DETAILED DESCRIPTION OF THE INVENTION

[0098] Embodiment 1

[0099] A method for etching an organic film according to Embodiment 1 of the invention will now be described with reference to FIGS. 1, 2A through 2 D, 3 A through 3 E, 5 A and 5 B.

[0100] In the method for etching an organic film of Embodiment 1, a mixed gas including, as principal constituents, a CH ₄ gas and a N ₂ gas is used as the etching gas, so as to etch an organic film with plasma generated from the mixed gas. Exemplified etching conditions in Embodiment 1 are:

[0101] Plasma etching system: NLD plasma etching system Type of etching gas and flow rates per minute in standard condition:

CH ₄ :N ₂ =30 ml:70 ml

[0102] Antenna power: 1000 W (13.56 MHz)

[0103] Bias power: 200 W (2 MHz)

[0104] Pressure: 0.4 Pa

[0105] Substrate cooling temperature: 0° C.

[0106] Etching time: 180 seconds

[0107] FIG. 1 is a diagram for explaining the RIE lag characteristic of the method for etching an organic film of this embodiment, which is obtained by etching an organic film with a thickness of approximately 1.2 μm by using a mask pattern of a silicon oxide film with a thickness of approximately 240 nm. FIG. 1 also shows the RIE lag characteristics obtained when the ratio in the flow rate (ml) per minute in the standard condition between the CH ₄ gas and the N ₂ gas included in the etching gas, namely, CH ₄ :H ₂ , is 50 ml:50 ml, 70 ml:30 ml or 0 ml:100 ml.

[0108] As is understood from FIG. 1 , the RIE lag characteristic is abrupt in forming a hole with a diameter of 0.2 μm or less when CH ₄ :N ₂ is 0 ml:100 ml.

[0109] Also, when CH ₄ :N ₂ is 70 ml:30 ml, the RIE lag characteristic is observed and the etching rate is lowered as a whole because of a high concentration of the CH ₄ gas. The reason why the etching rate is lowered when the concentration of the CH ₄ gas is high will be described below with reference to FIGS. 4A and 4B .

[0110] When CH ₄ :N ₂ is 50 ml:50 ml, the so-called inverse RIE lag that the etching rate is increased as the diameter of a hole is smaller is observed.

[0111] On the other hand, when CH ₄ :N ₂ is 30 ml:70 ml, no RIE lag is caused, substantially a constant etching rate is obtained in all the holes having a diameter of 0.16 μm through 0.4 μm, and the etching rate is improved. In general, it is understood that a reaction is the most efficiently proceeded in an etch target surface when the etching rate is maximum, namely, when CH ₄ :N ₂ is 30 ml:70 ml.

[0112] FIGS. 2A through 2D are cross-sectional SEM photographs of holes formed in an organic film by the etching method for Embodiment 1, and the holes of FIGS. 2A through 2D have diameters of 0.16 μm, 0.18 μm, 0.24 μm and 0.40 μm, respectively. In FIGS. 2A through 2D , a reference numeral 1 denotes a silicon substrate, a reference numeral 2 denotes an organic film with a thickness of approximately 1.2 μm, and a reference numeral 3 denotes a mask pattern of a silicon oxide film with a thickness of approximately 240 nm. At the beginning of the etching, a resist pattern with a thickness of approximately 0.4 μm is present on the mask pattern 3 , and is eliminated during the etching of the organic film 2 .

[0113] As is understood from FIGS. 2A through 2C , when a hole has a diameter of 0.16 μm, 0.18 μm or 0.24 μm, a vertical cross-section can be obtained, and as is understood from FIG. 2 D, when a hole has a diameter of 0.40 μm, a forward taper cross-section can be obtained.

[0114] FIGS. 3A through 3E are cross-sectional SEM photographs of holes for showing the relationship, obtained in the pattern formation method for Embodiment 1, between the mixing ratios of the CH ₄ gas and the N ₂ gas included in the etching gas and the cross-sectional shape of a hole with a diameter of 0.24 μm. FIGS. 3A through 3E show the cross-sections of the holes formed when the ratios in the flow rate (ml) per minute in the standard condition between the CH ₄ gas and the N ₂ gas in the etching gas, namely, CH ₄ :N ₂ , are 0 ml:100 ml, 30 ml:70 ml, 50 ml:50 ml, 70 ml:30 ml and 100 ml:0 ml, respectively. In any cases, the etching is carried out under the aforementioned etching conditions. In FIGS. 3A through 3E , a reference numeral 1 denotes a silicon substrate, a reference numeral 2 denotes an organic film, a reference numeral 3 denotes a mask pattern of a silicon oxide film, a reference numeral 4 denotes a resist pattern and reference numerals 5 and 6 denote depositions.

[0115] As is understood from comparison of FIG. 3B with FIGS. 3A, 3C , 3 D and 3 E, the resist pattern 4 is removed through the etching only when CH ₄ :N ₂ is 30 ml:70 ml.

[0116] As is understood from FIGS. 3C and 3D , when the etching gas includes 50% or more of the CH ₄ gas, the resist pattern 4 remains and the etching rate is lowered.

[0117] As is understood from FIG. 3 E, when 100% of the etching gas is the CH ₄ gas, the resist pattern 4 is not etched and the depositions 5 and 6 are formed. Specifically, after a hole is formed by etching the organic film 2 to some extent, the deposition 6 is deposited within the hole.

[0118] The phenomena shown in FIGS. 3A through 3E reveal the following: If the resist pattern 4 can remain after completing the etching of the organic film 2 , the etching conditions can be selected from a wide range where the amount of the CH ₄ gas to be added to the etching gas is approximately 70% or less. Alternatively, when the resist pattern 4 cannot remain after completing the etching of the organic film 2 , the amount of the CH ₄ gas to be added to the etching gas is set to a range having approximately 30% as the center for not allowing the resist pattern 4 to remain.

[0119] The holes of FIGS. 3A through 3E are obtained when the etching time is 180 seconds. In actual fabrication of a semiconductor device, namely, when over-etching is carried out, the etching time is longer than 180 seconds. Therefore, the range of the amount of the CH ₄ gas to be added for not allowing the resist pattern 4 to remain is further enlarged. Depending upon an actually employed etching process, the amount of the CH ₄ gas to be added may be 0% (shown in FIG. 3A ) or 50% (shown in FIG. 3C ).

