Surface acoustic wave (SAW) device with antireflective structure
A device including a piezoelectric substrate, an interdigital transducer (IDT), and an antireflective structure is disclosed herein. The piezoelectric substrate has a front-side surface and a smoothed back-side surface. The IDT is on the front-side surface of the piezoelectric substrate. The antireflective structure is over at least a portion of the smoothed back-side surface of the piezoelectric substrate. By having the antireflective structure on at least a portion of the smoothed back-side surface of the piezoelectric substrate, reflection of spurious bulk acoustic waves toward the front-side surface of the piezoelectric substrate can be reduced and/or eliminated to lessen interference with surface acoustic waves. The reduction and/or elimination of spurious bulk acoustic waves allows the device to forego conventional roughening of the back-side surface of the piezoelectric substrate, thereby reducing fractures at the back-side surface and allowing for singulation techniques capable of producing smaller die sizes.
1. A device comprising:
a piezoelectric substrate comprising:
a front-side surface; and
a back-side surface;
an interdigital transducer (IDT) on the front-side surface of the piezoelectric substrate;
an antireflective structure on at least a portion of the back-side surface of the piezoelectric substrate; and
a dielectric compound on the antireflective structure.
2. The device of claim 1 , wherein the antireflective structure comprises a first thin-film layer having a first thin-film thickness less than a period of the IDT.
3. The device of claim 1 , wherein:
the IDT is configured to generate longitudinal bulk acoustic waves in the piezoelectric substrate; and
the antireflective structure comprises a first thin-film layer having a first thin-film thickness approximately equal to one-quarter of a wavelength of the longitudinal bulk acoustic waves.
4. The device of claim 1 , wherein the antireflective structure comprises a first thin-film layer having a first thin-film thickness approximately equal to one quarter of a wavelength of a longitudinal mode in the first thin-film layer.
5. The device of claim 1 , wherein the antireflective structure comprises a first thin-film layer having a first thin-film thickness less than a period of the IDT and approximately equal to one quarter of a wavelength of a longitudinal mode in the first thin-film layer.
6. The device of claim 1 , wherein the antireflective structure comprises a first thin-film layer having a first thin-film thickness approximately equal to one quarter of a wavelength of a longitudinal mode in the first thin-film layer at twice a center frequency of the device.
7. The device of claim 3 , further comprising:
a second IDT on the front-side surface of the piezoelectric substrate, wherein:
the device is a surface acoustic wave (SAW) filter having a passband that has a center frequency; and
the wavelength of the longitudinal bulk acoustic waves is defined at twice the center frequency of the passband of the device.
8. The device of claim 1 , wherein:
the IDT is configured to generate bulk acoustic waves in the piezoelectric substrate; and
the antireflective structure comprises a first thin-film layer having a first thin-film thickness based on a wavelength of the bulk acoustic waves.
9. The device of claim 8 , further comprising:
a second IDT on the front-side surface of the piezoelectric substrate, wherein:
the device is a surface acoustic wave (SAW) filter having a passband that has a center frequency; and
the wavelength of the bulk acoustic waves is defined at twice the center frequency of the passband of the device.
10. The device of claim 1 , wherein:
the dielectric compound has a dielectric acoustic impedance;
the piezoelectric substrate has a piezoelectric acoustic impedance; and
the antireflective structure comprises a first thin-film layer having a first thin-film acoustic impedance approximately equal to the square root of the product of the dielectric acoustic impedance and the piezoelectric acoustic impedance.
11. The device of claim 1 , wherein:
the dielectric compound has a dielectric acoustic impedance; and
the antireflective structure comprises a first thin-film layer having a first thin-film acoustic impedance based on the dielectric acoustic impedance.
12. The device of claim 11 , wherein:
the piezoelectric substrate has a piezoelectric acoustic impedance; and
the first thin-film acoustic impedance is further based on the piezoelectric acoustic impedance.
13. The device of claim 1 , wherein:
the IDT is configured to generate longitudinal bulk acoustic waves in the piezoelectric substrate;
the antireflective structure comprises a first thin-film layer having a first thin-film thickness approximately equal to one-quarter of a wavelength of the longitudinal bulk acoustic waves;
the dielectric compound has a dielectric acoustic impedance;
the piezoelectric substrate has a piezoelectric acoustic impedance; and
the first thin-film layer has a first thin-film acoustic impedance approximately equal to the square root of the product of the dielectric acoustic impedance and the piezoelectric acoustic impedance.
14. The device of claim 1 , wherein:
the IDT is configured to generate bulk acoustic waves in the piezoelectric substrate;
the dielectric compound has a dielectric acoustic impedance;
the antireflective structure comprises a first thin-film layer having a first thin-film thickness based on a wavelength of the bulk acoustic waves; and
the first thin-film layer has a first thin-film acoustic impedance based on the dielectric acoustic impedance.
15. The device of claim 1 , wherein the antireflective structure comprises:
a first thin-film layer; and
a second thin-film layer.
16. The device of claim 15 , wherein:
the IDT is configured to generate longitudinal bulk acoustic waves in the piezoelectric substrate;
the first thin-film layer has a first thin-film thickness approximately equal to one-quarter of a wavelength of the longitudinal bulk acoustic waves; and
the second thin-film layer has a second thin-film thickness approximately equal to one-quarter of the wavelength of the longitudinal bulk acoustic waves.
