A scalable, resource-efficient process for synthesis of self-supporting germanium nanomembranes

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Introduction
Germanium (Ge) thin films are used across a wide spectrum of scientific and technological applications.Due to its unique and advantageous properties, Ge is an attractive choice of material for electronics [1], photonics [2], optoelectronics [3,4], and next-generation solar cell technology [5] due to its higher carrier mobility and a smaller band gap as compared to the primary semiconductor material silicon (Si) [1].Optically, Ge is semitransparent in the infrared part of the electromagnetic spectrum which makes it important for applications in infrared optics [6].Due to its excellent electrical properties and its ability to absorb a broad range of light, including the near-infrared spectrum, Ge is constantly studied as a potential material for high-efficiency photovoltaic cells [7,8].The emerging field of silicon photonics attains attention to Ge properties for the development of optoelectronic devices compatible with the current Si technology, such as photodetectors, LEDs, and lasers [9,10].Understanding and leveraging these properties is crucial for advancing the utilization of germanium in various scientific and industrial domains.Several deposition techniques are used to achieve desired Ge film properties with precise control over thickness, uniformity, and crystal structure such as physical vapor deposition (PVD) [11], chemical vapor deposition (CVD) [12], and molecular beam epitaxy (MBE) [13].However, thin films on a substrate may have their properties influenced or altered by the substrate materials.On the other hand, self-supporting Ge membranes and Ge alloys are free from substrate-induced effects like strain, stress, or interfacial reactions, leading to purer and more representative behaviors and allowing for a more accurate understanding, characterization and utilization of the intrinsic properties of the membrane material [14,15].Self-supporting membranes offer additional advantages such as being lightweight, flexible and potentially resource effective [4,[16][17][18] and their stacking permits to explore physical properties of systems not readily achievable within conventional synthesis [19].Furthermore, bulk Ge has been shown to develop into a highly dense network of nanoscale pores under ion irradiation [20][21][22].By inducing the same porosity effect on self-supporting Ge nanomembranes, the nanoporous membranes can be used for multiple applications in gas/liquid phase separation, nanofiltrations, and simultaneous filtration-catalysis. Similar applications have been demonstrated using self-supporting graphene [23], molybdenum disulfide [24] and hybrid graphene/carbon nanotubes [25], but have not yet been demonstrated using self-supporting Ge membranes.
Still, transitioning to scalable, cost-effective manufacturing processes for commercial applications is challenged by process complexity, high cost of processing, substrates damage and contamination of the final product [4].Developing production methods that maintain film quality at large scale is a key area of research.One such cost-effective approach for synthesizing large area self-supporting layers is the Porous Germanium Efficient Epitaxial LayEr Release (PEELER) process.In this process, high-quality ultrathin semiconductors are grown on a porous Ge substrate and then later peeled off from the substrate to produce wafer-scale single crystalline membranes with the possibility to recondition the Ge substrate [17].As research progresses, addressing these challenges will contribute to unlocking the full potential of germanium thin films in various applications.In this paper we present a guide to craft self-supporting Ge membranes with sub μm thickness spanning many hundreds of μm based on a strategy which employs hard Al mask coating combined with highly selective reactive ion etching (RIE) of Si substrates plus a final wet etching of the underlying oxide layer.The fabrication procedure enables large-scale processing on full-scale Si wafers.Hence, scalability for practical applications is feasible.As Ge is a scarce element, ranked 50th in relative abundance, our method employing the sputtering of ultrathin Ge layers from a Ge target is also very resource-efficient, and hence leads to low-cost.The process in its fundamental steps employs different plasmas in vacuum environment to deposit inorganic layers, or to remove the material from the surface when combined with chemical reactions.An optimal balance is discussed between anisotropy, selectivity, and fast etching of the substrate in order to increase the fabrication yield while maintaining high quality of the end product.The obtained Ge self-supporting membranes characterized using optical microscopy, scanning electron microscopy (SEM)/Energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and nuclear microprobe analysis.These techniques collectively provide a comprehensive understanding of the structure, morphology, and composition of the membrane, aiding in optimizing the fabrication processes and tailoring the physic-chemical properties of the membrane.

