# Patent

## Title
Removal of native oxide with high selectivity

## Abstract

Provided are methods and systems for removing a native silicon oxide layer on a wafer. In a non-sequential approach, a wafer is provided with a native silicon oxide layer on a polysilicon layer. An etchant including a hydrogen-based species and a fluorine-based species is introduced, exposed to a plasma, and flowed onto the wafer at a relatively low temperature. The wafer is then heated to a slightly elevated temperature to substantially remove the native oxide layer. In a sequential approach, a wafer is provided with a native silicon oxide layer. A first etchant including a hydrogen-based species and a fluorine-based species is flowed onto the wafer. Then the wafer is heated to a slightly elevated temperature, a second etchant is flowed towards the wafer, and the second etchant is exposed to a plasma to complete the removal of the native silicon oxide layer and to initiate removal of another layer such as a polysilicon layer.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of priority to U.S. Provisional Application No. 61/667,329, titled “POLYSILICON ETCH WITH HIGH SELECTIVITY,” filed Jul. 2, 2012, and is a continuation-in-part of U.S. patent application Ser. No. 13/916,387, titled “POLYSILICON ETCH WITH HIGH SELECTIVITY,” filed Jun. 12, 2013, all of which are incorporated herein by reference for all purposes.


BACKGROUND
The present disclosure relates generally to the etching of native oxides on a wafer, and more particularly to plasma-based etching of native oxides and polysilicon with a high selectivity on a wafer.
Plasma-based etching can be an important processing step in the fabrication of semiconductor devices and integrated circuits. However, the presence of a native oxide layer on the surfaces of many materials, including semiconductor substrates containing silicon and metals, can adversely affect the patterning of such materials. This can be an important part in the fabrication of semiconductor chips or memory devices. For example, a native oxide layer on polysilicon can substantially suppress and increase the non-uniformity of the etching of polysilicon. A native oxide layer may form when a silicon-containing surface is exposed to ambient conditions or oxygen.
Typically, removal of native oxides can be performed using wet processes, such as treating the native oxide with dilute hydrofluoric acid (HF). However, the use of such a wet etching process for removing native oxides may be expensive, may pose serious safety concerns, may not achieve a high selectivity over other materials, and may cause additional exposure to ambient conditions to allow native oxides to regrow prior to etching polysilicon. Wet processes may also be problematic for devices involving high-aspect-ratio features.
Typically, removal of polysilicon can be performed using wet or dry reactive-ion-etch (RIE) processes. However, a wet etching process for removing polysilicon can result in a low etch rate of polysilicon, which leads to a low throughput. Furthermore, a wet etching process for removal of polysilicon may not achieve as high of selectivity over other materials as dry etching processes.
A dry RIE process can result in greater cost due at least in part to complicated hardware for controlling ion direction and energy using an external bias. In addition, the use of a dry RIE process can damage surrounding structures due to exposure to ion and photon fluxes. The surrounding structures can be sidewalls made of, for example, exposed nitrides and/or oxides. Such surrounding structures can include silicon nitride (Si3N4), titanium nitride (TiN), and silicon oxide (SiO2) including thermal oxide.
SUMMARY
This disclosure pertains to methods of removing a native silicon oxide layer on a wafer. One method includes providing a wafer having a native silicon oxide layer on a polysilicon layer and introducing an etchant including a hydrogen-based species and a fluorine-based species, where the hydrogen-based species or the fluorine-based species includes nitrogen. The method further includes exposing the etchant to a plasma to activate the hydrogen-based species and the fluorine-based species, flowing the etchant onto the wafer to contact the native silicon oxide layer, and heating the wafer to a temperature greater than about 60° C. to substantially remove the native silicon oxide layer from the polysilicon layer. In some implementations, the hydrogen-based species is hydrogen and the fluorine-based species is nitrogen trifluoride. In some implementations, the wafer includes an exposed nitride and/or oxide structure. An etch rate of the exposed nitride and/or oxide structure can be less than about 3 Å per minute during removal of the native silicon oxide layer. In some implementations, exposing the etchant to a plasma includes exposing the etchant to a remote plasma. In some implementations, flowing the etchant towards the wafer can occur at a temperature less than about 40° C.
Another method of removing a native silicon oxide layer on wafer includes providing a wafer having a native silicon oxide layer on a polysilicon layer and flowing a first etchant including a first hydrogen-based species and a first fluorine-based species onto the wafer to contact the native silicon oxide layer, where the first hydrogen-based species or the first fluorine-based species includes nitrogen. The method further includes heating the wafer to a temperature greater than about 60° C. and flowing a second etchant including a second hydrogen-based species and a second fluorine-based species towards the wafer. The method further includes exposing the second etchant to a plasma and removing the native silicon oxide layer and at least partially the polysilicon layer from the wafer. In some implementations, each of the steps occurs without breaking vacuum. In some implementations, the first etchant is identical to the second etchant. The first and the second hydrogen-based species can be hydrogen, and the first and the second fluorine-based species can be nitrogen trifluoride. In some implementations, the wafer includes an exposed nitride and/or oxide structure. The etch rate of the exposed nitride and/or oxide structure is less than about 3 Å per minute during removal of the native silicon oxide layer. In some implementations, flowing the first etchant onto the wafer can occur at a temperature less than about 40° C.
This disclosure also pertains to an apparatus for removing native silicon oxide from a wafer. The apparatus includes a reaction chamber comprising the wafer, where the wafer includes a polysilicon layer, a native silicon oxide layer over the polysilicon layer, and an exposed nitride and/or oxide structure. The apparatus further includes a plasma source coupled to the reaction chamber and configured to generate a plasma outside the reaction chamber, and a controller with instructions for performing a series of operations. This includes flowing a first etchant including a first hydrogen-based species and a first fluorine-based species onto the wafer to contact the native silicon oxide layer, where the first hydrogen-based species or the first fluorine-based species includes nitrogen. The instructions also perform heating the wafer to a temperature greater than about 60° C. and flowing the second etchant including a second hydrogen-based species and a second fluorine-based species towards the wafer. The instructions also perform exposing the second etchant to the plasma, removing the native silicon oxide layer from the polysilicon layer where the etch rate of the exposed nitride and/or oxide structure is less than about 3 Å per minute, and removing at least partially the polysilicon layer where the removal of the polysilicon layer occurs at a selectivity over the exposed nitride and/or oxide structure of greater than about 1000:1. In some implementations, the apparatus includes a first station and a second station, where the first station is configured to perform the operation of flowing the first etchant onto the wafer, and the second station is configured to perform the rest of the operations. In some implementations, the controller includes instructions for heating the wafer to a temperature of less than about 200° C. In some implementations, flowing the first etchant onto the wafer can occur at a temperature less than about 40° C.
These and other embodiments are described further below with reference to the figures.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified schematic showing a top view of a multi-station processing apparatus.
FIG. 2 illustrates a downstream plasma device according to certain embodiments.
FIG. 3 illustrates a cross-section of an example of a structure with a polysilicon layer over an underlying layer and having multiple vertical structures.
FIG. 4A illustrates a graph showing the polysilicon etch rate as a function of temperature for pure H2 plasma
FIG. 4B illustrates a graph showing the polysilicon etch rate as a function of pressure for pure H2 plasma.
FIG. 4C illustrates a graph showing the polysilicon etch rate as a function of RF power for pure H2 plasma.
FIG. 5A illustrates a graph showing the polysilicon etch rate as a function of temperature for H2 and NF3 plasma.
FIG. 5B illustrates a graph showing a polysilicon etch rate as a function of pressure for H2 and NF3 plasma.
FIG. 6 illustrates a cross-section of an example of a structure with a polysilicon layer and a native silicon oxide layer over the polysilicon layer.
FIG. 7 illustrates an exemplary flow diagram illustrating a method of removing native silicon oxide on a wafer for non-sequential implementations.
FIG. 8A illustrates a graph showing an etch rate of tetraethyl orthosilicate (TEOS) as a function of temperature.
FIG. 8B illustrates a graph showing an etch rate of TEOS as a function of pressure.
FIG. 8C illustrates a graph showing an etch rate of TEOS as a function of NF3 concentration.
FIG. 9 illustrates an exemplary flow diagram illustrating a method of removing native silicon oxide on a wafer for sequential implementations.



