Silicon micromachining often requires the selective removal of either surface or bulk layers of material from substrates. Often it is advantageous to protect one side of a substrate while the other side is exposed to the etchant. Sometimes an etch can take as long as 8-10 h at temperatures as high as 80-degrees-C, such as in wet etching of bulk silicon in potassium hydroxide (KOH) solution. Other times, the etch may be at room temperature, and there may be prolonged periods of exposure to concentrated hydrofluoric acid (HF) when removing sacrifical layers of oxide. Both KOH and concentrated HF are effective in etching silicon and silicon dioxide respectively, but there is difficulty in finding suitable materials that mask these etchants for long periods of time. Silicon nitride is a good mask for KOH, but is deposited at a relatively high temperature (800-degrees-C), while concentrated HF tends to delaminate or etch most materials. This makes it difficult to protect layers on silicon wafers after any metal deposition. This paper describes a system designed to wet etch substrates from one side only, protecting the front side to a high degree. The one-sided etching concept is introduced, followed by the design of the apparatus with variations on the basic concept. This is followed by a particular implementation in a KOH etching system. Also described as part of the KOH etching system is a simple and economical method for solution level detection based on the conductance of the solution. This detection system compensates for loss of water in the solution due to evaporation when etching at high temperatures. Finally, an example of the usefulness of the one-sided apparatus is given.
