Cross Reference to Related Applications

This application is based upon and claims the benefit of priority to US Provisional Application No. 62/964,331, filed January 22, 2020, and hereby incorporated by reference in its entirety.

Field of the Invention

This invention relates to methods for forming anti-reflection coatings on surfaces. Background

An anti-reflection coating comprises one or more optical layers positioned on a substrate to create multiple light reflecting interfaces which generate reflected waves between the interfaces. Using the principle of wave superposition, and with the proper combination of material indices of refraction and layer thicknesses, it is possible to minimize the reflected light (which represents lost energy) and maximize the light transmitted to the substrate by the phenomenon of destructive interference. The choice of refractive indices is crucial for the selection of the range of wavelengths of interest. Anti reflection coatings can be used at ultra violet wavelengths, as well as in the infrared and visible regions of the electromagnetic spectrum. The visible portion of the spectrum is particularly important for photovoltaic applications. Indeed, an element common to most photovoltaic cells is the presence of an anti-reflection coating to reduce the amount of light reflected at the cell’s surface, thus increasing the total energy available for the photovoltaic conversion process. There is clearly an opportunity to improve the efficiency of photovoltaic cell operation by improvements in anti-reflection coatings.

Summary

The invention concerns a method of forming an anti-reflection coating on a substrate within a control volume. In one example embodiment the method comprises: creating a partial vacuum within the control volume; creating a plasma from a mixture of hydrogen gas and a gaseous hydrocarbon within the control volume; establishing a first plurality of parameters within the control volume, the first plurality of parameters including pressures of the hydrogen gas and the gaseous hydrocarbon, a ratio of the hydrogen gas to the gaseous hydrocarbon, flow rates of the hydrogen gas and the gaseous hydrocarbon into the control volume, a gas mixture rate of the hydrogen gas and the gaseous hydrocarbon or a predetermined gas mixture delivered at a desired flow rate, a temperature of the substrate, a voltage of the substrate, a voltage of an anode within the control volume, an electrical current through the anode, a voltage of a cathode within the control volume; for a first duration of time using the first plurality of parameters, depositing a first layer comprising diamond-like carbon on the substrate, the first layer having a first thickness and a first index of refraction; establishing a second plurality of parameters within the control volume by changing at least one of the first plurality of parameters within the control volume while maintaining the partial vacuum within the control volume; for a second duration of time, depositing a second layer comprising diamond-like carbon on the first layer while maintaining the partial vacuum within the control volume, the second layer having a second thickness and a second index of refraction different from the first index of refraction.

By way of example, the second thickness may be different from the first thickness. In an example method the gaseous hydrocarbon comprises methane and/or higher order hydrocarbons than methane. In an example the first index of refraction ranges from 1.6 to 2.8, and the second index of refraction ranges from 1.6 to 2.8. Further by way of example, the first thickness ranges from 0.1 pm to 0.5 pm and the second thickness ranges from 0.1 pm to 0.5 pm.

In an example embodiment the base vacuum ranges from 1X10 ⁸ Torr to 5xl0 ⁷ Torr. Further by way of example, partial pressures of the hydrogen gas and the gaseous hydrocarbon range from 50X10 ³ Torr to 200X10 ³ Torr. In an example embodiment the ratio of the hydrogen gas to the gaseous hydrocarbon varies from 0/100 to 20/80 to 40/60 to 50/50 to 60/40 by volume.

In a further example, the flow rates of the hydrogen gas and the gaseous hydrocarbon into the control volume range from 2 seem to 15 seem.

In an example embodiment, the gas mixture rate ranges from 2 seem to 15 seem. Further by way of example, the temperature of the substrate ranges from 200°C to 300°C. In an additional example, the voltage of the substrate ranges from 0 volts to 100 volts.

The voltage of an anode within the control volume ranges from 400 volts to 800 volts and the voltage of the cathode ranges from 0 volts to +/- 300 volts by way of example. Additionally by way of example, the electrical current through the anode ranges from 20 mA to 300 mA.

In an example anti-reflection coating formed on the substrate according to the invention, the first plurality of parameters comprises: pressures of the hydrogen gas and the gaseous hydrocarbon of 150X10 ³

Torr; the ratio of the hydrogen gas to the gaseous hydrocarbon of 0/100; the flow rates of the hydrogen gas and the gaseous hydrocarbon into the control volume of 5 seem; the temperature of the substrate of 200° C; the voltage of the substrate of 0 volts; the voltage of the anode within the control volume of 650 +/- 50 volts; the voltage of the cathode within the control volume of 0 volts; the electrical current through the anode of 20+/- 2 mA.

