Developing Reliable Device Simulation Models using ADS

Slide 1
Developing Reliable
Device Simulation Models
using ADS
ADS User Meeting Böblingen, 14.05.2009
ADS User Meeting Böblingen
May 2009
Dr. Thomas Gneiting
AdMOS GmbH
Frickenhausen
thomas.gneiting@admos.de
Dr. Franz Sischka
Agilent Technologies
Böblingen
franz.sischka@agilent.com
1
(C) Bildarchiv der Stadt Böblingen
Slide 2
Developing reliable device simulation models using ADS
AGENDA
- The difference between selecting a model and extracting
model parameters.
- Trusted, verified measurements.
- contact resistance
- self-heating effects
- linear S-parameter
- de-embedding
- Example of a device modeling sequence
2
Slide 3
Which model
to select ?
Semiconductor Device Models (Nonlinear Models):
- BJT Models: HICUM 2, HICUM 0, MEXTRAM, VBIC, Gummel-Poon, ST BJT,
Agilent EEBJT2
- Diode Models: PN diodes, Philips JUNCAP, Agilent Root, PIN
- MESFET/HEMT Models: TOM3, Angelov, Agilent EEFET3 and EEHEMT1,
Agilent Root MESFET/HEMT (HPFET), TOM scaleable, Curtice quadratic
and cubic, Advanced Curtice, Statz et al. (Raytheon), Materka, Modified
Materka (Rizzoli), Tajima
- MOSFET Models: BSIM3, BSIM4, BSIM3SOI, BSIM4SOI, PSP, HiSIM2,
HiSIM-HV, Philips MOS Model 11, Philips MOS Model 30, Philips MOS,
Model 9, Agilent Root MOSFET, Agilent EEMOS1, BSIM1, BSIM2,
MOS Level 1,2,3
3
There are many models available, even for the same device !
Slide 4
Which model is better ?
Model A
?
Model B
Only reliable measurements
provide good simulation models
4
You may think of using rather model_A than model_B.
However, as we will show with the following slide set, the main topic is to perform reliable, verified
measurements. This is the very base of any good model implemented in a design kit of a manufacturer.
Of course, some models feature equations to better fit a certain effect (thermal self-heating, non-linear
large-signal RF behavior).
But also here applies the statement of above: if the measurements are not available, the ‘better’ model
does not help.
And if the measurements are not ‘qualified and verified’, the ‘better’ mdoel does not help too.
Slide 5
iD
What is a model ?
RS
RS =
∂(v D − v D int )
∂iD
Ohmic Losses
DC
space
charge
vD
diffusion
capacitance
vDint
Model equations
 v D int

iD = IS ⋅  e N* vt − 1


DC
vt = 8.6171E - 5 ⋅ (TEMP + 273.15 )
Cs =
C JO
 v D int 
1 −

VJ 

Junction Cap
M
CD = TT ⋅
∂iD
1
= TT ⋅
⋅ iD
∂v D int
N ⋅ vt
Diff.Cap (Transit Time)
Is
... Model Parameter
5
A set of equations describing the principal electrical behavior of a device represents what is commonly
called a device model.
NOTE: A ‘SPICE simulation model’ is a name for a model available in UCB Spice (University of
California, Berkeley), and thus typically available in all of today’s simulators.
For a real device, only the model parameters like e.g. IS, NF are adjusted to get a simulation results
close to the real behavior.
Additionally, the instance parameters (e.g. L, W) describe the geometry and structure of a device.
If no model parameters or instance parameters are specified, the simulator takes default parameter
values. They shall prevent the simulator from numeric errors (e.g. divide by zero) and represent an
ideal device (the specific model effect is ‘switched-off’).
Caution: Default parameter are in most cases DC bias and frequency independent!!
Slide 6
What is model parameter extraction ?
Plot diode_basics/DC/forward/ia_va
1E-1
1E-2
 vD

