RU2807779C1 - Method for forming polycrystalline silicon - Google Patents

Abstract

FIELD: semiconductor technology.

SUBSTANCE: invention can be used in the manufacture of micro-, nanoelectronic and optoelectronic devices, such as thin-film field-effect transistors, non-volatile memory cells and solar cells. A thin layer of Au gold is formed on a pre-cleaned substrate using thermal vacuum deposition, and on this layer there is a layer of non-stoichiometric silicon suboxide α-SiO_x applied by magnetron sputtering. Fused quartz or monocrystalline silicon (c-Si) with a sublayer of thermal silicon oxide (t-SiO₂) is used as a substrate. The Au layer thickness is selected from 15 nm to half the thickness of layer α-SiO_x, selected from the range from double the thickness of the gold layer to several microns. The resulting composite is treated with radiation from a solid-state pulsed Nd-YAG laser with a wavelength of 1064 nm and a pulse duration of 11 ns with a Gaussian spatial profile and with an energy density from 0.33 to 1.3 J/cm², causing a phase transition in the amorphous silicon layer with the formation of continuous film of polycrystalline silicon in the region of radiation exposure at a given layer thickness of α-SiO_x. In this case, the number of pulses is 50, and the frequency varies in the range from 0 to 3000 Hz.

EFFECT: homogeneous films of polycrystalline silicon without contamination are obtained.

9 cl, 3 dwg

Description

The invention relates to semiconductor technology and can be used in the manufacture of micro-, nanoelectronic and optoelectronic devices, in particular, thin-film field-effect transistors, non-volatile memory cells, and solar cells.

Polycrystalline silicon (poly-Si) thin film has high carrier mobility, light absorption rate and stable photovoltaic performance, so it is widely used in various electronic devices such as thin film transistors, image sensors, solar cells and others.

Currently, to obtain thin films of polycrystalline silicon, chemical vapor deposition (CVD), solid-phase crystallization (SPC), laser-induced crystallization (LIC) and metal-induced crystallization are used. crystallization (Metal-induced crystallization, MIC).

Solid phase crystallization is one of the main methods for producing high-quality polysilicon thin films, however, due to the high heating temperature (usually above 600°C), only expensive quartz substrates are required. In addition, the solid-phase crystallization method usually takes about ten hours, which increases the cost of production.

Laser-induced crystallization and metal-induced crystallization are low-temperature methods that can be used to produce flat-panel displays.

Known, for example, method of metal-induced crystallization of thin films of amorphous silicon suboxide [N.A. Lunev, A.O. Zamchiy, E.A. Baranov et al., Gold-induced crystallization of thin films of amorphous silicon suboxide / Letters to ZhTP, 2021, volume 47, issue. 14]. This work demonstrates for the first time the possibility of producing poly-Si using the AuIC a-SiOx method. Thin films of gold about 20 nm thick were deposited by thermal vacuum sputtering onto fused quartz (SiO2) substrates. Next, thin films of α-SiOx with a thickness of 150 nm were deposited onto the samples using gas-phase plasma-chemical deposition. The resulting samples were annealed in a furnace at temperatures of 285, 335 and 370°C for 10 hours in a high (~ 10-5 Pa) vacuum.

The main disadvantage of the metal-induced crystallization method is the long annealing time of several tens of hours, and the quality of the poly-Si film is not very high due to metal diffusion.

Laser-induced crystallization is superior to other low-temperature crystallization methods because it has high efficiency and low cost for producing polysilicon films. During laser-induced crystallization, the grain size of poly-Si is 300-600 nm, and the carrier mobility reaches 200 cm ² /V-s.

The use of an excimer laser makes it possible to obtain polycrystalline silicon in a short time without damaging the substrate, since the laser wavelength used (308 nm) has a high absorption rate by amorphous silicon.

There is a known method for laser-induced crystallization of thin films of polycrystalline silicon, poly-Si [Bronnikov K.A. Formation of laser-induced periodic surface structures on films of metals and semiconductors. Dissertation for the degree of candidate of physical and mathematical sciences. Novosibirsk - 2022]. Laser modification is based on the transition of silicon from the amorphous phase to the polycrystalline phase (poly-Si). Considering the processing parameters used ( F = 150 mJ/cm2, λ = 1026 nm, τ = 230 fs, f = 200 kHz), the maximum temperature on the film surface is approximately 700°C, which is significantly lower than the melting point of silicon (1400°C) and corresponds to the temperature associated with solid-phase crystallization processes. Another advantage of laser annealing is the speed of movement of the crystallization front, which is V ≈ 1 m/s. This value significantly exceeds typical crystal growth rates.

