The effect of gate-oxide process variations on the long-term fading of PMOS dosimeters

Sensors and Actuators A, 37-38 (1993) 370-374

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The effect of gate-oxide process variations on the long-term fading of PMOS dosimeters A Kelleher, N McDonnell, B O’Nelll and W Lane Narronal Mtcroelecwomcs Research Centre, Lee Maltmgs, Prospect Row, Cork (Ireland)

L Adams ESTEC, Noordnyk (Netherlands)

Abstract Radlatlon-sensltwe field-effect transistors (RADFETs) have appbcations as Integrating doslmeters m spacecraft, me&cme and laboratones to measure the amount of dose absorbed from various radlatlon sources The prmapal advantages that the RADFET has over more tramtlonal forms of doslmeters IS that it IS small and provides contmuous readout However, all doslmetrrc m&a undergo some anneahng or fadmg subsequent to irradiation Fading effects m PMOS doslmeters may be negative or pontlve, depending on the occurrence of reverse annealing The aim of tlus paper 1s to examme the effect of gate-oxide process vanatlons on the long-term fadmg of the NMRC’s PMOS doslmeter which has a dry/wet/dry gate oxide The necessity for this evaluation arises because exlstmg published results do not address the fadmg charactenshcs of dry/wet/dry oxides m the range lOO- 1000 nm This paper clearly shows that for the NMRC’s RADFET gate oxide an optimum set of processing conditions exists to nummlze the dosmeter fadmg The optimum processmg condlhons for fade reduction are compared with those required to maxumze the radlatlon sensltlvlty of the doslmeter and the lmphcatlons of fading at kgh doslmeter operating temperatures are dlscussed

1. Iutroduetlon

The pnnaple of the PMOS doslmeter depends upon hole trapping and interface trap generation in the gate oxide on exposure to lonmng radiation This bmld-up of oxide charge causes the dosnneter threshold voltage to change A measure of this change m threshold voltage allows the total absorbed dose to be determined The basic prmaples of the PMOS doslmeter and build-up of trapped oxide charge are urldely discussed m the hterature [ 1 - 51 and are not outhned here The mam advantage of the PMOS doslmeter is that it IS small and provides contmuous readout This type of dosnneter has apphcatlons m spacecraft, medlcme and personnel dosunetry However, the dosnneter smtablhty for a particular apphcatlon depends on Its sensltlvlty and its ability to retam mfocmatlon about the absorbed dose The fadmg (or long-term anneahng) of the PMOS dosnneter depends on the gate-oxide thickness [6,7], the gate-oxide processmg and the post-lrradlatlon electnc field [S, 61 In the PMOS doslmeter fadmg may cause either an Increase or a decrease m the threshold voltage, dependmg on the relative values of the trapped hole annealing and the bmld-up of interface traps Typlcal values of dosnneter fading published to date are that after eight years with zero bias exposure, the

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doslmeters had faded less than 10% and less than 20% for positive has [5,6] The dependence of the dosnneter sensltlvlty on the gate-oxide growth condltlons was discussed m ref 8 and IS not outlined here The read-time stablhty [ 7, 91 IS also an Important charactenstlc of the PMOS doslmeter However, this issue IS currently under mvestlgatlon for the NMRC’s PMOS doslmeter and IS not discussed as part of this work The aim of this paper IS to examme the effect of gate-oxide growth condmons on the longterm fading of the PMOS radiation-sensitive field-effect transistor (RADFET) Pubhshed results already exist for the effect of dry gate-oxide thickness on doslmeter fading [6, IO] However, the NMRC’s RADFET gate oxide consists of a dry/wet/dry oxide combmatlon, so It IS necessary to evaluate the effect of gate-oxide processmg on the fading of the doslmeter The evaluation of fading was camed out both at room temperature and at 100 “C These results allow the processing condltlons for mmlmum fadmg to be obtained for the NMRC’s PMOS dosnneter The mmnnum-fade processmg condlttons are then compared with the ‘optimum’ condmons required to obtain maxnnum doslmeter senatlvlty [ 81 The fadmg that occurs at both room temperature and at 100 “C is compared and the nnphcatlons of fadmg m doslmeters operating at tigh temperatures are discussed

