Patent Application
20070039924
The present invention relates to semiconductor processing, and more particularly, to a low-temperature process for oxide removal from a substrate and subsequent formation of a film on the substrate.
Silicon-containing films are used for a wide variety of applications in the semiconductor industry. Silicon-containing films include silicon films such as epitaxial silicon, polycrystalline silicon (poly-Si), amorphous silicon, epitaxial silicon germanium (SiGe), silicon germanium carbide (SiGeC), silicon carbide (SiC), silicon nitride (SiN), silicon carbonitride (SiCN), and silicon carboxide (SiCO). As circuit geometries shrink to ever smaller feature sizes, lower processing temperatures are preferred, for example because of introduction of new materials into semiconductor devices and reduction of thermal budgets of shallow implants in source and drain regions. Moreover, it is evident that non-selective (blanket) and selective deposition of silicon-containing films will be needed for future devices.
Epitaxial silicon deposition is a process where the crystal lattice of the bulk silicon is extended through growth of a new silicon-containing film that may have a different doping level than the bulk. Matching target epitaxial film thickness and resistivity parameters is important for the subsequent fabrication of properly functioning devices. Prior to depositing a silicon-containing film, e.g., epitaxial silicon or epitaxial silicon germanium films, on a silicon substrate, it may be required to remove a native oxide layer from the surface of the substrate in order to prepare a proper starting growth surface (i.e., a seed layer) to deposit a high quality epitaxial film. A native oxide layer, which may be a few to several Angstroms (Å) thick, for example, forms easily on clean silicon surfaces when exposed to an oxygen-containing environment (e.g., air), even at room temperature and atmospheric pressure. If the substrate is not cleaned prior to depositing a silicon-containing film on the substrate, i.e., all oxygen and other contaminants have not been properly removed from the substrate surface, then the silicon-containing film subsequently deposited on the substrate will not grow epitaxially and may contain defects that can lead to a high leakage current through the film and cause the microelectronic device to not perform optimally.
Similarly, a poly-Si film can be deposited directly on a poly-Si film to form an electrical contact. However, because other processing typically occurs between the poly-Si deposition steps, the substrates (wafers) can be removed from the processing system between the deposition steps, in which case a native oxide layer can form on the substrates. If the native oxide layer is not removed prior to depositing the poly-Si film, the resulting contact can have high electrical resistance.
It may also be necessary to remove a native oxide layer from a substrate, for example, prior to depositing a high dielectric constant (high-k) layer on the substrate, where the high-k film is a part of a gate stack. Examples of high-k films include HfO2, HfSiOx, HfSiOxNy, ZrO2, ZrSiOx, and ZrSiOxNy. The presence of an oxide layer can reduce the effective dielectric constant of the gate stack since the oxide layer normally has a lower dielectric constant than the high-k film. Thus, a higher dielectric constant and higher level of control over the overall dielectric constant can be achieved if the oxide layer is effectively removed before depositing a high-k film.
Traditionally, a high-temperature annealing of above 900° C. in a hydrogen atmosphere has been used in (vertical) batch processing systems to remove a native oxide layer from substrates and clean the substrates of other impurities prior to a deposition process. However, such a high-temperature process does not meet current or future thermal budget needs for many advanced processes. For example, current gate lengths and modern microelectronic structures limit devices to a reduced thermal budget.
Plasma processing has been found to allow lowering of the substrate temperature during processing and thus offers an alternative to high-temperature annealing in a hydrogen atmosphere. However, exposure of the substrate to a plasma source can damage the substrate as a result of the interaction of excited species in the plasma with the substrate. Another oxide removal method is based on hydrogen fluoride (HF), but the use of HF can result in incomplete oxide removal and unwanted erosion of the substrate and various films on the substrate.
Embodiments of the invention address the above-described problems associated with removing an oxide layer from a substrate. Embodiments of the invention can allow removal of an oxide layer from a substrate at a low process temperature while reducing damage to the substrate, thereby providing flexibility in the temperature of an oxide removal step. Embodiments of the invention can also allow removal of an oxide layer from a substrate using an etchant gas that is not as aggressive as, and much safer to handle than, conventional etchant gases including, for example, NF3 or CLF3.