[0120] FIGS. 4A and 4B are diagrams for explaining an effect of the etching method for this embodiment, wherein FIG. 4A shows the etching mechanism obtained in using a conventional etching gas of a mixed gas including N ₂ and H ₂ or a NH ₃ gas, and FIG. 4B shows the etching mechanism obtained in using the etching gas of Embodiment 1 of the mixed gas including CH ₄ and N ₂ . In FIGS. 4A and 4B , a reference numeral 1 denotes a silicon substrate, a reference numeral 2 denotes an organic film to be etched, a reference numeral 3 denotes a mask pattern of a silicon oxide film, a reference numeral 7 denotes a radical flux isotropically reaching the organic film 2 , and a reference numeral 8 denotes deposition film formed on the wall and the bottom of a recess formed in the organic film 2 by the etching.

[0121] In general, in conducting anisotropic etching by using plasma, the etching is mainly realized by proceeding an ion assisted etching reaction, and is minimally proceeded by chemical sputtering, physical sputtering and a thermochemical reaction as compared with the ion assisted etching reaction. In the ion assisted reaction, when ions are released from the plasma to reach an etch target film, the ions are accelerated by an electric field of a plasma sheath region formed between a plasma generation region and the etch target film so as to collide with the etch target film, resulting in proceeding a surface chemical reaction in the vicinity of collision portions by collision energy. The mechanism of the etching through the ion assisted reaction is roughly divided into the following two types:

[0122] (First Etching Mechanism)

[0123] In the first mechanism, reactive radicals participating in the etching reaction are physically or chemically adsorbed onto the etch target surface, and this mechanism is further classified into the following three cases:

[0124] In the first case, ions collide with the vicinity of the portion where the radicals are adsorbed, so as to cause a chemical reaction among the ions, the adsorbed substance and the material of the etch target film.

[0125] In the second case, the adsorption is further proceeded so as to form a thin deposition film on the etch target surface, and also in this case, the ion assisted reaction can be efficiently proceeded through the ion collision, resulting in attaining a high etching rate.

[0126] In the third case, the deposition film has a large thickness, and in this case, most of ions reaching the etch target surface are consumed in removing the deposition film, and hence, the etching rate is excessively lowered. Also, when the thickness of the deposition film is larger than a predetermined value, namely, too large to remove through the ion collision, the deposition film cannot be removed by the ions. Therefore, the chemical reaction among the ions reaching the etch target surface, the adsorbed substance and the material of the etch target film is terminated, resulting in stopping the etching.

[0127] (Second Etching Mechanism)

[0128] In the second mechanism, no reactive radicals participating in the etching reaction is adsorbed onto the etch target surface. In this case, ions collide with the etch target surface and cause a chemical reaction with the material of the etch target film directly by the energy of the ions themselves, so as to proceed the ion assisted etching reaction.

[0129] In the case where the plasma generated from the mixed gas including N ₂ and H ₂ , radicals (hereinafter, reactive neutral particles with activity including atoms are generally designated as radicals) generated in the plasma are considered to be N, N ₂ , H and H ₂ , and in the case where the plasma of a NH ₃ gas is used, generated radicals are considered to be not only N, N ₂ , H and H ₂ but also NH, NH ₂ and NH ₃ . Therefore, when the NH ₃ gas is used, the amount of generated radicals is larger as compared with the case where the mixed gas of N ₂ and H ₂ is used, and hence, the amount of substances adhered onto the etch target surface is probably increased. However, the adsorbed substance is not resistant to the collision of ions emitted from the plasma to the etch target surface, and hence, a deposition film is not formed on the etch target surface.

[0130] Accordingly, the etching reaction occurring on the etch target surface on the bottom of the recess is probably dominantly a reaction to etch a small amount of atoms or molecules adhered onto the etch target surface and atoms present on the surface of the organic film by the ion assisted reaction caused by the ions emitted from the plasma (by the mechanism of the first case of the first etching mechanism), or an etching reaction between the ions and the etch target surface (by the second etching mechanism). In particular, in the etching by using the conventional plasma of N ₂ and H ₂ , the etching is probably dominantly proceeded by the second etching mechanism.

[0131] Since a hydrogen ion is small in its atomic radius and inertial mass, it probably enters the inside of the organic film without causing a reaction when it reaches the etch target surface. Therefore, it seems that nitrogen ions and ions of ammonia fragments (molecules and atoms generated through dissociation and decomposition from ammonia molecules) are the prime cause for proceeding the ion assisted reaction.

[0132] In general, an organic film includes, as a principal constituent, a polymer consisting of carbon atoms and hydrogen atoms, and the organic film is etched by nitrogen or hydrogen radicals and ions reaching the organic film as in Conventional Example 1. Therefore, it seems that a principal reaction product generated in the etching is volatile HCN and that the etching is proceeded by releasing the HCN from the etch target surface.

[0133] In general, the radical flux is lowered in the bottom of a recess having a higher aspect ratio. In contrast, ions comparatively constantly reach the bottom of a recess substantially regardless of the aspect ratio of the recess.

[0134] Accordingly, the etching rate is lowered in a recess having a higher aspect ratio where the radical flux is lowered, namely, a recess having a smaller diameter. This is the prime cause of the RIE lag. To be extract, the RIE lag may also be caused by the ion flux lowered depending upon the aspect ratio. on the contrary, when the plasma generated from the mixed gas including CH ₄ and N ₂ is used as in Embodiment 1, radicals of N, N ₂ , C, H, CH, CH ₂ , CH ₃ and CH ₄ are generated. Specifically, Embodiment 1 is different from Conventional Example 1 in a first point that there exist radicals of CH _x (wherein x is 1, 2, 3 or 4). These CH _x radicals tend to form a polymer on the etch target surface, and hence, the deposition film 8 is formed on the etch target surface, so as to fix atoms required for the etching reaction onto the etch target surface. The deposition film 8 with an appropriate thickness has a function as a reaction layer, so as to serve as the prime cause for efficiently causing the ion assisted reaction (by the mechanism of the second case of the first etching mechanism).

