METHOD FOR DETERMINING STEPPER FOCUS AND OFFSETS
Original Publication Date: 1992-Aug-01
Included in the Prior Art Database: 2002-Jan-15
Individual semiconductor layer features must be the right size and have tight critical dimension (CD) con- trol, or have low CD variation, for integrated circuits to function to performance and yield specs. One of the factors governing CD control is the stepper focus set- ting or the stepper focus offset (see Figure 1; ref. 1). A baseline stepper focus setting is determined for every stepper every day using the stepper self-metrology sys- tem, In-situ (2). Since this focus value is determined on a perfect substrate, namely flat silicon wafers, it may not be an appropriate value for real product wafers at some layers, where a significant amount of vertical fabrication layer depositions have occurred, or where due to reduced feature size required, a better centering of the focus set- ting is necessary to ensure good overall CD control within wafer and wafer to wafer. When these effects occur the data of Figure 1 would sbii? on the focus axis, the amount of focus offset induced by the substrate (see ref. 1 for an example). Even worse, the data of Figure 1 can be asymmetrical in defocus. A better method of obtaining the required layer-dependent focus offset value has been developed and is described here.