Silicon thin films produced by CVD using liquid silanes with an adjustable carbon content

Abstract

The invention relates to a method for producing a silicon-containing layer, the method being characterized by the method steps of providing a solution, which contains trisilane and at least one organic solvent; and evaporating the solution and depositing silicon on a substrate surface.

Description

[0001] Thin silicon films produced by CVD using liquid silanes with adjustable carbon content

[0002] Description

[0003] The present invention relates to a method for producing thin films of silicon and in particular to a method for producing thin films of silicon having a defined carbon content.

[0004] Semiconductor devices are electronic components made from semiconductor materials. Semiconductor materials have the ability to control the flow of electrical current depending on external influences such as voltage, temperature, or light intensity. This allows semiconductor devices to be used for a wide variety of applications, from simple switches to complex integrated circuits.

[0005] There are different types of semiconductor devices, each with different functions and properties. Some of the most common semiconductor devices are described below:

[0006] A diode is a simple semiconductor device that allows current to flow in only one direction. It consists of a pn junction, where a p-doped semiconductor (with positive charge carriers) borders an n-doped semiconductor (with negative charge carriers). When the diode is forward-biased, i.e., with a positive voltage across the p-doped region, current flows. In reverse bias, the current flow is blocked.

[0007] The transistor is a key component of modern electronics and can function as a switch or amplifier. There are several types of transistors, but the most common are bipolar transistors (NPN and PNP) and field-effect transistors (FETs). Transistors consist of several layers of semiconductor materials arranged in such a way that they can control the flow of current. By applying a control voltage to specific terminals, transistors can amplify or switch the current.

[0008] A thyristor is a semiconductor device used as an electronic switch or rectifier. It consists of several layers of pn junctions connected by common doping. Thyristors have the property of blocking current flow in one direction until a control pulse is applied. Once switched on, the thyristor conducts current until the current drops below a certain threshold or the voltage is reversed.

[0009] A rectifier is a semiconductor device that converts alternating current to direct current. Rectifiers are commonly used in power supply applications to provide a constant direct voltage. They consist of diodes that allow current flow in one direction and are arranged in a bridge circuit to reverse the polarity of the alternating current. Optoelectronic components include light-emitting diodes (LEDs) and photodiodes. LEDs convert electrical energy into light and are used in many applications such as lighting, scoreboards, and display backlighting. Photodiodes, on the other hand, convert light into electrical energy and are used in applications such as photodetectors, optical communication systems, and solar cells.

[0010] This is just a selection of the many different types of semiconductor components that exist. Semiconductor components are the foundation of modern electronics and play an important role in many areas of everyday life, from communications technology to power generation and consumer electronics. They enable the efficient use of electrical energy and the implementation of complex circuits in extremely small spaces.

[0011] Thin silicon films are widely used in various areas of semiconductor technology, particularly microelectronics and photovoltaics. These films are typically produced using various deposition techniques, including chemical vapor deposition (CVD), physical vapor deposition (PVD), and atomic layer deposition (ALD).

[0012] Silicon is a semiconductor material with outstanding electronic properties. It has a crystal structure in which each silicon atom forms four bonds, thus forming a lattice of silicon atoms. Depending on the conditions during deposition, the silicon layers can have an amorphous or partially crystalline structure.

[0013] The thickness of thin silicon layers can vary depending on the application, typically ranging from a few nanometers to a few micrometers. These layers can be deposited on various substrates, such as silicon wafers, glass, plastics, or metals.

[0014] The deposition of thin silicon films is often performed by chemical vapor deposition (CVD). In this process, a gaseous silicon precursor material, such as silane (SiH4) or dichlorosilane (SiH2Ch), is introduced into a reaction chamber heated to a specific temperature. In the reaction chamber, the silicon precursor reacts to deposit silicon atoms. The reaction takes place on the surface of the substrate and gradually forms a thin layer of silicon.

[0015] Another method for depositing thin silicon films is physical vapor deposition (PVD). In this process, silicon is evaporated in a vacuum chamber and then condenses on the surface of the substrate to form a thin film. This process is often referred to as vapor deposition.

[0016] Atomic layer deposition (ALD) is a precise method for forming thin films, including silicon films. In this process, precursor materials are alternately introduced into the reaction chamber to deposit an atomic layer. Each step consists of a chemical reaction followed by a purge with an inert gas to remove excess precursors. This process is repeated until the desired film thickness is achieved. ALD enables precise control over the film thickness and chemical composition of the thin silicon films.

