Silicon X-ray sensor with modified electrode structures

:I r

Nuclear Instruments and Methods 215 (1983) 213-217 North-Holland Publishing Company

SILICON X-RAY SENSORS WITH MODIFIED ELECTRODE STRUCTURES Y. NARUSE and M. SEKIMURA

Toshiba Research and Development Center, Toshiha Corporaoon, Kaff-asaki 2111, Japan Received 18 February 1983

New surface barrier silicon sensors, which work under zero-bias conditions, have been developed to incesttgate the applrcatn rn, t . " X-radiation, especially to energy monitoring and position sensing . Four kinds of prototype sensors have been fabricated using high purity n-type silicon to meet these requirements by changing the configuration of electrode structures . They are designed for purelti X-ray energy monitoring, X-ray beam position detecting for both linear and X--1 area categories, as well is .omhincd encrg'. monitoring and linear position sensing function. respectively . The zero-bias condition has enabled all these sensors, with modified electrode structures. t o directly couple to the 1 t Lomerter " and stable operational results were obtained .

1. Introduction Several kinds of silicon surface barrier radiation sensors with modified electrode structures have been reported from the early days of the silicon detector . They are, for example, the single surface barrier strip

category [11, the checker board category [21, the position-sensitive category [31 and the double surface barrier category [4]. All these sensors serve different purposes .

but their fundamental operational modes are under reverse biasing. Recently, however, silicon surface barrier radiation sensors, fabricated from high purity n-type

silicon, were developed for medical systems in the zerobiasing operational modes [5-10] . As high purity silicon has a long minority carrier lifetime, a large amount of radiation induced diffusion currents can be obtained as sensor signal currents, even under zero-biasing conditions. Additionally, the zero-biasing operation leads to

on both sides of a silicon wafer. while an alunimum ohmie electrode was evaporated only on the hack surface. In the detecting operations . )`;-rags are pro-

jected perpendicularly to the gold electrckile, and the X-rays attenuate in the silicon depending on the elfc,tive X-ray energy . Accordingly, the output signal :urrent ratio I F : ;'I B is a function of effective X-ray :ncrg% and is given by IF _ IF

drif

+ I f-

'101

IB

drif

+ IB

d , ff ,

rB

where It d',f* l 8 .ir :f' i f ,i ~r and 1i, i , . aic dritt .'i' rents from the depletion region and dtffuslc1n ~urrem , from the neutral region for front electrode f~ and ha,k electrode B, respectively . Theoretical er.pre-wn" for thr above four currents component, are given in Xppcndis A.

electronic circuit simplicity and sensor signal temperature stability . From the above point of view, four kinds of proto-

Au

type surface barrier radiation sensors with modified electrode structures were fabricated from high purity n-type silicon, whose doping concentration Nt,, resistivity p and hole lifetime r r are around 2 X 10 1 ` cm ', 2.5 kil cm and 400 ps, respectively . In the following sections, the four kinds of sensors will he described sep-

(Energy)

AI

arately.

2. X-ray effective energy monitoring sensor The fundamental structure of this sensor is shown in fig. 1. Gold surface barrier electrodes were evaporated (1167-5087/83/0000- 00t)n/$03 .00 " 1983 North-Holland

n-Si Fig. I . Fundamental structure for effective X-ray energy nron,toring sensor . F is the front electrode and II i " the h.r,k electrode .

Y. Naruce, M. Sekimura j Si x" -ray sensors

214

X - ray (Position)

A B

Fig. 2. Photograph of the effective X-ray energy monitoring sensor. 15

Au m w

,

D

E

F

G

Fig. 4. Schematic structure of the strip X-ray beam position sensor.

A! filtration : Q6cm Temperature 20*C

F, 3mmt® X-ray

C

Al

14

0 w v 0 C? v c sr

én

12

Fig. S. Photograph of the strip X-ray beam position sensor .

Theory o Experiment

(fig. S) has 3 mm wide gold signal electrodes with 1 mm 1

60

70 80 90 100 110 X-ray Tube Voltage (kVp)

120

Fig. 3. Theoretically calculated and experimentally measured values of current ratio IF/la versus :X-ray tube voltage.

gaps between them . Experiments were made by sweeping with a 1 mm diameter X-ray beam, collimated from

an X-ray source with 120 kVp tube voltage and 4 mA tube current. The J',/V converters output voltage versus

X-ray beam position is shown in fig. 6. Because of the

Experiments were performed for a prototype 3 mm thick sensor (fig. 2) by changing the X-ray tube voltage,

that is, by changing the effective X-ray energy . Fig. 3 shows the current ratio IF/I B versus X-ray tube voltage

between 60 and 120 kVp. Theoretical values are included in this figure using the equations derived in Appendix A. There is reasonable agreement between experimental and theoretical values.