[0135] In Embodiment 1, the etching gas of the mixed gas including the CH ₄ gas and the N ₂ gas is used, the deposition film 8 is formed on the etch target surface, and the deposition film 8 fixes atoms required for the etching reaction onto the etch target surface. Therefore, the deposition film 8 has a function to compensate the phenomenon that the radical flux is lowered on the bottom of a recess with a higher aspect ratio.

[0136] Also, as described above, the ions comparatively constantly reach the bottom of a recess substantially regardless of the aspect ratio.

[0137] Accordingly, substantially without depending upon the aspect ratio, the ion assisted reaction is caused on the bottom of the recess, so that the constant etching rate can be obtained regardless of the aspect ratio, namely, the diameter of the opening.

[0138] When the aforementioned phenomena are more strictly observed, the radical flux reaching the bottom of a recess is actually lowered as the aspect ratio of the recess is higher, and hence, the thickness of the deposition film formed on the bottom of the recess is varied in accordance with the proceeding of the etching. Accordingly, as is shown in FIG. 1 , different RIE lag characteristics are observed depending upon the mixing ratios of the CH ₄ gas and the N ₂ gas.

[0139] Now, the influence of the mixing ratios of the CH ₄ gas and the N ₂ gas in the etching gas upon the RIE lag characteristic will be described in more detail.

[0140] First, in the case where no CH ₄ gas is added, namely, when the etching gas includes the N ₂ gas alone, a conspicuous RIE lag characteristic is observed for the same reason as in Conventional Example 1.

[0141] Next, in the case where the CH ₄ gas is included in approximately 30%, the RIE lag characteristic is not observed for the following reason: Since the radical flux reaching the bottom of a recess is lowered as the aspect ratio of the recess is higher, although the thickness of the deposition film 8 depends upon the diameter of the recess, the deposition film 8 in an amount equal to or sufficiently larger than an amount consumed in the ion assisted reaction is formed on the bottom of a recess having a small diameter. Therefore, a substantially constant etching rate is obtained without depending upon the diameter of the recess, namely, the aspect ratio. This fact paradoxically reveals that the surface adsorption coefficient (surface adsorption probability) of the CH _x radicals is small and that the transfer loss to the inside of a hole or interconnect groove with a high aspect ratio is considerably small.

[0142] Next, in the case where the CH ₄ gas is included in approximately 50%, the amount of radicals that can reach the etch target surface on the bottom of a recess basically from the uppermost face of the substrate (namely, the surface of the mask pattern 3 ) depends upon the aspect ratio, and when the amount of radicals supplied onto the uppermost face of the substrate is increased, the amount of the radicals reaching the bottom of the recess is naturally increased. At this point, part of ions reaching the bottom of the recess are consumed in removing an excessive deposition, and hence, the etching rate is lowered as a whole. Furthermore, as the aspect ratio is lower, namely, as the diameter of the opening is larger, the deposition film 8 is formed on the bottom of the recess in an amount larger than the amount necessary for the ion assisted reaction. Accordingly, as the deposition film 8 has a larger thickness, a larger part of the ions reaching the bottom of the recess is consumed in removing the excessive deposition film 8 . As a result, the amount of ions consumed in removing the deposition film 8 is smaller in a recess with a smaller diameter than in a recess with a larger diameter, and therefore, the so-called inverse RIE lag phenomenon that the etching rate of a recess with a smaller diameter is larger than the etching rate of a recess with a larger diameter is observed.

[0143] Next, the case where the CH ₄ gas is included in approximately 70% will be examined. As described with respect to the etching method for Conventional Example 1 referring to FIG. 4 A, ions of nitrogen atoms or nitrogen molecules are the prime cause for proceeding the ion assisted reaction. When the etching gas includes 70% of the CH ₄ gas, the supply amount of the N ₂ gas is naturally reduced, and hence, the amount of generated ions of nitrogen atoms or nitrogen molecules serving as the prime cause of the ion assisted reaction is reduced. Accordingly, the reduced amount of the ions of nitrogen atoms or nitrogen molecules also lowers the etching rate when the amount of the CH ₄ gas to be added is increased.

[0144] Furthermore, when the CH ₄ gas is included in approximately 70%, the thickness of the deposition film 8 is further increased, and hence, a larger amount of ions are consumed in removing the deposition film 8 . In addition, since the supply amount of the N ₂ gas indispensable to the etching reaction is reduced, the etching rate is further lowered as a whole. Simultaneously, as is understood from FIG. 3 D, the resist pattern 4 formed on the mask pattern 3 of the silicon oxide film is minimally etched. Accordingly, when the etching is proceeded to some extent, the aspect ratio is excessively increased in a recess with a small diameter, so that the amounts of radicals and ions supplied to the bottom of the recess can be abruptly reduced. Therefore, the etching rate of the recess with a small diameter is excessively lowered. As a result, in the case where the CH ₄ gas is included in 70% in which the resist pattern 4 is minimally etched, the RIE lag occurs again.

[0145] Embodiment 2

[0146] A method for etching, an organic film according to Embodiment 2 of the invention will now be described.

[0147] In the method for etching an organic film of this embodiment, a mixed gas including, as principal constituents, a CH ₄ gas and a NH ₃ gas is used as the etching gas, so as to etch an organic film with plasma generated from the mixed gas. Exemplified etching conditions in Embodiment 2 are:

[0148] Plasma etching system: NLD plasma etching system Type of etching gas and flow rates per minute in standard condition:

CH ₄ :NH ₃ =30 ml:70 ml

[0149] Antenna power: 1000 W (13.56 MHz)

[0150] Bias power: 200 W (2 MHz)

[0151] Pressure: 0.4 Pa

[0152] Substrate cooling temperature: 0° C.

[0153] An effect of the etching method for Embodiment 2 will now be described.

[0154] Embodiment 2 is different from Embodiment 1 in using the NH ₃ gas instead of the N ₂ gas, which results in attaining a higher etching rate in Embodiment 2 than in Embodiment 1.

[0155] The effect attained by adding the CH ₄ gas to the etching gas is the lowering of the RIE lag characteristic as described in Embodiment 1.

[0156] Since the mixed gas including the CH ₄ gas and the NH ₃ gas as the principal constituents is used for etching an organic film in Embodiment 2, the improvement of the etching rate and the lowering the RIE lag characteristic can be both realized.

