Michael Tchagaspanian

A broadband THz imager in a low-cost CMOS technology

Terahertz (THz) technology has become of large interest over the last 10 years. THz rays are an alternative to X-rays for imaging through thin materials and their non-ionizing character makes them inherently health-safe. The THz domain is also suitable for heterodyne detection and the use of radar techniques to perform 3D imaging. Commercial applications range from non-destructive testing, security screening of objects or persons, and medical imaging to secure communications.

Smart imagers of the future

This paper presents the evolutions of CMOS image sensors. From the early works, highly image processing oriented, the main research effort has then emphasized on image acquisition. To overcome the rising limitations of standard approaches and to promote new functionalities, several research directions are underway with promising results.

A 533pW NEP 31×31 pixel THz image sensor based on in-pixel demodulation

A THz 31×31 pixel, 100 fps image sensor integrated in a 130 nm CMOS process is presented. Taking advantage of the possibility to modulate the active source that lights the scene, a significant improvement in sensitivity and NEP is achieved by shifting the modulated THz radiation, by means of an antenna/MOSFET, then filtering the signal band using an in-pixel 16-paths passive SC-filter combined with a CT Gm-C filter resulting in a high Q factor of 100. This THz imager features a measured NEP of 533 pW at 270 GHz and 732 pW at 600 GHz respectively, and a sensitive readout chain with an input referred noise of 0.2 μVRMS.

A 25μm pitch LWIR focal plane array with pixel-level 15-bit ADC providing high well capacity and targeting 2mK NETD

CEA Leti has recently developed a new readout IC (ROIC) with pixel-level ADC for cooled infrared focal plane arrays (FPAs). It operates at 50Hz frame rate in a snapshot Integrate-While-Read (IWR) mode. It targets applications that provide a large amount of integrated charge thanks to a long integration time. The pixel-level analog-to-digital conversion is based on charge packets counting. This technique offers a large well capacity that paves the way for a breakthrough in NETD performances. The 15 bits ADC resolution preserves the excellent detector SNR at full well (3Ge-). These characteristics are essential for LWIR FPAs as broad intra-scene dynamic range imaging requires high sensitivity. The ROIC, featuring a 320x256 array with 25μm pixel pitch, has been designed in a standard 0.18μm CMOS technology. The main design challenges for this digital pixel array (SNR, power consumption and layout density) are discussed. The IC has been hybridized to a LWIR detector fabricated using our in-house HgCdTe process. The first electro-optical test results of the detector dewar assembly are presented. They validate both the pixel-level ADC concept and its circuit implementation. Finally, the benefit of this LWIR FPA in terms of NETD performance is demonstrated.

A 25μm pitch LWIR staring focal plane array with pixel-level 15-bit ADC ROIC achieving 2mK NETD

CEA-Leti MINATEC has been involved in infrared focal plane array (IRFPA) development since many years, with performing HgCdTe in-house process from SWIR to LWIR and more recently in focusing its work on new ROIC architectures. The trend is to integrate advanced functions into the CMOS design for the purpose of applications demanding a breakthrough in Noise Equivalent Temperature Difference (NETD) performances (reaching the mK in LWIR band) or a high dynamic range (HDR) with high-gain APDs. In this paper, we present a mid-TV format focal plane array (FPA) operating in LWIR with 25μm pixel pitch, including a new readout IC (ROIC) architecture based on pixel-level charge packets counting. The ROIC has been designed in a standard 0.18μm 6-metal CMOS process, LWIR n/p HgCdTe detectors were fabricated with CEA-Leti in-house process. The FPA operates at 50Hz frame rate in a snapshot integrate-while-read (IWR) mode, allowing a large integration time. While classical pixel architectures are limited by the charge well capacity, this architecture exhibits a large well capacity (near 3Ge-) and the 15-bit pixel level ADC preserves an excellent signal-to-noise ratio (SNR) at full well. These characteristics are essential for LWIR FPAs as broad intra-scene dynamic range imaging requires high sensitivity. The main design challenges for this digital pixel array (SNR, power consumption and layout density) are discussed. The electro-optical results demonstrating a peak NETD value of 2mK and images taken with the FPA are presented. They validate both the pixel-level ADC concept and its circuit implementation. A previously unreleased SNR of 90dB is achieved.

HgCdTe-based APD focal plane array for 2D and 3D active imaging: first results on a 320 x 256 with 30 µm pitch demonstrator

CEA-Leti has developed a new 320x256 hybrid focal plane array (FPA) for flash LADAR imaging. The detector array consists of 30μm pixel pitch MWIR HgCdTe avalanche photodiodes operating at 80K and the readout integrated circuit (ROIC) is fabricated on a standard 0.18μm CMOS process. The custom ROIC can operate as a passive thermal imager or a flash LADAR imager. In this second mode, each pixel will provide the time of flight measurement (3D) and the returned intensity (2D) of one laser pulse. For the first laboratory trials the e-APD photodiode array performances were measured in passive mode and the same FPA was then tested in one shot LADAR mode. This paper describes the readout IC pixel architecture and reports the first electro-optical test results in both passive and active modes. This new prototype takes advantage of the latest developments of the partnership between Sofradir and CEA-Leti.

