We demonstrate a back illuminated colour image sensor with a 1.4μm pixel pitch. A novel backside adapted pinned deep diode on n-substrates has been developed and characterized in order to achieve an improved colour separation. High quantum efficiency around 60% in the visible light spectrum has been attained whereas the other parameters stay in line with standard front side illuminated image sensors. Introduction: Backside illuminated (BSI) CMOS image sensors have been reported as a possible solution for the 1.4μm pixel pitch and below [1]. Beside the multipl e advantages, like 100% fill factor, lower optical st ack and higher metal packaging density [2-4], the BSI sensors also point out several design and process challenges, like the backside to frontside alignmen t and the electrical crosstalk [3-4]. We have realize d a colorized BSI CMOS image sensor demonstrator with a 1.4μm pixel pitch in 1T5 architecture and a deep pinned photodiode on n-substrates for effectiv interpixel junction isolation, resulting in crossta lk improvement. The quantum efficiency (QE) has been improved by a factor of 2 in comparison to a simple frontside conventional diode, which is illuminated from the backside. The other parameters are in line with standard front side illuminated image sensors. Pixel Design: The front-end design of the backside illuminated image sensor is based on a 0.13μm CMOS process. The backend metallization has been realized in 90nm copper based design rules. The pixel architecture consists of a pinned photodiode with transfer gate for reset noise cancellation due to correlated double sampling (CDS). Each pixel has a diode and a transfer gate transistor (TG). In order to increase the fill factor, four pixels are regrouped to one sensi ng node, which is connected to a RESET transistor and to a source follower (SF) transistor. With the READ transistor placed in the column circuitry, the aver ge number of transistors per pixel is 1.5 (1T5 architecture, see figure 1). Process& Experiments: The process flow is summarized in figure 2: After t he CMOS imager frontend and backend realisation on SOI substrates a waferbonding layer (WBL) is deposited and prepared for wafer bonding on a support wafer. Once the wafers are bonded and backside thinned, an anti-reflective coating is deposited, the pads are reopened from the backside and subsequently colour filters (CFA) and microlenses are processed. The alignment of the backside CFA and microlenses to front side layers was controlled by standard production FRAME-inFRAME overlay controls (figure 3) and mappings (figure 4). The circuit transfer process (bonding a nd thinning) has been improved in order to decrease th e layer deformation after transfer. A layer deformati on, which is compatible with the colorization alignment specs, has been achieved thanks to these improvements (figure 4). Three process approaches have been tested in order to improve electrical crosstalk, which is inherent to backside illuminated image sensors: thin p-epi laye rs, graded p-epi layers and a novel backside adapted pinned deep diode, realized on n-substrates. If the photo sensitive layer (epi-layer) gets thinn er, the photo generated electrons have to cover a short er distance in order to reach the diode. Thus they hav e a lower possibility to diffuse into the neighbouring pixels. Figure 5 shows the comparison between a 1.0 and 1.5μm epitaxial layer . We observe a decrease of crosstalk for thinner substrates for the benefit of higher quantum efficiency. Nonetheless this approac h of frontside conventional photodiode still induces too much QE losses. Likewise in combination with frontside conventional photodiodes, doping gradients have been realized by adding boron during the epitaxial growth process. The boron doping gradient (see figure 6), creates a n electric built-in field, which pushes the photogenerated electrons to the photodiode. The introduction of the graded p-doping profile results in a limited crosstalk improvement. The modulation transfer function (MTF) at Nyquist/4 has been improved by at least a factor of 2 in the visible spectrum. A novel backside adapted pinned deep diode architecture on n-substrates has been simulated (figure 7) and processed. This deep diode can be subdivided into two parts: A first collecting and storing diode on the illuminated side, with a natur l built-in electrical field. This first diode guarant ees the collection of photogenerated electrons to the stora ge diode (figure 8). The storage diode is located at t he opposite of the illuminated side. The interpixel isolation has been realized by p-implants at multip le energies and doses, in order to create a junction isolation wall. These isolation walls are 4-11 time s more effective by replacing the standard p-substrat e by an n-substrate (figure 9). Pixel performances of the pinned deep diode on nsubstrates: The conversion gain of the image sensor is 70μV/e . We notice a significant improvement of the QE by the introduction of the deep diode on n-substrates. The QE increases to 60% in the blue spectrum and to 50% in the green spectrum, attended by a corresponding decrease of crosstalk, as can be seen in (figure 10). We attribute these enhanced performances to the effective interpixel isolation and to a very good drift of the photogenerated electron s t the diode. The red signal can be improved by 10% QE by means of a broadband antireflective filter (figure 11). Figure 11 also shows the extrapolation of QE gain in case of a thicker epitaxial layer at 4μm instead of 2μm. It was assumed that the crosstalk increases in the same relation as the absorption. T his simulation shows that the QE can be higher than 65% for the blue, green and red spectrum due to an improved antireflective coating layer and a higher absorption in thicker epitaxial layers. The full we ll diode saturation charge is as good as for standard frontside diode. The dark current is 1e /s at 25°C. The low dark current has been achieved thanks to dedicated frontside and backside process steps such as p pinning layer and thermal treatment. Thanks to a very good charge transfer, the lag is below the measurement threshold. Figure 12 shows a colour picture, taken with the discussed image sensor demonstrator with deep diode on n-substrate at 500lux, 15 frames per second, f-number 2.8. The main image sensor parameters are summarized in table 1. Conclusion: We have demonstrated a back illuminated colour image sensor with a 1.4μm pixel pitch. Due to the introduction of a deep diode in n-substrates, resul ting in an effective interpixel isolation and an enhance d drift of photogenerated electrons to the diode, we obtained a significant improvement of QE with 60% in the blue spectrum and 50% in the green spectrum. The application of a broadband antireflective filte r and thicker epi substrates will lead to a QE higher than 65% in the blue, green and red spectrum. Acknowledgements: We would like to thank TraciT Technologies for wafer bonding studies. Likewise we would like to express our gratitude to the CEA Leti process teams and the STMicroelectronics process and characterization teams for fruitful discussions, wa fer processing and characterization. References: [1] JungChak Ahn et al, “Advanced image sensor technology for pixel scaling down toward 1.0μm”, p.275, IEDM 2008 [2] S. Iwabuchi et al., “A Back-Illuminated High Sensitivity Small-Pixel Color CMOS Image Sensor with Flexible Layout of Metal Wiring”, p.302, ISSCC 2006 [3] Tom Joy et al, “Development of a ProductionReady, Back-Illuminated CMOS Image Sensor with Small Pixels”, p. 1007, IEDM 2007 [4] Jens Prima et al, “A 3Mega-Pixel backilluminated image sensor in 1T5 architecture with 1.45μm pixel pitch”, p.5, IISW 2007 Figure. 1: 1T5 pixel schematic SOI wafer CMOS Imager Process – FE 130nm – BE 90nm Cu Wafer Bonding Layer (WBL) Deposition and Preparation Wafer bonding and backside grinding Anti-reflective-coating (ARC) Pad opening Color Filters and Micro-Lens processing Figure 2: Process flow Precedent layer Subsequent layer Figure 3: Left: Principle of FRAME-in-FRAME overlay control; Right: Overlay control of backside layer to frontsi de layer 1μm Figure 4:Mapping of overlay control, showing a laye r deformation of the bonded wafer, which is compatib le to colorization alignment 0 5 10 15 20 25 30 35 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700 wavelength [nm] Q E [% ] Blue, 1.0μm Gr_R, 1.0μm Red, 1.0μm Blue, 1.5μm Gr_R, 1.5μm Red, 1.5μm Figure 5: Comparison of QE of 1.0 and 1.5μm thick e pitaxial SOI substrates (conventional diode) 1E+15 1E+16 1E+17 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Depth [μm] C on ce nt ra tio n [a t/c m 3] ... pixel specific PWELL grad Boron= 0.5 decades/2μm Figure 6: SIMS of sample with a boron gradient of 0 .5 decades. The corresponding electrical drift field is 20 mV/μ m

