Photolithography machine is the core equipment of IC. Its function is to transfer the mask pattern bearing IC layout information to the silicon wafer photoresist.
During the operation of the lithography machine, the light output from the system is reflected on the mask, and the unblocked light carries the mask graphic information, and the optical image of the mask graphic is formed after being collected by the projection objective lens. Photochemical reaction takes place on the photoresist which is reflected by the optical image to transfer the mask pattern.
The phenomenon that the material of the projection objective absorbs the energy of light and causes the temperature change inside the objective is called the thermal effect of the projection objective. When the photolithography machine is working, the laser continues to project the objective lens, the heat accumulates in the projection objective lens, the local temperature of the lens changes significantly, resulting in the refractive index of the lens material and the surface shape changes accordingly, and then increases the aberration of the projection objective lens. Thermal aberration caused by the thermal effect of the projection objective lens is called thermal aberration.
With the development of technology nodes, the range of aberration allowed by lithography machine is smaller and smaller. The factors such as the increase of light source power and the application of bright field mask with higher transmittance make the light energy of the projection objective lens larger. The application of advanced computational lithography techniques such as combined optimization of the source mask makes the distribution of light intensity to the projection objective more uneven. The thermal aberration of the lithographic projection objective lens in advanced nodes is larger. Thermal aberration has a more serious effect on the image quality of lithography machine. In addition, the thermal aberration will change dynamically with the time of exposure, and the effect on the image quality is more complicated. If not controlled, the thermal aberration size can be far beyond the range of aberration size allowed by the normal operation of the lithography machine, resulting in the lithography machine can not work properly.
The thermal aberration can be reduced by optimizing the objective lens design. The thermal aberration compensation technique can be used to reduce the effect of thermal aberration on image quality.
The thermal effect model of projection objective lens is mainly used for thermal aberration