IEEE T R A " r m N S ON NUCLEAR SCIENCE, VOL. 39, NO.4,1992 619 Large Area Cylindrical Silicon Drift Detector W. Chen, H. Kraner, Z. Li, P. Rehak Brookhaven National Laboratory, Upton NY 11973 E. Gatti, A. Longoni, M. Sampietro Politecnico di Milano, 32 Piaeza Leonard0 da Vinci, 20133 Milano, Italy* P. Holl, J. Kemmer TU Miinchen, 8046 Garching and KETEK GmbH, Hauptstrafle 41d, 8048 Haimhausen, Germany U. Faschingbauer, B. Schmitt, A. Worner, J. P. Wurm Max-Planck-Institut for Nuclear Physics, D-6900 Heidelberg 1, Germany Abstract I. INTRODUCTION The semiconductor drift detector [I] uses the transport of electrons in a direction parallel to the large detector surface. The direction of the transport is imposed by means of rectifying junctions at different potentials on both sides of the wafer. The shape of the junctions as well as the applied potential on them can vary to achieve different drift geometries. Up to now only linear geometries and cylindrical geometries with the drift of electrons toward the center were realized [2-31. An ideal detector for the experiment NA45 at the CERN SPS [4] should measure x and y coordinates of several hundred charged particles in an unambiguous way. The required resolution is about 20 pm in both coordinates within a circular area with a radius of 3 cm. A cylindrical silicon drift detector can fulfill those requirements of the experiment. In reality the cylindrical geometry of the detector is very close to the ideal geometry for most of fixed target experiments with unpolarieed beams. Fig. 1 shows the detector and its supporting assembly. The active part is practically the entire 3 inch silicon wafer with a small hole in the center. Signal electrons produced by fast particles drift inside the silicon radially towards the outside edge of the detector. Close to the outside edge there is an array of 360 anodes a t the same radius each collecting electrons from 1 deg segments. Anodes are bonded to traces on a ceramic board to which the silicon wafer is glued. The traces on the ceramic board contact traces on an FR-4 (G10) mother board leading to the inputs of charge sensitive preamplifiers. The contacts are accomplished by a layer of elastomer interconnectors [5]. The mechanical forces for all connectors are provided by 12 screws. The ceramic An advanced silicon drift detector, a large area cylindrical drift detector, was designed, produced, tested and installed in the NA45 experiment. The active area of the detector is practically the total area of a 3 inch diameter wafer. Signal electrons created in the silicon detector by fast charged particles drift radially outside toward an array of 360 anodes located on the periphery of the detector. The drift time measures the radial coordinate of the particle's intersection; the charge sharing between anodes measures the azimuthal coordinate. The detector provides unambiguous pairs of r,d coordinates for events with multiplicities up to several hundred. Its use in the experiment aims at a position resolution of 20 pm (rms) in each direction giving about 2 loe two-dimensional elements. There is a small hole in the center of the detector to allow the passage of the noninteracting particle beam. The longest drift distance is about 3 cm. The nominal value of the drift field is 500 V/cm resulting in a maximum drift time of 4 ps. This manuscript has been authorized under contract number DE-AC02-76CH00016 with the U.S. Department of Energy. Accordingly, the U. S. Government retains a non-exclusive, royalty-free licence to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. (*) This research is also supported by the Italian INFN, MURST and CNR. 0018-9499/92$03.00 0 1992 IEEE .Si02 interface due to anticipated radiation damage effects less the detector. The net flux of electrons in the X direction thus can be written as: X an j x = nu. A list of conclusions closes the paper.X ) is the electron density and D is the diffusion constant of electrons in silicon. The well known expression for the diffusion in a linear field . The solution of the above differential equation is a Gaussian distribution of n(t. ) Let us define direction X = ~4 perpendicular to the radius direction at the center of the electron cloud. Combined with the continuity equation we obtain the diffusion equation for electrons in X direction where TO is the initial radial coordinate of the electron cloud. The consequences of the analysis of the transport for the pitch of anodes and for the number of read out channels is shown. The detector is the silicon wafer of 3 inch diameter with a small hole in the center to allow the passage of the noninteracting beam. (Moving away from the center with a constant radial