FIELD OF THE INVENTION:
The present invention relates to a novel wafer carrier for rapid heating system
for quick heating of “n” type crystalline silicon wafers in the fabrication of
silicon heterojunction (HIT) solar cells using Plasma Enhanced Chemical Vapor
Deposition (PECVD) process
BACKGROUND OF THE INVENTION:
Silicon heterojunction solar cell technology is a technology which is a
combination of crystalline silicon and amorphous silicon technology. Highest
efficiency achieved in crystalline silicon solar cell on large area is manufactured
from silicon heterojunction technology. In silicon heterojunction technology a
high bulk lifetime "n" type wafer is taken as base on top of which amorphous
silicon layer are deposited by Plasma Enhanced Chemical Vapor Deposition
(PECVD) process. PECVD is a process by which thin films of various materials
can be deposited on substrates. The deposition temperatures in PECVD process
are critical in many applications like silicon heterojunction solar cells. In
PECVD process wafer are first loaded in a wafer carrier which is made of
stainless steel with slots in it for holding the wafers. The wafer is heated in load
lock/ isolation chamber to attain the temperature required for deposition
process. The carrier is 9mm thick, time require to heat the carrier is quite long
which is around 20 minutes in isolation chamber. The wafers require to be
heated quickly to avoid the oxidation of cleaned silicon wafer. More the time of
heating the more the chance of contamination and growth of oxide layer on
wafer surface. Also uniform heating of all the wafers on the carrier is required
for uniform deposition of a-Si layers.
In one of the prior art US 2010/0151680A1, Fabrication of thin film
semiconductor devices is achieved by mounting a substrate against a back
plate, With the substrate juxtaposed against the back plate. The back plate is
transported, with the substrate, through a deposition chamber and exposed to
a deposition process. Heating or cooling of the substrate occurs through the
back plate during the deposition process, so as to provide temperature

uniformity during a deposition process. This patent describes about the method
to achieve uniform heating of substrate carrier but do not suggest anything
about quick heating of substrate/ wafers.
In another prior art CN202297766U, an embedded PECVD (Plasma Enhanced
Chemical Vapor Deposition) silicon wafer carrier is described. The embedded
PECVD silicon wafer carrier compare a square frame, the side length of the
frame is equal to the side length of a silicon wafer carrier device of the first
specification, one face of the frame is a first square, the other face of the frame
is a second square, and the first square and the second square form a step in
height. The size of the first square is larger than the size of a silicon wafer to be
processed and the four corners of the first square are all processed into process
fillets; and the second square is a hollowed square. The wafer to be processed
just stepped into the slot, the process gas directly on a silicon wafer surface
deposition, to avoid the flaw next slide is coated on a silicon wafer covered in
hooks produced. In this patent method to hold wafer in a carrier is explained
but there is no reference to the quick heating of wafers.
In another prior art CN104465465A, the invention provides a graphite carrier
for plate type PECVD. The graphite carrier is provided with multiple cell type
silicon wafer supporting areas, wherein each cell type silicon wafer supporting
area is of a 156 mm*156 mm structure, the distances between the hooking pins
and the distances between the blocking pins are re-designed, silicon wafers in
various specifications can be supported under the condition that a machine is
not changed.
In another prior art US20160002774, a wafer carrier for carrying solar cell
wafers during a deposition process is described. The carrier is coated with
pyrolytic carbon, silicon carbide or a ceramic material, and is adapted to receive
and support the wafers.
In above mentioned two patents viz., CN 104465465A and US20160002774A1,
the wafer carrier are fabricated from new type of material to hold and support
the wafers but there is no reference to the quick and uniform heating of wafers.

Hence, there is always a long felt need to provide a rapid heating arrangement
and methodology for deposition of Si-layers on Silicon wafers carrier quick and
uniform heating can be achieved without chance of oxidation.
The present invention meets the long felt need.
SUMMARY OF THE INVENTION:
A novel wafer carrier for rapid heating of silicon wafer is made up of aluminum
material with varying cross section thickness such as 2 mm beneath the wafer
holding slots as lower thickness and 9 mm elsewhere, for quick heating of
silicon wafer.
OBJECTS OF THE INVENTION:
It is therefore, the primary object of the present invention to provide a wafer
carrier used in the HIT solar cell is made of aluminum material with varying
cross section thickness is designed for quick heating of silicon wafer to the
required temperature before deposition.
Yet another object of the present invention to provide a wafer carrier, where the
thickness of lower beneath of slots is optimized and then the wafer will get
heated faster due to less thermal mass beneath the slot.
Further object of the present invention to provide a wafer carrier, which is
made up of aluminum which has a much higher thermal conductivity atleast 5
to 7 times that of stainless steel and hence aluminum carrier will get heated
faster than stainless steel carrier.
Yet another object of the present invention to provide the wafer design, which is
economic and simple yet rapid.
Another object of the present invention to provide a fabrication method of
silicon heterojunction solar cells, where the thickness of 2 mm is optimized for
the slot designed to hold the Si wafer to ensure rapid heat transfer while
maintaining carrier structural integrity during the PECVD deposition process.
Yet another object of the present invention to provide a fabrication method of
silicon heterojunction solar cells, where heating of wafer occurs quickly to avoid

