## 1 Introduction  
The deterioration of the retinal pigment epithelium and cone cells can cause retinal degenerative diseases, such as age-related macular degeneration and retinitis pigmentosa [1, 2, 3]. These diseases damage the outer retinal optic cells, leading to vision impairment; however, the inner retinal neurons, such as ganglion and bipolar cells, remain viable for transmitting neural impulses [4, 5]. Therefore, vision can be restored by electrically stimulating the surviving inner retinal cells. The primary issue faced by conventional retinal prostheses using an external camera is the inconvenience caused to the patient by the need to carry external devices [6, 7]. Moreover, retinal prosthesis systems that utilize external cameras face inherent interconnection challenges and limitations in scalability pathways because the camera and microelectrode array (MEA) are physically separated. As an alternative to these limitations, a full-CMOS solution that integrates the photosensitive devices, stimulation circuit, and MEA has been proposed [8, 9, 10, 11]. However, existing research has indicated that the direct implementation of an integrated system faces several challenges. In recent research, integrated solutions using micro-photodiode arrays as photo sensors have been developed [12, 13]. These systems offer the advantage of eliminating interconnection challenges and providing a scalable pathway instead of using external devices. However, while co-locating the photosensor and circuit achieves high resolution and wide dynamic range in photoresponsivity, the larger size of the system poses a significant limitation. The bulk of the device makes implantation difficult, emphasizing the need for miniaturization, especially to maintain high resolution in a smaller form factor. In retinal prosthesis systems, high-density photodetector arrays are essential to achieve adequate spatial resolution [14; 15]. To this end, silicon nanowires(SiNWs) have been utilized as photonic sensors by several researchers due to their nanoscale dimensions and high surface-to-volume ratios [16; 17; 18; 19; 20]. SiNWs are fabricated using a top-down approach because of the ease of integration with existing semiconductor processes and the advantage of producing uniform structures [21; 22]. To enhance the density and surface-to-volume ratio of SiNWs, a horizontal arrangement can increase the dimensions of photodetectors. However, this approach presents limitations in forming high-density photodetectors within a confined area. Consequently, vertically stacked SiNW arrays can simultaneously achieve high integration density and an improved surface-to-volume ratio. Nevertheless, this requires advanced nanolithography techniques such as electron beam lithography (EBL), along with wet oxidation and wet etching processes to precisely define SiNW diameters [16; 23; 24; 25]. The high processing temperatures involved in wet processes impose constraints on the fabrication, and the wet etching process using HF cleaning can cause damage to the SiNWs. To overcome these issues, our research team fabricated vertically stacked silicon nanowire (SiNW) arrays using only the deep-reactive-ion etching process and conventional lithography methods. We fabricated SiNW arrays with a diameter of 300 nm in multiple layers, achieving high-density configurations within finite dimensions. This approach eliminates the need for wet processes and advanced nanolithography. SiNWs are fabricated in multiple layers vertically using a simplified process, increasing the density within a finite dimension. As the number of vertically stacked SiNWs increases, the photocurrent response also increases, exhibiting more sensitive characteristics at low light intensities. This approach demonstrates the potential for developing high-resolution retinal prostheses by integrating high-density photodetector arrays into a single chip, thereby minimizing size and increasing photosensitivity.