[0157] Embodiment 3

[0158] A method for etching an organic film according to Embodiment 3 of the invention will now be described.

[0159] In the method for etching an organic film of Embodiment 3, a mixed gas including, as principal constituents, a CH ₄ gas, a N ₂ gas and a H ₂ gas is used as the etching gas, so as to etch an organic film with plasma generated from the mixed gas. Exemplified etching conditions in Embodiment 3 are:

[0160] Plasma etching system: NLD plasma etching system Type of etching gas and flow rates per minute in standard condition:

CH ₄ :N ₂ :H ₂ =30 ml:35 ml:35 ml

[0161] Antenna power: 1000 W (13.56 MHz)

[0162] Bias power: 200 W (2 MHz)

[0163] Pressure: 0.4 Pa

[0164] Substrate cooling temperature: 0° C.

[0165] An effect of the etching method for this embodiment will now be described.

[0166] Since the mixed gas including the CH ₄ gas, the N ₂ gas and the H ₂ gas as principal constituents is used in Embodiment 3, not only a higher etching rate than in Embodiment 1 can be obtained but also the amounts of N ₂ and H ₂ included in the etching gas can be more precisely controlled than in Embodiment 2. Accordingly, the etching can be carried out under more preferable conditions than in Embodiments 1 and 2. Specifically, Embodiment 3 can realize a process with a large process window.

[0167] The effect attained by adding the CH ₄ gas to the etching gas is the lowering of the RIE lag characteristic as described in Embodiment 1.

[0168] Since the mixed gas including the CH ₄ gas, the N ₂ gas and the H ₂ gas as the principal constituents is used for etching an organic film in Embodiment 3, not only the improvement of the etching rate and the lowering of the RIE lag characteristic can be both realized but also the controllability of the etching and the process window can be increased.

[0169] Embodiment 4

[0170] A method for etching an organic film according to Embodiment 4 of the invention will now be described with reference to FIGS. 5A, 5B , 6 A, 6 B, 7 A through 7 C and 8 .

[0171] In the method for etching an organic film of Embodiment 4, a mixed gas including, as principal constituents, a CH ₄ gas, a N ₂ gas and a CH ₃ NH ₂ (methylamine) gas is used as the etching gas, so as to etch an organic film with plasma generated from the mixed gas. Exemplified etching conditions in Embodiment 4 are:

[0172] Plasma etching system: NLD plasma etching system

[0173] Type of etching gas and flow rates per minute in standard condition:

CH ₄ :CH ₃ NH ₂ :N ₂ =15 ml:50 ml:35 ml

[0174] Antenna power: 1000 W (13.56 MHz)

[0175] Bias power: 200 W (2 MHz)

[0176] Pressure: 0.4 Pa

[0177] Substrate cooling temperature: 0° C.

[0178] Now, an effect of the method for etching an organic film of Embodiment 4 will be described in contradiction to etching of an organic film by using an etching gas including a CH ₃ NH ₂ gas as a principal constituent (in a flow rate per minute in the standard condition of the CH ₃ NH ₂ gas of 100 ml) and etching of an organic film by using an etching gas including a CH ₄ gas and a N ₂ gas as principal constituents (in a ratio in the flow rate per minute in the standard condition between CH ₄ and N ₂ of 30 ml:70 ml). The plasma etching system, the antenna power, the bias power, the pressure and the substrate cooling temperatures are the same in all the etching as those employed in Embodiment 4.

[0179] FIG. 5A is a cross-sectional SEM photograph of a hole formed by singly using the CH ₃ NH ₂ gas, and FIG. 5B is a cross-sectional SEM photograph of a hole formed by using the mixed gas of CH ₄ and N ₂ . In FIGS. 5A and 5B , a reference numeral 1 denotes a silicon substrate, a reference numeral 2 denotes an organic film, a reference numeral 3 denotes a mask pattern of a silicon oxide film, and the bottom of an opening formed in the mask pattern 3 has a diameter of 0.24 μm. A resist pattern with a thickness of approximately 0.4 μm formed on the mask pattern 3 is eliminated during the etching of the organic film 2 .

[0180] FIG. 6A shows the RIE lag characteristic obtained by singly using the CH ₃ NH ₂ gas and FIG. 6B shows the RIE lag characteristic obtained by using the mixed gas of CH ₄ and N ₂ .

[0181] When the CH ₃ NH ₂ gas is singly used, the hole has a good forward taper cross-section as is understood from FIG. 5A but the RIE lag characteristic is not very good as is understood from FIG. 6A .

[0182] On the other hand, when the mixed gas of CH ₄ and N ₂ is used, the hole has a substantially vertical cross-section as is understood from FIG. 5 B and the RIE lag characteristic is good as is understood from FIG. 6B . In other words, a constant etching rate is obtained without depending upon the diameter of the hole.

[0183] In Embodiment 4, the ratio between the constituents of the etching gas (CH ₄ :CH ₃ NH ₂ :N ₂ =15 ml:50 ml:35 ml) is obtained by mixing a half of the flow rate per minute of the singly used CH ₃ NH ₂ gas (namely, 100 ml) and a half of the flow rates per minute of the mixed gas of CH ₄ and N ₂ (namely, CH ₄ : NH ₂ =30 ml:70 ml). Accordingly, Embodiment 4 can attain a characteristic intermediate between the characteristic obtained by singly using the CH ₃ NH ₂ gas and the characteristic obtained by using the mixed gas of CH ₄ and N ₂ .

[0184] FIG. 7A shows a shape of a hole 9 A expected to be formed by setting the volume flow ratio between the CH ₃ NH ₂ gas and the (CH ₄ +N ₂ ) gas to CH ₃ NH ₂ <<(CH ₄ +N ₂ ), FIG. 7B shows a shape of a hole 9 B expected to be formed by setting the volume flow ratio between the CH ₃ NH ₂ gas and the (CH ₄ +N ₂ ) gas to CH ₃ NH ₂ >> (CH ₄ +N ₂ ), and FIG. 7C shows a shape of a hole 9 C expected to be formed by setting the volume flow ratio between the CH ₃ NH ₂ gas and the (CH ₄ +N ₂ ) gas to CH ₃ NH ₂ =(CH ₄ +N ₂ ), namely, expected in Embodiment 4.