According to an embodiment of the invention, a method is provided for removing an oxide layer from a substrate at low substrate temperature and subsequently forming a low defect film on the substrate.
According to an embodiment of the invention, the method includes (1) providing a substrate in a process chamber of a processing system, the substrate having an oxide layer formed thereon, and (2) exposing the substrate to an etching gas containing F2 at a first substrate temperature to remove the oxide layer from the substrate. According to an embodiment of the invention, the method may further include (3) heating the substrate to a second temperature greater than the first substrate temperature, and, optionally, (4) forming a film on the substrate at the second substrate temperature.
According to an embodiment of the invention, the substrate can contain Si, SiGe, Ge, a glass substrate, a LCD substrate, or a compound semiconductor. According to another embodiment of the invention, the film can be a Si-containing film, such as Si or SiGe, or a high-k dielectric film, such HfO2, HfSiOx, HfSiOxNy, ZrO2, ZrSiOx, or ZrSiOxNy.
According to an embodiment of the invention, the method includes (1) providing a Si substrate in a process chamber of a processing system, the Si substrate having an oxide layer formed thereon and (2) exposing the Si substrate to an etching gas containing F2 at a first substrate temperature lower than about 500° C. to remove the oxide layer from the Si substrate. According to an embodiment of the invention, the method may further include (3) heating the Si substrate to a second temperature between about 100° C. and about 900° C., and, optionally, (4) forming a Si film on the substrate at the second substrate temperature. According to an embodiment of the invention the Si substrate and the Si film can be epitaxial Si. According to another embodiment of the invention, the Si substrate and the Si film can be poly-Si.
The features, advantages thereof, and technical and industrial significance of embodiments of the present invention will be better understood by reading the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings, in which:
Embodiments of the invention may be used for native oxide removal (NOR) from a substrate prior to depositing a film onto the substrate. For example, the native oxide removal can be carried out prior to forming an epitaxial Si film, a poly-Si film, or a high-k film on a Si substrate. Embodiments of the invention may also be used to remove other types of oxides than native oxides, such as thin chemical oxides film grown or deposited on substrates, for example.
In the following description, the terms native oxide layer and oxide layer are used interchangeably to refer to any oxide layer to be removed from a substrate prior to forming a film on the substrate. For a Si substrate, an oxide layer or a native oxide layer can, for example, be a SiO2 layer or a SiOx (x<2) layer. Further, in order to facilitate a thorough understanding of the invention and for purposes of explanation, specific details are set forth, such as a particular geometry of the batch processing system and descriptions of various components. However, it should be understood that the invention may be practiced in other embodiments that depart from these specific details which are not limiting in any way.
A gas delivery system 97 is configured to introduce gases into the process chamber 10. A plurality of gas supply lines can be arranged around the manifold 2 to supply a plurality of gases into the process tube 25 through the gas supply lines. For simplicity, only one gas supply line 45 among the plurality of gas supply lines is shown in
Alternatively, or in addition, one or more of the gases can be supplied from the (remote) plasma source 95 that is operatively coupled to a second gas source 96 and to the process chamber 10 by the gas supply line 45. The plasma-excited gas is introduced into the process tube 25 by the gas supply line 45. The plasma source 95 can, for example, be a microwave plasma source, a radio frequency (RF) plasma source, or a plasma source powered by light radiation. In the case of a microwave plasma source, the microwave power can be between about 500 Watts (W) and about 5,000 W. Examples of microwave frequencies that could be used are 2.45 GHz or 8.3 GHz. In one example, the remote plasma source can be a Downstream Plasma Source Type AX7610, manufactured by MKS Instruments, Wilmington, Mass., USA.
A cylindrical heat reflector 30 is disposed so as to cover the reaction tube 25. The heat reflector 30 has a mirror-finished inner surface to suppress dissipation of radiation heat radiated by a main heater 20, a bottom heater 65, a top heater 15, and an exhaust pipe heater 70. A helical cooling water passage (not shown) is formed in the wall of the process chamber 10 as a cooling medium passage. The heaters 20, 65, and 15 can, for example, maintain the temperature of the substrates 40 between about 20° C. and about 900° C.
