JP2629773B2 - Method of forming multilayer thin film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for forming a multilayer thin film suitably used for forming an amorphous semiconductor superlattice thin film and the like.

[Problems to be solved by conventional technology and invention]

Although amorphous semiconductor thin films are being put to practical use in solar cells, thin film transistors, and the like, research results in performance tend to be saturated. The main reason for this is that carrier mobility is low in an amorphous semiconductor having many defects. As a new material to overcome this situation, amorphous semiconductor superlattices have attracted attention.

The amorphous semiconductor superlattice is obtained by stacking two types of amorphous semiconductors (hereinafter, referred to as an A layer and a B layer) having different compositions, band gaps, or impurities to be doped at a period of several to several tens of degrees. Conventionally, as a method for producing this amorphous semiconductor superlattice thin film, plasma or photo-excited CVD (chemical vaper deposition) has been used.
A method is adopted in which the A layer and the B layer are alternately laminated by mechanically switching the introduction of the raw material gas. That is, A
A source gas for depositing a layer is introduced into the reaction chamber, and the source gas is excited and decomposed by light or plasma to deposit an A layer to a desired thickness. Next, the supply of the raw material gas is stopped, and after the reaction chamber is evacuated, a raw material gas for depositing the B layer is introduced, and the B layer is similarly deposited to a desired thickness. Next, the reaction chamber is evacuated again so that both layers are not contaminated with impurities at the boundary between the layer A and the layer B. A method of manufacturing a superlattice thin film by repeating this operation a predetermined number of times is general. For example, when laminating a-Si (amorphous-Si) and a-siC (amorphous-SiC), a source gas containing silicon is decomposed when depositing a-Si, and when a-SiC is deposited. Decomposes a mixed gas of a source gas containing silicon and a source gas containing carbon, and a superlattice thin film is produced by repeating the above operations.

However, the conventional method of producing a superlattice thin film by plasma or photoexcited CVD essentially requires mechanical switching of the source gas, and the reaction layer is formed every time a reaction for forming a film of each layer is performed. Since the operation of evacuating the source gas and evacuating the system is required, the productivity of the superlattice film is very low.

Therefore, in the above method, the source gas is required to be switched, and one layer of deposition is essentially one operation step. Therefore, when a superlattice thin film as a multilayer laminated film is manufactured, the number of operation steps becomes enormous, and There was a problem that it became a big obstacle.

In addition, a reaction chamber that forms the layer A and a reaction chamber that forms the layer B are adjacent to each other via a partition wall, and a rotary body (rotary) that alternately rotates between the two chambers at a predetermined speed on the partition wall. There is also a method in which a substrate is placed on the rotating body, and the substrate is rotated integrally with the rotating body, so that the A layer and the B layer are alternately laminated on the substrate. ineffective.

The present invention has been made to improve the above situation,
An object of the present invention is to provide a method for forming a multilayer thin film such as an amorphous semiconductor superlattice thin film, which can efficiently, simply and reliably form a multilayer thin film in one reaction chamber without repeatedly introducing and exhausting a source gas. .

[Means and actions for solving the problem]

In order to achieve the above object, the present invention provides a first raw material gas capable of only or preferentially causing any one of a plasma-excited chemical reaction and a photo-excited chemical reaction, A second source gas having a significantly different reactivity from the source material, for example, a mixed gas with a source gas capable of performing both reactions is introduced into the reaction chamber, and one of the plasma-excited chemical reaction and the photo-excited chemical reaction is independently performed in the reaction chamber. Alternatively, one of the chemical reactions is performed on the substrate preferentially and intermittently, and the other chemical reaction is performed intermittently or continuously so that the reaction partially overlaps with one of the chemical reactions. The multilayer thin film is formed by sequentially and alternately stacking the thin film when it is applied and the thin film when one of the chemical reactions is stopped.

