This article presents a detector system consisting of three components, a
CMOS imaging array, a gaseous-detector structure with a Micromegas layout and a
UV-photon sensitive CsI reflective photocathode. All three elements have been
monolithically integrated using simple post-processing steps. The Micromegas
structure and the CMOS imaging chip are not impacted by the CsI deposition. The
detector operated reliably in He/isobutane mixtures and attained charge gains
with single photons up to a level of 6 \cdot 10^4. The Timepix CMOS array
permitted high resolution imaging of single UV-photons. The system has an MTF50
of 0.4 lp/pixel which corresponds to app. 7 lp/mm.Comment: 4 pages with 8 figures. Submitted to Nucl. Instr. and Meth. A
  (Elsevier) for proceedings of VCI 2010

Amos Breskin

Bellazzini

Blanco Carballo

Breskin

Chechik

Coelho

Cora Salm

Derré

Escada

Harry van der Graaf

Holy

Jan Visschers

Joost Melai

Jurriaan Schmitz

Llopart

Lyashenko

Marco Cortesi

Martin Fransen

Victor Blanco Carballo

Yevgen Bilevych

English

arXiv

Crossref

A UV sensitive integrated Micromegas with Timepix readout

This article presents a detector system consisting of three components, a CMOS imaging array, a gaseous-detector structure with a Micromegas layout, and a UV-photon sensitive CsI reflective photocathode. All three elements have been monolithically integrated using simple post-processing steps. The Micromegas structure and the CMOS imaging chip are not impacted by the CsI deposition. The detector operated reliably in He/isobutene mixtures and attained charge gains with single photons up to a level of 6x104. The Timepix CMOS array permitted high resolution imaging of single UV-photons. The system has an MTF50 of 0.4lp/pixel which corresponds to approximately 7lp/mm