The vacuum pumping system 88 comprises a vacuum pump 86, a trap 84, and an automatic pressure controller (APC) 82. The vacuum pump 86 can, for example, include a dry vacuum pump capable of a pumping speed up to 20,000 liters per second (and greater). During processing, gases can be introduced into the process chamber 10 via the gas supply line 45 of the gas delivery system 97 and the process pressure can be adjusted by the APC 82. The trap 84 can collect unreacted precursor material and by-products from the process chamber 10.
The process monitoring system 92 comprises a sensor 75 capable of real-time process monitoring and can, for example, include a mass spectrometer (MS), a Fourier Transform Infrared (FTIR) spectrometer, or a particle counter. A controller 90 includes a microprocessor, a memory, and a digital I/O port capable of generating control voltages sufficient to communicate and activate inputs to the processing system 1 as well as monitor outputs from the processing system 1. Moreover, the controller 90 is coupled to, and can exchange information with, gas delivery system 97, motor 28, process monitoring system 92, heaters 20, 15, 65, and 70, and vacuum pumping system 88. The controller 90 may be implemented as a DELL PRECISION WORKSTATION 610™. The controller 90 may also be implemented as a general purpose computer, processor, digital signal processor, etc., which causes a substrate processing apparatus to perform a portion or all of the processing steps of the invention in response to the controller 90 executing one or more sequences of one or more instructions stored in a computer readable medium. The computer readable medium or memory is configured to hold instructions programmed according to the teachings of the invention and to store data structures, tables, records, or other data described herein. Examples of computer readable media are compact discs, hard disks, floppy disks, tape, magneto-optical disks, PROMs (EPROM, EEPROM, flash EPROM), DRAM, SRAM, SDRAM, or any other magnetic medium, compact discs (e.g., CD-ROM), or any other optical medium, punch cards, paper tape, or other physical medium with patterns of holes, a carrier wave (described below), or any other medium from which a computer can read.
The controller 90 may be locally located relative to the processing system 1, or it may be remotely located relative to the processing system 1 via the internet or an intranet. Thus, the controller 90 can exchange data with the processing system 1 using at least one of a direct connection, an intranet, and the internet. The controller 90 may be coupled to an intranet at a customer site (i.e., a device maker, etc.), or coupled to an intranet at a vendor site (i.e., an equipment manufacturer). Furthermore, another computer (i.e., controller, server, etc.) can access controller 90 to exchange data via at least one of a direct connection, an intranet, and the internet.
It is to be understood that the batch processing system 1 depicted in
The inventors of the present invention have realized that traditional high-temperature hydrogen anneals of above 900° C. that are performed to remove an oxide layer and other impurities from a substrate prior to processing do not meet current or future thermal budget needs. Further, plasma assisted processes, while performed at lower temperatures, can result in damage to a substrate. Thus, lower-temperature non-plasma processes for removing an oxide layer are needed that are compatible with advanced device processing requiring a low thermal budget. Based on this recognition, the present inventors studied the use of fluorine containing gas in a low-temper non-plasma etch process. Fluorine containing gasses are known to have aggressive etch characteristics that were thought to be suitable only for aggressive etch processes. For example, NF3 and CLF3 have been used in chamber cleaning processes wherein deposited materials are removed from interior surfaces of the chamber. Further, U.S. Pat. No. 6,194,327 discloses use of a fluorine gas for rapid thermal etching of thick sacrificial oxide layers. However, this process is performed at temperatures of 800-1200° C., which as with annealing discussed above, is too high for current and future thermal budget requirements. Moreover, the present inventors have recognized that this process is too aggressive, and can result in etching of the underlying substrate or in a rough substrate surface unsuitable for subsequent film deposition.
