Microelectromechanical systems (hereinafter MEMS) have been developed for movable micro devices, such as hygroscopes, accelerometers, tunable RF capacitors, digital mirrors, sensors and the like. They are used for forming small electrical and mechanical structures on a substrate, particularly a substrate of silicon or a silicon-containing material. These devices are made using conventional semiconductor processing techniques, such as chemical vapor deposition and plasma etching for example.
FIG. 1 illustrates a conventional three-layer substrate that can be used to make a MEMS device. A substrate layer of silicon 10 is covered with a sacrificial layer of silicon oxide 11 and a layer of polysilicon 12 deposited thereover. In accordance with a prior art method, the silicon oxide layer 11 is etched away to at least partially separate the layer of polysilicon 12 from the substrate 10. This etch step is known as release. FIG. 2 illustrates a partial etch of the sacrificial silicon oxide layer 11, as by using an isotropic wet HF etch. Now the polysilicon feature 12 can move, e.g., bend toward and away from the silicon substrate 10. Suitable etchants are anhydrous and aqueous hydrogen fluoride (HF).
The above method however requires the steps of deposition and removal of the sacrificial layer, and requires a wet etch, as of HF, to etch through the sacrificial silicon oxide layer.
In addition to the extra steps required for depositing and isotropically etching away the sacrificial layer, release is also a problem. Release is a complex process wherein the silicon oxide layer must be controllably etched. If too much silicon oxide is removed, the desired structure is undercut; if too little silicon oxide is etched away, frozen microstructures are formed that are not able to move as intended. Further, such an etch produces residues which adhere to the substrate.
Another part of the problem is that different silicon oxides have different etch rates. Doped oxides, such as PSG, BPSG and doped TEOS oxide, attain a high and fairly stable etch rate quite rapidly. However, dense oxides, such as thermal oxides, TEOS oxides and high temperature oxides, have an etch initiation period, and a much slower etch rate, even after initiation. Etch rates can also vary with device design. Thus it is difficult to know how long the etch needs to be carried out for the desired release.
Aqueous HF has traditionally been used to isotropically etch silicon oxides. However, this etchant has a high surface tension that causes capillary and van der Waals forces that pull the released features toward the substrate until they contact each other. This results in a generally permanent adhesion of the feature to the substrate, a result known as stiction.
Anhydrous HF has also been tried as a release etchant. However, it is a very strong acid, and will damage materials used in the interior of the etch chamber, adding to the costs of the process. A special chamber must be made, one that is at least partially impervious to anhydrous HF, a difficult and expensive challenge.
Thus a method of releasing MEMS devices that does not cause stiction, and that can be done in situ in a single chamber with fewer steps and high throughput that results in reduced costs, would be highly desirable.