 N* vt

iD = IS ⋅  e
− 1




1E-3
1E-4
1E-5
1E-6
Fitting the model
equation to the
measured data
by varying the model
parameters.
ia.m [LOG]
1E-7
1E-8
1 decade
1E-9
2.3*N*vt
1E-10
1E-11
1E-12
1E-13
1E-14
1E-15
0.0
Is
0.2
0.4
0.6
va [E+0]
0.8
1.0
NOTE: The model
equation specifies the
measurement setup
(what to measure)!
6
Slide 7
Model Parameters and Instance Parameters
IMPORTANT NOTE:
when not specified, the instance and model parameter values used in the simulation will be the ‘default values‘,
i.e. the effect is switched off (CDS=0F, RTH=0Ω
Ω , VAF=infinite), or the simulation is standardized (L=1).
SA, SB
Device Instance
(L, W, geometry, lithography, size of the production masks)
L
W
Device Model
(typically process-dependent Model Parameters: VTH, U0, CDS etc.)
7
Slide 8
Developing reliable device simulation models using ADS
AGENDA
- The difference between selecting a model and extracting
model parameters.
- Trusted, verified measurements.
- contact resistance
- self-heating effects
- linear S-parameter
- de-embedding
- Example of a device modeling sequence
8
Slide 9
Contact resistance of on-wafer measurements
IC-CAP/ADS
(C) Cascade Microtech
9
One of the most important preconsiderations regarding accurate device modeling is to characterize the
contact resistance of the probes to the contact pads on the wafer. This is especially important for silicon
processes, where the on-wafer contacts are made from aluminum (III/IV processes use typically gold).
When not considering these contact resistances, their effect will be included in the ohmic parameters of
the model, (they will become too big). This will badly affect any chip design.
Slide 10
The effect of contact resistance
including
Rcontact
id.M id
id without
simulatedRcontact
without Rc ididwith
simulated
Rc [E-3] [1E-3]
Plot ADS_simul_types/DC_mdlg/idvd/idvsvd
50
Behavior of the
final model in design kit
= real device behavior !
(without contact resistance)
40
30
20
10
Drain
0
0.0
0.5
1.0
1.5
Rcontact2
2.0
vd [E+0]
IMPORTANT NOTE:
Including the contact
resistance is mandatory during
the modeling process
Measurement and
Simulation
including contact
resistance
Gate
MOSFET1
Rcontact1
10
NOTE: do not confuse these measurement-related contact resistances with the device‘s inner, ohmic
model parameters (e.g. RD, RS etc.)
Slide 11
Self-heating with DC measurements
Silicon Bipolar Transistor on wafer
Ic (mA)
Measurements
at various pulse
widths
infinite
4 µsec
2 µsec
1 µsec
Only a few models can
handle self-heating:
MOS:
BSIMSOI3, BSIMSOI4,
HiSIM-HV
BJT:
VBIC, HiCUM
MESFET/HEMT:
Angelov, ...
Model Parameters
to model the self-heating:
RTH, CTH
pulse period: 1msec
VCE (V)
Hint: 50mA device current: beginning of self-heating
11
As a general observation, currents above ~25mA will cause self-heating effects with transistors.
Rule of thumb: 25mA with typically 2V equals already 50mΩ for that tiny transistor on the wafer!
Self-heating can only be avoided when applying pulsed DC measurements. However, the max. duration
for such a DC pulse is 100ns, followed by typically 1ms wait time.
If you have to live with self-heating, make sure to have repeatable measurements for the DC Settings
(currents for the transfer and for the output characteristics must be identical for identical bias
conditions !!). Furthermore, the self-heating during the (very often fast measured (!)) DC
measurements must be the same as with the (slowly performed) S-parameter measurements !!
-> measure your DC curves as slowly as later the DC-biased S-parameters.
Slide 12
S-parameter measurements
Transistor and Diode S-Parameter
measurements are performed
including a DC bias.
Due to the nonlinear transistor
behavior, a too big AC amplitude
results in a distorted output signal.
These distortions will shift the DC
operating point !
iD
DC oper.point shifted by too big an RF signal
DC oper.point w/o RF signal
vGS
12
The energy for the harmonics of a non-linear behavior is provided
- by the DC bias
- by the applied RF signal.
In any case, when harmonics occur, the DC bias is affected. The currents provided by the DC power
supply will become a function of the distortion of the RF signals.
Slide 13
Find Out The Max. Applicable RF Signal to obtain
linear S-parameters
Simultaneous measurement of DC behavior
with overlaid S-parameter signal
If the RF signal is small enough,
it does not disturb the DC
biasing of the S-parameter
measurements !
-20dBm: RF signal disturbs DC traces
-30dBm: correct RF signal level for this transistor
13
From the plot above, we can determine 2 important things for obtaining linear S-parameters:
the max. applicable RF signal level for the S-parameter measurements for a given DC operating point,
or, inversely, the minimum applicable DC bias for a given RF signal.
