TITLE:
An etching solution of acids for etching of Si wafers.
FIELD OF INVENTION
This invention relates to an etching solution of acids for etching of SI wafers.
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
POC3 vapours in the presence of O2 at more than 850oC in a quartz tubular
furnace. In a product setup about 400 large size (6"x 6") Si wafers are loaded
on a quartz boa, 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
the diffusion tube is filled with POCI3 vapours and O2 The phosphorous (P)
diffuses into the Si wafer from ail 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. A new setup has
been developed and is being patented separately. The setup uses a mixture of
HNO3, HF and water. H2SO4 is added to control the reaction. The etching
reaction is exothermic and produces corrosive fumes. The temperature also
needs to be controlled precisely. In the present patent application a process is
described where one side of the diffused emitter can be removed with a
throughput of one wafer every few seconds.
US Patent no. 4165252 published in 1979, describes a method for chemically
treating a single side of a workpiece, such as for etching or anodizing a
semiconductor wafer, comprising, placing such a workpiece face down on a flat
centrally apertured, relatively level table having a top or work surface of a size
and shape commensurate with the dimensions of the workpiece and introducing
the liquid for the chemical treatment between the top surface and side of the
workpiece to be treated where the liquid passes over the entire surface to be
treated and then returns to its source. The concept of this method is different

from the concept given in the proposed patent. In the proposed patent the Si
wafer is transported on continuously moving Teflon rollers and the etching liquid
is supplied by capillary action.
In another prior art described in US Patent no. 0041526 A1 published in 2008,
describes a method and apparatus for single-sided etching. 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 differentia.
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
Though the setup etches single side of Si wafers but the concept is entirely
different from what is conceived in the proposed patent.

OBJECTS OF THE INVENTION:
An object of the present invention is to propose an etching solution of acids for
etching of St wafers.
Another object of the present invention is to propose an etching solution which is
capable of removing the diffused emitter from one side of the Si wafer.
Still another object of the present invention is to propose an etching solution
which upon etching the Si wafer increases its efficiency.
Further object of the present invention is to propose an etching solution which is
capable of removing diffused emitter from one side of the Si wafer without
affecting the other side.
BRIEF DESCRIPTION OF THE PRESENT INVENTION:
According to this invention there is provided an etching solution of acids for
etching of Si wafers comprising:
HNO3:HF:H2O:H2SO4 at a ratio of 15:0.62:1.7:28.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
Fig 1: the schematic showing components of Single side etcher.
DETAILED DESCRIPTION OF THE INVENTION:
A new setup has been developed using a number of Teflon rollers to transport
the 6"x 6" delicate Si wafers. These are special rollers and have very fine slits
along the length of the rollers. The set of rollers is arranged in a shallow tank and
a specific level of acid solution is maintained in the shallow tank. Due to the
capillary action the acid solution rises in the fine slits and wets the lower surface
of the Si wafer. The setup is fitted with a storage tank of PVDF material to store
the mixture of HNO3 and HF safely at a constant temperature. Acid mixture of
required strength is prepared and pumped into the shallow tank using a VFD
driven pump. A number of Si wafer have been etched successfully by varying
acid concentrations, rollers speed and temperature of the acid solution
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 acid 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 acid solution rises in the fine slits and wets the lower surface
of the Si wafer when it rolls on the rollers. A separate patent application has been
sent for the setup. The setup is fitted with a storage tank of PVDF material to
store the mixture of HNO3 and HF safely. It is also fitted with a chilling coil of
Teflon material to control the temperature of the acid solution by circulating
chilled water. Initially 15 lit of water is added to the storage tank and circulated
using the acid proof recirculation pump. This is a VFD driven pump and the acid
circulation speed can be controlled with precision. Concentrated HNO3 acid is
added slowly to the shallow tank with circulation ON. This ensures that high
temperatures are not produced locally due to the reaction of acid and water.
Desired amount of concentrated HF is also added to the shallow tank. After
about a few hours of circulation, the acid temperature is reduced to room
temperature (<30 oC). A number of diffused Si wafers are put on the rollers and

taken out from the other end fully dried up. Due to chemical reaction of Si with
acid solution, the diffused emitter is dissolved and removed. The various reaction
products are given as below.
Si + 4HNO3^ Si02 + 4N02 + 2H2O
Si02 + 6HF -» H2SiF6 + 2H2O
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. In one go, the emitter could not be removed completely.
The concentration of the HNO3 and HF is increased by adding more quantity of
acids into the shallow tank slowly and maintaining their relative ratios. After
enough cooling of the acid solution again diffused Si wafer are passed and sheet
resistance measured. The emitter starts going away and the sheet resistance
increases. It is observed that the etched surface becomes patchy with blue or
black colored lines. These patches are of porous silicon and are soluble in dilute
NaOH solution at room temperature very easily as shown in following reaction.
2Si + 2NaOH + 4H2O -> 2NaSi03 + 5H2
A dilute solution of NaOH is prepared in a flat bottom tank of plastic material.
The Si wafers are removed from the etching setup before they enter into the
rinsing section and are rinsed in Dl water manually. The Si wafers are dipped in
dilute NaOH solution for about 30 s and are removed and put again on rollers of

etching solution for rinsing in running Dl water and drying. The concentration of
the acid solution is further increased by adding HNO3 and HF. The etch rate
increases further but the uniformity of emitter etch is not good enough. About 2.5
lit of H2SO4 is added to the acid mixture. This acid retards the reaction. A number
of diffused Si wafers are etched with this solution show uniform etching. 2.5 lit of
H2SO4 is further again added and the Si wafer are etched again. The uniformity
of etching improves significantly. More experiments are performed by varying the
acid bath temperature from 15 °C to 25 °C by passing chilled water at fixed

temperatures ranging from 10°C to 20 °C. A number of experiments are carried
out and the results and observations are shown in the table below.


WE CLAIM:
1. An etching solution of acids for etching of Si wafers comprising:
HNO3:HF:H2O:H2SO4 at a ratio of 15:0.62:1.7:28.
2. The etching solution as claimed in claim 1 wherein the temperature of the
etching acid solution is maintained preferably at 22°C.
3. A process for the etching only one side of the Si wafers comprising:
placing the Si wafers on the rollers and passing it over the etching solution,
exposing only one side of the wafers to the acid solution,
removing the dried Si wafers from the other end having no diffused emitter,
maintaining the concentration of the acid and the temperature inside the tank,
removing the patches using dilute alkali solution.
4. The process as claimed in claim 3, wherein the acid solution rises for etching
the wafer through the slits present in the rollers due to the capillary action.

5. The process as claimed in claim 3, wherein the said diffused emitter is
dissolved and removed by the reaction between the Si and the etching acid
solution.
6. The process as claimed in claim 3, wherein after removing the diffused emitter
from the surface of the wafer it leaves behind some patches which are finally
rinsed using dilute NaOH solution at room temperature.
7. The process as claimed in claim 3, wherein the said wafers are passed over
the etching acid solution preferably at a linear speed of 22 cm/min.