Patent Application
20060251828
The present invention relates to a plasma-assisted deposition method of depositing an insulating film, such as a fluorine-containing carbon film (fluorocarbon film), using a plasma and to a plasma-assisted deposition system.
A multilevel wiring structure is one of methods of raising the level of integration of semiconductor devices. In -a multilevel wiring structure, wiring layers on the nth and the (n+1)th layer are interconnected by conductors embedded in a thin film called a interlayer insulation film. A silicon dioxide film is a representative interlayer insulation film. The interlayer insulation film must have a small relative dielectric constant to increase the operating speed of the device.
Such circumstances take a great interest in the fluorine-containing carbon film. The relative dielectric constant of the fluorine-containing film is far smaller than that of the silicon dioxide film. A fluorine-containing film deposition method is disclosed in JP 11-162960 A (Patent document 1). According to Paragraphs 0016 to 0018 of the specification of JP-A 11-162960, cyclic C5F8 gas is used as a source gas, electron cyclotron resonance (ECR) is caused by the interaction of a 2.45 GHz microwave and a magnetic field of 875 G to produce a plasma of a plasma forming gas, such as Ar gas, ionizes C5F8 gas by the plasma to produce a plasma so that a fluorine-containing carbon film is deposited on a semiconductor wafer (hereinafter, referred to simply as “wafer”).
C4F8 gas is a known source gas. However, a film of three-dimensional structure as shown in
If the process pressure of a plasma-assisted deposition process is high, a reaction occurs in a gas phase, particles of a product produced by the reaction adhere to the wafer. It is feared that the particles adhering to the wafer fly in all directions while the wafer is being carried. The density of electrons in the plasma is low when the process pressure is high. Consequently, deposition rate decreases, the throughput of the process decreases to a level unsuitable for a mass production process. The process pressure, namely, the pressure of a processing atmosphere, must be lower than 100 Pa during the deposition process.
When the process pressure is low, the electron temperature of the plasma is high and the source gas is decomposed excessively. Consequently, small molecules are formed and a film of a composition greatly different from a desired composition is deposited, so that the composition and structure of the material cannot be effectively utilized. For example, when C5F8 gas is used as a source gas, small particles of a decomposition product deposit on the wafer in an amorphous film. Such an amorphous film does not have a small relative dielectric constant, and has electric characteristics permitting the flow of large leakage current, inferior mechanical characteristics, such as low strength and low stress resistance, and inferior chemical characteristics, such as low water resistance. For example, the advantage of C5F8 gas is its capability to form a three-dimensional chain structure of CF2, namely, a fluorine-containing carbon film having a small relative dielectric constant and excellent mechanical characteristics. However, the best use of such an advantage of C5F8 gas has not been made until today. If the process pressure is raised, the electron temperature of the plasma decreases and the excessive decomposition of the source gas can be avoided. However, in this case, problems owing to particles and low deposition rate arises. Thus those parameters are incompatible with each other. The incompatibility of those parameters is one of factors that obstruct the formation of excellent fluorine-containing carbon films.
It is an object of the present invention to provide a method and a system capable of forming an insulating film having intrinsic molecular structure of a material by using a source gas plasma (a source gas activated by a plasma). Another object of the present invention to provide a film deposition method and a film deposition system capable of forming an insulating film having a low dielectric constant and excellent electric characteristics.
A plasma-assisted deposition method in a first aspect of the present invention for forming an insulating film on a substrate placed on a support device in an airtight processing vessel by activating a source gas by a plasma characterized in that electron temperature in a plasma producing space extending between a source gas supply opening and a surface of the substrate in terms of mean square velocity is 3 eV or below and electron density in the plasma producing space is 5×1011 electrons per cubic centimeter or above.
Since the electron temperature is 3 eV or below, the excessive decomposition of the source gas can be suppressed, and an insulating film having a molecular structure effectively utilizing the characteristics of the source gas, such as an insulating film having a small relative dielectric constant and excellent electric characteristics, can be deposited.
In the plasma-assisted deposition method in the first aspect of the present invention, a microwave is guided to a flat antenna member disposed opposite to the support device by a waveguide, and the microwave is radiated from a plurality of slots formed in a circumferential arrangement in the flat antenna member to activate the source gas by the energy of the microwave.
