Formation of metal oxide gate dielectric

Details Formation of metal oxide gate dielectric Formation of metal oxide gate dielectric

2002-08-26

2003-08-19

Ho, Hoai (Department: 2818)

Active solid-state devices (e.g., transistors, solid-state diode

Housing or package

Insulating material

C204S192100, C204S192320

Reexamination Certificate

active

06608378

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to semiconductor fabrication methods and apparatus for implementing such methods. More particularly, the present invention relates to metal oxide gate structures for semiconductor devices, e.g., MOSFET devices, memory devices, etc., and other structures including metal oxide dielectric material.
BACKGROUND OF THE INVENTION
Semiconductor devices such as field effect transistors are commonly used in the electronics industry. Such devices may be formed with extremely small dimensions, such that thousands or even millions of these devices may be formed on a single crystal silicon substrate or “chip” and interconnected to perform useful functions in an integrated circuit such as a microprocessor, a memory device, etc. For example, metal oxide semiconductor (MOS) devices are widely used in memory devices that comprise an array of memory cells that include field effect transistors and capacitive structures.
Although transistor design and fabrication are generally complex, a simplified field effect transistor is described below. In such a field effect transistor, a portion of a substrate near the surface is designated as a channel of the transistor. The channel is electrically connected to a source and a drain such that when a voltage difference exists between the source and the drain, current will tend to flow through the channel. The semiconducting characteristics of the channel are altered such that its resistivity may be controlled by the voltage applied to a gate, which generally includes a conductive layer or gate electrode overlying the channel. By changing the voltage on the gate, more or less current can be made to flow through the channel. The gate electrode and the channel are separated by a gate dielectric. Generally, the gate dielectric is insulating, such that between the gate and channel little or no current flows during operation, although tunneling currents observed within certain dielectrics. The gate dielectric allows the gate voltage to induce an electric field in the channel.
Generally, integrated circuit performance may be enhanced by scaling. In other words, performance and density are enhanced by decreasing the size of the individual semiconductor devices on the chip. This has been accomplished by decreasing the thickness of the gate dielectric, thus bringing the gate in closer proximity to the channel. As modern silicon device size becomes smaller or has been scaled to smaller and smaller dimensions, with a corresponding size reduction of the gate length of MOS devices, the gate dielectric thickness has continued to decrease, for example, to less than 2 nm (20 Å) and as thin as 1 nm (10 Å).
However, the most commonly used gate dielectric material, silicon dioxide, exhibits high leakage current density in this thickness range because of direct band-to-band tunneling current or Fowler-Nordheim tunneling current. Further, because such silicon dioxide layers are formed from a few layers of atoms, complex process control is required to repeatably produce such silicon dioxide layers. Further, uniformity of coverage is also critical because device parameters may change dramatically based on the presence or absence of even a single monolayer of dielectric material. Because of the limitations of silicon dioxide, alternative high dielectric constant (K) films such as TiO
2
, Ta
2
O
5
, HfO
2
, and other high dielectric films have received a lot of interest as substitutions for very thin silicon dioxide gate dielectrics. Such alternate dielectric materials can be formed in a thicker layer than silicon dioxide and yet still produce the same field effect performance. Such performance is often expressed as “equivalent oxide thickness.” In other words, although the alternate material layer may be thick, it has the equivalent effect of a much thinner layer of silicon dioxide. Most of the interest in alternate materials for silicon dioxide have employed the use of metal oxides.
Various methods have been described for the formation of metal oxides, e.g., formation of metal oxide gate dielectrics. For example, in Haraguchi et al., “A TiO
2
Gate Insulator of a 1-nm Equivalent Oxide Thickness Deposited by Electron-Beam Evaporation,”
Extended Abstracts of
1999
International Conference on Solid State Devices and Materials
, pps. 376-377 (1999), fabrication of thin dielectric films by electron beam evaporation was described. As described in Haraguchi et al., one of the more common methods of forming metal oxide films, e.g., titanium dioxide (TiO
2
), is by chemical vapor deposition. However, for example, impurities such as carbon and chlorine originating from titanium precursors in such chemical vapor deposition processes may cause undesirable influence on the TiO
2
film properties. To achieve the preparation of high purity TiO
2
films, electron beam evaporation (as described in Haraguchi et al.) has been used instead of chemical vapor deposition.
For example, as described in Haraguchi et al., electron beam evaporation for forming metal oxides was performed in the ambient of ozone plasma minimizing the effect of oxygen depletion, resulting in pure TiO
2
films. Further, by optimizing TiO
2
deposition thickness and TiO
2
annealing conditions, TiO
2
films with 1 nm equivalent oxide thickness which showed low leakage current and interface trap density were realized.
However, even though electron beam evaporation methods have been found to produce metal oxides which show low leakage current and have suitable equivalent oxide thickness, optimization of such film formation processes are necessary. The optical properties for most vacuum evaporated thin films change when the films are exposed to moisture, and they are unstable in air since the properties are dependent on the relative humidity. Such properties are attributed to microstructure of the films, which have been reported to include approximately cylindrical columns several tens of nanometers in diameter with voids between them. As a result, the density of the films is less than that of the bulk material. Upon contact with the moisture, the internal surfaces of the columns adsorb a monolayer of water. On exposure to a humid atmosphere, the voids act as capillaries and fill with water, upon bringing the relative humidity above a certain threshold, which depends upon the diameter of the pores. Consequently, the refractive indices of the films when deposited are less than those of the bulk material and change when the film is exposed to a humid atmosphere. The extent of the change is dependent upon the relative humidity. Typical packing densities for such films have been found to be between 0.75 to 1.0.
Higher packing densities for films and, hence, increased stability were reported to be achieved as described in an article by Martin et al., “Ion-beam-assisted deposition of thin films,”
Applied Optics
, Vol. 22, No. 1 (Jan. 1, 1983), where the adatoms had greater mobility on the substrate surface. The article indicates they can be produced by heating a substrate or by increasing the energy of the arriving atoms or molecules as occurs in sputtering or ion beam deposition. Additional activation energy can be added to the growing film if it is bombarded with low energy ions during deposition, as reported therein.
In addition, an article by Souche et al., entitled “Visible and infrared ellipsometry study of ion assisted SiO
2
films,”
Thin Solid Films
, Vol. 313-314, pps. 676-681 (1998), described the study of oxygen ion-assisted silica thin films by means of in situ visible spectroscopic ellipsometry and infrared spectroscopic ellipsometry in air. The article discusses the transition from porous evaporated films to compact films, with emphasis on compaction of silicon dioxide films by ion-assisted deposition.
Further, ion-assisted deposition of silver thin films was described in an article by Lee et al., entitled “Ion-assisted deposition of silver thin films,”
Thin Solid Films
, Vol. 359, pps. 95-97 (2000). The article describes silver films deposited with ion bombardme

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