JP2003001598A - ETCHING METHOD OF Si FILM - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an etching method of an Si film which can precisely detect an etching end point when XeF2 gas is used to subject the Si film to an etching. SOLUTION: The method is an etching method of the Si film according to an isotropic dry etching method using a gas of xenon difluoride (XeF2 ). In the present method, a substrate having an Si film to be etched is put into an etching chamber to subsequently introduce the XeF2 gas into the etching chamber. When the etching proceeds while the pressure in the etching chamber is maintained at a predetermined pressure, the pressure maintained at the predetermined pressure value in the etching chamber temporarily and rapidly increases at a certain point of time. This point of time is determined as the one at which a natural oxidation film generated on the Si film is eliminated and the etching of the Si film is started, thereby enabling the time control of the etching of the Si film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for etching a Si film, and more specifically, when etching a Si film in which a natural oxide film is formed, a natural oxide film removal end point, that is, a Si film is removed. Reliably detect the etching start point,
The present invention relates to a Si film etching method capable of accurately controlling the Si film etching time. In particular, the present invention is directed to a metal film and a SiN film forming a micromachine element.
_This is the most suitable method for etching the Si film which is interposed as a sacrificial layer between the _X film and the _X film.

[0002]

2. Description of the Related Art With the progress of fine technology, silicon substrates,
A fine structure formed on a substrate such as a glass substrate is electrically and mechanically coupled to a semiconductor integrated circuit,
So-called Micro Machine (MEMS: Micro Electric M
The echanical system and ultra-compact electrical / mechanical composite devices are attracting attention.

Here, as an example of the mechanical portion of the micromachine element, a micro switching element will be taken as an example, and its configuration will be described with reference to FIG. FIG. 1 is a perspective view showing a configuration of a microminiature switching element which is an example of a micromachine element. As shown in FIG. 1, the microminiature switching element 10 includes an insulating substrate 12 such as a glass substrate and a C
A substrate-side electrode 14 formed of an r thin film or the like on the insulating substrate 12 and a bridge electrode portion 16 that intersects the substrate-side electrode 14 and bridges in a bridge shape. The board-side electrode 14 and the bridge electrode portion 16 are electrically insulated from each other by the void portion 18 between them.

The bridge electrode portion 16 is a bridge member 20 formed of a dielectric film such as a SiN _x film as an electrode supporting member that stands on the substrate 12 while straddling the substrate side electrode 14 in a bridge shape, and the substrate side electrode 14 And a bridge-side electrode 22 formed of an ITO film or the like on the bridge member 20 in parallel to each other. That is,
The bridge member 20 is provided so as to face the substrate-side electrode 14 and be separated by a predetermined distance, and to support the bridge-side electrode 22 in parallel with each other. When a minute voltage is applied between the substrate-side electrode 14 and the bridge-side electrode 22 facing the substrate-side electrode 14, the bridge electrode portion 16 approaches the substrate-side electrode 14 due to an electrostatic phenomenon, and When the application is stopped, they are separated and the original state is restored. The switching operation can be performed by utilizing the operation of approaching and separating the bridge electrode portion 16 with respect to the substrate side electrode 14.

Next, with reference to FIG. 2, a method of manufacturing the above-mentioned ultra-small switching element 10 will be described. 2A to 2E are cross-sectional views taken along the line I-I in FIG. 1 for each step in manufacturing the microminiature switching element 10. As shown in FIG. 2A, a metal film such as a Cr film is formed on the substrate 12 and patterned to form the substrate-side electrode 14. Next, as shown in FIG. 2B, a Si film or a polysilicon film is formed on the entire surface of the substrate 12 and patterned to form a sacrificial layer 24 on the substrate-side electrode 14. The sacrificial layer 24 functions as a support layer for forming the next bridge member 20, and is finally removed as described later.
Therefore, the sacrificial layer 24 is formed of a single crystal Si film, a polysilicon film, or the like having an excellent etching selection ratio with respect to the oxide film, the nitride film, and the metal film that form the substrate-side electrode 14 and the bridge electrode portion 16. There is.

Subsequently, a dielectric film such as a SiN _x film is formed on the entire surface of the substrate 12 and patterned, and as shown in FIG. A bridge member 20 standing on the substrate 12 is formed. Next, as shown in FIG. 2D, a bridge-side electrode film is formed of an ITO film or the like on the entire surface of the substrate 12 including the substrate-side electrode facing portion 20a of the bridge member 20, and patterning is performed to form the bridge member 20. The bridge-side electrode 22 is formed on the substrate-side electrode facing portion 20a. Next, the sacrificial layer 24 made of the Si film or the polysilicon film is removed by the dry etching method to form the switching element 10 as shown in FIG.

