The bond energy is characterized > 200 % of non-activated reference wafer annealed at the same temperature. The maximal bond strength is achieved with nitrogen and oxygen as process gases and is sufficiently high with a homogeneous dispersion over the wafers after annealing at 250 ☌. The plasma ignites in the RIE-reactor (shown in figure "Scheme of a plasma reactor for low pressure plasma activated bonding"). Following, the surfaces of the wafers charge up negatively caused by the electrons and attract the positive ions of the plasma. The electrode attached to the HF-Generator is used as carrier of the wafer. HF power, this method is usable for surface activation. The RIE mode is used in dry etching processes and through reduction of parameters, i.e. Scheme of a plasma reactor for low pressure plasma activated bonding The surface activation with plasma at low pressure is processed in the following steps: Within those environment the surface activation is based on the striking ions and radicals interacting with the surface of the wafer (compare to figure "Scheme of a plasma reactor for low pressure plasma activated bonding"). ĭue to its positive orientation the massive ions, that are not able to follow the HF field, move to the negatively charged electrode, where the wafer is placed. The moving electrons of the atmosphere are banging into the plasma gas atoms and hit out electrons. The gap between the electrode and the chuck is filled with plasma gas. The most established frequency of the HF electrode is 13.56 MHz.įurther, the electrons are not able to leave the electrode within the positive half wave of applied voltage, so the negative electrode is charged up to 1000 V ( bias voltage). Electrons of the atmosphere move towards the HF electrode during its positive voltage. The plasma exposed surface is activated by ion bombardment and chemical reactions through radicals. High frequency (HF) electrical field between two electrodes.The Low Pressure-Plasma Activated Bonding operates in fine vacuum (0.1 – 100 Pa) with a continuous gas flow. Low Pressure-Plasma Activated Bonding (LP-PAB) Plasma activated wafer bonds can achieve fracture toughnesses that are comparable to bulk material. The bond strength is characterized by fracture toughness determined by micro chevron tests. If using glass, based on the high surface roughness, a chemical-mechanical planarization (CMP) step after rinsing is necessary to improve the bonding quality. Furthermore, treatment with plasma is suitable to prevent bond defects during the annealing procedure. The optimal gas mixture for the plasma treatment is depending on the annealing temperature. Process gases used for glass or LiTaO 3.Surface activation at atmospheric pressure.The wafer pairs pass the following process flow: Between the two electrodes plasma gas is ignited via alternating voltage. Ītmospheric Pressure-Plasma Activated Bonding enables the possibility to ignite plasma at specific local areas or the whole surface of the substrate. This method is to ignite plasma without using a low pressure environment, so no expensive equipment for vacuum generation is needed. ICP RIE ( inductively coupled plasma reactive ion etching)Ītmospheric Pressure-Plasma Activated Bonding (AP-PAB).Low Pressure-Plasma Activated Bonding (LP-PAB).Atmospheric Pressure-Plasma Activated Bonding (AP-PAB).Plasma activated bonding is based on process pressure divided into: To establish maximum surface energy at low temperatures (< 100 ☌) numerous parameters for plasma activation and annealing need to be optimized according to the bond material. Based on ambient pressure, two main surface activation fields using plasma treatment are established for wafer preprocessing to lower the temperatures during annealing. Further, the increase is caused by elevation in amount of Si-OH groups, removal of contaminants on the wafer surface, the enhancement of viscous flow of the surface layer and the enhanced diffusivity of water and gas trapped at the interface. The decrease of temperature is based on the increase of bonding strength using plasma activation on clean wafer surfaces. Surface activation prior to bonding has the typical advantage that no intermediate layer is needed and sufficiently high bonding energy is achieved after annealing at temperatures below 400 ☌. The main requirements for lowering temperatures of direct bonding are the use of materials melting at low temperatures and with different coefficients of thermal expansion (CTE). Plasma-activated bonding is a derivative, directed to lower processing temperatures for direct bonding with hydrophilic surfaces.
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