JPH07273071A - Plasma etching method - Google Patents

Abstract

PURPOSE:To reduce the damages of a foundation insulating film due to plasma radiation or VUV radiation by a method wherein a pattern is formed while heat-conducting gas which has a photon energy smaller than that of the main spectrum line of He plasma radiation is supplied. CONSTITUTION:A gate insulating film 2 made of SiO2 and an Si-base layer 3 made of polycrystalline silicon containing impurities are deposited on a semiconductor substrate 1 successively by a heat oxidation method and a CVD method respectively. Then a resist mask is formed by a negative-type chemical amplifying resist and KrF excimer laser lithography method to obtain a substrate to be etched. The substrate to be etched is placed on the substrate stage of a substrate bias application type ECR plasma etching apparatus. Then a pattern is formed while heat-conducting gas which has a smaller photon energy than that of the main spectrum line of He plasma radiation, for instance heat- conducting Xe gas, is supplied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plasma etching method, and more particularly to a plasma etching method for a gate electrode or the like in which the underlying insulating film is less likely to be damaged by irradiation.

[0002]

2. Description of the Related Art As semiconductor device design rules such as LSI are miniaturized from a level of half micron to quarter micron, requirements for microfabrication techniques such as plasma etching are becoming more severe. Above all,
In patterning the gate electrode of MISFET,
There is a demand for a plasma etching method capable of satisfying various requirements such as high anisotropy, high selectivity, high etching rate, low pollution, and low damage to the underlying insulating film at a high level.

Highly anisotropic etchant without side etching
Etching gas that achieves a high selectivity between the insulating film and the underlying insulating film
As a Br-based gas and O₂Mixed gas with or Cl-based gas
And O ₂The mixed gas with is effective. This is Br
Radical (Br^*) And Cl radicals (Cl^*) Chemical
CF activity is usually used_FourGenerated from equal F-based gas
F radical (F^*) Is smaller than
This is because it can be suppressed. O₂Is selected as a base oxide film.
In addition to the effect of improving the selection ratio, the effect of improving the etching rate
It

SiB which is a reaction product of etching
This is also because r _x or SiCl _x adheres to the side surface of the gate electrode where a small amount of ions are incident to form a side wall protective film, and therefore the effect of protecting the side surface of the pattern from attack of radicals or incident ions from directions other than the vertical direction is expected. Since these side wall protection films are partially oxidized even in the plasma atmosphere in the chamber, the side wall protection function can be adjusted by changing the mixing ratio of O ₂ gas, and it is necessary not only to form a vertical pattern. It is also possible to add a forward taper to the pattern or control the taper angle in accordance with the above.

However, when the substrate to be etched is taken out into the atmosphere or the resist pattern is ashed after the etching is completed, this side wall protective film is converted into a strong oxide-based side wall altered film and the substrate to be etched or the etching is performed. There are problems that the particles are contaminated inside the chamber and that the step coverage of the insulating film for forming the LDD sidewall is reduced in the next step.

Therefore, it is necessary to remove the side wall protective film or the side wall altered film after the gate electrode etching is completed and before proceeding to the next step. Therefore, a wet treatment with dilute hydrofluoric acid or a buffered hydrofluoric acid aqueous solution is performed, but these etching solutions simultaneously etch the underlying gate insulating film and the like. The etching of the gate insulating film can be dealt with by controlling the etching time or the like if the amount of film reduction can be predicted. However, recent research has revealed that the etching rate of the gate insulating film after the end of the gate electrode etching is abnormally large as compared with the etching rate of an unprocessed ordinary thermal oxide film, etc., and its control is difficult. became. Regarding this issue, "Etch
Rate Acceleration of SiO
₂ during Wet Treatment af
ter Gate Etching ”by Jpn.
J. Appl. Phys. Part 1, 32 (199
3) 6114 reported by the present inventors. The main point of this report is that plasma irradiation during etching induces compound damage on the exposed gate insulating film due to crystal disordering on the surface of the gate insulating film, contamination by Br atoms, etc. and roughening of the surface. However, these factors overlap and accelerated etching is performed during wet processing.

