Table 2.

Classification, principle, and characteristics of the various bonding methods.

Bonding methods	Classifications	Principles	Characteristics	References
Direct bonding	Thermal bonding	Heat to near the vitrification temperature of the material and apply pressure to achieve bonding	Without auxiliary binder, no contaminants produced; good mechanical and thermal properties after bonding; high quality for surface, complex post-treatment	(Bhattacharyya and Klapperich, 2007; Brown et al., 2006; Gan et al., 2011; Hsu and Chen, 2007; Koesdjojo et al., 2008; Sun et al., 2007; Thompson and Abate, 2013; Wang et al., 2011; Wu et al., 2005; Zhang et al., 2010b; Zhou et al., 2010, 2012)
	Surface modification bonding	The surface of the material is modified by radiation treatment (X-ray, UV light) or plasma treatment (O2, O3, N2) to achieve bonding	Low bonding temperature, high requirements for equipment	(Bhattacharyya and Klapperich, 2007; Brown et al., 2006; Gan et al., 2011; Hsu and Chen, 2007; Koesdjojo et al., 2008; Sun et al., 2007; Thompson and Abate, 2013; Wang et al., 2011; Wu et al., 2005; Zhang et al., 2010b; Zhou et al., 2010, 2012)
	Ultrasonic bonding	Bonding is achieved by melting the material using ultrasonic energy	Fast bonding speed, high efficiency, part-bonding; suitable for mass production of polymer chips; does not use adhesives, reduces pollution; simple process and low cost; increases the difficulty of chip manufacturing	(Bhattacharyya and Klapperich, 2007; Brown et al., 2006; Gan et al., 2011; Hsu and Chen, 2007; Koesdjojo et al., 2008; Sun et al., 2007; Thompson and Abate, 2013; Wang et al., 2011; Wu et al., 2005; Zhang et al., 2010b; Zhou et al., 2010, 2012)
Indirect bonding	Solvent bonding	A special solvent, such as acetone, is used to slightly dissolve the surface of the material and then pressure is applied to achieve permanent bonding	Room temperature bonding technique; small microchannel deformation, suitable for small batch production; high requirements for the chip surface cleanliness and flatness; suitable solvent selection	(Bhattacharyya and Klapperich, 2007; Brown et al., 2006; Gan et al., 2011; Hsu and Chen, 2007; Koesdjojo et al., 2008; Sun et al., 2007; Thompson and Abate, 2013; Wang et al., 2011; Wu et al., 2005; Zhang et al., 2010b; Zhou et al., 2010, 2012)
	Adhesive bonding	Bonding is achieved by adding an intermediate medium between the substrate and the cover (epoxy, UV glue, etc.)	Simple, low cost, suitable for chips of most materials; room temperature bonding technique; poor stability, adhesives may block microchannels	(Bhattacharyya and Klapperich, 2007; Brown et al., 2006; Gan et al., 2011; Hsu and Chen, 2007; Koesdjojo et al., 2008; Sun et al., 2007; Thompson and Abate, 2013; Wang et al., 2011; Wu et al., 2005; Zhang et al., 2010b; Zhou et al., 2010, 2012)