G.-F. Dalla Betta SCIPP 16 sept. 2005
Maurizio Boscardina, Claudio Piemontea, Alberto Pozzaa, Sabina Ronchina,
Nicola Zorzia, Gian-Franco Dalla Bettab
a ITC-irst, Microsystems Division, via Sommarive, 18 38050 Povo di Trento, Italy b DIT, University of Trento, Via Sommarive, 14, 38050 Povo di Trento, Italy
Development of 3D detectors
at ITC-irst
G.-F. Dalla Betta SCIPP 16 sept. 2005
Introduction Single-Type Column 3D detectors Simulation results Technological tests Fabrication of the first batch Preliminary electrical results Conclusion
Outline
G.-F. Dalla Betta SCIPP 16 sept. 2005
IntroductionIntroduction
ITC-irst is developing the process for the fabrication of 3D sensors.
Framework:• RD50 collaboration - for the development of radiation tolerant detectors• INFN-PAT agreement
We have started with a simplified process, since all necessary clean-room equipment is not available yet
G.-F. Dalla Betta SCIPP 16 sept. 2005
““Standard” 3D detectorsStandard” 3D detectors - concept - conceptFirst 3D architecture, proposed by S.I. Parker et al. [1] in 1997: columnar electrodes of both doping types
n-columns p-columns
wafer surface
ionizing particle
Short distance between electrodes:• low full depletion voltage• short collection distance more radiation tolerant than planar detectors!!n-type substrate
[1] S.I. Parker et al., Nucl. Instr. Meth. Phys. Res. A 395 (1997) 328
DRAWBACK: Fabrication process rather long and not standard, so mass production of 3D devices very critical and very expensive.
G.-F. Dalla Betta SCIPP 16 sept. 2005
““Standard” 3D detectorsStandard” 3D detectors - fabrication - fabrication
Kenney et al. IEEE TNS, vol. 46, n. 4 (1999)
1) wafer bonding
support wafer
detector wafer
2) n+ hole definition and etching
resist
oxide
3) hole doping and filling
n+ polysilicon
4) p+ hole definition and etching
resist
5) hole doping and filling
p+ polysilicon
6) Metal deposition and definition
metal
G.-F. Dalla Betta SCIPP 16 sept. 2005
Single-Type-Column 3D detectorsSingle-Type-Column 3D detectors - concept- concept
Sketch of the detector:
grid-like bulk contact
ionizing particle
cross-sectionbetween twoelectrodes
n+ n+
electrons are swept away by the transversalfield
holes drift in the central region and diffuse towards p+contact
n-columns
p-type substrate
Etching and column doping performed only once
Functioning:
G.-F. Dalla Betta SCIPP 16 sept. 2005
3DSTC detectors3DSTC detectors - concept (2) - concept (2)
Further simplification: not passing-through holes
p-type substraten+ electrodes
Uniform p+ layer
Bulk contact is provided by a Backside uniform p+ implant single side process.
No need of support wafer.
G.-F. Dalla Betta SCIPP 16 sept. 2005
TCAD Simulations - static (1)TCAD Simulations - static (1)
3D simulations are necessary DEVICE3D tool by Silvaco
Important to exploit the structuresymmetries to minimize the regionto be simulated
n+
cell50m
• p-type substrate
• Wafer thickness: 300m
• Holes: 5m-radius
250m-deep
G.-F. Dalla Betta SCIPP 16 sept. 2005
TCAD Simulations - static (2)TCAD Simulations - static (2)
Potential distribution(vertical cross-section)
0V
-10V
-5V
50m
300
m
Potential distribution(horizontal cross-section)
null field regions
-15V
G.-F. Dalla Betta SCIPP 16 sept. 2005
TCAD Simulations - static (3)TCAD Simulations - static (3)
Potential and Electric field along a cut-line from the electrode to the center of the cell
To increase the electric field strength one can act on the substrate doping concentration
Na=1e12 1/cm3
Na=5e12 1/cm3
Na=1e13 1/cm3
Na=1e12 1/cm3
Na=5e12 1/cm3
Na=1e13 1/cm3
G.-F. Dalla Betta SCIPP 16 sept. 2005
TCAD Simulations - dynamic (1)TCAD Simulations - dynamic (1)
Current signal in response to an ionizing particle.
