Indian Patents. 266058:IN-SITU DEPOSITION OF GRADED INDEX SILICON NITRIDE ANTIREFLECTION LAYER DURING MANUFACTURE OF C-SILICON SOLAR CELLS

Title of Invention	IN-SITU DEPOSITION OF GRADED INDEX SILICON NITRIDE ANTIREFLECTION LAYER DURING MANUFACTURE OF C-SILICON SOLAR CELLS
Abstract	A method for in-situ deposition of graded-index silicon nitride comprising; Making silicon nitride (SiN) coatings in a PECVD tubular furnace on mono-silicon wafers; Depositing at least two layers of SiN at different flow rates in a monolithic design; Subjecting said deposited mono-silicon wafers to various steps of solar cell fabrication.

Full Text	This invention relates to a method for in-situ deposition of graded-index Silicon Nitride antireflection coating in the manufacture of c-Silicon solar cells. BACKGROUND OF THE INVENTION: Silicon Nitride (SiN) antireflection coating (arc) is conventionally deposited as a single-layer with thickness and refractive index best matched to obtain high efficiency Silicon solar cells. These are realized in line manufacturing process by Plasma-Enhanced Chemical Vapour Deposition (PECVD) employing direct or remote plasma with silane and ammonia precursor gases. The resultant coating on the silicon wafer (substrate) has characteristics such as composition, thickness and refractive index that are a function of pressure, substrate temperature and the relative ratio of reactant gases. Typical value of layer thickness and refractive index of the silicon nitride is 80-nm and 2.1 respectively. The antireflection layer minimizes the reflection from the solar cell surface by destructive interference of light reflected from top surface and the silicon-arc interface. This enhances the conversion efficiency of the solar cell. If a double or multi-layer is employed, the number of reflection minima increases for the wavelength range incident on the material surface. Such layers possess refractive index so as to render the light absorption process more effective [1]. Simulations have been made to model multi-layer antireflection coatings on silicon substrate [2]. Analysis of these designs revealed that by increasing the number of layer materials with appropriate refractive index and thickness in a judicious manner, the maximum and average reflectance from the surface can be decreased to very low magnitudes. These configurations, mainly used in filters, included Si/LaF3, Si/MgF2/ZrO2, Si/MgF2/ZrO2/CdTe, etc. By exploiting the porosity control available through dynamic electrochemical etching, porous Silicon (PSi) films with various index gradients have been simulated and fabricated [3]. It was determined that the optimal design for thin film PSi-antireflection coatings combines the features of a single-layer arc with an index gradient. Graded-index coatings of Silicon Oxynitride were reported in the literature [4] formed by ion-assisted deposition (IAD). Here, the chemical composition of the coating was changed in a controlled fashion. Stoichiometry of SiN(X)O(y) varied continuously from Silicon Nitride to Silicon Oxide when the index of refraction changed from 2.1 and 1.45. The process was used to deposit graded-index antireflection coatings and rugate filters. Several patents are published on the formation of single-layer SiN arc by Chemical Vapor Deposition method. OBJECTS OF THE INVENTION: An object of this invention is to propose a method for in-situ deposition of graded- index silicon nitride antireflection layer that possesses compositional homogeneity and ease in synthesis with no additional material inputs in the manufacture of c-Silicon solar cells. Another object of this invention is to propose a method to reduce the reflection below 5% and, in turn, increase absorption of a wide range of wavelengths in the solar spectrum. Further object of this invention is to propose a method, which will enhance the conversion efficiency of industrial solar cells beyond 16%. Yet another object of this invention is to propose a method, which is easily adaptable to the manufacture of industrial solar cells. DESCRIPTION OF THE ACCOMPANYING DRAWINGS: Fig 1: shows that optical paths in 2-layer (also called a step-layer) arc system with high-index (n2) and low-index (n1) SiN materials. Fig 2: shows that refractive index of SiN single-layers with Silane flow: -o- 500- sccm, -♦- 700-sccm, -■- 900-sccm and Step-layer, fitted: -x- High-n layer (840- sccm); -0- Low n-layer (500-sccm). Fig 3: shows that power output dispersion of solar cells with step- and single- layer Silicon Nitride in production runs. Fig 4: shows that current-voltage curve of PV module with SiN step-layer solar cells after outdoor exposure of 100-days. DETAILED DESCRIPTION OF THE INVENTION: According to this invention there is provided a method for in-situ deposition of graded-index silicon nitride comprising: - Making Silicon Nitride (SiN) coatings in a PECVD tubular furnace on mono- silicon