using SONNET software
to go to our main CAD page
to go to our EM analysis page
This page is now officially sponsored
by SONNET Software!
New for April
2007! Here's a separate page analyzing
the loss of microstrip on silicon, a case where substrate resistivity
is not at its ideal infinite value!
We installed SONNET
on a Dell Dimension 4700 computer (low end hardware!), and waded
through the first two tutorials. We like how fast it runs, and look
forward to posting some examples that we will design ourselves (not
just stuff from their manuals!)
Note that in spite
of advanced age, the Unknown Editor is a neophyte at EM software,
a regular Luddite. So if he can
use this tool right out of the box, so can you.
analysis uses finite element math. At best it is a good approximation
of how a circuit will behave, and is more accurate than linear analysis
software. At worst, it is bad approximation, and a big waste of
a your time because it can run very slowly. The "art"
of EM analysis is knowing when you have a good approximation, and
how to take shortcuts that save time but don't corrupt the analysis.
So we will start with some very simple circuits that will help us
(and you) check the accuracy of the analysis.
In SONNET, the "finite
element" that is being analyzed zillions of times is a rectangle
that you define, which they call a "box". Choose a box
that is too big, and your approximation will be off. Choose a box
size that is too small, and you'll be waiting a long time to analyze
even the simplest structures. We'll discuss this more in Project
#1: loss of fifty-ohm line on 2-mil GaAs
We're not huge fans
of reading manuals or analyzing example files, so after an hour
of following the tutorial, we jumped right in and created our own
circuit. In this case, it was a fifty ohm line! Not that interesting
you say, unless you are curious about the loss of fifty ohm lines
up at 100 GHz, on 2-mil GaAs, due to skin depth.
Without too much
fuss, we built up the following geometry:
We really like SONNET's
cool 3D viewer, as well as the "quick start guide", which
sits on screen and prods you through each step of the analysis.
We created a 2 mil GaAs (ER=12.9) substrate, with a "cover"
height of 50 mils. We used a metal thickness of 0.15 mils, choosing
gold as the conductor (which automatically sets the conductivity
to 40900000 Siemens/meter). The line width is 1.4 mils, and length
is 160 mils (experience told us that 1.4 mils is close to fifty
ohms). Originally we set the "box size" to 0.1 mils in
both X and Y directions, but soon learned that the circuit was gonna
take all day to analyze. So we switched to 1.0 mils in X and 0.2
mils in Y, and it ran in a couple of minutes. Here are the results:
Looks like our width
of 1.4 mils was a good guess for fifty ohms (-30 dB match is good
enough for government work!) The loss at 100 GHz is -1.12 dB. Because
the circuit is well matched, we can ignore the mismatch loss, and
compute the loss of this medium at 100 GHz, in dB/inch, and dB/cm:
Loss of 2-mil GaAs 50-ohm line
at 100 GHz= -7 dB/inch, or -2.75 dB/cm
Why is there a loss of -0.1 dB
down at DC (0 Hertz)? That's because this particular fifty ohm line
has 6 milliohms/square DC sheet resistance (you can do the math
for homework!), and there are a total of 133 squares of metal. This
is certainly something you need to take into account if you want
to pass some DC current through it.
Let's check our "skin
depth rule of thumb" that says you need only five skin
depths to achieve maximum conductivity. Using our skin depth calculator,
we find that at 100 GHz, the skin depth is just under 10 microinches.
The line above was 150 microinches... what happens if we decrease
it down to 50 uin? Here's the new response:
The DC loss is has increased
to -0.2 dB, but the loss at 100 GHz is exactly the same at 1.12
dB, so 50 uin is as good as 150 uin at this frequency. Suppose you
were a cheapskate, and tried to get away with 1 skin depth at 100
GHz (10 uin). Below is the result (we had to change the right Y-axis,
it went off the page!)
The loss only increased to -1.21
dB at 100 GHz, but no microwave engineer wants to give up even a
tenth of a dB if he doesn't have to. Thus SONNET EM analysis supports
our rule that five skin depths is all you need!
Let's take a look at the loss
that is predicted by the next iteration of our RF
sheet resistance spreadsheet: for the same geometry (160 mil
50-ohm line, metal thickness 10 microinches). Looks like it is off
a bit, now that we have SONNET we can figure out what is the problem!
#2: millimeterwave terminations
While we are on
2-mil GaAs, we decided to look at "fan-style" terminations.
An ideal termination acts like a simple shunt 50 ohm resistor, the
center of the Smith chart. Using via
holes for ground adds unwanted series reactance due to via
inductance which increases with frequency. An open stub can
achieve a "perfect short" over a moderate bandwidth, but
no one trusts linear models (such as ADS or Eagleware) at millimeterwave
frequencies to get the exact dimensions. So we used SONNET to analyze
some fifty-ohm termination structures.
The first thing
we had to do was come up with a SONNET metalization layer that provides
nominal 50 ohms per square sheet resistance. By selecting tantalum
in the "metal types" pull down, the bulk conductivity
is given as 6450000 S/m. Using Excel as a scratchpad, we calculated
that a metal thickness of 0.000122 mils would give us 50.0 ohms/mm
(sheet resistance is (1/(conductivity*metal thickness, just make
sure the units are consistent).
In a new project
we created a fan stub, from the "tools", "add metalization"
menu. We chose a length of ten mils, which is a quarterwave somewhere
around 50 GHz. The box size initially was still set to 0.2 x 1.0
mils; the actual geometry that Sonnet analyzes is shown in the staircase
outline below. The green feature is the fifty-ohm resistor, which
in this case measures 1 mil x 1 mil. This analysis took 21 seconds.
After we analyzed
the case shown above, we changed the box size down to 0.2 x 0.2
mils, which gives a much more accurate outline of the fan stub.
This is shown in the following figure. This analysis took 51 seconds.
We ran both cases
to see what error is introduced by using the cruder approximation;
the two traces below show that an error of about 2 GHz (4%) in center
frequency is the result. In this case, it is apparent that the box
size in the first run was too large for a "final design",
but might be just fine for preliminary design work where you want
speedy, if not 100% accurate answers.
By the way, what
happens if we merely ground the 1x1 mil fifty-ohm resistor with
a via hole? Below is the response computed by Sonnet. It looks great
down at X-band, but the fan stub termination does much better at
This page is entirely
sponsored by SONNET! More to come!