HgCdTe APD- focal plane array development at CEA Leti-Minatec

We report the latest developments of MW HgCdTe electron initiated avalanche photo-diodes (e-APDs) focal plane arrays (FPAs) at CEA-LETI. The MW e-APD FPAs are developed in view of ultra-sensitive high dynamic range passive starring arrays, active 2D/3D and dual-mode passive-active imaging, which is why both the passive imaging performance and the gain characteristics of the APDs are of interest. A passive mode responsivity operability of 99.9% was measured in LPE and MBE e-APDs FPAs associated with an average NETD=12mK, demonstrating that dual mode passive-active imaging can be achieved with LETI e-APDs without degradation in the passive imaging performance. The gain and sensitivity performances were measured in test arrays and using a low voltage technology (3.3V) CTIA test pixel designed for 3D active imaging. The CTIA and test arrays measurements yielded comparable results in terms of bias gain dependence (M=100 at Vb=-7V), low excess noise factor (=1.2) and low equivalent input current (Ieq_in<1pA). These results validated the low voltage CTIA approach for integrating the current from a HgCdTe e-APD under high bias. The test array measurements demonstrated a relative dispersion below 2% in both MBE and LPE e- APDs for gains higher than M>100, associated with an operability of 99%. The operability at Ieq_in<1pA at M=30 was 95%. A record low value of Ieq_in=1fA was estimated in the MBE e-APDs at M=100, indicating the potential for using the MW e-APDs for very low flux applications. The high potential of the MW e-APDS for active imaging was demonstrated by impulse response measurements which yielded a typical rise time lower than 100ps and diffusion limited fall time of 900ps to 5ns, depending on the pixel pitch. This potential was confirmed by the demonstration of a 2ns time of flight (TOF) resolution in the CTIA e-APD 3D pixel. The combined photon and dark current induced equivalent back ground noise at f/8 with a cold band pass filter at λ=1.55μm was 2 electrons rms for an integration time of 50ns.

Advanced pixel design for infrared 3D LADAR imaging

CEA Leti has demonstrated the good performances of its MWIR HgCdTe avalanche photodiode arrays. Gains above 20 at a moderate bias voltage of 5V have typically been measured with an excess noise factor of only 1.2. The next generation of infrared focal plane arrays will take advantage of these characteristics to address new applications, reduce system complexity and enhance performances. One of the main opportunities offered by avalanche photodiode detectors concerns long range active imaging. This paper reports the development of two novel pixel architectures for 3D active imaging based on flash LADAR technology. Both pixels have been designed in a standard 0.35μm CMOS process and perform time-of-flight measurement in addition to 2D intensity imaging with a single emitted laser pulse. The analog input circuits have been optimized to allow fast pulse detection while providing robustness to process variability. A small readout IC demonstrator has been fabricated and coupled to a 10x10 avalanche photodiode array at 40μm pixel pitch. The first test results in lab conditions show good electro-optical performances with a ranging resolution around 30cm (2ns).

Status of p-on-n HgCdTe technologies at DEFIR

This paper presents recent development made at CEA-LETI on manufacturing and characterization of planar p-on-n HgCdTe photodiodes on long-, mid- and short-wavelength. HgCdTe (MCT) layer was grown both by liquid-phase epitaxy (LPE) and by molecular beam epitaxy (MBE) on lattice matched CdZnTe (CZT). The n-type MCT base layer was obtained by indium doping. Planar p-on-n photodiodes were manufactured by arsenic doping, which has been activated by post-implanted annealing in Hg overpressure. As incorporation is achieved either by implantation or by incorporation (during MBE growth). Electro-optical characterizations on these p-on-n photodiodes were made on FPAs. Results show excellent operabilities (99.95% with ±0.5×mean value criterion) in responsivity and NETD and background limited photodetectors. For long-wavelength FPAs, dark current is very low, leading to a R0A product comparable to the state of the art at cut-off wavelength of λc = 9.2 μm. MBE mid-wavelength FPAs present very low responsivity dispersion, reaching 1.1%. Comparisons are made between implantation and growth incorporation As doping technologies.

An 88dB SNR, 30µm pixel pitch Infra-Red image sensor with a 2-step 16 bit A/D conversion

A new readout IC (ROIC) with a 2 step A/D conversion for cooled infrared image sensors is presented in this paper. The sensor operates at a 50Hz frame rate in an Integrate-While-Read snapshot mode. The 16 bit ADC resolution preserves the excellent detector SNR at full well (~3Ge-). The ROIC, featuring a 320×256 array with 30μm pixel pitch, has been designed in a standard 0.18μm CMOS technology. The IC has been hybridized (indium bump bonding) to a LWIR (Long Wave Infra Red) detector fabricated using our in-house HgCdTe process. The first measurement results of the detector assembly validate both the 2-step ADC concept and its circuit implementation. This work sets a new state-of-the-art SNR of 88dB.