Method of controlling semiconductor device fabrication
A semiconductor wafer fabrication metrology method in which process steps are characterised by a change in wafer mass, whereby during fabrication mass is used as a measurable parameter to implement statistical process control on the one or more of process steps. In one aspect, the shape of a measured mass distribution is compared with the shape of a predetermined characteristic mass distribution to monitor the process. An determined empirical relationship between a control variable of the process and the characteristic mass change may enable differences between the measured mass distribution and characteristic mass distribution to provide information about the control variable. In another aspect, the relative position of an individual measured wafer mass change in a current distribution provides information about individual wafer problems independently from general process problems.
1. A method of statistical process control (SPC) for semiconductor wafer fabrication comprising a plurality of fabrication processes, the method including the steps of:
obtaining a characteristic mass change distribution for each one of the plurality of fabrication processes;
measuring a change in mass for each fabrication process for one or more monitored semiconductor wafers subjected to the plurality of fabrication processes;
comparing each measured change in mass to the characteristic mass change distribution of its respective fabrication process; and
generating for each monitored semiconductor wafer process control data representative of the position of each measured change in mass relative to its respective characteristic mass change distribution for each one of the plurality of fabrication processes completed for that monitored semiconductor wafer.
2. A method according to claim 1 , wherein obtaining the characteristic mass change distribution at a point in time for each one of the plurality of fabrication processes includes the steps of:
measuring a change in mass for each of a plurality of semiconductor wafers subjected to the fabrication process in a period immediately before the point in time;
generating a current measured mass change distribution consisting of the changes in mass which were measured in the period immediately before the point in time; and
using the current'measured mass distribution as the characteristic mass change distribution for monitored semiconductor wafers subjected to the fabrication process after the point in time.
3. A method according to claim 2 , wherein the current measured mass change distribution consists of a predetermined number of measured mass changes.
4. A method according to claim 2 , wherein the current measured mass change distribution consists of all mass changes measured in a predetermined period immediately before the point in time.
5. A method according to claim 2 , including periodically updating the measured mass change distribution.
6. A method according to claim 1 including determining whether or not a monitored semiconductor wafer is to progress to a future fabrication step based on the generated process control data.