DETAILED DESCRIPTION
Introduction
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the presented concepts. The presented concepts may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail so as to not unnecessarily obscure the described concepts. While some concepts will be described in conjunction with the specific embodiments, it will be understood that these embodiments are not intended to be limiting.
Plasma-based etching can be used in the fabrication of integrated circuits. For a variety of technology nodes, such as in the 1×-nm or the 2×-nm node, new classes of materials for structures such as memory-device stacks can offer tremendous advantages. Fabrication processes such as etching of a particular layer may need to be relatively benign to such new materials while also etching at a high efficiency. While it may be desirable to achieve etching of certain materials like polysilicon at a high efficiency for high throughput, it may also be desirable to minimize losses to surrounding exposed materials to avoid adversely affecting device performance.
A remote or downstream plasma can provide acceptable etch rates while minimizing losses of surrounding materials. In some implementations, for example, the materials can include silicon nitride (Si3N4) and/or titanium nitride (TiN). The Si3N4 can be used as spacers and/or etch stop layers, and the TiN can be used as metal-gate structures or electrodes. A remote or downstream plasma can provide conditions that can minimize the damage caused by direct plasma exposure, including ion impact damage, charging damage, and defects introduced by high fluxes of energetic photons.
Plasma Reactor and Process Chamber
The process chamber can include any suitable reaction chamber for the etching operation being performed. It may be one chamber of a multi-chambered apparatus or it may simply be a single chamber apparatus. The chamber may also include multiple stations where different wafers are processed simultaneously. FIG. 1 is a simplified schematic showing a top view of a multi-station apparatus including stations 1, 2, 3, 4, 5, and 6. Wafers enter the apparatus at station 1 via chamber 1001, are transferred to each station in sequence for a processing operation at that station and exit from station 6 via chamber 1002 after the process is complete. Some of the stations may be configured with a remote plasma source for performing the operations described herein.
Plasma reactor apparatus include apparatus configured to remove photoresist materials and other materials from device structures such as integrated circuits. Examples of such apparatus include the Gamma 2100, 2130 I2CP (Interlaced Inductively Coupled Plasma), G400, GxT, and the SIERRA, offered by Lam Research Corporation of Fremont, Calif. Other systems include the Fusion line from Axcelis Technologies Inc. of Rockville, Md., TERA21 from PSK Tech Inc. in Korea, and the Aspen from Mattson Technology Inc. in Fremont, Calif. Additionally, various plasma reaction chambers may be associated with cluster tools. For example, a strip chamber may be added to a Centura cluster tool available from Applied Materials of Santa Clara, Calif.
FIG. 2 is a schematic illustration of a downstream plasma device 200 according to certain embodiments. The device 200 has a downstream plasma source chamber 202 and an exposure chamber 204 separated by a showerhead assembly 206. The showerhead assembly 206 includes a showerhead 208. Inside the exposure chamber 204, a wafer 212 rests on a platen, stage, or work piece support 214. In some embodiments the surface of the showerhead 208 facing the platen 214 is about 0.5 to 2 inches from the surface of a wafer on the platen 214. In further embodiments the surface of the showerhead 208 facing the platen 214 is about 1.2 inches from the surface of a wafer on the platen 214. In yet further embodiments the surface of the showerhead 208 facing the platen 214 is about 0.1 to 0.9 inches from the surface of a wafer on the platen. The thicknesses of wafers is generally less than about 1 mm, so the distance between the surface of the showerhead and the surface of a wafer can generally be considered the distance between the surface of the showerhead and the platen when the wafer rests directly on the platen. In cases where the wafer rests on a support structure (e.g., lift pins or a gas flow blanket) on the platen, the distance between the surface of the showerhead and the surface of a wafer can generally be considered the distance between the surface of the showerhead and the support structure.
In some embodiments the platen 214 is fitted with a heating/cooling element. In some embodiments, a radio frequency (RF) power source (not shown) can be configured to apply RF power to the platen 214 to provide a bias source. However, many of the embodiments discussed hereinafter below do not employ a bias source to the platen 214. The RF power source can be a low frequency (LF) power source in some embodiments, and the RF power source can be a high frequency (HF) power source in other embodiments. For example, the low frequency power source has a frequency of about 50 kilo-Hertz (kHz) to 1 mega-Hertz (MHz) and the high frequency power source has a frequency of about 2 to 200 mega-Hertz (MHz), in some embodiments. In other embodiments the low frequency power source has a frequency of about 400 kHz and the high frequency power source has a frequency of about 13.56 MHz. In further embodiments the RF power source includes both a LF and a HF power source.
Low pressure is attained in the exposure chamber 204 via a vacuum pump (not shown) via a conduit 218. The pressure in the exposure chamber 204 can be about 300 milli-torr (mtorr) to 3.5 torr in some embodiments, and about 5 mtorr to over 200 mtorr in further embodiments.
Gas sources (not shown) provide a flow of process gas via an inlet 220 into the plasma source chamber 202 of the device 200. The plasma source chamber 202 is surrounded in part by induction coils 222, which are in turn connected to a power source 224. Various configurations and geometries of the plasma source chamber 202 and the induction coils 222 may be used. For example, the induction coils 222 may loop around the plasma source chamber 202 in an interlaced pattern. In another example, the plasma source chamber 202 may be shaped as a dome instead of a cylinder. A controller 226 may be connected to components of the device 200 to control the operation of device 200. For example, the controller 226 may be connected to the power supply 224. The controller 226 may also be connected to other components of the device 200 to control, for example, the process gas composition, the pressure, and the temperature of the platen 214. Machine-readable media may be coupled to the controller 224 and contain instructions for controlling process conditions for the operations in the device 200.
Various types of plasma sources may be used in accordance with the invention, including RF, DC, and microwave based plasma sources. In a preferred embodiment, a downstream RF plasma source is used. Typically, the RF plasma power for a 300 mm wafer ranges between about 300 Watts to about 10 Kilowatts. In some embodiments, the RF plasma power is between about 2000 Watts and 5000 Watts, e.g., 3500 W.
In some embodiments, the plasma source chamber 202 can be a dome made of various materials, including quartz, ceramic, or aluminum nitride (AlN) material. Etching materials such as polysilicon in a quartz dome may lead to the formation of silicon oxide. However, etching polysilicon in a ceramic dome can minimize the formation silicon oxide. The dome surface can also be coated with films such as yttrium oxide or yttrium fluoride in order to reduce or otherwise prevent the degradation of dome material.
During operation, gas mixtures are introduced into the plasma source chamber 202 and the induction coils 222 are energized with power source 224 to generate a plasma (i.e., the induction coils generate an inductively coupled plasma in the plasma source chamber 202). The gas mixtures introduced into the plasma source chamber 202 contain chemically active species that will be ionized and radicalized to form the plasma. The showerhead 208 includes a plurality of holes or passageways through which plasma species from the plasma may pass and enter the exposure chamber 204. The showerhead 208, with a voltage applied to it, in some embodiments, terminates the flow of ions from the plasma and allows the flow of radicals and other neutral species from the plasma into the exposure chamber 204. The showerhead 208 may be grounded or have an applied voltage to attract some charge species while not affecting the flow of neutral species to the wafer, e.g., 0-1000 Watt bias. Many of the electrically charged species in the plasma recombine at the showerhead. The showerhead may be a metal plate having holes to direct the plasma and inert gas mixture into the reaction chamber. The number and arrangement of the showerhead holes may be set to optimize the etching operation. The plasma species that enter the exposure chamber 204 may remove material from the wafer 212.
Controller
The controller 226 may contain instructions for controlling process conditions for the operation of the device 200. The controller 226 will typically include one or more memory devices and one or more processors. The processor may include a CPU or computer, analog and/or digital input/output connections, stepper motor controller boards, etc. Instructions for implementing appropriate control operations are executed on the processor. These instructions may be stored on the memory devices associated with the controller 226 or they may be provided over a network. Machine-readable media containing instructions for controlling process operations in accordance with the present invention may be communicatively coupled to the controller 226.
