Performing Phase Assignments To Reduce Transmission Imbalances Between Phase Shifters
Publication Date: 2002-Sep-27
The IP.com Prior Art Database
In alternating aperature phase shifting masks, there can be a transmission imbalance between the zero and 180 degree phase shifters. A phase assignment strategy is described to help reduce the effects of the imbalance.
PERFORMING PHASE ASSIGNMENTS TO REDUCE TRANSMISSION IMBALANCES BETWEEN PHASE SHIFTERS
between zero-degree and 180-degree phase shifters on a phase shifting mask.
180-degree phase shifters are generally formed by cutting deeper into the quartz of the phase shifting mask. These deeper cuts give rise to "side- wall scattering," which reduces the amount of light transmitted through 180- degree phase shifting regions. This reduction in transmission can create a
"transmission imbalance" between equally sized zero-degree and 180-degree phase shifters.
Field of the Invention
The invention relates to the process of fabricating semiconductor chips.
More specifically, the invention relates to a method and an apparatus for performing phase assignments in a manner that facilitates reducing transmission imbalances between zero-degree phase shifters and 180-degree phase shifters during an optical lithography process for manufacturing a semiconductor chip.
As integration densities continue to increase, it is becoming desirable to use phase shifters on a phase shifting mask to define more and more features within a layout of a semiconductor chip. This can lead to a number of problems. One problem arises as a consequence of the transmission imbalance
In order to compensate for this transmission imbalance problem, it is possible to increase the size of 180-degree phase shifters relative to the size of corresponding zero-degree phase shifters. Unfortunately, in many situations, the tight spacing between circuit elements makes it hard to increase the size of 180- degree phase shifters.
Hence, what is needed is a method and an apparatus that facilitates compensating for the transmission imbalance problem in the presence of tight spacing between circuit elements.
FIG. 1 illustrates an exemplary layout in accordance with an embodiment of the present invention. In this layout, phase shifters 104 and 106 are used to define a gate region 105 of a first transistor. This means that phase shifter 104 must be opposite in phase from phase shifter 106. Additionally, phase shifters 106 and 108 are used to define a gate region 107 of a second transistor.
This means that phase shifter 106 must be opposite in phase from phase shifter
108. Hence, phase shifters 104 and 108 must have the same phase, and phase shifter 106 must have the opposite phase.
Note that two possible phase assignments will work in this example. (1) Phase shifters 104 and 108 can be 180-degree and phase shifter 106
can be zero-degree. Alternatively, phase shifter 106 can be 180-degree and phase shifters 104 and 108 can be zero-degree.
However, note that phase shifter 106 is sandwiched between gate regions 105 and 107, which leaves little room for phase shifter 106 to expand to compensate for the transmission imbalance problem. In this case, it is desirable
for phase shifters 104 and 108 to be...