Slide 14
NWA Calibration, applpying the identified RF power
After the standard calibration,
the calibration plane is at the end of the GSG probes.
Probes and calibration substrate must match !
ISS calibration substrate
Cal.Plane
14
Slide 15
S=
B
NWA Cal Plane
G
S=
B
D
Unfortunately, our transistor is not directly
at the probe contact location …
15
In the above slide, in the magnified view, is the inner transistor, which we want to characterize. All the
rest has to be de-embedded, i.e. to be stripped-off.
In other words, the NWA calibration plane, obtained by the NWA calibration so far, has to be shifted
from the GSG probe contact location down to the beginning of the device.
Slide 16
De-embedding:
Shifting NWA Calibration Plane vs. DUT
eliminate
parasitic
effects
Reference
plane of
NWA por t 1
Ground
Ground
Signal
Signal
Ground
Ground
Reference
plane of
NWA por t 1
through deem bedding
Source + Substrat
Gate
Drain
Source + Substrat
Ground
Ground
The simulation model shall only
describe the behavior of the DUT
without any influence of the
surrounding pads
-> The influence of the parasitic
components between the NWA
calibration plane and the
transistor is eliminated by
de-embedding.
Signal
Signal
Ground
Ground
Reference plane of
cir cuit library elem ent
(C) www.ihp-microelectronics.com
16
Slide 17
De-embedding: Measurement Data
after de-embedding
measured data of device and pad
17
The two diagrams demonstrate the effect of the de-embedding.
The left one shows the input and output reflection S11 and S22 before and after the de-embedding.
Looking at S22, the difference in both, phase and magnitude can be seen very clearly.
A similar behavior can be observed in the transmission behavior. The original measured data of the
forward transmission S21 clearly has an increased phase shift and a decreased magnitude with
increasing frequency, compared to the de-embedded data.
As a summary, it is absolutely necessary to perform a correct de-embedding to get the real transistor
data as a base for accurate modeling.
Slide 18
De-embedding: Principal Teststructure Layouts
OPEN
SHORT
OPEN and
SHORT are
a must for
de-embedding
THRU
The THRU device
is for verifying the
de-embedding quality:
-> After de-embedding
from OPEN and SHORT,
the THRU S-parameters
must represent a short
transmission line.
These facts should be widely
known and are explained in
many publications.
However, we still see many
cases, where de-embedding
cannot be done correctly due
to bad de-embedding
structures !!
(C) www.ihp-microelectronics.com
18
The pre requisite for a correct de-embedding is that certain test structures are available on a wafer
together with the device under test (DUT) itself.
Depending on the selected de-embedding method, an OPEN, a SHORT and a THROUGH dummy pad
structure must be available and must be measured.
The principle layout of these structures are given above.
RECOMMENDATION: the on-wafer THROUGH is an ideal, golden de-embedding verification
device. Compared to a diode or transistor, the de-embedded THRU is
- DC bias independent
- not suffering from self-heating
- not susceptible to large RF signals etc.
In other words, when the THRU, de-embedded from the OPEN or the OPEN/SHORT represents a
short strip line, with its characteristic appearance in both, the Smith Chart (S11, S22) and the Polar
Diagram (S21, S12), it can be assumed that when replacing that little piece of aluminum by the diode
or the transistor, that the de-embedding will work OK also for these very devices.
Slide 19
Wrap-up:
De-embedding, the step after NWA calibration
The measured S-parameters
of the DUT describe the behavior
of the device plus the pads
measuredDUT
G
G
S
S
G
G
The DUT to-be-modeled
is without the pads
de-embedded DUT
the DUT itself (!)
Layout screen shots:
(C) www.ihp-microelectronics.com
19
Slide 20
Device Modeling:
The extrapolated starting points of the S-par (0Hz)
are determined by the DC fitting !!
If this starting point check fails,
verify:
-> too much RF signal
-> self-heating
-> voltage drop in S-par testset
-> accurate DC contact resistance
-> DC modeling was performed at
different bias range than the S-par
modeling !
measured Spar
fmin=100MHz
DC data converted to S-parameter, f=0Hz
20
Since S-parameter modeling means (black-and-white) characterizing ‘the speed’ from going from the
DC biasing to freq->infinite, the capacitors and the transit time are the only parameters to adjust. And
these capacitors and the transit time, although bias dependent, describe ‘just’ the phase(freq) and
magnitudes(freq) of the S-parameters with increasing frequency.
-> The extrapolated 0Hz starting points are 100% determined by the DC fitting !
Slide 21
Developing reliable device simulation models using ADS
AGENDA
