiC using SF 6 /O 2 plasma material with different ratios have been studied, resulting in an improved etch rate of 3050 Å/min. We have also found Ni to be a robust etch mask material, allowing a high selectivity of 100:1. Bevel structures with a large angle ranging from 40° to 80° were studied particularly under different gas mixtures, including BCl 3 /Cl 2 and BCl 3 /N 2 , in which BCl 3 /N 2 shows the best results in terms of obtaining controllable angles through adjusting the ratio of BCl 3 with (BCl 3 + N 2 ). Large-angle bevels in SiC etching are significant for the growth of the epitaxial layer of AlGaN/GaN/SiC diodes, transistors, and switches. According to a preferred embodiment of the concept, an angle of approximately 60° optimizes the performance of the epitaxial layer. For other applications, smaller angles of approximately 40° are preferable, as they are less prone to peripheral breakdown 26 .
Small-angle bevel often encounters problems with early edge breakdown, as a locally enhanced electric field occurs on the etched junction surface at the active region of APDs during the mesa etching 27 . The formation of a small (~7°) SiC sidewall bevel structure is favoured to prevent such a premature edge breakdown 3 . Small-angle bevel structures (from 7° to 15°, in particular) are achieved using Cl 2 /O 2 gas chemistry, which has been found in miniature, highly reliable, low power consumption, SiC-based APDs.

Methods
This study combined various experiments to optimize the etching rate and selectivity on the 4H-SiC substrate. The performance of two different metal masks (Ni and Cr) was compared under a fixed SiC etching rate of 3050 Å/min. The more robust Ni mask, which allows for improved selectivity, was used as an etching mask in our other experiment. These masks were built to a thickness of 11 μm through e-beam evaporation at a deposition rate of 5 Å/sec and under a vacuum of 3.5 E −6 Torr.
Another experiment investigated the effects of different gas mixtures on the formation of various angles and bevel structures, ranging from as low as 7° (which can be effectively applied to APDs), 40°-80° (applicable for PIN diodes, Schottky diodes, and JFETs' bevel mesa), and up to as high as 90° (interpreted as vertical etching, for application in MMICs through backside via-hole etching).
Further experimentation evaluated the impact of mixture rations (from 0 to 100%) in SF 6 /O 2 gas on vertical bevel structure, using a fixed coil power of 2000 W and platen power of 200 W. The applied pressure was set up constantly at 20 mTorr, and the etching time was 5 h for all samples. Six samples were tested, with SF 6 gas flow rates varying from 0 sccm to 25 sccm at 5 sccm intervals, while the O 2 flow rate was inversely adjusted.
This study also developed an effective formation process for large-angle bevel-structured etching using BCl 3 + N 2 and BCl 3 + Cl 2 gas mixtures. Different conditions for SiC etching were studied using BCl 3 with ratios ranging from 0 to 100%. Fixed process conditions (coil power = 900 W, platen power = 300 W, process pressure = 5 mTorr, and etching time = 30 min) were used for the ICP dry etching process with both BCl 3 + N 2 and BCl 3 + Cl 2 . Furthermore, different mixtures of gases-including BCl 3 , BCl 3 + Cl 2 , Cl 2 , and Cl 2 + O 2 at different ratios-were studied as a means to obtain small-angle bevel structures. In total, six different tests were conducted, including BCl 3 alone at a volume of 6 sccm, BCl 3 + Cl 2 at volumes of 4.8 sccm BCl 3

Results and Discussion
Vertical Etching. Figure 1(a) demonstrates the changes in SiC etching rate when performed with different ratios of SF 6 + O 2 . The highest SiC removal rate, 3050 Å/min, is achieved at the SF 6 mixing ratio of 80%, whereas the etching rate tends to decrease for mixture ratios beyond 80%. The in reactive F + -ions, reactive gas dilution, removal efficiency of the etching products, decreased sulphur reaction efficiency, and competition from forming SiO 2 likely all combine to result in the increase and eventual decline of the etching rate 28 . Although the addition of O 2 to the SF 6 plasma provides another pathway for volatilizing C in the forms CO, CO 2 , etc.-thereby increasing Scientific RepoRts | 7: 3915 | DOI:10.1038/s41598-017-04389-y the SiC etching rate-it also produces SiO 2 on the surface, which can limit the etching process. As a result of this competition, an optimum O 2 ratio in the SF 6 /O 2 gas mixture of around 20% is obtained.