wafers; - Depositing at least two layers of SiN at different flow rates; - Subjecting said deposited mono-silicon wafers to various steps of solar cell fabrication. The direct relationship between silicon content and the refractive index in the SiN can be exploited to form dual or multi-layers of graded-index in a homogeneous stack. In this configuration, the conjoint layers are likely to possess good interfacial and optical properties for minimum reflection criteria. In the present invention, an engineering solution is applied to obtain a graded- index profile of SiN in monolithic design during continuous PECVD process. Optical analysis and further application of the resultant layer in industrial solar cells is carried out. Outdoor reliability tests are performed on photovoltaic modules to confirm that the layer composition is stable. Deposition of SiN coatings: Formation of single-layer: Silicon Nitride coatings are made in a PECVD tubular furnace on chemically textured, mono-silicon wafers (1-ohm.cm, 125-mm pseudo-square) after n- diffusion. The single-layer recipe uses 500-sccm silane (4.7N) and 4.2-slm ammonia (5N) precursor gases. Thickness and refractive index (wavelength range 300-1100-nm) of silicon nitride films measured in an ellipsometer (Sentech 850E) on polished silicon substrates correspond to 95-nm and 2.03 (@600-nm) respectively. SiN single-layers are also deposited at two higher flow rates of silane namely, 700-sccm and 900-sccm. Solar cells are fabricated from each SiN-deposition cycle by metallisation in a line printer and are tested. Matched cells are encapsulated in 36-cell configuration using textured glass and tedlar composite back sheet. Module characteristics are evaluated in Spire simulator (calibrated with reference module) at STC. All encapsulated photovoltaic modules were monitored (in short-circuit conditions) at regular intervals during sunlight (roof-top) exposure for over 100 days. Formation of a step-layer: The 2-layer SiN system is as shown in Fig. 1 where the step-layer is grown by modifying the gas flow composition. The high-refractive index layer is indicated as a thin component. For the given materials, there is usually a combination of layer reflectance at the design wavelength. Thickness of the film can be controlled by the deposition time and is measured by a 2-layer program in the ellipsometer on polished silicon wafer. Step-layer properties and results on solar cells: The wavelength dispersion curve of refractive index is shown in Fig. 2. The fitted profiles of refractive index values of n1 and n2 (@600-nm) are 2.29 and 2.03 respectively for layer thickness, 22-nm and 75-nm. Solar cell data representing 392-cells is included in Table-1. Maximum distribution (7.7%) of solar cells with highest efficiency (16%) is observed in the 2-layer SiN lot. The power output dispersion in production runs is graphically represented in Fig-3, which indicates that nearly 90% of 6000 processed solar cells belong to premium category. Stability and outdoor performance of 15 photovoltaic modules with SiN-coated solar cells after 100-day exposure is consistent at about 97-98% of the nominal power output. EXAMPLE: Typical values of refractive index, layer thickness and solar cell operating current (lop at 470-mV) are listed in Table-1. The refractive index increases with increasing ratio of silane-to-ammonia gases. From the derived deposition rate of the single-layers, an estimate of the thickness and refractive index in the step- layer is made. The quantities (%) of solar cells and values of solar cell operating current (lop) are progressively higher in high-index SiN layers and in the step-layer. WE CLAIM: 1. A method for in-situ deposition of graded-index silicon nitride comprising; - Making silicon nitride (SiN) coatings in a PECVD tubular furnace on mono-silicon wafers; - Depositing at least two layers of SiN at different flow rates in a monolithic design; - Subjecting said deposited mono-silicon wafers to various steps of solar cell fabrication. 2. The method as claimed in claim 1 wherein the said silicon nitride coatings are made in a PECVD tubular furnace on chemically textured mono-silicon wafers after n-diffusion. 3. The method as claimed in claim 1 wherein the thickness and refractive index of said silicon nitride coatings on polished silicon substrates correspond to 22-nm & 75-nm and 2.29 & 2.03 respectively. 4. The method as claimed in claim 1, wherein the said SiN coatings result in high efficiency (16%) industrial solar cells which demonstrate good stability in photovoltaic modules. A method for in-situ deposition of graded-index silicon nitride comprising; Making silicon nitride (SiN) coatings in a PECVD tubular furnace on mono-silicon wafers; Depositing at least two layers of SiN at different flow rates in a monolithic design; Subjecting said deposited mono-silicon wafers to various steps of solar cell fabrication.

