JP3559429B2 - Plasma processing method - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a plasma processing method using high-frequency discharge.
[0002]
[Prior art]
A plasma processing method using high-frequency discharge is used in various places in a semiconductor manufacturing method such as dry etching for fine processing, sputtering for forming a thin film, plasma CVD, and ion implantation.
Hereinafter, dry etching suitable for fine processing will be described as an application example of the plasma processing method. Reactive ion etching (so-called RIE), which is most widely used as a dry etching technique, causes an etching reaction by exposing a sample to a high-frequency discharge plasma of an appropriate gas, thereby removing unnecessary portions on the surface of the sample. Things.
[0003]
In reactive ion etching, it is necessary to make the directionality of ions uniform in order to promote miniaturization. For that purpose, it is indispensable to reduce scattering of ions in plasma. In order to make the directionality of ions uniform, it is effective to lower the pressure in the plasma generator and increase the mean free path of the ions, but there is a problem that the plasma density decreases and the etching rate decreases. .
[0004]
As a countermeasure, a high-density plasma device such as an inductively coupled plasma device or a helicon plasma device is being introduced. A high-density plasma apparatus can generate plasma at a density of about 10 to 100 times as high as that of a conventional parallel-plate RIE apparatus. An equivalent or higher etching rate can be obtained.
[0005]
[Problems to be solved by the invention]
However, even the conventional plasma processing method improved in this way has problems such as abnormal etching shape, generation of a microloading effect, and deterioration or destruction of the gate insulating film. These still have a narrow control range of the chemical reaction in the plasma against the progress of miniaturization, and furthermore, because of the high-density plasma, the charge-up is remarkable, and the radical density decreases due to excessive dissociation. Cause. As one method for solving these problems, a pulse plasma process (for example, see Japanese Patent Application Laid-Open No. 6-267900) has been proposed.
[0006]
This pulsed plasma process supplies a plasma-generating high-frequency power (RF) in a pulsed form, and provides an off-period during the high-frequency power supply period to control the dissociation reaction in the plasma and the charge accumulation process on the substrate. The purpose was, but the detailed mechanism is unknown, and the problem has not been sufficiently solved.
An object of the present invention is to provide a plasma processing method which is particularly focused on controlling the radical density among these problems and has excellent fine processing properties even under a low pressure. In particular, in the etching of an oxide film, it is necessary to control radical species and density involved in an etching reaction and formation of a deposited protective film in order to obtain the selectivity.
[0007]
Accordingly, an object of the present invention is to provide a plasma processing method capable of controlling the radical density with high accuracy.
[0008]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the present invention is characterized in that a dissociation reaction is controlled by a pulse modulation parameter in a pulsed plasma process, and a highly accurate control of radical density, which cannot be realized by a continuous discharge CW process, is realized.
[0009]
According to the first aspect of the present invention, radicals are generated by converting a reaction gas supplied to a vacuum chamber into plasma using high frequency power for plasma generation supplied to the vacuum chamber, and the plasma generates a substrate bias in the vacuum chamber. A plasma processing method for etching a sample to which high-frequency power for power is applied, wherein the on- off modulation of the high-frequency power for plasma generation controls the radical density for each radical species to control the etching rate and etching selectivity , The sample is processed by reducing the duty ratio in the over-etching step after the main etching step with respect to the ratio (duty ratio) of the on-time to the on-off modulation cycle of the high frequency power for plasma generation in the main etching step. It is characterized by the following.
[0010]
Generally, by increasing the supplied high frequency power, the electron density in the plasma increases, and the electron impact causes the dissociation reaction of the reaction gas to proceed. In the process of this dissociation reaction, radicals necessary for etching are generated, but in the case of a high-density plasma source, under high RF power input, even radicals necessary for etching are decomposed by excessive dissociation reaction. There is a concern. In order to achieve both a high etching rate and a high selectivity, it is necessary to obtain a high density for a required radical species under a high RF power supply at which a high ion current density can be obtained.
[0011]
In the present invention, by utilizing the time dependency of the dissociation reaction process, it is possible to obtain a high radical density that cannot be obtained during CW by pulse modulation of on / off of an effective time interval with respect to high-frequency power, thereby achieving high selection and high speed. An etching process is realized. In the present invention, the effective ON time and OFF time, that is, ON time / modulation period (= duty ratio) are found.
