1. Field
The present invention relates to semiconductor devices, and, more particularly, to such devices having nanoclusters.
2. Description of the Related Art
Some devices such as memories (e.g. non volatile memories) utilize discrete charge storage elements called nanoclusters (e.g. of silicon, silicon germanium, aluminum, gold, or germanium) for storing charge in a charge storage location of a transistor. In some examples, the nanoclusters are located between two dielectric layers, a bottom or tunnel dielectric and a top or control dielectric. Examples of such transistors include thin film storage transistors. A memory typically includes an array of such transistors. Examples of nanocluster types include doped and undoped semiconductor nanoclusters such as silicon nanocrystals, germanium nanocrystals and their alloys. Other examples of nanocluster types include various conductive structures such as metal nanoclusters (e.g., gold nanoclusters and aluminum nanoclusters), and metal alloy nanoclusters. In some examples, nanoclusters are from 1-10 nanometers (nm) in size.
Some memories that have charge storage transistors with nanoclusters are implemented on integrated circuits that also include high voltage transistors in the circuitry used for charging and discharging the charge storage locations of the charge storage transistors. Charging or discharging the charge storage locations is used to store one or more bits of information, and may be referred to as programming or erasing. These high voltage transistors typically include a relatively thick gate oxide. This gate oxide may be formed under severe oxidizing conditions. This oxidizing ambient may penetrate the control dielectric of the charge storage transistors thereby undesirably oxidizing the nanocrystals and undesirably increasing the bottom dielectric thickness. Accordingly, an improved method for making a device with nanoclusters is desirable.
The use of the same reference symbols in different drawings indicates similar or identical items. Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.