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Anyone who has spent hours transferring parametric data
into a database (or spreadsheet) and plotted graphs of
parametric data across a wafer will realise this can be
a very time-consuming process. The subsequent analysis of
this parametric data across the wafer, in rows, columns
or concentric rings again can become very difficult to interpret
to get an insight to the underlying cause of yield loss.
- Is the yield loss a process function?
- Is the variation in certain areas of the wafer?
- Is the variation caused by damaged probe needles?
- Is the variation tester dependent or process related?
These questions may be very difficult to answer. Most wafer
probers will have built-in wafer maps of the bin result
to help this diagnostic process. One difficulty is that
you need to know which parameter is of interest in advance,
and set-up a number of bins to analyse the parameter range.
In response to this the ipTEST developed wafer map
utility is a valuable tool to aid yield analysis for wafer
testing. It gives a visual depiction (a colour map) of parametric
trends across the wafer showing the variation of:
- Bin Result
- Category Results
- Parametric Distribution test data
ipTEST has developed the tools for transferring
the XY wafer co-ordinates into the Datalog file and the
Run environment. The wafer map can then be used either online
while testing, or off-line to view stored data files. The
wafer map window contains all the information necessary
to plot the wafer. The Bin, Category or Parametric map views
may be selected as required.
Bin Map
The Bin map is most important for determining which devices
are good or bad. ipTEST offers an off-line post processor
function which allows devices tested good but which are
out side a 3 sigma variation from the average to be rejected.
Category Map
The category map is useful especially for analysing the
rejects across a wafer.
Parametric Map
The parametric map shows the variation of the measured parameters
across the wafer.
The measure parameter to map is selected from a drop down
pick list:
The maximum and minimum range for the map
is set automatically, but may be overwritten by the user:
The parametric map view showing the RDS(ON)
distribution:
The mapping software allows the user to customise the view
with user defined colour schemes which may be saved in a
wafer profile.
Current device data
If a die on the wafer is selected using the mouse or arrow
keys, the device data is displayed in the CURRENT DEVICE
section:
The associated Bin and Category colours are highlighted:
Zoom function
A Zoom function exists for large numbers of die on a wafer
to easily select a die:
Colour scheme
The colour scheme may be defined by the user and stored
in the wafer profile:
Wafer profile
The wafer profile allows the view of the wafer to be altered
such that the maximum and minimum X and Y values may be
entered. This may be useful if only a small section of the
wafer is of interest.
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