Patent Application
20050029226
1. Field of the Invention
This invention relates to the field of semiconductor devices and, more particularly, to anisotropic plasma etching of silicon.
2. Description of the Prior Art
Plasma etch processes currently used to etch materials for semiconductor device fabrication consist of an electrical discharge of halogen bearing gases. Halogens typically encountered in these processes are fluorine, chlorine and bromine. The process begins with application of a masking material, such as photoresist, to protect the desired geometry of the device from the etch process. The device in process is then placed in a plasma reactor and etched. The subsequent steps are determined by the type of device being fabricated. This process is especially valuable for the definition of small geometry on the order of less than one to five microns. For definition of geometry of less than one micron, it is essential that the etching proceeds only in the vertical direction. The fragile nature of the small geometry structures cannot have a reasonable degree of reliability if any significant amount of undercutting takes place during the process.
A very common silicon etch process is based on fluorine. When mixtures such as CF4—O2 are dissociated in an electrical discharge, fluorine atoms are liberated, and volatilize the silicon as SiF4. Such processes are isotropic; they etch in all directions at the same rate. Anisotropic, or vertical, etches in silicon are not observed when fluorine is the sole etchant.
In U.S. Pat. No. 4,226,665, Mogab describes etch chemistries which yield vertical etches. For vertical etching of silicon a chemistry such as C2F6—Cl2 is indicated. The C2F6 serves as a source of “recombinants”, such as CF3. The recombinants suppress etching in the horizontal direction by recombining with Cl atoms, which have been adsorbed on the etched walls. Etching can proceed in the vertical direction because ion bombardment from the plasma suppresses the recombination mechanism.
Chemistries based on chlorine have been considered to be necessary for vertical etching of silicon, and discharges of pure Cl2 have been found useful for this purpose. However, some silicon materials, such as highly doped polysilicon, still experience some undercutting if etch conditions are not closely controlled.
Furthermore, the use of chlorocarbons as the Cl source for etching polysilicon is not desirable. Selectivity to the underlying SiO2 layer is a constant problem because the C is a good reducer of SiO2, combining with the O to form CO and CO2. A practical limit on the selectivity of doped polysilicon to SiO2 appears to be roughly 20:1.
In U.S. Pat. No. 4,490,209, Hartman describes the addition of HBr to etch polysilicon. HBr is a poisonous and ozone depleting gas.
In addition, in the production of most MOS integrated circuits, the polysilicon to be etched overlays a thin silicon dioxide layer. It is essential that the etching of the polysilicon does not continue very far into this thin layer. The preferred embodiment of this invention includes a reasonable selectivity to silicon dioxide.
Accordingly, there is a need to develop a plasma etch chemistry which provides a safe and accurate anisotropic etch of silicon and selectivity to silicon dioxide without the prior art problems such as undercutting.
In one embodiment, there is provided a method for selectively etching a layer of a silicon-bearing material in producing a semiconductor device. A patterned masking material is provided over the silicon-bearing material to expose portions of the silicon-bearing material to be etched. A gaseous mixture is passed over the layer. The gaseous mixture comprises dibromomethane. Preferably, the method further includes creating a plasma discharge in the gaseous mixture adjacent the layer of silicon-bearing material.
In one aspect of the present invention, there is provided a plasma etch chemistry which has been shown to be capable of nearly perfectly vertical etching of silicon or materials having a significant silicon content, yet it does not present the problems noted above by the prior art. In a preferred embodiment, dibromomethane is added thereto, and anisotropic etching is provided. This is accomplished by the adsorption of a Si—Br compound on the sidewalls of the etch cavity, while the ion bombardment on the flat surface inhibits the formation of the compound in that area. The silicon may be etched as Si—Cl—Br compound.
Dibromomethane is a relatively safe gas and not ozone depleting, yet giving substantially similar results in plasma etching of silicon, silicon nitride, and other materials, and therefore is an improvement over the prior art.
The etch process includes selectively etching a layer of silicon or silicon compound. The material is first masked then placed in a plasma reactor, where the plasma chemistry consists of dibromomethane and chlorine gaseous compounds and/or fluorine gaseous compounds.
The present invention teaches that a highly anisotropic etch can be achieved by the addition of dibromomethane to the silicon etching chemistry. Ion bombardment overcomes the resistance to etching on flat surfaces, but lateral etching can be suppressed by the formation of a Si—Br compound on the sidwalls.
A five (5) wafer Reactive Ion Etch plasma reactor was used for the following experiments. The wafers rested on the unpowered 12 inch diameter quartz plate which in turn rested on an anodized aluminum electrode.
The counter electrode was stainless steel. The RF frequency was 13.56 MHz. The process conditions for the etching of heavily phosphorus doped polysilicon were as follows:
SiCl4=200 sccm
Cl2=20 sccm
Power=200 watts
Temp.=60.degree. C.
Plate Spacing=1 inch
Pressure=20 milliTorr.
The polysilicon etch rate was about 500 angstroms/min. After 50% overetch, the polysilicon was severely undercut, indicating essentially isotropic etching was occurring, additionally, 320 angstroms of gate oxide was lost during the overetch.
The same process as above was used except CH2Br2 was added at a flow rate of 20 sccm. The polysilicon etch rate was about 400 angstroms/min. After 50% overetch, SEM pictures indicated the process was highly anisotropic. Vertical wall profiles coincident with the photoresist pattern were obtained. In addition, only 20 angstroms of gate oxide was removed, compared to 320 angstroms for the process without CH2Br2.
