Schottky Diode RF-Detector and
Focused Ion Beam Post-Processing
MURI Annual Review
Woochul Jeon,
Todd Firestone, John Rodgers & John Melngailis
University of Maryland.
(consultations with Jake Baker Boise State &
Michael Gaitan NIST)
Outline
• Operation and characteristics of Schottky power
detector
• Mask layout for Schottky diodes
• Fabricated Schottky diodes with n+ substrate with n-epi
layer on top
• Schottky diodes by CMOS process
• RF radiation test
• Schottky diodes by using Focused ion beam technology
• Schottky diodes designed for MOSIS standard CMOS
process
• Conclusion and future work
Original Project Objectives:
- Direct analog microwave level measurement on a chip using
a) Schottky diodes
b) Thermal detectors
- Incorporation of RF detectors on chips, including
FIB diode fabrication on existing chips
- Focused ion beam diagnosis circuit restructuring and
device diagnosis by burned out element sectioning
Changes to Objectives:
- Thermal detectors not pursued
Example of FIB Circuit Rewiring:
Cut and Jumper
FIB-Milled Circuit Cross Section
Operation of RF Power Detector
50pF
1kΩ
+
-
RF
Output
100ns
RF input
DC
Key factors limiting maximum frequency
• Junction capacitance
Cj = A
ε qN d
2 (V a + V d )
Î A: contact area, Nd: doping concentration
Va: applied voltage, Vd: built in voltage
Rj
• Junction resistance:
dI x
1
=
= AI
Rj
dV a
s
q
 qV a 
exp 

nkT
nkT


Rn
Cj
Rn+
• Series resistance = Rn + Rn+ , Rn=Ro/A + R1/A1/2
• Equivalent circuit
Î Rn >> Rn+ ( >> Rj , after turning on)
Î Series resistance mainly determined by Rn(n layer resistance).
• RC time constant ∝ A1/2(due to spreading resistance), Rn(Nd), etc.
• Objective Î Reduce junction capacitance(Cj) => decrease contact area
Reduce series resistance => minimize n layer thickness
Schottky diode fabricated in MOSIS
V. Milanovic, M. Gaitan, J.C. Marshall M. E. Zaghloul,
IEEE Trans. Electr. Devices 43, 2210 (Dec. 1996)
Coplanar Schottky Diode
Developed for Rectifying
Antennas
K.M. Strohm, J. Buecher, & E. Kasper ,
Daimler Benz Research, Ulm
IEEE Trans. MTT Vol.46, 669, (May, 1998)
Proposed structure using n+
substrate with n-epi layer on top
• Reduce series resistance => use n+ substrate
• Reduce contact capacitance => decrease contact area
2 µm
n-epi (0.3 µm )
n+
Ohmic contact
Schottky contact
Resistivity Ω-cm
Resistivity vs. Depth of n on n+ Layer
Depth µm
Mask layout
9 mm
8.5 mm
Schottky diode with clock tree
600µm
Schottky diode with 150µm pitch pads
G
4 – 25 µm
Schottky diode
150µm
S 130µm
100µm
G
24 – 50 µm
Schottky Diode layout
A
Ohmic contact
2µm
Metal
(Al)
Schottky Contact
n+
A’
SiO2
Etch SiO2 and 0.3um n silicon layer with n-high mask
(SiO2: wet etching, n-Si: RIE)
n+
Etch SiO2(wet etching) with n-low mask
n+
Deposit 0.5um Al with E-Beam evaporator
n+
Etch Al(wet etching) with metal mask
n+
Measured result I (DC)
• DC Characteristics(2µm x 2µm by RIE, I-V curve)
6
2500
5
2000
4
1500
Iout
Io u t(u A)
3
2
1000
500
1
0
0
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5
1
-1
-500
Vin
ÎExponential change of
contact resistance
Rj >> Rs
Vin
Îlinear series resistance
Rs >> Rj
Measured result II (RF)
• 2µm x 2µm contact area diodes are tested.
Î These diodes worked at the power level from -10 dBm to 10 dBm
Î DC output was linearly changed by changing power level.
Î Observed diode response up to 5GHz
Î These diodes could detect RF power level, but because of the direct
capacitance connection between anode and cathode of diode, the
output DC voltage substantially depended on frequency.
This huge capacitance (1.2 pF by calculation) comes from pad
structure for connecting diode to RF source.
Al-Si Pad
SiO2
n
n+
1.2pF
Equivalent circuit
Rj
Î
Cj
Rs
Rj: Junction Resistance
Co Cj: Junction capacitance
Rs: Series resistance (Rsn + Rs n+ )
Co : Overlay capacitance between
Al-Si pad and n+ layer
• Overlay capacitance gives direct path between anode
and cathode of Schottky diode.
Schottky diode Design for
CMOS process
Î To remove the effect of Co, different substrate which has
higher resistivity rather than n+ substrate should be used.
Î Design new diode structure to minimize series resistance of
n layer without using Silicon Molecular Beam Epitaxy(Si-MBE)
Î Minimize contact area
Schottky Contact
SiO2
Ohmic Contact
Al
2um
Al
n+
n - Substrate
Measured result (DC)
2x2 patch I-V
30
25
• Voltage
range: -5V ~ 5V.
