JP2697501B2 - Thin film formation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a thin film such as a silicon film on the surface of a substrate by a plasma CVD method using high-frequency discharge.

[0002]

2. Description of the Related Art FIG. 3 is a schematic view showing an example of a conventional plasma CVD apparatus. This apparatus is of a so-called parallel plate type (also called a capacitive coupling type), and holds a substrate (for example, a substrate) 2 on which a film is to be formed in a vacuum vessel 4 which is evacuated by a not-shown evacuation apparatus. The holder / electrode 6 and the discharge electrode 8 are housed facing each other. The base 2 on the holder / electrode 6 is heated by, for example, a heater 10.

[0003] The holder / electrode 6 is grounded, and the high-frequency power source 1 is connected to the discharge electrode 8 via a matching box 12.
The high-frequency power supply 14 supplies high-frequency power between the electrodes 6 and 8. This high-frequency power is conventionally a continuous sine wave, and its frequency is usually 13.56 MHz.

In such an apparatus, the vacuum vessel 4 is evacuated, a required raw material gas (for example, a mixed gas of silane (SiH ₄ ) gas and hydrogen (H ₂ ) gas) is introduced therein, and the electrode 6 When a high-frequency power is supplied from a high-frequency power source 14 between the first and second electrodes 8, a high-frequency discharge is generated between the electrodes 6 and 8 and the source gas 16 is turned into plasma (18 indicates the plasma). (For example, a silicon thin film) is formed.

[0005]

However, the conventional film forming method as described above has the following problems.

[0008] Since a mere high-frequency power is simply supplied between the electrodes 6 and 8, the state of the plasma 18 and, in particular, the radicals (active species) therein cannot be controlled. The generation of particles (dust) due to the generation of unnecessary radicals cannot be suppressed.

It is not possible to prevent the negatively charged particles in the plasma 18 from collecting and adhering to the substrate 2 as particles.

In low-temperature film formation, crystallization of the film cannot be expected because sufficient energy is not given to the film to cause crystallization of the film formed on the surface of the substrate 2.
In order to obtain a crystallized film, heat treatment such as high-temperature annealing and laser annealing is required after film formation, and the number of steps increases accordingly.

Accordingly, the present invention is based on the plasma CVD method, and has as its main object to provide a thin film forming method capable of suppressing the generation of particles and promoting crystallization of the film even at a low temperature. And

[0010]

In order to achieve the above object, a thin film forming method according to the present invention is characterized in that an original high-frequency signal is intermittently interposed between the discharge electrode and a holder / electrode. In addition to supplying high-frequency power that has been subjected to the first modulation and the second modulation that is intermittently performed at a shorter cycle than the first modulation, the holder / electrode is synchronized with the second modulation of the high-frequency power. It is characterized in that an intermittent negative bias voltage is applied.

[0011]

In the plasma, radicals contributing to the formation of a high-quality film and radicals unnecessary for film formation and causing particles are mixed. Generally, the former has a relatively long life and the latter has a relatively short life. Thus, as described above, by applying the first modulation to the high-frequency power, it is possible to preferentially generate radicals contributing to high-quality film formation and suppress unnecessary radicals, thereby suppressing generation of particles. Can be.

Further, by subjecting the high-frequency power to the second modulation, the generation of radicals in the plasma is greatly reduced by utilizing the electron temperature transition at the initial stage of plasma generation when the high-frequency power is turned on by the second modulation. It is possible to control the contributing electron temperature, thereby increasing the required radicals and suppressing unnecessary radicals in the early stage of radical generation, which cannot be controlled by the first modulation alone, in a shorter time schedule. Can be formed and maintained.

When a negative bias voltage is applied to the holder / electrode as described above, ions in a sheath region formed near the surface of the substrate are accelerated by the bias voltage and collide with the surface of the substrate. Thereby, crystallization of the film can be promoted even in low-temperature film formation.

[0014]

FIG. 1 shows a plasma C used in the embodiment of the present invention.
It is the schematic which shows an example of a VD apparatus. Parts that are the same as or correspond to those in the conventional example of FIG. 3 are denoted by the same reference numerals, and differences from the conventional example will be mainly described below.

In this embodiment, instead of the conventional high-frequency power supply 14, a high-frequency signal generator 20 that can generate a high-frequency signal having an arbitrary waveform, and a high-frequency power amplifier 22 that amplifies the high-frequency signal from the high-frequency signal generator 20 Is used. And this, for example, as shown in FIG. 2, with respect to the underlying high-frequency signal, a first modulation for intermittently it with a period T _1, the is intermittently a short period T ₂ than the first modulation The high frequency power that has been subjected to the modulation of 2 (that is, has been subjected to the double modulation) is
The power is supplied between the discharge electrode 8 and the holder / electrode 6 described above.

The original high frequency signal is, for example, a 13.56 MHz sine wave signal as in the conventional example, but is not limited to this.

Further, a bias power supply 24 is inserted between the holder / electrode 6 and the ground, thereby providing the holder / electrode 6
For example, as shown in FIG. 2, a negative bias voltage intermittently applied in synchronization with the second modulation of the high-frequency power is applied. The ON period of the bias voltage is in the on period t ₃ of the high frequency power according to a second modulation, the bias voltage is turned off at the same time as the high frequency power-off by the second modulation.

