A large area circular position sensitive Si detector

NUCLEAR

INSTRUMENTS

AND

METHODS

I34 (I976) 71-76; ©

NORTH-HOLLAND

PUBLISHING

CO.

A LARGE AREA CIRCULAR POSITION SENSITIVE Si DETECTOR* J . E . L A M P O R T , G. M. M A S O N , M . A . P E R K I N S and A . J . T U Z Z O L I N O

Laboratory for Astrophysics and Space Research, Enrico Fermi Institute, UniL'ersity of Chicago, Chicago, Illinois 60637, U.S.A. Received 23 January 1976 Circular position sensitive detectors with sensitive areas o f 17 cm 2 have been constructed by connecting gold strips on the front surface o f a Li-drifted silicon detector to an external resistor network. Detector responses to alpha-particle sources and pulsedlight sources are presented which demonstrate the position and energy response characteristics o f the detectors. Measurements with a pulsed-light source equivalent of 140 MeV show that a resolution (fwhm) of 0.19 m m (the width of the gold strips) can be achieved for energy deposits of ~ 4 0 MeV. The position resolution (fwhm) obtained from these detectors m o u n t e d in a telescope exposed to 95-350 MeV/nucleon 4°Ar nuclei is 0.82 m m for a single detector.

1. Introduction

Silicon position-sensitive detectors (PSD) have many applications in nuclear physics measurements, and a n umber of designs have been described in the literature. One approach has used the principle of charge division in which resistive surfaces were deposited on the surface of the detector itself 1'2). These resistive-divider detectors have the advantages of good position resolution with comparatively simple supporting electronics, along with the disadvantages that (1) depositing a stable resistive film on the Si wafer is quite difficult, and (2) the detector shape is usually rectangular or square, which is undesirable in some applications. Another approach has been the "checkerboard" detector in which each parallel electrode strip on the detector is individually connected to its own amplifier and associated electronics3). The checkerboard design features simple silicon-wafer preparation, but has the disadvantage of complex electronic support, and the problem of "crosstalk" between detector strips. In this paper we describe a silicon detector which incorporates the simple silicon wafer preparation of the "checkerboard" design with an external resistor network to achieve a large area, circular resistive-divider detector which measures position in one dimension. The detector described here was designed for use in a high-resolution cosmic ray telescope in which 4 or more such detectors are used to determine the trajectory of an incoming particle and thereby make possible the elimination of path length uncertainties in a dE/dx versus residual energy telescope. In the present design the detector area has been maximized in order to achieve the largest feasible geometrical factor needed * This work was supported in part by N A S A grants N G L 14-001258 and N G L 14-001-006.

to measure the low-intensity quiet time cosmic ray flux. In order to keep the detector capacitance at a reasonable level, the thickness of the detector is chosen to be approximately 0.60mm. Other applications stressing low energy measurements (such as solar flare particle studies) could dictate minimizing detector thickness in order to lower the energy threshold of the system, at a cost of smaller detector area +. The development of these PSD arose from the need to build high-resolution space-flight cosmic ray telescopes - which requires minimizing the path length uncertainties in the system. In a previous approach to this problem, this laboratory developed spherically curved dE/dx detectors 4) which greatly improved telescope resolution over previous designs and have allowed the first separation of cosmic ray isotopes heavier than helium in a space flight instrumentS). The present system will permit at least a factor of 10 improvement in resolution in future experiments. 2. Detector fabrication

Large diameter ( > 5 0 r a m ) p-type silicon with electrical resistivity in the range 2000-4000 f2 cm and carrier lifetime > 200 #s is used as the starting material. After wafer slicing and lapping, a lithium evaporation and diffusion is carried out in a vacuum oven with the wafer heated to approximately 400°C. Gold is evaporated over the Li-diffused surface to a nominal thickness of 400 ,~. The wafer diameter is then reduced to a pre-determined "drift diameter". The Li-diffused side is protected with a suction holder and the wafer is etched in a standard CP-4 solution. After etching, the + Specifically, we have also fabricated fully depleted silicon position sensitive detectors with sensitive areas of ~ 2 . 5 cm 2 and thickness of 50/t with resolution characteristics comparable to those reported here for the larger area detectors.

