Electrically-alterable read-only-memory using Si-rich SiO₂ injectors and a floating polycrystalline silicon storage layer

Abstract

Currently, electrically-alterable read-only-memory (EAROM) has become increasingly important for memory and logic operations. A novel EAROM device in a field-effect transistor (FET) configuration, which uses a floating polycrystalline silicon (poly-Si) layer on top of thermal SiO2 and a dual electron injector structure (DEIS) between this floating poly-Si and a control gate poly-Si contact, is described. The DEIS stack consists of sequentially chemically vapor deposited (CVD) layers of Si-rich SiO2 (46% atomic Si), SiO2, and Si-rich SiO2 (46% atomic Si) between the poly-Si layers. Electrons from either poly-Si layer can move to the other poly-Si layer biased at the higher voltage with moderate applied voltages. Thus, the floating poly-Si storage layer can be charged with electrons ("write" operation) or with positive charge ("erase" operation) in milliseconds with negative and positive control gate voltages, respectively. The average electric fields in the intervening CVD SiO2 layer during writing and erasing are 5-6 MV /cm and 4-5 MV/cm, respectively, and voltages from ±10 V to ±40 V can be used depending on the device configuration. The enhanced electron injection in these devices is believed to be controlled by localized electric field distortion at the Si-rich SiO2-SiO2 interface caused by the two phase (Si and SiO2) nature of the Si-rich SiO2. The electrical asymmetry of the DEIS is believed to be due to differences in the interfaces of the bottom and top Si-rich SiO2 injectors with the intervening SiO 2 layer. At the low voltages used for the "read" operation in which the charge state of the floating poly-Si layer is sensed by the FET drain current, no read perturb effects are observed. These structures also show excellent charge retention at low voltages, characteristic of a floating poly-Si storage layer surrounded by SiO2. This excellent retention is due to a characteristic of the Si-rich SiO2 in which it builds up a reversible space-charge layer which collapses the field at the interface with the poly-Si layers and minimizes charge loss or further injection. This field screening phenomenon of the Si-rich SiO2 layers also prevents low-voltage breakdowns (which are believed to be due to irregularities in the field at the cathode) from occurring. These DEIS EAROMs operate at lower power due to the small injected SiO2 currents involved and low applied voltages compared to most commercially available devices which use Si junction avalanche injection which requires large Si currents to charge the floating poly-Si storage layer. These devices are capable of being cycled at least 104 times before pronounced threshold-voltage window collapse due to trapped electron space-charge buildup in the intervening CVD SiO2 layer.