Jan Putzeys

Fully integrated silicon probes for high-density recording of neural activity

Sensory, motor and cognitive operations involve the coordinated action of large neuronal populations across multiple brain regions in both superficial and deep structures. Existing extracellular probes record neural activity with excellent spatial and temporal (sub-millisecond) resolution, but from only a few dozen neurons per shank. Optical Ca2+ imaging offers more coverage but lacks the temporal resolution needed to distinguish individual spikes reliably and does not measure local field potentials. Until now, no technology compatible with use in unrestrained animals has combined high spatiotemporal resolution with large volume coverage. Here we design, fabricate and test a new silicon probe known as Neuropixels to meet this need. Each probe has 384 recording channels that can programmably address 960 complementary metal–oxide–semiconductor (CMOS) processing-compatible low-impedance TiN sites that tile a single 10-mm long, 70 × 20-μm cross-section shank. The 6 × 9-mm probe base is fabricated with the shank on a single chip. Voltage signals are filtered, amplified, multiplexed and digitized on the base, allowing the direct transmission of noise-free digital data from the probe. The combination of dense recording sites and high channel count yielded well-isolated spiking activity from hundreds of neurons per probe implanted in mice and rats. Using two probes, more than 700 well-isolated single neurons were recorded simultaneously from five brain structures in an awake mouse. The fully integrated functionality and small size of Neuropixels probes allowed large populations of neurons from several brain structures to be recorded in freely moving animals. This combination of high-performance electrode technology and scalable chip fabrication methods opens a path towards recording of brain-wide neural activity during behaviour.

Low-noise low-power readout electronics circuit development in standard CMOS technology for 4 K applications

In the framework of the Photodetector Array Camera and Spectrometer (PACS) project IMEC designed the Cold Readout Electronics (CRE) for the Ge:Ga far-infrared detector array. Key specifications for this circuit were high linearity, low power consumption and low noise at an operating temperature of 4.2K. We have implemented this circuit in a standard CMOS technology which guarantees high yield and uniformity, and design portability. A drawback of this approach is the anomalous behavior of CMOS transistors at temperatures below 30-40K. These cryogenic phenomena disturb the normal functionality of commonly used circuits. We were able to overcome these problems and developed a library of digital and analog building blocks based on the modeling of cryogenic behavior, and on adapted design and layout techniques. We will present the design of the 18 channel CRE circuit, its interface with the Ge:Ga sensor, and its electrical performance. We will show how the library that was developed for PACS served as a baseline for the designs used in the Darwin-far-infrared detector array, where a cryogenic 180 channel, 30μm pitch, Readout Integrated Circuit (ROIC) for flip-chip integration was developed. Other designs and topologies for low noise and low power applications will be equally presented.

Time Multiplexed Active Neural Probe with 1356 Parallel Recording Sites

We present a high electrode density and high channel count CMOS (complementary metal-oxide-semiconductor) active neural probe containing 1344 neuron sized recording pixels (20 µm × 20 µm) and 12 reference pixels (20 µm × 80 µm), densely packed on a 50 µm thick, 100 µm wide, and 8 mm long shank. The active electrodes or pixels consist of dedicated in-situ circuits for signal source amplification, which are directly located under each electrode. The probe supports the simultaneous recording of all 1356 electrodes with sufficient signal to noise ratio for typical neuroscience applications. For enhanced performance, further noise reduction can be achieved while using half of the electrodes (678). Both of these numbers considerably surpass the state-of-the art active neural probes in both electrode count and number of recording channels. The measured input referred noise in the action potential band is 12.4 µVrms, while using 678 electrodes, with just 3 µW power dissipation per pixel and 45 µW per read-out channel (including data transmission).

Time multiplexed active neural probe with 678 parallel recording sites

We present a high density CMOS neural probe with active electrodes (pixels), consisting of dedicated in-situ circuits for signal source amplification. The complete probe contains 1356 neuron sized (20×20 μm2) pixels densely packed on a 50 μm thick, 100 μm wide and 8 mm long shank. It allows simultaneous high-performance recording from 678 electrodes and a possibility to simultaneously observe all of the 1356 electrodes with increased noise. This considerably surpasses the state of the art active neural probes in electrode count and flexibility. The measured action potential band noise is 12.4 μVrms, with just 3 μW power dissipation per electrode amplifier and 45 μW per channel (including data transmission).

Qualification status of the stressed photoconductor arrays for the PACS instrument aboard Herschel

The photoconductor detector arrays for the PACS instrument (Photoconductor Array Camera and Spectrometer) aboard the future ESA telescope Herschel have been developed during the engineering phase in 1999. In early 2000 the construction of the qualification models began for both, the highly and low stressed Ge:Ga arrays, which consist of 12 linear modules each. These two types of photoconductor arrays are dedicated for different wavelengths bands in the spectrometer section of the instrument. While the performance of a few engineering arrays has been studied and presented earlier, additional data are meanwhile available on the absolute responsivity and quantum efficiency of the detectors. Furthermore, experience has been obtained during manufacture of a larger series of arrays giving better statistics on performance aspects, such as uniformity of the cutoff wavelengths and of the responsivity or the maximum stress obtainable within such arrays. Considerable progress has also been made in the development and manufacture of the 4 Kelvin Cold Read-out Electronics (CRE), which will integrate and multiplex the signals generated in each linear array with its 16 detector pixels. Manufacture of the detector arrays for the qualification model is scheduled to be completed by this summer, and manufacture of the flight model has already started. The qualification model will be delivered to the test facilities, where absolute spectral performance of the 24 linear modules will be determined. In this paper we give a summary of the related activities and results as obtained during manufacturing and testing.