This problem will be further described with reference to FIGS. 7 and 8. First FIGS. 7 (a) after successively depositing a Si-based material layer 3 of polycrystalline silicon or the like as the gate insulating film 2 of SiO ₂ or the like is formed on the semiconductor substrate 1 of Si or the like as shown, to form a resist mask 4 . Next, main etching with high anisotropy and high etching rate is performed using Cl-based gas, Br-based gas, or the like to pattern the Si-based material layer 3. Next in FIG.
As shown in (b), the underlying gate insulating film 2 is partially exposed and the remaining portion 3a of the Si-based material layer is partially left, and the overetching with a high selectivity is performed to form the Si-based material layer 3. Complete the gate electrode. This state is shown in Figure 7.
It is (c). Resist mask 4 and Si-based material layer 3
On the side surface of the pattern, a side wall protective film 5 made of a decomposition product of the resist or a reaction product of etching is formed. As shown in FIG. 8, switching to over-etching is performed using Si.
The emission spectrum of the reaction products such as Cl _x and SiBr _x in plasma is monitored and set at the time when the intensity starts to drop. At this point, 1 of the underlying gate insulating film 2
However, the damaged layer 7 is exposed to strong plasma irradiation during main etching for a short time. In the actual etching process, a method of performing waveform processing such as time differentiation of the emission spectrum intensity signal to increase the detection sensitivity and shortening the plasma irradiation time of the base insulating film 2 during the main etching is taken. Not a solution.

Further, according to a recent study by the present inventor, it has been found that the damage to the gate insulating film is also induced by vacuum ultraviolet light (VUV) having a wavelength of 200 nm or less during plasma emission. Especially, the band gap energy of SiO ₂ often used as a gate insulating film is 8.8 eV.
In the region on the shorter wavelength side than 140.9 nm corresponding to
The absorption coefficient increases remarkably as shown in. That is,
When directly exposed to VUV light having a wavelength significantly shorter than 140.9 nm, the damage to the gate insulating film increases. FIG. R. Philip et. al. "Ha
ndbook of Optical Constant
s of Solids (Academic Pres
S., Orlando, 1985) Part 2 p. 7
It was created from 49.

By the way, in plasma etching in recent semiconductor processes, strict control of etching conditions is required, and control of the substrate temperature to be etched is no exception. In particular, when controlling the substrate temperature at a low temperature to reduce the radical reaction, the substrate to be etched is brought into close contact with the substrate stage cooled at a low temperature, and then heat such as He is directed from the upper surface of the substrate stage toward the rear surface of the substrate to be etched. It is common practice to flow a conductive gas to enhance the thermal interaction between the substrate stage and the substrate. Since the heat transfer gas is released into the etching chamber, it is turned into plasma here. The main emission spectra of plasma-excited He are 58 nm and 54 nm for He neutrally excited species and 30 nm and 26 nm for He ^{+, which} is a very short wavelength V in the millican region.
It has UV light and the photon energy level is high. Therefore, damage to the gate insulating film cannot be ignored.

Recent sub-half micron MISFE
At T, the thickness of the gate insulating film itself is 10 nm.
If the following are required and such ultra-thin gate insulating film is damaged and accelerated etching occurs, the semiconductor substrate is exposed, and the impurity diffusion layer may be damaged or contaminated.
In particular, if damage occurs immediately below the side edge of the gate electrode, L
Even after the DD sidewall is formed, the deterioration of the gate breakdown voltage becomes a problem. Even if the semiconductor substrate is not exposed, if the gate insulating film is reduced to a thickness of 2 to 3 nm, there is no means for confirming the presence or absence of a residual film, which leaves a problem in process control.

[0011]

Therefore, an object of the present invention is to perform plasma irradiation or VUV in a plasma etching method for patterning a Si-based material layer on a base insulating film.
It is an object of the present invention to provide a plasma etching method that reduces damage to a base insulating film due to light irradiation.

Another object of the present invention is to provide a plasma etching method which does not cause abnormal reduction of the gate insulating film or damage to the semiconductor substrate due to accelerated etching during wet processing after completion of plasma etching.

Still another object of the present invention is to provide a plasma etching method capable of manufacturing a MIS type semiconductor device having a fine gate electrode width and an extremely thin gate insulating film with good controllability and without deterioration in withstand voltage. Other problems of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0014]

The plasma etching method of the present invention was devised in order to solve the above-mentioned problems, and a thermal conductive gas is supplied from the upper surface of the substrate stage toward the rear surface of the substrate to be etched, while In the plasma etching method for patterning the Si-based material layer on the insulating film, the photon energy of the main spectrum line of plasma emission generated by diffusing the heat conductive gas in the etching chamber is the main spectrum line of He plasma emission. Plasma etching is performed while supplying a heat conductive gas smaller than photon energy.

