R&D on Counting Pixel Chips
Yunpeng Lu

Outline•••••

Motivation
Fundamental Issues
Current Mirror & Amp-Sha-Disc
Nested-Wells & DSOI
Summary

Motivation(1)• Shielding is a key issue in SOI Pixel Technology, and Counting Pixel is aneffective measure to study it.
– Necessity of shielding was recognized and understood by F. X. Pengg– Integrating pixel works fine with BPW suppressing back gate effect(Why no charge injection observed? Slow slew rate/ Cancelled by integration?)– But in counting pixel, charge injection messed up the counting results.
Shielding-well proposed by F. X. Pengg
in his dissertation “Monolithic Silicon
Pixel Detectors in SOI Technology”
Good concept but not implemented
successfully.

Motivation(2)• Counting Pixel is getting more and more popular in synchrotron radiation
application.
• Particularly interested in the area detector proposed by Prof. Kishimoto.
– 30 um2 pixel size
– 1k frames/s– 14 bit counter– Low energy X-ray 2~4 keV

Very compact pixel circuit
and good S/N required!

Fundamental Issues• On-chip circuit– Amp-Sha-Disc system– Counter and register in pixel• Shielding– Nested-wells– Double SOI• Leakage current– Low temperature would mitigate it• Radiation damage– Should be fine if back-illuminated by low energy X-ray

Review of CPIXTEG2 results•

Amp-Sha-Disc system
Counter and Register
Bias and Aobuf
Current Source variation

 Shielding between analog
and sense node Response of Light stimulus• Shielding between counterand sense node not reported

Continuing efforts of Nested-wells on CPIXTEG3
New nested-wells layout:• Expand to full pixel• More BNW contacts
CPIXTEG3TEG04C_N

Findings though CPIXTEG2/3• Low current source variation (double checked)• Insufficient shielding efficiency if the sense node overlaps withDiscriminator or Counter (measured on CPIXTEG3)– Shielding between shaper and sense node is good (double checked)• A new chip CPIXTEG3b designed on basis of above findings.

Display Mode: Persist

PREAMPSHAPERCounter_CLKThe Krummenacher Scheme and current source• The kummenacher scheme was adopted for Preamp and Shaper incpixteg2/3 design.
– Baseline of output can be set by Vref, which is good for DC coupling to the next
discrimination stage.
– However, its operation relies on the exact ratio of Isource_h and Isource_l.

••

CPIXTEG3 Measurement
PMOS current increased dramatically!
VH_FB_AMP_N/VL_FB_AMP_N=1.0nA/0.55nA (SPICE)VH_FB_AMP_P/VL_FB_AMP_P=0.53nA/1.1nA (SPICE)––54.543.5
32.5
21.5
10.50VH_FB_AMP_N
VL_FB_AMP_N
VH_FB_AMP_P
VL_FB_AMP_P

123456789 10 11 12 13 14 15•PREAMP is not supposed to work withVH_FB_AMP_P/VL_FB_AMP_P=3.14nA/1.95nA (0.53nA/1.1nA by SPICE).
– Feedback current ~ (VL_FB_AMP_P - VH_FB_AMP_P)•Some PREAMPs among the 13 TEG elements did fail to operate.
– Others showed quite different falling edge, which implies different feedback current

ARITEG_13C_P
6.1us
ARITEG_11C_P
350ns

ARITEG_12C_P
48.4usARITEG_21C_P
1.545us

ARITEG_22C_P
6.42usARITEG_23C_P
failedARITEG_31C_P
failedARITEG_32C_P
failedARITEG_33C_P
504ns

350ns falling edge ~ 4.6nA feedback current
48.4us falling edge ~ 0.033nA feedback current
6.1us falling edge ~ 0.26nA feedback current

ARITEG_14C_P
10.8usARITEG_24C_P
456ns

ARITEG_15C_P
1.14usARITEG_25C_P
failedConstant Current Feedback• Constant current feedback structure is less dependent on the precision of
low current source.
– Variation of current source only changes falling edge but the amplifier would still
operate.
– Vout depends on the Vth of input transistor, leakage current and DC operating point offeedback transisitor.
– DC coupling to discriminator is not a good choice any more.

Differential-pair discriminator• DC coupled discriminator used in CPIXTEG2/3.
– Differential pair with composite load(a diode-connected transistor
and another one operated in linear region)– Hysteresis– DAC coded current to adjust the local threshold

Diode-biased-inverter discriminator• Used in the PILATUS chip.
• AC coupling is compatible with Amp-Sha that adopted the constantcurrent feedback.
• 120e- threshold dispersion without threshold trim reported, very
attractive.
• Threshold voltages set by Vdiodeaccording to Ithr = Is(eVD/VT -1)

Amp-Sha-Disc System designed for CPIXTEG3b• Constant current feedback Preamp, 5fF feedback capacitor• AC coupled to shaper, voltage gain of 6
• AC coupled diode-biased inverter discriminator, 4-bit local tunningIthreshold (I0) =40nA, input charge = 750e- to 2000e-

input charge = 1500e-, Ithreshold (I0) = 40nA to 66nA

I thresholdinput charge (e-)

Transition charge (e-)

Average input charge (e-)

25nA625-812.5

712.5-775 (62.5)

743.7575nA

1500-1687.5

1575-1662.5 (87.5)

1618.75

125nA

2375-2562.5

2437.5-2537.5 (100)2487.5150nA

2812.5-3000

2875-2975 (100)2925

Average input charge (e-)
3500
3000

y = 17.441x + 308.69

2500

2925

2487.52000

1nA ~ 17.4eIthreshold Adjustment Range: 25-150nA
I threshold tuning step: 2-5nA
I threshold tuning DAC: 4 bits or 3 bits

