Black Silicon CMOS extends the range for industrial, medical, and scientific applications

Imaging in the near infrared (near-IR) is becoming more prevalent for a variety of defense, commercial, medical, and scientific uses. SIONYX, the company that pioneered black silicon CMOS image sensors, has used its patented ultra-low-light sensor technology to commercialize many digital night-vision (NV) products.

In 2019, SIONYX was selected for the US Army’s integrated visual augmentation system (IVAS) goggle program owing to its ability to image at below moonless starlight (0.5 mLux or about 6 photons per pixel) and SXGA (super extended graphics array) resolution with low latency. Because the technology enables digital night vision, it supports the digital soldier initiative of interconnecting information gathering and sharing in real time.

This performance is attributable to a pixel design that utilizes a micro- and nano-texturing technology to increase the photon free path. It is also back illuminated, and pixels are optically isolated to prevent crosstalk, providing sensitivity beyond the traditional 800 nm range of sCMOS/CMOS. Consequently, SIONYX devices can enable low-noise imaging with high quantum efficiencies (QE) from 400 nm to beyond 1,150 nm without support illumination. The higher the QE, the greater the sensor’s ability to detect and image at that wavelength.

Black silicon is a modified form of silicon that exhibits unique optical and electrical properties. SIONYX’s co-founders Eric Mazur and Jim Carey discovered and developed this technology at Harvard University. In that original work, they used femtosecond laser pulses to modify the surface of silicon—laser-induced periodic surface structuring—to create nanoscale structures resulting in black silicon. The process roughens the silicon surface, creating a micro- and nanostructure that traps incoming light with reduced reflectance. SIONYX has demonstrated that black silicon can be implemented using a variety of different processing techniques, including standard CMOS fabrication processes.

This increased light absorption enables black silicon CMOS to detect visible and near-IR light with increased sensitivity and image quality compared to traditional CMOS image sensors. As industrial, scientific, and medical imaging applications continue expanding into the infrared-A (IR-A, 780 nm–1,400 nm), the accessibility and cost benefits of black silicon CMOS becomes increasingly attractive as compared to expensive III-V semiconductor SWIR cameras.

A crossover use of black silicon CMOS for both industrial and military applications is See-Spot. On the modern battlefield many visible and invisible light sources are used, whether for marking locations, for simple range finding, lidar geo-mapping, or for designating targets for hostile fire—“seeing the spot.” In military targeting applications it is common that a 1,064 nm laser emission is used to pinpoint an intelligence or munitions target. Similarly, the ability to detect this wavelength in industrial uses enables beam profiling and precision alignments, for example, to characterize the spatial intensity distribution or location of a laser’s emission. Such characterization is essential for optical metrology and medical systems, for example. In field testing, SIONYX’s OPSIN monocular has proven capable of detecting low-power 1,064 nm laser emission at distances exceeding 2.5 km in moonless starlight conditions.

In medical imaging, SIONYX sensors have demonstrated efficacy in detecting skin cancer lesions using laser speckle perfusion imaging, a noninvasive technique that measures blood flow in tissues by analyzing the speckle pattern produced by scattering of laser light.

Malignant lesions exhibit significantly different perfusion patterns compared to benign lesions. The black silicon CMOS camera is able to capture sufficiently high-resolution images to allow for detailed analysis of blood flow dynamics in the skin. With a growing body of evidence, the technology may usher in a simpler method for broadening the range of dermatology diagnostics or screening.

Another medical imaging area of promise for black silicon is indocyanine green-fluorescent imaging, commonly encountered in fluorescence-guided surgery. Because of black silicon’s heightened near-IR sensitivity and low noise properties, the camera can simultaneously capture bright and dim fluorescence signals without sacrificing image quality. This facilitates identification of subtle variations in tissue fluorescence and more precise image interpretation by the surgeon.

An aerial drone image captured in the late evening by OPSIN. Photo credit: Sionyx

Raman spectroscopy presents yet another scientific instrumentation domain in which SIONYX’s core camera technology is well suited. For applications involving low signals or weak Raman scattering effects, high sensitivity delivers the gain function to assist with counterfeit medication detection or food contamination quality monitoring. Black silicon CMOS can improve accuracy and selectivity of detection. 

Black silicon CMOS sensors or cameras also offer distinct cost-performance advantages over visible/SWIR cameras relying on InGaAs (indium, gallium, arsenide) or compound semiconductor sensors owing to a broad spectral range at a lesser cost. Enhanced light sensitivity, low noise for better signal-to-noise ratio, and greater sensor resolution allow finer detailing inspection and broader contaminant detection without external illumination where lighting can be uneven. Black silicon also operates efficiently without thermal electric cooling.

For example, thru silicon inspection has been traditionally a domain of InGaAs-based image sensors because traditional CMOS is unable to effectively reach 1,150 nm. However, in December 2022, a paper cited the successful use of SIONYX’s XQE image sensor for thru silicon alignments in advanced 3D packaging. For traditional low-cost screening of photovoltaic defect detection or general impurity-crystalline anomalies in silicon wafers, black silicon-enhanced performance CMOS presents a compelling lower-cost alternative.

As with silicon, the 1,150 nm band is also valuable for food inspection. The 1,100 nm to 1,450 nm water absorption bands present opportunity to deliver a single image sensor module for the entire visible spectrum to beyond 1,150 nm. And while not suitable for all applications, black silicon CMOS cameras facilitate lower cost sorting points—looking for damaged or contaminated items—which is applicable to an array of agricultural uses. Unmanned aerial vehicles are regularly deployed for crop assessment, algae monitoring, and mine tailing pond containment. These applications take advantage of the extended working range of black silicon.

SIONYX will soon introduce a 10 MP high-resolution 4K platform featuring 4.7 µm pixels in monochrome and color versions. This new sensor provides the same high QE and near-IR range as it’s XQE-1350 platform, but suitably increases the resolution capability to satisfy voice-of-the-customer requests in the scientific instrumentation, medical, and industrial vision markets. Like the pre-existing XQE-1350 that is field proven for military applications, RoHS (restriction of hazardous substances) compliant, and AEC-Q100 G2 certified, the new XQE-9950 platform carries the same high quality and reliability performance. SIONYX black silicon CMOS is dual use and not specifically controlled for export (classified EAR99). SIONYX is a US-based imaging technology company and a member of the CHIPS Consortium.

Bob Wadja is Global Product Manager, Sensors and Camera Components for SIONYX, which is headquartered in Beverly, Massachusetts. www.sionyx.com