Experiments
The synthesis process for the self-supporting membrane developed for this work consists of 15 main steps, described below in detail.The outline is summarized in Table 1.The separate steps of the process are illustrated in sequential order by cross section view in Fig. 1.Additional details on the employed equipment as well as the processing are available in the supplementary material.
The process starts with cleaning single-side polished Si (100) wafers using the standard RCA process to remove all organic and inorganic contaminants from the silicon wafer surface (step 1).The native oxide layer residing on the Si substrates is subsequently removed using hydrofluoric acid (buffered oxide etch BOE:water 7:1).In step 2 the cleaned substrates are oxidized in a furnace at a temperature of 1000 • C exposing it to an ultra-high purity pyrogenic steam of water.After 3 h, ~500 nm thick SiO 2 layers are formed on both sides of the Si substrates.The thickness of the oxide layer is controlled by the temperature and the duration of the process, which is subsequently checked using spectroscopic ellipsometry.Due to the high selectivity of the reactive ion etching (RIE) of Si against SiO 2 , this oxide layer will provide a good shielding for the Ge membrane during the backside etching of the Si substrates.For the backside etching, a 900 nm layer of aluminum (Al) is deposited on the oxide layer using the Von Ardenne CS730S sputter (step 3).The deposition conditions are provided in the supplementary material.The Al mask is very resistant to deep inductively coupled plasma (ICP-RIE) etching of Si.
Next, a positive-tone photolithography is used to create an opening for the backside etching.A 1.2 μm photoresist (PR) layer is spin-coated (step 4) onto the Al layer, and then selectively exposed (step 5) through a glass mask with ultra-violet light.After developing, a periodic square pattern of 1 mm × 1 mm with a pitch of 10 mm is defined (Fig. 2a).This PR pattern determines the opening of the backside etching and sizes of the final membranes.The Al parts where the layer is no longer protected by the PR layer (Fig. 2b), are removed using ICP-RIE with chlorine-based chemistry (step 6).The plasma working conditions are provided in the supplementary material.A quick visual inspection with an optical microscope shows that the color at the bottom of the square pattern changed from silver (Al) to blue (SiO 2 ) in Fig. 2c.The SiO 2 layer exposed after the Al etching, is subsequently removed using BOE (step7).This side of the substrate is now ready for the back-side Si dry etching (Fig. 2d).A second inspection can be done by measuring the groove depth with a stylus profilometer which should be equal to the thickness of the layers removed: the photoresist + Al layer + the SiO 2 (1200 + 900 + 500 = 2600 nm, approximately).The photoresist masking layers are removed (step 8) by dipping the wafer into an acetone ultra-sonic bath for 3 min.
For the upcoming deposition of the Ge layer, organic residues or any type of trace impurities on the surface have been shown to weaken the adhesion of the Ge film to the substrate.Therefore, it is important that the samples are exposed to oxygen plasma cleaning (step 9) to remove such contamination.The cleaning process appears to strongly enhance the adhesion of the Ge membranes as delamination of the Ge layer is no longer observed.The process is done with a Tepla 300 system at the power of 50 W for 5 min.This cleaning process is gentle and does not affect the Al and or SiO 2 layers.In step 10, the Ge layer is deposited with a PREVAC magnetron sputtering system.Additional information and insights on the thin film deposition are presented in the supplementary material.During the sputtering step, the deposition rate was monitored by the built-in quartz crystal microbalance (QCM).With the chosen sputtering parameters, the QCM sensor built in the sputtering chamber measured a deposition rate of 15 nm/min.The film thickness of ~300 nm was achieved by timing the deposition.The post-sputtering thickness was later identified by measuring with the spectroscopic ellipsometry and the SEM (Fig. 3).Now that the Ge membrane is deposited, it is advantageous to coat it with a PR layer to protect it from the subsequent steps and to initially provide extra mechanical support when the Ge layer becomes a selfsupporting membrane.The resist layer with a thickness of approximately 1200 nm is spin coated (step 12) on the Ge side of the wafer and only soft baked at 100 • C for 3 min to make the subsequent resist removal easy.It is worth noting that the Al mask (step 3-8) and the Ge deposition steps are interchangeable as long as the Ge film is covered with a protecting resist layer which can be removed later using acetone.
The wafer is then introduced into the ICP-RIE system, where deep etching of Si (step 13) with reactive ions takes place.All details on the plasma etching conditions are mentioned in the supplementary material.The Al mask is used to transfer the pattern into Si.Using the Bosch etching process, the repeated sequences of etching and passivation steps result in anisotropic etching with vertical side walls.The surface morphology does not change with the ongoing etching until it reaches the silicon oxide layer.The etching rate is highest in the center of the groove, which is why the breakthrough of the Si substrates starts from the center towards the edges of the cavity.(Fig. 4 a and b).The reflective violet/blue color is characteristic of thin SiO 2 layer and it can be even detected by eye.This observation determines the end-point of the Si etching.For the present membranes, the etching procedure took 28 min (± 2 min) at an average etching rate of 19 μm/min.
Before proceeding with the SiO 2 etching step using BHF, it is crucial