However, using the laser-induced crystallization method, it is difficult to ensure the uniformity of a polycrystalline silicon film, since the location and order of grains in the film are non-uniform due to the uneven deflection of laser energy, which reduces the electrical characteristics of the resulting polysilicon film, such as carrier mobility and threshold voltage uniformity .

Methods for producing polycrystalline silicon are known from the state of the art, combining the advantages of laser-induced crystallization and metal-induced crystallization methods, which makes it possible to reduce the characteristic process times, reduce the characteristic temperature of the process, and therefore eliminate the destruction of the material due to breakdown absorption of radiation in the film.

The main stages of these methods:

1. sequential formation of a metal film and an amorphous silicon film on a pre-cleaned substrate;

2. laser annealing of the resulting composite.

Typically, the metal layer is prepared by sputtering or evaporation, but these two methods have serious disadvantages: high cost, inability to control the amount of deposited metal. Residual metals have a great influence on the stability of manufactured devices. Therefore, it is necessary to add steps to remove contaminants.

There is a known method for producing polycrystalline silicon [TW200421618, 2004-10-16, H01L21/77; H01L21/84; H01L27/12; H01L29/04], which allows you to control the position and size of the grains. The method includes the following steps:

1. sequential formation on a substrate (panel) having a peripheral area and a display area:

- a buffer layer containing a layer of silicon nitride and a layer of silicon oxide;

- a layer of amorphous silicon on top of the buffer layer;

- a masking layer with a hole revealing part of the amorphous silicon layer in the peripheral region;

- a layer of nickel on top of an exposed layer of amorphous silicon inside the hole

2. transformation of a layer of amorphous silicon in the peripheral region into a layer of polycrystalline silicon by metal-induced crystallization (MIC);

3. removing the mask after forming a layer of polycrystalline silicon in the peripheral area;

4.laser annealing using an excimer laser with a wavelength of 157 nm (F2), 193 nm (ArF), 248 nm (KrF), 308 nm (XeCl), 351 (XeF).

Due to differences in processing conditions, the polycrystalline silicon layer in the display region has a grain size smaller than the polycrystalline silicon layer in the peripheral region. The method requires additional steps, which complicates the process. In this method, as a result of annealing, nickel silicide is formed, which requires removal, for example, by dry etching, which, however, does not completely remove the contamination.

There is a known method for producing polycrystalline silicon [US2005136612; 2005-06-23, H01L21/20; H01L21/268; H01L21/336; H01L29/786], including:

1. formation of an a-Si layer on a glass substrate, for example, using plasma-enhanced chemical vapor deposition (PECVD) or physical vapor deposition (PVD);

The preferred thickness of the a-Si layer is about 50 nm.

2. dehydrogenation of the a-Si layer to prevent hydrogen explosion during subsequent laser annealing;

3. sequential formation of a protective layer and a reflective layer on the a-Si layer;

The protective layer is a non-metallic material such as silicon oxide (SiOx). The preferred thickness of the protective layer is about 100 nm.

The reflective layer is a metal material such as molybow frame (MoW). The preferred thickness of the reflective layer is about 100 nm.

4. first laser annealing with an excimer XeCl laser;

5. Second laser annealing with lower laser energy to more completely crystallize polycrystalline silicon (poly-Si) and obtain a smooth surface of the poly-Si layer.

The method achieves control of the position and size of grains, obtaining a smooth surface of the polysilicon film. Laser annealing using an excimer laser allows the production of polycrystalline silicon in a short time without damaging the substrate, since the laser wavelength used has a high absorption rate in amorphous silicon.

However, the laser annealing method using an excimer laser has limitations because it is difficult to ensure uniformity of the entire polycrystalline silicon layer. The method requires additional steps, which complicates the process of obtaining polysilicon film.

There is a known method for manufacturing a thin-film polysilicon transistor [KR20100065701, 2010-06-17, G02F1/136, H01L29/786], which includes the sequential formation of a protective layer, an amorphous silicon layer and a heat transfer metal layer on the substrate. Heat transfer metal layer - Mo, MoTi, Cr. A layer of polycrystalline silicon is obtained through laser annealing with a diode laser with a wavelength from 800 to 810 nm. In this case, a metal silicide film is formed between the crystalline silicon layer and the heat transfer metal layer. To clean the surface of metals and their derivatives, the dry etching method is used.

The method ensures the production of a homogeneous layer of polycrystalline silicon in a short time without damaging the substrate. In this method, to remove the metal silicide film, the dry etching method is used, which does not completely remove contaminants. Additional steps, namely, the formation of a protective layer on the substrate and cleaning the surface using dry etching, complicate the production of a polycrystalline silicon film.