[0185] When the CH ₃ NH ₂ gas and the (CH ₄ +N ₂ ) gas are mixed in substantially the same volume flow ratio, it is obvious that the hole 9 C has a forward taper cross-section as is shown in FIG. 7C . Also, it is understood that the angle of the forward taper cross-section can be adjusted to a desired angle by controlling the mixing ratios of the CH ₃ NH ₂ gas and the (CH ₄ +N ₂ ) gas.

[0186] FIG. 8 shows a RIE lag characteristic expected to be obtained when 100% of the etching gas is the (CH ₄ +N ₂ ) gas (shown with a broken line marked with ∘), a RIE lag characteristic expected to be obtained when 100% of the etching gas is the CH ₃ NH ₂ gas (shown with a broken line marked with ▴) and a RIE lag characteristic expected to be obtained when the etching gas includes the CH ₃ NH ₂ gas and the (CH ₄ +N ₂ ) gas in the same ratio (shown with a solid line). In all the cases, the values are standardized to a hole diameter of 0.24 μm.

[0187] The RIE lag characteristic obtained by using 100% of the CH ₃ NH ₂ gas is not very good, but the RIE lag characteristic can be improved by mixing the CH ₃ NH ₂ gas with the (CH ₄ +N ₂ ) gas. Also, a desired RIE lag characteristic can be obtained by controlling the mixing ratios of the CH ₃ NH ₂ gas and the (CH ₄ +N ₂ ) gas.

[0188] As described so far, since the etching gas including the CH ₃ NH ₂ gas and the (CH ₄ +N ₂ ) gas is used in Embodiment 4, a forward taper cross-section of a hole and a small RIE lag characteristic can be both realized. Also, the angle of the forward taper cross-section and the RIE lag characteristic can be controlled by adjusting the mixing ratios of the CH ₃ NH ₂ gas and the (CH ₄ +N ₂ ) gas.

[0189] There is a relationship between the angle of the forward taper cross-section and the RIE lag characteristic that one is improved when the other is degraded, namely, there is a reciprocal relationship therebetween. Specifically, under conditions for increasing the angle of the forward taper cross-section, the RIE lag characteristic is increased, and under conditions for reducing the angle of the forward taper cross-section, the RIE lag characteristic is reduced.

[0190] However, an actually required taper cross-section is a vertical cross-section or a slightly forward taper cross-section (an ideal taper angle is generally considered to be 89 degrees). Therefore, when the (CH ₄ +N ₂ ) gas is mixed with the CH ₃ NH ₂ gas in an amount slightly smaller than that of the (CH ₄ +N ₂ ) gas, a taper cross-section derived from the CH ₃ NH ₂ gas and a very small RIE lag characteristic (shown with the solid line in FIG. 8 ) derived from the (CH ₄ +N ₂ ) gas can be both realized.

[0191] Although the CH ₃ NH ₂ (methylamine) gas is mixed with the CH ₄ gas and the N ₂ gas in Embodiment 4, the same characteristic as that obtained by mixing methylamine can be obtained by mixing one of or a combination of gases containing, as a principal constituent, a compound including carbon, hydrogen and nitrogen, such as dimethylamine ((CH ₃ ) ₂ NH), trimethylamine ((CH ₃ ) ₃ N) and triethylamine (C ₂ H ₅ NH ₂ )- Methylamine, dimethylamine, trimethylamine and triethylamine can be very conveniently used because they can be taken out as a gas at a pressure of 1 atm and a temperature of 25° C. (room temperature). The boiling points of methylamine, dimethylamine, trimethylamine and triethylamine are −6.3° C., +7.4° C., +2.9° C. and +16.6° C. respectively.

[0192] Alternatively, the CH ₃ NH ₂ (methylamine) gas may be replaced with any of propylamine (C ₃ H ₇ NH ₂ ), a gas of the nitrile family such as C ₂ H ₃ N, C ₃ H ₃ N and C ₃ H ₅ N, a gas of the diamine family such as C ₂ H ₈ N, and a gas including four or more carbon atoms such as C ₄ H ₅ N, C ₄ H ₇ N, C ₄ H ₁₁ N, C ₅ H ₇ N and C ₅ H ₉ N.

[0193] The etching method for any of Embodiments 1 through 4 provides a high performance process. A process optimal to the environment is preferably selected in comprehensive consideration of the requirement for fine processing (the degree of refinement) in semiconductor fabrication, a fabrication cost (such as an employed gas supply system and cost of employed gases) and fabrication environments (such as maintenance of a gas line and use of a system for exhausting a gas).

[0194] Embodiment 5

[0195] A method for etching an organic film according to Embodiment 5 of the invention will now be described.

[0196] In the method for etching an organic film of Embodiment 5, a mixed gas including, as principal constituents, a CH ₄ gas, a N ₂ gas and a rare gas (such as an Ar gas) is used as the etching gas, so as to etch an organic film with plasma generated from the mixed gas. Exemplified etching conditions in Embodiment 5 are:

[0197] Plasma etching system: NLD plasma etching system

[0198] Type of etching gas and flow rates per minute in standard condition:

CH ₄ :N ₂ :Ar=30 ml:70 ml:200 ml

[0199] Antenna power: 1000 W (13.56 MHz)

[0200] Bias power: 200 W (2 MHz)

[0201] Pressure: 0.4 Pa

[0202] Substrate cooling temperature: 0° C.

[0203] An effect of the etching method for Embodiment 5 will now be described.

[0204] In the etching method for any of Embodiments 1 through 4, a CH ₄ gas is mixed in the etching gas so as to realize a small RIE lag characteristic, and owing to the CH ₄ gas, a deposition is easily adhered onto the inner walls of a reaction chamber of the etching system, which causes a problem of generation of particles.

[0205] When the etching gas further includes a rare gas as in Embodiment 5, the deposition rate of the deposition formed on the inner walls of the reaction chamber can be lowered.

[0206] Now, the mechanism for lowering the deposition rate of the deposition obtained by mixing a rare gas in the etching gas will be described.