Table 1
Outline of the multi-step process for fabricating the self-supporting Ge membranes.
Step  to etch the remaining layers of the Al mask, because the HF acid solution can otherwise react with Al to form Al-related microdeposits.These deposits attach to the back side of the Ge membrane and hence severely contaminate the material.The Al mask is removed with the Al etch ICP-RIE recipe, leaving behind the SiO 2 /Si substrate.Note that the Al etching will cause no noticeable damage to the underneath SiO 2 /Si substrate nor to the SiO 2 layer at the bottom of the groove.Up to this point, SiO 2 is the only remaining layer before the Ge membrane.The droplet-by droplet etching technique with BHF allows for accurate and controlled removal of the SiO 2 (step 14).The BHF solution selectively dissolves the SiO 2 while leaving the Si and Ge relatively unaffected.A small volume of the solution is loaded onto the surface of the substrate with a dropper or a pipette, creating a droplet sufficient for the groove dimensions.Subsequent droplets of the etchant need to be added to sustain the etchant reaction with the surface until all of the SiO 2 is dissolved and the desired cleanliness of the underlying Ge membrane is achieved.The progression of the SiO 2 removal is monitored with optical microscopy.The change in color of the central membrane into silver (Fig. 4c) determines that the underlying Ge film is released from the substrate and thus can be referred to as fully selfsupporting.Sudden movements of the membrane can induce stress beyond its mechanical strength.Therefore, careful handling is required.Finally, the PR layer used for strengthening the Ge membrane is dissolved using acetone (15).We reckon that dipping the whole sample into the acetone batch can break the membrane due to, perhaps, a rapid and local dissolution of the photoresist.A gentler approach is to apply the acetone again droplet-by-droplet using a pipette to avoid exerting an excessive force on the membranes.For wafer-scale applications, this approach can be replaced by spraying the solvent or by dipping the wafers into the diluted solution, which can also uniformly dissolve the resists.The final rinsing with droplets of isopropanol and soft blow drying leaves the Ge-self-supporting extremely clean.
The present fabrication process provides high flexibility in designing the size and the pitch of the membranes.These parameters are determined by the patterns of the glass mask during the lithography step.The only condition limiting the size of the membranes is the mechanical strength of the membranes.Larger membranes are more prone to mechanical rupture.Practically, it is possible to increase both the effective areas and the mechanical strength of the membranes by reducing the size and increasing the number of the membranes on the wafers.This implementation can be done by redesigning the pattern of the glass mask for photolithography.The minimal pitch can be as small as a few tens of micrometers.