@ 1993- Elsemer Sequoia All nghts reserved

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2. Experimental 2 I Dosuneter technology The NMRC’s PMOS doslmeter 1s fabncated usmg a metal-gate PMOS technology and the gate oxide conslsts of a dry/wet/dry sandwich The expernnental work outlined in ths paper consists of evaluating the effect on RADFET fadmg of varying the gate-oxide thickness m the range 250- 1000 nm, varying the oxldatlon temperature, varying the oxldatlon anneal tnne and the oxldatlon anneal temperature Fmally, the effect of the metal smtermg temperature 1s also evaluated The results of the expernnents are then used to indicate the optnnum processing condltlons required to mlmmlze the fading of the PMOS dosnneter 2 2 Doslmeter measurement procedure One of the most convement methods used m measurmg the shift m threshold voltage 1s to bias the PMOS dosuneter m saturation [8] In this method a constant dram current I,,, 1s supplied to the dosnneter and the change m source voltage, VO, reqmred to gve that current 1s measured The shift m V0 approxnnates the change m the threshold voltage and 1s the dosunetic parameter The post-lrradlatlon fading IS obtained by evaluating the ratio VTR/VTO,where V,, IS the post-lrradlatlon change m the threshold voltage and VW is the maximum change m dosuneter threshold voltage due to radiation All dosnneters were irradiated urlth 6oCo gamma n-radlatlon at ESTEC at room temperature The dometers were irradiated wth either positive has ( +5 V) on the gate or zero bias on the gate Followmg irradiation, one set of samples was annealed wth ather zero or positive bias at room temperature or at 100 “C The remaining samples were allowed to anneal unbiased for two years and were then annealed wth positive bias both at room temperature and at 100 “C

3. Results 3 1 Dosrmeter fadnag dependence on gate-oxide thickness The post-u-radiation fading for PMOS dosnneters urlth oxides grown at 1000 “C m the range 250- 1000 nm 1s shown m Rg 1 These results were obtamed for dosnneters which were (1) annealed for 280 h after irradiation at room temperature or at 100 “C with posltlve bias, or (2) annealed for SK months at room temperature without bias, or (3) annealed for two years at room temperature mthout bias The samples exammed were irradiated mth either zero (ZIR) or positive lrradlatlon (PIR) has

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tox (nm) Fig 1 Fading as a function of gate-oxide thickness m PMOS doslmeters The dosuneters which were annealed for SIXmonths and two years were unbiased at room temperature The other doslmeters were annealed mth a constant voltage at the mdrcated anneal temperature

After irradiation, annealing at 100 “C causes the worst-case fading and gves an mdlcatlon of the longterm fading to be expected from a dosuneter annealed at room temperature The long-term fadmg, after two years, is larger than that of the 100 “C annealing for 280 h Complete saturation of the fadmg had not been achieved at 280 h, so for longer anneal times it 1s expected that the 100 “C fading would be equivalent to that of the two year room-temperature anneal The samples which received ZIR bias faded more when annealed for 280 h at 100 “C than the PIR bias dosuneters annealed vvlth the same condltlons This suggests that the holes trapped durmg zero-bias lrradlatlon are not as permanent as those trapped durmg positive-bias lrradlatlon Exammmg the fadmg of the ZIR and PIR samples which received an unbiased anneal for SIXmonths at room temperature shows that for oxides tlncker than 400 nm, the fading IS less than 30% Exanumng the fadmg as a function of oxide thickness shows that the fadmg for the 250 mn and 1OCQnm oxides 1s greater than that of the 500 mn and 850 mn oxides However, these oxides were irradiated and annealed at a constant bias voltage, rather than at a constant electnc field, and this explains the difference m the trend of these results compared to those of ref 7 Sarrabayrouse et al [ 71 show that for a constant elccttrc field, the tticker the oxide the smaller the fading that occurs A constant has voltage was used for ths work because for certam apphcatlons, regardless of the dosuneter oxide thickness, the maxlmum power supply 1s set, so a constant voltage 1s available for the doslmeter rather than a constant electnc field

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Rg 2 The effect of oxide-growth temperature on PMOS dostmeter fadmg The doslmeters were fabncated with a 500 nm dry/wet/ dry gate oxide

Fig 3 ShlFt m threshold voltage as a fun&on of anneal tme for three dtierent oxide anneal condlfions The doslmeters were fabncated wth a 500 nm dry/wet/dry gate oxide grown at 950 “C

