Methods for inspecting semiconductor wafers
Methods and systems are presented for analysing semiconductor materials as they progress along a production line, using photoluminescence images acquired using line-scanning techniques. The photoluminescence images can be analysed to obtain spatially resolved information on one or more properties of said material, such as lateral charge carrier transport, defects and the presence of cracks. In one preferred embodiment the methods and systems are used to obtain series resistance images of silicon photovoltaic cells without making electrical contact with the sample cell.
1. A method for obtaining series resistance or carrier transport information across a semiconductor material selected from the group consisting of: a photovoltaic cell; a partially metallised photovoltaic cell precursor; a photovoltaic cell precursor having an emitter layer on a base; and a photovoltaic cell precursor having a selective emitter layer on a base, said method comprising the steps of:
illuminating a first portion of said material with a first illumination from a light source suitable for generating a photoluminescence response from said material;
detecting photoluminescence emitted from a second portion of said material with an image capture device;
scanning said first and second portions across a substantial area of said material, said substantial area corresponding to an entire surface of said semiconductor material; and
interrogating said image capture device while said first and second portions are being scanned across said substantial area to acquire an image of photoluminescence emitted from said substantial area,
wherein an intensity of said first illumination is selected such that there is a significant lateral flow of photo-generated charge carriers out of said first portion, and wherein said method further comprises the step of interpreting said image to identify defects that impede carrier transport in said substantial area.
2. A method according to claim 1 , wherein said first and second portions are scanned in a scanning direction across the substantial area of said material and wherein said second portion is wider than said first portion in the scanning direction.
3. A method according to claim 1 , wherein the intensity of said first illumination is selected such that the lateral flow rate of photo-generated charge carriers out of said first portion is at least 10% of the photo-generation rate.
4. A method according to claim 3 , wherein the intensity of said first illumination is selected such that the lateral flow rate of photo-generated charge carriers out of said first portion is at least 50% of the photo-generation rate.
5. A method according to claim 4 , wherein the intensity of said first illumination is selected such that the lateral flow rate of photo-generated charge carriers out of said first portion is at least 80% of the photo-generation rate.
6. A method according to claim 1 , wherein the scanning step comprises moving said material relative to said light source and said image capture device.
7. A method according to claim 1 , wherein said method is applied in-line in a photovoltaic cell manufacturing process for quality control or process control purposes.
8. An article of manufacture comprising a non-transitory computer usable medium having a computer readable program code configured to implement the method according to claim 1 .
9. A system for obtaining series resistance or carrier transport information across a semiconductor material selected from the group consisting of: a photovoltaic cell; a partially metallised photovoltaic cell precursor; a photovoltaic cell precursor having an emitter layer on a base; and a photovoltaic cell precursor having a selective emitter layer on a base, said system comprising:
a light source adapted to illuminate a first portion of said material with a first illumination suitable for generating a photoluminescence response from said material;
an image capture device adapted to detect photoluminescence emitted from a second portion of said material;
a mechanism for scanning said first and second portions across a substantial area of said material, said substantial area corresponding to an entire surface of said semiconductor material; and
a computer programmed to interrogate said image capture device while said first and second portions are being scanned across said substantial area to acquire an image of photoluminescence emitted from said substantial area,
wherein an intensity of said first illumination is selected such that there is a significant lateral flow of photo-generated charge carriers out of said first portion, and wherein said computer is programmed to interpret said image to identify defects that impede carrier transport in said substantial area.
10. A system according to claim 9 , wherein said mechanism is configured to scan said first and second portions in a scanning direction across the substantial area of said material and wherein said second portion is wider than said first portion in the scanning direction.
11. A system according to claim 9 , wherein the intensity of said first illumination is selected such that the lateral flow rate of photo-generated charge carriers out of said first portion is at least 10% of the photo-generation rate.
12. A system according to claim 11 , wherein the intensity of said first illumination is selected such that the lateral flow rate of photo-generated charge carriers out of said first portion is at least 50% of the photo-generation rate.
13. A system according to claim 12 , wherein the intensity of said first illumination is selected such that the lateral flow rate of photo-generated charge carriers out of said first portion is at least 80% of the photo-generation rate.
14. A system according to claim 9 , wherein the scanning mechanism comprises a mechanism for moving said semiconductor material relative to said light source and said image capture device.
15. A system according to claim 9 , wherein said image capture device comprises a line camera or a time delay integration camera.
16. A system according to claim 9 , wherein the pixels within said image capture device comprise silicon, InGaAs or InGaAsP.
17. A system according to claim 9 , wherein said image capture device comprises a silicon sensor in combination with an InGaAs or InGaAsP photocathode.
18. A system according to claim 9 , when used in-line in a photovoltaic cell manufacturing process for quality control or process control purposes.
19. An article of manufacture comprising a non-transitory computer usable medium having a computer readable program code configured to operate the system according to claim 9 .