30 Mrad(SiO2) radiation tolerant pixel front- end for the BTEV experiment A. Mekkaoui, J. Hoff Fermilab, Batavia IL Fermilab
FPIX History • 1997: FPIX0, a 12X64 HP 0.8u process – Two stage front-end, analog output digitized off chip – A data driven non-triggered RO – Successfully used in beam tests • 1998: FPIX1, a 18X160 Hp 0.5u process – Two stage front-end, with one 2b FADC/cell. – Fast triggered/non triggered RO – successfully used in beam tests • 1999: preFPIX2_T, 2X160 TSMC 0.25u (to be presented today) – Radiation tolerant techniques forced us to design a new front- end with a new leakage compensation strategy. • 2000: preFPIX2_I, 18X32 0.25u CERN process (In fab) – Same as FE cell as in preFPIX2_T but with compelete fast non- triggered RO. 6/16/00 2 A. Mekkaoui
FPIX1 front-end Vdda Thresholds Token Out Iff ADC Flash Latch Mf Resets Row Address Bus Sensor Command Interpreter Controller 00 - idle 01 - reset Kill 10 - output Inject 11 - listen Test Threshold 4 pairs of RFastOR Throttle HFastOR Command Lines Token In Read Clock See the proceedings of the 1999 workshop on the electronics for the LHC (Snowmass) and references therein. 6/16/00 3 A. Mekkaoui
Main radtol design constraints • The feedback structure used two NMOS devices and a biasing PMOS device (as a current source Iff). • In the previous design: stability, noise and proper shaping relied on having a long (W/L << 1) N-channel device in the feedback (Mf). • Leakage current tolerance insured by the feedback structure. • Problems to implement present DSM radtol design : – NMOS in 0.25 µ has higher transconductance than in 0.5 µ process. – Minimum enclosed NMOS has W/L around 2.5. (See the RD49 reports.) – Enclosed NMOS with its required guard ring occupy large area. It’s quasi impossible to implement the present design in the available area. 6/16/00 4 A. Mekkaoui
Feedback solution • One NMOS feedback transistor biased by a global voltage VFF. • VFF generated such as to track (to Vff the 1st order) the preamp DC level shifts due to global changes (process, temperature…) Vbp • Feedback is current controlled as One bias cell per chip before. This current can be much Sensor Iff Vbp higher than in the previous scheme. Vff • It is more reliable to work with Vbn Inject higher currents. Test • Leakage current compensation assured by a separate scheme (next slide). 6/16/00 5 A. Mekkaoui
Leakage current compensation scheme A Vdda = 0 c H ( s ) Very low bandwith diff. Amp. Ideally: s - gmc Simplistic analysis yields: + Vff Vo 1 ( s ) 1 = A g ( ) Rf Ii s + + 0 c mc Cfs gf s Cf Sensor Vo 1 ( s ) s -A = Vo1 Iin Ileak 1 Ii ( s ) + + � � 2 C s g s f f � LC � f Inject Test S = Laplace variable => Compensates only one polarity. => The new scheme, though more complexe, occupy a modest area 6/16/00 6 A. Mekkaoui
Leakage current compensation scheme A = 0 c H ( s ) Vdda Vff s - Rf gmc + Vff Cf -A Rf Vo1 Cf Lc Inductor -A Vo1 6/16/00 7 A. Mekkaoui
TSMC && the “CERN process” • After some comparative work we decided to constrain our design to work well whether implemented in the “CERN” or TSMC process. • Minor additional layout is required to submit to both processes (mostly through automatic generation) • TSMC is offered by MOSIS (4 runs/year). 6 runs/yr is planned. • It is wise to have a 2nd source for production. => CERN process is the process selected by CERN to implement their deep sub- micron radtol designs. 6/16/00 8 A. Mekkaoui
preFPIX2 • preFPIX2 is the first prototype we designed to investigate our ideas and to test the radiation hardness of the TSMC 0.25 µ process. It contains 8 pixel front-end cell and several isolated transistor. 8 prefpix2 front-end cells Test structures 6/16/00 9 A. Mekkaoui
Typical front-end response Buffered output of the second stage 6/16/00 10 A. Mekkaoui
Feedback control ) F F I ( t n e r r u c k c a b d e e f g n i s a e r c e d 6/16/00 11 A. Mekkaoui
A. Mekkaoui Feedback control 6/16/00 12
A. Mekkaoui Feedback control 6/16/00 13
Leakage current compensation After the first nA no change in the response is observed ! 6/16/00 14 A. Mekkaoui
Leakage current compensation II 6/16/00 15 A. Mekkaoui
A. Mekkaoui Response to large signals 6/16/00 16
