Surface activated bonding

Surface activated bonding (SAB) is a non-high-temperature wafer bonding technology with atomically clean and activated surfaces. Surface activation prior to bonding by using fast atom bombardment is typically employed to clean the surfaces. High-strength bonding of semiconductors, metals, and dielectrics can be obtained even at room temperature.^[1][2]

Overview

In the standard SAB method, wafer surfaces are activated by argon fast atom bombardment in ultra-high vacuum (UHV) of 10⁻⁴–10⁻⁷ Pa. The bombardment removes adsorbed contaminants and native oxides on the surfaces. The activated surfaces are atomically clean and reactive for formation of direct bonds between wafers when they are brought into contact even at room temperature.

Researches on SAB

The SAB method has been studied for bonding of various materials, as shown in Table I.

Table I. Studies of standard SAB for various materials

	Si	Ge	GaAs	SiC	Cu	Al2O3	SiO2
Si	[3][4]		[5]	[6]		[7][8]
Ge		[9]
GaAs	[5]			[10]
SiC	[6]		[10]	[11]
Cu					[12][13]
Al2O3	[7][8]					[7]
SiO2							Failure[7]

The standard SAB, however, failed to bond some materials such as SiO₂ and polymer films. The modified SAB was developed to solve this problem, by using a sputtering deposited Si intermediate layer to improve the bond strength.

Table II. Modified SAB with Si intermediate layer

	Bonding intermediate layer	References
SiO2-SiO2	Sputtered Fe-Si on SiO2	[14]
Polymer films	Sputtered Fe-Si on both sides	[15][16][17]
Si-SiC	Sputtered Si on SiC	[18]
Si-SiO2	Sputtered Si on SiO2	[19]

The combined SAB has been developed for SiO₂-SiO₂ and Cu/SiO₂ hybrid bonding, without use of any intermediate layer.

Table III. Combined SAB using Si-containing Ar beam

	Bond interface	References
SiO2-SiO2	Direct bond interface	[20]
Cu-Cu, SiO2-SiO2, SiO2-SiNx	direct bond interface	[21]

Technical specifications

Materials	Semiconductor: Si-Si,[3][4] Ge-Ge,[9] GaAs-SiC,[10] SiC-SiC,[11] Si-SiC,[6][18] etc. Metal: Al-Al, Cu-Cu,[12][13] etc. Dielectric: Polymer films,[16][17] SiO2,[20][22] etc. Cu/Dielectric Hybrid: Cu/SiO2 and Cu/SiO2/SiNx[21]
Advantages	Low process temperature: room temperature–200 °C No concerns of thermal stress and damages High bonding quality Semiconductor and metal bonding interfaces without oxides Completely dry process without wet chemical cleaning Process compatibility to semiconductor technology
Drawbacks	High vacuum level (10−4–10−7 Pa)