In the plasma-assisted deposition method in the first aspect of the present invention, the slots have a length between half the wavelength of the microwave at the side of the waveguide with respect to a part of the flat antenna member and half the wavelength of the microwave at the side of the plasma producing space with respect to a part of the flat antenna member.
In the plasma-assisted deposition method in the first aspect of the present invention, the plurality of slots are arranged on concentric circles having their centers at the center of the flat antenna member or on a spiral around the center of the flat antenna member.
In the plasma-assisted deposition method in the first aspect of the present invention, the microwave radiated from the flat antenna member is a circularly polarized wave or a linearly polarized wave.
In the plasma-assisted deposition method in the first aspect of the present invention, the pressure of a processing atmosphere is 19.95 Pa or below.
When the pressure in the processing vessel is 19.95 Pa (150 mTorr) or below, an insulating film having a small relative dielectric constant and excellent electric characteristics can be deposited by selectively determining process conditions.
In the plasma-assisted deposition method in the first aspect of the present invention, the insulating film to be deposited on the substrate is a fluorine-containing carbon film.
In the plasma-assisted deposition method in the first aspect of the present invention, the source gas is C5F8 gas.
A plasma-assisted deposition system in a second aspect of the present invention comprising: an airtight processing vessel internally provided with a support device for supporting a substrate thereon; a source gas supply system for supplying a source gas for forming an insulating film on the substrate into the processing vessel; a microwave generator for generating a microwave for activating the source gas to produce a plasma; a waveguide for guiding the microwave generated by the microwave generator into the processing vessel; and a flat antenna member provided with a plurality of slots formed therein in a circumferential arrangement; characterized in that electron temperature in a plasma producing space extending between a source gas supply opening and a surface of the substrate in terms of mean square velocity is 3 eV or below and an electron density in the plasma producing space is 5×1011 electrons per cubic centimeter or above.
In the plasma-assisted deposition system in the second aspect of the present invention, the slots have a length between half the wavelength of the microwave at the side of the waveguide with respect to a part of the flat antenna member and half the wavelength of the microwave at the side of the plasma producing space with respect to a part of the flat antenna member.
In the plasma-assisted deposition system in the second aspect of the present invention, the plurality of slots are arranged on concentric circles having their centers at the center of the flat antenna member or on a spiral around the center of the flat antenna member.
In the plasma-assisted deposition system in the second aspect of the present invention, the microwave radiated from the flat antenna member is a circularly polarized wave or a linearly polarized wave.
In the plasma-assisted deposition system in the second aspect of the present invention, the insulating film to be deposited on the substrate is a fluorine-containing carbon film.
In the plasma-assisted deposition system in the second aspect of the present invention, the source gas is C5F8 gas.
A plasma-assisted deposition system in a preferred embodiment according to the present invention will be described with reference to FIGS. 1 to 7. Referring to
A support table 11 for supporting a substrate, such as a wafer W is disposed in a substantially central part of the bottom wall of the processing vessel 1. An insulating sheet 11a is interposed between the support table 11 and the bottom wall of the processing vessel 1. The support table 11 is made of, for example, aluminum nitride (AIN) or aluminum oxide (Al2O3). The support table 11 is provided with a coolant passage 11b through which a coolant flows. A heater, not shown, for heating the support table 11 is embedded in the support table 11. An electrostatic chuck is formed on the upper surface of the support table 11. A high-frequency power source 12 for generating a bias high-frequency power of, for example, 13.56 MHz is connected to the support table 11. A bias high-frequency power is applied to charge the surface of the support table at a negative potential so that ions of a plasma can be attracted to the support table in high perpendicularity to the surface of the support table 11.
The processing vessel 1 has an open upper end. A first gas discharge head 2 substantially circular in a plane is placed in the open upper end of the processing vessel 1 opposite to the support table 11. A sealing member, not shown, such as an O ring, is placed between the upper end of the processing vessel 1 and the first gas discharge head 2. The first gas discharge head 2 is made of, for example, Al2O3 and is provided many first gas discharge holes 21 opening in a surface thereof facing the support table 11. A gas passage 22 is formed in the gas discharge head 2 so as to connect to the first gas discharge holes 21. A first gas supply line 23 has one end connected to the gas passage 22 and the other end connected to a plasma gas source 24 for supplying a plasma gas, such as Ar gas or Kr gas and a hydrogen gas source 25 for supplying H2 gas. The first gas supply line 23 carries the gases into the gas passage 22. The gases are discharged through the gas discharge holes 21 and are distributed uniformly in a space extending under the first gas discharge head 2. In this embodiment, the distance between the lower surface of the first gas discharge head 2 and a surface of a wafer W supported on the support table 11 is 50 mm.
