US20060194404A1

US20060194404A1 - Method and system for fabricating and cleaning free-standing nanostructures

Info

Publication number: US20060194404A1
Application number: US11/325,631
Authority: US
Inventors: Audrey Dupont; Ronald Hoyer
Original assignee: Infineon Technologies AG
Current assignee: Infineon Technologies AG
Priority date: 2005-02-25
Filing date: 2006-01-05
Publication date: 2006-08-31
Also published as: TW200633016A; CN1825538A; KR20060094925A; TWI281198B; KR100810808B1; JP2006237620A; US7008853B1

Abstract

Systems and methods include introducing a semiconductor wafer into a process chamber. An etching chemistry is injected into the process chamber to etch a patterned layer and to release free-standing nanostructures on the semiconductor wafer. The etching chemistry includes a supercritical or liquid carbon dioxide fluid and an etching solution. The semiconductor wafer is rinsed by flooding a supercritical or liquid carbon dioxide fluid into the process chamber. The semiconductor wafer is dried by venting out supercritical or liquid carbon dioxide fluid from the process chamber.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part application of U.S. application Ser. No. 11/066,320, entitled “Method and System for Fabricating Free-Standing Nanostructures” and filed Feb. 25, 2005 which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to systems and corresponding methods for fabricating and cleaning free-standing nanostructures on a semiconductor wafer. In particular, the invention relates to the field of etching, cleaning, and drying a semiconductor wafer with a patterned layer to fabricate bottom electrode structures on a semiconductor wafer and to cleaning and/or drying the bottom electrode structures.