That is, the present invention is to continuously form a multilayer thin film such as a superlattice structure without intermittent supply of the source gas,
This utilizes the difference in chemical reactivity between two or more source gases, and two types of excitation sources are used simultaneously to excite the reaction gas. For example, a light source that generates excitation energy with a narrow distribution such as a low-pressure mercury lamp or an excimer laser and the generation of high-speed electrons having a wide energy distribution, for example, a discharge plasma that generates high energy of 10 eV or more are used.

Thus, the present invention provides one of plasma excitation energy and photoexcitation energy discontinuously to a reaction chamber filled with a mixture of gases having optical reactivity different from each other, and applies the other excitation energy to one of the reaction chambers. The film is applied intermittently or continuously so as to partially overlap with the excitation energy, and a thin film when one of the excitation energies is used and a thin film when one of the excitation energies are stopped are alternately formed on the substrate. For example, as a source gas, one that can be decomposed by light or plasma and one that can be decomposed by plasma but not decomposed by light are selected, and a mixed gas of these source gases is constantly guided to the reaction chamber. By irradiating the light continuously and turning the discharge on and off periodically to pulsate the plasma, it is possible to perform CVD using only light and composite excitation of light and plasma. Continuously switched in the same reaction chamber of CVD that, whereby the discharge is only light when off CV
A film of D is formed, and when the discharge is on, a film of an alloy composition from both source gases is formed.

Therefore, according to the present invention, desired various multilayer thin films can be formed by variously selecting the mixed gas and the reaction mode, and in this case, the mixed gas as a raw material can be continuously supplied as described above, The introduction and exhaust do not need to be repeated, and the number of reaction chambers may be one, and the reaction chambers do not need to be switched. Therefore, a multilayer thin film can be formed efficiently and simply, and the apparatus becomes complicated and the cost is high. Inconvenience such as becoming does not occur.

Further, the superlattice film manufactured according to the present invention has better semiconductor properties than a single-layer film having the same composition on average,
For example, compared to a conventional bulk film having the same optical bandgap, and having a very high photoelectricity as compared with each laminated single layer. Further, since the plasma excitation and the light excitation are repeated with the phase changed, the structure and characteristics of the obtained superlattice film are improved as compared with those obtained by the conventional gas exchange method or the single excitation method. Therefore, according to the present invention, a solar cell,
An amorphous semiconductor superlattice film having excellent characteristics can be obtained by being used for an optical sensor, a photosensitive drum, a light emitting element, a thin film transistor and the like.

 Hereinafter, the present invention will be described in more detail.

The method for forming a multilayer thin film according to the present invention is to sequentially form thin films having different compositions or band gaps alternately in the same reaction chamber using a plasma-excited chemical reaction and a photo-excited chemical reaction. Only one of the chemical reaction and the photo-excited chemical reaction is possible or occurs preferentially (the plasma-excited chemical reaction occurs preferentially over the photo-excited chemical reaction, or the photo-excited chemical reaction occurs over the plasma-excited chemical reaction). A mixed gas of a first raw material gas (which occurs preferentially) and a second raw material gas having a significantly different reactivity from the above raw materials for these chemical reactions is introduced.

Here, specific examples of the mixed gas include the following.

(1) A raw material gas capable of only or preferentially causing a plasma-excited chemical reaction (hereinafter referred to as P reaction) and a raw material gas capable of only or preferentially occurring a photoexcited chemical reaction (hereinafter referred to as L reaction) Mixed gas.

(2) A mixed gas of a raw material gas capable of performing only the P reaction or which occurs preferentially and a raw material gas capable of performing both the P reaction and the L reaction.

(3) A mixed gas of a source gas capable of performing only the L reaction or occurring preferentially and a source gas capable of performing both the P reaction and the L reaction.

Here, the layer obtained from the first source gas is different from the layer obtained from the second source gas.