In certain embodiments, the controller 226 controls all or most activities of the downstream plasma device 200 described herein. The controller 226 may execute system control software and include sets of instructions for controlling the timing, gas composition, gas density, flow rates, chamber pressure, chamber temperature, RF power levels, wafer position, and/or other parameters. Other computer programs, scripts, or routines stored on memory devices associated with the controller 226 may be employed in some embodiments. To provide appropriate process conditions at the environment adjacent to the wafer 212, parameters such as the RF power levels, gas density, and timing of the plasma can be adjusted by controller 226.
The process conditions and the process flow itself can be controlled by the controller 226 which comprises program instructions for a monitoring, maintaining and/or adjusting certain process variables. For example, instructions specifying flow rates may be included. Instructions may also include parameters of pre-clean, passivation, protective layer formation, non-protective layer, pinning operations, other post-deposition treatments, etching, partial filling, and the like. The controller 226 may comprise different or identical instructions for different apparatus stations, thus allowing the apparatus stations to operate either independently or synchronously.
In some embodiments, the controller 226 may include instructions for performing a series of operations. This includes flowing an etchant including a hydrogen-based species and a fluorine-based species towards a wafer, where the wafer includes a polysilicon layer and an exposed nitride and/or oxide structure. The instructions further include exposing the etchant to a plasma source to activate the hydrogen-based species and the fluorine-based species, and removing the polysilicon layer at a selectivity over the exposed nitride and/or oxide structure of greater than about 500:1.
In some embodiments, there may be a user interface associated with controller 226. The user interface may include a display screen, graphical software displays of the apparatus and/or process conditions, and user input devices such as pointing devices, keyboards, touch screens, microphones, etc.
The computer program code for controlling the above operations can be written in any conventional computer readable programming language: for example, assembly language, C, C++, Pascal, Fortran, or others. Compiled object code or script is executed by the processor to perform the tasks identified in the program.
Signals for monitoring the process may be provided by analog and/or digital input connections of the system controller. The signals for controlling the process are output on the analog and digital output connections of the processing system.
Device Structure
FIG. 3 illustrates a cross-section of an example of a device structure with a polysilicon layer. As illustrated in the device structure 300 in FIG. 3, the polysilicon layer 310 can be over an underlying layer 320, which can include Si3N4. The polysilicon layer 310 can also be separated by multiple vertical structures 330 that can each include TiN and/or Si3N4, for example. In some embodiments, the polysilicon layer 310 can include polysilicon that is annealed. Annealed polysilicon can be more crystalline and relaxed than non-annealed polysilicon, and can etch at a different rate than non-annealed polysilicon. It will be understood by a person of ordinary skill in the art that the polysilicon layer 310 can be surrounded and/or separated by any number of different materials.
In the example in FIG. 3, the device structure 300 may be a memory device. The underlying Si3N4 layer 320 can serve as an etch stop and the TiN and Si3N4 vertical structures 330 can be electrodes. In some embodiments, the polysilicon layer 310 is etched and then a space between the TiN and Si3N4 vertical structures 330 is filled with a dielectric material to create a capacitor in between the TiN and Si3N4 vertical structures 330.
In the example in FIG. 3, the thickness of the polysilicon layer 310 can be between about 1 μm and about 2 μm, such as between about 1.10 μm and about 1.35 μm. In addition, the TiN and Si3N4 vertical structures 330 can also be between about 1 μm and about 2 μm, such as between about 1.10 μm and about 1.35 μm. It will be understood by a person of ordinary skill in the art that the memory device structure 300 can have varying thicknesses and orientations.
In the example in FIG. 3, the dimensions of the polysilicon and other features can depend on the application and technology node. In some embodiments, the thickness of the polysilicon for removal can be about 1.3 μm, which can correspond to a 2×-nm node. For a technology node that is the 2×-nm, this can correspond to features (e.g., gate width) of about 22 nm or less. In some embodiments, the thickness of the polysilicon for removal can be about 1.5 μm, which can correspond to a 1×-nm node. For a technology node that is the 1×-nm node, this can correspond to features (e.g., gate width) of about 16 nm or less.
Process Conditions
The disclosed implementations described herein involve a method of removing polysilicon at a high etch rate with a high selectivity over exposed nitride and/or oxide structures. The method includes providing a wafer, where the wafer includes a polysilicon layer and an exposed nitride and/or oxide structure. The method further includes flowing an etchant including a hydrogen-based species and a fluorine-based species towards the wafer, exposing the etchant to a remote plasma to activate the hydrogen-based species and the fluorine-based species, and removing the polysilicon layer. The removal of the polysilicon layer is performed at a selectivity over the exposed nitride and/or oxide structure of greater than about 500:1. In some implementations, the selectivity over the exposed nitride and/or oxide structure is greater than about 1000:1, or greater than about 75000:1.
The wafer can include any semiconductor wafer, partially integrated circuit, printed circuit board, or other appropriate work piece. Process conditions may vary depending on the wafer size. Typically, many fabrication facilities are configured for 200 mm wafers, 300 mm, or 450 mm wafers. The disclosed implementations described herein are configured to operate on any suitable wafer size, such as 300 mm and 450 mm wafer technologies.
In some implementations, the removal of polysilicon can be carried out by a remote or downstream plasma reactor, such as the reactor described with respect to FIG. 2. The gases introduced into the reactor described in FIG. 2 can vary with the application. In some embodiments, an etch reaction can be carried out using a hydrogen-based etchant. The hydrogen-based etchant can include, for example, hydrogen (H2). Another example can include ammonia (NH3). In some embodiments, an etch reaction can be carried out using a combination of H2 and a fluorinated species, such as nitrogen trifluoride (NF3) or carbon tetrafluoride (CF4). Gases such as H2 and NF3 are non-toxic and generally do not have adverse effects on the environment.
Using a hydrogen-based species as an etchant effectively etches polysilicon while acting as a reducing agent that minimizes the oxidation and loss of other exposed materials, such as TiN, Si3N4, and SiO2. An oxidizing agent such as oxygen may increase the etch rate of polysilicon but may also oxidize and increase the losses to other exposed materials. Adding a fluorine-based species as an etchant with a hydrogen-based species can increase the etch rate of polysilicon, but can also increase the losses to other exposed materials if the concentration of the fluorinated-based species exceeds a certain limit.
As discussed earlier herein, the hydrogen-based species can include hydrogen gas (H2) or ammonia (NH3), and the fluorine-based species can include nitrogen trifluoride (NF3) or carbon tetrafluoride (CF4). Other examples of fluorine-based species can include sulfur hexafluoride (SF6), hexafluoroethane (C2F6), trifluoromethane (CHF3), difluoromethane (CH2F2), octofluoropropane (C3F8), octofluorocyclobutane (C4F8), octofluoro[1-]butane (C4F8), octofluoro[2-]butane (C4F8), octofluoroisobutylene (C4F8), fluorine (F2), and the like. Exposure to a remote plasma can activate the hydrogen-based species and the fluorine-based species to form plasma-activated species, radicals, and charged species.
In some embodiments, removing the layer of polysilicon can occur at an etch rate of greater than about 1000 Å per minute. In some embodiments, the exposed nitride can include Si3N4. In some embodiments, the exposed nitride can include TiN. In some embodiments, the exposed oxide can include SiO2, such as thermal oxide. The etch rates of the exposed nitride and/or oxide structure can be less than about 5 Å per minute, such as less than about 1 Å per minute. In some embodiments, the etch rates of the exposed nitride and/or oxide structure can be less than about 0.01 Å per minute. These etch rates may be achieved while the polysilicon etch rate is greater than about 1000 Å per minute.
The process conditions of the plasma etch can affect the etch rates of polysilicon and exposed nitrides and/or oxides. Various process parameters such as surface temperature, pressure, source power, flow rates of the gases, wafer size, and relative concentrations of the etchant gases can affect the process conditions and, hence, the etch rates of polysilicon and exposed nitrides and/or oxides. Such process parameters may be optimized within a “process window” to maximize the etch rate of polysilicon while limiting the etch rates of the exposed nitrides and/or oxides.
