Process for depositing a silica coating with an irregular surface on a glass substrate

The present invention relates to a method for forming an uneven surface silica layer on a substrate of the plate-like glass and the substrate thus coated, useful in particular in solar batteries.

Such a layer irregular surface may be used to form a matte surface, an anti-reflection surface a surface treated for solar cell, etc.

It is known to produce low cost solar cells using amorphous silicon a-Si deposited by vacuum techniques from silane. Such a solar cell Amorphous silicon has a low photoelectric conversion rate compared to other batteries made from other semiconductors (Si, GaAs, etc.).

To improve the battery performance, taking various measures to the manufacturing process and structure of said cells were taken and yet to be taken.

Among the possible measures, a very important and is to reduce the yield loss due to the reflection of light on the surface and to increase the short-circuit current.

For this, it was proposed to create a structure of inequality in a solar cell, to a multi-reflection refraction of the incident light in the cell and thus improve the collection efficiency of the light.

Thus, Chemical Abstracts Vol 103 No. 12, 23-09-85, abstract n ° 90 447 g indicates that particulate silica antireflection layer is obtained on a glass substrate by dipping the substrate in a dilute alkaline solution Si0₂ colloidal.

EP-A-0137291 describes an electrically conductive layer deposited on a glass substrate to form a transparent electrode useful in solar batteries. This electrically conductive layer (for example indium oxide and / or tin oxide) has an uneven surface in contact with the amorphous silicon layer, the irregular surface being due to induim oxide particles and / or tin oxide. When the amorphous silicon layer is formed on said conductive layer having an irregular surface, the interstices between the particles are filled with silicon, constitaunt as an intermediate layer for the reflection of light.

The document "EMPTY" -vol No. 227 40-May June-1985 pages 353-357 describes a monocrystalline silicon substrate covered with a layer of silica. This layer is obtained by projecting, at ambient temperature, the gas SiCl₄ and water vapor for hydrolysis of SiC1₄.

DE-A-1301188 describes the formation of a silica layer on a substrate, such as quartz, by gas decomposition such as SiC1₄ and water vapor for hydrolysis of SiC1₄ and forming a layer of SiO₂ on the substrate surface. The conditions are such that water vapor is adsorbed on the substrate for forming the silica layer on the substrate and not the gas reaction by formation of powder before reaching the substrate.

These methods may have disadvantages due in particular to the use of gas SiC1₄ with glass substrates.

In addition, when the inequality is too large, the thickness of the layer is no longer regular, pinholes causing short-circuits are created, so that the performance instead of being improved , drop, the voltage of a short circuit is reduced, the percentage of acceptable products manufactured is reduced.

The present invention aims to remedy this problem by controlling very precisely the creation of inequalities that make the anti-reflection surface.

The method according to the invention, to form an irregular surface silica layer on a glass substrate, comprising particles of silica embedded in the layer, is characterized in that the blowing on the heated substrate, from ' a nozzle having a plurality of separate outlets, gaseous molecules containing silicon atoms and an oxidizing gas diluted in inert gas and additionally an inert gas separation and in that control the concentration X of gaseous molecules comprising silicon atoms in the inert gas and the concentration Y of the oxidizing gas in the inert gas and, furthermore, the flow of each gas such that the ratio B / A is between 0.5 and 2 and the ratio C / A + B is between 0.1 and 1.5, "A" represents the gas flow mixture silicon- + inert gas, "B" represents the flow rate of the oxidizing gas mixture + inert gas and "C" represents the flow inert gas separation, so as to obtain web formation silica grains by reaction between gaseous molecules containing silicon and the oxidizing gas in the space between the nozzle outlets and the substrate and simultaneously depositing a layer of silica on the substrate from the components that have not yet reacted, the silica grains become encrusted in said layer so as to form an uneven surface layer of silica.

The concentration control the proportions, flow rates of the raw materials so that the deposition of the layer on the substrate surface and the formation of silica particles in the space between the nozzle and the substrate are simultaneously, which facilitates the entry of these grains in the deposited layer.

silica deposition techniques of sending on a substrate heated gas containing silicon and an oxidizing gas (eg, CVD method described in "Solid State Technology, October 1981) result in a smooth layer and even as smooth and even as possible.

