ESD Verification

Below two examples from our automated test suite showcasing how our approach reverse solving the HBM model math as elaborated in Physics-Based_Wire_Sizing_for_I/O_Pad_Cells actually leads to simulation results in ngspice which show that our ESD diodes we chose actually protect our internal circuitry.

Oh wow. When you solve Ohm's law in one direction and then the other, you end up with the current you originally have defined at a certain voltage. Who would have thought this... anyway. Here some examples.

After running

./tests/test_all_padcells.sh

you end up with a folder called "generated_output"

We're looking at "generated_output/SG13G2/padcell/1.2V" and "generated_output/SG13G2/padcell/3.3V" here as examples

As described here the test setup is really simple:

The deck models a basic HBM discharge setup with:

• a 100 pF capacitor
• a 1.5 kΩ series resistor
• a switched discharge path

This is just the driver stage test. The input buffer will be having additional tests

ESD Tests

The energy which would otherwise go through the internal digital logic takes the alternative path of least resistance, which is the two diodes (we're not generating Schottky diodes yet, see the list of devices in LibrePDK), which we put as close to the bonding pad as possible.

Here are the SPICE decks

Here a quick drawing to illustrate the configuration.

Running the detailed tests for IHP with

./tests/run_single_test.sh SG13G2:1.2 padcell

yields

Peak PAD voltage: 2.467 V
Peak current: 1.330122 A
Peak resistor power: 2.654 kW
Final dissipated energy: 168.225485 µJ
HBM plots written to /run/media/leviathan/dde9e2a1-4a6e-4d1b-afec-7c557d080ef8/LibrePDK/librepdk/generated_output/SG13G2/1.2V/padcell/out/io_cell_20mA_1p2V/plots/hbm_pd_forward
Waveform data: /run/media/leviathan/dde9e2a1-4a6e-4d1b-afec-7c557d080ef8/LibrePDK/librepdk/generated_output/SG13G2/1.2V/padcell/out/io_cell_20mA_1p2V/hbm_waveforms_hbm_pd_forward.dat
Peak PAD voltage: 2.310 V
Peak current: 1.330248 A
Peak resistor power: 2.654 kW
Final dissipated energy: 168.240327 µJ
HBM plots written to /run/media/leviathan/dde9e2a1-4a6e-4d1b-afec-7c557d080ef8/LibrePDK/librepdk/generated_output/SG13G2/1.2V/padcell/out/io_cell_20mA_1p2V/plots/hbm_ns_forward
Waveform data: /run/media/leviathan/dde9e2a1-4a6e-4d1b-afec-7c557d080ef8/LibrePDK/librepdk/generated_output/SG13G2/1.2V/padcell/out/io_cell_20mA_1p2V/hbm_waveforms_hbm_ns_forward.dat
Peak PAD voltage: -0.029 V
Peak current: 1.323457 A
Peak resistor power: 2.627 kW
Final dissipated energy: 166.729178 µJ
HBM plots written to /run/media/leviathan/dde9e2a1-4a6e-4d1b-afec-7c557d080ef8/LibrePDK/librepdk/generated_output/SG13G2/1.2V/padcell/out/io_cell_20mA_1p2V/plots/hbm_nd_reverse
Waveform data: /run/media/leviathan/dde9e2a1-4a6e-4d1b-afec-7c557d080ef8/LibrePDK/librepdk/generated_output/SG13G2/1.2V/padcell/out/io_cell_20mA_1p2V/hbm_waveforms_hbm_nd_reverse.dat
Peak PAD voltage: -0.029 V
Peak current: 1.323461 A
Peak resistor power: 2.627 kW
Final dissipated energy: 166.782671 µJ

HBS Forward

Simulating forward ESD discharge basically simulates the path drawn below

The forward discharge results in a cummulative energy of 100-150 micro Jules.

The things won't even get warm

HBS Reverse

For the reverse test, it's backwards...

The same graph results from simulating the reverse ESD, because that's how the diodes have been dimensioned

Not matter which direction, things won't even get warm

The IHP SG13G2 case

Comparing those two you will notice that the pad with the lower operational voltage needs more area.

While at first counter intuitive, this is because we're not dealing with Ohm's law here but with solid state physics, such as hot carriers and the such.

It's a balance between electron mobility and thermal budget... took a while to code that. It's really complicated physics, but it's now all done in Python so you won't have to solve those partial differential equations yourself.

30mA SG13G2@1V2

30mA SG13G2@3V3

Lets look at those two pad cells here. One is dimensionsed for 30mA at 1.2V and the other at 3.3V

One would assume that the 3.3V transistors would be larger.

However.

The width of a channel (length of the gate) is being determined by $L_{gate}=I_{target}/I_{sat}$ with $\left[I_{sat}\right]=\frac{mA}{uA}$

That however has to be combined with the thermal budget, which gives a minimum area for the energy to be dissipated on.

In the case of the 1.2V transistors the saturation current of the channel isn't high enough anyway for violating the minimum area requirement because it doesn't have additional doping like the IHP transistor lifted to 3.3V by additional HV doping.

This doping improves the conductivity of the channel, so the gates had to be scaled up in order to cover enough space for dissipating the energy they conduct.