Fabrication method of SPAD sensor for automotive LiDAR to compensate the process fluctuation by feedforward system

Abstract

There have been many developments in Light Detection And Ranging (LiDAR) sensors used in Autonomous Driving (AD) and Advanced Driver Assistance Systems (ADAS) to measure the precise distance to an object, recognize the shape of an intersection, and classify road types. These LiDAR sensors can achieve fantastic results day and night without any loss of performance. In the past, Time-Correlated Single Photon Counting (TCSPC) and complete digital signal processing (DSP) have been used in to achieve a 100m range Time-of-Flight (ToF) sensor [1]. Background (BG) noise-rejection techniques [2] have been used to improve the signal-to-noise ratio (SNR), leading to detection of objects at a 6km range. Single Photon Avalanche Diode (SPAD)-based architectures implement per-pixel level histogramming, Time-to-Digital Conversion (TDC) and signal processing [3,4]. Another ToF sensor has been shown that enables significantly higher resolution, 1200×900 pixels [5].On the other hands, for the automotive use, stable characteristics are needed over an automotive-grade temperature range of -40 to 125℃.In addition to its high light sensitivity, it is also highly sensitive to process fluctuations, and its characteristic variations due to manufacturing are large, which is a problem for mass-producing SPAD sensors for automotive applications.
First, we introduce the temperature dependence of the VBD and PDE of our SPAD pixels (Fig. 1). The VBD increases at high temperatures and varies by more than 4 V from -40°C to 125°C. The PDE decreases at higher temperatures. This value is typical, and characteristics may decrease due to process fluctuations during manufacturing. We adopted APC (Advanced Process Control) as the process flow for manufacturing SPAD sensors and constructed a manufacturing method to suppress characteristics variations against process fluctuations during manufacturing. The process forming the avalanche region is the most demanding process control in SPAD pixels. The avalanche region is located near the surface of the SPAD pixel on the wiring layer side and in the center of the pixel, and is formed by the insulator stacking process, the impurity doping process, and the impurity activation process in the front end of line (FEOL) (FIG. 2). We have found at least two critical processes with particularly large characteristic effects among the avalanche region forming processes based on our previous manufacturing results. A reference process is defined as a process with a fast construction order from these processes, and the construction results are measured during the process. From the measured results, the construction conditions of the subsequent process are adjusted and determined. The process to be adjusted is called target process. In order to determine the construction conditions to be adjusted by APC, the characteristic effects of reference process and target process were investigated (Fig. 3). The characteristic to be adjusted was set to PDE at 125°C, which has the largest variation. From this result, the amount of adjustment of target process to compensate the fluctuation of reference process was determined (Fig. 4).
In the prototype manufactured by the compensation process using APC, the variation was suppressed in the temperature range of the automotive standard. The VBD has the largest variation of 125°C, and the variation of 681 mV was reduced to 427 mV by using the compensation process (Fig. 5). The variation itself is small at -40°C, but it has a similar suppression effect. In addition, the variation of PDE was large at 125°C. Normalized by the standard value, it decreased from -0.43 to -0.18. (Figure 6). At -40°C, we could keep the almost same variation. We have achieved that suppression of PDE and VBD variation on the 125℃ without increasing any other characteristics variation.
We proposed a manufacturing method using APC for SPAD pixels with strong sensitivity to the variation of manufacturing process.

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