velocity v . Outputs of the shapers are sampled at 50 MHz by fast 6 bit nonlinear flash ADCs [7]. This is the simple geometrical projection of the initial sigma to the final radius. Theoretical considerations Figure 1: Photograph of the n-side of the cylindrical detector within its assembly. The second section studies the transport of electrons in a cylindrical geometry. Results from the experiment are very preliminary and more beam results will be published later.X) with respect to the coordinate x 1 n ( t . Moreover. in the 4 direction the effect of the diffusion is combined with the defocusing caused by the radial divergence of the drift field. The mean radial velocity of an electron a t X has a component of velocity v x = v $ in the X direction. The general problem of the electron transport in a radial field can be simplified in our case of a constant radial drift velocity v. X ) = -e 2~ -q &U where the time dependence enters through U The second term is the square of the initial spread of the electron cloud multiplied by the square of the radii ratio.The fourth section shows some results obtained from laboratory tests and from the run of the detector at the NA45 experiment at CERN. However. Anodes are connected to the outside preamplifiers. The outside diameter of the assembly is 160 mm. We treat the motion of electrons in a coordinate system having the origin a t the center of gravity of the entire electron cloud. The third section treats the flow of the leakage current generated at the Si Si02 interface into a sink anode. A mother board for the preamplifiers fills the outside radius. (160 During the drift time the electrons diffiise in the drift direction (r) and in a direction perpendicular to the drift (4). the increase of the current generation on the Si . ax where n(t. The first term is due to the diffusion of electrons during the drift. The diffusion in the drift direction is not influenced by the drift field. The photograph shows the sockets into which the preamplifier units are plugged. TRANSPORT OF ELECTRONS 1 A. The present paper treats mainly the design aspects of the detector and is organized as follows. The preamplifiers are of a hybrid type [6] with 3 channels in each unit. 1 .D T " ) board holding the detector can be thus quickly disconnected from the FR-4 mother board and heated to as high a temperature as 3 O O O C for a possible annealing of the radiation damage. The detector is thus expected to be radiation harder than one would first estimate. Each preamplifier drives about 5 m long 50S2 cable with a gaussian shaper [6] a t the receiving end. The diversion of the leakage current away from the signal anodes improves the performance of' the detector. However. Position Resolution The difference between the the diffusion during the drift in cylindrical and linear geometry is larger when the electrons were created closer to the center. There are several resolution curves on Fig. Figure 2: Optimal resolution of an ideal cylindrical detector in the X coordinate obtainable by the charge division as a function of the anode pitch. The resolution obtained a t the anode ring is thus improved by the ratio of the initial and the anode radii.0 I 800.between modes o .10% of anode pitch X . The position along the anode direction is determined by a charge division method. There is aot enough signal on the neighbour anodes if the anode pitch is larger than the spread of the electrons.0 I 400.0 1000.0 Anode pitch Iuml 1000.62 1 0 I I D . 00 40U. 3. That means that the full signal from every anode receiving the electron cloud is used in an optimal way to determine precisely the anode coordinate X. The charge division gives the position of the center of the cloud with a very good accuracy even when the anode pitch is larger then the spread of the cloud. The lowest curve (the best resolution) corresponds to a case where the cloud of signal electrons arrives in between two neighbour anodes. 2 and Fig. in the cylindrical case. 3 show the optimum resolution along the x coordinate as a function of the anode pitch in a cylindrical and linear geometry respectively. Signal electrons were produced 6 mm from the center and drifted toward anode ring located a t radius of 32 mm. If we project the measured diffusion at the radius of anodes back to the origin of the ionization. the width of diffusion is smaller than the diffusion width in a linear geometry. Fig. No such projection is done in the case of a linear geometry and the anode resolution is the resolution of the measurement.0 I 3.40% of anode pitch 0 I 9 200.0 Anode pitch [uml Figure 3: Optimal resolution in a linear geometry under the same signal and the same drift path as at previous Fig. Physically it means that the diffusion during the drift is multiplied by an average geometrical factor smaller than the factor corresponding to a full drift length. 