BACKGROUND

One goal in the manufacture of integrated circuits is to continuously decrease feature sizes of the fabricated components. For certain components, like capacitors, shrinking adversely affects the properties of the component. In order to achieve a desired value of capacitance, it is therefore necessary to keep the surface area of the capacitor above a threshold value. This is particularly important for dynamic random access memory cells (DRAM) which call for high integration densities.
As the surface area for a single memory cell decreases, the capacity of the storage capacitor also decreases. For proper operation of the memory cell, a certain minimum capacity (typically on the order of about 30 femto farad) is mandatory for the storage capacitor. If the capacity of the storage capacitor is too small, the charge stored in the storage capacitor is not sufficient to produce a detectable signal. In such a case, the information stored in the memory cell is lost and the memory cell is not operating in a desired manner.
Several methods have been developed to overcome the problems associated with shrinking feature sizes by integrating capacitors of DRAM cells in a three dimensional manner. For example, one method introduces deep trench capacitors which are formed in the substrate of a semiconductor wafer to maintain a large capacitor area with a high capacity while using only a small amount of the surface of the substrate. The selection or access transistor is usually formed on the planar surface of the substrate in this method.
In another example, stacked capacitors are used which are formed on top of a planar surface on the substrate. The selection transistors are formed below the planar surface in this method. The stacked capacitor includes a first electrode and a second electrode with a dielectric layer between the two electrodes. The first electrode (also called bottom electrode) is usually formed as a cylindrical structure on the surface of the substrate by lining trenches of a patterned sacrificial mold layer with the electrode material. Afterwards, the bottom electrodes are released by etching the sacrificial mold layer. Subsequently, the surface of the bottom electrodes are cleaned to be prepared for further processing, including deposition of the dielectric layer and the second or top electrode.
However, with the ever decreasing feature sizes of structures, etching and/or cleaning steps become increasingly difficult. Etching and/or cleaning steps are usually performed by wet processing. Standard wet processing, e.g. rinsing the wafer with ultra pure de-ionized water for cleaning purposes, introduces capillary forces between neighboring structures (e.g., between adjacent bottom electrodes). With reduced feature sizes, this may lead to adhesion of neighboring structures. This is described in Legtenberg et al., “Stiction of surface macromachined structures after rinsing and drying: model and investigation of adhesion mechanisms”, Sensors and Actuators A, 43 (1994), pages 230-238. Adhesion of neighboring structures is mediated by the cleaning liquid, usually referred to as “stiction”.
For semiconductor processing, stiction is primarily important during drying steps which usually follow the etching and cleaning steps in semiconductor wafer processing. There, capillary forces induced by the liquid lead to adhesion of adjacent bottom electrodes. The adjacent bottom electrodes remain stuck to each other even after being completely dried, particularly when the adhesion force between the contacting adjacent bottom electrodes is larger than the elastic restoring force of the deformed bottom electrodes.
Exposing wafers to an air-liquid interface during transfer between etching, cleaning and drying process modules is detrimental to obtaining stiction free process performance. Failing to achieve stiction free process performance ultimately results in a low yield of the produced circuits. One potential solution to this problem is to completely avoid wet processing and perform etching steps using gas phase processing, e.g. in a hydrogen fluoride vapor. However, such gas phase processing leads to etching residues and to silicon surface termination states which in turn hinders further processing.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a system and corresponding method for fabricating free-standing nanostructures on a semiconductor wafer which overcomes the above mentioned problems associated with stiction.
It is another object of the invention to provide such a system and method that includes etching, cleaning, and drying of a semiconductor wafer with a patterned hard mask layer for fabrication of bottom electrode structures.
The aforesaid objects are achieved individually and in combination in accordance with the present invention, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the invention, a method for fabricating and cleaning free-standing nanostructures includes providing a semiconductor wafer comprising a substrate and a patterned layer above the substrate, the patterned layer comprising a plurality of openings extending from an upper surface of the patterned layer to an upper surface of the substrate, and structural elements being arranged within the openings, providing a process chamber, the process chamber being configured to receive the semiconductor wafer, introducing the semiconductor wafer into the process chamber, injecting an etching chemistry into the process chamber to etch the patterned layer and to release the structural elements as free-standing nanostructures on the semiconductor wafer, the etching chemistry comprising a carbon dioxide fluid and an etching solution, injecting a cleaning chemistry into the process chamber in order to remove particles from the surface of structural elements being free-standing nanostructures on the semiconductor wafer, the cleaning chemistry comprising a supercritical carbon-dioxide fluid and cleaning solution, rinsing the semiconductor wafer by flooding a carbon dioxide fluid into the process chamber, and drying the semiconductor wafer by injecting a supercritical carbon dioxide fluid into the process chamber and by venting out the supercritical carbon dioxide fluid from the process chamber.
Accordingly, stiction free processing is achieved by employing unique properties of carbon dioxide in the supercritical or liquid state. The supercritical state is a high density phase characterized by a low viscosity and a zero surface tension, thus enabling better solubility and efficiency of the etching chemistry. On the other hand, properties similar to a gas phase presents high diffusion capabilities, allowing for easy solvent removal and greater drying efficiency. Another feature of the invention is that all process steps are performed in the same process chamber. This ensures that no air-liquid interfaces during transfer between etching, cleaning and drying process modules can occur. Accordingly, capillary forces are eliminated. This is achieved by employing carbon dioxide in its various states, i.e. supercritical, liquid and gas. Furthermore, a cleaning step is applied which removes residues on the surface of the free-standing nanostructures. The cleaning step is performed in the same process chamber thus utilizing a supercritical process sequence which allows for adding a cleaning solution. As a result, contaminants on the surface of the free-standing nanostructures are largely eliminated.
In accordance with another embodiment of the invention, a method for fabricating and cleaning free-standing nanostructures includes providing a semiconductor wafer having a substrate and a patterned layer above the substrate, the patterned layer comprising structural elements as free-standing nanostructures, providing a process chamber, the process chamber being configured to receive the semiconductor wafer, introducing the semiconductor wafer into the process chamber, injecting a cleaning chemistry into the process chamber in order to remove particles from the surface of structural elements being free-standing nanostructures on the semiconductor wafer, the cleaning chemistry comprising a supercritical carbon-dioxide fluid and a cleaning solution, rinsing the semiconductor wafer by flooding a supercritical carbon-dioxide fluid into the process chamber, and drying the semiconductor wafer by venting out the supercritical carbon-dioxide fluid from the process chamber.
In accordance with another embodiment of the invention, a system for fabricating and cleaning free-standing nanostructures comprises: a semiconductor wafer comprising a substrate and a patterned layer disposed above the substrate, the patterned layer comprising a plurality of openings extending from the surface of the patterned layer to the surface of the substrate and structural elements being arranged within the openings; a process chamber, the process chamber being configured to receive the semiconductor wafer; means for introducing the semiconductor wafer into the process chamber; means for injecting an etching chemistry into the process chamber to etch the patterned layer and to release the structural elements as free-standing nanostructures on the semiconductor wafer, the etching chemistry comprising a liquid or supercritical carbon dioxide fluid and an etching solution; means for injecting a cleaning chemistry into the process chamber in order to remove particles from the surface of structural elements being free-standing nanostructures on the semiconductor wafer, the cleaning chemistry comprising a supercritical carbon-dioxide fluid and cleaning solution; means for rinsing the semiconductor wafer by flooding supercritical carbon dioxide fluid into the process chamber; and means for drying the semiconductor wafer by venting out supercritical carbon dioxide fluid from the process chamber.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a side view in partial cross-section of a semiconductor wafer including a plurality of stacked capacitor DRAM-cells.
FIG. 2 depicts a side view in partial cross-section of a semiconductor wafer including a plurality of surrounding gate transistors.
FIGS. 3A-3D depict a side view in partial cross-section of parts of a stacked capacitor DRAM-cell formed in accordance with an exemplary method of the invention.
FIGS. 4A-4B depict a side view in partial cross-section of free-standing nano-structures formed in accordance with another exemplary method of the invention.
FIG. 5 depicts a side view in partial cross-section of a wafer drying, rinsing and cleaning system in accordance with the invention.
FIGS. 6A-6B are images showing a plurality of bottom electrodes of stacked capacitor DRAM-cells formed according to conventional wet etching techniques.
FIG. 7 is an image showing parts of a plurality of surrounding gate transistors formed according to conventional wet etching techniques.
FIG. 8 is a flow chart of method steps for a further embodiment of the invention.
FIG. 9 is a flow chart of method steps for a further embodiment of the invention.