1618.75

1500
1000
743.75
500
0020

40

60

80

100

120

140

160

Ithreshold (nA)Total Noise Spectrum @ shaper output
Primary noise source: NN1&PP1 of preamp around 2 MHz;
NN1&PP1 of shaper at low frequency;
SUM = NN1_preamp + PP1_preamp + NN1_shaper + PP1_shaper(Total – SUM) is mainly contributed by the Feed-back Tr. (P1) in shaper
V2/Hz• Noise @ PREAMP Output

Cd=100fF;
Cf=5fF;
Ccouple = 30fF
Ifb_preamp = Ifb_shaper = 1nA;

– no = 1.4mV– equivalent to 70 e-• Noise @ SHAPER Output– no = 6.8mV– equivalent to 57 eENC (e-)

ENC @ Shaper Output

120100y = 0.4089x + 14.856
8060

Leakage Current Noise not Included
1/f noise and channel thermal noise included

40200050100150200250Cd (fF)

First Test Results of CPIXTEG3b (1)• Bias current measurement (VIO_BPW = 0.85V)– Discrepancy between Iout and Iout_pixel1,2,3 affected by VIO_BPW, but required
different VIO_BPW to compensate NMOS and PMOS respectively.
– Error on Ifb_shaper not understood.
– Should be OK to operate the chip on the basis of measurement results.
CurrentSource

Iin(nA)

CurrentRatio

Iout(nA)

Iout_pixel1(nA)

Iout_pixel2(nA)

Iout_pixel3(nA)Ifb_preamp

-10.5

10:1

-1-0.49

-0.5

-0.47

Ifb_shaper

1.6

10:1

1.4

1.3

1.4

1.4

Iref_preamp-3000

4:1-791

-726

-709

-696

Iref_shaper

3000

4:1

1180

1140

1200

1170

Ithr-1500

8:1

-223

-172

-167

-172

Ithr_tunning

-40

8:1

-4

First Test Results of CPIXTEG3b (2)•Analog out of Preamp–

Vtest = 80mV, equivalent to 1900 e-

First Test Results of CPIXTEG3b (3)•Analog out of Shaper–

Vtest = 80mV, equivalent to 1900 e-

First Test Results of CPIXTEG3b (4)•Analog out of Discriminator–Vtest = 80mV, equivalent to 1900 e-–

2
1.8
1.6
1.4
1.2
V 1
0.8
0.6
0.4
0.2
0Vtest=50mV

Vtest=110mV
-0.83
-0.82
-0.81
-0.8
-0.79
-0.78
-0.77
-0.76
-0.75
-0.74
-0.73
-0.72
-0.71
-0.7
-0.69
-0.68
-0.67

2
1.8
1.6
1.4
1.2
V 1
0.8
0.6
0.4
0.2
0-1.42
-1.4
-1.38
-1.36
-1.34
-1.33
-1.32
-1.31
-1.3
-1.29
-1.28
-1.27
-1.26
-1.25
-1.24
-1.23
-1.22

-0.51
-0.5
-0.49
-0.48
-0.47
-0.46
-0.45
-0.44
-0.43
-0.42
-0.41
-0.4
-0.39
-0.38
-0.37
-0.36
-0.35

•

-1.12
-1.11
-1.1
-1.09
-1.08
-1.07
-1.06
-1.05
-1.04
-1.03
-1.02
-1.01
-1
-0.99
-0.98
-0.97
-0.96

First Test Results of CPIXTEG3b (5)S curve measurement
ENC ~ 27e-2
1.8
1.6
1.4
1.2
V 1
0.8
0.6
0.4
0.2
0uA2
1.8
1.6
1.4
1.2
V 1
0.8
0.6
0.4
0.2
0Vtest=80mVuA

Vtest=140mVuAuA

Shielding Measurement on CPIXTEG3 (1)1.25VPREAMPSHAPER

Disable SHAPER
via VH2

1.8V

DISC_HDISC_L

2. Oscillation
stopped after
shaper disabled

1. Oscillation
Pixel Layout

Vtest = 300mV;
VL_AMP_N = 300mV;
Vdet = +5V

3. Response toCharge Injection
300mV

26

Shielding Measurement on CPIXTEG3 (2)Vdet = +5V;
Adjust VL_AMP_N = 0.2V;
Vtest = 300mV;
Vthh = 1250mV;
Vthl = 800mV;
Glitches can be removed by disabling
Disc. via DO_EN.

PREAMPSHAPER

DISC

Pixel Layout

Shielding Measurement on CPIXTEG3 (3)Disable Disc.
via VH2

TEG04C_N

1. Charge
Injection with
Discr. working
Vtest = 600mV;
VL_AMP_N = 280mV;
Vdet = +5V

2. Charge
injection with
Discr. stopped

Shielding Measurement on CPIXTEG3 (4)PREAMPSHAPER

DO_14
LO_X

Counter
Hit logic
operation operation

X011_1111_1111_1111
X100_0000_0000_0000

16 clks

X100_0000_0000_0111
X100_0000_0000_1000

16 clks

X100_0000_0001_0111
X100_0000_0001_1000

X100_0000_0010_0111
X100_0000_0010_1000

X111_1111_1111_1111
X000_0000_0000_0000

Preliminary resutls on CPIXTEG3bFirst CPIXTEG3b dilivered
was fabricated on normal
SOI wafer. No DSOI at all.
Clear crosstalk seen.

ShaperCounter_clk

Summary• Shielding is a critical issue in counting pixel chips.
• A new Amp-Sha-Disc system works, which is less dependent
on the precision of very low current source.
• Nested-Wells provided shielding between analog and sensenode, but no sufficient for shielding digital part in the pixel.
• Small sense node and small pixel is being pursued.