Vertical tracks in 3 different positions Electron cloud evolution (b)
1 2
3 4
n+ n+
n+ n+
a b c
G.-F. Dalla Betta SCIPP 16 sept. 2005
TCAD Simulations - dynamic (2)TCAD Simulations - dynamic (2)
Case b.
Case c.
Nsub=1e12 cm-3
Nsub=5e12 cm-3
Nsub=1e13 cm-3
Nsub=1e12 cm-3
Nsub=5e12 cm-3
Nsub=1e13 cm-3
1ns
10ns
Cur
rent
(A
)C
urre
nt (
A)
In the worst casethe induced currentpeak is within 10ns
For a hit 5m from theelectrode the peak is within 1ns.
G.-F. Dalla Betta SCIPP 16 sept. 2005
TCAD Simulations TCAD Simulations – – critical points evidencedcritical points evidenced
• Long low tail in the current signal due to hole diffusion. Simulation have shown that this effect can be strongly minimized using passing-through electrodes.
• High electric field at the bottom of the hole. Anyway we should not reach critical values because of the low bias voltages needed.
• In case of p-spray isolation, high electric field at the column/p-spray junction. Again, may be of concern depending on the bias voltage needed.
G.-F. Dalla Betta SCIPP 16 sept. 2005
Technological tests - lithographyTechnological tests - lithography
as-deposited: Photoresist tends to penetrate inside the hole forming a thicker layer in these region
after exposure and development: residual photoresist in the hole region
Important to note that resist can be defined in the proximity of the hole.
The resist trapped in the hole is removed after the process
G.-F. Dalla Betta SCIPP 16 sept. 2005
Technological tests - depositionTechnological tests - deposition
Surface
Top botton
Poly 1m 0.8m 0.7m
TEOS 1m 0.7m 0.6m
Poly and oxide deposition
top
bottom
hole
holesurface
oxide
poly
poly
oxide
G.-F. Dalla Betta SCIPP 16 sept. 2005
Technological tests – oxide growthTechnological tests – oxide growth
Oxide Growth (500nm) in a 5m diameter hole
metal
oxide
oxide
hole
top bottom
G.-F. Dalla Betta SCIPP 16 sept. 2005
hole
Aluminum does not penetrate inside the hole
contacts have to be made on the wafer surface
aluminum
Technological tests – metal sputteringTechnological tests – metal sputtering
G.-F. Dalla Betta SCIPP 16 sept. 2005
initial oxide p+-doping of back Isolation: p-stop or p-spray masking for deep- RIE process
deep-RIE(CNM, Barcelona-Spain)
Fabrication process (1)
oxidep-stop
p+ doping
holes
G.-F. Dalla Betta SCIPP 16 sept. 2005
P-diffusion
(no hole filling)
Oxidation (hole passivation) Opening contacts Metal and sintering
Fabrication process (2)
P- diffused regions
Hole passivation
contacts
metal
G.-F. Dalla Betta SCIPP 16 sept. 2005
Mask layoutMask layout
Small version of strip detectors
Planar and 3D test structures
“Low density layout”to increase mechanicalrobustness of the wafer
“Large” strip-like detectors
G.-F. Dalla Betta SCIPP 16 sept. 2005
Mask Layout-Test structures
3D-Diode
Pitch 80 µm
10x10 matrixØ hole 10
µm44 holes GR
Standard (planar) test structures
G.-F. Dalla Betta SCIPP 16 sept. 2005
Mask layout - strip detectors layout - strip detectors
metal
p-stop
hole
Contact opening n+
•AC and DC coupling•Inter-columns pitch 80-100 m•Two different p-stop layouts•Holes Ø 6 or 10 m
Inner guard ring (bias line)
G.-F. Dalla Betta SCIPP 16 sept. 2005
Fabrication run:main characteristics
Substrate: Si High Resistivity, p-type, <100> FZ (500 m) >5.0 kcm
Cz (300m) >1.8 kcm
Wafers with lower resistivity are not easy to find
Surface isolation: p-stop p-spray
G.-F. Dalla Betta SCIPP 16 sept. 2005