20000 electrons were produced at radius of 6 mm and drifted towards an anode ring a t a radius of 32 mm. is multiplied with a factor. B. the position X determined at the anode ring by charge division is projected to the initial radius. The highest curve (the worst precision) corresponds to the arrival of the signal electrons right a t the center of an anode. 2 and Fig. which is the square root of the geometrical factor for the original spread.O 600. that is. The . We can see that for an anode pitch of 500 p m (selected in the design) and for the origin of ionization TO equal to 6 mm the cylindrical case has about three times better resolution than the linear case. Due to the defocusing effects during the diffusion in the cylindrical geometry the electrons arrive at the anode ring with a Y = 400pm to be compared with a spread I in a linear case of 170 p m only. for TO smaller. The difference between the resolution in cylindrical and linear geometry decreases with an increase of PO.0 I I ~~ I 600.0 800. The worsening of the resolution can be avoided by integrating the first transistor onto the wafer of the detector [8]. 4 shows the coexistence of the linear and the cylindrical geometry on the detector. about 125 p m from the surface. When the capacitance of the first transistor is made equal to the anode capacitance the noise of Figure 5: Photography of the part of the n-side of the detector under a magnification of 8 times. The total noise considered for the position determination depends only on the spread of electrons and there is no deterioration of the resolution with a finer anode pitch. The amplifier noise of an individual anode channel is thus also independent of the anode pitch. 2 is the worsening of resolution with a smaller anode pitch (region between an anode pitch of 200 p m to 500 p m ). For smaller anode pitch the signal is read through more channels adding noise from more preamplifiers into the position determination. There are other features visible on Fig. the read out channel is proportional to the anode size. . C. The total preamplifier input capacitance is dominated by the capacitance of the first transistor and the capacitance of the connection and is independent of the anode size and pitch.6 times. In this detector the preamplifiers are external as opposed to ones integrated on the silicon wafer. The r. that is. curves between these two extremes have the center of the electrons shifted a t 10% of the anode pitch. The electric field i n the middle of the wafer does not have a sharp angle and the field under a 177" angle between two sectors is rounded. Due to the limitations in the production of masks for the detector. The electric field is applied on 241 concentric rings (or polygons with 120 sides) just resolved in Fig. the cylindrical detector is in reality a polygon with 120 sides.622 Figure 4: Closer look on the n-side of the cylindrical detector and its assembly. 4. Anode pitch is 500 p m .s of the electron cloud is 400 p m in the Fig.The effect is due to the contribution of the series noise of the preamplifiers to the final resolution. 4. The signal electrons are transported in the middle of the wafer. The magnification is about 2. Implementation Fig. 2 and we start to see the dependence of the resolution on the coordinate relative to the anode position for the anode pitch above 500 p m The other interesting detail shown in Fig. At small radii where the linear dimension of a 3 O sector is less than 200 p m the drift field inside the wafer is practically cylindrical. The detector is linear within 3" sectors and then there is a 177' angle between two neighbor sectors. The black spots almost randomly scattered across the silicon .m. The right values of the drift voltages must be applied to the rings at the boundary of each group. 6 shows the n-side of the detector under larger magnifications 8 and 25 times respectively. the dissipation is constant for 12 neighbouring rings and then changes to some other value for the next 12 rings. The area covered by individual rings depends on the radius of the ring. Individual rings are well visible. 4 are connections between the anodes and the gold plated traces on a ceramic board. Fig. 4 bring the drift voltages from an outside divider to every 12-th ring. Potential a t this surface depends on the global design of the detector. An effort was made to place the resistors on the wafer in such a way that the power dissipated in the resistors is distributed uniformly across the whole wafer area. The total leakage current collected on the detector anodes is the sum of the bulk generation current and junction diffusion currents only. There are 240 resistors implanted on each side of the wafer. 