Melai, Joost

Breskin, Amos

Cortesi, Marco

Bilevych, Yevgen

Fransen, Martin

van der Graaf, Harry

Visschers, Jan

Blanco Carballo, Victor

Salm, Cora

Schmitz, Jurriaan

University of Twente Research Information

Nuclear Instruments and Methods in Physics Research A 628 (2011) 133–137Contents lists available at ScienceDirectNuclear Instruments and Methods inPhysics Research A0168-90doi:10.1 CorrE-mjournal homepage: www.elsevier.com/locate/nimaA UV sensitive integrated Micromegas with Timepix readoutJoost Melai a, Amos Breskin b, Marco Cortesi b, Yevgen Bilevych c, Martin Fransen c, Harry van der Graaf c,Jan Visschers c, Victor Blanco Carballo c, Cora Salm a,, Jurriaan Schmitz aa MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlandsb Weizmann Institute of Science, Rehovot, Israelc NIKHEF, Amsterdam, The Netherlandsa r t i c l e i n f oAvailable online 4 July 2010Keywords:Gaseous radiation detectorMicromegasUV-photon detectionInGridCsI photocathodeCMOS post-processing02/$ - see front matter & 2010 Elsevier B.V. A016/j.nima.2010.06.301esponding author. Tel.: +31 53 489 2648.ail address: c.salm@utwente.nl (C. Salm).a b s t r a c tThis article presents a detector system consisting of three components, a CMOS imaging array, agaseous-detector structure with a Micromegas layout, and a UV-photon sensitive CsI reflectivephotocathode. All three elements have been monolithically integrated using simple post-processingsteps. The Micromegas structure and the CMOS imaging chip are not impacted by the CsI deposition.The detector operated reliably in He/isobutane mixtures and attained charge gains with single photonsup to a level of 6104. The Timepix CMOS array permitted high resolution imaging of singleUV-photons. The system has an MTF50 of 0.4 lp/pixel which corresponds to approximately 7 lp/mm.& 2010 Elsevier B.V. All rights reserved.1. IntroductionWe present a novel integrated gaseous UV-photon detectorthat is made by post-processing a charge-sensitive CMOS pixeldetector. It is based on the InGrid technology [1] which usesmicrotechnology techniques to construct a Micromegas detectorstructure on top of the readout chip. This method allows aligningthe different components of the detector with the anode plane.The pillars are placed at the intersections of four adjacent pixels;the circular holes in the grid are centered on the input pad of eachpixel. The hole pitch equals that of the readout matrix, here55mm. The hole diameter can be varied, typical values are20230mm. This affects the optical transparency of the grid, forthe range mentioned it is 10–23%. The photosensitivity isobtained by coating the grid with a reflective CsI photocathode[2]. CsI is a very robust photocathode that has been used incombination with many gaseous photon detectors [3]. In Ref. [4]a Micromegas device with a single anode is coupled to a CsIreflective photocathode. A CMOS imaging chip coupled to a non-integrated gaseous electron multiplier (GEM) with a CsI photo-cathode is described in Ref. [5]. This work presents the first fullymonolithic embodiment of such a UV-photon imaging detector.2. ExperimentalThe detectors are composed of three elements: the imagingchip, the integrated Micromegas grid, and a reflective photo-cathode. Fig. 1 shows the full detector assembly. The Micromegasll rights reserved.structure, the photocathode and some of the experimentalmethods discussed below are similar to our earlier workreported in Ref. [6].The Timepix chip used here [7] is a variant of the Medipix2chip [8]. It has been designed for the Medipix2 collaboration byCERN, within the EUDET framework. The Timepix chip contains amatrix of 256256 square pixels with a size of 55mm. The totalimaging area is  14 14 mm2. Each of the pixels has a largemetal input pad connected to an input capacitor. The collectedcharge is read out with a charge sensitive pre-amplifier capable ofdetecting charges of less than 1000e . The charge is comparedwith a threshold value which can be changed per pixel. There aredifferent readout modes. Just as Medipix2, the chip can be used tocount the number of hits above a selected threshold during adefined shutter period. In Timepix-mode the arrival time of thefirst hit above threshold is recorded per pixel, and in TOT-mode(Time Over Threshold) the 14 bit counter is incremented as longas the signal remains above threshold. Because the dischargecurrent of the input capacitor is kept constant at a known value,the TOT readout can be used to measure the input charge.2.1. Chip post-processingThe detector is built directly on the surface of the imaging chipas described in detail in Ref. [1]. The present devices have beenbuilt on single chips, but the process can also be performed onclusters of several chips (for instance a square of 3 by 3 chips) oron entire wafers.The first step is to cover the anode surface of the pixel matrixwith a highly resistive layer, in our case this is a 7mm Si-rich SixNylayer deposited by plasma-enhanced CVD. This layer protects theFig. 