- The difference between selecting a model and extracting
model parameters.
- Trusted, verified measurements.
- contact resistance
- self-heating effects
- linear S-parameter
- de-embedding
- Example of a device modeling sequence
21
Slide 22
Typical Device Modeling Flow
Test model
- in dedicated simulators
- for specified operating conditions
- outside specifications (model robustness)
Perform all
necessary
measurements
Documentation
Quality Assurance
Generate a
final report with
a comparison
measurement /
simulation
Statistical Modeling
Parameter
Extraction
Measurements
for Modeling
Parametric Tests
topics discussed
in this presentation
Extract model
parameters
Adjust model to
process targets.
Adding corner cases
and statistical
distributions
- Golden Die selection
- Mismatch & statistical analysis
- Process stability
22
Slide 23
IC-CAP: Dedicated Software to Support Modeling
• Parameter extraction for state-of-the-art CMOS processes (e.g. 65nm)
is a highly complex task.
• To fulfill all requirements with respect
to accuracy and
statistical behavior
dedicated software
is a must.
• The screenshot
shows the PSP
Modeling Package
of IC-CAP
23
Slide 24
Examples of Parameter Extractions (RF CMOS)
• The following slides show different examples how
parameters are adjusted.
• A typical RF CMOS process with a minimum feature
length of 130nm was taken as a test vehicle.
• We demonstrate from which characteristics certain model
parameters are extracted.
• In the same simulation setup, the influence of the
extracted parameter to other regions of operation are
visualized.
• For this purpose, the following measurements/simulations
are taken into account:
– DC, S-parameter, Harmonic Balance, Minimum Noise Figure
24
Slide 25
Influence of Contact Resistance on DC, S21, ...
Simplified MOS model
Rd
Gate
Rcontact1
Drain
Rgate
Cgd
gm
Rearly
Rcontact2
Cgs
Rs
25
Slide 26
Influence of Contact Resistance on DC, S21, ...
Rcontact:
3Ω
(=correct value)
10Ω
Extraction:
From DC
Measurements
of an on-wafer
SHORT dummy
26
Slide 27
Influence of Gate Resistance on S11, NFMIN
Simplified MOS model
Rd
Gate
Rcontact1
Drain
Rgate
Cgd
gm
Rearly
Rcontact2
Cgs
Rs
27
Slide 28
Influence of Gate Resistance on S11, NFMIN
Rshg:
4.45Ω/
(=correct value)
10Ω/
Extraction:
From S11, Y11
NOTE:
Rshg is the
Gate Sheet Resistor.
28
Slide 29
Influence of DC Parameter
to Id-Vd, gds, S21, S22, Pout, NFmin
Simplified MOS model
Rd
Gate
Rcontact1
Drain
Rgate
Cgd
gm
Rearly
Rcontact2
Cgs
Rs
PCLM 0.3 .. 1.3
29
Slide 30
Influence of DC Parameter
to Id-Vd, gds, S21, S22, Pout, NFmin
PCLM:
1.3
(=correct value)
0.1
Extraction from:
Id-Vd, gds
NOTE:
PCLM is a DC
related parameter
(Early voltage).
It affects all regions
of operations !
30
Slide 31
Influence of Capacitance Parameter
Simplified MOS model
Rd
Gate
Rcontact1
Drain
Rgate
Cgd
gm
Rearly
Rcontact2
Cgs
Rs
31
Slide 32
Influence of Capacitance Parameter
CGDL:
493pF/m
(=correct value)
5nF/m
Extraction from:
S12 and CV
32
Slide 33
Modeling Services
CMOS Technology
Design libraries for CMOS and other
technologies:
- Advanced Processes down to 45nm
- RF CMOS
- High voltage devices
- Silicon on Insulator
- Packaged devices
Passive Devices
Simulation models for:
- On-chip passives (inductors, ...)
- PCB elements for high speed circuits
- Connectors
- IC packages
The principal services and products of Adanced Modeling Solutions are in the area of device modeling
and connector design (signal integrity).
Slide 34
AdMOS Company Information
•
•
AdMOS GmbH
Advanced Modeling Solutions
In den Gernaeckern 8
D-72636 Frickenhausen/Germany
Phone: +49 (7025) 911698-0
Fax:
+49 (7025) 911698-99
•
email: info@admos.de
http://www.admos.de
•
AdMOS was founded in 1997 by
Dr. Thomas Gneiting.
AdMOS is focused on:
– Software development of tools
for model parameter extraction
of CMOS and other devices.
–
Modeling and simulation service
for complex devices and systems.
–
Engineering service for design and test
of RF and high speed components.
Actually, we employ 5 highly qualified engineers
for our modeling service
and software development activities.
Due to our ongoing expansion, we moved to a
new office building in December 2007.
34
Advanced Modeling Solutions is a young startup company, which focuses on the aspects of device
modeling for circuit simulation. It was founded in 1997 by Dr. Thomas Gneiting.
Advanced Modeling Solutions is located pretty close to Stuttgart, which is well known for its famous
car manufacturers and a lot of high tech industry.
Slide 35
Developing reliable device simulation models using ADS
Wrap-Up
- The difference between selecting a model
and extracting model parameters.
- Trusted, verified measurements.
- contact resistance
- self-heating effects
- linear S-parameter
- de-embedding
- Example of a device modeling sequence
35