4. MEMS application of PECVD a-SiC layers

#### 4.1. Patterning of SiC layers

Micro patterning of thin α-SiC film layer deposited in PECVD reactor is similar to the patterning of crystalline or polycrystalline SiC layers. The patterning can be done in Reactive lon Etching (RIE) or Inductive Coupled Plasma (ICP) deep RIE (DRIE) equipments by fluorine chemistry, i.e. using a fluorine-containing gas (CF "SFç or even CHF3) and O2 and using photoresist as masking layer. It must be mentioned that the selectivity of the etching process to Si (used as substrate), SiO2, or even to photoresist is not outstanding (ranging from 1.2 up to 0.5) [16]. Metal masking layers have thus been initially been choosen [38, 39]. However, metal masks cause a major problem, namely the micromasking effect: as dry etching progresses, metallic particles are extracted from the mask and re-deposited onto the film/substrate where they can continue to have a protective role, resulting in a very uneven and rough final surface. For instance, aNF JO2 chemistry was tested for etching a SiC layer [40] using to photoresist is mask, achieving etching rates of 0.135um/min. Similarly, HBr/Cl2 etching chemistries using SiO2 etch masks have been developed [41]. This latter chemistry allowed a high selectivity (20:1) of etching SiC with respect to SiO2, but the price to pay was a very low etching rate (0.02um/min). In a more recent work, Senesky and Pisano reported the usage of AIN as masking layer for SiC structures in an ICP DRIE reactor using SF / O2 chemistry [42]. The etching process yielded a SiC etch rate of 0.4um/min, having a high selectivity (SiC/AlN) of 16:1. Moreover, the anisotropy of the process was very good, as features with a sidewall angle of 10° were reported. In another work,Cl2 chemistry was used in an ICP DRIE reactor (by Pandraud et al.) for uniform patterning of a PECVD a-SiC layer for wave guide applications [43].

#### 4.2. PECVD α-SiC as masking layer

The intrinsic chemical inertness of SiC makes PECVD a-SiC an interesting candidate as masking layer for harsh wet and dry etching.

#### 4.2.1. Masking layer for orientation dependent etching of Si in alkaline solutions

A low etching rate of 78nm/h of low stress PECVDa-SiC in a 30%KOH solution was reported [19, 24, 37], while etching rates lower than 2nm/h in both 33%KOH at 85°C as well as in 25% TMAH are reported by Sarro in [16]. As expected, the etching rate depends on both the composition of the deposited a-SiC layer and its density. However, in general, the reported results are in the same range with the etch rate values of PECVD Si.JN. (13 nm/h) in KOH 20% at 85℃ [36] but much better than the etch rate of thermal SiO2 (462 nm/h)in the same etchant /44). As the reported etch rates of PECVD a-SiC have such relatively low values, we can conclude that PECVD α-SiC can be successfully used as a mask in silicon bulk micromachining processes. Furthermore, we have practically tested this hypothesis and demonstrated the feasibility of using PECVD a-SiC as masking layer, as is detailed in the next section.

#### 4.2.2. Masking layer for etching in HF based solutions

Deep wet etching of glass is an important technology for microfluidic applications [45]. The main etchant for glass materials is highly concentrated HF [46] (sometimes with a small amount of HCl [47] or HyPO4 added [48]). PECVD α-SiC is almost an inert material in these solutions, exhibiting etching rates lower than 10A/h [37]. Such an extremely low etching rate in highly concentrated HF solutions combined with the reduced value of the stress are the main request of a good masking layer for deep wet etching of glasses [46, 49-51]. In our tests, a compound masking multilayer 'sandwich' made to low stress α-Si/low stress α-SiC/photoresist seemed to be the best solution in terms of depth of the etch achieved without any damage of the mask (through- etching of 1 mm thick Pyrex glass wafers, equivalent of a 2.5 hour exposure to 49% HF). The experimental results of processing Pyrex glass wafers (Corning 7740) in such a manner are presented in Fig. 6. It can be noticed that the mask is fully intact after the etching process while the shape of the etched hole describes a perfect isotropic process. The a-Si was used in this case as an adhesion layer. If only PECVD a-SiC is used as masking layer, a hugely isotropical etching process can still be observed [50].

Figure 6. (a) Optical image of a glass wafer coated with PECVD a-SiC/Photoresist after etching through in HF 49%, and (b) a SEM picture of one of the resulted etch-through holes in the glass wafer.

