Special detectors have been developed for the NASA ISEE-C Cosmic Ray Telescope. These are Li-drifted silicon detectors 5 mm thick and 1500 mm/sup 2/ area with the criteria that thickness variations on the whole area be less than +-10 ..mu..m and that the Li-diffused contact dead-layer not exceed 15 ..mu..m. Techniques used to fabricate and test these detectors are presented

Bieser, F. S.

Greiner, D. E.

Sommer, H. A.

Walton, J. T.

UNT Digital Library

THIN WINDOW S i ( L i ) DETECTORS FOR THE _ISEE-C TELESCOPE* LBL-6434 J. T. Walton, l i . A. Sonmer, D. E. Greiner, and F. Lawrence Berkeley Laboratory University of California Berkeley, California 94720 Abstract Special deteUors have been developed for the NASA ISEE-C Cosmic Ray Telescope. These are Li-dr i f ted sil icon detectors 5 mm thick and 1500 mm* area with the cr i ter ia that thickness variations on the whole area be lass than tlO urn and that the Li-diffused contact dead-layer not exceed 15 urn. Techniques used to fabricate and test these detectors are presented. Introduction An experiment or the NASA ISEE-C Heliocentric sate l l i te is designed to determine the Isotopic compo­si t ion of cosmic rays. A multi-detector telescope of l i thium-drifted sil icon <S1(Lf)) detectors is to be used for this purpose. To provide the necessary ident i f icat ion, the telescope must contain ten detectors 1* 1 of large area (1500 ram1), 5 mm thick with thickness variations of less than t0.2X (tlO um). Furthermore to minimize the dead-layer effects between the detectors In the telescope, the L1-d1ffused con­tact must be less than 15 urn. Finally, the detectors were required to have less than 10 nA leakage and less than 300 keV no1s& at an RC shaping time constant of 5 vs. These requirements represent a substantial step beyond existing techniques for the fabrication of Si(Li) detectors* necessitating the new procedures to be described. In addition the testing of the devices to assure that the specifications were met and to meet the testing requirements for a space f l i gh t have required the development of special test procedures which w i l l also be discussed. Fabrication Figure 1 shows the cross-section of a typical Si(Li) detector using the "grooved" geometry. The de­vice has a Li-diffused contact on one face and a Au-evaporatod contact on the opposite face. The recessed or "well" region of the Au-contact serves a practical purpose in protecting the. sensitive Au-cont?ct from mechanical damage during processing and actual use. The "well" is normally etched in with a fast chemical etch and "ripples" and non-uniformities introduced by this etching process are a major source of any varia­tion in the thickness of the f inal detector, To avoid these problems, the planar etching technique of Madden3 was successfully adapted to the etching uf this "well" to provide the necessary surface flatness. Achieving a thin Li-Diffused contact presents d i f f i cu l t ies as this contact must provide the source of l ithium for the dr i f t ing nrocess. Furthermore since l i thium diffuses from the original layer during the d r i f t , thicknesses of 120-I50iiin, as shown in Fig. 1, are typical for this layer o>; the completion of the*dr i f t ­ing process. To achieve the thin contact desired, the *This work was done with support from the United States Department of Cnergy. Honct— — -•pui upon w* rnp*»4 u m tccwM of %oek •poMorad by Ku Ufcwd SulM b W M H*hw * • Veto* 5ui» »er * • I M M * W M D * P « N M I * EMtv, not wry oHhrit«Mftpyw.MiMy O f * * cantneton, Mteoawcten, ot * * —»l«r««, mltK my wiim»ty, apm v tafNH, r MMNMy l*d mm or t*ron+*ty tor * * acc««c)r.«Mflmitt to tMhlMMorMy Hfon«UM,«ff«ntM,rto4iictor poo* tedoMJ. * n p m f t *M Hi I M * M M •* Wrt»iHfnMlyow»rtrt#». _ _ _ _ lDI»m LI DIFFUSION WITH I Al IVA'OHATION ON 11 (LI] POLIIHED SURFACE F1g. T •WILL' WITH Au •LOCKING CONTACT 1 Normal and modified S1(L1) detectors. Shown are the cross-sections of the devices with the S1(L1) compensated regions, the Li-contact and the Au-blocking contact. thick contact was removed at the completion of the dr i f t ing process and the thin contact was produced by evaporating l i thium onto the surface and carrying out a low temperature diffusion (18Q'C for 30 minutes). + This diffusion cycle produces a 10 um L1-d1ffused N contact. Measurements on the depletion characteristics of the devices before and after the thin contact pro­cessing Indicate that there Is a negligible disturb­ance of the bulk-l i thium compensation by this process. The f inal processing of the L1-contact Involves polish­ing the surface using "Syton"1* with a precision polish­ing machine and then evaporating aluminum onto the polished surface. The s tab i l i t y requirements for a space mission can only be met 1f the surfaces ( i . e . , the grooves) of the detectors are adequately protected. Several types of protection were examined for their ab i l i t y to protect the detector surfaces, The protection coating selected is under addftional examination with a long-term evaluation of detector performance In ultra-high vacuum being conducted. Tests Electronics An initial selection of detectors was performed by recording ths leakage current and noise at a long shaping time (5 us RC). By using such a long measure­ment