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Long term semiconductor storage Disclosure Number: IPCOM000010432D
Original Publication Date: 2002-Dec-25
Included in the Prior Art Database: 2002-Dec-25

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Differences in cycle times between semiconductor industry and for example, automotive industry, lead to storage problems. Automotive industry may use ASICs in Engine Management Computers that may be for whatever reason discontinued by semi-conductors manufacturers. As redesign of EMC for replacement of discontinued ASIC may not be economically feasible, and further, in view of obligation for the automotive equipment manufacturer to provide spare parts during a long period (i.e. 10 years after the last series vehicle), it may be necessary to store a huge quantity of said ASICs for a long period of time (up to 10 years). In order to avoid any solderability and delamination problems on packaged components and to reduce storage volume, the choice is made to store wafers. To keep the full integrity of the wafers during storage, it is mandatory to put them in a neutral atmosphere, with a minimum of oxygen and humidity. In addition, a mechanical protection against harsh environment (fire, flood, earthquake...) is mandatory. Usually, the solution chosen by the semiconductor industry for small stored volumes is to put the wafers in non hermetic boxes and to store the boxes containing the wafers in cabinets with constant nitrogen flow. In this case, because of the required holes to flow the nitrogen, the mechanical protection is not guaranteed. This is the reason why atmosphere control and mechanical protection have to be considered separately. Wafers are placed in closed plastic boxes containing 24 wafers (one manufacturing lot), each wafer separated by paper separators (appropriate paper sheet), ended by a foam aimed to absorb mechanical shocks during handling (see fig.1). The plastic boxes are put in double sealed bags filled with nitrogen. These bags are stacked up in safe watertight cabinets (vaults). In these conditions, a constant neutral atmosphere is guaranteed around the wafers and vaults ensure a safe mechanical protection. The main problem that has to be solved, is nitrogen saturation inside the bags. By construction, the foam retains gas and is saturated by standard atmosphere. Therefore, the first operation is to remove this atmosphere and to replace it by nitrogen. After several tests and trials the selected solution comprises the following steps :