[0017] Thin silicon films are used in many applications. In microelectronics, for example, they are used to manufacture transistors and integrated circuits. In photovoltaics, they serve as the active layer in solar cells to convert sunlight into electrical energy. Thin silicon films are also used in optoelectronics, sensor technology, and many other areas of semiconductor technology.

[0018] The properties of silicon thin films depend, among other things, on the composition of the film material. According to the invention, the carbon content has proven particularly important. According to the invention, it was discovered that the carbon content influences, among other things, the hydrogen content in such silicon thin films and the conductivity of the silicon thin films.

[0019] There are applications, for example, in the field of amorphous silicon thin films, where the carbon content should be as low as possible in the silicon thin film. However, in other applications, a certain, precisely defined carbon content can also be advantageous.

[0020] DE 10 2010 030 696 A describes a method for producing at least one silicon layer arranged on a substrate, wherein at least one of the silicon layers has suboxide structures on its surface or overall suboxide structures, comprising the steps: (a) providing a substrate, (b) providing a formulation containing at least one silicon compound, (c) applying the formulation to the substrate, (d) irradiating and / or thermally treating the coated substrate, (e) treating the layer obtained after step (d) with oxygen in the form of elemental oxygen, carbon dioxide, one or more oxygen-containing compounds or a mixture thereof in pure form or in the form of a liquid or gaseous mixture,and (f) irradiating and / or thermally treating the coated substrate obtained after step (e) to form a layer consisting predominantly of silicon and at least partially polymorphic, which has suboxide structures on its surface.

[0021] DE 10 2014 223 465 A describes a method for producing highly doped polycrystalline semiconductor layers on a semiconductor substrate, wherein a first Si precursor composition containing at least a first dopant is applied to one or more regions of the surface of the semiconductor substrate; optionally a second Si precursor composition containing at least a second dopant is applied to one or more other regions of the surface of the semiconductor substrate, wherein the first dopant is an n-type dopant and the second dopant is a p-type dopant or vice versa; and the coated regions of the surface of the semiconductor substrate are each converted so that polycrystalline silicon is formed from the Si precursor.Therefore, a first object of the present invention is to provide a method for producing silicon thin films with which the resulting carbon content can be adjusted.

[0022] Furthermore, the method according to the invention should be as cost-effective and simple to implement as possible. Conventional CVD processes such as PECVD or HWCVD are typically used to produce thin silicon layers. Plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin layers from a gaseous (vapor) state into a solid state on a substrate. This process involves chemical reactions that occur after the generation of a plasma from the reacting gases. The plasma is usually generated by radio frequency (RF) (alternating current (AC) or direct current (DC)) between two electrodes whose space is filled with the reacting gases.Hot Wire Chemical Vapor Deposition (HWCVD) or Catalytic Chemical Vapor Deposition (CAT-CVD) is a process in which the reaction gases are decomposed by catalytic or pyrolytic reactions using a heated catalyst. PECVD and HWCVD rely on expensive vacuum technology and are therefore specifically avoided in the process according to the invention.

[0023] An economically advantageous option, which avoids the use of complex vacuum technology, is the deposition of silanes from the liquid phase, whereby the liquid silanes are preferably used diluted in an organic solvent.

[0024] Liquid-phase silicon thin film deposition refers to a process for creating thin silicon films on a substrate using a liquid silicon source. This process is also known as liquid-phase deposition (LPD).

[0025] The process begins with a solution containing a silicon compound, such as liquid trisilane, in a solvent. This solution is vaporized and deposited from the vapor phase onto the substrate, which is typically made of glass, crystalline silicon, or another material. The vapor-phase silicon compound has the ability to bond with the substrate and form a thin layer of silicon.

[0026] The deposition takes place in several steps. The substrate is first positioned in a process chamber, the process chamber is sealed, and the substrate is heated to the desired temperature to achieve thermal equilibrium. The silicon precursor is then introduced into the process chamber in liquid form, optionally with the aid of an inert gas, and then evaporates under the influence of heat. During this time, the silicon forms a thin silicon layer on the surface of the substrate.

[0027] Process parameters such as temperature, silicon compound concentration, and deposition time can be varied to control the thickness and quality of the deposited layer. Liquid-phase deposition of silicon thin films offers several advantages. It enables the production of thin, uniform layers on large substrate surfaces. The process is comparatively cost-effective because it does not require complex vacuum systems, as used in other deposition techniques such as chemical vapor deposition (CVD) or physical vapor deposition (PVD).