A second gas discharge head 3 is disposed between the support table 11 and the first gas discharge head 2 in the processing vessel 1 so as to divide a space between the support table 11 and the first gas discharge head 2 into upper and lower spaces. The second gas discharge head 3 has, for example, a flat, substantially circular shape and is made of a conducting material, such as a Mg-containing Al alloy or an Al-containing stainless steel. The second gas discharge head 3 is provided in its surface facing the support table 11 with many second gas discharge holes 31. Gas passages 32 are formed, for example, in a grid inside the second gas discharge head 3 as shown in
The second gas supply line 33 connects the second gas discharge head 3 to a source gas source 35 that supplies a source gas containing fluorine and carbon, such as C5F8. The second gas supply line 33 carries the source gas into the gas passages 32. The source gas is discharged through the gas discharge holes 31 into the lower space under the second gas discharge head 3 to distribute the source gas uniformly in the lower space. In
A cover plate 13 made of a dielectric material, such as Al2O3, is put on the first gas discharge head 2 with a sealing ember, not shown, such as an O ring, held between the cover plate 13 and the first gas discharge head 2. An antenna 4 is placed in close contact with the upper surface of the cover plate 13. As shown in
A lagging plate 43 made of a low-loss dielectric material, such as Al2O3, SiO2 or Si3N4 is sandwiched between the antenna body 41 and the flat antenna member 42. The lagging plate 43 shortens the wavelength of a microwave to shorten the wavelength in the waveguide. In this embodiment, the antenna body 41, the flat antenna member 42 and the lagging plate 43 constitute a radial line slot antenna.
The antenna 4 is placed with the flat antenna member 42 in close contact with the cover plate 13 with a sealing member, not shown, held between the cover plate 13 and the flat antenna member 42. The antenna 4 is connected to an external microwave generator 45 by a coaxial waveguide 44. A microwave of 2.45 GHz or 8.3 GHz is transmitted to the antenna 4. The coaxial waveguide 44 has an outer waveguide 44A and a central conductor 44B. The outer waveguide 44A is connected to the antenna body 41 and the central conductor 44B is passed through a through hole formed in the lagging plate 43 and is connected to the flat antenna member 42.
The flat antenna member 42 is a copper plate of a thickness, for example, on the order of 1 mm. As shown in
The T-shaped slot sets each consisting of the slots 46a and 46b are arranged at intervals corresponding to the wavelength of the microwave compressed by the lagging plate 43 on concentric circles having their centers at the center of the flat antenna member 42 or a spiral around the center of the flat antenna member 42. Therefore, the flat antenna member 42 radiates a substantially plane microwave. The slots 46a and 46b are formed in a length L1 between half the wavelength of the microwave on the side of the waveguide 44 with respect to the flat antenna 42 and half the wavelength of the microwave on the side of a plasma producing space with respect to the flat antenna 42; that is the length L1 of the slots 46a and 46b is not longer than half the wavelength of the microwave on the side of the coaxial waveguide 44 with respect to the flat antenna 42 and longer than the wavelength of the microwave on the side of the plasma producing space, i.e., a space in the processing vessel 2, with respect to the antenna member 42. Thus the microwave is able to propagate through the slots 46 into the plasma producing space and unable to propagate back from the plasma producing space into the coaxial waveguide 44. When the slots 46 are arranged on concentric circles, the distance L2 between the slot 46 on an inner circle and the slot 46 on an outer circle adjacent to the inner circle is, for example, equal to half the wavelength of the microwave on the side of the coaxial waveguide 44.
An exhaust pipe 14 provided with a pressure regulator 51 has one end connected to the bottom wall of the processing vessel 1 and the other end connected to a vacuum pump, namely, an evacuating means. The vacuum pump can evacuate the processing vessel 1 at a predetermined vacuum.
A film deposition method to be carried out by this plasma-assisted deposition system will be described by way of example. A wafer W, namely, a substrate, provided with aluminum wiring lines on a surface thereof is carried through the gate valve into the processing vessel 1. The wafer W is placed on the support table 11. Then, the processing vessel 1 is evacuated to a predetermined vacuum. Subsequently, a plasma gas, such as Ar gas, is supplied to the first gas discharge head 2 through the first gas supply line 23 at a flow rate of, for example, 300 sccm, and a source gas, such as C5F8 gas, is supplied to the second gas discharge head 3 through the second gas supply line 33 at a predetermined flow rate of, for example, d150 sccm. The interior of the processing vessel 1 is kept at a precess pressure of, for example, 13.3 Pa and the support table 11 is heated to a surface temperature of 350° C.
The microwave generator generates a high-frequency wave, namely, a microwave, of 2.45 GHz and 2000 W. The microwave propagates in the coaxial waveguide 44 in the TM, the TE or the TEM mode to the flat antenna member 42 of the antenna 4. Then, the microwave propagates through the central conductor 44B to a central part of the flat antenna member 42 and propagates radially from the central part of the flat antenna member 42 toward the circumference of the flat antenna member 42. While the microwave thus propagates radially, the microwave radiates from the slots 46a and 46b, propagates through the cover plate 13 and the first gas discharge head 2. Then the microwave is radiated into the processing space under the first gas discharge head 2. The cover plate 13 and the first gas discharge head 2 made of a material capable of transmitting the microwave, such as Al2O3 serve as a microwave transmission window. The microwave penetrates the cover plate 13 and the first gas discharge head 2 efficiently.
Since the slots 46a and 46b are arranged in the arrangement mentioned above, the circularly polarized wave is radiated uniformly from the flat antenna member 42 and creates an electric field having a uniform electric-field distribution. A uniform, high-density plasma is produced over the entire processing space by the energy of the microwave. The plasma flows through the through holes 34 of the second gas discharge head 3 into the processing space under the second gas discharge head 3. The plasma activates the C5F8 gas discharged from the second gas discharge head 3 to produce active species. The C5F8 gas plasma (the C5F8 gas activated by the plasma) has a low electron temperature of, for example, 1.2 eV. Sine the process pressure is low, the plasma has an electron density on the order of 1012 electrons per cubic centimeter.
Electron temperature is defined in terms of mean square velocity. Any measuring means may be used for measuring electron temperature. A measuring point for measuring electron temperature is in a space between the gas discharge hole 31 of the second gas discharge head 3 and the wafer W and is not in spaces near the wall of the processing vessel 1 and below the circumference of the support table 11. Electron temperature is defined in terms of mean square velocity on an assumption that the relation between electron temperature and the number of electrons can be represented by a Maxwell-Boltzmann distribution as shown in
Although the reason a plasma having a high electron density and a low electron temperature together is not clearly grasped, it is inferred that electrons follow up the electric field satisfactorily when the source gas is activated to produce a plasma by the microwave radiated through the circumferential slots 46 of the flat antenna member 42. If the performance of electrons to follow up the electric field is unsatisfactory, the field intensity of the electric field needs to be increased. However, it is inferred that the source gas can be activated without increasing the field intensity of the electric field and only few electrons come off the electric field, collide against the wall of the processing vessel 1 and disappear when electrons follow up the electric field satisfactorily.
Active species reached the surface of the wafer W deposit in a CF film on the wafer W. The sputter etching effect of Ar ions attracted to the wafer W by the bias voltage for attracting the ions of the plasma etches off a CF film deposited on corners of lines of a pattern formed on the surface of the wafer W expands openings and deposits CF films in recesses in the pattern to fill up the recesses with the CF film. The wafer W coated with the CF film is carried out of the processing vessel through the gate valve, not shown.
The plasma-assisted deposition system in this embodiment forms the insulating film by plasma having the high electron density and a low electron temperature. The CF film thus formed has satisfactory electric characteristics including a small relative dielectric constant and capability of suppressing leakage current. It is inferred that such a CF film having satisfactory electric characteristics can be formed because all the cyclic bonds of C5F8 gas, namely, a source gas, are not broken by excessive decomposed, only some of the cyclic bonds of C5F8 gas are broken, and the broken C5F8 molecules link together to form a three-dimensional structure of long CF chains. Thus the CF film has molecular bonds intrinsic to the composition of the source gas. The CF film has excellent mechanical characteristics including high strength and high stress resistance, and excellent chemical characteristics including high water resistance. Since the plasma keeps a high electron density, the deposition rate will not decrease a level unsuitable for a mass production process.
