In this work the interelement (interstrip) resistance of the microstrip detector is studied. A few detectors with a different construction are investigated. The dependence of the interstrip resistance on the dose of detector irradiation with electrons is obtained. The possibility of application interstrip resistance measurement for the determination of the good strip yield is shown

Kulibaba, V.

Maslov, N.

Potin, S.

Starodubtsev, A.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

INTERSTRIP RESISTANCE OF A SEMICONDUCTOR MICROSTRIP DETECTORV. Kulibaba, N. Maslov, S. Potin, A. StarodubtsevNSC KIPT, Kharkov, UkraineIn this work the interelement (interstrip) resistance of the microstrip detector is studied. A few detectors with a dif-ferent construction are investigated. The dependence of the interstrip resistance on the dose of detector irradiation with electrons is obtained. The possibility of application interstrip resistance measurement for the determination of the good strip yield is shown.PACS number: 29.40.Wk.1  INTRODUCTIONInterelement (interstrip) resistance is one of the most important parameters characterizing the quality of a mi-crostrip  as  well  as  other  multielement  semiconductor detectors.  The  value  of  the  interstrip  resistance  along with the interstrip capacitance determines a number of strips over  which the charge produced by an ionizing particle  is  distributed  (cluster)  and,  consequently,  the spatial resolution of the detector [1]. One can conclude from the value of the interstrip resistance and its varia-tion while a detector is affected by different factors on the  state  of  its  surface,  defect  content  in  silicon  etc. Apart  from this,  the interstrip  resistance at  the ohmic side of the detector shows the quality of performance of the p+-stop structure [2]. Measuring the interstrip resis-tance enables one to determine such detector parameters as depletion voltage, n+-strip separation voltage, as well as a large number of technological defects these being the short-circuited strips in the simplest case.2  METHOD OF MEASUREMENTThe problem of determining the interstrip resistance is not a trivial one because the measurements must be performed at the voltage of total depletion of the detec-tor. While measuring the interstrip resistance one should provide for minimum distortions of electrostatic fields within the interstrip volume of the detector being under the voltage of total depletion. Usually the value of the interstrip resistance falls into the range from hundreds M Ω  to tens G Ω  depending on the detector design. In order to determine the interstrip resistance a method is used conventionally that permits to determine the inter-strip  resistance  from  strip  leakage  currents  [3].  The essence of the method is in that first one measures the leakage current of one strip according to the scheme of fig. 1 a) at the voltage values U exceeding those of total depletion.  One  determines  the  voltage  range  within which the leakage current experiences small variation. Then an additional supply unit V is switched into the scheme such that V<<U’ and U’+V=U and again leak-age currents are measured according to the scheme pre-sented in Fig. 1b). Then one plots the graphs from these measurements (see Fig. 3 and Fig. 4), from which the interstrip resistance is determined as a tangent of the in-clination angle of the straight line.It should be noted that these measurements furnish the accurate value of the interstrip resistance only in the absence of the bias resistor on the strip under measure-ment. In the presence of all resistors whose resistance does not exceed several tens M Ω , we will determine the total resistance of the circuit made up by two bias resistors and one interstrip resistors switched in parallel. Therefore for physical studies of the detector it is neces-sary to develop special microstrip test structures.3  STUDING THE INTERSTRIP RESIS-TANCEThe  studies  were  performed on  four  types  of  test structures. Test microstrip structures were manufactured simultaneously with the main detector and they differed from it only in the diminished number of strips. Test mi-crostrip structures possess 64 strips instead of 768 for the  main  detector.  Other  parameters  of  the  test mi-crostrip structures are the same as the parameters of the main  detector:  thickness  of  300 μm,  strip  length  of 40 mm, step of 100 μm.а) b) Fig. 1. Schemes employed for the determination of  the interstrip resistance: а) measuring the strip  leakage current; b) measuring the sum of currents (leakage current and interstrip currents).ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.Серия: Ядерно-физические исследования (39), с. 180-182.1806Fig. 2. Corner of multichannel microstrip detectors (a- p+ detector, b- n+ detector): 1 - contact pad of  integrated capacitors, 2 - polysilicon resistors, 3 - basing pad of microstrip active elements,  4 -  p+-guard ring, 5 - p+-stop structure, 6 - contact pads of microstrip active zone.Fig. 3. Interstrip currents of the test 64-strip P-detector.Fig. 4. Interstrip currents of the test 64-strip N-detectorTo ensure the accurate measurement of the interstrip resistance the test structures 1 and 2 are made without bias resistors, and the structures 3 and 4 lack 3 resistors each. Structures 1 and 2 are the test structures of one-sided р+ microstrip detector.  The surface of silicon of test structure 1 is covered with a silicon oxide layer, be-sides, structure 2 has an additional layer of Si3N4 insula-tion. Structure 3 is a test structure of р+  side, and struc-ture 4 is a test structure of the n+  side of a double-sided microstrip detector. Structures 3 and 4 possess single-layered SiO2  insulation. Fig. 2 shows the view of the 3 and 4 test structures.The studies were performed to reveal the effect of design peculiarities of detectors on the interstrip resis-tance. The behavior of the interstrip resistance under ir-radiation of detectors was also performed. To this end the  test  detectors  were  irradiated  with  a  beam  of 20 MeV electrons. Detectors 1 and 2 were irradiated in 4 stages up to the dose of 2.1 Mrad. Detectors 3 and 4 were irradiated once up to the dose of 200 krad.Fig. 3 and Fig. 4 show the interstrip resistance of de-tectors 3 and 4 before and after irradiation. Consider the difference of the interstrip resistance for p+- (detector 3) and n+- (detector 4) sides. As is seen from the figures, the resistance of detector 3 (p+-side) is higher than the resistance of the detector 4 (n+-side). This is attributed to the design peculiarities of the n+-side. Owing to the presence of the positive static charge at the Si-SiO2 in-terface, a layer of electrons is formed under SiO2 at the surface of the n-silicon. At the p-side these electrons are pushed  away  by  the  depletion  regions  and  disappear completely with the growth of the depletion region size. At the n+-side this layer simply short-circuits n+-strips between  themselves.  