This invention relates to a method for etching/polishing a single side of silicon wafer with an alkali in single side etcher.
BACKGROUND OF THE INVENTION:
Crystalline Si solar cells are produced by making a pn-junction (positive-negative junction) on a thin (-200 micron) Si wafer. This pn-junction is made by exposing a p-type Si wafer to POCb vapours in the presence of O2 at more than 850oC in a quartz tubular furnace. In a production setup, about 400 large size (6" x 6") Si wafers are loaded on a quartz boat and pushed into the preheated tube of diffusion furnace for a predetermined time (-40 min). POCL3 vapours are carried in by bubbling N2 gas in a bubbler filled with POCb solution. O2 is also passed into the diffusion tube. During this process the diffusion tube is filled with POCb 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 from one side leaving the other side untouched and un-etched.
Earlier, a patent was filed by the same inventor entitled "Method for single side chemical etching of Si wafers." Application No: 331/KOL/2013 dated. 22.03.2013. A modified concept has been used in the setup to enhance the throughput with better control on process parameters. Another patent was filed by the same inventor titled "Process for single side etching of Si wafers on pilot scale" application no 1120/KOL/2015 date: 29.10.2015. In this invention, the etching is done by mixture of acids which is far more hazardous than the proposed patent.
EP 2079856B1 teaches us method and apparatus for single-sided etching. WO2008020974A2 relates to single -sided etching.
The patent in Reference No. 1. The etcher includes a vacuum chamber; a perforated belt positioned against the vacuum chamber; and an etch chamber positioned on an opposing side of the perforated belt relative to the vacuum chamber. The etch chamber has an

opening through which an etchant is released. The vacuum chamber is configured to create a pressure differential which protects the back side of the wafer from the etchant. In use, a back side of a wafer is disposed against the perforated belt. The front side of the wafer is exposed to the released etchant. The pressure differential secures the back side of the wafer to the belt and/or extracts through a perforation of the belt etchant not deposited on the front side of the wafer.
The patent in Reference No. 2 describes method to etch a single side of a wafer comprising: disposing a back side of a wafer against a perforated belt; exposing a front side of the wafer to an etchant; and creating a pressure differential between opposing sides of the belt, the pressure differential extracting through a perforation of the belt etchant not deposited on the front side of the wafer to protect the back side of the wafer from the etchant.
Further, the etching/polishing are being done with HF/HN03 acid mixture in solar cell industry. This method has many disadvantages. For example, polishing using acid releases lot of hazardous fumes, whereas NaOH, when used is less toxic. The acid produces some black marks on the wafer surface, while polishing with NaOH is a clean process.

The setup uses a less-toxic and less-hazardous NaOH solution with 30% concentration. The etching reaction is exothermic and produces bubbles and fumes which are less-corrosive in nature. Further, if it is done after antireflective coating (ARC) on one side, the front surface does not deteriorate due to spillage of NaOH from edges. This is because SiNx does not react with NaOH and keeps the front surface intact. The temperature of NaOH solution is set at 80°C for fast reaction and better throughput. In the present patent application a process is described, wherein one side of the diffused emitter can be removed with a throughput of one wafer every few seconds.
OBJECTS OF THE INVENTION:
An object of the present invention is to propose a method for etching/polishing a single side of silicon wafer with an alkali.
Another object of the present invention is to propose a method to remove diffused emitter from one side of the Si wafer without affecting the other side using an inline etching setup.
Still another object of the present invention is to propose a method for etching/ polishing a single side by using a less-hazardous chemical.