1. SEM image of a complete detector with Timepix substrate, highly resistivespark protection layer and InGrid structure with isolating pillars supporting aperforated Al grid.Mesh cathodeEaUV photonLow E-field transfer of primary e high E field multiplicationAl grid with CsITo readout of TimepixSU-8 pillarFig. 2. Illustration of the device and its operation. Light enters through a windowfrom the top; photoelectrons from a CsI photocathode on the grid are focused intothe multiplication gap; the resulting avalanche induces charge onto the CMOSpixel array of the Timepix chip, via a discharge-protecting film. Operation was ingas flow mode at 1 atm.Field strength [kv/cm]0 1 2 3 4 5Ar/CH4 (95/5)Ar (100)He (100)isobutane (100)He/isobutaneAr/isobutane (90/10)Extraction efficiency10.90.80.70.60.50.40.30.20.10Fig. 3. Extraction efficiency of photoelectrons from a CsI photocathode intovarious gas mixtures (reference to vacuum). All gases were maintained at 1 atm.J. Melai et al. / Nuclear Instruments and Methods in Physics Research A 628 (2011) 133–137134chip by quenching discharges that may occur in the gas duringdetector operation.In a next phase the InGrid (Integrated Grid) structure is built.First a photoimagable SU-8 polymer layer is spin cast onto thesubstrate. This layer is baked to render it solid. A 1mm thick Allayer is sputtered onto the SU-8. To form the perforated grid themetal is patterned using standard lithography and wet etchtechniques. The SU-8 layer, which has already been exposedearlier, is then developed in acetone, removing the unexposed, notcrosslinked material from below the grid through the holes in themetal. The structure is rinsed by immersion in iso-propyl alcohol.After drying, the structure appears as shown in Fig. 1. Our presentdetector has 25mm diameter holes at a pitch of 55mm; themultiplication gap (grid-to-anode) is  80mm.The chip is bonded to a readout board. To operate the Timepixchip we further need a small, USB-connected readout module anda PC with dedicated software [9]. The readout board has anintegrated gas chamber of approximately 10101.5 cm3. Insidethis volume an additional steel cathode mesh is mounted5–10 mm above the grid. Above this mesh, centered on thelocation of the chip, is a UV-transparent Suprasil quartz window.After mounting the chip can be coated with CsI. For this thewhole assembly—without the top cover, window and cathodemesh—is placed inside the evaporation chamber. The depositionis done by thermal evaporation in vacuum from a solid CsI source,using resistive heating [2]. In some cases the CsI layer is patternedusing a simple Al foil shadow-mask during deposition. The typicalCsI thickness is 220 nm.2.2. Measurement methodsThe different elements of the system and the detectoroperation principle are shown in Fig. 2. Light impinges onto thephotocathode layer on the top surface of the grid. The chip surface(anode) is kept at ground potential. The cathode mesh and thegrid are typically biased at the same negative potential, to preventelectrons from drifting upwards. The electrons follow theevanescent field of the high field region below the grid. Afterdrifting through the holes the charge is multiplied towards theanode. The charge cloud then falls onto one of the pixels of thematrix.Charge signals from the detector are recorded from thegrid, using a charge preamplifier, a shaping amplifier and amulti-channel analyzer (MCA). The amplifier chain is calibratedby injecting a known charge from a test capacitor. The images arerecorded with the Timepix chip, using the readout methodsdiscussed above.The devices can be operated with different gas mixtures; bestresults were obtained with a He/isobutane (80/20) mixture.Device operation is very stable in He/isobutane, several detectorshave been operated for up to several weeks without any signs ofdegradation. Lower gains were reached in Ar-based mixtures.There are two drawbacks related to the use of He and isobutane.First, isobutane absorbs UV radiation. For our setup this effect islimited because of the low isobutane concentration and the smallthickness of the gas layer ð  1 cmÞ. Secondly, in He there is ratherhigh backscattering of photoelectrons on gas molecules comparedto other noble gases [10]. This reduces the effective