Full Text

This invention relates to a method for in-situ deposition of graded-index Silicon
Nitride antireflection coating in the manufacture of c-Silicon solar cells.
BACKGROUND OF THE INVENTION:
Silicon Nitride (SiN) antireflection coating (arc) is conventionally deposited as a
single-layer with thickness and refractive index best matched to obtain high
efficiency Silicon solar cells. These are realized in line manufacturing process by
Plasma-Enhanced Chemical Vapour Deposition (PECVD) employing direct or
remote plasma with silane and ammonia precursor gases. The resultant coating
on the silicon wafer (substrate) has characteristics such as composition,
thickness and refractive index that are a function of pressure, substrate
temperature and the relative ratio of reactant gases. Typical value of layer
thickness and refractive index of the silicon nitride is 80-nm and 2.1 respectively.
The antireflection layer minimizes the reflection from the solar cell surface by
destructive interference of light reflected from top surface and the silicon-arc
interface. This enhances the conversion efficiency of the solar cell.
If a double or multi-layer is employed, the number of reflection minima increases
for the wavelength range incident on the material surface. Such layers possess
refractive index so as to render the light absorption process more effective [1].
Simulations have been made to model multi-layer antireflection coatings on
silicon substrate [2].

Analysis of these designs revealed that by increasing the number of layer
materials with appropriate refractive index and thickness in a judicious manner,
the maximum and average reflectance from the surface can be decreased to
very low magnitudes. These configurations, mainly used in filters, included
Si/LaF3, Si/MgF2/ZrO2, Si/MgF2/ZrO2/CdTe, etc.
By exploiting the porosity control available through dynamic electrochemical
etching, porous Silicon (PSi) films with various index gradients have been
simulated and fabricated [3]. It was determined that the optimal design for thin
film PSi-antireflection coatings combines the features of a single-layer arc with an
index gradient.
Graded-index coatings of Silicon Oxynitride were reported in the literature [4]
formed by ion-assisted deposition (IAD). Here, the chemical composition of the
coating was changed in a controlled fashion. Stoichiometry of SiN(X)O(y) varied
continuously from Silicon Nitride to Silicon Oxide when the index of refraction
changed from 2.1 and 1.45. The process was used to deposit graded-index
antireflection coatings and rugate filters.
Several patents are published on the formation of single-layer SiN arc by
Chemical Vapor Deposition method.

OBJECTS OF THE INVENTION:
An object of this invention is to propose a method for in-situ deposition of graded-
index silicon nitride antireflection layer that possesses compositional
homogeneity and ease in synthesis with no additional material inputs in the
manufacture of c-Silicon solar cells.
Another object of this invention is to propose a method to reduce the reflection
below 5% and, in turn, increase absorption of a wide range of wavelengths in the
solar spectrum.
Further object of this invention is to propose a method, which will enhance the
conversion efficiency of industrial solar cells beyond 16%.
Yet another object of this invention is to propose a method, which is easily
adaptable to the manufacture of industrial solar cells.
DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig 1: shows that optical paths in 2-layer (also called a step-layer) arc system
with high-index (n2) and low-index (n1) SiN materials.

