Actually, due to this feature, wet etching of crystalline silicon carbide is extremely difficult as SiC is totally inert to all aqueous etching solutions at room temperature [ 76 ]. To our knowledge, the lowest etching temperature referenced in the literature was mentioned by Chu and Campbell in [ 77 ]. The use of other solutions like KOH is also feasible but it requires higher temperatures [ 78 ].
In addition, wet etching is often isotropic and, due to the severe conditions required for silicon carbide etching, difficult to localize. Consequently, due to the huge difficulties of wet silicon carbide etching, many efforts were enforced to develop a more user-friendly method. This is the case for plasma etching, which has since been the subject of intense research for decades. Historically, reactive ion etching RIE using a capacitively coupled plasma CCP reactor was massively investigated during the ss.
In this configuration usually just called RIE , a RF electromagnetic field is applied between the two electrodes located on both sides of the reactor. Then electrons are accelerated by the high-frequency electric field and ionize the molecules of the gas, leading to a plasma. Consequently, the ions produced can react with the material to etch. For that matter, this behavior is the source of the RIE appellation. Typically, this chemical reaction is isotropic, leading to sloped sidewall profiles. In contrast, according to ion energy, a sputtering effect of the material can also be observed, which mainly results in an anisotropic etching, then to vertical sidewalls.
The two effects coexist and the predominance of one effect compared with the other depends on the etching parameters power, pressure, etc. Therefore, in the s, another configuration was developed with the emergence of inductively coupled plasma ICP reactors.
In such a reactor, the plasma, generated by a RF magnetic field as previously shown RIE power , is also contained inside the chamber, which is encircled by an inductive coil ICP power. The great advantage of this configuration, in comparison with a RIE reactor, is the possibility to independently control the ion energy with the RIE power and their densities with the ICP power. Silicon carbide plasma etching has been largely investigated. However, as previously mentioned, silicon carbide is a material that is difficult to etch.
This is also the case using plasma etching. Then, the success of SiC plasma etching involves the use of severe conditions, which are rarely compatible with the masking materials. For example, photoresist, which is a classical masking material for plasma etching, only operates for thin SiC layers according to the etching selectivity, classically around 1. Silicon dioxide films have also been investigated to act as a mask for plasma etching but it requires thick layers, typically several microns [ 80 , 81 ]. As a consequence, due to the poor selectivity of these materials, a metallic mask is usually preferred.
Among them, aluminum has been largely investigated and results to a selectivity at least one order of magnitude higher than the one observed with photoresist [ 82 , 83 ].
- Silicon Carbide Micro Electromechanical Systems for Harsh Environments (2006, Hardcover).
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Unfortunately, the use of an aluminum mask induces a micromasking effect [ 84 ]. This phenomenon, which can lead to a grass-like surface of the SiC film, is explained by the formation of Al 2 O 3 , which is a nonvolatile species [ 82 ].
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Thus, nickel is widely used as a hard mask instead of aluminum as it presents the interesting detail of being chemically inert towards the chemical species of the plasma. Then, as nickel is only etched by ion bombardment, no micromasking effect is observed using such a metal, except if the mask design is not spaced out enough, which prevents the evacuation of nonvolatile species, as explained in [ 85 ]. In terms of chemistry, silicon carbide plasma etching has been largely investigated using fluorinated gases as it is generally admitted that fluor atoms react with both silicon and carbon to form, respectively, SiF x and CF y species [ 86 ].
These volatile species are then eliminated by pumping. In some studies, an additional gas, which could be argon or oxygen, is added to the fluorinated gas. Argon is attributed to promoting physical sputtering and also to increasing the dissociation of the plasma gas into reactive species, which therefore increase the etch rate [ 87 ]. In contrast, the role of oxygen is controversial.
Some authors suggest that oxygen atoms participate directly in the etching of the SiC film by the formation of CO and CO 2 species [ 88 ]. However, for higher fractions, it leads to a dilution of the fluorinated gas and then to a decrease of the etching rate [ 89 ]. This behavior was also observed by Jiang et al. Beheim et al. This behavior was hypothesized by the formation of a SiF x O y layer that could have a greater tendency to charge than SiC.
As a consequence, the charges on the sidewalls lead to the deflection of the incident ions, resulting in a microtrenching phenomenon. This same behavior has been observed by other groups [ 92 , 93 ].
3C-SiC — From Electronic to MEMS Devices
The aim is to promote the formation of a nickel oxide, which is more resistant to the plasma treatment [ 68 ]. It can be helpful for the plasma etching of thick silicon carbide layers as it increases selectivity. In conclusion, the benefit of using oxygen for silicon carbide plasma etching is still debated. Some examples of already completed 3C-SiC-based microsystems are presented in Fig.
