As dimensions get smaller and circuits get more complex, the demand for comprehensive measurements of reticule geometries increases. 3D characterization of phase shift mask (PSM) is required to understand the quality of the transferred image. To avoid anomalies between the measurements, the structures on both mask/reticule and wafer should be measured using the same technique. The technique used should be insensitive to differences in the intrinsic characteristics of the materials (chromium on quartz, resist on conductive or non-conductive layers). Scanning probe microscopy (SPM) is ideally suited to make these characterizations on both masks/reticule and wafers. It quantitatively profiles lines and trenches in three dimensions. SPM is a nondestructive technique, allowing for the preservation of the integrity of mask and wafers. The profiles of features on a phase shift mask (PSM) are measured with SPM. Some undesirable effects such as micro loading versus structure size during quartz etch, positive slope of the quartz sidewall, and CD differential between chromium and quartz are characterized. Some of the corresponding features on the wafer are measured with SPM and the correlation between the mask anomalies and their effect on wafer features are established.
Decreasing dimensions of features in semiconductor device manufacturing makes it imperative to control the sidewall, line and line-edge roughness. The roughness contributes to the variation in critical dimension (CD) and it might affect device functions and reliability. Roughness of vertical surfaces is needed in order to understand its effect on the performance, especially in the case of structures such as optical wave-guides. One of the ways to measure the sidewall, line and line-edge roughness is to use a scanning probe microscope. By using specific techniques in operating the scanning probe microscope and special analysis, we obtain the sidewall, line and line-edge roughness. We also use high-resolution image of the sidewall to characterize its roughness with various techniques including spatial frequency analysis. Both qualitative and quantitative evaluations are demonstrated. These measurements are made with an automated tool in a non-destructive fashion and are useful in production control.
Foot (bottom corner) characterization of a trench or a line in semiconductor processing is of high interest to set and follow processes. The scanning probe microscopes (SPM) available at the present time are not capable of obtaining this measurement. Conventional atomic force microscopes (AFM) are not able to measure the shape of the foot of a trench or a line due to scanning algorithm and probe shape. Even CD-AFM performed with 2 dimensional servo code and boot shaped tips is limited in its ability to make this measurement if the corner is sharper than the radius of the corner of the boot tip used for measurement. We use an extra sharp probe (full cone angle 5 degrees or less) and a technique to tilt the sample to get at the foot of the structure to be measured. We are able to scan this corner of the structure and are able to characterize it by various techniques such as surface roughness. In addition, sidewall, line and line edge roughness can be addressed using the same technique. This characterization can be performed automatically and set as a production control.
ULSI processing for the manufacture of devices such as DRAMs involves fabrication of several high aspect ratio structures. The determination and control of depth of these structures is crucial for device performance. We report the utilization of Atomic Force Microscopy to characterize 0.2 micrometers ground rule products. Features with 2.1 micrometers depth and 0.2 micrometers top nominal top width dimensions can be consistently measured. TO accomplish this a recently developed Deep Trench scan mode is employed and used in conjunction with new Super Angle Tapered CD and Super Angle COne tips. New analysis algorithms are developed to extract eh data in a repeatable from width decreased sensitivity to changes in tip size, top shape, and sample.
Mask making in the coming years will face major challenges. Feature measurements in the mask will no longer be restricted to 2D but will require information on the third dimension as well. High precision AFMs will be needed not only for measuring feature dimensions but also as a diagnostic tool for the fabrication of high quality masks.
Focus exposure matrices (FEMs) are a critical tool for evaluating the performance of lithographic processes. Any change in any process component, including critical dimension (CD) targets, chemistry, optics, or processing times requires that an FEM be run to verify process performance. Scheduled FEMs are also used as part of regular process monitoring. The CD-AFM is a powerful tool in evaluating FEMs. Unlike standard AFMs it quantitatively profiles lines and trenches in three dimensions. Further, none of the tedious and time consuming sample preparation required by cross-sectional TEM or SEM is needed; since samples need not be cleaved, profiles can conveniently be measured anywhere on the wafer and in any order. A CD-FEM is used to characterize an FEM wafer and the results are compared with those obtained with electron microscopy. First, the CD-AFM is calibrated, which includes the characterization of the tip geometry. Then, the measurements on the FEM wafer is made and the results computed taking the tip width into account. The measurements thus obtained are compared with SEM measurements.
Ever-smaller dimensions and more complex circuits demand ever more accurate and precise characterization of mask geometry. Feature must be characterized non-destructively for attributes that include width, undercut, centering, shorting, rounding, optical proximity correction (OPC) and seriff formation. Once characterized, the transfer function of these mask features to wafer features must be determined. The CD-AFM is a uniquely powerful tool for performing these measurements on masks and wafers. It is non-destructive and provides data unobtainable with standard AFMs or electron microscopes. Unlike standard AFMs, it quantitatively profiles lines and trenches in three dimensions. It does not require any of the tedious and time- consuming sample preparation required by cross-sectional TEM or SEM. Another advantage of the CD-AFM is that the samples need not be cleaved and profiles can conveniently be measured anywhere on the wafer and in any order. CD-AFM is used to characterize the mask and techniques for setting the lithographic process are developed. The CD-AFM is calibrated, which includes the characterization of CD-AFM tip-geometry. The effect of tip-geometry on measurement-precision and accuracy are analyzed. Measurement throughput is explored including the benefits of automated data acquisition and analysis.