[0207] When a rare gas is mixed in the etching gas, partial pressures in the plasma of the etching gas and atoms or molecules dissociated from the etching gas can be lowered in the vicinity of the inner walls of the reaction chamber, and hence, the deposition rate of the deposition is lowered. From this point of view, any of He, Ne, Ar, Kr, Xe and Rn may be used as the rare gas.

[0208] Furthermore, when a rare gas is mixed in the etching gas, the mixed rare gas is ionized in the plasma, and the ions of the rare gas are accelerated by a plasma sheath electric field formed in the vicinity of the inner walls of the reaction chamber so as to collide with the inner walls of the reaction chamber. Therefore, the deposition deposited on the inner walls of the reaction chamber is removed through physical sputtering. From this point of view, any of Ne, Ar, Kr, Xe and Rn may be used as the rare gas. Since He is small in its inertial mass, it cannot effectively remove the deposition through the physical sputtering.

[0209] <Reason Why Straight Chain Saturated Hydrocarbon is Preferred as Molecule Including Carbon and Hydrogen>

[0210] In each of Embodiments 1 through 5, CH ₄ is used as a molecule including carbon and hydrogen to be mixed in the etching gas, but the molecule including carbon and hydrogen is not limited to CH ₄ but may be widely selected from straight chain saturated hydrocarbons.

[0211] Now, the reason why the straight chain saturated hydrocarbon is preferred as the molecule including carbon and hydrogen will be described.

[0212] In weakly ionized nonequilibrium plasma used in the fine processing, dissociation within the plasma is proceeded in proportion to the number N of collisions between molecules and electrons. Specifically, as the collision number N is larger, the dissociation is further proceeded. The collision number N is expressed as a value obtained by multiplying residence time r of the gas molecules by a collision frequency V (the number of times of collisions per unit time (one second)), namely, N=τ×ν, wherein τ is a value in proportion to the volume V and the pressure P of a reaction chamber and in inverse proportion to an exhaust amount Q and τ=P·V/Q; and the collision frequency v is expressed by a product of the electron density Ne of the plasma and the dissociation rate (σ _dis ·v _e ) peculiar to a molecule. Therefore, N=τ×Ne×(σ _dis ·v _e ). The dissociation rate (σ _dis ·v _e ) peculiar to a molecule is determined on the basis of the dissociation cross-sectional area σ _dis peculiar to the molecule and convolution of the electron speed (energy) distribution v _e .

[0213] Accordingly, in plasma with the same electron density and the same electron speed distribution, the proceeding of the dissociation is uniquely determined depending upon the residence time τ of a gas.

[0214] In a straight chain saturated hydrocarbon, when n→∞, H/C=2. Specifically, the H/C ratio in a straight chain saturated hydrocarbon always has a value larger than 2, namely, H/C>2.

[0215] The effects of this invention obtained because the molecule including carbon and hydrogen satisfies H/C > 2 is understood as follows:

[0216] The first effect is the following: The composition ratio between carbon and hydrogen in plasma generated from a gas of the molecule including carbon and hydrogen and satisfying H/C>2 also satisfies H/C>2. Accordingly, on a time average basis, carbon and hydrogen are supplied onto an etch target surface in a ratio of H/C>2. Thus, a state where carbon is excessively supplied can be avoided on the etch target surface. As a result, a phenomenon of stopping the etching, namely, the etch stop, never occurs.

[0217] The second effect is the following: Molecules generated through dissociation in plasma are supplied onto an etch target surface in the form of ions or radicals. A molecule C _x H _y (wherein x and y are positive integers, 1≦x≦n and 1≦y≦2n+2) generated from a gas of the molecule satisfying H/C>2 has a straight chain single bond alone.

[0218] Now, as the premise of the description of the reason why a straight chain saturated hydrocarbon exhibits the effects in the etching according to any of Embodiments 1 through 5, dissociation of a molecule proceeded in plasma will be examined.

[0219] In order to practically supply a gas to a plasma etching system, it is necessary to generate a vapor pressure minimally required for enabling a mass flow at 1 atm and room temperature. For this purpose, in a molecule represented by C _x H _y (wherein x and y are positive integers), it is effective that x is equal to or smaller than approximately 5.

[0220] Therefore, by exemplifying a gas of a molecule in which the composition ratio of C is 5 or less, it will be examined how the dissociation is proceeded and what a dissociation product is generated as well as how the dissociation product affects the etching characteristics (the RIE lag characteristic and the cross-section of a hole).

[0221] (1) In straight chain saturated hydrocarbons (such as CH ₄ , C ₂ H ₆ , C ₄ H ₁₀ and C ₅ H ₁₂ ), a C—C bond or a C—H bond is cut through collision between molecules and electrons, and hence, an introduced gas is successively decomposed into a molecule having a smaller composition ratio of C. As a result, molecules having a straight chain C—C bond, such as C _x H _y , CH, CH ₂ , C and H, are generated. In particular, in the case of molecules of straight chain saturated hydrocarbons where the composition ratio x is approximately 5 or less, most of the molecules are probably decomposed into CH, CH ₂ , C and H. Such low molecular weight radicals have much lower probability to adhere onto a substance than a higher molecular weight radicals such as C _x H _y , and hence are supplied (transported) to an etch target surface on the bottom of a recess with a high aspect ratio. As a result, a deposition film is uniformly formed on the wall and the bottom of the recess. Accordingly, a vertical etch shape can be realized, and the etching rate does not depend upon the opening diameter or the aspect ratio, namely, etching with small RIE lag can be realized.

[0222] (2) In C ₂ H ₂ , C ₃ H ₄ , C ₄ H ₂ , C ₄ H ₄ , C ₄ H ₆ and C ₅ H ₈ (hereinafter referred to as a first group of hydrocarbons for convenience), H/C≦1.5, and thus the H/C ratio is smaller than in the straight chain saturated hydrocarbon, and hence, a large number of molecules generated through dissociation in the plasma include a double bond. When introduced gas molecules include a double bond or a triple bond, the rate of decomposing the gas molecules is lowered in accordance with the number of the double or triple bonds. Therefore, as compared with the straight chain saturated hydrocarbon, the ratio of the amount of generated CH, CH ₂ , C and H to the amount of generated large molecules represented by C _x H _y is lowered. Also, the radicals of the molecules including the double or triple bonds have larger probability to adsorb onto a substance, and hence are not supplied (transported) to the bottom of a recess with a high aspect ratio but are mainly deposited on the upper portion of the wall of the recess. Therefore, the effective aspect ratio of the recess is further increased, so that the amount of radicals supplied to the bottom of the recess can be largely reduced. As a result, the recess attains an excessively taper cross-section, which leads to shape defect, or large RIE lag is caused. Therefore, in an extreme case, the phenomenon where the etching is terminated, namely, the etch stop, is caused. Accordingly, none the first group of hydrocarbons is preferred for realizing the etching with small RIE lag.