The cross-sectional images of the etched SiC, based on the SF 6 gas ratio of 40%, 60%, and 80%, are illustrated in Fig. 1(b) to show well-defined vertical etch structures, which verify the success of our proposed experiments and demonstrate the observed differences dependent on our proposed work from all of the etching conditions. Figure 1(c) summarizes detailed measurement results of the Ni and Cr applied in this work. The etching rates observed when using Ni and Cr masks are 30 Å/min and 77 Å/min, respectively, when all other parameters are held constant to allow a SiC etching rate of 3050 Å/min. This discrepancy results in a selectivity of 100:1 for SiC:Ni and 40:1 for SiC:Cr. Because of this, the SF 6 -optimized etching conditions for SiC exhibit higher etching rates when Ni metal masks are used, approximately 3050 Å/min with a selectivity of 100:1. This was experimentally  Table 1. Summarization of the detailed information of all experiments conducted in this work. All of the experiment is based on 4H-SiC substrate, Ni mask is applied in the process of vertical etching and large-angle bevel etching, and AZ4620 photoresist is used as the etching mask for small-angle bevel etching. verified using a gas mixture of SF 6 (20 sccm) + O 2 (5 sccm). This preferential selectivity occurs because both the Ni and Cr metals react with fluoride gas, respectively forming NiF 2 and CrF 3 , and the by-product NiF 2 is more stable than CrF 3 in air. As NiF 2 is less volatile (the sublimation temperature of NiF 2 is 1474 °C, whereas the anhydrous form of CrF 3 sublimates at 1100-1200 °C), it causes less damage to the surface during etching, resulting in the higher selectivity 29,30 . The slow etching rate observed with the Ni mask demonstrates that Ni is versatile enough be used as mask under various etching conditions. In contrast, Cr masks work only in fluorine-free environments, which minimize mask erosion. Atomic force microscopy (AFM) images are exhibited in Fig. 1(d), illustrating surface roughness after the following etching of gas mixtures with different ratios. It can be observed that surface roughness becomes less smooth with increasing of the SF 6 proportion in SF 6 + O 2 . As the ratio of SF 6 increases, the surface roughness gradually deteriorates from 6.7 nm to 11.3 nm-14.0 nm before suddenly rebounding to 22.1 nm when the flow rate of SF 6 is 25 sccm. The deterioration in the surface roughness occurs primarily because low-volatility reaction products, such as CF x (CF 2 , CF 3 ), are generated and scattered across the etching surface as a secondary mask. Generally, when more fluorine ions participate in the reaction, more products are created and etching surface becomes rougher. In addition, serious roughness deterioration occurs when the gas ration is over 80%, this is due to the fluorine atoms are of primary interest and they act as the main chemical reactants. The fraction of free surface increases rapidly, providing favourable conditions for both the physical sputtering etching and the chemical reaction. The optimized parameters used to achieve the 3050 Å/min etching rate with ICP dry etching are: coil power = 2000 W, platen power = 200 W, processing mixture = SF 6 (20 sccm) + O 2 (5 sccm), and processing pressure = 20 mTorr. Figure 2 shows a series of scanning electron microscopy (SEM) images of a 70-µm via-hole array formed on a SiC substrate, as obtained in this study.
Large-angle Bevel Etching. Even though the SF 6 /O 2 gas mixture can achieve a relative high etching rate, it is not the ideal solution for bevel etching; the SiC etching profile remains unchanged through variations in component ratio of the SF 6 /O 2 gas mixture. This is because the most impactful species in SF 6 /O 2 plasma processing is created when electrons collide with neutral gas molecules. These collisions result in dissociation (leading to radical formation), ionization (SF 5 + ), and excitation, in accordance with the energy required for each process 31 . In an SF 6 /O 2 -SiC etching system, the O 2 passivates the SiC surface with a SiO 2 layer; subsequently, SF 5 + ions etch these passivations and allow the F + radicals to etch the SiC substrate beneath. Thus, anisotropic SiC etching is achieved.
