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Simulation and Design at the MPG HLL



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  Simulation and Design at the MPG HLL  
  The development of new detector concepts requires a deep insight into device physics and technology. The new device ideas have to undergo a thorough examination through detailed simulations of the fabrication process as well as the electrical behaviour before becoming structures in silicon. A profound detector simulation is mandatory to reduce the number of fabrication cycles until the final application in an experiment. All silicon detectors invented, designed, fabricated and tested at the MPG HLL are optimized by means of two and three dimensional device and process simulation.   text fig A
Figure A
Simulated two-dimensional potential distribution of six pixels of a pnCCD during transfer of an electron charge pocket. View from the radiation entrance side (back side).
  A large variety of simulation tools is used to optimize design and technology of new semiconductor structures. But they are also helpful to gain an insight into the structures, e.g. regarding their operation principles, as well as their advantages and limitations. By studying the potential distribution in a completely depleted detector bulk (Figure A), a very instructive basic picture is obtainable. The programs POSIBIN and POSEIDON solve the Laplace equation in two and three dimensions, respectively. Both codes have been developed by the Politecnico di Milano and Brookhaven National Lab (BNL) in collaboration with the MPG HLL. They are very fast and robust due to their semi-analytical approach. They help gaining a first “impression” of the potential properties of the new devices.    
  A full simulation cycle is composed of a semiconductor fabrication process computation (a) and a semiconductor device simulation (b). In (a) the technical feasibility is analyzed, in (b) the electrical and physical properties are studied.    
  Our technology simulator uses DIOS ISETCAD which provides geometry, topology and the doping profiles of a structure in two dimensions. The properties of the base material are implemented as well as photolithography, ion implantation, thermal treatments and deposition of dielectric and conductive layers. The result of this simulation also serves as an input for the device simulator.    
  The device simulator of the Weierstraß-Institut in Berlin (WIAS-Berlin), WIAS-TeSCA, solves the van Roosbroeck’s equations (Poisson, continuity and transport equations) time dependently in two dimensions. On each node (out of up to 100,000 nodes) of a large triangular grid the electrical potential and the densities of electrons and holes are calculated. Electrical currents and fields are derived from these variables. The robust finite element code allows an efficient simulation of large area detectors with highly refined surface regions like pnCCDs (To internal topicpnCCD Detectors), silicon drift detectors (To internal topicSilicon Drift Detectors made by MPG HLL) and active pixel sensors (To internal topicDEPFET Active Pixel Sensors).  
Figure B
Layout of a circular DEPFET (prototype design for XEUS). The design includes two polysilicon and two metal layers.
  The ionization process of incident photons or particles can be simulated by a local and time dependent increase of the generation rate of electron-hole pairs. They are separated by the electric field and drift towards the readout or collection electrodes, respectively. Avalanche multiplication is introduced in the code through the parameterized ionization coefficients.    
  An example for a two-dimensional simulation of a pnCCD is shown in Figure C. A charge cloud of 1,600 electrons was generated in the detector bulk and drifted into a CCD pixel well representing the incidence of a 55Fe photon with energy of 5.9 keV. After the collection process in the pixel well (within 5 ns) the charge is transferred along the CCD pixel structure (five pixels are displayed). Doping profiles and geometries were optimized in a way that no transfer loss occurs even for a wide variation of operation voltages (To internal topicpnCCD Detectors).

More recently three-dimensional simulation packages became available. They are needed to get correct results e.g. on pixel detectors where the third dimension cannot be neglected anymore. Much more than in the twodimensional case an efficient meshing of the simulation domain is necessary in order to discretize the large volume detector structures by tetrahedron elements.
  Three-dimensional electron density distribution within a rectangular DEPFET immediately after the collection of signal electrons

Figure C
Three-dimensional electron density distribution within a rectangular DEPFET of signal electrons. The grey shaded area depicts a twodimensional cross section along the transistor channel of the DEPFET.
  First results obtained with a three-dimensional device simulator were also developed at WIAS-Berlin. The test code was used to design the next generation of DEPFET prototypes where the pixel size is in the range of 25 µm by 25 µm. To the top Figure A shows an example for the charge collection process in a rectangular shaped DEPFET prototyped for International Linear Collider (To internal topicThe Vertex Detector at the ILC) at CERN. In order to identify less sensitive regions a signal charge was generated at different positions in the detector bulk while monitoring the current response of the DEPFET.    
  There is commercial design software available for our detector layout structures, also in use by microelectronics. We are applying the Virtuoso layout editor by Cadence Design Systems, equipped with special libraries which have been adapted to our detector technology. To the top Figure B shows a layout example of a 75 µm by 75 µm DEPFET pixel which was fabricated within a 64 by 64 array in a double-polysilicon double-metal layer technology at the MPG Halbleiterlabor   (To internal topicDEPMOSFET Active Pixel Sensors for the XEUS Mission).    
  Along with WIAS-Berlin we are continuously expanding the use and the capabilities of the two-dimensional and three-dimensional simulators.    

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Three-dimensional electron density distribution within a rectangular DEPFET of signal electrons