Results
Fig. 5 shows the optical microscope and the SEM images of the resulting membranes.The optical and electron microscope tools specifications are described in the supplementary material.The membrane maintained its structural integrity without the support of the substrate.The Ge membrane has a very smooth surface and appears to wrinkle which can be caused by the relief of stress in the membrane (Fig. 5a).Stress between the Ge layer and the substrate can occur during the sputtering of the incompatible material on the Si substrate.Although the Ge layers were deposited on amorphous SiO 2 and hence are not epitaxially grown, stress in Ge might be caused by thermal expansion/ contraction and the high density of defects in the material.
The backside of the same membrane shows the cavity through which the etching proceeded (Fig. 5b).The back side shows the same surface characteristic as the front side but this time it is surrounded by the etched Si layer which appears rough with distinct pits.A closer look at the edges between the Ge and Si layers shows no cracks, voids, or other structural irregularities.The surface is extremely clean without any traces of residual SiO 2 or Al microdeposits.The SEM images in Fig. 5c) and d) at much higher magnification reveal similar surface characteristics of a flat and clean Ge surface with no strongly contrasting features.
EDX in the SEM system is used to analyze the elemental composition of the Ge membrane before and after releasing it from the substrate to reveal the distribution of germanium and any impurities if present.The spots selected for the EDX analysis are in the central region of the Ge membrane.For both membranes it displays peaks at characteristic energy levels associated with the Ge Lα (1.2 eV), Ge Kα (9.8 keV) and Ge K β (11 keV) lines.The X-ray spectrum in Fig. 6a also shows peaks at the characteristic energies for Si, O, C and Ar.For the sample after etching, the intensity of the Si and O peaks drop close to zero.This result demonstrates that the membrane is composed of mostly Ge (~99 at%) and miniscule percentage of other elements.The elemental mapping of the self-supporting membrane is shown in Fig. 6b and the atomic concentration deduced from the EDX data for both membranes is reported in Table 2.
The chemical composition of the membrane is depth profiled by nondestructive micro-Rutherford Backscattering Spectrometry (μ-RBS, see supplementary material for a detailed description) at the scanning Fig. 3. SEM cross section image of a 300 nm Ge thin film sputter deposited on a SiO 2 /Si substrate.nuclear microprobe beamline [26] of the Tandem Laboratory, Uppsala University [27].A beam of 2 MeV He + ions is focused to a 5 μm spot and raster-scanned over the whole membrane area.The backscattered ions are detected at θ = 168 • .The μ-RBS map, based on the backscattered particle signal of the SiO 2 and the Si substrate, is illustrated in Fig. 7a.This map gives an excellent contrast between the frame and membrane region, already suggesting the absence of Si and O in the membrane, and allowing for an easy selection of their spatial regions of interest.The μ-RBS spectra of the frame and membrane regions are shown in Fig. 7b.These spectra are processed using the SIMNRA simulation software [28] resulting in absolute values for the areal density of the chemical elements that are present in the sample.The μ-RBS simulation results show only subtle amounts of O contamination on the two surfaces, and less than 0.5 % Ar in the film as a consequence of the Ge sputtering process.This μ-RBS result is in good agreement with the EDX analysis.The areal density of the membrane is found to be 1492 × 10 15 atoms/cm 2 .Considering the thickness value of 330 nm provided by the TEM, the density of the membrane deduced from the areal density matches the density of bulk Ge within the uncertainty of the measurement.
The microstructure of the self-supporting Ge membrane was analyzed with TEM.The specifications of the TEM instrument are detailed in the supplementary material.The cross-section image in Fig. 8a provides a detailed view of the internal structure, thickness, and morphology of the membrane.The surface appears very smooth at this high magnification, without any surface features or roughness.The membrane has a uniform thickness across different regions.Occasional hills occur with heights less than 10 nm.No interface between the Ge membrane and the supporting substrate could be captured, which   confirms the successful elimination of the substrate and the high purity of the released membrane.Above the Ge membrane are Pt layers deposited after the synthesis for protecting the surface of the Ge membrane during TEM preparation using focused ion beam.The high-resolution TEM (HRTEM) image (Fig. 8b) clearly shows a uniform, featureless appearance with lack of regular, repeating atomic structure, indicating a highly disordered arrangement of the atoms (or amorphous).The Fourier transform pattern of the HR-TEM image in Fig. 8b is made of continuous diffuse rings of radial symmetry representing the disordered atomic arrangement of the amorphous Ge.The lack of sharp, well-defined spots is consistent with the disordered noncrystalline nature of the grown material.The amorphous characteristics make the membrane have much smoother and flatter surface than what would otherwise appear in polycrystalline membranes.For certain applications, the crystal structures can be changed using thermal annealing so that the material is suitable for the specific purposes.