The introduction of the hydrogen-based species provides active species that will be ionized and radicalized in a remote plasma source to form a plasma. Without being limited by any theory, the etching of polysilicon can occur by successive addition of adsorbed hydrogen atoms to silicon atoms to form Si—Hx complexes, where the number of chemisorbed hydrogen atoms grows from x=1, 2, and 3, i.e., SiH, SiH2, and SiH3. Such a reaction mechanism occurs at least in the presence of pure H2 plasma. The addition of a hydrogen atom to SiH3 promotes formation of volatile silane, SiH4, which facilitates etching of polysilicon. The following equation can describe the overall Si-etch reaction: Si(s)+4H→SiH4(g).
Other reactions may also occur to facilitate the removal of polysilicon. Activated fluorine atoms may react with silicon atoms and form volatile tetrafluorosilane, SiF4 in the following reaction: Si(s)+4F(g)→SiF4(g). Also, the mixture of the hydrogen-based species and the fluorine-based species could form gas phase reactants, such as HF, NH4F.HF, and NH4F. These gas phase reactants and other plasma-activated species can potentially react with silicon atoms to form a solid byproduct (NH4)2SiF6. The solid byproduct is sublimed at slightly elevated temperatures (e.g., greater than about 75° C.) to form gaseous byproducts and thereby remove polysilicon. Thus, plasma-activated species as well as gas phase reactants from the etchant can collectively facilitate the etching of polysilicon.
For pure H2 plasma, the polysilicon etch rate can be dependent on temperature. FIG. 4A illustrates a graph showing the polysilicon etch rate as a function of temperature for pure H2 plasma. At temperatures of less than about 40° C., the etch rate of polysilicon increases with increasing temperature. At temperatures greater than about 40° C., the etch rate of polysilicon decreases with increasing temperature. As such, the maximum etch rate of polysilicon can exhibit a temperature maximum (Tmax), such as Tmax ̃40° C. Below Tmax, increasing the temperature increases the thermal activation for the volatile silane formation, thereby increasing the polysilicon etch rate. Above the Tmax, there can be recombination of H atoms on the polysilicon surface that consumes the chemisorbed H atoms needed for the formation of volatile SiH4.
For pure H2 plasma, the polysilicon etch rate can be dependent on pressure. FIG. 4B illustrates a graph showing the polysilicon etch rate as a function of pressure for pure H2 plasma. As pressure increases, the etch rate of polysilicon decreases steadily. However, it is possible that when the pressure goes below 0.6 Torr, a maximum in etch rate as a function of pressure can be observed. At pressures greater than about 0.6 Torr and less than about 3.5 Torr, the etch rate of polysilicon decreases. When pressure increases, the physisorption of hydrogen atoms on the polysilicon surface is expected to increase. The rate of surface recombination of physisorbed hydrogen atoms and chemisorbed hydrogen atoms can increase to form H2 gas, resulting in the depletion of SiHx precursors. In addition, the rates of surface (wall) and volume recombination of hydrogen atoms also increase with pressure, which could give rise to lower concentrations of hydrogen radicals that are available for reaction with the polysilicon surface.
The addition of a fluorinated gas species, such as NF3, with H2 can change the etch behavior of polysilicon. Fluorine atoms can be generated by the dissociation of NF3. The remote plasma can cause the dissociation of the fluorinated gas species to produce fluorine radicals. The fluorine radicals react with silicon to form gaseous by-products, including a stable form of SiF4. Thus, the polysilicon surface is not only etched by hydrogen atoms, but can also be etched by fluorine atoms. As such, the addition of a fluorinated gas species can generally increase the etch rate of polysilicon. There may also be reactions between the activated species generated from the dissociation of H2 and those from the dissociation of NF3 to form more complex plasma species. The latter species can then react with and etch the polysilicon surface. NF3, H2, and the plasma-activated species could also react in gas phase to generate a mixture of gas-phase reactants such as HF, NH4F.HF, and NH4F, which in turn react and ultimately remove the polysilicon surface.
The relative concentration of the fluorinated gas species can reach a certain limit to maintain desirable selectivity over the exposed nitride and/or oxide. In some embodiments, the concentration of the fluorinated gas species can be less than about 50% per volume. In some embodiments, the concentration of the fluorinated gas species can be less than about 20% per volume. In some embodiments, the concentration of fluorinated gas species is between about 0.7% and about 10% per volume.
Furthermore, the addition of an inert gas carrier may be used in some embodiments. Such carrier can increase the etch rate of polysilicon. It is believed that an inert gas carrier may reduce the likelihood of recombination of radicals in the gas phase. Examples of inert gas carriers can include noble gases, such as helium (He), neon (Ne), and argon (Ar). In some embodiments, for example, dilutions of about 5:1 of inert gas carrier to H2 can be used.
For plasma with a gas mixture of H2 and NF3, the polysilicon etch rate exhibits a strong dependence on temperature. FIG. 5A illustrates a graph showing a polysilicon etch rate as a function of temperature. For a gas mixture with a concentration of about 5% NF3, the etch rate of polysilicon increases to a maximum at Tmax between about 60° C. and about 80° C. When the temperature is greater than about Tmax, the etch rate of polysilicon decreases. For a gas mixture with a concentration of about 0.7% NF3, when the temperature is greater than about 30° C., the etch rate of polysilicon decreases. In this case, Tmax can occur at a temperature that is less than about 30° C. Additionally, for temperatures greater than about 40° C., the etch rate of polysilicon is higher for 5% NF3 than for 0.7% NF3. Thus, an increase in the concentration of a fluorinated gas species can lead to a higher etch rate of polysilicon.
For plasma with a gas mixture of H2 and NF3, the polysilicon etch rate also exhibits a strong dependence on pressure. FIG. 5B illustrates a graph showing a polysilicon etch rate as a function of pressure. When the temperature is fixed at about 100° C. for a gas mixture with about 5% NF3, the etch rate of polysilicon increases with pressure between about 0.6 Torr and about 1.5 Torr. However, the etch rate of polysilicon decreases with pressure at pressures beyond about 1.5 Torr. When the temperature is fixed at about 60° C. for a gas mixture with about 5% NF3, the etch rate of polysilicon also increases with pressure between about 0.6 Torr and about 0.8 Torr. At a high pressure of about 2 Torr, the etch rate is found to drop substantially. Thus, the maximum etch rate of polysilicon can be realized at a pressure of Pmax between about 1.0 Torr and about 1.5 Torr at about 5% NF3 for both 60° C. and 100° C.
Process conditions such as temperature, pressure, flow rates, and relative concentrations of gas mixtures may be adjusted within a certain process window to optimize the etch rate of polysilicon. In some embodiments, the temperature can be between about 10° C. and about 160° C., such as between about 20° C. and about 110° C. or between about 35° C. and about 100° C. In some embodiments, the pressure can be between about 0.5 Torr and about 3.5 Torr, such as between about 1.5 Torr and about 3.5 Torr. In some embodiments, the relative concentration of the fluorinated gas species can be less than about 50% by volume, such as less than about 20% or less than about 10% by volume.
As the data suggests, to achieve etch rates of polysilicon in excess of about 1000 Å per minute, process conditions can be established such that the temperature is between about 60° C. and about 100° C., the pressure is between about 0.6 Torr and about 3.0 Torr, and the concentration of NF3 is about 2% to 10% (e.g., about 5%) by volume. When the temperature is about 100° C. and the pressure is about 1 Torr, the etch rate of polysilicon can be greater than about 1100 Å per minute.
In addition to varying the process conditions to achieve a relatively efficient etch rate of polysilicon, the process conditions can be varied to achieve a low loss of exposed materials, such as Si3N4 and TiN. Other materials that may be used with polysilicon include SiO2 and various metals.
Table I provides etch rates of Si3N4 and TiN expressed as relative losses of material as a function of temperature. The etch was performed for a duration of about 4.2 minutes. In this example, Si3N4 was formed by low-pressure chemical vapor deposition (LPCVD). The concentration of NF3 was fixed at about 5% by volume and the pressure can be fixed at 0.6 Torr. To achieve the 5% by volume concentration of NF3, the flow rate of the NF3 was set to about 106 standard cubic centimeters per minute (sccm), where the flow rate of H2 was set to about 2 standard liters per minute (slm). As the temperature increases between about 60° C. and about 250° C., the change in Si3N4 loss is extremely negligible. However, TiN loss is not as negligible at temperatures greater than about 100° C., and increases substantially with temperatures beyond 100° C. Thus, in polysilicon—TiN and/or polysilicon—Si3N4 structures, a temperature of between about 60° C. and about 100° C. may be appropriate. Under these process conditions, the loss of Si3N4 and TiN is less than about 10 Å.









