A shadow mask is used for defining the areas of a device and creating microstructures on it with precision by masking or covering part of the target surface. Shadow masks, also known as stencils or deposition masks, are used in a wide range of different vacuum-chamber evaporation and sputtering processes to fabricate both simple and complex semiconductor, micro-engineered electronic components and variety of products in the consumer and life science world. Compared to photolithography, masking method has advantages of lower cost and simpler process, although it may not achieve the same level of precision as electron beam lithography (EBL). However, one easily overlooked advantage of masking method is its ability to be heated, which is particularly important in cases where substrate heating is required.

Let's take an example of fabricating complex devices using masking method with a crossbar array of memristors based on vertical heterojunctions.

Firstly, let's refer to a paper published in Advanced Materials. In this paper, each intersection point of the crossbar array consists of a 4-layer vertical heterojunction structure: bottom electrode Pd-Ta2O5-TaOx-top electrode Pd. If we were to use masking method to fabricate such a device array, what would be our approach?

Figure 1：Pd-Ta2O5-TaOx-Pd heterojunction-based memristor crossbar array.

Step 1：Shadow mask design. Here we need at least four shadow masks: Shadow mask #1 for the deposition of bottom electrode, shadow mask #2 for the deposition of Ta2O5, shadow mask #3 for TaOx, and shadow mask #4 for top electrode. As shown in Figure 2, we should make sure that when these four shadow masks are overlapped or positioned at the same place, they form the memristor crossbar arrays.

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Figure 2: Shadow masks for fabrication of memristor crossbar arrays.

Step 2：Device fabrication. After the completion of shadow masks design and processing, we can start fabricating devices, as shown in Figure 3.

1) Deposition of bottom electrode through shadow mask #1

2) Deposition of Ta2O5 through shadow mask #2

3) Deposition of TaOx through shadow mask #3

4) Deposition of top electrode through shadow mask #4

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Figure 3：Fabrication of memristor crossbar arrays by shadow mask technique.

The process of fabricating devices using the mask method may seem simple, but in reality, there are many factors that need to be considered. These factors not only determine the success or failure of device fabrication, but also determine the minimum size of the devices. These factors include：

1. Mask Alignment Error or Positioning Error: Taking the preparation of memristors as an example, in the fabrication process illustrated in Figure 3, it is crucial that the mask used in each step of the process is aligned as accurately as possible to the same position. Only then can each material deposited ultimately form the vertical heterostructure array we require. The error in the deviation of the mask from the predetermined position is one of the key factors determining the minimum device size. Currently, the error in commercial mask alignment devices is around ±5 μm. However, a significant drawback is that they often cannot be used at high temperatures and have a large volume, making them unsuitable for most vacuum systems.

2. Mask Feature Size: The feature size refers to the minimum processing size of gaps, circles, square holes, etc. Generally, this is determined by the mask plate processing equipment and processes, such as laser power and spot size. The feature size of the mask plate is also one of the important factors determining the minimum device size. Currently, advanced femtosecond laser cutting can achieve feature sizes below 1 μm.

3. Mask Processing Error: The processing of the mask plate, such as laser processing or etching, may have positioning errors, especially in XY positioning errors and repeated positioning errors of laser cutting devices. This results in a discrepancy between the actual size of the processed mask plate and the design size. Currently, the mainstream fiber laser devices on the market have an error of ±15 μm, while femtosecond laser devices can achieve errors as low as ±1 μm.

4. Spacing Between Mask Plate and Substrate – Shadow and Diffusion Effects: Due to the presence of a certain gap between the mask plate and the substrate, during the process of material deposition through the pores, shadow and diffusion effects can lead to the actual size of the prepared structure being slightly larger than the design size. Generally, the smaller the distance between the mask plate and the substrate, the weaker the shadow and diffusion effects, and vice versa.

Therefore, when using the mask method to fabricate complex microelectronic devices, especially those involving multilayer heterostructures, attention should be paid to the following aspects: 1. Reduce mask alignment errors or positioning errors; 2. Reduce mask pattern feature sizes; 3. Minimize mask processing errors; 4. Reduce shadow and diffusion effects.

Currently, a widely used method is to create an opening on a stainless steel plate, weld the mask plate on one side of the opening, and form a groove on the other side where the substrate is mounted, as shown in Figure 4a. This setup can only accommodate single-step processes and cannot be used for multi-step material deposition or the fabrication of complex devices, such as the memristor crossbar array mentioned earlier. Here, we recommend a new product – the shadow mask support and alignment device (Model X-S04, Jiangsu Ximai Technology Co., Ltd., http://www.ximai-tech.com), as shown in Figure 4b. This device consists of a sample holder, an elastic substrate housing, a mask support frame, and a mask alignment tool. The substrate is mounted in the substrate housing, the mask plate is installed on the mask support frame, and the mask alignment tool ensures precise alignment of the mask plate at the same position.

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Figure 4：(a) Mainstream mask panel support device. (b) Model X-S04: The shadow mask support and alignment device that combines mask support and alignment.

This device has the following advantages:

Zero alignment error/positioning error: The unique alignment and locking device of the mask ensures that after replacing different mask plates, the mask plate is precisely positioned in the same location, completely eliminating alignment errors or positioning errors caused by the inability to position different mask plates in the same location.

Elastic substrate holding device: The built-in substrate holding device with tiny springs can provide appropriate elasticity to secure the substrate. Its compression rebound design is compatible with substrates of different thicknesses, ensuring good thermal conductivity while fixing the substrate, avoiding errors caused by substrate slippage, and preventing problems such as substrate fragmentation or deformation of mask alignment devices that may occur when using rigid substrate holding devices.

Minimization of shadowing and diffusion effects:The distance between the mask plate and the substrate is reduced to as small as 50 μm, resulting in a tested size broadening due to shadowing effects of less than 2 μm.

Plug-and-play design for mask plates: Replacing the mask plate is simple and convenient. It can be easily positioned by a simple "insert-lock" mechanism, and can be quickly removed by "unlock-pull out". This allows for rapid replacement of different mask plates required for various process steps.

High-temperature resistance: The key components are made of high-temperature resistant aerospace materials, capable of withstanding temperatures up to 550 ℃. They are particularly suitable for processes requiring high temperatures in thin film growth and device fabrication, such as MBE, PLD, ALD, sputtering, and thermal evaporation, etc.

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