CROSS -REFERENCE
Reference is hereby incorporated to Indian patent application 331/KOL/2013 dated 22.03.2013 by the same inventor entitled "Method for single side chemical etching of Si wafers". A modified concept has been proposed in the present invention to enhance the throughput with better control on process parameters.
FIELD OF THE INVENTION
The present invention generally relates to solar photovoltaics and semiconductor devices. More particularly, the present invention relates to an inline etching system to remove emitter form one side of the diffused Si wafer without affecting the other side.
RACKCROUND OF THE INVENTION
Crystalline Si solar cells are known to be produced by making a p-n junction on a thin (~200 micron) Si wafer. This p-n junction is made by exposing a p-type Si wafer to POCI3 vapours in the presence of 02 at more than 850°C in a quartz tubular furnace. In a production setup about 400 large size (6"x6") Si wafers are loaded on a quartz boat and pushed into the preheated tube furnace for a predetermined time (~40 min). POCL3 vapours are carried by bubbling N2 gas in a bubbler filled with POCI3. 02 is also passed into the diffusion tube. During this process the diffusion tube is filled with POCI3 vapours and 02. The phosphorous (P) diffuses into the Si wafer from all sides making a shallow emitter of about 0.3 micron depth. For solar cell operation diffused emitter is required only on one side of Si wafer. At that high temperature and to avoid any contamination, it is not possible to cover one side of the Si wafer for diffusing P only on one side.

For developing high efficiency solar cells, diffused emitter is nevertheless required to be etched from one side leaving the other side untouched and un-etched. Although it is known that the diffused emitter can be removed by using a mixture of HN03 and HF, the main disadvantage is however, as to how only one side of the thin Si wafer can be wetted without affecting the other side. It must be remembered that this reaction is exothermic and produces corrosive fumes. Further, the temperature also needs to be controlled precisely. In a solar cell industry, thousands of Si wafers are to be processed daily and the time permitted for each process is less than 2 sec per wafer. This requires an inline etching system where Si wafers are fed from one end and removed from the other end after drying.
OBJECT THE INVENTION
It is therefore an object of the invention to propose an inline etching system to remove emitter form one side of the diffused Si wafer without affecting the other side.
SUMMARY OF THE INVENTION
Accordingly, there is provided an inline etching system to remove emitter form one side of the diffused Si wafer without affecting the other side. The thin Si wafers are transported on an assembly of rollers. The rollers made of known material have slits of very narrow width and are arranged in a shallow tank. The rollers are rotated using a worm gear at a very low rpm. A mixture of known concentration of HF, HNO3 and water is filled in a tank and pumped into the shallow tank. When the level of the liquid is maintained at a height of 3mm below the top of the roller, the acid mixture rises into the slits due to capillary

action and when the wafer comes on the roller, the lower side of the Si wafer is wetted with acid mixture. The wafers keep moving from one roller to the next and react with acid solution selectively on the lower side only. In this process the emitter on the lower side of the Si wafer gets etched. This is an inline system and the operator can easily keep feeding Si wafers from one side and taking out from the other.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
Figure 1 Shows schematic of an inline etching system according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in figure 1, the thin Si wafers are transported on an assembly of rollers. The rollers have slits of narrow width of about 0.9 mm. About 60 rollers made of known material of about 40mm diameter are arranged in a shallow tank of ~80 mm height. The rollers are rotated using a worm gear (Fig 1) at very low (20-40) rpm. The speed of rotation of the rollers is controllable by adjusting the VFD control. A mixture of known concentration of HF, HN03 and water is filled in a tank and pumped into a shallow tank using an electrical pump where the pumping speeds can be controlled using a VFD. The level of the acid mixture can be regulated at the desired height. When the level of the liquid is maintained at a height of 3mm below the top of the roller, the acid mixture rises into the slits due to capillary action and when the wafer comes on the roller, the lower side of the Si wafer is wetted with acid mixture. The wafers keep moving from one roller to the next and react with acid solution selectively on the lower side only. During this movement the Si wafer comes in contact with next roller and fresh acid

replaces the old reactants. The reaction also produces gas bubbles. These are automatically removed as the Si wafer goes form one roller to the other. The rotational speed of the roller is controlled in such a way that the time taken by the wafers to cross all the rollers is sufficient for the emitter to get etched completely. Hundreds of Si wafers have been etched selectively from single side using this equipment. The diffused emitter has been successfully removed and the same has been confirmed by measuring the sheet resistance. The sheet resistance of the diffused emitter has increased from 30-40ohm/sq to more than 1000 ohm/sq.
ADVANTAGES OF THE INVENTION
• The invention is unique wherein only one side is wetted selectively of the
diffused Si wafer. . Property of rising of liquid due to capillary action has been exploited in
this system. . The thin Si wafers are transported safely on the rollers and only one side
of the Si wafers is wetted. . The system has a facility to vary the rotation speed suitable for
optimizing the process conditions. . The technique is simple to use and does not require any specialized
manpower for operation. . Use of rollers for transporting Si wafers helps in avoiding any
contamination to the Si wafer. . The Inline system of the invention can be easily integrated into a manufacturing process line of solar cells production.

WE CLAIM :
1. An inline etching system to remove emitter form one side of the diffused
Si wafer without affecting the other side, the system comprising a plurality
of rollers for transporting silicon wafers for production of solar cells, the
rollers have slits of narrow width and disposed inside a shallow tank; a
worm gear rotating the rollers under VFD Control; an electric pump for
delivering a mixture of known concentration of HF, HN03 and water into
said shallow tank, the speed of the pump is controlled by a VFD whereby
the level of the acid mixture in the tank regulated to a desired height to
allow the acid mixture to rise in the slits of the rollers by capillary action
and wet the lower side of the silicon wafers with the acid mixture, the
upper side of the silicon wafers remaining free of contact by the acid
mixture.
2. The system as claimed in claim 1, wherein the rollers are made of known material used in the industry and having diameter about 40 mm.
3. The system as claimed in claim 1, wherein the rollers have a slit of narrow width for example 0.9 mm.
4. The system as claimed in claim 1, wherein the rollers are rotated at a low speed for example 20-40 r.p.m.
5. The system as claimed in any of the preceding claims, wherein the acid-mixture in the shallow tank is regulated at a height of about 3 mm below the top of the rollers.

6. The system as claimed in claim 1, further comprising moving the s4 Si wafers from one roller to the next to enable contacting the lower side of the wafer with acid mixture.
7. The system as claimed in claim 6, wherein the rotational speed of the rollers as controlled such that the time taken by the wafers to cross all the rollers is sufficient for the etching to be completed.
8. The system as claimed in claim 1, wherein the system is integratable into a manufacturing facility of solar cells.