Marc Christophersen

Gray-tone lithography using an optical diffuser and a contact aligner

This paper describes a simple method for the three-dimensional (3D) microfabrication of complex high-aspect structures in a one mask lithography process. The method relies on an unconventional way of performing gray-tone lithography. The main idea is to randomize the collimated light by using an optical diffuser to generate intensity distributions in the photoresist. The resist topography is determined by the density of open and opaque squares in the photomask. The resulting 3D resist is then transferred into 3D silicon structures by using reactive ion etching and deep reactive ion etching.

Beam test studies of 3D pixel sensors irradiated non-uniformly for the ATLAS forward physics detector

Abstract Pixel detectors with cylindrical electrodes that penetrate the silicon substrate (so called 3D detectors) offer advantages over standard planar sensors in terms of radiation hardness, since the electrode distance is decoupled from the bulk thickness. In recent years significant progress has been made in the development of 3D sensors, which culminated in the sensor production for the ATLAS Insertable B-Layer (IBL) upgrade carried out at CNM (Barcelona, Spain) and FBK (Trento, Italy). Based on this success, the ATLAS Forward Physics (AFP) experiment has selected the 3D pixel sensor technology for the tracking detector. The AFP project presents a new challenge due to the need for a reduced dead area with respect to IBL, and the in-homogeneous nature of the radiation dose distribution in the sensor. Electrical characterization of the first AFP prototypes and beam test studies of 3D pixel devices irradiated non-uniformly are presented in this paper.

Laser-micromachining for 3D silicon detectors

We laser-micromachined hole arrays into silicon with aspect ratios up to 100:1. Direct laser-drilling and trepanning were used. Since laser-machining leaves a damaged silicon region that is not suitable for detector fabrication, we removed the damaged silicon with an isotropic xenon difluoride etch step. Hole arrays with trepan drilled holes had lower leakage currents. We successfully collected a Co-57 photon spectrum (energy resolution: 3.0 keV FWHM at 122 KeV) with a 3D silicon detector based on trepan-drilled holes. Our results show that silicon laser-machining can be used for 3D detector fabrication with good charge collection propeties.

Curved radiation detector

As vertex detectors for smaller for smaller inner beam pipe diameters are required, the planar nature of the detector becomes more and more of a problem. We present a curved silicon vertex detector, whose radius of curvature can be adjusted to the beam pipe. The advantage of these curved detectors over conventional planar ones is twofold: The first advantage is that these detectors are curved to a specific curvature and shaped directly for the specific application (e.g. beam pipe radius), and second, the curvature of the backside is independent from the front surface, which allows thinning of the detector using standard semiconductor processing. Both strip detectors and pixel arrays (with Indium bump bonds) have been realized on the curved topography. The key micro-fabrication technique for curved topography, so called “gray-tone lithography”, will be introduced and discussed. We demonstrated low-noise performance by successfully detecting low-energy gamma-rays with a curved strip detector. The energy resolution was ∼ 1.73 keV FWHM at 59.5 keV for the pixel detector. There is excellent charge collection at the curved surface. The general fabrication method could also be applied for curved focal plane arrays to improve optical systems.

Alumina, Al 2 O 3 , layers as effective p-stops for silicon radiation detectors

Inter-strip shortening due to electron accumulation is a problem for any segmented p-type and double-sided n-type detectors. The standard approach for inter-strip or interpixel isolation is an implanted and annealed p-type layer, called p-stop. We show that alumina layer can be used as effective p-stops due to a negative surface charge at the silicon-alumina interface. We used ALD (atomic layer deposition) and e-beam evaporated alumina layers as inter-strip dielectrics for n-on-n strips. We fabricated a double sided strip detector (DSSD) on n-type silicon with alumina as a p-stop and tested the DSSD under gamma-ray irradiation. We compare ALD alumina layers with standard silicon oxide for isolating n-on-n strips. Equivalents to “p-stop” and “p-spray” were fabricated and evaluated with respect to their leakage currents. Finite element simulations support our experimental findings.

