Title of Invention	SELECTIVE ETCHING OF SILICON DIOXIDE COMPOSITIONS
Abstract	A process for selectively etching a material comprising SiO2over silicon, the method comprising the steps of: placing a silicon substrate comprising a layer of a material comprising Si02 within a reactor chamber equipped with an energy source; i creating a vacuum within the chamber; introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a polymerIzable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of added oxygen; activating the energy source to form a plasma activated reactive etching gas mixture within the chamber; and selectively etching the material comprising SiO2 preferentially to the silicon substrate.

Title of Invention

SELECTIVE ETCHING OF SILICON DIOXIDE COMPOSITIONS

Abstract

A process for selectively etching a material comprising SiO2over silicon, the method comprising the steps of: placing a silicon substrate comprising a layer of a material comprising Si02 within a reactor chamber equipped with an energy source; i creating a vacuum within the chamber; introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a polymerIzable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of added oxygen; activating the energy source to form a plasma activated reactive etching gas mixture within the chamber; and selectively etching the material comprising SiO2 preferentially to the silicon substrate.

Full Text	TITLE OF THE INVENTION: SELECTIVE ETCHING OF SILICON DIOXIDE COMPOSITIONS CROSS REFERENCE TO RELATED APPLICATIONS This patent application claims the benefit of US Provisional Patent Application Serial Number 61/092,916 filed 29 August 2008. BACKGROUND OF THE INVENTION [0001] The present invention is directed to selectively dry etching a material comprising silicon dioxide over silicon. In particular, the present invention is directed to selectively dry etching phosphorous silicate glass (PSG) over crystalline silicon (doped or undoped) in the manufacture of photovoltaic solar cells. [0002] PSG is formed in solar cell processing during emitter diffusion by exposing a crystalline silicon substrate to phosphorous oxychloride (POCI3) gas. Under an oxygen atmosphere phosphorous is driven into the silicon to form the n* emitter of the solar cell. After the phosphorous diffusion process, the PSG is removed. Prior art processes for PSG removal typically employ wet chemicals such as, for example, hydrofluoric acid (HF), which are extremely harmful to environment and to the handlers. [0003] Dry plasma etching processes have also been developed for PSG removal that do not suffer from the drawbacks of wet chemical processes. Such processes typically rely on fluorocartxjn gases that, in the plasma state, form a polymer layer on the surface. Selectivity between the PSG and silicon is mainly attributed to the formation of this polymer layer because its growth on silicon surfaces is much faster thereby preventing further etching of the silicon. Such prior art dry plasma processes typically employ oxygen in the plasma to limit the amount of polymer formation. Oxygen plasmas, however, are problematic for several reasons. For example, in semiconductor applications, oxygen plasmas are known to damage the dielectric properties of low dielectric materials. Moreover, in solar cell applications where a PSG layer is being - 1 etched from a layer of phosphorous doped silicon, oxygen plasma tends to form SiO2 on the doped silicon surface which acts as an insulator by hindering the flow of electrons through the layer. Accordingly, there is a need in the art for a process for selectively etching a material comprising SiO2 over silicon that does not suffer from the above-mentioned drawbacks. BRIEF SUMMARY OF THE INVENTION [0004] The present invention satisfies this need in the art by providing a process for selectively etching a material comprising SiO2 preferentially to silicon, the process comprising the steps of: placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with an energy source; creating a vacuum within the chamber; introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a poiymerizable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of oxygen; activating the energy source to form a plasma activated reactive etching gas mixture within the chamber; and selectively etching the material comprising SiOz preferentially to the silicon substrate. [0005] In another aspect the present invention provides a process for selectively etching a material comprising Si02 preferentially to silicon, the method comprising the steps of: placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with electrodes; creating a vacuum within the chamber; introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a poiymerizable fluorocartwn, and an inert gas, wherein the reactive gas mixture is substantially free of oxygen; supplying a high frequency electrical energy to the electrodes to form a plasma activated reactive etching gas mixture within the chamber; and selectively etching the material comprising SiO2 preferentially to the silicon substrate. BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS [0006] FIGURE 1 is an illustration of a layer of a silicon substrate comprising a layer of a material comprising SiOg; [0007] FIGURE 2 is a graphic illustration of the conditions evaluated in connection with the etch selectivity ofSiO2 over silicon with C4F6/NF3 according to the process of the present invention; [0008] FIGURE 3 is a graphic illustration of the conditions evaluated in connection with the etch selectivity of PSG over silicon with C4F6/NF3 according to the process of the present invention; [0009] FIGURE 4 is a graphic summary of selectivity data forSiO2/Si and 4% PSG/Si for various etch chemistries evaluated; [0010] FIGURE 5 is a graphic summary of etch rate data forSiO2, 4% PSG, and Si for various etch chemistries evaluated; [0011] FIGURE 6 illustrates a series of mass spectral data demonstrating the effect of NF3 on C4F6 in a plasma while etching SiOz and Si; and [0012] FIGURE 7 illustrates the in situ optical emission spectral data monitoring F and CFa species during a selective etch according to the present invention. DETAILED DESCRIPTION OF THE INVENTION [0013] The present invention provides a process for selectively etching a material comprising SiOz preferentially to silicon, the process comprising the steps of: placing a silicon substrate having a layer of a material comprising SiOz within a reactor chamber