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To address the scalability of phase change memory (PCM), we study a 1-D thickness scaling effect on threshold switching voltage (Vth), Vth drift, high resistance state (RESET) resistance (RRESET) drift, and crystallization temperature (Tcrys). We use a pseudo three-terminal device to accurately correlate the amorphous region thickness to the observed characteristics. The pseudo 3-terminal device is a fully functional PCM cell and enables 1-D thickness scaling study down to 6 nm without the need for ultrafine lithography. Vth scales down to 0.65-0.5 V (at 25°C-75°C) for 6-nm-thick Ge2Sb2Te5 (GST), showing that stable read operation is possible in scaled PCM devices. The Vth drift measurement suggests that Vth drift can be attributed to threshold switching field (Eth) drift, whereas Vth0, i.e., Vth at zero thickness, stays almost constant. RRESET drift shows no dependence on the amorphous GST thickness. Tcrys is ~175°C for the device with 6-nm-thick GST, compared with ~145°C of thick GST. From the 1-D scaling study, no significant hurdles against scaling are found down to 6 nm. Further study of scaling effect on endurance and development of scalable selection device is needed to assess the ultimate scalability of PCM.