Semiconductor wafers comprising micro-mechanical components, for example, micro-mirrors, are known. The micro-mechanical components are formed in a membrane layer, and where the micro-mechanical components are micro-mirrors, the micro-mirrors are of thickness similar to the depth of the membrane layer. In order to minimise the mass of the micro-mirrors, and in turn, the drive voltage required for tilting the micro-mirrors, the micro-mirrors should be relatively thin, and typically, of thickness in the range of 3 μm to 10 μm, and preferably, closer to 3 μm for minimising the drive voltage required to tilt the micro-mirrors. Thus, the membrane layer in which the micro-mirrors are to be formed should be of depth similar to the desired thickness of the micro-mirrors, and thus, in the range of 3 μm to 10 μm, and preferably, in the order of 3 μm.
The membrane layer is supported on a handle layer, and in general, a buried insulating layer is disposed between the handle layer and the membrane layer. Generally, both the handle layer and the membrane layer are of silicon, and the buried insulating layer is an oxide layer. The oxide layer may be formed on either the handle layer or the membrane layer and laminated to the other of the handle layer and the membrane layer by a suitable bonding process. The buried oxide layer may be either grown, or may be deposited by, for example, chemical vapour deposition. After the membrane layer and the handle layer have been laminated together with the buried oxide layer disposed between the respective layers, the membrane layer is machined to the desired depth, which as mentioned above is typically in the range of 3 μm to 10 μm. The handle layer typically is of depth in the range of 300 μm to 400 μm while the buried oxide layer is of depth typically less than 500 nm.
After lamination and machining of the membrane layer and the handle layer, a photo-resist layer is deposited on the exposed surface of the membrane layer, which is patterned to define the micro-mechanical components. The membrane layer is etched to form a plurality of trenches which extend through the membrane layer to the buried oxide layer for defining the micro-mechanical components. Where the micro-mechanical components are micro-mirrors, two arcuate trenches are typically formed to define each micro-mirror. The two trenches together define the micro-mirror and a pair of tethers. The tethers extend between the membrane layer and the micro-mirror on respective opposite sides of the micro-mirror for supporting the micro-mirror in the membrane layer. The tethers of each micro-mirror define a pivot axis about which the micro-mirror is tiltable. After the membrane layer has been etched to form the micro-mirrors, the handle layer is then etched to form communicating openings to the respective micro-mirrors. The communicating openings are typically formed by through bores which extend through the handle layer and the buried oxide layer. Typically, the through bores are of cross-sectional area greater than the area of the micro-mirrors, and typically, each through bore is of cross-sectional area similar to the area of the micro-mirror plus the width of the trenches on opposite sides of the micro-mirror. At an appropriate time one or both faces of the micro-mirrors are metallised for providing a reflective surface or surfaces on each micro-mirror.
The actual etching of the trenches in the membrane layer to the buried oxide layer for defining the micro-mirrors and their tethers is a relatively straightforward procedure. However, once the trenches have been formed in the membrane layer, the portions of the buried oxide layer bridging the trenches are unsupported on the membrane layer side of the buried oxide layer. Additionally, during etching of the through bores through the handle layer, the buried-oxide layer acts as an etch stop layer. Some of the through bores etch faster than others, due to variation of the etch rate across the semiconductor wafer, and thus, the portions of the buried oxide layer at the end of some of the through bores will be exposed to the through bore etch longer than others. The through bore etch has an etching effect on the portions of the buried oxide layer exposed by the through bores, and those portions of the buried oxide layer which are subjected to the through bore etch for a relatively lengthy duration are etched to a relatively thin depth. Additionally, the through bore etch induces stresses in the portions of the buried oxide layer which are exposed by the through bores, and which have been subjected to the through bore etch for relatively lengthy durations.
It has been found that the magnitude of the stresses increases as a function of the duration for which the portions of the buried oxide layer are subjected to the through bore etch and the amount by which the buried oxide layer has been thinned. It has been found that when the buried oxide layer has been thinned to a thickness of less than 200 nm, it is no longer able to withstand the induced stresses, and the unsupported portions of the buried oxide layer commence to curl up at the periphery of the through bores, and thus rupture from the buried oxide layer which is buried between the membrane and handle layers. In other words, the portions of the buried oxide layer adjacent the peripheral edge of some of the through bores which are unsupported by virtue of the arcuate trenches having been formed in the membrane layer curl up into the through bore, thus rupturing from the remainder of the buried oxide layer. Since the tethers which are supporting the micro-mirrors are of relatively small transverse cross-section and are attached to the buried oxide layer, the curling up of the peripheral edges of the portions of the buried oxide layer can cause one or both of the tethers to curl up with the buried oxide layer. This, thus, causes the tether or tethers to rupture from the membrane layer, thus leaving the micro-mirror partly or wholly unsupported in the membrane layer.
Even if the buried oxide layer does not curl up adjacent the periphery of the through bore, thus leaving the tethers intact, it has been found that the stresses induced in the portions of the buried oxide layer at the end of the through bores, which have been exposed to the through bore etch for a relatively lengthy duration, cause the oxide layer to bow. Since the micro-mirrors are relatively thin, in the range of 3 μm to 10 μm and are attached to the buried oxide layer, the bowing of the oxide layer causes similar bowing in the micro-mirrors. Such bowing leaves the micro-mirrors unsuitable for use.
The problem of rupturing of the tethers is particularly common in semiconductor wafers where the depth of the membrane layer is at the lower end of the range of 3 μm to 10 μm, and in turn the micro-mirrors or other micro-mechanical components which are of similarly small thickness. Increasing the thickness of the buried oxide layer, while it may overcome the problem of rupturing of the unsupported bridging portions of the oxide layer, results in another problem. It has been found that the stresses induced in portions of thicker buried oxide layers at the end of the through bores which have been exposed to the through bore etch for relatively lengthy durations are such as to cause bowing of the buried oxide layer at the end of the through bore. Since the micro-mirrors or other micro-mechanical components are attached to the buried oxide layer, bowing of the buried oxide layer causes bowing of the micro-mirrors or other micro-mechanical components, which in many cases leads to rupturing of the component, and where the component is a micro-mirror, the micro-mirror may rupture or one or both of its tethers may rupture.
The problem of bowing of micro-mirrors resulting from bowing of the buried oxide layer also increases as the depth of the membrane layer, and thus the thickness of the micro-mirror or micro-mechanical component is reduced, and the thickness of the buried oxide layer is increased, since the thinner the micro-mirror or micro-mechanical component is, the more susceptible it is to bowing, resulting from bowing of the buried oxide layer.
There is therefore a, need for a method for forming a micro-mirror and for forming other micro-mechanical components in a semiconductor wafer wherein the risk of rupturing or deforming of the micro-mechanical component or a part thereof is minimised.
The present invention is directed towards providing such a method, and the invention is also directed towards providing a semiconductor wafer comprising a micro-mechanical component formed therein.