DETAILED DESCRIPTION

Exemplary embodiments of methods and systems according to the invention are discussed in detail below. It is appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to apply the method and the system of the invention, and do not limit the scope of the invention.
In particular, the following embodiments are described in the context of fabricating bottom electrode structures for stacked capacitor DRAM cells and surrounding gate transistors for a vertical cell technology. In both technologies, free-standing nanostructures are formed as protruding elements on the surface of a semiconductor wafer having, in accordance with present technologies, a height to width ratio in excess of 20. It should be noted, however, that the inventive methods and systems can be applied for other high aspect ratio nanostructures as well.
Referring to FIG. 1, stacked capacitor DRAM-cells are shown in a side view. It should be appreciated that FIG. 1 merely serves as an illustration of fabricating stacked capacitor DRAM-cells, and the individual components shown in FIG. 1 are not true to scale.
In FIG. 1, a semiconductor wafer 2 is shown including a substrate 4 of semi-conductive material (e.g., silicon). The semiconductor wafer 2 is used for fabricating a plurality of stacked capacitor DRAM cells 6. Each DRAM cell 6 includes a selection transistor 10 and a stacked capacitor 12. The stacked capacitor 12 is located above the substrate surface 8.
The transistor 10 is located in the substrate 4. The transistor 10 is formed by a first junction 14 and a second junction 16. Between the first junction 14 and the second junction 16 a gate 20 is disposed above a gate dielectric layer 18. The gate 20 can include a stack of several layers (e.g., silicon and tungsten). The stack of several layers reduces the resistance of the gate 20. The gate 20 also serves as a word line for addressing a specific DRAM cell 6 of the plurality of DRAM cells during operation.
The first junction 14 is connected to a bit-line contact 22. The bit-line contact 22 is disposed above the first junction 14. The bit-line contact 22 is connected to a bit-line 24 which is located above the bit-line contact 22. The bit-line 24 serves as a write or read line during operation.
The second junction 16 is connected to a contact plug 26. The contact plug 26 is disposed above the second junction 16. The contact plug 26 serves as a connection to a bottom electrode 28 of the capacitor 12. The bottom electrode 28 of the capacitor 12 is located above the substrate surface 8.
In FIG. 1, the bottom electrodes 28 are shown in a cross sectional side view. For the three dimensional shape of the bottom electrode 28, many different geometries can be used including cylindrical, elliptical or rectangular.
As shown in FIG. 1, the bottom electrode 28 is formed with vertical side walls above the contact plug 26. However, non-vertical side walls or side walls being laterally recessed are also conceivable.
The bottom electrode 28 of the capacitor 12 is covered by a dielectric layer 30 which serves as the dielectric of capacitor 12. Above the dielectric layer 30 a top electrode 32 is disposed. The top electrode 32 of the capacitor 12 is usually common to all DRAM cells 6 thus providing a connection between adjacent DRAM cells 6.
To a person skilled in the art, it is known that other elements might be necessary to fabricate working DRAM cells 6. For example, an insulation between adjacent transistors 10 might be necessary to avoid electrical shorts to adjacent junction regions. In addition, a barrier layer between the contact plug 26 and the bottom electrode 28 can be provided, if necessary, to eliminate diffusion of different materials.
Furthermore, an isolation material can be disposed around the contact plugs 26 to avoid shorts to adjacent DRAM cells 6. It is also possible to dispose the bit-line 24 on top of the top electrode 32 of the capacitor 12 by using a modified bit-line contact 22.
Referring now to FIG. 2, a second semiconductor wafer 2 is shown including a substrate 4 of semi-conductive material (e.g., silicon). The semiconductor wafer 2 is used for fabricating a plurality of surrounding gate transistors 10.
Each transistor 10 is located on pillars 21 formed on the substrate 4, where the transistor is formed by a first junction 14 on the lower side of the pillar 21 and a second junction 16 on the top side of the pillar 21. A gate dielectric layer 18 and a gate 20 are disposed on the side walls of the pillar 21 between the first junction 14 and the second junction 16.
In an exemplary method of the invention for forming DRAM cells 6 of FIG. 1, a semiconductor wafer 2 is first provided as shown in FIG. 3A. The semiconductor wafer 2 has the substrate 4, with transistors 10 in the substrate (not shown) that have been formed for each DRAM cell 6.
In FIG. 3A, contact plugs 26 are shown on the surface 8 of the substrate 4. As described above, the contact plugs 26 are used to contact the bottom electrodes with the junction regions of the transistors. The contact plugs 26 usually have a low resistance and are made of, e.g., arsenic doped amorphous silicon. As an example, in a technology providing 70 nm minimal feature size, adjacent contact plugs 26 are spaced from each other at a distance ranging from 50 nm to 200 nm.
On the surface 8 of substrate 4, a hard mask 40 as is deposited a mold layer. The hard mask 40 may be composed of, e.g. silicon oxide. The hard mask 40 serves later as a mold for the bottom electrode 28 of the stacked capacitor DRAM cell 6. Accordingly, the thickness 41 of the hard mask directly affects the height of the bottom electrode 28 to be formed and, as a consequence, the capacitance of stacked capacitor 12. Accordingly, the hard mask 40 must have a certain thickness which can be about 2 μm, e.g., for the 70 nm technology. However, other values for the thickness 41 might also be used, e.g., thicknesses in the range of 1 μm to 20 μm.
After the hard mask 40 has been deposited, a plurality of openings or trenches 48 are formed in the mask layer 40, as shown in FIG. 3B. A photo lithographic patterned resist can be applied, e.g., to define the regions of the trenches 48. Each trench 48 is arranged above a respective contact plug 26, as shown in FIG. 3B. The trenches 48 are formed using a reactive ion etching step.
During the etching step, material of the mask layer 40 is removed, thus forming the trench 48 from the top surface of the contact plugs 26. As a result, the bottom of the trenches 48 are formed by the contact plugs 26 while the side walls 49 of the trench are formed within hard mask 40. Etching of trenches 48 preferably leads to side walls 49 which have a width in the range of 50 nm to 200 nm as measured at the center of the trench 48.