quantumefficiency of the detector, even when mixed with isobutane. Fig. 3shows the measured extraction efficiency into various gases; thisefficiency is the ratio between the extraction levels in gas andin vacuum, at the same bias and irradiation conditions. InHe/isobutane (80/20) we reach an extraction level of around50% for normal operating conditions ð  2 kV=cmÞ. For thebackscattering in other gas mixtures see Ref. [11].J. Melai et al. / Nuclear Instruments and Methods in Physics Research A 628 (2011) 133–137 1353. Results and discussion3.1. Gain as determined from pulse measurementsThe gain of the detector was determined by irradiating it withtwo different sources (UV light and 55Fe). In both cases the gridand the cathode mesh were biased at equal potentials and theanode was grounded; the measurements were done inHe/isobutane (80/20) at atmospheric pressure. In one experimentwe used an Ar(Hg) lamp with a pinhole aperture placed far fromthe detector. The light flux was further reduced by means ofabsorbers. The response of the detector to this irradiation wasmeasured with the pre-amplifier attached to the grid. Theexponential pulse-height spectra (typical for single-photon gasavalanche detectors) for a range of bias voltages are shown inFig. 4a. The distributions are accurately described by Eq. (1),where N is the number of hits, Q is the charge, G is the averagegain and C is a constant:NðQ Þ ¼ C  1=G  expðQ=GÞ: ð1Þcount [-]1.E+071.E+061.E+051.E+041.E+031.E+021.E+011.E+000.0+E00 5.0+E05 1.0+E06increasing V500 V525 V540 V550 V555 V560 V565 V570 V575 VQ [e-]Fig. 4. (a) Pulse-height spectra for a range of bias voltages, recorded on the grid underUV-photons and of 55Fe 5.9 keV X-rays.aFig. 5. (a) An image of a slanted metal sheet, the data of a certain ROI are considered, thsame conditions—providing the step response on the right (b).By making fits to the measured spectra we determined theaverage gain G for all voltages. We have also determined the gainby measuring the average charge cloud arriving after multi-plication of the initial charge from the conversion of 55Fe photons(irradiating through a thin Kapton window) in the gas. The resultsof both gain measurements are plotted in Fig. 4b. There is goodagreement between the two methods; with 55Fe we obtained aslope of 100 V/decade and with UV-photons it is 110 V/decade.The maximum gain that we reached was 6104.3.2. Spatial resolution using the slanted edge methodTo determine the spatial resolution we followed the ISO 12233procedure [12]. Under UV irradiation images were recorded of theedge of a thin steel sheet placed at a slight angle with respect tothe pixel orientation. The response was corrected for fixedpatterns, dividing by the data of an open frame (flat-field)measurement taken under the same conditions. The step responsewas analyzed within a certain Region Of Interest (ROI), as shown1.E+061.E+051.E+041.E+03Gain with UVGain with Fe55Fit to pulse data425 450 475 500 525 550 575Vgrid [V]Gain [-]UV irradiation and (b) Gain vs. bias voltage derived from pulse-height spectra of0 10 2030 40 5060 70 8005010000.511.5e raw image data are corrected with an open frame measurement taken under theJ. Melai et al. / Nuclear Instruments and Methods in Physics Research A 628 (2011) 133–137136in Fig. 5a. For each line of pixels we took the derivative of the pixelcounter value and determined the centroid; this defined the edgelocation within each line of pixels. A linear fit was performedESF [-]-0.2500.250.50.7511.250 200 400 600ESFLSFLSF [-]00.10.20.3300 310 320 330 340 350 360bin (8 per pixel)LSF dataGaussian fitFWHM 1.13 pixel 62.2 µmbin (8 per pixel)Fig. 6. (a) ESF as derived from the rebinned data within the ROI. (b) LSF data andGaussian fit.Fig. 7. 2D images of flat-field UV irradiation of the detector shown in Fig. 2 using He/isobthe 55mm pitch detector and (b) direct imaging of the mesh when the mesh pitch is ithrough the centroid locations to derive a description of the edge.The data of all lines were rebinned (in this case with 8 bins perpixel) and simultaneously shifted according to the slant that wasdetermined before. In this way we could combine all data in theROI in one Edge Step Function (ESF). The derivative of the ESF isthe Line Spread Function (LSF). A Gaussian curve was fitted tothe LSF; the 1s spread is 0.48 pixels or 26:4mm and the FWHM is1.13 pixels or 62:2mm. The results are shown in Fig. 6.By performing a Fourier transform of the LSF we obtainedthe Modulation Transfer Function (MTF) of the entire system.The MTF reaches 50% (MTF50) at 0.4 lp/pixels ðC7 lp=mmÞ. Theresolution limit is below the pixel pitch, at the