As described above, in the fabrication of the micromachine element, the surface micromachining technology based on the fabrication process of the semiconductor integrated circuit for forming the thin film structure on the silicon substrate is applied to the silicon substrate or the glass substrate. Forming a fine structure. Then, in order to form the above-described fine structure applying the elasticity of the beam or the like, it is necessary to form the void layer under the beam, and as described above, the sacrifice layer is provided in advance, Another layer forming the beam is formed on the sacrificial layer, and then the sacrificial layer is etched and removed to provide a void layer under the beam.

As the sacrificial layer, a single crystal Si film or a polysilicon film (hereinafter, referred to as a polysilicon film) having an excellent etching selection ratio with respect to the oxide film, the nitride film and the metal film is used. Further, XeF ₂ gas is used as an etching gas for the sacrificial layer, and Si is etched by the reaction of XeF ₂ + Si → SiF ₄ ↑ + Xe. The XeF ₂ gas is used for the materials such as LPCVD-SiN, thermal oxide film, aluminum (Al), titanium (Ti), etc.
Since it has an etching selection ratio of 000 or more, it is a very effective process when etching a polysilicon film to form a micromachine element.

[0009] In general the dry etching, using a plasma or a microwave or the like, it is necessary to excite the etching gas, etching of the polysilicon film by XeF ₂ gas, without exciting the XeF ₂ gas That is, the polysilicon film can be etched by using so-called raw gas. Therefore, since the polysilicon film can be etched by simply introducing the XeF ₂ gas into the etching chamber of the etching apparatus from the XeF ₂ gas supply source, the etching apparatus can have a very simple structure, and the RF Since it does not require electricity,
Low etching cost.

The structure of the etching apparatus using XeF ₂ gas will be described with reference to FIG. FIG. 3 is a flow sheet showing the structure of the etching apparatus. The etching apparatus 30 has a substrate stage 32 on which a substrate W having a polysilicon film is placed, an etching chamber 34 for introducing a XeF ₂ gas to etch the polysilicon film, and a XeF ₂ gas source. A cartridge mounting table 37 for mounting the XeF ₂ solid cartridge 36, and a XeF
And XeF ₂ gas plenum 38 provided as a buffer tank in order to stabilize the flow of ₂ gas, and a pressure controller 40 for controlling the pressure of the XeF ₂ gas. The etching chamber 34 includes a precise pressure gauge 42, and is connected to a vacuum suction device (not shown) via an exhaust port 44.
Maintained under vacuum. As the pressure gauge 42, a Baratron vacuum gauge or the like is used.

[0011] In vacuum etch chamber 34 by the vacuum suction device, when the the XeF ₂ gas plenum 38 for example 4.5 Torr, the XeF ₂ gas sublimed from XeF ₂ gas solid cartridge 36 XeF ₂ is. XeF
_{The 2} gas flows into the etching chamber 34 while being pressure controlled by the pressure controller 40, etches the polysilicon film, and then flows out from the exhaust port 44.

However, no matter how high the etching selection ratio is, if the etching is overetched, the SiN film or the like forming the bridge member 20 may be etched and damaged, and if the etching time becomes unnecessarily long, Since the work efficiency of etching is lowered, it is necessary to detect the end point (end point) of etching. In the dry etching method in which the etching gas is excited by RF or microwave, the emission spectrum can be detected to detect the etching end point. However, in the etching of the polysilicon film using XeF ₂ gas, the etching gas is used. Because it does not excite
The end point of etching cannot be detected by detecting the emission spectrum. Therefore, conventionally, when forming a micromachine element, when etching a sacrificial layer formed of a polysilicon film with XeF ₂ gas, the etching amount is adjusted by controlling the etching time.

[0013]

However, in forming a micromachine element, XeF ₂ gas is used,
When the polysilicon film forming the sacrificial layer is actually etched, the time required for etching varies, and it is difficult to control the etching time, and it is difficult to uniformly and evenly etch the polysilicon film.

