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We discuss the use of light scattered from a latent image to control photoresist exposure dose and focus conditions which results in improved control of the critical dimension (CD) of the developed photoresist. A laser at a non-exposing wavelength is used to illuminate a latent image grating. The light diffracted from the grating is directly related to the exposure dose and focus and thus to the resultant CD in the developed resist. Modeling has been done using rigorous coupled wave analysis to predict the diffraction from a latent image as a function of the substrate optical properties and the photoactive compound (PAC) concentration distribution inside the photoresist. It is possible to use the model to solve the inverse problem: given the diffraction, to predict the parameters of the latent image and hence the developed pauem. This latent image monitor can be implemented in a stepper to monitor exposure in situ, or prior to development to predict the developed CD of a wafer for early detection of bad devices.Experimentation has been conducted using various photoresists and substrates with excellent agreement between theoretical and experimental results. The technique has been used to characterize a test pattern with a focused spot as small as 36j.tm in diameter. Using diffracted light from a simulated closed-loop control of exposure dose, CD control was improved by as much as 4 times for substrates with variations in underlying film thickness, compared to using fixed exposure time. The latent image monitor has also been applied to wafers with rough metal substrates and focus optimization.

In this paper we discuss an optical meirology technique f the determination of optimum lithography parameters through an interrogation of the latent image. This technique, called the Lithography Process Monitor (LPM), involves illuminating a latent image grating with a laser beam. The intensity of the orders diffracted from the grating has been shown to be directly related to the photoactive compound (PAC) concentration profile, and consequently, to the proffle of the developed resist. The 0th order intensity is not influenced by focus and exposure conditions, and indicates the thickness of the resist and underlying films. The 1st order intensity is sensitive to both focus and exposure; however, it reaches a maximum at the optimum focus for all exposure conditions. We have developed a method of modeling the intensity in the diffracted orders by using lithography simulation software in conjunction with rigorous coupled wave diffraction analysis. The lithography simulator provides a PAC concentration proffle for given wafer compositions and lithography conditions. Diffraction analysis is then performed on this PAC proffle to determine the intensity of each diffracted order.Experiments have been conducted with both positive and negative resists. Three substrate types have currently been investigated: bare Si, poly-Si on oxide on Si, and amorphous-Si on oxide on Si. Using the LPM we have been able to determine the optimum stepper focus for all of these samples. In addition, we have been able to determine the "absolute" location of the top of the photoresist with respect to the stepper focal reference and determine film thickness variations on the wafer.