A floating gate (110) of a nonvolatile memory cell is formed in a trench (114) in a semiconductor substrate (220). A dielectric (128) covers the surface of the trench. The wordline (140) has a portion overlying the trench. The cell's floating gate transistor has a first source/drain region (226), a channel region (224), and a second source/drain region (130). The dielectric (128) is stronger against leakage near at least a portion of the first source/drain region (122) than near at least a portion of the channel region. The stronger portion (128.1) of the additional dielectric improves data retention without increasing the programming and erase times if the programming and erase operations do not rely on a current through the stronger portion. Additional dielectric (210) has a portion located below the top surface of the substrate between the trench and a top part of the second source/drain region (130). The second source/drain region has a part located below the additional dielectric and meeting the trench. The additional dielectric can be formed with shallow trench isolation technology. The additional dielectric reduces the capacitance between the second source/drain region (130) and the floating gate.