72

J.E. LAMPORT et al. THICK" FILM GOLD LINE

0.001 400 ,~ - EVAP. GOLD STRIP

INCH GOLD WIRE

TYPICAL CONNECTION POSITION

.--~ -

SCHEME

SIGNAL ~ ' "

GOLD STRIP

E N E R G Y SIGNAL

Fig. 1. Perspective view of silicon position sensitive detector. The total area of the gold strip pattern is 17.0 cm2. The pattern contains 144 gold strips with a spacing between gold strips of 0.318 ram. The characteristics of the detector are summarized in table 1. wafer is rinsed for two minutes in de-mineralized water. Gold is then evaporated onto the etched " p " surface to a n o m i n a l thickness of 400/~. If the detector at this stage exhibits good diode characteristics, it is placed in a '~drift" oven and heated to 100-120°C. A " d r i f t " voltage is then applied to the detector until the entire silicon wafer (thickness of ~ 0 . 6 0 ram) is compensated with lithium, as calculated from the drift parameters6). The detector is removed from the drift chamber and electrical and chargedparticle measurements are carried out. The '' p" side of the wafer is then lapped to 0.60 mm thickness, removing the gold surface. The lithium diffused side is protected with a suction holder while the edges and opposite side are etched and rinsed as described above. Next, strips on the front surface of the silicon detector are applied by an evaporative gold process. The gold is evaporated to a thickness of 400,~ through a chemically milled beryllium-copper shadow mask which insures pattern uniformity. During the gold evaporation process, as well as in all subsequent processing and handling of the detector, care must be exercised to insure that the required degree of electrical isolation between adjacent gold strips, which is provided by the "'gap" region, is maintained. M o u n t i n g rings for the detector, shown in fig. I, are made of C o r n i n g Machinable Glass Ceramic material onto which a resistive-divider network is fired using conventional thick film circuitry techniques. External lead m o u n t i n g pads are palladium-silver to provide

TABLE

l

Typical characteristics of Li-drifted silicon position sensitive detectors. Characteristic

Value

Silicon wafer thickness 0.600 mm Silicon wafer area 18.4 cmz Intrinsic depth (0.45-0.55) mm Lithium-window thickness (0.15-0.05) mm Number of gold strips 144 Gold strip width 0.191 mm "Gap" width 0.127 mm Gold strip pattern area 17.0 cm2 Total detector capacitance (400 450) pF Total resistance of resistance-divider network (8.2-8.8) k£2 Operating bias voltage 30 V Detector leakage current (I-2) HA Electronic noise at energy amplifier (fwhm) (70-100) keV Electronic noise at position amplifier (fwhm) (I 00-150) keV for soldering. Clear glyptal is used to m o u n t the wafer in the ceramic ring. C o n n e c t i o n of the detector strip pattern to the resistive-divider network is made by means of ultrasonic wire b o n d i n g of 0.001" gold wire. Table 1 lists typical characteristics for the PSD which have been fabricated. A total of fourteen such PSD have had their electrical and charged-particle characteristics monitored over a seven m o n t h period. In all cases, the PSD have shown stable characteristics over this time interval.

POSITION SENSITIVE

3. Detector performance characteristics -q,.1. SIGNAL AND NOISE CHARACTERISTICS

The signal and noise characteristics of position sensitive detectors based upon the principle of resistive charge division have been extensively treated in the literature~'Z'V-11). The PSD has been treated as a 3.5 --~

3.5

(A)

3.0

3.0

2,5

2.5

"6 2.0

2.0

1.5

1.5

a

1.0

rn

0.5

0.5

I

]

I

0.5

I

I

I

I

I

1.0

I

]

I

I

1.5 T/RC

............ .; ................ 1,o

E

ooI i

0.1

o.oo,1, 1.0

......

0.1

,

. . . . . . .

,

........

,

I0 I00 I000 Energy Depo~tim (MeV)

Fig. 2. (A) Calculated energy ballistic deficit for particles incident at the center o f the position sensitive detector vs T/RC for a detector with characteristics given in table 1. Single integrationsingle differentiation of time constant T is assumed for the pulse shaping. The external resistor network is assumed to have a (uniform) total resistance R and the detector is assumed to have a (uniform) total capacitance C. (B) Curve 1: calculated position resolution for the detector in table 1 with noise levels of 100 and 150 keV at the energy and position amplifier outputs, respectively. The dashed line shows the width of a single gold strip, which is the limiting resolution for the detector in table 1. Curve 2: calculated position resolution for a detector with noise levels of 50 and 75 keV at the energy and position amplifier outputs, respectively, which could be typical for detectors with a smaller area than lhose described in table 1.