Linearity (small signals) => Ideal gain = (1/cf)(Cc2/cf2) = (1/8fF)*4 = 80 µ V/e- => Spice predicted gain = 76 µ V/e- 6/16/00 17 A. Mekkaoui
A. Mekkaoui Linearity (larger signals) 6/16/00 18
A. Mekkaoui Threshold control and matching 6/16/00 19
Threshold control and matching II => 25 channels from 5 different boards. 6/16/00 20 A. Mekkaoui
Noise ( measured from efficiency curves ) 6/16/00 21 A. Mekkaoui
PreFPIX2_T PreFPIX2_T is 2X160 pixel array. Each pixel cell contains all the functions needed for the BTEV experiment: kill and Inject logic, 3bit FADC, hit buffering, fast sparse RO. => EOC logic implemented off chip. => The analog and digital outputs of the two upper cells are available for direct test and characterization. 6/16/00 22 A. Mekkaoui
Top cell buffered outputs Rl Analog Out pad A=gmRl Vdda - + Vff Digital Out pad - drv + Sensor Inject Test Vref Threshold 6/16/00 23 A. Mekkaoui
preFPIX2_T front-end Vdda - + Vff Rf Cf2 Cf - -A - + Sensor 4Cf2 + Main discriminator PMOS Inject Test Vref Threshold Same FE as preFPIX2 except that the injection transitor is a PMOS and the injection cap is realized with m1/m2 sandwich (2.6fF) instead of m1/poly (4fF). 2nd stage feedback “resistor” not shown. 6/16/00 24 A. Mekkaoui
preFPIX2_T pixel cell 3b FADC Vdda Thresholds Token Out - + ADC Thermometer Flash Latch to Binary Vff Encoder Resets Row Address Bus Command Interpreter Controller - + Sensor 00 - idle 01 - reset Kill 10 - output Inject 11 - listen Test Vref Threshold 4 pairs of RFastOR Throttle Read Clock HFastOR Command Lines Read Reset Token In Token Reset 6/16/00 25 A. Mekkaoui
PreFPIX2_T: pulse shapes Qin=3260e- channel R. 3 different feedback currents. 6/16/00 26 A. Mekkaoui
Irradiation of the PreFPIX2_T We have irradiated several test structures from two 0.25 µ processes, from TSMC • and a domestic vendor. • Besides the individual devices we have irradiated also the prefPIX2 and preFPIX2_T pixel circuits The irradiation took place at the Co 60 irradiation facility of the Argonne National • Lab. • A complete report on the results is still under preparation. • Partial and VERY preliminary results from the test of the preFPIX2_T will be presented today. • Dosimetry accurate to 20%. • No filter for low energy particles was used. • All the results shown are after 1 to 7 days of annealing at room temperature. In all subsequent slides rad should read rad(SiO 2 ) • 6/16/00 27 A. Mekkaoui
General effects after 33 Mrad • Chip fully functional • No degradation in speed (as inferred from the kill/inject shift register operation). • Less than 10% change in “analog” power. Power was less after irradiation. Understandable from circuit point of view and is due to small VT change in the PMOS (<50 mV). 6/16/00 28 A. Mekkaoui
Total dose effects on front-end 6/16/00 29 A. Mekkaoui
Total dose effects on front-end 6/16/00 30 A. Mekkaoui
Total dose effects on front-end Before irradiation After 33 Mrad => 3 mV DC offset shift (due mainly to output buffer) => < 4% Rise time difference => < 5% change in fall time. 6/16/00 31 A. Mekkaoui
Linearity before and after 33 Mrad => 7 % max gain error. Believed to be due to output buffer only. 6/16/00 32 A. Mekkaoui
Rise and fall time before and after 33 Mrad => Changes are minimal and may disappear after annealing. 6/16/00 33 A. Mekkaoui
Noise and threshold distributions => Practically no change in noise and threshold dispersion. => 200 e- change in the threshold voltage. 6/16/00 34 A. Mekkaoui
A. Mekkaoui Effects at higher threshold 6/16/00 35
A. Mekkaoui Readout typical output 6/16/00 36
A. Mekkaoui Readout Max speed 6/16/00 37
Conclusions • We successfully migrated our design from 0.5 µ process to 0.25 µ using radiation tolerant techniques. • The design can be submitted to two different vendors. • Chip performed as expected before and after 33 Mrad. • We are still working on the radiation results. • DSM is the way to go for radiation hardness (if you can). 6/16/00 38 A. Mekkaoui
Acknowledgements • William Wester co-organizer of the irradiation “week”. • Tory Steed and Al. Al Svirmickas from ANL for their precious help. • Al Deyer and Kelly Knickerbocker for preparing the boards and the 100’s of feet of cable. • Ray Yarema for his advice and encouragements. 6/16/00 39 A. Mekkaoui
Recommend
More recommend
