FIELD OF THE INVENTION:
This invention relates to a single side etcher of silicon wafer for laboratory use.
This invention further relates to a single side etcher which can be used in solar photovoltaic and semiconductor devices. Etching or polishing of single side of diffused Si wafer is required using chemical processes for rear surface preparation in developing high efficiency Si solar cells.
BACKGROUND OF THE INVENTION:
Crystalline Si solar cells are produced by making a pn junction on a thin (~200 micron) Si wafer. This pn junction is made by exposing a p-type Si wafer to POCl3 vapours in the presence of O2 at more than 850ºC 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 POCl3. O2 is also passed into the diffusion tube. During this process the diffusion tube is filled with POCl3 vapours and O2. 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 required to be etched or polished from one side leaving the other side untouched and un-etched. This process step has to be tried at various

concentrations and temperatures of mixture corrosive acids (HNO3 and HF). Production worthy single side etchers are available but the process needs to be optimised more precisely using small quantities of acids under very controlled conditions. As the acid mixture is highly corrosive, any other setup, having a vacuum chuck connected to a vacuum pump is not advisable due to safety reasons. In case of accidental breakage of fragile Si wafer, the corrosive acid solution can be sucked by the pump and discharged dangerously.
US 9355867 B2 discloses a process for the solar cells having a local back surface field (LBSF) using an alkaline etching paste which allows the back surface to be polished and the back surface edges to be insulated in a single process step.
WO 2013/169208A1 entitled ‘Non-acidic isotropic etch-black for silicon wafer cells (en)’ discloses a method for solar cell fabrication. The method includes etching a doped surface of a silicon wafer solar cell using a solution including potassium hydroxide (KOH) and sodium hypochlorite (NaOCl).
However, the need exists, to propose a process for rear surface preparation of silicon wafers using any chemical while keeping the other side untouched. Further, it would be useful to develop an equipment for such purpose, whereby the Si wafer to be etched can be held firmly using vacuum from a venturivacuum unit and any accidental suction of acid mixture will not harm the operator due to its unique design.
OBJECTS OF THE INVENTION:
It is therefore an object of this invention to propose a single side etcher of diffused silicon wafer.

It is a further object of this invention to propose a single side etcher of diffused silicon wafer by which etching/polishing is effected on a single side of the silicon wafer, leaving the other side untouched and unetched.
Another object of this invention is to propose a single side etcher of diffused silicon wafer whereby accidental breakage of Si-wafer or suction of corrosive acid mixture does not cause damage to persons.
Yet another object of this invention is to propose a single side etcher of diffused silicon wafer, whereby metal contamination can be avoided.
A further object of this invention is to propose a single side etcher of diffused silicon wafer, which uses simple inexpensive chemicals and is cost effective.
These and other objects and advantages of the invention will be apparent from the ensuing description, when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1 shows a schematic diagram of the arrangement of a single side etcher.
DETAILED DESCRIPTION OF THE INVENTION:
Thus according to this invention is provided a single side etcher (1) of silicon wafer.
In accordance with this invention, a single Si wafer (2) is placed on a chemically inert Teflon chuck (3) and held with the help of vacuum in a

chuck, the chuck will be dipped in the acid mixture (8) in the acid tank (7) for a pre determined time. The acid mixture (8) is held upto height l in the acid tank (7). After the time is over, the Teflon chuck (3) will be removed from the acid tank (7) and dipped in water tank to remove traces of acid. After satisfactory removal of acid traces from the wafer and chuck, the vacuum will be stopped and the Si wafer will be taken out for further processing.
The venturivacuum unit includes a compressed air input (9) and a compressed air output (10) and is releaseably connected via channel means (11) to the outlet (12) of the chuck by a suction tube (6) which is a flexible hose, preferably a flexible Teflon hose.
Unique feature of this setup is that that it can hold large area (6” x 6”) Si wafer on a chemically inert material chuck of Teflon material. The vacuum is created using a venturi vacuum unit of Teflon material which uses compressed air to create vacuum and not the vacuum pump. This has the advantage that if accidently the corrosive acid is sucked by the vacuum unit, the discharge is released inside the tank only. Hence the chance of any injury to the person doing experiment is totally ruled out. The silicon wafer is releasably held by vacuum at the back of the wafer to said chuck by a pressure in the range of 0.20 to 0.30 kg/cm2. The vacuum is preferably about 0.25 times of the atmospheric pressure i.e. about 0.25 kg/cm2
This pressure will be along a very fine slit close to the border of the wafer. Therefore the force of this pressure will be sufficient to hold the fragile wafer tightly against a polished Teflon surface and still not break it.
The acid mixture comprises a mixture of nitric acid, hydrofluoric acid and water, wherein the ratio of acids in the acid mixture is in the range of HNO3: HF: H2O is 40-45: 5-10: 50, typically HNO3: HF: H2O is 45: 5: 50. The acid mixture will be prepared with (volume/volume) 45% HNO3, 5% HF and 50% water. The Si wafer will be dipped in the solution at room temperature or below (25degree C). The etching time will in the range of from half a minute to a few minutes.

We Claim:
1. A single side etcher (1) of silicon wafer comprising a tank (7), a chemically
inert chuck (3) adapted to releasably hold a silicon wafer (2) to be etched with
the help of vacuum,
said chuck (3) being connected to a venture vacuum unit (5) by means of flexible tubing to enable free movement of the chuck (3),
said chuck being adapted to dip into an acid mixture (8) contained in said tank (7), said venturivacuum unit (5) including a compressed air input (9) and a compressed air output (10) and releasably connected via channel means (11) to the outlet (12) of the chuck (3) by a suction tube (6).
2. The single side etcher (1) as claimed in claim 1, wherein said chuck (3) and suction tube (6) are made from flexible inert material such as Teflon.
3. The single side etcher (1) as claimed in claim 1, wherein said silicon wafer is releasably held by vacuum at the back of the wafer.
4. The single side etcher (1) as claimed in claim 1, wherein said silicon wafer is held to said chuck by a pressure in the range of 0.20 to 0.30 kg/cm2.
5. The single side etcher (1) as claimed in claim 1, wherein said acid mixture comprises a mixture of nitric acid, hydrofluoric acid and water.
6. The single side etcher (1) as claimed in claim 1 &5, wherein the ratio of acids in the acid mixture is in the range of HNO3: HF: H2O is 40-45: 5-10: 50.