GaAs Membranes

During the work of thesis, two approaches to the fabrication of free-standing GaAs membranes were pursued. One approach was the so-called back-etching release, in which a membrane was fabricated through the removal of the substrate from the back of the sample. The other was the release of a membrane using a focused ion beam (FIB) from the frontside of the sample, in combination with wet-etching to remove part of the sacrificial layer. The advantages and disadvantages of both approaches are discussed in the corresponding sections. From the experimental point of view, the approach via back-etching was more feasible, the FIB approach was more reliable and faster from the fabrication point of view.

The base material for both approaches was a GaAs-Al0.5Ga0.5As layer system on a GaAs substrate, which was grown at ETHZ² in the group of Jerome Faist. The sample was grown by means of molecular beam epitaxy as a layer system as shown in Figure 3.3.

The following layer parameters were obtained:

Top Layer 200 nm GaAs, undoped Sacrificial Layer 1000 nm Al0.5Ga0.5As Bottom Layer 200 nm GaAs, undoped

Substrate 500µmGaAs, 3 inch substrate in (100)±0.01 degree orientation The Al0.5Ga0.5-layer or sacrificial layer is used in both processes as a selective etch-stop, i.e. as etch-stop in the back-etching process and solely as etching-material in the focused ion beam approach.

2Eidgenössische Technische Hochschule, Zürich, Switzerland

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3.2 Sample Preparation

200 nm GaAs 1 µm

Al_0.5Ga_0.5As

520 µm GaAs

hydrofluoric acid - etch C₆H₈O₇ : H₂O - etch

H₂SO₄ : 8 H₂O₂ : H₂O - etch

Figure 3.3: Scheme of the GaAs-Al0.5Ga0.5As layer system which was used for the GaAs membrane prepa-ration. Details of the three steps of the wet-etching process are given in the text.

Back-Etching

The process of back-etching was developed in the context of the bachelor thesis of Gehrke, and is described in detail there [39]. The main challenge in this etch process was the protection of the top-layer while etching the substrate, as both are of the same material.

To prevent etching of the top layer, the sample was placed top-down in a plastic container and fixed by hot glue in a way that only a ∼1 x 1 mm hole was left. Using two different etchants, see Table 3.1 and Figure 3.3, the substrate was then removed down to the sacrificial layer. The first etchant (H2SO4:8 H2O2:H20) was a fast etch at 14µm/min [48], and was used to remove the substrate to some micrometer before the sacrificial layer. Due to the high etch rate and the small selectivity against Al0.5Ga0.5As , it was not suitable to etch directly until reaching the etch stop. This was done using a second etch (4 C6H8O7:H2O), having a rather small etch rate of 0.5µm/min [48], but a high selectivity of 260 towards Al0.5Ga0.5As. To remove the Al0.5Ga0.5As-layer, a final etch using 50 % hydrofluoric acid was needed, as this has a very high selectivity against GaAs (>1000). The result was a free-standing membrane of 200 nm thickness.

Etchant Target Material Etch-Rate Selectivity H2SO4 : 8 H2O2 : H20 GaAs 14 µm/min

-4 C6H8O7 : H2O GaAs 0.5 µm/min 260 towards AlGaAs hydroflouric acid 50% Al0.5Ga0.5As 10 µm/min >1000 towards GaAs

Table 3.1: Etchants used in the fabrication of the GaAs membranes.

The advantage of this etch process is the possibility to etch large membranes, and to completely remove the substrate underneath the membrane itself. This is particularly convenient for the pump-probe measurements, as one can use backlight illumination, and no pump or probe light is scattered back from the substrate.

In Figure 3.4, two scanning electron micrographs of the GaAs membrane, discussed in the low temperature measurements of this thesis, are shown. The direction of the view

3 Experimental Set-Up and Sample Fabrication

is from below the substrate onto the backside of the membrane, as indicated in the inset of Figure 3.4a. In Figure 3.4b a close up to the membranes surface shows the flatness of the membrane. The total dimensions of this membrane are around 780 µm length with a maximum width of 20 µm. The shape of the membrane corresponds roughly to the shape of the hole that was formed with the glue at the beginning of the preparation process. Since the shape and the size of the holes were difficult to control, is was not possible to define the exact shape of the membrane at the beginning of the etching. The close up of the membrane reveals the efficiency of the selective etching, as no residuals of the intermediate layer can be seen, and the surface roughness is below the resolution of the SEM. Comparing measurements using an atomic force microscope were not possible due to the deep etching through the substrate.

direction of view

100 µm

(a)SEM of full membrane.

membrane edge AlGaAs Buffer Layer

10 µm

(b)SEM close up from the center of the mem-brane.

