An unpackaged microchip is used as the sensing element in a miniaturized gaseous proportional chamber. Thisletter reports on the fabrication and performance of a complete radiation imaging detector based on this principle. Our fabrication schemes are based on wafer-scale and chip-scale postprocessing.\ud
Compared to hybrid-assembled gaseous detectors, our microsystem shows superior alignment precision and energy resolution, and offers the capability to unambiguously reconstruct 3-D radiation tracks on the spot.\u

Blanco Carballo, V.M.

Chefdeville, M.A.

Fransen, Martin

Melai, J.

Salm, Cora

Schmitz, Jurriaan

Timmermans, J.

Timmermans, Jan

van der Graaf, Harry

English

An unpackaged microchip is used as the sensing element in a miniaturized gaseous proportional chamber. Thisletter reports on the fabrication and performance of a complete radiation imaging detector based on this principle. Our fabrication schemes are based on wafer-scale and chip-scale postprocessing. Compared to hybrid-assembled gaseous detectors, our microsystem shows superior alignment precision and energy resolution, and offers the capability to unambiguously reconstruct 3-D radiation tracks on the spot

Blanco Carballo, Victor Manuel

Chefdeville, Maximilien

Melai, Joost

University of Twente Research Information

IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 6, JUNE 2008 585A Radiation Imaging Detector Made byPostprocessing a Standard CMOS ChipVictor Manuel Blanco Carballo, Maximilien Chefdeville, Martin Fransen, Harry van der Graaf, Joost Melai,Cora Salm, Member, IEEE, Jurriaan Schmitz, Senior Member, IEEE, and Jan TimmermansAbstract—An unpackaged microchip is used as the sensingelement in a miniaturized gaseous proportional chamber. Thisletter reports on the fabrication and performance of a complete ra-diation imaging detector based on this principle. Our fabricationschemes are based on wafer-scale and chip-scale postprocessing.Compared to hybrid-assembled gaseous detectors, our microsys-tem shows superior alignment precision and energy resolution,and offers the capability to unambiguously reconstruct 3-D radia-tion tracks on the spot.Index Terms—Microsensors, nuclear imaging, radiation detec-tors, SU-8, wafer postprocessing, wafer-scale integration.I. INTRODUCTIONFOR RADIATION imaging applications in high-energy andnuclear physics, gas gain grids [1], [2] are commonly used.These grids are punctured metal membranes suspended somemicrometers over an anode plane inside a gas volume filledwith a gas mixture, such as helium/isobutane. When ionizingradiation (e.g., a cosmic ray particle or an X-ray) crosses thegas volume above the grid, electrons are liberated and driventoward the anode by a moderate electric field (∼1 kV/cm).By applying a high electric field (∼100 kV/cm) between thegrid and the anode, each free electron will create an ionizationavalanche in this region, yielding an exponential increase in thenumber of free electrons. The avalanche electrons are collectedat the anode. Typically, a charge-sensitive amplifier is usedto record arrival time, position, and pulse height. This typeof radiation detector is relatively cheap and of low mass, andconsumes little power. It finds application in nuclear, high-energy, and astrophysics, as well as biology, medicine, andindustrial radiology [3].When a microchip is used as the anode [4], [5], the signal isdirectly picked up at the origin, reaching very high sensitivity[schematic in Fig. 1(a)]. The microchip typically has an arrayof bond pads (for picking up the charge) each connected toa preamplifier and buffer. With a manually mounted grid,Manuscript received March 27, 2008. This work was supported in part by theDutch Foundation for Fundamental Research on Matter (FOM) and in part bythe Dutch Technology Foundation STW under project TET 6630. The reviewof this letter was arranged by Editor P. Yu.V. M. Blanco Carballo, J. Melai, C. Salm, and J. Schmitz are withthe MESA+ Institute for Nanotechnology, University of Twente, 7500 AEEnschede, The Netherlands (e-mail: j.schmitz@utwente.nl).M. Chefdeville, M. Fransen, H. van der Graaf, and J. Timmermans are withthe National Institute for Nuclear Physics and High Energy Physics, NIKHEF,1098 SJ Amsterdam, The Netherlands.Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LED.2008.925649Fig. 1. (Top) Schematic view of the detector. An ionizing particle createsseveral free electrons that drift toward the CMOS chip and create an avalanchebetween the grid and the chip. (Bottom) SEM picture of an integrated device.misalignment between the holes in the grid and the sensingarray on the chip leads to Moiré effects. To overcome these,we pursue the integration of the grid on a chip through waferpostprocessing [6], which is inspired by other microsystemsreported in the literature [7]–[11]. After early trials by Key et al.[12], we showed the first functional grid made in postprocessingplanar technology [13] on bare silicon wafers. In this letter,we present the first results of fully integrated detectors madeby microfabrication of a grid on top of CMOS microchips:Medipix2 [14] and Timepix [15], [16] chips were employed.Both wafer postprocessing and chip postprocessing were pur-sued. Integration of detecting device and readout electronicsleads to superior performance at lower projected cost.II. MATERIAL AND FABRICATION PROCESSThe system is built by suspending a conductive mesh over apixel readout chip supported by 50-µm-high insulating pillars.0741-3106/$25.00 © 2008 IEEE586 IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 6, JUNE 2008We chose SU-8 negative-tone resist [17] as pillar material. Thiswell-documented resist can be used as both a sacrificial layerand a structural support [18], it can be spin coated in a thicknessranging from a few micrometers to a millimeter [19], and it ispatterned with standard lithography. Its low temperature budget(below 95 ◦C) makes it CMOS postprocessing compatible, andit is radiation hard [20].Sputtered aluminum was used as metal for the conductivegrid. It exhibits low residual stress, can easily be etched andbonded, and shows good adhesion with SU-8 [21]. However,metal deposition over unexposed SU-8 may lead to cross-linking, through