The prototype is a method for forming polycrystalline silicon, described in [A.O. Zhamchiy et al. Fabrication of polycrystalline silicon films by gold-induced crystallization of amorphous silicon suboxide (Vacuum, 2021, v. 192), including the formation of a thin layer of gold on a previously cleaned substrate, the subsequent formation of a layer of non-stoichiometric silicon suboxide on it.

The main disadvantage of the metal-induced crystallization method is the long annealing time of several tens of hours, and the quality of the poly-Si film is not very high due to metal diffusion.

The goal is to create a simple method for forming high-quality polycrystalline silicon films.

The technical result is a simple way to obtain high-quality polycrystalline silicon films, homogeneous and free of contamination.

A method has been proposed for the formation of polycrystalline silicon, which consists in forming a layer of amorphous silicon and a metal layer on a pre-cleaned substrate to obtain a composite, and then treating the resulting composite with laser radiation with an infrared wavelength and energy density that causes a phase transition in the amorphous silicon layer with the formation of a layer of polycrystalline silicon.

According to the invention, first a thin layer of gold, Au, is formed on a pre-cleaned substrate, then a layer of amorphous non-stoichiometric silicon suboxide α-SiO _x is formed on the Au layer.

According to the invention, laser processing of the resulting composite is carried out by laser radiation with a wavelength of the infrared range and a nanosecond pulse duration with a Gaussian spatial profile and with an energy density that ensures the formation of a continuous layer of polycrystalline silicon in the region of radiation exposure at a given thickness of the layer of amorphous non-stoichiometric silicon suboxide α-SiO _x .

According to the invention, the formation of the Au layer is carried out on a pre-cleaned substrate made of fused quartz or single-crystalline silicon (c-Si) with a sublayer of thermal silicon oxide (t-SiO ₂ ).

According to the invention, the formation of an Au layer on a pre-cleaned substrate is carried out by thermal vacuum deposition, and the formation of a layer of amorphous non-stoichiometric silicon oxide α-SiO _x on the Au layer is carried out by magnetron sputtering.

According to the invention, the thickness of the Au layer is selected from the range from 15 nm to half the thickness of the amorphous non-stoichiometric silicon suboxide layer, and the thickness of the amorphous non-stoichiometric silicon suboxide layer is selected from the range from twice the thickness of the gold layer to several microns.

According to the invention, laser processing of the resulting composite is carried out with a solid-state pulsed Nd-YAG laser with a wavelength of 1064 nm in a pulsed mode with a pulse duration of 11 ns.

According to the invention, when laser processing of the resulting composite with a solid-state pulsed Nd-YAG laser with a wavelength of 1064 nm and a pulse duration of 11 ns, the number of pulses is 50, the frequency varies in the range from 0 to 3000 Hz, and the energy density of pulsed radiation is from 0.33 to 1.3 J /cm².

Thus, polycrystalline silicon is formed as follows:

1. obtain the “c-Si/t-SiO ₂ /Au α-SiO _x ” composite;

2. carry out laser processing of the resulting composite with radiation from a solid-state pulsed Nd-YAG laser with a wavelength of 1064 nm and a pulse duration of 11 ns with a Gaussian spatial profile.

The composite is produced by sequential deposition of a thin layer of gold (Au) and a thin layer of amorphous non-stoichiometric silicon suboxide (α- _SiOx ) onto a pre-cleaned substrate of single-crystalline silicon (c-Si) with a sublayer of thermal silicon oxide (t-SiO ₂ ). The gold layer is deposited using thermal vacuum sputtering. A thin layer of amorphous non-stoichiometric silicon suboxide α-SiO _x is deposited onto the gold layer using magnetron sputtering.

At laser processing of the resulting composite with a solid-state pulsed Nd-YAG laser with a wavelength of 1064 nm and a pulse duration of 11 ns; the number of pulses is 50, and the frequency varies in the range from 0 to 3000 Hz. Energy DensityF ₀ on the surface of the composite vary in the range of 0.33 - 1.3 J/cm².

Usage gold as a metal makes it possible to obtain high-quality (pure and homogeneous) polycrystalline silicon, since gold does not form silicides during laser processing, which makes it possible to eliminate additional stages of cleaning the composite from contaminants from the process of producing polycrystalline silicon. It should be noted that known methods for removing residual metals from a composite are ineffective, since they do not completely remove all contaminants.

By using a gold layer, the uniformity of the polycrystalline silicon film is also achieved, since the metal layer reduces the energy threshold of crystallization and, due to high thermal conductivity, ensures uniform heating of the silicon suboxide layer.