In this case, in the present invention, the P reaction is preferably a discharge plasma capable of exciting high energy of 10 eV or more, particularly 10 to 20 eV by high-speed electrons, and inductively coupled or capacitively coupled plasma by direct current or high frequency. It can be performed using a CVD apparatus, a microwave CVD apparatus, or the like. Also L
The reaction is excellent in monochromaticity of excitation energy, and a light source having a narrow distribution can be effectively used.For example, an ultraviolet light source such as a low-pressure mercury lamp, an excimer laser, a deuterium discharge tube, a rare gas discharge tube, etc. It can be performed by utilizing a photochemical reaction induced by light of 300300 nm. Therefore, as a raw material gas capable of P reaction or L reaction or occurring preferentially, it reacts by these reaction methods,
Those capable of forming a thin film can be used.

Specifically, considering plasma CVD as the P reaction and mercury-sensitized CVD as the L reaction, as a raw material gas that can only perform the P reaction or is significantly more preferred than the L reaction,
Examples thereof include CF ₄ , CH ₄ , SiF ₄ , BF ₃ , NF ₃ , and N ₂ , and one or more of these can be selectively used depending on the type of a thin film to be formed. In addition, examples of the source gas capable of only the L reaction or that occurs preferentially include GeF ₄ and GeH ₄ , and one or more of these may be selectively used. Further, as a source gas capable of performing both the P reaction and the L reaction, SiH ₄ , Si
₂ H ₆ , Si ₃ H ₈ , SiHnR _(4-n) , SiHnF _(4-n) , AlR ₃ , GeR ₃ , ZnR ₂ , SnR
₄ (however, n = 1, 2, or 3, R represents an alkyl group or an aryl group, and R has 1 to 7 carbon atoms, particularly 1 to 4 when R is an alkyl group, and R represents an aryl group. In this case, the compound preferably has 6 to 7 carbon atoms).
Species or two or more may be used. In the present invention, the L reaction includes a dark reaction in which the reaction is excited by light irradiation, and the reaction continues even if the light irradiation is stopped. Can be. Such a source gas capable of dark reaction includes, for example, SiH ₄ + O ₂ , Al (CH ₃ ) _{3 and the} like.

In the present invention, the mixed gas of any of the above (1) to (3) is introduced into the reaction chamber at a pressure of, for example, 0.01 to 100 Torr, and then the reaction between the plasma-excited chemical reaction and the photo-excited chemical reaction is performed in the reaction chamber. At least one of the chemical reactions is performed intermittently, and the other chemical reaction is performed intermittently or continuously so that the one chemical reaction and the reaction partially overlap.

That is, the following are examples of the reaction mode.

(A) The P reaction and the L reaction are each performed intermittently so that the reactions partially overlap.

(B) As shown in FIG. 1, the P reaction is performed intermittently, while the L reaction is performed continuously.

(C) As shown in FIG. 2, the L reaction is performed intermittently,
During this time, the P reaction is continuously performed.

In this case, the reaction time (or no reaction time) interval is usually
The interval is 0.1 second to 20 minutes, preferably 1 second to 10 minutes, but this interval is not particularly limited, and varies depending on the multilayer thin film.

In the reaction modes (b) and (c), the continuous reaction may be performed continuously during the formation of the multilayer thin film, and an appropriate pause period is provided between the formations of the multilayer thin film (ie, May be performed intermittently). Further, various modifications of the above (a) to (c) are possible, for example, the P reaction or the L reaction can be performed during this rest period.

Thus, the reactor is configured such that any of these modes of reaction take place in the reaction chamber.

More specifically, an apparatus for forming a multilayer thin film can be constituted by the following. For example, a reaction chamber,
A device for holding a substrate in a reaction chamber, a first source gas capable of only or preferentially causing only one of a plasma-excited chemical reaction and a photo-excited chemical reaction, and the above-mentioned source material for these chemical reactions. Is a device for introducing a mixed gas with a second source gas having different reactivity into a reaction chamber, a device for plasma generation, a device for light generation, and one of the device for plasma generation and the device for light generation operates intermittently. In addition, a reaction control device that controls the other to operate intermittently or continuously in a different phase from the other can be configured. Specifically, an apparatus shown in FIG. 3 to be described later can be used, but is not limited to this.

In this case, such a reaction control can be performed by a computer or a known electric circuit. For example, by using continuous irradiation light and pulsed plasma as an excitation source, the reaction mode of (b) can be performed. it can. Here, the order of the pulses can be constant or changed during the reaction.

