They're similar. However, an AFM directly measures the deflection of a probe you're rastering, while a STM measures the current caused by quantum tunneling betw

They're similar. However, an AFM directly measures the deflection of a probe you're rastering, while a STM measures the current caused by quantum tunneling between the probe and the sample. This is extremely sensitive to height because an electron's wavefunction degrades as e^(-sqrt(2*m_e*E)*x/hbar) with distance x and energy barrier E (the measured current will scale with the wavefunction^2). Additionally, a STM allows you to probe electron states as well as just topography.