Fig 2: shows that refractive index of SiN single-layers with Silane flow: -o- 500-
sccm, -♦- 700-sccm, -■- 900-sccm and Step-layer, fitted: -x- High-n layer (840-
sccm); -0- Low n-layer (500-sccm).
Fig 3: shows that power output dispersion of solar cells with step- and single-
layer Silicon Nitride in production runs.
Fig 4: shows that current-voltage curve of PV module with SiN step-layer solar
cells after outdoor exposure of 100-days.
DETAILED DESCRIPTION OF THE INVENTION:
According to this invention there is provided a method for in-situ deposition of
graded-index silicon nitride comprising:
- Making Silicon Nitride (SiN) coatings in a PECVD tubular furnace on mono-
silicon wafers;
- Depositing at least two layers of SiN at different flow rates;
- Subjecting said deposited mono-silicon wafers to various steps of solar cell
fabrication.
The direct relationship between silicon content and the refractive index in the SiN
can be exploited to form dual or multi-layers of graded-index in a homogeneous
stack. In this configuration, the conjoint layers are likely to possess good
interfacial and optical properties for minimum reflection criteria.

In the present invention, an engineering solution is applied to obtain a graded-
index profile of SiN in monolithic design during continuous PECVD process.
Optical analysis and further application of the resultant layer in industrial solar
cells is carried out. Outdoor reliability tests are performed on photovoltaic
modules to confirm that the layer composition is stable.
Deposition of SiN coatings:
Formation of single-layer:
Silicon Nitride coatings are made in a PECVD tubular furnace on chemically
textured, mono-silicon wafers (1-ohm.cm, 125-mm pseudo-square) after n-
diffusion. The single-layer recipe uses 500-sccm silane (4.7N) and 4.2-slm
ammonia (5N) precursor gases. Thickness and refractive index (wavelength
range 300-1100-nm) of silicon nitride films measured in an ellipsometer (Sentech
850E) on polished silicon substrates correspond to 95-nm and 2.03 (@600-nm)
respectively. SiN single-layers are also deposited at two higher flow rates of
silane namely, 700-sccm and 900-sccm. Solar cells are fabricated from each
SiN-deposition cycle by metallisation in a line printer and are tested. Matched
cells are encapsulated in 36-cell configuration using textured glass and tedlar
composite back sheet. Module characteristics are evaluated in Spire simulator
(calibrated with reference module) at STC. All encapsulated photovoltaic
modules were monitored (in short-circuit conditions) at regular intervals during
sunlight (roof-top) exposure for over 100 days.

Formation of a step-layer:
The 2-layer SiN system is as shown in Fig. 1 where the step-layer is grown by
modifying the gas flow composition. The high-refractive index layer is indicated
as a thin component. For the given materials, there is usually a combination of
layer reflectance at the design wavelength. Thickness of the film can be
controlled by the deposition time and is measured by a 2-layer program in the
ellipsometer on polished silicon wafer.
Step-layer properties and results on solar cells:
The wavelength dispersion curve of refractive index is shown in Fig. 2.
The fitted profiles of refractive index values of n1 and n2 (@600-nm) are 2.29 and
2.03 respectively for layer thickness, 22-nm and 75-nm.
Solar cell data representing 392-cells is included in Table-1. Maximum
distribution (7.7%) of solar cells with highest efficiency (16%) is observed in the
2-layer SiN lot. The power output dispersion in production runs is graphically
represented in Fig-3, which indicates that nearly 90% of 6000 processed solar
cells belong to premium category.
Stability and outdoor performance of 15 photovoltaic modules with SiN-coated
solar cells after 100-day exposure is consistent at about 97-98% of the nominal
power output.

EXAMPLE:
Typical values of refractive index, layer thickness and solar cell operating current
(lop at 470-mV) are listed in Table-1. The refractive index increases with
increasing ratio of silane-to-ammonia gases. From the derived deposition rate of
the single-layers, an estimate of the thickness and refractive index in the step-
layer is made.
The quantities (%) of solar cells and values of solar cell operating current (lop) are
progressively higher in high-index SiN layers and in the step-layer.