5 and Fig. The bonds at the left hand side of Fig. Fig. The white appearance of the junction is due to the aluminum which covers the junctions. To dissipate the same power per unit area the power dissipated per resistor should be proportional to the radius of the ring. 7 shows the negative potential in a radial cross section of the detector. As a compromise. The drift field is constant and therefore the voltage difference between the neighbour rings is constant.Si02 interface i on a guard anode. detector are integrated resistors of the voltage divider. The white rings are the rectifying p+n junctions. The second important feature of the design of the cylindrical drift detector is the collection of the current generated on the Si . Holes in a FR-4 board above the ceramics are exit holes for the forced radial air flow which cools the detector.U1 Figure 8: The same part of the detector n-side under a magnification about 25 times. The bond wires visible a t the left bottom of Fig. The picture shows very clearly the realization of the main . Figure 7: Negative potential in a radial cross section (called “Y”) of the detector. Surface between rectifying junctions is covered by S O z . The designed detector collects all electrons generated at the S . In total. 1 1 SURFACE PHENOMENA 1. Equipotentials imposed at both surfaces of the detector are the rectifying junctions. The design of drift detector was made in such a way that the electrons generated on the interface are not collected at the detector anode. The remaining silicon areas are covered by thermally grown SiOz. there are 20 different groups of resistors on each side of the detector. Equipotentials imposed at both surfaces of the detector are the rectifying junctions.Si02 interface on the guard (sink) anode. Si02 interface [9]. To follow these electrons we have to know the shape of the interface in the coordinate not shown in Fig. to secondary valleys located right below surface covered by thermally grown SiOz. Electrons falling onto the interface have no way to escape. The existence of the secondary valleys is a consequence of fixed posi. Holes generated at the interface (one can visualize holes as bubbles on figures where the negative potential is plotted) are immediately absorbed by surrounding p + n junctions. tive charges in the Si02 close to the Si . 7. Potential shown in Fig. Now let us concentrate our attention.on of the detector close to the surface. This is one extreme condition which is hard to realize and is not very desirable. The presence of these charges bends the energy band in such a way that electrons are held close to the interface (simple electrostatic attraction between mobile electrons within the silicon and fixed positive charges in the oxide close to the interface). and a relatively large current is generated on the Si . After a while an equilibrium situation is reached where there is the maximum density of electron in the accumulation layer at the interface. Electrons. the potential is identical at each cross section independent of the azimuthal angle 4. Mobile electrons in .Si02 interface to compensate positive charge of the SiOz. The surface of silicon under areas covered by Si02 is depleted. The potentid is valley in the potential energy for electrons (negative potential). They start to accumulate at the regions of secondary valleys forming an accumulation layer at the interface. The boundary between the secondary valley and the main valley has disappeared at one point along the secondary valley. (one can visualize the motion of electrons as heavy balls) fall to the minimum of the potential energy right at the interface. Due to the negative electric charge of electrons the positive oxide charges are partially compensated and the band bending at the interface decreases. 7 was calculated assuming that there are no electrons a t the Si . however. however.Si02 interface.Figure 8: Detail of negative potential in a cross secti. In a case of cylindrical symmetry or in a fully symmetrical geometry of polygon of 120 sides. In this valley the signal electrons are transported from the point of creation by the ionization of fast particle to the anode. The number of “rivers” increases from 2 at the center to 40 at the largest radius of the detector. At a small radius of i the detector.SiOa interface. Fig. would deplete larger part of the interface surface and increase the amount of the surface leakage current. however. the surface is small and there is only little current which was generated a t a smaller radius to be carried away. Flow of electrons ia towards left bottom of the picture. At the bottom left there is a structure which allows “rivers” current to reach the sink anode at the outside radius while avoiding signal anodes. To prevent the inclusion of electrons generated a t S . 