On-chip p-FETs were developed t o monitor the r a d i a t i o n dose o f n-well CMOS ICs by monitoring threshold voltage s h i f t s due t o r a d i a t i o n induced oxide and i n t e r f a c e charge. The design employs closed geometry FETs and a zero-bi ased n-we1 1 t o e l i m i nate 1 eakage currents. The FETs are operated using a constant c u r r e n t chosen t o g r e a t l y reduce the FET's temperature s e n s i t i v i t y. The dose s e n s i t i v i t y o f these p-FETs i s about-2.6 mV/krad(Si) and the o f f-c h i p instrumentation resolves about 400 r a d (S i) / b i t. When operated w i t h a c u r r e n t a t t h e temperatureindependent point, i t was discovered t h a t the p r e-i r r a d i a t i o n output voltage i s about-1.5 V which depends o n l y on design-independent s i l i c o n m a t e r i a l parameters. The temperature s e n s i t i v i t y i s l e s s than 63 pV/'C over a 70°C temperature range centered about the temperature i n s e n s i t i v e p o i n t. This appears t o be the f i r s t time a p-FET dosimeter was f a b r i c a t e d i n a f i n e-l i n e , thin-oxide semi conductor technology. The p-FET i s biased t o about-1.5 V during measurement and i s unbiased when n o t being measured. This approach t o b i a s i n g i s intended t o provide a known b i a s environment a t a l l times. I n c e r t a i n applications, t h e avai 1 abi 1 i t y o f spacecraft power i s unpredi c t a b l e. Thus, being unbiased d u r i ng i r r a d i a t i o n provides a known b i a s scenario. On-chip dosimetry provides t h e advantage t h a t the dose i s measured d i r e c t l y next t o the I C. This reduces t h e u n c e r t a i n t y inherent i n dosimetry c a l c u l a t i o n s which are complicated e s p e c i a l l y f o r h i g h l y shielded e l e c t r o n i c s .
This paper describes a 3-sm CMOS timing samlpler wllich is a test circuit designied inito the JPL CRRES chip to be flown on the Coombinied Release and Radiation Effects Satellite (CR-RES). The timing sampler consists of 64 inverter-pair stages with samiiplinig latches anid decoder circuitry. The samiipler is used to measure inverter-pair propagation delays, whicli are iiominally 2.5 iianosecoiids, with a resolutioni of 100 picoseconids. A siiimple iiiodel was developed to explaini the radiationiiniduced iniverter-pair delay shifts in termiis of radiationi-iniduced MOSFET-thleslhold voltage shifts aiid effective iiodal capacitanices. The imiagnlitude of the shift in pair delay with radiation was estimuiated at the poinit where the ni-MOSFET tlhreshold voltage became zero. For a 0.7-volt threslhold slhift, the pairdelay increased from its preradiationi value by 360 picosecolids for a rising step iniput and decreased by 190 picoseconids for a falling step input.
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