The magnitude of the negative bias voltage is, for example, in the range of 10 V to 1 KV.

When a mixed gas of, for example, SiH ₄ + He is used as the source gas 16, the plasma 18 contains SiH ₃ radicals having a relatively long life and contributing to the formation of a high quality silicon film, Necessary and relatively short-lived SiH ₂ radicals and SiH radicals which cause particles are mixed. Therefore, when the first modulation is applied to the high-frequency power as described above, of the radicals generated during the on-time t ₁ (see FIG. 2) of the high-frequency power, the SiH ₃ radical having a relatively long life is generated during the off-period t ₂ . However, the SiH ₂ radicals and SiH radicals having relatively short lifetimes disappear in a short time in the off period t ₂ . This enables preferential generation of radicals contributing to high-quality film formation and suppression of unnecessary radicals, thereby suppressing generation of particles.

Further, by subjecting the high-frequency power to the second modulation, the generation of radicals in the plasma can be largely eliminated by utilizing the electron temperature transition at the initial stage of plasma generation when the high-frequency power is turned on by the second modulation. Control of the contributing electron temperature becomes possible. That is, the temperature of the electrons in the plasma rises sharply when the high-frequency power is turned on (the manner of the rise is mainly determined by the temporal change rate (dE / dt) of the electric field intensity).
It has a time transition of falling thereafter, and controls the high-frequency power in this transition region, more specifically, by controlling the cycle T ₂ (that is, frequency) and the duty ratio of the second modulation, so that the Control of electron temperature becomes possible. As a result, it is possible to form and maintain a plasma state in which the number of necessary radicals is increased and unnecessary radicals are suppressed in the initial stage of radical generation having a shorter time schedule, which cannot be controlled only by the first modulation. Therefore, generation of particles can be further suppressed. The above will be described with reference to the drawings.
FIG. 4 shows that the period T2 of the _second modulation is constant and the duty
It shows the change in electron temperature when the ratio is small and when it is large.
Yes, as shown by the broken line in FIG.
When the second modulation is turned off, the electron temperature
After several cycles, the period during which
Electron temperature is higher than when the duty ratio is small.
It becomes. Therefore, depending on the duty ratio of the second modulation,
It is clear that control of the electron temperature in the plasma is possible.
Call FIG. 5 shows that the duty ratio of the second modulation is constant,
Indicates the change in electron temperature when the period is small and when it is large
And the period is reduced as shown by the solid line in FIG.
When the second modulation is off, the electron temperature increases
Since the falling period is short, the electron temperature after several cycles
The degree is higher than when the period is large. Therefore,
Control of the electron temperature in the plasma also depends on the cycle of modulation 2.
It turns out that control is possible. Radiation in plasma
The rate of cull formation depends on the electron temperature. For example,
If the electron temperature is too low, the decomposition of the molecule will not progress very much,
If the degree is too high, the decomposition of the molecule will proceed too much,
Many desired radicals cannot be obtained. The electron temperature
Control to match the desired radical as described above.
In the radical generation stage,
This allows preferential generation of dicals. As described above,
The modulation of 1 depends on the difference in the lifetime of radicals.
To enable the preferential generation (preferential survival) of
The second modulation is required for the radical generation stage.
That allow for the preferential generation of
Contributes to high-quality film formation by using modulation
Preferential generation of radicals and suppression of unnecessary radicals
More reliably possible, further suppressing particle generation
can do.

By applying the above-mentioned negative bias voltage to the holder / electrode 6, ions (eg, He ions) in a sheath region formed near the surface of the base 2 are accelerated by the bias voltage, and Collides with the surface of the substrate 2, i.e., acts like ion irradiation, and the energy of the ions can promote crystallization of the film on the surface of the substrate 2 even in low-temperature film formation.

In summary, the following four regions A, B, C, and D are formed by using the high frequency power and the bias voltage that have been subjected to the double modulation as described above. This corresponds to A, B, C, and D in FIG.

Area A: Plasma annihilation area for annihilating unnecessary radical components B Area: Film formation area of only good-quality radicals in which unnecessary radical components are suppressed C area: Ion irradiation by negative bias voltage, crystallization area D area: Plasma annihilation region for electron temperature control

By the continuation of the four regions, the disappearance of the unnecessary radical component in the region A, the film formation of, for example, 1 nm or less in the region B, and the crystallization of the film formation layer in the region C are repeated. .

In the above case, the frequency (1 / T ₁ ) of the first modulation of the high-frequency power is such that the lifetime of the radical is generally msec.
Since it is in the order of c, it is preferable to select within the range of 100 Hz to 1 KHz. The duty ratio (t ₁ / T _{1 in} FIG. 2) is preferably selected within a range of 10 to 90%.

Further, the frequency (1 / T ₂ ) of the second modulation of the high-frequency power is 5
It is preferable to select within the range of KHz to 5 MHz. Also,
The duty ratio (t ₃ / T _{2 in} FIG. ₂ ) is 10 to
It is preferable to select within the range of 90%.