In forming this example anti-reflection coating the first layer comprising diamond-like carbon is deposited for the first duration of time of 10 minutes on the substrate. The first layer has a first thickness of approximately 0.2 pm and a first index of refraction of 1.6. The second plurality of parameters is established by changing the voltage of the substrate to 300 volts, and the second layer comprising diamond-like carbon is deposited for the second duration of time of 10 minutes on the first layer. The second layer has a second thickness of approximately 0.1 pm and a second index of refraction of 1.9.

In a further example, a method of forming an anti-reflection coating on a substrate within a control volume may comprise: creating a partial vacuum within the control volume; creating a plasma from a gaseous feedstock within the control volume; establishing a first plurality of parameters within the control volume, the first plurality of parameters including pressures of the gaseous feedstock, a ratio of constituents of the gaseous feed stock, flow rates of the constituents into the control volume, a gas mixture rate of the constituents (or a predetermined gas mixture delivered at a desired flow rate), a temperature of the substrate, a voltage of the substrate, a voltage of an anode within the control volume, an electrical current through the anode, a voltage of a cathode within the control volume; for a first duration of time using the first plurality of parameters, depositing a first layer on the substrate, the first layer having a first thickness and a first index of refraction; establishing a second plurality of parameters within the control volume by changing at least one of the first plurality of parameters within the control volume while maintaining the partial vacuum within the control volume; for a second duration of time, depositing a second layer on the first layer while maintaining the partial vacuum within the control volume, the second layer having a second thickness and a second index of refraction different from the first index of refraction. Brief Description of the Drawings

Figure 1 is a flow chart illustrating an example method of forming an anti- reflection coating on a substrate according to the invention;

Figure 2 is a schematic diagram of an apparatus used to form an anti-reflection coating according to the method illustrated in Figure 1 ; and

Figure 3 is a schematic representation of an anti-reflection coating on a substrate. Detailed Description

An example method of forming an anti-reflection coating on a substrate is illustrated in the flow diagram of Figure 1. The example method according to the invention is advantageously accomplished using saddle field glow discharge (SFGD) techniques, a type of plasma enhanced vapor deposition (PECVD). PECVD techniques are well established and not described in detail in this specification. The method is executed within a control volume 10 defined by the vacuum chamber 12 of a PECVD device 14 which contains one or more substrates 16, an example of which is illustrated schematically in Figure 2. By way of example, the substrates may be photovoltaic cells, optical lenses, mirrors, optical filters and the like.

The example method according to the invention uses hydrogen and gaseous hydrocarbon compounds such as methane and/or higher order hydrocarbons (precursor gases) to form the anti-reflection coating of diamond-like carbon. The example method illustrated in Figure 1 comprises: creating a partial vacuum within the control volume (18); creating a plasma from a precursor gas mixture of hydrogen gas and a gaseous hydrocarbon within the control volume (20); and establishing a first plurality of parameters within the control volume (22). The first plurality of parameters are the conditions within the control volume 10 of the PECVD device 14 which will result in the formation of the anti -reflection coating on the substrate 16. The parameters include: 1. pressures of the hydrogen gas and the gaseous hydrocarbon;

2. a ratio of the hydrogen gas to the gaseous hydrocarbon;

3. flow rates of the hydrogen gas and the gaseous hydrocarbon into the control volume;

4. a gas mixture rate of the hydrogen gas and the gaseous hydrocarbon or a predetermined gas mixture delivered at a desired flow rate;

5. a temperature of the substrate;

6. a voltage of the substrate;

7. a voltage of an anode and a cathode within the control volume;

8. an electrical current through the anode.

The first plurality of parameters are used to deposit a first layer comprising diamond-like carbon on the substrate (24). The first layer has a first thickness and a first index of refraction which are tuned to one or more particular optical wavelengths of interest based upon the purpose of the substrate. For a photovoltaic cell operating in ambient air for example (see Figure 3), light in the visible spectrum is of interest and the first layer thickness and index of refraction are determined by the relations ί=l/4hi and n ₂ =sqrt(noxni) where t=layer thickness, / =wavclcngth of interest, is the index of refraction of the substrate (typically 3.8 for crystalline silicon photovoltaic cells), n ₂ is the index of refraction of the first layer and no is the index of refraction of ambient air.