The thickness of the gold layer affects the fraction of absorbed radiation and the heating rate of silicon suboxide. The thickness of the gold layer can vary from 15 nm to half the thickness of the amorphous non-stoichiometric silicon suboxide layer. The lower threshold (15 nm) is due to the fact that at smaller thicknesses gold does not form a continuous layer, and there is a high probability of detecting a clean substrate or islands of gold.

The formed composite is “substrate/Au/α-SiO _x ”, i.e. a layer of amorphous non-stoichiometric silicon suboxide, α- _SiOx , is formed on the Au layer.

If the amorphous non-stoichiometric silicon suboxide α-SiO _x layer is the top layer, as in the present invention, the laser beam will freely pass through the amorphous silicon suboxide layer and be partially absorbed by the gold, heating it. If gold is the top layer, part of the radiation energy will be reflected from the gold and irretrievably lost from the system.

The use of a solid-state pulsed laser with an infrared wavelength, nanosecond pulse duration and a Gaussian spatial profile ensures absorption of radiation throughout the entire thickness of the metal layer, which eliminates the destruction of the composite. In the infrared range, silicon is almost completely transparent, therefore, the radiation is absorbed by the metal, which heats up and transfers heat to the silicon. When a certain temperature is reached, the crystallization process begins. In the UV range, silicon is opaque, therefore, it absorbs all incident radiation and heats up, which causes tension between the layers, the gold layer comes off the substrate and the entire composite breaks and curls.

The number of laser pulses is 50, and the frequency can vary from 0 to 3000 Hz. When using a smaller number of pulses, the crystallization process will not be completed and there is a high probability of detecting inclusions of gold in the polycrystalline silicon film. When changing the frequency from 0 to 3000 Hz, the only condition is that the silicon film must have time to cool. Before each new pulse, the silicon must be at the same temperature, otherwise the crystallization process will develop into continuous heating and the film will melt.

Laser processing at an energy density F ₀ on the surface of the sample in the range of 0.33 - 1.3 J/cm ² allows, at a given thickness of the layer of amorphous non-stoichiometric silicon suboxide α-SiO _x , to form a continuous film of polycrystalline silicon in the region of exposure to radiation, eliminating the destruction of the composite. Leaving this interval will either not produce results or will lead to film ablation.

The thickness of the amorphous non-stoichiometric silicon suboxide layer can vary from twice the thickness of the gold layer (the reason is stated above) to several microns.

The simplicity of the method is ensured by using the thermal vacuum deposition method to form a layer of Au and the magnetron sputtering method to form a layer of amorphous non-stoichiometric silicon oxide α-SiO _x . Any other methods that ensure uniform thickness and the absence of foreign impurities are also suitable.

Experimental studies were performed.

In the experiments, a composite “c-Si/t-SiO ₂ /Au/α-SiO _x ” was obtained on a single-crystalline silicon (c-Si) substrate with a sublayer of thermal silicon oxide (t-SiO ₂ ). The composites were analyzed by scanning electron microscopy (SEM) using a JEOL JSM-6700F scanning microscope.

In fig. Figure 1 shows an SEM image of a cross section of the structure of the “c-Si/t-SiO ₂ /Au/α-SiO _x ” composite before laser exposure.

In fig. 1 it can be seen that the gold layer is continuous, its thickness is 30 nm. The α-SiO _x layer is also continuous, its thickness is 130 nm.

According to the results of a spectrophotometric study, the absorption capacity of α-SiO _x in the IR region of the spectrum is quite small. And the absorption capacity of gold in the IR region of the spectrum, according to the Beer-Lambert-Bragg law [Starinskiy SV, Shukhov YG, Bulgakov AV Laser-induced damage thresholds of gold, silver and their alloys in air and water // Appl. Surf. Sci. Elsevier BV, 2017. Vol. 396. P. 1765-1774], up to 85% of energy. This indicates that radiation is absorbed by the gold layer, after which heat is transferred to the upper layer of the α-SiO _x composite (Fig. 1).

After laser exposure, the degree of crystallinity of the material was studied using Raman spectroscopy (RS). Raman spectra were obtained on a T64000 Horiba Jobin-Yvon instrument using laser excitation at a wavelength of 514 nm. The laser beam was focused into a beam with a diameter of less than 2 μm.

In fig. Figure 2 shows a snapshot of the composite surface after exposure to 50 laser pulses and the corresponding laser radiation energy density profile. The scale of the abscissa axis of the energy density profile and the image of the composite surface are identical. Red lines indicate the threshold of material modification, black lines indicate the threshold of destruction.