3. Strip X-ray beam position sensor The strip X-ray beam position sensor is for detecting the X-ray beam position . The signal electrodes are strips, as shown in fig. 4. The prototype 0.3 mm thick sensor

Electrode A

Electrode 8

Position

Electrode C

Pig. 6. I/ V converters (feedback resistance is 10 MS;t) output voltage versus X-ray beam position .

Y. Narase, M. Sekimuru / Si X-raY sensors

215

hole diffusion from the gaps in the electrodes and the X-ray photon scattering in the silicon, the output signal trace does not have a rectangular shape. However, by processing the output signals, this sensor can be applied to a one-dimensional X-ray beam position sensor . 4. X- Y output category X-ray beam position sensor The third sensor is also for detecting the X-ray beam position . However, in contrast to the strip sensor mentioned above, this sensor can detect the X-ray beam position continuously. The structure and a photograph of a prototype sensor are shown in figs. 7 and 8, respectively . Gold was evaporated onto the surface of the silicon wafer, to form a resistive thin film (- 70 Â) by which X-ray induced signal currents were divided towards four gold electrodes A - D (- 300 A,). Experiments were made by sweeping a 1 mm diameter X-ray beam, collimated from an X-ray source with 120 kVp tube voltage and 4 mA tube current. An example of the I/V converters output voltages from electrodes B and D versus X-ray beam position is shown in fig. 9. An approximate linear change in the output voltages was obtained.

Fig. 8. Photograph of the X- Y output X-rah beam po .mon sensor.

5. X-ray beam position and effective energy monitor sensor The fourth sensor is the X-ray beam position and effective energy monitor sensor, which has functions of both the strip position sensor and the effective energy monitor sensor . The prototype sensor's structure and a photograph are shown in figs. 10 and 11, respectively . There are three separated gold signal electrodes and one

x- ~ay

Bear,

POS , " )r

Fig. 9. An example of experimentally measureu l t con%erter (feedback resistance is 10 MS?) Output witages t, , m electrode . B and D versus X-ra% beam position . X-ray (Position Ek Energy)

Fig. 7. Structure of the X- Y output X-ray sensor .

beam

position

Fig. 10 . The structure of a prototype X-ray heam position and effective energy monitor sensor .

Y 1Varwe, M. Sek- ;Mura / Si X-rav sensors

I A + 113) : (1 B + IF,) : (1 c, a !F). More complicated operations, such as aetecting the X-ray beam incident directions, may be performed by processing signal currents IA - IF . After the experiments were made on the position detection and energy monitor by projecting the X-ray beam perpendicular to the signal electrodes, the X-ray beam was projected parallel to the signal electrodes . The X-ray tube voltage was changed in order to examine the effective X-ray energy monitoring functions for this sensor . Experimental results for (IA, + 'D)/(ID + 10 and ( IA + ID)/(IC + 1F) are shown in fig. 12 . Theoretical values are also included in the figure, using the approximate expressions given in Appendix B. Reasonable agreement between experimental and theoretical values are obtained . (

Fig. 11 . Photograph of the prototype X-ray beam effective energrv monitor sensor .

position and

aluminum ohmic electrode on both s.Jes of the silicon wafer. The sensor is 1 mm thick The oth°r dimensions are included to fig. 10.

If the X-ray beam is projected perpendicularly to the signal electrodes, the X-ray beam position can be detected by the signal current ratio 1, : IF, : I(.- or Ir, : It- : IF , and the effective X-ray energy can be monitored by the signal current ratio I, : It) , 11, : I E or 1, : I F. On th .- other hand . if the X-ray beam is projected parallel to the signal electrodes, the effective X-ray energy can he monitored by the signal current ratio

6. Summary Four kinds of prototype silicon surface barrier X-ray sensors with modified electrode structures were fabricated from high purity n-type silicon. The sensors are the effective X-ray energy monitoring sensor using double surface barrier structures, tlae strip X-ray beam position sensor, using strip surface barrier electrodes, the X- Y output X-ray beam position sensor. using a thin film surface barrier electrode. and the X-ray beam position detecting and energy monitoring sensor, using strip double surface barrier electrodes. All experiments were performed under zero-biasing conditions while coupled to I/ V converters, and stable operation was obtained (temperature coefficient of sensitivity is less than 0.5%/°C between 0-40°C). Additionally, some theoretical analysis was accomplished for comparison to the experimental results and reasonable agreement was obtained . Appendix A: Theoretical expressions for IF/18 and

IF-drir IF-drof -

f

s.

fE

IB .dr.r

U

me .

are given by

ES( E)ft en ( E )Fty '(E)

x (i - exp[ --p,(E) W])dE

(A .1)

in A/cm2, and IB

d,,r=es,

f0 E " , ES(E)ps  (E)pt i (E)

xexp [- p,(E)(L ,,- W)] x(1-exp [ - p,(E)W])dE,

Fig. 12. Theoretically calculated and experimentally measured alues for (L,, + 1001B + 10 versus X-ray tube voltage and (1 ., + ID)/(1(: + IF ) versus X-ray tube voltage.

