In contrast to semiconductor production of silicon wafers, which are processed individually, the manufacture of devices such as micromechanical sensors is accomplished using relatively small substrates. In this context, the base substrate is present in the form of a small cylindrical steel element. In order for the manufacturing process and for application of a circuit produced in thin-film technology onto such substrates to be cost-effective, the substrates are processed in multiple fashion. The individual base substrates are therefore retained in large numbers on a workpiece carrier, and processed together.
High-pressure sensors that are utilized in many automotive engineering systems and in automation technology are fabricated on such substrates. Applications include, for example, direct fuel injection, common-rail technology for diesel vehicles, electrohydraulic brake systems, vehicle dynamics control systems, and many more.
Pressure is sensed via the deflection of the sensor membrane, which is coated with a Wheatstone measurement bridge using thin-film technology. The circuit is patterned using a photolithographic manufacturing process. Contacting and passivation can also be performed using a production step of this kind. The quality of high-pressure sensors depends substantially on the accuracy with which the resistance features of the Wheatstone measurement bridge are positioned on the pressure membrane. The resistances must be centered as much as possible on the pressure membrane. This positional accuracy substantially influences the electrical properties of the measurement bridge, for example, the range of the signal furnished by the sensor.
It is believed that the photolithographic process of patterning the functional layer to form the resistances is therefore essential in the production of a high-pressure sensor. A preconditioning of the surface of the functional layer, application of a photosensitive resist onto the functional layer, an oven step to condition the resist for exposure, exposure of the resist through an exposure mask, development of the exposed features, an oven step to condition the developed resist structure for etching, mapping of the resist image into the functional layer using an etching process, and subsequent stripping of the photoresist mask from the completed etched functional structure, are usually performed in this context.
The location of the resistances may be determined in the exposure step. The substrates, previously diced and retained on a workpiece carrier, may then be exposed with sufficient positional accuracy relative to an exposure mask. In contrast to the exposure of silicon wafers in semiconductor production, in which the wafers can be registered as one piece with the exposure mask (e.g. a quartz mask) to be imaged, in the exposure of the previously diced plurality of substrates there may be a positional inaccuracy for each substrate with respect to the imaging exposure mask.
It is believed that the substrates are often handled in immovably bolted-down workpiece carriers that must each be individually registered with respect to the exposure mask. This registration operation is performed manually.
For example, in the projection exposure method of the Nagano Keiki Co. Ltd., a special workpiece carrier that is automatically positionable is provided in each case. This workpiece carrier is not identical for the subsequent processes.