Further, the plasma etching method of the present invention is
In the plasma etching method of patterning the Si-based material layer on the underlying insulating film while supplying the heat conductive gas from the upper surface of the substrate stage to the back surface of the substrate to be etched, plasma generated by diffusing the heat conductive gas in the etching chamber The wavelength of the main spectrum line of light emission is larger than the wavelength of the main spectrum line of He plasma light emission, and plasma etching is performed while supplying a heat conductive gas. Xe, K is used as the heat transfer gas that satisfies this condition.
Noble gases such as r, Ar and Ne can be exemplified.

Furthermore, in the plasma etching method of the present invention, when the gate electrode is multi-stage etched using the above-mentioned heat conductive gas, switching to the low ion energy over-etching step is performed by exposing the underlying insulating film. It is characterized by setting before.

[0017]

The point of the present invention is to use He, which has been commonly used as a heat transfer gas, instead of Xe, Kr, Ar,
The point is that a rare gas such as Ne is used. Table 1 shows the main spectral lines in the VUV region of plasma emission of each of these rare gases in the order of intensity. Plasma light emission is classified into a neutral excited species in an atomic state and a monovalent ion. In terms of emission intensity, the neutral excited species may be focused. The numerical values and intensities of the emission spectrum line are shown in D. R. Lid
e "CRC Handbook of Chemist
ry and Physics "71st. Editi
on (CRC Press, Boston, 1990-
1991).

[0018]

[Table 1]

FIG. 6 is a diagram showing the positions and intensities of the spectral lines of plasma emission of He, Ar, Kr and Xe. This figure shows A. R. Striganov and N.M.
S. Sventitskii "Tables of
Neutral and ionization Atom
s "(IFI / PLENUM, New York, 19
68) p. It was created by referring to No. 19.

As seen in Table 1 and FIG. 6, Xe,
The main emission spectrum lines of the neutral excited species of Kr, Ar, and Ne are all on the longer wavelength side than the main emission spectrum line of He. Therefore, the photon energy of the main emission spectrum of Xe, Kr, Ar, and Ne is smaller than the photon energy of the main emission spectrum of He. That is, it is expected that the energy absorbed by the gate insulating film is small and the damage given to the gate insulating film is small. Above all, Xe
When used as a heat transfer gas, the wavelength corresponding to the band gap energy of SiO ₂ of 8.8 eV is 140.9 n.
It is theoretically supported that the value is close to m and the absorption into SiO ₂ is extremely small and the damage is also slight.

In order to confirm this, the following experiment was conducted. That is, a sample obtained by thermally oxidizing a Si substrate under the same conditions is subjected to VU by the plasma of Xe, Kr, Ar and He.
Irradiate V light under a certain condition to reduce the thickness of the surface damage layer to XP.
S (X-ray photo-electron sp
It was analyzed by means of electron microscopy. The plasma was prevented from coming into direct contact with the sample to prevent plasma irradiation damage. The results are shown in Table 2.

[0022]

[Table 2]

As is clear from Table 2, Xe, Kr, A
The damage of the thermal oxide film due to the VUV light irradiation by the r plasma is less than the damage due to the VUV light irradiation by the He plasma, and the effect of the present invention is experimentally verified.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described below with reference to the drawings.

Example 1 This example is an example in which Xe is used as a heat conduction gas and polycrystalline silicon on a gate oxide film is etched in one step. First, the substrate stage portion of the plasma etching apparatus used in this example is first described. Will be described with reference to FIG.

FIG. 3 is a schematic sectional view of the substrate stage portion of the substrate bias application type ECR plasma etching apparatus. Cooling pipe 13 for circulating and supplying a refrigerant such as ethanol
The substrate to be etched 11 is placed on the substrate stage 12 containing
On the substrate stage 12 with the mechanical clamper 14.
Crimp on. Further, a monopolar electrostatic chuck 15 is attached to the substrate stage 12, and the substrate 11 to be etched can be chucked also by the electrostatic chuck 15. Further, a substrate bias power supply 17 is introduced into the substrate stage via a blocking capacitor, and a heat conducting gas introducing hole 16 is formed so that a heat conducting gas of Xe is supplied from the upper surface of the substrate stage 12 to the rear surface of the substrate 11 to be etched. . The heat conducting gas flows out from the peripheral portion of the substrate 11 to be etched into the etching chamber and diffuses therein, where it is mixed with the etching gas to form the ECR plasma 18. Although there is a gap between the substrate 11 to be etched and the substrate stage 12 in FIG. 4, this is for convenience of description, and in reality, they are in close contact with each other.

