END POINT DETECTION FOR BACK CONTACT SOLAR CELL LASER VIA DRILLING
Methods and structures for fabricating photovoltaic back contact solar cells having multi-level metallization using laser via drilling end point detection are provided.
1 . A method for forming a back contact solar cell, comprising:
depositing a first metal layer of electrically conductive metal on a backside surface of a semiconductor solar cell substrate, said first metal layer comprising base electrodes and emitter electrodes connected to base regions and emitter regions on said semiconductor solar cell substrate;
attaching an electrically insulating backplane layer on said semiconductor solar cell substrate comprising said first metal layer;
drilling via holes in said backplane layer to said first metal layer, said via holes laser drilled through said backplane layer and said first metal layer at specified positions to expose conductive metal on said first metal layer to form base contacts and emitter contacts to said first metal layer;
detecting the via hole endpoint during said laser via drilling process to extend said via hole through said electrically insulating backplane layer to said first metal layer and prevent breaching or punching through said first level metal;
forming a second metal layer of electrically conductive metal on said backplane layer, said second metal layer contacted to said first metal layer through said via holes and providing conductive leads for electrical connections to said back-contact solar cell.
2 . The method of claim 1 , wherein said laser via holes are formed using a CO2 continuous wave laser.
3 . The method of claim 2 , wherein said laser via holes are formed using a CO2 continuous wave laser pulsed using an AOM.
4 . The method of claim 1 , wherein said laser via holes are formed using a laser having a wavelength in the range of UV to infrared and a pulse length in the range of approximately 1 nanosecond up to continuous wave.
5 . The method of claim 1 , wherein said laser via holes are formed using a laser having a wavelength in the range of approximately 9.4 to 10 microns and a pulse length in the range of approximately 1 to 100 microseconds.
6 . The method of claim 1 , wherein said via hole endpoint detection is performed using laser induced breakdown spectroscopy or plasma emission.
7 . The method of claim 6 , wherein said laser induced breakdown spectroscopy or plasma emission detects the presence of an element, said element present in said first metal layer.
8 . The method of claim 6 , wherein said laser induced breakdown spectroscopy or plasma emission detects the absence of an element, said element present in said electrically insulating backplane layer.
9 . The method of claim 1 , wherein said backplane layer is a polymeric material.
10 . The method of claim 1 , wherein said via hole endpoint detection is performed using laser reflectrometry.
11 . The method of claim 1 , wherein said via hole endpoint detection is performed using laser interferometry.
12 . The method of claim 1 , wherein said via hole endpoint detection is performed using the photoacoustic signal of ablated materials.
13 . The method of claim 1 , wherein said via hole endpoint detection is performed using the Raman spectroscopy signal of ablated materials.
14 . The method of claim 1 , wherein the end point detection signal is collinear with the laser ablation/drilling beam and synchronized to pick up the detection signal from the spot being drilled.
15 . The method of claim 1 , wherein laser reflectometry is used off-line to monitor the consistency and quality laser of drilled via holes through said electrically insulating backplane layer.
16 . The method of claim 1 , wherein Raman spectroscopy is used off-line to monitor the quality of drilled holes or vias in the backplane layer.