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Good morning. My name is Rick Poore from Agilent Technologies and I'm here to talk about an
integrated electrothermal solution that delivers thermally aware circuit simulation.
What's the problem with today's RF and microwave designs? Heat and temperature. Electrical
power in a chip is dissipated and turns into heat heating up the devices. The electrical
outputs of the components change over temperature. Designers have to worry about a maximum safe
temperature for reliability purposes; so accurate simulations results require knowing the correct
temperature of every device in the design.
Several contributors to temperature in the circuit: Each device creates its own power
and heats itself up, but there's also coupling between devices. A device dissipating a lot
of power will spread its heat out and heat up the neighboring components around it; that
needs to be considered in your design. Self-heating models make it very difficult to account for
this sort of thermal coupling.
Heat transfer occurs from the die out to the package and then out to the board level; we
need to simulate that entire flow. And it's a nonlinear problem. The semiconductor thermal
properties -- the thermal conductivity, the heat capacity -- are functions of the temperature
itself so we have to do some nonlinear modeling here and that requires iteration.
And obviously the device electrical properties, transistors and FETs; their performance is
a function of temperature as well.
So in the past the traditional way of doing this -- one way was to use one of the general
purpose multi-physics tools. These are great for analyzing the thermal properties of an
integrated circuit for analyzing the airflow in the convention center, for simulating a
jet aircraft engine; they're very general purpose. But that also makes them very difficult
to use for a very specific application like IC Design. The user has to do a lot of manual
transfer of data back and forth between the two environments.
The other approach has been to use self-heating models and to build some sort of equivalent
circuit network to model the thermal transfer between devices on a chip. Every time you
change your layout you have to go and re-extract this, it's very difficult to extract a large
network for a very large chip.
So we're offering a new approach here. We start with a circuit schematic and with a
layout at the chip level -- at the package -- and some technology files. We take these
into a circuit simulator and a thermal simulator. The circuit simulator converges -- does a
normal simulation -- and computes the power dissipation of every device.
The thermal simulator takes the layouts, reads the power dissipation, creates the heat source
and solves the full temperature problem over the entire system. It then writes out the
temperatures, sends them back to the circuit simulator. The circuit simulator will read
the temperatures, update the device models, simulate again and write out a new set of
powers. And the iterate back and forth; power, temperature, automatically iterating and stopping
when the powers and the temperatures stop changing and we've reached a self-consistent
solution.
At that point then we have the full results from the circuit simulator and the results
from the thermal simulator.
To do this we're working with a proven thermal solver. We're working with a company called
Gradient Design Automation, who's providing the thermal engine for this; a company that's
been focused on IC-level thermal simulation for the last few years. They have a full chip,
high-capacity, 3-dimensional, finite element thermal simulator. This works down to the
device level; we can model the heat sources in the individual fingers and cells of each
individual transistor. We also model the heat flow through the individual wires, all the
metallization in the chip and in the package.
It's been proven for use in large, digital mixed-signal applications in the past and
we're extending this and integrating this into the ADS environment. And it's capable
of working with any type of integrated circuit process; it's a very general-purpose simulator
focused on thermal simulation of IC's.
So the benefits of having thermally-aware circuit simulation results are we know the
temperature for each device -- so we can look for things like maximum temperatures -- reliability
groups will typically tell the designer to keep junction temperatures below a certain
level -- and this makes sure we can get that criteria met.
We can also simulate things like thermal runaway, current crowding in multi-finger drivers,
we can simulate the memory effect in the time domain that's due to thermal heating and thermal
time constants; and being able to simulate and make sure that these aren't causing us
any problems gives us better confidence in producing a good design, and we also end up
with the temperature distribution everywhere in the system.
So let's look at what goes into a simulation. We start from a design. In this design we
have a schematic view an amplifier. This amplifier has 80-plus transistors, 50 resistors, a few
other passive components. Associated with it is a full layout. And we set up a harmonic
balance situation. And we have the options to control thermal simulation. Just a few
values that need to be provided -- primarily just a simple boundary condition for thermal
resistance to the outside world -- and then we perform a regular simulation.
At this point the two simulators iterate back and forth exchanging power and temperature
information. And in the end we're left with a full set of electrical simulation results
and thermal simulation results.
It takes three iterations for this example to go back and forth. It does take a little
bit more time because we're doing a full 3-dimensional thermal simulation, but it provides information
which is not available in any other way. So it provides a whole new level of fidelity
to simulation. When we're done we have different types of
ways we can view the thermal simulation results. We can look at the flat view and look at the
temperature of the individual transistors in our driver cell. We can take a 3-dimensional
view and slice off reference planes and look at a cross-section of the temperature anywhere
in the device. We can look at the -- similar to the 2-dimensional view -- but here, besides
coloring the temperatures with color we can also mark them with height, so we can quickly
see the hotspots on the chip and see how hot things are in the driver's stage.
The other type of simulation we can do is a time-domain simulation. This is a simulation
where we looked at what happened when we made a step in the Vcc supply. So we increased
the voltage and the power dissipation goes up very quickly and responds with the electrical
signal, but then it slowly decays. And if we look at the details we can see why.
These other eight traces here are the eight individual fingers in the output driver and
we can look at the temperature of each transistor individually here. The two lowest temperatures
are the two devices on the outside that have the ability to cool themselves the most. The
two at the top are the highest temperatures -- the ones that are stuck in the middle and
find it more difficult to cool themselves off -- and what we see is the temperatures
come up but not in step response directly with the electrical result; there is a thermal
time constant which slowly heats up the devices on the order of a couple of microseconds.
And we can see that ten microseconds into the pulse the circuit's temperatures are just
starting to stabilize.
And then when turn off the Vcc -- when we lower it -- we see a similar decay. It takes
about ten microseconds for it to get back to a steady-state temperature.
So these are the sort of things we can look at with time-domain simulation using the thermal
and electrical coupling between the two.
Now if this works what we're playing here is the time-domain movie of the temperatures
of the circuit we just looked at. So from zero to 20 microseconds -- at 2 microseconds
the power goes up and the temperature slowly went up, now we're on the downside, and now
we're repeating the cycle. Temperature is going up, reaching a peak and then at 12 microseconds
we turn the voltage down and the temperatures go down. So we can get these sorts of animations
from the electrical thermal simulation.
In conclusion the ADS Electrothermal Solution is targeted at high-power RF and IC designs
-- MMIC designs -- power amplifiers are the initial target for this. We deliver thermally
aware circuit simulation results. We include all of the effects of on-chip temperature
rise; we model everything in a full 3-dimensional system. We're able to incorporate the effects
of not only the chip, but the package and the printed circuit board that it's mounted
on.
It's integrated into the ADS environment, so it's easy to set up and use. And it works
with all of the simulation types that are available from circuit simulation: DC, AC,
S-parameter, harmonic balance, transient and envelope simulation. And this is available
in the new ADS 2012 release. If you'd like more information about this product come over
to the Agilent booth; we have this on display and we can talk to you in more detail about
this.
Thank you.