equipped with an energy source; creating a vacuum within the chamber; introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a polymerizable fluorocartwn, and an inert gas, wherein the reactive gas mixture is substantially free of oxygen; activating the energy source to form a plasma activated reactive etching gas mixture within the chamber; and selectively etching the material comprising SiOz preferentially to the silicon substrate. The process of the present invention as described in detail herein employs a polymer forming fluorocarbon material mixed in a plasma with a fluorine source to aid in the selective etch of PSG or SiOz over silicon. The unique advantage of the present invention is that the selectivity is achieved in a plasma environment that is substantially free of oxygen. As used herein, the phrase "substantially free of added oxygen" as it relates to a plasma environment refers to a plasma environment where no oxygen is added even though some oxygen may be inherently present depending on the level of vacuum present in the chamber, or oxygen -3- may be produced as a byproduct of the etching process. A surprising aspect of the present invention is the discovery that the addition of a fluorine species to the plasma without oxygen will also result in the favorable fluorocarbon fragmentation and polymer formation which results in the selective etching of PSG or SiOg preferentially to silicon. The process of the present invention is applicable, for example, to the photovoltaic industry in the manufacture of multicrystalline solar cells, and to the electronics industry in the manufacture of semiconductor devices. [0014] The process of the present invention comprises the step of placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with an energy source. As used herein, the term "silicon substrate" refers to silicon in its various forms such as monocrystalline silicon, microcrystalline silicon, polysilicon, amorphous silicon, and epitaxial silicon. The silicon substrate can be doped or undoped. As used herein, the term "doped" as it refers to the silicon substrate means an added impurity that may lower the resistance of the layer. Typical such impurities include Group III elements such as, for example, B (P-type dopants), and Group V elements such as, for example. As, P, and N (N-type dopants). As used herein, the term "material comprising SiO2" refers to Si02 or any SiOz-containing material such as, for example, any organosilicate glass (OSG), phosphorous silicate glass (PSG), boron phosphorous silicate glass (BPSG), and fluorosillcate glass (FSG). Figure 1 provides an example of a silicon substrate comprising a layer of a material comprising SiO2 wherein silicon substrate 12 is, for example, monocrystalline silicon, and layer 10 is, for example, either SiOz or PSG. The reactor chamber in which the silicon substrate having a layer of a material comprising SiOg is placed is any reaction chamber suitable for use with a vacuum in a plasma process that is equipped with an energy source sufficient to form a plasma. [0015] The process of the present invention also comprises the step of creating a vacuum within the chamber once the silicon substrate having a layer of a material comprising SiO2 is placed within the chamber and the chamber is sealed. In preferred emtxDdiments, a vacuum is created such that the operating pressure is anywhere from 0.1 to 10,000 mTorr, preferably from 1 to 10,000 mTorr, and more preferably from 1 to 1000 mTorr. The vacuum can be created by any pumping means known to those skilled in the art for creating a vacuum in a vacuum chamber. -4- [0016] The process of the present invention also comprises the step of introducing into the reactor chamber a reactive gas mixture comprising, consisting essentially of, or consisting of, a fluorine compound, a poiymerizable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of oxygen. In the reactive gas mixture the fluorine compound functions to provide fluorine atoms that control the rate of polymer formation as well as etch the SiOa. Preferred fluorine compounds include NF3, F2, F2 created in situ, and mixtures of Fg in an inert gas such as, for example, helium, argon, or nitrogen. [0017] Preferably, the fluorine compound is present in the reactive gas mixture at a concentration of from 1 to 40% by volume, more preferably from 5 to 15% by volume and, most preferably, from 5 to 10% by volume. [0018] As used herein, the term "poiymerizable fluorocarbon" refers to a fluorocarbon that is capable of polymerizing under plasma conditions to form a polymer layer on the substrate being etched. In the reactive gas mixture the poiymerizable fluorocarbon functions to form a polymer layer on silicon surfaces at a faster rate relative to SIO2-containing surfaces thereby preventing further etching of the silicon. Preferred poiymerizable fluorocart)ons include, for example, a perfluorocarixin compound having the formula ChFj wherein h is a number ranging from 4 to 6 and i is a number ranging from h to 2h+2. Examples of perfluorocarbons having the formula ChFj include, but are not limited to, C4F8 (octafluorocyclobutane), CsFs (octafluorocyclopentene), CBFS (hexafluorobenzene), and C4F6 (hexafluoro-1,3-butadiene). In some embodiments of the present invention, the poiymerizable fluorocartjcn is a hydrofluorocarbon having the fomiula CxFyHz wherein x is a number ranging from 1 to 4 and z is a number ranging from 1 to (2x+1) and y is ((2x+2)-z). Examples of hydrofluorocart)ons having the formula CxFyFz include, but are not limited to, CHF3 (trifluoromethane), C2F5H (1,1,1,2,2-pentafluoroethane), and C3F7H (1,1,1,2,3,3,3 heptafluoropropane). In preferred embodiments, the poiymerizable fluorocarbon is hexafluoro-1,3-butadiene. [0019] Preferably, the poiymerizable fluorocarbon is present in the reactive gas mixture at a concentration of from 1 to 25% by volume, more preferably from 5 to 15% by volume and, most preferably, from 5 to 10% by volume. [0020] The inert gas component of the reactive gas mixture typically comprises the remainder of the volume percent of the mixture and functions as a diluent/carrier for the 5- fluorine compound and the polymerizable fluorocarbon. Examples of suitable inert gases include argon, helium, nitrogen, and mixtures thereof. The preferred inert gas is argon. [0021] Preferably, the ratio of the fluorine compound to the polymerizable fluorocarbon in the reactive gas mixture is from 0.1 to 20, more preferably from 0.5 