Question

Which are the three orientations of silicon wafers most commonly employed in micromachining? Arrange them in order of most common usage and briefly describe their benefits and drawbacks.

Answer 1

Silicon bulk micromachining – processes that involve partial removal of bulk material in order to create three-dimensional structures or free devices. Bulk silicon micromachining is one of the simplest possible, and most obvious, structures is the patterning of insulated electrical conductors.

The orientation of a surface or a crystal plane may be defined by considering how the plane (or indeed any parallel plane) intersects the main crystallographic axes of the solid. The application of a set of rules leads to the assignment of the Miller Indices, (h,k,l) a set of numbers that quantify the intercepts and thus may be used to uniquely identify the plane or surface.

As demonstrated, a rectangular mask window will result in a mask undercut. When off-axis cut is present, both the off-axis angle and in-plane orientation of the mask windows to the crystal orientation determine this mask underetching. Assuming zero off-axis cut and an infinitesimal deep initial cavity in the silicon to initialize sideways etching, a fully developed mask underetch is established, bounded by the out-of-the-wafer-plane-oriented –planes.

The etch cavity can be determined by moving lines parallel to these orientations from the outside toward the mask center until they intersect with the mask window. The information, added to the mask layout by the crystallographic orientation, is substantial.

The following properties of silicon have made micro machining feasible

(a) The possible application of this could be to use electric fields to manipulate individual cells.

(b) Silicon can be readily oxidized by exposing it to steam or dry oxygen. It allows silicon wafer to be masked selectively during chemical etching.

(c) Single crystal silicon is brittle and can be cleaved like diamond but it is harder than most metals. It is resistant to mechanical stresses and the elastic limit of Silicon is greater than that of steel.

(d) A single crystal silicon can withstand repeated cycles of compressive and tensile stresses.

(e) The crystal orientation of single crystal silicon wafer decides the rate of chemical etching in certain etching solutions, which is important in creating various structures. Bulk micromachining and surface micromachining are the two distinctly different approaches of micromachining silicon for realizing micro sensors and actuators. Isotropic and anisotropic etching of silicon has been used for realizing micro mechanical parts from bulk silicon wafer and forms the basis of “bulk micromachining”. In another approach for micro machining called “surface micro machining”, the silicon substrate is primarily used as a mechanical support upon which the micro mechanical elements are fabricated. The bulk of the silicon wafer itself is not etched in surface micro machining. There are no holes through the wafer and No cavities on the backside. When bulk micro machining silicon, the backside of the wafer is conventionally protected against an etchant with an oxide or nitride layer in which Windows are opened where the micro mechanical structures are to emerge. An accurate alignment of the etch windows is essential to obtain the structures at a proper position with respect to the photolithographic patterns at the front. In surface micro machining a sacrificial layer is deposited on the silicon substrate, which may be coated first with an isolation layer. Windows are opened in the sacrificial layer and the micro structural thin film is deposited and etched. Selective etching of the sacrificial layer leaves a freestanding micro mechanical structure. There are two main methods of etching – wet etching and dry etching. Wet etching is done with the use of chemicals. A batch of wafers is dipped into a highly concentrated pool of acid and the exposed areas of the wafer are etched away. Dry etching refers to any of the methods of etching that use gas instead of chemical etchants. Bulk micromaching of silicon uses wet and dry etching techniques in conjunction with etch masks and etch stops to sculpt micromechanical devices from silicon substrate. The selective etching of silicon can be carried by using isotropic and anisotropic etchant. The isotropic etchant under-etch large area in lateral direction than the area defined by mask opening. On the other hand, anisotropic etchant, which are also known as orientation dependent or crystallographic etchant, etch the silicon surface at different rates in different directions in the crystal lattice. They can form well-defined shapes with sharp edges and corners