An innovative process development for sub-2μm CMOS imager sensors is described, leading to tremendous improvements on main pixel parameters like conversion gain, saturation charge, sensitivity, dark current and noise, A full 3MP demonstrator with 1.75μ pixel pitch and 1.45μm pixel pitch have been successfully designed, fabricated and characterized

A 30 frames/s SXGA 5.6 /spl mu/m pinned photodiode pixel column parallel CMOS image sensor achieves 340 /spl mu/V noise floor and 40 pA/cm/sup 2/ dark current. Performance is limited by pixel 1/f noise, not by the ADC noise floor of 140 /spl mu/V. The column ADC memory employs a custom DRAM to save area. The sensor utilizes a 0.35 /spl mu/m 1P 3M CMOS process.

An 8 bit 20 MHz flash ADC using a radiation hardened SOI process is presented. The circuit is capable of operating at up to 20 MHz, even after a total dose exposure of 100 MRad (SiO2) (10 Kev X-ray).

Ionizing environment effects on CCD test devices are studied. Like MOS technology, CCD technology exhibits active and field oxide threshold voltage shift. Resulting from this shift, transfer efficiency degradation is observed on two phase CCD registers. On the other hand, 4 phase CCD registers working in specific conditions retain charge storage and transfer efficiency performances for a large total dose range. The dark current increase comes from peripheral insulation contribution. This degradation can result from a combination of mechanical stress induced by the bird's beak and hole accumulation induced by the ionizing radiation in the peripheral insulator. These two effects contribute to increase the interface states density and, by field effect, to extend the area of the active interface states along the bird's beak.<>

The next generation of image sensors for use in industrial applications require high resolution, high signal speed, low noise, good sensitivity and high resistance to blooming at over-exposure. Automated functional testing of sensors as well as image processing are important. ESPRIT Project 1572 has addressed the most limiting areas of the available technology with the aim of creating better solutions for advanced designs. With transparent conductive electrodes made of Indium-Tin-Oxide in exchange with polycrystalline silicon the overall sensitivity can be increased by a factor of two. A prototype of an automated functional test system for important operating parameters of sensors has been developed. By optimizing the peripheral electronic and output stage of the sensor, output amplifiers on chip were achieved functioning at a frequency of 20 MHz with a temporal noise of lower than 70 electrons. Antiblooming structures for smaller pixel dimensions have been simulated and realized for the horizontal structure. Blooming is suppressed up to 60 times the saturation light level. The study of image processing resulted in the proposal for preprocessing methods within the camera head, to relieve the main image processing host of significant computational load.

A 2/3-inch format 768(H) x 576 (V) pixel CCD image sensor has been developed, fabricated and tested. It has a frame transfer organization and is incorporating a lateral built in anti-blooming system. A horizontal resolution of more than 500 TV lines has been obtained and photosensitivity reaches 30 mv/lux. By the possible use of two video outputs, an image mirror function is provided. Thanks to its windowing device, the sensor is well suited for tracking applications.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.