any oxidation of chemically cleaned silicon wafer and chances of contamination
and growth of oxide layer is almost nil.
Further, object of the present invention to provide a fabrication method of
silicon heterojunction solar cells, where uniform heating of wafers is possible to
have uniform deposition of a Si-layers.
DETAILED DESCRIPTION OF THE INVENTION:
The present subject matter relates to a modified wafer carrier which is designed
for rapid heating of silicon wafer to the required temperature before deposition
to ensure that the contamination and oxygen pick-up before PECVD process is
substantially minimum.
The wafer carrier is heated in the Isolation chamber and then placed in “I”
chamber for deposition of intrinsic amorphous silicon “I” layer in moving mode.
The load lock and Isolation chamber comprises infrared (IR) heaters and I
chamber consists resistive type heaters.
The originally designed carrier is made of Stainless Steel material having 9 mm
thickness, the time required to heat the carrier to a process temperature of
150°C is around 20 minutes in isolation chamber, however, wafers need to be
heated more quickly to avoid oxidation of chemically cleaned silicon wafer.
Shorter the heating time, lesser are the chances of contamination and growth of
oxide layer on wafer surface. Also uniform heating of all the wafers on the
carrier is very essential for uniform deposition of a-Si layers. Hence a new
carrier is designed for rapid heating of silicon wafers.
In accordance with the embodiment of the present invention, there is provided
an innovative wafer carrier (1), which is made up of aluminum material with
varying cross section thickness, which comprises plurality of slots or more
particularly four number of round slots (2) for holding the wafers (as shown in
fig.1).
The slots are of round impression on the corner of each slot for easy withdrawal
of wafers after the deposition. The thickness of carrier is varying across the

cross section; it has lower thickness of 2 mm in the slots for holding wafers and
9 mm elsewhere (fig 2.) (as illustrated in fig 2.). So that the quick heating of
silicon wafer to the required temperature can be achieved before deposition. Fig
2 illustrations the wafer carrier, and it appears that the thickness of carrier is
lower beneath the slots where the wafer is held. The concept is that during the
IR heating the wafer will get heated faster due to less thermal mass beneath the
slot. While in normal carrier first the carrier gets heated then the wafer reaches
that temperature which takes a longer time.
Aluminum has a much higher thermal conductivity, 5-7 times that of stainless
steel, hence aluminum carrier will get heated faster than stainless steel carrier.
In accordance with another embodiment of the present invention, there is
provided a method for fabrication of HIT solar cell including the wafer carrier,
comprises the steps of :
i) cleaning of the wafers through wet chemical process before deposition
on a-Si layers;
ii) dipping of wafer in Hydrofloric (HF) solution for removing the oxide
layer for deposition of amorphous silicon layers;
iii) loading of wafers in a novel wafer carrier and then it is introduced in
the load lock chamber and then transferred to isolation chamber for heating;
iv) heating of wafers to attain optimum temperature of 130°-180° C for
proper deposition of a-Si Layers.
The cleaning process of the wafers consists of saw damage removal process,
texturisation process, polishing & RCA cleaning process.
The non limiting advantages of the present invention are given below:
i) the Silicon wafer carrier design is suitably modified by varying cross
sectional thickness for rapid heating of wafers;
ii) the rapid heating of silicon wafer prevents oxidation of the cleaned
wafer;

iii) the design of carrier results in uniform heating of all the wafers on the
carrier as required to obtain uniform deposition of a-Si layers on the wafer;
iv) the wafer carrier with reduced thickness beneath the slots for holding
wafers is beneficial for rapid heating;
v) the material used for the process is of high thermal conductivity having
desired strength at the temperature of operation of the PECVD process;
vi) the material chosen is easy to machine (kindly confirm);
vii) the low cost wafer carrier design for improving quick and uniform
heating of carriers.
Although embodiments for the present subject matter have been described in
language specific to structural features, it is to be understood that the present
subject matter is not necessarily limited to the specific features described.
Rather, the specific features are disclosed as embodiments for the present
subject matter. Numerous modifications and adaptations of the method of the
present invention will be apparent to those skilled in the art, and thus it is
intended by the appended claims to cover all such modifications and
adaptations which fall within the scope of the present subject matter.

WE CLAIM:
1. A novel wafer carrier for rapid heating of silicon wafer is made up of
aluminum material with varying cross section thickness such as 2 mm beneath
the wafer holding slots as lower thickness and 9 mm elsewhere, for quick
heating of silicon wafer.
2. The wafer carrier as claimed in claim 1, wherein the wafer carrier comprises
plurality of slots (2) for holding the wafers and same round impression (3).
3. The wafer carrier as claimed in claim 1, wherein the slots are round shaped
wafer carrier and placed on the corner of each slot for easy withdrawal of wafer
after the deposition of Si layers.
4. The wafer carrier as claimed in claim 1, wherein the thickness of the corner
is optimized as 2 mm so that the slot designed to hold the Si wafer to ensure
rapid heat transfer while maintaining corner structural integrity during PECVD
deposition process.
5. The wafer carrier as claimed in claim 1, wherein the material used for the
making carrier is of high thermal conductivity, have desired strength and is
easy to machine.
6. A method for fabrication of silicon layers by using the wafer carrier as
claimed in claim 1, comprises the steps of:
i) cleaning of the wafers through wet chemical process before deposition
on a-Si layers;

ii) dipping of wafer in Hydrofluoric (HF) solution for removing the oxide
layer for deposition of amorphous silicon layers;
iii) loading of wafers in a novel wafer carrier and then it is introduced in
the load lock chamber and then transferred to isolation chamber for heating;
iv) heating of wafers to attain optimum temperature of 130°-180°C for
proper deposition of Si Layers.