Shallow p+n junction silicon nuclear radiation detectors

5.58

Sensor and Actuators A, 41-42 (1994) 558-561

Shallow p +n junction silicon nuclear radiation detectors J. Dobrovodsw,

I. Be#eb, L Hrubh”

and P. KovhE”

‘Depament of Nuclear Phystcs and Technologv Slovak Techmcal Unrwsuy, IlkovtEova 3, 812 I9 Bnatlrhava (Slovak Re@bc) bInshtute of Phyucal ELectmmcs, Slovak Academy of Scaenceq V&n&i cesta 102, 921 01 heWany (Slovak Repubhc) %stxtite of Electracal Engmeenng, Slovak Academy of Scrences,Wbmvskd cesta 9, &12 39 Brahshava (Slovak Repltbbc)

Abstract Si planar technology and ton unplantatlon were used for manufacturmg shallow p+n lunctlon nuclear rachatlon detectors Theu charactenstics were mvestlgated as a function of alternatwe unplantation steps mtb and v&out preamorphlsatlon by Si (SifB, Sl+BF,, B, BF,) The influence of Ar lmplantatlon on the Increase of the breakdown voltage was exammed The tluckness of the msensltwe entrance wmdow (dead layer) was determmed by the tlltmg method A descnptlon of the detector manufacturmg process, and results of the measured detector performance (currenthroltage charactenshcs, detector resolution, dead wmdow thickness) ISpresented and discussed

1. Introduction The workmg prmciple of the nuclear radiation detector under consideration 1s that the deep depletion layer of a reverse-bIased p’n Junction diode 1s used for the absorption and detection of radlatlon The energy or mtenslty of radiation can be detemuned by measurement of the number of charge carriers produced by the rachatlon witlun this layer Semiconductor detectors are used widely as nuclear radiation and charged particle sensors They are applied for the detetion and spectrometry of rachatlon m research and technology as well as m enwomnental momtormg The high energy resolution IS necessary for apphcation 111ion beam analysis techmques such as Rutherford backscattenng spectrometry (RBS) m the lower energy range (below 1 MeV for alpha particles) In recent years, modem technologes of semiconductor devxe manufactunng have been used for the fabncation of Si radiation detectors [l] They have enabled the production of the passwated nnplanted planar sihcon detector [2] that exhibits a lower leakage current and a better energy resolution than commonly used silicon surface barner detectors 2. Tbemctical considerations Several vanatlons of the technologxal process of the detector manufactunng can be denvcd from the theoretical pnnclples of the detector performance The breakdown voltage IS an nnportant parameter for the depth of the depletion region d (cm) determmed by the formula [3]

0924-4247/94/$07 00 0 1994 Elscvler Sequoia All nghts reserved SSDI 0924-4247(93)00571-K

where U (V) 1s the reverse voltage and p (0 cm) IS the specdic resistance An mcrease of the breakdown voltage could be achieved by dopant Implantation through a bevelled onde layer A bevelled wmdow m the Si02 layer could be created by Ar implantation before etchmg The detector resolution IS greatly affected by two factors - electronic noise and the energy straggling of the detected partxles 111the detector dead layer The electronic nose is determmed mamly by the reverse current It 1s the sum of the d&slon current, surface leakage current and bulk generation current The surface leakage current can be suppressed by a lugh quality oxide layer surroundmg the p’ region In this case the total reverse current IS determmed by the generatIon current I=eAdn,l(2r)

where e (C) 1s the elementary charge, n, (cm-‘) the mttmslc concentration, r(s) the mmonty earner hfebme, A (ctr~“) the actlve area, d (cm) the depth of the depletion region For n,=const the most nnportant parameter 7 is determmed prnnanly by defects m the &con Thus technologcal steps which leave the mmlmum of damage are mandatory The dommant contnbution to the detector resolution hes m the energy straggling of the particles m the msensltlve detector entrance window The followmg formula [4] determmes the relation between the energy resolution R (keV) and the depth of this wmdow (dead layer) b (pm)

559

n=25(Ax)‘” Consequently a technology leadmg to a shallow and sharp p+n Junction is desired m order to reach the small depth of the dead layer and therefore good energy resolution of the detectors