In addition, the introduction of the mixed gas as a raw material can be continuously performed without interruption, and thus a multilayer thin film can be continuously formed. Often, the composition of the mixed gas can be changed during the reaction.

Known conditions can be used for the conditions of the P reaction and the L reaction, and the P reaction and the L reaction can be carried out by ordinary methods. For example, although not particularly limited, the temperature of the substrate can be from room temperature to 500 ° C., and the reaction gas can be diluted with hydrogen gas or a rare gas, but the total pressure of the gas is 0.01 to It can be 100Tonn. In addition, pulsed high frequency oscillator <RF> (13.56M
Hz) power density of 0.01-100 W / m ² , mercury vapor pressure 0-0.2
Conditions such as Torr can be employed.

In the present invention, the type of the substrate is not particularly limited and variously selected. However, if a superlattice film is obtained, a flexible substrate such as glass, ceramic, metal, or a polymer film may be used. it can.

Thus, the present invention provides the following methods (1) to (3).
By combining the reaction modes of (a) to (c) with the mixed gas as the raw material of the above, the thin film when one chemical reaction is performed on the substrate and the thin film when one chemical reaction is stopped on the substrate By laminating them sequentially and alternately, various desired multilayer thin films in which thin films having different compositions or band gaps are laminated can be obtained.

For example, when the mixed gas of (2) is used and the reaction mode of (b) is adopted, the source gas capable of performing both the P reaction and the L reaction when the plasma discharge is stopped is light. And the P based on this L reaction
A thin film is formed by a source gas capable of reacting and reacting L,
Then, when the plasma discharge is turned on, both the source gas capable of performing only the P reaction or the source gas capable of performing the P reaction and the L reaction can be excited, and the composition or band gap of the thin film is different from that of the thin film. A thin film is formed by the two different source gases.

The number of layers varies depending on the type of multilayer thin film,
Usually, it can be 2 to 400 layers, preferably 2 to 200 layers. For example, it is possible to form a 100-layer multilayer thin film in which each layer obtained by alternately laminating 50 layers obtained by one reaction and 50 layers obtained by the other reaction has a thickness of about 50 °. In this case, the thickness of each layer is not particularly limited.
In particular, it is preferable that the angle be 10 ° to 200 °.

〔The invention's effect〕

As described above, according to the present invention, a multilayer thin film such as an amorphous semiconductor superlattice thin film can be formed efficiently and simply and reliably in five reaction chambers continuously without repeatedly introducing and exhausting a source gas. can do. Further, according to the present invention, a superlattice film having excellent semiconductor characteristics can be easily formed.

Hereinafter, the present invention will be described in detail with reference to Examples of the present invention, but the present invention is not limited to the following Examples.

Example 1 A multilayer thin film was formed using the reactor shown in FIG.

Here, in FIG. 3, 1 is a tubular reactor, 2 is a reaction chamber formed therein, 3 is a mixed gas introduction path, 4
Reference numeral denotes an exhaust port, and a rotating body 5 is rotatably disposed in the reaction chamber 2, and a substrate 7 is placed on a disk 6 of the rotating body 5. A pulse high-frequency oscillator (13.56 MHz) 8 which is provided outside the reactor 1 and controlled on and off by a computer is provided beside the plasma forming space (upper part of the reaction chamber 2) 2a above the substrate 7. (Inductive coupling type)
When the oscillator 8 is turned on, the plasma forming space 2a
Plasma is formed in the plasma forming space
Above 2a, a low-pressure mercury lamp 10 that is always on during the thin film formation period and irradiates light through a synthetic quartz window 9
Is provided. The arrows in the figure indicate the flow direction of the mixed gas. In the figure, reference numeral 11 denotes an inert gas introduction port, and reference numeral 12 denotes a flow path thereof. The introduction of the inert gas prevents the quartz window 9 from fogging.

Thus, a multilayer thin film was formed using the above apparatus under the following conditions.

