Electrical Characterization of the Access Transistor of Deep Trench Based DRAM Products via Backside Contacting

Dynamic Random Access Memory (DRAM) is the one most widespread commodity product of the microelectronic industry. Although the basis structure is quite simple, an indepth electrical characterization of the single cell is mostly correlated with huge efforts in terms of test patterns due to the multiple possibilities for leakage of the cell itself [1]. A direct characterization of the access transistor is not possible because of the missing contact on the drain side (Deep Trench side). A tentative method to overcome this problem has been reported by G. Zimmermann, by using a front side Focused Ion Beam (FIB) contact to access the drain [2]. Unfortunately this method is limited to “coarse” technologies down to 0.15µm due to the resolution of the FIB probe. In addition, the backside contacting via trench allows the measurement of resistance and/or leakage elements at the interface buried strap, Poly 1-Poly 2 within DT (process conditioned). This paper presents an innovative way to contact the access transistor from the backside of the die, using the deep trench of the cell itself as connection to the drain of the investigated device. The backside contact to the polysilicon filled DT is the key aspect of the method and is realised by backside Focused Ion Beam.