Si

DETECTOR

73

distributed RC transmission line where, as a consequence of this model, the total charge collected at the "position" contact resulting from a charged particle is directly proportional to the product of the position of incidence and energy deposition in the detectorV'8). The total charge collected at the "energy" contact is directly proportional to the energy deposition in the detector so that the ratio of the total charge collected at the position contact to that collected at the energy contact will determine the position or incidence of the particle. Since the time development of the charge at the position and energy contacts depends upon the position of incidence of the charged particle, the total resistance of the resistive-divider network, and the total capacitance of the detector, pulse shaping of the position and energy signals may give rise to a non-linear relationship between the position of incidence of a charged particle and the corresponding measurements of energy and position obtained from the amplitudes of the energy contact and position contact shaping amplifier pulse outputsV'8). The difference of the output voltage amplitude resulting from a current pulse of finite duration and the amplitude resulting from an infinitely short current pulse carrying the same total charge has been termed the "ballistic deficit" of the measurement12). The "ballistic deficits" of the energy and position measurements depend upon position of incidence, the total resistance R of the resistive-divider network, the total capacitance C of the detector, and the characteristics of the pulse-shaping networks t'v'8). To establish the extent to which the measured characteristics of the circular PSD are consistent with those predicted from the idealized model treated by Kalbitzer and MelzerT), we have taken the equations for the time dependence of the charge delivered to the position and energy contacts given by Doehring et al. 8) and obtained solutions for the time dependence of the output pulses from the position and energy shaping amplifiers. Fig. 2A shows a plot orthe expected energy ballistic deficit for a particle incident at a distance x from the position contact equal to 0.5 of the distance l between the position contact and the ground contact versus the parameter T/RC, where T is the shaping amplifier time constant, R is the total resistance of the detector resistance-divider network and C is the total detector capacitance. For example, for T/RC = 0.5, a measured energy ballistic deficit of 2.5% is expected for particles incident at the center of the detector (x/t=0.5). An estimate of the position resolution which may be obtained from the ratio of position signal amplitude

74

J.E. LAMPORT et al.

H (A)

(S)

Light Pulser 1140 MeV EquivJ

\

"/130

6.00 MeV Pulser 241

Am

.~ 6 0 0

(5.33 MeV)

OB

I

500

,.,, 400 "8

o•1000

I I I 1

O.080mm~

3 0 0 (FWHM) g E

==

I l I

¢n

I

I I

t~--

I

I

~d

t

2oo

I

I00

I I I

~-

"6 5 0 0

I /

~1 ( F , H M I

/

/

~

86 key

120 keV

z

I

I

t

I

I

IOO

.+.,. 0.490

I I t

F~

t_fd___ 0.495

0.500

Position Amplitude/Energy Amplitude

90

I00 I10 Chonnel Number

120

Fig. 3. (A) D i s t r i b u t i o n o f p o s i t i o n a m p l i t u d e / e n e r g y a m p l i t u d e e v e n t s f o r a p o s i t i o n sensitive d e t e c t o r e x p o s e d to a p u l s e - l i g h t s o u r c e c o l l i m a t e d a t t h e c e n t e r o f t h e d e t e c t o r to ~ 1 m m d i a m e t e r . T h e s i g n a l g e n e r a t e d b y t h e l i g h t s o u r c e w a s set to c o r r e s p o n d to a n e n e r g y d e p o s i t i o n in t h e d e t e c t o r o f a p p r o x i m a t e l y 140 M e V . (B) D i s t r i b u t i o n o f Z4~Am a l p h a p a r t i c l e e v e n t s o b t a i n e d w i t h t h e d e t e c t o r e x p o s e d to t h e s o u r c e w h i c h w a s c o l l i m a t e d a t t h e c e n t e r o f t h e d e t e c t o r to a 2 m m d i a m e t e r . T h e r a t i o o f m e a s u r e d e n e r g y (5.33 M e V ) to t r u e e n e r g y (5.48 M e V ) c o r r e s p o n d s to a b a l l i s t i c deficit o f 2 . 7 % .