TABLE I





Temperature (° C.)
Si3N4 Loss (Å)
TiN Loss (Å)

















60
~0
~0


100
~0
2.9


160
0.4
42.3


250
1.3
56.7









At process conditions where the temperature is about 100° C. and the pressure is about 1 Torr for a concentration of NF3 at about 5%, the etch rate of Si3N4 is about 1.0 Å per minute and the etch rate of TiN is about 0 Å per minute. At such process conditions, the etch selectivity of polysilicon over Si3N4 can be greater than 1000, and the etch selectivity of polysilicon over TiN can also be greater than 1000.
In addition to process parameters such as temperature, pressure, and concentrations of gas mixtures, the RF power of the plasma source can also affect etch rates of polysilicon, Si3N4, and TiN. FIG. 4C illustrates a graph showing the etch rate of polysilicon as a function of RF power for pure H2 plasma. The etch rate of polysilicon increases with increasing RF power for pure H2 plasma. For a single 300 mm wafer, the etch rate of polysilicon approximately plateaus at a power, which can be about 3000 W for a Lam Research Corporation Gamma GxT tool. Beyond this maximum power, the recombination of hydrogen atoms may be substantial enough so that the etch rate of polysilicon does not appreciably change. Additionally, the etch rate of Si3N4 also increases with increasing RF power and plateaus at a relatively high RF power.
In some embodiments, an apparatus can be provided with multiple stations, as illustrated in FIG. 1, to provide different process conditions to achieve varying etch rates of materials. For example, some stations can have process conditions tailored to provide for a relatively high etch rate of polysilicon and a relatively low selectivity over exposed nitrides and/or oxides. Some stations can have process conditions tailored for a relatively low etch rate of polysilicon and a relatively high selectivity over exposed nitrides and/or oxides. Some stations can have process conditions tailored to remove a native oxide layer on top of a bulk polysilicon layer, with subsequent stations configured for removal of the bulk polysilicon layer. In certain embodiments, an apparatus can have a plurality of stations within a low temperature range (e.g., between about 20° C. and about 100° C.) and a high RF power (e.g., greater than about 3000 W for Gamma GxT) for a high etch rate of polysilicon and a high selectivity to Si3N4 and TiN. The apparatus can also have another plurality of stations within a high temperature range (e.g., between about 100° C. and about 250° C.) and a low RF power (e.g., less than about 3000 W for Gamma GxT) for a low etch rate of polysilicon and a low selectivity to TiN.
A remote plasma device as described earlier herein can provide radicals or other activated species adjacent to a wafer surface to react with the polysilicon. The remote plasma device minimizes ion bombardment at the wafer surface, and so minimizes ion impact damage, charging damage, and defects introduced by high fluxes of energetic photons that is otherwise caused by direct plasma exposure. Etching by direct plasma exposure can include, for example, dry reactive ion etching (RIE), which applies an external bias to control the direction of ions. Typically, RIE produces more ions than remote plasma etching. RIE apparatus employs a plasma source within the substrate chamber and uses complicated hardware in order to control the direction and energy of ions using an external bias. The complicated hardware of RIE is quite expensive in comparison to remote plasma device. For example, the cost of a typical RIE reactor is greater than a cost of a typical downstream plasma reactor (e.g., Gamma GxT) by about a factor of two. In addition, RIE can lead to higher loss amounts of sidewalls in a device structure as illustrated in the example in FIG. 2, as such sidewalls are subjected to ion and electron bombardment as the polysilicon is being etched.
For removal of polysilicon, a bias voltage need not be applied in the remote or downstream plasma reactor. However, a bias voltage may need to be applied for the removal of residue that is left behind post polysilicon etch. To effectively remove residue left behind after the bulk polysilicon has been etched, a bias voltage may be used with the remote or downstream plasma reactor.
Native Oxide Removal—Introduction
Removal of polysilicon or any other silicon-containing structure can be hindered by the presence of native oxides. A native silicon oxide layer can form on a polysilicon layer or other silicon-containing layer when exposed to ambient conditions or oxygen. FIG. 6 illustrates a cross-section of an example of a structure with a native silicon oxide layer over a polysilicon layer.
In FIG. 6, a device structure 600 can be similar to the device structure 300 provided earlier in FIG. 3. A polysilicon layer 610 can be separated by vertical structures 630 that can each include TiN and/or Si3N4. The polysilicon layer 610 can also be disposed over an underlying layer 620, which can include Si3N4. In some implementations the device structure 600 can be a memory device, where the underlying layer 620 is an etch stop and the vertical structures 630 are electrodes. A native silicon oxide layer 640 can be formed over polysilicon layer 610. In some implementations, an oxygen-containing layer (e.g., silicon oxynitride) 640 can form on the Si3N4 in the vertical structures 630.
When oxygen reacts with silicon at the surface of silicon-containing structure, the native silicon oxide layer 640 can form. The native silicon oxide layer 640 can have a thickness between about 5 Å and about 50 Å, or between about 10 Å and about 30 Å. Since the native silicon oxide layer 640 is not intentionally fabricated or synthesized, but formed upon exposure to any environment that contains oxygen, the structure of the native silicon oxide layer 640 may be non-uniform and highly amorphous.
The presence of the native silicon oxide layer 640 can hinder chemical reactions when trying to perform chemical reactions on underlying materials. Specifically, the native silicon oxide layer 640 can hinder the etching of the polysilicon layer 610, suppressing and increasing the non-uniformity of polysilicon removal. This can adversely impact throughput and device performance.
It is desirable to remove the native silicon oxide layer 640 prior to removing the polysilicon layer 610 while incurring minimal loss of surrounding materials, such as the vertical structures 630 containing TiN and/or Si3N4. In some implementations, it is desirable to remove both the native silicon oxide layer 640 and the polysilicon layer 610 while incurring minimal loss of surrounding materials.
Typically, removal of a native silicon oxide layer on a wafer is achieved by a wet etching processing, such as dipping the wafer into a bath containing dilute HF and then transferring to another reaction chamber for further processing. This wet etching process can have several drawbacks, such as allowing native oxides to regrow during queue time while transferring the wafer, a relatively high cost of ownership, and utilizing solvents that are toxic, dangerous, and non-environmentally friendly. In addition, wet processing may compromise the integrity of high-aspect-ratio structures found in devices. However, the disclosed implementations described herein may alleviate at least some of the drawbacks by applying a method of removing a native silicon oxide layer on a wafer with high selectivity and using a dry plasma etching process. In some instances, the dry plasma etching process for removing the native silicon oxide layer can be incorporated in the same apparatus as the polysilicon etching process.
Removal of Native Oxide—Non-Sequential Approach
In some implementations, the removal of a native silicon oxide layer occurs in a separate apparatus from subsequent processing steps (e.g., removal of polysilicon). This approach can be used for any number of reasons, such as to replace the HF wet etching process to reduce costs, and/or to remove native silicon oxide where the subsequent processing step occurs in an apparatus that is incompatible with the removal of the native silicon oxide.
FIG. 7 illustrates an exemplary flow diagram illustrating a method of removing native silicon oxide from a wafer for non-sequential implementations. The process 700 begins at block 705, where a wafer is provided having a native silicon oxide layer on a polysilicon layer. The wafer can include any semiconductor wafer, partially integrated circuit, printed circuit board, or other appropriate work piece. In some implementations, the wafer can be part of a memory device. In some implementations, the wafer can include an exposed nitride and/or oxide structure. The exposed nitride structure can include silicon nitride and/or titanium nitride. The exposed oxide structure can include silicon dioxide, such as thermal silicon oxide. The wafer can be provided into a reaction chamber at a relatively cool temperature below about 60° C., such as between about 20° C. and about 40° C.
The process 700 continues at block 710, where an etchant including a hydrogen-based species and a fluorine-based species is introduced, and where the hydrogen-based species or the fluorine-based species includes nitrogen. In some implementations, the hydrogen-based species includes hydrogen and the fluorine-based species includes nitrogen trifluoride. The etchant may be introduced without contacting the wafer. For example, the etchant may flow into a plasma chamber or plasma source, where the plasma source is a remote plasma source. The wafer during block 710 may be maintained at the relatively cool temperature of below about 60° C., such as between about 20° C. and about 40° C.
