As shown in Fig. 2.5, a typical SNSPDs is constituted by a single nanowire that forms a meander covering the photon input area. For single mode fibre (SMF) at 1550 nm, a round area of approximately 8 µm diameter is necessary to cover the the optical gaussian mode. The fill-factor is defined as ff = gap+widthwidth and represents the portion of active area of the detector. The typical nanowire width and thickness are 150 nm and 6.5 nm, respectively, while the fill-factor is commonly between 0.4 and 0.7. The extremely small feature of the meander makes it difficult to fabricate and a lot of optimization and fine tuning steps are crucial to ensure the detectors quality. Ultimately, one would like a meander as clean as the one presented in Fig. 2.6. But before achieving this quality, a lot of issues had to be addressed and techniques improved. In the next part of this section, the major issues that happened during the course of this thesis are presented together with their solution.
Oxygen plasma
A common way to remove EBL and photolithography resists is to place the wafer in an O2
plasma for few minutes. This step strips the resist in a clean and efficient manner. However,
2.3. Nanowire patterning
Figure 2.5: SEM image of a MoSi nanowire (in blue) forming a meander. The active area is16×16µm2. The dashed circle is the±3-sigma of the SMF gaussian mode, it represents where 99.7% of the photon are absorbed. Colours are added for clarity purposes.
when done repetitively, it can seriously damage metallic layers and more particularly MoSi. A systematic study of the effect of the O2 plasma on the nanowire shape was performed by imaging (SEM) and measuring the depth profile (AFM) before and after O2 process. Fig. 2.7 shows a SEM image and the corresponding AFM measurement after an O2 plasma step. During the plasma, the resist being stripped is still present on top of the nanowire and protects the MoSi from oxidation. The problem arises only when all the resist is gone, leaving the MoSi unprotected. Despite being capped by a 3 nm thick layer of Si, MoSi is not protected enough for oxidation. While the meander is still electrically connected, the internal quantum efficiency of such device is clearly limited and yields to non-saturating devices, see Chapter 4. This problem can be avoided by stripping the resists in a clean bath of NMP-based stripper, heated up for few hours.
Fences
A common etching technique used in nano-fabrication is called ion beam etching (IBE). In this process, argon ions are accelerated in a plasma and physically hit the wafer, while the resist acts like a shield and stops the material from being fired. Argon ions acceleration being very directional, this technique normally yields to very well defined edges, which is desired for SNSPD fabrication. In first attempts, this techniques was used to pattern the nanowire but had different drawbacks. Firstly, due to the high energy of the argon ions, the EBL resist can be transformed in a polymer-like structure, making it very difficult to remove without O2 plasma. Secondly, a redeposition process can happen. Due to its amorphous properties, MoSi adhesion is usually good on numerous materials. While this is normally a desired feature in nano-fabrication, it becomes a problem in this particular case. When the Mo and Si atoms are being hit, they have a significant probability to be
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Figure 2.6: (a) SEM image of a nanowire after the etching process. (b) AFM images of the same devices shown in (a).
2.3. Nanowire patterning
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Figure 2.7: (a) SEM image of damaged nanowire after O2 plasma process. (b) AFM images of the same devices shown in (a). The nanowire shape is clearly affected with bumps rising up from the edges. Despite being capped by a 3 nm thick layer of Si, MoSi is not protected enough from the O2 plasma oxidation.
redeposited on the side of the resist, building up fences. When the resist is stripped off, the fence stays in shape, covering the nanowire and affecting the critical current and the photon absorption. This effect is visible in Fig. 2.8. Interestingly, the height of the fences corresponds exactly to the thickness of the EBL resist.
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Figure 2.8: SEM image after IBE etching showing large fences due to the redeposition process.
To tackle this issue, a different etching approach consisting of reactive ion etching (RIE) was taken. This technique involves a chemically-reactive etching, which is known to be more isotropic. In the case of very thin films though, the process time is so short that this feature was not observed to be an issue. Different gases and mixture of gases can be selected depending on the material to etch, SF6 and CF4 are commonly used in the SNSPDs fabrication. The device presented in Fig. 2.6 was etched with SF6 gas.
Patterning optimization
Another technique worth mentioning concerns the optimization of the electron beam exposition. As described in more details in Chapter 4 the current crowding effect arises at the maximum inflection point of the turns, limiting the device critical current. The turns are a particularly important limiting factor and they have to be properly designed to create the smoothest line possible. One technique is to over expose on purpose each pixel step of the electron beam shot, as depicted in Fig. 2.9. The blue pattern is the exposed part, the blue circles represent the step size of 5 nm where the electron beams will be shot. The black line draws the border of the pattern after exposition. The size of the pattern is adjusted accordingly.