Subsequently, a conductive layer 50 is conformably deposited on the semiconductor wafer 2. The conductive layer 50 covers the top side of the hard mask 40, the trench side walls 49, and the bottom part of the trench 48, (including the contact plugs 26). A suitable material for forming the conductive layer 50 by deposition can be, e.g., doped amorphous silicon. The conductive layer 50 serves later as the bottom electrode 28 of the stacked capacitor 12 of DRAM cell 6 once the hard mask 40 has been removed and the bottom electrodes are released. Therefore, the thickness of the conductive layer 50 affects the stability of the free-standing bottom electrodes 28.
In the next process step, a portion of the conductive layer 50 is removed from the surface of the hard mask 40, as shown in FIG. 3C. This portion of the conductive layer 50 can be removed, e.g., by etching using a plasma etcher. Alternatively, the trenches 48 can be filled with a fill material, followed by removing the portion of the conductive layer 50 from the top surface of the hard mask 40 by chemical mechanical polishing, and then removing the fill material from within the trenches 48. According to these process steps, the fill material protects the inner sides of the trenches 48 from residues which could be a problem during later process steps (e.g., deposition of the dielectric layer 30).
Referring to FIG. 3D, the next process step includes removing the hard mask layer 40 in order to release the remaining part of conductive layer 50 as free-standing nanostructures. The free-standing nanostructures form bottom electrodes of a stacked capacitor DRAM-cell 6, as described above and depicted in FIG. 1.
In FIGS. 4A and 4B, another process sequence of fabricating free-standing nanostructures in accordance with the invention is shown. Referring to FIG. 4A, the surface 8 of semiconductor wafer 2 is covered with a patterned resist layer or any other suitable hard mask layer which has been structured by, e.g., a photo lithographic process step. Referring to FIG. 4B, the next process step includes releasing free-standing nanostructures 52 by etching. For example, the free-standing nanostructures 52 of FIG. 4B are used as silicon pillars for surrounding gate transistors, such as the gate transistors described above and depicted in FIG. 2.
According to an embodiment of the invention, both releasing of the free-standing pillars and the bottom electrodes, as described in above methods and depicted in FIGS. 3A-3D and 4A-B, are performed by an etching step. In a first exemplary embodiment, the etching step employs an etching chemistry with a carbon dioxide fluid being in the liquid phase and an etching solution. The carbon dioxide fluid acts as a carrier for an etching solution which is chosen with respect to the material to be removed during the etching step. Referring to the embodiment shown in FIG. 3C, the mold layer can be composed of silicon-dioxide, such that a fluorine based etching chemistry (e.g., a formulation of a hydrofluoric acid) can be used to etch the mold layer.
In addition, the etching chemistry is a mixture of carbon dioxide, the etching solution and a co-solvent. For the co-solvent, an alcohol/de-ionized water-mixture can be chosen, but alcohol, alkane, ketone, amine or fluorine containing mixtures can be used as well. Suitable substances are specified below.
Furthermore, a surfactant can be incorporated in the etching chemistry in order to enhance water incorporation. The surfactant should be compatible with the carbon dioxide fluid. Again, suitable substances are specified below.
In a second exemplary embodiment, the process steps to release the free-standing structures are performed by an etching step mediated by a supercritical carbon dioxide fluid. In general, a supercritical fluid are compounds above the so-called critical point in the pressure/temperature phase diagram at a certain critical temperature and critical pressure. The supercritical state is often called the fourth state of matter. Supercritical fluids exhibit properties of both liquids and fluids. For example, transport properties like viscosity are similar to gases while solvating properties like density are similar to those of liquids.
According to an embodiment of the invention, a system is provided, as shown in FIG. 5, that includes a process chamber 60. The process chamber 60 is configured (e.g., suitably dimensioned) to accommodate the semiconductor wafer 2. As shown in FIG. 5, the process chamber 60 is connected to a reservoir 62 capable of delivering carbon dioxide in the supercritical phase. In addition, the system shown in FIG. 5 allows for adding extra substances with controlled concentrations, such as etching agents and/or co-solvents, as described below. Furthermore, the process chamber 60 can be pressurized and is operated with a controlled temperature. This is achieved by a control unit 64, which is schematically connected to reservoir 62 and process chamber 60.
For the supercritical fluid system shown in FIG. 5 many commercially available systems can be used. As an example, TELSSI, Inc. and SC Fluids Inc. provide supercritical fluid tools. Tools from other vendors might be used as well.
Before processing is started, the semiconductor wafer 2 is introduced into the process chamber 60. In the next step, an etching chemistry is injected into the process chamber 60. The etching chemistry is used to etch the hard mask layer 40 and to release the structural elements 52 as free-standing nanostructures on the semiconductor wafer 2. The etching chemistry includes the supercritical or liquid carbon dioxide fluid and an etching solution. The etching solution is chosen with respect to the material to be removed during the etching step. According to the embodiment shown in FIG. 3C, the hard mask layer 40 or mold layer is composed of silicon-dioxide. Accordingly, a fluorine based etching chemistry might be used.
The etching chemistry can be a formulation of a hydrofluoric acid in mixture of carbon dioxide and a co-solvent. Preferably, the hydrofluoric acid is added to the etching chemistry on the level of a few micro-liter per liter of the etching chemistry.
For the co-solvent, an alcohol/de-ionized water-mixture can be chosen, but alcohol, alkane (e.g., hexane), ketone (e.g., acetone), amine and/or fluorine containing mixtures can be used as well. Suitable alcohol substances include methanol, ethanol, propanol, iso-propanol, butanol and/or pentanol. Suitable amine substances include n-methylpyrrolidone, di-glycol amine, di-isopropyl amine and/or tri-isopropyl amine. Suitable fluorine containing substances include ammonium fluoride and/or 1,1,1-fluoro methane.
In addition, one or more surfactants can be incorporated in the etching chemistry in order to enhance water incorporation. The surfactant should be compatible with the supercritical carbon dioxide fluid. For example, an anionic surfactant such as sodium dioctyl sulfosuccinate and its derivatives (e.g., AEROSOL-OT and its derivatives, available from Cytec Industries, West Paterson, NJ, www.cytec.com) and/or a nonionic surfactant can be used. Suitable non-ionic surfactants include ethylene oxide; octylphenol ethoxylates, alkyl polyglucosides, and derivatives of each (e.g., TRITON series of nonionic surfactants, available from Dow Chemical, Midland, Mich., www.dow.com); and/or acetylenic diols, alkoxylated acetylenic diols, and derivatives of each (e.g., SURFYNOL and DYNOL surfactants available from Air Products and Chemicals, Allentown, Pa., www.airproducts.com).