#### 4.2.3. Masking layer for dry release structure in XeF2

Dry release in XeF2 is an emerging technology in surface and bulk micromachining of MEMS free-standing structures. The PECVD a-SiC films present a low etching rate in XeE2 gas (around 7 A/min) which makes the α-SiC very suitable as a structural layer for any dryrelease processes using a-Si or polysilicon as a sacrificial layer. Fig. 7 presents SEM images with PECVD a-SiC cantilevers fabricated using dry released in XeE2 [24].

Figure 7. PECVD a-SiC cantilevers fabricated using dry-released process in XeF5,

#### 4.2.4. Protective layer for harsh environment

PECVD a-SiC films are also very suitable for structures intended to operate in harsh environments, due to a-SiC 's large hardness (2.48kg/m²) [52], high fracture strength, high modulus [53], excellent wear resistance [54] and chemical inertness in acid or based solutions, low oxidation rate and strong covalent Si-C bonds [52]. Early work proved the potential of PECVD a-SiC as a potential material for encapsulation of micromachined transducers due to its good resistance in a large range of media such as piranha solution, HF and KOH [55].

The good mechanical strength and anti-stiction surface properties of PECVD a-SiC [56] as well as its inertness in corrosive environment recommends this material for diverse applications. An illustrating example is that of a 1um-thick PECVD α-SiC combined together with Teflon like fluoro-polymer coatings to reduce the demolding energy by a factor of about 10, compared to a bare silicon mold [57]. In another similar application PECVD α-SiC was used for its hardness and for its good step-coverage (improving the wall roughness generated during the deep RIE process) while the Teflon layer acted as an anti-stiction layer [58]. Decreasing the demolding energy, in this application, is equivalent with the increasing of the life-time of the Si mold (used usually for rapid prototyping on hot embossing tools).

In another application [30], a 1 µm-thick PECVD α-SiC layer was used as an anti-erosion coating layer of a piezoresistive pressure sensor. In order to reduce the residual stress in the a-51C protection layer (initially evaluated at -450MPa), annealing was performed at 450°C for 1h. The resulting stress value was of only +60MPa. The a-SiC protective layer also showed a good coverage of the Al metallization layer. The erosion testing was performed in 45%KOH solution at 80℃. The final PECVD a-SiC-coated pressure sensor showed a small decrease in sensitivity, but exhibited a high erosion resistance and less temperature dependence. A similar application was also reported [59], in which low stress PECVD at-SiC layer was used for a capacitive pressure sensor fabricated using surface micromachining. Aluminum was used as material for electrodes while polyimide was selected as a sacrificial layer.

#### 4.3. Fabrication of free standing structuresof PECVD x-SiC using to surface micromachining

The opportunity of using thick a-51 (amorphous silicon) layers [22] (up to 20um thick, from our own experience) in surface micromachining gives rise to new processing opportunities, especially for microfluidic applications. The α-Si sacrificial layer can be easily removed by wet etching in an alkaline solution (TMAH or KOH) or by dry release in XeF2 The PECVD a-SiC presents a high chemical inertness to all the etchants of the above mentioned processes, and, therefore, is a very attractive candidate as a structural layer for surface micromachining processes in which amorphous silicon is used as a sacrificial layer. An example of such a process used a 3um-thick free standing structure fabricated from PECVD α-SiC on a glass substrate using 9 µm-thick PECVD α-Si as sacrificial layer [22]. The structure was released using wet etching in 30% KOH at 80°C. The structure (presented in Fig. 8) shows also that the PECVD deposition process ensured a very good step coverage, the thickness of the vertical wall being identical with that of the layer on the horizontal surfaces [22]. In another example of using PECVD a-SiC as structural material for surface micromachining, 1μmthick self-sustaining microbridges and microtunnels were fabricated from PECVD a-SiC films (deposited at 320°C using CH4 and SiH4 as reactive gases), using a SiOxNy"%()z/zz/¥ rificial layer [60]. In a similar manner, self-sustained grids have also been fabricated [61].

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As an illustrative example, chips with 2.5um-thick PECVD α-SiC membranes were fabricated by bulk micromachining in order to test the biocompatibility of the a-SiC [37]. Fibroblast NIH3T3 cells were used in this study as the model cell line. It was observed that the presence of CH2 groups on the surface of the membranes improved the cell adhesion. Moreover, dipping for 1 minute in 40% NH f, which was performed mainly to reduce the density of native silicon oxide groups on the α-SiC surface, also improved the adhesion of the cells on the membranes' surfaces. Fig. 9 shows cell culture images taken 24 hours and 48 hours after starting the culturing.
