Toward Photoconductive Switch Fabrication —— Metal Issues

The principle of photoconductive switches consists in reducing the resistance of the semiconductor placed in the Micro-Wave (MW) gap made in PhC, according to the repetition rate of the clock laser. Given this, the generated carriers in the PhC cavities have to be collected by the metallic pads for current circulation. Carriers exhibit a short diffusion length in PhC, therefore, the closer the metal is from the cavity, the better it is. Given this, the metallization is performed on top of the self-suspended membrane, where the mechanical strains of the structure are weak. Performing the metal deposition between step 3 and 4 (c.f.table 2.1) allows the stack structure to be mechanically

Table 2.1 | Self-suspended photonic crystal membrane fabrication summary (case of GaAs).

Step Remarks Schematic Structure

1. Resist patterning e-beam lithography 2. Hard mask transfer ICP CHF₃−O₂ 3. I I I-V etching ICP SiCl₄ 4. Under-etching GaInP HCl−H3PO₄

GaAs GaInP Resist SiO₂

strength enough for handling the rest of the process-flow as the sacrificial layer is still present.

However, the under-etching step is done while metal is on top of the membrane.

Two steps of metal deposition are performed to fabricate the microwave lines: a small area providing ohmic contact close to the PhC and, above it, the MW line. The ohmic contact is made by alternating the deposition of Au/Ge, Ni and Au layers to a total height of 100 nm. The sample after undergoes a a flash annealing at 380°C (30 sec, 2 min time raise) to transform the different metal layers into an alloy of GaAs-Ga/Au-Ni/As/Ge-Au/Ga-NiGaGe-AuGa -Ni [91]. In this situation, Germanium is considered to dope the GaAs, providing a transition region between the semi-conductor and the metal for better ohmic contacts [92].

The microwave pads are made by the deposition of 200 nm gold on top of the ohmic contact.

5μm 5μm

(a) (b)

Fig. 2-3 | SEM im-age of two structures from the same sam-ple. (a) without any metal – (b) with ohmic contact and microwave pads. The circle point out the fact that in (a) the under-etch was effi-cient, not in (b).

Depositing metal at the step previously mentioned leads to a crippling issue. Indeed, it was observed that the chemical sacrificial layer removal is prevented by the metallic pads. This problem was reproduced with several samples (Fig. 2-3(b)), even though self-suspended membranes are freed on the same sample where no metal is present close to the PhC (Fig. 2-3(a)). It is clear from

2.1. SELF-SUSPENDED MEMBRANE 37

the halo around the structure without metal that under-etching was well performed, whereas walls, made of GaInP, in the opening are still present in the structure with metallic pads. No sacrificial layer removal is highly problematic as no vertical confinement is provided.

On the samples made for testing the under-etching process, two designs have been imple-mented. The first one is the PhCs for the photoconductive switch application. The second consists of several beams patterned in the membrane material between the two electrodes. This structure makes it easier to observe with Scanning Electron Microscopy (SEM) if the GaInP was effectively or not removed under GaAs.

1μm 1μm

GaInP (sacrificial layer)

GaAs

(a) Au (b)

Au

Fig. 2-4 |SEM image of FIB cut of the sample af-ter under etching trying a) for a beam nanopat-terning b) photonic crys-tal nanopatterning.

To further investigate the behavior of the under-etching process, a Focused Ion Beam (FIB) cut was performed to reveal the I I I-V stack on structure with metallic pads. As seen in Fig. 2-4, the layer of GaInP was partially removed from the bottom. However, the etching depends on the distance between the pads, as far away from them, the etching seems to have been more efficient.

Several hypotheses could explain such chemical etching behavior. 1) First, the metal could change the solution wettability and/or the solution tends to form bubble over the PhC because of the metal. Therefore, HCl might not go through the holes and etch the GaInP layer. 2) Secondly, the metal would modify the electrochemical potential of either the mixture and/or the material, changing the reaction behavior and preventing the etching GaInP. 3) Finally, HCl and/or H₃PO₄ could react with the metal. Reactors are then used and wasted with the metal reaction, decreasing the effective concentration of the etching solution for the sacrificial layer removal, thus limiting its removal.

