Subject: Process parameters

From: Stephen Peters <[email-masked]>
Date: Wed, 16 Jul 1997 08:53:44 -0700

Hello All:

   Mr. Singhal raises a good point, and I thought I'd respond.
In my experience the "min" and "max" process corners in a semicondutor
model (especially for the early version processes I deal with) could
be better labled "really, really min" and "absloluty, positivly,
completly max".  These corners are usually so far out there
that a model built at those corners is unrealistic.  By this I mean that 
by the time a part is speced 
and the useful yeilds determined the customer probably never ever sees 
a truly "min" and "max" part.  There is also the problem of having to 
figure out how to correlate the parameters that are used in production
testing of a part (Tco, power dissipation, etc.) with the parameters an 
I/O buffer model cares about (edge rate and drive strength).  For
example, does the min and max Tco really correspond to min and max
drive strength?  Given all that, one can resonable use an imperical
% derating factor to determine the corners of an IBIS model.  
   This is not to say that varying the process corners is not a
useful way to make min and max models.  One just has to understand
that "min" and "max" process corners do not necesarly represent the
worst and best case I/O buffers the customer receives.

            Best Regards,
            Stephen Peters
            Intel Corp.

> On Wed, 16 Jul 1997 03:48:15 -0500, Vipul K. Singhal wrote:

Hello all,

          I have a doubt regarding the method for obtaining the curves
by simulation as outlined in IBIS  COOKBOOK. The cookbook says ( under
the  section  3.3.1  :  "OBTAINING CURVES  BY SIMULATION -  Simulation
Specifics" ) that :

       "For both the rise/fall   time and I-V curve measurements,
        use "typical" process parameters."

        "For CMOS:
        min = min VCC, max temperature, typ process parameters
                                        ---
        max = max VCC, min temperature, typ process parameters"
                                        ---

Later it says that  " To account for  process variation, decrease  the
current  values taken at  min conditions,  increase the current values
taken  at max, and   derate  the rise  and  fall  time values  by  the
appropriate percentages. "

	This seems to be a  roundabout way of doing  it.  Why can't we
directly  use  "typ" ,  "min"   ,  and  "max" process  parameters,  as
appropriate, directly in  the simulations ?  For  example, if SPICE is
used for  simulation, we can   have three different SPICE  models  for
transistors,  corresponding  to the  process-variation  corners.   Why
should "typical" be used in each case ?

Regards,
Vipul K. Singhal
Texas Instruments 

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