We present an optoacoustic method to non-destructively measure the average dimensions of a periodic array of simple structures with aspect ratios greater than 10:1, which are inaccessible to AFM techniques. The technique that we describe could be used as the basis of an inline metrology tool for wafer inspection. The samples examined were test structures with high precision lithographically defined lines of silicon dioxide deposited on a silicon substrate. The thickness of the silicon dioxide was around 400 nm, and the gaps between the lines ranged from 100 nm down to smaller than 40 nm. A drop of water was placed on the sample, and an optoacoustic transducer was placed on top; measurements were taken with a water thickness less than 1 micron between the optoacoustic transducer and the sample. The water filled the spaces between the lines due to the hydrophilic nature of the sample surface. Using the picosecond ultrasonics technique, acoustic pulses are generated in a special optoacoustic transducer, transmitted through a coupling fluid (water), scattered off of the sample being examined and then return to the transducer. The returning acoustic signal shows nanometer sensitivity to the height of the lines and the specific details of their profile.