The process 700 continues at block 715 where the etchant is exposed to a plasma to activate the hydrogen-based species and the fluorine-based species. In some implementations, exposing the etchant to a plasma includes exposing the etchant to a remote plasma. The remote plasma can be generated from a remote downstream plasma reactor. Exposure to the plasma can form reactive species, including plasma-activated species and radicals from the hydrogen-based species and the fluorine-based species. The wafer during block 715 may be maintained at the relatively cool temperature of below about 60° C., such as between about 20° C. and about 40° C.
The process 700 continues at block 720 where the etchant is flowed onto the wafer to contact the native silicon oxide layer. Reactive species in the etchant may flow from the remote plasma source to the wafer to react with the native silicon oxide layer. Dissociation of the NF3 and H2 in the plasma may generate activated species and radicals that include F*, N*, NFx*, and H*. Without being limited by any theory, the above species together with the molecular NF3 and H2 can all react in the plasma to generate reactive species such as NH4F, and NH4F.HF, which can in turn adsorb onto and react with silicon dioxide to form ammonium hexafluorosilicate ((NH4)2SiF6). Ammonium hexafluorosilicate is a solid product that decomposes into a volatile gas by heating. Prior to heating, the wafer during block 720 may be maintained at the relatively cool temperature of below about 60° C., such as between about 20° C. and about 40° C.
The process 700 continues at block 725 where the wafer is heated to a temperature greater than about 60° C. to substantially remove the native silicon oxide layer from the polysilicon layer. The wafer is heated to cause the ammonium hexafluorosilicate byproduct to be sublimed. The ammonium hexafluorosilicate may be removed by the following sublimation reaction: ((NH4)2SiF6)(s)→SiF4(g)+2NH3(g)+2HF(g). Each of the reaction products from heating the ammonium hexafluorosilicate are gases. In some implementations, the wafer is heated to a temperature greater than about 75° C., or greater than about 100° C. Moreover, the wafer is heated to a temperature of less than about 200° C., or less than about 300° C., so as to not initiate chemical reactions to remove or otherwise adversely impact surrounding materials. While the wafer is being heated, the plasma source may be turned off. In other words, the etchant is not exposed to a plasma when the wafer is being heated. Gases may continue to flow to stabilize chamber pressure.
The etch rate of the exposed nitride and/or oxide structure may be less than 3 Å per minute, or less than about 1 Å per minute, during the removal of the native silicon oxide layer. The removal of the native silicon oxide layer may also be selective over exposed metals, such as tungsten. The high selectivity over exposed materials may be due in part because the wafer is maintained at a relatively low temperature during plasma exposure and the concentration of the fluorine-based species is kept low. In addition, the plasma is turned off when the wafer is being heated at block 725.
In some implementations, the process 700 may further include transferring the wafer to another reaction chamber for subsequent processing. For example, the wafer may be transferred to another apparatus for removing polysilicon. Other processing steps can include cleaning, deposition, etching, doping, annealing, and patterning, among many others. In some instances, the transferring process can break vacuum and cause the wafer to be exposed to ambient conditions for a period of time. This can cause some native silicon oxides to regrow.
Removal of Native Oxide—Process Window
The process conditions for removing native silicon oxide can affect the etch rates of native silicon oxide and surrounding exposed materials. Various process parameters such as wafer temperature, reaction chamber pressure, source power, flow rates of the etchant gases, wafer size, and relative concentrations of the etchant gases can be adjusted with a process window to optimize the etch rate of the native silicon oxide while limiting the etch rates of exposed nitride, oxide, and/or metal structures.
Due to challenges in accurately measuring the amount of native silicon oxide removal, especially since the thickness of native silicon oxides are typically less than about 20 Å, experimental data was gathered using tetraethyl orthosilicate (TEOS) with the understanding that TEOS mimics the chemical and physical properties of native silicon oxides. In fact, TEOS may be more difficult to remove than the native oxide layer, as the former is associated with a more ordered film structure with fewer defects. Hence, the TEOS loss should serve as an overestimate for the amount of the native oxide removed. The error bars in FIGS. 8A-8C refer to the standard deviation of loss measurements across multiple wafers.
FIG. 8A illustrates a graph showing an etch rate of TEOS as a function of temperature. The TEOS was exposed to an H2/NF3 plasma with an NF3 concentration of 4.1% by volume, RF power of 3500 W, and a chamber pressure of 3 Torr. In FIG. 8A, the temperature refers to the surface temperature of the wafer during plasma exposure and before heating the wafer. After plasma exposure, the wafer was heated to 100° C. in the absence of plasma (i.e., RF power=0 W) to complete the reaction for removal of TEOS. The TEOS loss refers to the amount of TEOS removed after heating the wafer. For surface temperatures between 20° C. and 40° C., the amount of TEOS removed increased, while for surface temperatures greater than 60° C., the amount of TEOS removed decreased substantially. A maximum amount of TEOS removal can occur at a surface temperature between 40° C. and 60° C. Without being limited by any theory, the relatively low temperature window for optimizing removal of TEOS may be attributed to the low temperature that is desired for adsorption of reactive species on the oxide surface. Higher surface temperatures may inhibit the adsorption of the reactants on the silicon oxide surface and, hence, suppress the reaction rate.
FIG. 8B illustrates a graph showing an etch rate of TEOS as a function of pressure. The TEOS was exposed to an H2/NF3 plasma with an NF3 concentration of 4.1% by volume, RF power of 3500 W, and a surface temperature of 20° C. After plasma exposure, the wafer was heated to 100° C. in the absence of plasma to complete the reaction for the removal of TEOS. In FIG. 8B, the pressure refers to the chamber pressure, which is held constant throughout the plasma exposure and heating of the wafer. As pressure increased, the amount of TEOS loss continuously decreased. Thus, a pressure of less than about 3 Torr, or less than about 2 Torr, may be desired to optimize TEOS removal. Without being limited by any theory, the increased chamber pressure may increase wall and volume recombination of plasma-activated species and, hence, reduce the concentration of reactants that can react with the oxide surface.
FIG. 8C illustrates a graph showing an etch rate of TEOS as a function of NF3 concentration. The TEOS was exposed to an H2/NF3 plasma with RF power of 3500 W, a surface temperature of 20° C., and a chamber pressure of 3 Torr. After plasma exposure, the wafer was heated to 100° C. in the absence of plasma to complete the reaction for the removal of TEOS. In FIG. 8C, the NF3 concentration refers to the concentration by volume of the NF3 gas species in the downstream H2/NF3 plasma. As the NF3 concentration increased, the amount of TEOS loss continuously increased. Thus, an NF3 concentration of greater than about 3.0% by volume may be desired to optimize TEOS removal. Without being limited by any theory, the increased NF3 concentration may provide more fluorine or fluorine-based radicals, which in turn increases the concentrations of the reactive species that adsorb onto and react with the oxide surface.
As discussed earlier herein, the presence of a native silicon oxide layer can hinder and suppress the removal of polysilicon or silicon-containing materials. Table II provides the etch rates of polysilicon under various conditions. In each of the conditions, an etch was performed on a wafer coated with polysilicon and with native silicon oxide on the polysilicon. The native oxide layer was removed first, followed by the removal of the underlying polysilicon. The native oxide etch included one of four conditions: (1) HF dip, (2) H2/NF3 plasma followed by heating in the absence of plasma at 100° C., (3) H2/NF3 without plasma (i.e., only H2 and NF3 gases were flowed and RF power was set to 0 W) followed by heating in the absence of plasma at 100° C., and (4) no etch (i.e., as is). After performing one of the four conditions, the wafer was subjected to ambient conditions for less than about 4 hours to minimize regrowth of the native silicon oxide. Then the wafer was subjected to a polysilicon removal process using a plasma etching process described earlier herein. The process conditions employed to remove the polysilicon layer were as follows: RF power of 3500 W, 4.1% NF3 in H2, chamber pressure of 3 Torr, and a wafer temperature of 100° C.










