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218 Fiber Optic Communications

W
100 10 1 0.1 0.01 0.001
10 10
RCE max RCE max RCE max
R 2 = 1 no RCE R 2 = 0.9 no RCE R 2 = 0.8 no RCE
RCE min RCE min RCE min
1 R 1 = 0 R = 0 R = 0 1
Quantum efficiency 0.1 R 1 = 0.70 R 1 = 0.70 R 1 = 0.70 0.1
1
1
R 1 = 0.50
R 1 = 0.50
R 1 = 0.50
R 1 = xx
R 1 = xx
R 1 = xx
0.01 R 1 = xx R 1 = xx R 1 = xx 0.01

0.001 0.001
100 10 1 0.1 0.01 0.001 100 10 1 0.1 0.01 0.001
W W

Figure 5.20 Resonant cavity enhancement (RCE max, non-monotonic thin lines) of photodetectors with Fabry–Perot
resonator as a function of the product × W of the absorption coefficient and the absorption layer width W compared
with non-resonant photodetectors (thick lines, R = 0) and suppression in RCE (RCE min, dashed lines with symbols)
2
for several reflections of the front mirror (R = R , as indicated by labels on the lines) and back mirror (R = 1, 0.9 and
1 S 2
0.8, in the plots from left to right).

˜10 μm

Figure 5.21 Cross-section of the oxide stack in a six-metal CMOS, around year 2000.

broad-spectrum photosensors for imager arrays, or by temperature and fabrication variations. Note that not
all the curves in Fig. 5.20 exceed unity, which means that RCE can only remedy incomplete absorption in
thin layers.
Submicrometer Si technologies usually have a thick stack of oxide layers, in the range of 3–10 μmasshown
in Fig. 5.21 from Ref. [43], since many metal layers need to be accommodated for electrical interconnections.