[0012]
According to a second aspect of the present invention, in the first aspect, the duty ratio in the main etching step or the over-etching step is in a range of 0.1 to 0.8.
According to a third aspect of the present invention, in the first aspect, the duty ratio in the main etching step or the over-etching step is in the range of 0.3 to 0.7.
[0013]
In the present invention, a condition for obtaining a high radical density has been found in order to obtain a high selectivity, but a high selectivity is not necessarily required during all processing times for processing the same sample. Rather, depending on the film thickness ratio and the structure, it may be desirable to partially provide time for processing under conditions of a low selectivity and a high etching rate. In the present invention, by continuously changing the on / off modulation condition over time, it is possible to perform continuous processing under a low selective high etching rate condition and a high selective condition.
[0014]
According to a fourth aspect of the present invention, in the first, second, or third aspect, the on / off modulation of the high frequency power supply for plasma generation is synchronized with the on / off modulation of the high frequency power supply for substrate bias.
According to a fifth aspect of the present invention, in the first, second, third or fourth aspect, the on / off modulation period of the high frequency power for plasma generation or the high frequency power for the substrate bias is set to 20 μsec to 200 μsec. It is characterized by.
[0015]
According to a sixth aspect of the present invention, in the first, second, third, fourth or fifth aspect, the reaction gas is a gas containing fluorine, and the sample has an oxide film formed on Poly-Si. It is characterized by being a sample.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[First Embodiment]
FIG. 1 is a schematic diagram showing the structure of a plasma etching apparatus (dry etching apparatus) according to the first embodiment of the present invention. This apparatus is constituted by a plasma generation chamber 1 of an inductive coupling system, and the chamber is grounded. From the gas inlet 2, a reactive gas, for example, a mixed gas of CHF ₃ (50%) / C ₄ F ₈ (50%) is introduced at about 50 sccm and 5 Pa in the case of etching an oxide film.
[0016]
The plasma 6 can be generated by applying high-frequency power from the high-frequency power supply 5 for plasma generation via the matching circuit 4 to the multi-spiral coil 3 mounted above the plasma generation chamber 1 with a quartz plate interposed therebetween. . A wafer 7 serving as a sample is placed on the lower electrode 8 so as to face the multi-spiral coil 3, and high frequency bias power is supplied from a high frequency bias power supply 10 via a matching circuit 9. A pulse signal for on / off modulation is sent from the pulse generator 11 to the high frequency power supply 5 for plasma generation and the high frequency power supply 10 for bias.
[0017]
FIG. 2 is an explanatory diagram of a time change of the on / off pulse modulation parameter and various parameters. FIG. 2A shows high-frequency power (for example, 13.56 MHz) at the time of CW. This high-frequency power is modulated on and off by the modulation pulse shown in FIG. 2B as shown in FIG. You. The modulation parameters include an on-time, an off-time, and a modulation period, and the duty ratio is a ratio of the on-time during the modulation period.
[0018]
When the modulation period is set to 10 μsec to 1000 μsec, the electron density changes as shown in FIG. 2D according to the on / off state of the plasma, but the lifetime of radicals in the plasma is 1 msec or more, which is far longer than the modulation period. Long. Therefore, the radical density is constant throughout the modulation period as shown in FIG. In FIG. 3E, the solid line indicates the density of CF ₂ radicals, and the dashed line indicates the density of CF radicals.
[0019]
FIG. 3 is a characteristic diagram showing a relative change in the density of CF radicals and CF ₂ radicals in CHF ₃ (50%) / C ₄ F ₈ (50%) plasma in the first embodiment of the present invention. This figure shows the relative change in radical density due to the pulse modulation period under the condition that the instantaneous input power is 1500 W and the duty ratio is constant at 50%, that is, the time average input power is 750 W. From this figure, it can be seen that the radical density, particularly the CF ₂ radical density, increases as the modulation period becomes shorter than in the CW mode.
[0020]
This change in radical density is thought to be due to the time dependence of the multi-step dissociation reaction process from the mother gas to each radical / atom. That is, in the high-density plasma of CW, the dissociation reaction tends to progress to a higher order, and the CF ₂ radical generated in the first-stage reaction tends to be further decomposed and damaged. It is considered that the higher-order reaction process was suppressed due to the time dependency, and as a result, the CF ₂ radical density was relatively increased.
