Integrated Circuit yield
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Circuit Yield
The operating margin of a circuit is defined as the range of
device parameters for which the circuit is functional. Margins
can be determined by simulating the circuit with a circuit
simulator such as SPICE, while varying different device
parameters. Assume that such an analysis yields for a single
parameter, P, a minimum value, Pmin,
and maximum value, Pmax,
for which the circuit is functional. Let us also assume that
the actual distribution of this device parameter is gaussian,
so that the optimal average value of the parameter P is halfway
between Pmin and Pmax
as shown in the figure below.
yield1.gif
Fig. Gaussian distribution of a parameter P.
The parameter range for which the circuit is functional is
between Pmin and Pmax
The probability that for N devices their parameter P
lies within
the two boundaries is then given by:
(eqy1)
where we assumed their is no statistical correlation between
the parameter P of two different devices. Using the
first two
terms of the binomial expansion this can be approximated for
large values of N as:
(y2)
Both expressions are plotted in Fig.2 for different values of N.
A simple rule of thumb can be derived from equation y2 namely
that a yield of 80 % occurs for:
(y3)
This transcendental equation can be easily solved by iteration
yielding the following values for a as calculated for different values
of N:
| N |
103 |
104 |
105 |
106 |
| a |
3.73 |
4.28 |
4.76 |
5.21 |
This
means that in
order to obtain an 80 % circuit yield for a circuit in which
only one parameter varies, and assuming the distribution of
this parameter to be gaussian with spread
s, the circuit margin
must be at least 3.73s for a circuit with one thousand devices
and 5.21s for a circuit with one million devices. This explains
the common 3s design rule for LSI (Large Scale Integration)
circuits which has become a 5s design rule for VLSI (Very Large
Scale Integration) circuits.
yield.xls - yield.gif
Fig. Circuit yield as a function of the
normalized parameter range
(Pmax - Pmin)/2s
Problem:
For an NMOS aluminum gate transistor on a 1016 cm-3.
substrate with 10 nm gate oxide and a threshold voltage of
150 mV, find the spread of the oxide thickness corresponding
to a threshold variation of 20 mV.
Process Yield
Process yield is the more commonly quoted yield associated with
the fabrication of integrated circuits. Referred to is the yield
reduction caused by physical defects on the wafer circuit which
cause the adjacent device and therefore also the circuit to fail.
The probability that one chip contains exactly k defects, while
the wafer contains N circuits as well as n defects, is given
by:
(y4)
This expression is obtained by finding the total number of
combinations of putting the remaining n-k defects in the
remaining N-1 chips devided by the total number of combinations
to put the n defects into N chips. This does not account for the
possibility of exchanging defects while maintaining the same
number k on the chip of interest. We therefore multiply that
ratio with the total number of ways to exchange any of the n
defects so that the same number, k, remain on the one chip so
that n-k can be found on the rest of the wafer.
The above equation, for large n and N and a small number of
defects k, using the Stirling approximation yielding:
(y5)
which also equals the Poisson distribution. The corresponding
yield, Y, is given by the probability that the circuit has no
defect (k = 0) so that
(y6)
This result can be further extended to non-uniform defect
distributions by summing the expression for a constant
distribution applied to a section
DD and weighted by the
probability of having a particular defect density, f(D),
yielding:
(y7)
which in the limit where
DD
goes to zero can be expressed as a
function of the following integral:
(y8)
© Bart J. Van Zeghbroeck, 1997