Development of a Si:As blocked impurity band detector for far IR detection

This paper reports on the fabrication and characterization of a linear array of Blocked Impurity Band (BIB) far infrared detectors and of the related Cryogenic Readout Electronics (CRE). It is part of the ESA DARWIN project which aims at the study of exoplanets by means of null interferometry and requires high performance infrared detector arrays in the 6 18μm range. Si:As BIB detectors have been fabricated on an infrared transparent Silicon substrate enabling backside illumination. The buried contact, the active and the blocking layers are deposited by epitaxy; the doping profile is controlled by adjusting the growth parameters. Access to the buried contact is provided by anisotropic silicon etch of V-grooves in the epi layers. Spray coating of photoresist is used for the lithography of the wafers with high topography. The CRE is composed of an input stage based on an integrating amplifier in AC coupled feedback with selectable integrator capacitors, of a sample and hold stage which provides isolation between input and sampling capacitance, and of an output buffer with multiplexing switch. The readout is optimized for low noise with minimum operating temperature of 4K. Linear arrays made of 42 and 88 detectors and having 30μm pixel pitch with various active areas are fabricated. Detector arrays are coupled to the CRE by Indium bumps using flip-chip technology. Measurements on the readout show reduced noise, good linearity and dynamic range. First detector characterization results are presented.

A low-noise low-power readout electronics circuit at 4 K in standard CMOS technology for PACS/Herschel

IMEC has designed, in the framework of the PACS project (for the European Herschel Space Observatory) the Cold Readout Electronics (CRE) for the Ge:Ga far-infrared detector array. Key specifications for the CRE were high linearity (3 %), low power consumption (80 μW for an 18 channel array), and very low noise (200 e-) at an operating temperature of 4.2 K (LHT - Liquid Helium Temperature). IMEC has implemented this circuit in a standard CMOS technology (AMIS 0.7 μm), which guarantees high production yield and uniformity, relatively easy availability of the technology and portability of the design. However, the drawback of this approach is the anomalous behavior of CMOS transistors at temperatures below 30-40K, known as kink and hysteresis effects and under certain conditions the presence of excess noise. These cryogenic phenomena disturb the normal functionality of commonly used circuits or building blocks like buffer amplifiers and opamps. We were able to overcome these problems and developed a library of digital and analog building blocks based on the modeling of cryogenic behavior, and on adapted design and layout techniques. These techniques have been validated in an automated cryogenic test set-ups developed at IMEC. We will present here in detail the full design of the 18 channel CRE circuit, its interface with the Ge:Ga sensor, and its electrical performance and demonstrate that all major specifications at 4.2 K were met. Future designs and implementations will be equally presented.

Why not record from every channel with a CMOS scanning probe

Neural recording devices normally require one output connection for each electrode. This constrains the number of electrodes that can be accommodated by the thin shafts of implantable probes. Sharing a single output connection between multiple electrodes relaxes this constraint and permits designs of ultra-high density neural probes. Here we report the design and in vivo validation of such a device, a complementary metal-oxide-semiconductor (CMOS) scanning probe with 1344 electrodes and 12 reference electrodes along an 8.1 mm × 100 μm × 50 μm shaft; the outcome of the European research project NeuroSeeker. This technology presented new challenges for data management and visualization, and we also report new methods addressing these challenges developed within NeuroSeeker. Scanning CMOS technology allows the fabrication of much smaller, denser electrode arrays. To help design electrode configurations for future probes, several recordings from many different brain regions were made with an ultra-dense passive probe fabricated using CMOS process. All datasets are available online.

Cold performance tests of blocked-impurity-band Si:As detectors developed for Darwin

We report first results of laboratory tests of Si:As blocked-impurity-band (BIB) mid-infrared (4 to 28 μm) detectors developed by IMEC. These prototypes feature 88 pixels hybridized on an integrated cryogenic readout electronics (CRE). They were developed as part of a technology demonstration program for the future Darwin mission. In order to be able to separate detector and readout effects, a custom build TIA circuitry was used to characterize additional single pixel detectors. We used a newly designed test setup at the MPIA to determine the relative spectral response, the quantum efficiency, and the dark current. All these properties were measured as a function of operating temperature and detector bias. In addition the effects of ionizing radiation on the detector were studied. For determining the relative spectral response we used a dualgrating monochromator and a bolometer with known response that was operated in parallel to the Si:As detectors. The quantum efficiency was measured by using a custom-build high-precision vacuum black body together with cold (T ~ 4K) filters of known (measured) transmission.

Flight Qualification and Circuit Development of Sensor Front-End Electronics for PACS/Hershel at Liquid Helium Temperature

In the framework of the development of the European Space Agencys Herschel Space Observatory (HSO), IMEC designed the cold-readout electronics (CRE) for the PACS instrument. Key specifications for this circuit were high linearity, low power consumption, high uniformity, and very low noise at an operating temperature of 4.2K (liquid helium temperature, LHT). To ensure high production yields and uniformity, relatively easy availability of the technology, and portability of the design, the circuit was implemented in a standard CMOS technology. The circuits are functional at room temperature, which allows screening prior to integration and qualification and has an important impact on the production yield and time. The circuit was mounted on an Al2O3 substrate for optimum electrical performance. On the same substrate, bias signal generation, short-circuit protection circuitry, and decoupling capacitors for the power lines were integrated. This led to a relatively complex substrate containing over 30 passives ...