A method that uses Fourier Transform Infrared (FTIR) Reflectance spectroscopy to determine the depths of poly silicon filled trenches is described. These trenches, which form the cells for trench DRAM, are arranged in arrays that are periodic in both directions. The method is non-contact and non-destructive. Large number of points per wafer can be easily measured to determine etch uniformity performance. Unlike cross section SEM based metrology, the wafer does not need to be cleaved, and thereby destroyed. The technique is thus suited for in-line metrology of product wafers. The FTIR technique was found t be very robust and provided excellent correlations with SEMs have been observed for 110 nm trenches and are reported in the paper. The method is a viable manufacturing solution for inline, non-destructive, rapid metrology on product wafers.
The silylation step in the top surface imaging process has been difficult to monitor and characterize for lack of appropriate metrology tools. Utilizing scatterometry to measure silylated wafers, we report successful monitoring of processing effects. Wafers were manufactured under nominally identical processing conditions. Applying scatterometry, we are able to discern location dependent variations within wafers. In addition, wafer to wafer variations are also observe. Both these variations are detrimental to yield. Variations in processing conditions cause modifications and perturbations in the gratings. Different gratings diffract light in a dissimilar manner. Processing conditions and their effects on the wafers are deduced from these measurements using computational analysis. This information is used to detect unwanted variations in processing conditions so that corrective responses can be implemented. This technique is rapid, non-destructive and sensitive to changes introduced by the silylation process.
We have initiated an effort to develop metrology tools that isolate the effect of each process step. Light scattered from diffracting structures is analyzed to determine characteristics of the structure. The technique is rapid, non-destructive, and extremely sensitive to variations in the samples that were examined. Through our technical collaboration with Texas Instruments Inc. we obtained wafers coated with surface imaging resists and exposed under varying focus and exposure conditions. We present results that utilize scatterometry to monitor the exposure step to determine defocus and exposure variations in the latent image. We also report using scatterometry to monitor the post-exposure bake (PEB) process for chemically amplified resists. Wafer-to-wafer variations in resist and underlying film thicknesses result in CD variations for constant exposure. The PEB time can be adjusted for each wafer to account for some of the parameter variations. We present experimental data supporting the concept of a scatterometer PEB monitor.
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