1 P+/deep n-well versus p+/n-well junction SPAD
The circular device consists of a p+/n-well junction, a p-well guard-ring, and a poly guard-ring. A cross-section of the SPAD structure is shown in Fig. 1. The SPAD has a narrow depletion layer and a strong electric field, which raises the avalanche breakdown probability. The spectral response of the device is biased towards the blue band. The p-well, as a guard ring, merges with the depletion layer when the p+/n-well SPAD is relatively small in size. As the breakdown voltage of the device increases, the p+/n-well loses its efficacy as the diameter of the source area is 5 µm
. In this study, we used a source area of 10 µm.
The p+/deep n-well junction SPAD has a wide wavelength response to suit the wide-spectrum detector, because electrons generated in the wide junction trigger the avalanche with high impact ionization possibilities. A cross-section of the SPAD structure is shown in Fig. 2. The deep n-well with a small doping concentration in the structure replaces the n-well, and the p+/deep n-well forms an active multiplication junction. The device has a wide depletion layer which allows for extended spectrum response. The p+/deep n-well and p+/n-well junction of SPAD structure we tested are described in table 1.
Shape PN junction Circular p+/n-well SPAD 2.1 10 3 2.2 2.5 1.6 2.9 Circular p+/n-well Circular p+/deep n-well SPAD 2.1 10 3 2.2 2.5 1.6 2.9 Circular p+/deep n-well
The two devices retain the STI around the protection ring to reduce the width of the protection ring. The Silvaco TCAD simulation of electric field are shown in Fig.3, indicating that the protection ring prevents edge breakdown. Compared with P+/Deep n-well junction SPAD, the doping concentration of P+/N-well junction SPAD has been increased, the depletion layer has been narrowed, and the breakdown voltage has been decreased.
Fig. 3 Electric field in p+/n-well junction SPAD(a) and p+/deep n-well junction SPAD(b)
As shown in Fig.4, the photoreceptor depth of the p+/n-well junction SPAD and p+/deep n-well junction SPAD is 0.2-0.6 µm and 0.2-0.9 µm, respectively. Deeper junctions absorb photons at longer wavelengths providing improved PDE toward the red band of the spectrum. The 2D Silvaco TCAD simulation results show that the p+/deep n-well junction SPAD has the wider spectral response between them.
2 Octagonal versus circular SPAD
Two shapes of SPAD were tested in this study as per their effects on the edge breakdown. As shown in Fig.5, we used a Silvaco TCAD 3D device to simulate the octagonal and circular p+/deep n-well junction SPADs. The electric field of the octagonal SPAD edge is greater than that of the circular SPAD edge, which is mainly due to the large curvature of the p-n junction and sharp SPAD shape. The octagonal and circular SPAD structure we tested are described in table 2.
Fig. 5 Silvaco TCAD 3D device simulation for octagonal (a) and circular (b) p+/deep n-well junction SPADs
图5 八边形（a）和圆形p+/深n阱结SPAD（b）的Silvaco TCAD 3D 器件仿真结构图
Shape PN junction Circular p+/deep n-well SPAD 2.1 10 3 2.2 2.5 1.6 2.9 Circular p+/deep n-well Octagonal p+/deep n-well SPAD 2.1 10 3 2.2 2.5 1.6 2.9 Octagonal p+/deep n-well
We found that the electric field of the octagonal SPAD edge is greater than that of the circular SPAD edge (red circle, Fig.6). The circular shape SPAD reduces the electric field concentration and prevents premature edge breakdown.
3 Experiments and discussion
To optimize the structural shape and junction doping concentration of the SPAD device, we fabricated different SPADs in 0.18 µm CIS technology. A photomicrograph of the chip is shown in Fig.7. The circular p+/n-well junction SPAD, circular p+/deep n-well junction SPAD, and octagonal p+/deep n-well junction SPAD are shown from left to right in Fig.7.
Fig. 7 Micrograph of SPADs fabricated in 0.18 µm CIS technology of the circular p+/n-well junction (a), the circular p+/deep n-well junction (b), the octagonal p+/deep n-well junction (c)
图7 圆形p+/ n阱结SPAD（a），圆形p+/深n阱结SPAD（b）和八边形p+/深n阱结SPAD（c）的0.18 µm CIS技术芯片显微照片
A passive quench circuit was used to measure the dark count rate (DCR) of the SPADs; the measurement circuit is shown in Fig.8. The cathode of the SPAD was connected to the voltage source V
Excess+V Breakdownand the anode to the ground at a 51 kΩ quenching resistance. The SPAD test platform and quenching circuits (Fig.9) were measured for DCR in darkness.
Figure 9 shows a test platform which consists of three templates representing a DC power supply, digital oscilloscope, and the proposed SPAD plus quenching circuits, respectively.
The inverse I-V characteristics of the three SPADs were measured as shown in Fig.10. The breakdown voltage of the circular p+/deep n-well junction SPAD, the octagonal p+/deep n-well junction SPAD, and the circular p+/n-well junction SPAD are approximately 15.14 V, 15.12 V, and 10.62 V, respectively (Fig.10). The breakdown voltage of the circular p+/deep n-well junction SPAD is approximately 15.14 V, which exceeds the 13.9 V reported in the literature circular devic
edue to the 0.13-µm CMOS technology, in which the doping concentration is relatively large and the breakdown voltage is small. Under the same bias voltage and non-breakdown edge condition, the photocurrent of the circular p+/n-well junction SPAD is particularly large.
