Patent Grant
7491653
This invention pertains to methods for forming thin dielectric films. More specifically, the invention pertains to methods of depositing a conformal film of dielectric material with a high degree of surface smoothness particularly suited to high aspect ratio gap fill applications in semiconductor device fabrication.
Conformal, uniform dielectric films have many applications in semiconductor manufacturing. In the fabrication of sub-micron integrated circuits (ICs) several layers of dielectric film are deposited. Four such layers are shallow trench isolation (STI), premetal dielectric (PMD), inter-metal dielectric (IMD) and interlayer dielectric (ILD). All four of these layers require silicon dioxide films that fill features of various sizes and have uniform film thicknesses across the wafer.
In particular, it is often necessary in semiconductor processing to fill a high aspect ratio gap with insulating material. As device dimensions shrink and thermal budgets are reduced, void-free filling of high aspect ratio (AR) spaces (AR>3.0:1) becomes increasingly difficult due to limitations of existing deposition processes. The deposition of doped or undoped silicon dioxide assisted by high density plasma CVD, a directional (bottom-up) CVD process, is the method currently used for high aspect ratio (AR) gap-fill. Evolving semiconductor device designs and dramatically reduced feature sizes have resulted in several applications where HDP processes are challenged in filling the high aspect ratio structures (AR>7:1) using existing technology (see, for example, U.S. Pat. No. 6,030,881). For structures representative of the 65 nm and 45 nm technology nodes, engineering the gap-fill process becomes structure dependent, hence the process requires re-optimization, a task of considerable complexity, every time a new structure needs to be filled.
An alternative to CVD is atomic layer deposition (ALD). ALD methods involve self-limiting adsorption of reactant gases and can provide thin, conformal dielectric films within high aspect ratio features. The ALD process involves exposing a substrate to alternating doses of, usually two, reactant gasses. As an example, if reactants A and B are first and second reactant gases for an ALD process, after A is adsorbed onto the substrate surface to form a saturated layer, B is introduced and reacts only with adsorbed B. In this manner, a very thin and conformal film can be deposited. One drawback, however, to ALD is that the deposition rates are very low. Films produced by ALD are also very thin (i.e., about one monolayer); therefore, numerous ALD cycles must be repeated to adequately fill a gap feature. These processes are unacceptably slow in some applications in the manufacturing environment.
Another more recently developed technique useful in gap fill and other dielectric deposition applications in semiconductor processing is referred to as pulsed deposition layer (PDL) processing, sometimes also referred to as rapid surface-catalyzed vapor deposition (RVD). PDL is similar to ALD in that reactant gases are introduced alternately over the substrate surface, but in PLD the first reactant A acts as a catalyst, promoting the conversion of the second reactant B to a film. In ALD the reaction between A and B is approximately stoichiometric, meaning that a monolayer of A can only react with a similar amount of B before the film-forming reaction is complete. The catalytic nature of A in PDL allows a larger amount of B to be added, resulting in a thicker film. Thus, PDL methods allow for rapid film growth similar to using CVD methods but with the film conformality of ALD methods.
Current PDL-type processes involve the use of a metal or metalloid containing catalyst. As an example of the use of PDL to deposit silicon dioxide on silicon, the first reagent can be trimethylaluminum (TMA) and the second tirs(pentoxy)silanol (TPOSL). The heated silicon substrate is first exposed to a dose of TMA, which is thought to react with the silicon surface to form a thin layer of surface-bound aluminum complex. Excess TMA is pumped or flushed from the deposition chamber. A large dose of TPOSL is then introduced. The aluminum complex catalyzes the conversion of the silanol to silicon oxide until the silanol is consumed, or the growing film covers or otherwise inactivates the catalytic complex. When excess silanol is used, the film growth is usually self-limiting and a thick and uniform film results. Unreacted silanol may now be removed from the chamber and the growth cycle repeated. Other metal-containing precursors that can be deposited to activate or reactivate the catalytic surface include, but are not limited to, precursors containing zirconium, hafnium, gallium titanium, niobium, tantalum, and their oxides or nitrides.
In front-end of line (FEOL) applications the metal content of the dielectric films is very tightly controlled, and the presence of metals in concentrations of 1-2% atomic may not be desirable. The metal or metalloid compound results in the formation of second phase (e.g., Al2O3 in the case of an Al-containing precursor) in PDL silicon oxide, which can affect film properties. Moreover, prior work in the field indicates a phosphorus incorporation scheme that implies that phosphorus concentration in the film will be increasing proportionally to that of the metal or metalloid. Since P-doped silica films contain typically >5 wt. % P, similar concentrations of metals/metalloids will be incorporated into the film, which is likely to impact subsequent process steps (CMP, contact etch, etc).
