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Center for Polymer Microfabrication (CPM)

MIT Laboratory for Manufacturing Productivity

NTU Biochemical Process Engineering Laboratory

Research:
systems, the factory and enterprise

Micro-hot-forging/embossing Production Systems

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An important activity is to examine the issues in creating practical factories for polymer microfabrication. Figure 1 shows a hypothetical factory design for a micro-hot-forging/embossing process. Raw material enters in the form of 400mm by 400mm sheets of 1 millimeter thick polymer. Each sheet is put into a vacuum die press to make channels in one side of one layer of a device. The die press must be operated with a precise temperature and pressure profile. The resulting sheets are called wafers. Many wafers could be pressed in a single batch, depending on the design of the machine. The wafers are then functionalized that is, the channels are treated physically or chemically so that they have certain specified properties. This is done by means of chemical baths, vapor deposition, or other process.

 

Figure 1. The micro-forging/embossing process.

 

The wafers are then inspected and unsatisfactory wafers are discarded. The acceptable wafers are bonded to one another sequentially. Alignment is critical so that the closed channels have the required geometries. In this example, three layers are bonded together. Next, the bonded sheets are cut into chips (approximately 25 x 25 mm) and they are inspected again. Connections and packaging are added to the chips, and they are inspected and tested one last time. Testing is destructive so only a sample is tested. The finished chip is sent to the endcustomer.

Many systems issues arise in considering this process. The first is whether this product will be produced in a make-to-stock or make-to-order basis. Make-to-stock production is appropriate when the product diversity is limited. Factories are operated in as steady a manner as possible. There can be substantial finished goods inventory, and often there is substantial work-in-process inventory, to avoid stockouts. Machines are heavily utilized. Make-to-order production often requires low in-process inventory, high raw material inventory, and excess capacity to keep lead times short. These considerations influence the amount and location of inventory.

We investigate the reliability and flexibility of the production machines. The required in-process inventory (and therefore the cost and lead time) decreases with increasing reliability and flexibility. The operation time of each step determines how many machines are needed in parallel to satisfy the expected demand. When operation times are long and machines are relatively cheap, the factory can be designed as a flow line with many machines in parallel at each process step. When operation times are short and machines are expensive, reentrant operation such as in semiconductor fabrication may appear to be necessary. This would lead to similar factory management difficulties that semiconductor manufacturers face.

Inspection is an important part of this process, and inspection always has two purposes: to reject or rework unsatisfactory items; and to monitor the performance of the machines. Inspection is particularly important when quality failures result from changes that occur in the machines. Inspection and testing strategies are very important, particularly since the final test is destructive.

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