to energy signal amplitude for electronic noise levels at the shaping amplifier outputs as listed in table 1 may be obtained by assuming that the output noise signals from the position and energy shaping amplifiers are independent. Curve I in fig. 2B is a plot of the calculated position resolution versus energy deposition in the detector for particles incident at the center of such a detector. This curve shows that position resolutions of ,,~0.2 mm may be expected for energy depositions in the detectors of ~ 4 0 MeV. 3.2. PULSED-LIGHTAND ALPHA PARTICLERESPONSE A number of PSD have had their "position" and "energy" responses determined by applying the signals at the position and energy contacts to charge sensitive preamplifiers and then to pulse shaping amplifiers. The amplifiers provided single integration - single differentiation pulse shaping l'2'8) with a time constant T of 2/ts. Fig. 3A shows the measured distribution of position amplitude/energy amplitude events obtained from one of the PSD using a pulsed-light source collimated at the center of the detector to ~ 1 mm diameter. The charge generated in the detector by the pulsed-light was set to be equal to that which would be generated by an energy deposition in the detector of approximately 140 MeV. The measured resolution fwhm of 0.080 mm shown in

fig. 3A, is determined soMy by the electronic noise contributions from the energy and position signals and is consistent with that predicted from fig. 2B for an energy deposition of ~ 100 MeV. Fig. 3B shows the event distribution measured at the detector energy amplifier output for 241Am alpha particles collimated at the center of the detector to a 2 mm diameter. The detector has a total R of 8.25 k[2 and a total C of 425 pF, and thus an RC product of 3.5 lls. For a 2/~s shaping amplifier, T/RC for this detector is 0.57 and fig. 2A predicts an energy ballistic deficit of 1.9%. The measured energy of 5.33 MeV shown in fig. 3B corresponds to a ballistic deficit for the 241Am alpha particles (5.48 MeV) of 2.7%, which, within errors, is consistent with the predicted value. Fig. 4A shows the relative energy, measured at the energy amplifier output, for 24¢Cm alpha particles versus relative position of incidence (x/l) of the alpha particles for a PSD with an RC product of 3.6/~s, and thus a value for T/RC of 0.56. The dashed curve shown in fig. 4A is the calculated dependence of the energy ballistic deficit as a function of x/l for T/RC=0.50. The measured and calculated curves are seen to be in good qualitative agreement. Fig. 4B shows the average position amplitude/energy amplitude ratio as a function of relative position measured using the pulsed-light source for the same PSD

POSITION

SENSITIVE

of fig. 4A and illustrates the typical position linearity of the position amplitude/energy amplitude ratio which has been obtained for the PSD. We note that detailed studies of the responses of the PSD to particles incident on the " g a p " regions of the detector surface ]have shown that there is no "loss of charge" for ]particles incident anywhere within a " g a p " region so that the entire gold strip area is "sensitive" for position and energy measurements. 3.3.

RESPONSE TO 4 ° A r NUCLEI

In order to measure the position resolution of these detectors for penetrating heavy nuclei we used 95350MeV/nucleon 4°Ar nuclei accelerated at the Lawrence Berkeley Laboratory Bevalac accelerator. Two identical detectors, separated by 5.0 cm, were (A) Cm

E < ~J

1.0'

244

= - particles

f 5 . 8 0 MeV --.----,----.-----.'.---.----.----.----.----,

0,99 0.98 0.97

\•~

~--

•-

---~---•--

•/ o~ ~

• .........

/ Colculoted for T / R C =0.5

0,96 ._> "6 0 . 9 5

75

Si D E T E C T O R

mounted one in back of the other, and then exposed to the beam. The Ar nuclei were able to easily penetrate both detectors, leaving energy deposits of between about 100 MeV to 310 MeV in each detector - depending on the energy of the incident nucleus. The "position" and "energy" signals from each detector were analyzed for each Ar nucleus crossing the detectors. Because of the mounting geometry, each detector should have nominally produced the same PIE ratio for a given Ar event. Fig. 5 shows the preliminary experimental data, where we have plotted the difference in the PIE ratios for one detector versus the other. The fwhm of this difference is 1.165 mm. Under the assumption that each of the two detectors is contributing equally to the width of this peak, we deduce a fwhm of 1.165/x/2=0.82 mm for a single detector. This figure is larger than what would be expected from fig. 2B. This relatively large fwhm is most probably the result of a spread of ,~ 1° in the angle of incidence of particles hitting the telescope due to scattering in absorbers in front of the detectors. In any case, the position resolution shown in fig. 5 is sufficient to improve cosmic ray telescope resolution to the point of separating isotopes as heavy as Fe. 4. Conclusions

(B)

Circular one dimensional position sensitive silicon detectors with sensitive area of 17.0 cm 2 have been