Next, the plasma etching method according to the present embodiment will be described with reference to FIGS. Si
A gate insulating film made of SiO ₂ having a thickness of 10 nm is deposited on the semiconductor substrate 1 by thermal oxidation, and a Si-based material layer 3 made of polycrystalline silicon containing impurities is deposited to a thickness of 0.4 μm by CVD. Next, a negative type chemically amplified resist (SAL-601 manufactured by Shipley) and KrF excimer laser lithography are used to form a resist mask 4 having a width of 0.35 μm as an example. The sample formed up to this point shown in FIG. 1A is used as a substrate to be etched.

This substrate to be etched was set on the substrate stage 12 of the above-mentioned substrate bias application type ECR plasma etching apparatus, and as an example, polycrystalline silicon was patterned under the following conditions. Xe as heat transfer gas
Was maintained at 1 kPa (~ 8 Torr). Cl ₂ 70 sccm HBr 20 sccm O ₂ 10 sccm Gas pressure 1.0 Pa Microwave power 850 W (2.45 GHz) RF bias power 40 W (2 MHz) Substrate temperature 0 ° C. In this etching process, decomposition products of the resist are generated. The reaction products containing SiCl _x and SiBr _x form the side wall protective film 5 and the substrate temperature is set to 0 ° C., so that the radical reaction is suppressed, and as shown in FIG. Patterning is possible. Also, the addition ratio of a small amount of O ₂ makes it possible to obtain a selectivity with respect to the underlying SiO _2, and the gate insulating film 2 is less damaged by plasma irradiation. Further, since Xe is used as the heat conduction gas, damage by the short wavelength VUV light does not occur.

Next, when the resist mask 5 is removed by ashing, the side wall protective film 5 is oxidized and remains as the side wall altered film 6 as shown in FIG. 1 (c). This is removed with a 100: 1 diluted hydrofluoric acid aqueous solution to complete the gate electrode of the Si-based material layer 3 made of polycrystalline silicon as shown in FIG. In wet treatment with dilute hydrofluoric acid solution,
The gate insulating film is not abnormally thinned by the accelerated etching.

Example 2 This example is an example in which the same substrate to be etched as in Example 1 was etched in two steps using Xe as a heat transfer gas.
This will be described with reference to FIGS. 2 (a) to 2 (c) and FIG.

The substrate to be etched shown in FIG. 2A is shown in FIG.
Since it is the same as (a), duplicate description will be omitted. This substrate to be etched was set on the substrate stage 12 of the substrate bias application type ECR plasma etching apparatus used in Example 1, and as an example, main etching with a high etching rate was performed under the following conditions. The pressure of Xe as a heat transfer gas was maintained at 1000 Pa (up to 8 Torr). Cl ₂ 70 sccm O ₂ 10 sccm Gas pressure 1.0 Pa Microwave power 850 W (2.45 GHz) RF bias power 40 W (2 MHz) Substrate temperature 0 ° C. In this etching process, the radical reaction by Cl ^* is C
High-speed anisotropic etching progresses in a form assisted by the incidence of ions of l ⁺ and O ⁺ . SiCl _x adheres to the pattern side surface of the polycrystalline silicon together with the decomposition products of the resist to form the side wall protective film 5, which contributes to the improvement of the anisotropy. In the main etching, most of the Si-based material layer 3 in the layer thickness direction is etched, and further, the underlying gate insulating film 2 is stopped before being exposed at any place on the substrate to be etched, and the main overetching is performed under the following overetching conditions. Switch. The state at the end of the main etching is shown in FIG.

The timing of step switching to overetching is, for example, S which is an etching reaction product.
The emission spectrum 391 nm of iCl _x is monitored and set before the emission spectrum intensity during the main etching starts to drop. For this, the time when the emission spectrum intensity starts to drop on the same dummy substrate as the substrate to be etched is obtained in advance, and for example, 90% of the time may be set. FIG. 3 shows how the emission spectrum intensity changes with time and the timing of step switching.