Therefore, a method is needed to eliminate the presence of metals and metalloid elements from the silica films (both doped and undoped), and at the same time maintain conformality, growth rate, and other film properties.
The present invention eliminates the need for a metal or metalloid comprising catalyst in a pulsed deposition layer (PDL) dielectric formation process, while maintaining film properties including conformality, growth rate, and dielectric material composition. The invention provides doped or un-doped PDL silica glass (silicon oxide) films with improved performance characteristics where metal content is disfavored and applies to IMD/ILD, PMD, and STI gap-fill, among other types of dielectric formation. A metal- and metalloid-free compound, is used to catalyze the reaction of silica deposition by surface reaction of alkoxysilanols. Films can be grown at rates faster than 30 nm per exposure cycle. The invention can be used for the deposition of both doped (e.g., PSG) and undoped silica (USG) films. The elemental composition of the films is that of silicon oxide. The films deposited are conformal, hence, the method can accomplish void free gap-fill in high aspect ratio gaps encountered in advanced technology nodes (e.g., the 45 nm technology node and beyond).
This method involves two principal, sequential operations: 1) exposing the substrate surface to a metal- and metalloid-free compound to form a catalytic layer; and 2) exposing the catalyst-activated substrate surface to a silicon-containing precursor. Operations 1) and 2) can be repeated a number of times, for example a total of 2-5 times, to build up a silica nanolaminate structure forming the completed metal and metalloid-free dielectric film. A preferred class of catalytic precursors are acidic compounds (e.g., an organic acid such as acetic acid (CH3COOH) or an inorganic acid such as phosphoric acid (H3PO4). Furthermore, any suitable metal- and metalloid-free compound that can sufficiently adsorb onto or react with the substrate surface and prepare it to sufficiently react with the subsequently added silicon-containing precursor to form a dielectric layer that is more than a monolayer thick may be used. Other potential catalysts include other organic or inorganic acids, organic bases, salts, organophosphorus compounds and precursors to these.
Between operations, inert gas purges may be performed. Any suitable inert gas may be used. A typical purge lasts about 5 seconds. Additionally, there may be a post-deposition treatment to remove water from the deposited film.
In preferred embodiments of the invention, the substrate is a partially fabricated semiconductor wafer including shallow trench isolation (STI) features, over which the dielectric film is to be deposited.
Exposure to the catalytic precursor and the silicon-containing precursor may occur in different chambers in preferred embodiments of the invention. Further, additional precursor gases may be used.
In the following detailed description of the present invention, numerous specific embodiments are set forth in order to provide a thorough understanding of the invention. However, as will be apparent to those skilled in the art, the present invention may be practiced without these specific details or by using alternate elements or processes. In other instances well-known processes, procedures and components have not been described in detail so as not to unnecessarily obscure aspects of the present invention.
The formation of silica films by a pulsed layer deposition (PDL) can be catalyzed by metal and metalloid-free compounds, such as acidic compounds. A specific example is phosphoric acid (H3PO4); several other examples are provided below. Referring to
The deposition process 100 begins with operation 101, wherein a substrate is placed into a deposition chamber. For many embodiments of the invention, the substrate is a semiconductor wafer. A “semiconductor wafer” as discussed in this document is a semiconductor substrate at any of the various states of manufacture/fabrication in the production of integrated circuits. As mentioned previously, one commercially important application of the present invention is in various dielectric gap-fill applications, such as filling of STI or PMD features. The process continues with operation 103, where a catalyst that does not contain a metal or metalloid element, or a precursor for such a compound (referred to herein alternatively as catalyst or catalytic precursor) is supplied into the deposition chamber so as to substantially saturate the substrate surface with the catalyst. Any compound or precursor that can sufficiently adsorb onto or react with the substrate surface and prepare it to sufficiently further react with the subsequently added silicon-containing precursor may be used. In addition, the catalyst or precursor should be capable of aiding the catalytic polymerization of the subsequently added silicon-containing precursor to produce a film thicker than a monolayer.
