Table 2.

Comparison of fabrication methods.

Comparison Criteria	Fabrication Technique
		Wafer Direct Bonding			Adhesive Bonding
	Sacrificial Layer-Based [19]	SOI Wafer-Based [17,21]	Non-SOI Wafer-Based [20]	Anodic Bonding [5]	Other Methods [22,28]	* PMMA-Based
Wafer pair	N/A	SOI	LPCVD SiN	SOI	LPCVD SiN	${\mathrm{SiO}}_2$
		Si	LPCVD SiN	Si	Si	${\mathrm{SiO}}_2$
Wafer cost	low	high	low	high	low	low
Special requirements	N/A	N/A	CMP required	Electric field	BCB as adhesive	PMMA as adhesive
Surface-quality restriction	N/A	Restrict	Restrict	Restrict	low	low
Max Temp	$785{}^{\circ }\mathrm{C}$	$1100{}^{\circ }\mathrm{C}$	$1000{}^{\circ }\mathrm{C}$	$350{}^{\circ }\mathrm{C}$	$240{}^{\circ }\mathrm{C}$	$180{}^{\circ }\mathrm{C}$
Deposition required	yes	yes	yes	yes	yes	no *
# of lithography steps	5	4	3	3	3	1 *
# of wet etching steps	3	3	1	3	2	0 *
# of dry etching steps	4	2	2	1	2	1 *
Simplicity	low	high	high	low	high	highest

* The presented CMUT was specifically used for low frequencies. Higher frequencies can also be achieved at the cost of additional etching and metal-deposition steps as discussed earlier in Section 1.