8 shows the detail of the negative potential a t the interface where there are still some electrons left at the interface. At larger radii there is larger quantity of the generated current and also current generated upstream must flow in “rivers”. 6 and Fig. The negative potential in the next region is similar to the previous one but the potential difference of one ring (6 V). These interruptions form “rivers” where the surface current flows.Si02 interface is . the accumulation layer form almost an equipotential surface and an arrival of one electron anywhere along the interface leads to the emission of one electron from the secondary valley into the main valley which is collected later a t the signal anode of the detector. Several “rivers” are visible as radial lines in Fig.Si02 interface from the secondary valley in the i signal anode of the detector a part of the interface charge must be drained away from the interface before the maximum electron density is reached. One way of draining is by breaking the cylindrical symmetry by incorporating of small openings into the rectifying rings. Thus electrons generated a t the interface move from one inter-ring region into the next region.Si02 interface.625 Figure 10: Microphotography of the part of the n-side of the detcctor under a magnification of 170 times. The draining of surface generated current should be done at a correct rate. The following ring is interrupted as well. These “rivers” have to carry away all current generated a t the S . Figure 11: Microphotography of the “river” region under a rnagniflcatisn of 600 times. 6 . Not enough drainage would let the surface leakage current reach the detector anode. The potential barrier to prevent electrons generated at Si . Entrance opening of the river is always smaller to prevent excess drainage of electrons from the Si . Small openings let the electrons accumulated in a region between two rectifying rings fall into the next inter-ring region. Too much drainage. The non sintered aluminum covers Si02 and bridges above the “river” to connect the sintered aluminum (rougher surface) covering rectifying junctions. Two different aluminum layers are visible. 10 shows a “river” terminating at the sink anode outside signal anodes. The second layer of non sintered aluminum covering partially Si02 is visible as a smooth metallization layer. still present. 12. Resistance of the implant is about 8kn/U. Fig. Quality of the alignment and of the photolithography can be seen from this picture and from the microphotography shown in Fig. Surface electrons flow toward the left bottom of the picture.x .~ Slope=7.x . The electric field along the interface is small enough that it does not introduce any azimuthal component of the drift field for signal electrons moving iu the middle of the main valley in the radial direction. Changing the width of the connection between two rings different values of resistance can be obtained.2hA/cmZ t I I I I I I T I I I ‘ 13 I 25 37 I 49 61 RING # 1 5 73 85 97 1 0 9 1 1 133145157 2 I I 5 10 s (cm2) 20 25 Figure 12: Microphotography of one resistor of the voltage divider under a magnification of 600 times.. The entrance opening of the “river” is the part of the river to avoid the excessive drainage of the surface electrons. Fig. 11 shows details of ring interruption to form a “river”. Tests in Laboratory Several cylindrical drift detectors were produced at BNL and TU Munich. Electrons generated a t the upper (bottom) half of the picture are transported downwards (upwards) towards the “river”.RESULTS A. 13 shows the functioning of the designed collection mechanism for the surface current qualitatively.626 a e E i z 0 X 0 a 0 0 O 4 I A L L ANODES 1 ~ x y ~ x ~ ~ x ~ ~ ~ x . . IV. 9. The second aluminum layer was non sintered. This layer defines the potential a t the top of Si02 surface and also makes bridges across “rivers”. Conductivity due to the electrons left on the interface is just right to carry generated electrons from location of origin to “rivers”. thereby eliminating the possibility of “spiking” at the oxide cut corners. The leakage current flowing to the all signal anodes as well as the leakage current flowing to the sink . This direction corresponds to a vertical flow between rectifying junctions as shown in Fig.x . are needed to put the entire ring a t the same potential. The rectifying electrode circumvallating signal anodes is interrupted between