Use of diffracted light from latent images to improve lithography control

摘要

As the microelectronics industry strives to achieve smaller device design geometries, control of linewidth, or critical dimension (CD), becomes increasingly important. Currently, CD uniformity is controlled by exposing large numbers of samples for a fixed exposure time which is determined in advance by calibration techniques. This type of control does not accommodate variations in optical properties of the wafers that may occur during manufacturing. In this work, a relationship is demonstrated between the intensity of light diffracted from a latent image consisting of a periodic pattern in the undeveloped photoresist and the amount of energy absorbed by the resist material (the exposure dose). This relationship is used to simulate exposure dose control of photoresist on surfaces which have different optical properties chosen to represent surfaces typical of those found in operating process lines. Samples include a variety of photoresist materials and substrates with a wide variety of optical properties. The optical properties of the substrates were deliberately varied to determine the effect of these properties on CD (in the presence and absence of an exposure monitor) during lithography. It was observed that linewidth uniformity of the developed photoresist can be greatly improved when the intensity of diffracted light from the latent image is used to control the exposure dose. Diffraction from the latent image grating structures was modeled using rigorous coupled wave analysis. The modeling is used to predict the diffraction from a latent image as a function of the substrate optical properties and the parameters of the latent image (i.e., linewidth, sidewall angle). Good agreement is obtained between theoretical and experimental observations. Conversely, the inverse problem is solved in which the parameters of the diffracting structure (the latent image) are determined from a measurement of the diffracted power. Therefore, the diffracted power can be monitored for the purpose of determining when the latent image will produce the proper CD upon development.