[0028] Furthermore, the process only deposits silicon on the hot areas of the substrate, whereas prior art processes also involve (undesired) deposition on other areas of the process chambers used. According to the invention, this results in increased efficiency (increased desired yield).

[0029] The liquid deposition process is used in various fields, such as solar cell manufacturing, where thinner layers can help reduce material costs and improve cell performance. It is also used in microelectronics, sensors, and other areas where thin silicon layers are required.

[0030] However, it is not known what influence the dilution of liquid silanes with organic solvents such as cyclooctane has on the thin film and, in particular, at which mixing ratio carbon is incorporated into the film.

[0031] Therefore, a further object of the present invention is to provide a method for producing silicon thin films with which the resulting carbon content can be adjusted and which, if possible, does not require the use of complex vacuum and plasma technology.

[0032] These objects underlying the present invention are achieved by a method for producing a silicon-containing layer, which is characterized by the following process steps: a. Providing a solution containing trisilane and at least one organic solvent; b. Evaporating the solution and depositing silicon on a carrier surface.

[0033] In particular, the objects underlying the present invention are achieved by a process for producing a silicon-containing layer with a defined carbon content, which comprises the following process steps: a. Providing a solution containing trisilane and at least one organic solvent; b. Evaporating the solution and depositing silicon on a carrier surface, wherein the carbon content is adjusted by the ratio of trisilane to organic solvent in the solution used in process step a.

[0034] According to the invention, it was found that the carbon content of the resulting thin film can be adjusted by varying the mixing ratio of trisilane and organic solvent.

[0035] According to the invention, it has also been found that the incorporation of hydrogen into the silicon-containing layer is simultaneously controlled,

[0036] In addition, the use of trisilane as a silicon precursor compound has the advantage of having a higher decomposition temperature compared to its higher homologues.

[0037] The process according to the invention is particularly suitable for the production of a-Si:H thin films.

[0038] In the context of the present invention, thin films are understood to mean layers of amorphous silicon having a layer thickness of 1 nm to 10 pm, preferably of 1 nm to 1 pm, even more preferably 1 nm to 100 nm. In the context of the present invention, the silicon thin films obtained by the process according to the invention have a homogeneous layer thickness.

[0039] The solution used to produce the silicon thin films contains trisilane (SiS) and at least one organic solvent. In particular, the solution used to produce the silicon thin films consists of trisilane (SiS) and at least one organic solvent.

[0040] The content of trisilane in the solution used is preferably at least 0.10 vol.%, preferably at least 0.20 vol.%, even more preferably at least 0.25 vol.%.

[0041] Furthermore, it has been found to be advantageous that the content of trisilane in the solution is at most 40 vol.%, preferably at most 37.5 vol.%, even more preferably at most 35 vol.%.

[0042] This results in particular in contents of trisilane of 0.10 vol.% to 40 vol.%, preferably 0.20 vol.% to 37.5 vol.%, even more preferably 0.25 to 35 vol.%, in the starting solution.

[0043] The organic solvent content in the starting solution is preferably at least 5 vol.%, more preferably at least 10 vol.%, more preferably at least 20 vol.%, more preferably at least 30 vol.%, more preferably at least 40 vol.%, more preferably at least 50 vol.%, 60 vol.%, preferably at least 62.5 vol.%, even more preferably at least 65 vol.%. The organic solvent content in the starting solution is preferably at most 99.90 vol.%, preferably at most 99.80 vol.%, even more preferably at most 99.75 vol.%.

[0044] This results in particular in organic solvent contents of 5 vol.% to

[0045] 99.90 vol.%, more preferably 10 vol.% to 99.90 vol.%, more preferably 20 vol.% to

[0046] 99.90 vol.%, more preferably 30 vol.% to 99.90 vol.%, more preferably 40 vol.% to

[0047] 99.90 vol.%, more preferably 50 vol.% to 99.90 vol.%, more preferably 60 vol.% to

[0048] 99.90 vol.%, more preferably 62.5 vol.% to 99.80 vol.%, even more preferably 65 to

[0049] 99.75 vol%, in the starting solution.

[0050] The organic solvent used in the process according to the invention is preferably a solvent comprising exclusively carbon and hydrogen. Furthermore, the organic solvent is preferably selected from the group consisting of n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, cyclohexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,2-dimethylpentane,

[0051] 2,3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, 3-ethylpentane, 2,2,3-trimethylbutane, cycloheptane, n-octane, 2-methylheptane, 3-methylheptane, 4-methylheptane, 2,2-dimethylhexane, 2,3-dimethylhexane, 2,4-dimethylhexane, 2,5-dimethylhexane, 3,3-dimethylhexane, 3,4-dimethylhexane, 3-ethylhexane, 2,2,3-trimethylpentane, 2,2,4-trimethylpentane (iso-octane),

[0052] 2.3.3-Trimethylpentane, 2,3,4-trimethylpentane, 3-ethyl-2-methylpentane, 3-ethyl-3-methylpentane, 2,2,3,3-tetramethylbutane and cyclooctane.