The etching chemistry containing the supercritical carbon dioxide fluid and the etching solution is injected at a temperature and pressure above the critical point of carbon dioxide, which is located above about 34° C. and about 1050 psi in the phase diagram.
Preferably, the etching chemistry is injected into the process chamber 60 at a single transparent phase in a temperature and pressure range selected between 50° C. and 100° C. and at about 1100 psi. In this temperature and pressure range, the density of the supercritical carbon dioxide fluid is in the range between 0.4 and 0.8 g/mL (g/mL=gram/milliliter). The supercritical state is further characterized by a low viscosity and negligible zero surface tension.
When injecting the etching chemistry in the process chamber, the semiconductor wafer 2 is etched. As a result, the mold layer 40 is completely removed from the surface 8 of the semiconductor wafer 2. The bottom electrodes 28 are released as free-standing nanostructures 52 on the surface 8 of the semiconductor wafer 2.
It should be noted, that the same process chamber can be used for processing the non-supercritical liquid carbon dioxide fluid as described above by selecting proper processing conditions, i.e. a temperature and pressure below the critical point of carbon dioxide.
After etching the free-standing nanostructures, contaminants may be present on the surface of the free-standing nanostructures. As an example, particles from the mold layer 40 or particles from other materials present on semiconductor wafer 2 aggregate on the surface of the free-standing nanostructures. In order to remove these particles from the surface of the free-standing nanostructures 52, a cleaning chemistry or composition is injected into the process chamber 60. As a result, particles aggregated to the surface of the structural elements of the free-standing nanostructures 52 on the semiconductor wafer 2 are removed.
The cleaning chemistry can be a mixture of a supercritical carbon-dioxide fluid and a cleaning solution. As described previously with respect to the etching step, the cleaning chemistry containing the supercritical carbon dioxide fluid and the etching solution is injected at a temperature and pressure above the critical point of carbon dioxide, which is located above about 34° C. and about 1050 psi in the phase diagram.
According to a first exemplary embodiment, a further etching solution similar to the previously described etching solution is used for the cleaning solution. The etching solution is chosen with respect to the material to be removed during the cleaning step. According to the embodiment shown in FIG. 3B, the free-standing nanostructures 52 are composed of silicon. Accordingly, an acid based etching chemistry (for instance HF, H₂SO₄or HCl based chemistries) or a dilute basic chemistry such as ammonia based chemistry might be used. An etching chemistry combining the use of surfactants can also be used. Various classes of surfactants may be used in SCCO₂, such as anionic surfactants like AEROSOL-OT (AOT, “branched” AOTs with methylated groups, fluorinated AOTs), nonionic surfactants such as TRITON and all its derivates.
In this case, etch residues are removed by the surfactants via transport in micelle structures. Micelle structures can exist in different shapes including spherical, cylindrical and the like. Generally, surfactants have a hydrophilic and a hydrophobic part of the molecules. The hydrophilic part forms in case of non-polar solvents the core of the micelle while the hydrophobic part remain on the surface of the micelle. Micelles assist in cleaning by incorporating the contaminants into their core.
As a result, the cleaning solution etches the surface of the free-standing nanostructures 52 up to a thickness between 5 Å and 100 Å in order to facilitate the separation of particles from the surface of the free-standing nanostructures 52.
In a second exemplary embodiment, a cleaning solution being capable of reducing the aggregation of particles to the free-standing nanostructures 52 is used. Aggregation of particles is generally described by the so-called zeta-potential.
In D. J. Shaw, Introduction to Colloid and Surface Chemistry, Butterworth-Heinemann Oxford (1993), page 183, which is incorporated herein by reference, the concept of the zeta potential is described more detailed. As particles dispersed in a solution, in this case the cleaning solution, are surrounded by oppositely charged ions. The layer of opposite charge ions is called the fixed layer and is accompanied by a cloud-like area being formed of a ions having opposite polarities so that the whole area appears electrically neutral. The zeta-potential is considered to be the electrical potential within the cloud-like area. When this potential is overcome by the Van-der-Waals binding force, particles tend to aggregate. Accordingly, the cleaning solution should have a zeta-potential leading to a sufficient repulsive force between particle and wafer surface. As a consequence, the cleaning solution changes the zeta-potential of the surface of the free-standing nanostructures to promote the separation of the particles from the surface of the free-standing nanostructures 52.
To further enhance the removal of particles, it is also conceivable to apply a mechanical force to the particles. The mechanical force promotes to fully remove the particles from the surface of the free-standing nanostructures 52.
In a first exemplary embodiment, the mechanical force is applied as a megasonic sound wave. Accordingly, a megasonic source (or transducer) is provided which is preferably arranged inside the process chamber 60 with an electrical power feed through. The transducer is immersed into the fluid for an effective transfer of the megasonic energy to the wafer surface. Optionally, the megasonics may also be directed to the wafer surface by a quartz rod acoustically coupled to a transducer, as disclosed in U.S. Pat. No. 6,684,890, incorporated herein by reference in its entirety.
In a second exemplary embodiment, the mechanical force is applied by periodically increasing and decreasing of the pressure of process chamber 60. It is conceivable to operate the process chamber 60 in a range between 1000 and 7000 psi for 1 to 20 cycles, for example. In order to increase the efficiency of the cycles, the pressure may be increased and especially decreased within less than 2 seconds, but the speed of pressure increase or decrease may be limited by the mechanical force applied to the components in the process chamber, especially the wafer substrate. Other increase and decrease sequences of the pressure in process chamber 60 might be used as well.