3 2 Dependence of dosrmeter fadmg on the gate-oxzde temperature In tis set of exigent the dosnneters were fabncated with a SOOnm gate oxide The effect of three dBerent oxidation temperatures, 950, 1000 and 1025 “C, on dosnneter fadmg IS evaluated Figure 2 shows the fading obtamed when the samples were annealed at room temperature and at 100 “C for 280 h From these results it may be seen that for a dry/wet/dry oxide to obtam the least fadmg an oxldatlon temperature of 1025 “C IS reqmred and 1000 “C e;lves the largest percentage fadmg It IS mte~estmg to note that m ref 8, 1000 “C gave the least seusztmty for these oxides and that 1025 “C gave the largest oxide sensltlvlty of the three temperatures From this It may be concluded that the greater the radlatlon sensltlvlty of the oxide, the lower the fading which occurs m that oxide This may be attributed to deep hole traps that occur m the extrasensltlve dosnneter oxides These results are m agreement wrth those of ref 5, which showed that rad~~onha&s& oxtdes faded more rapidly than soft oxtdes Oxldatlon temperatures greater than 1025 “C reqmre mvestlgatlon for dry/wet/dry oxides, however, Sarrabayrouse et al [6] showed that for dry oxides grown at 1125 “C, fading at room temperature for oxides greater than 500 nm IS not detectable at tunes less than lo3 h

anneal for 30 mm and the 1000 “C anneal for 15 mm gve approximately the same shift m threshold voltage However, the 1000 “C anneal for 30 mm gives a smaller shift m threshold voltage and hence lower fading In ref 8 it 1s shown that the 1000 “C anneal for 30 mm improves the sensitlvlty of the oxide Other published results [ 11, 121 state that the hardness of an oxide is dependent on the post-oxidation anneal and the effect of the anneal very much depends on the temperature and time of the anneal, as well as the oxtdatlon temperature Schlesler and Beynon [12] show that at high tem~ratures the hardness of the oxide 1s degraded as the anneal tune Increases Conversely, these results show that at an anneal temperature greater than the oxidation temperature, as the anneal time 1s lengthened the fadmg 1s reduced as the oxide becomes softer

3 3 L)ependmce of fadmg cm the gate-oxtde anneal The dosnneters for this expenment were fabncated ulth a 500nm gate oxide grown at 950 “C and the post-oxldatlon anneahng was camed out m mtrogen The three anneal condltlons exammed are 30 mm at 950 “C, 15 mm at 1000 “C! and 30 mm at 1000 “C Figure 3 shows the post-lrradlatlon stift m threshold voltage as a function of time when the samples were annealed at 100 “C From these results, both the 950 “C

3 4 Dependence of doslmeter f&mg on the metal sznter In these experiments two smtermg temperatures, 400 and 460 “C, are exammed The fadmg obtained after anneahng the samples under positive bias at room temperature and at 100 “C 1s shown m Fig 4 The 400 “C smter g?ves reduced fadmg for all oxidation temperatures, with the effect bemg greatest for the oxldatlon temperature of 1025 “C Pubhshed results for metal smtermg show that decreasmg the smtermg temperature m the range 550-400 “C causes the oxides to become more ra&ation sensltzve [13] Agam, these results suggest that the more sensltlve the oxide, the smaller the fading that occurs 4 Discussion Fading IS both bias and temperature dependent These results show that the amount of fadmg that

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Ox&ion Temperature (“C) Rg 4 The effect of metal smtermg temperature on the fadmg of PMOS dosmeters after a 280 h anneal The doslmeters were fabncated with a 500 nm dry/wet/dry gate oxide

