## 2 Materials and Methods
### Tungsten MEMS Hotplates  
The MEMS hotplates are fabricated out of a single material, in this case, sputtered tungsten, such that the central hotplate and heater arms form a single structure. Optical lithography is used to pattern the precise shape of the heater elements. The hotplates are suspended above the substrate by a plurality of heaters, as shown in Figure 1. The simplified schematic shows the top view of a tungsten microplate (reproduced from the patent US20230131181A1). The emitting plate has an area of 0.02 mm\({}^{2}\). The arm length may vary; in this case, it is 80 \(\upmu\)m, and the thickness is 2 \(\upmu\)m. The width of the narrowest part of the arms is 3 \(\upmu\)m by design but may vary after the fabrication process is complete [24]. The dimensions of the hotplate and heater arms studied in this article are marked in Figure 1. The devices are operated in vacuum, estimated to be \(<\)10\({}^{-5}\) mbar, such that there are no chemical interactions between an atmosphere and the hotplates, important for longer operations. Even a short operation is not possible in the presence of a gas, including inert gases, as the devices would be thermally shorted to the package.  
The devices are heated by applying a voltage in the range of 0.5-0.8 V (either DC or AC) to the electric pads, which heat the central plate to temperatures in excess of 1700 \({}^{\circ}\)C. The temperature is measured using a visible infrared (VIS/IR) spectrometer by fitting the emission intensity between 400 nm and 950 nm to [PERSON]'s equation. The clamps remain cool, close to ambient temperature, while the center plate reaches a high, uniform temperature, close to the maximum temperature. Thermal pathways are limited to conduction through the material and radiative cooling, which starts to become significant above 1700 \({}^{\circ}\)C (typically, radiative power is on the order 5-15% of the total power, depending on the exact design). The radiative cooling of the center means that the hottest point of the emitters is on the heaters, close to the attachment point to the hotplate.  
The emitters were tested using a power source (GW Instek GPP-4323, China), amplifier (Texas Instruments OPA548 EVM, TX, USA), waveform generator (Keysight 33220A, CA, USA), and a multimeter (Keysight 34460A, CA, USA). A CMOS camera (IDS Imaging Industrial camera UI-6240 SE-M-G, Germany) was used for capturing live images during operation and used to track how the device aged. The setup is presented in Figure 2.  
Figure 3 illustrates the hottest areas of the emitters located close to the attachment point of the hotplate. Figure 3a shows a cross-section of the schematic of the studied microhotplates, with the hottest area indicated by the orange-colored arrow. Figure 3b-e show the aging of the heater elements. The testing conditions of the device shown in Figure 3 can be found in Table 2.  
Figure 1: A simplified schematic of the studied tungsten microhotplates (the top view of the structure). The numbered labels correspond to components as defined in the original patent: 1—radiator device, 2—emitter plate, 3—electric pads, 4—heater arms [24].  
Figure 2: Testing bench.  
The images depicted in Figure 3 are taken at a wavelength of 900 nm, where the light observed is emitted from the device itself. Figure 3b shows a pristine device operating at 1577 \({}^{\circ}\)C. Figure 3c is an image of the same heater after 224 h. Notching, or void formation, is clearly visible, a result of atomic migration driven by electromigration, stress gradients or concentration gradients.  
The purpose of this work is to quantify the different atomic migration drivers and simulate which failure mechanism dominates. Figure 3d,e illustrate the heaters after failure. In Figure 3e, a break is visible, resulting in a colder (darker) heater. The hotplate is still hot as the remaining heater elements are still functional and in part hotter than before due to the increase in current Figure 3d.