The previously discussed DC and RF measurement procedures are done for four categories of diodes that are available at NXP. These four categories are explained in this section.
Firstly, the diodes are varied in anode area. Increasing the anode area aims at decreasing Ron, but increasing Cof f. Table 2 shows an overview of the diodes that are varied in Anode width (WA) and Anode Length (LA). Additionally, the Cathode Width (CW), the STI width (SW) and number of DTI’s (DR) are listed.
Secondly, the number of parallel structures is varied. Using parallel PiN diodes aims at decreas-ing Ron with the factor of parallel diodes, whereas Cof f will increase with the factor of parallel diodes. NXP has proposed the so-called Multi-Finger diode to achieve a parallel structure. Figure 3.11 and 3.12 shows the difference between a single square diode and a multi-finger diode in the top view and cross section respectively. A multi-finger diode consist of multiple parallel cells and parallel fingers inside the cell, the effect of parallel fingers and parallel cells is evaluated separately.
Table 3 shows an overview of the diodes that are varied in the number of parallel fingers or cells.
Thirdly, the thickness of the epitaxial (EPI thickness) layer is varied. Increasing the EPI thickness aims at decreasing Cof f but increases Ron. The EPI thickness is by default 0.63 µm, but is varied to 0.73, 0.78, and 0.83 µm in these measurements. All diodes from Table 2 and 3 are increased in EPI thickness, however; one diode will be chosen for further evaluation. In the results will be explained which is chosen and why.
Fourthly, the difference between the PPLUS and PSB diode is of interest. The difference be-tween a PPLUS and PSB diode was explained in section 2.2.2; the PSB diode consists of a poly-Si anode and an effective thicker EPI. Table 4 shows an overview of the PPLUS and PSB diodes that are available. The PSB diodes are only available in the so-called PCM configuration, whereas the GSG configuration is required to perform RF measurements. Therefore, the PSB diodes can only be compared to the PPLUS diodes for DC measurements. Figure 3.13 shows again the GSG configuration but now compared to the PCM configuration.
In addition, also temperature measurements are done in DC to check the temperature depen-dent behavior of the diode compared to theory. This is done for a temperature range of -30◦C to 105◦C
Figure 3.11: A schematic top view of the square diode (left) and the Multi-finger diode (right), with the anode area in green, the cathode area in red and the STI area in grey
Figure 3.12: A schematic cross section of the square diode (left) and the Multi-finger diode (right), with the anode area in green, the cathode area in red.
Figure 3.13: The PCM (left) and GSG (right) structure. The GSG configuration consists of an anode and cathode patch and four extra patches for grounding purposes. The PCM configuration only consists of an anode and cathode patch.
Table 2: The parameters of the diodes that are used for analyzing the area scaling of the PiN diode.
With Anode Width (WA), Anode Length (LA), Cathode Width (CW), STI width (SW) and number of DTI’s (DR)
Layout WA [µm] LA [µm] CW [µm] SW [µm] DR
Square 10 10 1.5 0.8 3
Square 8 8 1.5 0.8 3
Square 7 7 1.5 0.8 3
Square 6 6 1.5 0.8 3
Square 5 5 1.5 0.8 3
Square 4 4 1.5 0.8 3
Square 2 2 1.5 0.8 3
Table 3: The parameters of the diodes that are used for analyzing the parallel structure of the PiN diode. With Anode Width (WA), Anode Length (LA), the Number of Fingers (NF), the Number of Cells (NC), Cathode Width (CW), STI width (SW) and number of DTI’s (DR)
Layout WA [µm] LA [µm] NF NC CW [µm] SW [µm] DR
Multi-Finger 0.5 5 5 1 1.5 0.8 3
Multi-Finger 0.5 5 4 1 1.5 0.8 3
Multi-Finger 0.5 5 3 1 1.5 0.8 3
Multi-Finger 0.5 5 2 1 1.5 0.8 3
Multi-Finger 0.5 5 1 1 1.5 0.8 3
Multi-Finger 0.5 10 1 10 1.5 0.8 3
Multi-Finger 0.5 10 1 8 1.5 0.8 3
Multi-Finger 0.5 10 1 6 1.5 0.8 3
Multi-Finger 0.5 10 1 4 1.5 0.8 3
Multi-Finger 0.5 10 1 2 1.5 0.8 3
Table 4: The parameters of the diodes that are used for comparing PPLUS and PSB diodes. With Anode Width (WA), Anode Length (LA), Cathode Width (CW), STI width (SW), number of DTI’s (DR), the number of fingers (NC) and number of cells (NC) if applicable.
Layout PPLUS or PSB WA [µm] LA [µm] NF NC CW [µm] SW [µm] DR
Square PPLUS 10 10 - - 1.5 0.8 1
Square PPLUS 6 6 - - 1.5 0.8 1
Square PPLUS 2 2 - - 1.5 0.8 1
Multi-Finger PPLUS 0.5 10 2 3 1.5 0.8 1
Multi-Finger PPLUS 0.5 6 2 3 1.5 0.8 1
Multi-Finger PPLUS 0.5 2 2 3 1.5 0.8 1
Square PSB 10 10 - - 1.5 0.8 1
Square PSB 6 6 - - 1.5 0.8 1
Square PSB 2 2 - - 1.5 0.8 1
Multi-Finger PSB 0.5 10 2 3 1.5 0.8 1
Multi-Finger PSB 0.5 6 2 3 1.5 0.8 1
Multi-Finger PSB 0.5 2 2 3 1.5 0.8 1
4 Simulation Methodology
In the previous chapter the experimental methodology has been discussed. The experimental characterization has two main disadvantages. Firstly, the device is represented as a black box and thus unexpected behaviour cannot be investigated properly. Secondly, in practical circumstances not all devices are available. For example; the EPI thickness is only varied from 0.63 µm to 0.83 µm
Therefore, it is beneficial to support the experimental measurements with diodes simulated in the dedicated simulation tool Technology Computer-Aided-Design (TCAD). TCAD is based on the Finite Element Method (FEM), which provides the possibility to extract the electrical parameters and to look into the diode. The main disadvantage of using TCAD is that it requires extensive knowledge and time for inexperienced users.
In this research, Sentaurus from Synopsis is used as the dedicated TCAD tool. In this chapter, the different steps of simulation are discussed; the process simulation, the device simulation and the analysis routine for this research specifically.
4.1 Process simulation
In the process simulation the fabrication of the PiN diode is simulated. This process starts with a silicon substrate and ends with depositing the electrical contacts on the anode and cathode. This process was already explained in section 2.2.2. The simulation of the fabrication the PiN diode is thus done in the same manner as it is done in reality. However, in reality the PiN diode is fabricated in 3D and in the simulation the fabrication is done in 2D. Another important difference is that the bias is directly applied to the anode and cathode connection instead via the metal contacts on top of the wafer. Therefore, the simulation does not take the induction L into account of Figure 3.7.
In TCAD, two PiN diode structures can be established; the square diode and a Multi-Finger diode with one cell and 1 or 2 fingers (see two examples in Figure 4.1).
Figure 4.1: A diode structure with an anode width of 4µm (left) and a diode structure with two so-called finger anodes, both having a width of 0.5µm and a STI in-between of 0.25µm (right)