BRIEF DESCRIPTION OF THE INVENTION:
This invention relates to a method for etching/polishing a single side of silicon wafer with an alkali comprising: subjecting both sides of Si wafer to the step of diffusion, etching one side of the said diffused Si wafer in an alkaline solution, introducing the etched Si wafer to the step of rinsing and drying in infrared(lR) lamps.
BREF DESCRIPTION OF THE ACCOMPANYING DRAWING:
Fig 1: shows the schematic components of single side etcher.
DETAILED DESCRIPTION OF THE INVENTION:
During the diffusion process of Si wafers for making solar cells, emitter is diffused on both sides of the Si wafer. For solar cell operation diffused emitter is required only on one side of the wafer. For developing high efficiency solar cells, diffused emitter is required to be etched from one side leaving the other side untouched and un-etched. Using a new setup, developed recently, a process has been developed for etching the diffused emitter selectively from one side of the Si wafer in NaOH solution. In the present invention, a process has been described, wherein one side of the diffused emitter can be removed with a throughput of one wafer every few seconds.

A new setup has been developed, which uses a number of Teflon rollers to transport the 6" x 6" delicate Si wafers of thickness of about 180 microns. These are special rollers and have very fine slits along the length of the rollers. The rollers are designed to move with a variable speed resulting in a linear speed of a few cm/min to 50 cm/min. The set of rollers is arranged in a shallow tank so that using minimum amount of NaOH solution, rollers can be submerged up to 90% of their height in acid solution. Fig 1 shows the schematic of the setup. Due to the capillary action, the NaOH solution rises in the fine slits and wets the lower surface of the Si wafer, when it rolls on the rollers. The setup is fitted with a storage tank of PVDF material to store the NaOH solution safely. It is also fitted with a heating coil coated with Teflon to heat the NaOH solution. A variable frequency driven pump is provided to circulate the solution in the shallow tank, where the circulation speed can be controlled with precision. Two heaters are inserted into the tank to heat the solution up to 80°C. It is done to increase the throughput of polishing to one wafer every few seconds, as high temperature leads to faster polishing. The advantage of using NaOH is that it does not release any harmful and corrosive by-product
The chemical reaction used is:

The Sodium Silicate (Na2Si03) is non-corrosive and dissolves in water when the wafer is rinsed after polishing.

Further, while using it on wafer with an anti-reflective coating on one side ensures no harmful effect on that side as NaOH does not reacts with SiNx. A number of diffused Si wafers with antireflective coating on front side are put on the rollers and taken out from the other end fully dried up. The sheet resistance of the diffused emitter is measured before and after the etching process using a four probe setup. Initially the etch rate is found to be very low with dilute solution and less temperature. In one go, the emitter could not be removed completely. The concentration of the NaOH is increased by adding more quantity NaOH pellets into the shallow tank slowly. Temperature is also increased slightly to increase the throughput. Si wafer are passed and then sheet resistance is measured. The emitter starts going away and the sheet resistance increases. The polished surface is free from any patches and lines. The Si wafers are then passed into the rinsing section, where they are rinsed with deionized (DI) water. The drying section dries it up using infrared (IR) lamps. The best result was achieved at speed of 35cm/min at a 30% concentration of NaOH at 80 deg C.

WE CLAIM:
1. A method for etching/polishing a single side of silicon wafer with an alkali
comprising:
subjecting both sides of Si wafer to the step of diffusion,
etching one side of the said diffused Si wafer in an alkaline solution,
introducing the etched Si wafer to the step of rinsing and drying in infrared(IR)
lamps.
2. The method as claimed in claim 1, wherein the said alkaline solution is NaOH
3. The method as claimed in claim 1, wherein the concentration of NaOH is 30% at 80X.

4. The method as claimed in claim 1, wherein the said etched wafer an rinsed using deionized(DI) water.
5. A system for etching /polishing a single side of silicon wafer with an alkali
comprising:
a number of Teflon rollers to transport the "6 x 6" delicate Si wafers said rollers have fine very fine slits along the length of the rollers, rollers move with a variable speed resulting in a linear speed,

the set of rollers are arranged in a shallow tank having NaOH solution,
storage tank containing NaOH solution is also fitted with a heating coil to heat the
solution,
Si wafer after the NaOH treatment is passed into the rinsing section and then to
the drying section having infrared(IR) lamps.