Linear and non-linear properties of slab drift waves in the negative sheared slab configuration modelling of the qmin surface region in negative shear tokamaks are studied, where qmin is the minimum value of the safety factor q. Linear calculations show that both the slab ion temperature gradient (ITG) driven mode and the slab electron temperature gradient (ETG) driven mode become strongly unstable around the qmin surface. Non-linear simulations are performed for ETG turbulence, which evolves on a much faster timescale than ITG turbulence. It is found that quasi-steady Er × B zonal flows are generated by an inverse wave energy cascade process. Linear stability analyses of the electrostatic Kelvin-Helmholtz (KH) mode show that the quasi-steady Er × B zonal flow profile is closely related to the q profile or to the magnetic shear, which has a stabilizing effect on the KH mode. It is shown that the microscopic quasi-steady Er × B zonal flows arising from ETG turbulence have a strong stabilizing effect on the slab ITG mode.