These webpages relate to research carried out between 2001 and 2004 into applying new micromachining techniques to the development of gas-avalanche type radiation detectors. Three-dimensional microstructures fabricated with SU-8 photosensitive epoxy showed promise in overcoming electrical breakdowns and other problems encountered in high radiation fluxes with the early generation of gas-avalanche micropattern radiation detectors (MPGDs). The motivation behind the work was to produce stable detectors with fast response principally for X-ray imaging and synchrotron diffraction applications. The work was funded by a Marie Curie Fellowship awarded by the European Community.

A large amount of research has been devoted to the development of micropattern gas detectors (MPGDs), most of which has been undertaken by the High Energy Physics community, and many ingenious gas-avalanche devices have been introduced. These types of detectors are now used in an ever widening range of applications, including X-ray crystallography, neutron diffraction, industrial imaging, and X-ray astronomy. Increasingly, new technological developments are focussed towards smaller scale applications, a shift fuelled by the fact that the microfabrication processes implied with advanced micropattern devices often limit the practical size of detectors. This limitation is highly unattractive for detectors requiring surface areas up to hundreds of square metres, although the the introduction of polyimide sheet PCB technology, used for example in the gas electron multiplier (GEM), has contributed greatly to the production of larger area detectors. However, where smaller area detectors are concerened, size restrictions imposed by fabrication technologies originally developed for silicon wafer processing are not always an issue, for example in many X-ray imaging applications. This allows for problems associated with microstructure detectors to be addressed using advanced micromaching techniques, the subject of this research.

Non-planar microstrip detectors ("microtrench" geometry)

Regarding X-ray imaging applications, certain important short-term effects are observed in gas microstrip detectors when they are subjected to high photon irradiation rates. Firstly, some of the positive ions produced in the avalanche accumulate on the insulating substrate between the anode and cathode strips, causing a modification of the applied electric field and resulting in a steady reduction in the gas gain. Secondly, as the radiation flux is increased, the probability of electrical breakdown increases dramatically, effectively limiting the safe operating potential and therefore the maximum achievable gain of the detector. These breakdowns appear to be initiated by a corona discharge mechanism at the cathode, with streamer propagation aided by the presence of a dielectric surface between the electrodes. If the streamer is able to bridge the anode-cathode gap, a low resistivity plasma channel may be formed, leading to a spark event. Additionally, the practice of lowering the resistivity of the substrate surface to alleviate the accumulation of positive ions serves to increase the vulnerability of the detector to streamer-mediated breakdowns.

microtrench detector
microtrench detector
Basic structure of non-planar microstrip (microtrench) detectors, with electrode strips patterned on top of 50 micron high SU8 pillars (sectioned with wafer saw)

The philosophy of the non-planar microstrip (microtrench) configuration is to firstly eliminate the effect of positive ion accumulation on the substrate by removing the substrate surface from the microstrip plane, and secondly, to enable electrodes to be patterned directly onto the substrate to perform various functions depending on the operating regime of the detector. To this end, the standard microstrip configuration has been modified such that the anode and cathode strips are supported above the substrate surface, with deep trenches separating the electrodes. This is achieved by photolithographically etching a layer of SU-8 photosensitive epoxy, forming pillars on top of which the electrode structure is patterned. In high gain detectors, electrodes (not shown in images above) can be patterned on the substrate surface to control the behaviour of surface streamers and neutralize them before plasma channel spark precursors develop between the anode and cathode strips. Detectors fabricated with different trench electrode structure also enable us to study the nature of breakdown mechanisms at high irradiation rates

Of particular interest in this reseach is the development of an imaging plate based on microtrench technology, where an orthogonal electrode pattern is patterned on the substrate, as in the diagram below (purple structures), to acheive a structure capable of providing two-dimensional readout. This configuration affords less protection against spark damage and limits the maximum gain of the detector. However, the proximity of the orthogonal pick-up electrode to the avalanche region implies a lower gain requirement for two-dimensional imaging compared to backplane readout schemes, particularly with detectors constructed on high dielectric constant substrates e.g. Schott S8900. Faster signals can be obtained, and better positional resolution inthe case where the strip pitch is significantly lower than the substrate thickness.

microtrench detector schematic
Schematic of microtrench detector with orthogonal trench electrode
microtrench detector micrograph
Micrograph of microtrench detector with gold electrodes

This geometry is well suited for delay-line readout, with the anode providing the "start" signal, and the cathode and trench electrodes providing the positional information. The lower gain operation compared to standard 2D microstrips also serves to limit aging (or "radiation damage") phenomena. Certain types of radiation damage occur when organic quench gases additives, used to prevent non-local photoionisation caused by UV photons formed in the avalanche, disassociate into free radicals which may be able to attach to electrode surfaces. Thus, insulating deposits can build up on the electrodes over a period of prolonged irraditation, causing secondary phenomena such as gain loss and "Malter discharge". By reducing the operating gain of the detector, quench gas breakdown is reduced and the detector life prolonged.

microtrench detector electric field
Typical electric field distribution in a non-planar gas microstrip (microtrench) detector

Monolithic GEM-type detectors ("microwell" geometry)

While the focus at present (writing in 2001) is on the microtrench devices, a second type of microstructure detector using SU-8 is also under development. This device is related to the evolution of the gas electron multiplier (GEM) device, originally introduced in order to separate the avalanche and readout regions of gas avalanche detectors to increase stability and resistance to damaging spark events. The original GEM device consists of a thin polyimide sheet clad with copper on both sides; a matrix of holes is patterned through the sheet and a potential applied across the holes. Gas amplification then takes place as electrons pass through the holes and high gains can be achieved by using a series of GEM stages separated by pillars at a distance of about 1 mm from each other. Unfortunately, some charge up of the polyimide sidewalls of the holes causes a gain reduction, caused by the conical profile of the sidewalls arising from the hole etching process.

We are developing a process whereby several ampification stages are stacked together in a monolithic structure with the additional possibility of being patterned directly onto a pixel readout chip (ASIC) for smaller area devices.

microwell detector structure
A monolithic multiple "microwell" structure. The blue layers represent the multiple electron accelerating electrodes, the green layers the dielectric SU-8 epoxy, the purple dots are the readout anodes. The whole structure is patterned onto a silicon or glass substrate (grey).
microwell detector micrograph
Planar view of a single-layer microwell detector, well diameter 100 microns
microwell detector scanning em
Single-layer microwell structure sectioned with wafer saw

This structure has the advantage of vertical sidewalls, flexibility in the depth of the amplification holes, and the monolithic structure will preserve spatial information otherwise degraded by misalignment between polyimide layers and electron diffusion between layers. Electron diffusion increases with gas pressure, and X-ray detection gas mixtures are also difficult to optimise for low electron diffusion, so the monolithic structure is especially suited to hard X-ray detection. In addition, signal loss resulting from electron collection on the lower GEM electrodes will be reduced. A disadvantage of this device is that the signal at the readout electrode contains the slow positive ion contribution eliminated in the polyimide sheet configuration due the induction gap between avalanche and readout planes. Thus, in the absence of a universal detector solution, the trade-offs necessarily made when selecting imaging detector technologies for a particular application will take this effect into consideration.

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