Photon imaging using post-processed CMOS chips

This thesis presents our work on an integrated photon detector made by post-processing of CMOS sensor arrays. The aim of the post-processing is to combine all elements of the detector into a single monolithic device. These elements include a photocathode to convert photon radiation into electronic signals (in the extreme case this conversion is from a single photon into a single electron), an electron multiplication structure to increase the magnitude of the signal pulse and a position sensitive recording element to register the place, time and size of the electron charge pulse.\ud \ud The recording element is a CMOS imaging IC called Timepix. The rest of the detector structure is processed directly onto the CMOS substrate. It is essential that the post-processing steps are compatible with the underlying CMOS devices.\ud \ud The multiplication structure that we use is InGrid, an integrated version of a socalled Micromegas detector structure. The device consists of a thin metal grid that is elevated 50-80 µm above the chip surface with dielectric pillars. These are made of SU-8, a photoimagable polymer. We have investigated the dielectric strength of SU-8. The value of remarkably high for a polymer: 4.4 MV/cm.\ud \ud A large potential is applied between the grid and the anode, the structure is filled with a gas mixture. When electrons enter the space below the grid they are accelerated and upon collision with a gas atom secondary electrons are generated. This process is repeated a number of times before tht total charge cloud arrives to the anode plane (the chip) where it is detected.\ud \ud The InGrid structure is coupled to a CsI photocathode to make it sensitive for UV photons. The system is capable of imaging UV photons with high selectivity and with a high resolution of 0.8 lp/pixel

Considerations on using SU-8 as a construction material for high aspect ratio structures

This paper discusses two material aspects of SU-8 that have up till now been insufficiently documented. We present initial results on the outgassing behavior and a study on the dielectric properties of SU-8 at high bias voltage. The dielectric strength is determined to be at least 2 MV/cm. These elements are investigated in the light of plans to manufacture an SU-8 based Micro-Channel Plate (MCP). Although the outgassing properties and dielectric strength are favorable the patterning capabilities are expected to limit the use of such an MCP

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/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

An integrated Micromegas UV-photon detector

Preliminary results of a photon detector combining a Micromegas like multiplier coated with a UV-sensitive CsI photocathode are described. The multiplier is made in a CMOS compatible InGrid technology, which allows to post-process it directly on the surface of an imaging IC. This method is aimed at building light-sensitive imaging detectors where all elements are monolithically integrated. We show that the CsI photocathode deposited in the InGrid mesh does not alter the device performance. Maximum gains of ~6000 were reached in a single-grid element operated in Ar/CH4, with a 2% Ion Back Flow fraction returning to the photocathode.Comment: 5 pages with 6 figures. This paper is submitted to Nuclear Instruments and Methods A (Elsevier) for the proceedings of IWORID 2009

Qualitative and quantitative characterization of outgassing from SU-8

SU-8 is often used as a structural material in Microsystems. In this work, the outgassing characteristics from such cross-linked SU-8 layers are studied using mass spectrometry and gas-chromatography techniques.\ud With these methods the composition of the released matter can be identified, also the outgassing rate can be quantitatively characterized. A qualitative estimate of outgassing from SU-8 is given for crosslinked layers. The effect of hard-bakes is studied in situ by measuring at typical hard-bake temperatures.\ud These tests indicate that a hard-bake is needed to provide good performance in UHV environments. Using gas-chromatography the outgassing rate from SU-8 is determined. The total outgassing rate is inversely proportional with time which further illustrates the effect of a hard-bake step.\u

The electrical conduction and dielectric strength of SU-8

This paper presents a study on the dielectric behavior of SU-8 photoresist. We present measurements on the leakage current levels through SU-8 layers of varying thickness. The leakage current is dominated by thermionic emission. We have further determined the dielectric strength of SU-8 to be 4.4 MV cm−1. The remarkably high dielectric strength allows the material to be used for high-voltage applications

Reliability aspects of a radiation detector fabricated by post-processing a standard CMOS chip

This paper describes various reliability concerns of the newly developed INGRID detector. This radiation detector is fabricated by waferscale CMOS post-processing; fresh detectors show excellent performance. Since the microsystems will be used unpackaged they are susceptible to all kinds of environmental conditions.\ud The device passed tests of micro-ESD, radiation hardness, dielectric strength; but humidity tests show one weakness of SU-8 as a structural material. Already after 1 day of exposure to a humid condition the structural integrity, as measured by a shear stress test, is dramatically lowered. Dry storage of these devices is therefore a necessity. KMPR photoresist shows promising results as an alternative structural material.\u

Further outgassing studies on SU-8

SU-8 is often used as a structural material in microsystems. In this work the outgassing characteristics from such cross-linked SU-8 layers are studied using Mass Spectrometry and Gas Chromatography techniques. With these methods the composition of the released matter can be identified, also the outgassing rate can be quantitatively characterized. A qualitative estimate of outgassing from SU-8 is given for cross-linked layers. The effect of Hard-Bakes is studied in situ by measuring at typical Hard-Bake temperatures. These tests indicate that a Hard-Bake is needed to provide good performance in UHV environments. Using Gas-Chromatography the outgassing rate from SU-8 is determined. The total outgassing rate is inversely proportional with time which further illustrates the effect of a hard-bake step

A Radiation Imaging Detector Made by Postprocessing a Standard CMOS Chip

Abstract-An unpackaged microchip is used as the sensing element in a miniaturized gaseous proportional chamber. This letter 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