Hi all
The goal is to partially open the anti-reflective cover-layer (glass of 80-100 µm thick) from silicon wafer based solar cells, ideally by selective/damage-less ablation with a IR femtosecond laser source.
I've been doing literature survey for a little while. From what I understand so far, glass being transparent in the near IR and silicon having an ablation threshold way smaller than that of glass, the so called "damage-less" ablation is in fact superficial ablation of silicon and the cover glass is indeed blown away by silicon spallation. Am I correct there? As consequence, i guess this would cause certain amount of silicon damage/amorphorization, hence downgrading cell performance.
Is there a smarter way to remove this glass top layer without undermining underlying silicon at all? using microscope objectives? another wavelength? any other "out-of-the-box" ideas?
Cheers
Dear Sedao,
The anti reflective coating using SIO2 has a thickness of lamda/4 where lambda is the wavelength of the incident radiation. The wavelength of light in the material = lambda/sq root of epsilonr , Assuming lambda= 550 nm, sq root of epsilonr= 2, then the thickness will be in approximately = 550/8= 70 nm. Then you mean the thickness is from 80:100 nm.
The second comment is you said that you plan to use IR femto second laser for ablation. in the literature , 532nm laser is used for ablation of anti reflective coating of silicon, see the attached link. This is understandable as this wavelength will path to silicon where the laser beam must be absorbed to locally melt the silicon at the surface and gets the anti reflective coating film separated from the silicon surface after rapid solidification. So the wavelength must be also as small as possible to restrict its absorption at the surface area of the silicon. Accordingly IR laser will be less suitable than the given wavelength above.
HF dos not etch silicon, it etches sio2 and some other metals..
https://www.spiedigitallibrary.org/proceedings/Download?fullDOI=10.1117/12.2269681&SSO=1
Best wishes
Hi Sedao,
there will always be some damage, but depending on your sample and process sequence, the impact of the laser process on the performance can be low and even comparable to mask and etch techniques. From our experience, an NIR pulse is not a good choice if deep damage needs to be avoided, but the shortest we tried was 180fs. As Mr Zekry already pointed out, the absorption length is too high. We use 355nm for ablation on the emitter side of the cell to keep the damage shallow. Because the SiO2 layer is still transparent (as long as nonlinear absorption can be neglected), silicon ablation will take place at the interface and I would expect a thin layer of silicon to become amorphized with detremental impact on sheet resistance (see Fig. 6 in Article Reduction of picosecond laser ablation threshold and damage ...
)Kind regards, Fabian
Thanks Fabian for sharing your valuable experience!
By the way, very interesting approach used in your paper there. A side question: does the ns pre-pulse alone at that fluence alter the brightness of your "SiNx on Silicon" sample? I image a slightly-below-ablation-threshold ns pulse would still cause silicon to melt and re-solidify. If I am not mistaken there, these transitions induced by ns laser pulse do not cause significant silicon microstructure change, do they?
There was no change visible under the bright field microsope, however contours were visible in dark-field. So yes, I think the pre-pulse is melting a layer of silicon, while the nitride remains intact. There must be some microstructure change, otherwise I would not have observed it under the microscope, but it did not have a measureable effect on the electronic properties of our sample.
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