[0021]
This increase in radical density has a large effect on etching characteristics. FIG. 4 shows changes in the BPSG etching rate in CHF ₃ (50%) / C ₄ F ₈ (50%) plasma and the selectivity with respect to Poly-Si (polysilicon) in the pulse modulation period under the same conditions as in FIG. FIG. As the modulation period becomes shorter, the radical density, in particular, the CF ₂ radical density increases, and the etching rate decreases, but the poly-Si selectivity increases.
[0022]
Previously, CF radicals as a parameter for determining selectivity, CF ₂ radicals, and the correlation between the density or density ratio of F have been discussed, the presence of CF ₂ radical density from the result of the pulse modulation where It is suggested to be important. In addition, quantitatively, it turns out that a modulation period of 20 μsec to 200 μsec is particularly effective.
[0023]
FIG. 5 shows the change in density of CF radicals and CF ₂ radicals in CHF ₃ (50%) / C ₄ F ₈ (50%) plasma depending on the duty ratio under the condition of an instantaneous input power of 1500 W and a constant pulse modulation period of 100 μsec. It is a characteristic diagram. The change in radical density with respect to the duty ratio is very remarkable, but this change also includes a change in the time-average input power. FIG. 6 shows a change in density of CF radicals and CF ₂ radicals during pulse modulation and during CW as viewed from the time-average input ICP power in order to determine the effect of only pulse on-off modulation. Note that ICP is an abbreviation for inductively coupled plasma (inductively coupled plasma).
[0024]
As can be seen from these comparisons, the duty ratio range effective for increasing the radical density, that is, the duty ratio range in which a higher radical density can be obtained as compared with CW, is 10 to 80%.
FIG. 7 is a characteristic diagram showing changes in the BPSG etching rate in CHF ₃ (50%) / C ₄ F ₈ (50%) plasma and the selectivity with respect to Poly-Si with the duty ratio under the same conditions as in FIG. . Due to the change in the duty ratio, the radical density changes in a form in which the pulse modulation effect is added to the radical density change during CW, and the etching characteristics change correspondingly. Basically, CF radicals and CF ₂ radicals act as a deposition protection film, and the etching characteristics change depending on the balance between ions that cause an etching reaction and F atom density. That is, in the high duty ratio region where the radical density is small, the etching rate is high but the selectivity is small, and in the low duty ratio region where the radical density is large, film deposition occurs. Highly selective conditions exist in these intermediate regions. The region to be used can be determined in accordance with the ion and F atom densities. Basically, on / off modulation is performed within a range of on time / period = 0.1 to 0.8, which has a larger radical density than CW. Is effective. In addition, in consideration of the superiority to the CW time and the substantial power use efficiency, a practically particularly effective range is ON time / period = 0.3 to 0.7.
[0025]
[Second embodiment]
In the first embodiment, it has been clarified that the etching characteristics can be controlled by on / off modulation of the high-frequency power, and in particular, it is possible to improve the selectivity. There is no need to process with high selectivity over time. Therefore, in consideration of the processing speed, it may be desirable to provide a part of the processing time under the condition that the selectivity is low and the etching rate is high depending on the film thickness ratio and the structure. For example, as shown in the time change of the ON / OFF pulse modulation parameter and various parameters in FIG. 8, the duty ratio is temporally changed, and a high selection is performed only in the over-etching process out of both the main etching and the over-etching processes. It is important to use conditions. 8A shows a change in the duty ratio at the time of pulse modulation, FIG. 8B shows a change of the radical density at the time of pulse modulation, and FIG. 8C shows the change of the etching characteristic and the selectivity at the time of pulse modulation. The change is shown.
[0026]
In this embodiment, it is described that the ON time / modulation cycle is changed over time in the range of 0.1 to 0.8, but the ON time / modulation cycle is set to 0.1 to 0. The ON time / modulation period need not be limited to the range of 8, and may be changed to a value outside the above range. In other words, this embodiment explains that the etching rate can be changed by changing the ON time / modulation period with time.