To  overcome  this  difficulty  the n+-strips  are  separated  with  р+-regions  called  р+-stop structures [5]. In this case the interstrip resistance is de-termined by the width of the р+-stop structures and their design, whereas at the p+-side it  is  determined by the distance between adjacent strips. As the width of the р+-stop layer is less than the distance between the strips, the interstrip resistance at the n+-side is less than that at the p+-side.Fig. 5 depicts the dependence of the interstrip resis-tance on the irradiation dose for detectors 1 and 2. As is seen from the figure, the interstrip resistance of detec-tors 1 (squares) and 2 (crosses) differed strongly.The lesser  interstrip resistance of  detector  2  is  at-tributed to the additional Si3N4 insulating layer increas-ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.Серия: Ядерно-физические исследования (39), с. 180-182.181ing the contribution of the detector surface into the total leakage current and into interstrip currents [4]. This is probably associated with the  existence  of  the  genera-tion-recombination  centers  at  the  boundary  dividing SiO2  and Si3N4.  Under irradiation the difference in the interstrip resistance values for the detectors with single-layer and double-layer insulation disappears practically. In order to explain the dose dependence of the interstrip resistance, let us consider the variation in leakage cur-rents under irradiation of detectors with single-layer and double-layer insulation.Fig. 5. Interstrip resistance of 64-strip detectors  with (2) and without (1) Si3N4 against the irradiation dose by 20 MeV electrons.Fig. 6. Leakage currents of the detectors with Si3N4 (triangles) and without Si3N4 (squares).Fig. 6 shows the variation of leakage currents under irradiation of detectors with an additional Si3N4 insula-tion layer  and without  it.  The difference between the leakage  currents  for  the  detectors  with  an  additional Si3N4 insulation layer and without it before irradiation was about of 25 nA. It is seen from the figure that the difference  between  the  leakage  currents  is  constant within the total range of irradiation doses. Under irradi-ation the leakage currents of the detectors with the addi-tional Si3N4 insulation layer and without it are increased due  to  the  increase  of  the  volume  component  of  the leakage current. This is attributed to the increase of the concentration  of  generation-recombination  centers  in the silicon volume. The density of surface generation-recombination centers does not change within the dose range under study. Therefore the difference between the leakage  currents  for  the  detectors  with  an  additional Si3N4 insulation layer and without it is constant.With large irradiation doses the total leakage current of the detector and, consequently, the interstrip resist-ance is determined by the increased volume generation-recombination current [5]. Therefore with the dose in-creasing the interstrip resistance values of both detectors become practically equal.Apart from the physical studies of the interstrip re-sistance that require obtaining the accurate value of the resistance, other measurements are possible. Specifical-ly, in the process of technological measurement of the good strip yield one employs measuring the interstrip resistance. As these measurements are made not on test structures but on main detectors, all  strips possess the bias resistors. As was already mentioned above, in this case we measure the resistance of the circuit made up of two bias resistors and a single interstrip resistance. As the resistance of the bias resistor is much less than the interstrip one, the resulting resistance is equal approxi-mately to the double value of the bias  resistor  (some tens M Ω ). Usually in the presence of a defect between the strips the value of the resulting resistance doe not exceed some hundreds k Ω  and it may be used for dis-covering defect strips.4  CONCLUSIONSThe interstrip resistance is one of the most important parameters indicating the quality of a microstrip detec-tor. The value of the interstrip resistance may give the information  on  the  spatial  resolution  of  a  detector. Studying  the  behavior  of  the  interstrip  resistance  one can determine such electrophysical characteristics of the detector as the depletion voltage, the n+-strip separation voltage and the quality of  the performance of  the p+- stop  structure.  While  determining  the  yield  of  good strips the measurement of the interstrip resistance en-ables one to reveal technological defects of a detector.REFERENCES1.F.Retiere at al. Performances of Double-Sided Silicon Strip Detectors for the ALICE experiment at LHC. AL-ICE/99-36 Internal Note/SIL 21 July 1999.2.A.P. de Haas,  P.Kuijer,  V.I.Kulibaba,  N.I.Maslov, V.L.Perevertailo, V.D.Ovchinnik, S.M.Potin, A.F.Staro-dubtsev.  Characteristics  and  radiation  tolerance  of  a double-sided microstrip detector with polysilicon bias-ing resistors //  Problems of Atomic Science and Tech-nology. Issue: Nuclear-Physics Research (36). 2000, v. 2, p. 26-33.3.N.L.Bruner at all. Characterization procedure for dou-ble-sided silicon microstrip detectors // NIM. 1995, v. A 362, p. 315-337.4.N.Maslov,  V.Kulibaba,  S.Potin,  P.Kuijer,  A.Staro-dubtsev, A.P. de Haas,  V.Perevertailo.  Radiation toler-ance of single-sided microstrip detector with Si3N4 in-sulator // Nuclear Physics B (Proc. Suppl.). 1999, № 78, p. 689-694.5.A.G.Chilingarov. Coordinate semiconductor detectors  in  the  elementary  particle  physics.  Preprint  90-113, ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.Серия: Ядерно-физические исследования (39), с. 180-182.182Novosibirsk, 1990 (In Russian).ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.Серия: Ядерно-физические исследования (39), с. 180-182.183

Interstrip resistance of a semiconductor microstrip detector

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Interstrip resistance of a semiconductor microstrip detector

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