OWE (Overall Wafer Effectiveness) and MFU (Mask Field Utilization) are two closely related terms and both are about how ASIC can be effectively and efficiently manufactured at foundry. This post tries to describe what they are and it is based on a the patent of Method for enhancing wafer exposure effectiveness and efficiency.
The wafer exposure effectiveness, the accounting of the gross number of dies per wafer is determined by the chip size and the exposure layout. In addition to the wafer exposure efficiency, the shot number of exposure equipment (i.e. stepper or scanner) for wafer exposure per layer is determined by the utilization of mask field which is also influenced by chip size.
In the wafer fabrication, a wafer is exposed to a multiple-step photolithography (exposure) process, each of the steps with one mask. One of the mask defines one layer on which the electronic device is fabricated. A stepper (or a scanner) is the equipment used to perform the photolithography process. The stepper steps from one location to another on the wafer until the entire wafer is exposed by projecting light through the mask. Each movement (step) of the stepper
is called “one exposure shot’.
In above figure, on the wafer, each die 21 on the wafer is defined and separated by scribe lines 22. By breaking or cutting on the scribe lines 22, the dies (chips) can be separated from the wafer 2. The figure also shows multiple incomplete exposure dies 23 along the wafer border and they are not exposed completely and cannot function well.
The gross die number and exposure shot number are mainly influenced by the chip area, the product of the chip length and the chip width. The chip length and the chip width are denoted as L and W. For a given chip size, by changing the chip length l and the chip width w, various combinations of the gross die
number (HGD) and exposure shot number (fishot) can be achieved.
OWE can be represented as:
MFU can be represented as:
where lmf, is the length of the mask filed, wmf is the width of the mask filed, lef is the length of the exposure field and wef is the width of the exposure field.
The scanner is a machine that creates die images on wafers. It starts by shining a light through a reticle (sometimes referred to as a mask) onto the wafer – which has been coated with a protective photoresist – to imprint an image of the reticle pattern. The light that hits the wafer causes some of the photoresist to be removed, so that when the wafer is given a chemical bath, the silicon that no longer has the photoresist coating gets etched away to create the circuit layer. Then, after cleaning the wafer and recoating it with another photoresist, the scanner creates the rest of the circuit layers by repeating this process, one mask layer at a time, which is referred to as photolithography.
The scanner’s effectiveness is determined primarily by the time duration before the actual scan of the die image (which is referred to as the pre-scan) and after the scan is complete (the over-scan). Hence, a reticle that has a larger percentage of its area occupied by the die image – that is, a reticle layout with a high MFU – translates to less pre-scan and over-scan times at each shot because it requires fewer overall shots to step the die image across the entire wafer
The relationship of OWE and MFU can be shown as: