This application is a continuation-in-part of commonly-owned U.S. patent application Ser. No. 09/225,922, filed Jan. 5, 1999, which is incorporated herein by this reference to the extent consistent herewith.
This invention pertains generally to the fabrication of semiconductor devices and, more particularly, to a method and apparatus for generating important chemical species in the deposition, etching, cleaning, and growth of various materials and layers.
It is in general an object of the invention to provide a new and improved chemical generator and method for generating chemical species at or near the location where they are to be used.
Another object of the invention is to provide a chemical generator and method of the above character which are particularly suitable for generating chemical species for use in the fabrication of semiconductor devices.
These and other objects are achieved in accordance with the invention by providing a chemical generator and method for generating a chemical species at or near a point of use such as the chamber of a reactor in which a workpiece such as a semiconductor wafer is to be processed. The species is generated by creating free radicals, and combining the free radicals, alone or with other materials, to form the chemical species.
FIG. 1 is a diagrammatic view of a chemical generator incorporating aspects of-the invention.
FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1.
FIG. 3 is a diagrammatic view of another version of the chemical generator incorporating aspects of the invention.
FIG. 4 is a diagrammatic view of a remote ICP torch incorporating aspects of the invention.
As illustrated in FIG. 1, a chemical generator includes a free radical source 11 which has one or more chambers in which free radicals are created and delivered for recombination into stable species. In the embodiment illustrated, the source has three chambers which are formed by elongated, concentric tubes 12-14. Those chambers include a first annular chamber 16 between the outermost tube 12 and the middle tube 13, a second annular chamber 17 between middle tube 13 and the innermost tube 14, and a third chamber 18 inside the innermost tube 14. The tubes are fabricated of a material such as ceramic or quartz.
The number of tubes which are required in the generator is dependent upon the chemical species being generated and the reaction by which it is formed, with a separate chamber usually, but not necessarily, being provided for each type of free radical to be used in the process.
Gases or other precursor compounds from which the free radicals are formed are introduced into the chambers from sources 21-23 or by other suitable means. Such precursors can be in gaseous, liquid and/or solid form, or a combination thereof.
As previously explained, although a separate chamber may be used for providing each type of free radicals, it is also contemplated for certain chemical reactions such as described below that a single chamber may also be used for providing more than one type of free radicals. In such a case, gases or other precursor compounds from which the more than one type of free radicals are formed are introduced into the single chamber from corresponding sources.
A plasma is formed within the one or more chambers to create the free radicals, and in the embodiment illustrated, the means for generating the plasma includes an induction coil 26 disposed concentrically about the one or more tubes, a radio frequency (RF) power generator 27 connected to the coil by a matching network 28, and a Tesla coil 29 for striking an arc to ignite the plasma. The plasma can, however, be formed by any other suitable means such as RF electrodes or microwaves.
In the embodiment illustrated, the free radicals are recombined to form the desired species downstream of the tubes. In this case, recombination takes place in a chamber 31 which is part of a reactor 32 in which a semiconductor wafer 33 is being processed. Recombination can be promoted by any suitable means such as by cooling 36 and/or by the use of a catalyst 37.
Cooling can be effected in a number of ways, including the circulation of a coolant such as an inert gas, liquid nitrogen, liquid helium or cooled water through tubes or other suitable means in heat exchange relationship with the reacting gases.
A catalyst can be placed either in the cooling zone or downstream of it. It can, for example, be in the form of a thin film deposited on the wall of a chamber or tube through which the reacting gases pass, a gauze placed in the stream of gas, or a packed bed. The important thing is that the catalyst is situated in such a way that all of the gas is able to contact its surface and react with it.
If desired, monitoring equipment such as an optical emission spectrometer can be provided for monitoring parameters such as species profile and steam generation.
In the embodiment illustrated in FIG. 1, the chemical generator is integrated with the reactor, and the species produced is formed in close proximity to the wafer being processed. That is the preferred application of the generator, although it can also be used in stand-alone applications as well. It can be added to existing process reactors as well as being constructed as an integral part of new reactors, or as a stand-alone system.
FIG. 3 illustrates another version of the chemical generator. In the embodiment illustrated, a chemical generator includes a free radical source 300 which has two chambers in which free radicals are created and delivered for recombination into stable species. In the embodiment illustrated, the source has two chambers which are respectively formed within elongated tubes 312 and 313. Those chambers include a first chamber inside tube 312 and a second chamber inside tube 313. The tubes are preferably fabricated of a material such as quartz or ceramic.
In the tubes depicted in FIG. 3, a separate chamber is provided to generate each type of free radical to be used in the process. This approach ensures that the free radicals will not recombine to form the desired chemical species until after they are introduced into the reactor 331. In the tube depicted in FIG. 4, however, more than one type of free radical may be generated in the tube. In this latter approach, recombination of free radicals to form the desired chemical species may occur within the tube as well.