Figure 3.4:SEM micrographs of the GaAs membrane fabricated using the back-etching process.

The pictures are taken from the substrate side, as indicated in the inset in (a). The membrane shows a slit-like shape, having a very high aspect ratio between, due to the lack of control of the final shape of the etching mask. The efficiency of the etching and the etch-stop at the GaAs membrane is illustrated in (b), showing a clean surface at the bottom of the membrane.

Focused Ion Beam

The main drawback of the back-etching fabrication of GaAs membranes is the difficulty to control the size and shape of the membranes. This can be circumvented by an etching process from the front-side of the membrane. The schematics of this approach are depicted in Figure 3.5a. Using a focused beam of Ga-ions, two small slits are cut from the top into the GaAs top layer. By then placing the sample in a hydrofluoric acid etch, it is possible to partly remove the underlying buffer layer without affecting the membrane, as GaAs is inert to hydrofluoric-acid etching. This produces a free-standing membrane which is supported on two sides only.

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3.2 Sample Preparation The great advantage of the approach using the focused ion beam (FIB) is the possibility to directly define the size and the shape of the desired membrane, i.e. due to the versatility of the ion beam, it is possible to define any arbitrary shape. The drawback of this approach is twofold: i) The ion beam implants a lot of Ga ions in the vicinity of the cuts, changing the composition of the GaAs membrane, and the existence of the substrate inhibited the control of the etching quality, i.e. the control of residuals on the membranes backside; ii) Using this approach it is not possible to fabricate a membrane, which is supported from all sides. This might result in lateral stress distribution along the membrane, which then again might influence the properties (e.g. lifetimes) of the membrane’s vibrational modes. This effect is known as acusto-elastic effect, and can be described with the use of the higher order elastic constants of Equation 2.4 [49].

200 nm GaAs 1 µm

Al_0.5Ga_0.5As

520 µm GaAs

HF etching membrane

FIB cut ^Ga FIB cut

+ Ga⁺

(a) Schematics of the FIB assisted etch.

membrane

end of etching area

FIB cuts

(b)SEM close up on the membrane.

Figure 3.5: Fabrication process and SEM micrographs of the resulting GaAs membrane. The top layer of GaAs is cut using a focused beam of Ga⁺-ions. Using hydrofluoric acid the underlying AlGaAs layer is washed out until a free-standing membrane is released between both slits.

3.2.2 Metal-Semiconductor Membranes Fabricated by Means of Wet-Etching

In this section, the preparation of the silicon and silicon nitride (Si3N4) membranes with metal transducers is described. It is a back-etching process, similar to the etching of the GaAs membranes, in which the membrane is released by removing the substrate from the backside of the wafer. Base material for the silicon membranes is a silicon on insulator (SOI) wafer which is produced using the smart-cut technique. The resulting layer system is a 340 nm thick crystalline silicon layer on top of a 400 nm SiO2 layer on a silicon substrate. The backside is covered with 200 nm Si3N4, which serves as an etching mask.

3 Experimental Set-Up and Sample Fabrication

In the case of the Si3N4 membranes, the layer system is similar to that of the silicon membranes, but the silicon top layer is replaced by a silicon nitride layer. The fabrication of these wafers is different in a way that the silicon substrate is thermally oxidized on both sides, and the Si3N4 layer is deposited by low-pressure chemical vapor deposition (LPCVD) on both sides of the silicon substrate. The thickness values of the individual layers are given in Chapter 3.2.4, with varying nitride thickness of 57 nm, 107 nm, and 157 nm.

In both cases the silicon nitride etch mask on the back side is opened using a laser marker, removing squared shapes windows in the silicon nitride. The wafers are then placed inside a KOH wet-etch, protecting the membrane side from the etch. The etch is stopped when the silicon substrate is completely removed, leaving just the dioxide and the membrane layer behind (Si or Si3N4). The dioxide is removed in a final hydrofluoric acid etch, releasing the final membrane.

346 nm silicon on the example of a silicon membrane. First the protection mask on the backside is opened. Then the substrate is removed with an KOH etch until the SiO2 etch stop is reached.

The SiO2 is removed by a HF etch. Finally the metallization layer is evaporated.