generation of ultraviolet light, or heat transfer.The literature reports the use of thermal evaporation [18], [22],and the use of a “light absorption layer” [23] or a metal mask[24] to circumvent this problem. We have chosen to deposita layer of standard positive resist (Fujifilm OiR 907) over theSU-8 and expose both at the same time. The positive resistprotects the SU-8 during sputter deposition, avoiding the cross-linking of the SU-8. It is selectively removed by wet etch in alater stage.Fig. 1(b) shows a scanning electron microscopy (SEM) im-age of the final integrated device, where a punctured aluminumgrid is suspended over a Medipix2 chip by SU-8 pillars placedin the middle of four pixels. More details about the fabricationprocess can be found in [25].The fabricated prototypes are screened by optical inspec-tion and electrical test (isolation between grid and chip, andcapacitance–voltage measurement to verify the rigidity of thesuspended membrane). Wafer-level fabrication in our (univer-sity) laboratory presently reaches a yield of above 80%, whichis similar in chip-level fabrication. Our process allows for chipreprocessing in case of grid imperfections.III. RESULTThe postprocessed chips were wire bonded on a printedcircuit board (PCB) and placed inside a sealed chamber (notethat no package is required for this microsystem). The chamberis flushed with a helium/iC4H10 (80/20) gas mixture, whichleads to a low risk of sparks. The resistance of the microsys-tem against sparking was significantly improved with a sparkprotection layer (amorphous silicon directly deposited on thechip) [26], also allowing operation in other gas mixtures, suchas argon/iC4H10 (95/5). The PCB was connected to a computerto read out the chip. The Medipix2 chip stores the number ofelectron avalanches that have reached every pixel; the Timepixchip, in addition, provides the time or charge information ofthe avalanche (this avalanche must contain more electrons thanthe threshold of the pixel, calibrated about 1000 electrons,to be detectable). The performance of the digital-to-analogconverters and the number of dead pixels in the chips did notchange after the postprocessing.The devices were irradiated with 6-keV photons producedby an 55Fe radioactive source. The integral image of the hitsper pixel is shown in [Fig. 2 (inset)]. To (quickly) illustrateimaging capability, a circular metal nut was placed on top ofthe chamber, locally absorbing the 55Fe photons. We obtaineda homogeneous response as the Moiré effect is eliminatedFig. 2. Detector performance illustrated from X-ray radiation response, oper-ating at 335 V on the grid in Ar/iC4H10 (95/5). The histogram shows a countspectrum of 55Fe radioactive decays reconstructed from summing up single-electron avalanches. The insets show the imaging capability of the detector:(a) shadow image of a circular nut being irradiated with 55Fe and (b) a similarimage disturbed by an alpha particle crossing the detector area during datacollection. The chip’s sensitive area is 14 × 14 mm2.Fig. 3. Tracks of high-energy ionizing particles crossing the detector. (Top)An electron originating from 90Sr radioactive decay, diagonally crossing, andemitting a horizontal delta-ray on its way. The X and Y coordinates are knownfrom the pixel array; the Z coordinate is reconstructed from the signal arrivaltime. (Bottom) 2-D cosmic rays tracks (left: 2 tracks, right: 4 tracks). Thefigures illustrate the detector’s imaging capability for high-energy ionizingparticles, such as cosmic rays.through the alignment between the holes in the grid and thepixels in the chip. During 55Fe irradiation, tracks of alphaparticles also crossed the detector area, as shown in [Fig. 2(inset b)].BLANCO CARBALLO et al.: RADIATION IMAGING DETECTOR MADE BY POSTPROCESSING STANDARD CMOS CHIP 587Using a distance of 10 cm between the cathode andthe grid, and a gas mixture with high transverse diffusion(Ar/iC4H10 95/5), the primary charge of a 55Fe photon con-version can be spread across the chip surface. The numberof activated pixels is then a direct measure of the numberof primary electrons. The 55Fe spectrum reconstructed in thismanner is shown in Fig. 2. A single-electron efficiency of 80%is deduced from this spectrum at 335 V on the grid.With such a single-electron efficiency, cosmic rays passingthe detector area can be visualized, and their trajectory can bereconstructed. Typical tracks of ionizing particles, both froma 90Sr radioactive source and from cosmic rays, are displayedin Fig. 3. Using the Timepix microchip, the arrival time ofelectrons is also recorded, allowing the reconstruction of 3-Dtracks [Fig. 3 (top)].Several prototype detectors were continuously operated forseveral months in He/iC4H10 and Ar/iC4H10 while recordingcosmic ray events and other types of radiation. We foundstable and unchanged operation. Various reliability tests on themicrosystem, such as temperature and moisture cycling, andtests of mechanical integrity, are ongoing.IV. CONCLUSIONWe have designed, realized, and tested an integrated gaseouspixel detector on several different pixel chips from 0.25-µmfoundry CMOS. It consists of a punctured metal mesh sup-ported by insulating pillars placed over the unpackaged chip.This integrated radiation imaging detector exhibits state-of-the-art tracking capabilities for ionizing radiation, goodsingle-electron efficiency, and stable long-term operation. Thestraightforward CMOS postprocessing scheme allows massproduction of this new detector at low cost.ACKNOWLEDGMENTThe authors would like to thank T. Aarnink, D. Altpeter,A. Boogaard, and B. Rajasekharan for their help during clean-room processing; S. M. Smits for the support in mask design;J. Rovekamp for the mechanical work; and J. Visschers and theMedipix consortium for the supply of chips and wafers, andsupport in data acquisition.REFERENCES[1] Y. Giomataris, P. Rebourgeard, J. P. Robert, and G. Charpak, “Mi-cromegas: A high-granularity position-sensitive gaseous detector for high-particle-flux environments,” Nucl. Instrum. Methods Phys. Res. A, Accel.Spectrom. Detect. Assoc. 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A Radiation Imaging Detector Made by Postprocessing a Standard CMOS Chip

