  Superthin galaxies are observed to have stellar disks with extremely small
minor-to-major axis ratios. In this work, we investigate the formation of
superthin galaxies in the TNG100 simulation. We trace the merger history and
investigate the evolution of galaxy properties of a selected sample of
superthin galaxies and a control sample of galaxies that share the same joint
probability distribution in the stellar-mass and color diagram. Through making
comparisons between the two galaxy samples, we find that present-day superthin
galaxies had similar morphologies as the control sample counterparts at higher
redshifts, but have developed extended flat `superthin' morphologies since $z
\sim 1$. During this latter evolution stage, superthin galaxies undergo
overwhelmingly higher frequency of prograde mergers (with orbit-spin angle
$\theta_{\rm orb} \leqslant 40^\circ$). Accordingly the spins of their dark
matter halos have grown significantly and become noticeably higher than that of
their normal disk counterparts. This further results in the buildup of their
stellar disks at larger distances much beyond the regimes of normal disk
galaxies. We also discuss the formation scenario of those superthin galaxies
that live in larger dark matter halos as satellite galaxies therein.
