Karin Ljungberg

Spontaneous bonding of hydrophobic silicon surfaces

The initial attraction of silicon surfaces etched in aqueous and buffered HF solutions have been studied, by observing the spontaneity and velocity of the contact wave. Also, the effect of a following water rinse was investigated. The bond strengths were determined by measuring the surface energy of the bonds at room temperature. Surfaces etched in an aqueous HF solution, with no subsequent water rinse before drying and contacting, bond spontaneously. If water rinsing is performed after the etch, the spontaneity is lost and the surfaces bond only if a pressure is applied. Surfaces etched in a buffered HF solution did not bond.

Modification of silicon surfaces with H2SO4:H2O2:HF and HNO3:HF for wafer bonding applications

Two combinations of oxidizing and etching agents, H2SO4:H2O2:HF and HNO3:HF, have been used to modify silicon surfaces prior to wafer bonding. The chemical oxide thickness can be adjusted between 0 and 10 Angstrom by tuning the HF content of the mixtures

Improved direct bonding of Si and SiO2 surfaces by cleaning in H2SO4:H2O2:HF

A method for silicon surface preparation prior to wafer bonding in presented. By cleaning thp wafers in a H2SO4:H2O2 mixture in which a small amount of HF is added, and then rinsing in H2O, the bon ...

A suggested mechanism for silicon direct bonding from studying hydrophilic and hydrophobic surfaces

Silicon direct bonding (SDB) between both hydrophilic and hydrophobic surfaces was studied by measuring the surface energies. Without annealing, roughly the same energies were measured for both types of surfaces. The strongest bonds were achieved for hydrophobic surfaces, annealed at 600 degrees C and above. The established mechanism for SDB, where OH groups are held responsible for the initial attraction force, cannot explain bonding between hydrophobic surfaces where almost no OH groups are present. By calculating the surface energy/mole, and comparing this to data for hydrogen bonds and van der Waals forces, a novel mechanism for SDB is proposed where the initial attraction is due to van der Waals forces.

The Effects of HF Cleaning Prior to Silicon Wafer Bonding

The effects of preparation of silicon surfaces in hydrofluoric acid (HF) solutions, prior to direct wafer bonding, is investigated. Surface analysis with atomic force microscopy, electron spectroscopy for chemical analysis, and estimation of the surface particle density is made. This is related to results from room temperature bonding experiments. A diluted (1-10%) HF solution is most favorable for hydrophobic silicon wafer bonding. The subsequent water rinse should be omitted, or performed in a careful way, to avoid particle contamination. HF:NH 4 F solutions generally are not favorable for bonding. The initial room temperature bonding is attributed to the relatively weak van der Waals forces, which makes the bonding sensitive to the surface roughness and particle density. The surface chemistry appears to have a second order influence in hydrophobic bonding

The influence of wafer dimensions on the contact wave velocity in silicon wafer bonding

The contact wave velocity in silicon wafer bonding is experimentally found to decrease with wafer thickness and to be only weakly dependent on wafer diameter. Wafers of different thicknesses ranging from 270 to 5000 μm, were dipped in HF:H2O before bonding to give the surfaces hydrophobic properties. A model based on energy conservation can explain the main characteristics of the experimental results. The contact wave velocity is determined by the amount of energy available as kinetic energy for the entrapped gas in the gap between the wafers. By increasing wafer thickness, the elastic energy stored in the material is increased, and the contact wave velocity is decreased.

Buried Cobalt Silicide Layers in Silicon Created by Wafer Bonding

A buried conductive layer in silicon has been created using wafer bonding technique, with a cobalt interfacial layer.Co-coated silicon wafers were brought into contact with either similar or uncoated wafers at room temperature. CoSi2 wasthen formed through a solid-phase reaction, during an anneal at 700 to 900°C. A 700 A buried CoSi2-layer, with a resistivityof approximately 21 µ cm, was achieved. Good adhesion, as measured by tensile strength testing, between the wafers wasachieved. Transmission electron microscopic investigations (Co-coated wafer bonded to bare silicon) showed that thesilicide has not grown into the opposite wafer, and that an amorphous layer exists between the silicide and the siliconsurface. The presence of such a layer has been confirmed by electrical characterization.

Optically trapped non-linear particles as probes for scanning near-field optical microscopy

Abstract We use the frequency doubled light from an optically trapped lithium niobate particle for non-intrusive scanning near-field optical microscopy. The detected power from this 50–100 nm diameter probe is currently tens of pW and is expected to approach nW with an improved detection system. The current experimental resolution is approximately 0.5 μm, while the ultimate theoretical resolution is 70–90 nm. An acoustic trap which potentially allows higher resolution imaging is briefly described.

Characterization of Spontaneously Bonded Hydrophobic Silicon Surfaces

The choice of surface treatment prior to silicon wafer bonding is crucial both for the reliability of the bonding process and the electrical properties of the bonded structure. In the case of silicon direct bonding for manufacturing of buried electrical junctions, the bonding step is particularly critical for accurate and reproducible results. In this work, the attraction between HF-etched surfaces and the effect of a following water rinse were investigated by studying the bonding spontaneity, velocity of the initial bonding wave, and the electrical properties of the bonded junctions. Spontaneous bonding occurred when no subsequent water rinse was made after the HF-etch, while water-rinsed wafers did bond only with the help of an applied pressure and also ended up with more voids

Formation of directly bonded Si/Si interfaces in ultra-high vacuum

Abstract The formation of Si/Si interfaces by surface cleaning and direct bonding in ultra-high vacuum, at a pressure in the 10 −10 Torr range, is presented. High bond strengths have been achieved for bonding temperatures above 450°C in the last heating step, by applying mechanical pressure. Spreading resistance measurements show features similar to conventional direct bonded interfaces. The increasing resistance at the location of the interface corresponds to approximately 5 × 10 10 positive elementary charges per cm 2 . UHV bonded interfaces with contamination levels below the SIMS detection limit can be made.