[0053] Cyclooctane is particularly preferred as the organic solvent. Even more preferred is the use of cyclooctane as the sole organic solvent.

[0054] When the process according to the invention is carried out with the above-described mixing ratio of trisilane and organic solvent, the process is carried out with incorporation of carbon into the silicon thin film.

[0055] During silicon deposition, the consumption of silicon during formation of the silicon thin film leads to a decrease in the concentration of trisilane in the solution. To ensure homogeneous deposition of silicon and homogeneous incorporation of carbon into the silicon thin film, in one embodiment of the process according to the invention, process steps a. and b. are repeated multiple times, preferably at least twice, even more preferably at least three times.

[0056] In the context of the present invention, the deposition time is preferably 10 seconds to 24 hours, preferably 30 seconds to 12 hours, even more preferably 60 seconds to 2 hours.

[0057] The process according to the invention is preferably carried out under a protective gas. Suitable protective gases include nitrogen or argon, but especially nitrogen. In contrast to prior art processes, the process according to the invention is preferably carried out at essentially atmospheric pressure. This makes the process according to the invention particularly simpler in terms of equipment and economically advantageous.

[0058] The solution in process step a. is preferably prepared in a dosing device that can implement different mixing ratios of trisilane to organic solvent. The dosing device preferably implements the above-mentioned mixing ratios of trisilane and organic solvent.

[0059] The deposition of the solution of trisilane and organic solvent in process step b. is preferably carried out by evaporating the solution in the process chamber. The support surface used in process step b., onto which the solution is deposited, is heated. The support surface is heated to a temperature of 300°C to 700°C, preferably 350°C to 650°C, and even more preferably 400°C to 600°C.

[0060] The dosing device and the carrier surface are preferably located in a closed process chamber, wherein the closed process chamber in process step b. has a protective gas atmosphere as described above.

[0061] The silicon thin films obtained by the process according to the invention can be used in particular in solar cells, diodes, rectifiers, transistors, thyristors and optoelectronic components.

[0062] The present invention is explained in more detail with reference to the following examples.

[0063] Example

[0064] The present invention is explained in more detail using the following embodiments.

[0065] In the context of the exemplary embodiments, the carbon content in the produced silicon thin film is determined using Si-C bonds and CH bonds in the FTIR spectrum.

[0066] Example 1: a-Si:H thin films are deposited using a self-built APCVD system.

[0067] The APCVD system used is shown in Figure 1. Figure 1 illustrates:

[0068] 1 : APCVD system

[0069] 2: Heating device with temperature control

[0070] 3: Heating plate for heating the carrier surface (not shown) 4: Dosing device

[0071] 5: Trial Chamber

[0072] 6: Tripod arm

[0073] The process chamber 5 is height-adjustable with the dosing device 4 on the stand arm 6 and is lowered onto the heating plate 3 to form a closed process chamber. The carrier surface is then located in this process chamber during the implementation of the method according to the invention.

[0074] To protect against contamination, the system is installed in an MBraun glove box filled with nitrogen and a chlorine content of less than 1 ppm.

[0075] The heat source is an Isotemp heating plate from Fisherbrand. The heating plate heats a 6 x 6 cm 2 large stainless steel block and on top of it a 10 x 10 cm 2 large quartz glass plate with a thickness of 1 mm to reduce contamination of the sample substrates by the plate. The 6 x 6 cm 2A large steel block is used to concentrate the heat on this area of ​​the glass plate and achieve greater utilization of the raw materials. A thermocouple within the stainless steel block controls the plate temperature. A height-adjustable head can be lowered onto the heating plate, creating a closed coating chamber. Special sealing is not required, as no vacuum is needed.

[0076] Trisilane and cyclooctane are pumped into a tube on the chamber head using a dosing pump with at least 5 pl dosing strokes. They are then flushed into the chamber by opening a solenoid valve for 0.1–1 s. The dosing process is software-controlled. To ensure that the silicon precursor remains in the liquid phase during pumping, the inlet block is cooled to temperatures below 30 °C by a Peltier element, as the boiling point of trisilane is 53 °C.