Once the first anti-reflection layer is formed a second plurality of parameters is established within the control volume by changing at least one of the first plurality of parameters within the control volume (26). This is accomplished while maintaining the partial vacuum within the control volume. Upon establishment of the second plurality of parameters a second layer comprising diamond-like carbon is deposited on the first layer for a second duration of time while maintaining the partial vacuum within the control volume (28). The second layer has a second thickness (which may or may not be different from the thickness of the first layer) and a second index of refraction different from the first index of refraction. By way of a practical example the parameters of the example method may have the following values:

1. both the first and second indices of refraction may range from 1.6 to 2.8;

2. the first and second thicknesses may range from 0.1 pm to 0.5 pm;

3. the base vacuum before introducing the precursor gas mixture may range from 1X1 O ⁸ Torr to 5x1 O ⁷ Torr;

4. the partial vacuum within the control volume with the precursor gas mixture may range from 10X10 ³ Torr to 300X10 ³ Torr

5. the ratio of the hydrogen gas to the gaseous hydrocarbon may vary from 0/100 to 20/80 to 40/60 to 50/50 to 60/40 by volume;

6. the flow rates of the hydrogen gas and the gaseous hydrocarbon into the control volume may range from 2 seem to 15 seem;

7. the gas mixture rate may range from 2 seem to 15 seem;

8. the temperature of the substrate may range from 200°C to 300°C;

9. the voltage of the substrate may range from 0 volts to 100 volts;

10. the voltage of the anode within the control volume may range from 400 volts to 800 volts;

11. the electrical current through the anode may range from 30 mA to 50 mA; and

12. the electrical current of the substrate may range from 0 mA to +/- 10 mA.

Figure 3 illustrates an experimental anti-reflection coating 30 formed on the substrate 16 using the example method according to the invention. Substrate 16 comprises a silicon wafer having an index of refraction of 3.8 and the anti-reflection coating 30 comprises first and second respective layers 32 and 34. First layer 32 has an index of refraction of 1.6 and a thickness of approximately 0.1 pm; the second layer 34 has an index of refraction of 1.9 and a thickness of on the order of approximately 0.1 pm. Both layers are tuned to operate over a range of wavelengths from 200 nm to 600 nm which encompasses a portion of the visible spectrum. The anti-reflection coating 30 was formed on the substrate 16 using SFGD techniques using a PECVD device similar to the device 14 shown schematically in Figure 2.

The example method to form the anti-reflection coating 30 was pursued by first using a pump 36 to create a base vacuum of lxl O ⁷ Torr within the vacuum chamber 12 comprising the control volume 10. Next, the first plurality of parameters were established within the control volume 10. Hydrogen gas and gaseous hydrocarbon (methane in this example) were fed into the control volume 10 from respective reservoirs 38 and 40 using respective mass flow controllers 42 and 44 to set the ratio of the hydrogen gas to the gaseous hydrocarbon of 20/80, the flow rates of the gas mixture of the hydrogen gas and the gaseous hydrocarbon of 5 seem. (Alternately, the precursor gases could be mixed according to the desired ratio in a mixing bottle and the gas mixture input at a particular flow rate.) The temperature of the substrate 16 of 250° C was established and maintained using thermal conditioning elements 46 (for example, electrical resistance heaters) and the substrate was maintained at a voltage of 0 volts using a voltage source 48. The gases within the control volume 10 were formed into a plasma using a Spellman DC power supply SL600 (54) and the voltage of the anode 52 within the control volume was set to 560 volts +/- 20 volts, with a corresponding electrical current through the anode 50 of 200 mA+/- 2 mA being maintained. Cathode 56 within the control volume 10 was grounded at 0 volts. The plasma ions were accelerated toward the substrate 16 and by the voltage difference between the anode 50 and cathode 54 and the first layer 32 comprising diamond-like carbon was deposited on the substrate for the first duration of time of 5 minutes resulting in the first layer having a first thickness of approximately 0.1 pm and a first index of refraction of 1.6. Control of device 14 is accomplished via a microprocessor 58 executing resident control algorithms in response to feedback from temperature and pressure sensors 60 and 62 as is understood for PECVD devices.

While maintaining vacuum conditions within the control volume 10 the second plurality of parameters were established by changing the voltage of the substrate 16 to 300 volts. The second layer 34 comprising diamond-like carbon was deposited on the on the first layer 32 for the second duration of time of 5 minutes. This resulted in the second layer having a second thickness of approximately 0.1 pm and a second index of refraction of 1.9. Reflectivity measurements conducted on the experimental substrate 16 having the anti-reflection coating 30 showed a percent reflectivity below 0.1% for wavelengths from 5 200 nm to 350 nm and a percent reflectivity below 0.25% for wavelengths from 350 nm to 600 nm.