In a third exemplary embodiment, the mechanical force is applied by agitation of the cleaning chemistry. The agitation device may be operated in a rotational movement and may be formed by metallic wings being located within the process chamber 60 above the semiconductor wafer 2. For reducing metal contamination, the agitation device might be coated with a polymer (e.g., polyether ether ketone (PEEK)) so that it remains resistant to chemical attack under high pressure and temperature conditions. The shape of the wings as well as distance of the wings to the surface of semiconductor wafer 2 might be adjusted to generate a optimized fluid streaming profile so as to achieve most efficient removal of particle and reduced damage to the structures on the surface of semiconductor wafer 2.
In order to remove etching residues during the etching step or cleaning residues during the cleaning step, the semiconductor wafer 2 is rinsed in the next step. This is achieved by flooding a supercritical carbon dioxide fluid into the process chamber 60. Flooding is performed with a controlled gradient of flow of the supercritical carbon dioxide fluid which ensures that the etching chemistry is progressively removed from the reaction mixture.
After the rinse step, an optional flushing of the free-standing nanostructures is performed within the process chamber 60. Again, a supercritical carbon dioxide fluid is used. The flow of the supercritical carbon dioxide fluid is selected in the range from 0.1 L/min to 5 L/min, preferably between 0.5 L/min and 2 L/min (L/min=liters per minute). Again, both the rinse step and the flushing step can also be performed with non-supercritical liquid carbon dioxide fluid. Preferably, the same process chamber 60 is used for performing the etching, rinse and flushing steps by selecting proper processing conditions, i.e. a temperature and pressure above or below the critical point of the carbon dioxide fluid.
Afterwards a drying step of the semiconductor wafer is performed. This is achieved by venting out supercritical or liquid carbon dioxide fluid from the process chamber 60. During this step the pressure created by the venting supercritical carbon dioxide fluid within the process chamber is released. Since the supercritical carbon dioxide fluid has negligible surface tension properties, capillary forces do not occur during drying. Accordingly, the process sequence according to this embodiment of the invention is free of stiction.
According to the embodiment of the invention, stiction free processing is achieved by employing unique properties of carbon dioxide in the supercritical state and/or liquid state. The supercritical state is a high density phase characterized by a low viscosity and a zero surface tension, thus enabling better solubility and efficiency of the etching chemistry. On the other hand, the gas phase presents high diffusion capabilities, allowing for easy solvent removal and greater drying efficiency. Another feature of the embodiment of the invention is that the process is run entirely in the same process chamber 60, ensuring that the nanostructures remain permanently wetted, and thus eliminating all together capillary forces. This is possible by making good use of the carbon dioxide various states, i.e. supercritical, liquid and gas. The process sequence according to the invention also offers additional advantages as carbon dioxide is a non-flammable and non-toxic substance and can easily be recycled.
Still another feature of the embodiment of the invention is that a cleaning step is performed after etching the free-standing nanostructures. Again, the cleaning step runs in the same process chamber 60 and ensures that the nanostructures are essentially particle free before further processing continues.
Accordingly, problems encountered in the prior art with respect to high aspect ratio nanostructures are circumvented, as shown below.
Referring now to FIGS. 6A and 6B, bottom electrodes 28 of stacked capacitor DRAM-cells are shown which are fabricated by applying prior art wet etching and drying techniques. In FIG. 6A, a first SEM-picture 80 shows bottom electrodes 28 of stacked capacitor DRAM-cells in a side view. In FIG. 6B, a second SEM-picture 82 shows bottom electrodes 28 of stacked capacitor DRAM-cells in a top view. Stiction between the neighboring cylinders of bottom electrodes 28 is clearly visible in the first SEM-picture 80 and in the second SEM-picture 82.
Referring now to FIG. 7, free-standing nanostructures 52 of surrounding gate transistors are shown which are fabricated by applying prior art wet etching and drying techniques. In FIG. 7, a third SEM-picture 84 shows nanostructures 52 in a side view. Stiction between the neighboring nanostructures 52 is clearly visible in the third SEM-picture 84.
Referring now to FIG. 8, a flowchart of method steps is provided for utilizing the system of FIG. 5. Referring to the flowchart, a semiconductor wafer 2 is provided in step 100. In step 102, a process chamber 60 is provided, which is configured to accommodate the semiconductor wafer 2. In step 104, the semiconductor wafer 2 is introduced into the process chamber 60. In step 106, an etching chemistry is injected into the process chamber 60 to etch the patterned layer 40 and to release free-standing nanostructures on the semiconductor wafer 2. The etching chemistry includes a supercritical or liquid carbon dioxide fluid and an etching solution. In step 108, the semiconductor wafer is rinsed by flooding a supercritical or liquid carbon dioxide fluid into the process chamber 60. In step 110, the semiconductor wafer 2 is dried by injecting a supercritical carbon dioxide fluid into the process chamber 60 and by venting out the supercritical carbon dioxide fluid from the process chamber 60.
Referring now to FIG. 9, a flowchart of method steps is provided for utilizing the system of FIG. 5 to perform cleaning of free-standing nanostructures. Referring to the flowchart, a semiconductor wafer 2 is provided in step 200. The semiconductor wafer 2 has a substrate and a patterned layer above the substrate. The patterned layer includes structural elements as free-standing nanostructures 52 on the substrate of semiconductor wafer 2. In step 202, a process chamber 60 is provided, which is configured to accommodate the semiconductor wafer 2. In step 204, the semiconductor wafer 2 is introduced into the process chamber 60. In step 206, a cleaning chemistry/composition is injected into the process chamber 60 to remove particles from the surface of free-standing nanostructures 52 on the semiconductor wafer 2. The cleaning chemistry includes a supercritical carbon-dioxide fluid and a cleaning solution. In step 208, the semiconductor wafer is rinsed by flooding a supercritical dioxide fluid into the process chamber 60. In step 210, the semiconductor wafer 2 is dried by venting out the supercritical carbon dioxide fluid from the process chamber 60.
Having described embodiments for a method and a system for fabricating free-standing semiconductor structures, it is believed that other modifications, variations and changes will be suggested to those skilled in the art in view of the teachings set forth herein. It is therefore to be understood that all such variations, modifications and changes are believed to fall within the scope of the present invention as defined by the appended claims and their equivalents.

