The Sacrificial Layer Technique

A combined fabrication technology made possible through a sacrificial layer technique was developed to obtain partly or totally movable microstmctures together with fixed structures on a single substrate. Many practitioners consider this ability to be the hall mark of the LIGA process. 

The substrate is first coated, for instance, via electroless or physical vapor deposition with a thin ( 1 pm) metallic layer, which in turn is patterned by photolithography and wet etching. This layer serves two roles, as a plating base and as an electrically conducting layer for the finished structures. In the subsequent step a sacrificial layer, of about 5 pm in tliickness, is deposited on the substrate and also patterned by photolithography and wet etching. Titanium is used most often as the sacrificial material because it adheres well to the resist and to the electrodeposited layer and can be etched with hydrofluoric acid that does not attack other materials such as chromium, silver, nickel, copper, and which are usually used in the LIGA process. 

The standard LIGA process is then followed polymerization of the thick X-ray resist directly on to the substrate, exposure to synchrotron radiation through a precisely adjusted mask, development of the resist, and electrodeposition. Some parts of the metallic microstructures are built up on the first metal layer, while other parts lie on top of the sacrificial layer. After stripping the resist, the sacrificial layer is etched selectively against all the other materials. 

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The sacrificial layer techniques can be difficult to apply as they require a delicate choice of materials and etching methods that might not be compatible with the CPs. Also, the method introduces a step and thus a weak spot in the actuator (see Fig. 4A4). Therefore, a new method was developed that utilizes the poor adhesion between two materials, e.g., between Au and Si, the so-called differential adhesion method (Smela et al. 1995). First, an adhesive frame is patterned on the substrate that surrounds the actuator, except for an anchoring point (Fig. 4D). In the case of Si and... 

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