Although scanning electron microscopes (SEM) are usually used for measuring submicrometer critical dimensions (CD), the shortcomings of using an SEM for this purpose have prompted an investigation of the use of a new alternative, the confocal scanning laser microscope (CSLM). A SiScan-I instrument manufactured by SiScan Systems was used in this study. The most attractive features of a CSLM are its reliability, ease of use, and high precision. CDs as small as 0.5 pm can be measured on most samples with an instrumental precision of about 0.03μm (3 σ ). However, variable systematic errors may reduce the practical precision of the instrument if certain parameters are not tightly controlled from sample to sample. Results are presented that show how CD measurements are affected by the following key parameters: 1) thickness of the resist, 2) thickness of an underlying transparent layer, 3) size of the feature, and 4) slope of the edge wall of the resist. Although all of these parameters are found to affect the CSLM measurements, errors due to variations in parameters 1, 3, and 4 will probably be insignificant for a well controlled lithographig process.. Variations in parameter 2 are often less well controlled and systematic CD errors on the order of 100 to 200 A per 100 A of variation in the film thickness can be expected. Suggestions are made whereby operators can recognize and compensate for these types of systematic errors. It appears that CDs of most samples can be measured on a CSLM with an effective precision of about 0.05 μm (3σ ), which is comparable to that of an SEM based measuring system. This precision estimate includes 0.03 μm for random instrumental errors and up to 0.04 μm for systematic errors due to random variations in the sample.