to 2.0 and, most preferably the ratio is 1 to 1. [0022] The components of the reactive gas mixture can be delivered to the reaction chamber by a variety of means, such as, for example, conventional cylinders, safe delivery systems, vacuum delivery systems, solid or liquid-based generators that create the chemical reagent and/or the gas mixture at the point of use (POU). [0023] The process of the present invention also includes the step of activating the energy source to form a plasma activated reactive etching gas mixture within the chamber. Here, the reactive gas mixture of the present invention is exposed to one or more energy sources sufficient to generate active species to at least partially react with the dielectric material and form volatile species. The energy source for the exposing step may include, but not be limited to, a-particles, p-particles, y-rays, x-rays, high energy electron, electron beam sources of energy, ultraviolet (wavelengths ranging from 10 to 400 nm), visible (wavelengths ranging from 400 to 750 nm), infrared (wavelengths ranging from 750 to 101 nm), microwave (frequency > 10® Hz), radio-frequency wave (frequency > 10* Hz) energy; thermal, RF, DC, arc or corona discharge, sonic, ultrasonic or megasonic energy, and combinations thereof. [0024] In one embodiment, the reactive gas mixture is exposed to an energy source sufficient to generate a plasma having active species contained therein. Specific examples of using the plasma for etching processes include, but are not limited to, plasma etching, reactive ion etch (RIE), magnetically enhanced reactive ion etch (MERIE), a inductively coupled plasma (ICP) with or without a separate bias power source, transformer coupled plasma (TCP), hollow anode type plasma, helical resonator plasma, electron cyclotron resonance (ECR) with or without a separate bias power source, RF or microwave excited high density plasma source with or without a separate bias power source, etc. In embodiments wherein a RIE process is employed, the etching process is conducted using a capacitively coupled parallel plate reaction chamber. In these embodiments, the layered substrate (e.g., a patterned wafer) may be placed onto a RF powered lower electrode within a reaction chamber. In embodiments wherein a plasma etching process is employed, the etching process is conducted using a capacitively coupled parallel plate reaction chamber. In these embodiments, the layered substrate (e.g., a patterned wafer) may be placed onto a grounded lower electrode within a reaction chamber. The substrate is held onto the electrode by either a mechanical clamping ring or an electrostatic chuck. The backside of the substrate may be cooled with an inert gas such as helium. The RF power source may be, for example, an RF generator operating at a frequency of 13.56 MHz, however other frequencies can also be used. The RF power density can vary from 0.3 to 30 W/cm^, preferably from 1 to 16 W/cm^. The flow rate of the mixture into the reaction chamber ranges from 10 to 50,000 standard cubic centimeters per minute (seem), preferably from 20 to 10,000 seem, and more preferably from 25 to 1,000 seem. [0025] The process of the present invention also includes the step of selectively etching the material comprising Si02 0ver the silicon substrate. As used herein, the term "selectivel/' or "selectivity" as It refers to etching means a ratio of the etch rate of the material comprising Si02 preferential to the etch rate of the silicon substrate that is greater than 1.0. Although in theory the higher the selectivity, the better the selectivity of the process, typical selectivlties achieved by the process of the present invention range from about 1 to about 100, more preferably from about 5 to at)out 20 and, most preferably about 10. The selective etching is described in more detail in the examples that follow. [0026] Once the layer of material comprising SiOz is etched from the silicon substrate, the silicon substrate is ready for additional process steps. For example, if the silicon substrate is a silicon substrate in the manufacture of a solar cell, a layer of silicon nitride may be deposited on the silicon substrate. In preferred emtiodiments of the process of the present invention, the material comprising SiOa is etched and a layer of silicon nitride is deposited in the same plasma chamber without breaking vacuum. [0027] The invention will be illustrated in more detail with reference to the following Examples, but it should be understood that the present invention is not deemed to be limited thereto. EXAMPLES [0028] The reactor used is the Gaseous Electronics Conference (GEC) RF reference plasma reactor configured for capacitively coupled plasmas (CCP). The standard GEC -7 Cell lower electrode was replaced by a custom built electrostatic chuck/powered electrode assembly (Model CP100, Electrogrip Inc.) for 100 mm (4 inch) wafer processing. Helium backside cooling pressure was set at 4 Torr during plasma processing. The electrostatic chuck assembly was cooled by a recirculating coolant at 20°C inlet temperature. The entire RF powered electrode/electrostatic chuck assembly had a diameter of 150 mm (6 inch). During the experiments, 300 W of RF power at 13.56 MHz was delivered from a RF generator and a matching network to the lower electrode to generate plasmas. The center part of the grounded top electrode was a standard GEC electrode (100 mm diameter) with a feed-gas distribution showerhead. The RF conductor of the top electrode was connected to the grounded chamber wall through a copper strap outside of the vacuum chamber. The showerhead/top electrode assembly was also cooled by a recirculating coolant flow at 20°C inlet temperature. In reactive ion etch (RIE), the etch rate depends strongly on the DC self-bias and RF voltage at the powered (wafer) electrode. At a given input power, a higher ratio between grounded and powered surface areas typically leads to increased DC self-bias voltage and consequently increased etch rates. To increase the DC self-bias voltage at the powered (wafer) electrode, the top grounded electrode was extended by a grounded annular ring. With the extension, the grounded electrode had a diameter of 230 mm. The spacing between the grounded and powered electrode was 25 mm (1 inch). The flow of process gases was controlled by a set of mass flow controllers and the gases were fed into the reactor through the showerhead on the top electrode. After passing between the