time, incipient surface breakdown problems are usually revealed. Those detectors meeting the criteria at 600 volts bias of having less than 10 pA leakage and less than 300 keV equivalent rms noise were accepted for further testing. Reliability The selected devices were placed 1n a ultra-high vacuum (2 x 10"* Torr.) and subjected to an automated test cycle for a period of 14 days. The test schedule involved 4 days at 25°C and 10 days at 35°C with continuous sampling measurements of leakage current blSTRiaUTIONDR THiSTOCUMEMT IS OTttK/ & •nd m s noise. Noise Impulses or bursts were also counted and recorded. A block diagram of the test system 1s shown 1n Fig. 2. Testing of this type5 has previously been employed 1n qualifying detectors for space IHghts and is Intended to assess the stability and reliability of the devices. A typical result of thermally cycling a detector In high vacuum is shown in Fig. 3. Three features were sought 1n detectors during this test: <) absence of bursts of Impulse noise, b] constant or near constant leakage and noise following the temperature Increase and a return close to the initial values at the end of the test, and c) less than 20 uA leakage at 35'C. LBL-6434 VMIAN fC-ll-F VNCION IVITEM Fig. ? Block diagram of the thermal-vacuum test system. The HP 9B15 Cslculator interfaces with the detectors through the detector selector module and relay selector mo"jle. Sixty detectors can be tested in each run. JiA - 5 0 0 400 keV OR COUNTS 800 VOLTS RMS NOISE IN k.V ^^-A DAYS f i g . 3 Thermal-vacuum test results. Leakage current, rms noise, and number of Impulse bursts in two minutes are plotted versus time 1n days. The time at which the device was at 25°C and 35"C fs indicated. Detectors passing this test were then subjected to tests for determining the thickness of the L1-diffused contact and for assessing the uniformity of the entire detector. Contact Thicknesses ha2"1 alpha particles were used to measure the thickness of the L1-diffused and Au-barrier contacts. As expected the results showed that the Au-contact thickness was negligible in comparison to the L i -dlffused contact thickness. The thickness and uniformity of the Li-diffused contact was determined by scanning across the contact in 4 mn ^.teps. Figure 4 shows the results of one scan. The contact thickness was determined from the observed sh i f t in the position of the alpha-particle peak as compared with I ts posi­t ion as ipeasured through the Au-contact. As seen in Fig. 4, the Li-diffused contact In this case was 13 urn in the center with a 2 urn radial taper which is believed to be caused by the polishing of this surface on the precision polisher. Fig. 4 t" DETECTOH NO. 2R92 k'i : r ~1 • i i t r • M i | U — i n -t ' IBL jna-ioio] Alpha scan results across' the Li contact. The dead layer is shown 1n ym of s i l icon. The shape of the lead layer is shown against the cross-section of the detector. References to a company or product name does not im­ply approval or recommendation of the product by the University of California or the United States Department of Energy to the exclusion of others that •ay be suitable. Detector Thickness Uniformity Two types of measurements were used to determfne the detector thickness uniformity. As a f inal test, the detectors were subjected to high energy l , 0 Ar ions and the pattern of signals across the detector area was obtained and examined. Since this process 1s very expensive and time consuming, an optical measurement was used to screen for dimensionally non-uniform de­tectors before the Ar ion test. Of course, factors other than thickness variation may affect the signal amplitudes produced by the Ar Ions. For example, charge loss due to trapping is one such factor. Therefore, while the optical measurement will measure dimensional uniformity, the Ar ion measurement will measure both dimensional and charge collection uniformity. AgreeiiiC-rst between the two would indicate that charge loss mechanisms 1n the detector are not Important. Optical Measurements. The primary source of thickness variations 1n the detector, as Indicated earlier, 1s the "well" on the Au-barr1er side of the detector. Since the Au-barr1er is sensitive to mechanical damage, an optical method provides the means for examining this surface. Fortunately, an optical scanner developed for nuclear emulsion track studies was available and Its characteristics were quite adequate to measure variation of less than 10 urn on the gold surface. Scans were also performed on the L1-contact side of the detector. The data was reduced to produce uniformity profiles; as shown 1n F1g. 6, for each detector. X 46*1 l~ g « 7 1 ~ 90O00 r j - rr ' i n - i 1 1 1 I I00OO 70000 60000 50000 40000 30000 VARIATION FROM CENTER DETECTOR 2802 H i "10-10)0? Fig. 5 Uniformity scan across the "welt" side of the detector. Shown 1s the results of scan number 14 of the fifteen scans made across this surface. The crosses Indicate the measured profile points. The symbol "G" shown both on the profile and on the inset depicting the detector denotes the device active region within which the detector uniformity was required. Ar Ion Beam Measurement. Figure 6 shows the experimental arrangement used for these tests. A broad ""Ar BEVALAC beam (482 HeV/Amu) at