5.2 SH-SAW delay line with SiO 2 micro-ridges
5.2.2 Effect of SiO 2 ridges on mass sensitivity, insertion loss and stress
mental surface mass density on the delay path containing SiO2 ridges. Two identical SH-SAW devices containing ridges of the same height are considered, one with added mass and the other without mass loading. Mass loading causes time delay (or phase shift) between the output volt- age waveforms of the two devices. Fig. 5.8aand 5.8b shows the time shift due to mass loading with a surface mass density of 2×10−5 kg m−2 for two different ridge heights, 2000 and 1050 nm respectively. In the absence of coupled resonance at ht = 2000 nm, mass loading causes negligible reduction in the peak output voltage and a time delay of 0.01 ns (equivalent to ∆φ of 0.02 rad). However, for the same mass loading in the presence of coupled resonance at ht
= 1050 nm, the output voltage reduces by about 0.27 V and a considerable time delay of 0.55 ns (equivalent to ∆φ of 1.12 rad) is obtained. Fig. 5.9a displays the normalized phase shift (∆φ/kD) caused by incremental mass loading (∆m) for different ridge heights. The slope of the plot represents the phase mass sensitivitySφand it increases greatly near the coupled resonant height (1050 and 2850 nm). Fig. 5.9bshows the variation in frequency mass sensitivitySf with ridge height. For most of the ridge heights, Sf remain between 30–70 m2kg−1, which is close to the Sf of a plain LW device. However, at the coupled resonant heights 1050 and 2850 nm, a high mass sensitivities of 1295 and 1084 m2kg−1 are obtained respectively. The values of Sf
obtained for the SiO2 ridge based SH-SAW delay line device is similar to the calculated values ofSf for the resonator case discussed in section 5.1.2.
The variation in area-averaged total stress σT as defined in (5.2) at the interface between ridge and substrate with time is plotted in Fig. 5.10 for different ridge heights. The stress at the interface increases with time and tends to stabilize after 30 ns. It is noted that coupled resonance causes a substantial increase in the contact stress. Atht = 1050 and 2850 nm, in the presence of coupled resonance, the contact stress increases to a value of 8.6 and 5.4 MPa. On
1.0x10 -5
2.0x10 -5
3.0x10 -5
4.0x10 -5
5.0x10 -5
6.0x10 -5
7.0x10 -5 0.00
0.02 0.04 0.06 0.08 0.10 0.12 0.14
/kD(rad/rad)
m (kg/m 2
) 500 nm
1050 nm
2000 nm
2850 nm
3500 nm
(a)
500 1000 1500 2000 2500 3000 3500 0
200 400 600 800 1000 1200 1400
Sf
(m
2 /kg)
h t
(nm)
(b)
Figure 5.9: (a) The change in normalized phase shift (∆φ/kD) with increasing surface mass density (∆m) plotted for different ridge heights. (b) Variation in mass sensitivity Sf of device with ridge height ht.
0 5 10 15 20 25 30 35 40 45 50
0.1 1 10
T
(MPa)
t (ns)
500 nm
1050 nm
2000 nm
2850 nm
3500 nm
Figure 5.10: Time response of the area averaged total stressσT of the SH-SAW delay line device with SiO2 ridge height.
the other hand, in the absence of coupled resonance, atht= 500, 2000 and 3500 nm the contact stress remains close to or lower than 1 MPa.
The frequency response of the delay line was acquired by taking the Fourier transform of the impulse response voltage obtained at the output IDT. Since the time response simulation contained a limited number of data points, the frequency response signal was improved by zero padding the output voltage signal. Fig. 5.11a shows the insertion loss (IL) of the device considering ridge heightsht= 800, 1050, 2000, and 2850 nm. In the absence of coupled resonance, atht = 800 and 2000 nm, a minimum insertion loss of about -26.1 dB at 358 MHz is obtained.
However, in the presence of coupled resonance, atht= 1050 nm, minimum insertion loss of -32.3 dB is obtained at a decreased frequency of 326.5 MHz. Also, when ht = 2850 nm, minimum insertion loss of -31 dB is obtained at an increased frequency of 360.3 MHz. An increase in the insertion loss by about 5 dB along with positive and negative frequency shifts are observed near the coupled resonance. The variation in minimum insertion loss ILmin of the device with
5.2. SH-SAW delay line with SiO2 micro-ridges
100 150 200 250 300 350 400 450 500 550 600 -60
-55 -50 -45 -40 -35 -30 -25
Insertionloss(dB)
f (MHz)
800 nm
1050 nm
2000 nm
2850 nm
(a)
500 1000 1500 2000 2500 3000 3500
-33 -32 -31 -30 -29 -28 -27 -26
Insertionloss(dB)
h t
(nm)
w ith ridge
w ithout ridge
(b)
Figure 5.11: (a) Insertion loss of SH-SAW delay line device considering different ridge heights ht= 800, 1050, 2000 and 2850 nm. (b) Variation in minimum insertion loss of delay line device with ridge height.
∆m (kg m−2) ∆t (ns) for different values ofht (nm) 500 nm 1050 nm 2000 nm 2850 nm 3500 nm
2×10−5 0.006 0.55 0.013 0.468 0.014
3×10−5 0.009 0.745 0.019 0.665 0.022
4×10−5 0.013 0.95 0.025 0.825 0.031
5×10−5 0.016 1.16 0.032 0.987 0.04
6×10−5 0.02 1.34 0.039 1.141 0.048
Sφ (m2kg−1) 35 1995 65 1668 86 Sf (m2kg−1) 23 1295 42 1084 56
σT (MPa) 0.8 8.6 0.9 5.4 1.1
ILmin (dB) -28.2 -32 -26.5 -31 -27.2
Table 5.1: Time delay due to mass loading, mass sensitivities, area averaged total stress, and minimum insertion loss for different ridge heights.
ridge height is plotted in Fig. 5.11b. For the plain SH-SAW device without ridges, the device showsILmin of -28.2 dB. As ridge height is increased, theILstarts to deteriorate and becomes maximum near the coupled resonant height. The insertion loss increases by about 5 dB near the coupled resonant height. Further increase in the height of the ridges causes mode transition (from mode-0 to mode-1) andILagain starts to improve. The rise and dip in the insertion loss are again observed when the transition occurs from mode-1 to mode-2.
The time delay due to mass loading, mass sensitivities, stable values of area-averaged total stress, and insertion loss for different ridge heights are listed in Table 5.1. In the presence of coupled resonance, at ht = 1050 and 2850 nm, significant time delays of about 0.55 and 0.47 ns (upon mass loading with 2×10−5 kg m−2) along with high Sf of 1295 and 1084 m2kg−1 are obtained respectively. Coupled resonance also leads to increase in the area averaged stress values. Although coupled resonance provides high mass sensitivity, it comes at the cost of increased insertion losses in the device.