NARCIS 

A fully electronic label-free DNA sensor chip,”

A MEMS-based projection display,”

A miniaturized multiwire proportional chamber using CMOS wafer scale post-processing,” in

Adding functionality to microchips by wafer postprocessing,”

An electron-multiplying ‘Micromegas’ grid made in silicon wafer post-processing technology,”

CMOS image sensors: Electronic camera-on-a-chip,”

Electronic Imaging of Ionizing Radiation With Limited Avalanches

Erratum to ‘Timepix, a 65 k programmable pixel readout chip for arrival time, energy and/or photon counting measurements’,”

Fabrication of miniaturized ﬂuidic devices using SU-8 based lithography and low temperature wafer bonding,” Sens.

Fabrication ofgasampliﬁcation microstructureswith SU8 photosensitive epoxy,”

First test measurements of a 64 k pixel readout chip working in single photon counting mode,”

Geblade damascene process for post-CMOS integration of nano-mechanical resonators,”

GEM: A new concept for electron ampliﬁcation in gas detectors,”

Investigation of the bond strength between the photo-sensitive polymer SU-8 and gold,”

Micromegas:Ahigh-granularityposition-sensitivegaseousdetectorforhighparticle-ﬂux environments,”

On the radiation tolerance of SU-8, a new material for gaseous microstructure radiation detector fabrication,”

Reading a GEM with a VLSI pixel ASIC used as a direct charge collecting anode,”

Results from MPGDs with a protected TimePix or Medipix-2 pixel sensor as active anode,” in

Rotational driven RF variable capacitors with postCMOS processes,”

Selection of mold materials for electroforming of monolithic two-layer microstructure,”

SU-8: A low-cost negative resist for

The readout of a GEM or Micromegas-equipped TPC by means of the Medipix2 CMOS sensor as direct anode,”

thick photoresist processing as a functional material for

Three-dimensional microfabrication in negative resist using printed masks,”

Timepix, a 65 k programmable pixel readout chip for arrival time, energy and/or photon counting measurements,”

Uncrosslinked SU-8 as a sacriﬁcial material,”

https://ris.utwente.nl/ws/files/6427627/blancocarballo_edl.pdf

A Radiation Imaging Detector Made by Postprocessing a Standard CMOS Chip

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