Chris Yang

On the residual stress and fracture strength of crystalline silicon wafers

This letter reports on residual stress measurement in thin crystalline silicon wafers with a full-field near-infrared polariscope. Residual stress is analyzed in combination with observed surface defects, and the results are related to measured fracture strength variation in the wafers. Measurements indicate that there is a sawing process-related residual stress in the as-cut wafers, and that etch-removal of ∼5 μm from the wafer surface eliminates a damage layer that can significantly reduce the residual stress in the wafer, and therefore increases the observed fracture strength. There is a corresponding 2 to 3 μm reduction in the observed characteristic defect size after etching. Fracture strength anisotropy observed in the wafers is related to defect orientation (scratching grooves and microcracks) caused by the sawing process.

On the Fracture Toughness Measurement of Thin Film Coated Silicon Wafers

This paper reports on the effect of residual stress on the microcrack propagation of thin-film coated silicon wafers and the fracture toughness measurement using the micro indentation technique. Two types of silicon nitride films with different stress states, a high tensile vs. a low compressive stress, deposited by a LPCVD method were studied. It is found that a high tensile residual stress in the film leads to longer microcracks and a lower fracture toughness of silicon, while a compressive stress depresses the microcrack propagation and increases the toughness of silicon. To improve the mechanical performance of silicon devices, a slightly compressive stress in the film is suggested.

Characteristic Study of Silicon Nitride Films Deposited by LPCVD and PECVD

This paper analyzes and compares the characteristics of silicon nitride films deposited by low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), with special attention to the hydrogenation and chemical composition of silicon nitride films. Three different LPCVD processes at various DCS and NH3 gas flow rates and deposition temperatures, together with PECVD using SiH4 and NH3 and ICP CVD using SiH4 and N2, were compared. The silicon nitride film deposition rate decreases with an increasing NH3/DCS ratio in LPCVD, which also leads to an increase in the refractive index and a decrease in the residual stress in the film. There is nearly no hydrogen incorporated in the LPCVD films, which differs from PECVD and ICP CVD that show significant Si-H and N-H bonds. The chemical composition of silicon nitride films is mostly Si-rich, except for the LPCVD process at high NH3/DCS ratio with near stoichiometric chemistry.

Tribologically-induced Damage in Cutting and Polishing of Silicon

This paper discusses the influence of tribology on the mechanical properties of cutting, shaping and forming silicon wafers. These processes, such as multi wire slurry sawing, diamond wire sawing, lapping and grinding, Chemical Mechanical Polishing (CMP), and dicing, utilize either a two-body or a three-body material removal, where the fundamental cutting process results from micro-fracturing of silicon by the hard abrasives. There are specific types of defects and related fracture strength characteristics for each process. The associated surface and subsurface damage, especially microcracks, have a dominant effect on the fracture strength of the silicon substrates, even playing a more significant role than edge chipping. There is a need to reduce the surface and subsurface damage, possibly through ductile regime machining/polishing, to improve the mechanical strength. KeywordsFracture Strength; Defects; Microcracks; Silicon

A multi-purpose wafer scanning system for PV inspection

This paper describes a PV wafer scanning system that can perform multiple measurements on one platform. The scanning system consists of two probes (a Kelvin probe and a fiber optic probe), a light source, and a monochromator. The Kelvin probe is used to measure the surface potential at different illumination conditions. Surface uniformity and defects, particularly shunting, can be identified from various types of surface potential maps. A monochromator is incorporated with the Kelvin probe to yield the spectral response of solar cells. The fiber optic probe is used to measure the crystalline orientation - related reflectivity and stress-induced wafer curvature. Applications for the scanning system in PV wafer and cell inspections are discussed.

Fracture strength analysis of slurry cut mc-Si wafers

The fracture strength variation across the surface of multi-crystalline silicon (mc-Si) wafers cut by multi-wire slurry sawing (MWSS) was studied in this paper. As-sawn 156×156mm2 mc-Si wafers were diced in two orthogonal orientations relative to the wire speed direction to yield small rectangular samples whose fracture strength was measured using the four line bending method. It is shown that the fracture strength of the wafer increases gradually from wire entry to wire exit along the wire speed direction whereas the variation in strength in the wire feed direction is small. Wafer samples bent in the orientation perpendicular to the wire speed direction show a slightly higher fracture strength than those bent parallel to the wire speed direction. Wafer samples from an interior and a corner brick show similar fracture strength variation. The results also reveal the critical role of abrasive-induced surface damage and the associated fracture strength of the silicon wafers.

Crack induced surface potential variation on Si PV cells

This paper describes measurements of the surface potential (SP) of silicon surfaces that contain cracks. The impact of cracks on the PV performance is also discussed using light illumination as compared to the dark condition. The surface potential was measured using the Kelvin probe technique, in both vibrating and non-vibrating modes, and the data were collected on bare silicon wafers and monocrystalline PV cells. It is found that there is almost no surface depletion on the newly cracked interfaces, which is different from the uncracked surface. The electrical field discontinuity at the crack surface brings about contact potential difference (CPD) signals in the non-vibrating mode. The SP at the crack surfaces is reversible and experimentally measured to be 23mV and 44mV for the light and dark conditions respectively. There is a decreasing surface potential at the cracks in the PV cells, which is similar to that on bare silicon wafers. The SP in a PV cell is normally at 4.6 to 4.8V in the dark condition, but only at about 4.4V at a crack. The impact of the cracks in PV cells varies with the status of the surface, which may behave as an open circuit or a current drain. The average of SP difference between the light and dark conditions in a PV cell is at 350mV. However, the SP difference reduces to 250mV at an open crack or less than 70mV at a shunted crack. The cracks in PV cells would lead to a power loss in both cases.

Surface Characterization with an Ionization Probe

This paper discusses the application of an ionizing source coupled with galvanic differences between metals in a measure of the work function difference between the metal surfaces. The electrical field generated from the contact potential difference (CPD) between two electrodes will cause the gaseous ions to discharge at both surfaces, creating a measurable current. The current depends on the surface size, spacing, and ionizing source power. One of the surfaces (probe) can vary in shape and size, and if inert, can be used to obtain the work function or surface potential of the second surface. The ionic current is proportional to ion mobility, ion generation rate, CPD, and the probe size, but inversely proportional to the spacing between the probe and the sample. It is found, as expected, that there is an approximate linear relationship between the ionization probe signal and the work function of the surfaces of metals.