(From –5V to 0V , current
output was 0)
Iout(mA)
20
15
10
• Series resistance(between
4V and 5V) ≅ 83Ω
5
4
4.5
Vin(V)
3.5
3
2.5
2
1.5
1
0.5
0
0
5
RF direct injection test
(50µm x 50 µm contact area)
RF input:
Cascade
probe
G
S
G
DC
output
Output voltage(V)
DC output vs RF Power level
4
3
2
1
0
0
5
10
15
RF power(dBm)
20
25
RF direct injection test
(2µm x 2 µm contact area)
DC output vs. Power level
Output Voltage [V]
Output voltage(V)
DC output vs. Frequency
2.5
2
1.5
1
0.5
0
1
3
5
7
9
Frequency(GHz)
Î Flat response at high frequency range
2
1.5
5 GHZ
6 GHZ
1
0.5
0
0
5
10
Power [dBm]
15
20
RF direct injection test
(50µm x 50 µm contact area)
10
Rectified Voltage (V)
Diode size 50X50 microns
1
0.1
0.01
1
10
Frequency (GHz)
100
Output Voltage Pulse in Response to 20 GHz. RF Burst
0.05
Rectified
Voltage
Output (V)
0.04
0.03
20 GHz RF Pulse
Envelope
0.02
0.01
0
-0.01
0
1
2
3
Time (µsec)
4
5
Output (V)
Output Voltage Pulse in Response to 2GHz.RF Burst
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1
Rectified
Voltage
2GHz RF Pulse
Envelope
0
1
2
3
Time (µ sec)
4
5
RF radiation test on a patch antenna structure
12cm x 10cm x 6cm size box is used for radiation test
RF radiation
Horn structure
• Top view
Ground plane
Output port
6 cm
12 cm
10 cm Bonding to ground plane
Patch antenna
RF radiation test result
Frequency vs. DC output (RF power = 40dBm)
Rf_in vs. Vdc_out (frequency = 12 GHz)
120
DC output(m V)
80
60
40
20
18.8
17.6
14.6
12.6
6.6
2.8
1.8 1.8
0
0.4
2
1.4
0
0.4
1
0
0.8 0.8
0.4
0
RF radiation input(dBm)
49
45
47
41
43
37
39
33
35
29
31
0
25
27
DC output(mV)
100
20
18
16
14
12
10
8
6
4
2
0
Frequency(GHz)
Roll-off frequency ≅ 12 GHz
0
0
Fabricating Schottky diodes by FIB
Measured result(IV curve)
SiO2
Al
n+
3000
n substrate
2500
FIB milling
2000
Iout ( uA )
Al
n
1500
1000
FIB Metal(Pt) deposition
500
Vin ( V )
2
1. 6
1. 2
0. 8
0. 4
0
.4
-0
.8
-0
.2
-1
-500
-1
Schottky contact n
.6
0
-2
Al
Fabrication of Schottky diode by FIB
N+ doped area
Al
SiO2
FIB Pt deposition
FIB milling (2µm x 3 µm)
RF direct injection test of FIB diode
FIB power sweep at 8GHz and 10GHz
10
Vout vs. Frequency at 15dBm RF power
2.5
8GHz
10GHz
1.5
1
Vout (V)
Vout (V)
2
1
0.5
0.1
0
1
3
5
7
9
11
13
15
17
Frequency(GHz)
Vout vs. Frequency sweep
19
21
0
5
10
15
20
Injected RF power(dBm)
Vout vs. RF power sweep
25
Schottky diodes with capacitor load and MOSFET amp
for amplifying small output signal
One Schottky diode
Ohmic contact
150 µm
100 µm
Schottky contact
Contact area:
2 µm x 2 µm - 40 µm x 40 µm
Fabricating Schottky diode by FIB
(Future work)
FIB milling
Al
SiO2
n+
Al
Al
Î
n
n
FIB milling(0.1-0.5 µm)
and Metal deposition
Al
Al
FIB SiO2 deposition
Al
Al
Al
Í
n
Schottky contact
n
FIB Tungsten Vias
Through FIB Deposited Oxide Plugs
500nm
Summary
• Schottky diodes on n-epi and n+ substrate were
•
•
•
•
fabricated and tested
CMOS process Schottky diodes were designed,
fabricated and tested with RF radiation up to
12.5GHz (50X higher than previous CMOS result)
and by direct injection up to 20GHz
Schottky diodes were fabricated by FIB techniques
and tested up to 17.5 GHz
Various Schottky diodes have been designed and
submitted to MOSIS for standard CMOS processing
Paper will be presented at the 2003 International
Semiconductor Device Research Symp. in DC
Future work
• MOSIS chips now being built will be tested by
RF radiation and direct injection
• Post processing MOSIS chips for FIB diodes
• Diodes with in-situ amplifiers on chip
• Diodes with built in DC bias will be designed
for MOSIS and built
• Diodes will be incorporated into test chips
designed by colleagues to verify various
RF propagation models
Understand what limits frequency
& push toward 100GHz without MBE