occurs also depends on the irradiation b=s Mechanisms for device annealing m 45 mn oxides m the temperature range 25-125 “C were Qscussed m ref 14, and the device annealmg 1s attnbuted to electrons tunnelmg mto the oxide from the &con and neutrahzmg the trapped oxide charge Increasmg the anneal temperature increases the probability that an electron ~111tunnel mto the oxide and thus increases the amount of trapped-oxide charge neutralization Schwank et al [14] show that for n~hannel tran~sto~ the annealing saturatton IS independent of the anneal temperature, but the ttme to reach saturation vanes accordmg to the anneal temperature This was not evaluated for PMOS transistors, however, comparing the results from the 100 “C anneal and the two year anneal in Fig 1 suggests that these results also apply to PMOS doslmeters For reduced fading of the dosnneter operatmg at a particular electnc field, the followmg conclusions may be made about the processmg reqmrements (1) increase the gate oxide thickness, (2) oxide growth temperature should be 1025 “C or greater, (3) mcrease the oxide anneal temperature to be greater than the oxldatlon temperature and mcrease the anneal tune, (4) reduce the metal smtermg temperature to 400 “C Applymg the rmmmum fadmg conditions to the doslmeter process wtth a gate oxide thickness of 500 nm, the long-term fading was reduced to 7% for a biased room-temperature anneal and to 31% at a biased 100 “C anneal Comparmg the conditions for nnmmum fadmg unth those for maxmuzmg the radiation senntivlty discussed m ref 8 shows that the optimum condo-

tlons are identical m both cases The more sensitive the oxide, the lower the fading which occurs m that oxide, suggestmg for these radiation-senwtive oxides the amount of electron tunnelhng is reduced However, for higher anneal temperatures the amount of fadmg takmg place IS increased Thu+ ts attrsbuted to an increased amount of electron tunnelhng rather than thermal annealing, because the temperature required for thermal annealing IS typically m the range 150-300 “C [ 151 Another charactenstlc of fading at 100 “C 1s that it takes place rapidly In these samples saturation of the fading began after the first 50 h at 100 “C From the fadmg results of this work one major concern 1s how the doslmeter behaves at ddferent dose rates Blarmres et al [5] exammed &fferent dose rates from 0 001 to 1 Gy h-’ and did not find any significant dfierence m the response of samples annealed at room temperature ms IS reasonable for samples annealed at room temperature because the fadmg 1s very low In tbs work the mmmum saturation fadmg obtamed 1s 7%, suiting that this doslmeter 1s s&able for all dose rates However, If the doslmeter is to operate at higher temperatures, e g , 100 “C, then It IS expected that different dose rates wdl gve different responses m the doslmeters because the fadmg occurs very rapidly It 1s also expected that temperatures m the range 25-100 “C wfll produce d&rent rates of fading Thus for tigher temperatures rt IS necessary to know the operation tenets and the fading rate at that ~~1~~1~ temperature to obtam an accurate estimate of the absorbed dose from a doslmeter readmg

5. Conel~ons

In this work we have Investigated the effect of process variations on the long-term fading of PMOS dostmeters both at room temperature and at 100 “C under posltlve bias The necesstty for this evaluation arose because results published to date do not address the issues of the effect of process vatlations or hlghtemperature annealing on the fadmg of PMOS dosnneters The results show that a certain set of processmg conditions exists which mmmuzes the fadmg of PMOS dosnneters and these conditions are identical to those that maxumze the sensltxvlty of the dosmeters This study also shows that the operatmg temperature of the donmeter has a maJor nnpact on the amount and rate of fadmg that occurs For a doslmeter that IS to operate at vatred temperatures m the nuhtary range, then the fading must be characterized as a function of tune and temperature to allow an accurate estJmalon of the absorbed dose

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Acknowledgements

6 G Sarrabayrouse, A Bellaouar and P Rossel, Electrical propertles of MOS radlatlon dosnneters, Rev Phys Appl , 21

This work was sponsored and technically supported by the European Space Agency as part of the MTSL-01 work programme The authors thank Bengt Johlander of ESTEC for his advice and assistance Hrlth this project Thanks are also due to Mlnam Curtm for performing device measurements and the staff of the NMRC Fab Laboratory for fabricating the devices used m this study

(1986) 283-287 7 G Sarrabayrouse, A Bellaouar and P Rosscl, Denve tem-

porelle post-lrradlatlon dans les dosrmetres MOS de rayonnement, Rev Phys AppI, 21 (1986) 131-137 8 A Kelleher, M O’Sulhvan, J Ryan, B O’Nell and W Lane, Development of the radlatlon sensltlvlty of PMOS doslmeters, IEEE Tram Nucl Scz , NS-39 (3) (1992) 343-346 9 A G Holmes-Sledle and L Adams, The mechanisms of small mstablhhes m irradiated MOS transistors, IEEE Trans Nucl Scr , NS-30 (6) (1983) 4135-4140

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