FIELD OF INVENTION
The present invention relates to a shower head electrode for uniform distribution
of process gases in a plasma vacuum chamber to produce texturized silicon
wafers. The invention further relates to a method for uniform distribution of
process gases for deposition of thin film silicon semiconductors.
BACKGROUND OF INVENTION
The plasma based process is extensively used for cracking gas modules into its
species in a vacuum chamber either for deposition of thin films of
semiconductors or for plasma etching of semiconductor surfaces for example,
silicon wafers over the large area electrodes. The etching process of silicon
wafers produces a rough surface termed as texturised surface which reduces the
reflection losses of the incident light on the solar cells. In plasma process, a gas
is introduced into the vacuum chamber and the plasma is struck between the
two electrodes using a radio frequency (RF) power supply. The uniformity of the
etching process depends on the uniformity of plasma which in turn depends on
the uniformity of the distribution of process gases.
Texturisation of silicon wafers is a critical process step in the production of silicon
solar cells. In this plasma based reactive ion etching process, the vacuum
chamber is evacuated and process gases are introduced into the vacuum
chamber in a controlled manner using a plurality of mass flow controllers in the
range of 100-1000 sccm. The gases are premixed in a gas blender and allowed
to flow into the vacuum chamber through a vacuum feed-through. The gas is
allowed to expand in a stainless steel hollow cylinder. Twenty numbers of very
fine capillaries of stainless steel material each are welded to this cylinder and the
premixed gas is allowed to travel through these capillaries to reach a plurality of
zones and is further allowed to expand in a perforated box to reach a
corresponding number of silicon wafers. This apparatus consisting of a stainless
steel cylinder, capillaries and perforated box is termed as the shower head
electrode. Plasma is struck between the two electrodes, the shower head and the
ground electrode, using a radio frequency (RF) power supply. The substrate,
which is silicon wafer in this case, is placed in the plasma on the ground
electrode. The process gases break into its various etching species and etch or
texturise the silicon surface. The uniform texturisation of the silicon surface is a
prerequisite for marking an efficient solar cell which depends on the uniformity of
plasma which in turn depends on the uniformity of the distribution of process
gases over the entire area of the electrode. This is a critical process requirement.
The uniform texturisation of silicon wafer is a prerequisite for producing high
efficiency solar cells. The high throughput requirement of solar cell industry
necessitates a large plasma reactor to accommodate large number (at least 20 in
one run) of big size (~6 sq. inch area) silicon wafers. At low process pressure of
about 100-500 mTorr, maintaining an uniform gas distribution in a large area
vacuum chamber to maintain uniform plasma, is quite difficult.
To the best of out 'inventors' knowledge, the texturisation process of crystalline
silicon wafers using plasma etching process is still under R&D stage globally.
There are various technical issues to be resolved for developing this process at
industrial scale. To match the throughput requirement of solar cell industry, it is
necessary to develop a high throughput industrial process including rugged
hardware for trouble free operation. This necessitates configuration of a large
plasma reactor to accommodate large number (at least 20 in one run) of big size
(~6 sq. inch area) silicon wafers. This further requires a large vacuum chamber
with large electrode. The process pressure has to be maintained at about 100-
500 mTorr. At this low pressure, maintaining uniform gas distribution to maintain
uniform plasma is a big challenge.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a shower head electrode to
distribute process gases uniformly in a vacuum chamber over a large area
ground electrode.
Another object of the invention is to propose a shower head electrode to
distribute process gases uniformly in a vacuum chamber over a large area
ground electrode, which enables to achieve a uniform plasma density for uniform
deposition on thin films of silicon semiconductors or for etching silicon
semiconductor surfaces.
A further object of the invention is to propose a method to distribute process gas
uniformly in a vacuum chamber over a large area ground electrode to produce
texturised Si semiconductor wafers.
SUMMARY OF INVENTION
According to the invention, a shower head electrode is provided which employs
capillary tubes of equal lengths for different zones of the electrode to ensure an
equal flow of gases over the entire electrode area using the capillary tubes.
Though the silicon wafers are placed at different distances from the gas feed
through, the gas however, experiences the same resistance to flow through the
capillary of equal length for all the wafers. As the process gases is further
diffused into another box and reach the silicon wafers through nine holes, it
allows the gas coming out from the capillaries at the centre of the silicon wafer
to get divided for uniform spread on each silicon wafer. The distribution of gas is
uniform over the large area of about 0.7 sq. m. The process pressure inside the
vacuum chamber is maintained in the range of 100 to 500 m Torr using a
continuous pumping of the process gases by vacuum pumps.
The uniformity of plasma has been confirmed by texturing at least twenty
numbers Si wafers uniformly over the entire area of the electrode.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - schematically shows a shower head electrode according to the
invention.
DETAIL DESCRIPTION OF THE INVENTION
Figure -1 shows an improved shower head electrode (SHE) for uniform
distribution of process gases in a process chamber. For simplicity reasons, only
four capillaries (2) and four silicon wafers (1) are shown in the figure. In the
actual shower head, at least twenty such capillaries (2) are used for twenty
silicon wafers (1). The process gases are premixed in a gas blender (not shown)
and are fed to a vacuum chamber through a vacuum feed through (3) via a first
tube (10). Inside the vacuum chamber, a Teflon tube (4) connected to the first
tube (10) is used to feed the premixed gases into the shower head electrode
(SHE) through a stainless steel hollow cylinder (5). A plurality of fine capillaries
(2), are connected to this cylinder (5). The silicon wafers (1) are kept on a flat
stainless steel carrier (6) which is at ground potential. The carrier (6) is placed
exactly below the shower head electrode (SHE) so that the centre of each Si
wafer (1) matches exactly with outlets (11) of respective capillary tube (2). The
shower head electrode (SHE) is connected to an RF power supply (not shown).