Therefore, the present inventors have studied in order to study the progress of etching of a polysilicon film by XeF ₂ gas.
An etching experiment as described below was conducted. As a wafer sample 50, as shown in FIG.
An SiO ₂ film 54 and then a polysilicon film 56 were formed thereon, and a mask 58 made of a SiN film was further formed on the polysilicon film 56 to prepare an etching sample. still,
FIG. 4 is a perspective view showing the configurations of the wafer sample 50 and the mask 58. The mask 58 has a slit width of 0.5 μm.
A number of slit openings 60 having a length of 50 mm to 200 mm are arranged in parallel at the same intervals. There are two masks having a gap between slits of 2 μm and 4 μm.

FIGS. 5 and 6 are plan views seen from above the mask in order to show the time dependence when the wafer sample shown in FIG. 4 is etched with XeF ₂ gas. Figure 5
(A) to (d) show 3 minutes after 3 minutes when the mask 58 having the slit openings 60 in which the distance between the slits is 2 μm is used and the polysilicon film 56 is etched using XeF ₂ gas. Minutes later, 8 minutes later, and 12
Showing the etching state of the polysilicon film 56 after
It is the top view seen from the mask. Further, FIGS. 6A to 6D respectively use a mask 58 having a slit opening 60 in which the interval between the slits is 4 μm, and X
Plan view seen from above the mask showing the etching state of the polysilicon film 56 after 3, 6, 8 and 12 minutes when the polysilicon film 56 is etched using eF ₂ gas. Is.

As shown in FIGS. 5 (a) and 5 (b) and FIGS. 6 (a) and 6 (b), both of the cases using the mask 58 in which the intervals between the slit openings 60 are 2 μm and 4 μm, respectively, The etching of the polysilicon film 56 has not started until 6 minutes after the start of etching. Then, as shown in FIGS. 5 (c) and 6 (c), after 8 minutes,
Suddenly, the etching of the polysilicon film 56 progresses, and FIG.
As shown in (d) and FIG. 6 (d), after 12 minutes,
The polysilicon film 56 is in a state of being significantly etched.

The present inventor has further researched and found the following. A natural oxide film having a film thickness of several nm is formed on the polysilicon film, for example, about 2 months after the polysilicon film is formed on the SiO ₂ film. Since the unexcited XeF ₂ gas is used in the etching of the polysilicon film, the XeF ₂ gas easily passes through the natural oxide film even if the thickness of the natural oxide film is several nm. It cannot reach the polysilicon film. as a result,
As shown in FIGS. 5A and 5B and FIGS. 6A and 6B, the natural oxide film is etched for at least 6 minutes from the start of etching, and the etching proceeds to the polysilicon film. Not not. Then, as soon as the XeF ₂ gas erodes the thin portion of the natural oxide film and reaches the polysilicon film, as shown in FIGS. 5C and 6C, the polysilicon film is etched at a high etching rate. Begin to.

That is, the natural oxide film formed on the polysilicon surface delays the reaction between the XeF ₂ gas and the polysilicon, and the polysilicon surface is not uniformly etched. This is different from normal dry etching using RF or microwave in which gas molecules are excited and has large molecular energy, unlike XeF ₂
This is because XeF ₂ gas molecules are not supplied with energy from the outside during etching. That is, XeF ₂
Since the gas molecules do not have the ability to ion bombard the natural oxide film, the natural oxide film delays the reaction between the XeF ₂ gas and the polysilicon.

As described above, the etching amount of the polysilicon film does not depend on the etching time after the start of etching because of the presence of the natural oxide film. Therefore,
It was difficult to control the etching time of the polysilicon film. Therefore, conventionally, in order to prevent the etching of the polysilicon film from becoming non-uniform due to the natural oxide film, the natural oxide film formed on the polysilicon film is removed by wet etching or the like. Therefore, in the conventional etching of the polysilicon film, the process becomes complicated, and it is difficult to improve the etching productivity.

Therefore, an object of the present invention is to accurately detect the etching start point when etching a Si film and a polysilicon film by using XeF ₂ gas.
A method of etching a Si film and a polysilicon film is provided.