The free radicals are generated from precursor materials which are introduced into the chambers of tubes 312 and 313 respectively from, for example, first and second precursor sources 322 and 323. The precursor materials can be in gaseous, liquid and/or solid form, or a combination thereof.
Plasmas are formed within the chambers to create the free radicals, and in the embodiment illustrated, the means for generating the plasmas includes an induction coil 332 disposed concentrically about tube 312, another induction coil 333 disposed concentrically about tube 313, and a radio frequency (RF) power generator 327 connected to the coils 332 and 333 by a matching network 328. Although this embodiment shows the coils 332 and 333 sharing the same RF power generator 327 and matching network 328, an alternative embodiment, fully contemplated but not shown herein to avoid unnecessary duplication or straightforward elaboration of details, includes each of the coils 332 and 333 having its own RF power generator and/or matching network. A Tesla coil (not shown) for striking an arc to ignite each of the plasmas may also be included if useful. Although shown as being generated through RF energized induction coils, the plasmas can also be formed by any other suitable means such as RF electrodes or microwaves.
Insulation housings 342 and 343 conventionally protect adjacent computer and other circuitry from electromagnetic fields induced by energized coils 332 and 333, as well as preventing such induced electromagnetic fields from interfering or otherwise interacting with each other or the plasmas generated therefrom.
In the embodiment illustrated, the free radicals are recombined to form the desired species downstream of the tubes. In this case, recombination takes place in a chamber 331 which is part of a reactor 332 in which a semiconductor wafer 333 is being processed. Recombination can be promoted, if necessary, by any suitable means such as by cooling (not shown) and/or by the use of a catalyst (not shown).
As an example of the use of this embodiment of a chemical generator, the formation of steam (H2O) is described. In this example, the first precursor source 322 provides H2 gas which is admitted into the chamber of tube 312 and the second precursor source 323 provides O2 gas which is admitted into the chamber of tube 313. Plasmas are created in both chambers, and as a result, hydrogen and oxygen free radicals are respectively generated and provided to the chamber 331. Within the reactor chamber 331, these free radicals recombine to form steam (H2O), which in turn, may be used, for example, to produce SiO2 on the exposed surface of the semiconductor wafer 333.
As illustrated in FIG. 4, a remote inductively coupled plasma (ICP) source (or “torch”) includes a free radical source 400 having a tube 401 with a closed end 411 and an open end 412. The open end (or outlet port) 412 is to be fluidically connected to a reactor chamber for processing semiconductors. The torch is referred to as being “remote” in this case, because it creates a plasma that is outside of the reactor chamber. The tube is preferably made of ultra-pure quartz (such as GE 214), or alternatively, of some other material commonly used for such purposes, such as ceramic.
A coil 430 is disposed concentrically about the tube 401 and aligned such that the high voltage or “hot” side of the coil 430 is closest to the closed end 411 of the torch, and the grounded end of the coil 430 is closest to the open end 412 of the torch. In this example, the coil 430 is depicted as a 4-turn coil made of suitable material such as gold-plated copper tubing.
A radio frequency (RF) power generator 480 is connected to the coil 430 by a matching network 481. The matching network 481 is used to adjust the overall impedance of the torch and coil assembly to couple (i.e., resonate in phase) with the 50 ohm output impedance of the RF power generator 480. The RF power generator 480 delivers, as an example, up to 5 kW of forward power to the matching network 481 at a fixed frequency of approximately 27.12 MHz.
Inlet ports 440 and 450 made from similar material as the tube 401 are fused into the tube's inner chamber walls between its closed end 411 and the “hot” side of the coil 430, and inlet ports 460 and 470 also made from similar material as the tube 401 are fused into the tube's inner chamber walls between its open end 412 and the grounded end of the coil 430. Connectors 441, 451, 561, and 471 made of, for examples, Teflon, PFA, or ceramic, are clamped to the ends of respective inlet ports 440, 450, 460, and 470 to serve as connectors for respective delivery hose lines 443, 453, 463, and 473.
The connectors 441, 451, 461, and 471 cause turbulence in the flow of precursor materials passing through them as the flow of molecules collide with and scatter from the inner walls 442, 452, 462, and 472 of their L-shaped bends. As a result of such turbulence, the density of the precursor materials flowing into and through the chamber of tube 401 has high uniformity, which is useful for controlling plasma generation in the tube 401. Although the connectors 441, 451, 461, and 471 depict 90 degree bends, it is to be appreciated that the angle of the bend may be other values as long as the molecules in the flow of precursor material strike at least one wall in the connector/inlet port combination so as to increase the turbulence in the flow before entering the chamber of the tube 401.