Preparation conditions Mixed gas Si ₂ H ₆ : 3.0sccm CF ₄ : 60.0sccm Pressure 700mTorr Substrate temperature 300 ℃ Plasma power 30W Plasma ON state: 33 seconds 10 sets repetition OFF state: 58 seconds UV wavelength 185nm, 254nm Substrate Corning 7059 glass silicon wafer (Single crystal) In the above method, when the plasma is off, Si ₂ H
Only ₆ is decomposed by light to form a thin film composed of Si and H. When the plasma is on, both Si ₂ H ₆ and CF ₄ are excited to form a thin film composed of Si, C, H, and F. Was done.

 The physical properties of the obtained film were as follows.

Film when plasma is off (L reaction only, Comparative example) Film thickness (1 hour reaction) 3100Å Composition (amorphous silicon) Si: 96% H: 4% Optical band cap 1.80 eV Dark conductivity 6.04 × 10 ^-11 S · cm ^-1 Photoconductivity 8.56 × 10 ^-6 S · cm ^-1 Film when plasma is on (film by L reaction and P reaction, comparative example) Film thickness (1 hour reaction) 5500Å Composition (Amorphous SiC) Si: 57% C: 11%: 28%: 4% Optical band cap 2.35 eV Dark conductivity 1.12 × 10 ^-8 S ・ cm ^-1 Photoconductivity 2.40 × 10 ^-7 S ・ cm ^-1 A film obtained by laminating a total of 20 layers of a film formed by only the L reaction and a film formed by the L reaction and the P reaction, each having a thickness of 50 mm and a total of 20 layers each (a superlattice film, an example of the present invention). Cap 2.05eV Dark conductivity 1.69 × 10 ^-8 S · cm ^-1 Photoconductivity 6.18 × 10 ^-5 S · cm ^-1 X-ray diffraction results of the superlattice film obtained in Fig. 4 (a-Si
(X-ray diffraction spectrum of (50 °) 41 layers / a-SiC (50 °) 40 layers). The results in FIG. 5 clearly show that the superlattice films are regularly stacked at the set film thickness.

Based on the above results, a combination of a gas that is excited by plasma and is not decomposed by light and a gas that decomposes by light is always irradiated with light, and by generating plasma intermittently, it is possible to continuously operate without switching gas. It is recognized that a superlattice film can be produced by the process.
The superlattice film manufactured by the above method has better semiconductor characteristics than a single-layer film having the same composition on average. That is, compared to a conventional bulk a-SiC having the same optical band gap, the photoconductivity is higher by one digit or more. Moreover, the photoconductivity of each a-Si, a-SiC single layer is better than that of the monolayer, and therefore, the present invention ensures a superlattice film having excellent semiconductor properties based on the quantum effect unique to the superlattice. It is found that it can be obtained.

Example 2 A multilayer thin film was formed using the above-described apparatus under the following conditions.

Preparation conditions Mixed gas Si ₂ H ₆ : 3.0sccm CH ₄ : 60.0sccm Pressure 700mTorr Substrate temperature 300 ℃ Plasma power 30W Plasma On state: 94sec Off state: 151sec Ultraviolet wavelength 185nm, 254nm Substrate Corning 7059 glass silicon wafer (single crystal In the above method, when the plasma is on, Si ₂ H
Only ₆ is decomposed by light (only the L reaction proceeds) to form a thin film composed of Si and H (amorphous Si). When the plasma is on, both Si ₂ H ₆ and CH ₄ are excited (L and P reaction both proceed), thin film composed of Si, C, H (amorphous Si
C) formed.

 The physical properties of the obtained film were as follows.