(A .2)

where y is electronic charge, es, is average energy expended to generate an electron-hole pair in silicon (about 3.6 eV at room temperature), E is X-ray in A/cm2,

x2 ),diffsensor -following one-dimensional 1A,P(x)-Ai 1),(A pES(E),u (electrodes, carrier eq and (eV), =P(Lx-W)=0 solving, and (E ffenergy E) isfBfand W layers conditions Dpe thickness ),(A Em f'marIgDp(dp/dx)rac,-n-IdE (Ic+IF) is4/1 isf I,diff energy Approximate fEES(E)exp[-p,(E)(1,+12)] Pen(E) (A jiit S(E) concentration attenuation sidepletion IBthe equations Ifor (eV), 1(E)(1 11(xDp diapproximate P(x) isare and carrier ff=isand p(x) (cm) absorption can denotes calculated at,En,ax W, diffusion X-ray given (A -exp[-p,(E)I2])dE, the 1(E)(1 layer S(E) Lbe from expressions coefficient diffusion isp(x) edges -calculated by spectrum (E)tt the maximum thickness In W) are eq expressions -exp[-li,(E)I,])dE from equation coefficient the (cm-3 of hole based (Ain 1(E) the numerical of for eqparallel (photons/cm from diffusion silicon (cm) front on eVand X-ray (IA (1) Naruse, for are refs the of +using 1)and and eq the (cm ID), given with and calculaphoton followsilicon IcoeffiM Lx 1(A back (B hole -and eqs 2the the (I1, Sekimura 1 is),)s by /'Si 12] p1A, ff, ]-can XJY =H YCEW EYat and B, 2Y~DPrP Lr, the eq ray IR' 2W+ =p(Lx-W)=0 diffusion Kim Naruse Store =Naruse Naruse Sugimoto the Cbe Bateman Ludwig t)carrier isHofker sensors is 11, width Lx (1981) (1982) Radiation above! or (B 134 173 depletion and rdges By calculated given (1977) and 12exp(l/Lp) --LI,, toetpand D, 2W and and solving et K(x) collection Weff alet etH 251 the E, and 141 equation GP equations, P(x)+L alof by Husimi Tin aland TF, M Detectors, 13I/LP) IEEE (cm the Japanese Israel denotes W effective is layer IEEE Kobayashi, Kobayashi are Hoshasenxo respectively IEEE Kobayashi, Sekimura from for given LP -+depletion Trans Trans effective 3Hogg, Proc -LA3753 Trans thickness width p(x) exp( )for PTrans exp( kGI, and Japan, the by the carrier Japan is NS-27(1),(1980) NS-28(1) the -Proc Nucl =0, Nucl from NS-12(1) Proc Kenkyu inNS-13(3) following I/LP) -1/LP Proc the alayers hole (1967) length The (1981) hole (cm) the constant Soot 1981 Instr Instr boundary eq 2nd collection Ist effective generation (1981) -neutral )Appl (4) (x excess (1965) 249 for and (B INS 2(1966) Sensor an and one-dimen(1974) =electrodes L47 252 which W Int Ph% Steth %leth aregion and carrier 208 condiwidth Svmp Sv_ is211 L,,rate Symp mp the eq146 14R -79 isin

Y. photon energy eV lten(E) (cm the IF ing excess boundary depletion dz d

.

 DPrP

.en(E)exp[-1,,(E)x]=0,

.

(A .4) .

.3)

.

.1.IgDP(dp/dx)x_wldE

.4),

(A/cm2) (A .5)

and

(A/cm2 .

.

Finally, (A .1), .2), tion, ., 12. Apendix +IE)

(A,3)

.

.3),

IF-diff= . IB

217

.

.

+ p(W) In cient. By IF-d,ff

.

.6) .

.5)

(A .6) .

. .

:

Neglecting signal the

:

IA+I,=kfE"",ES(E)l'en a X(1-exp[-p,(E)Ij)dE, IB+1,=k

.E~-ES(E)exp[-IL,(E)111uen(E)

X

.1) (B .2)

1C+IF=k

u XAen(E)tit

. .3) (B

In proportional W. of collection We,f, where effective Lp sional concentration tions W). 2

. (BA) .

. (B .5)

dx2P(x)-Dh PP p(W) In . .5), (cm-3s-" . (B .6), L

.

(B .6) .

.5)

.

exp(

where References [l] .E. . (1970) [2] .K. [3] .J. [4] . October [5] . [6] . [7] . Japan [8] . Japan [9] . Nucl. 1101 . . (1982) . [I . [ Japanese .

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