SEM micrographs
Portion of a strip detector
Top-side of a column
strip
Column-section
100 m
6 m
metal
oxide
G.-F. Dalla Betta SCIPP 16 sept. 2005
Electrical Characterization (1)
Parameter Unitp-spray p-stop
Nd [1E12 cm-3]Vdep [V]Ileak [nA/cm2]Vbreak [V] 60 - 140 155 - 175Tox [nm] 570 - 585 860 - 875Nox [1E10 cm-2] 9.5 - 11 6 - 9.6So [cm/s] 1.3 - 1.7 7 - 7.5
typical range
1 - 3.5200 - 500
1 - 20
Different sub-typesand thicknesses
2% to 13% variationon single wafer
Ileak measured below full depletiondue to breakdown
Standard (planar) test structures
electrical parameters compatible with standard planar processes
DRIE does not endanger device performances
G.-F. Dalla Betta SCIPP 16 sept. 2005
1.0E-12
1.0E-11
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
0 20 40 60 80 100Vbias [V]
I lea
k [A
]
1.0E-12
1.0E-11
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
0 20 40 60 80 100
Vbias [V]
I lea
k [A
]
Electrical Characterization (2)IV measurements
3D-Diode
P-stop
P-stop
P-sprayP-spray
Ileak/column @ 20V 0.67 ±0.12 pA
Ileak/column @ 20V 0.33 ±0.24 pA
CZ FZ
diode
─ guard ring
10x10 matrixØ hole 10 µmPitch 80 µm
Active area ~0.64 mm2
diode
─ guard ring
G.-F. Dalla Betta SCIPP 16 sept. 2005
0
5
10
15
20
25
30
0 5 10 15 20 25 30 35 40 45 50 >50
I bias line [nA]
Det
ecto
rs c
ou
nt
Electrical Characterization (3)Strip detectors
Current distribution @ 40V of 70 different devices
1detector:230 columns x 64
strips on 1 cm2
~ 15000 columns
average current per column < 1pA
Good process yield
FZ
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
0 50 100 150 200 250
p-spray
p-stop
I BL I GR
Reverse Voltage [V]
Lea
kag
e C
urr
ent
[A]
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
0 50 100 150 200 250
p-spray
p-stop
I BL I GR
Reverse Voltage [V]
Lea
kag
e C
urr
ent
[A]
G.-F. Dalla Betta SCIPP 16 sept. 2005
Conclusions
A new type of 3D detector has been conceived whichleads to a significant simplification of the process: hole etching performed only once no hole filling no wafer bonding
First production is completed: Good electrical parameters (DRIE does not endanger device performance) Low leakage currents < 1pA/column and BD ~ 50V for p-spray and >100V for p-stop in 3D diodes Good yield for strip detectors (Current/hole < 1pA/column for 93% of detectors)
Accurate analysis of CV measurement results is in progress with the aid of TCAD simulations …
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 2 4 6 8 10|Vback| [V]
Cin
t [p
F]
Lateral depletion-voltageLateral depletion-voltagePreliminary “3d-diode”/GR capacitance measurements
Cint-Vback
Weak capacitive coupling at very low voltages (conductive substrate layer between columns)
~ 5V lateral full depletion voltage (100µm pitch)
Vback
Capacitive coupling between 40 columns (100um=pitch, 150um depth)
Cint
Backplane full-depletion-voltageBackplane full-depletion-voltage
Preliminary “3d-diode”/back capacitance measurements
0
2
4
6
8
10
12
0 10 20 30 40 50 60Vrev [V]
Cd
iod
e [p
F]
D4_4
0.00
0.50
1.00
1.50
2.00
2.50
0 10 20 30 40 50 60Vrev [V]
C-2
[p
F-2
]
C-V
1/C2-V
Lateral depletion contribution to measured capacitance at low voltages
Linear 1/C2 vs V region corresponding to the same doping level of planar diodes
Saturation capacitance corresponding to a depleted width of ~ 150µm) Column depth ~ 150µm
~ 40V full depletion voltage (300µm wafer)
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