REFERENCE NUMERALS

- 2 wafer
- 4 substrate
- 6 DRAM cell
- 8 substrate surface
- 10 transistor
- 12 capacitor
- 14 first junction
- 16 second junction
- 18 gate dielectric layer
- 19 pillar
- 20 gate
- 22 bitline contact
- 24 bitline
- 26 contact plug
- 28 bottom electrode
- 30 dielectric layer
- 32 top electrode
- 40 hard mask
- 41 width of hard mask
- 48 trench
- 49 trench sidewall
- 50 conductive layer
- 52 free-standing nanostructures
- 60 process chamber
- 62 reservoir
- 64 control unit
- 80 first SEM picture
- 82 second SEM picture
- 84 third SEM picture
- 100-110 process steps
- 200-210 process steps

Claims

1. A method for fabricating and cleaning free-standing nanostructures, comprising the steps of:

providing a semiconductor wafer comprising a substrate and a patterned layer above the substrate, the patterned layer comprising a plurality of openings extending from an upper surface of the patterned layer to an upper surface of the substrate, and structural elements being arranged within the openings;

providing a process chamber, the process chamber being configured to receive the semiconductor wafer;

introducing the semiconductor wafer into the process chamber;

injecting an etching chemistry into the process chamber to etch the patterned layer and to release the structural elements as free-standing nanostructures on the semiconductor wafer, the etching chemistry comprising a carbon dioxide fluid and an etching solution;

injecting a cleaning chemistry into the process chamber in order to remove particles from the surface of structural elements being free-standing nanostructures on the semiconductor wafer, the cleaning chemistry comprising a supercritical carbon-dioxide fluid and cleaning solution;

rinsing the semiconductor wafer by flooding a carbon dioxide fluid into the process chamber; and

drying the semiconductor wafer by injecting a supercritical carbon dioxide fluid into the process chamber and by venting out the supercritical carbon dioxide fluid from the process chamber.

2. The method of claim 1, wherein the carbon dioxide fluid is in a liquid state in the etching chemistry injected into the process chamber.

3. The method of claim 1, wherein the carbon dioxide fluid is in a supercritical state in the etching chemistry injected into the process chamber.

4. The method of claim 3, wherein the injection of the supercritical carbon dioxide fluid containing etching chemistry into the process chamber is performed at a single transparent phase of the supercritical carbon dioxide fluid.

5. The method of claim 4, wherein the step of injecting the etching chemistry into the process chamber is performed at a pressure of the supercritical carbon dioxide fluid within the process chamber being above 1000 psi.

6. The method of claim 5, wherein the step of injecting the etching chemistry into the process chamber is performed at a temperature of the supercritical carbon dioxide fluid within the process chamber being above 40° C.

7. The method of claim 1, wherein the carbon dioxide fluid is in a liquid state during the rinsing of the semiconductor wafer.

8. The method of claim 1, wherein the carbon dioxide fluid is in a supercritical state during the rinsing of the semiconductor wafer.

9. The method of claim 8, wherein the rinsing of the semiconductor wafer is performed at constant pressure and constant temperature of the supercritical carbon dioxide fluid within the process chamber.