3. Detector technology The startmg material was a high resistance n-type slhcon with a reslstnnty of 1.5-18 kiI m, onented m the (111) dtrectlon, prepared m Tesla Roznov The SI wafers were processed at Tesla PleSt’any Detds of the manufacturmg process are as follows (see also Fig 1) (I) On cleaned slices the thermal omdatlon and growmg of the passmation 175 nm SiO, layer was performed (ii) Ar implantation* at two energes 100 and 40 keV at the same dose of 1X1014 cm-’ was used so that m the next etchmg step the bevelled ‘walls’ of the window m the S10, layer could be created (m) The desired wmdows for a 5 x 5 mm’ active area detector were opened by standard photohthography and n-St wafer oxide passlvat0n and Ar Implantation’

chemical etching The total size of the detector chips was 8x8 mm2 (IV) A 40 mn thm screenmg oxide was grown m order to avoid channellmg of low energy ions and outdffislon of the unplanted Impunties durmg thermal actwation (v) Preamorphsation of Sl* was achieved by S1 nnplantation at an energy of 150 keV at a dose of 2 x 101’ cm-2 and 60 keV at a dose of 7X 1Ol4 cmm2 The thickness of the amorphous layer was between 180 and 200 nm The successwe nnplanted dopant range was lower than this amorphlsed layer (vl) The shallow p+n Junction on the front side of the detectors was created by nnplantatlon of either 20 keV llB at a dose of 5 X 1014 cm-’ or 60 keV 4gBF2 at the same dose of 5X1014 cmm2 (vn) The rear contact was nnplanted usmg As The energy was 120 keV and the dose 5 x 10” cm-* (vm) After nnplantatlon the wafers were thermally annealed at 950 “C for 40 mm m a dry nitrogen amblent (IX) Photolithography and etching were used to form a front contact wmdow m S102 (x) The mput side was coated with 24 mn of alummmm by vacuum deposltlon (XI) The front side Al contacts were formed by photolithography and etchmg (xu) The backside of the wafers was metalhzed mth a 1 1 pm thick Al layer (xm) The Al contacts were alloyed at 450 “C for 20 mm m nitrogen ambient

openmg of wmdows growth of thm screentng oxide

4. Detector properties

and preamorphlzatior?

S or BF2 and As Implantation

opening of windows

Al metallzatlon p-side

front Al-contact formation Al metalltzation back side

n SI

q

S or BF21mplanted

q 80~

I

AS

implanted

I

Al

Fig 1 Slmphfied procesang steps of Ion-Implanted s~hcondetector fabncatlon * are altematlve technology steps

*An akematwe step

The mfluence of the technolo@cal process on some of the unportant detector parameters was ‘thoroughly mvestlgated by variation of one manufacturmg step while mamtammg the rest of the technology Depending on the various fabrrcatron steps the detectors were placed mto 8 groups Detectors of groups 5-8 were preamorphlsed, detectors of groups l-4 were not In the case of samples of groups 3, 4, 7 and 8 the Ar unplantatlon step was apphed, m groups 1, 2, 5 and 6 it was not The last vanatlon was B lmplantatlon m groups 1, 3, 5 and 7, and BF, unplantatlon m groups 2, 4, 6 and 8 Electrical and spectrometry tests were performed for each detector group 4 1 Electncal pmperttes The reverse current and breakdown voltage were derived from the current-voltage charactetrstics measurements For comparmn of the technological steps and as a quahty test of the detectors the reverse current Z, at -30 V was determmed, Fig 2 This voltage

Fig 2 Comparison of reverse current statistics of the detector groups prepared by ddferent tecImolo@cal steps The has voltage IS -30v TABLE 1 Summary of detector performance alternative technology steps GKXlp no

Implantation spccles

I,

B

71 82 112 122 12 1 13 8 109 217

BFz Ar+B Ar + BFI B+B !h + BFZ !%+Ar+B B+Ar+BF*

Fig :3 Cnmparlson of breakdown voltage statlstlcs of detector groups prepared by Merent te-chnolo~cal steps

- dependence on

FWHM

Ax (nm)