WE CLAIM:
1. A method for in-situ deposition of graded-index silicon nitride comprising;
- Making silicon nitride (SiN) coatings in a PECVD tubular furnace on
mono-silicon wafers;
- Depositing at least two layers of SiN at different flow rates in a monolithic
design;
- Subjecting said deposited mono-silicon wafers to various steps of solar
cell fabrication.

2. The method as claimed in claim 1 wherein the said silicon nitride coatings
are made in a PECVD tubular furnace on chemically textured mono-silicon
wafers after n-diffusion.
3. The method as claimed in claim 1 wherein the thickness and refractive
index of said silicon nitride coatings on polished silicon substrates
correspond to 22-nm & 75-nm and 2.29 & 2.03 respectively.
4. The method as claimed in claim 1, wherein the said SiN coatings result in
high efficiency (16%) industrial solar cells which demonstrate good
stability in photovoltaic modules.

A method for in-situ deposition of graded-index silicon nitride comprising; Making silicon nitride (SiN) coatings in a PECVD tubular furnace on mono-silicon wafers; Depositing at least two layers of SiN at different flow rates in a monolithic design; Subjecting said deposited mono-silicon wafers to various steps of solar cell fabrication.

Documents:

0878-kol-2006-correspondence others.pdf

0878-kol-2006-description (provisional).pdf

0878-kol-2006-form1.pdf

0878-kol-2006-form2.pdf

0878-kol-2006-form3.pdf

0878-kol-2006-g.p.a.pdf

878-KOL-2006-(06-01-2014)-ABSTRACT.pdf

878-KOL-2006-(06-01-2014)-CLAIMS.pdf

878-KOL-2006-(06-01-2014)-CORRESPONDENCE.pdf

878-KOL-2006-(06-01-2014)-DESCRIPTION (COMPLETE).pdf

878-KOL-2006-(06-01-2014)-DRAWINGS.pdf

878-KOL-2006-(06-01-2014)-FORM-1.pdf

878-KOL-2006-(06-01-2014)-FORM-2.pdf

878-KOL-2006-(06-01-2014)-OTHERS.pdf

878-KOL-2006-(15-11-2011)-CORRESPONDENCE.pdf

878-kol-2006-abstract.pdf

878-kol-2006-claims.pdf

878-kol-2006-correspondence.pdf

878-kol-2006-description (complete).pdf

878-kol-2006-drawings.pdf

878-kol-2006-form 1.pdf

878-kol-2006-form 18.pdf

878-kol-2006-form 2.pdf

878-kol-2006-form 3.pdf

878-kol-2006-form 5.pdf

878-kol-2006-gpa.pdf

878-kol-2006-specification.pdf

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Patent Number

266058

Indian Patent Application Number

878/KOL/2006

PG Journal Number

14/2015

Publication Date

03-Apr-2015

Grant Date

30-Mar-2015

Date of Filing

30-Aug-2006

Name of Patentee

BHARAT HEAVY ELECTRICALS LIMITED

Applicant Address

REGIONAL OPERATIONS DIVISION (ROD), PLOT NO. 9/1, DJ BLOCK, 3RD FLOOR, KARUNAMOYEE, SALT LAKE CITY, KOLKATA

Inventors:

#	Inventor's Name	Inventor's Address
1	VENKATARAMU. M	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026
2	FELIX MENEZES	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026
3	SIVARAM. S	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026
4	RAVI. S	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026
5	PRABHAKARAN. K. S	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026
6	PREMACHANDRAN. S. K	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026
7	SAJI SALKALACHEN	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026
8	CHANDRAPPA. R.C.	BHARAT HEAVY ELECTRICALS LIMITED, ELECTRONICS DIVISION, SEMICONDUCTORS AND PHOTOVOLTAICS DEPT, MYSORE ROAD, BANGALORE-560026

PCT International Classification Number

H01L 31/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA