Insulation film on semiconductor substrate and method for forming same

[0001] This is a continuation-in-part of U.S. patent application Ser. No. 10/288,641 filed Nov. 5, 2002, which is a continuation-in-part of U.S. patent application Ser. No. 09/827,616 filed Apr. 6, 2001 now U.S. Pat. No. 6,514,880, which is a continuation-in-part of (i) U.S. patent application Ser. No. 09/243,156 filed Feb. 2, 1999, now abandoned, which claims priority to Japanese patent application No. 37929/1998 filed Feb. 5, 1998, (ii) U.S. application Ser. No. 09/326,847 filed Jun. 7, 1999, now U.S. Pat. No. 6,352,945, (iii) U.S. patent application Ser. No. 09/326,848 filed Jun. 7, 1999, now U.S. Pat. No. 6,383,955, and (iv) U.S. patent application Ser. No. 09/691,376 filed Oct. 18, 2000, now U.S. Pat. No. 6,432,846, all of which are incorporated herein by reference in their entirety. This application claims priority to all of the foregoing under 35 U.S.C. §119 and §120.

[0002] 1. Field of the Invention

[0003] This invention relates generally to a semiconductor technique and more particularly to a silicone polymer insulation film on a semiconductor substrate and a method for forming the film by using a plasma CVD (chemical vapor deposition) apparatus.

[0004] 2. Description of the Related Art

[0005] Because of the recent rise in requirements for the large-scale integration of semiconductor devices, a multi-layered wiring technique attracts a great deal of attention. In these multi-layered structures, however, capacitance among individual wires hinders high speed operations. In order to reduce the capacitance it is necessary to reduce relative dielectric constant of the insulation film. Thus, various materials having a relatively low relative dielectric constant have been developed for insulation films.

[0006] Conventional silicon oxide films SiO_x are produced by a method in which oxygen O₂ or nitrogen oxide N₂O is added as an oxidizing agent to a silicon material gas such as SiH₄ or Si(OC₂H₅)₄ and then processed by heat or plasma energy. Its relative dielectric constant is about 4.0.

[0007] Alternatively, a fluorinated amorphous carbon film has been produced from C_xF_yH_z as a material gas by a plasma CVD method. Its relative dielectric constant ε is as low as 2.0-2.4.

[0008] Another method to reduce the relative dielectric constant of insulation film has been made by using the good stability of Si—O bond. A silicon-containing organic film is produced from a material gas under low pressure (1 Torr) by the plasma CVD method. The material gas is made from P-TMOS (phenyl trimethoxysilane, formula 1), which is a compound of benzene and silicon, vaporized by a babbling method. The relative dielectric constant ε of this film is as low as 3.1.

[0009] A further method uses a porous structure made in the film. An insulation film is produced from an inorganic SOG material by a spin-coat method. The relative dielectric constant ε of the film is as low as 2.3.

[0010] However, the above noted approaches have various disadvantages as described below.

[0011] First, the fluorinated amorphous carbon film has lower thermal stability (370° C.), poor adhesion with silicon-containing materials and also lower mechanical strength. The lower thermal stability leads to damage under high temperatures such as over 400° C. Poor adhesion may cause the film to peel off easily. Further, the lower mechanical strength can jeopardize wiring materials.

[0012] Oligomers that are polymerized using P-TMOS molecules do not form a linear structure in the vapor phase, such as a siloxane structure, because the P-TMOS molecule has three O—CH₃ bonds. The oligomers having no linear structure cannot form a porous structure on a Si substrate, i.e., the As a result, the relative dielectric constant of the film cannot be reduced to a desired degree.

[0013] In this regard, the babbling method means a method wherein vapor of a liquid material, which is obtained by having a carrier gas such as argon gas pass through the material, is introduced into a reaction chamber with the carrier gas. This method generally requires a large amount of a carrier gas in order to cause the material gas to flow. As a result, the material gas cannot stay in the reaction chamber for a sufficient length of time to cause polymerization in a vapor phase.

[0014] Further, the SOG insulation film of the spin-coat method has a problem in that the material cannot be applied onto the silicon substrate evenly and another problem in which a cure system after the coating process is costly.

[0015] It is, therefore, a principal object of this invention to provide an improved insulation film and a method for forming it.

[0016] It is another object of this invention to provide an insulation film that has a low relative dielectric constant, high thermal stability, high humidity-resistance and high adhesive strength, and a method for forming it.

[0017] It is a further object of this invention to provide a material for forming an insulation film that has a low relative dielectric constant, high thermal stability, high humidity-resistance and high adhesive strength.

[0018] It is a still further object of this invention to provide a method for easily forming an insulation film that has a low relative dielectric constant without requiring an expensive device.

[0019] One aspect of this invention involves a method for forming an insulation film on a semiconductor substrate by using a plasma CVD apparatus including a reaction chamber, which method comprises a step of directly vaporizing a silicon-containing hydrocarbon compound expressed by the general formula Si_αO_βC_xH_y (α, β, x, and y are integers) and then introducing it to the reaction chamber of the plasma CVD apparatus, a step of introducing an additive gas, the flow volume of which is substantially reduced, into the reaction chamber and also a step of forming an insulation film on a semiconductor substrate by plasma polymerization reaction wherein mixed gases made from the vaporized silicon-containing hydrocarbon compound as a material gas and the additive gas are used as a reaction gas. It is a remarkable feature that the reduction of the additive gas flow also results in a substantial reduction of the total flow of the reaction gas. According to the present invention, a silicone polymer film having a micropore porous structure with low relative dielectric constant can be produced.

[0020] The present invention is also drawn to an insulation film formed on a semiconductor substrate, and a material for forming the insulation film, residing in the features described above.

[0021] For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

[0022] Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.

[0025] In an embodiment of the present invention, a method is provided to form an insulation film on a semiconductor substrate by plasma reaction and comprises the steps of: (i) vaporizing a silicon-containing hydrocarbon compound to provide a source gas; (ii) introducing the source gas into a reaction space for plasma CVD processing wherein a semiconductor substrate is placed; (iii) optionally introducing an additive gas selected from the group consisting of an inert gas and an oxidizing gas, said oxidizing gas being used in an amount less than the source gas, said source gas and said additive gas constituting a reaction gas; and (iv) forming an insulation film on the semiconductor substrate by activating plasma polymerization reaction in the reaction space, wherein the plasma polymerization reaction is activated while controlling the flow of the reaction gas to lengthen a residence time, Rt, of the reaction gas in the reaction space, wherein 100 msec≦Rt,
Rt[s]=9.42×10⁷(Pr·Ts/Ps·Tr)r_w²d/F
wherein Pr: reaction space pressure (Pa); Ps: standard atmospheric pressure (Pa); Tr: average temperature of the reaction gas (K); Ts: standard temperature (K); r_w: radius of the silicon substrate (m); d: space between the silicon substrate and the upper electrode (m); F: total flow volume of the reaction gas (sccm).

[0026] In the present invention, the silicon-containing hydrocarbon compound expressed as the general formula Si_αO_βCxH_y (α, β, x, and y are integers) can be any suitable compounds having structures accomplishing polymerization or oligomerization of the basal structures of the compounds under prolonged residence time conditions. The basal structure includes, but is not limited to, (i) a cyclosiloxan compound which may preferably have the formula Si_nO_nR_2n−m wherein n is an integer of 3-6, m represents the number of a unsaturated bond between Si and C and is an integer of 0-6 (m≦n), and R is C_1-6 saturated or unsaturated hydrocarbon attached to Si, and (ii) a linear siloxan compound which may preferably have the formula Si_αO_α−1R_2α−β+2 wherein α is an integer of 1-3, β is 0, 1, or 2 (β≦α), n is an integer of 1-3, and R is C_1-6 hydrocarbon attached to Si.

[0027] Reactive groups which form oligomers using the basal structures include, but are not limited to, alkoxy group such as —O—CH₃, unsaturated hydrocarbon such as —CH═CH₂, amino group such as —NH₂, and acid radical such as carboxylic radical —COOH and acetoxyl group —OOCCH₃. The reactive group(s) and the basal structure can be included in a single compound or different compounds. That is, as long as the reactive group(s) and the basal structure exist and forms oligomers, compounds can be used singly or in any combination.

[0028] For example, the silicon-containing hydrocarbon can be a mixture of a cyclosiloxan compound (precursor 1) and an unsaturated hydrocarbon-containing compound (precursor 2). By oligomerization of precursor 1 using precursor 2, wherein precursor 1 is linked to each other using vinyl groups of precursor 2, a film comprised of oligomers can be formed as described earlier. This film has a low dielectric constant.

[0029] In the above, any suitable cyclosiloxan compound can be used, but the cyclosiloxan compound may preferably have the formula Si_nO_nR_2n−m wherein n is an integer of 3-6, m represents the number of a unsaturated bond between Si and C and is an integer of 0-6 (m≦n), and R is C_1-6 saturated or unsaturated hydrocarbon attached to Si. The compound has the structure —(SiR_2-m/nO_)n— and may include hexamethylcyclotrisiloxane

[0030] In the above, the unsaturated hydrocarbon-containing compound has at least one vinyl group. Such a unsaturated hydrocarbon may be selected from the group consisting of compounds of the formula R¹_ySi_xR²_2x−y+2 and compounds of the formula C_nH_2(n−m)+2, wherein R¹ is C_1-6 unsaturated hydrocarbon attached to Si, R² is C_1-6 saturated hydrocarbon attached to Si, x is an integer of 1-4, y is an integer of 1-2, n is an integer of 1-6, and m represents the number of unsaturated carbon bonds and is an integer of 1-5 (n≧m). The above unsaturated hydrocarbon includes, but are not limited to, unsaturated hydrocarbon-containing organosilicon such as (CH₃)₂Si(C₂H₃)₂, (C₂H₃)₂SiH₂, (C₆H₅)SiH₃, (C₆H₅)Si(CH₃)₃, (C₆H₅)₂Si(CH₃)₂, and (C₆H₅)₂Si(OCH₃)₂, and unsaturated hydrocarbon compounds such as C₂H₄, C₃H₄, C₃H₆, C₄H₈, C₃H₅(CH₃), and C₃H₄(CH)₂. The flow ratio of precursor 1 (sccm) to precursor 2 (sccm) may be in the range of 0.1 to 10 (including 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 5, and a range of any two of the foregoing such as a range of 0.5 to 2).

[0031] In the above, oligomerization of the cyclosiloxan compound can be performed with the reactive groups in a vaporous phase, thereby depositing an insulation film comprised of oligomers of the cyclosiloxan compound. According to the above embodiments, the insulation film can have a dielectric constant of 2.7 or less, preferably 2.4 or less.

[0032] In another embodiment, the aforesaid basal structure and the reactive group(s) are included in a single compound. For example, in the general formula (Si_nO_nR_2n−m)X_m, wherein X is —O—C_pH_2p+1 wherein p is an integer of 1-4 (preferably 1 or 2) such as —O—CH₃, or —C₂H_2(z-w)+2 wherein z is an integer of 1-4 (preferably 1 or 2), and w represents the number of unsaturated carbon bonds and is an integer of 1-3 (preferably 1) such as —CH═CH₂. These compounds include 1,3,5-trimethyl-1,3,5-trimethoxycyclotrisiloxane

[0033] and 1,3,5-trimethyl-1,3,5-trivinyilycyclotrisiloxane.

[0034] In the above, oligomerization of the cyclosiloxan compound can be performed using their reactive groups in a vaporous phase, thereby depositing an insulation film comprised of oligomers of the cyclosiloxan compound. According to the above embodiments, the insulation film can have a dielectric constant of 2.7 or less, preferably 2.4 or less.

[0035] In still another embodiment of the present invention, the silicon-containing hydrocarbon compound may have, as the basal structure, a linear siloxan compound which may preferably have the formula (Si_αO_α−1R_2α−β+2)X_β or a cyclo-siloxan compound which may preferably have the formula (Si_nO_nR_2n−m)X_m, wherein X is a reactive group including, but not limited to, an amino group such as —NH₂, and an acid radical such as carboxylic radical —COOH and acetoxyl group —OOCCH₃. These compounds includes H₂N—Si(CH₃)₂—NH₂ and CH₃COO—Si(CH₃)₂—OOCCH₃. In the above, if the amino group-containing compound and the acid radical-containing compound are mixed, polymerization can be enhanced by acid-alkali reaction.

[0036] In the above, the reactive groups are attached to the basal structure in a single compound, but the reactive groups and the basal structure can be different compounds wherein a reactive group gas is separately added to a basal structure compound to cause oligomization by acid-alkali reaction. For example, the reactive group gas can be any suitable gas including N such as dimethylamine ((CH₃)₂NH), N,N-dimethylhydrazine ((CH₃)₂NNH₂), ethylazide (C₂H₅N₃), methylamine (CH₃NH₂), and methylhydrazine (CH₃NHNH₂).

[0037] A film formed from a precursor having an amino group and/or an acid radical can suitably be used in formation of a wiring structure. When a device uses a low dielectric constant film, a resist is applied on a low dielectric constant film which is then subjected to patterning and etching. Thereafter, the remaining resist is removed and washed out with a chemical solution. In addition to a resist, a sacrificial film may be applied on the low dielectric film, which may also be removed with a chemical solution simultaneously with the resist. The resist and the sacrificial film are organic, and thus their characteristics are similar to those of a low dielectric film (e.g., k<3) which often contains organic materials. Thus, a chemical solution used for removing a resist and/or a sacrificial film may dissolve the low dielectric film. Accordingly, if a strong chemical solution is used in order to increase the removal of the resist and/or sacrificial film, it is difficult to protect the low dielectric film. Many chemical solutions are alkali or acidic. Thus, by rendering the low dielectric film the opposite characteristic, i.e., acidic against an alkali solution and alkali against an acidic solution, decomposition of the low dielectric film can effectively be prevented and protected. This is an example of effective use of a film formed using a precursor having an amino group and/or acid radical as described above.

[0038] In the above, oligomerization of the cyclosiloxan compound can be performed with the reactive groups in a vaporous phase, thereby depositing an insulation film comprised of oligomers of the cyclosiloxan compound. According to the above embodiments, the insulation film can have a dielectric constant of 2.7 or less, preferably 2.4 or less.

[0039] Additionally, all of the aforesaid compounds and the reactive groups can be used singly or in a combination of at least two of any compounds and/or at least two of any reactive groups.

[0040] Compounds which can be mixed include a compound having at least one Si—O bond, two or less O—C_nH_2n+1 bonds and at least two hydrocarbon radicals bonded with silicon (Si). A preferable silicon-containing hydrocarbon compound has formula:
Si_αO_α−1R_2α−β+2(OC_nH_2n+1)_β
wherein α is an integer of 1-3, β is 0, 1, or 2, n is an integer of 1-3, and R is C_1-6 hydrocarbon attached to Si.

[0041] More specifically, the silicon-containing hydrocarbon compound includes at least one species of the compound expressed by the chemical formula (2) as follows:
wherein R1 and R2 are one of CH₃, C₂H₃, C₂H₅, C₃H₇ and C₆H₅, and n are any integer.

[0042] Except for the species indicated above, the silicon-containing hydrocarbon compound can include at least one species of the compound expressed by the chemical formula (3) as follows:
wherein R1, R2 and R3 are one of CH₃, C₂H₃, C₂H₅, C₃H₇ and C₆H₅, and n is any integer.

[0043] Except for those species indicated above, the silicon-containing hydrocarbon compound can include at least one species of the compound expressed by the chemical formula (4) as follows:
wherein R1, R2, R3 and R4 are one of CH₃, C₂H₃, C₂H₅, C₃H₇ and C₆H₅, and m and n are any integer.

[0044] Further, except for those species indicated above, the silicon-containing hydrocarbon compound can include at least one species of the compound expressed by the chemical formula (5) as follows:
wherein R1, R2, R3, R4, R5 and R6 are one of CH₃, C₂H₃, C₂H₅, C₃H₇and C₆H₅, and the additive gases are argon (Ar), Helium (He) and either nitrogen oxide (N₂O) or oxygen (O₂).

[0045] Furthermore, except for those species indicated above, the silicon-containing hydrocarbon compound can include at least one species of the compound can include at least one species of the compound expressed by the chemical formula (6) as follows:
wherein R1, R2, R3 and R4 are one of CH₃, C₂H₃, C₂H₅, C₃H₇ and C₆H₅, and the additive gases are argon (Ar), Helium (He) and either nitrogen oxide (N₂O) or oxygen (O₂).

[0046] Still further, the material gas can include at least one of said silicon-containing hydrocarbon compounds indicated above.

[0047] In accordance with another aspect of this invention, an insulation film is formed on a substrate and the film is polymerized with plasma energy in a plasma CVD apparatus by using a material gas including a silicon-containing hydrocarbon compound expressed by formula 2.

[0048] Additionally, the insulation film is formed on a substrate and the film is polymerized with plasma energy in a plasma CVD apparatus by using a material gas including a silicon-containing hydrocarbon compound expressed by formula 3.

[0049] Further, the insulation film is formed on a substrate and the film is polymerized with plasma energy in a plasma CVD apparatus by using a material gas including a silicon-containing hydrocarbon compound expressed by formula 4.

[0050] Furthermore, the insulation film is formed on a substrate and the film is polymerized with plasma energy in a plasma CVD apparatus by using a material gas including a silicon-containing hydrocarbon compound expressed by formula 5.

[0051] Still further, the insulation film is formed on a substrate and the film is polymerized with plasma energy in a plasma CVD apparatus by using a material gas including a silicon-containing hydrocarbon compound expressed by formula 6.

[0052] In accordance with a further aspect of this invention, a material for forming an insulation film is supplied in a vapor phase in the vicinity of a substrate and is treated in a plasma CVD apparatus to form the insulation film on the substrate by chemical reaction, and the material is further expressed by formula 2.

[0053] Additionally, a material for forming an insulation film is supplied in a vapor phase in the vicinity of a substrate and is treated in a plasma CVD apparatus to form the insulation film on the substrate by chemical reaction, and the material is further expressed by formula 3.

[0054] Further, a material for forming an insulation film is supplied in a vapor phase in the vicinity of a substrate and is treated in a plasma CVD apparatus to form the insulation film on the substrate by chemical reaction, and the material is further expressed by formula 4.

[0055] Furthermore, a material for forming an insulation film is supplied in a vapor phase with either nitrogen oxide (N₂O) or oxygen (O₂) as an oxidizing agent in the vicinity of a substrate and is treated i on said substrate by chemical reaction, and this material can be the compound expressed by formula 5.

[0056] Still further, a material for forming an insulation film is supplied in a vapor phase with either nitrogen oxide (N₂O) or oxygen (O₂) as the oxidizing agent in the vicinity of a substrate and is treated on said substrate by chemical reaction, and this material further can be the compound expressed by formula 6.

[0057] The residence time of the reaction gas is determined based on the capacity of the reaction chamber for reaction, the pressure adapted for reaction, and the total flow of the reaction gas. The reaction pressure is normally in the range of 1-10 Torr, preferably 3-7 Torr, so as to maintain stable plasma. This reaction pressure is relatively high in order to lengthen the residence time of the reaction gas. The total flow of the reaction gas is important to reducing the relative dielectric constant of a resulting film. It is not necessary to control the ratio of the material gas to the additive gas. In general, the longer the residence time, the lower the relative dielectric constant becomes. The material gas flow necessary for forming a film depends on the desired deposition rate and the area of a substrate on which a film is formed. For example, in order to form a film on a substrate [r(radius)=100 mm] at a deposition rate of 300 nm/min, at least 50 sccm of the material gas is expected to be included in the reaction gas. That is approximately 1.6×10² sccm: per the surface area of the substrate (m²). The total flow can be defined by residence time (Rt). When Rt is defined described below, a preferred range of Rt is 100 msec≦Rt, more preferably 200 msec≦Rt≦5 sec. In a conventional plasma TEOS, Rt is generally in the range of 10-30 msec.
Rt[s]=9.42×10⁷(Pr·Ts/Ps·Tr)r_w²d/F

• • [0058] wherein:
  • [0059] Pr: reaction chamber pressure (Pa)
  • [0060] Ps: standard atmospheric pressure (Pa)
  • [0061] Tr: average temperature of the reaction gas (K)
  • [0062] Ts: standard temperature (K)
  • [0063] r_w: radius of the silicon substrate (m)
  • [0064] d: space between the silicon substrate and the upper electrode (m)
  • [0065] F: total flow volume of the reaction gas (sccm)

[0066] In the above, the residence time means the average period of time in which gas molecules stay in the reaction chamber. The residence time (Rt) can be calculated at Rt=αV/S, wherein V is the capacity of the chamber (cc), S is the volume of the reaction gas (cc/s), and α is a coefficient determined by the shape of the reaction chamber and the positional relationship between the inlet of gas and the outlet of exhaust. The space for reaction in the reaction chamber is defined by the surface of the substrate (πr²) and the space between the upper electrode and the lower electrode. Considering the gas flow through the space for reaction, α can be estimated as ½. In the above formula, α is ½.

[0067] In this method, the material gas is, in short, a silicon-containing hydrocarbon compound including at least one Si—O bond, at most two O—C_nH_2n+1 bonds and at least two hydrocarbon radicals bonded to the silicon (Si). Also, this material gas is vaporized by a direct vaporization method. The method results in an insulation film having a low relative dielectric constant, high thermal stability and high humidity-resistance.

[0068] More specifically, the source gas vaporized by the direct vaporization method can stay in the plasma for a sufficient length of time. As a result, a linear polymer can be formed so that a linear polymer having the basic structure (formula 7), wherein the “n” is 2 or a greater value, forms in a vapor phase. The polymer is then deposited on the semiconductor substrate and forms an insulation film having a micropore porous structure.
wherein X1 and X2 are O_nC_mH_p wherein n is 0 or 1, m and p are integers including zero.

[0069] The insulation film of this invention has a relatively high stability because its fundamental structure has the Si—O bond having high bonding energy therebetween. Also, its relative dielectric constant is low because it has a micropore porous structure. Further, the fundamental structure (—SiO—)_n has, on both sides, dangling bonds ending with a hydrocarbon radical possessing hydrophobicity, and Furthermore, the bond of a hydrocarbon radical and silicon is generally stable. For instance, both the bond with a methyl radical, i.e., Si—CH₃, and bond with benzene, i.e., Si—C₆H₅, have a dissociation temperature of 500° C. or higher. Since above semiconductor production requires thermal stability to temperatures above 450° C., that property of the film is advantageous for production of semiconductors.

[0070] Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred examples which follows.

[0071] FIG. 1 diagrammatically shows a plasma CVD apparatus usable in this invention. This apparatus comprises a reaction gas-supplying device 12 and a plasma CVD device 1. The reaction gas-supplying device 12 comprises plural lines 13, control valves 8 disposed in the lines 13, and gas inlet ports 14, 15 and 16. A flow controller 7 is connected to the individual control valves 8 for controlling a flow of a material gas of a predetermined volume. A container accommodating liquid reacting material 18 is connected to a vaporizer 17 that directly vaporizes liquid. The plasma CVD device 1 includes a reaction chamber 6, a gas inlet port 5, a susceptor 3 and a heater 2. A circular gas diffusing plate 10 is disposed immediately under the gas inlet port. The gas diffusing plate 10 has a number of fine openings at its bottom face and can inject reaction gas to the semiconductor substrate 4 therefrom. There is an exhaust port 11 at the bottom of the reaction chamber 6. This exhaust port 11 is connected to an outer vacuum pump (not shown) so that the inside of the reaction chamber 6 can be evacuated. The susceptor 3 is placed in parallel with and facing the gas diffusing plate 10. The susceptor 3 holds a semiconductor substrate 4 thereon and heats it with the heater 2. The gas inlet port 5 is insulated from the reaction chamber 6 and connected to an outer high frequency power supply 9. Alternatively, the susceptor 3 can be connected to the power supply 9. Thus, the gas diffusing plate 10 and the susceptor 3 act as a high frequency electrode and generate a plasma reacting field in proximity to the surface of the semiconductor substrate 4.

[0072] A method for forming an insulation film on a semiconductor substrate by using the plasma CVD apparatus of this invention comprises a step of directly vaporizing silicon-containing hydrocarbon compounds expressed by the general formula Si_αO_βC_xH_y (α, β, x, and y are integers) and then introducing it to the reaction chamber 6 of the plasma CVD device 1, a step of introducing an additive gas, whose flow is substantially reduced, into the reaction chamber 6 and also a step of forming an insulation film on a semiconductor substrate by plasma polymerization reaction wherein mixed gases, made from the silicon-containing hydrocarbon compound as a material gas and the additive gas, are used as a reaction gas. It is a remarkable feature that the reduction of the additive gas flow also renders a substantial reduction of the total flow of the reaction gas. This feature will be described in more detail later.

[0073] The additive gases used in this embodiment, more specifically, are argon gas and helium gas. Argon is principally used for stabilizing plasma, while helium is used for improving uniformity of the plasma and also uniformity of thickness of the insulation film.

[0074] In the method described above, the first step of direct vaporization is a method wherein a liquid material, the flow of which is controlled, is instantaneously vaporized at a vaporizer that is preheated. This direct vaporization method requires no carrier gas such as argon to obtain a designated amount of the material gas. This differs greatly with the babbling method. Accordingly, a large amount of argon gas or helium gas is no longer necessary and this reduces the total gas flow of the reaction gas and then lengthens the time in which the material gas stays in the plasma. As a result, sufficient polymerizing reactions occur in the vapor so that a linear polymer can be formed and a film having a micropore porous structure can be obtained.

[0075] In FIG. 1, inert gas supplied through the gas inlet port 14 pushes out the liquid reacting material 18, which is the silicon-containing hydrocarbon compound, to the control valve 8 through the line 13. The control valve 8 controls the flow of the liquid reacting material 18 with the flow controller 7 so that it does not exceed a predetermined volume. The reduced silicon-containing hydrocarbon compound 18 goes to the vaporizer 17 to be vaporized by the direct vaporization method described above. Argon and helium are supplied through the inlet ports 15 and 16, respectively, and the valve 8 controls the flow volume of these gases. The mixture of the material gas and the additive gases, which is a reaction gas, is then supplied to the inlet port 5 of the plasma CVD device 1. The space between the gas diffusing plate 10 and the semiconductor substrate 4, both located inside of the reaction chamber 6 which is already evacuated, is charged with high frequency RF voltages, which are preferably 13.4 MHz and 430 kHz, and the space serves as a plasma field. The susceptor 3 continuously heats the semiconductor substrate 4 with the heater 2 and maintains the substrate 4 at a predetermined temperature that is desirably 350-450° C. The reaction gas supplied through the fine openings of the gas diffusing plate 10 remains in the plasma field in proximity to the surface of the semiconductor substrate 4 for a predetermined time.

[0076] If the residence time is short, a linear polymer cannot be deposited sufficiently so that the film deposited on the substrate does not form a micropore porous structure. Since the residence time is inversely proportional to the flow volume of the reaction gas, a reduction of the flow volume of the reaction gas can lengthen its residence time.

[0077] Extremely reducing the total volume of the reaction gas is effected by reducing the flow volume of the additive gas. As a result, the residence time of the reaction gas can be lengthened so that a linear polymer is deposited sufficiently and subsequently an insulation film having a micropore porous structure can be formed.

[0078] In order to adjust the reaction in the vapor phase, it is effective to add a small amount of an inert gas, an oxidizing agent, or a reducing agent to the reaction chamber. Helium (He) and Argon (Ar) are inert gases and have different first ionization energies of 24.56 eV and 15.76 eV, respectively. Thus, by adding either He or Ar singly or both in combination in predetermined amounts, the reaction of the material gas in the vapor phase can be controlled. Molecules of the reaction gas undergo polymerization in the vapor phase, thereby forming oligomers. The oligomers are expected to have a O:Si ratio of 1:1. However, when the oligomers form a film on the substrate, the oligomers undergo further polymerization, resulting in a higher oxygen ratio. The ratio varies depending on the relative dielectric constant or other characteristics of a film formed on the substrate.

[0079] The use of an oxidizing agent or a reducing agent is determined depending on the target relative dielectric constant (3.30 or less, preferably 3.10 or less, more preferably 2.80 or less) of a silicone polymer film and other characteristics such as stability of dielectric constant and thermal stability. The O:Si ratio in the material gas is also considered to select an oxidizing agent or a reducing agent, as described above. Preferably, if the ratio is lower than 3:2, an oxidizing agent is used, whereas if the ratio is higher than 3:2, a reducing agent is used. Further, an inert gas such as Ar and He is for controlling plasma reaction, but is not indispensable to form a silicone polymer film. The flow of material gas and the flow of additive gas can also vary depending on the plasma CVD apparatus. The appropriate flow can be determined by correlating the relative dielectric constant of the silicone polymer film with the residence time of the reaction gas (composed of the material gas and the additive gas). The longer the residence time, the lower the dielectric constant becomes. A reduction rate of dielectric constant per lengthened residence time is changeable, and after a certain residence time, the reduction rate of dielectric constant significantly increases, i.e., the dielectric constant sharply drops after a certain residence time of the reaction gas. After this dielectric constant dropping range, the reduction of dielectric constant slows down. This is very interesting. In the present invention, by lengthening residence time until reaching the dielectric constant dropping range based on a predetermined correlation between the dielectric constant of the film and the residence time of the reaction gas, it is possible to reduce the relative dielectric constant of the silicone polymer film significantly.

[0080] An experiment was conducted as described below. In this experiment, octamethylcyclotetrasiloxane as a precursor 1 and (CH₃)₂Si(C₂H₃)₂ as a precursor 2 were used as a source gas. An ordinary plasma CVD device (Aurora™, ASM Japan K.K.) was used as an experimental device. The conditions for forming the film are as follows:

• • [0081] OMCTC (octamethylcyclotetrasiloxane): 160 sccm
  • [0082] DVDVS ((CH₃)₂Si(C₂H₃)₂): 80 sccm
  • [0083] He: 100 sccm
  • [0084] O₂: 0 sccm
  • [0085] RF power supply: 1500W (by overlaying 13.4 MHz power and 430 kHz power)
  • [0086] Pr (reaction chamber pressure): 532 Pa
  • [0087] Tr (average temperature of the reaction gas): 673 K
  • [0088] r_w (radius of the silicon substrate): 0.1 m
  • [0089] d (space between the silicon substrate and the upper electrode): 0.024 m
  • [0090] F (total flow volume of the reaction gas): 340 sccm
  • [0091] Ps (standard atmospheric pressure): 1.01×10⁵ Pa
  • [0092] Ts (standard temperature): 273 K
  • [0093] Rt (residence time; Rt[s]=9.42×10⁷(Pr·Ts/Ps·Tr)r_w²d/F): 160 ms

[0094] The dielectric constant of the resultant film was as low as 2.65, and the RI (Refractive index) was 1.61.

[0095] Although this invention has been described in terms of certain examples, other examples apparent to those of ordinary skill in the art are within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims that follow. The present invention includes various embodiments and are not limited to the above examples. The present invention particularly includes, but are not limited to, the following embodiments, and any combination of the forgoing embodiments and the following embodiments can readily be accomplished:

[0096] 1) A method is for forming an insulation film on a semiconductor substrate by plasma reaction and comprises the steps of: (i) vaporizing a silicon-containing hydrocarbon compound to provide a source gas; (ii) introducing the source gas into a reaction space for plasma CVD processing wherein a semiconductor substrate is placed; (iii) optionally introducing an additive gas selected from the group consisting of an inert gas and an oxidizing gas, said oxidizing gas being used in an amount less than the source gas, said source gas and said additive gas constituting a reaction gas; and (iv) forming an insulation film on the semiconductor substrate by activating plasma polymerization reaction in the reaction space, wherein the plasma polymerization reaction is activated while controlling the flow of the reaction gas to lengthen a residence time, Rt, of the reaction gas in the reaction space, wherein 100 msec≦Rt,
Rt[s]=9.42×10⁷(Pr·Ts/Ps·Tr)r_w²d/F
wherein Pr: reaction space pressure (Pa); Ps: standard atmospheric pressure (Pa); Tr: average temperature of the reaction gas (K); Ts: standard temperature (K); r_w: radius of the silicon substrate (m); d: space between the silicon substrate and the upper electrode (m); F: total flow volume of the reaction gas (sccm).

[0097] In the above, the reaction space should not be limited to a physically defined single section, but should include any suitable space for plasma reaction. That is, as one of ordinary skill in the art readily understands, the space is a functionally defined reaction space. The space may be comprised of a physically defined single section such as the interior of a reactor, or physically defined multiple sections communicated with each other for plasma reaction, such as the interior of a remote plasma chamber and the interior of a reactor. Further, the space includes the interior of piping connecting multiple sections through which a reaction gas passes. The interior of the reactor includes only the space used for plasma reaction. Thus, if only a part of the reactor interior is used for plasma reaction where the reactor is composed of multiple sections, only the part used for plasma reaction constitutes a reaction space. Further, the plasma reaction includes a preliminary reaction for plasma polymerization. For example, upstream of a reactor, heating a reaction gas (e.g., 150° C. to 500° C., preferably 200° C. to 300° C., in a pre-heater chamber), exciting a reaction gas (e.g., in a remote plasma chamber), or mixing an excited additive gas and a source gas (e.g., in a pre-heater chamber) is included in a preliminary reaction.

[0098] 2) In the method according to Item 1, the source gas and the additive gas are separately introduced into the reaction space. The additive gas and the source gas can be mixed upstream of a reactor and introduced into the reactor. However, they can be introduced separately, depending on the configuration of a reactor. As long as the gases are not in a reactive state, regardless of whether the additive gas and the source gas are mixed or separated, the space where the gases are present does not constitute a reaction space. At a point where additive gas and the source gas are in contact in a reactive state, the reaction space begins. The reactive state includes states where the reaction gas is heated or excited, or the excited additive gas and the source gas are mixed, for example.

[0099] 3) In the method according to Item 1 or 2, the plasma polymerization reaction comprises exciting the reaction gas and depositing the film on the substrate. As described above, the plasma polymerization reaction includes a preliminary reaction such as excitation of the reaction gas.

[0100] 4) In the method according to any one of Items 1-3, the reaction space comprises a space for exciting the reaction gas and a space for depositing the film. In this embodiment, the reaction gas can be excited in a remote plasma chamber installed upstream of a reactor, and the film is deposited on the substrate in the reactor. The source gas and the additive gas can be introduced into the remote plasma chamber. In this case, the reaction space is composed of the interior of the remote plasma chamber, the interior of the reactor, and the interior of the piping connecting the remote plasma chamber and the reactor. Because of using the interior of the remote plasma chamber, the interior of the reactor can be significantly reduced, and thus, the distance between the upper electrode and the lower electrode can be reduced. This leads to not only downsizing the reactor, but also uniformly controlling a plasma over the substrate surface. Any suitable remote plasma chamber and any suitable operation conditions can be used in the present invention. For example, usable are the apparatus and the conditions disclosed in U.S. patent application Ser. Nos. 09/511,934 filed Feb. 24, 2000 and No. 09/764,523 filed Jan. 18, 2001, U.S. Pat. No. 5,788,778, and U.S. Pat. No. 5,788,799. The disclosure of each of the above is incorporated herein by reference in its entirety.

[0101] 5) In the method according to Item 3 or 4, the excitation of the reaction gas comprises exciting the additive gas and contacting the excited additive gas and the source gas. The excitation of the reaction gas can be accomplished in the reactor or upstream of the reactor. As described above, both the source gas and the additive gas can be excited in a remote plasma chamber. Alternatively, the excitation of the reaction gas can be accomplished by exciting the additive gas in a remote plasma chamber and mixing it with the source gas downstream of the remote plasma chamber.

[0102] 6) In the method according to any one of Items 1-3, the reaction space comprises a space for heating the reaction gas and a space for exciting the reaction gas and depositing the film. In this embodiment, the reaction gas can be heated in a pre-heat chamber installed upstream of a reactor, the reaction gas is excited in the reactor, and film is deposited on the substrate in the reactor. The source gas and the additive gas can be introduced into the pre-heater chamber. In this case, the reaction space is composed of the interior of the pre-heater chamber, the interior of the reactor, and the interior of the piping connecting the pre-heater chamber and the reactor. Because of using the interior of the pre-heater chamber, the interior of the reactor can be significantly reduced, and thus, the distance between the upper electrode and the lower electrode can be reduced. This leads to not only downsizing the reactor, but also uniformly controlling a plasma over the substrate surface. Any suitable remote plasma chamber and any suitable operation conditions can be used in the present invention. For example, usable are the apparatus and the conditions disclosed in the aforesaid references.

[0103] 7) In the method according to Item 6, the excitation of the reaction gas comprises exciting the additive gas and contacting the excited additive gas and the source gas. In this embodiment, the additive gas can be excited in a remote plasma chamber, and the source gas is heated in a pre-heater chamber where the excited additive gas and the source gas are in contact, and then the reaction gas flows into the reactor for deposition of a film. In this case, deposition of unwanted particles on a surface of the remote plasma chamber, which causes a failure of ignition or firing, can effectively be avoided, because only the additive gas is present in the remote plasma chamber. The source gas is mixed with the excited additive gas downstream of the remote plasma chamber. The reaction space may be composed of the interior from a point where the excited additive gas and the source gas meet to an entrance to the reactor, and the interior of the reactor.

[0104] 8) In the method according to any one of Items 1-7, the additive gas can be selected from the group consisting of nitrogen, argon, helium, and oxygen, but should not be limited thereto.

[0105] 9) In the method according to any one of Items 1-8, the plasma polymerization reaction is conducted at a temperature of 350-450° C. However, the suitable temperature varies depending on the type of source gas, and one of ordinary skill in the art could readily select the temperature. In the present invention, polymerization includes any polymerization of two or more units or monomers, including oligomerization.

[0106] 10) In the method according to any one of Items 1-9, the formation of the insulation film is conducted while maintaining a gas diffusing plate at a temperature of 150° C. or higher (e.g., 150° C. to 500° C., preferably 200° C. to 300° C.), through which the reaction gas flows into the reaction space, so that the reaction is promoted. In the above, the gas diffusing plate (or showerhead) may be equipped with a temperature control device to control the temperature. Conventionally, the temperature of the showerhead is not positively controlled and is normally 140° C. or lower when the temperature of the reaction space is 350-450° C., for example.

[0107] 11) In the method according to any one of Items 1-10, the residence time is determined by correlating the dielectric constant with the residence time. This embodiment has been described earlier. The following embodiments also have been described earlier:

[0108] 12) In the method according to any one of Items 1-11, the flow of the reaction gas is controlled to render the relative dielectric constant of the insulation film lower than 3.10.

[0109] 13) In the method according to any one of Items 1-12, Rt is no less than 165 msec or 200 msec.

[0110] It will be understood by those of skill in the art that numerous and various modifications can be made without departing from the spirit of the present invention. Therefore, it should be clearly understood that the forms of the present invention are illustrative only and are not intended to limit the scope of the present invention.