580 622 800 7.5 6 776 615 894 796

129 134 14 6 148 142 140 170 14 3

131 117 141 142 165 136 135 176

I, = reverse current, C&,=breakdown voltage, FWHM= detector resolution, Ax= dead layer thckness

corresponds to a 100 w depletion layer for used &con wafers Generally, the reverse current of preamorptused samples 1s about half an order of magmtude lugher than that of non-preamorpbed samples Accordmg to the measurements the mmlmum leakage current, less than 4 nA/cm’, was observed for B unplanted detectors with no 8 preamorphlsatlon and no Ar implantation The values of breakdown voltage U,, represented m Table 1 and Fig 3 represent the pomt where the reverse current oversteps 80 nA/cm2 The mcrease of the breakdown voltage of Ar nnplanted samples was lugher than 10 V as shown on Figs 3 and 4 4 2 Detector pe$nmance The well-known tiltmg technique [4] was used for measurement of the detector wmdow thxlcness The energy loss m the wmdow IS dtierent when the detector IS hlted Hnth respect to the madent particles This wmdow thickness was calculated from the peak shift

lE-101 0

! 10

! 20

(a)

! ao

40

50

0

BIAS VOLTAGE

[iq

30

60

! 70

8o

90

70

80

eo

1E

lE-101. (b)

1 0 IO

20

40

50

BIASVOLTAGE[VI

l%g 4 Example of leakage current-voltage cbaractenstwa (a) detectors of group 5 (.% preamorphsed and B Implanted), (b) detectors of group 7 (SI preamorpiused and B Implanted wth Ar lmplantatlon for breakdown voltage enhancement)

of the alpha spectra taken at the normal and 60“ mcident angles 5 5 MeV ?‘u alpha particles pulse-height spectra were used for entrance wmdow thickness and detector

561

/

ecwl

The followmg can be deduced from the obtamed results l Ion unplantatlon through a thm oxide layer creates a shallow, abrupt mput p+n Junction with low dark reverse current l Ar nnplantation mcreases the breakdown voltage of the detectors by about 10 V l There was no slgmficant dtierence III the properties of detectors prepared by B or BF, lmplantatlon l Preamorphlsatlon by Sl unplantatmn mcreases the reverse current but this dtierence 1s not sigmficant 6. Conclusions

Cl-RNNEL

Fig 5 5 5 MeV =Pu alpha partxles pulse-he@ at the normal and 60” nmdent angles

spectra taken

resolution (FWHM) determmatlon, Fig 5 The dead layer thxkness was found to be approxunately 130 nm A standard spectrometxy lme was used, consstmg of an ORTEC 142A low noise preamphfier, ORTEC 572 ampltier and 4k multichannel analyzer The results are gwen m Table 1

Detector performance. as a function of altematlve unplantatlon steps ~th and without preamorphlsatlon showed that at the present stage of mvestigatlon the most sultable technique for preparmg the detectors 1s B or BF, unplantatlon for creatmg the mput wmdow combined ~th Ar Implantation mto SlO, for mcreasmg the breakdown voltage It 1s possiile to fabncate detectors of various shapes ~rlth good energy resolution usmg the described technology References

5.

Discussion

The sun of the present work was to mvestlgate the mfluencc of altematlve fabrrcatlon steps on the detector performance The energy resolution of the prepared detectors corresponds to the overall standard An FWHM better than 13 keV for 5.5 MeV alpha pticles was achxved for the detectors of group 1. Lower leakage current and better energy resolution for detectors fabrrcated by the described technology could be reached when usmg higher quahty Sl shces

J Kemmer, Advanced concepts for seuuconductor nuclear ra&ation detectors, NucL Ins&um. Methoak, Phys Res B, 45 (1990) 247-251 W de Caster, B BIIJS, W Vandervorst and P Burger, HeavyIon uxxitation effects on passwated unplanted planar sdwn detectors, Nud Instrum. Methoak, Alrys Res B, 64 (1992) 287-291 J Kemmer, Improvement of detector fabncatmn by planar process, NucL Inqhum Methods,PAYSRes, 226 (1984) 89-93 T Ma~scb, R Gtizler, M Welscr, S Kalbltzer, W Welser and J Kemmer, Ion-nnplanted SI pm~unctmn detectors wth ultrathin wmdows, Nucl. Inrmun Methods, Phys Res A, 288 (1990) w-23