two anodes to let the surface current to pass signal anodes and flow into the sink anode. Resistors are made during the same production step as the rectifying junctions.x . Microphotography of Fig. Registration of alignment to 5 2 pm is required in this design. The bridges connect sintered aluminum a t the top of rectifying junctions of the same ring after being interrupted by “rivers”. Connections Figure 13: Dependence of the leakage current flowing into all signal anodes and the leakage current flowing to the sink anode on the active sise of the B11 detector. anode are plotted as function of the active size of the a detector. The improvement (decrease) of the leakage current is a factor of 25.events with no hit in an aeimuthal sector in the pad detector were entered. The position resolution of the detector of 20 p m (rms) in each direction provides about 2 10’ two-dimensional elements. Only. produced. 3 sectors are O well visible. 118. Fig. We have other indications that the surface leakage current was drained too hard by about a factor of 3 which gives a more realistic improvement factor of 8.169 OL -7OOV r g “229 oL -5OOV 04-04-1991 Sun anode Is O. . We would like to stress that the diversion of the surface generated leakage current into the sink anode made the detector less sensitive to potential radiation damage of the Si . 14 shows the leakage current of individual anodes in B12 detector.2 O. grd 0. V.OOaOE+W 200 160 120 80 LO 0 CHANNELS FIRED 320 3 0 Figrire 14: Leakage current for each of the 360 anodes of a cylindrical detector B12.. 16 and Fig.3154E+04 nR 3M) 320 240 280 1 F 5 I9 Envia Me07 863 21703 177.Si02 interface. Although somewhat complicated. Ring . 17) are preliminary. 16.+ coordinates for events with multiplicities up to several hundred. An advanced silicon drift detector. it has been possible to produce nearly fully operational devices on the whole area of 3 inch diameter wafers.‘L282E+04 nR. was designed. The observed anode current is primarily bulk generated current at an average level of 7nA/cm2. The upper curve (left linear scale) shows the current flowing into the sink anode. Beam P e r f o r m a n c e In summer of 1991 the B12 cylindrical detector was installed into the NA45 Experiment a t the CERN SPS.2 nA/cm2. The slope of the leakage current to the sink anode is 180 nA/cm2. Three figures presented here (Fig. 90 % of the detector surface was active and most of the anodes have the leakage current below 1 nA under operating field and bias. The detector provides unambiguous pairs of r. a large area cylindrical drift detector. tested and installed in the NA45 experiment. The surface current is efficiently collected by a sink anode as designed. This detector has no defect between rings number 13 and number 169 a t the inner radius. Figure 15: Frequency of hits as a function of the anode number or the azimuthal coordinate. The measurement w s done with the detector B11. The entries into histogram were selected according to the information from a silicon pad detector right in front of the cylindrical silicon drift detector.DOOOE+W RHS UOFLW OVRW O.627 r812. CONCLUSIONS B. Fig. The ring # 169 was biased a t -700 V. A hole in the frequency of hits at arround the anode number 230 corresponds to the location of the selected pad. The slope of the leakage current to all signal anodes is 7. Rvrg. E. Instr. 36. The entries into the histogram were selected according to the pad detector. Faschingbauer et al. Preamplifiers and shapers were developed at BNL and produced by ASTER TECHNOLOGY Inc. 3. 9. This time only events with no hits between two radii were plotted. New York. Germany. REFERENCES 1. 367 (1986). 1967. Inc. Box 729.203 (1989). Rehak et a .. NY 11960. Science . Instr. 2000 Tangstedt/Hamburg. 248. 225. Proposal to the SPSC CERN. CA 94587. DRl. Instr. J. and Meth.. U.. No selection of entries into the histogram. A. Rehak et al.r[TIUE DISTR ALL Figure 16: Frequency of hits as a function of drift time. Physics and Technology of Semicondmctor Devices. P. Struck. P. 168 6.. and Meth. Nucl. Rehak et al. Union City. Page 298. GD type low resistance connectors from Shin-Etsu Polymer America. Nucl.. P. Nucl. 34135 7-th Street. Gatti and P. Inc. and Meth. IEEE Transactions on Nucl. Rehak. (1990). SPSC/P 237. m. Grove. l.628 160 k - I ID 70 - w 50 40 - DRllTlYE Dl5lR ALL Figure 17: Frequency of hits as a function of drift time. Ramsenburg.S.. 608 (1984).O. . 8. P. 4. 2. 5. SYSC 88-25. 7. ADCs were produced by Dr. Wiley and Sons. The increase of number of events per bin with the increase of the drift time is due to the physics of the particle production in the experiment. B.
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