Contact Formation and Gettering of Precipitated Impurities by Multiple Firing During Semiconductor Device Fabrication
Photovoltaic (PV) solar cell fabrication often involves the process of “co-firing” where metallic electrical conductors are mechanically and electrically coupled to semiconductor materials to create an electrical p-n semiconductor PV device. For example, during fabrication, an aluminum “ink” would be applied to a back surface (e.g. a p-type region) of a wafer of semiconductor material (such as a silicon wafer). The opposing front surface (e.g. an n-type region layer) of the wafer would have a coating of silicon nitride applied and a pattern of silver ink applied over the SiN. The device is then fired such that that the Ag ink dissolves through portions of the SiN layer to make electrical contact with the n-type region and the Al and Si at the back surface dissolve and then regrow to form a good electrical and mechanical connection.
One problem associated with the fabrication of a PV solar cell involves the purity level of a semiconductor (e.g. Si) material. The purity level is often a function of the production facility used. That is, production of semiconductor wafers with little to no impurities usually requires tightly controlled cleanroom environments. For industrial facilities attempting to produce Si wafers on a mass-production scale for PV solar cell devices, maintaining strict cleanroom environments is economically prohibitive. Therefore, some degree of impurities can be expected to exist in the final fabricated wafers. Such industrial facilities may therefore employ one or more post-processing techniques to attempt to rid impurities from the wafers. For example, if Si layers of a PV device include dissolved (non-precipitated) impurities within the Si material, then optical processing can be used to cleanse the wafer. Under optical processing, a diffusion process called gettering can be initiated that will cause the dissolved impurities to migrate into the Al contact material. For example, by initiating diffusion, dissolved iron impurities can rapidly be removed from the Si layers. This serves to clean impurities from the Si wafer. A problem occurs however, when the impurities include precipitated material. In that case, diffusion will not cause movement of the precipitated impurities from the Si layers. Further, the durations and high temperatures required to dissolve the precipitate impurities may not be compatible with other co-firing process steps, and have the potential to increase manufacturing expenses.
Engineers at the National Renewable Energy Laboratory (NREL) have developed a system that separates a single-step co-firing process for metallization of silicon solar cells into two steps – one for front contact and the other for back contact. This approach allows for the incorporation of efficient impurity gettering and formation of a deep back surface field into the back contact formation step. Thus, front and back contacts can be optimized separately. This process can also simultaneously provide impurity gettering and a deep back surface field – both necessary for high efficiency solar cells.
This two-step process has many advantages and separates strong alloying required for the back contact from gentle sintering required for the front side. While increasing cell processing by one step, the increased cell efficiency is large enough to merit a small increase in cell cost.
· Optimized gettering and deep back surface field
· Higher cell efficiency
Applications and Industries
· Multicrystalline silicon solar cells
· Back surface field
· Impurity gettering
· Vacancy injection
· Interstitial injection
Patents and Patent Applications
|Title and Abstract||
Contact formation and gettering of precipitated impurities by multiple firing during semiconductor device fabrication
Methods for contact formation and gettering of precipitated impurities by multiple firing during semiconductor device fabrication are provided. In one embodiment, a method for fabricating an electrical semiconductor device comprises: a first step that includes gettering of impurities from a semiconductor wafer and forming a backsurface field; and a second step that includes forming a front contact for the semiconductor wafer, wherein the second step is performed after completion of the first step.
|National Renewable Energy Laboratory||05/27/2014
|Technology ID||Development Stage||Availability||Published||Last Updated|
|NREL ROI 10-65||Development||Available||04/26/2016||04/26/2016|