The delay time t from the time when the high-frequency power is turned on by the second modulation to the time when the bias voltage is turned on.
₅ (see FIG. 2) is preferably selected within the second 10% to 90% of the range of RF power on period t ₃ by the modulation.

The features of the film forming method as described above are as follows.

A crystallized thin film can be formed at a low film formation temperature that cannot be formed by a conventional plasma CVD method.

Since the radicals can be controlled, a crystallized thin film with few particles can be formed.

The control of the electron temperature and further the electron density in the plasma, which cannot be controlled only by the first modulation, is made possible by the second modulation, so that the selectivity of radicals is further improved and the ion excitation by the bias voltage is performed. In addition to the above, electronic excitation that further promotes crystallization of the film becomes possible.

Post-processing (high-temperature annealing, laser annealing, etc.) for obtaining a polycrystalline film becomes unnecessary, and the process can be simplified accordingly.

If a continuous bias voltage is applied to the holder / electrode 6 and the substrate 2 or the film is an insulator, the surface of the film is charged by the incidence of ions and ion irradiation cannot be performed. In this case, the charge on the surface of the film can be released, whereby stable ion irradiation becomes possible.

By selecting the magnitude of the negative bias voltage applied to the holder / electrode 6, the ion irradiation energy necessary for crystallization of the film is secured, and the plasma 18
Damage in the film due to high-speed electrons existing therein can be prevented.

Since the formation of a very thin film (for example, several to several tens of atomic layers) and the crystallization thereof are repeated, the smoothness of the film surface is greatly improved as compared with the crystallization by heat treatment. .

To describe a more specific example, a silicon film was formed on the surface of the substrate 2 under the following conditions.

Substrate 2: 100 mm square substrate Size of electrodes 6, 8: 300 mm square Distance between substrate and electrode 8: 50 mm Source gas 16: 10% SiH ₄ / He Gas pressure in vacuum vessel during film formation: 5 × 10 ^-2 Torr Substrate temperature: 250 ° C. Original high frequency frequency: 13.56 MHz First modulation frequency: 800 Hz, duty ratio: 2
0% Second modulation frequency: 100 KHz, duty ratio:
50% High-frequency power level: 200 W Negative bias voltage delay time t ₅ : 2.5 μsec Negative bias voltage level: 100 V

As a result, the smoothness is reduced to about 1/100 of the conventional one.
(The smaller the smaller, the better the smoothness), and a polycrystalline silicon film with good film quality was formed.

[0039]

As described above, according to the present invention, preferential generation of radicals contributing to high-quality film formation and suppression of unnecessary radicals are achieved by using high-frequency power subjected to double modulation as described above. And generation of particles can be suppressed.

In addition, by applying the above-mentioned negative bias voltage to the holder / electrode, the energy of ions in the sheath region formed near the surface of the base, which collides with the film, can be used even in low-temperature film formation. In addition, crystallization of the film can be promoted. As a result, post-processing for obtaining a polycrystalline film becomes unnecessary, and the process can be simplified accordingly.

Further, since the formation of a very thin film and the crystallization thereof are repeated, a crystallized thin film having a better film surface smoothness can be formed as compared with the crystallization by heat treatment. .

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of a plasma CVD apparatus used for carrying out the present invention.

FIG. 2 is a diagram showing an example of a high-frequency power and a bias voltage in the device of FIG.

FIG. 3 is a schematic view showing an example of a conventional plasma CVD apparatus.

FIG. 4 The period of the second modulation is constant and the duty ratio is
Schematic diagram showing examples of changes in electron temperature for small and large cases
It is.

FIG. 5 The duty ratio of the second modulation is constant and the period is
Schematic diagram showing examples of changes in electron temperature for small and large cases
It is.

[Explanation of symbols]

 2 Base 4 Vacuum container 6 Holder / electrode 8 Discharge electrode 14a High frequency power supply 18 Plasma 24 Bias power supply

Claims (2)

(57) [Claims] 1. A thin film forming method for forming a thin film on a surface of a substrate by a plasma CVD method in which plasma is generated by a high-frequency discharge between a holder / electrode holding the substrate and a discharge electrode facing the holder, the discharge electrode comprising: A high-frequency power obtained by applying a first modulation for intermittently transmitting an original high-frequency signal and a second modulation for intermittently transmitting the original high-frequency signal at a period shorter than the first modulation between the original high-frequency signal and the holder / electrode; And applying a negative bias voltage intermittently in synchronization with the second modulation of the high-frequency power to the holder / electrode. 2. The frequency of the first modulation of the high-frequency power is in the range of 100 Hz to 1 KHz and the duty ratio is 10
９０90% and the frequency of the second modulation is 5 KH
The duty ratio is 10 to 90% within the range of z to 5 MHz.
And the on-period of the bias voltage is within the on-period of the high-frequency power by the second modulation, and the delay time from the on-time of the high-frequency power by the second modulation to the on-time of the bias voltage. 2. The thin film forming method according to claim 1, wherein the ON period of the high frequency power by the second modulation is within a range of 10 to 90%.