Patent Application
20050268855
This invention relates to the field of deposition of thin films composed of multiple materials by thermal evaporation.
Evaporative deposition techniques are extremely important in the semiconductor industry where there is a necessity for highly uniform and very thin films of various materials. In the semiconductor industry, evaporative deposition is useful in forming a material layer of a desired stoichiometry from a plurality of different materials.
In thermal evaporation techniques, vapor particles can be generated in high vacuum by sublimation or vaporization of a material via a variety of heating sources and then condensed on a substrate. Heating sources include resistive heating sources, lasers, and electron beam sources. Typically, a material source is placed in an evaporation crucible or boat and a heat source, such as resistive heating coils, applies thermal energy to the crucible or boat (indirect resistive heating) causing the material source to melt and vaporize. Upon contacting a cooler surface the vaporized material condenses and forms a film.
Formation of a homogenous thin film having high uniformity and desired stoichiometry by thermal evaporation of a single material is a simple procedure because a homogenous material source will have only a single boiling point, a single freezing point, and there is no opportunity for dissociation. Therefore, under appropriate conditions, a very thin film that is useful for various purposes can be easily formed. However, when a binary (or tertiary or greater) film is desired, problems are presented because of the differing physical characteristics (e.g., melting and boiling points) of the multiple source materials and the ever-present problem of dissociation. Often, when forming binary films by thermal evaporation for semiconductor industrial purposes, a material gradient is unintentionally formed in the thin film where the initial material deposited does not have the desired stoichiometry. This requires longer formation times to reach the desired or required stoichiometric levels and can lead to films that are not as uniform as desired. Such problems increase and are exaggerated as the physical characteristics of the different source materials become increasingly divergent.
This invention provides a method for improving the stoichiometric character of a thermal-vapor-deposited material layer formed of materials having different physical (e.g., melting and boiling points) and chemical properties. An inert medium is added to the source materials within an evaporation container (e.g., a crucible) that are to form a binary (or greater) film upon vaporization and condensation. By this method, films of increased uniformity and maintained stoichiometry are achievable.
These and other advantages and features of the invention will be more clearly understood from the following detailed description which is provided in conjunction with the accompanying drawings.
The invention relates to thin films that are at least binary in nature and their deposition by evaporative techniques. In the semiconductor industry it is often important to maintain both the stoichiometry in thin films and as well as the uniformity of the films. Thermal evaporation is an inexpensive and commonly used method of forming such films. This invention utilizes a method of increasing the surface area of an evaporation container, preferably by using an inert medium added to source materials held by the container that are to form the binary (or greater) film. By this method, films of increased uniformity and maintained stoichiometry are achievable.
In the following detailed description, reference is made to various specific embodiments in which the invention may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that structural and electrical changes may be made without departing from the spirit or scope of the present invention.
The terms “substrate” and “wafer” can be used interchangeably in the following description and may include any foundation surface, but preferably a semiconductor-based structure. The structure should be understood to include silicon, siicon-on insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. The semiconductor need not be silicon-based. The semiconductor could be silicon-germanium, germanium, or gallium arsenide. When reference is made to the substrate in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation.
Now referring to the figures, where like reference numbers denote like features,
In accordance with the invention, the problems associated with the prior art techniques can be mitigated, as shown in
Typically the inert medium 18 consists of solid material similar in shape and size to the source materials 14 and 16 (e.g., pellets); however, it will be readily apparent to those of skill in the art that a multitude of variations in size and shape of the inert medium 18 are possible and, depending on the circumstances, desirable. Though the shape of the inert medium 18 can vary, generally spherical shapes are preferred because such a design achieves the maximum relative surface area without interfering with the evaporation process (because of folds, sharp corners, etc.). Further, the added inert medium 18 are preferably large enough to effectively maximize evaporation container 10 surface area by contacting the container 10 itself, as well as the source materials 14 and 16. However, the size of the inert medium 18 should not be so large as to interfere with the evaporation process (e.g., by blocking the evaporation container 10 opening).
As shown in
As shown in
The uneven heating, melting, and evaporation of the source materials 14 and 16 found in the prior art is diminished so that the two source materials 14 and 16 melt and vaporize more quickly and more synchronously. The result is that the resultant film deposits in less time, leading to more uniform films, and has a more desirable stoichiometry due, in part, to less dissociation.
As illustrated in
Though this invention has been described primarily with reference to binary films utilizing two source materials 14 and 16, it can also achieve thin films 22 of desired uniformity and stoichiometry utilizing three or more source materials.
The following supporting data was obtained in experiments using actual embodiments of the invention. Table I below shows experimental results. The experiments are explained in reference to
Each experimental run was conducted in a vacuum chamber 11 and used a standard ceramic crucible 108 as an evaporation container 10 and standard resistive heating coils 110 for a heat source 12, as is known in the art. As a deposition target, a 3500 Å layer of TEOS oxide over a 200 mm silicon (Si) wafer having a (111) crystalline orientation served as a substrate 104 upon which to condense the thin film. The source material used in all runs were pellets 100 formed of silver and selenium (Ag2Se), manufactured on site to be of known stoichiometry. The target stoichiometry for the deposited thin films was Ag66Se33 and the initial stoichiometry of the source material reflected this desired film stoichiometry in a 2:1 ratio (with Ag being no greater than 2). For each run, thermal energy was applied to the crucible 108 and its contents by the resistive heating coils 110 as a function of the % total power. The Ag2Se source pellets 100 were heated for a minimum of 60 seconds to vaporize. Time to boiling was subjective and a function of the % power used. The desired thickness for each deposited experimental film was 500 Å.
For the Control Run (reflecting prior art techniques), no inert medium was added to the Ag2Se source pellets 100. The power used was about 11% of total power. As is shown in Table I, the resulting stoichiometry of the deposited film did not achieve the target 2:1 Ag to Se ratio, but the resulting 3:2 ratio did reflect results common to techniques used in the prior art. The undesired stoichiometry was due to the dissimilar physical characteristics of the silver and selenium, uneven heating, and dissociation, resulting in uneven deposition rates and amounts between the source materials.
As shown in Table 1, Run 1 utilized the same Ag2Se source pellets 100, but inert silicon (Si) media 102 was added in accordance with the invention. Thermal energy was applied by the resistive heating coils at about 13% total power. The 500 Å film was deposited and determined by subsequent analysis to have close to target stoichiometry. Run 2 also utilized inert silicon (Si) media 102 in accordance with the invention. For Run 2, thermal energy was applied at about 16% total power. The resulting film was not as close to the target stoichiometry as with Run 1, but was still closer than the Control Run, which used no inert media.
The above description, examples, and accompanying drawings are only illustrative of exemplary embodiments, which can achieve the features and advantages of the present invention. It is not intended that the invention be limited to the embodiments shown and described in detail herein. The invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, but is only limited by the scope of the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10128349 | Apr 2002 | US |
| Child | 11202139 | Aug 2005 | US |