Film when plasma is off (L reaction only, Comparative example) Film thickness (90 min reaction) 2900Å Optical band cap 1.78 eV Dark conductivity 8.67 × 10 ^-11 S · cm ^-1 Photoconductivity 2.50 × Film when 10 ^-6 S · cm ^-1 plasma is on (film by L reaction and P reaction, comparative example) Film thickness (120 minute reaction) 2400Å Optical band cap 2.13 eV Dark conductivity 3.07 × 10 ^-11 S ・ cm ^-1 Photoconductivity 1.22 × 10 ^-7 S ・ cm ^-1 One layer of L reaction only and one layer of L reaction and P reaction are alternately 10 layers each with a thickness of 50mm, totaling 20 layers Laminated film (superlattice film, example of the present invention) Thickness 1000Å Optical band cap 1.90 eV Dark conductivity 7.77 × 10 ^-9 S · cm ^-1 Photoconductivity 3.25 × 10 ^-5 S · cm ⁻¹ [Example 3) A multilayer thin film was formed using the above-described apparatus under the following conditions.

Create conditions mixed gas _{Si 2 F 4: 78sccm H 2} : 7.35sccm GeH 4: 0.15sccm Ar: 15sccm pressure 1.0mTorr substrate temperature 300 ° C. plasma power 40W plasma ON state: 67 seconds 10 pairs repeatedly turned off: 455 seconds UV wavelength 185nm 254nm substrate Corning 7059 glass silicon wafer (single crystal) In the above method, when plasma is off, SiF ₄
Is not excited by light, forms only a thin film of Ge and H (amorphous Ge), and when plasma is on, SiF ₄
Is excited to form a thin film composed of Si, Ge, H, and F (amorphous SiG
e) formed.

 The physical properties of the obtained film were as follows.

Film when plasma is off (film by L reaction only, comparative example) Film thickness (120 minute reaction) 1500Å Optical band cap 0.82eV Dark conductivity 2.33 × 10 ^-4 S · cm ^-1 Photoconductivity 2.35 × Film when 10 ^-4 S · cm ^-1 plasma is on (film by L reaction and P reaction, comparative example) Thickness (60 minute reaction) 7200 反 応 Optical band cap 1.56 eV Dark conductivity 1.27 × 10 ^-11 S ・ cm ^-1 Photoconductivity 3.60 × 10 ^-7 S ・ cm ^-1 A film made only by L reaction and a film made by L reaction and P reaction are alternately 10 layers each with a thickness of 50 mm, totaling 20 layers Laminated film (superlattice film, example of the present invention) Thickness 1000 Å Optical band cap 1.25 eV Dark conductivity 1.56 × 10 ^-7 S · cm ^-1 Photoconductivity 2.76 × 10 ^-7 S · cm ^-1 Example 2 From the results of 3 and 3, the gas which is not decomposed by light while being excited by the plasma is combined with the gas which decomposes, and the light is constantly irradiated and the plasma is cut off. It is recognized that the superlattice film can be produced in a continuous process without switching the gas by continuously generating the gas. Also, the superlattice film produced by the above method,
It has better semiconductor characteristics than a single-layer film having the same composition on average.

[Brief description of the drawings]

1 and 2 are diagrams each illustrating an example of a reaction mode of the present invention, FIG. 3 is a schematic sectional view showing an example of a reaction apparatus used for carrying out the present invention, and FIG. 4 is obtained by the present invention. 7 is an X-ray diffraction spectrum of an example of a multilayer thin film. DESCRIPTION OF SYMBOLS 1 ... Reaction device, 2 ... Reaction chamber, 2a ... Plasma formation space, 3 ... Mixed gas introduction path, 4 ... Exhaust port, 7 ... Substrate, 8 ... Pulse high frequency oscillator, 10 ... Low pressure mercury lamp.

Claims (1)

(57) [Claims] 1. A first raw material gas capable of performing only one of a plasma-excited chemical reaction and a photo-excited chemical reaction or occurring preferentially, and said raw material reacting with these chemical reactions. A mixed gas with a second source gas of different type is introduced into the reaction chamber, and one of the plasma-excited chemical reaction and the photo-excited chemical reaction is independently or preferentially and intermittently performed in the reaction chamber, and When the reaction is performed intermittently or continuously so that the reaction partially overlaps with one of the chemical reactions, and the thin film and one of the chemical reactions are stopped when one of the chemical reactions is performed on the substrate Characterized in that the thin films are sequentially and alternately laminated.