Nyquist frequencyMTF¼0.32. The resolution limit is reached at 0.8 lp/pixelsðC14 lp=mmÞ.3.3. Imaging resultsThe cathode mesh (see Fig. 2), needed to shape the field abovethe grid, modulates the light reaching the photocathode. In Fig. 7awe see a Moiré pattern which is the result of using a cathodemesh with a wire pitch of 56mm and a detector pitch (both gridand readout pads) of 55mm. Using a coarser mesh, with a pitchof 500mm, the mesh wires are directly imaged by the detector(Fig. 7b). The high resolution permits observing mesh defects.The images in Fig. 7 were taken with a partial CsI coating. Forexperimental reasons not discussed here, the bottom 15% of thematrix were not coated with CsI, which naturally resulted in alower response in that region.Fig. 8 shows two images obtained in He/isobutane (80/20)using TOT counting under UV-photon irradiation. Fig. 8(a) is animage recorded with a steel mask of the logo of the University ofTwente placed in front of the detector; Fig. 8(b) is an image of afingerprint intentionally left on the detector window. The latterwas obtained by correcting the raw image with the data of anopen frame reference image recorded under the same conditions.The fingerprint image was recorded without CsI on the Al grid.This shows that, in spite of the (very) low quantum efficiency,even without CsI the detector performs as a UV imaging device.utane (80/20) at 1 atm. (a) Moiré pattern resulting from using a 56mm mesh abovencreased to 500mm.Fig. 8. (a) Image of the logo of the University of Twente and (b) image of a fingerprint left on the detector window.J. Melai et al. / Nuclear Instruments and Methods in Physics Research A 628 (2011) 133–137 1374. ConclusionsWe have presented a monolithic gaseous UV-photon imagingdetector with a CsI photocathode. The photocathode depositionand its operation are found to be adequate in combination withboth the Micromegas multiplier and with the imaging chip. Mostexperiments have been performed in He/isobutane (80/20), inwhich the extraction efficiency is around 50%. The maximum gainthat was reached amounts to 6104.The imaging experiments indicate a good spatial resolution,the LSF has a spread of 26:4mm, MTF50 is reached at 0.4 lp/pixelðC7 lp=mmÞ. High quality 2D images were recorded.These encouraging results suggest further investigationsdirected towards the search for operation conditions and forgas-mixtures assuring better photoelectron collection efficiencies(lower backscattering); investigations of absolute photondetection efficiencies and of ion-blocking (ion-feedback) areforeseen. The new device could find applications in high-resolution imaging of UV-photons. Furthermore, being madeof UHV-compatible materials, it could be applicable as amultiplication and readout element also in cascaded visible-lightimaging gas photomultipliers [13].AcknowledgmentsThe authors acknowledge Sander Smits (of the Universityof Twente), Max Chefdeville, Fred Hartjes, Joop Rövekamp andJan Timmermans (all with NIKHEF) and Moshe Klin and AlexeyLyashenko (both from the Weizmann Institute) for their helpduring the manufacturing of the test devices and the measure-ments. This research is funded by the Dutch technology founda-tion STW through project TET-6630. A. Breskin is the W.P. ReutherProfessor of Research in The Peaceful Use of Atomic Energy.References[1] V.M. Blanco Carballo, et al., IEEE Electron. Dev. Lett. 29 (2008) 585.[2] A. Breskin, Nucl. Instr. and Meth. A 371 (1996) 116.[3] R. Chechik, A. Breskin, Nucl. Instr. and Meth. A 595 (2008) 116.[4] J. Derré, et al., Nucl. Instr. and Meth. A 449 (2000) 314.[5] R. Bellazzini, et al., Nucl. Instr. and Meth. A 581 (2007) 246.[6] J. Melai, A. Lyashenko, A. Breskin, H. van der Graaf, J. Timmermans, J.Visschers, C. Salm, J. Schmitz, Nucl. Instr. and Meth. A, in press, doi:10.1016/j.nima.2010.06.165.[7] X. Llopart, et al., Nucl. Instr. and Meth. A 581 (2007) 485 and its erratum inNucl. Instr. and Meth. A 585 (2008) 106.[8] X. Llopart, et al., IEEE Trans. Nucl. Sci. NS-49 (2002) 2279.[9] T. Holy, et al., Nucl. Instr. and Meth. A 563 (2006) 254.[10] L.C.C. Coelho, et al., Nucl. Instr. and Meth. A 581 (2007) 190.[11] J. Escada, et al., J. Instr. 4 (2009) P11025.[12] P. Burns, D. Williams, in: Proceedings of the PICS 1999, Imaging Science andTechnology, 1999, pp. 51–53.[13] A. Lyashenko, et al., J. Instr. 4 (2009) P07005.

A UV sensitive integrated micromegas with timepix readout

https://doi.org/10.1016/j.nima.2010.06.301

A UV Sensitive Integrated Micromegas with Timepix Readout

A UV Sensitive Integrated Micromegas with Timepix Readout

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