electrodes, the process gases and plasma byproducts were pumped out of the reactor through an 8-inch conflat side port by a 510 liter/second turbomolecular pump backed by a multistage dry mechanical pump. The chamber base pressure was atwut 10^ Torr. During plasma processing, the chamber pressure was measured by a capacitance manometer (MKS Baratron) and controlled by an electronic throttle valve between the reactor and the turbomolecular pump. 5 sIm of Ng was injected into the dry mechanical pump through an interstage pump purge. [0029] The examples below employ mixtures of C4F6 and NF3 at various concentrations to etch SiOa, 4% PSG, and polysilicon (or Si). [0030] Table 1 is the full design of experiments (DOE) for C4F6/NF3 etching of SiOs and Si. The information in Table 1 suggests that the best conditions for highest selectivity are when %C4F6 is equal to 5% and the % NF3 is equal to 5% at the stated plasma conditions. Figure 2 represents graphically a model that predicts selectivity over the -8- range of conditions based upon the information in Table 1. The model was prepared by employing Origin Scientific Graphing and Analysis Software™ (version 7.5 SR6) by OrlginLab Corporation (Northampton, MA). As is evident, there appears to be a channel of conditions that could provide enhanced SiO/Si selectivity. [0031] Table 2 is a smaller DOE for C4F6/NF3 etching of 4% PSG and Si based on the information in Table 1 and Figure 2. The information in Table 2 suggests that the best conditions for highest selectivity are when the concentration of C4F6 is equal to 5% and the concentration of NF3 is equal to 5% at the stated plasma conditions. Figure 3 represents graphically a model that predicts selectivity over the range of conditions based upon the information in Table 2. The model was prepared by employing Origin -9- Scientific Graphing and Analysis Software™ (version 7.5 SR6) by OriginLab Corporation (Northampton, MA). As is evident, there appears to be a channel of conditions that could provide enhanced SiO/Si selectivity. [0032] Figure 4 presents a summary plot of all etch chemistries evaluated and the resultant best selectivities for SiOa/Si and 4% PSG/Si. The increase in selectivity with the addition of C4F6 to a fluorocarbon plasma (CF4) supports the enhanced selectivity as a result of the process of the present invention. Also, the increase in selectivity observed between CF4/NF3 and C4F6/NF3, strongly suggests that the increased selectivity is due to the superior polymer forming ability of C4F6 under plasma conditions relative to CF4 without the presence of oxygen. [0033] Figure 5 presents a summary plot of all etch chemistries evaluated and the resultant best etch rates (related to the best selectivity) for SiOa, 4% PSG, and Si. It is important to note that the silicon etch rate in relation to both PSG and SIO2 is diminished in the C4F6 chemistry due to the higher rate of polymer formation. [0034] Figures 6 and 7 present C4F6/NF3 SiOz and Si data as measured by in-situ instoimentation. The data within these plots illustrates that the addition of NF3 to C4F6 in a plasma (free of added oxygen) alters the C4F6 fragmentation within the plasma. Without intending to be bound by any particular theory, the foregoing examples suggest that the use of NF3 decreases the quantity of smaller C4F6 fragments which, in turn, -10- effects how C4F6polymerizes on the surface of an SiOs or Si film, and thus affects the etch rate and selectivity. CLAIMS 1. A process for selectively etching a material comprising SiO2 preferentially to silicon, the method comprising the steps of: placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with an energy source; creating a vacuum within the chamber; introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a polymerizable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of added oxygen; activating the energy source to form a plasma activated reactive etching gas mixture within the chamber; and selectively etching the material comprising SiO2 preferentially to the silicon substrate. 2. The process of Claim 1 wherein the polymerizable fluorocarbon and the fluorine compound in the reactive gas mixture are present at a ratio of 1:1. 3. The process of Claim 1 wherein the material comprising SiO2 is selected from the group consisting of: SiO2 and an organosilicate glass. 4. The process of Claim 3 wherein the organosilicate glass is selected from the group consisting of phosphorous silicate glass and fluorosilicate glass. 5. The process of Claim 4 wherein the material comprising SiOsis phosphorous silicate glass. 6. The process of Claim 3 wherein the material comprising SiO2 is substantially Si02. 12- 7. The process of Claim 1 wherein the fluorine compound is selected from the group consisting of: NF3, F2, F2 created in situ, and a mixture of F2 in an inert gas. 8. The process of Claim 7 wherein the fluorine compound is NF3. 9. The process of Claim 1 wherein the polymerizable fluorocarbon is a compound having the formula ChFj wherein h is a number ranging from 4 to 6 and i is a number ranging from h to 2h+2. 10. The process of Claim 9 wherein the polymerizable fluorocarbon is selected from the group consisting of: octafluorocyclobutane, octafluorocyclopentene, hexafluorobenzene, hexafluoro-1,3-butadiene and mixtures thereof. 11. The process of Claim 10 wherein the polymerizable fluorocarbon is hexafluoro-1,3-butadiene. 12. The process of Claim 1 wherein the polymerizable fluorocarbon is a compound having the formula CxFyHz wherein x is a number ranging from 1 to 4 and z is a number ranging from 1 to (2x+1) and y is {(2x+2)-z). 13. The process of Claim 12 wherein the polymerizable fluorocarbon is selected from the group consisting of: CHF3 (trifluoromethane), C2F5H (1,1,1,2,2-pentafluoroethane), and C3F7H (1,1,1,2,3,3,3 heptafluoropropane) and mixtures thereof. 14. A process for selectively etching a material comprising Si02 preferentially to silicon, the method comprising the steps of: placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with electrodes; creating a vacuum within the chamber; 13 introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a polymerizable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of added oxygen; supplying a high frequency electrical energy to the electrodes to form a plasma activated reactive etching gas mixture v11ithin the chamber; and selectively etching the material comprising SiOg preferentially to the silicon substrate.