Subsequently, the remaining polycrystalline silicon is over-etched under the following conditions. The pressure of Xe as a heat transfer gas is constant at 1000 Pa (up to 8 Torr). HBr 120 sccm O ₂ 4 sccm Gas pressure 1.0 Pa Microwave power 1000 W (2.45 GHz) RF bias power 20 W (2 MHz) Substrate temperature 0 ° C. In the etching process, SiBr _x was formed on the side surface of the polycrystalline silicon pattern. Adheres together with the decomposition products of the resist to form the side wall protective film 5, which contributes to the improvement of anisotropy.
Also, in polycrystalline silicon, the radical reaction due to Br ^* is Br.
Etching is assisted by very weak ions of ⁺ and O ⁺ . Therefore, even if the underlying gate insulating film 2 is exposed, it is not irradiated with plasma having high ion energy, and plasma irradiation damage is small. Further, since Xe is used as the heat transfer gas, it is not damaged by the short wavelength VUV light. The state after the completion of overetching is shown in FIG.

Next, as in Example 1, the resist mask 4 is removed by ashing, and the side wall altered film 6 is removed with a dilute hydrofluoric acid aqueous solution to complete the gate electrode of the Si-based material layer 3 made of polycrystalline silicon.

According to the present embodiment, in addition to using Xe as the heat conduction gas, the timing of switching to overetching is set before the exposure of the underlying gate insulating film, so that damage to the gate insulating film is caused. The preventive effect is more thorough, and accelerated etching of the gate insulating film during wet processing is hardly observed. Since most of the layer thickness of the Si-based material layer 3 is patterned by high-speed main etching, the throughput of the entire process is improved.

The present invention has been described above with reference to the two examples, but the present invention is not limited to these examples.

For example, Xe was used as the heat transfer gas,
Alternatively, a rare gas such as Kr, Ar or Ne may be used. The use of the heat conductive gas is useful not only for cooling the substrate but also for improving the efficiency of the heat exchange action between the substrate stage and the substrate to be etched not only when the substrate is heated.

Although Cl ₂ and HBr were used as etching gases, CCl ₄ , SiCl ₄ , BCl ₃ and CHC were used.
Other Cl-based gas such as l ₃ and HCl, Br ₂ , BBr ₃ and C
Other Br-based gas such as HBr ₃ may be used. Further, in order to improve the etching rate, it is effective to use or add an F type gas such as SF ₆ Use of an I-based gas such as HI can be expected to further improve the selection ratio with respect to the gate insulating film.

Although a dilute hydrofluoric acid aqueous solution was used for the wet treatment for removing the side wall altered film after the etching, buffered hydrofluoric acid (BHF), ammonia hydrogen peroxide solution or the like may be used.

The resist mask was removed by ashing with O ₂ or O ₃ , but it may be removed with a resist stripping solution.

Polycrystalline silicon has been exemplified as the Si-based material layer, but it may have a laminated structure such as silicide of other refractory metal such as W and Mo, polycide of refractory metal layer or the like. In this case, providing an antireflection layer such as SiON on the surface of the Si-based material layer is effective for fine gate electrode patterning.

Although the substrate bias application type ECR plasma etching apparatus is used as the etching apparatus, a parallel plate type RIE apparatus or a magnetron RIE apparatus may be used. If a high-density plasma etching apparatus such as a helicon wave plasma etching apparatus, a TCP etching apparatus, an ICP etching apparatus is used, further low damage, a high etching rate, and uniformity within a substrate to be etched can be expected.

Although the spectroscopic analysis of the emission spectrum is used as a process monitor for switching to the over-etching conditions, other monitor methods such as laser interferometry and mass spectrometry may be used as appropriate.

[0044]

As is apparent from the above description, according to the present invention, Xe is used as the heat transfer gas, and Kr, A
Since a rare gas such as r or Ne is used, the damage to the gate insulating film due to the short-wavelength VUV light is reduced as compared with the case where the conventional He is used as the heat conductive gas. Further, since the timing of switching to overetching is set before the underlying gate insulating film is exposed, the effect of reducing damage to the gate insulating film due to plasma irradiation can be obtained.