To verify each hypothesis, several chemical etchings were performed under different conditions.

1. To answer the first assumption, MaO₂ has been added to the solution. Mainly, this would raise the wettability of the solution (Fig. 2-5(a)). Unfortunately, no improvement has been noticed using this technique. The metal does not impact the ability of the solution to go

With Light

5μm

5μm 5μm

5μm Heated HCl HCl + MaO₂

Several HCl

(a) (b)

(c) (d)

Fig. 2-5 | Several different attempts to under etch the GaInP sacrificial layer.

through the PhC holes.

2. About the chemical potential change induced by the metal, two experiments have been tried.

The first one consists of chemically etching the GaInP layer under illumination using a white light source. It has been proven that lightening during chemical etching can change the material selectivity of the solution [93]. This first attempt (Fig. 2-5(b)) did not etch the GaInP but instead provided a chemically deposited material on top of the PhC. The second attempt consisted of heating the solution, as this reaction between HCl and GaInP is highly dependent on the temperature [94]. Therefore, following the Arrhenius law, this would change the activation energy of the chemical reaction and could etch more effectively the sacrificial layer. It is clear that heating the solution does not provide an efficient under-etching (Fig. 2-5(c)).

3. Finally, if the reaction is limited by other reactions between HCl and metal, immersing the sample in several newly mixed HCl−H3PO₄solution would lead to an improvement in the sacrificial layer removal. In Fig. 2-5(d), 5 solutions of HCl solution have been used, each of them freshly mixed before the etching attempt. The try corresponds to a total immersing time of 100 min. The SEM image reveals that this process could have led to an improvement, removing more material on the bottom of the GaInP layer. However, as seen in Fig. 2-4, no etching is performed close to the GaAs. Even if the chemical reaction is limited by non-desired

2.1. SELF-SUSPENDED MEMBRANE 39

one between HCl and the metals, multiplying the etching attempt does not free efficiently the GaAs slab.

Currently, the metal used for both metal deposition is gold (an alloy of gold for the ohmic contact, gold only for the microwave pad). As the under-etching issue is due to the presence of such metal near the PhC, by changing it, the electrochemical potential of the contact changes, then the etching behavior can be altered.

The ohmic contact has been changed to an alloy of titanium and platinum, and the microwave pads by platinum only. Under-etching was processed as usual using HCl−H₃PO₄ for 20 min, which removed the GaInP sacrificial layer. A FIB cut was performed to obtain a closer look at the self-suspended membrane (Fig. 2-6). The membrane is obviously well released. However, another issue appeared: the GaAs is etched around the ohmic contact. This is a crucial drawback because, even with a self-suspended membrane providing light confinement vertically, and therefore enabling carrier generation in the cavity, those carriers cannot be collected as the contact is separated from the structure.

10μm

Fig. 2-6 | SEM image of FIB cut of the sample after under etching for a Ti/Pt contact.

Finally, depositing metal between the steps 3 and 4 of the original self-suspended process flow (c.f.table 2.1) could be avoided. Ohmic contact and microwave pad can be deposited after under-etching over the already freed self-suspended membrane. Unfortunately, the mechanical strength of the PhC slab is too weak for further process steps, especially resist deposition. It is evident in Fig. 2-7 that after only one spin-coating, the PhC membrane are already cracked (mostly all structures of the sample were broken). Even if during this step, enough structures would have been rescued, it means that all of these nonbroken structures still have to go through metal deposition, metal lift-off, and the second step of metal layer. The position in the process flow for the metal layer does not provide a reliable enough method for fabricating photoconductive switch.

10μm

3μm

Fig. 2-7 |Cracked mem-brane after resist deposi-tion prior to metal depo-sition.