Full Text

TITLE OF THE INVENTION: SELECTIVE ETCHING OF SILICON DIOXIDE COMPOSITIONS
CROSS REFERENCE TO RELATED APPLICATIONS
This patent application claims the benefit of US Provisional Patent Application Serial Number 61/092,916 filed 29 August 2008.
BACKGROUND OF THE INVENTION
[0001] The present invention is directed to selectively dry etching a material comprising silicon dioxide over silicon. In particular, the present invention is directed to selectively dry etching phosphorous silicate glass (PSG) over crystalline silicon (doped or undoped) in the manufacture of photovoltaic solar cells.
[0002] PSG is formed in solar cell processing during emitter diffusion by exposing a crystalline silicon substrate to phosphorous oxychloride (POCI3) gas. Under an oxygen atmosphere phosphorous is driven into the silicon to form the n* emitter of the solar cell. After the phosphorous diffusion process, the PSG is removed. Prior art processes for PSG removal typically employ wet chemicals such as, for example, hydrofluoric acid (HF), which are extremely harmful to environment and to the handlers.
[0003] Dry plasma etching processes have also been developed for PSG removal that do not suffer from the drawbacks of wet chemical processes. Such processes typically rely on fluorocartxjn gases that, in the plasma state, form a polymer layer on the surface. Selectivity between the PSG and silicon is mainly attributed to the formation of this polymer layer because its growth on silicon surfaces is much faster thereby preventing further etching of the silicon. Such prior art dry plasma processes typically employ oxygen in the plasma to limit the amount of polymer formation. Oxygen plasmas, however, are problematic for several reasons. For example, in semiconductor applications, oxygen plasmas are known to damage the dielectric properties of low dielectric materials. Moreover, in solar cell applications where a PSG layer is being
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etched from a layer of phosphorous doped silicon, oxygen plasma tends to form SiO2 on the doped silicon surface which acts as an insulator by hindering the flow of electrons through the layer. Accordingly, there is a need in the art for a process for selectively etching a material comprising SiO2 over silicon that does not suffer from the above-mentioned drawbacks.
BRIEF SUMMARY OF THE INVENTION
[0004] The present invention satisfies this need in the art by providing a process for selectively etching a material comprising SiO2 preferentially to silicon, the process comprising the steps of: placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with an energy source; creating a vacuum within the chamber; introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a poiymerizable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of oxygen; activating the energy source to form a plasma activated reactive etching gas mixture within the chamber; and selectively etching the material comprising SiOz preferentially to the silicon substrate.
[0005] In another aspect the present invention provides a process for selectively etching a material comprising Si02 preferentially to silicon, the method comprising the steps of: placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with electrodes; creating a vacuum within the chamber; introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a poiymerizable fluorocartwn, and an inert gas, wherein the reactive gas mixture is substantially free of oxygen; supplying a high frequency electrical energy to the electrodes to form a plasma activated reactive etching gas mixture within the chamber; and selectively etching the material comprising SiO2 preferentially to the silicon substrate.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0006] FIGURE 1 is an illustration of a layer of a silicon substrate comprising a layer of a material comprising SiOg;