Discussion
Understanding the compatibility between different materials and reactive gases is essential for the success of the process development.Some chemical reactions can be very selective towards certain materials.Different materials exhibit varying etching characteristics depending on their chemical stability and the etching method used.This is described as the etch selectivity, which is the ability to etch the film while the mask or the stop layer is relatively unaffected due to significant difference between the corresponding etching rates [29].Accordingly, it is crucial to select the appropriate reactive chemicals (gases or liquids) which favors etching of the film over the mask and the underlying layers.For instance, photoresist masks are appropriate to transfer the pattern to an underlying thin Al layer when processed with chlorine-based plasma.
Whereas the Al metal on its own is resistant to fluorine-based plasma due to the formation of involatile AlF 3 [30].This property makes Al mask suitable for etching Si, SiO 2 , and silicon nitride [31].However, Al can be dissolved by HF acid on the long run.That is why in the present process it is essential to etch the leftover parts of the Al mask using ICP-RIE with chlorine-based chemistry (step 13) before proceeding with the wet etching of the SiO 2 layer using BHF (step 14).As another example, SiO 2 is much less affected by the SF 6 etching gas than Si (the selectivity between SiO 2 and Si is about 200 [32]), but SiO 2 quickly dissolves in BHF solution.Hence, SiO 2 is used as an etch stopping layer during the deep.
When a thin layer is observed with an optical microscope, thin-film interference occurs due to light waves reflecting off the top and bottom surfaces and interfering with each other constructively or destructively [33].The interference depends on the thickness of the film and the wavelength of the incident light and as a result vibrant colors can be observed.In other words, different colors correspond to different thicknesses of the observed thin film.This phenomenon is used to track the thickness of the SiO 2 thin film which can be roughly estimated by comparing its color to the color/thickness calibration chart as shown in Fig. 9.
The SiO 2 layer which appeared by the end of the Si etching is shown in Fig. 5b.The image of the surface was then taken by a camera attached to an optical microscope under white light.The color of the membrane in the center was blue/violet which indicates about 4800 Å of SiO 2 [34].The ongoing wet etching of SiO 2 with BHF reduced the thickness of the layer and resulted in a change of color.This is clearly illustrated in Fig. 9, where images of the membrane were taken at different stages of the wet etching.As the thickness diminished from (a) to (f), the color changed from blue/violet, to green/yellow, to orange/melon, and to light gold slightly metallic.The last few hundreds Å of oxide were almost colorless, however it was possible to capture the appearance of this  extremely thin layer together with free regions of the underlying Ge membrane (Fig. 9f).
After several trials, it was noticed that dipping the sample in BHF bath affects its mechanical strength.At first, it was suspected that HF is dissolving Ge at a high rate, but a simple test denied this speculation.A thin film of Ge (200 nm thick) was sputter deposited on SiO 2 /Si substrate and dipped in BHF for 15 min.The etching rate was determined to be 7 Å/min by measuring the Ge film thickness with ellipsometry before and after dipping it in BHF solution.This test reflects a fairly good compatibility between the membrane material and the etching solution.There are several factors which can compromise the membranes integrity during the dipping step such as tensile pressure and mechanical stress causing it to expand or contract rapidly.This tensile stress can exceed the capacity of the membrane to withstand the capillary forces from the liquid, resulting in cracks at weak spots like the border between the self-supporting membrane and the Si substrate.The cracks can propagate along the membrane over long etching time and destroy the self-supporting film.
Fig. 11 a) and c) show an illustration of a Si-groove etched with ICP-RIE on a wafer which was entirely soaked in BHF solution.By principal, the wetting properties of Si and SiO 2 surface are determined from water contact angle and can be used to indicate qualitatively the presence or absence of the oxide on top of the silicon [35,36].Note that at the beginning of the etching, the hydrophilic SiO 2 was present on top of the hydrophobic Si layer.Therefore, water could easily wet the oxide and form a thin sheet on the surface.This condition makes it possible for the BHF solution to slide inside of the etching groove.However, after a few minutes the oxide was dissolved in HF acid, and the solution on the silicon surface started to form discrete drops.When the surface is not wetted, gas bubbles are likely to be trapped within the groove.The BHF solution cannot accumulate inside of the groove, resulting in extremely long etching time to promote the dissolution of the oxide.As a consequence, the photoresist protective layer above the Ge film peeled off and the membrane was subject to higher tensile stress and the final etching result was not acceptable as shown in Fig. 10.The membrane either broke before SiO 2 was completely removed (Fig. 10 a), or flakes of SiO 2 precipitated on the Ge surface due to bad feed of BHF solution into the groove (Fig. 10 b), or the membrane survived but residual SiO 2 particles were impossible to remove since longer etching time will eventually break the membrane (Fig. 10 c).
Fig. 11 b) and d) show an illustration of a similar Si-groove, this time etched dropwise with BHF solution.Since the surface was readily hydrophilic, the droplets had a higher probability to accumulate inside of the groove and came into contact with the SiO 2 at the bottom to dissolve it.The droplet-by-droplet technique improved the outcome of the etching to a great extent.While 70% of the membranes were lost after BHF-dipping, almost all of the membranes survived upon the etching with BHF-droplets.Potentially, Isopropanol and EtOH can also be added into the etching solution to reduce the surface tension and hence improve the wettability.
In our experiments, reactive ion etching (RIE) was applied to craft the membrane.It involves both chemical components (like free radicals) and physical components (like directional ions) at each point of the process which yields both good selectivity and highly anisotropic  The second C 4 F 8 gas plasma produces CF x radicals which are adsorbed on the surface and forms a uniform passivation layer of fluorocarbon to retard the Si-etching.This passivation layer deposited at the bottom of the trench is etched much faster than that on the sidewalls all due to the anisotropy character of the physical etching.Consequently, the combination of chemical etching, physical etching and the passivation layer enables the etch profiles with vertical sidewall, such as in Fig. 12a.Strong deviation from the vertical sidewalls might occur as being shown in Fig. 12b if the process parameters are not optimized, such as the undercooling of the samples.Due to the heat induced by the high-density plasma, unproperly cooled samples might have less effective passivation layer, which then leads to the strong isotropic etch profiles indicted by the bowing shape of the Si trench.