Precursor and/or other materials are provided to one or more of the inlet ports 440, 450, 460, and 470 by corresponding of the sources 444, 454, 464, and 474 through corresponding delivery hose lines and connectors. The type or types of materials to be provided and the inlet ports through which they are to be provided generally depend upon the reaction used to generate a desired chemical species.
As one example, steam (H2O) can be generated in the chemical generator (or further down the line of flow towards or in the reactor chamber) by providing O2 gas at inlet port 440 and H2 gas at inlet port 450, with no materials provided to inlet ports 460 and 470. In this case, hydrogen and oxygen free radicals are generated by the induced plasmas respectively from the H2 and O2 gases, and then recombined to form the desired chemical species of steam (H2O).
As another example, steam (H2O) can also be generated in the chemical generator (or further down the line of flow towards or in the reactor chamber) by providing O2 gas at inlet port 440 (and optionally, also at inlet port 450 to improve uniformity of the gas density in the tube 401) and H2 gas at inlet port 460 (and optionally, also at inlet port 470). In this case, oxygen free radicals are generated by the induced plasma from the O2 gas, and then combined with the H2 gas molecules provided just outside the induced plasma to form the desired chemical species of steam (H2O).
A ground strap 490 is mounted in direct contact with the tube 401 at a strategic position between the grounded end of the coil and the open end of the tube 401 to inhibit plasma generation in the chamber beyond the ground strap 490 and preferably restrict plasma generation to the immediate or near vicinity of the coil 430. The ground strap 490 is preferably made of copper or other highly conductive material.
The chemical generators described herein can be employed in a wide variety of applications for generating different species for use in the fabrication of semiconductor devices, some examples of which are given below.
Steam for use in a wet oxidation process for producing SiO2 according to the reaction
can be generated in accordance with the invention by admitting H2 and O2 into one of the plasma generating chambers. When the plasma is energized, the H2 and O2 react to form steam in close proximity to the silicon wafer. If desired, oxygen admitted alone or with N2 and/or Ar can be used to produce ozone (O3) to lower the temperature for oxidation and/or improve device characteristics.
It is known that the use of NO in the oxidation of silicon with O2 can improve the device characteristics of a transistor by improving the interface between silicon and silicon oxide which functions as a barrier to boron.
Conventionally, NO is supplied to the reactor chamber from a source such as a cylinder, and since NO is toxic, special precautions must be taken to avoid leaks in the gas lines which connect the source to the reactor. Also, the purity of the NO gas is a significant factor in the final quality of the interface formed between the silicon and the silicon oxide, but it is difficult to produce extremely pure NO.
With the invention, highly pure NO can be produced at the point of use through the reaction
by admitting N2 and O2 to one of the chambers and striking a plasma. When the plasma is struck, the N2 and O2 combine to form NO in close proximity to the wafer. Thus, NO can be produced only when it is needed, and right at the point of use, thereby eliminating the need for expensive and potentially hazardous gas lines.
NO can also be produced by other reactions such as the cracking of a molecule containing only nitrogen and oxygen, such as N2O. The NO is produced by admitting N2O to the plasma chamber by itself or with O2. If desired, a gas such as Ar can be used as a carrier gas in order to facilitate formation of the plasma.
N2O can also be cracked either by itself or with a small amount of O2 to form NO2, which then dissociates to NO and O2. In rapid thermal processing chambers and diffusion furnaces where temperatures are higher than the temperature for complete dissociation of NO2 to NO and O2 (620° C.), the addition of NO2 will assist in the oxidation of silicon for gate applications where it has been found that nitrogen assists as a barrier for boron diffusion. At temperatures below 650° C., a catalyst can be used to promote the conversion of NO2 to NO and O2. If desired, nitric acid can be generated by adding water vapor or additional H2 and O2in the proper proportions.
Similarly, NH3 and O2 can be combined in the plasma chamber to produce NO and steam at the point of use through the reaction
By using these two reagent gases, the efficacy of NO in the wet oxidation process can be mimicked.
It is often desired to include chlorine in an oxidation process because it has been found to enhance oxidation as well as gettering unwanted foreign contaminants. Using any chlorine source such as TCA or DCE, complete combustion can be achieved in the presence of O2, yielding HCl+H2O+CO2. Using chlorine alone with H2 and O2 will also yield HCl and H2O.
When TCA or DCE is used in oxidation processes, it is completely oxidized at temperatures above 700° C. to form HCl and carbon dioxide in reactions such as the following:
The HCl is further oxidized in an equilibrium reaction:
Decomposition of various organic chlorides with oxygen at elevated temperatures provides chlorine and oxygen-containing reagents for subsequent reactions in, e.g., silicon processing. Such decomposition is generally of the form
where x and y are typically 2, 3 or 4.
All of the foregoing reactions can be run under either atmospheric or subatmospheric conditions, and the products can be generated with or without a catalyst such as platinum.
- X=1, 2, . . .