Therefore, it is possible to avoid abnormal film reduction due to accelerated etching of the gate insulating film and damage to the semiconductor substrate itself, which occurred during wet processing with dilute hydrofluoric acid or the like after completion of the gate electrode patterning, and it is possible to avoid the MIS type semiconductor. The device can be manufactured with good controllability. The effect of improving the gate breakdown voltage is also remarkable.

The plasma etching method of the present invention has a great effect particularly when it is used for patterning a fine gate electrode in the sub-half micron class, and has great significance for high integration of MIS type semiconductor devices.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view illustrating a plasma etching method according to a first embodiment of the present invention in the order of steps thereof,
(A) A state in which a gate insulating film, a Si-based material layer and a resist mask are sequentially formed on a semiconductor substrate, (b) is a state in which the Si-based material layer is patterned, and (c) is a resist mask ashed to form a side wall modified film. The remaining state, (d), is a state in which the side wall altered film is removed by wet processing to complete the gate electrode made of the Si-based material layer.

FIG. 2 is a schematic cross-sectional view illustrating a plasma etching method according to a second embodiment of the present invention in the order of steps thereof,
(A) A state in which a gate insulating film, a Si-based material layer and a resist mask are sequentially formed on a semiconductor substrate, (b) a state in which most of the Si-based material layer in the layer thickness direction is patterned by main etching, (c) Is a state in which the remaining portion of the Si-based material layer in the layer thickness direction is patterned by overetching.

FIG. 3 is a diagram showing a change over time in plasma emission intensity of a reaction product in the plasma etching method of Example 2 to which the present invention is applied.

FIG. 4 is a schematic sectional view of a substrate stage portion of the plasma etching apparatus used in Examples 1 and 2 to which the present invention is applied.

FIG. 5 is a light absorption spectrum diagram of SiO _{2 in} a VUV light region.

FIG. 6 is a diagram showing positions and intensities of spectral lines of plasma emission of He, Ar, Kr, and Xe.

FIG. 7 is a schematic cross-sectional view illustrating a conventional plasma etching method in the order of steps, (a) a state in which a gate insulating film, a Si-based material layer and a resist mask are sequentially formed on a semiconductor substrate, and (b) is a main state Etching is completed, and (c) is overetching.

FIG. 8 is a diagram showing a time change of plasma emission intensity of a reaction product in a conventional plasma etching method.

[Explanation of symbols]

 1 Semiconductor Substrate 2 Gate Insulating Film 3 Si-based Material Layer 3a Remaining Si-based Material Layer 4 Sidewall Protection Film 5 Sidewall Altered Film 6 Sidewall Protection Film 7 Damaged Layer 11 Etched Substrate 12 Substrate Stage 13 Cooling Pipe 14 Mechanical Clamper 15 Electrostatic Chuck 16 Heat conduction gas introduction hole 17 Substrate bias power supply 18 ECR plasma

Claims (6)

[Claims] 1. A thermal conductive gas is supplied from the upper surface of the substrate stage toward the rear surface of the substrate to be etched, and S on the base insulating film is supplied.
In the plasma etching method for patterning an i-based material layer, the photon energy of the main spectrum line of plasma emission generated by diffusing the heat conduction gas in the etching chamber is smaller than the photon energy of the main spectrum line of He plasma emission. A plasma etching method comprising patterning while supplying a heat conduction gas. 2. The S on the base insulating film is supplied from the upper surface of the substrate stage toward the rear surface of the substrate to be etched while supplying a heat transfer gas.
In a plasma etching method for patterning an i-based material layer, the wavelength of a main spectrum line of plasma emission generated by diffusing the heat conductive gas in an etching chamber is larger than that of a main spectrum line of He plasma emission. A plasma etching method comprising patterning while supplying a gas. 3. The plasma etching method according to claim 1, wherein the heat conduction gas is at least one selected from the group consisting of Xe, Kr, Ar and Ne. 4. A plasma etching method for patterning by multi-step etching including a low ion energy over-etching step, wherein switching to the over-etching step is set before exposure of a base insulating film. 3. The plasma etching method according to claim 1 or 2. 5. The plasma etching method according to claim 1, wherein the base insulating film is a gate insulating film of a MISFET. 6. The Si-based material layer is at least one selected from the group consisting of polycrystalline silicon, refractory metal silicide, and refractory metal polycide, and is characterized in that: Plasma etching method.