Dibromomethane is a liquid at room temperature with sufficient vapor pressure to be directly metered by conventional flow devices such as mass flowmeters without pressure regulation, as will be apparent to those skilled in the art.
Thus, when used under the proper conditions, it is apparent that CH2Br2 can prevent lateral etching and increase the selectivity to SiO2. In an analogous reaction to Br2, the CH2Br2 can scavenge free Cl atoms, i.e.
4Cl+CH2Br2→2HCl+2Br+CCl2+ΔG ° C.298=−30.8 kcal/mole,
that apparently are the species participating in the lateral etching.
In addition to the prevention of undercutting in any Cl-containing etch, CH2Br2 addition can be used in a F-containing plasma. CH2Br2 reacts spontaneously with F atoms, i.e.
6F+CH2Br2→2HF+2Br+CF4+ΔG ° C.298=−95.8 kcal/mole.
This can be used to advantage to prevent undercutting caused by F atoms. The F/C ratio is an important parameter that can be related to the SiO2 to silicon etch-rate ratio during SiO2 etching. Since CH2Br2 can scavenge F atoms, it can be used to control the F/C ratio and thus the selectivity during SiO2 etching.
The results of the tests conducted indicate that the addition of dibromomethane to certain halogen-bearing gases contributes significantly to a nearly perfect vertical etch of silicon. In particular, the combination of SiCl4, Cl2 and CH2Br2 yields highly desirable results in that the etched surface of the silicon is very smooth. Therefore, the apparent improvements to the plasma etch process, in the form of increased etch rates and nearly perfect vertical etch (substantially no undercutting), without the need for poisonous gasses such as HBr, afforded by the present invention, will be immediately recognized by those skilled in the art.
It should be noted that, while the use of dibromomethane may be crucial in achieving the advantage of the present invention, the use of SiCl4 and/or Cl2 is not critical at all. A wide variety of other chlorine-bearing etchants such as carbon tetrachloride, phosphorus trichloride, chlorine, hydrogen chloride, boron trichloride, molecular chlorine, and combinations thereof can be substituted for the SiCl4 and/or Cl2. Moreover, a fluorine-bearing gas such as tetrafluoromethane, hexafluoromethane, octofluoropentane, sulfur hexafluoride, and combinations thereof, or a fluorine and chlorine-bearing species such as trifluoromethane, trifluorochloromethane, difluorodichloromethane, fluorotrichloromethane, and combinations thereof can also alternatively be used as the etchant gas. Thus, for example, use of dibromomethane in combination with such etchant gas species as tetrafluoromethane, hexafluoromethane, octofluoropentane, trifluoromethane, trifluorochloromethane, difluorodichloromethane, fluorotrichloromethane, or a wide variety of others are in the scope of the present invention.
To the dibromomethane bearing etchant gas, oxygen, nitrogen and/or, inert gases such as helium, neon, argon, krypton, and xenon can be added to the mixture.
Since the crucial advantage of the present invention may be derived in large part from the function of dibromomethane as a safe and environmentally friendly source for a sidewall passivation agent, the etch process taught by an embodiment of the present invention is applicable not only to single crystalline silicon or polysilicon, but is also applicable to other compositions which contain a large silicon content, for example, tungsten silicide and silicon carbide which are as much as one half silicon by atomic composition. Thus, the etch mixture taught by an embodiment of the present invention is not only applicable to anisotropic etching of silicon nitride, but also is applicable to anisotropic etching of a wide variety of metal silicides. Where the present invention is applied to etching silicides, the etchant gas is preferably a gas containing both chlorine and fluorine-bearing species, to accomplish transport of the metal away from the etch site. However, a mixture of dibromomethane with an etchant gas containing only chlorine-bearing species, or only fluorine-bearing species, is alternatively possible.
In the above example as applied to anisotropic etching of polysilicon, there is used a pressure of about 20 milliTorr, RF power of about 200 watts, a silicon tetrachloride flow rate of about 200 sccm, a chlorine flow rate of about 10 sccm, and a dibromomethane flow rate of about 10 sccm. However, polysilicon etching may be practiced with a broad range of substitutions in this preferred formula, including pressure in the range of about 0.005 Torr to about 5.0 Torr, power density in the range of about 1 to about 30 watts per square inch, halogen-bearing gas flow in the range of about 1 to about 1000 sccm, chlorine gas flow rate in the range of 0 to 30 times the flow rate of the other halogen-bearing etchant, and dibromomethane flow rate in the range of about 1% to about 25%, more preferably 9%. The flow rate of the halogen-bearing etchant is based upon the weight ratio of the bromine in the dibromomethane to the halogen or halogens in the etchant chloride and/or fluoride.
Thus, the present invention provides the advantage of an anisotropic etch for silicon.
The present invention provides the further advantage of an anisotropic etch for silicon or polysilicon, which is insensitive to doping level.
The present invention provides the further advantage of a highly anisotropic etch for silicon-bearing materials, which is highly insensitive to doping level.
The present invention provides the further advantage of a highly anisotropic etch for silicon, which leaves a clean etched surface.
The present invention provides the further advantage of a highly selective etch of polysilicon to silicon dioxide, for example, at least about 10:1.
The present invention provides the further advantage of an environmentally friendly etch.
Though the invention has been described with respect to specific preferred embodiments thereof, many variations and modifications will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.
This application claims priority from U.S. Provisional Application No. 60/493,686, filed on Aug. 7, 2003, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60493686 | Aug 2003 | US |