[0021]

The inventor of the present invention produced a sample having the same structure as that of the above-described ultra-small switching element 10 and
The experiment of etching the sacrificial layer of the polysilicon film with eF ₂ gas was repeated many times. In the process, when the etching progresses from the natural oxide film to the polysilicon film,
As shown in FIG. 7, it was found that the pressure in the etching chamber rapidly rises. This increase in pressure is due to the fact that Sie ₄ gas and Xe gas are generated as a result of the reaction between XeF ₂ and Si due to the rapid etching of the polysilicon film. FIG. 7 is a graph showing the relationship between the elapsed etching time and the pressure in the etching chamber, where the horizontal axis represents the elapsed time of the etching time and the vertical axis represents the pressure in the etching chamber. From this experimental result, it was found that the time for controlling the etching of the polysilicon film should be controlled with the starting point of the etching of the polysilicon film at the time when the pressure in the etching chamber suddenly rises.

In order to achieve the above object, based on the above findings, the Si film etching method according to the present invention is a Si film by an isotropic dry etching method using xenon difluoride (XeF ₂ ) gas. The etching method of
When the XeF ₂ gas is introduced into the etching chamber accommodating the substrate having the Si film to be etched to maintain the pressure in the etching chamber at the predetermined pressure value and the etching is advanced, the pressure is maintained at the predetermined pressure value in the etching chamber. The moment when the applied pressure suddenly rises is recognized as the moment when the etching shifts from the etching of the natural oxide film generated on the Si film to the etching of the Si film, that is, the etching start point of the Si film. , Si film etching is characterized by time control.

When the etching of the Si film is started,
This is the point in time when the pressure maintained at the predetermined pressure value in the etching chamber temporarily and suddenly rises. The pressure difference at that time is a pressure difference with which the pressure gauge can detect an increase in the pressure with respect to the pressure maintained at a predetermined pressure value. For example, the pressure maintained at 60mTorr is 80m
It is the pressure difference when rising to Torr. In the method of the present invention, the substrate is not limited to a plate-like one, but broadly refers to an object having a Si film to be etched. The Si film is a Si film including a single crystal Si film, an amorphous Si film, and a polysilicon film.

The Si film etching method according to the present invention comprises:
The Si film formed by the natural oxide film on the Si film is replaced with XeF ₂
It can be applied when etching with gas, but the Si film is a sacrificial layer interposed between the non-Si film and the non-Si film.
When forming an air gap between the non-Si film and the non-Si film by etching the i film, the Si film is particularly a micromachine element (MEMS: Micro Electric Mechanical System). Film and SiN _x
A sacrificial layer interposed between the film and the Si film is etched in forming the gap between the metal film and the SiN _X film, it can be suitably applied.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described specifically and in detail with reference to the accompanying drawings by way of example embodiments. Embodiment Example In this embodiment example, the etching method of the Si film according to the present invention is applied to the removal of the sacrificial layer made of the polysilicon film of the micromachine element (micro switching element 10) using the etching apparatus 10 described above. It is an example of the embodiment. First, the substrate having the sacrificial layer 24 (see FIGS. 2B and 2D) made of a polysilicon film is placed on the substrate stage 32 of the etching chamber 34, and the solid cartridge 36 is mounted on the cartridge mounting base 37. Then, the etching chamber 34 was vacuum-evacuated to maintain the etching conditions under the following pressures. Etching conditions Chamber pressure: 50 mTorr Chamber pressure set value: 1.0 Torr Gas plenum internal pressure: 4.5 Torr

Explaining further, Xe of the method of this embodiment
In the etching of the polysilicon film with F ₂ gas, as shown in FIG.
As shown in, the etching sample was fed into the etching chamber 34, vacuum suction was performed, and after 44 seconds, XeF ₂ gas was introduced. In FIG. 8, the time point of etching control is started when the etching sample is fed. The pressure in the etching chamber 34 is approximately constant at 60 mTorr until 345 seconds have passed after the etching sample was fed. During this time, the natural oxide film is being etched. After the elapse of 345 seconds, the pressure in the etching chamber 34 rapidly increased to 80 mTorr and was maintained at 80 mTorr for about 20 seconds. The pressure then gradually dropped back to 60 mTorr after about 40 seconds. It was confirmed that the etching of the Si film was started when the pressure suddenly increased to 80 mTorr. Further, the etching of the polysilicon film 24 progressed, and the etching of the polysilicon film 24 was completed about 200 seconds after the pressure was increased.

In the method of this embodiment, the pressure inside the etching chamber 34 rises as shown by the circles in FIG. 8 during the etching after the start of the etching. The time when the pressure rise starts is the time when the natural oxide film on the polysilicon film 24 is removed and the etching of the polysilicon film 24 is started. Therefore, by controlling the elapsed time from the start time, it is possible to always perform stable etching of the polysilicon film. In this embodiment, the etching of the polysilicon film is taken as an example, but a single crystal Si film may be used.