[0027]
[Third Embodiment]
In the second embodiment, in addition to the configuration of the device of the first embodiment, by changing the on / off modulation condition over time, continuous under a low selective high etching rate condition and a high selective condition. Processing is possible. In this case, it is preferable that only the on-time of the modulation parameter be changed and the modulation period be constant so as not to disturb the matching condition of the high-frequency discharge. In addition, by providing a function of synchronizing the high-frequency power supply for plasma generation with the high-frequency power supply for substrate bias, and having a function of changing the ON time / modulation cycle with time, it is preferable for high-precision etching.
[0028]
【The invention's effect】
As described above, according to the present invention, the radical generation density can be controlled by performing pulse modulation in the range of ON time / modulation period = 0.1 to 0.8, and highly accurate etching can be realized.
In addition, by changing the ON time / modulation cycle with time, continuous processing can be performed under a low selective high etching rate condition and a high selective condition.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating a configuration of a plasma etching apparatus used in a plasma processing method according to a first embodiment of the present invention.
FIG. 2 is an explanatory diagram of a time change of an on / off pulse modulation parameter and various parameters in the plasma processing method according to the first embodiment of the present invention.
FIG. 3 shows a change in density of CF radicals and CF ₂ radicals in CHF ₃ (50%) / C ₄ F ₈ (50%) plasma due to a pulse modulation period in the plasma processing method according to the first embodiment of the present invention. FIG.
FIG. 4 shows a BPSG etching rate in CHF ₃ (50%) / C ₄ F ₈ (50%) plasma and a selectivity ratio to Poly-Si in a CHF ₃ (50%) / C ₄ F ₈ (50%) plasma in the plasma processing method according to the first embodiment of the present invention. FIG. 6 is a characteristic diagram showing changes in
FIG. 5 shows a change in density of CF radicals and CF ₂ radicals in CHF ₃ (50%) / C ₄ F ₈ (50%) plasma depending on a pulse modulation duty ratio in the plasma processing method according to the first embodiment of the present invention. FIG.
FIG. 6 shows CHF ₃ (50%) / C ₄ F ₈ (50%) plasma during pulse modulation and CW in the plasma processing method according to the first embodiment of the present invention viewed from the time-average input ICP power. FIG. 3 is a characteristic diagram showing a change in density of CF radicals and CF ₂ radicals of FIG.
FIG. 7 shows a BPSG etching rate in CHF ₃ (50%) / C ₄ F ₈ (50%) plasma and a selection with respect to Poly-Si according to a pulse modulation duty ratio in the plasma processing method according to the first embodiment of the present invention. FIG. 4 is a characteristic diagram showing a change in a ratio.
FIG. 8 is an explanatory diagram of an on-off pulse modulation parameter and a change over time of various parameters by a plasma processing method according to a second embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Plasma generation chamber 2 Gas inlet 3 Multi spiral coil 4 Matching circuit 5 High frequency power supply 6 Plasma 7 Wafer 8 Lower electrode 9 Matching circuit 10 High frequency power supply 11 Pulse generator

Claims (6)

A sample in which a reactive gas supplied to a vacuum chamber is converted into plasma by high-frequency power for plasma generation supplied to the vacuum chamber to generate radicals, and the plasma is applied with high-frequency power for substrate bias in the vacuum chamber. A plasma processing method for etching a plasma, wherein the on-off modulation of the high-frequency power for plasma generation controls the radical density for each radical species to control an etching rate and an etching selectivity , and the plasma in a main etching step. The sample is processed by reducing a duty ratio in an over-etching step after the main etching step with respect to a ratio (duty ratio) of an on-time to an on-off modulation cycle of the high-frequency power for generation. Plasma treatment method. 2. The plasma processing method according to claim 1, wherein a duty ratio in the main etching step or the over-etching step is in a range of 0.1 to 0.8. 2. The plasma processing method according to claim 1, wherein a duty ratio in the main etching step or the over-etching step is in a range of 0.3 to 0.7. 4. The plasma processing method according to claim 1, wherein on / off modulation of the high frequency power supply for plasma generation is synchronized with on / off modulation of the high frequency power supply for substrate bias. 5. The plasma processing method according to claim 1, wherein an on / off modulation cycle of the high frequency power for plasma generation or the high frequency power for substrate bias is set to 20 to 200 [mu] sec. 6. The plasma processing according to claim 1, wherein the reaction gas is a gas containing fluorine, and the sample is a sample in which an oxide film is formed on Poly-Si. Method.

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