Using Python to Access IC-CAP Objects and Data Structures

IC-CAP comes with a powerful API for accessing internal variables and data structures. In what follows, we will illustrate an example of accessing a Setup's Inputs and Outputs to obtain parameter values for the bias conditions for our custom Python measurement routines using the TableVar object.

Then use some Python statements to read the parameter values from the table.

# query setup variables
vg_start = SVar("vgstart").get_val()
vg_stop = SVar("vgstop").get_val()
vg_step = SVar("vgstep").get_val()
vg_pts = SVar("vgpts").get_val().split(".")

But instead, I'm going to show you how to read the measurement configuration values entered in the Measure/Simulate tab and use these values in your Python code.

This allows you to set parameters in the Measure/Simulate tab just like when using the native IC-CAP instrument drivers in normal Measure mode.  Now you can use the built-in IC-CAP user interface to modify the parameters. In using this method, you will have to select the Extract/Optimize tab, select the transform and then click the Execute button to invoke your custom Python transform.

Prerequisites

If you have not already setup your Python virtual environment and have IC-CAP installed, please refer to my previous article Measuring Sub-Threshold MOSFET Characteristics Using a Quasi-Static I-V Method.

Depending on whether you are using an older Agilent 4156B/C or a newer Keysight E5270B for your SMU, you may download and open one of the IC-CAP model files included at the end of this post.

• py_api_demo_5270_v.2.0.mdl
• py_api_demo_4156_v.2.0.mdl 

The Python code included for the E5270B can be applied to the B1500A with almost no modification as their command sets are very similar. Similarly, the code for the 4156B/C can be used with a 4142B with minor modification because we are using the US42 command mode using Flex commands. 

I recommend saving the *.mdl to C:\Keysight\ICCAP_2018\examples\python.  

Load the IC-CAP Model File

Assuming you saved the model file to the location above, click File/Examples... and select: 

python\py_api_demo_5270_v.2.0.mdl.

Go to the DUT/Setup called dc/idvg_quasi. Select the Measure/Simulate tab and observe the settings for the Inputs and Outputs. Notice that we have set conditions to make the Id vs. Vg measurement using IC-CAP's built-in drivers. Now select the Setup Variables tab and review the variable table.

# add variables for instrument options and other variables
 polarity = MVar("POLARITY").get_val() 

 # get setup variables
 hold=float(SVar("hold").get_val())
 delay=float(SVar("delay").get_val())
 step_delay=float(SVar("step_delay").get_val())

...

In our previous IC-CAP model file, the bias conditions for the gate sweep, drain, source and substrate values were stored in the Setup Variables table then read from table and assigned to local variables.  In this latest version we have removed these values from the SVars table and will define them in the _init Python transform, as shown below.

Note: You may need to change the SMU channel assignments ('drain', 'gate', 'source', 'substrate') to match the configuration of your instrument. For example, your Drain SMU Channel may be slot 5, not slot 1.

Exploring the _init transform

Select the Extract/Optimize tab and then select the _init transform.

New initialization code has been added to the _init transform relative to previous versions of the example mdl files. In this example, we will use functions provided by IC-CAP to query the settings of the Inputs from the user interface in the Measure/Simulate tab to set the measurement conditions for our quasi-static "Id vs. Vg" MOSFET measurement. 

Variable Name   Description
vgstart         Gate sweep start voltage
vgstop          Gate sweep stop voltage
vgstep          Gate step size
vgpts           Gate points
igcompl         Gate current compliance
vdvalue         Drain voltage (constant)
idcompl         Drain current compliance
vsvalue         Source voltage (constant)
iscompl         Source current compliance
vbvalue         Bulk voltage (constant)
ibcompl         Bulk current compliance

We will use these variables the instrument specific SMU commands to programmatically control the quasi-static sweep. 

Setup Object and Methods

Instead of using the IC-CAP function call SVar(“Varname”).get_val(), we will be using the TableVar object provided by IC-CAP's Python environment.  Thanks to the iccap python module, we can programmatically define, manipulate and query DUTs, Setups, Transforms, Inputs and Outputs.  In order to access the TableVar object of the Input and query its values, we will need a reference to its parent Setup object.

Let’s define an object representing the idvg_quasi Setup by name. You will find these two statements in the _run transform.

s = Setup("/py_api_demo_5270/dc/idvg_quasi")
setupName = s.get_fullname()

We can now query this object for its properties (assuming debug = 1). 

# query setup
if debug: print "fullname:",s.get_fullname()
if debug: print "name:", s.name
if debug: print "mytype:", s.mytype

Setup query response:

fullname: /py_api/demo/5270/dc/idvg_quasi

name: idvg_quasi

mytype: Setup

# query sweeps with order
if debug:
   for sweepid.range(1,2):
      inputobj = set.get_sweep(sweepid)
      if inputobj == None: break
      print "Sweep ", sweepid, s.get_fullname()

In this case, we have only a 1^st order sweep.

Sweep query response:

Sweep 1 /py_api_demo_5270/dc/idvg_quasi

Now, I will show you how to query some of the other parent/child hierarchies so that you get an idea of the type of information that is available to your Python programs.

We can query the setup object for children objects in its hierarchy.

# query child objects and type
if debug:
   print "child objects of Setup:"
   for objchildtype in s.childtypetuple:
      print objchildtype
      for objchild in s.get_child_objects(objchildtype):
         print " ", objchild.name
         print

Child object query response:

child objects of Setup:

Input

      vd

      vg

      vs

      vb

Output

      id

Transform

      _run

      _init

      _setup_iv

      _meas_iv

      _close

      _calc_vth

      README

SVar

      fileName_PATH

      fileName_PATH

      drain

      gate

      source

      substrate

      interface

      visaResponse

      error_status

      error_command

      hold_bias

      hold

      delay

      step_delay

      MDM_FILE_PATH

      MDM_FILE_NAME

Plot

      ThresholdVoltageIdsvsVgs

One may query any Setup for these properties and then assign them to local variables for use in your Python code.

Input/Output Objects

We will read the Input values from the IC-CAP TableVar objects and assign them to a Python string variable. Let's start with the Sweep parameters for the vg source.

vg = Input(setupName + "/vg")

The full statement will expand to Input("/py_api_demo_5270/dc/idvg_quasi/vg").  Like the Setup object,  the Input object also provides functions for returning information like full_name, name, and type.

# build full name for the sweep input
sweepName = vg.get_fullname()

Using the Input object vg, we set a variable sweepName for the fullname returned from the Input, we can read each of the vg Input's sweep TableVar parameters including: Compliance, Start, Stop, Step Size and # of Points.

igcompl = TableVar(sweepName + "/Compliance")
vgstart = TableVar(sweepName + "/Start")
vgstop = TableVar(sweepName + "/Stop")
vgstep = TableVar(sweepName + "/Step Size")
vgpts = TableVar(sweepName + "/# of Points")

We now have pointers to the following:

We can now query various properties of our newly created TableVar object.

# get the device polarity from the model variable table
polarity = MVar("POLARITY").get_val()