1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory, and in particular, to resistors in a nonvolatile semiconductor memory having a stack structure and a method of manufacturing the resistive elements.
2. Description of the Related Art
Nonvolatile semiconductor memories use elements of a fixed resistance to generate internal voltages. For example, NAND flash memories use a polysilicon layer serving as a floating gate electrode layer, as a resistor.
As design rule dimensions decrease, the effect of, for example, a mutual interference resulting from the crosstalk between adjacent memory cell transistors becomes serious. Accordingly, to reduce the effect of such a mutual interference or the like, the floating gate electrode layer tends to have an increasingly reduced film thickness.
The reduced film thickness of the floating gate electrode layer significantly varies a resistance. The resistors elements may thus fail to provide appropriate functions, affecting circuit operations.
For example, a nonvolatile semiconductor memory has already been disclosed in which a gate oxide film material and a first gate electrode material (floating gate electrode layer) are sequentially deposited on a semiconductor substrate in this order, then an inter-gate insulating film material and a second gate electrode material (control gate electrode layer) are sequentially deposited on the first gate electrode in this order, and the second gate electrode material (control gate electrode layer) is formed into a resistor in a peripheral circuit portion (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-110825).
When the control gate electrode layer is used for the resistor, the resistor disadvantageously has an increased area if its resistance remains unchanged. This is because the control gate electrode layer comprises a stack structure of silicide with metal such as tungsten or cobalt and polysilicon and thus has a lower resistivity than the floating gate electrode layer.