the Lawrence Berkeley laboratory Bevatron was directed through the two, three-plane multi-wire chambers to the detector stack. The multi-wire chambers define the beam trajectories to within 1 mm through the stack. Only t,°Ar ions which passed completely through the silicon detectors and were detected in the plastic scintillator were recorded. The detector stack consisted of A planes each contain­ing a square array of 4 detectors, thereby, allowing 16 detectors to be tested simultaneously. Data obtained from the experiment was analyzed for each detector in terms of 90 equal area segments. LBL-6434 The average signa: amplitude from each s&gment and the average signal for the entire detector was calculated. The percentage deviation of each segment signal &ver&^ from the entire detector average was calculated. The number of segments and '.heir percentage deviation from the total detector average energy signal is shown for two detectors in Fig. 7. OITICIOH KRRHV5 *U«i:.IER MWPC *1 J PLASTIC 5CIMT/PMT —11 *—! n / ^\j*0 Fig. 6 Experimental arrangement at the Bevatron used to measure detector uniformity. The three layer Multl-wire Proportional Chamber (MWPC) define the beam location to within i nun. I«NCE at MfAfUKfMENrt •ltV*l*C DAT* % OEVIATION FROM MEAN WL mo-iouo Fig. 7 BEVALAC uniformity measurement results. Ordinate gives the number of detector segments within a band of 0,0535 with the percentage deviation from the mean average energy given on the abscissa. Detectors that meet the specifications are typified by (a) while those that exceed the specifications are typified by (b). Normal distribution function fits to the data are given by curves 1 and 2. The number of events in each segment was used to determine the accuracy of the average energy determina­tion of the segment. Many of the segments had insuf­ficient counts for the averages to be determined at the level of precision required. For all the detectors measured", there were segments whose accuracy was so poor due to insufficient counts that the results in those segments had to be discarded. Unfortunately, the BEVALAC beam intensity was very low during thpsi measurements and the availability of accelerator time has not permitted repetition of the experiment. Consequently in assessing the BEVALAC data, the accuracy of the measurements was a big factor in selecting detectors for the flight telescope. In Fig. 7. a normal distribution curve whose variance Is equal to the calculated variance (i.e., accurtcy) in the experimental data is plotted as curve 1, while a second normal distribution curve 2 indicates the expected results 1f the accuracy of the measurements 1s folded 1n with the allowed dimensional variation of tO.ZX («10 um). Figure1 7(a) presents data where the measurements were considered sufficiently good for the detector to be selected as a flight detector since it is clear that the BEVALAC histogram falls well within the calculated expectations. For this detector, the thickness spread must be smaller than the J0.2K requirement. Figure 7(b) presents similar data on a detector not selected as a flight detector since the BEVALAC histogram falls outside the calculated expectations. Conclusions This work has demonstrated the feasibility of fabricating large area .11 (Li) detectors meeting stringent requirements en the thickness uniformity •nd on the L1-diffused contact thickness. While the BEVALAC measurements have been complicated by the low Intensity of the BEVALAC beams during the experiments, the data obtained has been adequate to select 10 detectors that meet the uniformity and other specifications required for the ISIE-C Telescope. Finally, the BEVALAC data would indicate that no good evidence exists to suggest that signal amplitude variations are caused by any factor other than thick­ness variations. Where the BEVALAC measurement has shown thickness variations substantially greater than the expected Inaccuracy of the measurement, these variations have been confirmed by the optical Measurements. Acknowledgments The authors gratefully acknowledge the support of F. S. Gouldlng who not only lent encouragement, but did the basic design work and all the programming for the thermal-vacuum tester; to D. A. Landls and R. A. Adachl who did the detailed design work on the tester, and to S. D. Wright, R. C. Davis, and R. C. Zink who assisted in the assembly. The work in optical scanning and plotting of results from the thermal-vacuum tests by M. Banks and H. Vee is appreciated. Finally, the assistance of E. E. Haller in interpreting the BEVALAC data 1s also gratefully acknowledged. References LBL-6434 1. D. E. Greiner, Nucl. Inst, and Meth., 103, 291-308 (1972). 2. 0. E. Greiner, Proc. 15th International Cosmic Ray Conference, Plovdiv, Bulgaria (1977). 3. T. C. Madden and U. H. Gibson, 9th Scintallation and Semiconductor Counter Symposium, UasMjintnn. 0. C , February 1964. t. Obtainable from the Remet Chemical Corp., P. 0. Box 278, Chadwick, NV 13519. j. Alan Cummi.igs, Bill Althaus, J.P.L., Cal. Tech., Pasadena, CA, private communication. 

Thin window Si(Li) detectors for the ISEE-C Telescope

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Thin window Si(Li) detectors for the ISEE-C Telescope

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