In fig. 2 indicated:

1 - photo of the composite surface after exposure to 50 laser pulses;

2 - profile of laser radiation energy density, J/cm ² , as the dependence of laser energy density on the distance to the center of the beam, mm;

3 - modified region when achieving local energy density = 0.33 J/cm ² ;

4 - modified region at local energy density 0.33<F< 1.3 J/cm ² ;

5 - region at local energy density F = 1.3 J/cm ² .

It can be seen that when the local energy density F = 0.33 J/cm ² is achieved, the original surface (area 3 in image 1) is significantly modified.

The modification region (region 4) corresponds to the range of local energy density F = 0.33 - 1.3 J/cm ² .

When the local energy density exceeds 1.3 J/ ^cm2 , destruction of the composite occurs (region 5), namely, complete removal of material from the surface of the substrate is observed.

In fig. Figure 3 shows the Raman spectra of the original α-SiO _x film and the modified film (poly-Si); fused quartz (SiO ₂ ), where:

6 - Raman scattering of the original film (α-SiO _x ), which corresponds to region 3 in Fig. 2;

7 - Raman modified film (poly-Si), which corresponds to area 4 in Fig. 2;

8 - Raman fused quartz (SiO ₂ ), which corresponds to area 5 in Fig. 2.

Raman spectra of the original α-SiO _x 6 layers reproduce the effective density of vibrational states of Si-Si bonds and are characteristic of the amorphous structure of the material [Starinskiy SV, Shukhov YG, Bulgakov AV Laser-induced damage thresholds of gold, silver and their alloys in air and water / /Appl. Surf. Sci. Elsevier BV, 2017. Vol. 396. P. 1765–1774].

In the spectra of the modified film (poly-Si) 7, obtained in a spot after exposure to laser radiation (region 4 in Fig. 2) limited by the energy density range of 0.33 - 1.3 J/cm ² , there is a characteristic peak at 520 cm ^-1 , which indicates that as a result of laser exposure, the process of crystallization of amorphous silicon suboxide took place with the formation of polycrystalline silicon.

Spectra 8 are characteristic of fused quartz (SiO ₂ ), which confirms the destruction of the coating.

Thus, it is shown that radiation absorption occurs in the gold layer, and not in the α-SiO _x silicon suboxide layer.

It is also shown that laser processing of the composite, which ensures the formation of polycrystalline silicon, must be carried out with beams with a peak energy density in the range of 0.33 - 1.3 J/cm².

Claims (9)

1. A method for forming polycrystalline silicon, including the formation of a thin layer of Au on a previously cleaned substrate, the subsequent formation of a layer of non-stoichiometric silicon suboxide α-SiO on it_x, characterized in that the resulting composite is treated with laser radiation with an infrared wavelength and a nanosecond pulse duration with a Gaussian spatial profile and an energy density from 0.33 to 1.3 J/cm², causing a phase transition in a layer of amorphous silicon with the formation of a continuous film of polycrystalline silicon in the region of exposure to radiation at a given thickness of the layer of amorphous non-stoichiometric silicon suboxide α-SiO_x. 2. Method according to claim 1, characterized in that The formation of the Au layer is carried out on a pre-cleaned substrate made of fused quartz or single-crystalline silicon (c-Si) with a sublayer of thermal silicon oxide (t-SiO₂). 3. Method according to claim 1, characterized in that The formation of the Au layer on a pre-cleaned substrate is carried out by thermal vacuum deposition. 4. Method according to claim 1, characterized in that formation of a layer of amorphous non-stoichiometric silicon oxide α-SiO on the Au layer_x carried out by magnetron sputtering. 5. The method according to claim 1, characterized in that the thickness of the Au layer is selected from the range from 15 nm to half the thickness of the amorphous non-stoichiometric silicon suboxide layer. 6. The method according to claim 1, characterized in that the thickness of the layer of amorphous non-stoichiometric silicon suboxide α-SiO _x is selected from the range from double the thickness of the gold layer to several microns. 7. Method according to claim 1, characterized in that Laser processing of the resulting composite is carried out with a solid-state pulsed Nd-YAG laser with a wavelength of 1064 nm in pulsed mode. 8. Method according to claim 1, characterized in that Laser processing of the resulting composite is carried out with a solid-state pulsed Nd-YAG laser with a wavelength of 1064 nm and a pulse duration of 11 ns. 9. Method according to claim 1, characterized in that when laser processing of the resulting composite with a solid-state pulsed Nd-YAG laser with a wavelength of 1064 nm and a pulse duration of 11 ns; the number of pulses is 50, and the frequency varies in the range from 0 to 3000 Hz.