10. The method of claim 1, wherein the step of drying the semiconductor wafer further includes releasing the pressure created by the venting supercritical carbon dioxide fluid within the process chamber.

11. The method of claim 1, wherein, prior to the step of drying the semiconductor wafer, the method further comprises:

flushing the free-standing nanostructures with a supercritical carbon dioxide fluid at flow ranging from about 0.1 L/min to about 5 L/min.

12. The method of claim 1, wherein adjacent openings of the plurality of openings of the semiconductor wafer are spaced at a distance of about 200 nm or less.

13. The method of claim 12, wherein the patterned layer of the semiconductor wafer has a thickness in the range of about 1 μm to about 20 μm.

14. The method of claim 13, wherein openings of the plurality of openings of the semiconductor wafer have a width of about 200 nm or less.

15. The method of claim 1, wherein the cleaning solution of the cleaning chemistry changes the zeta-potential of the surface of the free-standing nanostructures to promote the separation of the particles from the surface of the free-standing nanostructures.

16. The method of claim 1, wherein the cleaning solution of the cleaning chemistry etches the surface of the free-standing nanostructures up to a thickness between 5 Å and 100 Å so as to promote the separation of the particles from the surface of the free-standing nanostructures.

17. The method of claim 16, wherein the cleaning solution of the cleaning chemistry comprises an etchant being selected from the group consisting of HF, H₂SO₄, HCl, H₂O₂, and NH₃.

18. The method of claim 16, wherein the cleaning solution comprises one of an anionic surfactant and a non-ionic surfactant that allows for etch residues and particles to be transported away from the semiconductor in micellar structures.

19. The method of claim 18, wherein the anionic surfactant comprises one of a sodium dioctyl sulfosuccinate and a sodium dioctyl sulfosuccinate derivative.

20. The method of claim 18, wherein the non-ionic surfactant is selected from the group consisting of ethylene oxide, octylphenol ethoxylates, alkyl polyglucosides, acetylenic diols, alkoxylated acetylenic diols, derivatives of each, and combinations thereof.

21. The method of claim 15, further comprising:

applying a mechanical force to the particles in order to fully remove the particle from the surface of the free-standing nanostructures.

22. The method of claim 21, wherein the mechanical force comprises application of megasonics.

23. The method of claim 21, wherein the mechanical force comprises periodical increasing and decreasing of the chamber pressure.

24. The method of claim 21, wherein the mechanical force comprises agitation of the cleaning chemistry.

25. A method for cleaning free-standing nanostructures, comprising the steps of:

providing a semiconductor wafer having a substrate and a patterned layer above the substrate, the patterned layer comprising structural elements as free-standing nanostructures;

introducing the semiconductor wafer into the process chamber;

injecting a cleaning chemistry into the process chamber in order to remove particles from the surface of structural elements being free-standing nanostructures on the semiconductor wafer, the cleaning chemistry comprising a supercritical carbon-dioxide fluid and a cleaning solution;

rinsing the semiconductor wafer by flooding a supercritical carbon-dioxide fluid into the process chamber; and

drying the semiconductor wafer by venting out the supercritical carbon-dioxide fluid from the process chamber.

26. The method of claim 25, wherein at least part of the free-standing nanostructures have a height-to-width-ratio of 20 or larger.

27. The method of claim 26, wherein the free-standing nanostructures are aligned with respect to each other and the substrate to facilitate forming of bottom electrodes of a stacked capacitor memory cell.

28. The method of claim 26, wherein the free-standing nanostructures are aligned with respect to each other and the substrate to facilitate forming of active transistors of a surrounding gate transistor.

29. The method of claim 25, wherein the cleaning solution of the cleaning chemistry changes the zeta-potential of the surface of the free-standing nanostructures to promote the separation of the particles from the surface of the free-standing nanostructures.

30. The method of claim 25, wherein the cleaning solution etches the surface of the free-standing nanostructures up to a thickness between 5 Å and 100 Å so as to promote the separation of the particles from the surface of the free-standing nanostructures.

31. The method of claim 29 further comprising:

applying a mechanical force to the particles in order to fully remove the particles from the surface of the free-standing nanostructures.

32. The method of claim 31, wherein the mechanical force comprises application of megasonics.

33. The method of claim 31, wherein the mechanical force comprises periodical increasing and decreasing of the chamber pressure.

34. The method of claim 31, wherein the mechanical force comprises agitation of the cleaning chemistry.

35. A system for fabricating and cleaning free-standing nanostructures, comprising:

a semiconductor wafer comprising a substrate and a patterned layer disposed above the substrate, the patterned layer comprising a plurality of openings extending from the surface of the patterned layer to the surface of the substrate and structural elements being arranged within the openings;

a process chamber, the process chamber being configured to receive the semiconductor wafer;

means for introducing the semiconductor wafer into the process chamber;

means for injecting an etching chemistry into the process chamber to etch the patterned layer and to release the structural elements as free-standing nanostructures on the semiconductor wafer, the etching chemistry comprising a liquid or supercritical carbon dioxide fluid and an etching solution;

means for injecting a cleaning chemistry into the process chamber in order to remove particles from the surface of structural elements being free-standing nanostructures on the semiconductor wafer, the cleaning chemistry comprising a supercritical carbon-dioxide fluid and cleaning solution;

means for rinsing the semiconductor wafer by flooding supercritical carbon dioxide fluid into the process chamber; and

means for drying the semiconductor wafer by venting out supercritical carbon dioxide fluid from the process chamber.