The premixed process gases are allowed to travel to the plurality of silicon
wafers (1) through a corresponding number of capillaries (2). Though the linear
distance of all the silicon wafers (1) is different from the gas input feed through
(3), an equal length and identical inner diameter of the capillaries (2) ensure that
the gas molecules reaching the silicon wafers (1) experience the same resistance
and allow supply of exactly same amount of gas to each silicon wafers (11). The
gases are further allowed to diffuse into another rectangular box (7) and reach
the silicon wafers (1) through at least nine holes on the bottom plate of box (7).
This ensures that the gas coming out from the capillaries (2) at the centre of the
silicon wafer (1) further gets divided for uniform spread on each silicon wafer
(1). The gas flows are very small and its measurement is difficult. An innovative
technique has been developed to confirm gas flows through various capillaries
(2) at atmospheric pressure. A copper strip (8) is connected between the hollow
cylinder (5) and a second tube (9) for RF-supply. The uniformity of plasma has
been confirmed by testing the texturisation results of at least twenty wafers per
run.
WE CLAIM
1. A shower head electrode for uniform distribution of process gases in a
plasma vacuum chamber to produce texturised silicon wafers, comprising:
a vacuum chamber having a vacuum feed-through (3) and receiving
premixed process gases from a gas blender via a first tube (10), the first
tube (10) being connected to a first end of a Teflon tube (4) disposed
inside the vacuum chamber;
- a stainless steel hollow cylinder (5) comprising a top end, a bottom end,
and an intermediate section, the top end operably connected to a second
end of the Teflon tube (4), the bottom end releasably disposed over a
correspondingly sized rectangular box (7) and the intermediate section
connected to a plurality of capillaries (2) having outlets (11) in registration
with the number of capillaries (2) adapted;
- a plurality of silicon wafers (1) placed on the carrier (6) such that the
centre of the silicon wafers (1) matches the outlets (11) of the capillaries
(2); and
- the silicon wafers (1) are enabled to receive identical amount of premixed
process gases through said capillaries (2) via at least nine holes which
allow an uniform plasma deposition / etching on each wafer with shower
head electrode connected to an RF supply with a copper strip (8).
2. A method in a shower head electrode for uniform distribution of process
gases for deposition of thin films silicon semiconductors the method
comprising the steps of:-
- premixing process gases in a gas blender and feeding the gas to a
vacuum chamber via a vacuum feed-through;
- providing a shower head electrode connected to an RF power supply;
- feeding the premixed process gases into a hollow cylinder via a Teflon or
metal tube, the hollow cylinder provided with a plurality of capillary;
- disposing a plurality a silicon wafer or other substrates on a carrier
positioned at ground potential, the carrier being placed exactly below the
shower head electrode;
- allowing the process gases to flow through the capillaries enabling
impingement of the process gases on the wafers or other substrates
through the capillaries, wherein each of the silicon wafers or other
substrates receive an identical amount of gas uniformly spreaded.
3. The shower head electrode as claimed in claim 1, wherein the linear
distance of the silicon wafers from the gas input feed-through is different
from one another.
4. The shower head electrode as claimed in claim 1, wherein the capillaries
each has an identical inner diameter including identical length.
5. The shower head electrode as claimed in claim 1, comprising a
rectangular box (7) disposed below the hollow cylinder (5).
6. The shower head electrode as claimed in claim 5, wherein the rectangular
box (7) comprising at least nine holes on the bottom plate of box (7).
7. A shower head electrode for uniform distribution of process gases in a
plasma vacuum chamber to produce texturised silicon wafers, as
substantially described and illustrated herein with reference to the
accompanying drawings.
8. A method in a shower head electrode for uniform distribution of process
gases for deposition of thin films of silicon semiconductors/other
substrates as substantially described and illustrated herein with reference
to the accompanying drawings.

The invention relates to a shower head electrode for uniform distribution of
process gases in a plasma vacuum chamber to produce texturised silicon wafers
comprising a vacuum chamber having a vacuum feed-through (3) and receiving
premixed process gases from a gas blender via a first tube (10), the first tube
(10) being connected to a first end of a Teflon tube (4) disposed inside the
vacuum chamber; a stainless steel hollow cylinder (5) comprising a top end, a
bottom end, and an intermediate section, the top end operably connected to a
second end of the Teflon tube (4), the bottom end releasably disposed over a
corresponding sized rectangular box (7), and the intermediate section connected
to a plurality of capillaries (2) having outlets (11) in registration with the number
of capillaries (2) adapted; a plurality of silicon wafers (1) placed on the carrier
(6) such that the centre of the silicon wafers (1) matches with the outlets (11) of
the capillaries (2); and the silicon wafers (1) are enabled to receive the premixed
process gases through said capillaries (2) to impinge on silicon wafers (1) with
the shower head electrode (SHE) connected to an RF-power supply such that
each of the silicon wafers (1) are enabled to receive identical amount of process
gas via at least nine holes which allows uniform plasma deposition/etching on
each wafer (1).