Conclusion
This work demonstrates large-area self-supporting Ge nanomembranes and provides an empirical guide for their synthesis.The process applies dry etching with ICP-RIE fluorine-based plasma to attack the Si substrate and wet etching of oxide layers with buffered hydrofluoric acid solutions.The fabrication method enables large-scale production of the self-supporting membrane for practical applications in electronics, sensors and nanofiltration.Using sputtering from a Ge target is resource effective as most sputtered material is deposited on the substrate to synthesize the ultrathin membrane.Since Ge is a relatively rare and expensive material, the efficiency in using the material leads to significant reduction in the production cost.Finally, our results show a large-area self-supporting Ge nanomembrane of amorphous structure and extremely clean and flat surfaces.These properties are confirmed by comprehensive characterization using SEM/EDX, TEM and microbeam RBS.

Fig. 1 .
Fig. 1.Graphic illustrations (not to scale) of each step of the process in cross section view of the wafer.

Fig. 4 .
Fig. 4. Optical microscope top view images of the square mask window where (a) represents 5 min after RIE etching of Si through the Al mask, the groove deepens and the Si layer develops etching pits; b) 26 min after Si-etching, the SiO 2 layer split-opens in the center surrounded by the remaining rough Si layer; and c) the Ge self-supporting membrane is released after etching SiO 2 .

Fig. 5 .
Fig. 5. Optical microscope images of a clean self-supporting Ge membrane: a) top-side view, b) back-side view.c) and d): SEM images at different magnifications of Ge membrane at the bottom of the etching groove.The surface is extremely flat, clean and contamination free.

Fig. 6 .
Fig. 6. a) EDX spectrum of Ge-self-supporting membrane and Ge on SiO 2 /Si substrate.The inset in the graph shows a magnification of the area between 0 and 2 keV.b) Elemental mapping of the self-supporting Ge membrane.

Fig. 7 .
Fig. 7. (a) μ-RBS map and (b) spectra of the frame and membrane region recorded using a microbeam of 2 MeV He + ions.

Fig. 8 .
Fig. 8. (a) Cross-section TEM micrographs of a self-supporting Ge membrane, and (b) a high-resolution TEM image of the membrane showing the disordered atomic structure.The inset to the left is a Fourier transform image of the HR-TEM image showing a diffused ring pattern indicative of an amorphous, noncrystalline structure.

Fig. 9 .
Fig. 9. Optical microscope images of the Si-groove, where SiO 2 is completely exposed and surrounded by the rough Si etched layer.The color of the SiO 2 is changing due to the change in its thickness as function of the etching time.From a) to f) the SiO 2 thickness decreases as the BHF etching time increases.The table to the left is a color chart for thermally grown silicon dioxide films of different thickness observed perpendicularly under daylight fluorescent lighting [27].

Fig. 10 .
Fig. 10.Optical microscope images of self-supporting Ge-membranes which were entirely soaked in BHF to dissolve the SiO 2 .Figure a) shows a broken membrane at the edges before complete etching of the SiO 2 reflecting with blue and gold color.Figure b) shows large precipitation of SiO 2 on the Ge membrane.Figure c) shows small particles of SiO 2 remaining on the Ge surface.
Fig.11.a) and c): Illustration of Si-groove on a wafer which is entirely soaked in BHF and where gas bubbles are likely to be formed.b) and d): Graphic illustration of Si-groove etched dropwise with BHF, the small droplets have a higher probability to reach the SiO 2 at the bottom of the groove.

Fig. 12 .
Fig. 12. Cross section SEM images of the Si trench with different etching profiles.

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Table 2
Atomic composition of Ge membranes on substrate and self-supporting.
ElementComposition of the Ge on substrate, at.% Composition of the self-supporting Ge, at.%