Given this process, a BCl 3 -based etching atmosphere is considered for bevel SiC etching. Figure 3(a) demonstrates the SiC etching rate changes according to the contents of the processing gas; as the total flow rate of the processing gas is kept constant at 40 sccm, the content ratio of the N 2 or Cl 2 gas is adjusted to obtain various ratios (from 0 to 100%) of the whole gas mixture. During the BCl 3 + Cl 2 gas mixing process, the etch rate is observed to be inversely proportional to the content of BCl 3 . This occurs because the addition of chloride gas reduces the densities of positive ions and electrons. As a consequence of this higher dissociation threshold energy, BCl 3 can absorb more energy before molecular dissociation than Cl 2 32 . The highest SiC removal rate (1330 Å/min) is observed when the BCl 3 + Cl 2 gas mixture is held with a BCl 3 content ratio of 0%. The removal rate decreases dramatically to 407 Å/min at a BCl 3 content ratio of 100%. In contrast, the SiC etching rate when using a BCl 3 + N 2 gas mixture does not vary consistently with BCl 3 ratio. Instead, the etching rate increases almost linearly as the BCl 3 content ratio increases from 0 to 60%, but decreases significantly and abruptly beyond this point. The highest removal rate (860 Å/min) was observed at a 60% content ratio, a rate which falls to 407 Å/min as the BCl 3 content ratio reaches 100%. The addition of BCl 3 to N 2 plasma initially results in a significant increase in etching rate, caused by increased BCl 3 dissociation. The dissociation of BCl 3 stems from an increase in electron temperature, the result of energy transferred from the N 2 metastable. As such, the N 2 metastable is responsible for the both the increased dissociation and enhanced production of the etch species. However, the continuous increase in BCl 3 content ends up reducing the etching rate, which is most likely the result of a decrease in the effective volume density of the BCl 3 -ions and reactive Cl − . Such a decrease reduces the ion bombardment and associated chemical reaction between the SiC substrate and Cl − -atoms while the ICP power is maintained; thereby reducing the number of reactive species and ultimately leading to a decrease in etching rate.
In accordance with previously reported quadrupole mass spectrometry results 33 , BCl 2 , BCl, Cl 2 , and Cl will exist as a function of the percentage of N 2 in the flow. The Cl 2 intensity can increase up to several times the initial value for BCl 3 in the presence of 60% N 2 . Under these conditions, the Cl − intensity will decrease, but continues to exhibit a local maximum at 80%. The recombination tendency of Cl − , reforming Cl 2 during transport to the mass spectrometer, may account for the low measurements Cl − intensity that result in the observed high SiC etching rate. Figure 3(b) shows the obtained etching profile angle as a function of the gas mixture ratio. Large etching angles-ranging from 70° to 77°-are achieved by applying the processing gas mixtures BCl 3 + Cl 2 , whereas in the case of BCl 3 + N 2 the angle tends to increase with the gas mixture ratio. An etching profile angle of approximately 40° is noted at a BCl 3 + N 2 gas mixture ratio of 20%, and it increases to approximately 75° at a BCl 3 gas mixture ratio of 100%. However, the use of a BCl 3 + Cl 2 gas mixture plasma results in (almost perfect) smooth vertical walls with relatively good anisotropy, because of the continual existence of the diboron tetrachloride (B 2 Cl 4 ) thin polymer layer, which produces sidewall passivation. We believe that the narrow-range profile angle changes are caused by the generation of chloropolymers on the freshly etched SiC surface, resulting in B 2 Cl 4 that cannot be easily etched with BCl 3 and/or Cl 2 . The controllable angle characteristics of BCl 3 + N 2 gas mixtures can be attributed to the results of anisotropic etching and N 2 -promoted passivation. First of all, the mask edges might be oxidized by the pristinely etched SiC surface generated by the O 2 residual, resulting in SiO x thin films. The oxygen sources can be residual oxygen gases or reaction by-products from the hard mask 34 . As soon as N 2 (<40%) is added to the BCl 3 mixture gas, a slight compound-nitride-like passivation layer is deposited. At this point, the SiC sidewall will remain close to vertical, showing the SiO x layer is thick and dense enough to prevent chemical etching reactions between neutral chlorine species and the sidewall. With the continuous increase of N 2 (>40%), passivation deposition becomes excessive, the etch rate decreases, and the sidewall becomes more extensively profiled. Moreover, a grass-like roughness appears at the bottom of the etched area, caused by the formation of a compound-nitride-like passivation layer, which requires a 1:6 buffered oxide etching process for thorough removal from the thin SiO x film. Similar SiO x and nitride-based passivation layers are observed on InP wafers, etched by BCl 3 /N 2 and Cl 2 /N 2 gas mixtures 35,36 .