[0223] (3) C ₃ H ₆ , C ₄ H ₈ and C ₅ H ₁₀ are classified into two types: straight chain hydrocarbon and cyclic hydrocarbon.

[0224] In straight chain C ₃ H ₆ , C ₄ H ₈ and C ₅ H ₁₀ (hereinafter referred to as a second group of hydrocarbons for convenience), the H/C ratio is 2, which accords with the H/C ratio of the straight chain saturated hydrocarbon. However, since each of these molecules includes one double bond, the ratio of the amount of generated CH, CH ₂ , C and H to the amount of generated large molecules represented by C _x H _y is lowered similarly to the first group of hydrocarbons, and molecules including a double bond, such as C ₂ H ₄ and C ₂ H ₂ , are generated. Accordingly, the supply (transportation) of the radicals to the bottom of a recess with a high aspect ratio and the formation of a deposition film are similar to those obtained by the first group of hydrocarbons. In particular, when C ₂ H ₄ is generated and ions reach from the plasma with a large number of C ₂ H ₄ adhered onto an etch target surface, C ₂ H ₄ are bonded through ion collision, so as to easily form a polymer represented by (CH ₂ ) _n . Therefore, similarly to the first group of hydrocarbons, none of the second group of hydrocarbons is preferred for realizing etching with small RIE lag.

[0225] On the other hand, cyclic C ₃ H ₆ , C ₄ H ₈ and C ₅ H ₁₀ (hereinafter referred to as a third group of hydrocarbons for convenience) are chemically bonded through a single bond alone to form saturated hydrocarbons. Also, in the third group of hydrocarbons, the H/C ratio is 2. In these molecules, the cyclic C—C bond can be easily cut through dissociation caused by electron collision, and molecules whose C—C bond is cut can easily form C ₂ H ₄ or CH ₂ . In particular, C ₄ H ₈ has a dissociation path to be decomposed into two C ₂ H ₄ at a time, and hence, a larger amount of C ₂ H ₄ or CH ₂ are generated than in the first or second group of hydrocarbons. As described above, since the radicals of C ₂ H ₄ not only have high probability to adsorb onto a substance but also are easily bonded to each other, they can be easily deposited on the upper portion of the wall of a recess without being supplied (transported) to the bottom of the recess with a high aspect ratio. Accordingly, none of the third group of hydrocarbons is preferred for realizing etching with small RIE lag.

[0226] (4) As described above, C ₂ H ₄ is a molecule working as a fundamental cause for shape defect and a large RIE lag characteristic. Although it depends upon the degree of dissociation of plasma, the concentration of a supplied gas is high in plasma generally used. Accordingly, C ₂ H ₄ exhibits the most serious effect to deposit on the upper portion of the wall of a recess without being supplied (transported) to the bottom of the recess with a high aspect ratio among the various hydrocarbons. Accordingly, C ₂ H ₄ is not preferred not only because a vertical or forward taper cross-section cannot be obtained but also because a small RIE lag characteristic cannot be attained.

[0227] As described above, when one of or a combination of straight chain saturated hydrocarbons represented by C _n H _2n+1 (wherein n is a positive integer), such as CH ₄ , C ₂ H ₆ , C ₄ H ₁₀ and C ₅ H ₁₂ , is used as the molecule including carbon and hydrogen, a recess with a vertical or forward taper cross-section can be formed with a very small RIE lag characteristic.

[0228] A gas used in the plasma etching can be optimally selected basically depending upon the method or system for exciting the plasma. As a plasma system can more highly excite the plasma, a higher molecular weight gas can be used, and hence, a gas to be used can be selected from a larger range. Specifically, when a plasma etching system capable of high excitement, such as an inductively coupled plasma etching system, a surface wave plasma etching system, an NLD plasma etching system, a capacity coupled parallel plate etching system using RF and an ECR plasma etching system, is used, a gas to be used can be selected in accordance with the actually used power (energy).

[0229] Furthermore, as the C/H ratio (composition ratio) of the molecule including carbon and hydrogen is larger, the ability to generate a deposition on the bottom of a recess is increased. Accordingly, the molecular weight and the C/H ratio are optimally selected in accordance with the density of plasma to be used.

[0230] Moreover, an organic film to be etched by the etching method for any of Embodiments 1 through 5 may include atoms such as N, O, B and S or halogen atoms such as F, Cl and Br as far as the organic film mainly has a framework of carbon and hydrogen.

[0231] Also, the effects of this invention are described in each of Embodiments 1 through 5 on the basis of the result obtained by using the etching gas in the NLD plasma etching system. However, the method for etching an organic film of any of Embodiments 1 through 5 is applicable to use of any plasma etching system, such as a parallel plate reactive ion etching system, a narrow-gap or two-frequency type parallel plate reactive ion etching system, magnetron enhanced reactive ion etching system, an inductively coupled plasma etching system, an antenna coupled plasma etching system, an electron cyclotron resonance plasma etching system and a surface wave plasma etching system.

[0232] Embodiment 6

[0233] A method for fabricating a semiconductor device according to Embodiment 6 of the invention will now be described with reference to FIGS. 9A through 9C and 10 A through 10 C.

[0234] First, as is shown in FIG. 9A, a laminated metal interconnect consisting of a first barrier metal layer 12 , a metal film 13 and a second barrier metal layer 14 is formed on a semiconductor substrate 11 , and an organic film 15 is formed on the metal interconnect. Thereafter, a silicon oxide film 16 is formed on the organic film 15 as is shown in FIG. 9B .