TABLE II







Native Oxide Etch Condition
Polysilicon Etch Rate (Å/min)



















F dip
562.5



H2/NF3 plasma
647.7



H2/NF3 (RF = 0 W)
281.8



No etch (control)
76.3










As Table II demonstrates, subjecting the wafer to a native oxide etch condition prior to etching polysilicon substantially increases the etch rate of polysilicon. Without treating the wafer for removal of native oxides, the etch rate of polysilicon is less than 80 Å per minute. However, treating the wafer with an HF wet etch or H2/NF3 plasma prior to etching polysilicon can achieve an etch rate of polysilicon of greater than 500 Å per minute. In fact, applying an H2/NF3 plasma to the wafer prior to etching polysilicon can achieve an etch rate of polysilicon of greater than 600 Å per minute, which is comparable to applying an HF wet etch prior to etching polysilicon. To remove the native oxide, the H2/NF3 plasma uses a concentration of NF3 of 4.1% by volume, RF power of 3500 W, a chamber pressure of 3 Torr, and a surface temperature of 40° C. The wafer was then heated to 100° C. following plasma exposure. When the H2/NF3 gas mixture is not exposed to a plasma (i.e., RF power=0 W) during the native-oxide removal step, the removal of the native oxide is presumably incomplete, which ultimately results in a depressed polysilicon etch rate of less than 300 Å per minute. This finding suggests that the wafer needs to be exposed to a H2/NF3 plasma in order to ensure a substantial or complete removal of the native oxide layer.
In addition, applying the H2/NF3 plasma to the wafer prior to etching polysilicon not only provides a high etch rate of polysilicon, but is also highly selective over surrounding materials. The etch rates of surrounding exposed nitride and/or oxide structures can be less than about 3 Å per minute. In this instance, the H2/NF3 downstream plasma etch process demonstrated very negligible etch rates for surrounding Si3N4 and TiN structures. This observation is mainly attributed to the low temperatures (<60° C.) at which the wafers are exposed to the H2/NF3 plasma during the native-oxide removal step and the relatively low concentration of the fluorine-based species. Si3N4 had an etch rate of about 1 Å per minute and TiN had an etch rate of about 0 Å per minute. This means that if a process time of about 2 minutes is applied for removal of native silicon oxide, the loss of Si3N4 is only about 2 Å and the loss of TiN is less than 1 Å.
Removal of Native Oxide—Sequential Approach
In some implementations, the removal of a native silicon oxide layer occurs in the same apparatus as for the removal of polysilicon. This integrated approach may utilize a dry plasma etching process for both the removal of native silicon oxide and polysilicon without breaking vacuum. As a result, there is no queue time between the native silicon oxide removal and the polysilicon removal to allow the regrowth of native silicon oxide. Additionally, the removal processes are combined into one tool to further reduce costs of ownership.
FIG. 9 illustrates an exemplary flow diagram illustrating a method of removing native silicon oxide from a wafer for sequential implementations. The process 900 begins at block 905, where a wafer is provided having a native silicon oxide layer, as generally described at block 705 of the process 700. In some implementations, the wafer includes a polysilicon layer, where the native silicon oxide layer is over the polysilicon layer. In some implementations, the wafer includes an exposed nitride and/or oxide structure. The exposed nitride structure can include silicon nitride and/or titanium nitride. The exposed oxide structure can include thermal silicon oxide.
The process 900 continues at block 910, where a first etchant including a first hydrogen-based species and a first fluorine-based species is flowed onto the wafer to contact the native silicon oxide layer, where the first hydrogen-based species or the first fluorine-based species includes nitrogen. The first hydrogen-based species and the first fluorine-based species as well as any reactive products formed from the reactions between the above species may adsorb onto the native silicon oxide surface and initiate its removal reaction. In some implementations, the first hydrogen-based species is hydrogen and the first fluorine-based species is nitrogen trifluoride. While flowing the etchant onto the wafer, the wafer may be maintained at the relatively cool temperature of below about 60° C., or between about 20° and about 40° C. In contrast to the non-sequential approach of the process 700 in FIG. 7, the etchant is not exposed to a plasma when contacting the wafer. Without applying a plasma, any material loss incurred from exposed films during these steps is expected to be negligible.
In some implementations, the process 900 further includes transferring the wafer to a station for removal of polysilicon after flowing the first etchant onto the wafer but prior to heating the wafer.
The process 900 continues at block 915, where the wafer is heated to a temperature greater than about 60° C. In some implementations, the wafer is heated to a temperature of less than about 200° C., or less than about 300° C. By heating the wafer to only a slightly elevated temperature, additional thermal energy is not imparted to the wafer to initiate undesirable chemical reactions for the removal of materials other than native silicon oxide and polysilicon, which may be detrimental to device performance.
As the wafer is heated, the process 900 continues at block 920, where a second etchant including a second hydrogen-based species and a second fluorine-based species is flowed towards the wafer. In some implementations, the first etchant may be identical with the second etchant. The concentration of the first and the second fluorine-based species may be greater than about 3% by volume. For a volume of 2 L of hydrogen, a flow rate of NF3 can be between about 50 sccm and about 500 sccm. In some implementations, the concentration of the first and the second fluorine-based species may be the same.
The process 900 continues at block 925, where the second etchant is exposed to a plasma. In some implementations, exposing the second etchant to a plasma includes exposing the second etchant to a remote plasma. The remote plasma can be generated from a remote downstream plasma reactor, as discussed earlier herein. Exposure to the plasma can generate substantially higher concentrations of reactive species that can adsorb onto and react quickly with the native oxide layer as the wafer is being heated up slowly to a temperature that is greater than 60° C. Additionally, the plasma can help activate the surface reactions that occur between the native oxide layer and the reactive species, such that the rates of the surface reactions are accelerated. In contrast to the non-sequential approach of the process 700 in FIG. 7, the exposure to the plasma occurs as the wafer is being heated to a temperature greater than about 60° C.
Without being limited by any theory, the first hydrogen-based species and the first fluorine-based species may react to form reactive species such as NH4F, and NH4F.HF. The reactive species may adsorb onto the native oxide surface, as the wafer temperature is maintained at a relatively low temperature (e.g., less than about 40° C.). The reactive species may react with the native oxide to form (NH4)2SiF6. When a plasma is ignited as the wafer is slowly being heated, the reaction rate of forming (NH4)2SiF6 may substantially increase and at the same time, the sublimation of (NH4)2SiF6 may occur.
The process 900 continues at block 930, where the native silicon oxide layer is removed from the wafer when the wafer temperature reaches greater than about 60° C. The relatively high wafer temperature allows the byproduct of the native oxide removal reaction, (NH4)2SiF6, to be sublimed; thus, completing the native oxide removal. In some implementations, the polysilicon layer is at least partially removed concurrently due to the exposure of the wafer to the plasma of the second etchant. In some implementations, an etch rate of an exposed nitride and/or oxide structure is less than about 3 Å per minute during removal of the native silicon oxide layer. In some implementations, the removal of at least partially the polysilicon layer occurs at a selectivity over the exposed nitride and/or oxide structure of greater than about 1000:1. The removal of the native silicon oxide layer and the polysilicon layer may occur in the same apparatus without breaking vacuum.
The aforementioned processes for native silicon oxide removal and polysilicon removal may be performed with reference to FIGS. 1 and 2. In some implementations, the process 900 in FIG. 9 may be performed with reference to the multi-station apparatus in FIG. 1, which includes a plurality of stations 1, 2, 3, 4, 5, and 6. A controller (not shown) may be coupled to the multi-station apparatus and configured with instructions to control the operations and process conditions of the multi-station apparatus. A plasma source, such as a remote downstream plasma reactor, may be coupled to the multi-station apparatus and configured to generate a plasma outside a reaction chamber housing a wafer. The wafer may be provided in station 1 of the multi-chamber apparatus, where the wafer includes a native silicon oxide layer. In some implementations, the wafer includes a polysilicon layer and an exposed nitride and/or oxide structure. The exposed nitride structure can include silicon nitride and/or titanium nitride, and the exposed oxide structure can include thermal silicon oxide.
In some implementations, station 1, stations 1-2, stations 1-3, or any other combination of stations may be configured to perform operations for the removal of the native silicon oxide layer. The controller coupled to the multi-station apparatus may be configured with instructions for performing each of the operations. For example, stations 1-3 may be configured to flow a first etchant including a first hydrogen-based species and a first fluorine-based species onto the wafer to contact the native silicon oxide layer, where the first hydrogen-based species or the first fluorine-based species includes nitrogen. During the operations in stations 1-3, the wafer may be maintained at a relatively low temperature of less than about 60° C., such as between about 20° C. and about 40° C. The first hydrogen-based species can be hydrogen and the first fluorine-based species can be nitrogen trifluoride. The concentration of the first fluorine-based species can be between about 3% and about 20% by volume, such as about 4% by volume. During the operations in stations 1-3, the plasma source is turned off (i.e., RF power=0 W). With a relatively low surface temperature, the first etchant and reactive species from the reactions between the hydrogen-based species and fluorine-based species can adsorb onto an oxide surface of the wafer. This enables the initiation of chemical reactions for the removal of the native silicon oxide layer.
In some implementations, station 4, stations 4-5, stations 4-6, or any other combination of stations may be configured to perform operations for the removal of the polysilicon layer or for a processing step compatible with the etch chemistry of the native oxide removal step. For example, stations 4-6 may be configured to heat the wafer to a temperature greater than about 60° C., flow a second etchant including a second hydrogen-based species and a second fluorine-based species towards the wafer, expose the second etchant to a plasma, and remove the native silicon oxide layer. In some implementations, a polysilicon layer may be at least partially removed after the second etchant is exposed to a plasma. After the native oxide has been removed, stations 4-6 can be dedicated to etching the polysilicon layer. In some implementations, another processing step (e.g., etching a different material) may be performed after the second etchant is exposed to a plasma.
During the operations at stations 4-6, the second etchant may be the same as the first etchant flowed to the wafer at stations 1-3. Therefore, the operations at stations 1-3 and 4-6 are highly compatible and minimize cross-contamination. The concentration of the second fluorine-based species may be the same as the first fluorine-based species, or may be varied to optimize removal of a polysilicon layer or another layer. During the operations at stations 4-6, the temperature of the wafer is increased to greater than about 60° C., and the plasma source is turned on (e.g., RF power=3500 W). Plasma generation at stations 4-6 allows the native oxide removal reaction to proceed quickly to completion and, thereafter, allows the underlying polysilicon to be etched.
The multi-station apparatus in FIG. 1 enables integration of native oxide removal and a subsequent processing step (e.g., polysilicon removal) without breaking vacuum. The process conditions for the native oxide removal and the subsequent processing step can be similar. In some implementations, the etchant gases can be the same to minimize cross-contamination (e.g., H2 and NF3), the pressure can be the same (e.g., 3 Torr), the process time can be the same (e.g., 90 seconds per station), and the concentration of the fluorine-based species can be the same (e.g., about 4% by volume). However, in some implementations, the etchant gases and the concentration of the fluorine-based species can be different. For example, each station in the multi-chamber apparatus may have its own gas supply to deliver different reactant gases. Nevertheless, process conditions that change from the native oxide removal to the polysilicon removal operations are the wafer temperatures (e.g., from about 40° C. to about 100° C.) and the RF powers of the plasma source (from about 0 W to about 3500 W).
Table III shows a comparison of the polysilicon etch rates for the following conditions: (1) H2/NF3 native oxide removal step at low temperatures followed by H2/NF3 plasma, (2) HF wet etch for native oxide removal followed by H2/NF3 plasma, and (3) H2/NF3 native oxide removal step at high temperatures followed by H2/NF3 plasma. The process time in each of the stations was about 90 seconds per station.