To investigate the effects of N 2 and Cl 2 in a BCl 3 plasma, gas flow ratio of the two gases are separately varied from 80% to 0%. Under this procedure, an evaluation of improvements in sidewall etching profile control can be conducted, observing the separate effects of N 2 and Cl 2 gas. Corresponding SEM images of etching profile angles are shown in Fig. 4(a 1 -a 4 ) and Fig. 4(b 1 -b 4 ). A wider angle range, from 41° to 78°, is observed when using N 2 + BCl 3 as the etching gas mixture; in comparison, using Cl 2 + BCl 3 results in larger angles within a more narrow range, from 70° to 77°. These findings show the BCl 3 + N 2 mixture is an optimal choice for wide-range, tuneable, large-angle SiC bevel etching.

Small-angle Bevel Etching.
Small-angle bevelled mesa etching demands that focus be placed simultaneously on achieving small angles (~7°) and ensuring surface smoothness, a requirement for effective application in APDs. This prevents the premature breakdown around the mesa edge termination referred to at the beginning of the paper. Aiming for the smallest possible bevelled sidewall increases the depletion width at the surface of the APD, in comparison to bulk. As such, the electric field is lower at the edges than the centre, further contributing to quite high breakdown characteristics and allowing bulk breakdown to precede surface breakdown. In an SF 6 + O 2 mixture, the chance of polymer formation during photoresist etching increases greatly with the increase in O 2 ratio. These polymers, once formed by plasma, function as very tough microetching masks that transfer to the SiC surface and eventually obstruct SiC etching. Additionally, by using SF 6 , more fluoride ions participate in the reaction, creating more by-products. These by-products are then responsible for generating roughness in the polymer surface, because of scattering on the etching surface as a secondary mask 37,38 . This effect restricts the use of SF 6 + O 2 as a gaseous mixture for small-angle bevel etching. BCl 3 could be another choice for achieving small-angle bevels; however, the anisotropy characteristics of BCl 3 only allow large-angle bevels (≥70°), unlike some other mixture gases. As mentioned before, if a gas such as N 2 is added to the BCl 3 atmosphere, a relatively small-angle bevel (almost as low as 40°) may be formed. However, for the preparation of ultra-small SiC sidewall bevel angles, a larger etching selectivity between the etching mask and the etched object must be obtained, properly adjusted, and controlled.