According to the aforementioned conventional CVD method, it is normal to use separate nozzle outlets to separately bring the various materials and inert gas between them and blow them separately from separate outputs so that the reaction between gaseous molecule com taking the silicon atom and the oxidizing gas may occur mainly on the surface of the substrate and the nozzle is not clogged by a reaction of the gas molecule with the oxidizing gas to the proximity of outlets of the nozzle. According to this invention have the same uses separate outputs nozzle and also an inert gas separation.

In addition, according to the invention we control the ratio Y / X of the raw materials, namely the ratio of the concentration "Y" oxidizing gas in the oxidizing gas mixture and inert gas, concentration "X" gas containing silicon in the mixed gas to silicon and of inert gas, controls the flow rate of each raw material gas including the flow rate of inert gas, so that the B / A ratio of the oxidizing gas B flow diluted in the inert gas to the gas flow A silicon diluted in the inert gas is 0.5 to 2 and also that the ratio C / (A + B) is between 0.1 and 1.5 C is the gas flow inert separation. As used gas are:<ul><li>. NOT₂ for inert gas,</li><li>. SiH₄ for the gas containing silicon,</li><li>0₂ for oxygen.</li></ul>

With these aforementioned gases the ratio y: x is advantageously between 10 and 40.<ul><li>. "X" represents the volume proportion of SiH₄ SiH in the mixture₄N +₂ is of the order of 2.5 to 4%,</li><li>. "Y" representing the proportion by volume of 0₂ in the mixture 0₂N +₂ is of the order of 20 to 40%,</li><li>. "A" represents the flow of SiH₄N +_Z is of the order of 1.8 to 2.5 I / min at room temperature,</li><li>. "B" represents the O throughput_ZN +₂ is of the order of 2 to 2.5 L / min at room temperature,</li><li>. "C" represents the rate of N₂ is of the order of 0.5 to 5 l / min at room temperature.</li></ul>

Throughput "C" of inert stripping gas is very important. When the flow rate of inert separation gas is low, one can not obtain a stable continuous deposition of the layer because the clogging of the nozzle outlets is common. For against, when the flow rate of inert separation gas is very important, can not get either a layer having an uneven surface because the silica grains produced between the nozzle and the substrate do not enter the layer.

As gaseous molecule including the silicon atom, the silane gas is used (SiH₄) And the gas of disilane (SiH₂H₆) Preferentially. But it is possible to use other products. Also, if the silane gas is mixed with PH₃ or Ti (OC₂H₅)₄Can be deposited mixed layers of Si0₂ and P₂0₁ or Si0₂ and Ti0₂.

It is also possible to deposit another layer in the same or another method of depositing the layer thus deposited by the above method.

The present method esttrès useful for minutely control the shape of the surface of a layer at the time of deposition when a very high shape accuracy is required as is the case for a substrate for a solar cell.

It is preferable that the diameter of the silica grains produced between the nozzle and the substrate surface is 20 to 500 nm. Grains above 500 nm in diameter and below 20 nm in diameter are not easily incorporated into the layer, making it difficult to obtain a layer having an uneven surface.

The invention will now be described in more detail using examples, with reference to the accompanying drawings which represent:<ul><li>. Figure 1 is an overall view of a device for depositing the layer of the invention,</li><li>. 2: a copy of a photo of a electron micrograph showing a surface layer portion in which silica grains are incrustrés.</li></ul>

The device shown in Figure 1 in section, is mainly composed of a heating furnace 2 of a substrate 6 such as a glass plate made of soda and lime, a coating zone 3, an oven annealing 4 and a conveyor 5 as the substrate 6 placed on the conveyor belt 5 can be driven in the direction of the arrow F in succession in the heating furnace in the coating zone and the annealing lehr.