200 mm silicon wafer processing for large area strip detectors

We developed large silicon single-sided strip detectors made of 200 mm float-zone Si wafers. The single-sided silicon strip detectors have an effective active area of 156 cm2 and 725 μm in thickness and were fully depleted. Basic performance was measured using Am-241 and Co-57 sources. The leakage current varied from strip to strip due to some contaminations during the processing.

Thick silicon drift detectors

A new concept of silicon drift detector is presented that potentially allows much thicker devices. The detector is based on a trench array, which penetrate the bulk with different depths. Finite element (FEM) simulations of the detector structure will be presented and discussed. The key micro-fabrication technique for different depth trenches, so called “gray-tone lithography”, will be introduced and discussed in a feasibility study.

Multiple-order staircase etalon spectroscopy

Traditional Fabry-Perot (FP) spectroscopy is bandwidth limited to avoid mixing signals from different transmission orders of the interferometer. Unlike Fourier transformation, the extraction of spectra from multiple-order interferograms resulting from multiplexed optical signals is in general an ill-posed problem. Using a Fourier transform approach, we derive a generalized Nyquist limit appropriate to signal recovery from FP interferograms. This result is used to derive a set of design rules giving the usable wavelength range and spectral resolution of FP interferometers or etalon arrays given a set of accessible physical parameters. Numerical simulations verify the utility of these design rules for moderate resolution spectroscopy with bandwidths limited by the detector spectral response. Stable and accurate spectral recovery over more than one octave is accomplished by simple matrix multiplication of the interferogram. In analogy to recently developed single-order micro-etalon arrays (Proc. of SPIE v.8266, no. 82660Q), we introduce Multiple-Order Staircase Etalon Spectroscopy (MOSES), in which micro-arrays of multiple order etalons can be bonded to or co-fabricated with a sensor array. MOSES enables broader bandwidth multispectral and hyperspectral instruments than single-order etalon arrays while keeping a physical footprint insignificantly different from that of the detection array.

Neutron/gamma pulse shape discrimination (PSD) in plastic scintillators with digital PSD electronics

Pulse shape discrimination (PSD) is a common method to distinguish between pulses produced by gamma rays and neutrons in scintillator detectors. This technique takes advantage of the property of many scintillators that excitations by recoil protons and electrons produce pulses with different characteristic shapes. Unfortunately, many scintillating materials with good PSD properties have other, undesirable properties such as flammability, toxicity, low availability, high cost, and/or limited size. In contrast, plastic scintillator detectors are relatively low-cost, and easily handled and mass-produced. Recent studies have demonstrated efficient PSD in plastic scintillators using a high concentration of fluorescent dyes. To further investigate the PSD properties of such systems, mixed plastic scintillator samples were produced and tested. The addition of up to 30 wt. % diphenyloxazole (DPO) and other chromophores in polyvinyltoluene (PVT) results in efficient detection with commercial detectors. These plastic scintillators are produced in large diameters up to 4 inches by melt blending directly in a container suitable for in-line detector use. This allows recycling and reuse of materials while varying the compositions. This strategy also avoids additional sample handling and polishing steps required when using removable molds. In this presentation, results will be presented for different mixed-plastic compositions and compared with known scintillating materials

Photo-patterned silicone bump bonds for sensor interconnects

Bump bonding is the standard approach for connecting pixel sensors with read-out ASICS. Current fine-pitch bump bonds lead to permanent bonds. We use photo-patternable silicone (polydimethylsiloxane, or PDMS) in combination with a metal coating to generate flexible and reversible bump bonds. This process requires only a relatively low temperature, <150 °C, and leads to very homogeneous bump height, because the PDMS is applied by spin-coating. The technique is also applicable to brittle materials, where wire-bonding is problematic, such as gamma-ray detectors based on CdZnTe and CdTe. This paper describes the general concept and presents first results.