[0235] Next, after forming a resist pattern 17 on the silicon oxide film 16 by a know lithography technique as is shown in FIG. 9 C, the silicon oxide film 16 is subjected to plasma etching (dry etching) by using the resist pattern 17 as a mask, thereby forming a mask pattern 16 A from the silicon oxide film 16 as is shown in FIG. 10A . The type of etching gas to be used in the plasma etching is not herein specified, and a gas including fluorocarbon may be used.

[0236] Then, the organic film 15 is subjected to plasma etching using plasma generated from the etching gas used in the etching method for any of Embodiments 1 through 5, namely, an etching gas including, as a principal constituent, a mixed gas of a molecule including carbon and hydrogen and a molecule including nitrogen, and by using the resist pattern 17 and the mask pattern 16 A as masks. In this manner, recesses 18 each having a vertical or forward taper cross-section to be used as a via hole or an interconnect groove are formed in the organic film 15 as is shown in FIGS. 10B and 10 C. This etching is carried out under etching conditions of any of Embodiments 1 through 5. Since the resist pattern 17 is made from an organic compound, it is removed during the etching of the organic film 15 .

[0237] FIG. 10B shows a state in the middle of the etching of the organic film 15 , and under the etching conditions of any of Embodiments 1 through 5, the etching can be carried out at a constant etching rate regardless of the diameters of the openings of the recesses 18 . A dashed line shown in FIG. 10B corresponds to a reference line for indicating a predetermined depth in the organic film 15 .

[0238] FIG. 10C shows a state where the etching of the organic film 15 is completed, and under the etching conditions of any of Embodiments 1 through 5, the etching is proceeded at substantially the same timing to the surface of the second barrier metal layer 14 regardless of the diameters of the openings of the recesses 18 . Therefore, desired over-etching can be carried out regardless of the diameters of the openings of the recesses 18 .

[0239] Accordingly, the organic film can be stably processed without causing excessive or insufficient etching and without depending upon the diameters of the openings of the recesses 18 , and hence, a large process margin (process window) can be realized.

[0240] Thereafter, although not shown in the drawings, after cleaning the surfaces of the recesses 18 and the mask pattern 16 A, a third barrier metal layer of TiN or TaN is formed on the walls of the recesses 18 by sputtering or CVD. Then, a metal material film is deposited on the entire surface of the mask pattern 16 A by the CVD or plating so as to fill the recesses 18 , and a portion of the metal material film exposed on the mask pattern 16 A is removed by the CMP. Thus, a connection plug or metal interconnect can be formed from the metal material film. Thereafter, when a connection plug and a metal interconnect are alternately formed by a single damascene method, a multi-level interconnect structure can be obtained.

[0241] According to Embodiment 6, it is possible to prevent the problem of the excessive or insufficient etching caused in Conventional Example 2 because the etching rate of a recess with a small diameter is lower than the etching rate of a recess with a large diameter. Therefore, the reliability of a semiconductor device can be improved, and the semiconductor device can be fabricated with a large process window.

[0242] Although the connection plug or metal interconnect is formed by the single damascene method in Embodiment 6, the etching method for any of Embodiments 1 through 5 is naturally applied to a dual damascene method for simultaneously forming a connection plug and a metal interconnect. Also in this case, the reliability of a semiconductor device can be improved, and the semiconductor device can be fabricated with a large process window.

[0243] Furthermore, in Embodiment 6, the metal film 13 may be formed from, for example, a W film, an AlCu film, a Cu film, an Ag film, an Au film or a Pt film. Alternatively, the metal film 13 may be replaced with a conducting film such as a polysilicon film.

[0244] Also, in Embodiment 6, the materials for the first barrier metal layer 12 and the second barrier metal layer 14 may be selected so as to accord with the metal film 13 , and for example, a laminated film including a Ti film and a TiN film or a Ta film and a TaN film may be used.

[0245] Moreover, an insulating film of a Si ₃ N ₄ film or the like may be used as a barrier layer instead of the second barrier metal layer 14 . In this case, the Si ₃ N ₄ film is additionally etched after the etching of the organic film for forming the recesses.

[0246] Although the mask pattern 16 A is formed from a silicon oxide film in Embodiment 6, a silicon nitride film may be used instead, whereas the silicon nitride film is preferably formed from a material having a smaller dielectric constant than a silicon oxide film. From this point of view, a material with a small dielectric constant such as a-SiC:H is preferably used.

[0247] Furthermore, in the case where the mask pattern 16 A is also removed in removing the portion of the metal material film deposited on the mask pattern 16 A exposed outside the recesses 18 by the CMP, the mask pattern 16 A may be formed from a material with a large dielectric constant, such as a metal film of titanium or the like, a silicon nitride film, and a metal nitride film of titanium nitride or the like.

[0248] Embodiment 7

[0249] A pattern formation method (top surface imaging process) according to Embodiment 7 of the invention will now be described with reference to FIGS. 11A through 11C , 12 A and 12 B.

[0250] First, as is shown in FIG. 11 A, an organic film 22 is formed on a semiconductor substrate 21 , and a silylation target layer 23 is formed on the organic film 22 .

[0251] Next, as is shown in FIG. 11 B, the silylation target layer 23 is irradiated with exposing light 25 through a photomask 24 for selectively allowing the light to pass, thereby selectively forming a decomposed layer 26 in the silylation target layer 23 .

[0252] Then, as is shown in FIG. 11 C, with the substrate temperature increased, a gaseous silylation reagent 27 is supplied onto the surface of the silylation target layer 23 , so as to selectively silylate a non-decomposed portion (a portion excluding the decomposed layer 26 ) of the silylation target layer 23 . Thus, a silylated layer 28 serving as a mask layer is formed.

[0253] Instead of silylating the non-decomposed portion, the decomposed layer 26 may be silylated to form the silylated layer 28 , or the silylated layer 28 may be directly formed on the organic film 22 without forming the silylation target layer 23 .

[0254] Next, the organic film 22 is subjected to etching by using plasma generated from the etching gas used in the etching method for any of Embodiments 1 through 5, namely, the etching gas including, as a principal constituent, a mixed gas of a molecule including carbon and hydrogen and a molecule including nitrogen, by using the silylated layer 28 as a mask. Thus, openings 29 are formed in the organic film 22 as is shown in