17. The device of claim 15 , wherein:
the IDT is configured to generate bulk acoustic waves in the piezoelectric substrate;
the first thin-film layer has a first thin-film thickness based on a wavelength of the bulk acoustic waves; and
the second thin-film layer has a second thin-film thickness based on the wavelength of the bulk acoustic waves.
18. The device of claim 15 , wherein:
the dielectric compound has a dielectric acoustic impedance;
the first thin-film layer has a first thin-film acoustic impedance based on the dielectric acoustic impedance; and
the second thin-film layer has a second thin-film acoustic impedance based on the dielectric acoustic impedance.
19. The device of claim 15 , wherein:
the IDT is configured to generate bulk acoustic waves in the piezoelectric substrate;
the first thin-film layer has a first thin-film thickness based on a wavelength of the bulk acoustic waves;
the second thin-film layer has a second thin-film thickness based on the wavelength of the bulk acoustic waves;
the dielectric compound has a dielectric acoustic impedance;
the first thin-film layer has a first thin-film acoustic impedance based on the dielectric acoustic impedance; and
the second thin-film layer has a second thin-film acoustic impedance based on the dielectric acoustic impedance.
20. The device of claim 1 , further comprising a high-impedance layer on the antireflective structure.
21. The device of claim 20 , further comprising the dielectric compound on the high-impedance layer, wherein:
the dielectric compound has a dielectric acoustic impedance; and
the high-impedance layer has a high-impedance acoustic impedance greater than the dielectric acoustic impedance.
22. The device of claim 21 , wherein:
the IDT is configured to generate longitudinal bulk acoustic waves in the piezoelectric substrate;
the antireflective structure comprises a first thin-film layer having a first thin-film thickness approximately equal to one-quarter of a wavelength of the longitudinal bulk acoustic waves;
the piezoelectric substrate has a piezoelectric acoustic impedance; and
the first thin-film layer has a first thin-film acoustic impedance approximately equal to the square root of the product of the dielectric acoustic impedance and the piezoelectric acoustic impedance.
23. The device of claim 21 , wherein:
the IDT is configured to generate bulk acoustic waves in the piezoelectric substrate;
the antireflective structure comprises a first thin-film layer having a first thin-film thickness based on a wavelength of the bulk acoustic waves; and
the first thin-film layer has a first thin-film acoustic impedance based on the dielectric acoustic impedance.
24. The device of claim 21 , wherein the antireflective structure comprises:
a first thin-film layer; and
a second thin-film layer.
25. The device of claim 24 , wherein:
the IDT is configured to generate bulk acoustic waves in the piezoelectric substrate;
the first thin-film layer has a first thin-film thickness based on a wavelength of the bulk acoustic waves;
the second thin-film layer has a second thin-film thickness based on the wavelength of the bulk acoustic waves;
the first thin-film layer has a first thin-film acoustic impedance based on the dielectric acoustic impedance; and
the second thin-film layer has a second thin-film acoustic impedance based on the dielectric acoustic impedance.
26. The device of claim 1 , wherein:
the device is configured to receive an input signal and generate a filtered output signal based on the received input signal.
27. The device of claim 26 , further comprising:
a second IDT on the front-side surface of the piezoelectric substrate, wherein the device is a surface acoustic wave (SAW) filter having a passband that has a center frequency.
28. The device of claim 1 , wherein the back-side surface has a roughness average less than 20 nanometers (nm).
29. The device of claim 1 , wherein:
the piezoelectric substrate comprises lithium niobate; and
a dielectric film comprising silicon oxide is on the IDT to reduce a temperature sensitivity of the device.
30. The device of claim 1 , wherein:
the piezoelectric substrate is on a front-side surface of a second substrate; and
the antireflective structure is on a back-side surface of the second substrate.
31. The device of claim 30 , wherein the second substrate comprises a non-piezoelectric material.
32. The device of claim 30 , wherein the second substrate comprises silicon.
33. The device of claim 30 , wherein the second substrate comprises quartz.
34. A method of fabricating a device comprising:
providing a piezoelectric substrate comprising:
a front-side surface; and
a smoothed back-side surface;
forming an interdigital transducer (IDT) on the front-side surface of the piezoelectric substrate;
forming an antireflective structure on at least a portion of the back-side surface of the piezoelectric substrate; and
forming a dielectric compound on the antireflective structure.
35. The method of claim 34 , wherein forming the antireflective structure on the at least the portion of the back-side surface of the piezoelectric substrate comprises forming the antireflective structure the at least the portion of the back-side surface of the piezoelectric substrate before forming the dielectric compound on the antireflective structure.
36. The method of claim 34 , wherein providing the piezoelectric substrate comprising the front-side surface and the back-side surface comprises:
providing the piezoelectric substrate with a smoothed back-side surface; and
backgrinding the smoothed back-side surface of the piezoelectric substrate.
37. The method of claim 36 , wherein backgrinding the smoothed back-side surface of the piezoelectric substrate comprises:
backgrinding the smoothed back-side surface of the piezoelectric substrate; and
not roughening the smoothed back-side surface of the piezoelectric substrate in a subsequent step.
38. The method of claim 34 , further comprising singulating the device, wherein forming the antireflective structure on the at least the portion of the back-side surface of the piezoelectric substrate comprises forming the antireflective structure on the at least the portion of the back-side surface of the piezoelectric substrate after singulating the device and before forming the dielectric compound on the antireflective structure.
39. The device of claim 1 , wherein back-side surface of the piezoelectric substrate comprises a smoothed back-side surface.