1.0 xOx 0.9

g

o.o

E

0,7

~

0.6

~

0,5

w

Light Pulser

\• \•

I

\•

\

I

I

I

t

I

I

I

I

I000 900

•\•\

800 Ideol Response

~

I

•\e

0.4

\

i~ 700 °\o

~, 0.3

\o

o_

-=

600

\

.= ,.>, 50o

0.2-

\,. 0.1 I 0,1

L 0.2

I 0.3

I 0.4

I 0.5

[ 0.6

\-

I

i

I

0,7

0,8

0,9

.,=--- 1.165 mm (FWHM)

400 x

I.O

E

3oo

Relative Position

Fig. 4. (A) Measured relative energy amplitude vs relative position o f incidence for a position sensitive detector exposed to 2 4 4 C m alpha particles. T h e source was collimated to a 2 m m diameter. T h e dashed curve is the calculated dependence o f the energy ballistic deficit as a function ofx/l for T/RC = 0.50. T h e value o f T/RC for the detector is 0.56. (B) M e a s u r e d average position amplitude/energy amplitude ratio as a function o f relative position x/1 obtained for a position sensitive detector exposed to a pulsed-light source collimated to a 1 m m diameter. T h e dashed line corresponds to the ideal response.

200 I00 I

I

r--I

_~

--1__~ I

Difference in Position M e o s u r e m e n t

Imm)

Fig. 5. M e a s u r e d distribution o f the difference in m e a s u r e d position for energetic 4°Ar nuclei penetrating two identical position sensitive detectors (see text for details).

76

J.E.

LAMPORT

constructed by employing an external resistive-divider network. The signal and noise characteristics of the detectors are well described by the model which treats the detector as a homogeneous RC transmission line. Position sensing detectors such as the ones described here, or straightforward variations of this concept, can have many applications in nuclear physics measurements where large detector area and cylindrical symmetry are desired. Their ability to measure the position of heavily ionizing particles - without crosstalk problems - and their long-term stability makes them ideal for use in a number of studies including cosmic ray work. The demonstrated position resolution of these detectors will make it possible to correct for pathlength variations to the accuracy required for the resolution of nuclei at least as heavy as iron - thereby improving significantly on present instrument capabilities. Without the active encouragement and support of Dr J. A. Simpson, this work would not have been completed. We thank J. J. Kristoff and G. C. Ho of LASR for their contributions to the detector fabrication. We are grateful to R. Chin of the Chicago Office of Naval Research and E. Riggs of the Naval Ammunition Depot, Crane, Indiana, for arranging the early ultrasonic bonding experiments, and we thank J. Horton and H. Hieshima of the Enrico Fermi Institute electronics shop for their help in the detector

et al.

ring construction and bonding. We are grateful to many people at the Lawrence Berkeley Laboratory, especially Dr E. J. Lofgren, F. H. G. Lothrop, Dr H. H. Heckman and their groups for making the 4°Ar calibration possible. References ~) J. R. Gigantc, Nucl. Instr. a n d Meth. 59 (1973) 345, and references therein. 2) E. Elad a n d R. Sareen, IEEE Trans. Nucl. Sci. NS-21 (1974) 75. 3) W . K . Hofker, D . P . Oosthoek, A . M . E . Hoeberechts, R. van Dantzig, K. Mulder, J. E . J . Oberski, L. A. Ch. Koerts, J . H . Dieperink, E. K o k and R . F . R u m p h o r s t , IEEE Trans. Nucl. Sci. NS-13 (1966) 208. 4) M. A. Perkins, J. J. Kristoff, G. M. M a s o n and J. D. Sullivan, Nucl. Instr. a n d Meth. 68 (1969) 149. 5) M. G a r c i a - M u n o z , G . M . M a s o n and J . A . Simpson, Astrophys. J. (Lett.) 201 (1975) L145. 6) j. L. Blankenship and C. J. Borkowski, I R E Trans. Nucl. Sci. NS-9 (1963) 184. 7) S. Kalbitzer and W. Melzer, Nucl. Instr. and Meth. 56 (1967) 301. 8) A. Doehring, S. Ka[bitzer and W. Melzer, Nucl. Instr. and Meth. 59 (1968) 40. 9) R . B . Owen and M . L . Awcock, IEEE Trans. Nucl. Sci. NS-15 (1968) 290. 1o) E. Mathieson, Nucl. Instr. and Meth. 97 (1971) 171. 11) E. Mathieson, K. D. Evans, W. Parkes and P. F. Christie, Nucl. Instr. and Meth. 121 (1974) 139. 12) E. Baldinger and W. Franzen, Adv. Electronics Electron Phys. 8 (1956) 256.