In studying the use if fluorine gas for etching oxides, the present inventors have discovered that F2 gas applied at substrate temperature of less than about 400° C. can provide acceptable etching characteristics for thin native oxides that can be 10 angstroms or less in thickness, and are typically as thin as 2-3 angstroms. The upper limit originates from the fact that the underlying Si substrate becomes rough if the substrate temperature is too high. The inventors have seen this effect, even at about 300° C. However, roughening effects at this temperature may be acceptable for some applications. This effect is (in part) due to faster etching rate of the underlying Si substrate compared to the overlying native oxide layer. This faster etching of Si vs oxide has been observed when etching oxide layers formed on a Si substrate in a deposition process from TEOS gas. At 200° C. the (poly) Si etch rates were about 2.5 angstrom/min and the oxide etch rates were about 0.07 angstrom/min. At 300° C. the (poly) Si etch rates were about 8 angstrom/min and the oxide etch rates were about 0.1 angstrom/min.
Embodiments of the invention further provide formation of a film on the substrate following removal of the oxide layer. The oxide layer is removed at first substrate temperature and the substrate can be heated to a second substrate temperature, greater than the first substrate temperature, at which a film can be formed on the substrate by exposing the substrate to a deposition gas. Thus, the oxide removal step according to embodiments of the invention is not restricted based on processing temperatures of other process steps.
In one embodiment, the film is formed on the substrate at a second substrate temperature greater than the first substrate temperature to achieve deposition rates that are high enough for device manufacturing and to ensure that the deposited film has the desired material properties. The desired material properties can, for example, include a crystal structure (i.e., epitaxial, polycrystalline, or amorphous), and elemental composition. Furthermore, the second substrate temperature can be selected to provide selective film deposition on exposed silicon-containing surfaces of the substrate, or non-selective (blanket) film deposition on the whole substrate. In order to prevent formation of a new oxide layer on the substrate, the silicon-containing film can be formed on the substrate following the oxide removal step without exposing the substrate to ambient air or other oxygen-containing ambients.
After providing a substrate in the process chamber in 210, the substrate is heated to a first substrate temperature T1 during time period t1 as shown in
At 212, during time period t2, the substrate is exposed to a flow of an etching gas containing F2 gas at the first substrate temperature T1 lower than about 400° C. to remove the oxide layer from the substrate. The first substrate temperature T1 can be selected in consideration of the overall thermal budget and/or to allow efficient removal of the oxide layer from the substrate while minimizing damage such as etching of the substrate material or other materials formed on the substrate material. For example, where a thermal budget allows for higher temperatures and substrate etching is of little concern, then T1 may approach about 400° C. However, with lower thermal budgets or damage is a concern, the substrate temperature can be much lower, although the etch rate may also be lowered. Exemplary run times for the F2 native oxide removal stop are about 15 minutes at 300° C., or 90 minutes at 200° C. According to an embodiment of the invention, the first substrate temperature can be less than about 400° C. and greater than about 20° C., or between about 100° C. and about 300° C. or between 200° C. and 300° C. According to an embodiment of the invention, the first substrate temperature can be about 200° C.
According to an embodiment of the invention, the processing conditions for the oxide removal can include a gas pressure between about 0.1 Torr and about 100 Torr in the process chamber. Alternatively, the gas pressure can be between about 1 Torr and about 10 Torr in the process chamber. The etching gas for the oxide removal step contains F2 gas and can further contain an inert gas. The inert gas can, for example, contain N2, argon (Ar), helium (He), neon (Ne), krypton (Kr), or xenon (Xe), or a combination of two or more thereof. According to one embodiment of the invention, the etching gas contains F2 gas, an inert gas, and a reducing gas. The reducing gas can, for example, contain H2, H, or NH3, or other hydrogen-containing gases. The reducing gas can aid in the decomposition of F2 on the substrate at low substrate temperature. According to an embodiment of the invention, the reducing gas can be plasma-excited in a remote plasma source. A gas flow rate between about 0.010 standard liters per minute (slm) and about 20 slm can be used for the etching gas. As those skilled in the art will readily appreciate, the inert gas can be used to control the concentration of F2 in the etching gas.