[0007] FIGURE 2 is a graphic illustration of the conditions evaluated in connection with the etch selectivity ofSiO2 over silicon with C4F6/NF3 according to the process of the present invention;
[0008] FIGURE 3 is a graphic illustration of the conditions evaluated in connection with the etch selectivity of PSG over silicon with C4F6/NF3 according to the process of the present invention;
[0009] FIGURE 4 is a graphic summary of selectivity data forSiO2/Si and 4% PSG/Si for various etch chemistries evaluated;
[0010] FIGURE 5 is a graphic summary of etch rate data forSiO2, 4% PSG, and Si for various etch chemistries evaluated;
[0011] FIGURE 6 illustrates a series of mass spectral data demonstrating the effect of NF3 on C4F6 in a plasma while etching SiOz and Si; and
[0012] FIGURE 7 illustrates the in situ optical emission spectral data monitoring F and CFa species during a selective etch according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] The present invention provides a process for selectively etching a material comprising SiOz preferentially to silicon, the process comprising the steps of: placing a silicon substrate having a layer of a material comprising SiOz within a reactor chamber equipped with an energy source; creating a vacuum within the chamber; introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a polymerizable fluorocartwn, and an inert gas, wherein the reactive gas mixture is substantially free of oxygen; activating the energy source to form a plasma activated reactive etching gas mixture within the chamber; and selectively etching the material comprising SiOz preferentially to the silicon substrate. The process of the present invention as described in detail herein employs a polymer forming fluorocarbon material mixed in a plasma with a fluorine source to aid in the selective etch of PSG or SiOz over silicon. The unique advantage of the present invention is that the selectivity is achieved in a plasma environment that is substantially free of oxygen. As used herein, the phrase "substantially free of added oxygen" as it relates to a plasma environment refers to a plasma environment where no oxygen is added even though some oxygen may be inherently present depending on the level of vacuum present in the chamber, or oxygen
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may be produced as a byproduct of the etching process. A surprising aspect of the present invention is the discovery that the addition of a fluorine species to the plasma without oxygen will also result in the favorable fluorocarbon fragmentation and polymer formation which results in the selective etching of PSG or SiOg preferentially to silicon. The process of the present invention is applicable, for example, to the photovoltaic industry in the manufacture of multicrystalline solar cells, and to the electronics industry in the manufacture of semiconductor devices.
[0014] The process of the present invention comprises the step of placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with an energy source. As used herein, the term "silicon substrate" refers to silicon in its various forms such as monocrystalline silicon, microcrystalline silicon, polysilicon, amorphous silicon, and epitaxial silicon. The silicon substrate can be doped or undoped. As used herein, the term "doped" as it refers to the silicon substrate means an added impurity that may lower the resistance of the layer. Typical such impurities include Group III elements such as, for example, B (P-type dopants), and Group V elements such as, for example. As, P, and N (N-type dopants). As used herein, the term "material comprising SiO2" refers to Si02 or any SiOz-containing material such as, for example, any organosilicate glass (OSG), phosphorous silicate glass (PSG), boron phosphorous silicate glass (BPSG), and fluorosillcate glass (FSG). Figure 1 provides an example of a silicon substrate comprising a layer of a material comprising SiO2 wherein silicon substrate 12 is, for example, monocrystalline silicon, and layer 10 is, for example, either SiOz or PSG. The reactor chamber in which the silicon substrate having a layer of a material comprising SiOg is placed is any reaction chamber suitable for use with a vacuum in a plasma process that is equipped with an energy source sufficient to form a plasma.
[0015] The process of the present invention also comprises the step of creating a vacuum within the chamber once the silicon substrate having a layer of a material comprising SiO2 is placed within the chamber and the chamber is sealed. In preferred emtxDdiments, a vacuum is created such that the operating pressure is anywhere from 0.1 to 10,000 mTorr, preferably from 1 to 10,000 mTorr, and more preferably from 1 to 1000 mTorr. The vacuum can be created by any pumping means known to those skilled in the art for creating a vacuum in a vacuum chamber.
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[0016] The process of the present invention also comprises the step of introducing into the reactor chamber a reactive gas mixture comprising, consisting essentially of, or consisting of, a fluorine compound, a poiymerizable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of oxygen. In the reactive gas mixture the fluorine compound functions to provide fluorine atoms that control the rate of polymer formation as well as etch the SiOa. Preferred fluorine compounds include NF3, F2, F2 created in situ, and mixtures of Fg in an inert gas such as, for example, helium, argon, or nitrogen.
[0017] Preferably, the fluorine compound is present in the reactive gas mixture at a concentration of from 1 to 40% by volume, more preferably from 5 to 15% by volume and, most preferably, from 5 to 10% by volume.
[0018] As used herein, the term "poiymerizable fluorocarbon" refers to a fluorocarbon that is capable of polymerizing under plasma conditions to form a polymer layer on the substrate being etched. In the reactive gas mixture the poiymerizable fluorocarbon functions to form a polymer layer on silicon surfaces at a faster rate relative to SIO2-containing surfaces thereby preventing further etching of the silicon. Preferred poiymerizable fluorocart)ons include, for example, a perfluorocarixin compound having the formula ChFj wherein h is a number ranging from 4 to 6 and i is a number ranging from h to 2h+2. Examples of perfluorocarbons having the formula ChFj include, but are not limited to, C4F8 (octafluorocyclobutane), CsFs (octafluorocyclopentene), CBFS (hexafluorobenzene), and C4F6 (hexafluoro-1,3-butadiene). In some embodiments of the present invention, the poiymerizable fluorocartjcn is a hydrofluorocarbon having the fomiula CxFyHz wherein x is a number ranging from 1 to 4 and z is a number ranging from 1 to (2x+1) and y is ((2x+2)-z). Examples of hydrofluorocart)ons having the formula CxFyFz include, but are not limited to, CHF3 (trifluoromethane), C2F5H (1,1,1,2,2-pentafluoroethane), and C3F7H (1,1,1,2,3,3,3 heptafluoropropane). In preferred embodiments, the poiymerizable fluorocarbon is hexafluoro-1,3-butadiene.
[0019] Preferably, the poiymerizable fluorocarbon is present in the reactive gas mixture at a concentration of from 1 to 25% by volume, more preferably from 5 to 15% by volume and, most preferably, from 5 to 10% by volume.
[0020] The inert gas component of the reactive gas mixture typically comprises the remainder of the volume percent of the mixture and functions as a diluent/carrier for the
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fluorine compound and the polymerizable fluorocarbon. Examples of suitable inert gases include argon, helium, nitrogen, and mixtures thereof. The preferred inert gas is argon.
[0021] Preferably, the ratio of the fluorine compound to the polymerizable fluorocarbon in the reactive gas mixture is from 0.1 to 20, more preferably from 0.5 to 2.0 and, most preferably the ratio is 1 to 1.
[0022] The components of the reactive gas mixture can be delivered to the reaction chamber by a variety of means, such as, for example, conventional cylinders, safe delivery systems, vacuum delivery systems, solid or liquid-based generators that create the chemical reagent and/or the gas mixture at the point of use (POU).
[0023] The process of the present invention also includes the step of activating the energy source to form a plasma activated reactive etching gas mixture within the chamber. Here, the reactive gas mixture of the present invention is exposed to one or more energy sources sufficient to generate active species to at least partially react with the dielectric material and form volatile species. The energy source for the exposing step may include, but not be limited to, a-particles, p-particles, y-rays, x-rays, high energy electron, electron beam sources of energy, ultraviolet (wavelengths ranging from 10 to 400 nm), visible (wavelengths ranging from 400 to 750 nm), infrared (wavelengths ranging from 750 to 101 nm), microwave (frequency > 10® Hz), radio-frequency wave (frequency > 10* Hz) energy; thermal, RF, DC, arc or corona discharge, sonic, ultrasonic or megasonic energy, and combinations thereof.
[0024] In one embodiment, the reactive gas mixture is exposed to an energy source sufficient to generate a plasma having active species contained therein. Specific examples of using the plasma for etching processes include, but are not limited to, plasma etching, reactive ion etch (RIE), magnetically enhanced reactive ion etch (MERIE), a inductively coupled plasma (ICP) with or without a separate bias power source, transformer coupled plasma (TCP), hollow anode type plasma, helical resonator plasma, electron cyclotron resonance (ECR) with or without a separate bias power source, RF or microwave excited high density plasma source with or without a separate bias power source, etc. In embodiments wherein a RIE process is employed, the etching process is conducted using a capacitively coupled parallel plate reaction chamber. In these embodiments, the layered substrate (e.g., a patterned wafer) may be placed onto a RF powered lower electrode within a reaction chamber. In embodiments wherein a plasma etching process is employed, the etching process is conducted using a