The coating zone 3 is equipped with a chamber 7 for extracting and discharging the gases. A nozzle 8 associating a plurality of outlets is mounted in said coating zone 3. The nozzle 8 includes a channel 9 for feeding the gas containing Si atom, e.g. SiH₄, A channel 10 for the separation of gas, a channel 11 for the oxidizing gas such O₂. These different paths 9, 10, 11 thus separated are used to separately blow the raw material gas including the separation gas in the form of jets from the outlets for example in the shape of slot 12, 13 and 14 at the respective ends lanes 9, 10, 11.

This deposition equipment was used in the following way for forming on a glass substrate a layer of silica.

As samples of glass 6 used was a square glass containing soda-lime of 100 mm square and 1 mm thick. This écahntil-Ion is washed and dried.

We put this glass substrate 6 of the conveyor 5 which moves at a speed of 30 cm / min, heated to 520 ° C in the heating furnace 2. Before entering this glass substrate 6 in the region of coating 3, gas blowing is started SiH₄ diluted in N₂To 4.0% by volume of SiH₄, Gas separation and O2 gas diluted in N₂ to 43% by volume of 0₂With respective flow rates of 2.3 l / min, 1.0 l / min and 2.4 l / min (flow rates measured at room temperature). These gases are ejected from the slot exits 12, 13 and 14 after being routed through 9 for the gas SiH₄, Channel 10 for gas separation and channel 11 for the O gas_Z. At the same time we started blowing these gases, it starts extracting the excess gas and waste gas from the chamber 7 discharge gas with a flow rate of 22 l / min.

And a silica layer 15 is deposited on the glass substrate 6 drawn into the coating zone 3.

In the case of this example, the distance between the end of the nozzle 8 and the glass substrate 6 is approximately 2 cm. The reaction gas SiH₄ with the gas 0₂ occurs between the nozzle 8 and the glass substrate and silica grains with a diameter of approximately 100 to 400 nm are formed.

The substrate 6 is cooled to room temperature in the annealing furnace 4 after passing through the coating zone 3. The thickness of this silica layer thus deposited on the substrate is about 200 nm and is observed to using an electron microscope, of the convex semi-spheres formed on this layer corresponding to the embedded silica particles.

2 shows a photograph, taken on the electronic microscope. This photo shows the shape of the grains formed on the silica layer.

The shape and number of these convex parts formed by grains on the layer to a unit area can be expressed as degree of disturbance or interference ratio of a light incident perpendicularly to the substrate. This interference ratio of the obtained sample was measured and evaluated in about 5% (interference measurement method of the acrylic sheet used in aerospace industries Japanese JIS K-6714-1977). A jamming rate of 0.2% was obtained on a sample coated with a layer of Si0₂ having no inequalities on its surface; the value obtained by the interference layer with inequalities according to the invention is very important and satisfactory.

A layer free of irregularities was manufactured with the same device as the layer according to the invention; were changed only the compositions of gases and their flow rates.

So:<ul><li>. um mixture of SiH₄ and N₂ 0.9% by volume of SiH₄ with a flow rate of 2.3 l / min,</li><li>. a mixture of 0₂ and N₂ to 26% by volume of 0₂ with a flow rate of 2.4 l / min,</li><li>. N gas₂ with a flow rate of 10 l / min, are directed towards the heated substrate.</li></ul>

With these flows and these modified compositions compared with the operating conditions of the invention, the silica grains are not formed in the space between the nozzle and the layer. We only got a smooth layer of 60 nm thick.

According to this invention, one can deposit a silica layer having an uneven surface using a cuoche deposition equipment providers based on the CVD method and generally used as conventional deposition equipment with a layer comprising silica and having a smooth surface. Also, as is clear from the example, you can deposit a layer having an uneven surface on a substrate continuously.

As already said, on the uneven surface layer thus formed, it is possible to deposit other layers, eg conductive and transparent, these other subsequent filings pouvent be obtained in the same coating zone 3, so almost simultaneously.

In the description which preceded it was said that the substrate out of the coating zone undergoing annealing. But other treatments, instead of the annealing or in addition are also possible.