A source gas other than C5F8 gas may be used for forming the CF film. Possible source gases are, for example, C3F6 gas, C4F6 gas and C4F8 gas. The insulating film to be formed by the present invention is not limited to the CF film; the insulating film may be a SIOF film, namely, a film of a compound of silicon, oxygen and fluorine. Those insulating films, other than the CF film, having excellent electric characteristics and molecular bonds intrinsic to the composition of the source gas can be formed.
The slots 46a and 46b of the flat antenna member 42 do not necessarily need to be combined in the substantially T-shaped slot sets. The flat antenna member 42 may be provided with slots formed and arranged so as to radiate a linearly polarized microwave instead the circularly polarized microwave. The present invention may be embodied in ECR plasma deposition systems, parallel-plate plasma deposition systems and inductively coupled plasma deposition systems capable of producing a plasma having an electron density of 5×1011 electrons per cubic centimeter or above and an electron temperature of 3 eV or below for depositing an insulating film.
CF films were formed under various process conditions by the plasma-assisted deposition system shown in
In
Electron temperature was preliminarily measured in a space between the source gas discharge holes and the wafer with a Langmuir probe. Electron temperature defined by mean square velocity was in the range of 1.1 to 2.0 eV. Since the process pressure is in the low process pressure range of 6.65 to 19.95 Pa (50 to 150 mTorr), electron density was on the order of 1012 electrons per cubic centimeter. Electron density was confirmed by measurement using a Langmuir probe. As a Method of measuring electron temperature and electron density during a film is deposited under certain process conditions using C3F6 gas and the like as a source gas, Ar gas plasma (Ar gas activated by plasma) under the same process conditions was used instead of the source gas plasma and measured the electron temperature and the electron density of the plasma with a Langmuir probe. Thus, Ar gas was used to determine electron temperature and electron density because there is the possibility that a CF gas corrodes the Langmuir probe. Values of electron temperature and electron density are substantially the same for different gases and hence there is no problem in the method of evaluation.
Process conditions for forming the CF films respectively having relative dielectric constants in the range of 1.9 to 2.1 represented by the data indicated by three blank rhombuses will be described. The process pressure was 13.3 Pa (100 mTorr), the high-frequency power is 2000 W, the flow rate of Ar gas is 300 sccm, the flow rate of the source gas is 100 sccm, the distance between the wafer W and the lower surface of the flat antenna member is 50 mm and electron temperature is 1.1 eV. Data shown herein is the mean of measured data on three parts of the wafer.
Blank circles indicate data on CF films formed by the ECR plasma deposition system mentioned in Patent document 1 using C5F8 gas and a plasma having an electron temperature in the range of 5 to 6 eV.
An excellent interlayer insulation film must have a small relative dielectric constant and must be permitting only a low leakage current. The inventors of the present invention intend to provide interlayer insulation films having a relative dielectric constant of 2.2 or below and permitting a leakage current of 10−8 A/cm2 or below. As obvious from
It is clearly known from this data that data on CF films formed at an electron temperature between 2 and 3 eV, as compared with data on CF films formed at an electron temperature of 5 eV, lie in lower left-hand region of the graph shown in
Data on CF films formed by using plasmas having an electron density lower than 5×1011 electrons per cubic centimeter was not obtained. When the conventional ECR plasma deposition system is used in an ordinary manner, electron temperature can be decreased by increasing pressure, but electron density decreases when pressure is increased. The present invention specifies an electron density to discriminate plasma-assisted deposition method of the present invention from such a plasma-assisted deposition method. It is known experientially that a film can be deposited at a sufficiently high deposition rate when the electron density is 5×1011 electrons per cubic centimeter or above with the dissociation of the source gas. The present invention is premised on this condition and carries out a deposition process using a low electron temperature.
CF films were formed under various process conditions by the plasma-assisted deposition system shown in
As obvious from
It is known through the examination of the experimental results shown in
The dependence of leakage current and relative dielectric constant on process conditions will be described. There is a tendency that leakage current decreases as the temperature of the wafer is decreased, and that relative dielectric constant decreases as the distance between the second gas discharge head 3 and the wafer is shortened. To deposit CF films having properties represented by data distributed in a lower left-hand region of the graph shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-083292 | Mar 2003 | JP | national |
| 2004-076958 | Mar 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP04/04070 | 3/24/2004 | WO | 6/6/2006 |
