Page 151 - High Power Laser Handbook

120 Diode Lasers Semiconductor Laser Diodes 121

where E is a thermal activation energy and m and n are exponents of
a
current and power accelerations, respectively. To derive model
parameters E , m, and n, the maximum likelihood method is used.
a
This method finds the model parameters by maximizing the likeli-
hood of observing actual number of failures on the test. For a random
exponential distribution of failures, the time likelihood function L is

cellls
,, )) − ∑
λ
IP
ln()L = n i ln((TI i P i λ (T ji , ,)t (5.7)
i
ji
i
i
i
where n and t are the number of failures and total device hours in
i
i
cell i. Application of this method to the results of Table 5.2 extracts the
following model parameters:
.
E = 078 eV
a
m = 27. (5.8)
n = 0
With Eq. (5.6), the estimated failure rate at operating conditions of
–1
–9
400 mW and 25ºC is 53 failures-in-time (FIT) (53 × 10 hr ). This type
of multicell methodology can be extended to packaged laser diodes,
which is the subject of a subsequent section. But first we provide
background on various package designs and processes.
5.11 Submount Design and Assembly
The chip-on-submount (COS) serves as a building block that may be
used alone or integrated into a higher level of assembly. The semicon-
ductor laser chip is soldered to the submount (or carrier) to provide a
means for mechanical handling, burn-in testing, and thermal dissipa-
tion. The COS is typically clamped or bolted down (e.g., C-mount) or
soldered to reduce thermal impedance (R ). Hermetic designs (e.g.,
th
transistor outline [TO] cans) are sealed by a cap and window to pro-
tect against dust and moisture. Examples of these package types are
shown in Fig. 5.19.
More than a decade ago, when high-reliability submarine compo-
nents were first deployed, suppliers needed to eliminate field failures
over a 25-year lifetime. Creep-resistant, high-shear-strength “hard
solders,” such as AuSn (80/20 percent weight) with a melting point
(MP) of 280ºC, became universally established for die bonding single-
mode, 980-nm lasers. The advantage of flux- and whisker-free bonding
is even more compelling when applied to high-power diodes with
p-side-down bonds, where the laser facet is located only several microm-
eters from the solder interface. In this case, the facet must also be placed
within plus or minus several micrometers from the submount edge