Y=0, 1, . . .
Z=0, 1, . . .
Q=0, 1, . . .
The invention can also be employed in the cleaning of quartz tubes for furnaces or in the selective etching or stripping of nitride or polysilicon films from a quartz or silicon oxide layer. This is accomplished by admitting a reactant containing fluorine and chlorine such as a freon gas or liquid, i.e. CxHyFzClq, where
- Dielectric Films
and the amount of fluorine is equal to or greater than the amount of chlorine. It is also possible to use a mixture of fluorinated gases (e.g., CHF3, CF4, etc.) and chlorinated liquids (e.g., CHCl3, CCl4, etc.) in a ratio which provides effective stripping of the nitride or polysilicon layer.
Other dielectric films can be formed from appropriate precursor gases. Polysilicon can be formed using SiH4 and H2, or silane alone. The silane may be introduced downstream of the generator to avoid nucleation and particle formation.
Silicon nitride can be formed by using NH3 or N2 with silane (SiH4) or one of the higher silanes, e.g. Si2H6. The silane can be introduced downstream of the generator to avoid nucleation and particle formation.
- Metal and Metal Oxide Films
In addition to gases, the chemical generator is also capable of using liquids and solids as starting materials, so that precursors such as TEOS can be used in the formation of conformal coatings. Ozone and TEOS have been found to be an effective mixture for the deposition of uniform layers.
Metal and metal oxide films can be deposited via various precursors in accordance with the invention. For example, Ta2O5 films which are used extensively in memory devices can be formed by generating a precursor such as TaCl5 via reduction of TaCl5, followed by oxidation of the TaCl5 to form Ta2O5. In a more general sense, the precursor from which the Ta2O5 is generated can be expressed as TaXm, where x is a halogen species, and m is the stoichiometric number.
Copper can be deposited as a film or an oxide through the reaction
- Wafer and Chamber Cleaning
and other metals can be formed in the same way. Instead of a gaseous precursor, a solid precursor such as Cu or another metal can also be used.
With the invention, organic residue from previous process steps can be effectively removed by using O2 to form ozone which is quite effective in the removal of organic contaminants. In addition, reacting H2 with an excess of O2 will produce steam and O2 as well as other oxygen radicals, all of which are effective in eliminating organic residue. The temperature in the chamber should be below about 700° C. if a wafer is present, in order to prevent oxide formation during the cleaning process.
Sulfuric acid, nitric acid and hydrofluoric acid for use in general wafer cleaning are also effectively produced with the invention. Sulfuric acid (H2SO4) is generated by reacting either S, SO or SO2 with H2 and O2 in accordance with reactions such as the following:
then quickly quenching the free radicals thus formed with or without a catalyst.
Nitric acid (HNO3) is generated by reacting NH3 with H2 and O2, or by a reaction such as the following:
- X=1, 2,
Hydrofluoric acid is generated by co-reacting H2 and O2 with a compound containing fluorine such as NF3 or CxHyFz, where
Mixed acids can be generated from a single precursor by reactions such as the following:
These are but a few examples of the many reactions by which mixed acids can be generated in accordance with the invention. Including more H2 and O2 in the reactions will allow steam to be generated in addition to the mixtures of acids.
- Native Oxide Removal
In order to devolitize the various resultant products of the reaction of HCl, HF, H2SO4 or HNO3, either H2O or H2 and O2 can be co-injected to form steam so that the solvating action of water will disperse in solution in the products. The temperature of the water must be cool enough so that a thin film of water will condense on the wafer surface. Raising the temperature of the water will evaporate the water solution, and spinning the wafer will further assist in the removal process.
- Photoresist Stripping
The native oxide which is ever present when a silicon wafer is exposed to the atmosphere can be selectively eliminated by a combination of HF and steam formed by adding a fluorine source such as NF3 or CF4 to the reagent gases H2 and O2. In order for the native oxide elimination to be most effective, the reaction chamber should be maintained at a pressure below one atmosphere.
H2 and O2 can also be reacted to form steam for use in the stripping of photoresist which is commonly used in patterning of silicon wafers in the manufacture of integrated circuits. In addition, other components such as HF, H2SO4 and HNO3 which are also generated with the invention can be used in varying combinations with the steam to effectively remove photoresist from the wafer surface. Hard implanted photoresist as well as residues in vias can also be removed with steam in combination with these acids.
SO3 for use in the stripping of organic photoresist can be generated by adding O2 to SO2. Similarly, as discussed above, N2O can be converted to NO2, a strong oxidizing agent which can also be used in the stripping of photoresist.
Hydrofluoric acid for use in the stripping of photoresist can be generated in situ in accordance with any of the following reactions:
It is apparent from the foregoing that a new and improved chemical generator and method have been provided. While only certain presently preferred embodiments have been described in detail, as will be apparent to those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.