Generally, the F2 etchant gas is diluted to 20% F2 and 80% N2 from a gas source. However, the mixture may be diluted to provide from 3-20% F2, with the remaining gas N2. Exemplary compositions and flows of the etching gas include 8.8 slm N2 and 0.2 slm F2 (8 slm N2+1 slm of 20% F2 in N2), and 8.6 slm N2 and 0.4 slm F2 (7 slm N2+2 slm of 20% F2 in N2), but embodiments of the invention are not limited to those compositions and gas flows.
While not required to practice the present invention in one embodiment, following removal of the oxide layer from the substrate, at 214, the substrate is heated during time period t3 from the first substrate temperature T1 to a second substrate temperature T2 greater than the first substrate temperature T1. The process chamber may be evacuated prior to or during the time period t3 to minimize etch damage to the substrate. Time period t3 is a transition step and may be variable in length depending on system design and processing temperature differences between the oxide removal step at substrate temperature T1 and the substrate temperature T2. The time period t3 can, for example, be between about 5 min and about 45 min, but this is not required in embodiments of the invention. According to an embodiment of the invention, the second substrate temperature T2 can be between about 100° C. and about 900° C. According to another embodiment of the invention, the second substrate temperature T2 can be between about 650° C. and about 750° C. According to yet another embodiment of the invention, the second substrate temperature T2 can be between about 750° C. and about 850° C.
At 216, a film is formed on the substrate at the second substrate temperature T2. The film is formed on the substrate following the removal of the oxide layer without exposing the substrate to ambient air that can form an oxide layer on the substrate. According to an embodiment of the invention, the film can be a silicon-containing film that is formed on the substrate by exposing the substrate to a gas containing a silicon-containing gas, for example, SiH4, SiCl4, Si2H6, SiH2Cl2, or Si2Cl6, or a combination of two or more thereof. According to an embodiment of the invention, the silicon-containing gas can further contain a germanium-containing gas, including, for example, GeH4, GeCl4, or a combination thereof, for depositing a SiGe film on the substrate. As described above, the silicon-containing film can be formed by providing a silicon-containing gas from a non-plasma gas source such as the gas source 94 in
When a film with a desired thickness has been formed on the substrate, flow of the deposition gas is stopped, the substrate is allowed to cool down during time period t5, and the substrate is subsequently removed from the process chamber. Like time periods t1 and t3, the time period t5 is a transition step and may be variable in length. Time period t5 can, for example, be between about 2 min and about 15 min, but this is not required in embodiments of the invention.
Although not shown in
As would be appreciated by those skilled in the art, each of the steps or stages in the flowchart of
Comparison of the depth profiles shown in
The selective deposition of the epitaxial silicon-containing film 550 allows for subsequent removal of the patterned film 520 from the Si substrate 510 using methods known to those skilled in the art, to form a raised epitaxial silicon-containing film 550 on the silicon substrate 510. In general, the patterned film 520 can include at least one of an oxide mask (e.g., SiO2) and a nitride mask (e.g., Si3N4). The use of selective deposition of epitaxial silicon-containing films can be used for manufacturing silicon-on-insulator (SOI) devices with a raised source and drain regions. During SOI device fabrication, processing may consume an entire Si film in source and drain regions, thereby requiring extra Si in these regions that can be provided by selective epitaxial growth (SEG) of silicon-containing films. Selective epitaxial deposition of silicon-containing films can reduce the number of photolithography and etch steps that are needed, which can reduce the overall cost and complexity involved in manufacturing a device.
Although only certain embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.
The present invention is related to U.S. patent application Ser. No. 10/673,375, titled “DEPOSITION OF SILICON-CONTAINING FILMS FROM HEXACHLORODISILANE,” U.S. patent application Ser. No. 11/094,462, titled “A METHOD AND SYSTEM FOR REMOVING AN OXIDE FROM A SURFACE,” and U.S. patent application Ser. No. 10/647,534, titled “MULTIPLE GROW-ETCH CYCLIC SURFACE TREATMENT FOR SUBSTRATE PREPARATION,” the entire contents of all of which are hereby incorporated by reference.