In this study, a well-patterned photoresist is used as the etching mask due to its chemical activity, as shown in Figure S1 and S2 (supplementary section). Given that Cl 2 is more inert to photoresist than BCl 3 , it enable etching of smaller angles (<20°); and with the addition of a regular photoresist reactant gas (i.e. O 2 ), the mixture gas is likely to allow even smaller bevel angles. The smallest bevel etching angle (as low as 7.63°), is achieved by using a Cl 2 /O 2 gas mixture with a gas flow ratio of 2.4 sccm for Cl 2 and 3.6 sccm for O 2 (shown in Fig. 5). This indicates that when the majority of the gaseous mixture is O 2 , photoresist erosion is significantly accelerated without inducing further SiC etching. The etching depth, etching rate, and selectivity thus obtained are 1.23 µm, 411 Å/ min, and 1:3.03, respectively. The use of Cl 2 gas alone at 6 sccm results in a maximum etching depth of 2.28 µm and an etching rate of 711 Å/min, whereas the minimum etching depth of 0.33 µm and an etching rate of 109 Å/ min are obtained by using BCl 3 gas at 6 sccm. The use of Cl 2 as an etchant results in a small angle, whether alone or mixed with O 2 . As the ratio of O 2 is increased with respect to Cl 2 (from 0 to 3.6 sccm of O 2 ), the bevel angle is found to decrease from 17.91° to 7.63°, at the expense of decreasing both the etching depth and etching rate, which decrease from 2.28 to 1.23 µm and from 761 to 411 Å/min, respectively. With the continuous increase of O 2 from 4.8 sccm to 6.0 sccm, the etch selectivity of all samples increases monotonically with the increasing O 2 flow rate, which shows that the patterned photoresist is not applicable as a suitable etching mask for SiC ICP-RIE. The surface roughness of the SiC substrate depends on the etching conditions, and therefore remains another important parameter when evaluating the small-angle bevel etching quality, particularly during the APD mesa termination fabrication. The SiC layer surface morphologies produced by different Cl 2 -related gas mixtures are shown in Fig. 6-via AFM-revealing that the use of Cl 2 + O 2 gas mixtures causes a slight change of surface morphology. In the absence of SiC etching, surface roughness is found to be 36.0 nm, and increases slightly in the presence of the various Cl 2 + O 2 gas mixtures. Using Cl 2 alone, the surface roughness is found to be 62.8 nm. As the amount of O 2 increases, the SiC surface roughness deteriorates from 67.4 nm to 78.4 nm. The surface roughness worsens with the formation of oxide films in the presence of excessive O 2 , greatly reducing the etching rate in certain areas 39 . However, when the O 2 content increases to 3.6 sccm, the surface roughness improves to 71.3 nm. Unevenly etched surfaces occur primarily because of either splits in the metal mask material and plasma polymer residues, which scatter throughout the etching region and form a microscopic mask. Oxygen plasma can help removing this scattered layer. Therefore, adding an appropriate amount of O 2 can improve the surface roughness.

Conclusions
In this work, different gas chemistries were used to obtain a variety of bevel angles for SiC substrates, and their performance for ICP-RIE etching was investigated and analysed. Results showed that a high etching rate and improved selectivity for vertical etching can be obtained with the use of a fluorine-based gas mixture (i.e. SF 6 ) in conjunction with different ratios of O 2 and an Ni mask. Additionally, it was found that a wide, controllable  A surface morphological study of the effects of different Cl 2 and O 2 ratios, using atomic force microscopy data. The results suggest that on using an appropriate gas mixture ratio, the resultant surface roughness is acceptable for APD applications. range of bevel angles can be achieved in large-angle bevel structure formation by using a BCl 3 + N 2 gas mixture. It was also demonstrated that Cl 2 gas can effectively achieve small-angle bevel structures, and that the addition of different ratios of O 2 can further reduce the small-angle bevel angle to as low as 7°. Furthermore, it was shown that surface morphology is not significantly affected by the use of Cl 2 and O 2 etching gas mixtures, with a smooth surface being maintained on the SiC layer.
The vertical SiC etching process developed here can be applied to large volume manufacturing for future MMIC backside via etching and source grounding applications. Large-angle bevels in SiC etching are significant for the bevel mesa of diodes, transistors, and switches. Moreover, the issue caused by electric field concentration could be overcome through large-angle bevel structure, so that high performance and high reliability can be obtained for the devices including PIN diodes, Schottky diodes, SITs, and JFETs. Finally, with optimal small-angle SiC bevel structures and a smooth surface morphology, the leakage current of the APDs could be effectively reduced, and greatly improved breakdown properties will be achieved as predicted in other works.