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For most applications, the idea is to take advantage of the SiC physical properties. For example, the resonant frequencies of the vertical resonators presented in Fig. Indeed, state of the art silicon-based technology is not compatible with conditions encountered by most devices. To become sufficient, some silicon-based devices require the use of cooling system or radiation shielding. Moreover, for specific applications in the field of spatial or aeronautics, an increase of the weight leads to a severe rise of the cost [ 96 ].
To do that, two ways are possible as, as opposed to silicon carbide, silicon can be easily etched by means of wet etching.
Silicon Carbide Micro Electromechanical Systems for Harsh Environments (2006, Hardcover)
However, it is also possible to directly etch the silicon substrate by means of plasma etching [ 80 ]. Indeed, according to the design of the microsystem, this step can even be completed in the same run that the plasma etching of the SiC film as both materials require fluorinated gases to be etched.
It must also be noted that a modification of the plasma parameters can be helpful to favor the isotropic etching of the silicon substrate, in order to liberate the microsystems, as presented in Fig. As a consequence, many research works were focused on this mechanical property. The first method consists of penetrating the SiC material by using a hard tip whose mechanical properties are known. Usually, the geometry of the indenter is known with high precision, which is the case for the Berkovic tip presenting a three-sided pyramid geometry.
The indenter tip progressively penetrates the investigated material with the applications of increasing load. During the indentation process, depth penetration is recorded as a function of the applied load, resulting in a load vs. As this method cannot be used to provide an elastic modulus value in a particular direction, nanoindentation is more fitting for polycrystalline materials [ ].
The second method, mainly used for the determination of 3C-SiC mechanical properties, consists of determining the resonance frequency of clamped-free cantilevers, as illustrated in Fig. Indeed, as mentioned previously, the use of this method has highlighted the fact that 3C-SiC cantilevers are bended downwards whereas 3C-SiC cantilevers are bended upwards, which is clearly visible on submicron-thick cantilevers, and reveal opposite residual stress effects [ 68 , ].
Equation 1 assumes that the cantilever is free at one end and fixed to the bulk material at the other. Nonetheless, consecutive to the etching of the silicon substrate used to release the beams, an undercutting of the attachment region can be obtained. Consequently, the anchorage point is not totally fixed.
However, since , Tong et al. As the crystalline quality is closely dependent of the 3C-SiC deposition method, the dispersion could be explained by the defect density. In , Mastropaolo et al. In their work, cantilever resonators were fabricated from the two types of materials using films deposited by CVD. That same year, Locke et al. For a 2. Based on these studies, it was then difficult to clearly determine the influence of the defects towards the mechanical properties of 3C-SiC films. This result was in contradiction with the literature data obtained on thicker 3C-SiC materials.
This behavior was attributed to the defect density and the evidence that the mechanical properties of 3C-SiC films were severely affected by the defect density, which has been suggested since , was finally highlighted. More recently, Anzalone et al.
In , the same group also investigated the dependence of mechanical properties of 3C-SiC film with defect densities artificially induced by ion implantation [ ]. To conclude on this part, even if the defect density is not deeply detrimental for the functioning of MEMS devices, as defects can affect the 3C-SiC mechanical properties, their influence has to be taken into account. The chemical inertia and the temperature resistance of this material are also huge benefits to achieve microsystems that can operate in harsh environments.
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In , Michaud et al. The process was based on the use of the sandwiched silicon film acting as a sacrificial layer. Indeed, this result seems promising as, in , Anzalone et al. For example, using a thick 3C-SiC epilayer, the silicon substrate could be completely etched in order to obtain a self-supporting monocrystalline 3C-SiC structure. Such a feasibility could be very helpful for medical applications or for devices functioning in harsh environments for which the presence of a silicon substrate is restraining.
In addition, thanks in large part to the efforts engaged in controlling the doping level of 3C-SiC films, new MEMS devices could be achievable with, for example, the use of a highly doped layer acting as an electrode. For decades, silicon carbide has been the subject of intensive research activities. This material exists in more than identified structures called polytypes, but only 4H, 6H and 3C-SiC are commercially available. Among these polytypes, only the cubic one, 3C-SiC, can be grown on silicon substrates. This feasibility is a huge benefit to reducing the cost of the devices but, whereas SiC-based devices are more and more present in the market, 3C-SiC-based ones are lacking.
However, important headways have been reached for electrical and MEMS applications using this material. Then, the purpose of this chapter was to summarize the noticeable results obtained on this material.