As indicated earlier, forming a catalyst-activated layer is a self-limiting process. Relevant process conditions can include substrate (e.g., wafer) temperature, reactor pressure, reactant partial pressure, and combinations thereof. Pressures can range broadly, e.g., between about 1 mTorr and 760 Torr. Typical pressures range between about 500 and 1250 mTorr and typical temperatures range between about 0 and 300° C., for example between about 20 and 250° C., such as 20, 50, 100, 200, 230 or 250° C. in various implementations. Flow rates of catalyst reactants can range broadly, e.g., between about 1 and 10000 sccm in the gas phase. Preferred gas phase flow rates of the catalyst range between about 1 and 100 sccm. Catalysts may also alternatively be delivered to the reactor and/or the substrate surface in the liquid phase. Suitable liquid phase delivery flow rates can be between 0.01 and 100 mL/min; or the liquid catalysts can be delivered to the substrate surface by dipping, painting, spraying, etc. Suitable liquid phase catalyst concentrations can vary widely, for example from about 10−3 to 12M, such as about 0.01, 0.1, 1, 2, 5, 7 or 10M. Exposure times suitable for forming a saturated layer are typically only seconds, e.g., about 1 to 10 seconds. In some embodiments, for example, an exposure time of about 2 seconds or 5 seconds is found to be sufficient. Given the directions and parameters provided herein, one of skill in the art will be able to readily determine the appropriate conditions for effective use of a given catalyst in accordance with the present invention.
After a metal and metalloid-free catalyst-activated layer is formed an inert gas is preferably used to purge the substrate surface and reaction chamber (not shown). It should be noted that introduction of a purge gas can be used in between operations wherein contact between reactant gases should be avoided, including between each process cycle. Further, the purge gas can be flowed continuously during any of these operations and a period or periods between the operations. Purge gases are generally inert. Examples include the noble gases (e.g., argon) and nitrogen. The reaction chamber may additionally be evacuated following inert gas purge.
Returning to
Any suitable silicon-containing precursor that can sufficiently adsorb onto and react with the catalytic layer to form a dielectric film may be used. In addition, the silicon-containing precursor should be capable of polymerization when exposed to the adsorbed catalyst to produce a film thicker than a monolayer. Preferred silicon-containing precursors include silanols and silanediols, such as alkoxysilanols, alkyl alkoxysilanols, alkyl alkoxysilanediols and alkoxysilanediols. Examples of suitable precursors include tris(tert-butoxy)silanol ((C4H9O)3SiOH), tris(tert-pentoxy)silanol((C5H11O)3SiOH), di(tert-butoxy)silandiol ((C4H9O)2Si(OH)2) and methyl di(tert-pentoxy)silanol.
Process parameters during exposure to the silicon-containing precursor including temperature, gas pressure, flow rate, dose and exposure times will vary depending on the types of precursors used, reaction chamber configuration and desired final film thickness, deposition rate and dielectric characteristics, among other variables. As noted, the process conditions in accordance with the present invention are dynamic, at first selected to optimize surface smoothness, then selected to optimize conformality and deposition rate. Generally, temperatures can range from about 200 to 300° C. A typical initial deposition temperature is about 200° C.; and 230° C. for subsequent deposition stages. Pressures can range broadly, e.g., between about 1 mTorr and 760 Torr. Typical pressures range between about 500 mTorr and 20 Torr. Flow rates of silicon-containing precursor gas can range broadly, e.g., between about 1 and 10000 sccm. Preferred flow rates of silicon-containing precursor gas range between about 200 and 1000 sccm. Exposure times can range broadly, e.g., between about 1 milliseconds and 200 seconds. Preferred exposure times typically range between about 10 and 60 seconds. The number of separate exposures to silicon-containing precursor will primarily depend upon the desired final film thickness. Typical numbers of silicon-containing precursor exposure cycles for a STI feature 0.1 micron in diameter range between about 2 and 4. Again, as with the temperature process condition, these other process conditions may be modulated alone or in combinations in the dynamic process of the invention in accordance with the parameters noted above.
While the invention is not limited to this theory of operation, as mentioned previously, it is believed that the catalyst is adsorbed onto or reacts with the substrate surface, and the accumulation of dielectric film is achieved via a polymerization process. The activated substrate surface layer can catalytically polymerize the silicon-containing precursor to produce growing chains of silica. After a period of growth determined by the substrate temperature, the silica polymer can “gel” or “cross-link” to form a solid silicon oxide. Thus, it is believed that the first PDL precursor forms a catalyst-activated surface. This could be accomplished by physical adsorption of the catalyst on the surface, in which case the active species in the catalyst itself, as delivered. It is likely that in some cases the catalyst will react chemically with the surface. In this scenario this surface becomes the actual active species in the silanol polymerization and gel formation. For example, phosphoric acid is envisioned to react with silanol moieties on the surface of the silicon wafer. This surface-bound phosphate can now further react with silanols resulting in a siloxane. After repeated insertions of silanol, silica-like chains grow and eventually gel at the elevated temperature.