[0077] The individual process steps of the deposition according to the invention are as follows:

[0078] The hot plate is heated to 490 °C and the lines for dosing trisilane and cyclooctane are purged with nitrogen.

[0079] The substrate is placed on the heating plate and the process chamber is closed.

[0080] The process is first waited until thermal equilibrium is established. Then, 400 μl of a solution of 2 vol% trisilane and 98 vol% cyclooctane is fed into the dosing pump, and the Arduino-controlled (microcontroller) deposition process begins by pumping the solution into the chamber via the dosing pump.

[0081] Subsequently, a nitrogen purge (0.3 bar; 0.2 s) is applied to inject the remaining liquid in the line between the pump and the deposition chamber into the deposition chamber. The deposition process is completed after 10 minutes, after which the chamber is lifted and the substrate, along with the deposited a-Si:H thin film, is removed from the process chamber and the glove box.

[0082] The a-Si:H thin film thus produced is then examined spectroscopically.

[0083] Figure 2 shows various FTIR spectra for different concentrations of trisilane in cyclooctane. Characteristic of the incorporation of carbon into the a-Si:H thin film are the stretching modes of the CH bond at a wavelength of approximately 2850 cm -1 The figure shows that at a concentration of approximately 5 vol% or less of trisilane, there is an increased incorporation of carbon into the a-Si:H thin film.

[0084] Figure 3 also shows various FTIR spectra for different concentrations of trisilane in cyclooctane. The bands shown at approximately 2100 cm -1 correspond to Si-H bonds, whereby the incorporation of hydrogen into the a-Si:H thin film is accompanied by the incorporation of carbon into the a-Si:H thin film. A comparison of the bands at 2100 cm -1 shows that increased carbon incorporation occurs when the concentration of trisilane in cyclooctane is below 5 vol%.

[0085] Figure 4 shows FTIR spectra for an a-Si:H thin film deposited at 490 °C. The characteristic Si-C peak at 780 cm -1 (possibly slightly shifted) illustrates an increased incorporation of carbon in the a-Si:H thin film in the case of a concentration of trisilane in cyclooctane of less than 5 vol%.

[0086] Figure 5 shows that the incorporation of carbon into the a-Si:H thin film increases with increasing deposition temperature.

Claims

Claims 1 . A method for producing a silicon-containing layer, characterized by the following process steps: a. Providing a solution containing trisilane and at least one organic solvent; b. Evaporating the solution and depositing silicon on a carrier surface.

2. A method for producing a silicon-containing layer with a defined carbon content, comprising the following process steps: a. Providing a solution containing trisilane and at least one organic solvent; b. Evaporating the solution and depositing silicon on a support surface, characterized in that the carbon content is adjusted by the ratio of trisilane to organic solvent in the solution.

3. Process according to claim 1 or 2, characterized in that the content of trisilane in the solution is at least 0.10 vol.%, preferably at least 0.20 vol.%, even more preferably at least 0.25 vol.%.

4. Process according to one of claims 1 to 3, characterized in that the content of trisilane in the solution is at most 40.00 vol.%, preferably at most 37.50 vol.%, even more preferably at most 35.00 vol.%.

5. The method according to any one of claims 1 to 4, characterized in that the content of organic solvent in the solution is at least 5.00 vol.%, more preferably at least 10.00 vol.%, more preferably at least 20.00 vol.%, more preferably at least 30.00 vol.%, more preferably at least 40.00 vol.%, more preferably at least 50.00 vol.%, more preferably at least 60.00 vol.%, more preferably at least 62.50 vol.%, even more preferably at least 65.00 vol.%.

6. Process according to one of claims 1 to 5, characterized in that the content of organic solvent in the solution is at most 99.90 vol.%, preferably at most 99.80 vol.%, even more preferably at most 99.75 vol.%.

7. The method according to any one of claims 1 to 6, characterized in that process steps a. and b. are repeated several times, preferably at least twice, even more preferably at least three times.

8. Process according to one of claims 1 to 7, characterized in that the process duration is 10 hours to 24 hours, preferably 20 hours to 12 hours, even more preferably 30 hours to 2 hours.

9. The method according to any one of claims 1 to 8, characterized in that the carrier surface is heated to a temperature of 300 °C to 700 °C, preferably 350 °C to 650 °C, even more preferably 400 °C to 600 °C.

10. Use of a silicon-containing layer obtainable according to any one of claims 1 to 9 in solar cells, diodes, rectifiers, transistors, thyristors and optoelectronic components.

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