capacitively coupled parallel plate reaction chamber. In these embodiments, the layered substrate (e.g., a patterned wafer) may be placed onto a grounded lower electrode within a reaction chamber. The substrate is held onto the electrode by either a mechanical clamping ring or an electrostatic chuck. The backside of the substrate may be cooled with an inert gas such as helium. The RF power source may be, for example, an RF generator operating at a frequency of 13.56 MHz, however other frequencies can also be used. The RF power density can vary from 0.3 to 30 W/cm^, preferably from 1 to 16 W/cm^. The flow rate of the mixture into the reaction chamber ranges from 10 to 50,000 standard cubic centimeters per minute (seem), preferably from 20 to 10,000 seem, and more preferably from 25 to 1,000 seem.
[0025] The process of the present invention also includes the step of selectively etching the material comprising Si02 0ver the silicon substrate. As used herein, the term "selectivel/' or "selectivity" as It refers to etching means a ratio of the etch rate of the material comprising Si02 preferential to the etch rate of the silicon substrate that is greater than 1.0. Although in theory the higher the selectivity, the better the selectivity of the process, typical selectivlties achieved by the process of the present invention range from about 1 to about 100, more preferably from about 5 to at)out 20 and, most preferably about 10. The selective etching is described in more detail in the examples that follow.
[0026] Once the layer of material comprising SiOz is etched from the silicon substrate, the silicon substrate is ready for additional process steps. For example, if the silicon substrate is a silicon substrate in the manufacture of a solar cell, a layer of silicon nitride may be deposited on the silicon substrate. In preferred emtiodiments of the process of the present invention, the material comprising SiOa is etched and a layer of silicon nitride is deposited in the same plasma chamber without breaking vacuum.
[0027] The invention will be illustrated in more detail with reference to the following Examples, but it should be understood that the present invention is not deemed to be limited thereto.
EXAMPLES
[0028] The reactor used is the Gaseous Electronics Conference (GEC) RF reference plasma reactor configured for capacitively coupled plasmas (CCP). The standard GEC
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Cell lower electrode was replaced by a custom built electrostatic chuck/powered electrode assembly (Model CP100, Electrogrip Inc.) for 100 mm (4 inch) wafer processing. Helium backside cooling pressure was set at 4 Torr during plasma processing. The electrostatic chuck assembly was cooled by a recirculating coolant at 20°C inlet temperature. The entire RF powered electrode/electrostatic chuck assembly had a diameter of 150 mm (6 inch). During the experiments, 300 W of RF power at 13.56 MHz was delivered from a RF generator and a matching network to the lower electrode to generate plasmas. The center part of the grounded top electrode was a standard GEC electrode (100 mm diameter) with a feed-gas distribution showerhead. The RF conductor of the top electrode was connected to the grounded chamber wall through a copper strap outside of the vacuum chamber. The showerhead/top electrode assembly was also cooled by a recirculating coolant flow at 20°C inlet temperature. In reactive ion etch (RIE), the etch rate depends strongly on the DC self-bias and RF voltage at the powered (wafer) electrode. At a given input power, a higher ratio between grounded and powered surface areas typically leads to increased DC self-bias voltage and consequently increased etch rates. To increase the DC self-bias voltage at the powered (wafer) electrode, the top grounded electrode was extended by a grounded annular ring. With the extension, the grounded electrode had a diameter of 230 mm. The spacing between the grounded and powered electrode was 25 mm (1 inch). The flow of process gases was controlled by a set of mass flow controllers and the gases were fed into the reactor through the showerhead on the top electrode. After passing between the electrodes, the process gases and plasma byproducts were pumped out of the reactor through an 8-inch conflat side port by a 510 liter/second turbomolecular pump backed by a multistage dry mechanical pump. The chamber base pressure was atwut 10^ Torr. During plasma processing, the chamber pressure was measured by a capacitance manometer (MKS Baratron) and controlled by an electronic throttle valve between the reactor and the turbomolecular pump. 5 sIm of Ng was injected into the dry mechanical pump through an interstage pump purge.
[0029] The examples below employ mixtures of C4F6 and NF3 at various concentrations to etch SiOa, 4% PSG, and polysilicon (or Si).
[0030] Table 1 is the full design of experiments (DOE) for C4F6/NF3 etching of SiOs and Si. The information in Table 1 suggests that the best conditions for highest selectivity are when %C4F6 is equal to 5% and the % NF3 is equal to 5% at the stated plasma conditions. Figure 2 represents graphically a model that predicts selectivity over the
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range of conditions based upon the information in Table 1. The model was prepared by employing Origin Scientific Graphing and Analysis Software™ (version 7.5 SR6) by OrlginLab Corporation (Northampton, MA). As is evident, there appears to be a channel of conditions that could provide enhanced SiO/Si selectivity.