Fig. 10 SPAD breakdown voltage versus diameter for circular p+/n-well SPAD and circular p+/deep n-well SPAD and octagonal p+/deep n-well SPAD
The DCR and bias voltage have linear relationship at room temperature (Fig.11) due to the probability of avalanche breakdown and the generation of tunnel carriers. An excess bias voltage ranging from 0.1 V to 1 V was applied with a 0.1V step. The DCR of the circular p+/n-well junction SPAD is much greater than that of the circular p+/deep n-well junction SPAD at the same excess bias voltage to the photosensitive junction concentration and high avalanche breakdown probability. At room temperature, the octagonal and circular p+/deep n-well junction SPAD DCRs are 112 Hz and 105 Hz, respectively, when the excess bias voltage is 1 V.
The photon detection probability (PDP), which is the ratio of the quantity of detected photons over the quantity of incident photons, was measured over a wavelength range from 400 nm to 1100 nm at room temperature with an excess bias voltage of 500 mV. As shown in Fig.12, the p+/n-well junction SPAD detected a wavelength range with maximum PDP of 565 nm, or 16%. The circular p+/deep n-well junction SPAD has longer wavelength response with a peak value wavelength up to 775 nm. The device achieves spectral response sensitivity with above 40% PDP from 490 to 775 nm wavelength, and the spectral response is enlarged.
The proposed circular SPAD device with p+/deep n-well junction allows active multiplying junctions with deep n-well shallow n-type doping, increases the depletion layer width compared to the other SPADs, and detects light over a wide spectrum and long wavelength; however, its PDP is relatively low. As shown in the diagram, the loss region is relatively thick and it is to some extent beneficial to enhancing the spectral response.
A circular p+/deep n-well junction SPAD implemented in a 0.18 µm CMOS technology was introduced in this paper. The circular device consists of a p+/deep n-well junction, a p-well guard-ring, and a poly guard-ring. The circular p+/deep n-well junction SPAD, octagonal p+/deep n-well junction SPAD, and circular p+/n-well junction SPAD were compared to find that the p+/deep n-well junction SPAD has extended spectrum response characteristics relative to the p+/n-well junction SPAD; the former device achieves wide spectral sensitivity with above 40% PDP from 490 to 775 nm wavelength at 0.5 V excess bias. Theoretical analysis, simulation via Silvaco TCAD 3D device, and measurements together indicated that the avalanche breakdown voltage of the circular SPAD device with 10-µm diameter is 15.14 V. It also shows high premature edge breakdown and low DCR of 638 Hz at 25℃.
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This paper presents a 0.18 µm complementary metal-oxide-semiconductor (CMOS) technology high premature edge breakdown, extended spectrum and low dark count rate circular single-photon avalanche diode (SPAD) which together form a novel wide spectrum fluorescence correlation spectroscopy (FCS) detector. The circular device consists of a p+/deep n-well junction, a p-well guard-ring, and a poly guard-ring. Simulations on a Silvaco TCAD 3D device also show that the 10 µm-diameter circular p+/deep n-well SPAD device has high premature edge breakdown characteristics. Moreover，compared to the SPAD p+/n-well junction, the p+/deep n-well junction has a longer wavelength response and spectral expansion. The device achieves wide spectral sensitivity enabling greater than 40% photon detection probability from 490 to 775 nm wavelength at 0.5 V excess bias. The circular p+/deep n-well SPAD has fine avalanche breakdown is 15.14 V and a low dark count rate of 638 Hz at 25℃.
介绍了一种0.18 µm互补金属氧化物半导体（CMOS）技术的新型宽光谱荧光相关谱探测器,其为高边缘击穿、扩展光谱和低暗计数率圆形单光子雪崩二极管（SPAD）。该器件由p+/deep n-well结，p-well保护环和多晶硅保护环组成。通过Silvaco TCAD 3D器件仿真，直径为10 µm的圆形p+/deep n-well SPAD器件具有较高边缘击穿特性。此外，p+/deep n-well结SPAD比p+/n-well结SPAD具有更长的波长响应和扩展光谱响应范围。该器件在0.5 V过量偏压下，可在490~775 nm波长范围内实现超过40％的光子探测率。该圆形p+/deep n-well SPAD器件在25℃时具有较好雪崩击穿为15.14 V，具有较低暗计数率为638 Hz。
Single-photon avalanche detectors (SPADs) are PN junction devices biased by Geiger Mode, i.e., devices with bias voltage beyond their breakdown voltage. The electric field in the SPAD multiplication region is so intense that a single photon carrier is sufficient to begin a self-maintained avalanche by impact ionization due to the generation of photons, which creates high pulse signaling. A single photon can thus be detected by the associated electronic circuitry
Recent scholars have focused on developing new SPADs with low DCR
A variety of SPAD shapes (e.g., circular, octagonal, ellipsoidal) have been proposed to improve the device performance