Table 1 lists a number of metal- and metalloid-free compounds suitable as catalysts in a PDL process in accordance with the present invention. Note that these are not limited to acids, but include aprotic compounds with acidic character as well as organic bases and salts and organophosphorus compounds.
It should be noted that one skilled in the art may choose to introduce the precursor to a catalyst into a deposition chamber instead of the actual catalyst, if it is advantageous to do so. The precursor can then be transformed into the catalyst by any number of convenient means, such as chemical, thermal or light-activated reactions. As an example, P2O5 may be delivered to a wafer. Following reaction with water P2O5 may be converted to phosphoric acid. The water may be present in the film or may be separately delivered to the processing chamber. In another example, acetic anhydride may be converted to acetic acid, also by reaction with water Table 1, above, also contains examples of compounds that can be used as precursors to active catalysts.
As indicated by the arrow in
This method applies to the deposition of silica (USG). However, this method may also be used for depositing doped silica films, such as fluorine-doped silicate glass (FSG), phosphosilicate glass (PSG), boro-phospho-silicate glass (BPSG), or carbon doped low-k materials. The use of a P-containing catalyst can lead to the formation of P-doped silica films. In a similar manner, the use of appropriate compounds can lead to the formation of F, B, N, or C-doped silicon oxide films. Examples of corresponding compounds can be CF3COOH, H2BO3, and organic acids.
Other deposition co-reactants, such as silanols with varying substituents (e.g., more than one kind of alkoxy substituent) may be used to improve the film characteristics. For an example, see U.S. patent application Ser. No. 10/874,814, filed Jun. 22, 2004, titled “Mixed Alkoxy Precursors and Methods of Their Use for Rapid Vapor Deposition of SiO2 Films.”
Apparatus
As shown, a reactor 201 includes a process chamber 203, which encloses components of the reactor and serves to contain the reactant gases and provide and area to introduce the reactant gases to substrate 209. The chamber walls may be made of or plated with any suitable material, generally a metal that is compatible with the deposition and associated processes conducted therein. In one example, the process chamber walls are made from aluminum. Within the process chamber, a wafer pedestal 207 supports a substrate 209. The pedestal 207 typically includes a chuck 208 to hold the substrate in place during the deposition reaction. The chuck 208 may be an electrostatic chuck, a mechanical chuck or various other types of chuck as are available for use in the industry and/or research.
A heat transfer subsystem including resistive heaters and/or lines 211 for supplying a heat transfer fluid to the pedestal 207 controls the temperature of pedestal 207. In some embodiments, the heat transfer fluid comprises water or another liquid. The reactant gases, as well as inert gases during purge, are introduced individually into the reactor via inlet 217. A showerhead 227 may be used to distribute the gas flow uniformly in the process reactor. Reactant gases are introduced through a gas supply inlet mechanism including orifices. There may be multiple reactant gas tubes and inlets. A vacuum pump connected to outlet 219 can draw out gases between PDL cycles.
Various details of the apparatus have been omitted for clarity's sake, and various design alternatives may be implemented.
The following example provides details relating to performance advantages of the present invention. It should be understood the following is representative only, and that the invention is not limited by the detail set forth in this example.
The feasibility of the method was demonstrated using phosphoric acid (H3PO4) as a catalyst for a metal and metalloid-free pulsed layer deposition (PDL) process for dielectric film deposition in accordance with the present invention. Si substrates were exposed to the 7.1M phosphoric acid for 5 seconds and subsequently to TPOSL (tris t-pentoxy silanol) under conditions used for the deposition of PDL oxide (230° C., 20 Torr reactor pressure).
Film growth as a function of exposure cycles is shown in
The conformality of a film deposited by a metal- and metalloid-free catalyzed PDL process was tested using cantilever structures. A scanning electron microscope (SEM) image of a typical structure with a minimal amount of PDL oxide deposition is shown in
The disclosed method applies to the gap fill deposition of metal- and metalloid-free silica films using the pulsed deposition layer (PDL) process. It pertains to IMD, STI and PMD applications. While described primarily with reference to silicon dioxide herein, the method may also be used for depositing doped silica films, such as FSG, PSG, BPSG, SiON, or carbon doped low-k materials. Films can be grown at rates faster than 30 nm per exposure cycle. The films deposited are conformal, hence the method can accomplish void free gap-fill in high aspect ratio gaps encountered in advanced technology nodes (e.g., the 45 nm technology node and beyond), and can be used in other applications requiring conformal dielectric deposition.
Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there may be alternative ways of implementing the present invention. Also, while the invention has been described primarily in terms of preparing integrated circuits, it is not so limited. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.
The entire disclosures of all references cited herein are incorporated by reference for all purposes.
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