[0031] Table 2 is a smaller DOE for C4F6/NF3 etching of 4% PSG and Si based on the information in Table 1 and Figure 2. The information in Table 2 suggests that the best conditions for highest selectivity are when the concentration of C4F6 is equal to 5% and the concentration of NF3 is equal to 5% at the stated plasma conditions. Figure 3 represents graphically a model that predicts selectivity over the range of conditions based upon the information in Table 2. The model was prepared by employing Origin
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Scientific Graphing and Analysis Software™ (version 7.5 SR6) by OriginLab Corporation (Northampton, MA). As is evident, there appears to be a channel of conditions that could provide enhanced SiO/Si selectivity.

[0032] Figure 4 presents a summary plot of all etch chemistries evaluated and the resultant best selectivities for SiOa/Si and 4% PSG/Si. The increase in selectivity with the addition of C4F6 to a fluorocarbon plasma (CF4) supports the enhanced selectivity as a result of the process of the present invention. Also, the increase in selectivity observed between CF4/NF3 and C4F6/NF3, strongly suggests that the increased selectivity is due to the superior polymer forming ability of C4F6 under plasma conditions relative to CF4 without the presence of oxygen.
[0033] Figure 5 presents a summary plot of all etch chemistries evaluated and the resultant best etch rates (related to the best selectivity) for SiOa, 4% PSG, and Si. It is important to note that the silicon etch rate in relation to both PSG and SIO2 is diminished in the C4F6 chemistry due to the higher rate of polymer formation.
[0034] Figures 6 and 7 present C4F6/NF3 SiOz and Si data as measured by in-situ instoimentation. The data within these plots illustrates that the addition of NF3 to C4F6 in a plasma (free of added oxygen) alters the C4F6 fragmentation within the plasma. Without intending to be bound by any particular theory, the foregoing examples suggest that the use of NF3 decreases the quantity of smaller C4F6 fragments which, in turn,
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effects how C4F6polymerizes on the surface of an SiOs or Si film, and thus affects the etch rate and selectivity.

CLAIMS
1. A process for selectively etching a material comprising SiO2 preferentially to silicon, the method comprising the steps of:
placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with an energy source;
creating a vacuum within the chamber;
introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a polymerizable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of added oxygen;
activating the energy source to form a plasma activated reactive etching gas mixture within the chamber; and
selectively etching the material comprising SiO2 preferentially to the silicon substrate.
2. The process of Claim 1 wherein the polymerizable fluorocarbon and the fluorine compound in the reactive gas mixture are present at a ratio of 1:1.
3. The process of Claim 1 wherein the material comprising SiO2 is selected from the group consisting of: SiO2 and an organosilicate glass.
4. The process of Claim 3 wherein the organosilicate glass is selected from the group consisting of phosphorous silicate glass and fluorosilicate glass.
5. The process of Claim 4 wherein the material comprising SiOsis phosphorous silicate glass.
6. The process of Claim 3 wherein the material comprising SiO2 is substantially Si02.
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7. The process of Claim 1 wherein the fluorine compound is selected from the group consisting of: NF3, F2, F2 created in situ, and a mixture of F2 in an inert gas.
8. The process of Claim 7 wherein the fluorine compound is NF3.
9. The process of Claim 1 wherein the polymerizable fluorocarbon is a compound having the formula ChFj wherein h is a number ranging from 4 to 6 and i is a number ranging from h to 2h+2.
10. The process of Claim 9 wherein the polymerizable fluorocarbon is selected from the group consisting of: octafluorocyclobutane, octafluorocyclopentene, hexafluorobenzene, hexafluoro-1,3-butadiene and mixtures thereof.
11. The process of Claim 10 wherein the polymerizable fluorocarbon is hexafluoro-1,3-butadiene.
12. The process of Claim 1 wherein the polymerizable fluorocarbon is a compound having the formula CxFyHz wherein x is a number ranging from 1 to 4 and z is a number ranging from 1 to (2x+1) and y is {(2x+2)-z).
13. The process of Claim 12 wherein the polymerizable fluorocarbon is selected from the group consisting of: CHF3 (trifluoromethane), C2F5H (1,1,1,2,2-pentafluoroethane), and C3F7H (1,1,1,2,3,3,3 heptafluoropropane) and mixtures thereof.
14. A process for selectively etching a material comprising Si02 preferentially to silicon, the method comprising the steps of:
placing a silicon substrate having a layer of a material comprising SiO2 within a reactor chamber equipped with electrodes;
creating a vacuum within the chamber;
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introducing into the reactor chamber a reactive gas mixture comprising a fluorine compound, a polymerizable fluorocarbon, and an inert gas, wherein the reactive gas mixture is substantially free of added oxygen;
supplying a high frequency electrical energy to the electrodes to form a plasma activated reactive etching gas mixture v11ithin the chamber; and
selectively etching the material comprising SiOg preferentially to the silicon substrate.

Documents:

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Patent Number

270989

Indian Patent Application Number

2045/CHE/2009

PG Journal Number

05/2016

Publication Date

29-Jan-2016

Grant Date

29-Jan-2016

Date of Filing

26-Aug-2009

Name of Patentee

AIR PRODUCTS AND CHEMICALS, INC

Applicant Address

7201 HAMILTON BOULEVARD, ALLENTOWN, PA 18195-1501

Inventors:

#	Inventor's Name	Inventor's Address
1	GLENN MICHAEL MITCHELL	330 WASHINGTON AVENUE, SELLERSVILLE, PA 18960
2	STEPHEN ANDREW MOTIKA	817 NOBLE STREET, KUTZTOWN, PA 19530
3	ANDREW DAVID JOHNSON	110 SOUTH FRANKLIN STREET, DOYLESTOWN, PA 18901

PCT International Classification Number

B01F 5/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	12/504,064	2009-07-16	U.S.A.
2	60/092/916	2008-08-29	U.S.A.