[0028]

According to the method of the present invention, a substrate having a Si film to be etched is placed in an etching chamber, and then XeF ₂ gas is introduced into the etching chamber to maintain the pressure in the etching chamber at a predetermined pressure. When the etching progresses, the natural oxide film formed on the Si film is removed at the time when the pressure maintained at the predetermined pressure value in the etching chamber temporarily and rapidly increases.
The following effects can be obtained by recognizing the time when the etching of the i film is started and controlling the etching of the Si film for a time. (1) Since the etching start point of the Si film can always be detected stably without depending on the film thickness and quality of the natural oxide film formed on the Si film, the etching start point of the Si film can be used as a reference. As a result, the etching of the Si film can be precisely controlled. In other words, based on the etching start point of the Si film, the etching can be stopped during the etching of the Si film, and the residual film management can be accurately performed. (2) Since the step of removing the natural oxide film, which was conventionally necessary to keep the etching amount of the Si film constant, can be omitted, the process can be simplified and the etching cost can be reduced. (3) Instead of expensive endpoint detection means required when detecting the etching end point based on the emission spectrum, the etching start point of the Si film is detected simply by reading the pressure gauge, and the etching is accurately performed. Can be time controlled.

[Brief description of drawings]

FIG. 1 is a perspective view showing a configuration of a micro switching element which is an example of a micromachine element.

2 (a) to 2 (e) are each a line I-I of FIG. 1 for each step in manufacturing a microminiature switching element.
FIG.

FIG. 3 is a flow sheet showing the configuration of an etching apparatus.

FIG. 4 is a perspective view showing configurations of a wafer sample and a mask.

5 (a) to 5 (d) are 3 minutes and 6 minutes, respectively, when using a mask having slit openings with a slit spacing of 2 μm and etching with XeF ₂ gas. FIG. 6 is a plan view showing the state of etching of the polysilicon film after 8 minutes and 12 minutes as seen from above the mask.

6 (a) to 6 (d) are 3 minutes and 6 minutes, respectively, when using a mask having slit openings with a slit spacing of 4 μm and etching with XeF ₂ gas. FIG. 6 is a plan view showing the state of etching of the polysilicon film after 8 minutes and 12 minutes as seen from above the mask.

FIG. 7 is a graph showing the relationship between the elapsed etching time and the pressure inside the etching chamber.

FIG. 8 is a graph showing the relationship between the elapsed etching time and the pressure in the etching chamber when the method according to the embodiment is performed.

[Explanation of symbols]

10 ... Ultra-small switching element, 12 ... Insulating substrate, 14 ... Substrate side electrode, 16 ... Bridge electrode part, 1
8 ... Void portion, 20 ... Bridge member, 22 ... Bridge side electrode, 24 ... Sacrificial layer, 30 ... Etching device,
32 ... Substrate stage, 34 ... Etching chamber,
36 …… XeF ₂ solid cartridge, 37 …… Cartridge mount, 38 …… XeF ₂ gas plenum, 40 ……
Pressure controller, 42 ... pressure gauge, 44 ... exhaust port,
50 ... Wafer sample, 52 ... Si substrate, 54 ...
SiO ₂ film, 56 ... Polysilicon film, 58 ... SiN
Membrane, 60 ... slit opening.

Claims (3)

[Claims] 1. A method for etching a Si film by an isotropic dry etching method using a xenon difluoride (XeF ₂ ) gas, wherein an XeF ₂ gas is contained in an etching chamber containing a substrate having a Si film to be etched. Is introduced, and when the etching is advanced while maintaining the pressure in the etching chamber at a predetermined pressure value, the etching is performed when the pressure maintained at the predetermined pressure value in the etching chamber temporarily rises rapidly. The etching of the Si film is time-controlled by recognizing the time point when the etching of the natural oxide film formed on the Si film shifts to the etching of the Si film, that is, the etching start point of the Si film. Etching method of Si film. 2. The Si film is a sacrificial layer interposed between the non-Si film and the non-Si film, and the Si film is etched to form a gap between the non-Si film and the non-Si film. The method for etching a Si film according to claim 1, wherein: 3. The micromachined device (M
3. The etching of the Si film according to claim 2, which is a sacrificial layer interposed between a metal film and a SiN _x film forming an EMS (Micro Electric Mechanical System). Method.