TABLE III








Temperature















Station 2
Station 3







(native
(native
Station 4
Station 5
Station 6




oxide
oxide
(polysilicon
(polysilicon 
(polysilicon



Polysilicon
removal)
removal)
removal)
removal)
removal)
Polysilicon


Wafer
(RF = 0 W)
(RF = 0 W)
(RF = 3500 W)
(RF = 3500 W)
(RF = 3500 W)
Etch Rate





As Is
40° C.
40° C.
100° C.
100° C.
100° C.
713 Å/min


HF-dipped
40° C.
40° C.
100° C.
100° C.
100° C.
806 Å/min


As Is
100° C. 
100° C. 
100° C.
100° C.
100° C.
289 Å/min









The results in Table III demonstrate that applying a native oxide removal step at a low temperature with H2 and NF3 gases yields a polysilicon etch rate that is comparable with applying a native oxide removal step using an HF wet etch process. Furthermore, applying the native oxide removal step at a low temperature yields a substantially higher polysilicon etch rate. Without being limited by any theory, it is believed that the lower temperature is necessary for the adsorption of the reactive species derived from the H2 and NF3 gases on the oxide surface to promote native oxide removal.
Photolithography
The apparatus/process described hereinabove may be used in conjunction with lithographic patterning tools or processes (for example, for the fabrication or manufacture of semiconductor devices, displays, LEDs, photovoltaic panels and the like). Typically, though not necessarily, such tools/processes will be used or conducted together in a common fabrication facility. Lithographic patterning of a film typically includes some or all of the following steps, with each step enabled with a number of possible tools: (1) application of photoresist on a work piece (i.e., substrate, using a spin-on or spray-on tool); (2) curing of photoresist using a hot plate or furnace or UV curing tool; (3) exposing the photoresist to visible, UV, or x-ray light with a tool such as a wafer stepper; (4) developing the resist so as to selectively remove resist and thereby pattern it using a tool such as a wet bench; (5) transferring the resist pattern into an underlying film or work piece by using a dry or plasma-assisted etching tool; and (6) removing the resist using a tool such as an RF or microwave plasma resist stripper.
Although the foregoing has been described in some detail for purposes of clarity and 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 are many alternative ways of implementing the processes, systems, and apparatus described. Accordingly, the described embodiments are to be considered as illustrative and not restrictive.

## Claims

1. A method of removing a native silicon oxide layer from a wafer, the method comprising:
providing a wafer having a native silicon oxide layer on a polysilicon layer;
flowing a first etchant onto the wafer to contact the native silicon oxide layer without exposure to a plasma and including a first hydrogen-based species and a first fluorine-based species, wherein the first hydrogen-based species is hydrogen and the first fluorine-based is nitrogen trifluoride;
heating the wafer to a temperature greater than about 60° C.;
removing at least partially the native silicon oxide layer during the heating of the wafer;
flowing a second etchant including a second hydrogen-based species and a second fluorine-based species towards the wafer;
exposing the second etchant to a plasma; and
removing the remainder of the native silicon oxide layer and removing at least partially the polysilicon layer after exposing the second etchant to the plasma.

2. The method of claim 1, further comprising:
transferring the wafer to a station for removal of polysilicon and completion of the native oxide removal after flowing the first etchant onto the wafer but prior to heating the wafer.

3. The method of claim 2, wherein each of the steps occur without breaking vacuum.

4. The method of claim 1, wherein the first etchant is identical to the second etchant.

5. The method of claim 1, wherein heating the wafer includes heating the wafer to a temperature of less than about 200° C.

6. The method of claim 1, wherein the wafer includes an exposed nitride and/or oxide structure.

7. The method of claim 6, wherein an etch rate of the exposed nitride and/or oxide structure is less than about 3 Å per minute during removal of the native silicon oxide layer.

8. The method of claim 6, wherein the exposed nitride structure includes silicon nitride and/or titanium nitride.

9. The method of claim 6, wherein the exposed oxide structure includes thermal silicon oxide.

10. The method of claim 1, wherein removal at least partially of the polysilicon layer occurs at a selectivity over the exposed nitride and/or oxide structure of greater than about 1000:1.

11. The method of claim 1, wherein exposing the second etchant to a plasma includes exposing the second etchant to a remote plasma.